Tubulin‐Perturbing Naphthoquinone Spiroketals

Several natural and synthetic naphthoquinone spiroketals are potent inhibitors of the thioredoxin–thioredoxin reductase redox system. Based on the antimitotic and weak antitubulin actions noted for SR‐7 ([8‐(furan‐3‐ylmethoxy)‐1‐oxo‐1,4‐dihydronaphthalene‐4‐spiro‐2′‐naphtho[1″,8″‐ de ][1′,3′][dioxin]), a library of related compounds was screened for tubulin‐perturbing properties. Two compounds, TH‐169 (5′‐hydroxy‐4′ H ‐spiro[1,3‐dioxolane‐2,1′‐naphthalen]‐4′‐one) and TH‐223 (5′‐methoxy‐4′ H ‐spiro[1,3‐dioxane‐2,1′‐naphthalen]‐4′‐one), had substantial effects on tubulin assembly and were antiproliferative at low micromolar concentrations. TH‐169 was the most potent at blocking GTP‐dependent polymerization of 10  μ m tubulin in vitro with a remarkable 50% inhibitory concentration of ca. 400 n m . It had no effect on paclitaxel‐induced microtubule assembly and did not cause microtubule hypernucleation. TH‐169 failed to compete with colchicine for binding to β‐tubulin. The 50% antiproliferative concentration of TH‐169 against human cancer cells was at or slightly below 1  μ m . Flow cytometry showed that 1  μ m TH‐169 caused an increase in G 2 /M and hypodiploid cells. TH‐169 eliminated the PC‐3 cells’ polyploid population and increased their expression of p21 WAF1 and Hsp70 in a concentration‐dependent manner. The antiproliferative effect of TH‐169 was irreversible and independent of changes in caspases, actin, tubulin, glyceraldehyde phosphate dehydrogenase or Bcl‐x S/L . This structurally simple naphthoquinone spiroketal represents a small molecule, tubulin‐interactive agent with a novel apoptotic pathway and attractive biological function.