"Chemically related is not chemically identical." - Dr. Diana Blithe, Contraceptive Program Director, NICHD
Well, in a word: yes! As a career medicinal chemist, I completely concur that tiny changes - say, a methyl here or a deuterium there - can have huge effects on drug potency, targeting, clearance, recognition, plasma concentration, etc, etc.
Dr. Blithe's statement rebuffed a remark made by another researcher, who said that since abortifacient RU-486 (mifepristone) and ella contained similar core skeletal structures ("chemical cousins"), they must function similarly at the same dose.
Actually, are those two drugs really that similar?
But what about at C-17? For one, they're locally diastereomeric - the alcohol points "up" (beta) for RU-486, but the related acetate "down" (alpha) for ella. The various moieties dangling off the right side subtly change drug targeting, explains Dr. Blithe, directing effects toward the ovary (ella) or the uterine lining (RU-486). Finally, different uses (delayed ovulation vs. pregnancy termination) combined with different pharmacokinetics result in different dosing: ella sells in 30 mg tablets, while RU-486 utilizes 200 mg of active ingredient...quite a large dose for a steroid.