Live Organoid Cyclic Imaging

Organoids are becoming increasingly relevant in biology and medicine for their physiological complexity and accuracy in modeling human disease. To fully assess their biological profile while preserving their spatial information, spatiotemporal imaging tools are warranted. While previously developed imaging techniques, such as four-dimensional (4D) live imaging and light-sheet imaging have yielded important clinical insights, these technologies lack the combination of cyclic and multiplexed analysis. To address these challenges, bioorthogonal click chemistry is applied to display the first demonstration of multiplexed cyclic imaging of live and fixed patient-derived glioblastoma tumor organoids. This technology exploits bioorthogonal click chemistry to quench fluorescent signals from the surface and intracellular of labeled cells across multiple cycles, allowing for more accurate and efficient molecular profiling of their complex phenotypes. Herein, the versatility of this technology is demonstrated for the screening of glioblastoma markers in patient-derived human glioblastoma organoids while conserving their viability. It is anticipated that the findings and applications of this work can be broadly translated into investigating physiological developments in other organoid systems.

Image Denoising of Low-Dose PET Mouse Scans with Deep Learning: Validation Study for Preclinical Imaging Applicability

PURPOSE

Positron emission tomography (PET) image quality can be improved by higher injected activity and/or longer acquisition time, but both may often not be practical in preclinical imaging. Common preclinical radioactive doses (10 MBq) have been shown to cause deterministic changes in biological pathways. Reducing the injected tracer activity and/or shortening the scan time inevitably results in low-count acquisitions which poses a challenge because of the inherent noise introduction. We present an image-based deep learning (DL) framework for denoising lower count micro-PET images.

PROCEDURES

For 36 mice, a 15-min [18F]FDG (8.15 ± 1.34 MBq) PET scan was acquired at 40 min post-injection on the Molecubes β-CUBE (in list mode). The 15-min acquisition (high-count) was parsed into smaller time fractions of 7.50, 3.75, 1.50, and 0.75 min to emulate images reconstructed at 50, 25, 10, and 5% of the full counts, respectively. A 2D U-Net was trained with mean-squared-error loss on 28 high-low count image pairs.

RESULTS

The DL algorithms were visually and quantitatively compared to spatial and edge-preserving denoising filters; the DL-based methods effectively removed image noise and recovered image details much better while keeping quantitative (SUV) accuracy. The largest improvement in image quality was seen in the images reconstructed with 10 and 5% of the counts (equivalent to sub-1 MBq or sub-1 min mouse imaging). The DL-based denoising framework was also successfully applied on the NEMA-NU4 phantom and different tracer studies ([18F]PSMA, [18F]FAPI, and [68 Ga]FAPI).

CONCLUSION

Visual and quantitative results support the superior performance and robustness in image denoising of the implemented DL models for low statistics micro-PET. This offers much more flexibility in optimizing preclinical, longitudinal imaging protocols with reduced tracer doses or shorter durations.

A genetically encoded protein tag for control and quantitative imaging of CAR T cell therapy

Chimeric antigen receptor (CAR) T cell therapy has been successful for hematological malignancies. Still, a lack of efficacy and potential toxicities have slowed its application for other indications. Furthermore, CAR T cells undergo dynamic expansion and contraction in vivo that cannot be easily predicted or controlled. Therefore, the safety and utility of such therapies could be enhanced by engineered mechanisms that engender reversible control and quantitative monitoring. Here, we use a genetic tag based on the enzyme Escherichia coli dihydrofolate reductase (eDHFR), and derivatives of trimethoprim (TMP) to modulate and monitor CAR expression and T cell activity. We fused eDHFR to the CAR C terminus, allowing regulation with TMP-based proteolysis-targeting chimeric small molecules (PROTACs). Fusion of eDHFR to the CAR does not interfere with cell signaling or its cytotoxic function, and the addition of TMP-based PROTACs results in a reversible and dose-dependent inhibition of CAR activity via the proteosome. We show the regulation of CAR expression in vivo and demonstrate imaging of the cells with TMP radiotracers. In vitro immunogenicity assays using primary human immune cells and overlapping peptide fragments of eDHFR showed no memory immune repertoire for eDHFR. Overall, this translationally-orientied approach allows for temporal monitoring and image-guided control of cell-based therapies.

