What Are The Differences Between FaSSIF-V2 And FaSSIF?

FaSSIF-V2 and FaSSIF are types of fasted state Biorelevant Media simulating small intestinal fluid. ’Fasted state’ simply means without food or before a meal (pre-prandial).

How similar are FaSSIF and FaSSIF-V2 to the fluids actually inside the human body?
FaSSIF-V2 and FaSSIF are good simulations of Fasted State Human Intestinal Fluid (often abbreviated to ‘FaHIF’). Although they do not reflect the exact content of phospholipids and bile salts and omit cholesterol, they are a quantum leap forward from earlier dissolution media (Williams et al., 1999) (3) which they were intended to replace.

In 2014 Augstijns et al. (5) published data correlating FaSSIF with Fasted State Human Intestinal Fluid. The review data, based on about  102 solubility values, correlated FaSSIF to FaHIF with a R2 = 0.85. This indicates that the predictive solubility of FaSSIF to actual human intestinal fluid is quite good. This is also confirmed in a different study that examined nine HIV protease inhibitors and their solubility in FaSSIF. This study by Wuyts et al. (5) correlated the solubility of the set of compounds in FaSSIF and actual human intestinal fluid with an R = 0.817 showing the correlation was again good.

A similar study has not yet been published for FaSSIF-V2 but Soderlind et al. (6) of AstraZeneca (2010) presented a data set of 24 compounds that tended to favour FaSSIF-V2. FaSSIF-V2 was also suggested as their medium of choice for internal studies.

Why is it so important to test drugs in fasted state media?
FaSSIF and FaSSIF-V2 are the most popular and widely used Biorelevant Media. This is because when new orally administered drugs are being designed for humans, it is desirable that they can be taken on an empty stomach for maximum dosing flexibility.

What are the main differences between FaSSIF and FaSSIF-V2?
FaSSIF and FaSSIF-V2 share the same type of bile salt and lecithin so from a qualitative composition they are quite similar. However, FaSSIF-V2 has a much lower lecithin concentration and uses a different buffer system. This variation in lecithin content results in a difference in colloidal structures formed in these media which in some cases can influence drug solubility. The structures in FaSSIF are slightly larger due to the higher lecithin content and probably tend towards larger, more bilayered (liposomal or mixed bilayer) vesicular structures. FaSSIF-V2 particles are slightly smaller due to the lower lecithin content and the colloidal structures tend more towards smaller mixed micelles. This why measures (using photon correlation spectroscopy) the particle size of the particles to check they are as expected.

The lower lecithin level in FaSSIF-V2 means the bile salt to lecithin ratio is 15:1. For FaSSIF the bile salt to lecithin ratio is 4:1. In fact as can be seen from the review of Fuchs and Dressman (7), these ratios actually bracket the levels found in humans.

pH and buffer capacity are close but the buffers are different
The pH is the same and buffer capacity of FaSSIF and FaSSIF-V2 are quite similar. However, the buffer system is slightly different. Phosphate (used for FaSSIF) has the advantage of not interfering with UV whereas maleic acid (used for FASSIF-V2) has quite a large response in the UV range in which most drugs are analyzed. Therefore, maleic acid can limit the analysis of samples by direct UV measurement and if using HPLC please check it does not elute at the same time as the peak of your drug. The osmolarity of FaSSIF-V2 is also lower.

Comparisons of the composition and properties of FaSSIF and FaSSIF-V2 are provided in Table 1 and Table 2 respectively.

Table 1 Comparison of FaSSIF and FaSSIF-V2 compositions

Component  Concentration in FaSSIF (mM unless otherwise stated) Concentration in FaSSIF-V2 (mM unless otherwise stated) Purpose 
Sodium Taurocholate 3 3 Natural bile salt
Lecithin 0.75 0.2 Natural surfactant
Sodium Chloride 105.9 68.6 Adjusts osmolarity
Sodium Phosphate 28.4 Buffer
Maleic acid 19.1 Buffer
Sodium Hydroxide 8.7 101 Adjusts pH
Pancreatin (Optional) 10.0 mg/L 10.0 mg/ml Added to digest gelatin of capsules

Table 2 Comparison of FaSSIF and FaSSIF-V2 properties

Property FaSSIF FaSSIF-V2
pH 6.5 6.5
Osmolarity (mOsm/kg) 270 180
Buffer capacity (mM/dpH) 12 10

What equipment do I need when using FaSSIF or FaSSIF-V2?
For solubility testing just normal vials are needed. HPLC is the recommended analytical method of choice though UV can be used if the absorption wavelength of your drug is outside of that of the media and a control of just Biorelevant Media is taken into account. For dissolution testing, standard pharmacopeia equipment can be used. It depends on the objective of your study but USP 2 is most widely used.

What do the media look like?
FaSSIF is a slightly opalescent liquid. The opalescence is totally normal and is due to the slightly larger colloidal structures as described previously. FaSSIF-V2 is a clear liquid. Both liquids foam a little after shaking. Instructions how to make FaSSIF-V2 are here and make FaSSIF here.

  1. Galia, E., et al. “Evaluation of various dissolution media for predicting in vivo performance of class I and II drugs.” Pharmaceutical research 15.5 (1998): 698-705.
  2. Jantratid, Ekarat, and Jennifer Dressman. “Biorelevant dissolution media simulating the proximal human gastrointestinal tract: an update.” Dissolut Technol 16.3 (2009): 21-5.
  3. Williams GC, Sinko PJ. 1999. Oral absorption of the HIV protease inhibitors: A current update. Adv Drug Deliv Rev 39:211–238
  4. Augustijns, Patrick, et al. “A review of drug solubility in human intestinal fluids: implications for the prediction of oral absorption.” European Journal of Pharmaceutical Sciences 57 (2014): 322-332.
  5. Wuyts, Benjamin, et al. “Solubility profiling of HIV protease inhibitors in human intestinal fluids.” Journal of pharmaceutical sciences 102.10 (2013): 3800-3807.
  6. Söderlind, Erik, et al. “Simulating fasted human intestinal fluids: understanding the roles of lecithin and bile acids.” Molecular pharmaceutics 7.5 (2010): 1498-1507.
  7. Fuchs, Alexander, and Jennifer B. Dressman. “Composition and Physicochemical Properties of Fasted‐State Human Duodenal and Jejunal Fluid: A Critical Evaluation of the Available Data.” Journal of pharmaceutical sciences 103.11 (2014): 3398-3411.
  8. Fuchs, Alexander, et al. “Advances in the design of fasted state simulating intestinal fluids: FaSSIF-V3.” European Journal of Pharmaceutics and Biopharmaceutics 94 (2015): 229-240.
  9. Fagerberg, Jonas H., and Christel AS Bergström. “Intestinal solubility and absorption of poorly water soluble compounds: predictions, challenges and solutions.” Therapeutic delivery 6.8 (2015): 935-959.
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