FTIR Spectroscopic Investigation of Drug–Excipient Interactions in a Lipid-Based Formulation of a Calcium Channel Blocker

Efonidipine hydrochloride ethanolate (EF) is a dihydropyridine calcium channel blocker that acts on both L- and T-type calcium channels. Its aqueous solubility is very low, which restricts dissolution and oral absorption and makes it a suitable candidate for a lipid-based drug delivery system. Before such a system is optimised, the chemical compatibility of the drug with the selected oil, surfactant and co-surfactant must be confirmed. In the present work, Fourier transform infrared (FTIR) spectroscopy was used to study the compatibility of Efonidipine with Capmul MCM (oil), Tween 80 (surfactant) and Transcutol HP (co-surfactant). Physical mixtures were prepared by trituration and stored in sealed glass vials at 25–28 °C and about 55% relative humidity for one month. Spectra of the pure drug, each excipient and the physical mixture were recorded between 4000 and 400 cm⁻¹ at a resolution of 4 cm⁻¹, and each excipient was examined separately against the drug. Pure EF showed characteristic bands at 3410.87 cm⁻¹ (dihydropyridine N–H), 1704.01 cm⁻¹ (conjugated ester C=O), 1525.49 and 1348.35 cm⁻¹ (asymmetric and symmetric NO2), 1221.23 cm⁻¹ (P=O) and 1058.40 cm⁻¹ (P–O–C). Each excipient showed bands consistent with its chemical composition. The spectrum of the physical mixture showed no new absorption band, and the resolvable drug bands did not shift beyond the instrumental resolution; the symmetric nitro band appeared at 1349.09 cm⁻¹. Weaker drug bands were partly masked by intense excipient absorptions, which was attributed to dilution and spectral overlap. Within the limits of FTIR spectroscopy, EF was found to be compatible with Capmul MCM, Tween 80 and Transcutol HP, and these excipients can be carried forward to the formulation and optimisation of an EF-loaded lipid-based formulation.