Validated Stability-Indicating RP-HPLC Method for Nirmatrelvir in Self-Emulsifying Drug Delivery Systems: Formulation Characterization and Permeability Enhancement

This study presents a novel analytical and formulation strategy to enhance the oral delivery and quality assessment of Nirmatrelvir, a poorly water-soluble antiviral agent. A self-emulsifying drug delivery system (SEDDS) was developed using Labrafac MC 60, ethanol, and Transcutol HP (55:25:20), resulting in a nanoemulsion with a droplet size of 145.23 ± 3.23 nm, low polydispersity index (0.189 ± 0.023), and high transmittance (98.97 ± 0.25%). The formulation exhibited rapid emulsification (< 90 s) and significantly improved permeability, achieving a fivefold increase (Papp: 4.20 × 10⁻⁶ cm/s) across Caco-2 cell monolayers compared to the tablet formulation. A stability-indicating reverse-phase HPLC method was developed using a mobile phase of 5 mM potassium dihydrogen phosphate buffer (pH 4.0) and acetonitrile (40:60, v/v), and validated per ICH Q2(R1) guidelines. The method showed excellent linearity (R² = 0.9999), accuracy (98.6%–100.2%), precision (%RSD < 0.3%), and robustness. An optimized sample preparation protocol ensured efficient extraction of Nirmatrelvir from the SEDDS matrix with minimal interference. Forced degradation studies under ICH Q1A(R2) demonstrated that Nirmatrelvir remained stable under oxidative (98.44%), thermal (98.45%), and photolytic (98.50%) conditions. Maximum degradation was observed under alkaline stress (20.56% at 0.5 N NaOH), followed by acidic stress (13.53% at 5 N HCl). The major alkaline degradant (Rt 2.7 min) was characterized by LC-TQ/MS (m/z 518.2 [M+H]⁺). The method's sustainability was supported by an AGREE score of 0.64 and a Whiteness score of 85.4, offering a validated platform for routine analysis of Nirmatrelvir in lipid-based formulations.