drug biotransformation n.
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Drug Biotransformation. Elimination of the drugs. Drug Biotransformation. Metabolism or biotransformation - complex of processes which provide decreasing of toxicity and accelerate excreting of the molecule of a drug or other foreign substance after its incoming into the organism

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Drug Biotransformation

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drug biotransformation

Drug Biotransformation

Elimination of the drugs

drug biotransformation1
Drug Biotransformation

Metabolism orbiotransformation -

complex of processes which provide decreasing of toxicity and accelerate excreting of the molecule of a drug or other foreign substance after its incoming into the organism

(Chemical alteration of the drug in the body )

metabolism of drugs
Metabolism of Drugs
  • Aim: to convert non-polar lipid soluble compounds to polar lipid insoluble compounds to avoid reabsorption in renal tubules
  • Most hydrophilic drugs are less biotransformed and excreted unchanged – streptomycin, neostigmine and pancuronium etc.
  • Biotransformation is required for protection of body from toxic metabolites
results of biotransformation
Results of Biotransformation
  • Active drug and its metabolite to inactive metabolites – most drugs (ibuprofen, paracetamol, chlormphenicol etc.)
  • Active drug to active product (phenacetin – acetminophen or paracetamol, morphine to morphine-6-glucoronide, digitoxin to digoxin etc.)
  • Inactive drug to active/enhanced activity (prodrug) – levodopa - carbidopa, prednisone – prednisolone and enalapril – enalaprilat)
  • No toxic or less toxic drug to toxic metabolites (Isonizide to Acetyl isoniazide)
biotransformation of drugs into active or more active metabolites
Initial drug



Acetylsalicylic acid










Active metabolite



Salicylic acid










Biotransformation of drugs into active (or more active) metabolites
organs of drugs metabolism
  • liver
  • kidneys
  • muscle tissue
  • intestinal wall
  • lungs
  • skin
  • blood
reactions of biotransformation
Reactions of biotransformation
  • Nonsynthetic - І phase – metabolite may be active or inactive
  • Synthetic - ІІ phase – metabolites are inactive (Morphine – M-6 glucoronide is exception)

І phase (nonsynthetic reactions):

(oxydation, reduction, hydrolysis)

  • 1) microsomal reactions
  • 2) nonmicrosomal reactions

Reactions of І phase - transformation in molecule with formation of functional groups with active hydrogen atom

phase i oxidation
Phase I - Oxidation
  • Most important drug metabolizing reaction – addition of oxygen or (–ve) charged radical or removal of hydrogen or (+ve) charged radical
  • Various oxidation reactions are – oxygenation or hydroxylation of C-, N- or S-atoms; N or 0-dealkylation
  • Examples – Barbiturates, phenothiazines, paracetamol and steroids
phase i oxidation1
Phase I - Oxidation
  • Involve – cytochrome P-450 monooxygenases (CYP), NADPH and Oxygen
  • More than 100 cytochrome P-450 isoenzymes are identified and grouped into more than 20 families – 1, 2 and 3 …
  • Sub-families are identified as A, B, and C etc.
  • In human - only 3 isoenzyme families important – CYP1, CYP2 and CYP3
  • CYP 3A4/5 carry out biotransformation of largest number (30–50%) of drugs. In addition to liver, this isoforms are expressed in intestine (responsible for first pass metabolism at this site) and kidney too
  • Inhibition of CYP 3A4 by erythromycin, clarithromycin, ketoconzole, itraconazole, verapamil, diltiazem and a constituent of grape fruit juice is responsible for unwanted interaction with terfenadine and astemizole
  • Rifampicin, phenytoin, carbmazepine, phenobarbital are inducers of the CYP 3A4
the catalytic cycle of cytochrome p450
CYP-450 – hemoprotein, which is able to interact with substrate of oxydation, to activate oxygen and combine it with substrate. Specifically on CYР-450 reactions of hydroxydation are performed

large amount of isoforms of this enzyme – possibility of its binding with different substrates and taking part in their metabolism

There are 24 isoforms of CYР-450 in microsomes of human liver

Multiplicity of the enzyme has a group character: one isoform of CYР-450 interacts not only with one substrate but with a group of substances

The catalytic cycle of cytochrome P450
microsomal enzyme system
Microsomal enzyme system

Oxydoreductases, esterases, enzymes of proteins, lipids, glycerophosphatides, lipo- and glycoproteids, bile acids, cholesterol, prostaglandins biosynthesis, enzyme systems of biosynthesis of couple compounds, ethers of glucuronic and sulfur acids


