2-Methoxyestradiol and analogs as novel antiproliferative agents: Analysis of three-dimensional quantitative structure-activity relationships for DNA synthesis inhibition and estrogen receptor binding

2-Methoxyestradiol (2-MEO), a metabolite of estrogen, is an attractive lead compound for the development of novel antitumor and anti-inflammatory agents, because it embodies antiproliferative and antiangiogenic activities in one molecule. However, the affinity of 2-MEO for the estrogen receptor would lead to undesirable side effects. As a prelude to the design of 2-MEO-like compounds with an optimal activity profile, we assayed 2-MEO and a series of analogs for their ability to cause G₁ cell-cycle arrest (by measuring inhibition of DNA synthesis in human cultured airway smooth muscle) and to inhibit binding of [³H]estradiol at the estrogen receptor (ER; from rat uterine smooth muscle). One compound, a diacetoxy enediol derivative, was identified with reasonable potency for DNA synthesis (plC₅₀ = 5.97) but showed negligible affinity for the ER (plC₅₀ < 5). Three-dimensional quantitative structure-activity relationships were developed for these activities using comparative molecular field analysis (CoMFA) techniques. Comparison of optimized CoMFA models revealed distinct structural requirements for DNA synthesis inhibition and ER binding. For example, DNA synthesis inhibition is enhanced by electropositive substitutions in the 2-position below the plane of the steroid A-ring, whereas ER binding is favored by electronegative substitution in this position. Similarly, DNA synthesis inhibition correlates negatively with increased steric bulk in regions clustered around the A and B rings; changes in steric bulk in these regions has little correlation with ER binding. These observations will guide the design of new analogs with improved potency for desired characteristics (e.g., DNA synthesis inhibition) with minimal unwanted activities (e.g., ER binding).