P15: Myocardial necrosis related to hydrochlorothiazide-induced hypokalemia in cynomolgus monkeys

Myocardial necrosis in cynomolgus monkey is one of the most difficult findings to get interpreted in the toxicity study. Here, we examined the role of hypokalemia, one of the possible causes to induce myocardial necrosis, using a diuretic hydrochlorothiazide. Four female cynomolgus monkeys were orally administrated 50 mg/kg/day hydrochlorothiazide for 1 or 2 weeks. The animals were tested for electrocardiography (ECG), plasma electrolytes and urinalysis, then necropsied on the next day after the final dosing. Two animals showed marked hypokalemia (plasma K level exhibited 2.1–2.9 mEq/L after Day 3 (control animal remained between 3.6–4.4 mEq/L); average from Day 3 was less or equal than 2.6 mEq/L) and flattening of the T wave in the ECG, thus necropsied after 1 week of dosing. The hearts of these animals showed focal red areas in the ventricle macroscopically, which turned out to be hemorrhage with myocardial coagulative necrosis in the histopathology. Additionally, there were multifocal myocardial necroses with inflammatory cell infiltration in the subendocardium or subepicardium of both ventricles. Further investigations revealed that necrotized myocardium lost cross striation, displayed abnormal staining for hERG channel immunohistochemstry, and degeneration of the myofibrils and cellular organelles in the electron microscopy. Further 2 of the animals displayed slight hypokalemia (2.4 or 2.5 mEq/L at minimum; average from Day 3 was more than 2.9 mEq/L) and no ECG abnormalities, then necropsied after 2 weeks of dosing. In the histopathology, one of these animals showed a few small necrotic foci, however no changes were noted in another animal. In the present study, myocardial necroses were observed in the cynomolgus monkeys when plasma potassium level kept lower level, around 2.6 mEq/L, for several days.