Validation of the calcineurin phosphatase assay

Monitoring of Ex Vivo Cyclosporin a Activity in Healthy Volunteers Using T Cell Function Assays in Relation to Whole Blood and Cellular Pharmacokinetics

Therapeutic drug monitoring (TDM) of calcineurin inhibitors (i.e., tacrolimus and cyclosporin A) is standard of care after solid organ transplantation. Although the incidence of acute rejection has strongly decreased, there are still many patients who experience severe side effects or rejection after long-term treatment. In this healthy volunteer study we therefore aimed to identify biomarkers to move from a pharmacokinetic-based towards a pharmacodynamic-based monitoring approach for calcineurin inhibitor treatment. Healthy volunteers received a single dose of cyclosporine A (CsA) or placebo, after which whole blood samples were stimulated to measure ex vivo T cell functionality, including proliferation, cytokine production, and activation marker expression. The highest whole blood concentration of CsA was found at 2 h post-dose, which resulted in a strong inhibition of interferon gamma (IFNy) and interleukin-2 (IL-2) production and expression of CD154 and CD71 on T cells. Moreover, the in vitro effect of CsA was studied by incubation of pre-dose whole blood samples with a concentration range of CsA. The average in vitro and ex vivo CsA activity overlapped, making the in vitro dose–effect relationship an interesting method for prediction of post-dose drug effect. The clinical relevance of the results is to be explored in transplantation patients on calcineurin inhibitor treatment.

Calcineurin Activity Assay Measurement by Liquid Chromatography–Tandem Mass Spectrometry in the Multiple Reaction Monitoring Mode

BACKGROUND

Blood concentrations of the calcineurin inhibitors (CNIs) cyclosporine and tacrolimus are currently measured to monitor immunosuppression in transplant patients. The measurement of calcineurin (CN) phosphatase activity has been proposed as a complementary pharmacodynamic approach. However, determining CN activity with current methods is not practical. We developed a new method amenable to routine use.

METHODS

Using liquid chromatography–multiple reaction monitoring mass spectrometry (LC-MRM-MS), we quantified CN activity by measuring the dephosphorylation of a synthetic phosphopeptide substrate. A stable isotope analog of the product peptide served as internal standard, and a novel inhibitor cocktail minimized dephosphorylation by other major serine/threonine phosphatases. The assay was used to determine CN activity in peripheral blood mononuclear cells (PBMCs) isolated from 20 CNI-treated kidney transplant patients and 9 healthy volunteers.

RESULTS

Linearity was observed from 0.16 to 2.5 μmol/L of product peptide, with accuracy in the 15% tolerance range. Intraassay and interassay recoveries were 100.6 (9.6) and 100 (7.5), respectively. Michaelis–Menten kinetics for purified CN were Km = 10.7 (1.6) μmol/L, Vmax = 2.8 (0.3) μmol/min · mg, and for Jurkat lysate, Km = 182.2 (118.0) μmol/L, Vmax = 0.013 (0.006) μmol/min · mg. PBMC CN activity was successfully measured in a single tube with an inhibitor cocktail.

CONCLUSIONS

Because LC-MRM-MS is commonly used in routine clinical dosage of drugs, this CN activity assay could be applied, with parallel blood drug concentration monitoring, to a large panel of patients to reevaluate the validity of PBMC CN activity monitoring.

Molecular Diagnostics of Calcineurin-Related Pathologies

BACKGROUND

The Ca2+-dependent protein phosphatase enzyme calcineurin (Cn) (protein phosphatase 3) is best known for its role as director of the adaptive immune response. One of its principal substrates is the nuclear factor of activated T cells (NFAT), which translocates to the nucleus after dephosphorylation to mediate gene transcription. Drugs targeting Cn (the Cn inhibitors tacrolimus and cyclosporin A) have revolutionized posttransplantation therapy in allograft recipients by considerably reducing rejection rates.

CONTENT

Owing primarily to intensive study of the side effects of the Cn inhibitors, the unique importance of Cn and Cn/NFAT signaling in the normal physiological processes of many other cell and tissue types is becoming more evident. During the last decade, it has become clear that an extensive and diverse array of clinical conditions can be traced back, at least in part, to a disturbed Cn-signaling axis. Hence, both diagnostics and therapeutic monitoring could benefit from a technique that conveniently reads out Cn/NFAT operative status.

