Pharmacokinetics and Toxicology of the Neuroprotective e,e,e-Methanofullerene(60)-63-tris Malonic Acid [C3] in Mice and Primates.

BACKGROUND AND OBJECTIVES: Fullerene-based compounds are a novel class of molecules being developed for a variety of biomedical applications, with nearly 1000 publications in this area in the last 4 years alone. One such compound, the e,e,e-methanofullerene(60)-63-tris malonic acid (designated C3), is a potent catalytic superoxide dismutase mimetic which has shown neuroprotective efficacy in a number of animal models of neurologic disease, including Parkinsonian Macaca fascicularis monkeys. The aim of this study was to characterize its toxicity and pharmacokinetics in mice and monkeys. METHODS: To assess pharmacokinetics in mice, we synthesized and administered 14C-C3 to mice using various routes of delivery, including orally. To assess potential toxicity in primates, serial blood studies and electrocardiograms (ECGs) were obtained from monkeys treated with C3 (3 or 7 mg/kg/day) for 2  months. RESULTS AND CONCLUSIONS: The plasma half-life of C3 was 8.2 ± 0.2 h, and there was wide tissue distribution, including uptake into brain. The compound was cleared by both hepatic and renal excretion. C3 was quite stable, with minimal metabolism of the compound even after 7 days of treatment. The LD50 in mice was 80 mg/kg for a single intraperitoneal injection, and was > 30 mg/kg/day for sustained administration; therapeutic doses are 1-5 mg/kg/day. For primates, no evidence of renal, hepatic, electrolyte, or hematologic abnormalities were noted, and serial ECGs demonstrated no alteration in cardiac electrical activity. Thus, doses of C3 that have therapeutic efficacy appear to be well tolerated after 2 years (mice) or 2 months (non-human primates) of treatment.

SOD activity of carboxyfullerenes predicts their neuroprotective efficacy: a structure-activity study.

Superoxide radical anion is a biologically important oxidant that has been linked to tissue injury and inflammation in several diseases. Here we carried out a structure-activity study on six different carboxyfullerene superoxide dismutase (SOD) mimetics with distinct electronic and biophysical characteristics. Neurotoxicity via N-methyl-D-aspartate receptors, which involves intracellular superoxide, was used as a model to evaluate structure-activity relationships between reactivity toward superoxide and neuronal rescue by these drugs. A significant correlation between neuroprotection by carboxyfullerenes and their ki toward superoxide radical was observed. Computer-assisted molecular modeling demonstrated that the reactivity toward superoxide is sensitive to changes in dipole moment, which are dictated not only by the number of carboxyl groups but also by their distribution on the fullerene ball. These results indicate that the SOD activity of these cell-permeable compounds predicts neuroprotection, and establishes a structure-activity relationship to aid in future studies on the biology of superoxide across disciplines.

A biologically effective fullerene (C60) derivative with superoxide dismutase mimetic properties.

Superoxide, a potentially toxic by-product of cellular metabolism, may contribute to tissue injury in many types of human disease. Here we show that a tris-malonic acid derivative of the fullerene C60 molecule (C3) is capable of removing the biologically important superoxide radical with a rate constant (k(C3)) of 2 x 10(6) mol(-1) s(-1), approximately 100-fold slower than the superoxide dismutases (SOD), a family of enzymes responsible for endogenous dismutation of superoxide. This rate constant is within the range of values reported for several manganese-containing SOD mimetic compounds. The reaction between C3 and superoxide was not via stoichiometric "scavenging," as expected, but through catalytic dismutation of superoxide, indicated by lack of structural modifications to C3, regeneration of oxygen, production of hydrogen peroxide, and absence of EPR-active (paramagnetic) products, all consistent with a catalytic mechanism. A model is proposed in which electron-deficient regions on the C60 sphere work in concert with malonyl groups attached to C3 to electrostatically guide and stabilize superoxide, promoting dismutation. We also found that C3 treatment of Sod2(-/-) mice, which lack expression of mitochondrial manganese superoxide dismutase (MnSOD), increased their life span by 300%. These data, coupled with evidence that C3 localizes to mitochondria, suggest that C3 functionally replaces MnSOD, acting as a biologically effective SOD mimetic.