The mechanism of action of trifluridine has not been fully determined, but appears to involve the inhibition of viral replication. Trifluridine does this by incorporating into viral DNA during replication, which leads to the formation of defective proteins and an increased mutation rate. This drug also reversibly inhibits thymidylate synthetase, an enzyme that is necessary for DNA synthesis.
Trifluridine is a fluorinated pyrimidine nucleoside with in vitro and in vivo activity against herpes simplex virus, types 1 and 2 and vacciniavirus. Some strains of adenovirus are also inhibited in vitro. Trifluridine is also effective in the treatment of epithelial keratitis that has not responded clinically to the topical administration of idoxuridine or when ocular toxicity or hypersensitivity to idoxuridine has occurred. In a smaller number of patients found to be resistant to topical vidarabine, trifluridine was also effective. Trifluridine interferes with DNA synthesis in cultured mammalian cells. However, its antiviral mechanism of action is not completely known.
One major metabolite, 5-carboxy-2'-deoxyuridine found on the endothelial side of the cornea, indicating localized metabolism.
Overdosage by ocular instillation is unlikely because any excess solution should be quickly expelled from the conjunctival sac. Acute overdosage by accidental oral ingestion has not occurred. However, should such ingestion occur, the 75 mg dosage of trifluridine in a 7.5 mL bottle of trifluridine is not likely to produce adverse effects. Single intravenous doses of 1.5 to 30 mg/kg/day in children and adults with neoplastic disease produce reversible bone marrow depression as the only potentially serious toxic effect and only after three to five courses of therapy. The acute oral LD50 in the mouse and rat was 4379 mg/kg or higher.