Quantitative Determination of Decamethoxine in Pharmaceutical Preparations by its Inhibitory Effect on the Enzyme Cholinesterase

A simple and sensitive kinetic-spectrophotometric method was developed for quantifying decamethoxine (DM) in ear/eye drops and antiseptic solutions. The assay is based on inhibition of acetylcholinesterase (AChE) by DM, followed by kinetic measurement of the residual acetylcholine via peracetic acid formation and its reaction with p-phenetidine, monitored at 358 nm (logε = 4.2). Reaction rate constants were determined from kinetic curves in the presence and absence of the inhibitor, and inhibition degree (U, %) was calculated and used for calibration. A linear relationship was observed in the range 1–6 ng/mL: U % = (11.1 ± 1.3) × C + (7.5 ± 4.5) (r = 0.996). The LOQ, corresponding to 20 % inhibition, was 1.0 ng/mL. The method was validated for selectivity, precision (RSD ≤ 2.5 %), and accuracy, and demonstrated suitability for routine analysis of pharmaceutical formulations.