Nanoparticle-based formulation of metallacarboranes with bovine serum albumin for application in cell cultures

We report on the unique self-assembling properties of one molybdacarborane (1) and two ruthenacarborane complexes (2 and 3) to spontaneously form nanoparticles alone or in combination with bovine serum albumin (BSA) in a 10:1 molar ratio (BSA:metallacarborane). The maverick behaviour of the metallacarboranes in aqueous media was investigated using several spectroscopic techniques, including nanoparticle tracking analysis, UV-Vis, fluorescence, Rayleigh light scattering and NMR spectroscopy, as well as MALDI-TOF mass spectrometry, from which a molecular picture of the nanoparticles was developed. The metallacarborane–albumin nanoparticles showed optimal stability in phosphate buffer (pH 7.4), retaining a mostly monomodal dispersion over 24 h, in contrast to the highly polydispersed nanoparticles of 1–3 alone. The three metallacarboranes were tested in vitro against the MCF-7 (breast carcinoma) cell line, using a BSA-free and a BSA-containing formulation. Surprisingly, the latter induced a significant increase in the cytotoxicity of 1, whereby it did not greatly affect the activity profile of 2 and 3. Finally, label-free confocal Raman imaging was applied to visualise the uptake of the metallacarboranes into single cells. The size of intracellular aggregates in the cytoplasm ranged from 300 nm to 5 μm. The discussed formulation concept is proposed as a new method to increase the bioactivity and the biological stability of poorly water-soluble (metalla)carboranes.