Bioequivalence of an ezetimibe/simvastatin combination tablet and coadministration of ezetimibe and simvastatin as separate tablets in healthy subjects

OBJECTIVE

To assess the bioequivalence of an ezetimibe/simvastatin (EZE/SIMVA) combination tablet compared to the coadministration of ezetimibe and simvastatin as separate tablets (EZE + SIMVA).

METHODS

In this open-label, randomized, 2-part, 2-period crossover study, 96 healthy subjects were randomly assigned to participate in each part of the study (Part I or II), with each part consisting of 2 single-dose treatment periods separated by a 14-day washout. Part I consisted of Treatments A (EZE 10 mg + SIMVA 10 mg) and B (EZE/SIMVA 10/10 mg/mg) and Part II consisted of Treatments C (EZE 10 mg + SIMVA 80 mg) and D (EZE/SIMVA 10/80 mg/mg). Blood samples were collected up to 96 hours post-dose for determination of ezetimibe, total ezetimibe (ezetimibe + ezetimibe glucuronide), simvastatin and simvastatin acid (the most prevalent active metabolite of simvastatin) concentrations. Ezetimibe and simvastatin acid AUC(0-last) were predefined as primary endpoints and ezetimibe and simvastatin acid Cmax were secondary endpoints. Bioequivalence was achieved if 90% confidence intervals (CI) for the geometric mean ratios (GMR) (single tablet/coadministration) of AUC(0-last) and Cmax fell within prespecified bounds of (0.80, 1.25).

RESULTS

The GMRs of the AUC(0-last) and Cmax for ezetimibe and simvastatin acid fell within the bioequivalence limits (0.80, 1.25). EZE/ SIMVA and EZE + SIMVA were generally well tolerated.

CONCLUSIONS

The lowest and highest dosage strengths of EZE/SIMVA tablet were bioequivalent to the individual drug components administered together. Given the exact weight multiples of the EZE/SIMVA tablet and linear pharmacokinetics of simvastatin across the marketed dose range, bioequivalence of the intermediate tablet strengths (EZE/SIMVA 10/20 mg/mg and EZE/SIMVA 10/40 mg/mg) was inferred, although these dosages were not tested directly. These results indicate that the safety and efficacy profile of EZE + SIMVA coadministration therapy can be applied to treatment with the EZE/SIMVA tablet across the clinical dose range.

Characterization of the solubility of a poorly soluble hydroxylated metabolite in human urine and its implications for potential renal toxicity

The solubility, in human urine, of the major hydroxylated metabolite (M1) of an experimental cognition enhancer was characterized through a series of in vitro experiments in an effort to estimate the probability of crystalluria occurring following oral administration of the parent compound. The aim of these experiments was to determine if a safety margin existed between clinically observed urine concentrations and the solubility of M1. The mean urine concentrations of M1 in young and elderly subjects following oral administration of the parent compound at the highest doses tested, were 4865 +/- 2368 ng/mL and 2764 +/- 791 ng/mL, respectively. In vitro solubility experiments with M1 were conducted in drug-free human urine (37 degrees C) from four male and four female healthy subjects under conditions of high and low urine osmolality. Mean concentrations (n = 16) of M1 in human urine to which solid M1 was added, were 3656 +/- 621 ng/mL, 4678 +/- 1169 ng/mL and 5378 +/- 2474 ng/mL after stirring for 24, 48 and 72 h, respectively, indicating that the ex vivo mean solubility of M1 in human urine is no greater then approximately 5 microg/mL. Addition of solid M1 to urine from human subjects dosed with the parent compound resulted in mean urine M1 concentrations 23.5% greater than those observed in vivo. The results from both experiments indicated a significant overlap between urine concentrations of M1 in vivo following the highest oral administration of the parent drug and M1 solubility measured in vitro, suggesting a high potential for in vivo saturation of urine with M1 with subsequent precipitation, crystalluria, and nephrotoxicity. Consequently, the results of these studies have placed restrictions on the dose that could be administered during clinical development of this compound.

High-throughput sample preparation procedures for the quantitation of a new bone integrin ανβ3 antagonist in human plasma and urine using liquid chromatography–tandem mass spectrometry

High throughput LC-MS/MS assays to quantitate a new alpha(nu)beta(3) bone integrin antagonist (I) in human plasma and urine have been developed using instruments programmed to automate sample preparation procedures. Packard liquid handling system-MultiPROBE II EX was programmed for preparing calibration standards in control plasma and urine, acidifying all standards, quality control (QC), and clinical samples with necessary dilutions, and adding the internal standard to the acidified samples. TOMTEC Quadra 96 was programmed to perform the solid phase extraction (SPE) process on a 3M 96-well mixed phase cation standard density (MPC-SD) plate to isolate the analytes from the sample matrix. The extract collected from both types of matrices was directly injected into reversed-phase LC-MS/MS system with a Turbo Ion Spray (TIS) interface in the positive ionization mode. The plasma and urine assays have the calibration range of 0.5-1500 and 2-6000 ng/mL, respectively. Validation of the automated and the manual plasma assays showed that application of MultiPROBE II to sample preparation gave comparable accuracy and precision. Overall, the automated approaches with minimum manual intervention enhanced the throughput of sample preparation.

