Addition of DNA dependent protein kinase (DNA-PK) inhibitor peposertib enhances efficacy of 177Lu-based radioimmunotherapy in pre-clinical models of prostate and renal cell carcinoma

Introduction: Radiotherapy targeted to cell surface receptors expressed by cancer cells is being used increasingly in the treatment of solid tumors. In an effort to increase the apoptotic response of tumor cells to β radiation, trials are being pursued using molecularly targeted radiotherapy in combination with next generation orally available inhibitors of the cellular DNA damage response. DNA-PK activates the non-homologous end joining mechanism of DNA double strand break repair. We investigated radioimmunotherapy (RIT) alone or in combination with the orally available small molecule DNA-PK inhibitor, peposertib, in pre-clinical models of renal cell carcinoma or prostate cancer.

Methods: ¹⁷⁷Lu labelled monoclonal antibodies Girentuximab (chimeric antibody recognizing carbonic anhydrase IX, (CAIX)) and Rosopatamab (humanized antibody recognizing prostate-specific membrane antigen (PSMA)), were used to deliver b radiation via a single intravenous dose (3MBq or 6MBq) to BALB/c nude mice bearing subcutaneous SK-RC-52 renal cell carcinoma, or LNCaP prostate cancer xenografts, respectively. Peposertib was delivered by daily oral gavage (50 mg/kg) for 14 days commencing the day after dosing with RIT. Tumor growth rates and whole mouse bodyweights were recorded. A subset of mice were imaged in vivo on day 7 after commencement of therapy using whole-body single photon computed tomography (SPECT)/MRI and biodistribution was determined ex vivo using gamma counting. Assessment of DNA damage response pathways by phosphoDNA-PK and phosphoKAP1 levels and assessment of DNA double strand breaks by immunofluorescence analysis of gammaH2AX foci formation was also conducted on a subset of tumors ex vivo at day 7.

Results: The combination of RIT plus peposertib was well tolerated. Ex vivo biodistribution and in vivo SPECT/MRI demonstrated excellent tumor uptake of each radiopharmaceutical with minimal blood pool activity and no effect of peposertib on tumor targeting. Addition of peposertib to RIT showed enhanced anti-tumor efficacy compared with RIT alone in both CAIX and PSMA pre-clinical models. The enhanced efficacy by the addition of peposertib to RIT was evidenced by the 100% complete response (CR) rate (4/4) in 6MBq ¹⁷⁷Lu-anti-CAIX plus peposertib arm compared to 25% CR (1/4) in the 6MBq ¹⁷⁷Lu-anti-CAIX RIT alone arm. Furthermore, the addition of peposertib to 3MBq ¹⁷⁷Lu-anti-CAIX RIT demonstrated enhancement of anti-tumor activity following treatment to at least that of 6MBq ¹⁷⁷Lu-anti-CAIX RIT used as single agent. Similarly, in the prostate model 6MBq ¹⁷⁷Lu-anti-PSMA plus peposertib had 75% CR (3/4) vs 25% CR (1/4) in the 6MBq ¹⁷⁷Lu-anti-PSMA 6MBq alone arm. A radiation dose-dependent activation of phosphoDNA-PK and phosphoKAP1 was found in SK-RC-52 tumors ex vivo at day 7, suggesting increased DNA damage at 6MBq compared with the 3MBq dose.

Conclusions: Addition of peposertib to RIT was well tolerated in pre-clinical mouse models of cancer. The synergy between the two therapies has the potential to enable enhanced response to RIT. These are promising novel combination therapies that warrant further investigation.

Disclosures: This study was funded by a research partnership between Telix Pharmaceuticals and Merck, and by the healthcare business of Merck KGaA, Darmstadt, Germany (CrossRef Funder ID: 10.13039/100009945) who provided peposertib free of charge.