Solubility-dependent complexation of active pharmaceutical ingredients with trimethyl-β-cyclodextrin under supercritical fluid condition

The effect of the solubility of active pharmaceutical ingredients (APIs) in supercritical carbon dioxide (SC-CO₂) on their complexation behavior with trimethyl-β-cyclodextrin (TM-β-CD) has been investigated. Flurbiprofen or naproxen, the solubility of which is lower than that of ibuprofen, was mixed with TM-β-CD and the complexation phenomena on SC-CO₂ processing was evaluated using powder X-ray diffraction, differential scanning calorimetry and IR measurement. Drug complexation depended both on SC-CO₂ treatment time and on drug solubility in CO₂. The inclusion complex formation of flurbiprofen with TM-β-CD proceeded slowly compared with the case of ibuprofen. The slower complexation behavior was also observed when naproxen was used as the guest molecule. These results indicate that dissolution of drug molecules in SC-CO₂ is a rate-determining step for the inclusion complex formation with TM-β-CD and that complexation proceeds after dissolving the both components in SC-CO₂.     

Improvement of Steroid Biotransformation with Hydroxypropyl-β-Cyclodextrin Induced Complexation

The inclusion complexes induced by cyclodextrins and its derivates have been shown previously to enhance the biotransformation of hydrophobic compounds. Using hydroxypropyl-β-cyclodextrin (HP-β-CD; 20% w/v), the water solubility of cortisone acetate increased from 0.039 to 7.382 g L⁻¹ at 32 °C. The solubilization effect of HP-β-CD was far superior to dimethylformamide (DMF) and ethanol. The dissolution rate also significantly increased in the presence of HP-β-CD. The enzymatic stability of Δ¹-dehydrogenase from Arthrobacter simplex TCCC 11037 was not influenced by the increasing concentrations of HP-β-CD contrary to the organic cosolvents which negatively influenced in the order DMF > ethanol. The activity inhibition effect caused by HP-β-CD was not so conspicuous as ethanol and DMF. Inactivation constants of ethanol, DMF, and HP-β-CD were 5.832, 4.541, and 1.216, respectively. The inactivation energy (E _a) was in the order of HP-β-CD (55.1 kJ mol⁻¹) > ethanol (39.9 kJ mol⁻¹) > DMF (37.1 kJ mol⁻¹).     

Chiral separation of newly synthesized arylpropionic acids by capillary electrophoresis using cyclodextrins or a glycopeptide antibiotic as chiral selectors

Summary The chiral separation of two newly synthesized arylpropionic acids of pharmaceutical interest, namely 2-[(5′-benzoil-2′-hydroxy)phenyl]-propionic acid (DF-1738y) and 2-[(4′-benzoiloxy-2′-hydroxy)phenyl]-propionic acid (DF-1770y), was performed by Capillary Zone Electrophoresis (CZE) using either cyclodextrins or antibiotics as chiral selectors in coated capillary. In order to optimize the separation, the effect on the migration time and resolution of type and concentration of the chiral selector, the buffer pH and the capillary temperature were studied. Several cyclodextrins, namely the charged 6^A-monomethylamino-β-cyclodextrin (MeNH-β-CD) and the neutral methyl-β-cyclodextrins (M-β-CD) and heptakis-2,3,6-tri-O-methyl-β-cyclodextrin (TM-β-CD), were tested for the enantiomeric separation of aryl propionic acids (APAs) compounds. Of these TM-β-CD provided the highest enantiomeric resolution at pH 5, however only DF-1738y optical isomers were baseline resolved. Using background electrolytes (BGEs) at higher pHs (pH=6–7) supported with the above listed CDs, an enantioresolution increase was recognized only for compound DF-1738y. In contrast DF-1770y exhibited the highest resolution at the lowest pH value studied (pH 4). The macrocyclic antibiotic vancomycin was therefore added to the BGE and tested as chiral selector using the partial filling counter current mode in order to obtain a sensitive analysis, high resolution and reduced antibiotic adsorption on the capillary wall. 5 mM vancomycin dissolved in the BGE at pH 5 and 25°C provided relatively high enantiomeric resolution (R _DF-1738y=3.4,R _DF-1770y=2.22) of both compounds.     

