A PET imaging agent for evaluating PARP-1 expression in ovarian cancer

AKAP8 promotes ovarian cancer progression and antagonizes PARP inhibitor sensitivity through regulating hnRNPUL1 transcription

Ovarian cancer (OC) is the highest worldwide cancer mortality cause among gynecologic tumors, but its underlying molecular mechanism remains largely unknown. Here, we report that the RNA binding protein A-kinase anchoring protein 8 (AKAP8) is highly expressed in ovarian cancer and predicts poor prognosis for ovarian cancer patients. AKAP8 promotes ovarian cancer progression through regulating cell proliferation and metastasis. Mechanically, AKAP8 is enriched at chromatin and regulates the transcription of the specific hnRNPUL1 isoform. Moreover, AKAP8 phase separation modulates the hnRNPUL1 short isoform transcription. Ectopic expression of the hnRNPUL1 short isoform could partially rescue the growth inhibition effect of AKAP8-knockdown in ovarian cancer cells. In addition, AKAP8 modulates PARP1 expression through hnRNPUL1, and AKAP8 inhibition enhances PAPR inhibitor cytotoxicity in ovarian cancer. Together, our study uncovers the crucial function of AKAP8 condensation-mediated transcription regulation, and targeting AKAP8 could be potential for improvement of ovarian cancer therapy.

Positron Emission Tomography Imaging of Tumor Proliferation and DNA Repair

ABSTRACT

Positron emission tomography (PET) is an established tool for molecular imaging of cancers, and its role in diagnosis, staging, and phenotyping continues to evolve and expand rapidly. PET imaging of increased glucose utilization with 18F-fluorodeoxyglucose is now entrenched in clinical oncology practice for improving prognostication and treatment response assessment. Additional critical processes for cancer cell survival can also be imaged by PET, helping to inform individualized treatment selections for patients by improving our understanding of cell survival mechanisms and identifying relevant active mechanisms in each patient. The critical importance of quantifying cell proliferation and DNA repair pathways for prognosis and treatment selection is highlighted by the nearly ubiquitous use of the Ki-67 index, an established histological quantitative measure of cell proliferation, and BRCA mutation testing for treatment selection. This review focuses on PET advances in imaging and quantifying cell proliferation and poly(ADP-ribose)polymerase expression that can be used to complement cancer phenotyping approaches that will identify the most effective treatments for each individual patient.

Nuclear Medicine and Molecular Imaging Applications in Gynecologic Malignancies: A Comprehensive Review

Gynecologic malignancies, consisting of endometrial, cervical, ovarian, vulvar, and vaginal cancers, pose significant diagnostic and management challenges due to their complex anatomic location and potential for rapid progression. These tumors cause substantial morbidity and mortality, often because of their delayed diagnosis and treatment. An estimated 19% of newly diagnosed cancers among women are gynecologic in origin. In recent years, there has been growing evidence supporting the integration of nuclear medicine imaging modalities in the diagnostic work-up and management of gynecologic cancers. The sensitivity of fluorine-18 fluorodeoxyglucose positron emission tomography (18F-FDG PET) combined with the anatomical specificity of computed tomography (CT) and magnetic resonance imaging (MRI) allows for the hybrid evaluation of metabolic activity and structural abnormalities that has become an indispensable tool in oncologic imaging. Lymphoscintigraphy, using technetium 99m (99mTc) based radiotracers along with single photon emission computed tomography/ computed tomography (SPECT/CT), holds a vital role in the identification of sentinel lymph nodes to minimize the surgical morbidity from extensive lymph node dissections. While not yet standard for gynecologic malignancies, promising therapeutic nuclear medicine agents serve as specialized treatment options for patients with advanced or recurrent disease. This article aims to provide a comprehensive review on the nuclear medicine applications in gynecologic malignancies through the following objectives: 1) To describe the role of nuclear medicine in the initial staging, lymph node mapping, response assessment, and recurrence/surveillance imaging of common gynecologic cancers, 2) To review the limitations of 18F-FDG PET/CT and promising applications of 18F-FDG PET/MRI in gynecologic malignancy, 3) To underscore the promising theragnostic applications of nuclear medicine, 4) To highlight the current role of nuclear medicine imaging in gynecologic cancers as per the National Comprehensive Cancer Network (NCCN), European Society of Surgical Oncology (ESGO), and European Society of Medical Oncology (ESMO) guidelines.

The Current and Future Roles of Precision Oncology in Advanced Breast Cancer

Breast cancer is a common but heterogeneous disease characterized by several biologic features, including tumor grade, hormone receptor status, human epidermal growth factor receptor 2 status, and gene expression assays. These biologic and genomic features drive treatment decisions. In the advanced disease setting, inter- and intrapatient tumor heterogeneity is increasingly recognized as a challenge for optimizing treatment. Recent evidence and the recent approval of novel radiopharmaceuticals have increased recognition and acceptance of the potential of molecular imaging as a biomarker to impact and guide management decisions for advanced breast cancer.

Poly(ADP-ribose) polymerase (PARP)-targeted PET imaging in non-oncology application: a pilot study in preclinical models of nonalcoholic steatohepatitis

Poly(ADP-ribose) polymerase (PARP) activation often indicates a disruptive signal to lipid metabolism, the physiological alteration of which may be implicated in the development of non-alcoholic fatty liver disease. The objective of this study was to evaluate the capability of [68Ga]DOTA-PARPi PET to detect hepatic PARP expression in a non-alcoholic steatohepatitis (NASH) mouse model. In this study, male C57BL/6 mice were subjected to a choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD) for a 12-week period to establish preclinical NASH models. [68Ga]DOTA-PARPi PET imaging of the liver was conducted at the 12-week mark after CDAHFD feeding. Comprehensive histopathological analysis, covering hepatic steatosis, inflammation, fibrosis, along with blood biochemistry, was performed in both NASH models and control groups. Despite the induction of severe inflammation, steatosis and fibrosis in the liver of mice with the CDAHFD-NASH model, PET imaging of NASH with [68Ga]-DOTA-PARPi did not reveal a significantly higher uptake in NASH models compared to the control. This underscores the necessity for further development of new chelator-based PARP1 tracers with high binding affinity to enable the visualization of PARP1 changes in NASH pathology.

[123I]CC1: A PARP-Targeting, Auger Electron-Emitting Radiopharmaceutical for Radionuclide Therapy of Cancer

Poly(adenosine diphosphate ribose) polymerase (PARP) has emerged as an effective therapeutic strategy against cancer that targets the DNA damage repair enzyme. PARP-targeting compounds radiolabeled with an Auger electron-emitting radionuclide can be trapped close to damaged DNA in tumor tissue, where high ionizing potential and short range lead Auger electrons to kill cancer cells through the creation of complex DNA damage, with minimal damage to surrounding normal tissue. Here, we report on [123I]CC1, an 123I-labeled PARP inhibitor for radioligand therapy of cancer. Methods: Copper-mediated 123I iododeboronation of a boronic pinacol ester precursor afforded [123I]CC1. The level and specificity of cell uptake and the therapeutic efficacy of [123I]CC1 were determined in human breast carcinoma, pancreatic adenocarcinoma, and glioblastoma cells. Tumor uptake and tumor growth inhibition of [123I]CC1 were assessed in mice bearing human cancer xenografts (MDA-MB-231, PSN1, and U87MG). Results: In vitro and in vivo studies showed selective uptake of [123I]CC1 in all models. Significantly reduced clonogenicity, a proxy for tumor growth inhibition by ionizing radiation in vivo, was observed in vitro after treatment with as little as 10 Bq [123I]CC1. Biodistribution at 1 h after intravenous administration showed PSN1 tumor xenograft uptake of 0.9 ± 0.06 percentage injected dose per gram of tissue. Intravenous administration of a relatively low amount of [123I]CC1 (3 MBq) was able to significantly inhibit PSN1 xenograft tumor growth but was less effective in xenografts that expressed less PARP. [123I]CC1 did not cause significant toxicity to normal tissues. Conclusion: Taken together, these results show the potential of [123I]CC1 as a radioligand therapy for PARP-expressing cancers.

