A hybrid approach for fusing 4D-MRI temporal information with 3D-CT for the study of lung and lung tumor motion.

PURPOSE: Accurate visualization of lung motion is important in many clinical applications, such as radiotherapy of lung cancer. Advancement in imaging modalities [e.g., computed tomography (CT) and MRI] has allowed dynamic imaging of lung and lung tumor motion. However, each imaging modality has its advantages and disadvantages. The study presented in this paper aims at generating synthetic 4D-CT dataset for lung cancer patients by combining both continuous three-dimensional (3D) motion captured by 4D-MRI and the high spatial resolution captured by CT using the authors' proposed approach. METHODS: A novel hybrid approach based on deformable image registration (DIR) and finite element method simulation was developed to fuse a static 3D-CT volume (acquired under breath-hold) and the 3D motion information extracted from 4D-MRI dataset, creating a synthetic 4D-CT dataset. RESULTS: The study focuses on imaging of lung and lung tumor. Comparing the synthetic 4D-CT dataset with the acquired 4D-CT dataset of six lung cancer patients based on 420 landmarks, accurate results (average error <2 mm) were achieved using the authors' proposed approach. Their hybrid approach achieved a 40% error reduction (based on landmarks assessment) over using only DIR techniques. CONCLUSIONS: The synthetic 4D-CT dataset generated has high spatial resolution, has excellent lung details, and is able to show movement of lung and lung tumor over multiple breathing cycles.

Four-month chemotherapy in the treatment of smear-negative pulmonary tuberculosis: results at 30 to 60 months.

INTRODUCTION: Most patients with active pulmonary tuberculosis (PTB) are treated with a 6-month short course regimen. The purpose of the present study was to assess the efficacy of using 4 months of chemotherapy to treat patients with smear-negative PTB. METHODS: A total of 314 patients were randomised to a daily or combined (daily and intermittent) regimen as follows: (1) 2HRZ/2HR--2 months of isoniazid (H), rifampicin (R) and pyrazinamide (Z), followed by 2 months of H and R or (2) 2HRZ/2H3R3--2 months of HRZ as in regimen 1, followed by H and R given 3 times weekly for 2 months or 4 months if initial sputum specimens were culture positive. RESULTS: One hundred and fifty-eight patients were assigned to the daily regimen and 156 to the combined regimen. Of the 158 patients, 99 had negative cultures and 59 had positive cultures. There was no relapse among 96 culture-negative patients assessed at 30 months and 68 patients at 60 months. However, 6 patients had no radiological response while 1 was considered on review to have non-tuberculous disease. There was no relapse among 57 culture-positive patients assessed at 30 months and 41 at 60 months. In the combined regimen group, 102 had negative cultures and 54 had positive cultures. There was 1 relapse in the culture-negative group of 100 patients assessed at 30 months and 74 at 60 months. There was no radiological response in 5 patients. One patient in the culture-positive group failed therapy but there were no relapses during follow-up to 60 months. CONCLUSION: A 4-month daily or combined regimen appears to be highly effective in the treatment of non-immunocompromised patients with smear- and culture-negative PTB.

Thalidomide is distributed into human semen after oral dosing.

As part of a double-blind placebo-controlled study of the effect of thalidomide on body weight and the viral load of human immunodeficiency virus-seropositive patients, plasma and semen samples were analyzed for the presence of thalidomide. Patients were orally dosed with 100 mg of thalidomide/day for 8 weeks. Blood samples were obtained at baseline and weeks 4, 8, and 12, and semen was obtained at baseline and weeks 4 and 8. Samples were extracted with solid-phase cartridges and analyzed by liquid chromatography/tandem mass spectrometry using atmospheric pressure chemical ionization in the negative ion mode. Two of four patients taking thalidomide were able to provide semen samples. Both had detectable levels of thalidomide in their plasma (10-350 ng/ml) and semen (10-250 ng/g) at weeks 4 and 8. There was an apparent correlation between plasma and semen levels. Semen levels could be significantly greater for therapeutic doses of more than 100 mg/day. Since the threshold dose for birth defects and thalidomide exposure is not known, male patients are advised to use barrier contraception.

