Pharmacodynamic Monitoring of Mammalian Target of Rapamycin Inhibition by Phosphoflow Cytometric Determination of p70S6 Kinase Activity

Pharmacodynamic Effect of mTOR Inhibition-based Immunosuppressive Therapy on T- and B-cell Subsets After Renal Transplantation

Background

The mammalian target of rapamycin inhibitor (mTORi) therapy after kidney transplantation is solely monitored pharmacokinetically, not necessarily reflecting PI3K-Akt-mTOR pathway blockade efficacy leading to potential under-or overimmunosuppression.

Methods

In this cross-sectional study, phosphoflow cytometry was used to determine the efficacy of mTOR inhibition in peripheral T- and B-lymphocyte subsets by assessing p70S6 kinase (p70S6K) phosphorylation in renal transplant recipients upon treatment with a combination of either mTORi and calcineurin inhibitors (n = 18), or mTORi with mycophenolic acid (n = 9). Nine dialysis patients with end-stage renal disease and 17 healthy age-matched volunteers served as controls.

Results

mTORi treatment reduced p70S6K phosphorylation in CD4+, CD8+ T, and CD19+ B cells compared with healthy controls (HCs). Subpopulation analysis of CD4+ T cells and CD19+ B cells revealed a significant reduction of p70S6K phosphorylation in CD4+CD45RA-CD25- Th cells (P < 0.05), CD24hiCD38hi transitional B cells (P < 0.001), CD24+CD38- memory B cells (P < 0.001), and CD24intCD38int-naive B cells (P < 0.05) upon mTORi treatment, whereas CD4+CD45RA-CD25++CD127- regulatory T cells and CD24-CD38hi plasmablasts were not affected. Compared with mTORi + mycophenolic acid therapy, mTORi + calcineurin inhibitor treatment exhibited an even stronger inhibition of p70S6K phosphorylation in CD4+CD45RA-CD25- Th cells and CD8+ T cells. However, trough levels of mTORi did not correlate with p70S6K phosphorylation.

Conclusions

mTORi selectively inhibited p70S6K phosphorylation in select lymphocyte subtypes. Assessing p70S6K phosphorylation by phosphoflow cytometry may serve as an approach to understand cell subset specific effects of mTORi providing detailed pharmacodynamic information for individualizing immunosuppression.

CXCR4 blockade reduces the severity of murine heart allograft rejection by plasmacytoid dendritic cell-mediated immune regulation

Allograft-specific regulatory T cells (T_reg cells) are crucial for long-term graft acceptance after transplantation. Although adoptive T_reg cell transfer has been proposed, major challenges include graft-specificity and stability. Thus, there is an unmet need for the direct induction of graft-specific T_reg cells. We hypothesized a synergism of the immunotolerogenic effects of rapamycin (mTOR inhibition) and plerixafor (CXCR4 antagonist) for T_reg cell induction. Thus, we performed fully-mismatched heart transplantations and found combination treatment to result in prolonged allograft survival. Moreover, fibrosis and myocyte lesions were reduced. Although less CD3⁺ T cell infiltrated, higher T_reg cell numbers were observed. Noteworthy, this was accompanied by a plerixafor-dependent plasmacytoid dendritic cells-(pDCs)-mobilization. Furthermore, in vivo pDC-depletion abrogated the plerixafor-mediated T_reg cell number increase and reduced allograft survival. Our pharmacological approach allowed to increase T_reg cell numbers due to pDC-mediated immune regulation. Therefore pDCs can be an attractive immunotherapeutic target in addition to plerixafor treatment.

