Pathology and Neurotoxicity in Dogs after Repeat Dose Exposure to a Serotonin 5-HT1B Inhibitor

Abstract

AZD3783, a cationic amphiphilic drug and a potent inhibitor of the 5-hydroxytryptamine (5-HT1B) receptor, was explored as a potential treatment for depression. To support clinical trials, repeat dose toxicity studies in rats and dogs were conducted. Here we report toxicity findings in dogs after dosing from 1 to 3 months. In the 1-month study, there were minimal neuronal vacuolation in the brain, a marked increase in liver enzymes accompanied by hepatocellular degeneration/necrosis and phospholipidosis (PLD), and PLD/cholecystitis in the gallbladder of animals dosed at 47 mg/kg/day. In the 3-month study, neurotoxicity resulted in euthanasia of one animal dosed at 30 mg/kg/day after 86 days. Extensive pathologic changes were seen in all animals in retina epithelium (inclusion bodies), brain (neuronal vacuolation, degeneration, or necrosis and nerve fiber degeneration), spinal ganglia (vacuolation, degeneration, or necrosis), as well as sciatic and optic nerves (degeneration). Pigment-laden macrophages were observed in the lung, kidney, liver, gallbladder, bone marrow, gastrointestinal tract, and lymphoid tissues. Also seen were vitrel and retinal hemorrhage in the eyes. A brain concentration and pathology study showed that the concentration of AZD3783 in the brain was approximately 4 times higher than in the plasma after 4 weeks of dosing, however, they were similar in all regions examined, and did not correlate with areas with pathologic findings. Our findings with AZD3783 in dogs have not been reported previously with other CNS compounds that effect through serotonergic pharmacology.

Keywords: dog; pathology; neurotoxicity; preclinical safety assessment; phospholipidosis; cationic amphiphilic drugs (CADs).

Fig. 1.

Treatment with AZD3783 for 1 month at 47 mg/kg/day caused marked loss of hepatocytes with fibrosis (thick arrow), bile duct proliferation (star), and pigments in Kupffer cells (thin arrow) in male dog (M20). H&E staining, 200×.

Fig. 2.

Treatment with AZD3783 for 1 month at 47 mg/kg/day caused phospholipidosis in the liver. Shown here are lamellar bodies in the hepatocyte as seen by electron microscopy. Bar = 2 µm.

Fig. 3.

Treatment with AZD3783 for 3 months at 15 or 30 mg/kg/day caused degenerative and inflammatory lesions in the central and peripheral nervous tissues. (A) Ganglion cell vacuolation and mononuclear cell infiltration in the DRG; (B) Necrosis and inflammation in the cochlear nuclei; (C) Neuronal degeneration/necrosis and inflammation in the CA2 region of the hippocampus; (D) Perivascular mononuclear cell infiltration in the retina. H&E staining, 200×. In the insert, the arrow points to eosinophilic inclusion body.

Fig. 4.

Compound-induced lesions in the brain of dogs treated with AZD3783 are irreversible. (A) Hippocampus CA4 region of vehicle-treated control dog; (B) Extensive neuronal loss and prominent astrogliosis in the hippocampus CA4 region of a dog treated with AZD3783 at 30 mg/kg/day for 3 months and then being dose-free for 5 weeks. H&E staining, 200×.