What Is Radioimmunotherapy and How Does It Work?

Radioimmunotherapy uses antibodies armed with radioactive atoms to hunt down and irradiate cancer cells from the inside. The antibody finds a specific protein on the tumor’s surface, binds to it, and delivers a lethal dose of radiation directly to the cancer while largely sparing surrounding tissue. Two radioimmunotherapy drugs won FDA approval in the early 2000s for a type of blood cancer, and research has since expanded into solid tumors, brain cancers, and even infectious diseases.

How a Radioactive Antibody Finds Its Target

The core idea is straightforward. Antibodies are proteins the immune system naturally produces to latch onto specific molecular targets. In radioimmunotherapy, scientists take a monoclonal antibody designed to recognize a protein that sits on the surface of cancer cells and attach a radioactive isotope to it. Once injected into a patient’s bloodstream, the antibody circulates until it encounters cells displaying the right surface marker, binds to them, and irradiates the tumor from point-blank range.¹

The radioactive cargo does the killing. Unlike a naked antibody, which might slow tumor growth by blocking a signal or flagging the cell for the immune system, a radioimmunotherapy agent carries its own weapon. The radiation damages the DNA of cancer cells so severely that they cannot repair themselves and die. And because radiation travels a short distance through tissue, even neighboring cancer cells that the antibody did not directly bind can be caught in the crossfire. This “crossfire effect” is one of the advantages over therapies that require every single cancer cell to be directly engaged.

The Radioactive Payload Matters

Not all radiation behaves the same way, and the choice of isotope shapes what a radioimmunotherapy drug can do. The two broad categories used in this field are beta emitters and alpha emitters. Beta particles travel a few millimeters through tissue, making them useful against larger tumor masses because their radiation can reach cells several layers deep. Alpha particles travel only a fraction of a millimeter but deposit far more energy along that short path, making them extremely destructive to individual cells or small clusters.

A physics comparison found that alpha particles can deposit roughly a thousand times more dose to single cells than beta emitters, giving them a decisive edge against isolated cancer cells and tiny clusters.² That concentrated energy is especially relevant after surgery, when microscopic disease may be scattered through a body cavity. Alpha-emitter immunoconjugates look particularly promising for mopping up disseminated cancer cells and small tumor clusters left behind during resection of a primary tumor.³

In a head-to-head preclinical study comparing an alpha emitter (bismuth-213) and a beta emitter (lutetium-177) attached to the same antibody fragment, both significantly slowed tumor growth in mice bearing colon cancer grafts. Mice treated with the higher alpha-emitter dose survived about twice as long as untreated controls, and the beta-emitter group showed a comparable survival benefit.⁴ Both types have a place; the question is matching the right emitter to the right clinical scenario.

Keeping the Isotope Attached to the Antibody

If the radioactive atom falls off the antibody before it reaches the tumor, you get radiation in the wrong place and less at the target. This is where chemistry becomes critical. Metal isotopes are bonded to antibodies using a molecular cage called a chelator. The chelator has to grip the metal tightly under the warm, chemically messy conditions of the bloodstream, and it has to do so without damaging the antibody’s ability to recognize its target.⁵

Different chelator designs vary enormously in stability. Ring-shaped (macrocyclic) chelators, for instance, held onto their copper-64 payload in blood serum with less than six percent loss over 48 hours, while an older, open-chain chelator design performed much worse.⁶ Better chelators translate directly into less radiation going to the liver, kidneys, and bone marrow, so this seemingly minor chemical detail has real consequences for patient safety.

Lymphoma and the Rise and Fall of Two Approved Drugs

Radioimmunotherapy’s biggest clinical milestone came in blood cancers. Two drugs targeting CD20, a protein found on the surface of B cells, were approved by the FDA for non-Hodgkin lymphoma: Zevalin (yttrium-90-ibritumomab tiuxetan) in 2002 and Bexxar (iodine-131-tositumomab) in 2003.⁷ Both targeted the same surface protein, but they used different isotopes and had somewhat different side-effect profiles.

In a direct comparison involving patients with relapsed or treatment-resistant low-grade lymphoma, about 71 percent of Bexxar-treated patients and about 78 percent of Zevalin-treated patients had an objective response. Complete responses were seen in roughly a third of the Bexxar group and about 42 percent of the Zevalin group. The main toxicity for both was temporary drops in blood cell counts, which were more frequent in the Zevalin group but reversible.⁸

Despite these impressive response rates, both drugs were severely underused. Bexxar was voluntarily withdrawn from the market in 2014 due to low demand, and Zevalin survived commercially but barely. The reasons were largely logistical and economic rather than scientific. Referring physicians were often unfamiliar with nuclear medicine procedures, and the introduction of rituximab maintenance therapy gave oncologists a more convenient option that did not require coordination with a nuclear medicine department. Recent commentary has described the field as a “phoenix” potentially ready to rise again, with new combinations and next-generation agents reigniting interest.⁹