Regulation of eDHFR-tagged proteins with trimethoprim PROTACs

Temporal control of protein levels in cells and living animals can be used to improve our understanding of protein function. In addition, control of engineered proteins could be used in therapeutic applications. PRoteolysis-TArgeting Chimeras (PROTACs) have emerged as a small-molecule-driven strategy to achieve rapid, post-translational regulation of protein abundance via recruitment of an E3 ligase to the target protein of interest. Here, we develop several PROTAC molecules by covalently linking the antibiotic trimethoprim (TMP) to pomalidomide, a ligand for the E3 ligase, Cereblon. These molecules induce degradation of proteins of interest (POIs) genetically fused to a small protein domain, E. coli dihydrofolate reductase (eDHFR), the molecular target of TMP. We show that various eDHFR-tagged proteins can be robustly degraded to 95% of maximum expression with PROTAC molecule 7c. Moreover, TMP-based PROTACs minimally affect the expression of immunomodulatory imide drug (IMiD)-sensitive neosubstrates using proteomic and biochemical assays. Finally, we show multiplexed regulation with another known degron-PROTAC pair, as well as reversible protein regulation in a rodent model of metastatic cancer, demonstrating the formidable strength of this system. Altogether, TMP PROTACs are a robust approach for selective and reversible degradation of eDHFR-tagged proteins in vitro and in vivo.

Visualizing Bacterial Infections With Novel Targeted Molecular Imaging Approaches

Although nearly a century has elapsed since the discovery of penicillin, bacterial infections remain a major global threat. Global antibiotic use resulted in an astounding 42 billion doses of antibiotics administered in 2015 with 128 billion annual doses expected by 2030. This overuse of antibiotics has led to the selection of multidrug-resistant "super-bugs," resulting in increasing numbers of patients being susceptible to life-threatening infections with few available therapeutic options. New clinical tools are therefore urgently needed to identify bacterial infections and monitor response to antibiotics, thereby limiting overuse of antibiotics and improving overall health. Next-generation molecular imaging affords unique opportunities to target and identify bacterial infections, enabling spatial characterization as well as noninvasive, temporal monitoring of the natural course of the disease and response to therapy. These emerging noninvasive imaging approaches could overcome several limitations of current tools in infectious disease, such as the need for biological samples for testing with their associated sampling bias. Imaging of living bacteria can also reveal basic biological insights about their behavior in vivo.

Monitoring Therapeutic Response to Anti-FAP CAR T Cells Using [18F]AlF-FAPI-74

PURPOSE

Despite the success of chimeric antigen receptor (CAR) T-cell therapy against hematologic malignancies, successful targeting of solid tumors with CAR T cells has been limited by a lack of durable responses and reports of toxicities. Our understanding of the limited therapeutic efficacy in solid tumors could be improved with quantitative tools that allow characterization of CAR T-targeted antigens in tumors and accurate monitoring of response.

EXPERIMENTAL DESIGN

We used a radiolabeled FAP inhibitor (FAPI) [18F]AlF-FAPI-74 probe to complement ongoing efforts to develop and optimize FAP CAR T cells. The selectivity of the radiotracer for FAP was characterized in vitro, and its ability to monitor changes in FAP expression was evaluated using rodent models of lung cancer.

RESULTS

[18F]AlF-FAPI-74 showed selective retention in FAP+ cells in vitro, with effective blocking of the uptake in presence of unlabeled FAPI. In vivo, [18F]AlF-FAPI-74 was able to detect FAP expression on tumor cells as well as FAP+ stromal cells in the tumor microenvironment with a high target-to-background ratio. We further demonstrated the utility of the tracer to monitor changes in FAP expression following FAP CAR T-cell therapy, and the PET imaging findings showed a robust correlation with ex vivo analyses.

