Role of central biogenic amines on the protection afforded by clonidine against the toxicity of soman, an irreversible cholinesterase inhibitor

A phase II clinical trial to assess the safety of clonidine in acute organophosphorus pesticide poisoning

BACKGROUND

An estimated 2-3 million people are acutely poisoned by organophosphorus pesticides each year, mostly in the developing world. There is a pressing need for new affordable antidotes and clonidine has been shown to be effective in animal studies. Our aim was to determine the safety of clonidine given as an antidote in adult patients presenting with signs or symptoms of acute organophosphate ingestion.

METHODS

This study was a dose finding, open-label, multicentre, phase II trial. Forty eight patients with acute organophosphate poisoning were randomized to receive either clonidine or placebo: Four to receive placebo and twelve to receive clonidine at each dose level. The first dose level was an initial loading dose of 0.15 mg followed by an infusion of 0.5 mg of clonidine over 24 hours. The initial loading dose was increased to 0.3 mg, 0.45 and 0.6 mg. at all dosing levels however the subsequent infusion remained at 0.5 mg of clonidine over 24 hours.

RESULTS

The baseline characteristics of both groups were similar. The trial was stopped after completion of the 3rd dosing level. At the 1st and 2nd dosing level there were no reported adverse drug reactions. At the 3rd dosing level 5 patients (42%) developed significant hypotension during clonidine treatment that responded to intravenous fluids. There were no statistical differences in ventilation rate, pre and post GCS, and mortality rates over all levels.

CONCLUSION

Our findings suggest use of moderate doses of clonidine in acute organophosphate poisoning can be used without causing frequent clinical problems but that higher doses are associated with a high incidence of hypotension requiring intervention. Further studies are needed to study the efficacy of clonidine as an antidote in organophosphate poisoning.

Modification of spino-bulbar autonomic cholinergic systems by activation of alpha-adrenergic receptors

Intrathecal (i.t.) injection of neostigmine was employed to activate spinal cholinergic neurons mediating a hypertensive response in freely-moving rats. Our earlier studies have demonstrated that stimulation of central alpha-adrenergic receptors inhibits the pressor response following inhibition of brain cholinesterase. Clonidine (0.5-5 micrograms) pretreatment by i.t. injection did not alter the magnitude of the pressor response to i.t. injection of neostigmine, but did significantly delay the onset of the response. Since systemic administration of clonidine abolished the pressor response to i.t. neostigmine, clonidine was administered by the intracisternal (i.c.) route to determine whether higher centers mediated the inhibitory response. In this situation, clonidine pretreatment significantly inhibited the pressor response to i.t. injection of neostigmine. I.t. injection of norepinephrine (1-10 micrograms) was more effective, but i.c. pretreatment less effective than clonidine in inhibiting the pressor response to i.t. neostigmine. In order to confirm the ascending nature of the spinal cholinergic system, hemicholinium-3 was injected i.c. to deplete medullary levels of acetylcholine. Carbachol was then injected i.t. (since carbachol is a direct-acting agonist, it is not affected by local depletion of acetylcholine). Depletion of medullary acetylcholine significantly blocked the pressor response to i.t. injection of carbachol. These findings are consistent with the concept of an ascending spinal cholinergic pressor pathway. The pressor response to activation of spinal cholinergic receptors is not sensitive to local injection of clonidine, but, the medullary cholinergic component of the system is inhibited by alpha-receptor stimulation.