Two third-generation
aromatase inhibitors,
letrozole and
anastrozole, and the
antiestrogen tamoxifen, were compared for growth-inhibiting activity in two
estrogen receptor (ER)-positive
aromatase-overexpressing human
breast cancer cell lines, MCF-7aro and T-47Daro. Inhibition of
hormone (1 nM
testosterone)-stimulated proliferation was evaluated in both monolayer cultures and in three-dimensional spheroid cultures.
Letrozole and
anastrozole were also compared for effectiveness of
aromatase inhibition, and relative affinity for
aromatase, under both monolayer and spheroid growth conditions.
Letrozole was an effective inhibitor of MCF-7aro monolayer cell proliferation, with an estimated 50% inhibitory concentration (IC50) of 50-100 nM, whereas an IC50 was not reached with
anastrozole at any concentration tested (100-500 nM). An IC50 of
tamoxifen was 1000 nM. Proliferation of T-47Daro monolayer cells was more sensitive to inhibition by all three agents; as with MCF-7aro cells,
letrozole was the most effective inhibitor. MCF-7aro spheroids were slightly less sensitive than monolayer cells proliferation-inhibiting effects of
letrozole (IC50 about 200 nM), and there was no significant inhibition with 100-200 nM
anastrozole or 200-1000 nM
tamoxifen.
Letrozole and
anastrozole significantly inhibited T-47Daro spheroid cell proliferation, at 15-25 and 50 nM, respectively, consistent with the greater sensitivity of T-47Daro monolayer cells to inhibition of proliferation by these agents.
Tamoxifen failed to significantly inhibit T-47Daro spheroid cell proliferation over a 100-500 nM concentration range. Determination of
aromatase inhibition in monolayers of both cell lines by a direct-access microsomal assay and an intact-cell assay revealed that
letrozole was more active than
anastrozole in monolayers of both cell lines and in both assays. In MCF-7aro spheroids following cell lysis, only
letrozole significantly inhibited
aromatase activity, supporting the conclusion that
letrozole binds stronger to
aromatase than
anastrozole does. Our results demonstrate that MCF-7aro and T-47Daro spheroids could be a suitable model for evaluation of growth-inhibitory effects of agents used in hormonal
therapy of
breast cancer.