Based on the results of studies
Based on the results of studies done to-date, there are three characteristics that make a letrozole-based protocol attractive for use in cattle: 1) it is steroid-free, 2) it is cost-effective and requires a shorter time and fewer animal handlings than conventional methods (Table 1), and 3) it may have a positive effect on fertility (pregnancy and reduced embryonic loss) due to its luteotrophic action.
According to available figures from the National Animal Health Monitoring System in the United States, AI is used in only 8% of beef operations . Although more common in the dairy industry, a surprising 33% of heifers and 22% of cows are still bred naturally at first service, and only 7% of heifers and 22% of cows are induced into estrus . At the time of this writing, the results of the 2017 NAHMS survey were not published, but since 2009 there has been a dramatic increase in the use of FTAI in both dairy and beef cattle.
Recent economic studies have confirmed that dairy herd net return is strongly associated with reproductive performance , . While extensive use of AI in the dairy industry since the 1950s resulted in an astounding 328% increase in average herd milk production , it came at the cost of fertility. The first-service conception rate in lactating dairy cattle decreased by 0.5–1% per year world-wide between 1957 and 2001 resulting in a dramatic increase in days open and cull rate. A recent reversal of the long decline in fertility in dairy cattle has been attributed to increasing use of programmed breeding systems that permit FTAI, and has resulted in a decrease in days open of approximately 18 days , . Increasing the 21-day pregnancy rate by 1% increases the economic net return ($US per cow per year) by $14.40 when pregnancy rates are between 10% and 15% or by $10 when pregnancy rates are between 15% and 20% . Currently, the national average 21-day pregnancy rate in dairy cattle is between 14 and 18% in North America ,  so clearly there is room for improvement.
The impact of FTAI in beef cattle is no less remarkable. In a study of a herd of 1197 suckled beef cows , FTAI resulted in an increase in calf value of $49.17 per calf as a result of an additional 17.5 kg at weaning from proven AI sires. Over a 5-year MHY1485 of study of a cow/calf operation with 300 cows in transition from a traditional breeding system to FTAI , the duration of the breeding season was reduced from 120 days to 72 days and the mean calving day (from first calf) was reduced from 79 to 38 days. This translated into an increase in calf value of $169 per calf, or $51,000 for the 300-cow herd.
In North America, current populations of breedable beef and dairy cattle are 41 and 15.5 million, respectively . If synchronization and FTAI were used on 50% of these animals and 50% became pregnant, the beef industry would benefit by $1.7 billion per year ($169.17/calf; ) and the dairy industry by $147 million per year ($37.87/cow increase from natural service; ). The potentiating effects of an aromatase inhibitor-based protocol on superstimulatory response and CL development hold promise for further broadening the use of synchronization treatments by optimizing both in vivo- and in vitro-derived embryo production and reducing the rate of early embryonic loss in embryo recipients and in high-producing dairy cows. Further, the use of aromatase inhibitor-based protocols to control ovarian function in other production and companion animal species has not yet been explored.
Introduction Breast cancer, which is well known as a common disease in women, remains a high rate of death for female patients worldwide , . It is widely accepted that the level of estrogens and the profile of estrogen receptors are two measurable indicators towards a risk assessment for breast tumors . Since nearly 70% of the patients with breast cancer are estrogen receptor-positive (ER+) and may be estrogen dependent, estrogen deprivation has been considered an attractive therapeutic strategy for ER-positive breast cancer . Aromatase, otherwise called CYP19, is a rate-limiting enzyme in the biosynthesis of estrogens that is responsible for the conversion of androgens including androstenedione and testosterone into estrone and estradiol . For the postmenopausal women with ER-positive breast cancer, aromatase is usually overexpressed in the breast tissues and is the main source of local estrogen production in neoplastic tissues . Interfering with aromatase activity and reducing the level of estrogens in tumor tissues may slow down the growth of breast cancer cells, eventually extending the lifespan of patients .