Determination of rofecoxib, a cyclooxygenase-2 specific inhibitor, in human plasma using high-performance liquid chromatography with post-column photochemical derivatization and fluorescence detection

Comprehensive Assessment of the Stability of Selected Coxibs in Variable Environmental Conditions along with the Assessment of Their Potential Hepatotoxicity

Determining the influence of environmental factors on the stability of drugs is very helpful when choosing excipients, storage conditions or packaging materials. In addition, information about possible toxic degradation products enables detecting and avoiding the harmful side effects of the drug. We used the thin-layer chromatographic-densitometric procedure for the assay of five coxibs, conducted degradation studies in various environments and at different temperatures along with the determination of pharmacokinetic parameters. The results were subjected to chemometric analysis, to investigate and visualize the similarities and differences of the studied coxibs. Samples of the tested drug were also analyzed by UPLC-MS/MS in order to identify degradation products, and determine possible drug degradation pathways. Using the human liver cancer HepG2 cell line, the hepatotoxic effect of the degradation products was also determined. It was observed that all substances were relatively stable under the analyzed conditions and degraded more in acidic than alkaline environments. Robenacoxib is the drug that decomposes the fastest, and cimicoxib turned out to be the most stable. Robenacoxib also showed significant hepatotoxicity at the highest tested concentration, which correlates with the high degree of its degradation, and the probable formation of a more hepatoxic product. The obtained mass spectra of compounds formed as a result of hydrolysis of the protonated drug leading to the formation of several product ions, which enabled us to propose probable degradation pathways.

Maytansinoid-BODIPY Conjugates: Application to Microscale Determination of Drug Extinction Coefficients and for Quantification of Maytansinoid Analytes

Determining drug to antibody ratios (DAR) for antibody-drug conjugates (ADCs) in early research and development can be hampered by difficulties in accurate weighing of the effector payload and subsequent determination of its extinction coefficient. Two maytansinoids, DM1 and DM4, potent antimitotic agents used in clinical ADCs, were derivatized with the compact fluorophore BODIPY FL using two different linker designs. We identified DM1-mal-BODIPY as a conjugate with little through-space interaction between the maytansinoid and BODIPY chromophores. The 1:1 stoichiometry between the maytansinoid and BODIPY makes the molar concentration of both components equal and the extinction coefficient of the maytansinoid in proportion with the known BODIPY chromophore according to Beer's Law. By only derivatizing 50 μg of unpurified DM1 and analyzing about 25 μg of DM1-mal-BODIPY by UV-vis, we determined εDM1 252 nm and εDM1 280 nm as 26 355 ± 360 and 5230 ± 160 cm(-1) M(-1), respectively. These values are nearly identical to those accepted for DM1 based on weighing >100 mg of pure sample. Surprisingly, some of the maytansinoid-BODIPY conjugates that were synthesized were partially or completely fluorescence-quenched. The green fluorescence of quenched DM4-acetamide-BODIPY could be fully restored in the presence of an antibody designed to tightly bind maytansine. We exploited this observation to develop a simple "mix and read" fluorogenic immunoassay for detection of nanogram quantities of maytansinoids.

Current perspectives in NSAID-induced gastropathy

Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most highly prescribed drugs in the world. Their analgesic, anti-inflammatory, and antipyretic actions may be beneficial; however, they are associated with severe side effects including gastrointestinal injury and peptic ulceration. Though several approaches for limiting these side effects have been adopted, like the use of COX-2 specific drugs, comedication of acid suppressants like proton pump inhibitors and prostaglandin analogs, these alternatives have limitations in terms of efficacy and side effects. In this paper, the mechanism of action of NSAIDs and their critical gastrointestinal complications have been reviewed. This paper also provides the information on different preventive measures prescribed to minimize such adverse effects and analyses the new suggested strategies for development of novel drugs to maintain the anti-inflammatory functions of NSAIDs along with effective gastrointestinal protection.

