Annoglabrins D–F, Three New ent -Kaurane Diterpenoids from the Annona glabra Fruit Pulp

Abstract

Annona glabra L. is a tropical fruit tree distributed across the Americas and Asia, renowned for its diverse traditional therapeutic uses and rich phytochemical profile. Despite its medicinal potential, research on its fruit remains limited, with ambiguities surrounding the distinct constituents of the seeds versus the pulp. To address this gap, the present study focused on the fruit pulp of A. glabra, resulting in the isolation of two new nor-ent-kaurane diterpenoids, annoglabrin D (1) and annoglabrin F (3), and one new ent-kaurane diterpenoid, annoglabrin E (2), along with five known compounds (4–8) of the same skeletal type. Their structures were elucidated via spectroscopic analyses (1D, 2D NMR, HR-ESI-MS) and comparison with previous literature. DP4 probability and ECD spectrum were deployed to identify the absolute configuration of compounds 1–3. The in vitro nitric oxide production inhibitory assay indicated 3 (IC50 48.1 μM) and 8 (IC50 51.8 μM) as moderate inhibitors in comparison with L-NMMA (IC50 12.9 μM), while the others were inactive. Compound 8 demonstrated potent cytotoxicity against KB (IC50 19.7 μM) and HepG2 (IC50 16.4 μM) cancer cell lines, in comparison with ellipticine (IC50 1.91, 1.95 μM, respectively). These findings were corroborated by molecular docking and molecular dynamics simulations, revealing the efficient and stable binding interactions of 3 and 8 with proteins associated with key inflammatory mediators (iNOS, IL-6, TNF-α, COX-2, IL-1β), transcription factors (NF-κB, Nrf2), and the KB and HepG2 cancer cell lines. The simulations predicted the contribution of specific functional groups in compounds 3 and 8, particularly the C-4 methyl, C-16 ketone, and acetoxymethyl carbonyl in 3, as well as the methyl ester moiety at C-16 and the carboxyl group (−COOH) at C-4 in 8, in mediating these compounds’ stable and effective interactions.