An efficient mixed solid–liquid phase synthesis of a heterobifunctional amphiphilic PEG–NH2 derivative and its conjugation to folic acid

A very efficient, versatile as well as simple to perform procedure was developed in order to prepare a heterobifunctional amphiphilic PEG–NH2 derivative which can be used for conjugation to a targeting ligand (such as folic acid). This method proceeds by a mixed solid–liquid phase strategy using a TentaGel® PAP resin, a copolymer consisting of a polystyrene matrix on which a PEG (Mw 3400 Da) terminated by an amino function has been grafted. Solid phase chemistry was used for the conjugation of a highly hydrophobic moiety. After release from the resin, the amphiphilic PEG–OH conjugate was converted into its corresponding amphiphilic PEG–NH2 derivative (four steps in 77% overall yields). This procedure allowed the preparation of ∼330 mg batches. This derivative was then coupled to folic acid, a ligand that is used for the targeting of drug (gene) carrier and delivery systems to cells over-expressing the folate receptor. The low and high molecular weight of folic acid and its amphiphilic PEG–folate conjugate, respectively, allowed easy purification by dialysis and led to the targeted compound with high recovery.