Chapter 14 Pharmacokinetics

Getting a drug to the target in the body

DRUG DESIGN AND DEVELOPMENTDRUG DESIGN AND DEVELOPMENT

StagesStages 1) Identify target disease 2) Identify drug target 3) Establish testing procedures 4) Find a lead compound 5) Structure Activity Relationships (SAR) 6) Identify a pharmacophore 7) Drug design- optimizing target interactions 8) Drug design - optimizing pharmacokinetic properties 9) Toxicological and safety tests10) Chemical development and production11) Patenting and regulatory affairs12) Clinical trials

PHARMACOKINETICS - DRUG DESIGNPHARMACOKINETICS - DRUG DESIGN

Aims•To improve pharmacokinetic properties of lead compound

•To optimize chemical and metabolic stability

•To optimize hydrophilic / hydrophobic balance

•To optimize solubility

•To optimize drug half life

•To optimize distribution characteristics

Notes•Drugs must be sufficiently polar to be soluble in aqueous conditions

•Drugs must be sufficiently polar to interact with molecular targets

•Drugs must be sufficiently ‘fatty’ to cross cell membranes

•Drugs must be sufficiently ‘fatty’ to avoid rapid excretion

•Drugs must have both hydrophilic and lipophilic characteristics

•Many drugs are weak bases with pKa’s 6-8

PHARMACOKINETICS - DRUG DESIGNPHARMACOKINETICS - DRUG DESIGN

Receptor interaction& water solubility

Crossesmembranes

+ H

HN N H

H

- H

Vary alkyl substituentsVary alkyl substituents

Rationale•Varying the size of alkyl groups varies the hydrophilic / hydrophobic balance of the structure•Larger alkyl groups increase hydrophobicity

DisadvantageMay interfere with target binding for steric reasons

Methods•Often feasible to remove alkyl groups from heteroatoms and replace with different alkyl groups•Usually difficult to remove alkyl groups from the carbon skeleton - full synthesis often required

Solubility and membrane permeabilitySolubility and membrane permeability

Vary alkyl substituents

Methylene shuffleMethylene shuffle

Solubility and membrane permeabilitySolubility and membrane permeability

O

CH3

S OO

N

N

CH3

N

HNN

N

CH3

CH3

O

Viagra

Extra bulkExtra bulk

O

CH3

S OO

N

N

CH3

N

HNN

N

CH3

O

NO

CH3

S OO

N

N

N

HNN

N

H3C

O

N

H3C

UK343664

Methyleneshuffle

•Second-generation anti-impotence agent•Increased selectivity•Excess lipophilicity

Reduced lipophilicityBetter in vivo activity

Extra bulkExtra bulk

O

CH3

S OO

N

N

CH3

N

HNN

N

CH3

O

N

‘‘Masking’ or removing polar groupsMasking’ or removing polar groups

Rationale Masking or removing polar groups decreases polarity and increases hydrophobic character

Disadvantages•Polar group may be involved in target binding•Unnecessary polar groups are likely to have been removed already (simplification strategy)•See also prodrugs

Methods

Solubility and membrane permeabilitySolubility and membrane permeability

R OH R OMe

CH3I

R NHR

CH3COCl

R

HN

O

CH3

RC

OH

O

H+ / R'OH RC

OR'

O

Systemic antifungal agent improved blood solubility

F

F

C

OH NN

NNNN

Fluconazole

Adding polar groupsAdding polar groupsRationale •Adding polar groups increases polarity and decreases hydrophobic character•Useful for targeting drugs vs. gut infections•Useful for reducing CNS side effects

Disadvantage of adding polar groupsMay introduce unwanted side effects

Solubility and membrane permeabilitySolubility and membrane permeability

Antifungal agent with poor solubility - skin infections only

Cl

Cl

C O

HN

NS

Cl

Tioconazole

Systemic antifungal agent improved blood solubility

F

F

C

OH NN

NNNN

Fluconazole

Vary pKVary pKaa

Rationale •Varying pKa alters percentage of drug which is ionised•Alter pKa to obtain required ratio of ionised to unionised drug

