Drug Metabolism Phase 1 Quiz

Drug Metabolism

• Animals have evolved intricate systems to detoxify foreign chemicals (xenobiotics), including carcinogens and toxins from poisonous plants.
• Drugs are a specialized category of xenobiotics, often exhibiting chirality, meaning they exist as different stereoisomers.
• Drug chirality influences metabolism.
• Drug metabolism involves two types of reactions, phase 1 and phase 2, typically occurring sequentially.
• Phase 1 and phase 2 reactions decrease the lipid solubility of the compounds, enhancing renal elimination of xenobiotics.

Phase 1 Reactions

• Phase 1 reactions (e.g., oxidation, reduction, hydrolysis) are categorized as catabolic.
• Products of phase 1 reactions are often more chemically reactive and potentially more toxic or carcinogenic than the original drug.
• Phase 1 reactions introduce reactive groups (e.g., hydroxyl) into the molecule, a process termed functionalization.
• These reactive groups act as attachment points for phase 2 conjugation systems (e.g., glucuronide attachment).
• Phase 1 reactions commonly precede phase 2 reactions due to the functionalization requirement.
• The liver is a crucial organ for phase 1 reactions.
• Hepatic drug-metabolizing enzymes, including CYP enzymes, are localized in the smooth endoplasmic reticulum (microsomal fraction).
• Drugs must cross the plasma membrane to reach these metabolizing enzymes.
• Polar molecules cross the plasma membrane less readily than non-polar molecules, unless specific mechanisms exist.

Cytochrome P450s

• CYP enzymes, haem proteins, are part of a large superfamily of related but distinct enzymes involved in numerous reactions.
• Each enzyme is designated as CYP followed by specific numbers and a letter.
• CYP enzymes vary in amino acid sequences, sensitivity to inhibitors and inducing agents, and reaction specificity.
• P450 enzymes possess distinct but overlapping substrate specificities.
• Classification of P450 enzymes is based on amino acid sequence similarities determined through purification and cloning.
• CYP enzyme family 1-3 mediate around 70-80% of all phase 1 dependent metabolism of clinically used small molecule drugs.
• 12 CYP enzymes account for approximately 93% of drug metabolism in 1839 known drug metabolising reactions.

CYP Reaction Mechanism

• The CYP mechanism requires several components: the drug substrate (DH), P450 enzyme, molecular oxygen, NADPH, and NADPH-P450 reductase (a flavoprotein).
• The mechanism is a cyclical process.
• The reaction outcome is the addition of one oxygen atom from molecular oxygen to the drug, resulting in hydroxylation to form DOH.
• The other oxygen atom forms water.
• Hydroxylation introduces a hydroxyl group into an organic molecule.
• Reduced forms of CYP bind carbon monoxide, resulting in distinct absorbance peaks.
• Drug metabolism can be influenced by factors such as the existence of inducing agents.

P450 Biological Variation

• Species differences influence the choice of species for toxicity and carcinogenicity testing during drug development.
• Variations in P450 enzymes are significant in humans, impacting drug therapies.
• Variations arise from genetic polymorphisms (alterations in DNA sequences) and environmental factors.
• Examples of environmental factors include grapefruit juice (inhibitor), brussel sprouts, and cigarette smoke (inducers), and St. John's Wort (induces CYP450 isoenzymes).
• Drug interactions often occur as a result of one drug influencing another's metabolism.
• Predicting drug interactions and individualizing treatment depend on understanding an individual's phenotype.
• Drugs' effects may not be limited to hepatic enzymes, with plasma, lung, and gut enzymes also involved.

Hydrolytic Reactions

• Hydrolytic reactions are chemical reactions where water functions as a nucleophile to break a chemical bond.
• These reactions occur in plasma and other tissues.
• Both ester and amide bonds are susceptible to hydrolytic cleavage.
• Esters are generally less readily cleaved compared to other bonds.

Reduction Reactions

• Reduction reactions are less common than oxidation reactions in phase 1 metabolism.
• An example of reduction is the conversion of keto groups (in warfarin) to hydroxyl groups.
• CYP2A6 catalyzes this reduction, producing inactive alcohols.
• Warfarin is also oxidized by CYP2C9, resulting in the formation of inactive hydroxylated metabolites.
• The primary route of warfarin inactivation is through oxidation by CYP2C9.

