Detection of a Tadalafil Analogue as an Adulterant in a Dietary Supplement for Erectile Dysfunction

TL;DR

A dietary supplement marketed in Argentina for erectile dysfunction was found to be adulterated with the unapproved PDE-5 inhibitor analogue aminotadalafil. The identification was made by TLC/HPLC-DAD and confirmed by isolation and spectral analysis (NMR, MS, IR, UV).

Key findings

• TLC and HPLC-DAD detected one major compound in the supplement extract: Rf values 0.81 and 0.74 in two TLC systems, and an HPLC retention time of 11.79 minutes, eluting before tadalafil at 16.31 minutes; its UV spectrum (λmax 291.4, 284.3, and 220.6 nm) was similar to tadalafil.
• Purification by column chromatography yielded 97.5 mg of an amorphous yellowish powder; ESI-MS gave a molecular formula C21H18N4O4 with m/z 391.1 [M+H]+, indicating one more nitrogen but one less carbon and hydrogen than tadalafil.
• NMR showed replacement of tadalafil’s N-methyl group, evidenced by a two-proton signal at δH 5.11 and disappearance of the δH 2.93 methyl signal, leading to identification as (6R,12aR)-2-amino-6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydropyrazino[1′,2′:1,6]pyrido[3,4-b]indole-1,4-dione, or aminotadalafil.
• The authors conclude this is the first report in Latin America and one of the few independent studies of adulteration with an unapproved PDE-5 inhibitor in an herbal product for ED treatment.

Methods

• The suspected dietary supplement (sample A) was submitted by the Urology Department at Hospital de Clínicas José de San Martín, University of Buenos Aires; three additional lots were purchased from the local market. The label claimed Panax ginseng, Astragalus membranaceus, Schizandra chinensis, Ginkgo biloba, and vitamins.
• The contents of four capsules (brown powder) were ground and extracted with dichloromethane (50 mL/g) by ultrasonic shaking at 25°C for 1 hour, filtered, and evaporated under vacuum at 40°C to yield the dried extract (DESA). Cialis (tadalafil 20 mg), Viagra (sildenafil 50 mg), and Levitra (vardenafil 20 mg) tablets were used as standards and treated similarly.
• Analysis used TLC with two mobile phases, HPLC-DAD on a C18 column with gradient elution, purification by open column chromatography over silica gel, and structural identification by UV, FT-IR, ESI-MS, 1H NMR, 13C NMR, DEPT-135, and HSQC NMR.

Limitations

• Stated: the exact market penetration of such dietary supplements is difficult to assess because they are frequently sold informally and even online; the safety profiles and toxic effects of PDE-5 inhibitor analogues have not been fully established.
• Evident from the text: although three additional lots were purchased, the reported chromatographic and spectroscopic results focus on sample A/DESA, and no quantitative adulterant level or clinical adverse-event data are reported.
• Evident from the text: no formal method validation or confirmation against an authentic aminotadalafil reference standard is described; identification relies on spectral data compared with published data.