Solid-state characterization of 17β-estradiol co-crystals presenting improved dissolution and bioavailability

17β-estradiol (E2) is a steroid hormone with low oral bioavailability due to its low aqueous solubility (0.2–5 μg mL−1). To improve its solubility and thus bioavailability, two co-crystals of E2 with isonicotinamide (INA) (1) and piperazine (PIP) (2) were successfully designed and synthesized and their structures were successfully determined. In vivo PK studies revealed that 1 possesses improved bioavailability in an animal model.