Isoform separation and structural identification of mono-PEGylated recombinant human growth hormone (PEG-rhGH) with pH gradient chromatography

Human growth hormone plays an essential role in the treatment of dwarfism diseases, but it is limited in its short circulating half-life. Nowadays, some manufacturers are trying to take advantage of polyethylene glycol (PEG) conjugated with recombinant human growth hormone (rhGH) to improve its half-life and efficacy. However, the modified products are heterogeneous mixtures composed of reaction products with different modification sites. It is generally known as a challenging task to separate and characterize a PEGylated product, especially for its positional isoforms. In this study, cation exchange high performance liquid chromatograph (IEC-HPLC) based on a pH gradient separation method was presented to separate five position isomers of rhGH conjugated with a 40-kDa branched PEG N-hydroxysuccinimidyl (NHS) functional group. Then Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALD-TOF MS) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) indicated that each of five materials collected by IEC-HPLC was conjugated with only one branched PEG chain. Furthermore, rhGH and PEG-rhGH were digested by trypsin and peptides were collected by reversed phase high performance liquid chromatography (RP-HPLC). Following MALDI-TOF MS, PEG modification sites were determined through comparative analysis of peptide mapping between PEG-rhGH and rhGH. Finally, biological activities of those positional isomers were performed in vivo and very small variations were observed. This method was shown to be suitable for heterogeneity analysis of PEGylated biopharmaceutical products.