Thalidomide dose proportionality assessment following single doses to healthy subjects.

Thalidomide is approved in the United States for treating erythema nodosum leprosum, a complication of leprosy. The present study determined the single-dose oral pharmacokinetics and dose proportionality from 50 to 400 mg of Celgene's commercial Thalomid thalidomide formulation in an open-label, single-dose, three-way crossover study. Fifteen healthy subjects were given 50, 200, and 400 mg of thalidomide on three occasions, and blood samples were collected over 48 hours. Pharmacokinetic parameters were determined using noncompartmental methods, and dose proportionality was assessed by linear regression of dose-normalized Cmax and AUC0-infinity. No serious or unexpected adverse events occurred. The most common adverse events were dizziness, somnolence, headache, and nausea. One patient was discontinued because of pharyngitis. There was a significant deviation from proportionality for Cmax with increases being less than proportional than changes in dose. AUC0-infinity increased proportionally with dose, suggesting that the overall amount of thalidomide absorbed, as well as its clearance, is independent of dose over the range used. V/F was found to increase with dose. This was most likely due to the terminal rate constant, which is used to calculate V/F, actually representing the absorption process rather than elimination (i.e., flip-flop phenomenon). The terminal rate constant (absorption rate constant) for the highest dose was 50% less than for the other two lower doses. The less than proportional increases in Cmax were most likely due to thalidomide's low aqueous solubility. Thalidomide shows reasonable dose proportionality with respect to AUC from 50 to 400 mg.

Safety profile of thalidomide after 53 weeks of oral administration in beagle dogs.

Fifty-six adult beagle dogs (28 male, 28 female) were orally administered thalidomide at 43, 200, or 1000 mg/kg/day for 53 weeks. Sixteen (2/sex/dose group) and 32 (4/sex/dose group) dogs were euthanized and necropsied after 26 and 53 weeks of dosing, respectively. The remaining 8 animals (2/sex/group; high-dose and control groups) were dosed for 53 weeks, euthanized, and necropsied at 58 weeks after a 5-week recovery period. There were no deaths during the study. The only observed clinical signs attributable to thalidomide administration were green-colored urine, white-colored fecal residue presumed to be unchanged thalidomide, enlarged and/or blue coloration of female mammary tissue, and prolonged estrus. There were no thalidomide-related changes in body weights, food consumption, electrocardiography, ophthalmoscopy, neurological function, and endocrine function. The mostly slight and/or transient variations observed in some hematology and blood chemistry values of dosed dogs were considered to be toxicologically insignificant and were supported by the lack of histopathologic correlates. The only gross finding attributable to thalidomide was a yellow-green discoloration of the femur, rib, and/or calvarium that was observed at each euthanization interval including recovery. There was no microscopic correlate for this finding. No thalidomide-related microscopic changes were seen in any of the organs and tissues at 26 weeks. Mammary duct dilatation and/or glandular hyperplasia observed in females at 53 and 58 weeks and hepatic bile pigment exhibited by high-dose males at 53 weeks were microscopic changes considered to be thalidomide-related. There was no gross and histopathologic evidence of any tumors. In summary, thalidomide at up to 1000 mg/kg/day for 53 weeks did not induce any major systemic toxicity or tumors in dogs. The NOAEL was 200 mg/kg/day.

Effect of a high-fat meal on thalidomide pharmacokinetics and the relative bioavailability of oral formulations in healthy men and women.

