Targeting the internal epitope of prostate-specific membrane antigen with 89Zr-7E11 immuno-PET

Intratumoral distribution and pharmacokinetics of the radiolabeled ICAM-1 targeting monoclonal antibody R6.5 in a prostate cancer mouse model

Despite new therapies, castration-resistant prostate cancer (CRPC) is still incurable. Intercellular Adhesion Molecule 1 (ICAM-1) is a well-characterized cell surface protein involved in prostate cancer pathogenesis, differentially expressed during transition from hormone-sensitive to CRPC. This study aimed to investigate ICAM-1 as a target for imaging and radioimmunotherapy of CRPC.Anti-ICAM-1 antibody R6.5 was labeled with 111In or 177Lu, and a non-specific antibody with 177Lu. In vitro uptake of R6.5 was tested in PC-3 prostate cancer cells. Biodistribution studies, SPECT/CT imaging, and autoradiography were performed in a PC-3 xenograft model.In vitro uptake of R6.5 ([177Lu]Lu-R6.5) increased during 6 h of incubation. The uptake was higher at lower mAb concentration and could be blocked by 500 nM of unlabeled R6.5. In vivo and ex vivo biodistribution showed that [111In]In-R6.5 and [177Lu]Lu-R6.5 targeted the xenograft tumors better than the control Ab, however [111In]In-R6.5 had better tumor uptake than [177Lu]Lu-R6.5, probably due to less aggressive conjugation with chelator and smaller tumor sizes. From 24 h post-injection, the tumors in mice injected with [111In]In-R6.5 and [177Lu]Lu-R6.5 were visible on SPECT, optimal contrast at 48 h. Uptake was low in normal organs except the spleen and liver for all mAbs. Autoradiography showed [111In]In-R6.5 and [177Lu]Lu-R6.5 accumulated along the edges of viable tumor. The control Ab tended to accumulate in partly necrotic areas.This study demonstrates ICAM-1 as a potential target for theragnostics in CRPC.

Isopeptidic Desferrioxamine Analogues: The Role of Hydroxamate Spacing for Chelation of Zr4

[89Zr]Zr4+ is a radionuclide of increasing clinical relevance for PET (positron emission tomography). However, an ideal chelator for stable Zr-chelation remains to be discovered. This study describes the solid-phase synthesis of octadentate Zr-chelators based on an isopeptidic (ip) scaffold derived from the natural siderophore desferrioxamine (DFOB). Several analogues with different spacers separating the chelating hydroxamates have been prepared and converted to [89Zr]Zr-complexes. The stability of these complexes was evaluated in human serum and in competition to excess of competing chelators. The assays revealed a beneficial effect of long hydroxamate spacing (9 atoms). Shorter spacing led to a decrease in complex stability. The most stable [89Zr]Zr-ipDFO complex had a high stability in challenging competition experiments with a large excess of EDTA for 72 h as determined by radio TLC and LC/MS. The straightforward synthesis, high complex stability and a modular character make ipDFO derivatives promising chelators for applications in targeted PET.

Biological determinants of PSMA expression, regulation and heterogeneity in prostate cancer

Prostate-specific membrane antigen (PSMA) is an important cell-surface imaging biomarker and therapeutic target in prostate cancer. The PSMA-targeted theranostic 177Lu-PSMA-617 was approved in 2022 for men with PSMA-PET-positive metastatic castration-resistant prostate cancer. However, not all patients respond to PSMA-radioligand therapy, in part owing to the heterogeneity of PSMA expression in the tumour. The PSMA regulatory network is composed of a PSMA transcription complex, an upstream enhancer that loops to the FOLH1 (PSMA) gene promoter, intergenic enhancers and differentially methylated regions. Our understanding of the PSMA regulatory network and the mechanisms underlying PSMA suppression is evolving. Clinically, molecular imaging provides a unique window into PSMA dynamics that occur on therapy and with disease progression, although challenges arise owing to the limited resolution of PET. PSMA regulation and heterogeneity - including intertumoural and inter-patient heterogeneity, temporal changes, lineage dynamics and the tumour microenvironment - affect PSMA theranostics. PSMA response and resistance to radioligand therapy are mediated by a number of potential mechanisms, and complementary biomarkers beyond PSMA are under development. Understanding the biological determinants of cell surface target regulation and heterogeneity can inform precision medicine approaches to PSMA theranostics as well as other emerging therapies.

Current Perspectives on 89Zr-PET Imaging

⁸⁹Zr is an emerging radionuclide that plays an essential role in immuno-positron emission tomography (PET) imaging. The long half-life of ⁸⁹Zr (t_1/2 = 3.3 days) is favorable for evaluating the in vivo distribution of monoclonal antibodies. Thus, the use of ⁸⁹Zr is promising for monitoring antibody-based cancer therapies. Immuno-PET combines the sensitivity of PET with the specificity of antibodies. A number of studies have been conducted to investigate the feasibility of ⁸⁹Zr immuno-PET imaging for predicting the efficacy of radioimmunotherapy and antibody therapies, imaging target expression, detecting target-expressing tumors, and the monitoring of anti-cancer chemotherapies. In this review, we summarize the current status of PET imaging using ⁸⁹Zr in both preclinical and clinical studies by highlighting the use of immuno-PET for the targets of high clinical relevance. We also present ⁸⁹Zr-PET applications other than immuno-PET, such as nanoparticle imaging and cell tracking. Finally, we discuss the limitations and the ongoing research being performed to overcome the remaining hurdles.

Development of zirconium-89 PET for in vivo imaging of alpha-klotho

Alpha-klotho is a single-pass membrane protein primarily expressed by the kidneys. Klotho deficiency in chronic kidney disease contributes to an accelerated aging phenotype. We report here development of [⁸⁹Zr]DFO-anti-klotho positron emission tomography (PET) imaging as a novel non-invasive method for assessing whole-body alpha-klotho distribution. Rat monoclonal anti-mouse klotho antibody was reacted with SCN-Bn-deferoxamine (DFO) and was radiolabeled using Zirconium-89. In vitro testing of [⁸⁹Zr]DFO-anti-mKlotho was done in a distal convoluted tubule kidney cell line and with 40-micron whole kidney sections from C57BL/6J mice. Competitive binding was assessed in co-incubation studies with unlabeled anti-mKlotho antibody. For in vivo testing, C57BL/6J mice were injected retro-orbitally with [⁸⁹Zr]DFO-anti-mKlotho and were scanned using Inveon PET/CT. Autoradiographs of kidney sections were obtained post-imaging on select animals. Radiochemical yield of [⁸⁹Zr]DFO-anti-mKlotho was >70% and radiochemical purity was confirmed by iTLC. Specific binding in the kidney cell line was reduced by 60% in the presence of unlabeled anti-mKlotho. In the PET/CT scans, initial uptake of [⁸⁹Zr]DFO-anti-mKlotho was observed in the intestines and liver. Selective retention of radioactivity was observed in the kidneys in the subsequent 24, 48, and 72 hrs scans with cortical binding of [⁸⁹Zr]DFO-anti-mKlotho clearly visualized. Sites of lower alpha-klotho expression were not visualized. In summary, we have successfully synthesized [⁸⁹Zr]DFO-anti-mKlotho and our initial in vitro and in vivo studies in mice demonstrate selective binding in the kidney cortex, which is known to express high levels of alpha-klotho. PET imaging promises to be a novel tool for in vivo evaluation of alpha-klotho distribution.

