Medicinal Chemistry Lecture Drug Metabolism Lectures 11 & 13 Chemical Delivery Systems Joseph O. Oweta | PHS 2201

Objectives 1.General pathways of drug metabolism. 2.Sites of drug biotransformation. 3.Role of Cyp450 monoxygenases. 4.Oxidative reactions. 5.Reductive Reactions. 6.Hydrolytic reactions. 7.Phase II or conjugation reactions. 8.Factors affecting Drug Metabolism.

Metabolism Overview Metabolism aims at changing the structural configuration in order to promote xenobiotic excretion. Xenobiotic are usually lipophilic with a natural propensity for renal reabsorption in the renal tubules. Various enzymes systems thus aim at altering the water solubilities to promote excretion. Subsequently the xenobiotics have terminal activity, altered toxicities. Consequently, metabolic reactions have also been described as detoxification processes.

Metabolism Overview Importance Metabolism is responsible for the in vivo formation of biologically active molecules. Pharmacological effects for the prodrugs Toxic effects:

Sites of drug biotransformation. The Liver Is the most important organ in metabolism and detoxification of endo- and exogenous cpds. It is well perfused. Very rich in metabolising enzymes (most of them!) Orally administered drugs are usually susceptible to the first pass metabolism. May be significant and result in reduced oral bioavailability.

Sites of drug biotransformation. The Intestines

Sites of drug biotransformation. Other Organs Kidneys Lungs Adrenal Glands Brains Placenta Brain Skin

Role of Cyp450 monoxygenases. Nomenclature CYP-ARABIC NUMBER-CAPITAL LETTER ARABIC NUMBER CYP 3A4 CYP: cytochrome P-450 enzymes Arabic Number : Family (CYP 1, CYP 3….) 40% identical amino acid sequence Capital Letter: Sub Family (CYP 1A, CYP 2C, CYP 3A ….) 55% identical amino acid sequence Arabic Number: Individual Enzyme (CYP 1A2, CYP 2C9, CYP 3A4 ….) >90% identical amino acid sequence

Role of Cyp450 monoxygenases. They are membrane bound proteins. Have an approximate molecular weight of 50 kD, and contain a haeme moiety. There are about 30 human cytochrome P450 enzymes. Six, CYP1A2 CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4 are the metabolising enzymes. They are involved in: – The metabolism of many drugs and dietary substances. – Synthesis of steroid hormones and other extracellular lipid signalling molecules.

General Stoichiometry RH + NADPH + O 2 + H +  ROH + NADP + + H 2 O Requires both molecular oxygen and reducing agent NADPH One atom of molecular oxygen is (O 2 ) is introduced to the xenobiotic RH  ROH The other atom is incorporated in water. CYP450 are a super family of mixed function oxidases resp. for the above transfer. Other important factors: – NADPH-dependant cytochrome P450 Reductase – NADH-linked cytochrome b5 They supply the electrons needed in oxidation of xenobiotics Assignement: Read and make notes on Catalytic reaction cycle involving CYP450 in oxidation of xenobiotics

General pathways of drug metabolism Phase I Reactions Drug metabolism may be divided into two categories: Phase I or Functionalization reactions. Phase II or Conjugation reactions.

Phase I/ Functionalization Reactions Include oxidative, reductive and hydrolytic biotransformation rxns. Introduce polar groups into the xenobiotic to produce a more water soluble molecule. – COOH – OH – NH2 – SH The products may not be sufficiently hydrophilic but are suitable precursors for phase II/ Conjugation.

Phase I/ Functionalization Reactions Modalities: a)Direct introduction of the functional group. – E.g. aromatic or aliphatic hydroxylation reactions b)Modifying or ‘Unmasking’ of existing polar functional groups. Existing functional GroupNew functionality Ketones, aldehydes (Reduction)Alcohols Alcohols(Oxidation)Carboxylic Acids Esters, Amide (Hydrolysis)COOH, NH 2, OH Azo, Nitro (Reduction)NH 2 N-, O-, S- (O-dealkylation)NH 2, OH, SH

Oxidation. Are by far, the most common and important in drug metabolism. Normally the first step of drug metabolism Mainly carried out by mixed function oxidases.

