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

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 in the simulated gastrointestinal media under fed and fasted conditions.

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


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

Lipolysis is allowed to proceed for a fixed time (30–60 min), the reaction is then subjected to high‐speed ultracentrifugation, and further drug analysis in the various phases allows predicting whether the drug will remain solubilized in the intestinal lumen after digestion of the formulation. However, if the drug is partially precipitated, then drug will be found in the pellet, which may be still in solution. The rate and extent of lipolysis can be quantified by the data generated from the pH‐stat. This technique was recently applied in LFCS Types I–IV formulations to predict the effect of formulation on the fate of a number of drug compounds and assumed that surfactants are subjected to digestion, probably for SMEDDS and SNEDDS, where water soluble surfactants are used predominantly. Lipolysis experiments may play a vital role in the near future for establishing strong methods for *in vitro in vivo* correlations (*IVIVCs*).
