β-Glucuronidase-responsive prodrugs for selective cancer chemotherapy: an update

A first-in-class β-glucuronidase responsive conjugate for selective dual targeted and photodynamic therapy of bladder cancer

In this report, we present a novel prodrug strategy that can significantly improve the efficiency and selectivity of combined therapy for bladder cancer. Our approach involved the synthesis of a conjugate based on a chlorin-e6 photosensitizer and a derivative of the tyrosine kinase inhibitor cabozantinib, linked by a β-glucuronidase-responsive linker. Upon activation by β-glucuronidase, which is overproduced in various tumors and localized in lysosomes, this conjugate released both therapeutic modules within targeted cells. This activation was accompanied by the recovery of its fluorescence and the generation of reactive oxygen species. Investigation of photodynamic and dark toxicity in vitro revealed that the novel conjugate had an excellent safety profile and was able to inhibit tumor cells proliferation at submicromolar concentrations. Additionally, combined therapy effects were also observed in 3D models of tumor growth, demonstrating synergistic suppression through the activation of both photodynamic and targeted therapy.

Targeted Glioma Therapy-Clinical Trials and Future Directions

Glioblastoma multiforme (GBM) is the most common type of glioma, with a median survival of 14.6 months post-diagnosis. Understanding the molecular profile of such tumors allowed the development of specific targeted therapies toward GBM, with a major role attributed to tyrosine kinase receptor inhibitors and immune checkpoint inhibitors. Targeted therapeutics are drugs that work by specific binding to GBM-specific or overexpressed markers on the tumor cellular surface and therefore contain a recognition moiety linked to a cytotoxic agent, which produces an antiproliferative effect. In this review, we have summarized the available information on the targeted therapeutics used in clinical trials of GBM and summarized current obstacles and advances in targeted therapy concerning specific targets present in GBM tumor cells, outlined efficacy endpoints for major classes of investigational drugs, and discussed promising strategies towards an increase in drug efficacy in GBM.

Antibody-drug conjugates in cancer therapy: innovations, challenges, and future directions

The emergence of antibody-drug conjugates (ADCs) as a potential therapeutic avenue in cancer treatment has garnered significant attention. By combining the selective specificity of monoclonal antibodies with the cytotoxicity of drug molecules, ADCs aim to increase the therapeutic index, selectively targeting cancer cells while minimizing systemic toxicity. Various ADCs have been licensed for clinical usage, with ongoing research paving the way for additional options. However, the manufacture of ADCs faces several challenges. These include identifying suitable target antigens, enhancing antibodies, linkers, and payloads, and managing resistance mechanisms and side effects. This review focuses on the strategies to overcome these hurdles, such as site-specific conjugation techniques, novel antibody formats, and combination therapy. Our focus lies on current advancements in antibody engineering, linker technology, and cytotoxic payloads while addressing the challenges associated with ADC development. Furthermore, we explore the future potential of personalized medicine, leveraging individual patients' molecular profiles, to propel ADC treatments forward. As our understanding of the molecular mechanisms driving cancer progression continues to expand, we anticipate the development of new ADCs that offer more effective and personalized therapeutic options for cancer patients.

Combination of Targeted Therapies for Colorectal Cancer Treatment

The design of innovative therapeutic strategies enabling the selective destruction of tumor cells while sparing healthy tissues remains highly challenging in cancer therapy. Here, we show that the combination of two targeted therapies, including bevacizumab (Bev), and a β-glucuronidase-responsive albumin-binding prodrug of monomethyl auristatin E (MMAE), is efficient for the treatment of colorectal cancer implanted in mice. This combined therapy produces a therapeutic activity superior to that of the association of FOLFOX and Bev currently used to treat patients with this pathology. The increased anticancer efficacy is due to either a synergistic or an additive effect between Bev and MMAE selectively released from the glucuronide prodrug in the tumor microenvironment. Since numerous drug delivery systems such as antibody-drug conjugates employ MMAE as a cytotoxic payload, this finding may be of great interest for improving their therapeutic index by combining them with Bev, particularly for the therapy of colorectal cancer.

Tethered Indoxyl-Glucuronides for Enzymatically Triggered Cross-Linking

Indoxyl-glucuronides, upon treatment with β-glucuronidase under physiological conditions, are well known to afford the corresponding indigoid dye via oxidative dimerization. Here, seven indoxyl-glucuronide target compounds have been prepared along with 22 intermediates. Of the target compounds, four contain a conjugatable handle (azido-PEG, hydroxy-PEG, or BCN) attached to the indoxyl moiety, while three are isomers that include a PEG-ethynyl group at the 5-, 6-, or 7-position. All seven target compounds have been examined in indigoid-forming reactions upon treatment with β-glucuronidase from two different sources and rat liver tritosomes. Taken together, the results suggest the utility of tethered indoxyl-glucuronides for use in bioconjugation chemistry with a chromogenic readout under physiological conditions.

Induced-volatolomics for the design of tumour activated therapy

The discovery of tumour-associated markers is of major interest for the development of selective cancer chemotherapy. Within this framework, we introduced the concept of induced-volatolomics enabling to monitor simultaneously the dysregulation of several tumour-associated enzymes in living mice or biopsies. This approach relies on the use of a cocktail of volatile organic compound (VOC)-based probes that are activated enzymatically for releasing the corresponding VOCs. Exogenous VOCs can then be detected in the breath of mice or in the headspace above solid biopsies as specific tracers of enzyme activities. Our induced-volatolomics modality highlighted that the up-regulation of N-acetylglucosaminidase was a hallmark of several solid tumours. Having identified this glycosidase as a potential target for cancer therapy, we designed an enzyme-responsive albumin-binding prodrug of the potent monomethyl auristatin E programmed for the selective release of the drug in the tumour microenvironment. This tumour activated therapy produced a remarkable therapeutic efficacy on orthotopic triple-negative mammary xenografts in mice, leading to the disappearance of tumours in 66% of treated animals. Thus, this study shows the potential of induced-volatolomics for the exploration of biological processes as well as the discovery of novel therapeutic strategies.