Inclusion complexation of diclofenac with natural and modified cyclodextrins explored through phase solubility, 1H-NMR and molecular modeling studies

Guest–host interactions were examined for neutral diclofenac (Diclo) and Diclofenac sodium (Diclo sodium) with each of the cyclodextrin (CD) derivatives: α-CD, β-CD, γ-CD and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), all in 0.05 M aqueous phosphate buffer solution adjusted to 0.2 M ionic strength with NaCl at 20 °C, and with β-CD at different pHs and temperatures. The pH solubility profiles were measured to obtain the acid–base ionization constants (pK _as) for Diclo in the presence and absence of β-CD. Phase solubility diagrams (PSDs) were also measured and analyzed through rigorous procedures to obtain estimates of the complex formation constants for Diclo/CD and Diclo sodium/CD complexation in aqueous solutions. The results indicate that both Diclo and Diclo sodium form soluble 1:1 complexes with α-, β-, and HP-β-CD. In contrast, Diclo forms soluble 1:1 Diclo/γ-CD complexes, while Diclo sodium forms 1:1 and 2:1 Diclo/γ-CD, but the 1:1 complex saturates at 5.8 mM γ-CD with a solubility product constant (pK _sp = 5.5). Therefore, though overall complex stabilities were found to follow the decreasing order: γ-CD > HP-β-CD > β-CD > α-CD, some complex precipitation problems may be faced with aqueous formulations of Diclo sodium with γ-CD, where the overall concentration of the latter exceeds 5.8 mM γ-CD. Both ¹H-NMR spectroscopic and molecular mechanical modeling (MM⁺) studies of Diclo/β-CD indicate the possible formation of soluble isomeric 1:1 complexes in water.     

Enhancement of aqueous solubility of tricyclic acyclovir derivatives by their complexation with hydroxypropyl-β-cyclodextrin

Solubilities of tricyclic acyclovir derivatives in buffered aqueous solutions of hydroxypropyl-β-cyclodextrin (HP-β-CD) at pH 5.5 and 7.0 were determined at 25 and 37 °C. Complexation of these compounds with HP-β-CD resulted in a noticeable increase of their solubility; nevertheless it was limited to tricyclic derivatives of acyclovir carrying an aryl substituent. Combination of ¹H NMR and DSC techniques demonstrated the existence of inclusion complexes between acyclovir derivatives and HP-β-CD. The stability constants, estimated using the Higuchi–Connors method, were found in the range of 10–100 M⁻¹. Additionally, the pK _a values at 25 °C and molar extinction coefficients in aqueous buffered solutions were also determined for all studied compounds.     

Selective Pseudo-Rotaxane Type Complex Formation of Zinc(II) (t-butylatedtetraphenyl) porphyrin-Viologen Linked Compounds with tri-O-methyl-β-Cyclodextrin

Inclusion complexation behavior of 2,3,6−tri-O-methyl-β-cyclodextrin (TM-β-CD) with zinc(II) 5,10,15-tri-(4-t-butyl-phenyl)-20-(4-(n-alkyloxy)phenylporphyrin covalently linked with violgen by a polymethylene chain (Zn-t- bu-PC _n V²⁺; n=4, 6, 8, 10 and 12) was investigated by means of ¹H NMR, UV/Vis absorption spectroscopies in acetonitrile-water (1:1, v/v). The ¹H NMR spectra indicated that Zn-t-bu-PC _n V²⁺ presumably existed as a mixture of a dimer and a monomer in high concentration (>1×10⁻³ mol dm⁻³), and the dimer was degraded by the complex formation with TM-β-CD. The ¹H NMR spectra of these compounds as a function of [TM-β-CD] showed the selective formation of 1:1 (=Zn-t-bu-PC _n V²⁺: TM-β-CD) pseudo-rotaxane type complexes. The chemical modification by t-butyl groups on porphyrin showed a good protective effect on inclusion of benzene groups into the TM-β-CD cavity. These rotaxane formation constants (K) were determined by titration studies using UV/Vis absorption spectroscopy. These complex formation constants were somewhat affected by the spacer methylene chain between the porphyrin and viologen. The value of K for Zn-t-bu-PC₄V²⁺·TM-β-CD is 1.0×10³ M⁻¹ which is the smallest whereas those for Zn-t-bu-PC _n V²⁺·TM-β-CD (n=8, 10, 12) were similar (1.0×10⁴ M⁻¹).     

