Activaton in Drugs

7/25/2019 Activaton in Drugs

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Drug Mechanochemical Activation

I. COLOMBO,

1

G. GRASSI,

2,3

M. GRASSI

4

1Eurand S.p.A., Physical Pharmacy Laboratory, via Martin Luther King, 13-20060 Pessano con, Bornago, Milano, Italy

2Department of Internal Medicine, University Hospital of Trieste, Cattinara I-34149, Trieste, Italy

3Department of Life Sciences, University of Trieste, Trieste, Italy

4Department of Chemical, Environmental and Raw Materials Engineering DICAMP, University of Trieste,Piazzale Europa 1, I-34127, Trieste, Italy

Received 30 September 2008; revised 12 December 2008; accepted 28 January 2009

Published online 31 March 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.21733

ABSTRACT: The aim of this review is to describe the theoretical background lyingbehind the solid drug mechanochemical activation by cogrinding pointing out its

advantages and drawbacks. A brief historical introduction precedes the discussion about

the mechanisms leading to solid drug activation. This allows to clarify the concept of

solid activation whose main effect is to improve drug solubility and, thus, drug bioavail-

ability. Then, the attention is focused on the experimental tools used to evaluate drug

activation before the in vivo use. This, of course, permits to properly modulate the

milling conditions (milling time, mill revolution speed, drug/carrier ratio and so on) in

the light of the optimisation of milling process and activated system properties. There-

after, the discussion shifts on the different kinds of mills that can be used and on mills

classification based on the energy transferred to the materials. Fundamental tool to

perform this task is the mathematical modelling of mill dynamics that is here shown for

different mills kinds. Finally, some examples of activated systems performance bothin vitro and in vivo are presented and discussed. In conclusion, mechanochemical

activation improves drug bioavailability. Interestingly, this activation does not require

the use of solvents whose elimination from the activated product can be difficult and

expensive but a relatively simple mechanical treatment. On the other hand, this

approach, usually, works only for poorly water soluble drugs (solubility


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been the first type of energy deliberately employed

by the human mind as witnessed by the fabrica-

tion of prehistoric weapons and fire production by

friction. Nevertheless, modern mechanochemistry

was born in between the end of the 19th century

and the beginning of the 20th century when the

first books on this topic were published andthe term mechanochemistry was, introduced in

the scientific literature.3 Undoubtedly, the

middle of the 20th century sees a considerable

development of solids mechanochemistry due to

the studies about explosion excitation in both

initiating and brisant explosive substances under

mechanical action. France, England and Russia

are the leading countries in this field and the

theory ofhot spots, explaining the initiation and

development of explosion by local increase of

temperature at the contacts of two solid under-

going mechanical action, dates back to this period.The development of new methods for mineral raw

processing, preparation of new construction

materials, mineral fertilizers and functional

ceramics, represented the goal of mechanochem-

istry researchers in Germany, Japan, Israel and

USSR in the 1960s.2 The end of the 1960s sees the

ingress of mechanochemistry in the material

science field with the ball milling production of

nickel- and iron based super-alloys, impossible to

be obtained by conventional melting and casting

techniques.4 Interestingly, the most important

mechanochemistry development, occurring in the

1980s,510 sees the end of the leading role played

by eastern block countries due to the renewed

interest of Japanese researchers and the rapid

progresses in mechanical alloying. The constitu-

tion of the International MechanochemicalAssociation in 1988 and the 1st InternationalConference on Mechanochemistry (Kosice, SlovakRepublic, 1993), definitively consecrated Mechan-

ochemistry all over the word. In the light of this

frame, it is not surprising that the pharmaceutical

interest for mechanochemistry developed only in

the 1980s.11 In addition, initially, the severe

requirements needed by pharmaceutical produc-tion (Good Manufacturing Practice and FDAapproval) hindered the clear affirmation of

mechanochemstry in this newfield. Nevertheless,

once one of the most important problems (milled

material contamination by grinding media and

mill walls) was solved by adopting proper mill

lining and high abrasion resistant materials for

grinding media, the pharmaceutical doors were

definitively opened to mechanochemistry.12 Since

the beginning, this synergistic cooperation has

considerably developed and led to patents and

industrial applications.13,14 Basic reasons for this

fruitful cooperation rely on the possibility of

producing pharmaceutical products avoiding the

use of solvents (whose elimination can be difficult,

expensive and can alter drug activated status) and

the possibility of increasing the bioavailability ofpoorly water soluble drugs.

MECHANICAL ACTIVATION

One Component Systems

A one component solid material being processed in

a mill receives mechanical energy in pulse form

every times it is trapped between two (or more)

colliding grinding media or between mill wall and

one (or more) wall-impacting grinding medium(see Fig. 1). In particular, mechanical energy is

transferred by means of normal and shear

stresses acting on solid material surfaces. The

effect of this externally imposed stressfield is the

growth of a strain field in the solid bulk. The strain

field manifests through different phenomena such

as (1) atoms shifts from equilibrium stable

positions at lattice nodes, (2) changes of bond

lengths and angles and, sometimes, excitation of

electron subsystem.2 Despite its apparent, macro-

scopic, simplicity, mechanical energy transfer into

solid material is very complicated. Among the

many theories developed to address this problem,

one of the most popular and interesting is the so-

called triboplasma approach.1 Basically, it

assumes that an impact of sufficient intensity

results in a quasi-adiabatic local energy accumu-

lation (temperature can grow up to 104 K in

submicroscopic zones of the impact). This, in turn,

gives origin to a metastable structure that must

release part of the accumulated energy to get a

more stable thermodynamic condition. According

to this approach, a multistage pattern of energy

dissipation takes place. Within the first 1011

107 s stochastic reactions dissipate the energypertaining to the triboplasma highly excited

energy states. After 107 s numerous elementary

excitation processes such as the recombination of

plasma products, the propagation and interaction

of dislocations, the propagation and emission of

electrons and photons and the formation of hot

spots during heat release occur. It is in this period

that phase transformations and mechanochemical

reactions proceed. Of course, part of the mechan-

ical energy provided is retained by the solid

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 98, NO. 11, NOVEMBER 2009 DOI 10.1002/jps

3962 COLOMBO, GRASSI, AND GRASSI


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material in the form of the so-called excess Gibbs

energy. Basically this energy is linked to long-

living mestable states that are preserved in the

activated material after mechanical treatment.

This excess energy may be released very slowly

according to sequence of irreversible processes

approaching thermodynamic equilibrium. From

macroscopically point of view, energy relaxation

takes place according to three mechanisms: (a)

heat, (b) plastic deformation and (c) rupture of

chemical bonds (mechanochemical reaction).2

Due to the periodic impulsive action of grinding

media, the externally imposed stress field ispulsating so that milling process sees an alterna-

tion of strain field formation and relaxation.

Obviously, the relative speed of the energy

relaxation processes and the stress field applica-

tion plays an important role in determining the

final properties of the ground product.15

The main part of the supplied energy is

converted into heat. The concentration of the

strain field in particular crystal sites can lead to

crystal crushing and thus to the formation of new

surface. The iteration of this phenomenon induces

crystal size reduction (particle size reduction) to

some critical threshold. Further energy supply

yields to the accumulation of defects into crystal

volume or on its surface to finally lead to a

complete amorphisation. Although the molecular

solids amorphisation process remains unclear,16

in general, crystal transformation to amorphous

phase can be explained on the basis of two leading

theories: mechanical and thermodynamic desta-

bilisation. According to the first theory,17 crystal

lattice collapse occurs for too high anharmonicity

of lattice vibrations (phonons) induced by com-pression. Indeed, in this condition, Born stability

criteria for crystal lattice are violated. The

second theory18 affirms that mechanical energy

continuously increases crystal defects concentra-

tion up to a critical threshold beyond which

amorphous phase is thermodynamically more

stable than the crystal one. Anyway, amorphisa-

tion process implies the formation of defects such

as point defects (e.g. interstitial occupancies and

vacancies), line defects (e.g. edge and screw

Figure 1. Schematic representation of what happens inside mill bowl during grind-

ing. Grinding media transfer mechanical energy to the charge (solid drug) in pulse form

as, in each collision, only a small fraction k of the total charge is involved.

DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 98, NO. 11, NOVEMBER 2009

DRUG MECHANOCHEMICAL ACTIVATION 3963


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dislocations), plane defects (e.g. grain boundaries

and crystal surfaces), and anomalous distribution

of punctual defects1921 that can decrease mate-

rial true density due to the formation of the

so-called activation volume.20 Experimental

evidences22 indicate that crystal amorphisation

starts on a thin surface layer (0.11 mm20) andthen propagates into the bulk. Undoubtedly, this

internal propagation can be favoured by intrinsic

crystal disorder (due to molecules crystallising in

a single conformation but in a different orienta-

tion relative to other molecules in the crystal)

typical of pharmaceutical solids such as nonster-

oidal antiiflammatory drugs, salicylsalicylic acid

and antiarrhythmic compound flecainide.23

If defects formation is not random but it follows

particular orders, a metastable polymorph struc-

ture occurs.5When the preferred relaxation way is

the third one (rupture of chemical bonds), amechanochemical reaction occurs.10 The term

mechanochemical activationmeans the accumu-

lation of defects (amorphisation process), the

formation of polymorphs and chemical reactions

occurrence. Accordingly, the intrinsic energy

content of an activated system is higher than

that of a not-activated one. Thus, the reason for

the adjective activation relies on the potential

ability of the system of doing something. Indeed,

transformations of a high energy content system

are facilitated as the energetic barriers hindering

the evolution to a new equilibrium condition are

lower than those of a not-activated system which

is thermodynamically more stable. Accordingly,

the termactivationexpresses an intrinsic poten-

tiality of the activated system.

Comminution

In the light of what above discussed about

mechanochemistry, the grinding of a solid (crys-

talline) material leads to: (1) particle size reduc-

tion (plastic deformation leading to comminution),

(2) mechanical activation and (3) heat release.Whereas the last two aspects have been pre-

viously discussed, a basic interpretationpredic-

tion of comminution can be easily given according

to the Griffith theory.7,24 The rupture of an ideally

brittle material implies the interruption of intera-

tomic bonds with the consequent formation of two

new surfaces. Obviously, the breaking energy Beper unit surface (as2/E, where a is the distancebetween two consecutive crystalline planes, s is

the applied stress and E is Young modulus) must

be equal to the surface energy, 2gsv, competing to

the two newly formed surfaces. Accordingly, the

critical stress sc leading to rupture is:

scffiffiffiffiffiffiffiffiffiffiffiffiffi

2Egsva

r (1)

Griffith explained the huge equation (1) over-

estimation of experimental sc values affirming

that each material contains many elliptical cracks

reducing mechanical resistance. Indeed, he pos-

tulated that at the tip of such cracks (correspond-

ing to the end of the major axis) a strong

concentration of the stress occurs. On this basis,

he found that the energy per unit length, Le,released due to crack propagation is:

Le pc2s2

E plane stress;

Le 1 m2E

pc2s2 plane strain(2)

where m is the Poisson modulus (0.5 for incom-

pressible materials), c is the half length of thecrack major axis and sis the stress applied to the

bulk. Obviously, crack propagation (enlargement)

implies the formation of new surfaces and,

consequently, an increase of system surface

energySe 4cgsv. According to Griffith, the crackwill propagate, producing brittle fracture, if crack

propagation does not imply system energy

(Se

Le) increase (d(Se

Le)/dc

0). This leads

to the following Eq. (1) improvement:

seffiffiffiffiffiffiffiffiffiffiffiffiffi

2Egsvpc

r plane stress;

seffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2Egsvp1 m2c

s plane strain

(3)

As real crystals do not show ideally brittle fracture

(crack propagation is accompanied by material

plasticisation near the crack tip), Eq. (3) still

detaches to real behaviour. Nevertheless, ifgsv is

replaced by the effective surface energygsve, sum

ofgsv and gpl (local plastic deformation energy7),Eq. (3) yields satisfactory agreement with experi-

mental data. gsve depends on internal and external

factors. Among the external factors, magnitude

and mode of stress application (normal or shear),

rate of strain and temperature must be men-

tioned. For what concerns internal factors,

material structure, impurity concentration, grain

and particle size and the extent of preceding

plastic strain must be remembered.7 It is

worth mentioning that for brittle materials, gsve




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coincides with the material solidvapour surface

tension whose determination is, usually, not so

cumbersome.25 The opposite occurs for the tough

material (gsvegpl).

The importance of comminution does not

exhaust in enhancing drug dissolution kinetics

due to the increase of solid surface per unit area.Indeed, it is well known25 that solid solubility

depends also on crystals size and the reference

equation for this effect is the OstwaldFreundlich

relation:26,27

rRT

M ln

Sr

S1

2gsl

r (4)

whererandMare, respectively, solid density andmolecular weight,R is the universal gas constant,T is the absolute temperature, Sr and S1 are,respectively, the solubility of a spherical crystal

characterised by radius r and infinite radius (i.e. aplane) and gsl is the solidliquid surface tension. It

is now important to spend few words on the

concept of crystal radius. Ground material

appears as a distribution of different radius

particles (secondary grains) each one constituted

by an ensemble of crystals (primary grains) bound

together by amorphous connecting phase. Indeed,

mechanical action produces an increase in lattice

defect density leading to the formation of coherent

crystalline domains (crystallites) inside each

crystal. Accordingly, in Eq. (4), r refers to thecrystallite radius. Of course, the frame is more

complicated by the fact that real crystallites are

not necessary spherical and, in this case, rshouldrefer to crystallite local curvature radius.28

Although Eq. (4) has been subject to severe

criticism,29,30 experimental evidences show that

solid solubility increases for sufficiently small

crystallites (usually r


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of the delivery system is a prompt action, the

inclusion complex situation is not to be preferred

as, reasonably, drug release will be slower with

respect to the external drugcyclodextrin inter-

action situation. Figure 3 reports the microscopic

aspect of a coground system composed by

the polymeric stabilizer (micronised crosslinked

polyvinylpyrrolidone particles) and an anti-

inflammatory drug constituted by oxygen, fluor-

ine, carbon and sulphur. Interestingly, an energy

dispersive spectroscopy (EDS) analysis, permit-

ting to identify the EDS compound spectrum

(determined by its constituent atoms), allows to

verify that the drug is present both outside andinside polymeric matrix (globular structures in

Fig. 3). Indeed, EDS spectrum reveals that

sulphur peak (an element that the drug and the

polymer do not share) is present in both needle

structures (point N, thick line, Fig. 3) and in

polymeric matrix (point M, thin line, Fig. 3)

where no needles (drug crystals) are visible.

Accordingly, not only drug crystals are stabilised

by surface interactions with the polymer, but part

of the drug, lying inside particles, is stabilised by

the polymeric network hindering drug transfor-

mations. Finally, it is worth mentioning that asdrug crystals (needles) depicted in Figure 3 are

almost cylindrical shaped, they can be considered

nanocrystals in correspondence of the cylinder

basis where the surface curvature radius is lower

than 300 nm.

EXPERIMENTAL VERIFICATIONOF ACTIVATION

The problem of the experimental verification of

mechanochemical activation falls in the broad

field of solid-state characterisation. Accordingly,

techniques relying on differences in periodicites of

atoms in crystals (X-ray powder diffraction

XRPD), energies of bond stretching/bending

vibrations and lattice vibration (IR, Raman),

electronic environments of nuclei (nucleic mag-

netic resonance, NMR), heat flow or weight

change (thermal analysis: differential scanning

calorimetry (DSC) and thermal gravimetric ana-

lysis (TGA)) and morphology (optical microscopy),

can be useful at this purpose.34,35 In particular, in

the mechanochemical activation context, solid

state characterisation serves to exclude theformation of new chemical entities (occurrence

of chemical reactions) and drug polymorphs. At

the same time, this characterisation is needed to

estimate the residual amount of drug crystallinity

(Xrc) after mechanical treatment. Indeed, thisparameter can be roughly elected as a measure of

drug activation. To be more precise, a deeper

evaluation of activation not only requires the

determination ofXrcbut also the size distributionof drug crystals (it is worth remembering that

Figure 2. When cyclodextrin is used as carrier in

cogrinding, its stabilising action manifests through

surface interactions with amorphous or macro

nano-crystalline drug or through the formation of inclu-sion complexes.

Figure 3. SEM picture of a coground mixture com-

posed by a drug (needles) and a crosslinked polymer

(particles). Drug needles can stand alone on polymeric

particles surface or inserted in the polymeric network.

Figure insert shows the EDS spectrum referring to the

drug (thick line, point (N), drug needles) and to poly-

meric particle where drug needles cannot be detected

(thin line, point (M)). As both the spectra are qualita-

tively similar, the drug presence inside the polymeric

matrix (M) is confirmed.




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drug solubility increases for smaller crystals, see

Eq. 4). In addition to these techniques, other

approaches can provide estimation of mechan-

ochemical activation. Among others, solution

calorimetry, water sorption36 and release tests

(RT) can be mentioned.25 As the description of all

the above mentioned approaches for the evalua-tion of mechanochemical activation is out of the

aim of this review, we focus the attention on those

commonly used in the industrial field.

DSC

Some nano-sized crystals (or crystallites) peculiar

behaviour depend on the fact that their properties

are influenced by their surface atoms rather than

by their bulk atoms. Indeed, it is well known that

surface atoms have fewer interatomic bonds than

those in the bulk, this making them more loosely

bound than bulk atoms.37Huang et al.38 revealed,

by experimental data and molecular dynamics

simulations, neat differences in the structural

dynamics of nanocrystals surface and bulk atoms.

