PARP-1 Expression Quantified by [18F]FluorThanatrace: A Biomarker of Response to PARP Inhibition Adjuvant to Radiation Therapy

PARP inhibitor resistance mechanisms and PARP inhibitor derived imaging probes

INTRODUCTION

Poly(ADP-ribose) polymerase 1 (PARP1) inhibition has become a major target in anticancer therapy. While PARP inhibitors (PARPi) are approved for homologous recombination (HR) deficient cancers, therapeutic resistance is a challenge and PARPi are now being investigated in cancers lacking HR deficiencies. This creates a need to develop molecular and imaging biomarkers of PARPi response to improve patient selection and circumvent therapeutic resistance.

AREAS COVERED

PubMed and clinicaltrials.gov were queried for studies on PARPi resistance and imaging. This review summarizes established and emerging resistance mechanisms to PARPi, and the current state of imaging and theragnostic probes for PARPi, including fluorescently labeled and radiolabeled probes.

EXPERT OPINION

While progress has been made in understanding PARPi therapeutic resistance, clinical evidence remains lacking and relatively little is known regarding PARPi response outside of HR deficiencies. Continued research will clarify the importance of known biomarkers and resistance mechanisms in patient cohorts and the broader utility of PARPi. Progress has also been made in PARPi imaging, particularly with radiolabeled probes, and both imaging and theragnostic probes have now reached clinical validation. Reducing abdominal background signal from probe clearance will broaden their applicability, and improvements to molecular synthesis and radiation delivery will increase their utility.

PARP1 expression predicts PARP inhibitor sensitivity and correlates with metastatic potential and overall survival in melanoma

Metastatic melanoma is still a difficult-to-treat cancer type owing to its frequent resistance mechanisms to targeted and immunotherapy. Therefore, we aimed to unravel novel therapeutic strategies for melanoma patients. Preclinical and clinical studies show that melanoma patients may benefit from a treatment with poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi). In this study, we focus on PARP1 as a potential biomarker to predict the response of melanoma cells to PARPi therapy. We found that melanoma cells with high basal PARP1 expression exhibit significantly increased cell death after PARPi treatment owing to higher PARP1 trapping compared with melanoma cells with low PARP1 expression. In addition, we could demonstrate that PARP1 expression levels are low in nonmalignant skin cells, and metastatic melanomas show considerably higher PARP1 levels compared with primary melanomas. Most strikingly, we found that high PARP1 levels correlate with worse overall survival of late stage metastasized melanoma patients. In conclusion, we show that PARP1 might act as a biomarker to predict the response to PARPi therapy, and that in particular the late stage metastasized melanoma patients are especially sensitive to PARPi therapy owing to elevated PARP1 expression. Our data suggest that the PARPi cytotoxicity primarily will affect the high PARP1 expressing melanoma cells, rather than the low PARP1 expressing nonmalignant skin cells resulting in only low side effects.

[18F]F-Poly(ADP-Ribose) Polymerase Inhibitor Radiotracers for Imaging PARP Expression and Their Potential Clinical Applications in Oncology

Including poly(ADP-ribose) polymerase (PARP) inhibitors in managing patients with inoperable tumors has significantly improved outcomes. The PARP inhibitors hamper single-strand deoxyribonucleic acid (DNA) repair by trapping poly(ADP-ribose)polymerase (PARP) at sites of DNA damage, forming a non-functional "PARP enzyme-inhibitor complex" leading to cell cytotoxicity. The effect is more pronounced in the presence of PARP upregulation and homologous recombination (HR) deficiencies such as breast cancer-associated gene (BRCA1/2). Hence, identifying HR-deficiencies by genomic analysis-for instance, BRCA1/2 used in triple-negative breast cancer-should be a part of the selection process for PARP inhibitor therapy. Published data suggest BRCA1/2 germline mutations do not consistently predict favorable responses to PARP inhibitors, suggesting that other factors beyond tumor mutation status may be at play. A variety of factors, including tumor heterogeneity in PARP expression and intrinsic and/or acquired resistance to PARP inhibitors, may be contributing factors. This justifies the use of an additional tool for appropriate patient selection, which is noninvasive, and capable of assessing whole-body in vivo PARP expression and evaluating PARP inhibitor pharmacokinetics as complementary to the currently available BRCA1/2 analysis. In this review, we discuss [18F]Fluorine PARP inhibitor radiotracers and their potential in the imaging of PARP expression and PARP inhibitor pharmacokinetics. To provide context we also briefly discuss possible causes of PARP inhibitor resistance or ineffectiveness. The discussion focuses on TNBC, which is a tumor type where PARP inhibitors are used as part of the standard-of-care treatment strategy.

Prudent tactics to sail the boat of PARP inhibitors as therapeutics for diverse malignancies

INTRODUCTION

PARP inhibitors block the DNA-repairing mechanism of PARP and represent a promising class of anti-cancer therapy. The last decade has witnessed FDA approvals of several PARP inhibitors, with some undergoing advanced-stage clinical investigation. Medicinal chemists have invested much effort to expand the structure pool of PARP inhibitors. Issues associated with the use of PARP inhibitors that make their standing disconcerting in the pharmaceutical sector have been addressed via the design of new structural assemblages.

