Week 8 - Conjugation Reactions + Metabolic Elimination of Drugs Flashcards
What are the 2 types of metabolic reactions
- Oxidation (Phase 1)
- drug binds to CYP (CYP450)
- forms drug-CYP complex
- oxidation occurs and metabolite is formed
- metabolite formed can also go through phase 2
- metabolite is renally excreted in urine - Conjugation (Phase 2)
- conjugate metabolite can be formed directly from drug or from its metabolite
- need to have certain functional group for this to occur
- conjugate metabolites can be renally excreted (urine) or biliary excreted (liver ~ stools)
What is a conjugation reaction
A type of metabolism
- Require specific functional group (-OH, -COOH, -NH2)
- group that is an electrophilic acceptor = accepts e-
- all listed groups can perform glucuronidation (most common conjugating reaction) - Drug/metabolite joins with conjugating molecule
- conjugating molecules inc. glucose, amino acid - Produces conjugate metabolite (= terminal metabolite)
- metabolite is very hydrophilic, polar and large - Conjugate metabolite is excreted (renal or biliary) or can go into systemic circulation (via transporters)
Glucuronidation will form glucuronide metabolites
Conjugates:
- most conjugates are inactive
- some can be renally toxic
- some can contribute to DDIs
How does glucuronidation via UGT enzymes occur
UGT are glycoproteins that catalyse the addition of glucuronic acid to drug (containing specific group)
- e.g. UGT1A and UGT2B
- UGT enzymes are mainly present in liver (also found in kidney + small intestines)
- UGT mediates glucuronidation of many drugs + endogenous compounds
- e.g. bilirubin, bile acids, steroid hormones etc.
- unconjugated bilirubin build up can cause jaundice - Glucuronidation eliminates drugs + endogenous compounds
How does the inter-individual variability in UGT activity impact on pharmacokinetics and pharmacodynamic effect of drugs
- UGT1A1 activity + expression is low in new borns
- ↓ activity leads to congenital jaundice due to build up of unconjugated bilirubin (in blood)- UGT1A1 deficiency in adults leads to jaundice as well
- Variants of UGT1A1 can also cause reduced conversion SN-38
- SN-38 (active) gets glucuronidated to SN-38G (inactive) by UGT
- ↑ conc. of SN-38 = neutropenia (↓ neutrophils in blood = infection)
How does sulphation (SULT) reactions work (Phase 2)
Another conjugating reaction
- Has overlapping substrates with UGT (-OH, COOH)
- SULT enzymes are present in liver + small intestines
- metabolise drugs, sulphates
- drug metabolised will form sulphate (metabolite) - SULT require activation via ATP
Describe the properties of Phase 1 metabolic enzymes (CYP450)
Oxidation = most common / important Phase 1 reaction
- Drug binds to CYP forming drug-CYP complex
- CYP catalyses oxidation
- Change in chemical structure of drug = metabolite formed
- may add functional group to allow glucuronidation to occur
Classification of CYP (CYP = Cytochrome P450)
1st number = the family (it belongs to ~ 18 families)
2nd letter = the subfamily
3rd number = the individual gene (/ member of the subfamily)
- CYP3A4 / CYP3A5 are most abundant in liver + intestines
- CYP2D6 has smaller expression but many frugs are metabolised via this enzyme
- Each enzyme has its own degree of specificity
- CYP3A4 isn’t specific = any lipophilic molecule will bind + be metabolised
What are the differences in the properties of the parent drug and its metabolites
- Metabolites are usually inactive + less toxic than parent drug
- active metabolites inc. diazepam, codeine
- diazepam can be metabolised to form 3 active metabolites
- pro-drugs (inactive drug) can produce active metabolites - Metabolites are large, more polar, more hydrophilic, more ionised = excreted easier than unchanged drug
- Metabolites can have ↑ conc. than parent drug = can contribute to therapeutic effects
What are the potential efficacy and toxicities of metabolites (ADRs)
Reactive metabolites can cause ADRs
Types of ADRs
1. Type A (majority of ADRs)
- is reversible
- can be avoided by modifying dose or not co-administering drugs
- can occur if high dose is given or DDIs occur
- effects of ADR can be linked to the pharmacological effect of drug (A1) or effects can be unrelated (Type A2)
- Type B/C/D
- are irreversible + more problematic
- can occur if have reactive metabolite or genetic factors
- i.e. correct dose but lack gene
Report ADRs with Yellow Card System
How does paracetamol overdose happen
Paracetamol has 3 metabolism reactions that occur at the same time
- CYP (oxidation) pathway - produces NAPQI (toxic)
- Glucuronidation pathway - produces paracetamol glucronide
- Sulphation pathway - produces paractemal sulphate
- NAPQI is quickly metabolised into gluthionine conjugation (non-toxic)
- Increase paracetamol intake = glucuronidation + sulphation pathway becomes saturated
= ↑ NAPQI formed - Some NAPQI will be converted into gluthionine but some will bind to proteins + nucleic acid = liver cell death / liver failure
Give N-acetyl cysteine to eliminate reactive NAPQI
What is the impact of disease on enzymes (CYP)
- In cancer + HIV get ↓ activity of CYP3A4, CYP2D6
- Coeliac disease, CKD, liver cirrhosis effects expression of CYPs
- Illness can cause a change in the release of pro-inflammatory mediators (e.g. interleukin ~ IL)
- IL causes downregulation of CYP = lower expression = ↓ activity = metabolism impacted
What is the impact of genetic polymorphism of enzymes (CYP2D6) on drug pharmacokinetics
- Need to know what drugs are metabolised by what enzymes
- impact pKa of drug, efficacy and safety - Some people may be poor metabolisers due to lack of CYP2D6
- drug accumulates as can’t eliminate it
- drugs made should NOT only be metabolised by this gene
Genotypes of CYP2D6
1. Poor Metaboliser (PM)
- lack CYP2D6 gene = can’t metabolise / remove drug
- ↑ risk of side effects due to ↑ plasma conc. of drug
= adjust dose regimen (lower)
- Extensive Metaboliser (EM)
- normal wild type genotype (have normal expression of gene) - Ultra-rapid Metabolisers (UM)
- have high expression of gene / duplication of gene
- drug metabolised extensively = no therapeutic effect with standard dose (low plasma conc. of drug)
= adjust dose regimen (increase dose)
What causes DDIs
Inhibition or induction of drug metabolising enzymes in liver or small intestines
- Metabolites can cause inhibition of enzymes = DDIs
- Drugs with severe DDIs may be removed from market
DDIs
- When developing drugs there are specific enzymes which need to be tested
- identify if drug is a substrate, inhibitor or inducer
