Targeting an MMP-9-activated prodrug to multiple myeloma-diseased bone marrow: a proof of principle in the 5T33MM mouse model

Recent Advances in the Enzyme-Activatable Organic Fluorescent Probes for Tumor Imaging and Therapy

The exploration of advanced probes for cancer diagnosis and treatment is of high importance in fundamental research and clinical practice. In comparison with the traditional "always-on" probes, the emerging activatable probes enjoy advantages in promoted accuracy for tumor theranostics by specifically releasing or activating fluorophores at the targeting sites. The main designing principle for these probes is to incorporate responsive groups that can specifically react with the biomarkers (e. g., enzymes) involved in tumorigenesis and progression, realizing the controlled activation in tumors. In this review, we summarize the latest advances in the molecular design and biomedical application of enzyme-responsive organic fluorescent probes. Particularly, the fluorophores can be endowed with ability of generating reactive oxygen species (ROS) to afford the photosensitizers, highlighting the potential of these probes in simultaneous tumor imaging and therapy with rational design. We hope that this review could inspire more research interests in the development of tumor-targeting theranostic probes for advanced biological studies.

Bio-vehicles of cytotoxic drugs for delivery to tumor specific targets for cancer precision therapy

Abnormal structural and molecular changes in malignant tissues were thoroughly investigated and utilized to target tumor cells, hence rescuing normal healthy tissues and lowering the unwanted side effects as non-specific cytotoxicity. Various ligands for cancer cell specific markers have been uncovered and inspected for directional delivery of the anti-cancer drug to the tumor site, in addition to diagnostic applications. Over the past few decades research related to the ligand targeted therapy (LTT) increased tremendously aiming to treat various pathologies, mainly cancers with well exclusive markers. Malignant tumors are known to induce elevated levels of a variety of proteins and peptides known as cancer "markers" as certain antigens (e.g., Prostate specific membrane antigen "PSMA", carcinoembryonic antigen "CEA"), receptors (folate receptor, somatostatin receptor), integrins (Integrin αvβ3) and cluster of differentiation molecules (CD13). The choice of an appropriate marker to be targeted and the design of effective ligand-drug conjugate all has to be carefully selected to generate the required therapeutic effect. Moreover, since some tumors express aberrantly high levels of more than one marker, some approaches investigated targeting cancer cells with more than one ligand (dual or multi targeting). We aim in this review to report an update on the cancer-specific receptors and the vehicles to deliver cytotoxic drugs, including recent advancements on nano delivery systems and their implementation in targeted cancer therapy. We will discuss the advantages and limitations facing this approach and possible solutions to mitigate these obstacles. To achieve the said aim a literature search in electronic data bases (PubMed and others) using keywords "Cancer specific receptors, cancer specific antibody, tumor specific peptide carriers, cancer overexpressed proteins, gold nanotechnology and gold nanoparticles in cancer treatment" was carried out.

Stimulus-cleavable chemistry in the field of controlled drug delivery

This review comprehensively summarises stimulus-cleavable linkers from various research areas and their cleavage mechanisms, thus provides an insightful guideline to extend their potential applications to controlled drug release from nanomaterials.

Plasma Cell Leukemia - Facts and Controversies: More Questions than Answers?

Plasma cell leukemia (PCL) is an aggressive hematological malignancy characterized by an uncontrolled clonal proliferation of plasma cells (PCs) in the bone marrow and peripheral blood. PCL has been defined by an absolute number of circulating PCs exceeding 2.0 × 10⁹/L and/or >20% PCs in the total leucocyte count. It is classified as primary PCL, which develops de novo, and secondary PCL, occurring at the late and advanced stages of multiple myeloma (MM). Primary and secondary PCL are clinically and biologically two distinct entities. After the diagnosis, treatment should be immediate and should include a proteasome inhibitor and immunomodulator-based combination regimens as induction, followed by stem cell transplantation (SCT) in transplant-eligible individuals who have cleared the peripheral blood of circulating PCs. Due to the rarity of the condition, there have been very few clinical trials. Furthermore, virtually all of the myeloma trials exclude patients with active PCL. The evaluation of response has been defined by the International Myeloma Working Group and consists of both acute leukemia and MM criteria. With conventional chemotherapy, the prognosis of primary PCL has been ominous, with reported overall survival (OS) ranging from 6.8 to 12.6 months. The use of novel agents and autologous SCT appears to be associated with deeper response and an improved survival, although it still remains low. The PCL prognostic index provides a simple score to risk-stratify PCL. The prognosis of secondary PCL is extremely poor, with OS of only 1 month.