The effect of food on the oral pharmacokinetics of thalidomide and the relative bioavailability of two oral thalidomide formulations were determined in an open label, single dose, randomized, three-way crossover study. Five male and eight female healthy volunteers received a single oral dose of 200 mg Celgene thalidomide capsules under fasted and non-fasted conditions, and a single dose of 200 mg tablets of Serral thalidomide under fasted conditions. The high-fat meal resulted in a 0.5-1.5 h absorption lag time, an increased mean C(max), a decreased mean AUC and a delay in mean T(max). The Serral tablet formulation resulted in a lower mean C(max), and slower terminal decline in plasma thalidomide concentrations compared with both Celgene treatments. Mean C(max) concentrations were 1.99+/-0.41 microg/mL (range 1.28-2.76) within 4.00+/-1.13 h (2-5) for the Celgene formulation fasted, 2.17+/-0.51 microg/mL (1.43-3.01) within 6.08+/-2.33 h (3-12) for the Celgene formulation with food, and 1. 05+/-0.31 microg/mL (0.62-1.65) within 6.23+/-1.88 h (5-10) for the Serral formulation fasted. Mean terminal half-lives were 13.50+/-6. 77 h for the Serral product, compared with 5.80+/-1.72 h and 5. 09+/-1.03 h for Celgene fasted and fed, respectively. Celgene's formulation exhibited slightly greater bioavailability than Serral's formulation, with mean ratios of 122% and 110% for Ln-transformed AUC(0-t) and AUC(0-infinity), respectively. The mean C(max) for the Celgene formulation was approximately two times greater than Serral's. Food delayed the onset of absorption of by 0.5-1.5 h, but had little effect on the extent of absorption from the Celgene capsule. Under fasted conditions, the Celgene thalidomide resulted in a two-fold greater C(max) and 10% greater AUC(0-infinity) than the Serral formulation.

Metabolism of thalidomide in human microsomes, cloned human cytochrome P-450 isozymes, and Hansen's disease patients.

Previous in vitro studies in rat microsomal preparations suggested that thalidomide is metabolized by the cytochrome P450 system (CYP). In this study, we examined the extent of thalidomide metabolism by preparations of pooled human microsomes, microsomes containing cloned human CYP isozymes (CYPIA2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4), and Hansen's disease patients. Results indicated that thalidomide was a poor substrate for CYP isozymes. Alteration of incubation buffer, pH, incubation time, and microsome and thalidomide concentrations did not increase the production of any metabolites. Thalidomide also did not inhibit metabolism of CYP-specific substrates and therefore any interactions with other drugs that are metabolized by the same enzyme system are unlikely. Hansen's patients were given a single oral dose of thalidomide (400 mg), and their blood and urine were collected at time points up to 72 hours, processed, and analyzed by tandem mass spectrometry. Although thalidomide was present in the plasma and urine, no metabolites were found in the plasma and very low amounts of the 5-OH thalidomide metabolite were present in the urine. These results suggest that thalidomide does not undergo significant metabolism by human CYP and that clinically important interactions between thalidomide and drugs that are also metabolized by this enzyme system are unlikely. The major route of thalidomide breakdown in humans and animals is through spontaneous hydrolysis with subsequent elimination in the urine.

Single-dose oral pharmacokinetics of three formulations of thalidomide in healthy male volunteers.

Thalidomide was recently approved in the United States for the treatment of erythema nodosum leprosum, a complication of leprosy. The present study determined the bioequivalence and pharmacokinetics of Celgene's commercial and clinical trial thalidomide formulations and the Brazilian Tortuga formulation in an open-label, single-dose, three-way crossover design. Seventeen healthy subjects were given 200 mg of thalidomide on three occasions, and blood samples were collected over 48 hours. Pharmacokinetic parameters were determined using compartmental methods for the two Celgene formulations and using noncompartmental methods for all three formulations. All subjects reported adverse events, none of which was serious or unexpected. Celgene formulations were bioequivalent when comparing Cmax, tmax, and AUC. There was significant variability in plasma levels from the Tortuga formulation, giving a mean profile that was distinctly different from the two Celgene formulations with a lower Cmax value and a longer terminal phase. The lower Cmax was probably due to slower absorption. The terminal rate constant for the Tortuga formulation was significantly less, giving rise to a terminal half-life of 15 hours compared to about 5 to 6 hours for the Celgene formulations. Confidence intervals for Cmax between the Tortuga and the Celgene formulations were outside the 80% to 125% range, indicating a lack of bioequivalence. Extent of absorption, as measured by AUC0-infinity, was approximately equal for all three formulations. Terminal half-life for Tortuga was two to three times longer compared to the Celgene formulations and is clear evidence for absorption rate limitations. The two Celgene formulations showed similar pharmacokinetic parameters with profiles that were best described by a one-compartment model with first-order absorption and elimination. The authors conclude that Celgene's clinical trial and commercial thalidomide formulations are similar to each other and distinctly different from the Tortuga formulation and that all three formulations exhibited absorption rate-limited elimination.

