Non-invasive PET Imaging of PARP1 Expression in Glioblastoma Models

Design, Synthesis, and Evaluation of [18F]BIBD-300 as a Positron Emission Tomography Tracer for Poly(ADP-Ribose) Polymerase-1

Compounds 8a-j were designed to adjust the mode of interaction and lipophilicity of FTT by scaffold hopping and changing the length of the alkoxy groups. Compounds 8a, 8d, 8g, and BIBD-300 were screened for high-affinity PARP-1 through enzyme inhibition assays and are worthy of further evaluation. PET imaging of MCF-7 subcutaneous tumors with moderate expression of PARP-1 showed that compared to [18F]FTT, [18F]8a, [18F]8d, and [18F]8g exhibited greater nonspecific uptake, a lower target-to-nontarget ratio, and severe defluorination, while [18F]BIBD-300 exhibited lower nonspecific uptake and a greater target-to-nontarget ratio. PET imaging of 22Rv1 subcutaneous tumors, which highly express PARP-1, confirmed that the uptake of [18F]BIBD-300 in normal organs, such as the liver, muscle, and bone, was lower than that of [18F]FTT, and the ratio of tumor-to-muscle and tumor-to-liver [18F]BIBD-300 was greater than that of [18F]FTT. The biodistribution results in mice with MCF-7 and 22Rv1 subcutaneous tumors further validated the results of PET imaging. Unlike [18F]FTT, which mainly relies on hepatobiliary clearance, [18F]BIBD-300, which has lower lipophilicity, undergoes a partial shift from hepatobiliary to renal clearance, providing the possibility for [18F]BIBD-300 to indicate liver cancer. The difference in the PET imaging results for [18F]FTT, [18F]BIBD-300, and [18F]8j in 22Rv1 mice and the corresponding molecular docking results further confirmed that subtle structural modifications in lipophilicity greatly optimize the properties of the tracer. Cell uptake experiments also demonstrated that [18F]BIBD-300 has a high affinity for PARP-1. Metabolized and unmetabolized [18F]FTT and [18F]BIBD-300 were detected in the brain, indicating that they could not accurately quantify the amount of PARP-1 in the brain. However, PET imaging of glioma showed that both [18F]FTT and [18F]BIBD-300 could accurately localize both in situ to C6 and U87MG tumors. Based on its potential advantages in the diagnosis of breast cancer, prostate cancer, and glioma, as well as liver cancer, [18F]BIBD-300 is a new option for an excellent PARP-1 tracer.

Late-stage gem-difluoroallylation of phenol in bioactive molecules and peptides with 3,3-difluoroallyl sulfonium salts

An efficient method for the late-stage selective O-fluoroalkylation of tyrosine residues with a stable yet highly reactive fluoroalkylating reagent, 3,3-difluoroallyl sulfonium salts (DFASs), has been developed. The reaction proceeds in a mild basic aqueous buffer (pH = 11.6) with high efficiency, high biocompatibility, and excellent regio- and chemoselectivity. Various oligopeptides and phenol-containing bioactive molecules, including carbohydrates and nucleosides, could be selectively O-fluoroalkylated. The added vinyl and other functional groups from DFASs can be valuable linkers for successive modification, significantly expanding the chemical space for further bioconjugation. The synthetic utility of this protocol has been demonstrated by the fluorescently labeled anti-cancer drug and the synthesis of O-link type 1,4,7,10-tetraazacyclododecane-N,N',N,N'-tetraacetic acid-tyrosine3-octreotate (DOTA-TATE), showing the prospect of the method in medicinal chemistry and chemical biology.

[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.

Nuclear Medicine and Cancer Theragnostics: Basic Concepts

Cancer theragnostics is a novel approach that combines diagnostic imaging and radionuclide therapy. It is based on the use of a pair of radiopharmaceuticals, one optimized for positron emission tomography imaging through linkage to a proper radionuclide, and the other bearing an alpha- or beta-emitter isotope that can induce significant damage to cancer cells. In recent years, the use of theragnostics in nuclear medicine clinical practice has increased considerably, and thus investigation has focused on the identification of novel radionuclides that can bind to molecular targets that are typically dysregulated in different cancers. The major advantages of the theragnostic approach include the elimination of multi-step procedures, reduced adverse effects to normal tissues, early diagnosis, better predictive responses, and personalized patient care. This review aims to discuss emerging theragnostic molecules that have been investigated in a series of human malignancies, including gliomas, thyroid cancer, neuroendocrine tumors, cholangiocarcinoma, and prostate cancer, as well as potent and recently introduced molecular targets, like cell-surface receptors, kinases, and cell adhesion proteins. Furthermore, special reference has been made to copper radionuclides as theragnostic agents and their radiopharmaceutical applications since they present promising alternatives to the well-studied gallium-68 and lutetium-177.

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.

PET imaging of PARP expression using 68Ga-labelled inhibitors

PURPOSE

Imaging the PARP expression using 18F probes has been approved in clinical trials. Nevertheless, hepatobiliary clearance of both 18F probes hindered their application in monitoring abdominal lesions. Our novel 68Ga-labelled probes aim for fewer abdominal signals while ensuring PARP targeting by optimizing the pharmacokinetic properties of radioactive probes.

METHODS

Three radioactive probes targeted PARP were designed, synthesized, and evaluated based on the PARP inhibitor Olaparib. These 68Ga-labelled radiotracers were assessed in vitro and in vivo.

