Telomerase inhibition as a potential new therapy for colorectal cancer

Colorectal cancer represents the second leading cause of cancer-related deaths. Despite local tumour control, patients die from disseminated disease and improved therapy is clearly required. One possible new approach is inhibition of telomerase, a reverse-transcribing enzyme thought to be essential to prevent senescence of cells by synthesizing chromosomal telomeres, which is reactivated in 85-95% of colorectal cancers. The purpose of this study was to determine the degree of telomerase inhibition by known retroviral reverse-transcriptase inhibitors using concentrations that are not acutely toxic to cells. The concentrations of three drugs (azidothymidine (AZT); dideoxythymidine (ddT); and dideoxyguanidine (ddG)) needed to reduce the proliferation (ID50 ) of the colorectal cell line HT29 by 50% were determined. Extracts were made of cells exposed for 24-48 h to these concentrations of each prodrug, telomerase activity determined using the Telomerase Repeat Amplification Protocol (TRAP), quantifying polymerase chain reaction products generated by telomerase with a phosphorimager and ImageQuant(TM) software. The drug treatments reduced the activity of telomerase in these cells by 97% for AZT, 38% for ddT and 47% for ddG, compared with control extracts. In order to confirm that the drugs used were directly inhibitory to telomerase, extracts of control cells were exposed to the active (phosphorylated) drugs and telomerase activity determined: greater than 60% and 80% inhibition occurred at 0.02 m M and 0.04 m M concentration of ddT and ddG, respectively. (The phosphorylated form of AZT was not available.) Control experiments demonstrated that the action of the active drug was not at the PCR stage of the TRAP assay and so was directly exerted on telomerase. We conclude that reverse transcriptase inhibitors can directly inhibit telomerase in cells exposed to prodrug concentrations, which are not acutely toxic, and that the active drug does directly inhibit telomerase. We propose that such inhibitors may have a role in reducing the survival, by inducing senescence, of remaining malignant cells after potential curative surgery, thus reducing recurrence and improving the prognosis of the disease. In addition they may be used in high-risk susceptible patients and in early-stage cancers.