New monoclonal antibodies in renal transplantation

A decade of spectacular innovation in maintenance immunosuppression drugs has resulted in dramatic reductions in acute rejection and improvement in short and long term outcome after renal transplantation. However the new drugs continue to lack specificity, many require frequent therapeutic drug monitoring and all are associated with acute and chronic toxicities. The new biologic agents, monoclonal antibodies (chimeric, humanized, and fully human) and receptor-fusion proteins, lack immunogenicity, have long half-life and prolonged biologic effects, require intermittent administration and have minimal toxicity. The specificity and selectively of the targets of the new biologic agents render them less toxic than the oral maintenance drugs and thus could possibly replace the maintenance drugs most associated with long-term toxicity such as the corticosteroids and the calcineurin inhibitors. The recently introduced anti-interleukin 2 receptor (IL-2R) monoclonal antibodies (mAbs) are the prototype of future biologic agents; selective, safe, and inducing prolonged biologic effects. The IL-2R mAbs have been used with a variety of maintenance immunosuppression regimens double therapy with cyclosporine and prednisone, triple therapy with cyclosporine, azathioprine and prednisone and with newer regimens such as cyclosporine or tacrolimus, mycophenolate mofetil (MMF) and prednisone, and most recently with sirolimus, MMF and prednisone. The major thrust of the new biologics in clinical development is to block the co-stimulatory pathway. The first attempt at blockade of the CD40-CD154 with anti-CD154 mAbs was disappointing. Anti-CD 154 therapy was associated with thromboembolic events and acute rejection. Attempts at blocking the CD28-B7s (CD80-CD86) pathway are currently underway with the receptor fusion protein, LEA29Y a second generation CTL4Aig, and humanized mAbs to CD 80 and CD86. LFA1, an adhesion molecule that also participates in the co-stimulatory pathway, has also been targeted with a mAb that binds to the CD11a chain of LFA1. Efalizumab, a humanized anti-CD11a mAb, was shown in a phase I trial to be potentially effective in renal transplantation. A humanized anti-CD45 RB mAb is currently in pre-clinical studies and will likely be tested in a phase I trial of renal transplantation within 1 year. While excellent results with anti-CD45 RB mAbs have been published in experimental transplantation, the mechanism of action of anti-CD45 RB mAbs remains to be determined. Several antibodies that are currently approved for non-transplant indications are currently used in single center clinical trials in renal transplantation including Campath 1 H, a humanized anti-CD52 mAb, Rituxamab, an anti-CD20 chimeric mAb, and Infliximab an anti-TNFa chimeric mAb. In addition, several humanized mutagenized anti-CD3 mAbs, huOKT3g1, aglycosyl CD3 and HuM291 have been used in limited trials in renal transplantation but have yet to have a formal clinical development. Humanized mAbs and receptor fusion proteins offer the potential of providing renal transplant recipients with a novel algorithm for immunosuppression that relies on chronic intermittent intravenous administration of safe, non-toxic agents replacing oral drug therapy maintenance.