PET of CXCR4 Expression by a 68Ga-Labeled Highly Specific Targeted Contrast Agent

First-in-Human Experience of CXCR4-Directed Endoradiotherapy with 177Lu- and 90Y-Labeled Pentixather in Advanced-Stage Multiple Myeloma with Extensive Intra- and Extramedullary Disease

Chemokine receptor 4 (CXCR4) is a key factor for tumor growth and metastasis in several types of human cancer. Based on promising experiences with a radiolabeled CXCR4 ligand ((68)Ga-pentixafor) for diagnostic receptor targeting, (177)Lu- and (90)Y-pentixather were recently developed as endoradiotherapeutic vectors. Here, we summarize the first-in-human experience in 3 heavily pretreated patients with intramedullary and extensive extramedullary manifestations of multiple myeloma undergoing CXCR4-directed endoradiotherapy.

METHODS

CXCR4 target expression was demonstrated by baseline (68)Ga-pentixafor PET. Each treatment was approved by the clinical ethics committee. Pretherapeutic (177)Lu-pentixather dosimetry was performed before (177)Lu-pentixather or (90)Y-pentixather treatment. Subsequently, patients underwent additional chemotherapy and autologous stem cell transplantation for bone marrow rescue.

RESULTS

A remarkable therapeutic effect was visualized in 2 patients, who showed a significant reduction in (18)F-FDG uptake.

CONCLUSION

CXCR4-targeted radiotherapy with pentixather appears to be a promising novel treatment option in combination with cytotoxic chemotherapy and autologous stem cell transplantation, especially for patients with advanced multiple myeloma.

Positron emission tomographic imaging of CXCR4 in cancer: challenges and promises

Molecular imaging is an attractive platform for noninvasive detection and assessment of cancer. In recent years, the targeted imaging of the C-X-C chemokine receptor 4 (CXCR4), a chemokine receptor that has been associated with tumor metastasis, has become an area of intensive research. This review article focuses on positron emission tomography (PET) and aims to provide useful and critical insights into the application of PET to characterize CXCR4 expression, including the chemical, radiosynthetic, and biological requirements for PET radiotracers. This discussion is informed by a summary of the different approaches taken so far and a comparison of their clinical translation. Finally, our expert opinions as to potential future advances in the field are expressed.

Radiosynthesis of clinical doses of ⁶⁸Ga-DOTATATE (GalioMedix™) and validation of organic-matrix-based ⁶⁸Ge/⁶⁸Ga generators

INTRODUCTION

68Ga-DOTATATE is a radiolabeled peptide-based agonist that targets somatostatin receptors overexpressed in neuroendocrine tumors. Here, we present our results on validation of organic matrix 68Ge/68Ga generators (ITG GmbH) applied for radiosynthesis of the clinical doses of 68Ga-DOTATATE (GalioMedixTM).

METHODS

The clinical grade of DOTATATE (25 μg±5 μg) compounded in 1 M NaOAc at pH=5.5 was labeled manually with 514±218 MBq (13.89±5.9 mCi) of 68Ga eluate in 0.05 N HCl at 95°C for 10 min. The radiochemical purity of the final dose was validated using radio-TLC. The quality control of clinical doses included tests of their osmolarity, endotoxin level, radionuclide identity, filter integrity, pH, sterility and 68Ge breakthrough.

RESULTS

The final dose of 272±126 MBq (7.35±3.4 mCi) of 68Ga-DOTATATE was produced with a radiochemical yield (RCY) of 99%±1%. The total time required for completion of radiolabeling and quality control averaged approximately 35 min. This resulted in delivery of 50%±7% of 68Ga-DOTATATE at the time of calibration (not decay corrected).