Disadvantage•May affect binding interactions

Method •Vary alkyl substituents on amine nitrogens•Vary aryl substituents to influence aromatic amines or aromatic carboxylic acids

Solubility and membrane permeabilitySolubility and membrane permeability

AntithromboticAntithromboticToo basicToo basic

Decreased basicityDecreased basicityNitrogen locked into heterocyclic ringNitrogen locked into heterocyclic ring

Vary pKVary pKaa

Solubility and membrane permeabilitySolubility and membrane permeability

H2N NH

NH

O

N

O

N

N

(I)

amidine

N NH2

NH

O

N

O

N

N

PRO3112N NH2

NH

O

N

O

N

N

PRO3112

StericStericshieldshield

Steric ShieldsSteric Shields

Rationale : •Used to increase chemical and metabolic stability•Introduce bulky group as a shield •Protects a susceptible functional group (e.g. ester) from hydrolysis•Hinders attack by nucleophiles or enzymes

Blocks hydrolysis of Blocks hydrolysis of terminal amideterminal amide

Drug stabilityDrug stability

Antirheumatic agentAntirheumatic agentD1927D1927

HSNH

HN CONHMe

O

O

C

NOO

H3C CH3CH3

Terminal amideTerminal amideAntirheumatic agentAntirheumatic agentD1927D1927

HSNH

HN CONHMe

O

O

C

NOO

H3C CH3CH3

‘ ‘Electronic shielding’ of NHElectronic shielding’ of NH22

Rationale •Used to stabilise labile functional groups (e.g. esters)•Replace labile ester with more stable urethane or amide •Nitrogen feeds electrons into carbonyl group and makes it less reactive•Increases chemical and metabolic stability

Drug stabilityDrug stability

ISOSTERE

H3CC

O

O O

C

O

H2NR R

ISOSTERE

NH

C

O

CH3H3CC

O

OR R

‘ ‘Electronic shielding’ of NHElectronic shielding’ of NH22

Drug stabilityDrug stability

R NH

C

R'

O

R NH

C

R'

O

•ortho Methyl groups act as steric shields •Hinder hydrolysis by esterases•Amide more stable than ester (electronic effect)

Stereoelectronic EffectsStereoelectronic Effects

Rationale •Steric and electronic effects used in combination•Increases chemical and metabolic stability

Drug stabilityDrug stability

•Local anaesthetic•Susceptible to esterases•Short duration

CH2N

O

O CH2CH2NEt2ProcaineProcaine

CH3

CH3

NH

C

CH2NEt2

O

LidocaineLidocaineCH3

CH3

NH

C

CH2NEt2

O

LidocaineLidocaine

Rationale •Replace susceptible group with a different group without affecting activity•Bio-isostere shows improved pharmacokinetic properties•Bio-isosteres are not necessarily isosteres

Pyrrole ring = Pyrrole ring = bioisostere for amide bioisostere for amide

Examples •Amides and urethanes for esters (see earlier)•Du122290 (dopamine antagonist)

Bio-isosteresBio-isosteres Drug stabilityDrug stability

OMe

EtSO2

NH

NEt

Du122290

OMe

EtSO2

NHO

NEt

Sultopride

Rationale:•Metabolism of drugs usually occurs at specific sites. •Introduce groups at a susceptible site to block the reaction•Increases metabolic stability and drug lifetime

•Oral contraceptive•Limited lifetime

Metabolic blockersMetabolic blockers Drug stabilityDrug stability

MetabolicOxidation

6 MegestrolAcetate

CO

C

H

O

Me

Me

H H

Me OMe

O

MetabolismBlocked

6

Me

Me

O

Me

CO C

H

H H

Me O Me

O

Rationale•Remove susceptible group or replace it with a metabolically stable group e.g. modification of tolbutamide (antibiotic)

Susceptible Susceptible groupgroup

Remove / replace susceptible metabolic groupsRemove / replace susceptible metabolic groups Drug stabilityDrug stability

