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Clinical perspective on pharmacogenetic labeling

Clinical perspective on pharmacogenetic labeling. Some drugs have such a wide therapeutic range that individualizing the dose is not important. Predicated upon the assumption that getting the right dose of the drug for this disease is important.

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Clinical perspective on pharmacogenetic labeling

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  1. Clinical perspective on pharmacogenetic labeling

  2. Some drugs have such a wide therapeutic range that individualizing the dose is not important

  3. Predicated upon the assumption that getting the right dose of the drug for this disease is important • Probability of response and/or adverse effects related to drug dosing • Titrating drug dose to response is not an optimal approach • Disease too serious to risk period of under-tx • Adverse effects too serious to risk them • Response or adverse effects are delayed or too difficult to monitor

  4. Phenotypes in ALL • All: myelosuppression • VCR: peripheral neuropathy • Prednisone: avascular necrosis • MTX: Delayed neurotoxicity • Cyclophosphamide: sterility, obesity, growth • All: Cure vs relapse • Etoposide: 2nd tumors onset

  5. E. Vessell

  6. Decision-Making • What do I want to know? • How sure do I need to be? • What am I willing to assume?

  7. What do I want to know? • Do specific genetic polymorphisms influence the probability of response or adverse effects? • A general effect of genetics on drug response, without any target genes ID’d, unlikely to be of use for individual prescribing

  8. What do I want to know? • How do polymorphisms affect drug response? (change absorption, metabolism, excretion, distribution, or pdy of the drug) • Tell me this info in the context of other info on factors that affect probability of response/adverse effects (e.g. if metabolism is subject to polymorphism plus other metabolized-drugs on board….)

  9. What do I want to know? • Tell me what doses/routes were tested • Low doses/long exposures don’t saturate enzymes • Hepatic metabolism may be more relevant for oral or prolonged exposures (e.g. MTX, teniposide)

  10. What am I willing to assume? In vitro, preclinical data can be helpful…. • Enzymes saturate: There will likely be competition, and thus possible saturation, if > 1 drug share the same gene products (e.g. low CYP3A activity might be more problematic in a pt taking > 1 CYP3A-substrate/drug than in a pt taking 1 CYP3A-substrate/drug---e.g. VCR, erythromycin, azole) • Effects of polymorphisms gleaned from one drug may have relevance for an independent drug that shares the same gene product (e.g. CYP2D6 PM status should be mentioned for ~all CYP2D6 substrates)

  11. Dose recommendations in relation to P450 genotype Drug Average PM EM UM Enzyme dose (mg) Propafenon 450 40% 130% CYP2D6 Amitriptyline 150 50 % 120% -”- Tropisetron 10 30% 130% -”- Nortriptyline 150 50% 140% 230% -”- Metoprolol 100 30% 140% -”- Lansoprazole 40 20% 110% CYP2C19 Omeprazole 40 20% 110% CYP2C19 S-Warfarin 3 20% 130% CYP2C9 Kirchheiner et al., Acta Psyciatr Scand 104: 173-192, 2001.

  12. What do I want to know? • What is the frequency of the specific genotypes in the 3 largest ethnic/racial groups (whites, blacks, Asians)? • AA, homozygous common or wild-type • Aa, heterozygotes • aa, homozygous variant or defective • Could give allele frequencies, but most clinicians won’t be familiar with calculating genotypic frequencies

  13. Hardy-Weinberg P = frequency of wild-type allele(s) Q = frequency of variant allele(s) 1 = p + q P2 = frequency of wild-type genotype Q2 = frequency of variant genotype 2pq = frequency of heterozygote genotype

  14. What is the difference between phenotype and genotype? • Phenotype is the bottom line, but phenotype can be influenced by concurrent drugs, diet etc • DNA is DNA is DNA (except for allogeneic BMT recipients and if source of DNA is tumor) • Genotype has to be studied only once (unless technology changes to capture more variants and pt was originally genotyped as < homoz. Variant) • Genotype more susceptible to false negatives than phenotype

  15. Genotyping Tests (1) • Multiple types of “variant” and “wild-type” alleles exist for every gene • False negatives: A genotyping test can’t reveal any information about areas of the gene not interrogated by the test (e.g. one can only know that the pt is “wild-type” at the loci tested) • Number of false negatives depends on proportion of inactivating variants accounted for by the tested variants (must be disclosed by the test)