Pharmacokinetics of total and unbound ertapenem in healthy elderly subjects

Ertapenem is a new once-a-day parenteral carbapenem antimicrobial agent. The pharmacokinetics of unbound and total concentrations of ertapenem in plasma were investigated in elderly subjects and compared with historical data from young adults. In a single- and multiple-dose study, healthy elderly males and females (n = 14) 65 years old or older were given a 1-g intravenous (i.v.) dose once daily for 7 days. Plasma and urine samples collected for 24 h on days 1 and 7 following administration of the 1-g doses were analyzed by reversed-phase high-performance liquid chromatography. Areas under the concentration-time curve from 0 h to infinity (AUC(0- infinity )) for elderly females and males were similar following administration of 1-g single i.v. doses, and thus, the genders were pooled in subsequent analyses. Concentrations in plasma and the half-life of ertapenem were generally higher and longer, respectively, in elderly subjects than in young adults. The mean AUC(0- infinity ) of total ertapenem in the elderly was 39% higher than that in young subjects following administration of a 1-g dose. The differences were slightly greater for the mean AUC(0- infinity ) of unbound ertapenem (71%). The unbound fraction of ertapenem in elderly subjects ( approximately 5 to 11%) was generally greater than that in young adults ( approximately 5 to 8%). As in young adults, ertapenem did not accumulate upon multiple dosing in the elderly. The pharmacokinetics of ertapenem in elderly subjects, while slightly different from those in young adults, do not require a dosage adjustment for elderly patients.

LC-MS/MS determination of a farnesyl transferase inhibitor in human plasma and urine

To support clinical pharmacokinetic studies in cancer patients, sensitive and specific methods for measuring 4-[1-(4-cyanobenzyl)-5-imidazolylmethyl]-1-(3-chlorophenyl) piperazinone (I), a farnesyl transferase inhibitor (FTI), in human plasma and urine were developed and validated. The methods are based on high-performance liquid chromatography (HPLC) with atmospheric pressure chemical ionization (APCI) and tandem mass spectrometric (MS/MS) detection in the positive ion mode using a heated nebulizer interface. Drug and internal standard were isolated from plasma or basified urine using automated solid-phase extraction on cyano cartridges. The organic extracts were dried, reconstituted in aqueous acetonitrile and injected into the system. Chromatographic separation of I and internal standard (IS) was achieved using a BDS Hypersil C8 analytical column, with a mobile phase consisting of acetonitrile:methanol:water (50:4:46) and trifluoroacetic acid (0.05%) at a flow rate of 0.6 ml/min. MS/MS detection was performed on a PE-Sciex API 300 tandem mass spectrometer operated in selected reaction monitoring mode. The parent-->product ions monitored were m/z 406-->195 for analyte I and m/z 448-->195 for the internal standard. Unusual in this method is that quantitation is accomplished using a secondary product ion, m/z 195, of drug I and IS. The assays were validated over the concentration range of 0.5-1000 ng/ml (1.2 nM to 2.5 microM, respectively) in plasma, and 2.5-500 ng/ml (6.2 nM to 1.23 microM) in urine. Accuracy was within +/-10% of nominal concentration at all levels in urine, and all but the lowest standard in plasma (+/-14% at 0.5 ng/ml). Intraday precision (expressed as coefficients of variation, CVs) for standard replicates and interday precision for quality control (QC) samples were less than 8% at all concentrations in both matrices. Detailed descriptions of the extraction procedure and analytical methodology used in the assay of I in plasma and urine are presented. This procedure may have utility in the quantitation of other imidazole-based FTIs with cyanobenzyl substructures.

Pharmacokinetics of ertapenem in healthy young volunteers

Ertapenem (INVANZ) is a new once-a-day parenteral beta-lactam antimicrobial shown to be effective as a single agent for treatment of various community-acquired and mixed infections. The single- and multiple-dose pharmacokinetics of ertapenem at doses up to 3 g were examined in healthy young men and women volunteers. Plasma and urine samples collected were analyzed using reversed-phase high-performance liquid chromatography with UV detection. Ertapenem is highly bound to plasma protein. The protein binding changes from approximately 95% bound at concentrations of <50 micro g/ml to approximately 92% bound at concentrations of 150 micro g/ml (concentration at the end of a 30-min infusion following the 1-g dose). The nonlinear protein binding of ertapenem resulted in a slightly less than dose proportional increase in the area under the curve from 0 h to infinity (AUC(0- infinity )) of total ertapenem. The single-dose AUC(0- infinity ) of unbound ertapenem was nearly dose proportional over the dose range of 0.5 to 2 g. The mean concentration of ertapenem in plasma ranged from approximately 145 to 175 micro g/ml at the end of a 30-min infusion, from approximately 30 to 34 micro g/ml at 6 h, and from approximately 9 to 11 micro g/ml at 12 h. The mean plasma t(1/2) ranged from 3.8 to 4.4 h. About 45% of the plasma clearance (CL(P)) was via renal clearance. The remainder of the CL(P) was primarily via the formation of the beta-lactam ring-opened metabolite that was excreted in urine. There were no clinically significant differences between the pharmacokinetics of ertapenem in men and women. Ertapenem does not accumulate after multiple once-daily dosing.