CONCLUSIONS

This noninvasive imaging approach to interrogate the tumor microenvironment represents an innovative pairing of a diagnostic PET probe with solid tumor CAR T-cell therapy and has the potential to serve as a predictive and pharmacodynamic response biomarker for FAP as well as other stroma-targeted therapies. A PET imaging approach targeting FAP expressed on activated fibroblasts of the tumor stroma has the potential to predict and monitor therapeutic response to FAP-targeted CAR T-cell therapy. See related commentary by Weber et al., p. 5241.

Abstract B19: FAP CAR T cell therapy for solid tumors with PET imaging

Two of the major obstacles that adoptive cell transfer immunotherapy needs to overcome to be successful in solid tumors are 1) the immunosuppressive tumor microenvironment (TME) and 2) the lack of robust biomarkers that allow for the identification of patients that would benefit from the therapy and for monitoring of the treatment response. Here we paired a CAR T cell therapy with a companion Positron Emission Tomography (PET) imaging approach that allows for serial, non-invasive, whole-body visualization of a biological target of interest -in this specific case, Fibroblast Activating Protein (FAP). FAP is a cell surface serine protease that is highly expressed by cancer associated fibroblasts in the TME that participate in the generation of the immunosuppressive stromagenic response in solid tumors. We designed a novel FAP CAR construct based on the scFv of the 4G5 antibody developed, in house, against canine FAP that cross-reacts against mouse and human FAP. In this study, we utilized the 18F-radiolabeled FAP inhibitor (FAPI), [18F]AlF-FAPI-74, to image FAP in two different mouse tumor models. First, the probe specificity was evaluated in the I45 human mesothelioma tumor model (a line which does not induce FAP+ fibroblasts). I45 WT and I45 cells transduced with human FAP were injected s.c. into the opposite flanks of a mouse and imaged following 2 weeks of tumor growth. We observed a 7.5-fold higher uptake of [18F]AlF-FAPI-74 in the I45 huFAP tumor compared to the WT, demonstrating the high specificity of the probe for FAP. Next, we used a more clinically-relevant A549 model where the tumor cells do not express FAP but induce a stromagenic response and drive the recruitment of FAP+ stromal cells in the TME. [18F]AlF-FAPI-74 PET/CT following 3 weeks of tumor growth showed a 6.5-fold increased radiotracer uptake in the tumor relative to the muscle at the baseline scan. Using this model, we evaluated the potential of our new CAR T cells to reduce tumor burden, as well as evaluating the potential of the [18F]AlF-FAPI-74 tracer as a tool to monitor the clearance of FAP-expressing cells in response to FAP CAR T cell therapy. Immediately after the baseline scan, we injected 5x106 FAP CAR T cells iv. At day 14 post-T cell injection, mice treated with FAP CAR T cells had significantly smaller tumors relative to the control group, which were treated with T cells that do not express the CAR, highlighting the therapeutic efficacy of the FAP CAR T cell therapy. Moreover, [18F]AlF-FAPI-74 PET/CT imaging showed no detectable tracer uptake in the tumors treated with FAP targeted CAR T. These findings were confirmed by immunofluorescence, indicating successful clearance of the FAP+ stroma by the injected FAP CAR T cells. In conclusion, the new 4G5 FAP CAR shows specific targeting toward mouse stroma infiltrating lung adenocarcinoma xenografts. PET imaging of FAP could be a highly useful approach to stratify patients prior to FAP CAR T therapy, as well as to monitor the pharmacodynamic response for FAP-targeted therapies.

Citation Format: Estela Noguera-Ortega, Iris K Lee, Zebin Xiao, Leslie Todd, John Scholler, Decheng Song, Maria Liousia, Katheryn Lohith, Kexiang Xu, Kimberly J Eduards, Michael D Farwell, Carl H June, Steven M Albelda, Ellen Puré, Mark A Sellmyer. FAP CAR T cell therapy for solid tumors with PET imaging [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr B19.

