Multinuclear 1H/13C/15N chemical shift assignment of therapeutic octreotide acetate performed at natural abundance

Octreotide acetate, the active pharmaceutical ingredient in the long-acting release (LAR) drug product Sandostatin®, is a cyclic octapeptide that mimics the naturally occurring somatostatin peptide hormone. Modern NMR can be a robust analytical method to identify and quantify octreotide molecules. Previous ¹H chemical shift assignments were mostly performed in organic solvents, and no assignments for heteronuclear ¹³C, ¹⁵N, and aromatic ¹H nuclei are available. Here, using state-of-the-art 1D and 2D homo- and heteronuclear NMR experiments, octreotide was fully assigned, including water exchangeable amide protons, in aqueous buffer except for ¹³CO and ¹⁵NH of F1, ¹⁵NH of C2, and ¹⁵NζHζ of K5 that were not observed because of water exchange or conformational exchange. The solution NMR spectra were then directly compared with 1D ¹H/¹³C/¹⁵N solid-state NMR (SSNMR) spectra showing the potential applicability of ¹³C/¹⁵N SSNMR for octreotide drug product characterization.