A rapid and sensitive liquid chromatography/negative ion tandem mass spectrometry method for the determination of an indolocarbazole in human plasma using 96-well diatomaceous earth plates for solid-liquid extraction [correction of using internal standard (IS) 96-well diatomaceous earth plates for solid-liquid extraction]

A sensitive liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for the quantitation of a novel topoisomerase I inhibitor (indolocarbazole derivative I) in human plasma was developed to support clinical studies. Drug and internal standard were isolated from plasma by solid-liquid extraction using 96-well diatomaceous earth plates. Various extraction solvents were evaluated for extraction of I and 9% isopropyl alcohol (IPA) in methyl-tert-butyl ether (MtBE) was chosen as the optimal extraction solvent. The sensitivity of this LC/MS/MS method is 10x higher in negative ion mode using alkaline conditions than in positive ion mode using a wide range of pH's. A mobile phase with 2 mM ammonium hydroxide enhanced the sensitivity in negative ion mode over other volatile bases. The calibration curve for compound I is linear over the range 0.05-200 ng/mL in plasma and the lower limit of quantification (LLOQ) of the assay is 0.05 ng/mL, when 0.25 mL of plasma is processed. The method was fully validated and successfully applied to plasma samples from clinical studies. Performing chromatography at high pH, for enhanced negative ion sensitivity, eliminates the need for post-column addition of base. Furthermore, the 96-well diatomaceous earth plate extraction offers the following advantages over liquid-liquid extraction (LLE) or solid-phase extraction (SPE): clean sample extracts with reduced sample preparation time; increased sample throughput; no conditioning or washing steps; and a neutral eluate applicable to acid/base labile compounds.

Liquid chromatographic-tandem mass spectrometric urine assay for a highly metabolized cyclic ureidobenzenesulfonamide: issues concerning assay specificity and quality control preparation

An LC-MS-MS method was validated for the quantitation of a beta(3) agonist (A) in human urine to support Phase I studies. A was designed to accelerate metabolism for weight reduction. During assay development a significant loss of A was apparent from frozen urine quality control samples. The addition of 0.75% bovine serum albumin (BSA) in urine (v/v) was required to maximize the recovery of A from urine. Urine samples were basified and extracted into methyl t-butyl ether-isopropyl alcohol (90:10, v/v). The organic layer was washed, evaporated, reconstituted, and injected onto a 5 cm, C8 HPLC column prior to MS-MS analysis. The standard curve was linear from 5 to 500 ng/ml. Intraday precision for peak area ratios from BSA urine samples at seven separate concentrations over a range of 5-500 ng/ml (n=5) was <4.0% and calculated concentrations were within 91-115% of nominal concentrations. Interday precision for BSA urine quality control (QC) samples at four separate concentrations (n=10 of each) was <5.0% and individual calculated concentrations were within 90-111% of nominal concentrations. This work emphasizes that potential metabolites and quality control standards should be prepared and assayed as early as possible in method development, especially before the sample collection section of the clinical protocol is prepared. The methods described here have wide utility to other compounds containing basic benzene sulfonamides and to beta3 agonist candidates.

Determination of a beta(3)-agonist in human plasma by LC/MS/MS with semi-automated 48-well diatomaceous earth plate

Methods for the determination of a beta(3)-agonist (A) in human plasma were developed and compared based on high-performance liquid chromatography (HPLC) with tandem mass spectrometric (MS/MS) detection using a turbo ion spray (TIS) interface. Drug and internal standard were isolated from plasma by three sample preparation methods, liquid-liquid extraction, Chem Elut cartridges and 48-well diatomaceous earth plates, that successively improved sample throughput for LC/MS/MS. MS/MS detection was performed on a PE Sciex API 365 tandem mass spectrometer operated in positive ion mode and using multiple reaction monitoring (MRM). The precursor/product ion combinations of m/z 625/607 and 653/515 were used to quantify A and internal standard, respectively, after chromatographic separation of the analytes. Using liquid-liquid extraction and Chem Elut cartridges, the assay concentration range was 0.5-100 ng/ml. Using diatomaceous earth plates, the concentration range of the assay was extended to 0.5-200 ng/ml. For all three assays, the statistics for precision and accuracy is comparable. The assay accuracy ranged from 91-107% and intraday precision as measured by the coefficient of variation (CV) ranged 2-10%. The sample throughput was tripled when the diatomaceous earth plate method was compared with the original liquid-liquid extraction method.

Pharmacokinetics of rizatriptan in healthy elderly subjects

OBJECTIVE

Rizatriptan is a serotonin 5-HT1B/1D receptor agonist for acute treatment of migraine. Its pharmacokinetics were assessed in healthy elderly males and females receiving a single 10 mg tablet oral dose. The pharmacokinetic data (AUC(0-infinity) and Cmax) for the elderly in this study were compared with historical data from previous studies for healthy young adults (n = 65).

