5. Biopharmaceutical aspects

In humans, the rectal ampulla is characterized by the presence of a small quantity of fluid (1 to 3 mL) with a pH close to neutral (pH = 7-8) and a low buffering capacity. The expanded surface area of the rectum is smaller than that of the upper gastrointestinal tract, as it contains neither villi nor microvilli. The epithelium of the rectal mucosa is composed of a single layer of absorptive cells and caliciform cells (responsible for mucin secretion). Histologically, the rectum is similar to the upper digestive tract, with a comparable capacity to absorb active substances. The rectum is drained by hemorrhoidal veins. The lower and middle veins enable direct absorption of active substances into the general circulation without passing through the liver. This effect is particularly sought-after for active molecules with a high hepatic first-pass effect, i.e. a high level of degradation in...