Buflomedil for intermittent claudication.

BACKGROUND: Intermittent claudication is pain, caused by chronic occlusive arterial disease, that develops in a limb during exercise and is relieved with rest. Buflomedil is a vasoactive agent claimed to have beneficial effects on the microcirculation. It is used chiefly to treat peripheral vascular disease and to a lesser extent for cerebrovascular arterial disease. However, its clinical efficacy for intermittent claudication has not yet been critically examined. OBJECTIVES: To evaluate the available evidence on the efficacy of buflomedil for intermittent claudication. SEARCH STRATEGY: We searched Medline, International Pharmaceutical Abstracts (IPA) and the Cochrane Controlled Trials Register. Abbott Laboratories, the distributor of buflomedil, was asked to provide reports of controlled clinical trials. Reference lists of retrieved articles were checked, and enquiries sent to authors of known trials, to identify additional trials. Finally, we conducted a Science Citation Index search. SELECTION CRITERIA: Trial reports had to be double-blinded, randomized, and conformed to our PIO-criteria (Patients, Intervention, Outcome) to be considered for inclusion. Patients were required to have proven intermittent claudication (Fontaine stage II); the intervention was to be oral administration of buflomedil compared to placebo; and outcomes had to include pain-free walking distance (PFWD) and maximum walking distance (MWD) analysed by standardized exercise test. DATA COLLECTION AND ANALYSIS: Searches of bibliographic databases yielded three eligible randomized controlled trials (RCTs) and a meta-analysis referring to nine eligible trials. Two of these nine trials had already been identified; two had been published in journals not referenced in traditional bibliographic indexes; and five were unpublished. Despite multiple requests, only one of the five unpublished trials was provided by the author of the meta-analysis, the other four could not be retrieved. Four of the six eligible trials retrieved were subsequently excluded after quality evaluation. Data on walking distances were extracted from the two remaining trials. Differences in incremental gain between active and placebo groups for PFWD and MWD with their confidence intervals were calculated. MAIN RESULTS: Both RCTs showed moderate improvements in PFWD for patients on buflomedil. In one trial this improvement (75 m, 95% CI 37-114) was statistically significant, but in the other, with a wholly diabetic population, it was non-significant (81m, 95% CI -9-170) compared to placebo. For both RCTs the gains in MWD were statistically significant, but with wide confidence intervals (81 m, 95% CI 30-131; and 171 m, 95% CI 27-316 respectively). Pooling of the data was not attempted. REVIEWER'S CONCLUSIONS: There is little evidence available to evaluate the efficacy of buflomedil for intermittent claudication. Most available trials are of poor quality and were excluded. The two trials included showed moderately positive results but these are undermined by publication bias since we know of another four unpublished, irretrievable, and inconclusive studies. There is a lack evidence for the efficacy of buflomedil in intermittent claudication.

Oral vasoactive medication in intermittent claudication: utile or futile?

OBJECTIVE: To evaluate the role of orally administered vasoactive medication in the management of intermittent claudication. SETTING: We limited our study to the products on the market in Belgium: cinnarizine, cyclandelate, isoxsuprine, naftidrofuryl, pentoxifylline, xanthinol nicotinate and buflomedil. DATA SOURCES: We conducted a systematic literature search involving Medline, International Pharmaceutical Abstracts, the Cochrane Library, direct contact with marketing companies and key authors, snowballing and Science Citation Index search. We looked for randomised placebo-controlled trials (RCTs) in patients with Fontaine stage II, in which pain-free and/or maximal walking distance were measured using a standardised exercise test. For isoxsuprine and xanthinol nicotinate, no trials conforming to these criteria were found. Thirty-six trials on cinnarizine, cyclandelate, buflomedil, naftidrofuryl and pentoxifylline met our inclusion criteria. STUDY SELECTION: After quality assessment, 26 trials were excluded, mainly because of short trial duration (less than 12 weeks), small sample size (less than 30 patients) and/or failure to report details on variability (standard deviation or confidence limits). For cinnarizine and cyclandelate, none of the three selected RCTs was included. DATA EXTRACTION: For buflomedil, of six published RCTs, two were included after quality assessment, each showing a marginally positive effect of buflomedil versus placebo. For naftidrofuryl, nine RCTs were selected; six were included of which five showed a significant positive result. The likelihood of publication bias and the heterogeneity of the results within and between trials precluded a meta-analysis. For pentoxifylline, of the 18 selected RCTs, only two could be included, both with inconclusive results. CONCLUSION: A national consensus conference, based on this review, concluded that health resources should be allocated to prevention and rehabilitation of intermittent claudication rather than to reimbursement of these products with doubtful efficacy.

