What Is 17α-Ethinylestradiol 3-benzoate

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Last updated: April 14, 2026

Quick Answer: 17α-Ethinylestradiol 3-benzoate is a synthetic estrogen ester used primarily in research, with a molecular weight of 408.51 g/mol and first synthesized in the 1930s. It functions as a long-acting prodrug of ethinylestradiol after hydrolysis in the body.

Key Facts

First synthesized in 1931 by Adolf Butenandt and colleagues
Molecular formula: C₂₆H₂₈O₃, molecular weight: 408.51 g/mol
Half-life extends to over 24 hours due to benzoate ester modification
Used experimentally to study estrogen receptor activation pathways
Not approved for human therapeutic use in any country

Overview

17α-Ethinylestradiol 3-benzoate is a synthetic derivative of estrogen, specifically an ester-modified form of ethinylestradiol. It was developed during early hormonal research to explore the pharmacokinetics of modified steroid compounds.

This compound combines the potent synthetic estrogen 17α-ethinylestradiol with a benzoate group at the 3-hydroxyl position, altering its solubility and metabolic stability. While not used clinically, it remains a subject of interest in endocrinology and pharmacology research.

First synthesized in 1931 by German chemist Adolf Butenandt, marking a milestone in steroid chemistry and hormone isolation techniques.
Molecular formula C₂₆H₂₈O₃ gives it a molecular weight of 408.51 g/mol, making it significantly heavier than unmodified ethinylestradiol.
The benzoate ester at position 3 increases lipid solubility, allowing for slower release and prolonged activity in animal models.
It acts as a prodrug, requiring enzymatic hydrolysis in vivo to release active 17α-ethinylestradiol, the biologically active form.
No approved medical use exists globally, and it is restricted to laboratory research due to insufficient safety and efficacy data.

How It Works

The mechanism of 17α-ethinylestradiol 3-benzoate centers on its conversion into active estrogen after administration. Its ester bond must be cleaved by esterase enzymes in tissues such as the liver or bloodstream.

Hydrolysis: The ester bond at the 3-position is cleaved by carboxylesterases, releasing benzoic acid and active 17α-ethinylestradiol within 6–12 hours post-injection.
Receptor binding: Once freed, ethinylestradiol binds to estrogen receptors ERα and ERβ with high affinity (Kd ≈ 0.1 nM), triggering genomic signaling pathways.
Lipophilicity: The benzoate group increases lipid solubility by over 40% compared to ethinylestradiol, enhancing tissue retention and depot formation.
Half-life: The modification extends the half-life to over 24 hours in rodent models, compared to 6–14 hours for unesterified ethinylestradiol.
Metabolism: Primarily metabolized in the liver via CYP3A4 enzymes, forming hydroxylated and conjugated metabolites excreted in bile and urine.
Bioavailability: When administered intramuscularly, bioavailability reaches up to 85% due to sustained release from the injection site.

Key Comparison

Compound	Molecular Weight (g/mol)	Half-Life (hrs)	Route of Administration	Primary Use
17α-Ethinylestradiol 3-benzoate	408.51	24–30	Intramuscular, subcutaneous	Experimental research
17α-Ethinylestradiol	296.40	6–14	Oral, transdermal	Birth control, hormone therapy
Estradiol valerate	432.58	12–16	Intramuscular	Hormone replacement therapy
Estradiol benzoate	330.45	12–24	Intramuscular	Veterinary and human estrogen therapy
Testosterone propionate	318.47	0.8–1.5	Intramuscular	Androgen replacement

This table highlights how 17α-ethinylestradiol 3-benzoate compares to other steroid esters in terms of pharmacokinetics and application. Its extended half-life and research-only status distinguish it from clinically used estrogens.

Key Facts

Understanding the key attributes of 17α-ethinylestradiol 3-benzoate helps clarify its niche role in scientific studies. These facts are drawn from historical literature and chemical databases.

Synthesized in 1931 during early steroid hormone research, contributing to foundational knowledge in endocrinology and medicinal chemistry.
LogP value of 4.32 indicates high lipophilicity, which enhances membrane permeability and depot formation at injection sites.
Not listed in FDA databases or EMA approvals, confirming its exclusion from human pharmaceutical use as of 2024.
Used in rodent studies to investigate estrogenic effects on bone density, with doses ranging from 10–100 µg/kg in published trials.
Stability at room temperature is approximately 18 months when stored under nitrogen, making it suitable for long-term lab storage.

Why It Matters

Though not used in medicine, 17α-ethinylestradiol 3-benzoate contributes to our understanding of estrogen pharmacology and prodrug design. Its structure informs the development of longer-acting hormonal therapies.

Advances prodrug research by demonstrating how esterification can modulate release kinetics and bioavailability of steroid hormones.
Helps map estrogen receptor dynamics in animal models, improving understanding of hormonal signaling in reproduction and disease.
Supports environmental studies on synthetic estrogens, as its breakdown products may mimic endocrine disruptors in aquatic systems.
Provides a historical benchmark for comparing modern estrogen formulations, such as those used in contraception and HRT.
Highlights regulatory caution in drug development, as potent estrogens require rigorous safety evaluation before clinical use.

While not a therapeutic agent, this compound remains a valuable tool in advancing hormonal science and refining drug delivery systems.