3.4
Assorbimento dei Farmaci: Fattori che Influenzano l'Assorbimento Gastrointestinale
Il processo di assorbimento dei farmaci per via orale può essere influenzato da diversi fattori. I farmaci debolmente acidi tendono ad essere assorbit…
L'assorbimento di un farmaco somministrato per via orale è influenzato da vari fattori.
I farmaci debolmente acidi hanno maggiori probabilità di essere assorbiti dallo stomaco, dove sono prevalentemente non ionizzati.
In confronto, l'assorbimento dall'intestino superiore, che contiene farmaci ionizzati, può essere basso.
In realtà, lo strato mucoso dello stomaco rende difficile la diffusione. La sua superficie più piccola rispetto all'intestino riduce ulteriormente il tasso di assorbimento dallo stomaco.
I villi intestinali forniscono una superficie più ampia, che unita a un flusso sanguigno più elevato, porta a un maggiore assorbimento del farmaco.
Se il cibo è nello stomaco, i farmaci possono essere scarsamente assorbiti in quanto formano complessi con i costituenti del cibo.
Il cibo ritarda anche lo svuotamento gastrico, cioè il movimento dei farmaci nell'intestino tenue, che rallenta l'assorbimento intestinale.
Il rapido movimento del contenuto intestinale durante la diarrea rallenta ulteriormente l'assorbimento del farmaco.
Inoltre, l'epitelio intestinale composto dai trasportatori della glicoproteina P spesso pompa i farmaci nel lume intestinale. Ciò riduce l'assorbimento del farmaco nel sangue.
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Q1: Why is the small intestine a better site for drug absorption than the stomach?
The small intestine provides superior drug absorption due to its larger surface area created by intestinal villi, combined with higher blood flow. In contrast, the stomach has a smaller surface area and a thick mucous layer that hinders diffusion. These anatomical and physiological features make the intestine the primary site for drug absorption.
Q2: How does food in the stomach affect oral drug absorption?
Food interferes with drug absorption through two mechanisms: drugs form complexes with food constituents, reducing their bioavailability, and food delays gastric emptying, slowing the movement of drugs into the small intestine. This delayed transit further reduces the rate of intestinal absorption, ultimately decreasing overall drug absorption.
Q3: What role do P-glycoprotein transporters play in drug absorption?
P-glycoprotein transporters in the gut epithelium actively pump drugs back into the gut lumen, reducing their absorption into the bloodstream. This efflux mechanism acts as a barrier to drug absorption, limiting the amount of drug that enters systemic circulation from the gastrointestinal tract.
Q4: Why are weakly acidic drugs absorbed more readily from the stomach?
Weakly acidic drugs are predominantly nonionized in the stomach's acidic environment, allowing them to cross the stomach lining more easily through passive diffusion. In contrast, drugs in the upper intestine are often ionized, making them less able to penetrate the intestinal epithelium, resulting in lower absorption rates.
Q5: How does diarrhea impact drug absorption from the gastrointestinal tract?
Diarrhea accelerates the movement of gut contents through the gastrointestinal tract, reducing the time available for drug absorption. This rapid transit limits drug contact with the intestinal epithelium, resulting in decreased absorption and lower drug bioavailability in the bloodstream overall.
Q6: What is the relationship between surface area and drug absorption in the GI tract?
Surface area directly influences absorption rates: the intestine's larger surface area, enhanced by villi, supports greater drug absorption compared to the stomach's smaller surface. Combined with higher intestinal blood flow, this increased surface area facilitates more efficient drug uptake into systemic circulation.
Q7: How do ionization states of drugs affect their absorption at different GI sites?
Drug ionization state determines absorption efficiency at specific GI locations. Nonionized drugs, like weakly acidic drugs in the acidic stomach, cross membranes readily. Ionized drugs in the upper intestine have reduced membrane permeability, though the intestine's superior surface area and blood flow often compensate for lower ionization-based absorption.