MIBG (meta-iodobenzylguanidine) is a compound used in nuclear medicine to detect and treat certain tumors, particularly neuroblastoma and pheochromocytoma. It works because its chemical structure closely resembles norepinephrine, a hormone produced by nerve and adrenal gland cells. This similarity allows MIBG to be taken up and stored by the same cells that absorb norepinephrine, making those cells visible on a scan or, at higher doses, targetable with radiation therapy.
How MIBG Works in the Body
MIBG contains a benzyl group and a guanidine group, giving it a shape similar enough to norepinephrine that the body’s cells can’t easily tell them apart. Neuroendocrine cells, the type found in certain tumors and in the nervous system of the heart, actively pull MIBG inside through the same transporter they use for norepinephrine. This process involves both an active, energy-dependent mechanism and, to a lesser extent, passive diffusion.
Once inside, the cells store MIBG in the same tiny compartments (called neurosecretory granules) where they would normally store norepinephrine. The key difference is that MIBG has been tagged with a radioactive form of iodine. Depending on which form of iodine is attached, the compound either lights up on a scanner or delivers a targeted dose of radiation to destroy the cell from within.
MIBG as a Diagnostic Scan
For imaging purposes, MIBG is labeled with iodine-123, a form of iodine that emits gamma rays a camera can detect. This version is used primarily to find neuroblastoma (a cancer most common in young children), pheochromocytoma (a rare adrenal gland tumor), and other neural crest tumors. Because these tumors are made up of cells that aggressively take up norepinephrine-like compounds, they absorb far more MIBG than surrounding tissue and show up as bright spots on the scan.
MIBG scans are also used to evaluate the heart. In heart failure, the sympathetic nerves supplying the heart can become damaged, and an MIBG scan can measure how well those nerves are functioning. Doctors compare the amount of tracer picked up by the heart to the amount in the chest (a measurement called the heart-to-mediastinum ratio). A ratio of 1.6 or higher identifies heart failure patients at lower risk of a serious cardiac event. The same type of scan has shown promise in distinguishing Parkinson’s disease from similar neurological conditions, since Parkinson’s tends to damage the heart’s sympathetic nerves in a characteristic pattern.
What the Scan Procedure Looks Like
An MIBG scan is done in stages over several days. During the first visit, a healthcare provider injects the tracer into a vein in your arm or hand. The tracer then needs roughly 24 hours to circulate through your body and concentrate in the cells that take it up. You return the next day for your first imaging session, and most people come back at least once more over the following days for repeat scans. In total, expect two to four visits, with each scan lasting one to two hours.
Before the scan, you may need to stop or switch certain medications that interfere with how cells absorb MIBG. These include some blood pressure medications, antidepressants, antipsychotics, diet pills, and most over-the-counter nasal decongestant sprays. Your doctor will give you specific instructions about which medications to pause and when.
Because the tracer contains radioactive iodine, your thyroid gland would naturally absorb some of it. To prevent unnecessary radiation exposure to the thyroid, you’ll be given potassium iodide (typically 100 mg twice daily) starting the day before the injection and continuing for one to three days afterward for iodine-123 scans, or up to a week for iodine-131 procedures. The potassium iodide saturates your thyroid with stable iodine so it doesn’t take up the radioactive form.
MIBG as a Cancer Treatment
When labeled with iodine-131 instead of iodine-123, MIBG becomes a targeted radiation therapy. Iodine-131 emits beta particles, which are potent enough to kill cells but travel only a short distance, concentrating the radiation dose on the tumor cells that absorbed the compound. This approach was first used to treat neuroblastoma in 1986 and has since become an established option for advanced or relapsed cases.
Therapeutic MIBG is used for stage III or IV neuroblastoma in patients whose tumors have already been confirmed to absorb MIBG on a diagnostic scan. The goals range from achieving complete remission to slowing tumor growth or relieving symptoms from primary or metastatic tumors. In a phase I study at the University of California, San Francisco, 30 patients with relapsed or treatment-resistant neuroblastoma received escalating doses. The overall response rate was 37%, with the best results seen at higher doses. At the highest doses studied, patients needed stem cell support afterward because the radiation can temporarily suppress bone marrow function.
Combination approaches, pairing MIBG therapy with chemotherapy and stem cell transplant, have improved response rates in patients with resistant disease. MIBG therapy has also been studied as part of initial treatment for newly diagnosed neuroblastoma, where early results have shown it to be feasible.
FDA-Approved Therapeutic Use
On July 30, 2018, the FDA approved a therapeutic MIBG product called Azedra (iobenguane I-131) for a specific group of patients: adults and children aged 12 and older with pheochromocytoma or paraganglioma that is scan-positive, cannot be surgically removed, and has spread locally or to distant sites. This made Azedra the first FDA-approved systemic treatment specifically for these rare adrenal and nerve-related tumors. Patients must first demonstrate that their tumors take up MIBG on a diagnostic scan before qualifying for therapy.