Pretreatment with metformin prevents microcystin-LR-induced tau hyperphosphorylation via mTOR-dependent PP2A and GSK-3β activation

Microcystin-leucine-arginine (MC-LR) is a toxin secreted by freshwater cyanobacteria that is considered a potential environmental risk factor for Alzheimer's disease (AD). A previous study indicated that tau protein hyperphosphorylation via protein phosphatase 2A (PP2A) and GSK-3β inhibition was the mechanism by which MC-LR induces neurotoxicity; however, how MC-LR-induced neurotoxicity can be effectively prevented remains unclear. In this study, the reversal effect of metformin on MC-LR-induced neurotoxicity was investigated. The results showed that metformin effectively prevented tau hyperphosphorylation at Ser202 caused by MC-LR through PP2A and GSK-3b activity. The effect of metformin on PP2A activity was dependent on the inhibition of mTOR in MC-LR-treated SH-SY5Y cells. Metformin prevented spatial memory deficits in rats caused by intrahippocampal MC-LR administration. In sum, the results suggested that metformin can ameliorate the MC-LR-induced AD-like phenotype by preventing tau phosphorylation at Ser202, which was mainly mediated by mTOR-dependent PP2A and GSK-3β activation.

Reactivations of Latent Viral Infections Are Associated with an Increased Thr389 p70S6k Phosphorylation in Peripheral Lymphocytes of Renal Transplant Recipients

Reactivations of BK polyoma virus (BKPyV) and human cytomegalovirus (HCMV) frequently cause life- and graft-threatening complications after renal transplantation. Both viruses are dependent on the mTOR pathway for replication. In this study we investigated the association of viral replication with mTOR activity in peripheral lymphocytes of renal transplant recipients. A flow-cytometry based assay for the measurement of Thr389 p70S6k phosphorylation, a surrogate marker of the mTOR pathway was established. Forty-eight adult renal transplant recipients were recruited to measure p70S6k activity in their peripheral blood mononuclear cells. This data set in conjunction with information concerning previous replication of BKPyV and HCMV was examined for correlations. Episodes of BKPyV replication were significantly associated with increased p70S6k phosphorylation in CD4⁺ T lymphocytes (p = 0.0002) and CD19⁺ B lymphocytes (p = 0.0073). HCMV infection of patients with a high-risk HCMV constellation of donor and recipient (D+/R−) was associated with increased p70S6k phosphorylation in CD19⁺ B lymphocytes (p = 0.0325). These associations were found to be independent of the trough levels of the immunosuppressive drugs. Conclusion: P70S6k phosphorylation in peripheral lymphocytes is associated with BKPyV reactivations and to a lesser extent with HCMV infections in renal transplant recipients.

Intravenous Immunoglobulin Controls Th17 Cell-Mediated Osteoclastogenesis

The purpose of this study was to determine the regulatory role of intravenous Ig (IVIg) in Th17 cytokine–induced RANK ligand (RANKL) expression and osteoclast (OC) differentiation from OC precursors (pre-OC). Human CD14⁺ monocytes were isolated and stimulated by Th17 cytokines (IL-17, IL-21, and IL-22) and RANKL expression was investigated using a real-time PCR. CD14⁺ monocytes were incubated with RANKL, Th17 cytokines, and M-CSF, with/without IVIg, and OC differentiation was determined by counting tartrate-resistant acid phosphatase-positive multinucleated cells. OC differentiation was investigated after monocytes were cocultured with Th17 cells in the presence of IVIg. Th17 cell differentiation was determined using enzyme-linked immunosorbent assay and flow cytometry after CD4⁺ T cells were cultured with IVIg under Th17 condition. Th17 cytokines stimulated monocytes to express RANKL and IVIg suppressed the Th17 cytokine-induced RANKL expression. OCs were differentiated when pre-OC were cocultured with RANKL or Th17 cytokines and IVIg reduced the osteoclastogenesis. IVIg also decreased osteoclastogenesis when pre-OC were cocultured with Th17 cells. IVIg decreased both Th17 and Th1 cell differentiation while it did not affect Treg cell differentiation. In summary, IVIg inhibited Th17 cytokine-induced RANKL expression and OC differentiation. IVIg reduced osteoclastogenesis when monocytes were cocultured with Th17 cells. IVIg also reduced Th17 polarization. IVIg could be a new therapeutic option for Th17 cell–mediated osteoclastogenesis.

