Synthesis, structural studies, and cytostatic evaluation of 5,6-di-O-modified L-ascorbic acid derivatives

[18F]KS1, a novel ascorbate-based ligand images ROS in tumor models of rodents and nonhuman primates

Over production of reactive oxygen species (ROS) caused by altered redox regulation of signaling pathways is common in many types of cancers. While PET imaging is recognized as the standard tool for cancer imaging, there are no clinically-approved PET radiotracers for ROS-imaging in cancer diagnosis and treatment. An ascorbate-based radio ligand promises to meet this urgent need. Our laboratory recently synthesized [18F] KS1, a fluoroethoxy furanose ring-containing ascorbate derivative, to track ROS in prostate tumor-bearing mice. Here we report cell uptake assays of [18F]KS1 with different ROS-regulating agents, PET imaging in head and neck squamous cell carcinoma (HNSCC) mice, and doxorubicin-induced rats; PET imaging in healthy and irradiated hepatic tumor-bearing rhesus to demonstrate its translational potential. Our preliminary evaluations demonstrated that KS1 do not generate ROS in tumor cells at tracer-level concentrations and tumor-killing properties at pharmacologic doses. [18F]KS1 uptake was low in HNSCC pretreated with ROS blockers, and high with ROS inducers. Tumors in high ROS-expressing SCC-61 took up significantly more [18F]KS1 than rSCC-61 (low-ROS expressing HNSCC); high uptake in doxorubicin-treated rats compared to saline-treated controls. Rodent biodistribution and PET imaging of [18F]KS1 in healthy rhesus monkeys demonstrated its favorable safety, pharmacokinetic properties with excellent washout profile, within 3.0 h of radiotracer administration. High uptake of [18F]KS1 in liver tumor tissues of the irradiated hepatic tumor-bearing monkey showed target selectivity. Our strong data in vitro, in vivo, and ex vivo here supports the high translational utility of [18F]KS1 to image ROS.

Green solvent-free synthesis of new N-heterocycle-L-ascorbic acid hybrids and their antiproliferative evaluation

Aim: The authors' aim was to improve the application of copper-catalyzed azide-alkyne cycloaddition in the synthesis of hybrids containing biologically significant nucleobases and L-ascorbic acid scaffolds by introducing an environmentally friendly and waste-free ball mill. Results: Two series of hybrids with a purine, pyrrolo[2,3- d]pyrimidine or 5-substituted pyrimidine attached to 2,3-dibenzyl-L-ascorbic acid via a hydroxyethyl- (15a–23a) or ethylidene-1,2,3-triazolyl (15b–23b) bridge were prepared by ball milling and conventional synthesis. The unsaturated 6-chloroadenine L-ascorbic acid derivative 16b can be highlighted as a lead compound and showed strong antiproliferative activity against HepG2 (hepatocellular carcinoma) and SW620 (colorectal adenocarcinoma) cells. Conclusion: Mechanochemical synthesis was superior in terms of sustainability, reaction rate and yield, highlighting the advantageous applications of ball milling over classical reactions.

Understanding the Therapeutic Potential of Ascorbic Acid in the Battle to Overcome Cancer

Cancer, a fatal disease, is also one of the main causes of death worldwide. Despite various developments to prevent and treat cancer, the side effects of anticancer drugs remain a major concern. Ascorbic acid is an essential vitamin required by our bodies for normal physiological function and also has antioxidant and anticancer activity. Although the body cannot synthesize ascorbic acid, it is abundant in nature through foods and other natural sources and also exists as a nutritional food supplement. In anticancer drug development, ascorbic acid has played an important role by inhibiting the development of cancer through various mechanisms, including scavenging reactive oxygen species (ROS), selectively producing ROS and encouraging their cytotoxicity against tumour cells, preventing glucose metabolism, serving as an epigenetic regulator, and regulating the expression of HIF in tumour cells. Several ascorbic acid analogues have been produced to date for their anticancer and antioxidant activity. The current review summarizes the mechanisms behind ascorbic acid's antitumor activity, presents a compilation of its derivatives and their biological activity as anticancer agents, and discusses delivery systems such as liposomes, nanoparticles against cancer, and patents on ascorbic acid as anticancer agents.

