Measurement of tacrolimus (FK506) and its metabolites: a review of assay development and application in therapeutic drug monitoring and pharmacokinetic studies

Determination of tacrolimus, three mono-demethylated metabolites and a M1 tautomer in human whole blood by liquid chromatography - tandem mass spectrometry

The immunosuppressant tacrolimus is the primary drug used in kidney transplantation to prevent organ rejection. A sensitive ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed to measure tacrolimus and its three known mono-demethylated metabolites 13-O-desmethyl tacrolimus (M1), 31-O-desmethyl tacrolimus (M2), 15-O-desmethyl tacrolimus (M3). By generating the metabolites to use as standards after incubation of tacrolimus with rat liver microsomes, we discovered multiple M1 peaks which we identified as two tautomers of M1. The M1 tautomer II was also successfully validated in this method. The separation and purification of the metabolites and tautomers were performed by semi-preparative liquid chromatography with UV-detection, while confirmation was done by UPLC-MS/MS and Nuclear Magnetic Resonance. For quantification an easy sample preparation was performed with zinc sulfate and acetonitrile as cell lyses and precipitation. Detection was performed in positive electrospray ionization. By better characterization of the metabolites and the tautomers, we could possibly explain insight into the clinical condition and thus adjust the immunosuppressant therapy individually per patient. Calibration curves were linear for all compounds. Precision was assessed according to the NCCLS EP5-T guideline, being below 15 % and mean recoveries were between 93 and 110 % for tacrolimus, its three metabolites and the M1 tautomer II. The validated method was successfully applied in a cohort of 20 patients after kidney transplantation.

Longitudinal study on the use of dried blood spots for home monitoring in children after kidney transplantation

The use of DBSs for home monitoring has been limited due to unsatisfactory blood sampling and analytical difficulties. The aim of this longitudinal feasibility trial was to assess the utility of DBS to monitor TAC and Cr at home in transplant recipients. A total of 30 participants (2-21 years, mean±SD, 13.6±5.4 year) were enrolled over 12 months. Eighteen were males. Monthly DBS samples were obtained at home and mailed to the central laboratory for analysis of TAC and Cr. Nineteen patients completed the study, and 216 cards were received in the laboratory from a total of 279 cards expected, with 416/519 (80%) blood spots being suitable for analysis. We found a high correlation between blood TAC and Cr levels by DBS and the clinical laboratory, R² =.81 and .95, respectively. Fifteen parents and 15 youth completed measures of satisfaction with and preference for DBS testing. All but one parent/caregiver and youth reported satisfaction and preference for this method of testing over laboratory blood draws. We conclude that home DBS monitoring is a feasible method to monitor TAC and Cr in pediatric transplant recipients.

Effect of linker length between variable domains of single chain variable fragment antibody against daidzin on its reactivity

The peptide linker between variable domains of heavy (VH) and light (VL) chains is one of important factors that influence the characteristics of scFv, including binding activity and specificity against target antigen. The scFvs against daidzin (DZ-scFvs) with different linker lengths were constructed in the format of VH-(GGGGS)n-VL (n = 1, 3, 5, and 7). They were expressed in the hemolymph of silkworm larvae using the Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid DNA system, and their reactivity against daidzin and related compounds were evaluated using an indirect competitive enzyme-linked immunosorbent assay (icELISA), which is applicable for quantitative analysis of daidzin. The results showed that the reactivity of scFvs against daidzin was increased, whereas specificity slightly decreased when their peptide linker was lengthened. These results suggested that the linker length of DZ-scFvs contributes to its reactivity. In addition, the results emphasize that the linker length could control the reactivity of DZ-scFvs.

A population pharmacokinetic study of tacrolimus in healthy Chinese volunteers and liver transplant patients

AIM

To develop a population pharmacokinetic (PopPK) model of tacrolimus in healthy Chinese volunteers and liver transplant recipients for investigating the difference between the populations, and for potential individualized medication.

METHODS

A set of 1100 sparse trough concentration data points from 112 orthotopic liver transplant recipients, as well as 851 dense data points from 40 healthy volunteers receiving a single dose of tacrolimus (2 mg, p.o.) were collected. PopPK model of tacrolimus was constructed using the program NONMEM. Related covariates such as age, hepatic and renal functions that were potentially associated with tacrolimus disposition were evaluated. The final model was validated using bootstrapping and a visual predictive check.

RESULTS

A two-compartment model of tacrolimus could best describe the data from the two populations. The final model including two covariates, population (liver transplant recipients or volunteers) and serum ALT (alanine aminotransferase) level, was verified and adequately described the pharmacokinetic characteristics of tacrolimus. The estimates of V2/F, Q/F and V3/F were 22.7 L, 76.3 L/h and 916 L, respectively. The estimated CL/F in the volunteers and liver transplant recipients was 32.8 and 18.4 L/h, respectively. Serum ALT level was inversely related to CL/F, whereas age did not influence CL/F. Thus, the elderly (≥65 years) and adult (<65 years) groups in the liver transplant recipients showed no significant difference in the clearance of tacrolimus.

CONCLUSION

Compared with using the sparse data only, the integrating modeling technique combining sparse data from the patients and dense data from the healthy volunteers improved the PopPK analysis of tacrolimus.

Statistical tools for dose individualization of mycophenolic acid and tacrolimus co-administered during the first month after renal transplantation

AIM

To predict simultaneously the area under the concentration-time curve during one dosing interval [AUC(0,12 h)] for mycophenolic acid (MPA) and tacrolimus (TAC), when concomitantly used during the first month after transplantation, based on common blood samples.

