Yannis L. Loukas

Drugs, in cyclodextrins, in liposomes: A novel approach to the chemical stability of drugs sensitive to hydrolysis

The entrapment in multilamellar liposomes of the inclusion complexes of cyclodextrins with drugs sensitive to hydrolysis, is being examined. The anti-inflammatory drug indomethacin (IND), rapidly hydrolyzed in alkaline media, can be stabilized 75-fold when its inclusion complex with the hydroxypropylated derivative of β-cyclodextrin (HPβCD) is entrapped in multilamellar liposomes. These liposomes were composed of egg phosphatidylcholine and cholesterol of 1:1 molar ratio and were prepared according to the dehydration-rehydration method. The molar ratio for the IND:HPβCD inclusion complex was estimated to be 1:1 using the continuous variation method based on the 1H NMR data. Furthermore, the binding constant (K11) of the IND:HPβCD complex was estimated kinetically and by 1H NMR.

Novel liposome-based multicomponent systems for the protection of photolabile agents

Abstract A photosensitive drug (riboflavin) was entrapped as such or in the form of β- or γ-cyclodextrin complexes into the aqueous phase of multilamellar dehydration-rehydration vesicles (DRV liposomes) made of equimolar egg phosphatidylcholine or dipalmitoylphosphatidylcholine and cholesterol. Riboflavin-containing DRV were prepared in the absence or presence of one or more of the lipid-soluble UV absorbers oil red O, oxybenzone and dioxybenzone (entrapped into the lipid phase) and the water-soluble sulisobenzone (entrapped in the aqueous phase of liposomes together with riboflavin). In some experiments, lipid-soluble absorbers were supplemented with the antioxidant β-carotene. Entrapment values for free (41–47%) and complexed (19–23%) riboflavin were estimated fluorimetrically with additional data from NMR studies confirming that the complexes were entrapped as intact entities. Entrapment values for each of the UV light lipid-soluble absorbers (79–98%) and β-carotene (78 and 88%) were estimated by the use of the second-order derivative of their UV spectra to circumvent interference from overlapping absorption spectra of the other agents, when present. A number of conditions of entrapment were found to reduce values, for instance co-entrapment of sulisobenzone together with the vitamin in the case of riboflavin and, for all other materials, the absence (or reduced content) of cholesterol in DRV or certain variations in their manufacture. Exposure of a variety of riboflavin-containing DRV preparations to UV light revealed optimal protection with a formulation containing the γ-cyclodextrin complex of the vitamin, all three lipid-soluble light absorbers and β-carotene, increasing the half-life of riboflavin 266-fold. Results suggest that liposome-based multicomponent systems could be developed for the protection of photolabile agents in therapeutics and other uses.

Use of a nonlinear least-squares model for the kinetic determination of the stability constant of cyclodextrin inclusion complexes

Abstract The nonlinear least-squares model for the calculation of the stability constants (Kst) of drug:cyclodextrin complexes was used in kinetic studies. Complexation of riboflavin (R) with hydroxypropyl-β-cyclodextrin (HP:β-CD) was monitored kinetically by measuring the rate of photodegradation of R exposed to ultraviolet light in the presence of increasing concentrations of HP:β-CD. Formation of inclusion complex was confirmed in aqueous solution by proton nuclear magnetic resonance spectroscopy (1H-NMR). The experimental Kst value (3321 M−1), derived from the kinetic studies, appeared to fit well to a 1:1 drug: cyclodextrin molar ratio according to the nonlinear mathematical model. The model is particularly suitable for the study of cyclodextrin as a stabilizing system for compounds that are sensitive to light, oxygen, temperature or the media which contain them.

Novel non-acidic formulations of haloperidol complexed with β-cyclodextrin derivatives

Haloperidol (Hal), a highly hydrophobic drug, was complexed with two beta-cyclodextrin (beta-CD) derivatives. Hal solubility was increased 20-fold in the presence of a 10-fold excess of methyl beta-CD (Me beta-CD) and 12-fold in the presence of a 10-fold excess of 2-hydroxypropyl beta-CD (HP beta-CD). The stoichiometries and stability constants of Hal-Me beta-CD (1:1 and 2345 M-1 at 27 degrees C) and Hal-HP beta-CD (1:1 and 2112 M-1 at 27 degrees C) complexes were calculated by the continuous variation and phase solubility methods respectively. Differential scanning calorimetry and 1H-NMR were used to confirm the formation of inclusion complexes. Moreover, the enthalpy and entropy of the complexation process were calculated for both complexes in order to obtain such information as the main driving force and whether or not complex formation is thermodynamically favoured. This was achieved by monitoring the isothermic solubility lines at various temperatures.

