Our objective was to exploit a novel ligand-based delivery system for targeting diagnostic and therapeutic agents to cancers that express interleukin 13 receptor alpha 2 (IL13Rα2), a tumor-restricted plasma membrane receptor overexpressed in glioblastoma multiforme (GBM), meningiomas, peripheral nerve sheath tumors, and other peripheral tumors. On the basis of our prior work, we designed a novel IL13Rα2-targeted quadruple mutant of IL13 (TQM13) to selectively bind the tumor-restricted IL13Rα2 with high affinity but not significantly interact with the physiologically abundant IL13Rα1/IL4Rα heterodimer that is also expressed in normal brain. We then assessed the in vitro binding profile of TQM13 and its potential to deliver diagnostic and therapeutic radioactivity in vivo. Surface plasmon resonance (SPR; Biacore) binding experiments demonstrated that TQM13 bound strongly to recombinant IL13Rα2 (Kd∼5 nM). In addition, radiolabeled TQM13 specifically bound IL13Rα2-expressing GBM cells and specimens but not normal brain. Of importance, TQM13 did not functionally activate IL13Rα1/IL4Rα in cells or bind to it in SPR binding assays, in contrast to wtIL13. Furthermore, in vivo targeting of systemically delivered radiolabeled TQM13 to IL13Rα2-expressing subcutaneous tumors was demonstrated and confirmed non-invasively for the first time with 124I-TQM13 positron emission tomography imaging. In addition, 131I-TQM13 demonstrated in vivo efficacy against subcutaneous IL13Rα2-expressing GBM tumors and in an orthotopic synergeic IL13Rα2-positive murine glioma model, as evidenced by statistically significant survival advantage. Our results demonstrate that we have successfully generated an optimized biomarker-targeted scaffolding that exhibited specific binding activity toward the tumor-associated IL13Rα2 in vitro and potential to deliver diagnostic and therapeutic payloads in vivo.
glioblastoma multiforme; interleukin-13 receptor; PET; radioimmunotherapy
There is currently no imaging biomarker for metastatic prostate cancer. The bone scan index (BSI) is a promising candidate, being a reproducible, quantitative expression of tumor burden seen on bone scintigraphy. Prior studies have shown the prognostic value of a baseline BSI. This study tested whether treatment-related changes in BSI are prognostic for survival and compared BSI to prostate-specific antigen (PSA) as an outcome measure.
Patients and Methods
We retrospectively examined serial bone scans from patients with castration-resistant metastatic prostate cancer (CRMPC) enrolled in four clinical trials. We calculated BSI at baseline and at 3 and 6 months on treatment and performed univariate and bivariate analyses of PSA, BSI, and survival.
Eighty-eight patients were scanned, 81 of whom have died. In the univariate analysis, the log percent change in BSI from baseline to 3 and 6 months on treatment prognosticated for survival (hazard ratio [HR], 2.44; P = .0089 and HR, 2.54; P < .001, respectively). A doubling in BSI resulted in a 1.9-fold increase in risk of death. Log percent change in PSA at 6 months on treatment was also associated with survival (HR, 1.298; P = .013). In the bivariate analysis, change in BSI while adjusting for PSA was prognostic at 3 and 6 months on treatment (HR, 2.368; P = .012 and HR, 2.226; P = .002, respectively), but while adjusting for BSI, PSA was not prognostic.
These data furnish early evidence that on-treatment changes in BSI are a response indicator and support further exploration of bone scintigraphy as an imaging biomarker in CRMPC.
The development and evaluation of a computer-aided bone scan analysis technique to quantify changes in tumor burden and assess treatment effects in prostate cancer clinical trials.
We have developed and report on a commercial fully automated computer-aided detection system. Using this system, scan images were intensity normalized, then lesions identified and segmented by anatomic region-specific intensity thresholding. Detected lesions were compared against expert markings to assess the accuracy of the computer-aided detection system. The metrics Bone Scan Lesion Area, Bone Scan Lesion Intensity, and Bone Scan Lesion Count were calculated from identified lesions, and their utility in assessing treatment effects was evaluated by analyzing before and after scans from metastatic castration-resistant prostate cancer patients: 10 treated and 10 untreated. In this study, patients were treated with cabozantinib, a MET/VEGF inhibitor resulting in high rates of resolution of bone scan abnormalities.
Our automated computer-aided detection system identified bone lesion pixels with 94% sensitivity, 89% specificity, and 89% accuracy. Significant differences in changes from baseline were found between treated and untreated groups in all assessed measurements derived by our system. The most significant measure, Bone Scan Lesion Area, showed a median (interquartile range) change from baseline at week 6 of 7.13% (27.61) in the untreated group compared with −73.76% (45.38) in the cabozantinib-treated group (P = 0.0003).
