Stambler, Nancy

AbstractBackgroundProstate‐specific membrane antigen (PSMA) is a well‐established therapeutic and diagnostic target overexpressed in both primary and metastatic prostate cancers. PSMA antibody‐drug conjugate (PSMA ADC) is a fully human immunoglobulin G1 anti‐PSMA monoclonal antibody conjugated to monomethylauristatin E, which binds to PSMA‐positive cells and induces cytotoxicity. In a phase 1 study, PSMA ADC was well tolerated and demonstrated activity as measured by reductions in serum prostate‐specific antigen (PSA) and circulating tumor cells (CTCs). To further assess PSMA ADC, we conducted a phase 2 trial in metastatic castration‐resistant prostate cancer (mCRPC) subjects who progressed following abiraterone/enzalutamide (abi/enz) therapy.MethodsA total of 119 (84 chemotherapy‐experienced and 35 chemotherapy‐naïve) subjects were administered PSMA ADC 2.5 or 2.3 mg/kg IV q3w for up to eight cycles. Antitumor activity (best percentage declines in PSA and CTCs from baseline and tumor responses through radiological imaging), exploratory biomarkers, and safety (monitoring of adverse events [AEs], clinical laboratory tests, and Eastern Cooperative Oncology Group performance status) were assessed.ResultsPSA declines ≥50% occurred in 14% of all treated (n = 113) and 21% of chemotherapy‐naïve subjects (n = 34). CTC declines ≥50% were seen in 78% of all treated (n = 77; number of subjects with ≥5 CTCs at baseline and a posttreatment result) and 89% of chemotherapy‐naïve subjects (n = 19); 47% of all treated and 53% of chemotherapy‐naïve subjects had a transition from ≥5 to less than 5 CTCs/7.5 mL blood at some point during the study. PSA and CTC reductions were associated with high PSMA expression (CTCs or tumor tissue) and low neuroendocrine serum markers. In the chemotherapy‐experienced group, the best overall radiologic response to PSMA ADC treatment was stable disease in 51 (60.7%) subjects; 5.7% of subjects in the chemotherapy‐naïve group had partial responses. The most common treatment‐related AEs ≥Common Terminology Criteria for AE (CTCAE) grade 3 were neutropenia, fatigue, electrolyte imbalance, anemia, and neuropathy. The most common serious AEs were dehydration, hyponatremia, febrile neutropenia, and constipation. Two subjects who received 2.5 mg/kg died of sepsis.ConclusionsPSMA ADC demonstrated some activity with respect to PSA declines, CTC conversions/reductions, and radiologic assessments in abi/enz treated mCRPC subjects. Clinically significant treatment‐related AEs included neutropenia and neuropathy.

94 Background: Technetium (Tc99m) trofolastat (USANC) is a novel urea-based small molecule SPECT radiotracer with utility in imaging overexpression of PSMA in prostate cancer (PCa). Unlike earlier agents targeting PSMA, this ligand is rapidly internalized and retained in tumors. We conducted an open-label, multicenter phase 2 study in the US and Europe (NCT01667536). An interim analysis was performed at approximately 50% accrual. Methods: Patients (pts) with confirmed adenocarcinoma of the prostate scheduled for RP with EPLND at high risk for disease outside of the prostate gland were eligible. High risk pts were ≥cT3 or Kattan nomogram score ≥130. Within 30 days of screening, pts required a bone scan and pelvic MRI. After enrollment, pts received trofolastat followed by whole-body planar and SPECT/CT imaging 3 to 6 hrs later. Pts then underwent RP with EPLND within 21 days. SPECT/CT images were evaluated centrally by 3 readers blinded to clinical information and compared to on-site pathology assessments using a common scoring template. The primary endpoint was the ability of trofolastat to detect PCa within the gland. Secondary endpoints included detection of extent and location within the gland, pelvic lymph nodes and comparative performance against MRI. Results: 84 pts were enrolled to date from 16 centers. Interim data is available for 54 pts. A majority (≥2/3) of SPECT/CT readers correctly identified the presence or absence of primary PCa in 51/54 (94%, 85-98 95% CI) patients including 2 true-negative cases treated with neoadjuvant enzalutamide. Sensitivity and specificity were 94% (84-98 95% CI) and 100% (34-100 95% CI) respectively. Conclusions: Based on the interim data available, trofolastat has accurately detected primary prostate carcinoma within the gland with high sensitivity and specificity in high-risk pts prior to surgery. Updated results, analyses of secondary endpoints, pelvic lymph nodes, and comparative performance vs. MRI from this ongoing study will be presented. Clinical trial information: NCT01667536. [Table: see text]