RESULTS

Precursors that did not lose binding affinity for PARP were designed, synthesized, and then labelled with 68Ga in high radiochemical purity (> 97%). The 68Ga-labelled radiotracers were stable. Due to the increased expression of PARP-1 in SK-OV-3 cells, the uptake of the three radiotracers by SK-OV-3 cells was significantly greater than that by A549 cells. PET/CT imaging of the SK-OV-3 models indicated that the tumor uptake of 68Ga-DOTA-Olaparib (0.5 h: 2.83 ± 0.55%ID/g; 1 h: 2.37 ± 0.64%ID/g) was significantly higher than that of the other 68Ga-labelled radiotracers. There was a significant difference in the T/M (tumor-to-muscle) ratios between the unblocked and blocked groups as calculated from the PET/CT images (4.07 ± 1.01 vs. 1.79 ± 0.45, P = 0.0238 < 0.05). Tumor autoradiography revealed high accumulation in tumor tissues, further confirming the above data. PARP-1 expression in the tumor was confirmed by immunochemistry.

CONCLUSION

As the first 68Ga-labelled PARP inhibitor, 68Ga-DOTA-Olaparib displayed high stability and quick PARP imaging in a tumor model. This compound is thus a promising imaging agent that can be used in a personalized PARP inhibitor treatment regimen.

Preclinical evaluation of a brain penetrant PARP PET imaging probe in rat glioblastoma and nonhuman primates

PURPOSE

Currently, there are multiple active clinical trials involving poly(ADP-ribose) polymerase (PARP) inhibitors in the treatment of glioblastoma. The noninvasive quantification of baseline PARP expression using positron emission tomography (PET) may provide prognostic information and lead to more precise treatment. Due to the lack of brain-penetrant PARP imaging agents, the reliable and accurate in vivo quantification of PARP in the brain remains elusive. Herein, we report the synthesis of a brain-penetrant PARP PET tracer, (R)-2-(2-methyl-1-(methyl-11C)pyrrolidin-2-yl)-1H-benzo[d]imidazole-4-carboxamide ([11C]PyBic), and its preclinical evaluations in a syngeneic RG2 rat glioblastoma model and healthy nonhuman primates.

METHODS

We synthesized [11C]PyBic using veliparib as the labeling precursor, performed dynamic PET scans on RG2 tumor-bearing rats and calculated the distribution volume ratio (DVR) using simplified reference region method 2 (SRTM2) with the contralateral nontumor brain region as the reference region. We performed biodistribution studies, western blot, and immunostaining studies to validate the in vivo PET quantification results. We characterized the brain kinetics and binding specificity of [11C]PyBic in nonhuman primates on FOCUS220 scanner and calculated the volume of distribution (VT), nondisplaceable volume of distribution (VND), and nondisplaceable binding potential (BPND) in selected brain regions.

RESULTS

[11C]PyBic was synthesized efficiently in one step, with greater than 97% radiochemical and chemical purity and molar activity of 148 ± 85 MBq/nmol (n = 6). [11C]PyBic demonstrated PARP-specific binding in RG2 tumors, with 74% of tracer binding in tumors blocked by preinjected veliparib (i.v., 5 mg/kg). The in vivo PET imaging results were corroborated by ex vivo biodistribution, PARP1 immunohistochemistry and immunoblotting data. Furthermore, brain penetration of [11C]PyBic was confirmed by quantitative monkey brain PET, which showed high specific uptake (BPND > 3) and low nonspecific uptake (VND < 3 mL/cm3) in the monkey brain.

CONCLUSION

[11C]PyBic is the first brain-penetrant PARP PET tracer validated in a rat glioblastoma model and healthy nonhuman primates. The brain kinetics of [11C]PyBic are suitable for noninvasive quantification of available PARP binding in the brain, which posits [11C]PyBic to have broad applications in oncology and neuroimaging.

Synthesis and Preclinical Evaluation of a Novel Fluorine-18-Labeled Tracer for Positron Emission Tomography Imaging of Bruton's Tyrosine Kinase

Bruton's tyrosine kinase (BTK) is a target for treating B-cell malignancies and autoimmune diseases. To aid in the discovery and development of BTK inhibitors and improve clinical diagnoses, we have developed a positron emission tomography (PET) radiotracer based on a selective BTK inhibitor, remibrutinib. [18F]PTBTK3 is an aromatic, 18F-labeled tracer that was synthesized in 3 steps with a 14.8 ± 2.4% decay-corrected radiochemical yield and ≥99% radiochemical purity. The cellular uptake of [18F]PTBTK3 was blocked up to 97% in JeKo-1 cells using remibrutinib or non-radioactive PTBTK3. [18F]PTBTK3 exhibited renal and hepatobiliary clearance in NOD SCID (non-obese diabetic/severe combined immunodeficiency) mice, and the tumor uptake of [18F]PTBTK3 in BTK-positive JeKo-1 xenografts (1.23 ± 0.30% ID/cc) was significantly greater at 60 min post injection compared to the tumor uptake in BTK-negative U87MG xenografts (0.41 ± 0.11% ID/cc). In the JeKo-1 xenografts, tumor uptake was blocked up to 62% by remibrutinib, indicating the BTK-dependent uptake of [18F]PTBTK3 in tumors.

Preclinical Efficacy of a PARP-1 Targeted Auger-Emitting Radionuclide in Prostate Cancer

There is an unmet need for better therapeutic strategies for advanced prostate cancer. Poly (ADP-ribose) polymerase-1 (PARP-1) is a chromatin-binding DNA repair enzyme overexpressed in prostate cancer. This study evaluates whether PARP-1, on account of its proximity to the cell's DNA, would be a good target for delivering high-linear energy transfer Auger radiation to induce lethal DNA damage in prostate cancer cells. We analyzed the correlation between PARP-1 expression and Gleason score in a prostate cancer tissue microarray. A radio-brominated Auger emitting inhibitor ([77Br]Br-WC-DZ) targeting PARP-1 was synthesized. The ability of [77Br]Br-WC-DZ to induce cytotoxicity and DNA damage was assessed in vitro. The antitumor efficacy of [77Br]Br-WC-DZ was investigated in prostate cancer xenograft models. PARP-1 expression was found to be positively correlated with the Gleason score, thus making it an attractive target for Auger therapy in advanced diseases. The Auger emitter, [77Br]Br-WC-DZ, induced DNA damage, G2-M cell cycle phase arrest, and cytotoxicity in PC-3 and IGR-CaP1 prostate cancer cells. A single dose of [77Br]Br-WC-DZ inhibited the growth of prostate cancer xenografts and improved the survival of tumor-bearing mice. Our studies establish the fact that PARP-1 targeting Auger emitters could have therapeutic implications in advanced prostate cancer and provides a strong rationale for future clinical investigation.

