Prophylaxis against nerve agent toxicity: Physiological, behavioral, and neuroprotection of current and novel treatments

Compounds with another mechanism of action than enzyme inhibition alone were tested in their capability to counteract or prevent incapacitation and lethality after 2xLD50 soman when used as a pretreatment or as a post-intoxication therapy in guinea pigs. To increase the validity of extrapolating data to man, crucial experiments were performed in marmoset monkeys. The efficacy was evaluated in terms of survival, intoxication symptomatology, hypotermia, cortical electro-encephalogram (EEG) and behavioral incapacitation. The treatment scenarios in the guinea pigs were pyridostigmine (PYR), physostigmine (PHY), procyclidine (PC), PHY/scopolamine (SCO), PHY/PC, and (+)-PHY/PC. These treatments were given 30 minutes prior to soman intoxication. To test the efficacy of these compounds as a postintoxication therapy the treatments were administered on indication after the first sign appeared in a parallell group of guinea pigs The subacute pretreatment scenarios in marmosets were PYR, PHY/SCO, PHY/PC. In the marmoset the treatments were administered by using Alzet osmotic mini-pumps inserted twelve days prior to soman intoxication. All animals received atropine sulphate after soman. Pretreatments with PYR (0.05 mg/kg), PHY (0.3 mg/kg), and PC (3 mg/kg) did not protect sufficiently against lethality, intoxication symptoms, EEG and incapacitation. All animals pretreated with PYR deceased within 24 hrs. The most effective pretreatments in guinea pigs regarding survival and behavioral performance showed to be the combinations of PHY/PC and PHY/SCO(0.1 mg/kg) with a survival of 100%. These scenarios did also offer the best protection against hypothermia, seizures and convulsions. The PHY/PC combination showed to be even more protective, since none of these animals showed EEG seizure activity, in contrast to some PHY/SCO pretreated guinea pigs. Used as a therapy after soman intoxication, the PHY/PC combination was also the best treatment option, although not fully protective. (+)PHY (0.36 mg/kg), which does not bind to cholinersterase, in combination with PC, was the second best pretreatment group on survival. This means that also direct effects of PHY is responsible for survival. In marmoset monkeys, the pretreatment combinations PHY(0.0125 mg/kg/hr)/PC(0.125 mg/kg/hr) and PHY/SCO(0.01 mg/kg/hr) showed minor improvements compared to PYR (0.02 mg/kg/hr). In conclusion, the protective effects are based on the combination of 1) temporary cholinesterase inhibition, to prevent incapacitation and peripheral effects, and 2) direct effects on the cholinergic receptor, to prevent convulsive behavior, and 3) NMDA receptor antagonism, to prevent seizure activity. The sign free combinations of PHY/PC or PHY/SCO are promising alternatives for the PYR pretreatment. Furthermore, PHY/PC used as therapy increases chances of survival compared to PHY/SCO. Therefore, the combination of PHY/PC seems to be very promising and should be further investigated.