Cancer cell based Hsp90 dependent luciferase refolding assay. The extent of thermally denatured luciferase refolding in PC3-MM2 in the presence of N-terminal or C-terminal Hsp90 inhibitors was tested at 60 and 90 minutes (Figure 6A). The parent compound NB and an earlier, less potent, analogue F-4, along with KU174 and 17AAG was subsequently tested for their ability to inhibit luciferase refolding over 60 minutes (Figure 6B). Next we compared a second N-terminal inhibitor, radicicol, and an inactive novobiocin analog, KU298, determined in our laboratory to not bind Hsp90 as an additional positive and negative control for this assay, respectively (Figure 6C). Similar results were also seen in the androgen dependent cell line LNCaP; however, 17AAG, radiciol and KU174 exhibited increased potency (Figure 6D).