Enhanced nuclear-spin hyperpolarization of amino acids and proteins via reductive radical quenchers

Low-concentration photochemically induced dynamic nuclear polarization (LC-photo-CIDNP) has recently emerged as an effective tool for the hyperpolarization of aromatic amino acids in solution, either in isolation or within proteins. One factor limiting the maximum achievable signal-to-noise ratio in LC-photo-CIDNP is the progressive degradation of the target molecule and photosensitizer upon long-term optical irradiation. Fortunately, this effect does not cause spectral distortions but leads to a progressively smaller signal buildup upon long-term data-collection (e.g. 500 nM tryptophan on a 600 MHz spectrometer after ca. 200 scans). Given that it is generally desirable to minimize the extent of photodamage, we report that low-μM amounts of the reductive radical quenchers vitamin C (VC, i.e., ascorbic acid) or 2-mercaptoethylamine (MEA) enable LC-photo-CIDNP data to be acquired for significantly longer time than ever possible before. This approach increases the sensitivity of LC-photo-CIDNP by more than 100%, with larger enhancement factors achieved in experiments involving more transients. Our results are consistent with VC and MEA acting primarily by reducing transient free radicals of the NMR molecule of interest, thus attenuating the extent of photodamage. The benefits of this reductive radical-quencher approach are highlighted by the ability to collect long-term high-resolution 2D 1H-13C LC-photo-CIDNP data on a dilute sample of the drkN SH3 protein (5 μM).

Synthesis and Antibacterial Activity Against MRSA of Pleuromutilin Derivatives Possessing a Mercaptoethylamine Linker

BACKGROUND

Methicillin resistant Staphylococcus aureus (MRSA) usually invalidate powerful antibiotics in the clinic. Pleuromutilin derivatives have been reported to possess antibacterial activity against MRSA.

OBJECTIVE

The antibacterial activities against MRSA of a series of thirteen synthetic pleuromutilin derivatives were investigated through in vitro models.

METHODS

A series of novel thioehter pleuromutilin derivatives incorporating various aromatic substituents into the C14 side chain have been reported. The in vitro antibacterial activities of these derivatives against MRSA and Escherichia coli were tested by the broth dilution method.

RESULTS

Twelve pleuromutilin derivatives were designed, synthesized and evaluated for in vitro antibacterial activities against four Staphylococcus aureus strains. From structure-activity relationship studies, compound 11c was identified as promising compounds with the most potent in vitro antibacterial activity among the series (MIC = 0.0625-0.125 µg/ml) against Staphylococcus aureus strains. The binding of compound 11c to the 50s ribosome was investigated by molecular modeling.

CONCLUSION

It was found that there is a reasonable correlation between the binding free energy and the antibacterial activity.

Selective protection of zidovudine-induced DNA-damage by the antioxidants WR-1065 and tempol

The cytokinesis-block micronucleus cytome (CBMN) assay, introduced by Fenech, was used to demonstrate different types of DNA damage in MOLT-3 human lymphoblastoid cells exposed to 10 μM zidovudine (AZT). In addition, we explored the cytoprotective potential of two antioxidants, WR-1065 and Tempol, to decrease AZT-induced genotoxicity. Binucleated cells, arrested by Cytochalasin B (Cyt B), were evaluated for micronuclei (MN), caused by DNA damage or chromosomal loss, and chromatin nucleoplasmic bridges (NPBs), caused by telomere attrition. Additionally, nuclear buds (NBUDs), caused by amplified DNA, and apoptotic and necrotic (A/N) cells were scored. We hypothesized that AZT exposure would increase the frequency of genotoxic end points, and that the antioxidants Tempol and WR-1065 would protect against AZT-induced genotoxicity. MOLT-3 cells were exposed to 0 or 10 µM AZT for a total of 76 hr. After the first 24 hr, 0 or 5 µM WR-1065 and/or 0 or 200 µM Tempol were added for the remainder of the experiment. For the last 28 hr (of 76 hr), Cyt B was added to arrest replication after one cell division, leaving a predominance of binucleated cells. The nuclear division index (NDI) was similar for all treatment groups, indicating that the exposures did not alter cell viability. MOLT-3 cells exposed to AZT alone had significant (P < 0.05) increases in MN and NBs, compared to unexposed cells. Both Tempol and WR-1065 protected against AZT-induced MN formation (P < 0.003 for both), and WR-1065, but not Tempol, reduced the levels of A/N (P = 0.041). In cells exposed to AZT/Tempol there were significantly reduced levels of NBUDs, compared to cells exposed to AZT alone (P = 0.015). Cells exposed to AZT/WR-1065 showed reduced levels of NPBs, compared to cells exposed to AZT alone (P = 0.037). Thus WR-1065 and Tempol protected MOLT-3 cells against specific types of AZT-induced DNA damage.

