Pharmacological Factors Affecting Drug Metabolism

Glutathione+SWtransferase+(GST)+

• % Glutathione% >>% an% endogenous% protecGve% substance% (tripepGde,% GlyTCysTGlu)%for%removal%of%potenGally%toxic%electrophilic%compounds% • % Glutathione's% SH% (thiol)% group% aeacks% electrophilic% (reacGve)%%% CTatoms%>>%scavenger%of%electrophiles%>>%nonTtoxic%conjugates%% • %End%product,%a%derivate%of%mercapturic%acid%>>%excreted%in%urine%

Metabolism/+Biotransforma2on+

• % Phase% I% reacGons% convert% parent% drug% into% a% more% polar% metabolite% by%adding%funcGonal%groups%(TOH,%TSH,%TNH2,%TCOOH,%etc.)% • % Phase% II% reacGons:% ConjugaGon% with% endogenous% substrates% (e.g.% glucuronic%acid,%amino%acids)%%

AcGvaGon% of% inacGve% drug% (prodrug):%

• AcGvaGon% of% inacGve% drug% (prodrug):% needs% metabolic% conversion%in%the%body%to%one%or%more%acGve%metabolites% • Advantages%of%prodrugs%% • More%stable,%beeer%bioavailability,%less%side%effect/toxicity%%

Irreversible+CYP+Inhibi2on++

• BioacGvaGon%is%a%prerequisite%in%most%of%the%cases%% • Covalent%interacGon%of%metabolically%generated%reacGve% intermediate%>>%react%with%CYP%apoprotein/%heme%moiety% >>%Irreversible%inacGvaGon%of%CYP%enzyme% • AcGvity%of%the%enzyme%doesn't%recover%with%diluGon%or% removal%of%the%inhibitor%% • Examples% of% irreversible% inhibitors% (suicide% inhibitors% / mechanismTbased% inhibitors):% bergamotn% (CYP3A4),%% Gclopidine% (CYP2C19),% ritonavir% (CYP3A4),% clarithromycin% (CYP3A4)%

Rela2ve+Contribu2ons+of+CYP+Isoforms++ to+Metabolism+of+Drugs+in+Clinical+Use+

• CYP1A2,%CYP2C8/9,%CYP2D6%and%CYP3A4/5%are%the%major%enzymes% • CYP3A4%alone%is%responsible%for%the%metabolism%of%>50%%of%the% prescripGon%drugs%metabolized%by%the%liver%

Consequences+of+Inhibi2on+

• Decreased%rate%and%extent%of%metabolism% • ReducGon%in%metabolite%formaGon% • Increase%in%plasma%concentraGon%of%parent%drug%>>%↑%Bioavailability%% Therapeu2c+Implica2ons+of+Inhibi2on+ • Increased/prolonged%efficacy%or%parent%drugTinduced%toxicity%% - Dosing%rates%may%need%to%be%decreased%to%maintain%effecGve%plasma%conc.% • Drugs%with%acGve%metabolites%exert%lower%drug%effect%% • InhibiGon% of% reacGve% metabolite% formaGon% >>% lesser% metaboliteT mediated%toxicity%

Inhibi2on+of+Drug+Metabolism++

• Drug%metabolism,%an%enzymaGc%process,%is%subjected%to% inhibiGon% • Drugs,%natural%health%products,%and%dietary%consGtuents% can%inhibit%the%metabolism%of%other%drugs% • Types%of%inhibitors%% - CompeGGve%or%reversible%% - Irreversible%or%mechanismTbased%Inhibitors%%

Consequences+of+Altered+Drug+Metabolism+ • DrugTdrug%/%drugTherb%interacGons%

• DrugTdrug%/%drugTherb%interacGons% - Enzyme%inducGon%% - Enzyme%inhibiGon%% • Increased%adverse%effects%from%toxic%metabolites%% - Higher% level% of% reacGve% metabolites% can% arise% from% enzyme% inducGon% - Imbalance%between%formaGon%and%detoxificaGon%of%reacGve%% metabolites%%

