Table 4.2 An overview of the four levels of biorelevant media that can simulate the gastrointestinal luminal environment.
| Level | Purpose |
|---|---|
| Level 0 media | Replicates pH using a simple buffer |
| Level 1 media | Replicates pH and buffer capacity |
| Level 2 media | Replicates pH, buffer capacity, osmolality and solubilisation capacity using bile salts, dietary lipids and digestion products |
| Level 3 | Builds on level 2 by also incorporating proteins and enzymes In addition can be viscosity matched to in vivo fluid |
Table 4.3 Composition of commonly used simulated intestinal media: fasted state simulated gastric fluid (FaSSGF); fasted state simulated intestinal fluid (FaSSIF) and fed state simulated intestinal fluid (FeSSIF).
| FaSSGF | FaSSIF | FeSSIF | |
|---|---|---|---|
| pH | 1.6 | 6.5 | 5.0 |
| Bile salt (taurocholate) (mM) | 0.08 | 3 | 15 |
| Phospholipids (mM) | 0.02 | 0.75 | 3.75 |
| Sodium ions | 34 | 148 | 319 |
| Chloride ions | 59 | 106 | 203 |
| Phosphate ions | 29 | ||
| Acetic acid | 144 |
4.9.2 Buffer System – Phosphate vs Bicarbonate
In vivo, the gastrointestinal system is buffered by bicarbonate within the anaerobic conditions. Laboratory solubility and dissolution testing are technically difficult to conduct within anaerobic conditions which are required for bicarbonate buffers. Therefore, phosphate buffers are typically used instead. However, there is evidence that bicarbonate buffers are more representative and should be considered for certain drugs [12].
4.9.3 Solubilisation by Surfactants
Surfactants can improve the solubility of poorly soluble drugs. Surfactants are amphiphilic molecules that can form colloidal structures including micelles within an aqueous environment. Drug can then associate with these micelles, which improves the measured solubility. It is worth noting that the drug associated with micelles is not strictly dissolved but it is also no longer present as a solid. This distinction can be important for advanced modelling of drug solubilisation.
In biorelevant media, the bile salts and phospholipids form colloidal structures that include micelles that can enhance the measured solubility of the drug. Bile salt micelles will only form above the critical micelle concentration of the individual surfactant. Thus, small changes can have a large influence of the solubilisation capacity for a drug [13].
Studies have reported increased solubility of different drugs in FaSSIF compared to the same media without the bile salts [14]. This is shown in Figure 4.3.
Typically the solubility of drug in FaSSIF will be measured early in development as this can have implications for determining the most appropriate biopharmaceutics classification for the drug and to calculate the most relevant maximum absorbable dose.
Solubility increases are often also observed in FeSSIF compared to FaSSIF. However, the inherent variability associated with administration of drugs in the fed state makes reliance upon this solubility increase a risky strategy.
Figure 4.3 Comparison of the solubility of a series of drugs in blank FaSSIFf (left data point) and FaSSIF containing bile salts (right data point).
Source: Data from Fagerberg et al. [14].
4.9.4 Solubilisation During Digestion
The gastrointestinal tract is maintained in equilibrium via secretions and its composition has been well characterised in the fasted state. In the fed state the secretions change and in addition there is digestion of the consumed food. Thus it is not only the effect of endogenous surfactants but also ingested nutrients and digestion products that can affect drug solubility. For example, when short‐chain fatty acids are ingested, they are hydrolysed and then incorporated into the bile micelles to further solubilise certain drugs. Work has been conducted on simulated media that include lipolysis products within the media [15]. Oleic acid and monoolein are often used within the media to replicate the lipolysis products [16].
4.9.5 Excipients and Solubility
In cases where drug solubility is low and where it may limit exposure efforts are focussed on understanding the factors that limit solubility and identification of strategies to improve it. Typically different enabling formulations are used. Formulation solubilising vehicles including cosolvents, surfactants, complexation agents and oils/lipids are considered. In early stages of development, simple strategies are required to ensure that solubility does not limit the generation of data from an ascending dose study, thus a cosolvent approach can be a simple workaround at this stage yet would not be suitable for the commercial formulation. Typical cosolvents used include polyethylene glycol (PEG400), propylene glycol and ethanol.
For ionic drug compounds, commonly used approaches include pH adjustment and salt formulation. A salt screen can be performed to identify the most suitable counter ion. There are several examples of salt forms having large differences in solubility. For example, terfenadine salts formed with phosphoric acid, hydrochloric acid, methanesulphonic acid and lactic acid showed up to 10‐fold differences, ranging from 0.5 to 5 mg/mL [17]. There are some concerns about the common‐ion effect where the presence of ionic components, for example NaCl within the gastrointestinal media may affect the solubilisation rate (dissolution) where sodium or chloride was the counterion for the salt present.
Amorphous drug forms can be a useful strategy for drug compounds with a high melting temperature; the amorphous form disrupts the crystal packing and thus shows rapid dissolution, although there can be risks of precipitation. It is important that the amorphous form is stabilised during manufacture and storage. Often a solid dispersion, where the amorphous form of a drug is intimately mixed with a polymer provides a matrix to stabilise the amorphous form and prevent recrystallisation. There are many commercial examples including Kaletra which contains both lopinavir