METHODS

In a double-blind, parallel, placebo-controlled study, healthy elderly female and male subjects aged 65 or older (n = 8 each) received a single oral dose of 10 mg rizatriptan. Plasma and urine concentrations of drug were determined by HPLC with tandem mass spectrometry detection at several collection time points or intervals starting at predose and postdose over 24 h.

RESULTS

In elderly subjects, the geometric mean values for AUC(0-infinity) and Cmax were 77.7 ng/h/ml and 21.9 ng/ml; the average values for tmax, half-life (t 1/2), renal clearance (Clr), and percent urinary excretion of dose (Ue) were 1.2 h, 1.8 h, 197 ml/min and 9.3%, respectively. The AUC(0-infinity) and Cmax of rizatriptan were similar in elderly and young subjects. The geometric mean AUC ratio of elderly to young was 0.96 with 90% confidence interval (0.83, 1.11), p > 0.25. The geometric mean Cmax ratio was 0.89 with 90% confidence interval (0.72, 109), p > 0.25. No significant pharmacokinetic differences were observed between elderly males and females.

CONCLUSIONS

The plasma pharmacokinetics of rizatriptan appear to be similar in the elderly and young. In the elderly, the pharmacokinetics of rizatriptan do not appear to differ between male and female to a clinically significant extent.

The effects of moclobemide on the pharmacokinetics of the 5-HT1B/1D agonist rizatriptan in healthy volunteers

AIMS

The new 5-HT1B/1D agonist rizatriptan (MK-0462) has recently been registered for the treatment of migraine. Its primary route of metabolism is via monoamine oxidase-A (MAO-A). Antidepressants such as the MAO-A inhibitor moclobemide may be used in patients with chronic headache syndromes. Hence, this study aimed to investigate the interactions between rizatriptan and moclobemide.

METHODS

In a double-blind, randomized, placebo-controlled, two-period cross-over study 12 healthy, young volunteers (six males, six females) were treated with moclobemide (150 mg twice daily) or placebo for 4 days. On the fourth day, a single dose of rizatriptan (10 mg) was administered, and subsequently blood and urine samples were collected for assay of rizatripan and N-monodesmethyl rizatriptan. Plasma concentrates of 3,4-dihydroxyphenylglycol (DHPG), a marker of MAO-A inhibition, were also assessed. Supine and standing blood pressure were measured regularly.

RESULTS

Both treatments were well tolerated. During moclobemide, the increase in supine diastolic blood pressure following rizatriptan administration was augmented. Inhibition of MAO by moclobemide was inferred from a persistent decrease in DHPG level (43% on average). When rizatriptan was coadministered with moclobemide, the area under the plasma drug concentration-time profiles for rizatriptan and its N-monodesmethyl metabolite increased 2.2-fold (90% CI, 1.93-2.47) and 5.3-fold (90% CI, 4.81-5.91), respectively, when compared with placebo. Peak plasma drug concentrations for rizatriptan and its n-monodesmethyl metabolite increased 1.4-fold (90% CI, 1.11-1.80) and 2.6-fold (90% CI, 2.23-3.14), respectively, and half-lives of both were prolonged.

CONCLUSIONS

Moclobemide inhibited the metabolism of rizatriptan and its active N-monodesmethyl metabolite through inhibition of MAO-A. Thus, moclobemide may considerably potentiate rizatriptan action. Concurrent administration of moclobemide and rizatriptan is not recommended.

Lack of pharmacokinetic and pharmacodynamic interaction between rizatriptan and paroxetine

Rizatriptan is a potent, oral 5-HT(1B/1D) agonist with a rapid onset of action being investigated for the acute treatment of migraine. This study examined the clinical and pharmacolinetic interaction between rizatriptan and the selective serotonin reuptake inhibitor, paroxetine. In this two-period crossover study, 12 healthy young subjects (6 males and 6 females) received 1 mg rizatriptan following 14 days of treatment with placebo or paroxetine (20 mg once daily). Plasma was sampled for rizatriptan and N-monodesmethyl rizatriptan, a minor but active metabolite of rizatriptan. Safety evaluations included monitoring for adverse events, vital signs, and visual analog scale assessment of mood. Plasma levels of rizatriptan and N-monodesmethyl rizatriptan were not altered when rizatriptan was administered with paroxetine compared to the placebo. Clinically, coadministration of rizatriptan with paroxetine was well tolerated. Blood pressure, heart rate, and temperature changes during the observation period did not differ to a clinically significant degree when rizatriptan was administered with paroxetine compared to the placebo. No effects on mood occurred following treatment with the combination compared to rizatriptan alone. Adverse events following rizatriptan administration with paroxetine were similar to those reported when rizatriptan was given with the placebo.

