Therapeutic monitoring for cyclosporine: difficulties in establishing a therapeutic window

Initial dosage optimisation of cyclosporine in Chinese paediatric patients undergoing allogeneic haematopoietic stem cell transplantation based on population pharmacokinetics: a retrospective study

OBJECTIVE

Improved understanding of cyclosporine A (CsA) pharmacokinetics in children undergoing allogeneic haematopoietic stem cell transplantation (allo-HSCT) is crucial for effective prevention of acute graft-versus-host disease and medication safety. The aim of this study was to establish a population pharmacokinetic (Pop-PK) model that could be used for individualised therapy to paediatric patients undergoing allo-HSCT in China.

DESIGN, SETTING AND PARTICIPANTS

A retrospective analysis of 251 paediatric HSCT patients who received CsA intravenously in the early post transplantation period at Women and Children's Medical Center in Guangzhou was conducted.

ANALYSIS MEASURES

The model building dataset from 176 children was used to develop and analyse the CsA Pop-Pk model by using the nonlinear mixed effect model method. The basic information was collected by the electronic medical record system. Genotype was analysed by matrix-assisted time-of-flight mass spectrometry. The stability and predictability of the final model were verified internally, and a validation dataset of 75 children was used for external validation. Monte Carlo simulation is used to adjust and optimise the initial dose of CsA in paediatric allo-HSCT patients.

RESULTS

The typical values for clearance (CL) and volume of distribution ([Formula: see text]) were 14.47 L/hour and 2033.53 L, respectively. The body weight and haematocrit were identified as significant variables for V, while only body weight had an impact on CL. The simulation based on the final model suggests that paediatrics with HSCT required an appropriate intravenous dose of 5 mg/kg/day to reach the therapeutic trough concentration.

CONCLUSIONS

The CsA Pop-PK model established in this study can quantitatively describe the factors influencing pharmacokinetic parameters and precisely predict the intrinsic exposure to CsA in children. In addition, our dosage simulation results can provide evidence for the personalised medications TRIAL REGISTRATION NUMBER: ChiCTR2000040561.

Drug Carriers: A Review on the Most Used Mathematical Models for Drug Release

Carriers are protective transporters of drugs to target cells, facilitating therapy under each points of view, such as fast healing, reducing infective phenomena, and curing illnesses while avoiding side effects. Over the last 60 years, several scientists have studied drug carrier properties, trying to adapt them to the release environment. Drug/Carrier interaction phenomena have been deeply studied, and the release kinetics have been modeled according to the occurring phenomena involved in the system. It is not easy to define models’ advantages and disadvantages, since each of them may fit in a specific situation, considering material interactions, diffusion and erosion phenomena, and, no less important, the behavior of receiving medium. This work represents a critical review on main mathematical models concerning their dependency on physical, chemical, empirical, or semi-empirical variables. A quantitative representation of release profiles has been shown for the most representative models. A final critical comment on the applicability of these models has been presented at the end. A mathematical approach to this topic may help students and researchers approach the wide panorama of models that exist in literature and have been optimized over time. This models list could be of practical inspiration for the development of researchers’ own new models or for the application of proper modifications, with the introduction of new variable dependency.

Initial dosage optimization of ciclosporin in pediatric Chinese patients who underwent bone marrow transplants based on population pharmacokinetics

Bone marrow transplants (BMT) are an established therapeutic strategy for patients with severe aplastic anemia, acute lymphoblastic leukemia, acute myeloid leukemia or chronic myeloid leukemia. However, the successful application of BMT is limited by graft-vs.-host disease (GVHD). Ciclosporin has been widely used for treating GVHD in pediatric patients who underwent BMT. The present study aimed to optimize the dosage of ciclosporin for safety and effectiveness based on population pharmacokinetics. A non-linear mixed-effects model was used to analyze the clinical data of pediatric patients who underwent BMT between September 2016 and September 2019 at the Children's Hospital of Fudan University. Monte Carlo simulations were used to identify the optimal dose of ciclosporin. The final population pharmacokinetic model indicated that body weight and days post-transplant influenced the clearance of ciclosporin in pediatric patients who underwent BMT. The present study indicated that the optimal initial dose of ciclosporin for pediatric patients weighing 5-30 kg who underwent BMT was 6 mg/kg/day split into 2 doses.