A new example of reversal of the order of migration of enantiomers, as a function of cyclodextrin concentration and pH, by cyclodextrin-modified capillary zone electrophoresis: enantioseparation of 6,6′-dibromo-1,1′-binaphthyl-2,2′-diol

Enantioseparation of 6,6′-dibromo-1,1′-binaphthyl-2,2′-diol (DBBD) by cyclodextrin-modified capillary zone electrophoresis (CD-CZE) was studied using the three native α, β, and γ cyclodextrins, the three hydroxypropylated cyclodextrins (2-hydroxypropyl-α, β, and γ), heptakis-2,6-di-O-methyl-β-CD (DM-β-CD), and heptakis-2,3,6-tri-O-methyl-β-cyclodextrin (TM-β-CD). First, the acidity constants of DBBD were determined using capillary electrophoresis, before performing enantioseparation. The influence of the concentrations of the studied cyclodextrins on the enantioseparation was explored and the experimental optimal concentrations were determined and compared to the theoretical optimal concentrations. Moreover, the apparent complexation constants between each studied cyclodextrin and the two DBBD enantiomers were evaluated using a non-linear curve fitting method and three linear plotting methods (x-reciprocal, y-reciprocal and double reciprocal). For TM-β-CD, the order of migration of the enantiomers of DBBD reversed as a function of TM-β-CD concentration. The influence of the nature of methylated cyclodextrin derivatives (methyl-β-CD (M-β-CD) and DM-β-CD) was then studied. Inversion of the order of migration of the enantiomers of DBBD was observed for DM-β-CD, whereas the S enantiomer of DBBD always migrated first for M-β-CD.     

Spectrofluorimetric Study of an Inclusion Complex of 2-Hydroxypropyl-β-Cyclodextrin with the Antitumour 11-Methyl Benzophenothiazine. Analytical Application to Urine

The electronic absorption and fluorescence spectral properties of 11-methyl-12H-benzo[a]phenothiazine (11-MeBPHT) were investigated in various media (water, ethanol, β-cyclodextrin (β-CD) and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) aqueous solutions). Fluorescence quantum yields were respectively about 20 and 2 times larger in HP-β-CD and β-CD than in water. The formation of a 1:1 stoichiometry inclusion complex between 11-MeBPHT and HP-β-CD (association constant K _f=118±3 M⁻¹ at 20 °C) was studied in aqueous medium by fluorescence spectroscopy. Analytical figures of merit were satisfactory for 11-MeBPHT with linear dynamic ranges over at least two orders of magnitude and limits of detection (LODs) between 0.2 and 1 ng/ml according to the medium. An analytical application to the determination of 11-MeBPHT in human urine samples by the standard addition procedure led to satisfactory recovery percentages (91–108%).     

Study on the Association Phenomenon of Cyclodextrin to Porphyrin J-aggregates by NMR Spectroscopy

The nuclear magnetic resonance (NMR) spectroscopy demonstrated that the inclusion complexes of meso-tetrakis- (p-sulfonatophenyl) porphyrin (TPPS) with β-, Hydroxypropyl-β- and Methyl-β-cyclodextrin (β-, HP-β- and Me-β-CD) are formed, which resulted in the dissociation of TPPS J-aggregates efficiently under certain acidity. There are no significant differences in binding affinities and basic complexation mechanisms between TPPS and β-cyclodextrin (β-CD) or hydroxypropyl-β-cyclodextrin (HP-β-CD), i.e. porphyrin is included through the wide side of the cavity of β-CD or HP-β-CD. Alternatively, porphyrin is included through the narrow side of the Me-β-CD cavity.     