Antitumor and antiviral activities of 4-substituted 1,2,3-triazolyl-2,3-dibenzyl-L-ascorbic acid derivatives

Two series of 6-(1,2,3-triazolyl)-2,3-dibenzyl-l-ascorbic acid derivatives with the hydroxyethylene (8a−8u) and ethylidene linkers (10c−10p) were synthesized and evaluated for their antiproliferative activity against seven malignant tumor cell lines and antiviral activity against a broad range of viruses. Conformationally unrestricted spacer between the lactone and 1,2,3-triazole units in 8a−8u series had a profound effect on antitumor activity. Besides, the introduction of a long side chain at C-4 of 1,2,3-triazole that led to the synthesis of decyl-substituted 2,3-dibenzyl-l-ascorbic acid 8m accounted for a selective and potent antiproliferative activity on breast cancer MCF-7 cells cells in the nM range. Further analysis showed that compound 8m strongly enhanced expression of hypoxia inducible transcription factor 1 α (HIF-1α) and to some extent decreased expression of nitric oxide synthase 2 (NOS2) suggesting its role in regulating HIF-1α signalling pathway. The p-methoxyphenyl-substituted derivative 10g displayed specific anti-cytomegalovirus (CMV) potential, whereas aliphatic-substituted derivatives 8l and 8m had the most potent, yet relatively non-specific, anti-varicella-zoster (VZV) activity.

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Two series of 1,2,3-triazolyl 2,3-dibenzyl-l-ascorbic acid conjugates were synthesized.

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Conformationally unrestricted spacer had a major effect on antitumor activities.

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Decyl-substituted l-ascorbic acid 8m caused inhibition of breast cancer MCF-7 cells in the nM range.

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8m increased the expression of hypoxia inducible transcription factor HIF-1α.

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p-Methoxyphenyl-substituted derivative 10g had specific anti-CMV activity.

Therapeutic Perspective of Vitamin C and Its Derivatives

l-Ascorbic acid (ASA), vitamin C, is a ubiquitous carbohydrate-like compound that has an essential role in a number of cellular processes, such as collagen synthesis, cellular oxidation, and various hydroxylation reactions. ASA is a biomolecule of critical importance for protection of cellular components against oxidative damage caused by toxic free radicals and other reactive oxygen species (ROS) that are involved in the development of various types of chronic diseases. Vitamin C has a switchover role from being an antioxidant in physiological conditions to a prooxidant under pathologic conditions. Moreover, some l-ascorbic acid derivatives exhibit strong and selective antitumor and antiviral activity. This review emphasizes the advances on diverse and potent biological profiles of l-ascorbic acid and its derivatives, and their perspective in the development of new bioactive chemical entities in the future. The work is primarily addressed at antioxidant, anticancer, and antiviral potencies of l-ascorbic acid and compounds containing its butenolide structural motif.

Initial biological evaluations of 18F-KS1, a novel ascorbate derivative to image oxidative stress in cancer

Background

Reactive oxygen species (ROS)-induced oxidative stress damages many cellular components such as fatty acids, DNA, and proteins. This damage is implicated in many disease pathologies including cancer and neurodegenerative and cardiovascular diseases. Antioxidants like ascorbate (vitamin C, ascorbic acid) have been shown to protect against the deleterious effects of oxidative stress in patients with cancer. In contrast, other data indicate potential tumor-promoting activity of antioxidants, demonstrating a potential temporal benefit of ROS. However, quantifying real-time tumor ROS is currently not feasible, since there is no way to directly probe global tumor ROS. In order to study this ROS-induced damage and design novel therapeutics to prevent its sequelae, the quantitative nature of positron emission tomography (PET) can be harnessed to measure in vivo concentrations of ROS. Therefore, our goal is to develop a novel translational ascorbate-based probe to image ROS in cancer in vivo using noninvasive PET imaging of tumor tissue. The real-time evaluations of ROS state can prove critical in developing new therapies and stratifying patients to therapies that are affected by tumor ROS.

