Intracellular drug distribution-based targeting: Exploiting lysosomes to enhance the selectivity of drugs towards cancer cells

Abstract

Under the most ideal circumstances, anticancer agents should be minimally toxic to normal cells and maximally noxious to cancer cells. Unfortunately, an optimal degree of selectivity is not typically achieved and chemotherapy is often prematurely stopped due to potentially life threatening side effects. For this reason, various approaches have been explored in an attempt to enhance the selectivity of anticancer drugs. For the most part, these techniques are based on Paul Ehrlich's concept of a "magic bullet" which is the attempt to target drugs to a disease site while avoiding healthy tissues. These approaches therefore share a common requirement that the active drug achieves greater concentration in or around tumor cells relative to normal cells. However, many of these approaches have achieved limited success due to the difficulty of achieving site-specific accumulation of conventional anticancer agents. A rarely considered option in enhancing drug selectivity lies in optimizing the intracellular distribution of drugs to achieve favorable distribution in cancer cells (i.e. in compartments that allow drug-target interactions), and unfavorable distribution in normal cells (i.e. in compartments that diminish drug-target interactions), essentially an intracellular drug distribution-based (IDB) targeting approach. The IDB targeting approach presents a paradigm shift from the classical approaches to enhance selectivity, since the active drug is not expected to achieve higher concentrations in cancer cells relative to normal cells. Instead the drug accumulates to the same extent in both normal and cancer cells, but the aforementioned differences in intracellular drug distribution result in selectivity. In the work presented here, we investigated whether the defective lysosomal acidification associated with some cancer cells can be exploited to enhance selectivity of weakly basic anticancer agents. Normal cells typically have very acidic lysosomes, which provide a driving force for the accumulation of weakly basic drugs (with appropriate physicochemical properties) into lysosomes. Some cancer cells have been shown to have defective acidification of lysosomes, leading to a reduction in the extent of lysosomal trapping of such weakly basic drugs. Our hypothesis is that the reduced sequestration of weakly basic drugs in lysosomes of cancer cells would increase cytosolic drug concentration, thus enhancing drug-target interactions, compared to the case in normal cells, where extensive sequestration would diminish drug-target interactions. We proposed that these differences in drug localization patterns between normal and cancer cells, and the resultant difference in drug activity, would enhance selectivity of lysosomotropic anticancer drugs to cancer cells. In order to establish the potential for broad therapeutic application of this approach, we assessed the prevalence of defective lysosomal acidification in cancer cells, and whether lysosomal targeting of anticancer drugs could reduce their systemic toxicity. We also evaluated whether IDB selectivity can be optimized according to relevant physicochemical parameters of drug candidates, specifically the ionization constant (pKa). These evaluations provide a rationale for the design or modification of anticancer drugs with physicochemical properties that maximize lysosomal trapping in order to enhance selectivity. Collectively, our results demonstrate that drugs with optimal lysosomotropic properties are more selective to cells with defective lysosomal acidification. Therefore, intracellular drug-distribution based (IDB) targeting provides a viable approach to enhance anticancer drug selectivity. As mentioned previously, the major limitation to enhancing selectivity through site-directed targeting of conventional anticancer drugs to tumors is the difficulty of achieving site-specific localization. Enhancing selectivity through IDB targeting represents a rational approach that will not be subject to the limitations faced by site-directed targeting approaches since there is no requirement that drugs achieve tumor-specific localization.