Biological basis for chemo-radiotherapy interactions.

For over 10 years, chemo-radiotherapeutic combinations have been used to treat locally advanced epithelial tumours. The rationale for these combinations relies on spatial cooperation or interaction between modalities. Interactions may take place (i) at the molecular level, with altered DNA repair or modification of the lesions induced by drugs or radiation, (ii) at the cellular level, notably through cytokinetic cooperation arising from differential sensitivity of the various compartments of the cell cycle to the drug or radiation, and (iii) at the tissue level, including reoxygenation, increased drug uptake or inhibition of repopulation or angiogenesis. Some mechanisms underlying interaction of radiation with cis-diammino-platinum (II) (cis-Pt), 5-fluoro-2'-deoxyuridine (5-FU), taxanes and gemcitabine are described. It is shown how various mechanisms including cell synchronisation and reoxygenation concur to paclitaxel-induced radiosensitisation. In the future, specific targeting of tumours, for example, with the epidermal growth factor receptor (EGFR) or angiogenesis inhibitors, should be achieved in order to increase the therapeutic index.