Methods

We designed, synthesized, and characterized a novel ascorbate derivative (E)-5-(2-chloroethylidene)-3-((4-(2-fluoroethoxy)benzyl)oxy)-4-hydroxyfuran-2(5H)-one (KS1). We used KS1 in an in vitro ROS MitoSOX-based assay in two different head and neck squamous cancer cells (HNSCC) that express different ROS levels, with ascorbate as reference standard. We radiolabeled ¹⁸F-KS1 following ¹⁸F-based nucleophilic substitution reactions and determined in vitro reactivity and specificity of ¹⁸F-KS1 in HNSCC and prostate cancer (PCa) cells. MicroPET imaging and standard biodistribution studies of ¹⁸F-KS1 were performed in mice bearing PCa cells. To further demonstrate specificity, we performed microPET blocking experiments using nonradioactive KS1 as a blocker.

Results

KS1 was synthesized and characterized using ¹H NMR spectra. MitoSOX assay demonstrated good correlations between increasing concentrations of KS1 and ascorbate and increased reactivity in SCC-61 cells (with high ROS levels) versus rSCC-61cells (with low ROS levels). ¹⁸F-KS1 was radiolabeled with high radiochemical purity (> 94%) and specific activity (~ 100 GBq/μmol) at end of synthesis (EOS). Cell uptake of ¹⁸F-KS1 was high in both types of cancer cells, and the uptake was significantly blocked by nonradioactive KS1, and the ROS blocker, superoxide dismutase (SOD) demonstrating specificity. Furthermore, ¹⁸F-KS1 uptake was increased in PCa cells under hypoxic conditions, which have been shown to generate high ROS. Initial in vivo tumor uptake studies in PCa tumor-bearing mice demonstrated that ¹⁸F-KS1 specifically bound to tumor, which was significantly blocked (threefold) by pre-injecting unlabeled KS1. Furthermore, biodistribution studies in the same tumor-bearing mice showed high tumor to muscle (target to nontarget) ratios.

Conclusion

This work demonstrates the strong preliminary support of ¹⁸F-KS1, both in vitro and in vivo for imaging ROS in cancer. If successful, this work will provide a new paradigm to directly probe real-time oxidative stress levels in vivo. Our work could enhance precision medicine approaches to treat cancer, as well as neurodegenerative and cardiovascular diseases affected by ROS.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0513-x) contains supplementary material, which is available to authorized users.

Continuous flow-ultrasonic synergy in click reactions for the synthesis of novel 1,2,3-triazolyl appended 4,5-unsaturated l-ascorbic acid derivatives

A combination of flow chemistry and batch-based synthetic procedures has been successfully applied to the assembly of novel 4,5-unsaturated l-ascorbic acid series 6a–6n with diverse C-6-substituted 1,2,3-triazole moiety.

[Bicyclic furano[2,3-D] derivatives of pyrimidine nucleosides--synthesis and antiviral properties]

The methods of synthesis of furano- and pyrrolo[2,3-dlpyrimidine nucleosides as well as structure activity relationship of obtained compounds towards viruses of varicella zoster, hepatitis C, bovine viral diarrhea and some others are reviewed.

