Laboratory monitoring of patients with hereditary tyrosinemia type I

BACKGROUND

The prognosis of patients with Hereditary Tyrosinemia Type 1 (HT-1) has greatly improved with early detection through newborn screening and the introduction of nitisinone (NTBC) therapy. A recent guideline calls for periodic monitoring of biochemical markers and NTBC levels to tailor treatment; however, this is currently only achieved through a combination of clinical laboratory tests. We developed a multiplexed assay measuring relevant amino acids, succinylacetone (SUAC), and NTBC in dried blood spots (DBS) to facilitate treatment monitoring.

METHODS

Tyrosine, phenylalanine, methionine, NTBC and SUAC were eluted from DBS with methanol containing internal standards for each analyte and analyzed by liquid chromatography tandem mass spectrometry over 6.5 min in the multiple reaction monitoring positive mode.

RESULTS

Pre-analytical and analytical factors were studied and demonstrated a reliable assay. Chromatography resolved an unknown substance that falsely elevates SUAC concentrations and was present in all samples. To establish control and disease ranges, the method was applied to DBS collected from controls (n = 284) and affected patients before (n = 2) and after initiation of treatment (n = 29). In the treated patients SUAC concentrations were within the normal range over a wide range of NTBC levels.

CONCLUSIONS

This assay enables combined, accurate measurement of revelevant metabolites and NTBC in order to simplify treatment monitoring of patients with HT-1. In addition, the use of DBS allows for specimen collection at home to facilitate more standardization in relation to drug and dietary treatment.

Association of Neurodevelopmental Outcomes With Environmental Exposure to Cyclohexanone During Neonatal Congenital Cardiac Operations: A Secondary Analysis of a Randomized Clinical Trial

IMPORTANCE

Cyclohexanone is an industrial solvent used as a coupling agent in medical plastics. Perioperative exposure to cyclohexanone could play a role in lower scores on measures of neurodevelopmental outcomes after neonatal cardiac operations.

OBJECTIVE

To examine the presence and association of serum cyclohexanone level with neonatal cardiac operations and neurodevelopmental outcomes.

DESIGN, SETTING, AND PARTICIPANTS

This ad hoc secondary analysis used data from the Corticosteroid Therapy in Neonates Undergoing Cardiopulmonary Bypass randomized clinical trial. The cohort included neonates younger than 31 days and with at least 37 weeks postgestational age at surgical treatment who were enrolled at a single center between June 1, 2012, and October 31, 2016, and who had completed a neurodevelopmental assessment at age 12 months. Data were analyzed from July 8 to August 20, 2019.

EXPOSURES

Serum cyclohexanone and its metabolites were measured preoperatively (prior to skin incision), postoperatively (immediately after the surgical procedure was completed), and 12 hours postoperatively. Cyclohexanone and the molar sum of its metabolites were examined at each point and as a geometric mean of all 3 points.

MAIN OUTCOMES AND MEASURES

Neurodevelopment was assessed at age 12 months with the Bayley Scales of Infant and Toddler Development III, assessing cognitive, language, and motor function composite scores standardized to a population mean (SD) of 100 (15). Linear regression models were used to determine covariate-adjusted differences in 12-month cognitive, language, and motor composite scores per interquartile range increase in cyclohexanone level or summed metabolite molar concentrations.

RESULTS

Among 85 included neonates, mean (SD) age at surgical treatment was 9.7 (5.3) days, 49 (58%) were boys, and 54 (64%) underwent corrective repair. Mean (SD) Bayley Scales of Infant and Toddler Development III composite scores were 108.2 (12.2) for cognitive function, 104.7 (11.0) for language function, and 94.7 (15.7) for motor function. Median (interquartile range) cyclohexanone levels increased approximately 3-fold from immediately prior to surgical treatment to immediately after surgical treatment (572 [389-974] vs 1744 [1469-2291] μg/L; P = .001). In adjusted analyses, higher geometric mean cyclohexanone levels were associated with significantly lower composite scores for cognitive (-4.23; 95% CI, -7.39 to -1.06; P = .01) and language (-3.65; 95% CI, -6.41 to -0.88; P = .01) function. The difference in composite scores for motor function among infants with higher geometric mean cyclohexanone levels was not statistically significant(-3.93, 95% CI: -8.19 to 0.33, P = .07).

