Dose Of Racemate Research Articles

Multicenter Observational/Exploratory Study Addressed to the Evaluation of the Effectiveness and Safety of Pharmacological Therapy in Opioid-Dependent Patients in Maintenance Therapy in Southern Italy.

A multicenter-observational study was performed to assess the effectiveness of rac-methadone, levomethadone, and buprenorphine in opioid-dependent patients in polytherapy in Southern Italy. The primary endpoint was the reduction of urinary positivity to the substances and the maintaining doses. Patients (N = 266, age = 44.80 ± 5.65, male = 79.70%, female = 20.30%) have been recruited. At recruitment, 75% of them were on treatment with rac-methadone, levomethadone, and buprenorphine/naloxone. The patients were grouped into three clusters. The levomethadone patients of Cluster A (N patients = 211), after 180 days, showed stability in urinary methadone positivity, with a marked decrease in heroin −53 ± 4%, cannabinol’s −48 ± 2%, and cocaine −37 ± 6% positivity, with no differences between treatments. A lower QTcF value of 426 ± 8.4 ms was recorded in the levomethadone patients (delta = −19 ms) vs. rac-methadone, at significantly lower doses of levomethadone (−34%, −50.2% in males) (p < 0.05). The Cluster B data were collected from 37 patients, with a high prevalence of comorbidity infections (HIV/HCV/HPV), monitored for 21 months during COVID-19. High doses of levomethadone (58.33 ± 31.58 mg/day) were needed to stabilize those that were negative for opioids and cannabinoids, in contrast to the rac-methadone and buprenorphine/naloxone patients that showed positive toxicology. Eighteen patients of the Cluster C in double diagnosis (major depressive 38.90%, bipolar 27.78%, and schizophrenia 16.67%) were stabilized with high doses of racemate 97.5 ± 8 mg/day, 51.8 ± 5 mg/day of levomethadone (−46.8% vs. rac-methadone; −71% in men), and 2.5 ± 1 mg/day of buprenorphine/naloxone. Three patients in remission were treated with tapering doses of levomethadone. Significantly reduced QTcF values were recorded with levomethadone (delta −32 ms vs. rac-methadone) in the bipolar patients, as well as the schizophrenia patients in remission (delta −45.19 ms vs. rac-methadone). Our patients were safely stabilized. Levomethadone, compared to the racemate, contributes to reducing the illicit use, especially of opioids and cannabinoids at significantly lower doses with cardiovascular safety, which, in bipolar patients, is clinically significant.

Quiral Inversion of Ibuprofen after an Oral Administration Under Complete Fasting and Fed Conditions in Caucasian Healthy Subjects

Background: The aim was to determine the effect of feeding on chiral inversion of Ibuprofen isomers. Method: Six healthy volunteers participated in a two-treatment crossover study where a single dose of Ibuprofen racemate was given. One stage consisted in an 8-hour interval of fasting, while in the other, the drug was administered with a solution of saccharose and a standard food intake regimen was implemented.

Enantioselectivity in the Pharmacokinetic Interaction Between Fluvastatin and Lercanidipine in Healthy Volunteers

Hypertension and dyslipidemia are independent risk factors for cardiovascular mortality and are frequently present in the same patient. Fluvastatin (FV), used to reduce cholesterol levels, and lercanidipine (LER), used to control blood pressure, are marketed as racemic mixtures. Therapeutic activities are 30-fold higher for (+)-3R, 5S-FV and 100- to 200-fold higher for S-LER compared with their respective antipodes. The present study describes the enantioselective pharmacokinetic interaction between LER and FV in healthy volunteers. A crossover randomized study was conducted in 3 phases on 8 volunteers treated with a single oral racemic dose of LER (20 mg) or FV (40 mg) or LER plus FV. Serial blood samples were collected from 0 to 24 hours. Plasma concentrations of the LER and FV enantiomers were determined by liquid chromatography/tandem mass spectrometry, and pharmacokinetic parameters were evaluated using the WinNonlin software. The Wilcoxon and Mann-Whitney tests (P < .05) were used to analyze enantiomer ratios and the pharmacokinetic drug interaction. Data are expressed as medians. In monotherapy, the kinetic disposition of both FV and LER was enantioselective. AUC values were significantly higher for (-)-3S,5R-FV than for (+)-3R,5S-FV (358.20 vs 279.68 ng.h/mL) and for S-LER compared with R-LER (13.90 vs 11.88 ng.h/mL). The pharmacokinetic parameters of FV were not enantioselective when combined with LER (AUC: (-)-3S,5R-FV: 325.21; (+)-3R,5S-FV: 316.44 ng.h/mL). There was a significant reduction in S-LER (8.06 vs 13.90 ng.h/mL) and R-LER (6.76 vs 11.88 ng.h/mL) AUC values when FV was coadministered. In conclusion, the interaction between FV-LER might be clinically relevant because AUC values of (+)-3R,5S-FV were increased when LER was coadministered, and AUC values of the 2 LER enantiomers were reduced when FV was coadministered.

