A Synthetic Nucleoside Probe that Discerns a DNA Adduct from Unmodified DNA

Selective pairing of engineering nucleosides in DNA duplexes provides a potential means to probe structurally modified DNA bases (i.e., DNA adducts) and address challenges associated with correlating adduct chemical structure with biological impact. The current study provides the first example of a thermodynamically stable DNA base pair that is comprised of a biologically relevant carcinogen−DNA adduct and a synthetic nucleoside probe. O6-Benzylguanine is a mutagenic DNA adduct; molecular modeling indicates that a novel diaminonaphthyl-derived nucleoside (dNap):O6-benzylguanine base pair may be stabilized by a combination of hydrogen-bonding and hydrophobic interactions. The nucleoside dNap was synthetically incorporated into oligonucleotides, and a series of duplexes were evaluated by thermal denaturation studies. The bulky DNA adduct O6-benzylguanine forms a highly stable and orthogonal base pair with dNap. Data indicate π-stacking potential, self-pairing capacity, isomeric selectivity, 1:1 duplex stoichiometry, and a B-form DNA structure. Further studies are required to understand the physical determinants of adduct:probe pair stability for the design of probes for diverse forms of DNA damage and for the development of adduct−probe-based molecular techniques

Remote Substituents Influence Both the Thermodynamics and Kinetics of Zinc Binding to Tris-pyridyl Methanol Derivatives

Three families of tris-pyridyl methanol ligands were synthesized. An analysis of the Zn2+ binding properties of the ligands revealed that both steric and electronic properties of the pyridine substituents, as well as the nature of the group on the tertiary alcohol oxygen, control the thermodynamics and kinetics of complex formation

A Synthetic Nucleoside Probe that Discerns a DNA Adduct from Unmodified DNA

Selective pairing of engineering nucleosides in DNA duplexes provides a potential means to probe structurally modified DNA bases (i.e., DNA adducts) and address challenges associated with correlating adduct chemical structure with biological impact. The current study provides the first example of a thermodynamically stable DNA base pair that is comprised of a biologically relevant carcinogen−DNA adduct and a synthetic nucleoside probe. O6-Benzylguanine is a mutagenic DNA adduct; molecular modeling indicates that a novel diaminonaphthyl-derived nucleoside (dNap):O6-benzylguanine base pair may be stabilized by a combination of hydrogen-bonding and hydrophobic interactions. The nucleoside dNap was synthetically incorporated into oligonucleotides, and a series of duplexes were evaluated by thermal denaturation studies. The bulky DNA adduct O6-benzylguanine forms a highly stable and orthogonal base pair with dNap. Data indicate π-stacking potential, self-pairing capacity, isomeric selectivity, 1:1 duplex stoichiometry, and a B-form DNA structure. Further studies are required to understand the physical determinants of adduct:probe pair stability for the design of probes for diverse forms of DNA damage and for the development of adduct−probe-based molecular techniques

Quantitative Correlation of Drug Bioactivation and Deoxyadenosine Alkylation by Acylfulvene

Acylfulvenes (AFs) are a class of antitumor agents that exert their cytotoxic effects by forming covalent adducts with biomolecules, including DNA and proteins; clinical trials are ongoing for (−)-(hydroxymethyl)AF. Recently, depurinating DNA adducts N3-AF-deoxyadenosine (dAdo) and N7-AF-deoxyguanosine (dGuo) were identified from reactions of the parent compound, AF, with calf thymus DNA in the presence of the reductase enzyme alkenal/one oxidoreductase (AOR) and cofactor NADPH. We report here the development of a structure-specific quantitative analytical method for evaluating levels of the major base adduct N3-AF-adenine (Ade), which results from depurination of N3-AF-dAdo, and its utilization to further probe the relationship between AOR-mediated bioactivation and adduct formation in a cell-free system. As an internal standard, the isotopomer N3-AF-Ade-d3 was synthesized, and electrospray-ionization mass spectrometry coupled with high-performance liquid chromatography (HPLC-ESI-MS/MS) was used to detect and quantitate the adduct. This method was validated and found to be accurate (R2 ≥ 0.99) and precise (relative standard deviation 5.8–6.4%), with a limit of detection of 2 fmol. DNA samples, to which the stable-isotope-labeled internal standard was added, were subjected to neutral thermal hydrolysis yielding N3-AF-Ade. Adducts were isolated by a simple solid–liquid methanol extraction procedure, and adduct formation was examined in the presence of either high (1–3 µmol) or low (15 nmol) levels of DNA. Absolute amounts of N3-AF-Ade were measured in cell-free reaction mixtures containing varying levels of AOR as the only drug-activating enzyme. The increase in adduct formation (5–100 adducts per 105 DNA bases) over a range of enzyme concentrations (1–24 nM of AOR) showed saturation type behavior. This study reports a sensitive HPLC-ESI-MS/MS method for quantitation of the major DNA adduct induced by AF and illustrates a correlation between N3-AF-Ade formation and AOR-mediated enzymatic activation in a cell-free system, thus providing a template for further studies of drug toxicity in cells and in vivo

Chemical and Enzymatic Reductive Activation of Acylfulvene to Isomeric Cytotoxic Reactive Intermediates

