Tacrolimus in solid organ transplantation: an update

Design, Characterization, and In Vivo Pharmacokinetics of Tacrolimus Proliposomes

The objective of this study was to develop proliposomal formulation for a poorly bioavailable drug, tacrolimus. Proliposomes were prepared by thin film hydration method using different lipids such as hydrogenated soy phosphatidylcholine (HEPC), soy phosphatidylcholine (SPC), distearyl phosphatidylcholine (DSPC), dimyristoylphosphatidylcholine (DMPC), and dimyristoylphosphatidylglycerol sodium (DMPG) and cholesterol in various ratios. Proliposomes were evaluated for particle size, zeta potential, in vitro drug release, in vitro permeability, and in vivo pharmacokinetics. In vitro drug release was carried out in purified water using USP type II dissolution apparatus. In vitro drug permeation was studied using parallel artificial membrane permeation assay (PAMPA) and everted rat intestinal perfusion techniques. In vivo pharmacokinetic studies were conducted in male Sprague-Dawley (SD) rats. Among the different formulations, proliposomes with drug/DSPC/cholesterol in the ratio of 1:2:0.5 demonstrated the desired particle size and zeta potential. Enhanced drug release was observed with proliposomes compared to pure tacrolimus in purified water after 1 h. Tacrolimus permeability across PAMPA and everted rat intestinal perfusion models was significantly higher with proliposomes. The optimized formulation of proliposomes indicated a significant improvement in the rate and absorption of tacrolimus. Following a single oral administration, a relative bioavailability of 193.33% was achieved compared to pure tacrolimus suspension.

Solid self-nanoemulsifying drug delivery system (SNEDDS) for enhanced oral bioavailability of poorly water-soluble tacrolimus: physicochemical characterisation and pharmacokinetics

To develop a novel self-nanoemulsifying drug delivery system (solid SNEDDS) with better oral bioavailability of tacrolimus, the solid SNEDDS was obtained by spray-drying the solutions containing the liquid SNEDDS and colloidal silica. Its reconstitution properties were determined and correlated to solid state characterisation of the powder. Moreover, the dissolution and pharmacokinetics in rats was done in comparison to the commercial product. Among the liquid SNEDDS formulations tested, the liquid SNEDDS comprised of Capryol PGMC, Transcutol HP and Labrasol (10:15:75, v/v/v) presented the highest dissolution rate. In the solid SNEDDS, this liquid SNEDDS was absorbed in the pores and attached onto the surface of the colloidal silica. Drug was present in the amorphous state in it. The solid SNEDDS with 5% w/v tacrolimus produced the nanoemulsions and improved the oral bioavailability of tacrolimus in rats. Therefore, this solid SNEDDS would be a potential candidate for enhancing the oral bioavailability of tacrolimus.

Nerve allotransplantation as it pertains to composite tissue transplantation

Nerve allografts provide a temporary scaffold for host nerve regeneration and allow for the repair of significant segmental nerve injuries. From rodent, large animal, and nonhuman primate studies, as well as clinical experience, nerve allografts, with the use of immunosuppression, have the capacity to provide equal regeneration and function to that of an autograft. In contrast to solid organ transplantation and composite tissue transfers, nerve allograft transplantation requires only temporary immunosuppression. Furthermore, nerve allograft rejection is difficult to assess, as the nerves are surgically buried and are without an immediate functional endpoint to monitor. In this article, we review what we know about peripheral nerve allograft transplantation from three decades of experience and apply our current understanding of nerve regeneration to the emerging field of composite tissue transplantation.