Cyclosporin in cell therapy for cardiac regeneration

Stem cell therapy is a promising strategy in promoting cardiac repair in the setting of ischemic heart disease. Clinical and preclinical studies have shown that cell therapy improves cardiac function. Whether autologous or allogeneic cells should be used, and the need for immunosuppression in non-autologous settings, is a matter of debate. Cyclosporin A (CsA) is frequently used in preclinical trials to reduce cell rejection after non-autologous cell therapy. The direct effect of CsA on the function and survival of stem cells is unclear. Furthermore, the appropriate daily dosage of CsA in animal models has not been established. In this review, we discuss the pros and cons of the use of CsA on an array of stem cells both in vitro and in vivo. Furthermore, we present a small collection of data put forth by our group supporting the efficacy and safety of a specific daily CsA dosage in a pig model.

Comparison of architect I 2000 for determination of cyclosporine with axsym

BACKGROUND

Cyclosporine has been shown effective drug in suppressing acute rejection in recipients of allograft organ transplants.

METHODS

The cyclosporine concentration of 96 blood samples was determined using CMIA (chemiluminesecent microparticle immnoassay) Architect i 2000 and FPIA (fluorescence polarization immunoassay) AxSYM Abbott diagnostic. All patients have transplantation of kidneys and were hospitalized at Department of Nephrology at the Clinical center of University of Sarajevo. The reference serum range of cyclosporine for kidney organ transplantation for maintenance lies between 50 and 150 ng/mL. The quality control, precision and accurancy of Architect i 2000 were assessed.

RESULTS

The quality control was done using quality control serums for low (= 91 ng/mL), medium (= 328 ng/mL) and high (= 829 ng/mL). We have used commercial BIORAD controls and got reproducibility CV 5.83 % to 13 % for Architect i 2000. It was established that the main difference between Architect i 2000 and AxSYM and it was statistically significant for P < 0.05 according to Student t-test. Correlation coefficient was r = 0.903.

CONCLUSION

The CMIA Architect assay has significant reduced cyclosporine metabolite interference relative to other immunoassay and is a convenient and sensitive automated method to measure cyclosporine in whole blood.

Effect of a new functional CYP3A4 polymorphism on calcineurin inhibitors' dose requirements and trough blood levels in stable renal transplant patients

AIMS

CYP3A4 is involved in the oxidative metabolism of many drugs and xenobiotics including the immunosuppressants tacrolimus (Tac) and cyclosporine (CsA). The objective of the study was to assess the potential influence of a new functional SNP in CYP3A4 on the pharmacokinetic parameters assessed by dose requirements and trough blood levels of both calcineurin inhibitors (CNI) in stable renal transplant patients.

PATIENTS & METHODS

A total of 99 stable renal transplant patients receiving either Tac (n = 49) or CsA (n = 50) were genotyped for the CYP3A4 intron 6 C>T (rs35599367) and CYP3A5*3 SNPs. Trough blood levels ([Tac](0) or [CsA](0) in ng/ml), dose-adjusted [Tac](0) or [CsA](0) (ng/ml per mg/kg bodyweight) as well as doses (mg/kg bodyweight) required to achieve target concentrations were compared among patients according to allelic status for CYP3A4 and CYP3A5.

RESULTS

Dose-adjusted concentrations were 2.0- and 1.6-fold higher in T-variant allele carriers for the CYP3A4 intron 6 C>T SNP compared with homozygous CC for Tac and CsA, respectively. When CYP3A4/CYP3A5 genotypes were combined, the difference was even more striking as the so-defined CYP3A poor metabolizer group presented dose-adjusted concentration 1.6- and 4.1-fold higher for Tac, and 1.5- and 2.2-fold higher for CsA than the intermediate metabolizer and extensive metabolizer groups, respectively. Multiple linear regression analysis revealed that, taken together, both CYP3A4 intron 6 and CYP3A5*3 SNPs explained more than 60 and 20% of the variability observed in dose-adjusted [Tac](0) and [CsA](0), respectively.