Aligning germline and somatic mutations in prostate cancer. Are genetics changing practice?

OBJECTIVE

To review the current status of germline and somatic (tumour) genetic testing for prostate cancer (PCa), and its relevance for clinical practice.

METHODS

A narrative synthesis of various molecular profiles related to their clinical context was carried out. Current guidelines for genetic testing and its feasibility in clinical practice were analysed. We report the main identified genetic sequencing results or functional genomic scores for PCa published in the literature or obtained from the French PROGENE study.

RESULTS

The molecular alterations observed in PCa are mostly linked to disruption of the androgen receptor (AR) pathway or DNA repair deficiency. The main known germline mutations affect the BReast CAncer gene 2 (BRCA2) and homeobox B13 (HOXB13) genes, whereas AR and tumour protein p53 (TP53) are the genes with most frequent somatic alterations in tumours from men with metastatic PCa. Molecular tests are now available for detecting some of these germline or somatic alterations and sometimes recommended by guidelines, but their utilisation must combine rationality and feasibility. They can guide specific therapies, notably for the management of metastatic disease. Indeed, following androgen deprivation, targeted therapies for PCa currently include poly-(ADP-ribose)-polymerase (PARP) inhibitors, immune checkpoint inhibitors, and prostate-specific membrane antigen (PSMA)-guided radiotherapy. The genetic tests currently approved for targeted therapies remain limited to the detection of BRCA1 and BRCA2 mutation and DNA mismatch repair deficiency, while large panels are recommended for germline analyses, not only for inherited cancer predisposing syndrome, but also for metastatic PCa.

CONCLUSIONS

Further consensus aligning germline with somatic molecular analysis in metastatic PCa is required, including genomics scars, emergent immunohistochemistry, or functional pre-screen imaging. With rapid advances in knowledge and technology in the field, continuous updating of guidelines to help the clinical management of these individuals, and well-conducted studies to evaluate the benefits of genetic testing are needed.

Molecular imaging of PARP in cancer: state-of-the-art

INTRODUCTION

Poly-ADP-ribose-polymerase inhibitors (PARPi), which exploit the processes of so-called 'synthetic lethality,' have been successfully implemented in oncological practice. However, not all patients respond to PARPi, and there is an unmet need for noninvasive biomarkers suitable for patient selection and monitoring during PARPi therapy.

AREAS COVERED

The first clinical applications of molecular imaging with positron emission tomography/computed tomography (PET/CT) with [18F]-FluorThanatrace ([18F]-FTT) and [18F]-PARPi, highly effective PARP-ligands, in patients with several malignancies (head and neck, ovarian, prostate, and breast cancer) are covered, with a particular focus on its potential for pre-treatment selection and follow-up.

EXPERT OPINION

By a search made on the most common database, such as PubMed and Google Scholar in a period from January 2010 and 2023, first clinical evidence suggests that PET/CT with [18F]-FTT and [18F]-PARPi might represent a reliable tool for in vivo imaging and quantification of PARP-1 expression in ovarian, prostate, breast, head, and neck cancer, supporting their potential usefulness for patient selection before PARPi-therapies. In addition, a reduction in [18F]-FTT uptake has been registered after therapy initiation and seems to be correlated with patient outcome after PARPi-based regimens. Further studies are needed to better address the value of PARPI-radiolabeled PET imaging in these clinical settings, especially as it concerns technical features such as optimal scan modality (dynamic vs. static) and timing.

Preclinical studies of a PARP targeted, Meitner-Auger emitting, theranostic radiopharmaceutical for metastatic ovarian cancer

Advanced ovarian cancer currently has few therapeutic options. Poly(ADP-ribose) polymerase (PARP) inhibitors bind to nuclear PARP and trap the protein-inhibitor complex to DNA. This work investigates a theranostic PARP inhibitor for targeted radiopharmaceutical therapy of ovarian cancer in vitro and PET imaging of healthy mice in vivo.

METHODS

[77Br]RD1 was synthesized and assessed for pharmacokinetics and cytotoxicity in human and murine ovarian cancer cell lines. [76Br]RD1 biodistribution and organ uptake in healthy mice were quantified through longitudinal PET/CT imaging and ex vivo radioactivity measurements. Organ-level dosimetry following [76/77Br]RD1 administration was calculated using RAPID, an in-house platform for absorbed dose in mice, and OLINDA for equivalent and effective dose in human.

RESULTS

The maximum specific binding (Bmax), equilibrium dissociation constant (Kd), and nonspecific binding slope (NS) were calculated for each cell line. These values were used to calculate the cell specific activity uptake for cell viability studies. The half maximal effective concentration (EC50) was measured as 0.17 (95 % CI: 0.13-0.24) nM and 0.46 (0.13-0.24) nM for PARP(+) and PARP(-) expressing cell lines, respectively. The EC50 was 0.27 (0.21-0.36) nM and 0.30 (0.22-0.41) nM for BRCA1(-) and BRCA1(+) expressing cell lines, respectively. When measuring the EC50 as a function of cellular activity uptake and nuclear dose, the EC50 ranges from 0.020 to 0.039 Bq/cell and 3.3-9.2 Gy, respectively. Excretion through the hepatobiliary and renal pathways were observed in mice, with liver uptake of 2.3 ± 0.4 %ID/g after 48 h, contributing to estimated absorbed dose values in mice of 19.3 ± 0.3 mGy/MBq and 290 ± 10 mGy/MBq for [77Br]RD1 and [76Br]RD1, respectively.

CONCLUSION

[77Br]RD1 cytotoxicity was dependent on PARP expression and independent of BRCA1 status. The in vitro results suggest that [77Br]RD1 cytotoxicity is driven by the targeted Meitner-Auger electron (MAe) radiotherapeutic effect of the agent. Further studies investigating the theranostic potential, organ dose, and tumor uptake of [76/77Br]RD1 are warranted.

Advances in PET imaging of cancer

Molecular imaging has experienced enormous advancements in the areas of imaging technology, imaging probe and contrast development, and data quality, as well as machine learning-based data analysis. Positron emission tomography (PET) and its combination with computed tomography (CT) or magnetic resonance imaging (MRI) as a multimodality PET-CT or PET-MRI system offer a wealth of molecular, functional and morphological data with a single patient scan. Despite the recent technical advances and the availability of dozens of disease-specific contrast and imaging probes, only a few parameters, such as tumour size or the mean tracer uptake, are used for the evaluation of images in clinical practice. Multiparametric in vivo imaging data not only are highly quantitative but also can provide invaluable information about pathophysiology, receptor expression, metabolism, or morphological and functional features of tumours, such as pH, oxygenation or tissue density, as well as pharmacodynamic properties of drugs, to measure drug response with a contrast agent. It can further quantitatively map and spatially resolve the intertumoural and intratumoural heterogeneity, providing insights into tumour vulnerabilities for target-specific therapeutic interventions. Failure to exploit and integrate the full potential of such powerful imaging data may lead to a lost opportunity in which patients do not receive the best possible care. With the desire to implement personalized medicine in the cancer clinic, the full comprehensive diagnostic power of multiplexed imaging should be utilized.

Current status of contemporary diagnostic radiotracers in the management of breast cancer: first steps toward theranostic applications

BACKGROUND

Expanding therapeutic possibilities have improved disease-related prospects for breast cancer patients. Pathological analysis on a tumor biopsy is the current reference standard biomarker used to select for treatment with targeted anticancer drugs. This method has, however, several limitations, related to intra- and intertumoral as well as spatial heterogeneity in receptor expression as well as the need to perform invasive procedures that are not always technically feasible.

MAIN BODY

In this narrative review, we focus on the current role of molecular imaging with contemporary radiotracers for positron emission tomography (PET) in breast cancer. We provide an overview of diagnostic radiotracers that represent treatment targets, such as programmed death ligand 1, human epidermal growth factor receptor 2, polyadenosine diphosphate-ribose polymerase and estrogen receptor, and discuss developments in therapeutic radionuclides for breast cancer management.

