Formulation and simultaneous estimation of sustained release bilayer tablet containing polythiazide and reserpine

Hypertension management often requires combination therapy for optimal control. This study developed a bilayer tablet combining immediate-release Polythiazide (2 mg) with sustained-release Reserpine (0.5 mg) to enhance therapeutic efficacy and patient compliance. Six formulations each were prepared using wet granulation. PEG 6000 and Tween 80 facilitated immediate release, while HPMC K100LV/K100M controlled sustained release. Preformulation studies evaluated flow properties and compressibility. Tablets were assessed for physicochemical parameters. FTIR confirmed drug-excipient compatibility. A gradient RP-HPLC method using a C18 column, ammonium chloride buffer (pH 4.5), and acetonitrile was developed and validated per ICH guidelines. Granules exhibited good flow (angle of repose 25–30°, Carr's index 12–18%) and acceptable moisture (1.2–2.8%). F3 achieved optimal immediate release; F5 sustained Reserpine release for 12 hours. Tablets showed uniform weight, hardness, and friability <0.8%. RP-HPLC provided baseline separation (retention times 5.2 and 8.6 min; resolution 6.91) with excellent linearity (r² > 0.999), accuracy (99.6–99.9% recovery), and precision (RSD <1%). The optimized bilayer tablet successfully delivers immediate Polythiazide release and sustained Reserpine delivery over 12 hours. The validated RP-HPLC method ensures reliable quality control for this antihypertensive combination therapy.