**10.** *In vitro in vivo* **correlation (***IVIVC***) for lipid nanoformulations**

with log *P* values >6.0 and has been demonstrated to be crucial in a past study for the absorption

The mixed micelles substantially transport digestion products across the unstirred water layer and reach the vicinity of the aqueous‐microvillus interface to allow for lipid absorption through the mucosal cells. However, it is possible that digestion of a lipid formulation could reduce the solubility of the drug in the gut lumen, which would result in the precipitation of the drug and a decrease in the absorption rate. Therefore, more investigation on *in vitro* lipolysis is needed to clearly understand drug precipitation during digestion for better

The fate of the lipid carrier in the GI tract is essentially important for the absorption of the incorporated drug and therefore has to be closely analyzed. It is evident that the solvent capacity of the formulation can be lost on digestion, leading to drug precipitation [26, 50]. However, the investigation of the lipolysis by *in vivo* experiments is complex, costly and time‐ consuming. Thus, the *in vitro* model simulating the enzymatic degradation of lipid‐based formulations is highly significant as an alternative method of monitoring the digestion process

Lipolysis can be carried out as an *in vitro* test using a pH‐stat titration unit to maintain pH and using the lipase/co‐lipase content of porcine pancreatin to serve as model for human pancreatic juice. Bile salt lecithin‐mixed micelles are used in the reaction mixture to provide a sink for solubilization of degradation products. Composition of mixture that used in the *in vitro*

**Composition of the lipolysis buffer Concentration (fed state) Concentration (fasted state)**

**Table 2.** Composition of mixture for *in vitro* lipolysis experiments. \*Adapted with permission from Ref. [51].

Bile salt (BS, mM) 20 5 Phospholipid (PL, mM) 5 1.25 Trizma maleate (mM) 0.5 0.5

(mM) 0.05 0.05

(mM) 1.5 1.5

in the simulated gastrointestinal media under fed and fasted conditions.

of the antimalarial compound halofantrine [48, 49].

44 Advanced Technology for Delivering Therapeutics

**9.** *In vitro* **digestion (lipolysis): significance**

lipolysis studies is provided in **Table 2**.

**Substance of the mixture for 10 ml aqueous media**

**Pancreatic lipase 1 ml (800 TBU/ml)**

absorption.

**Lipid 250 mg**

Ca+

Na+

**Lipolysis buffer 9 ml**

The *IVIVCs* play a major role in drug development, particularly on the optimization of suitable formulations which is time‐consuming and a highly expensive process. Formulation optimi‐ zation requires modifications in composition, equipment, manufacturing process and batch sizes. If such changes applied to the formulation, the *in vivo* bioequivalence studies in human are necessary to be conducted to confirm the similarity of the new formulation. This process will increase the load of carrying out a number of bioequivalence studies and therefore will increase the cost of process optimization and marketing of the new formulation.

To overcome these issues, it is necessary to develop *in vitro* tests that can imitate the bioavail‐ ability data. The *IVIVC* can be used in the development of new pharmaceuticals to decrease the number of human trials during the formulation development and to support biowaivers.

In the beginning of 1980s, the *IVIVC* theory was established based on many published research studies, which can be used as a prediction tool for correlating *in vitro* and *in vivo* data. The *IVIVC* is usually used in the development stages of pharmaceuticals to enhance the formulation and dosage optimization with fewer trials in human [51–56] or additional bioavailability studies. The FDA defines *IVIVC* as "a predictive mathematical model describing the relation‐ ship between an *in vitro* property of a dosage form (usually the rate or extent of drug dissolution or release) and a relevant *in vivo* response (e.g., plasma drug concentration or amount of drug absorbed)." For drugs that are administered orally, dissolution and intestinal permeation are considered as the rate‐limiting steps for the absorption. 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 [57–59].

There are several tools which can be used to establish *IVIVC*. The *in vitro* drug release studies of the formulations can be performed using dissolution, dynamic dispersion and digestion tests, whereas the *in vivo* pharmacokinetic studies can be performed on various animal models. However, there are only a limited number of *IVIVC* studies so far have been conducted using lipid formulations. To obtain more robust *in vitro* and *in vivo* relationship, a large number of

model compounds should be explored along with more human clinical data sets and complete characterizations of *in vitro* and *in vivo* solubilization of PWSDs formulated in lipid vehicles.