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.

Solid phase radiosynthesis of an olaparib derivative using 4-[18F] fluorobenzoic acid and in vivo evaluation in breast and prostate cancer xenograft models for PARP-1 expression

INTRODUCTION

Solid-phase synthesis and conjugation reactions of acids and amines using coupling reagents are common in organic synthesis, but rare in 18F radiochemistry. 4-[18F]Fluorobenzoic acid (FBA) is a useful building block, but is seldom used directly with coupling reagents for the preparation of 18F radiopharmaceuticals. To overcome the inconveniences associated with using [18F]FBA in conjugation reactions, we have developed a non-covalent solid-phase synthesis (SPS) strategy for the radiosynthesis of [18F]PARPi, a derivative of olaparib as a Poly (ADP-ribose) polymerase-1 (PARP-1) radioligand.

METHODS

Fluoro-, bromo- and iodo-benzoic derivatives of olaparib were synthesized, and their PARP-1 affinities were measured using a recently developed cell culture-based competitive assay. To produce [18F]PARPi, [18F]FBA was radiosynthesized and purified using a cation-exchange cartridge, and then trapped by an anion-exchange resin cartridge, on which the solid-phase radiosynthesis was carried out to produce the desired product. [18F]PARPi was evaluated in vivo in breast and prostate xenograft tumor models by microPET imaging, biodistribution and autoradiography.

RESULTS

The best derivatives of olaparib were identified as compound 4, 7 and 8. [18F]4 ([18F]PARPi) was radiosynthesized in high radiochemical yield, high molar activity and high radiochemical purity using this SPS strategy. The in vivo evaluation of [18F]PARPi demonstrates the PARP-1 specific uptake of [18F]PARPi in the animal models.

CONCLUSIONS

This method is simple and efficient, having great potential for the synthesis of radiopharmaceuticals starting from [18F]FBA or other radiolabeled aromatic acids. Using [18F]PARPi prepared by this method, we demonstrated the promise of [18F]PARPi in the nuclear imaging of PARP-1 expression.

Imaging-guided targeted radionuclide tumor therapy: From concept to clinical translation

Since the first introduction of sodium iodide I-131 for use with thyroid patients almost 80 years ago, more than 50 radiopharmaceuticals have reached the markets for a wide range of diseases, especially cancers. The nuclear medicine paradigm also shifts from solely molecular imaging or radionuclide therapy to imaging-guided radionuclide therapy, which is deemed a vital component of precision cancer therapy and an emerging medical modality for personalized medicine. The imaging-guided radionuclide therapy highlights the systematic integration of targeted nuclear diagnostics and radionuclide therapeutics. Regarding this, nuclear imaging serves to "visualize" the lesions and guide the therapeutic strategy, followed by administration of a precise patient specific dose of radiotherapeutics for treatment according to the absorbed dose to different organs and tumors calculated by dosimetry tools, and finally repeated imaging to predict the prognosis. This strategy leads to significantly enhanced therapeutic efficacy, improved patient outcomes, and manageable adverse events. In this review, we provide an overview of imaging-guided targeted radionuclide therapy for different tumors such as advanced prostate cancer and neuroendocrine tumors, with a focus on development of new radioligands and their preclinical and clinical results, and further discuss about challenges and future perspectives.

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.

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.

Fully Automated, High-Dose Radiosynthesis of [18F]PARPi

[¹⁸F]PARPi is currently undergoing clinical trials as a PET tracer for many applications. However, only manual radiosynthesis was reported; this has several drawbacks, including an increased risk of contamination from the operator, and the need to limit the starting activity. The automation of the previously reported protocol for [¹⁸F]PARPi synthesis is challenging, as it requires transferring microvolumes of reagents, which many platforms cannot accommodate. We report a revised, high yield, and automated protocol for the radiosynthesis of [¹⁸F]PARPi, with final doses of over 20 GBq.

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.

Interest and Limits of [18F]ML-10 PET Imaging for Early Detection of Response to Conventional Chemotherapy

One of the current challenges in oncology is to develop imaging tools to early detect the response to conventional chemotherapy and adjust treatment strategies when necessary. Several studies evaluating PET imaging with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) as a predictive tool of therapeutic response highlighted its insufficient specificity and sensitivity. The [¹⁸F]FDG uptake reflects only tumor metabolic activity and not treatment-induced cell death, which seems to be relevant for therapeutic evaluation. Therefore, to evaluate this parameter in vivo, several cell death radiotracers have been developed in the last years. However, few of them have reached the clinical trials. This systematic review focuses on the use of [¹⁸F]ML-10 (2-(5-[¹⁸F]fluoropentyl)-2-methylmalonic acid) as radiotracer of apoptosis and especially as a measure of tumor response to treatment. A comprehensive literature review concerning the preclinical and clinical investigations conducted with [¹⁸F]ML-10 was performed. The abilities and applications of this radiotracer as well as its clinical relevance and limitations were discussed. Most studies highlighted a good ability of the radiotracer to target apoptotic cells. However, the increase in apoptosis during treatment did not correlate with the radiotracer tumoral uptake, even using more advanced image analysis (voxel-based analysis). [¹⁸F]ML-10 PET imaging does not meet current clinical expectations for early detection of the therapeutic response to conventional chemotherapy. This review has pointed out the challenges of applying various apoptosis imaging strategies in clinical trials, the current methodologies available for image analysis and the future of molecular imaging to assess this therapeutic response.