Pharmacodynamic Monitoring of mTOR Inhibitors

Pharmacodynamic (PD) monitoring may complement routine pharmacokinetic monitoring of mTOR inhibitors (mTORis) in an attempt to better guide individualized sirolimus (SRL) or everolimus (EVR) treatment after organ transplantation. This review focuses on current knowledge about PD biomarkers for personalized mTORi therapies. Different strategies have already been used in the evaluation of the pharmacodynamics of SRL and EVR as a proxy for their effects on the immune response after transplantation. These include measuring p70S6K (70 kDa ribosomal protein S6 kinase) activity, p70S6K phosphorylation (P-p70S6K), or P-S6 protein expression. Compared with Western blot and ELISA, phosphoflow cytometry can detect phosphorylated proteins and differentiate activation-induced changes of signaling molecules inside the cell from unstimulated populations of identical cells in the same sample. Alternatively, in patients receiving a combined therapy, the other PD approach is to consider biomarkers such as NFAT residual expression for calcineurin inhibitors or to evaluate nonspecific effects of the drugs such as lymphocyte proliferation, interleukin synthesis, specific peripheral blood T regulatory subsets, or lymphocyte surface antigens, which have the advantage to reflect the overall immunosuppressive status achieved. Although limited, the available data on mTOR pathway biomarkers seem promising. Before clinical implementation, the analytical methodologies must be standardized and cross-validated, and the selected biomarkers will have to demonstrate their clinical utility for SRL or EVR dose individualization in multicenter clinical trials.

Pharmacodynamic monitoring using biomarkers to individualize pharmacotherapy

Drug doses are often titrated upon their clinical effects (e.g., blood pressure). Unfortunately, for many drugs there is no direct, clinical read-out to estimate dose adequateness. Alternatively, drug dosing is based on the maximum tolerated dose approach or therapeutic drug monitoring. However, the concentration-response curves may be flattened or bell-shaped as suggested for some ‘biologicals’. Together, these aspects raise the question why drug dosing is not individualized by pharmacodynamic monitoring. Evaluating the effects of drugs at their pharmacological target or meaningful biomarkers might indicate nonresponders, objectively quantify the maximum molecular effect and thus restrict overdose and underdosing. This review outlines the theory and biological or technical prerequisites for biomarker-based pharmacodynamic monitoring, and highlights selected examples from different fields of clinical medicine.

Suppressive Effect of 1α,25-Dihydroxyvitamin D3 on Th17-Immune Responses in Kidney Transplant Recipients With Tacrolimus-Based Immunosuppression

BACKGROUND

The aim of this study was to investigate whether 1α,25-dihydroxyvitamin D3 can regulate Th17-related immune responses in kidney transplant recipients (KTRs) being treated with tacrolimus (Tac)-based immunosuppression.

METHODS

First, we evaluated the effect of 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) on Th17-immune responses in an in vitro study using peripheral blood mononuclear cells (PBMCs) from healthy volunteers or KTRs. Next, we investigated mammalian target of rapamycin/STAT3 signaling as a mechanism by which 1,25(OH)2D3 exerted its effect on T cells using the Jurkat cell line. Third, we investigated Th17-cytokine levels or Th17 cell percentage in PBMCs according to the serum 25-hydroxyvitamin D (25(OH)D) level in 81 KTRs, and we performed a prospective study to assess whether 1,25(OH)2D3 (calcitriol) treatment decreased Th17 cytokine levels (IL-17, IL-22) in 42 KTRs.

RESULTS

In the in vitro study, we observed that the addition of 1,25(OH)2D3 to Tac significantly inhibited the appearance of IL-17-positive cells in culture. The expression of IL-17 and IL-22 messenger RNA in PBMCs was also decreased by the addition of 1,25(OH)2D3. In the Jurkat cell line, the mTOR/STAT3 pathway was further downregulated with the addition of 1,25(OH)2D3 to Tac. In the 81 KTRs, the 25(OH)D level was inversely correlated with the Th17 cytokine levels or the proportion of Th17 cell out of CD4 T cells. Treatment with calcitriol for 6 months significantly decreased Th17 cytokine levels compared with the baseline values in another 42 KTRs.

