Imaging therapeutic PARP inhibition in vivo through bioorthogonally developed companion imaging agents

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

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

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

Purpose To demonstrate that positron emission tomography (PET) with fluorine 18 (¹⁸F) fluorthanatrace (FTT) depicts activated poly (adenosine diphosphate-ribose)polymerase (PARP) expression and is feasible for clinical trial evaluation. Materials and Methods All studies were conducted prospectively from February 2012 through July 2015 under protocols approved by the local animal studies committee and institutional review board. The area under the receiver operating characteristic curve (AUC, in g/mL· min) for ¹⁸F-FTT was assessed in normal mouse organs before and after treatment with olaparib (n = 14), a PARP inhibitor, or iniparib (n = 11), which has no PARP inhibitory activity. Murine biodistribution studies were performed to support human translational studies. Eight human subjects with cancer and eight healthy volunteers underwent imaging to verify the human radiation dosimetry of ¹⁸F-FTT. The Wilcoxon signed rank test was used to assess for differences among treatment groups for the mouse studies. Results In mice, olaparib, but not iniparib, significantly reduced the ¹⁸F-FTT AUC in the spine (median difference before and after treatment and interquartile range [IQR]: -17 g/mL· min and 10 g/mL · min, respectively [P = .0001], for olaparib and -3 g/mL · min and 13 g/mL · min [P = .70] for iniparib) and in nodes (median difference and interquartile range [IQR] before and after treatment: -23 g/mL · min and 13 g/mL · min [P = .0001] for olaparib; -9 g/mL · min and 17 g/mL · min [P = .05] for iniparib). The effective dose was estimated at 6.9 mSv for a 370-MBq ¹⁸F-FTT dose in humans. In humans, the organs with the highest uptake on images were the spleen and pancreas. Among five subjects with measurable tumors, increased ¹⁸F-FTT uptake was seen in one subject with pancreatic adenocarcinoma and another with liver cancer. Conclusion The results suggest that ¹⁸F-FTT uptake reflects PARP expression and that its radiation dosimetry profile is compatible with those of agents currently in clinical use. ^© RSNA, 2016 Online supplemental material is available for this article.

Dual-Modality Optical/PET Imaging of PARP1 in Glioblastoma

PURPOSE

The current study presents [(18)F]PARPi-FL as a bimodal fluorescent/positron emission tomography (PET) agent for PARP1 imaging.

PROCEDURES

[(18)F]PARPi-FL was obtained by (19)F/(18)F isotopic exchange and PET experiments, biodistribution studies, surface fluorescence imaging, and autoradiography carried out in a U87 MG glioblastoma mouse model.

RESULTS

[(18)F]PARPi-FL showed high tumor uptake in vivo and ex vivo in small xenografts (< 2 mm) with both PET and optical imaging technologies. Uptake of [(18)F]PARPi-FL in blocked U87 MG tumors was reduced by 84 % (0.12 ± 0.02 %injected dose/gram (%ID/g)), showing high specificity of the binding. PET imaging showed accumulation in the tumor (1 h p.i.), which was confirmed by ex vivo phosphor autoradiography.

CONCLUSIONS

The fluorescent component of [(18)F]PARPi-FL enables cellular resolution optical imaging, while the radiolabeled component of [(18)F]PARPi-FL allows whole-body deep-tissue imaging of malignant growth.

Novel Strategies for Breast Cancer Imaging: New Imaging Agents to Guide Treatment

The development of molecular therapies for cancer treatment has created a need to image biochemical and molecular processes to appropriately select tumors that express the drug target, thereby predicting a positive response to therapy. Biomarker-driven molecular imaging is complementary to pathologic analysis and offers a more direct measure of drug efficacy and treatment response, potentially providing early insight into therapeutic futility and allowing response-adapted treatment strategies. Imaging also allows a unique means of assessing the heterogeneity of both intra- and intertumoral targets as well as a mixed response to therapy; this information is important in the setting of metastatic disease. Here we review the development of novel molecular imaging probes and combinations of probes to guide therapy for two new targets and associated therapeutic agents: cyclin-dependent kinase inhibitors and poly(adenosine diphosphate-ribose) polymerase inhibitors.