Economic modeling has offered some vindication for the approach. A cost-effectiveness analysis comparing radioimmunotherapy consolidation, rituximab maintenance, and observation after first-line treatment for follicular lymphoma found that radioimmunotherapy provided about one additional quality-adjusted life-year compared with observation, at roughly $40,851 per year gained. That is within the range generally considered cost-effective in the United States and was similar to the figures for rituximab maintenance.¹⁰

Why Solid Tumors Are Harder

Blood cancers are an ideal setting for radioimmunotherapy because the cancer cells float freely in the blood and lymphatic system, making them easy for circulating antibodies to reach. Solid tumors are a different story. A large, dense tumor mass presents several physical barriers that antibodies struggle to overcome: poor blood flow in the tumor interior, high fluid pressure inside the tumor that pushes molecules back out, blood vessel walls that antibodies must squeeze through, and uneven distribution of the target protein across different parts of the tumor.¹¹

As a result, only a fraction of the injected antibody actually reaches the tumor. Most of it ends up in the blood, liver, and other normal organs, which then absorb radiation they were never meant to receive. Researchers have explored a range of strategies to improve delivery, from using smaller antibody fragments that penetrate tissue more easily to combining radioimmunotherapy with drugs that normalize the tumor’s blood vessels.¹² Progress has been real but incremental, and solid-tumor radioimmunotherapy remains largely in the experimental stage.

Pretargeting and the Two-Step Trick

One of the cleverest workarounds for the solid-tumor problem is pretargeting. Instead of attaching the radioactive atom directly to the antibody and injecting the whole package at once, you separate the process into two steps. First, you give the patient a “cold” (non-radioactive) antibody that binds to the tumor and waits there. After the unbound antibody clears from the bloodstream over a day or two, you inject a small, fast-moving radioactive molecule designed to snap onto the antibody already sitting on the tumor.

Because the radioactive piece is small, it zips through the body quickly, finds the pre-planted antibody at the tumor, and locks on. Any radioactive molecules that miss the target are rapidly filtered out by the kidneys. The result is higher radiation at the tumor and dramatically lower radiation to non-target organs.¹³ Dosimetry calculations from one pretargeting study confirmed that radiation doses to healthy organs were low and comparable to those reported by other research groups.¹⁴

The Theranostic Angle

A related concept that has gained momentum across nuclear medicine is theranostics, the pairing of a diagnostic and a therapeutic agent that target the same molecular feature. You image the patient first with one radioactive label to see whether and where the tumor lights up, then treat with a therapeutic isotope attached to the same targeting molecule. This “see and treat” principle is already standard practice in some cancers, with paired agents used in neuroendocrine tumors and prostate cancer.¹⁵

In radioimmunotherapy specifically, theranostic pairs are emerging. One group developed a single antibody conjugate that could be labeled with zirconium-89 for PET imaging or lutetium-177 for therapy, targeting immune cells in the tumor environment of brain cancers. The imaging version revealed where those immune cells clustered, and the therapeutic version reduced their numbers and improved the effectiveness of checkpoint immunotherapy in mouse models.¹⁶ This kind of dual-purpose agent could let clinicians confirm a tumor will respond before committing to therapy.

Radiation and the Immune System Working Together

Radiation does more than just shred DNA. When cancer cells die from radiation, they can die in a particularly “loud” way that alerts the immune system. This process, known as immunogenic cell death, involves the dying cell releasing alarm signals and displaying internal proteins on its surface that attract immune cells to the scene. The immune system then inspects the debris, learns to recognize the tumor, and in some cases mounts a broader attack.¹⁷

This immune awakening can occasionally produce an “abscopal effect,” where tumors that were never directly irradiated shrink because the immune response triggered at the irradiated site spreads systemically. The effect is rare with radiation alone, but combining radiation with immunotherapy drugs, particularly checkpoint inhibitors that take the brakes off the immune response, seems to make it more frequent. Preclinical and early clinical work suggests that radioimmunotherapy combinations can act against both the irradiated tumor and distant, non-irradiated metastases.¹⁸ Researchers are actively investigating how the molecular machinery inside tumor cells, including enzymes that tag damaged proteins for disposal, governs whether radiation triggers this immune cascade or whether the tumor suppresses it.¹⁹

Measuring the Dose to Bone Marrow

The dose-limiting side effect of most radioimmunotherapy is damage to the bone marrow. The marrow produces blood cells, and because it is a rapidly dividing tissue bathed in blood, it catches a significant share of the circulating radiation. Severe drops in white blood cells, platelets, or red blood cells are the main safety concern, and getting the dose right requires careful dosimetry.

