Peptide-amphiphiles are amphiphilic structures with a hydrophilic peptide headgroup that incorporates a bioactive sequence and has the potential to form distinct structures, and a hydrophobic tail that serves to align the headgroup, drive self-assembly, and induce secondary and tertiary conformations. In this paper we review the different self-assembled structures of peptide-amphiphiles that range from micelles and nanofibers, to patterned membranes. We also describe several examples where peptide-amphiphiles have found applications as soft bioactive materials for model studies of bioadhesion and characterization of different cellular phenomena, as well as scaffolds for tissue engineering, regenerative medicine, and targeted drug delivery.

Integrin α5β1 is expressed on several types of cancer cells, including colon cancer, and plays an important role in tumor growth and metastasis. The ability to target the integrin α5β1 using an appropriate drug delivery nano-vector can significantly help in inhibiting tumor growth, reducing tumor metastasis, and decreasing deleterious side effects associated with different cancer therapies. Liposomes are nano-sized phospholipid bilayer vesicles that have been extensively studied as drug delivery carriers. The goal of this study is to design stealth liposomes (liposomes covered with polyethylene glycol (PEG)) that will target colon cancer cells that express the integrin α5β1. The PEG provides a steric barrier allowing the liposomes to circulate in the blood and the functionalizing moiety, PR_b peptide, will specifically recognize and bind to α5β1 expressing cells. PR_b is a novel peptide sequence that mimics the cell adhesion domain of fibronectin, and includes four building blocks, RGDSP (the primary recognition site for α5β1), PHSRN (the synergy site for α5β1), a (SG)5 linker, and a KSS spacer. In this study we have demonstrated that by varying the amount of PEG (PEG750 or PEG2000) and PR_b on the liposomal interface we can engineer nano-vectors that bind to CT26.WT, HCT116, and RKOcolon cancer cells in a specific manner and are internalized through most likely α5β1-mediated endocytosis. GRGDSP-targeted stealth liposomes bind to colon cancer cells and internalize, but they have much lesser efficiency than PR_b-targeted stealth liposomes, and more importantly they are not as specific since many integrins bind to RGD peptides. PR_b-targeted stealth liposomes are as cytotoxic as free 5-Fluorouracil (5-FU) and exert the highest cytotoxicity on CT26.WT cells compared to GRGDSP-targeted stealth liposomes and non-targeted stealth liposomes. Thus, the proposed targeted delivery system has the great potential to deliver a therapeutic load directly to colon cancer cells, in an efficient and specific manner.

In recent years, there has been considerable effort in designing improved delivery systems by including site-directed surface ligands to further enhance their selective targeting. The goal of this study is to engineer alpha5beta1-targeted stealth liposomes (nanoparticles covered with poly(ethylene glycol) (PEG)) that will bind to alpha5beta1-expressing LNCaP human prostate cancer cells and efficiently release the encapsulated load intracellularly. For this purpose, liposomes (with and without PEG2000) were functionalized with a fibronectin-mimetic peptide (PR_b) and delivered to LNCaPs. The amount of PEG2000 and other liposomal components were characterized by 1H NMR, and the amount of peptide by the bicinchoninic acid protein assay. Fibronectin is the natural ligand for alpha5beta1, and a promising design for a fibronectinmimetic peptide includes both the primary binding site (RGD) and the synergy site (PHSRN) connected by a linker and extended off a surface by a spacer. We have previously designed a peptide-amphiphile, PRb, that employed a hydrophobic tail, connected to the N-terminus of a peptide headgroup composed of a spacer, the synergy site sequence, a linker mimicking both the distance and hydrophobicity/hydrophilicity present in the native protein fibronectin (thus presenting an overall "neutral" linker), and finally the primary binding sequence. We have examined different liposomal formulations, functionalized only with PR_b or with PR_b and PEG2000. For PR_b-targeted PEGylated liposomes, efficient cell binding was observed for peptide concentrations of 2 mol % and higher. When compared to GRGDSP-targeted stealth liposomes, PR_b functionalization was superior to that of GRGDSP as shown by increased LNCaP binding, internalization efficiency, as well as cytotoxicity after incubation of LNCaPs with tumor necrosis factor-alpha (TNFalpha)-encapsulated liposomes. More importantly, PR_b is alpha5beta1-specific, whereas many integrins bind to small RGD peptides. Thus, the proposed PR_b-targeted delivery system has the potential to deliver a therapeutic payload to prostate cancer cells in an efficient and specific manner.

pH-sensitive liposomes undergo rapid destabilization under mildly acidic conditions such as those found in endocytotic vesicles. Though this makes them promising drug carriers, their application is limited due to their rapid clearance from circulation by the reticulo-endothelial system. Researchers have therefore used pH-sensitive liposomes that are sterically stabilized by polyethylene glycol (PEG) molecules (stealth liposomes) on the liposome surface. The goal of this study is to bring bio-functionality to pH-sensitive PEGylated liposomes in order to facilitate their potential use as a targeted drug delivery agent. To improve the selectivity of these nanoparticles, we included a targeting moiety, PR_b which specifically recognizes and binds to integrin α(5)β(1) expressing cells. PR_b (KSSPHSRN(SG)(5)RGDSP) is a novel fibronectin-mimetic peptide sequence that mimics the cell adhesion domain of fibronectin. Integrin α(5)β(1) is expressed on several types of cancer cells, including colon cancer, and plays an important role in tumor growth and metastasis. We have thoroughly studied the release of calcein from pH-sensitive PEGylated liposomes by varying the lipid composition of the liposomes in the absence and presence of the targeting peptide, PR_b, and accounting for the first time for the effect of both pH and time (photo-bleaching effect) on the fluorescence signal of calcein. We have demonstrated that we can design PR_b-targeted pH-sensitive PEGylated liposomes, which can undergo destabilization under mildly acidic conditions and have shown that incorporating the PR_b peptide does not significantly affect the pH-sensitivity of the liposomes. PR_b-targeted pH-sensitive PEGylated liposomes bind to CT26.WT colon carcinoma cells that express integrin α(5)β(1), undergo cellular internalization, and release their load intracellularly in a short period of time as compared to other formulations. Our studies demonstrate that PR_b-functionalized pH-sensitive targeted delivery systems have the potential to deliver a payload directly to cancer cells in an efficient and specific manner.

Gene silencing by RNA interference (RNAi) has emerged as a powerful treatment strategy across a potentially broad range of diseases. Tailoring siRNAs to silence genes vital for cancer cell growth and function could be an effective treatment, but there are several challenges which must be overcome to enable their use as a therapeutic modality, among which efficient and selective delivery to cancer cells remains paramount. Attempts to use antibodies for siRNA delivery have been reported but these strategies use either nonspecific conjugation resulting in mixtures, or site-specific methods that require multiple steps, introduction of mutations, or use of enzymes. Here, we report a method to generate antibody–siRNA (1:2) conjugates (ARCs) that are structurally defined and easy to assemble. This ARC platform is based on engineered dual variable domain (DVD) antibodies containing a natural uniquely reactive lysine residue for site-specific conjugation to β-lactam linker-functionalized siRNA. The conjugation is efficient, does not compromise the affinity of the parental antibody, and utilizes chemically stabilized siRNA. For proof-of-concept, we generated DVD-ARCs targeting various cell surface antigens on multiple myeloma cells for the selective delivery of siRNA targeting β-catenin (CTNNB1). A set of BCMA-targeting DVD-ARCs at concentrations as low as 10 nM revealed significant CTNNB1 mRNA and protein knockdown.