DoE Optimization Empowers the Automated Preparation of Enantiomerically Pure [18F]Talazoparib and its In Vivo Evaluation as a PARP Radiotracer

Given the clinical potential of poly(ADP-ribose) polymerases (PARP) imaging for the detection and stratification of various cancers, the development of novel PARP imaging probes with improved pharmacological profiles over established PARP imaging agents is warranted. Here, we present a novel 18F-labeled PARP radiotracer based on the clinically superior PARP inhibitor talazoparib. An automated radiosynthesis of [18F]talazoparib (RCY: 13 ± 3.4%; n = 4) was achieved using a "design of experiments" (DoE) optimized copper-mediated radiofluorination reaction. The chiral product was isolated from the reaction mixture using 2D reversed-phase/chiral radio-HPLC (>99% ee). (8S,9R)-[18F]Talazoparib demonstrated PARP binding in HCC1937 cells in vitro and showed an excellent tumor-to-blood ratio in xenograft-bearing mice (10.2 ± 1.5). Additionally, a favorable pharmacological profile in terms of excretion, metabolism, and target engagement was observed. This synthesis of [18F]talazoparib exemplifies how DoE can enable the radiosyntheses of synthetically challenging radiolabeled compounds of high interest to the imaging community.

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.

Scalable 18F processing conditions for copper-mediated radiofluorination chemistry facilitate DoE optimization studies and afford an improved synthesis of [18F]olaparib

A convenient and scalable base-free method for processing [¹⁸F]fluoride as [¹⁸F]TBAF is reported and applied to copper-mediated radiofluorination radiosyntheses. A central feature of this method is that a single production of [¹⁸F]TBAF can be divided into small aliquots that can be used to perform multiple small-scale reactions in DoE optimization studies. The results of these studies can then be reliably translated to full batch tracer productions using automated synthesizers. The processing method was applied to the DoE optimization of [¹⁸F]olaparib, affording the tracer in high radiochemical yields via both manual (%RCY = 78 ± 6%, n = 4 (CMRF step only)) and automated (up to 80% (%RCY); 41% activity yield (%AY)) radiosynthesis procedures.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

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.

Sulfur [18F]Fluoride Exchange Click Chemistry Enabled Ultrafast Late-Stage Radiosynthesis

The lack of efficient [¹⁸F]fluorination processes and target-specific organofluorine chemotypes remains the major challenge of fluorine-18 positron emission tomography (PET). We report here an ultrafast isotopic exchange method for the radiosynthesis of novel PET agent aryl [¹⁸F]fluorosulfate enabled by the emerging sulfur fluoride exchange (SuFEx) click chemistry. The method has been applied to the fully automated ¹⁸F-radiolabeling of 25 structurally and functionally diverse aryl fluorosulfates with excellent radiochemical yield (83-100%, median 98%) and high molar activity (280 GBq μmol^-1) at room temperature in 30 s. The purification of radiotracers requires no time-consuming HPLC but rather a simple cartridge filtration. We further demonstrate the imaging application of a rationally designed poly(ADP-ribose) polymerase 1 (PARP1)-targeting aryl [¹⁸F]fluorosulfate by probing subcutaneous tumors in vivo.

[18F]AZD2461, an Insight on Difference in PARP Binding Profiles for DNA Damage Response PET Imaging

BACKGROUND

Poly (ADP-ribose) polymerase (PARP) inhibitors are extensively studied and used as anti-cancer drugs, as single agents or in combination with other therapies. Most radiotracers developed to date have been chosen on the basis of strong PARP1-3 affinity. Herein, we propose to study AZD2461, a PARP inhibitor with lower affinity towards PARP3, and to investigate its potential for PARP targeting in vivo.

METHODS

Using the Cu-mediated 18F-fluorodeboronation of a carefully designed radiolabelling precursor, we accessed the 18F-labelled isotopologue of the PARP inhibitor AZD2461. Cell uptake of [18F]AZD2461 in vitro was assessed in a range of pancreatic cell lines (PSN-1, PANC-1, CFPAC-1 and AsPC-1) to assess PARP expression and in vivo in xenograft-bearing mice. Blocking experiments were performed with both olaparib and AZD2461.

RESULTS

[18F]AZD2461 was efficiently radiolabelled via both manual and automated procedures (9 % ± 3 % and 3 % ± 1 % activity yields non-decay corrected). [18F]AZD2461 was taken up in vivo in PARP1-expressing tumours, and the highest uptake was observed for PSN-1 cells (7.34 ± 1.16 %ID/g). In vitro blocking experiments showed a lesser ability of olaparib to reduce [18F]AZD2461 binding, indicating a difference in selectivity between olaparib and AZD2461.

CONCLUSION

Taken together, we show the importance of screening the PARP selectivity profile of radiolabelled PARP inhibitors for use as PET imaging agents.

Preclinical and first-in-human-brain-cancer applications of [18F]poly (ADP-ribose) polymerase inhibitor PET/MR

Background

We report preclinical and first-in-human-brain-cancer data using a targeted poly (ADP-ribose) polymerase 1 (PARP1) binding PET tracer, [¹⁸F]PARPi, as a diagnostic tool to differentiate between brain cancers and treatment-related changes.

Methods

We applied a glioma model in p53-deficient nestin/tv-a mice, which were injected with [¹⁸F]PARPi and then sacrificed 1 h post-injection for brain examination. We also prospectively enrolled patients with brain cancers to undergo dynamic [¹⁸F]PARPi acquisition on a dedicated positron emission tomography/magnetic resonance (PET/MR) scanner. Lesion diagnosis was established by pathology when available or by Response Assessment in Neuro-Oncology (RANO) or RANO-BM response criteria. Resected tissue also underwent PARPi-FL staining and PARP1 immunohistochemistry.