Evaluation of In Vivo Prepared Albumin-Drug Conjugate Using Immunoprecipitation Linked LC-MS Assay and Its Application to Mouse Pharmacokinetic Study

There have been many attempts in pharmaceutical industries and academia to improve the pharmacokinetic characteristics of anti-tumor small-molecule drugs by conjugating them with large molecules, such as monoclonal antibodies, called ADCs. In this context, albumin, one of the most abundant proteins in the blood, has also been proposed as a large molecule to be conjugated with anti-cancer small-molecule drugs. The half-life of albumin is 3 weeks in humans, and its distribution to tumors is higher than in normal tissues. However, few studies have been conducted for the in vivo prepared albumin-drug conjugates, possibly due to the lack of robust bioanalytical methods, which are critical for evaluating the ADME/PK properties of in vivo prepared albumin-drug conjugates. In this study, we developed a bioanalytical method of the albumin-conjugated MAC glucuronide phenol linked SN-38 ((2S,3S,4S,5R,6S)-6-(4-(((((((S)-4,11-diethyl-4-hydroxy-3,14-dioxo-3,4,12,14-tetrahydro-1H-pyrano [3',4':6,7] indolizino [1,2-b] quinolin-9-yl)oxy)methyl)(2 (methylsulfonyl)ethyl)carbamoyl)oxy)methyl)-2-(2-(3-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)-N-methylpropanamido)acetamido)phenoxy)-3,4,5-trihydroxytetra-hydro-2H-pyran-2-carboxylic acid) as a proof-of-concept. This method is based on immunoprecipitation using magnetic beads and the quantification of albumin-conjugated drug concentration using LC-qTOF/MS in mouse plasma. Finally, the developed method was applied to the in vivo intravenous (IV) mouse pharmacokinetic study of MAC glucuronide phenol-linked SN-38.

Knowledge atlas of antibody-drug conjugates on CiteSpace and clinical trial visualization analysis

Objective

Antibody-drugs conjugates (ADCs) are novel drugs with highly targeted and tumor-killing abilities and developing rapidly. This study aimed to evaluate drug discovery and clinical trials of and explore the hotspots and frontiers from 2012 to 2022 using bibliometric methods.

Methods

Publications on ADCs were retrieved between 2012 and 2022 from Web of Science (WoS) and analyzed with CiteSpace 6.1.R2 software for the time, region, journals, institutions, etc. Clinical trials were downloaded from clinical trial.org and visualized with Excel software.

Results

A total of 696 publications were obtained and 187 drug trials were retrieved. Since 2012, research on ADCs has increased year by year. Since 2020, ADC-related research has increased dramatically, with the number of relevant annual publications exceeding 100 for the first time. The United States is the most authoritative and superior country and region in the field of ADCs. The University of Texas MD Anderson Cancer Center is the most authoritative institution in this field. Research on ADCs includes two clinical trials and one review, which are the most influential references. Clinical trials of ADCs are currently focused on phase I and phase II. Comprehensive statistics and analysis of the published literature and clinical trials in the field of ADCs, have shown that the most studied drug is brentuximab vedotin (BV), the most popular target is human epidermal growth factor receptor 2 (HER2), and breast cancer may become the main trend and hotspot for ADCs indications in recent years.

Conclusion

Antibody-drug conjugates have become the focus of targeted therapies in the field of oncology. The innovation of technology and combination application strategy will become the main trend and hotspots in the future.

Drug conjugate-based anticancer therapy - Current status and perspectives

Drug conjugates are conjugates comprising a tumor-homing carrier tethered to a cytotoxic agent via a linker that are designed to deliver an ultra-toxic payload directly to the target cancer cells. This strategy has been successfully used to increase the therapeutic efficacy of cytotoxic agents and reduce their toxic side effects. Drug conjugates are being developed worldwide, with the potential to revolutionize current cancer treatment strategies. Antibody-drug conjugates (ADCs) have developed rapidly, and 14 of them have received market approval since the first approval event by the Food and Drug Administration in 2000. However, there are some limitations in the use of antibodies as carriers. Other classes of drug conjugates are emerging, such as targeted drugs conjugated with peptides (peptide-drug conjugates, PDCs) and polymers (polymer-drug conjugates, PolyDCs) with the remaining constructs similar to those of ADCs. These novel drug conjugates are gaining attention because they overcome the limitations of ADCs. This review summarizes the current state and advancements in knowledge regarding the design, constructs, and clinical efficacy of different drug conjugates.

Pharmacologic characterization of TBP1901, a prodrug form of aglycone curcumin, and CRISPR-Cas9 screen for therapeutic targets of aglycone curcumin

Curcumin (aglycone curcumin) has antitumor properties in a variety of malignancies via the alteration of multiple cancer-related biological pathways; however, its clinical application has been hampered due to its poor bioavailability. To overcome this limitation, we have developed a synthesized curcumin β-D-glucuronide sodium salt (TBP1901), a prodrug form of aglycone curcumin. In this study, we aimed to clarify the pharmacologic characteristics of TBP1901. In β-glucuronidase (GUSB)-proficient mice, both curcumin β-D-glucuronide and its active metabolite, aglycone curcumin, were detected in the blood after TBP1901 injection, whereas only curcumin β-D-glucuronide was detected in GUSB-impaired mice, suggesting that GUSB plays a pivotal role in the conversion of TBP1901 into aglycone curcumin in vivo. TBP1901 itself had minimal antitumor effects in vitro, whereas it demonstrated significant antitumor effects in vivo. Genome-wide clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 screen disclosed the genes associated with NF-κB signaling pathway and mitochondria were among the highest hit. In vitro, aglycone curcumin inhibited NF-kappa B signaling pathways whereas it caused production of reactive oxygen species (ROS). ROS scavenger, N-acetyl-L-cysteine, partially reversed antitumor effects of aglycone curcumin. In summary, TBP1901 can exert antitumor effects as a prodrug of aglycone curcumin through GUSB-dependent activation.

