Polymer Degradation and Stability 92 (2007) 915e919www.elsevier.com/locate/polydegstab

Comparison of the sensitivity of 11 crosslinked hyaluronic acidgels to bovine testis hyaluronidase

Ibrahima Sall*, Georges Ferard

Laboratoire de Biochimie Appliquee, Faculte de Pharmacie, Universite Louis Pasteur, 74 Route du Rhin, BP 60024, F-67401 Illkirch Cedex, France

Received 12 October 2006; accepted 15 November 2006

Available online 27 January 2007

Abstract

Crosslinked hyaluronan gels are used in various applications where their stability is a prerequisite. The sensitivity of such gels to hyaluron-idase can be determined as an index of stability by several approaches: chromatography, electrophoresis, and viscometry. We describe here a testbased on the colorimetric determination of the N-acetyl-D-glucosamine released by hyaluronidase in standardized conditions. The sensitivities tobovine testicular hyaluronidase of 11 different gels used to fill skin wrinkles (Restylane; Perlane; Juvederm 18, 24, 24HV, 30, and 30HV;Surgiderm 18, 24XP, 30, and 30XP) were compared.

The method was reproducible, easy to perform, not time-consuming and allowed us to demonstrate that the sensitivity to testicular hyaluron-idase was dependent on the degree of crosslinking of the gels and also on their monophasic/biphasic nature. Under our conditions, Surgiderm 30,24XP and 30XP were the most resistant gels.

We propose to retain the hyaluronidase test to predict the in situ stability of a crosslinked gel used to fill skin wrinkles.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Crosslinked hyaluronan gels; Stability; Sensitivity to hyaluronidase; N-Acetyl-glucosamine reducing end assay

1. Introduction

Hyaluronic acid or hyaluronan (HA) [1e3] is a high-molec-ular mass linear, anionic polysaccharide without branchingside-chains, composed of 2000e25 000 disaccharide unitsformed by glucuronic acid and N-acetyl-D-glucosamine(NAG), which are linked by b(1,4)-glycosidic bond, and canreach 105e107 Da in molecular mass. This polymer representsa class of ubiquitous molecules [glycosaminoglycans (GAG)],the only one not linked to a core protein, non-synthesized in theGolgy apparatus, and the only non-sulfated GAG [4]. Inthe human body, HA is a major component of the extracellularmatrix, the skin, the synovial fluid, loose connective tissues,umbilical cord, vitreous body of the eye and the cartilage[5]. It exhibits a wide range of biological functions and plays

* Corresponding author. Fax: þ33 390 24 42 86.

E-mail address: ibrahima.sall@pharma.u-strasbg.fr (I. Sall).

0141-3910/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2006.11.020

a major role in the organization and integrity of the extracel-lular matrix, thereby participating in the preservation of theform and in the spatial arrangement of tissue components.Unlike collagen, it is present in an identical form in all animaland bacterial species with the largest amount in the skin. HA isinvolved in numerous biological and physiological functionssuch as cell motility, cell matrix adhesion, cell proliferation,water homeostasy of tissues, or joint lubrication [6]. The HAmolecules can absorb a large volume of water which expandsin the extracellular space, hydrates tissues and finally main-tains the moisture of the skin [7,8]. Degradation in the bodycan occur in any of the three ways: attack by free radicals, en-zymes (hyaluronidases), or thermal [9]. Natural HA providesa biological material with high viscoelastic and rheologicalproperties which, in addition to its non-immunogenicity, itsbiocompatibility, and its total biodegradability, makes it suit-able for various medical applications such as dermatology, sur-gery and wound healing, embryo implantation and drugdelivery [10]. Hyaluronidases are endo-glucosidases that can

