Genotoxicity studies on DNA-interactive telomerase inhibitors with application as anti-cancer agents.

Telomerase-targeted strategies have aroused recent interest in anti-cancer chemotherapy, because DNA-binding drugs can interact with high-order tetraplex rather than double-stranded (duplex) DNA targets in tumour cells. However, the protracted cell-drug exposure times necessary for clinical application require that telomerase inhibitory efficacy must be accompanied by both low inherent cytotoxicity and the absence of mutagenicity/genotoxicity. For the first time, the genotoxicity of a number of structurally diverse DNA-interactive telomerase inhibitors is examined in the Ames test using six Salmonella typhimurium bacterial strains (TA1535, TA1537, TA1538, TA98, TA100, and TA102). DNA damage induced by each agent was also assessed using the Comet assay with human lymphocytes. The two assay procedures revealed markedly different genotoxicity profiles that are likely to reflect differences in metabolism and/or DNA repair between bacterial and mammalian cells. The mutational spectrum for a biologically active fluorenone derivative, shown to be mutagenic in the TA100 strain, was characterised using a novel and rapid assay method based upon PCR amplification of a fragment of the hisG46 allele, followed by RFLP analysis. Preliminary analysis indicates that the majority (84%) of mutations induced by this compound are C --> A transversions at position 2 of the missense proline codon of the hisG46 allele. However, despite its genotoxic bacterial profile, this fluorenone agent gave a negative response in the Comet assay, and demonstrates how unwanted systemic effects (e.g., cytotoxicity and genotoxicity) can be prevented or ameliorated through suitable molecular fine-tuning of a candidate drug in targeted human tumour cells.

Design of telomerase inhibitors for the treatment of cancer.

Telomerase is a cellular ribonucleoprotein reverse transcriptase responsible for the maintenance of telomeres, the tandemly repeating guanine-rich nucleic acid sequences at the 3'-ends of eukaryotic chromosomes that serve to protect chromosomal stability and maintain integrity. Telomerase enzyme activity is essential for the sustained proliferation of most immortal cells, including cancer cells, and is currently an important recognised target for the development of novel and potentially tumour-specific anticancer chemotherapeutics. Herein, we review recent advances in the design and development of telomerase inhibitors for the treatment of cancer. To date, these have included antisense strategies, reverse transcriptase inhibitors, and agents capable of interacting with high-order telomeric DNA tetraplex (or "G-quadruplex") structures in such a way as to prevent enzyme access to its required linear telomeric DNA substrate. Critical appraisal of each distinct approach is provided together with highlighted areas for continued development necessary to further refine the present disparate classes of telomerase inhibitors for use in clinically viable therapies.