Variability of morphine disposition during long-term subcutaneous infusion in terminally ill cancer patients.

OBJECTIVE: To study the plasma concentrations of morphine and its glucuronides to assess the intra- and interindividual variability of the disposition of morphine administered by subcutaneous infusion in cancer patients. METHODS: Blood samples were taken repeatedly in eight patients with severe cancer pain who were being treated with morphine (60-3000 mg per day) via chronic (8-160 days) subcutaneous infusion. Venous blood samples were collected at least weekly and, when possible, on 3 consecutive days after dose adaptation or any other major change in the patients' treatment. Concentrations of morphine and its glucuronides in plasma were measured after solid-phase extraction using a validated high-performance liquid chromatography assay. The stability of the morphine solutions was determined by repeated measurement of the concentrations of morphine and its degradation products in the solutions. RESULTS: The morphine concentration in the infusion solutions remained unchanged during storage and infusion. The plasma concentrations of morphine and its glucuronides were within the ranges reported in the literature. There was, as expected, a large interindividual variability: from patient to patient, the mean of the normalised plasma concentrations ranged from 0.3 ng.ml(-1).mg(-1) to 0.8 ng.ml(-1).mg(-1) for morphine, from 1.0 ng.ml(-1).mg(-1) to 3.1 ng.ml(-1).mg(-1) for morphine-6-glucuronide and from 6.8 ng.ml(-1).mg(-1) to 24.3 ng.ml(-1).mg(-1) for morphine-3-glucuronide. Intraindividual variability was also important. The residual standard deviation of the mean normalised plasma concentrations calculated for each patient ranged from 26% to 56% for morphine, from 20% to 51% for morphine-6-glucuronide and from 20% to 49% for morphine-3-glucuronide. The normalised plasma concentrations of morphine and its glucuronides did not increase with dose or time, and no explanation for the pronounced pharmacokinetic intraindividual variability was found. CONCLUSION: During subcutaneous infusion of morphine, there is a large intra- and interindividual variability of the morphine disposition which could be of clinical relevance.

Attitude of physicians toward patient package inserts for medication information in Belgium.

In Belgium, the distribution of medications to outpatients in community pharmacies is almost exclusively by branded unit-of-use packages, with a package insert inside every package. At the time of the study (spring 1990), the implementation of legislation that mandated a shift from highly technical documents to patient package inserts (PPIs), understandable by the lay person, had begun. This study explores the attitude of practising physicians toward written medication information for patients. A mail questionnaire was sent to 1500 (8% random sample) Belgian general practitioners and to 500 (22% random sample) internal medicine specialists. A total of 543 usable questionnaires were returned (27.5% return rate). Ninety-two percent of the physicians stated that their patients seldom or never requested additional information on drug efficacy or side effects, during routine consultation; 30% estimated that more than half of their patients read the PPI; 75% expect that a patient would experience side effects after reading about them in the PPI; 59% agreed that the PPI could help the patient react more adequately in unforeseen situations. It was possible to cluster the respondents in a stable segmentation of three clusters: moderately positive physicians (20%), ambiguous to neutral physicians (44%), physicians overtly negative to written drug information (36%). The low response rate to this extensive postal questionnaire limits the conclusions to a qualitative description of relevant clusters of respondents. In contrast with the opinion of physicians about patient readership, results from other studies indicate that the vast majority of patients read the package inserts.

How do calcium channel blockers prevent cardiovascular events. Are they all alike?

The calcium channel blockers (calcium antagonists) have vasodilatory and cardiodepressant effects. These pharmacological properties explain, to a large extent, the antihypertensive, antianginal, and antiarrhythmic effects of these agents. Pharmacological differences between members of this class, e.g. differences in vascular selectivity, are well documented and can be exploited clinically. The efficacy of calcium channel blockers in the prevention of cardiovascular events may depend on factors other than their vasodilatory and cardiodepressive effects. Much interest, for example, has been shown in their possible antiatherosclerotic effects. It is, however, at present not possible to ascertain how important these and other ancillary effects (such as plaque stabilisation) are for the putative cardioprotective effects of calcium channel blockers. There are, moreover, few in vitro or in vivo (animal or clinical) studies allowing valid comparison of the different calcium channel blockers with regard to these ancillary properties.

