Cytotoxic Activity of Chemical Constituents of Clerodendrum glabrum and Combretum nelsonii Root Extracts Against Selected Cancer Cell Lines

Breast and colon cancers are leading causes of death worldwide. There is a need for improved treatment strategies. South African medicinal plants, including Clerodendrum glabrum (C. glabrum) and Combretum nelsonii (C. nelsonii), are known for their cytotoxic properties. This study aimed to isolate and characterize terpenoids and stilbenes from the roots of C. glabrum and C. nelsonii and evaluate their anticancer potential against colorectal adenocarcinoma (Caco-2) and hormone receptor-positive breast cancer (MCF-7) cell lines. Spectroscopic techniques including nuclear magnetic resonance spectroscopy (NMR) were used to characterize the isolated compounds. Repeated column chromatography of C. glabrum extract led to the isolation of ferruginol (1), royleanone (2), and β-amyrin palmitate (3). C. nelsonii extract afforded combretastatin A-1 (4), a mixture of combretastatin A-1-2′-O-β-D-glucopyranoside (5a) and combretastatin B-1-2′-O-β-D-glucopyranoside (5b). Compounds 1, 2, 4, 5a, and 5b were isolated for the first time from the plant species. C. glabrum extract showed good anticancer properties with LC50 of 1.30 × 103 µg/mL (CaCo-2) and 2790 µg/mL (MCF-7). Compound (1) exhibited high toxicity against the Caco-2 at LC50 of 24.3 µg/mL and moderate activity against MCF-7 at 48.4 µg/mL. Compound (4) and the mixture (5a and 5b) showed moderate activity against the MCF-7 at LC50 72.0 and 44.1 µg/mL, respectively. These findings highlight C. glabrum and C. nelsonii as promising sources of anticancer lead compounds.