Treatment efficacy in a soman-poisoned guinea pig model: added value of physostigmine?

Abstract

Current treatment of organophosphate poisoning is insufficient, and survivors may suffer from long-lasting adverse effects, such as cognitive deficits and sleep-wake disturbances. In the present study, we aimed at developing a guinea pig model to investigate the benefits of immediate and delayed stand-alone therapy on the development of clinical signs, EEG, heart rate, respiration and AChE activity in blood and brain after soman poisoning. The model allowed the determination of the therapeutic effects at the short-term of obidoxime, atropine and physostigmine. Obidoxime exerted the highest therapeutic efficacy at administration of the lowest dose (3.1 mg/kg i.m.), whereas two higher doses (9 and 18 mg/kg) were less effective on most parameters. Addition of atropine at 0.03 and 3 mg/kg (i.m.) to the treatment did not improve the therapeutic effects of obidoxime alone. Physostigmine (0.8 mg/kg im) at 1 min after poisoning increased mortality. Two lower doses (0.1 and 0.3 mg/kg i.m.) showed improvements on all parameters but respiration. The middle dose was most effective in preventing seizure development and therefore assessed as the most efficacious dose. Combined treatment of obidoxime and physostigmine shortened the duration of seizures, if present, from up to 80 min to ~10-15 min. In practice, treatment will be employed when toxic signs appear, with the presence of high levels of AChE inhibition in both blood and brain. Administration of physostigmine at that moment showed to be redundant or even harmful. Therefore, treatment of OP poisoning with a carbamate, such as physostigmine, should be carefully re-evaluated.

Fig. 1

Overview of the experimental design. Before and after poisoning, EEG, ECG, respiration and clinical signs were continuously monitored. Every 10 min after poisoning, blood was drawn from the ear vein to determine blood ChE activity

Fig. 2

Average onset times of clinical signs after different doses of soman. Results are presented as means ± SEM. Onset times of clinical signs in animals poisoned with 44 μg/kg soman were significantly shorter than in animals poisoned with 30 or 37 μg/kg (one-way ANOVA, within factor signs and between factor dose, P < 0.05 followed by Tukey’s post hoc test,*, P < 0.001)

Fig. 3

a Effects of different doses of soman on total power in EEG. b respiratory minute volume (RMV). Results are presented as mean ± SEM. No significant differences were observed between the different soman doses

Fig. 4

a AChE activity in hippocampus, striatum and medulla oblongata (HIPP, STR and MO) 90 min after poisoning with soman and subsequent treatment. The effect of obidoxime (3.1, 9 and 18 mg/kg) and atropine (3 mg/kg) 1 min after poisoning with 30 μg/kg soman. b Efficacy of addition of physostigmine (0.1 and 0.8 mg/kg). c the effects of treatments postponed to 10 min. Results are presented as mean ± SEM and analysed by MANOVA followed by Tukey’s HSD post hoc test. *indicates significant (P < 0.05) compared to soman alone

Fig. 5

a Treatment effect on total power in EEG before and after soman poisoning. Results are presented as 10-min averages ± SEM. The effect of obidoxime (3.1 mg/kg) and atropine (3 mg/kg) administered at 1 min after soman poisoning. b The effect of physostigmine (0.3 mg/kg) alone or as an addition to obidoxime and/or atropine. c The effect of similar treatments administered at 10 min after poisoning

Fig. 6

a Treatment effect on respiratory minute volume (RMV) before and after soman poisoning. Results are presented as 10-min averages ± SEM. The effect of obidoxime (3.1 mg/kg) and atropine (3 mg/kg) administered at 1 min after soman poisoning. b The effect of physostigmine (0.3 mg/kg) alone or as an addition to obidoxime and/or atropine. c The effect of similar treatments administered at 10 min after poisoning

Fig. 7

Continuous relationships of the occurrence of soman-induced signs obtained with logistic regression analysis. a The relationship between AChE activity in blood and clinical signs of animals poisoned with three doses of soman but no treatment (n = 18). b The relationship between the probability of seizures and the residual AChE of animals poisoned with 30 μg/kg soman treated at 1 min post-poisoning with single drugs or their combinations (n = 6–18 per treatment)