Last year the FDA approved a drug that is an alternative for Keytruda (pembrolizumab). It is called Jemperli (dostarlimab-gxly) and offered by GlaxoSmithKline (GSK). The FDA approved it for patients with mismatch repair deficient recurrent or advanced solid tumors, as determined by an FDA-approved test, that have progressed on or following prior treatment and who have no satisfactory alternative treatment options.
The FDA also approved the VENTANA MMR RxDx Panel as a companion diagnostic device to select patients with mismatch repair deficient solid tumors for treatment with dostarlimab-gxly.
Only about 3 to 5 percent of prostate cancer patients have a mismatch repair deficient tumor. The approval was based on the Garnet trial which included a prostate cancer patient.
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If your insurance does not cover this treatment, you can take part in one of the current trials with dostarlimab.
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Here are articles describing this new drug:
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New Clinical Trial. Clinical trial evaluates radiopharmaceutical as therapy for biochemically recurrent prostate cancer
UCLA is expanding access to the Ga-68-PSMA-11 PET scan
Trying a high dose testosterone clinical trial 
Apalutamide - SPARTAN versus Enzalutamide - PROSPER.
