Exploring versatile applications of cyclodextrins: an overview

Abstract

Context: Ever since the discovery of cyclodextrins, a family of cyclic oligosaccharides based on α (1 → 4) linkage among glucopyranose subunits, these versatile supramolecular hosts have received tremendous attention for scientific explorations. Due to their property of forming host–guest type inclusion complex, cyclodextrins and their synthetic derivatives exhibit wide range of utilities in different areas viz. pharmaceuticals, drug delivery systems, cosmetics, food and nutrition, textile and chemical industry etc.

Objective: The purpose of this review is to highlight properties, advantages, recent studies and versatile benefits of cyclodextrins and to re-strengthen their prospective applications in novel directions for future research.

Methods: This article summarizes a variety of applications of cyclodextrins in various industrial products, technologies, analytical and chemical processes and recent industrial advancements by extensively literature search on various scientific databases, Google and websites of various associated pharmaceutical industries and patenting authorities across the world.

Results and conclusion: Due to possibility of multidimensional changes in physical and chemical properties of molecules upon inclusion complexation in cyclodextrins, these compounds are of great commercial interest and may offer solution to many of the scientific problems of the current world.

• Bioavailability
• drug delivery
• inclusion complexes
• pharmaceuticals
• solubility

Introduction

Cyclodextrins (CDs) are defined as the class of cyclic oligosaccharides which contains glucopyranose units and which are linked to each other by α-(1,4) bonds (Del Valle, 2004). Three naturally occurring cyclodextrins, also known as first generation CDs or parent CDs (Singh et al., 2002), are: α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD), containing 6, 7 and 8 glucopyranose units, respectively.

When starch is reacted with amylase, which is obtained from Bacillus macerans, a crude mixture of α-CD (∼60%), β-CD (∼20%) and γ-CD (∼20%) together is formed with some traces of CDs containing more than eight glucose units. The purification of this mixture is very difficult because it contains very less amounts of CDs and it also have other linear and branched dextrins along with proteins and other impurities (Chordiya & Senthilkumaran, 2012). Various chronological in development of CDs are presented in Table 1. Finally, in the 1970s, several biotechnological advances led to major improvements in the production of pure α-CD, β-CD and γ-CD. Cyclodextrin glycosyl transferase (CGTase) enzyme was used for the production of highly pure α-CD, β-CD and γ-CDs (Loftsson & Duchene, 2007). Production of parent CDs and some of their commercially important derivatives are represented in Figure 1.

Figure 1. Parent and representative modified cyclodextrins of pharmaceutical interest [RAMEB: Randomly methylated β-cyclodextrin; HP-β-CD: Hydroxy propyl β-cyclodextrin; HE-β-CD: Hydroxy ethyl β-cyclodextrin; DIME-β-CD: Heptakis (2, 6-dimethyl)-β-cyclodextrin; TRIME-β-CD: Heptakis (2, 3, 6-trimethyl)-β-cyclodextrin].

Table 1. Chronological in development of cyclodextrins.

Year	Scientists	Advancements	References
1891	A. Villers	Crystalline substance isolated from bacterial digest of starch having resistance for acid hydrolysis and deficient in reducing property. Substance was found out to be dextrin and was termed as “cellulosine” by Villers.	Loftsson & Duchene (2007)
1903	Franz Schardinger	Fractionation of two more crystalline compounds, α-dextrin and β-dextrin, from bacterial digest of potato starch. Further, isolation of considerably more amount of β-dextrin, and recognized as Villers “cellulosine” was achieved. Later, these were better known as α-CD and β-CD.	Nitalikar et al. (2012)
1935	Freudenberg and Jacobi	γ-CD was discovered.	Nitalikar et al. (2012)
1935–1955	Freudenberg, Cramer and co-workers	Identification of the chemical structure of CDs, their physiochemical properties and their liability to form inclusion complexes.	Brewster & Loftsson (2007)
1938	Freudenberg and Cramer	CDs possess a ring structure containing glucose units, which are α (1 → 4)-linked and have a central cavity.	Loftsson & Duchene (2007)
1976		Prostaglandin E2/β-CD (Prostarmon E™ sublingual tablets), is the world’s first testified CD based pharmaceutical product sold in Japan.	Loftsson & Brewster (2010)
1997		The first US approved product was announced named as Itraconazole/2-hydroxypropyl-βCD oral solution (Sporanox®).	Kumar Das et al. (2013)
1998		The first European CD containing product Piroxicam/β-CD (Brexin® tablets) was introduced.	Brewster & Loftsson (2007)

Structure and properties of CDs

CDs, white crystalline powder, chemically are α-(1,4)-linked cyclic oligosaccharides containing glucopyranose units. The glucopyranose units possess chair conformation, due to which CDs are not cylindrical, and exhibit torus-like shape (truncated cone), in which the primary hydroxyl groups are present on narrow edge and secondary hydroxyl groups are on the wider edge of the cone in the outer surface. In the interior of the CD molecule, skeletal carbons with hydrogen atoms and oxygen bridges are present leading to hydrophilic outer surface and lipophilic central cavity of CD (Cal & Centkowska, 2008; Figure 2). Some of the structural and chemical properties of parent CDs are given in Table 2.

Figure 2. Torus-like shape of cyclodextrin.

Table 2. Structural and chemical properties of parent cyclodextrins.

Features	α-CD	β-CD	γ-CD	References
Glucopyranose units	6	7	8	Szejtli (2004)
Molecular weight (g/mol)	972	1135	1297	Singh et al. (2010)
Height (nm)	0.78	0.78	0.78	Loftsson & Brewster (2010)
Cavity diameter (nm)	0.50	0.62	0.80	Loftsson & Brewster (2010)
Outer diameter of cavity (nm)	1.46	1.54	1.75	Loftsson & Brewster (2010)
Cavity volume (Å^3)	174	262	427	Cal & Centkowska (2008)
Aqueous solubility at 25 °C (mg/ml)	129.5 ± 0.7	18.4 ± 0.2	249.2 ± 0.2	Loftsson & Brewster (2010)
Aqueous solubility at 25 °C (%w/v)	14.5	1.85	23.2	Del Valle (2004)

Cyclodextrin derivatives

Although, the natural CDs and their complexes are hydrophilic, but their aqueous solubility is reported to be limited, mainly in the case of β-CD. This is because CD molecules binds relatively strongly in their crystal state (Loftsson & Brewster, 2010). CDs contain glucopyranose units which further consists of three free hydroxyl groups differing on the basis of both functions as well as reactivity. The different reaction conditions like pH, temperature and reagents affect the relative reactivity of C-2 and C-3 secondary and the C-6 primary hydroxyls of the CD molecule. With the substitution of hydrogen and hydroxyl group with large number of substituting groups, such as alkyl, hydroxyl alkyl, carboxy alkyl, amino, thio, tosyl, glucosyl, maltosyl etc., and thousands of ethers, esters, anhydro, deoxy, acidic and basic etc., 21 hydroxyl groups of β-CD can be modified to obtain different CD derivatives by chemical and enzymatic reactions.

Such derivatizations are done to accomplish the following objectives:

• to enhance the solubility of different CD derivatives and their complexes;

• to improve the fitting and/or the association between the CD and its guest, with concomitant stabilization of the guest, reducing its reactivity and mobility;

• to achieve insoluble, immobilized CD-containing structures and polymers, e.g. for chromatographic purposes (Szejtli, 2004).

CD derivatives are mainly categorized as: (a) chemically modified CD derivatives and (b) natural enzymatically modified CD derivatives.

Enzymatically modified CD derivatives are formed with the help of pullulanase enzyme and are known as branched CDs. When the hydroxyls of β-CD are made to react with a chemical reagent, the substitution occurs and a heterogeneous product is generated (Szente & Szejtli, 1999). The extent to which substitution occurs can be defined and expressed in different ways as follows:

• Degree of substitution: It is defined as the number of the hydroxyl groups in a glucose moiety of CD molecule that has been substituted. Therefore, it can be ranged from 1 to 3. The average degree of substitution (DS) is the average number of hydroxyl groups being substituted in a glucose unit of CD. Its value can vary from 0 to 3 (Challa et al., 2005).

