br to be biologically stable as assessed by

to be biologically stable as assessed by fluorescence resonance energy transfer (FRET) (Fig. 15) and also inactive in the blood circulation. NucA was conjugated to the hydroxyl group at position 2′ of the drug via a dipeptide bond sensitive to Cathepsin B, which then got cleaved once inside the PFTα by Cathepsin B, thus triggering the anticancer mechanism.
The GFLG sequence was also embedded into a star-shaped peptidic prodrug structures that can be cleaved by Cathepsin B. This feature has been used to develop drug delivery vehicles for 2-methoxyestradiol (2ME) which is a natural metabolite of estradiol with antiproliferative and anti-angiogenic activities (Fig. 16) [140]. In the context of combination therapy, a dual-functionalized linker bearing Dox and Ptx, and comprising a maleimide moiety for its subse-quent coupling to albumin through its cysteine-34 position, was designed [141]. Each drug was linked by a self-immolative para-aminobenzyloxy carbonyl linker and a cleavable dipeptide (Phe-Lys) sensitive to Cathepsin B, leading to drug release at the tumor site (Fig. 17).A similar approach combining a polymer prodrug and a polymer-enzyme bioconjugate was used to selectively and rapidly deliver a cytotoxic drug to the target site [142].
Fig. 15. Schematic illustration of the in vivo tracking of the degraded Cathepsin B-labile dipeptide bond linker exploiting FRET with fluorescein amidate (FAM) and dual-labeled rhodamine B (Rh) NucA-Ptx bioconjugate. Adapted with permission from Ref. [139].
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
Fig. 16. Representation of the degradation of star-shaped peptidic prodrug structures that can be cleaved by Cathepsin B. Adapted with permission from Ref. [140].
Fig. 17. Structure of Cathepsin B-sensitive, dual-functionalized linker bearing Dox and Ptx, and comprising a maleimide moiety for its coupling to albumin. Adapted with permission from Ref. [141].
Pep42, which is a cyclic 13-mer oligopeptide, specifically binds to glucose-regulated protein 78 and translocates into the lysosomal compartment [143,144]. In this context, Pep42 was advantageously used to efficiently deliver Ptx and Dox into cancer cells for enhanced
cytotoxicity [145]. More specifically, Pep42-prodrug bioconjugates containing a Cathepsin B-sensitive linker were synthesized and facilitated the uptake of both cytotoxic agents for their delivery into cancer cells.
Fig. 18. Genetically engineered Cathepsin B-modulated bacteriophage conjugated to Dox. Adapted with permission from Ref. [148].
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
Nanoconstructs with methotrexate (Mtx) linked to a tuftsin-like peptide carrier via a GFLG spacer and several copies of a chemotactic targeting agent were designed [146]. These conjugates led to greater cy-totoxic effect than free Mtx and represented potential candidates for the
specific targeting of cancer cells. Similarly, Dox-based dipeptide conju-gates were designed and tethered to monoclonal antibodies (mAbs) recognizing tumor associated antigens on renal cell carcinoma and ana-plastic large cell lymphoma [147]. The dipeptides were substrates for
Fig. 19. Illustration of Dox-loaded, hollow mesoporous silica nanoparticles for in situ imaging of Cathepsin B and protease-mediated Dox release. (a) Nanoparticle synthesis.
(b) Nanoparticle disassembly mediated by enzyme cascade reactions with acid hyaluronidase (HAase) and Cathepsin B (Cat B). (c) Specific delivery, controlled Dox release and intracellular imaging: (i) specific uptake via receptor-mediated endocytosis; (ii) accumulation in endosomes; (iii) endosomal escape and intracellular imaging of Cat B; (iv) Dox release triggered by enzymes. Adapted with permission from Ref. [169].