Autophagy Inhibition Synergistically Enhances Anticancer Efficacy of RAMBA, VN/12-1 in SKBR-3 Cells, and Tumor Xenografts

Department of Pharmaceutical Sciences, Jefferson School of Pharmacy, Thomas Jefferson University, Philadelphia, Pennsylvania, USA.
Molecular Cancer Therapeutics (Impact Factor: 5.68). 02/2012; 11(4):898-908. DOI: 10.1158/1535-7163.MCT-11-0860
Source: PubMed


VN/12-1 is a novel retinoic acid metabolism blocking agent discovered in our laboratory. The purpose of the study was to elucidate the molecular mechanism of anticancer activity of VN/12-1 in breast cancer cell lines and in tumor xenografts. We investigated the effects of VN/12-1 on induction of autophagy and apoptosis in SKBR-3 cells. Furthermore, we also examined the impact of pharmacologic and genomic inhibition of autophagy on anticancer activity of VN/12-1. Finally, the antitumor activity of VN/12-1 was evaluated as a single agent and in combination with autophagy inhibitor chloroquine in an SKBR-3 mouse xenograft model. Short exposure of low dose (<10 μmol/L) of VN/12-1 induced endoplasmic reticulum stress, autophagy, and inhibited G(1)-S phase transition and caused a protective response. However, a higher dose of VN/12-1 initiated apoptosis in vitro. Inhibition of autophagy using either pharmacologic inhibitors or RNA interference of Beclin-1 enhanced anticancer activity induced by VN/12-1 in SKBR-3 cells by triggering apoptosis. Importantly, VN/12-1 (5 mg/kg twice weekly) and the combination of VN/12-1 (5 mg/kg twice weekly) + chloroquine (50 mg/kg twice weekly) significantly suppressed established SKBR-3 tumor growth by 81.4% (P < 0.001 vs. control) and 96.2% (P < 0.001 vs. control), respectively. Our novel findings suggest that VN/12-1 may be useful as a single agent or in combination with autophagy inhibitors for treating human breast cancers. Our data provides a strong rationale for clinical evaluation of VN/12-1 as single agent or in combination with autophagy inhibitors.

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    • "With regard to a number of other agents of varying mechanisms of action, enhanced effectiveness of a Src family kinase inhibitor was observed when combined with chloroquine in a xenograft model of prostate cancer (Wu et al., 2010), to interleukin-2 in a model of murine colorectal carcinoma (Liang et al., 2012), and to bevacizumab in glioblastoma (Hu et al., 2012); the latter two findings are of particular interest and relevance and will be addressed in more detail below. Conversely, only quite modest improvement in response to chemotherapeutic agents was detected for chloroquine in combination with an mTOR inhibitor in MCF-7 xenografts (Loehberg et al., 2012), with voranistat in colon carcinoma (Carew et al., 2010), with perifosine in lung cancer xenografts (Fu et al., 2009), with a dual phosphatidylinositide 3-kinase/ protein kinase B/mTOR inhibitor in non-small cell lung cancer xenografts (Xu et al., 2011), with a retinoic acid metabolism blocker in breast tumor cells (Godbole et al., 2012), and with panobinostat in breast cancer (Rao et al., 2012); in the latter studies, the lack of impact could be ascribed to the use of an unusually low dose of chloroquine of 10 mg/kg, which was apparently necessitated by the high degree of toxicity that was observed with chloroquine alone in this work. A particularly relevant issue is that it is frequently unclear whether chloroquine has achieved levels in the tumor cells that are likely to be therapeutically effective in inhibiting autophagy . "
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    ABSTRACT: Recognition of the cytoprotective functions of autophagy that occur in tumor cells exposed to various forms of chemotherapy or radiation has generated intense interest in the possibility that pharmacological interference with autophagy could provide a clinical strategy for overcoming therapeutic resistance. Multiple clinical trials are currently in progress to evaluate the antimalarial agent chloroquine (generally in its clinical formulation as hydroxychloroquine) and its impact on various forms of cancer therapy. In this commentary/review, we focus on the relatively limited number of studies in the literature where chloroquine has been tested in combination with chemotherapy or radiation in experimental tumor-bearing animal models. We also present recent data from our own laboratories, in cell culture experiments as well as in vivo studies, which demonstrate that neither chloroquine nor silencing of an autophagy regulatory gene was effective in conferring radiation sensitivity in an experimental model of breast cancer. The capacity for sensitization by chloroquine appears to be quite wide-ranging, with dramatic effects for some drugs/tumor models and modest or minimal effects in others. One possible caveat is that with only a few exceptions, experiments have generally been performed in xenograft models, thereby eliminating the involvement of the immune system, which might ultimately prove to play a central role in determining the effectiveness of autophagy inhibition in chemosensitization or radiosensitization. Nevertheless, a careful review of the current literature suggests that caution is likely to be warranted in translating preclinical findings relating to autophagy inhibition as an adjunctive therapeutic strategy.
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    ABSTRACT: Novel retinoic acid metabolism blocking agent (RAMBA), VN/12-1, is a highly potent anti-cancer agent that induces autophagy. Its combination with autophagy inhibitor chloroquine (CHL) has been shown to synergistically enhance apoptosis in breast cancer cells. The purpose of this study was to determine the toxicity and pharmacokinetic profile of VN/12-1 and its combination with CHL. Preliminary toxicology of VN/12-1 was determined using female SCID mice (n = 4 for each group). ATRA was used for comparison. We selected four different doses of VN/12-1 and ATRA. Two of the doses were low and less frequent (2.5 and 5 mg/kg twice a week), and the remaining doses were high and more frequent (10 and 20 mg/kg every day). The dose of CHL was 50 mg/kg twice a week. For pharmacokinetic (PK) study, 20 mg/kg of VN/12-1 was injected subcutaneously (s.c.) into the mice, and their plasma was collected at various intervals (n = 2) and analyzed by HPLC. The lower and less frequent doses of VN/12-1 and ATRA were found to be least toxic. However, high and more frequent doses of these compounds were toxic to the mice. PK results showed that VN/12-1 has a half-life of 6 h. The area under the curve (AUC) for VN/12-1 was 83.78 h μg/ml. VN/12-1 and ATRA are non-toxic when used as 5 mg/kg twice a week as single agents or in combination with CHL. The favorable PK properties of VN/12-1 can potentially be used for its further advanced pre-clinical and clinical development.
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