• Molar degree of substitution: It is the critical parameter, which is mainly defined as the average number of substituent’s that completely reacts with one glucopyranose repeat unit. If the molar degree of substitution (MS) value is equal to 0, then no substitution or up to 3, when two or more substituents react and form oligomeric or polymeric side chains (Loftsson & Brewster, 1996).

CD derivatives like hydroxypropyl derivatives of β and γ-CD (HP-β-CD and HP-γ-CD), randomly methylated β-CD (RM-β-CD), sulfobutylether β-CD sodium salt (SBE-β-CD) and the branched CDs, such as maltosyl-β-CD (M-β-CD) are of great pharmaceutical interest (Ogawa et al., 2012).

Formation of inclusion complexes

One of the most remarkable features of CDs is their tendency to form solid inclusion complexes with a wide variety of solid, liquid and gaseous compounds by the process of molecular complexation. The complexes formed are called host–guest type of complexes. Such type of complexes involves a guest molecule (drug) which is properly fitted into the lipophilic cavity of host (CD molecule; Del Valle, 2004). There is no breakage or formation of covalent bond during the formation of inclusion complex (Loftsson et al., 2005a,b). When the CD is added to an aqueous solution, the polar water molecules get fit into the lipophilic cavity of CD, but these are immediately replaced by the more favored guest molecule, which is less polar than replaced water molecules (Szejtli, 2004). The main driving force behind this complex formation is the replacement of high enthalpy water molecules with the guest moiety, Vander Waals interaction, hydrogen bonding and charge transfer interaction (Loftsson et al., 2005b). The ratio in which host and guest bind together is generally 1:1 (Loftsson et al., 2005a). The binding strength of the formed complex depends on the ability of the guest molecule to bind well with the host to form a stable complex. The formation of an inclusion complex depends upon following factors:

• Size of the CD to the size of the guest molecule or some main functional groups within the guest moiety. If the guest is of inappropriate size, it will not fit properly into the host cavity.

• Thermodynamic interactions between the various components of the system (CD, guest, solvent; Del Valle, 2004).

• Structure of added substituent to the CD derivative.

• Location of substituent within the CD molecule.

• Number of substituent per CD molecule.

Different techniques used for complex formation include physical mixing, kneading method, spray drying, lyophilization, co-precipitation, solvent evaporation method, neutralization precipitation method, milling method and supercritical anti-solvent technique (Patil et al., 2010).

Applications of cyclodextrins

Due to unique property of forming inclusion complexes with diverse molecules, CDs offers versatile benefits in many industries. Applications of CDs are summarized in Figure 3.

Figure 3. Versatile applications of cyclodextrins.

Modifications of existing drug molecules for improved efficacy

A new formulating agent serving dual purpose, i.e. improving the physical properties of new active pharmaceutical ingredient and reformulation of existing drugs is the need of hour in the pharmaceutical industries. CDs have been proven to be better than the existing standard formulating agents. The complexes formed between the active ingredients and CDs show improved stability, solubility and bioavailability with very few adverse effects. CDs are the agents which act as a drug delivery system and also have potential drug delivery profiles in many applications. These properties of CDs are due to the fact that CDs have tendency to modify the physical, chemical and biological properties of guest molecules by formation of inclusion complexes. Due to bio-adaptability and multi-functional property of CDs, there is improvement in the unwanted properties of drug molecules in various routes of administration, including oral, rectal, nasal, ocular, transdermal and dermal (Singh et al., 2002). Hence, CDs play a very significant role in the field of pharmaceuticals. The process of inclusion complex formation between the drug and CD and subsequent absorption of drug are depicted in Figure 4.

Figure 4. Schematic representation of cyclodextrins–drug complexation, subsequent release and absorption of drug.

Stability enhancement

CDs play an important role in the chemical stability of drugs. The following properties are concluded after studying the interaction between CDs and labile compounds: (a) CDs retard degradation, (b) CDs have no effect on reactivity or (c) CDs can accelerate drug degradation (Loftsson & Brewster, 1996).

CDs are widely known to speed up or slow down the different types of reactions such as hydrolysis, oxidation, photolysis, dehydration and isomerization, based on the characterization of the inclusion complex in aqueous and buffer solutions. This can be well understood by the given examples – in Vaseline and o/w emulsion, the stability of tixoxortol 17-butyrate 21-propionate – a dermocorticoid, was preserved by forming its inclusion complex with β-CD after storing it for 30 days at 40 °C (Matsuda & Arima, 1999). In addition, CD derivatives have been proven to better than the parent CDs in showing the stabilizing effects. Derivative of β-CD, O-carboxymethyl-O-ethyl-beta-CD (CME-β-CD) considerably maintained the stability of prostaglandin E (PGE) in an ointment containing fatty alcohol/propylene glycol (FAPG) and in aqueous solution, while the parent β-CD promoted its degradation in neutral and alkaline solutions (Matsuda & Arima, 1999).

Solubility and bioavailability improvement

CDs can enhance apparent water solubility by forming dynamic, non-covalent, water-soluble inclusion complexes (Davis & Brewster, 2004) as shown in Figure 5. Enhancement of solubility of the drug further helps in the better bioavailability and hence more therapeutic efficacy of the dosage form. Table 3 shows various reports on solubility enhancement of therapeutically important pharmaceutics by CD complexation.

Figure 5. Process of inclusion complex formation of drug and cyclodextrin.

Table 3. Various reports on solubility enhancement of therapeutically important pharmaceuticals by CD complexation.

Drugs	pKa	Log P	Water solubility	Enhancement in solubility	Complexation method	C:D	CD type	Binding constant (M^−1)	References
Aceclofenac	4.7	2.17	0.058 µg/ml	–	KN	1:1	HPβCD	221.11	Kasliwal & Negi (2011)
Acyclovir	2.27	−1.56	1.62 mg/ml	–	KN	1:1, 2:1, 3:1, 4:1	HPβCD, βCD	–	Sachan et al. (2010)
Amisulpride	9.37	1.10	16.89 µg/ml	3.74 times	KN	1:1	γCD	1166.65	Negi & Singh (2013)
Artemether	4.8	3.02	–	1.8 times	FD	1:1	HPβCD	220	Yang et al. (2009)
Albendazole	10.26	2.7	1.37 µg/ml	53.4 times	PM	1:1	βCD	1266	Moriwaki et al. (2008)
Atorvastatin	11.82	5.7	20.4 µg/ml	–	FD, PM, KN, CE	1:1	βCD	–	Palem et al. (2009)
Amiodarone	8.47	7.57	0.2–0.5 mg/ml	–	FD, SD, CE, PM	1:1	βCD	1957	Riekes et al. (2010)
Cefdinir	3.2	−0.2	0.46 mg/ml	101% βCD, 23.4% HPβCD	KN	1:1	HPβCD, βCD	–	Aleem et al. (2008)
Darifenacin	9.2	4.5	2.98 mg/ml	8.24–20.57 mg/ml	CE	1:1	HPβCD	465.30	Jagdale et al. (2012)
Daidzein	7.39	2.51	0.15 mg/ml	5.7-fold (βCD),7.2-fold (MβCD), 9.4-fold (HPβCD)	FD	1:1	HPβCD, βCD, MβCD	–	Borghetti et al. (2011)
Etodolac	4.65	2.5	16 µg/ml	–	FD, PM	–	HPβCD	–	Cappello et al. (2009)
Fexofenadin	13.2	5.6	3.6 mg/ml	–	–	1:1	βCD, HPβCD	1139, 406	Al Omari et al. (2007)
Gliclazide	5.98	2.6	55 mg/ml	20.31 times	KN	2:1	βCD	691.45	Ozkan et al. (2000)
Isoquercitin		2.92	5.6 mg/ml	12.3 times	–	1:1	DMβCD	783	Wang et al. (2009)
Itraconazole	3.7	5.66	∼1 ng/ml	12 times	–	2:1	HPβCD	–	Miyake et al. (1999)
Josamycin	13.5	3.47	0.35 mg/ml	13.14 times	CP	1:1	γCD	3060	El Harti et al. (2012)
Meloxicam	4.08	1.9	8 µg/ml	–	–	1:1	βCD	–	Awasthi et al. (2011)
Naringenin	9.46	2.47	4.38 µg/ml	1272.31 µg/ml	–	1:16	HPβCD	–	Wen et al. (2010)
Omeprazole	4.77	2.23	82.3 µg/ml	–	FD, SD	1:1	MβCD	77.4	Figueiras et al. (2007)
Piroxicam	6.3	3	23 µg/ml	–	FD	2:1	βCD	–	Jug et al. (2005)
Repaglinide	4.01	3.97	34 µg/ml	–	–	1:1	HPβCD, βCD, RMβCD	377.4	Nicolescu et al. (2010)
Rapamycin	13.37	4.3	2.6 µg/ml	61 times	FD	1:1	MβCD	579	Rouf et al. (2011)
Rofecoxib	19.7	3.2	–	–	KN	1:1	βCD	769	Rawat & Jain (2003)
Rifampicin	8.25	2.7	1.4 mg/ml	22 times	–	1:1	RMβCD	73.4	Tewes et al. (2008)
Rutin	8.45	0.15	0.125 mg/ml	–	–	1:1	HPβCD	405.3	Miyake et al. (2000)
Raloxifene HCl	9.55	5.2	0.25 µg/ml	–	KN	1:1	HPβCD	4.59	Wempe et al. (2008)
Telmisartan	4.45	7.7	0.09 µg/ml	–	KN	2:1	βCD, HPβCD	644.56	Kane & Kuchekar (2010)
Vanillin	7.78	1.21	10 mg/ml	–	FD	1:1	βCD	5.3	Karathanos et al. (2007)
Valdecoxib	9.8	3.2	10 µg/ml	–	KN	2:1	HPβCD	1.63	Rajendrakumar et al. (2005)
Sulfamethoxazole	5.8	0.7	610 µg/ml	–	CP	1:1	βCD	122.3	Ozdemir & Erkin (2012)

