Development and evaluation of an immunochromatographic strip for rapid screening of sildenafil-type compounds as illegal additives in functional foods

Sildenafil is a phosphodiesterase-5 inhibitor (PDE-5) for the treatment of erectile dysfunction. Undeclared sildenafil and related analogues adulterated in functional foods are a threat to public health. To screen these illegal drugs rapidly in herbal samples, an immunochromatographic (IC) assay was developed based on polyclonal antibodies specific to both sildenafil and its analogues. A group that is pharmacological necessary for sildenafil and its analogues was employed as a representative hapten for the generation antibodies against the target compounds. The desired antisera showed satisfactory specificities to sildenafil and major analogues with IC₅₀ values ranging from 19.3 to 34.6 ng ml^–1 in a referring enzyme-linked immunosorbent assay (ELISA). The optimised IC assay showed detection thresholds in the range 5.0–20 μg g^–1 for sildenafil and major analogues in herbal samples. Sixty herbal food supplements were screened and six were found to be positive using the IC strip. It was confirmed by ELISA and UPLC-PDA-MS/MS that positive samples contain target illegal additives in levels of 10–40 mg g^–1 (1–4%). In this range, sensitivity of the IC strip is adequate to screen sildenafil-type compounds in herbal commodities under a dilution ratio of 1:10³. Thus, the current IC assay is a suitable tool for screening sildenafil and its analogues as illegal additives in herbal food supplements.