Our system accurately and objectively identified and quantified metastases in bone scans, allowing for interpatient and intrapatient comparison. It demonstrates potential as an objective measurement of treatment effects, laying the foundation for validation against other clinically relevant outcome measures.
computer-assisted detection; bone neoplasms; computer-assisted image processing; bone scan; radionuclide imaging; prostate cancer
Despite intense efforts to develop radiotracers to detect cancers or monitor treatment response, few are widely used due to challenges with demonstrating clear clinical utility. We reasoned that a radiotracer targeting a validated clinical biomarker could more clearly assess the advantages of imaging cancer. The virtues and shortcomings of measuring secreted prostate specific antigen (PSA), an androgen receptor (AR) target gene, in prostate cancer (PCa) patients are well documented, making it a logical candidate for assessing whether a radiotracer can reveal new (and useful) information beyond that conferred by serum PSA. Therefore, we developed 89Zr-5A10, a novel radiotracer that targets “free” PSA. 89Zr-5A10 localizes in an AR-dependent manner in vivo to models of castration resistant prostate cancer, a disease state where serum PSA may not reflect clinical outcomes. Finally, we demonstrate that 89Zr-5A10 can detect osseous PCa lesions, a context where bone scans fail to discriminate malignant and non-malignant signals.
PET; prostate cancer; prostate specific antigen; androgen receptor; biomarker; PSA; PCa; AR; CRPC; 18F-FDG; fPSA; mAbs; PET; MRI; PSMA; ITLC
Dasatinib, a new-generation Src and platelet-derived growth factor receptor (PDGFR) inhibitor, is currently under evaluation in high-grade glioma clinical trials. To achieve optimum physicochemical and/or biologic properties, alternative drug delivery vehicles may be needed. We used a novel fluorinated dasatinib derivative (F-SKI249380), in combination with nanocarrier vehicles and metabolic imaging tools (microPET) to evaluate drug delivery and uptake in a platelet-derived growth factor B (PDGFB)-driven genetically engineered mouse model (GEMM) of high-grade glioma. We assessed dasatinib survival benefit on the basis of measured tumor volumes. Using brain tumor cells derived from PDGFB-driven gliomas, dose-dependent uptake and time-dependent inhibitory effects of F-SKI249380 on biologic activity were investigated and compared with the parent drug. PDGFR receptor status and tumor-specific targeting were non-invasively evaluated in vivo using 18F-SKI249380 and 18F-SKI249380-containing micellar and liposomal nanoformulations. A statistically significant survival benefit was found using dasatinib (95 mg/kg) versus saline vehicle (P < .001) in tumor volume-matched GEMM pairs. Competitive binding and treatment assays revealed comparable biologic properties for F-SKI249380 and the parent drug. In vivo, Significantly higher tumor uptake was observed for 18F-SKI249380-containing micelle formulations [4.9 percentage of the injected dose per gram tissue (%ID/g); P = .002] compared to control values (1.6%ID/g). Saturation studies using excess cold dasatinib showed marked reduction of tumor uptake values to levels in normal brain (1.5%ID/g), consistent with in vivo binding specificity. Using 18F-SKI249380-containing micelles as radiotracers to estimate therapeutic dosing requirements, we calculated intratumoral drug concentrations (24–60 nM) that were comparable to in vitro 50% inhibitory concentration values. 18F-SKI249380 is a PDGFR-selective tracer, which demonstrates improved delivery to PDGFB-driven high-grade gliomas and facilitates treatment planning when coupled with nanoformulations and quantitative PET imaging approaches.
Recent advances in the understanding of castrate-resistant prostate cancer (CRPC) have lead to a growing number of experimental therapies, many of which are directed against the androgen-receptor (AR) signaling axis. These advances generate the need for reliable molecular imaging biomarkers to non-invasively determine efficacy, and to better guide treatment selection of these promising AR-targeted drugs.
We draw on our own experience, supplemented by review of the current literature, to discuss the systematic development of imaging biomarkers for use in the context of CRPC, with a focus on bone scintigraphy, F-18 fluorodeoxyglucose (FDG)-positron emission tomography (PET) and PET imaging of the AR signaling axis.
The roadmap to biomarker development mandates rigorous standardization and analytic validation of an assay before it can be qualified successfully for use in an appropriate clinical context. The Prostate Cancer Working Group 2 (PCWG2) criteria for “radiographic” progression by bone scintigraphy serve as a paradigm of this process. Implemented by the Prostate Cancer Clinical Trials Consortium (PCCTC), these consensus criteria may ultimately enable the co-development of more potent and versatile molecular imaging biomarkers. Purported to be superior to single-photon bone scanning, the added value of Na18F-PET for imaging of bone metastases is still uncertain. FDG-PET already plays an integral role in the management of many diseases, but requires further evaluation before being qualified in the context of CRPC. PET tracers that probe the AR signaling axis, such as 18F-FDHT and 89Zr-591, are now under development as pharmacodynamic markers, and as markers of efficacy, in tandem with FDG-PET. Semi-automated analysis programs for facilitating PET interpretation may serve as a valuable tool to help navigate the biomarker roadmap.
