Failure to predict cyclosporine area under the curve using a limited sampling strategy

Bayesian estimation of cyclosporin exposure for routine therapeutic drug monitoring in kidney transplant patients

AIMS

AUC-based monitoring of cyclosporin A (CsA) is useful to optimize dose adaptation in difficult cases. We developed a population pharmacokinetic model to describe dose-exposure relationships for CsA in renal transplant patients and applied it to the Bayesian estimation of AUCs using three blood concentrations.

METHODS

A total of 84 renal graft recipients treated with CsA microemulsion were included in this study. Population pharmacokinetic analysis was conducted using NONMEM. A two-compartment model with zero-order absorption and a lag time best described the data. Bayesian estimation was based on CsA blood concentrations measured before dosing and 1 h and 2 h post dose. Predictive performance was evaluated using a cross-validation approach. Estimated AUCs were compared with AUCs calculated by the trapezoidal method. The Bayesian approach was also applied to an independent group of eight patients exhibiting unusual pharmacokinetic profiles.

RESULTS

Mean population pharmacokinetic parameters were apparent clearance 30 l h(-1), apparent volume of distribution 79.8 l, duration of absorption 52 min, absorption lag time 7 min. No significant relationships were found between any of the pharmacokinetic parameters and individual characteristics. A good correlation was obtained between Bayesian-estimated and experimental AUCs, with a mean prediction error of 2.8% (95% CI [-0.6, 6.2]) and an accuracy of 13.1% (95% CI [7.5, 17.2]). A good correlation was also obtained in the eight patients with unusual pharmacokinetic profiles (r(2) = 0.96, P < 0.01).

CONCLUSIONS

Our Bayesian approach enabled a good estimation of CsA exposure in a population of patients with variable pharmacokinetic profiles, showing its usefulness for routine AUC-based therapeutic drug monitoring.

Methods for clinical monitoring of cyclosporin in transplant patients

Cyclosporin was introduced into clinical practice in the early 1980s and has since been shown to prolong survival for transplant recipients. Because cyclosporin is a narrow therapeutic index drug and there are significant consequences associated with 'subtherapeutic' and 'supratherapeutic' concentrations, cyclosporin therapy is monitored as part of routine patient follow-up. However, the optimal method for the therapeutic drug monitoring of cyclosporin has yet to be defined. Currently, the most common method involves monitoring pre-dose trough concentrations, but this method is less than ideal. Other methods of monitoring cyclosporin therapy include monitoring the area under the concentration-time curve, limited sampling strategies, monitoring of single concentrations other than troughs and pharmacodynamic monitoring. Bayesian forecasting has been used successfully in clinical practice with other drugs with narrow therapeutic indices. However, few studies are available regarding Bayesian forecasting and cyclosporin. Existing studies are preliminary in nature and involve the old Sandimmun formulation rather than the Neoral formulation. Although these methods show promise, they have not gained widespread acceptance. This is because of their impracticality and the lack of prospective studies comparing other monitoring methods with trough concentration monitoring. Further comparative studies evaluating the impact of the specific monitoring method on definite patient outcomes are warranted.

Improving the therapeutic monitoring of cyclosporin A

The narrow therapeutic window of cyclosporin A (CsA) and the variable pharmacokinetics of the traditional preparation, CsA-SIM (Sandimmune), have made it difficult to establish its optimal use. The introduction of a microemulsion preparation, CsA-ME (Neoral), with less variable pharmacokinetics, has made it possible to attempt to define its use more closely. One hundred and one renal allograft recipients were converted from CsA-SIM to CsA-ME. Absorption was monitored using a standard pharmacokinetic 'profile' measuring 'whole blood' CsA levels at several time points following drug administration. Areas under the resulting time-versus-CsA concentration curve (AUC) were calculated. Surprisingly, many patients showed very little fluctuation in 'whole blood' levels after administration of the conventional preparation: 31% had a difference between trough (to) and maximal levels of < 100 micrograms/L. On microemulsion cyclosporin, there was much better correlation between AUC and several parameters incorporating elements of trough and peak values. The best correlation was with t0 + (t1/4), where t1 represents the concentration at 1 h following administration, (r2 = 0.779 for microemulsion cyclosporin). The use of this parameter is a practical possibility in an out-patient setting. The very common, but under-recognised, pattern of almost flat absorption profiles in patients on conventional CsA suggests that the use of CsA in numerous clinical contexts should be reviewed, since CsA immunosuppression may previously have been inadequately monitored.

