ImmunoPET Imaging with 89Zr-Labeled Atezolizumab Enables In Vivo Evaluation of PD-L1 in Tumorgraft Models of Renal Cell Carcinoma.

PURPOSE: Immune checkpoint inhibitors (ICI) targeting the programmed cell death protein 1 and its ligand (PD-1/PD-L1) have transformed the treatment paradigm for metastatic renal cell carcinoma (RCC). However, response rates to ICIs as single agents or in combination vary widely and predictive biomarkers are lacking. Possibly related to the heterogeneity and dynamic nature of PD-L1 expression, tissue-based methods have shown limited value. Immuno-positron emission tomography (immunoPET) may enable noninvasive, comprehensive, and real-time PD-L1 detection. Herein, we systematically examined the performance of immunoPET for PD-L1 detection relative to IHC in an RCC patient-derived tumorgraft (TG) platform. EXPERIMENTAL DESIGN: Eight independent RCC TGs with a wide range of PD-L1 expression (0%-85%) were evaluated by immunoPET. Uptake of 89Zr-labeled atezolizumab ([89Zr]Zr-DFO-ATZ) was compared with PD-L1 expression in tumors by IHC through double-blind analyses. Clinical outcomes of ICI-treated patients whose TGs were examined were analyzed to evaluate the clinical role of immunoPET in RCC. RESULTS: ImmunoPET with [89Zr]Zr-DFO-ATZ (day 6/7 postinjection) revealed a statistically significant association with PD-L1 IHC assays (P = 0.0014; correlation ρXY = 0.78). Furthermore, immunoPET can be used to assess the heterogeneous distribution of PD-L1 expression. Finally, studies in the corresponding patients (n = 4) suggest that PD-L1 signal may influence ICI responsiveness. CONCLUSIONS: ImmunoPET with [89Zr]Zr-DFO-ATZ may enable a thorough and dynamic assessment of PD-L1 across sites of disease. The power of immunoPET to predict ICI response in RCC is being explored in an ongoing clinical trial (NCT04006522).

Selective depletion of radiolabeled HER2-specific antibody for contrast improvement during PET.

The prolonged in vivo persistence of antibodies results in high background and poor contrast during their use as molecular imaging agents for positron emission tomography (PET). We have recently described a class of engineered Fc fusion proteins that selectively deplete antigen-specific antibodies without affecting the levels of antibodies of other specificities. Here, we demonstrate that these Fc fusions (called Seldegs, for selective degradation) can be used to clear circulating, radiolabeled HER2-specific antibody during diagnostic imaging of HER2-positive tumors in mice. The analyses show that Seldegs have considerable promise for the reduction of whole-body exposure to radiolabel and improvement of contrast during PET.

The hexosamine biosynthesis pathway is a targetable liability in KRAS/LKB1 mutant lung cancer.

In non-small-cell lung cancer (NSCLC), concurrent mutations in the oncogene KRAS and the tumour suppressor STK11 (also known as LKB1) encoding the kinase LKB1 result in aggressive tumours prone to metastasis but with liabilities arising from reprogrammed metabolism. We previously demonstrated perturbed nitrogen metabolism and addiction to an unconventional pathway of pyrimidine synthesis in KRAS/LKB1 co-mutant cancer cells. To gain broader insight into metabolic reprogramming in NSCLC, we analysed tumour metabolomes in a series of genetically engineered mouse models with oncogenic KRAS combined with mutations in LKB1 or p53. Metabolomics and gene expression profiling pointed towards activation of the hexosamine biosynthesis pathway (HBP), another nitrogen-related metabolic pathway, in both mouse and human KRAS/LKB1 co-mutant tumours. KRAS/LKB1 co-mutant cells contain high levels of HBP metabolites, higher flux through the HBP pathway and elevated dependence on the HBP enzyme glutamine-fructose-6-phosphate transaminase [isomerizing] 2 (GFPT2). GFPT2 inhibition selectively reduced KRAS/LKB1 co-mutant tumour cell growth in culture, xenografts and genetically modified mice. Our results define a new metabolic vulnerability in KRAS/LKB1 co-mutant tumours and provide a rationale for targeting GFPT2 in this aggressive NSCLC subtype.

