(M1330-06-39) Brexanolone Inclusion Complexes with Hydroxypropyl- and Sulfobutylether-β-Cyclodextrins for Intravenous Injections: Formulation and Stability Studies

Purpose: Brexanolone, a synthetic analog of the endogenous neurosteroid allopregnanolone, is the first FDA-approved drug specifically for the treatment of postpartum depression (PPD) [1]. It requires intravenous administration due to its low oral bioavailability. However, due to brexanolone's poor aqueous solubility, it presents significant formulation challenges for developing a stable and safe intravenous (IV) dosage form. Cyclodextrins (CDs) are cyclic oligosaccharides frequently used to enhance the solubility and stability of poorly water-soluble drugs by forming inclusion complexes with hydrophobic drugs [2]. Hydroxypropyl-β-cyclodextrin (HPβCD) and sulfobutylether-β-cyclodextrin (SBEβCD) are utilized in FDA-approved IV injection products. However, the FDA-approved IV formulation of brexanolone specifically only employs SBEβCD. Exploring the use of HPβCD as an alternative to SBEβCD in brexanolone formulations offers numerous potential advantages, including improved solubility, potentially reduced toxicity, enhanced stability, and greater versatility in formulation development. This study aimed to compare the inclusion complexes of brexanolone with HPβCD and SBEβCD in terms of liquid and solid-state characterizations and compare the formulation and stability of brexanolone IV injections with HPβCD and SBEβCD, providing valuable insights into the feasibility and advantages of using HPβCD in brexanolone formulations, potentially paving the way for safer and more effective PPD therapies.

Methods: This study involved the preparation of HPβCD and SBEβCD inclusion complexes of brexanolone specifically designed for intravenous administration. The phase solubility study was conducted to compare the inclusion complexes with and without citrate buffer pH 6.0 for their saturation solubility, stability rate constant, complexation efficiency, and change in Gibbs free energy. Solid-state characterizations of the inclusion complexes were determined using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC), comparing brexanolone, HPβCD, SBEβCD, physical mixture, and freeze-dried complex of brexanolone and βCDs. A 5 mg/mL brexanolone IV injection with HPβCD was aseptically prepared, containing 25% HPβCD. This preparation was compared to an FDA-approved brexanolone IV injection composition that uses SBEβCD. Both injection formulations were then compared for their visual appearance, pH, osmolality, and percentage of drug recovery. The HPLC method for brexanolone IV injection has been developed and validated to determine the limit of detection (LOD), the limit of quantitation (LOQ), precision, linearity, system suitability, accuracy, repeatability, and benchtop stability. The stability study was conducted by storing the sterile injection glass vials at three distinct storage temperatures: 40°C, ambient room temperature (~23°C), and 4°C. Subsequently, the samples were analyzed for their clarity, pH, osmolality, and percentage of drug recovery at intervals over a period of three months.

Results: The phase solubility study (Figure 1A and Table 1) demonstrated that brexanolone formed inclusion complexes with both HPβCD and SBEβCD in the presence and absence of citrate buffer (pH 6.0). All systems exhibited A_L- or A_N-type profiles, indicating 1:1 complexation. Among them, SBEβCD showed the highest stability constant (Kₛ = 156.73 M^-1 with citrate) and complexation efficiency (CE = 0.962), along with the most favorable Gibbs free energy (ΔG = –12.53), indicating spontaneous and efficient complexation. Although SBEβCD provided better complexation properties, HPβCD still showed acceptable solubilization performance. Thermal analysis using DSC (Figure 1B) showed that the brexanolone melting peak disappeared in freeze-dried complexes, confirming inclusion and reduced crystallinity. FTIR spectra (Figure 1C) also showed changes in hydroxyl and carbonyl peaks, indicating molecular interactions between brexanolone and the cyclodextrin cavity. A sensitive HPLC-UV method (Figure 2A and Table 2) was developed and validated for brexanolone quantification. The method demonstrated excellent linearity (R² = 0.9999), low LOD (0.34 mg/mL), and LOQ (1.04 mg/mL), with high precision ( < 1% RSD) and recovery ( >99%). Both HPβCD and SBEβCD formulations (Figure 2B and Table 3) were clear and physically stable for 3 months at accelerated conditions. However, HPβCD maintained a more physiologically acceptable pH (6.34 ± 0.03) and osmolality (203.5 ± 3.54 mOsm/kg), whereas SBEβCD had a lower pH (5.69 ± 0.01) and significantly higher osmolality (928 ± 2.83 mOsm/kg). Stability studies (Figures 3A and 3B) showed that both formulations maintained the pH, osmolality, and drug content above 95% over three months at 4°C, room temperature, and 40°C.

Conclusion: Although SBEβCD demonstrated superior complexation with brexanolone, HPβCD-based formulations offered more favorable pH and osmolality for intravenous administration. Both systems showed excellent chemical stability over time. Given HPβCD’s acceptable solubilization performance, lower osmolality, and established use in injectable products, it represents a promising and clinically viable alternative for IV brexanolone formulation. These findings support further development of HPβCD-based systems for safer and more practical PPD therapy.

References: [1] J. Faden and L. Citrome, “Intravenous brexanolone for postpartum depression: what it is, how well does it work, and will it be used?,” Therapeutic Advances in Psychopharmacology, vol. 10, p. 2045125320968658, Jan. 2020, doi: 10.1177/2045125320968658.
[2] T. Loftsson, “Cyclodextrins in Parenteral Formulations,” Journal of Pharmaceutical Sciences, vol. 110, no. 2, pp. 654–664, Feb. 2021, doi: 10.1016/j.xphs.2020.10.026.
Acknowledgements: This research was supported by funding from Ashland Specialty Ingredients.


Figure 1. Phase solubility study and solid-state characterizations of brexanolone with HP-/ SBE-β-Cyclodextrins inclusion complexes


Figure 2. Development and validation of HPLC-UV method development and specifications of brexanolone IV injections


Figure 3. Stability data of brexanolone IV injections