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pKa and Drug Solubility – Absorption and Distribution | Pharmacokinetics (PK)

by Pravin Shukle, MD
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    Okay. Let's talk about a new topic. Something called pKa. Now look at this question. Spend a little bit of time reading it. And then we will try and go through the answer together. Okay so hopefully you have read the question. What I want to do is I want to focus a skill that you need for your exam. When you are writing an exam and you see a long stem like this especially for the USMLE exam. Look at the last sentence first. Because what that's going to do it's gonna focus your attention. Now in this particular question, the last sentence says that it has a pKa of 3.8 and what percentage of the drug will be hydrophobic. Well, all of a sudden you have realised that the first half of that question is completely useless to getting your answer. So you can actually just ignore a lot of that clinical information and get to the answer much quicker. So now what we do is we want to pick between the answers. 100%, 50%, 10% and 1%. So what percentage of the drug will be hydrophobic at a pH of 4.8 if the pKa is 3.8. Well what is pKa? Let's go over that together. So, aspirin has a pKa of 3.8. So what is pKa? The pKa is that pH level at which a drug is 50% protonated and 50% non-protonated. So the ratio of protonated and non-protonated forms is 1:1. For weak acids and you can see that because the pKa is 3.8 which is less than 7, that's an acid. So, for all weak acids, the ratio changes to 1:10 or 10% at 1 pH unit more alkali than the pH. And the ratio changes to 1:100 or 1% at 2 pH units. So...

    About the Lecture

    The lecture pKa and Drug Solubility – Absorption and Distribution | Pharmacokinetics (PK) by Pravin Shukle, MD is from the course Pharmacokinetics and Pharmacodynamics.


    Included Quiz Questions

    1. Acidify the urine. This will protonate the drug, allowing it to be more soluble in the urine.
    2. Alkalinize the urine. This will protonate the drug, allowing it to be more soluble in the urine.
    3. Alkalinize the urine. This will deprotonate the drug, allowing it to be more soluble in the urine.
    4. Acidify the urine. This will deprotonate the drug, allowing it to be more soluble in the urine.
    1. 99%
    2. 10%
    3. 100%
    4. 1%
    1. 99%
    2. 90%
    3. 9%
    4. 1%
    1. 10%
    2. 90%
    3. 9%
    4. 1%
    5. 99%

    Author of lecture pKa and Drug Solubility – Absorption and Distribution | Pharmacokinetics (PK)

     Pravin Shukle, MD

    Pravin Shukle, MD


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