Background Prostate‐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. Methods A 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. Results PSA 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. Conclusions PSMA 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.
5019 Background: Chemotherapeutic options for HRPC are limited. S is a novel oral platinum compound. Methods: The SPARC trial is a multinational randomized double blind study comparing S 80 mg/m2/day x 5 q5weeks po + prednisone (P) vs placebo + P in HRPC patients (pts) who failed prior chemotherapy. In this analysis, progression-free survival (PFS) was the primary endpoint, defined as a composite endpoint of radiologic progression, symptomatic progression, skeletal events or death. All cases were blindly adjudicated for progression by an independent review committee (IRC). Results: 950 pts were accrued between Sept 2004 and Jan 2006. Baseline characteristics were well balanced between treatment arms. 51% of the pts had received prior docetaxel. 68% were ≥ 65 yrs old and 27% were ≥ 75 yrs old. Pts received a median of 4 courses in the S arm (range:1–28) vs 2 courses in the placebo arm (range 1–16). 802 pts had an IRC defined progression-free (PFS) event consisting in 80% of the cases of radiologic progression, pain progression or death. All analyses were conducted on an intent-to-treat basis. S was associated with a 31% reduction in the risk of PFS events (HR=0.69; 95% CI: 0.60–0.80; p<0.00001) and a 33% reduction in the risk of pain progression (HR=0.67; 95% CI: 0.54 - 0.83; p=0.00028). Consistent results in favor of S were found for PFS and time to pain progression in all subsets examined, including pts treated with prior docetaxel. Superior PSA response (25% vs.12%, p=0.00007), objective tumor response (7% vs. 1%, p<0.002), pain response (24% vs. 14%, p<0.005), and duration of pain response (HR=0.59; 95%CI: 0.35–1.00; p=0.049) were observed for S. Final analysis of overall survival awaits the occurrence of the pre-specified number of events. S was generally well tolerated - myelosuppression was the most frequent side effect, but grade 4 neutropenia was uncommon (4%) and a single patient had grade 4 thrombocytopenia. Grade 3/4 non-hematologic side effects included infection (4%), vomiting (2%) and diarrhea (2%). Conclusions: S is well tolerated and significantly reduces the risk of disease progression for HRPC pts who have failed prior chemotherapy. Supported by GPC Biotech and Pharmion No significant financial relationships to disclose.
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