Quantitation of geometric isomers of a phosphodiesterase inhibitor in rat and monkey plasma using liquid chromatography-tandem mass spectrometry.

Quantitation of geometric isomers of a phosphodiesterase inhibitor was required to determine the extent of interconversion following dosing of a single isomer in preclinical pharmacokinetic studies. Assays were developed for the simultaneous determination of Compound A (Fig. 1), 6-[1-methyl-1-(methylsulfonyl)ethyl-8(3-{(E)-2-(3-methyl-1,2,4-oxadiazol-5-yl)-2-[4-(methylsulfonyl)phenyl]ethenyl}phenyl)quinoline] and its geometric Z-isomer, Compound B, in plasma using liquid chromatography-tandem mass spectrometry. Sample clean-up was performed using a semi-automated liquid-liquid extraction procedure. Separation was achieved on a Phenomenex Synergi MAX-RP column. The method was validated in the linear range of 2-2000 ng/mL for Compound A and 0.5-500 ng/mL for Compound B in plasma and successfully applied to preclinical pharmacokinetic studies. Compound A was dosed in rats and Compound B in monkeys and the degree of conversion was determined by comparing the area under the curve. The relative amount of conversion was less than 1 and 10% in rats and monkeys, respectively. Because of the small amount of conversion and minor peak tailing of the dosed geometric isomer, the order of elution of the two analytes was important in order to achieve best quantitative results. The minor component needs to elute first; thus, a second assay was developed in which the order of elution was reversed. This was achieved by changing the mobile phase modifier.

Population pharmacokinetic modeling for enterohepatic recirculation in Rhesus monkey.

Enterohepatic recirculation (EHR) occurs via biliary excretion and intestinal reabsorption of a drug. Drug recycling through EHR can lead to a change in pharmacokinetic (PK) properties, such as reduced clearance (CL), extended half-life (T(1/2)) and increased plasma exposure (AUC). As a result, EHR may prolong the pharmacological effect of drugs. In the present study, the compound (Cpd A) was found to exhibit EHR in Rhesus monkeys associated with a reduction in CL (from 3.8 to 0.33 Lh(-1), IV; from 2.3 to 0.4 Lh(-1), PO), and an increase in T(1/2) (from 0.9 to 18 h, IV) and in AUC (from 1.5 to 17.4 microg h/mL, IV; from 2.8 to 16.3 microg h/mL, PO), by comparing the PK in the monkeys via the interruption of EHR (bile-duct cannulation) with that in the intact monkeys. A population four-compartment model was constructed based on recirculation loops incorporating all possible inputs (bile secretion, a lag-time model for gall bladder emptying, routes and amounts of a single dose administration) to fully evaluate the EHR of Cpd A. The plasma concentrations versus time profiles predicted from the model had a good fit to the values observed in the subjects and were further simulated with 90% confidence interval to demonstrate its utility. Thus, the model could be applied as a useful tool to evaluate the drugs or compounds that undergo EHR in different species.