Scaffold diversity-oriented synthesis of limonoid dimers: discovery of an axially chiral agent with in vivo anti-breast cancer activity

Starting from the natural monomeric limonoids 1 and 1, we designed and synthesized eight new limonoid dimers of four skeletons (two symmetric and two non-symmetric), containing a rotationally hindered C15?C15 central axis (2, 2), an embedded C15-spiro-fused 2H-pyran motif (3, 3), and an axially chiral C15-C15 central bond (4, 4, and 5a, 5b), respectively, by oxidative carbon-carbon radical coupling. The dimeric architectures of these compounds, particularly absolute configurations of C15?C15 and C15-C15 central axes, were unequivocally determined by extensive NMR investigations, single-crystal X-ray diffraction analyses, and electronic circular dichroism spectral comparison. The C-2-symmetric dimer 5b with a (M)-configured C15-C15 central axis exhibited selective and potent cytotoxicities against human triple-negative breast cancer (TNBC) MD-MBA-231 and MD-MBA-453 cells with IC50 values of 5.57 +/- 1.48 and 3.93 +/- 0.75 M, respectively. This induces cell-cycle arrest in the G2/M phase and apoptosis, and enhances the accumulation of reactive oxygen species (ROS) in cells. Importantly, 5b suppressed the growth of MDA-MB-453 tumor xenografts with no significant side effects in Balb/c nude mice. After intravenous injection with 5b at a dose of 30 mg kg(-1) every two days for four weeks, the inhibitory rate for TNBC MDA-MB-453 tumor xenografts was 63.83%. It was concluded that not only the presence of 6R-OH/6R-OH groups, but also the stable (M)-configuration of the C15-C15 central axis is pivotal for the in vivo anti-TNBC activity of 5b. This is the first report on an axially chiral limonoid dimer with in vivo anticancer activity.