916 I. Sall, G. Ferard / Polymer Degradation and Stability 92 (2007) 915e919

break down GAG such as HA, chondroitin, or chondroitins 4-and 6-sulfate [11]. These enzymes fall into three classes,according to their hydrolysis mechanism. Testicular orvenom-type hyalurono-glucosaminidases (EC 3.2.1.35) breakthe b(1,4) link [12]; they are vertebrate-type endo-b-acetyl-hexosaminidases generating tetrasaccharides as predominantend-products. They lack substrate specificity, because inaddition to HA, they can hydrolyze chondroitin sulfate anddermatan sulfate but at a much slower rate. Various enzymesof the mammalian group are involved in the pathophysiologyof many disorders like cancers, osteoarthritis, mucopolysac-charidosis, liver, and skin diseases [13,14]. Hyalurono-glucu-ronidases (EC 3.2.1.36) which are produced by salivaryglands of leeches, parasites and crustaceans break the b(1,3)link [15]; they are endo-b-glucuronidases generating tetra-and hexasaccharides. This group is specific mainly for HA.Hyaluronate-lyases (EC 4.2.99.1) are the third class of en-zymes found mainly in bacteria; they can degrade HA andto some extent chondroitin and dermatan sulfate. They areoperated by a b-elimination reaction yielding essentiallydisaccharides.

In order to increase the biomechanical properties and theresistance to enzymatic break down of natural HA containingpreparations (one-third of the human body’s HA is turnedover each day by lymphatics and the liver), various methodshave been developed for chemical modification or crosslink-ing of native HA by covalent links, resulting in larger andmost stable derivatives that retain HA biocompatibility andbiodegradability in vivo [16]. Crosslinking HA hydrogels re-sult in significant increase in tissue residence time, slow deg-radation, and the same bioresorbability. They are significantlymore stable than their non-crosslinked counterparts, becauseof the formation of a dense network by intermolecular bondsor bridges, that is much less susceptible to degradation bytissue enzymes such as the hyaluronidases [17,18], oxygen-derived free radicals [19] or the temperature [20]. This makesmodified HA derivatives obtained by bacterial cultures suit-able to be employed safely in most various medical andbiological applications [21], and also without any risk of aller-gic reactions. Crosslinked HA gels have multiple uses, such asaesthetic surgery (wrinkle smoothing), gynaecology, or drugtransport [22].

A wide number of techniques have been reported in the lit-erature to follow-up in vitro the degradation rate of crosslinkedHA hydrogels under the action of testicular hyaluronidase[5,23,24]: change in viscosity, in water content, in molecularmass by chromatography, gel plate assay in a Petri dish, or col-orimetric assays for the liberated glucuronic acid. All thesemethods are complex, time-consuming or they need sophisti-cated and expensive instruments.

The aim of this study was to develop a heterogeneous phaseapproach based on the measurement of the NAG release by animproved colorimetric method to evaluate the sensitivities ofdifferent crosslinked HA hydrogels to the action of beef testishyaluronidase. The test was then applied to 11 different com-mercially available gels, differing in their crosslinking degreeand their monophasic/biphasic nature.

2. Materials and methods

2.1. Chemicals and instruments

We tested the sensitivity of the following 11 crosslinkedgels to hyaluronidase: Restylane (lot 7345-1), Perlane (lot6380-1) are obtained from Q-Med (Uppsala, Sweden); Juve-derm 18 (lot IN180410), Juvederm 24 (lot IN240450), Juve-derm 24HV (lot HV240441), Juvederm 30 (lot IN3004126),Juvederm 30HV (lot HV300427), Surgiderm 18 (lot SG18210620), Surgiderm 24XP (lot XP24210626), Surgiderm 30(lot SG30210622), and Surgiderm 30XP (lot XP30210624)are from Corneal (Pringy, France). Restylane and Perlanegels are biphasic in constitution which means they are com-posed of distinct swollen particles in a continuous homoge-neous phase; Restylane has 100 000 particles per ml andPerlane has 10 000 particles per ml of gel. The other testedgels (Surgiderm and Juvederm families) are all monophasicand homogeneous without distinct particles. The Surgidermgels have the same features as those of Juvederm family,except that they are produced with another more efficientcrosslinking process. All these gels are used as dermal fillersin the treatment of aging skin and for soft tissue augmentation:facial lines, wrinkles, and lips.

The amount of crosslinking agent in the tested gels, notedas a number of (þ) is also variable: (þ) for Restylane and Per-lane; (þþ) for Juvederm 18, Surgiderm 18 and Juvederm 24;(þþþ) for Juvederm 30, Juvederm 24HV, Surgiderm 30, andSurgiderm 24XP; (þþþþ) for Juvederm 30HV and Surgi-derm 30XP.

