Background: Metastatic prostate cancer (mPCa) incidence is growing despite a decrease in the prevalence of prostate cancer (PCa). Androgen deprivation therapy (ADT), the mainstay treatment for mPCa, is accompanied by a number of side effects, such as decline in muscle mass and physical function, leading to the exacerbation of age-related conditions including frailty and sarcopenia. Frailty is characterized by a loss of functional reserve, where frail men with mPCa have a worse prognosis. Sarcopenia refers to a decrease in muscle mass, strength, and function, and is associated with increased mortality in men with mPCa. Notably, PCa patients who received ADT are 3 times more likely to develop frailty compared to patients who never received ADT and 2 times more likely to develop sarcopenia compared to men without cancer. Exercise plays a key role in ameliorating or preventing further deterioration of ADT-related side effects and improving muscle mass, fitness, and strength. However, while there is a number of exercise studies in men with PCa, those with mPCa have been vastly understudied, with a lack of studies focusing on frailty and sarcopenia and the mechanisms of how exercise could address such outcomes. Thus, we have designed the FIERCE Trial to assess the effects of a 16-week multicomponent exercise intervention, encompassing resistance, aerobic, and functional training on frailty and sarcopenic status and their potential mechanistic biomarkers, and disease progression. Methods: The FIERCE trial is a prospective study aiming to recruit eighty pre-frail/frail men with mPCa receiving ADT who will be randomized to an exercise or attention control group. The 16-week exercise intervention will include thrice weekly clinic supervised, resistance and functional exercise circuit training, and self-directed home-based aerobic exercise. The attention control group will receive a stretching program and will be offered the exercise program following the study period. The primary outcome will be frailty, measured by the Fried Frailty phenotype (i.e., muscle loss, exhaustion, physical activity, gait speed, and strength) and frailty associated biomarkers (IL-6, TNF-α, CRP). Secondary outcomes include sarcopenia, measured using computerized tomography scans and sarcopenia associated muscle biopsy-driven biomarkers (myokines and insulin pathway markers). An exploratory outcome will assess the effect of exercise on cancer cell line growth (LNCaP cell line). To date, eight out of planned 80 patients have been enrolled. This trial is registered in clinicaltrials.gov (NCT06040125). Clinical trial information: NCT06040125.