A Robust AQbD-Based RP-HPLC Method for Simultaneous Quantification of Acyclovir and Doxepin Hydrochloride in Bulk and Topical Cream Formulation

Abstract

The present study describes the development and validation of a simple, rapid, and reliable RP-HPLC method for the simultaneous estimation of acyclovir and doxepin hydrochloride in bulk and topical cream formulation using an Analytical Quality by Design (AQbD) approach. The combination of acyclovir, an antiviral agent, and doxepin, an antipruritic and anti-inflammatory drug, offers a combined topical approach for the management of herpes-associated skin conditions. The method development was guided by a predefined Analytical Target Profile (ATP), followed by systematic risk assessment to identify critical quality attributes (CQAs) and critical method parameters (CMPs). The central composite design (CCD) was employed to optimize key chromatographic variables, including mobile phase composition and flow rate. The chromatographic separation was achieved on a C18 column using an isocratic mobile phase of acetonitrile and water (40:60 v/v) at a flow rate of 1.0 mL/min, with UV detection at 240 nm. The retention times were found to be 2.58 min for acyclovir and 4.64 min for doxepin hydrochloride, indicating efficient separation with good peak symmetry. The method was validated according to ICH Q2(R2) guidelines and demonstrated excellent linearity over the concentration ranges of 2–10 µg/mL (acyclovir) and 12–28 µg/mL (doxepin hydrochloride), with correlation coefficients greater than 0.997. The method exhibited high precision (%RSD < 2%), accuracy (recovery within 99–101%), and good sensitivity with low LOD and LOQ values. The robustness studies confirmed the reliability of the method under small, deliberate variations in chromatographic conditions. The developed cream formulation showed uniform drug content and satisfactory in vitro drug release, with a sustained release profile reaching approximately 96% over 24 h. In conclusion, the proposed AQbD-based RP-HPLC method is suitable for routine quantitative analysis and quality control of acyclovir and doxepin hydrochloride in combined topical formulations.