CONCLUSION

Imaging of treatment targets with PET tracers may provide a more reliable precision medicine tool to find the right treatment for the right patient at the right time. In addition to visualization of the target of treatment, theranostic trials with alpha- or beta-emitting isotopes provide a future treatment option for patients with metastatic breast cancer.

Retinoic Acid Prevents α-Synuclein Preformed Fibrils-Induced Toxicity via Inhibiting STAT1-PARP1 Signaling

Parkinson's disease (PD), the second-most prevalent neurodegenerative disorder, is characterized by the aberrant deposition of α-synuclein (α-Syn) aggregation in neurons. Recent reports have shown that retinoic acid (RA) ameliorates motor deficits. However, the underlying molecular mechanisms remain unclear. In this article, we investigated the effects of RA on cellular and animal models of PD. We found that RA is beneficial for neuronal survival in PD-associated models. In α-Syn preformed fibrils-treated mice, RA administration relieved the formation of intracellular inclusions, dopaminergic neuronal loss, and behavioral deficits. α-Syn preformed fibrils-treated SH-SY5Y cells manifested decreased cell viability, apoptosis, α-Syn aggregation, and autophagy defects. All these negative phenomena were alleviated by RA. More importantly, RA could inhibit the neurotoxicity via inhibiting α-Syn preformed fibrils-induced STAT1-PARP1 signaling, which could also be antagonized by IFN-γ. In conclusion, RA could hinder α-Syn preformed fibrils-induced toxicity by inhibiting STAT1-PARP1 signaling. Thus, we present new insight into RA in PD management.

Loss of the receptors ER, PR and HER2 promotes USP15-dependent stabilization of PARP1 in triple-negative breast cancer

Poly(ADP-ribose) polymerase 1 (PARP1) is essential for the progression of several types of cancers. However, whether and how PARP1 is stabilized to promote genomic stability in triple-negative breast cancer (TNBC) remains unknown. Here, we demonstrated that the deubiquitinase USP15 interacts with and deubiquitinates PARP1 to promote its stability, thereby stimulating DNA repair, genomic stability and TNBC cell proliferation. Two PARP1 mutations found in individuals with breast cancer (E90K and S104R) enhanced the PARP1-USP15 interaction and suppressed PARP1 ubiquitination, thereby elevating the protein level of PARP1. Importantly, we found that estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) inhibited USP15-mediated PARP1 stabilization through different mechanisms. ER bound to the USP15 promoter to suppress its expression, PR suppressed the deubiquitinase activity of USP15, and HER2 abrogated the PARP1-USP15 interaction. The specific absence of these three receptors in TNBC results in high PARP1 levels, leading to increases in base excision repair and female TNBC cell survival.

Imaging genomics: data fusion in uncovering disease heritability

Sequencing of the human genome in the early 2000s enabled probing of the genetic basis of disease on a scale previously unimaginable. Now, two decades later, after interrogating millions of markers in thousands of individuals, a significant portion of disease heritability still remains hidden. Recent efforts to unravel this 'missing heritability' have focused on garnering new insight from merging different data types, including medical imaging. Imaging offers promising intermediate phenotypes to bridge the gap between genetic variation and disease pathology. In this review we outline this fusion and provide examples of imaging genomics in a range of diseases, from oncology to cardiovascular and neurodegenerative disease. Finally, we discuss how ongoing revolutions in data science and sharing are primed to advance the field.

CRISPR screens reveal genetic determinants of PARP inhibitor sensitivity and resistance in prostate cancer

Prostate cancer harboring BRCA1/2 mutations are often exceptionally sensitive to PARP inhibitors. However, genomic alterations in other DNA damage response genes have not been consistently predictive of clinical response to PARP inhibition. Here, we perform genome-wide CRISPR-Cas9 knockout screens in BRCA1/2-proficient prostate cancer cells and identify previously unknown genes whose loss has a profound impact on PARP inhibitor response. Specifically, MMS22L deletion, frequently observed (up to 14%) in prostate cancer, renders cells hypersensitive to PARP inhibitors by disrupting RAD51 loading required for homologous recombination repair, although this response is TP53-dependent. Unexpectedly, loss of CHEK2 confers resistance rather than sensitivity to PARP inhibition through increased expression of BRCA2, a target of CHEK2-TP53-E2F7-mediated transcriptional repression. Combined PARP and ATR inhibition overcomes PARP inhibitor resistance caused by CHEK2 loss. Our findings may inform the use of PARP inhibitors beyond BRCA1/2-deficient tumors and support reevaluation of current biomarkers for PARP inhibition in prostate cancer.

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.

Pre-clinical investigation of astatine-211-parthanatine for high-risk neuroblastoma

Astatine-211-parthanatine ([²¹¹At]PTT) is an alpha-emitting radiopharmaceutical therapeutic that targets poly(adenosine-diphosphate-ribose) polymerase 1 (PARP1) in cancer cells. High-risk neuroblastomas exhibit among the highest PARP1 expression across solid tumors. In this study, we evaluated the efficacy of [²¹¹At]PTT using 11 patient-derived xenograft (PDX) mouse models of high-risk neuroblastoma, and assessed hematological and marrow toxicity in a CB57/BL6 healthy mouse model. We observed broad efficacy in PDX models treated with [²¹¹At]PTT at the maximum tolerated dose (MTD 36 MBq/kg/fraction x4) administered as a fractionated regimen. For the MTD, complete tumor response was observed in 81.8% (18 of 22) of tumors and the median event free survival was 72 days with 30% (6/20) of mice showing no measurable tumor >95 days. Reversible hematological and marrow toxicity was observed 72 hours post-treatment at the MTD, however full recovery was evident by 4 weeks post-therapy. These data support clinical development of [²¹¹At]PTT for high-risk neuroblastoma.

The efficacy and toxicity of the PARP inhibitor parthanatine, radiolabeled with the alpha particle emitter astatine-211, in 11 patient derived neuroblastoma murine xenografts is investigated.

Role of PARP Inhibitors in Cancer Immunotherapy: Potential Friends to Immune Activating Molecules and Foes to Immune Checkpoints

Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) induce cytotoxic effects as single agents in tumors characterized by defective repair of DNA double-strand breaks deriving from BRCA1/2 mutations or other abnormalities in genes associated with homologous recombination. Preclinical studies have shown that PARPi-induced DNA damage may affect the tumor immune microenvironment and immune-mediated anti-tumor response through several mechanisms. In particular, increased DNA damage has been shown to induce the activation of type I interferon pathway and up-regulation of PD-L1 expression in cancer cells, which can both enhance sensitivity to Immune Checkpoint Inhibitors (ICIs). Despite the recent approval of ICIs for a number of advanced cancer types based on their ability to reinvigorate T-cell-mediated antitumor immune responses, a consistent percentage of treated patients fail to respond, strongly encouraging the identification of combination therapies to overcome resistance. In the present review, we analyzed both established and unexplored mechanisms that may be elicited by PARPi, supporting immune reactivation and their potential synergism with currently used ICIs. This analysis may indicate novel and possibly patient-specific immune features that might represent new pharmacological targets of PARPi, potentially leading to the identification of predictive biomarkers of response to their combination with ICIs.

Poly (ADP-ribose) polymerases as PET imaging targets for central nervous system diseases

Poly (ADP-ribose) polymerases (PARPs) constitute of 17 members that are associated with divergent cellular processes and play a crucial role in DNA repair, chromatin organization, genome integrity, apoptosis, and inflammation. Multiple lines of evidence have shown that activated PARP1 is associated with intense DNA damage and irritating inflammatory responses, which are in turn related to etiologies of various neurological disorders. PARP1/2 as plausible therapeutic targets have attracted considerable interests, and multitudes of PARP1/2 inhibitors have emerged for treating cancer, metabolic, inflammatory, and neurological disorders. Furthermore, PARP1/2 as imaging targets have been shown to detect, delineate, and predict therapeutic responses in many diseases by locating and quantifying the expression levels of PARP1/2. PARP1/2-directed noninvasive positron emission tomography (PET) has potential in diagnosing and prognosing neurological diseases. However, quantitative PARP PET imaging in the central nervous system (CNS) has evaded us due to the challenges of developing blood-brain barrier (BBB) penetrable PARP radioligands. Here, we review PARP1/2's relevance in CNS diseases, summarize the recent progress on PARP PET and discuss the possibilities of developing novel PARP radiotracers for CNS diseases.

