This is a very challenging issue in general, validating the use of repurposed drugs for treating X, much less for treating potentially advanced PCa. Why? Few medical professionals are willing to risk their reputations and sometimes careers to "swim upstream" against the medical establishment. It is generally against the interests of pharma and the medical establishment (AMA, NIH, FDA, etc) to pursue expensive trials to scientifically prove or disprove their efficacy. There is literally no money in it for them. So I personally admire those who go against the mainstream in medical issues like COVID vaccination, repurposing cheap drugs for serious illnesses, and so on.

This is an intriguing recent interview with one of those physicians, an oncologic surgeon, who is heading up an effort to develop some scientific evidence that ivermectin or similar meds have some efficacy against certain types of cancers. https://www.youtube.com/watch?v=-EcIurkanQg

I would also note that the STAMPEDE Platform -- called a platform instead of a trial because it is designed as an on-going, evolutionary series of trials -- is testing some repurposed drugs for their efficacy against prostate cancer. The most recent I read of was them randomizing recently for a trial of adding metformin to SOC versus SOC. Of course metformin is not a cancer drug, it's used to treat diabetes and metabolic syndrome.