CONCLUSIONS AND RELEVANCE

The findings of this secondary analysis of a randomized clinical trial suggest that infants who underwent neonatal cardiac surgical treatment with cardiopulmonary bypass had substantial cyclohexanone levels, which were associated with adverse neurodevelopmental function at age 12 months.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT01579513.

Daily variation of NTBC and its relation to succinylacetone in tyrosinemia type 1 patients comparing a single dose to two doses a day

INTRODUCTION

In hereditary tyrosinemia type 1 (HT1) patients, the dose of NTBC that leads to the absence of toxic metabolites such as succinylacetone (SA) is still unknown. Therefore, the aims of this study were to investigate the variation and concentrations of 2-(2-nitro-4-trifluormethyl-benzyl)-1,3-cyclohexanedione (NTBC) during the day in relation to the detection of SA, while comparing different dosing regimens.

METHODS

All patients were treated with NTBC (mean 1.08 ± 0.34 mg/kg/day) and a low phenylalanine-tyrosine diet. Thirteen patients received a single dose of NTBC and five patients twice daily. Home bloodspots were collected four times daily for three consecutive days measuring NTBC and SA concentrations. Statistical analyses were performed by using mixed model analyses and generalized linear mixed model analyses to study variation and differences in NTBC concentrations and the correlation with SA, respectively.

RESULTS

NTBC concentrations varied significantly during the day especially if NTBC was taken at breakfast only (p = 0.026), although no significant difference in NTBC concentrations between different dosing regimens could be found (p = 0.289). Momentary NTBC concentrations were negatively correlated with SA (p < 0.001). Quantitatively detectable SA was only found in subjects with once daily administration of NTBC and associated with momentary NTBC concentrations <44.3 μmol/l.

DISCUSSION

NTBC could be less stable than previously considered, thus dosing NTBC once daily and lower concentrations may be less adequate. Further research including more data is necessary to establish the optimal dosing of NTBC.

[Acute methoxetamine intoxication--a case report with serum and urine concentrations]

Methoxetamine (MXE) is a novel synthetic drug, structurally related to phencyclidine, with ketamine-like properties. Available in Poland since 2010, with no legal control, is adverti. sed as the "ideal dissociation drug". The aim of this study was to present a case of nasal methoxetamine acute poisoning in a 28-year-old man, the course of treatment, and the method of identification of this substance in serum and urine. In the course of this intoxication extreme agitation and aggression with slight response to benzodiazepines were observed. The patient was confused, hallucinated. In addition, the physical examination re. vealed tachycardia 120/min and normal blood pressure (130/80 mm Hg). The period of acute poisoning was covered by amnesia. The MXE concentrations in serum and urine were determined using liquid chromatography-mass spectrometry (LC-MS-MS) method, and were respectively 270 ng/ml and 660 ng/ml. Confirmed MXE poisoning increases our knowledge about this new substance, providing relevant clinical and analytical data.

Development and validation of a bioanalytical method for quantification of 2,6-bis-(4-hydroxy-3-methoxybenzylidene)-cyclohexanone (BHMC) in rat plasma

A sensitive and accurate high performance liquid chromatography with ultraviolet/visible light detection (HPLC-UV/VIS) method for the quantification of 2,6-bis-(4-hydroxy-3-methoxybenzylidene)-cyclohexanone (BHMC) in rat plasma was developed and validated. BHMC and the internal standard, harmaline, were extracted from plasma samples by a simple liquid-liquid extraction using 95% ethyl acetate and 5% methanol. Plasma concentration of BHMC and internal standard were analyzed by reversed phase chromatography using a C₁₈ column (150 × 4.6 mm I.D., particle size 5 µm) and elution with a gradient mobile phase of water and methanol at a flow rate of 1.0 mL/min. Detection of BHMC and internal standard was done at a wavelength of 380 nm. The limit of quantification was 0.02 µg/mL. The calibration curves was linear (R² > 0.999) over the concentration range of 0.02-2.5 µg/mL. Intra- and inter-day precision were less than 2% coefficient of variation. The validated method was then applied to a pharmacokinetic study in rats by intravenous administration of BHMC at a single dose of 10 mg/kg. Pharmacokinetic parameters such as half-life, maximum plasma concentration, volume of distribution, clearance and elimination rate constant for BHMC were calculated.