KN, kneading; FD, freeze drying; PM, physical mixing; CE, co-evaporation; SD, spray drying; CP, co-precipitation.

Safety and efficacy betterment

CDs are widely used to alleviate the irritation caused by drugs. The enhanced solubility of drug–CD inclusion complex results in better drug efficacy and potency, which further reduces the drug toxicity since the drug becomes effective at minimum doses. The soluble CD–drug inclusion complex also helps in lowering down the toxicities caused due to the crystallization of poorly water-soluble drugs in case of parenteral formulations. Moreover, drug entrapment in CD cavity at the molecular level protects the biological membranes from direct contact of the drug, which reduces the side effects associated with the drug as well as the local irritation without affecting the therapeutic efficacy of the drug (Rasheed et al., 2008).

Drug delivery through biological membranes

The CD permeability through the biological membranes is affected by various factors like its molecular weight, chemical structure and very low octanol/water partition coefficient (log P values) resulting in its inability to cross the biological membranes quickly. The free form of drug, which is not bound to the complex and is in equilibrium with the drug–CD complexes, has the tendency of penetrating the lipophilic membranes. CDs have no role in enhancing the permeability of hydrophilic water-soluble drugs through lipophilic biological membranes. Whether CD increases or decreases the drug delivery across the biological membrane will depend upon the factors like the physicochemical properties of the drug such as its aqueous solubility, the composition of the drug formulation, i.e. aqueous or non-aqueous and physiological composition of the membrane barrier like the presence of an aqueous diffusion layer (Rasheed et al., 2008). The drug delivery through aqueous diffusion-controlled barriers can be increased by the use of CDs, but it restricts the drug delivery through lipophilic membrane-controlled barriers. Still, there is an exception to it, i.e. hydrophobic CDs (e.g. methylated β-CDs) can easily pass through mucosa and helps to improve the drug delivery through biological membranes, like the nasal mucosa, by decreasing the barrier property of the membranes (Loftsson et al., 2005b).

CD-based drug delivery approaches

Drug delivery through oral route

Among a number of routes available for the drug delivery, oral route has always been the most preferred route for developing a drug delivery system. The release of the drug in oral drug delivery system can be either dissolution controlled, diffusion controlled, osmotically controlled, density controlled or pH controlled. The role of CDs in oral delivery system is as an excipient to carry the drugs across an aqueous phase to the lipophilic absorption surface in the gastrointestinal tract, i.e. forming the inclusion complexes with CDs helps in enhancing the dissolution rate of drugs with low aqueous solubility. Hydrophilic CDs are mostly used for this purpose. In the case of sublingual and buccal routes, the drug–CD complexes with fast dissolution profiles are prepared. A quick rise of drug concentration in the systemic circulation is achieved in sublingual and buccal administration since the issue of hepatic first pass metabolism is resolved via this route (Tiwari et al., 2010). The bioavailability of the drug is increased by CDs. This happens because of improved water solubility and drug dissolution rate due to drug–CD complexation. Moreover, they also play an important role as conventional penetration enhancers. However, there is a difference between administration of CD containing formulation via sublingual and oral route. For the drug to show its therapeutic action, it must be released from the inclusion complex before it gets absorbed. This is a bit difficult for sublingual application because a very small amount of aqueous saliva is available for the drug to get dissolve completely and also relatively short residence time. The time for the drug to release from the complex after administration is less since the dissolved drug is removed from the buccal region within few minutes (Rasheed et al., 2008).

CDs also help in achieving sustained or site-specific drug release. This can be explained by taking an example of hydrophobic (i.e. lipophilic) CDs like ethylated β-CD and triacetyl-β-CD which are used to get sustained drug release. In addition to this, CDs have also been used in osmotic pumps for controlled drug delivery and in matrix tablets to control solubility-dependent drug release (Okimoto et al., 1998). Drug–CD conjugates have also been used to obtain colon-specific drug delivery or sustained plasma levels (Loftsson et al., 2004). Many researchers have carried out studies recently to improve oral drug delivery of various drugs, which has been summarized in Table 4.

Table 4. Use of cyclodextrin in promoting oral drug delivery.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Furosemide (FS)	β-CD	β-CD complexes with the solid forms of FS.	Suitability for development of drug delivery system.	Garnero et al. (2014)
Econazole nitrate (ECN)	SBE-β-CD, HP-β-CD	Ternary complexes using β-CD derivatives and compounds (l-AAs, CA, hydrophilic polymers) for synergistic effect.	Effective in better treatment of oral candidosis.	Jug et al. (2014)
Tacrolimus (TL)	β-CD	Pluronic F127-modified liposome-containing CD inclusion complex.	Improved drug solubility and intestinal mucous membrane penetration.	Zhu et al. (2013)
Docetaxel (DTX)	SBE-β-CD	DTX–chitosan nanoparticles having CDs complexes.	Better candidates for oral administration in comparison to existing DTX formulations.	Wu et al. (2013)
Dihydroartemisinin (DHA)	HP-β-CD	Inclusion complexes	Improved pharmacokinetic parameters along with better oral delivery.	Ansari et al. (2011)
Praziquantel (PZQ)	α-, β-, γ-CD	Inclusion complexes	Enhanced solubility, bioavailability through oral route.	Becket et al. (1999)

Drug delivery through rectal route

One of the important applications of CDs is optimizing rectal delivery of drugs meant for systemic use. The action of CD on rectal drug delivery clearly depends upon following factors:

• Nature of vehicle used, i.e. hydrophilic or oleaginous.

• Physicochemical properties of inclusion complexes formed.

• Existence of tertiary excipients like viscous polymer, etc.

In the case of rectal absorption of lipophilic drugs, the improving effects of CDs are commonly dependent on the improved release of vehicles used and the dissolution rates of the drug in rectal fluids, while in the case of rectal delivery of poorly absorbed drugs such as antibiotics, peptides and proteins, the action of CD depends on its direct action on the rectal epithelial cells. Furthermore, the continuous effects of CDs on the levels of drug in blood is due to the sustained release from the vehicles, slower rates of dissolution of drug in the rectal fluid or the obstruction in the rectal absorption of drugs due complex formation having very poor absorption (Matsuda & Arima, 1999).