A limited sampling strategy for the estimation of 12-hour SangCya and neoral AUCs in renal transplant recipients

Neoral and SangCya are new cyclosporine formulations with more consistent pharmacokinetic profiles than the older formulation of cyclosporine, Sandimmune. Limited sampling strategies have been derived to estimate the full area under the curve (AUC) for both Neoral and Sandimmune. In addition, no limited sampling strategy has been derived for SangCya, and no rigorous prospective testing has been done on any of these formulas. The authors studied 32 renal transplant patients who received Neoral and then SangCya during a formulation conversion study. Full AUCs were drawn on all patients (twice while on Neoral, once while on SangCya). Abbreviated formulas were derived using linear regression models and then tested for the prediction error. The authors found that several abbreviated formulas offer excellent estimations of the full AUC for both SangCya and Neoral. The generated formulas worked equally well with either formulation. In addition, the authors found that limited sampling strategies using a 1.5-hour sample may predict a full AUC more accurately than those that use a 2-hour sample. The use of these abbreviated formulas allows for a convenient and accurate estimate of CsA exposure.

Abbreviated kinetic profiles in area-under-the-curve monitoring of cyclosporine therapy. Technical note

BACKGROUND

The new microemulsion formulation of cyclosporine (CsA-ME) displays more consistent pharmacokinetic properties than the original formulation and may allow successful implementation of an abbreviated area-under-the-curve (AUC) strategy.

METHODS

Here we compared two limited sampling strategies in order to define the one that best predicts AUC after CsA-ME in 51 renal transplant recipients with stable renal function. Pharmacokinetics were based on analysis of blood samples collected over 12 hours after drug administration by high-performance liquid chromatography (HPLC). Predicted AUC was estimated by using a three-point (0, 1 and 3 hr) or a two-point (2 and 6 hr or 0 and 2 hr) sampling strategy.

RESULTS

A simplified strategy with three time points of blood collection at 0, 1, and 3 hours after CsA-ME allowed adequate and accurate prediction of the daily exposure to CsA. AUC prediction with two-point sampling at 2 and 6 hours was less good with a very large error in prediction (only 59% of the estimated AUC were within the accepted range). This limitation was even more evident when the 0 and 2 hour time points were examined, in which only 51% of AUC estimates were included in the accepted range of variation (-10 to 10%).

CONCLUSIONS

A limited strategy of three-point sampling taken early after dosing allows an excellent and perfectly reliable prediction of the actual AUC.

Prevention of transplant rejection: current treatment guidelines and future developments

In the past 2 decades, progressive improvements in the results of organ transplantation as a therapeutic strategy for patients with end-stage organ disease have been achieved due to greater insight into the immunobiology of graft rejection and better measures for surgical and medical management. It is now known that T cells play a central role in the specific immune response of acute allograft rejection. Strategies to prevent T cell activation or effector function are thus all potentially useful for immunosuppression. Standard immunosuppressive therapy in renal transplantation consists of baseline therapy to prevent rejection and short courses of high-dose corticosteroids or monoclonal or polyclonal antibodies as treatment of ongoing rejection episodes. Triple-drug therapy with the combination of cyclosporin, corticosteroids and azathioprine is now the most frequently used immunosuppressive drug regimen in cadaveric kidney recipients. The continuing search for more selective and specific agents has become, in the past decade, one of the priorities for transplant medicine. Some of these compounds are now entering routine clinical practice: among them are tacrolimus (which has a mechanism of action similar to that of cyclosporin), mycophenolate mofetil and mizoribine (which selectively inhibit the enzyme inosine monophosphate dehydrogenase, the rate-limiting enzyme for de novo purine synthesis during cell division), and sirolimus (rapamycin) [which acts on and inhibits kinase homologues required for cell-cycle progression in response to growth factors, like interleukin-2 (IL-2)]. Other new pharmacological strategies and innovative approaches to organ transplantation are also under development. Application of this technology will offer enormous potential not only for the investigation of mechanisms and mediators of graft rejection but also for therapeutic intervention.

