Gel chromatographic analysis of cyclosporin and its metabolites in human blood compartments

Pharmacologically active drug metabolites: impact on drug discovery and pharmacotherapy

Metabolism represents the most prevalent mechanism for drug clearance. Many drugs are converted to metabolites that can retain the intrinsic affinity of the parent drug for the pharmacological target. Drug metabolism redox reactions such as heteroatom dealkylations, hydroxylations, heteroatom oxygenations, reductions, and dehydrogenations can yield active metabolites, and in rare cases even conjugation reactions can yield an active metabolite. To understand the contribution of an active metabolite to efficacy relative to the contribution of the parent drug, the target affinity, functional activity, plasma protein binding, membrane permeability, and pharmacokinetics of the active metabolite and parent drug must be known. Underlying pharmacokinetic principles and clearance concepts are used to describe the dispositional behavior of metabolites in vivo. A method to rapidly identify active metabolites in drug research is described. Finally, over 100 examples of drugs with active metabolites are discussed with regard to the importance of the metabolite(s) in efficacy and safety.

Liquid chromatographic–electrospray mass spectrometric determination of cyclosporin A in human plasma

A rapid, sensitive and selective liquid chromatography-mass spectrometry (LC-MS) assay has been developed for determination of cyclosporin A (CyA) in human plasma; cyclosporin B (CyB) was used as internal standard (IS). The method utilized a combination of a column-switching valve and a reversed-phase symmetry column. The mobile phase was a 25:75 (v/v) mixture of 10% aqueous glacial acetic acid and acetonitrile. Running time per single run was less than 10 min. Sample preparation included C8 SPE of human plasma spiked with the analyte and internal standard, evaporation of the eluate to dryness at 50 degrees C under N2 gas, and finally reconstitution in the mobile phase. Detection of cyclosporin A and the IS was performed in selected ion-monitoring mode at m/z 601.3 and 594.4 Da for CyA and IS, respectively. Quantitation was achieved by use of the regression equation of relative peak area of cyclosporin to IS against concentration of cyclosporin. The method was validated according to FDA guideline requirements. The linearity of the assay in the range 5.0-400.0 ng mL(-1) was verified as characterized by the least-squares regression line Y = (0.00268+/-1.9 x 10(-4))X+(0.00078+/-1.8 x 10(-3)), correlation coefficient, r = 0.9986+/-1.1 x 10(-3) (n = 48). Intra and inter-day quality-control measurements in the range 5.0-350.0 ng mL(-1) revealed almost 100% accuracy and < or = 9% CV for precision. The mean absolute recovery of CyA was found to be 84.01+/-9.9% and the respective relative recovery was 100.3+/-9.19. The limit of quantitation (LOQ) achieved was 5 ng mL(-1). Eventually, stability testing of the analyte and IS in plasma or stock solution revealed that both chemicals were very stable when stored for long or short periods of time at room temperature or -20 degrees C.

Evaluation and comparison of therapeutic monitoring of whole-blood levels of cyclosporin A and its metabolites in renal transplantation by HPLC and RIA methods

BACKGROUND

The aim of the work was to evaluate the possibility to estimate the level of cyclosporin A (CyA) metabolites as the difference of radioimmunoassay (RIA) non-specific and RIA specific methods.

METHODS

Blood samples of renal transplant patients were analyzed by three different methods: RIA specific method (CYCLO-Trac, DiaSorin, USA) (RIA(SP)), RIA non-specific method (Immunotech, Czech Republic) (RIA(NS)), and high performance liquid chromatography (HPLC) method.

RESULTS

Although values obtained by RIA(SP) correlated well those obtained by HPLC (RIA(SP)=0.995.HPLC+9.68; r(2)=0.962, n=448), the results of HPLC methods were lower by 8%. The values obtained by RIA(NS) were 2.57 times higher than the values obtained by RIA(SP) (RIA(SP)=0.356RIA(NS); r(2)=0.713, n=448). The ratio (CyA+CyA metabolites)/(CyA) calculated as the ratio RIA(NS)/RIA(SP) values for 42 renal transplant patients was relatively stable for each particular patient. The sum of selected CyA metabolites (M1+M17+M21) measured by HPLC correlated well with that estimated from the difference of RIA(NS)-RIA(SP): HPLC(metab)=0.921.(RIA(NS)-RIA(SP))+21.3; (r(2)=0.746, n=448).

