Validation and quality assurance program for monitoring tacrolimus (FK 506) concentrations in plasma and whole blood

Tacrolimus (FK506): validation of a sensitive enzyme-linked immunosorbent assay kit for and application to a clinical pharmacokinetic study

Tacrolimus (FK506) is a macrolide immunosuppressant approved for the prophylaxis of organ rejection in liver transplant. Immunoassays of low intra- and interday variability and high sensitivity are necessary to adequately characterize terminal elimination phase concentrations in pharmacokinetic studies. A new ELISA kit for the quantitation of tacrolimus in human whole blood has been validated for use in pharmacokinetic studies. Methanol sample extracts were dried and reconstituted in a horseradish peroxidase (HPR)-FK506 conjugate solution. The reconstituted samples and mouse anti-FK506 were added to a microplate, precoated with secondary antibody, and incubated, FK506 and the HPR-FK506 conjugate competed to bind with anti-FK506, which was immobilized by binding to the secondary antibody. Unbound FK506 was washed away, and substrate was added for color development. Once the reaction was stopped with 2 N H2SO4, the plate was read at 450 nm. The linear range was 0.5-60 ng/ml, with a limit of quantitation of 0.5 ng/ml. Interday precision and accuracy were < or = 10.4% C.V. and < or = 3% R.E. for quality control samples. The lack of interference from endogenous compounds was established by parallelism and recoveries of FK506 from six lots of control matrix. Cross-reactivity against the metabolites and analogs were not performed because the kit monoclonal antibody was from the same source as Kobayashi et al (1). The utility and sensitivity of the kit present a good method for the quantitation of tacrolimus in blood from pharmacokinetic studies. The method is robust and has been used to assay tacrolimus in several thousand whole blood samples by multiple analysts.

A highly sensitive enzyme-linked immunosorbent assay for the determination of tacrolimus in atopic dermatitis patients

A highly sensitive enzyme-linked immunosorbent assay method has been developed for the determination of tacrolimus in human blood samples. The assay is a modification of the previously published assay with improved sensitivity. Following extraction of tacrolimus with methanol and sulfosalicylic acid, the samples are incubated for 2 h at room temperature on a Nunc Maxisorb plate that has been treated for nonspecific binding by precoating with polyclonal antibody. The analysis of human blood following the standard addition of tacrolimus (0.02-5.0 ng/ml) demonstrated excellent precision and accuracy over a 6-day period. The interday and intraday co-efficients of variation were 6.0-28.9 and 3.9-15.2%, respectively. The limit of quantitation was 0.05 ng/ml. The method was used to quantitate blood concentration of tacrolimus in patients following the administration of tacrolimus ointment.

Quality assessment issues of new immunosuppressive drugs and experimental experience

There are established quality assessment schemes for the two immunosuppressive drugs that have entered routine clinical use: cyclosporine and tacrolimus. These two drugs, together with sirolimus and mycophenolic acid, have been the subject of recent consensus panel reports that have reached broad agreement on several issues relating to therapeutic monitoring of these agents. While the current quality assessment schemes are not based on validated reference methods, the data they yield on comparative assay performance are of value as a guide to patient management and for clinical studies of drug efficacy.

Quality assurance procedure for monitoring tacrolimus (FK506) concentrations in whole blood by IMx assay

A Quality Assurance Program for the IMx assay for FK506 in whole blood samples was established to monitor the performance of the assay in clinical sites enrolled by Fujisawa USA, Inc. Forty investigative sites participating in the program were required to perform assay to establish intraassay variability, interassay variability, and performance on blinded samples. Only two of the sites were required to repeat part of the program. The intraassay and interassay results at the sites were in good agreement with the target values obtained at Fujisawa Research Laboratory. Most of the coefficients of variation (CV) were within +/- 15%, well within the acceptance range of +/- 30%. Only a few values were outside the acceptance window. For the blinded samples, the CVs were variable and depended on the concentration of FK506 in the sample. At lower blood FK506 concentrations (5-10 ng/ml), the mean CVs were often outside the acceptance window, and many individual values were not acceptable. At concentrations of 15-50 ng/ml, the CVs were generally acceptable. Thus individual sites can quickly learn to perform the FK506 IMx assay and achieve good within- and between-day results. The assay of lower blood concentrations of FK506 may show higher variability. Patients are usually monitored for clinical signs of rejection and toxicity in addition to blood FK506 concentrations.