Design of a New Peptide Substrate Probe of the Putative Biomarker Legumain with Potential Application in Prostate Cancer Diagnosis Ex Vivo

The lysosomal endoprotease legumain (asparaginyl endoprotease) has been proposed as a putative biomarker in prostate tumours, in which the enzyme is markedly overexpressed. Overexpression, coupled with highly selective specificity for cleavage of substrates at the C-terminus of asparagine (Asn) residues, make legumain an attractive biochemical target for potential diagnosis, prognosis and treatment. We report the design, synthesis, characterisation and preliminary evaluation of a new rhodamine-B (Rho-B)-labelled legumain peptide substrate probe5[Rho-Pro-Ala-Asn-PEG-AQ(4-OH)] and its selective targeting to lysosomes in PC3 prostate cancer cells. Probe5was efficiently activated by recombinant human legumain to afford the high quantum yield reporter fluorophore tripeptide4b(Rho-Pro-Ala-Asn-OH) with concomitant release of intense fluorescence. Furthermore, probe5was activated upon incubation with homogenates derived from fresh-frozen tissue material of prostatectomy specimens. Probe5represents a new viable biochemical tool for probing the activity of legumain with the potential to be used in ex vivo diagnostics in the cancer pathology laboratory.

pH- and enzyme-triggered drug release as an important process in the design of anti-tumor drug delivery systems

It is necessary to design a reasonable drug delivery system(DDS) for targeted release to overcome the potential toxicity and poor selectivity of anti-tumor drug. How a drug is released from a DDS is a critical issue that determines whether the DDS is designed successfully. We all know that the microenvironment of tumors is quite different from normal tissues, such as its acidic environment, different expression levels of some enzymes, etc. These features are widely used in the design of DDSs and play an important role in the drug release process in vivo. Numerous DDSs have been designed and synthesized. This article attention to how drugs are released from DDSs. We summarizes and classify the characteristic enzymes and chemical bonds used in the drug release process by browsing a large number of papers, and describes how they are applied in DDSs with specific examples. By understanding these acid-sensitive chemical bonds and over-expressed enzymes in tumors, different DDSs can be designed for different drug structures to solve specific problems of anti-tumor drugs.

Improved Protease-Targeting and Biopharmaceutical Properties of Novel Prodrugs of Ganciclovir

The prodrug strategy has been frequently employed as a chemical approach for overcoming the disadvantages of existing parent drugs. In this report, we synthesized four monoester prodrugs of ganciclovir, an anticytomegalovirus drug, and demonstrated their potential advantages in protease-targeted activation and biopharmaceutical profiles over the parent compound. We demonstrated that these four prodrugs of ganciclovir, i.e., N-benzyloxycarbonyl-(L)-alanine-ganciclovir (CbzAlaGCV), N-benzyloxycarbonyl-(α,l)-aminobutyric acid-ganciclovir (CbzAbuGCV), N-acetyl-(l)-phenylalanine-(l)-alanine-ganciclovir (AcPheAlaGCV), and N-acetyl-(l)-phenylalanine-(α,l)-aminobutyric acid-ganciclovir (AcPheAbuGCV), are hydrolytically activated by the protease of human cytomegalovirus (hCMV), a serine protease that possesses intrinsic esterase activities. CbzAlaGCV and AcPheAlaGCV were found to be activated at a higher rate by the hCMV protease than CbzAbuGCV and AcPheAbuGCV. These ganciclovir prodrugs could potentially be targeted to selective activation by the hCMV protease which is only present at the viral infection sites, thereby achieving higher efficacy and lower systemic toxicity. The tissue stability, cellular uptake, and trans-epithelial transport of these ganciclovir prodrugs were also characterized. The N-acetylated dipeptide prodrugs of ganciclovir were found to be generally more stable than Cbz-amino acid prodrugs in various tissue matrices. Among the four prodrug candidates, AcPheAbuGCV was the most stable in human cell homogenates, plasma, and pooled liver microsomes. AcPheAbuGCV also possessed a superior cellular uptake profile and permeability across epithelial cell monolayers. Since the targeting and selective activation of a prodrug is determined by not only its rate of hydrolysis catalyzed by the hCMV protease target but also its biopharmaceutical properties, i.e., oral absorption and systemic availability, AcPheAbuGCV is considered the best overall candidate among the four ganciclovir prodrugs for further research and development for treatment of hCMV infection.

