1000/1000
Hot
Most Recent
Cathepsin B is a lysosomal cysteine protease, contributing to vital cellular homeostatic processes including protein turnover, macroautophagy of damaged organelles, antigen presentation, and in the extracellular space, it takes part in tissue remodeling, prohormone processing, and activation. However, aberrant overexpression of cathepsin B and its enzymatic activity is associated with different pathological conditions, including cancer. Cathepsin B overexpression in tumor tissues makes this enzyme an important target for smart delivery systems, responsive to the activity of this enzyme. The generation of technologies which therapeutic effect is activated as a result of cathepsin B cleavage provides an opportunity for tumor-targeted therapy and controlled drug release.
Different ADC were FDA-approved for cancer treatment and many of them use CtsB-sensitive linkers. Enhertu is an ADC composed of Transtuzumab (directed against tumor cells over-expressing Her-2) and was approved to treat breast [77], gastric [78] and non-small cell lung cancer [79]. The antibody is conjugated via the pentapeptide maleimide GGGPG (Gly-Gly-Gly-Pro-Gly) to the exatecan derivative Dxd. This linker is sensitive to CtsB and CtsL activity and is fundamental to control the release of Dxd where these proteases are overexpressed. Zynlonta comprises an antibody specific for CD19 conjugated via a VA linker to the DNA alkylating agent SG3199 and it was approved for treating B-cell non-Hodgkin lymphoma, over-expressing CD19. The ADC Adcetris was approved to treat Hodgkin lymphoma and systemic anaplastic large cell lymphoma and it is based on an antibody directed against CD30 (also known as the Reed–Sternberg cell-associated antigen). This antibody is conjugated to the drug monomethyl auristatin E (MMAE), through two chemical spacers (maleimidocaproyl and PABC groups), separated by the CtsB-sensitive VC dipeptide. The spacers have the primary function of physically conjugating the antibody, the linker and the MMAE, but also to favor the access of the enzyme to the peptide, that could be eventually affected by the drug [80]. This very similar technology was exploited to design the FDA-approved ADC Polivy and Padcev delivering MMAE. Polivy is used to treat diffuse large B-cell lymphoma by targeting CD79b while Padcev was approved to treat advanced or metastatic urothelial cancer and is composed of an antibody directed against Nectin-4 [81]. Tivdak comprised a human anti-TFIgG1κ antibody conjugated to MMAE via the CtsB-cleavable maleimidocaproyl-VC-PABC linker and was FDA-approved in 2021 to treat adult patients with recurrent or metastatic cervical cancer [82]. Disitamab vedotin, also known as RC48, with the same technology and CtsB sensitive linker, was approved by NMPA (China) to treat metastatic gastric cancer [83].
Interestingly, a deeper investigation of this linker showed that CtsB expression inhibition through different biomolecular techniques did not affect the cytostatic properties of the system. This phenomenon was due to compensating mechanisms originating from the activity of other proteases (i.e., CtsS) mitigating the lack of CtsB. It is also worth highlighting that linking MMAE via a non-cleavable enantiomer did not completely suppress the cytostatic properties of the system because a toxic catabolic product of MMAE can generate in the lysosomal compartment. On the other hand, the cytotoxic dependence on CtsB activity was restored by substituting MMAE with another drug (pyrrolo [2,1-c][1,4] benzodiazepine dimer) that did not generate any toxic catabolite in the lysosomes. This work is important because it showed that functional ADC can be generated despite the linker sensitivity for a specific protease [84].
A different category of ADC agents has been developed utilizing PBD dimers, which are roughly 50–100 times more potent than the standard drugs used in the creation of ADCs (such as MMAE). Two examples of such agents are SGN-CD33A (Vadastuximab talirine) [85] and SGN-CD70A [86], which both include the same talirine cleavable linker, a maleimidocaproyl linker with a CtsB-sensitive VA-PAB moiety, PBD dimer (SG 1882) and anti-CD33 and anti-CD70 antibodies, respectively. Unfortunately, the clinical trials for these drugs had to be terminated due to severe side effects and increased patient mortality [87]. Furthermore, SGN-CD123A, ADC with a similar structure based on an anti-CD213 antibody, was developed to treat acute myeloid leukaemia [88].
Another ADC drugs containing CtsB-cleavable tesirine linker (tesirine linker in its turn comprises Val-Ala peptide) are evaluated in the ongoing clinical trials: Rovalpituzumab tesirine is an anti-DLL3 ADC developed to treat small cell lung cancer (phase III) and loncastuximab tesirine [89] and camidanlumab tesirine [90] are anti-CD19 and anti-CD25 ADCs (phase II), respectively, indicated to treat B cell acute lymphoblastic leukemia and Hodgkin lymphoma.