SOD2-mediated adaptive responses induced by low-dose ionizing radiation via TNF signaling and amifostine

Manganese superoxide dismutase (SOD2)-mediated adaptive processes that protect against radiation-induced micronucleus formation can be induced in cells after a 2-Gy exposure by previously exposing them to either low-dose ionizing radiation (10cGy) or WR1065 (40μM), the active thiol form of amifostine. Although both adaptive processes culminate in elevated levels of SOD2 enzymatic activity, the underlying pathways differ in complexity, with the tumor necrosis factor α (TNFα) signaling pathway implicated in the low-dose radiation-induced response, but not in the thiol-induced pathway. The goal of this study was the characterization of the effects of TNFα receptors 1 and 2 (TNFR1, TNFR2) on the adaptive responses induced by low-dose irradiation or thiol exposure using micronucleus formation as an endpoint. BFS-1 wild-type cells with functional TNFR1 and 2 were exposed 24h before a 2-Gy dose of ionizing radiation to either 10cGy or a 40μM dose of WR1065. BFS2C-SH02 cells, defective in TNFR1, and BFS2C-SH22 cells, defective in both TNFR1 and TNFR2 and generated from BFS2C-SH02 cells by transfection with a murine TNFR2-targeting vector and confirmed to be TNFR2 defective by quantitative PCR, were also exposed under similar conditions for comparison. A 10-cGy dose of radiation induced a significant elevation in SOD2 activity in BFS-1 (P<0.001) and BFS2C-SH02 (P=0.005) but not BFS2C-SH22 cells (P=0.433), compared to their respective untreated controls. In contrast, WR1065 significantly induced elevations in SOD2 activity in all three cell lines (P=0.001, P=0.007, P=0.020, respectively). A significant reduction in the frequency of radiation-induced micronuclei was observed in each cell line when exposure to a 2-Gy challenge dose of radiation occurred during the period of maximal elevation in SOD2 activity. However, this adaptive effect was completely inhibited if the cells were transfected 24h before low-dose radiation or thiol exposure with SOD2 siRNA. Under the conditions tested, TNFR1 and 2 inhibition negatively affected the low-dose radiation-induced but not the thiol-induced adaptive responses observed to be mediated by elevations in SOD2 activity.

N,N-Bis(2-mercaptoethyl)methylamine: A New Coligand for Tc-99m Labeling of Hydrazinonicotinamide Peptides