Is the required therapeutic effect always achieved by racemic switch of proton-pump inhibitors?

Many of the drugs currently used in medical practice are racemates. The enantiomers of a racemic drug differ in pharmacodynamics and/or pharmacokinetics, thus in some cases it is preferable to develop pure enantiomers by racemic switch. In a recent study by Pai et al, dexrabeprazole [R(+)-rabeprazole] (10 mg) was found to be more effective than rabeprazole (20 mg) in the treatment of gastroesophageal reflux disease. We read with great interest in this study and discussed whether such racemic switch would be applicable to other proton-pump inhibitors (PPIs). A literature review indicates that stereoselective pharmacokinetics, rather than stereoselective pharmacological activity, is the main cause of differences in clinical efficacy between pure enantiomer and racemic PPI. Racemic switches of PPI provide the therapeutic advantages such as reducing metabolic load on the body, simplifying pharmacokinetics, providing benefit to the non-responders to standard dose of racemate, more homogenous response to treatment and better efficacy with equal safety. Further studies in quantitative structure-activity relationships (QSARs) are needed to address the fact that the preferred enantiomer of PPI is not always in the same absolute configuration, i.e. S-form is for omeprazole, pantoprazole and tenatoprazole whereas R-form is for lansoprazole and rabeprazole.

Pharmacokinetics of intravenous d- and l-methamphetamine in healthy volunteers

Background/Aims Methamphetamine has a chiral structure. The aim of the present study was to assess the pharmacokinetics of methamphetamine enantiomers in humans. Methods In a randomized, double-blind, six-period crossover study, methamphetamine (0.25 mg/kg d- or l-, 0.5 mg/kg d-, l- or racemic (1:1) methamphetamine and placebo) was given intravenously to 12 healthy methamphetamine users. Results d-Methamphetamine showed 30% smaller AUC compared to l-methamphetamine after administration of the racemic form. However, AUCs were similar between d- and l-methamphetamine when given on separate occasions at both doses (0.25 and 0.5 mg/kg). AUC ratios for d-methamphetamine/l-methamphetamine were 0.910 (90% CI; 0.837–0.984) for the 0.25 mg/kg dose, 0.894 (0.821–0.967) for the 0.5 mg/kg dose, and 0.679 (0.623–0.736) for the racemic dose. AUC and Cmax at 0.5 mg/kg dose were twofold greater than those at 0.25 mg/kg dose for both enantiomers. The elimination half-lives were slightly longer for l-methamphetamine. Vd was similar for both formulations. AUC ratios for amphetamine/methamphetamine were significantly higher for the d-enantiomer (16–17%) than the l-enantiomer (3–4%). Conclusions d- and l-Methamphetamine are apparently bioequivalent in terms of AUC when given separately, with linear pharmacokinetics between 0.25 and 0.5 mg/kg doses. However, administration of d-methamphetamine results in a higher metabolite concentration compared to the l-form. NIDA DA12521 and DA12393. Clinical Pharmacology & Therapeutics (2005) 77, P37–P37; doi: 10.1016/j.clpt.2004.12.034

Toxicokinetic assessment of methylphenidate (Ritalin) enantiomers in pregnant rats and rabbits.

