Pretargeting with Cucurbituril-Adamantane Host-Guest Pair in Xenograft Models.

The goal of reducing the total-body radiation dose of macromolecule-based nuclear medicine with a 2-step pretargeting strategy has been achieved with several pretargeting methodologies in preclinical and clinical settings. However, the lack of modularity, biocompatibility, and in vivo stability in existing pretargeting agents obstructs their respective platforms' wide clinical use. We hypothesized that host-guest chemistry would provide an optimal pretargeting methodology. A cucurbit[7]uril host and an adamantane guest molecule form a high-affinity host-guest complex (association constant, ∼1014 M-1), and in this work, we explored the use of this noncovalent interaction as the basis for antibody-based pretargeted PET. Along with the straightforward modularity of these agents, cucurbit[7]uril and adamantane are recognized to have high in vivo stability and suitability for human use, which is why we proposed this methodology as the ideal approach for pretargeted nuclear medicine. Methods: Three 64Cu-labeled adamantane guest radioligands were developed, and their in vitro stability, lipophilicity, and in vivo blood half-lives were compared. The adamantane radioligands were analyzed for pretargeting using a cucurbit[7]uril-modified carcinoembryonic antigen-targeting full-length antibody, hT84.66-M5A, as the macromolecule pretargeting agent with 2 different dosing schedules. These molecules were evaluated for pretargeting in human pancreatic cancer BxPC3 and MIAPaCa-2 mouse xenografts using PET and in vivo biodistribution studies. The dosimetry of the cucurbit[7]uril-adamantane (CB7-Adma) pretargeting approach in men was calculated and compared with that of the directly 89Zr-labeled hT84.66-M5A. Results: The adamantane radioligands possessed high in vitro stability up to 24 h (>90%). Pretargeted PET with CB7-Adma methodology resulted in specific tumor uptake (P < 0.05) with low background signal. The in vivo formed CB7-Adma complex was demonstrated to be stable, with high tumor uptake up to 24 h after radioligand injection (12.0 ± 0.9 percentage injected dose/g). The total-body radiation dose of the pretargeting strategy was only 3.3% that of the directly 89Zr-labeled hT84.66-M5A. Conclusion: The CB7-Adma strategy is highly suitable for pretargeted PET. The exceptional stability of the pretargeting agents and the specific and high tumor uptake of the pretargeted adamantane radioligands provide great potential for the platform.

Physicochemical Properties and Route of Systemic Delivery Control the In Vivo Dynamics and Breakdown of Radiolabeled Gold Nanostars.

The in vivo dynamics of nanoparticles requires a mechanistic understanding of multiple factors. Here, for the first time, the surprising breakdown of functionalized gold nanostars (F-AuNSs) conjugated with antibodies and 64 Cu radiolabels in vivo and in artificial lysosomal fluid ex vivo, is shown. The short-term biodistribution of F-AuNSs is driven by the route of systemic delivery (intravenous vs intraperitoneal) and long-term fate is controlled by the tissue type in vivo. In vitro studies including endocytosis pathways, intracellular trafficking, and opsonization, are combined with in vivo studies integrating a milieu of spectroscopy and microcopy techniques that show F-AuNSs dynamics is driven by their physicochemical properties and route of delivery. F-AuNSs break down into sub-20 nm broken nanoparticles as early as 7 days postinjection. Martini coarse-grained simulations are performed to support the in vivo findings. Simulations suggest that shape, size, and charge of the broken nanoparticles, and composition of the lipid membrane depicting various tissues govern the interaction of the nanoparticles with the membrane, and the rate of translocation across the membrane to ultimately enable tissue clearance. The fundamental study addresses critical gaps in the knowledge regarding the fate of nanoparticles in vivo that remain a bottleneck in their clinical translation.

Investigation of Copper-64-Based Host-Guest Chemistry Pretargeted Positron Emission Tomography.

