Since their identification over twenty-five years ago, the plethora of cell-penetrating peptides (CPP) and their applications has skyrocketed. These 5 to 30 amino acid in length peptides have the unique property of breaching the cell membrane barrier while carrying cargoes larger than themselves into cells in an intact, functional form. CPPs can be conjugated to fluorophores, activatable probes, radioisotopes or contrast agents for imaging tissues, such as tumors. There is no singular mechanism for translocation of CPPs into a cell, and therefore, many CPPs are taken up by a multitude of cell types, creating the challenge of tumor-specific translocation and hindering clinical effectiveness. Varying strategies have been developed to combat this issue and enhance their diagnostic potential by derivatizing CPPs for better targeting by constructing specific cell-activated forms. These methods are currently being used to image integrin-expressing tumors, breast cancer cells, human histiocytic lymphoma and protease-secreting fibrosarcoma cells, to name a few. Additionally, identifying safe, effective therapeutics for malignant tumors has long been an active area of research. CPPs can circumvent many of the complications found in treating cancer with conventional therapeutics by targeted delivery of drugs into tumors, thereby decreasing off-target side effects, a feat not achievable by currently employed conventional chemotherapeutics. Myriad types of chemotherapeutics such as tyrosine kinase inhibitors, antitumor antibodies and nanoparticles can be functionally attached to these peptides, leading to the possibility of delivering established and novel cancer therapeutics directly to tumor tissue. While much research is needed to overcome potential issues with these peptides, they offer a significant advancement over current mechanisms to treat cancer. In this review, we present a brief overview of the research, leading to identification of CPPs with a comprehensive state-of-the-art review on the role of these novel peptides in both cancer diagnostics as well as therapeutics.

Cell penetrating peptides are unique, 5-30 amino acid long peptides that are able to breach cell membrane barriers and carry cargoes intracellularly in a functional form. Our prior work identified a synthetic, non-naturally occurring 12-amino acid long peptide that we termed cardiac targeting peptide (CTP: APWHLSSQYSRT) due to its ability to transduce cardiomyocytes in vivo. Studies looking into its mechanism of transduction identified two lung targeting peptides (LTPs), S7A (APWHLSAQYSRT) and R11A (APWHLSSQYSAT). These peptides robustly transduced human bronchial epithelial cell lines in vitro and mouse lung tissue in vivo. This uptake occurred independently of clathrin mediated endocytosis. Biodistribution studies of R11A showed peak uptake at 15 minutes with uptake in liver but not kidneys, indicating primarily a hepatobiliary mode of excretion. Cyclic version of both peptides was ~100-fold more efficient in permeating cells than their linear counterparts. As proof of principle, we conjugated anti-spike and anti-envelope SARS-CoV-2 siRNAs to cyclized R11A and demonstrate anti-viral efficacy in vitro. Our work presented here identifies two novel lung-specific cell penetrating peptides that could potentially deliver myriad therapeutic cargoes to lung tissue.

Cell penetrating peptides are unique, 5-30 amino acid long peptides that are able to breach cell membrane barriers and carry cargoes intracellularly in a functional form. Our prior work identified a synthetic, non-naturally occurring 12-amino acid long peptide that we termed cardiac targeting peptide (CTP: APWHLSSQYSRT) due to its ability to transduce cardiomyocytes in vivo. Studies looking into its mechanism of transduction identified two lung targeting peptides (LTPs), S7A (APWHLSAQYSRT) and R11A (APWHLSSQYSAT). These peptides robustly transduced human bronchial epithelial cell lines in vitro and mouse lung tissue in vivo. This uptake occurred independently of clathrin mediated endocytosis. Biodistribution studies of R11A showed peak uptake at 15 minutes with uptake in liver but not kidneys, indicating primarily a hepatobiliary mode of excretion. Cyclic version of both peptides was ~100-fold more efficient in permeating cells than their linear counterparts. As proof of principle, we conjugated anti-spike and anti-envelope SARS-CoV-2 siRNAs to cyclized R11A and demonstrate anti-viral efficacy in vitro. Our work presented here identifies two novel lung-specific cell penetrating peptides that could potentially deliver myriad therapeutic cargoes to lung tissue.

Since their identification over twenty-five years ago, the plethora of cell penetrating peptides (CPP) and their applications has skyrocketed. These 5 to 30 amino acid long peptides have the unique property of breaching the cell membrane barrier while carrying cargoes larger then themselves into cells in an intact, functional form. CPPs can be conjugated to fluorophores, activatable probes, radioisotopes or contrast agents for imaging tissues, such as tumors. There is no singular mechanism for translocation of CPPs into a cell, and therefore, many CPPs are taken up by a multitude of cell types, creating the challenge of tumor specific translocation and hindering clinical effectiveness. Varying strategies have been developed to combat this issue and enhance their diagnostic potential by derivatizing CPPs for better targeting by constructing specific cell activated forms. These methods are currently being used to image integrin expressing tumors, breast cancer cells, human histiocytic lymphoma and protease secreting fibrosarcoma cells, to name a few. Additionally, identifying safe, effective therapeutics for malignant tumors has long been an active area of research. CPPs can circumvent many of the complications found in treating cancer with conventional therapeutics by targeted delivery of drugs into tumors, thereby decreasing off-target side effects, a feat not achievable by currently employed conventional chemotherapeutics. Myriad types of chemotherapeutics such as tyrosine kinase inhibitors, anti-tumor antibodies and nanoparticles can be functionally attached to these peptides leading to the possibility of delivering established and novel cancer therapeutics directly to tumor tissue. While much research is needed to overcome potential issues with these peptides, they offer a significant advancement over current mechanisms to treat cancer. In this review, we present a brief overview of the research leading to identification of CPPs with a comprehensive state of the art review on the role of these novel peptides in both cancer diagnostics as well as therapeutics.