4-Thio-5-bromo-2′-deoxyuridine: chemical synthesis and therapeutic potential of UVA-induced DNA damage

Crystal structures of (E)-2-amino-4-methyl-sulfanyl-6-oxo-1-(1-phenyl-ethyl-idene-amino)-1,6-di-hydro-pyrimidine-5-carbo-nitrile and (E)-2-amino-4-methyl-sulfanyl-6-oxo-1-[1-(pyridin-2-yl)ethyl-idene-amino]-1,6-di-hydro-pyrimidine-5-carbo-nitrile

The title compounds 3a, C14H13N5OS, and 3b, C13H12N6OS, both show an E configuration about the N=C bond and a planar NH2 group. The mol-ecules, which only differ in the presence of a phenyl (in 3a) or pyridyl (in 3b) substituent, are closely similar except for the different orientations of these groups. The amino hydrogen atoms form classical hydrogen bonds; in 3a the acceptors are the oxygen atom and the cyano nitro-gen atom, leading to ribbons of mol-ecules parallel to the b axis, whereas in 3b the acceptors are the oxygen atom and the pyridyl nitro-gen, leading to a layer structure perpendicular to (01).

Excited States of Thio-2'-deoxyuridine Bearing an Extended π-Conjugated System: 3',5'-Di-O-acetyl-5-phenylethynyl-4-thio-2'-deoxyuridine

A new thio-2'-deoxyuridine with an extended π-conjugated group was successfully synthesized: 3',5'-di-O-acetyl-5-phenylethynyl-4-thio-2'-deoxyuridine. The thio-2'-deoxyuridine derivative has a large red-shifted absorption band in the UVA region and also shows fluorescence, a rare photo-property among thionucleobases/thionucleosides. The triplet-triplet absorption spectrum and the rate constants (the intrinsic decay rate constant of the triplet state, the self-quenching rate constant, and the quenching rate constant of the triplet state by an oxygen molecule) of the thio-2'-deoxyuridine were obtained by transient absorption spectroscopy. The quantum yield of intersystem crossing and the quantum yield of singlet molecular oxygen formation (ϕ_Δ) under an oxygen atmosphere were also determined. The ϕ_Δ value of the new thio-2'-deoxyuridine was found to be substantially higher than all reported values of other thio-2'-deoxyribonucleosides in low oxygen concentrations similar to cancer cell environments. The fluorescence quantum yield depended on the excitation wavelength, revealing certain photochemical reactions in the higher excited singlet states. However, when excited into the higher excited state with non-resonant two-photon absorption, the ϕ_Δ of the thio-2'-deoxyuridine derivative was found to remain sufficiently large. These findings should be very useful for the development of thio-2'-deoxyribonucleoside-based pharmaceuticals as DNA-specific photosensitizers for photochemotherapy.

Toward Developing Therapies against Corona Virus: Synthesis and Anti-Avian Influenza Virus Activity of Novel Cytosine Thioglycoside Analogues

A series of pyrimidine and pyrimidine thioglycoside derivatives were newly synthesized using sodium 2-cyano-3-(arylamino)prop-1-ene-1,1-bis(thiolates) and urea to give the corresponding sodium 6-amino-5-aryl-2-oxo-1,2-dihydropyrimidine-4-thiolates. Stirring of the latter with peracetylated α-d-gluco- and galacto-pyranosyl bromides in DMF afforded the corresponding pyrimidine thioglycosides. On the other hand, treatment of 6-amino-5-aryl-2-oxo-1,2-dihydropyrimidine-4-thiolate salts with hydrochloric acid produced the corresponding pyrimidine-4-thioles, which on stirring with α-d-gluco- and galacto-pyranosyl bromides in sodium hydride and DMF gave the corresponding pyrimidine thioglycosides. Deacetylation of the protected pyrimidine thioglycosides gave the corresponding free pyrimidine thioglycosides. The synthesized compounds have been characterized by ¹³C NMR, ¹H NMR, and IR spectroscopy. The pyrimidine thioglycosides and free pyrimidine thioglycosides were tested against avian influenza H5N1 virus strain and exhibited highest to moderate activity.

Why Does the Type of Halogen Atom Matter for the Radiosensitizing Properties of 5-Halogen Substituted 4-Thio-2'-Deoxyuridines?