An Evaluation of the Dose-Dependent Inhibition of CYP1A2 by Rofecoxib Using Theophylline as a CYP1A2 Probe

This study was undertaken to determine whether rofecoxib can interfere with CYP1A2 activity in humans using theophylline as a probe substrate. Single oral doses of theophylline were administered to each of three panels of 12 healthy subjects receiving daily doses of rofecoxib for 7 days to examine the effect of rofecoxib administration on the absorption and disposition of theophylline. Each panel was administered doses of 12.5, 25, or 50 mg of rofecoxib or a matching placebo in a two-way, randomized, crossover fashion and administered a single oral 300-mg dose of theophylline on day 7 of rofecoxib or placebo administration. Plasma concentrations of theophylline were monitored for 48 hours postdose to assess differences in pharmacokinetics. All three commercially marketed doses of rofecoxib were found to slow the clearance of theophylline with no detectable effect on absorption. CL/F values for theophylline were estimated from AUC infinity and by point estimates from the concentrations of drug in plasma at 12 and 24 hours postdose. The point estimates of CL/F were found to be in agreement with those derived from AUC.

Structural Examination of 6-Methylsulphonylphenanthro- [9,10-C]-furan-1(3H)-one-A Rofecoxib Degradation Product

In the attempt to discover a new polymorph of rofecoxib (Vioxx®), an unexpected product resulted. The product was characterised by chemical composition, thermal behaviour and structure and found to be 6-methylsulphonylphenanthro-[9,10-C] furan-1(3H)-one, a photo-cyclization degradation product of rofecoxib. This is a significant finding because it indicates that without appropriate control of the recrystallisation procedures, the structural integrity of rofecoxib may be seriously compromised.

Automated liquid chromatography–tandem mass spectrometry method for the analysis of firocoxib in urine and plasma from horse and dog

A rugged, sensitive and efficient liquid chromatography-tandem mass spectrometry method was developed and validated for the quantitative analysis of firocoxib in urine from 5 to 3000 ng/mL and in plasma from 1 to 3000 ng/mL. The method requires 200 microL of either plasma or urine and includes sample preparation in 96-well solid phase extraction (SPE) plates using a BIOMEK 2000 Laboratory Automated Workstation. Chromatographic separation of firocoxib from matrix interferences was achieved using isocratic reversed phase chromatography on a PHENOMENEX LUNA Phenyl-Hexyl column. The mobile phase was 45% acetonitrile and 55% of a 2 mM ammonium formate buffer. The method was accurate (88-107%) and precise (CV<12.2%) within and between sets. Extraction efficiencies (recovery)>93% were achieved and ionization efficiencies (due to matrix effects) were >72%. Extensive stability and ruggedness testing was also performed; therefore, the method can be used for pharmacokinetic studies as well as drug monitoring and screening. The data presented here is the first LC-MS/MS method for the quantitation of firocoxib in plasma (LLOQ of 1 ng/mL), a 25-fold improvement in sensitivity over the HPLC-UV method and the first quantitative method for firocoxib in urine (LLOQ of 5 ng/mL). Additionally the sample preparation process has been automated to improve efficiency.

An overview of the recent developments in analytical methodologies for determination of COX-2 inhibitors in bulk drugs, pharmaceuticals and biological matrices

An extensive survey of the literature published in various analytical and pharmaceutical chemistry related journals has been conducted and the instrumental analytical methods which were developed and used for determination of COX-2 inhibitors in bulk drugs, formulations and biological fluids have been reviewed. This review covers the time period from 1995 to 2004 during which 138 analytical methods including all types of spectrophotometric and chromatographic techniques were reported. HPLC with UV detection was found to be the technique of choice for many workers and more than 100 methods were based on LC and UV. A critical analysis of the reported data has been carried out and the present state-of-art of the analytical techniques for determination of celecoxib, rofecoxib, etoricoxib, etodolac, nimesulide and meloxicam has been discussed.

Pre-column derivatization of rofecoxib for determination in serum by HPLC

An HPLC method for determination of rofecoxib in human serum is presented. The method is based on pre-column derivatization of analyte to a phenanthrene derivative of the drug. Rofecoxib and the internal standard were extracted from serum using liquid-liquid extraction. Upon exposure to UV light, the drug was found to undergo a photocyclization reaction, giving a species with high absorbance. Validation of the method has been studied in the concentration range 10-500 ng ml(-1).

