**3.1 Solubility and dissolution enhancement**

The most extensive use of the CDs is to improve the solubility of drug in aqueous solutions. An increase in solubility also aids in improving bioavailability and hence therapeutic efficiency. Cyclodextrins have the capability to form the inclusion complexes, which increases the solubility and dissolution of drug molecules in the solid state [25]. Even though solubilization effects of all the CD molecules can be found throughout the literature, methylated CDs have the greatest potential of increasing the solubility as they decrease the crystallinity of drugs, which also increases the dissolution [26]. Although the influences of the CD complexation on it are extremely empirical, yet a number of historical findings permit several inferences:


## **3.2 Permeability across biological membrane**

Remarkably, permeability across the biological membranes can be affected by several factors, which include molecular weight, molecular structure in addition to the partition coefficient. CDs have no part in increasing the permeation of hydrophilic

drugs, but in CD complexation, free drug has the affinity to penetrate biological membrane [27]. At this point, delivery across biological membrane is entirely dependent on the drug formulation as well as the barrier. Delivery across barrier controlled by water diffusion layers can be affected by cyclodextrins, but those across lipophilic membranes are limited. The only exception to this barrier is inclusion of hydrophobic cyclodextrins that can effortlessly cross the mucosa [28].