A Novel Fully Human Antibody targeting Extracellular Domain of PSMA Inhibits Tumor Growth in Prostate Cancer

Prostate cancer is the most commonly diagnosed malignancy in men and the second leading cause of cancer-related death. It is of vital importance to develop new strategies for prostate cancer therapy. PSMA (prostate-specific membrane antigen) is specifically expressed in prostate cancer and the neovasculature of certain cancer types, thus is considered to be an ideal target for cancer therapy. In our previous study, we have obtained a PSMA-specific single-chain variable fragment (scFv), named gy1, from a large yeast display naïve human scFv library. In this study, we reconstructed the PSMA scFv into a fully human antibody (named PSMAb) and evaluated its characterization both in vitro and in vivo We showed that PSMAb can specifically bind with and internalize into PSMA⁺ cells. The binding affinity of PSMAb is measured to be at nanomolar level, and PSMAb has very good thermostability. In vivo study showed that near IR dye-labeled PSMAb can specifically localize at PSMA⁺ tumors, and the application of PSMAb in vivo significantly inhibited the growth of PSMA⁺ tumors, but not PSMA^- tumors. At the studied doses, no obvious toxicity was observed when applied in vivo, as shown by the relative normal liver and kidney function and normal structure of important organs, shown by hematoxylin and eosin staining. In addition, PSMAb may inhibit tumor growth through antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity mechanisms. Our results indicated that the novel fully human antibody, PSMAb, deserve further study for PSMA-targeted diagnosis and therapy for prostate cancer and other cancer types with vascular PSMA expression.

Recent Advances in Prostate-Specific Membrane Antigen-Based Radiopharmaceuticals

BACKGROUND

Prostate cancer (PCa) is the most common sex-related malignancy with high mortality in men worldwide. Prostate-specific membrane antigen (PSMA) is overexpressed on the surface of most prostate tumor cells and considered a valuable target for both diagnosis and therapy of prostate cancer. A series of radiolabeled agents have been developed based on the featured PSMA ligands in the previous decade and have demonstrated promising outcomes in clinical research of primary and recurrent PCa. Furthermore, the inspiring response and safety of lutetium-177-PSMA-617 (177Lu-PSMA-617) radiotherapy represent the potential for expanded therapeutic options for metastatic castration-resistant PCa. Retrospective cohort studies have revealed that radiolabeled PSMA agents are the mainstays of the current success, especially in detecting prostate cancer with metastasis and biochemical recurrence.

OBJECTIVE

This review is intended to present a comprehensive overview of the current literature on PSMA ligand-based agents for both radionuclide imaging and therapeutic approaches, with a focus on those that have been clinically adopted.

CONCLUSION

PSMA-based diagnosis and therapy hold great promise for improving the clinical management of prostate cancer.

A radiopharmaceutical [89Zr]Zr-DFO-nimotuzumab for immunoPET with epidermal growth factor receptor expression in vivo

INTRODUCTION

The potential of the positron-emitting zirconium-89 (⁸⁹Zr) (t_1/2 = 78.4 h) has been recently reported for immune positron emission tomography (immunoPET) radioimmunoconjugates design. In our work, we explored the optimized preparation of [⁸⁹Zr]Zr-DFO-nimotuzumab, and evaluated ⁸⁹Zr-labeled monoclonal antibody (mAb) construct for targeted imaging of epidermal growth factor receptor (EGFR) overexpressed in glioma.

METHODS

To optimize the radiolabeling efficiency of ⁸⁹Zr with DFO-nimotuzumab, multiple immunoconjugates and radiolabeling were performed. Radiolabeling yield, radiochemical purity, stability, and activity assay were investigated to characterize [⁸⁹Zr]Zr-DFO-nimotuzumab for chemical and biological integrity. The in vivo behavior of this tracer was studied in mice bearing subcutaneous U87MG (EGFR-positive) tumors received a 3.5 ± 0.2 MBq/dose using PET/CT imaging. One group mice bearing subcutaneous U87MG (EGFR-positive) tumors received [⁸⁹Zr]Zr-DFO-nimotuzumab (3.5 ± 0.2 MBq, ~3 μg) (nonblocking) for immunoPET; the other group had 30 μg predose (blocking) of cold nimotuzumab 24 h prior to [⁸⁹Zr]Zr-DFO-nimotuzumab.

RESULTS

[⁸⁹Zr]Zr-DFO-nimotuzumab was prepared with high radiochemical yield (>90%), radiochemical purity (>99%), and specific activity (115 ± 0.8 MBq/mg). In vitro validation showed that [⁸⁹Zr]Zr-DFO-nimotuzumab had an initial immunoreactive fraction of 0.99 ± 0.05 and remained active for up to 5 days. A biodistribution study revealed excellent stability of [⁸⁹Zr]Zr-DFO-nimotuzumab in vivo compared with ⁸⁹Zr as a bone seeker. High uptake in the liver and heart and modest penetration in the brain were observed, with no significant accumulation of activity in other organs. ImmunoPET studies also indicated prominent image contrast that remarkably high uptake up to ~20%ID/g for nonblocking and ~2%ID/g for blocking in tumor between 12 and 120 h after administration.

CONCLUSION

These studies developed a radiopharmaceutical [⁸⁹Zr]Zr-DFO-nimotuzumab with optimized synthesis. The potential utility of [⁸⁹Zr]Zr-DFO-nimotuzumab in assessing EGFR status in glioma was demonstrated in this study.

Therapeutic effects of human monoclonal PSMA antibody-mediated TRIM24 siRNA delivery in PSMA-positive castration-resistant prostate cancer

Background and Aims: Prostate specific membrane antigen (PSMA) is specifically expressed on prostate epithelial cells and markedly overexpressed in almost all prostate cancers. TRIM24 is also up-regulated from localized prostate cancer to metastatic castration-resistant prostate cancer (CRPC). Because of the high relevance of TRIM24 for cancer development and the universal expression of PSMA in CPRC, we investigated the efficacy of human monoclonal PSMA antibody (PSMAb)-based platform for the targeted TRIM24 siRNA delivery and its therapeutic efficacy in CRPC in vivo and in vitro. Methods: The therapeutic complexes were constructed by conjugating PSMAb and sulfo-SMCC-protamine, and encapsulating TRIM24 siRNA. Flow cytometry, immunofluorescence, and fluorescence imaging were performed to detect the receptor-binding, internalization, and targeted delivery of PSMAb-sulfo-SMCC-protamine (PSP)-FAM-siRNA complex (PSPS) in vitro and in vivo. CCK-8, plate-colony formation, apoptosis, cell cycle, and Transwell assays were performed to evaluate the therapeutic potential of the PSP-TRIM24 siRNA complex in vitro, whereas the in vivo therapeutic efficacy was monitored by small animal imaging, radiography, and micro CT. Results: We confirmed that PSP could efficiently protect siRNA from enzymatic digestion, enable targeted delivery of siRNA, and internalize and release siRNA into PSMA-positive (PSMA+) prostate cancer cells in vitro and in vivo. Silencing TRIM24 expression by the PSP-TRIM24 siRNA complex could dramatically suppress proliferation, colony-formation, and invasion of PSMA+ CRPC cells in vitro, and inhibit tumor growth of PSMA+ CRPC xenografts and bone loss in PSMA+ CRPC bone metastasis model without obvious toxicity at therapeutic doses in vivo. Conclusion: PSMAb mediated TRIM24 siRNA delivery platform could significantly inhibit cell proliferation, colony-formation, and invasion in PSMA+ CRPC in vitro and suppressed tumor growth and bone loss in PSMA+ CRPC xenograft and bone metastasis model.

