Novel Analytical Methods For Sildenafil And Its Related Substances

Sildenafil is a kind of phosphodiesterase-5(PDE-5) inhibitors which is widelyused to treat erectile dysfunction because of its good effect. Since its function ofvasodilatory, it can cause adverse effect such as headache, indigestion, dizziness, skinrashes, and so on. Therefore, the establishment of method to analyze sildenafil and itsrelated substances including metabolites, intermediates, and analogues is ofsignificance. The present thesis focused on the development of novel method todetermine sildenafil and its related substances in different sample matrices. Its maincontents are as follows:1. Separation of sildenafil and its seven related substances was established.Reversed-phase high performance liquid chromatography (RP-HPLC) was used todetect and separate sildenafil and its metabolite desmethylsildenafil, one analoguevardenafil, one coproduct thiodimethylsildenafil and other four intermediates. By theoptimization of detection wavelength, types and ratios of mobile phase, the optimaldetection and separation conditions were established. Under the optimal conditions,separation of all analytes was completed within45min with good resolution. Themethod had good linearity, precision, accuracy, low limit of detection, which couldprovide an important basis for further analysis of sildenafil and its related substances.2. A novel method named dispersive liquid–liquid microextraction(DLLME)-back extraction based on ionic liquid (IL) was established fordetermination of PDE-5inhibitors including sildenafil, vardenafil and aildenafil inhuman plasma. In this method, the extractant (IL) and appropriate amount ofdispersant were added to the sample solution to achieve IL-DLLME; then the lowerlayer of IL was taken out and an HAc solution was used to retract the target analytesfrom IL. Several extraction parameters, including the type of IL, extraction approach,concentration of HAc in the back-extraction phase, pH of the sample solution, saltconcentration in the sample solution, type and volume of the dispersant and volume ofIL, were optimized and the optimum extraction condition was determined. The present method afforded wide linearity range (sildenafil1-1000ng/mL, vardenafil2-2000ng/mL, aildenafil2-2000ng/mL) with good coefficients (0.9988~9999),acceptable precision of intra-day and inter-day (RSD <6.38%), lower LODs（S/N=3）of0.17~0.43ng/mL and good spiked absolute recovery within87.9%~114.5%inhuman plasma. This method afforded a reference for the detection of the three PDE-5inhibitors in human plasma.3. A novel method named headspace single drop microextraction–gaschromatography (HS-SDME-GC) was established for determination of seven residualorganic solvents including methanol, ethanol, acetone, dichloromethane, n-hexane,ethyl acetate and toluene in sildenafil bulk drug. In this method, the analytes werevolatilized into the headspace above the sample solution; simutaneously, the analytesin the headspace was diffused into an extractant drop hanging in a GC syringe; thisdrop was finally sucked back into the syringe and was directly injected to GC foranalysis. Four extraction parameters, including the type and volume of extractant,extraction temperature and time, were optimized and the optimum extractioncondition was determined. The method afforded wide linearity range with goodcoefficients (0.9938~9999), acceptable precision of intra-day and inter-day (RSD<14.5%) and lower LODs(S/N=3)of0.05μg/mL~0.38μg/mL. This method wassuccessfully applied to detect these residual organic solvents in sildenafil bulk drug.The result indicated that amounts of them were all lower than the limitedrequirements of the Chinese Pharmacopoeia. This method can be used for thedetection and limited characterization of the seven organic residual solvents insildenafil bulk drugs.