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PET imaging of programmed cell death ligand 1 (PD-L1) may help to noninvasively predict and monitor responses to anti-programmed cell death 1/anti-PD-L1 immunotherapies. In this study, we compared the imaging characteristics of 3 radioligands derived from the anti-PD-L1 IgG1 complement 4 (C4). In addition to the IgG C4, we produced a fragment antigen-binding (Fab) C4, as well as a double-mutant IgG C4 (H310A/H435Q) with minimal affinity for the murine neonatal Fc receptor. Methods: The pharmacokinetics, biodistribution, and dosimetry of the 3 89Zr-labeled C4 ligands were compared by longitudinal PET/CT imaging in nude mice bearing subcutaneous human non-small cell lung cancer xenografts with positive (H1975 model) or negative (A549 model) endogenous PD-L1 expression. Results: The C4 radioligands substantially accumulated in PD-L1-positive tumors but not in PD-L1-negative tumors or in blocked PD-L1-positive tumors, confirming their PD-L1-specific tumor targeting. 89Zr-Fab C4 and 89Zr-IgG C4 (H310A/H435Q) were rapidly eliminated compared with 89Zr-IgG C4. Consequently, maximal tumor-to-muscle ratios were obtained earlier, at 4 h after injection for 89Zr-Fab C4 (ratio, ∼6) and 24 h after injection for 89Zr-IgG C4 (H310A/H435Q) (ratio, ∼9), versus 48 h after injection for 89Zr-IgG C4 (ratio, ∼8). Background activity in nontumor tissues was low, except for high kidney retention of 89Zr-Fab C4 and persistent liver accumulation of 89Zr-IgG C4 (H310A/H435Q) compared with 89Zr-IgG C4. Dosimetry estimates suggested that the C4 radioligands would yield organ-absorbed doses tolerable for repeated clinical PET imaging studies. Conclusion: This study highlights the potential of designing radioligands with shorter pharmacokinetics for PD-L1 immuno-PET imaging in a preclinical model and encourages further clinical translation of such radioligands.

PurposeThe swell of new and diverse radiotracers to predict or monitor tumor response to cancer immunotherapies invites the opportunity for comparative studies to identify optimal platforms. To probe the significance of antibody format on image quality for PD-L1 imaging, we developed and studied the biodistribution of a library of antibodies based on the anti-PD-L1 IgG1 clone C4.ProcedureA C4 minibody and scFv were cloned, expressed, and characterized. The antibodies were functionalized with desferrioxamine and radiolabeled with Zr-89 to enable a rigorous comparison with prior data collected using 89Zr-labeled C4 IgG1. The biodistribution of the radiotracers was evaluated in C57Bl6/J or nu/nu mice bearing B16F10 or H1975 tumors, respectively, which are models that represent high and low tumor autonomous PD-L1 expression.ResultsThe tumor uptake of the 89Zr-C4 minibody was higher than 89Zr-C4 scFv and equivalent to previous data collected using 89Zr-C4 IgG1. However, the peak tumors to normal tissue ratios were generally higher for 89Zr-C4 scFv compared with 89Zr-C4 minibody and 89Zr-IgG1. Moreover, an exploratory study showed that the rapid clearance of 89Zr-C4 scFv enabled detection of endogenous PD-L1 on a genetically engineered and orthotopic model of hepatocellular carcinoma.ConclusionIn summary, these data support the use of low molecular weight constructs for PD-L1 imaging, especially for tumor types that manifest in abdominal organs that are obstructed by the clearance of high molecular weight radioligands.

Aberrantly high mTORC1 signaling is a known driver of many cancers and human disorders, yet pharmacological inhibition of mTORC1 rarely confers durable clinical responses. To explore alternative therapeutic strategies, herein we conducted a proteomics survey to identify cell surface proteins upregulated by mTORC1. A comparison of the surfaceome from Tsc1-/-versus Tsc1+/+ mouse embryonic fibroblasts revealed 59 proteins predicted to be significantly overexpressed in Tsc1-/- cells. Further validation of the data in multiple mouse and human cell lines showed that mTORC1 signaling most dramatically induced the expression of the proteases neprilysin (NEP/CD10) and aminopeptidase N (APN/CD13). Functional studies showed that constitutive mTORC1 signaling sensitized cells to genetic ablation of NEP and APN, as well as the biochemical inhibition of APN. In summary, these data show that mTORC1 signaling plays a significant role in the constitution of the surfaceome, which in turn may present novel therapeutic strategies.