In particular they found that coherent diffraction

patterns recorded from individual nanocrystals

are very sensitive to the atomic structure of

nanocrystal surfaces. Assuming, for the sake of

simplicity, a spherical shaped crystal, all we know

that the surface/volume ratio becomes infinite for

vanishing crystal radius and this is the reason

why the importance of surface atoms becomes

more and more pronounced as crystal radiusdecreases. Typically, nano-crystal melting tem-

perature (Tmr), melting enthalpy Dhm, solidvapour (gsv) surface tension and grain growth

rate (GGR) depend on nano-crystal radius and

differ from those of the infinite crystal (macro-

crystal). Nano-crystals melting temperature

deviations are related to interfaces nature3941

and curvature.40 The melting point shifting in

nano-sized materials was originally predicted by

the thermodynamic nucleation theory in the early

1900s (GibbsThomson equation).27,42 More

recently, Brun et al.,

43

starting from the Laplaceand GibbsDuhem equations, found the following

relation between melting temperature and crys-

tallite radius:

ZTmrTm1

Dhmr

T dT 2

Z1=r0

glv1

rs 1

rl

d

1

Rlv

gsl

rsd

1

Rsl

5

where Tm1is the infinite radius crystal meltingtemperature, glvand gsl are, respectively, liquid

vapour and solidliquid surface tensions,rs and rlare, respectively, solid and liquid drug density,RlvandRsl are, respectively, liquidvapour and solidliquid interfaces curvature radii whiler is crystal-

lites radius. If drug crystallites are much moreabundant than amorphous boundaries (then they

are tightly packed), it can be assumed that

melting starts with each crystallite covered by a

liquid skin. Accordingly, we have thatRlvRsl r(crystallite radius). On the contrary, if the

amorphous drug fraction is much larger than

the crystalline one, crystallites melting takes

place in an amorphous (liquid) bath (weremember here that amorphous phase becomes

liquid as soon as its glass transition temperature

is exceeded and this happens much before crystal-

lite melting), and, thus, it can be assumed Rlv1andRsl r. On the basis of a thermodynamic cycleinterpreting crystallite melting process as the

sum offive steps ((i) crystallites clustering at Tmrto form the macro-crystal; (ii) macro-crystal

heating from Tmr to Tm1; (iii) macro-crystalmelting at Tm1; (iv) liquid disintegration intonano-sized droplets at Tm1; (v) droplets coolingfrom atTm1to Tmr) Zhang et al.

37 demonstrated

that Dhmr dependence on crystallite radius isgiven by:

Dhmr Dhm1 3r

gsvrs

glvrl

ZTm1

Tm

DCpdT (6)

where Dhm1 is macrocrystal melting enthalpywhile DCpis the difference between the liquid andthe solid specific heat capacity at constant

pressure. Eqs. (5) and (6) simultaneous numerical

solutions are shown in Figure 4 in the case of

nimesulide (Tm1 148.78C, Dhm1 108,720J kg1, rs 1490 kg m3, rl 1343.7 kgm3,glv 44.3 103 J m2, gsl 13.3 103 J m2,

Rlv

1and Rsl

r25). The fact that they predict

a reduction of both Tmr and Dhmrwith decreasingrcan be physically interpreted as the increased

facility of network braking due to the presence of

surface atoms that are more loosely bound than

bulk atoms.37 It is evident that for r 10 nm, asignificant melting temperature and enthalpy

reduction occur. Remembering that nimesulide

unit cell side is approximately 0.87 nm,25 Eqs. (5)

and (6) suggest that appreciable melting tem-

perature reductions take place when mean

crystallite radius (calculated on the volumetric




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distribution vs. radius) is constituted by approxi-

mately 20 unit cells. Figure 5 shows the experi-

mental DSC pattern referring to pure nimesulide

and to a nimesulide/crosslinked polyvinylpyrroli-

done mixture (1:3, w/w) after 1 h grinding in a

planetary mill. Melting temperature reduction

witnesses the absence of the original macro-

crystals (no thermal event is now detectable at

148.78C) and the presence of an ensemble of nano-

crystals whose mean melting temperature is

126.68C. It is worth mentioning that DSC can

be also very important for a full characterisation

of drug status after cogrinding. Indeed, it

allows the estimation of the fraction of original

macro-crystals, nano-crystals and amorphous

phase. This can be performed comparing the

specific melting enthalpy of an un-ground drug/carrier mixture and the specific melting enthalpy

of the same mixture after cogrinding. In addition,

DSC allows also the determination of nano-

crystals size distribution.

Magomedov,44 studying the surface energy

dependence on nano-crystal size and shape, found

that surface energy decreases with crystallite

radius and this behaviour becomes more and more

important for smaller nano-crystals. Interest-

ingly, he noticed that nanocrystal melts when

the surface energy decreases to a value at which it

becomes independent of the nanocrystal size andshape.

Another peculiar aspect of nano-crystals is the

GGR. Driving force for nucleation and growth of

crystallites is the free energy difference between

amorphous and crystalline phases and this

difference mainly depends on temperature and

crystallites diameter. For its practical and theo-

retical importance, researchers have deeply stu-

died this problem in order to derive equations able

to predict crystallites rate growth. Although more

refined approaches can be considered, GGR can be

retained proportional to the inverse of the crystal

diameter (Dc) powered to an exponent n greaterthan 1:45

dDcdt

kTDnc

;

Dc Dn1c0 n 1kTt1=n1(7)

where Dc0 is initial crystallite diameter. Anotherway of matching the problem of crystallites

growth is considering the JohnsonMehlAvrami

equation (JMA), or its modified form,46 providing

the increase of crystal fraction x versus time t:

x 1 expkt tn (8)

wherek is a temperature dependent rate constant,

tis an empirical parameter named scaling factor

and n is another empirical factor. The fact that Eq.(8) can also predict a very slow initial increase ofx,is motivated by the fact that strong hydrogen

bonding difference between amorphous and

crystalline states can delay crystallisation and

vice versa.47

Figure 4. Theoretical dependence of nimesulide

melting temperature (Tmr black line) and melting

enthalpy (D

Hmr

grey line) on crystals radius r.

Figure 5. Differential scanning calorimeter pattern

(DSC) referring to original nimesulide (melting tem-

perature 148.78C) and nimesulide coground with cross-

linked polyvinylpyrrolidone in a planetary mill for 1 h in

a ratio 1:3 (w/w) (melting temperature 126.68C).




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XRPD

When, upon melting, material decomposes and/or

solid phase interactions occur during heating,

DSC cannot be applied to check the presence and

abundance of crystals. In this case, a valid

alternative method is represented by XRPDprovided that the amount of amorphous phases

and/or adsorbed water is not excessive and

provided that the X-rays diffraction pattern is

not affected by severe peak overlaps and preferred

orientation of the powder sample is not signifi-

cant. Although a detailed description of crystals

abundance determination after drug/carrier

grinding goes behind the scope of this review, it

is interesting recalling that the working equation

is:48

XD Pi

IiDPi IiD0

fX

Da

(9)

whereXD is drug crystalline fraction in the groundmixture,IiD is the area of the ith peak belonging tothe ground drug X-rays pattern,

PiIiD andP

i IiD0 are, respectively, the sum of all the peakareas characterising the ground and native drug

X-rays pattern, ais a semi-empirical factor taking

into account micro absorption effects andf(XD) i s afunction of XD depending on the drugcarriermixture. The unknowns

Pi IiD0,a andf(XD) can

be calculated from a calibration procedure based

on the measurement of several physical mixtures

of the native crystalline drug and carrier.

It is important to remember that if amorphous

structures produce broad diffraction reflections

owing to the lack of any long range periodicity in

the atomic arrangement, nanocrystalline materi-

als comport a deviation from the X-ray diffraction

pattern competing to an ideal crystal as atomic

arrangement periodicity exists only on few

molecular distances. In particular, finite crystal-

lite size, strain and extended defects (stacking

faults and dislocations) lead to broadening of the

diffraction peaks. In this contest, whole pattern

profile modelling (peaks positions, intensities,width and shapes) can provide a complete

evaluation of grain size distribution and lattice

defect content.25 The use of the so-called radial

distribution function, determinable from the

experimental patterns by means of the Fourier

transform, allows determining the probability of

finding an atom at a given distance from the

centre of a reference atom of the system.49 Very

smooth and low noise experimental data are

needed to achieve reliable and reproducible

results. For these reasons synchrotron and

similarly intense X-ray sources are recommended.

