EXHIBIT 10.20 December 16, 1996 Re: Development and License Agreement between Pharmacia S.p.A. ("Pharmacia") and PDT, Inc. ("PDTI") dated July 1st, 1995, as amended July 25, 1995, September 11, 1995, March 20, 1996, July 10, 1996 and August 12, 1996...