Efficacy of injectable anticholinergic drugs against soman-induced convulsive/subconvulsive activity

Six FDA approved, injectable compounds [benztropine (BZT); biperiden (BIP); dicyclomine (DCL); l-hyoscyamine (HYO); orphenadrine (ORP); scopolamine (SCP)] were each compared to diazepam (DZ, the standard) in male guinea pigs against ongoing soman-induced convulsive or sub-CV (CV/sub-CV) activity. Three trained graders concurrently assigned CV/sub-CV scores to each animal based on signs of intoxication at various times post-soman. Animals received (im) pyridostigmine (26 micrograms/kg) 30 min before soman (56 micrograms/kg; 2 x LD50), atropine (2 mg/kg) admixed with 2-PAM (25 mg/kg) at one min after soman, and the candidate drug preparation at 5.67 min post soman, a time when CV activity was assured. BIP and SCP were effective over dosage ranges between 10 and 0.3, and 1.0 and 0.13 mg/kg, respectively, while the other preparations were less effective at their respective maximum dosages. At the most effective dosages of SCP (1.0 mg/kg) and BIP (10 mg/kg), the CV/sub-CV scores were significantly lower (p < 0.05) than those of DZ. Only 33% survival was observed at each of two doses of ORP and one dose of HYO; therefore, no further testing was done with these compounds. Using freshly prepared solutions, DCL (up to 40 mg/kg) and BZT (up to 96 mg/kg) were tested with mixed results; DCL lowered lethality while BZT increased lethality. CV/sub-CV scores for the most effective dose of DCL and BZT were, however, lower than those of DZ. SCP is an antimuscarinic drug devoid of antinicotinic activity, while BIP possesses antimuscarinic, antinicotinic, antispasmodic and anti-N-methyl-D-aspartate activity. Recent evidence suggests that, in late stages of intoxication by nerve agents, noncholinergic, excitatory amino acid receptors may become involved and necessitate the use of a multi-action drug like BIP. The findings herein suggest that SCP and BIP are superior to DZ, but further studies are needed to determine which drug or drug class should be pursued in more advanced testing.