CONCLUSIONS

68Ga eluted from the generator was directly applied for labeling of DOTA-peptide with no additional pre-concentration or pre-purification of isotope. The low acidity of 68Ga eluate allows for facile synthesis of clinical doses with radiochemical and radionuclide purity higher than 98% and average activity of 272±126 MBq (7.3±3 mCi). There is no need for post-labeling C18 Sep-Pak purification of final doses of radiotracer. Advances in knowledge and implications for patient care. The clinical interest in validation of 68Galabeled agents has increased in the past years due to availability of generators from different vendors (Eckert-Ziegler, ITG, iThemba), favorable approach of U.S. FDA agency to initiate clinical trials, and collaboration of U.S. centers with leading EU clinical sites. The list of 68Ga-labeled tracers evaluated in clinical studies should growth because of the sensitivity of PET technique, the simplicity of the shakebake approach for the dose preparation and reliability of 68Ge/68Ga generators. Our studies have confirmed the reproducible elution profile, and high reliability of ITG GmbH generators required for routine doses preparation according to FDA recommendations.

Evaluation of (68)Ga- and (177)Lu-DOTA-PEG4-LLP2A for VLA-4-Targeted PET Imaging and Treatment of Metastatic Melanoma

Malignant melanoma is a highly aggressive cancer, and the incidence of this disease is increasing worldwide at an alarming rate. Despite advances in the treatment of melanoma, patients with metastatic disease still have a poor prognosis and low survival rate. New strategies, including targeted radiotherapy, would provide options for patients who become resistant to therapies such as BRAF inhibitors. Very late antigen-4 (VLA-4) is expressed on melanoma tumor cells in higher levels in more aggressive and metastatic disease and may provide an ideal target for drug delivery and targeted radiotherapy. In this study, we evaluated (177)Lu- and (68)Ga-labeled DOTA-PEG4-LLP2A as a VLA-4-targeted radiotherapeutic with a companion PET agent for diagnosis and monitoring metastatic melanoma treatment. DOTA-PEG4-LLP2A was synthesized by solid-phase synthesis. The affinity of (177)Lu- and (68)Ga-labeled DOTA-PEG4-LLP2A to VLA-4 was determined in B16F10 melanoma cells by saturation binding and competitive binding assays, respectively. Biodistribution of the LLP2A conjugates was determined in C57BL/6 mice bearing B16F10 subcutaneous tumors, while PET/CT imaging was performed in subcutaneous and metastatic models. (177)Lu-DOTA-PEG4-LLP2A showed high affinity to VLA-4 with a Kd of 4.1 ± 1.5 nM and demonstrated significant accumulation in the B16F10 melanoma tumor after 4 h (31.5 ± 7.8%ID/g). The tumor/blood ratio of (177)Lu-DOTA-PEG4-LLP2A was highest at 24 h (185 ± 26). PET imaging of metastatic melanoma with (68)Ga-DOTA-PEG4-LLP2A showed high uptake in sites of metastases and correlated with bioluminescence imaging of the tumors. These data demonstrate that (177)Lu-DOTA-PEG4-LLP2A has potential as a targeted therapeutic for treating melanoma as well as other VLA-4-expressing tumors. In addition, (68)Ga-DOTA-PEG4-LLP2A is a readily translatable companion PET tracer for imaging of metastatic melanoma.

The chemokine receptors CXCR4/CXCR7 and their primary heterodimeric ligands CXCL12 and CXCL12/high mobility group box 1 in pancreatic cancer growth and development: finding flow

Novel therapies need to be developed for patients with pancreatic cancer because of the poor outcomes of current regimens. Pancreatic cancer cells respond to the C-X-C chemokine receptor type 4 (CXCR4)/C-X-C chemokine receptor type 7 (CXCR7)/C-X-C motif chemokine 12 (CXCL12)/high-mobility group box 1 signaling axis and this axis presents a novel target for therapy. C-X-C motif chemokine 12 stimulates CXCR4/CXCR7-bearing cells in a paracrine manner. C-X-C chemokine receptor type 4 and CXCR7 are transmembrane G protein-coupled receptors that, upon interaction with ligand CXCL12, activate downstream protein kinases that promote a more aggressive behavior. C-X-C chemokine receptor type 4 is expressed on most pancreatic cancer cells, whereas CXCR7 is primarily expressed on tumor-associated endothelium. High-mobility group box 1 promotes the CXCR4 and CXCL12 interaction, promoting angiogenesis and lymphangiogenesis. Hypoxia-inducible factor 1 is a potent stimulator of CXCR4 and CXCL12 expression, promoting more aggressive behavior. This receptor/ligand interaction can be disrupted by CXCR4 antagonists available and in clinical use to harvest bone marrow stem cells. Novel imaging strategies are now being developed at several centers to evaluate response to therapy and identify early recurrence. Thus, the CXCR4/CXCR7/CXCL12 interaction plays a critical role in cancer cell progression, proliferation, invasion, as well as metastasis and is a suitable target for therapy, imaging, as well as development of novel diagnostics.