Functional imaging of RAS pathway targeting in malignant peripheral nerve sheath tumor cells and xenografts.

BACKGROUND: Malignant peripheral nerve sheath tumor (MPNST) is an aggressive form of soft-tissue sarcoma (STS) in children. Despite intensive therapy, relatively few children with metastatic and unresectable disease survive beyond three years. RAS pathway activation is common in MPNST, suggesting MEK pathway inhibition as a targeted therapy, but the impact on clinical outcome has been small to date. PROCEDURE: We conducted preclinical pharmacokinetic (PK) and pharmacodynamic studies of two MEK inhibitors, trametinib and selumetinib, in two MPNST models and analyzed tumors for intratumor drug levels. We then investigated 3'-deoxy-3'-[18 F]fluorothymidine (18 F-FLT) PET imaging followed by 18 F-FDG PET/CT imaging of MPNST xenografts coupled to short-term or longer-term treatment with selumetinib focusing on PET-based imaging as a biomarker of MEK inhibition. RESULTS: Trametinib decreased pERK expression in MPNST xenografts but did not prolong survival or decrease Ki67 expression. In contrast, selumetinib prolonged survival of animals bearing MPNST xenografts, and this correlated with decreased pERK and Ki67 staining. PK studies revealed a significantly higher fraction of unbound selumetinib within a responsive MPNST xenograft model. Thymidine uptake, assessed by 18 F-FLT PET/CT, positively correlated with Ki67 expression in different xenograft models and in response to selumetinib. CONCLUSION: The ability of MEK inhibitors to control MPNST growth cannot simply be predicted by serum drug levels or drug-induced changes in pERK expression. Tumor cell proliferation assessed by 18 F-FLT PET imaging might be useful as an early response marker to targeted therapies, including MEK inhibition, where a primary effect is cell-cycle arrest.

PET imaging of occult tumours by temporal integration of tumour-acidosis signals from pH-sensitive 64Cu-labelled polymers.

Owing to the diversity of cancer types and the spatiotemporal heterogeneity of tumour signals, high-resolution imaging of occult malignancy is challenging. 18F-fluorodeoxyglucose positron emission tomography allows for near-universal cancer detection, yet in many clinical scenarios it is hampered by false positives. Here, we report a method for the amplification of imaging contrast in tumours via the temporal integration of the imaging signals triggered by tumour acidosis. This method exploits the catastrophic disassembly, at the acidic pH of the tumour milieu, of pH-sensitive positron-emitting neutral copolymer micelles into polycationic polymers, which are then internalized and retained by the cancer cells. Positron emission tomography imaging of the 64Cu-labelled polymers detected small occult tumours (10-20 mm3) in the brain, head, neck and breast of mice at much higher contrast than 18F-fluorodeoxyglucose, 11C-methionine and pH-insensitive 64Cu-labelled nanoparticles. We also show that the pH-sensitive probes reduce false positive detection rates in a mouse model of non-cancerous lipopolysaccharide-induced inflammation. This macromolecular strategy for integrating tumour acidosis should enable improved cancer detection, surveillance and staging.

Tumor Uptake of Triazine Dendrimers Decorated with Four, Sixteen, and Sixty-Four PSMA-Targeted Ligands: Passive versus Active Tumor Targeting.

