Bioavailability has been defined as the amount

Bioavailability has been defined as the amount of a drug given by any route, other than intravenously, that reaches general circulation and is available at the site of action [86]. The low bioavailability observed with the initial intravaginal devices (16%) was attributed to the melting point of the gel-vehicle used. This gel-vehicle is commonly used in intravaginal suppositories for women, but body temperature is lower in women than in cattle (37° vs 39 °C) [87], [88]. Rapid liquefaction of the letrozole-containing gel resulted in loss of the preparation through the vulvar opening during micturition, defecation, or ultrasound examinations. Based on these observations, we hypothesized that a letrozole device capable of a more sustained drug release may delay ovulation even further, while allowing more than one follicle to develop to a pre-ovulatory size when treatment is initiated prior to dominant follicle selection [85]. A subsequent study was done to develop an intravaginal device intended to provide biologically active circulating concentrations of an aromatase inhibitor for a minimum of 4 days [89]. Three inhibitors (letrozole, anastrozole and fenbendazole) were tested in vitro using a bovine granulosa cell culture, and letrozole was found to be the most efficient and potent inhibitor of estradiol production. Liposome-based and wax-based formulations were used to assess letrozole diffusion through bovine vaginal mucosa in a diffusion chamber study, and the wax-based vehicle was selected for further development. In an in vivo experiment in cattle, three different intravaginal devices containing 3 g of letrozole were tested. The wax-based vehicle with a higher melting point than the previous gel-based vehicle provided a more continuous delivery of letrozole over a 5-day treatment period in heifers, and the addition of a letrozole-containing gel coating resulted in more rapid initial Z-DQMD-FMK that hastened the increase in plasma concentrations of the active ingredient [89]. Rapid and prolonged suppression of estradiol induced by the new device was associated with increased follicular growth and follicle lifespan, and provided rationale for the development of a letrozole-based protocol for the synchronization of ovulation in cattle [89]. We then conducted an experiment designed to test the hypothesis that the addition of a letrozole-impregnated intravaginal device to a PGF and GnRH-based protocol, initiated at random stages of their estrous cycle, would increase the percentage and synchrony of ovulations in cattle over PGF and GnRH alone [90]. Heifers were treated with a luteolytic dose of PGF followed by 100 μg of GnRH 24 h later to serve as a comparison of the ovulatory response to a subsequent aromatase inhibitor protocol. At the time of ovulation (Day 0), heifers were assigned randomly to five day-groups (n = 8 to 10/group) and given an intravaginal device containing 3 g of letrozole for 4 days starting on Day 0, 4, 8, 12, or 16. At the time of device removal, heifers were given PGF followed by GnRH 24 h later. The degree of estradiol suppression and the diameter of the pre-ovulatory follicle were influenced by the status of the dominant follicle at the time of vaginal device insertion, but the day on which letrozole treatment was initiated did not affect the proportion of heifers that ovulated or the interval to ovulation. The percentage of heifers that ovulated following letrozole treatment was greater than controls (87.1% vs 69.4%, respectively: P < 0.05) as was the synchrony of ovulation (residuals: 0.24 ± 0.07 vs 0.68 ± 0.13; P < 0.01). Corpus luteum diameter profiles and progesterone production were not affected by day-group. Results suggest that a letrozole-based protocol may be initiated effectively at random stages of the estrous cycle, and provide impetus for continued development of a letrozole-based synchronization protocol for fixed-time insemination [90].
Is another synchronization protocol worth pursuing?