Pharmacokinetics and pharmacodynamics of mycophenolate mofetil under oral and intravenous therapy

Treatment of canine meningoencephalomyelitis of unknown aetiology with mycophenolate mofetil and corticosteroids: 25 cases (2007-2012)

The use of mycophenolate mofetil (MMF) for a variety of immune‐mediated diseases in veterinary medicine has been described. However, there is only a small number of cases documenting its use in dogs with meningoencephalomyelitis of unknown aetiology (MUE). We hypothesized that the use of MMF and corticosteroids in dogs with MUE results in comparable survival data to other published treatment protocols and is associated with limited adverse effects. A retrospective study of medical case records of dogs clinically diagnosed with MUE recorded signalment, neuroanatomic localization, magnetic resonance imaging findings, cerebrospinal fluid analysis results, medications administered, follow‐up neurologic examinations, survival and adverse events. Variables were compared between dogs which were treated with MMF within 30 days of diagnosis (immediate group) vs. dogs in which MMF therapy was started >30 days after diagnosis (delayed group). Twenty‐five cases of MUE were identified. The overall median survival time from diagnosis was 731 days (range 43–1672 days). After 1 month of MMF treatment, 92% of dogs showed improvement on a neurological examination. There was no significant effect of any recorded parameter on survival, including delayed vs. immediate initiation of MMF treatment. Dogs with delayed treatment had significantly lower clinical remission rates than dogs with immediate treatment at 6 months after starting MMF. Adverse events were identified in two cases (8%) and were characterized by mild gastrointestinal signs (vomiting and decreased appetite). Administration of MMF appears safe in dogs with MUE. The use of MMF results in comparable survival times to alternate immunosuppressive protocols.

Evaluation of inosin-5'-monophosphate dehydrogenase activity during maintenance therapy with tacrolimus

OBJECTIVES

The aim of this study was to identify a target range for inosin-5'-monophosphate dehydrogenase (IMPDH) activity in maintenance therapy with tacrolimus (TCL), and to apply the measurement of IMPDH activity to the therapeutic drug monitoring for mycophenolate mofetil (MMF).

METHODS

Eleven patients with renal transplants and 10 healthy volunteers were investigated. All patients were treated with a combination of TCL, steroid and MMF for 2 months after transplantation, and were in stable and good condition. IMPDH activity was determined indirectly by measuring xanthosine 5'-monophophate in cell lysates supplemented with IMP and beta-nicotine adenine dinucleotide using an high-performance liquid chromatography (HPLC) method.

RESULTS

The within-run reproducibility of the assay was excellent, with relative standard deviation (RSD) values of 0.41-4.08%. The mean differences between the spiked concentrations of xanthosine 5'-monophophate and their real values (mean relative errors; MREs) were within a range of 2.66-8.89%, showing good accuracy. The interday RSD values were 1.51-6.12% and MREs ranged from 2.10% to 8.89%. Cell lysates showed a 5-6 nmol/L IC(50) mycophenolic acid (MPA) concentration. TCL, cyclosporine and prednisolone did not affect IMPDH activity. The peak MPA concentration was achieved at 1 h after dosing. IMPDH activity decreased to 75% and 67% at 1 and 2 h after dosing respectively. Therefore, the inhibition rates of MPA against IMPDH activity may be adequate at 25-40% in TCL maintenance therapy.

CONCLUSION

Inosin-5'-monophosphate dehydrogenase activity in cell lysates could be reliably determined by HPLC. A 25-40% inhibition of IMPDH activity may be an appropriate range for preventing rejection with MPF but this requires further validation using larger studies with harder outcomes such as rejection episodes.

Neurocognitive-neurological complications of liver transplantation: a review

Neurological complications are common after liver transplantation (LT) and they are associated with a significant morbidity. Long-term effects of LT on cognitive and psychological outcomes are not clear. The objective of this study was to summarize the present knowledge on the neurological and cognitive complications of LT, resulting from a systematic review of the literature in the last 10 years. Several studies have investigated the incidence and the pathophysiology of neurological complications; in contrast, the knowledge of cognitive and psychological status after LT is poor. Currently, the effect of LT on mental performance is debated. Some studies have shown an improvement of cognitive function after OLTX and, at the same time, a persistence of different cognitive deficits. In addition, the quality of life (QoL) and the psychological status after LT seem to improve but LT recipients have significant deficiencies in most QoL domains. Consequently, future studies are necessary in order to investigate cognitive alterations and QoL in LT recipients.