A microbiome-dependent gut-brain pathway regulates motivation for exercise

Exercise exerts a wide range of beneficial effects for healthy physiology1. However, the mechanisms regulating an individual's motivation to engage in physical activity remain incompletely understood. An important factor stimulating the engagement in both competitive and recreational exercise is the motivating pleasure derived from prolonged physical activity, which is triggered by exercise-induced neurochemical changes in the brain. Here, we report on the discovery of a gut-brain connection in mice that enhances exercise performance by augmenting dopamine signalling during physical activity. We find that microbiome-dependent production of endocannabinoid metabolites in the gut stimulates the activity of TRPV1-expressing sensory neurons and thereby elevates dopamine levels in the ventral striatum during exercise. Stimulation of this pathway improves running performance, whereas microbiome depletion, peripheral endocannabinoid receptor inhibition, ablation of spinal afferent neurons or dopamine blockade abrogate exercise capacity. These findings indicate that the rewarding properties of exercise are influenced by gut-derived interoceptive circuits and provide a microbiome-dependent explanation for interindividual variability in exercise performance. Our study also suggests that interoceptomimetic molecules that stimulate the transmission of gut-derived signals to the brain may enhance the motivation for exercise.

Abstract A37: Evaluation of eDHFR/iTag PET reporter gene immunogenicity and application in GPC3 CAR T cells

Genetically engineered medicines such as chimeric antigen receptor (CAR) T cells have great potential to be the next pillar of medical therapy beyond chemo- and traditional biologic therapies. To develop genetic medicines, new methods to understand their pharmacokinetics (PK) in humans are crucial. It is not feasible to perform traditional PK analysis for “living drugs”, because the genes themselves (in the form of DNA or RNA), are not typically responsible for the therapeutic effect. Rather, the protein products of the genes or the cells harboring the engineered genes are the actuators, and thus cannot be measured using standard HPLC or ligand binding immunoassays for PK analysis. We used a positron emission tomography (PET) reporter gene or “imaging tag” based on the intracellular bacterial enzyme dihydrofolate reductase (eDHFR) that can be paired with radiolabeled versions of trimethoprim (TMP). In this work, we evaluate the potential for immunogenicity using primary human cells and assays geared to assess low affinity and rare T cell clones that may react to eDHFR. We used overlapping pools of 15-mer eDHFR peptides and found that across 9 patients, there was little reactivity compared to EBV and CMV peptide controls. Further, the relative strength of reactivity to the eDHFR peptides was less than that of the viral peptides. Next, we showed that eDHFR iTag harboring CAR T cells were functionally comparable to unlabeled CAR T cells in vitro, and demonstrated strong, selective [18F]-TMP uptake in the eDHFR-expressing CAR T cells. Finally, using a glypican 3 (GPC3) CAR T rodent model, we performed a feasibility study to non-invasively track proliferation in antigen-harboring xenograft tumors over time with ex vivo correlation to anti-CD3 immunohistochemistry. These data demonstrate the potential for non-invasive monitoring of CAR T cells using PET imaging and translational applicability of DHFR/TMP radiotracers.

Citation Format: Mark A Sellmyer, Iris K Lee, Kyle Kuszpit, Jyoti Roy, Alex Alfaro, Virginie Ory, Lily Cheng, Daniel Sutton, Emily Bosco, Christine Fazenbaker, Shabazz Novarra, Ryan Gilbreth, Nick Tschernia, Deborah Berry, Xiaoru Chen, Yuling Wu, Ryan Wong. Evaluation of eDHFR/iTag PET reporter gene immunogenicity and application in GPC3 CAR T cells [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr A37.

Imaging sensitive and drug-resistant bacterial infection with [11C]-trimethoprim

BACKGROUND

Several molecular imaging strategies can identify bacterial infections in humans. PET affords the potential for sensitive infection detection deep within the body. Among PET-based approaches, antibiotic-based radiotracers, which often target key bacterial-specific enzymes, have considerable promise. One question for antibiotic radiotracers is whether antimicrobial resistance (AMR) reduces specific accumulation within bacteria, diminishing the predictive value of the diagnostic test.