METHODS

Data were from two different sources, real patient pharmacokinetic (PK) profiles from 65 renal transplant recipients and 9000 PK profiles simulated from previously published models on MPA or TAC in the first month after transplantation. Multiple linear regression (MLR) and Bayesian estimation using optimal samples were performed to predict MPA and TAC AUC(0,12 h) based on two concentrations.

RESULTS

The following models were retained: AUC(0,12 h) = 16.5 + 4.9 × C1.5 + 6.7 × C3.5 (r(2) = 0.82, rRMSE = 9%, with simulations and r(2) = 0.66, rRMSE = 24%, with observed data) and AUC(0,12 h) = 24.3 + 5.9 × C1.5 + 12.2 × C3.5 (r(2) = 0.94, rRMSE = 12.3%, with simulations r(2) = 0.74, rRMSE = 15%, with observed data) for MPA and TAC, respectively. In addition, bayesian estimators were developed including parameter values from final models and values of concentrations at 1.5 and 3.5 h after dose. Good agreement was found between predicted and reference AUC(0,12 h) values: r(2) = 0.90, rRMSE = 13% and r(2) = 0.97, rRMSE = 5% with simulations for MPA and TAC, respectively and r(2) = 0.75, rRMSE = 11% and r(2) = 0.83, rRMSE = 7% with observed data for MPA and TAC, respectively.

CONCLUSION

Statistical tools were developed for simultaneous MPA and TAC therapeutic drug monitoring. They can be incorporated in computer programs for patient dose individualization.

Monitoring of nonsteroidal immunosuppressive drugs in patients with lung disease and lung transplant recipients: American College of Chest Physicians evidence-based clinical practice guidelines

OBJECTIVES

Immunosuppressive pharmacologic agents prescribed to patients with diffuse interstitial and inflammatory lung disease and lung transplant recipients are associated with potential risks for adverse reactions. Strategies for minimizing such risks include administering these drugs according to established, safe protocols; monitoring to detect manifestations of toxicity; and patient education. Hence, an evidence-based guideline for physicians can improve safety and optimize the likelihood of a successful outcome. To maximize the likelihood that these agents will be used safely, the American College of Chest Physicians established a committee to examine the clinical evidence for the administration and monitoring of immunosuppressive drugs (with the exception of corticosteroids) to identify associated toxicities associated with each drug and appropriate protocols for monitoring these agents.

METHODS

Committee members developed and refined a series of questions about toxicities of immunosuppressives and current approaches to administration and monitoring. A systematic review was carried out by the American College of Chest Physicians. Committee members were supplied with this information and created this evidence-based guideline.

CONCLUSIONS

It is hoped that these guidelines will improve patient safety when immunosuppressive drugs are given to lung transplant recipients and to patients with diffuse interstitial lung disease.

A simultaneous d-optimal designed study for population pharmacokinetic analyses of mycophenolic Acid and tacrolimus early after renal transplantation

Mycophenolic acid (MPA) and tacrolimus (TAC) are immunosuppressive agents used in combination with corticosteroids for the prevention of acute rejection after solid organ transplantation. Their pharmacokinetics (PK) show considerable unexplained intraindividual and interindividual variability, particularly in the early period after transplantation. The main objective of the present work was to design a study based on D-optimality to describe the PK of the 2 drugs with good precision and accuracy and to explain their variability by means of patients' demographics, biochemical test results, and physiological characteristics. Pharmacokinetic profiles of MPA and TAC were obtained from 65 stable adult renal allograft recipients on a single occasion (ie, day 15 after transplantation). A sampling schedule was estimated based on the D-optimality criterion with the POPED software, using parameter values from previously published studies on MPA and TAC modeling early after transplantation. Subsequently, a population PK model describing MPA and TAC concentrations was developed using nonlinear mixed-effects modeling. Optimal blood-sampling times for determination of MPA and TAC concentrations were estimated to be at 0 (predose) and at 0.24, 0.64, 0.98, 1.37, 2.38, and 11 hours after oral intake of mycophenolate and TAC. The PK of MPA and TAC were best described by a 2-compartment model with first-order elimination. For MPA, the absorption was best described by a transit compartment model, whereas first-order absorption with a lag time best described TAC transfer from the gastrointestinal tract. Parameters were estimated with good precision and accuracy. While hematocrit levels and CYP3A5 genetic polymorphism significantly influenced TAC clearance, the pharmaceutical formulation and MRP2 genetic polymorphism were retained as significant covariates on MPA absorption and elimination, respectively. The prospective use of the simultaneous D-optimal design approach for MPA and TAC has allowed good estimation of MPA and TAC PK parameters in the early period after transplantation characterized by a very high unexplained variability. The influence of some relevant covariates could be shown.

Challenges of developing a bioanalytical method for a macrolide immunosuppressant compound by LC-MS/MS

The quantification of tacrolimus in human whole blood was developed by LC-MS/MS for a range of 50.0 to 50,000.0 pg/ml. Different challenges were faced during method development due to ion-suppression, lack of sensitivity and low recovery. The optimization of the extraction procedure played a crucial role as tacrolimus had to be isolated from red blood cells, to which it is strongly bound. Another particular challenge arose from the freeze-thaw stability where the extracted samples from fresh blood always showed a lower recovery. Finally, matrix effect was observed in some matrices over time, which resulted in a failed long-term stability in whole blood. In order to resolve the matrix effect issue, the sample procedure had to be improved. The final assay showed good recovery, low matrix effect, linearity, blood stability and good precision and accuracy.

