High-performance liquid chromatographic method for sensitive determination of the alkylating agent CB1954 in human plasma

A high-performance liquid chromatography (HPLC) method is described for the measurement of the weak alkylating agent CB1954 in human plasma. CB1954 can be used as an innocuous prodrug designed for activation by bacterial nitroreductases in strategies of gene-directed enzyme–prodrug therapy, and becomes activated to a potent bifunctional alkylating agent. The HPLC method involves precipitation and solvent extraction and uses Mitomycin C (MMC) as an internal standard, with a retention time for MMC of 5.85±0.015 min, and for CB1954 of 10.72±0.063 min. The limit of detection for CB1954 is 2.9 ng/ml, and this compares favourably with systems involving direct analysis of plasma (limit of detection 600 ng/ml, approximately). The method is now being used for pharmacokinetic measurements in plasma samples from cancer patients entering phase I clinical trials of CB1954. Results using serial plasma samples from one patient are presented. The patient was treated intravenously with CB1954 (6 mg/m²), and plasma clearance of the drug showed biphasic kinetics with α half-life 14.6 min, and β half-life 170.5 min.     

Crystal structure of quinone reductase 2 in complex with resveratrol

Resveratrol has been shown to have chemopreventive, cardioprotective, and antiaging properties. Here, we report that resveratrol is a potent inhibitor of quinone reductase 2 (QR2) activity in vitro with a dissociation constant of 35 nM and show that it specifically binds to the deep active-site cleft of QR2 using high-resolution structural analysis. All three resveratrol hydroxyl groups form hydrogen bonds with amino acids from QR2, anchoring a flat resveratrol molecule in parallel with the isoalloxazine ring of FAD. The unique active-site pocket in QR2 could potentially bind other natural polyphenols such as flavonoids, as proven by the high affinity exhibited by quercetin toward QR2. K562 cells with QR2 expression suppressed by RNAi showed similar properties as resveratrol-treated cells in their resistance to quinone toxicity. Furthermore, the QR2 knockdown K562 cells exhibit increased antioxidant and detoxification enzyme expression and reduced proliferation rates. These observations could imply that the chemopreventive and cardioprotective properties of resveratrol are possibly the results of QR2 activity inhibition, which in turn, up-regulates the expression of cellular antioxidant enzymes and cellular resistance to oxidative stress.     

Structure-activity relationships and docking studies of synthetic 2-arylindole derivatives determined with aromatase and quinone reductase 1

In our ongoing effort of discovering anticancer and chemopreventive agents, a series of 2-arylindole derivatives were synthesized and evaluated toward aromatase and quinone reductase 1 (QR1). Biological evaluation revealed that several compounds (e.g., 2d, IC₅₀?=?1.61?μM; 21, IC₅₀?=?3.05?μM; and 27, IC₅₀?=?3.34?μM) showed aromatase inhibitory activity with half maximal inhibitory concentration (IC₅₀) values in the low micromolar concentrations. With regard to the QR1 induction activity, 11 exhibited the highest QR1 induction ratio (IR) with a low concentration to double activity (CD) value (IR?=?8.34, CD?=?2.75?μM), while 7 showed the most potent CD value of 1.12?μM. A dual acting compound 24 showed aromatase inhibition (IC₅₀?=?9.00?μM) as well as QR1 induction (CD?=?5.76?μM) activities. Computational docking studies using CDOCKER (Discovery Studio 3.5) provided insight in regard to the potential binding modes of 2-arylindoles within the aromatase active site. Predominantly, the 2-arylindoles preferred binding with the 2-aryl group toward a small hydrophobic pocket within the active site. The C-5 electron withdrawing group on indole was predicted to have an important role and formed a hydrogen bond with Ser478 (OH). Alternatively, meta-pyridyl analogs may orient with the pyridyl 3′-nitrogen coordinating with the heme group.     

