Which one of the opioids has maximum plasma protein binding capacity ?
**Core Concept**
Opioids have varying plasma protein binding capacities, which affect their distribution, potency, and potential for drug interactions. The degree of plasma protein binding influences the free fraction of the opioid available to exert pharmacological effects.
**Why the Correct Answer is Right**
Nalorphine has the highest plasma protein binding capacity among the opioids. This is due to its high affinity for plasma proteins, particularly albumin. The binding of nalorphine to plasma proteins reduces its free fraction, which in turn decreases its potency and potentially affects its analgesic efficacy. The high plasma protein binding capacity of nalorphine is also responsible for its relatively low volume of distribution.
**Why Each Wrong Option is Incorrect**
* **Option A:** Morphine has moderate plasma protein binding capacity, but it is not the highest among the opioids.
* **Option B:** Fentanyl has a high lipid solubility, which allows it to penetrate the brain and exert analgesic effects rapidly, but its plasma protein binding capacity is not the highest.
* **Option D:** Methadone has a high volume of distribution due to its lipophilicity, but its plasma protein binding capacity is not the highest among the opioids.
**Clinical Pearl / High-Yield Fact**
Opioids with high plasma protein binding capacities, such as nalorphine, may have reduced efficacy in patients with hypoalbuminemia or liver disease, as the free fraction of the opioid is decreased.
**Correct Answer:** C.