UV photodegradation of murine growth hormone: chemical analysis and immunogenicity consequences

During manufacturing, therapeutic proteins may be exposed to ultraviolet (UV) radiation. Such exposure is of concern because UV radiation may cause photooxidative damage to proteins, which in turn could lead to physical changes such as aggregation and enhanced immunogenicity. We exposed murine growth hormone (mGH) to controlled doses of UV radiation, and examined the resulting chemical, physical and immunogenic changes in the protein. mGH chemical structure was analyzed by mass spectrometry after UV irradiation. Photooxidation products detected by mass spectrometry included methionine sulfoxide formed at Met[127] and Met[149] residues, and, tentatively assigned by MS/MS analysis, ether cross-links between original Ser[78] and Cys[188], and Cys[206] and Ser[213], and a thioether cross-link between Cys[17] and Cys[78] residues, transformation of Cys[189] into Ala, and various hydrolytic fragments. Physical damage to UV-irradiated mGH was monitored by infrared spectrometry, chromatographic analyses, and particle counting by micro-flow imaging. UV radiation caused mGH to aggregate, forming insoluble microparticles containing mGH with non-native secondary structure. When administered subcutaneously to Balb/c or Nude Balb/c mice, UV-irradiated mGH provoked antibodies that cross-reacted with unmodified mGH in a fashion consistent with a T-cell dependent immune response. In wildtype Balb/c mice, titers for anti-mGH IgG1 antibodies increased with increasing UV radiation doses.