The proliferative state of a given tissue is a major determinant of its sensitivity to both phase-specific and cycle-specific chemotherapeutic agents. To study the extent of injury induced by antitumor agents to normal and tumorous tissues, a technique for following DNA synthesis as reflected in the incorporation of tritiated thymidine (3H-TdR) into DNA was compared to the conventional radioautographic technique of the labeling index (LI) and to the functional kinetic technique of granulocyte colony formation in vitro. Alterations in DNA synthesis induced by a single dose of cyclophosphamide in normal and tumorous tissues in vivo paralleled in many respects the changes seen when the more time-consuming techniques of the LI or granulocyte colony formation were employed. However, the recovery of granulocyte colony formation after cyclophosphamide therapy laged behind the recovery of DNA synthesis in the bone marrow, obscuring a kinetic event of potential therapeutic significance. The determination of DNA synthesis simultaneously in normal and tumorous tissues in vivo was easy to perform and supplied therapeutically pertinent results comparatively quickly.

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