drug disposition & excretion

DRUG DISPOSITIONAND EXCRETION

PRESENTED by k.b.GAUThAmI

m-PhARmACy 1ST yEAR II SEm

(PhARmACEUTICS)

ROll NO: 256213886009.

bIOTRANSFORmATION

• Chemical alteration of a substance within the body, as by the action of enzymes

• Vital to survival

• Key in defense mechanism….

METABOLISM OR BIOTRANSFORMATION The conversion from one chemical form of a substance to another.

The term metabolism is commonly used probably because products of drug transformation are called metabolites.

Metabolism is an essential pharmacokinetic process, which renders lipid soluble and non-polar compounds to water soluble and polar compounds so that they are excreted by various processes.

This is because only water-soluble substances undergo excretion, whereas lipid soluble substances are passively reabsorbed from renal or extra renal excretory sites into the blood by virtue of their lipophilicity.

Metabolism is a necessary biological process that limits the life of a substance in the body.

Biotransformation: It is a specific term used for chemical transformation of xenobiotics in the body/living organism.

• A series of enzyme-catalyzed processes—that alters the physiochemical properties of foreign chemicals (drug/xenobiotics) from those that favor absorption across biological membranes (lipophilicity) to those favoring elimination in urine or bile (hydrophilicity )

Metabolism : It is a general term used for chemical transformation of xenobiotics and endogenous nutrients (e.g., proteins, carbohydrates and fats) within or outside the body.

Xenobiotics : These are all chemical substances that are not nutrient for body (foreign to body) and which enter the body through ingestion, inhalation or dermal exposure.

They include : drugs, industrial chemicals, pesticides, pollutants,

plant and animal toxins, etc.

Site/Organs of drug metabolism

The major site of drug metabolism is the liver (microsomal enzyme systems of hepatocytes)

Secondary organs of biotransformation • kidney (proximal tubule) • lungs (type II cells) • testes (Sertoli cells)• skin (epithelial cells); plasma. nervous tissue

(brain); intestines

Hepatic microsomal enzymes (oxidation, conjugation)

Extrahepatic microsomal enzymes (oxidation, conjugation)

Hepatic non-microsomal enzymes (acetylation, sulfation,GSH, alcohol/aldehyde dehydrogenase,hydrolysis, ox/red)

Drug Metabolism

UPTAKE

ORGAN

EXCRETION

ORGAN

UPTAKE

EXCRETION

Primarily biotransformation makeslipophilic compounds more hydrophilic

UPTAKE

EXCRETION

BIOTRANS-FORMATION

Uptake and excretion of hydrophilic and lipophilic compounds

REACTIONS

• PHASE I : modification

• PHASE II : conjugation

TYPES OF BIOTRANSFORMATION

Phase 1 reaction. (Non synthetic phase)

A change in drug molecule. generally results in the introduction of a

functional group into molecules or the exposure of new functional groups of

molecules : Phase I (non-synthetic or non-

conjugative phase) includes reactions which catalyse oxidation, reduction and hydrolysis of drugs.

In phase I reactions, small polar functional groups like-OH, -NH2. -SH, -COOH, etc. are either added or unmasked (if already present) on the lipid soluble drugs so that the resulting products may undergo phase II reactions.

• result in activation, change or inactivation of drug.

Phase II reaction. (Synthetic phase)• Last step in detoxification reactions

and almost always results in loss of biological activity of a compound.

• May be preceded by one or more of phase one reaction

• Involves conjugation of functional groups of molecules with hydrophilic endogenous substrates- formation of conjugates - is formed with (an endogenous substance such as carbohydrates and amino acids. )with drug or its metabolites formed in phase 1 reaction.

Involve attachment of small polar endogenous molecules like glucuronic acid, sulphate, methyl, amino acids, etc., to either unchanged drugs or phase I products.

Products called as 'conjugates' are water-soluble metabolites, which are readily excreted from the body.

• Phase I metabolism is sometimes called a “functionalization reaction,”

• Results in the introduction of new hydrophilic functional groups to compounds.

• Function: introduction (or unveiling) of functional group(s) such as –OH, –NH2, –SH, –COOH into the compounds.

• Reaction types: oxidation, reduction, and hydrolysis

• Enzymes:• Oxygenases and oxidases: Cytochrome P450 (P450

or CYP), flavincontaining• monooxygenase (FMO), peroxidase, monoamine

oxidase(MAO), alcohol dehydrogenase, aldehyde dehydrogenase, and xanthine 0xidase. Reductase: Aldo-keto reductase and quinone reductase.

