Bioequivalence and Bioavailability Forum

¡Hola Ricardo!

❝ […] a drug which binds 75% to plasma proteins and 98% to erythrocyte carbonic anhydrase, […] excreted unchanged at 70% in urine.

Let’s call the drug chlorthalidone.

❝ Do i have to use plasma, whole blood or urine […]?

If my guess is right – whole blood. Sorry, one of the rare exceptions. Yes, I know, it’s a mess, but check the literature yourself.
Hints for analytical method development: 1 mL thawed whole blood + 3 mL H₂0, centrifuge at 2000×g at +4 ℃ for 20 minutes, C18 SPE… It should be possible to get inaccuarcy ~±5% at a precision of ~5%, no problems with freeze-thaw-cycles, and storage at -20 ℃ for at least three months. Don’t worry about a recovery of ~70%.

Good luck!