Important Genotypes in DrugEnzyme Interactions

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

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
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 http//www.healthanddna.com/professional/depre
  ssion.html

7
New Genotypes Added

• MTHFR
• DPYD
• HLA-B5701
• HER2/NEU
• 5HTT

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 0.34 Japanese 0.42 Africans 0.08

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

10
Folate pathway
MTX

• X

Image downloaded from http//www.pharmgkb.org
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

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

13
HLA-B5701

• 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-B5701 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

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.
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/
  overexpressionstd 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)

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

18

• Thank you Genelex!
• Any questions???

19
References

• MTHFR
• Kremer JM. Methotrexate pharmacogenomics. Ann
  Rheum Dis. 2006 Sep65(9)1121-3.
• 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. 2006
  Sep65(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 methylenetetrahydrof
  olate reductase.Nat Genet. 1995 May10(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.
  2008 Feb8(1)4-15
• Maitland ML, Vasisht K, Ratain MJ. TPMT, UGT1A1
  and DPYD genotyping to ensure safer cancer
  therapy? Trends Pharmacol Sci. 2006
  Aug27(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. 1996
  Aug 198(3)610-5
• HLA-B5701
• Saag M, Balu R, Phillips E, et al Study of
  Hypersensitivity to Abacavir and Pharmacogenetic
  Evaluation Study Team. High sensitivity of human
  leukocyte antigen-b5701 as a marker for
  immunologically confirmed abacavir
  hypersensitivity in white and black patients.
  Clin Infect Dis. 2008 Apr 146(7)1111-8.
• Mallal S, Phillips E, Carosi G, et al PREDICT-1
  Study Team. HLA-B5701 screening for
  hypersensitivity to abacavir. N Engl J Med. 2008
  Feb 7358(6)568-79.
• 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.
  2008 Jun28(6)822-32.
• www.fda.gov/cder/drug/InfoSheets/HCP/abacavirHCP.h
  tm
• HER2/Neu
• Lin A, Rugo HS.The role of trastuzumab in early
  stage breast cancer current data and treatment
  recommendations. Curr Treat Options Oncol. 2007
  Feb8(1)47-60.
• Yamauchi H, Hayes D. HER2 and predicting response
  to therapy in breast cancer. Downloaded from
  www.uptodate.com accessed 2008 July 29.
• 5HTT
• Lesch KP, Gutknecht L. Pharmacogenetics of the
  serotonin transporter. Prog Neuropsychopharmacol
  Biol Psychiatry. 2005 Jul29(6)1062-73.
• Gelernter J, Cubells JF, Kidd JR, Pakstis AJ,
  Kidd KK. Population studies of polymorphisms of
  the serotonin transporter protein gene. Am J Med
  Genet. 1999 Feb 588(1)61-6.