High-performance liquid chromatographic methods for the determination of a new carbapenem antibiotic, L-749,345, in human plasma and urine

A column-switching, reversed-phase high-performance liquid chromatographic (HPLC) method for the determination of a new carbapenem antibiotic assay using ultraviolet detection has been developed for a new carbapenem antibiotic L-749,345 in human plasma and urine. A plasma sample is centrifuged and then injected onto an extraction column using 25 mM phosphate buffer, pH 6.5. After 3 min, using a column-switching valve, the analyte is back-flushed with 10.5% methanol-phosphate buffer for 3 min onto a Hypersil 5 microm C18 BDS 100x4.6 mm analytical column and then detected by absorbance at 300 nm. The sample preparation and HPLC conditions for the urine assay are similar, except for a longer analytical column 150x4.6 mm. The plasma assay is specific and linear from 0.125 to 50 microg/ml; the urine assay is linear from 1.25 to 100 microg/ml.

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.

An in vitro comparison of the permeability of prednisolone, prednisolone sodium phosphate, and prednisolone acetate across the NZW rabbit cornea

Controversy and ambiguity in the literature concerning the corneal penetration of prednisolone acetate over Prednisolone Sodium phosphate in NZW rabbits has recently prompted comparative studies using specific chromatographic assays. In vitro, corneal penetration studies were performed using the Ussing Chambers to compare the permeability and flux of both esters and prednisolone at 0.5% using a reversed phase HPLC-UV assay. Chromatograms of samples from the receiver chambers show primarily the presence of prednisolone from both esters; only prednisolone phosphate penetrated the cornea intact. Flux measurements were similar for prednisolone and both salt forms in terms of the metabolite prednisolone. Permeability coefficient calculations give the relative comparison: prednisolone acetate greater than prednisolone greater than prednisolone sodium phosphate.

Assay methodology for prednisolone, prednisolone acetate and prednisolone sodium phosphate in rabbit aqueous humor and ocular physiological solutions

Prednisolone, prednisolone acetate and prednisolone sodium phosphate are glucocorticoids used for ocular, anti-inflammatory therapy. A reversed-phase high-performance liquid chromatographic assay using ultraviolet detection has been developed that affords baseline resolution of the above analytes in balanced salt solutions and rabbit aqueous humor. The drugs can be quantified at 0.025-0.05 micrograms/ml in the above matrices; 6 alpha-methylprednisolone is used as the internal standard. Both esters of prednisolone are vulnerable to chemical and enzymatic hydrolysis giving prednisolone. Analysis of aqueous humor samples shows prednisolone acetate penetrating/metabolizing primarily to prednisolone; prednisolone sodium phosphate penetrates the cornea giving the ester and alcohol.

Comparative corneal penetration of prednisolone sodium phosphate and prednisolone acetate in NZW rabbits

Comparative corneal penetration studies in the literature with prednisolone sodium phosphate solution and prednisolone acetate suspension administered to rabbit eyes give conflicting results concerning the greater bioavailability of prednisolone acetate. A recent in vitro penetration study shows similar fluxes for both salt forms in terms of prednisolone, quantified by a specific HPLC assay. An in vivo comparison has been conducted in NZW rabbit eyes and shows similar bioavailability for both drugs. Prednisolone sodium phosphate, prednisolone acetate and the primary metabolite prednisolone are quantified by a specific HPLC assay in aqueous humor.

Quantification of imipenem's primary metabolite in plasma by postcolumn chemical rearrangement and UV detection

Imipenem (thienamycin formamidine) is an antibiotic active against a broad spectrum of bacteria. Its primary metabolite arises from cleavage of the lactam ring. The metabolite can be formed in-vitro by acid-catalyzed or enzymatic hydrolysis. In animals and man, this metabolite can be generated systemically as well as in the kidneys following the excretion of imipenem into the urine. In man, this dehydropeptidase-catalyzed renal metabolism is minimized by the coadministration of cilastatin, a competitive inhibitor. A specific HPLC assay has been developed to evaluate the disposition of this metabolite in humans having normal or end-stage renal function. The assay employs ion-pair, reversed-phase chromatography, and post-column acid treatment of the analyte for ultraviolet detection.

Simultaneous high-performance liquid chromatographic analysis of carbidopa, levodopa and 3-O-methyldopa in plasma and carbidopa, levodopa and dopamine in urine using electrochemical detection

Two assay procedures are described for the analysis of levodopa, carbidopa and 3-O-methyldopa in plasma and levodopa, carbidopa and dopamine in urine. The methods are suitable for quantifying the analytes following therapeutic administration of levodopa and carbidopa. Both were based on reversed-phase high-performance liquid chromatography (HPLC) with electrochemical detection and with methyldopa as the internal standard. Plasma samples were prepared by perchloric acid precipitation followed by the direct injection of the supernatant. Urine was prepared by alumina adsorption, and the analytes were desorbed with perchloric acid solution containing disodium EDTA and sodium metabisulfite prior to injection into the HPLC system. The methods have been utilized to evaluate the pharmacokinetics and bioavailability of oral dosage forms containing levodopa and carbidopa.

