Nefopam enantiomers: preclinical pharmacology/toxicology and pharmacokinetic characteristics in healthy subjects after intravenous administration

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.

Rediscovery of nefopam for the treatment of neuropathic pain

Nefopam (NFP) is a non-opioid, non-steroidal, centrally acting analgesic drug that is derivative of the non-sedative benzoxazocine, developed and known in 1960s as fenazocine. Although the mechanisms of analgesic action of NFP are not well understood, they are similar to those of triple neurotransmitter (serotonin, norepinephrine, and dopamine) reuptake inhibitors and anticonvulsants. It has been used mainly as an analgesic drug for nociceptive pain, as well as a treatment for the prevention of postoperative shivering and hiccups. Based on NFP's mechanisms of analgesic action, it is more suitable for the treatment of neuropathic pain. Intravenous administration of NFP should be given in single doses of 20 mg slowly over 15-20 min or with continuous infusion of 60-120 mg/d to minimize adverse effects, such as nausea, cold sweating, dizziness, tachycardia, or drowsiness. The usual dose of oral administration is three to six times per day totaling 90-180 mg. The ceiling effect of its analgesia is uncertain depending on the mechanism of pain relief. In conclusion, the recently discovered dual analgesic mechanisms of action, namely, a) descending pain modulation by triple neurotransmitter reuptake inhibition similar to antidepressants, and b) inhibition of long-term potentiation mediated by NMDA from the inhibition of calcium influx like gabapentinoid anticonvulsants or blockade of voltage-sensitive sodium channels like carbamazepine, enable NFP to be used as a therapeutic agent to treat neuropathic pain.

Compartmental pharmacokinetics of nefopam during mild hypothermia

BACKGROUND

Nefopam is a non-opioid, non-steroidal, centrally acting analgesic which has an opioid-sparing effect. It also reduces the threshold (triggering core temperature) for shivering without causing sedation or respiratory depression. The drug is therefore useful as both an analgesic and to facilitate induction of therapeutic hypothermia. However, compartmental pharmacokinetics during hypothermia are lacking for nefopam.

METHODS

We conducted a prospective, randomized, blinded study in eight volunteers. On two different occasions, one of two nefopam concentrations was administered and more than 30 arterial blood samples were gathered during 12 h. Plasma concentrations were determined using gas chromatography/mass spectrometry to investigate the pharmacokinetics of nefopam with non-linear mixed-effect modelling.

RESULTS

A two-compartment mammillary model with moderate inter-individual variability and inter-occasional variability independent of covariates was found to best describe the data [mean (SE): V(1)=24.13 (2.8) litre; V(2)=183.34 (13.5) litre; Cl(el)=0.54 (0.07) litre min(-1); Cl(dist)=2.84 (0.42) litre min(-1)].

CONCLUSIONS

The compartmental data set describing a two-compartment model was determined and could be implemented to drive automated pumps. Thus, work load could be distributed to a pump establishing and maintaining any desired plasma concentration deemed necessary for a treatment with therapeutical hypothermia.

The effects of nefopam on the gain and maximum intensity of shivering in healthy volunteers

BACKGROUND

Mild hypothermia has been shown to improve neurologic outcome after cardiac arrest. Nefopam, a centrally acting, nonsedative analgesic, decreases the threshold of shivering, but not vasoconstriction, and thus might be a suitable drug for induction of therapeutic hypothermia. However, not only the threshold but also the gain and maximum intensity of shivering define the thermoregulatory properties of a drug and thus are clinically important. Therefore, we evaluated the gain and maximum intensity of shivering at 2 different doses of nefopam and placebo.

METHODS

Seven healthy volunteers were randomly assigned to 3 study days: (1) control (saline), (2) small-dose nefopam (50 ng/mL), and (3) large-dose nefopam (100 ng/mL). On all study days volunteers were cooled using central venous infusion of cold IV fluid while mean skin temperature was maintained at 31 degrees C. Core temperature was recorded at the tympanic membrane. Threshold, gain, and maximum intensity of shivering were evaluated using oxygen consumption.

