Drug half-lifes: a question for debate

[Richenda]

I have a question about drug half-lifes. To convey the point I'll need to use a fictional measurement; it's fictional so if you reply using it please don't put a real measurement. This is an important query to me and I don't want a mod's chop.

Okay, so take a drug like Cypreterone Acetate. It has a half-life of 39 hours + or - 12 hours. http://www.medsafe.govt.nz/profs/datasheet/s/Siteronetab.pdf

Suppose your target dose is 1000 fmf (fmf = fictional measurement figure).

Some people might suggest to take that 1000 fmf every other day.

But surely, because of the near two day half life, the same bioavailability occurs in your system if you were to take 500 fmf a day? By the second day your serum level will be identical to if you were taking the full amount every other day, and will remain like that.

I just want to repeat that my question is about half lifes and their effect on bioavailability.

[Dena]

The problem with dosing every other day is initially you would be way above the targeted level and at the end of two days, you would be below it. Every day would reduce the peaks and valley and without the valleys as low, the drug would be far more effective at blocking T production. If a drug is more effective, that allows you to use less of it.  Some drugs are are administered with a pump that delivers the dose over a period of days in order to maintain a uniform level in the blood. A 40 hour half life wouldn't require a constant drip but every other day isn't as good as every day.

[Richenda]

Thanks Dena so much. That confirms what I was wondering.

[Randi]

I once ran a spreadsheet with a half-life of 7 days, and administration every 7 days.

The amount from the first dosage is 1/2 after one week, 1/4 the next, 1/8 the next ad infinitum.

Even without calculus, it becomes apparent the the serum levels APPROACH twice the dosage amount (without ever actually getting there).

The "take away" is that after a dozen or so iterations, the weekly fluctuations are greatly reduced.  After several months, even if a dose were missed or delayed, there is a substantial reserve circulating because of all the other earlier doses.