A prospective pharmacogenetic study of thymidylate synthase (TS) polymorphisms in high risk stage II or stage III colon cancer patients treated with 5-fluorouracil (5-FU) and leucovorin (LV)

13018 Background: Retrospective studies suggest TS polymorphisms predict toxicity from TS inhibitors as well as therapeutic response. The TS promoter has a variable number of tandem repeats (VNTR) polymorphism containing putative E-box binding sites that bind upstream stimulatory factor (USF) -1 and -2. One E-box binding site exists in TSER*2 and 2 binding sites exist in TSER*3. A single nucleotide polymorphism (SNP) at position 12 (G→C) in TSER*3’s 2nd repeat abolishes binding of USF-1/2. Combined effects of VNTR and SNP means individuals may have 2, 3 or 4 enhancer regions. We hypothesized that decreased enhancer numbers predicts for patients at risk for grade ≥3 mucositis, diarrhea, neutropenia, and overall toxicity and performed a prospective pharmacogenetic study of TS polymorphisms in adjuvant colon cancer patients treated with 5-FU/LV. Methods: High risk Stage II or Stage III colon cancer patients treated with 5FU/LV (425 mg/m2/ 20 mg/m2) daily for 5 days every 4 weeks had blood drawn for genotyping prior to starting therapy. Patients were assessed for toxicity using NCI CTC 2.0 in cycle 1. Three year disease-free survival will also be assessed. Results: 103 patients were enrolled and genotyped with 95 evaluable for toxicity. Patient characteristics: median age 61yrs (36–79): M 52%/F 48%; Stage III 77%, Stage II 23%. Frequency of genotypes containing 2 enhancers (TSER*2/ TSER*2, TSER*2 / TSER*3C, TSER*3C/ TSER*3C), 3 enhancers (TSER*2/ TSER*3G, TSER*3C/ TSER*3G) and 4 enhancers (TSER*3G/ TSER*3G) was 0.64, 0.31, and 0.05 respectively. No significant difference was seen in overall toxicity, mucositis, diarrhea and neutropenia based on TS polymorphism genotypes. Frequency of recurrence in 2 enhancer patients was 13% versus 26% in 3/4 enhancer patients but this did not reach statistical significance (p = 0.16). Conclusions: This prospective study does not confirm previously published retrospective studies suggesting that TS VNTR and SNP predict toxicity from TS inhibitors in patients treated with 5-fluorouracil for colon cancer. There is a trend for prediction of recurrence which requires further follow-up. No significant financial relationships to disclose.