Glycosaminoglycans (GAGs) are key elements of the pericellular space, where they are involved in key signalling pathways determining cell fate but also progression of some malignant tumours.1,2 The main transmitters of GAGs bioactivity are their cell surface receptors, such as CD44 for hyaluronan (HA) and tyrosine-protein phosphatase S, PTPσ, for chondroitin sulfate (CS).3,4 While overexpression of CD44 is a hallmark of various malignant tumours, the interaction of CS with PTPσ is associated with inhibition of the axons growth and reduced neural regeneration.
In this study, we modified HA and CS with a short (C9) alkane thiol. The functionalization was performed in an end-on fashion using oxime click chemistry at the reducing end of the GAG thus, preserving its bioactivity. The obtained novel amphiphilic GAGs (HAC9 and CSC9) self-assemble into micelles at physiological conditions. We explored the redox-sensitivity (due to the presence of the thiol group) and the encapsulation ability in both hydrophobic and hydrophilic compartments of the generated micelles.
The synthesis of the aminooxy-alkylthiol was preformed following previously described procedures5. The following oxime coupling was carried out in 1/1 mixtures of ethanol or DMSO/phospohate buffer (pH 4.5) right after in situ deprotection of the aminooxy group from the n hidroxyphatalimide precursor. The obtained amphiphilic GAGs were purified by dyalisis against ethanol and characterized by 1H NMR (modification degree and purity). Critical micellar concentration (CMC) was determined by fluorescence spectroscopy using Nile Red (Table 1). Size and charge of the micelles was determined through dynamic and electrophoretic light scattering. Redox-sensitivity was evaluated by monitoring the particle size upon addition of redox agent 1,4-Dithiothreitol (DTT) and confirmed by Nile Red fluorescence quenching (Figure 1). The ability to encapsulate both hydrophobic and hydrophilic compounds was demonstrated by determining the encapsulation efficacy of Nile Red and Dextran-FITC respectively.
RESULTS AND DISCUSSION
Both HAC9 (91% modified) and CSC9 (63% modified) formed aggregates with ~150 nm and 200 nm of diameter, respectively, and a negative superficial charge -30 mV. However, the CMC for the HA was higher (Table 1). Importantly, the CMC did not change significantly upon increasing the temperature to the physiological one. Upon the addition of DTT, we observe a micelle disaggregation of HAC9 as a result of the break of thiol groups between polymers (Fig. 1). This effect was not observed when CSC9 was used instead. Finally, we were able to entrap either hydrophilic (shell) or hydrophobic (core) compound in the HAC9 micelles and release them upon stimulus (redox environments) relevant for tumour therapeutics.
The obtained HAC9 micelles are prominent as tumour therapeutics as they (i) can target cells overexpressing the CD44 via the HA presented on the surface; (ii) response to the tumour relevant redox stimulus; and (iii) allow entrapment of either hydrophilic or hydrophobic drugs.
1. Fuster M. M. and Esko J. D., Nature Rev Cancer. 5:526-542, 2005; 2. Bulow H. E. and Hobert O, Annu Rev Cell Dev Biol. 22:375-407, 2006; 3. Shen Y. J. et al., Science. 326:592-596, 2009; 4. Isacke, C. M. and Yarwood, H., Int J Biochem Cell Biol. 34:718–721, 2002; 5. Chan E. et al., Langmuir 18:311–313, 2001