Single dose NTBC-treatment of hereditary tyrosinemia type I

NTBC (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3cyclohexanedione) is the mainstay of treatment in tyrosinemia type 1 (HT 1). The current recommendation is to divide the total daily dose of NTBC into two doses. We monitored the plasma NTBC concentrations in a series of seven patients who were changed from multiple divided doses to a single daily dose of NTBC. Two additional patients were started on a single daily dose of NTBC after the diagnosis of HT 1 was established. In three patients, NTBC kinetics were performed over 6 and 24 hours, respectively. The use of multiple divided doses or a single daily dose did not significantly affect plasma NTBC concentrations or the mean daily dose needed to attain therapeutic plasma NTBC concentrations. Moreover, kinetic studies demonstrated that plasma NTBC concentrations were completely stable over a period of 24 hours with a single dose regimen, as expected given the known NTBC plasma half life of 54 hours. Although these preliminary results need to be confirmed in more patients, our findings show that administration of NTBC in a single daily dose may be as effective as a multiple-dose regimen in reaching therapeutic plasma NTBC concentrations and suppressing succinylacetone formation in patients with HT 1. In fact, single dose treatment may increase patients' compliance with the drug treatment and improve metabolic control.

Increase of CSF tyrosine and impaired serotonin turnover in tyrosinemia type I

OBJECTIVE

Psychomotor impairment has been described in hypertyrosinemia types II and III (HT III). Only recently cognitive deficits have also been reported in hypertyrosinemia type I (HT I). The pathogenic mechanisms responsible are unknown. Since implementation of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC, Nitisinone (Swedish Orphan International)) in the treatment of HT I, plasma tyrosine elevation is a common finding as known from the other hypertyrosinemias.

PATIENTS AND METHODS

With elevated tyrosine as suspected pathogenic factor in the development of cognitive deficits, we here investigated tyrosine in the cerebrospinal fluid (CSF) and serotonergic and dopaminergic neurotransmitter levels in three patients with HT I during long-term treatment with Nitisinone. In addition, Nitisinone concentrations in plasma and CSF were measured. We also assessed psychomotor and cognitive development by standardized test systems and brain morphology by magnetic resonance imaging.

RESULTS

All patients presented with high tyrosine concentrations in CSF correlating with increased plasma tyrosine levels and a reduced CSF serotonin turnover. MRI revealed no structural abnormalities in the brain. All patients presented with either impaired cognitive development or behavioural abnormalities.

CONCLUSIONS

We here outline the need to further study the exact pathogenic mechanisms responsible for the neurotransmitter changes observed in HT type I in order to possibly prevent cognitive dysfunction. Nitisinone has significantly improved outcome and quality of life in HT type I; however, it is also accompanied by elevated plasma and CSF tyrosine. Further studies are essential to identify the necessary dietary tyrosine restriction and the optimal Nitisinone dose.

Monitoring tyrosinaemia type I: Blood spot test for nitisinone (NTBC)

BACKGROUND

Quantification of nitisinone, 2-(nitro-4-trifluoromethylbenzoyl)1,3-cyclohexanedione (NTBC) has been repeatedly described. Nevertheless monitoring of NTBC has not yet become part of routine therapy surveillance in tyrosinaemia type I (OMIM 276700). We developed a blood spot test to facilitate collection and transport of samples. Furthermore, the test material can be used for determination of other parameters like tyrosine and succinylacetone.

METHOD

For quantification of NTBC in blood spots filter paper discs of 3.2mm diameter were extracted with 150 μL methanol containing mesotrione as internal standard (IS). Analysis was done by UPLC-MS/MS on a Xevo mass spectrometer (ESI+), (MRM). Parent ions were 330.05 for NTBC and 340.05 for IS, daughter ions were m/z 217.95 and m/z 125.95 for NTBC, and m/z 227.95 and m/z 103.95 for IS.

RESULTS

The calibration curve for NTBC in blood spots was linear from 0.1 μmol/L to 100 μmol/L. Recovery exceeded 73.1%, CV intraday and interday were below 9.6%. Instrumental run time was 2.5 min. Sensitivity of the method was 0.1 μmol/L. NTBC concentrations in plasma were higher than in blood spots by a factor of 1.56 ± 0.13.