Direct high-performance liquid chromatography determination of diastereomeric oxprenolol glucuronides.

The beta-blocking agent oxprenolol is used therapeutically as the racemate. In humans and animals it is metabolized i.a. to ether glucuronide diastereomers. A stereoselective HPLC assay was developed to determine directly, without hydrolysis to their parent enantiomers, the oxprenolol glucuronides in biological samples. The glucuronide standards for this direct assay are prepared by incubation of rabbit liver microsomes with RS-oxprenolol. The glucuronides obtained are purified and concentrated with solid-phase extraction, and their concentration is measured by an indirect method, i.e. HPLC assay of the oxprenolol enantiomers after enzymatic hydrolysis with beta-glucuronidase. The direct assay involves separation by HPLC using a C18-reversed-phase column, with UV detection at 274 nm; nalorphine is used as internal standard. On injection onto the column, without previous hydrolysis, the limit of detection is 20 ng for both glucuronides. The assay is sensitive, accurate and reproducible. The method is suitable for the assay of glucuronides in liver microsomal incubates and plasma.

Cytochrome P450: genetic polymorphism and drug interactions.

In this review, after a short discussion of our knowledge about cytochrome P450 isoenzymes, two important sources of variability in the metabolism of drugs by cytochrome P450 are described, i.e. genetic factors and drug-drug interactions. Many hepatic cytochrome P450 enzymes play an important role in the oxidative biotransformation of numerous drugs and other foreign compounds, and of many endogenous substrates. In humans more than 20 different isoenzymes of cytochrome P450 responsible for the hepatic metabolism of drugs, have been identified. They are classified into families and subfamilies on the basis of the degree of amino acid similarity. Cytochrome P450 isoenzymes are regulated by both genetic and environmental factors. Of particular interest is genetic polymorphism in drug oxidation. Two genetic polymorphisms in drug oxidation are well known, the sparteine/debrisoquine (CYP2D6) polymorphism and the mephenytoin oxidation (CYP2C19) polymorphism. As a result of these polymorphisms, two phenotypes exist in the population, poor and extensive metabolizers. Poor metabolizers may be prone to adverse reactions towards drugs with a narrow therapeutic range. In extensive metabolizers clinically significant drug interactions between drugs metabolized by the same isoenzyme can occur.

Effect of probenecid on the enantioselective pharmacokinetics of oxprenolol and its glucuronides in the rabbit.

PURPOSE: To study the effect of probenecid on the stereoselective pharmacokinetics of oxprenolol and its glucuronides in the rabbit. METHODS: An oral dose of 50 mg/kg racemic oxprenolol was given to nine rabbits twice, in random sequence with and without the concurrent administration of probenecid. Oxprenolol enantiomers were determined in plasma and urine by an enantioselective HPLC method. Oxprenolol glucuronides were measured in plasma and urine after enzymatic hydrolysis. RESULTS: The disposition of the oxprenolol enantiomers in rabbits is stereoselective, mainly due to a difference in metabolism. Renal excretion is only a minor elimination route for unchanged oxprenolol, and the renal clearances of the enantiomers are similar. Pretreatment with probenecid did not affect the plasma concentrations of the oxprenolol enantiomers, but there was a slight decrease in their urinary excretion. The plasma concentrations of the oxprenolol glucuronides are much higher than those of the parent enantiomers, and those of (S)-glucuronide are about twice those of its antipode. About 10% of the oxprenolol dose is excreted in the urine as glucuronides. The renal clearances of both glucuronides are similar, and markedly higher than the creatinine clearance. After probenecid, the mean glucuronide plasma levels were markedly higher, with for both glucuronides a more than twofold increase in mean AUC. Probenecid decreased the renal clearance of both glucuronides to about 30%. Moreover, it decreased slightly the formation clearance of (S)-glucuronide, while the formation clearance of (R)-glucuronide was not significantly influenced. CONCLUSIONS: Our results show that in the rabbit, both oxprenolol glucuronide diastereomers are actively secreted by the kidney, and that this process is inhibited by probenecid.

