METABOLISM

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BIOTRANSFORMATIONOF

DRUGS

NIHAL.PFIRST YEAR M.PHARMNATIONAL COLLEGE OF PHARMACY

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-Biotransformation/Xenobiotic metabolism/ drug metabolism/detoxification.-Xenobiotics: a wide variety of foreign compounds to which humans get exposed in day to day life. -It includes unknown compounds, drugs, environmental pollutants, toxins.-Many xenobiotics can evoke biological responses.

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DEFINITION

• The biochemical alteration of drug or xenobiotic in the presence of various enzymes that acts as a catalyst which themselves not consumed in the reaction and there by may activate or deactivate the drug is called biotransformation.

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Why Biotransformation is necessary?:

• To easily eliminate the drug• To terminate drug action by

inactivating it By changing its physicochemical properties from:

Active /inactiveLipophilic UnionisedNonpolarPlasma protein bound

Inactive /activeHydrophilic IonisedPolarFree

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Consequences of Biotransformation

• Active to Inactive: Phenobarbitone----

Hydroxyphenobarbitone• Inactive (prodrug) to Active : L-Dopa ---- Dopamine Parathion -- Paraoxon Talampicillin -- Ampicillin

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• Active to equally active: Diazepam -- Oxazepam Amitriptyline -- Nortriptyline Imipramine -- Des-

imipramine Codeine -- Morphine

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Sites of biotransformation

• In the body: Liver, small and large intestines, lungs, skin, kidney, nasal mucosa & brain.

• Liver is considered “metabolite clearing house” for both endogenous substances and xenobiotics.

• Intestines are considered “initial site of drug metabolism”.

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FIRST PASS METABOLISM:• First pass metabolism or

presystemic metabolism or ‘first pass effect’

• After oral administeration many drugs are absorbed from the small intestine - transported first via portal system to the liver, where they undergo extensive metabolism before reaching systemic circulation.

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LIVE

R

INTE

STIN

AL M

UCO

SA

BRO

NCH

IAL

MU

COSA • NICOTINE

• ISOPRINALINE

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fundamental concepts in drug biotransformation

Lipid soluble drugs are poorly excreted in the urine. They tend to store in fat and/or circulate until they are converted (phase I biotransformation) to more

water soluble metabolites or metabolites that conjugate (phase II biotransformation) with water

soluble substances.Water soluble drugs are more readily excreted in the

urine. They may be metabolized, but generally not by the CYP enzyme systems.

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Drug metabolism reactions :

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Enzymes catalyzing phase I biotransformation reactions

Enzymes catalyzing phase I biotransformation reactions include:

cytochrome P-450aldehyde and alcohol dehydrogenase deaminases esterases amidasesepoxide hydratases

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PHASES OF DRUG METABOLISM

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DRUG METABOLIZING ENZYMES

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PHASE-I REACTIONS

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PHASE-I REACTIONS

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PHASE-I REACTIONS

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Hydrolysis

• Addition of water– Cleavage of R-O or R-N bond accompanied by addition of

H2O

Ether

Amide

R

O

R'

+ H2O R OH HO R'+

R

N

R'

+ H2O R N HO R'+

H

H

H

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Reduction

• Nitro to amino group

• Chromium VI to Chromium III

NO NOHNO2 NHH H

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Flavin Monooxygenases

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EPOXIDE HYDROLASE

•Highly reactive electrophile•Binds to DNA,RNA.PROTEINS•Cell toxicity

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CYTOCHROME P450

The cytochrome P-450 families are referred to using an arabic numeral, e.g., CYP1, CYP2, etc.

Each family has a number of subfamilies denoted by an upper case letter, e.g., CYP2A, CYP2B, etc.

The individual enzymes within each subfamily are denoted by another arabic numeral, e.g., CYP3A1, CYP3A2, etc.