Advancing biomedical imaging

Imaging reveals complex structures and dynamic interactive processes, located deep inside the body, that are otherwise difficult to decipher. Numerous imaging modalities harness every last inch of the energy spectrum. Clinical modalities include magnetic resonance imaging (MRI), X-ray computed tomography (CT), ultrasound, and light-based methods [endoscopy and optical coherence tomography (OCT)]. Research modalities include various light microscopy techniques (confocal, multiphoton, total internal reflection, superresolution fluorescence microscopy), electron microscopy, mass spectrometry imaging, fluorescence tomography, bioluminescence, variations of OCT, and optoacoustic imaging, among a few others. Although clinical imaging and research microscopy are often isolated from one another, we argue that their combination and integration is not only informative but also essential to discovering new biology and interpreting clinical datasets in which signals invariably originate from hundreds to thousands of cells per voxel.

Detection and delineation of oral cancer with a PARP1 targeted optical imaging agent

Earlier and more accurate detection of oral squamous cell carcinoma (OSCC) is essential to improve the prognosis of patients and to reduce the morbidity of surgical therapy. Here, we demonstrate that the nuclear enzyme Poly(ADP-ribose)Polymerase 1 (PARP1) is a promising target for optical imaging of OSCC with the fluorescent dye PARPi-FL. In patient-derived OSCC specimens, PARP1 expression was increased 7.8 ± 2.6-fold when compared to normal tissue. Intravenous injection of PARPi-FL allowed for high contrast in vivo imaging of human OSCC models in mice with a surgical fluorescence stereoscope and high-resolution imaging systems. The emitted signal was specific for PARP1 expression and, most importantly, PARPi-FL can be used as a topical imaging agent, spatially resolving the orthotopic tongue tumors in vivo. Collectively, our results suggest that PARP1 imaging with PARPi-FL can enhance the detection of oral cancer, serve as a screening tool and help to guide surgical resections.

[(18)F]FluorThanatrace uptake as a marker of PARP1 expression and activity in breast cancer

The nuclear enzyme PARP1 plays a central role in sensing DNA damage and facilitating repair. Tumors with BRCA1/2 mutations are highly dependent on PARP1 as an alternative mechanism for DNA repair, and PARP inhibitors generate synthetic lethality in tumors with BRCA mutations, resulting in cell cycle arrest and apoptosis. Zhou et al. recently synthesized an (18)F-labeled PARP1 inhibitor ([(18)F]FluorThanatrace) for PET, and demonstrated high specific tracer uptake in a xenograft model of breast cancer [1]. In the current study, we characterize the level of baseline PARP expression and activity across multiple human breast cancer cell lines, including a BRCA1 mutant line. PARP expression and activity, as measured by levels of PAR and PARP1, is correlated with in vitro [(18)F]FluorThanatrace binding as well as tracer uptake on PET in a xenograft model of breast cancer. Radiotracer uptake in genetically-engineered mouse fibroblasts indicates [(18)F]FluorThanatrace is selective for PARP1 versus other PARP enzymes. This motivates further studies of [(18)F]FluorThanatrace as an in vivo measure of PARP1 expression and activity in patients who would benefit from PARP inhibitor therapy.

Optical Imaging of PARP1 in Response to Radiation in Oral Squamous Cell Carcinoma

Targeting and inhibiting DNA repair pathways is a powerful strategy of controlling malignant growth. One such strategy includes the inhibition of PARP1, a central element in the intracellular DNA damage response. To determine and visualize the expression and intercellular distribution of PARP1 in vivo, and to monitor the pharmacokinetics of PARP1 targeted therapeutics, fluorescent small probes were developed. To date, however, it is unclear how these probes behave in a more realistic clinical setting, where DNA damage has been induced through one or more prior lines of therapy. Here, we use one such imaging agent, PARPi-FL, in tissues both with and without prior DNA damage, and investigate its value as a probe for PARP1 imaging. We show that PARP1 expression in oral cancer is high, and that the uptake of PARPi-FL is selective, irrespective of whether cells were exposed to irradiation or not. We also show that PARPi-FL uptake increases in response to DNA damage, and that this increase is reflected in higher enzyme expression. Our findings provide a framework for measuring exposure of cells to external beam radiation, and could help to elucidate the effects of such treatments non-invasively in mouse models of cancer.

