Triptorelin

Triptorelin is a synthetic analogue of gonadotropin-releasing hormone (GnRH) in which D-tryptophan replaces the glycine at position 6 of the native decapeptide. This vial holds 2 mg of it as a white freeze-dried powder. Research on triptorelin and its relatives covers why a D-amino acid at position 6 raises potency, how GnRH analogues act at the pituitary receptor, how triptorelin is cleared in people, and how it degrades in aqueous solution.

One residue changed

The certificate gives the sequence as pGlu-HWSY-[D-Trp]-LRPG-NH₂: the ten residues of native GnRH with the glycine at position 6 replaced by D-tryptophan, the mirror-image form of the amino acid. The formula is C64H82N18O13 and the theoretical mass 1311.5 Da, the figure to read the LC-MS identity result against. The chain now carries two tryptophans, at positions 3 and 6, which matters for its chemistry in solution.

Mass alone cannot tell D-tryptophan from L-tryptophan, because mirror-image residues weigh the same; the triptorelin guide discusses how the configuration at position 6 is confirmed. In molar terms, 2 mg of a 1311.5 Da peptide is about 1.5 µmol, slightly less once the certificate's water content is allowed for. Triptorelin is also sold in solution, as a triptorelin preloaded pen and a triptorelin nasal spray.

What the research covers

Why position 6

Triptorelin grew out of structure-activity work on LH-RH in the 1970s. Coy and colleagues reported in 1976 that putting simple D-amino acids in place of the glycine at position 6 produced analogues with far greater gonadotropin-releasing activity than the natural hormone, both in vivo and in vitro¹. Activity rose with the lipophilic character, and perhaps the size and aromaticity, of the new side chain: measured as LH release in immature male rats over six hours, [D-Ala6]-LH-RH was 7 times as active as LH-RH, [D-Phe6] 10 times and [D-Trp6] 13 times¹. Replacing the C-terminal glycinamide with an ethylamide, which had raised the potency of the D-Ala6 and D-Leu6 analogues, made the D-Phe6 and D-Trp6 analogues less potent instead¹.

Receptor binding and clearance

Müller and colleagues summarized triptorelin as binding GnRH receptors with higher affinity than the native hormone and as having a prolonged half-life because it resists enzymatic degradation². Its target, the pituitary type I GnRH receptor, signals mainly through the G protein Gq, and Millar's review of GnRH receptors noted that differences in ligand structure may direct which intracellular pathway the receptor engages³. Müller's group measured triptorelin pharmacokinetics in an open study of 24 men: healthy volunteers, two groups with renal insufficiency of differing severity and one group with hepatic insufficiency². Peak concentrations were similar in all four groups, total clearance fell as renal impairment worsened and was lowest with hepatic insufficiency, and serum levels fitted a three-compartment model². Elimination half-lives in the three impaired groups were between 6.6 and 7.7 hours².

Degradation in solution

Helm and Müller studied gonadorelin and triptorelin in aqueous solution across pH, temperature, buffer species, buffer concentration and ionic strength, using stability-indicating HPLC⁴. The two peptides degraded in similar ways and were most stable at about pH 5; acetate buffer was the most favorable, phosphate caused more degradation, and ionic strength had no significant effect⁴. For triptorelin in acetate buffer at pH 5 with mannitol, they predicted a t90 of 7.7 years at 20 °C⁴. Hoitink and colleagues later traced the degradation of triptorelin, buserelin and goserelin to a small number of residues, with the L-serine (position 4 in triptorelin) involved in both backbone hydrolysis and epimerization⁵. The two tryptophans add an oxidation route. In a hydrogen peroxide model, Simat and Steinhart found that free tryptophan degraded fastest, short tryptophan peptides more slowly and the protein lysozyme slowest, with the pyrrole part of the indole ring the most susceptible and oxindolylalanine and N-formylkynurenine the main products identified⁶.

What is not established

The human data cited here are pharmacokinetic only. The clinical pharmacology, regulatory status and safety record of triptorelin are covered in the triptorelin guide.

Frequently asked questions

References

1. Coy DH, Vilchez-Martinez JA, Coy EJ, et al. Analogs of luteinizing hormone-releasing hormone with increased biological activity produced by D-amino acid substitutions in position 6. J Med Chem. 1976;19(3):423-5. DOI · PubMed
2. Müller FO, Terblanchè J, Schall R, et al. Pharmacokinetics of triptorelin after intravenous bolus administration in healthy males and in males with renal or hepatic insufficiency. Br J Clin Pharmacol. 1997;44(4):335-41. DOI · PubMed
3. Millar RP, Lu ZL, Pawson AJ, et al. Gonadotropin-releasing hormone receptors. Endocr Rev. 2004;25(2):235-75. DOI · PubMed
4. Helm VJ, Müller BW. Stability of gonadorelin and triptorelin in aqueous solution. Pharm Res. 1990;7(12):1253-6. DOI · PubMed
5. Hoitink MA, Beijnen JH, Boschma MU, et al. Degradation kinetics of three gonadorelin analogues: developing a method for calculating epimerization parameters. Pharm Res. 1998;15(9):1449-55. DOI · PubMed
6. Simat TJ, Steinhart H. Oxidation of Free Tryptophan and Tryptophan Residues in Peptides and Proteins. J Agric Food Chem. 1998;46(2):490-498. DOI · PubMed