3.4
薬物吸収：胃腸吸収に影響を与える要因
口からの薬物吸収のプロセスには、いくつかの要因が影響を与えることがあります。弱酸性の薬物は、非イオン状態であるため、胃から容易に吸収される傾向があります。しかし、薬物は頻繁にイオン化されるため、上部腸での吸収効率は劣る場合があります。興味深いことに、胃が薬物吸収に有利に見えるにもかかわらず、その粘液…
経口投与された薬物の吸収は、さまざまな要因の影響を受けます。
弱酸性の薬物は、主に非イオン化されている胃から吸収される可能性が高くなります。
それに比べて、イオン化された薬物を含む上部腸からの吸収は低いかもしれません。
実際には、胃の粘膜層が拡散を困難にしています。腸に比べて表面積が小さいため、胃からの吸収率がさらに低下します。
腸絨毛はより大きな表面積を提供し、それがより高い血流と相まって、より高い薬物吸収につながります。
食物が胃の中にある場合、薬物は食物成分と複合体を形成するため、吸収されにくい可能性があります。
食物はまた、胃内容排出、つまり小腸への薬物の移動を遅らせ、腸の吸収を遅らせます。
下痢中の腸内容物の急速な動きは、薬物の吸収をさらに遅くします。
また、P-糖タンパク質トランスポーターで構成される腸上皮は、多くの場合、薬物を腸内腔に戻します。これにより、血液への薬物の吸収が減少します。
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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.