Lysosomotropic Properties of Weakly Basic Anticancer Agents Promote Cancer Cell Selectivity In Vitro

Abstract

Drug distribution in cells is a fundamentally important, yet often overlooked, variable in drug efficacy. Many weakly basic anticancer agents accumulate extensively in the acidic lysosomes of normal cells through ion trapping. Lysosomal trapping reduces the activity of anticancer drugs, since anticancer drug targets are often localized in the cell cytosol or nucleus. Some cancer cells have defective acidification of lysosomes, which causes a redistribution of trapped drugs from the lysosomes to the cytosol. We have previously established that such differences in drug localization between normal and cancer cells can contribute to the apparent selectivity of weakly basic drugs to cancer cells in vitro. In this work, we tested whether this intracellular distribution-based drug selectivity could be optimized based on the acid dissociation constant (pKa) of the drug, which is one of the determinants of lysosomal sequestration capacity. We synthesized seven weakly basic structural analogs of the Hsp90 inhibitor geldanamycin (GDA) with pKa values ranging from 5 to 12. The selectivity of each analog was expressed by taking ratios of anti-proliferative IC50 values of the inhibitors in normal fibroblasts to the IC50 values in human leukemic HL-60 cells. Similar selectivity assessments were performed in a pair of cancer cell lines that differed in lysosomal pH as a result of siRNA-mediated alteration of vacuolar proton ATPase subunit expression. Optimal selectivity was observed for analogs with pKa values near 8. Similar trends were observed with commercial anticancer agents with varying weakly basic pKa values. These evaluations advance our understanding of how weakly basic properties can be optimized to achieve maximum anticancer drug selectivity towards cancer cells with defective lysosomal acidification in vitro. Additional in vivo studies are needed to examine the utility of this approach for enhancing selectivity.

Amine-Containing Molecules and the Induction of an Expanded Lysosomal Volume Phenotype: A Structure–Activity Relationship Study

“…Despite this, the drug‐induced ELVP can be expected to significantly influence both the activity and pharmacokinetic behavior of the drug perpetrating the phenotype as well as those that may be subsequently administered. The degree to which a therapeutic drug is sequestered in lysosomes can influence its activity and selectivity by influencing the degree of binding interactions with intended therapeutic targets . In addition, the degree of drug accumulation in lysosomes can have a profound impact on the pharmacokinetics and distribution properties of drugs that are weakly basic and lysosomotropic .…”

“…For example, the Golgi apparatus, early endosomes, and late endosomes are relatively acidic (∼pH 6.4, 6.2, 5.5, respectively) and would theoretically accumulate weakly basic compounds. In relation to these organelles however, the lysosomes would be subjected to the greatest ion‐trapping‐based accumulation of weakly basic compounds by virtue of their vastly more acidic luminal pH (4.0–4.5) . This hyperaccumulation of drugs within lysosomes can make their resident proteins, enzymes, and lipid bilayers particularly susceptible to drug‐induced alterations.…”

Time‐Dependent Effects of Hydrophobic Amine‐Containing Drugs on Lysosome Structure and Biogenesis in Cultured Human Fibroblasts

“…For drugs that are designed to target lysosomal proteins, medicinal chemists have imparted lysosomotropic properties to their drugs in an effort to improve potency . In addition, we have shown that it is possible to exploit lysosomal sequestration properties to increase the preferential toxicity of cancer drugs to cancer cells that possess a lysosomal acidification defect . Finally, the sequestration of drugs in lysosomes can have a profound impact on the pharmacokinetic properties of drugs.…”

“…12,13 In addition, we have shown that it is possible to exploit lysosomal sequestration properties to increase the preferential toxicity of cancer drugs to cancer cells that possess a lysosomal acidification defect. 4,14,15 Finally, the sequestration of drugs in lysosomes can have a profound impact on the pharmacokinetic properties of drugs. Despite the fact that lysosomes typically only account for a small percentage of the total cell volume, the total cellular accumulation of most lysosomotropic drugs can be attributed to lysosomal sequestration.…”

Pathways and Progress in Improving Drug Delivery Through the Intestinal Mucosa and blood–brain Barriers

“…There has been limited work carried out to overcome the problem of drug sequestration [109,116–118]. One possible solution is to derivatize the amine group of the drug with a promoiety (e.g., to make an amide prodrug) so the amino group cannot be protonated or charged.…”