This review aims to provide an update of the literature on the pharmacology and toxicology of mycophenolate in solid organ transplant recipients. Mycophenolate is now the antimetabolite of choice in immunosuppressant regimens in transplant recipients. The active drug moiety mycophenolic acid (MPA) is available as an ester pro-drug and an enteric-coated sodium salt. MPA is a competitive, selective and reversible inhibitor of inosine-5'-monophosphate dehydrogenase (IMPDH), an important rate-limiting enzyme in purine synthesis. MPA suppresses T and B lymphocyte proliferation; it also decreases expression of glycoproteins and adhesion molecules responsible for recruiting monocytes and lymphocytes to sites of inflammation and graft rejection; and may destroy activated lymphocytes by induction of a necrotic signal. Improved long-term allograft survival has been demonstrated for MPA and may be due to inhibition of monocyte chemoattractant protein 1 or fibroblast proliferation. Recent research also suggested a differential effect of mycophenolate on the regulatory T cell/helper T cell balance which could potentially encourage immune tolerance. Lower exposure to calcineurin inhibitors (renal sparing) appears to be possible with concomitant use of MPA in renal transplant recipients without undue risk of rejection. MPA displays large between- and within-subject pharmacokinetic variability. At least three studies have now reported that MPA exhibits nonlinear pharmacokinetics, with bioavailability decreasing significantly with increasing doses, perhaps due to saturable absorption processes or saturable enterohepatic recirculation. The role of therapeutic drug monitoring (TDM) is still controversial and the ability of routine MPA TDM to improve long-term graft survival and patient outcomes is largely unknown. MPA monitoring may be more important in high-immunological recipients, those on calcineurin-inhibitor-sparing regimens and in whom unexpected rejection or infections have occurred. The majority of pharmacodynamic data on MPA has been obtained in patients receiving MMF therapy in the first year after kidney transplantation. Low MPA area under the concentration time from 0 to 12 h post-dose (AUC0-12) is associated with increased incidence of biopsy-proven acute rejection although AUC0-12 optimal cut-off values vary across study populations. IMPDH monitoring to identify individuals at increased risk of rejection shows some promise but is still in the experimental stage. A relationship between MPA exposure and adverse events was identified in some but not all studies. Genetic variants within genes involved in MPA metabolism (UGT1A9, UGT1A8, UGT2B7), cellular transportation (SLCOB1, SLCO1B3, ABCC2) and targets (IMPDH) have been reported to effect MPA pharmacokinetics and/or response in some studies; however, larger studies across different ethnic groups that take into account genetic linkage and drug interactions that can alter a patient's phenotype are needed before any clinical recommendations based on patient genotype can be formulated. There is little data on the pharmacology and toxicology of MPA in older and paediatric transplant recipients.

Pharmacology and toxicology of mycophenolate in organ transplant recipients: an update.

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.

A Rationale for Age-Adapted Immunosuppression in Organ Transplantation.

When mycophenolic acid (MPA) was originally marketed for immunosuppressive therapy, fixed doses were recommended by the manufacturer. Awareness of the potential for a more personalized dosing has led to development of methods to estimate MPA area under the curve based on the measurement of drug concentrations in only a few samples. This approach is feasible in the clinical routine and has proven successful in terms of correlation with outcome. However, the search for superior correlates has continued, and numerous studies in search of biomarkers that could better predict the perfect dosage for the individual patient have been published. As it was considered timely for an updated and comprehensive presentation of consensus on the status for personalized treatment with MPA, this report was prepared following an initiative from members of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT). Topics included are the criteria for analytics, methods to estimate exposure including pharmacometrics, the potential influence of pharmacogenetics, development of biomarkers, and the practical aspects of implementation of target concentration intervention. For selected topics with sufficient evidence, such as the application of limited sampling strategies for MPA area under the curve, graded recommendations on target ranges are presented. To provide a comprehensive review, this report also includes updates on the status of potential biomarkers including those which may be promising but with a low level of evidence. In view of the fact that there are very few new immunosuppressive drugs under development for the transplant field, it is likely that MPA will continue to be prescribed on a large scale in the upcoming years. Discontinuation of therapy due to adverse effects is relatively common, increasing the risk for late rejections, which may contribute to graft loss. Therefore, the continued search for innovative methods to better personalize MPA dosage is warranted.

Personalized Therapy for Mycophenolate: Consensus Report by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology

LC-MS/MS as a tool for TDM services: Where are we?

Background:Idiopathic nephrotic syndrome (INS) necessitates administration of corticosteroids or corticoid-sparing agents in 60% of the cases for prolonged periods resulting in serious adverse effects.Methods:To avoid these complications, we investigated the efficacy and safety of mycophenolate mofe

Mycophenolate Mofetil Therapy in Children With Idiopathic Nephrotic Syndrome: Does Therapeutic Drug Monitoring Make a Difference?