AGuIX® are sub-5 nm nanoparticles made of a polysiloxane matrix and gadolinium chelates. This nanoparticle has been recently accepted in clinical trials in association with radiotherapy. This review will summarize the principal preclinical results that have led to first in man administration. No evidence of toxicity has been observed during regulatory toxicity tests on two animal species (rodents and monkeys). Biodistributions on different animal models have shown passive uptake in tumours due to enhanced permeability and retention effect combined with renal elimination of the nanoparticles after intravenous administration. High radiosensitizing effect has been observed with different types of irradiations in vitro and in vivo on a large number of cancer types (brain, lung, melanoma, head and neck…). The review concludes with the second generation of AGuIX nanoparticles and the first preliminary results on human.

The swell of experimental imaging technologies to noninvasively measure immune checkpoint protein expression presents the opportunity for rigorous comparative studies toward identifying a gold standard. 89Zr-atezolizumab is currently in man, and early data show tumor targeting but also abundant uptake in several normal tissues. Therefore, we conducted a reverse translational study both to understand if tumor to normal tissue ratios for 89Zr-atezolizumab could be improved and to make direct comparisons to 89Zr-C4, a radiotracer that we showed can detect a large dynamic range of tumor-associated PD-L1 expression. PET/CT and biodistribution studies in tumor bearing immunocompetent and nu/nu mice revealed that high specific activity 89Zr-atezolizumab (∼2 μCi/μg) binds to PD-L1 on tumors but also results in very high uptake in many normal mouse tissues, as expected. Unexpectedly, 89Zr-atezolizumab uptake was generally higher in normal mouse tissues compared to 89Zr-C4 and lower in H1975, a tumor model with modest PD-L1 expression. Also unexpectedly, reducing the specific activity at least 15-fold suppressed 89Zr-atezo uptake in normal mouse tissues but increased tumor uptake to levels observed with high specific activity 89Zr-C4. In summary, these data reveal that low specific activity 89Zr-atezo may be necessary for accurately measuring PD-L1 in the tumor microenvironment, assuming a threshold can be identified that preferentially suppresses binding in normal tissues without reducing binding to tumors with abundant expression. Alternatively, high specific activity approaches like 89Zr-C4 PET may be simpler to implement clinically to measure the broad dynamic range of PD-L1 expression known to manifest among tumors.

Oncogenic lesions up-regulate bioenergetically demanding cellular processes, such as protein synthesis, to drive cancer cell growth and continued proliferation. However, the hijacking of these key processes by oncogenic pathways imposes onerous cell stress that must be mitigated by adaptive responses for cell survival. The mechanism by which these adaptive responses are established, their functional consequences for tumor development, and their implications for therapeutic interventions remain largely unknown. Using murine and humanized models of prostate cancer (PCa), we show that one of the three branches of the unfolded protein response is selectively activated in advanced PCa. This adaptive response activates the phosphorylation of the eukaryotic initiation factor 2-α (P-eIF2α) to reset global protein synthesis to a level that fosters aggressive tumor development and is a marker of poor patient survival upon the acquisition of multiple oncogenic lesions. Using patient-derived xenograft models and an inhibitor of P-eIF2α activity, ISRIB, our data show that targeting this adaptive brake for protein synthesis selectively triggers cytotoxicity against aggressive metastatic PCa, a disease for which presently there is no cure.

While there have been tremendous efforts to target oncogenic RAS signaling from inside the cell, little effort has focused on the cell-surface. Here, we used quantitative surface proteomics to reveal a signature of proteins that are upregulated on cells transformed with KRASG12V, and driven by MAPK pathway signaling. We next generated a toolkit of recombinant antibodies to seven of these RAS-induced proteins. We found that five of these proteins are broadly distributed on cancer cell lines harboring RAS mutations. In parallel, a cell-surface CRISPRi screen identified integrin and Wnt signaling proteins as critical to RAS-transformed cells. We show that antibodies targeting CDCP1, a protein common to our proteomics and CRISPRi datasets, can be leveraged to deliver cytotoxic and immunotherapeutic payloads to RAS-transformed cancer cells and report for RAS signaling status in vivo. Taken together, this work presents a technological platform for attacking RAS from outside the cell.

