Clinical perspective on pharmacogenetic labeling

Individualizing Therapy is Important for Drugs witha Narrow Therapeutic Range

e.g. penicillin

Safe to prescribe dose effective for>90% of population

e.g. anticancer drug

Difficult to prescribe effectivenon-toxic dose

narrowtherapeutic range

Dose

100

efficacy

toxicity100

efficacy

toxicity

wide therapeuticrange

Dose

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

• 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

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

E. Vessell

Decision-Making

• What do I want to know?• How sure do I need to be?• What am I willing to assume?

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

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….)

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)

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)

Dose recommendations in relation to P450 genotypeDrug 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.

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

Hardy-Weinberg

P = frequency of wild-type allele(s)Q = frequency of variant allele(s)1 = p + q

P2 = frequency of wild-type genotypeQ2 = frequency of variant genotype2pq = frequency of heterozygote

genotype

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

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)

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

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

Frequency

Low Enzyme activity/ drug clearance High

• Stop codons

• Deletions

• Missense SNPs

• Splice defects

• Heterozygous deleterious SNPs

• Unstable protein

• Conserved aa substitutions

• Promoter/3´-5´SNPs

• Gene duplication

• Induction

Possible Drug Metabolism Phenotypes and Genotypes

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

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)

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)

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

Greater oral mucositis index (OMI) after low-dose MTX among BMT patients with mutant MTHFR C677T genotypes than among patients with C677C genotypes

Ulrich et al Blood 98:231-4, 2001

50% 40% 10%

Effect of the CYP2C9 genotype on the daily dose of warfarin to achieve target

INR

0

2

4

6

8

10

0 1 2 3 4

War

fari

n,

mg

/day

wt/wt wt/mutwt/var var/var

Wadelius et al., 2002

Cure rates for H. pylori infections may depend upon CYP2C19

genotypes

0

20

40

60

80

100

genotype

% c

ured

wild-type

heterozygote

homozygousvariant

• 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

Thiopurine Methyltransferase (TMPT)Genetic Polymorphism and

6MP Dose Requirement

10

8

6

4

2

00 5 10 15 20 25 30

TPMT Activity (units/ml pRBC)

wt/wt

wt/m

m/m

500

250

0m/m wt/m wt/wt

TPMT Genotypes Evans et al, SJCRH, 2000

Polymorphism in TPMT leads to 3 distinct phenotypes,Who differ in their 6MP dosage requirements

Cumulative incidence (CI) of Dosage Decrease based on Genotype

Relling et al JNCI, 1999

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

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)

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

Decision Tree for Pgenetics

N o Y esp h eno type vs g eno type

N oresp o nse re la ted to p gen?

Y esu se th em

R espo n se rela ted to s im ple lab tes ts?

N o* la te e ffec ts

* in vas ive m o n itor ing

Y esad jus t do ses based on resp on de

ab ility to titra te d ose intra -in d ivid u alT yp e title h ere