Radiosensitizing properties of substituted uridines are of great importance for radiotherapy. Very recently, we confirmed 5-iodo-4-thio-2'-deoxyuridine (ISdU) as an efficient agent, increasing the extent of tumor cell killing with ionizing radiation. To our surprise, a similar derivative of 4-thio-2'-deoxyuridine, 5-bromo-4-thio-2'-deoxyuridine (BrSdU), does not show radiosensitizing properties at all. In order to explain this remarkable difference, we carried out a radiolytic (stationary and pulse) and quantum chemical studies, which allowed the pathways to all radioproducts to be rationalized. In contrast to ISdU solutions, where radiolysis leads to 4-thio-2'-deoxyuridine and its dimer, no dissociative electron attachment (DEA) products were observed for BrSdU. This observation seems to explain the lack of radiosensitizing properties of BrSdU since the efficient formation of the uridine-5-yl radical, induced by electron attachment to the modified nucleoside, is suggested to be an indispensable attribute of radiosensitizing uridines. A larger activation barrier for DEA in BrSdU, as compared to ISdU, is probably responsible for the closure of DEA channel in the former system. Indeed, besides DEA, the XSdU anions may undergo competitive protonation, which makes the release of X- kinetically forbidden.

New synthetic strategies for acyclic and cyclic pyrimidinethione nucleosides and their analogues

Pyrimidinethione nucleosides are effective compounds and have significant and pivotal effects in several fields. New synthetic strategies for many pyrimidinethione nucleosides including acyclic and cyclic derivatives have been reported.

Oxidatively-generated damage to DNA and proteins mediated by photosensitized UVA

UVA accounts for about 95% of the solar ultraviolet (UV) radiation that reaches Earth and most likely contributes to human skin cancer risk. In contrast to UVB, which comprises the remaining 5% and is absorbed by DNA nucleobases to cause direct photodamage, UVA damages DNA indirectly. It does this largely through its interactions with cellular chromophores that act as photosensitisers to generate reactive oxygen species. Exogenously supplied chemicals, including some widely-prescribed medicines, may also act as photosensitisers and these drugs are associated with an increased risk of sun-related cancer. Because they amplify the effects of UVA on cells, they provide a means to investigate the mechanisms and effects of UVA-induced photodamage. Here, we describe some of the major lesions induced by two groups of UVA photosensitisers, the DNA thionucleotides and the fluoroquinolone antibiotics. In thionucleotides, replacement of the oxygen atoms of canonical nucleobases by sulfur converts them into strong UVA chromophores that can be incorporated into DNA. The fluoroquinolones are also UVA chromophores. They are not incorporated into DNA and induce a different range of DNA damages. We also draw attention to the potentially important contribution of photochemical protein damage to the cellular effects of photosensitised UVA. Proteins targeted for oxidation damage include DNA repair factors and we suggest that UVA-mediated protein damage may contribute to sunlight-induced cancer risk.

Modified Nucleotides as Substrates of Terminal Deoxynucleotidyl Transferase

The synthesis of novel modified nucleotides and their incorporation into DNA sequences opens many possibilities to change the chemical properties of oligonucleotides (ONs), and, therefore, broaden the field of practical applications of modified DNA. The chemical synthesis of nucleotide derivatives, including ones bearing thio-, hydrazino-, cyano- and carboxy groups as well as 2-pyridone nucleobase-containing nucleotides was carried out. The prepared compounds were tested as substrates of terminal deoxynucleotidyl transferase (TdT). The nucleotides containing N⁴-aminocytosine, 4-thiouracil as well as 2-pyridone, 4-chloro- and 4-bromo-2-pyridone as a nucleobase were accepted by TdT, thus allowing enzymatic synthesis of 3’-terminally modified ONs. The successful UV-induced cross-linking of 4-thiouracil-containing ONs to TdT was carried out. Enzymatic post-synthetic 3’-modification of ONs with various photo- and chemically-reactive groups opens novel possibilities for future applications, especially in analysis of the mechanisms of polymerases and the development of photo-labels, sensors, and self-assembling structures.

NMR studies on 4-thio-5-furan-modified and 4-thio-5-thiophene-modified nucleosides

Systematic NMR characterization of 4-thio-5-furan-pyrimidine nucleosides or 4-thio-5-thiophene-pyrimidine nucleosides (ribonucleosides and 2'-deoxynucleosides) was performed. All proton and carbon signals of 4-thio-5-thiophene-ribouridine and related analogues were unambiguously assigned. The orientations of the base (4-thiouridine or its deoxy analogue) relative to the ring (furan or thiophene) are explored by a NMR approach and further supported by X-ray crystallographic studies. The procedures presented here would be applicable to other modified nucleosides and nucleotides. Copyright © 2016 John Wiley & Sons, Ltd.

Synthesis of 4-thio-5-(2′′-thienyl)uridine and cytotoxicity activity against colon cancer cells in vitro

Antiproliferative effect of 4-thio-5-(2′′-thienyl)uridine on MC-38 cells, HT-29 cells and normal human fibroblasts cells.

UVA photoactivation of DNA containing halogenated thiopyrimidines induces cytotoxic DNA lesions

•

Growing cells incorporate thio-iodo-deoxyuridine and thio-bromo-deoxyuridine into DNA.

