Encouraging results for belatacept immunosuppression in renal transplantation

Quantitative analysis predicts the relative therapeutic efficacy of different forms of CTLA4Ig

Modulating the activities of costimulatory molecules controlling immune responses holds considerable promise for immunotherapy. CTLA4Ig (abatacept), a soluble version of the T cell-expressed membrane receptor CTLA-4, is approved for the treatment of rheumatoid arthritis. Like natural CTLA-4 molecules, CTLA4Ig ligates B7-1 and B7-2 on antigen presenting cells, preventing CD28-mediated costimulation of T cells. However, CTLA4Ig can also prevent ligation of CTLA-4, potentially blocking vital inhibitory signals, thereby augmenting immunity. There have been no quantitative analyses of the likely effects of CTLA4Ig on costimulatory interactions at the immunological synapse. We present a mathematical model, based on rigorous biophysical and expression data, for simulating the effects of abatacept and a mutated derivative, LEA29Y, on the synaptic interactions of CD28 and CTLA-4. The simulations reveal an unexpectedly large window within which CD28, but not CTLA-4, ligation is blocked by CTLA4Ig, perhaps explaining the efficacy of abatacept at the recommended therapeutic dose (10mg/kg) and its relative safety. However, the simulations suggest that the present dosing regimen is close to the maximum theoretically safe dose. The simulations also show that, within the therapeutic window, LEA29Y enhances the interaction of CTLA-4 with the more potent of its two native ligands, B7-1. They also suggest that CTLA-4 ligation by B7-1 could, in principle, be enhanced by further decreasing the off-rate of CTLA4Ig for binding to B7-2. Our findings therefore offer molecular explanations for why LEA29Y might prove to be more effective than abatacept in a clinical setting, and suggest ways in which its therapeutic efficacy could be further optimised.

Biological agents in kidney transplantation: belatacept is entering the field

IMPORTANCE OF THE FIELD

Kidney transplantation is the best treatment for end-stage kidney-disease patients. However, despite major breakthroughs in the last decades, and the progresses made with immunosuppressants, the long-term results still need to be improved. This is related to the increased risk of cardiovascular mortality, de novo post-transplant malignancies, and chronic kidney disease within the allograft. The last is multifactorial and includes immunological and non-immunological factors. Amongst the last is the calcineurin inhibitor (CNI) (cyclosporine A (CsA) and tacrolimus)-related nephrotoxicity. Kidney-allograft function at 1-year post-transplantation is a good surrogate marker of long-term allograft survival.

AREAS COVERED IN THIS REVIEW

Cytotoxic T-lymphocyte-associated antigen 4 (CTLA4)-Ig, a fusion protein, presents as abatacept, which conserves the natural structure of CTLA4, and belatacept, which has enhanced activity thanks to two amino-acid substitutions. Abatacept and belatacept block CD86-CD28 interaction, but belatacept blocks them more powerfully. Abatacept is already approved for the treatment of rheumatoid arthritis and is marketed as Orencia(®) (Bristol-Myers Squibb, Princeton, NJ, USA), whereas belatacept is not yet approved. Herein, we review the current data available on the use of belatacept in Phase II and III kidney-transplantation trials. Note, though, that data from belatacept Phase II liver transplantation trials are not yet available.

WHAT THE READER WILL GAIN

The results show in de novo kidney transplant patients that as compared to CsA-treated patients, belatacept-treated patients showed: i) a significant better allograft function both at 1- and 2- year post-transplantation and ii) better cardiovascular and metabolic profiles. Regarding the safety data, Epstein-Barr virus (EBV) seronegative belatacept-treated patients experience more post-transplant lymphoproliferative disorders than the EBV seropositive belatacept-treated patients and the CsA-treated patients.

TAKE HOME MESSAGE

CNIs are potent immunosuppressants but have some degree of nephrotoxicity. Therefore, it is important to have strong data showing that belatacept-based therapy is as efficient as CsA-based therapy, but displaying at both 1- and 2-year post-transplantation a better allograft function, which might translate in the long-term into longer allograft survival.