Photosensitized Oxidative DNA Damage: From Hole Injection to Chemical Product Formation and Strand Cleavage

Oxidatively generated damage to DNA induced by a pyrenyl photosensitizer residue (Py) covalently attached to a guanine base in the DNA sequence context 5‘-d(CAT[G1Py]CG2TCCTAC) in aerated solutions was monitored from the initial one-electron transfer, or hole injection step, to the formation of chemical end-products monitored by HPLC, mass spectrometry, and high-resolution gel electrophoresis. Hole injection into the DNA was initiated by two-photon excitation of the Py residue with 355 nm laser pulses, thus producing the radical cation Py•+ and hydrated electrons; the latter are trapped by O2, thus forming the superoxide anion O2•-. The decay of the Py•+ radical is correlated with the appearance of the G•+/G(−H)• radical on microsecond time scales, and O2•- combines with guanine radicals at G1 to form alkali-labile 2,5-diamino-4H-imidazolone lesions (Iz1Py). Product formation in the modified strand is smaller by a factor of 2.4 in double-stranded than in single-stranded DNA. In double-stranded DNA, hot piperidine-mediated cleavage at G2 occurs only after G1Py, an efficient hole trap, is oxidized thus generating tandem lesions. An upper limit of hole hopping rates, khh 3 s-1 from G1•+−Py to G2 can be estimated from the known rates of the combination reaction of the G(−H)• and O2•- radicals. The formation of Iz products in the unmodified complementary strand compared to the modified strand in the duplex is ∼10 times smaller. The formation of tandem lesions is observed even at low levels of irradiation corresponding to “single-hit” conditions when less than ∼10% of the oligonucleotide strands are damaged. A plausible mechanism for this observation is discussed

Base Selectivity and Effects of Sequence and DNA Secondary Structure on the Formation of Covalent Adducts Derived from the Equine Estrogen Metabolite 4-Hydroxyequilenin

Equilenin, an important component of a widely prescribed hormone replacement formualtion for postmenopausal women, is metabolized by mammalian P450 enzymes to the catechol 4-hydroxyequilenin (4-OHEN). The oxidized o-quinone derivative of 4-OHEN is known to form cyclic covalent adducts with DNA [Bolton, J. (1998) Chem. Res. Toxicol. 11, 1113] in vitro and in vivo. The characteristics of 4-OHEN−DNA adduct formation were investigated with the oligonucleotides 5‘-d(CCATCGCTACC) (I), its complementary strand 5‘-d(GGTAGCGATGG) (II), one rich in C and the other in G, and the duplexes I·II. The identities of the modified bases were elucidated in terms of four stereoisomeric 4-OHEN−2‘-deoxynucleoside standards described earlier [Shen et al. (2001) Chem. Res. Toxicol. 11, 94; Embrechts et al. J. Mass Spectrom. 36, 317). The reactions of 4-OHEN with C are favored overwhelmingly in both single-stranded I and II with no guanine adducts observed in either case, and only minor proportions of A adducts were detected in sequence II. However, guanine adducts are observed in oligonucleotides that contain only G and unreactive T residues. The relative levels of cyclic covalent adducts observed in single-stranded I, II, and duplex I·II are ∼54:21:5, with only the end C groups in I modified in the I·II duplex. When 4-OHEN is reacted with calf thymus DNA, the reaction yield of cyclic adducts is more than ∼103-fold lower than in I. The cyclic 4-OHEN adducts lead to a pronounced thermal destabilization of duplexes I·II. Overall, cyclic adduct formation is markedly dependent on the sequence context and secondary structure of the DNA. The latter effect is attributed to the poor accessibilities of 4-OHEN to the reactive nucleotide Watson−Crick hydrogen-bonding interface in the interior of the duplex. In the single-stranded oligonucleotides I and II, the strikingly different selectivities of adduct formation are attributed to the formation of noncovalent preassociation complexes that favor reaction geometries with C, rather than with A or G. Finally, the levels of several typical biomarkers of oxidative DNA damage (including 8-oxo-2‘-deoxyguanosine) are formed in I in aqueous solutions with a yield at least 10 times smaller than the yield of cyclic 4-OHEN−dC adducts under identical reaction conditions

Light-Responsive Pt(IV) Prodrugs with Controlled Photoactivation and Low Dark Toxicity

Light-induced release of cisplatin from Pt­(IV) prodrugs represents a promising approach for precise control over the antiproliferative activity of Pt-based chemotherapeutic drugs. This method has the potential to overcome crucial drawbacks of conventional cisplatin therapy, such as high general toxicity toward healthy organs and tissues. Herein, we report two Pt­(IV) prodrugs with BODIPY-based photoactive ligands Pt-1 and Pt-2, which were designed using carbamate and triazole linkers, respectively. Both prodrugs demonstrated the ability to release cisplatin under blue light irradiation without the requirement of an external reducing agent. Dicarboxylated Pt-2 prodrug turned out to be more stable in the dark and more sensitive to light than its monocarbamate Pt-1 counterpart; these observations were explained using DFT calculations. The investigation of the photoreduction mechanism of Pt-1 and Pt-2 prodrugs using DFT modeling and ΔG0 PET estimation suggests that the photoinduced electron transfer from the singlet excited state of the BODIPY axial ligand to the Pt­(IV) center is the key step in the light-induced release of cisplatin from the complexes. Cytotoxicity studies demonstrated that both prodrugs were nontoxic in the dark and toxic to MCF-7 cells under low-dose irradiation with blue light, and the observed effect was solely due to the cisplatin release from the Pt­(IV) prodrugs. Our research presents an elegant synthetic approach to light-activated Pt­(IV) prodrugs and presents findings that may contribute to the future rational design of photoactivatable Pt­(IV) prodrugs