Simultaneous LC determination of tizanidine and rofecoxib in tablets

A reverse phase high performance liquid chromatographic method to determine tizanidine (TZ) and rofecoxib (RF) in combination is proposed and applied to the pharmaceuticals. This method allows the determination of 0.1-0.5 microg/ml of TZ and 1.2-6.0 microg/ml of RF along with 10 microg/ml of nimesulide (internal standard), in a mobile phase consisting of 1% (v/v) triethylamine (pH adjusted to 2.5 using dilute orthophosphoric acid):acetonitrile in the ratio 55:45% (v/v). Detection wavelength of 303 nm and flow rate of 0.8 ml/min were fixed for the study. The limit of detection (LOD) for TZ and RF were found to be 10 and 1 ng/ml, respectively. The limit of quantification (LOQ) for TZ and RF were found to be 80 and 12 ng/ml, respectively. The amount of drug present in the tablet and the recovery studies were also carried out. The % R.S.D. of recovery studies for TZ and RF were found to be 0.0673 and 0.0146, respectively. The method is validated for accuracy, precision, ruggedness and robustness.

Application of HPLC and HPTLC for the simultaneous determination of tizanidine and rofecoxib in pharmaceutical dosage form

Two methods are described for the simultaneous determination of tizanidine and rofecoxib in binary mixture. The first method was based on HPTLC separation of the two drugs followed by densitometric measurements of their spots at 311 nm. The separation was carried out on Merck HPTLC aluminium sheets of silica gel 60 F254 using toluene:methanol:acetone (7.5:2.5:1.0, v/v/v) as mobile phase. The linear regression analysis data was used for the regression line in the range of 10-100 and 100-1500 ng/spot for tizanidine and rofecoxib, respectively. The second method was based on HPLC separation of the two drugs on the reversed phase kromasil column [C18 (5 microm, 25 cm x 4.6 mm, i.d.)] at ambient temperature using a mobile phase consisting of phosphate buffer pH 5.5 and methanol (45:55, v/v). Flow rate was 1.0 ml/min with an average operating pressure of 180 kg/cm2. Quantitation was achieved with UV detection at 235 nm based on peak area with linear calibration curves at concentration ranges 10-200 and 100-2000 microg/ml for tizanidine and rofecoxib, respectively. Both methods have been successively applied to pharmaceutical formulation. No chromatographic interference from the tablet excipients was found. Both methods were validated in terms of precision, robustness, recovery and limits of detection and quantitation. The analysis of variance (ANOVA) and Student's t-test were applied to correlate the results of tizanidine and rofecoxib determination in dosage form by means of HPTLC and HPLC method.

Determination of rofecoxib in human plasma and breast milk by high-performance liquid chromatographic assay

A rapid and simple HPLC assay was developed for the determination of rofecoxib in human plasma and breast milk. After solid-phase extraction, rofecoxib was resolved on a C18 column and detected by UV detection at 272 nm. Standard curves were linear over the concentration range 10-2000 microg/L (r2 >0.99). Intra- and inter-day coefficients of variation for both matrices were <10% and the limit of quantification was around 10 microg/L.

Pharmacokinetics of rofecoxib: a specific cyclo-oxygenase-2 inhibitor

Rofecoxib is a commonly used specific cyclo-oxygenase-2 (COX-2) inhibitor. Rofecoxib has high bioavailability, poor aqueous solubility, an elimination half-life suitable for daily administration and a volume of distribution approximating body mass. Species-specific, predominantly hepatic, metabolism occurs, with novel enterohepatic circulation in rats and O-glucuronidation by uridine diphosphate-glucuronosyl transferase (UGT) 2B7 and 2B15 in human liver microsomes. Discrepancies in studies of postoperative analgesia can be putatively explained by known pharmacokinetics. Changes in rofecoxib disposition and pharmacokinetics are evident between races, in elderly patients, in patients with chronic renal insufficiency and in patients with mild to moderate hepatic impairment. Despite the selective action of COX-2 inhibitors, there remains the potential for significant drug interactions. Rofecoxib has been shown to have interactions with rifampicin (rifampin), warfarin, lithium and angiotensin converting enzyme (ACE) inhibitors and theophylline. COX-2 inhibitors represent a major therapeutic advance in terms of gastrointestinal safety; however, long-term safety in other organ systems and with concomitant drug administration still remain to be proven.