CONCLUSION

As demonstrated in patients with tyrosinaemia type I quantification of NTBC by UPLC-MS/MS in blood spots is feasible and gives valuable information for monitoring NTBC treatment.

Use of nitisinone in patients with alkaptonuria

Alkaptonuria, a rare autosomal recessive disorder caused by mutations in the HGD gene and deficiency of homogentisate 1,2 dioxygenase, is characterized by ochronosis, arthritis, and daily excretion of gram quantities of homogentisic acid (HGA). Nitisinone, an inhibitor of the enzyme 4-hydroxyphenylpyruvate dioxygenase, can drastically reduce urinary excretion of HGA in individuals with alkaptonuria. We investigated the safety and the HGA-depleting efficacy of nitisinone in an open-label, single-center study of 9 alkaptonuria patients (5 women, 4 men; 35-69 years of age) over the course of 3 to 4 months. Each patient received nitisinone in incremental doses, 0.35 mg bid followed by 1.05 mg bid, and remained on this dosage and a regular diet for 3 months. Nitisinone reduced urinary HGA levels from an average of 4.0 +/- 1.8 (SD) g/day to 0.2 +/- 0.2 g/day ( P < .001). The average plasma tyrosine concentration, initially 68 +/- 18 mmicro mol/L, rose to 760 +/- 181 micro mol/L ( P < .001). During the final week of the study, 5 patients adhered to a protein-restricted diet (40 g/day), and their mean plasma tyrosine level fell from 755 +/- 167 to 603 +/- 114 mu mol/L. Six of the 7 patients who received nitisinone for more than 1 week reported decreased pain in their affected joints. Weekly ophthalmologic examinations showed no signs of corneal toxicity. Adverse events included the passing of kidney stones, the recognition of symptoms related to aortic stenosis, and elevation of liver transaminase levels. We conclude that low-dose nitisinone effectively reduced urinary HGA levels in patients with alkaptonuria. Future long-term clinical trials are planned to determine the benefits of nitisinone in preventing joint deterioration and providing pain relief, and its long-term side effects.

LC/MS determination of the enaminones E139, DM5 and DM27 in rat serum

Enaminones, E139, DM5 and DM27, have been recently recognized as potential anticonvulsant compounds. The molecular masses of these enarminones were proven using ion trap Finnigan mass spectrometer. For conduction of biological studies in animals, a sensitive and selective high-performance liquid chromatography-mass spectrometry (LC/MS) was developed for the determination of the selected enaminones in rat serum. A simple protein precipitation procedure was followed for cleaning up the serum samples before analysis. LC/MS determinations were performed using an APCI probe at 430 degrees C. Positive ions (M+1)(+) were acquired in MS/MS-SRM mode at m/z 308.1 (parent m/z 340.2) for E139 and m/z 262.1 (parent m/z 294.1) for DM5. On the other hand, DM27 and E118 (internal standard) were measured in SIM mode at m/z 236.5 and 222.5, respectively. Quantitation was based on measurement of the peak area ratio of enaminones (E139, DM5, DM27) and E118 as an internal standard. Calibration curves were linear (r>0.9989) over the concentration range 100-1000 ngml(-1) and were free from serum interference. Precision and accuracy studies of control samples showed intra-day and inter-day %RSD <10.1 and % deviation from nominal concentrations (%DEV) from -4.3 to +10.1. Recoveries of E139, DM5 and DM27 from quality control rat serum samples using protein precipitation method were 92.3, 89.4 and 89.6%, respectively. The reported data suggest the utility of this developed method for structural elucidation and for performing pharmacokinetics studies on the selected enaminones in rats.

Determination of the enaminone DM5, an anti-epileptic agent, in mouse plasma and brain tissue by high-performance liquid chromatography with ultraviolet detection