Results

In a preclinical mouse model, we illustrated that [¹⁸F]PARPi crossed the blood–brain barrier and specifically bound to PARP1 overexpressed in cancer cell nuclei. In humans, we demonstrated high [¹⁸F]PARPi uptake on PET/MR in active brain cancers and low uptake in treatment-related changes independent of blood–brain barrier disruption. Immunohistochemistry results confirmed higher PARP1 expression in cancerous than in noncancerous tissue. Specificity was also corroborated by blocking fluorescent tracer uptake with an excess unlabeled PARP inhibitor in patient cancer biospecimen.

Conclusions

Although larger studies are necessary to confirm and further explore this tracer, we describe the promising performance of [¹⁸F]PARPi as a diagnostic tool to evaluate patients with brain cancers and possible treatment-related changes.

Advancements in PARP1 Targeted Nuclear Imaging and Theranostic Probes

The central paradigm of novel therapeutic approaches in cancer therapy is identifying and targeting molecular biomarkers. One such target is the nuclear DNA repair enzyme Poly-(ADP ribose) polymerase 1 (PARP1). Sensitivity to PARP inhibition in certain cancers such as gBRCA^mut breast and ovarian cancers has led to its exploitation as a target. The overexpression of PARP1 in several types of cancer further evoked interest in its use as an imaging target. While PARP1-targeted inhibitors have fast developed and approved in this past decade, determination of PARP1 expression might help to predict the response to PARP inhibitor treatment. This has the potential of improving prognosis and moving towards tailored therapy options and/or dosages. This review summarizes the recent pre-clinical advancements in imaging and theranostic PARP1 targeted tracers. To assess PARP1 levels, several imaging probes with fluorescent or beta/gamma emitting radionuclides have been proposed and three have advanced to ongoing clinical evaluation. Apart from its diagnostic value in detection of primary tumors as well as metastases, this shall also help in delivering therapeutic radionuclides to PARP1 overexpressing tumors. Henceforth nuclear medicine has now advanced towards conjugating theranostic radionuclides to PARP1 inhibitors. This paves the way for a future of PARP1-targeted theranostics and personalized therapy.

A one-pot radiosynthesis of [18 F]PARPi

In this paper, we disclose a new strategy for the radiosynthesis of [¹⁸ F]PARPi from the corresponding, boc-protected, nitro-precursor. Using a two-step procedure, [¹⁸ F]PARPi could be isolated in radiochemical yields up to 9.6%. The reaction proceeds via an efficient one-pot, two-step process, allowing for simplification over previous methods that require complex multi-step, multi-pot strategies to be implemented.

Fluorine-18 labeled poly (ADP-ribose) polymerase1 inhibitor as a potential alternative to 2-deoxy-2-[18F]fluoro-d-glucose positron emission tomography in oral cancer imaging

OBJECTIVES

The evaluation of disease extent and post-therapy surveillance of head and neck cancer using 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) PET is often complicated by physiological uptake in normal tissues of the head and neck region, especially after surgery or radiotherapy. However, irrespective of low positive predictive values, [18F]FDG PET remains the standard of care to stage the disease and monitor recurrences. Here, we report the preclinical use of a targeted poly (ADP-ribose) polymerase1 (PARP1) binding PET tracer, fluorine-18 labeled poly (ADP-ribose) polymerase1 inhibitor ([18F]PARPi), as a potential alternative with greater specificity.

METHODS

Using an orthotopic xenograft mouse model injected with either FaDu or Cal 27 (human squamous cell carcinoma cell lines) we performed PET/CT scans with the 2 tracers and compared the results. Gamma counts and autoradiography were also assessed and correlated with histology.

RESULTS

The average retained activity of [18F]PARPi across cell lines in tumor-bearing tongues was 0.9 ± 0.3%ID/g, 4.1 times higher than in control (0.2 ± 0.04%ID/g). Autoradiography and histology confirmed that the activity arose almost exclusively from the tumor areas, with a signal/normal tissue around a ratio of 42.9 ± 21.4. In vivo, [18F]PARPi-PET allowed delineation of tumor from healthy tissue (p < .005), whereas [18F]FDG failed to do so (p = .209).

CONCLUSIONS AND IMPLICATIONS FOR PATIENT CARE

We demonstrate that [18F]PARPi is more specific to tongue tumor tissue than [18F]FDG. [18F]PARPi PET allows for the straightforward delineation of oral cancer in mouse models, suggesting that clinical translation could result in improved imaging of head and neck cancer when compared to [18F]FDG.

Safety and Feasibility of PARP1/2 Imaging with 18F-PARPi in Patients with Head and Neck Cancer

PURPOSE

We performed a first-in-human clinical trial. The aim of this study was to determine safety and feasibility of PET imaging with ¹⁸F-PARPi in patients with head and neck cancer.

PATIENTS AND METHODS

Eleven patients with newly diagnosed or recurrent oral and oropharyngeal cancer were injected with ¹⁸F-PARPi (331 ± 42 MBq), and dynamic PET/CT imaging was performed between 0 and 25 minutes postinjection. Static PET/CT scans were obtained at 30, 60, and 120 minutes postinjection. Blood samples for tracer concentration and metabolite analysis were collected. Blood pressure, ECG, oxygen levels, clinical chemistry, and complete blood count were obtained before and after tracer administration.

RESULTS

¹⁸F-PARPi was well-tolerated by all patients without any safety concerns. Of the 11 patients included in the analysis, ¹⁸F-PARPi had focal uptake in all primary lesions (n = 10, SUV_max = 2.8 ± 1.2) and all ¹⁸F-FDG-positive lymph nodes (n = 34). ¹⁸F-PARPi uptake was seen in ¹⁸F-FDG-negative lymph nodes of 3 patients (n = 6). Focal uptake of tracer in primary and metastatic lesions was corroborated by CT alone or in combination with ¹⁸F-FDG. The overall effective dose with ¹⁸F-PARPi PET was 3.9 mSv - 5.2 mSv, contrast was high [SUV_max(lesion)/SUV_max(trapezius muscle) = 4.5] and less variable than ¹⁸F-FDG when compared with the genioglossus muscle (1.3 vs. 6.0, P = 0.001).