Phase 2 Reactions

• Phase 2 reactions are synthetic (anabolic), involving a substituent group conjugation.
• These reactions often produce inactive products.
• Exceptions include active sulphate metabolites of minoxidil.
• Phase 2 reactions primarily occur in the liver.
• Phase 1 products (e.g., with hydroxyl, thiol, or amino groups) are prone to conjugation.
• Common phase 2 conjugates include glucuronyl, sulphate, methyl, and acetyl groups.
• Examples:
• Glutathione conjugates drugs or phase 1 metabolites.
• Glucuronidation involves transferring glucuronic acid to the substrate.
• UDP-glucuronyl transferase catalyzes these reactions and has broad substrate specificity.
• Acetylation (using acetyl-CoA) and methylation (using SAM) are additional types of Phase 2 reactions.
• Although primarily located in the liver, these reactions also occur in other tissues such as lungs and kidneys.

Stereoselectivity

• Many clinically important drugs, such as sotalol, warfarin, and cyclophosphamide, are mixtures of stereoisomers.
• Stereoisomers can display different pharmacological effects and metabolic pathways.
• Clinically important drug interactions often involve stereospecific inhibition of one drug's metabolism by another.
• Drug toxicity is frequently linked to a single stereoisomer, not necessarily the active form.
• Regulatory authorities recommend, wherever practical, drugs consisting of pure active isomers to reduce complications.
• The practicality of using single isomers is debated, especially when dealing with well-established racemates.
• Enzymatic interconversion of stereoisomers can detract from the benefits of designing drugs as single isomers.

Inhibition of P450s

• P450 inhibitors exhibit selectivity for different isoforms.
• Inhibition mechanisms vary.
• Some inhibitors compete for the active site but are not substrates.
• Non-competitive inhibitors form tight complexes with iron, causing reversible enzyme inhibition.
• Mechanism-based inhibitors bind covalently to the enzyme, leading to self-destruction (suicide inhibition).
• Examples include ketoconazole, and gestodene.

Induction of Microsomal Enzymes

• Enzyme induction, especially with repeated administration, increases the activity of microsomal oxidase and conjugating systems.
• Induction is a result of increased synthesis and/or decreased breakdown of microsomal drug metabolizing enzymes, such as CYP enzymes.
• Induction can increase drug toxicity and carcinogenicity.
• Inducing agents can be used therapeutically (e.g., phenobarbital in premature babies with potential for kernicterus).
• Examples of inducing agents include specific chemicals.
• Some agents influence drug interactions involving transcription factors, like CAR and PXR.

Pre-Systemic Metabolism

• After oral administration some drugs undergo presystemic metabolism in the liver or gut wall.
• Presystemic metabolism reduces the bioavailability of the drug.
• Challenges associated with pre-systemic metabolism include needing higher oral doses, variability among individuals, and potential interactions with other drugs affecting the liver or intestine.

Pharmacologically Active Drug Metabolites

• Some drugs are not active until after metabolism (pro-drugs).
• Examples include azathioprine and enalapril.
• Drug metabolism can change how a drug acts.
• Metabolites of drugs can sometimes have similar effects or have different effects than the original drug (e.g., aspirin produces salicylic acid which has anti-inflammatory effects).
• Metabolites can cause toxicity (e.g., methanol and ethylene glycol).

Interactions Caused by Enzyme Induction

• Enzyme induction is a major cause of drug interactions.
• Enzyme induction can cause variability in drug response.
• Induction may be slower and more complicated.
• Interactions can have adverse effects.
• Pharmacokinetics of some interactions can improve or decrease tolerance/effectiveness.

Interactions Caused by Enzyme Inhibition

• Enzyme inhibition, especially of CYP enzymes, can affect drug metabolism and lead to increased concentrations of other drugs.
• Protease inhibitors from HIV treatment are potent CYP inhibitors, impacting combination therapy.
• Examples of enzyme inhibitors include metronidazole, omeprazole, and amiodarone.
• These can inhibit certain drugs' metabolism.
• Some drugs can inhibit metabolic pathways resulting in their own increased effectiveness.
• Co-prescribing drugs with inhibition potential needs careful assessment to avoid unwanted side effects.