The cerebral radioprotective effect of alternative barbiturates to pentobarbital.

The potential for normal brain tissue injury is one of the limiting factors in the use of radiotherapy for brain tumors. As attempts to enhance brain tumor radiation sensitivity have been unsuccessful, the use of cerebral radioprotectants provides an attractive alternative. Pentobarbital has recently been shown to be a cerebral radioprotectant in the rodent and primate models of single fraction radiation injury. Because daily high doses of pentobarbital bring certain significant risks, the potential usefulness of alternative barbiturates was explored. Seven groups of rats received 70 Gy of whole-brain-only irradiation in the single fraction. Group 1 was treated while awake. Groups 2, 3, and 4 received pentobarbital, thiopental, and methohexital, respectively. Groups 5, 6, and 7 received increasing doses of phenobarbital. Mean group survival at 30 days after treatment was determined and compared with the survival of animals treated while awake. Thiopental enhanced survival, similar to pentobarbital. Methohexital and phenobarbital were of no radioprotective value. The differences in the hypnotic effects of these barbiturates is based on dissimilar effects on the kinetics of chloride ion channel patency. We propose that these differences also influence their radioprotective properties. Thiopental is a shorter acting alternative to pentobarbital for cerebral radioprotection. Use of it should permit safer and easier investigation of this radioprotective effect in human trials.