Determination of neostigmine and pyridostigmine in the urine of patients with myasthenia gravis.

A method has been described for the estimation of neostigmine and pyridostigmine in urine by ion exchange treatment and colorimetric estimation of the blue complex produced when either of the drugs is made to react with bromophenol blue. Urine containing 2 mug/ml. or more of neostigmine or 3 mug/ml. or more of pyridostigmine can be quantitatively estimated. After intramuscular injection of neostigmine to patients with myasthenia gravis, up to 67% of the drug is excreted, whilst after oral administration less than 5% is excreted. When pyridostigmine is given by mouth, the amount of drug excreted in the urine varies between approximately 2 and 16%. It has been established by chromatographic analysis that the blue complexes formed under these conditions are due only to neostigmine and pyridostigmine respectively and that the quantitative estimation described is a true measure of the amount of these drugs excreted in the urine. The significance of these results is discussed in relation to the absorption and metabolism of the two drugs.