METHODS

Using a PET radiotracer based on the antibiotic trimethoprim (TMP), [¹¹C]-TMP, we performed in vitro uptake studies in susceptible and drug-resistant bacterial strains and whole-genome sequencing (WGS) in selected strains to identify TMP resistance mechanisms. Next, we queried the NCBI database of annotated bacterial genomes for WT and resistant dihydrofolate reductase (DHFR) genes. Finally, we initiated a first-in-human protocol of [¹¹C]-TMP in patients infected with both TMP-sensitive and TMP-resistant organisms to demonstrate the clinical feasibility of the tool.

RESULTS

We observed robust [¹¹C]-TMP uptake in our panel of TMP-sensitive and -resistant bacteria, noting relatively variable and decreased uptake in a few strains of P. aeruginosa and E. coli. WGS showed that the vast majority of clinically relevant bacteria harbor a WT copy of DHFR, targetable by [¹¹C]-TMP, and that despite the AMR, these strains should be “imageable.” Clinical imaging of patients with [¹¹C]-TMP demonstrated focal radiotracer uptake in areas of infectious lesions.

CONCLUSION

This work highlights an approach to imaging bacterial infection in patients, which could affect our understanding of bacterial pathogenesis as well as our ability to better diagnose infections and monitor response to therapy.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03424525.

FUNDING

Institute for Translational Medicine and Therapeutics, Burroughs Wellcome Fund, NIH Office of the Director Early Independence Award (DP5-OD26386), and University of Pennsylvania NIH T32 Radiology Research Training Grant (5T32EB004311-12).

Using CD69 PET Imaging to Monitor Immunotherapy-Induced Immune Activation

ABSTRACT

Immune checkpoint inhibitors (ICI) have been effective in treating a subset of refractory solid tumors, but only a small percentage of treated patients benefit from these therapies. Thus, there is a clinical need for reliable tools that allow for the early assessment of response to ICIs, as well as a preclinical need for imaging tools that aid in the future development and understanding of immunotherapies. Here we demonstrate that CD69, a canonical early-activation marker expressed on a variety of activated immune cells, including cytotoxic T cells and natural killer (NK) cells, is a promising biomarker for the early assessment of response to immunotherapies. We have developed a PET probe by radiolabeling a highly specific CD69 mAb, H1.2F3, with Zirconium-89 (89Zr), [89Zr]-deferoxamine (DFO)-H1.2F3. [89Zr]-DFO-H1.2F3 detected changes in CD69 expression on primary mouse T cells in vitro and detected activated immune cells in a syngeneic tumor immunotherapy model. In vitro uptake studies with [89Zr]-DFO-H1.2F3 showed a 15-fold increase in CD69 expression for activated primary mouse T cells, relative to untreated resting T cells. In vivo PET imaging showed that tumors of ICI-responsive mice had greater uptake than the tumors of nonresponsive and untreated mice. Ex vivo biodistribution, autoradiography, and IHC analyses supported the PET imaging findings. These data suggest that the CD69 PET imaging approach detects CD69 expression with sufficient sensitivity to quantify immune cell activation in a syngeneic mouse immunotherapy model and could allow for the prediction of therapeutic immune responses to novel immunotherapies.

18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography Following Chimeric Antigen Receptor T-cell Therapy in Large B-cell Lymphoma

PURPOSE

18F-Fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) is a well-established imaging modality to assess responses in patients with B-cell neoplasms. However, there is limited information about the utility of FDG PET/CT after chimeric antigen receptor T-cell (CART) therapies for large B-cell lymphomas. In this retrospective analysis, we aimed to evaluate how FDG PET/CT performs in patients receiving commercially available anti-CD19 CART therapies for relapsed/refractory (r/r) large B-cell lymphomas. In addition, we examined the time to repeat scan and the rate of pseudoprogression within this population. Lastly, the rates of radiographic response to CART therapy using FDG PET/CT are reported.

