A huge challenge in clinical pharmacy is determining what information is actually relevant. By relevant, I mean what information actually impacts patient care.
I see gabapentin most frequently used for neuropathy. The pharmacokinetics of gabapentin are a little unique in that gabapentin has an inverse dose dependent absorption. What does this mean?
This means that as you increase the dose, you actually get less (percentage) of the medication absorbed. Clinically this means that as you get to higher doses, you may not get as great of an anticipated response in neuropathy treatment given an increase in dose.
Why does this happen? Gabapentin requires a transporter to get across the GI tract. As the dose increases, this transporter gets “saturated” with drug. It can’t get all that drug transported across before it moves further down the gut. Think about one lane on a busy New York City bridge, if only a few cars are there, a higher percentage will get through. If a lot of cars are there, less will get through.
From Lexi-comp, here are the bioavailability numbers:
- 900mg/day 60% is absorbed
- 1,200 mg/day 47% is absorbed
- 2,400 mg/day 34% is absorbed
Doing the math here on how much is actually absorbed
- 900X0.6 = 540mg
- 1,200X0.47 = 564mg
- 2,400X0.34 = 816mg
Always keep in mind that these are approximations, but I think these numbers demonstrate what I’m talking about. So if you see someone already on moderate to high doses, and continue to push the dose, a 100 mg increase at low dose will likely cause more of a response than a 100 mg increase at higher doses. Also remember that once absorbed, the kidneys come into play which can change the game again 🙂
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