Using genetic and clinical factors to predict tacrolimus dose in renal transplant recipients

AIMS

Tacrolimus has a narrow therapeutic window and shows significant interindividual difference in dose requirement. In this study we aim to first identify genetic factors that impact tacrolimus dose using a candidate gene association approach, and then generate a personalized algorithm combining identified genetic and clinical factors to predict individualized tacrolimus dose.

MATERIALS & METHODS

We screened 768 SNPs in 15 candidate genes in metabolism, transport and calcineurin inhibition pathways of tacrolimus, for association with tacrolimus dose in a discovery cohort of 96 patients.

RESULTS

Four polymorphisms in CYP3A5 and one polymorphism in CYP3A4 were identified to be significantly associated with tacrolimus stable dose (p < 8.46 × 10(-5)). The same SNPs were identified when dose-normalized trough tacrolimus concentration was analyzed. The CYP3A5*1 allele was associated with significantly higher stable dose, bigger dose increase, higher risk of being underdosed and lower incidence of post-transplant hyperlipidemia. ABCB1 polymorphisms were not associated with stable dose. No significant difference was found between CYP3A5 expressers and nonexpressers in incidence of acute rejection and time to first rejection. Age, ethnicity and CYP3A inhibitor use could predict 30% of tacrolimus dosing variability. Adding the identified genetic polymorphisms to the algorithm increased the predictability to 58%. In two validation cohorts of 77 and 64 patients, the algorithm containing both genetic and clinical factors produced correlation coefficients of 0.63 and 0.42, respectively. This algorithm gave a prediction of the stable doses closer to the actual doses when compared with another algorithm based only on the CYP3A5 genotype.

CONCLUSION

CYP3A5 genotype is the most significant genetic factor that impacts tacrolimus dose among the genes studied. This study generated the first pharmacogenomics model that predicts tacrolimus stable dose based on age, ethnicity, genotype and comedication use. Our results highlight the importance of incorporating both genetic and clinical, demographic factors into dose prediction.

Childhood abuse, nonadherence, and medical outcome in pediatric liver transplant recipients

OBJECTIVE

The study assessed the relationship between a history of child abuse, nonadherence to medications, and medical outcome in children who had a liver transplant.

METHOD

Abuse history for children and adolescents ages 8 to 21 who underwent a liver transplantation at Mount Sinai Medical Center in New York was obtained in interviews in 2002. Adherence to tacrolimus was assessed from January 1 to December 31, 2003 by computing the SD of a series of medication blood levels for each patient. Biopsy-proven rejection episodes, degree of fluctuation of alanine aminotransferase (ALT), and maximal ALT levels were recorded as indicators of medical outcome.

RESULTS

Of 72 eligible patients, 56 were evaluated. Five had documented abuse. Abused children were less adherent to their medication regimen (p = .02; 95% confidence interval [CI] -2.66 to -0.24), had poor disease control (higher maximal ALT, p <.01; 95% CI -613.72 to -249.55), had greater fluctuation in ALT levels (p <.01; 95% CI -151.19 to -65.91), and suffered more biopsy-proven rejection episodes (two episodes in the abused cohort versus none in the rest) in 2003.

CONCLUSIONS

A history of child abuse is a significant risk factor for poor outcome posttransplantation and should be evaluated routinely. Adherence to medications can be a target for intervention in patients with a history of abuse.

Interaction and transport characteristics of mycophenolic acid and its glucuronide via human organic anion transporters hOAT1 and hOAT3

The immunosuppressant mycophenolate mofetil (MMF) is frequently administered with calcineurin inhibitors and corticosteroids to recipients of organ transplantations. However, the renal handling of the active metabolite mycophenolic acid (MPA) and 7-O-MPA-glucuronide (MPAG) has been unclear. The purpose of the present study was to assess the interaction of MPA and MPAG with the human renal organic anion transporters hOAT1 (SLC22A6) and hOAT3 (SLC22A8), by conducting uptake experiments using HEK293 cells stably expressing these transporters. MPA and MPAG inhibited the time-dependent uptake of p-[(14)C]aminohippurate by hOAT1 and that of [(3)H]estrone sulfate by hOAT3. The apparent 50% inhibitory concentration (IC(50)) of MPA for hOAT1 and hOAT3 was estimated at 10.7 and 1.5 microM, respectively. In the case of MPAG, the IC(50) values were calculated at 512.3 microM for hOAT1 and 69.1 microM for hOAT3. Eadie-Hofstee plot analyses showed that they inhibited hOAT1 noncompetitively and hOAT3 competitively. No inhibitory effects of tacrolimus, cyclosporin A and azathioprine on transport of p-[(14)C]aminohippurate by hOAT1 and of [(3)H]estrone sulfate by hOAT3 were observed. No transport of MPA by these transporters was observed. On the other hand, the uptake of MPAG into cells was stimulated by the expression of hOAT3, but not hOAT1. These findings propose the possibility that the administration of MMF decreases the renal clearance of drugs which are substrates of hOAT1 and hOAT3. Present data suggest that hOAT3 contributes to the renal tubular secretion of MPAG.

Toxicology: Then and now

Toxicology is "the science of poisons"; more specifically the chemical and physical properties of poisons, their physiological or behavioral effects on living organisms, qualitative, and quantitative methods for their analysis and the development of procedures for the treatment of poisoning. Although the history of poisons dates to the earliest times, the study and the science of toxicology can be traced to Paracelsus (1493-1541) and Orfila (1757-1853). Modern toxicology is characterized by sophisticated scientific investigation and evaluation of toxic exposures. The 20th century is marked by an advanced level of understanding of toxicology. DNA and various biochemicals that maintain cellular functions were discovered. Our level of knowledge of toxic effects on organs and cells is now being revealed at the molecular level. This paper will review the historical progress of clinical and forensic toxicology by exploring analytical techniques in drug analysis, differing biological matrices, clinical toxicology, therapeutic drug management, workplace drug testing, and pharmacodynamic monitoring and pharmacogenetics.