CONCLUSION

The CYP3A4 intron 6 C>T polymorphism is associated with altered Tac and CsA metabolism. CYP3A4 intron 6 C>T along with CYP3A5*3 (especially for Tac) pharmacogenetic testing performed just before transplantation may help identifying patients at risk of CNI overexposure and contribute to limit CNI-related nephrotoxicity by refining the starting dose according to their genotype. Original submitted 5 May 2011; Revision submitted 29 June 2011.

Pharmacologic monitoring and outcomes of cyclosporine

Cyclosporine was introduced into clinical transplantation as an immunosuppressive agent 20 years ago. Cyclosporine is a critical dose drug with a narrow therapeutic index and requires monitoring through blood levels to avoid rejection through underexposure or toxicity through overexposure. Traditional monitoring was by measuring the trough level taken 12 hours after an oral dose, but the results correlated poorly with drug dose, toxicity, and outcome. Monitoring cyclosporine by calculating total drug exposure correlated better with outcome but was time consuming and labour intensive. An abbreviated measure of exposure over the first 4 hours after administration was found to predict outcome and allow dose adjustment. This was based on the observation that the majority of variability in the absorption of the drug was during the first 4 hours after administration--the absorption phase--and this was not reflected in the trough level. Cyclosporine exerts its immunosuppressive action by inhibition of calcineurin. The peak of this inhibition occurs during the peak concentration of the drug, which occurs during the absorption phase. On the basis of the fact that as a single time point the 2-hour level was the best surrogate marker of the maximum level, a strategy for monitoring by 2-hour levels (C2) evolved. It was shown in all organ types to be the best single point predictor of exposure and has led to an improvement in outcome both in de novo and in maintenance transplant patients.

Therapeutic drug monitoring of cyclosporine: 20 years of progress

Since its introduction 20 years ago, cyclosporine (CyA), a powerful immunosuppressant with a narrow therapeutic window, remains the cornerstone of many drug regimens in renal transplantation. However, attempts to balance its therapeutic value with its pleiotropic side effects continue to challenge clinicians. To address the wide intraindividual and interindividual differences in absorption, distribution, metabolism, and elimination of the oil-based formulation of CyA (Sandimmune), a microemulsion (Neoral) was introduced; it displayed better absorption and lower intraindividual variability. Neoral also improves the utility of therapeutic monitoring of CyA to estimate exposure to the drug and predict patient outcomes. Drug monitoring techniques are undergoing continual refinement: today, a limited sampling strategy--C2 monitoring--shows great promise as a comparatively simple, safe, and effective method to optimize patient outcomes during both short-term and maintenance CyA therapy. However, it is not clear whether this method is useful for treatment optimization with generic formulations of CyA. Although generic substitutes meet federal bioequivalence criteria, they may not display the same pharmacokinetic properties. Further, preliminary data have shown a 10% lower 1-year graft survival rate among patients treated with generic versus Neoral CyA. Current challenges in optimizing CyA therapy include determining pretransplant patient characteristics relevant to selection of the appropriate exposure or the development of a Bayesian forecasting technique that predicts dose adjustments necessary to achieve the optimal drug regimen during the critical period immediately posttransplant.