Hydrazinonicotinamide (HYNIC) forms stable coordination complexes with Tc-99m when reacted with Tc(V)oxo species such as Tc-mannitol or other Tc-polyhydric complexes. However, radio-HPLC of [Tc-For-MLFK-HYNIC] labeled via Tc-polyhydric ligands demonstrated multiple radiochemical species each with unique biodistribution patterns. This is likely due to the fact that Tc can bind to the hydrazino moiety, as well as polyhydric ligands, in a variety of coordination geometries. Tridentate ligands, such as bis(mercaptoethyl)methylamine (NS2), may constrain the possible coordination geometries and improve overall stability. To investigate this, we synthesized NS2, converted the [Tc-mannitol-For-MLFK-HYNIC] to the corresponding NS2-containing complex [Tc-NS2-For-MLFK-HYNIC], and compared its infection imaging and biodistribution properties with [Tc-mannitol-For-MLFK-HYNIC]. Conversion to the NS2 complex was confirmed by HPLC which showed a single unique hydrophobic species with retention time greater than the [Tc-mannitol-For-MLFK-HYNIC] complex. Imaging experiments with both preparations were performed in rabbits with E. coli infections in the left thigh. Tissue radioactivity measurements demonstrated that compared to Tc-mannitol-peptide, accumulation of Tc-NS2-peptide was lower in blood, heart, and normal muscle and higher in spleen, infected muscle, and pus (p < 0.01). These results indicate that the Tc-NS2-peptide complex is chemically more homogeneous and exhibits improved infection localization and biodistribution properties. In an effort to model the interactions of the metal-HYNIC core with NS2 and related ligand types, the reactions of [ReCl3(NNC5H4NH)(NHNC5H4N)] and [99TcCl3(NNC5H4NH)(NHNC5H4N)], effective structural analogues for the [M(NNC5H4NH(x))2] core, with NS2, C5H3N-2,6-(CH2SH)2, O(CH2CH2SH)2, and S(CH2CH2SH)2 were investigated and the compounds [M[CH3N(CH2CH2S)2](NNC5H4N)(NHNC5H4N] (M = 99Tc (5a), Re (5b)), [Re[C5H3N-2,6-(CH2S)2](NNC5H4N)(NHNC5H4N)].CH2Cl2.0.5MeOH (7), [Re[SCH2CH2)2O] (NNC5H4N)(NHNC5H4N)] (8), and [Re[(SCH2CH2)2S](NNC5H4NH)(NHNC5H4N)]Cl (9) were isolated. Similarly, the reaction of [ReCl3(NNC5H4NH)(NHNC5H4N)] with the bidentate ligands pyridine-2-methanethiol and 3-(trimethlysilyl)pyridine-2-thiol led to the isolation of [ReCl(C5H4N-2-CH2S) (NNC5H4N)(NHNC5H4N)] (10) and [Re(2-SC5H3N-3-SiMe3)2 (NNC5H4N)(NHNC5H4N)] (11), respectively, while reaction with N-methylimidazole-2-thiol yielded the binuclear complex [Re(OH)Cl(SC3H2N2CH3)2(NNC5H4N)2 (NHNC5H4N)2] (12). The analogous metal-(HYNIC-OH) precursor, [ReCl3[NNC5H3NH(CO2R)] [NHNC5H3N(CO2R)]] (R = H, 13a; R = CH3, 13b) has been prepared and coupled to lysine to provide [RCl3[NNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)] [NHNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)]].2HCl (14.2HCl), while the reaction of the methyl ester 13b with 2-mercaptopyridine yields [Re(2-SC5H4N)2[NNC5H3N(CO2Me)][NHNC5H3N(CO2Me)]] (15). While the chemical studies confirm the robustness of the M-HYNIC core (M = Tc, Re) and its persistence in ligand substitution reactions at adjacent coordination sites of the metal, the isolation of oligomeric structures and the insolubility of the peptide conjugates of 13, 14, and 15 underscore the difficulty of characterizing these materials on the macroscopic scale, an observation relevant to the persistent concerns with reagent purity and identity on the tracer level.

Comparative study between the interaction of dephosphorylated amifostine (WR-1065) and amoxicilline with pBR322 in absence and presence of cisplatin by AFM

The effect of the amoxicilline as a possible cisplatin-action protector on pBR322 DNA has been visually studied by atomic force microscopy in comparison with the modifications caused by the controversed cisplatin protector amifostine. Incubation of amoxicilline with the plasmid DNA showed aggregation and compaction of DNA. Cisplatin incubated in the same conditions with DNA produced kinks and super-coiling of the circular form. In the case of previous treatment of DNA with amoxicilline, the characteristic effect of cisplatin is only partially observed. The amoxicilline seemed to control the action of cisplatin on DNA. The initial effect of dephosphorylated amifostine (WR-1065) when this protector was incubated with the plasmid was also the formation of aggregates and the compaction of DNA. However, addition of cisplatin successively after 1 and 2 h showed the characteristic modification caused by cisplatin but only in a decreasing percentage of molecules of DNA. Both molecules, amoxicilline and WR-1065 seem to control the strong direct action of cisplatin on DNA. This effect can justify the role as protecting agent of amifostine on side effects caused by cisplatin and can open new possibilities to other agents like amoxicilline.