CONCLUSIONS

Treatment with 1,25(OH)2D3 may have immunologic benefits by effectively suppressing the Th17-related immune responses in KTRs on Tac-based immunosuppression.

Protective effect of 1α,25-dihydroxyvitamin D3 on effector CD4+ T cell induced injury in human renal proximal tubular epithelial cells

Background

The aim of this study was to investigate the protective effect of 1α,25-dihydroxyvitamin D3 [1,25(OH)₂D3] on effector CD4⁺ T cells or on inflammatory cytokine-induced injury in human renal proximal tubular epithelial cells (HRPTEpiC).

Methods

First, we investigated the effect of 1,25(OH)₂D3 on CD4⁺ T cell proliferation. Second, we examined the effect of 1,25(OH)₂D3 on inflammatory cytokine secretion or fibrosis in HRPTEpiC induced by inflammatory cytokines or activated CD4⁺ T cells using ELISA and real-time PCR. Lastly, we compared urine inflammatory-cytokine (IL-6, IL-8) or KIM-1 levels in kidney transplant recipients low serum 25-hydroxyvitamin D (25(OH)D) group (< 20 ng/mL) (n = 40) and normal 25(OH)D group (n = 50).

Results

Pre-incubation with 1,25(OH)₂D3 significantly reduced the percentages of Th1 and Th17 cells compared to that of Th0 condition (P < 0.05 for each). In contrast, 1,25(OH)₂D3 increased the proportion of Th2 and Treg cells in a dose-dependent manner (P < 0.05 for each). Treatment of HRPTEpiC with inflammatory cytokines (TNF-α, IL-17, and TGF-β) or effector CD4⁺ T cells resulted in increased production of IL-6, IL-8, or KIM-1 from HRPTEpiC in a dose-dependent manner. However, treatment with 1,25(OH)₂D3 significantly reduced the level of these cytokines (P < 0.05 for all). Western blot analysis demonstrated that the mTOR/STAT3/ERK pathway was downregulated by 1,25(OH)₂D3 in HRPTEpiC. Furthermore, the concentrations of urine IL-6/creatinine (P < 0.05) and Kim-1/creatinine (P < 0.05) were higher in the low 25(OH)D group than in the normal 25(OH)D group in kidney transplant recipients.

Conclusion

The results of this study suggests that vitamin D may have a significant role in the regulation of inflammation in allograft tissue in kidney transplant recipients.

Trial registration

All participants provided written informed consent in accordance with the Declaration of Helsinki. This study was approved by the Institutional Review Board of Seoul St. Mary’s Hospital (KC13TNMI0701).

Barcelona Consensus on Biomarker-Based Immunosuppressive Drugs Management in Solid Organ Transplantation

With current treatment regimens, a relatively high proportion of transplant recipients experience underimmunosuppression or overimmunosuppression. Recently, several promising biomarkers have been identified for determining patient alloreactivity, which help in assessing the risk of rejection and personal response to the drug; others correlate with graft dysfunction and clinical outcome, offering a realistic opportunity for personalized immunosuppression. This consensus document aims to help tailor immunosuppression to the needs of the individual patient. It examines current knowledge on biomarkers associated with patient risk stratification and immunosuppression requirements that have been generally accepted as promising. It is based on a comprehensive review of the literature and the expert opinion of the Biomarker Working Group of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology. The quality of evidence was systematically weighted, and the strength of recommendations was rated according to the GRADE system. Three types of biomarkers are discussed: (1) those associated with the risk of rejection (alloreactivity/tolerance), (2) those reflecting individual response to immunosuppressants, and (3) those associated with graft dysfunction. Analytical aspects of biomarker measurement and novel pharmacokinetic-pharmacodynamic models accessible to the transplant community are also addressed. Conventional pharmacokinetic biomarkers may be used in combination with those discussed in this article to achieve better outcomes and improve long-term graft survival. Our group of experts has made recommendations for the most appropriate analysis of a proposed panel of preliminary biomarkers, most of which are currently under clinical evaluation in ongoing multicentre clinical trials. A section of Next Steps was also included, in which the Expert Committee is committed to sharing this knowledge with the Transplant Community in the form of triennial updates.

