The Mechanism of Nucleophilic Displacements at Phosphorus in Chloro-Substituted Methylphosphonate Esters: P−O vs P−C Bond Cleavage: A DFT Study

Towards catch-up therapy: evaluation of nucleophilic active pharmaceutical ingredients for the treatment of percutaneous VX poisoning, in-vial and in-vitro studies

Dermal exposure to low volatility organophosphorus chemical warfare agents (OP CWA) poses a great risk to the exposed person. Due to their lipophilic nature, these compounds rapidly absorb into the skin, leading to the formation of a "dermal reservoir" from which they slowly enter the bloodstream causing prolonged intoxication. Traditionally, strategies to counter the toxicity of such substances consist of chemical decontamination/physical removal of the residual agent from the skin surface (preferably as soon as possible following the exposure) and administration of antidotes in the case of intoxication signs. Hence, these strategies are unable to counter a substantial amount of the agent, which accumulates inthe dermal reservoir. More than a decade ago, the concept of a "catch-up therapy" intended to neutralize the dermal reservoir was suggested. Herein, we describe examples of potential "catch-up therapy" lotions - vehicles designed to deliver small nucleophilic molecules into the skin and potentially decompose the remaining CWA before it reaches the blood stream. Eleven nucleophilic compounds, based on approved drugs, were initially screened. They were then tested in various binary solutions, for their detoxification efficacy and degradation ability towards lipophilic OP CWA models such as dibutylphosphofluoridate and o-nitro-phenyl diphenyl phosphate, as well as the nerve agent VX, by means of kinetic ³¹P NMR and UV-Vis spectroscopy. Of these, the potassium and diethyl ammonium salts of acetohydroxamic acid (AHAK and AHA DEA) in (DMSO/H₂O 1:4) were found to be the most active nucleophiles, hydrolyzing VX in practical time scales (t_1/2 = 5.28 and 6.78 min, respectively). The vehicle solution DMSO/H₂O 1:4 promoted the penetration of substantial amounts of AHA K and AHA DEA through excised pig skin in in-vitro studies, suggesting that such formulations may serve as useful CWA nucleophilic scavengers for both on and within -skin detoxification. These findings may pave the way to a more efficacious treatment against low volatility OP CWA percutaneous poisoning.

The intriguing biology and chemistry of fosfomycin: the only marketed phosphonate antibiotic

Recently infectious diseases caused by the increased emergence and rapid spread of drug-resistant bacterial isolates have been one of the main threats to global public health because of a marked surge in both morbidity and mortality. The only phosphonate antibiotic in the clinic, fosfomycin, is a small broad-spectrum molecule that effectively inhibits the initial step in peptidoglycan biosynthesis by blocking the enzyme, MurA in both Gram-positive and Gram-negative bacteria. As fosfomycin has a novel mechanism of action, low toxicity, a broad spectrum of antibacterial activity, excellent pharmacodynamic/pharmacokinetic properties, and good bioavailability, it has been approved for clinical use in the treatment of urinary tract bacterial infections in many countries for several decades. Furthermore, its potential use for difficult-to-treat bacterial infections has become promising, and fosfomycin has become an ideal candidate for the effective treatment of bacterial infections caused by multidrug-resistant isolates, especially in combination with other therapeutic drugs. Here we aim to present an overview of the biology and chemistry of fosfomycin including isolation and characterization, pharmacology, biosynthesis and chemical synthesis since its discovery in order to not only help scientists reassess the role of this exciting drug in fighting antibiotic resistance but also build the stage for discovering more novel phosphonate antibiotics in the future.

Screening for Improved Nerve Agent Simulants and Insights into Organophosphate Hydrolysis Reactions from DFT and QSAR Modeling

The effective detoxification of chemical warfare agents, specifically nerve agents, is a pressing issue in the modern world. Due to the high toxicity of these molecules, simulants are often used in experiments as substitutes for the agents. However, there is little reason to believe that the current simulants used in the literature are optimal predictors of nerve agent reactivity. Density functional theory calculations were performed on the alkaline hydrolysis of over 100 organophosphate molecules to identify improved simulants for the G-series nerve agents soman and sarin, based on low toxicity and similarity to nerve agent hydrolysis energetics and degradation mechanism. This screening highlighted 5 molecules that have nearly identical reaction barriers to the actual agents, while being far less toxic. Quantitative structure-activity relationship (QSAR) models were also derived to determine the most significant molecular descriptors for describing the hydrolysis free energy barriers of these reactions. The optimal QSAR model was subjected to a thorough statistical analysis and validation procedure to confirm its predictive capacity, showing excellent quantitative and ranking accuracy. It was further shown that the model trained on G-series agents can reliably predict energetics for other organophosphate classes as well, including VX. Through these computational insights, experimentalists may be aided in accurately and safely studying these reactions with less toxic simulants.

Nucleophilic Substitution (SN 2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

The reaction potential energy surface (PES), and thus the mechanism of bimolecular nucleophilic substitution (SN 2), depends profoundly on the nature of the nucleophile and leaving group, but also on the central, electrophilic atom, its substituents, as well as on the medium in which the reaction takes place. Here, we provide an overview of recent studies and demonstrate how changes in any one of the aforementioned factors affect the SN 2 mechanism. One of the most striking effects is the transition from a double-well to a single-well PES when the central atom is changed from a second-period (e. g. carbon) to a higher-period element (e.g, silicon, germanium). Variations in nucleophilicity, leaving group ability, and bulky substituents around a second-row element central atom can then be exploited to change the single-well PES back into a double-well. Reversely, these variations can also be used to produce a single-well PES for second-period elements, for example, a stable pentavalent carbon species.

Phosphonate Biochemistry

Organophosphonic acids are unique as natural products in terms of stability and mimicry. The C-P bond that defines these compounds resists hydrolytic cleavage, while the phosphonyl group is a versatile mimic of transition-states, intermediates, and primary metabolites. This versatility may explain why a variety of organisms have extensively explored the use organophosphonic acids as bioactive secondary metabolites. Several of these compounds, such as fosfomycin and bialaphos, figure prominently in human health and agriculture. The enzyme reactions that create these molecules are an interesting mix of chemistry that has been adopted from primary metabolism as well as those with no chemical precedent. Additionally, the phosphonate moiety represents a source of inorganic phosphate to microorganisms that live in environments that lack this nutrient; thus, unusual enzyme reactions have also evolved to cleave the C-P bond. This review is a comprehensive summary of the occurrence and function of organophosphonic acids natural products along with the mechanisms of the enzymes that synthesize and catabolize these molecules.

Extensive hydrolysis of phosphonates as unexpected behaviour of the known His6-organophosphorus hydrolase

The catalytic activity of hexahistidine-tagged organophosphorus hydrolase (His6-OPH) in hydrolytic reactions of methylphosphonic acid (MPA) and its monoesters and diesters being decomposition products of R-VX was demonstrated for the first time. The catalytic constants of enzyme in such reactions were determined. The mechanism of C-P bond cleavage in the MPA by His6-OPH was proposed. Such reaction was estimated to be carried out with the soluble and nanocapsulated forms of His6-OPH. His6-OPH was demonstrated to be capable of degrading the key organophosphorus components of reaction masses (RMs) that are produced by the chemical detoxification of R-VX and RMs are multi-substrate mixtures for this enzyme. The kinetic model describing the behaviour of His6-OPH in RMs was proposed and was shown to adequately fit experimental points during degradation of the real samples of RMs.