Introduction Thymidylate synthase inhibition remains a central therapeutic strategy in colorectal cancer, most commonly achieved with fluoropyrimidines such as 5-fluorouracil (5-FU). Raltitrexed, a direct thymidylate synthase inhibitor, was developed to provide more selective pathway inhibition and potentially improve efficacy or tolerability compared with indirect fluoropyrimidine-based approaches.
Introduction
Thymidylate synthase inhibition remains a central therapeutic strategy in colorectal cancer, most commonly achieved with fluoropyrimidines such as 5-fluorouracil (5-FU). Raltitrexed, a direct thymidylate synthase inhibitor, was developed to provide more selective pathway inhibition and potentially improve efficacy or tolerability compared with indirect fluoropyrimidine-based approaches.
Problem Statement
Whether direct thymidylate synthase inhibition can improve outcomes in advanced colorectal cancer, particularly after prior fluoropyrimidine exposure, has remained uncertain. Raltitrexed was hypothesized to offer clinical benefit through more specific target inhibition and to potentially overcome resistance to 5-FU-based therapy, but its true efficacy in advanced colorectal cancer required prospective evaluation.
Summary
This phase II ECOG-ACRIN study found that raltitrexed has minimal clinical activity in advanced colorectal cancer, both in treatment-naïve patients and in those previously exposed to 5-FU-based therapy. Across all treatment strata, objective response rates were very low and insufficient to justify further trial expansion, with only limited disease control and short progression-free survival observed. Survival outcomes were modest and did not suggest a meaningful advantage over existing fluoropyrimidine-based approaches. Toxicity was consistent with the known safety profile of raltitrexed, with no unexpected safety signals, but this did not offset its limited antitumor activity. Importantly, the study also failed to establish thymidylate synthase expression as a useful predictive biomarker, largely due to low response rates and limited discriminatory value. These findings suggest that direct thymidylate synthase inhibition alone is insufficient to meaningfully improve outcomes in advanced colorectal cancer and reinforce the limitations of relying on isolated antifolate pathway targeting in fluoropyrimidine-exposed disease. The study also underscores the broader need for more effective biomarker-driven and mechanistically distinct strategies in advanced colorectal cancer therapeutics.
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