Enaminone derivatives of the 4-carbomethoxy-5-methylcyclohexane-1,3-dione series represent a new and potentially active series of compounds for the treatment of Epilepsy. Enaminone esters have been previously evaluated as compounds with potent oral anticonvulsant activity similar to class 1 anticonvulsants phenytoin, carbamazepine, and lamotrigine. DM5, a member of this class with -Cl in the para-substituted position, has been assessed to have the most potent pharmacological activity (ED50) in both the mouse and rat. A selective and specific high-performance liquid chromatography method was developed to quantitate DM5 in plasma and brain tissue in mice. Reverse phase chromatography with ultraviolet (lambda = 307 nm) detection was utilized to quantitate eluate. A C18 analytical column was used and the mobile phase consisted of acetonitrile and 0.05 M NaH2PO4 buffer (60:40; v/v). Liquid-liquid extraction with ether was used to extract the DM5 from plasma or brain homogenates. DM5 and carbamazepine (internal standard) eluted at approximately 6.0 and 9.0 min without any interfering peaks. The calibration curves were found to be linear (r > or = 0.9999) in the range of 0.1-5.0 microg/ml or microg/g. Intra-run precision's were in all in the range of 90%. The absolute recovery of the analyte in brain and plasma samples was < or = 90%. The valid method accurately quantified DM5 in plasma and brain tissue samples collected from a pharmacokinetic study consisting of an intravenous bolus in the tail vein of wild type and genetically altered mice.

Tissue distribution of 2-(2-nitro-4-trifluoromethylbenzoyl)-cyclohexane-1,3-dione (NTBC) and its effect on enzymes involved in tyrosine catabolism in the mouse

Administration of a single oral dose of 2-(2-nitro-4-trifluoromethyl-benzoyl)-cyclohexane-1,3-dione (NTBC) to mice increases the concentration of tyrosine in the plasma and aqueous humour. The tyrosinaemia is both time and dose-dependent with a single dose of 30 micromol NTBC/kg (10 mg/kg) producing maximal concentrations of tyrosine in plasma of about 1200 nmol/ml and in aqueous humour of about 2200 nmol/ml at 16 h after dosing. Analysis of the key hepatic enzymes involved in tyrosine catabolism, following a single dose of 30 micromol NTBC/kg, showed that 4-hydroxyphenylpyruvate dioxygenase (HPPD) was markedly inhibited soon after dosing and that the activity recovered very slowly. In response to the tyrosinaemia, the activity of hepatic tyrosine aminotransferase (TAT) was induced about two-fold, while the activity of hepatic homogentisic acid oxidase (HGO) was reduced at 4 and 5 days after dosing. Daily oral administration of NTBC at doses up to 480 micromol NTBC/kg (160mg/kg/day) to mice produced a maximal tyrosinaemia of about 600-700nmol/ml plasma, showing some adaptation relative to a single dose. Unlike the rat, no treatment-related corneal lesions of the eye were seen at any dose levels up to 6 weeks. Administration of a single oral dose of [14C]-NTBC at 30 micromol/kg led to selective retention of radiolabel in the liver and to a lesser extent the kidneys. Our studies show that NTBC is a potent inhibitor of mouse liver HPPD, which following repeat exposure produces a marked and persistent tyrosinaemia, which does not result in ocular toxicity.

Determination of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione in plasma by direct injection into a coupled column liquid chromatographic system

The chemical substance 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) is in clinical use for the treatment of hereditary tyrosinemia type 1. In the present study, the plasma concentration of NTBC was determined by a coupled column liquid chromatographic method. A 20-microl volume of plasma was diluted with phosphate buffer, pH 2, and injected into a small precolumn (BioTrapAcid C18) with a mobile phase containing sulfuric acid. The precolumn was based on the restricted access principle, i.e., retention of NTBC within the lipophilic pores, while polar and large endogenous compounds were eluted with the void volume. NTBC was transferred to the analytical column using a mobile phase with a high content of acetonitrile. The compound was monitored by UV detection at 278 nm. The standard curve was linear between 0.3 and 69 microM, and the between-day precision (RSD) was 3% (n=6 days) at 13.8 microM and 14% (n=6 days) at 0.3 microM NTBC in plasma. The quantitation limit was approximately 0.3 microM using 20 microl of plasma.

