Synthesis and characterisation of oligodeoxynucleotides containing thio analogues of (6-4) pyrimidine-pyrimidinone photo-dimers

Thionated organic compounds as emerging heavy-atom-free photodynamic therapy agents

This minireview focuses on recent progress in developing heavy-atom-free photosensitizers based on the thionation of nucleic acid derivatives and other biocompatible organic compounds for prospective applications in photodynamic therapy. Particular attention is given to the use of thionated nucleobase derivatives as "one-two punch" photodynamic agents. These versatile photosensitizers can act as "Trojan horses" upon metabolization into DNA and exposure to activating light. Their incorporation into cellular DNA increases their selectivity and photodynamic efficacy against highly proliferating skin cancer tumor cells, while simultaneously enabling the use of low irradiation doses both in the presence and in the absence of molecular oxygen. Also reviewed are their primary photochemical reactions, modes of action, and photosensitization mechanisms. New developments of emerging thionated organic photosensitizers absorbing visible and near-infrared radiation are highlighted. Future research directions, as well as, other prospective applications of heavy-atom-free, thionated photosensitizers are discussed.

Detection of the thietane precursor in the UVA formation of the DNA 6-4 photoadduct

Notwithstanding the central biological role of the (6-4) photoadduct in the induction of skin cancer by sunlight, crucial mechanistic details about its formation have evaded characterization despite efforts spanning more than half a century. 4-Thiothymidine (4tT) has been widely used as an important model system to study its mechanism of formation, but the excited-state precursor, the intermediate species, and the time scale leading to the formation of the (6-4) photoadduct have remained elusive. Herein, steady-state and time-resolved spectroscopic techniques are combined with new and reported quantum-chemical calculations to demonstrate the excited state leading to the formation of the thietane intermediate, its rate, and the formation of the (6-4) photoadduct using the 5’-TT(4tT)T(4tT)TT-3’ DNA oligonucleotide. Efficient, sub-1 ps intersystem crossing leads to the population of a triplet minimum of the thietane intermediate in as short as 3 ps, which intersystem crosses to its ground state and rearranges to form the (6-4) photoadduct.

The mechanisms of formation of the (6-4) photoproducts in DNA damage by sunlight is still debated. Here the authors show, by optical spectroscopies and computations, the details of the formation of a (6-4) photoadduct via the thietane intermediate in a single-stranded DNA oligonucleotide.

Theoretical studies on photo-induced cycloaddition and (6-4) reactions of the thymidine:4-thiothymidine dimer in a DNA duplex

Photo-induced cycloaddition and (6-4) reactions of the thymidine:4-thiothymidine dimer in a DNA duplex.

Mechanistic insight into photocrosslinking reaction between triplet state 4-thiopyrimidine and thymine

Multiple nonadiabatic pathways greatly facilitate the proceeding of photocrosslinking reactions between 4-thiopyrimidine and thymine.

UVA photoactivation of DNA containing halogenated thiopyrimidines induces cytotoxic DNA lesions

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

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

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

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

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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 the interaction between 4-thio-5-methyluridine and human serum albumin by spectroscopy and molecular modeling

The interaction between 4-thio-5-methyluridine and human serum albumin (HSA) under simulative physiological conditions has been studied by the methods of fluorescence, UV-VIS absorbance and circular dichroism (CD) spectroscopy. The results show that 4-thio-5-methyluridine has a static fluorescence quenching on human serum albumin (HSA). The main forces of both interactions have typical static interaction from thermodynamic data to the determination. The experimental result was in correspondence with molecular modeling theory.

4-Thiothymidine sensitization of DNA to UVA offers potential for a novel photochemotherapy

Photochemotherapy, in which ultraviolet radiation (UVR: 280-400 nm) or visible light is combined with a photosensitizing drug to produce a therapeutic effect that neither drug or radiation can achieve alone, is a proven therapeutic strategy for a number of non-malignant hyperproliferative skin conditions and various cancers. Examples are psoralen plus UVA (320-400 nm) radiation (PUVA) and photodynamic therapy (PDT). All existing photochemotherapies have drawbacks - for example the association of PUVA with the development of skin cancer, and pain that is often associated with PDT treatment of skin lesions. There is a clear need to develop alternative approaches that involve lower radiation doses and/or improved selectivity for target cells. In this review, we explore the possibility to address this need by exploiting thionucleoside-mediated DNA photosensitisation to low, non toxic doses of UVA radiation.

