Drug Metabolism.pdf

B.Pharm 4sem

1. Presented by:
Dr. Joohee Pradhan
Assistant Professor, Dept. of Pharmaceutical Sciences, MLSU,
Udaipur.

2. Outline of Presentation:
Definition
Consequences
Types
Metabolizing Enzymes
Phase-I Metabolism with examples
Phase-II Metabolism with examples
Factors affecting drug metabolism

3. BIOTRANSFORMATION (Metabolism)
Biotransformationmeanschemicalalterationofthedrug
inthebody.Itisneededtorendernon-polar(lipid-
soluble)compoundspolar(lipidinsoluble/watersoluble)
sothattheyarenotreabsorbedintherenaltubulesand
areexcreted.
Mosthydrophilicdrugs,e.g.streptomycin,neostigmine,
pancuronium,etc.arelittlebiotransformedandarelargely
excretedunchanged.
Mechanismswhichmetabolizedrugs(essentiallyforeign
substances/xenobiotics)havedevelopedtoprotectthebody
fromingestedtoxins.
Theprimarysitefordrugmetabolismisliver;othersare—
kidney,intestine,lungsandplasma.
Biotransformationofdrugsmayleadtothefollowing
consequences:

4. 1.Inactivation: Mostdrugsandtheiractive metabolitesare
renderedinactiveorlessactive,e.g.ibuprofen,
paracetamol,lidocaine,chloramphenicol,propranololand
itsactivemetabolite4-hydroxypropranolol.
2.Activemetabolitefromanactivedrug: Manydrugs
havebeenfoundtobepartiallyconvertedtooneormore
activemetabolite;theeffectsobservedarethesumtotalof
thatduetotheparentdruganditsactivemetabolite(s)thatduetotheparentdruganditsactivemetabolite(s)
(seeTable1).
3.Activationofinactivedrug: Fewdrugsare inactiveas
suchandneedconversioninthebodytooneormoreactive
metabolites.Suchadrugiscalleda prodrug(seeTable2).
Theprodrugmay offeradvantagesovertheactiveformin
beingmorestable,havingbetterbioavailabilityorother
desirablepharmacokineticpropertiesorlesssideeffectsand
toxicity.Someprodrugsareactivatedselectivelyatthesite
ofaction.

5. Table 1: Active metabolites from active drugs

6. Table 2: Prodrugsand their active froms

7. Types of Biotransformation
reactions
Phase-I/Nonsynthetic
reactions
Phase-
II/Synthetic/Conjugation
reactions
1. Oxidation
2. Reduction
3. Hydrolysis
4.Cyclization
5. Decyclization
1. Glucuronideconjugation
2.Acetylation
3.Methylation
4.Sulfate Conjugation
5.Glycine Conjugation
6.Glutathion Conjugation
7.Ribonucleotide/Ribonucle
osideSynthesis

8. The drug metabolizing enzymes
The drug metabolisingenzymes are divided into two types:
A.Microsomalenzymes
B.Non-microsomalenzymes
A. Microsomalenzymes
Thesearelocatedonsmoothendoplasmicreticulum(a
systemofmicrotubulesinsidethecell),primarilyinliver,also
inkidney,intestinalmucosaandlungs.
Themonooxygenases,cytochromeP450,UGTs,epoxide
hydrolases,etc.aremicrosomalenzymes.
Theycatalysemostoftheoxidations,reductions,hydrolysis
andglucuronideconjugation.
Microsomalenzymesare induciblebydrugs,dietandother
agencies.

9. B.Non-microsomalenzymes:
Thesearepresentinthecytoplasmandmitochondriaof
hepaticcellsaswellasinothertissuesincludingplasma.
Theesterases,amidases,someflavoproteinoxidasesand
mostconjugasesarenonmicrosomal.
Reactionscatalysedare:Someoxidationsandreductions,Reactionscatalysedare:Someoxidationsandreductions,
manyhydrolyticreactionsandallconjugationsexcept
glucuronidation.
The nonmicrosomalenzymes are not inducible but many
show genetic polymorphism (acetyl transferase,
pseudocholinesterase).