Correlation between molar activity, injection mass and uptake of the PARP targeting radiotracer [18F]olaparib in mouse models of glioma

PURPOSE

Radiopharmaceuticals targeting poly(ADP-ribose) polymerase (PARP) have emerged as promising agents for cancer diagnosis and therapy. PARP enzymes are expressed in both cancerous and normal tissue. Hence, the injected mass, molar activity and potential pharmacological effects are important considerations for the use of radiolabelled PARP inhibitors for diagnostic and radionuclide therapeutic applications. Here, we performed a systematic evaluation by varying the molar activity of [18F]olaparib and the injected mass of [TotalF]olaparib to investigate the effects on tumour and normal tissue uptake in two subcutaneous human glioblastoma xenograft models.

METHODS

[18F]Olaparib uptake was evaluated in the human glioblastoma models: in vitro on U251MG and U87MG cell lines, and in vivo on tumour xenograft-bearing mice, after administration of [TotalF]olaparib (varying injected mass: 0.04-8.0 µg, and molar activity: 1-320 GBq/μmol).

RESULTS

Selective uptake of [18F]olaparib was demonstrated in both models. Tumour uptake was found to be dependent on the injected mass of [TotalF]olaparib (µg) but not the molar activity. An injected mass of 1 μg resulted in the highest tumour uptake (up to 6.9 ± 1.3%ID/g), independent of the molar activity. In comparison, both the lower and higher injected masses of [TotalF]olaparib resulted in lower relative tumour uptake (%ID/g; P < 0.05). Ex vivo analysis of U87MG xenograft sections showed that the heterogeneity in [18F]olaparib intratumoural uptake correlated with PARP1 expression. Substantial upregulation of PARP1-3 expression was observed after administration of [TotalF]olaparib (> 0.5 µg).

CONCLUSION

Our findings show that the injected mass of [TotalF]olaparib has significant effects on tumour uptake. Moderate injected masses of PARP inhibitor-derived radiopharmaceuticals may lead to improved relative tumour uptake and tumour-to-background ratio for cancer diagnosis and radionuclide therapy.

Imaging PARP with [18F]rucaparib in pancreatic cancer models

PURPOSE

Rucaparib, an FDA-approved PARP inhibitor, is used as a single agent in maintenance therapy to provide promising treatment efficacy with an acceptable safety profile in various types of BRCA-mutated cancers. However, not all patients receive the same benefit from rucaparib-maintenance therapy. A predictive biomarker to help with patient selection for rucaparib treatment and predict clinical benefit is therefore warranted. With this aim, we developed [18F]rucaparib, an 18F-labelled isotopologue of rucaparib, and employed it as a PARP-targeting agent for cancer imaging with PET. Here, we report the in vitro and in vivo evaluation of [18F]rucaparib in human pancreatic cancer models.

METHOD

We incorporated the positron-emitting 18F isotope into rucaparib, enabling its use as a PET imaging agent. [18F]rucaparib binds to the DNA damage repair enzyme, PARP, allowing direct visualisation and measurement of PARP in cancerous models before and after PARP inhibition or other genotoxic cancer therapies, providing critical information for cancer diagnosis and therapy. Proof-of-concept evaluations were determined in pancreatic cancer models.

RESULTS

Uptake of [18F]rucaparib was found to be mainly dependent on PARP1 expression. Induction of DNA damage increased PARP expression, thereby increasing uptake of [18F]rucaparib. In vivo studies revealed relatively fast blood clearance of [18F]rucaparib in PSN1 tumour-bearing mice, with a tumour uptake of 5.5 ± 0.5%ID/g (1 h after i.v. administration). In vitro and in vivo studies showed significant reduction of [18F]rucaparib uptake by addition of different PARP inhibitors, indicating PARP-selective binding.

CONCLUSION

Taken together, we demonstrate the potential of [18F]rucaparib as a non-invasive PARP-targeting imaging agent for pancreatic cancers.

Upregulation of PARG in prostate cancer cells suppresses their malignant behavior and downregulates tumor-promoting genes

Post-translational modification of nuclear proteins through the addition of poly(ADP-ribose) (pADPr) moieties is upregulated in many metastatic cancers, where the high levels of pADPr have often been associated with poor cancer prognosis. Although the inhibitors of poly(ADP-ribose) polymerases (PARPs) have been utilized as potent anti-cancer agents, their efficacy in clinical trials varied among patient groups and has often been unpredictable. Such outcome cannot be interpreted solely by the inability to keep PARP-driven DNA repair in check. The focus of studies on PARP-driven tumorigenesis have recently been shifted toward PARP-dependent regulation of transcription. Here we utilized the controlled overexpression of poly(ADP-ribose) glycohydrolase (PARG), a sole pADPr-degrading enzyme, to investigate pADPr-dependent gene regulation in prostate cancer PC-3 cells. We demonstrated that PARG upregulation reduces pADPr levels and inhibits the expression of genes in key tumor-promoted pathways, including TNFα/NF-kB, IL6/STAT3, MYC, and KRAS signaling, the genes involved in inflammation response, especially chemokines, and endothelial-mesenchymal transition. The observed effect of PARG on transcription was consistent across all tested prostate cancer cell lines and correlates with PARG-induced reduction of clonogenic potential of PC-3 cells in vitro and a significant growth inhibition of PC-3-derived tumors in nude mice in vivo.

Predictive biomarkers for molecularly targeted therapies and immunotherapies in breast cancer

Globally, breast cancer is the most common malignancy in women. Substantial efforts have been made to develop novel therapies, including targeted therapies and immunotherapies, for patients with breast cancer who do not respond to standard therapies. Consequently, new targeted therapies, such as cyclin-dependent kinase 4 and 6 inhibitors, poly (ADP-ribose) polymerase inhibitors, phosphoinositide 3-kinase inhibitor, and antibody-drug conjugates targeting human epidermal growth factor receptor 2 or trophoblast cell surface antigen-2, and immune checkpoint inhibitor targeting programmed cell death-1, have been developed and are now in clinical use. However, only some patients have benefited from these novel therapies; therefore, the identification and validation of reliable or more accurate biomarkers for predicting responses to these agents remain a major challenge. This review summarizes the currently available predictive biomarkers for breast cancer and describes recent efforts undertaken to identify potential predictive markers for molecularly targeted therapies and immune checkpoint inhibitors.

Current status and progress in using radiolabelled PARP-1 inhibitors for imaging PARP-1 expression in tumours

Poly(ADP-ribose) polymerase-1 (PARP-1) is a key enzyme in the DNA repair process, and the overexpression of PARP-1 in several tumours makes this enzyme a promising molecular target. Recently, several PARP-1 inhibitors, such as olaparib, rucaparib, niraparib and talazoparib, have been clinically approved as anticancer drugs. Several of these inhibitors have been radiolabelled for noninvasive imaging of PARP-1 expression in several types of tumours. In this review, the background and progress for using various radiolabelled PARP-1 inhibitors for cancer diagnosis are discussed and future development directions are proposed.

