Intrinsic sympathomimetic activity: physiological reality or marketing phenomenon

Intrinsic sympathomimetic activity (ISA) describes the partial beta-adrenergic agonist responses elicited by a series of beta-adrenergic antagonists. The dual effect on the beta-adrenergic receptor appears to be related to structural specificity of the drugs allowing competitive binding to the receptor (antagonist activity) and partial interaction at the receptor's activation site (agonist activity). The clinical effects of a beta-adrenergic antagonist with ISA depend on the relative balance of the drug's inherent antagonist and agonist activity and on the degree of underlying sympathetic tone in the patient. Theoretically, the agonist activity may be beneficial in the patient in whom beta-adrenergic antagonists are indicated, but who has concomitant bradycardia and/or mild to moderate congestive heart failure or compromised pulmonary function, or in the patient being withdrawn from beta-adrenergic antagonist therapy. There is positive evidence from clinical trials that in select patient populations a few of these benefits of ISA are afforded without compromise to beta-adrenergic antagonist activity. However, predisposing factors such as acute illness and individual idiosyncrasies may interfere with the manifestations of the agonist effects. Further, maximal response to full beta-adrenergic agonists will be diminished by concurrent therapy with beta-adrenergic antagonists regardless of ISA presence. In summary, ISA does have a physiological basis and increased experience in larger patient populations will help to place it in proper clinical perspective.