Development and validation of a simple and sensitive HPLC–UV method for the determination of captopril in human plasma using a new derivatizing reagent 2-naphthyl propiolate

• A sensitive HPLC–UV method for determination of captopril in plasma was developed.
• The method is based on pre-column derivatization of captopril with a new reagent.
• For the first time 2-naphthyl propiolate was used for thiol derivatization in plasma.
• The method sensitivity is better than previous and comparable to LC/MS methods.
• The method may be applicable to the analysis of other thiols in biological medium.

In this study, a simple, sensitive and reliable HPLC–UV method applying rapid sample preparation technique for the determination of captopril in human plasma was developed and validated. The method is based on pre-column derivatization of captopril and 2-propene-1-thiol (internal standard) with a new reagent 2-naphthyl propiolate. Sample clean-up, derivatization and extraction were carried out in two steps, totally less than 30 min. The extracts were chromatographed on a C18 column (5 μm, 150 mm × 4.6 mm i.d.). The mobile phase consisted of methanol (75%, v/v) and phosphate buffer (25%, pH = 8, 0.01 M). UV detection was performed at 290 nm. To obtain the best reaction yield, the factors that could influence the derivatization process, including the concentration of derivatization reagent, pH of sample solution and temperature were investigated in detail and optimized using Box–Behnken response surface methodology. Under optimized conditions the average extraction recovery of captopril and internal standard were >86%. The achieved lower limit of quantification (LLOQ) was 3 ng/mL; the assay exhibited a linear dynamic range of 3–2000 ng/mL with correlation coefficient (r2) of ≥0.99. The precision was satisfactory in the whole calibration range with RSD of 5.9–12.4% (accuracy: from 97.5% to 93.6%) and of 6.4–12.8% (accuracy: from 97.3% to 95.2%) for intra- and inter-assay, respectively. The method stability was confirmed in a series of experiments including: freeze–thaw, short- and long-term stability testing. Lastly, the developed method was successfully applied to the bioequivalence study of captopril administrated as a single oral dose (50 mg) to 12 healthy male volunteers.