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((R)), 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 H-1 chemical shift assignments were mostly performed in organic solvents, and no assignments for heteronuclear C-13, N-15, and aromatic H-1 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 (CO)-C-13 and (NH)-N-15 of F1, (NH)-N-15 of C2, and N-15 zeta H zeta of K5 that were not observed because of water exchange or conformational exchange. The solution NMR spectra were then directly compared with 1D H-1/C-13/N-15 solid-state NMR (SSNMR) spectra showing the potential applicability of C-13/N-15 SSNMR for octreotide drug product characterization.