Normally, hydrophilic CDs help in improving the release of drugs with very less water solubility from oleaginous suppository bases because of the minimum interaction of the resulting complexes with the vehicles. The lipophilic drugs become insoluble in hydrophobic vehicles on complexation with hydrophilic CDs, although the complex exists in the form of well-dispersed fine particles in the vehicles. This step serves the dual purpose as it improves the drug dissolution at an interface between the molten bases and the surrounding fluid as well as hinders the reverse diffusion of the drugs back into the vehicles (Rasheed et al., 2008). Table 5 illustrates representative work done in enhancing the rectal drug delivery by using CDs.

Table 5. Representative work done in enhancing the rectal drug delivery using cyclodextrins.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Diazepam and Flubiprofen	TM-β-CD	Inclusion complexes	Better drug release and permeability.	Lakshmi et al. (2012)
Ethyl 4-biphenylylacetate (EBA)	β-CD, HP-β-CD, DM-β-CD	Inclusion complexes	Better stability, release rate and hence improved rectal absorption.	Arima et al. (1992)
Morphine	α-CD	Pharmacokinetic studies of morphine and its glucuronides in rabbit plasma upon rectal administration from hollow type suppositories with xanthan gum.	Improved rectal bioavailability of morphine.	Kondo et al. (1996)

Drug delivery through nasal route

Nasal drug delivery has become an appealing approach for the purpose of systemic delivery of highly potent drugs, which have the issue of low oral bioavailability because of the huge amount of gastrointestinal breakdown and excessive hepatic first-pass metabolism. CDs help in enhancing the nasal absorption of the drugs like peptide and proteins, which have less nasal bioavailability due to their large size and hydrophilic nature, making their transport across the nasal mucosa tough. CD enhances their water solubility by increasing their nasal absorption (Merkus et al., 1999). The hydrophobic CDs also have a role as penetration enhancers, particularly in the nasal delivery of peptides. Very less local toxicity of CDs after nasal administration has been reported (Rasheed et al., 2008).

CD also serves as a pharmaceutical excipient and acts as a solubilizer and absorption promoter in nasal drug delivery. It is very effective even in a very low concentration and also is inert from a pharmacological–toxicological point of view (Chordiya & Senthilkumaran, 2012). Some representative studies, in which CD has played a major role in the drug delivery via nasal route, are given in Table 6.

Table 6. Role of cyclodextrins in nasal drug delivery.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Dihydroergotamine (DHE)	RM-β-CD	Development of nasal formulation containing inclusion complex.	Better stability with enhanced concentration and improved nasal drug delivery.	Rasheed et al. (2008)
Estradiol (ES)	DM-β-CD	Inclusion complex formation.	Better absorption hence enhanced bioavailability.	Ali et al. (2012)
Melatonin (MT)	HP-β-CD, RM-β-CD	Inclusion complexes.	HP-β-CD as well as RM-β-CD increased the nasal permeability of MT.	Babu et al. (2008)
Pirodavir	HP-β-CD	Formulation development for nasal delivery	Improved efficacy.	Rasheed et al. (2008)

Drug delivery through ophthalmic route

The basic remedy to treat the ocular diseases is mainly the drug application topically in the form of aqueous eye drop solutions. Recent reports have proved CDs as useful additives in ophthalmic formulations as they increase the water solubility, aqueous stability and bioavailability of ophthalmic drugs, and helps in decreasing the drug irritation (Loftsson & Jarvinen, 1999). The hydrophilic CDs, primarily 2-HP-β-CD and SB-β-CD, have been reported to be nontoxic to the eye and also are well tolerated in aqueous eye drop formulations (Loftsson & Stefansson, 2002). Hydrophilic CDs do not have access to the tight biological barriers such as the eye cornea, but increase the ocular bioavailability of lipophilic drugs by keeping the drugs in solution and increasing their availability at the surface of the corneal barrier (Rasheed et al., 2008). Very less amount (i.e. <5%) of the drug, which is delivered topically is absorbed into the eye. By the means of CD, solubilization increase in the dose-to-solubility ratio is feasible thereby, facilitating the application of the drugs topically, which were earlier delivered only by systemic routes. CD solubilization of the drug results in the increase in the amount of dissolved drug at the lipophilic membrane surface, but at the same time, if excess of the CD is used, then it will lead to the lowering of the ability of the drug molecules to penetrate into the lipophilic barrier. As a result of this, drug delivery through the cornea is decreased (Loftsson et al., 2005b). Some of the recent studies conducted for improvement in drug delivery through ophthalmic route by applying CD–drug complexes are listed in Table 7.

Table 7. Application of cyclodextrins in ophthalmic drug delivery.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Dexamethasone	CDs	Inclusion complex.	More patient compliance in comparison to un-complexed drug-based eye drops.	Loftsson & Stefansson (2002)
Econazole	SBE-β-CD	Chitosan nanoparticles containing complex CD were prepared and monitored.	Chitosan/SBE-β-CD nanoparticles developed had the tendency to act as a carrier for controlled delivery of drug to eyes.	Mahmoud et al. (2011)
Methazolamide	2-HP-β-CD	Effect on intra ocular pressure estimated in a double-blind randomized trial in humans.	Better lowering of intraocular pressure due to CD complexation.	Gudmundsdottir et al. (2000)
Zinc diethyldithiocarbamate (Zn-DDC)	HP-β-CD	Formulation optimization and anti-cataract effects of aqueous eye drop having high concentration of Zn-DDC.	Drug solubility and permeability were increased through the rabbit cornea.	Wang et al. (2004)

Drug delivery through dermal route

There is a remarkable safety margin of CD in dermal and transdermal application of drugs meant either for local or systemic use. They have a significant role in improving the solubility as well as stability of drugs in the topical formulations, augmenting the transdermal absorption of drugs, helps in the sustain release of the drug from the vehicle and thus, circumventing the unwanted after effects related with dermally applied drugs. Stratum corneum is the outermost layer of the skin and is the principal hurdle for the absorption of drugs given dermally (Rasheed et al., 2008). To reduce its barrier properties, penetration enhancers such as alcohols, fatty acids etc. are used. Due to hydrophobic characteristics, CD-complexes are able to deliver the drug through aqueous diffusion layer only. These complexes cannot penetrate the drug through lipophilic barriers like stratum corneum. Hence, an appropriate aqueous vehicle must be selected for aqueous vehicle must be selected for maximizing the utility of CDs as penetration enhancers. The rate of drug permeability through skin by CD-complexes depend upon two factors; release of drug from an aqueous vehicle; and transport across aqueous diffusion layer on the outer surface of skin. However, if drug permeation and subsequent absorption depend on transport across lipophilic stratum corneum as rate determining factor, then cyclodextrins are not capable to improve the drug delivery (Rasheed et al., 2008). Applications of CDs for improving dermal and transdermal drug delivery are shown in Table 8.

Table 8. Applications of CDs for improving dermal and transdermal drug delivery.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Bupranolol (BPL)	HPβ-CD, PM β-CD	Inclusion complexes.	Improved solubility and penetration.	Babu & Pandit (2004)
Glipizide (GPZ)	β-CD, DM-β-CD, HP-β-CD and HP-γ-CD.	Percutaneous formulations of the drug complex in different bases (o/w emulsion, PEG, CMC and Carbopol).	Effective means for transdermal delivery in management of type 2 diabetes.	Ammar et al. (2006)
Gliquidone	HP-β-CD	Effect of pH and complexation on drug delivery.	Higher transdermal delivery of gliquidone was achieved due to improved solubility and permeability of the drug.	Sridevi & Diwan (2002)
Isotretinoin	HP-β-CD	Elastic liposomes loaded with complexes.	Possibility for skin targeting, extended drug release, less photo degradation and least possibility of skin irritation with better topical delivery.	Kaur et al. (2010)

CDs are also known to enhance the release of drugs from the vehicles by the complexation with the drug. CDs help in easing the skin irritation instigated by drugs by decreasing the extent of free drug occurring due to the inclusion equilibrium (Chordiya & Senthilkumaran, 2012). CD discharges certain constituents like proteins, cholesterol and phospholipids with regard to the cutaneous irritation of CDs themselves to skin, from the stratum corneum of skin. As the result of this, a change may occur in the barrier function of skin and permeability of drugs and other xenobiotics. Therefore, special attention should be paid to the possible irritation effects of skin (Matsuda & Arima, 1999).

