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.