Technology - Clinical Imaging of Cell Kinetics
- CellSight scientists received the first Investigational New Drug approval from United States FDA for an imaging reporter probe: [18F]FHBG.
- [18F]FHBG has been used to image expression of therapeutic transgenes and location of therapeutic cells in cancer patients.
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The PRG Herpes Simplex Virus 1 thymidine kinase (HSV1-tk) or its mutants (example: HSV1-sr39tk) can be delivered into cells by viral transduction, plasmid transfection or electroporation/nucleofection. HSV1-sr39tk encodes the HSV1-sr39TK enzyme that can catalyze phosphorylation of PRPs such as [18F]FHBG (9-[4-[18F]fluoro-3-(hydroxymethyl)butyl]guanine). HSV1-tk or HSV1-sr39tk can also serve as therapeutic transgenes (suicide gene) or safety genes, because cells expressing them can be killed when exposed to a pharmacological dose of antiviral drugs, Ganciclovir and Penciclovir.
Examples of [18F]FHBG Clinical Studies
Imaging Biodistribution of [18F]FHBG in a Human
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The figure to the right illustrates [18F]FHBG imaging in a human whose cells do not express a PET reporter gene; Hence normal biodistribution of [18F]FHBG. As shown, [18F]FHBG does not cross the blood brain barrier hence it can only accumulate within the central nervous system when the blood brain barrier is compromised. [18F]FHBG clearance is rapid from cells not expressing its PET reporter genes, resulting in low background. [18F]FHBG clears through the hepatobilliary and urinary pathways. Therefore declining activity can be seen in the liver and kidneys and increasing activities can be seen in the gall bladder, intestines and bladder.
[18F]FHBG Imaging of Targeted Cytolytic T Cells (CTLs) In Glioma Patients
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Autologous CD8+ T cells expanded with IL-2 treatment were electroporated with a plasmid to express Hygromycin resistant gene, Herpes Simplex virus type 1 thymidine kinase (HSV1-tk, serves as a PET reporter gene and a safety gene), and IL-13 Zetakine (to target CTLs to glioma cells). Clones were selected with Hygromycin and verified. The clone was expanded in culture to obtain > 109 CTLs. CTLs were infused over a period of 5 weeks into the tumor resection site of recurrent glioma patients.
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This image shows MRI and [18F]FHBG PET over MRI superimposed brain images of a patient who had received full CTL regimen. Images were acquired approximately two hours after [18F]FHBG injection. The patient had a surgically resected tumor (1) in the right parietal lobe and a new non-resected tumor in the center (2), near corpus callosum of his brain. The infused cells had localized at the site of tumor 1 and also trafficked to tumor 2. [18F]FHBG activity is higher than the brain background at both sites. Background [18F]FHBG activity is low within the Central Nervous System due to its inability to cross the blood brain barrier. Background activity is relatively higher in all other tissues. Activity can also be observed in the meninges. The tumor 1/meninges and tumor 2/meninges [18F]FHBG activity ratio in this patient was 1.75 and 1.57, respectively. Whereas the average resected tumor site/meninges and intact tumor site to meninges [18F]FHBG activity ratio in control patients was 0.86 and 0.44, respectively.
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[18F]FHBG PET images of another recurrent glioma patient. This patient had been scanned both prior to initiation of CTL infusions and three days after completion of all CTL infusions. There was a 60% increase in [18F]FHBG activity at the tumor resection site, where CTLs had been infused.
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[18F]FHBG head PET images superimposed over corresponding MRI images of the same patient in the image to the left, illustrating increased [18F]FHBG accumulation after CTL infusions at the recurrent tumor resection site. Images acquired approximately 2 hours after bolus intravenous [18F]FHBG injection.