Effect of continuous vs intermittent application of 3-OH-tamoxifen or tamoxifen on the proliferation of the human breast cancer cell line MCF-7 M1.

The antiproliferative potency of 5 x 10(-7) M tamoxifen (TAM) and 3-hydroxytamoxifen (3-OH-TAM) was investigated during continuous (8 days) or intermittent (2 h every 2nd or 3rd day, respectively) application to the oestrogen-receptor-positive, estradiol-sensitive human mammary carcinoma cell line MCF-7 M1, a variant of MCF-7 wild type. Growth modulation was evaluated in parallel by counting cells and by measuring DNA content. Continuous incubation resulted in a growth inhibition to 21.8 +/- 3.2% by 3-OH-TAM and to 39.5 +/- 4.8% by TAM when compared with control cultures defined as 100%. Intermittent addition induced a growth reduction to 23.0 +/- 2.1% by 3-OH-TAM and to 41.2 +/- 2.4% by TAM in relation to 100% controls. Addition of 3-OH-TAM for 2 h only at day 1 resulted in an inhibition to 70.3 +/- 3.2%, again in relation to 100% controls. When TAM was administered once for 2 h at day 1 it induced an inhibition to 79.0 +/- 4.9% at day 8. The in vitro results indicate that (a) at 5 x 10(-7) M 3-OH-TAM has a better antiproliferative effectiveness than TAM, (b) the intermittent application is as effective as continuous application (no significant difference), and (c) the addition once a week reveals only a slight growth reduction after 8 days of culture. Application of the long-living TAM results in continuously high serum concentrations, which have been shown to create resistant cell clones. Compared to TAM the 3-OH metabolite has a considerably shorter half-life and its application in vivo reveals rise and fall of its serum concentrations. Since the presented data demonstrate that 3-OH-TAM is more potent than TAM and that the intermittent application is as effective as the continuous form, interval therapy with 3-OH-TAM may slow down the process of acquiring resistance to antioestrogens.