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Old 02-26-2015
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Default Pharm - pKa and absorption

An 85-year old man was recently admitted to a nursing facility. Medications taken orally by the patient include:

- Sertraline (base, pka = 9.5)
- Diazepam (base, pka = 3.0)
- Amiodarone (base, pka = 7.4)

- Theophylline (acid, pka = 8.8)
- Ibuprofen (acid, pka = 4.8)

Shortly after administration, which of the following drugs was most likely concentrated inside the gastric cells?

i.e. which drug best crosses the stomach and enters the blood.

My understanding is that all the bases will be protonated and ionized at the gastric pH of 1.5 and thus unable to be absorbed. This leaves the acids. By the Henderson-Hasselbach equation, the acid with the pka higher and further from the pH will have more drug in protonated (unionized) form. This would be Theophylline. The answer, however, is Ibuprofen.

The reasoning I've heard is something along the lines of Ibuprofen's pKa being closer to the pH of the solution and thus more of it being unionized and able to cross the stomach. I'm having a hard time making sense of this... what am I missing?

Thank you!!
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