INHIBITORY EFFECTS OF BESTRABUCIL, A CONJUGATE OF CHLORAMBUCIL AND ESTRADIOL, ON THE PRODUCTION OF ANDROGEN‐INDUCED GROWTH FACTOR(S) BY SHIONOGI CARCINOMA 115 CELLS

It is known that diffusible trophic factors play an important role in both the normal and cancerous growth regulatory processes of hormone‐responsive cells such as are found in the prostate and mammary glands. Consequently, it is important to identify whether the production of such growth factors is affected by administration of therapeutic agents. We examined the effect of bestrabucil, a benzoate of an estradiol‐chlorambucil conjugate, on the production of growth factor(s) by Shionogi carcinoma 115 (SC‐115) cells, an androgen‐responsive cultured cancer cell line. We then investigated whether the inhibitory effect found was specific to bestrabucil, or if it was also produced by a mixture of the 2 compounds, estradiol and chlorambucil. Bioassay employing BALB/3T3 cells demonstrated the presence of two kinds of growth factor in the conditioned medium obtained by culturing SC‐115 cells in medium containing 10‐ 8 M testosterone; these factors could be separated by heparin‐sepharose column chromatography using 0.5 M NaCI and 1.1 M NaCI. When the SC‐115 cells were cultured in medium containing bestrabucil, at a concentration of 10 −5 M, no growth factor activity was detected in the fraction eluted from the heparin‐sepharose column by 1.1 M NaCI. At bestrabucil concentrations of 10 5 ‐10 7 M, concentration‐dependent inhibition of growth factor production by SC‐115 cells could be demonstrated by 3 H‐thymidine uptake assay. However, this inhibitory effect could not be demonstrated using only a mixture of estradiol and chlorambucil. The finding that bestrabucil specifically inhibits the production of growth factor(s) may help in the development of a new class of drugs which will be clinically useful in treating hormone‐responsive cancer.