Methods for treating a telomerase-expressing cell characterized by an over-activation of telomerase in a subject in need thereof comprising administering a pharmaceutical composition comprising a first amount of a 6-mercaptopurine analog, and a second amount of a telomerase inhibitor, wherein the first amount and the second amount together comprises an amount effective to reduce stability of telomere length and to induce cell death in the telomerase-expressing cell are provided. The methods can be used to treat patients with lung cancer, colon cancer, or cervical cancer.