Nutlin-3 radiosensitizes hypoxic prostate cancer cells independent of p53

Nutlin-3 is a small-molecule inhibitor that acts to inhibit MDM2 binding to p53 and subsequent p53-dependent DNA damage signaling. Whether Nutlin-3 alters cell toxicity following DNA damage under oxic versus hypoxic conditions has not been studied. The potential radio-sensitization (0-10 Gy) properties of Nutlin-3 (dose range, 2-10 μmol/L for up to 24 h) were investigated in vitro using three prostate cancer cell lines, 22RV1 [wild-type p53 (WTp53)], DU145 (mutated p53), and PC-3 (p53-null) under oxic (21% O₂), hypoxic (0.2% O ₂), and anoxic (0% O₂) conditions. As a single agent, Nutlin-3 (2-10 μmol/L) stabilized p53 and p21^WAF levels and was toxic to WTp53-22RV1 cells (IC₅₀, 4.3 μmol/L) but had minimal toxicity toward p53-deficient cells (IC₅₀, > 10 μmol/L). When combined with radiation under oxic conditions, Nutlin-3 decreased clonogenic survival in all three cell lines: 22RV1 [sensitizing enhancement ratio (SER), 1.24], DU145 (SER, 1.27), and PC-3 (SER, 1.12). Anoxia induced p53 protein expression in 22RV1 cells and this was augmented by Nutlin-3 treatment. Furthermore, Nutlin-3 was more effective as a radiosensitizer under hypoxic conditions particularly in WTp53-expressing cells: 22RV1 (SER, 1.78), DU145 (SER, 1.31), and PC-3 (SER, 1.28). The decrease in clonogenic survival with Nutlin-3 was not correlated to altered levels of radiation-induced apoptosis within the three cell lines. Our results indicate that Nutlin-3 can act as a radiosensitizer via p53-independent mechanisms under low O₂ levels. Nutlin-3 may be a useful adjunct to improve the therapeutic ratio using precision radiotherapy targeted to hypoxic cells and warrants further study in vivo.