An alternative approach to the analysis of

diffraction data of amorphous materials has been

recently proposed.50 In this framework, the

crystallite size, jointly to microstrain, has been

introduced to account for the broadening of thediffraction peaks of an amorphous structure. By

reducing the coherently scattering domains, the

corresponding Bragg reflections will broaden to

such an extent to give place to a typical amorphous

diffraction pattern (halo).

Finally, we would like to conclude this section

reminding an important function of XRPD in the

field of mechanical activated systems. Discussing

the meaning of activated systems (see One

Component Systems Section), we said that

mechanical energy supply can lead to the occur-

rence of chemical reactions. Whereas often this isthe aim of the mechanical treatment, in the

pharmaceuticalfield this is not desirable. What is

needed is drug activation without the formation of

new chemical entities or polymorphs that would

lead to huge regulatory problems. Indeed, FDA

approval for the native drug is absolutely not

extensible to other chemical compounds gener-

ated by the mechanochemical treatment. In this

frame, XRPD plays a very important role as the

formation of drug polymorphs or the occurrence of

drug chemical transformations upon grinding

reflect in evident differences between the XPRD

pattern of native and ground drug.

Release

Drug release test from activated systems is

another excellent method to evaluate the activa-

tion grade even if the determination of the

crystalline, nano-crystalline and amorphous drug

abundance is not direct and can be indirectly

determined interpreting experimental data by

proper mathematical models.25 It does not exist a

unifying model describing drug release from everyactivated systems (but all of them must share

some common characteristics) as release kinetics

strongly depends also on the carrier (stabiliser)

considered. In this light, carriers can be roughly

subdivided into two main classes: crosslinked

polymers and the rest of the world. This subdivi-

sion is motivated by the fact that while in the

second case drugcarrier interactions develop

superficially through more or less complex

adsorption/desorption mechanisms, in the first




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case (crosslinked polymers) also topological car-

rier properties can play a very important role as

the drug can be embedded inside the (stable)

polymeric network. Accordingly, both drug diffu-

sion in the (swelling) network and drug-polymer

interactions affect release kinetics. Beside these

mechanisms, also the crystalline, nano-crystallineand amorphous drug dissolution into release

medium heavily rules release kinetics. Indeed,

upon contact with the external release medium,

the stabilising action exerted by the carrier

vanishes (obviously, with different kinetics

depending on carrier type) and the drug starts

dissolving with the possibility of macro-crystals

and/or polymorphs formation. As drug solubility

depends on crystal radius (amorphous phase can

be regarded as a crystal of vanishing radius, see

Comminution Section), the release process takes

place as the drug were characterised by a timedependent solubility.25One of the most commonly

used equations to describe this particular beha-

viour is:51

Cst Ca;nc Cmc eKrt Cmc (10)whereCsis the time (t) dependent drug solubility,

Kr is the re-crystallisation constant, Ca,nc repre-sents the amorphous or nanocrytsalline solubility

while Cmc indicates the solubility of macrocrys-tals. This equation simply states that at the

beginning (t 0) Cs is equal to Ca,nc and then itexponentially decreases to get Cmc. Obviously, Eq.(10) intrinsically accounts also for macro-crystalsdissolution (those who do not undergo re-crystal-

lisation phenomena). Indeed, their dissolution is

represented by Eq. (10) setting Kr 1 (Cs isalways equal to Cmc). Accordingly, the generaldissolution equation becomes:

@Ca;nc

@t Ka;nct Cst C (11)

whereKa;nct indicates the amorphous or nanocryt-salline drug dissolution constant. Of course, three

Eq. (11) dissolution types exist: the first referring

to the amorphous drug, the second referring tonano-crystals and the third referring to macro-

crystals. The contribution of each equation to the

global dissolution flux depends on amorphous,

macro and nano-crystals relative abundance.

Coupling Eq. (11) with a proper equation account-

ing for drugcarrier interaction and, eventually,

drug diffusion through the polymeric network,

yields thefinal mathematical model formulation.

From an experimental point of view, release

tests can be performed in sink or un-sink

conditions according to usual dissolution testing.

If in sink conditions the higher activation grade

reflects into higher release kinetics, the adoption

of un-sink allows seeing a peculiar release

pattern. Indeed, due to the conversion of amor-

phous drug into the more stable macro-crystalline

form, implying drug solubility reduction, therelease concentration curve can show a rapid

increase followed by a decrease as shown in

Figure 6 in the case of an anti-inflammatory drug

(Cmc 2.5 mg/cm3; amorphous drug fraction 0.6,nanocrystalline drug fraction 0.4) release from

coground drug/crosslinked polyvinylpyrrolidone

(1:2, w/w) system under un-sink conditions.

Stability

An essential prerequisite for the pharmaceuticaluse of mechanochemical activated systems is their

physical stability over months or years. At this

purpose, specific stability tests are performed in

temperature and humidity controlled environ-

ments. Typically, DSC, XRPD and release tests

are conducted just after sample preparation and

at fixed times after exposure to constant tem-

perature and humidity environments. Tempera-

ture can span from 5 to 408C while humidity can

be up to 90%. If activation characteristics such as

Figure 6. Amorphous drug re-crystallisation can

yield, in nonsink conditions, to the appearance of a

maximum in the release curve (release environment

concentration C vs. time t). This picture refers to the

release of a poor soluble drug (solubility Cs 2.5mg/cm3,water, 378C) coground with crosslinked polyvinylpirro-

lidone (1:2 (w/w) ratio). Amorphous drug fraction 0.6;nanocrystalline drug fraction 0.4.




11/26

amorphous, nano-crystalline drug contents and

release kinetics are substantially stable over time,

the carrier (stabiliser) is approved. Obviously, in

particular cases, essentially dependent on drug

and carrier chemical characteristics, also other

chemical tests can be performed. The possibility of

adding other substances enhancing drug stabili-sation can be also considered.

BIOAVAILABILITY ENHANCEMENTS

Bioavailability, defined as the rate and extent to

which the active drug is absorbed from a

pharmaceutical form and becomes available at

the site of drug action,52 depends on several

factors, among which drug solubility in an

aqueous environment and drug permeability

through lipophilic membranes play the role ofkey parameters.53 Indeed, only solubilised mole-

cules can be absorbed by the cellular membranes

subsequently reaching the site of drug action

(vascular system for instance). According to the

high or low values assumed by these parameters

(solubility and permeability), drugs can be classi-

fied into four different classes54 and a drug can be

defined bioavailable if it belongs to the fourth class

(high solubility and permeability). Many different

techniques are commonly used to improve the

bioavailability of poorly water-soluble but perme-

able drugs.55 In this sense, drug particle size

reduction, drug conglobation inside the lipidic

matrix of nano- or microspheres56 or drug

solubilisation in the dispersed lipophilic phase

of an O/W emulsion or microemulsion5760 can be

mentioned. A similar task can be achieved

complexing the drug with cyclodextrins by mixing

in solution or in presence of the melted drug.61

Finally, by means of solvent swelling it is possible

to load a drug into a polymeric carrier in a

nanocrystalline or amorphous state, thus con-

siderably increasing its bioavailability.32,62 For

example, the simple particle size reduction (by

means of a milling process) allowed reducingfenofibrate dose (Tricor1) from a 300 mg capsule

(standard drug) to a bioequivalent 145 mg tablet

containing nano-particulate drug.55 Interesting

examples of lipid based formulations (either self-

emulsifying or emulsifying due to the presence of

bile salts) regards antivirals (Norvir1 (ritonavir)

and Fortonase1 (sanquinavir)) and immune

suppressant cyclosporine (Sandimmune1 and

Sandimmune Neoral1). If Fortonase1 increased

sanquinavir bioavailability up to threefold with

respect to the original Invirase1 (sanquinavir

mesylate in powder form),63 the reduction of

emulsion particle size allowed Neoral1 to be more

bioavailable than the original Sandimmune1.64

Cyclodextrins can be found in several marketed

products such as Vfend1 (voriconazole), Geodon1

(zispradisone mesylate) and Sporanox1 (itraco-nazole). These solutions are intended for injection

or oral use55 and all of them are characterised by

high cyclodextrin/drug ratio (from around 15:1 to

40:1). Prograf1 and Sporanox1 capsules are

successful examples of commercial application of

the solvent swelling technique.65,66 Obviously,

each approach shows advantages and drawbacks

and it is more suitable for a determined drug or

for a specified administration route. Mechano-

chemical activated systems, in particular, can be

administered in the form of tablets or capsules, as

both formulations do not modify the activatedstatus. In general, any formulation that does not

require the use of solvents or high temperature

can in principle be considered. For these reasons,

mechanochemical activated systems are suitable

for oral administration, the most common route

for drug delivery into the human body because it

leads to a better patient compliance and is very

versatile for what concerns dosing conditions.

Nevertheless, mechanochemical activation and, of

course, solvent swelling activation, comport a

considerable bioavailability improvement if drug

solubility in aqueous medium is approximately

lower than 100 mg/cm3.

