We have devised and tested a new strategy for selectively delivering molecules to tumor cells. Cellular association of polyargininebased, cell-penetrating peptides (CPPs) is effectively blocked when they are fused to an inhibitory domain made up of negatively charged residues. We call these fusions activatable CPPs (ACPPs) because cleavage of the linker between the polycationic and polyanionic domains, typically by a protease, releases the CPP portion and its attached cargo to bind to and enter cells. Association with cultured cells typically increases 10-fold or more upon linker cleavage. In mice xenografted with human tumor cells secreting matrix metalloproteinases 2 and 9, ACPPs bearing a far-red-fluorescent cargo show in vivo contrast ratios of 2-3 and a 3.1-fold increase in standard uptake value for tumors relative to contralateral normal tissue or control peptides with scrambled linkers. Ex vivo slices of freshly resected human squamous cell carcinomas give similar or better contrast ratios. Because CPPs are known to import a wide variety of nonoptical contrast and therapeutic agents, ACPPs offer a general strategy toward imaging and treating disease processes associated with linker-cleaving activities such as extracellular proteases.cancer ͉ molecular imaging ͉ polycation ͉ transduction

High-resolution imaging of molecules intrinsically involved in malignancy and metastasis would be of great value for clinical detection and staging of tumors. We now report in vivo visualization of matrix metalloproteinase activities by MRI and fluorescence of dendrimeric nanoparticles coated with activatable cell penetrating peptides (ACPPs), labeled with Cy5, gadolinium, or both. Uptake of such nanoparticles in tumors is 4-to 15-fold higher than for unconjugated ACPPs. With fluorescent molecules, we are able to detect residual tumor and metastases as small as 200 μm, which can be resected under fluorescence guidance and analyzed histopathologically with fluorescence microscopy. We show that uptake via this mechanism is comparable to that of other near infrared protease sensors, with the added advantage that the approach is translatable to MRI. Once activated, the Gd-labeled nanoparticles deposit high levels (30-50 μM) of Gd in tumor parenchyma with even higher amounts deposited in regions of infiltrative tumor, resulting in useful T 1 contrast lasting several days after injection. These results should improve MRI-guided clinical staging, presurgical planning, and intraoperative fluorescence-guided surgery. The approach may be generalizable to deliver radiationsensitizing and chemotherapeutic agents.Molecular navigation | dendrimeric nanoparticles | molecular amplification | targeted imaging agent | transgenic tumor model C linical cancer staging currently depends mainly on anatomical imaging with x-ray computed tomography (CT) and MRI. Some tumors can be imaged by PET of glucose uptake, but modest spatial resolution, high cost, exposure to radiation, and imperfect correlation of glucose uptake with malignancy limit the usefulness of PET and its more recent combination with CT. MRI is a particularly attractive imaging modality due to its moderate cost, relatively widespread availability, high spatial resolution tomography, excellent anatomical detail, and lack of radioactivity. Most clinical MRI is either T 1 -or T 2 -weighted, for which the standard contrast agents are, respectively, gadolinium (Gd) chelates and superparamagnetic iron oxide particles. The difficulty in using MRI for molecular imaging of specific biomolecules rather than for anatomy is sensitivity, because the detection limit is on the order of 10 −5 M Gd chelate or Fe, respectively (1, 2). Therefore several orders of magnitude of molecular amplification are necessary to detect tumor markers at low nanomolar abundance. T 2 -weighted MRI has the additional disadvantages that contrast is usually negative and the iron oxide particles are largely confined to the intravascular and reticuloendothelial compartments. Recently, there has been interest in designing T 1 magnetic resonance (MR) contrast agents that give information beyond that of a standard blood pool agent and detect tumor neovascularization (3, 4), folate receptor (5), and various antigens (6-8). Recent attempts at in vivo MRI of matrix metalloproteinase (MMP) activity have been based on...

The completeness of tumor removal during surgery is dependent on the surgeon's ability to differentiate tumor from normal tissue using subjective criteria that are not easily quantifiable. A way to objectively assess tumor margins during surgery in patients would be of great value. We have developed a method to visualize tumors during surgery using activatable cell-penetrating peptides (ACPPs), in which the fluorescently labeled, polycationic cell-penetrating peptide (CPP) is coupled via a cleavable linker to a neutralizing peptide. Upon exposure to proteases characteristic of tumor tissue, the linker is cleaved, dissociating the inhibitory peptide and allowing the CPP to bind to and enter tumor cells. In mice, xenografts stably transfected with green fluorescent protein show colocalization with the Cy5-labeled ACPPs. In the same mouse models, Cy5-labeled free ACPPs and ACPPs conjugated to dendrimers (ACPPDs) delineate the margin between tumor and adjacent tissue, resulting in improved precision of tumor resection. Surgery guided by ACPPD resulted in fewer residual cancer cells left in the animal after surgery as measured by Alu PCR. A single injection of ACPPD dually labeled with Cy5 and gadolinium chelates enabled preoperative wholebody tumor detection by MRI, intraoperative guidance by real-time fluorescence, intraoperative histological analysis of margin status by fluorescence, and postoperative MRI tumor quantification. Animals whose tumors were resected with ACPPD guidance had better long-term tumor-free survival and overall survival than animals whose tumors were resected with traditional bright-field illumination only.intraoperative fluorescence imaging | molecular navigation | long-term survival | molecular imaging | surgical margin