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Electron flow in microsomal drug oxidizing system

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• When phase I reactions are not producing sufficiently hydrophilic (water soluble) or inactive metabolites to be eliminated from the body, the drugs or metabolites formed from phase I reaction undergoes phase II reactions.

• Generally phase I reactions provide a functional groups or handle in the molecule that can undergo phase II reactions. Thus, phase II reactions are those in which the functional groups of the original drug (or metabolite formed in a phase I reaction) are masked by a conjugation reaction.

• Phase II conjugation reactions are capable of converting these metabolites to more polar and water soluble products.

• Many conjugative enzymes accomplish this objective by attaching small, polar, and ionizable endogenous molecules such as glucuronic acid , sulfate, glycine, glutamine and glutathione to the phase I metabolite or parent drug. The resulting conjugated products are very polar (water soluble), resulting in rapid drug elimination from the body.

PHASE 2 REACTIONS

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• These reactions require both a high-energy molecule and an enzyme.

• The high-energy molecule consists of a coenzyme which is bound to the endogenous substrate and the parent drug or the drug’s metabolite resulted from phase I reaction.

• The enzymes that catalyzed conjugation reactions are called transferases, found mainly in the liver and to a lesser extent in the intestines and other tissues.

• Most conjugates are biologically inactive and nontoxic because they are highly polar and unable to cross cell membrane.

• Exceptions to this are acetylated and methylated conjugates because these phase II reactions (methylation and acetylation) does not generally increase water solubility but serve mainly to terminate or reduce pharmacological activity (they are usually pharmacologically inactive).

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CONJUGATION MOLECULES:

• 1- Glucuronic acid conjugation:• It forms O-glucuronides with phenols Ar-OH, alcohols R-

OH, hydroxylamines H2N-OH,and carboxylic acid RCOOH.• It can form N-glucuronides with sulfonamides, amines,

amides, and S-glucuronides with thiols.• 2-Sulfate conjugation:• It is less common.• It is restricted to phenols, alcohols, arylamines, and N-

hydroxyl compounds.• But primary alcohols and aromatic hydroxylamines can

form unstable sulfate conjugates which can be toxic.

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• 3-Amino acid conjugation:• By the formation of peptide link. With glycine

or glutamine.• 4- Glutathione conjugation:• It reacts with epoxides, alkylhalides,

sulfonates, disulfides, radical species. • These conjugates are converted to

mercapturic acid and mostly are excreted in bile. It is important in detoxifying potentially dangerous environmental toxins.

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• 5,6- Methylation and acetylation reactions:• These decrease the polarity of the drugs except tertiary

amines which are converted to polar quaternary salts. • The groups susceptible for these reactions are phenols,

amines, and thiols.• O-methylation of meta-phenolic OH in catecholamines• does not generally increase water solubility but serve

mainly to terminate or reduce pharmacological activity (they are usually pharmacologically inactive).

• 7- Cholesterol conjugation:• For carboxylic acids by ester link formation or for drug with

ester group by trans esterification.• 8- Fatty acid conjugation:• For drugs with alcohol functional groups by ester link

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There are six conjugation pathways:

1)-Glucuronidation: by glucuronyl transferase.O

HOHO

HOOCH

O UDPOH

R X+

O

HOHO

HOOCH

XOH R

X = OH, NR2, CO2H, SH, acidic carbon atomGlucuroinc acid UDP

2)-Sulfate conjugation

OAdenine

P

O

OS

O

HO

O OH

HOH2O3PO

PAPS

+ R X S X

O

HORO

X = OH, arylamine, NHOH

Glucuronyl Transferease catalyses this conjugation reaction

Sulfotransferease catalyses this conjugation reaction

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There are six conjugation pathways:

3)-Amino acid conjugation:

