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Effect of Cyclodextrin Derivatization on Solubility and Efficacy of Drugs

Cyclodextrins (CDs) possess cyclic structure having (α-1,4)-linked glucopyranose units making them less vulnerable to enzymatic degradation as than the linear dextrins. Commonly used natural CDs are α-CD, β-CD, and ɣ-CD with truncated cone-like appearance having lipophilic central cavity and hydrophilic exterior surface. The problem of low aqueous solubility of natural CDs can be addressed by reacting them with various reagents to produce water-soluble derivatives. CD derivatives can be categorized in many ways depending upon their substituents, biological activity, polarity, and size. The derivatization of natural CDs produces noncrystalline and amorphous forms with higher water solubility that are physically and microbiologically stable for prolonged time period. Variety of methods can be used to determine average degree of substitution for a modified CD. Dissociation by dilution is considered as major release mechanism of drugs from complex. It is essential to optimize the amount of CDs for a given preparation because they can either retard or promote drug delivery through biological membrane.


Introduction
Cyclodextrins (CDs), first isolated by Antoine in 1891, are categorized as cyclic oligosaccharides in which glucose units are repeatedly connected through α-1,4glycosidic linkages. Their unique property is the availability of various hydroxyl groups that serve as active site to form a variety of derivatives and linkages. CDs possess the capability of forming inclusion complexes with various molecules by incorporating them in their inner hydrophobic cavity, which not only alters the biological and physicochemical properties but also expands their magnitude of application [1][2][3].
Inclusion complexation is basically the formation of hydrogen bonds and Van der Waals' and hydrophobic interactions along with removal of water molecules [4,5]. CDs are getting popular as food additives, owing to their solubilizing and protecting properties; they can be effectively used for incorporating vitamins, flavors, fragrances, essential oils, and dyes. They not only provide the controlled release of drugs and other incorporated molecules but also mask the obnoxious taste and odor [6][7][8][9].
The order of water solubility for commonly used cyclodextrins is as follows: β-CD (18.5 gL −1 ) < α-CD (145gL −1 ) < γ-CD (232 gL −1 ). The crystal state of natural CDs indicates strong molecular bonding (high crystal lattice energy) which in turn results in low aqueous solubility. β-CD has limited application as a solubilizing agent due to its low aqueous solubility, despite low cost, ease of availability, and appropriate cavity size. The most likely justification of lower solubility of β-CD is inadequate hydration by water molecule due to intramolecular hydrogen bond interaction offered by optimally arranged secondary hydroxyl groups [10]. Another possible elucidation of low aqueous solubility is the formation of aggregates that leads to unfavorable interactions with the hydrogen bonded structure of water molecule as proposed by Colman and coworkers [11]. β-CD shows Bs-type behavior in phase solubility graph when added to the aqueous drug solution or suspension due to precipitation of respective CD inclusion complex.
The solubilizing power of parent CDs can be enhanced by adopting many different strategies, but the most interesting of them is the derivatization of CDs. The hydroxyl groups of CDs can be substituted to yield a variety of derivatives with significantly high aqueous solubility [12][13][14]. This chapter discusses the various strategies to functionalize the CDs and the resultant improvement in the aqueous solubility of formed complexes.

Functionalized CDs
Parent CDs can be modified through structural modifications by incorporating hydrophilic moieties that will ultimately result in significant increase in aqueous solubility. Furthermore, the inclusion complexes formed by modified CDs have higher complexation efficiencies than that of parent CDs. Rekharsky and Inoue reported various examples of thermodynamic parameters of inclusion complexes involving derivatized CDs [5].
It is noteworthy that derivatization of CDs does not always enhance the complexation. The inhibition and promotion of complexation solely depends upon the type of substituents. Although the strong electrostatic force of attraction between the cationic substituents and organic anion was predicted to promote complexation, paradoxical effect was observed when the complexation of 2-naphthalene sulfonate was reduced with polyamine derivatives of β-CD. An unfavorable entropic effect may be involved that results in decrease in complexation [15]. On the other hand, a favorable electrostatic interaction occurs that leads to formation of zwitterionic corona by substituting both cationic and anionic groups at the primary face that will ultimately enhance the complexation of amino acids [16].
The interdependence of numerous molecular parameters including type of substituent at the CDs, contribution of hydrophobic, and charge character of the guest moiety and competitive self-complexation (possible inclusion complexation of substituent moiety inside the core of CD) was discussed in detail by Kean and his coworkers [17]. While dealing with ionic species, electrostatic effects are usually dominated which exert a paradoxical effect. Based on this status quo, the inclusion complexation of charged CD with organic ions must be evaluated on the basis of electrostatic interactions. The properties of modified CDs are governed by the location of hydroxyl groups on the parent CDs that are going to be substituted. Three different hydroxyl groups that exert different reactivities are located on the glucose repeating unit of a CD molecule, including one primary hydroxyl group attached to Effect of Cyclodextrin Derivatization on Solubility and Efficacy of Drugs DOI: http://dx.doi.org /10.5772/intechopen.90364 C6 (at the narrow side) and two secondary hydroxyl groups attached to C2 and C3 (at wider side).
Most common derivatives of CDs are discussed in this section.