Determination of a cyclooxygenase II inhibitor in human plasma by capillary gas chromatography with mass spectrometric detection

Sensitive methods based on capillary gas chromatography (GC) with mass spectrometric (MS) detection in a selected-ion monitoring mode (SIM) for the determination of a cyclooxygenase II (COX-II) inhibitor (3-isopropoxy-4-(4-methanesulfonylphenyl)-5,5'-dimethyl-5H-furan-2-one, I) in human plasma, in two concentration ranges of 0.1-20 and 5-1000 ng/ml, are described. Following liquid-liquid extraction, the residue, after evaporation of the organic phase to dryness, was reconstituted in acetonitrile (20 l) and part of the extract (1 l) was analyzed by GC/MS/SIM. The drug (I) and internal standard (II) were separated on a 25 mx0.2 mm capillary column with HP Ultra 1 (100% dimethylpolysiloxane, 0.33 m) phase and analyzed by MS/SIM monitoring ions at m/z 237 and 282 for I and II, respectively. The standard curve was linear within the lower concentration range of 0.1-20 ng/ml and the lower limit of quantification (LLOQ) in plasma was 0.1 ng/ml. Intraday coefficients of variation (CV, n=5) were 8.9, 4.2, 5.7, 3.1, 1.9, 1.9, and 4.4% at 0.1, 0.2, 0.5, 1.0, 5.0, 10, and 20 ng/ml, respectively. The standard curve was also linear within the higher concentration range of 5-1000 ng/ml and the LLOQ in plasma was 5 ng/ml. Intraday coefficients of variation (CV, n=5) were all below 9% at all concentrations within the standard curve range. The accuracy for I in human plasma was 91-112% and the recovery of I and II was greater than 70% at all concentrations within both standard curve ranges. The details of the assay methodology are presented.

Derivative spectrophotometric and fluorimetric methods for determination of rofecoxib in tablets and in human plasma in presence of its photo-degradation product

Rofecoxib (I) has been determined in the presence of its photo-degradation product (II) using first derivative spectrophotometry ((1)D) and first derivative of the ratio spectra ((1)DD) by measuring the amplitude at 316.3 and 284 nm for (1)D and (1)DD, respectively. (I) can be determined in the presence of up to 70% and 80% of (II) by the (1)D and (1)DD, respectively. The linearity range of both the methods was the same (5.8-26.2 microg ml(-1)) with mean percentage recovery of 100.08 +/- 0.84 and 100.06 +/- 1.06 for (1)D and (1)DD, respectively. (1)D method was used to study kinetics of (I) photo-degradation that was found to follow a first-order reaction. The t(1/2) was 20.2 min while K (reaction rate constant) was 0.0336 mol min(-1). Both methods were applied to the analysis of (I) in bulk powder and in pharmaceutical formulations. Also a spectrofluorimetric method is described to determine (I) at very low concentrations (25-540 ng ml(-1)) where (I) is converted to its photo-degradate (II), which possesses a native fluorescence that could be measured. The proposed method was applied for the analysis of tablets containing rofecoxib as well as to rofecoxib-spiked human plasma.

Rofecoxib: an update on physicochemical, pharmaceutical, pharmacodynamic and pharmacokinetic aspects