• Hydrolytic enzymes: esterase, amidase, aldehyde oxidase, and alkylhydrazine

• oxidase.• Enzymes that scavenge reduced oxygen:

Superoxide dismutases, catalase,• glutathione peroxidase, epoxide hydrolase, y-

glutamyl transferase,• dipeptidase, and cysteine conjugate β-lyase

• Phase II metabolism includes what are known as conjugation reactions.

• Generally, the conjugation reaction with endogenous substrates occurs on the metabolite( s) of the parent compound after phase I metabolism; however, in some cases, the parent compound itself can be subject to phase II metabolism.

• Function: conjugation (or derivatization) of functional groups of a compound or its metabolite(s) with endogenous substrates.

• Reaction types: glucuronidation, sulfation, glutathione-conjugation, Nacetylation, methylation and conjugation with amino acids (e.g., glycine, taurine, glutamic acid).

• Enzymes: Uridine diphosphate-Glucuronosyltransferase (UDPGT): sulfotransferase (ST), N-acetyltransferase, glutathione S-transferase (GST),methyl transferase, and amino acid conjugating enzymes.

• Glucuronidation by uridine diphosphate-glucuronosyltransferase; Sulfation by sulfotransferase

• 3. Acetylation by N-acetyltransferase; Glutathione conjugation by glutathione S-transferase;. Methylation by methyl transferase; Amino acid conjugation

A small polar group is either exposed on the toxicant or added to the toxicant…

Oxidation

Reduction

Hydrolysis

Acetylation

PHASE I REACTION

OXIDATION substrate loses electrons

addition of oxygen, dehydrogenation, or

simply transfer of electrons…

alcohol dehydrogenation aldehyde dehydrogenation alkyl/acyclic hydroxylation aromatic hydroxylation deamination desulfuration N-dealkylation N-hydroxylation N-oxidation O-dealkylation sulphoxidation

Aliphatic hydroxylation

Aromatic hydroxylation

R R OH

R - CH CH - R’ R - CH - CH - R’

Epoxidation

O

N-, O-, or S-dealkylation

R - (N, O, S) - CH3

H

N - hydroxylation

Deamination

R - C - H + NH3

O

OO

R - NH - C – CH3 R - NOH - C – CH3

R - C - H R - C - OH R - C - H + HX

OX X

H H

R - S - R’ R - S - R’

Sulphur oxidation

S

R1R2P - X R1R2P - X + S

O

De-sulphurnation

Oxidative dehalogenation

R - CH2 – CH2 – CH3 R – CH2 – CHOH – CH3

R – (NH2, OH, SH) + CH2O

R – CH2 – NH2

PHASE I REACTION

REDUCTION Substrate gains electrons Occurs when oxygen content is low Common reaction

azo reduction dehalogenation disulfide reduction nitro reduction N-oxide reduction sulfoxide reduction

PHASE I REACTION

HYDROLYSIS Addition of water splits the molecule into

two fragments or smaller molecules

-OH gp to one fragment and –H to other

Eg : Larger chemicals such as esters, amines, hydrazines, and carbamates

Conjugation

Endogenous substance is added to the reactive site of the Phase I metabolite

more water-soluble

TYPE I

• Methylation

• Glucuronidation

• Sulfation

• Acetylation

TYPE II

• Peptide conjugation

• Glutathione conjugation

• Glycosylation

glucuronide conjugation

sulfate conjugation

acetylation

amino acid conjugation

glutathione conjugation

methylation

COFACTORSCOFACTORSTYPE 1- REacTIvE/ acTIvaTEd

cofacToR a)UDP- Glucuronic acid

b)PAPS

c)Acetyl CoA

d)SAM

TYPE 2- REacTIvE XEnobIoTIc

a)Glutathione

b)Aminoacids(glycine,glutamine,

taurine)

Glucuronosyltransferase

Sulfotransferase

Glutathione-S-transferase

Acetyltransferase

GLUCURONIDE CONJUGATION glucuronic acid from glucose Sites involve substrates having O2, N2 or S

bonds Includes xenobiotics as well as endogenous

substances Reduces toxicity..(sometimes produce

carcinogenic substances) Excreted: kidney or bile depending on

conjugate size

GLUcURonIdE conJUGaTIon

R – OH + UDPO

HO

O

OH

OH

COOH

Glucuronyltransferase O

HO

O

OH

OH

COOH

R + UDP

SULPHATE CONJUGATION Decreases toxicity readily excreted by urine Sulphotransferase PAPS limits the pathway