Synthesis of carbon-11-labeled casimiroin analogues as new potential PET agents for imaging of quinone reductase 2 and aromatase expression in breast cancer

Carbon-11-labeled casimiroin analogues were first designed and synthesized as new potential PET agents for imaging of quinone reductase (QR) 2 and aromatase expression in breast cancer. [¹¹C]casimiroin (6-[¹¹C]methoxy-9-methyl-[1,3]dioxolo[4,5-h]quinolin-8(9H)-one, [¹¹C]11) and its carbon-11-labeled analogues 5,6,8-trimethoxy-1-[¹¹C]methyl-4-methylquinolin-2(1H)-one ([¹¹C]17), 8-methoxy-1-[¹¹C]methyl-4-methylquinolin-2(1H)-one ([¹¹C]21a), 6,8-dimethoxy-1-[¹¹C]methyl-4-methylquinolin-2(1H)-one ([¹¹C]21b), and 5,8-dimethoxy-1-[¹¹C]methyl-4-methylquinolin-2(1H)-one ([¹¹C]21c), were prepared from their corresponding precursors with [¹¹C]methyl triflate ([¹¹C]CH₃OTf) under basic conditions (NaH) through either O- or N-[¹¹C]methylation and isolated by semi-preparative HPLC method in 40-50% radiochemical yields decay corrected to end of bombardment (EOB), based on [¹¹C]CO₂, and 111-185 GBq/μmol specific activity at the end of synthesis (EOS).     

Crystal structure of human guanosine monophosphate reductase 2 (GMPR2) in complex with GMP

Guanosine monophosphate reductase (GMPR) catalyzes the irreversible and NADPH-dependent reductive deamination of GMP to IMP, and plays a critical role in re-utilization of free intracellular bases and purine nucleosides. Here, we report the first crystal structure of human GMP reductase 2 (hGMPR2) in complex with GMP at 3.0 A resolution. The protein forms a tetramer composed of subunits adopting the ubiquitous (alpha/beta)8 barrel fold. Interestingly, the substrate GMP is bound to hGMPR2 through interactions with Met269, Ser270, Arg286, Ser288, and Gly290; this makes the conformation of the adjacent flexible binding region (residues 268-289) fixed, much like a door on a hinge. Structure comparison and sequence alignment analyses show that the conformation of the active site loop (residues 179-187) is similar to those of hGMPR1 and inosine monophosphate dehydrogenases (IMPDHs). We propose that Cys186 is the potential active site, and that the conformation of the loop (residues 129-133) suggests a preference for the coenzyme NADPH over NADH. This structure provides important information towards understanding the functions of members of the GMPR family.     

The crystal structure of d-mandelate dehydrogenase reveals its distinct substrate and coenzyme recognition mechanisms from those of 2-ketopantoate reductase

d-Mandelate dehydrogenases (d-ManDHs), belonging to a new d-2-hydroxyacid dehydrogenase family, catalyze the conversion between benzoylformate and d-mandelate using NAD as a coenzyme. We determined the first d-ManDH structure, that of ManDH2 from Enterococcus faecalis IAM10071. The overall structure showed ManDH2 has a similar fold to 2-ketopantoate reductase (KPR), which catalyzes the conversion of 2-ketopantoate to d-pantoate using NADP as a coenzyme. They share conserved catalytic residues, indicating ManDH2 has the same reaction mechanism as KPR. However, ManDH2 exhibits significant structural variations in the coenzyme and substrate binding sites compared to KPR. These structural observations could explain their different coenzyme and substrate specificities.     

Cellular knock-down of quinone reductase 2: a laborious road to successful inhibition by RNA interference

NRH:quinone oxidoreductase 2 (QR2) is a long forgotten oxidoreductive enzyme that metabolizes quinones and binds melatonin. We used the potency of the RNA interference (RNAi)-mediated gene silencing to build a cellular model in which the role of QR2 could be studied. Because standard approaches were poorly successful, we successively used: (1) two chemically synthesized fluorescent small interfering RNA (siRNA) duplexes designed and tested for their gene silencing capacity leading to a maximal 40% QR2 gene silencing 48h post-transfection; (2) double transfection and cell-sorting of high fluorescent siRNA-transfected HT22 cells further enhancing QR2 RNAi silencing to 88%; (3) stable QR2 knock-down HT22 cell lines established with H1and U6 promoter driven QR2 short hairpin RNA (shRNA) encoding vectors, resulting in a 71-80% reduction of QR2 enzymatic activity in both QR2 shRNA HT22 cells. Finally, as a first step in the study of this cellular model, we observed a 42-48% reduction of menadione/BNAH-mediated toxicity in QR2 shRNA cells compared to the wild-type HT22 cells. Although becoming widespread and in some cases effective, siRNA-mediated cellular knock-down proves in the present work to be of marginal efficiency. Much development is required for this technique to be of general application.     

