Characterization of xenobiotic-induced hepatocellular enzyme induction in rats: anticipated thyroid effects and unique pituitary gland findings.

During routine safety evaluation of RO2910, a non-nucleoside reverse transcriptase inhibitor for HIV infection, histopathology findings concurrent with robust hepatocellular induction occurred in multiple organs, including a unique, albeit related, finding in the pituitary gland. For fourteen days, male and female rats were administered, by oral gavage vehicle, 100, 300, or 1000 mg/kg/day of RO2910. Treated groups had elevated serum thyroid-stimulating hormone and decreased total thyroxine, and hypertrophy in the liver, thyroid gland, and pituitary pars distalis. These were considered consequences of hepatocellular induction and often were dose dependent and more pronounced in males than in females. Hepatocellular centrilobular hypertrophy corresponded with increased expression of cytochrome P450s 2B1/2, 3A1, and 3A2 and UGT 2B1. Bilateral thyroid follicular cell hypertrophy occurred concurrent to increased mitotic activity and sometimes colloid depletion, which were attributed to changes in thyroid hormone levels. Males had hypertrophy of thyroid-stimulating hormone-producing cells (thyrotrophs) in the pituitary pars distalis. All findings were consistent with the well-established adaptive physiologic response of rodents to xenobiotic-induced hepatocellular microsomal enzyme induction. Although the effects on the pituitary gland following hepatic enzyme induction-mediated hypothyroidism have not been reported previously, other models of stress and thyroid depletion leading to pituitary stimulation support such a shared pathogenesis.

Spontaneous cardiomyopathy in cynomolgus monkeys (Macaca fascicularis).

A previously undescribed spontaneous cardiomyopathy was identified by routine light microscopic examination of the heart from four clinically healthy purpose-bred cynomolgus monkeys that ranged from four to nine years of age and included 2 males and 2 females. Special stains of Sirius red, Masson's trichrome, and Mallory's phosphotungstic acid hematoxylin (PTAH); and immunohistochemistry using anti-CD68, troponin-I, and desmin antibodies were used to facilitate lesion characterization and assess cardiomyocyte viability. Microscopically, the apical to mid-ventricular myocardium to subendocardium had foci of cardiomyocyte disarray with cytoplasmic pallor to stippling and karyomegaly, vacuolization of the perimyseal connective tissue, a meshwork of fibrous tissue that concentrated around medium-sized blood vessels and dissected between or less often replaced affected cardiomyocytes; and a minimal, predominantly macrophage infiltrate. The disrupted cardiomyocytes were immunoreactive to desmin and troponin-I antibodies and had a normal cross-striation pattern by PTAH, indicating the chronic cardiomyopathy was not associated with active cardiomyocyte damage. The consistent distribution and morphology of the cardiomyopathy suggested a common etiology and pathogenesis. The features were reminiscent of chronic catecholamine-induced experimental cardiomyopathy and stress cardiomyopathy in monkeys and humans, respectively. This report documents another spontaneous heart lesion in clinically healthy monkeys for consideration during interpretation of toxicology studies.

Susceptibility of rats to corneal lesions after injectable anesthesia.

Corneal injury is not a commonly reported side effect after injectable or inhalation anesthesia in rats, but a number of surgery studies at our facility resulted in a high incidence of these injuries. To explore the potential association of various anesthetic protocols with the development of corneal lesions in rats, we retrospectively evaluated clinical records and sections of eyes from 215 male and 187 female Wistar rats used in eight intravenous infusion toxicology studies. None of the studied compounds was associated with eye toxicity. For placement of jugular vein vascular access ports, rats were anesthetized with enflurane, isoflurane, ketamine-xylazine, or Hypnorm-midazolam. Histologically, corneal changes were scored from 0 to 4 in light of degree of mineralization, leukocytic infiltrates, neovascularization, fibrosis, and ulceration. Prestudy (postsurgical) ophthalmic examination findings of corneal opacities were correlated with mineralization of the anterior limiting membrane and corneal ulceration. Corneal lesions were more severe in animals anesthetized with ketamine-xylazine, and minimal changes occurred after anesthesia with either enflurane or isoflurane. The results of further analysis suggest that corneal lesions can be observed within 24 h after injectable anesthetic administration and are not reversible. The severity of corneal changes was reduced when ketamine-xylazine anesthesia was reversed with yohimbine. Compared with Sprague-Dawley and Lewis rats, Wistar, Long-Evans, and Fischer 344 rats had increased incidence and severity of corneal lesions after anesthesia with ketamine-xylazine, suggesting that these three strains are at increased risk for developing postanesthetic corneal lesions with this regimen.

Differentiating spontaneous from drug-induced vascular injury in the dog.

When vascular injury is observed in dogs used in preclinical toxicology studies, careful evaluation of the lesions is warranted, especially when differentiating drug-induced vascular changes from spontaneous findings, such as idiopathic canine polyarteritis. The clinical signs as well as the nature and distribution of lesions can often be distinguishing, as is the case with vasoactive drugs, including vasodilators and/or positive inotropes (hydralazine, minoxidil, endothelin receptor antagonists, and phosphodiesterase III inhibitors). For most types of vasodilator-induced vascular injury, the lesion is often restricted to coronary arteries, whereas in idiopathic canine polyarteritis, arterial lesions not only involve coronary arteries, but also medium to small arteries of other organs. In addition, the nature of the changes in vessels yields important clues. Medial and adventitial hemorrhage is generally associated with vasodilator-induced arterial lesion, whereas hemorrhage is generally absent in idiopathic polyarteritis. Although idiopathic canine polyarteritis can generally be differentiated from vasoactive-induced vascular injury in dogs, there are increasing incidences of this type of polyarteritis in dogs receiving any 1 of a number of unrelated classes of compounds, suggestive of an exacerbation of the spontaneous disease. Therefore, in order to differentiate drug-induced injury from idiopathic canine polyarteritis, it is critical that examination of the vascular pathology be conducted with good understanding of clinical, pharmacological, and mechanistic data associated with the drug.