AREA COVERED

In this review, the authors present a detailed account of the medicinal chemistry campaigns conducted in the recent past for the construction of PARP1/PARP2 inhibitors, PARP1 biased inhibitors, and PARP targeting bifunctional inhibitors as well as PARP targeting degraders (PROTACs). Limitations associated with FDA-approved PARP inhibitors and strategies to outwit the limitations are also discussed.

EXPERT OPINION

The PARP inhibitory field has been rejuvenated with numerous tractable entries in the last decade. With numerous magic bullets in hand coupled with unfolded tactics to outwit the notoriety of cancer cells developing resistance toward PARP inhibitors, the dominance of PARP inhibitors as a sagacious option of targeted therapy is highly likely to be witnessed soon.

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.

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.

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.

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.

DNA damage response inhibitors: An avenue for TNBC treatment

The DNA damage response (DDR) is critical for the maintenance of genomic stability by sensing DNA damage, regulating cell cycle and initiating DNA repair. Drugs targeting DDR pathways have been increasingly exploited in treating various tumors. Triple negative breast cancer (TNBC) is a highly heterogeneous and aggressive tumor with constitutive activation of oncogenes, inducing replication stress and DNA damage, which require the DDR for survival. In addition, emerging studies have demonstrated that TNBC harbors aberrant genetic alterations in DDR pathways, such as a high frequency of p53 dysfunction and BRCA1/2 mutations. DDR alterations force TNBC to rely on the existing DDR pathways for survival, and make TNBC particularly sensitive to specific DDR inhibitors, such as high sensitivity of TNBC with BRCA1/2 mutations to PARP inhibitors. This review first and comprehensively covers the current status of the development of DDR inhibitors and discusses the mechanism of targeting the DDR in TNBC. Preclinical and clinical studies on inhibitors of the ATR-CHK1-WEE1 pathway and PARP inhibitors, the most studied inhibitors, and some other DDR inhibitors as monotherapy or combination therapy in TNBC are summarized. We also highlight the possible predictive biomarkers for these DDR inhibitors and their potential combination strategies with chemotherapy, radiotherapy or other targeted agents to optimize the efficacy of DDR inhibitors in TNBC treatment. In conclusion, this review discussed the recent considerations related to the use of DDR inhibitors for TNBC and provides a perspective to address future directions and potential therapeutic strategies for patients with TNBC.

Advances in the Treatment of Ovarian Cancer Using PARP Inhibitors and the Underlying Mechanism of Resistance

The standard treatment for advanced ovarian cancer is cytoreductive surgery followed by cytotoxic chemotherapy. However, it has high risk of recurrence and poor prognosis. Poly(ADPribose) polymerase (PARP) inhibitors selectively target DNA double-strand breaks (DSBs) in tumor cells that cannot be repaired and induce the synthetic lethality of BRCA1/2 mutation cancers. PARP inhibitors are clinically used to treat recurrent ovarian cancer and show significant efficacy in ovarian cancer patients with homologous recombination repair (HRR) pathway defects. PARP inhibitors also have significant clinical benefits in patients without HR defects. With the increasingly extensive clinical application of PARP inhibitors, the possibility of acquiring drug resistance is high. Therefore, clinical strategies should be adopted to manage drug resistance of PARP inhibitors. This study aims to summarize the indications and toxicity of PARP inhibitors, the mechanism of action, targeted treatment of drug resistance, and potential methods to manage drug-resistant diseases. We used the term "ovarian cancer" and the names of each PARP inhibitor as keywords to search articles published in the Medical Subject Headings (MeSH) on Pubmed, along with the keywords "clinicaltrials.gov" and "google.com/patents" as well as "uspto.gov." The FDA has approved olaparib, niraparib, and rucaparib for the treatment of recurrent epithelial ovarian cancer (EOC). Talazoparib and veliparib are currently in early trials and show promising clinical results. The mechanism underlying resistance to PARP inhibitors and the clinical strategies to overcome them remain unclear. Understanding the mechanism of resistance to PARP inhibitors and their relationship with platinum resistance may help with the development of antiresistance therapies and optimization of the sequence of drug application in the future clinical treatment of ovarian cancer.

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.

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.

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.

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.

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.

PARP inhibitors: Clinical utility and possibilities of overcoming resistance

PARP inhibitors represent a major breakthrough in ovarian cancer care. Almost half of all ovarian cancers have deficiencies in the homologous recombination (HR) DNA repair pathway, namely BRCA1/2 mutations. Given the limited therapeutic options for recurrent ovarian cancer patients there has been a significant effort to develop novel therapies to exploit DNA repair deficiencies. In 2005 and 2006, inhibiting PARP enzymes was first observed to be highly effective against cancers with HR deficiencies. PARP inhibitors are being utilized in the clinic to manage recurrent ovarian cancers that display defects in the HR repair pathway. However, PARP inhibitors also show significant clinical benefit in patients without HR deficiencies. There are currently three FDA-approved PARP inhibitors for recurrent ovarian cancer and an additional two PARP inhibitors being evaluated in late stage clinical trials. Given the expanding clinical use of PARP inhibitors and the high likelihood of acquired resistance, there is a significant need for clinical strategies to manage PARP inhibitor resistant disease. This review will examine PARP inhibitors in the context of: indications and toxicities, novel biomarkers to predict response, targeted-therapy resistance, and potential approaches to manage resistant disease.

Molecular Imaging of PARP

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