CONCLUSION

The combination of both the specific and non-specific methods for the determination of CyA presents an improved means for the TDM of CyA and CyA metabolites in renal transplant patients. Moreover, a combination of both methods can help to elucidate some unexpected events, such as the persistence of high cyclosporin blood levels.

High-performance liquid chromatographic method for therapeutic drug monitoring of cyclosporine A and its two metabolites in renal transplant patients

A novel fast HPLC method was developed for the determination of cyclosporine A (CyA) and its two metabolites M17 (AM1) and M21 (AM4N) in blood. Whole blood was precipitated with zinc sulphate, extracted with diethyl ether, evaporated, dissolved in aqueous methanol and partitioned twice with n-hexane. Chromatography was carried out using a microbore RP-column under isocratic elution with acetonitrile-methanol-water (200:80:140, v/v/v) at 70 degrees C and a detector set at 205 nm. Linearity for all three compounds was tested in the range of 1-1000 ng/ml. Recovery was 97-109%, and a coefficient of variation was 1.6-8.8% depending on the particular compound and its concentration. The method was used for a group of renal transplant patients having an inadequate response to CyA therapy in order to evaluate the possible role of CyA and its metabolites on the occurrence of hypertension and other toxicological events.

In-vivo distribution and erythrocyte binding characteristics of cyclosporin in renal transplant patients

The pharmacokinetic parameters of cyclosporin, a potent immunosuppressive agent, show large intra- and inter-individual variability, possibly because of the different analytical methods used. A recently developed cyclosporin-specific radioimmunoassay has been used to study the in-vivo distribution and binding characteristics of cyclosporin in whole blood, plasma and erythrocytes of fifteen renal transplant patients. The profiles of cyclosporin concentration-time curves after an oral dose of cyclosporin had either one peak (ten patients, group A) or two (five patients, group B). Essentially no difference was observed between the two groups in the relationship between equilibrium cyclosporin concentrations in erythrocyte and plasma as a function of whole-blood concentration. The equilibrium in-vivo cyclosporin concentrations in erythrocyte and plasma were, however, markedly lower than those previously observed under in-vitro conditions. The ratio of cyclosporin concentration in erythrocytes (CE) to that in plasma (CP) changed with time, in inverse proportion to the change in cyclosporin concentration in blood, over the range 0.63-2.80 in individual patients with an average of 1.36 +/- 0.07 (mean +/- s.e.m.) for group A and 1.42 +/- 0.23 for group B. The apparent cyclosporin binding affinity (Kd) to erythrocytes under in-vivo conditions averaged 452.2 +/- 47.6 nM (543.5 +/- 57.2 ng mL-1) for group A and 419.4 +/- 41.2 nM (504.1 +/- 49.5 ng mL-1) for group B, whereas apparent cyclosporin binding capacity (Bmax) of the blood cell averaged 0.83 +/- 0.07 nmol mL-1 for group A and 0.78 +/- 0.07 nmol mL-1 for group B. Significantly reduced average Kd (262.7 +/- 40.2 nM or 315.8 +/- 48.9 ng mL-1, P < 0.01) and Bmax (0.56 +/- 0.08 nmol mL-1, P < 0.05) values were observed during the period after Tmax (4-12 h after the drug ingestion) in group A patients. Apparent Kd and Bmax, determined by a nonlinear regression technique, were 131.6 +/- 29.4 and 1088.0 +/- 114.7 nM (158.2 +/- 35.4 and 1307.8 +/- 137.9 ng mL-1) and 0.178 +/- 0.024 and 0.814 +/- 0.078 nmol mL-1, respectively, during the 4-12 h period in group A patients. These findings reveal distinct differences in in-vivo distribution of cyclosporin and the binding characteristics of the compound to erythrocytes from those previously observed under in-vitro conditions. The significantly lower Kd of cyclosporin binding to erythrocytes during the elimination phase suggests a potential effect of cyclosporin-containing erythrocytes or of cyclosporin contained in erythrocytes during cyclosporin treatment.