The ‘Other’ Telomerase Inhibitors: Non-G-Quadruplex Interactive Agent, Non-Antisense, Non-Reverse Transcriptase Telomerase Inhibitors

Human telomeres are several kilobases of repeated (TTAGGG)n sequences at the ends of chromosomes, a short fragment of which is lost with each cell division. This shortening serves as a “mitotic clock” which limits the number of divisions that a normal somatic cell can undergo. Cells undergoing continuous division need some method of bypassing this clock. One such method is the expression of telomerase. This ribonucleoprotein is an enzyme that rebuilds the lost portion of the telomeres. Between 80- 95% of tumors are telomerase-positive, including ovarian carcinoma, hepatocellular carcinoma, neuroblastoma, leukemia / lymphoma, and cancers of the breast, prostate, lung, kidneys and bladder, as well as many immortalized cell lines . While absent in most normal tissues, this enzyme is expressed at higher levels in germline tissues, bone marrow, and lymphocytes.Due to the expression of telomerase in most tumor cells and its absence in most normal tissues, telomerase inhibitors are being investigated as possible anticancer agents. This review focuses on non-reverse transcriptase inhibitor, non-oligonucleotide and non-G-quartet interactive agent telomerase inhibitors. These agents include: differentiating agents, kinases and phosphatases, cell cycle and apoptosis regulating agents, immunotherapeutic agents, antibiotics, steroids, bisindole derivatives, and a variety of other compounds. These agents hold much promise for the future treatment of malignancies.