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

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.

High-level De novo biosynthesis of arbutin in engineered Escherichia coli

Arbutin is a hydroquinone glucoside compound existing in various plants. It is widely used in pharmaceutical and cosmetic industries owing to its well-known skin-lightening property as well as anti-oxidant, anti-microbial, and anti-inflammatory activities. Currently, arbutin is usually produced by plant extraction or enzymatic processes, which suffer from low product yield and expensive processing cost. In this work, we established an artificial pathway in Escherichia coli for high-level production of arbutin from simple carbon sources. First, a 4-hydroxybenzoate 1-hydroxylase from Candida parapsilosis CBS604 and a glucosyltransferase from Rauvolfia serpentina were characterized by in vitro enzyme assays. Introduction of these two genes into E. coli led to the production of 54.71mg/L of arbutin from glucose. Further redirection of carbon flux into arbutin biosynthesis pathway by enhancing shikimate pathway genes enabled production of 3.29g/L arbutin, which is a 60-fold increase compared with the initial strain. Final optimization of glucose concentration added in the culture medium was able to further improve the titer of arbutin to 4.19g/L in shake flasks experiments, which is around 77-fold higher than that of initial strain. This work established de novo biosynthesis of arbutin from simple carbon sources and provided a generalizable strategy for the biosynthesis of shikimate pathway derived chemicals. The high titer achieved in our engineered strain also indicates the potential for industrial scale bio-manufacturing of arbutin.

Targeting the tumour microenvironment with an enzyme-responsive drug delivery system for the efficient therapy of breast and pancreatic cancers

The development of novel therapeutic strategies allowing the destruction of tumour cells while sparing healthy tissues is one of the main challenges of cancer chemotherapy. Here, we report on the design and antitumour activity of a low-molecular-weight drug delivery system programmed for the selective release of the potent monomethylauristatin E in the tumour microenvironment of solid tumours. After intravenous administration, this compound binds covalently to plasmatic albumin through Michael addition, thereby enabling its passive accumulation in tumours where extracellular β-glucuronidase initiates the selective release of the drug. This targeting device produces outstanding therapeutic efficacy on orthotopic triple-negative mammary and pancreatic tumours in mice (50% and 33% of mice with the respective tumours cured), leading to impressive reduction or even disappearance of tumours without inducing side effects.

Annexin-directed β-glucuronidase for the targeted treatment of solid tumors

Enzyme prodrug therapy has the potential to remedy the lack of selectivity associated with the systemic administration of chemotherapy. However, most current systems are immunogenic and constrained to a monotherapeutic approach. We developed a new class of fusion proteins centered about the human enzyme β-glucuronidase (βG), capable of converting several innocuous prodrugs into chemotherapeutics. We targeted βG to phosphatidylserine on tumor cells, tumor vasculature and metastases via annexin A1/A5. Phosphatidylserine shows promise as a universal marker for solid tumors and allows for tumor type-independent targeting. To create fusion proteins, human annexin A1/A5 was genetically fused to the activity-enhancing 16a3 mutant of human βG, expressed in chemically defined, fed-batch suspension culture, and chromatographically purified. All fusion constructs achieved >95% purity with yields up to 740 μg/l. Fusion proteins displayed cancer selective cell-surface binding with cell line-dependent binding stability. One fusion protein in combination with the prodrug SN-38 glucuronide was as effective as the drug SN-38 on Panc-1 pancreatic cancer cells and HAAE-1 endothelial cells, and demonstrated efficacy against MCF-7 breast cancer cells. βG fusion proteins effectively enable localized combination therapy that can be tailored to each patient via prodrug selection, with promising clinical potential based on their near fully human design.

A self-immolative and DT-diaphorase-activatable prodrug for drug-release tracking and therapy

A DT-diaphorase-activatable theranostic prodrug has been developed for visualizing the release of active drug and enhancing the therapeutic effect.

Prodrug approach: An overview of recent cases

In this review we highlight the most modern trends in the prodrug strategy. In drug research and development, the prodrug concept has found a number of useful applications. Selected examples of this approach are provided in this paper and they are classified according to the aim of their design.

Glycosidase activated release of fluorescent 1,8-naphthalimide probes for tumor cell imaging from glycosylated 'pro-probes'

Glycosylated 4-amino-1,8-naphthalimide derivatives possess a native glycosidic linkage that can be selectively hydrolysed in situ by glycosidase enzymes to release the naphthalimide as a fluorescent imaging or therapeutic agent. In vitro studies using a variety of cancer cell lines demonstrated that the naphthalimides only get taken up into cells upon enzymatic cleavage from the glycan unit; a mechanism that offers a novel approach for the targeted delivery of probes/drugs.