Methyl derivatives of CDs
There is variety of ways to perform methylation of native cyclodextrins: Formerly methyl derivatives were synthesized by using either methyl iodide or dimethyl sulfate [18]. These two reagents are highly toxic and unsafe so may be detrimental to the environment as well as human being. Based on this status quo, it is mandatory to seek new and novel synthetic method to replace these toxic chemicals.
Dimethyl carbonate being eco-friendly could be an attractive alternate in this scenario [19]. The synthesis involves addition of β-CD in dimethylformamide solution followed by stirring until clear solution is obtained. Potassium carbonate is added as catalyst followed by dropwise addition of dimethyl carbonate, and mixture is allowed to stir for the next 48 hours. The catalyst and dimethyl carbonate are removed to produce syrupy consistency. At the end, this concentrate is treated with acetone followed by its removal by filtration. Finally, the product is treated with ether two to three times, and after filtration white powdered product is obtained as indicated in Figure 1.
An important feature of methyl β-CD is its degree of substitution that has marked effect on drug solubilization, so it must be carefully investigated [20]. Table 1 gives the use of methyl derivatives of β-CD with effect on solubility and efficacy of various drugs.
This pharmaceutically significant methylated derivative of β-CD has the following advantages: • Easy availability (as dry powder/aqueous solution 50%) • Good water solubility • Cost-effective • High inclusion capacity for hydrophobic drugs

Hydroxypropyl CDs
The replacement of hydroxyl groups of CD by 2-hydroxypropyl moiety can be done by the reaction of CD with propylene oxide in an alkaline aqueous solution (Figure 2a) [31]. An isopropylene (oligomeric hydroxypropylene) side chain is formed for high degree of substitution (Figure 2b). The characterization of finally synthesized hydroxypropyl derivatives involves determination of average number of substituents on the cyclodextrin molecule (degree of substitution). According to latest US and European pharmacopoeias, the acceptable range for DS is 2.8-10.5 for HPβCD. Degree of substitution can be measured by a variety of techniques including near-infrared reflectance spectroscopy, nuclear magnetic resonance (NMR), microcalorimetric titration [33], mass spectrometry (MS), differential scanning calorimetry (DSC) [34], and reductivecleavage and methylation analysis.
Encapsin™ and Molecusol™ are the trade names of commercially available form of hydroxyalkyl derivative (2-hydroxypropyl-β-CD). Various clinical trials have been performed with this derivative besides its use in technological, toxicological, and pharmaceutical experiments [35]. Being the most thoroughly studied derivative, FDA has approved Sporanox™ by Janssen using the same derivatives as molecular carrier. The hydroxypropyl derivative of β-CD and ɣ-CD have been widely used for solubility enhancement and leading to increase in efficacy of various drugs as illustrated in Tables 2 and 3.

Sulfoalkylated CDs
Almost more than 180 articles had focused on the preparation and use of charged (anionic) CD derivatives by the end of July 1998.The glucopyranose unit present in the native CD ring could be directly substituted with charged moiety, or a neutral spacer group may be used for the insertion [49]. Such functional groups can be inserted at different degrees of substitution (DS) and have variable sizes, so the final product of modified CD derivative may be influenced by electronic and steric factors.
The sulfopropyl and sulfobutyl derivatives of beta-CDs are produced by reacting native CD with propane sultone and butane sultone, respectively, in an alkaline aqueous solution as shown in Figure 3. Sulfobutylated β-CD with seven substituents is considered as suitable derivative and is commercially available under the trade name of Captisol™ by CyDex. Resveratrol is complexed with sulfobutylether derivatives of β-CD and increased the anticancer activity with increase in solubility of resveratrol as well [51].

Sulfated CDs
A class of water-soluble CD derivatives having anti-angiogenic, biological, and anticancer properties is sulfated CDs. As the tumor growth is dependent on angiogenesis, the use of these derivatives could be a unique approach for the treatment of solid tumors, as reported by Folkman et al. in early 1970 [52]. In addition to these properties, they also possess antilipemic, antiviral, and anti-inflammatory effects [53].
Reaction of CD in absolute dimethylformamide with pyridine sulfur trioxide gives better yield of sulfated derivatives of CDs [54]. Besides their use as solubility enhancers, these derivatives also provide protection against gentamicin-induced nephrotoxicity and have no hemolytic properties, so can be effectively used in clinical studies [55]. Figure 4 presents the scheme of preparation of sulfated β-CD.  Table 3.
Effects of HPɣCD on solubility and efficacy of drugs.

Guidelines regarding synthesis of CD derivatives
The following guidelines should be strictly followed during the synthesis of CD derivatives in order to get the better solubility and complexing ability: • Although the substituents at primary hydroxyl side may influence other uses, they do not exert much effect on the complexation. The pH of the medium is decisive that either the secondary hydroxyl side or primary side will be substituted. Moderately basic medium will favor secondary hydroxyl substitution, whereas primary hydroxyl side will get substituted in strong basic medium [57].
• To avoid the deformation of cavity shape, avoid the bulky substitution which may crowd each other. Although the CD torus is made of anhydroglucose repeating units which are quite rigid, they are connected to each other through single glycosidic bonds. As torus is stabilized by hydrogen bonds between the secondary hydroxyls on adjacent glucose unit, the stabilizing effect of hydrogen bonding is diminished by secondary hydroxyl substitution. Finally, the anhydroglucose units may tilt out the defined torus shape due to introduction of steric strain between substituting units present on the adjacent anhydroglucose residues [35].
• The water-soluble CDs are used either in solid form or in concentrated solution state. Some substituting groups have the tendency to incorporate in the core of CDs, so they compete with active drug molecules for inclusion complexation. Therefore, it is mandatory to select those substituents that are unable to fit in the cavity of CDs [58].
• A glucopyranose unit of cyclodextrin ring contains how many substituted hydroxyl groups are defined as its degree of substitution. One mole of glucopyranose unit contains three reactive hydroxyl groups so the maximum possible numbers of substituents are 18, 21, and 24 for α-, β-, and γ-CDs, respectively. Controlling the degree of substitution is important in producing the desired properties in functionalized CDs.

Conclusion
Using different functional groups, modified CDs could play a pivotal role in improving limited drug stability and boosting aqueous solubility and dissolution behavior of drugs with poor water solubility. In order to use full potential of CDs as