Matrix Metalloproteinases: A challenging paradigm of cancer management

Matrix metalloproteinases (MMPs) are members of zinc-dependent endopeptidases implicated in a variety of physiological and pathological processes. Over the decades, MMPs have been studied for their role in cancer progression, migration, and metastasis. As a result, accumulated evidence of MMPs incriminating role has made them an attractive therapeutic target. Early generations of broad-spectrum MMP inhibitors exhibited potent inhibitory activities, which subsequently led to clinical trials. Unexpectedly, these trials failed to meet the desired goals, mainly due to the lack of efficacy, poor oral bioavailability, and toxicity. In this review, we discuss the regulatory role of MMPs in cancer progression, current strategies in targeting MMPs for cancer treatment including prodrug design and tumor imaging, and therapeutic value of MMPs as biomarkers in breast, lung, and prostate cancers.

Secreted Matrix Metalloproteinase-9 of Proliferating Smooth Muscle Cells as a Trigger for Drug Release from Stent Surface Polymers in Coronary Arteries

Cardiovascular diseases are the leading causes of death in industrialized countries. Atherosclerotic coronary arteries are commonly treated with percutaneous transluminal coronary intervention followed by stent deployment. This treatment has significantly improved the clinical outcome. However, triggered vascular smooth muscle cell (SMC) proliferation leads to in-stent restenosis in bare metal stents. In addition, stent thrombosis is a severe side effect of drug eluting stents due to inhibition of endothelialization. The aim of this study was to develop and test a stent surface polymer, where cytotoxic drugs are covalently conjugated to the surface and released by proteases selectively secreted by proliferating smooth muscle cells. Resting and proliferating human coronary artery smooth muscle cells (HCASMC) and endothelial cells (HCAEC) were screened to identify an enzyme exclusively released by proliferating HCASMC. Expression analyses and enzyme activity assays verified selective and exclusive activity of the matrix metalloproteinase-9 (MMP-9) in proliferating HCASMC. The principle of drug release exclusively triggered by proliferating HCASMC was tested using the biodegradable stent surface polymer poly-l-lactic acid (PLLA) and the MMP-9 cleavable peptide linkers named SRL and AVR. The specific peptide cleavage by MMP-9 was verified by attachment of the model compound fluorescein. Fluorescein release was observed in the presence of MMP-9 secreting HCASMC but not of proliferating HCAEC. Our findings suggest that cytotoxic drug conjugated polymers can be designed to selectively release the attached compound triggered by MMP-9 secreting smooth muscle cells. This novel concept may be beneficial for stent endothelialization thereby reducing the risk of restenosis and thrombosis.

New approaches to selectively target cancer-associated matrix metalloproteinase activity

Heightened matrix metalloproteinase (MMP) activity has been noted in the context of the tumor microenvironment for many years, and causal roles for MMPs have been defined across the spectrum of cancer progression. This is primarily due to the ability of the MMPs to process extracellular matrix (ECM) components and to regulate the bioavailability/activity of a large repertoire of cytokines and growth factors. These characteristics made MMPs an attractive target for therapeutic intervention but notably clinical trials performed in the 1990s did not fulfill the promise of preclinical studies. The reason for the failure of early MMP inhibitor (MMPI) clinical trials that are multifold but arguably principal among them was the inability of early MMP-based inhibitors to selectively target individual MMPs and to distinguish between MMPs and other members of the metzincin family. In the decades that have followed the MMP inhibitor trials, innovations in chemical design, antibody-based strategies, and nanotechnologies have greatly enhanced our ability to specifically target and measure the activity of MMPs. These advances provide us with the opportunity to generate new lines of highly selective MMPIs that will not only extend the overall survival of cancer patients, but will also afford us the ability to utilize heightened MMP activity in the tumor microenvironment as a means by which to deliver MMPIs or MMP activatable prodrugs.

Moving targets: Emerging roles for MMPs in cancer progression and metastasis

Matrix metalloproteinases have long been associated with cancer. Clinical trials of small molecule inhibitors for this family of enzymes however, were spectacularly unsuccessful in a variety of tumor types. Here, we discuss some of the newer roles that have been uncovered for MMPs in cancer that would not have been targeted with those initial inhibitors or in the patient populations analyzed. We also consider novel ways of using cancer-associated MMP activity for clinical benefit.