Molecular imaging strategies, particularly those that probe the AR signaling axis, have the potential to accelerate drug development in CRPC. The development and use of analytically valid imaging biomarkers will increase the likelihood of clinical qualification, and ultimately lead to improved patient outcomes.
In contrast to normal cells, cancer cells avidly take up glucose and metabolize it to lactate even when oxygen is abundant, a phenomenon referred to as the Warburg effect. This fundamental alteration in glucose metabolism in cancer cells enables their specific detection by Positron Emission Tomography (PET) following intravenous injection of the glucose analogue 18F-fluorodeoxy-glucose (18FDG). However, this useful imaging technique is limited by the fact that not all cancers avidly take up FDG. To identify molecular determinants of 18FDG-retention, we interogated the transcriptomes of human cancer cell lines and primary tumors for metabolic pathways associated with 18FDG radiotracer uptake. From 95 metabolic pathways that were interrogated, the glycolysis and several glycolysis-related pathways (pentose-phosphate, carbon fixation, aminoacyl-tRNA biosynthesis, one-carbon-pool by folate) showed the greatest transcriptional enrichment. This “FDG signature” predicted FDG-uptake in breast cancer cell lines and overlapped with established gene expression signatures for the “basal-like” breast cancer subtype and MYC-induced tumorigenesis in mice. Human breast cancers with nuclear MYC staining and high RNA expression of MYC target genes showed high 18FDG-PET uptake (p < 0.005). Presence of the FDG signature was similarly associated with MYC gene copy gain, increased MYC transcript levels, and elevated expression of metabolic MYC target genes in a human breast cancer genomic dataset. Together, our findings link clinical observations of glucose uptake with a pathologic and molecular subtype of human breast cancer. Further, they suggest related approaches to derive molecular determinants of radiotracer retention for other PET-imaging probes.
FDG-PET; breast cancer; MYC; basal-like; metabolism; imaging
We propose a standardized approach to quantitative molecular imaging (MI) in cancer patients with multiple lesions.
Twenty castration-resistant-prostate-cancer patients underwent 18F-FDG and 18F-16β-fluoro-5α-dihydrotestosterone (18F-FDHT) PET/CT scans. Using a 5-point confidence-scale, two readers interpreted co-registered scan-sets on a PET-VCAR (General Electric) workstation. 203 sites/scan (specified in a lexicon) were reviewed. 18F-FDG-positive lesion bookmarks were propagated onto 18F-FDHT studies, then manually accepted or rejected. Discordant-positive 18F-FDHT lesions were similarly bookmarked. Lesional SUVmax was recorded. Tracer and tissue-specific background correction-factors were calculated via receiver-operating-characteristic analysis of 65 scan-sets.
Readers agreed on >99% of 18F-FDG and 18F-FDHT negative-sites. Positive-site agreement was 84% and 85%, respectively. Consensus-lesion SUVmax was highly reproducible (CCC>0.98). Receiver-operating-characteristic curves yielded four correction-factors (SUVmax 1.8-2.6). A novel scatter (“LFG”) plot depicted tumor burden and ΔSUVmax for response assessments.
Multi-lesion MI is optimized with a five-step approach incorporating a confidence scale, site lexicon, semi-automated PET software, background-correction and LFG-graphing.
Molecular imaging; PET/CT; 18F-FDG; 18F-FDHT; semi-automated
To compare the diagnostic and prognostic value of FDG PET and bone scans (BS) in the assessment of osseous lesions in patients with progressing prostate cancer.
In a prospective imaging trial, 43 patients underwent FDG PET and BS prior to experimental therapies. Bone scan index (BSI) and standardized uptake value (SUV) on FDG PET were recorded. Patients were followed until death (n=36) or at least 5 years (n=7). Imaging findings were correlated with survival.
Osseous lesions were detected in 39 patients on BS and 32 on FDG PET (p=0.01). Follow-up was available for 105 FDG-positive lesions, and 84 (80%) became positive on subsequent BS. Prognosis correlated inversely with SUV (median survival 14.4 vs. 32.8 mos if SUVmax > 6.10 vs. ≤ 6.10, p=0.002) and BSI (14.7 vs. 28.2 mos if BSI >1.27 vs. < 1.27; p=0.004). Only SUV was an independent factor in multivariate analysis. In castrate resistant patients combining a nomogram for progressive prostate cancer with SUV dichotomized patients into a high vs. low risk group (median survival 14.4 vs. 34.6 mos, p=.015) more prognostic than either nomogram or SUV alone.
