Simultaneous determination of carisoprodol and aspirin in human plasma using liquid chromatography-tandem mass spectrometry in polarity switch mode: application to a human pharmacokinetic study.

A simple, sensitive and rapid LC-MS/MS-ESI method has been developed and validated for simultaneous quantification of the carisoprodol and aspirin in human plasma. Carisoprodol was detected in positive ion mode, whereas aspirin was detected in negative ion mode. Carbamazepine and furosemide were used as internal standards (IS) for quantification of carisoprodol and aspirin, respectively. The extraction procedure involves a liquid-liquid extraction method with ter-butyl methyl ether. Chromatographic separation was achieved on a Zorbax XDB-Phenyl (4.6 × 75 mm, 3.5 µm) column using an isocratic mobile phase (5 mm ammonium acetate:methanol, 20:80, v/v) at a flow rate of 0.8 mL/min with a total run time of 2.2 min. A detailed method validation was performed as per the FDA guidelines. The standard curves found to be linear in the range of 25.5-4900 and 15.3-3000 ng/mL for carisoprodol and aspirin, respectively. The results met the acceptance criteria. Carisoprodol and aspirin were found to be stable in various stability studies. The validated method was successfully applied to a pharmacokinetic study following co-administration of carisoprodol (250 mg) and aspirin (75 mg) tablets by oral route to human volunteers.

CYP2C19 genetics in fatal carisoprodol intoxications.

INTRODUCTION: Carisoprodol, a frequently used muscle relaxant, can cause potentially fatal intoxications. Conversion to its active metabolite meprobamate is almost solely mediated by cytochrome P450 2C19 (CYP2C19), and mutations in this enzyme could have significant effects on serum concentrations. The objective of this study was to investigate the role of CYP2C19 genetics in mortalities due to carisoprodol intoxication. METHODS: The frequencies of CYP2C19 variant alleles were compared between the study group (n = 75) and two control groups, i.e. (1) deaths where carisoprodol was detected in the blood of the deceased, but intoxication was not the cause of death (control group A, n = 38), and (2) a healthy population not using carisoprodol (control group B, n = 185). In the study group and control A, the concentrations of carisoprodol and meprobamate were compared between the different genotype subgroups. RESULTS: The variant allele frequencies of CYP2C19 did not differ significantly between the study group and control groups. Moreover, no statistically significant difference in the concentrations of carisoprodol and meprobamate between the different genotype subgroups was found. CONCLUSIONS: This study finds no evidence for an important association between CYP2C19 genetics and mortality risk of carisoprodol. Other factors, such as co-administration with other drugs, likely play a more important role.

Carisoprodol pharmacokinetics and distribution in the nucleus accumbens correlates with behavioral effects in rats independent from its metabolism to meprobamate.

Carisoprodol (Soma®) is a centrally-acting skeletal-muscle relaxant frequently prescribed for treatment of acute musculoskeletal conditions. Carisoprodol's mechanism of action is unclear and is often ascribed to that of its active metabolite, meprobamate. The purpose of this study was to ascertain whether carisoprodol directly produces behavioral effects, or whether metabolism to meprobamate via cytochrome P450 (CYP450) enzymatic reaction is necessary. Rats were trained to discriminate carisoprodol (100 mg/kg) to assess time course and whether a CYP450 inhibitor (cimetidine) administered for 4 days would alter the discriminative effects of carisoprodol. Additionally, pharmacokinetics of carisoprodol and meprobamate with and without co-administration of cimetidine were assessed via in vivo microdialysis combined with liquid-chromatography-tandem mass spectrometry from blood and nucleus accumbens (NAc). The time course of the discriminative-stimulus effects of carisoprodol closely matched the time course of the levels of carisoprodol in blood and NAc, but did not match the time course of meprobamate. Administration of cimetidine increased levels of carisoprodol and decreased levels of meprobamate consistent with its interfering with metabolism of carisoprodol to meprobamate. However, cimetidine failed to alter the discriminative-stimulus effects of carisoprodol. Carisoprodol penetrated into brain tissue and directly produced behavioral effects without being metabolized to meprobamate. These findings indicate that understanding the mechanism of action of carisoprodol independently of meprobamate will be necessary to determine the validity of its clinical uses.

