Assessing the therapeutic efficacy of oxime therapies against percutaneous organophosphorus pesticide and nerve agent challenges in the Hartley guinea pig

Abstract

Given the rapid onset of symptoms from intoxication by organophosphate (OP) compounds, a quick-acting, efficacious therapeutic regimen is needed. A primary component of anti-OP therapy is an oxime reactivator to rescue OP-inhibited acetylcholinesterases. Male guinea pigs, clipped of hair, received neat applications of either VR, VX, parathion, or phorate oxon (PHO) at the 85th percentile lethal dose, and, beginning with presentation of toxicosis, received the human equivalent dose therapy by intramuscular injection with two additional follow-on treatments at 3-hr intervals. Each therapy consisted of atropine free base at 0.4 mg/kg followed by one of eight candidate oximes. Lethality rates were obtained at 24 hr after VR, VX and PHO challenges, and at 48 hr after challenge with parathion. Lethality rates among symptomatic, oxime-treated groups were compared with that of positive control (OP-challenged and atropine-only treated) guinea pigs composited across the test days. Significant (p ≤ 0.05) protective therapy was afforded by 1,1-methylene bis(4(hydroxyimino- methyl)pyridinium) dimethanesulfonate (MMB4 DMS) against challenges of VR (p ≤ 0.001) and VX (p ≤ 0.05). Lethal effects of VX were also significantly (p ≤ 0.05) mitigated by treatments with oxo-[[1-[[4-(oxoazaniumylmethylidene)pyridin-1-yl] methoxymethyl]pyridin-4-ylidene]methyl]azanium dichloride (obidoxime Cl2) and 1-(((4-(aminocarbonyl) pyridinio)methoxy)methyl)-2,4-bis((hydroxyimino)methyl)pyridinium dimethanesulfonate (HLö-7 DMS). Against parathion, significant protective therapy was afforded by obidoxime dichloride (p ≤ 0.001) and 1,1′-propane-1,3-diylbis{4-[(E)-(hydroxyimino)methyl]pyridinium} dibromide (TMB-4, p ≤ 0.01). None of the oximes evaluated was therapeutically effective against PHO. Across the spectrum of OP chemicals tested, the oximes that offered the highest level of therapy were MMB4 DMS and obidoxime dichloride.

Toxicity and median effective doses of oxime therapies against percutaneous organophosphorus pesticide and nerve agent challenges in the Hartley guinea pig

“…Interestingly, while this same oxime completely prevented lethality against both percutaneously (Snider et al, 2015) or subcutaneously (Wilhelm et al, 2014) administered VX, it did not significantly promote overall survival against PHO in either previous reports. Efficacy in those studies was evaluated at 24-hr after a LD 85 challenge followed by a single administration of therapy that was given either 1 min or at onset of clinical toxic signs.…”

“…Figure 1 shows that MMB4, HLö-7, and obidoxime are bis-quaternary oximes, whereas 2-PAM is mono-functional. As discussed in Snider (2015), the bis nature of the larger molecules gives them an advantage over 2-PAM in protecting AChE from OP attack, as one end can interact with the rim of the AChE gorge while the other end can extend into the gorge (Harel et al, 1993). Such dual binding may help to shield AChE from OP attack, as well as outcompete and/or dislodge OP molecules interacting at sites within the gorge.…”

“…Although the mechanism of toxicosis via cholinolergic hyperstimulation is the same between OP insecticides/pesticides and OP CWNAs, the relative potencies can be radically different. In general, CWNAs such as sarin, soman, cyclosarin, VX and Russian VX (VR), have been shown to exhibit greater toxicity than bioactivated OP pesticides such as paraoxon, chlorpyrifos-oxon and phorate oxon or PHO (Balali-Mood and Shariat, 1998;Wilhelm et al, 2014;Snider et al, 2015;Misik et al, 2015).…”

“…In a previously published work, we described an experimental model using topical OP challenges at the LD 85 level on clipped guinea pigs to assess the relative efficacies of eight candidate oximes, including 2-PAM Cl, in terms of 24-hr survivability and mitigation of blood cholinesterase (ChE) inhibition (Snider et al, 2015). The objectives of the work presented herein were to determine the median effective dose (ED 50 ) for the four most efficacious oximes down-selected from our prior screen against each of three topical threat OPs: VR, VX, or PHO.…”

Kinetic analysis of oxime-assisted reactivation of human, Guinea pig, and rat acetylcholinesterase inhibited by the organophosphorus pesticide metabolite phorate oxon (PHO)

“…In the subcutaneous challenge study, 2-PAM Cl, MMB4 DMS, obidoxime Cl 2 , and HLö-7 all significantly reduced PHO-induced mortality when administered approximately one minute after challenge [37]. However, no oxime significantly enhanced survival following a percutaneous challenge with PHO, when oxime administration was withheld until the onset of clinical signs (average oxime administration was approximately 5 hr after challenge) [38]. One obvious difference between the study designs is the timing of the oxime administration, which was almost concurrent with PHO challenge in the subcutaneous study but was delayed for several hours in the percutaneous study in order to mimic a real-world scenario, where a patient might not be aware of a small percutaneous exposure without the presence of clinical signs or symptoms.…”