PET imaging of thymidine kinase gene expression in the liver of non-human primates following systemic delivery of an adenoviral vector

A brief review of reporter gene imaging in oncolytic virotherapy and gene therapy

Reporter gene imaging (RGI) can accelerate development timelines for gene and viral therapies by facilitating rapid and noninvasive in vivo studies to determine the biodistribution, magnitude, and durability of viral gene expression and/or virus infection. Functional molecular imaging systems used for this purpose can be divided broadly into deep-tissue and optical modalities. Deep-tissue modalities, which can be used in animals of any size as well as in human subjects, encompass single photon emission computed tomography (SPECT), positron emission tomography (PET), and functional/molecular magnetic resonance imaging (f/mMRI). Optical modalities encompass fluorescence, bioluminescence, Cerenkov luminescence, and photoacoustic imaging and are suitable only for small animal imaging. Here we discuss the mechanisms of action and relative merits of currently available reporter gene systems, highlighting the strengths and weaknesses of deep tissue versus optical imaging systems and the hardware/reagents that are used for data capture and processing. In light of recent technological advances, falling costs of imaging instruments, better availability of novel radioactive and optical tracers, and a growing realization that RGI can give invaluable insights across the entire in vivo translational spectrum, the approach is becoming increasingly essential to facilitate the competitive development of new virus- and gene-based drugs.

Monitoring of biodistribution and persistence of conditionally replicative adenovirus in a murine model of ovarian cancer using capsid-incorporated mCherry and expression of human somatostatin receptor subtype 2 gene

A significant limiting factor to the human clinical application of conditionally replicative adenovirus (CRAd)-based virotherapy is the inability to noninvasively monitor these agents and their potential persistence. To address this issue, we proposed a novel imaging approach that combines transient expression of the human somatostatin receptor (SSTR) subtype 2 reporter gene with genetic labeling of the viral capsid with mCherry fluorescent protein. To test this dual modality system, we constructed the Ad5/3Δ24pIXcherry/SSTR CRAd and validated its capacity to generate fluorescent and nuclear signals in vitro and following intratumoral injection. Analysis of 64Cu-CB-TE2A-Y3-TATE biodistribution in mice revealed reduced uptake in tumors injected with the imaging CRAd relative to the replication-incompetent, Ad-expressing SSTR2 but significantly greater uptake compared to the negative CRAd control. Optical imaging demonstrated relative correlation of fluorescent signal with virus replication as determined by viral genome quantification in tumors. Positron emission tomography/computed tomography studies demonstrated that we can visualize radioactive uptake in tumors injected with imaging CRAd and the trend for greater uptake by standardized uptake value analysis compared to control CRAd. In the aggregate, the plasticity of our dual imaging approach should provide the technical basis for monitoring CRAd biodistribution and persistence in preclinical studies while offering potential utility for a range of clinical applications.

Noninvasive assessment of gene transfer and expression by in vivo functional and morphologic imaging in a rabbit tumor model

Purpose

To evaluate the importance of morphology in quantifying expression after in vivo gene transfer and to compare gene expression after intra-arterial (IA) and intra-tumoral (IT) delivery of adenovirus expressing a SSTR2-based reporter gene in a large animal tumor model.

Materials and Methods

Tumor directed IA or IT delivery of adenovirus containing a human somatostatin receptor type 2A (Ad-CMV-HA-SSTR2A) gene chimera or control adenovirus (Ad-CMV-GFP) was performed in VX2 tumors growing in both rabbit thighs. Three days later, ¹¹¹In-octreotide was administered intravenously after CT imaging using a clinical scanner. ¹¹¹In-octreotide uptake in tumors was evaluated the following day using a clinical gamma-camera. Gene expression was normalized to tumor weight with and without necrosis. This procedure was repeated on nine additional rabbits to investigate longitudinal gene expression both 5 days and 2 weeks after adenovirus delivery. CT images were used to evaluate tumor morphology and excised tissue samples were analyzed to determine ¹¹¹In-octreotide biodistribution ex vivo.

Results

VX2 tumors infected with Ad-CMV-HA-SSTR2 had greater ¹¹¹In-octreotide uptake than with control virus (P<0.05). Intra-arterial and intra-tumoral routes resulted in similar levels of gene expression. Longitudinally, expression appeared to wane at 2 weeks versus 5 days after delivery. Areas of necrosis did not demonstrate significant uptake ex vivo. Morphology identified areas of necrosis on contrast enhanced CT and upon excluding necrosis, in vivo biodistribution analysis resulted in greater percent injected dose per gram (P<0.01) and corresponded better with ex vivo biodistribution(r = 0.72, P<0.01, Coefficient of the x-variable = .72) at 2 weeks than without excluding necrosis (P<0.01).