Oxidation of Aromatic Moieties Aromatic hydroxylation refers to mixed function oxidation of aromatic cpds (arenes). They corresponding end products being arenols. Proceed initially through an epoxide intermediate known as an “arene oxide” This undergoes spontaneous rearrangement to form an arenol in most cases.

Oxidation of Aromatic Moieties Arene oxides are important in metabolic, toxicological and formation of arenols.

Oxidation of carbon-heteroatom systems. Carbon-heteroatom systems (N, O, S) are commonly present in many drugs. They are metabolized by any of the following oxidation processes : 1.Oxidation or hydroxylation of heteroatom: Ex: N- oxidation, N-hydroxylation, S – oxidation. 2.Hydroxylation of carbon atom attached to the heteroatom followed by cleavage of carbon-heteroatom bond. Ex: N-dealkylation, S- dealkylation, O-dealkylation.

N-Hydroxylation Drugs containing non-basic nitrogen atom (amides), non-basic aromatic amines and basic amines are metabolized by N-hydroxylation.

N-Oxidation Compounds possessing of basic nitrogen are metabolized by N-oxidation process

S-Oxidation Compounds possessing of carbon-sulfur bonds are metabolized to sulfoxides by S-oxidation. The sulfoxides may be excreted as urinary metabolites or oxidized to sulfones (—SO2—).

DEALKYLATIONS.

S-Dealkylation S-Dealkylation involves oxidative cleavage of alkyl carbon-sulfur bonds.

N-Dealkylation (1 O / 2 O Amines ) In the case of primary or secondary amines, dealkylation of an alkyl group starts at the carbon adjacent to the nitrogen;

N-Dealkylation. (3 O Amines ) In the case of tertiary amines, with hydroxylation of the nitrogen. The intermediate products are labile and break up into the dealkylated amine and aldehyde.

O-Dealkylation O-Dealkylation of drugs possessing C—O bond involves hydroxylation of α-carbon to form an unstable hemiacetal or hemiketal intermediates. These intermediates spontaneously cleave to form alcohol and carbonyl compound.

Oxidation of benzylic carbons The carbons directly attached to aromatic rings are oxidized to aldehydes and carboxylic acids via alcohols. Read about oxidation of olefins R R R

Reductive Reactions Drugs containing carbonyl, nitro, and azo groups are metabolized by reduction to alcohols and amines respectively. The reduced compounds are conjugated and eliminated from the body. Read about the reduction of chloral hydrate

Phase II/ Conjugation Reactions Overview Attach small, polar and ionisable, endogenous, functionalities to – handles of phase metabolites. – Parent compounds with existing suitable functional groups. Functionalities include: – Glucuronic acid – Sulphate – Glycine – Amino Acids

Phase II/ Conjugation Reactions Over View The conjugated metabolites are: – Easily excreted. – Generally devoid of pharmacological activity. – Generally devoid of toxicities. Some phase II reactions may attenuate or terminate biological activity. – Methylation. – Acetylation Glutathione (GSH) conjugation protects the body against toxic metabolites.

Glucuronidation Glucuronidation involves conjugation of metabolite or drug molecule with glucuronic acid. In these reactions glucuronic acid molecule is transferred to the substrate from a cofactor. Glucuronides are generally inactive and are rapidly excreted into the urine and bile. Molecules that undergo glu. Rxns are associated with: – phenolic hydroxyl, – Alcoholic hydroxyl, and – carboxylic acid groups. ( Look at general reaction pathways!!)

Sulfation Sulfate conjugation involves transfer of a sulphate molecule from a cofactor (to the substrate (metabolite or drug moiety) by the enzymes (sulfotransferases). Substrate molecules include: – Alcoholic hydroxyl, – phenolic hydroxyl and – aromatic amine groups.

Hydrolysis. Hydrolysis is also observed for a wide variety of drugs. The enzymes involved in hydrolysis are esterases, amidases, and proteases. These reactions generate hydroxyl or amine groups, which are suitable for phase II conjugation.

Read and make notes about factors affecting metabolism

The end.