PROCEDURES

The pre-treatment and post-treatment scans were analyzed from a selected cohort of 43 patients from a single institution. Patients were stratified by diagnosis of either a first occurrence of diffuse large B-cell lymphoma: de novo diffuse large B-cell lymphoma (DLBCL); or a transformed diffuse large B-cell lymphoma arising from indolent non-Hodgkin lymphoma (t-iNHL).

RESULTS

More patients received CART therapy for DLBCL than t-iNHL (65 % vs 35 %). FDG PET/CT had a 99 % sensitivity and 100 % specificity for detecting recurrent disease in this group. The median time to initial response assessment was 86 days (IQR 79-91; full range 24-146) after infusion. There were no biopsy-proven cases of pseudoprogression identified. In this selected group of patients, the overall response rate by Lugano 2014 criteria was 56 %. All patients with a partial response (N = 6) eventually progressed despite additional therapy.

CONCLUSIONS

Due to its excellent test characteristics and ability to detect asymptomatic disease, routine surveillance with PET/CT at 3 months after CART infusion is supported by our data. Earlier PET/CT may be of value in select situations as we did not find any cases of pseudoprogression.

Building the Bridge: Molecular Imaging Biomarkers for 21st Century Cancer Therapies

Precision medicine, where the molecular underpinnings of the disease are assessed for tailored therapies, has greatly impacted cancer care. In parallel, a new pillar of therapeutics has emerged with profound success, including immunotherapies such as checkpoint inhibitors and cell-based therapies. Nonetheless, it remains essential to develop paradigms to predict and monitor for therapeutic response. Molecular imaging has the potential to add substantially to all phases of cancer patient care: predicative, companion diagnostics can illuminate therapeutic target density within a tumor, and pharmacodynamic imaging biomarkers can complement traditional modalities to judge a favorable treatment response. This "Focus on Molecular Imaging" article discusses the current role of molecular imaging in oncology and highlights an additional step in clinical paradigm termed a "therapeutic biomarker," which serves to assess whether next generation drugs reach their target to elicit a favorable clinical response.

A Chemical Approach for Programmable Protein Outputs Based on Engineered Cell Interactions

Cell-cell interactions and communication are crucial to the proper function of complex mammalian physiology including neurocognitive and immune system functions. While many tools are available for observing and perturbing intracellular processes, relatively few exist to probe intercellular processes. Current techniques for studying interactions often rely on direct protein contact, and few can manipulate diverse, functional outputs with tunable protein expression. To address these limitations, we have developed a small-molecule approach based on a trimethoprim prodrug-enzyme pair capable of reporting the presence of two different engineered cell populations with programmable protein outputs. The approach relies on bacterial nitroreductase enzyme catalysis, which is orthogonal to normal mammalian biology, and diffusion of trimethoprim from "activator" cells to "receiver" cells. We test this strategy, which can theoretically regulate many different types of proteins, using biochemical and in vitro culture assays with optical and cytokine protein readouts. This describes the first small-molecule approach capable of detecting and controlling engineered cell-cell outputs, and we anticipate future applications that are especially relevant to the field of immuno-oncology.

Radiochemical Approaches to Imaging Bacterial Infections: Intracellular versus Extracellular Targets

The discovery of penicillin began the age of antibiotics, which was a turning point in human healthcare. However, to this day, microbial infections are still a concern throughout the world, and the rise of multidrug-resistant organisms is an increasing challenge. To combat this threat, diagnostic imaging tools could be used to verify the causative organism and curb inappropriate use of antimicrobial drugs. Nuclear imaging offers the sensitivity needed to detect small numbers of bacteria in situ. Among nuclear imaging tools, radiolabeled antibiotics traditionally have lacked the sensitivity or specificity necessary to diagnose bacterial infections accurately. One reason for the lack of success is that the antibiotics were often chelated to a radiometal. This was done without addressing the ramifications of how the radiolabeling would impact probe entry to the bacterial cell, or the mechanism of binding to an intracellular target. In this review, we approach bacterial infection imaging through the lens of bacterial specific molecular targets, their intracellular or extracellular location, and discuss radiochemistry strategies to guide future probe development.

