Herein we describe the synthesis and in vitro and in vivo activity of thiazolo [5, 4-d] pyrimidines as a novel class of immunosuppressive agents, useful for preventing graft rejection after organ transplantation. This research resulted in the discovery of a series of compounds with potent activity in the mixed lymphocyte reaction (MLR) assay, which is well- known as the in vitro model for in vivo rejection after organ transplantation. The most ...