Biotransformation

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Biotransformation

• Prepared by
• Prof .Abdulkader.H.El Daibani

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Biotransformation

• Biotransformation (metabolism) of drugs
• As most drugs are lipid soluble and be
  reabsorbed, biotransformation is essential for
  termination of action and elimination of drugs.

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Biotransformation

• Drug metabolism lead to
• 1) Increase polarity of drugs by converting lipid
  soluble drugs into water soluble drugs and
  therefore enhance excretion.
• 2) Inactivation of drugs
• a) most drugs form inactive metabolites.
• b) With some drugs both the parent drug and the
  metabolite are active.
• e.g.
• Diazepam ? oxazepam
• Herion ? Morphine

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Biotransformation

• c) Other drugs, the parent drug is inactive
  (prodrug) and the metabolite is active and formed
  by hydrolysis of an ester or amide bond e.g.
• enalapril ? enalaprilate
• L-dopa ? dopamine
• Talampicillin ? ampicillin
• Cyclophosphamide ? 4-ketocyclophosphamide

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Biotransformation

• Loss of activity or formation of active
  metabolites occurs mainly in phase I reaction
  where a functional group is introduced or
  exposed. if not rapidly excreted in urine, the
  metabolites of phase I undergo conjugation in
  phase II reactions to form highly polar
  conjugates which are generally inactive and
  rapidly excreted renally.

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Biotransformation

• Sites of drug metabolism
• The liver is the major organ of drug metabolism,
  although other sites (lungs, kidneys, plasma,
  small intestine. Placenta, skin and brain) may
  contribute.
• Cytochrome P450 monooxygenase or mixed function
  oxidase (M.F.O) system is the major catalyst of
  drug biotransformation reaction and located in
  smooth endoplasmic reticulum (SER) of liver and
  other tissues.

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Biotransformation

• Enzymes presented in S.E.R are called Microsomal
  enzyme.
• Non-microsomal enzymes are located in
  mitochondria cytosole phase I enzyme are located
  in S.E.R. while phase II enzyme are located in
  the cytosol.
• Most drugs are metabolized by phase I (oxidation)
  followed by phase II (conjugation) with
  glucouronic acid in some cell.
• e.g.
• PI PII
• paracetamol ? P-OH ?
  P-OH-Gu
• ( p) Oxi Conj.

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Biotransformation

• In human 12 cytochrome P450 gene families have
  been identified.
• The cyto.P450 1, P4502, P4503,(CYP1, CYP2,CYP3).
• Encode the enzyme involved in the majority of
  drug transformation.
• CYP3A4,CYP2D6 are involved in the metabolism of
  all drugs. as a result of low substrate
  specificity among cyt.P450 proteins, two or more
  enzymes may catalyze a given metabolic reaction.

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Biotransformation

• Biochemical pathways of drug biotransformation
• 1)phase I (Non synthetic) reactions
• loss of activity or formation of active
  metabolites occur in phase I ,enzyme of phase I
  located in S.E.R.
• a) Oxidation
• Most important phase I reactions and involve
  (M.O.F) system.
• Aliphatic oxidation tolbutamide,Ibuprofen.
• Aromatic oxidation propranolol, phenytoin
• S oxidation cimetidine, chloropromazine(CPZ)
• N-Oxidation paracetamol,dapsone.
• N-dealkylation morphine,imipramine.
• O-dealkylation codeine,indomethacin.
• Oxidation deamination diazepam,amphetamine.

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Biotransformation

• Cytochrome P450 independent oxidation
• Amine oxidases (M.AO) sympathomimetic
  (adrenaline, Noradrenaline)
• Dehydrogenase-ethanol.
• Oxidative stress
• is a harmful consequence of drug oxidation
  leading to toxicity or carcinogenicity due to
  formation of highly reactive intermediates(FR)
  (free radicals) capable of damaging specific
  cellular components, also some diseases e.g.
  parkinsonism may be due to oxidative stress.

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Biotransformation

• b)Reduction
• Microsomal and non microsomal.
• Nitroreduction Chloramphenicol, clorazepate .
• c) hydrolysis
• ester-succinylcholine, aspirin, procaine by non
  specific estrase in liver and other tissue.
• Amide Lidocaine, Indomethacin by specific
  estrase in liver.
• Peptide-insulin, vasopressin by protease and
  peptidases in plasma, erythrocyte and other
  tissue.

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Biotransformation

• II) phase II (conjugation) reactions
• Also called synthetic reaction because drug or
  metabolite binds with polar endogenous substance
  to form water soluble conjugate which is readily
  eliminated by kidney or in bilegt300 in M.W.
• The drug must possess a chemically active group
  (mainly-OH introduced-phase I) to which the
  conjugation substance is attached.
• The most imp. conjugation reactions are
• a) Glucuronidation.
• b) Sulphation.
• c) Acetylation.
• d) Methylation.
• e) Conjugation with glutathione.
• f) Conjugation with amino acids.

