STUDIES ON THE SAFETY OF CHRONICALLY ADMINISTERED INTRATHECAL NEOSTIGMINE METHYLSULFATE IN RATS AND DOGS

Background: The spinal delivery of the cholinesterase inhibitor neosti gmine yields analgesia in rats and augments the analgesic effects of a lpha(2)-agonists in sheep, To assess its activity in humans, preclinic al toxicology studies to define its safety were required in two specie s. Methods: Rats with chronic intrathecal catheters received daily inj ections of saline (vehicle) or 5 mu g/10 mu l or 10 mu g/10 mu l neost igmine HCl (n = 6/group) for 4 days and were observed for general beha vior and nociception (52.5 degrees C hot plate), On day 6, rats were a nesthetized and submitted to whole body perfusion/fixation. For dog st udies, male beagles were prepared following rigid aseptic precautions with catheters passed from the cisterna magna to the lumbar intratheca l space, Catheters were connected to an external vest-mounted pump. Ba sed on preliminary studies, ten implanted dogs were randomly assigned to receive infusions of neostigmine for 28 days (4mg/4ml/day; n = 6) o r saline (4ml/day; n = 4), At 28 days, dogs were anesthetized, cistern al cerebrospinal fluid was obtained, and dogs were submitted to perfus ion-fixation. Rat and dog spinal cords were embedded, sectioned, stain ed, and assessed by the pathologist without knowledge of treatment. Re sults: In rats, neostigmine produced a dose-dependent increase in hot plate latency, and no tolerance was observed, Mild tremor was observed but was not debilitating, Histopathology revealed a mild fibrotic rea ction to the catheter with mixed signs of moderate, acute, and chronic inflammation with no differences between saline or drug groups, In do gs, neostigmine had no effect on blood pressure or on the skin twitch response but produced bradycardia and an increase in muscle tone, At s acrifice, cerebrospinal fluid protein, specific gravity, and glucose w ere elevated in both saline and neostigmine groups. Histopathology dis played a local reaction to the spinal catheter and a mixed acute and c hronic inflammatory reaction, No group differences were observed, Thes e results suggest that, at the neostigmine concentration of 1 mg/ml in the rat and dog and in doses up to 4 mg/day in the dog, there is no e vidence of spinal tissue toxicity that can be attributed to the drug, This result, observed in two species, suggests that intrathecal neosti gmine given in this manner is without distinguishable toxicity in thes e two models,