Quantification of protein concentration by the Bradford method in the presence of pharmaceutical polymers

We investigated how the Bradford assay for measurements of protein released from a drug formulation may be affected by a concomitant release of a pharmaceutical polymer used to formulate the protein delivery device. The main result is that polymer-caused perturbations of the Coomassie dye absorbance at the Bradford monitoring wavelength (595 nm) can be identified and corrected by recording absorption spectra in the region of 350–850 mm. The pharmaceutical polymers Carbopol and chitosan illustrate two potential types of perturbations in the Bradford assay, whereas the third polymer, hydroxypropylmethylcellulose (HPMC), acts as a nonperturbing control. Carbopol increases the apparent absorbance at 595 nm because the polymer aggregates at the low pH of the Bradford protocol, causing a turbidity contribution that can be corrected quantitatively at 595 nm by measuring the sample absorbance at 850 nm outside the dye absorption band. Chitosan is a cationic polymer under Bradford conditions and interacts directly with the anionic Coomassie dye and perturbs its absorption spectrum, including 595 nm. In this case, the Bradford method remains useful if the polymer concentration is known but should be used with caution in release studies where the polymer concentration may vary and needs to be measured independently.