Imaging CAR T Cell Trafficking with eDHFR as a PET Reporter Gene

Cell-based therapeutics have considerable promise across diverse medical specialties; however, reliable human imaging of the distribution and trafficking of genetically engineered cells remains a challenge. We developed positron emission tomography (PET) probes based on the small-molecule antibiotic trimethoprim (TMP) that can be used to image the expression of the Escherichia coli dihydrofolate reductase enzyme (eDHFR) and tested the ability of [¹⁸F]-TMP, a fluorine-18 probe, to image primary human chimeric antigen receptor (CAR) T cells expressing the PET reporter gene eDHFR, yellow fluorescent protein (YFP), and Renilla luciferase (rLuc). Engineered T cells showed an approximately 50-fold increased bioluminescent imaging signal and 10-fold increased [¹⁸F]-TMP uptake compared to controls in vitro. eDHFR-expressing anti-GD2 CAR T cells were then injected into mice bearing control GD2^- and GD2⁺ tumors. PET/computed tomography (CT) images acquired on days 7 and 13 demonstrated early residency of CAR T cells in the spleen followed by on-target redistribution to the GD2⁺ tumors. This was corroborated by autoradiography and anti-human CD8 immunohistochemistry. We found a high sensitivity of detection for identifying tumor-infiltrating CD8 CAR T cells, ∼11,000 cells per mm³. These data suggest that the [¹⁸F]-TMP/eDHFR PET pair offers important advantages that could better allow investigators to monitor immune cell trafficking to tumors in patients.

Molecular imaging of bacterial infections: Overcoming the barriers to clinical translation

Clinical diagnostic tools requiring direct sample testing cannot be applied to infections deep within the body, and clinically available imaging tools lack specificity. New approaches are needed for early diagnosis and monitoring of bacterial infections and rapid detection of drug-resistant organisms. Molecular imaging allows for longitudinal, noninvasive assessments and can provide key information about infectious processes deep within the body.

Inflammation and DNA damage: Probing pathways to cancer and neurodegeneration

Cancer and neurodegeneration represent two opposite ends of the biological spectrum but contain many common biological mechanisms. Two such mechanisms include the elevated levels of oxidative stress and DNA damage. In this brief review, we describe current approaches for imaging these biological pathways with the molecular imaging technique, Positron Emission Tomography (PET), and the potential of PET imaging studies to measure the efficacy of anticancer drugs and strategies for delaying the progression of neurodegenerative disorders.

Quantitative PET Reporter Gene Imaging with [11C]Trimethoprim

There is a need for improved methods to image genetically engineered cells, including immune cells used for cell-based therapy. Given the genetic manipulation inherent to gene therapy, the use of a reporter protein is a logical solution and positron emission tomography (PET) can provide the desired sensitivity and spatial localization. We developed a broadly applicable PET imaging strategy based on the small bacterial protein E. coli dihydrofolate reductase (Ec dhfr) and its highly specific small molecule inhibitor, trimethoprim (TMP). The difference in TMP affinity for bacterial compared to mammalian DHFR suggests that a TMP radioligand would have a low background in unmodified mammalian tissues and high retention in Ec dhfr engineered cells, providing high contrast imaging. Here, we describe the in vitro properties of [¹¹C]TMP and show over 10-fold increased signal in transgenic Ec dhfr cells compared to control. In a mouse xenograft model, [¹¹C]TMP rapidly accumulated in Ec dhfr carrying cells within minutes of intravenous administration. Moreover, [¹¹C]TMP can identify less than a million xenografted cells in a small volume in tissues other than the abdominal compartment. This limit of detection is a clinically relevant number and bodes well for clinical translation especially given that [¹¹C]TMP is an isotopologue of clinically approved antibiotic.