10. Sitesof drug metabolism: Hepatic and Extra-hepatic enzymes

11. Phase-I Metabolic reactions
1.Oxidation:
Thisreactioninvolvesadditionofoxygen/negatively
chargedradicalorremovalofhydrogen/positively
chargedradical.Oxidationsarethemostimportantdrug
metabolizingreactions.
Oxidationisthemainprocessofmetabolism.Oxidationisthemainprocessofmetabolism.
Producesunstableintermediates-Epoxides,
Superoxides,Quinones

12. The Role of CytochromeP450 (CYP) in Oxidative reactions
ThecytochromeP450(CYP)enzymesarealsoknownas
microsomalmixedfunctionoxidases.
TheCYPenzymesaremembrane-boundproteins,presentin
thesmoothendoplasmicreticulumofliverandothertissues.
TheyarethemostimportantenzymesforPhaseI
biotransformationofdrugs.biotransformationofdrugs.
Theseenzymescontainahemeprostheticgroup,where
hemegroupistheiron-porphyrinunit.
Theoxidizingsiteintheseenzymesisthehemecentre,and
isresponsiblefortheoxidationofhydrophobiccompoundsto
hydrophilicormorepolarmetabolitesforsubsequent
excretion.

13. ThesearecalledCYP450becausetheironinreducedstate
canbindwithhighaffinitytocarbonmonoxideandthisCO-
boundCYPcomplexshowsalargeabsorbanceat450nm.
CYPscatalyzethetransferofoneatomofoxygentoa
substrateproducinganoxidisedsubstratealongwitha
moleculeofwater,asshownintheequationbelow:moleculeofwater,asshownintheequationbelow:

14. The catalytic cycle of CYP450, leading to the oxidation of substrate.

15. 1. Oxidation Reactions–9 types
1.OXIDATION AT NITROGEN ATOM
RNH
2
RNHOH
Examples:
•Chlorpheniramine
•Dapsone
O
NH
2
N
H OH
S OO
NH
2
S OO
NH
2
Dapsone Dapsone Hydroxylamine

16. 2.OXIDATION AT SULPHUR ATOM:
R1 R1
R2 R2
Examples:
•Chlorpromazine
•Chloramphenicol
SH
2
O
S=O
•Chloramphenicol
N
S
Cl
N
CH
3H
3C
Chlorpromazine
N
S
Cl
N
CH
3H
3C
Chlorpromazine-S-Oxide
O

17. 3.ALIPHATIC HYDROXYLATION: Hydroxyl group added to drug:
RCH
2
CH
3
RCHOHCH
3
Examples:
•Salicylic acid to Gentisicacid
•Ibuprofen
•Tolbutamide, Chlorpropamide
O

18. 4.AROMATIC HYDROXYLATION:
Examples:
•Acetanilide
•Phenytoin
R R
OH
O
•Phenytoin
•Phenobarbitone
•Propranolol

19. 5.DEALKYLATON AT OXYGEN ATOM:
ROCH
3
ROH + CH
2
O
Examples:
Phenacetinto Paracetamol
O
O
O
O
OH
O
Phenacetin
N
H
O
Paracetamol
O
N
H
O

20. 6.DEALKYLATON AT NITROGEN ATOM:
RNHCH
3
RNH
2
+ CH
2
O
Examples:
Methamphetamine to Amphetamine
O

21. 7.DEALKYLATON AT SULPHUR ATOM:
RSCH
3
RSH +CH
2
O
Examples:
6-Methyl mercaptopurineto Mercaptopurine
O

22. 8.OXIDATIVE DEAMINATION:
RCHNH
2
R RCOR +NH
3
Example:
Amphetamine
O

23. 9.DESULFURATION:
R1 R1
R2 R2
Examples:
Parathion to Paraoxon
P=S
O
P=O

24. 2. Reduction Reactions
Thisreactionistheconverseofoxidationandinvolvescytochrome
P-450enzymesworkingintheoppositedirection.Alcohols,
aldehydes,quinonesarereduced.
1.NitroReduction:
R-NO
2
R-NH
2
Example:ClonezepamandNitrazepamExample:ClonezepamandNitrazepam

25. 2.KetoReduction:
R-CO-R R-CHOH-R
Example:Phenylacetone

26. 3.AzoReduction:
Example:Prontosil

27. 3. Hydrolytic Reactions
Thisiscleavageofdrugmoleculebytakingupamoleculeofwater.
esterase
Ester+H2O Acid+Alcohol
Similarly,amidesandpolypeptidesarehydrolysedbyamidasesand
peptidases.Inaddition,thereareepoxidehydrolaseswhichdetoxify
epoxidemetabolitesofsomedrugsgeneratedbyCYPoxygenases.
Hydrolysisoccursinliver,intestines,plasmaandothertissues.Hydrolysisoccursinliver,intestines,plasmaandothertissues.
Examplesofhydrolyseddrugsarecholineesters,procaine,
lidocaine,procainamide,aspirin,carbamazepine-epoxide,pethidine,
oxytocin.