Acylfulvenes (AFs), a class of semisynthetic analogues of the sesquiterpene natural product illudin S, are cytotoxic toward cancer cells. The minor structural changes between illudin S and AFs translate to an improved therapeutic window in preclinical cell-based assays and xenograft models. AFs are, therefore, unique tools for addressing the chemical and biochemical basis of cytotoxic selectivity. AFs elicit cytotoxic responses by alkylation of biological targets, including DNA. While AFs are capable of direct alkylation, cytosolic reductive bioactivation to an electrophilic intermediate is correlated with enhanced cytotoxicity. Data obtained in this study illustrate chemical aspects of the process of AF activation. By tracking reaction mechanisms with stable isotope-labeled reagents, enzymatic versus chemical activation pathways for AF were compared for reactions involving the NADPH-dependent enzyme prostaglandin reductase 1 (PTGR1) or sodium borohydride, respectively. These two processes resulted in isomeric products that appear to give rise to similar patterns of DNA modification. The chemically activated isomer has been newly isolated and chemically characterized in this study, including an assessment of its relative stereochemistry and stability at varying pH and under bioassay conditions. In mammalian cancer cells, this chemically activated analogue was shown to not rely on further cellular activation to significantly enhance cytotoxic potency, in contrast to the requirements of AF. On the basis of this study, we anticipate that the chemically activated form of AF will serve as a useful chemical probe for evaluating biomolecular interactions independent of enzyme-mediated activation

Depurinating Acylfulvene−DNA Adducts: Characterizing Cellular Chemical Reactions of a Selective Antitumor Agent

Acylfulvenes (AFs) are a class of semisynthetic agents with high toxicity toward certain tumor cells, and for one analogue, hydroxymethylacylfulvene (HMAF), clinical trials are in progress. DNA alkylation by AFs, mediated by bioreductive activation, is believed to contribute to cytotoxicity, but the structures and chemical properties of corresponding DNA adducts are unknown. This study provides the first structural characterization of AF-specific DNA adducts. In the presence of a reductive enzyme, alkenal/one oxidoreductase (AOR), AF selectively alkylates dAdo and dGuo in reactions with a monomeric nucleoside, as well as in reactions with naked or cellular DNA, with 3-alkyl-dAdo as the apparently most abundant AF−DNA adduct. Characterization of this adduct was facilitated by independent chemical synthesis of the corresponding 3-alkyl-Ade adduct. In addition, in naked or cellular DNA, evidence was obtained for the formation of an additional type of adduct resulting from direct conjugate addition of Ade to AF followed by hydrolytic cyclopropane ring-opening, indicating the potential for a competing reaction pathway involving direct DNA alkylation. The major AF-dAdo and AF-dGuo adducts are unstable under physiologically relevant conditions and depurinate to release an alkylated nucleobase in a process that has a half-life of 8.5 h for 3-alkyladenine and less than approximately 2 h for dGuo adducts. DNA alkylation further leads to single-stranded DNA cleavage, occurring exclusively at dGuo and dAdo sites, in a nonsequence-specific manner. In AF-treated cells that were transfected with either AOR or control vectors, the DNA adducts identified match those from in vitro studies. Moreover, a positive correlation was observed between DNA adduct levels and cell sensitivity to AF. The potential contributing roles of AOR-mediated bioactivation and adduct stability to the cytotoxicity of AF are discussed

Quantitative Analysis of Polyethylene Glycol (PEG) and PEGylated Proteins in Animal Tissues by LC-MS/MS Coupled with In-Source CID

The covalent conjugation of polyethylene glycol (PEG, typical MW > 10k) to therapeutic peptides and proteins is a well-established approach to improve their pharmacokinetic properties and diminish the potential for immunogenicity. Even though PEG is generally considered biologically inert and safe in animals and humans, the slow clearance of large PEGs raises concerns about potential adverse effects resulting from PEG accumulation in tissues following chronic administration, particularly in the central nervous system. The key information relevant to the issue is the disposition and fate of the PEG moiety after repeated dosing with PEGylated proteins. Here, we report a novel quantitative method utilizing LC-MS/MS coupled with in-source CID that is highly selective and sensitive to PEG-related materials. Both ^40KPEG and a tool PEGylated protein (ATI-1072) underwent dissociation in the ionization source of mass spectrometer to generate a series of PEG-specific ions, which were subjected to further dissociation through conventional CID. To demonstrate the potential application of the method to assess PEG biodistribution following PEGylated protein administration, a single dose study of ATI-1072 was conducted in rats. Plasma and various tissues were collected, and the concentrations of both ^40KPEG and ATI-1072 were determined using the LC-MS/MS method. The presence of ^40kPEG in plasma and tissue homogenates suggests the degradation of PEGylated proteins after dose administration to rats, given that free PEG was absent in the dosing solution. The method enables further studies for a thorough characterization of disposition and fate of PEGylated proteins

Characterization of ADME properties of [¹⁴C]asunaprevir (BMS-650032) in humans

<div><p></p><p>1. Asunaprevir (ASV, BMS-650032), a highly selective and potent NS3 protease inhibitor, is currently under development for the treatment of chronic hepatic C virus infection. This study describes <i>in vivo</i> biotransformation in humans and the identification of metabolic enzymes of ASV.</p><p>2. Following a single oral dose of [¹⁴C]ASV to humans, the majority of radioactivity (>73% of the dose) was excreted in feces with <1% of the dose recovered in urine. Drug-related radioactivity readily appeared in circulation and the plasma radioactivity was mainly attributed to ASV. A few minor metabolites were observed in human plasma and are not expected to contribute to the pharmacological activity because of low levels. The area under the curve (AUC) values of each circulating metabolite in humans were well below their levels in animals used in the long-term toxicological studies. In bile and feces, intact ASV was a prominent radioactive peak suggesting that both metabolism and direct excretion played important roles in ASV clearance.</p><p>3. The primary metabolic pathways of ASV were hydroxylation, sulfonamide hydrolysis and the loss of isoquinoline. <i>In vitro</i> studies with human cDNA expressed CYP enzymes and with human liver microsomes (HLM) in the presence of selective chemical inhibitors demonstrated that ASV was primarily catalyzed by CYP3A4 and CYP3A5.</p></div