[18F]-Labeled PARP-1 PET imaging of PSMA targeted alpha particle radiotherapy response

The growing interest and clinical translation of alpha particle (α) therapies brings with it new challenges to assess target cell engagement and to monitor therapeutic effect. Noninvasive imaging has great potential to guide α-treatment and to harness the potential of these agents in the complex environment of disseminated disease. Poly(ADP) ribose polymerase 1 (PARP-1) is among the most abundantly expressed DNA repair enzymes with key roles in multiple repair pathways-such as those induced by irradiation. Here, we used a third-generation PARP1-specific radiotracer, [18F]-PARPZ, to delineate castrate resistant prostate cancer xenografts. Following treatment with the clinically applied [225Ac]-PSMA-617, positron emission tomography was performed and correlative autoradiography and histology acquired. [18F]-PARPZ was able to distinguish treated from control (saline) xenografts by increased uptake. Kinetic analysis of tracer accumulation also suggests that the localization of the agent to sites of increased PARP-1 expression is a consequence of DNA damage response. Together, these data support expanded investigation of [18F]-PARPZ to facilitate clinical translation in the ⍺-therapy space.

Homologous Recombination Deficiency in Ovarian, Breast, Colorectal, Pancreatic, Non-Small Cell Lung and Prostate Cancers, and the Mechanisms of Resistance to PARP Inhibitors

Homologous recombination (HR) is a highly conserved DNA repair mechanism that protects cells from exogenous and endogenous DNA damage. Breast cancer 1 (BRCA1) and breast cancer 2 (BRCA2) play an important role in the HR repair pathway by interacting with other DNA repair proteins such as Fanconi anemia (FA) proteins, ATM, RAD51, PALB2, MRE11A, RAD50, and NBN. These pathways are frequently aberrant in cancer, leading to the accumulation of DNA damage and genomic instability known as homologous recombination deficiency (HRD). HRD can be caused by chromosomal and subchromosomal aberrations, as well as by epigenetic inactivation of tumor suppressor gene promoters. Deficiency in one or more HR genes increases the risk of many malignancies. Another conserved mechanism involved in the repair of DNA single-strand breaks (SSBs) is base excision repair, in which poly (ADP-ribose) polymerase (PARP) enzymes play an important role. PARP inhibitors (PARPIs) convert SSBs to more cytotoxic double-strand breaks, which are repaired in HR-proficient cells, but remain unrepaired in HRD. The blockade of both HR and base excision repair pathways is the basis of PARPI therapy. The use of PARPIs can be expanded to sporadic cancers displaying the “BRCAness” phenotype. Although PARPIs are effective in many cancers, their efficacy is limited by the development of resistance. In this review, we summarize the prevalence of HRD due to mutation, loss of heterozygosity, and promoter hypermethylation of 35 DNA repair genes in ovarian, breast, colorectal, pancreatic, non-small cell lung cancer, and prostate cancer. The underlying mechanisms and strategies to overcome PARPI resistance are also discussed.

Examples of Inverse Comorbidity between Cancer and Neurodegenerative Diseases: A Possible Role for Noncoding RNA

Cancer is one of the most common causes of death; in parallel, the incidence and prevalence of central nervous system diseases are equally high. Among neurodegenerative diseases, Alzheimer’s dementia is the most common, while Parkinson’s disease (PD) is the second most frequent neurodegenerative disease. There is a significant amount of evidence on the complex biological connection between cancer and neurodegeneration. Noncoding RNAs (ncRNAs) are defined as transcribed nucleotides that perform a variety of regulatory functions. The mechanisms by which ncRNAs exert their functions are numerous and involve every aspect of cellular life. The same ncRNA can act in multiple ways, leading to different outcomes; in fact, a single ncRNA can participate in the pathogenesis of more than one disease—even if these seem very different, as cancer and neurodegenerative disorders are. The ncRNA activates specific pathways leading to one or the other clinical phenotype, sometimes with obvious mechanisms of inverse comorbidity. We aimed to collect from the existing literature examples of inverse comorbidity in which ncRNAs seem to play a key role. We also investigated the example of mir-519a-3p, and one of its target genes Poly (ADP-ribose) polymerase 1, for the inverse comorbidity mechanism between some cancers and PD. We believe it is very important to study the inverse comorbidity relationship between cancer and neurodegenerative diseases because it will help us to better assess these two major areas of human disease.

Evaluation of a CD13 and Integrin αvβ3 Dual-Receptor Targeted Tracer 68Ga-NGR-RGD for Ovarian Tumor Imaging: Comparison With 18F-FDG

Ovarian cancer has the highest mortality rate of gynecologic malignancy. ¹⁸F-FDG positron emission tomography (PET) adds an important superiority over traditional anatomic imaging modalities in oncological imaging but has drawbacks including false negative results at the early stage of ovarian cancer, and false positives when inflammatory comorbidities are present. Aminopeptidase N (APN, also known as CD13) and integrin α_vβ₃ are two important targets overexpressed on tumor neo-vessels and frequently on ovarian cancerous cells. In this study, we used subcutaneous and metastatic models of ovarian cancer and muscular inflammation models to identify ⁶⁸Ga-NGR-RGD, a heterodimeric tracer consisting of NGR and RGD peptides targeting CD13 and integrin α_vβ₃, respectively, and compared it with ¹⁸F-FDG. We found that ⁶⁸Ga-NGR-RGD showed greater contrast in SKOV3 and ES-2 tumors than ¹⁸F-FDG. Low accumulation of ⁶⁸Ga-NGR-RGD but avid uptake of ¹⁸F-FDG were observed in inflammatory muscle. In abdominal metastasis models, PET imaging with ⁶⁸Ga-NGR-RGD allowed for rapid and clear delineation of both peritoneal and liver metastases (3-6 mm), whereas, ¹⁸F-FDG could not distinguish the metastasis lesions due to the relatively low metabolic activity in tumors and the interference of intestinal physiological ¹⁸F-FDG uptake. Due to the high tumor-targeting efficacy, low inflammatory uptake, and higher tumor-to-background ratios compared to that of ¹⁸F-FDG, ⁶⁸Ga-NGR-RGD presents a promising imaging agent for diagnosis, staging, and follow-up of ovarian tumors.

Perspective on the Use of DNA Repair Inhibitors as a Tool for Imaging and Radionuclide Therapy of Glioblastoma

Simple Summary

The current routine treatment for glioblastoma (GB), the most lethal high-grade brain tumor in adults, aims to induce DNA damage in the tumor. However, the tumor cells might be able to repair that damage, which leads to therapy resistance. Fortunately, DNA repair defects are common in GB cells, and their survival is often based on a sole backup repair pathway. Hence, targeted drugs inhibiting essential proteins of the DNA damage response have gained momentum and are being introduced in the clinic. This review gives a perspective on the use of radiopharmaceuticals targeting DDR kinases for imaging in order to determine the DNA repair phenotype of GB, as well as for effective radionuclide therapy. Finally, four new promising radiopharmaceuticals are suggested with the potential to lead to a more personalized GB therapy.

Abstract

Despite numerous innovative treatment strategies, the treatment of glioblastoma (GB) remains challenging. With the current state-of-the-art therapy, most GB patients succumb after about a year. In the evolution of personalized medicine, targeted radionuclide therapy (TRT) is gaining momentum, for example, to stratify patients based on specific biomarkers. One of these biomarkers is deficiencies in DNA damage repair (DDR), which give rise to genomic instability and cancer initiation. However, these deficiencies also provide targets to specifically kill cancer cells following the synthetic lethality principle. This led to the increased interest in targeted drugs that inhibit essential DDR kinases (DDRi), of which multiple are undergoing clinical validation. In this review, the current status of DDRi for the treatment of GB is given for selected targets: ATM/ATR, CHK1/2, DNA-PK, and PARP. Furthermore, this review provides a perspective on the use of radiopharmaceuticals targeting these DDR kinases to (1) evaluate the DNA repair phenotype of GB before treatment decisions are made and (2) induce DNA damage via TRT. Finally, by applying in-house selection criteria and analyzing the structural characteristics of the DDRi, four drugs with the potential to become new therapeutic GB radiopharmaceuticals are suggested.

