Post-extraction disulfide bond cleavage for MS/MS quantification of collision-induced dissociation-resistant cystine-cyclized peptides and its application to the ultra-sensitive UPLC-MS/MS bioanalysis of octreotide in plasma.

Disulfide-cyclized peptides, including the somatostatin receptor agonist octreotide, are usually resistant against collision-induced dissociation (CID) complicating their bioanalysis by MS/MS. Post-extraction reductive cleavage of the disulfide bridge of such peptides utilizing tris(2-carboxyethyl)phosphine generates the corresponding linear dithiol-peptides. This procedure enables monitoring of larger, specific fragments in CID which usually avoid matrix interference present for single amino acid iminium ions abundant in CID of the intact peptides. To assist formulation development for oral administration of the cystine-cyclized therapeutic peptide octreotide, we applied this methodology to the development of an ultra-sensitive UPLC-MS/MS assay for plasma octreotide and validated it according to FDA's and EMA's pertinent guidelines. Octreotide was extracted from plasma by fast and simple protein precipitation with acetonitrile and subsequently reduced to linear dithiol-octreotide for the monitoring of specific fragments in selected reaction monitoring. The calibrated concentration range of 10 (9.8 pM) to 20,000 pg mL-1 was linear with correlation coefficients > 0.99. Interday accuracy ranged between 100.0 and 108.9% with corresponding precision of <8.1%. The assay was successfully applied to the quantification of octreotide plasma concentrations after intravenous administration in beagle dogs. The presented procedure of disrupting the cyclic structure of cystine-cyclized peptides by reductive cleavage of the intramolecular disulfide bond enables the generation of more specific and intense fragments in CID can serve as a general methodology for the sensitive bioanalysis of cystine-bearing cyclic peptides.