High-dose carisoprodol during pregnancy and lactation.

OBJECTIVE: To report a case of use of high-dose carisoprodol during pregnancy and breast-feeding. CASE SUMMARY: A 28-year-old woman with severe back muscle spasm took carisoprodol 2800 mg/day before and throughout an uncomplicated pregnancy and while exclusively breast-feeding her infant during the first month after birth. Serum drug concentrations of carisoprodol and the active metabolite meprobamate were measured in the mother and infant. Concentrations of these agents also were measured in breast milk. Developmental toxicity was not observed in the near-term infant, whose birth weight was at the 10th percentile for gestational age. Only slight sedation was noted in the infant during breast-feeding, and no signs or symptoms of withdrawal were noted when nursing was stopped. DISCUSSION: Carisoprodol and meprobamate are excreted into breast milk. Although the published human pregnancy data are limited to 15 cases, carisoprodol does not appear to cause developmental toxicity (growth restriction, structural anomalies, functional/neurobehavioral deficits, or death), even when the mother is taking high doses. No signs or symptoms of withdrawal were noted in our infant or in a previously published case when breast-feeding was stopped. Long-term follow-up has not been conducted in exposed infants, and the possibility of functional/neurobehavioral l deficits appearing later in life cannot be excluded. CONCLUSIONS: Except for mild sedation, no other toxicity was observed in a near-term infant exposed to carisoprodol throughout gestation and during breast-feeding in the first month after birth.

Pharmacokinetic modeling of carisoprodol and meprobamate disposition in adults.

Carisoprodol is a widely prescribed muscle relaxant and is also a drug known to be a subject to abuse. Despite the fact that carisoprodol has been available for prescription since 1959, a number of gaps in our knowledge of the toxicokinetics of this common drug exist. For example, the volume of distribution (Vd) for carisoprodol in humans has not been reported. A two-compartment pharmacokinetic model describing carisoprodol metabolism and that of the primary metabolite, meprobamate, was developed to better understand the pharmacokinetics of this drug. The model accounts for first pass metabolism of carisoprodol and was able to replicate the data from several previously reported data sets. Based on an analysis of four different data sets, the Vd for carisoprodol ranged from 0.93 to 1.3 L/kg, while that for meprobamate ranged from 1.4 to 1.6 L/kg. The model was also used to estimate the probable dose of this drug in an individual where questions concerning the drug's role in her death had been posed. The model may, therefore, have significant utility for estimating doses of carisoprodol in medicolegal cases.

Formation of meprobamate from carisoprodol is catalysed by CYP2C19.

Carisoprodol is a muscle relaxant analgesic, which has an active metabolite i.e. meprobamate. We conducted an open three-panel single-dose administration study with 15 healthy volunteers: five poor metabolizers of mephenytoin, five poor metabolizers of debrisoquine and five extensive metabolizers of both substrates. The aim was to investigate if the elimination of carisoprodol and meprobamate is dependent on the two metabolic polymorphisms of mephenytoin and debrisoquine. The subjects were given single oral doses of 700 mg carisoprodol and 400 mg meprobamate on separate occasions. The disposition of carisoprodol was clearly correlated to the mephenytoin hydroxylation phenotype. The mean serum clearance of carisoprodol was four times lower in poor metabolizers of mephenytoin than in extensive metabolizers, which confirms the hypothesis from our previous study that N-dealkylation of carisoprodol cosegregates with the mephenytoin hydroxylation polymorphism. However, mean serum clearance of meprobamate did not differ between the two groups. Also, polymorphic debrisoquine hydroxylation did not influence the elimination of carisoprodol or meprobamate. Poor metabolizers of mephenytoin thus have a lower capacity to metabolize carisoprodol and may therefore have an increased risk of developing concentration dependent side-effects such as drowsiness and hypotension, if treated with ordinary doses of carisoprodol.

Carisoprodol concentrations from different anatomical sites: three overdose cases.

