Vorozole and letrozole are third-generation aromatase (cytochrome P450 19A1) inhibitors. [11C]-Vorozole can be used as a radiotracer for aromatase in living animals but when administered by IV, it collects in the liver. Pretreatment with letrozole does not affect the binding of vorozole in the liver. In search of finding the protein responsible for the accumulation of vorozole in the liver, fluorometric high-throughput screening assays were used to test the inhibitory capability of vorozole and letrozole on a series of liver cytochrome P450s (CYP1A1, CYP1A2, CYP2A6, and CYP3A4). It was determined that vorozole is a potent inhibitor of CYP1A1 (IC50 = 0.469 μM) and a moderate inhibitor of CYP2A6 and CYP3A4 (IC50 = 24.4 and 98.1 μM, resp.). Letrozole is only a moderate inhibitor of CYP1A1 and CYP2A6 (IC50 = 69.8 and 106 μM) and a very weak inhibitor of CYP3A4 (<10% inhibition at 1 mM). Since CYP3A4 makes up the majority of the CYP content found in the human liver, and vorozole inhibits it moderately well but letrozole does not, CYP3A4 is a good candidate for the protein that [11C]-vorozole is binding to in the liver.

Gonadal hormones like estrogen acts as a metabolic signal of energy sufficiency and also regulates the stage of reproductive cycle. Apart from hormonal changes, feeding circuits also affects the estrous cycle. Estrogen, a hormone secreted by the ovaries regulates reproductive cycle and has a negative effect on food intake. Leptin, a hormone secreted by the adipose tissue has an inhibitory effect on food intake. In this study we tried to analyze the estrous cycle in relation with leptin and estrogen levels in two mouse lines ob/ob and Ay/a, both of which are obese, hypometabolic and infertile but with contrasting leptin levels. To assess the progression of estrous cycle, vaginal smears was performed on female WT, ob/ob and Ay/a (n=4) mice. In WT mice at proestrous; nucleated cells were round to hexagonal shaped, estrous; cornified cells and metaestrous; leukocytes with many nucleated cells, diestrous; the cells were all scattered leukocytes. We found that both ob/ob and Ay/a mice were not cycling and were at a persistent estrous phase with cornified cells. The number of cornified cells however was not similar at all stages. Quantitization of beta‐estradiol showed both ob/ob and Ay/a compared to WT had same or higher b‐estradiol levels. Thus, this study leads us to believe the estrous cycle of these mouse models is not only dependent on estrogen levels but interplay of gonadal hormones and feeding circuits.