N-(2-Mercaptoethyl)picolylamine as a diaminomonothiolate ligand for the "fac-[Re(CO)3]+" core

N-(2-Mercaptoethyl)picolylamine (MEPAH) was studied as a potentially biologically relevant ligand for the "fac-[M(CO)(3)](+)" core (M = Re, (99)Tc, (99m)Tc). To this end, the complex Re(CO)(3)(MEPA) was synthesized. The reaction of MEPAH with fac-[Re(CO)(3)(MeCN)(3)](+) took place over the course of seconds, showing the high affinity possessed by this ligand for the "fac-[Re(CO)(3)](+)" core. A single-crystal X-ray diffraction study was performed confirming the nature of Re(CO)(3)(MEPA), a rare mononuclear rhenium(I) thiolate complex. Additional exploration into derivatization of the ligand backbone has afforded the analogous N-ethyl complex, Re(CO)(3)(MEPA-NEt). The high affinity of the ligand for the metal coupled with the ease of its derivatization implies that utilization of this ligand system for the purposes of (99m)Tc-radiopharmaceutical development is promising.

Kinetic analysis of the reactions of 4-hydroperoxycyclophosphamide and acrolein with glutathione, mesna, and WR-1065

The kinetics of the reactions of glutathione (GSH) with 4-hydroperoxycyclophosphamide (4OOH-CP) and acrolein, a metabolite of 4OOH-CP, were investigated in a cell-free medium (pH approximately 7.5) and peripheral blood mononuclear cells. The ability of the thiol drugs, sodium 2-mercaptoethane sulfonate (mesna) and S-2-(3-aminopropylamino)ethanethiol (WR-1065), to affect the reactions of cellular GSH with the alkyalting agents was also studied. The amount of unreacted thiols in the various reactions was determined by derivatization with monobromobimane, followed by separation of fluorescent-labeled thioether adducts using high-pressure liquid chromatography. The second-order rate constants (k(2)) for reactions of GSH, mesna, and WR-1065 with 4OOH-CP in solution were 38 +/- 5, 25 +/- 5, and 880 +/- 50 M(-1) s(-1), respectively. The corresponding k(2) for reactions of GSH, mesna, and WR-1065 with acrolein were 490 +/- 100, 700 +/- 150, and >2000 M(-1) s(-1), respectively. The apparent rate constants for reactions of cellular GSH with acrolein and 4OOH-CP were smaller than those obtained in solution. Assuming that the k(2) is the same inside and outside cells, we estimate the first-order rate constant (k(1)) for transfer of 4OOH-CP and acrolein across the cell membrane as approximately 0.01 and approximately 0.04 s(-1), respectively. WR-1065 was more effective than mesna in blocking depletion of cellular GSH (because it passes into the cell more quickly and has higher reaction rates with the alkylators than the latter compound). When WR-1065 and mesna were used together, the protection against cellular depletion of GSH was additive. Our results are relevant to the administration of thiol drugs with high-dose alkylating agents.

Properties of selected S-nitrosothiols compared to nitrosylated WR-1065

WR-1065 ([N-mercaptoethyl]-1-3-diaminopropane), the active form of the aminothiol drug Ethyol/Amifostine, protects against toxicity caused by radiation, chemotherapy and endotoxin. Because WR-1065 and other thiols readily bind nitric oxide (NO), injurious conditions or therapies that induce the production or mobilization of NO could alter the effects of WR-1065. S-Nitrosothiols were prepared from various thiols by a standard method to compare properties and stability. Heteromolecular quantum correlation 2D nuclear magnetic resonance was used to characterize nitrosylated glutathione (GSH) and WR-1065; both S- and N-nitrosothiols were observed, depending on the experimental conditions. Three categories of S-nitrosothiol stability were observed: (1) highly stable, with t(1/2) > 8 h, N-acetyl-L-cysteine nitrosothiol (t(1/2) 15 h) > GSH nitrosothiol (t(1/2) 8 h); (2) intermediate stability, t(1/2) approximately 2 h, cysteamine nitrosothiol and WR-1065 nitrosothiol; and (3) low stability, t(1/2) < 1 h, cysteine nitrosothiol and Captopril nitrosothiol. Similar relative rates were observed for Hg(+2)-induced denitrosylation: WR-1065 reacted faster than GSH nitrosothiol, while GSH nitrosothiol reacted faster than N-acetyl-L-cysteine nitrosothiol. Mostly mediated by mixed-NPSH disulfide formation, the activity of the redox-sensitive cysteine protease, cathepsin H, was inhibited by the S-nitrosothiols, with WR-1065 nitrosothiol > cysteine nitrosothiol > N-acetyl-L-cysteine nitrosothiol and GSH nitrosothiol. These observations indicate that, relative to other nitrosylated non-protein thiols, the S-nitrosothiol of WR-1065 is an unstable non-protein S-nitrosothiols with a high reactive potential in the modification of protein thiols.

