In this context, the primary objective of the present study was to evaluate the efficacy of doxycycline as an adjunctive therapy, compared to a placebo, in patients with metastatic prostate cancer, specifically assessing its impact on biochemical response, measured as a reduction in PSA levels, as well as its impact on patients’ quality of life and cognitive function. The gene discussed is KLK3; the disease is metastatic prostate carcinoma.