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5055 Background: AVPCa shares clinical features with small cell carcinoma (SCC) such as resistance to androgen signaling inhibition and frequent visceral metastases. A prospective clinical trial of...

149 Background: Unusually aggressive PC behavior is linked to small cell carcinoma (SCC) morphology. We observed SCC clinical features in association with morphologically heterogeneous PC and, in a prospective clinical trial (NCT00514540), also with chemotherapy responsiveness. Our previous studies in patient derived xenografts (PDX) revealed a distinct molecular profile for SCC. Here we sought to support the hypothesis that clinically defined aggressive variant PCs also share relevant biology with SCC. Methods: 59 PC samples, and 8 PDX, from 42 men registered to NCT00514540 were stained for RB, p53, AR, NKX3-1, ASCL1, AURKA, UBE2C and Ki67. Labeling indices (LI) were calculated as the ratio of positive epithelial cells to total of epithelial cells, at 200x. We determined copy number alterations (CNA) by Onco Scan® in 36 of 59 samples and 8 PDX lines. We used Western Blot and qRT-PCR to expand pathway analyses and their associations in PDX models. Results: Donor patients were similar to non-donor patients except for higher ECOG PS and LDH values. Strong positive correlations between nuclear AR (AR-N) and NKX3.1, AR-N and RB, AR-N and nuclear AURKA (AURKA-N), NKX3.1 and RB, p53 and nuclear UBE2C (UBE2C-N), Ki67 and UBE2C-N, Ki67 and cytoplasmic UBE2C (UBE2C-C) as well as a strong negative correlation between NKX3.1 and Ki 67 were observed. Frequent copy number losses were found for PTEN (largely gene-specific) and RB1 (often associated with regional-CNA). Gene-specific AURKA amplifications were not observed. Only RB1 CNA showed a positive correlation with its LI. Samples were segregated by quantity of CNA. PDX models shared these features and showed that only AR-negative tumors expressed pro-neural transcription factors, albeit heterogeneously. Conclusions: Our results support the hypothesis that clinically defined aggressive variant PC (including SCC and non-SCC morphologies) are characterized by frequent Rb, p53 and PTEN alterations, aberrant expression of mitotic, neural development and AR-signaling pathways, and high rates of CNA. Ongoing studies will confirm whether these molecular features distinguish this variant from the more common bone-homing, bone-forming, AR-driven adenocarcinomas of the prostate.

277 Background: Androgen deprivation, anti-androgen and androgen biosynthesis inhibitor treatment can initially control the metastatic prostate cancer (PCa), but treatment-refractory progression frequently follows, with the loss of tumor suppressors (TSPs) and increased expression of cell cycle proteins. Inhibition of the nuclear export protein, Exportin 1 (XPO1) leads to nuclear accumulation of cargo proteins such as TSPs & cell-cycle regulators implicated in castration-resistant PCa (CRPC) progression. XPO1 and specific cargo genes are overexpressed in metastatic CRPC relative to benign & primary prostate tumors, implicating XPO1 activity as playing a role in disease progression. Selinexor (KPT-330), a novel, oral SINE currently in Phase 1/2 for both hematological and solid tumors, has potent activity against CRPC. We hypothesized this activity is due selinexor induced nuclear expression of TSPs. Methods: To test this hypothesis, we treated selected PCa cell lines and patient-derived xenografts (PDXs, two adenocarcinomas and one small cell carcinonoma) with selinexor to determine the effect on survival and cargo protein localization. Results: Treatment with selinexor markedly inhibited PCa cell proliferation in vitro, activated the tumor suppressor TP53 & inhibited cell-cycle regulators. Also, treatment of the PDXs with selinexor for at least 3 weeks significantly inhibited tumor growth & reduced the prostate-specific antigen level in the adenocarcinomas. Selinexor increased cell death in all three PDX tumors and reduced cell proliferation in the adenocarcinomas, but not in the small-cell tumor. Expression analyses demonstrated that selinexor induced nuclear accumulation of different cargo proteins unique to the PCa model, accounting for PDX-specific regression. Conclusions: These results point to an anti-tumorigenic effect of selinexor treatment across a spectrum of hormone-refractory PCa that may provide insight into the drivers of PCa treatment resistance and heterogeneity.

158 Background: CRPC typically responds to anti-androgen therapy but resistance is common. CYP17 inhibitors that block lyase (L) and not hydroxylase (H), do not require prednisone, and may delay tumor resistance are needed. VT-464 is an oral, non-steroidal inhibitor of CYP17 L in clinical development for CRPC. The present studies characterized: 1) the kinetic mechanism of VT-464 inhibition of h-CYP17 L and H, and 2) VT-464 effects compared to abiraterone acetate (AA) in CRPC models. Methods: CYP17 Enzyme Assays: r-h-CYP17 inhibition studies were conducted using substrates pregnenolone (for H) or 17-a-hydroxypregnenolone (for L). Product rates (17-a-hydroxypregnenolone (H) and DHEA (L)) were assessed by LC/MS/MS. Data were fit to inhibition models using SigmaPlot 11.2. In vitro CRPC studies: VT-464 and AA effects on AR transcription (luciferase) and PSA and NKX3.1 gene expression (QRT-PCR) were compared in C4-2B cells. Mouse xenograft model: Effects of oral VT-464 or AA on tumor growth and tumoral steroids (T, DHT, P) were compared in the MDA-PCa-133 castrate mouse model. Results: VT-464 was a reversible uncompetitive inhibitor of CYP17 L (Ki = 84 nM, K’i=200 nM) and a competitive inhibitor of H (Ki = 620 nM). VT-464 CYP17 L/H selectivity was 7.4 at low [S] but selectivity increased with increasing [S]. VT-464 L/H selectivity was 60-x greater than AA. In C4-2B cells VT-464 and AA inhibited AR transactivation but VT-464 suppressed PSA and NKX3.1 more potently than AA. In the xenograft model, VT-464 decreased tumor volume as effectively as AA. VT-464 more potently decreased T and DHT, while AA increased P. Conclusions: VT-464 demonstrated much greater CYP17 L selectivity than AA and equivalent or superior activity in several CRPC models. Less tumor resistance may arise from treatment with the CYP17 lyase-selective inhibitor VT-464 than with AA, since it more effectively blocked androgen biosynthesis, did not cause an accumulation of progesterone, and should not require co-administration of prednisone.