Oxydoreductases of microsomes (oxygenases of microsomes, microsomal hydroxydating system,NADPH-hydroxylase system, monooxygenases of mixed functions)

– these are enzymes which activate molecular oxygen and catalize including of one (monooxygenase) or two (dioxygenases) atoms of oxygen into molecule of substrate (R)Reaction is presented as follows:

R + O2 + DН = ROH + H2O +D

One atom of О2is included into molecule of the substrate, other is reduced to Н2О, therefore enzyme performs oxygenase and oxydase functions simultaneously. That’s why monooxygenases ate also called oxydases of mixed function. Along with this hydroxyl group (-ОН) forms in molecule of substrate, that’s why monooxygenase is also calles hydroxylating system, and reaction of oxydation – oxydating hydroxylation

nonmicrosomal enzyme oxidation
Nonmicrosomal Enzyme Oxidation
  • Some Drugs are oxidized by non-microsomal enzymes (mitochondrial and cytoplsmic) – Alcohol, Adrenaline, Mercaptopurine
  • Alcohol – Dehydrogenase
  • Adrenaline – MAO
  • Mercaptopurine – Xanthine oxidase
phase i reduction
Phase I - Reduction
  • This reaction is conversed of oxidation and involves CYP 450 enzymes working in the opposite direction.
  • Examples - Chloramphenicol, levodopa, halothane and warfarin
  • Levodopa (DOPA) Dopamine DOPA-decarboxylase
phase i hydrolysis
Phase I - Hydrolysis
  • This is cleavage of drug molecule by taking up of a molecule of water. Similarly amides and polypeptides are hydrolyzed by amidase and peptidases. Hydrolysis occurs in liver, intestines, plasma and other tissues.
  • Examples - Choline esters, procaine, lidocaine, pethidine, oxytocin
phase ii metabolism
Phase II metabolism
  • Conjugation of the drug or its phase I metabolite with an endogenous substrate - polar highly ionized organic acid to be excreted in urine or bile - high energy requirements
  • Glucoronide conjugation - most important synthetic reaction
  • Compounds with hydroxyl or carboxylic acid group are easily conjugated with glucoronic acid - derived from glucose
  • Examples: Chloramphenicol, aspirin, morphine, metroniazole, bilirubin, thyroxine
  • Drug glucuronides, excreted in bile, can be hydrolyzed in the gut by bacteria, producing beta-glucoronidase - liberated drug is reabsorbed and undergoes the same fate - enterohepatic recirculation (e.g. chloramphenicol, phenolphthalein, oral contraceptives) and prolongs their action
phase ii metabolism contd
Phase II metabolism – contd.
  • Acetylation: Compounds having amino or hydrazine residues are conjugated with the help of acetyl CoA, e.g.sulfonamides, isoniazid
  • Genetic polymorphism (slow and fast acetylators)
  • Sulfate conjugation: The phenolic compounds and steroids are sulfated by sulfokinases, e.g. chloramphenicol, adrenal and sex steroids
phase ii metabolism contd1
Phase II metabolism – contd.
  • Methylation: The amines and phenols can be methylated. Methionine and cysteine act as methyl donors.
  • Examples: adrenaline, histamine, nicotinic acid.
  • Ribonucleoside/nucleotide synthesis : activation of many purine and pyrimidine antimetabolites used in cancer chemotherapy
main ways of biotransformation of drugs
Main ways of biotransformation of drugs
  • Iphase
  • Oxydation:diazepam, pentazocin, sydnocarb, phenotiazin, phenobarbital, aspirin, butadion, lidokain, morphin, codein, ethanol, rifampicin
  • Reduction:hestagens, metronidazol, nitrazepam, levomycetin, chlozepid
  • Hydrolysis: levomycetin, novocain, cocain, glycosides, ditilin, novocainamid, xycain, fentanyl
  • II phase
  • Conjugation with sulfate:morphin, paracetamol, isadrin
  • Conjugation with glucuronic acid: teturam, sulfonamides, levomycetin, morphin
  • Conjugation with remains of  - aminoacids:nicotinic acid, paracetamol
  • Acetylation:sulfonamides, isoniasid, novocainamid
  • Methylation:morphin, unitiol, ethionamid, noradrenalin
metabolism in the intestinal wall
Metabolism in the intestinal wall

