Thiol WR-1065 and disulphide WR-33278, two metabolites of the drug Ethyol (WR-2721), protect DNA against fast neutroninduced strand breakage

The main metabolites of the cytoprotective drug Ethyol (Amifostine, WR-2721) are the thiol WR-1065 and the disulphide WR-33278 (formed by the oxidation of WR-1065). Both metabolites are well-known protectors against DNA damage induced by gamma -rays. Using supercoiled plasmid DNA and restriction fragments we show that they protect efficiently also in the case of fast neutrons. In anoxic conditions WR-1065 (Z 2) protects by scavenging of OH and by 'chemical repair' (by H donation from its SH function). WR-33278 (Z 4) protects by scavenging of OH and, in the case of the supercoiled plasmid DNA, by reducing the accessibility of radiolytic attack sites via the induction of packaging of DNA in liquid-crystalline condensates (observed by circular dichroism). Because of this second mechanism, the plasmid DNA is more efficiently protected by WR-33278 than by WR-1065, at concentration ratios 1 drug/4 nucleotides. Moreover, using sequencing gel electrophoresis of irradiated fragments of known sequence, we show that the protection by the two metabolites is non-homogeneously distributed along the DNA sequence, with 'hot spots' of protection and with unprotected regions. Based on presented molecular modelling results we explain the sequence dependence of radioprotection by structural variations induced by the binding of the drugs.