Pyridine-4(1H)-one Alkaloids from Waltheria indica as Antitrypanosomatid Agents

Twelve new pyridine-4(1H)-one derivatives, namely, 8-demethoxywaltherione F (1), waltheriones R-V (2, 6, 7, 10, and 11), 1-methoxywaltherione O (3), (S)-15-hydroxywaltherione G (4), (8R)-8-hydroxywaltherione M (5), (9S,13S)-2-hydroxymethylwaltherione C (8), (9S,10S,13S)-10-hydroxywaltherione C (9), and (S)-13-methoxywaltherione V (12), as well as melovinone (13) and 5'-methoxywaltherione A (14) were isolated from the CH₂Cl₂ extract of the aerial parts of Waltheria indica. Their chemical structures were determined by means of a comprehensive analysis including ¹H NMR, DEPTQ, HSQC, HMBC, ¹H-¹H COSY, ROESY, and HRESIMS data. The absolute configurations were assigned via comparison of the experimental and calculated ECD data. In addition, the isolated constituents as well as the known waltheriones M-Q were evaluated for their in vitro antitrypanosomal activity. Compounds 2, 5, and 7 as well as waltheriones M, P, and Q showed potent growth inhibition toward Trypanosoma cruzi with IC₅₀ values of 2.1, 0.8, 2.1, 1.3, 0.5, and 0.1 μM, respectively, and selectivity indices of >12, >33, >13, 5, 25, and 14. These findings further demonstrate that the waltheriones are a promising class of antichagasic compounds worthy of further investigations.