Heparosan-based self-assembled nanocarrier for zinc(II) phthalocyanine for use in photodynamic cancer therapy

Zinc(II) phthalocyanine (ZnPc) is a promising photosensitizer in photodynamic therapy (PDT) for melanoma treatment. However, the poor solubility of ZnPc limits its application. To overcome this limitation, heparosan (HP)-based nanoparticles were prepared by anchoring the l-lysine-linked α-linolenic acid branch to the carboxylic acid group to produce amphiphilic conjugates named heparosan with an l-lysine-linked α-linolenic acid branch (HLA). HLA conjugates could self-assemble into spherical nanoparticles in aqueous media and encapsulate ZnPc to form HLA-ZnPc nanoparticles. The cellular uptake of ZnPc could be improved by HLA carriers. These nanoparticles presented excellent photodynamic-mediated toxicity against mouse melanoma cells (B16) by markedly upregulating the intracellular reactive oxygen species (ROS) levels while showing no cytotoxicity to either B16 or normal cells (L02 and HK-2 cells) in the dark. Furthermore, HLA-ZnPc displayed excellent stability in both powder and Roswell Park Memorial Institute (RPMI) 1640 medium, indicating its promise for application in drug delivery and PDT. These results revealed a strategy for HP-based enhancement of ZnPc in PDT efficacy.

Polyphenolic promiscuity, inflammation-coupled selectivity: Whether PAINs filters mask an antiviral asset

The Covid-19 pandemic has elicited much laboratory and clinical research attention on vaccines, mAbs, and certain small-molecule antivirals against SARS-CoV-2 infection. By contrast, there has been comparatively little attention on plant-derived compounds, especially those that are understood to be safely ingested at common doses and are frequently consumed in the diet in herbs, spices, fruits and vegetables. Examining plant secondary metabolites, we review recent elucidations into the pharmacological activity of flavonoids and other polyphenolic compounds and also survey their putative frequent-hitter behavior. Polyphenols, like many drugs, are glucuronidated post-ingestion. In an inflammatory milieu such as infection, a reversion back to the active aglycone by the release of β-glucuronidase from neutrophils and macrophages allows cellular entry of the aglycone. In the context of viral infection, virions and intracellular virus particles may be exposed to promiscuous binding by the polyphenol aglycones resulting in viral inhibition. As the mechanism's scope would apply to the diverse range of virus species that elicit inflammation in infected hosts, we highlight pre-clinical studies of polyphenol aglycones, such as luteolin, isoginkgetin, quercetin, quercetagetin, baicalein, curcumin, fisetin and hesperetin that reduce virion replication spanning multiple distinct virus genera. It is hoped that greater awareness of the potential spatial selectivity of polyphenolic activation to sites of pathogenic infection will spur renewed research and clinical attention for natural products antiviral assaying and trialing over a wide array of infectious viral diseases.

Urolithin A targets the AKT/WNK1 axis to induce autophagy and exert anti-tumor effects in cholangiocarcinoma

Urolithin A (UA; 3,8-dihydroxybenzo[c]chromen-6-one), a metabolite generated by intestinal bacteria during the biotransformation of ellagitannins, has gained considerable attention in treating several cancers. Cholangiocarcinoma (CCA) remains one of the most lethal cancers; it grows in a special environment constantly exposed to both blood and bile. Since UA is known to undergo enterohepatic recirculation, we hypothesized that UA might have significant antitumor effects in CCA. Here, we investigated the therapeutic potential of UA in CCA and aimed to elucidate its mechanisms, including autophagy. UA treatment inhibited cell proliferation and induced G2/M phase cell cycle arrest in CCA cells. UA also suppressed cell migration and invasion, but did not cause apoptosis. Furthermore, Western blotting and immunocytochemistry demonstrated increased LC3-II accumulation, while electron microscopy demonstrated induced autophagosomes after UA treatment, suggesting that UA upregulated autophagy in CCA cells. In xenograft mice treated with UA, tumor growth was inhibited with increased LC3-II levels. On the other hand, phospho-kinase array demonstrated downregulation of the AKT/WNK1 pathway. LC3-II expression was elevated in WNK1 knocked down cells, indicating that WNK1 is the key signal for regulating autophagy. Thus, UA exerted antitumor effects by suppressing the AKT/WNK1 signaling pathway and inducing autophagy. In conclusion, UA, a natural, well-tolerated compound, may be a promising therapeutic candidate for advanced CCA.

Bambus[4,6]urils as Dual Scaffolds for Multivalent Iminosugar Presentation and Ion Transport: Access to Unprecedented Glycosidase-Directed Anion Caging Agents

Bambusurils, BU[4] and BU[6], were used for the first time as multivalent scaffolds to link glycosidases inhibitors derived from 1-deoxynojirimycin (DNJ). Two linear DNJ ligands having six or nine carbon alkyl azido linkers or a trivalent DNJ dendron were grafted onto octapropargylated BU[4] and dodecapropargylated BU[6] using copper-catalyzed cycloaddition (CuAAC) to yield corresponding neoglycobambus[4] and neoglycobambus[6]urils bearing 8 to 24 iminosugars. The inhibition potencies of neoglycoBU[4], neoglycoBU[6] and neoglycoBU[6] caging anions were evaluated against Jack Bean α-mannosidase and compared to monovalent DNJ derivatives. Strong affinity enhancements per inhibitory head were obtained for the clusters holding trivalent dendrons with inhibitory constants in the nanomolar range (K_i = 24 nM for BU[4] with 24 DNJ units). Interestingly, the anion (bromide or iodide) encapsulated inside the cavity of BU[6] does not modify the inhibition potency of neoglycoBU[6], opening the way to water-soluble glycosidase-directed anion caging agents that may find applications in important fields such as bio(in)organic chemistry or oncology.