Systematic review of clinical efficacy of topical treatments for head lice.

OBJECTIVES: To collect and evaluate all trials on clinical efficacy of topical treatments for head lice. DESIGN: Systematic review of randomised trials identified from following data sources: Medline, International Pharmaceutical Abstracts, Science Citation Index, letters to key authors and companies, and hand search of journals. SETTING: Trials in schools or communities. SUBJECTS: Patients infested with lice. MAIN OUTCOME MEASURE: Cure rate (absence of live lice and viable nits) on day 14 after treatment. RESULTS: Total of 28 trials were identified and evaluated according to eight general and 18 lice specific criteria. Of the 14 trials rated as having low to moderate risk of bias, seven were selected as they used the main outcome measure. These seven trials described 21 evaluations of eight different compounds and placebo (all but two evaluations were of single applications). Only permethrin 1% creme rinse showed efficacy in more than two studies with the lower 95% confidence limit of cure rate above 90%. CONCLUSIONS: Only for permethrin has sufficient evidence been published to show efficacy. Less expensive treatments such as malathion and carbaryl need more evidence of efficacy. Lindane and the natural pyrethrines are not sufficiently effective to justify their use.

Stereoselective pharmacokinetics of oxprenolol and its glucuronides in humans.

OBJECTIVE: To study the pharmacokinetics of R(+)- and S(-)-oxprenolol and their corresponding glucuronide conjugates in healthy subjects. METHODS: An oral dose of 80 mg racemic oxprenolol was given to eight male volunteers. Venous blood samples and urine were collected as a function of time. Oxprenolol enantiomers in plasma and urine were determined by an enantiospecific HPLC method. Oxyprenolol glucuronides in plasma and urine were measured as oxprenolol equivalents after enzymatic hydrolysis. RESULTS: For R-oxprenolol the area under the plasma concentration-time curve was slightly higher (R/S ratio, 1.19) and the oral clearance slightly lower (R/S ratio, 0.84) than those parameters for S-oxprenolol. The free fraction of R-oxprenolol in plasma was 4% higher than that of S-oxprenolol. The intrinsic clearance of S-oxprenolol was 1.5 times larger than that of R-oxprenolol, and a maximum of 3% of the dose was excreted as unchanged enantiomers in the urine. The plasma concentrations of S-oxprenolol glucuronide were more than three times higher than those of R-oxprenolol glucuronide. Twenty-five percent of the dose of the R-enantiomer was excreted in the urine as R-oxprenolol glucuronide; 29% of the S-enantiomer dose was excreted as S-oxprenolol glucuronide. The renal clearance of R-oxprenolol glucuronide was, on average, 172 ml/min, suggesting active tubular secretion. In contrast, the renal clearance of S-oxprenolol glucuronide was only 49 ml/min, which can be explained by the plasma binding of the compound. CONCLUSIONS: Our results show small differences in disposition between R- and S-oxprenolol but a marked difference in disposition between the glucuronides. The difference in plasma concentrations between the oxprenolol glucuronides is mainly attributable to the stereoselectivity of the renal excretion.

Protein binding of propranolol and verapamil enantiomers in maternal and foetal serum.

The protein binding of the enantiomers of propranolol and verapamil was measured in 19 pairs of maternal and foetal serum obtained at delivery. The binding of the enantiomers of both drugs was lower in foetal than in maternal serum. In maternal serum the mean (+/- s.d.) unbound percentages were 22.4 +/- 6.2 and 20.7 +/- 6.6 for R- and S-propranolol, and 16.8 +/- 5.5 and 22.5 +/- 6.2 for R- and S-verapamil; in foetal serum the values were 38.8 +/- 8.6 and 40.4 +/- 10.6 for R- and S-propranolol, and 34.7 +/- 10.5 and 44.8 +/- 10.7 for R- and S-verapamil. For propranolol, in maternal, but not in foetal serum, the difference between the binding of the R- and S-enantiomers was significant; the R/S ratio was significantly (P < 0.01) larger in the mother (1.099 +/- 0.072) than in the foetus (0.973 +/- 0.068). For verapamil, the difference in binding between the R- and S-enantiomers was significant in both maternal and foetal serum, but the R/S ratio was similar in mother (0.735 +/- 0.098) and foetus (0.763 +/- 0.070). Serum alpha 1-acid glycoprotein (AAG) concentrations were markedly higher and albumin concentrations slightly lower in maternal than in foetal samples. The binding of the four enantiomers in maternal and foetal serum was correlated (P < 0.001) with the AAG concentration (r propranolol: R 0.749, S 0.746; r verapamil: R 0.753, S 0.782). Our findings show that measurement of concentrations of total, unresolved drug allow a reasonably accurate assessment of transplacental gradients of individual isomers.