The apoptotic and autophagic properties of two natural occurring prodrugs, hyperoside and hypoxoside, against pancreatic cancer cell lines

Pancreatic cancer is only the 12th most common cancer, but the fourth leading cause of cancer-related deaths in the world. This is due to late prognosis, poor response to chemotherapy and early metastases. Natural prodrugs may play an important role in the treatment of pancreatic cancer. The main aim of this study was to determine the cytotoxicity of five natural prodrugs, namely harpagoside, hyperoside, hypoxoside, oleuropein and polydatin, by investigating apoptosis and autophagy as possible mechanism/s of action. Hypoxoside and hyperoside have shown selective cytotoxicity at IC50 values of ∼25 and 50μM against INS-1 and MIA PaCa-2 pancreatic cancer cells, respectively. Hypoxoside and hyperoside induced G2/M phase arrest and caspase-3 activation in INS-1 and MIA PaCa-2 cells, respectively. Hoechst/phalloidin staining confirmed morphological changes, including condensed chromatin multinucleation, membrane blebbing and loss of cytoskeletal arrangement in INS-1 and MIA PaCa-2 cells. Acridine orange staining was absent in INS-1 (hypoxoside) and MIA PaCa-2 (hyperoside) treated cells, whereas LC3B expression was not significantly increased. INS-1 and MIA PaCa-2 treated cells favour the cell death pathway, apoptosis, over the cell survival pathway, autophagy.

Flow Cytometric Method for the Detection of Flavonoids in Cell Lines

Here, we describe an easy-to-use flow cytometric method using diphenylboric acid 2-amino ethyl ester (DPBA) stain for the detection of flavonoids in cells from human/animal origin. Flavonoid bioavailability and bioactivity depend on structure, conjugation and the cell type to which they are presented. We have studied cellular uptake of five flavonoids with different structures and conjugation forms. First, parameters including fixation method, technical and batch variability, and concentration were optimized. Second, uptake of two aglycones—quercetin and hesperetin—and their corresponding glycosides—rutin and hesperidin—in Caco-2 cells was compared. Third, the aglycone quercetin, glycoside rutin, and glucuronide baicalin were added to the Caco-2, HepG2, and CHO-K1 cell lines at 1, 10, and 20 µM concentrations and cellular uptake was measured after 1, 4, and 7 h. We conclude that quercetin was taken up by cells in a dose-dependent way, and that HepG2 cells had the highest uptake factors, followed by CHO-K1 and Caco-2 cells. Confocal microscopy showed cell type–dependent localization of quercetin in the cell membrane and cytoplasm. No uptake of flavonoid glycosides was detected. This flow cytometric method can be used for future research unravelling mechanisms behind flavonoid bioactivity in health and disease at the cellular level.

Discovery of BC-01, a novel mutual prodrug (hybrid drug) of ubenimex and fluorouracil as anticancer agent

We designed and synthesized a novel mutual prodrug, named BC-01 (3), by integrating ubenimex and Fluorouracil (5-FU) into one molecule based on prior research results that showed that a combination of the aminopeptidase N (CD13) inhibitor, ubenimex, and the cytotoxic antitumor agent, 5-FU, exhibited improved in vitro and in vivo antitumor efficiency. 3 showed potent inhibitory activity against CD13 enzymatic activity. Compared with ubenimex, 3 exhibited more potent anti-angiogenesis effects, and compared with the approved 5-FU prodrug, capecitabine, 3 exhibited more potent tumor growth inhibitory and anti-metastasis effects. Additionally, compared with 5-FU or 5-FU plus ubenimex, 3 also exhibited a superior antitumor efficiency even in our 5-FU-resistant mice model. Other antitumor agents could be conjugated with ubenimex using this strategy to obtain novel mutual prodrugs with promising antitumor potency.