Design and evaluation of self-microemulsifying drug delivery system (SMEDDS) of tacrolimus

The objective of present investigation was to formulate self-microemulsifying drug delivery systems (SMEDDS) of tacrolimus (FK 506), a poorly water soluble immunosuppressant that exhibits low and erratic bioavailability. Solubility of FK 506 in various oils, surfactants cosurfactants and buffers was determined. Phase diagrams were constructed at different ratios of surfactant/cosurfactant (K(m)) to determine microemulsion existence region. The effect of oil content, pH of aqueous phase, dilution, and incorporation of drug on mean globule size of resulting microemulsions was studied. The optimized SMEDDS formulation was evaluated for in vitro dissolution profile in comparison to pure drug and marketed formulation (Pangraf capsules). The in vivo immunosuppressant activity of FK 506 SMEDDS was evaluated in comparison to Pangraf capsules. Area of o/w microemulsion region in phase diagram was increased with increase in K(m). The SMEDDS yielded microemulsion with globule size less than 25 nm which was not affected by the pH of dilution medium. The SMEDDS was robust to dilution and did not show any phase separation and drug precipitation even after 24 h. Optimized SMEDDS exhibited superior in vitro dissolution profile as compared to pure drug and Pangraf capsules. Furthermore, FK 506 SMEDDS exhibited significantly higher immunosuppressant activity in mice as compared to Pangraf capsules.

Neuroregeneration in Composite Tissue Allografts: Effect of Low-Dose FK506 and Mycophenolate Mofetil Immunotherapy

BACKGROUND

The immunosuppressant FK506 has been reported to increase the rate of peripheral nerve regeneration in nerve crush injury and nerve allograft models. The purpose of this study was to determine whether low doses of FK506 and mycophenolate mofetil had a neuroregenerative effect in revascularized peripheral nerve allografts in a rat hind limb transplantation model.

METHODS

Wistar Furth rat recipients received limbs from syngeneic Wistar Furth donors (group 1, n = 4) or from allogeneic August X Copenhagen Irish rat donors (group 2, n = 6). Wistar Furth recipients received limbs from August X Copenhagen Irish donors and were treated with FK506/mycophenolate mofetil for 5 months (group 3, n = 7). At the end of the follow-up period, histomorphometric analysis of sciatic and tibial nerves from transplanted and intact hind limbs was conducted. Sciatic and tibial nerves were examined at the level of coaptation and near the neuromuscular junction, respectively.

RESULTS

Transplanted limbs in groups 1 and 3 completed the study without rejection, while the limbs in group 2 were rejected within a few days. Sciatic and tibial nerve analysis in groups 1 and 3 limbs showed myelinated axons of various diameters but in significantly fewer numbers than in nontransplanted contralateral nerves. The number and size of myelinated axons of transplanted nerves at corresponding levels were not significantly different between syngeneic and allogeneic (FK506/mycophenolate mofetil-treated) transplants.

CONCLUSIONS

The authors conclude that long-term neuroregeneration of revascularized peripheral nerves using low-dose FK506/mycophenolate mofetil was similar to that of syngeneic transplants. The occurrence of acute rejection episodes with low-dose FK506/mycophenolate mofetil did not appear to benefit nor impair neuroregeneration.

Clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplantation

The aim of this review is to analyse critically the recent literature on the clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplant recipients. Dosage and target concentration recommendations for tacrolimus vary from centre to centre, and large pharmacokinetic variability makes it difficult to predict what concentration will be achieved with a particular dose or dosage change. Therapeutic ranges have not been based on statistical approaches. The majority of pharmacokinetic studies have involved intense blood sampling in small homogeneous groups in the immediate post-transplant period. Most have used nonspecific immunoassays and provide little information on pharmacokinetic variability. Demographic investigations seeking correlations between pharmacokinetic parameters and patient factors have generally looked at one covariate at a time and have involved small patient numbers. Factors reported to influence the pharmacokinetics of tacrolimus include the patient group studied, hepatic dysfunction, hepatitis C status, time after transplantation, patient age, donor liver characteristics, recipient race, haematocrit and albumin concentrations, diurnal rhythm, food administration, corticosteroid dosage, diarrhoea and cytochrome P450 (CYP) isoenzyme and P-glycoprotein expression. Population analyses are adding to our understanding of the pharmacokinetics of tacrolimus, but such investigations are still in their infancy. A significant proportion of model variability remains unexplained. Population modelling and Bayesian forecasting may be improved if CYP isoenzymes and/or P-glycoprotein expression could be considered as covariates. Reports have been conflicting as to whether low tacrolimus trough concentrations are related to rejection. Several studies have demonstrated a correlation between high trough concentrations and toxicity, particularly nephrotoxicity. The best predictor of pharmacological effect may be drug concentrations in the transplanted organ itself. Researchers have started to question current reliance on trough measurement during therapeutic drug monitoring, with instances of toxicity and rejection occurring when trough concentrations are within 'acceptable' ranges. The correlation between blood concentration and drug exposure can be improved by use of non-trough timepoints. However, controversy exists as to whether this will provide any great benefit, given the added complexity in monitoring. Investigators are now attempting to quantify the pharmacological effects of tacrolimus on immune cells through assays that measure in vivo calcineurin inhibition and markers of immunosuppression such as cytokine concentration. To date, no studies have correlated pharmacodynamic marker assay results with immunosuppressive efficacy, as determined by allograft outcome, or investigated the relationship between calcineurin inhibition and drug adverse effects. Little is known about the magnitude of the pharmacodynamic variability of tacrolimus.