Assessment of a combined preparation of isoniazid, rifampicin and pyrazinamide (Rifater) in the initial phase of chemotherapy in three 6-month regimens for smear-positive pulmonary tuberculosis: a five-year follow-up report.

SETTING: Singapore Tuberculosis Service. OBJECTIVE: To assess the acceptability, efficacy and relapse rate of a combined formulation of three drugs--isoniazid, rifampicin and pyrazinamide (Rifater)--given in the initial phase of chemotherapy in three 6-month regimens (2SHRZ/4H3R3, 1SHRZ/5H3R3 and 2HRZ/4H3R3) under direct observation for all patients. DESIGN: A randomised, controlled, unblinded study comparing a group of patients treated with Rifater and another given the three component drugs as separate formulations. RESULTS: The 310 patients admitted to the study were divided into two groups of 155 patients. The frequency of side effects was similar in both groups. Of 271 patients with drug-sensitive strains who had completed treatment without interruption, sputum cultures converted in all patients. At the end of 5 years, there were 15 relapses: three (2.2%) in the separate drugs group and 12 (9.3%) in the Rifater group. Exclusion of two cases in the Rifater group, one with silicotuberculosis and another with no bacteriological confirmation of diagnosis, gave a relapse rate of 7.9% (P = 0.03 for the comparison of relapse rates in the two groups). CONCLUSION: A combined formulation of three drugs given daily in the initial phase of 6-month short-course therapy, followed by intermittent treatment with isoniazid and rifampicin given three times a week under direct observation for all patients, appears to be less effective than treatment with the component drugs given as separate formulations.

The spectrum of cutaneous and internal malignancies in chronic arsenic toxicity.

We report 3 patients of chronic arsenic poisoning with characteristic skin changes. All 3 patients had a past history of asthma and were treated with Traditional Chinese Medication. We believe that the Chinese medications were the source of arsenic poisoning. Two of the 3 patients also had internal malignancy. The association of arsenic with internal malignancy is reviewed.

Are antacids necessary as routine prescriptives with non-steroidal anti-inflammatory drugs?

In Singapore, there exists a local habit to routinely prescribe antacids with non-steroidal anti-inflammatory drugs (NSAIDs) perhaps in the belief that gastrointestinal (GI) symptoms and complications are common, and that antacids protect from them. We prospectively studied 140 adults in an orthopaedic clinic who were prescribed a short course of NSAIDs (1 to 4 weeks) without antacids to determine the frequency and severity of GI symptoms. Symptomatic patients were then given antacids to determine their effect on the GI symptoms and followed up by telephone interview. These patients had mild inflammation, soft tissue rheumatism, injury or degenerative disease. All were otherwise well with no known peptic ulcer disease or major illness and were not on ulcerogenic drugs. Only 13 (9.3%) had significant GI symptoms, of which 6 (4.2%) of the total took antacid and 5 (3.5%) had partial or total relief. In this study, GI symptoms were not common with short course NSAIDs in otherwise well patients. Antacids may afford symptomatic relief for GI symptoms. However, because antacids may offer no significant protection against NSAID-induced peptic ulcer, may dangerously mask symptoms of GI irritation, may be troublesome to take and costly on a large scale, we should stop routine prescription of antacids in patients requiring only short-term NSAIDs and not at risk for peptic ulcer disease.

