Selective precipitation of interleukin-4 using hydrophobic ion pairing: a method for improved analysis of proteins formulated with large excesses of human serum albumin.

In order to ensure the stability of protein pharmaceuticals, human serum albumin (HSA) is often added as an excipient, frequently in large excess. This makes chromatographic analysis of the stability of the active protein difficult. In the case of interleukin-4 (IL-4), separation from HSA can be achieved to some degree by size exclusion chromatography, but some HSA co-elutes with the IL-4. Hydrophobic ion pairing provides a method for selective precipitation of IL-4 from HSA. Hydrophobic ion pairing involves the electrostatic interaction of ionic detergents with oppositely charged polypeptides. Even when HSA is present in fifty-fold excess (w/w), the resulting precipitate contains greater than 70% of the IL-4. Selective precipitation with SDS produces enhancements in IL-4 over HSA of more than 2000-fold. This approach permits subsequent facile analysis of IL-4 by conventional reverse phase HPLC.