Abstract
Myostatin, also known as growth differentiation factor 8 (GDF-8), is a protein acting as a negative regulator in skeletal muscle growth. Inhibition of myostatin by therapeutic agents provides opportunities for current unmet medical needs. In order to better understand drug engagement to aid the drug development, we have developed a hybrid LC-MS/MS method which can differentially measure myostatin and another protein from the same GDF family, GDF-11. Although the two proteins share high homology, the LC-MS/MS assay provided the specificity based on monitoring of unique surrogate peptide generated from enzymatic digestion. An automated sample preparation platform, Agilent AssayMap Bravo, was used for automated immunocapture. Capture antibody that is non-competing with our investigational drug and has similar binding affinity to both myostatin and GDF-11 was used. Therefore, total myostatin and GDF-11 including both free form and drug-bound form were captured and measured. The enriched sample was digested after reduction and alkylation. Two surrogate peptides (IPAMVVDR for myostatin and IPGMVVDR for GDF-11) were monitored and the lower limit of quantitation (LLOQ) was established at 1.0 ng/mL for myostatin and 0.1 ng/mL for GDF-11. The accuracy was demonstrated with recovery for IPAMVVDR between 99.2% and 103.1% and for IPGMVVDR between 90.3% and 114.5%. The developed hybrid assay exhibits sufficient sensitivity, accuracy and specificity to differentiate between the highly structurally similar myostatin and GDF-11. This analytical approach was successfully applied to a rat toxicology study, and was demonstrated to be a powerful tool for biomarker measurement in the present of a therapeutic agent.