Anyone using metformin with jevtana (cabazitaxel) in treating advanced stage prostate cancer? If so is it holding down tumor growths.

Yes. Metformin is being used in advanced cancer treatment including prostate. It may interfere with tumor's metabolism to slow growth. Not sure if there is an active clinical trial now.
Investigative stage. Cabozitaxel ( jevtana) is aproved for use in metastatic carcinoma of prostate, diabetic drug metfomin shown to have chemo therapeutic property , slowing the cancer cell division, use is under investigation.