Three cases involving overdoses of carisoprodol are presented. Concentrations of carisoprodol and its major metabolite meprobamate, were determined in urine, vitreous humor, heart, and femoral blood. All drugs were quantified by gas chromatography/mass spectrometry (GC/MS).

Factors affecting carisoprodol metabolism in pain patients using urinary excretion data.

Carisoprodol is a skeletal muscle relaxant prescribed to treat pain. Carisoprodol is metabolized to meprobamate, an active metabolite with anxiolytic effects, by the genetically polymorphic CYP2C19 enzyme. Concomitant use of CYP2C19 substrates or inhibitors may alter carisoprodol metabolism, with therapeutic and/or toxic implications for effectively treating patients with pain. This was a retrospective analysis of urinary excretion data collected from patients with pain from March 2008 to May 2011. Carisoprodol and meprobamate urine concentrations were measured by liquid chromatography-tandem mass spectrometry, and the metabolic ratio (MR) of meprobamate to carisoprodol concentrations was determined in 14,965 subjects. The MR geometric mean and 95% confidence interval (95% CI) of the young group (105, 95% CI = 99.1-113) were ∼47.4% higher than the middle-aged group (71.9, 95% CI = 70-73.8) and nearly two times higher than the elderly group (54.4, 95% CI = 51.3-57.6). Females had a 20.7% higher MR compared with males. No significant change in the MR was observed with overall CYP2C19 inhibitor or substrate use. However, evaluation of individual inhibitors showed co-administration with esomeprazole or fluoxetine was associated with a 31.8 and 24.6% reduction in MR, respectively, compared with controls (P < 0.05). Omeprazole did not significantly affect the MR. Patient-specific factors such as age, sex and co-medications may be important considerations for effective carisoprodol therapy.

Determination of carisoprodol and meprobamate in oral fluid.

The determination of carisoprodol and its metabolite meprobamate in oral fluid using solid-phase extraction and liquid chromatography with tandem mass spectral detection (LC-MS-MS) and its application to authentic specimens is described. The method employs collection of oral fluid with the Quantisal device, extraction using cation exchange/hydrophobic solid-phase columns, and LC-MS-MS in positive ion electrospray mode. The method was fully validated using various parameters, including selectivity, linearity, accuracy, intra-day and inter-day imprecision, drug recovery from the collection pad, limit of quantitation and matrix effects. The method was applied to both routine research specimens and an authentic specimen taken from an individual prescribed a daily dose of 350 mg carisoprodol following surgery.

Evaluating the relationship between carisoprodol concentrations and meprobamate formation and inter-subject and intra-subject variability in urinary excretion data of pain patients.

Using urinary carisoprodol data from pain patients, our objectives were to determine the relationship between carisoprodol concentration and its conversion to meprobamate, and quantify the intra-subject and inter-subject variability in carisoprodol metabolism. Liquid chromatography-tandem mass spectrometry was used to quantitate carisoprodol and meprobamate concentrations in urine specimens. The log creatinine-corrected carisoprodol versus log creatinine-corrected meprobamate showed a marginal positive relationship (R(2) = 0.395), with a 29.1-fold variance between subjects at the mean carisoprodol concentration. The geometric mean carisoprodol and meprobamate urine concentrations were 0.519 ± 3.38 mg and 28.2 ± 2.34 mg analyte per gram creatinine, respectively. The log metabolic ratio (MR) versus log creatinine-corrected carisoprodol displayed a marginal positive correlation. A subpopulation of outliers with higher carisoprodol and lower meprobamate levels were considered poor metabolizers and represented 0.483% (n = 21) of the study population. Using a curve-fit mathematical model, we estimated 0.318% (n = 10) to be ultra-rapid metabolizers. The inter-subject population geometric standard deviation (SD) of the MR was 3.64. The intra-subject geometric median and mean SD of the MR were 1.60 (interquartile range: 1.28, 2.07) and 1.72 ± 1.60, respectively. Inter-subject variability was 2.27 times greater than the median intra-subject variability. With a better understanding of urine carisoprodol and meprobamate concentrations and variability, urine drug testing provides a useful monitoring reference for clinicians.

