Background: Over-expression of thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD) in tumor tissue is associated with insensitivity to 5-fluorouracil (5-FU). Over-expression of ERCC1 correlates with insensitivity to oxaliplatin (OX) therapy, while high thymidine phosphorylase (TP) levels predict for increased sensitivity to capecitabine (Xel).
Methods: Biopsies of metastatic tumor were taken before OX (130 mg/m2 day 1) given with Xel (1200-3000 mg/m2 in two divided doses days 1-5 and 8-12) every 3-weeks. Micro-dissected metastatic and primary tumors were analyzed for relative gene expression by real-time quantitative polymerase chain reaction. The clinical protocol prospectively identified the molecular targets of interest that would be tested. Endpoints for the molecular analyses were correlation of median, first and third quartiles for relative gene expression of each target with response, time to treatment failure (TTF), and survival.
Results: Among 91 patients participating in this trial; 97% had colorectal cancer. The median number of prior chemotherapy regimens was 2, and most had prior 5-FU and irinotecan. In paired samples, median mRNA levels were significantly higher in metastatic versus primary tumor (-fold): TS (1.9), DPD (3.8), ERCC1 (2.1) and TP (1.6). A strong positive correlation was noted between DPD and TP mRNA levels in both primary (r = 0.693, p < 0.0005) and metastatic tissue (r = 0.697, p < 0.00001). There was an association between TS gene expression and responsive and stable disease: patients whose intratumoral TS mRNA levels were above the median value had significantly greater risk of early disease progression (43% vs 17%), but this did not translate into a significant difference in TTF. ERCC1 gene expression above the third quartile was associated with a shorter TTF (median 85 vs 162 days, p = 0.046). Patients whose TS mRNA levels in metastatic tumor tissue were below the median had a longer overall survival (median 417 vs 294 days, p = 0.042).
Conclusion: Target gene expression in primary tumor was significantly lower than that in paired metastatic tissue. High ERCC1 mRNA levels in metastatic tumor was associated with a shorter TTF. Lower expression of TS mRNA correlated with a lower chance of early PD with XelOX therapy and improved overall survival.