Medarex is developing MDX-RA, an immunotoxin, for the potential prevention of secondary cataracts. It entered phase III clinical trials in December 1997. The phase III studies will involve 680 patients undergoing primary cataract surgery at approximately 60 sites in the US [272814]. The immunotoxin consists of a murine monoclonal antibody (4197X) that binds specifically to human lens epithelial cells conjugated to a polypeptide toxin, ricin A (RA) chain toxin. MDX-RA is formulated as a salt solution which will be injected into the eye following the removal of the natural opaque lens and the insertion of a clear plastic intraocular lens. The specificity of the antibody directs the agent to the target where RA will exert its cytotoxic activity. The linkage technology used in the manufacture of MDX-RA is proprietary to Sanofi SA and was licensed exclusively to Medarex for use in ophthalmic therapeutics [193106]. Results from a placebo-controlled phase I/II clinical trial demonstrated that patients treated with 50 units of MDX-RA experienced substantially less opacification than patients on placebo [272814]. There was less lens capsule opacification in the treated group than in the placebo group even at this early stage. The company expected to complete additional interim analyses during the two-year follow-up. MDX-RA has also been in phase II trials in patients undergoing bilateral cataract surgery. Santen will fund the clinical development of MDX-RA, at a cost of some US $8 million over six years, in return for an exclusive marketing licence for Japan [204883]. Houston Biotechnology, which was acquired by Medarex in 1997, holds an exclusive worldwide license to patents from the Baylor College of Medicine, to which most of the early patents relating to the immunotoxin are assigned [193106]. Medarex estimates that there will be 1.8 million primary cataract procedures performed in the US in 1998, with the number performed elsewhere equalling this figure. The projected worldwide market potential for MDX-RA, based on these figures, is US $450 to 600 million [193106].