X-ray structural studies of quinone reductase 2 nanomolar range inhibitors

Quinone reductase 2 (QR2) is one of two members comprising the mammalian quinone reductase family of enzymes responsible for performing FAD mediated reductions of quinone substrates. In contrast to quinone reductase 1 (QR1) which uses NAD(P)H as its co‐substrate, QR2 utilizes a rare group of hydride donors, N‐methyl or N‐ribosyl nicotinamide. Several studies have linked QR2 to the generation of quinone free radicals, several neuronal degenerative diseases, and cancer. QR2 has been also identified as the third melatonin receptor (MT3) through in cellulo and in vitro inhibition of QR2 by traditional MT3 ligands, and through recent X‐ray structures of human QR2 (hQR2) in complex with melatonin and 2‐iodomelatonin. Several MT3 specific ligands have been developed that exhibit both potent in cellulo inhibition of hQR2 nanomolar, affinity for MT3. The potency of these ligands suggest their use as molecular probes for hQR2. However, no definitive correlation between traditionally obtained MT3 ligand affinity and hQR2 inhibition exists limiting our understanding of how these ligands are accommodated in the hQR2 active site. To obtain a clearer relationship between the structures of developed MT3 ligands and their inhibitory properties, in cellulo and in vitro IC₅₀ values were determined for a representative set of MT3 ligands (MCA‐NAT, 2‐I‐MCANAT, prazosin, S26695, S32797, and S29434). Furthermore, X‐ray structures for each of these ligands in complex with hQR2 were determined allowing for a structural evaluation of the binding modes of these ligands in relation to the potency of MT3 ligands.     

Design,synthesis, biological and structural evaluation of functionalized resveratrol analogues as inhibitors of quinone reductase 2

Resveratrol (3,5,4′-trihydroxylstilbene) has been proposed to elicit a variety of positive health effects including protection against cancer and cardiovascular disease. The highest affinity target of resveratrol identified so far is the oxidoreductase enzyme quinone reductase 2 (QR2), which is believed to function in metabolic reduction and detoxification processes; however, evidence exists linking QR2 to the metabolic activation of quinones, which can lead to cell toxicity. Therefore, inhibition of QR2 by resveratrol may protect cells against reactive intermediates and eventually cancer. With the aim of identifying novel inhibitors of QR2, we designed, synthesized, and tested two generations of resveratrol analogue libraries for inhibition of QR2. In addition, X-ray crystal structures of six of the resveratrol analogues in the active site of QR2 were determined. Several novel inhibitors of QR2 were successfully identified as well as a compound that inhibits QR2 with a novel binding orientation.     

Insights into the redox cycle of human quinone reductase 2

Abstract

NRH:quinone oxidoreductase 2 (QR2) is a cytosolic enzyme that catalyzes the reduction of quinones, such as menadione and co-enzymes Q. With the aim of understanding better the mechanisms of action of QR2, we approached this enzyme catalysis via electron paramagnetic resonance (EPR) measurements of the by-products of the QR2 redox cycle. The variation in the production of oxidative species such as H₂O₂, and subsequent hydroxyl radical generation, was measured during the course of QR2 activity under aerobic conditions and using pure human enzyme. The effects on the activity of the following were compared: (i) synthetic (N-benzyldihydronicotinamide, BNAH) or natural (nicotinamide riboside, NRH) co-substrates; (ii) synthetic (menadione) or natural (co-enzyme Q0, Q2) substrates; (iii) QR2 modulators and inhibitors (melatonin, resveratrol and S29434); (iv) a pro-drug activated via a redox cycle [CB1954, 5-(aziridin-1-yl)-2,4-dinitrobenzamide]. The results were also compared with those obtained with human QR1. The production of hydroxyl radicals is: (i) observed whatever the substrate/co-substrate used; ii) quenched by adding catalase; (iii) not observed with the specific QR2 inhibitor S29434; (iv) observed with the pro-drug CB1954. While QR2 produced free radicals with this pro-drug, QR1 gave no EPR signal showing the strong reducing capacity of QR2. In conclusion, EPR analysis of QR2 enzyme activity through free radical production enables modulators and effective inhibitors to be distinguished.     

