Effects of N-demethylated carbachol on iris and ciliary muscles

The tertiary nitrogen derivative of carbachol, N-demethylated carbachol, was synthesized to produce a drug that could be used as an alternative to pilocarpine in the treatment of open-angle glaucoma. Unlike pilocarpine, N-demethylated carbachol appeared to be stable in alkaline solutions. Since pilocarpine has to be dissolved in an acidic medium (pH 5.5) to prevent its destruction, less than 10% of its molecules are in an unionized form. On the other hand, N-demethylated carbachol is very stable at pH values higher than 7.4 and thus has more molecules (greater than 10%) in an unionized, penetrable form than does pilocarpine in high pH media. N-Demethylated carbachol acted as a full agonist on isolated dog iris and ciliary muscle preparations, whereas pilocarpine produced only 49 and 38% of the maximum tissue responses, respectively. These findings suggest that N-demethylated carbachol may be able to affect the outflow facility to a greater extent than is achievable with pilocarpine at high doses.