Statistical properties of thermodynamic quantities for cyclodextrin complex formation

Bupivacaine hydrochloride complexation with some α- and β-cyclodextrins studied by potentiometry with membrane electrodes

Membrane electrodes selective to bupivacaine cations were developed and those with PVC-dibutylphthalate membrane containing sparingly soluble bupivacaine phosphotungstate appeared to be the most suitable. Inclusion complexation of bupivacaine cations with cyclodextrins was studied by potentiometric measurements of the free bupivacaine cation concentration in aqueous solutions of bupivacaine hydrochloride with cyclodextrin using the prepared electrodes. Native α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD), as well as their random-substituted derivatives hydroxypropyl-α-cyclodextrin (HP-α-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD) and methyl-β-cyclodextrin (M-β-CD), were chosen for the study. The measured potentiometric data processed both by a linear and nonlinear regression corroborated the formation of weak 1:1 bupivacaine cation-cyclodextrin complexes and the corresponding complexation constants K11 ∼50–155m−1 were evaluated by the non-linear least-squares method. The mutual order of K11 values, especially α-CD > β-CD, suggested that the bupivacaine butyl group was mainly responsible for the inclusion complexation; the highest K11 was exhibited by M-β-CD followed by α-CD. The observed complexation may substantially modify properties of bupivacaine hydrochloride dosage forms with sufficient concentration of cyclodextrin but bupivacaine cations are readily released from the weak cyclodextrin complexes by dilution.

The utility of cyclodextrins for enhancing oral bioavailability

Cyclodextrins (CD) have been utilized extensively in pharmaceutical formulations to enhance oral bioavailability. A critical review of the literature in which cyclodextrins were utilized for this purpose was conducted. The goal of this review was to determine if quantitative guidelines for drug and cyclodextrin properties necessary for bioavailability enhancement using cyclodextrins could be extracted. Twenty-eight studies were examined in which the focus was on the use of cyclodextrins as solubilizers to enhance bioavailability. Commonly observed factors included: utilization of pre-formed complex rather than physical mixtures, drug hydrophobicity (logP > 2.5), low drug solubility (typically< 1 mg/ml), moderate binding constant (< 5000 M(-1)), low dose (< 100 mg), and low CD:drug ratio (< 2:1). These general guidelines, however, did not apply to all studies. Quantitative guidelines useful to a formulation scientist considering the use of cyclodextrins were difficult to develop due to missing information and the complicated manner in which drug and cyclodextrin properties interact to influence key drug delivery processes (e.g., dissolution, absorption). The mechanisms by which cyclodextrins influence these processes, again emphasizing solubilization capabilities, are discussed to provide further insight into why cyclodextrins will increase bioavailability in certain cases but not influence or possibly decrease bioavailability in others.

Practical considerations in development of solid dosage forms that contain cyclodextrin

The following is a review of the literature that addresses the use of cyclodextrin in solid dosage forms. Care was taken to exclude physical and chemical characteristics of cyclodextrin, which have been discussed in the literature. A flow diagram is provided to outline the decision-making steps that are involved in the development process. Both preparation of physical mixtures and inclusion complexes are considered. Analytical techniques to determine the presence of inclusion complexes, the effect of other excipients on complex formation, the effect of size limitation of solid dosages forms, powder processing, and storage of solid dosage forms are discussed.

Trends in Solubility of Polymorphs

Polymorphism of drug substances has been the subject of intense investigation in the pharmaceutical field for over 40 years. Considering the multitude of reports on solubility or dissolution of polymorphs in the literature, an attempt is made in this study to answer the question: How big is the impact of polymorphism on solubility? A large number of literature reports on solubility or dissolution of polymorphs were reviewed and the data were analyzed for trends in solubility ratio of polymorphs. The general trend reveals that the ratio of polymorph solubility is typically less than 2, although occasionally higher ratios can be observed. A similar trend is also observed for anhydrate/hydrate solubility ratios, although anhydrate/hydrate solubility ratios appear to be more spread out and higher than the typical ratio for nonsolvated polymorphs. An attempt is also made in this commentary to estimate the ratio of solubilities of polymorphs from thermal data. The trend in estimated solubility ratio shows good agreement with the one observed with experimentally determined solubility values.

