Germinal BRCA-mutation significance in the tumor microenvironment formation Efficacy of PARP inhibition in late-line therapy of metastatic castration-resistant prostate cancer

Metastatic castration-resistant prostate cancer is a difficult problem for a clinical oncologist. In addition, mutations in genes of homologous DNA recombination, including BRCA1/2, suggest an aggressive behavior and therapy resistance. Treatment options for such patients were significantly limited until new drugs - PARP inhibitors have been registered. Nevertheless, there is evidence that BRCA1/2 gene mutations are associated with increased mutational load, neoepitopes formation, increased number of tumor-infiltrating lymphocytes and a response to the immune response checkpoints blockade. Studies have shown that BRCA2-mutated prostate cancer demonstrates high level of immune cells infiltration compared to tumors without mutation, in particular with respect to CD4⁺, CD8⁺ and FOXP3⁺ T-lymphocytes. It should be noted that studies have shown a tendency of CD8+ T-lymphocytes/FOXP3⁺ T-cells ratio decreasing in BRCA2-mutated tumors. Thus, the mutational status of BRCA2 presumably forms the immune phenotype of prostate cancer with an increase of intratumoral immune cells, but with immunosuppressive properties. At the same time, the use of immune checkpoint blockers in advanced prostate cancer has been unsuccessful in terms of overall survival. Despite the fact that immune checkpoint blocker's efficacy is often associated with a high intracellular CD4⁺ and CD8⁺ T lymphocytes, their presence is clearly insufficient for response. Studies showed that PARP inhibitors effect tumor microenvironment significantly. Anti-PD-1/PD-L1 combination with PARP inhibitors is being actively studied due to their properties of modulating the tumor microenvironment. Thus, future immunooncological strategies for primary prostate cancer therapy may include not only an increase in mutational load, but also an impact on the immunosuppressive microenvironment. The article presents clinical cases of 3 brothers, carriers of the germinal BRCA2 c.9371A>T mutation, suffering from prostate cancer with a burdened family history. The disease development under standard therapies was studied and markers of the tumor microenvironment were immunohistochemically evaluated. PARP inhibitor Olaparib efficacy in prostate cancer of older brother in late-line therapy for metastatic castration-resistant disease was analyzed.