Increased/Targeted Brain (Pro)Drug Delivery via Utilization of Solute Carriers (SLCs)

Membrane transporters have a crucial role in compounds’ brain drug delivery. They allow not only the penetration of a wide variety of different compounds to cross the endothelial cells of the blood–brain barrier (BBB), but also the accumulation of them into the brain parenchymal cells. Solute carriers (SLCs), with nearly 500 family members, are the largest group of membrane transporters. Unfortunately, not all SLCs are fully characterized and used in rational drug design. However, if the structural features for transporter interactions (binding and translocation) are known, a prodrug approach can be utilized to temporarily change the pharmacokinetics and brain delivery properties of almost any compound. In this review, main transporter subtypes that are participating in brain drug disposition or have been used to improve brain drug delivery across the BBB via the prodrug approach, are introduced. Moreover, the ability of selected transporters to be utilized in intrabrain drug delivery is discussed. Thus, this comprehensive review will give insights into the methods, such as computational drug design, that should be utilized more effectively to understand the detailed transport mechanisms. Moreover, factors, such as transporter expression modulation pathways in diseases that should be taken into account in rational (pro)drug development, are considered to achieve successful clinical applications in the future.

Targeting the tumor microenvironment by an enzyme-responsive prodrug of tubulin destabilizer for triple-negative breast cancer therapy with high safety

In response to the long-term potential toxicity concerns of tubulin destabilizer, an enzyme-responsive prodrug therapy for triple-negative breast cancer was developed based on the different β-glucuronidase levels between tumor and normal tissues in this study. All the prodrugs synthesized herein showed remarkable stability in phosphate buffer and bovine serum solution, among which 17a was found to be more susceptible to enzymatic cleavage. 17a exhibited excellent selectivity between the in vitro antiproliferative activities against β-glucuronidase-pretreated and -untreated cancer cells (IC₅₀ (+Enz) = 8.9-15.7 nM, IC₅₀ (-Enz) > 50 μM), along with favorable liver microsomal metabolic stability and improved aqueous solubility. Furthermore, as a candidate prodrug 17a showed potent antitumor efficacy in MDA-MB-231 xenograft mouse model without causing perceptible injury to organs. Importantly, 17a exhibited superior safety profiles with higher LD₅₀ value and no perceivable cardiotoxicity, which was a major dose-limiting adverse effect for the parent compound 1. These salient toxicity-reduced effects of 17a would merit further in-depth assessment of this compound for preclinical therapeutic usages.

Chloromethyl Glycosides as Versatile Synthons to Prepare Glycosyloxymethyl‐Prodrugs

This work investigates the addition of monosaccharides to marketed drugs to improve their pharmacokinetic properties for oral absorption. To this end, a set of chloromethyl glycoside synthons were developed to prepare a variety of glycosyloxymethyl‐prodrugs derived from 5‐fluorouracil, thioguanine, propofol and losartan. Drug release was studied in vitro using β‐glucosidase confirming rapid conversion of the monosaccharide prodrugs to release the parent drug, formaldehyde and the monosaccharide. To showcase this prodrug approach, a glucosyloxymethyl conjugate of the tetrazole‐containing drug losartan was used for in vivo experiments and showed complete release of the drug in a dog‐model.

Linkers: An Assurance for Controlled Delivery of Antibody-Drug Conjugate

As one of the major therapeutic options for cancer treatment, chemotherapy has limited selectivity against cancer cells. Consequently, this therapeutic strategy offers a small therapeutic window with potentially high toxicity and thus limited efficacy of doses that can be tolerated by patients. Antibody-drug conjugates (ADCs) are an emerging class of anti-cancer therapeutic drugs that can deliver highly cytotoxic molecules directly to cancer cells. To date, twelve ADCs have received market approval, with several others in clinical stages. ADCs have become a powerful class of therapeutic agents in oncology and hematology. ADCs consist of recombinant monoclonal antibodies that are covalently bound to cytotoxic chemicals via synthetic linkers. The linker has a key role in ADC outcomes because its characteristics substantially impact the therapeutic index efficacy and pharmacokinetics of these drugs. Stable linkers and ADCs can maintain antibody concentration in blood circulation, and they do not release the cytotoxic drug before it reaches its target, thus resulting in minimum off-target effects. The linkers used in ADC development can be classified as cleavable and non-cleavable. The former, in turn, can be grouped into three types: hydrazone, disulfide, or peptide linkers. In this review, we highlight the various linkers used in ADC development and their design strategy, release mechanisms, and future perspectives.

Nanomedicine from amphiphilizedprodrugs: Concept and clinical translation

Nanomedicines generally consisting of carrier materials with small fractions of active pharmaceutical ingredients (API) have long been used to improve the pharmacokinetics and biodistributions, augment the therapeutic efficacies and mitigate the side effects. Amphiphilizing hydrophobic/hydrophilic drugs to prodrugs capable of self-assembly into well-defined nanostructures has emerged as a facile approach to fabricating nanomedicines because this amphiphilized prodrug (APD) strategy presents many advantages, including minimized use of inert carrier materials, well-characterized prodrug structures, fixed and high drug loading contents, 100% loading efficiency, and burst-free but controlled drug release. This review comprehensively summarizes recent advances in APDs and their nanomedicines, from the rationale and the stimuli-responsive linker chemistry for on-demand drug release to their progress to the clinics, clinical performance of APDs, as well as the challenges and perspective on future development.