29. 4. Cyclization
This is formation of ring structure from a straight chain compound,
e.g. proguanil.
5. Decyclization
Thisisopeningupofringstructureofthecyclicdrugmolecule,e.g.
barbiturates,phenytoin.Thisisgenerallyaminorpathway.

30. Synthetic/Conjugation/ Phase-II Reactions
ThePhaseIIreactionsfollowPhaseIreactions,andoccur
mostlyintheproductsderivedfromPhaseIreactions.
Inthesereactions,asuitablemoietysuchasglucuronicacid,
glutathione,sulphate,glycine,etc.,getconjugatedtothe
metabolitesofPhaseIreaction.
ThePhaseIIreactionsaretherealdrugdetoxification
pathways.pathways.
Thesearealsotermedasconjugationreactions,becausethe
metabolitesareconjugatedwiththeabove-mentioned
moietieswhicharelargeinsizeandstronglypolarinnature.
Thesereactionsarecatalyzedbyavarietyoftransferase
enzymes,suchasuridinediphosphate(UDP)-
glucoronsyltransferases,sulfotransferases,glutathione
transferasesetc.

31. 1.Glucuronideconjugation
Thisisthemostimportantsyntheticreactioncarriedoutbya
groupofUDP-glucuronosyltransferases(UGTs).
Compoundswitha hydroxylorcarboxylicacidgroupare
easilyconjugatedwithglucuronicacidwhichisderivedfrom
glucose.
GlucuronidationincreasesthemolecularweightofthedrugGlucuronidationincreasesthemolecularweightofthedrug
whichfavoursitsexcretioninbile.

32. 2. Acetylation
Compoundshaving aminoorhydrazine residuesare
conjugatedwiththehelpofacetylcoenzyme-A,e.g.
sulfonamides,isoniazid,PAS,dapsone,hydralazine,
clonazepam,procainamide.
MultiplegenescontroltheN-acetyltransferases(NATs),and
rateofacetylationshowsgeneticpolymorphism(slowandfast
acetylators).

33. 3.Methylation
Theaminesandphenolscanbemethylatedbymethyl
transferases(MT);methionineandcysteineactingasmethyl
donors,e.g.adrenaline,histamine,nicotinicacid,
methyldopa,captopril,mercaptopurine.
COMT-Catechol-O-methyl transferase

34. 4.Sulfate conjugation
Thephenoliccompoundsand steroidsaresulfatedby
sulfotransferases(SULTs),e.g.chloramphenicol,
methyldopa,adrenalandsexsteroids.

35. 5. Glycineconjugation
Salicylates,nicotinicacidandotherdrugshaving carboxylic
acidgroup areconjugatedwithglycine,butthisisnota
majorpathwayofmetabolism.

36. 6. Glutathione conjugation
Thisiscarriedoutbyglutathione-S-transferase(GST)
formingamercapturate.
Itisnormallyaminorpathway.However,itservesto
inactivatehighlyreactivequinoneorepoxideintermediates
formedduringmetabolismofcertaindrugs,e.g.
paracetamol.
Whenlargeamountofsuchintermediatesareformed(in
poisoningorafterenzymeinduction),glutathionesupplyfalls
short—toxicadductsareformedwithtissueconstituents→short—toxicadductsareformedwithtissueconstituents→
tissuedamage.

37. Ribonucleoside/nucleotide synthesis
Thispathwayisimportantfortheactivationofmanypurine
andpyrimidineantimetabolitesusedincancerchemotherapy
.
Metabolic activation of 5-fluorouracil to 5-FdUMP (5-Fluoro deoxyuridinemono
phosphate).

38. Fig.Simultaneousand/orsequentialmetabolismofadrugby
phaseIandphaseIIreactions

39. Factors affecting Drug Metabolism
Biotransformationis significantly affected by a number
offactors, these include:
Enzyme Induction
Enzyme Inhibition
PresystemicMetabolism/First pass effect/Route of
AdministrationAdministration
Genetic Variations
Species Differences
Exposure to Pollutants from Environment or Industry
Age
Sex

