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The androgen receptor plays a critical role in the progression of prostate cancer. Here, we describe targeting the prostate-specific membrane antigen using a lipid nanoparticle formulation containing small interfering RNA designed to silence expression of the messenger RNA encoding the androgen receptor. Specifically, a Glu-urea-Lys PSMA-targeting ligand was incorporated into the lipid nanoparticle system formulated with a long alkyl chain polyethylene glycol-lipid to enhance accumulation at tumor sites and facilitate intracellular uptake into tumor cells following systemic administration. Through these features, and by using a structurally refined cationic lipid and an optimized small interfering RNA payload, a lipid nanoparticle system with improved potency and significant therapeutic potential against prostate cancer and potentially other solid tumors was developed. Decreases in serum prostate-specific antigen, tumor cellular proliferation, and androgen receptor levels were observed in a mouse xenograft model following intravenous injection. These results support the potential clinical utility of a prostate-specific membrane antigen–targeted lipid nanoparticle system to silence the androgen receptor in advanced prostate cancer.

The androgen receptor (AR) plays a critical role in the progression of prostate cancer. Silencing this protein using short-hairpin RNA (shRNA) has been correlated with tumor growth inhibition and decreases in serum prostate specific antigen (PSA). In our study, we have investigated the ability of lipid nanoparticle (LNP) formulations of small-interfering RNA (siRNA) to silence AR in human prostate tumor cell lines in vitro and in LNCaP xenograft tumors following intravenous (i.v.) injection. In vitro screening studies using a panel of cationic lipids showed that LNPs containing the ionizable cationic lipid 2,2-dilinoleyl-4-(2-dimethylaminoethyl)-[1,3]-dioxolane (DLin-KC2-DMA) exhibited the most potent AR silencing effects in LNCaP cells. This is attributed to an optimized ability of DLin-KC2-DMA-containing LNP to be taken up into cells and to release the siRNA into the cell cytoplasm following endocytotic uptake. DLin-KC2-DMA LNPs were also effective in silencing the AR in a wild-type AR expressing cell line, LAPC-4, and a variant AR expressing cell line, CWR22Rv1. Importantly, it is demonstrated that LNP AR-siRNA systems containing DLin-KC2-DMA can silence AR gene expression in distal LNCaP xenograft tumors and decrease serum PSA levels following i.v. injection. To our knowledge, this is the first report demonstrating the feasibility of LNP delivery of siRNA for silencing AR gene expression in vivo.

Prostate cancer is the most commonly diagnosed non-skin cancer in men and one of the leading causes of cancer-death. While frequently curable by surgical or radiation ablation, recurrent or metastatic disease is usually managed with androgen withdrawal therapies (medical castration), which block the growth promoting effects of androgens on the androgen receptor (AR). Unfortunately, the cancers eventually progress to the lethal castration-resistant state. Previously, we reported that knocking down the AR by RNA interference using small hairpin RNA (shRNA) inhibits this progression and, in about 50% of tumours, causes complete regression. In this study, we investigated the ability of small interfering RNA (siRNA) encapsulated in lipid nanoparticle (LNP) to silence AR in human prostate cancer cell lines and xenograft tumours following intravenous injections. In vitro screening studies with prostate cancer cell lines using a panel of cationic lipids demonstrated that siRNA formulated in LNPs containing the ionizable cationic lipid 2,2-dilinoleyl-4-(2-dimethylaminoethyl)-[1,3]-dioxolane (DLin-KC2-DMA) exhibited the most potent AR silencing effects. This is attributed to an optimized ability of DLin-KC2-DMA-containing LNP to enter into cells and to release the siRNA into the cell cytoplasm following endocytotic uptake. DLin-KC2-DMA LNPs were also effective in silencing the AR in a wild-type AR expressing cell line, LAPC-4, and a variant AR expressing cell line, CWR22Rv1. Importantly, we found that LNP AR-siRNA systems containing DLin-KC2-DMA could silence AR gene expression in distal LNCaP xenograft tumors and decrease serum PSA levels following intravenous injection. The 5′RACE experiments further confirmed that siRNA-induced specific cleavage of AR mRNA was the primary mechanism of action. In conclusion, these results indicate that in vivo delivery of siRNA to knockdown AR using our LNP formulations causes a decrease in serum PSA and provides proof in principle that this approach may offer a therapeutic strategy for treating advanced prostate cancer. (Funded by Canadian Institutes for Health Research and a PCF-STAR Project from Prostate Cancer Canada with the support of Safeway Canada) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1970. doi:1538-7445.AM2012-1970