In this work, studies were performed to investigate the toxicological, biochemical, vasotropic and radiomodifying properties of the new nitric oxide synthase (NOS) inhibitor, compound T1023. Toxicological studies included the estimation of acute toxicity in mice after i.p. administration of T1023. Radiometric analysis and electron paramagnetic resonance spectroscopy were used to study NOS-inhibitory properties of T1023 in vitro and in vivo, respectively. T1023 vasoactive properties were studied in rat central hemodynamics. Radiobiological experiments were performed using endogenous and exogenous spleen colony formation as well as 30-day survival tests. The morphological changes in peripheral blood and bone marrow (BM) induced with T1023 were analyzed in mice during hematopoietic acute radiation syndrome (H-ARS). It was shown that T1023 is a sufficiently safe compound (LD10 of 317 mg/kg; LD50 of 410 mg/kg). It is an effective competitive NOS-inhibitor that is 10-to-15-fold selective to endothelial and inducible NOS (IC50 for nNOS, iNOS, eNOS: 52.3, 3.2 and 5.1 μM, respectively). Its NOS-inhibitory activity is realized in vivo and is accompanied by an increase in vascular tone. Its single i.p. administration in doses greater than 1/8 LD10 provides significant (40–50%) and long-lasting (more than 90 min) weakening of cardiac output, which can cause transient hypoxia. In radiobiological studies, T1023 proved to be a hypoxic radioprotector. Its radioprotective effect was observed only when administered prophylactically [single i.p dose, 5–120 min before total-body irradiation (TBI)] and only in doses that reduced cardiac output (1/8 LD10 and more, 40 mg/kg for mice), and was correlated in time with the dynamics of circulatory depression. Its radioprotective effect was not observed when administered in vitro and in the first 4 h after TBI. The optimal radioprotective doses of T1023 are relatively safe (1/5–1/4 LD10). In addition, T1023 effectively prevents H-ARS and gastrointestinal acute radiation syndrome (G-ARS) in experimental animals in vivo: dose modifying factor of 1.6–1.9. In the H-ARS mouse model, the prophylactic effect of T1023 (75 mg/kg, single i.p. injection) was accompanied by clinically significant effects. There was an express decrease in the degree of indicators of early BM devastation (by 40%) and maximal neutropenia and thrombocytopenia (2–2.5 times), in addition to a reduction in recovery time (by 30–40%). The obtained experimental results and literature data indicate that NOS inhibitors are an independent class of vasoactive radioprotectors with a specific hypoxic mechanism of action. NOS inhibitors provide new opportunities for developing effective and safe tools for the prevention of ARS.

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