Effect of pH Variation on Cross-Linking of Water-Soluble and Acid- Soluble Chitosan with Sodium Tripolyphosphate and Gallium-67

Background: Chitosan is a cationic biopolymer obtained from deacetylating chitin, a natural compound present in crustacean shell, fungi and exoskeleton of insects. Chitosan involves various applications, including as drug and gene delivery systems, as wound dressing material and scaffolds for tissue engineering, agriculture, textile, food and feed nanotechnology, and in wastewater treatments. Chitosan-TPP particle has been figured out as the most important and stable nanoparticle for chitosan application in various fields. Objective: In this study, chitosan was chemically modified by sodium tripolyphosphate (TPP). Afterward, TPP-chitosan was radiolabeled with the gallium-67 radionuclide. The effect of several factors on labeling yield, such as chitosan solubility, acidity and concentration of TPPchitosan solution, and incubation time with gallium-67, were investigated. Methods: To prepare [⁶⁷Ga] gallium-chitosan complex, chitosan (0.5 ml) was dissolved in 2.2 mCi of [⁶⁷Ga] gallium chloride solution. The obtained solution was stirred for 5 min and then kept for 30 min at room temperature. The radiochemical purity and radiolabeling yield were measured via radiochromatography, which was performed by using a radio thin-layer chromatography (TLC) scanner instrument. To investigate the effect of chitosan kind and concentration on the labeling yield, two kinds of chitosan (acid-soluble chitosan and water-soluble chitosan) at two different concentrations (1% and 0.5%) and different pH were used. In addition, labeling efficiency and stability of the ⁶⁷Ga-TPP-chitosan complex (acidic/water soluble chitosan) at both concentrations (0.5 and 1%) and at room temperature were assessed for 30, 45 and 60 min. Results: The incubation time did not have any significant effect on labeling yield. The acidic soluble chitosan exhibited the highest radiolabeling yield at pH=9.3-10.4, while water-soluble chitosan showed the highest radiolabeling yield at pH > 5. Also, the prepared complex was stable in the final solution at room temperature and could even be used 24 hours after preparation for further application. Conclusion: Taken together, the TPP-modified water-soluble chitosan at the concentration of 0.5 % depicted the highest radiochemical yield (>95 %) in the optimized condition (pH= 6.2– 7.6). Therefore, TPP modified water-soluble chitosan can prove to be an effective carrier for therapeutic radionuclides in tumor treatment.