**9.** *In vitro-in vivo* **correlation (***IVIVC***) for self nanoemulsion**

New drug synthesis and design, optimization of new drug and its formulations are very costly and time consuming processes and *IVIVC* play prime importance in it. Optimization demands *in vivo* bioequivalence data in human to ascertain similarity of the new formulation. Due to this load importance of performing bioequivalence studies significantly increase hence coast of formulation and optimization process increases. To solve this problem it is necessary to perform in vitro tests that can interpret bioavailability data. *IVIVC* concept was established in scientific research studies which give a prediction of in-vitro parameter with in vivo activity [26, 27]. The *IVIVC* predominantly used in the product development to decrease the human trials during the formulation development and to support biowaivers. *IVIVC* acts as vital tool for correlating *in vitro* and *in vivo* data. Core balance of *IVIVC* study in development and optimization of new formulation with minimum human trials [28]. *IVIVC* is a mathematical model which predicts the relation between rate and extent of drug release and plasma drug concentration. For drugs that are administered orally, dissolution and intestinal permeation are considered as the rate-limiting steps for the absorption. Bioavailability is calculated by controlling dissolution profile, therefore, if an excellent correlation exists between *in vitro* dissolution test and a bioavailability parameter, then controlling the dissolution profile will permit the evaluation of bioavailability [29]. The *in vitro* drug release studies of the formulations can be performed using dissolution, disintegration tests on t other hand *in vivo* pharmacokinetic study performed on animal models. Apart from this, there is a limited number of *IVIVC* studies conducted using lipid formulations. To get strong *IVIVC* relationship a large no. of nanoemulsions of PWSDs administered orally. Clinical data of more human volunteers along with in detail evaluation of in-*vitro* and *in vivo* solubility of poorly water soluble drugs entrapped in lipid vehicles [30].

### **10. Solid SNEF**

As Nanoemulsions facing problems related to stability, handling, formulation and development scientist decided to overcome these hurdles related to stability and converted nanoemulsions into solid-state and concept of solid SNEF coined. Solid dosage forms are most convenient and stable for handling and they are dry solid powder overcomes hurdles of stability. Adsorption on a solid carrier is the best technique involved to convert liquid nanoemulsion to solid SNEF apart from other techniques available such as spray drying and freeze-drying. The choice right process for the formulation of solid SNEF Mainly depends on the properties of API, such as solubility, stability, and compatibility with other ingredients and nature of the oil phase of the formulation. Adsorption on the surface of adsorbent act as a binder and it is coast effective, accurate, optimizable, uncomplicated and large scale manufacturing is possible with ease. Adsorption on solid carrier adsorbs heat and moisture sensitive drugs can be formulated into SNEF also it has an advantage over other methods. Commonly used adsorbents in SNEF are Neusilin-US2, Fujicalin, Aerosil etc. [31].

**Figure 4.** *SEM of SNEF adsorbed on neusilin-US2 adsorbant.*
