A New Analytical Method for Quantifying Acid-End-Cap PLGA in Sub-Milligram Quantities

Characterization of PLGA polymers used in FDA-approved drug products is critical for quality control and qualitative/quantitative (Q1/Q2) evaluation of potential generic formulations. Various techniques have been developed and used to characterize the molecular properties of PLGA polymers, such as molecular weight, molecular composition, and molecular structure. Commonly used techniques include gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), semisolvent methods, and GPC-based intrinsic viscosity measurement. It is noted that the existing analytical methods may not be able to separate and quantify PLGA polymers when used as a mixture in a drug product (e.g., Durysta and Ozurdex). In particular, one assay method still lacking is quantitating the PLGA polymer with acid-end-cap (PLGA-A) in the mixture containing PLGA with ester-end-cap (PLGA-E), especially when the sample quantity is below the submilligram level. The total PLGA quantities available in Durysta and Ozurdex formulations are too small (<1 mg) to use existing assay methods to quantify the PLGA-A content. A new assay method was developed to quantitate PLGA-A in the mixture with PLGA-E. The acid end-cap was modified with pyrene methylamine (a UV dye) to enhance the signal and compared with the total PLGA quantity measured with the refractive index (RI) after a sample was run through a GPC. This GPC-UV/RI approach is based on measuring the total acid number (TAN) of PLGA-A and converting it to the PLGA-A quantity to compare with the total PLGA. Unlike conventional methods of measuring TAN, the GPC-UV/RI methods enables TAN measurements of submilligram PLGA quantities. Application of this method to Ozurdex-similar samples showed the expected acid:ester ratio of PLGAs. This new approach provides another powerful tool for characterizing PLGA polymers in FDA-approved drug products. This is especially significant considering that the PLGAs of commercial products are likely to have molecular properties different from those of the raw PLGAs before going through the manufacturing process.