The trisubstituted acridine derivative BRACO-19 has been designed to interact with and stabilize the quadruplex DNA structures that can be formed by folding of the single-stranded repeats at the 3′ end of human telomeres. We suggest that the BRACO-19 complex inhibits the catalytic function of telomerase in human cancer cells and also destabilizes the telomerase-telomere capping complex so that cells enter senescence. Here, we present evidence showing that the inhibition of cell growth caused by BRACO-19 in DU145 prostate cancer cells occurs more rapidly than would be expected solely by the inhibition of the catalytic function of telomerase, and that senescence is accompanied by an initial up-regulation of the cyclin-dependent kinase inhibitor p21, with subsequent increases in p16^INK4a expression. We also show that treatment with BRACO-19 causes extensive end-to-end chromosomal fusions, consistent with telomere uncapping.