CS

CoA

O

R+

H2N CO2H

YH

N CO2H

YH

CR

O

HY = H or CH2CH2CO2H

Acyl coenzyme A

N-acyltransferase catalyses the conjugation reaction

4)-Glutathione conjugation

CS

CoA

O

R+

H2N CO2H

YH

N CO2H

YH

CR

O

HY = H or CH2CH2CO2H

Acyl coenzyme A

Glutathione S-transferase catalyses this conjugation reaction

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5)-Methylation

N-acyltransferase catalyses the conjugation reaction

Methyltransferase catalyses this conjugation reaction

OAdenine

S+

NH2

HO2C

CH3

HOH2O3PO

R X+ R X CH3

SAM

X = OH, NH2, SH

Aceyl CoA Y =NH2, NHNH2, SO2NH2, CONH2

RX

CR

O

+CS

CoA

O

H3CR X

6)-Acetylation

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Non-Enzymatic Biotransformation

• Skeletal muscle relaxants like ATRACURIUM are metabolised in the plasma spontaneously through molecular rearrangement without involvement of any enzyme action.

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FACTORS AFFECTING BIOTRANSFORMATION OF DRUGS

1. Physicochemical properties of the drug2. Chemical factors a. Induction of drug metabolising enzymes b. Inhibition of drug metabolising enzymes c. Environmental chemicals3. Biological factors a. Species differences b. Strains differences c. Sex differences d. Age e. Diet f. Altered physiological factors: (pregnancy, hormonal imbalance, disease states) g. Temporal factors: ( circadian rhythm, circannual rhythm)

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Induction of Drug Metabolism

• Enzyme induction is the process by which exposure to certain substrates (e.g., drugs, environmental pollutants) results in accelerated biotransformation with a corresponding reduction in unmetabolized drug.

• Many currently used drugs are well known to induce their own metabolism or the metabolism of other drugs. Some examples are the anticonvulsant medications phenobarbital and carbamazepine

• Cigarette smoking can cause increased elimination of theophylline and other compounds.

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Consequences of Induction• Increased rate of metabolism• Decrease in drug plasma concentration • Enhanced oral first pass metabolism• Reduced bioavailability• If metabolite is active or reactive, increased drug effects or

toxicity

Therapeutic Implications of Induction

-Most drugs can exhibit decreased efficacy due to rapid metabolism but drugs with active metabolites can display increased drug effect and/or toxicity due to enzyme induction

-Dosing rates may need to be increased to maintain effective plasma concentrations

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Inhibition of Drug Metabolism

• Drug metabolism is an enzymatic process can be subjected to inhibition.

• Drugs and other substances can inhibit the metabolism of other drugs.

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Some types of inhibition

• Competition between substrates for enzyme active siteConcentration of substratesAffinity for binding site (drug with hi affinity for an

enzyme will slow the metabolism of any low affinity drug)• Irreversible inactivation of enzyme

Complex with heme iron of CYP450 (cimetidine, ketoconazole)

Destruction of heme group (secobarbital)• Depletion of cofactors such as NADH2 for phase II

enzymes

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Consequences of Inhibition

• Increase in the plasma concentration of parent drug• Reduction in metabolite concentration• Exaggerated and prolonged

pharmacological effects• Increased liklihood of drug-induced

toxicity

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Therapeutic Implications of Inhibition

• May occur rapidly with no warning • Particularly effects drug prescribing for

patients on multidrug regimens • Knowledge of the CYP450 metabolic pathway

provides basis for predicting and understanding inhibition

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Reference a. Milo gibaldi, Biopharmacetics and clinical pharmacokinetics, 4th edition, page No:213-230 b. Leon shargel,susanna wu-pong,Andrew.B.C.YU, Applied biopharmaceutics & pharmacokinetics,5th edition, page No:303-347 c.D.M.Brahmankar,Sunil.B.Jaiswal, Biopharmaceutics & pharmacokinetics A Treatise, 2nd edition, page No: 139-192 d. CVS Subrahmanyam, Text book of biopharmaceutics & pharmacokinetics-concepts & applications,1st edition 2010, page No:339-364

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