Comparison of the toxicity of methylcyclopentadienyl manganese tricarbonyl with that of its two major metabolites

Manganese neurotoxicity: behavioral, pathological, and biochemical effects following various routes of exposure

The human central nervous system is an important target for manganese intoxication, which causes neurological symptoms similar to those of Parkinson's disease. With the increasing use of methylcyclopentadienyl manganese tricarbonyl (MMT) as an octane-improving additive to unleaded gasoline, the prospect of worldwide manganese exposure is once again attracting attention as increases in environmental manganese concentrations have been recorded relative to traffic density. One crucial question is whether a small increase of manganese contamination resulting from the widespread use of MMT could have neurotoxic effects. In this review we concentrate on central nervous system abnormalities and neurobehavioral disturbances. Most experimental animal studies on manganese neurotoxicity have been conducted in nonhuman primates and rodents. Most studies performed in rodents used oral manganese administration and did not assess bioaccumulation or central nervous system changes. The major effect found was transient modification of spontaneous motor activity. Very few inhalation toxicological studies were carried out. As manganese intoxication in humans usually occurs via inhalation, more studies are required using the respiratory route of administration. Given the proven neurotoxic effects of manganese and the prospect of worldwide MMT usage, this metal should be considered a new environmental pollutant having potentially widespread public health consequences.

Chronic oral administration of methylcyclopentadienyl manganese tricarbonyl altered brain biogenic amines in the mouse: comparison with inorganic manganese

This study was conducted to investigate the effects associated with high dose administration of organic manganese to mice and to compare these effects with those of inorganic manganese. The disposition and toxicity of methylcyclopentadienyl manganese tricarbonyl (MMT; a potential substitute for lead in gasoline) in the brains of ddY mice was studied after 12-months administration (at 0.5 g/kg of MMT) in food. Mice exposed to inorganic manganese received 2.0 g/kg of MnCl2 in food for the same period. There was no significant difference in food intake between the control mice and the MMT-exposed mice or MnCl2-exposed mice. Normetanephrine level in the cerebellum of the MMT-exposed group was significantly increased compared with the control, and correlated with the manganese concentration. The manganese concentration was significantly increased in the cerebellum of the MMT-exposed group compared with the control and MnCl2-exposed groups. On the whole, methoxylation from the 3-hydroxyl of catecholamine tended to be promoted by manganese.

Comparative pneumotoxicity of cyclopentadienyl manganese tricarbonyl and methylcyclopentadienyl manganese tricarbonyl

The acute pneumotoxic effects of cyclopentadienyl manganese tricarbonyl (CMT) and methylcyclopentadienyl manganese tricarbonyl (MMT) were compared to delineate the role of the methyl side chain in the toxicity of these organomanganese compounds and to further our understanding of the mechanisms by which these compounds act. Specifically, lung manganese (Mn) burdens and the pneumotoxic response, as measured by bronchoalveolar lavage parameters, were determined in male Sprague-Dawley rats 24 hr after sc administration of 0.5, 1.0, or 2.5 mg Mn/kg as CMT or MMT. The pneumotoxic response to either compound was characterized by large increases in lavage albumin and protein content with smaller increases in lactate dehydrogenase levels. CMT was approximately twice as potent as MMT. This difference in potency may be due to methyl side chain oxidation, a metabolic detoxification pathway unavailable to CMT. Lung Mn content was significantly elevated after treatment with either CMT or MMT. Heptane extraction studies revealed that Mn was accumulated in a nonlipid soluble form, suggesting the accumulation of metabolites rather than heptane soluble parent MMT or CMT. A strong correlation between pulmonary Mn content and toxicity was observed, suggesting a causal relationship between the accumulation of CMT or MMT metabolites and toxicity. Piperonyl butoxide diminished both the pneumotoxicity and Mn accumulation resulting from CMT or MMT, suggesting both phenomena are due to monooxygenase metabolites. Pulmonary nonprotein sulfhydryl (NPSH) levels were increased twofold 24 hr after administration of either CMT or MMT. Depletion of NPSH was not observed 1.5 or 6 hr after administration. The mechanisms of this response are unclear but may be due to the metabolism of CMT or MMT to unstable compounds which release inorganic Mn within pulmonary cells.