New small molecule immunosuppressants for transplantation: review of essential concepts

More immunosuppressive drugs than ever have recently graduated from the laboratory to extensive clinical trials of their safety and efficacy in patients undergoing transplantation. Although none of these drugs is perfect, they control different forms of rejection in stringent animal models more effectively than other immunosuppressants; yet these novel molecules suppress the immune system far more specifically than steroids and regimens that cause lymphopenia. Cyclosporin G and IMM 125 (analogues of cyclosporine) and FK506 are the only drugs that selectively inhibit T-cell proliferation by blocking cytokine synthesis. The primary action of rapamycin and leflunomide appears to be an inhibition of the actions of cytokines and growth factors on T, B, and some nonimmune cells. T and B cells are more sensitive than nonimmune cells to the depletion of purines and pyrimidines caused by mizoribine, mycophenolic acid, and brequinar sodium. Nucleotide depletion causes interruption of DNA synthesis and glycosylation of adhesion molecules in immune cells. Further differentiation of T and B cells after proliferation into fully functional immune cells is inhibited by unknown mechanisms by brequinar and deoxyspergualin. On the basis of preclinical studies, these drugs may effectively suppress clinical rejection that is (1) acute (all drugs), (2) chronic (rapamycin, leflunomide, and mycophenolic acid), or (3) antibody-mediated (brequinar, deoxyspergualin, mycophenolic acid, and rapamycin). Some drugs (FK506, deoxyspergualin, mycophenolic acid, rapamycin, and leflunomide) may reverse acute rejection refractory to conventional immunosuppression. These new drugs not only block different biochemical steps that normally lead to fully functional T and B cells after stimulation by alloantigen, but their toxicity profiles also differ. Results from preclinical studies predict that use of selected combinations of these drugs in patients will be more effective, less nephrotoxic, less myelotoxic, and less broadly immunosuppressive than current regimens based on cyclosporine, T-cell depletion, steroids, and azathioprine ... at least, that's the idea! Or as former Vice President Dan Quayle said, "It's a question of whether we're going to go forward into the future, or past to the back."