How does the concept of 'intrinsic activity' relate to the difference between a full opioid agonist and a partial opioid agonist?

Intrinsic activity, also known as efficacy, is a measure of the ability of a drug to activate a receptor and produce a maximal biological response once bound. It explains the difference between full opioid agonists and partial opioid agonists. A full opioid agonist, such as morphine or fentanyl, has high intrinsic activity at the mu (μ) opioid receptor. This means that when a full agonist binds to the mu receptor, it can elicit a maximal response, such as full pain relief or maximal respiratory depression. Even if more receptors are available, the drug cannot produce a greater effect because it has already reached its maximum capacity to activate the receptor. A partial opioid agonist, such as buprenorphine, has lower intrinsic activity at the mu opioid receptor compared to a full agonist. When a partial agonist binds to the mu receptor, it can only elicit a submaximal response, even when all available receptors are occupied. This means that even at high doses, a partial agonist cannot produce the same level of effect as a full agonist. In other words, a partial agonist has a 'ceiling effect' on its maximal response. Furthermore, a partial agonist can act as an antagonist in the presence of a full agonist. If a partial agonist is bound to the receptor, it prevents the full agonist from binding and producing its maximal effect. The concept of intrinsic activity is essential for understanding the differing clinical effects and safety profiles of full and partial opioid agonists.