MILLS

Although it is often believed that a good mill is also

a good mechanical activator, this is not always

true as these two devices are intended for different

purposes. Indeed, if a mill is aimed to maximise

ground material specific surface (particle size

reduction) and to realise a good mixing with the

minimum energy expenditure in the shortest time

possible, mechanical activator target is to inducedefects (plastic deformation) in the ground mate-

rial structure (see Comminution Section). Accord-

ingly, an optimal mechanical treatment (OMT)

should imply an initial reduction of particle size

(milling) followed by the mechanical activation.

This is the reason why OMT requires the

sequential use of two different machines or the

same machine working with different operating

conditions.15 In order to meet all these require-

ments, many different grinding devices exist and




12/26

they can be subdivided into three main categories

on the basis of how energy is transferred to the

material to be ground (mill charge):2,15 (1) ball

mills, (2) shear action mills and (3) shock action

mills. Although an exhaustive and complete

description of all the existing mills goes beyond

the scope of this review, some of the most relevantexamples will be discussed in the following

section.

Ball Mills

In ball mills, energy is transferred to grindingmedia and mill charge (the mixture to be ground)

by mill body or by impellers. Mechanical action

takes place through both shear and normal

stresses and their relative importance can be

modulated in a wide range, acting on mill building

features and operating regime.67

Tumbling ballmills, Planetary, vibrational, Spex mills and

attritors belong to this category.

Tumbling ball mills, relatively cheap, reliable

and easy to control and maintain, are constituted

by a rotating cylinder (drum), characterised by a

high length/diameter ratio, containing charge and

grinding media (balls). As charge grinding is due

to balls movement provoked by drum rotation, the

bulk energy supplied depends on drum diameter

and speed. They are primarily used for large-scale

industrial applications (for example cement

industry).

Planetary mill, deriving its name from the

planet-like movement of its vials, is essentially

made up by a circular basement rotating around

its main symmetry axis and carrying two or more

(even number) vials. These vials, containing

grinding media (balls), can be co- or counter-

rotating with respect to the basement. This

assembly induces grinding balls to run down

the inside wall of the vial (friction phase) to finally

collide against the opposing inside wall (impact

phase). Energy transfer to charge takes place both

in the friction and impact phase. Laboratory and

pilot-plant planetary mills can perform accelera-tions up to 100 times gravitational acceleration.

These mills are suitable for both ultrafine grind-

ing and mechanical activation.7 Grinding vials

and balls are available in agate, silicon nitride,

sintered corundum, zirconia, chrome steel, CrNi

steel, tungsten carbide, and plastic polyamide.68

A typical high-energy vibrational mill is made

up of a bowl having, approximately, a toroidal

shape and containing the grinding media. The

upper part of the bowl is cut and covered by the

mill lid, while the lower part is connected by

springs to get a resiliently supported rigid body to

a metallic basefixed to the ground. Alternatively,

the bowl can be replaced by a metallic disc

carrying separate cylindrical grinding vials. The

bowl or the metallic disc are rigidly connected to

an (electrical) engine (placed approximately onthe bowl symmetry axis) determining the motion

of a shaft carrying, in its upper and lower parts,

two eccentrics. When the engine runs, the rotation

of the shaft provokes the motion of the two

eccentrics, which, in turn, give origin to a torque

determining the complex dynamics of the

bowl (metallic disc)/engine system. In the bowl

case, grinding media vibrate and undertake

a spiral movement around the toroidal bowl

axis.69 Grinding media acceleration essentially

depends on vibration motion frequency and

amplitude. Typically, frequency spans from 25revolution/s down to 16 revolution/s while ampli-

tude ranges between 2 and 12 mm. In these

conditions, grinding media acceleration does not

exceed 20 times gravitational acceleration. Beside

acceleration, grinding effect depends on grinding

media shape (typically balls or cylinders), density

and grinding media/charge ratio. Vibrational

mills, operating batch-wise or continuously,

are suitable for both grinding and mechanical

activation in pilot-plant and industrial applica-

tions.7 Mill bowl can be constituted, for example,

by stainless steel (with or without an internal

lining usually made up by rubber or polyurethane)

or polyurethane while grinding media are

typically made up by high abrasion resistant

material such as alumina (Al2O3), zirconia (ZrO2)

and agate.

SPEX mill, essentially used for laboratory

purposes, consists of one vial, containing the

charge and grinding balls, secured in the clamp

and swung energetically back and forth several

thousand times a minute. The back-and-forth

shaking motion is combined with lateral move-

ments of the ends of the vial, so that the vial

movement resembles an8or 1. For each swingof the vial, the balls clash against the sample and

the end of the vial. Because of the amplitude

(about 5 cm) and speed (about 1200 rpm) of the

clamp motion, ball velocities are on the order of

5 m/s and, consequently, the energy involved in

each impact is very high. Therefore, these are

high-energy mills. Hardened steel, alumina,

tungsten carbide, zirconia, stainless steel, silicon

nitride, agate, plastic, and methacrylate are used

for vials building and balls.68




13/26

Attritors are made up by a vertical or horizontal

drum carrying a series of impellers inside it and

positioned at right angles to each other. A proper

engine induces impellers rotation that is trans-

ferred to grinding media and charge. Mechanical

energy supply occurs because of impact between

balls, balls and drum wall and balls and impellers.Nevertheless also charge interparticle collisions

and balls sliding can be responsible for mechanical

energy supply. Stainless steel or stainless steel

coated inside with alumina, silicon carbide, silicon

nitride, zirconia, rubber, and polyurethane con-

stitute drum. Grinding media can be made up by

glass, flint stones, steatite ceramic, mullite, silicon

carbide, silicon nitride, sialon, alumina, zirconium

silicate, zirconia, stainless steel, carbon steel,

chrome steel and tungsten carbide. Laboratory

and industrial attritors can be found.68

Shear Action Mills

In shear action mills energy is given to crushingelements (solid surfaces in relative motion) among

which mill charge lies. Rollers mill is a typical

example of shear action mill where mill charge is

forced to pass through two parallel counter-

rotating cylinders.

Shock Action Mills

In shock action mills energy is directly given tomill charge. Jet and high peripheral-speed pin

mills belong to this category. In jet mills, no

moving parts exist as charge particles collisions

are due to a gas jet (compressed air or superheated

steam). Indeed, the charge is carried in a gas

stream flowing at high velocity where single

charge particles, colliding at approximately

1001000 m s1, undergo mutual attrition and

collisions. Both vertical and horizontal grinding

chambers may exist and the nozzles introducing

carrier and charge are located in different

positions. Sometimes (coarser grinding) they canbe positioned on opposite sides in order to

originate countercurrent streams. If comminution

mainly takes place near the nozzles where

particles collisions are more probable and ener-

getic, the effect of particle collisions with chamber

wall and rigid reflection bodies cannot be

neglected. The most important advantages of jet

mills lie in their reduced dimensions easy of

maintenance and the possibility of coating the

grinding chamber with different liners. Typically,

liners are constituted by polyurethane or dense

ceramic.

High peripheral-speed pin mills are constituted

by two counter-rotating rotors carrying, in con-

centric rows of circles, pin-breakers. Charge is

centrally fed so that, in their motion from centre to

periphery of rotors, particles collide with pinbreakers and with each other. Relative circumfer-

ential velocity can be up to 200300 m s1.

Number of concentric rows of circles, distance

among pins and their geometry, are key factors for

the product final characteristics.7

Cryogenic Mills

Cryomilling consists in milling materials at

cryogenic temperatures and/or it consists in

milling in presence of cryogenic media such asliquid nitrogen (wet grinding).68 Despite the

considerable costs connected to the use of cryo-

genicfluids (e.g. nitrogen), a cost analysis proved

that cryomilling is an economically feasible

processing approach for the commercial fabrica-

tion of nanostructured materials.70 One of the

most interesting aspects of this approach is that

cryogenic temperatures make brittle the material

to be milled and this implies that the specific

energy required for milling is reduced. Addition-

ally, cryogenic milling prevents the materials

from thermal damage, hinders the occurrence of

undesirable chemical reactions between phases70

and reduces particles aggregation.16 Obviously,

other particular reasons can suggest the use of

cryogenic milling. For example, Feng et al.71

found that cryogenic milling of griseofulvin

mainly implies drug crystallinity reduction due

to the increase of crystal defects, rather than the

formation of amorphous drug. This, in turn,

implies having a defective crystal whose bulk

properties differs from the amorphous form

(significant decrease of melting enthalpy as a

function of milling time but absence of glass

transition temperature). Jayasankar et al.,72studying the reaction of co-crystal formation

during cogrinding, needed cryogenic conditions

to avoid that the reaction proceeded through the

melt phase forming in normal cogrinding.