Spectroscopic properties,structure, and photoinduced motion of 4-(2-naphthyl)pyridine in cyclodextrin cavities

Spontaneous and photoinduced protonation of 4-(2-naphthyl)pyridine (1) in solutions and in complexes with β-cyclodextrin (β-CD) and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) was studied using the absorption and fluorescence spectroscopies. The structures and stabilities of complexes of compound 1 and its quaternized derivative, 1-methyl-4-(2-naphthyl)pyridinium perchlorate (3), with β-CD and HP-β-CD were examined by ¹H NMR titration (logK = 1.5–2.3). The molecule of naphthylpyridine 1 is always in the cyclodextrin cavity, regardless of the pH value of the solution. 2-Hydroxypropyl-β-cyclodextrin binds better the neutral form of compound 1 than does β-CD, while naphthylpyridinium salts exhibit nearly equal affinities to both cavitands. According to spectroscopic data, pK _a (1) is 5.12 in water, which favors protonation of the N atom both in the ground and excited states; as a result, the fluorescence spectrum exhibits only the band of the protonated form with a lifetime of 15 ns. The addition of HP-β-CD to a solution of naphthylpyridine 1 results in the formation of inclusion complex 1@HP-β-CD, lowers pK _a to 4.62, and gives rise to a fluorescence band of the nonprotonated form of compound 1 with a lifetime of 1.25 ns. Therefore, the presence of compound 1 in the HP-β-CD cavity precludes its protonation in the excited state. The initial portions of the fluorescence curves for compound 1 in solution and in its complex with HP-β-CD obtained upon pulsed excitation were compared to propose the initiation mechanism of short-lived fluorescence of the nonprotonated form of naphthylpyridine 1. Quantum chemical modeling of the protonation and complexation of compound 1 in the presence of water was performed. Based on the results obtained, a reversible photoinduced mechanical motion of naphthylpyridine 1 in the HP-β-CD cavity was suggested.     

Study on inclusion complexes of meso-tetrakis(2-thienyl)porphyrin and Cu-meso-tetrakis(2-thienyl)porphyrin with cyclodextrins by spectroscopy method

In phosphate buffer solution of pH5.4, the interaction of meso-tetrakis(2-thienyl)porphyrin(H₂TTP) and Cu-meso-tetrakis(2-thienyl)porphyrin(Cu-TTP) with α-cyclodextrin(α-CD), β-CD, γ-CD, heptakis(2,3,6-tri-O-methyl)-β-CD(TM-β-CD) has been studied by means of UV-vis, fluorescence and ¹HNMR spectroscopy, respectively. The H₂TTP and Cu-TTP can form 1:2 inclusion complexes with TM-β-CD and 1:1 inclusion complexes with the other three cyclodextrins. In this paper, the inclusion constants (K) of H₂TTP and Cu-TTP for the formation of the inclusion complexes have been estimated from the changes of absorbance and fluorescence intensity in phosphate buffer solution. The inclusive capabilities of different kinds of cyclodextrins are compared. The result shows that the inclusion ability of α-CD with H₂TTP and Cu-TTP is the strongest among the three native CDs. The inclusion ability of modified β-CD with H₂TTP and Cu-TTP is stronger, compared to the native β-CD, which indicates that the capacity matching plays a crucial role in the inclusion procedure except for the hydrophobic effect. In addition ¹HNMR spectra supports the inclusion conformation of the TM-β-CD-Cu-TTP inclusion complex, indicating the interaction mechanism of inclusion processes.     

A study of the interaction between enantiomers of zolmitriptan and hydroxypropyl-beta-cyclodextrin by capillary electrophoresis

The enantioresolution of zolmitriptan was performed using cyclodextrin (CD)-modified capillary zone electrophoresis (CZE) with hydroxypropyl-β-CD (HP-β-CD) as the chiral selector. The influence of experimental conditions on the enantioseparation of zolmitriptan, such as pH, temperature, applied voltage, and concentrations of running electrolyte and CD, was systematically investigated, obtaining a baseline separation of two enantiomers by the use of a 25 mM sodium dihydrogen phosphate (SDPH) running electrolyte (pH 2.4) containing 30 mM HP-β-CD at 15 °C. Binding constants for each enantiomer–HP-β-CD pair at different temperatures, as well as thermodynamic parameters for binding, were calculated. A nonlinear van’t Hoff plot was obtained, indicating that the thermodynamic parameters of complexation were temperature-dependent for zolmitriptan enantiomers. The significant contribution of the enthalpy difference to the Gibbs free energy change suggested a stereomeric barrier mechanism for chiral recognition. Figure Resolution of zolmitriptan enantiomers was achieved by using CD-modified CZE Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pseudorotaxane complexes of naphthylpyridines and naphthylbipyridyl with β-cyclodextrin and hydroxypropyl-β-cyclodextrin

