i)Kinetics and Metabolism of MMT

i)Kinetics and Metabolism of MMT

i.1Absorption

i.2Distribution

A single subcutaneous injection of MMT (4 and 10 mg/kg bw) to male Sprague-Dawley rats resulted in a significant increase in lung Mn content. After 24 hours rats that had not received MMT had 0.7 g Mn per lung. By comparison rats that received 1 mg/kg bw MMT had 6.5 g Mn per lung, and those that received 2.5 mg/kg bw MMT had 20.1 g Mn per lung. An additional study was performed assessing Mn lung burden resulting from MMT administration in the presence of piperonyl butoxide, a cytochrome P450 monooxygenase inhibitor. A 1 hour pre-treatment with piperonyl butoxide (400 mg/kg bw) was found to protect against lung Mn accumulation resulting from a single subcutaneous injection of 4 mg/kg bw MMT (approximately 5 g Mn/lung compared with 10 g Mn/lung). Heptane extraction of lung homogenates from MMT-treated rats indicated less than 2% of the pulmonary Mn was extractable, suggesting the presence of metabolites as opposed to MMT. Furthermore the decrease in Mn lung content in the presence of piperonyl butoxide suggests the presence of cytochrome P450 dependent monoxygenase metabolites. Each treatment group contained between 4 and 9 rats (Clay and Morris 1989).

The level of Mn in the brain of CD-1 mice was significantly increased by subcutaneous injection of MMT. In an acute study mice received a single MMT injection and were sacrificed after 24 hours. Mice in the control group were found to have 0.61 g Mn/g brain. By comparison mice that received 11 mg Mn/kg bw MMT injection had 0.93 g Mn/g brain, and those that received 22 mg Mn/kg bw MMT injection had 1.35 g Mn/g brain. In a separate chronic study mice received 10 injections (given on alternate days) and were sacrificed 24 hours after the tenth injection. At the end of the study mice in the control group had 0.64 g Mn/g brain, while mice receiving 11 mg Mn/kg bw MMT had 1.37 g Mn/g brain, and those receiving 22 mg Mn/kg bw MMT had 3.33 g Mn/g brain. On a temporal basis the concentration of brain Mn reached a level approximately twice that of controls within 4-8 hours post treatment and remained elevated for the length of the study. The number of animals per treatment group was not stated (Gianutsos et al., 1985).

The disposition of MMT was assessed in male ddY mice after chronic oral administration of MMT in food (0.5 g Mn/kg food) for 12 months. Daily food intake per mouse was reported as 3.6 g for controls and 3.1 g in the MMT-treated group. At the end of the exposure period there was significantly more Mn in the liver, kidney, pancreas, sublingual gland, lung and muscle in the MMT-treated group than in the control group. The highest level of Mn in MMT-treated mice was observed in the kidney (approximately 12.5 g Mn/g ww), followed by the thyroid gland (12 g Mn/g ww), then the liver (10.5 g Mn/g ww), prostate (7.5 g Mn/g ww) and sublingual gland (7 g Mn/g ww). The level of Mn in blood was significantly elevated in MMT-treated mice when compared to controls. The blood Mn concentration was 1.12 ± 0.19 g/mL creatinine in the MMT group and 0.14 ± 0.05 g/mL creatinine for the control. The weight gain of MMT-treated mice was significantly reduced after 9 months of the treatment and remained depressed until the study was concluded. Each treatment group contained between 4 and 6 animals (Komura and Sakamoto 1992).

An increase in the concentration of Mn in the brain of mice was reported following either a 12 months treatment with MMT at 0.5 g Mn/kg in food or a single IP MMT injection (100-2000 mg/kg bw in propylene glycol or corn oil) (Komura and Sakamoto 1994; Fishman et al., 1987) (See Section 11.11).

The disposition of MMT was investigated in male Charles River rats following oral and intravenous (IV) administration of [⁵⁴Mn]MMT. One day following oral administration of 2.5 mg [⁵⁴Mn]MMT in 0.2 mL Wesson oil, the highest ⁵⁴Mn concentrations were observed in the liver, lungs, kidneys, urinary bladder, pancreas, and abdominal fat. Nine days following oral administration, the highest ⁵⁴Mn concentrations were observed in the kidneys, liver, pancreas, and lungs. Each treatment group contained 12 animals. The concentration of ⁵⁴Mn in individual tissues was not reported (Moore et al., 1974).

