The chinchilla microdialysis model for the study of antibiotic distribution to middle ear fluid.

In cases of slow or limited penetration of an antibiotic to the site of infection such as in acute otitis media (the middle ear), plasma levels of the agent may not reflect the concentrations that are relevant in determining clinical outcome. There is a need for a model that allows prediction of the time-course of unbound, pharmacologically active drug levels in middle ear fluid (MEF). This article introduces microdialysis as a sampling tool to measure unbound antibiotic concentrations in the MEF of the chinchilla, and briefly summarizes the results of studies of MEF penetration of a cephalosporin, a macrolide, and a ketolide antibiotic using this technique. The general concurrence of preliminary results of the chinchilla studies with clinical findings suggests that the chinchilla microdialysis model may be useful in predicting efficacy in patients.

Microdialysis studies of the distribution of antibiotics into chinchilla middle ear fluid.

For conditions such as acute otitis media, in which antibiotic penetration into middle ear fluid (MEF) may be slow or limited, antibiotic plasma levels may not reflect the concentrations at the site of infection that are relevant to clinical outcome. In such cases, a model is needed that will enable prediction of the time course of unbound, pharmacologically active antibiotic levels in MEF. We describe the use of microdialysis as a sampling tool for measurement of unbound antibiotic concentrations in the MEF of the awake, freely moving chinchilla. Results of studies of MEF penetration of the beta-lactam antibiotic, cefdinir, with use of this technique are also described. Preliminary results of studies of the penetration of antibiotics into MEF of the chinchilla appear consistent with clinical findings and suggest that the chinchilla microdialysis model may prove to be a useful tool for predicting antibiotic efficacy in patients.

The application of sample pooling methods for determining AUC, AUMC and mean residence times in pharmacokinetic studies.

For high-throughput screening in drug development, methods that can reduce analytical work are desirable. Pooling of plasma samples from an individual subject in the time domain to yield a single sample for analysis has been used to estimate the area under the concentration-time curve (AUC). We describe a pooling procedure for the estimation of the area under the first moment curve (AUMC). The mean residence time (MRT), and where intravenous dosing has been used, the steady-state volume of distribution can then be determined. Plasma samples from pharmacokinetic studies in dogs and humans analyzed in our laboratory were used to validate the pooling approach. Each plasma sample containing a prokinetic macrolide and three of its metabolites was first analyzed separately, and AUCs and AUMCs were calculated using the linear trapezoidal rule. The procedures for the estimation of AUC by sample pooling have been reported by Riad et al. [Pharm. Res. (1991) vol. 8, pp. 541-543]. For the estimation of AUMC, the volume taken from each of n samples to form a pooled sample is proportional to t(n)(t(n+1) - t(n-1)), except at t0 where the aliquot volume is 0 and at t(last) where the aliquot volume is proportional to t(last)(t(last) - t((last)-1)). AUMC to t(last) is equal to C(pooled) x T2/2, where T is the overall experimental time (t(last) - t0). The ratio between AUMC and AUC yields the mean residence time (MRT). Bivariate (orthogonal) regression analysis was used to assess agreement between the pooling method and the linear trapezoidal rule. Bias and root mean square error were used to validate the pooling method. Orthogonal regression analysis of the AUMC values determined by pooling (y-axis) and those estimated by the linear trapezoidal rule (x-axis) yielded a slope of 1.08 and r2 of 0.994 for the dog samples; slope values ranged from 0.862 to 0.928 and r2 values from 0.838 to 0.988 for the human samples. Bias, expressed as percentage, ranged from -25.1% to 14.8% with an overall average of 1.40%. The results support the use of a pooled-sample technique in quantitating the average plasma concentration to estimate areas under the curve and areas under the first moment curve over the sampling time period. Mean residence times can then be calculated.

Simultaneous intravenous and intramiddle-ear dosing to determine cefditoren influx and efflux clearances in middle ear fluid in freely moving chinchillas.

This study was conducted to determine cefditoren (CDTR) transport kinetics between plasma and middle ear fluid (MEF) by characterizing influx (CLin) and efflux (CLout) clearances expressed in terms of unbound concentrations and their ratio. Simultaneous intravenous bolus and intramiddle-ear dose were administered to two groups of chinchillas: normal control and infected. In vivo microdialysis was employed to determine protein-unbound CDTR levels in MEF. Compartmental and noncompartmental analysis was performed. Parameters determined in both groups were compared to assess the effect of infection and inflammation on CDTR distribution kinetics. CLin and CLout estimates obtained by compartmental and noncompartmental analysis agreed well. The calculated CLin/CLout ratio was 0.76 +/- 0.23 and 0.56 +/- 0.25 in normal (n = 9) and infected (n = 6) animals, respectively. The 95% confidence interval of this ratio in both groups does not include unity. Statistical tests showed no difference (p > 0.05) in CLin, CLout, and their ratio between the two groups. In conclusion, middle ear infection and inflammation does not affect CDTR distribution. The CLin/CLout ratio determined in chinchillas compares well with values estimated from data in pediatric patients. An active efflux mechanism in middle ear mucosa may be involved in CDTR distribution in MEF.

Intestinal absorption and presystemic elimination of the prokinetic agent, EM574, in the rabbit.

The purpose of this study was to characterize the pharmacokinetics and dose proportionality of the prokinetic macrolide, EM574, in rabbits following intravenous dosing, and to determine the intestinal absorption and intestinal and hepatic first-pass elimination of EM574 in rabbits. Two doses (0.05 and 0.25 mg/kg) of EM574 were given to rabbits intravenously in a crossover study. In a separate gut perfusion study, rabbit duodenal or jejunal segments were perfused with EM574 solution at 0.2 mL/min for 130 min. Plasma levels of EM574 were determined by a validated LC-MS/MS assay, and concentrations in perfusate were determined by HPLC with UV detection. The absorptive clearance (PeA) of EM574 was calculated from the steady-state rate of disappearance from the gut lumen during perfusion. The cumulative amount (A(app)) of drug appearing in the systemic circulation was calculated by deconvolution, where the input response was the plasma concentration-time profile during intestinal perfusion and the unit impulse response was the mean profile following intravenous bolus dosing to sham-operated rabbits in a separate experiment. F(g)F(h) was calculated from the ratio of A(app) to the total amount disappeared from gut lumen during perfusion. Hepatic first-pass elimination was measured by intraportal venous infusion. EM574 exhibits linear kinetics over the dose range studied. CL, V(ss), and terminal half-life (mean +/- SD) of EM574 were 68.6 +/- 15.5 mL/min/kg, 13.4 +/- 3.0 L/kg, and 2.7 +/- 0.8 h, respectively. EM574 is expected to be absorbed completely from the rabbit small intestine based on its high jejunal PeA values (8.1 +/- 2.2, and 5.5 +/- 1.5 microL/min/cm following low and high dose perfusion, respectively). The first-pass extraction of EM574 was substantial and dose independent. Mean F(g) and F(h) were 0.14 and 0.20, respectively, suggesting that the intestinal and hepatic first-pass elimination of EM574 were comparable. Deconvolution was successfully applied in the determination of gut wall and hepatic first-pass elimination of EM574.