Issue of Holdback Shares pursuant to Xanthus Acquisition

London, UK, and Cambridge, MA: 17 December 2009 - Antisoma plc (LSE: ASM; USOTC: ATSMY) ("Antisoma" or the "Company") announces the issue of 9,568,960 shares ("Holdback Shares") to certain former shareholders of Xanthus Pharmaceuticals, Inc., a company which Antisoma acquired in June 2008. As described in the prospectus published in connection the acquisition at the time, the Xanthus shareholders were entitled to receive the Holdback Shares 18 months after the closing date of the acquisition, subject to reductions in the event of any claims for indemnification by Antisoma. The 18 month period has now expired and all the Holdback Shares are now being issued to these shareholders.

Application has been made to the UKLA and to the London Stock Exchange for admission to the Official List and to trading on the Main Market of the London Stock Exchange. It is expected that trading in the Holdback Shares will commence on or around Tuesday 22 December. Following admission, there will be 625,285,385 shares in issue.

Enquiries:

Antisoma plc

Glyn Edwards, CEO                         

Daniel Elger, VP Marketing & Communications     +44 (0) 7909 915 068