Intrinsic Activity (alpha)

A quantitative measure of a drug's ability to produce a maximal response upon binding to a receptor, expressed as a fraction relative to a full agonist (alpha = 1 for full agonist, 0 < alpha < 1 for partial agonist, alpha = 0 for antagonist). Originally defined by Ariens, intrinsic activity relates to efficacy but is an empirical measure that depends on the tissue and assay system used.

Exemples

• Isoproterenol: alpha = 1.0 at beta-adrenergic receptors (full agonist reference)
• Buprenorphine: alpha ~0.4-0.6 at mu-opioid receptors (partial agonist)
• Naloxone: alpha = 0 at mu-opioid receptors (pure antagonist)