The delta agent comes of age.

Abstract

Combretastatin-A4 (CA-4) is a natural derivative of the African willow tree Combretum caffrum. CA-4 is one of the most potent anti-mitotic components of natural origin but it is however intrinsically unstable. A novel series of CA-4 analogues incorporating a 3,4-diaryl-2-azetidinone (β-lactam) ring were designed and synthesised with the objective to prevent cis-trans isomerisation and improve the intrinsic stability without altering the biological activity of CA-4. Evaluation of selected β-lactam CA-4 analogues demonstrated potent anti-tubulin, anti-proliferative and anti-mitotic effects in human leukemia cells. A lead β-lactam analogue, CA-476 displayed comparable antiproliferative activities with CA-4. CA-476 induced rapid apoptosis in HL-60 acute myeloid leukemia (AML) cells which was accompanied by depolymerisation of the microtubular network, PARP cleavage, caspase-3 activation and Bcl-2 cleavage. A prolonged G2M cell cycle arrest accompanied with a sustained phosphorylation of mitotic spindle checkpoint protein, BubR1, and the anti-apoptotic proteins Bcl-2 and Bcl-xL preceded apoptotic events in K562 chronic myeloid leukemia (CML) cells. Molecular docking studies in conjunction with comprehensive cell line data rule out CA-4 and β-lactam derivatives as P-glycoprotein substrates. Furthermore, both CA-4 and CA-476 induced significantly more apoptosis compared to imatinib mesylate in ex-vivo CML patient samples including those positive for the T315I mutation displaying resistance to imatinib mesylate and dasatinib. In summary, synthetic intrinsically stable analogues of CA-4 have been designed and synthesised which display significant clinical potential as anti-leukemic agents.