Introduction: Macroscale drug delivery vehicles are applied to locally deliver various drugs. The challenges for these vehicles to optimally function are their injectability, tunability of mechanical properties and spatiotemporal control of drug release[1].
In our group, supramolecular hydrogels have been developed based on poly(ethylene glycol) PEG polymers functionalized on both ends with quadruple hydrogen bonding ureido-pyrimidinone (UPy) units (UPy-PEG). These UPy-PEG hydrogels have been demonstrated to be biocompatible after implantation in the kidney and to erode over time, presumably due to dissolution of the supramolecular polymer aggregates[2]. Uniquely, this system shows a pH-responsiveness that allows for catheter-injection of the hydrogelator as viscous solution at pH > 8.5 with a fast sol-gel transition upon contact with tissues at physiological pH. We showed the feasibility of this approach by injection of the UPy-PEG hydrogel in combination with growth factors in the infarcted region of a pig heart[3],[4]. Theoretically, any drug or bioactive compound can be dissolved in the hydrogelator solution before injection. More recently we expanded our research to incorporation of RNAi therapeutics and chemotherapeutic compounds (Fig 1). A convenience of the UPy system is that new features can be added by supramolecular incorporation of functionalities. One example is the incorporation of amines to induce electrostatic interactions with RNAi molecules to prevent free diffusion from the hydrogel and to obtain a sustained release. Furthermore, we propose enhanced transfection efficiencies because of the presence of charges.
Fig. 1| PEG polymers functionalized with UPy units form supramolecular injectable hydrogels that can serve as delivery depots after mixing with desired drugs or RNAi therapeutics.
Materials and Methods: UPy-hydrogelator powder is dissolved in PBS pH 11.7 by heating to 70 °C for 1 hour. After cooling to RT the mixture reaches a pH of 9.0. A drug or biomolecule is subsequently added and gelation is triggered by adding HCl to decrease the pH to 7.4. Release studies are performed in Transwell 8 µm inserts at 37 ˚C with PBS as release medium. Release of molecules and degradation of hydrogel is quantified via fluorescence spectroscopy and UV-Vis spectroscopy. Material properties of the viscous solution (shear viscosity) and of the hydrogel (storage and loss moduli) are recorded on an Anton-Paar Rheometer. Fluorescence recovery after photobleaching (FRAP) experiments were performed on a Leica confocal microscopy system.
Results: Drugs can be conveniently entrapped within the hydrogel by employing the pH switch method. Rheology measurements show that at pH 9.0 the solution has a shear viscosity below 1 Pa.s which allows for injection via a catheter. When brought to neutral pH, the material shows a solid-like response observed by a storage modulus G′ which is larger than the loss modulus G″. In vitro release studies show that the hydrogel slowly degrades and that RNAi therapeutics and chemotherapeutics are released from the hydrogel. With fluorescent model molecules a correlation was observed between FRAP recovery time and release from the hydrogel.
Conclusion: UPy supramolecular hydrogels hold promise for in vivo application as sustained drug delivery depot. Unlike many other materials, it shows catheter compatibility, which allows minimal invasive treatments. New features can be added by supramolecular incorporation of functionalities, for example to slow down drug release.
References:
[1] David J. Mooney, Cathal J. Kearney, Nature Materials, 2013.
[2] Patricia Y.W. Dankers et al, Biomaterials, 2012.
[3] Maartje M.C. Bastings et al, Advanced Healthcare Materials, 2013.
[4] A.C.H. Pape, Maarten H. Bakker et al, JoVe, 2015.