Investigating the Influence of Excipients on the Stability of Levothyroxine Sodium Pentahydrate

A range of tablet excipients were evaluated for their influence on the physical form and chemical stability of levothyroxine sodium pentahydrate (LSP; C₁₅H₁₀I₄NNaO₄·5H₂O). LSP–excipient binary powder blends were stored under two conditions: (a) in hermetically sealed containers at 40 °C and (b) at 40 °C/75% RH. By use of synchrotron X-ray diffractometry, the disappearance of LSP could be quantified and the appearance of crystalline levothyroxine (free acid) could be identified.

Under hermetically sealed conditions (40 °C) hygroscopic excipients such as povidone induced partial dehydration of LSP to form levothyroxine sodium monohydrate. When stored at 40 °C/75% RH, acidic excipients induced measurable disproportionation of LSP resulting in the formation of levothyroxine (free acid). HPLC analyses of drug–excipient mixtures revealed that lactose monohydrate, microcrystalline cellulose, and croscarmellose sodium caused pronounced chemical decomposition of LSP. On the other hand, magnesium stearate, sodium stearyl fumarate, and alkaline pH modifiers did not affect the physical and chemical stability of the API following storage at 40 °C/75% RH.

HPLC, being a solution based technique, revealed chemical decomposition of the API, but the technique was insensitive to physical transformations. Excipient properties such as hygroscopicity and microenvironmental acidity were identified to be critical determinants of both physical and chemical stability of LSP in a drug product. For drugs exhibiting both physical and chemical transformations, simultaneous solid-state and solution based analyses will enable comprehensive stability evaluation.

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Article information: Navpreet Kaur and Raj Suryanarayanan, Molecular Pharmaceutics 2021 18 (7), 2683-2693, DOI:10.1021/acs.molpharmaceut.1c00217

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