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The role of aldehyde dehydrogenase (ALDH) in carcinogenesis and resistance to cancer therapies is well known. Mounting evidence also suggests a potentially important role for ALDH in the induction and function of regulatory T (Treg) cells. Treg cells are important cells of the immune system involved in promoting immune tolerance and preventing aberrant immune responses to beneficial or non-harmful antigens. However, Treg cells also impair tumor immunity, leading to the progression of various carcinomas. ALDH expression and the subsequent production of retinoic acid by numerous cells, including dendritic cells, macrophages, eosinophils and epithelial cells, seems important in Treg induction and function in multiple organ systems. This is particularly evident in the gastrointestinal tract, pulmonary tract and skin, which are exposed to a myriad of environmental antigens and represent interfaces between the human body and the outside world. Expression of ALDH in Treg cells themselves may also be involved in the proliferation of these cells and resistance to certain cytotoxic therapies. Hence, inhibition of ALDH expression may be useful to treat cancer. Besides the direct effect of ALDH inhibition on carcinogenesis and resistance to cancer therapies, inhibition of ALDH could potentially augment the immune response to tumor antigens by inhibiting Treg induction, function and ability to promote immune tolerance to tumor cells in multiple cancer types.

Leelamine is an anticancer chemotherapeutic agent inhibiting intracellular cholesterol transport. Cell death mediated by leelamine occurs due to the lysosomotropic property of the compound, its accumulation in the lysosome, and inhibition of cholesterol transport leading to lack of availability for key processes required for functioning of cancer cells. The present study dissects the structure-activity-relationship of leelamine using synthesized derivatives of leelamine and abietic acid, a structurally similar compound, to identify the moiety responsible for anti-cancer activity. Similar to leelamine, all active derivatives had an amino group or a similar moiety that confers a lysosomotropic property to the compound enabling its accumulation in the lysosome. Active derivatives inhibited intracellular cholesterol transport and hindered xenografted melanoma tumor development without obvious systemic toxicity. In silico studies suggested that active derivatives accumulating in lysosomes bound to NPC1, a protein responsible for cholesterol export from the lysosome, to inhibit its activity that then caused accumulation, and lack of cholesterol availability for other key cellular activities. Thus, active derivatives of leelamine or abietic acid maintained lysosomotropic properties, bound to NPC1, and disrupted cellular cholesterol transport as well as availability to retard tumor development.

Drug resistance and toxicity are major limitations of cancer treatment and frequently occurs during melanoma therapy. Nanotechnology can decrease drug resistance by improving drug delivery, with limited toxicity. This study details the development of nanoparticles containing arachidonyl trifluoromethyl ketone (ATK), a cytosolic phospholipase A inhibitor, which can inhibit multiple key pathways responsible for the development of recurrent resistant disease. Free ATK is toxic, limiting its efficacy as a therapeutic agent. Hence, a novel nanoliposomal delivery system called NanoATK was developed, which loads 61.7% of the compound and was stable at 4C for 12 weeks. The formulation decreased toxicity-enabling administration of higher doses, which was more effective at inhibiting melanoma cell growth compared to free-ATK. Mechanistically, NanoATK decreased cellular proliferation and triggered apoptosis to inhibit melanoma xenograft tumor growth without affecting animal weight. Functionally, it inhibited the cPLA, AKT, and STAT3 pathways. Our results suggest the successful preclinical development of a unique nanoliposomal formulation containing ATK for the treatment of melanoma.

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The aldehyde dehydrogenases (ALDH) are a major family of detoxifying enzymes that contribute to cancer progression and therapy resistance. ALDH overexpression is associated with a poor prognosis in many cancer types. The use of multi-ALDH isoform or isoform-specific ALDH inhibitors as anticancer agents is currently hindered by the lack of viable candidates. Most multi-ALDH isoform inhibitors lack bioavailability and are nonspecific or toxic, whereas most isoform-specific inhibitors are not effective as monotherapy due to the overlapping functions of ALDH family members. The present study details the development of a novel, potent, multiisoform ALDH inhibitor, called KS100. The rationale for drug development was that inhibition of multiple ALDH isoforms might be more efficacious for cancer compared with isoform-specific inhibition. Enzymatic IC 50 s of KS100 were 207, 1,410, and 240 nmol/L toward ALDH1A1, 2, and 3A1, respectively. Toxicity of KS100 was mitigated by development of a nanoliposomal formulation, called NanoKS100. NanoKS100 had a loading efficiency of approximately 69% and was stable long-term. NanoKS100 was 5-fold more selective for killing melanoma cells compared with normal human fibroblasts. NanoKS100 administered intravenously at a submaximal dose (3-fold lower) was effective at inhibiting xenografted melanoma tumor growth by approximately 65% without organ-related toxicity. Mechanistically, inhibition by KS100 significantly reduced total cellular ALDH activity to increase reactive oxygen species generation, lipid peroxidation, and accumulation of toxic aldehydes leading to apoptosis and autophagy. Collectively, these data suggest the successful preclinical development of a nontoxic, bioavailable, nanoliposomal formulation containing a novel multi-ALDH isoform inhibitor effective in the treatment of cancer.