•

They are non-toxic but act as powerful UVA photosensitisers.

•

UVA lesions include DNA-protein and DNA–DNA crosslinks.

•

Singlet oxygen is involved in the formation of this potentially lethal damage.

•

Thio-halo-deoxynucleosides offer a potential selective therapeutic option.

Photochemotherapy, the combination of a photosensitiser and ultraviolet (UV) or visible light, is an effective treatment for skin conditions including cancer. The high mutagenicity and non-selectivity of photochemotherapy regimes warrants the development of alternative approaches. We demonstrate that the thiopyrimidine nucleosides 5-bromo-4-thiodeoxyuridine (SBrdU) and 5-iodo-4-thiodeoxyuridine (SIdU) are incorporated into the DNA of cultured human and mouse cells where they synergistically sensitise killing by low doses of UVA radiation. The DNA halothiopyrimidine/UVA combinations induce DNA interstrand crosslinks, DNA-protein crosslinks, DNA strand breaks, nucleobase damage and lesions that resemble UV-induced pyrimidine(6-4)pyrimidone photoproducts. These are potentially lethal DNA lesions and cells defective in their repair are hypersensitive to killing by SBrdU/UVA and SIdU/UVA. DNA SIdU and SBrdU generate lethal DNA photodamage by partially distinct mechanisms that reflect the different photolabilities of their C–I and C–Br bonds. Although singlet oxygen is involved in photolesion formation, DNA SBrdU and SIdU photoactivation does not detectably increase DNA 8-oxoguanine levels. The absence of significant collateral damage to normal guanine suggests that UVA activation of DNA SIdU or SBrdU might offer a strategy to target hyperproliferative skin conditions that avoids the extensive formation of a known mutagenic DNA lesion.

Study on Interaction between 5-Bromo-4-thio-2'-deoxyuridine and human serum albumin by spectroscopy and molecular docking

The interaction between 5-Bromo-4-thio-2'-deoxyuridine (4-SBrdU) and human serum albumin (HSA) was investigated by the methods of UV-vis absorbance, fluorescence and circular dichroism (CD) spectroscopy and molecular docking under simulative physiological conditions. The results showed that the quenching mechanism of HAS by 4-SBrdU was dynamic fluorescence quenching, hydrophobic interaction was the main intermolecular force based on thermodynamic data, the fluorescence experimental results were in agreement with results obtained by the molecular docking study.

Systematic assignment of NMR spectra of 5-substituted-4-thiopyrimidine nucleosides

Unambiguous characterization of 5-substituted-4-thiopyrimidine nucleosides (ribonucleosides and 2'-deoxynucleosides) was performed using NMR spectroscopy. Assignments of all proton and carbon signals of 5-bromo-4-thiouridine and related nucleosides were systematically carried out and firmly established by COSY and HMQC techniques. The NMR data of various 4-thiopyrimidine nucleosides are compared, and the key contributing factors discussed. The approach presented here is applicable to other modified nucleosides and nucleotides, as well as nucleobases.

NMR and UV studies of 4-thio-2'-deoxyuridine and its derivatives

5-Substituted-4-thio-2'-deoxyuridine nucleosides have been chemically synthesized and studied by NMR and UV spectroscopy. The results have been analyzed and discussed in connection with the previous data. The imino proton signal and the carbon signal of the thiocarbonyl group in the 5-substituted-4-thio-2'-deoxyuridines were found to be at much lower field, offering a potential for monitoring these modified bases at the DNA level. All 4-thionucleosides have strong absorptions at around 340 nm and consequently would be useful as potential UVA-induced anticancer agents.

Triplet Formation of 4-Thiothymidine and Its Photosensitization to Oxygen Studied by Time-Resolved Thermal Lensing Technique

Excited-state dynamics of 4-thiothymidine (S4-TdR) and its photosensitization to molecular oxygen in solution with UVA irradiation were investigated. Absorption and emission spectra measurements revealed that UVA photolysis of S4-TdR gives rise to a population of T1(pipi*), following S2(pipi*) --> S1(npi*) internal conversion. In transient absorption measurement, the 355 nm laser photolysis gave broad absorption (380-600 nm) bands of triplet S4-TdR. The time-resolved thermal lensing (TRTL) signal of S4-TdR containing the thermal component due to decay of triplet S4-TdR was clearly observed by the 355 nm laser excitation. The quantum yield for S1 --> T1 intersystem crossing was estimated to be unity by a triplet quenching experiment with potassium iodide. In the presence of molecular oxygen, the photosensitization from triplet S4-TdR gave rise to singlet oxygen O2 (1Deltag) with a quantum yield of 0.50. Therapeutic implications of such singlet oxygen formation are discussed.