Ritalin or methylphenidate (MPH) is prescribed for the treatment of attention deficit hyperactivity disorder (ADHD) and narcolepsy. The present report describes the determination of plasma concentrations of D-threo- and L-threo-enantiomers of MPH in toxicokinetic (TK) studies in pregnant Wistar Hannover rats and New Zealand white rabbits following repeated daily oral dosing of D,L-MPH (racemate). A previously reported chiral liquid chromatography tandem mass spectrometric (LC-MS/MS) method with a lower limit of quantification (LLOQ) of 1.09 ng/mL was utilized. Oral (gavage) doses of 7, 25 and 75 mg/kg/day of racemic MPH were selected for the rat study. An over-proportional increase in exposure was observed with increasing doses of MPH racemate, the effect being more profound with the D- than the L-enantiomer. In contrast, Cmax values of both enantiomers were approximately proportional to the dose. Oral (gavage) doses of 20, 60 and 200 mg/kg were selected for the rabbit study. In general, for the D-isomer, an over-proportional increase in exposure was observed with increasing doses of MPH racemate. Conversely, for the L-isomer, a slight under-proportionality was detected in exposure with increasing doses of D,L-MPH. For mean Cmax, while L-isomer exhibited dose proportionality with increasing doses of MPH racemate, the D-isomer appeared to be over-proportional. Herein, the experimental design and observed TK parameters in each study are presented.

Pain Treatment With NSAIDs, Primary Focus on Ibuprofen

For the past 30 years ibuprofen has been known as one of the safest NSAIDs and in some countries, including The Netherlands, it is available as an over-the-counter medicine. Nevertheless, all NSAIDS may show adverse effects, such as gastrointestinal toxicity, sodium/water retention, reduced kidney perfusion and allergic responses. In developing safer NSAIDs two new types of drug have been introduced: the COX-2 inhibitors and the single enantiomer NSAIDs, in particular S(+)-ibuprofen. S(+) ibuprofen shows an equipotency with half of the racemic ibuprofen dose , and the introduction of S(+) ibuprofen (dexibuprofen, Seractil) permits the prescription of lower doses. The new COX-2 inhibitors have recently been compared to the racemic ibuprofen. Day et al. found no significant difference between rofecoxib and racemic ibuprofen concerning overall incidence rates of clinical adverse experiences. In the study by Silverstein et al. the annualised incidence rates of upper GI ulcer complications alone and combined with symptomatic ulcers for celecoxib vs ibuprofen showed no statistical difference (P = 0.09), but for patients not taking aspirin, a difference was found in favour of celecoxib (P = 0.04). Studies comparing the COX-2 inhibitors and dexibuprofen need to be performed.

Pharmacokinetics and pharmacodynamics of methadone enantiomers after a single oral dose of racemate.

The pharmacokinetics and dynamics of methadone are characterized by high interindividual variability. This study aimed to examine a number of factors that may contribute to this variability. Eight healthy drug-free women were administered 0.2 mg/kg of R,S-methadone orally. The concentrations of methadone's enantiomers in plasma and urine were monitored for 96 hours. Vital signs, blood biochemical parameters, and pupillary diameter were monitored frequently during this period. Cytochrome P450 3A (CYP3A) activity and alpha1-acid glycoprotein (alpha1-AGP) concentrations and phenotypes were determined. Pharmacokinetic and pharmacodynamic modeling was used to assess the influence of the above-mentioned covariables on methadone enantiomer disposition and actions. The pharmacokinetic profile of the active enantiomer of methadone, R -methadone, showed a relatively normal distribution with 38% to 90% of the interindividual variability in modeled pharmacokinetic parameters being explained by their individual variability in CYP3A activity, the cumulative amount of the main CYP3A4 metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrolidine, excreted in the urine, the fraction unbound in plasma, and the alpha1-AGP orosomucoid 2 (ORM2) variant plasma concentration. S-Methadone showed an idiosyncratic distribution with largely unpredictable pharmacokinetics. Pupillary constriction response was highly variable between individuals. The disposition of the active enantiomer, R -methadone, can be predicted in part by CYP3A activity and protein binding to alpha1-AGP (ORM2), whereas S-methadone disposition is not well explained by the factors examined in this study. Central nervous system effects were difficult to interpret on the basis of plasma R-methadone pharmacokinetics.