Postmortem carisoprodol and meprobamate concentrations in blood and liver: lack of significant redistribution.

Carisoprodol is a therapeutic and occasionally abused centrally acting muscle relaxant. We compare central blood and liver concentrations of carisoprodol and the metabolite meprobamate to concentrations in peripheral blood in 11 medical examiner cases. Specimens were initially screened for alcohol and simple volatiles by gas chromatography (GC)-flame ionization detection headspace analysis, enzyme-linked immunosorbent array for drugs of abuse, and therapeutic drugs by GC-mass spectrometry (MS). Carisoprodol, when detected by the therapeutic drug screen, was confirmed and quantified by a specific GC-MS procedure. The results suggest that when ingested with other medications, carisoprodol may be a contributing factor in death, even when present at therapeutic concentrations. Considering the cases studied, together with previously published therapeutic and fatal concentrations, blood carisoprodol concentrations greater than 15 mg/L and liver concentrations greater than 50 mg/kg may be considered excessive and potentially fatal. Carisoprodol central blood to peripheral blood ratios averaged 1.31 + 0.33 (mean ± standard deviation), and liver to peripheral blood, 2.83 ± 1.51. Meprobamate central blood to peripheral blood ratios averaged 0.92 ± 0.22, and liver to peripheral blood, 1.25 ± 0.69. The low liver to peripheral blood ratio (less than 5), taken together with the low central blood to peripheral blood ratio, is an indicator that both carisoprodol and meprobamate lack the potential to exhibit postmortem redistribution.

Potential impact of drug effects, availability, pharmacokinetics, and screening on estimates of drugs implicated in cases of assault.

Drug-facilitated sexual assault (DFSA) is a serious and troubling crime. It is important to know if and how different drugs might be used to facilitate assault in order to deter such crime. There are a number of ways in which drugs that are used for DFSA might not be detected by routine screens. The purpose of this analysis was to draw reasonable inferences regarding drugs with a high likelihood of being used for DFSA and not being detected by routine screens. National data from poison control centres, hospital emergency rooms, and law enforcement seizures were used to evaluate the relative magnitude of problems and illicit availability associated with different classes of drugs. General drug classes were examined to include additional drugs that might be used for DFSA on the basis of their amnesic effects, widespread availability, and pharmacokinetics (i.e. short half-life). The benzodiazepine-site ligands zolpidem and eszopiclone, 'club drugs' GHB and ketamine, muscle relaxants such as carisoprodol, and antihistamines such as diphenhydramine were identified as drugs that might be used for DFSA and remain undetected by routine screens. Future studies that are designed to examine the role of these drugs in DFSA cases could provide better estimates of their use for DFSA. A better understanding of what is being missed in DFSA cases might help prioritize the development of new assays, provide rationale for the availability of particular assays for routine testing, and inform practitioners and the general public of the potential DFSA risks of certain drugs.

Pharmacogenetics and forensic toxicology.

Large inter-individual variability in drug response and toxicity, as well as in drug concentrations after application of the same dosage, can be of genetic, physiological, pathophysiological, or environmental origin. Absorption, distribution and metabolism of a drug and interactions with its target often are determined by genetic differences. Pharmacokinetic and pharmacodynamic variations can appear at the level of drug metabolizing enzymes (e.g., the cytochrome P450 system), drug transporters, drug targets or other biomarker genes. Pharmacogenetics or toxicogenetics can therefore be relevant in forensic toxicology. This review presents relevant aspects together with some examples from daily routines.

The CYP2C19 genotype and the use of oral contraceptives influence the pharmacokinetics of carisoprodol in healthy human subjects.