Progress and problems with the use of suicide genes for targeted cancer therapy

Among various gene therapy methods for cancer, suicide gene therapy attracts a special attention because it allows selective conversion of non-toxic compounds into cytotoxic drugs inside cancer cells. As a result, therapeutic index can be increased significantly by introducing high concentrations of cytotoxic molecules to the tumor environment while minimizing impact on normal tissues. Despite significant success at the preclinical level, no cancer suicide gene therapy protocol has delivered the desirable clinical significance yet. This review gives a critical look at the six main enzyme/prodrug systems that are used in suicide gene therapy of cancer and familiarizes readers with the state-of-the-art research and practices in this field. For each enzyme/prodrug system, the mechanisms of action, protein engineering strategies to enhance enzyme stability/affinity and chemical modification techniques to increase prodrug kinetics and potency are discussed. In each category, major clinical trials that have been performed in the past decade with each enzyme/prodrug system are discussed to highlight the progress to date. Finally, shortcomings are underlined and areas that need improvement in order to produce clinical significance are delineated.

Systems metabolic engineering of Escherichia coli to enhance the production of flavonoid glucuronides

Through modulating UDPGA biosynthetic pathway and introducting SbUGT, an engineered strain was constructed to enhance the production of flavonoid glucuronides.

Bisphenol A at the reference level counteracts doxorubicin transcriptional effects on cancer related genes in HT29 cells

Human exposure to Bisphenol A (BPA) results mainly from ingestion of food and beverages. Information regarding BPA effects on colon cancer, one of the major causes of death in developed countries, is still scarce. Likewise, little is known about BPA drug interactions although its potential role in doxorubicin (DOX) chemoresistance has been suggested. This study aims to assess potential interactions between BPA and DOX on HT29 colon cancer cells. HT29 cell response was evaluated after exposure to BPA, DOX, or co-exposure to both chemicals. Transcriptional analysis of several cancer-associated genes (c-fos, AURKA, p21, bcl-xl and CLU) shows that BPA exposure induces slight up-regulation exclusively of bcl-xl without affecting cell viability. On the other hand, a sub-therapeutic DOX concentration (40 nM) results in highly altered c-fos, bcl-xl, and CLU transcript levels, and this is not affected by co-exposure with BPA. Conversely, DOX at a therapeutic concentration (4 μM) results in distinct and very severe transcriptional alterations of c-fos, AURKA, p21 and CLU that are counteracted by co-exposure with BPA resulting in transcript levels similar to those of control. Co-exposure with BPA slightly decreases apoptosis in relation to DOX 4 μM alone without affecting DOX-induced loss of cell viability. These results suggest that BPA exposure can influence chemotherapy outcomes and therefore emphasize the necessity of a better understanding of BPA interactions with chemotherapeutic agents in the context of risk assessment.

Drug delivery and release systems for targeted tumor therapy

Most toxic agents currently used for chemotherapy show a narrow therapeutic window, because of their inability to distinguish between healthy and cancer cells. Targeted drug delivery offers the possibility to overcome this issue by selectively addressing structures on the surface of cancer cells, therefore reducing undesired side effects. In this broad field, peptide-drug conjugates linked by intracellular cleavable structures have evolved as highly promising agents. They can specifically deliver toxophores to tumor cells by targeting distinct receptors overexpressed in cancer. In this review, we focus on these compounds and describe important factors to develop a highly efficient peptide-drug conjugate. The necessary properties of tumor-targeting peptides are described, and the different options for cleavable linkers used to connect toxic agents and peptides are discussed, and synthetic considerations for the introduction of these structures are reported. Furthermore, recent examples and current developments of peptide-drug conjugates are critically evaluated with a special focus on the applied linker structures and their future use in cancer therapy.

Microbial shifts in the aging mouse gut

BACKGROUND

The changes that occur in the microbiome of aging individuals are unclear, especially in light of the imperfect correlation of frailty with age. Studies in older human subjects have reported subtle effects, but these results may be confounded by other variables that often change with age such as diet and place of residence. To test these associations in a more controlled model system, we examined the relationship between age, frailty, and the gut microbiome of female C57BL/6 J mice.

RESULTS

The frailty index, which is based on the evaluation of 31 clinical signs of deterioration in mice, showed a near-perfect correlation with age. We observed a statistically significant relationship between age and the taxonomic composition of the corresponding microbiome. Consistent with previous human studies, the Rikenellaceae family, which includes the Alistipes genus, was the most significantly overrepresented taxon within middle-aged and older mice. The functional profile of the mouse gut microbiome also varied with host age and frailty. Bacterial-encoded functions that were underrepresented in older mice included cobalamin (B12) and biotin (B7) biosynthesis, and bacterial SOS genes associated with DNA repair. Conversely, creatine degradation, associated with muscle wasting, was overrepresented within the gut microbiomes of the older mice, as were bacterial-encoded β-glucuronidases, which can influence drug-induced epithelial cell toxicity. Older mice also showed an overabundance of monosaccharide utilization genes relative to di-, oligo-, and polysaccharide utilization genes, which may have a substantial impact on gut homeostasis.