RESULTS

Both 50 and 100 ng/mL nefopam significantly reduced the shivering threshold as well as the gain of shivering: shivering threshold: 35.6 degrees C + or - 0.2 degrees C (control); 35.2 degrees C + or - 0.3 degrees C (small dose); 34.9 degrees C + or - 0.5 degrees C (large dose), P = 0.004; gain of shivering: 597 + or - 235 mL x min(-1) x degrees C(-1) (control); 438 + or - 178 mL x min(-1) x degrees C(-1) (small dose); 301 + or - 134 mL x min(-1) x degrees C(-1) (large dose), P = 0.028. Maximum intensity of shivering did not differ among the 3 treatments.

CONCLUSIONS

Nefopam significantly reduced the gain of shivering. This reduction, in combination with a reduced shivering threshold, will allow clinicians to cool patients even further when therapeutic hypothermia is indicated.

[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.

Nefopam is more potent than carbamazepine for neuroprotection against veratridine in vitro and has anticonvulsant properties against both electrical and chemical stimulation

Nefopam (NEF) is a known analgesic that has recently been shown to be effective in controlling both neuropathic pain and convulsions in rodents. In this study we compared nefopam to carbamazepine (CBZ), a reference antiepileptic drug (AED), for their ability to protect cerebellar neuronal cultures from neurodegeneration induced by veratridine (VTD). Furthermore, we tested nefopam for protection against both, maximal electroshock-induced seizures (MES), and isoniazid-induced seizures in mice. Both NEF and CBZ were effective in preventing both signs of excitotoxicity and neurodegeneration following exposure of cultures to 5 microM veratridine for 30 min and 24 h, respectively. Concentrations providing full neuroprotection were 500 microM CBZ and 50 microM NEF, while the concentration providing 50% neuroprotection was 200 microM for CBZ and 20 microM for NEF. Neither NEF nor CBZ reduced excitotoxicity following direct exposure of cultures to glutamate, but CBZ failed to reduce increases in intracellular calcium following stimulation of L-type voltage sensitive calcium channels. In vivo, NEF (20 mg/kg i.p.) significantly reduced MES and fully prevented MES-induced terminal clonus (TC). In comparison, NEF was significantly more effective than CBZ in preventing MES, although both drugs were equally effective against MES-induced TC. Furthermore, nefopam provided protection against isoniazid-induced seizures at doses similar to those protecting against MES.

Palladium-Mediated Intramolecular Aryl Amination on Furanose Derivatives: An Expedient Approach to the Synthesis of Chiral Benzoxazocine Derivatives and Tricyclic Nucleosides

Pd-catalyzed intramolecular arylamination on sugar derivatives has been accomplished by using bulky biaryl phosphine ligands. An application of this methodology on a variety of D-glucose-derived substrates, 2a-f, led to the synthesis of highly functionalized cis-fused tricyclic oxazocines, 3a-e. The products could subsequently be transformed to the optically active benzoxazocine derivative 4 and tricyclic nucleoside 6. This is the first example of the synthesis of eight-membered rings via intramolecular cycloamination of furanose derivatives, which provides a very useful method for the catalytic synthesis of medium-ring heterocycles.

Spectrophotometric microdetermination of nefopam, mebevrine and phenylpropanolamine hydrochloride in pharmaceutical formulations using alizarins

Simple and rapid spectrophotometric procedures have been established for quantitation of nefopam hydrochloride (NF) mebevrine hydrochloride (MB) and phenylpropanolamine hydrochloride (PP). The procedures are based on the reaction between the examined drugs (NF, MB and PP) and alizarin (I), alizarin red S (II), alizarin yellow G (III) and quinalizarin (IV) producing ion-pair complexes which can be measured at the optimum wavelength. The optimization of the reaction conditions is investigated. Beer's law is obeyed in the concentration ranges 0.5-30.0 microg ml(-1). The molar absorptivity, Sandell sensitivity, detection and quantification limits are also calculated. The correlation coefficient was > or =0.9988 (n=6) with a relative standard deviation (R.S.D.) of < or =1.3, for six determinations of 20 microg ml(-1). The methods are successfully applied to the determination of NF, MB and PP in their pharmaceutical formulations.

