A Phthalocyanine-ortho-Carborane Conjugate for Boron Neutron Capture Therapy: Synthesis, Physicochemical Properties, and in vitro Tests

Boronated molecular systems can be applied to boron neutron capture therapy (BNCT). Among these systems, carborane-containing phthalocyanines (Pcs) are the most promising BNCT agents. Herein we report the new zinc (II) complex of the hexacationic Pc 6, which has been obtained as iodide salt through quaternization of the neutral precursor with methyl iodide. Compound 6 was synthesized over a sequence of four steps. The complex, and its precursors as well, were characterized by a combination of spectroscopic techniques, and their structures assessed by ¹ H, ¹³ C, ¹¹ B, and two-dimensional NMR spectroscopy experiments. Together with a marked tendency to aggregate, 6 showed appreciable solubility in water. Singlet oxygen quantum yield (Φ _Δ ) of 0.38, and fluorescence quantum yield (Φ _F ) of 0.13 were obtained for 6 in a DMF solution. The complex proved to be very effective in enriching UMR-106 cells with ¹⁰ B, showing very good performance even in case of very low concentrations exposure, i. e. 1 ppm, that moreover resulted in a mild cytotoxic effect. Such a feature can be related to the polycationic nature of the complex, and hence to the well-known propensity of positively charged species to enter the cellular membrane or to adhere to its external surface.