CYTOCHROME P-450 IN THERAPEUTICS

Dr. DIVYA G KRISHNAN CALICUT MEDICAL COLLEGE

CASE HISTORY…………………………. A 74 year old woman on warfarin & metoprolol for AF brought

with symptoms of depression.The treating doctor prescribed Fluoxetine.

3 days later patient brought to casualty dizzy & complained of difficulty in urinating.Cathetrisation of bladder yielded 2litres of dark urine.Her PT/INR was 4.

On discussion with a colleague,the treating doctor learned that Fluoxetine inhibits CYTOCHROME P 450 enzymes responsible for the metabolism of patient’s other medications.

CONTENTS……………………

Introduction History Location Structure Mechanism of action Nomenclature Properties Clinical relevance Summary

INTRODUCTION

Humans constantly exposed to xenobiotics(usually non polar) that can cause harm if not eliminated.

Biotransformation renders non polar compounds polar and helps in their elimination.

Biotransformation occurs in 2phases(phase1 &phase 2)

Cytochrome p-450 is a superfamily of enzymes that catalyse most of the oxidation reactions of phase 1.

HISTORY…………………………………….

1955 : First evidence of, presence of an enzyme in the liver endoplasmic reticulum, capable of oxidising xenobiotics.

1964 : Enzyme demonstrated to be a hemoprotein and was named CYTOCHROME P 45O

1985 : Full structure of P 450 cam from bacteria Pseudomonas putida obtained.Now crystal structures of most enzymes available.

Origin of the name CYTOCHROME P 450

Cytochrome=cellular pigment P 450 denotes the

Being a hemoprotein capable of absorption peak at

oxidation reactions,it came to be 450nm(SORET PEAK)

called a cytochrome.

CYTOCHROME P 450

HISTORY CONTINUED…………………….

CYP 450

LOCATION……………………………

HEPATOCYTE

LOCATION……………………….

STRUCTURE……………………………

MECHANISM OF ACTION………………

Microsomal oxidations require:-

CYP 450

CYP 450 REDUCTASE

NADPH

OXYGEN Hydroxylation reactions most common oxidation reactions

reduced CYP 450 oxidised CYP 450

RH + Oշ R-OH + HշO

MECHANISM OF ACTION CONTINUED…………..

OTHER OXIDATION REACTIONS…………….. N Dealkylation

Eg: morphine to normorphine O Dealkylation

Eg: phenacetin to paracetamol S Dealkylation

Eg: 6 methylthiopurine to 6 mercaptopurine N Oxidation

Eg: CPM , Dapsone S Oxidation

Eg: CPZ to CPZ sulfoxide Deamination

Eg:amphetamine to phenylacetone Desulfurisation

Eg:parathion to paraoxon

NOMENCLATURE…………………………

GENE IS DENOTED IN ITALICS. EG:-CYP 3A4

CYPS IMPORTANT IN MAN……………..

50 cyps grouped into 17 families and 30 sub families

Cyps belonginging to families 1 to 3 involved in drug metabolism.

PROPERTIES OF CYP 450ENZYMES……………………

1.CYPs involved in xenobiotic metabolism as well as metabolism of endogenous substances.Xenobiotic metabolising CYPs Endogenous substances

metabolising CYPs

Substrate overlapping Specific substrate specificities

Can metabolise multiple substrates at a time

Metabolise only a single,specific substrate at a time.

Slower rate of metabolism Faster rate of metabolism

PROPERTIES CONTD…………….

2. INDUCTION Drugs on continuous administration induce CYPs by

increased synthesis or decreased degradation

MECHANISMS OF INCREASED SYNTHESIS

-transcription of genes coding for CYP enzymes

-drugs act as ligands for receptors involved in transcription

of genes

Cytosolic receptor(AHR)

Receptors------

Nuclear receptors(PXR,CAR,PPAR)

INDUCTION BY AHR

Polycyclic hydrocarbons,omeprazole etc bind to AHR

Ligand receptor complex translocated into nucleus where

it forms a dimer with nuclear protein Arnt.

Dimer results in transcription of genes coding for

CYP1A1,CYP1A2,CYP1B1. The 3 CYPs are involved in conversion of procarcinogens to carcinogens.Hence their induction leads to increased risk of carcinogenicity.

INDUCTION BY NUCLEAR RECEPTORS PXR : CYP3A4 induction by ligands like

rifampicin,atorvastatin,St John’s wort CAR : CYP2B6,CYP2C9,CYP3A4 induction

by phenobarbitone. PPARα : CYP4A induction by fibrate group of drugs.

OUTCOMES OF ENZYME INDUCTION

Decreased intensity of action of drugs that are metabolised to inactive metabolites.

eg:-failure of OCP by rifampicin

-higher doses of warfarin needed if administered with barbiturates

Increased intensity of action of drugs activated by metabolism-toxicity

eg:-paracetamol to N-acetyl benzoiminoquinone in alcoholics due to induction of CYP2E1.

Carcinogenesis due to induction of CYP2E1,CYP1A1/2

Pharmacokinetic tolerance Increased metabolism of endogenous substances

----------------------------------------------------------------------------------- 3.ENZYME INHIBITION Occurs by either of the following mechanisms:-

-competitive inhibition(reversible)

-irreversible inactivation of enzyme Outcomes of enzyme inhibition

-augmentation of plasma drug levels of drug whose metabolism is inhibited--toxic effects

Eg:- terfenadine with CYP3A4 inhibitor ketoconazole/erythromycin fatal arrythmias.

- cisapride with CYP3A4 inhibitors cardiotoxicity

4.GENETIC POLYMORPHISM CYP2D6 Polymorphisms

slow metabolisers ultrarapid metabolisers(Debrisoquin sparteine Oxidation type polymorphism)

CYP2C19 in mephenytoin metabolism polymorphic form produces metabolism of both (S) & (R)Mephenytoin to nirvanol-profound sedation & ataxia

CYP2A6 polymorphism Individuals with null alleles are proteceted against smoking

CLINICAL RELEVANCE OF CYPS Drug interactions can be explained by induction & inhibition. Knowledge about CYPs helps in :-

-avoiding potentially dangerous interactions

-dosage adjustments when certain drugs are co- administered

-dose adjustments in elderly,disease states,alcoholics Explains the risk of carcinogenicity with smoking,alcohol

consumption of charcoal broiled meat etc Genetic polymorphisms help in understanding interindividual

variations and atypical responses

-Genotyping of P 450 profiles to detect polymorphisms may help in individualisation of therapy

AMPLI CHIP CYP450 ARRAY FIRST PHARMACOGENOMICS DIAGNOSTIC TOOL

SUMMARY

CYPs involved in phase 1 oxidation of 50% of drugs Hemoproteins located in liver ER(microsomal

enzymes)/mitochondria Broad substrate specificity Many are inducible,resulting in one cause of drug

interaction Many are inhibited by drugs/metabolites,another cause

od drug interaction In some cases the products of metabolism are

carcinogenic Some exhibit genetic polymorphism.Genotyping will help

in individualisation of therapy

THANK YOU