br Targeting inflammatory joints in
Targeting inflammatory joints in rheumatoid arthritis (RA) was achieved by AWO54, a new prodrug that binds to endogenous albumin and was composed of Mtx, a spacer based on lysine and an enzyme-sensitive peptide linker linked to a maleimide moiety for further linkage
Fig. 21. Structure of dual-functionalized PHPMA nanocarriers with Cy5 and Cy7 dyes for further Cathepsin B-mediated release of Cy7. 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
to albumin . The prodrug was cleaved by two enzymes, Cathepsin B and plasmin, that exist in high concentrations in synovial effusion under RA condition, thus releasing Mtx lysine derivatives. The in situ coupling of endogenous albumin, AWO54 was found to be better in terms of dosage and efficacy than administration of the parent drug for treating collagen-induced arthritis.
2.3. Immune cell-targeting drug delivery systems
Lysosomal peptidases are part of innate and adaptive immune re-sponses [158–160]. Hence, modulation of such responses with Cathepsin-sensitive prodrugs can further enhance the immunological action and regulate cytotoxicity issues related to NK and T cells. For in-stance, influence of superparamagnetic iron oxide nanoparticles (SPIONs) from both a physiological and immunological point of view was investigated on cell function and their interaction with oxysterol laden Lipo2000 . Iron-loaded nanoparticles upregulated Cathepsin, membranous ferroportin (cellular efflux channel for iron) and ferritin degradation, which further altered cellular immune functions, resulting in secretion of pro- (TNF-α) and anti-inflammatory (IL-10) cytokines and ferritin. Importantly, this study highlighted a specific relationship between SPION metabolism and atheroma cellfunction that might con-duct to innovative approaches to treat atherosclerotic plaques.
Immunoconjugates were also prepared from cytotoxic agents using a valine-alanine-p-aminobenzyl-amine linker which was well-adapted for the bioconjugation to monoclonal antibody and further specific cleavage by proteases . The linker efficiently released amino-geldanamycin and streptonigrin upon protease-mediated hydrolysis, emphasizing the activity and specificity of the conjugates in vitro and in vivo. In another study, different immunoconjugates comprising lysosomally cleavable peptides (i.e., Phe-Lys and Val-Cit), were synthe-sized . The monoclonal antibody BR96 that is known to bind to Lewisy-related tumor-associated antigen expressed at the surface of cancer cells was linked to Dox via a p-aminobenzyloxycarbonyl (PABC) spacer. Interestingly, the conjugates bearing the Phe-Lys
sequence exhibited a 30-fold greater drug release kinetics in the pres-ence of Cathepsin B than its counterpart with the Val-Cit linker.
2.4. Cathepsins as probes for imaging and theranostic
Different types of enzymes (e.g., caspases, secretases, furinases, phosphatases, etc.) have been exploited for cancer diagnosis. Further-more, imaging probes utilizing these proteases have rapidly evolved [164–167]. It has been shown that the monitoring of protease activity was closely related to cancer progression especially in case of Cathepsin B . Among the numerous studies on proteases for such a purpose, hollow mesoporous silica nanoparticles loaded with Dox and conferred with a dual-enzyme sensitivity were conceived for the in situ imaging of Cathepsin B and the release of Dox mediated by proteases (Fig. 19) . The peptide-based satellite/shell structures secured Dox inside the nanoparticles thus acting as three-dimensional gatekeepers and Dox release subsequently occurred upon incubation with Cathepsin B.
It was also recently discovered that indocyanine green (ICG)–con-taining PGA nanoparticles can be digested byCathepsin B and induce a sentization of the endo-lysosomal membrane mediated by the NIR prop-erties of the released ICG (Fig. 20) . The system was combined with a ribosome-inactivating protein (saporin) which showed synergistic cytotoxicity because of the photo-induced release of saporin from endosomes or lysosomes.
An enzymatically dependent FRET process was also used to monitor the payload release from PHPMA prodrug nanocarriers . PHPMA was functionalized with donor Cy5 and acceptor Cy7, thus inducing FRET. However, since only Cy7 was linked to the polymer via the GFLG sequence, presence of Cathepsin B was accurately measured because of the change in the FRET signal during the Cathepsin B-mediated Cy7 release (Fig. 21). The in vitro results showed that the high level of expression of Cathepsin B in cancer cells induced effective release of the dye while in vivo observations resulted in a faster release in the ovarian tumor as compared to normal tissues.