Evaluation of the toxicity of ISIS 2302, a phosphorothioate oligonucleotide, in a 4-week study in CD-1 mice

The subchronic toxicity of ISIS 2302 and ISIS 3082, phosphorothioate oligonucleotides with antisense activity against human and murine ICAM-1 mRNA, respectively, was investigated in CD-1 mice. ISIS 2302 is currently in clinical trials as an anti-inflammatory agent. Because of the differences in mRNA sequence targets between humans and mice, ISIS 2302 has no pharmacologic activity in mice. ISIS 3082 was specifically designed to inhibit murine ICAM-1 and was included in this study to evaluate the effects of prolonged ICAM-1 inhibition. The oligonucleotides were administered by bolus i.v. injection (via tail vein) every other day for 27 days (14 doses) at dose levels of 0, 0.8, 4, 20, and 100 mg/kg per injection ISIS 2302 or 20 mg/kg per injection ISIS 3082. The basic group size consisted of 10 male and 10 female mice, which were sacrificed 2 days after the last dose and an additional 5 mice per sex in vehicle control and 100 mg/kg ISIS 2302 dose groups, which remained on study for a 28-day treatment-free period. No treatment-related deaths occurred during this study, and there were no effects of either oligonucleotide on body weight gain or food consumption. The most common changes observed in this study included a mixed mononuclear cell infiltrate seen in a number of organs or tissues, splenomegaly, and lymphoid hyperplasia at dose levels of > or = 20 mg/kg ISIS 2302. In the group that received the highest dose level of ISIS 2302 (100 mg/kg), there were alterations in serum chemistry parameters that appeared to be related to perturbations in the liver, including 3- to 4-fold increases in aspartate and alanine aminotransferase and smaller changes in bilirubin, alkaline phosphatase, cholesterol, triglycerides, and albumin levels. Treatment-related effects on hematologic parameters were limited to the 100 mg/kg ISIS 2302 dose group and included slight monocytosis and thrombocytopenia. None of the effects observed appeared to be life threatening. Complete or partial reversal of all effects was evident in the remaining high-dose ISIS 2302 animals at the end of the 4-week recovery period. Comparison of the effects produced by the same dose level (20 mg/kg) of ISIS 2302 and ISIS 3082 did not reveal any differences that could be attributed to exaggerated pharmacology. In conclusion, treatment-related alterations were observed primarily at the 100 mg/kg dose level, including immune stimulation and hepatic alterations, which were partially reversed following a 4-week treatment-free period.