Pharmacokinetics and bioavailability of Sinemet CR: a summary of human studies

The pharmacokinetics of Sinemet CR, a controlled-release formulation containing carbidopa and levodopa, were investigated in healthy young and elderly volunteers and in patients with Parkinson's disease. Sinemet CR produced more sustained plasma levels of levodopa, carbidopa, and 3-O methyldopa than did conventional Sinemet. In elderly subjects, the corresponding steady-state plasma levels fluctuated in narrower ranges with Sinemet CR than those following the administration of Sinemet. Results indicate a levodopa bioavailability of 71% for Sinemet CR, in contrast to a bioavailability of 99% for Sinemet for these subjects. The carbidopa bioavailability of Sinemet CR was 58% relative to that of Sinemet. Systemic decarboxylase inhibition was comparable between the 2 regimens as indicated by the renal clearance of levodopa. The absorption of levodopa was slower and more protracted with Sinemet CR than with Sinemet. Food increased the levodopa bioavailability of Sinemet CR. This increase was attributed to an increased gastric retention time. No dose-dumping occurred with Sinemet CR in either the nonfasting or the fasting state. Levodopa bioavailability was lower in young volunteers than in elderly volunteers. This was attributed to an age-related decrease in gastric emptying and in 1st-pass metabolic decarboxylation in the gastrointestinal (GI) tract. In parkinsonian patients, as in healthy subjects, the Sinemet CR formulation produced more sustained levodopa plasma levels. These patients required a higher total daily dosage of Sinemet CR than of Sinemet for control of parkinsonian symptoms, but less frequent dosing was required during chronic therapy. Peak plasma levodopa levels increased proportionately with increasing Sinemet CR dosage. These observations were consistent with the pharmacokinetic characteristics of the formulation.

A sensitive method for assay of a novel tricyclic compound using coulometric electrochemical detection

A reversed-phase high-performance liquid chromatographic method using coulometric electrochemical detection in the oxidative mode has been developed for the analysis of 3-(9-chloro-5,6-dihydro-11-H-pyrrolo[2,1-b][3]benzazepine-11-ylidene- N,N-dimethyl-1-propanamine(E)-Z-butenedioate hydrogen maleate (1) in plasma of patients dosed with 2-8 mg/kg/d of the drug. Concentrations as little as 0.1 ng/mL of 1 in plasma can be estimated with a mean coefficient of variation of 7.4 +/- 1.08%. The utility of the procedure was demonstrated by the analysis of 500 patient samples from a rising multiple-dose study.

A sensitive gas chromatographic mass spectrometric assay for a novel dopamine agonist (+)-trans-3,4,4A,5,6,10B-hexahydro-4-propyl-2H-naphth(1,2-B)(1,4) oxazin-9-ol in human plasma

(+)-trans-3,4,4A,5,6,10B-Hexahydro-4-propyl-2H-naphth(1,2-B)(1,4) oxazine-9-ol is a novel potent dopamine agonist. A sensitive and specific gas chromatographic/mass spectrometric assay procedure has been developed for the determination of the dopamine agonist at low picogram per millilitre levels in human plasma. The method comprises an extraction of the agonist from human plasma and subsequent derivatization of the phenolic functionality with pentafluoropropionic anhydride. The derivative is quantified by gas chromatographic and mass spectrometric detection using selected ion monitoring. The assay is linear over the concentration range 10-1000 pg ml-1.

Occurrence of beta-hydroxylated asparagine residues in non-vitamin K-dependent proteins containing epidermal growth factor-like domains

Vitamin K-dependent bovine protein S has been shown to contain a posttranslationally hydroxylated asparagine within a conserved sequence in three of its epidermal growth factor (EGF)-like domains. In a review of amino acid sequences deduced from cDNA data, we have observed that a conserved sequence containing a potential asparagine hydroxylation site exists within EGF-like domains of a variety of functionally diverse proteins. We have studied a number of these and report the presence of erythro-beta-hydroxyasparagine (e-beta Hyn) in three non-vitamin K-dependent proteins: the plasma complement proteins C1r and C1s (where overbar indicates activated form) and the urinary protein uromodulin. For each protein, e-beta Hyn was identified in enzyme digests following the initial observation of erythro-beta-hydroxyaspartic acid (e-beta Hya) in acid hydrolysates of the proteins. e beta Hya and e-beta Hyn residues are detected by a postcolumn derivatization cation-exchange HPLC method herein described. HPLC isolation of the presumptive e-beta Hyn residue from enzyme digests of intact C1r allowed confirmation of its structure by GC/MS. Based upon available cDNA sequence data and observation of e-beta Hya in acid hydrolysates, we suggest other proteins in which e-beta Hyn may occur.

A sensitive analytical method for the determination of a novel lipoxygenase inhibitor, 4-bromo-2,7-dimethoxy-3H-phenothiazin-3-one, in plasma using reductive electrochemical detection

A reversed-phase high-performance liquid chromatographic assay using electrochemical detection in the reductive mode has been developed for the analysis of 4-bromo-2,7-dimethoxy-3H-phenothiazin-3-one (1) in plasma to determine drug absorption. Free drug in plasma in concentrations as little as 0.25 ng/mL can be estimated with a mean coefficient of variation (CV) of 6.3 +/- 2.6%. Metabolites which can be converted to the parent drug by acid hydrolysis can be quantified in concentrations of 10 ng/mL or more, with a mean CV of 4.3 +/- 1.9%. To test the procedure, plasma was obtained from dogs receiving 14C-labeled 1. After acid hydrolysis of plasma, the electrochemical assay for parent drug showed good agreement with the radioactive equivalents in plasma, suggesting that parent drug and metabolites can be satisfactorily analyzed by this procedure.

