PET of Poly (ADP-Ribose) Polymerase Activity in Cancer: Preclinical Assessment and First In-Human Studies

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.

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.

[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.

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.

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.

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.

Polyethylene Glycol 3350 (PEG 3350) as a Practical Vehicle for Rapid Reconstitution of PARPi-FL Formulations for Clinical Use

PARPi-FL is a molecularly specific fluorescent agent that targets poly ADP-ribose polymerase 1, a DNA repair enzyme overexpressed in the nuclei of tumor cells. This imaging agent is being investigated in a clinical trial (NCT03085147) for the detection of oral cancer. The PARPi-FL mouthwash formulation currently being used in the phase I/II clinical trial comprises 1,000 nM of PARPi-FL dissolved first in 4.5 ml of polyethylene glycol (PEG) 300 and then in 9.5 ml of water. This formulation requires a 2-step process that can be cumbersome for routine clinical use. To minimize errors and simplify the formulation process, we have developed a new one-step formulation, which requires only the direct addition of water into a vial containing a mixture of the PARPi-FL and PEG 3350, which is also a powder. In a series of analytical and preclinical studies, we demonstrate that the new formulation of PARPi-FL is stable over 365 days, sustains its characteristics, and performs similar to the previous formulation. Moving forward, the new formulation of the PARPi-FL will be used for patients accrued in the phase II clinical trial.

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.

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 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.

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.

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.

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.

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.

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.

Novel Tracers and Radionuclides in PET Imaging

The use of PET imaging agents in oncology, cardiovascular disease, and neurodegenerative disease shows the power of this technique in evaluating the molecular and biological characteristics of numerous diseases. These agents provide crucial information for designing therapeutic strategies for individual patients. Novel PET tracers are in continual development and many have potential use in clinical and research settings. This article discusses the potential applications of tracers in diagnostics, the biological characteristics of diseases, the ability to provide prognostic indicators, and using this information to guide treatment strategies including monitoring treatment efficacy in real time to improve outcomes and survival.

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.

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.

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.

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.

Targeting the Versatile Wnt/β-Catenin Pathway in Cancer Biology and Therapeutics: From Concept to Actionable Strategy

This expert review offers a critical synthesis of the latest insights and approaches at targeting the Wnt/β-catenin pathway in various cancers such as colorectal cancer, melanoma, leukemia, and breast and lung cancers. Notably, from organogenesis to cancer, the Wnt/β-catenin signaling displays varied and highly versatile biological functions in animals, with virtually all tissues requiring the Wnt/β-catenin signaling in one way or the other. Aberrant expression of the members of the Wnt/β-catenin has been implicated in many pathological conditions, particularly in human cancers. Mutations in the Wnt/β-catenin pathway genes have been noted in diverse cancers. Biochemical and genetic data support the idea that inhibition of Wnt/β-catenin signaling is beneficial in cancer therapeutics. The interaction of this important pathway with other signaling systems is also noteworthy, but remains as an area for further research and discovery. In addition, formation of different complexes by components of the Wnt/β-catenin pathway and the precise roles of these complexes in the cytoplasmic milieu are yet to be fully elucidated. This article highlights the latest medical technologies in imaging, single-cell omics, use of artificial intelligence (e.g., machine learning techniques), genome sequencing, quantum computing, molecular docking, and computational softwares in modeling interactions between molecules and predicting protein-protein and compound-protein interactions pertinent to the biology and therapeutic value of the Wnt/β-catenin signaling pathway. We discuss these emerging technologies in relationship to what is currently needed to move from concept to actionable strategies in translating the Wnt/β-catenin laboratory discoveries to Wnt-targeted cancer therapies and diagnostics in the clinic.