FK506 (tacrolimus) and its immunoreactive metabolites in whole blood of liver transplant patients and subjects with mild hepatic dysfunction

PURPOSE

To determine the concentrations of FK506 and its metabolites in blood from liver transplant patients and subjects with hepatic dysfunction.

METHODS

HPLC was combined with an enzyme-linked immunosorbent assay (ELISA) to determine the concentrations of FK506 and its immunoreactive metabolites in human whole blood.

RESULTS

In four liver transplant patient, most of the immunoreactivity was seen in the HPLC fractions where unchanged FK506 eluted. FK506 accounted for about 95% or more of the total immunoreactivity in the first days of posttransplant. Immunoreactivity observed in the nonFK506 fractions ranged from 1.6% to 10.7% of the total immunoreactivity; about 30% of the nonFK506 immunoreactivity was due to M-III(15-O-demethyl FK506). Blood from subjects with mild hepatic dysfunction was examined at 1.5 and 6 hours after an oral and intravenous dose, respectively, by HPLC-ELISA. Regardless of the route of administration, more than 96% of the total immunoreactivity was recovered in the FK506 fraction. M-III was detected in the blood of 3 of 6 subjects after an oral dose, but in none of these after an intravenous dose.

CONCLUSIONS

ELISA is an appropriate method for therapeutic drug monitoring of FK506.

Clinical pharmacokinetics of tacrolimus

Tacrolimus, a novel macrocyclic lactone with potent immunosuppressive properties, is currently available as an intravenous formulation and as a capsule for oral use, although other formulations are under investigation. Tacrolimus concentrations in biological fluids have been measured using a number of methods, which are reviewed and compared in the present article. The development of a simple, specific and sensitive assay method for measuring concentrations of tacrolimus is limited by the low absorptivity of the drug, low plasma and blood concentrations, and the presence of metabolites and other drugs which may interfere with the determination of tacrolimus concentrations. Currently, most of the pharmacokinetic data available for tacrolimus are based on an enzyme-linked immunosorbent assay method, which does not distinguish tacrolimus from its metabolites. The rate of absorption of tacrolimus is variable with peak blood or plasma concentrations being reached in 0.5 to 6 hours; approximately 25% of the oral dose is bioavailable. Tacrolimus is extensively bound to red blood cells, with a mean blood to plasma ratio of about 15; albumin and alpha 1-acid glycoprotein appear to primarily bind tacrolimus in plasma. Tacrolimus is completely metabolised prior to elimination. The mean disposition half-life is 12 hours and the total body clearance based on blood concentration is approximately 0.06 L/h/kg. The elimination of tacrolimus is decreased in the presence of liver impairment and in the presence of several drugs. Various factors that contribute to the large inter- and interindividual variability in the pharmacokinetics of tacrolimus are reviewed here. Because of this variability, the narrow therapeutic index of tacrolimus, and the potential for several drug interactions, monitoring of tacrolimus blood concentrations is useful for optimisation of therapy and dosage regimen design.

Tacrolimus analysis: a comparison of different methods and matrices

We determined the through blood and plasma concentrations of tacrolimus from the day of transplantation through 30 days posttransplantation in four liver and four kidney transplant patients by three different methods. The first method involved a solid phase extraction of the blood or plasma using Sep-Pak columns (SPs) followed by quantitation of tacrolimus using an enzyme-linked immunosorbent assay (ELISA); the second method involved a liquid-liquid extraction using methylene chloride (MC) followed by quantitation of tacrolimus using the ELISA, and the third method involved a high-performance liquid chromatography (HPLC) fractionation of the extract obtained from the solid-phase extraction and quantitation of tacrolimus in the fractions by ELISA. The trough plasma tacrolimus concentrations ranged from 0.1 to 5.2 ng/ml. While the trough plasma concentrations of tacrolimus were similar and independent of the method of analysis in kidney transplant patients and in liver transplant patients with normal biochemical profile, in patients with liver dysfunction, tacrolimus plasma concentrations were higher when measured by SP-ELISA and MC-ELISA methods as compared to the HPLC-ELISA method. In plasma samples obtained from liver transplant patients with liver dysfunction, the presence of some metabolites that cross-reacted with the antibody used in the ELISA could be documented in the HPLC fraction corresponding to the metabolites. This indicates that while tacrolimus metabolites that cross-react significantly with the antibody used in the ELISA do not accumulate in kidney transplant patients, they can appear in the plasma of patients, they can appear in the plasma of patients with liver dysfunction. The trough blood tacrolimus concentrations in patients were significantly higher than the corresponding plasma concentrations and ranged from 1.4 to 107 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)