A 3D-QSAR Study on the Antitrypanosomal and Cytotoxic Activities of Steroid Alkaloids by Comparative Molecular Field Analysis.

As part of our research for new leads against human African trypanosomiasis (HAT), we report on a 3D-QSAR study for antitrypanosomal activity and cytotoxicity of aminosteroid-type alkaloids recently isolated from the African medicinal plant Holarrhena africana A. DC. (Apocynaceae), some of which are strong trypanocides against Trypanosoma brucei rhodesiense (Tbr), with low toxicity against mammalian cells. Fully optimized 3D molecular models of seventeen congeneric Holarrhena alkaloids were subjected to a comparative molecular field analysis (CoMFA). CoMFA models were obtained for both, the anti-Tbr and cytotoxic activity data. Model performance was assessed in terms of statistical characteristics (R², Q², and P² for partial least squares (PLS) regression, internal cross-validation (leave-one-out), and external predictions (test set), respectively, as well as the corresponding standard deviation error in prediction (SDEP) and F-values). With R² = 0.99, Q² = 0.83 and P² = 0.79 for anti-Tbr activity and R² = 0.94, Q² = 0.64, P² = 0.59 for cytotoxicity against L6 rat skeletal myoblasts, both models were of good internal and external predictive power. The regression coefficients of the models representing the most prominent steric and electrostatic effects on anti-Tbr and for L6 cytotoxic activity were translated into contour maps and analyzed visually, allowing suggestions for possible modification of the aminosteroids to further increase the antitrypanosomal potency and selectivity. Very interestingly, the 3D-QSAR model established with the Holarrhena alkaloids also applied to the antitrypanosomal activity of two aminocycloartane-type compounds recently isolated by our group from Buxus sempervirens L. (Buxaceae), which indicates that these structurally similar natural products share a common structure⁻activity relationship (SAR) and, possibly, mechanism of action with the Holarrhena steroids. This 3D-QSAR study has thus resulted in plausible structural explanations of the antitrypanosomal activity and selectivity of aminosteroid- and aminocycloartane-type alkaloids as an interesting new class of trypanocides and may represent a starting point for lead optimization.

Evaluation of ethnomedical claim III: anti-hyperglycemic activities of Gongronema latifolium root and stem.

BACKGROUND: The insulinotropic activity of the combined root and stem of Gongronema latifolium (Asclepiadaceae) was evaluated to justify its African ethnomedicinal use in the management of diabetes. METHODS: A methanolic extract and its chromatographic fractions (A1 -A6 ) were tested for glucose-reducing and in vitro insulin-stimulating abilities using glucose-loaded rats and INS-1 cells, respectively. In vivo insulin-releasing activities for the significantly (P<0.05) active antihyperglycemic A5 and A6 and in vitro insulinotropic activity of the C1 fraction and its isolated constituents were also similarly determined. RESULTS: The extract (100 mg/kg) had higher in vivo antihyperglycemic activity than the individual A1 -A6 , indicating a synergistic effect of the plant constituents. Higher in vivo insulin release in response to A5 (100 mg/kg) than A6 , agreed with their in vivo antihyperglycemic activities and confirmed insulin release as a mechanism of action of the plant. Compared with 5.6 mmol/L glucose (negative control), the extract and the A3 , A6 , and C1 (all at 100.0 µg/mL) elicited significantly higher in vitro insulin release that was similar to that of glibenclamide (1.0 µg/mL; P>0.05). Fraction C1 yielded a 1:1 mixture of α-amyrin and ß-amyrin cinnamates (1a:1b), lupenyl cinnamate (2), lupenyl acetate (3), and two unidentified triterpenoids, Y and Z. The 1a:1b mixture (100.0 µg/mL) demonstrated the highest insulinotropic activity that was comparable (P>0.05) to that of glibenclamide (1.0 µg/mL). CONCLUSIONS: The results confirm pancreatic activity as a mechanism underlying the antidiabetic action of G. latifolium and justify its ethnomedical use.