Given that radium-223 is a radiopharmaceutical that induces DNA damage, and olaparib is a PARP inhibitor that interferes with DNA repair mechanisms, we hypothesized their synergy in metastatic castration-resistant prostate cancer (mCRPC). We sought to demonstrate the safety and efficacy of olaparib + radium-223. We conducted a multicenter phase I 3+3 dose escalation study of olaparib with fixed dose radium-223 in patients with mCRPC with bone metastases. The primary objective was to establish the RP2D of olaparib, with secondary objectives of safety, PSA response, alkaline phosphatase response, radiographic progression-free survival (rPFS), overall survival, and efficacy by homologous recombination repair (HRR) gene status. Twelve patients were enrolled; all patients received a prior androgen receptor signaling inhibitor (ARSI; 100%) and 3 patients (25%) prior docetaxel. Dose-limiting toxicities (DLT) included cytopenias, fatigue, and nausea. No DLTs were seen in the observation period however delayed toxicities guided the RP2D. The RP2D of olaparib was 200 mg orally twice daily with radium-223. The most common treatment-related adverse events were fatigue (92%) and anemia (58%). The rPFS at 6 months was 58% (95% confidence interval, 27%-80%). Nine patients were evaluable for HRR gene status; 1 had a BRCA2 alteration (rPFS 11.8 months) and 1 had a CDK12 alteration (rPFS 3.1 months). Olaparib can be safely combined with radium-223 at the RP2D 200 mg orally twice daily with fixed dose radium-223. Early clinical benefit was observed and will be investigated in a phase II study.

Background

CDK12 inactivation leading to increased neoantigen burden has been hypothesized to sensitize tumors to immune checkpoint inhibition. Pan-cancer data regarding the frequency of CDK12 alterations are limited. We aimed to characterize CDK12 alterations across all cancer types through real-world clinical-grade sequencing.

Methods

This was a single-center retrospective analysis of 4994 cancer patients who underwent tissue or blood genomic profiling, including CDK12 assessment, conducted as part of routine care from December 2012 to January 2020. Prevalence, clinical characteristics, and treatment outcomes of patients with tumors with pathogenic CDK12 alterations were described.

Results

In all, 39 (0.78%, n = 39/4994) patients had pathogenic CDK12 alterations. Among CDK12-altered tumors, the most common organ site was prostate (n = 9, 23.1%) followed by colorectal (n = 5, 12.8%). Adenocarcinoma was the most common histology (n = 26, 66.7%). Median follow-up from time of diagnosis was 4.02 years. Median overall survival from time of metastasis was 4.43 years (95% CI: 3.11-5.74). Ten patients with CDK12-altered tumors received at least one immune checkpoint inhibitor-containing regimen. The majority of patients (n = 6/10, 60%) experienced an objective response. Progression-free survival for patients who had metastatic disease and received a checkpoint inhibitor-containing regimen was 1.16 years (95% CI: 0.32-2.00).

Conclusion

CDK12 alterations are rare events across hematologic and solid tumor malignancies. They represent a clinically distinct molecular cancer subtype which may have increased responsiveness to checkpoint inhibition. Prospective studies are warranted to investigate checkpoint inhibition in CDK12-altered tumors.