Unusual potency of BN 80915, a novel fluorinated E-ring modified camptothecin, toward human colon carcinoma cells

BN 80915 is the lead compound from a novel class of E-ring modified camptothecin analogues, the homocamptothecins, which show potent antitumor activities in animal models. Here, we report that BN 80915 induces up to 2-fold more cleavable complexes between plasmid DNA and purified human topoisomerase I than SN-38 and camptothecin. BN 80915 also induces DNA-topoisomerase I complexes in living HT-29 colon carcinoma cells, as shown by the in vivo link assay. BN 80915 is an extremely potent inducer of DNA-protein complexes in these cells starting at a concentration of 5 nM in the media. BN 80915 is clearly more potent than SN-38, because at least 20 times more SN-38 is needed to induce comparable levels of cleavable complexes. Kinetic experiments show that BN 80915 induces cleavable complexes within minutes that remain stable for at least 6 h in the presence of drug. Whereas the majority of the complexes are reversed within 15 min after drug removal, a substantial fraction (30%) persists for at least 4 h, in contrast with SN-38-treated cells, where all complexes have disappeared by this time. BN 80915 shows strong antiproliferative effects toward HT-29 cells with an IC50 of 0.3 nM compared with 20 nM for SN-38 and 40 nM for topotecan. BN 80915 is also potent against other colon carcinoma cells as well as toward cells growing in three dimensions as multicellular spheroids. HL-60 cells expressing functional P-glycoprotein or multidrug resistance protein show no cross-resistance toward BN 80915. Taken together, our results show that BN 80915 is unusually potent toward human colon carcinoma cells because of the formation of high levels of stable, covalent DNA-topoisomerase complexes.