Methyl iodide-induced fetal hypothyroidism implicated in late-stage fetal death in rabbits.

Methyl iodide (MeI) induces fetotoxicity in New Zealand White (NZW) rabbits when maternal exposure occurs during a susceptible window late in gestation (gestation days [GD] 23-26). To identify the possible mode of action, comprehensive maternal and fetal bioanalysis and thyroid structure/function assessments were conducted in MeI-exposed (25 ppm by whole-body inhalation) and unexposed time-mated NZW rabbits (10/group) during GD 21-27. Key developmental events were observed within this window in unexposed fetuses, including the appearance of colloid in the thyroid follicular lumen and the detection of serum T(3) beginning on GD 22. Fetal T(4) and T(3) levels were diminished following maternal MeI exposure compared to baseline values. Fetal TSH was significantly increased following 4 days of maternal MeI exposure. MeI-induced changes in the fetal thyroid included reduced colloid formation, epithelial follicular hypertrophy, and epithelial cytoplasmic vacuolation. Time-course investigations using 20 ppm MeI revealed highly concentrated levels of iodide in fetal versus maternal serum. Direct maternal administration of sodium iodide by intravenous infusion during GD 23-26 induced similar effects on fetal thyroid structure and function as MeI, identifying iodide as the putative agent. Elevated S-methylcysteine adduct concentrations were noted in fetal hemoglobin, indicating that some unreacted MeI may be delivered directly to the fetus. However, the weight of evidence from these studies suggests that late-stage fetal death following maternal exposure to MeI during GD 23-26 is the result of preferential accumulation of iodide in the fetal compartment causing disruption of the fetal hypothalamic-pituitary-thyroid axis at a critical time in the development of the rabbit fetal thyroid.

Two-day inhalation toxicity study of methyl iodide in the rat.

The effects of inhaled methyl iodide (MeI) on clinical pathology parameters, glutathione (GSH) tissue levels, serum thyroid hormone and inorganic iodide concentrations, S-methylcysteine hemoglobin concentrations, and liver UDP-glucuronyltransferase activity were studied in the rat. Male rats were exposed by whole-body inhalation to 0, 25, or 100 ppm MeI, 6 h/day for up to 2 days. Serum cholesterol concentrations (both high-density lipoprotein [HDL] and low-density lipoprotein [LDL] fractions) were increased and triglycerides were decreased at both exposure levels. Serum thyroid-stimulating hormone (TSH) concentrations were increased at 25 and 100 ppm, and serum triiodothyronine (T(3)) and thyroxine (T(4)) concentrations were decreased at 100 ppm. There was no change in either reverse triiodothyronine (rT(3)) or UDP-glucuronyltransferase activity at either exposure level. A dose- and time-dependent reduction in GSH levels in blood, kidney, liver, and nasal tissue was observed, with the greatest reduction in nasal tissue (olfactory and respiratory epithelium). MeI exposure also resulted in a substantial dose- and time-dependent increase in both serum inorganic iodide and red blood cell S-methylcysteine hemoglobin adducts. These results indicate that following inhalation exposure, MeI is rapidly metabolized in blood and tissue of rats, resulting in methylation products and release of inorganic iodide.

Evaluation of respiratory parameters in rats and rabbits exposed to methyl iodide.

Laboratory animals exposed to methyl iodide (MeI) have previously demonstrated lesions of the olfactory epithelium that were associated with local metabolism in the nasal tissues. Interactions of MeI in the nasal passage may, therefore, alter systemic toxicokinetics. The current study used unrestrained plethysmographs to determine the MeI effect on the breathing frequency and minute volume (MV) in rats and rabbits. Groups of 4 rats each were exposed to 0, 25, or 100 ppm and groups of 4 rabbits each were exposed to 0 and 20 ppm MeI for 6 h. Breathing frequency and MV were measured and recorded during the exposure. Blood samples were collected for inorganic serum iodide and the globin adduct S-methylcysteine (SMC) as biomarkers of systemic kinetics immediately following exposure. No significant reductions in breathing frequency were observed for either rats or rabbits. Significant changes in minute volume were demonstrated by both rats and rabbits; however, the changes observed in rats were not concentration dependent. The MeI-induced changes in MV resulted in significant differences in the total volume of test substance atmosphere inhaled over the 6-h period. Rats demonstrated a concentration-dependent increase in both inorganic serum iodide and SMC. Rabbits exposed to 20 ppm MeI demonstrated a significant increase of inorganic serum iodide; SMC was also increased but was not statistically significant. The results of this study are consistent with previous kinetic studies with MeI, and the data presented here can be integrated into a computational fluid dynamics physiologically based pharmacokinetic model for both rats and rabbits.

In vitro glutathione conjugation of methyl iodide in rat, rabbit, and human blood and tissues.