In principle, many of the mills presented in

previous sections can be used for cryomilling

although some of them better adapt to cryogenic

conditions. Among them attritors, ball mills and

Pin mills can be remembered. In particular, a

widely used71,73,74 mill is SPEX model 6750. It




14/26

consists of stainless steel vessel immersed in

liquid nitrogen, within which a stainless steel rod

was vibrated by means of a magnetic coil.

GRINDING MODELLING

Once practical evidences underline the effective-

ness and the reliability of the mechanochemical

activation process, from both a theoretical and

industrial point of view, the necessity of process

optimisation arises. Obviously, this target can be

achieved by a classical trial and error procedure or

by the adoption of proper mathematical models

able to yield a mathematical metaphor (repre-

sentation) of the entire process.25 While the first

approach is more convenient in the case of

particular mill types and considering small

variations of the operating conditions, the secondis to be preferred for the attainment of general

principles working for a wide range of operating

conditions and different mills. Regardless the

strategy adopted, however, the main question is

always the same: how do fixed operating condi-

tions reflect on ground material properties? Or,

conversely: which are the operating conditions

leading to fixed ground material properties? In the

light of the trial and error approach, a valuable

help in answering to these specular questions is

given by the application of artificial neural

network (ANN). ANN is a theoreticalmathema-

tical tool mimicking the learning processes of the

human brain. Indeed, it is constituted by elabora-

tion units (ANN neurons or nodes) that are each

other interconnected.75 This means that, as real

neurons do, the elaboration unit receives informa-

tion from and sends information to the other

elaboration units. On the basis of the inter-

neurons connections, ANN assumes different

architectures.76 Feed forward ANN can be used

to predict output values after a proper training,

called learning, is performed. Briefly, ANN is

presented many input/output sets represented by

operating conditions (e.g. mill revolution speed,charge and milling time) and the corresponding

ground material properties (e.g. mean particle

size and amorphous fraction). Accordingly, ANN

learns the relation between input and output

data. When new operating conditions are pre-

sented, ANN should be able to yield a reasonable

evaluation of output data (mean particle size and

amorphous fraction, in this example). Obviously,

the successful use of ANN strongly relies on the

quality and reliability of the learning step. One of

the most important advantages of ANN consists in

establishing a relation between input and output

data without the necessity of knowing the exact

mechanisms of the process allowing input data

transformation into output data. On the contrary,

the second approach (mathematical modelling) is

exactly aimed to the identification of the mechan-isms leading from input to output data. Thus, in

order to answer to the two cardinal questions

above presented, we need to model mill dynamics

(enabling the evaluation of collisions frequency

and energy), how collision energy is transferred to

the ground material and, finally, how the energy

received modifies ground material properties in

terms of, for example mean particle size, amor-

phous fraction and so on.

Mill DynamicsTkacova7 shows a very interesting approach for

the estimation of the energy transferred to mill

charge working, in principle, for any kind of mill.

Starting point is the recognition that mechanical

energy is not continuously supplied to the charge

but it is discontinuously administered through

periodic, impulsive, events (grinding media

collisions). Iftis the grinding duration, t1 is theimpulse duration andTis the impulse period, theintrinsic grinding time tint is given by tint (t1/T)t. Definingm and v as single grinding medium

mass and velocity, respectively, the associatedkinetics energy E is given by E (1/2)mv2.Accordingly, the intrinsic mill power, Pint, canbe calculated as follows:

PintXJj1

dE

dtXJj1

mvdv

dtXJj1

Fv (12)

where J, the number of impulses per unit time,depends on mill operating conditions. Accord-

ingly, mechanical energy W supplied to millcharge is given by:

Wint PinttintXJj1

Fvt1t

T FNt1v

T (13)

where NJt is the number of all impulses. Thespecific energy of grindingWsegis given by Wint/M,whereMis mill charge mass (mass of the materialto be ground). On the basis of this analysis, Table 1

reports the expressions of N, v and Wseg fordifferent mills. According to this approach, balls

mills ranking according to Wseg sees planetary




15/26

mills in pole position, followed by vibrational,

attritors and tumbling mills.

Although Eqs. (12) and (13) represent very nice

and simple tools for a rough evaluation of theenergy transfer, a more precise evaluation ofWsegrequires a detailed modelling approach to mill

dynamics.77,78 For example some authors7981

apply the discrete element method (DEM) to

describe balls dynamic in tumbling mill. Inter-

estingly, they find good agreement between

predictions and experimental observations for

what concerns balls movement and energy trans-

fer. The analysis of planetary mill dynamics has

been undertaken by many researchers for its

large use in common practice. Particle element

method,82 analytical/numerical models83 and

DEM84 have been used to simulate balls dynamics

in planetary mills. Mio et al.84 found that specific

impact energy of balls increases with increasing

the ratio between vials rotation speed and circular

basement rotation speed, but it falls about the

critical speed ratio due to rolling motion. Among

the various attempts to model planetary mill

dynamics, it is worth mentioning that by Magini

and coworkers85 as it yields an analytical solution

for the power P transferred to the unit mass ofcharge:

P bNbmbtvp vn v

3v Rv db=2

vp vpvvRp

Rv db=22pp

(14)

wherepis the charge mass,tis the time,Rpis thedistance between the circular basement axis and

vial centre,Rvis the vial radius,vpis the circularbasement angular velocity, vvis the vial angular

velocity, mb is the grinding medium (ball) mass, dbis the grinding medium (ball) diameter,Nbis thenumber of grinding media in each vial and wb is

the correcting factor depending onNb. This modelallows discovering that energy transfer to mill

charge takes place both in the friction and in the

impact phase. Despite its simplicity, Eq. (14)

proved to be reliable.86 Castillo et al.69 success-

fully compared the calculated (Visual Nastran

software) dynamics of a vibrational mill with

the experimental detected one. In addition, he

evaluated grinding media collision frequency (N).Wang,78 in his PhD thesis, built up a mathema-

tical model simulating the 3D milling dynamics of

SPEX-8000 mills. Accordingly, he could evaluate

ball positions, velocities and impacts frequency. In

particular, he found that for different number of

balls, the frequency of impacts between balls and

the vial wall is proportional to the number of balls

and the number of impacts between balls is nearly

proportional to the square of the number of balls

and the square of radius of balls. Model simula-

tions show good agreement with the results

coming from a probability analysis, indicating

that the model is reasonable. Always applying

DEM approach, Wang78 also considered attritors

modelling getting interesting preliminary results.

It is important to underline that the main problem

arising in ball mills modelling is the hugecomputational duty due to the high number of

balls involved. We would like to finally mention an

interesting mathematical model devoted to

describe grinding in jet mills. Briefly, the authors

subdivided mill chamber in two zones: (1) grinding

and (2) central zone.87 An additional third zone is

represented by the external classificatory. Parti-

cles size reduction in zone 1 (grinding) is modelled

by considering the selectivity and the breakage

functions. The first one accounts for the particle

Table 1. Evaluation of Impulses NumberN, Grinding Media Colliding Velocityv

and Specific Grinding Work Wsegfor Different Mills

Mill N v Wseg

Planetary vt

ffiffiffiffiffiffiffiffiffiffi2AD

p Wseg (Mgm/M)AvtD

Vibrational vt 4pva Wseg

(Mgm/2M)vt(4pva)2

Attritor vt pvD Wseg (Mgm/M)vtgpDm(v)Jet N vgas Wseg N=2v2gasPheripheral speed N pvD Wseg (N/2)(pvD)2Tumbling vt

ffiffiffiffiffiffiffiffiffi2gD

p Wseg (Mseg/2M)(pvtgpD)2

vis the rotational speed, tis the grinding time,g is the gravitational acceleration,A is theacceleration,D is drum diameter,a is the amplitude,m(v) is the friction coefficient,Mgmis thegrinding media mass andMis the charge mass (adapted from Ref. 7).




16/26

fraction not undergoing diameter reduction while

the second one describes how particle size

modification occurs. The stochastic particlesflow

from zone 1 to zone 2 (and vice versa) and from

zone 2 to zone 3 is described by three similar

probability functions. The model is completed by

two mass balances over zone 1 and 2 and referringto each particles class. Model simulations led to

realistic results.

Energy Transfer to Mill Charge

Once mill dynamics has been properly accounted

for, the second important step to consider is how

mechanical energy is transferred to mill charge.

