Losartan is an angiotensin receptor blocker, therefore it decreases the effect of angiotensin II. That means, theoretically, losartan would be good for prostate cancer.
It is possible losartan can cause dry cough, but highly unlikely. ACE-Is (angiotensin converting enzyme inhibitors) are the drugs implicated most with dry cough. That is why drugs like losartan and Benicar are often substituted in patients exhibiting that side effect with ACE-Is (lisinopril, captopril, ramipril, enalapril, etc).
Losartan is a very good drug for blood pressure and congestive heart failure. It may have a beneficial effect on prostate cancer.
Good luck!
Paul