All the tested gels are made of crosslinked non-animal HAfrom a streptococcus bacterial fermentation source, swollen inphosphate buffer at a defined concentration. The crosslinkingprocess (also named stabilization by QMED) uses 1,4-butane-diol diglycidylether as the crosslinker.

Hyaluronidase from bovine testis was purchased from SigmaChemical Co. (St Louis, USA; ref H3506, 608 USP Units/mg).All other chemicals were of the highest purity available.

The absorbances were measured, using an Uvikon 922(Kontron, Milano, Italy) dual-beam spectrophotometer.

2.2. Preparation of gel pellets

The gel syringes were stored in sealed plastic bags at a tem-perature of þ4 �C until use. They were brought to laboratoryroom temperature for 1 h before use.

Gel samples’ weights between 0.1 and 0.5 g were placed inthe bottom of stoppered glass tubes. These tubes were thencentrifuged for 5 min at 1000 g in a Beckman Allegro benchtop refrigerated centrifuge equipped with a swinging bucketrotor. At the end of the centrifugation, thin pellets firmly at-tached at the bottom of the tubes were obtained.

2.3. Hyaluronidase sensitivity test

A solution of hyaluronidase was prepared as required ata concentration of 10 mg/ml (6080 U/ml) in an isotonic

917I. Sall, G. Ferard / Polymer Degradation and Stability 92 (2007) 915e919

phosphateeNaCl buffer (10e155 mmol/l) at a pH of 7.4. Tomeasure the degradation power of the hyaluronidase on thegels, a heterogeneous method with no prior dilution of thegel was used. The glass tubes containing the gel pellets andthe hyaluronidase solution were pre-incubated separately for10 min at 37 �C. Then 100 ml of the enzyme solution was in-troduced gently, without stirring, onto the surface of the gels.After incubation for 120 min at 37 �C, the enzymatic reactionwas stopped by the addition of 0.1 ml of a potassium tetrabo-rate solution (0.8 mol/l, at a pH of 9.1), followed by immediatestirring on a vortex mixer and heating for 3 min at 100 �C. Thetubes were then cooled at room temperature and the releasedNAG was assayed.

2.4. Assay of the released NAG

We used Ehrlich’s reagent, in accordance with the methoddescribed by Reissig et al. [25]. To each tube we added 3 mlof freshly prepared reagent. The contents of the tubes weremixed by vigorous stirring on a vortex mixer. The tubeswere then stoppered and incubated for 20 min at 37 �C, whichallowed the development of a violet colour. The tubes werethen centrifuged at 1000 g for 15 min to remove at the sametime the turbidity in the reaction medium and also the gel frag-ments that were still present. After confirming previously thatgels incubated without addition of enzyme did not produceany violet colour with Ehrlich’s reagent, the reagent blankswere prepared with only the phosphate buffer and the colourreagent.

3. Results

3.1. Variation of the amount of NAG released asa function of gel amount

Results which were obtained with increasing weights of Ju-vederm 30 crosslinked gel (Fig. 1), showed that the NAGabsorbance curve at 585 nm as a function of gel amount washyperbolic, tending toward a plateau from 0.3 g of gel and ex-hibiting MichaeliseMenten kinetics. Between 0.3 and 0.5 g ofgel, the increase in absorbance reached hardly 3e4%. Thismeans that the test sample containing 0.3 g of gel was

0.60.50.40.30.20.10.00.0

0.5

1.0

1.5

Weight of gel per tube (g)

Abso

rban

ce (5

85 n

m)

Fig. 1. Effect of Juvederm 30 gel weight on the amount of NAG released.

sufficient to obtain the maximum hydrolysis rate (correspond-ing to enzyme substrate saturation) for the added hyaluroni-dase activity. Moreover, with such gel amount, only thinpellets were formed at the bottom of the tubes after centrifu-gation. Therefore, to compare the sensitivities of the 11 gelsto hyaluronidase, a fixed test sample consisting of 0.3 g foreach of the gels was used.

3.2. Sensitivities of the 11 gels to hyaluronidase

Under our conditions, (a pellet consisting of 0.3 g of gel,enzyme volume of 100 ml, pH 7.4, temperature 37 �C, incuba-tion time of 120 min), we compared the absorbance values at585 nm that corresponded to the amounts of the released NAGdue to the effect of the hyaluronidase on the 11 gels (threesamples for each gel). The mean absorbance values obtainedat 585 nm varied according to the gel with decreasing valuesfrom Restylane to Juvederm 24HV (Fig. 2).

