Pharmacology question HELP

[aspiringmdgirl]

For doxycycline, more than 90% of the drug is bound to plasma proteins. It is also known that this drug is almost completely absorbed from the GI tract.
I thought that drugs bound to plasma proteins cannot be absorbed to be distributed to the tissues. If more than 90% are bound to plasma proteins, how can doxycycline be almost completely absorbed from GI tract?

Thanks in advance!

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[Ra1nMak3r]

This is more of a pharmacokinetics type question. The absorption of doxy comes from its lipophilicility. Usually lipophilic drugs have greater absorption through the GI. Protein binding occurs during distribution phase of the drug, once its systemically absorbed.

 

[Lnsean]

The absorption of drugs across the GI mucosa is dependent on the amount ionized versus the amount unionized. For weak acid drugs, like doxy, doxy remains in its acidic form (unionized) in the stomach and in the small intestine. Thus, it is readily absorbed mainly in the stomach and upper intestine. It is also the reason you should not take antacids with doxy because of this phenomenon. It is described by the Henderson-Hasselbalch equation. Plasma protein binding is after the drug has been absorbed through the GI mucosa and into the bloodstream.

 

[aspiringmdgirl]

So when we describe volume of distribution, the concentration of drug in plasma is referring to the systemic blood after absorption via GI tract?

Also, if the drugs are protein bound in the systemic blood, how can they be distributed to the tissues?

 

[Ra1nMak3r]

Protein binding is reversible process. The unbound drugs will distribute to the tissues first. Once drug concentration reaches steady state, the unbound drugs circulating in the plasma will be eliminated through excretion or metabolism. The drugs that are protein bound will start to unbound due to concentration gradients and will either be distributed to the tissues or eliminated.

 

[CetiAlphaFive]

So when we describe volume of distribution, the concentration of drug in plasma is referring to the systemic blood after absorption via GI tract?
Also, if the drugs are protein bound in the systemic blood, how can they be distributed to the tissues?

You're getting your ADME mixed up.

How could protein in the blood effect absorption from the lumen?

Also, you're looking at protein binding incorrectly.
It's not "X% of a static amount of the drug is bound to proteins"

 

[aspiringmdgirl]

You're getting your ADME mixed up.

How could protein in the blood effect absorption from the lumen?

Also, you're looking at protein binding incorrectly.
It's not "X% of a static amount of the drug is bound to proteins"

I think you are looking at my question incorrectly.

 

[CetiAlphaFive]

I think you are looking at my question incorrectly.

Ohhh, I think you're trying to understand a kinetics concept that you don't how to articulate.

Do they teach you about fraction unbound?