Liver and Advanced Drug Metabolism, University of Portsmouth Presentation

Pharmacology Overview

Neuroscience, Endocrine & Gastrointestinal Pharmacology +/- Therapeutics Liver and Advanced Drug Metabolism Dr. Jeremy Mills Pharmacology University of Portsmouth

Pro-drugs (bioactivation)

• Takes advantage of metabolism to produce pharmacologically active drug metabolites
• Often drugs which are too toxic or may be inactivated before reaching the target tissue
• Given as a parent compound - lacks activity of its own
• 5-fluorouracil - anti-cancer drug activated by cellular enzymes to form 5-FdUMP, then to 5-FdUTP before incorporation into DNA
• Valcyclovir - pro-drug of acyclovir, gives better pharmacokinetics (tissue dose)
• Codeine ...

Inducers and Inhibitors

• Polypharmacy issue
• Phenobarbitone increases the metabolism of warfarin - underdose and reduced blood-thinning effect.
• Alcohol increases paracetamol metabolism - overdose and liver toxicity.

Inducers & Drug Metabolism

metabolite drug metabolite drug Enzyme drug metabolite drug metabolite drug metabolite drug metabolite drug metabolite Inducers increase enzyme levels resulting in higher drug metabolism. Depending on the drug, this can reduce therapeutic effects or lead to toxic builup of metabolites.

Centrilobular P450 induction

e G C C 1A G 1B C C 1D

Inducers and Inhibitors (Continued)

• Cimetidine blocks metabolism of carbamazepine - overdose and side - effects.
• Paroxetine inhibits tamoxifen metabolism - underdose and therapeutic failure.

Inhibitors & Drug Metabolism

Enzyme metabolite drug drug inhibitor metabolite drug metabolite Inhibiting compounds block drug metabolism enzymes, Depending on the drug, inhibition can lead to reduced therapeutic effects or toxic buildup of unmetabolized compounds

Genetic Polymorphisms

• A genetic polymorphism is a monogenic trait that is caused by the presence in the population of more than one allele at the same locus (gene) and more than one phenotype in the organism.
• Polymorphisms do not normally cause illness or other problems that would decrease reproductive efficiency.
• That is why a polymorphism can be maintained in a population at such a high frequency (> 1-50%).
• Single nucleotide polymorphisms (SNPs)

SNP or Mutation?

A ATAGCC G ATAGCC father carrier mother carrier cystic fibrosis carrier carrier unaffected

Possible Polymorphic Changes

• Change the DNA sequence but do not change protein sequence
• Change protein sequence but do not change function
• Create a protein with a different activity
• Create a mutant protein which is non-functional

Metabolic Polymorphisms

• Like other proteins, many drug metabolising enzymes are polymorphic.
• The non-essential nature of some drug metabolising enzymes means that such polymorphisms can have marked effects.
• In most cases, the PM phenotype comprises of several alleles
• Some alleles can produce intermediate phenotypes.

CYP2D6 Polymorphism

• At least 24 different allelic variations possible, giving distinct metaboliser populations - UM (ultra) - EM (extensive) - IM (intermediate) - PM (poor) UM EM IM PM Concentration Concentration Concentration Concentration Time Time Time Time Therapeutic Window Lack of response Expected response Exaggerated response Adverse effects Frequency 0.01 Urinary Parent-Metabolite Ratio of a CYP2D6 Substrate 100

Affected Therapeutic Agents

• 30-40 commonly prescribed drugs - ß-blockers - timolol, pindolol, propanolol - Antidepressants - imipramine, fluoxetine - Antihistamine - chlorpheniramine, promethazine - Also - antiarrhythmics, antitussives, analgesics, antiperistaltics, antipsychotics, nicotine, tamoxifen - Psychedelics - amphetamines, MDMA (xtc)

Codeine Metabolism

-0 H3C Codeine -0 HO. HaC 0 H NH I H N H CH3 HO Normorphine 0 H A 1 CHy OH HO 0 0 H N H CHa 0 H N H H 1 H N H IT CH3 HOWwww Morphine-3- glucuronide Morphine-6- glucuronide OH 0 OH 0 MIOH C HO Codeine-6- glucuronide OH Q OH ICH D HO H H H NH Norcodeine OH H Morphine HO. CH3 0 0 HOW

Impact of CYP2D6 SNPs

Table 1 Assignment of likely codeine metabolism phenotypes based on CYP2D6 diplotypes Likely phenotypeª Activity score Genotypes Examples of diplotypes Ultrarapid metabolizer (~1-2% of patients) >2.0 An individual carrying more than two copies of functional alleles *1/*1xN, *1/*2xN Codeine CYP3A4 10-15% Norcodeine Morphine* Normorphine I 50-70% UGT2B7 60% UGT1A1 Morphine-3-glucuronide UGT2B7 5-10% UGT1A1 Codeine-6-glucuronide Morphine-6-glucuronide* Poor metabolizer (~5-10% of patients) 0 An individual carrying no functional alleles *4/*4, *4/*5, *5/*5, *4/*6 ªThe frequency estimates are based on data from Caucasians and may differ substantially for other ethnicities. See Supplementary Data online for estimates of phenotype frequencies among different ethnic/geographic groups. bNote that some investigators define patients with an activity score of 0.5 and 1.0 as intermediate metabolizers and define patients with an activity score of 1.5 and 2.0 as extensive metabolizers. Classifying patients with an activity score of 1.0 as extensive metabolizers in this guideline is based on data specific for formation of morphine from codeine in these patients. 13 *4/*10, *5/*41 Intermediate metabolizer (~2-11% of patients) 0.5b An individual carrying one reduced and one nonfunctional allele 1.0-2.0b An individual carrying *1/*1, *1/*2, *2/*2, two alleles encoding full *1/*41,*1/*4,*2/*5, or reduced function or *10/*10 one full function allele together with either one nonfunctional or one reduced-function allele UGT2B7 CYP2D6 5-15% Extensive metabolizer (~77-92% of patients)

