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Labeling of antibodies and antibody derivatives for molecular imaging

Maarten Brom, PhD

Antibodies are used for immunotherapy of cancer. After radiolabeling the antibody, the targeting of the antibody to the tumor can be visualized by PET or SPECT. The imaging data gives information on the targeting properties of the antibody and can predict whether immunotherapy would be successful. When you label the antibody with a therapeutic radionuclide (e.g., alpha or beta-emitters), you can use it to destroy the tumor with radioimmunotherapy.

In short

In this article we’ll discuss the use of antibodies, antibody fragments, single-chain variable fragments (scFv), diabodies, nanobodies and scaffold proteins. Although this article focuses on tumor targeting, at TRACER we also apply labeling techniques with other compound structures and/or therapeutic areas. With a radiolabeled compound the pharmacokinetics and biodistribution of a new drug can be studied in the target population before going into the Phase-I-III trials.

Book your TRACER knowledge session to discuss antibody labeling for your pipeline.


You can develop and produce antibodies against any target of choice on tumor cells. Antibodies usually bind strong and selective to the target. Due to their size (150 kilodalton (kDa)) they circulate in the blood for a long time after injection. When you use a radiolabeled antibody for tumor targeting, the optimal time point for imaging is between 4-7 days.

Disadvantages of antibodies

  • Because of the long circulation in blood, the radiation burden to the patient is high
  • The large size of the antibody makes it more difficult to leave the bloodstream (extravasation)
  • Also the size of the antibodies has a negative effect on penetrating tissue to reach the target on the tumor

Antibody fragments can help to overcome these issues.

Antibody fragments

To overcome long circulation time of antibodies, you can generate antibody fragments ((Fab’)2 or Fab fragments). You can digest the antibody with an enzyme and isolate the binding fragments of the antibody. The antibody fragments are smaller than the complete antibody (55 kDa or 110 kDa for Fab and (Fab’)2, respectively).

Advantages of antibody fragments

  • Because the antibody fragments are smaller, they are cleared from the blood much quicker
  • Extravasation is similar to complete antibodies
  • Antibody fragments still have strong binding to the target, although it is not as strong as the complete antibody
  • With radiolabeled antibody fragments you can acquire images on the same day or 1 day after injection
  • Radiation burden to the patient is significantly lower because the antibody fragments clear much faster

Engineered antibody fragments: chain variable fragments (scFv), diabodies and nanobodies

You can also engineer even smaller fragments of the antibody. Some examples are single chain variable fragments (scFv), diabodies and nanobodies. Since these compounds are much smaller, they are excreted from the blood rapidly. Also, they can extravasate quicker than complete antibodies. As a result, you can acquire images within hour(s). This also reduces the radiation exposure to the patient.

Engineered scaffold protein

All the agents mentioned above are based on the structure of an antibody. It is also possible to design a protein that doesn’t look like an antibody structurally. Nevertheless, it is also a protein with specific and strong binding to a tumor target. Some examples are Affibodies and DARPins. They have the same optimal imaging properties as the engineered antibody fragments. A major advantage is that scaffold proteins can be engineered to target a different receptor on the tumor cell, using the same scaffold platform.

Choosing the right radionuclide

The type of radionuclide for labeling your compound depends on the type of antibody (fragment) you want to use and your imaging modality. At TRACER, we can assist you in the labeling of your compound and the consecutive (first-in-human) imaging study.

TRACER knowledge session

Contact TRACER to get an in-depth overview of the different labeling options for your compound. This meeting takes approximately 30 minutes. During this one-on-one we’ll advice you on what radiolabel best matches your antibody. We also inform you about how Phase 0 clinical trials positively affect funding, development time and development cost. After this meeting you’ll be able to design your trial to answer your study question.

Book your TRACER knowledge session

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