MEDI 500 (G 022, anti-TCR-&agr;&bgr; monoclonal antibody T10B9, T10B9, T10B9.1A31, T-10B9 monoclonal antibody, T12A10) is a murine monoclonal antihuman T cell antibody which is specific for the T cell receptor (TCR)&agr;&bgr; heterodimer found on the surface of most T cells. It is believed that when MEDI 500 binds to these T cells, it suppresses their function, thereby reversing acute organ rejection which is thought to be mediated largely by T cells.MedImmune (USA) began developing MEDI 500 using exclusive rights acquired from the University of Kentucky Medical Center (USA). The drug is now being codeveloped by MedImmune and BioTransplant (USA) after the 2 companies formed a strategic alliance to develop products for the prevention and treatment of organ transplant rejection. MedImmune will fund and assume responsibility for clinical testing and commercialisation of any resulting product.In the USA, MEDI 500 is undergoing clinical development for the treatment of liver transplant rejection, acute heart transplant rejection (phase I) and acute kidney transplant rejection (phase II), and forex vivoT cell depletion for the prevention of acute graft-versus-host disease following bone marrow transplant (phase III). The University of Kentucky has completed phase II trials for the treatment of kidney transplant rejection. MEDI 500 has been used in over 500 patients under physician INDs. The primary potential benefit of MEDI 500 over muromonab CD3 (the only marketed monoclonal antibody for the reversal of acute renal rejection) is an improved adverse effects profile. However, the main apparent disadvantage of MEDI 500 is a short half-life necessitating a more frequent dosing schedule than muromonab CD3.A humanised version of MEDI 500 is under development for the treatment of autoimmune disorders and T cell cancers.