Hypoglycaemia in the elderly.

AIM: To determine the prevalence, presentation, causes and consequences of hypoglycaemia in the elderly, and to make preventive recommendations. METHOD: Retrospective review of case records. RESULTS: The definition of hypoglycaemia is defined as symptoms with a capillary blood sugar of less than 3 mmol/L measured on the Reflolux II or Accutrend glucometer. Out of 1,919 admissions to our department from November 1993 to January 1996, there were 45 cases of hypoglycaemia. The average age was 76.2 years (range 66 to 89 years); 32 were females, 13 males, 35 had diabetes mellitus and 10 were non-diabetics. Forty patients presented with neuroglycopaenic symptoms and 5 patients presented with adrenergic symptoms. Thirteen patients presented were solely due to drugs (mainly glibenclamide); 9 cases were due only to disease (mainly psychiatric illnesses with poor intake); 23 cases were due to both drugs and diseases (mainly a combination of glibenclamide, tolbutamide and psychiatric illness with poor intake, renal failure, gastroenteritis and sepsis). All were easily reversed with an intravenous bolus of 50% glucose or continuous 10% glucose infusions. Forty-three patients did not suffer any morbidity, one suffered a stroke and another fell because of giddiness. CONCLUSION: We recommend that: (1) the importance of having regular meals be emphasised to elderly patients and their carers, especially if they are taking hypoglycaemic agents; (2) regular home glucose monitoring for diabetic patients; (3) assessment and monitoring of renal function before prescribing hypoglycaemic agents; (4) avoidance of the use of long or medium acting sulphonylureas eg. chlorpropamide, glibenclamide in the elderly; (5) adjustment of hypoglycaemic agents (in consultation with a trained nurse/doctor) if the patient suffers from gastroenteritis and (6) less stringent blood glucose control in those with psychiatric illnesses who may have variable food intake.

Refinement and verification of the physiologically based dosimetry description for acrylonitrile in rats.

The physiologically based dosimetry description for acrylonitrile (ACN) and its mutagenic epoxide metabolite 2-cyanoethylene oxide (CEO) in F-344 rats (M. L. Gargas, M. E. Anderson, S.K.O. Teo, R. Batra, T. R. Fennell, and G. L. Kedderis, 1995, Toxicol. Appl. Pharmacol. 134, 185-194) has been refined to include a physiological stomach compartment and the reactions of ACN with tissue glutathione (GSH). The second-order rate constant for reaction of ACN and GSH at pH 7.3 was measured and included in the dosimetry description. Metabolic parameters for ACN and CEO were estimated from oral bolus pharmacokinetic studies and previously obtained iv bolus data (3.4, 47, 55, or 84 mg ACN/kg). Rats were given bolus oral doses of 3, 10, or 30 mg ACN/kg in water, and blood samples were collected at selected time points. ACN and CEO blood concentrations were determined by gas chromatography. The brain and liver concentrations of ACN and CEO were also measured after 10 mg ACN/kg po. ACN elimination from blood was described by saturable P450 epoxidation (Vmax of 5.0 mg/hr/kg and K(M) of 1.5 mg/liter) and first-order GSH conjugation (73 hr(-1)/kg). CEO elimination was described by first-order GSH conjugation (500 hr(-1)/kg). The pharmacokinetic data were well simulated, although CEO blood concentrations after bolus oral dosing were somewhat overestimated. Sensitivity analysis of the dosimetry description indicated that the inhalation exposure route was much more sensitive to changes in metabolic and physiological parameters than either the iv or oral bolus routes. Therefore, inhalation pharmacokinetic data were obtained and compared to simulations of the dosimetry description. Rats were exposed to 186, 254, or 291 ppm ACN for 3 hr. ACN and CEO concentrations were measured in blood, brain, and liver at selected postexposure time points. The dosimetry description accurately simulated the ACN inhalation pharmacokinetic data, providing verification of the parameter estimates. The verified rat dosimetry description for ACN and CEO will be used as the basis for development of a dosimetry description for ACN in people.