As, obviously, energy transfer strictly depends on

mill type, for their wide use, we can focus the

attention on ball mills. Among many othervaluable approaches, we believe that the one

proposed by Delogu and Cocco88 is very interest-

ing. These authors, assuming that the mill charge

is kept homogeneous during grinding, affirmed

that energy transfer to mill charge is gradual and

progressive. Indeed, after thefirst collision, only a

small fraction k (typically of the order of 105106) of the whole mill charge will be modified due

the collision energy. Thus, mill charge can be

subdivided into two classes, x0(1) and x1(1),

representing, respectively, the mill charge frac-

tion that has never been impacted and the millcharge fraction that underwent one impact after

n 1 impact. Obviously, x0 1 k and x1k.After the second impact, three mill charge classes

will exist: x0(2), x1(2) and x2(3). They represent,

respectively, the mill charge fraction that has

never been impacted and the fractions that

underwent, respectively, one and two collisions

after n 2 impacts. Obviously, charge classesincreases with n and this process continuesindefinitely until grinding stop. The authors

suggested the following relation for the evaluation

of each class variation (Dxi(n)) due to increasingcollisions number:

Dx0n kx0n;Dxin kxin kxi1n 8i> 0

(15)

Since the number of impacts occurring during a

milling process, n, is very large and the massfraction of powder processed in each collision, k, isreasonably quite small, all the previous discrete

equations can be safely written in the following

continuous form:

dx0n kx0n dn;dxin kxin dn kxi1n dn

(16)

Their solutions read:

x0n ekn; xin kni

i! ekn (17)

This means that x0 decreases exponentially with

n, while all the other classes xi increase, reach amaximum (whose occurrence increases with i) andthen exponentially decrease (see Fig. 7). Once mill

dynamics is known, grinding media impact

frequency can be evaluated so that it is possible

to convertn into time. Accordingly, this approachrepresents the connection between mill operating

condition, determining mill dynamics, and grind-

ing time.

Energy Effect and Activation Yield

The final step regards the theoretical evaluation

of mechanical energy effect on mill charge.

Despite this is a key point of the entire grinding

process, the intrinsic difficulty of this topic put its

discussion out of the aim of this review.1,15,89 In

addition, in the light of the energetic cogrinding

Figure 7. According to the model proposed by Delogu

and Cocco,88 mill charge does not homogeneously

receive mechanical energy during milling. Accordingly,

he supposes that after n collisions, mill charge can be

subdivided into n classes. Class zero (x0), represents

never impacted mill charge fraction, class 1 (x1) repre-

sents mill charge fraction involved only in one collision,

class 2 (x2) represents mill charge fraction involved in

two collisions and so on up to classn (xn). According to

this theory, x0 decreases exponentially, while all other

classes show a maximum.




17/26

process optimisation, this knowledge is not

strictly required. Indeed, the energy balance

made up around the mill reads:

hE ET Ek Ea (18)where h is the mill engine yield (

0.97 for

electrical engine), hE is the energy effectivelysupplied by the mill engine to the mill,ET,Ekand

Ea are, respectively, the thermally dissipatedenergy (due to the attrition between mill moving

parts and colliding grinding media), the kinetic

energy owned by all the mill moving parts (in this

term it is comprised the potential energy associ-

able with possible mill springs) and the energy

causing mill charge activation. In the case of one

component grinding, Ea can be subdivided intotwo terms:Eas and Eab. Eas is connected with theincrease of surface area (comminution) and sur-

face properties modifications. Accordingly, wehave:

Ea Eas Eab AggsvgAgsv Eab (19)where Ag and A represent, respectively, powdersurface area after and before grinding, while gsvgand gsv represent, respectively, powder surface

energy after and before grinding. Eab, instead, isconnected with bulk properties modifications

(namely, introduction of defects in the crystalline

network as discussed in Mechanical Activation

Section). Obviously, similar considerations can be

done for the cogrinding case, but the expression ofEas is not equally straightforward as both the drugand stabilising agent surface properties need to be

considered. Anyway, regardless of the grinding or

cogrinding situation, Eqs. (18) and (19) give the

opportunity of defining a global activation yield,

hga, as follows:

hgaEa

E h Ea

ET Ek Ea(20)

As, in general,Ea is the most difficult term to beestimated, it is convenient rewriting Eq. (20) in

the light of Eq. (18) which allowsEaexpression as

a function of more convenient quantities (hE, ETand Ek):

hgahE ET Ek

E (21)

IfhE can be easily evaluated on the basis of millengine nominal power, Ek estimation descendsfrom the knowledge of mill dynamics while ETimplies a mill thermal analysis. AlthoughETcanbe theoretically estimated, it is usually more

convenient to build up a mill thermal model whose

fitting on mill temperature increase allows ETdetermination.90 As in Eq. (18) E and ET arecomparable, we should expect low values for hga.

APPLICATIONS

After the discussion about mechanical-activation

process and the tools needed for its realisation

(mills), it is interesting to present some results

descending from this approach. As drug activation

can be determined either directly on the coground

system by means of different techniques (see

Experimental Verification of Activation Section)

or it can be evaluated on the basis of its in vivoeffect (bioavailability enhancement), this section

is divided into two parts: in vitro and in vivo.While thefirst part is essentially devoted to show

and discuss in vitro evidences of activation, thesecond one deals with the in vivo evidences ofmechanical activation. In the impossibility of

giving an exhaustive presentation of all the many

examples regarding drug mechanochemical acti-

vation, but aimed to provide a rational presenta-

tion of them, in vitro applications are presentedaccording to the carrier used, while in vivoapplications are shown according to the pharma-

cological class of the drug (obviously, many other

possible criteria, such as milling type, energy

involved in milling and so on, could have been

adopted). Accordingly, in vitro section compre-hends inorganic, polymeric and cyclodextrins

carriers.In vivosection considers anti-inflamma-tory, anti-tumoural, antihypertensive, antispas-

modic and antifungal drugs.

In Vitro

Inorganic Carriers

Examples of inorganic carriers are calcium

silicate and silicon dioxide.91 For example, Bahl

and Bogner91 studied the indomethacin (g-poly-

morph) (IM) activation process recurring to its(low energy) cogrinding with Neusilin US2

(amorphous magnesium aluminometasilicate,

specific surface are 300 m2/g) in a rolling jarmill consisting of a cylindrical porcelain jar

(internal volume 1000 mL) hosting zirconia balls.

Different IM-Neusilin US2 weight ratios were

considered (1:5, 1:4, 1.1, 1:0.5). Interestingly,

these authors found that, whatever the drug

carrier ratio considered, the relative humidity

(RH) of the cogrinding environment highly




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influences amorphisation kinetics. For example

when RH 75% and IM:Neusilin US2 1:5, 60%of the whole drug was converted into amorphous

phase after 0.5 days while similar conversions for

RH 0% and the same weight ratio occurred after2 days. Watanabe et al.92 used a vibrational mill

made up by a cylindrical zirconia bowl (100 mL)hosting 74 zirconia balls (diameter 10 mm) toget IM (g-polymorph) amorphisation. Four differ-

ent carriers were used: talc (3MgO4SiO2H2O),

SiO2, Mg(OH)2 and a 0.42:0.58 (w/w) Mg(OH)2:-

SiO2 mixture (PMS). The drug:carrier weight

ratio was constantly equal to 1:1. The neatly

higher energetic process used by Watanabe in

comparison to that of Bahl and Bogner91 made

possible an almost complete amorphisation in

10 min in the case of IMPMS system. IMSiO2and IMtalc systems yielded to a complete

amorphisation after 60 and 30 min, respectively.On the contrary, IMMg(OH)2 system did not

yield a complete amorphisation in the maximum

experimental time range considered (60 min).

According to the authors, this behaviour was due

to optimal interaction of IM with talc or SiO2.

Shakhtshneider et al.93 employed a planetary mill

(vial volume 40 mL; ball diameter 6 mm;grinding media:mill charge 20:1 (w/w)) to studythe activation process of Ibuprofen (IB) in

presence of talc (1:10, w/w). XRPD and DSC

analysis demonstrated that the authors could

obtain complete drug amorphisation. Interest-

ingly, dissolution studies (a known amount of

coground material was put in 100 mL water at

378C under mixing) revealed that the coground

system yielded, in about 30 min, to a IB

concentration of 500 mg/mL which is approxi-

mately 12.5 times native IB solubility in water

(see Tab. 2).

Polymeric Carriers

Although different polymeric carriers can be used

(dextrans, chitin, chitosan, gelatin, polyethylene

glycol, methyl cellulose, hydroxypropyl cellu-

lose91), polyvinylpyrrolidone (PVP) is one of the

most used. For example, Shakhtshneider et al.93

employed this carrier to activate sulfathiazole and

Piroxicam adopting the same milling conditions

above reported for Ibuprofen activation with talc.