Radiochemistry and Preclinical PET Imaging of 68Ga-Desferrioxamine Radiotracers Targeting Prostate-Specific Membrane Antigen

Radiotracers incorporating the urea-based Glu-NH-C(O)-NH-Lys group have gained prominence due to their role in targeting prostate-specific membrane antigen (PSMA)-a clinical biomarker of prostate cancer. Here, the synthesis, radiolabeling, and in vitro and in vivo characterization of two ⁶⁸Ga-radiolabeled Glu-NH-C(O)-NH-Lys radiotracers conjugated to the desferrioxamine B (DFO) chelate were evaluated. Two linker groups based on amide bond and thiourea coupling chemistries were employed to develop ⁶⁸Ga-DFO-Nsucc-PSMA (⁶⁸Ga-4) and ⁶⁸Ga-DFO- pNCS-Bn-PSMA (⁶⁸Ga-7), respectively. Radiosynthesis proceeded quantitatively at room temperature with high radiochemical yields, chemical/radiochemical purities, and specific activities. Pharmacokinetic profiles of ⁶⁸Ga-4 and ⁶⁸Ga-7 were assessed using positron-emission tomography (PET) in mice bearing subcutaneous LNCaP tumors. Data were compared to the current clinical benchmark radiotracer ⁶⁸Ga-HBED-CC-PSMA (⁶⁸Ga-1) (HBED = N,N'-Bis(2-hydroxy-5-(ethylene-beta-carboxy)benzyl)ethylenediamine N,N'-diacetic acid). Results indicated that the target binding affinity, protein association, blood pool and background organ clearance properties, and uptake in PSMA-positive lesions are strongly dependent on the nature of the chelate, the linker, and the spacer groups. Protein dissociation constants ( K_d values) were found to be predictive of pharmacokinetics in vivo. Compared to ⁶⁸Ga-1, ⁶⁸Ga-4 and ⁶⁸Ga-7 resulted in decreased tumor uptake but enhanced blood pool clearance and reduced residence time in the kidney. The study highlights the importance of maximizing protein binding affinity during radiotracer optimization.

Current advances in ligand design for inorganic positron emission tomography tracers 68Ga, 64Cu, 89Zr and 44Sc

A key part of the development of metal based Positron Emission Tomography probes is the chelation of the radiometal. In this review the recent developments in the chelation of four positron emitting radiometals, ⁶⁸Ga, ⁶⁴Cu, ⁸⁹Zr and ⁴⁴Sc, are explored. The factors that effect the chelation of each radio metal and the ideal ligand system will be discussed with regards to high in vivo stability, complexation conditions, conjugation to targeting motifs and complexation kinetics. A series of cyclic, cross-bridged and acyclic ligands will be discussed, such as CP256 which forms stable complexes with ⁶⁸Ga under mild conditions and PCB-TE2A which has been shown to form a highly stable complex with ⁶⁴Cu. ⁸⁹Zr and ⁴⁴Sc have seen significant development in recent years with a number of chelates being applied to each metal - eight coordinate di-macrocyclic terephthalamide ligands were found to rapidly produce more stable complexes with ⁸⁹Zr than the widely used DFO.

High resolution digital autoradiographic and dosimetric analysis of heterogeneous radioactivity distribution in xenografted prostate tumors

PURPOSE

The first main aim of this study was to illustrate the absorbed dose rate distribution from ¹⁷⁷Lu in sections of xenografted prostate cancer (PCa) tumors using high resolution digital autoradiography (DAR) and compare it with hypothetical identical radioactivity distributions of ⁹⁰Y or 7 MeV alpha-particles. Three dosimetry models based on either dose point kernels or Monte Carlo simulations were used and evaluated. The second and overlapping aim, was to perform DAR imaging and dosimetric analysis of the distribution of radioactivity, and hence the absorbed dose rate, in tumor sections at an early time point after injection during radioimmunotherapy using ¹⁷⁷Lu-h11B6, directed against the human kallikrein 2 antigen.

METHODS

Male immunodeficient BALB/c nude mice, aged 6-8 w, were inoculated by subcutaneous injection of ∼10⁷ LNCaP cells in a 200 μl suspension of a 1:1 mixture of medium and Matrigel. The antibody h11B6 was conjugated with the chelator CHX-A″-DTPA after which conjugated h11B6 was mixed with ¹⁷⁷LuCl₃. The incubation was performed at room temperature for 2 h, after which the labeling was terminated and the solution was purified on a NAP-5 column. About 20 MBq ¹⁷⁷Lu-h11B6 was injected intravenously in the tail vein. At approximately 10 h postinjection (hpi), the mice were sacrificed and one tumor was collected from each of the five animals and cryosectioned into 10 μm thick slices. The tumor slices were measured and imaged using the DAR MicroImager system and the M3Vision software. Then the absorbed dose rate was calculated using a dose point kernel generated with the Monte Carlo code gate v7.0.

RESULTS

The DAR system produced high resolution images of the radioactivity distribution, close to the resolution of single PCa cells. The DAR images revealed a pronounced heterogeneous radioactivity distribution, i.e., count rate per area, in the tumors, indicated by the normalized intensity variations along cross sections as mean ± SD: 0.15 ± 0.15, 0.20 ± 0.18, 0.12 ± 0.17, 0.15 ± 0.16, and 0.23 ± 0.22, for each tumor section, respectively. The absorbed dose rate distribution for ¹⁷⁷Lu at the time of dissection 10 hpi showed a maximum value of 2.9 ± 0.4 Gy/h (mean ± SD), compared to 6.0 ± 0.9 and 159 ± 25 Gy/h for the hypothetical ⁹⁰Y and 7 MeV alpha-particle cases assuming the same count rate densities. Mean absorbed dose rate values were 0.13, 0.53, and 6.43 Gy/h for ¹⁷⁷Lu, ⁹⁰Y, and alpha-particles, respectively.

CONCLUSIONS

The initial uptake of ¹⁷⁷Lu-h11B6 produces a high absorbed dose rate, which is important for a successful therapeutic outcome. The hypothetical ⁹⁰Y case indicates a less heterogeneous absorbed dose rate distribution and a higher mean absorbed dose rate compared to ¹⁷⁷Lu, although with a potentially increased irradiation of surrounding healthy tissue. The hypothetical alpha-particle case indicates the possibility of a higher maximum absorbed dose rate, although with a more heterogeneous absorbed dose rate distribution.

The therapeutic and diagnostic potential of the prostate specific membrane antigen/glutamate carboxypeptidase II (PSMA/GCPII) in cancer and neurological disease

Prostate specific membrane antigen (PSMA) otherwise known as glutamate carboxypeptidase II (GCPII) is a membrane bound protein that is highly expressed in prostate cancer and in the neovasculature of a wide variety of tumours including glioblastomas, breast and bladder cancers. This protein is also involved in a variety of neurological diseases including schizophrenia and ALS. In recent years, there has been a surge in the development of both diagnostics and therapeutics that take advantage of the expression and activity of PSMA/GCPII. These include gene therapy, immunotherapy, chemotherapy and radiotherapy. In this review, we discuss the biological roles that PSMA/GCPII plays, both in normal and diseased tissues, and the current therapies exploiting its activity that are at the preclinical stage. We conclude by giving an expert opinion on the future direction of PSMA/GCPII based therapies and diagnostics and hurdles that need to be overcome to make them effective and viable.

