Methoxy-substituted 3-formyl-2-phenylindoles inhibit tubulin polymerization

The aim of this study was the identification of the essential structural el ements in the 12-formyl-5,6-dihydroindolo[2,1-a]isoquinoline system require d for the inhibition of tubulin polymerization which is understood to be th e predominant mode of action of this class of cytostatics. Since 2-phenylin dole forms the main fragment of this tetracycle, it was used as the basic s tructure and modified with respect to the number and positions of the oxyge n functions in the aromatic rings. Further modifications related to the nit rogen, which was both replaced by oxygen and sulfur and alkylated. All deri vatives were tested for cytostatic activity in human breast cancer cells (M DA-MB 231, MCF-7) and inhibition of tubulin polymerization. The spectrum of activity ranged from inactive to IC50 values of 35 nM (cell growth inhibit ion) and 1.5 mu M (tubulin polymerization), respectively, for the most acti ve derivative 3e (3-formyl-6-methoxy-2-(4-methoxyphenyl)indole). Although t he correlation between antiproliferative activity and inhibition of tubulin polymerization was not very pronounced, all of the potent cytostatic agent s in this study disrupted microtubule assembly completely at the standard c oncentration of 40 mu M. By fluorescence microscopy it was demonstrated tha t the derivative 3e degrades the cytoskeleton in a similar fashion as colch icine does leading to the condensation of the microtubules around the nucle us after treatment. The comparison between hydroxy and methoxy derivatives revealed st striking difference between the 2-phenylindole derivatives and the indoloisoquinolines. In the 2-phenylindole series, the methoxy compound s were much more effective than the free phenols, whereas in the tetracycli c system the effect of the hydroxy derivatives exceeded that of the methyla ted compounds by I order of magnitude. Preliminary studies on the binding m ode showed that both the 2-phenylindole derivatives and the indoloisoquinol ines bind to the colchicine site on tubulin.