Imidazopyridine CB2 agonists: optimization of CB2/CB1 selectivity and implications for in vivo analgesic efficacy

A new series of imidazopyridine CB2 agonists is described. Structural optimization improved CB2/CB1 selectivity in this series and conferred physical properties that facilitated high in vivo exposure, both centrally and peripherally. Administration of a highly selective CB2 agonist in a rat model of analgesia was ineffective despite substantial CNS exposure, while administration of a moderately selective CB2/CB1 agonist exhibited significant analgesic effects.     

Crystal structure of a biliverdin IXalpha reductase enzyme-cofactor complex

Biliverdin reductase (BVR) catalyzes the last step in heme degradation by reducing the gamma-methene bridge of the open tetrapyrrole, biliverdin IXalpha, to bilirubin with the concomitant oxidation of a beta-nicotinamide adenine dinucleotide (NADH) or beta-nicotinamide adenine dinucleotide phosphate (NADPH) cofactor. Bilirubin is the major bile pigment in mammals and has antioxidant and anticompliment activity. We have determined X-ray crystal structures of apo rat BVR and its complex with NADH at 1.2 A and 1.5 A resolution, respectively. In agreement with an independent structure determination of the apo-enzyme, BVR consists of an N-terminal dinucleotide-binding domain (Rossmann-fold) and a C-terminal domain that contains a six-stranded beta-sheet that is flanked on one face by several alpha-helices. The C-terminal and N-terminal domains interact extensively, forming the active site cleft at their interface. The cofactor complex structure reported here reveals that the cofactor nicotinamide ring extends into the active site cleft, where it is adjacent to conserved amino acid residues and, consistent with the known stereochemistry of the reaction catalyzed by BVR, the si face of the ring is accessible for hydride transfer. The only titratable side-chain that appears to be suitably positioned to function as a general acid in catalysis is Tyr97. This residue, however, is not essential for catalysis, since the Tyr97Phe mutant protein retains 50% activity. This finding suggests that the dominant role in catalysis may be performed by hydride transfer from the cofactor, a process that may be promoted by proximity of the invariant residues Glu96, Glu123, and Glu126, to the nicotinamide ring.     

Carboxin resistance in Paracoccus denitrificans conferred by a mutation in the membrane-anchor domain of succinate:quinone reductase (complex II)

Succinate:quinone reductase is a membrane-bound enzyme of the citric acid cycle and the respiratory chain. Carboxin is a potent inhibitor of the enzyme of certain organisms. The bacterium Paracoccus denitrificans was found to be sensitive to carboxin in vivo, and mutants that grow in the presence of 3′-methyl carboxin were isolated. Membranes of the mutants showed resistant succinate:quinone reductase activity. The mutation conferring carboxin resistance was identified in four mutants. They contained the same missense mutation in the sdhD gene, which encodes one of two membrane-intrinsic polypeptides of the succinate:quinone reductase complex. The mutation causes an Asp to Gly replacement at position 89 in the SdhD polypeptide. P. denitrificans strains that overproduced wild-type or mutant enzymes were constructed. Enzymic properties of the purified enzymes were analyzed. The apparent K _m for quinone (DPB) and the sensitivity to thenoyltrifluoroacetone was normal for the carboxin-resistant enzyme, but the succinate:quinone reductase activity was lower than for the wild-type enzyme. Mutations conferring carboxin resistance indicate the region on the enzyme where the inhibitor binds. A previously reported His to Leu replacement close to the [3Fe-4S] cluster in the iron-sulfur protein of Ustilago maydis succinate:quinone reductase confers resistance to carboxin and thenoyltrifluoroacetone. The Asp to Gly replacement in the P. denitrificans SdhD polypeptide, identified in this study to confer resistance to carboxin but not to thenoyltrifluoroacetone, is in a predicted cytoplasmic loop connecting two transmembrane segments. It is likely that this loop is located in the neighborhood of the [3Fe-4S] cluster. Received: 18 November 1997 / Accepted: 13 February 1998     

MicroRNA-665 is involved in the regulation of the expression of the cardioprotective cannabinoid receptor CB2 in patients with severe heart failure

The myocardial endocannabinoid system has been linked to stress response and cardioprotection. In chronic heart failure (CHF), protective CB2 receptors are markedly up-regulated while CB1 receptors are slightly down-regulated. We here provide evidence that myocardial CB receptors are subject to microRNA regulation. By a combined computational and experimental approach we show that CB1 receptors are regulated by miR-494, and CB2 receptors are targeted by miR-665. Moreover, we demonstrate that in CHF, miR-665 expression is significantly decreased while miR-494 is slightly increased, which is concordant with the previously reported alterations of CB receptors. These results suggest that in CHF, altered expression of specific miRNAs may contribute to a compensatory response of the diseased myocardium.     

