Farnesyltransferase inhibitors as radiation sensitizers

Purpose: The inhibition of activated Ras combined with radiotherapy was identified as a potential method for radiosensitization. Materials and methods: Immunoblotting was used to control for prenylation inhibition of the respective Ras isoforms and for changes in activity of downstream proteins. Clonogenic assays with human and rodent tumour cell lines and transfected cell lines served for the testing of radiosensitivity. Xenograft tumours were treated with farnesyl transferase inhibitors and radiation and assayed for ex vivo plating efficiency, regrowth of tumours and EF5 staining for detection of hypoxia. Concurrent treatment with L-778,123 and radiotherapy was performed in non-small cell lung cancer (NSCLC) and head and neck cancer ( HNC) patients. Results: Blocking the prenylation of Ras proteins in cell lines with Ras activated by mutations or receptor signalling resulted in radiation sensitization in vitro and in vivo. The PI3 kinase downstream pathway was identified as a contributor to Ras-mediated radiation resistance. Additionally, increased oxygenation of xenograft tumours was observed after FTI treatment. Combined treatment in a phase I study was safe and effective in NSCLC and HNC. Conclusions: Tumour cells with activated Ras were sensitized to radiation. Unravelling the underlying mechanisms promises to lead to even more specific drugs with higher potency and safety.