Reversible pegylated drugs are provided by derivatization of free functionalgroups of the drug selected from amino, hydroxyl, mercapto, phosphate and/orcarboxyl with groups sensitive to mild basic conditions such as 9-fluorenylmethoxycarbonyl (Fmoc) or 2-sulfo-9-fluorenylmethoxycarbonyl (FMS),to which group a PEG moiety is attached. In these pegylated drugs, the PEGmoiety and the drug residue are not linked directly to each other, but ratherboth residues are linked to different positions of the scaffold Fmoc or FMSstructure that is highly sensitive to bases and is removable underphysiological conditions. The drugs are preferably drugs containing an aminogroup, most preferably peptides and proteins of low or medium molecular weight.