SULFATE CONJUGATION

glucuronidation or sulfation can conjugate the same xenobiotics

Primary, secondary, phenols, catechols, N-oxides, amines undergo this…

GLUTATHIONE CONJUGATION Conjugate loses glutamic acid and glycine

Cysteine is N-acetylated to give stable mercapturic acid derivatives

N

O

H

O

H

N

S

H

O

N

O

O

H H

OH

H

H+

+

N

OO

O

H H

O

H

H

N

O

S

H H

O

H

N

O

HH

O

H

Glutamicacid

Cysteine

GlycineGlutathione

ACETYLATION the water solubility of parent molecule

and their excretion Masks the functional group of parent

from participating in conjugations Acetyl transferases Aromatic amines or hydrazine group to

amides or hydrazides

Methylation Makes slightly less soluble Masks available functional groups Types

O- methylation

N- methylation

S- methylation

PHASE II REACTIONS

• Aminoacid conjugation

GENETICSNfr2- nuclear factor erythroid derived

Inactive oxidative stress active

CP nucleus

ENZYMES

ENZYMESmicrosomal…. Phase I and glucuronidation enzymesCytosolic enzymes….phase II and oxidation and

reductionMitochondrial, nuclei and lysosomes contain a little

transforming activity….

MICROSOMALPhase I reactions

– Most oxidation and reduction

– Some hydrolysis

Phase II reactions– ONLY Glucuronide

conjugation• Inducible

– Drugs, diet, etc.

• SER

NONMICROSOMALPhase I reactions

– Most hydrolysis– Some oxidation and

reductionPhase II reactions

ALL except Glucuronide conjugation

• Not inducible• CP, MT etc

ENZYMES

• High molecular weight proteins..

MONOOXYGENASESCYTOCHROME P450 ENZYME SYSTEM

• Mixed function oxidase

• Commonly in microsomes

• Important in plant terpenoid biosynthesis

• In phase I reactions

• Contains 2 enz NADPH CYP reductase and cyp 450

CYTOCHROME P450 ENZYME SYSTEMCYTOCHROME P450 ENZYME SYSTEM

superfamily of heme-dependent proteinsexpressed in mammals mainly in the liver,

with lower levels of expression in the small intestine, lungs, kidneys, brain and placenta

In man, to date 57 different P450 isoforms have been identified, which were assigned to 18 families and 43 subfamilies based on their protein sequences

REDUCTASE

P-450P-450

TYPES

• Microsomal P450 systems: electrons are transferred from NADPH via cytochrome P450 reductase.

• Mitochondrial P450 systems: employ adrenodoxin reductase and adrenodoxin to transfer electrons from NADPH to P450.

CYTOCHROME P 450 ENZYME ACTION

HC(inducer)

Ah receptor-hsp90

HC

P450 mRNAP450 protein

• Bioactivation• Detoxification

Toxicity

Elimination

Cell

HC-AhR

hsp90

HC-AhR

XRE

P450 gen

Nucleus

HC: Hydrocarbon (inducer)XRC: Regulator gene (stimulates transcription of P-450 gene)

P450 family Function

CYP1, CYP2, CYP3 Metabolism of drugs and xenobiotics

CYP4, CYP5, CYP8

Fatty acids hydroxylation, biosynthesis of prostaglandins, prostacyclins and thromboxanes

CYP7, CYP11, CYP17, CYP19 (=steroid aromatase), CYP21, CYP24, CYP27, CYP39, CYP46, CYP51 Biosynthesis and metabolism of cholesterol, steroid

hormones and bile acids

CYP26 Retinoic acid hydroxylation

CYP20 Unknown

FLAVIN MONO OXYGENASE

• Microsomal enzyme

• mixed function amine oxidase

• Cofactors: NADPH, molecular O₂

• Do not contain heme

• Broad specificity

• Nicotine detoxification

OTHER ENZYMES

• Monoamine oxidases- breakdown of neurotransmitters and antidepressant drugs

• Alcohol and aldehyde dehydrogenases

TYPES OF FACTORS

• Chemical

• Biological

• Altered Physiological

• Temporal

• Route of Drug Administration

• Enviornmental

Chemical Factors

1. Enzyme Induction

2. Enzyme Inhibition

Enzyme Induction

It is a process in which a drug induces or enhances the expression of an enzyme.

Rifampicin If taken by female patients taking contraceptives, causes decreased therapeutic effect, leading to pregnancy.

Phenobarbitone If administered to patients taking warfarin, may cause therapeutic failure, leading to increased bleeding tendency.

Auto induction: The phenomenon in which a

drug induces metabolism of other drugs as well

as its own E.g. carbamazepine-antiepileptic.

Enzyme Inhibition

Decrease in the drug metabolizing ability of

enzymes. Competition for the active sites

takes place between the inhibitor and the

drugs. When enzyme inhibitor attaches,

less metabolism occurs. E.g. Sulfonamides decrease the metabolism of phenytoin so

that its blood levels become toxic. Cimetidine decreases the metabolism of propanolol

leading to enhanced bradycardia. Oral contraceptives inhibit metabolism of antipyrine.