The current study of progressive prostate cancer confirms earlier work that BSI is a strong prognostic factor. Most FDG-only lesions at baseline become detectable on follow-up BS, suggesting their strong clinical relevance. FDG SUV is an independent prognostic factor and provides complementary prognostic information.
prostate cancer; positron emission tomography; FDG; bone scan; prognosis
The primary aim of this analysis was to examine the quantitative features of antibody–antigen interactions in tumors and normal tissue after parenteral administration of antitumor antibodies to human patients.
Humanized anti-A33 antibody (10 mg) labeled with the positron-emitting radionuclide 124I (124I-huA33) was injected intravenously in 15 patients with colorectal cancer. Clinical PET/CT was performed approximately 1 wk later, followed by a detailed assay of surgically removed tissue specimens including radioactivity counting, autoradiography, immunohistochemistry, and antigen density determination.
PET/CT showed high levels of antibody targeting in tumors and normal bowel. In tissue specimens, the spatial distribution of 124I-huA33 conformed to that of A33 antigen, and there was a linear relationship between the amount of bound antibody and antigen concentration. Antibody uptake was high in 1- to 2-mm regions of antigen-positive tumor cells (mean, ~0.05 percentage injected dose per gram) and in antigen-positive normal colonic mucosa (mean, ~0.03 percentage injected dose per gram). The estimated binding site occupancy for tumor and normal colon was 20%–50%.
The in vivo bio-distribution of 124I-huA33 in human patients 1 wk after antibody administration was determined by A33 antigen expression. Our data imply that the optimal strategy for A33-based radioimmunotherapy of colon cancer will consist of a multistep treatment using a radionuclide with short-range (α- or β-particle) emissions.
huA33; colorectal cancer; 124I; immuno-PET; radioimmunotherapy
Bone marrow is usually dose-limiting for radioimmunotherapy. In this study, we directly estimated red marrow activity concentration and the self-dose component of absorbed radiation dose to red marrow based on PET/CT of 2 different 124I-labeled antibodies (cG250 and huA33) and compared the results with plasma activity concentration and plasma-based dose estimates.
Two groups of patients injected with 124I-labeled monoclonal antibodies (11 patients with renal cancer receiving 124I-cG250 and 5 patients with colorectal cancer receiving 124I- huA33) were imaged by PET or PET/CT on 2 or 3 occasions after infusion. Regions of interest were drawn over several lumbar vertebrae, and red marrow activity concentration was quantified. Plasma activity concentration was also quantified using multiple patient blood samples. The red marrow–to–plasma activity concentration ratio (RMPR) was calculated at the times of imaging. The self-dose component of the absorbed radiation dose to the red marrow was estimated from the images, from the plasma measurements, and using a combination of both sets of measurements.
RMPR was observed to increase with time for both groups of patients. Mean (±SD) time-dependent RMPR (RMPR(t)) for the cG250 group increased from 0.13 ± 0.06 immediately after infusion to 0.23 ± 0.09 at approximately 6 d after infusion. For the huA33 group, mean RMPR(t) was 0.10 ± 0.04 immediately after infusion, 0.13 ± 0.05 approximately 2 d after infusion, and 0.20 ± 0.09 approximately 7 d after infusion. Plasma-based estimates of red marrow self-dose tended to be greater than image-based values by, on average, 11% and 47% for cG250 and huA33, respectively, but by as much as −73% to 62% for individual patients. The hybrid method combining RMPR(t) and plasma activity concentration provided a closer match to the image-based dose estimates (average discrepancies, −2% and 18% for cG250 and huA33, respectively).
These results suggest that the assumption of time-independent proportionality between red marrow and plasma activity concentration may be too simplistic. Individualized imaged-based dosimetry is probably required for the optimal therapeutic delivery of radiolabeled antibodies, which does not compromise red marrow and may allow, for some patients, a substantial increase in administered activity and thus tumor dose.
dosimetry; marrow; PET
Advanced human thyroid cancers, particularly those that are refractory to treatment with radioiodine (RAI), have a high prevalence of BRAF (v-raf murine sarcoma viral oncogene homolog B1) mutations. However, the degree to which these cancers are dependent on BRAF expression is still unclear. To address this question, we generated mice expressing one of the most commonly detected BRAF mutations in human papillary thyroid carcinomas (BRAFV600E) in thyroid follicular cells in a doxycycline-inducible (dox-inducible) manner. Upon dox induction of BRAFV600E, the mice developed highly penetrant and poorly differentiated thyroid tumors. Discontinuation of dox extinguished BRAFV600E expression and reestablished thyroid follicular architecture and normal thyroid histology. Switching on BRAFV600E rapidly induced hypothyroidism and virtually abolished thyroid-specific gene expression and RAI incorporation, all of which were restored to near basal levels upon discontinuation of dox. Treatment of mice with these cancers with small molecule inhibitors of either MEK or mutant BRAF reduced their proliferative index and partially restored thyroid-specific gene expression. Strikingly, treatment with the MAPK pathway inhibitors rendered the tumor cells susceptible to a therapeutic dose of RAI. Our data show that thyroid tumors carrying BRAFV600E mutations are exquisitely dependent on the oncoprotein for viability and that genetic or pharmacological inhibition of its expression or activity is associated with tumor regression and restoration of RAI uptake in vivo in mice. These findings have potentially significant clinical ramifications.
