Antitumor potential of conjugable valinomycins bearing hydroxyl sites: in vitro studies

Following our pioneering studies on the direct and efficient introduction of derivatizable hydroxyl handles into the valinomycin (VLM, 1) structure, a K(+)-ionophore with potent antitumor activity, the ensuing conjugable analogues (HyVLMs 2, 3, and 4) have herein been compared to the parent macrocycle for their potential antiproliferative effects on a panel of cancer cell lines, namely, human MCF-7, A2780, and HepG2, as well as rat C6 cells. On the basis of IC50 values, we find that hydroxyl analogues 3 and 4 are only moderately less active than 1, while analogue 2 experiences a heavily diminished activity. Cytofluorimetric analyses of MCF-7 cells treated with HyVLMs suggest that the latter depolarize mitochondria, thus retaining the typical VLM behavior. It is likely that C6 cells, for which the exceptionally potent cytotoxicity of VLM has never reported previously, follow the same fate, as evidenced by alteration of mitochondrial morphology upon incubation with each ionophore.