Bioavailability and First-Pass Effect
Why oral drugs often deliver less active compound than the administered dose, and how first-pass metabolism reduces bioavailability.
Defining Bioavailability
Bioavailability (F) is the fraction of an administered drug dose that reaches the systemic circulation in unchanged form. By definition, intravenous administration has F = 1.0 (100%). For all other routes, bioavailability is less than or equal to 100%.
The standard measure compares AUC (area under the plasma concentration-time curve) after oral versus IV administration:
F = AUC(oral) / AUC(IV)
The First-Pass Effect
When a drug is swallowed, it is absorbed from the gastrointestinal tract into the portal vein, which delivers it directly to the liver before it reaches the systemic circulation. The liver may metabolize a substantial fraction of the drug during this first passage, dramatically reducing the amount available to exert a therapeutic effect.
Sequential Barriers to Oral Bioavailability
- Gut lumen: degradation by gastric acid, digestive enzymes, or gut bacteria
- Gut wall: CYP3A4 and efflux transporters (P-glycoprotein) in enterocytes metabolize or pump drug back into the lumen
- Portal circulation: drug travels to the liver
- Hepatic extraction: CYP enzymes metabolize drug on first pass
- Systemic circulation: only the surviving fraction is bioavailable
Drugs with High First-Pass Metabolism
| Drug | Oral Bioavailability | Primary Enzyme |
|---|---|---|
| Nitroglycerin | < 1% | Hepatic esterases |
| Morphine | ~25% | UGT2B7 |
| Propranolol | ~25% | CYP1A2, CYP2D6 |
| Verapamil | ~20% | CYP3A4 |
| Lidocaine | ~35% | CYP3A4 |
| Atorvastatin | ~14% | CYP3A4 |
Strategies to Bypass First-Pass
- Sublingual/buccal: nitroglycerin tablets dissolve under the tongue, absorbing directly into systemic veins
- Transdermal patches: fentanyl, nicotine bypass the portal system entirely
- Rectal (lower rectum): drains into inferior rectal veins, partially bypassing the liver
- Inhalation: pulmonary absorption directly into systemic circulation
- Prodrugs: designed to survive first-pass and activate later (e.g., enalapril to enalaprilat)
Clinical Significance
When switching a patient from IV to oral dosing, the oral dose must be increased to compensate for first-pass losses. Morphine requires roughly 3x the IV dose when given orally. Liver disease may paradoxically increase oral bioavailability by reducing hepatic extraction capacity.
Key Takeaways
- Bioavailability measures the fraction of drug reaching systemic circulation unchanged
- First-pass metabolism in gut wall and liver is the main cause of low oral bioavailability
- IV bioavailability is always 100% by definition
- Alternative routes (sublingual, transdermal, rectal) can partially or fully bypass first-pass
- Dose conversion between routes must account for bioavailability differences