Folate-mediated intracellular drug delivery increases the anticancer efficacy of nanoparticulate formulation of arsenic trioxide

Arsenic trioxide (As₂O₃) is a frontline drug for treatment of acute promyelocytic leukemia and is in clinical trials for treatment of other malignancies, including multiple myeloma; however, efforts to expand clinical utility to solid tumors have been limited by toxicity. Nanoparticulate forms of As₂O₃ encapsulated in 100-nm-scale, folate-targeted liposomes have been developed to lower systematic toxicity and provide a platformfor targeting this agent. The resultant arsenic "nanobins" are stable under physiologic conditions but undergo triggered drug release when the pH is lowered to endosomal/lysosomal levels. Cellular uptake and antitumor efficacy of these arsenic liposomes have been evaluated in folate receptor (FR)-positive human nasopharyngeal (KB) and cervix (HeLa) cells, aswell as FR-negative human breast (MCF-7) tumor cells through confocal microscopy, inductively coupled plasma mass spectroscopy, and cytotoxicity studies. Uptake of folate-targeted liposomal arsenic by KB cells was three to six times higher than that of free As₂O₃ or nontargeted liposomal arsenic; the enhanced uptake occurs through folate-mediated endocytosis, leading to a 28-fold increase in cytotoxicity. In contrast, tumor cells with lower FR density on the surface (HeLa and MCF-7) showed much less uptake of the folate-targeted drug and lower efficacy. In cocultures of KB and MCF-7 cells, the folate-targeted arsenic liposomes were exclusively internalized by KB cells, showing high targeting specificity. Our studies further indicate that folate-targeted delivery of As₂O ₃ with coencapsulated nickel(II) ions (as a nontoxic adjuvant) potentiates the As₂O₃ efficacy in relatively insensitive solid tumor-derived cells and holds the promise of improving drug therapeutic index.