Claudia Bello

Granulocyte colony stimulating factor (G‐CSF) is a cytokine used to treat neutropenia. Different glycosylated and non‐glycosylated variants of G‐CSF for therapeutic application are currently generated by recombinant expression. Here, we describe our approaches to establish a first semisynthesis strategy to access the aglycone and O ‐glycoforms of G‐CSF, thereby enabling the preparation of selectively and homogeneously post‐translationally modified variants of this important cytokine. Eventually, we succeeded by combining selenocysteine ligation of a recombinantly produced N‐terminal segment with a synthetic C‐terminal part, transiently equipped with a side‐chain‐linked, photocleavable PEG moiety, at low concentration. The transient PEGylation enabled quantitative enzymatic elongation of the carbohydrate at Thr133. Overall, we were able to significantly reduce the problems related to the low solubility and the tendency to aggregate of the two protein segments, which allowed the preparation of four G‐CSF variants that were successfully folded and demonstrated biological activity in cell proliferation assays.