Pretargeting is a technique that uses macromolecules as targeting agents for nuclear imaging and therapy with the goal of reducing the radiation toxicity to healthy tissues often associated with directly radiolabeled macromolecules. In pretargeting, a macromolecule is radiolabeled in vivo at the target site using a radiolabeled small molecule (radioligand) that interacts with the macromolecule with high specificity. We report an investigation of host-guest chemistry-driven pretargeting using copper-64 radiolabeled ferrocene (Fc; guest) compounds and a cucurbit[7]uril (CB7; host) molecule functionalized carcinoembryonic antigen targeting hT84.66-M5A monoclonal antibody (CB7-M5A). Two novel ferrocene-based radioligands ([64Cu]Cu-NOTA-PEG3-Fc and [64Cu]Cu-NOTA-PEG7-Fc) were prepared, and their in vitro stability, pharmacokinetic in vivo profile in healthy mice, and pretargeting performance in a subcutaneous BxPC3 human pancreatic cancer cell xenograft mouse model were compared. The antibody dosing was optimized using a zirconium-89 radiolabeled M5A antibody ([89Zr]Zr-DFO-M5A) in a BxPC3 xenograft model, and the dosimetry of [89Zr]Zr-DFO-M5A and the pretargeting approach were compared. Finally, the effects of varying lag times up to 9 days between CB7-M5A and radioligand injection were investigated. In vivo pretargeting studies with both ferrocene radioligands resulted in specific tumor uptake (p = 0.0006 and p = 0.003) and also showed that the host-guest-based pretargeting approach excels with extended lag times up to 9 days with good tumor localization, suggesting that host-guest pretargeting may be suitable for use without clearing agents which have complicated clinical application of this technique. To our knowledge, the reported lag time of 9 days is the longest investigated lag time in any reported pretargeting studies.

Cucurbituril-Ferrocene: Host-Guest Based Pretargeted Positron Emission Tomography in a Xenograft Model.

Pretargeted positron emission tomography is a macromolecule-driven nuclear medicine technique that involves targeting a preadministered antigen target-bound macromolecule with a radioligand in vivo, aiming to minimize the overall radiation dose. This study investigates the use of antibody based host-guest chemistry methodology for pretargeted positron emission tomography. We hypothesize that the novel pretargeting approach reported here overcomes the challenges the current pretargeting platforms have with the in vivo stability and modularity of the pretargeting components. A cucurbit[7]uril host molecule modified, anti-carcinoembryonic antigen antibody (M5A; CB7-M5A) and a 68Ga-radiolabeled ferrocene guest radioligand ([68Ga]Ga-NOTA-PEG3-NMe2-Fc) were studied as potential host-guest chemistry pretargeting agents for positron emission tomography in BxPC3 xenografted nude mice. The viability of the platform was studied via in vivo biodistribution and positron emission tomography. Tumor uptake of [68Ga]Ga-NOTA-PEG3-NMe2-Fc was significantly higher in mice which received CB7-M5A prior to the radioligand injection (pretargeted) (3.3 ± 0.7%ID/g) compared to mice which only received the radioligand (nonpretargeted) (0.2 ± 0.1%ID/g).

Association between depression and HIV treatment outcomes in a US military population with HIV infection.

BACKGROUND: Depression is common among HIV-infected individuals and may contribute to suboptimal adherence to antiretroviral therapy (ART) and subsequent inability to attain viral load (VL) suppression. We evaluated associations between depression, self-reported adherence, and longitudinal HIV treatment outcomes in US Military HIV Natural History Study (NHS) participants with and without depression. METHODS: Male NHS participants with available ICD-9 data for mental health diagnoses, Center for Epidemiological Studies Depression (CES-D) measures, and self-reported adherence (SRA) were included. ART use was defined as ART initiation between 2006 and 2010, with follow-up through 2015. SRA was defined as taking 95% of ART doses and continuous ART was defined as longitudinal ART use with gaps < 30 days. Continuous VL suppression was defined as maintaining VLs < 200 c/mL on ART. To analyse the association between depression and HIV treatment outcomes, latent class analysis was used to create classes of depression trajectories: low depression (LD), recent onset depression (ROD) and high Depression (HD). RESULTS: Participants had a mean age of 32 (± 8.3) years at HIV diagnosis, and similar proportions were Caucasian (44.3%) or African American (40.8%). Overall, older participants at HIV diagnosis had greater odds of having 95% self-reported adherence (OR 1.06, 95% CI 1.02-1.12), and African Americans had lower odds (OR 0.41, 95% CI 0.22-0.76) compared to Caucasians (OR 1.49, 95% CI 0.52-4.28). However, there was no difference in SRA by depression trajectory. Participants with HD had an increased odds of taking ART continuously (OR 1.75, 95% CI 0.99-3.09), and those with ROD had significantly higher odds of virologic failure (OR 0.58, 95% CI 0.38-0.91). CONCLUSIONS: Although there was no observed association between depression and SRA, participants with ROD had lower odds of attaining the HIV treatment goal of VL suppression. Continued efforts to identify and aggressively manage mental health disorders is important to success along the HIV care continuum.