Synthesis and Evaluation of a Radioiodinated Tracer with Specificity for Poly(ADP-ribose) Polymerase-1 (PARP-1) in Vivo

Interest in nuclear imaging of poly(ADP-ribose) polymerase-1 (PARP-1) has grown in recent years due to the ability of PARP-1 to act as a biomarker for glioblastoma and increased clinical use of PARP-1 inhibitors. This study reports the identification of a lead iodinated analog 5 of the clinical PARP-1 inhibitor olaparib as a potential single-photon emission computed tomography (SPECT) imaging agent. Compound 5 was shown to be a potent PARP-1 inhibitor in cell-free and cellular assays, and it exhibited mouse plasma stability but approximately 3-fold greater intrinsic clearance when compared to olaparib. An (123)I-labeled version of 5 was generated using solid state halogen exchange methodology. Ex vivo biodistribution studies of [(123)I]5 in mice bearing subcutaneous glioblastoma xenografts revealed that the tracer had the ability to be retained in tumor tissue and bind to PARP-1 with specificity. These findings support further investigations of [(123)I]5 as a noninvasive PARP-1 SPECT imaging agent.

Single-cell pharmacokinetic imaging reveals a therapeutic strategy to overcome drug resistance to the microtubule inhibitor eribulin

Eribulin mesylate was developed as a potent microtubule-targeting cytotoxic agent to treat taxane-resistant cancers, but recent clinical trials have shown that it eventually fails in many patient subpopulations for unclear reasons. To investigate its resistance mechanisms, we developed a fluorescent analog of eribulin with pharmacokinetic (PK) properties and cytotoxic activity across a human cell line panel that are sufficiently similar to the parent drug to study its cellular PK and tissue distribution. Using intravital imaging and automated tracking of cellular dynamics, we found that resistance to eribulin and the fluorescent analog depended directly on the multidrug resistance protein 1 (MDR1). Intravital imaging allowed for real-time analysis of in vivo PK in tumors that were engineered to be spatially heterogeneous for taxane resistance, whereby an MDR1-mApple fusion protein distinguished resistant cells fluorescently. In vivo, MDR1-mediated drug efflux and the three-dimensional tumor vascular architecture were discovered to be critical determinants of drug accumulation in tumor cells. We furthermore show that standard intravenous administration of a third-generation MDR1 inhibitor, HM30181, failed to rescue drug accumulation; however, the same MDR1 inhibitor encapsulated within a nanoparticle delivery system reversed the multidrug-resistant phenotype and potentiated the eribulin effect in vitro and in vivo in mice. Our work demonstrates that in vivo assessment of cellular PK of an anticancer drug is a powerful strategy for elucidating mechanisms of drug resistance in heterogeneous tumors and evaluating strategies to overcome this resistance.

Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics

Pretargeted PET imaging has emerged as an effective strategy for merging the exquisite selectivity of antibody-based targeting vectors with the rapid pharmacokinetics of radiolabeled small molecules. We previously reported the development of a strategy for the pretargeted PET imaging of colorectal cancer based on the bioorthogonal inverse electron demand Diels-Alder reaction between a tetrazine-bearing radioligand and a transcyclooctene-modified huA33 immunoconjugate. Although this method effectively delineated tumor tissue, its clinical potential was limited by the somewhat sluggish clearance of the radioligand through the gastrointestinal tract. Herein, we report the development and in vivo validation of a pretargeted strategy for the PET imaging of colorectal carcinoma with dramatically improved pharmacokinetics. Two novel tetrazine constructs, Tz-PEG7-NOTA and Tz-SarAr, were synthesized, characterized, and radiolabeled with (64)Cu in high yield (>90%) and radiochemical purity (>99%). PET imaging and biodistribution experiments in healthy mice revealed that although (64)Cu-Tz-PEG7-NOTA is cleared via both the gastrointestinal and urinary tracts, (64)Cu-Tz-SarAr is rapidly excreted by the renal system alone. On this basis, (64)Cu-Tz-SarAr was selected for further in vivo evaluation. To this end, mice bearing A33 antigen-expressing SW1222 human colorectal carcinoma xenografts were administered huA33-TCO, and the immunoconjugate was given 24 h to accumulate at the tumor and clear from the blood, after which (64)Cu-Tz-SarAr was administered via intravenous tail vein injection. PET imaging and biodistribution experiments revealed specific uptake of the radiotracer in the tumor at early time points (5.6 ± 0.7 %ID/g at 1 h p.i.), high tumor-to-background activity ratios, and rapid elimination of unclicked radioligand. Importantly, experiments with longer antibody accumulation intervals (48 and 120 h) yielded slight decreases in tumoral uptake but also concomitant increases in tumor-to-blood activity concentration ratios. This new strategy offers dosimetric benefits as well, yielding a total effective dose of 0.041 rem/mCi, far below the doses produced by directly labeled (64)Cu-NOTA-huA33 (0.133 rem/mCi) and (89)Zr-DFO-huA33 (1.54 rem/mCi). Ultimately, this pretargeted PET imaging strategy boasts a dramatically improved pharmacokinetic profile compared to our first generation system and is capable of clearly delineating tumor tissue with high image contrast at only a fraction of the radiation dose created by directly labeled radioimmunoconjugates.

Radioiodinated PARP1 tracers for glioblastoma imaging

Background

Although the understanding of the genetic and molecular basis of cancer has advanced significantly over the past several decades, imaging and treatment options for glioblastoma patients have been more limited (N Engl J Med 359:492-507, 2008). This is in part due to difficulties in diagnosing this disease early, combined with its diffuse, infiltrative growth. This study was aimed at the development of a novel diagnostic tool for glioblastoma through the synthesis of a small molecule based on radioiodinated poly(ADP-ribose)polymerase 1 (PARP1) targeted tracers. This PARP1 is a biomarker that is overexpressed in glioblastoma tissue, but has only low expression levels in the healthy brain (Neoplasia 16:432-40, 2014).

Methods

A library of PARP1 inhibitors (iodo-PARPis) was synthesized. Based on their pharmacokinetic properties and nuclear PARP1 binding, the most successful inhibitor was radiolabeled with ¹³¹I and ¹²⁴I. Biodistribution as well as imaging experiments were performed in orthotopic and subcutaneous mouse models of glioblastoma.

Results

One member of our iodo-poly(ADP-ribose)polymerase 1 (PARP1) inhibitor library, I2-PARPi, shows promising biophysical properties for in vivo application. All synthesized tracers have IC₅₀ values in the nanomolar range (9 ± 2–107 ± 4 nM) and were able to inhibit the uptake of a fluorescent PARP1 inhibitor analog (PARPi-FL). I2-PARPi was able to reduce the uptake of PARPi-FL by 78 ± 4 % in vivo. In mouse models of glioblastoma, we show that the radioiodinated inhibitor analog has high uptake in tumor tissue (U251 MG xenograft, tumor, 0.43 ± 0.06 %ID/g; brain, 0.01 ± 0.00 %ID/g; muscle, 0.03 ± 0.01 %ID/g; liver, 2.35 ± 0.57 %ID/g; thyroid, 0.24 ± 0.06 %ID/g). PET and SPECT imaging performed in orthotopic glioblastoma models with [¹²⁴I]- and [¹³¹I]-I2-PARPi showed selective accumulation in the tumor tissue. These results were also verified using autoradiography of tumor sections, which displayed focal selective uptake of the tracer in the tumor regions as confirmed by histology. The uptake could be blocked through pre-injection of excess unlabeled PARP1 inhibitor (Olaparib).

