HIV protease inhibitors as cancer therapeutics: Is off-the-shelf right on target?

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HIV protease inhibitors (HIV PIs) target the protease responsible for processing of the budding HIV virion, and have significant clinical benefit for HIV-infected individuals. Recent evidence suggests that use of HIV PIs might affect the incidence and/or treatment of certain cancers. To determine whether these might have potential against lung cancer, we tested 7 FDA-approved HIV PIs: amprenavir, atazanavir, indinavir, lopinavir, nelfinavir, ritonavir and saquinavir and found that four (nelfinavir, lopinavir, ritonavir and saquinavir), inhibited the growth of A549 and H157 cells. Nelfinavir was most potent, with IC50<= 10μM. Three active HIV PIs were subsequently screened in the NCI60 cell line panel, and were found to uniformly inhibit growth and induce cytotoxicity, with nelfinavir again being most potent. To assess the mode of cell death induced by HIV PIs, we performed apoptosis assays and found that HIV PIs induced classic features of apoptosis including increased sub-G1 DNA content, condensed and fragmented nuclei, mitochondrial depolarization, caspase cleavage (8, 9, 3, 7), and PARP cleavage. HIV PI-induced death was caspase-dependent since the caspase inhibitor zVAD completely inhibited DNA fragmentation. Despite the induction of classic features of apoptosis, we also observed that active PIs caused prominent perinuclear vacuolization that preceded the onset of apoptosis. Transmission electron microscopy of nelfinavir-treated cells revealed 1. distorted mitochondria with loss of inner membrane integrity, 2. partial disintegration of nucleoli, and 3. the presence of membrane-bound vacuoles and decreased glycogen content in the cytoplasm. Because use of these drugs is associated with dyslipidemia and insulin resistance, which can be a sign of reduced signaling through the Akt pathway, we assessed Akt activation, and found that treatment with HIV PIs rapidly but transiently decreased endogenous Akt phosphorylation in H157 cells. In addition, pretreatment with HIV PIs inhibited growth factor-induced activation of receptor tyrosine kinase activity, as well as Akt and ERK in H157 cells. These studies identify HIV protease inhibitors as cancer cytotoxics that induce apoptosis through inhibition of key signaling pathways that promote cellular survival. Given their wide spectrum of activity, oral availability, and FDA approval, HIV PIs are examples of off-the-shelf drugs that have potential as targeted cancer therapeutics.