Methyl iodide (MeI) is an intermediate in the manufacture of some pesticides and pharmaceuticals, and is under review for US registration as a non-ozone depleting alternative for methyl bromide for pre-plant soil fumigation. MeI is primarily metabolized via conjugation with glutathione (GSH), with further metabolism to S-methyl cysteine and methanethiol. To facilitate extrapolations of animal pharmacokinetic data to humans, rate constants for the GSH metabolism of MeI were determined in cytosols prepared from the liver and kidneys of rats, human donors, female rabbits, and rabbit fetuses, from rabbit olfactory and respiratory epithelium, and from rabbit and rat blood using a headspace vial equilibration technique and two-compartment mathematical model. MeI was metabolized in liver and kidney from adults of all three species, but metabolism was not detectable in fetal rabbit kidney. Maximal metabolic rates (V(max)) were similar in liver from rat and human donors (approximately 40 and 47 nmol/min/mg, respectively) whereas the V(max) rates in kidney cytosols varied approximately three-fold between the three species. No difference was observed in the loss of MeI from active and inactive whole blood from either rats or rabbits. The metabolism in olfactory and respiratory epithelial cytosol had Michaelis-Menten constant (K(m)) values that were several times higher than for any other tissue, suggesting essentially first-order metabolism in the nose. The metabolism of MeI in human liver cytosol prepared from five individual donors indicated two potential populations, one high affinity/low capacity and one with a lower affinity but higher capacity.

Development and validation of an assay for iodide in serum using ion chromatography with pulsed amperometric detection.

We developed and validated an ion chromatography method to assay iodide in serum sampled from rats and rabbits that had been exposed to iodomethane. Iodomethane is of interest because it is a volatile liquid pre-plant soil crop protection fumigant that has been proposed as a non-ozone-depleting alternative to methyl bromide. Serum was prepared from whole blood collected on wet ice at the time of sacrifice and kept frozen at less than -65 degrees C. For analysis, serum samples were thawed unassisted at ambient temperature. Proteins were separated from the serum samples by ultrafiltration. A 100-microl filtered serum sample was then injected into the ion chromatograph without additional sample preparation. Iodide was separated in <20 min by anion-exchange chromatography using a 25-mM nitric acid eluent. The analyte of interest was detected by pulsed amperometry using a silver working electrode. The method showed linear response over the concentration range of 100 to 5000 ng/ml iodide (r2>.998) with a lower limit of quantitation of 100 ng/ml iodide. The accuracy of the procedure, determined by spiked recovery measurements at 100 ng/ml iodide, was between 90 and 110%. A method detection limit of 20 ng/ml for iodide in serum samples was demonstrated using the method of standard additions.

Quantification of dichloroiodomethane and bromochloroiodomethane in human blood by solid-phase microextraction coupled with gas chromatography-high-resolution mass spectrometry.

Iodine-containing trihalomethanes (iodo-THMs) are formed as disinfection byproducts when iodide-containing water is disinfected using chloramination process. Subsequent water use may lead to human exposure to iodo-THMs. Because of health concerns surrounding exposure to iodo-THMs, a rapid, reliable, and high-throughput analytical method was developed to quantify trace levels of two iodo-THMs: dichloroiodomethane (IDCM) and bromochloroiodomethane (IBCM) in human blood. These analytes from the headspace above blood samples were extracted using solid-phase microextraction. Analytes were then desorbed and separated by capillary gas chromatography and analyzed by high-resolution mass spectrometry with multiple ion monitoring. This method utilizes stable isotope dilution to quantify parts-per-trillion levels of all analytes, with excellent precision of < 9% coefficient of variation. At three spiked levels, method accuracy of IDCM and IBCM ranged between 6 and 20% difference when comparing spiked and measured amounts. The method limit of detection was 2 ng/L for both IDCM and IBCM. This selective, sensitive, and rapid method will help to assess human exposure to iodo-THMs and to study potential associations between exposure and adverse health outcomes.

Interspecies pharmacokinetic scaling of some iodinated organic acids.

We have investigated the possibility of interspecies scaling of relationships between the structure and total plasma clearance in a group of nine organic acids (iododerivatives of benzoic, phenylacetic and hippuric acids) in rabbits, rats and mice. The intercompound comparison established the dependence of total plasma clearance predominantly on the molecular structure in all the animals under study, but the dependence on drug lipophilicity was also meaningful. For interspecies scaling of total plasma clearance, the use of a biological clock with an effective renal plasma flow as the unit seemed most suitable and is probably connected with the principal role of the kidney in the elimination of the compounds under study.

A comparative investigation of the metabolism of methyl bromide and methyl iodide in human erythrocytes.

Human erythrocyte cytoplasm was incubated in head space vials with either methyl bromide or methyl iodide. The decline in concentration of the two methyl halides was monitored by gas chromatography. Simultaneously, the production of S-methylglutathione was determined by thin layer chromatography. In parallel experiments, boiled erythrocyte cytoplasm was used in order to determine non-enzymatic conjugation. Furthermore, inhibition experiments with sulfobromophthalein were performed. The results were compared with previous findings on the metabolism of methyl chloride. In contrast to methyl chloride, both methyl bromide and methyl iodide showed a significant non-enzymatic conjugation with glutathione. In addition, an enzymatic conjugation could be observed in the erythrocyte cytoplasm of the majority of the population, whereas a minority lacks this enzymatic activity. This is consistent with findings on methyl chloride. Inhibition experiments show that a minor form of the erythrocyte glutathione transferase may be responsible for the enzymatic conjugation. Of the three monohalogenated methanes, methyl bromide is the substrate with the highest affinity for the conjugating enzyme(s). In the case of methyl iodide, non-enzymatic reaction overweighs the enzymatic process. There are possible implications of the results for occupational health and the toxicity of the substances.