Conclusion

Tumor specificity and high transgene expression can be achieved in tumors via both tumor directed intra-arterial and intra-tumoral delivery in a large animal tumor model. Using clinical machines, morphologic imaging contributes to functional imaging for quantifying SSTR2-based reporter expression in vivo.

Oncolytic adenoviruses armed with thymidine kinase can be traced by PET imaging and show potent antitumoural effects by ganciclovir dosing

Replication-competent adenoviruses armed with thymidine kinase (TK) combine the concepts of virotherapy and suicide gene therapy. Moreover TK-activity can be detected by noninvasive positron emission-computed tomography (PET) imaging, what could potentially facilitate virus monitoring in vivo. Here, we report the generation of a novel oncolytic adenovirus that incorporates the Tat8-TK gene under the control of the Major Late Promoter in a highly selective backbone thus providing selectivity by targeting the retinoblastoma pathway. The selective oncolytic TK virus, termed ICOVIR5-TK-L, showed reduced potency compared to a non-selective counterpart. However the combination of ICOVIR5-TK-L with ganciclovir (GCV) induced a potent antitumoural effect similar to that of wild type adenovirus in a preclinical model of pancreatic cancer. Although the treatment with GCV provoked a reduction in the viral yield, both in vitro and in vivo, a two-cycle treatment of virus and GCV resulted in an enhanced antitumoral response that correlated with high TK-activity, based on microPET measurements. Thus, TK-expressing oncolytic adenoviruses can be traced by PET imaging providing real time information on the activity of the virus and its antitumoral potency can be optimized by GCV dosing.

Evaluation of adenovirus capsid labeling versus transgene expression

Adenoviral vectors have been utilized for a variety of gene therapy applications. Our group has incorporated bioluminescent, fluorographic reporters, and/or suicide genes within the adenovirus genome for analytical and/or therapeutic purposes. These molecules have also been incorporated as capsid components. Recognizing that incorporations at either locale yield potential advantages and disadvantages, our report evaluates the benefits of transgene incorporation versus capsid incorporation. To this end, we have genetically incorporated firefly luciferase within the early region 3 or at minor capsid protein IX and compared vector functionality by means of reporter readout.

Intensive pharmacological immunosuppression allows for repetitive liver gene transfer with recombinant adenovirus in nonhuman primates

Repeated administration of gene therapies is hampered by host immunity toward vectors and transgenes. Attempts to circumvent antivector immunity include pharmacological immunosuppression or alternating different vectors and vector serotypes with the same transgene. Our studies show that B-cell depletion with anti-CD20 monoclonal antibody and concomitant T-cell inhibition with clinically available drugs permits repeated liver gene transfer to a limited number of nonhuman primates with recombinant adenovirus. Adenoviral vector-mediated transfer of the herpes simplex virus type 1 thymidine kinase (HSV1-tk) reporter gene was visualized in vivo with a semiquantitative transgene-specific positron emission tomography (PET) technique, liver immunohistochemistry, and immunoblot for the reporter transgene in needle biopsies. Neutralizing antibody and T cell-mediated responses toward the viral capsids were sequentially monitored and found to be repressed by the drug combinations tested. Repeated liver transfer of the HSV1-tk reporter gene with the same recombinant adenoviral vector was achieved in macaques undergoing a clinically feasible immunosuppressive treatment that ablated humoral and cellular immune responses. This strategy allows measurable gene retransfer to the liver as late as 15 months following the first adenoviral exposure in a macaque, which has undergone a total of four treatments with the same adenoviral vector.

Modular strategies for PET imaging agents

In recent years, modular and simplified chemical and biological strategies have been developed for the synthesis and implementation of positron emission tomography (PET) radiotracers. New developments in bioconjugation and synthetic methodologies, in combination with advances in macromolecular delivery systems and gene-expression imaging, reflect a need to reduce radiosynthesis burden in order to accelerate imaging agent development. These new approaches, which are often mindful of existing infrastructure and available resources, are anticipated to provide a more approachable entry point for researchers interested in using PET to translate in vitro research to in vivo imaging.