Novel solid forms of insomnia drug suvorexant with improved solubility and dissolution: accessing salts from a salt solvate route

Abstract

Suvorexant (SRX) is a dual orexin receptor antagonist used for the treatment of insomnia. It belongs to the Biopharmaceutics Classification System (BCS) class II with high permeability and poor solubility in water (0.024 mg mL⁻¹). It is a very challenging molecule from the crystallization perspective because most of the crystallization experiments predominantly yield either form I or a semi-solid form. Increasing S-enantiomer impurity is another problem that was observed during the recrystallization. Further to this, SRX is known to exist in only two crystalline polymorphs (forms I and II). As far as we know, there is no report on the crystal structure analysis or the solid-state modification approaches pertaining to the suvorexant molecule. All these critical aspects of suvorexant have drawn our attention towards the development of novel solid forms by using crystal engineering principles. Here, we report two novel salts and one salt solvate of suvorexant with p-toluenesulfonic acid (PTSA) and benzenesulfonic acid (BSA) with the objective of enhancing the solubility and dissolution. All three solid forms were characterized by powder XRD, DSC, FT-IR, and single crystal XRD. Notably, the BSA salt crystallizes as an anhydrous form and the PTSA salt of suvorexant crystallizes as a methanol solvate. Furthermore, the methanol solvate of the PTSA salt shows an efficient single-crystal-to-single-crystal phase transformation to the anhydrous form upon heating, yielding X-ray diffraction quality single crystals. In view of the pharmaceutical importance of these salts, we have performed pH solubility and intrinsic dissolution rate (IDR) studies in comparison to the marketed form. The novel salts of suvorexant exhibit enhanced solubility in comparison to the marketed form II. Therefore, the novel salts are expected to show better formulation development.