** Shares of drugmaker PDS Biotech PDSB.O rise 4.7% to $1.04 premarket

** Co says it has asked U.S. FDA to approve changes to its late-stage trial of its experimental cancer drug, PDS0101

** Co's PDS0101 aims to treat advanced head and neck cancer caused by HPV16

** Amended plan focuses on progression-free survival to speed approval, co says

** PDSB says FDA held constructive meeting on potential accelerated approval

** "We believe that including progression-free survival as a main goal offers an important opportunity to shorten the trial," CEO Frank Bedu-Addo says

** As of last close, stock down ~53% over the past year

(Reporting by Sahil Pandey in Bengaluru)

((Sahil.Pandey@thomsonreuters.com))