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 (M_w 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.