What Is Ipamorelin?

Ipamorelin is a synthetic peptide that prompts the pituitary gland to release a burst of growth hormone. It belongs to a group of compounds known as growth hormone secretagogues, a term that simply means substances which cause growth hormone to be secreted.

Five Building Blocks

Its chain reads Aib-His-D-2-Nal-D-Phe-Lys, with an amide cap at the end. Only two of these, histidine and lysine, are among the twenty amino acids the body uses to make proteins in their usual form. The others were chosen for chemistry rather than biology:

• Aib (aminoisobutyric acid) has an extra methyl group that makes the start of the chain harder to break down.
• D-2-Nal is a mirror-image amino acid carrying a bulky naphthalene ring.
• D-Phe is the mirror-image form of phenylalanine.
• The amide cap on lysine removes a charged end that enzymes would otherwise recognise.

The Novo Nordisk chemists arrived at this sequence by removing the central pair of amino acids, alanine and tryptophan, from an older six-amino-acid peptide called GHRP-1 and then refining what remained.

The Receptor It Uses

Ipamorelin binds to the growth hormone secretagogue receptor, known as GHS-R1a. In 1998 its developers showed that blockers of the older GHRP peptides stopped its effect, while a GHRH blocker did not, so it acts through a different door from GHRH. The natural key to that door is ghrelin, a hormone made mainly in the stomach that also signals hunger.

Why "Selective"?

Earlier peptides acting on the same receptor raised ACTH and cortisol, the body's main stress hormones, alongside growth hormone. In pigs, ipamorelin did not, even at doses more than 200 times higher than those needed for half its maximum growth hormone effect. It also left FSH, LH, prolactin and TSH unchanged. That narrow profile is why the 1998 paper called it the first selective growth hormone secretagogue.

Selective here refers to those pituitary hormones. Because the same receptor is involved in appetite and gut movement, effects outside growth hormone are still possible, and later studies explored them.

A Short-Lived Signal

In healthy men given an intravenous infusion, growth hormone rose to a single peak about 40 minutes later and then fell back to very low levels. Ipamorelin itself had a terminal half-life of about two hours. This makes it a pulse-style stimulus rather than a long-acting one.

Key Points

• Ipamorelin is a five-unit peptide with three non-standard building blocks.
• It activates GHS-R1a, the ghrelin receptor, not the GHRH receptor.
• Animal studies found little effect on cortisol or other pituitary hormones.
• In people it produces one short growth hormone peak.