Difluoromethylornithine enhanced uptake of tritiated putrescine in 9L rat brain tumors

Abstract

Difluoromethylornithine (DFMO) depletes endogenous putrescine and enhances the uptake of and retention of [ 3H] putrescine in vitro. To determine if DFMO also enhances uptake of [ 3H] putrescine in vivo, DFMO and trace doses of [ 3H] putrescine, dissolved in artificial CSF, were infused into growing (6–9 day) 9L brain tumors by means of osmotic pumps. When 7-day osmotic pumps were loaded with 1 μCi[ 3H] putrescine, with or without 10 or 100 mM DFMO, pumped at 1 μl/h, the mean uptake after 3 days was 168 ± 62 cpm/mg tumor (17 rats) without DFMO, 300 ± 197 cpm/mg tumor (11 rats) with 10 mM DFMO and 1088 ± 421 cpm/mg tumor (11 rats) with 100 mM DFMO ( p ≤ 0.05 vs. control). Significantly less radioactivity was detected in the contralateral brain and in nonbrain tissues (0.5 ± 0.1 to 14 ± 5 cpm/mg). To measure the extent of [ 3H] putrescine distribution in the tumor, the same dose of drugs was delivered for a longer period of time, using 14-day pumps to allow tumors to become large enough to be divided into 1.4 mm thick transections. The mean radioactivity in the sections from eight control rats receiving [ 3H] putrescine without DFMO were not significantly different between the sections (174 ± 61 cpm/mg tumor for sections containing the cannulas, 273 ± 61 and 259 ± 91 cpm/mg for adjacent sections). In the six rats given 100 mM DFMO there was a significant increase in mean radioactivity in the cannula containing section (2251 ± 919 cpm/mg tumor). Mean counts from adjacent sections in these rats were 97 ± 44 and 33 ± 13 cpm/mg. Values for contralateral corpus striatum and nonbrain tissues ranged from 0.7 ± 0.3 to 4.3 ± 1.5 cpm/mg tissue. When DFMO was delivered directly to the tumors while [ 3H] putrescine was infused intraperitoneally, the uptake in the tumor slices was low (5–10 cpm/mg in different slices). These results demonstrate that infusion of DFMO directly into growing 9L brain tumors can selectively enhance the uptake of exogenous [ 3H] putrescine by rapidly dividing cells which are within a 1.4 mm diameter area at the cannula tip. Although these studies used [ 3H] putrescine at trace doses, it is estimated that infusion of higher doses of [ 3H] putrescine plus DFMO will selectively kill tumor cells.