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Important Genotypes in Drug-Enzyme Interactions

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Presentation on theme: "Important Genotypes in Drug-Enzyme Interactions"— Presentation transcript:

1 Important Genotypes in Drug-Enzyme Interactions
Nissa Abbasi-Shaffer University of Washington School of Pharmacy Doctoral Candidate, 2009 August 1, 2008

2 Outline Introduction Genotypes in GeneMedRx Additions to GeneMedRx

3 Drug-Enzyme Interactions
The activity an enzyme will have on a substrate is determined in part by the physical enzyme-substrate interaction (Nonsynonymous) Mutations in genes can affect the protein product interaction: Conformational changes pH changes H-bonding changes Patient response to drugs can be altered by their genotype Nonsynonymous = changes amino acid sequence This interaction is true for both the metabolizing enzymes and target enzymes…examples of both ahead “Drug response” includes both efficacy and toxicity

4 Genotypes in GeneMedRx
Function in GeneMedRx for genotype selection Users can select appropriate genotype and medication profile to determine if an “interaction” exists: Metabolic enzymes: Reduced efficacy: eg. CYP2D6 UM Increased toxicity: eg. CYP2C9 PM Target sites: Efficacy: eg. Her2/Neu Toxicity?

5 Updated Genes CYP2D6 CYP2C9 CYP2C19 NAT2 TPMT UGT1A1
Each phenotype was given a descriptor so that if selected, regardless of whether a known drug interaction exists, the significance of the genotype is displayed Phase I

6 e.g. CYP2D6 Poor Metabolizer updated note
Gene: 2D6 Poor Metabolizer Cytochrome P450 2D6 Poor Metabolizers (PM) have absent or greatly reduced CYP2D6 activity. They are identified by DNA testing and comprise up to 12% of the population. Drugs that are substrates of 2D6 will reach higher levels in 2D6 PMs and may cause toxicity. The increase is often substantial, roughly equal to adding a strong 2D6 inhibitor drug. Poor metabolizers may need to be started at 20-60% of standard doses for drugs metabolized by 2D6. Prodrugs that require activation by CYP2D6 may be less effective in PMs, including: tamoxifen, codeine, tramadol, risperidone and encainide. Recommendations for 2D6 genotype-based changes in dosage for many psychotropic drugs can be found at: An additional note will be displayed if, on the patient profile, a CYP2D6 metabolite was also chosen, eg. codeine--indicating that this drug has limited effect in a 2D6 PM.

7 New Genotypes Added MTHFR DPYD HLA-B*5701 HER2/NEU 5HTT Phase 2

8 MTHFR Methylene tetrahydrofolate reductase is an important enzyme in folate metabolism Allows for methyl group transfer needed for DNA synthesis and methylation Reduced activity is associated with homocysteine accumulation Folate status may be associated with malignancy (reduced -CH3) Position C677T mutation results in loss-of-function of reductase enzyme T/T 70% reduction: seen in ~10% of North Americans C/T 40% reduction: seen in ~40% of North Americans One study reports 677T allele frequencies of: Caucasians Japanese Africans 0.08 DNA Hypomethylation may allow for proto-oncogene activation The T/T, C/T, & C/C genotypes have been added to GeneMedRx

9 Methotrexate Folate analog that inhibits dihydrofolate reductase (also important in folate metabolism) MTX used to treat many cancers, rheumatoid arthritis & psoriasis T/T genotype associated with increased toxicity: Fever, malaise, GI, skin, pharyngeal, pulmonary mucositis & extended thrombocytopenia reported in leukemia pts Genotype is important in predicting response Recap: T/T genotype associated with increased levels of homocysteine & increased toxicity from MTX

10 Folate pathway MTX X Image downloaded from

11 DPYD Dihydropyrimidine dehydrogenase is the initial and rate-limiting enzyme in pyrimidine (uracil & thymine) catabolism Exon 14-skipping mutation GT to AT mutation located in the 5’-splicing consensus sequence Causes exon 14 of DPYD to be spliced out and not expressed in protein product, DPD 165 bp deletion = 55 amino acids absent Complete loss-of-function Mutation found in ~1% of European-based population DPD PM and NM added to database

12 Fluorouracil Analog of uracil
DPD is the initial enzyme in its metabolism Reduced DPD activity causes increase 5FU levels 5FU is widely used in cancer therapy (breast, colon, head & neck, etc.) Toxicity from 5FU is reported to occur in those who have even 50% of DPD activity--one null allele can be enough Toxicity can be severe, including mucositis, granulocytopenia In one study 24% of patients with WHO grade IV toxicity were found to have the exon 14-skipping mutation PM will need dose reduction Another example of a genotype that is an important predictor of drug response

13 HLA-B*5701 The HLA class I molecules present antigens to killer T cells to induce immunity Some allotypes within this class have a propensity to induce allergic-type reactions The HLA-B*5701 genotype is associated with allergic reactions to abacavir Found in ~5% of the European-based population (less common in African Americans) Abacavir Hypersensitivity Syndrome (AHS) Graft vs host-like response, generally appears within 6 wks Symptoms can include rash, fever, malaise, nausea, vomiting, diarrhea, respiratory problems, and can be fatal A reaction requires immediate discontinuation of the drug; do not rechallenge Abacavir is a reverse-transcriptase inhibitor used commonly in the treatment of HIV. It is a prodrug, and there is some discussion that the conversion to active drug produces the antigen that triggers the reaction. Genotypes added to GeneMedRx: HLA-B*5701 Positive or Negative