Synthetic and nonsynthetic reactions take place

  • Isadrin – conjugation with sulfate
  • Hydrlalasin - acetylation
  • Penicillin, aminazin – metabolism with nonspecific enzymes
  • Methotrexat, levodopa – metabolism with intestinal bacteria
factors affecting biotransformation
Factors affecting Biotransformation

Concurrent use of drugs: Induction and inhibition

  • Genetic polymorphism
  • Pollutant exposure from environment or industry
  • Pathological status
  • Age
enzyme inhibition
Enzyme Inhibition
  • One drug can inhibit metabolism of other – if utilizes same enzyme
  • However not common because different drugs are substrate of different CYPs
  • A drug may inhibit one isoenzyme while being substrate of other isoenzyme – quinidine
  • Some enzyme inhibitors – Omeprazole, metronidazole, isoniazide, ciprofloxacin and sulfonamides
microsomal enzyme induction
Microsomal Enzyme Induction
  • CYP3A – antiepileptic agents - Phenobarbitone, Rifampicin and glucocorticoide
  • CYP2E1 - isoniazid, acetone, chronic use of alcohol
  • Other inducers – cigarette smoking, charcoal broiled meat, industrial pollutants – CYP1A
  • Consequences of Induction:
  • Decreased intensity – Failure of OCPs
  • Increased intensity – Paracetamol poisoning (NABQI)
  • Tolerance – Carbmazepine
  • Some endogenous substrates are metabolized faster – steroids, bilirubin
influence of body weight on kinetics of drugs
Influence ofbody weighton kinetics of drugs
  • In exhaustedpatients – speeding up of elimination, that’s why it’ s appropriate to introduce the increased dose – 1+1/3
  • In patients with overweighting – retention of lipid-soluble drugs in the organism
  • For these patients it’s suitable to correct the dose according to “ideal”body weight:

For menІBW = 50 + [(Н - 150) : 2,5]

For women ІBW = 45 + [(Н - 150) : 2,5]

where Н – height in cm

  • in case of normal body weight the dose is calculated counting on 1 kg of patient’s body weight
drug drug interactions during metabolism
Drug-Drug Interactions during Metabolism

Many substrates are retained not only at the activesite of the enzyme but remain nonspecifically bound to the lipid membrane of the endoplasmicreticulum. In this state, they may induce microsomal enzymes; depending on the residual druglevels at the active site, they also may competitively inhibit metabolism of a simultaneously

administered drug.

drug drug interactions during metabolism1
Drug-Drug Interactions during Metabolism

Enzyme-inducing drugs include various sedative-hypnotics, tranquilizers, anticonvulsants, andinsecticides. Patients who routinely ingest barbiturates, other sedative-hypnotics, ortranquilizers may require considerably higher doses of warfarin (an oral anticoagulant) to maintaina prolonged prothrombin time. On the other hand, discontinuance of the sedative may result inreduced metabolism of the anticoagulant and bleeding—a toxic effect of the ensuing enhanced

plasma levels of the anticoagulant. Similar interactions have been observed in individuals receivingvarious combination drug regimens such as antipsychotics or sedatives with contraceptive agents,sedatives with anticonvulsant drugs, and even alcohol with hypoglycemic drugs (tolbutamide).

drug drug interactions during metabolism2
Drug-Drug Interactions during Metabolism

Simultaneous administration of two or more drugs may result in impaired elimination

of the more slowly metabolized drug and prolongation or potentiation of its pharmacologic effects

Both competitive substrate inhibition and irreversible substrate-mediated enzyme

inactivation may augment plasma drug levels and lead to toxic effects from drugs with narrowtherapeutic indices.

drug drug interactions during metabolism3
Drug-Drug Interactions during Metabolism

Allopurinol both prolongs the duration and enhances the

chemotherapeutic action of mercaptopurine by competitive inhibition of xanthine oxidase.