Mass methanol poisoning: a clinico-biochemical analysis of 10 cases.

Methanol is a common ingredient in many household products and intoxication can arise easily from inadvertent exposure through ingestion, inhalation or percutaneous absorption. We analysed ten cases of methanol poisoning who presented with visual, neurological and gastrointestinal symptoms, of whom one died and nine were successfully detoxified with ethanol and bicarbonate infusions. Clinical symptoms were not found to correlate with the severity of poisoning. Serum methanol level was found to correlate significantly with arterial pH (correlation coefficient -0.74, p = 0.014) and serum standard bicarbonate levels (correlation coefficient -0.87, p = 0.001). We found that an arterial pH of < 7.33 or a serum standard bicarbonate of < 20 mmol/L correlated well with a serum methanol level of > 45 mg/dL ie severe poisoning (X2 test with Yate's correction factor, p < 0.02). We conclude that arterial pH or serum standard bicarbonate levels can be used as surrogate indicators of the severity of methanol poisoning. They can be used to guide physicians in the method of detoxification (ie whether intravenous or oral ethanol or dialysis should be used) whilst awaiting serum methanol levels in cases where the index of suspicion for methanol poisoning is high. Some cases of severe poisoning can be successfully treated with oral ethanol if the intravenous form is not available.

Auto-antibodies in the hospitalised Oriental elderly.

Aches and pains are a common problem in the elderly and often a positive auto-antibody result is difficult to interpret in the given clinical setting. We studied 96 unselected hospitalised Oriental elderly aged 65 years and above to determine the prevalence of auto-antibodies and their clinical significance. Anti-nuclear antibody (ANA) was positive in 33%; commonest titres were low (i.e. 1:40); commonest patterns were speckled and homogeneous; 44% had no known cause for a positive result apart from their age; 37% were taking drugs known to be associated with a positive ANA and 19% had both a disease and a drug known to be associated with a positive ANA. Rheumatoid factor (RF) was positive in 16%; 47% had titres < or = 40 IU/ml and 87% had no known cause of positive RF. Other auto-antibodies tested were negative. We conclude that the prevalence of auto-antibodies in the hospitalised Oriental elderly are common and of no clinical significance if accompanying features of an auto-immune disease are absent.

A physiologically based dosimetry description of acrylonitrile and cyanoethylene oxide in the rat.

The cytochrome P450-mediated oxidation of acrylonitrile (ACN) to the mutagen 2-cyanoethylene oxide (CEO) is thought to be important for the carcinogenic effects of ACN in rats, while glutathione (GSH) conjugation of ACN and CEO is regarded as detoxication. A physiologically based dosimetry description for ACN and CEO in the male F-344 rat has been developed from in vitro data and studies of the iv pharmacokinetics of ACN and CEO. The dosimetry description includes tissue partition coefficients and in vitro estimates of the rates of reaction of ACN and CEO with hemoglobin and blood macromolecules and the reaction of CEO with tissue GSH. Metabolic parameters for ACN and CEO were estimated from iv pharmacokinetic studies. Rats were given bolus doses of 3.4, 47, 55, or 84 mg ACN/kg via the femoral vein and blood samples were collected at selected time points. ACN and CEO blood concentrations were determined by gas chromatography. The iv pharmacokinetics of CEO were also determined using 0.6 or 5.3 mg CEO/kg. ACN elimination from blood was described by saturable P450 epoxidation (Vmax of 6.5 mg/hr/kg and Km of 1.5 mg/liter) and first-order GSH conjugation (30 hr-1/kg). CEO elimination was described by first-order GSH conjugation (750 hr-1/kg). Calculation of hepatic clearance values shows first-pass hepatic extractions of 61 and 90% for ACN and CEO, respectively. The dosimetry description accurately simulated the dose-dependent urinary excretion of ACN metabolites derived from epoxidation to CEO and from direct GSH conjugation of ACN. The dose-dependent formation of hemoglobin adducts from ACN was also well simulated.