EnzymaGcally% formed% metabolites,% which% are% more% reacGve% than

• EnzymaGcally% formed% metabolites,% which% are% more% reacGve% than% the%parent%substance%>>%ReacGve%toxic%intermediate%or%metabolite% • The% most% significant% toxicological% effects% of% xenobioGcs% are% from% reacGve%metabolites%>>%Carcinogenesis,%hepatotoxicity%% - React%with%nucleophilic%sites% - NH2%and%-%COOH%groups%(DNA,%RNA,%proteins)%>>%Toxicity%% - SH%groups%(cysteines%of%glutathione)%>>%Neutralizes%reacGvity%%% • Specific% structural% features% lead% to% toxic% intermediate/metabolite% formaGon%% • Epoxides%and%free%radical%species%(unpaired%electrons)%are%shortTlived% and%hence%highly%reacGve% • Drugs,%solvents,%pesGcides,%environmental%toxicants%

Induc2on+of+Drug+Metabolism+ • Exposure% to% certain% substances%

• Exposure% to% certain% substances% (e.g.,% drugs,% environmental% pollutants,% diet)% results% in% increased% protein% expression% of% drug% metabolizing%enzymes%>>%Enzyme%inducGon%% - SGmulates%protein%synthesis%(enhance%transcripGon)%or%decrease%in%rate%of% protein%degradaGon%% • InducGon%>>%accelerated%biotransformaGon%with%a%corresponding% reducGon%in%parent%drug% • Drugs,% natural% health% products,% dietary% components,% environmental%factors,%lifestyle%% • AutoinducGon,% e.g.,% anGconvulsants% (e.g.% phenobarbital,% carbamazepine)%

Age+ • Fetal% metabolism% :

• Fetal% metabolism% :% % low% or% negligible% levels% of% the% CYP% ,% UGT,% SULT,% or% GST% enzymes% >>% increase% the% exposure% to% drug% (>% toxic% level)% • Newborns%and%infants:%metabolize%drugs%relaGvely%efficiently%but% at%a%rate%generally%slower%than%adults%(e.g.%glucuronidaGon)%%% • Slower% metabolism% with% aging% % >>% decreased% renal% clearance% % >>% ↑% bioavailability% and% prolonged% t1/2% >>% increased% pharmacologic/% toxic%acGvity%% - Reduced%enzymaGc%acGvity%or%availability%of%endogenous%cofactors% - Reduced%cardiac%output%%>>%decreased%hepaGc%blood%flow% - Diazepam,%propranolol,%morphine,%amitriptyline,%theophylline%

Gene2c+Varia2on+

• GeneGc% factors% that% influence% enzyme% levels% (polymorphism)% account%for%interindividual%differences%in%drug%metabolism%% • Polymorphisms% lead% to% subpopulaGons% with% different% drug% metabolizing%abiliGes% - Lack%of%acGvity% - ReducGon%in%catalyGc%ability% - Enhanced%acGvity% • GeneGc% polymorphisms% >>% altered% metabolism% and% pharmacokineGcs%>>%therapeuGc%failure%or%adverse%drug%reacGon% • Ethnic%differences%in%the%frequency%of%the%polymorphism%

Phase+II+Conjuga2on+Reac2ons+

• Glucuronida2on+ by+ UGT:+ on% TOH,% TCOOH,% TNH2,% TSH% groups% • Sulfa2on+by+SULT:+on%TNH2,%TSO2NH2,%TOH%groups% • Acetyla2on+by+NAT:%on%TNH2,%TSO2NH2,%TOH%groups% • Amino+acid+conjuga2on:+on%TCOOH%groups% • Glutathione+conjuga2on+by+GST:+on%epoxides%or%organic% halides%