Ascorbic acid derivatives as a new class of antiproliferative molecules

Ascorbic acid (AA) has long been described as an antiproliferative agent. However, the molecule has to be used at a very high concentrations, which necessitates i.v. injection, and the tight regulation of in-blood and in-cell AA concentrations making it impossible to hold very high concentrations for any substantial length of time. Here we report evidence that AA derivates are antiproliferative and cytotoxic molecules at an IC50 lower than AA itself. Among these new molecules, we selected K873 that has cytotoxic and antiproliferative effects on different human tumor cells at tenth micromolar concentration. In a further step, we demonstrated that K873 selectively to kills only cancer cells without being toxic for normal non-dividing (or poorly dividing) cells. Finally, we tested the effect of treatment with K873 (5-10 mg/kg/d by i.p. route) on tumor progression in xenografted immunodeficient mice (BALB/c Nude). Our data suggest that K873 administration strongly inhibits tumor progression. In a previous study using microarrays, we demonstrated that AA decreases the expression of two genes families involved in cell cycle progression, i.e. initiation factor of translation and tRNA synthetases. Here we show that K873 treatment also decreases the expression of four of these genes in xenografted tumors, in proportions similar to that previously observed with AA. Taken together, our data suggest that AA and K873 share similar action. Our findings suggest that AA derivatives could be a promising new class of anti-cancer drugs, either alone or in combination with other molecules.

Synthesis and antiviral and cytostatic evaluations of the new C-5 substituted pyrimidine and furo[2,3-d]pyrimidine 4',5'-didehydro-L-ascorbic acid derivatives

The novel C-5 alkynyl substituted pyrimidine (1-11) and furo[2,3-d]pyrimidine derivatives (12-22) of l-ascorbic acid were synthesized by coupling of 5-iodouracil-4',5'-didehydro-5',6'-dideoxy-l-ascorbic acid with terminal alkynes by using Sonogashira cross-coupling reaction conditions. The new compounds were evaluated for their cytostatic and antiviral activities. Among all evaluated compounds, the octynyl-substituted uracil derivative of l-ascorbic acid (3) exhibited the most pronounced cytostatic activities against all examined tumor cell lines (IC50 = 2-12 microM). Pyrimidine derivatives of l-ascorbic acid containing p-substituted phenylacetylene groups (8-11) displayed also a rather pronounced (IC50 = 3-37 microM) inhibitory effect toward all tumor cell lines. From the bicyclic series of compounds, 6-butylfuro[2,3-d]pyrimidine derivative (12) and 6-p-bromophenylfuro[2,3-d]pyrimidine derivative (19) showed the highest cytostatic activity (IC50 = 4.5-20 microM), particularly against malignant leukemia (L1210) and T-lymphocyte (Molt4/C8 and CEM) cells. Compounds 3 and 9 showed specific albeit moderate activity against cytomegalovirus (CMV, Davis strain, EC50 = 1.8 and 3.8 microM, respectively, for compounds 3 and 9) at a approximately 5-fold lower concentration than that required to show cytotoxicity.

The novel C-5 aryl, alkenyl, and alkynyl substituted uracil derivatives of L-ascorbic acid: synthesis, cytostatic, and antiviral activity evaluations

The novel C-5 substituted uracil derivatives of l-ascorbic acid were synthesized by coupling of 5-iodouracil-4,5-didehydro-5,6-dideoxy-l-ascorbic acid with unsaturated stannanes under Stille reaction conditions. The new compounds were evaluated for their antitumoral and antiviral activities. Among all compounds evaluated the 5-propynyl substituted uracil derivative of l-ascorbic acid (7) exhibited the most pronounced cytostatic activities against all examined tumor cell lines (IC(50): 0.2-0.78 microM). However, this compound was also cytotoxic to human normal fibroblasts WI 38. The 5-(phenylethynyl)uracil-2,3-di-O-benzylated l-ascorbic acid derivative (4) exhibited an albeit slight (IC(50): 55-108 microM), but selective inhibitory effect toward all tumor cell lines except for cervical carcinoma (HeLa), pancreatic carcinoma (MiaPaCa-2), laryngeal carcinoma (Hep-2), and colon carcinoma (SW 620), and no cytotoxicity to normal human fibroblast (WI 38). Compound 7 showed some, not highly specific, inhibitory potential against vesicular stomatitis virus, Coxsackie B4 virus, and Sindbis viruses (EC(50): 1.6 microM).