Determination of pilocarpic acid in human plasma by capillary gas chromatography with mass-selective detection.

A novel, highly sensitive method for the determination of pilocarpic acid (PA) in human plasma is described. In addition, the method provides for the conversion of the lactone, pilocarpine (P), to PA so that a total drug presence can be determined. Using novel high-performance liquid chromatographic conditions capable of separating P, isopilocarpine (I-P), PA and isopilocarpic acid (I-PA) from each other and from endogenous plasma impurities, it was confirmed that P exclusively and quantitatively converts to PA in heparinized human plasma during storage. For the determination of PA, the selective extraction of PA from protein-free plasma was accomplished using two different solid-phase extraction (SPE) cartridges in two consecutive SPE steps. After extraction, PA was lactonized with trifluoroacetic acid back to P, and both P and an internal standard were acylated using heptafluorobutyric anhydride (HFBA). The trifluoroacetylated derivatives were monitored using gas chromatography (GC) with mass spectrometric (MS) detection. This procedure allowed the sensitive and reliable determination of PA with a limit of quantification (LOQ) of 1 ng/ml, which could not be achieved using previously described methods. The assay was validated in the concentration range of 1 to 10 ng/ml with an intra-day precision (expressed as the coefficient of variation, C.V.) ranging from 9.9 to 0.5%. Inter-day precision for the quality control standard at 2.5 ng/ml showed a C.V. of 10.2%. Accuracy ranged from 94 to 102%. The assay was used to monitor the maximum systemic exposure to P, administered by the ocular route, in terms of total plasma PA (P and PA).

Stereoselective determination of R-(+)- and S-(-)-remoxipride, a dopamine D2-receptor antagonist, in human plasma by chiral high-performance liquid chromatography.

A stereoselective high-performance liquid chromatographic (HPLC) method is described for the selective and sensitive quantitation in human plasma of R-(+)- and S-(-)-enantiomers of remoxipride. Remoxipride was extracted from basified plasma into hexane-methyl-tert.-butyl ether (20:80, v/v), washed with sodium hydroxide (1.0 M), then back-extracted into phosphoric acid (0.1 M). A structural analog of remoxipride was used as an internal standard. The sample extracts were chromatographed using a silica-based derivatized cellulose chiral column, Chiralcel OD-R, and a reversed-phase eluent containing 30-32% acetonitrile in 0.1 M potassium hexafluorophosphate. Ultraviolet (UV) absorbance detection was performed at 214 nm. Using 0.5-ml plasma aliquots, the method was validated in the concentration range 0.02-2.0 microg/ml and was applied in the investigation of systemic inversion of remoxipride enantiomers in man.

Enantioselective high-performance liquid chromatographic determination of omeprazole in human plasma.

A new stereoselective HPLC assay was developed to isolate omeprazole enantiomers from human plasma using C2 solid-phase extraction cartridges and an analogue was used as internal standard. Recoveries of the (+)-isomer were 83.4 and 89.7% at 100 and 250 ng/ml, respectively. Recoveries of the (-)-isomer were 78.4 and 82.8%, respectively. Recovery of the internal standard averaged 77.2%. Direct chiral separation of the enantiomers is achieved on a Resolvosil BSA-7 chiral column (15 cm x 4 mm I.D.) and a matching guard column. The mobile phase is a variable amount of n-propanol (0.05-1.0%) in 0.05 M ammonium phosphate buffer (pH 7.0) and the flow-rate is 1.5 ml/min. Drug absorbance is monitored at 302 nm. Standard curves are linear from 15 to 250 ng/ml for each enantiomer. The coefficients of variation for intra-day precision at each concentration over the range of the standard curve were between 0.98 and 10.87%. The coefficients of variation for inter-day precision for the analyses of omeprazole enantiomers in plasma (30 and 175 ng/ml) were less than 10% over a four month interval.

Pharmacokinetics and bioinversion of ibuprofen enantiomers in humans.

An open, randomized, six-way crossover study was conducted in 12 healthy males to assess pharmacokinetics and bioinversion of ibuprofen enantiomers. The mean plasma terminal half-life (t1/2) of R(-)ibuprofen was 1.74 hr when intravenously infused as a racemic mixture and was 1.84 hr when intravenously infused alone. The mean t1/2 of S(+)ibuprofen was 1.77 hr when dosed as S(+)ibuprofen. Examination of values of both the absorption and disposition parameters of R(-)ibuprofen revealed that the kinetics of R(-)ibuprofen were not altered by concurrent administration of S(+)ibuprofen. In this study, there was little or no presystemic inversion of R(-)ibuprofen to its S(+)isomer. Also, 69% of the intravenous dose of R(-)ibuprofen was systemically inverted and 57.6% of the oral dose of R(-)ibuprofen lysinate was bioavailable as S(+)ibuprofen. These results indicate that the bioinversion of R(-)ibuprofen administered orally is mainly systemic. Because bioinversion of R(-)ibuprofen is not complete, S(+)ibuprofen produced higher bioavailability of S(+)ibuprofen (92.0%) than either racemic ibuprofen (70.7%) or R(-)ibuprofen (57.6%). However, bioavailability of R(-)ibuprofen (83.6%) when dosed alone was not significantly different from when dosed as racemic mixture (80.7%).

