Deciphering the phenotypic responses to ¹⁷⁷Lu-based radioligand therapy to identify efficacious combinations

Radioligand therapy (RLT) delivers radionuclides, such as Lutetium-177 (¹⁷⁷Lu), to selectively target cancer cells. Despite the favourable clinical outcomes of ¹⁷⁷Lu-based RLT, complete cure remains infrequent. A comprehensive understanding of its mode of action (MoA) is essential for strategic design of novel combination approaches that could improve therapeutic responses.

DNA damage response (DDR) markers were profiled by immunofluorescence in cancer cell lines treated with DOTA-chelated ¹⁷⁷Lu ([¹⁷⁷Lu]Lu-DOTA). Multiple DDR-deficient isogenic cell lines and DDR inhibitors were screened in viability assays with [¹⁷⁷Lu]Lu-DOTA to identify sensitizers. ¹⁷⁷Lu-RLT combination with an inhibitor of the non-homologous end joining (NHEJ) core factor DNA-PK, and its effect on viability and cell death, were further characterised in vitro and in vivo.

[¹⁷⁷Lu]Lu-DOTA treatment induced multiple DDR biomarkers indicative of DNA double-strand break (DSB) repair and genomic instability. Viability assays performed with DDR-deficient isogenic models and DDR inhibitors demonstrated that loss of DSB repair through NHEJ elicits the strongest sensitisation to [¹⁷⁷Lu]Lu-DOTA. Consistently, DNA-PK inhibition strongly sensitised PSMA-positive prostate cancer cells to [¹⁷⁷Lu]Lu-PSMA-617, exacerbated apoptosis and cell cycle arrest and improved efficacy in vivo, demonstrating good tolerability.

These results provide novel mechanistic insights into ¹⁷⁷Lu-based RLT and pinpoint NHEJ as a key pathway for the repair of ¹⁷⁷Lu-induced DNA damage. DNA-PK inhibition strongly sensitises cancer cells to ¹⁷⁷Lu-based RLT and results in stronger potentiation compared with other tested DDR inhibitor combinations, warranting the clinical exploration of ¹⁷⁷Lu-based RLT and DNA-PK inhibitor (DNA-PKi) combination.