Solubility improvement of an anthelmintic benzimidazole carbamate by association with dendrimers

Abstract

The improvement of aqueous solubility of methyl (5-[propylthio]-1H-benzimidazol-2-yl) carbamate, albendazole (ABZ) using polyamidoamine (PAMAM) dendrimers as solubility enhancers was investigated. Full generation PAMAM dendrimers with amine terminal groups, (G3), with hydroxyl terminal groups (G3OH) and half generation PAMAM dendrimers with carboxylate terminal groups (G2.5 and G3.5), were chosen for this study. The nature of dendrimer-ABZ association was investigated by UV absorption, fluorescence emission measurements and by ¹H-NMR spectroscopy. The results obtained show that these polymeric structures have the capacity to enhance the solubility of ABZ, both lipophilic and specific hydrogen bond interactions contributing to the guest-host association. Although all studied dendrimers have hydrophobic internal nanoenvironments with similar dimensions, their surfaces differ significantly and the nature and the localization of the interactions involved in ABZ-dendrimer association depend on the type of terminal groups.

Dendrimer; Solubility; Albendazole; Anthelmintic; Host-guest interaction

THERMODYNAMICS

Solubility improvement of an anthelmintic benzimidazole carbamate by association with dendrimers

L. Fernández^I; E. Sigal^I; L. Otero^II; J. J. Silber^II; M. Santo^I,*

^IDepartamento de Física, Facultad de Ciencias Exactas Fisicoquímicas y Naturales,Universidad Nacional de Río Cuarto, Phone: + 54 0358 4676146, Fax: [phone], Río Cuarto, (X5804BYA), Argentina. E-mail: [e-mail]

^IIDepartamento de Química. Facultad de Ciencias Exactas Fisicoquímicas y Naturales, Universidad Nacional de Río Cuarto, Río Cuarto, (X5804BYA), Argentina

ABSTRACT

The improvement of aqueous solubility of methyl (5-[propylthio]-1H-benzimidazol-2-yl) carbamate, albendazole (ABZ) using polyamidoamine (PAMAM) dendrimers as solubility enhancers was investigated. Full generation PAMAM dendrimers with amine terminal groups, (G3), with hydroxyl terminal groups (G3OH) and half generation PAMAM dendrimers with carboxylate terminal groups (G2.5 and G3.5), were chosen for this study. The nature of dendrimer-ABZ association was investigated by UV absorption, fluorescence emission measurements and by ¹H-NMR spectroscopy. The results obtained show that these polymeric structures have the capacity to enhance the solubility of ABZ, both lipophilic and specific hydrogen bond interactions contributing to the guest-host association. Although all studied dendrimers have hydrophobic internal nanoenvironments with similar dimensions, their surfaces differ significantly and the nature and the localization of the interactions involved in ABZ-dendrimer association depend on the type of terminal groups.

Keywords: Dendrimer; Solubility; Albendazole; Anthelmintic; Host-guest interaction.

INTRODUCTION

Albendazole (ABZ), methyl (5-[propylthio]-1Hbenzimidazol-2-yl) carbamate, (Scheme 1), is a broad spectrum anthelmintic agent (Casulli el al., 2006), that has been successfully used in clinics for the treatment of cerebral cysticercosis, a serious public health problem in underdeveloped countries (Zongde el al, 2005). Furthermore, ABZ is a potential anticancer agent that is currently under development for the treatment of cancer (Zhao et al., 2008), (Mohammad et al. 2010) A major problem associated with the formulation and effectiveness of ABZ is its poor aqueous solubility, 0.61 µg/ml, which may account for its low and irregular bioavailability in humans (Wu et al., 2005).

The absorption in the intestinal tract of most orally administered drugs depends on their solubility and permeability properties (Daniel-Mwambete et al., 2004). The solubility can be modified by drug complexation with different excipients and, through this process, the oral bioavailability of drugs with a low aqueous solubility can be greatly improved. Previous studies showed that complexation with povidone and cyclodextrins increases the solubility of ABZ, improving its bioavailability (Moriwaki el al., 2008; Pacioni et al., 2008).

In the last two decades, a novel polymeric nanoarchitecture for solubility enhancement has attracted the attention of many scientists. These compounds, named dendrimers, have been tried successfully for enhancing the solubility of hydrophobic compounds. Dendrimers with a hydrophobic core and hydrophilic periphery have been shown to exhibit micelle-like behavior and to have container properties in solution (Svenson, 2009).

