Nafarelin Acetate

At the initial stage of administration, nafarelin acetate produces a transient pituitary stimulatory response, clinically known as the "flare effect". This constitutes the first phase of its pharmacological action and is one of the key characteristics distinguishing it from LHRH antagonists. As a potent LHRH agonist, the drug rapidly binds to LHRH receptors in the anterior pituitary gland upon entering the body. This binding directly activates gonadotropin-secreting cells, triggering a short-term surge in the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which in turn causes a transient elevation in sex hormone levels (estrogen and testosterone).

The mechanism underlying this effect is closely related to the receptor-binding properties of the drug: The product has significantly higher affinity for LHRH receptors than endogenous LHRH and is resistant to degradation by endogenous peptidases. It rapidly occupies a large number of receptor sites, initiates receptor-mediated signal transduction, and activates the synthesis and release of gonadotropins. Temporally, this stimulatory effect typically occurs within 1–2 days after administration and lasts for a relatively short duration, generally 3–7 days, with slight variations depending on individual differences, dosage, and dosing frequency.

Long-term downregulation represents the core pharmacological action of the product and the key mechanism for its use in treating hormone-dependent diseases. This effect usually becomes evident after approximately 4 weeks of continuous administration and persists for some time after treatment cessation. The central principle is that sustained and regular administration of the product induces desensitization and downregulation of pituitary LHRH receptors, rendering the pituitary unresponsive to physiological LHRH stimulation. This ultimately results in a marked reduction in gonadotropin and sex hormone levels, achieving therapeutic effects of medical castration or medical menopause.

Specifically, the development of long-term downregulation involves two critical steps: first, receptor desensitization, whereby continuous drug stimulation gradually reduces the sensitivity of pituitary LHRH receptors.

Even with sufficient binding of the drug or endogenous LHRH, signal transduction pathways cannot be effectively activated, leading to a significant decline in gonadotropin synthesis and release. Second, receptor downregulation, where prolonged receptor activation triggers intracellular negative feedback mechanisms, reducing the number of LHRH receptors on the surface of pituitary cells. This further attenuates the effects of both the drug and endogenous LHRH, creating a dual effect of reduced receptor quantity and decreased sensitivity, ultimately achieving potent suppression of the gonadal axis.

Under this mechanism, three distinct physiological effects occur:

Inhibition of LH and FSH secretion: Following continuous administration, serum LH and FSH concentrations drop below basal levels and remain low, failing to effectively stimulate gonadal function.

Marked reduction in estrogen and testosterone levels: Suppressed gonadotropic stimulation impairs normal estrogen secretion by the ovaries in females and testosterone secretion by the testes in males, reducing serum sex hormone concentrations to levels approximating those in postmenopausal women or castrated men.

Induction of medical castration or medical menopause: Female patients develop menopause-related symptoms such as amenorrhea, hot flushes, and vaginal dryness, while males experience castration-related symptoms including decreased libido and erectile dysfunction. This state effectively inhibits hormone-dependent pathological tissues such as endometriotic lesions and prostate cancer cells, thereby achieving therapeutic goals.