Tadalafil and Sildenafil illicit association: Stability- indicating HPLC method, photodegradation kinetic and toxicological studies

Abstract The illicit market of counterfeit medicines containing sildenafil and tadalafil has been causing serious public health problems. Thus, further studies on this illicit association are needed. A stability-indicating HPLC method was developed for simultaneous determination of tadalafil (TAD) and sildenafil (SIL) using a C18 column (250 x 4.6 mm, 5 µm). Detection was achieved at 284 nm, for TAD, and 292 nm, for SIL. The method was considered to be specific, linear, precise, accurate, robust, and sensitive. In the photodegradation kinetic studies, the drugs showed a first-order reaction rate when isolated, and zero-order when associated. Toxicological assays demonstrated that the photodegraded drugs decreased cell viability in compared to non- degraded drugs, suggesting cytotoxic activity. Additional, mutagenic activity was not observed under the tested conditions. Photodegraded drugs, in association, depicted DNA damage index, suggesting genotoxic effects. The obtained results will be able to support the forensic intelligence laboratories, as well as to alert the population about the risk inherent to consuming counterfeit products.

Development and validation of alternative methods by non-aqueous acid-base titration and derivative ultraviolet spectrophotometry for quantification of sildenafil in raw material and tablets

Sildenafil citrate (SILC) is a potent phosphodiesterase-5 inhibitor used for erectile dysfunction and pulmonary hypertension. This study shows two simple, fast and alternative analytical methods for SILC determination by non-aqueous titration and by derivative ultraviolet spectrophotometry (DUS) in active pharmaceutical ingredient and/or dosage forms. The quantitation method of SILC active pharmaceutical ingredient by non-aqueous acid-base titration was developed using methanol as solvent and 0.1 mol/L of perchloric acid in acetic acid as titrant. The endpoint was potentiometrically detected. The non-aqueous titration method shows satisfactory repeatability and intermediate precision (RSD 0.70-1.09%). The neutralization reaction occurred in the stoichiometric ratio 1:1 in methanol. The determination of SILC active pharmaceutical ingredient or dosage forms by DUS was developed in the linear range from 10 to 40 µg/mL, in 0.01 mol/L HCl, using the first order zero-peak method at λ 256 nm. The DUS method shows selectivity toward tablets excipients, appropriate linearity (R2 0.9996), trueness (recovery range 98.86-99.30%), repeatability and intermediate precision in three concentration levels (RSD 1.17-1.28%; 1.29-1.71%, respectively). Therefore, the methods developed are excellent alternatives to sophisticated instrumental methods and can be easily applied in any pharmaceutical laboratory routine due to simple and fast executions.