Two general methods exist for estimating the radiation dose to bone marrow. One relies on blood samples, using the radioactivity in the blood as a proxy for what the marrow is absorbing. The other uses imaging, directly measuring radioactivity in the marrow with a camera. In a study of lymphoma patients treated with iodine-131-rituximab, the imaging-based method found a marrow dose of about 1.02 Gy, compared with 0.81 Gy estimated from blood sampling. The blood-based method underestimated the actual marrow exposure, and only the imaging-based measurements correlated with the degree of white blood cell toxicity the patients experienced.²⁰ Other work has confirmed that imaging-based dosimetry more reliably predicts blood toxicity and is the better method when marrow involvement by lymphoma is present.²¹

Whole-body dosimetry approaches using combined imaging and CT have been validated as clinically workable and safe, giving physicians a practical way to personalize treatment doses rather than relying on one-size-fits-all protocols.²²

Nanobodies and Smaller Delivery Vehicles

Full-size antibodies are large molecules, roughly 150 kilodaltons. Their bulk means they clear slowly from the blood, which extends the time healthy organs are bathed in radiation, and they struggle to penetrate deep into solid tumors. A growing area of research focuses on nanobodies, antibody fragments derived from camelid animals (llamas and camels produce a naturally miniaturized antibody type) that are roughly a tenth the size of a conventional antibody.

Clinical and preclinical work has shown that radiolabeled nanobodies clear the blood faster than full antibodies, penetrate tumors more effectively, and produce high-contrast images within hours rather than days.²³ When paired with alpha emitters, HER2-targeting nanobodies could deliver highly localized, lethal radiation to individual cancer cells with minimal exposure to surrounding healthy tissue.²⁴

One recently developed nanobody targeting a protein called CEA, which is overexpressed in many gastrointestinal cancers, showed rapid kidney clearance, low background uptake, and high tumor-to-background ratios in animal models. In a first-in-human study of healthy volunteers, the gallium-68-labeled version was well tolerated, and its overall radiation exposure was actually lower than a standard PET scan using the common tracer FDG.²⁵ The kidney, however, remains the main dose-limiting organ for nanobodies, since they are small enough to be filtered and partially retained there.

Beyond Cancer

One of the more surprising chapters in radioimmunotherapy research involves infectious diseases. The same principle, an antibody directing a radioactive payload to a specific target, works against pathogens if you have an antibody that recognizes the microbe’s surface. Over a stretch of experimental work, researchers adapted radioimmunotherapy for fungal infections (Cryptococcus neoformans, Histoplasma capsulatum), bacterial infections (Streptococcus pneumoniae, Bacillus anthracis), and even HIV-1-infected cells.²⁶

In one proof-of-concept study of pneumococcal infection, bacteria exposed to a bismuth-213-labeled antibody in the lab were killed in a dose-dependent fashion. In mice with pneumococcal infections, 60 percent more animals survived in the treated group compared to untreated controls, and the treatment did not cause blood cell toxicity at the doses used.²⁷ This line of research remains preclinical, but it hints at a potential tool for drug-resistant infections where conventional antibiotics fail.

Manufacturing and the Business Side

Getting a radioimmunotherapy drug from a research lab to a patient’s vein involves challenges that do not exist for a conventional pill. Radioactive isotopes decay on a fixed schedule, so the drug has a built-in expiration timer measured in hours or days rather than months. Manufacturing, quality testing, shipping, and administering the drug all have to happen within that window. Hospital radiopharmacy units operate under different regulations than large-scale pharmaceutical factories, and the recent expansion of nuclear medicine has put pressure on regulatory frameworks to keep up.²⁸

Commercially, the field has attracted serious investment. The FDA approvals of Lutathera (a lutetium-177 peptide therapy for neuroendocrine tumors) and NETSPOT (a gallium-68 imaging agent) triggered a wave of corporate acquisitions totaling billions of dollars. Theranostic agents are also proving useful in drug development itself, because imaging can quickly reveal whether a new targeting molecule actually reaches the tumor in patients, potentially reducing the cost and time of bringing new therapies through clinical trials.²⁹ The business model that once seemed untenable, given the logistical demands, now looks viable enough that major pharmaceutical companies are building dedicated nuclear medicine divisions.

Reaching the Brain

Brain tumors present a unique delivery challenge because the blood-brain barrier blocks most large molecules. One approach under investigation for diffuse intrinsic pontine glioma, a devastating childhood brain cancer with essentially no effective treatments, bypasses the bloodstream entirely. Convection-enhanced delivery uses a thin catheter placed directly into the brain to infuse a radioimmunotherapy agent under gentle pressure, distributing it through the tumor tissue. An iodine-124-labeled antibody called 8H9 has been proposed as a theranostic agent for this application, allowing clinicians to image the distribution of the drug with PET while simultaneously delivering therapeutic radiation.³⁰ The strategy is still experimental, but it represents the kind of creative engineering required to push radioimmunotherapy into cancers where systemic injection alone cannot deliver a meaningful dose.