Validation of the use of a fluorescent PARP1 inhibitor for the detection of oral, oropharyngeal and oesophageal epithelial cancers.

For oral, oropharyngeal and oesophageal cancer, the early detection of tumours and of residual tumour after surgery are prognostic factors of recurrence rates and patient survival. Here, we report the validation, in animal models and a human, of the use of a previously described fluorescently labelled small-molecule inhibitor of the DNA repair enzyme poly(ADP-ribose) polymerase 1 (PARP1) for the detection of cancers of the oral cavity, pharynx and oesophagus. We show that the fluorescent contrast agent can be used to quantify the expression levels of PARP1 and to detect oral, oropharyngeal and oesophageal tumours in mice, pigs and fresh human biospecimens when delivered topically or intravenously. The fluorescent PARP1 inhibitor can also detect oral carcinoma in a patient when applied as a mouthwash, and discriminate between fresh biopsied samples of the oral tumour and the surgical resection margin with more than 95% sensitivity and specificity. The PARP1 inhibitor could serve as the basis of a rapid and sensitive assay for the early detection and for the surgical-margin assessment of epithelial cancers of the upper intestinal tract.

Positron-Emission Tomographic Imaging of a Fluorine 18-Radiolabeled Poly(ADP-Ribose) Polymerase 1 Inhibitor Monitors the Therapeutic Efficacy of Talazoparib in SCLC Patient-Derived Xenografts.

INTRODUCTION: Inhibitors of poly-(ADP)-ribose polymerase (PARP) are promising therapeutics for SCLC. We tested whether PARP inhibitor (PARPi) target engagement as measured by a fluorine 18-radiolabeled PARPi ([18F]PARPi) has the potential to predict drug efficacy in vivo. METHODS: Tumor growth inhibition during daily talazoparib treatment was evaluated in mice engrafted with SCLC patient-derived xenografts to evaluate talazoparib efficacy at multiple doses. Mice were intravenously injected with [18F]PARPi radiotracer at multiple timepoints after single doses of oral talazoparib to quantitatively assess the extent to which talazoparib could reduce tumor radiotracer uptake and positron-emission tomographic (PET)/computer tomographic activity. Tumors were harvested and tumor poly-(ADP) ribose level was measured by enzyme-linked immunosorbent assay. RESULTS: A dose range of talazoparib with differential therapeutic efficacy was established, with significant delay in time to reach 1000 mm3 for tumors treated with 0.3 mg/kg (p = 0.02) but not 0.1 mg/kg talazoparib. On PET/computed tomography with [18F]PARPi, reduction in [18F]PARPi uptake after talazoparib dosing was consistent with talazoparib clearance, with reduction in PET activity attenuating over 24 hours. Talazoparib target engagement, measured by maximum tumor PET uptake, increased in a dose-dependent manner (3.9% versus 2.1% injected dose/g for 0.1 and 0.3 mg/kg at 3 hours post-talazoparib, p = 0.003) and correlated with PARP enzymatic activity among individual tumors as measured by total tumor poly-(ADP) ribose (p = 0.04, R = 0.62 at 1 hour post-talazoparib). CONCLUSIONS: PET imaging using [18F]PARPi has the potential to be a powerful tool in treatment monitoring by assessing PARPi target engagement in real-time.

Relationship between depression and risk behaviors in a US Military population with HIV infection.

This study evaluated the relationships between depression trajectories, depression diagnosis and sexual risk behaviors in the US Military HIV Natural History Study. Risk behavior survey data, a coded diagnosis of depression, available Center for Epidemiological Studies Depression measures, and self-reported depressive symptoms (n = 662) were utilized. Latent class analysis created 3 classes of depression trajectories, namely, low depression (LD, n = 378), recent-onset depression (ROD, n = 170), and high depression (HD, n = 114) trajectories. Overall, participants with clinically diagnosed depression were less likely to report often using condoms with new sexual partners in the past 3 months than those who have never been diagnosed with depression (OR 0.15, 95% CI 0.49-2.53). Participants with ROD (OR 0.52, 95% CI 0.28-0.97) and HD (OR 0.48, 95% CI 0.24-0.96) trajectories were less likely to report often using condoms with new sexual partners in the past 3 months than those with LD trajectories. Moreover, those with either ROD (OR 2.13, 95% CI 1.19-3.80) or HD (OR 2.74, 95% CI 1.43-5.24) trajectories were more likely to have had sex with ≥2 new sexual partners in the last 3 months than those with LD trajectories. Continued efforts targeting HIV-infected persons with mental health disorders are warranted to reduce sexual risk behaviors.

