GasGasGas - The FRCA Primary Exam Podcast Gas Gas Gas - pKa
Sep 5, 2024
Dive into the fascinating world of pKa and its critical role in drug absorption. Discover how pKa defines when a drug is ionized and why only unionized drugs can effectively cross membranes. Learn through examples like levobupivacaine and aspirin, revealing how pH influences drug solubility and absorption in different environments. Explore practical applications for local anesthetics and opioids, making complex concepts accessible and entertaining. Don't miss the insights on core principles that shape pharmacology!
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pKa Determines Membrane Passage
- pKa indicates the pH where a drug is 50% ionized and 50% unionized.
- Only the unionized portion crosses cell membranes to reach receptors on cells.
High pKa Reduces Unionized Fraction
- A drug with pKa above physiological pH will be mostly ionized at pH 7.4.
- Levobupivacaine (pKa 8.1) is only ~15% unionized at physiological pH, limiting initial membrane crossing.
What pKa Actually Means
- pKa breaks down into p (negative logarithm), K (equilibrium constant) and a (acid dissociation).
- The value reflects an acid's propensity to dissociate into ions in water.