SUMMARY

This review outlines the current knowledge on the pathologic conditions that have calcineurin as a common denominator and reports on the progress that has been made toward successfully applying Cn and Cn/NFAT activity markers in molecular diagnostics.

Cyclosporin and tacrolimus impair insulin secretion and transcriptional regulation in INS-1E beta-cells

BACKGROUND AND PURPOSE

Introducing the calcineurin inhibitors cyclosporin (CsA) and tacrolimus (Tac) has improved the outcome of organ transplants, but complications such as new onset diabetes mellitus after transplantation (NODAT) decrease survival rates.

EXPERIMENTAL APPROACH

We sought, in a beta-cell culture model, to elucidate the pathogenic mechanisms behind NODAT and the relative contribution of the calcineurin inhibitors. INS-1E cells were incubated at basal and stimulatory glucose concentrations, while exposed to pharmacologically relevant doses of CsA, Tac and vehicle for 6 or 24 h.

RESULTS

Tac inhibited basal (P < 0.05), but not glucose-stimulated insulin secretion (GSIS) after 6 h of exposure. After 24 h, both agents inhibited basal and GSIS (P < 0.05). Calcineurin phosphatase activity was decreased by both drugs during all conditions. Apoptosis was only seen with CsA treatment, which also induced a slight suppression of calcineurin and insulin mRNA, as well as increased levels of the sterol receptor element binding protein (SREBP)-1c, a transcription factor thought to suppress genes essential for beta-cell function and induce insulin resistance. Expression levels of nuclear factor of activated T-cells (NFAT)-c1, -c2, -c3 and -c4 were not decreased notably by either drug.

CONCLUSIONS AND IMPLICATIONS

Tac had acute inhibitory effects on basal insulin secretion, but prolonged exposure (24 h) to Tac or CsA revealed similar suppression of insulin secretion. These prolonged effects were mirrored by a total inhibition of calcineurin activity in beta-cells. CsA showed greater inhibition of beta-cell survival and transcriptional markers, essential for beta-cell function.

Pharmacodynamic monitoring of calcineurin inhibition therapy: principles, performance, and perspectives

The calcineurin inhibitors (CNIs) cyclosporin A and tacrolimus are immunosuppressive drugs used extensively in allograft recipients. These drugs show large interindividual pharmacokinetic variation and are associated with severe adverse affects, including nephrotoxicity and cardiovascular disease. In current practice, CNIs are combined with other immunosuppressive drugs such as steroids and mycophenolate mofetil. Dosage is titrated based on blood concentration measurement. For further optimization of calcineurin (CN) inhibition therapy, new monitoring strategies are required. Pharmacodynamic-monitoring strategies constitute novel approaches for optimization of CNIs therapy. This review focuses on the general aspects of immunosuppressive drug pharmacodynamic monitoring and describes the methodologies used for monitoring CN inhibition therapy. Two different types of pharmacodynamic-monitoring strategies can be distinguished: (1) enzymatic strategies, which monitor inhibition of drug-target enzyme activity, and (2) immunologic strategies, which measure cellular responsiveness after in vitro simulated immunologic responses. Enzymatic tests are drug type-specific markers in which CN activity is directly determined. Immunologic strategies measure immune responsiveness at several levels, such as mRNA transcripts (intracellular) concentrations/excretion of cytokines, expression of surface activation markers, and cell proliferation. This review also discusses analytical issues and clinical experience with these techniques. The call for new methodologies to evaluate immunosuppressive therapy has led to the development of a large variety of pharmacodynamic-monitoring strategies. The first reports of their clinical relevance are available, but further understanding of the analytical and clinical variables involved are required for the development of accurate, reproducible, and clinically relevant markers.

Temporal profile of calcineurin phosphatase activity during acute allograft rejection in the heterotopic rat heart transplantation model

BACKGROUND

Regardless of the extensive worldwide use of calcineurin inhibitors, little is known about the behavior of calcineurin phosphatase (CaN) during acute allograft rejection. The aim of this study was to investigate the temporal profile of CaN during acute allograft rejection and reveal if it can be utilized as a pharmacodynamic marker to identify and monitor the rejection process.

