Voltammetric behavior and assay of the contraceptive drug levonorgestrel in bulk, tablets, and human serum at a mercury electrode

The cyclic voltammograms of levonorgestrel (LNG) in Britton–Robinson buffers of pH 2–11 at the hanging mercury drop electrode showed a single two-electron irreversible cathodic peak over the whole pH range. This peak may be attributed to the reduction of the 3-keto-delta-4-group in the A-ring of the LNG molecule. The interfacial adsorptive character of levonorgestrel onto the surface of the hanging mercury drop electrode was identified by means of both cyclic voltammetry and chronocoulometry techniques. A simple, sensitive, and selective square-wave adsorptive cathodic stripping voltammetric procedure was developed for the quantitation of levonorgestrel. Under the optimized operational conditions, the maximum developed stripping voltammetric peak current showed a linear response with concentration of the bulk LNG substance. The achieved limits of detection (LOD) and quantitation (LOQ) were 6.7×10−10 and 2.2×10−9 M, or 4.8×10−10 and 1.6×10−9 M, following accumulation onto the hanging mercury drop electrode for 90 s or 150 s, respectively. The developed procedure was successfully applied to the determination of levonorgestrel in tablets, in spiked human serum, and in real plasma samples of healthy female volunteers following an oral administration of a 30-μg LNG single dose. The pharmacokinetic parameters (Cmax=1.05 ng ml−1, tmax=2.4 h and AUC0−t=16.5 ng h ml−1) were estimated and favorably compared to those reported in literature for equivalent dose.