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Molecular imaging in gene therapy

Prof. G.M. van Dam, MD, PhD

In many ways gene therapy is still an experimental treatment approach for oncology, metabolic and infectious diseases. So how do you know if your gene is incorporated in the DNA of the patient? Or that it is functional? With molecular imaging you can already evaluate the success of your gene therapy in vivo at an early stage.

What is gene therapy?

Simply said, gene therapy is a technique where you modify the (human) genome to treat or cure a disease. You can modify the genome by:

  1. Replacing a gene that causes a disease by a healthy copy of the gene (transgene)
  2. Inactivating a disease-causing gene
  3. Introducing a new or modified gene in the genome that can treat the disease (transgene)
  4. Cell based gene therapy: cells are removed from the patient, genetically modified (often using a viral vector) and then returned to the patient.

If you want to modify a gene in the human body, you need to use DNA or RNA products. Then to deliver the DNA product into human cells, you can use bacterial or viral vectors. However, the virus or bacteria can infect the cells in your body. That is why besides modifying the genetic information of the bacteria or virus with the gene of interest, you should also modify it in such a way that it cannot cause disease in humans.

How to determine the efficacy of your gene therapy.

The first thing you need to establish when you develop a (new) gene therapy approach is that your gene of interest incorporates in the DNA of the patient. Furthermore, you want to know if the incorporated gene of interest is functional. In the human body, genes are transcribed into proteins when a biological process is initiated. So, ideally you would also want to confirm that your gene of interest is transcribed in the organ/tissue of interest.

Molecular imaging in gene therapy.

With molecular imaging you can evaluate the success of your gene therapy in vivo. When applied to gene and cell therapy, molecular imaging can be categorized as follows:

  1. Direct imaging to monitor the delivery process of therapeutic genes/cells
  2. Indirect imaging to measure, localize, and quantify the function of therapeutic genes/cells.

Direct imaging of gene therapy.

With direct imaging you directly label the DNA or RNA. For example, it is possible to couple a chelator to the DNA to allow labeling with a radiometal (for example In-111, Zr-89, Ga-68). Another option is to directly label the DNA with F-18, radioisotopes of iodine or C-11. Although labeling of DNA and RNA is feasible, clinical use is limited.

You can also deliver the DNA to patients by encapsulating them in nanoparticles or liposomes. In that case you can label these particles with a radioisotope for PET or SPECT imaging. This direct labeling approach gives you information on the distribution of the gene therapy platform. However, you would expect that your gene therapy drug product will distribute throughout the body. So, it does not give you information on the incorporation of your transgene in the body.  Also, it does not give you information on the level of incorporation in the human genome or the transcription of your transgene. Therefore, you may still want to use “indirect imaging” to visualize the functionality of the transgene.

Indirect imaging of gene therapy with reporter genes.

By indirect labeling you visualize the incorporation of the target gene in the human genome. You can do this by using an additional transgene. Also referred to as a reporter gene. The reporter gene will be transcribed, and as a result a protein will be expressed on the cell surface. With a radiotracer you can visualize the presence of the protein on the cell surface.

An ideal reporter gene is one that is not endogenously expressed in the cell of interest. Further, you want it to be amenable to assays that are sensitive, quantitative, rapid, easy, reproducible, and safe. For example, I-124-FIAU), [F-18]-FHBG, and F-18]–FEAU serve as SPECT and PET imaging substrates for the (pre-)clinical evaluation of the expression of this enzyme in small- and large-animal models. This includes clinical applications in humans (Molecular Imaging. Ross B and Gambhir SS).

You can also target the protein transcribed by the therapeutic transgene with a radiotracer (without the use of a reporter gene). In that case your target needs to be accessible (i.e., on the cell surface). Furthermore, you need to have a radiotracer available (or need to develop a radiotracer).

A note on molecular imaging of cell-based therapy.

In the case of cell-based therapy you can i) label the cells directly with a radionuclide ii) target a specific marker on the cell surface with a radiotracer or iii) make use of a reporter gene. A detailed description on cell-based imaging can be found in our blog on CAR-T cell imaging.

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