Influence of different radioprotective compounds on radiotolerance and cell cycle distribution of human progenitor cells of granulocytopoiesis in vitro

Ficoll-separated mononuclear cells (MNC) of cryopreserved human bone marrow were incubated with isotoxic doses of diltiazem, N-acetylcysteine (NAC), glycopolysaccharide extract of spirulina platensis (SPE), tempol, thiopental, WR2721 and WR1065. After irradiation with a single dose of 0.73 Gy, survival of granulocyte/macrophage colony-forming cells (GM-CFC) was determined at d 10-14, using an agar culture system. Diltiazem, NAC, tempol and WR1065 significantly improved radiotolerance with protection factors (PF) between 1.21 and 1.36 (n = 5, P < 0.05) at 0.73 Gy (PF-0.73 Gy). The survival curves of diltiazem (D0 = 0.88 Gy, n = 1.00), NAC (D0 = 0.92 Gy, n = 1.10), tempol (D0 = 0.99 Gy, n = 1.10), WR1065 (D0 = 0.89 Gy, n = 1.16) and control (D0 = 0.78 Gy, n = 1.00) over 0.36-2.91 Gy showed a significant radioprotective effect for D0 only for tempol (P = 0.018) and for the extrapolation number 'n' only in the case of NAC (P = 0.023). Cell cycle analysis of the CD34⁺ cell subpopulation (control-0 h: G1 = 82.7%, S = 13.7%, G2/M = 3.6%) revealed that all compounds with a significant PF-0.73 Gy also caused a significant increase in CD34⁺ cells in S phase up to 48 h. Within the first 24 h, only NAC (26.7 ± 4.1%), tempol (14.3 ± 1.0%) and possibly WR1065 (15.5 ± 1.6%) had higher fractions of CD34⁺ S-phase cells compared with controls. This observation and the improvement of GM-CFC cloning efficiency indicated that only NAC was able to recruit progenitor cells in the cell cycle, whereas tempol and WR1065 possibly inhibited cell cycle progression by S and G2/M arrest. Of the radioprotectors tested, NAC, tempol and WR1065 may be suitable to support, alone or combined with cytokine therapy, accelerated haematopoietic recovery after irradiation.