Simultaneous quantification of cycloserine and its prodrug acetylacetonylcycloserine in plasma and urine by high-performance liquid chromatography using ultraviolet absorbance and fluorescence after post-column derivatization

D-Cycloserine is a broad-spectrum antibiotic used with other antibiotics to treat various forms of tuberculosis. Its prodrug sodium (R)-4-[(1-methyl-3-oxo-1-butenyl)amino]-3-isoxazolidinone hemihydrate, developed for better aqueous stability and solubility, is combined with another broad-spectrum antibiotic, fludalanine. An ion-pair, reversed-phase high-performance liquid chromatographic assay has been developed to simultaneously detect cycloserine and its prodrug in plasma and urine. The prodrug is detected directly by ultraviolet absorbance and cycloserine by fluorescence following post-column derivatization.

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.

Unexpected adduct ion formation under chemical ionization conditions

As part of our efforts to characterize the in vivo metabolic fate of the antihypertensive agent alpha-methyldopa, we have examined the urine of alpha-methyldopa-treated rats with the aid of a direct insertion probe chemical ionization mass spectral assay. The mass spectrum of the sample obtained by chromatographic purification followed by treatment with ethanolic hydrochloric acid and pentafluoropropionic anhydride displayed an intense ion at m/z 812, consistent with the beta-ethoxy-N,O,O,O-tetrakispentafluoropropionyl derivative of 6-hydroxy-alpha-methyl-norepinephrine, a potential aromatic hydroxylation product of the known alpha-methyldopa metabolite alpha-methyl-norepinephrine. Comparison of this spectrum with the spectrum obtained with the corresponding synthetic 6-hydroxy-alpha-methylnorepinephrine, however, ruled out this possibility. A more thorough examination of the mass spectral data established that the ion at m/z 812 observed with the metabolic species was due to the formation of an unexpected adduct ion between a known metabolite of alpha-methyldopa and an impurity ion formed from a common constituent of urine. This paper summarizes the characterization of this adduct ion.

Bioanalysis and disposition of alpha-fluoromethylhistidine, a new histidine decarboxylase inhibitor

A sensitive, selective, and rapid high-performance liquid chromatographic procedure was developed for the determination of alpha-fluoromethylhistidine (alpha-FMH) in human biological samples. The plasma assay required isolation of the drug using a weak cation-exchange resin prior to HPLC analysis with UV detection. The urine assay employed postcolumn derivatization with o-phthalaldehyde (without a thiol) and fluorescence detection. The extent of metabolism of alpha-FMH in humans was studied in four healthy volunteers using tritium-labeled material. No significant differences in the plasma and urine concentrations of radioactivity and unchanged drug were detected. In addition, the radiochromatograms of selected urine samples revealed a single peak with a retention time corresponding to the unchanged drug. The evidence presented suggests negligible biotransformation of alpha-FMH in humans.

Assay methodology for quantification of the ester and ether glucuronide conjugates of diflunisal in human urine

Diflunisal is a salicylate derivative with analgesic and anti-inflammatory properties. It is excreted in the urine as an ether glucuronide, a 1-O-acyl glucuronide and as unchanged drug. The 1-O-acyl glucuronide rearranges to isomeric esters of glucuronic acid under neutral to alkaline pH conditions. The development of a urine assay for the conjugates enables the elucidation of diflunisal non-linear pharmacokinetics. The assay quantitates the ether and ester glucuronides and free diflunisal in urine at 0.5-1.0 micrograms/ml. Analysis of the glucuronides does not require authentic standards.

Determination of imipenem (N-formimidoyl thienamycin) in human plasma and urine by high-performance liquid chromatography, comparison with microbiological methodology and stability

High-performance liquid chromatographic (HPLC) methods using ultraviolet (UV) detection have been developed for the assay of the antibiotic imipenem (N-formimidoyl thienamycin) in human plasma and urine. A reversed-phase analytical column is employed in the plasma assay method and a cation-exchange column is used in the urine assay method. Both methods use borate buffer in the mobile phase. The method of preparation of human fluid samples for HPLC injection has been optimized with respect to the stability of imipenem in aqueous buffers, in morpholine buffer--ethylene glycol stabilizer, and in urine and plasma. Preparation of the samples before injection into the HPLC systems involves deproteination/filtration of the plasma/urine samples. The open lactam metabolite and the coadministered dehydropeptidase inhibitor, cilastatin sodium, do not interfere with the 313-nm detection of imipenem in either the plasma or the urine assay. Thienamycin, the precursor of imipenem and an impurity in imipenem formulations, is separated from the drug using both of these methods. Concentrations generated from the HPLC analysis of plasma and urine samples from two healthy volunteers compare favorably with results using a microbiological assay method. Correlation of the two methods gives r greater than or equal to 0.990 for both fluids.