Pain Control Guidelines

Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for Codeine Therapy in the Context of Cytochrome P450 2D6 (CYP2D6) Genotype KR Crews1, A Gaedigk2, HM Dunnenberger3, TE Klein4, DD Shen5,6, JT Callaghan7,8, ED Kharasch9 and TC Skaar7 Codeine is bioactivated to morphine, a strong opioid agonist, by the hepatic cytochrome P450 2D6 (CYP2D6); hence, the efficacy and safety of codeine as an analgesic are governed by CYP2D6 polymorphisms. Codeine has little therapeutic effect in patients who are CYP2D6 poor metabolizers, whereas the risk of morphine toxicity is higher in ultrarapid metabolizers. The purpose of this guideline (periodically updated at http:// www.pharmgkb.org) is to provide information relating to the interpretation of CYP2D6 genotype test results to guide the dosing of codeine. FOCUSED LITERATURE REVIEW A systematic literature review was conducted that focused on CYP2D6 and its relevance in codeine use (see Supplementary Data online). This guideline was developed based on interpreta- tion of the literature by the authors and by experts in the field. Interpretation of genetic test results Most clinical laboratories report CYP2D6 genotype using the star (*) allele nomenclature and may provide interpretation of the patient's predicted metabolizer phenotype. Single-nucleotide polymorphisms (SNPs) and other sequence variations, includ- ing insertions and deletions, are determined by genetic labora- tory tests. The reference SNP number (rs number) for a given SNP defines the specific genomic nucleotide alteration. Each star (*) allele (or haplotype) is defined by the presence of a spe- cific combination of SNPs and/or other sequence alterations within the CYP2D6 gene locus. The key alleles are shown in Supplementary Table S1 online and the key allele-defining SNPs and their respective impacts on CYP2D6 enzyme function are provided in Supplementary Table S2 online. Genetic results are reported as a diplotype, which includes one maternal and one paternal allele (e.g., CYP2D6*1/*4; Supplementary Table S3 online). In some cases, patients have more than two copies

• Drug dose based on genotype.
• Consider the impact of taking other drugs at the same time.

Codeine Therapy Recommendations

Table 2 Codeine therapy recommendations based on CYP2D6 phenotype Classification of Phenotype Implications for codeine metabolism Recommendations recommendation for codeine for codeine therapyª therapy Ultrarapid Increased formation Avoid codeine use Strong metabolizer of morphine due to potential for following codeine administration, leading to higher risk of toxicity toxicity. Consider alternative analgesics such as morphine or a nonopioid. Consider avoiding tramadol.b Extensive Normal morphine 15-60 mg every Strong metabolizer formation 4 h as needed for pain (label recommendation) Intermediate Reduced morphine Begin with 15-60 metabolizer formation Moderate mg every 4 h as needed for pain. If no response, consider alternative analgesics such as morphine or a nonopioid. Monitor tramadol use for response. Poor Greatly reduced Avoid codeine Strong metabolizer morphine formation use due to lack of following codeine efficacy. Consider administration, alternative leading to analgesics such insufficient pain as morphine or a relief nonopioid. Consider avoiding tramadol.b aRating scheme is described in Supplementary Data online. Although detailed recommendations for using CYP2D6 phenotype in tramadol therapy are beyond the scope of this guideline, there is strong evidence for decreased efficacy of tramadol in poor metabolizers and a single case report of toxicity in an ultrarapid metabolizer with renal impairment following tramadol postsurgery. Use of other analgesics in CYP2D6 poor metabolizers and ultrarapid metabolizers may therefore be preferable. 18,19,21,23

Variable Metabolism

• Clopidogrel is an antiplatelet prodrug activated by polymorphic CYP2C19.
• The response to clopidogrel is highly variable among patients.
• Patients with low or incomplete platelet inhibition are at a greater risk of cardiovascular events COOCH3 <15% H COOCH3 0- S C Clopidogrel (Inactive in vitro) CYP1A2 CYP2B6 CYP2C19 Thiolactone metabolite (Inactive in vitro) >85% CYP3A4/A5 CYP2B6 CYP2C9 CYP2C19 ~50% SR26334 (Inactive in vitro) O H COOCH3 H COOH HO O H COOCH3 HO HS CI S CI SS CI Active metabolite (Platelet inhibition) Thiolactone acid metab. (Inactive in vitro) P2Y 12-Receptor N N H S C

Who to Test?

• The strongest association between CYP2C19 genotype and adverse cardiovascular outcomes appears to be reserved for those patients undergoing PCI with stenting.

1. Target population: ACS/PCI patients
2. Rapid bedside genetic test:
   • Determine CYP2C19 genotype
   • Predict CYP2C19 metabolic status
   • Predict the risk to have impaired response to clopidogrel

Metabolizer Status and Dosing

Rapid (*1/*17, *17/*17) and extensive (*1/*1) metabolizers Intermediate (*1/*2) metabolizers Poor (*2/*2) metabolizers Standard dosing of clopidogrel Adjusted dosing of clopidogrel or new thienopyridines? Switch clopidogrel to new thienopyridines if not contraindicated