Advances in PET Diagnostics for Guiding Targeted Cancer Therapy and Studying In Vivo Cancer Biology

Purpose of the Review

We present an overview of recent advances in positron emission tomography (PET) diagnostics as applied to the study of cancer, specifically as a tool to study in vivo cancer biology and to direct targeted cancer therapy. The review is directed to translational and clinical cancer investigators who may not be familiar with these applications of PET cancer diagnostics, but whose research might benefit from these advancing tools.

Recent Findings

We highlight recent advances in 3 areas: (1) the translation of PET imaging cancer biomarkers to clinical trials; (2) methods for measuring cancer metabolism in vivo in patients; and (3) advances in PET instrumentation, including total-body PET, that enable new methodologies. We emphasize approaches that have been translated to human studies.

Summary

PET imaging methodology enables unique in vivo cancer diagnostics that go beyond cancer detection and staging, providing an improved ability to guide cancer treatment and an increased understanding of in vivo human cancer biology.

The beginning of the end for conventional RECIST - novel therapies require novel imaging approaches

Owing to improvements in our understanding of the biological principles of tumour initiation and progression, a wide variety of novel targeted therapies have been developed. Developments in biomedical imaging, however, have not kept pace with these improvements and are still mainly designed to determine lesion size alone, which is reflected in the Response Evaluation Criteria in Solid Tumors (RECIST). Imaging approaches currently used for the evaluation of treatment responses in patients with solid tumours, therefore, often fail to detect successful responses to novel targeted agents and might even falsely suggest disease progression, a scenario known as pseudoprogression. The ability to differentiate between responders and nonresponders early in the course of treatment is essential to allowing the early adjustment of treatment regimens. Various imaging approaches targeting a single dedicated tumour feature, as described in the hallmarks of cancer, have been successful in preclinical investigations, and some have been evaluated in pilot clinical trials. However, these approaches have largely not been implemented in clinical practice. In this Review, we describe current biomedical imaging approaches used to monitor responses to treatment in patients receiving novel targeted therapies, including a summary of the most promising future approaches and how these might improve clinical practice.

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.

Accelerating anticancer drug development - opportunities and trade-offs

The traditional approach to drug development in oncology, with discrete phases of clinical testing, is becoming untenable owing to expansion of the precision medicine paradigm, whereby patients are stratified into multiple subgroups according to the underlying cancer biology. Seamless approaches to drug development in oncology hold great promise of accelerating the accessibility of novel therapeutic agents to the public but are also accompanied by important trade-offs, including the limited availability of information on the clinical benefit and safety of novel agents at the time of market entry. In this Perspectives article, we describe several opportunities, in the form of novel trial designs or modelling strategies, to improve the efficiency of drug development in oncology, as well as new mechanisms to obtain information about anticancer therapies throughout their life cycle, such as innovative functional imaging techniques or the use of real-world clinical data.

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.

Synthesis and evaluation of an AZD2461 [18F]PET probe in non-human primates reveals the PARP-1 inhibitor to be non-blood-brain barrier penetrant

Poly(ADP-ribose)polymerase-1 inhibitor (PARPi) AZD2461 was designed to be a weak P-glycoprotein (P-gp) analogue of FDA approved olaparib. With this chemical property in mind, we utilized the AZD2461 ligand architecture to develop a CNS penetrant and PARP-1 selective imaging probe, in order to investigate PARP-1 mediated neuroinflammation and neurodegenerative diseases, such as Alzheimer's and Parkinson's. Our work led to the identification of several high-affinity PARPi, including AZD2461 congener 9e (PARP-1 IC50 = 3.9 ± 1.2 nM), which was further evaluated as a potential 18F-PET brain imaging probe. However, despite the similar molecular scaffolds of 9e and AZD2461, our studies revealed non-appreciable brain-uptake of [18F]9e in non-human primates, suggesting AZD2461 to be non-CNS penetrant.

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.