Prospective Study of (68)Ga-NOTA-NFB: Radiation Dosimetry in Healthy Volunteers and First Application in Glioma Patients

Purpose: The chemokine receptor CXCR4 is overexpressed in various types of human cancers. As a specific imaging agent of CXCR4, ⁶⁸Ga-NOTA-NFB was investigated in this study to assess its safety, biodistribution and dosimetry properties in healthy volunteers, and to preliminarily evaluate its application in glioma patients.

Methods: Six healthy volunteers underwent whole-body PET scans at 0, 0.5, 1, 2 and 3 h after ⁶⁸Ga-NOTA-NFB injection (mean dose, 182.4 ± 3.7 MBq (4.93 ± 0.10 mCi)). For time-activity curve calculations, 1 mL blood samples were obtained at 1, 3, 5, 10, 30, 60, 90, 120, 150 and 180 min after the injection. The estimated radiation doses were calculated by OLINDA/EXM software. Eight patients with glioma were enrolled and underwent both ⁶⁸Ga-NOTA-NFB and ¹⁸F-FDG PET/CT scans before surgery. The expression of CXCR4 on the resected brain tumor tissues was determined by immunohistochemical staining.

Results:⁶⁸Ga-NOTA-NFB was safe and well tolerated by all subjects. A rapid activity clearance from the blood circulation was observed. The organs with the highest absorbed doses were spleen (193.8 ± 32.5 μSv/MBq) and liver (119.3 ± 25.0 μSv/MBq). The mean effective dose was 25.4 ± 6.1 μSv/MBq. The maximum standardized uptake values (SUV_max) and the maximum target to non-target ratios (T/NT_max) of ⁶⁸Ga-NOTA-NFB PET/CT in glioma tissues were 4.11 ± 2.90 (range, 0.45-8.21) and 9.21 ± 8.75 (range, 3.66-24.88), respectively, while those of ¹⁸F-FDG PET/CT were 7.34 ± 2.90 (range, 3.50-12.27) and 0.86 ± 0.41 (range, 0.35-1.59). The histopathological staining confirmed that CXCR4 was overexpressed on resected tumor tissues with prominent ⁶⁸Ga-NOTA-NFB uptake.

Conclusion: With a favorable radiation dosimetry profile, ⁶⁸Ga-NOTA-NFB is safe for clinical imaging. Compared to ¹⁸F-FDG PET/CT, ⁶⁸Ga-NOTA-NFB PET/CT is more sensitive in detecting glioma and could have potential in diagnosing and treatment planning for CXCR4 positive patients.

Radiopharmaceuticals as probes to characterize tumour tissue

Tumour cells exhibit several properties that allow them to grow and divide. A number of these properties are detectable by nuclear imaging methods. We discuss crucial tumour properties that can be described by current radioprobe technologies, further discuss areas of emerging radioprobe development, and finally articulate need areas that our field should aspire to develop. The review focuses largely on positron emission tomography and draws upon the seminal 'Hallmarks of Cancer' review article by Hanahan and Weinberg in 2011 placing into context the present and future roles of radiotracer imaging in characterizing tumours.

Disclosing the CXCR4 expression in lymphoproliferative diseases by targeted molecular imaging

Chemokine ligand-receptor interactions play a pivotal role in cell attraction and cellular trafficking, both in normal tissue homeostasis and in disease. In cancer, chemokine receptor-4 (CXCR4) expression is an adverse prognostic factor. Early clinical studies suggest that targeting CXCR4 with suitable high-affinity antagonists might be a novel means for therapy. In addition to the preclinical evaluation of [(68)Ga]Pentixafor in mice bearing human lymphoma xenografts as an exemplary CXCR4-expressing tumor entity, we report on the first clinical applications of [(68)Ga]Pentixafor-Positron Emission Tomography as a powerful method for CXCR4 imaging in cancer patients. [(68)Ga]Pentixafor binds with high affinity and selectivity to human CXCR4 and exhibits a favorable dosimetry. [(68)Ga]Pentixafor-PET provides images with excellent specificity and contrast. This non-invasive imaging technology for quantitative assessment of CXCR4 expression allows to further elucidate the role of CXCR4/CXCL12 ligand interaction in the pathogenesis and treatment of cancer, cardiovascular diseases and autoimmune and inflammatory disorders.