METHODS

The heterotopic cervical rat heart transplantation model was used (dark Agouti to Lewis). We performed 25 control isogeneic and 46 allogeneic transplantations. Rats were sacrificed at various postoperative time points. CaN activity was measured in isolated peripheral blood and spleen mononuclear cells and in graft heart homogenates. CaN activity was measured as the release of radiolabeled phosphate from a previously phosphorylated 19 amino acid peptide.

RESULTS

We have shown that CaN's activity levels are not significantly altered during acute allograft rejection in peripheral blood mononuclear cells and in spleen-isolated mononuclear cells. CaN's intragraft activity decreased with time in both rejectors and controls, and was significantly lower in the allogeneic group.

CONCLUSIONS

CaN failed as a pharmacodynamic biomarker of acute allograft rejection in the heterotopic rat heart transplantation model. Further research is required in order to reveal the precise role of CaN during acute allograft rejection.

[Therapeutic monitoring of immunosuppressive drugs: interest of calcineurin activity assessment in liver transplantation]

Therapeutic monitoring of calcineurin inhibitors (ciclosporin and tacrolimus) consists in pharmacokinetic monitoring. Pharmacodynamics based on calcineurin activity may be particularly interesting in liver transplantation due to the large intra- and interindividual variability of pharmacokinetics of ciclosporin and tacrolimus. A recent investigation on the pharmacokinetic-pharmacodynamic relationship of tacrolimus showed that monitoring of calcineurin activity in PBMC may be particularly relevant within the first three post-transplantation months. Thereafter, the monitoring of trough blood concentrations of tacrolimus remains adequate. Moreover, two clinical investigations carried out within the early and late post-transplantation periods reported a promising result which is a positive correlation between calcineurin activity and incidence of graft rejection, whatever graft type and calcineurin inhibitors. In each study, transplanted recipients with a graft rejection exhibited a greater trough calcineurin activity compared to patients without graft rejection. However, prospective investigations are required because of the small cohorts of patients enrolled in both studies. The aim of these investigations will be to confirm the interest of calcineurin activity monitoring as a marker of cellular immunity and its positive link with pharmacokinetic monitoring.

Variation in Leukocyte Subset Concentrations Affects Calcineurin Activity Measurement: Implications for Pharmacodynamic Monitoring Strategies

Background: In renal transplantation patients, therapeutic drug monitoring of the calcineurin (CN) inhibitor cyclosporin A (CsA) is mandatory because of the drug’s narrow therapeutic index. Pharmacodynamic monitoring of CN inhibition therapy could provide a tool to define and maintain the therapeutic efficacy of CsA therapy. We investigated the effect of variation in cell counts of leukocyte subsets on leukocyte CN activity measurement in renal transplant recipients.

Methods: We measured leukocyte CN activity, whole blood CsA concentrations, and leukocyte subset cell counts in 25 renal transplant recipients. Blood was collected before graft implantation and CsA therapy, 1 day before transplantation when CsA therapy was already started, and 5 days after transplantation. Monocyte, granulocyte, CD4+ T-cell, CD8+ T-cell, B-cell, and natural killer–cell CN activities and CsA inhibition sensitivities were determined in vitro by a spectrophotometric CN assay.

Results: Leukocyte CN activity was inhibited after drug intake. Inter- and intrapatient variation in leukocyte subset cell counts resulted in variation of sample composition. The mean (SD) CN activity varied among leukocyte cell subsets, ranging from 650 (230) to 166 (26) pmol/min/106 cells for monocytes and CD4+ T cells, respectively. CsA half maximal inhibitory concentration (IC50) values ranged from 15 to 78 μg/L for monocytes and B cells, respectively.

Conclusion: Inter- and intraindividual leukocyte subset cell count variation can affect measured CN activity independent of CsA concentration. Cell-specific activity and drug sensitivity should be considered for sample validation to optimize method specificity when pharmacodynamic monitoring strategies are applied in a clinical setting.

