Targeting redox-active pyridinium cations to mitochondria to inhibit proliferation of colon cancer cells

Despite the reliance of cancer cells on glycolytic metabolism, in many cancer types mitochondria are functional and important for cell survival and proliferation. This led to development of novel anticancer strategies, based on targeting cancer cell mitochondrial function. One of such approach is based on targeting bioactive compounds to mitochondria by conjugation to the triphenylphosphonium cationic moiety (TPP+) using alkyl linkers. It has been shown that TPP+-conjugated compound accumulate selectively in cancer cells in vitro and in tumor tissues in vivo. Several such compounds were shown to inhibit mitochondrial respiration with concomitant stimulation of mitochondrial oxidants. Inhibition of mitochondrial respiration, with concomitant decrease in cellular ATP was proposed to prevent the efflux of the compounds via the drug resistance proteins. Here, we report the synthesis and application of redox active pyridinium compounds linked to the TPP+ moiety, as efficient redox cycling agents, inducing superoxide production in colon cancer cells. Because of the presence of a positive charge on both the pyridinium and TPP+ moieties, the synthesized compounds carry two positive charges. The efficiency of superoxide production by the synthesized compounds is significantly higher than by any other mitochondria-targeted compound reported to date. The compounds exhibit a double mechanism of action, including inhibition of mitochondrial respiration and redox cycling, resulting in oxidation of mitochondrial peroxiredoxin-3, but not cytosolic peroxiredoxin-1, and depletion of cellular ATP level. This confirms that the synthesized compounds selectively affect mitochondrial respiration and redox status. The potency of the compounds to stimulate mitochondrial redox stress correlate with their antiproliferative effects against colon cancer cells. Thus, mitochondria-targeted pyridinium cations represent novel class of potential anticancer agents targeting both cancer cell bioenergetics and mitochondrial redox status.