Novel folated and non-folated pullulan bioconjugates for anticancer drug delivery

Abstract

Two new anticancer polymer therapeutics were designed for tumour cell targeting. The bioconjugates were synthesised by pullulan derivatisation with either doxorubicin or doxorubicin and folic acid. Pullulan was activated by periodate oxidation and functionalised by reductive conjugation with cysteamine and 1.9 kDa PEG(NH(2))(2). The cysteamine thiol groups were conjugated to doxorubicin through a pH-sensitive hydrazone spacer while the pending PEG-NH(2) functions of one derivatised pullulan batch were conjugated to folic acid to obtain one of the two polymer therapeutics. The reaction intermediates and the final products were characterised by mass spectrometry, UV-vis analysis and reverse phase and gel permeation chromatography. The folic acid-free derivative [(NH(2) PEG)-Pull-(Cyst-Dox)] contained 6.3% (w/w) doxorubicin while the folic acid-doxorubicin-coupled derivative [(FA-PEG)-Pull-(Cyst-Dox)] contained 6% (w/w) doxorubicin and 4.3% (w/w) folic acid. Photon correlation spectroscopy showed that (NH(2) PEG)-Pull-(Cyst-Dox) and (FA-PEG)-Pull-(Cyst-Dox) assembled into particles of about 150 and 100 nm diameter, respectively. The two bioconjugates displayed similar drug release profiles either at pH 7.4 buffer or in plasma, where less than 20% of doxorubicin was released within three days. At pH 5.5, both conjugates underwent complete drug release in about 40 h. In vitro studies carried out with KB tumour cells over-expressing folic acid receptor showed that both free doxorubicin and (FA-PEG)-Pull-(Cyst-Dox) were rapidly taken up by the cells, while the internalisation of the non-folated derivative was significantly slower. Cell viability studies did not show relevant difference between the two bioconjugates. After 72 h of incubation with folic acid receptor non-expressing MCF7 cells, the IC(50) values of doxorubicin, (NH(2)PEG)-Pull-(Cyst-Dox) and (FA-PEG)-Pull-(Cyst-Dox) were 0.3 μM, 1.2 μM and 3.1 μM, respectively. After incubation with KB cells over-expressing folic acid receptor, the IC(50) values were 0.4 μM, 1.8 μM and 1.1 μM, respectively. Pharmacokinetic studies showed that 4 h after intravenous administration of the conjugates to Balb/c mice about 40% of the administered drug equivalent dose was present in the bloodstream while in the case of unconjugated doxorubicin, 80% of the drug was cleared within 30 min. These findings suggest that the novel doxorubicin-pullulan bioconjugates possess suitable properties for passive tumour targeting. On the other hand, folic acid conjugation has been found to have limited effect on selective cell up-take.