Visualizing cell proximity with genetically encoded bioluminescent reporters

Cell-cell interactions underlie diverse physiological processes ranging from immune function to cell migration. Dysregulated cellular crosstalk also potentiates numerous pathologies, including infections and metastases. Despite their ubiquity in organismal biology, cell-cell interactions are difficult to examine in tissues and whole animals without invasive procedures. Here, we report a strategy to noninvasively image cell proximity using engineered bioluminescent probes. These tools comprise "split" fragments of Gaussia luciferase (Gluc) fused to the leucine zipper domains of Fos and Jun. When cells secreting the fragments draw near one another, Fos and Jun drive the assembly of functional, light-emitting Gluc. Photon production thus provides a readout on the distance between two cell types. We used the split fragments to visualize cell-cell interactions over time in vitro and in macroscopic models of cell migration. Further application of these tools in live organisms will refine our understanding of cell contacts relevant to basic biology and disease.

Chemical control of FGF-2 release for promoting calvarial healing with adipose stem cells

Chemical control of protein secretion using a small molecule approach provides a powerful tool to optimize tissue engineering strategies by regulating the spatial and temporal dimensions that are exposed to a specific protein. We placed fibroblast growth factor 2 (FGF-2) under conditional control of a small molecule and demonstrated greater than 50-fold regulation of FGF-2 release as well as tunability, reversibility, and functionality in vitro. We then applied conditional control of FGF-2 secretion to a cell-based, skeletal tissue engineering construct consisting of adipose stem cells (ASCs) on a biomimetic scaffold to promote bone formation in a murine critical-sized calvarial defect model. ASCs are an easily harvested and abundant source of postnatal multipotent cells and have previously been demonstrated to regenerate bone in critical-sized defects. These results suggest that chemically controlled FGF-2 secretion can significantly increase bone formation by ASCs in vivo. This study represents a novel approach toward refining protein delivery for tissue engineering applications.

A general method for conditional regulation of protein stability in living animals

The ability to rapidly and reversibly perturb protein levels in living animals is a powerful tool for researchers to determine protein function in complex systems. We recently designed a small protein domain based on the 12-kDa FKBP (FK506 binding protein) that can be fused at either the carboxyl or amino terminus of a protein of interest. This destabilization domain (DD) confers instability to fusion protein partners, allowing targeted degradation of the protein of interest. A small molecule called Shield-1 binds to the DD and protects the fusion protein from degradation. Small-molecule-mediated post-translational regulation of protein stability affords this system rapid, reversible, and tunable control of protein levels and functions in a variety of model systems. Theoretically, a number of transgene delivery methods (e.g., viral, liposomal, or stem cell) can be used for the analysis of a DD fusion protein in an animal model. This protocol uses tumor xenografts in mice as one such mechanism for delivering the fusion protein and presents a method for delivering Shield-1 to regulate the fusion proteins in vivo.

Engineering small molecule specificity in nearly identical cellular environments

Methotrexate (MTX), an inhibitor of dihydrofolate reductase, was tethered to an FKBP12 ligand (SLF), and the resulting bifunctional molecule (MTXSLF) potently inhibits either enzyme but not both simultaneously. MTXSLF is cytotoxic to fibroblasts derived from FKBP12-null mice but is detoxified 40-fold by FKBP12 in wild-type fibroblasts. These studies demonstrate that non-target proteins in an otherwise identical genetic background can be used to predictably regulate the biological activity of synthetic molecules.

Polymerization of diacetylenes by hydrogen bond templated adlayer formation

Adlayers were formed on self-assembled monolayers (SAMs) formed by alkanethiols on gold. Base SAMs exposing amide functional groups at the SAM surface were formed with 12-mercaptododecanamide. Adlayers of diacetylene-containing monomers were then formed via amide hydrogen bonding in decalin and decalin/toluene mixtures. Grazing angle FTIR, contact angle measurements, and ellipsometry suggest that these adlayer films exhibit ordering and packing similar to that of SAMs on gold. Resonance Raman spectroscopy showed that these diacetylene adlayers could be readily polymerized by exposure to UV light.