Direct measurement of pO 2 distribution and bioreductive enzymes in human malignant brain tumors

Abstract

Purpose : To measure the oxygen status of human malignant brain tumors in vivo and to determine the activities and expression of bioreductive enzymes in these same human brain tumor samples, as a means of assessing their suitability as targets for bioreductive drug therapy. Methods and Materials : A polarographic oxygen electrode was used to measure the intratumoral oxygen tension in twenty patients with malignant brain tumors during open brain surgery, performed under standard anaesthetic conditions. Six different tracks, each with a path length of 22 mm, were recorded per patient representing 192 readings. Following pO 2 measurements the tumors were resected and stored in liquid N 2 for subsequent bioreductive enzyme analysis. Eight human malignant brain tumors were assessed, by enzyme activity and western blot expression, for the presence of various bioreductive enzymes. These enzymes included DT-diaphorase, NADH cytochrome b5 reductase, and NADPH cytochrome P-450 reductase. Of these eight gliomas analyzed six samples were incubated with the bioreductive drug tirapazamine, in the presence of cofactor(s), to establish whether human brain tumors could metabolize this compound. Results : Both the high grade intrinsic and metastatic brain tumors showed significant regions of hypoxia. All the tumors subjected to enzyme profiling contained the bioreductive enzymes, DT-diaphorase, NADH cytochrome b5 reductase and NADPH cytochrome P-450 reductase. Also all six of the brain tumors investigated could metabolize tirapazamine to the two-electron reduction product. Conclusion : These findings would favor primary brain tumors as suitable targets for bioreductive therapy.