Dendrimers are highly branched and mono-disperse macromolecules with symmetrical architecture (Meredith and Grinstaff, 2011). They consist of a central core, branching units and terminal functional groups. This type of architecture induces the formation of a nano-environment, which determines its solubilizing or encapsulating properties; while the external groups primarily characterize the solubility and chemical behavior (Jang el al., 2009; Astruc et al, 2010). Neutral and negatively charged dendrimers do not interact with biological environments and hence are compatible for clinical applications, as elucidated by various studies reviewed by Jain et al. (2010).

The objective of this work is to examine the possible improvement of the aqueous solubility of ABZ using ethylenediamine core polyamidoamine (PAMAM) dendrimers as solubility enhancers. The use of dendrimers for solubility enhancement has been studied extensively (Wang et al., 2008). Namazi and Adeli (2005) used a new type of dendrimer with poly(ethylene glycol) as core and citric acid as the branching unit for the solubility enhancement of mefenamic acid and Diclofenac. Also, Generation 4 PAMAM dendrimers have been studied to enhance risperidone solubility. Temperature, pH and ionic strength conditions are evaluated in order to improve drug-dendrimer complexation (Prieto et al., 2011).

Over the last several years, substantial progress has been made towards the therapeutic applications of dendrimers for cancer treatment (Wolinsky and Grinstaff, 2008); diverse dendrimers were used to increase the solubility of anticancer drugs, as well as to reduce drug toxicity. Recently, Hai et al. (2011) have reported a PEGylated fourth generation PAMAM dendrimer as nanocarrier of the anticancer drug doxorubicin, encapsulated in the interior of PAMAM molecules; this study provides a potential application of this dendrimeric system for current brain cancer therapy.

Our previous studies using functionalized polyamide amine dendrimers were focalized on the analysis of the interactions between dendrimers and several biologically important guests (Santo and Fox, 1999). We analyzed the solubilization and release of different hydrophobic compounds by interaction with polyamidoamine and polypropylenimine dendrimers (Fernández et al., 2006). Moreover, we investigated a first generation of a new dendrimer as candidate for intravenous administration of an antichagasic compound. The results obtained show that guest-host specific interactions result in good drug solubilization and that they can be controlled by varying the solution pH, allowing drug delivery (Fernández et al., 2008).

In the present work, the effect of several PAMAM dendrimers on the aqueous solubility of the hydrophobic drug ABZ was studied. Full generation PAMAM dendrimers with amine terminal groups, G3, with hydroxyl terminal groups, G3OH, and half generation PAMAM dendrimers with carboxylate terminal groups, generation G2.5 and generation G3.5, were chosen for this study (Scheme 2). The influence of the different terminal groups on ABZdendrimer association was analyzed.

MATERIALS AND METHODS

Materials

PAMAM G3, G3OH, G2.5 and G3.5 dendrimers in methanol solution and ABZ were obtained from Sigma-Aldrich. The organic solvents (methanol, dimethylformamide, toluene, benzene, dimethyl sulfoxide) and water of HPLC quality were purchased from SINTORGAN and were used without further purification. The UV cut-off point of the solvents in a UV cell of 10 mm against air was used as the purity criterion.

Methods

UV visible spectroscopic measurements were performed using a Shimadzu U.V.2401PC spectrophotometer at 20.0 ± 0.2ºC. A Spex Fluoromax apparatus was employed for the fluorescence measurements at the same temperature. Dendrimer/ ABZ fluorescence spectra were taken using an excitation wavelength of λ=295nm, and the emission registered in the 300-500 nm range. NMR data were recorded using a Bruker 400 MHz Advance II. Semiempirical calculations were carried out using the HyperChem software, version 5.0, running on a Pentiun III personal computer.

EXPERIMENTAL

UV Absorption Spectroscopy

The electronic absorption spectra of ABZ were recorded in solvents of different polarity (methanol, dimethylformamide, toluene, benzene, dimethyl sulfoxide and water). The stock solution of ABZ was prepared by dissolving the solute in methanol at 1.0 x 10^-3 M. Appropriate aliquots of the solute stock solution were transferred into 5 mL volumetric flasks and the methanol evaporated off under a nitrogen atmosphere. The samples were then diluted to the final volume with the corresponding solvent to roughly 5.0 x 10^-5 M, except for water (less than 1.0x10^-6M), sonicated for 10 minutes, and stored at room temperature in darkness until further use.

Solubilization Enhancement of ABZ

Stock solutions of ABZ were prepared by dissolving the guest in methanol at 1.0 x 10^-3 M and stored in darkness. Appropriate aliquots of the solute stock solutions were transferred into 5 mL volumetric flasks and the solvent evaporated off under a nitrogen atmosphere. The samples were diluted to the appropriate volume with HPLC-grade water, sonicated for 10 minutes, and stored at room temperature in darkness. To perform a calibration graph, the absorbance corresponding to the λ_max, for solutions with different concentrations, was determined. Subsequently, a saturated solution of ABZ in HPLC-grade water was prepared and the absorbance corresponding to the λ_max for the saturated solution was determined to calculate the water solubility of ABZ.