Analytical methods for the determination of sulindac and metabolites in plasma, urine, bile, and gastric fluid by liquid chromatography using ultraviolet detection

A high-performance liquid chromatographic method using a linear elution gradient has been developed for the analysis of sulindac, sulindac sulfone, and sulindac sulfide in plasma, urine, bile, and gastric fluid. The methodology uses reverse-phase, radial compression chromatography with gradient elution, and UV detection. Sulindac and its metabolites in plasma can be quantitated at 0.25 microgram/mL with a mean CV of 6.0 +/- 2.9%; urine, bile, and gastric fluid (0.5 microgram/mL) yield a mean CV of 5.5 +/- 1.9%.

Sensitive high-performance liquid chromatographic assay using electrochemical detection for a novel prodrug ester of methyldopa, pivaloyloxyethyl 3-(3,4-dihydroxyphenyl)-2-methylalaninate, in plasma and urine

The pivaloyloxyethyl ester of methyldopa is an antihypertensive prodrug possessing improved bioavailability properties over methyldopa. A sensitive cation-exchange, high-performance liquid chromatographic assay using electrochemical detection has been developed for the ester in plasma and urine in order to determine the extent of its hydrolysis after oral administration. The chromatographic conditions involve two Altex Partisil 10 SCX columns (25 cm X 4.6 mm) in series; a mobile phase consisting of methanol, potassium phosphate buffer, pH 3.0, and EDTA disodium dihydrate; and an electrochemical detector set at 0.5 V. The pivaloyloxyethyl ester in plasma or urine is extracted into ethyl acetate, back-extracted into 0.1 M sulfuric acid, and analyzed directly by high-performance liquid chromatography. For urine, the ethyl acetate extract is washed with a buffer (pH 8.0) prior to the back-extraction step. The assay gives a linear response over the concentration range of 10-160 ng/ml in plasma and 20-400 ng/ml in urine.

Quantitation of norfloxacin, a new antibacterial agent in human plasma and urine by ion-pair reverse-phase chromatography

A specific and sensitive high-performance liquid chromatographic method for the analysis of norfloxacin in human plasma and urine is described. Norfloxacin was extracted from the sample matrix using dichloromethane under neutral conditions, followed by back extraction into dilute phosphoric acid for chromatographic analysis on a reverse-phase column with a mobile phase consisting of methanol, phosphate buffer, and ion-pairing reagent (pH 3.0) at a flow rate of 2.0 mL/min. The ability of this method to distinguish intact norfloxacin from its metabolites was demonstrated. The method is linear, quantitative, and reproducible for both plasma analysis (0.05-2.5 microgram/mL) and urinalysis (1.0-500 micrograms/mL) using peak area ratios (norfloxacin-internal standard) for quantitation. The stability of norfloxacin and its metabolites in dilute phosphoric acid was studied. To assess the presence of norfloxacin conjugates in the urine of dosed individuals, the effects of urine hydrolysis on drug quantitation were examined. Urine and plasma levels of norfloxacin at selected time points following the administration of single drug doses are presented.

Synthetic and preliminary hemodynamic and whole animal toxicity studies on (R,S)-, (R)-, and (S)-2-methyl-3-(2,4,5-trihydroxyphenyl)alanine

The synthesis, resolution, and absolute configuration assignment of 2-methyl-3-(2,4,5-trihydroxphenyl)alanine (6-OH-alpha-Me-Dopa) are reported. Hemodynamic studies in the rat have shown that this structural analogue and potential metabolite of the clinically useful drug (S)-alpha-Me-Dopa possesses weak hypotensive activity which resides in the R enantiomer. LD50 studies in mice have established that 6-OH-alpha-Me-Dopa is over four times more toxic than alpha-Me-Dopa. Chronic exposure to 6-OH-alpha-Me-Dopa leads to renal and hepatic lesions. The case of oxidation of this hydroquinone to the electrophilic quinone species may contribute to its enhanced toxicity compared to alpha-Me-Dopa.

Analysis of nitrosamines in aqueous and biological fluids based on measurement of photochemically liberated nitrite

A method is described for the analysis of nitrosamines in aqueous solution and in biological fluids (blood, plasma, and rat liver microsomal suspensions). The method is based on photochemical degradation of the nitrosamine in a controlled environment to yield the corresponding amine and nitrite ion, and the latter is subsequently used to form a chromophoric or fluorescent product. The analysis scheme is a modular three-component system consisting of a column to remove contaminating nitrite prior to photolysis, a photochemical reactor, and a chemical reactor. Additional modules are used to accommodate biological samples or large-volume (5--50 ml) aqueous samples. In this study, N-nitrosopyrrolidine, N-nitrosodimethylamine, and N,N-diethanolnitrosamine were utilized as substrates. Because of intersubstrate variability in the photochemical decomposition rate and overall nitrite yield, the structure (i.e., photochemical behavior) of the particular nitrosamine in the sample must be known prior to analysis. With a colorimetric readout, the sensitivity for analysis of N-nitrosopyrrolidine was 800 ng/ml for a 5-ml sample and the measurement precision was +/- 6% in the biological fluids. Fluorometric analysis improved sensitivity to 4 ng/ml with a precision of +/- 10% in biological media.