Over-expression of manganese superoxide dismutase (MnSOD) protects tissues from radiation. M40403 is a stable non-peptidyl mimetic of MnSOD that crosses cell membranes and is effective in reducing experimental inflammation. Male BALB/c mice were injected intraperitoneally (i.p.) and subcutaneously (s.c.) with M40403, 30 min before 6.5, 7.5 and 8.5 Gy total body irradiation (TBI). Whereas all control injected mice died after receiving 8.5 Gy TBI by day 17, 30 day survival of mice pre-treated i.p. with 40, 30, 20 or 10 mg/kg was 100%, 90%, 81% and 25%, respectively. The Dose Reduction Factor 50/30 for animals treated with 30 mg M40403 s.c. 30 min prior to TBI was 1.41. Decreased apoptosis of the large and particularly the small bowel and marked recovery of both lymphoid and hematopoietic tissues occurred in the M40403 pre-treated animals. M40403 is effective in reducing TBI-induced tissue destruction and has potential as a new radioprotective agent.

Ionizing radiation (IR) is a pro-oxidant that kills cells by both apoptotic and necrotic mechanisms. Pyrrolidine dithiocarbamate (PDTC) is a thiol-containing compound that may act either as a pro- or anti-oxidant depending on the experimental conditions. This study was designed to determine whether PDTC would reduce or enhance IR-induced cell death of freshly-isolated normal mouse B6/129 spleen cells (NMSC). We determined the effect of increasing doses of IR, PDTC alone and PDTC followed by IR on the viability of NMSC. Annexin V and propidium iodide (Annexin V/PI) staining demonstrated a dose and time-dependent relationship in which PDTC enhanced the percentage of IR-induced apoptotic/necrotic NMSC. Trypan blue dye inclusion confirmed that a loss of membrane integrity was occurring 1 h after incubation with PDTC plus IR. Reduction in the glutathione (GSH)/glutathione disulfide (GSSG) ratio and GSH demonstrated that both IR (8.5 Gy) and PDTC acted as pro-oxidants, but their mechanisms of action differed: In contrast to IR, which promoted p53 activation and caspase 3/7-mediated apoptosis, PDTC inhibited IR-induced p53 and caspase 3/7 activity. However, PDTC increased H(2)O(2) formation and necrosis, resulting in an overall increase in IR-induced cell death. Catalase prevented the PDTC-induced increase in IR cytotoxicity implicating the generation of H(2)O(2) as a major factor in this mechanism. These results demonstrate that in NMSC PDTC acts as pro-oxidant and enhances IR-induced cell cytotoxicity by increasing H(2)O(2)formation and thiol oxidation. As such, they strongly suggest that the use of PDTC as an adjunct to reduce radiation toxicity should be avoided.