Interactions of multitargeted kinase inhibitors and nucleoside drugs: Achilles heel of combination therapy?

Multitargeted tyrosine kinase inhibitors (TKI) axitinib, pazopanib, and sunitinib are used to treat many solid tumors. Combination trials of TKIs with gemcitabine, a nucleoside anticancer drug, in pancreas, renal, lung, ovarian, and other malignancies resulted in little benefit to patients. TKI interactions with human nucleoside transporters (hNT) were studied by assessing inhibition of [(3)H]uridine uptake in yeast producing recombinant hNTs individually and in cultured human cancer cell lines. Axitinib, pazopanib, and sunitinib inhibited hENT1 at low micromolar concentrations. In A549, AsPC-1, and Caki-1 cells, [(3)H]uridine, [(3)H]thymidine, [(3)H]gemcitabine, and [(3)H]fluorothymidine (FLT) accumulation was blocked by all three TKIs. Pazopanib > axitinib ≥ sunitinib inhibited hENT1 with IC50 values of 2, 7, and 29 μmol/L, respectively, leading to reduced intracellular gemcitabine and FLT accumulation. Pretreatment or cotreatment of Caki-1 cells with TKIs reduced cellular accumulation of [(3)H]nucleosides, suggesting that TKI scheduling with nucleoside drugs would influence cytotoxicity. In combination cytotoxicity experiments that compared sequential versus simultaneous addition of drugs in Caki-1 cells, cytotoxicity was greatest when gemcitabine was added before TKIs. In clinical settings, TKI inhibitor concentrations in tumor tissues are sufficient to inhibit hENT1 activity, thereby reducing nucleoside chemotherapy drug levels in cancer cells and reducing efficacy in combination schedules. An additional unwanted interaction may be reduced FLT uptake in tumor tissues that could lead to aberrant conclusions regarding tumor response.