How Letrozole (Femara) Works

Letrozole is a drug used as hormone therapy for the treatment of breast cancer, and included in a group of drugs called aromatase inhibitors due to its mechanism of action. Estrogen and progesterone are female sex hormones that promote cancer growth. While in menstruating women, these hormones are produced in the ovaries, in menopausal women, it is produced in the fat tissue through a process known as aromatase. Therefore, letrozole interferes with this process in order to decrease the levels of estrogen in the body and, consequently, stop or slow down cancer growth, or decrease the probability of cancer recurrence after surgery. Letrozole is only effective in hormone sensitive or hormone receptor positive breast cancer and its use is limited to women after menopause.

The drug is a “non-steroidal inhibitor of estrogen synthesis with antineoplastic activity,” as defined by the National Cancer Institute (NCI) Drug Dictionary. “As a third-generation aromatase inhibitor, letrozole selectively and reversibly inhibits aromatase, which may result in growth inhibition of estrogen-dependent breast cancer cells. Aromatase, a cytochrome P-450 enzyme localized to the endoplasmic reticulum of the cell and found in many tissues including those of the premenopausal ovary, liver, and breast, catalyzes the aromatization of androstenedione and testosterone into estrone and estradiol, the final step in estrogen biosynthesis.” Letrozole is available in the form of tablets to be taken daily.

Letrozole (Femara) for Breast Cancer

Letrozole was first approved by the U.S. Food and Drug Administration (FDA) on October 29, 2004, for extended adjuvant treatment, and a year after the approval was extended by the FDA for the drug to be used as an initial adjuvant treatment. It is commercialized under the brand name femara by Novartis Pharmaceuticals Corp. and it is currently approved for postmenopausal women diagnosed with hormone-receptor-positive, early-stage breast cancer right after surgery (or possibly chemotherapy and radiation) to reduce the risk of the cancer coming back, postmenopausal women diagnosed with hormone-receptor-positive early-stage breast cancer who have taken 5 years of tamoxifen to reduce the risk of the cancer coming back, or postmenopausal women diagnosed with advanced-stage or metastatic hormone-receptor-positive breast cancer.

“Safety and efficacy were evaluated in a double-blind, multi-center, international clinical study in postmenopausal women with hormone receptor positive early stage breast cancer who had received five years of adjuvant therapy with tamoxifen. A total of 5,187 women who were within three months of completion of adjuvant tamoxifen were randomly assigned to receive five years of either letrozole or placebo,” report the NCI about the investigations that backed the approval. The research also revealed that about 10% of the patients experienced hot flashes and sweats, pain in joints or bones, tiredness and weakness, and increased levels of cholesterol in the blood as adverse events associated with the treatment, while less common side effects include skin rashes, headache, dizziness, malaise, fluid retention, loss of appetite, indigestion, hair thinning, diarrhea, constipation, vaginal dryness or bleeding, reduced libido, pain, cough and breathlessness, and sadness or depression.

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