Rofecoxib (MK-966) is a new generation non-steroidal anti-inflammatory agent (NSAID) that exhibits promising anti-inflammatory, analgesic and antipyretic activity. It selectively inhibits cyclooxygenase (COX)-2 isoenzyme in a dose-dependent manner in man. No significant inhibition of COX-1 is observed with rofecoxib up to doses of 1000 mg. The pharmacokinetics of rofecoxib has been found to be complex and variable. Mean oral bioavailability after single dose of rofecoxib (12.5, 25 or 50 mg) is 93% with t(max) varying widely between 2 and 9 h. It is highly plasma-protein bound and is metabolized primarily by cytosolic reductases to inactive metabolites. Rofecoxib is eliminated predominantly by hepatic metabolism with a terminal half-life of approximately 17 h during steady state. Various experimental models and clinical studies have demonstrated rofecoxib to be superior, or at least equivalent, in anti-inflammatory, analgesic and antipyretic efficacy to comparator nonselective NSAIDs in osteoarthritis, rheumatoid arthritis and other pain models. Emerging evidence suggests that rofecoxib may also find potential use as supportive therapy in various pathophysiologic conditions like Alzheimer's disease, and in various malignant tumours and polyps, where COX-2 is overly expressed. Rofecoxib is generally well-tolerated. Analysis of data pooled from several trials suggests that rofecoxib is associated with fewer incidences of clinically symptomatic gastrointestinal ulcers and ulcer complications vis-à-vis conventional NSAIDs. However, this gastropreserving effect may be negated by concurrent use of low-dose aspirin for cardiovascular risk reduction. Rofecoxib tends to show similar tolerability for renal and cardiothrombotic events as compared with nonnaproxen nonselective NSAIDs. No clinically significant drug interaction has been reported for rofecoxib except with diuretics, where it reverses their salt-wasting effect and thus can be clinically exploited in electrolyte-wasting disorders. There is only modest information about the physicochemical and pharmaceutical aspects of rofecoxib. Being poorly water soluble, its drug delivery has been improved using varied formulation approaches. Although it is stable in solid state, rofecoxib is photosensitive and base-sensitive in solution form with its degradation mechanistics elucidated. Analytical determinations of rofecoxib and its metabolites in biological fluids employing HPLC with varied types of detectors have been reported. Isolated studies have also been published on the chromatographic and spectrophotometric assay of rofecoxib and its degradants in bulk samples and pharmaceutical dosage forms. The current article provides an updated overview on the physicochemical, pharmaceutical, pharmacokinetic and pharmacodynamic vistas of rofecoxib.

Effects of renal insufficiency and hemodialysis on the pharmacokinetics of rofecoxib

Rofecoxib (VIOXX, Merck & Co., West Point, PA) is a COX-2-selective inhibitor that combines anti-inflammatory and analgesic efficacy with improved gastrointestinal (GI) safety. The present open-label study investigated the pharmacokinetics, safety, and tolerability of a single, oral dose of rofecoxib (50 mg) in patients with end-stage renal failure (creatinine clearance <5 mL/min/1.73 m(2)) requiring hemodialysis. Rofecoxib AUC(0-48 h), AUC(0- infinity), C(max), T(max), and t(1/2) obtained from renal failure patients on hemodialysis were not significantly different from those obtained from healthy subjects. With hemodialysis initiated 48 hours postdose, rofecoxib AUC(0-48 h) adjusted mean ratio (renal failure/healthy subjects) was 0.81, with a corresponding 90% confidence interval (CI; 0.66, 1.00). Hemodialysis per se had no clinically meaningful effect on rofecoxib pharmacokinetics: plasma rofecoxib concentration-time curves were virtually superimposable when hemodialysis was initiated at 4 or 48 hours following rofecoxib dosing, although mean rofecoxib C(max) was 18% lower during the former (325 versus 395 ng/mL; P = 0.014). Overall, rofecoxib was well tolerated in end-stage renal disease patients. In this study, end-stage renal disease and hemodialysis had little effect on rofecoxib pharmacokinetics. Although there are no clinical data to support the use of rofecoxib in patients with severe renal insufficiency (creatinine clearance, 5-30 mL/min/1.73 m(2)), these data suggest that dosage adjustment of rofecoxib is not needed for patients with impaired renal function.

Opposite effects of rofecoxib on nuclear factor-kappaB and activating protein-1 activation

