GA 2017

Against the backdrop of our previous preliminary report on steroid alkaloids with promising antitrypanosomal activity from the leaves of West African Holarrhena africana [1], we have extended our search for such alkaloids to the stem bark. Bioactivity-guided isolation by repeated column chromatographic separation and purification of various alkaloidal fractions yielded six (1-6) steroidal alkaloids from the leaves [1], and eleven (7-17) steroidal alkaloids as well as one (18) nitrogen-free steroid from the stem bark. The structures were resolved by comparison of HRMS, 1D and 2D NMR spectra with reported data [2, 3]. All compounds were tested in vitro against Trypanosoma brucei rhodesiense and for cytotoxicity to mammalian cells (L6 cell line) and some structure-activity relationships became evident. The presence of a basic amino group at position 3 was found to be necessary for high antitrypanosomal activity of the steroid alkaloids. Pairwise comparison of the IC₅₀ values (µM) of 1 (0.40±0.28) and 3 (0.08±0.01), 12 (0.42±0.09) and 13 (0.17±0.11), 15 (1.66±0.39) and 14 (0.12±0.08) showed that monomethylation at the 3-amino group represents an optimum for antitrypanosomal activity. Comparison of the IC₅₀ values of 4 (1.20±0.75) and 5 (0.67±0.03) furthermore indicates that the C-3β amino substitution enhances activity. Overall, antitrypanosomal potency increases in the order O<NH₂<N(CH₃)₂<NHCH₃ with respect to substitution at C-3. The antitrypanosomal activities of these steroid alkaloids have not been reported before. Further studies on structure-activity relationships in terms of QSAR are currently in progress and will represent the basis for eventual lead optimization.

References

1. Nnadi CO, Nwodo NJ, Brun R, Kaiser M, Schmidt TJ. Planta Medica. 2016; 82(S 01):S1-S381.
2. Zirihi GN, Grellier P, Guede-Guina F, Bodo B, Mambu L. Bioorg Med Chem Lett. 2005; 15:2637–2640.
3. Leboeuf M, Cavé A, Goutarel R. Ann Pharm Fr. 1969; 27(3):217-28.