Thiazolidine prodrugs as protective agents against gamma-radiation-induced toxicity and mutagenesis in V79 cells

Representatives of two classes of thiazolidine prodrug forms of the well-known radioprotective agents L-cysteine, cysteamine, and 2-[(aminopropyl)amino]ethanethiol (WR-1065) were synthesized by condensing the parent thiolamine with an appropriate carbonyl donor. Inherent toxicity of the prodrugs was assessed in V79 cells using a clonogenic survival assay. Protection against radiation-induced cell death was measured similarly after exposure to 0--8 Gy gamma ((137)Cs) radiation. Antimutagenic activity was determined at the hypoxanthine-guanine phosphoribosyltransferase (HGPRT) locus. All thiazolidine prodrugs exhibited less toxicity than their parent thiolamines, sometimes dramatically so. Protection against radiation-induced cell death was observed for the 2-alkylthiazolidine, 2(R,S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)thiazolidine (RibCyst), which produced a protection factor at 8 Gy of 1.8; the cysteine analogue, 2(R,S)-D-ribo-(1',2',3',4'-tetrahydroxybutyl)thiazolidine-4(R)-carboxylic acid (RibCys), was less active. RibCyst also exhibited excellent antimutational activity, rivaling that of WR-1065. The 2-oxothiazolidine analogues showed little activity in either determination under the conditions tested, perhaps due to their enhanced chemical and biochemical stability.

Use of amifostine in hematologic malignancies, myelodysplastic syndrome, and acute leukemia

The phosphorylated thiol amine, amifostine (Ethyol; Alza Pharmaceuticals, Palo Alto, CA/US Bioscience, West Conshohocken, PA), is a cytoprotective agent for cisplatin-based chemotherapy. Recent investigations have given rise to new potential applications of amifostine in hematologic malignancies. Amifostine appears to exert a sustained mitogenic effect in primitive hematopoietic progenitors that results in a significant increase in colony-forming capacity. Amifostine also retards cell loss and delays commitment to apoptosis initiated by cytokine deprivation, suggesting that amifostine has trophic effects similar to the hematopoietic cytokines. The abilities to prolong progenitor survival and to delay apoptosis under conditions of cellular stress make amifostine an attractive agent for investigation in bone marrow failure states. Amifostine promotes more effective hematopoiesis in patients with myelodysplastic syndrome, although additional investigation is needed to further define the optimal dose and schedule of administration. Furthermore, amifostine may selectively enhance the cytotoxicity of chemotherapeutic agents in leukemia progenitors. When the sensitivity of leukemic and normal progenitors to mafosfamide was evaluated with and without amifostine pretreatment, amifostine effectively protected normal myeloid and erythroid progenitors while increasing leukemic cell kill. Thus, amifostine represents a unique agent with promising potential for therapeutic application in hematologic malignancies. Further investigation is needed to define its role in clinical practice.

Disulfide interchange by ionizing radiation

The irradiation of a soluition of two symmetric disulfides produces detectable amounts of the nonsymmetric mixed disulfide. The effect is abolished by the addition of ethylmaleimide. The finding indicates that radiation caulses a disulfide opening and recombining process which may be of radiobiological interest.