Pituitary down-regulation prior to in-vitro fertilization and embryo transfer: a comparison between a single dose of Zoladex depot and multiple daily doses of Suprefact.

In order to further evaluate the endocrinological, embryological and clinical efficacy of a single injection of the gonadotrophin-releasing hormone (GnRH) analogue Zoladex (goserelin), 142 women underwent pituitary down-regulation prior to in-vitro fertilization and embryo transfer: 71 with a single injection of Zoladex depot (group I) and 71 matched controls with multiple daily injections of Suprefact (buserelin; group II). Ovarian stimulation was performed with human menopausal gonadotrophin (HMG) and ovulation induction with human chorionic gonadotrophin (HCG). HMG and hydroxyprogesterone caproate depot were given for luteal phase support. The mean (+/- SD) age (34.01 +/- 4.42 versus 34.81 +/- 4.00 years), mean total dosage of HMG (61.25 +/- 26.87 versus 56.17 +/- 25.18 ampoules), mean daily dosage of HMG (4.74 versus 4.94 ampoules), duration of HMG stimulation (12.91 +/- 3.68 versus 11.31 +/- 3.46 days) and oestradiol concentration on the day of HCG (10,082 +/- 8007 versus 9440 +/- 7840 pmol/l) were similar in both groups but the mean total number of injections (GnRH and HMG) (13.55 +/- 3.35 versus 55.37 +/- 31.92) was significantly lower in group I. Furthermore, the proportion of women down-regulated by 2 weeks and pregnancy rate per embryo transfer were significantly higher in the Zoladex group, while miscarriage rates were similar. We conclude that a single dose of Zoladex is quicker, more convenient and should be investigated as an equally effective alternative to multiple doses of Suprefact for pituitary down-regulation prior to assisted conception. Further studies are required to test the teratogenicity and effectiveness of Zoladex.

Determination of tissue partition coefficients for volatile tissue-reactive chemicals: acrylonitrile and its metabolite 2-cyanoethylene oxide.

Partition coefficients (PCs) are chemical-specific parameters used in physiologically based pharmacokinetic models to describe chemical solubility in tissues. Tissue:air PCs for volatile chemicals can be estimated using vial equilibration techniques in which the chemical concentration in the vial headspace is measured by gas chromatography after equilibrium is reached between the chemical in tissue and air. However, equilibrium would not be expected with tissue-reactive chemicals such as acrylonitrile (ACN) or its epoxide metabolite 2-cyanoethylene oxide (CEO). Active uptake of ACN was observed in rat blood due to reaction with blood sulfhydryl groups, while CEO reacted with all tissues examined (rat blood, muscle, fat, liver, and brain). The active uptake processes were first order as evidenced by a linear decrease in the log of the vial headspace concentrations over time. Linear extrapolation of the log of the apparent PC to zero time, where the contribution of the active uptake process is zero, yielded an estimated PC of 487 for ACN in blood. Equilibrium was achieved with ACN after treatment of blood with diethyl maleate to modify blood sulfhydryl groups, with a PC of 512 +/- 29 (mean +/- SE, n = 14). These PC estimates were verified by direct measurement of ACN concentrations in both the air and the blood (stabilized by acidification). The directly measured ACN blood:air PC was 437 +/- 8 (n = 8), which compared well with the estimated values. Treatment of tissues with diethyl maleate or 2,4-dinitrofluorobenzene did not abolish the active uptake of CEO. However, pretreatment of tissues with CEO itself abolished subsequent CEO uptake. The CEO blood:air PC estimates obtained from zero time extrapolation of four CEO concentrations (1672 +/- 139) and from CEO pretreatment (1658 +/- 137, n = 8) were in good agreement. These data indicate that tissue:air PCs for volatile reactive chemicals can be estimated by extrapolation of a first-order uptake process to zero time or at equilibrium following chemical modification of reactive groups in tissues.