Potent Cytotoxicity of Novel L-Shaped Ortho-Quinone Analogs through Inducing Apoptosis.

Twenty-seven L-shaped ortho-quinone analogs were designed and synthesized using a one pot double-radical synthetic strategy followed by removing methyl at C-3 of the furan ring and introducing a diverse side chain at C-2 of the furan ring. The synthetic derivatives were investigated for their cytotoxicity activities against human leukemia cells K562, prostate cancer cells PC3, and melanoma cells WM9. Compounds TB1, TB3, TB4, TB6, TC1, TC3, TC5, TC9, TC11, TC12, TC14, TC15, TC16, and TC17 exhibited a better broad-spectrum cytotoxicity on three cancer cells. TB7 and TC7 selectively displayed potent inhibitory activities on leukemia cells K562 and prostate cancer cells PC3, respectively. Further studies indicated that TB3, TC1, TC3, TC7, and TC17 could significantly induce the apoptosis of PC3 cells. TC1 and TC17 significantly induced apoptosis of K562 cells. TC1, TC11, and TC14 induced significant apoptosis of WM9 cells. The structure-activity relationships evaluation showed that removing methyl at C-3 of the furan ring and introducing diverse side chains at C-2 of the furan ring is an effective strategy for improving the anticancer activity of L-shaped ortho-quinone analogs.

Flavoenzyme-mediated reduction reactions and antitumor activity of nitrogen-containing tetracyclic ortho-quinone compounds and their nitrated derivatives.

Nitrogen-based tetracyclic ortho-quinones (naphtho[1'2':4.5]imidazo[1,2-a]pyridine-5,6-diones, NPDOs) and their nitro-substituted derivatives (nitro-(P)NPDOs) were obtained by condensation of substituted 2,3-dichloro-1,4-naphthoquinones with 2-amino-pyridine and -pyrimidine and nitration at an elevated temperature. The structural features of the compounds as well as their global and regional electrophilic potency were characterized by means of DFT computation. The compounds were highly reactive substrates of single- and two-electron (hydride) - transferring P-450R (CPR; EC 1.6.2.4) and NQO-1 (DTD; EC 1.6.99.2), respectively, concomitantly producing reactive oxygen species. Their catalytic efficiency defined in terms of the apparent second-order rate constant (kcat/KM (Q)) values in P-450R- and NQO-1-mediated reactions varied in the range of 3-6 × 107 M-1 s-1 and 1.6-7.4 × 108 M-1 s-1, respectively. The cytotoxic activities of the compounds on tumor cell lines followed the concentration-dependent manner exhibiting relatively high cytotoxic potency against breast cancer MCF-7, with CL50 values of 0.08-2.02 µM L-1 and lower potency against lung cancer A-549 (CL50 = 0.28-7.66 µM L-1). 3-nitro-pyrimidino-NPDO quinone was the most active compound against MCF-7 with CL50 of 0.08 ± 0.01 µM L-1 (0.02 µg mL-1)) which was followed by 3-nitro-NPDO with CL50 of 0.12 ± 0.03 µM L-1 (0.035 µg mL-1)) and 0.28 ± 0.08 µM L-1 (0.08 µg mL-1) on A-549 and MCF-7 cells, respectively, while 1- and 4-nitro-quinoidals produced the least cytotoxic effects. Tumor cells quantified by AO/EB staining showed that the cell death induced by the compounds occurs primarily through apoptosis.

Discovery of quinone-directed antitumor agents selectively bioactivated by NQO1 over CPR with improved safety profile.

In this work, we mainly focused on discovering compounds with good selectivity for NQO1 over CPR. The NQO1-mediated two-electron reduction of compounds would kill cancer cells selectively, while CPR-mediated one-electron reduction would induce potential hepatotoxicity. Several novel quinone-directed antitumor agents were discovered as specific NQO1 substrates through structure-activity relationship studies. Among them, compound 3,7,8-trimethylnaphtho[1,2-b]furan-4,5-dione (12b) emerged as the most specific substrate of the two-electron oxidoreductase NQO1 and could hardly be reduced by CPR. It afforded the highest selectivity between NQO1/CPR (selectivity ratio = 6.37), much higher than the control ß-lapachone (selectivity ratio = 1.36), indicated 12b may possess superior safety profile. The electrochemical studies provided a reasonable explanation to the good selectivity toward NQO1. Molecular docking studies supported that 12b was capable of forming additional C-H … π interactions with Trp105 and Phe178 residues compared to the control ß-lap. In addition, compound 12b was shown to kill cancer cells efficiently both in vitro and in vivo model. This work gave us a promising and novel scaffold for further investigation.

A Catalyst-Controlled Aerobic Coupling of ortho-Quinones and Phenols Applied to the Synthesis of Aryl Ethers.

ortho-Quinones are underutilized six-carbon-atom building blocks. We herein describe an approach for controlling their reactivity with copper that gives rise to a catalytic aerobic cross-coupling with phenols. The resulting aryl ethers are generated in high yield across a broad substrate scope under mild conditions. This method represents a unique example where the covalent modification of an ortho-quinone is catalyzed by a transition metal, creating new opportunities for their utilization in synthesis.

