Novel cis-selective and non-epimerisable C3 hydroxy azapodophyllotoxins targeting microtubules in cancer cells

Podophyllotoxin (PT) and its clinically used analogues are known to be powerful antitumour agents. These compounds contain a trans fused strained γ-lactone system, a feature that correlates to the process of epimerisation, whereby the trans γ-lactone system of ring D opens and converts to the more thermodynamically stable cis epimer. Since these cis epimers are known to be either less active or lacking antitumour activity, epimerisation is an undesirable feature from a chemotherapeutic point of view. To circumvent this problem, considerable efforts have been reported, amongst which is the synthesis of azapodophyllotoxins where the stereocentres at C2 and C3 are removed in order to preclude epimerisation. Herein we report the identification of a novel C3 hydroxy, cis-selective γ-lactone configuration of ring C in the azapodophyllotoxin scaffold, through an efficient stereoselective multicomponent reaction (MCR) involving fluorinated and non-fluorinated aldehydes. This configuration releases the highly strained trans γ-lactone system in podophyllotoxin analogues into the more thermodynamically stable cis γ-lactone motif and yet retains significantly potent activity. These compounds were evaluated against the human cancer lines MCF-7 and 22Rv1 in vitro. Fourteen out of the seventeen tested compounds exhibited sub-micromolar activity with IC50 values in the range of 0.11-0.91 μM, which is comparable and in some cases better than the activity profile of etoposide in this assay. Interestingly, we obtained strong evidence from spectroscopic and X-ray data analyses that the previously reported structure of similar analogues is not accurate. Molecular modelling performed using the podophyllotoxin binding site on β tubulin revealed a novel binding mode of these analogues. Furthermore, sub-cellular study of our compounds using immunolabelling and confocal microscopy analyses showed strong microtubule disruptive activity, particularly in dividing cells.