Metabolism

TOLBUTAMIDE

Me S

O

O

NH C

O

NH CH2CH2CH2CH3 NH CH2CH2CH3C

O

NHS

O

O

Cl

Rapidly excreted - short lifetimeRapidly excreted - short lifetime

Metabolism

HOOC S

O

O

NH C

O

NH CH2CH2CH2CH3

Unsusceptible Unsusceptible groupgroup

NH CH2CH2CH3C

O

NHS

O

O

Cl

Rationale•Used if the metabolically susceptible group is important for binding•Shift its position to make it unrecognisable to metabolic enzyme •Must still be recognisable to target

e.g. Salbutamol

Shifting susceptible metabolic groupsShifting susceptible metabolic groups Drug stabilityDrug stability

CatecholO-MethylTransferase

ShiftGroup

Salbutamol

HO C

OH

OH

CH2 NH C

Me

Me

Me

H

C

Me

Me

Me

NHCHCH2

HO

OH

HO

CatecholO-MethylTransferase

HO CHCH2

OH

MeO

NH C

Me

Me

Me

Inactive

Susceptible group

C

Me

Me

Me

NHCHCH2

HO

OH

HO

Unsusceptible group

ShiftGroup

Salbutamol

HO C

OH

OH

CH2 NH C

Me

Me

Me

H

Metabolically Metabolically susceptiblesusceptible

Rationale•Used to decrease metabolic stability and drug lifetime•Used for drugs which ‘linger’ too long in the body and cause side effects•Add groups known to be susceptible to Phase I or Phase II metabolic reactions

Examples Anti-arthritic agents

Introducing susceptible metabolic groupsIntroducing susceptible metabolic groups Drug stabilityDrug stability

CH2OH

CO2H

SO2Me

N

Cl

NL787257

SO2Me

N

Cl

N CH3L791456

Resistant to metabolismResistant to metabolismExcessively long half lifeExcessively long half life

Examples Anti-asthmatic agents

Notes •Cromakalim produces cardiovascular side effects if it reaches the blood supply•Add metabolic instability such that compound is rapidly metabolised in blood•UK143220 - ester is quickly hydrolysed by esterases to an inactive acid•UK 157147- phenol is quickly conjugated and eliminated

Drug stabilityDrug stability Introducing susceptible metabolic groupsIntroducing susceptible metabolic groups

O

N

OH

Me

Me

NC

O

34

Cromakalim

O

O

OH

Me

Me

NN

NN

CO2Et

N

O

Me

34

UK143220 O

O

OH

Me

Me

SO2

MeN

N

HO

O

UK157147labile

O

O

OH

Me

Me

NN

NN

CO2Et

N

O

Me

34

UK143220

labileO

O

OH

Me

Me

SO2

MeN

N

HO

O

UK157147

Rationale•Used to decrease drug lifetime•Avoids reliance on metabolic enzymes and individual variations

Example Atracurium - i.v. neuromuscular blocking agent

Notes•Stable at acid pH, unstable at blood pH (slightly alkaline)•Self destructs by Hoffmann elimination and has short lifetime•Allows anaesthetist to control dose levels accurately•Quick recovery times after surgery

Introducing chemically susceptible groupsIntroducing chemically susceptible groups Drug stabilityDrug stability

NCH2 CH2

C

O

O

MeO

OMe

HN

(CH2)5MeO OC

OMe

O

CH2 CH2

Me

MeO

OMe

OMe

OMe

HoffmannElimination

Introducing chemically susceptible groupsIntroducing chemically susceptible groups Drug stabilityDrug stability

NMe

CH2

Ph

CH C

HO

R

ACTIVE

CHH2C C

OPh

RNMe-H

INACTIVE

Rationale:•Drug ‘smuggled’ into cell by carrier proteins for natural building block (e.g. amino acids or nucleic acid bases)•Increases selectivity of drugs to target cells and reduces toxicity to other cells

Example Anticancer drugs

Notes:•Alkylating group is attached to a nucleic acid base•Cancer cells grow faster than normal cells and have a greater demand for nucleic acid bases•Drug is concentrated in cancer cells - Trojan horse tactic

Linking a biosynthetic building blockLinking a biosynthetic building block Drug TargetingDrug Targeting

