Stopping treatment can reverse acquired resistance to letrozole from NIH study.

Using the intra tumoral aromatase xenograft model, we have observed that despite long lasting growth inhibition tumors eventually begin to grow during continued letrozole treatment. In cells isolated from these Long Term Letrozole Treated tumors (LTLT-Ca), ERα levels were decreased whereas signaling proteins in the MAPK cascade were upregulated along with Her-2. In the current study we evaluated the effect of discontinuing the letrozole treatment on the growth of letrozole resistant cells and tumors. The cells formed tumors equally well in the absence or presence of letrozole and had similar growth rates. After treatment was discontinued for six weeks, letrozole was administered again. Marked tumor regression was observed with this second course of letrozole treatment. Similarly in MCF-7Ca xenografts, a six-week break in letrozole treatment prolonged the responsiveness of the tumors to letrozole.

These results indicate that discontinuing treatment can reverse letrozole resistance. This could be a beneficial strategy to prolong responsiveness to AIs for breast cancer patients.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2491346/