Synthesis And Characterization Of Oleanolic Acid-PEG Conjugates

Oleanolic acid (OA; 3β-3-hydroxyolean-12-en-28-oic acid; molecular weight 456.7) is a triterpenoid compound that exists widely in medicinal herbs and natural plants in the form of free acid or aglycones for triterpenoid saponins. OA has many pharmaceutical functions, such as hepatoprotective effect, anti-inflammatory effect, anti-tumor activity, anti-hyperlipidemic effect, etc. Unfortunately, the bioavailability of OA is limited due to its poor solubility in water. Therefore it is important to find effective methods to improve the solubility and dissolution rate of OA.Polyethylene glycol (PEG) has been employed in many pharmaceutical applications. The technique of attaching PEG to drug, peptide, polymer or chemical moiety has been termed as PEGylation. The conjugation of insoluble drug and PEG can increase water solubility because of the exsistence of ethers which is known to be highly hydrophilic.In this study, OA-PEG conjugates of different molecular weights were synthesized by ring opening polymerization with OA as initiator, KOH andl8-crown-6 as catalyst and ethylene oxide (EO) as monomer in a stainless steel autoclave. The details are as follows:(1) The influencing factors of polymerization were investigated and the optimum reaction formula was identified. The conjugates:OA-PEG5 (OA:EO=1:5, m/m), OA-PEG10 (OA:EO=1:10, m/m) and OA-PEG15 (OA:EO=1:15, m/m) were synthesized under the optimum reaction formula.(2) The OA-PEG conjugates were characterized by UV, FTIR spectroscopy, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). The results of UV indicated that the maximum absorption peak of OA-PEG conjugates had no difference with that of OA. According to FTIR spectra analysis, it was the carboxyl group, not the hydroxyl group of OA that reacted with EO. The molecular weights of the synthesized OA-PEG conjugates as well as molecular weight distribution index were obtained by GPC. The results indicated that the molecular weights of conjugates increased with the increase of the mass ratio of EO and OA. The molecular weight distribution index was about 1.1.(3) The water solubility and partition coefficient of OA and OA-PEG conjugates were determined. The water solubility of OA-PEG conjugates increased incredibly compared with the parent drug OA. Meanwhile, the partition coefficient of OA-PEG conjugates was decreased. The solubility of OA-PEG15 was 3.65×104μg·mL-1, which was 7849 times of OA.(4) The dissolution profiles of OA-PEG conjugates and OA were determined. The results showed that dissolution rates of conjugates were obviously enhanced. The stability of OA-PEG conjugates was evaluated according to the appearance, content, or moisture absorption for OA. The results showed that the heat and light stability experiment suggested that the conjugates were stable. The OA-PEG conjugates absorb moisture in humid environment suggesting the conjugates should be packaged with barrier material.