Discriminating radiation injury from recurrent tumor with [18F]PARPi and amino acid PET in mouse models.

BACKGROUND: Radiation injury can be indistinguishable from recurrent tumor on standard imaging. Current protocols for this differential diagnosis require one or more follow-up imaging studies, long dynamic acquisitions, or complex image post-processing; despite much research, the inability to confidently distinguish between these two entities continues to pose a significant dilemma for the treating clinician. Using mouse models of both glioblastoma and radiation necrosis, we tested the potential of poly(ADP-ribose) polymerase (PARP)-targeted PET imaging with [18F]PARPi to better discriminate radiation injury from tumor. RESULTS: In mice with experimental radiation necrosis, lesion uptake on [18F]PARPi-PET was similar to contralateral uptake (1.02 ± 0.26 lesion/contralateral %IA/ccmax ratio), while [18F]FET-PET clearly delineated the contrast-enhancing region on MR (2.12 ± 0.16 lesion/contralateral %IA/ccmax ratio). In mice with focal intracranial U251 xenografts, tumor visualization on PARPi-PET was superior to FET-PET, and lesion-to-contralateral activity ratios (max/max, p = 0.034) were higher on PARPi-PET than on FET-PET. CONCLUSIONS: A murine model of radiation necrosis does not demonstrate [18F]PARPi avidity, and [18F]PARPi-PET is better than [18F]FET-PET in distinguishing radiation injury from brain tumor. [18F]PARPi-PET can be used for discrimination between recurrent tumor and radiation injury within a single, static imaging session, which may be of value to resolve a common dilemma in neuro-oncology.

PARP-1-Targeted Radiotherapy in Mouse Models of Glioblastoma.

The DNA repair enzyme poly(ADP-ribose) polymerase 1 (PARP-1) is overexpressed in glioblastoma, with overall low expression in healthy brain tissue. Paired with the availability of specific small molecule inhibitors, PARP-1 is a near-ideal target to develop novel radiotherapeutics to induce DNA damage and apoptosis in cancer cells, while sparing healthy brain tissue. Methods: We synthesized an 131I-labeled PARP-1 therapeutic and investigated its pharmacology in vitro and in vivo. A subcutaneous tumor model was used to quantify retention times and therapeutic efficacy. A potential clinical scenario, intratumoral convection-enhanced delivery, was mimicked using an orthotopic glioblastoma model combined with an implanted osmotic pump system to study local administration of 131I-PARPi (PARPi is PARP inhibitor). Results:131I-PARPi is a 1(2H)-phthalazinone, similar in structure to the Food and Drug Administration-approved PARP inhibitor AZD-2281. In vitro studies have shown that 131I-PARPi and AZD-2281 share similar pharmacologic profiles. 131I-PARPi delivered 134.1 cGy/MBq intratumoral injected activity. Doses to nontarget tissues, including liver and kidney, were significantly lower. Radiation damage and cell death in treated tumors were shown by p53 activation in U87-MG cells transfected with a p53-bioluminescent reporter. Treated mice showed significantly longer survival than mice receiving vehicle (29 vs. 22 d, P < 0.005) in a subcutaneous model. Convection-enhanced delivery demonstrated efficient retention of 131I-PARPi in orthotopic brain tumors, while quickly clearing from healthy brain tissue. Conclusion: Our results demonstrate 131I-PARPi's high potential as a therapeutic and highlight PARP's relevance as a target for radionuclide therapy. Radiation plays an integral role in brain tumor therapy, and radiolabeled PARP therapeutics could ultimately lead to improvements in the standard of care.

Direct Imaging of Drug Distribution and Target Engagement of the PARP Inhibitor Rucaparib.

Poly(ADP-ribose)polymerase (PARP) inhibitors have emerged as potent antitumor drugs. Here, we describe the intrinsic fluorescence properties of the clinically approved PARP inhibitor rucaparib and its potential to directly measure drug distribution and target engagement-a critical factor for understanding drug action and improving efficacy. Methods: We characterized the photophysical properties of rucaparib and determined its quantum yield and lifetime. Using confocal microscopy and flow cytometry, we imaged the intracellular distribution of rucaparib and measured uptake and release kinetics. Results: Rucaparib has an excitation/emission maximum of 355/480 nm and a quantum yield of 0.3. In vitro time-lapse imaging showed accumulation in cell nuclei within seconds of administration. Nuclear rucaparib uptake increased with higher PARP1 expression, and we determined an intracellular half-life of 6.4 h. Conclusion: The label-free, intrinsic fluorescence of rucaparib can be exploited to interrogate drug distribution and target binding, critical factors toward improving treatment efficacy and outcome.