Calorimetric measurement of phospholipid interaction with methyl-beta-cyclodextrin

Cyclodextrins are able to bind hydrophobic molecules in their interior cavity and as such have received a great deal of attention as carriers of cholesterol, lipophilic drugs, and other sparingly soluble compounds. Despite the importance of these biochemical applications, relatively little is known about the interactions of cyclodextrins with phospholipid membranes. Here we characterize the binding of randomly methylated beta-cyclodextrin (m beta CD) to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using right-angle light scattering and isothermal titration calorimetry. Existing models of lipophile-membrane interactions are inadequate to describe the observed binding; we introduce a modified chemical reaction model in which the chemical activity of the phospholipid is independent of its concentration. We find that an average of four m beta CD molecules bind to each POPC molecule with an enthalpy of reaction of 46 kJ mol(-1) and an equilibrium constant of 90 M(-3). These results are consistent with earlier qualitative observations and suggest that disruption of phospholipid membranes may be minimized if the concentration of m beta CD is kept below about 15 mM.

Inclusion Complexation of Carbethopendecinium Bromide with Some α- and β-Cyclodextrins Studied by Potentiometry with Membrane Electrodes

Inclusion complexation of an antimicrobial quaternary ammonium salt, carbethopendecinium bromide (SBr, Septonex), with five cyclodextrins was investigated potentiometrically using the prepared membrane electrodes selective to the surface-active carbethopendecinium cations (S+). Relatively strong complexation of the S+ cations with native α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), and their random-substituted derivatives, namely hydroxypropyl-α-cyclodextrin (HP-α-CD), methyl-β-cyclodextrin (M-β-CD), and hydroxypropyl-β-cyclodextrin (HP-β-CD), was evidenced in dilute aqueous solution. In the region where the potentiometrically determined concentration of the free S+ cations remained lower than the critical micelle concentration (cmc) of SBr by a factor of ca ten, formation of the 1:1 complexes with the complexation constants K11 ≈ 104-105 was evaluated by two respective methods, based on a modified linear and a non-linear regression. Deviations from the 1:1 complexation were observed when concentration of the free S+ cations approached cmc of SBr more closely and also in solutions with large excess of α-CD. Comparison of K11 values corroborated the inclusion of the n-C14H29 alkyl chain of the carbethopendecinium cation as the mechanism of its complexation with cyclodextrins. The well-soluble native α-CD with good complex-forming capability towards S+ cations may be especially suitable for possible blocking the undesirable residues of carbethopendecinium bromide.

When can cyclodextrins be considered for solubilization purposes?

This communication is intended to address the question: How can one determine, with minimum experimentation, if cyclodextrins (CDs) might be the right choice as solubilization enhancers for a given poorly water-soluble drug? The cyclodextrin utility number, U(CD), a dimensionless number, is introduced to assess the feasibility of the use of CDs in dosage forms. U(CD) is a lumped-parameter consisting of the dose of the drug, the workable amount of CD, the binding constant, and the drug solubility in the absence of CDs. U(CD) was been extended to ionizable drugs that show synergistic increase in solubility due to ionization and complexation. U(CD) is a guiding and not a predictive tool that the formulator can use.

Potentiometric Study of Carbisocaine Micellization and Inclusion Complexation with α-Cyclodextrin, β-Cyclodextrin, Methyl-β-cyclodextrin, and (Hydroxypropyl)-β-cyclodextrin

Micellization of the local anesthetic drug carbisocaine hydrochloride (BHCl) was studied by potentiometry with both cation- and anion-selective membrane electrodes in aqueous solution at 25 °C. The found critical micelle concentration 0.022 mol dm-3 and the concentration course of the free carbisocaine cation and chloride counterion in the micellar solution corresponded to the characteristic of cationic surfactants. In a more dilute aqueous solution, below critical micelle concentration, the complexation of carbisocaine cation BH+ with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and its random substituted methyl (M-β-CD) and hydroxypropyl (HP-β-CD) derivatives was followed using the prepared cation-selective electrodes. The potentiometric data corroborated formation of the carbisocaine-cyclodextrin complexes (BH+)CD (1 : 1) with all the cyclodextrins and the respective complexation constants K11 were estimated using a modified Scatchard method. Slight deviations from 1 : 1 plots were marginally observed with α-CD and HP-β-CD. Bigger K11 value of the complexation with α-CD in comparison with β-CD indicated inclusion of the carbisocaine C7 alkyl chain into the cyclodextrin cavity and the role of the hydrophobic interaction in complexation with β-CDs was emphasized by the increasing magnitude of K11 in the order of HP-β-CD < β-CD < M-β-CD.