Harnessing the power of cell-penetrating peptides: activatable carriers for targeting systemic delivery of cancer therapeutics and imaging agents

Targeted delivery of cancer therapeutics and imaging agents aims to enhance the accumulation of these molecules in a solid tumor while avoiding uptake in healthy tissues. Tumor-specific accumulation has been pursued with passive targeting by the enhanced permeability and retention effect, as well as with active targeting strategies. Active targeting is achieved by functionalization of carriers to allow specific interactions between the carrier and the tumor environment. Functionalization of carriers with ligands that specifically interact with overexpressed receptors on cancer cells represents a classic approach to active tumor targeting. Cell-penetrating peptides (CPPs) provide a non-specific and receptor-independent mechanism to enhance cellular uptake that offers an exciting alternative to traditional active targeting approaches. While the non-specificity of CPP-mediated internalization has the intriguing potential to make this approach applicable to a wide range of tumor types, their promiscuity is, however, a significant barrier to their clinical utility for systemically administered applications. Many approaches have been investigated to selectively turn on the function of systemically delivered CPP-functionalized carriers specifically in tumors to achieve targeted delivery of cancer therapeutics and imaging agents.

Digital switching of local arginine density in a genetically encoded self-assembled polypeptide nanoparticle controls cellular uptake

Cell-penetrating peptides (CPPs) are a class of molecules that enable efficient internalization of a wide variety of cargo in diverse cell types, making them desirable for delivery of anticancer drugs to solid tumors. For CPPs to be useful, it is important to be able to turn their function on in response to an external trigger that can be spatially localized in vivo. Here we describe an approach to turning on CPP function by modulation of the local density of arginine (Arg) residues by temperature-triggered micelle assembly of diblock copolymer elastin-like polypeptides (ELP(BC)s). A greater than 8-fold increase in cellular uptake occurs when Arg residues are presented on the corona of ELP(BC) micelles, as compared to the same ELP(BC) at a temperature in which it is a soluble unimer. This approach is the first to demonstrate digital 'off-on' control of CPP activity by an extrinsic thermal trigger in a clinically relevant temperature range by modulation of the interfacial density of Arg residues on the exterior of a nanoparticle.

Fusions of elastin-like polypeptides to pharmaceutical proteins

Elastin-like polypeptides (ELPs) are a class of stimulus-responsive biopolymers whose physicochemical properties and biocompatibility are particularly suitable for in vivo applications, such as drug delivery and tissue engineering. The lower critical solution temperature (LCST) behavior of ELPs allows them to be utilized as soluble macromolecules below their LCST, or as self-assembled nanoscale particles such as micelles, micron-scale coacervates, or viscous gels above their LCST, depending on the ELP architecture. As each ELP sequence is specified at its genetic level, functionalization of an ELP with peptides and proteins is simple to accomplish by the fusion of a gene encoding an ELP with that of the peptide or protein of interest. Protein ELP fusions, where the appended protein serves a therapeutic or targeting function, are suitable for applications in which the ELP can improve the systemic pharmacokinetics and biodistribution of the protein, or can be used as an injectable depot for sustained, local protein delivery. Here we describe considerations in the design of therapeutic protein ELP fusions and provide details of their gene design, expression, and purification.

Stimulus-responsive macromolecules and nanoparticles for cancer drug delivery

Nanoparticles and macromolecular carriers have been widely used to increase the efficacy of chemotherapeutics, largely through passive accumulation provided by the enhanced permeability and retention effect. Stimulus-responsive peptide and polymer vehicles can further enhance the efficacy of antitumor therapeutics compared with the administration of free drug by three mechanisms: increasing the overall accumulation within solid tumors; providing a homogeneous spatial distribution in tumor tissues; and increasing the intracellular localization of anticancer therapeutics. This article highlights recent developments in 'smart' - stimulus-responsive - peptide, polymer and lipid drug carriers designed to enhance the localization and efficacy of therapeutic payloads as compared with free drug.

T cell subset distributions following primary and secondary implantation at subcutaneous biomaterial implant sites

Synthetic biomaterials are considered to be nonimmunogenic. Therefore, the role that adaptive immunity may play in the host response to implanted synthetic biomaterials has not been extensively studied. Cardinal features of adaptive immunity include specificity and T cell responses, which are greater and more effective with upregulation of activation receptors upon rechallenge. We compared the primary and secondary in vivo host response to three synthetic biomaterials: Elasthane 80A, silicone rubber, and polyethylene terephthalate using a cage implant model in Sprague Dawley rats. The synthetic biomedical polymers were subcutaneously implanted in cages for 14 days. Following explantation of the cages and a 2 week healing period, rats were implanted with cages containing the biomedical polymers for an additional 2 weeks. The cellular exudates within the cages were analyzed 4, 7, and 14 days post primary and secondary implantation by flow cytometry for the following cell types: T cells (inclusive of CD8(+), CD4(+), and CD4(+)/CD25(+) subsets), B cells, granulocytes, and macrophages. At day 14 following secondary implantation, there was an increase in T cells, granulocytes, and macrophages in the exudates when compared with primary implantation for all groups inclusive of the empty cage control. However, CD4(+)/CD8(+) ratios, the percentage of CD4(+)CD25(+) T cells, and the macrophage surface adhesion/fusion did not vary significantly upon secondary implantation. Despite a quantitative increase in T cells following secondary biomaterial exposure, T cell subset distribution did not change, indicating nonspecific recruitment rather than an adaptive immune response.