A pharmacokinetic and clinical review of the potential clinical impact of using different formulations of cyclosporin A. Berlin, Germany, November 19, 2001

A meeting of 14 transplant and pharmacokinetic specialists from Europe and North America was convened in November 2001 to evaluate scientific and clinical data regarding the use of different formulations of cyclosporin A (CsA). The following consensus was achieved. (1) CsA is a critical-dose drug with a narrow therapeutic window. Clinical outcomes after transplantation are affected by the pharmacokinetic properties of CsA, particularly by its bioavailability, and by intrapatient variability in CsA exposure. (2) Standard bioequivalence criteria do not address differences in CsA pharmacokinetics between transplant recipients and healthy volunteers, or between subpopulations of transplant recipients. (3) In some circumstances, currently available formulations of CsA that meet standard bioequivalence criteria are likely to be nonequivalent with respect to pharmacokinetic characteristics. (4) The choice of CsA formulation can affect the short- and long-term clinical outcome. Currently, there is a lack of clinical comparisons between generic CsA formulations and the Neoral formulation (Novartis Pharmaceuticals Corporation, East Hanover, New Jersey). Initial retrospective data from the Collaborative Transplant Study suggest that use of generic CsA formulations may result in reduced graft survival at 1 year. (5) Management of transplant recipients by monitoring Neoral concentrations 2 hours after dosing (C(2)) reduces the incidence and severity of acute rejection compared with monitoring of trough concentrations with no increase in toxicity. C(2) monitoring has been developed based on the pharmacokinetics of Neoral only and has not been evaluated or validated for generic formulations of CsA. (6) The major costs of care after transplantation relate to the management of poor clinical outcomes and toxicity. CsA formulations with different pharmacokinetic properties may be associated with varying clinical outcomes, which would be expected to affect total health care costs. (7) The transplant physician is responsible for selecting immunosuppressive agents and formulations for his or her patients. Any switch between CsA formulations in a particular patient should take place only in a controlled setting with adequate pharmacokinetic monitoring.

Low intraindividual variability of cyclosporin A exposure reduces chronic rejection incidence and health care costs

The present study applied a receiver operating characteristic (ROC) analysis to assess the role of intraindividual variability of cyclosporin A (CsA) drug exposure in predisposing renal transplant recipients to the occurrence of chronic rejection, as well as to increased health care costs using a resource-based economic analysis. Two hundred and four adult renal transplant recipients were treated with tapering doses of prednisone (Pred) and with a concentration-controlled strategy that selected doses of the olive oil-based formulations of CsA (Sandimmune(R)) that achieved target concentrations based on serial pharmacokinetic profiles. The ROC analysis revealed an inflection point of plots of the coefficient of variation (%CV) of CsA exposure versus the risk of chronic rejection at >/=28.4% for the average concentration (C(av)), i.e., the dosing interval-corrected area under the concentration-time curves, and >/=36% for the trough concentration (C(0)). The incidence of chronic rejection over a period of 5 yr was 24% among the less variable (LV) versus 40% among the variable (V) cohort. The economic analysis revealed that the total mean facility and physician costs per patient were $48,789 versus $60,998, respectively (P < 0.01). The degree of variability displayed by any individual could only be predicted by serial measurements of CsA concentrations, and not by demographic features, laboratory determinations, clinical characteristics, individual or mean values of any observed CsA concentration, or other pharmacokinetic parameters calculated following a single drug exposure. Thus, strategies that reduce intrapatient variability of CsA exposure over time may lead to reductions in chronic allograft loss and in treatment costs.

Physiologically based pharmacokinetics of cyclosporine A: reevaluation of dose-nonlinear kinetics in rats