Biomarkers in preclinical cancer imaging

In view of the trend towards personalized treatment strategies for (cancer) patients, there is an increasing need to noninvasively determine individual patient characteristics. Such information enables physicians to administer to patients accurate therapy with appropriate timing. For the noninvasive visualization of disease-related features, imaging biomarkers are expected to play a crucial role. Next to the chemical development of imaging probes, this requires preclinical studies in animal tumour models. These studies provide proof-of-concept of imaging biomarkers and help determine the pharmacokinetics and target specificity of relevant imaging probes, features that provide the fundamentals for translation to the clinic. In this review we describe biological processes derived from the “hallmarks of cancer” that may serve as imaging biomarkers for diagnostic, prognostic and treatment response monitoring that are currently being studied in the preclinical setting. A number of these biomarkers are also being used for the initial preclinical assessment of new intervention strategies. Uniquely, noninvasive imaging approaches allow longitudinal assessment of changes in biological processes, providing information on the safety, pharmacokinetic profiles and target specificity of new drugs, and on the antitumour effectiveness of therapeutic interventions. Preclinical biomarker imaging can help guide translation to optimize clinical biomarker imaging and personalize (combination) therapies.

Structural characterization and in vivo evaluation of β-Hairpin peptidomimetics as specific CXCR4 imaging agents

The CXCR4 chemokine receptor is integral to several biological functions and plays a pivotal role in the pathophysiology of many diseases. As such, CXCR4 is an enticing target for the development of imaging and therapeutic agents. Here we report the evaluation of the POL3026 peptidomimetic template for the development of imaging agents that target CXCR4. Structural and conformational analyses of POL3026 and two of its conjugates, DOTA (POL-D) and PEG12-DOTA (POL-PD), by circular dichroism, two-dimensional NMR spectroscopy and molecular dynamics calculations are reported. In silico observations were experimentally verified with in vitro affinity assays and rationalized using crystal structure-based molecular modeling studies. [(111)In]-labeled DOTA conjugates were assessed in vivo for target specificity in CXCR4 expressing subcutaneous U87 tumors (U87-stb-CXCR4) through single photon emission computed tomography (SPECT/CT) imaging and biodistribution studies. In silico and in vitro studies show that POL3026 and its conjugates demonstrate similar interactions with different micelles that mimic cellular membrane and that the ε-NH2 of lysine(7) is critical to maintain high affinity to CXCR4. Modification of this group with DOTA or PEG12-DOTA led to the decrease of IC50 value from 0.087 nM for POL3026 to 0.47 nM and 1.42 nM for POL-D and POL-PD, respectively. In spite of the decreased affinity toward CXCR4, [(111)In]POL-D and [(111)In]POL-PD demonstrated high and significant uptake in U87-stb-CXCR4 tumors compared to the control U87 tumors at 90 min and 24 h post injection. Uptake in U87-stb-CXCR4 tumors could be blocked by unlabeled POL3026, indicating specificity of the agents in vivo. These results suggest POL3026 as a promising template to develop new imaging agents that target CXCR4.

CXCR4 as biomarker for radioresistant cancer stem cells

PURPOSE

Radioresistance of cancer cells remains a fundamental barrier for maximum efficient radiotherapy. Tumor heterogeneity and the existence of distinct cell subpopulations exhibiting different genotypes and biological behaviors raise difficulties to eradicate all tumorigenic cells. Recent evidence indicates that a distinct population of tumor cells, called cancer stem cells (CSC), is involved in tumor initiation and recurrence and is a putative cause of tumor radioresistance. There is an urgent need to identify the intrinsic molecular mechanisms regulating the generation and maintenance of resistance to radiotherapy, especially within the CSC subset. The chemokine C-X-C motif receptor 4 (CXCR4) has been found to be a prognostic marker in various types of cancer, being involved in chemotaxis, stemness and drug resistance. The interaction of CXCR4 with its ligand, the chemokine C-X-C motif ligand 12 (CXCL12), plays an important role in modulating the tumor microenvironment, angiogenesis and CSC niche. Moreover, the therapeutic inhibition of the CXCR4/CXCL12 signaling pathway is sensitizing the malignant cells to conventional anti-cancer therapy.