Therapeutic Drug Monitoring of Everolimus: A Consensus Report

In 2014, the Immunosuppressive Drugs Scientific Committee of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology called a meeting of international experts to provide recommendations to guide therapeutic drug monitoring (TDM) of everolimus (EVR) and its optimal use in clinical practice. EVR is a potent inhibitor of the mammalian target of rapamycin, approved for the prevention of organ transplant rejection and for the treatment of various types of cancer and tuberous sclerosis complex. EVR fulfills the prerequisites for TDM, having a narrow therapeutic range, high interindividual pharmacokinetic variability, and established drug exposure-response relationships. EVR trough concentrations (C0) demonstrate a good relationship with overall exposure, providing a simple and reliable index for TDM. Whole-blood samples should be used for measurement of EVR C0, and sampling times should be standardized to occur within 1 hour before the next dose, which should be taken at the same time everyday and preferably without food. In transplantation settings, EVR should be generally targeted to a C0 of 3-8 ng/mL when used in combination with other immunosuppressive drugs (calcineurin inhibitors and glucocorticoids); in calcineurin inhibitor-free regimens, the EVR target C0 range should be 6-10 ng/mL. Further studies are required to determine the clinical utility of TDM in nontransplantation settings. The choice of analytical method and differences between methods should be carefully considered when determining EVR concentrations, and when comparing and interpreting clinical trial outcomes. At present, a fully validated liquid chromatography tandem mass spectrometry assay is the preferred method for determination of EVR C0, with a lower limit of quantification close to 1 ng/mL. Use of certified commercially available whole-blood calibrators to avoid calibration bias and participation in external proficiency-testing programs to allow continuous cross-validation and proof of analytical quality are highly recommended. Development of alternative assays to facilitate on-site measurement of EVR C0 is encouraged.

Analytical Aspects of the Implementation of Biomarkers in Clinical Transplantation

In response to the urgent need for new reliable biomarkers to complement the guidance of the immunosuppressive therapy, a huge number of biomarker candidates to be implemented in clinical practice have been introduced to the transplant community. This includes a diverse range of molecules with very different molecular weights, chemical and physical properties, ex vivo stabilities, in vivo kinetic behaviors, and levels of similarity to other molecules, etc. In addition, a large body of different analytical techniques and assay protocols can be used to measure biomarkers. Sometimes, a complex software-based data evaluation is a prerequisite for appropriate interpretation of the results and for their reporting. Although some analytical procedures are of great value for research purposes, they may be too complex for implementation in a clinical setting. Whereas the proof of "fitness for purpose" is appropriate for validation of biomarker assays used in exploratory drug development studies, a higher level of analytical validation must be achieved and eventually advanced analytical performance might be necessary before diagnostic application in transplantation medicine. A high level of consistency of results between laboratories and between methods (if applicable) should be obtained and maintained to make biomarkers effective instruments in support of therapeutic decisions. This overview focuses on preanalytical and analytical aspects to be considered for the implementation of new biomarkers for adjusting immunosuppression in a clinical setting and highlights critical points to be addressed on the way to make them suitable as diagnostic tools. These include but are not limited to appropriate method validation, standardization, education, automation, and commercialization.

Performance of a phosphoflow assay to determine phosphorylation of S6 ribosomal protein as a pharmacodynamic read out for mTOR inhibition

OBJECTIVES

The S6 ribosomal protein (S6RP) is phosphorylated by the mammalian target of rapamycin (mTOR). The objective of this study was to assess the analytical suitability of a commercial kit-based phosphoflow cytometry protocol using whole blood (WBS) to measure the level of phosphorylated S6RP (p-S6RP) in T-cell subsets to study the pharmacodynamic effects of mTOR inhibitors (mTORi).

