Lorazepam Complexation with Hydroxypropyl ß-cyclodextrin (HPßCD) and Sulfobutylether ß-cyclodextrin (SBEßCD): Phase Solubility Parameters Evaluation

INTRODUCTION

Lorazepam (LZP) (figure 1) is a BCS class II drug with low water solubility (0.08mg/ml) and commercially available as a solid and liquid dosage form. The commercially available parenteral formulations are administered as solution in polyols in order to solve the solubility concern, but they can create serious patient compliance issues. We explored the feasibility of a parenteral alternative using lorazepam solubilization by cyclodextrin complexation as a viable and practical solution. Hydroxypropyl beta-cyclodextrin (HPßCD) and sulfobutylether ß-cyclodextrin (SBEßCD) possess a unique cyclic structure enabling the formation of host-guest composite by accommodating drug molecules inside their hydrophobic cavity resulting in inclusion complexes. The differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were used to conform the LZP inclusion complexes. Affinity constant (K_1:1) and complexation efficiency constant (CE) were calculated from phase solubility graphs as per Higuchi and Connor method. LZP complexes stability at 25 ℃ and 40 ℃ was evaluated. The lyophilized LZP-HPßCD and LZP-SBEßCD products were also analyzed. 

OBJECTIVES

• To evaluate the solubility increase, affinity constant (K_1:1) and complexation efficiency constant (CE) of lorazepam in hydroxypropyl beta-cyclodextrin (HPßCD, KLEPTOSE® HPB) and sulfobutylether ß-cyclodextrin (SBEßCD). 
• To investigate the stability of lorazepam in HPßCD and SBEßCD solutions. 
• To evaluate the reconstitution of lyophilized LZP-HPßCD and LZP-SBEßCD products as an alternative to liquid dosage form.

MATERIAL AND METHODS

Phase solubility evaluation of lorazepam (LZP, a present from TEVA) was performed by adding excess amounts of drug to deionized water (control) and HPßCD (KLEPTOSE® HPB hydroxypropyl beta-cyclodextrin, Roquette) or SBEßCD (Captisol®, CyDex) solutions (in deionized water) at concentrations of 10, 20, 30, 40, 50, 100 and 200 mM, respectively. All the samples were mixed for 7 days in a shaker at 25 ℃. At equilibrium, samples were filtered through Millipore (0.45 μm) disk filter. Aliquots of each sample were put on stability at 25 ℃ and 40 ℃ for 0, 30 and 60 days (as in figure 2). A separate set of samples were lyophilized. Drug concentration in each sample was analyzed by high performance liquid chromatography (HPLC) according to the USP official analytical methodology for LZP. The affinity constants and complexation efficiencies for LZP in HPßCD and SBEßCD respectively were calculated using the solubility increase diagram profile.

Figure 1. Lorazepam: Chloro-5-(2-chlorophenyl)-1,3-dihydro-3-hydroxy-2H-1,4-benzodiazepine-2-one 

Figure 2. Phase solubility and stability experiments design 

RESULTS AND DISCUSSIONS 

The phase solubility diagram (figure 3) shows a linear relationship between LZP solubility increase (mM) with cyclodextrin concentration increase (mM) in both cyclodextrin (HPßCD and SBEßCD) indicating a type AL complexation profile.

Figure 3. Phase-solubility profiles of LZP in HPßCD and SBEßCD solutions

Stability Constants (K_1:1) and Complexation Efficiency (CE) are calculated as follows: 

where m is the slope of the curve of the drug solubility versus cyclodextrin concentration determined by linear regression and S0 is the drug solubility in DI water as determined after 7 days of mixing. 

K_1:1 and CE were calculated from the experimental solubility graphs (table 1). 

Table 1. API-cyclodextrin affinity constants and complexation efficiencies 

K_1:1 value for lorazepam in HPßCD is higher than in SBEßCD, 625.4 M^-1 and 551.5 M^-1 respectively. 

The complexation efficiency in HPßCD was 9.54% and 8.41% in SBEßCD. A significant solubility increase even at low HPßCD or SBEßCD concentrations is observed (table 2).

Table 2. Solubility increase as a function of HPßCD or SBEßCD molarity 

Lorazepam solubilization in HPßCD outperform SBEßCD solubilization capability. 

Confirmation of LZP:CD complexes is shown in figure 4-A and 4-C (DSC thermograms) as the drug melting peak at 184.30 ℃ disappeared in both lyophilized lorazepam HPßCD and SBEßCD samples (indicating the drug was accommodated into the cavity of HPßCD and SBEßCD to form the inclusion complex). 

The same conclusion can be drawn from figure 4-B and 4-D (TGA), where the onset points of degradation of lyophilized samples were similar to that of cyclodextrin itself (339.81 and 273.22 ℃ for HPßCD and SBEßCD, respectively) and totally different from that of drug (188.79 ℃).

Figure 4. A: DSC for HPßCD; B: TGA for HPßCD; C: DSC for SBEßCD; D: TGA for SBEßCD. 

The drug content ratio at day 30 to day 1 for HPßCD solutions stored at 25 ℃ and 40 ℃ for 30 days was 93.5% and 89%, respectively (figure 5), showing a satisfactory stability of LZP:HPßCD complex. To be noted: 200 mM SBEßCD solution precipitated out during stability study. 

Figure 5. Relative drug contents at 25 ℃ and 40 ℃ after 30 and 60 days 

Figure 6-C shows that both the lyophilized HPßCD and SBEßCD powders dissolve completely, very fast, within 1 min. The HPßCD has more uniform porous structure (figure 6-A) than SBEßCD (figure 6-B).

Figure 6. Microscopic observation of lyophilized A) LZP:HPßCD and B) LZP:SBEßCD complexes. C) Reconstitution profiles of lyophilized LZP:HPßCD and LZP:SBEßCD complexes.

CONCLUSION

1. The HPßCD outperformed SBEßCD in Lorazepam solubilization and stability capabilities.
2. The significant enhanced solubility and satisfactory stability properties of lorazepam solubilized by HPßCD complexation may lead to future commercial development of intravenous drug delivery systems as a liquid or lyophilized form. This is a viable manufacturing formulation which addresses patient compliance by replacing organic solvents side effects.

REFERENCES  

1. Carmen Popescu, Hassan Almoazen, Wenli Lu, etc. Determination of the thermodynamic solubility and the affinity (binding) constants of Carbamazepine, Danazol and Albendazole in hydroxypropyl beta cyclodextrin (KLEPTOSE®HPB) Solutions. AAPS Annual Meeting 2011 (Washington, DC).

2. The United States Pharmacopeia 
https://www.usp.org/

3. Loftsson T, Másson M, Brewster ME. Self-association of cyclodextrins and cyclodextrin complexes. J Pharm. Sci. 2004 May; 93(5):1091-10999.
https://doi.org/10.1002/jps.20047

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