A series of
acridine-substituted bis(acridine-4-carboxamides) linked by a (CH2)3N(Me)(CH2)3 chain have been prepared by reaction of the isolated imidazolides of the substituted acridine-4-carboxylic
acids with N,N-bis(3-aminopropyl)methylamine. These dimeric analogues of the mixed
topoisomerase I/II inhibitor N-[2-(dimethylamino)ethyl]
acridine-4-carboxamide (DACA), currently in clinical trial, show superior potencies to the corresponding monomeric DACA analogues in a panel of cell lines, including wild-type (JLC) and mutant (JLA and JLD) forms of human Jurkat
leukemia. The latter mutant lines are resistant to
topoisomerase II targeted agents because of lower levels of the
enzyme. Analogues with small substituents (e.g., Me, Cl) at the
acridine 5-position were clearly superior, with IC50's as low
as 2 nM against the
Lewis lung carcinoma and 11 nM against JLC. Larger substituents at any position caused a steady decrease in potency, likely due to lowering of
DNA binding affinity. A small series of analogues of the most potent bis(5-methylDACA) compound, with second substituents (Me and Cl) in the 1- or 8- position had broadly similar potencies to the 5-Me compound, indicating that, while the 1- and 8-substituents are acceptable, they add little to the enhancing effect of the 5-methyl group. All of the compounds were at least equitoxic (some up to 4-fold more cytotoxic) against the mutant Jurkat lines than in the wild-type, consistent with a relatively greater effect on
topoisomerase I compared with
topoisomerase II. The bis(5-methylDACA) compound was found to inhibit the action of purified
topoisomerase I in a cell-free assay. Compounds were on average 10-fold less cytotoxic in an MCF7
breast cancer line overexpressing
P-glycoprotein than in the wild-type line and showed some selectivity for colon
tumor lines in the NCI human tumor cell line panel. Several analogues produced significant growth delays in the relatively refractory subcutaneous colon 38
tumor model in vivo at substantially lower doses than DACA. The bis(acridine-4-carboxamides) represent a new and interesting class of potent
topoisomerase inhibitors.