High sensitivity imaging tools could provide a more holistic view of target antigen expression to improve the identification of patients who might benefit from cancer immunotherapy. We developed for immunoPET a novel recombinant human IgG1 (termed C4) that potently binds an extracellular epitope on human and mouse PD-L1 and radiolabeled the antibody with zirconium-89. Small animal PET/CT studies showed that 89Zr-C4 detected antigen levels on a patient derived xenograft (PDX) established from a non-small-cell lung cancer (NSCLC) patient before an 8-month response to anti-PD-1 and anti-CTLA4 therapy. Importantly, the concentration of antigen is beneath the detection limit of previously developed anti-PD-L1 radiotracers, including radiolabeled atezolizumab. We also show that 89Zr-C4 can specifically detect antigen in human NSCLC and prostate cancer models endogenously expressing a broad range of PD-L1. 89Zr-C4 detects mouse PD-L1 expression changes in immunocompetent mice, suggesting that endogenous PD-1/2 will not confound human imaging. Lastly, we found that 89Zr-C4 could detect acute changes in tumor expression of PD-L1 due to standard of care chemotherapies. In summary, we present evidence that low levels of PD-L1 in clinically relevant cancer models can be imaged with immunoPET using a novel recombinant human antibody.

There is limited knowledge about the metabolic reprogramming induced by cancer therapies and how this contributes to therapeutic resistance. Here we show that although inhibition of PI3K-AKT-mTOR signaling markedly decreased glycolysis and restrained tumor growth, these signaling and metabolic restrictions triggered autophagy, which supplied the metabolites required for the maintenance of mitochondrial respiration and redox homeostasis. Specifically, we found that survival of cancer cells was critically dependent on phospholipase A2 (PLA2) to mobilize lysophospholipids and free fatty acids to sustain fatty acid oxidation and oxidative phosphorylation. Consistent with this, we observed significantly increased lipid droplets, with subsequent mobilization to mitochondria. These changes were abrogated in cells deficient for the essential autophagy gene ATG5 Accordingly, inhibition of PLA2 significantly decreased lipid droplets, decreased oxidative phosphorylation, and increased apoptosis. Together, these results describe how treatment-induced autophagy provides nutrients for cancer cell survival and identifies novel cotreatment strategies to override this survival advantage.

Noninvasive biomarkers that detect the activity of important oncogenic drivers could significantly improve cancer diagnosis and management of treatment. The goal of this study was to determine whether 68Ga-citrate (which avidly binds to circulating transferrin) can detect MYC-positive prostate cancer tumors, as the transferrin receptor is a direct MYC target gene. PET imaging paired with 68Ga-citrate and molecular analysis of preclinical models, human cell-free DNA (cfDNA), and clinical biopsies were conducted to determine whether 68Ga-citrate can detect MYC-positive prostate cancer. Importantly, 68Ga-citrate detected human prostate cancer models in a MYC-dependent fashion. In patients with castration-resistant prostate cancer, analysis of cfDNA revealed that all patients with 68Ga-citrate avid tumors had a gain of at least one MYC copy number. Moreover, biopsy of two PET avid metastases showed molecular or histologic features characteristic of MYC hyperactivity. These data demonstrate that 68Ga-citrate targets prostate cancer tumors with MYC hyperactivity. A larger prospective study is ongoing to demonstrate the specificity of 68Ga-citrate for tumors with hyperactive MYC.Implications: Noninvasive measurement of MYC activity with quantitative imaging modalities could substantially increase our understanding of the role of MYC signaling in clinical settings for which invasive techniques are challenging to implement or do not characterize the biology of all tumors in a patient. Moreover, measuring MYC activity noninvasively opens the opportunity to study changes in MYC signaling in patients under targeted therapeutic conditions thought to indirectly inhibit MYC. Mol Cancer Res; 15(9); 1221-9. ©2017 AACR.

Purpose: mTOR regulates many normal physiological processes and when hyperactive can drive numerous cancers and human diseases. However, it is very challenging to detect and quantify mTOR signaling noninvasively in clinically relevant animal models of disease or man. We hypothesized that a nuclear imaging tool measuring intracellular mTOR activity could address this unmet need.Experimental Design: Although the biochemical activity of mTOR is not directly amenable to nuclear imaging probe development, we show that the transferrin receptor can be used to indirectly measure intracellular changes in mTOR activity.Results: After verifying that the uptake of radiolabeled transferrin (the soluble ligand of the transferrin receptor) is stimulated by active mTORC1 in vitro, we showed that 89Zr-labeled transferrin (Tf) can measure mTORC1 signaling dynamics in normal and cancerous mouse tissues with PET. Finally, we show that 89Zr-Tf can detect the upregulation of mTORC1 by tumor cells to escape the antitumor effects of a standard-of-care antiandrogen, which is to our knowledge the first example of applying PET to interrogate the biology of treatment resistant cancer.Conclusions: In summary, we have developed the first quantitative assay to provide a comprehensive measurement of mTOR signaling dynamics in vivo, in specific normal tissues, and during tumor development in genetically engineered animal models using a nuclear imaging tool that is readily translatable to man. Clin Cancer Res; 23(12); 3045-52. ©2016 AACR.

