Sildenafil/cyclodextrin complexation: Stability constants, thermodynamics, and guest–host interactions probed by 1H NMR and molecular modeling studies

• Published in: Journal of pharmaceutical and biomedical analysis 2006-06, Vol.41 (3), p.857-865
• Main Authors: Al Omari, Mahmoud M., Zughul, Mohammad B., Davies, J. Eric D., Badwan, Adnan A.
• Format: Article
• Language: English
• Subjects: 1H NMR; Analysis; Analytical, structural and metabolic biochemistry; Biological and medical sciences; Calorimetry, Differential Scanning; Cyclodextrin; Cyclodextrins - chemistry; DSC; Fundamental and applied biological sciences. Psychology; General pharmacology; Isomeric inclusion complexes; Magnetic Resonance Spectroscopy - methods; Medical sciences; Models, Molecular; Molecular modeling; Pharmacology. Drug treatments; Phase solubility diagrams; Piperazines - chemistry; Protons; Purines; Sildenafil; Sildenafil Citrate; Spectrophotometry, Ultraviolet; Sulfones; Thermodynamics; X-Ray Diffraction
• ISSN: 0731-7085; 1873-264X
• DOI: 10.1016/j.jpba.2006.01.055

Guest–host interactions of sildenafil (Sild) with cyclodextrins (CyDs) have been investigated using several techniques including phase solubility diagrams (PSD), differential scanning calorimetry (DSC), X-ray powder diffractometry (XRPD), proton nuclear magnetic resonance ( 1H NMR) and molecular mechanical modeling (MM +). Estimates of the complex formation constant ( K 11) show that the tendency of Sild to complex with CyDs follows the order: β-CyD > HP-β-CyD > γ-CyD, α-CyD, where K 11 values at pH 8.7 and 30 °C were 150, 68 and 46, 43 M −1, respectively. Ionization of Sild reduces its tendency to complex with β-CyD, where protonated (at pH 3.6) and anionic Sild (at pH 12.1) species have K 11 values of 17 and 42 M −1, respectively, compared with 150 M −1 for neutral Sild (at pH 8.7). The hydrophobic character of Sild was found to provide 39% of the driving force for complex stability, while other factors including specific interactions contribute −7.9 kJ/mol. Complex formation of Sild with β-CyD (Δ G° = −22.9 kJ/mol) is largely driven by enthalpy (Δ H° = −19.8 kJ/mol) and slight entropy (Δ S° = 10.3 J/mol K) changes. 1H NMR and MM + studies indicate formation of two isomeric 1:1 complexes: one involving complete inclusion of the phenyl-moiety into the β-CyD cavity while the other pertaining to partial inclusion of the pyrimidinone moiety. The dominant driving force for complexation is evidently van der Waals with very little electrostatic contribution. DSC, XRPD and 1H NMR studies proved the formation of inclusion complex in solution and the solid state.