High-performance liquid chromatographic determination of cilastatin in biological fluids.

Cilastatin, a dehydropeptidase-I inhibitor, is coadministered with the beta-lactam antibiotic imipenem. The described procedure was developed for quantification of cilastatin in human plasma and urine. The assay involved sample purification on a C18 extraction cartridge, reversed-phase high-performance liquid chromatography with post-column derivatization and fluorescence detection. Standard curves were linear from 0.75 to 75.0 micrograms/ml in plasma and from 2.5 to 200.0 micrograms/ml in urine. Intra-day mean coefficients of variation at concentrations within the standard curve range were 4.2 +/- 2.4% and 3.1 +/- 1.7% in plasma and urine, respectively. The inter-day coefficients of variation for analyses of cilastatin in plasma (1.0 and 50.5 micrograms/ml) were less than 10% after 31 days of analysis while those for urine (5.0 and 74.1 micrograms/ml) were less than 11% after 44 days of analysis. The limits of reliable detection were 0.75 and 2.5 micrograms/ml in plasma and urine, respectively. This procedure met the sensitivity and specificity requirements for the analysis of samples from clinical pharmacokinetic studies.

Pharmacokinetics of imipenem and cilastatin in volunteers.

Imipenem/cilastatin sodium consists of imipenem, a broad-spectrum carbapenem antimicrobial agent, and cilastatin sodium, an inhibitor of dehydropeptidase I, the renal enzyme that catalyzes the metabolism of imipenem. When imipenem is administered alone by the intravenous route, the levels excreted in the urine are low and variable (6%-38% of the dose) between subjects. Kinetics of imipenem in plasma are less variable, and the half-life of imipenem in plasma is 1 hr. When imipenem is coadministered with an equal amount of cilastatin, the amount of imipenem excreted in the urine represents 70% of the plasma clearance and the plasma half-life remains at 1 hr. Whether administered alone or with imipenem, the urinary excretion of cilastatin is 70%-80% of the dose administered, and its plasma half-life is also 1 hr. Thus, the pharmacokinetics of both agents are linear across the therapeutic dose range, and no accumulation of these agents occurs for therapeutic regimens. Decreases in renal function slow the elimination of both compounds and require a reduction in dosage when the glomerular filtration rate is less than 30 ml/min per 1.73 m2.

Imipenem/cilastatin: pharmacokinetic profile in renal insufficiency.

The pharmacokinetics of 250 mg each of intravenous imipenem and cilastatin in combination and 250 mg of cilastatin alone were studied in subjects with varying degrees of renal clearance. Renal disease did not change the volume of distribution of either drug. When the glomerular filtration rate was greater than 100 ml per minute, 48.6 percent of imipenem and 57.1 percent of cilastatin were recovered unchanged in the urine. Consequently, as the glomerular filtration rate declined, the half-life of imipenem increased from 1.02 hours in normal subjects to 3.69 hours in those undergoing dialysis. For cilastatin, the comparable values were 0.86 and 17.08 hours, respectively. The kinetics of cilastatin were unaffected by imipenem. The greater effect of renal disease on the half-life of cilastatin was due to a concomitant 87 percent reduction in nonrenal clearance. Both drugs were removed by hemodialysis. Dialysis reduced the half-life of imipenem from 4.80 to 2.45 hours and that of cilastatin from 16.63 to 3.86 hours. Therefore, dose adjustments will be required in patients with markedly reduced renal function and supplemental dosing will be required after hemodialysis.

Disposition of radiolabeled imipenem and cilastatin in normal human volunteers.

In the first of two successive studies, four healthy male subjects received 500 mg of 14C-labeled imipenem alone and together with 500 mg of unlabeled cilastatin sodium. In the second study, the same subjects were given 250 mg of 14C-labeled cilastatin sodium alone and together with 250 and 1,000 mg of cold imipenem. Concentrations of imipenem and cilastatin in plasma, urine, and feces were assayed by high-pressure liquid chromatography and radiometry. Plasma concentrations of imipenem assayed radiometrically were higher than those measured by high-pressure liquid chromatography. In one subject studied at the end of drug administration, the open lactam metabolite of imipenem represented 9% of the radioactivity. Plasma levels of cilastatin determined by high-pressure liquid chromatography and radiometry were virtually identical. Urinary recovery of imipenem varied between 12 and 42% of the dose when that drug was given alone but increased to between 64 and 75% when administered with cilastatin sodium at a 1:1 ratio. Almost all radioactivity of imipenem was recovered in the urine within 96 h after drug administration. The open lactam metabolite, resulting from the metabolism of imipenem in the kidneys by a dipeptidase, dehydropeptidase-I, represented 80 to 90% of the effluent radioactivity when imipenem was given alone and about 20% when cilastatin sodium was coadministered. Renal excretion of cilastatin followed closely that of imipenem. Almost all of the administered radioactivity was recovered in 24 h, and about 75% of the dose was recovered as unchanged cilastatin within 6 h. The N-acetyl metabolite of cilastatin was found to represent about 12% of the total radioactivity.