A microemulsion of cyclosporine without intravenous cyclosporine in liver transplantation

A microemulsion formulation of cyclosporine (CsA) has improved absorption compared with the original form. The purpose of this case control study was to assess the safety and efficacy of the microemulsion without intravenous CsA for induction immunosuppression in adult liver transplantation. Twenty-one consecutive patients receiving induction immunosuppression with the microemulsion 15 mg/kg/day were compared with 20 patients receiving intravenous CsA and the original oral form. Both groups received the same dose of methylprednisilone. Twenty of 21 patients receiving the microemulsion required no intravenous CsA to achieve target CsA levels. All patients receiving the original form received initial intravenous CsA. There was no difference in trough CsA levels between the two groups at 24 and 48 hours. The microemulsion group had 24 hr and 48 hr trough CsA levels of 227+/-15 and 520+/-300 ng/ml by monoclonal RIA while the intravenous CsA group had 24 and 48 hr trough levels of 293+/-18 and 405+/-91 ng/ml. CsA levels analyzed by HPLC were 20% lower than by RIA. The frequency of adverse events resulting in reduction of drug dosage was similar for the microemulsion and the original form: neurotoxicity (23 vs. 40%, P=.30); nephrotoxicity (25 vs. 45%, P=.32), and no patients required dialysis. There was no difference in septic complications. One patient required discontinuation of the microemulsion in an attempt to reverse severe neurotoxicity. A total of 75% of microemulsion patients were rejection free at 3 months while only 35% of CsA patients remained rejection free (P=0.02). These data suggest that the use of the microemulsion without intravenous CsA in liver transplantation is safe and efficacious, and may result in decreased episodes of acute rejection.

Erythromycin breath test and clinical transplantation

The intravenous 14C-erythromycin breath test (ERMBT) appears to be a convenient and reproducible means of measuring the activity of a major drug-metabolizing enzyme present in the liver, termed CYP3A4. Interpatient differences in liver CYP3A4 activity, as measured by the ERMBT, seem to account, for the most part, for interindividual differences in the kinetics of cyclosporin A and FK506. The predictability of the test may be improved as an oral form of the test and improved oral formulations of transplant medications are developed. Although the ERMBT is a promising research tool, its role in clinical transplantation has yet to be defined.