The electronic absorption spectra and fluorescence spectra of 4-(2-naphthyl)pyridine (1), 2-(4-methyl-2-pyridyl)-4-(2-naphthyl)pyridine (2), and 4-(2-naphthyl)-2-phenylpyridine (3) in solutions and in complexes with β-cyclodextrin (β-CD) and well water-soluble hydroxy-propyl-β-cyclodextrin (HP-β-CD) were studied. Fluorescence near 475 nm observed in aqueous solutions of compounds 1–3 arises from protonated forms of these compounds produced in the excited state. Results of DFT quantum chemical calculations show an increase in proton affinity energies of excited-state naphthylpyridines 2 and 3. The formation of inclusion complexes with cyclodextrins makes protonation of compounds 2 and 3 more difficult, which manifests in large hypsochromic shifts of fluorescence band maxima. The stability constants of the complexes 1·HP-β-CD and 2·HP-β-CD determined from their fluorescence spectra are 3425 and 3760 L mol⁻¹, respectively. The stability constant of the complex 3·HP-β-CD (5500±600 L mol⁻¹) was found from the changes in the solubility of naphthylpyridine 3 in water upon complexation. Semiempirical quantum chemical calculations of the molecular structures and thermodynamic characteristics of pseudorotaxane inclusion complexes of trans-2, cis-2, and trans-2·H₂O with HP-β-CD were carried out. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 272–280, February, 2007.     

Chiral Separation of Duloxetine and Its R-Enantiomer by LC

The direct enantioseparation of duloxetine and its R-enantiomer was achieved by HPLC using hydroxypropyl-β-cyclodextrin (HP-β-CD) as a chiral selector and a vancomycin chiral stationary phase (Chirobiotic V). Operational parameters, such as the concentration of HP-β-CD, buffer pH, organic modifiers, temperature and flow rate, were varied in order to achieve the desired retention time and resolution. These two enantioseparation methods developed gave a baseline resolution of the enantiomers. Finally, the HPLC-CSP method was selected to determine the enantiomeric purity of duloxetine drug substance due to its much shorter analysis time and better resolution. The limit of detection of this method was 0.06 μg mL⁻¹.     

Enhancement of cyclosporine aqueous solubility using α- and hydroxypropyl β-cyclodextrin mixtures

Cyclodextrins (CDs) are cyclic oligosaccharides that form inclusion complexes with lipophilic molecules through their hydrophobic central cavity. In this study, the effect of α-CD, hydroxylpropyl-β-CD (HP-β-CD) and mixtures of these two CDs on the aqueous solubility of cyclosporine A (CyA) was investigated. Infrared spectroscopy and thermal analysis were used to confirm CyA-CD complex formation. CyA aqueous solubility was increased by 10 and 80 fold in the presence of α-CD and HP β-CD, respectively. The phase-solubility profile for HP-β-CD was linear while that for α-CD had positive deviation from linearity. In the presence of constant concentration of α-CD (15% w/v), aqueous solubility of CyA was further increased upon addition of HP-β-CD up to a concentration of 20% w/v. At higher HP-β-CD concentrations, aqueous solubility of CyA was observed to decrease. Addition of sodium acetate (up to 5% w/v) to aqueous solutions containing 20% w/v HP-β-CD and increasing concentrations of α-CD resulted in a significant reduction in CyA solubility. Complex formation between CyA and both α-CD and HP-β-CD was confirmed by differential scanning calorimetry (DSC). No significant changes were observed in the IR spectra of either CyA or CD following complex formation suggesting chemical interaction between CyA and the CD was unlikely. Phase-solubility studies showed that α-CD had a much greater effect on the solubility of CyA than HP-β-CD. Addition of HP-β-CD to aqueous solutions of α-CD affected the solubility of CyA in these systems. A mixture of 15% w/v α-CD and 20% w/v HP-β-CD was optimal for increasing aqueous solubility of CyA.     

