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Use of an aromatase inhibitor for ovarian synchronization in cattle

Date

2020-01-27

Journal Title

Journal ISSN

Volume Title

Publisher

ORCID

0000-0001-6666-5548

Type

Thesis

Degree Level

Doctoral

Abstract

Letrozole is a third-generation aromatase inhibitor approved for use in post-menopausal women to treat hormone-responsive breast cancer. Letrozole is used off-label in women for the induction of ovulation and for use in ovarian stimulation. The overall objective of this thesis was to determine the effect of letrozole on ovarian function using the bovine model. In the first study, letrozole was formulated into a silicone intravaginal device consisting of a small or a large surface area compared to a wax-based device or a control intravaginal device given for 8 days in pubertal heifers. The large-surface area device resulted in the greatest plasma letrozole concentration throughout the treatment period. The biological effect on the ovaries was also most profound in the large surface area group where the dominant follicle was largest at the time of device removal and plasma estradiol concentrations decreased the most during device insertion. In the second study, letrozole residues in milk were characterized in lactating dairy cattle. A silicone letrozole-releasing device (LRD) was inserted for 4 days, blood and milk sampling were performed at 12-hour intervals, and a partial validation for letrozole in milk was done using tandem mass spectrometry. Although letrozole was detected in milk, maximum concentration of letrozole was significantly lower in milk than in plasma. There were no differences in the elimination half-life or time to maximum concentration for letrozole in milk or plasma. The objective of the third study was to determine if letrozole would increase the superovulatory response in cattle. Non-lactating cows were given an LRD or a sham device at the time of transvaginal follicular ablation to induce wave emergence. At wave emergence, cows were given 8 doses of follicle-stimulating hormone at 12-hour intervals and were artificially inseminated 48 hours after prostaglandin (PGF). The LRD group exhibited estrus significantly later and estrus was more synchronous than the sham group. However, the LRD group had a lower proportion of quality one embryos and transferable embryos than the sham group. The objective of the fourth study was to characterize the effect of letrozole on follicular function and oocyte competence in cattle in three experiments. In the first experiment, transvaginal ablation was done to induce wave emergence and an LRD or sham device was given for 4 days followed by follicular aspiration to obtain granulosa cells for gene expression. In the second experiment following a 4-day LRD or sham device, frequent blood collections were done for analysis of plasma luteinizing hormone (LH) concentrations. Oocytes were collected that were either immediately denuded and stained or underwent 24 hours of in vitro maturation followed by staining. In the last experiment, following aspiration, oocytes were subjected to in vitro maturation, fertilization, and culture. There was no difference between treatments for gene expression from granulosa cells. Heifers in the LRD group had a significantly higher LH area under the curve and pulse amplitude than the sham group. The oocytes from the LRD group in vitro matured for 24 hours had the highest proportion of oocytes in the MII stage than the other groups. The LRD group following in vitro fertilization had a greater proportion of blastocysts and transferable embryos than the sham group. The objectives of the fifth study were to determine the interval to ovulation following LRD treatment, compare an LRD-based synchronization to commercially used protocols for fixed-time artificial insemination (FTAI), and to develop a new LRD designed to reduce vaginitis and increase letrozole release. In the first experiment, a 4-day LRD was given at random stages of the estrous cycle followed by PGF at device removal. Cattle (n=10/group; 6 groups) were given GnRH at 48- or 60-hours after PGF, or no GnRH. In the second experiment, the 48-hour interval from PGF to GnRH following a 4-day LRD treatment was chosen for a FTAI trial to compare pregnancy per AI (P/AI). In the third experiment, heifers (n=10/group) were given a newly designed LRD or an LRD designed in study one and frequent blood sampling was performed along with assessment of vaginitis. The 48-hour GnRH cow group ovulated more synchronous than the other groups. Following FTAI, the LRD group had significantly lower P/AI than the other two protocols. The newly designed LRD significantly increased plasma letrozole concentrations and numerically lowered the vaginitis score at device removal. The objectives of the last study were to determine the: interval to estrus following a 4-day LRD or sham device and PGF at device removal, effect of estrous cycle stage when letrozole treatment was initiated on P/AI, and to compare a newly developed LRD protocol to a progesterone-based protocol. Following PGF, no difference was detected in the interval to estrus between the LRD and sham groups. When LRD treatment was initiated during the proestrus period, it did not prevent ovulation. However, treatment during the metestrus period resulted in the highest P/AI. There was no difference in P/AI between a newly developed LRD-based protocol compared to a progesterone-based protocol. In summary, we have determined that intravaginal letrozole treatment can control ovarian function in cattle. Letrozole is not deleterious on the ability of the oocyte to undergo fertilization and can be used for FTAI protocols in cattle. Future studies should address the effect of letrozole during the proestrus period.

Description

Keywords

aromatase inhibitor, estrous cycle, letrozole

Citation

Degree

Doctor of Philosophy (Ph.D.)

Department

Veterinary Biomedical Sciences

Program

Veterinary Biomedical Sciences

Citation

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DOI

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