The absorbance values were related to the initial HAcontent of the gels and standardized, taking Restylane as thereference gel, for calculating relative rates of degradation(Table 1). These relative rates also decreased from Restylaneto Juvederm 24HV.

In another set of experiments, we compared the absorbancevalues at 585 nm for four Surgiderm gels to that of Restylane(Fig. 3). Surgiderm 30, 24XP and 30XP were the most resis-tant in this second group of gels.

4. Discussion

In this study, we first determined the amount of a Juvedermgel required to reach the maximum rate of hydrolysis pro-duced by a fixed activity of hyaluronidase from bovine testis,

0,00

0,05

0,10

0,15

0,20

0,25

0,30

Restylane

PerlaneJuv. 1

8Juv. 2

4Juv. 3

0

Juv. 24HV

Juv. 30HV

Abs

(585

nm

) per

mg

of in

itial

am

ount

of N

aHA

Gel wt = 0.3 g; Vol hyaluronidase = 100 µl; Conc hyaluronidase sample = 6080 U/ml;Temp= 37 ºC; pH = 7.4; Time = 120 min.Each point is the mean of three measurements ± 2 SD

Fig. 2. Comparison of the sensitivity to hyaluronidase for Juvederm gels vsRestylane/Perlane gels.

918 I. Sall, G. Ferard / Polymer Degradation and Stability 92 (2007) 915e919

Table 1

Comparison of the HA hydrolysis rates for the seven gels by hyaluronidase

Gel NaHA content (mg/g) Weight (g) NaHA (mg) Absorbance (585 nm) Absorbance per

g of NaHA

Relative rate

of degradationa

Restylane 20.0 0.300 6.00 1.185� 0.328 197.50� 54 100

Perlane 20 0.300 6.00 1.165� 0.175 194.17� 29 98.3

Juvederm 18 17.8 0.300 5.34 0.952� 0.172 178.28� 32 90.3

Juvederm 30 22.1 0.300 6.63 0.756� 0.215 114.03� 32 57.7

Juvederm 24 23.3 0.300 6.99 0.792� 0.172 113.30� 24 57.4

Juvederm 30HV 22.1 0.300 6.63 0.690� 0.109 104.07� 16 52.7

Juvederm 24HV 22.8 0.300 6.84 0.624� 0.078 91.23� 11 46.2

The indicated values correspond to the mean of three measurements � 2 SD for each gel.a Expressed in percent by taking Restylane as the reference gel.

which falls in the same mammalian group as the skin enzyme[26], with significant homology between them. This source ofenzyme was retained because of its frequent use for the studyof action of mammalian enzyme on HA [27]; its mechanism ofaction is similar to that of human hyaluronidases as from skin,and also for its easy commercial availability in pure form. Thedetermination of the optimum gel amount then allowed us tocompare the sensitivities of 11 crosslinked HA gels, from dif-ferent sources or grades, to hyaluronidase.

The variation curve of the amount of NAG produced asa function of the Juvederm 30 sample weight, showed a maxi-mum rate with 0.3 g sample. After initial centrifugation of thegel tubes, this sample amount finally yielded thin pellets thatwere fairly close to the in situ operating conditions. The het-erogeneous phase results, i.e. with no prior dilution of thegels, also appeared to be more reproducible than the resultsobtained in a homogeneous phase with prior dilution, that isafter gels were diluted in a buffer solution (data not shown).

The heterogeneous enzyme test was easy and rapid to per-form with only a small amount of gel, and required neitherspecialized reagents nor a particular measuring instrument.

0,30

0,00

0,05

0,10

0,15

0,20

0,25

Restylane

Surgiderm 18

Surgiderm 30

Surgiderm 24XP

Surgiderm 30XP

Abs

(585

nm

) per

mg

of in

itial

amou

nt o

f NaH

A

Gel wt = 0.3 g; Vol hyaluronidase = 100 µl; Conc hyaluronidase sample = 6080 U/ml;Temp= 37 ºC; pH = 7.4; Time = 120 min.Each point is the mean of three measurements ± 2 SD

Fig. 3. Comparison of the sensitivity to hyaluronidase for Surgiderm gels vsRestylane gel.

