Effect of route of administration on the pharmacokinetic behavior of enantiomers of nefopam and desmethylnefopam

A Simultaneous Mixed-Effects Pharmacokinetic Model for Nefopam, N-desmethylnefopam, and Nefopam N-Oxide in Human Plasma and Urine

BACKGROUND AND OBJECTIVE

Nefopam is a non-opioid, non-steroidal, central analgesic thought to act via multiple mechanisms including potent inhibition of serotonin-norepinephrine reuptake and modulation of voltage-sensitive calcium and sodium channels. There has been a resurgence in its use for postoperative pain and neuropathic pain. Dosing route-dependent metabolism and clinical effects have been described following intravenous and oral nefopam. N-desmethylnefopam and nefopam N-oxide are metabolites of clinical interest. We sought to develop a joint pharmacokinetic model to simultaneously describe the plasma and urinary pharmacokinetics of nefopam and the two metabolites following an oral pharmacological dose of [¹⁴C]-nefopam to healthy volunteers, and to estimate inter-individual variability in their pharmacokinetics.

METHODS

Pharmacokinetic data for the parent and metabolites were analyzed simultaneously using NONMEM^® (nonlinear mixed-effect modeling) v7.3. The modeling process evaluated, in part, one- and two-compartment linear pharmacokinetic models for nefopam and a single compartment for each of the two metabolites. Pathways for presystemic metabolism of both metabolites were explored.

RESULTS

The final structural model simultaneously described the plasma and urinary pharmacokinetics of nefopam and the two metabolites. It consists of a central compartment for nefopam and for each of the two metabolites, as well as a peripheral compartment for the parent, and the associated urine compartments. The rapid formation and appearance of the N-oxide in plasma, characterized by concentrations that peak earlier than the parent, could be described by presystemic formation in the gastrointestinal tract.

CONCLUSIONS

A descriptive, robust and predictive parent-metabolite model has been developed using a population mixed-effects approach to characterize the pharmacokinetics of nefopam and its metabolites simultaneously in healthy subjects following oral administration of nefopam. The model may be used for dose selection, analysis of sparse data, identification of intrinsic and extrinsic factors, and to model the clinical effects of each analyte.

Development and statistical optimization of nefopam hydrochloride loaded nanospheres for neuropathic pain using Box-Behnken design

Nefopam hydrochloride (NFH) is a non-opioid centrally acting analgesic drug used to treat chronic condition such as neuropathic pain. In current research, sustained release nefopam hydrochloride loaded nanospheres (NFH-NS) were auspiciously synthesized using binary mixture of eudragit RL 100 and RS 100 with sorbitan monooleate as surfactant by quasi solvent diffusion technique and optimized by 35 Box-Behnken designs to evaluate the effects of process and formulation variables. Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetric (DSC) and X-ray diffraction (XRD) affirmed absence of drug-polymer incompatibility and confirmed formation of nanospheres. Desirability function scrutinized by design-expert software for optimized formulation was 0.920. Optimized batch of NFH-NS had mean particle size 328.36 nm ± 2.23, % entrapment efficiency (% EE) 84.97 ± 1.23, % process yield 83.60 ± 1.31 and % drug loading (% DL) 21.41 ± 0.89. Dynamic light scattering (DLS), zeta potential analysis and scanning electron microscopy (SEM) validated size, charge and shape of nanospheres, respectively. In-vitro drug release study revealed biphasic release pattern from optimized nanospheres. Korsmeyer Peppas found excellent kinetics model with release exponent less than 0.45. Chronic constricted injury (CCI) model of optimized NFH-NS in Wistar rats produced significant difference in neuropathic pain behavior (p < 0.05) as compared to free NFH over 10 h indicating sustained action. Long term and accelerated stability testing of optimized NFH-NS revealed degradation rate constant 1.695 × 10-4 and shelf-life 621 days at 25 ± 2 °C/60% ± 5% RH.

Pharmacokinetics, distribution, metabolism, and excretion of the dual reuptake inhibitor [(14)C]-nefopam in rats

1. This study examined the pharmacokinetics, distribution, metabolism, and excretion of [(14)C] nefopam in rats after a single oral administration. Blood, plasma, and excreta were analyzed for total radioactivity, nefopam, and metabolites. Metabolites were profiled and identified. Radioactivity distribution was determined by quantitative whole-body autoradiography. 2. The pharmacokinetic profiles of total radioactivity and nefopam were similar in male and female rats. Radioactivity partitioned approximately equally between plasma and red blood cells. A majority of the radioactivity was excreted in urine within 24 hours and mass balance was achieved within 7 days. 3. Intact nefopam was a minor component in plasma and excreta. Numerous metabolites were identified in plasma and urine generated by multiple pathways including: hydroxylation/oxidation metabolites (M11, M22a and M22b, M16, M20), some of which were further glucuronidated (M6a to M6c, M7a to M7c, M8a and M8b, M3a to M3d); N-demethylation of nefopam to metabolite M21, which additionally undergoes single or multiple hydroxylations or sulfation (M9, M14, M23), with some of the hydroxylated metabolites further glucuronidated (M2a to M2d). 4. Total radioactivity rapidly distributed with highest concentrations found in the urinary bladder, stomach, liver, kidney medulla, small intestine, uveal tract, and kidney cortex without significant accumulation or persistence. Radioactivity reversibly associated with melanin-containing tissues.