Stock solutions of ABZ were prepared by dissolving the guest in methanol at 1.0 x 10^-3 M and stored in darkness. Then appropriate amounts of PAMAM dendrimers in methanol and ABZ stock solution were transferred into 5 mL volumetric flasks, the samples were diluted with 1 ml of methanol, sonicated for 20 min and allowed to equilibrate in darkness overnight. Afterwards, the solvent was evaporated off under nitrogen atmosphere and the samples were diluted to the appropriate volume with HPLC-grade water. A small amount of drug precipitated from solution and was removed via filtration through a 0.46 µm Millipore membrane. Samples for NMR analysis were prepared using deuterated solvent (D₂O).

RESULTS AND DISCUSSION

Solubility Enhancement of ABZ

The capacity of PAMAM dendrimers to enhance the solubility of ABZ was studied by spectroscopic methods for different ABZ-dendrimer systems in aqueous solution using full G3 and G3OH and half generation G2.5 and G3.5 PAMAM dendrimers.

UV Absorption Spectroscopy

The spectra of ABZ at different concentrations of aqueous G3 dendrimer are shown in Figure 1. The spectrum of ABZ in water is also included for reference. The G3 dendrimer concentration was varied from 5.2 x 10^-6 M to 1.1x10^-4M. ABZ was added in excess of its aqueous solubility limit, 2.4 x 10^-6M, so that the solubility enhancement could be observed. The analytical concentration of ABZ was 1.0x10^-4M, the real ABZ concentrations may be calculated depending on the extent of solubility enhancement in the presence of different dendrimer concentrations.

The observed increase in the absorbance, due to ABZ solubilization enhancement, is attributed to its association with the dendrimer. As expected, upon increasing the dendrimer concentration the solubilization increases (Figure 1). In addition, the ABZ absorption band, observed in pure water at 298.9 nm, shifts to lower wavelength with the increase in dendrimer concentration. To analyze the observed spectral shift, the electronic absorption spectra of ABZ were recorded in solvents of different polarity. The experimental λ_max shifted from 298.5 nm and 297.5 nm in polar solvents such as dimethyl sulfoxide and dimethylformamide, respectively, to 295.2 nm in nonpolar solvents like benzene and toluene. Thus, there is a blue shift with decreasing solvent polarity. Consequently the shift observed in the absorption band of ABZ with the increase of dendrimer concentration indicates a decrease in microenvironment polarity around ABZ. These results could indicate that ABZ solubility increases due to its association with lipophilic microenvironments defined within the dendrimeric structure. Similar results have been obtained using hydroxyl terminal dendrimers, G3OH.

The absorption spectra of ABZ in aqueous solutions of the PAMAM carboxylate terminal dendrimers G2.5 and G3.5 and the PAMAM hydroxyl terminal dendrimer G3OH also show that dendrimeric solutions improve drug solubility with respect to aqueous medium. The maximum solubility enhancement obtained for each dendrimer is listed in Table 1.

The study of different ABZ-dendrimer systems at equal concentration of the dendrimers indicated that the increase in solubility depends on both the size of the dendrimers and the type of end groups (Figure 2). In cationic (amine terminated) and neutral (hydroxyl terminated) dendrimers, the ABZ absorption band shifted to a wavelength lower than that observed in water, indicating that ABZ is surrounded by a lipophilic environment. The association may be due to the incorporation of ABZ in the hydrophobic microenvironment of dendrimers. In aqueous solutions of anionic half generation dendrimers, the ABZ band shifted to higher wavelength, indicating that ABZ is surrounded by a more polar environment. In these host-guest systems, ABZ may be preferentially located close to the terminal groups of the dendrimeric structures.

The increase in solubility was higher in G3 than in G2.5 and G3.5 PAMAM dendrimers. Dendrimermediated solubility enhancement depends mainly on factors such as generation size, dendrimer concentration, core, and terminal functionality (Gupta et al., 2006). In this system, we assume that the low encapsulation efficiency of half generation PAMAM dendrimers is a consequence of their anionic character.

Fluorescence Emission Spectroscopy

ABZ has a single broad fluorescence emission band in aqueous solution. Representative fluorescence spectra are shown in Figure 3. The fluorescence intensity with respect to pure water increases significantly with an augment in dendrimer concentration, indicating an enhancement in ABZ solubility.

