The potential usefulness of the DNA repair inhibitor, chloroquine, in radiation therapy has been investigated. Contrary to the suggestions made by others, it has been demonstrated that injection of chloroquine immediately after radiation exposure inhibits recovery as effectively in the normal tissues of the mouse as it does in the tumors they bear. Further, delay of the drug injection to 2 hr after exposure has similar effects on both tumor and normal tissues, i.e., radiation recovery is not inhibited in either case. We interpret these data as indicating that the tumor and normal tissues are equally sensitive to the recovery-inhibiting effects of chloroquine and that manipulation of the timing between exposure and injection does not alter this similarity. It is unlikely, therefore, that chloroquine will be able to improve the efficacy of radiotherapy.

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