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Biotransformation

• a) Glucuronidation
• Most common phase II reaction. Catalyzed by
  glucuronyl transferase in S.E.R. in liver, kidney
  and other tissues in presence of uridie
  diphosphate glucuronate (DP-G) as glucuronate
  donor to form glucaronides which are
• 1) Generally inactive
• But morphine glucuronide is more active than
  morphine as analgesic.
• 2) rapidly excreted by kidney and bile by an
  anion transport mechanism.
• 3) May be hydrolyzed by ß-glucuronidase of
  intestinal bacterial flora to produce the parent
  drug which is reabsorbed from S.I (E.H
  circulation).

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Biotransformation

• Examples
• Bilirubin, morphine, stilbesterol and
  chlormphenicol is C.I. in neonates because it may
  cause grey baby syndrome due to deficiency of
  glucuronyl transferase in neonates.
• Most biotransformation reaction are oxidation
  followed by glucuronidation.
• oxi. Glu.
• Paracetamol ? para-OH ? para-O-Glu
• phaseI phase II

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Biotransformation

• b) Sulphation
• In presence of 3-phosphadenosin,
  5-phosphosulphate, steroids, heparin and a methyl
  dopa.
• c) Acetylation catalyzed by N-acetyl transferase
  in presence of acetyl CoA as an acetyl group
  donnar limited to drug with primary-NH2.
• e.g. Isonaizid , sulphonamides, hydralazine.
• People who are genetically deficient in N-acetyl
  transferase are called slow acetylators and they
  may exhibit adverse effects from these drugs e.
• INH ? Peripheral neuropathy
• slow acetylator
• Hyralazine and procainamide ?S.L.E
• slow acetylator
  ( Systemic lupus erythrmatosus)

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Biotransformation

• d) Methylation
• Addition of methyl group via methionine in
  presence of COMT e.g. metabolism of adrenaline
  and histamine.
• e) Conjugation with glutathione
• Binds with unstable potentially toxic
  intermediate metabolites e.g. epoxide,
  paracetamol metabolite to form non harmful
  compounds.
• f) Conjugation with amino acids (Glycine and
  glutamine)
• e.g. conjugation of aromatic carboxylic acids
  such as salicylic acid, benzoic acid and
  nicotinic acid with glycine .

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Biotransformation

• Factors affecting biotransformation
• 1) Enzyme induction and enzyme inhibition.
• 2) Genetic polymorphism (oxidation,conjugation)gen
  etic differences in the metabolism of drugs e.g.
  Isoniazid (INH) slow acetylation and fast
  acetylation.
• 3) Environmental pollutants smoking,
  insecticides e.g D.D.T. (enzyme induction).
• 4) Pathological factors liver disease, heart
  failure and shock (? hepatic blood flow).
• 5) Age drug metabolism is reduced in extremes of
  age.

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Biotransformation

• 1)enzyme induction and enzyme inhibitionsome
  drugs stimulate (induction ) or inhibit
  microsomal enzymes in liver.
• a) Enzyme induction causes increased
  activity of microsomal enzyme.
• Involves de novo (synthesis of new protein).

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Biotransformation

• Characteristics of enzyme induction
• 1) reversible process.
• 2) Takes one week to appear and 2-3 weeks to
  disappear after stopping drug.
• 3) ? weight and size of liver due to hypertrophy
  of hepatocytes.
• Some drugs increase metabolism of other drugs and
  their own metabolism (Autoinduction) e.g.
  Carbamazepine.
• Inducing agents are specific for a given P450
  family e.g. polycyclic aromatic hydrocarbons,
  cigeratte smoking and barbequed meat ??CYP1A
  family.
• Glucocorticoids and anticonvulsants ??CYP3A4
  family .
• Acetone and chronic alcohol ?? CYP2E1 family.
• Many inducers of cyt. P450 induce enzyme involved
  in phase II reaction e.g. Glucuronyl transferase
  and glutathione transferase .

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Biotransformation

• Characteristics of inducing agent
• 1) lipid soluble.
• 2) Is a substrate for the enzyme it induces.
• 3) Has long t 1/2.
• 1) Clinically importance of enzyme inductions may
  occur e.g. failure of oral contraceptive or loss
  of anticoagulant, effect of warfarin following
  Rifampin.
• 2) Pharmacokinetic tolerance due to antiinduction
  with antiepileptic drug e.g. carbamazepine and
  others.

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Biotransformation

• 3) useful in therapy- phenobarbitone induces
  bilirubin conjugation so used in treatment of
  Hyperbilirubinemia.
• 4) Disease may result ( Itarogenic Effect)
  antiepilptics ? metabolism and elimination of
  vit.D ? leads to vit. D deficiency? osteomalasia
  in adults or rickits in children.

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Biotransformation

• B) Enzyme Inhibition
• Reduction in enzyme activity after exposure to
  some drugs e.g. cimetidine and ketoconazol
  inhibit oxidation of drugs by forming tight
  complex with heam-iron of cyp P450.
• Erythromycin inhibit CYP3A and its metabolites to
  form a complex with heam of iron of CYP 450.
• Other enzyme inhibitors include INH, sodium
  valproate, metronidazole and ciprofloxacin.
• Enzyme inhibition tend to be more selective than
  enzyme induction.

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Biotransformation

• Enzyme inhibitor ?metabolism of
  clinical effect
• Cimetidine warfarin
  bleeding
• Chloramphenicol tolbutamide
  hypoglycemia
• Erythromycin terfenadineor astrmizole
  arrthmias