Ar Buursma

[18F]FHPG Positron Emission Tomography for Detection of Herpes Simplex Virus (HSV) in Experimental HSV Encephalitis

ABSTRACT Herpes simplex virus type 1 (HSV-1) is one of the most common causes of sporadic encephalitis. The initial clinical course of HSV encephalitis (HSE) is highly variable, and the infection may be rapidly fatal. For effective treatment with antiviral medication, an early diagnosis of HSE is crucial. Subtle brain infections with HSV may be causally related to neuropsychiatric disorders such as Alzheimers dementia. We investigated the feasibility of a noninvasive positron emission tomography (PET) imaging technique using [18F]FHPG as a tracer for the detection of HSE. For this purpose, rats received HSV-1 (infected group) or phosphate-buffered saline (control group) by intranasal application, and dynamic PET scans were acquired. In addition, the distribution of tracer accumulation in specific brain areas was studied with phosphor storage imaging. The PET images revealed that the overall brain uptake of [18F]FHPG was significantly higher for the infected group than for control animals. Phosphor storage images showed an enhanced accumulation of [18F]FHPG in regions known to be affected after intranasal infection with HSV. High-performance liquid chromatography metabolite analysis showed phosphorylated metabolites of [18F]FHPG in infected brains, proving that the increased [18F]FHPG uptake in infected brains was due to HSV thymidine kinase-mediated trapping. Freeze lesion experiments showed that damage to the blood-brain barrier could in principle induce elevated [18F]FHPG uptake, but this nonspecific tracer uptake could easily be discriminated from HSE-derived uptake by differences in the tracer kinetics. Our results show that [18F]FHPG PET is a promising tool for the detection of HSV encephalitis.

Scintigraphic imaging of HSVtk gene therapy

The evolution of molecular biology has enabled the exploration of novel sophisticated gene-directed treating modalities for cancer. Suicide gene therapy - i.e. transfection of a so-called suicide gene that sensitizes target cells towards a prodrug - may offer an attractive approach to treat malignant tumors. For the development of effective clinical suicide gene therapy protocols, a non-invasive method to assay the extent, the kinetics and the spatial distribution of transgene expression is essential. This would allow investigators and physicians to assess the efficiency of experimental and therapeutic gene transfection protocols and would enable early prognosis of therapy outcome. Radionuclide imaging techniques like single photon emission computed tomography (SPECT) and positron emission tomography (PET), which can non-invasively visualize and quantify metabolic processes in vivo, are being evaluated for repetitive monitoring of transgene expression in living animals and humans. Transgene expression can be monitored directly by imaging the expression of the therapeutic gene itself, or indirectly using a reporter gene that is coupled to the therapeutic gene. Various radiopharmaceuticals have been developed and are now being evaluated for imaging of transgene expression. This review surveys the progress that has been made in the field of non-invasive nuclear imaging of transgene expression and focuses on the herpes simplex virus type 1 thymidine kinase (HSVtk) gene therapy approaches.

Monitoring HSVtk suicide gene therapy: the role of [(18)F]FHPG membrane transport.

Favourable pharmacokinetics of the prodrug are essential for successful HSVtk/ganciclovir (GCV) suicide gene therapy. [18F]FHPG PET might be a suitable technique to assess the pharmacokinetics of the prodrug GCV noninvasively, provided that [18F]FHPG mimics the behaviour of GCV. Since membrane transport is an important aspect of the pharmacokinetics of the prodrug, we investigated the cellular uptake mechanism of [18F]FHPG in an HSVtk expressing C6 rat glioma cell line and in tumour-bearing rats. The nucleoside transport inhibitors dipyridamol, NBMPR and 2-chloroadenosine did not significantly affect the [18F]FHPG uptake in vitro. Thymidine and uridine significantly decreased [18F]FHPG uptake by 84 and 58%, respectively, but an enzyme assay revealed that this decline was due to inhibition of the HSVtk enzyme rather than membrane transport. Nucleobase transport inhibitors, thymine and adenine, caused a 58 and 55% decline in tracer uptake, respectively. In vivo, the ratio of [18F]FHPG uptake in C6tk and C6 tumours decreased from 3.0±0.5 to 1.0±0.2 after infusion of adenine. Thus, in our tumour model, [18F]FHPG transport exclusively occurred via purine nucleobase transport. In this respect, FHPG does not resemble GCV, which is predominantly taken up via the nucleoside transporter, but rather acyclovir, which is also taken up via the purine nucleobase carrier.

Scintigraphic Imaging of HSVtk Expression in Gene Therapy

Suicide gene therapy is under investigation as a treatment for cancer. In this therapy, a suicide gene is introduced into tumor cells, enabling the conversion of a prodrug into a toxic metabolite that selectively kills the transfected tumor cells. In the most investigated strategy, the herpes simplex virus thymidine kinase (HSVtk) suicide gene is used in combination with the prodrug ganciclovir. To assess the efficiency and safety of gene therapy protocols, a noninvasive method to assay the magnitude, kinetics and spatial distribution of transgene expression is essential. Imaging methods for repetitive monitoring of HSVtk transgene expression in living animals and humans, using single photon emission computed tomography (SPECT) or positron emission tomography (PET), have been developed. For many therapeutic genes, however, no imaging method is available. In these cases, reporter genes can be applied. Expression of the therapeutic gene can be determined indirectly by imaging a reporter gene, like HSVtk, that is linked to the therapeutic gene. Reporter genes can also be applied to monitor the expression of endogenous genes and to track the fate of transplanted cells. This paper presents an updated review on the progress in the field of non-invasive nuclear imaging of HSVtk transgene expression in gene therapy.