Imaging of Tumor Characteristics and Molecular Pathways With PET: Developments Over the Last Decade Toward Personalized Cancer Therapy

PURPOSE

Improvements in personalized therapy are made possible by the advances in molecular biology that led to developments in molecular imaging, allowing highly specific in vivo imaging of biological processes. Positron emission tomography (PET) is the most specific and sensitive imaging technique for in vivo molecular targets and pathways, offering quantification and evaluation of functional properties of the targeted anatomy.

MATERIALS AND METHODS

This work is an integrative research review that summarizes and evaluates the accumulated current status of knowledge of recent advances in PET imaging for cancer diagnosis and treatment, concentrating on novel radiotracers and evaluating their advantages and disadvantages in cancer characterization. Medline search was conducted, limited to English publications from 2007 onward. Identified manuscripts were evaluated for most recent developments in PET imaging of cancer hypoxia, angiogenesis, proliferation, and clonogenic cancer stem cells (CSC).

RESULTS

There is an expansion observed from purely metabolic-based PET imaging toward antibody-based PET to achieve more information on cancer characteristics to identify hypoxia, proangiogenic factors, CSC, and others. ⁶⁴Cu-ATSM, for example, can be used both as a hypoxia and a CSC marker.

CONCLUSIONS

Progress in the field of functional imaging will possibly lead to more specific tumor targeting and personalized treatment, increasing tumor control and improving quality of life.

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

BACKGROUND

Poly(ADP-ribose) polymerase (PARP) inhibitors are effective in a broad population of patients with ovarian cancer; however, resistance caused by low enzyme expression of the drug target PARP-1 remains to be clinically evaluated in this context. We hypothesize that PARP-1 expression is variable in ovarian cancer and can be quantified in primary and metastatic disease using a novel PET imaging agent.

METHODS

We used a translational approach to describe the significance of PET imaging of PARP-1 in ovarian cancer. First, we produced PARP1-KO ovarian cancer cell lines using CRISPR/Cas9 gene editing to test the loss of PARP-1 as a resistance mechanism to all clinically used PARP inhibitors. Next, we performed preclinical microPET imaging studies using ovarian cancer patient-derived xenografts in mouse models. Finally, in a phase I PET imaging clinical trial we explored PET imaging as a regional marker of PARP-1 expression in primary and metastatic disease through correlative tissue histology.

RESULTS

We found that deletion of PARP1 causes resistance to all PARP inhibitors in vitro, and microPET imaging provides proof of concept as an approach to quantify PARP-1 in vivo. Clinically, we observed a spectrum of standard uptake values (SUVs) ranging from 2-12 for PARP-1 in tumors. In addition, we found a positive correlation between PET SUVs and fluorescent immunohistochemistry for PARP-1 (r2 = 0.60).

CONCLUSION

This work confirms the translational potential of a PARP-1 PET imaging agent and supports future clinical trials to test PARP-1 expression as a method to stratify patients for PARP inhibitor therapy.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02637934.

FUNDING

Research reported in this publication was supported by the Department of Defense OC160269, a Basser Center team science grant, NIH National Cancer Institute R01CA174904, a Department of Energy training grant DE-SC0012476, Abramson Cancer Center Radiation Oncology pilot grants, the Marsha Rivkin Foundation, Kaleidoscope of Hope Foundation, and Paul Calabresi K12 Career Development Award 5K12CA076931.

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.

Current status and future challenges for molecular imaging

Molecular imaging (MI), used in its wider sense of biology at the molecular level, is a field that lies at the intersection of molecular biology and traditional medical imaging. As advances in medicine have exponentially expanded over the last few decades, so has our need to better understand the fundamental behaviour of living organisms in a non-invasive and timely manner. This commentary draws from topics the authors addressed in their presentations at the 2017 Royal Society Meeting 'Challenges for chemistry in molecular imaging', as well as a discussion of where MI is today and where it is heading in the future.This article is part of the themed issue 'Challenges for chemistry in molecular imaging'.

Inflammation and DNA damage: Probing pathways to cancer and neurodegeneration

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

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

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