Buserelin is a synthetic luteinizing hormone–releasing hormone (LHRH) analog. Like naturally occurring LHRH that is produced by the hypothalamus, initial or intermittent administration of buserelin stimulates release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary.1 In humans, branded forms of buserelin are used primarily in the treatment of prostate cancer and endometriosis.
Clinical trials conducted several years ago demonstrated that treating mares with buserelin 9 or 10 days after ovulation increases pregnancy rates. "Early embryonic loss is a major cause of sub-fertility in mares. Progesterone produced in the ovary by the corpus luteum maintains the early pregnancy. The embryo must signal its presence and ensure the corpus luteum is maintained in order to survive the first few weeks of pregnancy. In other species, luteal function in pregnant animals starts to diverge from that of non-pregnant animals between 11 and 14 days after ovulation. It is therefore, considered to be a critical time for the maintenance of the pregnancy."2 Since then, buserelin has been offered under several trademarks and has become a popular medication for the prevention of embryonic loss in mares.
"We observed structural changes in the follicles and uterus of heavy draft mares during estrus and examined the effect of a single injection of the gonadotropin-releasing hormone analog buserelin on ovulation and endocrine profiles. Twenty-two heavy draft mares were divided into a buserelin-treated group (n=8) and a control group (n=14). Mares were given an intramuscular injection of 40 µg buserelin when they presented signs of estrus to a teaser stallion, had ≥45 mm diameter follicles, and presented decreased uterine edema compared with the previous examination. The follicles and uterus were monitored using transrectal ultrasound imaging and measurement of blood levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), progesterone, and estradiol-17β.
[R]esults indicate that a single injection of 40 µg buserelin when follicles are at least 45 mm in diameter and uterine edema is decreased is effective for inducing ovulation."3
"Buserelin treatment induced a rise in LH concentration during the 48 h period of the experiment, and LH concentrations before ovulation were significantly higher in buserelin treated cycles than in placebo cycles. These experiments demonstrated the usefulness of two new protocols of administration of buserelin, as an alternative to hCG for induction of ovulation. One hypothesis explaining the mechanism of action is that the persistant rise in LH concentration could modify the ratio of biological/immunological LH, as it occurs physiologically, thereby hastening ovulation."4
According to the available research, side effects in equines where buserelin is used are reported as minimal.