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Receptor Affinity

The potency of a pharmacologic agent is determined in part by its affinity—ie, the attractive force between the agonist and its receptor.

An agonist with high affinity binds readily to its receptor and remains bound, eliciting a prolonged response¹
An agonist with low affinity binds less tightly to its receptor and disassociates quickly, causing a physiologic response that terminates rapidly¹

High affinity low affinity

Lexiscan is the first selective A_2A adenosine receptor agonist.^2,3 The affinity of Lexiscan for the A_2A adenosine receptor determines its duration of coronary vasodilation.¹

Lexiscan has a relatively low affinity (K_i≈1.3 µM) for the A_2A adenosine receptor^1,3
Its affinity for the A₁ adenosine receptor is at least 10-fold lower, and it has weak, if any, affinity for the A_2B and A₃ adenosine receptors³

Click here for information on the effect of Lexiscan on intracoronary blood flow

REFERENCES
1. Gao Z, Li Z, Baker SP, et al. Novel short-acting A_2A adenosine receptor agonists for coronary vasodilation: inverse relationship between affinity and duration of action of A_2A agonists. J Pharmacol Exp Ther. 2001;298:209-218.
2. Palle VP, Elzein EO, Gothe SA, et al. Structure-affinity relationships of the affinity of 2-pyrazolyl adenosine analogues for the adenosine A_2A receptor. Bioorg Med Chem Lett. 2002;12:2935-2939.
3. Lexiscan (regadenoson) injection [package insert]. Deerfield, IL: Astellas Pharma US, Inc.

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Lexiscan (regadenoson) injection is a pharmacologic stress agent indicated for radionuclide myocardial perfusion imaging (MPI) in patients unable to undergo adequate exercise stress.

IMPORTANT SAFETY INFORMATION

Do not administer Lexiscan to patients with second- or third-degree AV block or sinus node dysfunction unless these patients have a functioning artificial pacemaker.

Fatal cardiac arrest, life-threatening ventricular arrhythmias, and myocardial infarction may result from the ischemia induced by pharmacologic stress agents. Hypersensitivity including anaphylaxis, angioedema, cardiac or respiratory arrest, respiratory distress, decreased oxygen saturation, hypotension, throat tightness, urticaria and rashes have occurred. Resuscitation equipment and trained staff should be immediately available before administering Lexiscan.

Adenosine receptor agonists, including Lexiscan, can depress the SA and AV nodes and may cause first-, second-, or third-degree AV block, or sinus bradycardia requiring intervention. In postmarketing experience, heart block (including third degree), and asystole within minutes of Lexiscan administration have occurred.

Adenosine receptor agonists, including Lexiscan, induce arterial vasodilation and hypotension. The risk of serious hypotension may be higher in patients with autonomic dysfunction, hypovolemia, left main coronary artery stenosis, stenotic valvular heart disease, pericarditis or pericardial effusions, or stenotic carotid artery disease with cerebrovascular insufficiency. In postmarketing experience, transient ischemic attack, seizures and syncope have been observed.

Adenosine receptor agonists, including Lexiscan, may result in clinically significant increases in blood pressure in some patients. In postmarketing experience, cases of potentially clinically significant hypertension have been reported, particularly in patients with underlying hypertension and when low-level exercise was included in the MPI.

Adenosine receptor agonists, including Lexiscan, may cause dyspnea, bronchoconstriction and respiratory compromise. Appropriate bronchodilator therapy and resuscitative measures should be available prior to Lexiscan administration.

The most common adverse reactions (≥5%) to Lexiscan are dyspnea, headache, flushing, chest discomfort, angina pectoris or ST-segment depression, dizziness, chest pain, nausea, abdominal discomfort, dysgeusia, and feeling hot. Most adverse reactions began soon after dosing, and generally resolved within approximately 15 minutes, except for headache, which resolved in most patients within 30 minutes. Aminophylline was used as a reversal agent in 3% of patients.

In postmarketing experience, QTc prolongation, tremor, abdominal pain in association with nausea, vomiting, or myalgias, and diarrhea, fecal incontinence, wheezing and musculoskeletal pain have occurred.

Intravenous Adenoscan is indicated as an adjunct to thallium-201 myocardial perfusion scintigraphy in patients unable to exercise adequately.

IMPORTANT SAFETY INFORMATION

Adenoscan is contraindicated in patients with second- or third-degree AV block, unless these patients have a functioning artificial pacemaker, sinus node disease, and known or suspected bronchoconstrictive or bronchospastic lung disease.

Fatal cardiac arrest, sustained ventricular tachycardia (requiring resuscitation), and nonfatal myocardial infarction have been reported coincident with Adenoscan infusion. Patients with unstable angina may be at greater risk. Appropriate resuscitative measures should be available.

Adenoscan is a potent peripheral vasodilator and can cause significant hypotension. The risk of hypotension may be higher in patients with cardiac or cerebrovascular insufficiency.

Adenoscan exerts a direct depressant effect on the SA and AV nodes and has the potential to cause first-, second- or third-degree AV block, or sinus bradycardia.

Increases in systolic and diastolic pressure have been observed.

Adenosine receptor agonists, including Adenoscan, may cause bronchoconstriction and respiratory compromise.

Atrial fibrillation has been reported in patients with Adenoscan infusion and may last from a few seconds to hours, however, patients spontaneously converted to normal sinus rhythm.

Most common adverse reactions (≥5%) to Adenoscan are flushing, chest discomfort, dyspnea, headache, discomfort of the throat, neck, or jaw, gastrointestinal discomfort, and lightheadedness/dizziness. Side effects with Adenoscan usually resolve quickly when the infusion is discontinued, although delayed or persistent effects have been observed.

Lexiscan is a registered trademark of Astellas US LLC. Other trademarks listed belong to their respective owners.
Lexiscan was developed in collaboration with Gilead Palo Alto, Inc. (formerly CV Therapeutics, Inc.).