Biological FactorsAGE

In infants microsomal enzyme system is not fully developed. The rate of metabolism is very low. Care should be taken in administering drugs in younger patients.

Chloramphenicol does not have great efficacy in infants. Toxic effects in the form of grey baby syndrome might occur. The baby may be cyanosed, hypothermic, flaccid and grey in color. Shock and even death might occur if toxic levels get accumulated.

Diazepam may result in floppy baby syndrome in which flaccidity of the baby is seen.

AGE

In elderly, most processes slow down which leads to decreased metabolism. Shrinkage of organs occurs as well along with decreased liver functions and decreased blood flow through the liver. All these factors decrease the metabolism.

The drug doses should be decreased in the elderly

GENDER

Gender related differences in the rate of metabolism are attributed to sex hormones and are generally observed following puberty.

Male have a higher BMR as compared to the females, thus can metabolize drugs more efficiently, e.g. salicylates and others might include ethanol, propanolol, benzodiazepines.

Women on oral contraceptives metabolize drugs at a slower rate

GENETICS

Drugs behave differently in different individuals due to genetic variations

Succinyl choline, which is a skeletal muscle relaxant, is metabolized by pseudocholine esterase. Some people lack this enzyme, due to which lack of metabolism of succinyl choline might occur. When administered in those individuals, prolonged Apnea might result.

Different groups of populations might be classified as fast metabolizers and poor metabolizers of drugs.For drugs, like Isoniazid, fast acetylators as well as slow acetylators are present. Fast acetylators cause rapid acetylation, while poor metabolizers metabolize less. Hepatic acetyl transferrase catalyzes acetylation. Slow acetylation might occur due to genetic malformation leading to decreased production.

RACE/SPECIES

Asians, Orientals, Blacks and Whites might have different drug metabolizing capacity. Examples include difference in drug metabolizing capacity of certain anti malarial.

Eskimos metabolize drugs faster than Asians.

Laboratory animals can metabolize drugs faster than man e.g. barbiturates.

DIET

The enzyme content and activity is altered by a number of dietary components.

Low protein diet decreases and high protein content in diet increases the drug metabolizing ability.

Dietary deficiency of vitamins and minerals retard the metabolic activity of enzymes.

ALTERED PHYSIOLOGICAL FACTORSPREGNANCYDuring pregnancy, metabolism of some

drugs is increased while that of others is decreased due to the presence of steroid hormones e.g.

Phenytoin Phenobarbitone Pethidine

HORMONAL IMBALANCE

Higher levels of one hormone may inhibit the activity of few enzymes while inducing that of others. E.g.

Hypothyroidism increases drug metabolizing capacity (increased half life of antipyrine, digoxin, methimazole, practolol) while hyperthyroidism decreases it.

DISEASE STATES

Liver disease such as hepatic carcinoma, cirrhosis, hepatitis, obstructive jaundice etc reduce the hepatic drug metabolizing ability and thus increase the half lives of almost all drugs.

In renal diseases conjugation of salycylates, oxidation of vitamin D and hydrolysis of Procaine are impaired.

Cardiovascular diseases, although have no direct effect, decrease the blood flow, which may slow down biotransformation of drugs like isoniazid, morphine and propanolol.

Pulmonary conditions may decrease biotransformation. Procaine and procainamide hydrolysis is impaired.

TEMPORAL FACTOR

Diurnal variations and variations in enzyme activity with light cycle is circadian rhythm.

Enzyme action is maximum during early morning and minimum in late afternoon which is probably due to high levels of coticosterone.

ROUTE OF ADMINISTRATION

Oral route can result in extensive hepatic metabolism of some drugs (first pass effect).

Lignocaine is almost completely metabolized if taken by oral route therefore the preferable route is Topical.

ENVIRONMENTAL FACTORS

Aromatic hydrocarbon contained in Cigarette smokers act as enzyme inducers.

Chronic alcoholism might lead to enzyme induction as well.

Pesticides or Organophosphate insecticides may act as enzyme inducers.

In hot and humid climate biotransformation is decreased and vice versa.

At high altitude decreased biotransformation occurs due to decreased oxygen leading to decreased oxidation of drugs.

REFERENCE

• http://www.eoearth.org/article/Biotransformation?topic=58074• profiles.nlm.nih.gov/ps/access/CCAAOR.pdf• www.slideshare.net/shishirkawde/biotransformation-10417087• www.eolss.net/sample-chapters/c17/e6-58-04-06.pdf• www.ncbi.nlm.nih.gov/pubmed/3116933• ingentaconnect.com RK Venisetty, V Ciddi - Current pharmaceutical

biotechnology, 2003 • web.squ.edu.om/med-Lib/MED_CD/E_CDs/.../020160r00.HTM• www.eoearth.org/article/Biotransformation