CONCLUSION

We have identified taxonomic and functional patterns that correlate with age and frailty in the mouse microbiome. Differences in functions related to host nutrition and drug pharmacology vary in an age-dependent manner, suggesting that the availability and timing of essential functions may differ significantly with age and frailty. Future work with larger cohorts of mice will aim to separate the effects of age and frailty, and other factors.

Quinone-methide species, a gateway to functional molecular systems: from self-immolative dendrimers to long-wavelength fluorescent dyes

Over the last 30 years, the quinone-methide elimination has served as a valuable tool for achieving various important molecular functions. Molecular adaptors based on quinone-methide or aza-quinone-methide reactivity have been designed, synthesized, and used in diagnostic probes, molecular amplifiers, drug delivery systems, and self-immolative dendritic/polymeric molecular systems. These unique adaptors function as stable spacers between an enzyme- or reagent-responsive group and a reporter moiety and can undergo 1,4-, 1,6-, or 1,8-type elimination reactions upon cleavage of the triggering group. Such reactivity results in the release of the reporter group through formation of a quinone-methide species. This type of elimination was applied to design distinct molecular adaptors capable of multiple quinone-methide eliminations. Using this chemistry, we have developed unique molecular structures that are known today as self-immolative dendrimers. These dendrimers disassemble upon a single triggering event in a domino-like manner from the focal point to their periphery with the consequent release of multiple end-groups. Such molecular structures are used in self-immolative dendritic prodrugs and in diagnostic probes to obtain a significant amplification effect. To further enhance amplification, we have developed the dendritic chain reaction, which uses simple molecules to achieve functionality of high-generation virtual self-immolative dendrimers. In addition, we harnessed the quinone-methide elimination reactivity to design polymers that disassemble from head-to-tail initiated by an analyte-responsive event. Following this example, other chemical reactivities were demonstrated by scientists to design such polymeric molecules. In a manner analogous to the quinone-methide elimination, electron rearrangement can lead to formation of conjugated quinone-methide-type dyes with long-wavelength emission of fluorescence. We have recently applied an intramolecular charge transfer to form a unique kind of quinone-methide type derivative based on a donor-two-acceptors molecular structure. This intramolecular charge transfer produces a new fluorochrome with an extended conjugation of π-electron system that is used for the design of long-wavelength fluorogenic probes with a turn-ON option. The rapidly expanding use of quinone-methide species, reflected in the increased number of examples reported in the literature, indicates the importance of this tool in chemistry. These species provide a useful gateway to functional molecular structures with distinct reactivities and spectroscopic characteristics.

Selective release of a cyclopamine glucuronide prodrug toward stem-like cancer cell inhibition in glioblastoma

Recent data suggest that inhibition of the Hedgehog pathway could be a therapeutic target for glioblastoma. Alkaloid cyclopamine inhibits Hedgehog signaling, depleting stem-like cancer cells derived from glioblastoma. However, this compound is toxic for somatic stem cells, preventing its use for clinical applications. In this study, we tested a derivatization product of cyclopamine in the form of cyclopamine glucuronide prodrug (CGP-2). This compound was used in vitro and in vivo toward glioblastoma-initiating cells (GIC). Results obtained in vitro indicate that CGP-2 is active only in the presence of β-glucuronidase, an enzyme detected in high levels in necrotic areas of glioblastomas. CGP-2 decreased proliferation and inhibited the self-renewal of all GIC lines tested. Hedgehog pathway blockade by 10 μmol/L of CGP-2 induced a 99% inhibition of clonogenicity on GICs, similar to cyclopamine treatment. Combination of CGP-2 with radiation decreased clonogenic survival in all GIC lines compared with CGP-2 alone. In a subcutaneous glioblastoma xenograft model, a two-week CGP-2 treatment prevented tumor growth with 75% inhibition at 8 weeks, and this inhibition was still significant after 14 weeks. Unlike cyclopamine, CGP-2 had no detectable toxic effects in intestinal crypts. Our study suggests that inhibition of the Hedgehog pathway with CGP-2 is more effective than conventional temozolomide adjuvant, with much lower concentrations, and seems to be an effective therapeutic strategy for targeting GICs.