The disposition kinetics of Cyclosporine A (CyA) in rat, based on measurement in arterial blood, appeared dose-linear over a wide i.v. dose range (1.2-30 mg/kg). Physiologically based pharmacokinetic (PBPK) analysis, however, demonstrated that this was an apparent observation resulting from counterbalancing nonlinear factors, such as saturable blood and tissue distribution, as well as clearance (CLb). A PBPK model was successfully developed taking into account these multiple nonlinear factors. Tissue distribution was distinctly different among various organs, being best described by either a linear model (muscle, fat; Model 1), one involving instantaneous saturation (lung, heart, bone, skin, thymus; Model 2), noninstantaneous saturation (kidney, spleen, liver, gut; Model 3), or one with saturable efflux (brain; Model 4). Overall, the whole body volume of distribution at steady state for unbound CyA (Vuss) decreased with increasing dose, due at least in part to saturation of tissue-cellular cyclophilin binding. Clearance, essentially hepatic, and described by the well-stirred model, was also adequately characterized by Michaelis-Menten kinetics, Km 0.60 microgram/ml. In model-based simulations, both volume of distribution at steady state (Vss,b) and CLb varied in a similar manner with dose, such that terminal t1/2 remained apparently unchanged; these dose responses were attenuated by saturable blood binding. CyA concentration measured in arterial blood was not always directly proportional to the true exposure, i.e., unbound or target tissue concentrations. The PBPK model not only described comprehensively such complicated PK relationships but also permitted assessment of the sensitivity of individual parameters to variation in local nonlinear kinetics. Using this approach, dose-dependent CyA uptake into brain was shown to be sensitive to both active and passive transport processes, and not merely the affinity of the active (efflux) transporter at the level of the blood-brain barrier.

Issues in cyclosporine drug substitution: implications for patient management

Substantial improvements in short-term and long-term outcomes for kidney transplant recipients have resulted from better use of existing immunosuppressive agents and newer treatment options. Calcineurin inhibitors (e.g., cyclosporine and tacrolimus) remain the foundation of immunosuppressive therapy. These agents are considered critical-dose drugs because of their narrow therapeutic range, variable pharmacokinetics, formulation-dependent bioavailability, and negative clinical consequences of underdosing or overdosing. With the recent introduction of a new cyclosporine formulation, concern exists that current bioequivalence guidelines for generic approval may not provide adequate assessment of the safety and efficacy of critical-dose drugs. Transplant experts at 2 recent conferences recommended more rigorous criteria for bioequivalence testing of critical-dose drugs and adoption of consistent drug substitution practices. Additional recommendations included specifying the intended formulation and instituting appropriate monitoring whenever formulations are switched. A summary of the outcomes of these conferences and practice implications for transplant coordinators is discussed.

Performance and Specificity of Monoclonal Immunoassays for Cyclosporine Monitoring: How Specific Is Specific?

Background: Immunoassays designed for the selective measurement of cyclosporin A (CsA) inadvertently show cross-reactivity for CsA metabolites. The extent and clinical significance of the resulting overestimation is controversial. A comprehensive assessment of old and new methods in clinical specimens is needed.

Methods: In a comprehensive evaluation, CsA was analyzed in 145 samples with the new CEDIA® assay and compared with the Emit® assay with the old and new pretreatments, the TDx® monoclonal and polyclonal assays, the AxSYM®, and HPLC. All samples were from patients with liver and/or kidney transplants.

Results: The CEDIA offered the easiest handling, followed by the AxSYM, which showed the longest calibration stability. The TDx monoclonal assay provided the lowest detection limit and the lowest CVs. The mean differences compared with HPLC were as follows: Emit, 9–12%; CEDIA, 18%; AxSYM, 29%; and TDx monoclonal, 57%. The CycloTrac® RIA paralleled the Emit results. In contrast to the mean differences, substantial (&gt;200%) and variable overestimations of the CsA concentration were observed in individual patient samples. Metabolic ratios, estimates of the overall concentrations of several cross-reacting metabolites (nonspecific TDx polyclonal/specific reference method), correlated with the apparent biases of the various monoclonal assays. Metabolic ratios varied up to 10-fold, which translated into biases for individual samples between −7% and +174%. The higher the cross-reactivity of an assay was, the higher was the range of biases observed. The interindividual differences markedly exceeded other factors of influence (organ transplanted, hepatic function).

Conclusion: Because assay bias cannot be predicted in individual samples, substantially erratic CsA dosing can result. The specificity of CsA assays for parent CsA remains a major concern.

Comparison of high performance liquid chromatography and immunoassay methods for measurement of cyclosporine A blood concentrations after feline kidney transplantation

OBJECTIVE

To compare two methods of whole blood cyclosporine A (CsA) measurement in cats.