Lintuzumab (HuM195), a humanized anti-CD33 antibody, targets myeloid leukemia cells and has modest single-agent activity against acute myeloid leukemia (AML). To increase the antibody’s potency without the nonspecific cytotoxicity associated with β-emitters, the α particle-emitting radionuclide bismuth-213 (213Bi) was conjugated to lintuzumab. This phase I/II trial was conducted to determine the maximum tolerated dose (MTD) and antileukemic effects of 213Bi-lintuzumab, the first targeted α-emitter, after partially cytoreductive chemotherapy.
Thirty-one patients with newly diagnosed (n = 13) or relapsed/refractory (n = 18) AML (median age, 67 years; range, 37–80) were treated with cytarabine 200 mg/m2/day for 5 days followed by 213Bi-lintuzumab 18.5–46.25 MBq/kg.
The MTD of 213Bi-lintuzumab was 37 MB/kg; myelosuppression lasting > 35 days was dose-limiting. Extramedullary toxicities were primarily limited to ≤ grade 2 events, including infusion-related reactions. Transient grade 3/4 liver function abnormalities were seen in 5 patients (16%). Treatment-related deaths occurred in 2 of 21 patients (10%) who received the MTD. Significant reductions in marrow blasts were seen at all dose levels. The median response duration was 6 months (range, 2–12). Biodistribution and pharmacokinetic studies suggested that saturation of available CD33 sites by 213Bi-lintuzumab was achieved after partial cytoreduction with cytarabine.
Sequential administration of cytarabine and 213Bi-lintuzumab is tolerable and can produce remissions in patients with AML.
Radioimmunotherapy; Alpha-particles; Acute myeloid leukemia; Monoclonal Antibodies; Bismuth-213
We compared 68Ga-DOTA-F(ab′)2-herceptin (DOTA is 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid [HER2 PET]) and 18F-FDG PET for imaging of tumor response to the heat shock protein 90 (Hsp90) inhibitor 17-allylamino-17-demethoxygeldanamycin (17AAG).
Mice bearing BT474 breast tumor xenografts were scanned with 18F-FDG PET and HER2 PET before and after 17AAG treatment and then biweekly for up to 3 wk.
Within 24 h after treatment, a significant decrease in HER2 was measured by HER2 PET, whereas 18F-FDG PET uptake, a measure of glycolysis, was unchanged. Marked growth inhibition occurred in treated tumors but became evident only by 11 d after treatment. Thus, Her2 downregulation occurs independently of changes in glycolysis after 17AAG therapy, and Her2 reduction more accurately predicts subsequent tumor growth inhibition.
HER2 PET is an earlier predictor of tumor response to 17AAG therapy than 18F-FDG PET.
Although the accurate detection of osseous metastases in the evaluation of patients with suspected metastatic breast cancer (MBC) has significant prognostic and therapeutic implications, the ideal diagnostic approach is uncertain. In this retrospective, single-institution study, we compare the diagnostic performance of integrated positron emission tomography/computed tomography (PET/CT) and bone scintigraphy (BSc) in women with suspected MBC.
Patients and Methods
Women with suspected MBC evaluated with PET/CT and BSc (within 30 days) between January 1, 2003 and June 30, 2008, were identified through institutional databases. Electronic medical records were reviewed, and radiology reports were classified as positive/negative/equivocal for osseous metastases. A nuclear medicine radiologist (blinded to correlative and clinical end points) reviewed all equivocal PET/CT and BSc images and reclassified some reports. Final PET/CT and BSc classifications were compared. Baseline patient/tumor characteristics and bone pathology were recorded and compared to the final imaging results.
We identified 163 women who had a median age of 52 years (range, 30 to 90 years); 32% had locally advanced breast cancer, 42% had been diagnosed with breast cancer less than 12 weeks before identification. Twenty studies were originally deemed equivocal (five with PET/CT, and 15 with BSc), and 13 (65%) of these studies were reclassified after radiology review. Overall, PET/CT and BSc were highly concordant for reporting osseous metastases with 132 paired studies (81%); 32 (20%) were positive, and 100 (61%) were negative. Thirty-one occurrences (19%) were discordant. Twelve of these (39%) had pathology confirming osseous metastases: nine (of 18) were PET/CT positive and BSc negative; one (of three) was PET/CT positive and BSc equivocal; and two (of two) were PET/CT equivocal and BSc negative.
This study supports the use of PET/CT in detecting osseous metastases for suspected MBC. Whether PET/CT may supplant BSc in this setting is unknown.