Activatable cell penetrating peptides (ACPPs) are novel in vivo targeting agents comprised of a polycationic cell penetrating peptide (CPP) connected via a cleavable linker to a neutralizing polyanion (Fig. 1A). Adsorption and uptake into cells are inhibited until the linker is proteolyzed1–3. An ACPP cleavable by matrix metalloproteinase-2 (MMP-2) in vitro was the first one demonstrated to work in a tumor model in vivo, but only HT-1080 xenografts and resected human squamous cell carcinomas were tested. Generality to other cancer types, in vivo selectivity of ACPPs for MMPs, and spatial resolution require further characterization. We now show that ACPPs can target many xenograft tumor models from different cancer sites, as well as a well thoroughly studied transgenic model of spontaneous breast cancer (mouse mammary tumor virus promoter driving polyoma middle T antigen, MMTV-PyMT). Pharmacological inhibitors and genetic knockouts indicate that current ACPPs are selective for MMP-2 and MMP-9 in the above in vivo models. In accord with the known local distribution of MMP activity, accumulation is strongest at the tumor-stromal interface in primary tumors and associated metastases, indicating better spatial resolution (<50 µm) than other currently available MMP-cleavable probes4. We also find that background uptake of ACPPs into normal tissues such as liver and kidney can be decreased by appending inert macromolecules of 30–50 KDa to the polyanionic inhibitory domain. Our results validate an approach that should generally deliver imaging agents and chemotherapeutics to sites of invasion, tumor-promoting inflammation, and metastasis.

Cell penetrating peptides (CPPs) have been developed as vehicles for payload delivery into cells in culture and in animals. However several biologic features limit their usefulness in living animals. Activatable cell penetrating peptides (ACPPs) are polycationic CPPs whose adsorption and cellular uptake is minimized by a covalently attached polyanionic inhibitory domain. Cleavage of the linker connecting the polyanionic and polycationic domains by specific proteases (tumor associated matrix metalloproteases discussed herein) dissociates the polyanion and enables the cleaved ACPP to enter cells. In contrast to CPP counterpart, ACPPs are relatively nonadherent and distributed uniformly to normal tissues. While nonaarginine (r 9 ) CPP administered intravenously into mice initially bind to the local vasculature and redistribute to the liver, where > 90% of the injected dose accumulates 30 min after injection. Regardless of the presence of the polyanionic inhibitory domain, confocal imaging of live tissues reveals that the majority of the ACPP and CPP remain in punctate organelles, presumably endosomes. Therefore further improvements in the efficiency of delivery to the cytosol and nucleus are necessary. In addition to improved target specificity, a major advantage of ACPPs over CPPs for potential clinical applications is reduced toxicity. Systemically administered r 9 CPP causes acute toxicity in mice at a dose 4 fold lower than the MMP cleavable ACPP, a complication not observed with an uncleavable ACPP presumably because the polycationic charge remains masked systemically. These data suggest that ACPPs have greater potential than CPPs for systemic delivery of imaging and therapeutic agents.

Mechanisms of ischemic neuronal and vascular injury remain obscure. Here we test the hypothesis that thrombin, a blood-borne coagulation factor, contributes to neurovascular injury during acute focal ischemia. Stroke was induced in adult Sprague Dawley rats by occluding the middle cerebral artery. Intra-arterial thrombin infusion during ischemia significantly increased vascular disruption and cellular injury. Intravenous infusion of argatroban, a direct thrombin inhibitor, alleviated neurovascular injury. Immunostaining showed thrombin on neurons in the ischemic core. Using an activatable cell penetrating peptide engineered to detect thrombin activity, we discovered that thrombin proteolytic activity was specifically associated with neuronal damage during ischemia. Protease activated receptor-1, the presumptive thrombin receptor, appeared to mediate ischemic neurovascular injury. Furthermore, rats receiving thrombin during ischemia showed cognitive deficit whereas rats receiving argatroban retained intact learning and memory. These results suggest a potential role for thrombin contributing to neurovascular injury and several potential avenues for neuroprotection.

We present a new class of ultrasound molecular imaging agents that extend upon the design of micromotors that are designed to move through fluids by catalyzing hydrogen peroxide (H 2 O 2 ) and propelling forward by escaping oxygen microbubbles. Micromotor converters require 62 mM of H 2 O 2 to move -1000-fold higher than is expected in vivo. Here, we aim to prove that ultrasound can detect the expelled microbubbles, to determine the minimum H 2 O 2 concentration needed for microbubble detection, explore alternate designs to detect the H 2 O 2 produced by activated neutrophils and perform preliminary in vivo testing. Oxygen microbubbles were detected by ultrasound at 2.5 mM H 2 O 2 . Best results were achieved with a 400-500 nm spherical design with alternating surface coatings of catalase and PSS over a silica core. The lowest detection limit of 10-100 µM was achieved when assays were done in plasma. Using this design, we detected the H 2 O 2 produced by freshly isolated PMA-activated neutrophils allowing their distinction from naïve neutrophils. Finally, we were also able to show that direct injection of these nanospheres into an abscess in vivo enhanced ultrasound signal only when they contained catalase, and only when injected into an abscess, likely because of the elevated levels of H 2 O 2 produced by inflammatory mediators.

Although multiple sclerosis (MS) has been associated with the coagulation system, the temporal and spatial regulation of coagulation activity in neuroinflammatory lesions is unknown. Using a novel molecular probe, we characterized the activity pattern of thrombin, the central protease of the coagulation cascade, in experimental autoimmune encephalomyelitis. Thrombin activity preceded onset of neurological signs, increased at disease peak, and correlated with fibrin deposition, microglial activation, demyelination, axonal damage, and clinical severity. Mice with a genetic deficit in prothrombin confirmed the specificity of the thrombin probe. Thrombin activity might be exploited for developing sensitive probes for preclinical detection and monitoring of neuroinflammation and MS progression.