Polymorphism, phase transition, and physicochemical property investigation of Ensifentrine

Abstract

Ensifentrine (ENSE) is a bifunctional dual phosphodiesterase 3/4 inhibitor that has both anti-inflammatory and bronchodilatory activities and is considered a new option for the therapy of chronic obstructive pulmonary disease (COPD) and various other respiratory inflammatory diseases for which it is currently undergoing clinical trials. This study presents a groundbreaking effort to evaluate the impact of ENSE polymorphs (forms I, II, and III) on ENSE solubility and chemical stability—both of which are critical factors for drug formulation. Different procedures for the induced crystallisation of ENSE were adopted, and the obtained polymorphic forms were investigated. All the newly obtained solid forms were characterized via single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM), nuclear magnetic resonance (¹H & ¹³C NMR), solid-state UV-visible spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC)). Interestingly, altering various crystallisation processes with varied solvent polarity in the solution affects the crystal structure and polymorphic forms. Form I (parent form) was converted into forms II and III using the reactive crystallization method with combinations of formic acid : H₂O (1 : 1) and formic acid : ethyl acetate (1 : 1), respectively. The reverse transformation was accomplished through gradual evaporation using different solvents. Similarly, variable-temperature PXRD (VT-PXRD) experiments revealed that form II converted into form I at 204 °C, whereas form III remained unaltered. In addition, the physicochemical properties of these three polymorphs were thoroughly discussed, and their solubility was tested in pH 1.2 and pH 7 media. A comprehensive examination of the residue that was recovered after solubility tests verified that form II and form III were converted into form I at both pH levels and that form I did not exhibit any phase change or dissociation at pH 7, whereas at pH 1.2, form I was converted into a new stable form and a thorough analysis verified that it was transformed into ENSE.Cl salt. To the best of our knowledge, this is the first study on the investigation of multiple forms of ENSE with varying physicochemical properties, and we hope that the current data will offer some valuable perspective prior to the development of ENSE as a medication.