Drug Interactions between iobenguane I 131 and methyclothiazide / reserpine

GENERALLY AVOID: Coadministration with drugs that reduce catecholamine uptake or deplete catecholamine stores may interfere with iobenguane I-131 uptake into neuroendocrine tumors such as pheochromocytoma and paraganglioma that express high levels of norepinephrine transporter on their cell surfaces. Since iobenguane is similar in structure to norepinephrine and is subject to the same uptake and accumulation pathways as norepinephrine, drugs that alter norepinephrine disposition in adrenergic nerve terminals and presynaptic storage vesicles will likewise affect iobenguane. Dosimetry calculations and efficacy of iobenguane I-131 may be altered. These drugs include central nervous system stimulants (e.g., amphetamines, cocaine, methylphenidate); norepinephrine and dopamine reuptake inhibitors (e.g., phentermine); norepinephrine and serotonin reuptake inhibitors (e.g., tramadol); central monoamine depleting drugs (e.g., reserpine); nonselective beta-adrenergic blockers (e.g., labetalol); alpha agonists or alpha/beta agonists (e.g., pseudoephedrine, phenylephrine, ephedrine, phenylpropanolamine, naphazoline); monoamine oxidase inhibitors; tricyclic antidepressants; norepinephrine reuptake inhibitors (e.g., bupropion, duloxetine, mirtazapine, venlafaxine); and botanicals that may inhibit reuptake of norepinephrine, serotonin, or dopamine (e.g., ephedra, ma huang, St John's wort, yohimbine). These drugs were not permitted in clinical trials that assessed the safety and efficacy of iobenguane I-131.

MANAGEMENT: Drugs that reduce catecholamine uptake or deplete catecholamine stores should be discontinued for at least five biological half-lives before administration of either the dosimetry dose or a therapeutic dose of iobenguane I-131. Do not administer these drugs until at least 7 days after each iobenguane I-131 dose. Patients should be monitored for the occurrence of clinically significant withdrawal symptoms, especially patients with elevated levels of circulating catecholamines and their metabolites.

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MONITOR: The hypotensive effects of thiazide diuretics and alpha-adrenergic blockers may be additive. Postural hypotension may occur.

MANAGEMENT: Hemodynamic responses should be monitored during coadministration, especially during the first few weeks of therapy. Patients should be advised to take the alpha-blocker at bedtime and to notify their physician if they experience dizziness or syncope while awake.

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