Finally, in the search for novel ligands, which might associate with tubulin outside the colchicine site, we generated a top-100 list of compounds from the NCI database, whose anticancer activity profiles correlated with griseofulvin, an antifungal drug, reported to bind tubulin with a low affinity (Kd ~ 80 μM) at a pocket not overlapping with the colchicine site. Our attention was captured by two griseofulvin-correlating ('gc') spiro-compounds (ranked #15 and # 97 in the list) (Fig. ). These compounds had relatively low average GI50 values (4 μM and 1.6 μM, respectively) and were both classified as tentative antitubulin compounds by our spectral clustering algorithm. The C-MST assay showed that both of these spiro-compounds interacted with the colchicine site of tubulin (Fig. , Fig. ), despite having a structure, clearly distinct from known colchicine site ligands. Of note, the binding of griseofulvin itself was beyond the sensitivity of the C-MST assay, consistent with a considerably higher tubulin-binding affinity of the gc-1 and gc-2 compounds (Fig. , Fig. ).