Measurement of Molecular Association in Drug: Cyclodextrin Inclusion Complexes with Improved 1H NMR Studies

The molecular association of haloperidol with hydroxypropyl‐β‐cyclodextrin, expressed by the binding constant of the inclusion complex formed, was calculated from the changes on the 1H NMR spectra of the drug in the presence of the cyclodextrin.

Inclusion complexes and stability studies of an organophosphorous insecticide with cyclodextrins: spectrophotometric and kinetic determination of stability constant

DCPE[α-(diethoxyphosphinoximino)dicyclopropylmethane] is a potent organophosphorous insecticide. It is an oily liquid and therefore the most suitable formulation is an ‘emulsifiable concentrate’. In an attempt to develop better formulations, complexes of this insecticide with the cyclodextrins (CDs)αCD, βCD and hydroxypropyl-βCD (HP-βCD) were examined. Stability constants were determined spectrophotometrically, in the case of DCPE–βCD, according to the spectral changes of DCPE due to the complexation and kinetically, in the case of DCPE–HP-βCD, according to the changes in the rate constant of DCPE degradation. HP-βCD has a stabilizing effect on DCPE by decelerating its degradation in aqueous solutions, but αCD and βCD have no effect. Finally, the thermodynamic parameters for the inclusion complex formation between DCPE and HP-βCD in aqueous solutions were calculated by correlating the changes in the stability constant with temperature.

Evaluation of the methods for the determination of the stability constant of cyclodextrin–chlorambucil inclusion complexes

The interaction of the antitumor agent chlorambucil (CHL) with three different cyclodextrins (CD), namely methyl-beta CD (Me beta CD) polymer-beta CD (poly-beta CD) and gamma CD, is examined kinetically and spectrophotometrically, monitoring the hydrolysis and the changes in the UV absorbance of CHL respectively, in the presence of increasing concentrations of the examined CD. The stoichiometry coefficient for all the CHL-CD complexes was calculated and found to be 1:1, using the continuous variation method based on the UV data. Also, the stability constant Kst for the CHL-CD complexes was calculated and evaluated using the above mentioned two methods, each one based on linear and nonlinear mathematical models. All studies demonstrate that the interaction of CHL with the methylated derivative (Me beta CD) is stronger (the highest Kst value), probably due to the enhanced hydrophobic character of this derivative.

Fluorimetric Studies of the Formation of Riboflavin-β-Cyclodextrin Inclusion Complex: Determination of the Stability Constant by use of a Non-linear Least-squares Model

The non‐linear least‐squares model for calculation of the stability constant (Kst) of a drug‐cyclodextrin complex has been used in fluorimetry studies.

A computer-based expert system designs and analyzes a 2(k - p) fractional factorial design for the formulation optimization of novel multicomponent liposomes.

A computer-based technique based on a 2(k - p) fractional factorial design was applied for the optimization of recently described multicomponent protective liposomal formulations. These formulations contain sodium ascorbate (vitamin C) as a model drug sensitive to photochemical oxidation, as well as oil red O and/or oxybenzone as oil soluble light absorbers, incorporated into the lipid bilayers and sulisobenzone as a water soluble light absorber incorporated into the aqueous phase of liposomes. The three light absorbers (present or absent) incorporated in multilamellar liposomes and the drug in free or in complexed with alpha-cyclodextrin form comprised the four factors of the system. The stabilization ratio and the percentage entrapment in the liposomes of the vitamin were the two response variables of the system to be optimized. The entrapment values were calculated for all the materials either spectrophotometrically or by using second order derivative spectrophotometry. The response variables were predicted by multiple regression equations comprising combinations of the four formulation factors. Both the higher entrapment and the higher protection for the drug should characterize the optimum formulation.

Multiple complex formation of unstable compounds with cyclodextrins : Efficient determination and evaluation of the binding constant with improved kinetic studies

Equations were derived that allow the determination of ncyclodextrin–compound complex primary and secondary binding nconstants by using kinetic analysis (by monitoring the degradation rate nconstant of the compound in the presence of cyclodextrins). The ndegradation rate constant is dependent on the concentration of ncyclodextrins in the solution of the compound and also on the nstoichiometry of the cyclodextrin–compound complex. It was found nthat riboflavin (a photosensitive compound) forms a 1:2 inclusion complex nwith α-cyclodextrin and a 1:1 complex with γ-cyclodextrin. nIndomethacin (sensitive to hydrolysis) also forms a 1:1 complex with nβ-cyclodextrin. Both compounds are unstable and can be stabilized nthrough complexation with cyclodextrins. The binding constants were also ncalculated fluorimetrically for riboflavin and by 1 nH NMR nspectrometry for indomethacin in order to compare the values from the ndifferent methods. The fact that the same compound (riboflavin) can nexhibit different binding behaviors with different cyclodextrins could nresult in new descriptive studies for each particular case.