BACKGROUND: Heat shock protein 90 (HSP90) is a molecular chaperone that is responsible for the conformational maintenance of several client proteins that play important roles in DNA damage repair, apoptosis following radiation, and resistance to radiation therapy. DS-2248 (tricyclic pyrazolopyrimidine derivative) is a newly-developed, orally available inhibitor of HSP90 with low adverse effects. We investigated the combined effects of radiation and DS-2248 in vitro and in vivo. METHODS: SCCVII squamous cell carcinoma cells and tumors transplanted in C3H/HeN mice were used. In vitro combined effects of X-ray radiation and DS-2248 were investigated using a colony assay. Phosphorylated histone H2AX (γH2AX) was quantified after 2-Gy irradiation with or without 24-hour pretreatment with DS-2248. The mice bearing SCCVII tumors received oral DS-2248 10 times over 2 weeks and received local irradiation with doses of 1, 2, 3, and 4 Gy delivered 6 times over 2 weeks. Then, tumor volumes were measured. RESULTS: Radiation plus pretreatment with 50 nM DS-2248 for 24 hours produced synergistic effects on SCCVII cells. γH2AX foci persisted after radiation for longer periods (6 and 24 hours) in DS-2248-treated cells than in control cells. In vivo, the combined effects appeared to be additive when 5 or 10 mg/kg DS-2248 was combined with total radiation doses of 6-18 Gy, but the effect was considered supra-additive when 15 mg/kg of DS-2248 was combined with a total dose of 24 Gy. CONCLUSIONS: The combined effects of DS-2248 and radiation were additive at low drug and radiation doses, but may have been supra-additive at higher doses. Inhibition of slow repair of DNA double strand breaks (i.e., homologous recombination) was considered to contribute to this combined effect.