Conclusions

We have successfully synthesized and radioiodinated the PARP1 selective tracer I2-PARPi. The novel tracer shows selective binding to tumor tissue, both in vitro and in models of glioblastoma, and has the potential to serve as a selective PET imaging agent for brain tumors.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0123-1) contains supplementary material, which is available to authorized users.

Single cell resolution in vivo imaging of DNA damage following PARP inhibition

Targeting DNA repair pathways is a powerful strategy to treat cancers. To gauge efficacy in vivo, typical response markers include late stage effects such as tumor shrinkage, progression free survival, or invasive repeat biopsies. These approaches are often difficult to answer critical questions such as how a given drug affects single cell populations as a function of dose and time, distance from microvessels or how drug concentration (pharmacokinetics) correlates with DNA damage (pharmacodynamics). Here, we established a single-cell in vivo pharmacodynamic imaging read-out based on a truncated 53BP1 double-strand break reporter to determine whether or not poly(ADP-ribose) polymerase (PARP) inhibitor treatment leads to accumulation of DNA damage. Using this reporter, we show that not all PARP inhibitor treated tumors incur an increase in DNA damage. The method provides a framework for single cell analysis of cancer therapeutics in vivo.

Cancer subclonal genetic architecture as a key to personalized medicine

The future of personalized oncological therapy will likely rely on evidence-based medicine to integrate all of the available evidence to delineate the most efficacious treatment option for the patient. To undertake evidence-based medicine through use of targeted therapy regimens, identification of the specific underlying causative mutation(s) driving growth and progression of a patient's tumor is imperative. Although molecular subtyping is important for planning and treatment, intraclonal genetic diversity has been recently highlighted as having significant implications for biopsy-based prognosis. Overall, delineation of the clonal architecture of a patient's cancer and how this will impact on the selection of the most efficacious therapy remain a topic of intense interest.

PARPi-FL--a fluorescent PARP1 inhibitor for glioblastoma imaging

New intravital optical imaging technologies have revolutionized our understanding of mammalian biology and continue to evolve rapidly. However, there are only a limited number of imaging probes available to date. In this study, we investigated in mouse models of glioblastoma whether a fluorescent small molecule inhibitor of the DNA repair enzyme PARP1, PARPi-FL, can be used as an imaging agent to detect glioblastomas in vivo. We demonstrated that PARPi-FL has appropriate biophysical properties, low toxicity at concentrations used for imaging, high stability in vivo, and accumulates selectively in glioblastomas due to high PARP1 expression. Importantly, subcutaneous and orthotopic glioblastoma xenografts were imaged with high contrast clearly defining tumor tissue from normal surrounding tissue. This research represents a step toward exploring and developing PARPi-FL as an optical intraoperative imaging agent for PARP1 in the clinic.

A photoactivatable drug-caged fluorophore conjugate allows direct quantification of intracellular drug transport

We report here a method that utilizes a photoactivatable drug-caged fluorophore conjugate to quantify intracellular drug trafficking processes at single cell resolution. Photoactivation is performed in labeled cellular compartments to visualize intracellular drug exchange under physiological conditions, without the need for washing, facilitating its translation into in vivo cancer models.

In vivo imaging of specific drug-target binding at subcellular resolution

The possibility of measuring binding of small-molecule drugs to desired targets in live cells could provide a better understanding of drug action. However, current approaches mostly yield static data, require lysis or rely on indirect assays and thus often provide an incomplete understanding of drug action. Here, we present a multiphoton fluorescence anisotropy microscopy live cell imaging technique to measure and map drug-target interaction in real time at subcellular resolution. This approach is generally applicable using any fluorescently labelled drug and enables high-resolution spatial and temporal mapping of bound and unbound drug distribution. To illustrate our approach we measure intracellular target engagement of the chemotherapeutic Olaparib, a poly(ADP-ribose) polymerase inhibitor, in live cells and within a tumour in vivo. These results are the first generalizable approach to directly measure drug-target binding in vivo and present a promising tool to enhance understanding of drug activity.

