Emerald MurrellPharmD Candidate 2011

8/27/10

About CYP2B6Located on chromosome 19Makes up about 2-10% of hepatic CYP

contentAlso expressed in brain (significantly higher

in smokers and alcoholics)Metabolizes 4% of the top 200 drugs Highly inducible (some substrates also

autoinduce, and it is also inducible in stressed conditions, e.g. fasting)

High interindividual variation

SubstratesArtemisin Methadone

Bupropion Nicotine

Cyclophosmamide Propofol

Efavirenz Pethidine

Ifosfamide Selegiline

Ketamine Sertraline

Meperidine Tamoxifen

Diazepam Testosterone

Mephenytoin Nevirapine

Aflatoxin b1 MDMA (ecstasy)

Sibutramine

Inhibitors and Inducers of 2B6Inhibitors

Clopidogrel- Potent inhibitor OC/HRT

Efavirenz Paroxetine

Fluoxetine Chlorpyrifos (insecticide)

Fluvoxamine Sertraline

Memantine Thiotepa

Nelfinavir Ticlopidine- Potent inhibitor

Clotrimazole & Itraconazole Raloxifene

Inducers

Lopinavir/Ritonavir Phenytoin

Phenobarbital Rifampin

Dexamethasone Carbamazepine

Metamizole Efavirenz (autoinduces)

Cyclophosphamide (auto) Artemisinin (autoinduces)

Endosulfan

DefinitionsCYPs (or Cytochrome P450 genes) produce enzymes

involved in synthesis and breakdown of molecules and are subdivided into groups and subfamilies (CYP3A4, CYP2C9, etc). Enzymes mostly found in liver, but also found throughout body (brain, intestine, etc). Different combinations of polymorphisms in these genes are haplotypes (CYP2B6*6, CYP2B6*1).

Haplotype= a combination of alleles (DNA sequences) at different places (loci) on the chromosome that are transmitted together

Non-synonymous= a SNP in which both wildtype and mutant forms lead to the same polypeptide sequence is termed synonymous (sometimes called a silent mutation) — if a different polypeptide sequence is produced they are nonsynonymous.

SNP nomenclature:

Haplotypes with decreased activity in vivo

CYP2B6*6, CYP2B6*16, CYP2B6*26

Haplotype with increased activity in vivo

CYP2B6*4

Where haplotypes are important in regard to therapy:Efavirenz for HIV treatmentCyclophosphamide for chemotherapyBupropion for smoking cessationCardiac side effects and death from

methadone

Effect on Efavirenz Therapy CYB2B6*6 haplotype

CYP2B6*6 has two nonsynonymous variants:CYP2B6:516G>T AND CYP2B6:785A>GWith or without additional promoter variants:-1456T>G AND -750T>C or -750T>C

Present in 15-40% Asian, >50% Black population, 26% Caucasian

• Successful Efavirenz dose reduction to 400mg or 200mg was done for CYP2B6*6/*6 and *6/*26 carriers whose EFV concentrations were extremely high (>6000 ng/ml).

Effect on Efavirenz Therapy516G>T

The 516G>T SNP that is present in CYP2B6*6 is also present in *7, *9, and *13

Presence is associated with 3-fold decrease in CYP2B6 activity

The TT genotype is associated with increased efavirenz exposure (resulting in CNS side effects). Also predicts increased risk of developing drug resistance among patients who discontinue efavirenz-containing regimens

983T>CDefining SNP of CYP2B6*18 allele, and found in linkage in

the CYP2B6*16 haplotypeNot observed in Asians, Caucasians. 4-9% Blacks, low

frequency HispanicsAssociated with increased efavirenz and nevirapine

exposure

Effect on Bupropion Therapy CYP2B6*4

Presence of 785A>G SNPHigh frequency in ethnic

groups (15% Asian, 50% Black)Clearance of bupropion increased 1.66-fold

• *1/*4 genotype had AUC of hydroxybupropion (active metabolite formed by 2B6) increased by 25% compared to *1/*1

Effect on Cyclophosphamide Therapy

CYP2B6*6 homozygotes have significantly higher clearance and shorter half-life of cyclophosphamide compared to CYP2B6*1 heterozygotes and homozygotes

Specific SNPs in the promoter region or introns (-2320T>C, -750T>C, 15582C>T of intron 3, or 18492T>C) of CYP2B6 affected cyclophosphamide activation to 4-hydroxycyclophosphamide (AUC of 4-hydroxycyclophosphamide is significantly related to leukocytopenia and neutropenia). This knowledge can help predict adverse effects.