A Novel Prostate-Specific Membrane-Antigen (PSMA) Targeted Micelle-Encapsulating Wogonin Inhibits Prostate Cancer Cell Proliferation via Inducing Intrinsic Apoptotic Pathway

Prostate cancer (PCa) is a malignant tumor for which there are no effective treatment strategies. In this study, we developed a targeted strategy for prostate-specific membrane-antigen (PSMA)-positive PCa in vitro based on 2-(3-((S)-5-amino-1-carboxypentyl)ureido) pentanedioic acid (ACUPA) modified polyethylene glycol (PEG)-Cholesterol micelles containing wogonin (WOG), which was named ACUPA-M-WOG. ACUPA-M-WOG was conventionally prepared using a self-assembling method, which produced stable particle size and ζ potential. Moreover, ACUPA-M-WOG showed good drug encapsulating capacity and drug release profiles. Fluorescence activated cell sorting (FACS) results suggested that ACUPA modified PEG-Cholesterol micelles could effectively enhance the drug uptake on PSMA(+) PCa cells, and the cytotoxicity of ACUPA-M-WOG was stronger than other controls according to in vitro cellular proliferation and apoptosis assays, separately through methyl thiazolyl tetrazolium (MTT) and Annexin V/Propidium Iodide (PI) staining. Finally, the molecular mechanisms of ACUPA-M-WOG’s effects on human PSMA(+) PCa were investigated, and were mainly the intrinsic or extrinsic apoptosis signaling pathways. The Western blot results suggested that ACUPA-M-WOG could enhance the WOG-induced apoptosis, which was mainly via the intrinsic signaling pathway rather than the extrinsic signaling pathway. In conclusion, ACUPA-M-WOG was successfully developed for WOG-selective delivery to PSMA(+) PCa cells and had stronger inhibition than free drugs, which might make it an effective strategy for PSMA(+) PCa.

Intraoperative Identification of Liver Cancer Microfoci Using a Targeted Near-Infrared Fluorescent Probe for Imaging-Guided Surgery

Difficulties in the highly sensitive detection of tumour microfoci represent a critical obstacle toward improved surgical intervention in liver cancer. Conventional preoperative imaging methods and surgeons’ subjective experience are limited by their inability to effectively detect tumour lesions measuring less than 2 mm; however, intraoperative fluorescence molecular imaging may overcome this limitation. Here, we synthesised an arginine-glycine-aspartic acid (RGD)-conjugated mesoporous silica nanoparticle (MSN) highly loaded with indocyanine green (ICG) dye that could accurately delineate liver cancer margins and provide excellent tumour-to-normal tissue contrast intraoperatively. The increased ICG loading capacity and tumour specificity enabled the identification of residual microtumours and satellite lesions measuring less than 1 mm in living mice. Histological analysis validated the sensitivity and accuracy of this approach. We believe this technique utilising a new fluorescent nanoprobe with intraoperative optical imaging may offer a more sensitive and accurate method for liver cancer resection guidance, resulting in better surgical outcomes.

(R)-NODAGA-PSMA: A Versatile Precursor for Radiometal Labeling and Nuclear Imaging of PSMA-Positive Tumors

PURPOSE

The present study aims at developing and evaluating an urea-based prostate specific membrane antigen (PSMA) inhibitor suitable for labeling with 111In for SPECT and intraoperative applications as well as 68Ga and 64Cu for PET imaging.

METHODS

The PSMA-based inhibitor-lysine-urea-glutamate-coupled to the spacer Phe-Phe-D-Lys(suberoyl) and functionalized with the enantiomerically pure prochelator (R)-1-(1-carboxy-3-carbotertbutoxypropyl)-4,7-carbotartbutoxymethyl)-1,4,7-triazacyclononane ((R)-NODAGA(tBu)3), to obtain (R)-NODAGA-Phe-Phe-D-Lys(suberoyl)-Lys-urea-Glu (CC34). CC34 was labeled with 111In, 68Ga and 64Cu. The radioconjugates were further evaluated in vitro and in vivo in LNCaP xenografts by biodistribution and PET studies. Biodistribution studies were also performed with 68Ga-HBED-CC-PSMA (HBED-CC: N,N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid) and 111In-PSMA-617 for comparison.

RESULTS

68Ga-CC34, 64Cu-CC34, and 111In-CC34 were prepared in radiochemical purity > 95%. 68/natGa-CC34, 64/natCu-CC34, 111/natIn-CC34, 68/natGa-HBED-CC-PSMA, and 111/natIn-PSMA-617 exhibited high affinity for the LNCaP cells, with Kd values of 19.3 ± 2.5 nM, 27.5 ± 2.7 nM, 5.5 ± 0.9 nM, 2.9 ± 0.6 nM and 5.4 ± 0.8 nM, respectively. They revealed comparable internalization profiles with approximately 75% of the total cell associated activity internalized after 3 h of incubation. 68Ga-CC34 showed very high stability after its administration in mice. Tumor uptake of 68Ga-CC34 (14.5 ± 2.9% IA/g) in LNCaP xenografts at 1 h p.i. was comparable to 68Ga-HBED-CC-PSMA (15.8 ± 1.4% IA/g) (P = 0.67). The tumor-to-normal tissue ratios at 1 and 2 h p.i of 68Ga-CC34 were also comparable to 68Ga-HBED-CC-PSMA (P > 0.05). Tumor uptake of 111In-CC34 (28.5 ± 2.6% IA/g) at 1 h p.i. was lower than 111In-PSMA-617 (52.1 ± 6.5% IA/g) (P = 0.02). The acquisition of PET-images with 64Cu-CC34 at later time points showed wash-out from the kidneys, while tumor uptake still remained relatively high. This resulted in an increased tumor-to-kidney ratio over time.

CONCLUSIONS

68Ga-CC34 is comparable to 68Ga-HBED-CC-PSMA in terms of tumor uptake and tumor to normal tissue ratios. 64Cu-CC34 could enable high contrast imaging of PSMA positive tissues characterized by elevated expression of PSMA or when delayed imaging is required. 64Cu-CC34 is currently being prepared for clinical translation.

Di-macrocyclic terephthalamide ligands as chelators for the PET radionuclide zirconium-89

The development of bifunctional chelators (BFCs) which can stably chelate zirconium-89 ((89)Zr) while being conjugated to targeting molecules is an area of active research. Herein we report the first octadentate terephthalamide ligands, which are easily radiolabeled with (89)Zr and are highly stable in vitro. They represent a novel class of chelators, which are worthy of further development as BFCs for (89)Zr.

Development, optimization, and validation of novel anti-TEM1/CD248 affinity agent for optical imaging in cancer

Tumor Endothelial Marker-1 (TEM1/CD248) is a tumor vascular marker with high therapeutic and diagnostic potentials. Immuno-imaging with TEM1-specific antibodies can help to detect cancerous lesions, monitor tumor responses, and select patients that are most likely to benefit from TEM1-targeted therapies. In particular, near infrared(NIR) optical imaging with biomarker-specific antibodies can provide real-time, tomographic information without exposing the subjects to radioactivity. To maximize the theranostic potential of TEM1, we developed a panel of all human, multivalent Fc-fusion proteins based on a previously identified single chain antibody (scFv78) that recognizes both human and mouse TEM1. By characterizing avidity, stability, and pharmacokinectics, we identified one fusion protein, 78Fc, with desirable characteristics for immuno-imaging applications. The biodistribution of radiolabeled 78Fc showed that this antibody had minimal binding to normal organs, which have low expression of TEM1. Next, we developed a 78Fc-based tracer and tested its performance in different TEM1-expressing mouse models. The NIR imaging and tomography results suggest that the 78Fc-NIR tracer performs well in distinguishing mouse- or human-TEM1 expressing tumor grafts from normal organs and control grafts in vivo. From these results we conclude that further development and optimization of 78Fc as a TEM1-targeted imaging agent for use in clinical settings is warranted.