AIMS: The aim of the present study was to investigate if subjects with one normal and one non-functional CYP2C19 allele (intermediate metabolizers; IMs) metabolized carisoprodol differently than individuals with two normal CYP2C19 alleles (extensive metabolizers; EMs) We also wanted to investigate whether the use of oral contraceptives influences the metabolism of carisoprodol in EMs and IMs. Impairing effects on psychomotor coordination and feelings of sedation were studied by comparing IMs with EMs following their ingestion of a single dose of 700 mg carisoprodol. METHODS: Thirty-seven healthy Caucasian volunteers participated in the study, of whom 25 were not using any drugs known to interact with CYP2C19, including two poor metabolizers (PMs) (CYP2C19 *2/*2 or CYP2C19 *2 /*4), 11 IMs (CYP2C19 *1/*2 or CYP2C19 *1/*4) and 12 EMs (CYP2C19 *1/*1); the remaining 12 participants were six EMs and six IMs using oral contraceptives. A single oral dose of 700 mg of carisoprodol was given, and blood drug concentrations were followed for 11 h and 45 min. During this time period, different pharmacodynamic measurements were made. RESULTS: IMs had a longer elimination half life (T(1/2)) (127 min; 95% confidence interval (CI) 95, 159) than EMs (96 min; 95% CI 84, 107) and a larger area under the concentration-time curve from 0 to infinity (AUC(0-infinity)) for carisoprodol (16.3 microg h ml(-1) ; 95% CI 11.9, 20.7) than EMs (11.3 microg h ml(-1) ; 95% CI 7.8, 14.8). The use of oral contraceptives was accompanied by larger AUC(0-infinity) for carisoprodol in both EMs (18.5 microg h ml(-1); 95% CI 10.7, 26.3) and IMs (26.0 microg h ml(-1) ; 95% CI 18.8, 33.2). EMs using oral contraceptives also had a longer T(1/2) (117 min; 95% CI 92, 143) and higher maximum carisoprodol concentration than EMs not using oral contraceptives. No significant differences in pharmacodynamic parameters were found between subjects in the different genotype groups or between users and non-users of oral contraceptives. CONCLUSIONS: Subsequent to a single-dose administration of carisoprodol, the carisoprodol AUC was approximately 45% larger in CYP2C19 IMs than in EMs. The use of oral contraceptives increased the AUC by approximately 60% in both EMs and IMs. Despite these pharmacokinetic effects, no significant differences with respect to the CYP2C19 IM and EM genotypes were observed in the acute impairing effects of a single dose of carisoprodol.

Carisoprodol elimination in humans.

The elimination of the muscle relaxant drug, carisoprodol, was examined in 10 healthy volunteers after an oral dose of 700 mg. In nine subjects, carisoprodol was rapidly eliminated, with a mean half-life of 99 +/- 46 min, and extensively converted to meprobamate. Within 2.5 h after carisoprodol intake, meprobamate serum concentrations exceeded those of carisoprodol. Serum levels of meprobamate recorded (15-25 mumol/L) indicate that meprobamate might contribute to the effect(s) of carisoprodol. One subject eliminated carisoprodol with an overall half-life of 376 min, and only small amounts of meprobamate were recorded. This subject was found to be a poor metabolizer of mephenytoin. In spiked human sera, protein binding of carisoprodol was in the range of 41-67%, whereas meprobamate was bound to a lesser extent, 14-24%.

Transfer of carisoprodol to breast milk.

There is no published information on the transfer of the centrally acting muscle relaxant carisoprodol and its active metabolite meprobamate into breast milk. The objective of this study was to quantify the excretion of carisoprodol and meprobamate in human milk and estimate the dose received by breast-fed infants. The concentrations of carisoprodol and meprobamate were measured in breast milk on 4 consecutive days at steady-state conditions in one woman using carisoprodol 2100 mg/d. The average milk concentrations were 0.9 microg/mL for carisoprodol and 11.6 microg/mL for meprobamate. Based on the milk concentrations measured, the absolute dose ingested by an exclusively breast-fed infant could be estimated at 1.9 mg/kg per day, and the relative dose would be 4.1% of the weight-adjusted maternal dose. No adverse effects were observed in the infant, but the infant was partly fed with formula because of insufficient maternal milk production. Thus, the authors consider that at least during prolonged use, lactation is generally inadvisable until more clinical data are available.

Carisoprodol (Soma): a new and cautious perspective on an old agent.

