br Cathepsin D conjugated peptides were self assembled into
Cathepsin D-conjugated peptides were self-assembled into nano-particles with the help of gelatin to bypass early nonspecific dissolution as well as off-target Dox release and is useful for optical imaging in animal models . Cathepsin D is an enzyme for breast cancer cell se-cretion, which got triggered by degrading the nanoparticles coated with peptide strands through hydrolytic cleavage, thus releasing Dox. The nanoparticles were evaluated under ultrasound imaging both in vitro and in vivo, and were found to be localized in the Protease Inhibitor Cocktail and the tumors of mice as a result of the fluorescent profile of Dox. Synthesis of Cathepsin B-sensitive, near-infrared fluorescent probe was also car-ried out (Fig. 24) . The probe was found to be water-soluble but still self-assembled into nanoparticles having potential for tumor-targeted imaging. A fluorescent molecule, DCPO (dicyanomethylene-4H-pyran), was released by Cathepsin B, leading to in vitro imaging Cathepsins of various tumor cells during incubation with different cell lines.
Similarly, a Cathepsin B-sensitive nanoparticulate probe compris-ing a Cathepsin B substrate peptidic probe linked to chitosan nanopar-ticles was reported . According to the study, this probe was successfully delivered into tumor cells after nanoparticle accumulation and exhibited fluorescent signals inside the cytosol in presence of Ca-thepsin B. It thus showed increased potential for the optical detection of biological activities especially related to tumor growth or metastasis (Fig. 25).
Recently, another strategy was used for Cathepsin imaging in breast cancer. It relied on a selective fluorogenic substrate and activity-based probe for the specific imaging of Cathepsin L . This approach en-abled to differentiate Cathepsin L activity from that of other Cathepsins such as Cathepsin B.
As shown in the previous sections, Cathepsins have been exploited in targeted drug delivery systems and imaging. However, Cathepsin in-hibitors can also be exploited in regard to their role in numerous dis-eased conditions, mainly cardiovascular diseases such as myocardial infarction, atherosclerosis, cardiac hypertrophy, cardiomyopathy and hypertension based on animal models . Cathepsin inhibitors are also used against immune responses, osteoporosis, arthritis, inflamma-tion and neurodegenrative disorders [36,188–190]. A selection of repre-sentative examples is discussed below.
Interestingly, peptide-based pseudosubstrates for Cathepsin G and elastase were developed. These substrates can decrease the activated interleukin-36 (IL-36) family cytokines especially in case of inflamma-tory diseases (e.g., psoriasis, arthritis)because such cytokines are pro-teolytically processed in the presence of Cathepsin G and other proteases . In another study, it was proven that amodiaquine, an antimalarial drug, inhibited host Cathepsin B to protect host cells against infection with multiple toxins or viruses . Cathepsin K has also been involved in diabetes-associated cardiac abnormalities. Wild-type as well as Cathepsin K knockout mice-induced diabetes ex-hibited severe cardiac dysfunctions in the form of dampened calcium handling intracellularly, cardiac morphology alterations and also in-crease in cardiomycyte apoptosis [193,194]. Hence, Cathepsin K may be a suitable target in the afore-mentioned conditions. One study also investigated cysteine Cathepsin inhibitors such as GB111-NH2 (that blocks the activity of Cathepsin B, L and S) as trigger in macrophage cell death especially in case of tumor-associated macrophages (TAMs) .
3. Clinical data on cathepsins
As seen throughout this review article, PHPMA has been extensively used for the design of Cathepsin-sensitive nanocarriers. This is ex-plained by the favorable properties of PHPMA regarding biomedical applications. PHPMA is indeed hydrophilic, non-immunogenic, chemi-cally inert, non-toxic (even at the dose of 30 g/kg rat), biocompatible and exhibits relatively long circulation time which is dependent on its molar mass . Among the different conjugates based on PHPMA that have been synthesized and evaluated so far, some of them entered clinical trials (Table 2). In particular,PK1 (Prague-Keele-1) has shown very promising results in oncology  and reachaed phase II trials in 2002 but clinical studies for both PK1 and PK2 were discontinued in 2008 because of lack of efficacy . Other polymeric systems
Fig. 25. Cathepsin B-sensitive nanoparticulate probes and tumor diagnosisin vivo. Adapted with permission from Ref. .
Please cite this article as: D. Dheer, J. Nicolas and R. Shankar, Cathepsin-sensitive nanoscale drug delivery systems for cancer therapy and other diseases, Adv. Drug Deliv. Rev., https://doi.org/10.1016/j.addr.2019.01.010