Metabolic interaction of ethanol and cyclohexanone in rabbits

The interaction of ethanol (EtOH) and cyclohexanone (CHN) metabolism was studied to demonstrate the influence of alcohol beverage in cyclohexanone poisoning. Rabbits were administered CHN and EtOH separately or together, and the plasma concentration of CHN and cyclohexanol (CHL), a metabolite of CHN, and blood concentrations of EtOH were measured at various times. When CHN alone was administered orally, the time to maximum plasma concentration (Tmax) of CHN was as short as 15 min, but that of CHL was 120 min. The short Tmax of CHN was considered to be due to the first-pass effect. The plasma concentration of CHL was much greater than that of CHN. On the other hand, when the same amount of molar CHL was administered in rabbits for sake of comparison, only small amounts of CHN were detected in plasma. The interconversion between CHN and CHL at the time of equilibration tended to shift predominantly toward the formation of CHL, whose plasma concentration ratio was about 1:6 to 1:8. The coadministration of EtOH decreased the plasma concentration of CHN and increased that of CHL more than the administration of CHN alone. The blood concentration of EtOH was also decreased with the coadministration of CHN. The coadministration of EtOH with CHN demonstrated an acceleration in the metabolism of both CHN and EtOH, which may be due to an increase in the concentrations of the alcohol dehydrogenase-NADH and -NAD complexes.

Disposition of acetone, methyl ethyl ketone and cyclohexanone in acute poisoning

A case of coma due to the drinking of a liquid cement for polyvinyl chloride resin, containing acetone, methyl ethyl ketone, cyclohexanone and polyvinyl chloride is described. The patient also simultaneously ingested the alcoholic beverage, sake. After gastric lavage, plasma exchanges and direct hemoperfusions, the patient recovered. The concentrations of these chemicals in plasma and urine were analyzed at various time intervals to estimate the clearance. The elimination half lives for acetone and methyl ethyl ketone were 18 hours and 10 hours, respectively. Although cyclohexanone made up the largest component in the solvents, the blood level was extremely low and a large amount of cyclohexanol, a metabolite of cyclohexanone was detected in the blood and urine. The glucuronide metabolite of cyclohexanol was also estimated after the hydrolysis with beta-glucuronidase. Since the conversion of cyclohexanone to cyclohexanol is known to be catalyzed by alcohol dehydrogenase, possible interactions between sake ingestion and cyclohexanone metabolism is proposed.

Assessment of the cataractogenic potential of cyclohexanone in guinea pigs and rabbits

The cataractogenic potential of cyclohexanone administered intravenously (0.5 and 5.0 mg/kg) and percutaneously (0.5 ml) was assessed in guinea pigs and rabbits. The negative control article was 0.9% sodium chloride. Positive control articles for guinea pigs were iv galactose and percutaneous dimethyl sulfoxide (DMSO), and for rabbits, iv poly(I) X poly(C) and percutaneous DMSO. An untreated group of guinea pigs was used as a sham control. Animals were treated three times a week for 3 consecutive weeks. Ophthalmic examinations performed monthly for 6 months for treated animals and 7 months for untreated animals revealed the presence of anterior subcapsular vacuoles in guinea pigs in all groups. These lesions consisted of swollen lens fibers that progressed to fiber disruption and protein liquefaction. There was no statistical difference in the incidence and severity of the lesions among treatment groups. No lenticular alterations were noted in any of the rabbits treated with cyclohexanone or any other chemical. It was concluded that these alterations are, apparently, an inherent characteristic of the guinea pig, making that animal an unsuitable model for the assessment of cataractogenic potential of xenobiotics.

Assessment of the toxicity of cyclohexanone administered intravenously to Wistar and Gunn rats

The toxicity of cyclohexanone, used as a solvent cement in polyvinyl chloride medical devices, was assessed in Wistar and Gunn rats. The Gunn rat was used because it has a negligible activity of UDP glucuronosyltransferase toward bilirubin and certain other aglycones. Cyclohexanone was administered iv for 28 consecutive days to Wistar and Gunn rats in two doses (50 and 100 mg/kg), using solutions containing 0.25 and 0.50 g per 100 ml, respectively, at a constant volume of 20 mg/kg. Saline (0.9% NaCl) was used as the control. Daily observations for signs of toxicity showed no adverse effects in Wistar or Gunn rats injected with either dose. Daily weight changes of control and test animals were similar. Ophthalmologic examinations revealed no treatment-related structural lesions. No adverse effects were noted when the data from the hemogram or clinical chemistry parameters were evaluated. Gross pathological and histopathologic assessment showed no alterations due to cyclohexanone treatment. Urinary excretions of total and glucuronide conjugates of cyclohexanol were similar for Wistar and Gunn rats; less than 1% was excreted as free cyclohexanone and cyclohexanol. It is concluded that the Gunn rat is capable of forming glucuronides of cyclohexanol and that cyclohexanone at these doses has a negligible toxic potential.