In vivo imaging of multidrug resistance using a third generation MDR1 inhibitor

Cellular up-regulation of multidrug resistance protein 1 (MDR1) is a common cause for resistance to chemotherapy; development of third generation MDR1 inhibitors-several of which contain a common 6,7-dimethoxy-2-phenethyl-1,2,3,4-tetrahydroisoquinoline substructure-is underway. Efficacy of these agents has been difficult to ascertain, partly due to a lack of pharmacokinetic reporters for quantifying inhibitor localization and transport dynamics. Some of the recent third generation inhibitors have a pendant heterocycle, for example, a chromone moiety, which we hypothesized could be converted to a fluorophore. Following synthesis and teasing of a small set of analogues, we identified one lead compound that can be used as a cellular imaging agent that exhibits structural similarity and behavior akin to the latest generation of MDR1 inhibitors.

Red Si-rhodamine drug conjugates enable imaging in GFP cells

Here we evaluated a series of Si-derivatized rhodamine (SiR) dyes for their ability to visualize a model drug in live cells. We show that a charge neutral SiR derivative (but not others) can indeed be used to follow the intracellular location of the model therapeutic drug in GFP cells.

Single cell imaging of Bruton's tyrosine kinase using an irreversible inhibitor

A number of Bruton's tyrosine kinase (BTK) inhibitors are currently in development, yet it has been difficult to visualize BTK expression and pharmacological inhibition in vivo in real time. We synthesized a fluorescent, irreversible BTK binder based on the drug Ibrutinib and characterized its behavior in cells and in vivo. We show a 200 nM affinity of the imaging agent, high selectivity, and irreversible binding to its target following initial washout, resulting in surprisingly high target-to-background ratios. In vivo, the imaging agent rapidly distributed to BTK expressing tumor cells, but also to BTK-positive tumor-associated host cells.

Building Blocks for the Construction of Bioorthogonally Reactive Peptides via Solid-Phase Peptide Synthesis

The need for post-synthetic modifications and reactive prosthetic groups has long been a limiting factor in the synthesis and study of peptidic and peptidomimetic imaging agents. In this regard, the application of biologically and chemically orthogonal reactions to the design and development of novel radiotracers has the potential to have far-reaching implications in both the laboratory and the clinic. Herein, we report the synthesis and development of a series of modular and versatile building blocks for inverse electron-demand Diels–Alder copper-free click chemistry: tetrazine-functionalized artificial amino acids. Following the development of a novel peptide coupling protocol for peptide synthesis in the presence of tetrazines, we successfully demonstrated its effectiveness and applicability. This versatile methodology has the potential to have a transformational impact, opening the door for the rapid, facile, and modular synthesis of bioorthogonally reactive peptide probes.

The inverse electron demand Diels-Alder click reaction in radiochemistry

The inverse electron-demand Diels-Alder (IEDDA) cycloaddition between 1,2,4,5-tetrazines and strained alkene dienophiles is an emergent variety of catalyst-free 'click' chemistry that has the potential to have a transformational impact on the synthesis and development of radiopharmaceuticals. The ligation is selective, rapid, high-yielding, clean, and bioorthogonal and, since its advent in 2008, has been employed in a wide variety of chemical settings. In radiochemistry, the reaction has proven particularly useful with (18)  F and has already been utilized to create a number of (18)  F-labeled agents, including the PARP1-targeting small molecule (18)  F-AZD2281, the αv β3 integrin-targeting peptide (18)  F-RGD, and the GLP-1-targeting peptide (18)  F-exendin. The inherent flexibility of the ligation has also been applied to the construction of radiometal-based probes, specifically the development of a modular strategy for the synthesis of radioimmunoconjugates that effectively eliminates variability in the construction of these agents. Further, the exceptional speed and biorthogonality of the reaction have made it especially promising in the realm of in vivo pretargeted imaging and therapy, and pretargeted imaging strategies based on the isotopes (111) In, (18)  F, and (64) Cu have already proven capable of producing images with high tumor contrast and low levels of uptake in background, nontarget organs. Ultimately, the characteristics of inverse electron-demand Diels-Alder click chemistry make it almost uniquely well-suited for radiochemistry, and although the field is young, this ligation has the potential to make a tremendous impact on the synthesis, development, and study of novel radiopharmaceuticals.