Nefopam blocks voltage-sensitive sodium channels and modulates glutamatergic transmission in rodents

In order to specify the nature of interactions between the analgesic compound nefopam and the glutamatergic system, we examined the effects of nefopam on binding of specific ligands on the three main subtypes ionotropic glutamate receptors: N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), or quisqualic acid (QA) and kainic acid (KA) in rat brain membrane preparations. Functionally, we investigated the effects of nefopam against the seizures induced by agonists of these excitatory glutamate receptors in mice. Since the synaptic release of glutamate mainly depends upon the activation of membrane voltage-sensitive sodium channels (VSSCs), the nature of interactions between nefopam and these ionic channels was studied by evaluating the effects of nefopam on binding of 3H-batrachotoxinin, a specific ligand of the VSSCs in rat brain membrane preparations. The functional counterpart of the binding of nefopam on VSSCs was evaluated by its effects on the 22Na uptake-stimulated by veratridine on human neuroblastoma cells and in the maximal electroshock test in mice. Nefopam showed no affinity for the subtypes of ionotropic glutamate receptors up to 100 microM. On the other hand, nefopam was effective against NMDA, QA and KA induced clonic seizures in mice. Nefopam displaced 3H-batrachotoxinin and inhibited the uptake of 22Na in the micromolar range and it protected mice against electroshock induced seizures. Nefopam may block the VSSCs activity: consequently, at the presynaptic level, this effect led to a reduction of glutamate release and at the postsynaptic level, it led to a decrease of the neuronal excitability following activation of the glutamate receptors.

Nefopam, a nonsedative benzoxazocine analgesic, selectively reduces the shivering threshold in unanesthetized subjects

BACKGROUND

The analgesic nefopam does not compromise ventilation, is minimally sedating, and is effective as a treatment for postoperative shivering. The authors evaluated the effects of nefopam on the major thermoregulatory responses in humans: sweating, vasoconstriction, and shivering.

METHODS

Nine volunteers were studied on three randomly assigned days: (1) control (saline), (2) nefopam at a target plasma concentration of 35 ng/ml (low dose), and (3) nefopam at a target concentration of 70 ng/ml (high dose, approximately 20 mg total). Each day, skin and core temperatures were increased to provoke sweating and then reduced to elicit peripheral vasoconstriction and shivering. The authors determined the thresholds (triggering core temperature at a designated skin temperature of 34 degrees C) by mathematically compensating for changes in skin temperature using the established linear cutaneous contributions to control of each response.

RESULTS

Nefopam did not significantly modify the slopes for sweating (0.0 +/- 4.9 degrees C. microg-1. ml; r2 = 0.73 +/- 0.32) or vasoconstriction (-3.6 +/- 5.0 degrees C. microg-1. ml; r2 = -0.47 +/- 0.41). In contrast, nefopam significantly reduced the slope of shivering (-16.8 +/- 9.3 degrees C. microg-1. ml; r2 = 0.92 +/- 0.06). Therefore, high-dose nefopam reduced the shivering threshold by 0.9 +/- 0.4 degrees C (P < 0.001) without any discernible effect on the sweating or vasoconstriction thresholds.

CONCLUSIONS

Most drugs with thermoregulatory actions-including anesthetics, sedatives, and opioids-synchronously reduce the vasoconstriction and shivering thresholds. However, nefopam reduced only the shivering threshold. This pattern has not previously been reported for a centrally acting drug. That pharmacologic modulations of vasoconstriction and shivering can be separated is of clinical and physiologic interest.

Randomized prospective study of the analgesic effect of nefopam after orthopaedic surgery † †Declaration of interest. This work has been sponsored by Biocodex Laboratories, in charge of nefopam (Acupan injectable™) commercialization

BACKGROUND

Balanced postoperative analgesia combines non-narcotic drugs and opioids. We organized a large study to evaluate nefopam analgesia and tolerance in combination with morphine for patient-controlled analgesia (PCA) after orthopaedic surgery.

