Letrozole (Femara) early response to treatment

Tumorigenic breast cancer cells characterized by high CD44 and low or undetectable CD24 levels (CD44+/CD24-/low) may be resistant to conventional therapies and responsible for cancer relapse. We defined a “signature” expression pattern of hundreds of genes associated with CD44+/CD24-/low, mammosphere-forming cells. In a panel of patient breast tumors, this tumorigenic gene signature was found exclusively manifested in tumors of the recently identified “claudin-low” molecular profile subtype characterized by overexpression of many mesenchymal-associated genes, suggesting that these tumors have pre-existing higher levels of tumorigenic cells. Furthermore, when comparing the expression profiles of paired breast cancer core biopsies before versus after hormone therapy or chemotherapy, both the tumorigenic and “claudin-low” signatures were more active in about half of tumors after treatment, indicative of a greater enrichment of tumorigenic cells as a result of treatments targeting the bulk tumor cells. Keywords: two group comparison Biopsies were taken from the same subjects both pretreatment and after 10-14 days Letrozol, 2.5 mg/day, oral.