CONCLUSIONS

Imaging of head and neck cancer with ¹⁸F-PARPi is feasible and safe. ¹⁸F-PARPi detects primary and metastatic lesions, and retention in tumors is longer than in healthy tissues.

Poly(ADP-Ribose)Polymerase (PARP) Inhibitors and Radiation Therapy

Poly(ADP-ribose)polymerase-1 (PARP1) is a DNA repair enzyme highly expressed in the nuclei of mammalian cells, with a structure and function that have attracted interest since its discovery. PARP inhibitors, moreover, can be used to induce synthetic lethality in cells where the homologous recombination (HR) pathway is deficient. Several small molecule PARP inhibitors have been approved by the FDA for multiple cancers bearing this deficiency These PARP inhibitors also act as radiosensitizing agents by delaying single strand break (SSB) repair and causing subsequent double strand break (DSB) generation, a concept that has been leveraged in various preclinical models of combination therapy with PARP inhibitors and ionizing radiation. Researchers have determined the efficacy of various PARP inhibitors at sub-cytotoxic concentrations in radiosensitizing multiple human cancer cell lines to ionizing radiation. Furthermore, several groups have begun evaluating combination therapy strategies in mouse models of cancer, and a fluorescent imaging agent that allows for subcellular imaging in real time has been developed from a PARP inhibitor scaffold. Other PARP inhibitor scaffolds have been radiolabeled to create PET imaging agents, some of which have also entered clinical trials. Most recently, these highly targeted small molecules have been radiolabeled with therapeutic isotopes to create radiotherapeutics and radiotheranostics in cancers whose primary interventions are surgical resection and whole-body radiotherapy. In this review we discuss the utilization of these small molecules in combination therapies and in scaffolds for imaging agents, radiotherapeutics, and radiotheranostics. Development of these radiolabeled PARP inhibitors has presented promising results for new interventions in the fight against some of the most intractable cancers.

Approaches to PET Imaging of Glioblastoma

Glioblastoma multiforme (GBM) is the deadliest type of brain tumor, affecting approximately three in 100,000 adults annually. Positron emission tomography (PET) imaging provides an important non-invasive method of measuring biochemically specific targets at GBM lesions. These powerful data can characterize tumors, predict treatment effectiveness, and monitor treatment. This review will discuss the PET imaging agents that have already been evaluated in GBM patients so far, and new imaging targets with promise for future use. Previously used PET imaging agents include the tracers for markers of proliferation ([11C]methionine; [18F]fluoro-ethyl-L-tyrosine, [18F]Fluorodopa,[18F]fluoro-thymidine, and [18F]clofarabine), hypoxia sensing ([18F]FMISO, [18F]FET-NIM, [18F]EF5, [18F]HX4, and [64Cu]ATSM), and ligands for inflammation. As cancer therapeutics evolve toward personalized medicine and therapies centered on tumor biomarkers, the development of complimentary selective PET agents can dramatically enhance these efforts. Newer biomarkers for GBM PET imaging are discussed, with some already in use for PET imaging other cancers and neurological disorders. These targets include Sigma 1, Sigma 2, programmed death ligand 1, poly-ADP-ribose polymerase, and isocitrate dehydrogenase. For GBM, these imaging agents come with additional considerations such as blood-brain barrier penetration, quantitative modeling approaches, and nonspecific binding.

Noninvasive PET Imaging of CDK4/6 Activation in Breast Cancer

The cell cycle is a progression of 4 distinct phases (G1, S, G2, and M), with various cycle proteins being essential in regulating this process. We aimed to develop a radiolabeled cyclin-dependent kinase 4/6 (CDK4/6) inhibitor for breast cancer imaging. Our transfluorinated analog (¹⁸F-CDKi) was evaluated and validated as a novel PET imaging agent to quantify CDK4/6 expression in estrogen receptor (ER)-positive human epidermal growth factor receptor 2 (HER₂)-negative breast cancer. Methods: ¹⁸F-CDKi was synthesized and assayed against CDK4/6 kinases. ¹⁸F-CDKi was prepared with a 2-step automated synthetic strategy that yielded the final product with remarkable purity and molar activity. In vitro and in vivo biologic specificity was assessed in a MCF-7 cell line and in mice bearing MCF-7 breast tumors. Nonradioactive palbociclib was used as a blocking agent to investigate the binding specificity and selectivity of ¹⁸F-CDKi. Results: ¹⁸F-CDKi was obtained with an overall radiochemical uncorrected yield of 15% and radiochemical purity higher than 98%. The total time from the start of synthesis to the final injectable formulated tracer is 70 min. The retention time reported for ¹⁸F-CDKi and ¹⁹F-CDKi is 27.4 min as demonstrated by coinjection with ¹⁹F-CDKi in a high-pressure liquid chromatograph. In vivo blood half-life (weighted, 7.03 min) and octanol/water phase partition coefficient (1.91 ± 0.24) showed a mainly lipophilic behavior. ¹⁸F-CDKi is stable in vitro and in vivo (>98% at 4 h after injection) and maintained its potent targeting affinity to CDK4/6. Cellular uptake experiments performed on the MCF-7 breast cancer cell line (ER-positive and HER₂-negative) demonstrated specific uptake with a maximum intracellular concentration of about 65% as early as 10 min after incubation. The tracer uptake was reduced to less than 5% when cells were coincubated with a molar excess of palbociclib. In vivo imaging and ex vivo biodistribution of ER-positive, HER₂-negative MCF-7 breast cancer models showed a specific uptake of approximately 4% injected dose/g of tumor (reduced to ∼0.3% with a 50-fold excess of cold palbociclib). A comprehensive biodistribution analysis also revealed a significantly lower activation of CDK4/6 in nontargeting organs. Conclusion: ¹⁸F-CDKi represents the first ¹⁸F PET CDK4/6 imaging agent and a promising imaging agent for ER-positive, HER₂-negative breast cancer.