Imaging PEG-like nanoprobes in tumor, transient ischemia, and inflammatory disease models

The iron chelator deferoxamine (DFO), approved for the treatment of iron overload, has been examined as a therapeutic in a variety of conditions which iron may exacerbate. To evaluate the potential of DFO-bearing PEG-like nanoprobes (DFO-PNs) as therapeutics, we determined their pharmacokinetics (PK) in normal mice, and imaged their accumulation in a tumor model and in models of transient brain ischemia and inflammation. DFO-PNs consist of a DFO, a Cy5.5, and PEG (5 kDa or 30 kDa) attached to Lys-Cys scaffold. Tumor uptake of a [(89)Zr]:DFO-PN(10) (30 kDa PEG, diameter 10 nm) was imaged by PET, surface fluorescence, and fluorescence microscopy. DFO-PN(10) was internalized by tumor cells (fluorescence microscopy) and by cultured cells (by FACS). [(89)Zr]:DFO-PN(4.3) (5 kDa PEG, diameter 4.3 nm) concentrated at incision generated inflammations but not at sites of transient brain ischemia. DFO-PNs are fluorescent, PK tunable forms of DFO that might be investigated as antitumor or anti-inflammatory agents.

Tripodal tris(hydroxypyridinone) ligands for immunoconjugate PET imaging with (89)Zr(4+): comparison with desferrioxamine-B

Due to its long half-life (78 h) and decay properties (77% electron capture, 23% β(+), Emax = 897 keV, Eav = 397 keV, Eγ = 909 keV, Iγ = 100%) (89)Zr is an appealing radionuclide for immunoPET imaging with whole IgG antibodies. Derivatives of the siderophore desferrioxamine-B (H3DFO) are the most widely used bifunctional chelators for coordination of (89)Zr(4+) because the radiolabeling of the resulting immunoconjugates is rapid under mild conditions. (89)Zr-DFO complexes are reportedly stable in vitro but there is evidence that (89)Zr(4+) is released in vivo, and subsequently taken up by the skeleton. We have evaluated a novel tripodal tris(hydroxypyridinone) chelator, H3CP256 and its bifunctional maleimide derivative, H3YM103, for coordination of Zr(4+) and compared the NMR spectra, and the (89)Zr(4+) radiolabeling, antibody conjugation, serum stability and in vivo distribution of radiolabelled immunoconjugates with those of H3DFO and its analogues. H3CP256 coordinates (89)Zr(4+) at carrier-free concentrations forming [(89)Zr(CP256)](+). Both H3DFO and H3CP256 were efficiently radiolabelled using [(89)Zr(C2O4)4](4-) at ambient temperature in quantitative yield at pH 6-7 at millimolar concentrations of chelator. Competition experiments demonstrate that (89)Zr(4+) dissociates from [(89)Zr(DFO)](+) in the presence of one equivalent of H3CP256 (relative to H3DFO) at pH 6-7, resulting largely in [(89)Zr(CP256)](+). To assess the stability of H3DFO and H3YM103 immunoconjugates radiolabelled with (89)Zr, maleimide derivatives of the chelators were conjugated to the monoclonal antibody trastuzumab via reduced cysteine side chains. Both immunoconjugates were labelled with (89)Zr(4+) in >98% yield at high specific activities and the labeled immunoconjugates were stable in serum with respect to dissociation of the radiometal. In vivo studies in mice indicate that (89)Zr(4+) dissociates from YM103-trastuzumab with significant amounts of activity becoming associated with bones and joints (25.88 ± 0.58% ID g(-1) 7 days post-injection). In contrast, <8% ID g(-1) of (89)Zr activity becomes associated with bone in animals administered (89)Zr-DFO-trastuzumab over the course of 7 days. The tris(hydroxypyridinone) chelator, H3CP256, coordinates (89)Zr(4+) rapidly under mild conditions, but the (89)Zr-labelled immunoconjugate, (89)Zr-YM103-trastuzumab was observed to release appreciable amounts of (89)Zr(4+)in vivo, demonstrating inferior stability when compared with (89)Zr-DFO-trastuzumab. The significantly lower in vivo stability is likely to be a result of lower kinetic stability of the Zr(4+) tris(hydroxypyridinone complex) relative to that of DFO and its derivatives.

Molecular imaging of prostate cancer: translating molecular biology approaches into the clinical realm

The epidemiology of prostate cancer has dramatically changed since the introduction of prostate-specific antigen (PSA) screening in the 1980's. Most prostate cancers today are detected at early stages of the disease and are considered 'indolent'; however, some patients' prostate cancers demonstrate a more aggressive behaviour which leads to rapid progression and death. Increasing understanding of the biology underlying the heterogeneity that characterises this disease has led to a continuously evolving role of imaging in the management of prostate cancer. Functional and metabolic imaging techniques are gaining importance as the impact on the therapeutic paradigm has shifted from structural tumour detection alone to distinguishing patients with indolent tumours that can be managed conservatively (e.g., by active surveillance) from patients with more aggressive tumours that may require definitive treatment with surgery or radiation. In this review, we discuss advanced imaging techniques that allow direct visualisation of molecular interactions relevant to prostate cancer and their potential for translation to the clinical setting in the near future. The potential use of imaging to follow molecular events during drug therapy as well as the use of imaging agents for therapeutic purposes will also be discussed.

KEY POINTS

• Advanced imaging techniques allow direct visualisation of molecular interactions in prostate cancer. • MRI/PET, optical and Cerenkov imaging facilitate the translation of molecular biology. • Multiple compounds targeting PSMA expression are currently undergoing clinical translation. • Other targets (e.g., PSA, prostate-stem cell antigen, GRPR) are in development.

The bioconjugation and radiosynthesis of 89Zr-DFO-labeled antibodies

The exceptional affinity, specificity, and selectivity of antibodies make them extraordinarily attractive vectors for tumor-targeted PET radiopharmaceuticals. Due to their multi-day biological half-life, antibodies must be labeled with positron-emitting radionuclides with relatively long physical decay half-lives. Traditionally, the positron-emitting isotopes 124I (t1/2=4.18 d), 86Y (t1/2=14.7 hr), and 64Cu (t1/2=12.7 hr) have been used to label antibodies for PET imaging. More recently, however, the field has witnessed a dramatic increase in the use of the positron-emitting radiometal 89Zr in antibody-based PET imaging agents. 89Zr is a nearly ideal radioisotope for PET imaging with immunoconjugates, as it possesses a physical half-life (t1/2=78.4 hr) that is compatible with the in vivo pharmacokinetics of antibodies and emits a relatively low energy positron that produces high resolution images. Furthermore, antibodies can be straightforwardly labeled with 89Zr using the siderophore-derived chelator desferrioxamine (DFO). In this protocol, the prostate-specific membrane antigen targeting antibody J591 will be used as a model system to illustrate (1) the bioconjugation of the bifunctional chelator DFO-isothiocyanate to an antibody, (2) the radiosynthesis and purification of a 89Zr-DFO-mAb radioimmunoconjugate, and (3) in vivo PET imaging with an 89Zr-DFO-mAb radioimmunoconjugate in a murine model of cancer.

