STUDY OF TOXICITY OF L-THYROXIN IMMOBILIZED CHITOSAN AND THE NATURE OF DRUG-POLYMER INTERACTION
Authors: Samira Safaraliyeva Samira Safaraliyeva 
Published: January 2022

It is known that L-thyroxine is used as a hormone substitute after goiter surgery. L-thyroxine contains sodium salt of levothyroxine, a synthetic substitute for the hormone of thyroxine. Absorption of the drug from the gastrointestinal tract is 48-79%. Its maximum concentration in plasma is reached after about 6 hours after acceptance. Levothyroxine, which is delivered mainly in combination with plasma proteins, is used in the liver, brain and muscles. Monoiodination of approximately 80% of sodium levothyroxine in various tissues occurs with the formation of triiodothyronine and inactive products. The half-life of the drug is 6-7 days. Approximately 15% of the drug is excreted unchanged by the kidneys and bile and in combination with conjugates. Frequent use of the drug is accompanied by an increase in its side effects. In this regard, it is necessary to create new forms in the blood that regulate the long-term therapeutic concentration of the drug. Polymer carriers play a very important role at this stage. In order to overcome these shortcomings, in recent years, the synthesis of hydrogels based on natural and synthetic polymers and immobilization of drugs and delivery to the appropriate organs are among the modern methods. Thus, the loading of the active drug to the polymer carrier safes its therapeutic concentration in the blood.
For this purpose, a combination of L-thyroxine with a chitosan polymer was designed and the toxicity of the new drug form was extensively studied in mice in vivo. There are very few studies in the literature on the use of L-thyroxine in combination with polymer based hydrogels. The study also examined the pharmacological basis of the complex of the L-thyroxine with a biological polymer, the possibility of its use in the treatment of hypothyroidism and the toxicity of the obtained L-thyroxine polymer. The nature of the physical and chemical interactions between L-thyroxine and chitosan has been identified by infrared spectroscopy. Simples - chitosan and chitosan complex with L-thyroxine were recorded on a Nicolet 6700 (USA) spectroscopic instrument in the range of 4000-400 cm-1. The molecular electron spectra of the liquid solutions of the samples, which are a highly sensitive method, were compared on a UV-Vis device in the range of 180-600 nm. Chitosan is a linear cationite-type polyaminosaccharide obtained from the deacetylation of chitin. Chitin is separated from mollusks, crustaceans and insects, which are layers of some chitin. Chitosan is composed of β-(1,4)-2-amino-2-deoxy-D-glucosamine and β-(1,4)-N-acetyl D-glucosamine residues. L-thyroxine, a hormone substitute, is a chemically organic molecule that contains 4 -J groups along with -NH2, -COOH and -OH groups. Toxicity of the L-thyroxine polymer compound at an initial dose of 5 mg/kg was studied. Two hours after use, sedation, grouping and grooming were observed in animals. According to the results, the application of L-thyroxine polymer did not cause any sedation and was proved to be non-toxic. When the mixture was first injected into animals, they were poisoned. Therefore, gastric juice, not CH3COOH, was used as a solvent. During the experiment, a solution of L-thyroxine dissolved in 0.4 ml of gastric juice was injected into the abdomen of animals, and after 5-10 minutes they did not show any serious adverse reactions. In animals, sedation, grouping, and locomotion predominated.
In the spectrum of the mixture of chitosan and L-thyroxine, 1643 and 1584 cm− 1 adsorption bands were subjected to chemical shift and were observed in the region of 1635 and 1544 cm− 1, respectively. Observation of a wide adsorption band around 3500 cm-1 is associated with an increase in hydrogen bonds in the system. A high-intensity peak is observed at 1544 cm-1, which involves electrostatic interactions. 1397 cm-1 characterizes the peak chitosan-L-thyroxine complex with changes in the region, which shows the interaction between the drug and chitosan. The mechanism of this type of interaction and the molecular structure of the product has also been confirmed by UV-Vis electron spectroscopy. In the UV-Vis spectrum of chitosan, a characteristic peak is observed for the >C=O carbonyl groups of non-deacetylated fragments around 208 nm. As can be seen, an intense peak around 250 nm, characteristic of the chitosan macromolecule, is observed, resulting in a change in the electron density in the polymer chains. This leads to a change in the shape of the spectrum and thus, it causes the formation of a wide absorption band in the region of 200-300 nm. If we look at the UV-Vis spectrum of L-thyroxine, a characteristic low-intensity band is observed in the range of 208-265 nm, which belongs to the aromatic cycle or phenyl group. In addition, the fact that the absorption band of the C-J group at 260 nm, together with the absorption bands of the other groups - COOH and >C=O, enter the same area and form a broad spectrum, shows both immobilization and interaction. In the UV-Vis spectrum of chitosan-L-thyroxine also occurs the formation of a broad absorption band around 300 nm, which is not observed in chitosan. This indicates a strong interaction of functional groups between the drug and chitosan. Such chemical shifts in the UV-Vis spectrum with bathochromic and hypsochromic effects are due to the occurrence of electrostatic attraction forces as well as hydrogen bonds between drugs and polymer macromolecules.This manifests itself both in the change in the shape of the spectrum and in the formation of a second adsorption at 300 nm. The attraction of the drug to the polysaccharide macromolecule due to functional groups causes changes in the electron density of both the chitosan and the L-thyroxine molecule. In our opinion, the method we have used to increase the effectiveness of L-thyroxine, which is widely used in the treatment of hypothyroidism, will increase the possibility of its application to humans in the future to solve this problem.
Keywords: chitosan; levothyroxine; immobilized; interaction; toxicity; in vivo; mice.