Comparison of methods to calculate cyclosporine A bioavailability from consecutive oral and intravenous doses

Explaining variability in ciclosporin exposure in adult kidney transplant recipients

Purpose

Optimal ciclosporin A (CsA) exposure in kidney transplant recipients is difficult to attain because of variability in CsA pharmacokinetics. A better understanding of the variability in CsA exposure could be a good means of individualizing therapy. Specifically, genetic variability in genes involved in CsA metabolism could explain exposure differences. Therefore, this study is aimed at identifying a relationship between genetic polymorphisms and the variability in CsA exposure, while accounting for non-genetic sources of variability.

Methods

De novo kidney transplant patients (n = 33) were treated with CsA for 1 year and extensive blood sampling was performed on multiple occasions throughout the year. The effects of the non-genetic covariates hematocrit, serum albumin concentration, cholesterol, demographics (i.e., body weight), CsA dose interval, prednisolone dose and genetic polymorphisms in genes encoding ABCB1, CYP3A4, CYP3A5, and PXR on CsA pharmacokinetics were studied using non-linear mixed effect modeling.

Results

The pharmacokinetics of CsA were described by a two-compartment disposition model with delayed absorption. Body weight was identified as the most important covariate and explained 35% of the random inter-individual variability in CsA clearance. Moreover, concurrent prednisolone use at a dosage of 20 mg/day or higher was associated with a 22% higher clearance of CsA, hence lower CsA exposure. In contrast, no considerable genotype effects (i.e., greater than 30–50%) on CsA clearance were found for the selected genes.

Conclusions

It appears that the selected genetic markers explain variability in CsA exposure insufficiently to be of clinical relevance. Therefore, therapeutic drug monitoring is still required to optimize CsA exposure after administration of individualized doses based on body weight and, as this study suggests, co-administration of prednisolone.

NONMEM population pharmacokinetic modeling of orally administered cyclosporine from routine drug monitoring data after heart transplantation

The population pharmacokinetics of cyclosporine (CsA) in adult recipients of cardiac transplants were determined from sparse, retrospective drug monitoring data accumulated for at least 3 months after surgery. All were receiving oral CsA twice daily, and morning trough levels in whole-blood were measured by high-performance liquid chromatography. Additional data included height, weight, gender, age, ethnicity, hematocrit, total bilirubin, and concurrent drug use. Population modeling was performed using NONMEM on 36 randomly selected patients, assuming a one-compartment model with first-order absorption and elimination. Improved fits were obtained by incorporating the following expression in the model to adjust oral bioavailability as a function of postoperative day (POD): F = 0.2 + 10 x ABS (POD - 7)/([POD + 10] x 60). Interpatient variability (CV%) in clearance (CL) was 20.2%. There was a mean bias of 8.5% at the average CsA concentration of 250 ng/ml when the predictive performance was assessed statistically in a reserved subset of 33 patients who received cardiac transplants. For the entire population (n = 69 patients), the average CsA CL and terminal half-life (T1/2) were, respectively: CL (l/h) = 0.256 x weight (kg); T1/2 = 11.0 hours, or CL (l/h) = 0.184 x weight (kg); T1/2 = 14.7 hours, if there was concomitant diltiazem administration. These results compared favorably with those reported elsewhere for studies of postcardiac transplant kinetics using the traditional multiple blood sampling approach.