Rofecoxib is a selective cyclooxygenase (COX)-2 inhibitor approved for the treatment of pain and inflammation in rheumatoid and osteoarthritis. Daily doses between 12.5 and 50 mg were found to reduce pain and inflammation, however, without a clear dose-effect relationship. Interestingly, rofecoxib treatment is associated with an unexpected incidence of renal adverse events compared with other COX inhibitors. Here, the effects of rofecoxib on the transcription factors nuclear factor-kappaB (NF-kappaB) and activating protein-1 (AP-1) were analyzed to find out whether transcriptional changes might explain the lack of clear dose dependency and the occurrence of renal side effects. In vitro, rofecoxib dose dependently inhibited DNA binding capacity of NF-kappaB at doses of 10 to 100 microM, whereas the binding activity of AP-1 was considerably increased at 100 microM. In vivo, the anti-inflammatory effect of rofecoxib was equal at 1 and 10 mg/kg, whereas 50 mg/kg caused a significant further reduction of a zymosan-induced paw edema. This was associated with a clear decrease of inducible nitric oxide synthase (iNOS) protein expression in the spinal cord at this dose. At 1 and 10 mg/kg, however, iNOS was increased but COX-2 was decreased. Thus, the expression of proinflammatory proteins was similarly inconsistent as transcription factor regulation. In conclusion, the opposite effects of rofecoxib on AP-1 and NF-kappaB may explain the lack of clear dose dependency with rofecoxib in clinical studies or animal experiments. The effects on AP-1 may possibly affect renal sodium transport because certain renal sodium channels are regulated through AP-1. Transcription factor regulation might therefore influence both wanted and unwanted effects of rofecoxib.

Determination of efavirenz, a selective non-nucleoside reverse transcriptase inhibitor, in human plasma using HPLC with post-column photochemical derivatization and fluorescence detection

Methods for the quantitative determination of efavirenz in human plasma and the qualitative assessment of the stereochemical integrity of efavirenz in post-dose human plasma samples are described. After the addition of an internal standard, plasma samples were extracted with hexane-methylene chloride (65/35, v/v%). The extracts were evaporated to dryness and reconstituted in mobile phase. Upon exposure to UV light, the analyte was found to form fluorescent products; the major fluorescent product was isolated and identified as a substituted quinoline. Thus, the plasma extracts were analyzed via HPLC with post-column photochemical derivatization and fluorescence detection. Reverse phase chromatography was used for the quantitative assay, whereas chromatography with a column containing a chiral stationary phase (dinitrobenzoyl leucine) was used for the stereochemical assessment. The quantitative assay has been validated in the concentration range of 50-1000 ng/ml using 0.5 ml samples. Analyte recovery was better than 89% at all points on the standard curve. Intra-day precision was better than 5% C.V., while accuracy was between 95 and 104% of nominal over the range of the assay. The selective detection method reduces the likelihood of interference by co-administered medications or endogenous species. The stereochemical configuration of efavirenz was confirmed to remain intact in post-dose human plasma samples. The quantitative method has been successfully utilized to support a study in which a possible drug interaction between co-administered HIV protease inhibitors and efavirenz was evaluated.

The disposition and metabolism of rofecoxib, a potent and selective cyclooxygenase-2 inhibitor, in human subjects

The disposition and metabolism of rofecoxib, a selective cyclooxygenase-2 inhibitor, were examined in healthy human subjects and in cholecystectomy patients. After oral administration of [(14)C]rofecoxib (125 mg, 100 micro Ci) to healthy subjects, the mean concentrations of total radioactivity and rofecoxib in plasma as a function of time indicated that the t(max) was achieved at 9 h postdose. After t(max), levels of both radioactivity and rofecoxib decreased in a parallel, exponential fashion (effective t(1/2) approximately equal 17 h). A similar result was obtained after oral administration of [(14)C]rofecoxib (142 mg, 100 micro Ci) to cholecystectomy patients equipped with an L-tube. In healthy subjects, radioactivity was recovered predominantly from the urine (71.5% of dose), with a small amount excreted in feces (14.2%). In patients with an L-tube, half the radioactive dose was recovered in feces, with a lesser amount excreted in urine (28.8%) and a negligible fraction in bile (1.8%). Rofecoxib underwent extensive metabolism in humans, and very little parent drug was recovered unchanged in urine (<1%). Products resulting from both oxidative and reductive pathways were identified by a combination of (1)H NMR and liquid chromatography-tandem mass spectrometry analyses, and included rofecoxib-3',4'-trans-dihydrodiol, 4'-hydroxyrofecoxib-O-beta-D-glucuronide, diastereomeric 5-hydroxyrofecoxib-O-beta-D-glucuronide conjugates, 5-hydroxyrofecoxib, rofecoxib-erythro-3,4-dihydrohydroxy acid, and rofecoxib-threo-3,4-dihydrohydroxy acid. Interconversion of rofecoxib and 5-hydroxyrofecoxib appeared not to be a quantitatively important pathway of rofecoxib disposition in human subjects, in contrast to previous findings in rats.