Spectrophotometric assay for calcineurin activity in leukocytes isolated from human blood

The calcineurin inhibitors, cyclosporine and tacrolimus, still constitute the cornerstone of immunosuppressive regimen after organ transplantation. An efficient and feasible way to measure calcineurin activity and inhibition by these drugs may improve therapeutic monitoring of these drugs in transplant recipients. Calcineurin activity was measured in leukocyte lysates isolated from human blood using spectrophotometric phosphate quantification. The dephosphorylation of a 19-amino acid peptide substrate of calcineurin was determined using the Malachite green phosphate reagent in the presence of okadaic acid and with and without the calcium chelator EGTA. Sample storage and lysis buffer components were among the variables optimized, and the inhibitory effect of calcineurin inhibitors was investigated. Observed loss of calcineurin activity during sample storage was eliminated by adding ascorbic acid to lysis buffer. The final inter- and intraassay variation coefficients were 10 and 4.5%, respectively, and the detection limit was 15 pmol min(-1)x10(6) WBC(-1), where WBC is white blood cells (leukocytes). In vitro IC50 values were 212 and 34 microg/L for cyclosporine and tacrolimus, respectively. In vivo calcineurin inhibition was observed when calcineurin activity was measured in transplant recipients on maintenance therapy with cyclosporine and tacrolimus.

The calcineurin activity profiles of cyclosporin and tacrolimus are different in stable renal transplant patients

Cyclosporin and tacrolimus remain the cornerstone immunosuppressive drugs in organ transplantation. Dosing and monitoring these drugs is based on pharmacokinetic protocols, but measuring a pharmacodynamic parameter, calcineurin phosphatase (CaN) activity, could be a valuable supplement in determining optimal doses. Forty stable renal transplant patients were investigated three times in a 6-month period. Blood samples were drawn at 0, 1, 2, 3 and 4 h after oral intake of tacrolimus (FK) or cyclosporin at days 1 and 180. At day 90, one blood sample at trough level (FK) or C2 level (cyclosporin A, CsA) was drawn. CaN activity was determined in whole blood as the release of 32P from a phosphorylated peptide. Activity of the 32P was quantitated by liquid scintillation and results converted to Units CaN, utilizing a calibration curve with CaN. We demonstrated that calcineurin activity profiles at days 1 and 180 were the same for both drugs. Furthermore, we found that patients treated with tacrolimus or cyclosporin displayed different calcineurin activity profiles. We found that cyclosporin displayed greater calcineurin inhibition than tacrolimus. We have demonstrated that the two drugs exert significantly different effects on calcineurin activity in renal transplant patients with stable, well-functioning grafts and that tacrolimus-treated patients can maintain good, stable graft function with minimal CaN inhibition.

Calcineurin Activity in Tacrolimus-Treated Renal Transplant Patients Early After and 5 Years After Transplantation

The pharmacodynamic (PD) action of tacrolimus (FK) within the T-cell is inhibition of calcineurin phosphatase (CaN). Determination of CaN activity provides us with an important PD marker. Eleven renal transplant patients treated with FK were investigated on day 14 following transplantation and 5 years later. Blood samples drawn before as well as 1, 2, 3, and 4 hours after oral intake of FK were analyzed for CaN activity and blood FK concentrations. Twenty healthy subjects had one blood sample drawn for CaN activity, which was measured as the release of (32)P from a phosphorylated peptide. Radioactivity of (32)P was quantitated by liquid scintillation counting with the results converted to units of CaN utilizing a calibration curve. On day 14, we observed significant inhibition of CaN activity at T:1, 2, and 3 compared with the predose level (P = .002; P = .015; P = .015). Furthermore, all measured CaN activities were significantly different from those observed in healthy nonmedicated subjects. In contrast, at 5 years posttransplant only the CaN activity at T:2 was significantly inhibited compared with the predose level (P = .02). Additionally, all CaN activities at this time were not significantly different from CaN activities in the healthy subjects. We were not able to demonstrate individual CaN activity profiles in the patients. The lack of CaN inhibition at 5 years after transplantation despite relevant drug concentrations, probably reflected the lower drug dose used long after transplantation. This result raises the question of whether CaN inhibition is necessary to hold graft function and whether FK possess CaN-independent mechanisms of action.