Pharmacokinetics and PET imaging properties of two recombinant anti-PSMA antibody fragments in comparison to their parental antibody

BACKGROUND

Radioimmunoimaging with disease-specific tracers can be advantageous compared to that with nonspecific tracers for the imaging of glucose metabolism and cell proliferation. Monoclonal antibodies (mAbs) or their fragments are excellent tools for immuno-positron emission tomography (PET). In this study, PSMA-specific mAb 3/F11 and its recombinant fragments were compared for the imaging of prostate cancer in xenografts.

METHODS

Recombinant anti-PSMA antibody fragments D7-Fc and D7-CH3 were constructed by genetically fusing the binding domains of mAb 3/F11 (D7) to the human IgG3 CH3 or CH2-CH3 (Fc) domain. The fragments and the mAb 3/F11 were DOTA conjugated, tested in vitro, and radiolabeled with (64) Cu. PSMA-positive C4-2 and PSMA-negative DU 145 prostate cancer xenografts were used for PET-MR imaging and for ex vivo biodistribution.

RESULTS

The constructs showed strong and specific binding to PSMA-positive C4-2 cells in vitro which did not decrease after DOTA conjugation. Both tested fragments showed stable accumulation in PSMA-positive C4-2 tumors at all measured time points but reduced uptake compared to the full-length antibody. Other organs and PSMA-negative tumors showed a very low tracer uptake only 3 hr after injection, with the exception of the kidneys, which demonstrated high radioactivity uptake due to rapid renal clearance of the mAb fragments.

CONCLUSION

Stable tumor uptake and fast serum clearance of the tested radiolabeled fragments was observed in this preclinical study compared to the full length mAb. Since the fragments show rapid and specific tumor uptake, the tested fragments might serve as tools for theranostic imaging with suitable isotopes for radioimmunotherapy.

Development of a 124I-labeled version of the anti-PSMA monoclonal antibody capromab for immunoPET staging of prostate cancer: Aspects of labeling chemistry and biodistribution

Correct staging of prostate cancer is an unmet clinical need. Radionuclide targeting of prostate-specific membrane antigen (PSMA) with 111In-labeled capromab pendetide (ProstaScint) is a clinical option for prostate cancer staging. We propose the use of 124I-labeled capromab to decrease the retention of radioactivity in healthy organs (due to the non-residualizing properties of the radiolabel). The use of 124I as a label should increase imaging sensitivity due to the advantages of PET as an imaging modality. Capromab targets the intracellular domain of PSMA; accumulation of radioactivity in the tumor should not depend on internalization of the antigen/antibody complex. Capromab was iodinated, and its targeting properties were compared with indium labeled counterpart in LNCaP xenografts in dual isotope mode. PSMA-negative xenografts (PC3) were used as a negative control. Radioiodinated capromab bound to PSMA specifically. Biodistribution of 125I/111In-capromab showed a more rapid clearance of iodine radioactivity from liver, spleen, kidneys, bones, colon tissue, as well as tumors. Maximum tumor uptake (13±8% ID/g for iodine and 29±9% ID/g for indium) and tumor-to-non-tumor ratios for both agents were measured 5 days post-injection (pi). High tumor accumulation and low uptake of radioactivity in normal organs were confirmed using microPET/CT 5 days pi of 124I-capromab.

Glypican-3-targeted 89Zr PET imaging of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a devastating malignancy in which imperfect imaging plays a primary role in diagnosis. Glypican-3 (GPC3) is an HCC-specific cell surface proteoglycan overexpressed in most HCCs. This paper presents the use of (89)Zr-conjugated monoclonal antibody against GPC3 ((89)Zr-αGPC3) for intrahepatic tumor localization using PET.

METHODS

Polymerase chain reaction confirmed relative GPC3 expression in cell lines. In vitro binding, in vivo biodistribution, and small-animal PET studies were performed on GPC3-expressing HepG2 and non-GPC3-expressing HLF and RH7777 cells and orthotopic xenografts.

RESULTS

(89)Zr-αGPC3 demonstrated antibody-dependent, antigen-specific tumor binding. HepG2 liver tumors exhibited high peak uptake (836.6 ± 86.6 percentage injected dose [%ID]/g) compared with background liver (27.5 ± 1.6 %ID/g). Tumor-to-liver contrast ratio was high and peaked at 32.5. The smallest HepG2 tumor (<1 mm) showed lower peak uptake (42.5 ± 6.4 %ID/g) and tumor-to-liver contrast (1.57) but was still clearly visible on PET. Day 7 tissue activity was still substantial in HepG2 tumors (466.4 ± 87.6 %ID/g) compared with control RH7777 tumors (3.9 ± 1.3 %ID/g, P < 0.01), indicating antigen specificity by (89)Zr-αGPC3. HepG2 tumor treated with unlabeled αGPC3 or heat-denatured (89)Zr-αGPC3 demonstrated tumor activity (2.1 %ID/g) comparable to that of control xenografts, confirming antibody dependency.

CONCLUSION

This study demonstrated the feasibility of using (89)Zr-αGPC3 to image HCC in the liver, as well as the qualitative determination of GPC3 expression via small-animal PET. The ability to clarify the identity of small liver lesions with an HCC-specific PET probe would provide clinicians with vital information that could significantly alter patient management, warranting further investigation for clinical translation.

Charting the mechanism and reactivity of zirconium oxalate with hydroxamate ligands using density functional theory: implications in new chelate design

DFT studies on Zr-oxalate reactivity with hydroxamic acids reveals new insight into the mechanism and coordination requirements of⁸⁹Zr⁴⁺ions.

In vitro and in vivo evaluation of a (18)F-labeled high affinity NOTA conjugated bombesin antagonist as a PET ligand for GRPR-targeted tumor imaging

Expression of the gastrin-releasing peptide receptor (GRPR) in prostate cancer suggests that this receptor can be used as a potential molecular target to visualize and treat these tumors. We have previously investigated an antagonist analog of bombesin (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂, RM26) conjugated to 1,4,7-triazacyclononane-N,N',N''-triacetic acid (NOTA) via a diethylene glycol (PEG₂) spacer (NOTA-P2-RM26) labeled with ⁶⁸Ga and ¹¹¹In. We found that this conjugate has favorable properties for in vivo imaging of GRPR-expression. The focus of this study was to develop a ¹⁸F-labelled PET agent to visualize GRPR. NOTA-P2-RM26 was labeled with ¹⁸F using aluminum-fluoride chelation. Stability, in vitro binding specificity and cellular processing tests were performed. The inhibition efficiency (IC₅₀) of the [^natF]AlF-NOTA-P2-RM26 was compared to that of the ^natGa-loaded peptide using ¹²⁵I-Tyr⁴-BBN as the displacement radioligand. The pharmacokinetics and in vivo binding specificity of the compound were studied. NOTA-P2-RM26 was labeled with ¹⁸F within 1 h (60-65% decay corrected radiochemical yield, 55 GBq/µmol). The radiopeptide was stable in murine serum and showed high specific binding to PC-3 cells. [^natF]AlF-NOTA-P2-RM26 showed a low nanomolar inhibition efficiency (IC₅₀=4.4±0.8 nM). The internalization rate of the tracer was low. Less than 14% of the cell-bound radioactivity was internalized after 4 h. The biodistribution of [¹⁸F]AlF-NOTA-P2-RM26 demonstrated rapid blood clearance, low liver uptake and low kidney retention. The tumor uptake at 3 h p.i. was 5.5±0.7 %ID/g, and the tumor-to-blood, -muscle and -bone ratios were 87±42, 159±47, 38±16, respectively. The uptake in tumors, pancreas and other GRPR-expressing organs was significantly reduced when excess amount of non-labeled peptide was co-injected. The low uptake in bone suggests a high in vivo stability of the Al-F bond. High contrast PET image was obtained 3 h p.i. The initial biological results suggest that [¹⁸F]AlF-NOTA-P2-RM26 is a promising candidate for PET imaging of GRPR in vivo.