Bioorthogonal chemistry: implications for pretargeted nuclear (PET/SPECT) imaging and therapy

Due to their rapid and highly selective nature, bioorthogonal chemistry reactions are attracting a significant amount of recent interest in the radiopharmaceutical community. Over the last few years, reactions of this type have found tremendous utility in the construction of new radiopharmaceuticals and as a method of bioconjugation. Furthermore, reports are beginning to emerge in which these reactions are also being applied in vivo to facilitate a novel pretargeting strategy for the imaging and therapy of cancer. The successful implementation of such an approach could lead to dramatic improvements in image quality, therapeutic index, and reduced radiation dose to non-target organs and tissues. This review will focus on the potential of various bioorthogonal chemistry reactions to be used successfully in such an approach.

Efficient acid-catalyzed (18) F/(19) F fluoride exchange of BODIPY dyes

Fluorine-containing fluorochromes are important validation agents for positron emission tomography imaging compounds, as they can be readily validated in cells by fluorescence imaging. In particular, the (18) F-labeled BODIPY-FL fluorophore has emerged as an important platform, but little is known about alternative (18) F-labeling strategies or labeling on red-shifted fluorophores. In this study we explore acid-catalyzed (18) F/(19) F exchange on a range of commercially available N-hydroxysuccinimidyl ester and maleimide BODIPY fluorophores. We show this method to be a simple and efficient (18) F-labeling strategy for a diverse span of fluorescent compounds, including a BODIPY-modified PARP-1 inhibitor, and amine- and thiol-reactive BODIPY fluorophores.

Effect of small-molecule modification on single-cell pharmacokinetics of PARP inhibitors

The heterogeneous delivery of drugs in tumors is an established process contributing to variability in treatment outcome. Despite the general acceptance of variable delivery, the study of the underlying causes is challenging, given the complex tumor microenvironment including intra- and intertumor heterogeneity. The difficulty in studying this distribution is even more significant for small-molecule drugs where radiolabeled compounds or mass spectrometry detection lack the spatial and temporal resolution required to quantify the kinetics of drug distribution in vivo. In this work, we take advantage of the synthesis of fluorescent drug conjugates that retain their target binding but are designed with different physiochemical and thus pharmacokinetic properties. Using these probes, we followed the drug distribution in cell culture and tumor xenografts with temporal resolution of seconds and subcellular spatial resolution. These measurements, including in vivo permeability of small-molecule drugs, can be used directly in predictive pharmacokinetic models for the design of therapeutics and companion imaging agents as demonstrated by a finite element model.

Synthesis, [¹⁸F] radiolabeling, and evaluation of poly (ADP-ribose) polymerase-1 (PARP-1) inhibitors for in vivo imaging of PARP-1 using positron emission tomography

Imaging of poly (ADP-ribose) polymerase-1 (PARP-1) expression in vivo is a potentially powerful tool for developing PARP-1 inhibitors for drug discovery and patient care. We have synthesized several derivatives of benzimidazole carboxamide as PARP-1 inhibitors, which can be (18)F-labeled easily for positron emission tomographic (PET) imaging. Of the compounds synthesized, 12 had the highest inhibition potency for PARP-1 (IC50=6.3 nM). [(18)F]12 was synthesized under conventional conditions in high specific activity with 40-50% decay-corrected yield. MicroPET studies using [(18)F]12 in MDA-MB-436 tumor-bearing mice demonstrated accumulation of [(18)F]12 in the tumor that was blocked by olaparib, suggesting that the uptake of [(18)F]12 in the tumor is specific to PARP-1 expression.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

Expanding room for tetrazine ligations in the in vivo chemistry toolbox

There is tremendous interest in developing and refining methods to predictably perform chemical reactions within the framework of living systems. Here we review recent advances in applying tetrazine cycloadditions to live cell and in vivo chemistry. We highlight new syntheses of the tetrazine and dienophile precursors useful for in vivo studies. We briefly overview the use of this reaction in combination with unnatural amino acid technology and discuss applications involving the imaging of glycans on live cells. An emerging area is the use of tetrazine ligations for the development of in vivo imaging probes such as those used for positron emission tomography. We summarize recent applications involving tetrazine cycloadditions performed in live mice for pretargeted imaging of cancer cell biomarkers.

