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
1. **Gut lumen**: degradation by gastric acid, digestive enzymes, or gut bacteria
2. **Gut wall**: CYP3A4 and efflux transporters (P-glycoprotein) in enterocytes metabolize or pump drug back into the lumen
3. **Portal circulation**: drug travels to the liver
4. **Hepatic extraction**: CYP enzymes metabolize drug on first pass
5. **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