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.

Labelled chemical probes for demonstrating direct target engagement in living systems

Demonstrating target engagement in living systems can help drive successful drug discovery. Target engagement and occupancy studies in cells confirm direct binding of a ligand to its intended target protein and provide the binding affinity. Combined with biomarkers to measure the functional consequences of target engagement, these experiments can increase confidence in the relationship between in vitro pharmacology and observed biological effects. In this review, we focus on chemically and radioactively labelled probes as key reagents for performing such experiments. Using recent examples, we examine how the labelled probes have been employed in combination with unlabelled ligands to quantify target engagement in cells and in animals. Finally, we consider future developments of this emerging methodology.

[18F]-SuPAR: A Radiofluorinated Probe for Noninvasive Imaging of DNA Damage-Dependent Poly(ADP-ribose) Polymerase Activity

Poly(ADP ribose) polymerase (PARP) enzymes generate poly(ADP ribose) post-translational modifications on target proteins for an array of functions centering on DNA and cell stress. PARP isoforms 1 and 2 are critically charged with the surveillance of DNA integrity and are the first line guardians of the genome against DNA breaks. Here we present a novel probe ([¹⁸F]-SuPAR) for noninvasive imaging of PARP-1/2 activity using positron emission tomography (PET). [¹⁸F]-SuPAR is a radiofluorinated nicotinamide adenine dinucleotide (NAD) analog that can be recognized by PARP-1/2 and incorporated into the long branched polymers of poly(ADP ribose) (PAR). The measurement of PARP-1/2 activity was supported by a reduction of radiotracer uptake in vivo following PARP-1/2 inhibition with talazoparib treatment, a potent PARP inhibitor recently approved by FDA for treatment of breast cancer, as well as ex vivo colocalization of radiotracer analog and poly(ADP ribose). With [¹⁸F]-SuPAR, we were able to map the dose- and time-dependent activation of PARP-1/2 following radiation therapy in breast and cervical cancer xenograft mouse models. Tumor response to therapy was determined by [¹⁸F]-SuPAR PET within 8 h of administration of a single dose of radiation equivalent to one round of stereotactic ablative radiotherapy.

PET Imaging of PARP Expression Using 18F-Olaparib

Poly(ADP-ribose) polymerase (PARP) inhibitors are increasingly being studied as cancer drugs, as single agents, or as a part of combination therapies. Imaging of PARP using a radiolabeled inhibitor has been proposed for patient selection, outcome prediction, dose optimization, genotoxic therapy evaluation, and target engagement imaging of novel PARP-targeting agents. Methods: Here, via the copper-mediated 18F-radiofluorination of aryl boronic esters, we accessed, for the first time (to our knowledge), the 18F-radiolabeled isotopolog of the Food and Drug Administration-approved PARP inhibitor olaparib. The use of the 18F-labeled equivalent of olaparib allows direct prediction of the distribution of olaparib, given its exact structural likeness to the native, nonradiolabeled drug. Results:18F-olaparib was taken up selectively in vitro in PARP-1-expressing cells. Irradiation increased PARP-1 expression and 18F-olaparib uptake in a radiation-dose-dependent fashion. PET imaging in mice showed specific uptake of 18F-olaparib in tumors expressing PARP-1 (3.2% ± 0.36% of the injected dose per gram of tissue in PSN-1 xenografts), correlating linearly with PARP-1 expression. Two hours after irradiation of the tumor (10 Gy), uptake of 18F-olaparib increased by 70% (P = 0.025). Conclusion: Taken together, we show that 18F-olaparib has great potential for noninvasive tumor imaging and monitoring of radiation damage.

Altering Nitrogen Heterocycles of AZD2461 Affords High Affinity Poly(ADP-ribose) Polymerase-1 Inhibitors with Decreased P-Glycoprotein Interactions

Poly(ADP-ribose) polymerase inhibitors (PARPi) are targeted therapeutics with enhanced selectivity and cytotoxicity in BRCA1/2 mutant cancer cells. AZD2461, a congener of FDA approved olaparib, is a potent PARPi with high affinity for PARP-1 and nonsubstrate for P-glycoprotein (P-gp), an attractive characteristic for cancer therapeutics. Analogues of AZD2461 were synthesized and profiled in BRCA1 functional and nonfunctional cell lines, revealing compounds (2, 3, and 5) of low cytotoxicity and excellent PARP-1 affinities (∼4-8 nM). In comparison to AZD2461, these agents were found to be less stimulating of P-gp, suggesting that these compounds may be excellent candidates for neurological applications where blood brain barrier penetrance is sought.

Preliminary evaluation of a novel 18F-labeled PARP-1 ligand for PET imaging of PARP-1 expression in prostate cancer

INTRODUCTION

Poly (ADP-ribose) polymerase-1 (PARP-1) plays many roles in prostate cancer (PC), such as mediating DNA damage repair, transcriptional regulation and nuclear hormone receptor signaling. Because of this, PARP-1 has been targeted for therapy in PC, and non-invasive imaging of PARP-1 could help predict which patients are likely to respond to such therapy. Several PARP-1 positron emission tomography (PET) imaging agents have been developed and show promise for imaging PARP-1 expression in breast, brain, and lung cancer in small animals, but not as yet in prostate cancer. [¹⁸F]WC-DZ-F is an analogue of [¹⁸F]FluorThanatrace (FTT) and [¹²⁵I]KX1, which are well-established PARP-1 ligands for measuring PARP-1 expression. Herein, we evaluated the potential of [¹⁸F]WC-DZ-F for the imaging PARP-1 expression in PC.

