A Radiolabeling Method for Precise Quantification of Polymers

Radiolabeling a protein, molecule, or polymer can provide accurate and precise quantification in biochemistry, biomaterials, pharmacology, and drug delivery research. Herein, we describe a method to ¹²⁵I label two different polymers for precise quantification in different applications. The surfaces of model contact lenses were modified with phenylboronic acid to bind and release the natural polymer, hyaluronic acid (HA); HA uptake and release were quantified by radiolabeling. In the second example, the in vivo distribution of a mucoadhesive micelle composed of the block copolymer of poly­(lactide)-b-poly­(methacrylic acid-co-acrylamidophenylboronic acid) was investigated. The presence of phenyl boronic acid groups (PBA), which bind to mucosal surfaces, was proposed to improve the retention of the micelle. ¹²⁵I labeling of polymers was examined for quantification of microgram amounts of HA present on a contact lens or to evaluate the enhanced retention of PBA micelles on mucosal surfaces in vivo. The introduction of phenol groups onto the polymers allowed for the labeling. HA was modified with phenol groups through a coupling reaction of its carboxylic acid with hydroxybenzylamine. Phenol functional block copolymer micelles with and without PBA were synthesized by including N-(4-hydroxyphenethyl)­acrylamide during polymerization. The phenol groups of HA and the block copolymers were labeled with ¹²⁵I using a modified ICl labeling method. ¹²⁵I labeling enabled quantification of HA loading and release including the effect of varying amounts of PBA on the contact lens surfaces. Micelles made from ¹²⁵I-labeled block copolymers with and without PBA were administered intranasally to Brown Norway rats. The animals were sacrificed either immediately after or 4 h after their last nasal instillation, and the nasopharyngeal tissues were removed and quantified. Radioactivity measurements demonstrated that the presence of the PBA mucosal binding groups led to approximately four times higher retention. The HA and block copolymer ¹²⁵I labeling presented in this article demonstrates the utility of the method for quantification and tracking of microgram quantities of polymers in diverse applications.