Why is caution required when trifluridine/tipiracil is used with zidovudine?

Zidovudine may compete with trifluridine in Trifluridine/Tipiracil Servier for activation by thymidine kinases, potentially reducing zidovudine efficacy, so monitor antiviral efficacy and consider an alternative antiviral that is not a human thymidine kinase substrate.