The effect of neostigmine on pyridostigmine bioavailability in myasthenic patients after oral administration.

Plasma levels of pyridostigmine and/or neostigmine were monitored in 8 myasthenic patients who were stabilised on oral pyridostigmine bromide only (60-540 mg per day), and in 9 patients who were stabilised on both neostigmine bromide (15-480 mg per day) and pyridostigmine bromide (240-1080 mg per day), over a period of 12 hr (8.00 a.m. - 8.00 p.m.). Maximum plasma concentrations of pyridostigmine in the first and second groups of patients ranged from 12.4 to 64.5 ng per ml and 15.3 to 144.0 ng per ml respectively. Despite this general intersubject variation in bioavailability of pyridostigmine, there was a direct relationship between the area under plasma concentration-time curves (AUC) and total daily dose in the first group of myasthenic patients (r = 0.95). However, no such observation was noticed neither in all 17 patients nor in the 9 patients who were treated with both drugs. Neostigmine was detected in only one of the second group of patients. It was suggested that neostigmine might interfere with the bioavailability of pyridostigmine when both drugs are concurrently administered orally.