Myeloma as a model for the process of metastasis: implications for therapy

Multiple myeloma (MM) is a plasma cell dyscrasia characterized by the presence of multiple myelomatous "omas" throughout the skeleton, indicating that there is continuous trafficking of tumor cells to multiple areas in the bone marrow niches. MM may therefore represent one of the best models to study cell trafficking or cell metastasis. The process of cell metastasis is described as a multistep process, the invasion-metastasis cascade. This involves cell invasion, intravasation into nearby blood vessels, passage into the circulation, followed by homing into predetermined distant tissues, the formation of new foci of micrometastases, and finally the growth of micrometastasis into macroscopic tumors. This review discusses the significant advances that have been discovered in the complex process of invasion-metastasis in epithelial carcinomas and cell trafficking in hematopoietic stem cells and how this process relates to progression in MM. This progression is mediated by clonal intrinsic factors that mediate tumor invasiveness as well as factors present in the tumor microenvironment that are permissive to oncogenic proliferation. Therapeutic agents that target the different steps of cell dissemination and progression are discussed. Despite the significant advances in the treatment of MM, better therapeutic agents that target this metastatic cascade are urgently needed.

Layer-by-layer nanoparticles with a pH-sheddable layer for in vivo targeting of tumor hypoxia

Inspired by the simplicity and versatility of layer-by-layer (LbL) assembly, we applied multilayered polyelectrolyte assemblies on nanoparticles to create viable systemic delivery systems. Focusing on tumor-specific delivery, LbL nanoparticles that exhibit a pH-sensitive outer stealth layer are demonstrated to target and be retained in hypoxic tumor regions. The neutral layers shed in response to acidity to reveal a charged nanoparticle surface that is readily taken up by tumor cells. The first in vivo demonstration of this mechanism of targeting is presented, as well as an initial examination of the mechanism of uptake of the nanoparticles. We further demonstrate that this concept for tumor targeting is potentially valid for a broad range of cancers, with applicability for therapies that target hypoxic tumor tissue.

A murine model of myeloma that allows genetic manipulation of the host microenvironment

Multiple myeloma, and the associated osteolytic bone disease, is highly dependent upon cellular interactions within the bone marrow microenvironment. A major limitation of existing myeloma models is the requirement for a specific host strain of mouse, preventing molecular examination of the bone marrow microenvironment. The aim of the current study was to develop a model of myeloma in which the host microenvironment could be modified genetically. The Radl 5T murine model of myeloma is well characterized and closely mimics human myeloma. In the current study, we demonstrate 5T myeloma establishment in recombination activating gene 2 (RAG-2)-deficient mice, which have improper B- and T-cell development. Importantly, these mice can be easily bred with genetically modified mice to generate double knockout mice, allowing manipulation of the host microenvironment at a molecular level. Inoculation of 5TGM1 myeloma cells into RAG-2(-/-) mice resulted in myeloma development, which was associated with tumor growth within bone and an osteolytic bone disease, as assessed by microcomputed tomography (microCT), histology and histomorphometry. Myeloma-bearing RAG-2(-/-) mice displayed many features that were similar to both human myeloma and the original Radl 5T model. To demonstrate the use of this model, we have examined the effect of host-derived matrix metalloproteinase 9 (MMP-9) in the development of myeloma in vivo. Inoculation of 5TGM1 myeloma cells into mice that are deficient in RAG-2 and MMP-9 resulted in a reduction in both tumor burden and osteolytic bone disease when compared with RAG-2-deficient wild-type myeloma-bearing mice. The establishment of myeloma in RAG-2(-/-) mice permits molecular examination of the host contribution to myeloma pathogenesis in vivo.

Tumour endoproteases: the cutting edge of cancer drug delivery?

Despite progression in anticancer drug development and improvements in the clinical utilization of therapies, current treatment regimes are still dependent upon the use of systemic antiproliferative cytotoxic agents. Although these agents are unquestionably potent, their efficacy is limited by toxicity towards 'normal' cells and a lack of tumour selective targeting, resulting in a therapeutic index which is modest at best. Consequently, the development of more tumour selective cancer treatments, with better discrimination between tumour and normal cells is unequivocally an important goal for cancer drug discovery. One such strategy is to exploit the tumour phenotype as a mechanism for tumour-selective delivery of potent therapeutics. An exciting approach in this area is to develop anticancer therapeutics as prodrugs, which are non-toxic until activated by enzymes localized specifically in the tumour. Enzymes suitable for tumour-activated prodrug development must have increased activity in the tumour relative to non-diseased tissue and an ability to activate the prodrug to its active form. One class of enzyme satisfying these criteria are the tumour endoproteases, particularly the serine- and metallo-proteases. These proteolytic enzymes are essential for tumour angiogenesis, invasion and metastasis, the major defining features of malignancy. This review describes the concept behind development of tumour-endoprotease activated prodrugs and discusses the various studies to date that have demonstrated the huge potential of this approach for improvement of cancer therapy.

