Self-assembly of small molecules affords multifunctional supramolecular hydrogels for topically treating simulated uranium wounds{ Zhimou Yang, a Keming Xu, b Ling Wang, a Hongwei Gu, a Heng Wei, c Mingjie Zhang c and Bing Xu* ab Received (in Cambridge, UK) 26th May 2005, Accepted 5th July 2005 First published as an Advance Article on the web 3rd August 2005 DOI: 10.1039/b507314f Two types of therapeutic agents, which have discrete yet complementary functions, self-assemble into nanofibers in water to formulate a new supramolecular hydrogel as a self- delivery biomaterial to reduce the toxicity of uranyl oxide at the wound sites. This communication reports the design and application of a supramolecular hydrogel, whose self-assembled networks of nanofibers consist of antiinflammatory molecules and a uranyl ion chelating ligand, as a biomaterial for topical treatment of simulated uranium wounds. Because of their biocompatibility, biodegradability, and resemblance to the extracellular matrix, hydrogels have attracted intensive research attention in recent years, particularly for tissue engineering and drug delivery. Following the successful applications of polymer-based hydrogels in biomedical engineering 1 and the successful studies on low molecular weight organogels, 2 supramolecular hydrogels, formed by self-assembly of small molecules, 3,4 have recently emerged as a new type of biomaterials that promise important biomedical applications (e.g., hydrogels based on the self-assembly of oligopeptides have been used as scaffolds to grow neurons 5,6 ). Inspired by the works reported by Stupp et al. 6,7 and Zhang et al. 5,8 on oligopeptide-based hydrogels, we have been developing hydrogels as potential biomaterials directly formed by self- assembly of pharmaceutical candidates or agents that are small molecules. 4 To further explore the in vivo activity of supramolecular hydrogels, we designed a multifunctional hydrogel that employs three small molecules as its structural components—two amino acid derivatives that can reduce inflammation 9 and a bispho- sphonate that coordinates with UO 2 2+ and lowers the toxicity of UO 2 2+ . These molecules self-assemble into networks of nanofibers that form the matrices of the hydrogel. We administered the hydrogel topically on wound sites that had been contaminated with (non-radioactive) uranyl nitrate on the skin of mice. After being treated with the hydrogel, the mice recovered to normal, while the control group of mice (whose wounds were contaminated and untreated) weighed 35% less or expired. Our results indicate that these small molecules maintain their therapeutic effects even when they serve as the structural components of the supramole- cular hydrogel. In addition to validating the approach of the direct use of drug molecules to form a hydrogel as a new type of biomaterial, this work, for the first time, demonstrates the in vivo activity of supramolecular hydrogels based on small molecules (i.e., molecular weight ,10 3 g mol 21 ). Scheme 1 shows the structures of the three small molecules. N-(Fluorenyl-9-methoxycarbonyl)-L-leucine (1) and N e -(fluorenyl- 9-methoxycarbonyl)-L-lysine (2) belong to a novel class of antiinflammatory agents reported by Burch et al., and 1 displays effective antiinflammatory activity in animal models. 9 Neither 1 nor 2 acts as a hydrogelator in a neutral aqueous solution. Pamidronate was chosen as the third component because, similarly to CO 3 22 , 4 it could form hydrogen bond networks with 1 and 2. The addition of pamidronate (3) to the suspension of 1 and 2 leads to formation of a hydrogel at pH 5 9–10.4 after a heating–cooling cycle, in which 3 probably acts as both a donor and an acceptor of hydrogen bonds to promote hydrogelation since changing the pH value alone (by NaOH and pH .9.0) only results in the solubilization of 1 and 2. Fig. 1A shows the optical images of the hydrogels containing 1 equiv. of 1, 1 equiv. of 2, and 1, 2, or 4 equiv. of 3, respectively. Heating the mixtures of 1, 2, and 3 to 70 uC and cooling them back to room temperature leads to hydrogelation in three minutes. When 1 or 2 equiv. of 3 are used, the formed hydrogel appears opaque, indicating that it contains insoluble microparticles due to the limited solubility of 1 and 2, which is confirmed by the electron micrographs.{ When 4 equiv. of 3 are used, the resulting hydrogel is transparent, indicating that most of 1 and 2 are utilized to form the matrices of the hydrogel. Fig. 1B displays the linear viscoelastic frequency sweep response of the three hydrogels. All of them exhibit a very weak frequency dependence from 0.1 to 100 rad s 21 , with G9 dominating G0, suggesting that all samples are highly elastic. The dynamic storage modulus of gel III is two orders of magnitude larger than that of gels I and II, suggesting that the a Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Hong Kong, China. E-mail: chbingxu@ust.hk; Fax: +852 2358 1594; Tel: +852 2358 7351 b Bioengineering Program, The Hong Kong University of Science & Technology, Clear Water Bay, Hong Kong, China c Department of Biochemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Hong Kong, China { Electronic Supplementary Information (ESI) available: The CD, 2D- NOSEY spectra, emission spectra, and the details of the in vivo test (3 pages). See http://dx.doi.org/10.1039/b507314f Scheme 1 Molecular structures of the components of the nanofibers as the matrices of the hydrogel. COMMUNICATION www.rsc.org/chemcomm | ChemComm 4414 | Chem. Commun., 2005, 4414–4416 This journal is ß The Royal Society of Chemistry 2005 Downloaded on 09 April 2011 Published on 03 August 2005 on http://pubs.rsc.org | doi:10.1039/B507314F View Online