DNA Repair Enzyme Poly(ADP-Ribose) Polymerase 1/2 (PARP1/2)-Targeted Nuclear Imaging and Radiotherapy

Simple Summary

In parallel to the successful clinical implementation of PARP1/2 inhibitors as anti-cancer drugs, which interfere with the DNA repair machinery, these small molecule agents have also gained attention as vehicles for molecular imaging and radiotherapy. In this review article, we summarize the development and preclinical evaluation of radioactively-labelled PARP inhibitors for positron emission tomography (PET) for many applications, such as selecting patients for PARP inhibitor treatment, response prediction or monitoring, and diagnosis of tumors. We report on early clinical studies that show safety and feasibility of PARP-imaging in humans. In addition, we summarize the latest developments in the field of PARP-targeted radiotherapy, where PARP inhibitors are studied as vehicles to deposit highly cytotoxic radioisotopes in close proximity to the DNA of tumor cells. Lastly, we look at synthetic strategies for PARP-targeted imaging and therapy agents that are compatible with large scale production and clinical translation.

Abstract

Since it was discovered that many tumor types are vulnerable to inhibition of the DNA repair machinery, research towards efficient and selective inhibitors has accelerated. Amongst other enzymes, poly(ADP-ribose)-polymerase 1 (PARP1) was identified as a key player in this process, which resulted in the development of selective PARP inhibitors (PARPi) as anti-cancer drugs. Most small molecule PARPi’s exhibit high affinity for both PARP1 and PARP2. PARPi are under clinical investigation for mono- and combination therapy in several cancer types and five PARPi are now clinically approved. In parallel, radiolabeled PARPi have emerged for non-invasive imaging of PARP1 expression. PARP imaging agents have been suggested as companion diagnostics, patient selection, and treatment monitoring tools to improve the outcome of PARPi therapy, but also as stand-alone diagnostics. We give a comprehensive overview over the preclinical development of PARP imaging agents, which are mostly based on the PARPi olaparib, rucaparib, and recently also talazoparib. We also report on the current status of clinical translation, which involves a growing number of early phase trials. Additionally, this work provides an insight into promising approaches of PARP-targeted radiotherapy based on Auger and α-emitting isotopes. Furthermore, the review covers synthetic strategies for PARP-targeted imaging and therapy agents that are compatible with large scale production and clinical translation.

Supraclavicular and Axillary Lymphadenopathy Induced by COVID-19 Vaccination on 18F-Fluorthanatrace, 68Ga-DOTATATE, and 18F-Fluciclovine PET/CT

ABSTRACT

COVID-19 vaccination has started in most countries, and postvaccination imaging is inevitable in the oncologic population. The immune response to the vaccination in the form of reactive lymphadenopathy has been well documented on 18F-FDG PET/CT. We present the imaging findings of 3 patients who have undergone non-FDG PET/CT imaging including 18F-fluorthanatrace, 68Ga-DOTATATE, and 18F-fluciclovine PET/CT. It is crucial to recognize the timing and laterality of immunization to avoid false-positive findings.

The Development of 18F Fluorthanatrace: A PET Radiotracer for Imaging Poly (ADP-Ribose) Polymerase-1

Fluorine 18 (¹⁸F) fluorthanatrace (¹⁸F-FTT) is a PET radiotracer for imaging poly (adenosine diphosphate-ribose) polymerase-1 (PARP-1), an important target for a class of drugs known as PARP inhibitors, or PARPi. This article describes the stepwise development of this radiotracer from its design and preclinical evaluation to the first-in-human imaging studies and the initial validation of ¹⁸F-FTT as an imaging-based biomarker for measuring PARP-1 expression levels in patients with breast and ovarian cancer. A detailed discussion on the preparation and submission of an exploratory investigational new drug application to the Food and Drug Administration is also provided. Additionally, this review highlights the need and future plans for identifying a commercialization strategy to overcome the major financial barriers that exist when conducting the multicenter clinical trials needed for approval in the new drug application process. The goal of this article is to provide a road map that scientists and clinicians can follow for the successful clinical translation of a PET radiotracer developed in an academic setting. Keywords: Molecular Imaging-Cancer, PET, Breast, Genital/Reproductive, Chemistry, Radiotracer Development, PARPi, ¹⁸F-FTT, Investigational New Drug © RSNA, 2022.

Pilot Study: PARP1 Imaging in Advanced Prostate Cancer

PURPOSE

PARP inhibitor (PARPi) therapy is approved for patients with metastatic castration-resistant prostate cancer (mCRPC) and homologous recombination repair (HRR) genomic aberrations. However, only a fraction of patients with BRCA1/2 mutations respond to PARPi therapy. In this pilot study, we assess PARP-1 expression in prostate cancer patients with and without HRR genomic alternations using a novel PARP-based imaging agent.

PROCEDURES

Nine advanced prostate cancer patients were studied with PET/CT and [¹⁸F]FluorThanatrace (FTT), an analogue of the PARPi rucaparib. Images were analyzed using maximum standardized uptake values (SUV_max). PARP expression was assessed by immunohistochemistry (IHC) when feasible (n = 4).

RESULTS

We found great variability in FTT uptake (SUV_max range: 2.3-15.4). Patients with HRR mutations had a significantly higher SUV_max (p = 0.0379) than patients with non-HRR mutations although there was an overlap in FTT uptake between groups. Three patients without HRR and one with HRR mutations had similarly high PARP1 IHC expression.

CONCLUSIONS

FTT-PET/CT may serve as an alternate biomarker for PARP1 expression and a potential method for PARPi treatment selection.

Application in Gene Editing in Ovarian Cancer Therapy

The onset and progression of ovarian cancer (OC) are closely related to dysregulated gene expression. Current treatments for OC are mainly limited to surgery and chemotherapy. However, due to low drug sensitivity, the prognosis OC is exceptionally poor and the recurrence rate remains high. Hence, it is vital to develop new treatment strategies. Gene editing for site-specific genomic modification is a powerful novel tool for the treatment of OC. In this article, current gene editing research for the treatment of OC is reviewed to provide a reference for the clinical application of new approaches to improve treatment outcomes and prognosis.

PARP inhibitor resistance in ovarian cancer: Underlying mechanisms and therapeutic approaches targeting the ATR/CHK1 pathway

Ovarian cancer (OC) constitutes the most common cause of gynecologic cancer-related death in women worldwide. Despite consistent developments in treatment strategies for OC, the management of advanced-stage disease remains a significant challenge. Recent improvements in targeted treatments based on poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) have provided invaluable benefits to patients with OC. Unfortunately, numerous patients do not respond to PARPi due to intrinsic resistance or acquisition of resistance. Here, we discuss mechanisms of resistance to PARPi that have specifically emerged in OC including increased drug efflux, restoration of HR repair, re-establishment of replication fork stability, reduced PARP1 trapping, abnormalities in PARP signaling, and less common pathways associated with alternative DNA sensing and repair pathways. Elucidation of the precise mechanisms is essential for the development of novel strategies to re-sensitize OC cells to PARPi agents. Additionally, novel potential concepts for preventing and combating resistance to PARPi under development and relevant clinical reports on treatment strategies have been reviewed, with emphasis on the exploitation of the ATR/CHK1 kinase pathway in sensitization to PARPi to overcome resistance-induced vulnerability in ovarian cancer.

Copper-Mediated Radiosynthesis of [18F]Rucaparib

The poly(ADP-ribose) polymerase (PARP) inhibitor rucaparib is used in the clinic to treat BRCA-mutated cancers. Herein, we report two strategies to access the 18F-isotopologue of rucaparib by applying a copper-mediated nucleophilic 18F-fluorodeboronation. The most successful approach features an aldehydic boronic ester precursor that is subjected to reductive amination post-18F-labeling and affords [18F]rucaparib with an activity yield of 11% ± 3% (n = 3) and a molar activity (Am) up to 30 GBq/μmol. Preliminary in vitro studies are presented.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

RESULTS

This commentary of highlights has resulted in 21 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals. Also the first contribution in relation to MRI-agents is included.