Nanoparticle-based materials, such as drug delivery vehicles and diagnostic probes, currently under evaluation in oncology clinical trials are largely not tumor selective. To be clinically successful, the next generation of nanoparticle agents should be tumor selective, nontoxic, and exhibit favorable targeting and clearance profiles. Developing probes meeting these criteria is challenging, requiring comprehensive in vivo evaluations. Here, we describe our full characterization of an approximately 7-nm diameter multimodal silica nanoparticle, exhibiting what we believe to be a unique combination of structural, optical, and biological properties. This ultrasmall cancer-selective silica particle was recently approved for a first-in-human clinical trial. Optimized for efficient renal clearance, it concurrently achieved specific tumor targeting. Dye-encapsulating particles, surface functionalized with cyclic arginine–glycine–aspartic acid peptide ligands and radioiodine, exhibited high-affinity/avidity binding, favorable tumor-to-blood residence time ratios, and enhanced tumor-selective accumulation in αvβ3 integrin–expressing melanoma xenografts in mice. Further, the sensitive, real-time detection and imaging of lymphatic drainage patterns, particle clearance rates, nodal metastases, and differential tumor burden in a large-animal model of melanoma highlighted the distinct potential advantage of this multimodal platform for staging metastatic disease in the clinical setting.
Zirconium-89 (t1/2 = 3.27 days) is a positron emitting radionuclide which displays excellent potential for use in the design and synthesis of radioimmunoconjugates for immunoPET. In these studies we report the preparation of 89Zr-DFO-J591, a novel 89Zr -labeled monoclonal antibody (mAb) construct for targeted immunoPET imaging and quantification of prostate-specific membrane antigen (PSMA) expression in vivo.
The in vivo behavior of [89Zr]Zr-chloride, [89Zr]Zr-oxalate and [89Zr]Zr-DFO was investigated by using PET imaging. High level computational studies using density functional theory (DFT) calculations have been used to investigate the electronic structure of [89Zr]Zr-DFO and probe the nature of the complex in aqueous conditions. J591 was functionalized with the hexadentate, tris-hydroxamate ligand desferrioxamine B (DFO) and radiolabeled with [89Zr]Zr-oxalate at room temperature. ImmunoPET imaging experiments in male, athymic nu/nu mice bearing sub-cutaneous LNCaP (PSMA positive) or PC-3 (PSMA negative) tumors were conducted. The change in 89Zr-DFO-J591 tissue uptake in response to high- and low-specific-activity formulations in the two tumor models was measured by using acute biodistribution studies and immunoPET.
Basic characterization of three important reagents, [89Zr]Zr-chloride and [89Zr]Zr-oxalate, as well as the complex, [89Zr]Zr-DFO, demonstrated that the nature of the 89Zr species has a dramatic effect on the biodistribution and pharmacokinetics. DFT calculations provide a rationale for the observed high in vivo stability of 89Zr-DFO-labeled mAbs and suggest that in aqueous conditions, [89Zr]Zr-DFO forms a thermodynamically stable, 8-coordinate complex by coordination of two water molecules. 89Zr-DFO-J591 was produced in high radiochemical yield (>77%) with radiochemical purity >99% and a specific-activity of 181.7±1.1 MBq/mg (4.91±0.03 mCi/mg). In vitro assays demonstrated that 89Zr-DFO-J591 had an initial immunoreactive fraction of 0.95±0.03 and remains active for up to 7 days. In vivo biodistribution experiments revealed high uptake of 89Zr-DFO-J591 in LNCaP tumors after 24, 48, 96 and 144 h (34.4±3.2 %ID/g; 38.0±6.2 %ID/g; 40.4±4.8 %ID/g; and 45.8±3.2 %ID/g, respectively). Specificity for PSMA expression was confirmed by biodistribution studies in PC-3 (PSMA negative) tumor models and by using low specific activity competitive inhibition studies. ImmunoPET studies also demonstrated that 89Zr-DFO-J591 provides excellent image contrast with tumor-to-muscle ratios >20 for delineation of LNCaP tumors between 48 – 144 h post-administration.
These experimental and computational studies demonstrate that 89Zr-DFO-labeled mAbs show exceptional promise as radiotracers for immunoPET imaging of human cancers. 89Zr-DFO-J591 displays tumor-to-background tissue contrast in immunoPET and can be used to delineate and quantify PSMA-positive prostate tumors in vivo.
ImmunoPET; zirconium-89; prostate-specific membrane antigen (PSMA); J591; monoclonal antibodies; density functional theory
3′-Deoxy-3′-18F-fluorothymidine (18F-FLT), a partially metabolized thymidine analog, has been used in preclinical and clinical settings for the diagnostic evaluation and therapeutic monitoring of tumor proliferation status. We investigated the use of 18F-FLT for detecting and characterizing genetically engineered mouse (GEM) high-grade gliomas and evaluating the pharmacokinetics in GEM gliomas and normal brain tissue. Our goal was to develop a robust and reproducible method of kinetic analysis for the quantitative evaluation of tumor proliferation.
