Despite strong biological rationale for the use of TKIs therapy for the treatment of PCa, Phase III clinical trials have produced disappointing results. As TKI strategies move forward, the failures of past trials need to be better understood. New approaches with these treatments will also have to take into account modern anti-androgens and a treatment landscape that now includes immunotherapy.

108 Background: Cabozantinib (C) is a multikinase inhibitor of c-Met, vascular endothelial growth factor receptor two and RET. C has shown activity in metastatic castrate resistant prostate cancer (mCRPC), with resolution of bone lesions on bone scan (BS), regression of soft tissue/visceral disease (STD), reductions in circulating tumor cells and bone biomarkers. Combining docetaxel (D) with other agents, without overlapping toxicities, can target different cellular signaling pathways necessary for tumor survival. Methods: Patients (pts), with no prior D for CRPC, receive a fixed dose of D (75 mg/m2 IV day one of each 21 day cycle) and prednisone (P) (5 mg po q12 hours) with C at three escalating dose levels: 20 mg, 40 mg, or 60 mg (all po daily). Using a standard three-plus-three design, three to six pts are treated at each dose level until the maximum tolerated dose (MTD) has been defined. Results: Thirteen pts have been accrued; four on dose level one, six on dose level two, and three on dose level three. Median age 69 (45 to 84). Four pts have an Eastern Cooperative Oncology Group Performance Status (ECOG PS) of zero and nine pts have a PS of one. Median Gleason score is nine (7 to 10). Median on-study prostate-specific antigen (PSA) is 129.2 ng/mL (0.01-508.5 ng/mL). Median cycles is six (1 to 17). Grade 1 adverse events (AEs), possibly related to C; dysgeusia (4/12), oral mucositis (4/12), increased ALT (3/12), and epistaxis (3/12). Grade 2 AEs; nausea (2/12), hand/foot syndrome (2/12), fatigue (2/12), dysgeusia (2/12), oral mucositis (2/12), hypophosphatemia (2/12), and anemia (2/12). Grade 3 AE is hypophosphatemia (2/12). Grade 4 AE is neutropenia (1/12). MTD of C is 60 mg. Of nine evaluable pts, six have bone only disease. Of these six, three pts have PSA declines of less than 30% with improvement on BS (two pts) or stable BS (one pt). The other three pts have PSA declines of greater than 30% and bone scan improvement. Three pts have STD and bone disease; one patient had a PSA decline of greater than 30% with improvement on BS and SD by CT scan. One patient had an increase in PSA of less than 30% with improvement on BS and CT. The third pt had PD by CT and an increase in PSA equal to 30%. PFS probability at six months is 90.0% and is 67.5% at eight months and beyond. Conclusions: The addition of C to D and P, has an acceptable toxicity profile. CT scan and BS improvements did not correlate with PSA declines in all pts. An expansion cohort will combine D plus P with C at the MTD (60 mg) to determine clinical benefit. Clinical trial information: NCT01683994.

Key Clinical MessagePresented herein is the case of a heavily pretreated patient with high‐grade neuroendocrine prostate cancer that achieved a complete metabolic response on platinum‐based chemotherapy after treatment with the dual CD‐47 and SIRP‐α inhibitor, RRx‐001, in a Phase II clinical trial.

218 Background: Trebananib is an angiopoietin1/2 antagonist peptibody. Androgens stimulate expression of VEGF via activation of hypoxia inducible factor-a (HIFa). Androgen deprivation therapy (ADT) is associated with lower HIF1a gene expression in prostate cancer tissue. Dual targeting of the androgen and angiogenic axis represents a potential synergistic anti-angiogenic therapeutic approach in metastatic castration resistant prostate cancer (mCRPC). In this preliminary safety study we hypothesize that trebananib in combination with abiraterone will have a favorable tolerability and efficacy profile. Methods: Patients with mCRPC were treated with abiraterone 1000mg daily and prednisone 5 mg twice daily. Trebananib was administered intravenously every week, in escalating doses from 15mg/kg to 30mg/kg on days 1, 8, 15, and 22 every 28-days. Results: A total of 9 patients were enrolled. Three of nine patients had prior chemotherapy. The median age was 63.8 (63-71yrs). No dose limiting toxicities were observed. The most common grade ≥ 2 toxicities included limb edema (3/9), hyperglycemia (1/9), gastrointestinal (2/9), fatigue (2/9), hypertension (1/9), confusion (1/9), weight gain (1/9) and insomnia (2/9). 5/9 of patients had an overall PSA decline of >30%. 8/9 patients were evaluable for response. Prior chemotherapy patients were on study for 1 and 3 months. No prior chemotherapy patients were treated for 1, 6, 9, 10, 10, and 17 months. Conclusions: Trebananib in combination with abiraterone is well tolerated and displayed an acceptable safety profile in patients with mCRPC. Based on this safety data a randomized phase II study randomizing chemotherapy-naïve mCRPC patients to either abiraterone/prednisone plus AMG 386 at 30mg/kg or abiraterone/prednisone is currently accruing at the NCI. Clinical trial information: NCIT01553188.

Poorly differentiated neuroendocrine carcinomas (NECs) are rare tumors that can arise anywhere along the gastrointestinal tract. They often present in advanced stage and portend a poor prognosis when compared to adenocarcinomas of the same stage. Characterization of these tumors is best accomplished with tissue biopsy, as peripheral tumor markers commonly used in NECs are of little utility. Therapeutic strategies often involve chemotherapeutic regimens that have been used to treat small-cell lung cancer. Recent studies have shown that programmed death-ligand 1 (PD-L1) expression within poorly differentiated NECs is a poor prognostic indicator. However, PD-L1 expression may represent a possible target for immunotherapy drugs, often called checkpoint inhibitors, such as anti-PD-1 inhibitors.