Improved assay for the nonradioactive determination of inosine 5'-monophosphate dehydrogenase activity in peripheral blood mononuclear cells

Mycophenolic acid (MPA) inhibits the enzyme inosine 5'-monophosphate dehydrogenase (IMPDH). Thus, the measurement of IMPDH activity could serve as a specific pharmacodynamic (PD) tool for monitoring MPA therapy. At present, however, monitoring of pharmacokinetic parameters is preferred over that of PD parameters because, in general, PD assays are labor-intensive and poorly reproducible. Currently, cell count or protein concentration is widely accepted as methods to normalize enzyme activity. In the present study, we have attempted to further improve a method for the determination of IMPDH activity to increase the robustness and reproducibility of the IMPDH activity assay itself, without making the assay more labor-intensive. Therefore, several aspects of the IMPDH method were investigated regarding their influence on the reproducibility and also modified to increase the feasibility and consistency of the assay. The isolation of peripheral blood mononuclear cells (PBMCs) of whole blood samples was found to be the most variable step. Normalization on cell count is labor-intensive and at the same time has a poor reproducibility. Determination of the protein content in cell extracts is impaired by contamination with extracellular proteins and non-PBMCs. Alternatively, the intracellular substance adenosine monophosphate (AMP) was investigated to normalize the newly generated xanthosine monophosphate. Among various subject groups, no significant differences in mean AMP concentration were found. To simplify the procedure, PBMCs were diluted to a fixed volume after isolation from sample of whole blood, and the IMPDH activity was normalized to the AMP concentration quantified in the same high-performance liquid chromatography run as xanthosine monophosphate was quantified. The within-run and total imprecision (coefficient of variation) ranged from 4.2% to 10.6% and from 6.6% to 11.9%, respectively. In conclusion, the modified method described here for the measurement of IMPDH activity can be used reliably in multicenter trials and in longitudinal studies to evaluate the additional value of any PD monitoring among a diversity of patients treated with MPA.

Neurological complications after orthotopic liver transplantation

BACKGROUND

The number of orthotopic liver transplantation performed each year is increasing due to increased safety and logistic facilities. Therefore, the importance of reducing adverse events is progressively growing.

AIM

To review present knowledge on the neurological complications of orthotopic liver transplantation.

METHODS

The epidemiology, the clinical features and the pathophysiology of the neurological complications of orthotopic liver transplants, resulting from a systematic review of the literature in the last 25 years, are summarized.

RESULTS AND CONCLUSIONS

The review highlights that a relevant variety of neurological adverse events can occur in patients undergoing orthotopic liver transplantation. The knowledge of neurological complications of orthotopic liver transplantation is important for transplantation teams to reduce their prevalence and improve their management. In addition, the likelihood of neurological adverse effects provides evidence for the need of a careful cognitive and neurological work up of patients in the orthotopic liver transplantation waiting list, in order to recognize and interpret neurological dysfunction occurring after orthotopic liver transplantation.

Clinical pharmacokinetics and pharmacodynamics of mycophenolate in solid organ transplant recipients

