Mabel Rodriguez
Introduction
Pharmacogenetics: Effect of heritability on response to drugs. Study of single genes & their effects on interindividual differences in drug metabolizing enzymes.
Pharmacogenomics: Effect of inherited & acquired genetic variation on drug response. Study of the functions & interactions of all genes in the genome & how the overall variability of drug response may be used to predict the right tx in individual pts & to design new drugs.
Polymorphisms: Common variations in a DNA sequence that may lead to ↓ or ↑ activity of the encoded gene (SNP, micro- & minisatellites)
SNPs: Single base interchange that may cause an amino acid exchange in the encoded protein, account for >90% of genetic variation in the human genome
Phase I Drug Metabolizing Enzymes
Genetic polymorphisms have been reported for CYP450 isoforms 2C8, 2C9, 2C19, 2D6, & 3A4
CYP3A4/5
CYP3A isoenzymes account for ∼1/2 of the metabolic activity of all CYP450 enzymes
Substrates include etoposide, paclitaxel, & vincristine
SNP frequencies of CYP3A4 relatively low in most populations (14% in Caucasians & 10% in Japanese individuals) making it difficult to link genotype to PK or PD of CYP3A4 substrates
One study conducted by the National Children’s CA Group Pediatric ALL trial set out to correlate the association between relapse & tox of 1204 pts w/CYP3A41B,CYP3A5*3, & CYP3A5*6 SNPs
Trial results were negative, however the first two variants previously mentioned had a statistical risk of peripheral neuropathy
CYP2C8
A/w paclitaxel metabolism by 6α-hydroxylation; out of its six alleles
CYP2C8*3 in Caucasians is less efficient in the metabolism of this chemotherapeutic agent
CYP2C19
A/w thalidomide activation by hydroxylation
CYP2C19*2 homozygotes showed poor metabolizing phenotype, not responding to thalidomide tx
CYP2D6
Involved in the conversion of tamoxifen to 4OH-tamoxifen (more potent antiestrogen)
CYP2D6 homozygous variant or heterozygous genotype individuals have ↓ plasma endoxifen conc. (a metabolite of tamoxifen) than homozygous wild-type subjects after 4 mos of Rx
Non-P450 Phase I Metabolizing Enzymes
Dihydropyrimidine dehydrogenase (DPD)
Represents the first & rate limiting step in the catabolism of pyrimidine bases, uracil & thymidine
DPD degradates the majority of 5-FU to 5–6-dihydro-5-FU in the liver
DPD deficiency: Prolonged 5-FU T1/2 → sev., fatal GI & neurologic tox
Thirty nine polymorphisms & Mt of the DPD gene are known. DPD*2A: Most common polymorphism → causes inactive protein in 0.9% of Caucasian pts.
Phase II Drug Metabolizing Enzymes
Thiopurine methyltransferase (TPMT)
Involved in the methylation rxns of 6-MP & its prodrug, AZA
Out of the 8 alleles identified, three specific ones (TMPT*2, TMPT*3A, & TMPT*3C), account for 95% of TMPT deficiency cases
Pts who carry TMPT polymorphisms are at a risk of sev. hematologic tox
Appropriate 6-MP dose reductions for TMPT-deficient pts have allowed for similar tox & survival outcomes when compared to pts w/nl levels
Tests for TMPT genotype & phenotype are commercially available
Uridine Diphosphate Glucuronosyltransferase 1A1 (UGT1A1)
A microsatellite Mt results in UGT1A1*28 polymorphism, & ↓ UGT1A1 expression levels & activity
UGT1A1*28 polymorphism: Typically found in pts w/Gilbert syndrome; frequency 40–43% in Africans, 32–39% in Caucasians, 16–33% in Asians
Important role in the metabolism of etoposide, epirubicin, & most importantly irinotecan
Irinotecan: Prodrug activated to ethyl-10 hydroxycampothecin (SN-38) via carboxyl esterase. SN-38 undergoes UGT1A1-catalyzed glucuronide conjugation to form inactive SN-38 glucuronide (SN-38G). ↑ exposure to SN-38 in pts w/this genetic variation → ↑ tox (diarrhea, myelosuppression) limiting optimal clinical use
Drug Transporters
P-gp (MDR-1)
P-gp & its encoding gene MDR-1 (ABCB1) are expressed in high levels in brain vessels, adrenal glands, kidney, liver, & GI tract
P-gp is involved in the transport of hydrophobic drugs including doxorubicin, paclitaxel, irinotecan, hormones, & carcinogens
Multidrug resistance proteins act as drug efflux pumps rendering cells resistant to cytostatic drugs
Three most common SNPs are P-gp*6 (22–56% in Caucasians), P-gp*7, & P-gp*8
Drug Targets
5,10 Methylenetetrahydrofolate reductase (MTHFR)
Reduces 5, 10 methylenetetrahydrofolate to 5-methyltetrafolate, methyl donor for the methylation of DNA, homocysteine, & DNA synthesis
Reduced MTHFR activity affects intracellular folate pools → ↑ tox in pts treated w/antifolates
Thymidylate synthase (TS)
Essential enzyme in de novo synthesis of thymidylate, precursor of thymidine triphosphate, essential for DNA synthesis & repair
TS gene promoter is polymorphic & has 2 (TSER*2) or 3 (TSER*3) 28 bp tandem repeat sequences
In vitro studies show TS promoters w/the TSER*3 sequence generate 3× higher mRNA than those w/the TSER*2 sequence
Inhibition of TS by 5-fluorodeoxyuridine monophosphate (FdUMP) of TS is one of the 1° mechanisms of action of 5-FU
TS expression levels inversely correlate w/the Sn of tumors to 5-FU, therefore TSER*2 homozygotes respond better to 5-FU than TSER*3 homozygotes or heterozygotes