Translational Molecular Imaging of Prostate Cancer

Prostate cancer is a heterogeneous disease, and its management is now evolving to become more personalized and to incorporate new targeted therapies. With these new changes comes a demand for molecular imaging techniques that can not only detect disease but also assess biology and treatment response. This review article summarizes current molecular imaging approaches in prostate cancer (e.g. 99mTc bone scintigraphy and 18F-fluorodeoxyglucose positron emission tomography) and highlights emerging clinical and preclinical imaging agents, with an emphasis on mechanism and clinical application. Emerging agents at various stages of clinical translation include radiolabeled analogs of lipid, amino acid, and nucleoside metabolism, as well as agents more specifically targeting prostate cancer biomarkers including androgen receptor, prostate-specific membrane antigen and others. We also highlight new techniques and targeted contrast agents for magnetic resonance imaging and spectroscopy. For all these imaging techniques, a growing and important unmet need is for well-designed prospective clinical trials to establish clear indications with clinical benefit in prostate cancer.

Mapping biological behaviors by application of longer-lived positron emitting radionuclides

With the technological development of positron emission tomography (PET) and the advent of novel antibody-directed drug delivery systems, longer-lived positron-emitting radionuclides are moving to the forefront to take important roles in tracking the distribution of biotherapeutics such as antibodies, and for monitoring biological processes and responses. Longer half-life radionuclides possess advantages of convenient on-site preparation procedures for both clinical and non-clinical applications. The suitability of the long half-life radionuclides for imaging intact monoclonal antibodies (mAbs) and their respective fragments, which have inherently long biological half-lives, has attracted increased interest in recent years. In this review, we provide a survey of the recent literature as it applies to the development of nine-selected longer-lived positron emitters with half-lives of 9-140h (e.g., (124)I, (64)Cu, (86)Y and (89)Zr), and describe the biological behaviors of radionuclide-labeled mAbs with respect to distribution and targeting characteristics, potential toxicities, biological applications, and clinical translation potentials.

(89)Zr, a radiometal nuclide with high potential for molecular imaging with PET: chemistry, applications and remaining challenges

Molecular imaging-and especially Positron Emission Tomography (PET)-is of increasing importance for the diagnosis of various diseases and thus is experiencing increasing dissemination. Consequently, there is a growing demand for appropriate PET tracers which allow for a specific accumulation in the target structure as well as its visualization and exhibit decay characteristics matching their in vivo pharmacokinetics. To meet this demand, the development of new targeting vectors as well as the use of uncommon radionuclides becomes increasingly important. Uncommon nuclides in this regard enable the utilization of various selectively accumulating bioactive molecules such as peptides, antibodies, their fragments, other proteins and artificial structures for PET imaging in personalized medicine. Among these radionuclides, 89Zr (t1/2 = 3.27 days and mean Eβ+ = 0.389 MeV) has attracted increasing attention within the last years due to its favorably long half-life, which enables imaging at late time-points, being especially favorable in case of slowly-accumulating targeting vectors. This review outlines the recent developments in the field of 89Zr-labeled bioactive molecules, their potential and application in PET imaging and beyond, as well as remaining challenges.

PET imaging in prostate cancer: focus on prostate-specific membrane antigen

Prostate cancer (PCa) is the second leading cause of cancer-related death in American men. Positron emission tomography/computed tomography (PET/CT) with emerging radiopharmaceuticals promises accurate staging of primary disease, restaging of recurrent disease, detection of metastatic lesions and, ultimately, for predicting the aggressiveness of disease. Prostate-specific membrane antigen (PSMA) is a well-characterized imaging biomarker of PCa. Because PSMA levels are directly related to androgen independence, metastasis and progression, PSMA could prove an important target for the development of new radiopharmaceuticals for PET. Preclinical data for new PSMA-based radiotracers are discussed and include new (89)Zr- and (64)Cu-labeled anti-PSMA antibodies and antibody fragments, (64)Cu-labeled aptamers, and (11)C-, (18)F-, (68)Ga-, (64)Cu-, and (86)Y-labeled low molecular weight inhibitors of PSMA. Several of these agents, namely (68)Ga- HBED-CC conjugate 15, (18)F-DCFBC 8, and BAY1075553 are particularly promising, each having detected sites of PCa in initial clinical studies. These early clinical results suggest that PET/CT using PSMA-targeted agents, especially with compounds of low molecular weight, will make valuable contributions to the management of PCa.

Imaging tumor burden in the brain with 89Zr-transferrin

A noninvasive technology that indiscriminately detects tumor tissue in the brain could substantially enhance the management of primary or metastatic brain tumors. Although the documented molecular heterogeneity of diseases that initiate or eventually deposit in the brain may preclude identifying a single smoking-gun molecular biomarker, many classes of brain tumors are generally avid for transferrin. Therefore, we reasoned that applying a radiolabeled derivative of transferrin ((89)Zr-labeled transferrin) may be an effective strategy to more thoroughly identify tumor tissue in the brain, regardless of the tumor's genetic background.

METHODS

Transferrin was radiolabeled with (89)Zr, and its properties with respect to human models of glioblastoma multiforme were studied in vivo.

RESULTS

In this report, we show proof of concept that (89)Zr-labeled transferrin ((89)Zr-transferrin) localizes to genetically diverse models of glioblastoma multiforme in vivo. Moreover, we demonstrate that (89)Zr-transferrin can detect an orthotopic lesion with exceptional contrast. Finally, the tumor-to-brain contrast conferred by (89)Zr-transferrin vastly exceeded that observed with (18)F-FDG, currently the most widely used radiotracer to assess tumor burden in the brain.

CONCLUSION

The results from this study suggest that (89)Zr-transferrin could be a broadly applicable tool for identifying and monitoring tumors in the brain, with realistic potential for near-term clinical translation.

Advances in immuno-positron emission tomography: antibodies for molecular imaging in oncology

Identification of cancer cell-surface biomarkers and advances in antibody engineering have led to a sharp increase in the development of therapeutic antibodies. These same advances have led to a new generation of radiolabeled antibodies and antibody fragments that can be used as cancer-specific imaging agents, allowing quantitative imaging of cell-surface protein expression in vivo. Immuno-positron emission tomography (immunoPET) imaging with intact antibodies has shown success clinically in diagnosing and staging cancer. Engineered antibody fragments, such as diabodies, minibodies, and single-chain Fv (scFv) -Fc, have been successfully employed for immunoPET imaging of cancer cell-surface biomarkers in preclinical models and are poised to bring same-day imaging into clinical development. ImmunoPET can potentially provide a noninvasive approach for obtaining target-specific information useful for titrating doses for radioimmunotherapy, for patient risk stratification and selection of targeted therapies, for evaluating response to therapy, and for predicting adverse effects, thus contributing to the ongoing development of personalized cancer treatment.