Relative Efficacies of Gastric Proton-Pump Inhibitors on a Milligram Basis: Desired and Undesired SH reactions. Impact of Chirality

Gastric proton-pump inhibitors (PPIs) are prodrugs. Their acid activation at pH 1.0 inside the canaliculus of a parietal cell should be fast relative to their serum elimination rate. Actually, all PPIs display chemical activation half-lives at pH 1.0 of a few minutes at the most, while being eliminated from serum with a half-life of about 1 h. This is the main reason they show similar antisecretory efficacies on a milligram basis. It is in line with about 5% to 15% higher healing rates in GERD, DU and GU when 40 mg is compared to 20 mg of either omeprazole or pantoprazole. The comparably large biological variation between patient samples explains why some studies show statistically significant differences between the two doses, while others do not. However, it would matter to the individual patient if s/he was the one additionally healed by a 40 mg dose within a defined treatment period. Chemical activation of PPI prodrugs is unwanted in weakly acidic tissue compartments such as lysosomes or secretory granules. However, the ratio of the serum elimination half-life (availability at the target) to the chemical activation half-life at a critical pH 5.0 is reversed only with pantoprazole, when compared to pH 1.0 (i.e. the ratio is <1 at pH 5.0 and >1.0 at pH 1.0). This is the basis of the high pH selectivity of pantoprazole. In contrast, rabeprazole is activated at pH 5.0 almost as quickly as it is at pH 1.0 and much faster than it is eliminated from serum. This unwanted reactivity of rabeprazole at pH 5.0 does not contribute to the antisecretory action at pH 1.0 and results in poor pH selectivity. Omeprazole and lansoprazole lie in between, as they are activated, at pH 5.0, about as quickly as they are eliminated from serum. The above activation rates refer to room temperature. At 37 °C, the activation rates of all PPIs further increase, by about the same factor of between 3 and 4. This renders their differential pH selectivities even more critical for drug safety. Biological consequences have been reported in the literature. It has been claimed that a dose of 40 mg of the S-enantiomer of omeprazole (esomeprazole) results in 10%—0;15% higher healing rates in GERD patients, compared to 20 mg omeprazole racemate. The same difference is found when the two doses of omeprazole racemate are compared to each other. This is not surprising, as the chiral PPI prodrug is converted by acid into an achiral cyclic sulfenamide which only then reacts with the proton pump. There is therefore no pharmacodynamic argument in favour of any single enantiomer formulation of any PPI. Moreover, potential pharmacokinetic differences between the enantiomers seem to be of little if any importance in the patient.

Pharmacokinetics of Clinafloxacin Enantiomers in Humans

The pharmacokinetics of R-clinafloxacin and S-clinafloxacin enantiomers of the broad-spectrum fluoroquinolone antibiotic, clinafloxacin, were characterized in selected volunteer subjects and patients after the administration of oral and intravenous doses of racemic drug. The absorption of each enantiomer was rapid and nearly complete after a single, oral 400 mg racemic dose. The mean (+/- SD) bioavailability of R-clinafloxacin was 87.5% +/- 4.8% compared to 86.2% +/- 5.8% for S-clinafloxacin. The mean Cmax of each enantiomer was 1.19 micrograms/mL, with plasma concentrations of each enantiomer remaining above 0.1 microgram/mL for at least 12 hours. No notable differences in the disposition of R-clinafloxacin and S-clinafloxacin were observed. After a single 400 mg intravenous dose of racemic drug, mean (+/- SD) t1/2 was 5.6 +/- 0.3 hours and 5.7 +/- 0.4 hours, plasma Cl was 329 +/- 49 mL/min and 314 +/- 45 mL/min, and Vdss was 138 +/- 18 L and 134 +/- 16 L for R- and S-clinafloxacin, respectively. Two healthy volunteers each received a single 400 mg oral dose of racemic clinafloxacin (alone) and with oral administration of 1 gm probenecid separated by a 1-week washout period between treatments. With probenecid coadministration, the increase in AUC0-infinity was 75% and 83% for R-clinafloxacin and was 71% and 75% for S-clinafloxacin in each subject, respectively. Probenecid increased the total exposure (AUC) of both R-clinafloxacin and S-clinafloxacin, although it had no stereo-selective effects on the disposition of either enantiomer. The antimicrobial potency of the isomers was also evaluated. In vitro susceptibility testing showed that the two compounds were comparable in their inhibitory activities, as all MICs were within twofold for each organism tested. These results demonstrate that in addition to their similar antimicrobial potency, R- and S-clinafloxacin have nearly identical disposition characteristics and are eliminated by similar mechanisms that display no apparent enantioselectivity in man.

