Cellular uptake, cytotoxicity and DNA-binding studies of the novel imidazoacridinone antineoplastic agent C1311

C1311 is a novel therapeutic agent with potent activity against experimenta l colorectal cancer that has been selected for entry into clinical trial. T he compound has previously been shown to have DNA-binding properties and to inhibit the catalytic activity of topoisomerase ii. In this study, cellula r uptake and mechanisms by which C1311 interacts with DNA and exerts cytoto xic effects in intact colon carcinoma cells were investigated. The HT29 col on cancer cell line was chosen to follow cellular distribution of C1311 ove r a time course of 24 h at drug concentrations that just inhibited cell pro liferation by 50% or 100%. Nuclear uptake of C1311 and co-localization with lysosomal or mitochondrial dyes was examined by fluorescence microscopy an d effects on these cellular compartments were determined by measurement of acid phosphatase levels, rhodamine 123 release or DNA-binding behaviour. Th e strength and mode of DNA binding was established by thermal melting stabi lization, direct titration and viscometric studies of host duplex length. T he onset of apoptosis was followed using a TUNEL assay and DNA-fragmentatio n to determine a causal relationship of cell death. Growth inhibition of HT 29 cells by C1311 was concomitant with rapid drug accumulation in nuclei an d in this context we showed that the compound binds to duplex DNA by interc alation, with likely A/T sequence-preferential binding. Drug uptake was als o seen in lysosomes, leading to lysosomal rupture and a marked increase of acid phosphatase activity 8 h after exposure to C1311 concentrations that e ffect total growth inhibition. Moreover, at these concentrations lysosomal swelling and breakdown preceded apoptosis, which was not evident up to 24 h after exposure to drug. Thus, the lysosomotropic effect of C1311 appears t o be a novel feature of this anticancer agent. As it is unlikely that C1311 -induced DNA damage alone would be sufficient for cytotoxic activity, lysos omal rupture may be a critical component for therapeutic efficacy. (C) 1999 Cancer Research Campaign.