Cannabinoid receptor antagonist-induced striated muscle toxicity and ethylmalonic-adipic aciduria in beagle dogs.

Ibipinabant (IBI), a potent cannabinoid-1 receptor (CB1R) antagonist, previously in development for the treatment of obesity, causes skeletal and cardiac myopathy in beagle dogs. This toxicity was characterized by increases in muscle-derived enzyme activity in serum and microscopic striated muscle degeneration and accumulation of lipid droplets in myofibers. Additional changes in serum chemistry included decreases in glucose and increases in non-esterified fatty acids and cholesterol, and metabolic acidosis, consistent with disturbances in lipid and carbohydrate metabolism. No evidence of CB1R expression was detected in dog striated muscle as assessed by polymerase chain reaction, immunohistochemistry, Western blot analysis, and competitive radioligand binding. Investigative studies utilized metabonomic technology and demonstrated changes in several intermediates and metabolites of fatty acid metabolism including plasma acylcarnitines and urinary ethylmalonate, methylsuccinate, adipate, suberate, hexanoylglycine, sarcosine, dimethylglycine, isovalerylglycine, and 2-hydroxyglutarate. These results indicated that the toxic effect of IBI on striated muscle in beagle dogs is consistent with an inhibition of the mitochondrial flavin-containing enzymes including dimethyl glycine, sarcosine, isovaleryl-CoA, 2-hydroxyglutarate, and multiple acyl-CoA (short, medium, long, and very long chain) dehydrogenases. All of these enzymes converge at the level of electron transfer flavoprotein (ETF) and ETF oxidoreductase. Urinary ethylmalonate was shown to be a biomarker of IBI-induced striated muscle toxicity in dogs and could provide the ability to monitor potential IBI-induced toxic myopathy in humans. We propose that IBI-induced toxic myopathy in beagle dogs is not caused by direct antagonism of CB1R and could represent a model of ethylmalonic-adipic aciduria in humans.

What do we need to know prior to thinking about incorporating an epigenetic evaluation into safety assessments?

The International Life Sciences Institute, Health and Environmental Sciences Institute sponsored a workshop entitled "State of the Science: Evaluating Epigenetic Changes," hosted by the National Institute of Environmental Health Sciences, Research Triangle Park, NC, 28-30 October 2009. The goal was to evaluate and enhance the scientific knowledge base regarding epigenetics and its role in disease, including potential relationships between epigenetic changes and transgenerational effects. A distinguishing aspect of the workshop was the highly interactive discussion session on the final morning. Meeting participants formed breakout groups (with representation from academia, industry, and government in each group) and were tasked with integrating their previous knowledge of epigenetics with what was learned during the workshop. The participants addressed the issue of what needs to be known prior to thinking about incorporating an epigenetic evaluation into safety assessment. To this end, the breakout groups were asked to address the following questions: (1) What model systems might be employed to evaluate the ability of a chemical to produce an epigenetic change (affecting the F1 and/or F3 generation); (2) What end points/targets might be evaluated; (3) What techniques might be employed; and (4) Regulatory Perspective: When is it appropriate to incorporate "new" science, in this case epigenetics, into the regulatory process? What does one need to know, what are the pitfalls and how might these be overcome/avoided? The basis of this paper is a synopsis of these discussions. The workshop highlighted the fact that the field of epigenetics is evolving at a very rapid pace and indicated that a great deal needs to be learned prior to being able to rationally incorporate an epigenetic evaluation into safety assessment. The value of the workshop is that it called attention to key data/knowledge gaps that should serve to focus attention on the areas where research and new thinking are needed to better understand epigenetics and its relationship to safety assessment.