Validated LC–MS/MS Methods for Quantification of AC02 and Porcine ACTH1–39 in Human Plasma: Application to a Phase I Study

Background/Objectives: AC02 is a novel 39-amino-acid adrenocorticotropic hormone (ACTH) analogue designed for the treatment of infantile spasms. To support its clinical study, in which porcine ACTH1–39 served as the positive-control drug, reliable methods for the determination of AC02 and porcine ACTH1–39 in human plasma were required. Reported analytical methods for ACTH analogues are mainly immunoassays, which are easily affected by cross-reaction and the hook effect, necessitating a more selective analytical approach. Methods: Two LC-MS/MS methods were developed for the determination of ACTH analogues AC02 and porcine ACTH1–39 in human plasma. Human ACTH1–39 was included as a selectivity marker to confirm that endogenous ACTH does not interfere with the quantification of AC02. Based on the distinct concentration requirements and matrix challenges, two sample-preparation procedures were established: micro-solid-phase extraction coupled with protein precipitation for porcine ACTH1–39 (LLOQ 0.100 ng/mL), and acid-mediated protein precipitation for AC02 (LLOQ 0.500 ng/mL). The [M+6H]6+ ions were selected as precursor ions, and the corresponding 5+ fragment ions, formed by loss of the C-terminal phenylalanine, were used for MRM detection. Results: Despite a mass difference of only 0.98 Da between AC02 and human ACTH1–39, which are indistinguishable by mass spectrometry, baseline chromatographic separation was achieved. Both methods were fully validated in accordance with current bioanalytical guidelines. Conclusions: The validated methods were successfully applied to the phase I clinical study of AC02 and porcine ACTH1–39, enabling reliable quantification of the drug candidate and its active comparator, porcine ACTH1–39.