Pharmacokinetic and Pharmacodynamic Characteristics and Safety of Inhaled Albuterol Enantiomers in Healthy Volunteers

The pharmacokinetics and pharmacodynamics of inhaled albuterol given as single or multiple doses of racemate (RS-) or single enantiomers (R-, S-) were determined. In an open-label, three-way crossover, parallel-dose study, 1.25 and 5 mg of (R)- and (S)-albuterol and 2.5 and 10 mg of (RS)-albuterol were given via nebulization to 15 healthy volunteers. The pharmacokinetic parameters of each enantiomer were determined by noncompartmental and model-fitting analyses. Both (R)- and (S)-albuterol showed rapid absorption and biexponential decline, with half-lives (t1/2) averaging 4 and 6 hours, respectively. There were no differences in pharmacokinetics of (R)-albuterol when administered as (R)- or (RS)-albuterol at the 5-mg dose with equivalent relative bioavailability as seen from maximum concentration (Cmax) and area under the concentration-time curve (AUC). The same was true for (S)-albuterol at the 1.25-mg and 5-mg doses. The data from 5-mg doses were considered to be more reliable due to assay sensitivity limitations, and indicated equivalent absorption and disposition of the individual enantiomers. There was no evidence of in vivo racemization, and (R)-albuterol did not interconvert to (S)-albuterol. Plasma potassium, plasma glucose, heart rate, and QTc interval were used in linear and Emax models to assess responses relating to (R)-albuterol concentrations. The Emax for potassium change was 1.32 meq/L, with an EC50 of 0.59 and 0.94 ng/mL after administration of (R)- and (RS)-albuterol, respectively. The slopes and intercepts for glucose and heart rate changes were similar after administration of (R)- and (RS)-albuterol. No concentration-effect relationships were evident for QTc interval or for (S)-albuterol. The extrapulmonary responses of (R)-albuterol and adverse effects were similar for single R-enantiomer or the racemic mixture.

Stereoselective pharmacokinetics and inversion of (R)- ketoprofen in healthy volunteers.

The pharmacokinetics of ketoprofen enantiomers were evaluated after 25-, 50-, and 100-mg doses of (R)- ketoprofen and 100 mg of racemic ketoprofen in 25 healthy volunteers (12 male and 13 female). The fractional inversion (Finv) of (R)- ketoprofen was 8.9 +/- 3.3% using plasma data and 10.0 +/- 2.2% using urine data. There were small (< 5%) but significant differences between the enantiomers for areas under the plasma concentration-time curve (AUC) after the racemic dose (P < 0.005). Half-lives were 130-144 minutes for (R)- ketoprofen and 132-209 minutes for (S)- ketoprofen. Dose proportionality in AUC and maximum plasma concentration (Cmax) values was noted for both enantiomers. A total of 69% of the dose was recovered in the urine as (R)- and (S)- ketoprofen and conjugates. The elimination rate constant of (R)- ketoprofen was significantly different (P < 0.05) between men and women. Exposure to cyclooxygenase inhibiting (S)- ketoprofen was approximately 10% of the dose after the administration of pure (R)- ketoprofen and was independent of gender.