DESIGN AND METHODS

A kit was used for fixation and permeabilization of mitogen-stimulated cells, and p-S6RP was assessed separately in CD3+CD4+ and CD3+CD8+ cells by employing an anti-phospho-Ser235/236 antibody. Specificity, linearity, within-run precision and stability were investigated in either WBS spiked with everolimus and non-mTORi immunosuppressants or in WBS from patients on immunosuppressive therapy (n=56). In addition, healthy controls (n=10) and patients without immunosuppression (n=10) were included. A comparison (n=15) with an established western blot method based on anti-phospho p70S6 kinase (Thr389) was made by splitting WBS.

RESULTS

Everolimus decreased p-S6RP in vitro concentration dependently (0.00-27.4μg/L). This effect was also confirmed in vivo after a single dose of everolimus to healthy volunteers (n=3). However, spiking WBS with 500μg/L cyclosporine also decreased p-S6RP. The within-run coefficient of variation was <18% in transplant patients and <27% in healthy controls for both cell subsets. Sample stability for p-S6RP analysis was limited (<24h). p-S6RP was significantly decreased in CD3+CD8+ cells of patients treated with sirolimus (p=0.02) but not with everolimus. No significant correlation between the phosphoflow- and western blot method was noted.

CONCLUSION

The phosphoflow assay of p-S6RP performed well analytically, but sample stability, specificity, and method comparison results question its fitness for clinical purposes.

A Rationale for Age-Adapted Immunosuppression in Organ Transplantation

Demographic changes are associated with a steady increase of older patients with end-stage organ failure in need for transplantation. As a result, the majority of transplant recipients are currently older than 50 years, and organs from elderly donors are more frequently used. Nevertheless, the benefit of transplantation in older patients is well recognized, whereas the most frequent causes of death among older recipients are potentially linked to side effects of their immunosuppressants.Immunosenescence is a physiological part of aging linked to higher rates of diabetes, bacterial infections, and malignancies representing the major causes of death in older patients. These age-related changes impact older transplant candidates and may have significant implications for an age-adapted immunosuppression. For instance, immunosenescence is linked to lower rates of acute rejections in older recipients, whereas the engraftment of older organs has been associated with higher rejection rates. Moreover, new-onset diabetes mellitus after transplantation is more frequent in the elderly, potentially related to corticosteroids, calcineurin inhibitors, and mechanistic target of rapamycin inhibitors.This review presents current knowledge for an age-adapted immunosuppression based on both, experimental and clinical studies in and beyond transplantation. Recommendations of maintenance and induction therapy may help to improve graft function and to design future clinical trials in the elderly.

Drug target molecules to guide immunosuppression

The individual and interindividual variability of response to immunosuppressants combined with the prevailing concept of lifelong immunosuppression following any organ transplantation motivates the search for methods to further individualize such therapy. Traditional therapeutic drug monitoring, adapting dose according to concentrations in blood, targets the pharmacokinetic variability. It has been increasingly recognized, however, that there is also a considerable variability in the response to a given concentration. Attempts to overcome this variability in response include the efforts to identify relevant targets and methods for pharmacodynamic monitoring. For several of the currently used immunosuppressants there is experimental data suggesting markers that are relevant as indicators for individual monitoring of the effects of these drugs. There are also some clinical data to support these approaches; however what is generally missing, are studies that in a prospective manner demonstrates the benefits and effects on outcome. The monitoring of antithymocyte globulin by lymphocyte subset counts is actually the only well established example of pharmacodynamic monitoring. For drugs such as MPA and mTOR inhibitors, there are candidates such as IMPDH activity expression and p70SK6 phosphorylation status, respectively. The monitoring of CNIs using assays for NFAT RGE, either alone or combined with concentration measurements, is already well documented. Even here, some further investigations relating to the categories of organ transplant, combination of immunosuppressants etc. will be requested. Although some further standardization of the assay is warranted and there is a need for specific recommendations of target levels and how to adjust dose, the NFAT RGE approach to pharmacodynamic monitoring of CNIs may be close to implementation in clinical routine.