Bioorthogonal approach to identify unsuspected drug targets in live cells

A proteomics method to pull down secondary drug targets from live cells is described. The drug of interest is modified with trans-cyclooctene (TCO) and incubated with live cells. Upon cell lysis, the modified drug bound to the protein is pulled down using magnetic beads decorated with a cleavable tetrazine-modified linker. Samples are then run on an SDS-PAGE gel and isolated bands are submitted for mass spectrometry analysis to identify drug targets.

Overcoming intratumor heterogeneity of polygenic cancer drug resistance with improved biomarker integration

Improvements in technology and resources are helping to advance our understanding of cancer-initiating events as well as factors involved with tumor progression, adaptation, and evasion of therapy. Tumors are well known to contain diverse cell populations and intratumor heterogeneity affords neoplasms with a diverse set of biologic characteristics that can be used to evolve and adapt. Intratumor heterogeneity has emerged as a major hindrance to improving cancer patient care. Polygenic cancer drug resistance necessitates reconsidering drug designs to include polypharmacology in pursuit of novel combinatorial agents having multitarget activity to overcome the diverse and compensatory signaling pathways in which cancer cells use to survive and evade therapy. Advances will require integration of different biomarkers such as genomics and imaging to provide for more adequate elucidation of the spatially varying location, type, and extent of diverse intratumor signaling molecules to provide for a rationale-based personalized cancer medicine strategy.

Single-cell and subcellular pharmacokinetic imaging allows insight into drug action in vivo

Pharmacokinetic analysis at the organ level provides insight into how drugs distribute throughout the body, but cannot explain how drugs work at the cellular level. Here we demonstrate in vivo single-cell pharmacokinetic imaging of PARP-1 inhibitors and model drug behaviour under varying conditions. We visualize intracellular kinetics of the PARP-1 inhibitor distribution in real time, showing that PARP-1 inhibitors reach their cellular target compartment, the nucleus, within minutes in vivo both in cancer and normal cells in various cancer models. We also use these data to validate predictive finite element modelling. Our theoretical and experimental data indicate that tumour cells are exposed to sufficiently high PARP-1 inhibitor concentrations in vivo and suggest that drug inefficiency is likely related to proteomic heterogeneity or insensitivity of cancer cells to DNA-repair inhibition. This suggests that single-cell pharmacokinetic imaging and derived modelling improve our understanding of drug action at single-cell resolution in vivo.

Single cell analysis of drug distribution by intravital imaging

Recent advances in the field of intravital imaging have for the first time allowed us to conduct pharmacokinetic and pharmacodynamic studies at the single cell level in live animal models. Due to these advances, there is now a critical need for automated analysis of pharmacokinetic data. To address this, we began by surveying common thresholding methods to determine which would be most appropriate for identifying fluorescently labeled drugs in intravital imaging. We then developed a segmentation algorithm that allows semi-automated analysis of pharmacokinetic data at the single cell level. Ultimately, we were able to show that drug concentrations can indeed be extracted from serial intravital imaging in an automated fashion. We believe that the application of this algorithm will be of value to the analysis of intravital microscopy imaging particularly when imaging drug action at the single cell level.

Efficient 18F-Labeling of Synthetic Exendin-4 Analogues for Imaging Beta Cells

A number of exendin derivatives have been developed to target glucagon-like peptide 1 (GLP-1) receptors on beta cells in vivo. Modifications of exendin analogues have been shown to have significant effects on pharmacokinetics and, as such, have been used to develop a variety of therapeutic compounds. Here, we show that an exendin-4, modified at position 12 with a cysteine conjugated to a tetrazine, can be labeled with 18F-trans-cyclooctene and converted into a PET imaging agent at high yields and with good selectivity. The agent accumulates in beta cells in vivo and has sufficiently high accumulation in mouse models of insulinomas to enable in vivo imaging.