A population pharmacokinetic model of cyclosporine in the early postoperative phase in patients with liver transplants, and its predictive performance with Bayesian fitting

The availability of personal computer programs to individualize drug regimens has stimulated interest in modeling population pharmacokinetics. This study used the NPEM2 software to determine cyclosporine population pharmacokinetic parameter values and distributions in a first group of 25 recipients of liver transplants during their first postoperative week. On a second group of 25 patients, the authors used these values to evaluate Bayesian predictive performance of cyclosporine blood concentrations with the USC*PACK PC program. During the study period, all the patients have been treated by continuous intravenous infusion. The one-compartment model pharmacokinetic parameter-the slope of volume to body weight (Vs) and the elimination rate constant (Kel) values found (mean values: Vs = 2.177 l/kg, Kel = 0.235 h(-1); median values: Vs = 1.559 l/kg, Kel = 0.163 h(-1); the percent coefficient of variation (Vs = 92%, Kel = 79%) appear reasonable and show the ability of NPEM2 to deal with sparse data. When the predictions were studied with day 1, day 2, or day 3 concentrations, predictive bias was respectively -0.030, -0.013, and 0.013 microg/ml, suggesting a greater clearance of cyclosporine immediately after surgery, the clearance decreasing in the days after. With the first three blood levels and the Bayesian fitting procedure, it was possible to predict at least half the subsequent measured blood levels of each patient accurately (within 20%) in more than three-quarters (76%) of the second group of recipients of transplants, and for 40% of patients the authors obtained accurate predictions in 100% of the subsequent blood levels. For a few patients (12%) they found quite poor predictions. The reason for this is unclear. The results suggest that this population model and the Bayesian fitting procedure using two or three blood levels can be reasonably and carefully used to control, in real time, cyclosporine blood levels in a majority of new patients with liver transplants.

Pharmacokinetics of cyclosporine after renal transplant in children

The pharmacokinetics of cyclosporine and the relationship between blood levels and average drug concentration were prospectively evaluated in 18 children 1 month after renal transplantation. All children had normal renal function and no hepatic or gastrointestinal dysfunction. Cyclosporine was administered after an overnight fast, and serial blood samples were drawn over a 24-hour period. Analysis of cyclosporine levels was performed by means of monoclonal radio immunoassay on whole blood. Children were divided into three age groups for comparison: 2-5 years, 5-10 years, and > 10 years. There were no differences between age groups in serum protein, serum lipids, or hemoglobin levels, or in the pharmacokinetic parameters of cyclosporine except as follows: significant differences were noted in cyclosporine dose based on body weight, apparent steady-state volume of distribution, and apparent blood clearance, with the youngest children (2-5) requiring higher doses, a relative greater distribution, and exhibiting more rapid drug clearance than those > 10 years of age. In addition, we observed diurnal variation in trough levels, with morning levels (0 hr) significantly higher than those obtained in the evening (12 hours after administration of cyclosporine). Trough levels demonstrated a fair correlation with area under the concentration-time curve (AUC) and average concentration (Cav), but an abbreviated kinetic profile using cyclosporine levels 1 and 3.5 hours after administration accurately predicted AUC.

Comparison of some practical sampling strategies for population pharmacokinetic studies

Using population analysis, sparsely sampled Phase 3 clinical data can be utilized to determine the pharmacokinetic characteristics of the target population. Data arising from such studies are likely to be constrained to certain sampling windows, i.e., the visiting hours at the study clinic. When the sampling window is narrow compared to the half-life of the drug, the advantage of taking more than one sample is not obvious. Study designs with one or two samples per visit have been compared with respect to (i) precision and bias of the population parameter estimates, (ii) the ability to identify the underlying pharmacokinetic model, and (iii) the estimation of individual parameter values. The first point was assessed using simulated data while the latter two were studied using a real data set. Results show: (i) Parameter estimates are more biased and imprecise when only one sample is taken compared to when two samples are obtained, this is true irrespective of the time span between the two samples. (ii) Ability to identify a more complex model is increased if two samples are taken. Specifically, the variability between occasions can be quantified. (iii) Two-sample designs are generally better with respect to prediction of individual parameter values. Even minor changes to commonly employed study designs, in this case the addition of one sample at each study occasion, can improve quality and quantity of the information obtained.