The concentration of tissue Mn was investigated in COBS rats following a single oral dose of MMT at 15-150 mg/kg bw in Wesson oil. A number of animals that received 45-150 mg/kg bw of MMT died within 6 days post treatment. The concentration of Mn in selected tissues (duodenum, kidney, liver, lung, heart and brain) of rats that died was significantly increased and generally in a dose dependant manner. At 14 days post treatment, with the exception of the lung, the levels of tissue Mn had fallen to approximately normal levels. Each treatment group contained 10 animals. No statistical analysis was performed (Hysell et al., 1974).

Repeated dermal contact of MMT (0.4-16 g/L) in gasoline solutions (with or without tetraethyl lead) in rabbits (5 days/week, for 14 weeks), did not result in increased organ Mn concentrations that could be directly attributed to MMT exposure. In contrast, there was an increase in lung Mn levels in rats dermally exposed to MMT for 5 days/week, for 6 months rats. The level of Mn in the brain, kidney and liver was normal. Each treatment group contained 3 rabbits and an unknown number of rats. The distribution of Mn was also examined following inhalation exposure in several species. When guinea pigs, rats, and cats were exposed to airborne MMT for 7 hours per day on each of 45 or 150 days, an increase in liver, kidney, and lung Mn levels were observed. The highest values were observed in the cat, then the rabbit, rat and guinea pig. The highest Mn levels were observed in the liver, followed by the kidney, lung, heart and urine. Results of 1 rabbit, post exposure, indicated rapidly decreasing levels of Mn in tissues, which were within normal range by day 21. Two dogs, 1-2 rabbits, and an unknown number of cats, rats and guinea pigs were used in each treatment group (Witherup et al Unknown date d).

The distribution of Mn in male golden hamsters and male outbred albino rats was investigated following exposure to MMT combustion products generated using a 1972 Chevrolet 350 CID (cubic inch displacement) engine dynamometer system. Emissions were derived by passing exhaust generated from the combustion of fuel consisting of indolene “clear” containing MMT at 0.25 g Mn/gallon through a muffler, followed by dilution (25:1) with clean conditioned air. The final diluted emissions were split in two with one half being irradiated prior to exposure to animals. Irradiated emissions typically contained 855 g/m³ particles (0.29 m) consisting of 117 mg/m³ Mn (13.7%). Nonirradiated emissions typically contained 635 g/m³ particles (0.26 m) consisting of 131 mg/m³ Mn (20.7%). Animals were fed a low Mn diet and exposed for 8 hours per day for 56 consecutive days. At the end of the exposure period, the concentration of Mn in the brain, liver and lung was significantly increased in the irradiated emission group when compared to controls. In the nonirradiated emission group, heart Mn levels were reduced while brain and liver Mn concentration were significantly increased. Kidney Mn levels were unaffected. Similar results were obtained with animals fed a regular (higher) Mn diet. No difference in Mn concentration was observed in the hamster in all tissues examined. The number of animals per treatment group was not stated (Moore et al., 1975b).

The distribution of Mn in various tissues has also been investigated in rats and monkeys following inhalation of the combustion products of MMT according to a procedure described by Rinehart, (1975) and Ulrich et al. (1979a). Briefly, the experimental procedure involves generating MMT combustion products by burning MMT vapours in a propane flame, which reportedly produces a solid product consisting of Mn oxide (Mn₃O₄) with an aerodynamic diameter of approximately 0.11(Ulrich et al., 1979a). The animals were exposed for 24 hours per day for nine months to 11.6-1152 g Mn/m³ as Mn oxide (Mn₃O₄) aerosol. After nine months the level of Mn in the kidney, lung, and blood were significantly increased when compared to controls. A significant increase in spleen Mn levels was also observed in the monkey. The level of Mn in the liver was unaffected in both species. Six months post exposure the level of Mn in the spleen of rats was slightly elevated at low dose and reduced in the blood at high dose in rats. The Mn content in all other tissues examined in both species was normal. Each treatment group contained 15 male and 15 female rats and 4 male and 4 female monkeys (Rinehart, 1975; Ulrich et al., 1979b,c).