Stereoselective pharmacokinetics of oxprenolol, propranolol, and verapamil: species differences and influence of endotoxin.

The influence of endotoxin-induced inflammation was studied on the pharmacokinetics of the enantiomers of the racemic drugs oxprenolol, propranolol, and verapamil in rabbits and dogs. Enantioselective pharmacokinetics were seen for oxprenolol and propranolol in the rabbit and for propranolol and verapamil in the dog. In the dog, the enantioselective differences in plasma concentrations are due to differences in both protein binding and metabolism, whereas in the rabbit the differences are due solely to differences in metabolism. In both species endotoxin treatment increases the plasma concentrations of the enantiomers of the three drugs; both protein binding and metabolism are influenced. In rabbits and in dogs, the influence of endotoxin on the disposition of the three drugs is less enantioselective than was previously observed in the rat.

Clinical pharmacokinetics of nitrates.

The pharmacokinetics of organic nitrates are discussed with emphasis on the possible clinical relevance. For glyceryl trinitrate, the measurement of plasma concentrations is very difficult. Its pharmacokinetics are unusual, with a very rapid disappearance from plasma, and large intraindividual and interindividual variations. After oral administration, there seems to be a very extensive first-pass hepatic extraction and the plasma concentrations are often below the detection limit; after sublingual administration, glyceryl trinitrate appears in plasma. With transdermal glyceryl trinitrate controlled-release systems, plasma concentrations of glyceryl trinitrate can be maintained over 24 hours, although with fluctuations and important intraindividual and interindividual variability. After administration of glyceryl trinitrate via different routes, glyceryl dinitrates and mononitrates are present in plasma. The pharmacokinetics of isosorbide dinitrate are somewhat easier to understand. The substance disappears less rapidly from the plasma than does glyceryl trinitrate. After oral administration, there is also a hepatic first-pass extraction; the plasma concentrations can be prolonged by administering slow-release products. Sublingual administration leads to higher plasma concentrations than oral administration. Isosorbide dinitrate is metabolized in the organism to isosorbide 5-mononitrate and isosorbide 2-mononitrate, which both have vasodilator activity: after administration of isosorbide dinitrate, the mononitrates, and mainly the 5-mononitrate, reach very high concentrations in plasma. Isosorbide 5-mononitrate has been studied in its own right as an antianginal agent: it is completely absorbed after oral administration; it has a half-life of around 4 hours, and oral standard and controlled-release formulations have been extensively studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of endotoxin on the stereoselective pharmacokinetics of oxprenolol, propranolol, and verapamil in the rat.

The influence of endotoxin-induced inflammation on the enantioselective pharmacokinetics of propranolol, oxprenolol, and verapamil, which bind to alpha 1-acid glycoprotein, was studied in the rat. The racemic mixtures were given orally. In the control animals, for propranolol and oxprenolol, the plasma concentrations of the (R)-enantiomer were higher than those of the (S)-enantiomer, while for verapamil the reverse was true. Protein binding and intrinsic clearance are the main factors responsible for this enantioselectivity. After endotoxin treatment, for the three drugs tested the plasma concentrations and the plasma binding of both enantiomers were significantly increased. This effect was more pronounced for (R)-propranolol, (R)-oxprenolol, and (S)-verapamil than for their respective antipodes. The enantioselective effect of endotoxin on the plasma concentrations of the drugs studied seems mainly due to the enantioselective increase in binding to alpha 1-acid glycoprotein.

Aging and the pharmacokinetics and metabolism of metoprolol enantiomers in the rat.