Improved procedure for the the determination of rofecoxib in human plasma involving 96-well solid-phase extraction and fluorescence detection

An improved assay for the determination of rofecoxib in human plasma samples is described. The analyte and an internal standard were extracted from the plasma matrix using solid-phase extraction in the 96-well format with an Empore C8-SD extraction plate. The analytes are chromatographed on a Waters Symmetry C18 analytical column (3.5 microm, 50x4.6 mm) with a mobile phase consisting of acetonitrile-water (35:65, v/v). Analyte detection was via fluorescence following post-column photochemical derivatization. Eight point calibration curves over the concentration range of 0.5-80 ng/ml yielded a linear response when a 1/y weighted linear regression model was employed. Based on the replicate analyses (n=5) of spiked standards, the within-day assay precision was better than 8% RSD at all points on the calibration curve, within-day accuracy was within 6% of nominal at all standard concentrations. The between-run precision and accuracy of the assay, as calculated from the results of the analysis of quality control samples, was better than 7% RSD and within 5% of nominal. Assay throughput was improved by a factor of three as compared to previously described methods. The method was partially automated using a combination of a Packard Multi-Probe liquid handling system and a TomTec Quadra 96 workstation.

LC determination of rofecoxib in bulk and pharmaceutical formulations

An isocratic reversed phase-liquid chromatographic (RP-LC) method has been developed for the determination and purity evaluation of rofecoxib in bulk and pharmaceutical dosage forms using photodiode array detection set at 225 nm. The method is simple, rapid and selective. The method is capable of detecting all process intermediates and other related compounds, which may be present at trace levels in finished products. Hence the method is very useful for process monitoring during the production of rofecoxib. Chlorophenyl methyl sulphone has been used as internal standard for the quantitative determination of rofecoxib. The method is linear in the range of 125-500 microg. The precision for inter- and intra-day assay variation of rofecoxib is below 1.6% relative standard deviation (R.S.D.). The accuracy determined as relative mean error (R.M.E.) for the intra-day assay is within +/-2.0%. The drug was extracted from tablets (Vioxx) using acetonitrile. The percentage recoveries from dosage forms were ranged from 98.2 to 102.6.

Selective and rapid liquid chromatography-mass spectrometry method for the quantification of rofecoxib in pharmacokinetic studies with humans

An easy, rapid and selective method for the determination of rofecoxib in human plasma is presented. The analytical technique is based on reversed-phase high-performance liquid chromatography coupled to atmospheric pressure chemical ionisation mass spectrometry (Finnigan Mat LCQ ion trap). The retention time of rofecoxib was 1.2 min. The method has been validated over a linear range from 1 to 500 microg/l using celecoxib as internal standard. After validation, the method was used to study the pharmacokinetic profile of rofecoxib in 12 healthy volunteers after administration of a single oral dose (12.5 mg). The presented method was sufficient to cover more than 95% of the area under the curve. The pharmacokinetic characteristics (mean+/-SD) were tmax: 2.4+/-1.0 h, Cmax: 147+/-34 microg/l, AUCinfinity: 2038+/-581 microg h/l and t 1/2: 11.3+/-2.1 h.

High-throughput, semi-automated determination of a cyclooxygenase II inhibitor in human plasma and urine using solid-phase extraction in the 96-well format and high-performance liquid chromatography with post-column photochemical derivatization-fluorescence detection