Non selective alkylating agentToxic

N

Cl

Cl

H3C

Uracil Mustard

HN

HN

O

O

N

Cl

Cl

Rationale•Useful for targeting drugs to cancer cells

•Identify an antigen which is overexpressed on a cancer cell

•Clone a monoclonal antibody for the antigen

•Attach a drug or poison (e.g. ricin) to the monoclonal antibody

•Antibody carries the drug to the cancer cell

•Drug is released at the cancer cell

Linking drugs to monoclonal antibodiesLinking drugs to monoclonal antibodies Drug TargetingDrug Targeting

Rationale•Design the antibacterial agent to be highly polar or ionised

•Agent will be too polar to cross the gut wall

•Agent will be concentrated at the site of infection

•Example - highly ionised sulfonamides

Targeting gut infectionsTargeting gut infections Drug TargetingDrug Targeting

Rationale•Increase polarity of the drug•Drug is less likely to cross the blood brain barrier

Targeting peripheral regions over CNSTargeting peripheral regions over CNS Drug TargetingDrug Targeting

Rationale•Toxicity is often due to specific functional groups

•Remove or replace functional groups known to be toxic e.g.•aromatic nitro groups•aromatic amines•bromoarenes•hydrazines•polyhalogenated groups•hydroxylamines

•Vary substituents

•Vary position of substituents

Reducing drug toxicityReducing drug toxicity

Varying substituents•Fluconazole (Diflucan) - antifungal agent

Reducing drug toxicityReducing drug toxicity

Cl

Cl

C

OH NN

NNNN

UK-47265

Substituents variedSubstituents variedLess toxicLess toxic

F

F

C

OH NN

NNNN

Fluconazole

Cl

Cl

C

OH NN

NNNN

UK-47265

Varying substituent position•Dopamine antagonists

Reducing drug toxicityReducing drug toxicity

Inhibits P450 enzymesInhibits P450 enzymes

N

HN

ONC

HN

No inhibition of P450 enzymesNo inhibition of P450 enzymes

N

HN

O

HN

NC

DefinitionInactive compounds which are converted to active compounds in the body

Uses •Improving membrane permeability•Prolonging activity•Masking toxicity and side effects•Varying water solubility•Drug targeting•Improving chemical stability•‘Sleeping agents’

ProdrugsProdrugs

Prodrugs to improve membrane permeabilityProdrugs to improve membrane permeability Esters•Used to mask polar and ionisable carboxylic acids•Hydrolysed in blood by esterases•Used when a carboxylic acid is required for target binding•Leaving group (alcohol) should ideally be non toxic

ExamplesEnalapril for enalaprilate (antihypertensive)

O

NH

O

RO

CO2H

N

CH3

R=Et EnalaprilR=H Enalaprilit

Prodrugs to improve membrane permeabilityProdrugs to improve membrane permeabilityExamplesCandoxatril for Candoxatrilat (protease inhibitor)

Notes•Varying the ester varies the rate of hydrolysis•Electron withdrawing groups increase rate of hydrolysis (e.g. 5-indanyl)•Leaving group (5-indanol) is non toxic

Candoxatrilat

HN

O

OCO2H

OMe

HO

O

HN

O

OCO2H

OMe

O

O

Candoxatril

5-indanyl group

Prodrugs to improve membrane permeabilityProdrugs to improve membrane permeabilityN-Methylation of amines•Used to reduce polarity of amines•Demethylated in liver

Examples - Hexobarbitone

N NH

Me

O O

O

Me

Dopamine •Useful in treating Parkinson’s Disease•Too polar to cross cell membranes and BBB

Levodopa•More polar amino acid•Carried across cell membranes by carrier proteins for amino acids•Decarboxylated in cell to dopamine

Prodrugs to improve membrane permeabilityProdrugs to improve membrane permeability Trojan Horse Strategy•Prodrug designed to mimic biosynthetic building block•Transported across cell membranes by carrier proteins

Example -Levodopa for dopamine

CH2

CH2

HONH2

HO HO

NH2

C

HO

CH2 CO2H

H

Prodrugs to improve membrane permeabilityProdrugs to improve membrane permeability

