انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة
الكلية كلية الصيدلة
القسم فرع الادوية والسموم
المرحلة 3
أستاذ المادة صباح نعمه كامل الثامر
5/22/2011 7:12:22 PM
pKa: More of the drug is in the ionized form
•?If a basic drug lives in a basic environment, then it will not want to accept a H+ and so it will exist preferably in its unionized form.
•?If pH = pKa: Equal amounts of the ionized and unionized forms
•?If pH < pKa: More of the drug is in the ionized form
•?If pH > pKa: More of the drug is in the unionized form
•?REMEMBER THAT A DRUG IS ABSORBED BETTER IN THE UNIONISED FORM
•?e.g. Aspirin is an acidic drug. In the stomach, the pH is from 1 - 3, which means that it is an acidic environment.
Hence most of the drug will exist in the unionized form and be better at passing through the lipid membranes (i.e. better absorbed - around 30% of the oral dose, the rest is absorbed in the small intestine). Do not confuse this with the solubility of aspirin. A drug in the ionized form is better soluble but not necessarily better absorbed.
•?Remember that the majority of a drug is absorbed in the small intestine, but acidic drugs also have a relatively large proportion of their concentration absorbed in the stomach.
•?On the other hand, basic drugs such as MIPRAMINE (an antidepressant), has 100% absorption in the small intestine.
pH changes in the gut
•?There is a 10 fold change in the pH as we move from the stomach to the small intestine:
•?Stomach pH: 1 - 3
•?Duodenum pH: 5
•?Small intestine pH 5 - 7
Factors affecting gastrointestinal absorption
•?Gastrointestinal contents
•?Generally, an empty stomach = better absorption since some drugs may interact with food.
•?Gastric retention time
•?Drugs which are well absorbed from the small intestine (e.g. ethanol) will benefit from an increased gastric emptying rate since more of the drug will be ejected from the stomach into the small intestine.
???????????????????????????????????????????????????????????????????????
•?Intestinal transit, GIT motility
•?First pass effect
•?Blood supply from the gut goes directly to the liver via the portal vein. Therefore, orally absorbed drugs will reach the liver before entering the systemic circulation. This will be bad if the drug is heavily metabolized by the liver, which in most cases inactivates the drug.
Bioavailabilty
•?The amount of drug reaching its site of action
•?If a drug is administered intravenously, and then the bioavailability will be 100% (provided we want the drug to be present in the plasma).
•?The bioavailability can be much less for other routes of administration.
•?e.g. 500?g of a drug is administered but only 100?g enters the circulation. That means 400?g is lost (wasted) in bodily excretions.
•?Factors affecting bioavailability:
•?Incomplete absorption
•?See the factors which affect absorption above
•?First pass effect
•?You should know what this is by now
•?Enterohepatic shunt
•?Lipid soluble drugs can make their way into bile and be excreted along with it. The drug can be excreted with the bile or reabsorbed with some bile which is reabsorbed via the enterohepatic circulation.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
|