STUDY DESIGN

Whole blood samples were analyzed for CsA concentrations with use of high performance liquid chromatography (HPLC) and monoclonal immunoassay methods.

ANIMALS

Blood (n = 36 samples) was obtained from six cats after renal transplantation.

METHODS

Results were compared by linear regression analysis using both pooled and individual patient data. Eight samples were off-scale on the immunoassay and were excluded.

RESULTS

There was significant correlation between CsA measured using HPLC and immunoassay methods (P < .001; r = .942; r2 = .887). However, individuals varied nonrandomly from the mean pooled patient data. Correlation between the assay methods was higher for individual patients using data only from that specific individual (mean r value = .976; r2 = .955). Clinical utility of the immunoassay (ie, results would prompt an appropriate CsA dosage adjustment) was good when based on individually derived conversion factors (27 of 28 [96.5%] of decision events).

CONCLUSION

HPLC is superior for measurement of blood CsA concentrations in cats after kidney transplantation. However, an immunoassay may provide reliable information for CsA management if a comparative database (HPLC v immunoassay) has been previously determined in a specific patient.

CLINICAL RELEVANCE

Locally available monitoring of CsA by immunoassay in cats may provide significant advantages when shipping of blood samples to distant locations is required to obtain analysis by HPLC. These advantages may include cost and timeliness of results in circumstances where daily blood CsA concentrations may be desired, such as when managing an acute rejection reaction.

Hypothermic storage of feline kidneys for transplantation: successful ex vivo storage up to 7 hours

OBJECTIVE

To describe the effect of hypothermic storage on transplanted feline kidneys.

STUDY DESIGN

Kidneys were stored in University of Wisconsin (UW) sodium gluconate (n = 3) or phosphate-buffered sucrose (n = 5) solutions before transplantation.

ANIMAL POPULATION

Eight cats with renal failure and seven normal cats as kidney donors.

METHODS

Kidneys were perfused through the renal artery with cold (10 degree C) storage solution and immersed in the solution on ice until transplantation.

RESULTS

Mean ex vivo storage time was 4.8 +/- 0.36 hours (range, 3.5 to 7 hours). Seven recipient cats survived surgery. Five of the cats had decreased serum creatinine concentrations from a mean of 8.2 mg/dL (range, 4.0 to 15.8 mg/dL) preoperatively to 1.7 mg/dL (range 1.3 to 2.2 mg/dL) within 4 days of surgery. In one cat, serum creatinine concentration dropped from 15.1 to 3.7 mg/dL in 3 days, but the cat developed a ureteral stricture that required revision. One graft did not function, and the cat died on day 19. The mean postoperative survival time of cats that were discharged from the hospital (n = 6) was 254 days (range, 49 to 717 days) at the time of this report. Long-term renal function (> 60 days postoperatively; n = 5) was excellent with mean serum creatinine concentrations of 1.6 +/- 0.15 mg/dL.

CONCLUSIONS

Hypothermic storage is feasible for short-term preservation of feline kidneys. The maximal length of feasible storage remains unknown.

CLINICAL RELEVANCE

Hypothermia protects against ischemia-induced nephron loss during ex vivo manipulation of the allograft and allows longer safe vascular anastomosis times. Short-term hypothermic storage also provides time to accommodate modifications in scheduling or anesthetic management of the recipient operation.