CONTENT

Within this review we are summarizing the role of the CXCR4/CXCL12 axis in the modulation of CSC properties, the regulation of the tumor microenvironment in response to irradiation, therapy resistance and tumor relapse.

CONCLUSION

In light of recent findings, the inhibition of the CXCR4/CXCL12 signaling pathway is a promising therapeutic option to refine radiotherapy.

Multimodality imaging of CXCR4 in cancer: current status towards clinical translation

CXCR4 has gained tremendous attention over the last decade, since it was found to be up-regulated in a wide variety of cancer types, in addition to its role in human immunodeficiency virus infection. Molecular imaging of CXCR4 with small molecules, peptides, and antibodies has been a vibrant research area over the last several years. In this review article, we will summarize the current status of imaging CXCR4 with fluorescence, bioluminescence, positron emission tomography, and single-photon emission computed tomography techniques. Since each molecular imaging modality has its own strengths and weaknesses, dualmodality probes that can be detected by more than one imaging techniques have also been investigated. Noninvasive visualization of CXCR4 expression has potential clinical applications in multiple facets of patient management. While big strides have been made over the last several years in the development of CXCR4- targeted imaging probes, clinical translation and investigation of these agents in cancer patients are eagerly awaited. Since CXCR4 is also involved in many other diseases beyond cancer, these clinically translatable probes can also play multiple roles in other pathological disorders such as myocardial infarction and several immunodeficiency disorders.

Bridged cyclams as imaging agents for chemokine receptor 4 (CXCR4)

Over-expression of chemokine receptor 4 (CXCR4) is present in a majority of cancers, has been linked to an aggressive phenotype, and may indicate the metastatic potential of primary tumor. Several CXCR4 targeted therapeutics are in clinical trials and the development of the corresponding imaging agents is an area of active interest. Previously, (64)Cu-labeled imaging agents for CXCR4 have provided clear images of CXCR4-bearing tissues in relevant experimental models but demonstrated fast washout from tissues harboring receptor. Addition of stabilizing bridges is known to provide more robust chelator-Cu(II) complexes. In addition, bridged cyclam-based CXCR4 binding agents demonstrated increased receptor residence times relative to existing agents. Based on that knowledge we synthesized several bridged cyclam analogs of AMD3465, a monocyclam-based CXCR4 imaging agent, to increase the retention time of the tracer bound to the receptor to allow for protracted imaging and improved target-to-non-target ratios. Specific accumulation of two radiolabeled, cross-bridged analogs ([(64)Cu] RAD1-24 and [(64)Cu]RAD1-52) was observed in U87-stb-CXCR4 tumors in both PET/CT imaging and biodistribution studies. At 90min post-injection of radiotracer, tumor-to-muscle and tumor-to-blood ratios reached 106.05±17.19 and 28.08±4.78, respectively, for cross-bridged pyrimidine analog [(64)Cu]RAD1-52. Receptor blockade performed in vivo denoted target binding specificity. The biodistribution and PET/CT imaging studies with the radiolabeled bridged cyclams demonstrated longer tumor retention and comparable uptake to [(64)Cu]AMD3465, though [(64)Cu]AMD3465 demonstrated superior overall pharmacokinetics.