Simultaneous determination of three isomeric metabolites of tacrolimus (FK506) in human whole blood and plasma using high performance liquid chromatography–tandem mass spectrometry

An ammonium-adduct based liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed and validated for the simultaneous determination of three isomeric metabolites of tacrolimus (FK506), 13-O-demethylated (M1), 31-O-demethylated (M2) and 15-O-demethylated (M3) tacrolimus in human whole blood and plasma. These metabolites and the internal standards were extracted from biological matrix by methylbutyl ether (MTBE). Separation was achieved on a Genesis C(18) column with a gradient mobile phase elution. Ammonium-adduct ions formed by a Turbo Ionspray in positive ion mode were used to detect each analyte and internal standard. The MS/MS detection was by monitoring the fragmentation of 807.5-->772.4 (m/z) for M1, 807.5-->754.5 (m/z) for both M2 and M3, 795.5-->760.5 (m/z) for IS1 (FR298701) and 961.5-->908.5 (m/z) for IS2 (FR290198) on a triple quadrupole mass spectrometer (Sciex API 3000). The retention times were approximately 4.1 min for M1, 6.8 min for M2, 6.0 min for M3, and 3.9 min for IS1 and 6.4 min for IS2, respectively. The validated dynamic range was 0.2-20 ng/ml for all three metabolites based on a sample volume of 0.25-ml. The linearity of calibration curves for M1, M2, and M3 in both matrices had a correlation coefficient of >/=0.9984. In whole blood, validation data showed intra-batch (n=6) CVs of </=5.9% and REs between -4.9 and 3.6% and inter-batch (N=18) CVs of </=4.9% and REs between -3.5 and 1.5% for all three metabolites. In human plasma, validation data showed intra-batch (n=6) CVs of </=7.3% and REs between -5.1 and 7.6% and inter-batch (N=18) CVs of </=6.6% and REs between -0.3 and 4.7% for all three metabolites. Extraction recoveries were 72% for M1, 87% for M2, 69% for M3, 79% for IS1, and 74% for IS2 from blood; and 94% for M1, 96% for M2, 98% for M3, 92% for IS1, and 93% for IS2 from plasma. All three metabolites in human blood and plasma were stable for three freeze-thaw cycles, or 24-h ambient storage, or 12 months storage at approximately -80 degrees C. Extracted samples were stable for at least 50h at room temperature (RT). This method has been successfully used to analyze whole blood and plasma samples from human pharmacokinetic studies. Several key factors affecting the performance of the assay methods have also been addressed briefly.

Determination of 13-O-demethyl tacrolimus in human liver microsomal incubates using liquid chromatography-mass spectrometric assay (LC-MS)

A simple, sensitive and specific liquid chromatography coupled electrospray ionization mass spectrometric (LC/ESI/MS) method for the determination of 13-O-demethylated metabolite (MI), one of the major metabolites of tacrolimus has been developed. The assay uses 32-demethoxyrapamycin (IS) as the internal standard; ethyl acetate as extraction solvent; a Hypersil-Keystone Beta Basic-18 reversed-phase column; and a gradient mobile phase of consisting 0.1% formic acid in water and methanol-acetonitrile (3:49, v/v). Mass detection is performed on a single quadrupole mass spectrometer equipped with an electrospray ionization (ESI) interface and operated in a positive ionization mode. MI in the microsomal incubates was quantitated by computing the peak area ratio (MI/IS) analyzed in single ion monitoring (SIM) mode (m/z: 804 and m/z: 901 for MI and IS, respectively). Precision of the assay was determined by calculating the intra-run and inter-run variation at three concentrations (15, 25, 80 ng/ml); the intra run relative standard deviation (R.S.D.) was less than 10% and ranged from 5.0 to 8.3%; and the inter-run R.S.D. was less than 10% and ranged from 4.6 to 9.6%. The limits of detection was 2 ng/ml. This assay has been used to evaluate the effect of three human immunodeficiency virus (HIV) protease inhibitors on the metabolism of tacrolimus in human liver microsomes.

Low Hematocrit and Serum Albumin Concentrations Underlie the Overestimation of Tacrolimus Concentrations by Microparticle Enzyme Immunoassay versus Liquid Chromatography–Tandem Mass Spectrometry

Background: Rapid liquid chromatography–tandem mass spectrometry (LC-MS/MS) methods are used increasingly for tacrolimus (TRL) monitoring but show a negative difference with respect to a microparticle immunoassay (MEIA). This report examines possible reasons for this difference between methods.

Methods: We collected 1156 blood samples from 277 adult and 121 pediatric recipients of liver, renal, and bone marrow grafts or hepatocyte or pancreatic islet cell implants. TRL was measured in whole blood by MEIA and LC-MS/MS, and hematologic and biochemical data were collected when available.

Results: LC-MS/MS was significantly more precise (P &lt;0.02) than the MEIA with increased sensitivity. The MEIA had a median difference of 16.2% vs LC-MS/MS overall, and this was significantly affected by patient cohort (P &lt;0.001). The difference was greater in adult or pediatric liver graft recipients while they were inpatients rather than outpatients (31.8% and 14.0% vs 7.5% and 6.5%, respectively). The difference was also greater in bone marrow than kidney graft recipients (32.8% vs 15.8%, respectively). Multiple linear regression analysis showed significant inverse relationships of this difference with hematocrit (packed cell volume) and plasma albumin (P &lt;0.001) in the total cohort and a positive relationship with plasma bilirubin in a subgroup of pediatric liver graft recipients.