In a pre-clinical work, it was discovered that Carboxylesterase 1C can break down VC linkers in the plasma [91] affecting the complex stability in the blood. Nevertheless, modifying the linker by introducing an aminocaproyl or additional Glu residue upstream of the VC dipeptide has been shown to mitigate this issue [92][93], while maintaining or increasing sensitivity to CtsB activity.
The most common CtsB-sensitive peptide linkers and their basic physicochemical properties are summarized in Table 1.
Table 1. Chemical structures and basic physicochemical properties of CtsB-cleavable linkers *.
Peptide Linker |
Chemical Structure |
Molecular Weight |
Net Charge |
Isoelectrical Point |
GFLG |
392.2 |
0 |
5.60 |
|
GAGRRAAG |
714.4 |
+2 |
12.49 |
|
VC |
220.1 |
0 |
4.95 |
|
DFK |
408.2 |
0 |
6.77 |
|
VA |
188.1 |
0 |
5.60 |
|
FFKF |
587.3 |
+1 |
9.93 |
|
PGFK |
447.2 |
+1 |
10.59 |
|
GIVRAK |
642.4 |
+2 |
11.56 |
* Based on data from www.pepdraw.com, accessed on 15 June 2023.
Ehrsam et al. [94] generated poly(dimethylsiloxane)-b-poly(methyloxazoline) (PDMS-PMOXA) NPs modified on their surface with the CtsB-responsive Fmoc-aminocaproic acid(Ahx)-GSGFLGSC peptide bearing PTX. A significant increase in OVCAR-3 and OVCAR-5 cell lines’ cytotoxicity was observed when the particle treatment was combined with purified CtsB, indicating that the enzyme could accelerate the release of the payload, while the addition of the CtsB inhibitor CA-074 decreased particle toxicity [103].
For cancer treatment, rare-earth doped upconversion nanocrystals (UCN) were embedded in a polymeric matrix bearing the photosensitizer Chlorin e6 (Ce6) and modified with the CtsB-sensitive peptide Ac-FKC(StBu)AC(SH)-CBT [104]. The peptide contained a protected reactive group comprising 2-cyanobenzothiazole that could covalently bind cysteine. CtsB cleavage of the peptide favored the covalent cross-linking between the exposed cysteine and 2-cyanobenzothiazole of adjacent particles, ultimately favoring their aggregation. UCN cross-linking enhanced their upconversion emission properties upon laser irradiation (wavelength of 808 nm) and the consequent generation of singlet oxygen by Ce6. The aggregation of UCN in an environment with a high content of CtsB was confirmed by electron microscopy and was accompanied by a shift in the UCN absorption peak. In vitro and in vivo fluorescence and photoacoustic imaging studies confirmed the success of this enzyme-induced cross-linking reaction.
Recently, self-assembling prodrug NPs have been proposed to generate a new concept of nanodelivery systems [105]. NPs assemble could occur by modifying the payload with hydrophobic or amphiphilic linkers inducing a particle self-assembly process. The prodrug molecules were modified with ester, thioether, thioketal and disulfide groups allowing for a selective activation of the system in the TME [106]. Proteolytic sensitivity against a particular enzyme could be imparted using sensitive peptides in the formula. It was known that Phe was the key amino acid residue for creating self-assembling nanostructures because of intermolecular hydrophobic and π-π interactions [107]. Peptides with the CtsB-cleavable sequence FFKF (Phe-Phe-Lys-Phe) demonstrated effective nanofiber formation and DOX loading and release efficacy [108]. The FRRL-DOX (Phe-Arg-Arg-Leu-DOX) self-assembled NPs (170 nm) showed a 16-times higher tumor targeting than free DOX [109].
Over the last year, only a few works have been published about CtsB-sensitive nanoparticles. One such work, by Huang et al. [110], involved the development of ferumoxytol nanoparticles that were linked to the MMAE drug by four polyethylene glycol linkers, a CtsB-cleavable VC dipeptide and a p-aminobenzylcarbamate spacer. These nanoparticles were tested in vitro using the U87-MG glioblastoma cell line and in vivo on nude mice that had been injected intracranially with these cells. The resulting nanoparticles were found to be effective in inducing the death of glioblastoma cells. The maximal anticancer effect was achieved through a combination of nanoparticle treatment and radiotherapy. Another work by Shi et al. [111] aimed to develop a complex of fluorocarbons linked with polyarginine and CtsB-sensitive GFLG peptides to anti-VEGF siRNA. Positively charged nanoparticles, about 90 nm in size, were prepared and the polyarginine and GFLG peptide sequences provided double cleavage mediated by GSH reduction and CtsB activity. The work resulted in efficient siRNA release and VEGF deregulation in HeLa cervical carcinoma cell line.
