From Inert to Active: Charge-Compensated Metallacarboranes with High Toxicity and Anticancer Potential

Since their initial synthesis by Hawthorne in the 1960s, inorganic metallacarboranes - most notably cobalt bis(dicarbollide), [CoSAN]⁻, have been considered chemically inert and harmless to eukaryotic cells. This reputation for inertness has supported their use in a wide range of biomedical applications, including as a boron source for boron neutron capture therapy, antifungal and Gram-positive antibacterial agents, and superchaotropic membrane carriers for impermeable biologically important molecules. In this study, we report a surprising twist: newly synthesized, iodinated and charge-compensated [CoSAN]⁻ derivatives with boron–nitrogen (-B–N-) bond show promising anticancer activity. The studied molecules display low-nanomolar IC₅₀ values across a range of human cancer cell lines, revealing cytotoxic effects far beyond what has previously been observed for this class. We also link their activity to specific structural motifs, identifying auspicious candidates for future therapeutic development. Their biological activity is further evaluated in a vertebrate in vivo model (Danio rerio embryos), where we characterize the developmental effects and toxicological profile. To the best of our knowledge, this is the first demonstration of simple [CoSAN]⁻-based molecules exhibiting such high cytotoxic potency and cancer cell selectivity. Our findings show that subtle chemical modifications -like incorporating B–N bond and adjusting charge - can unlock powerful bioactivity in molecules once thought to be inert, opening new possibilities for unconventional anticancer drug design.