Conclusions: Patients with a low packed cell volume and plasma albumin are likely to show artificially high concentrations of TRL when measured by MEIA. The increased risk of underimmunosuppression must be considered should doses be reduced to lower these seemingly high TRL concentrations.

Pharmacokinetics of tacrolimus after multidose oral administration and efficacy in the prevention of allograft rejection in cats with renal transplants

OBJECTIVE

To describe pharmacokinetics of multi-dose oral administration of tacrolimus in healthy cats and evaluate the efficacy of tacrolimus in the prevention of allograft rejection in cats with renal transplants.

ANIMALS

6 healthy research cats.

PROCEDURE

Cats received tacrolimus (0.375 mg/kg, PO, q 12 h) for 14 days. Blood tacrolimus concentrations were measured by a high performance liquid chromatography-mass spectrometry assay. Each cat received an immunogenically mismatched renal allograft and native kidney nephrectomy. Tacrolimus dosage was modified to maintain a target blood concentration of 5 to 10 ng/mL. Cats were euthanatized if plasma creatinine concentration exceeded 7 mg/dL, body weight loss exceeded 20%, or on day 50 after surgery. Kaplan-Meier survival curves were plotted for 6 cats treated with tacrolimus and for 8 cats with renal transplants that did not receive immunosuppressive treatment.

RESULTS

Mean (+/- SD) values of elimination half-life, time to maximum concentration, maximum blood concentration, and area under the concentration versus time curve from the last dose of tacrolimus to 12 hours later were 20.5 +/- 9.8 hours, 0.77 +/- 0.37 hours, 27.5 +/- 31.8 ng/mL, and 161 +/- 168 hours x ng/mL, respectively. Tacrolimus treated cats survived longer (median, 44 days; range, 24 to 52 days) than untreated cats (median, 23 days; range, 8 to 34 days). On histologic evaluation, 3 cats had evidence of acute-active rejection, 1 cat had necrotizing vasculitis, and 2 cats euthanatized at study termination had normal appearing allografts.

CONCLUSIONS AND CLINICAL RELEVANCE

Tacrolimus may be an effective immunosuppressive agent for renal transplantation in cats.

Mechanisms of clinically relevant drug interactions associated with tacrolimus

The clinical management of tacrolimus, a macrolide used as immunosuppressant after transplantation, is complicated by its narrow therapeutic index in combination with inter- and intraindividually variable pharmacokinetics. As a substrate of cytochrome P450 (CYP) 3A enzymes and P-glycoprotein, tacrolimus interacts with several other drugs used in transplantation medicine, which also are known CYP3A and/or P-glycoprotein inhibitors and/or inducers. In clinical studies, CYP3A/P-glycoprotein inhibitors and inducers primarily affect oral bioavailability of tacrolimus rather than its clearance, indicating a key role of intestinal P-glycoprotein and CYP3A. There is an almost complete overlap between the reported clinical drug interactions of tacrolimus and those of cyclosporin. However, in comparison with cyclosporin, only few controlled drug interaction studies have been carried out, but tacrolimus drug interactions have been extensively studied in vitro. These results are inconsistent and are of poor predictive value for clinical drug interactions because of false negative results. P-glycoprotein regulates distribution of tacrolimus through the blood-brain barrier into the brain as well as distribution into lymphocytes. Interaction of other drugs with P-glycoprotein may change tacrolimus tissue distribution and modify its toxicity and immunosuppressive activity. There is evidence that ethnic and gender differences exist for tacrolimus drug interactions. Therapeutic drug monitoring to guide dosage adjustments of tacrolimus is an efficient tool to manage drug interactions. In the near future, progress can be expected from studies evaluating potential pharmacokinetic interactions caused by herbal preparations and food components, the exact biochemical mechanism underlying tacrolimus toxicity, and the potential of inhibition of CYP3A and P-glycoprotein to improve oral bioavailability and to decrease intraindividual variability of tacrolimus pharmacokinetics.

Comparison of an ELISA and an LC/MS/MS method for measuring tacrolimus concentrations and making dosage decisions in transplant recipients

This study compared an enzyme-linked immunosorbent assay (ELISA) to a liquid chromatography-tandem mass spectrometry (LC/MS/MS) technique for measurement of tacrolimus concentrations in adult kidney and liver transplant recipients, and investigated how assay choice influenced pharmacokinetic parameter estimates and drug dosage decisions. Tacrolimus concentrations measured by both ELISA and LC/MS/MS from 29 kidney (n = 98 samples) and 27 liver (n = 97 samples) transplant recipients were used to evaluate the performance of these methods in the clinical setting. Tacrolimus concentrations measured by the two techniques were compared via regression analysis. Population pharmacokinetic models were developed independently using ELISA and LC/MS/MS data from 76 kidney recipients. Derived kinetic parameters were used to formulate "typical dosing" regimens for concentration targeting. Dosage recommendations for the two assays were compared. The relation between LC/MS/MS and ELISA measurements was best described by the regression equation ELISA = 1.02. (LC/MS/MS) + 0.14 in kidney recipients, and ELISA = 1.12. (LC/MS/MS) - 0.87 in liver recipients. ELISA displayed less accuracy than LC/MS/MS at lower tacrolimus concentrations. Population pharmacokinetic models based on ELISA and LC/MS/MS data were similar with residual random errors of 4.1 ng/mL and 3.7 ng/mL, respectively. Assay choice gave rise to dosage prediction differences ranging from 0% to 30%. ELISA measurements of tacrolimus are not automatically interchangeable with LC/MS/MS values. Assay differences were greatest in adult liver recipients, probably reflecting periods of liver dysfunction and impaired biliary secretion of metabolites. While the majority of data collected in this study suggested assay differences in adult kidney recipients were minimal, findings of ELISA dosage underpredictions of up to 25% in the long term must be investigated further.