Dynamic 18F-FLT PET imaging was performed for 60 min in glioma-bearing mice (n = 10) and in non–tumor-bearing control mice (n = 4) by use of a dedicated small-animal PET scanner. A 3-compartment, 4-parameter model was used to characterize 18F-FLT kinetics in vivo. For compartmental analysis, the arterial input was measured by placing a region of interest over the left ventricular blood pool and was corrected for partial-volume averaging. The 18F-FLT “trapping” and tissue flux model parameters were correlated with measured uptake (percentage injected dose per gram [%ID/g]) values at 60 min.
18F-FLT uptake values (%ID/g) at 1 h in brain tumors were significantly greater than those in control brains (mean ± SD: 4.33 ± 0.58 and 0.86 ± 0.22, respectively; P < 0.0004). Kinetic analyses of the measured time–activity curves yielded independent, robust estimates of tracer transport and metabolism, with compartmental model–derived time–activity data closely fitting the measured data. Except for tracer transport, statistically significant differences were found between the applicable model parameters for tumors and normal brains. The tracer retention rate constant strongly correlated with measured 18F-FLT uptake values (r = 0.85, P < 0.0025), whereas a more moderate correlation was found between net 18F-FLT flux and 18F-FLT uptake values (r = 0.61, P < 0.02).
A clinically relevant mouse glioma model was characterized by both static and dynamic small-animal PET imaging of 18F-FLT uptake. Time–activity curves were kinetically modeled to distinguish early transport from a subsequent tracer retention phase. Estimated 18F-FLT rate constants correlated positively with %ID/g measurements. Dynamic evaluation of 18F-FLT uptake offers a promising approach for noninvasively assessing cellular proliferation in vivo and for quantitatively monitoring new antiproliferation therapies.
18F-FLT; proliferation; brain tumors
To eliminate the variable of tumor heterogeneity from a novel in vivo model of tumor angiogenesis.
We developed a method to navigate tumor neovasculature in a living tissue microenvironment, enabling relocation of a cell- or microregion-of-interest, for serial in vivo imaging. Orthotopic melanoma was grown, in immunocompetent Tie2GFP mice. Intravital multiphoton fluorescence & confocal reflectance imaging was performed, on a custom microscope with motorized stage & coordinate navigation software. A point within a Tie2GFP+ microvessel was selected for relocation. Custom software predicted target coordinates based upon reference points (tissue-embedded polystyrene beads) & baseline target coordinates. Mice were removed from the stage to make previously-obtained target coordinates invalid in subsequent imaging.
Coordinate predictions always relocated target points, in vivo, to within 10-200 μm (within a single 40× field-of-view). The model system provided a virtual living histology of tumor neovascularization & microenvironment, with subcellular spatial resolution & hemodynamic information.
The navigation procedure, termed in vivo microcartography, permits control of tissue heterogeneity, as a variable. Tie2 may be the best reporter gene identified, to-date, for intravital microscopy of tumor angiogenesis. This novel model system should strengthen intravital microscopy in its historical role as a vital tool in oncology, angiogenesis research, and angiotherapeutic drug development.
We developed a novel approach to bioluminescent T cell imaging (BLI) using a membrane-anchored form of the Gaussia luciferase (GLuc) enzyme, termed extGLuc, which we could stably express in both mouse and human primary T cells. In vitro, extGLuc+ cells emitted significantly higher bioluminescent signal when compared to cells expressing GLuc, Renilla luciferase (RLuc), and membrane-anchored RLuc (extRLuc). In vivo, mouse extGLuc+ T cells exhibited higher bioluminescent signal when compared to GLuc+ and RLuc+ T cells. Application of this imaging approach to human T cells genetically modified to express tumor-specific chimeric antigen receptors (CARs) enabled us to demonstrate in vivo CAR-mediated T cell accumulation in tumor, T cell persistence over time, and concomitant imaging of T cells and tumor cells modified to express firefly luciferase (FFLuc). This sensitive imaging technology has application to many in vivo cell based studies in a wide array of mouse models.
J591, a monoclonal antibody that targets the external domain of the prostate specific membrane antigen (PSMA), has potential as an agent for radioimmunotherapy. A pilot trial was carried out in patients with prostate cancer using repetitive administrations of escalating masses of J591. An analysis was carried out to assess (1) lesion detectability by 111InJ591 gamma camera imaging compared to standard imaging methods and (2) the effect of increasing antibody mass on lesion detectability, biodistribution and dosimetry.