Gotov et al. [120] synthesized AuNPs modified with hyaluronic acid for targeted delivery of docetaxel (DTX). The hyaluronic acid was attached to the surface of the AuNPs using the CtsB-sensitive peptide GFLGC and allowed for increased circulation and targeting properties against the CD44 receptors overexpressed on cancer cells. The system showed greater cytotoxicity and higher tumor suppression efficacy in vivo than free DTX, providing a means to combine thermoablation and chemotherapy.
In a recent study conducted by Li et al. [121], it was demonstrated that resveratrol encapsulated in mesoporous silica nanoparticles linked to transferrin molecules with CtsB-cleavable DEGFLGED peptide, exhibited high anticancer properties. In this case, transferrin acted as a capping and targeting agent. The resulting nanoparticles effectively reduced the viability of MCF7 cells and increased the apoptosis rate up to 80.8%. The authors have also planned to conduct further research using a mouse xenograft model.
Peptide Linker |
Drug |
Delivery System |
Cancer |
Outcomes |
Ref. |
GFLG |
Doxorubicin |
Conjugate |
HepG2 cells |
The conjugate structure had an opposite effect on DOX release and tumor accumulation. The synergistic effect of these properties exhibited the highest antitumor efficacy |
[122] |
GFLG |
Paclitaxel and gemcitabine |
Conjugate with HPMA dendrimers |
A2780 human ovarian carcinoma cells |
The combination of PTX, GEM and diblock structures yielded the highest inhibition efficacy of tumor growth |
[123] |
GFLG |
Doxorubicin |
Conjugate with polymer |
Lung carcinoma, colorectal cancer and anthracycline-resistant breast cancer |
Antitumor activity in refractory cancers was demonstrated, and polymer-drug conjugation has been shown to reduce DOX dose-limiting toxicity |
[124] |
GFLG |
Doxorubicin |
Conjugate with polymer |
H22 mice tumor |
The conjugates were successfully internalized into the cell nuclei, resulting in an inhibition efficiency of ~90% for the tumor |
[125] |
GAGRRAAG |
Pheophorbide |
Conjugate |
Bone marrow cells |
The photodynamic effect was demonstrated to be greater than 60%, and the system could be used as a sensor for cathepsin activity |
[126] |
VC |
Doxorubicin |
Nanoparticle |
RKO colon carcinoma cells |
Conjugates can efficiently bind to and be internalized by EGFR-overexpressing cancer cells. This strategy could be used to reduce systemic toxicity |
[127] |
DFK |
SPION |
Nanoparticle |
MDA-MB-231 breast cancer cells |
The increased efficiency of NP internalization and spion release following exposure to CtsB were demonstrated |
[128] |
GSGFLGSC |
PTX |
Nanoparticle |
OVCAR-3 adenocarcinoma cells and OVCAR-5 ovarian cancer cells |
The time-dependent PTX release and a 25-fold reduction in IC50 compared to pure PTX were demonstrated |
[94] |
Ac-FKC(StBu)AC(SH)-CBT |
Chlorin e6 |
Nanoparticle |
H-29 human colorectal adenicarcinoma cells |
CtsB induced NPs self-assembly, resulting in an increased singlet oxygen generation and a significant enhancement of the photodynamic effect |
[104] |
FFKF |
Doxorubicin |
Self-assembled nanoparticle |
Tumor lysates |
A library of FFKF peptides with various N-terminal capping groups was studied, and their self-assembly and sensitivity to cathepsin B and L were analyzed. Cbz-FFKF-OH showed the highest potential and a release of 92% of DOX within 8 h |
[108] |
VA |
Duocarmycin and pyrrolobenzodiazepine |
ADC |
Hepatocellular carcinoma |
Using dipeptides, VC and VA, conjugating Duocarmycin SA and PBD dimers to antibodies targeting GPC3 on hepatocellular carcinoma cells advances in liver cancer therapy were achieved |
[129] |
VC |
(Pyrrolo [2,1-c][1,4]benzodiazepine dimer) |
ADC |
BT474 carcinoma cells, KPL-4 breast cancer cells and BJAM lymphoma cells |
The targeting agent used is of more importance for the effectiveness of ADC than the efficiency of linker cleavage |
[95] |
VC |
Auristatin-based |
ADC |
Expi293 cells |
Carboxylesterase 1C was identified as the enzyme responsible for the plasmatic hydrolysis of (VC-PABC)-based linkers |