False positive blood tacrolimus concentration in microparticle enzyme immunoassay

The difference in the blood concentration of tacrolimus between the assay methods, microparticle enzyme immunoassay (MEIA) and enzyme linked immunosorbent assay (ELISA) was observed in a liver transplant recipient with anemia. MEIA provided significantly higher concentration than those of ELISA (7.8+/-1.9 vs. 5.0 +/- 1.8nglml, p<0.05) while the patient had low haematocrit <25%. The difference, however, was not observed during the periods with haematocrit >25%. This observation suggested that unknown tacrolimus levels generated from difference in assay methods gave incorrect blood tacrolimus during anemia. False positive concentration of tacrolimus ranging 0.1-3.3 ng/ml was observed in MEIA applying to the blood samples obtained from the patients without receiving tacrolimus. The false positive tacrolimus increased in the samples with lower hematocrit, suggesting that MEIA gave incorrect blood tacrolimus during anemia. Since MEIA potentially overestimates the tacrolimus levels, ELISA should be used for blood tacrolimus monitoring in the patients with anemia.

Recommendations for the outpatient surveillance of renal transplant recipients. American Society of Transplantation

Many complications after renal transplantation can be prevented if they are detected early. Guidelines have been developed for the prevention of diseases in the general population, but there are no comprehensive guidelines for the prevention of diseases and complications after renal transplantation. Therefore, the Clinical Practice Guidelines Committee of the American Society of Transplantation developed these guidelines to help physicians and other health care workers provide optimal care for renal transplant recipients. The guidelines are also intended to indirectly help patients receive the access to care that they need to ensure long-term allograft survival, by attempting to systematically define what that care encompasses. The guidelines are applicable to all adult and pediatric renal transplant recipients, and they cover the outpatient screening for and prevention of diseases and complications that commonly occur after renal transplantation. They do not cover the diagnosis and treatment of diseases and complications after they become manifest, and they do not cover the pretransplant evaluation of renal transplant candidates. The guidelines are comprehensive, but they do not pretend to cover every aspect of care. As much as possible, the guidelines are evidence-based, and each recommendation has been given a subjective grade to indicate the strength of evidence that supports the recommendation. It is hoped that these guidelines will provide a framework for additional discussion and research that will improve the care of renal transplant recipients.

New developments in the immunosuppressive drug monitoring of cyclosporine, tacrolimus, and azathioprine

The calcineurin inhibitors cyclosporine and tacrolimus form the cornerstones of most immunosuppression protocols. Because of their variable pharmacokinetics, and their narrow therapeutic indices, post-transplant immunosuppressive drug monitoring is an essential part of patient care to minimize the risks of toxicity or acute rejection. Furthermore, a reduction in the rate of acute rejection has been shown to result in a lower rate of graft loss due to chronic rejection. The introduction of the microemulsion formulation of cyclosporine with its more consistent bioavailability has renewed interest in the use of alternative sampling strategies to the trough cyclosporine concentration. Both pharmacokinetic and pharmacodynamic considerations support the concept that determination of cyclosporine during the absorption phase (0-4 h) might offer a better prediction of cyclosporine immunosuppressive efficacy. Initial investigations suggest that monitoring a 2-h postdose concentration C(2) may provide a more efficacious alternative to trough monitoring for optimizing therapy with Neoral. Tacrolimus has a 10- to 100-fold greater in vitro immunosuppressive activity compared with cyclosporine. Consistent with its greater potency, therapeutic whole blood trough concentrations for tacrolimus are around 20-fold lower than the corresponding cyclosporine concentrations. The correlation between toxicity and tacrolimus trough concentrations appears to be stronger than that for acute rejection. The results from a concentration-ranging trial in primary kidney-transplantation and liver-transplantation trials all found a significant relationship between toxicity and tacrolimus trough levels. Azathioprine is converted in vivo to 6-mercaptopurine, which is subsequently metabolized to the pharmacologically active 6-thioguanine nucleotides. The latter are also responsible for the cytotoxic side effects. Reliance on blood counts to monitor azathioprine therapy can be misleading, and they do not provide information on immunosuppresive efficacy. More pertinent information can be obtained through the measurement of thiopurine S-methyltransferase activity and the quantification of intracellular 6-thioguanine nucleotides concentrations in red blood cells. Prospective studies have demonstrated the clinical utility of determining 6-thioguanine nucleotides to individualise immunosuppressive therapy with azathioprine not only in the field of transplantation, but also in inflammatory bowel disease.

Comparative Clinical Pharmacokinetics of Tacrolimus in Paediatric and Adult Patients

