Standard USP-SIF (sp) is a simple phosphate buffer that is an abbreviation for USP Simulated Intestinal Fluid (without enzyme). In contrast, FaSSIF is an acronym for Fasted State Simulated Intestinal Fluid, a biorelevant medium designed to better mimic the human intestinal environment in the fasted state. FaSSIF medium is prepared by adding 3F Powder® to FaSSIF biorelevant buffer.

Differences

• USP-SIF (sp) does not contain the natural surfactants present in the intestinal tract. In contrast, FaSSIF (biorelevant medium) contains surfactants (e.g., bile salts and lecithin) that simulate the physiological conditions of the gastrointestinal (GI) tract in the fasted state. These surfactants are essential for evaluating drug products containing poorly soluble drugs, as they aid in wetting and solubilization
• USP-SIF (sp) is a simple buffer with higher buffer capacity and does not contain sodium chloride for adjusting osmolarity. In comparison, FaSSIF has a buffer capacity that more closely resembles that of actual fasted state human intestinal fluid and includes sodium chloride to ensure physiological osmolarity
• The pH of USP-SIF (sp) is 6.8 and the pH of FaSSIF is 6.5

In summary, USP-SIF (sp) is a simpler, more traditional buffer system, whereas FaSSIF is specifically designed to simulate human intestinal fluid more closely in the fasted state, making it more suitable for biorelevant drug dissolution testing.