Folate pathway gene expression in metastatic colorectal cancer patients treated with arfolitixorin/5-FU-based chemotherapy.

99 Background: Arfolitixorin is the natural, biologically active form of the marketed folates and is expected to be efficacious in a larger proportion of patients with less inter- and intra-individual variability compared with e.g. leucovorin. We have previously found a positive correlation between survival and expression of folate pathway genes in stage III/IV CRC treated with 5-fluorouracil/leucovorin (5-FU/LV). Low expression of folate-related genes may lead to poor response to 5-FU/LV-based treatment, since suboptimal transport and metabolization of LV yield insufficient active [6R]-5,10-methylenetetrahydrofolate and weak inhibition of the target enzyme thymidylate synthase (TYMS). The aim of the present study was to investigate possible confounders and biomarkers of arfolitixorin/5-FU-based treatment in relation to safety and response in a phase I/IIa metastatic colorectal cancer (mCRC) trial. Methods: ISO-CC-005 is a multi-center, phase I/IIa study in mCRC patients eligible for 5-FU/folate therapy alone or in combination with irinotecan or oxaliplatin ± bevacizumab. Patients were also treated with different doses of arfolitixorin as a single or double bolus. The study investigated safety and tolerability of arfolitixorin, and anti-tumor activity was evaluated by overall response rate (ORR) per RECIST v1.1 after 4 cycles of chemotherapy. RNA was prepared from FFPE tumor tissue, reverse transcribed and used for gene expression profiling. The following genes of interest were evaluated: ABCC3, MTHFD2, SLC46A1, SLC19A1, SLC25A32 and TYMS. An ANOVA test was used to rule out potential biases in the baseline expression levels of the genes and to assess the potential association with clinical response. Results: Eighty-one (77.1%) of 105 patients provided material for this analysis. A lower pre-treatment expression of TYMS was associated with clinical benefit (PR and SD; p = 0.021). No clear association was identified between the gene expression markers and the number of adverse events. Gender was not significantly associated with differences in gene expression. Conclusions: Low pre-treatment expression levels of TYMS were associated with clinical benefit (PR and SD) following treatment. Given the role of this gene in the folate metabolic pathways we plan to further assess its predictive potential on a larger cohort during our ongoing global phase III AGENT study. In parallel an assessment of the expression of the other candidate genes on specific patient sub-groups is currently ongoing. These studies will provide additional cues on the use of these genes as predictive markers for treatment outcome and their role in the mode of action of the drug. Clinical trial information: NCT02244632.