Elsevier

Carbohydrate Research

Volume 345, Issue 17, 22 November 2010, Pages 2550-2556
Carbohydrate Research

Studies on trimethoprim:hydroxypropyl-β-cyclodextrin: aggregate and complex formation

https://doi.org/10.1016/j.carres.2010.08.018Get rights and content

Abstract

The present study is focused on the characterization of the interaction between trimethoprim, a dihydropteroate synthesase inhibitor, and hydroxypropyl-β-cyclodextrin (HP-β-CD) in aqueous solution and solid state. The freeze-drying method was used to prepare solid complexes, while simple blending was employed to obtain physical mixtures. The phase solubility was AN type, and demonstrated that trimethoprim solubility was significantly increased upon complexation with HP-β-CD. Conductivity experiments showed the presence of aggregates that explains the type profile for the solubility isotherm. The critical concentration for the aggregate formation was determined to be 69.3 mg/ml for pure HP-β-CD and 117.7 mg/ml in the presence of trimethoprim. Nuclear magnetic resonance spectroscopy provided evidence of trimethoprim:HP-β-CD molecular interaction in solution. Moreover, the complex was characterized in solid stated using Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The use of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed that the thermal stability of the drug is enhanced in the presence of HP-β-CD.

Introduction

Trimethoprim (Fig. 1) is a synthetic, broad-spectrum antimicrobial agent which acts as an inhibitor of bacterial dihydrofolate reductase. This drug is mainly used in combination with sulfonamides for prophylaxis and treatment of urinary tract and certain types of pneumonia. Trimethoprim is characterized by a very low aqueous solubility1, 2 fact that complicates the preparation of formulations such as those for the intravenous administration.

A strategy widely used to increase the solubility, stability, and bioavailability of drugs is the complex formation with cyclodextrins (CDs).3, 4, 5 CDs are macrocyclic oligosaccharides with six, seven or eight d-glucose units called α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin, respectively. Hydroxypropyl-β-cyclodextrin (HP-β-CD) (Fig. 2) is a hydroxyalkylated β-CD derivative that combines relatively high water solubility with low toxicity and satisfactory inclusion ability.6, 7 The cavities of CDs are relatively hydrophobic, while the external faces are hydrophilic. The most important structural feature of CDs is its capacity to form stable inclusion complexes with properly sized drug molecules,8, 9 prevailing this fact in dilute aqueous solutions.10 Moreover, it is known that CD molecules have the tendency to self-assemble to form aggregates which can also participate in the solubilization of poorly soluble drugs. It was observed that the aggregate formation depends on the concentration. Furthermore, the formation of inclusion complexes with lipophilic guests can convert a CD molecule from a simple oligosaccharide into a molecule capable of forming micellar-type aggregates. In addition, poorly soluble guests can participate in the formation of guest-CD co-aggregates where the molecules are kept together through non-inclusion complexes.10, 11

Studies involving the complexation of trimethoprim with natural CDs (α-, β- and γ-) and methylated β-CD have been reported in the literature,12, 13, 14 as well as the formation of a 1:1 complex in presence of HP-β-CD at various pH values, which could improve the aqueous solubility of the drug in trimethoprim/sulfamethoxazole parenteral solutions but could not prevent its precipitation.15 In addition, the chemical stability under oxidation stress of trimethoprim in co-trimoxazole (a 5:1 combination of sulfamethoxazole with trimethoprim) aqueous buffer solutions has been increased using HP-β-CD as a molecular inclusion excipient.16 Moreover, we have previously developed a method for the simultaneous quantification of trimethoprim and sulfamethoxazole in mixtures using HP-β-CD solutions.17 Nevertheless, none of the above mentioned works have characterized the inclusion complex of trimethoprim with HP-β-CD and investigated the possibility of aggregate formation, although there were observed results that may be indicative of non-linear complexation.16

Considering these previous investigations, the present study aimed to investigate the interactions between trimethoprim and HP-β-CD. The characterization has been performed using solubility analysis, nuclear magnetic resonance (1H NMR), infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Also, the formation of aggregates was investigated by conductivity measurements.

Section snippets

Chemicals and reagents

All the experiments were performed with analytical grade chemicals and solvents. HP-β-CD, degree of substitution 0.63, was kindly supplied by Ferromet agent of Roquette (France). NMR spectra were taken in deuterium oxide (D2O, deuterium content 99.9%) from Aldrich® USA. A Millipore Milli Q Water Purification System generated the water used in these studies.

Phase solubility studies

The solubility measurements were performed according to the method of Higuchi and Connors.18 An excess of trimethoprim was suspended in

Phase solubility analysis

The effect of HP-β-CD on the solubility of trimethoprim was investigated. The solubility of trimethoprim in water was determined to be 0.4 mg/ml. The solubility profile (Fig. 3) showed an increase in its solubility when the concentration of HP-β-CD increases. The solubility of trimethoprim was of 1.1 mg/ml, 2.1 mg/ml, and 3.2 mg/ml in the presence of 28.6, 71.4, and 142.9 mM HP-β-CD solution, respectively. Therefore, HP-β-CD increased the solubility of trimethoprim by approximately three, five, and

Conclusion

In conclusion, the results reported here revealed that the solubility of trimethoprim was enhanced in presence of HP-β-CD. At low HP-β-CD concentrations, the formation of an inclusion complex produced an increase in the solubility; whereas at concentrations higher than the critical concentration, aggregates capable of solubilizing trimethoprim through the formation of non-inclusion complexes were formed.

Acknowledgments

The financial support from Fondo para la Investigación Científica y Tecnológica (FONCYT) Préstamo BID 1728/OC-AR PICT 1376, the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Secretaría de Ciencia y Técnica de la Universidad Nacional de Córdoba (SECyT-UNC), are greatly acknowledged. We also thank the Ferromet S.A. (agent of Roquette in Argentina) for their donation of hydroxypropyl-β-cyclodextrin.

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