Effect on Doxorubicin and Cyclophosphamide Therapy

Higher dose delay (indicative of higher toxicity) was seen in CYP2B6*2 and CYP2B6*5

CYP2B6*2, CYP2B6*8, CYP2B6*9, CYP2B6*4 were associated with a worse outcome

Bray J, Sludden J, Griffin MJ, Cole M, Verrill M, Jamieson D, Boddy AVInfluence of pharmacogenetics on response and toxicity in breast cancer patients treated with doxorubicin and cyclophosphamide. Br J Cancer 2010; 102(6): 1003-1009.

Effect on Methadone TherapyMethadone is a racemic mixture

(R)-methadone: narcotic effect by activating µ-opiod receptor

(S)-methadone: inhibits cardiac potassium channelCYP2B6 is selective for (S)-methadone metabolismHaving decreased CYP2B6 activity increases

plasma concentrations of (S)-methadone (but not (R)-methadone) and increase risk of cardiac side effects and death.*6/*6 genotype had increased risk of prolonged QTc

References Bray J, Sludden J, Griffin MJ, Cole M, Verrill M, Jamieson D, Boddy

AVInfluence of pharmacogenetics on response and toxicity in breast cancer patients treated with doxorubicin and cyclophosphamide. Br J Cancer 2010; 102(6): 1003-1009.

Kirchheiner J, Klein C, Meineke I, et al. Bupropion and 4-OH-bupropion pharmacokinetics in relation to genetic polymorphisms in CYP2B6. Pharmacogenetics. 2003;13:619–26.

T.E. Klein, J.T. Chang, M.K. Cho, K.L. Easton, R. Fergerson, M. Hewett, Z. Lin, Y. Liu, S. Liu, D.E. Oliver, D.L. Rubin, F. Shafa, J.M. Stuart and R.B. Altman, "Integrating Genotype and Phenotype Information: An Overview of the PharmGKB Project" (220k PDF), The Pharmacogenomics Journal 2001; 1:167-170.

Thorn CF, Lamba JK, Lamba V, Klein, Teri E, Altman RB. PharmGKB summary: very important pharmacogene information for CYP2B6. Pharmacogenet Genomics 2010; 20(8):520-523.

Wang H, Tompkins LM. CYP2B6: new insights into a historically overlooked cytochrome P450 isozyme. Curr Drug Metab. 2008;9:598–610.

Wynn, Gary H, Jessica R Oesterheld, Kelly L Cozza, and Scott C Armstrong. Clinical Manual of Drug Interaction Principles for Medical Practice. Washington, DC: American Psychiatric Publishing, 2009.

References Eap CB, Crettol S, Rougier JS, Schlapfer J, Sintra Grilo L, Deglon JJ,

Besson J, Croquette-Krokar M, Carrupt PA, Abriel H. Stereoselective block of hERG channel by (S)-methadone and QT interval prolongation in CYP2B6 slow metabolizers. Clin Pharmacol Ther. 2007;81(5):719–28.

Ribaudo HJ, Haas DW, Tierney C, Kim RB, Wilkinson GR, Gulick RM, Clifford DB, Marzolini C, Fletcher CV, Tashima KT, Kuritzkes DR, Acosta EP. Pharmacogenetics of plasma efavirenz exposure after treatment discontinuation: an Adult AIDS Clinical Trials Group Study. Clin Infect Dis. 2006;42(3):401–7.

Gatanaga H, Hayashida T, Tsuchiya K, Yoshino M, Kuwahara T, Tsukada H, Fujimoto K, Sato I, Ueda M, Horiba M, Hamaguchi M, Yamamoto M, Takata N, Kimura A, Koike T, Gejyo F, Matsushita S, Shirasaka T, Kimura S, Oka S. Clin Infect Dis. 2007;45(9):1230–7.

Nakajima M, Komagata S, Fujiki Y, Kanada Y, Ebi H, Itoh K, Mukai H, Yokoi T, Minami H. Successful efavirenz dose reduction in HIV type 1-infected individuals with cytochrome P450 2B6 *6 and *26. Pharmacogenet Genomics. 2007;17(6):431–45.