CONCLUSIONS

Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Two experts and a newbie: [18F]PARPi vs [18F]FTT vs [18F]FPyPARP-a comparison of PARP imaging agents

Purpose

Imaging of PARP expression has emerged as valuable strategy for prediction of tumor malignancy. While [¹⁸F]PARPi and [¹⁸F]FTT are already in clinical translation, both suffer from mainly hepatobiliary clearance hampering their use for detection of abdominal lesions, e.g., liver metastases. Our novel radiotracer [¹⁸F]FPyPARP aims to bridge this gap with a higher renal clearance and an easily translatable synthesis route for potential clinical application.

Methods

We developed a less lipophilic variant of [¹⁸F]PARPi by exchange of the fluorobenzoyl residue with a fluoronicotinoyl group and automated the radiosyntheses of the three radiotracers. We then conducted a comparative side-by-side study of [¹⁸F]PARPi, [¹⁸F]FPyPARP, and [¹⁸F]FTT in NOD.CB17-Prkdc^scid/J mice bearing HCC1937 xenografts to assess xenograft uptake and pharmacokinetics focusing on excretion pathways.

Results

Together with decent uptake of all three radiotracers in the xenografts (tumor-to-blood ratios 3.41 ± 0.83, 3.99 ± 0.99, and 2.46 ± 0.35, respectively, for [¹⁸F]PARPi, [¹⁸F]FPyPARP, and [¹⁸F]FTT), a partial shift from hepatobiliary to renal clearance of [¹⁸F]FPyPARP was observed, whereas [¹⁸F]PARPi and [¹⁸F]FTT show almost exclusive hepatobiliary clearance.

Conclusion

These findings imply that [¹⁸F]FPyPARP is an alternative to [¹⁸F]PARPi and [¹⁸F]FTT for PET imaging of PARP enzymes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05436-7.

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.

PET Imaging for Gynecologic Malignancies

This review article summarizes the clinical applications of established and emerging PET tracers in the evaluation of the 5 most common gynecologic malignancies: endometrial, ovarian, cervical, vaginal, and vulvar cancers. Emphasis is given to 2-deoxy-2-[¹⁸F]fluoro-d-glucose as the most widely used and studied tracer, with additional clinical tracers also explored. The common imaging protocols are discussed, including standard dose ranges and uptake times, established roles, as well as the challenges and future directions of these imaging techniques. The key points are emphasized with images from selected cases.

Moving beyond PARP Inhibition: Current State and Future Perspectives in Breast Cancer

Breast cancer is the most frequent and lethal tumor in women and finding the best therapeutic strategy for each patient is an important challenge. PARP inhibitors (PARPis) are the first, clinically approved drugs designed to exploit synthetic lethality in tumors harboring BRCA1/2 mutations. Recent evidence indicates that PARPis have the potential to be used both in monotherapy and combination strategies in breast cancer treatment. In this review, we show the mechanism of action of PARPis and discuss the latest clinical applications in different breast cancer treatment settings, including the use as neoadjuvant and adjuvant approaches. Furthermore, as a class, PARPis show many similarities but also certain critical differences which can have essential clinical implications. Finally, we report the current knowledge about the resistance mechanisms to PARPis. A systematic PubMed search, using the entry terms “PARP inhibitors” and “breast cancer”, was performed to identify all published clinical trials (Phase I-II-III) and ongoing trials (ClinicalTrials.gov), that have been reported and discussed in this review.

Development of PARP inhibitor combinations for castration resistant prostate cancer unselected for homologous recombination repair mutations

Genetic instability is a hallmark of cancer and, with the introduction of poly (ADP-ribose) polymerase (PARP) inhibitors, is a targetable feature of many tumors. Currently, two PARP inhibitors, olaparib and rucaparib, have received approval as monotherapy by the Food and Drug Administration for the treatment of men with castration resistant prostate cancer with selected mutations involving the homologous recombination (HR) pathway. However, it is currently debated whether an HR mutation is a prerequisite for response or if patients with HR-proficient mCRPC may also benefit from their use when combined with other targeted or immunotherapeutic agents. Several large phase III trials of PARP inhibitors with novel androgen axis inhibitors in groups of unselected patients are underway. Additionally, there are several early phase trials combining PARP inhibitors with radioligands or immunecheckpoint inhibitors. Here we discuss the currently ongoing or recently concluded trials of PARP inhibitor based combinatorial therapies in unselected patients with mCRPC, the rationale behind these trials, and how these may impact the treatment paradigm in men with mCRPC.

Quantitation of multiple injection dynamic PET scans: an investigation of the benefits of pooling data from separate scans when mapping kinetics

Multiple injection dynamic positron emission tomography (PET) scanning is used in the clinical management of certain groups of patients and in medical research. The analysis of these studies can be approached in two ways: (i) separate analysis of data from individual tracer injections, or (ii), concatenate/pool data from separate injections and carry out a combined analysis. The simplicity of separate analysis has some practical appeal but may not be statistically efficient. We use a linear model framework associated with a kinetic mapping scheme to develop a simplified theoretical understanding of separate and combined analysis. The theoretical framework is explored numerically using both 1D and 2D simulation models. These studies are motivated by the breast cancer flow-metabolism mismatch studies involving15O-water (H2O) and18F-Fluorodeoxyglucose (FDG) and repeat15O-H2O injections used in brain activation investigations. Numerical results are found to be substantially in line with the simple theoretical analysis: mean square error characteristics of alternative methods are well described by factors involving the local voxel-level resolution of the imaging data, the relative activities of the individual scans and the number of separate injections involved. While voxel-level resolution has dependence on scan dose, after adjustment for this effect, the impact of a combined analysis is understood in simple terms associated with the linear model used for kinetic mapping. This is true for both data reconstructed by direct filtered backprojection or iterative maximum likelihood. The proposed analysis has potential to be applied to the emerging long axial field-of-view PET scanners.

Molecular Imaging: PARP-1 and Beyond

The genetic code to life is balanced on a string of DNA that is under constant metabolic and physical stress from environmental forces. Nearly all diseases have a genetic component caused by or resulting in DNA damage that alters biology to drive pathogenesis. Recent advancements in DNA repair biology have led to the development of imaging tools that target DNA damage response and repair proteins. PET has been used for early detection of oncogenic processes and monitoring of tumor response to chemotherapeutics that target the DNA repair machinery. In the field of precision medicine, imaging tools provide a unique opportunity for patient stratification by directly measuring drug target expression or monitoring therapy to identify early responders. This overview discusses the state of the art on molecular imaging of DNA damage and repair from the past 5 years, with an emphasis on poly[adenosine diphosphate ribose]polymerase-1 as an imaging target and predictive biomarker of response to therapy.

Radiosynthesis and Evaluation of Talazoparib and Its Derivatives as PARP-1-Targeting Agents

Poly (ADP-ribose) polymerase-1 (PARP-1) is a critical enzyme in the DNA repair process and the target of several FDA-approved inhibitors. Several of these inhibitors have been radiolabeled for non-invasive imaging of PARP-1 expression or targeted radiotherapy of PARP-1 expressing tumors. In particular, derivatives of olaparib and rucaparib, which have reduced trapping potency by PARP-1 compared to talazoparib, have been radiolabeled for these purposes. Here, we report the first radiosynthesis of [¹⁸F]talazoparib and its in vitro and in vivo evaluation. Talazoparib (3a″) and its bromo- or iodo-derivatives were synthesized as racemic mixtures (3a, 3b and 3c), and these compounds exhibit high affinity to PARP-1 (K_i for talazoparib (3a″): 0.65 ± 0.07 nM; 3a: 2.37 ± 0.56 nM; 3b: 1.92 ± 0.41 nM; 3c: 1.73 ± 0.43 nM; known PARP-1 inhibitor Olaparib: 1.87 ± 0.10 nM; non-PARP-1 compound Raclopride: >20,000 nM) in a competitive binding assay using a tritium-labeled PARP-1 radioligand [³H]WC-DZ for screening. [¹⁸F]Talazoparib (3a″) was radiosynthesized via a multiple-step procedure with good radiochemical and chiral purities (98%) and high molar activity (28 GBq/μmol). The preliminary biodistribution studies in the murine PC-3 tumor model showed that [¹⁸F]talazoparib had a good level of tumor uptake that persisted for over 8 h (3.78 ± 0.55 %ID/gram at 4 h and 4.52 ± 0.32 %ID/gram at 8 h). These studies show the potential for the bromo- and iodo- derivatives for PARP-1 targeted radiotherapy studies using therapeutic radionuclides.