14 FDA Alert Last week (7/24/2008) the FDA issued an alert recommending all patients be genotyped before starting or restarting abacavir due to the high correlation of AHS to this drug, the potential severity of the symptoms and the ease of preventing the reaction by DNA testing. Very important genotype in preventing toxicity

15 HER2/Neu HER2 belongs to the Epidermal Growth Factor Receptor family, associated w/ cellular growth and differentiation Overexpression of the HER2 protein found in 18-20% of breast cancer resulting in more aggressive tumors Clinical testing (IHC and FISH) to screen overexpressors determines course of therapy: targeted or not better response to anthracyclines Current targeted therapies: Trastuzumab (Herceptin) efficacy associated w/ overexpression—std treatmant for early stage Her2/Neu positive breast cancer (given in combo w/ chemo) Lapatinib is an oral TKI targeted to Her2/Neu in patients who are overexpressors who failed Herceptin therapy (given in combo with capecitabine) EGFR family = transmembrane tyrosine kinase receptors; transmit cellular signals for growth and differentiation Overexpressors: more aggressive tumors, more nodal involvement, poorer overall survival--determines course of therapy GeneMedRx: Added HER2/Neu positive or negative

16 5HTT/SLC6A4 The serotonin transporter (5HTT) reuptakes serotonin from the synaptic cleft into the presynaptic terminus 5HTT is the target of SSRIs--limiting reuptake allows for more serotonin transmission Variability in SSRI efficacy within the general population has lead the search for a genetic explanation Discovery of 5HTTLPR polymorphism

17 5HTTLPR The serotonin transporter-linked polymorphic region is located in the promoter region of 5HTT 44-bp insertion/deletion produces the long (L = 16 repeat units) and short (S = 14 repeat units) alleles S allele results in 50% of the transcriptional activity as the L allele ~40% of North Americans carry at least one S allele (varies from 10-70% globally) Other lengths have been discovered, but very rare in the population: 15, 18-20, 22 repeated units S allele is associated with reduced efficacy to SSRIs Hypothesis: if less transporter made, less target for SSRIs Carriers of the S allele often take longer to respond or do not respond at all Genotyping can be used to identify candidates GeneMedRx: added short allele and long allele

18 Thank you Genelex! Any questions???

19 References MTHFR: Kremer JM. Methotrexate pharmacogenomics. Ann Rheum Dis Sep;65(9): Hughes LB, Beasley TM, Patel H, et al.Racial or ethnic differences in allele frequencies of single-nucleotide polymorphisms in the methylenetetrahydrofolate reductase gene and their influence on response to methotrexate in rheumatoid arthritis. Ann Rheum Dis Sep;65(9):1213-8 Frosst P, Blom HJ, Milos R, Goyette P, Sheppard CA, Matthews RG, Boers GJ, den Heijer M, Kluijtmans LA, van den Heuvel LP, et al. A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase.Nat Genet May;10(1):111-3 DPYP: Tomalik-Scharte D, Lazar A, Fuhr U, Kirchheiner J. The clinical role of genetic polymorphisms in drug-metabolizing enzymes. Pharmacogenomics J Feb;8(1):4-15 Maitland ML, Vasisht K, Ratain MJ. TPMT, UGT1A1 and DPYD: genotyping to ensure safer cancer therapy? Trends Pharmacol Sci Aug;27(8):432-7 Wei X, McLeod HL, McMurrough J, Gonzalez FJ, Fernandez-Salguero P. Molecular basis of the human dihydropyrimidine dehydrogenase deficiency and 5-fluorouracil toxicity. J Clin Invest Aug 1;98(3):610-5 HLA-B*5701: Saag M, Balu R, Phillips E, et al; Study of Hypersensitivity to Abacavir and Pharmacogenetic Evaluation Study Team. High sensitivity of human leukocyte antigen-b*5701 as a marker for immunologically confirmed abacavir hypersensitivity in white and black patients. Clin Infect Dis Apr 1;46(7): Mallal S, Phillips E, Carosi G, et al; PREDICT-1 Study Team. HLA-B*5701 screening for hypersensitivity to abacavir. N Engl J Med Feb 7;358(6): Chessman D, Kostenko L, Lethborg T, et al. Human leukocyte antigen class I-restricted activation of CD8+ T cells provides the immunogenetic basis of a systemic drug hypersensitivity. Immunity Jun;28(6): HER2/Neu: Lin A, Rugo HS.The role of trastuzumab in early stage breast cancer: current data and treatment recommendations. Curr Treat Options Oncol Feb;8(1):47-60. Yamauchi H, Hayes D. HER2 and predicting response to therapy in breast cancer. Downloaded from: [accessed 2008 July 29]. 5HTT: Lesch KP, Gutknecht L. Pharmacogenetics of the serotonin transporter. Prog Neuropsychopharmacol Biol Psychiatry Jul;29(6): Gelernter J, Cubells JF, Kidd JR, Pakstis AJ, Kidd KK. Population studies of polymorphisms of the serotonin transporter protein gene. Am J Med Genet Feb 5;88(1):61-6. Heils A, Teufel A, Petri S, Stöber G, Riederer P, Bengel D, Lesch KP. Allelic variation of human serotonin transporter gene expression. J Neurochem Jun;66(6):


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