Unveiling the in vivo fate of albumin-bound paclitaxel nanoparticles by UPLC-MS/MS based on free, protein bound, and total drug

Ultrafiltration with UPLC-MS/MS assay simultaneously quantifies free, protein-bound, total paclitaxel in biosamples. • PTX-Nab in vivo shows sustained release, lower systemic exposure, longer half-life, lung/spleen accumulation. • The primary excretion pathway of paclitaxel shifts from renal to hepatobiliary following albumin nanoparticle formulation. Paclitaxel (PTX) has poor water solubility. Traditional formulations requiring solubilizers can induce allergic reactions and disrupt pharmacokinetics. Albumin-bound PTX nanoparticles (PTX-Nab) provide a solvent-free nanodelivery system that enhances solubility and may enable receptor-mediated tumor targeting. Comprehensive studies comparing its systemic pharmacokinetics, particularly the dynamics of free versus total drug concentrations and excretion routes, are still lacking. This study established a sensitive ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method coupled with ultrafiltration to simultaneously quantify free, protein-bound, and total PTX in rats following intravenous administration of PTX-Nab and conventional PTX-injection. PTX-Nab profoundly altered the pharmacokinetic profile. Despite a double dose (10 mg/kg), it exhibited significantly lower dose-normalized initial concentration ( C 0 /Dose) and systemic exposure (AUC/Dose) along with a longer elimination half-life (t 1/2 ) than the injection group (5 mg/kg), indicating sustained drug release and prolonged circulation. Tissue distribution showed preferential accumulation of PTX-Nab in lung and spleen.  Excretion studies showed a change in the primary excretion route. Unlike the renal-excreted conventional injection, PTX-Nab enhanced fecal excretion (56.14% versus 16.50%). These findings elucidate how the albumin carrier reshapes the in vivo fate of PTX through sustained release, tissue targeting, and clearance redirection, providing crucial pharmacokinetic evidence for its clinical application and future nanocarrier development.