40. 1.EnzymeInduction: Therateofmetabolismincreasesasenzyme
inductiontakesplace.Thedrugswhichbringaboutthesechangesare
knownasenzymeinducers .Someexamplesincludeanticonvulsants
likephenytoin,carbomycinandchronicalcoholism.Othersinclude
varioussedatives,hypnotics,tranquilizersandinsecticides.
Example:Phenobarbitoneisusedinseizuresandepilepsy.Itisalso
anenzymeinducer.Ifitisadministeredtopatientstakingwarfarin,
therapeuticfailuremightoccur,leadingtoincreasedbleeding
tendency.
2.EnzymeInhibition: Theprocessinwhichdrugmetabolizing
capacityofcytochromeisdecreasedisknownasenzymeinhibition.capacityofcytochromeisdecreasedisknownasenzymeinhibition.
Therateofmetabolismisdecreased.Drugsbringingaboutthese
changesareknownas enzymeinhibitors .Examplesinclude
ketoconazole-antifungaldrug,cimetidineandverapamil-calcium
channelblocker.
Example:
•Sulfonamidesdecreasethemetabolismofphenytoinsothatitsblood
levelsbecometoxic.
•Cimetidinedecreasesthemetabolismofpropanololleadingto
enhancedbradycardia.

41. 3.PresystemicMetabolism/Firstpasseffect/Routeof
Administration:
•Biotransformationofdrugbyliverorgutenzymesbeforecompound
reachessystemiccirculation
•Resultsinlowersystemicbioavailbilityofparentcompound,
diminishedtherapeuticresponse.
•Mostofthedrugsaremetabolizedwithintheliver.Changingthe
routeofadministration(IVorSublingually.)mightbypassthefirst
passmetabolism.
•Propanololis80%metabolizedbeforereachingsystemiccirculation.
4.GeneticVariations:
•Interindividualvariationsmightoccur;drugsbehavedifferentlyin
differentindividualsduetoabsentormalformedgenes.
•Mostlynonmicrosomalenzymeshowgeneticvariations.
•Examplesincludesuccinylcholine,whichisaskeletalmuscle
relaxant.Itismetabolizedbypseudocholineesterase.Somepeople
lackthisenzyme,duetowhichlackofmetabolismofsuccinylcholine
mightoccur.Whenadministeredinthoseindividuals,prolonged
apneamightresult.

42. Differentgroupsofpopulationsmightbeclassifiedasfast
metabolizersandpoormetabolizersofdrugs.Forcertaindrugs,like
isoniazid,fastacetylatorsaswellasslowacetylatorsarepresent.
Fastacetylatorscauserapidacetylation,whilepoormetabolizers
metabolizeless.Hepaticacetyltransferrasecatalyzesacetylation.
Slowacetylationmightoccurduetogeneticmalformationleadingto
decreasedproduction.
5.SpeciesDifferences: Mostestablishedinanimals.Some
metabolizedrugsrapidly.Ratsandrabbitsmetabolizedrugsmore
efficientlythanhumans.Certainspeciesofrabbitsfeedonefficientlythanhumans.Certainspeciesofrabbitsfeedon
Belladonna,andhaveatropinasetotoleratetheeffectsofatropine.
6.ExposuretoPollutantsfromEnvironmentorIndustry:
•Cigarettesmokersmightactasenzymeinducers.
•Chronicalcoholismmightleadtoenzymeinductionaswell.
Similarly,pesticidesorinsecticidesmayactasenzymeinducers.
•Inhotandhumidclimatebiotransformationisdecreasedandvice
versa.
•Athighaltitude,decreasedbiotransformationoccursdueto
decreasedoxygenleadingtodecreasedoxidationofdrugs.

43. 7.Age:
•Ageplaysaveryimportantrole.Extremeagegroups(veryyoungand
veryold)behavealmostthesame.
•Bothmicrosomalandnonmicrosomalenzymesaredeficientinthe
newborn,especiallypremature,makingthemmoresusceptibleto
manydrugs,e.g.chloramphenicol,opioids.
•Thisdeficitismadeupinthefirstfewmonths,morequicklyincaseof
oxidationandotherphaseIreactionsthanincaseofglucuronideand
otherconjugationswhichtake3ormoremonths.
8.Sex:
•MalehaveahigherBMRascomparedtothefemales,thuscan
metabolizedrugsmoreefficiently,e.g.salicylates(othersmight
includeethanol,propanolol,benzodiazepines).
•Females,duringpregnancy,haveanincreasedrateofmetabolism.
Thus,thedrugdosehastobeincreased.Afterthepregnancyisover,
thedosageisdecreasedbacktonormallevels.Exampleincludes
phenytoin,whosedosehastobeincreasedduringpregnancy
(speciallysecondandthirdtrimester).

44. Further Readings:
Wilson,C.O.,Gisvold,O.,&Doerge,R.F.(1982).Wilsonand
Gisvold'sTextbookoforganicmedicinalandpharmaceutical
chemistry.Philadelphia:Lippincott.