Target engagement imaging of PARP inhibitors in small-cell lung cancer.

Insufficient chemotherapy response and rapid disease progression remain concerns for small-cell lung cancer (SCLC). Oncologists rely on serial CT scanning to guide treatment decisions, but this cannot assess in vivo target engagement of therapeutic agents. Biomarker assessments in biopsy material do not assess contemporaneous target expression, intratumoral drug exposure, or drug-target engagement. Here, we report the use of PARP1/2-targeted imaging to measure target engagement of PARP inhibitors in vivo. Using a panel of clinical PARP inhibitors, we show that PARP imaging can quantify target engagement of chemically diverse small molecule inhibitors in vitro and in vivo. We measure PARP1/2 inhibition over time to calculate effective doses for individual drugs. Using patient-derived xenografts, we demonstrate that different therapeutics achieve similar integrated inhibition efficiencies under different dosing regimens. This imaging approach to non-invasive, quantitative assessment of dynamic intratumoral target inhibition may improve patient care through real-time monitoring of drug delivery.

Targeted PET imaging strategy to differentiate malignant from inflamed lymph nodes in diffuse large B-cell lymphoma.

Diffuse large B-cell lymphoma (DLBCL) is the most common lymphoma in adults. DLBCL exhibits highly aggressive and systemic progression into multiple tissues in patients, particularly in lymph nodes. Whole-body 18F-fluodeoxyglucose positron emission tomography ([18F]FDG-PET) imaging has an essential role in diagnosing DLBCL in the clinic; however, [18F]FDG-PET often faces difficulty in differentiating malignant tissues from certain nonmalignant tissues with high glucose uptake. We have developed a PET imaging strategy for DLBCL that targets poly[ADP ribose] polymerase 1 (PARP1), the expression of which has been found to be much higher in DLBCL than in healthy tissues. In a syngeneic DLBCL mouse model, this PARP1-targeted PET imaging approach allowed us to discriminate between malignant and inflamed lymph nodes, whereas [18F]FDG-PET failed to do so. Our PARP1-targeted PET imaging approach may be an attractive addition to the current PET imaging strategy to differentiate inflammation from malignancy in DLBCL.

Molecular Imaging of PARP.

The poly(adenosine diphosphate-ribose)polymerase (PARP) family of enzymes is an important factor in the cellular DNA damage response and has gained much attention for its role in many diseases, particularly cancer. Targeted molecular imaging of PARP using fluorescent or radiolabeled tags has followed on the success of therapeutic inhibitors and gained momentum over the past few years. This review covers PARP imaging from the very first imaging agents up to the current state of the technology, with a focus on the clinical applications made possible by these agents.

A Novel Technique for Generating and Observing Chemiluminescence in a Biological Setting.

Intraoperative imaging techniques have the potential to make surgical interventions safer and more effective; for these reasons, such techniques are quickly moving into the operating room. Here, we present a new approach that utilizes a technique not yet explored for intraoperative imaging: chemiluminescent imaging. This method employs a ruthenium-based chemiluminescent reporter along with a custom-built nebulizing system to produce ex vivo or in vivo images with high signal-to-noise ratios. The ruthenium-based reporter produces light following exposure to an aqueous oxidizing solution and re-reduction within the surrounding tissue. This method has allowed us to detect reporter concentrations as low as 6.9 pmol/cm2. In this work, we present a visual guide to our proof-of-concept in vivo studies involving subdermal and intravenous injections in mice. The results suggest that this technology is a promising candidate for further preclinical research and might ultimately become a useful tool in the operating room.

Evaluation of [18F]-ATRi as PET tracer for in vivo imaging of ATR in mouse models of brain cancer.

