Thymidine labeling index of human breast carcinoma. Enhancement of in vitro labeling by 5-fluorouracil and 5-fluoro-2'-deoxyuridine.

Inhibitors of thymidylate synthetase, 5-fluoro-2'-deoxyuridine (FUDR) and 5-fluorouracil (FU), enhanced in vitro thymidine labeling of human breast carcinoma cells. Their use resulted in an increase in the measured thymidine labeling index (TLI) of breast carcinomas by increasing detectability of labeled nuclei in autoradiographs. The TLI was measured with FU or FUDR enhancement in primary breast carcinomas from nine women younger than age 50, and from 30 women 50 years or older. The mean and geometric mean TLI were 8.0 and 6.3 respectively for the younger group, and 4.0 and 2.8 respectively for the older group. Similar significant age-associated differences were noted in a series of 133 TLI measurements without FU or FUDR. The TLI was not significantly correlated with primary breast carcinoma size or number of axillary nodal metastases. The capacity to form axillary metastases must be related to factors other than the rate of cell replication in breast carcinomas.