METHODS

Two hundred and one patients scheduled to undergo hip arthroplasty were included in this multicentre (n=24), double-blind, randomized study comparing nefopam (20 mg every 4 h for 24 h) with placebo, the first dose being infused peroperatively. The primary outcome measure was the cumulative morphine dose received postoperatively by PCA over 24 h. Secondary outcome measures were the amount of morphine received as a loading dose in the postanaesthesia care unit (PACU) and during the 24-h observation period, and pain assessments using a visual analogue scale (VAS) and a verbal pain scale (VPS), patient's satisfaction with analgesia and treatment tolerance.

RESULTS

The two groups were comparable with respect to their characteristics and preoperative pain assessment. PCA-administered morphine over 24 h was significantly less for the nefopam group than the control group (21.2 (15.3) and 27.3 (19.2) mg respectively; P=0.02). This morphine-sparing effect was greater (35.1%) for patients with severe preoperative pain (VAS>30/100). For the entire study period (loading dose and PCA), morphine use was less for the nefopam group (34.5 (19.6) vs 42.7 (23.6) mg; P=0.01). Pain VAS at PACU arrival and during the whole PACU period was significantly lower for the nefopam than for the placebo group (P=0.002 and 0.04 respectively). Patient satisfaction was similar for the nefopam and placebo groups.

CONCLUSION

In combination with PCA morphine, nefopam gives significant morphine-sparing with lower immediate postoperative pain scores without major side-effects. This analgesic effect seems to be particularly notable for patients with intense preoperative pain.

Comparative pharmacokinetics and pharmacodynamics of intravenous and oral nefopam in healthy volunteers

To determine the pharmacokinetic, subjective effects of a single 20 mg dose of nefopam administered either intravenously or orally in healthy volunteers, twenty-four healthy Caucasian men received 20 mg nefopam orally+placebo intravenous infusion and placebo orally+intravenous infusion of 20 mg nefopam with one week interval, in a double-blind, double-dummy cross-over study. Nefopam and desmethyl-nefopam plasma concentrations were measured by HPLC with UV detection up to 48 hr after drug administration. Self-rating questionnaires (Mood and vigilance Visual Analogue Scales, Addiction Research Centre Inventory) and drug safety were investigated. The F value (bioavailability) of the parent drug was 0.36+/-0.13. The AUCoral/AUCiv ratio of nefopam+desmethyl-nefopam was 0.62+/-0.23. The half-life of nefopam was similar whether administered orally (5.1+/-1.3 hr) or intravenously (5.1+/-0.6 hr). The half-life of desmethyl-nefopam was two to three times longer than that of the parent molecule (orally: 10.6+/-3.0 versus 5.1+/-1.3 hr, P<10(-4) and intravenously: 15.0+/-2.4 versus 5.1+/-0.6 hr, P<10(-4)). As assessed by the Addiction Research Centre Inventory, no evidence of abuse liability in healthy, drug-naive volunteers was observed. On visual analogue scales, volunteers rated themselves as more drowsy, less alert, less energetic and less anxious after oral compared to intravenous administration. The AUC0-->24 hr of anxiety and energy parameters were not different after oral and intravenous administration: 90+/-142 versus 35+/-84 (P=0.27) and 66+/-74 versus 46+/-54 mm x hr (P=0.36), respectively. The AUC0-->24 hr of drowsiness and alertness parameters were significantly greater after oral than after intravenous administration: 68+/-65 versus 27+/-30 (P=0.005) and 54+/-63 versus 28+/-48 mm x hr (P=0.03), respectively. A clockwise hysteresis loop was observed for drowsiness in 16 out of 24 volunteers after oral administration. The results suggest that in healthy volunteers desmethyl-nefopam may contribute to the pharmacodynamic effects of single dose nefopam solution administered orally. This study shows a rather low bioavailability of nefopam given in intravenous solution when administered orally. Nevertheless, when the main metabolite desmethyl-nefopam is taken into account, the ratio of the areas under the curves is almost doubled.