Thiol WR-1065 and disulphide WR-33278, two metabolites of the drug ethyol (WR-2721), protect DNA against fast neutron-induced strand breakage

The main metabolites of the cytoprotective drug Ethyol (Amifostine, WR-2721) are the thiol WR-1065 and the disulphide WR-33278 (formed by the oxidation of WR-1065). Both metabolites are well-known protectors against DNA damage induced by gamma-rays. Using supercoiled plasmid DNA and restriction fragments we show that they protect efficiently also in the case of fast neutrons. In anoxic conditions WR-1065 (Z = +2) protects by scavenging of OH. and by 'chemical repair' (by H donation from its SH function). WR-33278 (Z = +4) protects by scavenging of OH. and, in the case of the supercoiled plasmid DNA, by reducing the accessibility of radiolytic attack sites via the induction of packaging of DNA in liquid-crystalline condensates (observed by circular dichroism). Because of this second mechanism, the plasmid DNA is more efficiently protected by WR-33278 than by WR-1065, at concentration ratios > 1 drug/4 nucleotides. Moreover, using sequencing gel electrophoresis of irradiated fragments of known sequence, we show that the protection by the two metabolites is non-homogeneously distributed along the DNA sequence, with 'hot spots' of protection and with unprotected regions. Based on presented molecular modelling results we explain the sequence dependence of radioprotection by structural variations induced by the binding of the drugs.

Free-radical repair by a novel perthiol: reversible hydrogen transfer and perthiyl radical formation

2-(3-Aminopropyl-amino) ethaneperthiol (RSSH, the perthiol analogue of the thiol radioprotector, WR-1065) reacts with the alpha-hydroxy alkyl radical (CH3)2C.OH by donating a hydrogen atom as indicated by the characterization of perthiyl radicals (RSS.; lambda max approximately 374 nm, epsilon 374 approximately 1680 +/- 20 dm3 mol-1 cm-1) by pulse radiolysis. The perthiyl radical abstracts a hydrogen from the alcohol to establish a reversible hydrogen-transfer equilibrium. This equilibrium lies predominantly on the side of radical repair since the rate constants for the forward and reverse reactions at pH 4 are: kappa(RSSH+(CH3)2C.OH) = (2.4 +/- 0.1) x 10(9) dm3 mol-1 s-1 and kappa(RSS.+(CH3)2CHOH) = (3.8 +/- 0.3) x 10(3) dm3 mol-1 s-1 respectively. The pKa (RSSH<-->RSS(-)+H+) = 6.2 +/- 0.1 was determined from the pH dependence of the rate of perthiol repair. Identical experiments have been performed with WR-1065 allowing a direct comparison of free-radical repair reactivity to be made with the parthiol analogue. At pH approximately 7.4 the reactivities of the thiol and perthiol were similar, both repairing the alcohol radical with a rate constant of approximately (2.4 +/- 0.1) x 10(8) dm3 mol-1 s-1. However, at pH 5 whilst the hydrogen-donation rate of the thiol was 15-20% higher than at pH 7.4, the perthiol reactivity was over an order of magnitude higher. The thermodynamic driving force for the observed enhanced free-radical repair reactivity of RSSH compared to RSH is attributed to the resonance stabilization energy of 8.8 kJ mol-1 within the RSS. radical. These results indicate a possible application of RSSH/RSS- as DNA-targeted antioxidants or chemoprotectors.

Determination of the acid dissociation constants for WR-1065 by proton NMR spectroscopy

The acid dissociation constants for N-(2-mercaptoethyl)-1,3-diaminopropane (WR-1065) were determined in D2O by 360- and 500-MHz NMR spectroscopy. Results obtained at 0.21 M initial ionic strength and 26 degrees C were corrected to 25 degrees C yielding pKD1 = 8.28 +/- 0.04, pKD2 = 9.88 +/- 0.07, and pKD3 = 11.58 +/- 0.03. Correction of these values for the effect of the deuterium isotope upon the ionization reaction yielded dissociation constants in water of pKH1 = 7.69 +/- 0.09, pKH2 = 9.35 +/- 0.09, and pKH3 = 11.10 +/- 0.08. Analysis of the changes in chemical shift with pD indicated that the first ionization occurs largely through ionization of the thiol group (approximately 67%) and to a lesser extent the secondary ammonium group (approximately 30%), whereas the third ionization involves mainly the secondary ammonium group (approximately 65%) and to a lesser extent the primary ammonium group (approximately 30%). Estimates of the microscopic pK values for WR-1065 were also obtained from the results.