Sensitive chiral high-performance liquid chromatographic assay for labetalol in biological fluids

The four stereoisomers of the combined α- and β-adrenoceptor antagonist labetalol were separated and quantified at therapeutic concentrations by normal-phase high-pressure liquid chromatography using a chiral stationary phase and fluorescence detection. Drug in plasma or urine was recovered by solid-phase extraction with 83±5% efficiency. Limits of detection from biological samples (3 ml) were between 1.5–1.8 ng ml −1. Intra-day and inter-day variation at 25 ng ml −1 were ≤2.7% and ≤5.80% respectively for all stereoisomers. The assay was applied to an examination of the disposition of labetalol stereoisomers after a single oral dose of racemate to a human volunteer. Labetalol appears to undergo enantioselective metabolism leading to relatively low plasma concentrations of the pharmacologically active enantiomers.

Pharmacokinetics and Pharmacodynamics of Pirmenol Enantiomers in Coronary Artery Ligated Dogs

The pharmacokinetics and pharmacodynamics of pirmenol enantiomers were investigated in coronary artery ligated mongrel dogs. Reduction in frequency of premature ventricular complexes (PVCs) was determined following intravenous administration of 5-mg/kg doses of racemic pirmenol (n=5), (+)-pirmenol (n=4), and (−)-pirmenol (n=4), each given as a 5-min infusion. Electrocardiographic signals and blood samples were obtained serially over a 4-h period. Pirmenol enantiomer concentrations in plasma were determined by a stereospecific assay. Following the racemate dose, (−)-pirmenol had 47% lower clearance and 33% lower steady-state distribution volume than (+)-pirmenol. These differences could be mostly explained by stereoselective plasma protein binding, reflected in a 58% higher unbound fraction for (+)-pirmenol compared with (−)-pirmenol following racemate administration. Unbound pirmenol distribution volumes were nearly identical for both enantiomers, and unbound clearance was only 16% lower for (−)-pirmenol than (+)-pirmenol following administration of the racemate. Similar trends were observed for pirmenol enantiomers administered individually. Both pirmenol enantiomers were equally effective in arrhythmia suppression. The antiarrhythmic response of coronary artery ligated dogs to pirmenol was described by a sigmoid Emax model, and no statistically significant differences were observed in the pharmacodynamic parameters [i.e., EC50 (plasma concentration at 50% of maximum drug effect), S (constant that reflects the sigmoidal shape of the effect–concentration curve), and EC90 (plasma concentration at 90% of maximum drug effect)] for (+)-pirmenol, (−)-pirmenol, or pirmenol racemate.

Bi-directional chiral inversion of ketoprofen in CD-1 mice.

The R enantiomers of some of the 2-arylpropionic acid non-steroidal antiinflammatory drugs (NSAIDs) are known to undergo metabolic chiral inversion to their more pharmacologically active antipodes. This process is drug and species dependent and usually unidirectional. The S to R chiral inversion, on the other hand, is rare and has been observed, in substantial extents, only for ibuprofen in guinea pigs and 2-phenylpropionic acid in dogs. After i.p. administration of single doses of racemic ketoprofen or its optically pure enantiomers to male CD-1 mice and subsequent study of the concentration time-course of the enantiomers, we noticed substantial chiral inversion in both directions. Following racemic doses, no stereoselectivity in the plasma-concentration time courses was observed. After dosing with optically pure enantiomer, the concentration of the administered enantiomer predominated during the absorption phase. During the terminal elimination phase, however, the enantiomers had the same concentrations. Our observation is suggestive of a rapid and reversible chiral inversion for ketoprofen enantiomers in mice.

Genotyping of S-mephenytoin 4'-hydroxylation in an extended Japanese population.

