A gonadotropin-releasing hormone analogue (GnRH analogue or analog), also known as a luteinizing hormone releasing hormone agonist (LHRH agonist) or LHRH analogue is a synthetic peptide

Peptide

Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...

 drug

Medication

A pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease.- Classification :...

  modeled after the human hypothalamic

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

 gonadotropin-releasing hormone

Gonadotropin-releasing hormone

Gonadotropin-releasing hormone , also known as Luteinizing-hormone-releasing hormone and luliberin, is a tropic peptide hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the anterior pituitary. GnRH is synthesized and released from neurons within...

 (GnRH). A GnRH analogue is designed to interact with the GnRH receptor

Receptor (biochemistry)

In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...

 and modify the release of pituitary gonadotropin

Gonadotropin

Gonadotropins are protein hormones secreted by gonadotrope cells of the pituitary gland of vertebrates. This is a family of proteins, which include the mammalian hormones follitropin , lutropin , placental chorionic gonadotropins hCG and eCG and chorionic gonadotropin , as well as at least two...

s FSH

FSH

FSH may refer to:* Follicle-stimulating hormone* Fox Sports Houston, a regional Fox Sports Net affiliate station for the Houston, Texas area* Facioscapulohumeral muscular dystrophy* International Civil Aviation Organisation code for Flash Airlines...

 and LH

LH

LH, Lh, or lh may stand for:*Laurel and Hardy*Leasehold *Left-handed or left hand*Letterhead, the heading at the top of a sheet of letter paper*lh, a digraph for many languages...

 for therapeutic purposes. Shortly after the discovery of GnRH by Nobel laureates Guillemin

Roger Guillemin

Roger Charles Louis Guillemin received the National Medal of Science in 1976, and the Nobel prize for medicine in 1977 for his work on neurohormones, sharing the prize that year with Andrew Schally and Rosalyn Sussman Yalow.Completing his undergraduate work at the University of Burgundy, Guillemin...

 and Schally researchers tried to modify the GnRH decapeptide with the intent to synthesize stimulating and blocking variants.

Two types of analogues have to be distinguished:

Agonists

A gonadotropin-releasing hormone agonist (GnRH agonist) is an analogue that activates the GnRH receptor resulting in increased secretion of FSH and LH. Initially it was thought that agonists could be used as potent and prolonged stimulators of pituitary gonadotropin release, but it was soon recognized that agonists, after their initial stimulating action – termed a “flare” effect - eventually caused a paradoxical and sustained drop in gonadotropin secretion. This second effect was termed “downregulation” and can be observed after about 10 days. While this phase is reversible upon stopping the medication, it can be maintained when GnRH agonists use is continued for a long time.

Antagonists

A gonadotropin-releasing hormone antagonist (GnRH antagonist) is an analogue that blocks the GnRH receptor resulting in an immediate drop in gonadotropin (FSH, LH) secretion. The GnRH antagonist is primarily used in IVF treatments to block natural ovulation.

All GnRH analogues are contraindicated in pregnancy (pregnancy category

Pregnancy category

The pregnancy category of a pharmaceutical agent is an assessment of the risk of fetal injury due to the pharmaceutical, if it is used as directed by the mother during pregnancy. It does not include any risks conferred by pharmaceutical agents or their metabolites that are present in breast...

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