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The level of N7-(2-hydroxyethyl)guanine (N7-HOEtG), one of the DNA alkylation products formed by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) treatment, was measured in human brain tumor samples by high performance liquid chromatography with electrochemical detection. The tumors from 6 recurrent chemotherapy-naive patients with recurrent glioblastoma multiforme were analyzed as controls. The mean level of N7-HOEtG in DNA of these specimens was 0.42 pmol/mg DNA. Samples were also obtained from a patient with a recurrent glioblastoma multiforme after direct intratumoral therapy with BCNU in ethanol (DTI-015). The levels of N7-HOEtG in the samples distal, medial, and adjacent to the site of injection were 0.8, 2.6, and 369.5 pmol/mg DNA, respectively. Comparison of the level of N7-HOEtG detected in the distal sample after injection with BCNU in ethanol with the mean level of the untreated samples indicated that it was not sufficiently different to be ruled out as a chance occurrence. Comparison of the levels of N7-HOEtG in the medial and adjacent brain tumor samples with the mean level of the control samples showed values that were approximately 6- and 879-fold higher. These results demonstrate that intratumoral administration of BCNU in ethanol produces significant levels of DNA alkylation and suggest that DNA adduct measurements provide a unique molecular dosimeter to evaluate delivery of alkylating agents to brain tumors.