The use of therapeutic drug monitoring to optimise immunosuppressive therapy

Most experience of the therapeutic drug monitoring of immunosuppressive agents has been acquired in the field of solid organ transplantation; however, agents such as cyclosporin (cyclosporin A) are being increasingly utilised for the management of autoimmune diseases. Cyclosporin is the most widely studied immunosuppressant, but in spite of this many controversies still exist as to the optimum strategy for monitoring this drug. Owing to its widely variable pharmacokinetics and metabolism, and the absence of a simple method to measure therapeutic effectiveness, many factors should be considered. In most circumstances, measuring whole blood through concentrations of cyclosporin with a specific assay methodology is warranted. In addition, knowledge of other factors that may alter the pharmacokinetics (such as liver function, concomitant food or medications, gastrointestinal status, and time since transplantation) should be taken into account so that therapy can be appropriately adjusted. Other methods of monitoring have been investigated, such as AUC (area under the concentration-time curve) monitoring and immunological monitoring. However, further refinement of these techniques and greater experience with their efficacy must be accumulated before their role in the monitoring of cyclosporin can be defined. Tacrolimus, like cyclosporin, shares many of the difficulties in monitoring for efficacy and toxicity due largely to the variable pharmacokinetics; similarly to cyclosporin, whole blood through concentration monitoring should be utilised in combination with knowledge of the factors that may affect the pharmacokinetics. Muromonab CD3 (OKT3) is a monoclonal antibody used for the treatment and prophylaxis of acute allograft rejection. Several immunological monitoring techniques have been investigated for this agent. Monitoring CD3+ levels can assist clinicians in determining therapeutic efficacy, while measuring antimuromonab CD3 antibody titres can help determine if xenosensitisation has occurred, causing therapeutic ineffectiveness. The clinical monitoring of azathioprine, one of the first immunosuppressive agents used in transplantation, has historically been limited to monitoring complete blood counts for bone marrow suppression. However, newer techniques measuring intracellular DNA nucleotides appear to be promising. The new immunosuppressants on the horizon include mycophenolate mofetil and rapamycin. The clinical experience with therapeutic drug monitoring of these 2 compounds is scant in the literature; however, both agents have demonstrated efficacy in preventing or treating allograft rejection while maintaining a relatively well tolerated toxicity profile in recent clinical trials. Routine monitoring does not appear to be warranted for immunosuppressive therapy in autoimmune diseases.

Prediction of uptake of methyl mercury by rat erythrocytes using a two-compartment model

The uptake of methyl mercury (MeHg) by isolated rat erythrocytes was studied at 37 degrees C using MeHg-cysteine (MeHgCySH), MeHg-glutathione (MeHgGSH), MeHg-mercaptalbumin (MeHgMASH) and the mixture of MeHgCySH with MeHgGSH, MeHgCySH with MeHgMASH, MeHgGSH with MeHgMASH at different MeHg concentrations. The measured MeHg concentrations were analyzed according to the Akaike's information criterion in order to determine the suitable compartment model. After determining a two-compartment model, a model-independent two-compartment model was developed from the kinetics of uptake of MeHg at a concentration of 1 mmol MeHg/l packed erythrocytes using MeHgCySH, MeHgGSH and MeHgMASH, respectively. The developed two-compartment model was validated by predicting the kinetics of uptake of MeHg by rat erythrocytes at different MeHg concentrations and different mixtures of MeHg-complexes. Then, the predicted values were compared with the measured values. The results suggested: 1) MeHg uptake appeared suitable to be described by a two-compartment model, while using MeHgGSH, MeHgMASH, MeHgCySH at lower concentrations and the mixtures of MeHg-complexes; 2) MeHgCySH uptake was slowest among three kinds of MeHg-complexes, although a postulated cysteine-facilitated MeHgCySH transport system might exist in erythrocyte membrane; 3) the mixture of MeHg-complexes might facilitate MeHgCySH uptake; 4) there might be a second MeHg intracellular compartment in rat erythrocytes.

Development of limited sampling strategies for characteristics of a pharmacokinetic profile

New approaches for empirical and model-based development of constrained, limited sampling strategies for the estimation of one or more characteristics of a pharmacokinetic (PK) profile are evaluated. The methods (i) permit a specification of an overall risk function weighted by each estimation objective, (ii) require only a few sampling times for each of one or more new individuals, (iii) permit tight time constraints, such as those imposed by outpatients, and (iv) recognize variations across individuals, but determine optimal sampling times that are the same for all individuals. The methods are applied to a 4-way crossover pharmacokinetic study of two formulations of cyclosporin G, under fed and fasted conditions, in renal transplant patients. This application used average percentage absolute error for estimating AUC and Cmax with an overall risk function that first put all weight on the error for estimating AUC and second put equal weights on the errors for estimating AUC and Cmax. Empirical and model-based methods identified constrained, 3-point designs with acceptable precision for estimating either AUC alone or AUC and Cmax simultaneously. Model-based sampling times were obtained by software for minimizing a general objective function subject to linear constraints where the objective function was evaluated by computer-intensive sampling under a nonlinear mixed-effects model. The model-based approach permitted a direct comparison of the precision of limited and full sampling strategies.