Heterogeneity of serum albumin samples with respect to solid-state aggregation via thiol-disulfide interchange - Implications for sustained release from polymers

Serum albumin is a well-studied protein that has been used as a model for controlled release of proteins from polymeric matrices. Therefore, it is important to understand its stability under conditions approximating those within polymer matrices in vivo. Mimicking these conditions (protein lyophilized from pH 7.3 and incubated at 37 degrees C and 96% relative humidity), we have studied the solid-state aggregation of various bovine and human serum albumin preparations. Our findings demonstrate that the rate of aggregation (occurring via intermolecular thiol-disulfide interchange) varies significantly among commercially available protein samples. This is a reflection of the heterogeneity among these samples with respect to thiol content (free Cys(34)), presence of fatty acids bound to the albumin molecule, and dimers and higher order aggregates present in the albumin preparation prior to stability testing in the solid state. In order to ascertain the significance of these observations, we have compared the release of S-alkylated (thiol-blocked) albumin with that of the unmodified protein from the poly(fatty acid dimer:sebacic acid) matrix. Substantially less of the unmodified protein was released, consistent with aggregation occurring within the polymer. These findings illustrate that protein stability is an important consideration in evaluating the performance of protein:polymer depots.