Background The development of biomarkers describing the individual responses to the immunosuppressant mycophenolic acid (MPA) has focused on the target enzyme activity [inosine 5'-monophosphate dehydrogenase (IMPDH)]. An extended strategy is to quantify the metabolic consequences of IMPDH inhibition. The aim of this study was to develop an assay for quantification of IMPDH activity and related purine bases and to provide preliminary data on the behavior of these biomarkers during clinical exposure to MPA. Methods Liquid chromatography-mass spectrometry was used to determine xanthine (IMPDH activity in incubated cell lysate), hypoxanthine, guanine, and adenine derived from free nucleotides in lymphocytes. Analytical performance was assessed, and the biomarkers were examined in CD4⁺ cells from 2 groups: Healthy individuals in a single-dose MPA study (n = 5) and liver transplant recipients on MPA therapy (n = 15). Results Coefficients of variation between series were below 10% and 15% for measurement of the purines and IMPDH activity, respectively. Although IMPDH was inhibited, the purine levels increased in response to MPA in 3 of the 5 healthy individuals, and this positive response seemed to be associated with IMPDH1 c.579 + 119 G/G and c.580 - 106 G/G. In the liver transplant study, guanine was not reduced in response to the transient drop in IMPDH activity after MPA dosing. However, there were trends toward decrease in guanine and elevation of hypoxanthine during prolonged MPA therapy. The guanine/hypoxanthine ratio (median) was 37% lower and the adenine level was 21% lower at day 17 compared with day 4 after transplantation. Conclusions The assay allows precise quantification of IMPDH activity, hypoxanthine, guanine, and adenine in lymphocytes. Some individuals may possess a counteracting purine response to the MPA-mediated inhibition of IMPDH. Reduction of the guanine/hypoxanthine ratio may be related to prolonged inhibition of IMPDH and seems as an intriguing pharmacodynamic biomarker for MPA.

Simultaneous quantification of IMPDH activity and purine bases in lymphocytes using LC-MS/MS: assessment of biomarker responses to mycophenolic acid.

Background. The pharmacokinetics of the immunosuppressant mycophenolic acid (MPA) demonstrates high inter- and intra-patient variability. Variation in the binding of MPA to albumin has been postulated to be an important factor in this variability, and monitoring of free MPA has been suggested to improve therapeutic drug monitoring (TDM) of MPA. Inosine monophosphate dehydrogenase (IMPDH) is the target enzyme for MPA, therefore the IMPDH activity in lymphocytes can serve as a marker of the MPA-specific response. This study aimed to explore how the albumin concentration influences the free concentration of MPA in liver transplant recipients and to assess whether alteration in the free MPA influences IMPDH activity in CD4 + cells. Methods. Blood samples were taken from 20 liver transplant recipients on two separate occasions (days 3–5 and 16–21). Total and free concentrations of MPA, and IMPDH activity were measured during the first 4 h of each dose interval. Results. Albumin levels correlated with t...

Mycophenolate pharmacokinetics and inosine monophosphate dehydrogenase activity in liver transplant recipients with an emphasis on therapeutic drug monitoring.

Hemoglobingrenser og transfusjonspraksis blant norske anestesiologer

Background: In vitro and in vivo studies have indicated that stabilizers present in pharmaceutical-grade albumin influence the albumin-binding capacity for highly protein-bound drugs. However, the half-life of the stabilizers and the quantitative effect have been difficult to determine.



Method: A randomized crossover study including six healthy volunteers was performed. The study subjects received 750 mg of oral naproxen 2 h before the study. They were randomized to receive either 100 ml of 20% albumin or 100 ml of Ringer's acetate solution intravenously. Frequent blood samples were obtained. The experiment was repeated 4 weeks later with the alternate solution. The serum samples were analysed to determine the concentrations of albumin, N-acetyl-dl-tryptophan, caprylate, and naproxen.



Results: The free fraction of naproxen increased significantly after the infusion of albumin (P<0.05). The increase was concurrent with the appearance of N-acetyl-dl-tryptophan and caprylate in serum. The free fraction of naproxen declined rapidly after the albumin infusion was completed. N-acetyl-dl-tryptophan had a half-life of approximately 30 min. The half-life of caprylate was <15 min.



Conclusion: A transfusion of albumin results in an increase in the free fraction of naproxen. The transient increase in free-fraction naproxen decreased together with the detectable levels of the stabilizers N-acetyl-dl-tryptophan and caprylate. N-acetyl-dl-tryptophan and caprylate have a short half-life in serum.

Infusions of albumin increase free fraction of naproxen in healthy volunteers: a randomized crossover study

BACKGROUND Blood transfusions can be lifesaving, but definite transfusion triggers for different clinical situations need to be determined. MATERIAL AND METHODS We carried out a survey among Norwegian anaesthesiologists regarding haemoglobin levels and transfusion triggers during surgery and in an intensive care setting. Blood conservation techniques were also explored. The results for surgical patients were compared with those from a similar survey in 1996. RESULTS Compared with 1996, Norwegian anaesthesiologists now accept significantly lower transfusion triggers for surgical patients. Acceptable haemoglobin levels varied in the different patient examples in both the surgical and intensive care groups. With regard to surgery, junior doctors accept a lower transfusion trigger than do senior anaesthesiologists. A more liberal transfusion strategy is apparently used in patients with coronary disease. INTERPRETATION Norwegian anaesthesiologists have changed their attitude to acceptable transfusion triggers over the 1996-2002 period. In an intensive care setting, they appear to have a more liberal transfusion policy compared to anaesthesiologists in other western European countries.

Pål A Reine

Papers

Simultaneous quantification of IMPDH activity and purine bases in lymphocytes using LC-MS/MS: assessment of biomarker responses to mycophenolic acid.

Mycophenolate pharmacokinetics and inosine monophosphate dehydrogenase activity in liver transplant recipients with an emphasis on therapeutic drug monitoring.

Hemoglobingrenser og transfusjonspraksis blant norske anestesiologer

Infusions of albumin increase free fraction of naproxen in healthy volunteers: a randomized crossover study

Haemoglobin levels and transfusion practice among Norwegian anesthesiologists