Consequences+of+Induc2on+

• Increased%rate%and%extent%of%metabolism% • Enhanced%oral%first%pass%metabolism% • Decrease%in%drug%plasma%concentraGon%%>>%Reduced%bioavailability% % Therapeu2c+Implica2ons+of+Induc2on++ • Most%drugs%exhibit%decreased%efficacy%due%to%rapid%metabolism% - Dosing%rates%may%need%to%be%increased%to%maintain%effecGve%plasma%conc.% • Drugs%with%acGve%metabolites%may%display%increased%drug%effect%% • Drugs% metabolically% transformed% to% reacGve% metabolites,% enzyme% inducGon%exacerbate%metaboliteTmediated%toxicity% • Tolerance% occurs% if% the% drug% induces% its% own% metabolism% (autoinducGon),%e.g.%carbamazepine,%rifampin%%

Factors+Affec2ng+Biotransforma2on%

• Induc2on+of+drug+metabolizing+enzymes++ • Inhibi2on+of+drug+metabolizing+enzymes++ • Age%% • Gender%% • Pregnancy%% • Disease%states% • GeneGc%factors%%%

Reversible+CYP+Inhibi2on%

• Inhibitors%that%can%reversibly%bind%and%dissociate%from% enzyme%(usually%nonTcovalent%interacGon)%% - AcGvity% of% enzyme% recovers% when% inhibitor% diluted% out% % • Inhibitory% effect% can% be% reversed% by% increasing% the% substrate%conc./decreasing%the%conc.%of%inhibitor%% • Reversible% inhibitors:% ketoconazole% (CYP3A4),% sulfaphenazole%(CYP2C9),%quinidine%(CYP2D6)%%

DrugWDrug/+DrugWHerb+Interac2ons+ • Knowledge% of% metabolic% pathway% >>% PredicGon% of% drugTdrug% interacGons% • Nature%of%a%drug%or%herb%>>%Either%inducer,%inhibitor,%or%no%effect%

• Knowledge% of% metabolic% pathway% >>% PredicGon% of% drugTdrug% interacGons% • Nature%of%a%drug%or%herb%>>%Either%inducer,%inhibitor,%or%no%effect%% % • Increase% in% metabolism% as% a% result% of% sGmulaGon% of% protein% synthesis%(inducGon%)% - Decreased%AUC,%Cmax%>>%Decrease%in%efficacy%or%treatment%failure%% - Drug%regimen%with%inducing%agents%require%a%higher%dose%of%target%drug%% • Decrease%in%metabolism%as%a%result%of%inhibiGon%drug%metabolizing% enzymes%(inhibiGon)% - Increased%AUC,%Cmax%%(impaired%eliminaGon%of%the%target%drug)% - ProlongaGon/potenGaGon%of%its%pharmacologic%or%toxic%effects%% - CoTadministraGon%of%CYP%inhibitor%leads%to%lower%dose%or%contraindicaGon%%

Disease+Factors+

• Liver%Disease:%Cirrhosis,%carcinoma,%viralT%or%drugTinduced%hepaGGs%% ! Decreased%enzyme%acGvity%>>%impaired%metabolism%>>%↑%bioavailability% ! Exaggerated%pharmacological%response%and%adverse%effects% ! Increased%risk%to%hepatotoxic%drugs%(e.g.,%methotrexate,%valproic%acid)% • Cardiac% diseases% >>% limit% blood% flow% to% the% liver% >>% may% impair% disposiGon%of%drugs%(e.g.%imipramine,%propranolol,%morphine)% • Hypothyroidism% increases% the% t1/2% of% digoxin,% methimazole,% and% β% blockers,%whereas%hyperthyroidism%has%the%opposite%effect%%% • Inflammatory% mediators% (e.g.,% cytokines% and% NO% associated% with% bacterial/viral% infecGons,% cancer,% or% inflammaGon)% >>% impair% drug% metabolism%by%inacGvaGng%CYPs%and%enhancing%their%degradaGon%