COOHH2N

L-Dopa

COOHH2N

Enzyme

Dopamine

H2N

Bloodsupply

Braincells

BLOOD BRAIN BARRIER

Example: Azathioprine for 6-mercaptopurine

Suppresses immune responseShort lifetimeEliminated too quickly

Slow conversion to 6-mercaptopurineLonger lifetime

Prodrugs to prolong activityProdrugs to prolong activity Mask polar groupsMask polar groupsReduces rate of excretion

N

H

SH

NN

N

6-Mercaptopurine6-Mercaptopurine N

NN

N

S N

N

O2N

Me

HAzathioprineAzathioprine

Example: Valium for nordazepam

Prodrugs to prolong activityProdrugs to prolong activity

Valium Nordazepam

N-DemethylationN

NO

Me

Cl Cl

N

HO

N

Example: cycloguanil pamoate (antimalarial)

Add hydrophobic groups•Drug concentrated in fat tissue•Slow removal of hydrophobic group•Slow release into blood supply

Prodrugs to prolong activityProdrugs to prolong activity

LipophilicLipophilic

PamoateCycloguanil

N

N

N

Cl

Me

Me

NH3

H3N

CH2

CO2

OH

OH

CO2

Example: Hydrophobic esters of fluphenazine (antipsychotic)

Prodrugs to prolong activityProdrugs to prolong activity

•Given by intramuscular injection•Concentrated in fatty tissue•Slowly released into the blood supply•Rapidly hydrolysed in the blood supply

S

HN CF3

N

N

O (CH2)8CH3

O

fatty ester

Add hydrophobic groupsAdd hydrophobic groups

Example: Aspirin for salicylic acid

Prodrugs to mask toxicity and side effectsProdrugs to mask toxicity and side effects•Mask groups responsible for toxicity/side effects•Used when groups are important for activity

•Analgesic•Causes stomach ulcers •Due to phenol group

•Phenol masked by ester•Hydrolysed by esterases in bloodstream

OH

CO2H

Salicylic acidSalicylic acid

O

CO2H

O

H3C

AspirinAspirin

Example:Cyclophosphoramide for phosphoramide mustard (anticancer agent)

Prodrugs to mask toxicity and side effectsProdrugs to mask toxicity and side effects

CyclophosphoramideCyclophosphoramide Phosphoramide mustardPhosphoramide mustard

PhosphoramidasePhosphoramidase(liver)(liver)

O

P

NH O

N

Cl

Cl

HO

P

Cl

Cl

N

OH2N

Non toxicOrally active

Alkylating agent

Example: Antiviral drugs

Prodrugs to mask toxicity and side effectsProdrugs to mask toxicity and side effects

N

NN

N NH2HO

OHPenciclovir

Viralthymidinekinase

N

NN

N NH2P O

OH

Cell kinases

N

NN

N NH2O

OH

PPP

Notes•First phosphorylation requires viral thymidine kinase•Only activated in virally infected cells•Non-toxic to uninfected cells

LDZ for diazepam

Prodrugs to mask toxicity and side effectsProdrugs to mask toxicity and side effects

Avoids drowsy side effects of diazepam

Ar O

Cl

N

CH3

O

NH

O

NH2

NH2

LDZ

H a) Aminopeptidasea) Aminopeptidaseb) Cyclisationb) Cyclisation

DiazepamDiazepamCl

N

N

O

CH3

Ar

Prodrugs to mask toxicity and side effectsProdrugs to mask toxicity and side effects

Ar O

Cl

N

CH3

O

NH

O

NH2

NH2

LDZ

H

Enz

-lysine

Ar O

Cl

N

CH3

O

NH2

ClDiazepam

-HN

NAr O

CH3

Cl

N

NO

CH3

H

Ar

Cl

-HN

NAr O

CH3

HO

H

+H

Cl

N

N

ArO

CH3

H2O

H

Mechanism of activation

H

Enz

-lysine

Ar O

Cl

N

CH3

O

NH2

+H

Cl

N

N

ArO

CH3

H2O

H

Prodrugs to lower water solubilityProdrugs to lower water solubility•Used to reduce solubility of foul tasting orally active drugs •Less soluble on tongue•Less revolting taste