After i.v. administration of racemic metoprolol in the rat, the plasma concentrations of (R)- and (S)-metoprolol were comparable, and no differences in pharmacokinetic parameters between the two enantiomers were found. From the 3rd to the 12th month, comparable changes were seen for both enantiomers: there was an increase in the area under the plasma concentration-time curve (AUC) and a decrease in blood and plasma clearance. Half-life showed a significant prolongation, volume of distribution decreased between 3 and 12 months and increased between 12 and 24 months. After oral administration of the racemate, AUC and Cmax (maximum plasma concentration) were slightly higher, while oral clearance was slightly lower for (R)-metoprolol than for (S)-metoprolol. With aging, Cmax and AUC increased for both enantiomers, while oral clearance decreased. The change in oral clearance as a function of age is different between (S)- and (R)-metoprolol, and thus enantioselective. In vitro disappearance rate in 3-month-old rats was significantly higher for (S)-metoprolol than for (R)-metoprolol, although the difference was small. With aging, the disappearance rates of both enantiomers increased significantly, but not enantioselectively.

Stereoselective pharmacokinetics of atenolol in the rat: influence of aging and of renal failure.

The pharmacokinetics of R- and S-atenolol after intravenous administration of racemic atenolol were studied in 3-, 12- and 24-month-old rats and in 3-month-old rats with renal failure induced by uranyl nitrate. In all age groups, the area under the plasma concentration-time curves is higher for R- than for S-atenolol; volume of distribution, total clearance and renal clearance are lower for R-atenolol than for S-atenolol, but the differences are small. In function of age there is for both enantiomers a significant increase in AUC, due, at least in part, to a decreased renal clearance; the effect of aging is not stereoselective. In rats with renal failure, the AUC of both enantiomers increases, due mainly to a decrease in renal clearance, but to a lesser degree also to a decrease in nonrenal clearance. For both enantiomers, the volume of distribution decreases and the half-life increases in the uraemic rats. The total amount of both enantiomers excreted in the urine is decreased in the rats with renal failure. There are no stereoselective effects of treatment of the rats with uranyl nitrate.

Influence of human recombinant interleukin-1 beta on the enantioselective disposition of propranolol in rats.

The influence of i.v. administration of 10 micrograms/kg recombinant human interleukin-1 beta (rhIL-1 beta), a putative mediator of inflammation, on the pharmacokinetics and metabolism of the propranolol enantiomers was studied in rats aged 3, 12 and 24 months. After oral administration of rac-propranolol to control rats of the three age groups, the plasma concentrations of (R)-propranolol were higher than those of (S)-propranolol. Administration of IL-1 beta increased the plasma concentrations of the (R)-enantiomer markedly and significantly, those of the (S)-enantiomer only to a lesser degree. For both enantiomers an important increase in plasma binding was found in the IL-1 beta-treated rats, which was linked to the increase in alpha 1-acid glycoprotein levels. The in vitro clearance, measured in 3-month-old rats using the 9000 g liver fraction, was for neither of the propranolol enantiomers influenced by IL-1 beta treatment, which is in keeping with the unchanged cytochrome P450 content. The enantioselective influence of IL-1 beta treatment on the pharmacokinetics of propranolol was also present in 12- and 24-month-old rats, although somewhat less pronounced in the latter group. Our results show an enantioselective influence of IL-1 beta treatment on the pharmacokinetics of propranolol in the rat, favouring the (R)-enantiomer.

Influence of age on stereoselective pharmacokinetics and metabolism of hexobarbital in the rat.

The influence of age on stereoselective pharmacokinetics and in vitro metabolism of R- and S-hexobarbital was studied in the rat. After intravenous administration of the racemate, the plasma concentrations of S-hexobarbital are markedly lower than those of R-hexobarbital. For S-hexobarbital the half-life is somewhat shorter and the volume of distribution and plasma clearance is higher than for its antipode. For both enantiomers an increase in AUC and half-life, and a decrease in clearance are observed with aging. These changes occur mainly between the 3rd and the 12th month and are slightly more pronounced for R- than for S-hexobarbital, as appears from the S/R ratios. The volume of distribution shows no changes with aging. In vitro disappearance rate in 3-month-old rats is significantly higher for S- than for R-hexobarbital. There is for both enantiomers an increase in disappearance rate in 12-month-old rats as compared to younger or older rats, but this is significant only for the R-enantiomer. There are pronounced differences in the kinetics and metabolism of both hexobarbital enantiomers; changes with aging occur, but are only slightly and not always significantly more important for R- than for S-hexobarbital.