Protease-mediated arsenic prodrug strategy in cancer and infectious diseases: a hypothesis for targeted activation

A strategy for the selective in vivo activation of prodrugs by proteases is presented. The approach is based on the design of polythiol peptides able to neutralize the toxicity of As(III) through chelation, and contemporarily to be recognized as substrates of a disease-linked specific protease. Enzyme digestion implies conversion of such polythiol peptides into monothiol fragments with irreversible loss of the ability to chelate the metalloid, thus triggering the release in its free and pharmacologically effective form. The proteases whose activity appears dramatically up-regulated in various pathologies, ranging from cancer to infectious diseases, can be conveniently employed as prodrug activators in the disease microenvironment. The design of the representative peptide shown here has been assisted by molecular modeling in order to fulfill the dual characteristic to be an efficient As(III) chelator and simultaneously a substrate of the matrix metalloproteinase-9 (MMP-9) whose activity results dramatically increased at the surface of cells affected by several pathologies.

Matrix metalloproteases: underutilized targets for drug delivery

Pathophysiological molecules in the extracellular environment offer excellent targets that can be exploited for designing drug targeting systems. Matrix metalloproteases (MMPs) are a family of extracellular proteolytic enzymes that are characterized by their overexpression or overactivity in several pathologies. Over the last two decades, the MMP literature reveals heightened interest in the research involving MMP biology, pathology and targeting. This review describes various strategies that have been designed to utilize MMPs for targeting therapeutic entities. Key factors that need to be considered in the successful design of such systems have been identified based on the analyses of these strategies. Development of targeted drug delivery using MMPs has been steadily pursued; however, drug delivery efforts using these targets need to be intensified and focused to realize the clinical application of the fast developing fundamental MMP research.

The role of DNA hypomethylation in the control of stromelysin gene expression

Genome-wide DNA hypomethylation is a critical mechanism underlying neoplastic transformation. Thus, genes that are suppressed in normal tissues or in specific cell types may become aberrantly expressed in neoplasia. To determine whether DNA methylation can modulate matrix metalloproteinase (mmp) gene expression, we have used a genetically engineered cell line in which both key DNA methyltransferase genes, Dnmt-1 and Dnmt-3b, were removed by homologous recombination. We found that cells bearing a dual knock-out of both Dnmt-1 and Dnmt-3b genes induced de novo expression of mmp-3 gene, but not that of mmp-1 and mmp-2. Furthermore, treatment of the wild-type cells with DNA methylase inhibitors 5-aza-dC and zebularine also induced mmp-3 gene expression. On the other hand, in vitro methylation of the mmp-3 promoter suppressed its transcriptional activity. Finally, we found that induction of mmp-3 and mmp-10 gene expression by hypomethylation was cell-specific, suggesting that epigenetic changes may predispose cells to express stromelysin genes.

Importance of the bone marrow microenvironment in inducing the angiogenic response in multiple myeloma

Tumor microenvironment is essential for tumor cell proliferation, angiogenesis, invasion and metastasis through its provision of survival signals, secretion of growth and pro-angiogenic factors, and direct adhesion molecule interactions. This review examines its importance in the induction of an angiogenic response in multiple myeloma (MM). The encouraging results of preclinical and clinical trials in which MM has been treated by targeting the tumor microenvironment are also discussed.

Inflammation dampened by gelatinase A cleavage of monocyte chemoattractant protein-3

Tissue degradation by the matrix metalloproteinase gelatinase A is pivotal to inflammation and metastases. Recognizing the catalytic importance of substrate-binding exosites outside the catalytic domain, we screened for extracellular substrates using the gelatinase A hemopexin domain as bait in the yeast two-hybrid system. Monocyte chemoattractant protein-3 (MCP-3) was identified as a physiological substrate of gelatinase A. Cleaved MCP-3 binds to CC-chemokine receptors-1, -2, and -3, but no longer induces calcium fluxes or promotes chemotaxis, and instead acts as a general chemokine antagonist that dampens inflammation. This suggests that matrix metalloproteinases are both effectors and regulators of the inflammatory response.