PARP1 Is Targeted by miR-519a-3p and Promotes the Migration, Invasion, and Tube Formation of Ovarian Cancer Cells

Background: Poly ADP-ribose polymerase 1 (PARP1) has been discovered to be implicated in ovarian cancer (OC), but its interaction with microRNA (miR)-519a-3p remained poorly understood. This study aimed to uncover their roles and interactions in OC. Materials and Methods: Clinical tissue from OC patients and adjacent normal tissue were collected, and the survival rates of OC patients with high or low PARP1 expression were analyzed by Kaplan-Meier curve. After transfection, OC cell viability, migration, and tube formation were detected with cell counting kit-8 (CCK-8) assay, scratch assay, and tube formation assay, respectively. Target gene of miR-519a-3p and potential binding sites between them were predicted with TargetScan and confirmed using dual-luciferase reporter assay. Relative expressions of miR-519a-3p, PARP1, E-cadherin, N-cadherin, SNAIL, vascular endothelial growth factor (VEGF), and p53 were measured by quantitative real-time polymerase chain reaction and Western blot as needed. Results: PARP1 expression was upregulated in OC, which was related to poor prognosis of OC patients. Silencing PARP1 decreased PARP1 expression and suppressed viability, migration, invasion, and tube formation in OC cells, while overexpressed PARP1 did the opposite. PARP1 was the target gene of miR-519a-3p, and it reversed the effects of miR-519a-3p on the migration, invasion, and tube formation of OC cells by upregulating the expressions of PAR, PARP1, N-cadherin, SNAIL, and VEGF and downregulating those of E-cadherin and p53. Conclusion: PARP1, a target gene of miR-519a-3p, promoted the migration, invasion, and tube formation of OC cells, providing a possible therapeutic target for treatment of OC patients.

The Ubiquitin Ligase TRIP12 Limits PARP1 Trapping and Constrains PARP Inhibitor Efficiency

PARP inhibitors (PARPi) cause synthetic lethality in BRCA-deficient tumors. Whether specific vulnerabilities to PARPi exist beyond BRCA mutations and related defects in homology-directed repair (HDR) is not well understood. Here, we identify the ubiquitin E3 ligase TRIP12 as negative regulator of PARPi sensitivity. We show that TRIP12 controls steady-state PARP1 levels and limits PARPi-induced cytotoxic PARP1 trapping. Upon loss of TRIP12, elevated PARPi-induced PARP1 trapping causes increased DNA replication stress, DNA damage, cell cycle arrest, and cell death. Mechanistically, we demonstrate that TRIP12 binds PARP1 via a central PAR-binding WWE domain and, using its carboxy-terminal HECT domain, catalyzes polyubiquitylation of PARP1, triggering proteasomal degradation and preventing supra-physiological PARP1 accumulation. Further, in cohorts of breast and ovarian cancer patients, PARP1 abundance is negatively correlated with TRIP12 expression. We thus propose TRIP12 as regulator of PARP1 stability and PARPi-induced PARP trapping, with potential implications for PARPi sensitivity and resistance.

In vivo visualization of PARP inhibitor pharmacodynamics

BACKGROUND

[¹⁸F]FluorThanatrace ([¹⁸F]FTT) is a radiolabeled poly (adenosine diphosphate-ribose) polymerase inhibitor (PARPi) that enables noninvasive quantification of PARP with potential to serve as a biomarker for patient selection for PARPi therapy. Here we report for the first time to our knowledge noninvasive in vivo visualization of drug-target engagement during PARPi treatment.

METHODS

Two single-arm, prospective, nonrandomized clinical trials were conducted at the University of Pennsylvania from May 2017 to March 2020. PARP expression in breast cancer was assessed in vivo via [¹⁸F]FTT PET before and after initiation of PARPi treatment and in vitro via [¹²⁵I]KX1 (an analog of [¹⁸F]FTT) binding to surgically removed breast cancer.

RESULTS

Thirteen patients had baseline [¹⁸F]FTT PET. Nine of these then had resection and in vitro evaluation of [¹⁸F]FTT uptake with an analog and uptake was blocked with PARPi. Of the other 4 patients, 3 had [¹⁸F]FTT PET uptake, and all had uptake blocked with treatment with a therapeutic PARPi. Initial in vivo [¹⁸F]FTT tumor uptake ranged from undetectable to robust. Following initiation of PARPi therapy, [¹⁸F]FTT uptake was not detectable above background in all cases. In vitro tumor treatment with a PARPi resulted in 82% reduction in [¹²⁵I]KX1 binding.

CONCLUSION

[¹⁸F]FTT noninvasively quantifies PARP-1 expression. Early results indicate ability to visualize PARPi drug-target engagement in vivo and suggest the utility of further study to test [¹⁸F]FTT PET as a predictive and pharmacodynamic biomarker.

TRIAL REGISTRATION

ClinicalTrials.gov identifiers NCT03083288 and NCT03846167.

FUNDING

Metavivor Translational Research Award, Susan G. Komen for the Cure (CCR 16376362), Department of Defense BC190315, and Abramson Cancer Center Breakthrough Bike Challenge.

Human PARP inhibitor drug-target engagement can be visualized, with future possibilities including a precision diagnostic and drug discovery tool.

Kinetic and Static Analysis of Poly-(Adenosine Diphosphate-Ribose) Polymerase-1-Targeted 18F-Fluorthanatrace PET Images of Ovarian Cancer

The poly-(adenosine diphosphate-ribose) polymerase (PARP) family of proteins participates in numerous functions, most notably the DNA damage response. Cancer vulnerability to DNA damage has led to development of several PARP inhibitors (PARPi). This class of drugs has demonstrated therapeutic efficacy in ovarian, breast, and prostate cancers, but with variable response. Consequently, clinics need to select patients likely to benefit from these targeted therapies. In vivo imaging of ¹⁸F-fluorthanatrace uptake has been shown to correspond to PARP-1 expression in tissue. This study characterized the pharmacokinetics of ¹⁸F-fluorthanatrace and tested kinetic and static models to guide metric selection in future studies assessing ¹⁸F-fluorthanatrace as a biomarker of response to PARPi therapy. Methods: Fourteen prospectively enrolled ovarian cancer patients were injected with ¹⁸F-fluorthanatrace and imaged dynamically for 60 min after injection followed by up to 2 whole-body scans, with venous blood activity and metabolite measurements. SUV_max and SUV_peak were extracted from dynamic images and whole-body scans. Kinetic parameter estimates and SUVs were assessed for correlations with tissue PARP-1 immunofluorescence (n = 7). Simulations of population kinetic parameters enabled estimation of measurement bias and precision in parameter estimates. Results: ¹⁸F-fluorthanatrace blood clearance was variable, but labeled metabolite profiles were similar across patients, supporting use of a population parent fraction curve. The total distribution volume from a reversible 2-tissue-compartment model and Logan reference tissue distribution volume ratio (DVR) from the first hour of PET acquisition correlated with tumor PARP-1 expression by immunofluorescence (r = 0.76 and 0.83, respectively; P < 0.05). DVR bias and precision estimates were 6.4% and 29.1%, respectively. SUV_max and SUV_peak acquired from images with midpoints of 57.5, 110 ± 3, and 199 ± 4 min highly correlated with PARP-1 expression (mean ± SD, r ≥ 0.79; P < 0.05). Conclusion: Tumor SUV_max and SUV_peak at 55-60 min after injection and later and DVR from at least 60 min appear to be robust noninvasive measures of PARP-1 binding. ¹⁸F-fluorthanatrace uptake in ovarian cancer was best described by models of reversible binding. However, pharmacokinetic patterns of tracer uptake were somewhat variable, especially at later time points.