Validation of in vitro screening models for progestagenic activities: inter-assay comparison and correlation with in vivo activity in rabbits.

The PR CALUX® cell line is a stably transfected human U2-OS cell line expressing the human PR and a luciferase reporter construct containing three progesterone-responsive elements coupled to a minimal promoter. The validity of this assay has been studied as an alternative to the McPhail assay in rabbits, an in vivo assay to detect progestins. The PR CALUX assay was characterized by its stable expression of PR protein which leads to induction of endogenous PR target genes by progestins. It was found to have a highly selective response to low levels of different progestins, as well as an insignificant response to other nuclear hormone receptor ligands. As an important step in their validation, the PR CALUX bioassay was compared with another earlier described in vitro bioassay, a Chinese Hamster Ovary (CHO) cell-based PR-CHO reporter gene assay as well as with an in vitro PR-binding (PR-BIN) assay, and the in vivo McPhail assay. This was done using 35 (with the most accurate potency determinations in all tests) and 50 (with less reliable potency determinations in some tests) compounds tested in all assays. The correlation scores between PR CALUX and PR-CHO were r(2)=0.77, and 0.93, respectively; between PR CALUX and PR-BIN r(2)=0.69 and 0.80. Comparison between either the PR CALUX or the PR-CHO transactivation assay and the in vivo McPhail assay revealed very good correlations of r(2)=0.68 (n=35), and 0.85 (n=50). The transactivation assays can discriminate very potent, from potent, weak and inactive compounds rather easily. Besides testing the biological activity of pure chemicals and pharmaceuticals in vitro, the PR CALUX and PR-CHO transactivation assays proved to be relatively good predictors of in vivo progestagenic activity, allowing the use of these assays as prescreening methods or in vitro alternatives.