[130] |
VC |
Monomethyl auristatin E |
Nanoparticle |
U87 glioblastoma cells |
The system provided efficient cellular uptake and high toxic effect on glioblastoma cells. The maximal anticancer effect was achieved using NPs and radiotherapy |
[110] |
GFLG |
Anti-VEGF siRNA |
Nanoparticle |
HeLa cells |
Efficient siRNA release and VEGF deregulation in HeLa cells were achieved |
[111] |
GFLG |
Doxorubicin |
Functionalized nanoparticle |
HeLa cells |
80% of DOX release was observed in 24 h in the presence of CtsB |
[131] |
GIVRAKEAEGIVRAK |
Safranin O or DOX |
Functionalized nanoparticle |
Hela cells |
A 5-fold increase in the release of Safarin O was observed in the presence of lysosomal extract, leading to a CtsB-dependent cytotoxic effect |
[132] |
PGFK |
Doxorubicin |
Functionalized nanoparticle |
A549 human non-small cell lung cancer cells, NIH-3T3 mouse fibroblast cells, A2780 human ovarian cancer cells |
At acidic pH, CtsB led to a four-fold increase in DOX release and consequent higher toxicity |
[133] |
GFLG |
Doxorubicin |
Functionalized nanoparticle |
MCF-7 human breast cancer cell |
The nanoparticles represented a promising system to overcome MDR phenomena |
[134] |
GFLGC |
Docetaxel |
Functionalized nanoparticle |
HeLa and MCF-7 breast cells |
The systems showed higher circulation properties, efficacy and safety |
[120] |
DEGFLGED |
Resveratrol |
Functionalized nanoparticle |
MCF-7 breast cells |
Anticancer activity exceeded 80% |
[121] |
VA |
SG3199 |
ADC |
B-cell non-Hodgkin Lymphoma |
Zynlonta® is FDA-approved ADC for the treatment of large B-cell lymphoma (USA) |
|
VC |
Monomethyl auristatin E |
ADC |
Hodgkin lymphoma |
Adcetris® was approved ADC for the treatment of Hodgkin lymphoma (USA) |
|
VC |
Monomethyl auristatin E |
ADC |
Large B-cell lymphoma |
Polivy® was approved for the treatment of large B-cell lymphoma (USA) |
[139] |
VC |
Monomethyl auristatin E |
ADC |
Metastatic urothelial cancer |
Padcev® was approved for the treatment of metastatic urothelial cancer (USA) |
[140] |
VC |
Monomethyl auristatin E |
ADC |
Metastatic cervical cancer |
Tivdak® was approved for the treatment of metastatic cervical cancer (USA) |
[82] |
VC |
Monomethyl auristatin E |
ADC |
HER-2 positive solid tumors |
RC-48® was approved for the treatment of metastatic cervical cancer (China) |
[83] |
VA |
SGD-1882 |
ADC |
Positive acute myeloid leukemia |
Clinical trials were stopped because of severe adverse events and increased patient mortality |
[85] |
VA |
SGD-1882 |
ADC |
Non-Hodgkin Lymphoma and Renal Cell Carcinoma |
Clinical trials were stopped because of severe adverse events and increased patient mortality |
[86] |
VA |
SG3199 |
ADC |
Large B-cell lymphoma |
Phase 2 of clinical trials of ADCT-402 (Loncastuximab Tesirine), NCT05296070 NCT05249959 |
[141] |
VA |
SG3199 |
ADC |
Hodgkin lymphoma |
Phase 2 of clinical trials of Camidanlumab tesirine NCT04052997 |
[90] |
Here, the researchers have highlighted the potential of developing CtsB-cleavable technologies for targeting cancer cells and tumor microenvironments. Although ADCs have shown superior results in terms of translational purposes, the use of nanosystems may expand the portfolio of possible therapies, such as photodynamic and photothermal therapy. However, although CtsB-cleavable conjugates, nanoparticles, ADCs and surface modifications are extensively studied in preclinical and even clinical settings, more work is necessary to define the specificity of these systems, since cathepsin proteases are numerous and have redundant activity. Additionally, the malignant properties of cancer cells must be identified as a function of CtsB overexpression, even though this enzyme is ubiquitous and expressed at varying levels in all cancer cells. Finally, for future clinical translation purposes, these systems need to be simplified in their synthesis as they often consist of various components. Fine control of the manufacturing process may be challenging, particularly for large-scale production.