Compound I, 5-chloro-3-(4-methanesulfonylphenyl)-6'-methyl-[2,3']bipyridinyl, has been found to be a specific inhibitor of the enzyme cyclooxygenase II (COX II). The anti-inflammatory properties of this compound are currently being investigated. HPLC assays for the determination of this analyte in human plasma and human urine have been developed. Isolation of I and the internal standard (II) was achieved by solid-phase extraction (SPE) in the 96-well format. A C8 SPE plate was used for the extraction of the drug from human plasma (recovery >90%) while a mixed-mode (C8/Cation) SPE plate was used to isolate the analytes from human urine (recovery approximately 71%). The analyte and internal standard were chromatographed on a Keystone Scientific Prism-RP guard column (20 x 4.6 mm) connected to a Prism-RP analytical column (150 x 4.6 mm), using a mobile phase consisting of 45% acetonitrile in 10 mM acetate buffer (pH = 4); the analytes eluted at retention times of 5.2 and 6.9 min for I and II, respectively. Compounds I and II were found to form highly fluorescent products after exposure to UV light (254 nm). Thus, the analytes were detected by fluorescence (lambda(ex) = 260 nm, lambda(em) =375 nm) following post-column photochemical derivatization. Eight point calibration curves over the concentration range of 5-500 ng/ml for human plasma and human urine yielded a linear response (R2>0.99) when a 1/y weighted linear regression model was employed. Based on the replicate analyses (n = 5) of spiked standards, the within-day precision for both assays was better than 7% C.V. at all points on the calibration curve; within-day accuracy was within 5% of nominal at all standard concentrations. The between-run precision and accuracy of the assays, as calculated from the results of the analysis of quality control samples, was better than 8% C.V. and within 8% of nominal. I was found to be stable in human plasma and urine for at least 8 and 2 months, respectively. In addition, the human plasma assay was semi-automated in order to improve sample throughput by utilizing a Packard liquid handling system and a Tom-Tec Quadra 96 SPE system. The precision and accuracy of the semi-automated procedure were comparable to the manual procedure. Over 5000 clinical samples have been analyzed successfully using these methods.

Determination of rofecoxib (MK-0966), a cyclooxygenase-2 inhibitor, in human plasma by high-performance liquid chromatography with tandem mass spectrometric detection

A method for the determination of 4-(4-methanesulfonylphenyl)-3-phenyl-5H-furan-2-one (Rofecoxib, Vioxx, MK-0966, I) a cyclooxygenase-2 inhibitor, in human plasma has been developed. The method is based on high-performance liquid chromatography (HPLC) with atmospheric pressure chemical ionization tandem mass spectrometric (APCI-MS-MS) detection in negative ionization mode using a heated nebulizer interface. Drug and internal standard (II) were isolated from basified plasma using liquid-liquid extraction. The organic extracts were dried, reconstituted in mobile phase and injected into the HPLC-MS-MS system. Compounds I and II were chromatographed on a narrow bore (100 mm x 3.0 mm) C18 analytical column, with mobile phase consisting of acetonitrile:water (1:1, v/v) at a flow-rate of 0.4 ml/min. The MS-MS detection was performed on a PE-Sciex API III Plus tandem mass spectrometer operated in selected reaction monitoring mode. The parent-->product ion combinations of m/z 313-->257 and 327-->271 were used to quantify I and II, respectively, after chromatographic separation of the analytes. The assay was validated in the concentration range of 0.1 to 100 ng/ml of plasma. The precision of the assay (expressed as coefficient of variation) was less than 10% at all concentrations within the standard curve range, with adequate assay accuracy. The effect of HPLC mobile phase components on the ionization efficiency and sensitivity of detection in the positive and negative ionization modes, and the detailed description of all necessary steps involved in the assay for I in plasma are presented.

Determination of celecoxib in human plasma by normal-phase high-performance liquid chromatography with column switching and ultraviolet absorbance detection

A method is described for the determination of celecoxib in human plasma. Samples were extracted using 3M Empore membrane extraction cartridges and separated under normal-phase HPLC conditions using a Nucleosil-NO2 (150x4.6 mm, 5 microm) column. Detection was accomplished using UV absorbance at 260 nm. The HPLC method included a column switching procedure, in which late eluting compounds were diverted to waste, to reduce run-time to 12 min. The assay was linear in the concentration range of 25-2000 ng/ml when 1-ml aliquots of plasma were extracted. Recoveries of celecoxib were greater than 91% over the calibration curve range. Intraday precision and accuracy for this assay were 5.7% C.V. or better and within 2.3% of nominal, respectively. The assay was used to analyze samples collected during human clinical studies.