The incorporation of formate-C14 into human breast cancer in vitro.

The incorporation in vitro of formate-C14 into normal breast tissue protein, taken from different individuals and from the same individual at different times, was rela tively consistent. Administration of estradiol did not appreciably affect the rate of in corporation of formate-C14 into proteins or protein-bound serine of normal breast tis sue. However, surviving neoplastic breast tissue showed within a few days responses to estradiol therapy. Those patients whose tumor tissues showed an increased rate of for mate-C14 uptake subsequently had a more rapid progression of the disease, whereas those showing decreased uptake rates showed subsequent regression or no change in tumor size. Tumors which had a more rapid uptake of formate-C14 were observed in patients who showed a more rapid clinical progression of disease, whereas tumors with relatively slower uptakes of formate-C14 were observed in patients whose disease was clinically static or regressing. Estradiol therapy caused a stimulation of formate-C14 incorporation into RNA purines in both normal and neoplastic breast tissues. There was little or no incorpora tion of formate-C14 into DNA purines. Appreciable incorporation of formate-C14 into DNA thymine from normal and neoplastic tissues was observed.