Histologic transformation to small cell neuroendocrine prostate cancer occurs in a subset of patients with advanced prostate cancer as a mechanism of treatment resistance. Rovalpituzumab tesirine (SC16LD6.5) is an antibody-drug conjugate that targets delta-like protein 3 (DLL3) and was initially developed for small cell lung cancer. We found that DLL3 is expressed in most of the castration-resistant neuroendocrine prostate cancer (CRPC-NE) (36 of 47, 76.6%) and in a subset of castration-resistant prostate adenocarcinomas (7 of 56, 12.5%). It shows minimal to no expression in localized prostate cancer (1 of 194) and benign prostate (0 of 103). DLL3 expression correlates with neuroendocrine marker expression, RB1 loss, and aggressive clinical features. DLL3 in circulating tumor cells was concordant with matched metastatic biopsy (87%). Treatment of DLL3-expressing prostate cancer xenografts with a single dose of SC16LD6.5 resulted in complete and durable responses, whereas DLL3-negative models were insensitive. We highlight a patient with neuroendocrine prostate cancer with a meaningful clinical and radiologic response to SC16LD6.5 when treated on a phase 1 trial. Overall, our findings indicate that DLL3 is preferentially expressed in CRPC-NE and provide rationale for targeting DLL3 in patients with DLL3-positive metastatic prostate cancer.

Background: Delta-like protein 3 (DLL3) is expressed on the surface of small cell lung cancer (SCLC) tumor-initiating cells, and the DLL3 targeted antibody-drug conjugate, Rovalpituzumab tesirine (Rova-T™; SC16LD6.5), has shown promise for patients with SCLC. Neuroendocrine prostate cancer (NEPC) is an emerging late stage subtype of castration resistant prostate cancer with limited therapeutic options. Based on clinical and molecular similarities with SCLC, we investigated expression of DLL3 and the use of Rovalpituzumab tesirine in NEPC xenografts. Methods: We evaluated mRNA and/or protein expression of DLL3 in a cohort of 361 patients (552 samples) ranging from benign prostate (BEN), localized prostate adenocarcinoma (PCA), castration resistant adenocarcinoma (CRPC), and castration resistant NEPC and correlated with pathologic and genomic features. mRNA was assessed by RNAseq. Protein was assessed by immunohistochemistry (DLL3 SP347 antibody). Prostate cancer cell lines were engrafted in mice and treated with SC16LD6.5 in vivo. Results: DLL3 was expressed at the mRNA and/or protein level in 0/132 Benign (0%), 1/254 (0.4%) PCA, 10/90 (11.1%) CRPC and 47/76 (61.8%) NEPC samples. DLL3 IHC was of higher intensity in NEPC and co-localized with classical neuroendocrine (NE) markers. DLL3 was amongst the most differentially expressed genes by RNA-seq in NEPC versus CRPC (p=<0.0001, fold change=71), and positively correlated with ASCL1 expression (r= 0.88) and RB1 genomic loss (83%), and inversely with AR expression. siRNA knockdown of DLL3 did not alter AR signaling or NE score. In vivo treatment of the NEPC, NCI-H660, xenografts with vehicle, IgG1LD6.5, or SC16LD6.5 (0.3mg/Kg, intraperitoneal single dose) showed complete responses to SC16LD6.5 with no recurring tumors after 100 days; while the tumor developed from the CRPC line, DU145, which do not express DLL3, continued growing despite the treatment and were sacrified when tumors reached the maximum size allowed by our IACUC protocol. Conclusions: DLL3 is expressed on the surface of the majority of NEPC cases evaluated and is not expressed in primary prostate cancer or benign tissues. Modulation of DLL3 expression does not appear to affect AR or NEPC signaling in cell lines. SC16LD6.5 (Rova-T) demonstrates preferential preclinical activity in NEPC that express DLL3 compared to CRPC-adenocarcinoma that are DLL3 negative, in vivo. A Phase 1 Basket trial investigating Rova-T is now open with a dedicated NEPC arm (NCT02709889). Citation Format: Loredana Puca, Verena Sailor, Katie Gavyert, Etienne Dardenne, Kumiko Isse, Michael Sigouros, David M. Nanus, Scott T. Tagawa, Juan Miguel Mosquera, Laura Saunders, Himisha Beltran. Rovalpituzumab tesirine as a therapeutic agent for neuroendocrine prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1947.