Bioavailability of flumequine after semisimultaneous administration to veal calves

The absolute bioavailability of flumequine after semisimultaneous intramuscular administration as a water-based suspension to veal calves was 92 +/- 14%. The semisimultaneous experimental design provided a reliable determination of absorption rate and demonstrated flip-flop pharmacokinetics. No period or sequence effects were detected. Calculated elimination rate, clearance, and volume of distribution after intravenous administration were comparable to values obtained from traditional design studies. The semisimultaneous experimental design proved to be valuable for the assessment of bioavailability and pharmacokinetics of drugs in food-producing animals while preventing violation of basic clearance assumptions.

Absorption and in vivo dissolution of hydroxycholoroquine in fed subjects assessed using deconvolution techniques

1. Nine healthy subjects each received three doses of 155 mg rac-hydroxychloroquine, as a tablet, an oral solution and by intravenous infusion, in a randomised cross-over design study, 30 min after a standard high fat breakfast. 2. Four methods of deconvolution were used to assess the absolute bioavailability of the tablet and oral solution doses. These were the delta function method, the staircase approximation method, and two least squares methods using a single first-order input and a sequential first-order input. The mean (+/- s.d.) fraction absorbed estimated by the four methods was 0.64 +/- 0.14 after the tablet and 0.87 +/- 0.30 after the oral solution. Wide intersubject variability was observed (0.50-0.91 for the tablet; 0.30-1.37 for the solution). 3. The mean (+/- s.d.) absorption half-life was 3.7 +/- 2.0 h for the tablet and 3.3 +/- 1.6 h for the solution, suggesting that absorption following the tablet dose was not rate-limited by dissolution. 4. The in vivo dissolution rate, extent of release and lag-time were determined using cube-root law and first-order input functions. Dissolution was found to be rapid, after a significant lag-time, but incomplete in some subjects. 5. The rate and extent of absorption was similar to that reported previously for fasted subjects. The lag-time before absorption commenced in fed subjects (1.65 +/- 0.46 h) showed a significant three-fold increase over that reported previously in fasting subjects (0.63 +/- 0.33 h), but this difference is not likely to be of clinical significance.

Cyclosporin clinical pharmacokinetics

Cyclosporin is a powerful immunosuppressive drug used in transplantation medicine and to treat autoimmune diseases. It is a lipophilic molecule, with its bioavailability dependent on food, bile and other interacting factors. Cyclosporin is extensively metabolised in the liver by the cytochrome P450 3A system, which is subject to considerable interindividual variation. Distribution of cyclosporin depends not only on physicochemical characteristics, but also on biological carriers such as lipoproteins and erythrocytes in blood. Cyclophilin, a binding protein for cyclosporin, influences distribution of cyclosporin in the body. Despite its lipophilicity, cyclosporin does not appear in the brain. The distribution of metabolites in the body can differ from that of cyclosporin itself. Elimination of the drug is mainly via the bile as metabolites, other routes not being very important. Pharmacokinetic parameters of cyclosporin are highly variable and depend on factors such as age, the physical condition of the patient, type of organ transplant or comedication. Renal side effects of cyclosporin are dose-related, but the influence of the dosage regimen has not been thoroughly investigated. An important factor in the reported variability is the different analytical methods used. Following the recommendations of recent consensus documents to monitor blood concentrations, this source of variability may diminish in the future. Several metabolites are reported as having less immunosuppressive activity than the parent drug. Metabolites with renal side effects have been reported. These and other effects of metabolites have not been clearly defined in the literature, presumably because of the highly variable activity of cyclosporin-metabolising liver enzymes and the paucity of data available on metabolite pharmacokinetics. The therapeutic range and dosage of cyclosporin are therefore highly dependent on many individual parameters in patients. Dosages of less than 5 mg/kg/day, however, rarely cause renal side effects. Further studies to correlate the clinical pharmacokinetics of metabolites with their activity and adverse effects are needed.

In vitro-in vivo correlation for modified-release formulations

A correlation of in vitro dissolution rate measurements with in vivo pharmacokinetic results in a human study was obtained for a sustained-release formulation of bromocriptine. Different methods were applied to reach this correlation, and the various outcomes were compared.