Tacrolimus is a potent immunosuppressive agent used to prevent allograft rejection. The pharmacokinetics of tacrolimus have been studied in healthy volunteers and transplant recipients, mostly by using immunoassays to measure tacrolimus in plasma or blood. However, because of the cross-reactivity for certain tacrolimus metabolites of the antibodies used, these methods often lack specificity. This should be carefully taken into account when interpreting pharmacokinetic results for tacrolimus. In adult patients, tacrolimus is generally rapidly absorbed following oral administration (the time to reach maximum concentration is 1 to 2 hours), but in some patients absorption is slow or even delayed. Because of presystemic elimination, the oral bioavailability is low (around 20%) but may vary between 4 and 89%. Tacrolimus is highly bound to erythrocytes. Its binding to plasma proteins varies between 72 and 98% depending on the methodology used. Because of the extensive partitioning of tacrolimus into erythrocytes, its apparent volume of distribution (Vd) based on blood concentrations is much lower (1.0 to 1.5 L/kg) compared with values based on plasma concentrations (about 30 L/kg). Tacrolimus is metabolised by cytochrome P450 (CYP) 3A4 to at least 10 metabolites, some of which retain significant activity. Biliary excretion is the route of elimination of the tacrolimus metabolites. Systemic plasma clearance of tacrolimus is very high (0.6 to 5.4 L/h/kg), whereas blood clearance is much lower (0.03 to 0.09 L/h/kg). The terminal elimination half-life (t1/2beta) of tacrolimus is approximately 12 hours (with a range of 3.5 to 40.5 hours). Only limited information is available on the pharmacokinetics of tacrolimus in paediatric patients. The rate and extent of tacrolimus absorption after oral administration do not seem to be altered in paediatric patients. The Vd of tacrolimus based on blood concentrations in paediatric patients (2.6 L/kg) is approximately twice the adult value. Blood clearance of tacrolimus is also approximately twice as high in paediatric (0.14 L/h/kg) compared with adult (0.06 L/h/kg) patients. Consequently, t1/2beta does not appear modified in children, but oral doses need to be generally 2-fold higher than corresponding adult doses to reach similar tacrolimus blood concentrations. More pharmacokinetic studies in paediatric patients are, however, needed to rationalise the use of therapeutic drug monitoring for optimisation of tacrolimus therapy in this patient population.

Analysis of whole blood tacrolimus concentrations in liver transplant patients exhibiting impaired liver function

In transplant patients with impaired liver function, HPLC methodologies have been suggested for monitoring whole blood tacrolimus concentrations because of the reported inaccuracy of immunoassay for whole blood tacrolimus concentrations. One hundred fifty whole blood samples from 50 subjects enrolled in a multicenter liver transplant trial were chosen for HPLC/MS/MS analysis without consideration of the clinical status of the patient at the time of sampling. These samples were chosen to represent the sampling intervals during the 12-week posttransplantation period. Retrospectively, the authors identified a subset of 39 samples from 27 subjects exhibiting impaired liver function as demonstrated by bilirubin concentrations > 3.0 mg/dL (mean +/-SD = 7.5 +/- 5.6 mg/dL). The authors compared the agreement of concentrations obtained from the PRO-Trac II ELISA and HPLC/MS/MS by least squares linear regression analysis and Bland/Altman analysis, in this subset against the agreement of concentrations for 76 samples with normal bilirubin. In the samples obtained from patients with impaired liver function the resulting regression equation was: ELISA = 1.19(HPLC) + 0.7; r = 0.9. The mean difference (HPLC/MS/MS - ELISA) was -2.5 ng/mL +/- 2.9 ng/mL (mean +/- SD). While 71% of samples agreed within 3 ng/mL, 3% (n = 1) exhibited a difference >10 ng/ml. The corresponding evaluation of the samples with normal bilirubin concentrations resulted in the regression equation ELISA = 0.96(HPLC) + 0.9; r = 0.9, and a mean difference of -0.6 ng/mL +/- 2.3 ng/mL. The authors conclude that while a small subset of patients with cholestasis may require closer evaluation with a more specific methodology, the majority of the patients may be satisfactorily monitored with the PRO-Trac II ELISA.

Analytical Validation of the PRO-Trac II ELISA for the Determination of Tacrolimus (FK506) in Whole Blood

Background: The analytical validation of multiple lots of the PRO-TracTM II ELISA (DiaSorin) for the determination of tacrolimus in whole blood is described.

Methods: The analytical parameters assessed included analytical sensitivity, dilution linearity, functional sensitivity, values in samples containing no tacrolimus, intra- and interassay precision, supplementation and recovery, metabolite cross-reactivity, interference studies, and method comparisons HPLC-tandem mass spectrometry (HPLC/MS/MS) and the IMx® Tacrolimus II multiparticle enzyme immunoassay. Where appropriate, assessments were performed according to NCCLS guidelines.

Results: The mean analytical detection limit was &lt;0.25 μg/L for all lots, whereas the functional sensitivity was 1.0 μg/L. Excellent linear correlation (r = 0.985) was observed for dilution linearity. The intraassay imprecision was &lt;7%, and the total imprecision by ANOVA was &lt;10%. Recovery was 109% ± 11%. Metabolite cross-reactivity was consistent with previous reports for this antibody. No interference was observed for 35 tested drugs. Method comparison with HPLC/MS/MS showed no statistically significant differences. Samples exhibited stability through four freeze/thaw cycles and for 1 week at room temperature.

Conclusion: These data demonstrate that the PRO-Trac II ELISA is a robust, accurate, and precise tool for the assessment and management of tacrolimus blood concentrations in transplant patients.

P-glycoprotein-dependent disposition kinetics of tacrolimus: studies in mdr1a knockout mice

PURPOSE

This study was performed to evaluate the involvement of P-glycoprotein in disposition kinetics of tacrolimus (FK506), a substrate of P-glycoprotein, in the body.

METHODS

The blood and tissue concentrations of FK506 after i.v. or p.o. administration (2 mg/kg) to normal and mdr1a knockout mice were measured by competitive enzyme immunoassay.

RESULTS

The blood concentrations in knockout mice were significantly higher than those in normal mice. The value of the total clearance (CLtot) for knockout mice (19.3 mL/min/kg) was about 1/3 of that for normal mice (55.8 mL/min/kg)(P < 0.001), although there was no significant difference in the distribution volume at the steady-state (Vd(ss)) (about 4.6 L/kg) between both types of mice. FK506 rapidly penetrated the blood-brain barrier and the brain concentration reached a maximum, which was about 10 times higher in knockout mice than in normal mice, 1 hr after administration. The brain concentration in normal mice thereafter decreased slowly, whereas in knockout mice, an extremely high concentration was maintained for 24 hr.

