Despite significant strides in prevention, diagnosis, and treatment, cardiovascular diseases remain the number one cause of mortality in the United States, with rates climbing at an alarming rate in the developing world. Targeted delivery of therapeutics to the heart has been a lofty goal to achieve with strategies ranging from direct intra-cardiac or intra-pericardial delivery, intra-coronary infusion, to adeno-associated viral vectors which have preference, if not complete cardio-selectivity, for cardiac tissue. Cell penetrating peptides (CPP) are 5-30 amino acid long peptides that are able to breach cell membrane barriers while carrying cargoes up to several times their size, in an intact functional form. Identified nearly three decades ago, the first of these CPPs came from the HIV coat protein transactivator of transcription. Although a highly efficient CPP, its clinical utility is limited by its robust ability to cross any cell membrane barrier, including crossing the blood-brain barrier and transducing neuronal tissue non-specifically. Several strategies have been utilized to identify cell or tissue specific CPPs, one of which is phage display. Using this latter technique, we identified a cardiomyocyte targeting peptide (CTP) more than a decade ago, a finding that has been corroborated by several independent labs across the world that have utilized CTP for myriad different purposes in pre-clinical animal models. The goal of this publication is to provide a comprehensive review of the identification, validation, and application of CTP, and outlining its potential in diagnostic and therapeutic applications especially in the field of targeted RNA interference.

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.

Despite great strides in diagnosis and treatment, cardiac diseases remain the number one cause of mortality in the developed world, with rates catching up at an alarming rate in the developing world. Targeted delivery of therapeutics, specifically to cardiomyocytes, would open up new frontiers. Our prior work using a combinatorial in vitro and in vivo phage display methodology identified a 12-amino acid long peptide that targets cardiomyocytes selectively after an intravenous injection in as little as 5 mins, and hence was termed cardiac targeting peptide (CTP: APHLSSQYSRT). CTP has been used to deliver imaging agents, small drug molecules, photosensitizing nanoparticles, exosomes, and even miRNA to cardiomyocytes. As a natural extension for development of CTP as a clinically viable cardiac vector, we now present toxicity studies performed on the peptide. In vitro viability studies were performed in a human left ventricular myocyte cell line with 10µM of Cyanine-5.5 labeled CTP (CTP-Cy5.5). In vitro ion channel profiles were completed for CTP followed by more extensive studies performed by Eurofins in stably transfected cell lines for an extensive list of GPCR-coupled receptors. Positive data for GPCR-coupled receptors was interrogated further with RT-qPCRs performed on mouse heart tissue. In vivo studies consisted of pre- and post-blood pressure monitoring acutely after a single CTP (10 mg/Kg) injection. Further in vivo toxicity studies consisted of injecting 60, 6-week-old, wild-type CD1, male/female mice (1:1) injected with CTP (150 µg/Kg) intravenously. Mice were weighed immediately before injections and daily thereafter. Cohorts of mice were euthanized on day 0, 1, 2, 7 and 14 with inhalational CO2, followed by opening of the chest cavity, collection of blood via cardiac puncture, complete blood count analysis, metabolic profiling, and finally, liver, renal and thyroid studies. Lastly, mouse cardiac MRI was performed immediately before and after injections of CTP (150 µg/Kg) to assess changes in cardiac size or function. Human left ventricular cardiomyocytes showed no decrease in viability after a 30 min incubation with CTP-Cy5.5. In the Eurofins Cardiac Profiler Qube, no significant activation or inhibition of any of seventy-eight protein channels was observed other than OPRM1 and COX2 at the highest tested concentration, neither of which were expressed in mouse heart tissue as assessed by RT-qPCR. There was no change in blood pressures before and after injections with CTP at 10 mg/Kg. Blood counts and chemistries showed no evidence of significant hematological, hepatic or renal toxicities. Lastly, there was no significant difference in cardiac function, size or mass before and after CTP injections. Our studies with CTP showed no activation or inhibition of GPCR-associated receptors in vitro. We found no signals indicative of toxicity in vivo. Most importantly, cardiac functions remained unchanged acutely in response to CTP uptake. Further studies, in good laboratory practices fashion, are needed with more prolonged, chronic administration of CTP conjugated to a specific cargo of choice before human studies can be contemplated.