Real-Time Multi-Photon Tracking and Bioimaging of Glycosylated Theranostic Prodrugs upon Specific Enzyme Triggered Release

Real-time tracking of prodrug uptake, delivery and activation in vivo represents a major challenge for prodrug development. Herein, we demonstrate the use of novel glycosylated theranostics of the cancer pharmacophore Amonafide in highly-selective, enzymatic triggered release. We show that the use of endogenous enzymes for activated release of the therapeutic component can be observed, in real time, and monitored using one and two-photon bioimaging, offering unique insight into the prodrug pharmacokinetic profile. Furthermore, we demonstrate that the potent cytotoxicity of Amonafide is preserved using this targeted approach.

Enzyme-responsive smart nanocarriers for targeted chemotherapy: an overview

Nanocarriers play pivotal roles in the field of biomedical applications, particularly in anticancer therapy. One of the prominent strategies for the transport of anticancer drugs with site-specific release and improved therapeutic efficacy is the use of an enzyme-responsive drug delivery system. There is an emerging class of cancer therapeutics engineered to control the release of a drug via enzymatic degradation. Enzymes, being an essential component of bio-nanotechnology toolbox, hold exceptional biorecognition abilities as well as outstanding catalytic properties. Often, abnormal enzyme expression observed in cancer offers many opportunities in designing nanocarriers modified with enzyme-labile linkage. Through altered physical or chemical characteristics of these nanocarriers or cleavage of the drug in response to the bio-action of enzyme, an on-demand drug release can be obtained. In this review, several classes of enzymes performing critical roles in cancer such as hydrolases, lipases, and oxidoreductases are summarized. Insights on various approaches that interfere with the mechanism of these enzymes have also been included. Finally, various smart nanocarriers such as mesoporous silica nanoparticles, gold nanoparticles, carbon-nanotubes, micelles, liposomes, and dendrimers serving as excellent platforms for enzyme-responsive formulations have been discussed.

Advancements in folate receptor targeting for anti-cancer therapy: A small molecule-drug conjugate approach

Targeted delivery combined with controlled release of drugs has a crucial role in future of personalized medicine. The majority of cancer drugs are intended to interfere with one or more cellular events. Anticancer agents can also be toxic to healthy cells, as healthy cells may also need to proliferate and avoid apoptosis. The focus of this review covers the principles, advantages, drawbacks and summarize criteria that must be met for design of small molecule-drug conjugates (SMDCs) to achieve the desired therapeutic potency with minimal toxicity. SMDCs are composed of a targeting ligand, a releasable bridge, a spacer, and a therapeutic payload. We summarize the criteria for the effective design that influences the selection of tumor specific receptor and optimum elements in the design of SMDCs. We also discuss the criteria for selecting the optimal therapeutic drug payload, spacer and linker. The linker chemistries and cleavage strategies are also discussed. Finally, we review the folate receptor targeting SMDCs that are in preclinical development and in clinical trials.

Linker Hydrophilicity Modulates the Anticancer Activity of RGD-Cryptophycin Conjugates

Most anticancer agents are hydrophobic and can easily penetrate the tumor cell membrane by passive diffusion. This may impede the development of highly effective and tumor-selective treatment options. A hydrophilic β-glucuronidase-cleavable linker was used to connect the highly potent antimitotic agent cryptophycin-55 glycinate with the αv β3 integrin ligand c(RGDfK). Incorporation of the self-immolative linker containing glucuronic acid results in lower cytotoxicity than that of the free payload, suggesting that hydrophilic sugar linkers can preclude passive cellular uptake. In vitro drug-release studies and cytotoxicity assays demonstrated the potential of this small molecule-drug conjugate, providing guidance for the development of therapeutics containing hydrophobic anticancer drugs.

Antibody-Drug Conjugates: The New Frontier of Chemotherapy

In recent years, antibody-drug conjugates (ADCs) have become promising antitumor agents to be used as one of the tools in personalized cancer medicine. ADCs are comprised of a drug with cytotoxic activity cross-linked to a monoclonal antibody, targeting antigens expressed at higher levels on tumor cells than on normal cells. By providing a selective targeting mechanism for cytotoxic drugs, ADCs improve the therapeutic index in clinical practice. In this review, the chemistry of ADC linker conjugation together with strategies adopted to improve antibody tolerability (by reducing antigenicity) are examined, with particular attention to ADCs approved by the regulatory agencies (the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA)) for treating cancer patients. Recent developments in engineering Immunoglobulin (Ig) genes and antibody humanization have greatly reduced some of the problems of the first generation of ADCs, beset by problems, such as random coupling of the payload and immunogenicity of the antibody. ADC development and clinical use is a fast, evolving area, and will likely prove an important modality for the treatment of cancer in the near future.

Curcumin β-D-glucuronide exhibits anti-tumor effects on oxaliplatin-resistant colon cancer with less toxicity in vivo

The NF‐kappa B (NF‐κB) pathway plays a pivotal role in tumor progression and chemoresistance, and its inhibition has been shown to suppress tumor growth in a variety of preclinical models. Recently, we succeeded in synthesizing a water‐soluble injectable type of curcumin β‐D‐glucuronide (CMG), which is converted into a free‐form of curcumin by β‐glucuronidase in vivo. Herein, we aimed to clarify the efficacy, safety and pharmacokinetics of CMG in a xenograft mouse model. First, we confirmed that the presence of KRAS/TP53 mutations significantly increased the IC₅₀ of oxaliplatin (L‐OHP) and NF‐κB activity in HCT116 cells in vitro. Then, we tested the efficacy of CMG in an HCT116 colon cancer xenograft mice model. CMG demonstrated superior anticancer effects compared to L‐OHP in an L‐OHP‐resistant xenograft model. With regard to safety, significant bodyweight loss, severe myelosuppression and AST/ALT elevation were observed in L‐OHP‐treated mice, whereas none of these toxicity was noted in CMG‐treated mice. The combination of CMG and L‐OHP exhibited additive effects in these xenograft models without increasing toxicity. Pharmacokinetic analysis revealed that high levels of free‐form curcumin were maintained in the tumor tissue after 48 hours following CMG administration, but it was not detected in other major organs, such as the heart, liver and spleen. Immunohistochemistry revealed reduced NF‐κB activity in the tumor tissue extracted from CMG‐treated mice compared with that from control mice. These results indicated that CMG could be a promising anticancer prodrug for treating colon cancer with minimal toxicity.