Example: Palmitate ester of chloramphenicol (antibiotic)

Palmitate ester

O2N

OH

HN

O

O

Cl

ClH

H

O

Esterase

Chloramphenicol

O2N

OH

HN

O

OH

Cl

ClH

H

Prodrugs to increase water solubilityProdrugs to increase water solubility•Often used for i.v. drugs •Allows higher concentration and smaller dose volume•May decrease pain at site of injection

Example: Succinate ester of chloramphenicol (antibiotic)

Succinate ester

O2N

OH

HN

O

O

Cl

ClH

H

O

OHO

Esterase

Chloramphenicol

O2N

OH

HN

O

OH

Cl

ClH

H

Prodrugs to increase water solubilityProdrugs to increase water solubilityExample: Phosphate ester of clindamycin (antibacterial)

Less painful on injection

CO N

HC

C

Cl

CH3

O

H

HOOH

OPO32-

SCH3

H

H

H

MeN H

H

HH

CH3CH2CH2

Prodrugs to increase water solubilityProdrugs to increase water solubilityExample: Lysine ester of oestrone

Notes: •Lysine ester of oestrone is better absorbed orally than oestrone•Increased water solubility prevents formation of fat globules in gut•Better interaction with the gut wall•Hydrolysis in blood releases oestrone and a non toxic amino acid

H2NO O

NH2

OMe

Prodrug

HH

H

H

H2NO OH

NH2

OMe

HO

Lysine Oestrone

H

H HH

+H2N

O O

NH2

OMe

Prodrug

HH

H

H

Prodrugs used to target drugsProdrugs used to target drugs

Example: Hexamine

Notes: •Stable and inactive at pH>5•Stable at blood pH•Used for urinary infections where pH<5 •Degrades at pH<5 to form formaldehyde (antibacterial agent)

NN

N

N

'Locked'Nitrogen

Prodrugs to increase chemical stabilityProdrugs to increase chemical stability

Example: Hetacillin for ampicillin

Notes: •Ampicillin is chemically unstable in solution due to the-NH2 group attacking the-lactam ring•Nitrogen atom in heteracillin is locked up within a heterocyclic ring

HN N

H3C CH3

Ph O

N

S

CH3

CH3

OOH

O

Hetacillin

H2N HN

H3C CH3

Ph O

N

S

CH3

CH3

OOH

O

O

Ampicillin

Prodrugs activated by external influencesProdrugs activated by external influences-sleeping agents-sleeping agents

Example: Photodynamic therapy - Foscan

Notes: •Inactive and accumulates in cells•Activated by light - method of targeting tumour cells•Foscan is excited and reacts with oxygen to produce toxic singlet oxygen•Cell destruction is caused by singlet oxygen

NH N

HNN

OH

HO

HO

OH

HH

HH

Definition:Drugs which have a beneficial effect on the activity or pharmacokinetic properties of another drug

Drug Alliances - SynergismDrug Alliances - Synergism

Definition:A drug that is added to ‘protect’ another drugExample: Carbidopa

Notes•Carbidopa protects L-dopa•It inhibits the decarboxylase enzyme in the peripheral blood supply•It is polar and does not cross the blood brain barrier•It has no effect on the decarboxylation of L-Dopa in the CNS•Smaller doses of L-dopa can be administered - less side effects

Sentry DrugsSentry Drugs

Other examples: Other examples: Clavulanic acid and candoxatrilClavulanic acid and candoxatril

L-DOPA DOPAMINEENZYME

INHIBITION

CARBIDOPA

CNHNH2

HO

Me

HO

CO2H

Example:Adrenaline and procaine (local anaesthetic)•Adrenaline constricts blood vessels at the injection area•Procaine is localized at the injection area

Localizing drugs to a target areaLocalizing drugs to a target area

Increasing absorptionIncreasing absorption

Notes•Administered with analgesics in the treatment of migraine•Increases gastric motility and causes faster absorption of analgesics•Leads to faster pain relief

Example: Metoclopramide

Cl

NH2

OCH3

OHN

N(Et)2