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

Abstract

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.

A Review of the Antimalarial, Antitrypanosomal, and Antileishmanial Activities of Natural Compounds Isolated From Nigerian Flora

“…Related compounds from the stembark, Conessimine (0.17 ± 0.08) 19 ; isoconessimine (0.17 ± 0.11) 20 ; conessine (0.42 ± 0.09) 21 ; and holarrhesine (0.12 ± 0.08) 22 also showed very high antitrypanosomal activity, with high selectivity to the parasites [selectivity index (SI) >100] compared to mammalian L6 cells (Nnadi et al, 2017 ). A 3D-Quantitave Structure Activity Relationship (QSAR) study revealed that steric activity around the C-3 amino group tends to increase activity, while steric activity in the vicinity of the amino group of the pyrroline/pyrrolidine rings and the C-17β-acetyl or C-20 methyl groups tended to decrease activity (Nnadi et al, 2018 ). Following on from this, the active aminosteroids together with 22 , 3α- dihydroholaphyllamine ( 23 ), and holarrhetine ( 24 ) were also tested on animal trypanosome species using resazurin-based in vitro screening assay in our lab (Nnadi et al, 2019 ).…”

Potent Antitrypanosomal Activities of 3-Aminosteroids against African Trypanosomes: Investigation of Cellular Effects and of Cross-Resistance with Existing Drugs

“…Clearly, there is a need to explore this for further development. Following our previous findings on lead identification and subsequent refinement of the quantitative structure-antitrypanosomal (QSAR)/cytotoxic activities relationship of steroid alkaloids from Holarrhena africana (Apocynaceae) against T. brucei species and L6 mammalian myoblast [16], there is a need to further study their activities against AAT-causing trypanosomes, including their mode of trypanocidal activity and whether cross-resistance with diminazene is likely to occur.…”

“…The antimicrobial actions of natural compounds are usually complex and the reported cellular or biochemical effects are often not the primary causes of phenotypic observations, due to the compounds interacting with off-target proteins and multiple targets [18,19,20], especially in trypanosomatids, which are characterized by complex and unusual biochemical processes [21]. However, such mechanistic studies are necessary to give further insights into their antitrypanosomal action, which, supplemented with the theoretical findings on 3D-QSAR [16], may be used to optimize such compounds from hits to leads and further on to new and specific drugs. Thus, the compound’s potency, its target within the parasites’ physiology and the potential to withstand the development of resistance, especially cross-resistance with the existing set of drugs, have all to be considered to identify and develop new antiparasite chemotherapy.…”

Target-Guided Isolation of O-tigloylcyclovirobuxeine-B from Buxus sempervirens L. by Centrifugal Partition Chromatography

“…The target compound (tR 5.39 min, [M + H] + : m/z 497.4116, [M + 2H] 2+ : m/z 249.2087) was accompanied by small amounts of two congeneric alkaloids with [M + H] + at m/z 415 and 485 constituting about 3.9 and 4.9% (tR 3.77 and 5.13 min, Figure 3 ), as estimated by integration of extracted ion chromatograms for the respective [M + 2H] 2+ ions. The former of these impurities was unambiguously identified as unesterified cyclovirobuxeine-B by its exact mass ([M + H] + : m/z 415.3731, calcd for C 27 H 47 N 2 O + 415.3738; [M + 2H] 2+ : m/z 208.1923) and by comparison to authentic reference compound [ 13 , 14 ] The formation of this compound in small quantities by hydrolysis of 1 during the workup and isolation process cannot be excluded. The latter displayed an [M + H] + at m/z 485.4179 corresponding to C 31 H 53 N 2 O 2 + ([M + 2H] 2+ at 243.2134).…”