New immunosuppressive strategies.

Abstract

Discovery of novel biological and pharmaceutical agents directed against discrete molecular targets in the lympnocyte activation sequence has enabled the effective control of graft rejection by the use of combinatorial immunosuppressive therapy. Chimeric and humanized monoclonal antibodies against T-cell receptor CD3 complex chains or the IL-2 receptor block T-cell function without inducing activation, and do not cause the cytokine release syndrome of first generation products. Biological blockade of co-stimulatory molecules including CD40L and CD28 produces immunological allograft unresponsiveness in primates, though this effect is not yet proven in humans. Heterogeneity in clinical response to pharmaceutical agents is often explained by pharmacokinetic factors of absorption, metabolism and elimination. The use of microemulsion technology has increased the absorption and efficacy of cyclosporine in all organ transplants, so that there is little difference in efficacy between this agent and tacrolimus. Mycophenolate mofetil is not maximally effective alone, but significantly reduces the relative risk of acute rejection in combination with an immunophilin binding agent. It is also effective when introduced at the time of rejection. Whether it can replace other agents for maintenance immunosuppression is now under investigation. Sirolimus, the latest pharmaceutical agent to complete phase III trials, acts to inhibit IL-2 driven lymphocyte proliferation and reduces the risk of acute rejection to below 20%. Multiple pharmacokinetic interactions occur within and between these agents, so that pharmacokinetic monitoring is increasingly important. At present there are few tools to detect pharmacodynamic interactions, although reporter gene constructs and intracellular cytokine labeling offer exciting possibilities for biological monitoring. Despite these advances, none of these interventions confers demonstrable long-term benefit in graft survival or function. Acute rejection can not therefore be assumed to be a simple surrogate for chronic injury, and research must be re-focused to determine the relevant targets for long-term immunosuppression.