Molecular imaging of prostate cancer: PET radiotracers

OBJECTIVE

Recent advances in the fundamental understanding of the complex biology of prostate cancer have provided an increasing number of potential targets for imaging and treatment. The imaging evaluation of prostate cancer needs to be tailored to the various phases of this remarkably heterogeneous disease.

CONCLUSION

In this article, I review the current state of affairs on a range of PET radiotracers for potential use in the imaging evaluation of men with prostate cancer.

Measuring oncogenic signaling pathways in cancer with PET: an emerging paradigm from studies in castration-resistant prostate cancer

As parallel advances in cancer biology and drug development continue to elevate the role of targeted therapies in oncology, the need for imaging biomarkers that systematically measure the biology associated with therapeutic intervention has become more urgent. Although the molecular imaging community has a commitment to develop technologies to this end, few investigational radiotracers directly measure the biology of common oncogenic signaling pathways often addressed by targeted therapies. Visible progress has been achieved with a handful of radiotracers rationally designed to intercalate the pathobiology of prostate cancer, a molecularly heterogeneous disease nevertheless broadly defined by a fairly small repertoire of recurrent oncogenic lesions.

SIGNIFICANCE

That variable treatment responses or emergent resistance phenotypes are often documented in humans argues strongly for diagnostic technologies that can be realistically applied posttherapy to capture the dynamic patterns of disease response. The purpose of this review is to describe a collection of radiotracers developed to measure the pathobiology of prostate cancer for improved treatment monitoring, placing particular emphasis on the biologic rationale for their preparation. A chronologic description of radiotracer development programs is outlined, primarily to stress how an ongoing dialectic between earlier and more contemporary imaging technologies has accelerated discovery.

PET imaging with ⁸⁹Zr: from radiochemistry to the clinic

The advent of antibody-based cancer therapeutics has led to the concomitant rise in the development of companion diagnostics for these therapies, particularly nuclear imaging agents. A number of radioisotopes have been employed for antibody-based PET and SPECT imaging, notably ⁶⁴Cu, ¹²⁴I, ¹¹¹In, and (99m)Tc; in recent years, however, the field has increasingly focused on ⁸⁹Zr, a radiometal with near ideal physical and chemical properties for immunoPET imaging. In the review at hand, we seek to provide a comprehensive portrait of the current state of ⁸⁹Zr radiochemical and imaging research, including work into the production and purification of the isotope, the synthesis of new chelators, the development of new bioconjugation strategies, the creation of novel ⁸⁹Zr-based agents for preclinical imaging studies, and the translation of ⁸⁹Zr-labeled radiopharmaceuticals to the clinic. Particular attention will also be dedicated to emerging trends in the field, ⁸⁹Zr-based imaging applications using vectors other than antibodies, the comparative advantages and limitations of ⁸⁹Zr-based imaging compared to that with other isotopes, and areas that would benefit from more extensive investigation. At bottom, it is hoped that this review will provide both the experienced investigator and new scientist with a full and critical overview of this exciting and fast-developing field.

PET of signal transduction pathways in cancer

In this era of systems biology, the tide of information derived from "omic" technologies (genomics, proteomics, etc.) has sparked a revolution in drug design, with many industrial and academic programs now embracing the concepts of molecular medicine (i.e., targeting changes in specific proteins or pathways) as measures of treatment efficacy and outcome. This approach has yielded a plethora of new preclinical therapeutics directed at novel targets within oncology. In many ways, the evolution of molecular imaging agents as diagnostic probes mirrors that of chemotherapeutics; yet despite an increasing number of PET and SPECT radiotracers being evaluated in human trials, relatively few agents have found widespread use in clinical oncology. In light of this observation, is it time to reevaluate our strategies for radiopharmaceutical design and use? In this article, we argue that PET has enormous potential to deliver clinically relevant information on disease dynamics that extends beyond mapping the density and spatial distribution of a target. Recent developments in targeting pharmacodynamic biomarkers aim to exploit better the advantages of functional PET by detecting changes in signal transduction pathways, particularly in response to disease progression or treatment in cancer.

Monoclonal antibody RM2 as a potential ligand for a new immunotracer for prostate cancer imaging

OBJECTIVES

To investigate the potential of monoclonal antibody (mAb) RM2 as a ligand for a radioimmunotracer for prostate cancer imaging.

METHODS

Labeling was conducted with mAb RM2 and (125)I using the chloramine-T method. The cell study was conducted with PC-3 and LNCaP, which are prostate cancer cell lines, and MCF-7, which is a breast cancer cell line. The cells were treated or untreated with unlabeled mAb RM2 to block the haptoglobin-β chains expressed on the surface of the prostate cancer cells. (125)I-mAb RM2 was added into the cell culture media and cellular uptake of (125)I-mAb RM2 was evaluated at 1, 3 and 6 hours of incubation. For the in vivo biodistribution study, PC-3 cells were implanted in athymic male mice. The animals were injected intravenously with (125)I-mAb RM2. At 24, 48 and 72 hours after tracer injection, the animals were sacrificed and the activity levels of blood and tissue samples were determined.

RESULTS

The uptake of (125)I-mAb RM2 in the PC-3 and LNCaP cells increased according to the incubation time, while the uptake of (125)I-mAb RM2 in MCF-7 cells did not show any increase up to 6 hours. The increase of (125)I-RM2 uptake was not observed when the PC-3 and LNCaP cells were pre-treated with unlabeled RM2. In the biodistribution studies, (125)I-mAb RM2 showed marked uptake into the implanted PC-3 cells. In PC-3 tumor-bearing mice, the tumor muscle ratio of (125)I-RM2 was increased for up to 72 hours in a time-dependent manner.

CONCLUSIONS

(125)I-mAb RM2 showed excellent prostate cancer cell targeting in vitro and in vivo. Therefore, mAb RM2 seems to be a potential candidate for an immunoligand for prostate cancer imaging.

Molecular imaging in the management of cervical cancer

Positron emission tomography (PET), magnetic resonance imaging (MRI), and integrated 18-fluorodeoxyglucose ((18)F-FDG) PET/computed tomography are valuable techniques for assessing prognosis, treatment response after the completion of concurrent chemoradiation, suspicious or documented recurrence, unexplained post therapy elevations in tumor markers, and the response to salvage treatment when managing cervical cancer. However, PET plays a limited role in the primary staging of MRI-defined node-negative patients. Currently, (18)F-FDG is still the only tracer approved for routine use, but several novel targeting PET compounds, high-Tesla MRI machines, diffusion-weighted imaging without contrast, and dynamic nuclear polarized-enhanced (13)C-MR spectroscopic imaging may hold promising applications.

PET imaging with radiolabeled antibodies and tyrosine kinase inhibitors: immuno-PET and TKI-PET

During the last decade, the discovery of critical tumor targets has boosted the design of targeted therapeutic agents with monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs) receiving most of the attention. Immuno-positron emission tomography (immuno-PET) and TKI-PET, the in vivo tracking and quantification of mAbs and TKIs biodistribution with PET, are exciting novel options for better understanding of the in vivo behavior and efficacy of these targeted drugs in individual patients and for more efficient drug development. Very recently, current good manufacturing practice compliant procedures for labeling of mAbs with positron emitters have been described, as well as the preparation of some radiolabeled TKIs, while the first proof of principle studies has been performed in patients. In this review, technical developments in immuno-PET and TKI-PET are described, and their clinical potential is discussed. An overview is provided for the most appealing preclinical immuno-PET and TKI-PET studies, as well as the first clinical achievements with these emerging technologies.