[Nefopam by continuous intravenous injection and adverse drug reactions: which causality assessment?]

We report the case of a 77-year-old man, with nefopam postoperative analgesia, who developed subacute neurological symptoms, whereas he had profound hypoprotidemia and acute renal failure. Chronological, semiological and bibliographical criteria are in favour of causality assessment. The plasma nefopam concentration (135 ng/ml) during the neurological symptoms is another argument.

Specific and sensitive analysis of nefopam and its main metabolite desmethyl-nefopam in human plasma by liquid chromatography–ion trap tandem mass spectrometry

A specific and sensitive liquid chromatography-tandem mass spectrometric (LC-MS-MS) method using an ion trap spectrometer was developed for quantitation of nefopam and desmethyl-nefopam in human plasma. Nefopam, desmethyl-nefopam and the internal standard (ethyl loflazepate) were extracted in a single step with diethyl ether from 1 mL of alkalinized plasma. The mobile phase consisted of acetonitrile with 0.1% formic acid (50:50, v:v). It was delivered at a flow-rate of 0.3 mL/min. The effluent was monitored by MS-MS in positive-ion mode. Ionisation was performed using an electrospray ion source operating at 200 degrees C. Nefopam and desmethyl-nefopam were identified and quantified in full scan MS-MS mode using a homemade MS-MS library. Calibration curves were linear over the concentration range of 0.78-100 ng/mL with determination coefficients >0.996. This method was fast (total run time<6 min), accurate (bias<12.5%), and reproducible (intra- and inter-assay precision<17.5%) with a quantitation limit of 0.78 ng/mL. The high specificity and sensitivity achieved by this method allowed the determination of nefopam and desmethyl-nefopam plasma levels in patients following either intermittent or continuous intravenous administration of nefopam.

Liquid separation techniques coupled with mass spectrometry for chiral analysis of pharmaceuticals compounds and their metabolites in biological fluids

Determination of the chiral composition of drugs is nowadays a key step in order to determine purity, activity, bioavailability, biodegradation, etc., of pharmaceuticals. In this article, works published for the last 5 years on the analysis of chiral drugs by liquid separation techniques coupled with mass spectrometry are reviewed. Namely, chiral analysis of pharmaceuticals including, e.g., antiinflammatories, antihypertensives, relaxants, etc., by liquid chromatography-mass spectrometry and capillary electrophoresis-mass spectrometry are included. The importance and interest of the analysis of the enantiomers of the active compound and its metabolites in different biological fluids (plasma, urine, cerebrospinal fluid, etc.) are also discussed.

Simultaneous chiral analyses of multiple analytes: case studies, implications and method development considerations

The field of chiral separations had a modest beginning some two decades ago. However, due to rapid technological advancement coupled with simultaneous availability of innovative chiral stationary phases and novel chiral derivatization agents, the field of chiral separations has now totally outpaced many other separation fields. Keeping pace with rapid changes in the field of chiral separations, investigators continue to add stereoselective pharmacokinetic, pharmacodynamic, pharmacologic and toxicological data of new and/or marketed racemic compounds to the literature. Examination of the evolution of chiral separations suggests that in the beginning many investigators attempted to separate and quantify a single pair of enantiomers, adopting either direct (separation made on a chiral stationary phase) or indirect (separation made following precolumn conversion of enantiomers to corresponding diastereomers) approaches. However, more recent trends in chiral separations suggest that investigators are attempting to separate and quantify multiple pairs of enantiomers with available technologies. Added to this, some interesting trends have been observed in many of the recently reported chiral applications, including preferences regarding internal standard selection, mobile phase contents and composition, sorting out issues with mass spectrometric detection, determination of elution order, analytical manipulations of metabolite(s) without reference standards and addressing some specificity-related issues. This review mainly focuses on chiral separations involving multiple chiral analytes and attempts to justify the need for such chiral separations involving multiple analytes. In this context, several cases studies are described on the utility and applicability of such chiral separations under discrete headings to provide an account to the readership on the implications of such tasks. The topics of case studies covered in this review include: (a) therapy markers--differentiation from drug abuse and/or applicability in forensics; (b) role in pharmacogenetic/polymorphic evaluation; (c) monitoring and understanding the role of parent and active metabolite(s) in clinical and preclinical investigations; (d) exploration on the pharmacokinetic utility of an active chiral metabolite vis-a-vis the racemic parent moiety; (e) understanding the chirality play in delineating peculiar toxic effects; (f) exploration of chiral inversion phenomenon, and understanding the role of stereoselective metabolism. For the further benefit of readership, some select examples (n = 19) of the separation of multiple chiral analytes with appropriate information on chromatography, detection system, validation parameters and applicable conclusion are also provided. Finally, the review covers some useful considerations for method development involving multiple chiral analytes.