Chiral separation of pheniramine by capillary electrophoresis partial-filling technique using hydroxypropyl-β-cyclodextrin as chiral selector

A simple capillary electrophoresis partial-filling technique for the enantioseparation of pheniramine is presented. Phosphate buffer and hydroxypropyl-β-cyclodextrin (HP-β-CD) were used as the electrolyte and chiral selector, respectively. Several experimental parameters such as HP-β-CD concentration, HP-β-CD plug length, CE temperature and applied voltage were studied. Under the selected conditions, pheniramine enantiomers can be separated within less than 14 min. The assay was validated for linearity (5.0 × 10⁻⁶–5.0 × 10⁻⁴ M; R ² = 0.9987), limit of detection (5.0 × 10⁻⁷ M), limit of quantitation (5.0 × 10⁻⁶ M), analytical precision (%RSD ≤ 9.8) and accuracy (%recovery = 101 ± 3). The proposed methodology was then applied to the analysis of a commercially available pharmaceutical eye drop preparation. The results are in good agreement with that declared by the manufacturer. The proposed methodology provides adequate results in terms of simplicity, cost, sample throughput, repeatability and accuracy for quality control of pheniramine enantiomers in pharmaceutical preparations.     

Separation of chiral pharmaceuticals using ultrahigh pressure liquid chromatography

Summary Ultrahigh pressure liquid chromatography was demonstrated for fast and efficient chiral separations. Capillary columns approximately 13–24 cm in length packed with nonporous 1.0μm C₆-modified silical particles were used. β-Cyclodextrin (β-CD) and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) were added to the mobile phase as modifiers to produce transient diastereomeric complexes with the analytes. Pressures up to ≈42,000 psi were applied, and efficiencies in excess of 200,000 plates m⁻¹ were obtained for separations that were accomplished in less than 2 minutes.     

Determination of Enantiomeric Excess of Aromatic 1,2-Diols with HP-β-Cyclodextrin as Chiral Selector by CE

To determine optical purities of four aromatic 1,2-diol enantiomers synthesized by a Sharpless asymmetric dihydroxylation (AD) reaction of olefins, a simple and reliable separation method was achieved with high resolution (R _s > 2.2) by capillary zone electrophoresis (CZE) using hydroxypropyl-β-cyclodextrin (HP-β-CD) as chiral selector and borate combined with methanol additive as background buffer. Furthermore, the developed CZE method was successfully applied to the determination of enantiomeric excess of the tested enantiomers. RSD values of migration time and peak area fell within 1.0 and 3.8%, respectively. This method allowed for the determination of ee (%) values of targeted isomers as high as 99.6%. Impurities of undesired isomers could be detected at levels as low as 0.2% in the presence of the targeted isomers.     

Improvement of Solubility and Stability of Valsartan by Hydroxypropyl-\boldbeta-Cyclodextrin

Aim of the present work was to investigate the effect of hydroxypropyl-β-cyclodextrin (HP-β-CD) on the solubility, dissolution rate and stability of Valsartan (VAL), a drug used orally for the treatment of hypertension. Phase solubility studies demonstrated the ability of the HP-β-CD to complex VAL and to increase drug solubility. The dissolved amount of VAL increased linearly with the addition of HP-β-CD according to an A_L type plot. The apparent stability constant of the complex, calculated supposing a 1:1 stoichiometry, was 296±7 M⁻¹. VAL/HP-β-CD interactions were also studied by ¹³C-NMR spectroscopy. Equimolar VAL/HP-β-CD solid systems were prepared by physical-mixing and freeze-drying, and their properties in the solid state studied by DSC and FT-IR analysis. The results provided clear indications of the formation of a new solid phase corresponding to the inclusion complex in the freeze-dried sample. The dissolution profiles of the drug from each solid system were affected by its physico-chemical properties, the freeze-dried being the most rapidly dissolving form. The thermal stability of the complex was studied, also determining the number and identity of the decomposition products of the drug. The stability studies revealed that the VAL/HP-β-CD complex significantly decreases the rate of VAL degradation. These results suggest that CD technology would be a very useful method to overcome the solubility and the stability problems of VAL.