In vitro activity: Leucovorin administration increases the level of reduced folates in tissues, which promotes the inhibition of thymidylate synthase in two murine colon tumors. Leucovorin, Gemcitabine, Oxaliplatin, and 5-fluorouracil is a powerful antitumor and immunomodulating regimen that can make the tumor cells a suitable means to induce an Ag-specific CTL response. Leucovorin (20 mM) increases the cytotoxicity in all three replicate experiments in 10 of the 11 human colorectal carcinoma cell lines. Leucovorin enhances cytotoxicity of trimetrexate/fluorouracil, but not methotrexate/fluorouracil, in CCRF-CEM cells. Leucovorin (LV) is a reduced folate cofactor that competes with RTX for transport and polyglutamation in both tumor and normal tissues and thus has potential as a rescue agent. Leucovorin reverses the growth-inhibitory, and potentially cytotoxic, effects of RTX on populations of viable cells.

Cell Assay: Increasing concentrations of leucovorin (N5-CHO-THF) inhibit both zcSHMT and hcSHMT activities substantially, yet to a lesser extent than zmSHMT. The IC50 of leucovorin is approximately 30 μM for zcSHMT and higher than 70 μM for zmSHMT. The differential inhibition is evident with the presence of 10 μM leucovorin, the concentration estimated in serum in a high-dose leucovorin rescue regimen .