The aim of this work was to develop Eudragit (R) S100 (ES100) based pH-responsive microspheres of zaleplon (ZAL) as a potential delivery system for treatment of an insomnia characterised by premature awaking and inability to fall asleep again. Microspheres were prepared by spray-drying of ZAL/E-S100 dispersion from ethanolic (MS1) or 0.96% aqueous NH4HCO3 solution (MS2). In second case ES100 was first transformed into soluble, thermolabile ammonium salt and then regenerated by an additional thermal treatment at 85 degrees C for 3.5 h after spray-drying, as confirmed by FIR. Preparation from ethanolic medium (MS1) resulted in higher preparation yield (75.14% vs. 56.55%; p < 0.05), while in both cases high encapsulation efficiency (91.1 and 955%) of drug into microspheres of 1.71-1.76 mu m was achieved. However, MS1 were characterised by significant drug leaking in the simulated gastric medium (81.7 +/- 2.2% of drug dose in 2 h), while MS2 formulation released about 24.7 +/- 0.5% of ZAL (p < 0.01). DSC and XRPD analysis showed that this behaviour could be attributed to a thermally induced ZAL amorphization into ES100 matrix, but also to a sponge-like structure of MS1, while spherical MS2 offered better protection of encapsulated drug. Therefore MS2 were further optimized through addition of Eudragit (R) NE30D, glyceryl monostearate (GMS) and randomly methylated beta-cyclodextrin (RAMEB). The optimal formulation contained ES100, 10% of GMS and 50% of the drug dose in form of inclusion complex with RAMEB, resulting in an acceptable drug release level at pH 1.2 of about 10% of the ZAL dose after 2 h and a zero-order release (r(2) > 0.98, k = 155.0 mu g/min) at pH 6.8. (C) 2015 Elsevier B.V. All rights reserved.