Validation of methods to study the distribution and protein binding of tacrolimus in human blood

INTRODUCTION

Tacrolimus is a macrolide immunosuppressant that has a narrow therapeutic index, displays considerable variability in response, and has the potential for serious drug interactions. Therapeutic drug monitoring and dose individualisation for tacrolimus is complicated but essential. Few studies have investigated the blood distribution and protein binding of tacrolimus and the results of these studies are conflicting. The aim of the present study is to establish and validate methods to investigate the distribution of tacrolimus in human blood. To conduct these studies at clinically relevant concentrations the use of 3H-dihydro-tacrolimus instead of tacrolimus was investigated.

METHODS

The use of radiolabelled tacrolimus was validated by conducting studies with a mixture of both labelled and unlabelled drug where tacrolimus was analysed by LC-MS/MS. The in vitro distribution of tacrolimus and 3H-dihydro-tacrolimus was investigated in blood collected from healthy subjects using Ficoll-Paque reagent and density gradient ultracentrifugation, respectively. The unbound fraction of tacrolimus in plasma was studied using equilibrium dialysis conducted at 37 degrees C.

RESULTS

In blood, tacrolimus was found to be mainly associated with erythrocytes (85.3+/-1.5%), followed by diluted plasma proteins (14.3+/-1.5%) and lymphocytes (0.46+/-0.10%). In plasma, tacrolimus was found to mainly be associated with the soluble protein fraction (61.2+/-2.5%), high-density lipoproteins (HDL, 28.1+/-5.4%), low-density lipoproteins (LDL, 7.8+/-1.6%), and very low-density lipoproteins (VLDL, 1.4+/-0.3%). The unbound fraction of tacrolimus was found to be only 1.2+/-0.12%. Statistical comparison indicated that there was no significant difference in the blood distribution and plasma protein binding of 3H-dihydro-tacrolimus when compared with tacrolimus.

DISCUSSION

These results have important implications for therapeutic drug monitoring of tacrolimus and subsequent studies of tacrolimus distribution in transplant recipients.

Successful tacrolimus therapy for a severe recurrence of type 1 autoimmune hepatitis in a liver graft recipient

A 34-year-old woman underwent orthotopic liver transplantation (OLT) for decompensated type 1 autoimmune hepatitis (AIH). She was administered standard triple-drug immunosuppressive therapy (cyclosporine, steroids, and azathioprine). Ten years after OLT, she developed a recurrence of AIH, with emergence of serological markers of autoimmunity (high anti--smooth muscle antibody [ASMA] titer, high serum gamma globulin level), abnormal liver function test results, and characteristic histological features on liver biopsy. Despite intensified steroid therapy, her clinical and liver function deteriorated. The onset of cutaneous alternariosis led to a steroid dose reduction and cyclosporine replacement by tacrolimus. Clear-cut amelioration was observed, with an improvement in liver function test results and reduction in ASMA titer. One year after the recurrence of AIH, the patient has normal liver function and physical findings. Tacrolimus therefore may be effective in patients with severe recurrent autoimmune liver disease. Further studies are needed to assess tacrolimus therapy in patients who fail to respond to standard immunosuppressive therapy.