This review aims to provide an extensive overview of the literature on the clinical pharmacokinetics of mycophenolate in solid organ transplantation and a briefer summary of current pharmacodynamic information. Strategies are suggested for further optimisation of mycophenolate therapy and areas where additional research is warranted are highlighted. Mycophenolate has gained widespread acceptance as the antimetabolite immunosuppressant of choice in organ transplant regimens. Mycophenolic acid (MPA) is the active drug moiety. Currently, two mycophenolate compounds are available, mycophenolate mofetil and enteric-coated (EC) mycophenolate sodium. MPA is a potent, selective and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH), leading to eventual arrest of T- and B-lymphocyte proliferation. Mycophenolate mofetil and EC-mycophenolate sodium are essentially completely hydrolysed to MPA by esterases in the gut wall, blood, liver and tissue. Oral bioavailability of MPA, subsequent to mycophenolate mofetil administration, ranges from 80.7% to 94%. EC-mycophenolate sodium has an absolute bioavailability of MPA of approximately 72%. MPA binds 97-99% to serum albumin in patients with normal renal and liver function. It is metabolised in the liver, gastrointestinal tract and kidney by uridine diphosphate gluconosyltransferases (UGTs). 7-O-MPA-glucuronide (MPAG) is the major metabolite of MPA. MPAG is usually present in the plasma at 20- to 100-fold higher concentrations than MPA, but it is not pharmacologically active. At least three minor metabolites are also formed, of which an acyl-glucuronide has pharmacological potency comparable to MPA. MPAG is excreted into the urine via active tubular secretion and into the bile by multi-drug resistance protein 2 (MRP-2). MPAG is de-conjugated back to MPA by gut bacteria and then reabsorbed in the colon. Mycophenolate mofetil and EC-mycophenolate sodium display linear pharmacokinetics. Following mycophenolate mofetil administration, MPA maximum concentration usually occurs in 1-2 hours. EC-mycophenolate sodium exhibits a median lag time in absorption of MPA from 0.25 to 1.25 hours. A secondary peak in the concentration-time profile of MPA, due to enterohepatic recirculation, often appears 6-12 hours after dosing. This contributes approximately 40% to the area under the plasma concentration-time curve (AUC). The mean elimination half-life of MPA ranges from 9 to 17 hours. MPA displays large between- and within-subject pharmacokinetic variability. Dose-normalised MPA AUC can vary more than 10-fold. Total MPA concentrations should be interpreted with caution in patients with severe renal impairment, liver disease and hypoalbuminaemia. In such individuals, MPA and MPAG plasma protein binding may be altered, changing the fraction of free MPA available. Apparent oral clearance (CL/F) of total MPA appears to increase in proportion to the increased free fraction, with a reduction in total MPA AUC. However, there may be little change in the MPA free concentration. Ciclosporin inhibits biliary excretion of MPAG by MRP-2, reducing enterohepatic recirculation of MPA. Exposure to MPA when mycophenolate mofetil is given in combination with ciclosporin is approximately 30-40% lower than when given alone or with tacrolimus or sirolimus. High dosages of corticosteroids may induce expression of UGT, reducing exposure to MPA. Other co-medications can interfere with the absorption, enterohepatic recycling and metabolism of mycophenolate. Most pharmacokinetic investigations of MPA have involved mycophenolate mofetil rather than EC-mycophenolate sodium therapy. In population pharmacokinetic studies, MPA CL/F in adults ranges from 14.1 to 34.9 L/h (ciclosporin co-therapy) and from 11.9 to 25.4 L/h (tacrolimus co-therapy). Patient bodyweight, serum albumin concentration and immunosuppressant co-therapy have a significant influence on CL/F. The majority of pharmacodynamic data on MPA have been obtained in patients receiving mycophenolate mofetil therapy in the first year after kidney transplantation. Low MPA AUC is associated with increased incidence of biopsy-proven acute rejection. Gastrointestinal adverse events may be dose related. Leukopenia and anaemia have been associated with high MPA AUC, trough concentration and metabolite concentrations in some, but not all, studies. High free MPA exposure has been identified as a risk factor for leukopenia in some investigations. Targeting a total MPA AUC from 0 to 12 hours (AUC12) of 30-60 mg.hr/L is likely to minimise the risk of acute rejection and may reduce toxicity. IMPDH monitoring is in the early experimental stage. Individualisation of mycophenolate therapy should lead to improved patient outcomes. MPA AUC12 appears to be the most useful exposure measure for such individualisation. Limited sampling strategies and Bayesian forecasting are practical means of estimating MPA AUC12 without full concentration-time profiling. Target concentration intervention may be particularly useful in the first few months post-transplant and prior to major changes in anti-rejection therapy. In patients with impaired renal or hepatic function or hypoalbuminaemia, free drug measurement could be valuable in further interpretation of MPA exposure.

From Mechanisms to Long-Term Benefits

Mycophenolate mofetil (MMF), a widely used immunosuppressant in solid organ transplantation, is a prodrug to deliver mycophenolic acid (MPA). MMF has been shown to decrease rejection rates and to improve graft survival, and probably accomplishes this through a variety of mechanisms. Its most prominent mechanism is to inhibit lymphocyte proliferation by inhibition of inosine monophosphate dehydrogenase. However, just as important to its overall beneficial effects are other mechanisms that include inhibition of proliferation of smooth muscle and fibroblasts, inhibition of antibody production, inhibition of lymphocyte adhesion and migration, and inhibition of nitric oxide synthase.