Preclinical toxicological assessment of levothyroxine and liothyronine Maillard impurities

BACKGROUND: Following the introduction of new stability-indicating related substances methods, an unknown impurity was observed in levothyroxine (LeMI) and liothyronine (LiMI) tablets (ADVANZ PHARMA) in concentrations ≥1.0%, from 6 months of storage onwards. The impurity was identified as a Maillard condensation product between lactose and LeMI/LiMI in the LeMI and LiMI tablets, respectively. MATERIALS AND METHODS: To establish the toxicity profile of LeMI and LiMI in humans and to define appropriate shelf-life specification limits, a comprehensive nonclinical toxicological assessment was performed, including in silico (Leadscope and Derek Nexus analyses), in vitro (Ames test), and in vivo tests (7-day dose range finding and 90-day dose repeat studies in rats). In silico analyses indicated that potential LeMI and LiMI structures should not be considered bacterial mutagens or in vitro/in vivo clastogens, and that at the low oral exposure levels expected, the impurities are unlikely to cause harm. RESULTS: In vitro testing showed that neither LeMI nor LiMI were cytotoxic or mutagenic at up to 5000 μg/plate, both in the presence and absence of metabolic activation. The 2 in vivo studies further confirmed that no systemic toxicity or other notable negative effects were evident at up to 200 μg/kg/day for LeMI and 45 μg/kg/day for LiMI, the highest doses tested. These doses represent 120–122 times the maximum daily exposures of LeMI and LiMI, based on body surface area (μg/m(2)). CONCLUSIONS: Based on these results, a proposal has been formulated to increase the limits of Maillard condensation products to ≤8.0% for LeMI and ≤6.0% for LiMI at shelf life.