P70S6 kinase phosphorylation: a new site to assess pharmacodynamy of sirolimus

Background:

The phosphorylation of p70S6 kinase (p70S6K) represents an important target for sensitive detection on pharmacodynamic effects of sirolimus, but the methods of assessing p70S6K phosphorylation are still unclear. The aim of this study was to investigate p70S6K phosphorylation located down-stream of the mammalian target of rapamycin (mTOR) pathway in peripheral blood mononuclear cells (PBMCs) of liver transplant patients through different methods.

Methods:

Seventy-five liver transplant recipients from Beijing Chaoyang Hospital of the Capital Medical University were analyzed in this study. Patients were divided into three groups, patient treated with sirolimus (n = 22), patient treated with tacrolimus (n = 30), patient treated with cyclosporine (n = 23). The p70S6K phosphorylation of PBMCs in patients and healthy control (HC, n = 12) were analyzed by phospho-flow cytometry and Western blotting. A correlation analysis of data from phospho-flow cytometry and Western blotting was performed. Intra-assay variability of p70S6K phosphorylation in HC and different patients were measured.

Results:

Intra-assay variability of p70S6K phosphorylation in phospho-flow cytometry was from 4.1% to 8.4% and in Western blotting was from 8.2% to 18%. The p70S6K phosphorylation in patients receiving a sirolimus (19.5 ± 7.7) was significantly lower than in HC (50.1 ± 11.3, P < 0.001), tacrolimus (37.7 ± 15.7, P < 0.001) or cyclosporine treated patients (41.7 ± 11.7, P < 0.001). The p70S6K phosphorylation in HC (50.1 ± 11.3) was significantly higher than in tacrolimus (37.7 ± 15.7, P < 0.01) or cyclosporine-treated patients (41.7 ± 11.7, P < 0.01). There was correlation between data from phospho-flow cytometry and data from Western blotting (r = 0.88, P < 0.001).

Conclusions:

The degree of mTOR inhibition by assessing p70S6K phosphorylation was established by phospho-flow cytometry and Western blotting. Assessment of p70S6K phosphorylation may play an adjunct role to on pharmacodynamically guide and individualize sirolimus based on immunosuppression.

Pharmacokinetics and pharmacodynamics of immunosuppressive drugs in elderly kidney transplant recipients

Elderly patients are a fast growing population among transplant recipients over the past decades. Both the innate and adaptive immune reactivity decrease with age, which is believed to contribute to the decreased incidence of acute rejection and increased infectious death rate in elderly transplant recipients. In contrast to recipient age, donor age is associated with a higher incidence of acute rejection. Pharmacokinetic studies in renal transplant recipients show that CNI troughs are >5% higher in elderly compared to younger patients given the same dose normalized by body weight. This may impact the starting dose of tacrolimus and cyclosporine. Possibly in elderly patients the intracellular (in lymphocyte) concentrations are relatively high in relation to the whole blood concentration, resulting in a stronger pharmacodynamic effect at the same whole blood trough concentration. For cyclosporine this has been shown, but it is not clear if the same is true for other immunosuppressive drugs. Pharmacodynamic studies have compared the inhibition of target enzymes, or more downstream effects of immunosuppressive drugs, in younger and older patients. Measurement of nuclear factor of activated T-cell (NFAT)-regulated gene expression (RGE), a pharmacodynamic read-out of CNI, is a promising biomarker of immunosuppression. Low levels of NFAT RGE are associated with increased risk of infection and non-melanoma skin cancer in elderly patients. Clinical trials to evaluate the safety and efficacy of immunosuppression regimens in this specific patient population, which is underrepresented in published trials, are lacking. More studies in elderly patients are needed to investigate the impact of age on the pharmacokinetics or pharmacodynamics of immunosuppressive drugs, and to decide on the optimal regimen and target levels for elderly transplant recipients.