CDs in cosmetics, toiletries and personal care

Recent use of CDs has been discovered in the field of the cosmetic preparations, mainly in vaporization suppression of perfumes, room fresheners and detergents by obtaining the controlled release of fragrances from inclusion complex compounds. The crucial advantages of CDs in this regard are maintenance of stability, odor control and process improvement by converting a liquid ingredient to a solid form (Del Valle, 2004).

Following are the major roles of CD in the field of cosmetic preparations.

Shielding from light and oxidation

Presence of CD safeguards cosmetic products from the degredative effects of light and oxidation. In the absence of light and oxygen, pure tea tree oil remains unchanged for a month. Else the skin irritant, p-cymene, is formed because of the reaction between the terpenes present in the oil with light and oxygen. Due to the CD complexation, oil remains stable against light and oxygen. Hence, the possibility of the production of unwanted compounds is circumvented. In addition, there is no change in the antimicrobial and anti-inflammatory properties of tea tree oil after the complex formation (Buschmann & Schollmeyer, 2002).

Preventing the loss by evaporation

The phenomenon of complexation by CD aids in the stability of volatile compounds by decreasing their evaporation. The complexes can be added in either powders or liquid formulations. Now a days, even the solid form perfume complexes are used in perfumes. The application of these suspensions on the skin satisfies the demand of a long-lasting effect by slow release of the perfumes (Buschmann & Schollmeyer, 2002).

Removal of unpleasant odors

The dry form of CD powders even with the size less than 12 mm are used for odor control in products like diapers, menstrual products and paper towels etc. and are also widely used in hair care preparations for limiting the volatility of odorous mercaptans. Dishwashing and laundry detergents also contain CDs as important constituent, which helps in masking the odors in washed items (Del Valle, 2004).

CD complexation eliminates the undesirable odor of dihydroxyacetone which otherwise is very tough to be masked by perfumes. It is used as a tanning agent. Moreover, this CD complexation also causes the slow release of di-hydroxyacetone from the complex due to which there is more uniform tanning of the skin. The product containing this CD complex is marketed as Ultrasun Selftan© (Ultrasun) and in Self-Action Super Tan For Face© (Estee Lauder). CD is also used in deodorants to control the unpleasant smell caused by microbial degradation of sweat (Buschmann & Schollmeyer, 2002). Recent reports of cyclodextrins in cosmetics have been given in Table 9.

Table 9. Recent reports of cyclodextrins in cosmetics.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Eusolex® 4360 (Avobenzone), Eusolex® 9020 (Oxybenzone) and Eusolex® 232 (Ensulizole)	β-CD	Effects of β-CD complexation on transdermal penetration of sun blocking agents.	Reduction of systemic absorption of UV filters followed by toxicity and allergic reactions.	Shokri et al. (2013)
Lemongrass volatile oil	β-CD and HP-β-CD	Microparticles formed.	Efficacy improvement.	Weisheimer et al. (2010)
Octyl p-methoxycinnamate (OMC)	β-CD	Sunscreen preparations.	The lipo/OMC system proved as most beneficial release system.	Monteiro et al. (2011)
Phenylenediamine isomers (PDI)	β-CD	Detection of PDI in hair dyes.	Suitable method was developed.	Wu et al. (2011)

CDs in nutrition industry

The food/nutrition industry also demands use of CDs as food additives, for stabilization of flavors, for removal of offensive tastes and other undesirable compounds for instance cholesterol and to get rid of microbiological contaminations and browning reactions (Astray et al., 2009). The stability of colorants, flavors, unsaturated fats and vitamins can be elevated by the process of molecular encapsulation of lipophilic food ingredients with CD both in physical and chemical aspect causing prolonged product shelf life (Singh et al., 2002). Various experiments were conducted to monitor the effect of CD on the shelf life of the product and it was observed that almost all the natural and synthetic flavors and flavor substances show exceptionally increased shelf life throughout the long-term storage. Along with this, 12 various complexes of natural and synthetic flavor/β-CD have been kept in storage under normal conditions for the time span of 14 years and their actual flavor loads were examined from time to time by GC. Finally, the scientists concluded that the preserving power of the β-CD complexation relies on the structure, polarity and geometry of the different flavor substances. The strongest protection was seen with terpenoid, phenyl-propane and alkylsulfide-type flavors, whereas the flavors of phenolic structure were preserved comparatively to a much lesser amount (Szente & Szejtli, 2004).

The use of CDs in food processing and as food additives mainly focuses on the following goals:

• Preventing the lipophilic food components that may undergo oxygen and light or heat induced degradation.

• To terminate the unpleasant odors or tastes.

• Stabilization of fragrances, flavors, vitamins, and essential oils against the undesirable changes.

• To obtain a controlled release of certain food constituents.

• To promote the solubility of food colorants and vitamins (Astray et al., 2009).

β-CD improves the quality and stability of food products by acting as a cholesterol sesquestrant, i.e. it efficaciously removes the cholesterol from animal products thereby, enhancing their nutritional characteristics. In the case of milk, the immobilized β-CD glass beads were prepared by the process of silanization and β-CD immobilization reaction led to cholesterol removal in milk up to 41% and a recycling efficiency of almost 100% (Astray et al., 2009). Reduction in cholesterol and decreased oxidation were observed in phytosterol-supplemented mayonnaise with the help of crosslinked β-CD without any change in its physicochemical and sensory properties during storage (Jung et al., 2008). When lard was treated under various conditions with 5% crosslinked β-CD cholesterol removal was observed in the range of 91.2–93.0% (Kwak et al., 2004).

One of the most important applications of interest of CD is the CD-containing food-packaging materials. CD serves a dual purpose in food-packaging materials. First, all the residual organic volatile contaminants in packaging materials are minimized by CD. Second, the barrier properties of the packaging materials such as diffusion rate and transmission rate are improved by the use of CD further boosting the sensory properties by retaining the food quality and safety. Now a days many CD complexes are employed in foods as antiseptic or conserving agents. For instance, 0.1% iodine-β-CD obstructs the decomposition for 2 months at 20°C in fish paste or in frozen seafood products (Szente & Szejtli, 2004).

CD also helps in altering the taste and removal of undesirable bitter tastes and odors of food products. Certain food products have a very bad smell but, on addition of CDs during their manufacturing, inclusion complexes are formed between CD and food components thereby improving the smell of the resultant product. Using the same phenomenon the peculiar odors of food products like fish, soybean milk and soy protein was improved (Astray et al., 2009).

The unique property of CD of forming the inclusion complexes helps in the color modification of foods, since they too form complexes with the polyphenol-oxidase enzyme which is the accountable of catalyzing the browning reactions (Astray et al., 2009). Table 10 shows use of cyclodextrin in different food and flavors.

Table 10. Studies on use of cyclodextrins in food and flavors.

Food products	Cyclodextrin used	Work done	Conclusion	References
Cholesterol	β-CD	Study of inclusion properties and the factors affecting encapsulation and stability.	Successful elimination of cholesterol in the new food products.	Dos Santos et al. (2011)
Fat containing meal	α-CD	Effect of α-CD on the acute post-prandial responses in healthy adults.	Significant reduction of acute post-prandial blood triglyceride levels.	Jarosz et al. (2013)
Ginseng solution	β-, γ-CDs	Studies on taste masking and improvement of sensory properties of ginseng.	CDs in water and model energy drink base solutions containing ginseng, help in the formation of beverages suitable for consumers.	Tamamoto et al. (2010)
Goat milk and its yogurt	α-, β-, γ-CDs	Effect of CDs on the flavor of goat milk and its yogurt.	β-CD most effectively masked goaty flavor in yogurt with retention of nutritional benefits.	Young et al. (2012)
Mandarin juices enriched with pomegranate extract and goji berries juice	β-CD, HP-β-CD	Antioxidant efficacy of tested compounds was examined upon addition of β-CD or HP-β-CD.	Juices containing HP-β-CD showed more intense color, greater vitamin C content, retinol equivalents and antioxidant activity.	Navarro et al. (2011)

Environmental uses of CDs

CDs also have a crucial role to play in the field of environmental science with respect to the enhancement and elimination of organic pollutants and heavy metals from soil, water and atmosphere and solubility of organic contaminants (Gao & Wang, 1998). CDs are also put into application in water treatment for enhancing the stabilization aspect, encapsulation and adsorption of contaminants. One can get rid of extremely poisonous and lethal substances from industrial effluent by forming the inclusion complexes utilizing CD (Szejtli, 1989). All the aromatic toxic hydrocarbons like phenol, p-chlorophenol and benzene, which are present in wastewaters, are reduced to much lesser extent than its initial amounts after the treatment of wastewaters with β-CD (Singh et al., 2002). CDs are also employed in washing away the gaseous effluent from organic chemical industries (Szejtli, 1989). Different solubility enhancement techniques of CDs are used in the testing of soil improvement. The deterioration of various classes of hydrocarbons affecting the growth kinetics is promoted by β-CD, resulting in higher biomass yield and superior use of hydrocarbons as a carbon and energy source. β-CD has become the powerful tool for bioremediation process due to its properties such as the low cost, biocompatible and effective degradation (Singh et al., 2002).

