To gain more insight into the mechanistic processes controlling the kinetics of inotropic response of digoxin in the perfused whole heart, an integrated kinetic model was developed incorporating digoxin uptake, receptor binding (Na+-K+-ATPase inhibition), and cellular events linking receptor occupation and response. The model was applied to data obtained in the single-pass Langendorff-perfused rat heart for external [Ca2+] of 0.5 and 1.5 mM under control conditions and in the presence of the reverse-mode Na+/Ca2+ exchange inhibitor KB-R7943 (0.1 μM) in perfusate. Outflow concentration and left ventricular developed pressure data measured for three consecutive doses (15, 30, and 45 μg) in each heart were analyzed simultaneously. While disposition kinetics of digoxin was determined by interaction with a heterogeneous receptor population consisting of a high-affinity/low-capacity and a low-affinity/high- capacity binding site, response generation was >80% mediated by binding to the high-affinity receptor. Digoxin sensitivity increased at lower external [Ca2+] due to higher stimulus amplification. Coadministration of KB-R7943 significantly reduced the positive inotropic effect of digoxin at higher doses (30 and 45 μg) and led to a saturated and delayed receptor occupancy-response relationship in the cellular effectuation model. The results provide further evidence for the functional heterogeneity of the Na+-K+-ATPase and suggest that in the presence of KB-R7943 a reduction of the Ca2+ influx rate via the reverse mode Na+/Ca2+ exchanger might become the limiting factor in digoxin response generation.
The study aimed to characterize the pharmacokinetics of lacosamide, a new antiepileptic drug, in rats after intravenous and oral administration at doses of 1, 3, 10, and 30 mg/kg. Moreover, brain distribution and plasma protein binding were estimated. After intravenous injection, terminal half-life, systemic clearance, and steady state volumes of distribution remained unaltered as a function of dose with values in the range 3.01-3.53 h, 221-241 mL/h/kg and 702-732 mL/kg, respectively. Following oral administration, absolute oral bioavailability was not dose dependent and was at 93.3-106%. However, the time to peak concentration and the dose-normalized peak concentration for 30 mg/kg were significantly different with those for other doses. The extent of urinary excretion of lacosamide was 17.1% and 16.5% for intravenous and oral doses, respectively, whereas fecal excretion was negligible. The brain to plasma ratio of lacosamide was consistent regardless of post-dosing time and the brain to plasma partition coefficient was 0.553. Further, the plasma protein binding of lacosamide was concentration independent with free fraction at 95.9%. Lacosamide showed linear pharmacokinetics at an intravenous dose of 1-30 mg/kg and an oral dose of 1-10 mg/kg but non-linear pharmacokinetics at a 30 mg/kg oral dose.
A liquid chromatography-tandem mass spectrometric (LC/MS/MS) method was developed for the determination of an atypical antipsychotic drug, lurasidone, in rat plasma. The method involves the addition of acetonitrile and ziprasidone (internal standard) solution to plasma samples, followed by centrifugation. An aliquot of the supernatant was diluted with water and directly injected into the LC/MS/MS system. The separations were performed on a column packed with octadecylsilica (5 μm, 2.0 × 50 mm) with 0.1% formic acid and 0.1% formic acid in acetonitrile as mobile phase and the detection was performed using tandem mass spectrometry by multiple-reaction monitoring via an electrospray ionization source. The standard curve was linear (r = 0.9982) over the concentration range 0.002-1 μg/mL. The intra- and inter-assay precisions were 1.7 and 8.6%, respectively. The accuracy range was from 90.3 to 101.8%. The lower limit of quantification was 2.0 ng/mL using 50 μL of rat plasma sample. The developed analytical method was successfully applied to the pharmacokinetic study of lurasidone in rats.
A rapid, simple and sensitive liquid chromatography-tandem mass spectrometry (LC/MS/MS) was developed for the determination of an antiepileptic drug, lacosamide, in rat plasma. The method involves the addition of acetonitrile and internal standard solution to plasma samples, followed by centrifugation. An aliquot of the supernatant was diluted with water and directly injected into the LC/MS/MS system. The separations were performed on column packed with octadecylsilica (5 µm, 2.0 × 50 mm) with 0.1% formic acid and acetonitrile as mobile phase, and the detection was performed on tandem mass spectrometry by the multiple-reaction monitoring via an electrospray ionization source. The standard curve was linear over the concentration range from 0.3 to 1000 ng/mL. The lower limit of quantification was 0.3 ng/mL using 50 μL of rat plasma sample. The intra- and inter-assay precision and accuracy were found to be less than 11.7 and 8.8%, respectively. The developed analytical method was successfully applied to the pharmacokinetic study of lacosamide in rats.
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