** Shares of CytomX Therapeutics CTMX.O soar 62% to an over four-year high of $7.63

** Co on track to add about $500 million in market value, if gains hold

** Co says its experimental cancer drug continued to shrink tumors in patients with advanced form of colorectal cancer who had already tried several treatments, in an early-stage trial

** The drug, varsetatug masetecan, is designed to target EpCAM, a protein widely found on colorectal cancer cells

** A Cantor analyst said this is "a major win" for CTMX, saying the data "could set a new bar in late-line CRC" and position the drug as a likely new standard of care for patients who have already tried three or more prior treatments

** CytomX plans to meet with the U.S. FDA later this year to discuss the design of a larger trial

** Including session's moves, stock up 78% YTD

(Reporting by Kamal Choudhury in Bengaluru)

((Kamal.Choudhury@thomsonreuters.com;))