The Metabolic Fate of ortho-Quinones Derived from Catecholamine Metabolites.

ortho-Quinones are produced in vivo through the oxidation of catecholic substrates by enzymes such as tyrosinase or by transition metal ions. Neuromelanin, a dark pigment present in the substantia nigra and locus coeruleus of the brain, is produced from dopamine (DA) and norepinephrine (NE) via an interaction with cysteine, but it also incorporates their alcoholic and acidic metabolites. In this study we examined the metabolic fate of ortho-quinones derived from the catecholamine metabolites, 3,4-dihydroxyphenylethanol (DOPE), 3,4-dihydroxyphenylethylene glycol (DOPEG), 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylmandelic acid (DOMA). The oxidation of catecholic substrates by mushroom tyrosinase was followed by UV-visible spectrophotometry. HPLC analysis after reduction with NaBH4 or ascorbic acid enabled measurement of the half-lives of ortho-quinones and the identification of their reaction products. Spectrophotometric examination showed that the ortho-quinones initially formed underwent extensive degradation at pH 6.8. HPLC analysis showed that DOPE-quinone and DOPEG-quinone degraded with half-lives of 15 and 30 min at pH 6.8, respectively, and >100 min at pH 5.3. The major product from DOPE-quinone was DOPEG which was produced through the addition of a water molecule to the quinone methide intermediate. DOPEG-quinone yielded a ketone, 2-oxo-DOPE, through the quinone methide intermediate. DOPAC-quinone and DOMA-quinone degraded immediately with decarboxylation of the ortho-quinone intermediates to form 3,4-dihydroxybenzylalcohol (DHBAlc) and 3,4-dihydroxybenzaldehyde (DHBAld), respectively. DHBAlc-quinone was converted to DHBAld with a half-life of 9 min, while DHBAld-quinone degraded rapidly with a half-life of 3 min. This study confirmed the fact that ortho-quinones from DOPE, DOPEG, DOPAC and DOMA are converted to quinone methide tautomers as common intermediates, through proton rearrangement or decarboxylation. The unstable quinone methides afford stable alcoholic or carbonyl products.

In cellulo monitoring of quinone reductase activity and reactive oxygen species production during the redox cycling of 1,2 and 1,4 quinones.

Quinones are highly reactive molecules that readily undergo either one- or two-electron reduction. One-electron reduction of quinones or their derivatives by enzymes such as cytochrome P450 reductase or other flavoproteins generates unstable semiquinones, which undergo redox cycling in the presence of molecular oxygen leading to the formation of highly reactive oxygen species. Quinone reductases 1 and 2 (QR1 and QR2) catalyze the two-electron reduction of quinones to form hydroquinones, which can be removed from the cell by conjugation of the hydroxyl with glucuronide or sulfate thus avoiding its autoxidation and the formation of free radicals and highly reactive oxygen species. This characteristic confers a detoxifying enzyme role to QR1 and QR2, even if this character is strongly linked to the excretion capacity of the cell. Using EPR spectroscopy and confocal microscopy we demonstrated that the amount of reactive oxygen species (ROS) produced by Chinese hamster ovary (CHO) cells overexpressing QR1 or QR2 compared to naive CHO cells was determined by the quinone structural type. Indeed, whereas the amount of ROS produced in the cell was strongly decreased with para-quinones such as menadione in the presence of quinone reductase 1 or 2, a strong increase in ROS was recorded with ortho-quinones such as adrenochrome, aminochrome, dopachrome, or 3,5-di-tert-butyl-o-benzoquinone in cells overexpressing QR, especially QR2. These differences could originate from the excretion process, which is different for para- and ortho-quinones. These results are of particular interest in the case of dopamine considering the association of QR2 with various neurological disorders such as Parkinson disease.

2-Substituted 3-methylnaphtho[1,2-b]furan-4,5-diones as novel L-shaped ortho-quinone substrates for NAD(P)H:quinone oxidoreductase (NQO1).

A series of L-shaped ortho-quinone analogs were designed by analyzing the binding mode with NQO1. Metabolic studies demonstrated that compounds 2m, 2n and 2q exhibited higher metabolic rates than ß-lapachone. The docking studies, which supported the rationalization of the metabolic studies, constituted a prospective rational basis for the development of optimized ortho-quinone analogs. Besides, good substrates (2m, 2n and 2r) for NQO1 showed higher selective toxicity than ß-lapachone toward A549 (NQO1-rich) cancer cells versus H596 (NQO1-deficient) cells. Determination of superoxide (O2(•-)) production and in vitro cytotoxicity evaluation in the presence of the NQO1 inhibitor dicoumarol confirmed that the ortho-quinones exerted their antitumor activity through NQO1-mediated ROS production by redox cycling. It was suggested that the L-shaped quinone substrates for NQO1 possessed better specificity and safety than ß-lapachone.

A biomimetic catalytic aerobic functionalization of phenols.

The importance of aromatic C-O, C-N, and C-S bonds necessitates increasingly efficient strategies for their formation. Herein, we report a biomimetic approach that converts phenolic C-H bonds into C-O, C-N, and C-S bonds at the sole expense of reducing dioxygen (O2) to water (H2O). Our method hinges on a regio- and chemoselective copper-catalyzed aerobic oxygenation to provide ortho-quinones. ortho-Quinones are versatile intermediates, whose direct catalytic aerobic synthesis from phenols enables a mild and efficient means of synthesizing polyfunctional aromatic rings.

Mechanistic aspects of the tyrosinase oxidation of hydroquinone.

Contradictory reports on the behaviour of hydroquinone as a tyrosinase substrate are reconciled in terms of the ability of the initially formed ortho-quinone to tautomerise to the thermodynamically more stable para-quinone isomer. Oxidation of phenols by native tyrosinase requires activation by in situ formation of a catechol formed via an enzyme generated ortho-quinone. In the special case of hydroquinone, catechol formation is precluded by rapid tautomerisation of the ortho-quinone precursor to catechol formation.