ipamorelin mastery course
Unit 3 of 11

the pentapeptide design

five residues, four design tricks, one receptor

how Novo Nordisk got from GHRP-1 to NNC 26-0161

Ipamorelin is the result of a deliberate truncation-and-substitution exercise on the GHRP-1 scaffold. The Hansen/Ankersen/Raun team at Novo Nordisk removed the central Ala-Trp dipeptide, replaced the N-terminal histidine with Aib, and kept two D-amino acids and a C-terminal amide. The whole sequence is five residues -- H-Aib-His-D-2-Nal-D-Phe-Lys-NH2 -- and every choice traces back to one of two goals: keep GHS-R1a affinity, drop everything that triggered cortisol or prolactin.

the pentapeptide, annotated

each residue with its MW contribution, design role, and the amide cap that closes the chain.

residue derivation builder

walk the structure residue-by-residue, swap in natural vs unnatural amino acids, and see what changes.

residue derivation builder

key terms

tap to expand.

AAibamino acid
alpha-aminoisobutyric acid (2-methylalanine). non-proteinogenic. the alpha-methyl group sterically blocks aminopeptidase attack and locks the backbone into a 3-10-helical / alpha-helical N-cap turn. position 1 of ipamorelin.
DD-2-Nalamino acid
D-2-naphthylalanine. bulky aromatic non-natural D-amino acid. the naphthyl ring fills a hydrophobic pocket of GHS-R1a; the D-configuration makes the residue invisible to most proteases. position 3 of ipamorelin.
DD-Pheamino acid
D-phenylalanine. the mirror-image of natural L-Phe. proteases evolved against L-amino acids do not cleave at D-Phe efficiently. position 4 of ipamorelin.
LLys-NH2C-terminus
C-terminal lysine amide. the primary-amide cap (NH2 instead of COOH) blocks carboxypeptidase cleavage. the positively-charged lysine side chain contributes electrostatic interactions with the receptor pocket.
SSPPSchemistry
solid-phase peptide synthesis -- the standard Fmoc-protected amino-acid coupling strategy on a polymer resin. ipamorelin is synthesized C-to-N on Rink amide resin (which gives the C-terminal primary amide automatically) using HBTU activation.
Ddeletion sequenceQC
a peptide missing one of its residues, produced when a coupling step fails on the resin. the Aib residue couples sluggishly because of its alpha-methyl steric bulk -- it is the synthesis step most prone to deletion, and a known QC flag for gray-market ipamorelin product.

five residues -- the simple version

why each position is where it is.

Position 1 is Aib. It is not a natural amino acid. Its job is to be a wall at the N-terminus that aminopeptidases cannot cut through. It also bends the molecule's first turn into a shape the receptor likes.

Position 2 is histidine -- the only fully natural residue in the sequence. The imidazole ring of histidine is conserved across the GHRP family because it makes specific contacts inside the receptor pocket. Positions 3 and 4 are D-2-Nal and D-Phe, bulky aromatic D-amino acids that fill the receptor's hydrophobic core while staying hidden from proteases that only recognize L-amino acids.

Position 5 is lysine, capped as a primary amide instead of the usual carboxylic acid. The amide blocks carboxypeptidase enzymes. Together, the N-terminal Aib and the C-terminal amide protect both ends of the chain -- which is why ipamorelin survives in plasma long enough to act, despite being just five residues.

advanced: SAR notes from Raun and Ankersen 1998

truncation and substitution series

The Ankersen et al. J Med Chem 1998 paper (PMID 9733495) traced the evolution from GHRP-1 through ipamorelin. Removing the central Ala-Trp dipeptide cut the molecule from six residues to five without losing GH-releasing potency.

the selectivity-defining residue

Substituting Aib for the natural N-terminal histidine extended in vivo duration. Replacing L-Trp at position 3 with D-2-Nal preserved receptor engagement but eliminated the prolactin and ACTH release seen with natural-tryptophan analogs.

That single substitution at position 3 is the residue most responsible for the selectivity claim.

advanced: synthesis protocol and the Aib coupling problem

solid-phase peptide synthesis

Fmoc-SPPS on Rink amide resin, Applied Biosystems 431A FastMoc UV protocols, HBTU activation in NMP. Coupling order C-to-N: Fmoc-Lys(Boc)-OH, Fmoc-D-Phe-OH, Boc-D-2-Nal-OH, Boc-His(Trt)-OH, Boc-Aib-OH.

cleavage and purification

Final TFA cleavage with phenol/ethanedithiol/thioanisole/water scavengers (40:3:1:2:2). RP-HPLC purification typically delivers greater than 98% pure ipamorelin acetate.

the dominant impurity

The Aib coupling step has the highest deletion-sequence risk because its alpha-methyl group sterically hinders the activated ester approach. Des-Aib ipamorelin is the dominant impurity peak.

advanced: degradation pathways and stability

cleavage sites

Predicted in vivo cleavage sites are the Aib-His bond (resistant due to Aib) and the D-Phe-Lys bond. LC-MS of degraded product shows Aib-His and D-Phe-Lys cleavage peaks as the dominant signals.

lyophilized stability

The lyophilized powder is stable for years refrigerated and months at room temperature. Hygroscopic; protect from moisture and light.

reconstituted stability

Bacteriostatic 0.9% benzyl-alcohol water at 2.5 mg/mL gives refrigerated stability of 2-4 weeks. Plain sterile water shortens stability to about 7 days at 2-8 C. Degradation signals are turbidity, yellowing, and particulate formation.

three of ipamorelin's five residues are non-natural. that's an unusually high non-natural-residue density for a small peptide, and it is what makes the molecule both protease-resistant and impossible to confuse with anything your body produces endogenously.

where this has been studied

PK and structural characterization from the originator's program.

human IV PK
Gobburu et al. Pharm Res 1999 (PMID 10496658) -- 48 healthy male volunteers, 15-minute IV infusions across a 33-fold dose range. terminal half-life ~2 h, clearance 0.078 L/h/kg, volume of distribution at steady state 0.22 L/kg, dose-proportional across the full range.
in vitro affinity
Raun et al. Eur J Endocrinol 1998 (PMID 9849822) -- rat pituitary cell EC50 of 1.3+/-0.4 nM. in vivo ED50: 80+/-42 nmol/kg in anesthetized rats, 2.3+/-0.03 nmol/kg in conscious swine. comparable potency to GHRP-6 with markedly cleaner selectivity.
SAR series
Ankersen et al. J Med Chem 1998 (PMID 9733495) -- the medicinal-chemistry companion paper documenting the truncation and substitution series that led from GHRP-1 to ipamorelin. defines exactly which residue changes drive selectivity.
nasal absorption
Johansen et al. Xenobiotica 1998-1999 (PMID 9879640) -- intranasal absorption study in animals. published subcutaneous human PK at equivalent rigor does not exist; community subq estimates are extrapolated from IV data plus this nasal series.