EF24 (a curcumin analog) and ZSTK474 emphasizes the effect of cabozantinib in medullary thyroid cancer

XL184 is a small-molecule kinase inhibitor recently included in first-line systemic therapy for patients with advanced, progressive medullary thyroid cancer (MTC). EF24 is a curcumin analog with a high bio-availability, and ZSTK474 is an inhibitor of the PI3K signaling pathway. We investigated the effect of these compounds, alone and in combination, in two RET-mutated TT and MZ-CRC-1 MTC cell lines, and in six mostly RET wild-type human MTC primary cultures. Low IC50 values demonstrated the efficacy of the drugs, while the Combination Index revealed a significant synergistic effect of combinations of XL184+ZSTK474 and XL184+EF24. Cell cycle changes and the induction of apoptosis or necrosis were modulated by single compounds or combinations thereof. Both XL184 and EF24, alone or combined, were effective in reducing calcitonin secretion. Western blot and in-cell Western analysis showed that the compounds prompted a decrease in general reactivity to phosphorylated antibodies. Our data confirm XL184 alone as the reference drug for RET-mutated MTC, but we also demonstrated for the first time that EF24 alone is effective in inhibiting MTC cell viability. We tested the new combinations XL184+ZSTK474 and XL184+EF24 for the first time too, finding that they act synergistically irrespective of RET mutation status.