Various glutamate urea ligands have displayed high affinities to prostate specific membrane antigen (PSMA), which is highly overexpressed in prostate and other cancer sites. The multivalent versions of small PSMA-targeted molecules are known to be even more efficiently bound to the receptor. Here, we employ a well-known urea-based ligand, 2-[3-(1,3-dicarboxypropyl)-ureido] pentanedioic acid (DUPA) and triazine dendrimers in order to study the effect of molecular size on multivalent targeting in prostate cancer. The synthetic route starts with the preparation of a dichlorotriazine bearing DUPA in 67% overall yield over five steps. This dichlorotriazine reacts with G1, G3, and G5 triazine dendrimers bearing a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) group for 64Cu-labeling at the core to afford poly(monochlorotriazine) intermediates. Addition of 4-aminomethylpiperidine (4-AMP) and the following deprotection produce the target compounds, G1-(DUPA)4, G3-(DUPA)16, and G5-(DUPA)64. These targets include 4/16/64 DUPA groups on the surface and a DOTA group at the core, respectively. In vitro cell assay using PC3-PIP (PSMA positive) and PC3-FLU (PSMA negative) cells reveals that G1-(DUPA)4 has the highest PC3-PIP to PC3-FLU uptake ratio (10-fold) through the PSMA-mediated specific uptake. While G5-(DUPA)64 displayed approximately 12 times higher binding affinity (IC50 23.6 nM) to PC3-PIP cells than G1-(DUPA)4 (IC50 282.3 nM) as evaluated in a competitive binding assay, the G5 dendrimer also showed high non-specific binding to PC3-FLU cells. In vivo uptake of the 64Cu-labeled dendrimers was also evaluated in severe combined inmmunodeficient (SCID) mice bearing PC3-PIP and PC3-FLU xenografts on each shoulder, respectively. Interestingly, quantitative imaging analysis of positron emission tomograph (PET) displayed the lowest tumor uptake in PC3-PIP cells for the midsize dendrimer G3-(DUPA)16 (19.4 kDa) (0.66 ± 0.15%ID/g at 1 h. p.i., 0.64 ± 0.11%ID/g at 4 h. p.i., and 0.67 ± 0.08%ID/g at 24 h. p.i.). Through the specific binding of G1-(DUPA)4 to PSMA, the smallest dendrimer (5.1 kDa) demonstrated the highest PC3-PIP to muscle and PC3-PIP to PC3-FLU uptake ratios (17.7 ± 5.5 and 6.7 ± 3.0 at 4 h p.i., respectively). In addition, the enhanced permeability and retention (EPR) effect appeared to be an overwhelming factor for tumor uptake of the largest dendrimer G5-(DUPA)64 as the uptake was at a similar level irrelevant to the PSMA expression.

PD-L1 detection using 89Zr-atezolizumab immuno-PET in renal cell carcinoma tumorgrafts from a patient with favorable nivolumab response.

BACKGROUND: Programmed death-ligand 1 (PD-L1) expression in metastatic renal cell carcinoma (RCC) correlates with a worse prognosis, but whether it also predicts responsiveness to anti-PD-1/PD-L1 therapy remains unclear. Most studies of PD-L1 are limited by evaluation in primary rather than metastatic sites, and in biopsy samples, which may not be representative. These limitations may be overcome with immuno-positron emission tomography (iPET), an emerging tool allowing the detection of cell surface proteins with radiolabeled antibodies. Here, we report iPET studies of PD-L1 in a preclinical tumorgraft model of clear cell RCC (ccRCC) from a patient who had a favorable response to anti-PD-1 therapy. CASE PRESENTATION: A 49-year-old man underwent a cytoreductive nephrectomy in 2017 of a right kidney tumor invading into the adrenal gland that was metastatic to the lungs and a rib. Histological analyses revealed a ccRCC of ISUP grade 4 with extensive sarcomatoid features. IMDC risk group was poor. Within two hours of surgery, a tumor sample was implanted orthotopically into NOD/SCID mice. Consistent with an aggressive tumor, a renal mass was detected 18 days post-implantation. Histologically, the tumorgraft showed sarcomatoid differentiation and high levels of PD-L1, similar to the patient's tumor. PD-L1 was evaluated in subsequently transplanted mice using iPET and the results were compared to control mice implanted with a PD-L1-negative tumor. We labeled atezolizumab, an anti-PD-L1 antibody with a mutant Fc, with zirconium-89. iPET revealed significantly higher 89Zr-atezolizumab uptake in index than control tumorgrafts. The patient was treated with high-dose IL2 initially, and subsequently with pazopanib, with rapidly progressive disease, but had a durable response with nivolumab. CONCLUSIONS: To our knowledge, this is the first report of non-invasive detection of PD-L1 in renal cancer using molecular imaging. This study supports clinical evaluation of iPET to identify RCC patients with tumors deploying the PD-L1 checkpoint pathway who may be most likely to benefit from PD-1/PD-L1 disrupting drugs.

