Folinic Acid Calcium Salt Pentahydrate (Leucovorin Calcium Pentahydrate), a reduced folic acid, is used in combination with other chemotherapeutics.

Administering either 100 or 200 mg/kg of Leucovorin bi-daily for 4-6 days reduced the concentration of RTX in the liver by 2-4 fold compared to the control group in mice. Post severe weight loss and diarrhea, dosing mice with 200 mg/kg of Leucovorin twice daily for 5-7 days prevented further weight decline and initiated early recovery. Concurrent use of Leucovorin/5-FU with a gastric and colorectal cancer vaccine (200 mg/ml) enhanced the therapeutic effect on the growth of DHDK12 tumors in vivo.

In both tumor and normal tissues, Leucovorin acts as a reduced folate cofactor, competitively transporting and promoting polyglutamation with RTX, thereby serving as a potential rescue agent. In CCRF-CEM cells, Leucovorin enhances the toxicity of trimethoprim/fluorouracil but does not increase the toxicity of methotrexate/fluorouracil. It also enhances the cytotoxicity of RTX and reverses growth inhibition. Exposure to 20 mM Leucovorin in 11 human colon cancer cell lines results in increased cytotoxicity. The augmentation of Leucovorin slows the reduction of folate levels in tissues, thus enhancing the inhibitory effect of thymidylate synthase on murine colonic tumors.