The invention provides methods and compositions for modulating the activity of therapeutic agents for the treatment of a cancer by administering one or more agents that (either alone or in combination) induces telomere damage and inhibits telomerase activity in the cancer cell. The method initially uses, e.g., a telomere damage-inducing agent such as paclitaxel, and a telomerase inhibitory agent such as AZT. The invention also provides methods for identifying other agents with telomere damage-inducing activity and/or telomerase inhibitory activity (as well as and compositions having such activity), for use in the treatment of cancer.