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Dose-response relationships of RU 486
1Department of Medical Chemistry, University of Helsinki, Finland.
RU 486 exhibits dose-dependent and independent effects, with pregnancy termination being dose-independent. Its metabolism and restricted brain penetration influence its biological activity.
Area of Science:
- Pharmacology
- Endocrinology
Background:
- RU 486 (mifepristone) has demonstrated both dose-dependent and dose-independent effects in clinical use.
- Antiglucocorticoid actions are dose-dependent, while cervical dilatation and pregnancy termination appear dose-independent across tested regimens.
Purpose of the Study:
- To investigate the pharmacokinetic and pharmacodynamic properties of RU 486.
- To elucidate the factors influencing RU 486's dose-dependency and biological effects.
Main Methods:
- Serum and tissue concentrations of RU 486 were measured after oral administration.
- Metabolism pathways and receptor binding affinities of RU 486 and its metabolites were analyzed.
- Pharmacokinetics in humans and rats were compared, including blood-brain barrier penetration.
Main Results:
- RU 486 has a serum half-life of approximately 30 hours, with myometrial concentrations at one-third of serum levels.
- Serum levels are similar between 100-800 mg doses due to alpha 1-acid glycoprotein saturation.
- Metabolites retain significant progesterone and glucocorticoid receptor affinities, suggesting biological activity.
- Limited RU 486 diffusion across the blood-brain barrier in rats suggests central effects are dose-dependent.
Conclusions:
- Peripheral effects of RU 486, like pregnancy termination, are mediated by peripheral steroid receptors.
- Central effects may be dose-dependent due to restricted blood-brain barrier penetration.
- Lower doses might achieve similar biological effects, particularly for peripheral actions.
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