- Self-assembled peptide hydrogels comprise an important class of biomaterials that find application in tissue engineering and drug delivery. We have developed injectable in situ-forming hydrogel formulations based on self-assembled KLD motifs to control the delivery of a conventional cytotoxic drug (doxorubicin [DOX]) or a Smac-derived pro-apoptotic peptide (SDPP) for cancer therapy. The core KLD peptide ([KLDL]3) was separated into two β-sheet peptides by a three or four glycine-containing spacer (designated as 3G or 4G, respectively) or by a spacer containing four glycines and a phenylalanine (termed 4GF) to fine tune and influence the bulk properties. We found that the KLD-based hydrogel can effectively load DOX and SDPP within the hydrogel network. Addition of a three or four glycine-containing spacer arm decreased the time for gel formation, while inclusion of a single aromatic amino acid (F) into 4G sequence (4GF) significantly increased the rate of gel formation and improved its mechanical strength when compared to 3G- and 4G-containing gels. KLD16- and 4GF-based hydrogels presenting higher peptide concentrations can self-assemble very rapidly (in a few seconds) and were very effective at controlling the release of DOX and SDPP and inhibiting tumor cell growth in vitro. Pre-complexation of SDPP with polyanionic molecule (polyglutamic acid) further sustained in vitro drug release from 4GF-based formulations. This study is the first to test interactions between KLD-based hydrogel matrices and DOX or SDPP for local drug delivery applications. Copyright © 2014 John Wiley & Sons, Ltd.