Correlations between Calcineurin Phosphatase Inhibition and Cyclosporine Metabolites Concentrations in Kidney Transplant Recipients: Implications for Immunoassays

Cyclosporine exhibits a wide spectrum of metabolites that vary considerably in the extent to which they interfere with the various parent drug monitoring immunoassays. There is no consensus regarding the clinical significance of metabolites. Cyclosporine exerts its immunosuppressive action by inhibiting the enzyme calcineurin phosphatase. Determination of the enzyme's activity is one of the most promising pharmacodynamic markers. It is unknown how calcineurin phosphatase inhibition correlates with various cyclosporine monitoring assays and what is the potential impact of metabolites in this perspective? The aim of the present study was to determine the concentration of cyclosporine (by means of three different assay methods) and the four most significant metabolites (AM1, AM4N, AM9, and AM1C) in relation to calcineurin phosphatase inhibition. Twelve randomly selected cyclosporine-treated renal transplant patients were included in the study. Blood samples were drawn before, 1, 2, 3, 4, 6, 8, and 12 hr after oral intake of cyclosporine. Parent drug and metabolites were determined by liquid chromatography/tandem mass spectrometry (LC/MSMS). Additionally, cyclosporine concentration was determined by the enzyme multiplied immunoassay technique (EMIT) and by the polyclonal fluorescence polarization immunoassay (pFPIA). Calcineurin phosphatase activity was measured by its ability to dephosphorylate a previously phosphorylated 19-amino acid peptide. We found that calcineurin phosphatase inhibition correlates strongly with parent cyclosporine metabolites concentrations determined by all three assay methods. Determination methods that took metabolites into consideration exhibit stronger correlations with calcineurin phosphatase inhibition (sum of cyclosporin plus metabolites r=-0.93, LC/MSMS; pFPIA r=-0.94, P<or=0.001), compared with methods that measure exclusively the parent drug (EMIT: -0.84; LC/MS-MS: -0.81, P<or=0.05). Our results indicate that the immunosuppressive role of cyclosporines metabolites should not be considered valueless per se. Further research is required in order to verify the potential clinical importance of our observations.

24-h Monitoring of Calcineurin Phosphatase Activity in Healthy Subjects

The calcineurin inhibitors cyclosporine and tacrolimus are the cornerstone immunosuppressants used in solid organ transplantation. Studies investigating calcineurin (CaN) activity in renal transplanted patients have been published, but basic properties of the enzyme activity in healthy subjects remain to be described. The aim of this study was to investigate whether CaN displays circadian variation or sex difference is present in healthy subjects. Twenty subjects had blood samples drawn every 4 h for a 24-h period. CaN activity was determined in whole blood as the release of 32P from a phosphorylated peptide. Activity of the 32P was quantitated by liquid scintillation and results converted to units CaN utilizing a calibration curve. We found no circadian variation in CaN activity and no difference between the two sexes. The clinical importance of these findings is that blood samples for calcineurin activity can be drawn without taking the exact time of day into consideration, but only considering the time of drug intake.

Comparison of the Temporal Profile of Calcineurin Inhibition by Cyclosporine and Tacrolimus in Renal Transplant Patients

Calcineurin phosphatase (CaN) activity has been the focus of several recent studies on renal transplant patients as the calcineurin inhibitors tacrolimus (FK) and cyclosporine (CsA) are still the cornerstone in the immunosuppressive treatment. The aim of this study was to compare the temporal inhibition profiles of CaN using CsA or FK in two groups of renal transplant patients. Nineteen tacrolimus-treated and 10 cyclosporine-treated renal transplant patients had blood samples drawn before and at 1, 2, 3, 4, and 6 hours after ingestion of drug. CaN activity was measured as the release of 32P from a previously phosphorylated peptide and radioactivity quantitated by liquid scintillation counting. Results were expressed as units CaN. Blood concentrations of tacrolimus were determined with an IMx method and of CsA with an EMIT assay. FK-treated patients showed maximal inhibition of CaN activity at 1 to 3 hours, returning to the predose level 4 hours after drug intake. CsA-treated patients showed a gradual decrease in CaN activity with a nadir after 3 hours, failing to return to predose levels during the observation period. Both groups showed a significant rise in drug blood concentrations. To conclude, we have demonstrated that two widely used immunosuppressants, CsA and FK, show different CaN inhibitory profiles in renal transplant patients.