Identification of potentially cytotoxic lesions induced by UVA photoactivation of DNA 4-thiothymidine in human cells

Photochemotherapy-in which a photosensitizing drug is combined with ultraviolet or visible radiation-has proven therapeutic effectiveness. Existing approaches have drawbacks, however, and there is a clinical need to develop alternatives offering improved target cell selectivity. DNA substitution by 4-thiothymidine (S(4)TdR) sensitizes cells to killing by ultraviolet A (UVA) radiation. Here, we demonstrate that UVA photoactivation of DNA S(4)TdR does not generate reactive oxygen or cause direct DNA breakage and is only minimally mutagenic. In an organotypic human skin model, UVA penetration is sufficiently robust to kill S(4)TdR-photosensitized epidermal cells. We have investigated the DNA lesions responsible for toxicity. Although thymidine is the predominant UVA photoproduct of S(4)TdR in dilute solution, more complex lesions are formed when S(4)TdR-containing oligonucleotides are irradiated. One of these, a thietane/S(5)-(6-4)T:T, is structurally related to the (6-4) pyrimidine:pyrimidone [(6-4) Py:Py] photoproducts induced by UVB/C radiation. These lesions are detectable in DNA from S(4)TdR/UVA-treated cells and are excised from DNA more efficiently by keratinocytes than by leukaemia cells. UVA irradiation also induces DNA interstrand crosslinking of S(4)TdR-containing duplex oligonucleotides. Cells defective in repairing (6-4) Py:Py DNA adducts or processing DNA crosslinks are extremely sensitive to S(4)TdR/UVA indicating that these lesions contribute significantly to S(4)TdR/UVA cytotoxicity.

Thiothymidine combined with UVA as a potential novel therapy for bladder cancer

BACKGROUND

Thiothymidine (S(4)TdR) can be incorporated into DNA and sensitise cells to DNA damage and cell death following exposure to UVA light. Studies were performed to determine if the combination of S(4)TdR and UVA could be an effective treatment for bladder cancer.

METHODS

Uptake and incorporation of S(4)TdR was determined in rat and human bladder tumour cell lines. Measures of DNA crosslinking and apoptosis were also performed. In vivo activity of the combination of S(4)TdR and UVA was investigated in an orthotopic model of bladder cancer in rats.

RESULTS

Thiothymidine (200 μM) replaced up to 0.63% of thymidine in rat and tumour bladder cancer cells. The combination of S(4)TdR (10-200 μM) and UVA (1-5 kJ m(-2)) caused apoptosis and cell death at doses that were not toxic alone. Addition of raltitrexed (Astra Zeneca, Alderley Edge, Cheshire, UK) increased the incorporation of S(4)TdR into DNA (up to 20-fold at IC(5)) and further sensitised cells to UVA. Cytotoxic effect was associated with crosslinking of DNA, at least partially to protein. Intravenous administration of S(4)TdR, in combination with UVA delivered directly to the bladder, resulted in an antitumour effect in three of five animals treated.

CONCLUSION

These data indicate that the combination of S(4)TdR and UVA has potential as a treatment for bladder cancer, and give some insight into the mechanism of action. Further work is necessary to optimise the delivery of the two components.

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.

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

4-Thio-5-bromo-2'-deoxyuridine (3a) is prepared from 5-bromo-2'-deoxyuridine (BrdU) and its key properties are explored. The thionucleoside (3a) can react readily with monobromobimane and produces high fluorescence. 3a has UV maximum absorption at 340 nm and can be incorporated into cellular DNA. The cells containing 3a become sensitive to UVA light, offering therapeutic potential for UVA-induced cell killing.

Studies on the chemical synthesis of oligodeoxynucleotides containing the s5T(6-4)T photoproduct: side reactions derived from the methylsulfenyl thiol protection elucidated by MALDI mass spectrometry

Attempts to incorporate the phosphoramidite of the thymine-thymine (6-4) photoproduct C5 thiol analogue (s(5)T(6-4)T PP), whose sulfur atom was protected with the methylsulfenyl group, into oligodeoxynucleotides (ODNs), are reported. Using matrix-assisted laser desorption-ionisation mass spectrometry (MALDI-MS) coupled to enzymatic digestion, accurate mass measurements and tandem mass spectrometry experiments, we demonstrated that ODNs containing the (2-cyanoethylthio)(5)T(6-4)T PP were obtained. Supported by model reactions, these results were explained 1) by the incorporation, during oligonucleotide synthesis, of the sulfur deprotected phosphoramidite that arose from a Michaelis-Arbusov-type rearrangement, and 2) the Michael addition to the thiol of acrylonitrile released upon the cyanoethyl phosphotriester deprotection. To avoid the formation of the cyanoethyl adduct, the phosphotriester deprotection was carried out in the presence of a thiol in excess. This afforded the ODN containing the h(5)T(6-4)T PP.

