Estimating alloantibody levels in highly sensitized renal allograft candidates: Using serial dilutions to demonstrate a treatment effect in clinical trials

Abstract

Small reductions in calculated panel‐reactive antibody (cPRA) are associated with increased kidney transplantation in 100% cPRA patients. However, the high level of antibody in these patients is such that desensitization may reduce antibody but not cPRA, thus the cPRA change on undiluted serum with desensitization is an insensitive measure of effectiveness. We evaluated cPRA reduction, calculated per antibody titer, as a desensitization trial endpoint. To accomplish this, two serum samples from 20 kidney transplant candidates with cPRA ≥99.9% (100%) were obtained and serially diluted in triplicate to determine the titer of individual human leukocyte antigen (HLA) antibody specificities. CPRA was computed per dilution to identify the titer at which cPRA drops below 98%. Inter‐ and intra‐assay variability and changes overtime were determined. The dilution needed to reach a cPRA <98% was within 1 titer for replicates from the same sample, with 90% (36/40) concordance. This indicates that only changes >2 titers can be deemed clinically meaningful. The median (IQR) titer difference was 0 (0‐1) from baseline to follow‐up within 12 months. The cPRA per titer also risk‐stratified candidates for trial inclusion. In conclusion, determining the cPRA per titer is a reliable approach to simplify complex antibody data and an ideal endpoint for desensitization trials.

Current Approaches to Desensitization in Solid Organ Transplantation

“…We evaluated cPRA reduction per titer among 20 sensitized patients with a cPRA > 99.9% Figure 2 . We found that titer determination in a central laboratory using same lot reagents and batch testing leads to reproducible results and that the cPRA per dilution remains constant within approximately 1 titer if serum samples are obtained within 1 year ( 55 ). Transplant candidates with a cPRA of > 99.9% have vastly different quantities of antibody ( 55 ).…”

“…We found that titer determination in a central laboratory using same lot reagents and batch testing leads to reproducible results and that the cPRA per dilution remains constant within approximately 1 titer if serum samples are obtained within 1 year ( 55 ). Transplant candidates with a cPRA of > 99.9% have vastly different quantities of antibody ( 55 ). While the cPRA began to drop after only 1-2 dilutions for some candidates, other candidates remained at cPRA >99.9% even when their serum was diluted 1:4096.…”

“…Tambur et al Estimating Alloantibody levels in highly sensitized renal allograft candidates: Using serial dilutions to demonstrate a treatment effect in clinical trials. American Journal of Transplantation December 11, 2020 (55).…”

Apples, oranges, and anything in between: In search of the best desensitization therapy

“…Percent cPRA has been the common metric to assess patients' sensitization, however, as we have recently shown, patients with cPRA >99.9% can still exhibit substantial heterogeneity with regards to the strength (amount) of their antibodies. 3 For example, two patients may have the exactly same cPRA but have antibodies of different titer (e.g., I:8 vs. >1:16:384). This additional "dimension" of sensitization is critical when attempting to compare results between different treatments, assuring we are not comparing apples to oranges (especially when desensitization is used in the context of deceased donor transplantation).…”

“…This is a critical point as clinicians must remain thoughtful about when desensitization should be considered and how to personalize the approach. Percent cPRA has been the common metric to assess patients' sensitization, however, as we have recently shown, patients with cPRA >99.9% can still exhibit substantial heterogeneity with regards to the strength (amount) of their antibodies 3 . For example, two patients may have the exactly same cPRA but have antibodies of different titer (e.g., I:8 vs. >1:16:384).…”