METHODS

[¹⁸F]WC-DZ-F was synthesized by a two-step sequence. [¹⁸F]WC-DZ-F was evaluated by in vitro uptake studies in PC-3 cells and by in vivo biodistribution and microPET imaging using PC-3 tumor xenografts. Ex vivo autoradiography of PC-3 tumors after microPET imaging was also performed.

RESULTS

[¹⁸F]WC-DZ-F has high, PARP-1-specific uptake in PC-3 cells. In the microPET imaging study, [¹⁸F]WC-DZ-F accumulated in PC-3 xenograft tumors over 2 h, and the uptake was significantly reduced by blocking with olaparib. PC-3 tumors were clearly visualized in microPET images, and the imaging results were further confirmed by autoradiography of PC-3 tumors ex vivo. In the biodistribution study [¹⁸F]WC-DZ-F washed out quickly from most tissues within 2 h, except for the liver in which the uptake was not blockable by olaparib.

CONCLUSIONS

We synthesized a novel PARP-1 radioligand, [¹⁸F]WC-DZ-F. The preliminary evaluation of [¹⁸F]WC-DZ-F indicates that it is a suitable PET imaging agent for measuring PARP-1 expression in prostate cancer and should be applicable to other types of cancers.

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/cc_max ratio), while [¹⁸F]FET-PET clearly delineated the contrast-enhancing region on MR (2.12 ± 0.16 lesion/contralateral %IA/cc_max 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.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0399-z) contains supplementary material, which is available to authorized users.

Examination of Diazaspiro Cores as Piperazine Bioisosteres in the Olaparib Framework Shows Reduced DNA Damage and Cytotoxicity

Development of poly(ADP-ribose) polymerase inhibitors (PARPi's) continues to be an attractive area of research due to synthetic lethality in DNA repair deficient cancers; however, PARPi's also have potential as therapeutics to prevent harmful inflammation. We investigated the pharmacological impact of incorporating spirodiamine motifs into the phthalazine architecture of FDA approved PARPi olaparib. Synthesized analogues were screened for PARP-1 affinity, enzyme specificity, catalytic inhibition, DNA damage, and cytotoxicity. This work led to the identification of 10e (12.6 ± 1.1 nM), which did not induce DNA damage at similar drug concentrations as olaparib. Interestingly, several worst in class compounds with low PARP-1 affinity, including 15b (4397 ± 1.1 nM), induced DNA damage at micromolar concentrations, which can explain the cytotoxicity observed in vitro. This work provides further evidence that high affinity PARPi's can be developed without DNA damaging properties offering potential new drugs for treating inflammatory related diseases.

An 18F-Labeled Poly(ADP-ribose) Polymerase Positron Emission Tomography Imaging Agent

Poly(ADP-ribose) polymerase (PARP) is involved in repair of DNA breaks and is over-expressed in a wide variety of tumors, making PARP an attractive biomarker for positron emission tomography (PET) and single photon emission computed tomography imaging. Consequently, over the past decade, there has been a drive to develop nuclear imaging agents targeting PARP. Here, we report the discovery of a PET tracer that is based on the potent PARP inhibitor olaparib (1). Our lead PET tracer candidate, [18F]20, was synthesized and evaluated as a potential PARP PET radiotracer in mice bearing subcutaneous glioblastoma xenografts using ex vivo biodistribution and PET-magnetic resonance imaging techniques. Results showed that [18F]20 could be produced in a good radioactivity yield and exhibited specific PARP binding allowing visualization of tumors over-expressing PARP. [18F]20 is therefore a potential candidate radiotracer for in vivo PARP PET imaging.

Copper-Mediated Nucleophilic Radiobromination of Aryl Boron Precursors: Convenient Preparation of a Radiobrominated PARP-1 Inhibitor

The copper-mediated nucleophilic radiobromination of aryl boron precursors with a radiobromide ion is a novel radiolabeling method that is efficient and robust. High radiochemical conversion (RCC) was observed using a variety of solvents, temperatures and catalysts. The reaction is also clean and is feasible for purification to obtain high chemical and radiochemical purity. This method provides a very useful route for the preparation of radiobrominated pharmaceuticals, including a radiobromine labeled PARP-1 inhibitor, and it is a valuable addition to the family of copper-mediated radiolabeling processes.

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.

Avenues to molecular imaging of dying cells: Focus on cancer

Successful treatment of cancer patients requires balancing of the dose, timing, and type of therapeutic regimen. Detection of increased cell death may serve as a predictor of the eventual therapeutic success. Imaging of cell death may thus lead to early identification of treatment responders and nonresponders, and to “patient‐tailored therapy.” Cell death in organs and tissues of the human body can be visualized, using positron emission tomography or single‐photon emission computed tomography, although unsolved problems remain concerning target selection, tracer pharmacokinetics, target‐to‐nontarget ratio, and spatial and temporal resolution of the scans. Phosphatidylserine exposure by dying cells has been the most extensively studied imaging target. However, visualization of this process with radiolabeled Annexin A5 has not become routine in the clinical setting. Classification of death modes is no longer based only on cell morphology but also on biochemistry, and apoptosis is no longer found to be the preponderant mechanism of cell death after antitumor therapy, as was earlier believed. These conceptual changes have affected radiochemical efforts. Novel probes targeting changes in membrane permeability, cytoplasmic pH, mitochondrial membrane potential, or caspase activation have recently been explored. In this review, we discuss molecular changes in tumors which can be targeted to visualize cell death and we propose promising biomarkers for future exploration.

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.

Treatment of small-cell lung cancer remains a challenge due to multiple mechanisms of resistance to current therapies; measuring patient response is crucial in adapting and choosing adequate treatment. Here the authors develop a strategy to visualise in vivo dynamics of a class of widely used PARP inhibitors.