CDs play a major role in environmental protection, which is its usage in insecticide formulation. CDs are incorporated in the preparation of an insecticide from neem seed extract by forming water-soluble inclusion complex of neem seed kernel extract enclosing azadirachtin-A in a CD carrier molecule. In addition to all its above uses, CDs also helps in the photodegradation process of organophosphorous pesticides in humid water by catalyzing the reaction of pesticides with reactive radicals, which are produced by the humid photosensitizer and inclusion complexed CD (Singh et al., 2002).

CDs in textile and packing industry

Since CD has potential to form inclusion complexes with a variety of organic molecules. This property is responsible for its use in different textile applications. CDs can be taken into account as a new category of auxiliary substances for use in the textile industry. Since CD has natural origin and is environmentally safe, so it is considered for textile industry (Voncina, 2011).

CD performs the function as retarding reagent in much textile fiber dyeing such as cotton, polyester, polyamide, polypropylene, polyacrylonitrile by complexing with dye molecule, which are released slowly to the fiber thereby, slowing down the dye migration because of the competition for sites on the fiber between CD and dyes irrespective of its mechanism of action. Normally, when CD is used as a leveling agent or a retardant in the dyeing process, superior quality of dyeing together with the improved bath exhaustion is achieved and bath exhaustion in comparison to other already existing marketed auxiliaries; stimulating the better color levelness and some development in color depth when textile fibers are dyed in the presence of CDs. Besides its use in enhancing the quality of dyeing in textile industry, CD also aids in lowering down the environmental damage of exhausted baths. This mainly happens when covalently bonded CDs on textile support form inclusion complexes with organic pollutants. With this, all the adsorbed pollutants will change into water and carbon dioxide by the process known as incineration (Voncina, 2011). Superior quality fabrics with novel properties are generated with the help of CDs. The bad smell gained by the fabrics due to cigarette smoke and sweat is concealed by including CD to fabrics.

One of the critical roles of CDs is in the area of packing industry. The presence of CD inclusion complex in the oily antimicrobial and volatile agents helps in its coating on a water-absorbing sheet containing a natural resin binder, which is further used for wrapping fresh products. The usefulness of CD as food-packing material has been confirmed with the manufacturing of food-packaging bag using CD with ethylene-tetracyclo-3-dodecane copolymer and hinokitol as it was odorless and showed good antifungal properties when stored for 1 week at room temperature (Singh et al., 2002). Some of the uses of CD in textile industry have been listed in Table 11.

Table 11. Cyclodextrins applicability in textile industry.

Cyclodextrins	Work done	Conclusion	References
HP-γ-CDs	Use of CDs as a finishing agent of polyamide fibers in order to obtain inguinal meshes with enhanced antibiotic delivery properties. Polymerization between citric acid and CDs produced a cross-linked polymer that physically adhered to the surface of PA fibers.	CDs as efficient drug delivery systems for multiple applications in the fields of biomaterials and medical textiles.	El Ghoul et al. (2008)
MCT-βCD	Making of reactive cotton/wool and viscose/wool prints with outstanding UV-protection functions through inclusion of definite UV-absorbers/blockers like as a reactive additive, in the printing paste formulation with sodium alginate as a thickening agent.	Outstanding UV-protection efficacy even after 15 washing cycles.	Ibrahim et al. (2013b)
MCT-β-CD	Alteration in wool fabric structure.	Superior post-printing, using different dyestuffs and outstanding antibacterial activities.	Ibrahim et al. (2013a)
Monochlorotriazinyl-β-CD (MCT-β-CD)	The cotton grafted with an inclusion compound with natural anti-allergic active principles.	Anti-allergic knitted fabric/therapeutic pajamas were obtained.	Radu et al. (2013)

CDs in separation process

As the CDs efficiently differentiate between various positional isomers, functional groups, homologs and enantiomers, so it is broadly used in separation science. This capability makes them one of the most effective candidates worldwide for a variety of separations (Schneiderman & Stalcup, 2000). Their tendency to form inclusion complexes with smaller hydrophobic molecules also contributes to their usage in separation. The factor like shape, size and selectivity of CDs exert the major impact on the separation process (Singh et al., 2002). CDs are considered to be the ideal candidate by molecular recognition and further promote the complex forming ability and selectivity in various separations. One of the main uses of CDs is as chemically bonded or sorbed ligands in stationary phase or immobile phase during the separation process. Presently, CDs along with its derivatives are widely used in case of chiral separations. In addition, the hydrophilic CDs have been more commonly used as buffer modifiers in capillary electrophoresis to accomplish the chiral separation of drugs and specialty chemicals (Zhang et al., 2011).

Besides, CDs are also greatly used in high-performance liquid chromatography (HPLC) as stationary phases connected to solid support or even as mobile phase additives in HPLC and in capillary electrophoresis to achieve the separation of chiral compounds. CDs also have a role in various chromatographic techniques such as affinity chromatography, capillary zone electrophoresis, ion-exchange, thin layer chromatography, electrokinetic chromatography, microdialysis, isotachophoresis, gel electrophoresis, capillary gas chromatography and separation through membrane. Furthermore, CDs help in the stabilization of an analyte, prevention of non-specific absorption and promoting analyte detection. Besides all the above advantages, CD also aids in microscaling of already used separation technologies; for instance, different capillary techniques including capillary electrophoresis, microbore liquid chromatography and microdialysis. CDs are also employed in bulk scale preparations mainly extractions, dialysis, foam floatation, membrane separation and electrophoresis (De Boer et al., 2000). Table 12 lists few examples of the role of CDs in separation process of various drugs.

Table 12. Role of cyclodextrins in separation process.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Racemic trans-δ-viniferin (TVN)	HP-β-CD	Preparative chiral high-speed counter-current chromatography method, based on which induced circular dichroism spectrum was developed.	Effective separation of TVN enantiomers.	Han et al. (2014)
Iodiconazole (IA)	HP-γ-CD	Chiral separation of IA and 12 new structurally related triadimenol analogs using capillary electrophoresis by HP-γ-CD.	Development of proper strategy for chiral separation.	Li et al. (2012)
Resibufogenin and cinobufagin	γ-CD	RP-HPLC using CDs as mobile phase additives.	Effective separation.	Xing et al. (2012)

CDs as anti-infective agents

Apart from utility of CDs for efficacy enhancement of antibiotics through encapsulation, recently these compounds are reported for direct use as anti-microbial. Anti-infective properties of CDs are mainly attributed to their capability to efficiently block pore forming bacterial toxins because of same symmetry of these molecules as the target pores. Applicability of CDs as anti-infectives and mechanistic insights into inhibition of toxins has been reviewed and reported CD based inhibitors of various bacterial toxins are enlisted recently (Karginov, 2013). At least one CD derivative can specifically inhibit various pore forming toxins and thus allows possibilities to develop a novel range of CD derivatives as broad-spectrum antibiotics against various pathogens in near future. Table 13 shows some uses of CDs for complexation and efficacy improvement of existing anti-infective agents.

Table 13. Cyclodextrin as anti-infectives.