As previously pointed by Takahashi et al. [28], it was neces-sary to centrifuge the final medium reaction prior to absor-bance measurement in order to remove turbidity and smallgel fragments which were still present. Moreover, it was pos-sible to determine the degradation rate of HA hydrogels onseveral series with different samples. In our hands, the hyal-uronidase test was reproducible enough to show marked differ-ences in the sensitivity to the enzyme between the 11 gelstested here. Furthermore, in contrast to tests using chromatog-raphy, electrophoresis or viscometry, the present colorimetrictest is direct in the sense that it permits us to determine thenumber of b(1,4) links hydrolyzed by hyaluronidase.

For the heterogeneous phase measurement assay, we haveretained conditions (solid surface erosion effect, temperature37 �C, and neutral pH 7.4) very similar to the in vivo condi-tions on the skin. The obtained results indicated that the sen-sitivity of the gels to the effect of the enzyme decreased asa function of their amount of crosslinker. Restylane was themost sensitive, while Surgiderm 30, 24XP and 30XP gelswere the most resistant. The sensitivities of Juvederm 18, 24and 30 and Surgiderm 18 were intermediate. This differencein sensitivity among the 11 gels may be due to more limitedaccess by the enzyme to its HA substrate for Juvederm24XP and 30XP that are more highly crosslinked than Resty-lane/Perlane, and more efficiently crosslinked than Juvederm24HV and 30HV. Similarly, among the group of five studiedJuvederm gels, the Juvederm 18 (the least crosslinked gel) dis-played the greatest sensitivity to the effect of the enzyme(Table 2). In general the viscosity of tested gels increaseswith their concentration, but also with their reticulation de-gree; the latter being in the following increasing order: Juve-derm 18 and Surgiderm 18< Surgiderm 30 and Juvederm30< Surgiderm 24XP and Juvederm 24HV< Surgiderm30XP and Juvederm 30HV. The consequence for the mostheavily crosslinked gels is a greater stability in comparisonwith the other non-crosslinked gels as observed with anotheranalytical approach [29,30].

Gels such as Restylane and Perlane displayed greater sensi-tivity to the effect of the enzyme among all of the 11 studiedgels. Their biphasic nature can also explain their greatest sen-sitivity toward hyaluronidase: the distinct particles offera greater surface to the attack of the enzyme than monophasicgels like Juvederm or Surgiderm gels. In addition, we observed

919I. Sall, G. Ferard / Polymer Degradation and Stability 92 (2007) 915e919

Table 2

Comparison of the HA hydrolysis rates for five gels by hyaluronidase

Gel NaHA content (mg/g) Weight (g) NaHA (mg) Absorbance (585 nm) Absorbance per

g of NaHA

Relative rate

of degradationa

Restylane 20.0 0.300 6.00 1.391� 0.056 231.83� 9.3 100

Surgiderm 18 17.8 0.300 5.34 0.737� 0.010 138.01� 1.9 59.5

Surgiderm 30XP 24.6 0.300 7.38 0.747� 0.012 101.22� 1.6 43.7

Surgiderm 24XP 25.5 0.300 7.65 0.759� 0.079 99.21� 10.3 42.8

Surgiderm 30 24.3 0.300 7.29 0.707� 0.102 96.98� 14 41.8

The indicated values correspond to the mean of three measurements � 2 SD for each gel.a Expressed in percent by taking Restylane as the reference gel.

with the naked eye a much more rapid liquefaction of thesetwo gels after the addition of hyaluronidase to the incubationtubes. Meanwhile, the Juvederm 30XP gel required more timebefore starting to liquefy under the action of the enzyme. Itshould be later possible to compare the heterogeneous phasesensitivity results with viscosity measurements: measurementof elasticity change as a function of time.

The calculation of the relative rates referred to the initialHA content of the gels indicates that Restylane and Perlaneare the most sensitive gels, followed closely by Juvederm18. The least sensitive of the 11 studied gels were clearlythe Juvederm 24XP and the Juvederm 30XP.

This study also demonstrates that all tested Juvederm andSurgiderm gel implants compared to Restylane are indeedless degradable by hyaluronidase that means better stabilityand longer half-life. On the other hand Surgiderm gels havealso better stability than Juvederm gels.

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