CONCLUSIONS

The pharmacokinetic behavior of FK506 in the tissue distribution is related with the function of P-glycoprotein encoded by the mdrla gene. The brain distribution of FK506 is dominated by the P-glycoprotein-mediated drug efflux and presumably also by the binding to FK-binding proteins (immunophilins) in the brain.

Improvement in assay sensitivity for plasma dolastatin-10 using capillary electrophoresis at elevated temperatures

The very potent antimitotic and anticancer agent, dolastatin-10 (DOL-10), currently undergoing testing in a phase II clinical trial, has been quantitated previously in human plasma by high-performance capillary electrophoresis (HPCE). This method provides a lower limit of detection of 25 ng/ml DOL-10 from extracted patient samples. Without changes in preconcentration techniques, we report a significant improvement in the sensitivity of this method using elevated temperatures with conventional UV absorbance detection and liquid-liquid extraction which lowers the detection limit to 3 ng/ml of the drug. An elevated separation temperature of 50 degrees C was critical in achieving this 8x improvement in the detection limit. Partial validation of the method at this temperature gave excellent linearity (0-100 ng/ml; y=0.018x+0.085, r=0.993), limit of quantitation (5 ng/ml), and good overall recovery of the drug (>85%). We have applied this improved method towards the in vivo quantitation of DOL-10 in mice and in a patient receiving the drug in a phase I clinical study. From these analyses we conclude that this method is suitable for clinical studies where plasma levels of DOL-10 are > or = 5 ng/ml.

Modified pentamer formation assay for measurement of tacrolimus and its active metabolites: comparison with liquid chromatography–tandem mass spectrometry and microparticle enzyme-linked immunoassay (MEIA-II)

A modified pentamer formation assay (PFA) for quantification of tacrolimus and active metabolites after extraction from whole blood is described. The lower limit of detection was 2 μg/L. Intraassay precision (CV) was 5.7–13.7%, and the interassay CV was 6.1–14.9%. Tacrolimus trough concentrations in 104 whole blood specimens from liver and kidney transplant recipients were compared with results from HPLC–tandem mass spectrometry (LC/MS/MS) and microparticle enzyme immunoassay (MEIA-II). Data were analyzed by difference plots and are presented as median (95% confidence intervals) of the method differences. MEIA-II results were on average 2.00 μg/L (range, −0.08 to 5.17 μg/L) higher than LC/MS/MS, whereas PFA results were only 1.07 μg/L (range, −2.62 to 5.33 μg/L) higher. Of 104 specimens tested, 25 displayed differences ≥3 μg/L between MEIA-II and PFA: median difference, 4.65 μg/L (range, 3.01–8.79 μg/L). The corresponding median difference between PFA and LC/MS/MS was −0.91 μg/L (range, −4.11 to 0.85 μg/L), and the difference between MEIA-II and LC/MS/MS was 3.67 μg/L (range, 1.88–6.34 μg/L), suggesting the presence of inactive metabolites that caused a positive bias in the immunoassay. In contrast, similar median differences were observed for the remaining 79 specimens: MEIA-II minus LC/MS/MS, 1.78 μg/L (range, −0.45 to 4.11 μg/L); PFA minus LC/MS/MS, 1.90 μg/L (range, −1.70 to 5.50 μg/L). Active tacrolimus metabolites may have contributed to the higher apparent tacrolimus concentrations in these specimens.

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An HPLC/MS/MS assay for tacrolimus in patient blood samples. Correlation with results of an ELISA assay

An HPLC/MS/MS assay for tacrolimus in whole blood using FR900520 as an internal standard was validated over the standard curve range of 0.100-10.040 ng ml-1. The calibration curve for tacrolimus in human blood gave a slope of 0.2481, an intercept of 0.007, and a correlation coefficient (r) of 0.9996, with no interference noted from human blood, analyte, or internal standard stock solutions. Use of EDTA or heparin as the preservative in blood resulted in no significant differences. Samples were stable for at least the time required to assay the maximum number of samples that could be placed in the automated system. The limit of sensitivity of the assay was set at the concentration of the lowest nonzero standard tested, i.e., 0.100 ng ml-1. However, validation of the assay to a limit of 0.010 ng ml-1 is currently underway. The within-run and between-run precision and accuracy of the method were determined for four quality control samples. The highest CV was seen at 0.1 ng ml-1 (17.6% within-run and 15.9% between-run), with other CV < 5%. The recovery ranged 79.6-81.3% for tacrolimus over the range 0.3-8.0 ng ml-1 and was 63.10 +/- 1.37% for FR900520. There was a linear correlation (r2 = 0.963) between assay results by HPLC/MS/MS and ELISA in whole blood from atopic dermatitis patients treated with topical tacrolimus ointment. The difference between the means +/- S.D. determined by HPLC/MS/MS (1.22 +/- 1.46 ng ml-1) and ELISA (1.12 +/- 1.29 ng ml-1) was significant by a paired t-test (P < 0.001) Similarly, there was a linear correlation (r2 = 0.841) between assay results by HPLC/MS/MS and IMx in whole blood from solid organ transplant patients treated with tacrolimus. The difference between the means was significantly higher (P < 0.001) for the IMx (15.80 +/- 8.37 ng ml-1) than the HPLC/MS/MS (13.42 +/- 6.87 ng ml-1).