As can be observed in Figure 3, the emission intensity of ABZ in G3 is lower than that observed for G2.5 and G3.5, although the solubility enhancement of ABZ is higher using G3 dendrimers. The low intensity observed for the emission of ABZ in G3 dendrimers can be explained by considering the quenching capacity of tertiary amines present at the branching points in PAMAM dendrimers where the guest should be localized (Wade et al., 1999).

This result reinforces the assumption that the association between ABZ and full generation dendrimers G3 and G3OH occurs preferably in the interior of the dendrimeric structure, where the emission intensity could be quenched by the tertiary amines. Moreover, the association of ABZ with half generation dendrimers (G2.5 and G3.5) occurs preferably in the proximity of the carboxylate surface groups. The terminal groups of G2.5 and G3.5 dendrimers are unable to quench ABZ emission, so the contribution to the emission intensity is greater.

¹H NMR Studies

NMR spectroscopy is a very useful technique to investigate intermolecular interactions in solutions because it gives information on the formation of aggregates, ion pairing, encapsulation, and size variations (Biosselier el al., 2008). The ¹H-NMR technique was employed here in order to analyze further the proposed molecular interaction between ABZ and dendrimers. Among the different dendrimers studied, G3 was chosen for the NMR study because of its high solubilization effect on ABZ.

Five broad peaks are observed in the ¹H-NMR spectrum of a G3 dendrimer in D₂O, corresponding to: CONHCH₂ indicated as H_a, CONHCH₂CH₂NH₂, Hb, NCH₂CH₂CONH, H_c, CH₂CH₂NR₂, H_d, CH₂CONH, H_e. Scheme 3 (δ ppm) H_a=3.11; H_b=2.65; Η_c=2.59; H_d=2.48; H_e= 2.29. The ¹H-NMR spectrum of ABZ in D₂O shows seven kinds of protons (δ ppm) at 1.19 (t,3H), 1.86 (sext, 2H), 3.30 (m,2H), 4.1 (s,3H), 7.92 (dd,1H), 8.01(d.1H), 8.32(d,1H).

The NMR study of the host-guest system was performed using similar drug and dendrimer concentration. However, the signal strength of the protons of ABZ is much lower than that observed for the different protons of dendrimers, as the contribution of the number of protons per signal is higher in the latter. Therefore, the ABZ signals do not interfere in the analysis of the signal shifts in the dendrimer NMR spectrum.

The ¹H-NMR spectra obtained are shown in Figure 4. Changes in the proton peaks of the dendrimer induced by the incorporation of ABZ in the aqueous dendrimeric solutions were observed. Band broadening and chemical shifts are good indicators of ABZ-dendrimer complex formation. When the guest is added to the dendrimer solution, the Hc signal shifts downfield from 2.59 to 2.68 ppm. This is probably due to ring current effects produced by the aromatic ring of ABZ when this compound associates with the internal groups of the dendrimer (Scheme 3). No shift is observed for protons near the end groups (H_b). These results indicate that there are interactions with the internal nanoenvironment of the dendrimers, but that the ABZ-G3 interactions with surface groups, if any, are very weak.

Possible Mechanism of Interaction Between PAMAM Dendrimers and ABZ

The structure of dendrimers depends on their generation. Lower generations of dendrimers have an asymmetric shape and open structure, whereas the higher generations posses a nearly globular shape and densely packed end groups. In our studies, we chose to investigate third generation dendrimers which are almost spherical but have substantial flexibility. Molecular simulation of the structures of the PAMAM dendrimers showed that lower generation ones (G<4) possess an open structure and are well below De Gennes dense-packing transitions and, consequently, are accessible for guest inclusion (Meredith and Grinstaff, 2011; Fréchet and Tomalia, 2001).

Several studies suggest that ionic interactions, hydrogen bonding and hydrophobic drug-dendrimer interactions are the mechanisms by which dendrimers exert their solubilizing effect (Gupta et al., 2006). PAMAM dendrimers have primary amines on the surface and amide and tertiary amines in their interior, which could act as hydrogen bond donors and acceptors. It is clear from its structure that ABZ also has hydrogen bond donor and acceptor groups. So the formation of an intramolecular hydrogen bond between the carbonyl of the carbamoyl moiety and the proton of the amine group on the aromatic ring of ABZ is possible.

The predominant conformers and charge density distributions were predicted from theoretical calculations performed by semi- empirical molecular orbital methods AM1 (Dewar et al., 1985). Calculations were performed starting from standard bond lengths and bond angles. All geometries were fully optimised by minimising the energy with respect to the geometrical variables without symmetry constraint, using a gradient of 0.01 kcal/mol and the Polak–Ribierie algorithm as convergence criteria. This semi-empirical method does not describe the solvating processes, but it may provide some information about the interaction sites and the bond distances and changes in the charge distribution of the different atoms present in the main structure.