In the case of sulfathiazole, they found that part of

the drug was still crystalline after cogrinding

when the drug:PVP weight ratio is 3:1 or 1:1

(regardless milling time; 4, 6, 8 or 12 min), while

the drug was completely amorphous when this

ratio was 1:3 (12 min cogrinding). Dissolutionstudies performed on the 1:3 system (12 min

cogrinding) yielded to a drug concentration of

7000 mg/mL after 1 h. This concentration is

approximately 10 times the water solubility of

native sulfathiazole (see Tab. 2). In the case of

Piroxicam, the increase of apparent solubility was

approximately 2.5 and 4 times that of the

native drug when the drug:PVP weight ratio

was equal to 1:1 and 1:10, respectively. Obviously,

dissolution studies performed on simple physical

drugPVP mixtures (these systems did not

undergo cogrinding), did not imply any significant

improvement of drug solubility. Watanabe et al.92

used PVP to activate Indometacin (IM; g-poly-

morph) (IM:PVP 1:1, molar ratio) in a vibra-tional mill (cogrinding times: 30, 60, 120 and

180 min). XRPD revealed that IM diffraction

Table 2. Drugs Solubility in Aqueous Environment

Drug Class Solubility References

Glibenclamide Anti-diabetes 0.3 mg/cm3 (258C, water) 98

Gliquidone Hypoglycaemic agent 0.14 mg/cm3 (258C, water) 97

Glisentide Anti-diabetes 1.5 mg/cm3 (378C, artificial gastricmedium without enzymes)

94

Griseofulvin Antifungal 11.9 mg/cm3 (378C, water) 25

Ibuprofen Nonsteroidal anti-inflammatory 40 mg/cm3 (378C, water) 93

Indomethacin Nonsteroidal anti-inflammatory 35 mg/cm3 (258C, water) 105

Methylhydroxy-

progesterone

Low dose: progestinic activity

High doses: anticancer

1.2 mg/cm3 (378C, water pH 5.5) 31

Nifedipine Calcium-channel blocker 5 mg/cm3 (308C, water) 103

Nimesulide Nonsteroidal anti-inflammatory 12 mg/cm3 (378C, water, pH 5.5) 100Sulfathiazole Antimicrobial 600 mg/cm3 (378C, water) 93




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peaks decreased with increasing cogrinding time

to totally disappear after 180 min (totally amor-

phous IM). For this last system, apparent IM

solubility in water (378C) was raised up to 77 mg/

mL. Mura et al.94 studied glisentide (antidiabetic)

activation with PVP in a high energy vibrational

mill for different milling times (15180 min) anddrug:carrier weight ratios (1:3, 1:1 and 3:1). DSC

and XRPD confirmed that glisentide complete

amorphisation occurred after only 30 min in 1:3

systems. Interestingly, dissolution tests, per-

formed at 378C in 1000 mL of artificial gastric

medium without enzymes, indicated that after

60 min glisentide concentration in the release

environment was around 5 mg/mL while, after the

same time, the 1:3 physical mixture yielded to

drug concentration of about 1.5mg/mL. Shakhtsh-

neider et al.95,96 studied indomethacin (IM) and

piroxicam activation by cogrinding with PVP in acryogenic mill (6750 Freezer/Mill, Inc., Metuchen,

New Jersey). 1 g sample (PVP/drug mass/mass

ratio ranging from 0.1 to 0.8) was milled at an

impact frequency of 10 Hz alternating milling

periods of 2 min with 1 min cool-down period

(overall milling time spanned between 60 and

78 min). The authors were interested in evaluat-ing the stabilisation action of PVP by comparing

re-crystallisation kinetics relative to pure ground

drugs and to coground drugs. The necessity of

recurring to cryo-milling was dictated by the

impossibility of getting a complete amorphous by

simple drugs milling at room temperature in the

absence of the stabilising carrier. They found that

PVP exerts a good stabilising action on both drugs

even if better results were evidenced for IM.

Obviously, for both drugs, PVP stabilising action

increased in reason of its content in the coground

mixture. Finally, they found that coground cryo-

milled IM dissolution was neatly improved with

respect that of the same mixture that did not

undergo cryo-milling process.

CyclodextrinsFor their chemical and physical peculiarities,

cyclodextrins are widely used as stabilising agents

(carrier) in drug mechanochemical activation.

Indeed, a, b, g and substituted cyclodextrins

can be used. For example, Miro et al.97 coground

gliquidone (hypoglycaemic agent) with hydroxy-

propyl-b-cyclodextrin (HPbCD) in 1:2 molar ratio.

DSC and XRPD analyses revealed that the

authors obtained a system showing a very small

content of original crystalline drug, being the drug

majority in the amorphous state. Dissolution

studies, performed at 378C in 1000 mL of 0.1 M

phosphate buffer (pH 7.4), revealed that, after

60 min, drug concentration in the release envir-

onment was around 9 mg/mL, that is approxi-

mately, 64 times drug solubility in the same

environment and conditions (see Tab. 2). Inter-estingly, oral administration of this coground

system (rats, drug dose 300 mg/kg) caused a

reduction of plasma glucose concentration that

was approximately 1.52 times that of native

gliquidone in the first 15 h following administra-

tion. Fukami et al.98 focused the attention

on glibenclamide (anti-diabetes) bioavailability

enhancement recurring to cogrinding with highly

branched cyclodextrins (HBCD). HBCD is a cyclic

glucan produced from waxy corn starch by the

cyclisation reaction of branching enzyme. Ball

mill with drug:HBCD 1:5 weight ratio wasconsidered. Grinding time was fixed in 2 h and

150 rpm. Interestingly, DSC analysis revealed

that glibenclamide melting point in the coground

system was reduced of 5.48C respect to the native

drug that melts at 170.18C. This was the proof that

the original crystalline drug was completely

disappeared in favour of nano-crystalline one.

The authors verified that cogrinding increased

drug apparent solubility up to 12.4 mg/mL, being

native drug solubility equal to 0.3 mg/mL.

These examples clearly show that, apart from

the specific energy supplied by the mill, key

factors for the attainment of drug activation are

drug:carrier ratios >1:2 and good chemico-physi-cal interactions between drug and carrier.

In Vivo

While previous section was focussed on the in vitroevaluation of coground systems activation, this

section shows the in vivo evidences of activationand their relations with in vitro release tests.Indeed, what is often desired is to get information

about in vivo performance resorting to in vitrobehaviour. In order to rationalize the presenta-

tion, examples are subdivided according to drug

pharmacological class.

Anti-Inflammatory Drug

Perret and Venkatesh55 showed data referring to

an activated system composed by drug X (anti-inflammatory, poorly water-soluble crystalline

drug) and crosslinked polyvinylpyrrolidone




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(PVPclm) in a weightweight ratio equal to 1:2.

The system, once coground in a vibrational mill,

was characterised by 40% amorphous drug and

60% nanocrystalline drug (original crystalline

drug absent). Figure 8A clearly shows the better

release performance of the activated system (filled

circles) in comparison to that of an identical w/wratio drug-polymer physical mixture (open cir-

cles). While after about 10 min physical mixture

yields to a release environment concentration just

below 2 mg/mL, the activated system concentra-

tion is up to 8.5 mg/mL after about 50 s and then,

due to amorphous drug re-crystallisation, con-

centration decreases to 7.9 mg/mL. Interestingly,

the evident in vitro superiority of the activated

system reflects into a similar in vivo superiorityas shown in Figure 8B. Thisfigure reportsdrug Xplasma concentration (humans) following oral

administration of a 200 mg dose commercial

reference (open circles) and activated (filled

circles) tablet made by the same activated system

tested in vitro (Fig. 8A). Not only the maximumblood concentration (Cmax) relative to the acti-vated system is approximately two times that of

the reference, but the area under the curve (AUC)

relative to the activated system is approximately

1.3 times that of the reference. It is, thus, evident

the consistent increase ofdrug Xbioavailability.Magarotto and coworkers99 presented data

regarding the activation of nimesulide, a low

water soluble (100 mg/mL, pH 7.5, 378C,100melting temperature 148.78C) nonsteroidal anti

inflammatory drug. Cogrinding, performed in a

vibrational mill using b-cyclodextrin (bCD) ascarrier (nimesulide:bCD w/w ratio is 1:3), led to

the complete transformation of the original drug

crystals into nano-crystals melting at 145.78C.

This means that nano-crystals were characterised

by a radius of approximately 23 nm (see Fig. 4).

Thein vivobehaviour (see Fig. 9) of the activatedsystem (filled circles), showed a little improve-

ment with respect to a commercial reference

(open circles). Indeed, Cmax was increased from3.3 mg/mL (commercial reference) to 3.6 mg/mL

(coground system) and AUC passed from 20.4 mg

h/mL (commercial reference) to 22.4 mg h/mL

(coground system). The little improvement of the

in vivo performance of the coground system wascoherent with its low degree of activation as no

Figure 8. (A) In vitro test referring to drug X (poorwater soluble drug) release from activated drug Xcross-

linked polyvinylpyrrolidone (1:2 (w/w) ratio) system

(filled circles). Forty percent of the original drug is

amorphous while the remaining 60% is nanocrystalline.

Open circles indicate the release behaviour of an equal

w/w ratio physical mixtu

Activaton in Drugs

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