Drugs	Cyclodextrin used	Work done	Conclusion	References
Artemisinin (AN)	β-CD	Agglomerated powder dosage form for oral administration based on primary microparticles containing AN/β-CD complex.	Enhancement in drug bioavailability.	Balducci et al. (2013)
Norfloxacin	β-CD	The solid-state properties of complexes of β-CD and norfloxacin at the molecular level.	Better understandings of the molecular fragments.	Chattah et al. (2013)
Voriconazole (VA)	HP-β-CD, 2-O-methyl-β-CD	The influence of CD complexation on VA solubility, dissolution rate and chemical stability was studied.	2-O-methyl-β-CD was found as more efficient solubilizer.	Miletic et al. (2013)
Zaltoprofen (ZP)	2-HP-β-CD	Development of gel formulation using carbomer complexed drug and other excipients.	No dermal irritation.	Baek et al. (2013)

CDs in non-viral gene delivery

Gene delivery is the most promising area of biotechnological research, and has already proved its capability in biomedical applications. The parent CDs are inefficient in complexing with plasmid DNA (pDNA), this drawback of CD limits their transfection efficiency. To overcome this problem, CDs are normally derivatized before using it in gene transfer. This can be well exemplified by CD derivatives polycationic amphiphilic CDs (paCDs; built by the change in the facial anisotropy of the truncated-cone CD by the introducing cationic and hydrophobic elements). By controlling the various molecular parameters such as charge density, hydrophilic–hydrophobic balance, nature of the functional groups and spacer length, the complexing of DNA with CDs can be improved finally, regulating the transfection efficiency of paCDs (Lai, 2014).

CDs also have an important role in linking various polymers through covalent linkage forming large molecular structures similar to gene carriers, thereby behaving as a linking agent. This can be well explained by the example of polymers which were synthesized by linkage of low molecular weight poly(ethylenimine) (PEI), a cationic aziridine polymer displaying a high proton buffering capacity over a wide range of pH with β-CD using tosyl chloride, initially generating an amine reactive tosyl deoxy-β-CD, which was later reacted with PEI gives rise to the CD-PEI conjugate (PEI-β-CD; Neu et al., 2005). Besides the parent CDs, the derivatives of CDs have also been found to act as linking agents. Their activity as linking agent was confirmed when cross-linking of PEI was done by Huang et al. using 2-HP-β-CD and 2-HP-γ-CD. The two resultant polymers were better in properties as they showed lower cytotoxicity than PEI 25 kDa, with transfection efficiency in SKOV-3 cells approximately 20 and two times greater than that obtained by PEI 600Da and PEI 25 kDa, respectively (Lai, 2014).

CDs are also known for its functioning as a structural modulator since they were used to structurally modify already prevailing gene carriers. In this, CD can fulfill the function of both as a threading device as well as that of pendants. The role of CD as threading agent can be better explained by the experiments conducted by Li’s group, who have produced large number of supramolecular polyrotaxanes, which comprise of cationic α-CD rings threaded and blocked on a poly [(ethylene oxide)-ran-(propylene oxide)] [P (EO-r-PO)] random copolymer. The function of CDs as pendants can be explained by taking the example of vectors produced by this method is the polyamidoamine (PAMAM) dendrimer conjugates with all the three parent CDs (Lai, 2014). Some of the applications of CD in non-viral gene delivery are given in Table 14.

Table 14. Use of cyclodextrins in non-viral gene delivery.

Cyclodextrin used	Work done	Conclusion	References
Polycationic amphiphilic CDs (paCDs)	A novel targeted gene delivery system was developed and assessed.	Suitability for cost effective production at large scale.	Aranda et al. (2013)
α-CD	Investigation of in vitro and in vivo gene delivery efficiency of polyamidoamine starburst dendrimer conjugates with α-CD.	Establishment of Lac-α-CDE as a non-viral vector for gene delivery toward hepatocytes.	Arima et al. (2010)
β-CDs	Nanoparticles coated with β-CD coupled to amino acids with different surface charges through direct ligand-exchange reactions were synthesized and used to deliver siRNA.	New insights into the mechanisms of amino acid mediated delivery and monitoring of gene silencing studies.	Zhao et al. (2011)

CDs in bioconversion process

The proficiency of bioconversion process suffers from certain constraints due to the inhibitory or toxic influences from either the substrate or product on the biocatalyst. Likewise, another hindrance to this process is that the biocatalyst is most active in its natural environment, mainly an aqueous medium, despite the fact that the majority of organic substrates are lipophilic in nature and sparingly soluble in water. As a result, very less amount of substrate is available to the biocatalyst. Therefore, one of the most suitable approach to overcome these inadequacies involves the addition of CD in bioconversion. CDs have tendency of enhancing the solubilization of organic compounds, reducing the toxicity by complexation with toxins and the biocompatibility. Representative studies on efficiency enhancement of bioconversion processes using CDs are given in Table 15.

Table 15. Use of cyclodextrins in bioconversion process.

Cell culture	Substrate	Product	Inference	References
Saccharomyces cerevisiae	Androstenedione	Testosterone	Bioconversion increased in the presence of HP-β-CD from 27% to 78%.	Singer et al. (1991)
Mucuna pruriens	17-β-estradiol	4-hydroxyestradiol	The solubility of steroid and bioconversion efficiency was enhanced.	Woerdenbag et al. (1990)
Lactobacillus bulgaricus	Cholesterol	Testosterone	Increase in rate of bioconversion.	Singh et al. (2002)

CDs in plant cell technology

Precursor feeding has been an evident and widespread approach to increase secondary metabolite production in plant cell cultures. The concept is based upon the idea that any compound, which is an intermediate in or at the beginning of a secondary metabolite biosynthetic route, stands a good chance of increasing the yield of the final product (Rao & Ravishankar, 2002). The increase in product content is probably due to enhanced availability of precursor to the cells in a more dissolved state due to CD complexation which resulted in more conversion to product. However, most of these intermediates of biosynthetic pathway are poorly soluble in aqueous media and therefore are less accessible for bioconversion to the desired product due to their lipophilic nature in plant cell cultures. This property confines their use as precursors to enhance productivity. Many plant cell cultures barely convert precursors in the presence of organic phases, often as a result of dramatic decrease of cell viability and reduced enzymatic activities in these systems. In order to overcome these problems and enhance availability of water insoluble precursors, complexation with cyclodextrin has been efficaciously used as a novel approach. This method has advantages of both apolar systems (higher solubility of substrate) and aqueous systems (maintenance of cell viability). The culture characteristics, i.e. cell growth and pH does not differ when only CDs are added in cultures (Baldi, 2008). The CDs are also reported not to be metabolized by plant cell enzymes or to serve as a carbon source (Woerdenbag et al., 1990). Few of the studies involving CDs for precursor availability enhancement in plant cell cultures are given in Table 16.

Table 16. Cyclodextrins in plant cell technology.

Cell culture	Precursor	Product	Inference	References
Catharanthus roseus suspension culture	Methyl jasomate	Ajmalicine	Improvement in ajmalicine production	Almagro et al. (2011)
Artemisia annua suspension culture	Methyl jasomate	Artemisinin (AN)	300-fold enhancement in AN content.	Durante et al. (2011)
Vitis vinifera suspension culture	Methyl jasomate	Trans-Resveratrol	Increment in productivity was achieved.	Belchi Navarro et al., (2012)
Linum album suspension culture	Coniferyl alcohol	Podophyllotoxin, 6-methoxypodophyllotoxin	Increase in product content due to CDs by many folds.	Baldi (2008)

CDs in catalysis

One of the most promising role of CDs is in the catalytic reactions. They have the capability to function as enzyme mimics. These are fabricated by changing the naturally occurring CDs with the substitution on its primary or secondary face with different functional groups or attaching various reactive groups. The substituted groups attached to the CD molecule are responsible for the molecular recognition which makes the modified CDs beneficial as enzyme mimics (Szejtli, 1998). Morozumi et al. (1991) made use of modified β-CD in the catalysis and further employed it for the selective hydroxyl ethylation and hydroxyl methylation of phenol. This chemical modification significantly enhanced the catalytic activity with the resultant CD derivative acting as a transamine mimic and acting as a catalyst for the reaction in which the phenylpyruvic acid is converted to phenylalanine (Singh et al., 2002).