RATIONALE: Ataxia telangiectasia and Rad3-related (ATR) threonine serine kinase is one of the key elements in orchestrating the DNA damage response (DDR). As such, inhibition of ATR can amplify the effects of chemo- and radiation-therapy, and several ATR inhibitors (ATRi) have already undergone clinical testing in cancer. For more accurate patient selection, monitoring and staging, real-time in vivo imaging of ATR could be invaluable; the development of appropriate imaging agents has remained a major challenge. METHODS: 3-amino-N-(4-[18F]phenyl)-6-(4-(methylsulfonyl)phenyl)pyrazine-2-carboxamide ([18F]-ATRi), a close analogue of Ve-821, (a clinical ATRi candidate), was readily accomplished similarly to already established synthetic procedures. Structurally, 18F was introduced at the 4-position of the aromatic ring of Ve-821 for generating a labeled ATR inhibitor. In vitro experiments were conducted in U251 MG glioblastoma cell lines and ex vivo biodistribution were performed in subcutaneous U251 MG xenograft bearing athymic nude mice following microPET imaging. RESULTS: [18F]-ATRi has a similar pharmacokinetic profile to that of Ve-821. Using an U251 MG glioblastoma mouse model, we evaluated the in vivo binding efficiency of [18F]-ATRi. Blood and tumor showed a statistically significant difference between mice injected with only the probe or following blocking experiment with Ve-821 (1.48±0.40%ID/g vs. 0.46±0.12%ID/g in tumor and 1.85±0.47%ID/g vs. 0.84±0.3%ID/g in blood respectively). CONCLUSIONS: [18F]-ATRi represents the first 18F positron emission tomography (PET) ATR imaging agent, and is designed on a low nanomolar and clinically relevant ATR inhibitor.

Biomarker-Based PET Imaging of Diffuse Intrinsic Pontine Glioma in Mouse Models.

Diffuse intrinsic pontine glioma (DIPG) is a childhood brainstem tumor with a universally poor prognosis. Here, we characterize a positron emission tomography (PET) probe for imaging DIPG in vivo In human histological tissues, the probes target, PARP1, was highly expressed in DIPG compared to normal brain. PET imaging allowed for the sensitive detection of DIPG in a genetically engineered mouse model, and probe uptake correlated to histologically determined tumor infiltration. Imaging with the sister fluorescence agent revealed that uptake was confined to proliferating, PARP1-expressing cells. Comparison with other imaging technologies revealed remarkable accuracy of our biomarker approach. We subsequently demonstrated that serial imaging of DIPG in mouse models enables monitoring of tumor growth, as shown in modeling of tumor progression. Overall, this validated method for quantifying DIPG burden would serve useful in monitoring treatment response in early phase clinical trials. Cancer Res; 77(8); 2112-23. ©2017 AACR.

Near-Infrared Intraoperative Chemiluminescence Imaging.

Intraoperative imaging technologies recently entered the operating room, and their implementation is revolutionizing how physicians plan, monitor, and perform surgical interventions. In this work, we present a novel surgical imaging reporter system: intraoperative chemiluminescence imaging (ICI). To this end, we have leveraged the ability of a chemiluminescent metal complex to generate near-infrared light upon exposure to an aqueous solution of Ce(4+) in the presence of reducing tissue or blood components. An optical camera spatially resolves the resulting photon flux. We describe the construction and application of a prototype imaging setup, which achieves a detection limit as low as 6.9 pmol cm(-2) of the transition-metal-based ICI agent. As a proof of concept, we use ICI for the in vivo detection of our transition metal tracer following both systemic and subdermal injections. The very high signal-to-noise ratios make ICI an interesting candidate for the development of new intraoperative imaging technologies.

Non-invasive PET Imaging of PARP1 Expression in Glioblastoma Models.

PURPOSE: The current study presents [(18)F]PARPi as imaging agent for PARP1 expression. PROCEDURES: [(18)F]PARPi was generated by conjugating a 2H-phthalazin-1-one scaffold to 4-[(18)F]fluorobenzoic acid. Biochemical assays, optical in vivo competition, biodistribution analysis, positron emission tomography (PET)/X-ray computed tomography, and PET/magnetic resonance imaging studies were performed in subcutaneous and orthotopic mouse models of glioblastoma. RESULTS: [(18)F]PARPi shows suitable pharmacokinetic properties for brain tumor imaging (IC50 = 2.8 ± 1.1 nM; logPCHI = 2.15 ± 0.41; plasma-free fraction = 63.9 ± 12.6 %) and accumulates selectively in orthotopic brain tumor tissue. Tracer accumulation in subcutaneous brain tumors was 1.82 ± 0.21 %ID/g, whereas in healthy brain, the uptake was only 0.04 ± 0.01 %ID/g. CONCLUSIONS: [(18)F]PARPi is a selective PARP1 imaging agent that can be used to visualize glioblastoma in xenograft and orthotopic mouse models with high precision and good signal/noise ratios. It offers new opportunities to non-invasively image tumor growth and monitor interventions.