To assess the genotype pattern of S-mephenytoin 4'-hydroxylation in an extended Japanese population. One hundred eighty-six unrelated, healthy Japanese subjects were genotyped for S-mephenytoin 4'-hydroxylase (CYP2C19) according to a genotyping technique to identify the wild-type (wt) gene and two mutations, CYP2C19m1 in exon 5 and CYP2C19m2 in exon 4. Fourty-six of the 186 subjects genotyped were phenotyped with racemic mephenytoin using the conventional 8-hour urine analysis of 4'-hydroxymephenytoin. The frequency of poor metabolizers by the genotyping analysis was 18.8% (35 of the 186 subjects), consisting of 12 homozygous for CYP2C19m1 (m1/m1), three homozygous for CYP2C19m2 (m2/m2), and 20 heterozygous for the two defects (m1/m2). Thus the allele frequencies of CYP2C19m1 and CYP2C19m2 were calculated to be 0.29 and 0.13 (107 and 46 of the total of 372 alleles), respectively. Among the 46 subjects phenotyped, seven were identified as the poor metabolizers, with a log10 urinary excretion of 4'-hydroxymephenytoin of < 0.3% of the racemic dose. These seven subjects were genotyped as the individuals with the m1/m1 (two), m1/m2 (four) or m2/m2 (one) allele combination, indicating a complete concordance between the phenotyping and genotyping tests. The present genotyping test confirmed that the frequency of CYP2C19 mutant gene m1 is about 2.2 times greater than another mutant gene, m2, among Japanese poor metabolizers. The genotyping of CYP2C19 discriminates between the two S-mephenytoin 4'-hydroxylation phenotypes completely in the Japanese subjects.

Stereoselective determination of unchanged and glucuroconjugated eliprodil, a new anti-ischaemic drug, in human plasma and urine by precolumn derivatization and column-switching high-performance liquid chromatography with fluorescence detection

An HPLC method was developed and validated for the determination in human plasma and urine of the enantiomers of eliprodil, (±)-α-(4-chlorophenyl-4[(4-fluorophenyl)methyl]piperidine-1-ethanol hydrochloride, a new anti-ischaemic agent administered as a racemate. Both enantiomers are present in human plasma in unchanged and glucuroconjugated form, whereas only the glucuroconjugated form is excreted into urine; as a consequence, such metabolites in human plasma and urine should be submitted to enzymatic deconjugation with β-glucuronidase (Escherichia coli) before being extracted. The general method involves a liquid-liquid extraction of eliprodil and internal standard from alkalinized plasma or urine with n-hexane, evaporation of the organic phase and derivatization with ( S)-(+)-naphthylethyl isocyanate to give carbamate diastereoisomeric derivatives of ( S)-(+)- and ( R)-(−)-eliprodil and internal standard; after evaporation of the derivatizing mixture and dissolution of the residue in a small volume of phosphate buffer-acetonitrile (60:40, v/v), an aliqout is injected into a column-switching HPLC system. The derivatized sample extract is purified on a precolumn filled with C 8-bonded silica material, which is flushed with acetonitrile-water, then diastereoisomers of eliprodil and the internal standard are automatically transferred by the mobile phase to the analytical column. The analytical column is a C 8 type, specially deactivated for basic compounds, the mobile phase is 0.025 M phosphate buffer (pH 2.6)-methanol-acetonitrile (42:2:56) at a flow-rate of 1.2 ml min −1 and a fluorimetric detector operating at λ ex = 275 nm and λ em = 336 nm is used. The retention times, under these conditions, are about 16 and 17 min for ( S)-(+)- and ( R)-(−)-eliprodil diastereoisomers, respectively, and about 19 min for the first-eluted diastereoisomer of the internal standard. During the analysis time, the precolumn, reset in a different path from that of the analytical column, is back-flushed with different solvents, then re-equilibrated with acetonitrile-water before the next injection. Linearity in plasma, for unchanged eliprodil enantiomers, was assessed in the range 0.15–10 ng ml −1 and for total eliprodil enantiomers (unchanged + conjugated) in the range 0.75–500 ng ml −1; the limit of quantitation (LOQ) is 0.15 ng ml −1 for each unchanged enantiomer and 0.75 ng ml −1 for each total enantiomer. Linearity was also assessed in urine for total (conjugated) eliprodil enantiomers in the range 50–25 000 ng ml −1; the LOQ is 50 ng ml −1 for each enantiomer. The intra- and inter-day precision and accuracy of the method were investigated in plasma and urine and found to be satisfactory for pharmacokinetic studies. The method has been extensively used in pharmacokinetic studies in man treated with a 20-mg dose of eliprodil racemate and some results of this application are reported.

