GHRP-2 vs GHRP-6

How GHRP-2 and GHRP-6 differ

GHRP-2 and GHRP-6 are different molecules. Both are synthetic six-residue peptides with a free N-terminal amine and a C-terminal amide. Their compositions differ. Three of GHRP-2's six residues are D-configured, it carries one tryptophan, and one of its residues, 2-naphthylalanine, does not occur in proteins. Two of GHRP-6's six residues are D-configured, and it carries two tryptophans.

A small human study often cited for GHRP-6 tested GHRP-2 and a third peptide.^[6] GHRP-6 appears in it only in the introduction and was not tested.^[6] No finding from that study was obtained with GHRP-6. FDA's review cites reports of increased insulin requirement, infection, pancreatitis and deaths among critically ill study subjects given GHRP-2.^[7] Reports like these cannot show that GHRP-2 caused the events or how often they occur.

An author of the 1990 human study of GHRP-6 held an inventor interest.^[8] The literature reviewed for GHRP-6 reports no serious safety finding from its human studies.

Neither peptide contains cysteine, methionine, asparagine, glutamine or aspartate, so in each the routes that dominate most peptides' degradation are closed. What remains is oxidation of the tryptophan indole, driven by air, light and trace metals. In GHRP-6, the histidine coordinates transition metals, and trace metal contamination promotes tryptophan oxidation. Each is cationic and carries aromatic rings, so each can adsorb to surfaces both electrostatically and hydrophobically. In GHRP-2, those rings include the fused bicyclic naphthalene of its 2-naphthylalanine. In each, mass spectrometry cannot verify the configuration of the D-residues, since a D-residue and its L-form are identical in mass.

The FDA has not approved either GHRP-2 or GHRP-6 for human use.

Have GHRP-2 and GHRP-6 been compared directly?

A 1998 study tested GHRP-2 and GHRP-6 in conscious swine.^[3] Results are reported for each compound in that model. Five studies in cells or tissue tested GHRP-2 and GHRP-6 in the same system. Two used human material: pituitary and hypothalamic membranes (1998)^[4] and cell cultures from pituitary tumors (1996).^[9] Three used animal pituitary cells: rat (1997),^[10] sheep and rat (1996)^[11] and sheep (1997).^[12] Results are reported for each compound in each assay. A 2013 study tested both in mice.^[13] Results are reported for each compound in that model.