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PURPOSE In prostate cancer, inactivating CDK12 mutations lead to gene fusion–induced neoantigens and possibly sensitivity to immunotherapy. We aimed to clinically, pathologically, and molecularly characterize CDK12-aberrant prostate cancers. METHODS We conducted a retrospective multicenter study to identify patients with advanced prostate cancer who harbored somatic loss-of-function CDK12 mutations. We used descriptive statistics to characterize their clinical features and therapeutic outcomes (prostate-specific antigen [PSA] responses, progression-free survival [PFS]) to various systemic therapies, including sensitivity to poly (ADP-ribose) polymerase and PD-1 inhibitors. RESULTS Sixty men with at least monoallelic (51.7% biallelic) CDK12 alterations were identified across nine centers. Median age at diagnosis was 60.5 years; 71.7% and 28.3% were white and nonwhite, respectively; 93.3% had Gleason grade group 4-5; 15.4% had ductal/intraductal histology; 53.3% had metastases at diagnosis; and median PSA was 24.0 ng/mL. Of those who underwent primary androgen deprivation therapy for metastatic hormone-sensitive disease (n = 59), 79.7% had a PSA response, and median PFS was 12.3 months. Of those who received first-line abiraterone and enzalutamide for metastatic castration-resistant prostate cancer (mCRPC; n = 34), 41.2% had a PSA response, and median PFS was 5.3 months. Of those who received a first taxane chemotherapy for mCRPC (n = 22), 31.8% had a PSA response, and median PFS was 3.8 months. Eleven men received a PARP inhibitor (olaparib [n = 10], rucaparib [n = 1]), and none had a PSA response (median PFS, 3.6 months). Nine men received a PD-1 inhibitor as fourth- to sixth-line systemic therapy (pembrolizumab [n = 5], nivolumab [n = 4]); 33.3% had a PSA response, and median PFS was 5.4 months. CONCLUSION CDK12-altered prostate cancer is an aggressive subtype with poor outcomes to hormonal and taxane therapies as well as to PARP inhibitors. A proportion of these patients may respond favorably to PD-1 inhibitors, which implicates CDK12 deficiency in immunotherapy sensitivity.

15553 Background: 17-Allylamino-17-Demethoxygeldanamycin (17-AAG) is a benzoquinone ansamycin antibiotic with anti- proliferative activity in several mouse xenograft models including prostate cancer models. Serum IL-6, IL-8, and maspin are potentially important markers of prostate cancer biology. Methods: Patients (pts) with metastatic, hormone-refractory prostate cancer progressing on at least one prior systemic therapy with rising PSA were eligible. All pts received 17-AAG at a dose of 300 mg/m2 IV weekly for three out of four weeks. Primary objective was to assess the PSA response (50% decrease in PSA). Secondary objectives included Adverse Events (AEs) and correlative serum markers including IL-6, IL-8 and maspin levels. A Simon two-stage design required a total of 25 pts with early termination if < 2 responses occurred among the 1st 16 eligible patients. Results: Seventeen pts were enrolled of which 15 were deemed eligible. Median age was 68 and median PSA was 252 ng/mL. Pts received 17-AAG for a median number of 2 cycles. No pt had a PSA response. No grade 4/5 AEs occurred. Grade 3 AEs included fatigue (4 pts), lymphopenia (2 pts) and back pain (2 pts). The median PSA progression free survival was 1.8 months (95% CI: 1.3–3.4 months). The six-month overall survival was 61% (95% CI: 37%-100%). Due to the lack of PSA response, accrual was stopped per study design. At day 15, the median IL-6 and IL-8 increase from baseline was 0.4 pg/ml (p=0.57) and 3 pg/ml (p=0.73), respectively. Maspin levels had day 15 increase of 6-fold (p=0.44). At treatment failure (TF), the median IL-6 increase from baseline was 4.47 pg/ml (p=0.03) and IL-8 decrease was 1.8 pg/ml (p=0.31). Maspin levels had a 29-fold increase at TF (p=0.09). Conclusions: 17-AAG did not show any activity with regards to PSA response. Serum IL-6 was significantly increased at the time of TF. Further evaluation of 17-AAG at a dose of 300 mg/m2 IV weekly in this patient population is not warranted. No significant financial relationships to disclose.

259 Background: Vascular endothelial growth factor (VEGF) and platelet derived growth factor (PDGF) are produced by bladder cancer cell lines in vitro and expressed in human tumor tissues. Preclinical studies have also shown that bladder cancer cell lines express VEGF receptor 1 and 2 on their surface membrane. Pazopanib is a vascular endothelial receptor tyrosine kinase inhibitor with anti-angiogenesis and antitumor activity in several preclinical models. A two-stage phase II study was conducted to assess the activity and toxicity profile of pazopanib administered to patients with metastatic, urothelial carcinoma. Methods: Patients with one prior systemic therapy for recurrent, metastatic urothelial carcinoma were eligible. Patients received pazopanib at a dose of 800 mg orally daily for 4 week cycle. Results: Nineteen patients were enrolled. Median age was 66 years, with > 89% of patients presenting poorly differentiated bladder cancer. Adverse event data is available on 18 patients. No grade 4 or 5 events have been experienced. Nine patients have experienced 11 grade 3 adverse events of which 7 were deemed at least possibly related to treatment. Most common toxicities were anemia, thrombocytopenia, leucopenia and fatigue. For stage 1, none of the first 16 evaluable patients were deemed success (CR or PR) by the RECIST criteria during the first four 4-week cycles of treatment. Median progression- free survival was 1.9 months. This met the futility stopping rule of interim analysis, and therefore, the trial was recommended to be permanently closed. Correlative studies including measurement of VEGF levels in archived tissues and blood are pending. Conclusions: Pazopanib did not show activity in urothelial carcinoma patients. The role of anti-VEGF therapies in urothelial carcinoma may need further evaluation in rational combination strategies. [Table: see text]

Abstract Purpose: 17-Allylamino-17-demethoxygeldanamycin (17-AAG) is a benzoquinone ansamycin antibiotic with antiproliferative activity in several mouse xenograft models, including prostate cancer models. A two-stage phase II study was conducted to assess the activity and toxicity profile of 17-AAG administered to patients with metastatic, hormone-refractory prostate cancer. Experimental Design: Patients with at least one prior systemic therapy and a rising prostate-specific antigen (PSA) were eligible. Patients received 17-AAG at a dose of 300 mg/m2 i.v. weekly for 3 of 4 weeks. The primary objective was to assess the PSA response. Secondary objectives were to determine overall survival, to assess toxicity, and to measure interleukin-6, interleukin-8, and maspin levels and quality of life. Results: Fifteen eligible patients were enrolled. The median age was 68 years and the median PSA was 261 ng/mL. Patients received 17-AAG for a median number of two cycles. Severe adverse events included grade 3 fatigue (four patients), grade 3 lymphopenia (two patients), and grade 3 back pain (two patients). The median PSA progression-free survival was 1.8 months (95% confidence interval, 1.3-3.4 months). The 6-month overall survival was 71% (95% confidence interval, 52-100%). Conclusions: 17-AAG did not show any activity with regard to PSA response. Due to insufficient PSA response, enrollment was stopped at the end of first stage per study design. The most significant severe toxicity was grade 3 fatigue. Further evaluation of 17-AAG at a dose of 300 mg/m2 i.v. weekly as a single agent in patients with metastatic, hormone-refractory prostate cancer who received at least one prior systemic therapy is not warranted.