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

Nefopam hydrochloride is a non-narcotic analgesic used parenterally and orally as a racemic mixture for the relief of postoperative pain. However, no information is presently available on the oral kinetics of (+) and (-) nefopam in humans. Also, nefopam is metabolized by N-demethylation but it is not known whether the desmethylnefopam enantiomers (DES1 and DES2) are present in plasma following intravenous (I.V.) or oral administration of parent drug. To address these issues, 24 healthy white male subjects received two treatments using a double-blind, placebo-controlled crossover design: oral administration of 20 mg nefopam hydrochloride solution or a placebo solution on a sugar cube, simultaneously with a continuous infusion of 20 mg nefopam hydrochloride or placebo infusion. A chiral assay using LC-MS was developed for the simultaneous determination of both enantiomers of the parent drug and its metabolite in plasma and urine. Following I.V. administration, the kinetics of (+) and (-) nefopam could be fitted to a bi-exponential equation but exhibited no stereoselectivity. Both enantiomers had large clearances (53.7 and 57.5 L/hr) and volumes of distribution (390 and 381 L) and half-lives around 5 hours. Following oral administration, (+) and (-) nefopam were rapidly absorbed with bioavailabilities of 44% and 42%, respectively, probably due to a first-pass effect. After I.V. administration, the enantiomers of desmethylnefopam exhibited lower concentrations and longer half-lives (20.0 h for DES1 and 25.3 h for DES2) relative to nefopam enantiomers. Following oral administration, desmethylnefopam enantiomers' plasma concentrations peaked earlier and higher than after I.V. administration (P < 0.05). Following I.V. and oral administration, desmethylnefopam enantiomers showed stereoselectivity in AUC and Cmax values. Urinary excretion of parent and metabolite enantiomers was less than 5% of dose. This study shows that desmethylnefopam enantiomers can contribute to the analgesic effect of racemic nefopam only when it is administered orally.

Sensitive determination of nefopam and its metabolite desmethyl-nefopam in human biological fluids by HPLC

Nefopam (NEF) and desmethyl-nefopam (DMN) were assayed simultaneously in plasma, globule and urine samples using imipramine as internal standard. A liquid-liquid extraction procedure was coupled with a reverse phase high-performance liquid chromatography system. This system requires a mobile phase containing buffer (15 mM KH(2)PO(4) with 5 mM octane sulfonic acid: pH 3.7) and acetonitrile (77:33, v/v) through (flow rate=1.5 ml/min) a C(18) Symmetry column (150x4.6 I.D., 5 micrometer particle size: Waters) and a UV detector set at 210 nm. Internal standard was added to 1 ml of plasma or globule sample or 0.5 ml of urine sample, prior to the extraction under alkaline ambiance with n-hexane. The limits of quantification were 1 and 2 ng/ml for both molecules in plasma and globule, respectively; 5 and 10 ng/ml for NEF and DMN in urine, respectively. The method proved to be accurate and precise: the relative error at three concentrations ranged from -13.0 to +12.3% of the nominal concentration for all molecule and biological fluid; the within-day and between-day precision (relative standard deviation %) ranged from 1.0 to 10.1% for all the molecules and biological fluids. The method was linear between 1 and 60 ng/ml for both molecules in the plasma; 2 and 25 ng/ml for both molecules in the globule; 25 and 250 ng/ml for NEF and 50 and 500 ng/ml for DMN in the urine: correlation coefficients of calibration curves (determined by least-squares regression) of each molecule were higher than 0.992 whatever the biological fluid and during the pre-study and in-study validations. This method was successfully applied to a bio-availability study of NEF in healthy subjects.