Use of a genetically engineered E. coli overexpressing β-glucuronidase accompanied by glycyrrhizic acid, a natural and anti-inflammatory agent, for directed treatment of colon carcinoma in a mouse model

Bacteria-directed enzyme prodrug therapy (BDEPT), is an emerging alternative directed and tumor-specific approach. The basis of this method is the conversion of a non-toxic prodrug by a bacterial enzyme to a toxic drug within the tumor-microenvironment (TME). In the present study, the therapeutic efficacy of BDEPT was investigated based on the ability of Escherichia coli DH5α-lux/βG in activation of glycyrrhizic acid (GL), a natural and non-toxic compound purified from licorice, to glycyrrhetinic acid (GA) only in TME. To do so, the anti-bacterial effects of GL on bacteria and the cytotoxic effects of the produced GA on survival rate of CT26 mouse colon carcinoma cells were evaluated. The IC₅₀ values of the produced GA and cisplatin were determined as 210 μM and 100 μM, respectively. Comparing these values to GL treatment (1305 μM) indicates that bacteria could have efficiently activated GL to GA to inhibit the growth of tumor cells. Afterward, the anti-cancer effects of bacteria used in combination with GL was investigated in a mouse model of colon carcinoma. Results were indicative of targeted homing and even proliferation of luminescent bacteria in TME. Moreover, combined treatment greatly inhibited tumor growth. Histopathological analysis of dissected tissues also demonstrated increased apoptosis rate in tumor cells after combined treatment and interestingly, showed no obvious damage to the spleen and liver of treated mice. Accordingly, this BDEPT approach could be considered as an effective alternative tumor-specific therapy utilizing prodrug-activating enzymes expressing from tumor-targeting bacteria to allow the development of new tumor-specific pharmacotherapy protocols.

Development of an embedded multimodality imaging platform for onco-pharmacology using a smart anticancer prodrug as an example

Increasingly, in vivo imaging holds a strategic position in bio-pharmaceutical innovation. We will present the implementation of an integrated multimodal imaging setup enabling the assessment of multiple, complementary parameters. The system allows the fusion of information provided by: Near infrared fluorescent biomarkers, bioluminescence (for tumor proliferation status), Photoacoustic and Ultrasound imaging. We will study representative applications to the development of a smart prodrug, delivering a highly cytotoxic chemotherapeutic agent to cancer tumors. The results realized the ability of this embedded, multimodality imaging platform to firstly detect bioluminescent and fluorescent signals, and secondly, record ultrasound and photoacoustic data from the same animal. This study demonstrated that the prodrug was effective in three different models of hypoxia in human cancers compared to the parental cytotoxic agent and the vehicle groups. Monitoring by photoacoustic imaging during the treatments revealed that the prodrug exhibits an intrinsic capability to prevent the progression of tumor hypoxia. It is essential for onco-pharmacology studies to precisely document the hypoxic status of tumors both before and during the time course of treatments. This approach opens new perspectives for exploitation of preclinical mouse models of cancer, especially when considering associations between hypoxia, neoangiogenesis and antitumor activity.

Therapeutic significance of β-glucuronidase activity and its inhibitors: A review

The emergence of disease and dearth of effective pharmacological agents on most therapeutic fronts, constitutes a major threat to global public health and man's existence. Consequently, this has created an exigency in the search for new drugs with improved clinical utility or means of potentiating available ones. To this end, accumulating empirical evidence supports molecular target therapy as a plausible egress and, β-glucuronidase (βGLU) - a lysosomal acid hydrolase responsible for the catalytic deconjugation of β-d-glucuronides has emerged as a viable molecular target for several therapeutic applications. The enzyme's activity level in body fluids is also deemed a potential biomarker for the diagnosis of some pathological conditions. Moreover, due to its role in colon carcinogenesis and certain drug-induced dose-limiting toxicities, the development of potent inhibitors of βGLU in human intestinal microbiota has aroused increased attention over the years. Nevertheless, although our literature survey revealed both natural products and synthetic scaffolds as potential inhibitors of the enzyme, only few of these have found clinical utility, albeit with moderate to poor pharmacokinetic profile. Hence, in this review we present a compendium of exploits in the present millennium directed towards the inhibition of βGLU. The aim is to proffer a platform on which new scaffolds can be modelled for improved βGLU inhibitory potency and the development of new therapeutic agents in consequential.

