In vitro pentamer formation as a biomarker of tacrolimus-related immunosuppressive activity after liver transplantation.

Therapeutic drug monitoring of tacrolimus (FK) is widely performed to assist adjustments of drug dosage but may be an inadequate surrogate of the immunosuppression induced. The aim of this investigation was to develop an alternative method for measuring FK-related immunosuppressive activity in blood samples from liver transplant recipients. A pentamer formation assay (PFA) was devised based on the attachment of the 12 kDa FK-binding protein (FKBP12) to microtitre plates in the presence of calcineurin, calmodulin, Ca++ and FK. Pentamer formation could be detected at FK concentrations > or = 0.2 microg/l by optimising assay conditions, particularly by including Ca++ (0.5 mM) only during the formation of the pentameric complex. Three methods (blood lysis, proteolytic digestion and use of commercial solutions used in a microparticle enzyme immunoassay (MEIA) technique) were incompatible with PFA measurements after extracting immunosuppressive FK-related material from patients' blood samples. However, therapeutic amounts of FK-related material could be quantified by the PFA assay after extraction of blood samples with methanol. There was a moderate correlation (r = 0.689) of FK equivalents assayed by PFA with results using MEIA in 56 blood samples from 14 liver graft recipients, but no obvious relationship of results to variables reflecting their clinical status.