To study this hypothesis, low doses of the compounds were used which resulted in partial tumoursuppression. Thus a greater reduction in tumour growth may be achieved bycombining the two types of agent (Fig. 3). Since previous studies of 10 μg ofletrozole/mouse per day caused almost complete regression of tumours, dosesof 5 μg/day of letrozole and anastrozole were used in the combined treatmentswith tamoxifen at 3 μg/day. All compounds alone, or in combination at thesedoses, were effective in suppressing tumour growth in comparison to controlmice. Weights of tumours removed at the end of treatment were significantlyless for animals treated with the aromatase inhibitors letrozole and anastrozolethan with tamoxifen (P < 0.05). However, treatment with either anastrozole orletrozole combined with tamoxifen did not produce greater reductions intumour growth, as measured by tumour weight, than either aromataseinhibitor treatment alone, although tumour weights were reduced more thanwith tamoxifen alone . Oestrogen concentrations measured in tumourtissue of the letrozole-treated mice were markedly reduced from 460 to20 pg/mg of tissue. Studies in patients treated with tamoxifen and letrozolesuggest that the clearance rate of letrozole may be increased . This couldcontribute to the combination being rather less effective than letrozole alone.best price for generic viagra 3. Effects of letrozole and tamoxifen and their combination on the growth of MCF-7CAbreasttumour xenografts in female, ovariectomized, athymic nude mice. All mice received androstenedione(100 μg/day sc). Mice were divided into groups (n = 20 per group) and injected subcutaneously dailywith vehicle, letrozole (10 μg/day) and/or tamoxifen (100 μg/day). Tumour volumes were measuredweekly and are expressed as the percentage change relative to the initial tumour volume. Treatmentwith letrozole was statistically significantly better than the other treatments at 16 weeks. Tumour volumes were statistically significantly larger in the tamoxifen treatment group than in the letrozole treatment group at 28 weeks. Taken from .These results suggest that combining non-steroidal aromatase inhibitors withtamoxifen does not improve treatment. Similar results were obtained whenfulvestrant was combined with tamoxifen . Tamoxifen may have a weakagonistic effect on the tumours which overrides the reduction in oestrogenconcentrations by the aromatase inhibitors and which counteracts the effect ofthe pure antioestrogen. Subsequently, these results have been confirmed in theclinic by the Arimidex, Tamoxifen Alone or in Combination (ATAC) trial .Patients with early breast cancer were treated with anastrozole, tamoxifen, orthe combination. Treatment with anastrozole alone proved to be superior totamoxifen, indicating for the first time that aromatase inhibitors were moreeffective in treating breast cancer patients than tamoxifen. However, treatmentwith the combination of anastrozole and tamoxifen was no better than withtamoxifen alone.In recent studies, combining exemestane and tamoxifen showed that thecombination was better than either tamoxifen or exemestane alone. This mayreflect a dose-dependent effect by achieving a more complete oestrogen blockade .best price for generic viagra 4. Effects of letrozole and tamoxifen on the growth of MCF-7CAbreast tumour xenografts infemale, ovariectomized, athymic, nude mice. All mice received androstenedione (100 μg/day sc).Mice were divided into groups (n = 20 per group) and injected subcutaneously daily with vehicle,letrozole (10 μg/day) or tamoxifen (100 μg/day). Tumour volumes were measured weekly and areexpressed as the percentage change relative to the initial tumour volume. Treatment with vehicle,letrozole was statistically significantly better than the other treatments at 16 weeks. Tumour volumeswere statistically significantly larger in the tamoxifen treatment group than in the letrozole treatmentgroup at 28 weeks. Tumours were collected for analysis from some mice at weeks 4, 28, and 56 asindicated by the arrows.Phospho-mitogen-activated protein kinase (p-MAPK)was increased 2.3-fold in tumours that were responding to letrozole treatmentat week 4 compared to vehicle-treated tumours, but was increased up to 6-foldin tumours growing on letrozole at weeks 28 and 56 (Fig. 5) . These findings suggest that alternate signaling pathways are activated in tumours nolonger sensitive to the effects of letrozole and support ER-mediated transcription despite the depletion of ligand (oestrogen) .For further studies of the mechanisms involved in the loss of sensitivity toletrozole, tumour cells were isolated from the tumours of mice treated withletrozole for 56 weeks described above . The cells were maintained in thepresence of letrozole (1 nM) after isolation and designated long-term letrozole-treated, or LTLT, cells. The expression of signaling proteins in these cellswas compared to the parental MCF-7CAcell line and also to a variant cell linederived from MCF-7CAby culturing the latter in steroid-depleted medium for6 months (UMB-1CA) . In the latter cells, oestrogen deprivation resulted ina 2-fold increase in ER expression compared to the MCF-7CAcells. In contrast,ER expression was reduced in LTLT cells consistent with the decline inexpression observed in the tumours of long-term letrozole-treated mice .best price for generic viagra 5.