Design, synthesis and pharmacological evaluation of CVIB, a codrug of carvacrol and ibuprofen as a novel anti-inflammatory agent

The search for new drugs with anti-inflammatory properties remains a challenge for modern medicine. Among the various strategies for drug discovery, deriving new chemical entities from known bioactive natural and/or synthetic compounds remains a promising approach. Here, we designed and synthesized CVIB, a codrug developed by association of carvacrol (a phenolic monoterpene) with ibuprofen (a non-steroidal anti-inflammatory drug). In silico pharmacokinetic and physicochemical properties evaluation indicated low aqueous solubility (LogP ≥5.0). Nevertheless, the hybrid presented excellent oral bioavailability, gastrointestinal tract absorption, and low toxicity. CVIB did not present cytotoxicity in peripheral blood mononuclear cells (PBMCs), and promoted a significant reduction in IL-2, IL-10, IL-17, and IFN-γ cytokine levels in vitro. The LD₅₀ was estimated to be approximately 5000 mg/kg. CVIB was stable and detectable in human plasma after 24 h. In vivo anti-inflammatory evaluations revealed that CVIB at 10 and 50 mg/kg i.p. caused a significant decrease in total leukocyte count (p < 0.01) and provoked a significant reduction in IL-1β (p < 0.01). CVIB at 10 mg/kg i.p. efficiently decreased inflammatory parameters better than the physical mixture (carvacrol + ibuprofen 10 mg/kg i.p.). The results suggest that the codrug approach is a good option for drug design and development, creating the possibility of combining NSAIDs with natural products in order to obtain new hybrid drugs may be useful for treatment of inflammatory diseases.

Cleavable linkers in antibody–drug conjugates

This tutorial review summarises the advances in the field of cleavable linker technologies for antibody–drug conjugates (ADCs).

Monitoring glycosidase activity for clustered sugar substrates, a study on β-glucuronidase

Enzymatically-triggered probes to determine glucuronidase hydrolysis kinetics for clustered substrates.

Mitochondria-Centric Review of Polyphenol Bioactivity in Cancer Models

SIGNIFICANCE

Humans are exposed daily to polyphenols in milligram-to-gram amounts through dietary consumption of fruits and vegetables. Polyphenols are also available as components of dietary supplements for improving general health. Although polyphenols are often advertised as antioxidants to explain health benefits, experimental evidence shows that their beneficial cancer preventing and controlling properties are more likely due to stimulation of pro-oxidant and proapoptotic pathways. Recent Advances: The understanding of the biological differences between cancer and normal cell, and especially the role that mitochondria play in carcinogenesis, has greatly advanced in recent years. These advances have resulted in a wealth of new information on polyphenol bioactivity in cell culture and animal models of cancer. Polyphenols appear to target oxidative phosphorylation and regulation of the mitochondrial membrane potential (MMP), glycolysis, pro-oxidant pathways, and antioxidant (adaptive) stress responses with greater selectivity in tumorigenic cells.

CRITICAL ISSUES

The ability of polyphenols to dissipate the MMP (Δψm) by a protonophore mechanism has been known for more than 50 years. However, researchers focus primarily on the downstream molecular effects of Δψm dissipation and mitochondrial uncoupling. We argue that the physicochemical properties of polyphenols are responsible for their anticancer properties by virtue of their protonophoric and pro-oxidant properties rather than their specific effects on downstream molecular targets.

FUTURE DIRECTIONS

Polyphenol-induced dissipation of Δψm is a physicochemical process that cancer cells cannot develop resistance against by gene mutation. Therefore, polyphenols should receive more attention as agents for cotherapy with cancer drugs to gain synergistic activity. Antioxid. Redox Signal.

A β-glucuronidase-responsive albumin-binding prodrug programmed for the double release of monomethyl auristatin E

We report on the synthesis, in vitro and in vivo biological evaluations of a dimeric β-glucuronidase-responsive albumin-binding prodrug designed for the double release of MMAE upon a single enzymatic activation step. This prodrug produced a significant antitumour activity in mice bearing subcutaneous LS174T colorectal adenocarcinoma xenografts without inducing side effects.

Molecular Imaging of Hydrolytic Enzymes Using PET and SPECT

Hydrolytic enzymes are a large class of biological catalysts that play a vital role in a plethora of critical biochemical processes required to maintain human health. However, the expression and/or activity of these important enzymes can change in many different diseases and therefore represent exciting targets for the development of positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radiotracers. This review focuses on recently reported radiolabeled substrates, reversible inhibitors, and irreversible inhibitors investigated as PET and SPECT tracers for imaging hydrolytic enzymes. By learning from the most successful examples of tracer development for hydrolytic enzymes, it appears that an early focus on careful enzyme kinetics and cell-based studies are key factors for identifying potentially useful new molecular imaging agents.

The effects of urolithins on the response of prostate cancer cells to non-steroidal antiandrogen bicalutamide

BACKGROUND

Urolithins are bioavailable products of gut microbiota metabolism of ellagitannins. Their biological activity includes anti-cancer effects.

PURPOSE

The aim of this study was to explore the effects of urolithins on prostate cancer cells and activity of clinically used anti-androgen, bicalutamide.

METHODS

Prostate cancer cells were treated with urolithin A, urolithin B, urolithin C or their combinations with bicalutamide. Cell proliferation was determined by DNA fluorescence with Hoechst 33258. The combination index method was used to examine interactions. Apoptosis and androgen receptor (AR) localization were analysed by flow cytometry. Prostate specific antigen (PSA) secretion was measured by ELISA.

RESULTS

Urolithins inhibited proliferation of LNCaP prostate cancer cells. The mixtures of bicalutamide with uroA and uroB had additive anti-proliferative effect. All tested urolithins induced apoptosis of LNCaP cells. However, the combinations of bicalutamide with urolithin A and urolithin B had attenuated pro-apoptotic activity. UroA and uroC decreased DHT-induced PSA secretion. In contrast, uroB impaired PSA lowering effect of bicalutamide. UroA, individually and in combination with bicalutamide, promoted cytoplasmic localization of AR.

CONCLUSION

Urolithins might contribute to chemopreventive activity of ellagitannin rich preparations. Our results support use of ellagitannin rich preparations in prostate cancer chemoprevention, but advise caution in their potential use in complementary therapy of prostate cancer. The differences in activity profiles of urolithins indicate that possible health benefits and interactions will depend on the type of produced ellagitannins metabolite.