Carisoprodol (Soma, others) is a commonly prescribed, noncontrolled, skeletal muscle relaxant whose active metabolite is meprobamate. Patients for whom carisoprodol is prescribed are at risk for meprobamate dependence; several such cases have been reported. Toxicity and withdrawal associated with the use of meprobamate is high and has led to abandonment of this agent in clinical practice. Because carisoprodol possesses the same risks it should be avoided when possible, and it should be given schedule IV controlled substance status.

NTP toxicity studies of carisoprodol (CAS No. 78-44-4) administered by Gavage to F344/N rats and B6C3F1 mice.

[carisoprodol structure: see text] Carisoprodol is a widely used skeletal muscle relaxant and analgesic and is available as a prescription drug. Comparative studies were conducted to determine the toxicity of carisoprodol administered in corn oil and in 0.5% methylcellulose by gavage. Carisoprodol plasma concentrations of rats and mice were measured at the end of the 13-week studies; single-dose plasma carisoprodol analyses were also performed. Genetic toxicity studies were conducted in Salmonella typhimurium, L5178Y mouse lymphoma cells, cultured Chinese hamster ovary cells, and peripheral blood erythrocytes of mice. Groups of 10 male and 10 female F344/N rats received 0, 100, 200, 400, 800, or 1,600 mg carisoprodol per kilogram body weight in corn oil by gavage or 0, 100, 200, 400, or 800 mg/kg carisoprodol in 0.5% methylcellulose by gavage for 13 weeks. Groups of 10 male and 10 female B6C3F1 mice received 0, 75, 150, 300, 600, or 1,200 mg/kg carisoprodol in corn oil by gavage or 0, 600, 1,200, or 1,600 mg/kg carisoprodol in 0.5% methylcellulose by gavage for 13 weeks. Among rats that received carisoprodol in corn oil, survival was similar to that of the vehicle controls. Survival of rats administered carisoprodol in 0.5% methylcellulose was also similar to that of the vehicle controls after adjustment for deaths (two males and one female in the 800 mg/kg group and two females in the 400 mg/kg group). The final mean body weight gain of males administered 1,600 mg/kg carisoprodol in corn oil was significantly less than that of the vehicle controls; the final mean body weights and body weight gains of female rats in the 800 and 1,600 mg/kg groups were significantly greater. In the carisoprodol in 0.5% methylcellulose study, males in the 200 mg/kg group and females in the 100 and 800 mg/kg groups had significantly greater mean body weights and body weight gains than did the vehicle controls. Clinical findings in rats administered carisoprodol in corn oil or in 0.5% methylcellulose included lethargy, ataxia, diarrhea, and prostration; the incidences were dose-related, and females were more sensitive than males to the effects of carisoprodol. In the carisoprodol in corn oil study, differences in hematology and clinical chemistry parameters occurred with no consistent patterns. The effects of carisoprodol in 0.5% methylcellulose on hematology and clinical chemistry parameters were not studied. In the corn oil study, the kidney and liver weights of male and female rats administered 200 mg/kg carisoprodol or greater were generally significantly greater than those of the vehicle controls. In the 0.5% methylcellulose study, liver weights were significantly greater in male rats administered 400 or 800 mg/kg and in female rats administered 800 mg/kg carisoprodol compared to the vehicle controls; however, a consistent effect on the kidney weights was not observed. Nephropathy was observed in male rats administered 400 mg/kg carisoprodol or greater in corn oil; the livers of four males in the 1,600 mg/kg group had centrilobular hypertrophy of hepatocytes. No lesions were observed histopathologically in female rats administered carisoprodol in corn oil. In the carisoprodol in 0.5% methylcellulose study, the severity of nephropathy in males administered 200 mg/kg or greater was enhanced, and the incidence of nephropathy in female rats in the 800 mg/kg group was slightly greater than that in the vehicle controls. Plasma carisoprodol concentrations at the end of 13 weeks generally increased with increasing dose in rats administered carisoprodol in corn oil or in 0.5% methylcellulose. The plasma carisoprodol concentrations in rats administered a single gavage dose of carisoprodol in corn oil also increased with increasing dose. In the carisoprodol in corn oil mouse study, two females each in the vehicle