Determination of salbutamol enantiomers in human plasma and urine by chiral high-performance liquid chromatography

Enantiomers of salbutamol were directly separated ( R s =1.16) and quantitated at therapeutic concentrations after solid-phase extraction from human plasma and urine by normal-phase high-performance liquid chromatography on a chiral column with fluorescence detection. The assay was linear for each enantiomer between 1.25 and 500 ng ml −1 and had a minimum limit of detection of 250 pg ml −1. A 3-ml plasma or 1-ml urine sample was required for quantitation at therapeutic doses. Inter-day variation was 50% for S-(+)- and 6.5% for R-(−)-salbutamol. The assay was used to compare enantioselective disposition after single doses of racemate by the intravenous, oral and rectal routes.

Pharmacokinetics and pharmacodynamics of hydroxychloroquine enantiomers in patients with rheumatoid arthritis receiving multiple doses of racemate.

Hydroxychloroquine, a slow acting antirheumatic drug, is administered as the racemic mixture. Blood concentrations of the two enantiomers of hydroxychloroquine were measured in two studies, one study of eight patients, in whom blood and urine concentrations were measured during the first 6 months of therapy with rac-hydroxychloroquine, and one of 43 patients who had received rac-hydroxychloroquine therapy for at least 6 months. In the latter study rheumatoid disease activity was also measured. The pharmacokinetics of hydroxychloroquine were found to be enantioselective. The concentrations of (-)-(R)-hydroxychloroquine were higher than those of the (+)-(S)-antipode in all patients at all time points, although the ratios of the two enantiomers did display a two to three fold variability between patients. Both total and renal clearance were greater for the (+)-(S)-enantiomer. From the observational, cross-sectional study design used, it was not possible to differentiate concentration-effect relationships of the two enantiomers. The 11-fold range of drug concentrations swamped any effect of variability between patients in enantiomer proportions. Blood concentrations of both enantiomers were significantly higher in groups of patients with less active disease.

Stereoselective, competitive, and nonlinear plasma protein binding of ibuprofen enantiomers as determinedin vivo in healthy subjects

The plasma protein binding and competitive inhibition parameters of R(-)- and S(+)-ibuprofen were determined in vivo in 12 healthy subjects. Subjects participated in a 4 x 4 Latin square design in which oral solutions of drug were administered as 300 mg R(-)-ibuprofen, 300 mg S(+)-ibuprofen, 300 mg R(-)- + 300 mg S(+)-ibuprofen, and 300 mg R(-)- + 600 mg S(+)-ibuprofen. Unlabeled ibuprofen enantiomers were quantitated using a stereospecific reversed-phase HPLC assay, and plasma protein binding experiments were performed using radiolabeled 14C-enantiomers and an ultrafiltration method at 37C. At therapeutic drug concentrations, the protein binding of each enantiomer was greater than 99%. Furthermore, the binding of ibuprofen enantiomers was stereoselective and mutually competitive, as well as nonlinear. The bound-free data were fitted to a model in which the non-linearity of plasma protein binding and competition between enantiomers for binding sites could be accommodated. There were substantial differences in the affinity of ibuprofen enantiomers for protein binding sites (RP2 = 0.358 +/- 0.185 vs. SP2 = 0.979 +/- 0.501 micrograms/ml; mean +/- SD) but no differences in their binding capacity (RP1 = 160 +/- 86 vs. SP1 = 161 +/- 63 micrograms/ml). Although statistically significant, the differences in competitive inhibition parameters were more modest (SKI = 0.661 +/- 0.363 vs. RKI = 0.436 +/- 0.210 micrograms/ml). As a result, the intrinsic binding (i.e., P1/P2) of R(-)-ibuprofen was greater than S(+)-ibuprofen, and the unbound fraction was significantly greater for S-enantiomer vs. R-enantiomer after a given dose of R-ibuprofen or racemate.