Tuning HDAC Inhibition, Cell Uptake, and Antiproliferative Activity of Vorinostat-Derived Half-Sandwich Complexes with Phosphine Co-Ligands.

HDAC inhibitors are used as anticancer drugs, and the Zn2+-coordinating hydroxamic acid moiety in the drug vorinostat (SAHA) plays a key role in the mode of action. The introduction of metal centers in the structures of HDAC inhibitors (HDACis) allows for additional interactions between the inhibitor and the protein. To alter the interactions between HDACs and half-sandwich organometallic HDACis, we introduced amphiphilic 1,3,5-triaza-7-phosphaadamantane (PTA) or lipophilic triphenylphosphine (PPh3) as coligands. The half-sandwich compounds were found to be stable in aqueous solution, and the Rh(III) derivatives exhibited promising antiproliferative and HDAC inhibitory activity compared to their chlorido precursor. The lipophilic Rh-PPh3 compound was effectively taken up into human cancer cells, where it caused α-tubulin hyperacetylation with low impact on the histone H3 acetylation level. Molecular docking studies revealed additional interactions on the surface of the protein due to the presence of the P-donor ligands. Coordination of a CoIII(cyclen) chaperone to the hydroxamic acid moiety yielded reduction-responsive heterobimetallic compounds in which the bioactivity of the hydroxamic acid moiety was effectively masked.