Peptides represent a promising modality for the design of novel therapeutics that can potentially modulate traditionally non-druggable targets. Cell-penetrating peptides (CPPs) and antimicrobial peptides (AMPs) are two large families that are being explored extensively as drug delivery vehicles, imaging reagents, or therapeutic treatments for various diseases. Many CPPs and AMPs are cationic among which a significant portion is extremely basic and hydrophilic (e.g., nona-arginine). Despite their attractive therapeutic potential, it remains challenging to directly analyze and quantify these super cationic peptides from biological matrices due to their poor chromatographic behavior and MS response. Herein, we describe a generic method that combines solid phase extraction and LC-MS/MS for analysis of these peptides. As demonstrated, using a dozen strongly basic peptides, low μM concentration of perfluoropentanoic acid (PFPeA) in the mobile phase enabled excellent compound chromatographic retention, thus avoiding co-elution with solvent front ion suppressants. PFPeA also had a charge reduction effect that allowed the selection of parent/ion fragment pairs in the higher m/z region to further reduce potential low molecular weight interferences. When the method was coupled to the optimized sample extraction process, we routinely achieved low digit ng/ml sensitivity for peptides in plasma/tissue. The method allowed an efficient evaluation of plasma stability of CPPs/AMPs without fluorescence derivatization or other tagging methods. Importantly, using the widely studied HIV-TAT CPP as an example, the method enabled us to directly assess its pharmacokinetics and tissue distribution in preclinical animal models.
Keywords: Antimicrobial peptides; Cell-penetrating peptides; LC-MS; Peptide quantitation; Perfluoropentanoic acid.