The purpose of this work was to evaluate the effect of androgen receptor (AR) inhibition on prostate-specific membrane antigen (PSMA) uptake imaged using 68Ga-PSMA-11 PET in a mouse xenograft model and in a patient with castration-sensitive prostate cancer.MethodsWe imaged 3 groups of 4 mice bearing LNCaP-AR xenografts before and 7 d after treatment with ARN-509, orchiectomy, or control vehicle. Additionally, we imaged one patient with castration-sensitive prostate cancer before and 4 wk after treatment with androgen deprivation therapy (ADT). Uptake on pre- and posttreatment imaging was measured and compared.ResultsPSMA uptake increased 1.5- to 2.0-fold in the xenograft mouse model after treatment with both orchiectomy and ARN-509 but not with vehicle. Patient imaging demonstrated a 7-fold increase in PSMA uptake after the initiation of ADT. Thirteen of 22 lesions in the imaged patient were visualized on PSMA PET only after treatment with ADT.ConclusionInhibition of the AR can increase PSMA expression in prostate cancer metastases and increase the number of lesions visualized using PSMA PET. The effect seen in cell and animal models can be recapitulated in humans. A better understanding of the temporal changes in PSMA expression is needed to leverage this effect for both improved diagnosis and improved therapy.

New methodologies for site-specifically radiolabeling proteins with (18)F are required to generate high quality radiotracers for preclinical and clinical applications with positron emission tomography. Herein, we report an approach by which we use lipoic acid ligase (LplA) to conjugate [(18)F]-fluorooctanoic acid to an antibody fragment bearing the peptide substrate of LplA. The mild conditions of the reaction preserve antibody immunoreactivity, and the efficiency of LplA allows for >90% yield even with very small amounts of peptidic precursor (1-10 nmol). These features are advantageous compared to the current gold standard in the field. Moreover, the methodology introduces a new application for an important tool in chemical biology.

Chromatin modifying proteins are attractive drug targets in oncology, given the fundamental reliance of cancer on altered transcriptional activity. Multiple transcription factors can be impacted downstream of primary target inhibition, thus making it challenging to understand the driving mechanism of action of pharmacologic inhibition of chromatin modifying proteins. This in turn makes it difficult to identify biomarkers predictive of response and pharmacodynamic tools to optimize drug dosing. In this report, we show that (89)Zr-transferrin, an imaging tool we developed to measure MYC activity in cancer, can be used to identify cancer models that respond to broad spectrum inhibitors of transcription primarily due to MYC inhibition. As a proof of concept, we studied inhibitors of BET bromodomain containing proteins, as they can impart antitumor effects in a MYC dependent or independent fashion. In vitro, we show that transferrin receptor biology is inhibited in multiple MYC positive models of prostate cancer and double hit lymphoma when MYC biology is impacted. Moreover, we show that bromodomain inhibition in one lymphoma model results in transferrin receptor expression changes large enough to be quantified with (89)Zr-transferrin and positron emission tomography (PET) in vivo. Collectively, these data further underscore the diagnostic utility of the relationship between MYC and transferrin in oncology, and provide the rationale to incorporate transferrin-based PET into early clinical trials with bromodomain inhibitors for the treatment of solid tumors.

Solid tumors are hypoxic with altered metabolism, resulting in secretion of acids into the extracellular matrix and lower relative pH, a feature associated with local invasion and metastasis. Therapeutic and diagnostic agents responsive to this microenvironment may improve tumor-specific delivery. Therefore, we pursued a general strategy whereby caged small-molecule drugs or imaging agents liberate their parent compounds in regions of low interstitial pH. In this manuscript, we present a new acid-labile prodrug method based on the glycosylamine linkage, and its application to a class of positron emission tomography (PET) imaging tracers, termed [(18)F]FDG amines. [(18)F]FDG amines operate via a proposed two-step mechanism, in which an acid-labile precursor decomposes to form the common radiotracer 2-deoxy-2-[(18)F]fluoro-d-glucose, which is subsequently accumulated by glucose avid cells. The rate of decomposition of [(18)F]FDG amines is tunable in a systematic fashion, tracking the pKa of the parent amine. In vivo, a 4-phenylbenzylamine [(18)F]FDG amine congener showed greater relative accumulation in tumors over benign tissue, which could be attenuated upon tumor alkalinization using previously validated models, including sodium bicarbonate treatment, or overexpression of carbonic anhydrase. This new class of PET tracer represents a viable approach for imaging acidic interstitial pH with potential for clinical translation.