Interestingly, peripheral aromatization was very low in the control infusionsperformed 1 month after treatment, suggesting sustained effects of treatmentpossibly due to inactivation of aromatase by this steroidal inhibitor.Aromatization rates were reduced by up to 97% of control values. Additionalanalysis of the samples revealed no specific effects on the metabolic clearancerates of androstenedione and oestrone, the interconversion of the androgens oroestrogens, or on androstenedione conversion to dihydrotestosterone.In order to replicate this situation in the best price for generic viagra model, we have utilized MCF-7 cells stably transfectedwith aromatase (MCF-7CA) . As the rodent has no significant productionof oestrogen from non-ovarian tissue, MCF-7CAcells serve as a local source ofoestrogen to stimulate tumour formation in ovariectomized nude mice by aromatizing androstenedione. Thus inhibitors targeting aromatase and alsoantioestrogens that bind the ER can be studied in tumours formed from thesecells. Therefore, the model has been employed to provide information that predicts the effects of these agents in the clinic and also as a guide to the development of new protocols to optimize their use in treatment. For example, themodel has been used recently to investigate the effects of the non-steroidal aromatase inhibitors, letrozole and anastrozole and compared them with tamoxifen and Faslodex, the pure antioestrogen. As discussed below, combining aromatase inhibitors and antioestrogens was explored in the model. Inhibitingboth oestrogen synthesis and oestrogen action simultaneously might be moreeffective than using either type of agent alone .In the xenograft model, tumours are developed by inoculating ovariectomized, female Balb/c mice (aged 4–6 weeks) with MCF-7 cells (3 hh107cells/ml in Matrigel) stably transfected with the human aromatase gene (MCF-7CA). The cells were kindly provided by Dr. S. Chen (City of Hope, Duarte,CA, USA) . As production of adrenal steroids in athymic mice is deficient, animals are injected subcutaneously from the day of inoculation throughout the experiment with 0.1 mg of androstenedione/best price for generic viagra per day, the substrate for aromatization to oestrogens. Tumour growth is measured withcalipers weekly and tumour volumes are calculated. When all tumours reach ameasurable size (~300 mm3), usually 28–35 days after inoculation, animalsare assigned to groups with tumours of similar volume and treatment is begun.At autopsy, 4–6 h after the last injected dose, blood is collected and tumoursare removed, cleaned, and weighed.Letrozole (5 μg/day) was alsoable to cause marked regression, even of large tumours.The MCF-7CAtumours in the best price for generic viagra model synthesize sufficient amounts ofoestrogens to support oestrogen-dependent tumour growth and also to maintainthe uterus of these ovariectomized animals at a weight similar to that of intactmice during metoestrus. Letrozole and anastrozole caused a decrease in themean uterine weight compared to that of the control mice (P < 0.01). Neitherof the aromatase inhibitors had oestrogenic effects on the uterus. The uterineweights of mice treated with tamoxifen were not significantly different fromthose of the control mice, consistent with previous reported findings reflectingthe agonist/antagonists actions of tamoxifen . In contrast to tamoxifen,fulvestrant, considered to be a pure antioestrogen, blocked the actions ofoestrogen on the uterus. Thus the uterine weights of fulvestrant-treated micewere similar to those treated with aromatase inhibitors. This indicates a difference in sensitivity of the effects of the two antioestrogens on the tumour andthe uterus. Based on these results, it seems likely that aromatase inhibitors,even in long-term use, will not cause stimulation of the endometrium as reported in some women receiving tamoxifen. In recent clinical trials, no adverseeffects on the endometrium have been observed in patients treated with the aromatase inhibitors letrozole, anastrozole, and exemestane .Sequential treatment with aromatase inhibitors and antioestrogensWe observed previously that switching mice treated first with tamoxifen tosecond-line treatment with letrozole was effective in slowing tumour growthcompared to continuing treatment with tamoxifen . However, this strategyproved inferior to treatment with letrozole continued as first-line treatment.Unlike treatment with tamoxifen, tumours of mice treated with letrozole(10 μg/day) initially regressed. After extended treatment, tumours eventuallygrew during letrozole treatment. However, tumour-doubling time with letrozole was more than twice as long as with tamoxifen. Mice with tumours growing on letrozole treatment were then assigned to three groups so that each hadsimilar mean tumour volumes at the start of second-line treatment. The groupswere treated with either tamoxifen, a higher dose of letrozole (100 μg/day), orcontinued on letrozole (10 μg/day) treatment (Fig. 2) . However, althoughthe higher dose of letrozole slowed tumour growth, tumour volumes were notsignificantly different from those of groups treated with tamoxifen or continued on letrozole (10 μg/day). In another study, both antioestrogens, tamoxifenand fulvestrant, were ineffective as second-line therapy following letrozoletreatment . These results suggest that switching the animals from letrozole(10 μg/day) to antioestrogen treatment, is not beneficial for patients withtumours progressing on a therapeutically effective dose of letrozole.