Comparison of mRNA expression levels determined with TaqMan and competitive template RT-PCR

Phase II study of tailored chemotherapy for advanced colorectal cancer with either 5-fluouracil and leucovorin or oxaliplatin and irinotecan based on the expression of thymidylate synthase and dihydropyrimidine dehydrogenase

BACKGROUND

Thymidylate synthase (TS) and dihydropyrimidine dehydrogenase (DPD) are essential enzymes for 5-fluorouracil (5-FU) metabolism. In patients with advanced colorectal cancer (ACRC), retrospective studies have shown that low expression levels of TS and DPD correlated with response to 5-FU. We performed a prospective study in which the choice of first-line chemotherapy with either 5-FU or a non-5-FU containing regimen was based on TS and DPD expression.

PATIENTS AND METHODS

Fresh-frozen samples of metastases were obtained from 58 previously untreated patients with ACRC. mRNA expression of TS and DPD was quantified using an RT-PCR assay. Patients with low tumor expression of both TS and DPD received weekly bolus 5-FU/leucovorin (LV) 500 mg/m2 (group A); patients with high TS and/or DPD received 3-weekly oxaliplatin 85 mg/m2 and irinotecan 200 mg/m2 (group B). After progression, cross-over to the alternative regimen was attempted.

RESULTS

Of 53 eligible patients, 31 had tumors with both low TS and low DPD, and were treated in group A. A response was observed in 11 patients [35%; 95% confidence interval (CI) 19% to 54%]. Cross-over to second-line oxaliplatin/irinotecan resulted in a partial response in two out of 16 patients (13%; 95% CI 1% to 38%). In group B, four out of 22 patients responded (18%; 95% CI 5% to 40%), while no responses were observed in 12 patients after cross-over to 5-FU/LV (0%; 95% CI 0% to 28%).

CONCLUSIONS

Prospective selection of 5-FU/LV chemotherapy based on low TS and DPD expression in patients with ACRC did not confirm the high response rates reported in retrospective studies. The procedure of obtaining metastatic tissue and quantitation of enzymes appeared feasible but cumbersome. Before assessing the clinical utility of a predictive marker in a randomized trial, future studies should focus on prospective validation of the assay in a large and well defined population.

Prospective clinical trials using a pharmacogenetic/pharmacogenomic approach

The antitumor activity of most anticancer agents is limited by a number of different factors, such as their cellular targets and activating enzymes, while constitutive genetic polymorphisms may limit drug bioavailability and influence either antitumor efficacy or toxic side effects. An example of a drug for which clear predictive parameters have been identified is 5-fluorouracil (5FU): its antitumor activity is limited by either a high activity of the target enzyme thymidylate synthase (TS) and/or a high activity of its degrading enzyme, dihydropyrimidine dehydrogenase (DPD). Retrospective studies showed a clear correlation between a high expression of TS and a poor response, which was stronger when DPD was included in the evaluation (high DPD, poor response). Therefore we initiated a clinical prospective study in which we treated previously untreated patients with advanced colorectal cancer with tailored chemotherapy: at a low TS-mRNA and low DPD-mRNA patients were stratified to receive a standard weekly 5FU-leucovorin regimen. At a high TS and/or DPD, patients were stratified to receive a combination of oxaliplatin and irinotecan. Up to now this proof-of-principle study demonstrated that selection of patients is possible and can clearly improve the clinical outcome. The next step is to develop algorithms to select patients for combination chemotherapy with 5FU-leucovorin and new compounds, such as oxaliplatin or irinotecan, or novel targeted agents such as bevacizumab or cetuximab. For these combination schedules the optimal combination of predictive factors has to be explored.