Sites+of+Drug+Metabolism+ • Liver%and%intesGne:%Major%sites,%well%organized%with% several%enzyme%systems%

• Liver%and%intesGne:%Major%sites,%well%organized%with% several%enzyme%systems% • Other%organs%involved%in%drug%metabolism:%% • Lungs,%kidney,%skin,%brain%% • IntesGnal%esterases%and%lipases%carry%out%hydrolysis% of%many%ester%prodrugs% • First%Pass%Effect%(Metabolism)% • Drug% is% metabolized% in% the% gut% and% liver% before% it% reaches% the%systemic%circulaGon% • Reduces%bioavailability%%

Why+Drug+Biotransforma2on+is+Necessary?+% • Metabolism% generates% polar% and% ionized% molecules

• Metabolism% generates% polar% and% ionized% molecules% >>% less% able% to% cross% cellular% membranes% >>% decreased% tubular%reTabsorpGon%>>%readily%excreted%from%body% % • Facilitates%terminaGon%of%drug%acGon%% • A%criGcal%defense%mechanism%of%the%body% • Plays% an% important% role% in% the% design% and% therapeuGc% use%of%drugs

List+of+Phase+II+Enzyme+Inducers+

• Not%highly%inducible%like%phase%I%enzymes%% • Data% on% inducGon% of% human% phase% II% enzymes% are% relaGvely%sparse%and%is%an%ongoing%topic%of%research%

Biotransforma2on+in+Drug+Disposi2on++ Phase% I% metabolites% are% mostly% inacGve%

• Phase% I% metabolites% are% mostly% inacGve% (but% occasionally% acGvaGon% does% occur)%%>>%May%be%sufficiently%polar%to%be%excreted%readily% • Phase%II%reacGons%most%oaen%abolish%biological%acGvity%

Enzymes+in+Biotransforma2on+Reac2ons+

• Phase+I+ - Cytochrome++P450+(CYP)+ - Alcohol%dehydrogenase%and%aldehyde%dehydrogenase%% - Epoxide%hydrolase%% - Esterase,%amidase,%azoreductase%% - Xanthine%oxidase,%monoamine%oxidase% % • Phase+II++ - Uridine+diphosphate+(UDP)WGlucuronosyltransferases+(UGT)+ - Sulfotransferase%(SULT)% - Glutathione%STtransferase%(GST)% - NTacetyltransferase%(NAT)% - Methyltransferase%%

• RelaGve%contribuGons%of%different%phase%I%and%phase%II%enzymes

• RelaGve%contribuGons%of%different%phase%I%and%phase%II%enzymes%to% metabolism%of%drugs%in%clinical%use% • CYP% and% UGT% catalyze% the% bulk% of% the% hepaGc% drug% and% xenobioGc% metabolism%

Gender% • SexTdependent% differences% in% drug% metabolism% exist% in% humans%but%no%set%paeern%%

• SexTdependent% differences% in% drug% metabolism% exist% in% humans%but%no%set%paeern%% - Generally,%males%metabolize%compounds%faster%than%females% - Ethanol,%propranolol,%benzodiazepines,%estrogens,%salicylates% • HepaGc%adverse%drug%reacGons%are%more%common%in%females%% • Females% are% more% suscepGble% to% acetaminophen,% halothane,% nitrofurantoin,%diclofenac%and%sulindac% • GenderTspecific%differences%are%most%pronounced%at%the%Gme% of%ovulaGon%and%the%luteal%phase%of%menstruaGon% - Women%on%contracepGves%metabolize%drug%at%a%slower%rate%% - Phenytoin% metabolism% is% significantly% accelerated% just% before% the%beginning%of%menstrual%cycle%

Effect+of+CYP+Induc2on+on+Bioavailability++

• St%John's%wort%decreases%the%bioavailability%of%RT%and%STverapamil% through%inducGon%of%the%CYP3A4Tmediated%firstTpass%metabolism%