Cyclosporin pharmacokinetics after intravenous and oral administration in patients with Crohn's disease

Cyclosporin kinetics were estimated after single-dose intravenous and oral administration in 12 patients with Crohn's disease in accordance with a three-compartment model with zero-order inputs. Cyclosporin was measured in whole blood with a specific monoclonal radioimmunoassay. The median bioavailability (f) was 23.7% (range, 0-49.1%); the distribution volume at steady state, 2.3 l/kg (range, 1.0-3.5 l/kg); clearance (CL), 7.6 ml/min/kg (range, 4.8-10.8 ml/min/kg); and t1/2(z) 7.9 h (range, 3.2-13.9 h). Both the extent and rate of bioavailability were significantly lower in six of the patients, who had low or undetectable cyclosporin levels during a preceding therapeutic trial. After repeated oral administration significant correlations were found between the single-dose f/CL ratios and the steady-state blood concentrations, indicating that the kinetics did not change markedly with time. We conclude that the disposition kinetics of cyclosporin in patients with Crohn's disease are comparable to those of other groups, whereas the bioavailability may be decreased. It is suggested that cyclosporin levels should be monitored closely, and intravenous treatment should be considered in patients with a rapid gut transit time, because cyclosporin absorption seems to follow zero-order kinetics.

Chronopharmacokinetics of Cyclosporine A in the Wistar rat following oral administration

The pharmacokinetics of Cyclosporine A (CsA) was studied in male Wistar rats weighting 300 +/- 50 g trained to a 12:12 light-dark cycle. Oral administration (40 mg/kg) was performed at 1 of 4 different temporal stages: 09.00 h, 15.00 h, 21.00 h or 03.00 h (local time) i.e. 0200, 0800, 1400 or 2000 HALO (hours after light on). Blood samples were collected over 72-96 h after dosing, plasma was separated by centrifugation at 37 degrees C and stored frozen until assay, using radioimmunoassay (RIA). Two experiments were performed: the first with 4 groups of 48 rats and a non-specific polyclonal antibody (P-RIA); and the second with only 2 groups of 48 rats and a more specific monoclonal antibody (M-RIA). Plasma concentration data were evaluated with model-based linear pharmacokinetic concepts, with apparent zero-order or first-order absorption and n-exponential disposition (n = 1, 2 or 3): models MN0 or MN1. A compartment-independent approach was also conducted and led to area under the plasma concentration-time curve (AUC) and mean residence time (MRT) determinations. A comparison of the pharmacokinetic profiles across time of administration indicates that absorption, first-pass metabolism and tissue distribution of CsA in the rat are circadian-dosing stage dependent.

In vivo evaluation of the semi-simultaneous method for bioavailability estimation using controlled intravenous infusion as an 'extravascular' route of administration

A recently proposed method for bioavailability estimation, called the semi-simultaneous method, was evaluated in vivo in rats using methysergide as a test substance. In this method the test and the reference dose are administered with a short time interval and a model including the bioavailability parameters is fitted to the concentration-time profile. In the present study, in order to control the true bioavailability, intravenous infusion was used to mimic extravascular administration and various input profiles were produced. Mono-, bi- and triexponential disposition functions with the true and also various erroneous input models were fitted to the individual data sets. The models were also fitted to truncated data sets to mimic a situation where a long duration of sampling is precluded. A combined fitting-deconvolution procedure was also applied. The simi-simultaneous method gave precise and accurate estimates of the bioavailability in most groups and a robustness in the estimate concerning the model fitted was noted. The true input model could be identified for all data sets using common goodness-of-fit criteria. In the groups where a 'flip-flop' situation was created (slower input than elimination) a poorer precision and accuracy and a higher sensitivity concerning the model fitted was observed. The model fitting and the fitting-deconvolution procedure generally gave very similar results.