Background-These studies examined the early time course of plaque development and destabilization in the brachiocephalic artery of the apolipoprotein E-knockout mouse, the effects of pravastatin thereon, and the effects of pravastatin on established unstable plaques. Methods and Results-Male apolipoprotein E-knockout mice were fed a high-fat, cholesterol-enriched diet from the age of 8 weeks. Animals were euthanized at 1-week intervals between 4 and 9 weeks of fat feeding. Acutely ruptured plaques were observed in the brachiocephalic arteries of 3% of animals up to and including 7 weeks of fat feeding but in 62% of animals after 8 weeks, which suggests that there is a sharp increase in the number of plaque ruptures at 8 weeks. These acute plaque ruptures then appear to heal and form buried fibrous caps; after 9 weeks of fat feeding, mice had 1.05Ϯ0.15 buried fibrous caps at a single site in the brachiocephalic artery. Pravastatin (40 mg/kg of body weight per day for 9 weeks; resultant plasma concentration 16Ϯ4 nmol/L) had no effect on plasma cholesterol concentration in fat-fed apolipoprotein E-knockout mice but reduced the number of buried fibrous caps by 43% (PϽ0.0001). In longer-term experiments, the delay of pravastatin treatment until unstable plaques had developed reduced the incidence of acute plaque rupture by 36% (PϽ0.0001). Conclusions-Plaque rupture occurs at high frequency in the brachiocephalic arteries of male apolipoprotein E-knockout mice after 8 weeks of fat feeding. Pravastatin treatment inhibits early plaque rupture and is also effective when begun after unstable plaques have developed.

Abstract-The brachiocephalic arteries of fat-fed apolipoprotein E knockout mice develop plaques that frequently rupture and form luminal thromboses. The morphological characteristics of plaques without evidence of instability or with healed previous ruptures (intact) and vessels with acutely ruptured plaques (ruptured) have now been defined, to understand the process of plaque destabilization in more detail. Ninety-eight apolipoprotein E knockout mice were fed a diet supplemented with 21% lard and 0.15% cholesterol, for 5 to 59 weeks. Of these 98 mice, 51 had an acutely ruptured plaque in the brachiocephalic artery. See cover and page 713The apolipoprotein E (apoE) knockout mouse has become established as a model of hypercholesterolemia and atherosclerotic lesion development. 2-5 After 6 months on a high-fat diet, advanced fibrous plaques are observed with thin luminal elastin and collagen fibers capping small lipid-rich globular lesions. 5 Four studies have now reported that the lesions of apoE knockout mice can spontaneously become unstable. 6 -9 The brachiocephalic (also known as the innominate) artery, which is the first branch from the aortic arch, bifurcating to form the right common carotid artery and the right subclavian artery, is a site of predilection for the development of such vulnerable lesions. It provides a small and well-defined area in which to study the progressive loss of plaque stability and, eventually, rupture.In a previous study, 7 a small number of apoE knockout mice were examined. All of these had died suddenly, which meant that it was not possible to perfusion fix their arteries. In the current study, the development and morphology of brachiocephalic artery plaques have been characterized in a cohort of 98 apoE knockout mice. Ruptured and intact lesions have been compared both in animals dying suddenly and in those that were perfusion fixed after scheduled termination. Methods AnimalsHomozygous apoE knockout mice 10 were obtained from Charles River Laboratories (Manston, Kent, UK). The strain background of these animals was 50% C57BL/6, 50% 129SvJ, as determined by fingerprinting of tail-tip DNA. The housing and care of the animals and all the procedures used in these studies were performed in accordance with the guidelines and regulations of the University of Bristol and the United Kingdom Home Office. HusbandryStarting at 6 to 8 weeks of age, 98 apoE knockout mice (51 male) were fed a high-fat rodent diet containing 21% fat from lard and supplemented with 0.15% (wt/wt) cholesterol (Special Diets Services) for a period of 5 to 59 weeks. Cages were checked daily, and any animals that had suffered sudden death were immediately removed and processed for histological examination. TerminationAnimals were anesthetized by intraperitoneal injection of sodium pentobarbitone before exsanguination by arterial perfusion via the

Throughout the history of atherosclerosis research we have sought animal models of the disease process that exhibit high frequencies of the features that make human plaque a clinical risk: plaque rupture, mural thrombosis, and intra-plaque hemorrhage. This type of model is needed to determine the mechanisms by which plaques rupture and to design and test therapeutic interventions for stabilizing plaques. Studies of domestic and exotic animals have shown that most species will spontaneously develop fatty streaks and in some cases atheromatous lesions with sufficient time, but that rupture and thrombosis is exceedingly rare. Even with addition of fat and cholesterol to the diet, lesion development is accelerated but does not increase the frequency with which plaques rupture in most animal models. However, recently we have observed high frequencies of intra-plaque hemorrhage in the innominate/brachiocephalic arteries of older, chow-fed, hyperlipidemic, apolipoprotein E-deficient mice, and high frequencies of plaque rupture with mural thrombus in younger apolipoprotein E-deficient mice fed a high-fat diet. This suggests that plaque rupture and secondary thrombosis are frequent and reproducible occurrences at specific sites in apolipoprotein E-deficient mice, and that the timing and pathobiology of the ruptures are influenced by lipid status in this murine model.