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Aromatase Inhibitors: Effects, Uses & Side Effects

Aromatase inhibitors are medications that reduce estrogen production in the body. They are primarily used to treat hormone-sensitive breast cancer in postmenopausal women.

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Things worth knowing about "Aromatase inhibitor"

Aromatase inhibitors are medications that reduce estrogen production in the body. They are primarily used to treat hormone-sensitive breast cancer in postmenopausal women.

What Are Aromatase Inhibitors?

Aromatase inhibitors (AIs) are a class of medications that block the activity of the enzyme aromatase (also known as cytochrome P450 19A1). This enzyme is responsible for converting androgens (male sex hormones) into estrogens (female sex hormones). By inhibiting aromatase, these drugs significantly lower estrogen levels throughout the body.

Aromatase inhibitors belong to the category of endocrine (hormonal) therapies and are mainly used in the treatment of hormone receptor-positive breast cancer in postmenopausal women. After menopause, the body produces estrogen primarily through aromatase activity in fat tissue, muscle, and other peripheral tissues, since the ovaries are no longer active.

Mechanism of Action

Aromatase catalyzes the final step in estrogen biosynthesis, converting androgens such as testosterone and androstenedione into estrogens. Aromatase inhibitors block this enzyme in one of two ways:

Non-steroidal aromatase inhibitors (e.g., anastrozole, letrozole): These agents reversibly bind to the heme group of the aromatase enzyme, temporarily blocking its function.
Steroidal aromatase inhibitors (e.g., exemestane): These drugs bind irreversibly to aromatase, permanently inactivating the enzyme. They are sometimes referred to as aromatase inactivators.

By blocking aromatase, circulating estrogen levels are substantially reduced, slowing or stopping the growth of estrogen-dependent tumor cells.

Indications – When Are Aromatase Inhibitors Used?

Aromatase inhibitors are used in several clinical contexts:

Breast cancer (mammary carcinoma): Adjuvant therapy after surgery for hormone receptor-positive, HER2-negative breast cancer; palliative treatment for advanced or metastatic breast cancer; neoadjuvant therapy to shrink tumors before surgery.
Endometriosis: In combination with other hormonal agents to reduce estrogen-dependent tissue growth.
Gynecomastia: Enlargement of breast tissue in men caused by an imbalance between estrogen and androgen levels.
Infertility: In selected cases such as polycystic ovary syndrome (PCOS) to stimulate ovulation.
Early bone maturation in children: In select pediatric cases to delay premature skeletal maturation.

Key Active Substances and Preparations

The most commonly used aromatase inhibitors include:

Anastrozole (brand name: Arimidex): Non-steroidal, reversible; 1 mg once daily orally.
Letrozole (brand name: Femara): Non-steroidal, reversible; 2.5 mg once daily orally.
Exemestane (brand name: Aromasin): Steroidal, irreversible; 25 mg once daily orally.

All three agents are approved for the treatment of breast cancer in postmenopausal women and are considered equally effective, though they differ in their side effect profiles.

Dosage and Administration

Aromatase inhibitors are typically taken as oral tablets once daily. Treatment duration can span several years – in the adjuvant setting after breast cancer, therapy is commonly continued for 5 to 10 years. These medications are used exclusively in postmenopausal women, as premenopausal women have active ovaries that continue to produce estrogen through pathways unaffected by aromatase inhibition alone.

Side Effects

Because aromatase inhibitors substantially lower estrogen levels, many of their side effects are a direct consequence of estrogen deficiency:

Joint and muscle pain (arthralgia, myalgia): The most frequently reported side effect.
Osteoporosis and increased fracture risk: Reduced bone mineral density due to estrogen loss.
Hot flushes and night sweats: Common menopausal symptoms.
Vaginal dryness and discomfort during intercourse.
Fatigue: Persistent tiredness and low energy.
Elevated cholesterol levels: Particularly associated with non-steroidal agents.
Mood changes and depressive symptoms.
Headaches and dizziness.

To help prevent osteoporosis, concurrent supplementation with calcium and vitamin D is often recommended, along with bisphosphonate therapy if needed.

Contraindications and Precautions

Aromatase inhibitors should not be used in:

Premenopausal women (except in specific combination regimens).
Pregnant or breastfeeding women.
Patients with known hypersensitivity to the active ingredient.

Special caution is required in patients with pre-existing osteoporosis, elevated fracture risk, or significant liver or kidney impairment.

References

Leitlinienprogramm Onkologie (German Cancer Society): S3 Guideline for Early Detection, Diagnosis, Therapy and Follow-up of Breast Cancer, Version 4.4, 2021.
Nabholtz JM et al. – Anastrozole is superior to tamoxifen as first-line therapy for advanced breast cancer in postmenopausal women. Journal of Clinical Oncology, 2000.
World Health Organization (WHO): Model List of Essential Medicines, 23rd Edition, 2023.

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