The steric effects shown by CDs are held responsible for its role in biocatalytic reactions. It acts by improving the enantio-selectivity. The catalytic reaction can be completely stopped or can be improved in a better way by the participation of CD as it gets involved with the catalyst by forming inclusion complexes (Singh et al., 2002).

Toxicity considerations

It has been revealed from the various studies carried out on CD and its derivatives that very limited number of hydrophilic CDs and drug/CD complexes formed can pass through the hydrophobic membranes like gastrointestinal mucosa and skin. Due to this, they exhibit very less absorption hence having no toxic effects inside the body. Therefore, these are safe to administer orally. Unlike hydrophilic CDs, the lipophilic methylated β-CDs can permeate easily through the biological membranes thereby, show better absorption approximately equivalent to 10% from the GIT. As a result of this, oral formulations should contain less amount of hydrophobic CD derivatives to completely get rid of the possibility of adverse effects, which may be caused because of these lipophilic CDs. These CDs are incompatible for parenteral formulations (Loftsson & Duchene, 2007). The toxicity studies conducted on different CDs have disclosed that α-CD and β-CD are unfit for incorporation in the parenteral preparations so their utilization via this route should be strictly restricted (Irie & Uekama, 1997).

Furthermore, during the animal studies, it was observed that administration of γ-CD intravenously did not show any significant toxic effects. Large-scale toxicology studies were compiled on two main extensively used CD derivatives, i.e. 2-HP-β-CD and SBE β-CD. There are many marketed formulations available containing significant amount of these two CD derivatives (Loftsson & Duchene, 2007). 2-HP-β-CD is another better option other than α-, β- and γ-CD, which can be considered for the study as it possess improved aqueous solubility combined with very less toxic effects. Various animal studies done on HP-β-CD using animals such as rats, dogs and mice reached at a conclusion that the oral administration of HP-β-CD is well tolerated in these animals tested with negligible toxic effects even at daily oral dosing of <16 g for at least 14 days (Gould & Scott, 2005).

Market potential and future prospects of CDs

Besides the vast applications of CDs at the present time, CD has ample scope in future also involving the evolution of new CD derivatives as well as the demand of already existing derivatives. Till date, there are more than 50 marketed formulations, which contain CD complexes with drugs and a range of excipients are available commercially (Szejtli, 2004; Loftsson & Brewster, 2010; Kumar Das et al., 2013). Some representative examples are:

• The oral formulation of the drug limaprost complexed with α-CD is marketed under the trade names of Opalmon and Prorenal (Davis & Brewster, 2004).

• The sublingual preparation of nitroglycerin containing β-CD is in the market under the trade name of Nitropen in Japan (Zhang & Rees, 1999).

• The complex of piroxicam β-CD given via oral route is sold in Europe under the trade name of Brexin (Brewster & Loftsson, 2007).

• The intravenous formulation containing inclusion complex of voriconazole and sulfobutylether β-CD is sold in countries such as Europe, United States under trade name of Vfend (Loftsson & Duchene, 2007).

• Sporanox is the marketed formulation of Europe and United States taken orally as well as intravenously containing complex of drug, itraconazole and 2-HP-β-CD (Kumar Das et al., 2013).

For future prospective, CDs might prove to be beneficial to introduce some better changes with respect to formulation or it may help in drug delivery of protein, peptide and oligonucleotide dosage forms. The problem of agitation-associated aggregation in a mixture of a number of proteins, including human growth hormone, as well as lyophilization related aggregation of interleukin-2 (IL-2) can be overcome by the use of HP-β-CD together with other hydrophilic and lipophilic CD derivatives (Davis & Brewster, 2004). The physical and chemical deterioration of insulin formulation can be prevented by the use of CDs (Brewster et al., 1989). Therefore, it is responsible for the stabilization of insulin formulations. This is mainly related to the fact that by the interactions with readily available hydrophobic amino acid residues CDs stabilizes the protein conformation. CDs are also known to interact with aggregated/denaturated proteins resulting in natural refolding, thus acting like an artificial chaperones. One more future application of CD lies in its role in enhancing the cellular delivery of oligonucleotides. The uptake of phosphorothiolate oligodeoxynucleotides by human T-cell leukemia cells (H9) is upgraded several folds with the use of HP-β-CD. Moreover, ability of CD to complex is helpful in decreasing the immunogenicity of oligonucleotides as well other undesirable side effects of oligonucleotide administration comprising of the effects of macromolecules on platelet count. Besides its increasing demand as drug carriers, CDs are helpful in extraction of various compounds. Efforts have been made to merge CDs together with polymers and small molecule drugs to limit the amount of CD used along with the release rates of the drug. Biodegradable polymers including complexes of CDs and antibiotics have been formulated and these have showed promising results in the production of bacteria-resistant films and fibers. More advancement is occurring, most important being the discovery of many new CD derivatives. Recently, CDs consisting of quaternary amine groups have been found to act as multifunctional agents in ophthalmic formulations. Earlier also, CDs were included in eye drops as an important ingredient, e.g. eye drops of 2-HP-β-CD complexed indomethacin and RAME-β-CD complexed chloramphenicol, which contain as a complexing agent (Loftsson & Stefansson, 2002; Halim Mohamed & Mahmoud, 2011). Despite of advancements for more than 100 years and great commercial success, CDs still holds many unexplored potential in diverse facets of science and technology, which should be tapped in future by researcher across the globe.

Conclusion

CDs have been known worldwide and are very beneficial additive used in the pharmaceutical and other industries. All the advantages of the CD have been attributed to its tendency of complexing with varied drug molecules resulting in the formation of inclusion complexes. This interaction between CD and drug is host–guest type, i.e. it acts like a host and drug molecule (guest) gets partially or completely fit into its lipophilic cavity to form a complex. This single interaction is sufficient to change the physiochemical and pharmaceutical properties of the drug molecule in the form of enhanced water solubility of drugs further improving the bioavailability of the drug, better stabilization of drugs under the conditions such as light, heat and oxidizing conditions, i.e. avoids the deterioration of drugs in these sources and also reduction in the volatility of drugs. Therefore, all these improvements brought about in the drug characteristics depict the great potential of the CD as most advanced drug delivery system. Despite the fact that CDs are known and are used pharmaceutically since decades, in the recent times many more applications of CD have emerged apart from its usage in enhancing solubility and maintaining the stability of various drugs. CDs have a function as penetration enhancer. The hydrophobic CDs help in the permeation of the drugs through the biological membranes. One of the most important applications of CD known from the different experiments carried out in humans and animals as well is enhancing the drug delivery of majority of the formulations. CDs help in the delivery of sustained release preparations to be delivered via different possible routes. Most of the research conducted has revealed that the hydrophobic and ionizable CDs derivatives are mainly used as drug carriers in the delivery of sustained-release or delayed release formulations. The property of CDs to complex with various drugs is the ultimate solution to the various issues related to drug delivery. In addition to all these aspects, gene therapy continues to develop the interest of researchers. For the non-viral delivery of nucleic acids, CD containing polycations are used. Even though these agents have given promising results for gene delivery in animals, but no conclusions have been made in case of humans. CDs are also known for acting as molecular chelating agents with huge demand in the area of food, pharmaceuticals and chromatographic techniques established. CDs are also known worldwide for its role as toxicity modifier. CDs have improved biocatalysis reactions by increasing enantio-selectivity. Seeing such vast applications of CDs in number of bioprocesses, further survey and research on CDs should be done at small as well as on large scale to investigate its role in various other fields. Hence, it can be concluded that because of its distinctive structural features, CDs are the most demandable and important requirement of the researchers in the drug development, in various separation procedures, as enzyme mimics and most importantly as a complexing agent in pharmaceutical, food, cosmetic and many other industries.

Acknowledgements

We are thankful to Praveen Garg, Chairman, ISFCP, Moga, Punjab for providing necessary facilities.

Declaration of interest

The authors report no declarations/conflicts of interest. Financial assistance provided by Punjab State Council for Science and Technology, Chandigarh is gratefully acknowledged.