Effects of nefopam on the spinal nociceptive processes: a c-Fos protein study in the rat

We have evaluated the effects of nefopam on the spinal c-Fos protein expression in the model of acute (noxious heat) and persistent (intraplantar injection of formalin) nociception in the rat. One and two hours after i.pl. formalin injection, c-Fos immunoreactive (c-Fos-IR) nuclei were preferentially located in the superficial (I-II) and deep (V-VI) laminae of the spinal dorsal horn of segments L4-L5, i.e. spinal areas containing numerous neurons responding exclusively, or not, to peripheral nociceptive stimuli. The doses of 15 and 30 mg/kg (s.c.) of nefopam had significant reducing effects on the formalin-evoked spinal c-Fos protein expression (36+/-14% and 47+/-9% reduction of the total number of c-Fos-IR nuclei per section, respectively, P<0.05 for both). These reducing effects of nefopam were not detectable 2 h after formalin. These results provide evidence that the significant effects of nefopam are time-limited in the formalin model of persistent nociception. One hour after noxious heat stimulation (52 degrees C for 15 s), c-Fos-IR nuclei were principally located in the superficial laminae I-II of the spinal dorsal horn (about 90% of the total number of c-Fos-IR nuclei per section). Nefopam (15 mg/kg s.c.) significantly reduced the noxious heat-evoked spinal c-Fos protein expression (33+/-3% reduction of the total number of c-Fos-IR nuclei, P<0.0001). The present results provide first evidence for the reducing effects of nefopam on the noxiously evoked spinal c-Fos protein expression, principally in acute nociceptive processes. These results suggest that nefopam may produce antinociceptive effects mainly in acute pain states.

The enantiomer debate: current status and future directions

Psychopharmacology, and the treatment of depression in particular, is one area where there is currently an increasing awareness of chiral drug phenomena. It is now generally agreed that the pharmacological and pharmacokinetic properties of chiral drugs and their enantiomers should be assessed early on in the drug development process. However, issues surrounding the potential advantages and disadvantages of enantiomeric drugs, compared with racemates, continue to be debated. This article highlights some of the key issues raised during one such debate, the aim of which was to address the hypothesis that each enantiomeric drug should be considered on its own merit. The idea is considered both in general terms and specifically in the context of clinical psychiatry, with an emphasis on the treatment of depression. An update on recent developments in enantiomeric antidepressant therapy is also provided. Copyright 2001 John Wiley & Sons, Ltd.

An introduction to enantiomers in psychopharmacology

There is growing scientific, clinical, commercial and regulatory recognition that enantiomers offer benefits over racemates in the management of psychiatric diseases as well as in clinical medicine generally. However, relatively few studies consider enantiomers' individual characteristics. This review considers some of the clinical benefits associated with using stereochemically pure drugs in psychiatric conditions other than depression.A review of the evidence shows that enantiomers offer four main benefits. Firstly, using a single enantiomer may allow a reduction in total dose, while maintaining or improving outcomes. For example, (+)-nefopam's antinociceptive activity is greater than that produced by both the racemate and (-)-nefopam, but with the same level of acute toxicity. Thus, a single enantiomer may offer greater efficacy, dose for dose, than the racemate. Secondly, assessing dose-response relationships is simpler. There is no reason to suppose that a racemate will necessarily contain the isomers' optimum therapeutic ratio, that one of the isomers will be inactive or that the enantiomers' dose-response curves will coincide. For example, the dose-response relationship for the induction of catalepsy in the rat by thioridazine suggested that the racemate was around 12 times more potent than (+)-thioridazine and three times more potent than (-)-thioridazine, when considering the actual concentrations in the striatum. Thirdly, using a single enantiomer may reduce pharmacokinetic and pharmacodynamic variability between patients. For example, the coefficients of variation for some of methadone's pharmacokinetic parameters may reach 70%, which might have clinical consequences. Finally, using a single enantiomer may reduce toxicity arising from the therapeutically inactive stereoisomer. For example, the single enantiomers of bupivacaine and ropivacaine are significantly less cardiotoxic than their respective racemates.This review illustrates why stereochemistry should be considered when assessing the toxicology, pharmacokinetics, metabolism and efficacy of a racemate. Indeed, the differences may be so marked that achiral analyses may be misleading, and clinicians should consider prescribing an enantiomer whenever possible. In many cases, prescribing a single enantiomer improves the benefit:risk ratio. Finally, there is no reason to suppose that a racemate's characteristics will apply to the constituent enantiomers. Copyright 2001 John Wiley & Sons, Ltd.