Synthesis of the TT pyrimidine (6–4) pyrimidone photoproduct–thio analogue phosphoramidite building block

The phosphoramidite building block synthesis of the thio analogue at the 5,6-dihydropyrimidine C5 position of the thymidylyl(3'-5')thymidine (6-4) photoproduct 1 is presented. This compound was readily obtained from the appropriately protected dinucleotide P-methyl-5'-O-dimethoxytritylthymidilyl(3' --> 5')-4-thiothymidine 2 after irradiation at 366 nm, then S-sulfenylmethylation of the thiol function of the resulting (6-4) adduct, and phosphitylation of the 3'-hydroxyl group.

Pyrimidinethione nucleosides and their deaza analogues

The methods of preparation, structure, chemical properties and synthetic potentiality of pyrimidinethione nucleosides and their deaza analogues are reported.

Photoactivation of DNA thiobases as a potential novel therapeutic option

The thiopurines, 6-thioguanine and 6-mercaptopurine, are antileukemic agents that are incorporated into DNA following retrieval by the purine salvage pathway (see [1] for a review). Their toxicity requires active DNA mismatch repair (MMR), and thiopurine resistance is an acknowledged phenotype of MMR-defective cells [2, 3]. In addition to these direct cytotoxic effects, DNA thiobases have distinctive photochemical properties [4], the therapeutic potential of which has not been extensively evaluated. We report here that the thiopyrimidine nucleoside 4-thiothymidine is incorporated into DNA. It does not induce MMR-related toxicity, but it interacts synergistically with UVA light and dramatically sensitizes cultured human cells to very low, nonlethal UVA doses. 4-thiothymidine induced UVA dose enhancements of around 100-fold in DNA repair-proficient cells. Nucleotide excision repair-defective xeroderma pigmentosum cells were sensitized up to 1000-fold, implicating bulky DNA photoproducts in the lethal effect. The synergistic action of thiothymidine plus UVA required thymidine kinase, indicating a selective toxicity toward rapidly proliferating cells. Cooperative UVA cytotoxicity is a general property of DNA thiobases, and 6-thioguanine and 4-thiodeoxyuridine were also UVA sensitizers. Thiobase/UVA treatment may offer a novel therapeutic approach for the clinical management of nonmalignant conditions like psoriasis or for superficial tumors that are accessible to phototherapy.

Processing of UV damage in vitro by FEN-1 proteins as part of an alternative DNA excision repair pathway

Ultraviolet (UV) irradiation induces predominantly cyclobutane and (6-4) pyrimidine dimer photoproducts in DNA. Several mechanisms for repairing these mutagenic UV-induced DNA lesions have been identified. Nucleotide excision repair is a major pathway, but mechanisms involving photolyases and DNA glycosylases have also been characterized. Recently, a novel UV damage endonuclease (UVDE) was identified that initiates an excision repair pathway different from previously established repair mechanisms. Homologues of UVDE have been found in eukaryotes as well as in bacteria. In this report, we have used oligonucleotide substrates containing site-specific cyclobutane pyrimidine dimers and (6-4) photoproducts for the characterization of this UV damage repair pathway. After introduction of single-strand breaks at the 5' sides of the photolesions by UVDE, these intermediates became substrates for cleavage by flap endonucleases (FEN-1 proteins). FEN-1 homologues from humans, Saccharomyces cerevisiae, and Schizosaccharomyces pombe all cleaved the UVDE-nicked substrates at similar positions 3' to the photolesions. T4 endonuclease V-incised DNA was processed in the same way. Both nicked and flapped DNA substrates with photolesions (the latter may be intermediates in DNA polymerase-catalyzed strand displacement synthesis) were cleaved by FEN-1. The data suggest that the two enzymatic activities, UVDE and FEN-1, are part of an alternative excision repair pathway for repair of UV photoproducts.