  16. Genotyping Tests (2) • If a pt is “heterozygote” at > 1 polymorphic site in a gene, must understand whether those polymorphisms are allelic (and thus the pt is a likely “heterozygote”) or are likely on separate alleles (and thus the pts is homozygous variant)—but the genotyping test should disclose this • False positives: should not be a problem

  17. Some knowledge of genetics/molecular biology helpful… • Heterozygote phenotypes are generally in between those of the 2 homozygote genotypes • Homozygous variant defined by presence of 2 different variant alleles in many cases • Gene duplications are possible and may “offset” a variant allele

  18. Possible Drug Metabolism Phenotypes and Genotypes • Conserved aa substitutions • Promoter/3´-5´SNPs Frequency • Heterozygous deleterious SNPs • Unstable protein • Stop codons • Deletions • Missense SNPs • Splice defects • Gene duplication • Induction Low Enzyme activity/ drug clearance High

  19. Is this too much to expect? • To monitor effects of imatinib, we need to follow t(9;22) • Can be assessed by cytogenetics or FISH or RT-PCR • Presence of G6PD deficiency predisposes to MetHB for several drugs; • Activity affected by concurrent drugs, hematocrit • Hepatic dosing using Child-Pugh scores • Cardiac output vs CVP for pressors

  20. What do I want to know? • Phenotype: • in general how the test is done (with a blood sample, name of the test) • direction (e.g. low activity is associated with the variant allele and with greater risk of adverse effects) • what interferes with test • Genotype: • estimate of the number of inactivating variants • their approximate frequencies • proportion of phenotype accounted for by the genotypes (e.g. the *5, *19, and *22 inactive/variant alleles account for 85%, 92%, and 90% of low-activity alleles among whites, blacks, and Asians)

  21. What do I want to know? • Negative results can be helpful (e.g. this drug is not a substrate for the genetically regulated CYP2D6 or CYP2C19 enzymes)

  22. How sure do I need to be? Give me some real data…. • Avg/s.d. (median +/- CI) dose in AA vs Aa vs aa pts = 30, 50, and 80 mg/m2 • 50% (95% CI) of pts with vs 10% (95% CI) of pts without toxicity (e.g. QT widening) were AA vs Aa/aa pts • Given a dose of 50 mg, 10%, 30%, and 80% (95% CI) of AA, Aa, and aa pts displayed evidence of response/toxicity

  23. Greater oral mucositis index (OMI) after low-dose MTX among BMT patients with mutant MTHFR C677T genotypes than among patients with C677C genotypes 50% 40% 10% Ulrich et al Blood 98:231-4, 2001

  24. Effect of the CYP2C9 genotype on the daily dose of warfarin to achieve target INR Warfarin, mg/day wt/mut wt/wt wt/var var/var Wadelius et al., 2002

  25. Cure rates for H. pylori infections may depend upon CYP2C19 genotypes • 62 pts with duodenal or gastric ulcer • treated with omeprazole 20 mg and amoxicillin • 20% of Asian and 4% of whites are homo. variant N = 28 25 9 Ann Intern Med 1998;129:1027-30

  26. Polymorphism in TPMT leads to 3 distinct phenotypes, Who differ in their 6MP dosage requirements Evans et al, SJCRH, 2000

  27. Cumulative incidence (CI) of Dosage Decrease based on Genotype Relling et al JNCI, 1999

  28. Labeling • Description • Clinical Pharmacology • Indications and Usage • Contraindications • Warnings • Precautions (general, info for pts, lab tests, drug interxs, carcinogenesis, pregnancy, nursing mothers, pediatrics) • Adverse reactions • Overdosage • Dosage and administration (general, renal, hepatic) • How supplied • References

  29. Labeling: include cross references among sections • Clinical Pharmacology: include mechanism of how polymorphism affects the drug and some references • Warnings: if indicated • Precautions: include lab test information • Adverse reactions: include frequency among different genotypes • Overdosage: mention if genetics likely to affect • Dosage and administration (general, renal, hepatic, and genetic)

  30. Terminology • Use colloquial terms where relevant (EMs, PMs, ultra-rapid, fast, slow, etc) • Avoid “mutant” (prefer variant or defective) • Avoid “normal” (prefer wild-type or common or descriptive, e.g. high-activity allele) • Map HUGO nomenclature-designations to “wild-type/common” or “variant/defective” in the label

  31. Decision Tree for Pgenetics

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