N-vinylpyrrolidone dimer, a novel formulation excipient, causes hepatic and thyroid hypertrophy through the induction of hepatic microsomal enzymes in rats.

N-vinylpyrrolidone dimer (VPD) is a novel experimental formulation excipient intended for preclinical toxicology studies. In a previous 4-week toxicity study, VPD induced dose-dependent hepatocellular and thyroid gland hypertrophy in Sprague-Dawley (SD) rats. The objectives of the current investigation were to define the underlying molecular mechanisms of these changes. Two separate studies were conducted using male SD rats, daily doses of 300, 1000 or 3,000 mg/kg of VPD, and a positive control (phenobarbital at 75 mg/kg/day): (1) a 28-day study to monitor thyroid hormone levels after 7 and 28 days of dosing; (2) a 5-day study to evaluate hepatic and thyroid gland transcriptomic changes, as well as hepatic UGT activity levels. At VPD dosages of 300 mg/kg/day and higher, 2-fold increases of serum thyroid stimulating hormone (TSH) levels were observed in male SD rats after 28 days of dosing, while serum thyroxine (T4) and triiodothyronine (T3) levels were unchanged. Liver UGT enzyme activity levels were increased in VPD-treated rats after 5 days. In addition, in the 5-day study, VPD caused increased hepatic mRNA levels of a panel of drug metabolizing enzymes (DMEs) and transporters, including Cyp3a1, Cyp2b1, Ugt 2b1, and Abcc3. Similar patterns of induction were observed in primary rat hepatocytes exposed to VPD. Transcriptomic changes in the thyroid gland were identified for genes involved in thyroid hormone biosynthesis and in the FAK, PTEN, and Wnt/β-catenin signaling pathways. Collectively, these data indicate that VPD acts as an inducer of hepatic DMEs in SD rats and that this likely leads to enhanced peripheral metabolism of T3/T4, resulting in a feedback response characterized by increased serum TSH levels, and thyroid gland hypertrophy and hyperplasia.