Kinetic and structural characterisation of Escherichia coli nitroreductase mutants showing improved efficacy for the prodrug substrate CB1954

Escherichia coli nitroreductase (NTR) is a flavoprotein that reduces a variety of quinone and nitroaromatic substrates. Among these substrates is the prodrug 5-[aziridin-1-yl]-2,4-dinitrobenzamide (CB1954) that is activated by NTR to form two products, one of which is highly cytotoxic. NTR in combination with CB1954 has entered clinical trials for virus-directed enzyme-prodrug therapy of cancer. Enhancing the catalytic efficiency of NTR for CB1954 is likely to improve the therapeutic potential of this system. We previously identified a number of mutants at six positions around the active site of NTR that showed enhanced sensitisation to CB1954 in an E. coli cell-killing assay. In this study we have purified improved mutants at each of these positions and determined their steady-state kinetic parameters for CB1954 and for the antibiotic nitrofurazone. We have also made a double mutant, combining two of the most beneficial single mutations. All the mutants show enhanced specificity constants for CB1954, and, apart from N71S, the enhancement is selective for CB1954 over nitrofurazone. One mutant, T41L, also shows an increase in selectivity for reducing the 4-nitro group of CB1954 rather than the 2-nitro group. We have determined the three-dimensional structures of selected mutants bound to the substrate analogue nicotinic acid, using X-ray crystallography. The N71S mutation affects interactions of the FMN cofactor, while mutations at T41 and F124 affect the interactions with nicotinic acid. The structure of double mutant N71S/F124K combines the effects of the two individual single mutations, but it gives a greater selective enhancement of activity with CB1954 over nitrofurazone than either of these, and the highest specificity constant for CB1954 of all the mutations studied.     

Crystal Structure of the Human Cannabinoid Receptor CB2

1. Download high-res image (289KB)
2. Download full-size image

     

Optimisation of a novel series of selective CNS penetrant CB(2) agonists

A series of benzimidazole CB₂ receptor agonists were prepared and their properties investigated. Optimisation of the three benzimidazole substituents led to the identification of compound 23, a potent CB₂ full agonist (EC₅₀ 2.7 nM) with excellent selectivity over the CB₁ receptor (>3000-fold). Compound 23 demonstrated good CNS penetration in rat. Further optimisation led to the identification of compound 34 with improved selectivity over hERG and excellent CNS penetration in rat.     

Crystal structure of vestitone reductase from alfalfa (Medicago sativa L.)

Isoflavonoids are commonly found in leguminous plants, where they play important roles in plant defense and have significant health benefits for animals and humans. Vestitone reductase catalyzes a stereospecific NADPH-dependent reduction of (3R)-vestitone in the biosynthesis of the antimicrobial isoflavonoid phytoalexin medicarpin. The crystal structure of alfalfa (Medicago sativa L.) vestitone reductase has been determined at 1.4 A resolution. The structure contains a classic Rossmann fold domain in the N terminus and a small C-terminal domain. Sequence and structural analysis showed that vestitone reductase is a member of the short-chain dehydrogenase/reductase (SDR) superfamily despite the low levels of sequence identity, and the prominent structural differences from other SDR enzymes with known structures. The putative binding sites for the co-factor NADPH and the substrate (3R)-vestitone were defined and located in a large cleft formed between the N and C-terminal domains of enzyme. Potential key residues for enzyme activity were also identified, including the catalytic triad Ser129-Tyr164-Lys168. A molecular docking study showed that (3R)-vestitone, but not the (3S) isomer, forms favored interactions with the co-factor and catalytic triad, thus providing an explanation for the enzyme's strict substrate stereo-specificity.