Consequently, to avoid bone marrow toxicity, the dose of mercaptopurine is usually reduced inpatients receiving allopurinol. Cimetidine, a drug used in the treatment of peptic ulcer, has beenshown to potentiate the pharmacologic actions of anticoagulants and sedatives. The metabolism ofthe sedative chlordiazepoxide has been shown to be inhibited by 63% after a single dose of

cimetidine; such effects are reversed within 48 hours after withdrawal of cimetidine.

presystemic elimination

Presystemic elimination – extraction of the drug from blood circulatory system during it’s first going through the liver (first pass metabolism) – it leads to decreasing of bioavailability (and therefore, decreasing of biological activity) of drugs

propranolol (anaprilin), labetolol, aminazin, acetylsalicylic acid, labetolol, hydralasin, isadrin, cortizone, lidokain, morphin, pentasocin, organic nitrates, reserpin

clinical relevance of drug metabolism
Clinical Relevance of Drug Metabolism

The dose and the frequency of administration required to achieve effective therapeutic blood and

tissue levels vary in different patients because of individual differences in drug distribution and

rates of drug metabolism and elimination. These differences are determined by genetic factors and

nongenetic variables such as age, sex, liver size, liver function, circadian rhythm, body temperature,

and nutritional and environmental factors such as concomitant exposure to inducers or inhibitors of

drug metabolism.

elimination of the drugs
Elimination of the drugs

drugs can be excreted in forms of metabolites or unchanged forms through different ways:kidneys, liver, lungs, intestines, sweat and mammary glands etc.

Hydrophilic compounds can be easily excreted.

elimination through kidneys
Elimination through kidneys

filtration,canalicular secretion and

canalicular reabsorption

  • filtration(relative molecular weight of drugs is less than 90,

if 90-300 – with urine and bile):ampicillin, gentamicin, urosulfan, novokainamid, digoxin

  • Disorders of filtration – shock, collapse (due to decreasing of blood circulation and hydrostatic pressure of blood plasma in glomerular capillaries)
  • furosemide (closely connected with plamsa proteins) is not filtrated in glomerular capilaries
  • canalicular secretion – active process (with the aid of enzyme system and using energy):penicillins, furosemide, salicilates, chinin
  • Disorders of canalicular secretion – in case of disorders of energetic metabolism in kidneys: hypoxia, infections, intoxications
glomerular filtration
Glomerular Filtration
  • Normal GFR – 120 ml/min
  • Glomerular capillaries have pores larger than usual
  • The kidney is responsible for excreting of all water soluble substances
  • All nonprotein bound drugs (lipid soluble or insoluble) presented to the glomerulus are filtered
  • Glomerular filtration of drugs depends on their plasma protein binding and renal blood flow - Protein bound drugs are not filtered !
  • Renal failure and aged persons
tubular re absorption
Tubular Re-absorption
  • Back diffusion of Drugs (99%) – lipid soluble drugs
  • Depends on pH of urine, ionization etc.
  • Lipid insoluble ionized drugs excreted as it is – aminoglycoside (amikacin, gentamicin, tobramycin)
  • Changes in urinary pH can change the excretion pattern of drugs
  • Weak bases ionize more and are less reabsorbed in acidic urine.
  • Weak acids ionized more and are less reabsorbed in alkaline urine
  • Utilized clinically in salicylate and barbiturate poisoning – alkanized urine (Drugs with pKa: 5 – 8)
  • Acidified urine – atropine and morphine etc.
tubular reabsorbtion reversed absorbtion
Tubular reabsorbtion (reversed absorbtion)

lipid-solubledrugs are reabsorbedpassively

ionizeddrugs, which are weak acids or alkali are reabsorbed actively

regulation of level of reabsorbtion

- to speed up elimination of drugs – weak alkalis (antihistamine drugs, chinin, theophyllin) urine is made acidic (with ascorbinic acid, ammonium chloride)

- to speed up elimination of drugs –weak acids(NSAID, including ASA, barbiturates, sulfonamides) urine is made alkaline (introduction of sodium hydrocarbonate)

tubular secretion
Tubular Secretion
  • Energy dependent active transport – reduces the free concentration of drugs – further, more drug dissociation from plasma binding – again more secretion (protein binding is facilitatory for excretion for some drugs)
elimination of drugs cont d

with bile – drugs and their metabolites with relative MM over 3000

enterohepatic (intestinal-liver) recirculation:

cardiac glycosides, morphine, tetracyclines

are excreted with bile in unchanged condition (previously not metabolized): antibiotics of tetracyclines group, macrolides

through lungs – gases and volatilesubstances: ether for narcosis, ftorotan, N2O, partly – camphor, iodides, ethanol

through intestine:ftalasol, enteroseptol, magnesium sulfate

throughsweat glands:iodides, bromides, salicylates

through bronchial, salivary glands:bromides, iodides

withmilk:get into organism of the baby – levomycetin, fenilin, reserpin, lithium remedies, meprotan, tetracyclines, sulfonamides etc.