control and 75 mg/kg groups and one female each in the 150 and 600 mg/kg groups were accidentally killed; all males survived to the end of the study. One male and one female administered 1,600 mg/kg carisoprodol in 0.5% methylcellulose died; seven mice were accidentally killed. The mean body weights and body weight gains of mice administered carisoprodol in corn oil were generally similar to those of the vehicle controls. The final mean body weights and body weight gains of all groups of males and females administered carisoprodol in 0.5% methylcellulose were significantly less. Clinical findings in the carisoprodol in corn oil study included lethargy, ataxia, tremors, and prostration in male and female mice. Ataxia, lethargy, convulsions, and prostration were observed in all dosed groups of males and females administered carisoprodol in 0.5% methylcellulose. In the carisoprodol in corn oil study, liver weights were significantly greater in males administered 300 mg/kg or greater and in females administered 150 mg/kg or greater than in the vehicle controls. In the carisoprodol in corn oil study, no gross or microscopic lesions were considered related to carisoprodol administration. Minimal to mild centrilobular hypertrophy was observed in the liver of all dosed groups of males and in females in the 1,200 and 1,600 mg/kg groups in the carisoprodol in 0.5% methylcellulose study. The testis weights of males administered 1,200 mg/kg carisoprodol in corn oil were significantly less than those of the vehicle controls; the sperm motility of males in this group was also significantly less than that of the vehicle controls. There were no significant differences in vaginal cytology parameters between dosed and vehicle control females. At the end of the carisoprodol in corn oil study, the concentration of carisoprodol was above the limit of detection in the plasma of only one male mouse each in the 300 and 1,200 mg/kg groups and in four females in the 1,200 mg/kg group. In mice administered a single gavage dose of carisoprodol in corn oil, plasma concentrations increased with increasing dose; peak plasma concentrations occurred at 20 to 120 minutes in males and 60 to 120 minutes in females. In the carisoprodol in 0.5% methylcellulose study, plasma carisoprodol concentrations of female, but not male, mice increased with increasing dose; peak plasma carisoprodol concentrations occurred at 30 minutes postdosing in all groups of males and females. Results of proportionality and bioavailability studies indicated that single gavage doses of 200 to 800 mg/kg carisoprodol in 0.5% methylcellulose in rats or 300 to 1,200 mg/kg in mice were dose proportional; absolute bioavailability values increased with increasing dose, ranging from 15% to 32% for rats and from 18% to 38% for mice. For rats, the bioavailability of carisoprodol in 0.5% methylcellulose was approximately fivefold that of carisoprodol in corn oil; the Cmax values of the dose in 0.5% methylcellulose were approximately threefold those of the dose in corn oil. For mice, no significant difference was observed in the bioavailability of carisoprodol between the vehicles; however, the Cmax values of the dose in 0.5% methylcellulose were 1.5 to 1.75 times those of the dose in corn oil. Carisoprodol was not mutagenic in any of four strains of Salmonella typhimurium, with or without S9 metabolic activation. It did induce mutations in L5178Y mouse lymphoma cells in the absence of S9; with S9, no mutagenic activity was noted in this assay. Results of the sister chromatid exchange test with carisoprodol in cultured Chinese hamster ovary cells were considered equivocal with and without S9. Chromosomal aberrations in cultured Chinese hamster ovary cells were clearly increased by carisoprodol treatment, particularly in the presence of S9. No significant increases in the frequency of micronucleated erythrocytes were observed in peripheral blood samples from male and female mice administered carisoprodol by gavage for 13 weeks. In conclusion, carisoprodol induced ataxia and prostration in rats and mice, increases in liver weights in rats and mice, and nephropathy in male rats. The bioavailability of carisoprodol in 5% methylcellulose was greater than in corn oil. The no-observed-adverse-effect (NOAEL) level of carisoprodol administered in corn oil or in 0.5% methylcellulose was determined to be 100 mg/kg, compared to the clinical dose of 20 mg/kg per day for adults and 5 to 7.5 mg/kg per day for children.