To suggest a possible localization for the hydrogen-bond interactions, the calculated charge density distributions for different atoms of the active molecule were analyzed. (Scheme 4). A negative charge distribution is localized on the nitrogen of the aromatic ring, the carbonyl group of the carbamoyl moiety and the ester oxygen. All these groups are characterized as good hydrogen bond acceptors (Etter, 1990). Furthermore, a positive charge distribution localized on the proton of the amide group and the proton of the amine group on the aromatic ring shows that both protons could participate in hydrogen bonding interactions.

From the analysis of the possible conformers obtained, Scheme 4, we can conclude that both structures, with and without intramolecular hydrogen bonding, are possible and the contribution of the different structures will depend on the interactions of ABZ with its environment. It is possible that the intramolecular hydrogen bonding prevents stronger interaction with the environment. Moreover, the amide proton is a hydrogen bond donor site available in both conformations.

In summary, from these results it can be inferred that ABZ could participate in both inter- and intramolecular hydrogen bonding. Therefore, hydrogen bonding between ABZ and PAMAM dendrimers could contribute to facilitate the association in this host-guest system and be a possible mechanism contributing to the solubility enhancement.

In addition to hydrogen bonding, the solubilization properties of dendrimers could also be due to the hydrophobicity of their microenviroment. Previous studies have shown that the internal microenvironment in the dendrimeric structure is less polar than the bulk aqueous phase (Fernandez et al., 2006). The hydrophobic guest isolates itself from the outer interface of the host to afford minimum contact with polar and aqueous domains. The storage space of the dendrimers is controlled by geometrical parameters of the branch cell (branching angles, rotation angles and repeat-unit segment length) and by the shape and size of the available internal dendrimer microenvironment that influences the host-guest interactions (Tomalia el al., 2003). The maximum amount of entrapped guest molecules is proportional to the shape and size of the guest molecules. The molecular volume of ABZ (Scheme 4) allows accessibility into the dendrimer. Hence, in aqueous medium, highly hydrophobic ABZ could be solubilized within low polarity environments of the dendrimer. As observed in the spectroscopic studies, a decrease in the polarity of the environment causes a bathochromic shift in the absorption maximum of ABZ. The gradual blue shift of the ABZ band with the increase of dendrimer concentration with respect to pure water indicated a decrease of the microenvironment polarity around ABZ. Therefore, hydrophobic interactions help the incorporation of guests into the lipophilic interior of dendrimers and ABZ-dendrimer hydrogen bonding stabilizes the association of this host-guest system.

CONCLUSION

In the present work, we investigated different PAMAM dendrimers as solubility enhancers of ABZ. The results obtained show that these polymeric structures have the capacity to increase the solubility of poorly water soluble drugs such as ABZ. The association of the hydrophobic compound and the aliphatic dendrimer can be rationalized by the poor water solubility of the guest and by the affinity between ABZ and the lipophilic microenvironments of the dendrimeric structures. From our results, we can conclude that both lipophilic and specific hydrogen bond interactions contribute to the ABZ-dendrimer association, enhancing its solubility. The difference in solubility enhancement found with the different dendrimers studied could be attributed to the nature of the ABZ-dendrimer interactions, which depend on the identity of the surface functional groups.

ACKNOWLEDGEMENTS

Financial support from the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET-Argentina), Agencia Nacional de Promoción Científica y Tecnológica (FONCYT-Argentina), and the Secretaría de Ciencia y Técnica de la Universidad Nacional de Río Cuarto (SECYTUNRC) is gratefully acknowledged.

NOMENCLATURE

ABZ

Albendazole PAMAM Polyamidoamine dendrimer G2.5 Half generation 2.5 polyamidoamine dendrimer with carboxylate terminal groups G3 Full generation 3 polyamidoamine dendrimer with amine terminal groups G3.5 Half generation 3.5 polyamidoamine dendrimer with carboxylate terminal groups G3OH Full generation 3 polyamidoamine dendrimer with hydroxyl terminal groups ¹H-NMR Nuclear Magnetic Resonance of protons UV Ultraviolet-visible

(Submitted: November 3, 2010 ; Revised: June 16, 2011 ; Accepted: June 29, 2011)

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To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  24 Nov 2011
• Date of issue
  Dec 2011

History

• Accepted
  29 June 2011
• Reviewed
  16 June 2011
• Received
  03 Nov 2010