(Reuters) - They may be difficult to pronounce, but pharmaceutical companies eager to grab the attention of doctors and patients are returning to drug names starting with X and Z. Recent X-branded names include prostate cancer treatment Xtandi, lung cancer drug Xalkori, and Xgeva for cancer that has spread to bone. The letter Z is also becoming popular for new drugs like Zaltrap for colon cancer, melanoma drug Zelboraf and prostate cancer treatment Zytiga. ...