Combination of PSMA targeting alpha-emitting radioligand [(212)Pb]Pb-AB001 with BET bromodomain inhibitors in in vitro prostate cancer models.

The alpha-emitting radioligand [(212)Pb]Pb-AB001, targeting prostate-specific membrane antigen (PSMA), is a promising therapy approach for prostate cancer. Bromodomain and extra-terminal (BET) protein inhibitors, such as AZD5153 and JQ1, disrupt oncogenic transcriptional programs by altering chromatin structure. This study evaluated whether BET inhibition enhances the efficacy of radioligand therapy. Cytotoxic effects of [(212)Pb]Pb-AB001 alone and in combination with BET inhibitors were assessed in 2D monolayers and a 3D spheroid model of PSMA-positive C4-2 prostate cancer cells. Cell viability, cell cycle alterations, and DNA damage were assessed using viability assays and flow cytometry. Spheroid growth and viability were assessed by fluorescence microscopy. AZD5153 was more potent than JQ1 in reducing cell proliferation. [(212)Pb]Pb-AB001 induced activity- and time-dependent cytotoxicity with a delayed apoptotic response. BET inhibitors induced G1 arrest, while [(212)Pb]Pb-AB001 caused G2 arrest. Combination treatment reduced cell viability in an additive manner but did not further affect cell cycle distribution or increase apoptosis compared with [(212)Pb]Pb-AB001 alone. γH2AX staining in 2D models showed an activity- and time-dependent increase in DNA damage at 1, 3 and 6 days post-treatment with [(212)Pb]Pb-AB001. BET inhibitors alone induced minimal γH2AX, and combination treatments did not enhance DNA damage beyond [(212)Pb]Pb-AB001 alone. In 3D spheroids, combination treatment led to synergistic growth suppression. In conclusion, these findings indicate that therapeutic inhibition of BET bromodomain in combination with the alpha-emitting radioligand [(212)Pb]Pb-AB001 could significantly enhance tumour control and should be further evaluated in metastatic prostate cancer models.