Late pancreas allograft rejection. Preliminary experience with factors predisposing to rejection

A case series of 31 cadaveric pancreas transplant recipients who were insulin-independent at least for one year was analyzed for the factors predisposing to late acute rejection (> 12 months posttransplant). Sixty-two pancreas transplants were performed in 61 patients, of whom 53 had functioning allografts 3 months posttransplant; 31 of these had a follow-up > 12 months. Twenty had no evidence of late rejection, whereas 11 had evidence of acute rejection after 12 months. All patients received quadruple induction immunosuppression. No demographic or clinical factors-including donor age, organ cold time, HLA mismatch, age, sex, or race-could distinguish the late acute rejection group. The presence of acute rejection in the first year posttransplant was similar in the late rejectors (21 episodes in 9 of 11 patients) compared with patients without late rejection (31 episodes in 16 of 20 patients). Antilymphocyte induction therapy type had no influence, but the amount of immunosuppression with prednisone and cyclosporine (CsA) at 3 months posttransplant was significantly lower in those patients who experienced late rejection. After the first year posttransplant, CsA 12-hr trough levels were significantly lower in late rejection months (121 +/- 7 ng/ml) compared with each patient's own stable months (183 +/- 5 ng/ml, P < 0.0001). Neither prednisone nor azathioprine dosages differed between teh two groups after the first year posttransplant. Our preliminary results suggest that early under immunosuppression with prednisone and CsA in the first year and 12-hr trough CsA levels less than approximately 180 ng/ml after the first year posttransplant predispose to late pancreatic rejection.

Optimizing the use of cyclosporine in renal transplantation

OBJECTIVE

To review the existing data on the use of cyclosporine (CsA) in kidney transplantation, particularly with respect to therapeutic drug monitoring.

DATA SOURCES

A literature search was conducted of applicable articles related to therapeutic drug monitoring of cyclosporine in renal transplantation. Previous consensus guidelines were examined. Discussions on issues related to this topic convened in Toronto, ON, on June 15-16, 1994.

DATA SYNTHESIS

The literature was analyzed to examine patient factors and drug interactions affecting CsA concentrations, the effect of CsA concentrations on patient outcome, current methods of analysis, pharmacodynamic monitoring, and new immunosuppressants.

CONCLUSIONS

CsA has improved the success of kidney transplantation, reducing the incidence and severity of acute rejection and improving short-term patient and graft survival. The rate of graft loss after the first year (primarily due to chronic rejection) has remained largely unchanged. Sandimmune Neoral offers promise due to its better bioavailability and limited dependence on bile flow for absorption. Long-term studies are underway to determine its effectiveness and safety. Indications for therapeutic drug monitoring for CsA are provided.

A review of assay methods for cyclosporin. Clinical implications

Cyclosporin is a unique immunosuppressive agent with a narrow therapeutic range. The pharmacokinetics of the drug present substantial within- and between-patient variability and drug interactions can significantly alter blood cyclosporin concentrations. Monitoring of cyclosporin concentrations in blood is an invaluable and essential aid in adjusting dosage to ensure adequate immunosuppression while minimising toxicity. The principal rationale behind therapeutic monitoring of cyclosporin is the fact that the incidence of rejection is higher at low cyclosporin concentrations and toxicity occurs more often at high concentrations. In renal transplant recipients, cyclosporin concentrations help to discriminate between insufficient immunosuppression and cyclosporin-induced nephrotoxicity. There are several methods available, both specific and nonspecific, for the routine measurement of cyclosporin. Radioimmunoassay and fluorescence polarisation immunoassay are most widely employed, while high performance liquid chromatography remains the reference procedure. The allegedly specific immunoassays tend to slightly overestimate the actual blood cyclosporin concentrations. There is a need for assay systems capable of measuring the biological activity of cyclosporin. Cyclosporin concentrations should be determined by a specific method, using whole blood as the sample matrix. The routine monitoring of individual cyclosporin metabolites is not warranted, but characterising the metabolite pattern of cyclosporin by concomitant use of a nonspecific and a specific assay can be clinically useful in patients with cyclosporin-associated toxicity or impaired liver function. In organ transplantation, measurement of blood cyclosporin concentration should be continued periodically as long as the therapy continues, whereas monitoring is only indicated in special circumstances in patients with autoimmune and other nontransplant diseases. The assessment of a 'therapeutic window' for cyclosporin is complicated for several reasons and definite target ranges cannot be given. Cyclosporin concentrations should always be interpreted in conjunction with the recent blood concentration history and other relevant clinical and laboratory data.