Breast Tumor Microcalcification Induced by Bone Morphogenetic Protein-2: A New Murine Model for Human Breast Tumor Diagnosis.

Widespread use of screening mammography has recently increased the detection of breast microcalcifications. These nonpalpable microcalcifications with specific features in breast tissues are clinically considered an early indicator of breast carcinoma. Our goal in this study was to develop a murine breast microcalcification model for optimizing in vivo imaging. Recombinant human BMP-2 was expressed in E. coli, and the purified bioactive protein was used as inducing factor for the production of breast microcalcifications in a murine animal model. Syngeneic breast tumors were obtained by injection of MDA-MB-231 human breast cancer cells with Matrigel into the mammary fat pad of female nude mice. Different doses of bioactive rhBMP-2 were administered either as single or multiple intraperitoneal injections or directly into tumor on a weekly basis. Three weeks after the first injection of rhBMP-2, the microcalcification of breast tumor was detected by microcomputed tomography followed by intravenous injection of radiotracer [18F] Sodium fluoride for positron emission tomography imaging. Our findings indicate that rhBMP-2 induced microcalcifications of breast tumor by both systemic and direct injection of rhBMP-2 into tumors in a dose-dependent manner. Although little is known about the molecular mechanism of microcalcification, here we report a new murine model of human breast tumor induced microcalcification by rhBMP-2 to optimize in vivo imaging methods and to study the role of BMP-2 as a mediator of pathological mineralization and bone-like microcalcification formation in breast tumor. This BMP-2-induced microcalcification model may allow us to discriminate the type of microcalcification in tumors and to perform quantitative analysis on the calcification as a new detection strategy for early identification of pathological mineralization of breast tissues in women.

PD-L1 on host cells is essential for PD-L1 blockade-mediated tumor regression.

Programmed death-ligand 1 (PD-L1) expression on tumor cells is essential for T cell impairment, and PD-L1 blockade therapy has shown unprecedented durable responses in several clinical studies. Although higher expression of PD-L1 on tumor cells is associated with a better immune response after Ab blockade, some PD-L1-negative patients also respond to this therapy. In the current study, we explored whether PD-L1 on tumor or host cells was essential for anti-PD-L1-mediated therapy in 2 different murine tumor models. Using real-time imaging in whole tumor tissues, we found that anti-PD-L1 Ab accumulates in tumor tissues, regardless of the status of PD-L1 expression on tumor cells. We further observed that, while PD-L1 on tumor cells was largely dispensable for the response to checkpoint blockade, PD-L1 in host myeloid cells was essential for this response. Additionally, PD-L1 signaling in defined antigen-presenting cells (APCs) negatively regulated and inhibited T cell activation. PD-L1 blockade inside tumors was not sufficient to mediate regression, as limiting T cell trafficking reduced the efficacy of the blockade. Together, these findings demonstrate that PD-L1 expressed in APCs, rather than on tumor cells, plays an essential role in checkpoint blockade therapy, providing an insight into the mechanisms of this therapy.

An Adipose Tissue Atlas: An Image-Guided Identification of Human-like BAT and Beige Depots in Rodents.

[18F]Fluorodeoxyglucose-PET/CT (18F-FDG-PET/CT) imaging has been invaluable for visualizing metabolically active adipose tissues in humans with potential anti-diabetic and anti-obesity effects. To explore whether mice display human-like fat depots in anatomically comparable regions, we mapped fat depots using glucose or fatty acid imaging tracers, such as 18F-FDG through PET/CT or [123/125I]-ß-methyl-p-iodophenyl-pentadecanoic acid with SPECT/CT imaging, to analogous depots in mice. Using this type of image analysis with both probes, we define a large number of additional areas of high metabolic activity corresponding to novel fat pads. Histological and gene expression analyses validate these regions as bona fide fat pads. Our findings indicate that fat depots of rodents show a high degree of topological similarity to those of humans. Studies involving both glucose and lipid tracers indicate differential preferences for these substrates in different depots and also suggest that fatty acid-based visualized approaches may reveal additional brown adipose tissue and beige depots in humans.