The specific role of O6-methylguanine-DNA methyltransferase inhibitors in cancer chemotherapy

The DNA repair protein, O⁶-methylguanine DNA methyltransferase (MGMT), can confer resistance to guanine O⁶-alkylating agents. Therefore, inhibition of resistant MGMT protein is a practical approach to increase the anticancer effects of such alkylating agents. Numerous small molecule inhibitors were synthesized and exhibited potential MGMT inhibitory activities. Although they were nontoxic alone, they also inhibited MGMT in normal tissues, thereby enhancing the side effects of chemotherapy. Therefore, strategies for tumor-specific MGMT inhibition have been proposed, including local drug delivery and tumor-activated prodrugs. Over-expression of MGMT in hematopoietic stem cells to protect bone marrow from the toxic effects of chemotherapy is also a feasible selection. The future prospects and challenges of MGMT inhibitors in cancer chemotherapy were also discussed.

Antibody-Drug Conjugates for Cancer Therapy: Chemistry to Clinical Implications

Chemotherapy is one of the major therapeutic options for cancer treatment. Chemotherapy is often associated with a low therapeutic window due to its poor specificity towards tumor cells/tissues. Antibody-drug conjugate (ADC) technology may provide a potentially new therapeutic solution for cancer treatment. ADC technology uses an antibody-mediated delivery of cytotoxic drugs to the tumors in a targeted manner, while sparing normal cells. Such a targeted approach can improve the tumor-to-normal tissue selectivity and specificity in chemotherapy. Considering its importance in cancer treatment, we aim to review recent efforts for the design and development of ADCs. ADCs are mainly composed of an antibody, a cytotoxic payload, and a linker, which can offer selectivity against tumors, anti-cancer activity, and stability in systemic circulation. Therefore, we have reviewed recent updates and principal considerations behind ADC designs, which are not only based on the identification of target antigen, cytotoxic drug, and linker, but also on the drug-linker chemistry and conjugation site at the antibody. Our review focuses on site-specific conjugation methods for producing homogenous ADCs with constant drug-antibody ratio (DAR) in order to tackle several drawbacks that exists in conventional conjugation methods.

Advances in targeting the folate receptor in the treatment/imaging of cancers

The folate receptor (FR) is a recognised biomarker for tumour cells due to its overexpression on a large number of tumours. Consequently, the FR has been exploited by many diagnostic and therapeutic tools to allow targeted delivery to, and imaging of, cancer cells. Herein, we describe the many different approaches by which this has been achieved, including the attachment of folate to potent chemotherapeutic drugs to form FR-targeting small molecule-drug conjugates (SMDCs), FR-targeting antibodies (as antibody alone and as an antibody-drug conjugate), and in the form of complementary nanotechnology-folate platforms; as well as imaging variants thereof. The potential of exploiting the FR for targeted therapy/imaging has the potential to revolutionise the way several cancers are treated. These FR-targeted technologies can also pave the way for inspiring further sophisticated drug conjugates, especially as this receptor is being targeted by use of several complementary technologies: small molecule, nanoparticle and protein-based - thus providing broad and distinct knowledge in the area.

Prodrugs in medicinal chemistry and enzyme prodrug therapies

Prodrugs are cunning derivatives of therapeutic agents designed to improve the pharmacokinetics profile of the drug. Within a prodrug, pharmacological activity of the drug is masked and is recovered within the human body upon bioconversion of the prodrug, a process that is typically mediated by enzymes. This concept is highly successful and a significant fraction of marketed therapeutic formulations is based on prodrugs. An advanced subset of prodrugs can be engineered such as to achieve site-specific bioconversion of the prodrug - to comprise the highly advantageous "enzyme prodrug therapy", EPT. Design of prodrugs for EPT is similar to the prodrugs in general medicinal use in that the pharmacological activity of the drug is masked, but differs significantly in that site-specific bioconversion is a prime consideration, and the enzymes typically used for EPT are non-mammalian and/or with low systemic abundance in the human body. This review focuses on the design of prodrugs for EPT in terms of the choice of an enzyme and the corresponding prodrug for bioconversion. We also discuss the recent success of "self immolative linkers" which significantly empower and diversify the prodrug design, and present methodologies for the design of prodrugs with extended blood residence time. The review aims to be of specific interest for medicinal chemists, biomedical engineers, and pharmaceutical scientists.

Antibody Directed Enzyme Prodrug Therapy (ADEPT): Trials and tribulations

Antibody directed enzyme prodrug therapy has the potential to be an effective therapy for most common solid cancers. Clinical studies with CPG2 system have shown the feasibility of this approach. The key limitation has been immunogenicity of the enzyme. Technologies now exist to eliminate this problem. Non-immunogenic enzymes in combination with prodrugs that generate potent cytotoxic drugs can provide a powerful approach to cancer therapy. ADEPT has the potential to be non -toxic to normal tissue and can therefore be combined with other modalities including immunotherapy for greater clinical benefit.

Cell-mediated enzyme prodrug cancer therapies

Cell-directed gene therapy is a promising new frontier for the field of targeted cancer therapies. Here we discuss the current pre-clinical and clinical use of cell-mediated enzyme prodrug therapy (EPT) directed against solid tumors and avenues for further development. We also discuss some of the challenges encountered upon translating these therapies to clinical trials. Upon sufficient development, cell-mediated enzyme prodrug therapy has the potential to maximize the distribution of therapeutic enzymes within the tumor environment, localizing conversion of prodrug to active drug at the tumor sites thereby decreasing off-target toxicities. New combinatorial possibilities are also promising. For example, when combined with viral gene-delivery vehicles, this may result in new hybrid vehicles that attain heretofore unmatched levels of therapeutic gene expression within the tumor.