Plasma Estradiol Profile After Administration of Different Types of Estradiol Esters in Acyclic Mares.
Authors: Setoguchi Leticia S, Siqueira Leonardo M, Lucca Leticia K, Oquendo Pedro S, Oquendo Fabiana M G, Nogueira Guilherme P, Silva Elisa S M
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Reproductive specialists frequently employ estradiol esters to synchronise oestrous cycles in acyclic mares, yet evidence comparing different formulations in this species remains sparse—most protocols derive from cattle research where pharmacokinetics may differ substantially. Researchers administered identical total doses of estradiol benzoate (EB), estradiol cypionate (EC), and 17β-estradiol across three consecutive days to 14 acyclic mares, measuring plasma oestradiol concentrations and endometrial oedema scores daily until oedema resolved. Peak oestradiol occurred at different timepoints for each ester: benzoate peaked sharply at 24 hours post-first dose then declined rapidly, cypionate maintained elevated concentrations from 48 to 96 hours, whilst 17β-estradiol showed no distinct peak within the 24-hour sampling intervals. Notably, all three formulations produced clinically significant endometrial oedema within 24 hours, yet the relationship between plasma oestradiol concentration and oedema severity was inconsistent across groups—moderate correlation in EB and EC groups versus weak correlation in 17β-treated mares—suggesting oestrogen receptor sensitivity or endometrial responsiveness may vary independently of circulating hormone levels. For practitioners designing synchronisation protocols, these findings indicate that benzoate's rapid peak and decline may offer more predictable timing for insemination, whilst cypionate's prolonged elevation might suit mares requiring extended oestral simulation, though oedema scoring alone cannot reliably predict circulating oestradiol levels across different ester preparations.
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Practical Takeaways
- •Different estradiol esters produce markedly different plasma profiles even at equivalent doses and schedules; EB acts quickly (peak D1), EC has delayed onset (peak D2), and 17β produces steady state—choose based on desired timeline for breeding protocols
- •Endometrial edema response does not consistently track plasma E2 levels across ester types, so ultrasound monitoring remains essential and cannot be reliably predicted from hormone concentration alone
- •For acyclic mares requiring rapid endometrial preparation, EB may expedite edema resolution by D4-D5; for sustained hormonal effect, EC provides prolonged elevation through D4
Key Findings
- •Estradiol benzoate produced peak plasma E2 concentrations at 24 hours post-administration with sharp decline by 48 hours
- •Estradiol cypionate showed delayed peak at 48 hours with sustained elevation through 96 hours
- •Estradiol 17β demonstrated gradual, plateau-like concentrations without a distinct 24-hour peak
- •Endometrial edema persisted longest in EC group (until D7) compared to EB (D5) and 17β (D4) despite similar dosing schedules
- •Moderate to weak positive correlation between plasma E2 concentrations and endometrial edema score varied by ester type