Fourteen patients with metastatic prostate cancer received escalating amounts (10, 25, 50 and 100 mg) of J591 in a series of administrations each separated by 3 weeks. All antibody administrations included a fixed amount of radiolabeled antibody 111In-DOTA-J591 (2mg of J591 labeled with 185MBq (5 mCi) of 111In via the chelating agent DOTA). Three whole body gamma camera scans with at least one SPECT scan together with multiple whole body count-rate measurements and serum activity concentration measurements were obtained in all patients. Images were analyzed for distribution and lesion targeting. Estimates of clearance rates and liver and lesion uptake were made for each treatment cycle. These estimates were used to generate dosimetric projections for radioimmunotherapy with 90Y-labeled J591.
A total of 80 lesions in 14 patients were detected. Both skeletal and soft tissue disease was targeted by the antibody as seen on 111In-J591 scans. Antibody localized to 93.7% of skeletal lesions detected by conventional imaging. Clearance of radioactivity from whole body, serum and liver was dependent on antibody mass. Normalized average values of the ratio of residence times between lesion and liver for 10, 25, 50 and 100mg of antibody were 1.0, 1.9, 3.2 and 4.0 respectively.
Dosimetric projections for radioimmunotherapy with 90Y-labeled J591 suggested similar absorbed doses to lesions, for treatment at the maximally tolerated activity (MTA), irrespective of antibody mass. However absorbed doses to liver at MTA would be antibody mass-dependent with estimates of 20, 10, 7 and 5 Gy for 10, 25, 50 and 100mg of J591respectively.
The proportion of the amount of antibody increased in lesions and decreased in the liver with increasing mass of administered antibody up to a dose of 50 mg. Proportional hepatic uptake continued to decrease with increasing antibody mass up to 100 mg. The optimal antibody mass for radioimmunotherapy would therefore appear to be greater than or equal to 50mg.
J591 antibody; prostate cancer; radioimmunotherapy; dosimetry
Positron emission tomography (PET) identifies cancer deposits by detecting sites of gamma emissions that are released from radioactively labeled molecules targeting tumor to formulate a PET image. Correlating preoperative PET scans with intraoperative findings remains a challenge. We investigated whether high-energy gamma emissions detected by a novel hand-held PET probe would detect tumors and offer a real-time method to localize tumor intraoperatively. Furthermore, we investigated the novel beta probe, which detects emissions at a shorter range than gamma emissions, making them undetectable by PET scanners, but potentially valuable for close range intraoperative detection of tumor deposits.
Six-to-eight-week-old athymic mice were injected with one of four possible tumor cell lines: gastric, pancreas, squamous cell and breast cancer. After tumors reached at least 1 cm in size, they were euthanized and imaged with a micro-PET imager. Hand-held gamma and beta probes were then used in vivo and ex vivo to measure high-energy gamma and beta emissions.
The portable PET probes detected high-energy gamma and beta emissions from all tumors evaluated. These emissions were reproducible and we established that beta emissions correlate with high-energy gamma emissions and conventional PET scans. There was a strong positive correlation (R = 0.8) between gamma and beta counts. Beta emission showed a stronger correlation than gamma emission with overall tissue radioactivity.
This study is the first to demonstrate that gamma emission detected by conventional PET imaging correlates with beta emissions. This study shows that compared to detection of gamma emissions, beta counts may offer superior real-time localization of tumor deposits. Intraoperative portable PET probe may become a useful way to exploit tumor biology and PET technology to guide real-time tissue characterization during surgery.
Standard imaging studies are limited as outcome measures for patients with metastatic prostate cancer. We tested the hypothesis that serial fluorodeoxyglucose positron emission tomography (FDG-PET) scans can serve as an outcome measure for patients with castrate metastatic prostate cancer treated with antimicrotubule chemotherapy.
FDG-PET scans were done at baseline, 4, and12 weeks of treatment. The average maximum standardized uptake value (SUVmaxavg) was measured in up to five lesions and was tested as the quantitative outcome measure. Prostate-specific antigen (PSA) at 4 weeks and PSA, bone scan, and soft tissue imaging at 12 weeks were considered standard outcome measures. The change in SUVmaxavg that distinguished clinically assessed progression from non-progression was sought.
Twenty-two PET scans were reviewed and compared with PSA at 4 weeks; 18 PETs were compared at12 weeks with standard outcome measures. Applying the PSA Working Group Consensus Criteria guideline that a 25% PSA increase constitutes progression to the SUVmaxavg, PET correctly identified the clinical status of 20 of 22 patients (91%) at 4 weeks and 17 of 18 patients at 12 weeks (94%). The accuracy of PET could be further optimized if a >33% increase in PSA and SUVmaxavg were used to define progression.
FDG-PET is promising as an outcome measure in prostate cancer. As a single modality, it can show treatment effects that are usually described by a combination of PSA, bone scintigraphy, and soft tissue imaging. Preliminarily, a >33% increase in SUVmaxavg or the appearance of a new lesion optimally dichotomizes patients as progressors or nonprogressors.