Ligation of anti-cancer drugs to self-assembling ultrashort peptides by click chemistry for localized therapy† Michael R. Reithofer, * Kiat-Hwa Chan, Anupama Lakshmanan, Dang Hoang Lam, Archana Mishra, Began Gopalan, Mangesh Joshi, Shu Wang and Charlotte A. E. Hauser * Self-assembling ultrashort peptides from aliphatic amino acids were functionalized with platinum anti- cancer drugs by click chemistry. Oxaliplatin-derived hybrid peptide hydrogels with up to 40% drug loading were tested for localized breast cancer therapy. Stably injected gels showed significant tumor growth inhibition in mice and a better tolerance compared to the free platinum drug. Introduction Self-assembly, the spontaneous organization of molecules into ordered structures by non-covalent interactions is the most fundamental process for building supramolecular structures 1,2 from DNA, proteins and other biomolecules in living systems. 3 We have been able to harness the innate self-assembling capacity of rationally designed ultrashort peptides which contain only aliphatic amino acids. 4–6 The self-assembly process of these ultrashort peptides has been addressed in detail and a hypothesis of the underlying mechanism has been discussed. 4 In this study, we combine the use of these peptides as vehicles for sustained, local delivery of anti-cancer therapeutics and as scaffolds for replacing lost tissue and regenerating damaged tissue. Platinum-based anticancer therapeutics are amongst the most widely used drugs in clinics today for the treatment of different types of cancers. So far, three platinum(II) compounds have been approved by the Food and Drug Administration, namely cisplatin, carboplatin and oxaliplatin. 7–10 These drugs are used against a number of solid tumors including prostate, breast, colorectal, non-small-cell lung, and genitourinary cancers. 8,11,12 The drugs are administered intravenously, with only a small amount of the given dosage reaches the target. 13 The majority of the drug is excreted and causes severe side effects ranging from nausea and ototoxicity to nephro- and neurotox- icity. 14 Reducing side effects and enhancing drug uptake and efficacy is currently one of the biggest challenges in medicinal chemistry, especially in the development of metal based anti- cancer therapeutics. 15–25 Our aim is to address this challenge by using self-assembling ultrashort peptides as a platform for localized and sustained release of anti-cancer drugs. Localized treatment, using in situ gelling delivery systems injected directly into the tumor site, is a feasible strategy to overcome systemic effects and poorly directed uptake. In addi- tion, direct localized injection of a drug can be seen as a non- invasive therapeutic strategy, reducing hospitalization time and cost, thereby providing more comfort to patients. If surgical removal of the tumor is required, such systems could be implanted as biomimetic extracellular matrix, supporting tissue regeneration, in combination with providing a localized thera- peutic effect. For a number of malignancies localized treatment is already routine. 26–28 Several polymer-based hydrogels are currently in development for localized drug delivery. 28–32 A major drawback of many synthetic polymeric hydrogels is the requirement of a crosslinking step that necessitates the use of potentially harmful agents such as organic solvents or chemical initiators. The residual presence of such agents decreases the biocompatibility of the hydrogels. On the other hand, UV-cros- slinked polymers are oen incompatible with many anticancer drugs (e.g. doxorubicin, daunorubicin, cyclophosphamide). Although hydrogels containing cisplatin directly coordinated to an amino acid have been reported, such systems oen use an amino acid which is co-polymerized, yielding a hybrid peptide– polymer system, where the drug is loaded aer the polymeriza- tion step. This results in poor solubility and does not offer the possibility of in situ gelation aer a local injection. 33–37 Natural biomolecules such as peptides, which self-assemble into injectable hydrogels, offer a promising platform to over- come the above limitations. We recently reported a unique class of ultrashort peptides, which are able to form hydrogels by facile self-assembly without additional physico-chemical support or UV-crosslinking. 4,5 Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore. E-mail: chauser@ibn.a-star.edu.sg; mreithofer@ibn.a-star.edu.sg; Fax: +65-6478-9080; Tel: +65-6824-7108 † Electronic supplementary information (ESI) available: Experimental materials and methods and characterization details of all compounds. See DOI: 10.1039/c3sc51930a Cite this: Chem. Sci. , 2014, 5, 625 Received 10th July 2013 Accepted 17th October 2013 DOI: 10.1039/c3sc51930a www.rsc.org/chemicalscience This journal is © The Royal Society of Chemistry 2014 Chem. Sci. , 2014, 5, 625–630 | 625 Chemical Science EDGE ARTICLE