Solubility and permeability enhancement of levothyroxine sodium for oral administration
Levothyroxine (14), an endogenous hormone secreted by the thyroid gland is subject to complex biologic regulation. On conversion to active metabolite, triiodothyronine, 14 influences diverse metabolic processes including growth and maturation, cell respiration, total energy expenditure and the turnover of essentially all substrates. T4 is a narrow therapeutic index drug such that doses only about 20-25% outside the normal therapeutic window can put patients at risk of severe side effects of hyper or hypothyroidism. ^ Levothyroxine is a known example of classic bioequivalence problem between various available products. Dissolution of a drug is a crucial step in its oral absorption and bioavailability. A discriminative dissolution methodolgy with a sensitive inductively coupled plasma mass spectrometry (ICP-MS) analytical assay was used to study the dissolution behavior of available commercial products. The results showed that dissolution of such products is highly variable making it a rate-limiting step and a possible contributing factor to the bioequivalence concern. ^ Furthermore, the oral bioavailability of T4 is known to be incomplete and quite variable ranging from 50-80%. Two most important factors that can affect the oral absorption of a drug are solubility and permeability across the gastrointestinal tract. Among the many pharmaceutical interventions available to increase drug solubility, solid dispersions and solid solutions have found enormous attention. Vitamin E-TPGS, a water soluble amphiphilic polymer was used to enhance the solubility of 14 via solid dispersion technique. The formulations were subjected to solid state characterization in order to fully understand the mechanisms of solubility enhancement. ^ Levothyroxine is known to be a low permeability drug. Various straight chain fatty acids including capric acid (C10), lauric acid (C12) and oleic acid (C18) were employed to increase the permeability of T4 across Madin Darby Canine Kidney (MDCK) monolayer cell model. ^ The use of solubility and permeability enhancing excipients in a formulation for oral administration of T4 can lead to a more consistent and predictable bioavailability. The studies presented in this dissertation provide invaluable data to further use TPGS, C10 and/or C12 for oral administration of T4 and develop a more effective and stable formulation. ^
Health Sciences, Pharmacy
"Solubility and permeability enhancement of levothyroxine sodium for oral administration"
Dissertations and Master's Theses (Campus Access).