The mechanism of action of the antitumour drug 4'-(9-acridinylamino) methanesulphon-m-anisidide (mAMSA)

Abstract

1. Treating PY815 cells with mAMSA caused a decrease in the sedimentation rate of their DNA under both neutral and alkaline conditions, consistent with a reduction in size of the DNA due to drug-induced breakage. After lysis of cells a slow unfolding of the released DNA appeared to take place, the rate of unfolding or unwinding being slower for the larger control cell DNA than for the smaller DNA from mAMSA-treated cells. The unwinding process is facilitated by strong detergents and alkali. 2. There was no evidence that treating cells with either ethidium bromide or mAMSA damaged mitochondrial DNA under conditions that caused substantial breakage of nuclear DNA. 3. mAMSA-induced breakage of chromosomal DNA was observed after labeling cells with [3H] or [14C]-thymidine, confirming that inhibition of growth during [3H]-thymidine labelling was not responsible for any cell-cycle specific artefacts in the sedimentation profiles. 4. A simple method for detecting drug or X-ray effects on the DNA of intact cells was developed. This method used an easily constructed viscometer to measure changes in the viscosity of alkaline or neutral cell lysates. 5. The viscosity method has been tested by comparing the effects of mAMSA, oAMSA, daunomycin and adriamycin on cellular DNA. The results show that these drugs produce both single-strand and double-strand breaks in cellular DNA, even at concentrations of drug which produce minor effects on growth. 6. The termini of DNA fragments produced as a result of mAMSA treatment of PY815 cells were not available for phosphorylation, consistent with the view that they may be linked to protein. The 3' termini were sensitive to Exonuclease III action, indicating that they may be free from attached protein. 7. mAMSA and -oAMSA were equally effective in breaking the DNA in isolated nuclei in contrast to their different effects on growth and breakage of DNA in intact cells. 8. The topoisomerase inhibitors novobiocin and coumermycin inhibited the production of double-strand breaks in isolated PY815 cell nuclei. Novobiocin did not inhibit the resealing of DNA breaks induced by mAMSA. The results were consistent with mAMSA inducing the action of a type II topoisomerase.