A radiogallium-DOTA-based bivalent peptidic ligand targeting a chemokine receptor, CXCR4, for tumor imaging

We have developed a novel radiogallium (Ga)-DOTA-based bivalent peptidic ligand targeting a chemokine receptor, CXCR4, for tumor imaging. A CXCR4 imaging probe with two CXCR4 antagonists (Ac-TZ14011) on Ga-DOTA core, Ga-DOTA-TZ2, was synthesized, and the affinity and binding to CXCR4 was evaluated in CXCR4 expressing cells in vitro. The affinity of Ga-DOTA-TZ2 for CXCR4 was 20-fold greater than the corresponding monovalent probe, Ga-DOTA-TZ1. (67)Ga-DOTA-TZ2 showed the significantly higher accumulation in CXCR4-expressing tumor cells compared with (67)Ga-DOTA-TZ1, suggesting the bivalent effect enhances its binding to CXCR4. The incorporation of two CXCR4 antagonists to Ga-DOTA could be effective in detecting CXCR4-expressing tumors.

Preclinical evaluation of a CXCR4-specific (68)Ga-labelled TN14003 derivative for cancer PET imaging

Molecular imaging is an ideal platform for non-invasive detection and assessment of cancer. In recent years, the targeted imaging of CXCR4, a chemokine receptor that has been associated with tumour metastasis, has become an area of intensive research. In our pursuit of a CXCR4-specific radiotracer, we designed and synthesised a novel derivative of the CXCR4 peptidic antagonist TN14003, CCIC16, which is amenable to radiolabelling by chelation with a range of PET and SPECT radiometals, such as (68)Ga, (64)Cu and (111)In as well as (18)F (Al(18)F). Potent in vitro binding affinity and inhibition of signalling-dependent cell migration by unlabelled CCIC16 were confirmed by a threefold uptake in CXCR4-over-expressing cells compared to their isogenic counterparts. Furthermore, in vivo experiments demonstrated the favourable pharmacokinetic properties of the (68)Ga-labelled tracer (68)Ga-CCIC16, along with its CXCR4-specific accumulation in tissues with desirable contrast (tumour-to-muscle ratio: 9.5). The specificity of our tracer was confirmed by blocking experiments. Taking into account the attractive intrinsic PET imaging properties of (68)Ga, the comprehensive preclinical evaluation presented here suggests that (68)Ga-CCIC16 is a promising PET tracer for the specific imaging of CXCR4-expressing tumours.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Targets in small cell lung cancer

Recurrent small cell lung cancer is a recalcitrant malgnancy. The application of genomic technologies has begun to elucidate the large number of genetic abnormalities in SCLC. Several cell surface receptors are known to be overexpressed by SCLC in clinic specimens and cell in culture including GPCRs such as the bradykinin receptor, the chemokine receptor CXCR4, the vasopression receeptor and the three bomebsin receptors. The glucose transporter GLUT1, the tetraspanin family member PETA/CD151 and the immunoglobulin superfamily member ALCAM/CD166 are also overexpressed by SCLC. NCAM/CD56 is overexpressed by nearly all SCLC and is currently the target for an antibody drug conjugate in Phase II trial. Although SCLC is not considered a RTK driven disease, IGF1R and FGFRs are often overexpressed by SCLC. SCLC abberantly expresses several developmental transcription factors including ASCL1, SOX2, 4, and 11, OCT4, NANOG, PAX5; however, overexpression of MYC may be a driver in SCLC. Like other cancers, SCLC expresses survival factors and uses aerobic glycolysis as a major source of ATP. The drawback of many potential targets overexpressed by SCLC is expression of the same proteins by normal tissues. We are slowly learning more about the molecular abnormalities that occur in SCLC; however, therapeutic impact from new findings remains a goal to work toward.

Gallium-68: chemistry and radiolabeled peptides exploring different oncogenic pathways

Abstract Early and specific tumor detection and also therapy selection and response evaluation are some challenges of personalized medicine. This calls for high sensitive and specific molecular imaging such as positron emission tomography (PET). The use of peptides for PET molecular imaging has undeniable advantages: possibility of targeting through peptide-receptor interaction, small size and low-molecular weight conferring good penetration in the tissue or at cellular level, low toxicity, no antigenicity, and possibility of wide choice for radiolabeling. Among β(+)-emitter radioelements, Gallium-68 is a very attractive positron-emitter compared with carbon-11 or fluorine-18 taking into account its easy production via a (68)Ge/(68)Ga generator and well established radiochemistry. Gallium-68 chemistry is based on well-defined coordination complexes with macrocycle or chelates having strong binding properties, particularly suitable for linking peptides that allow resistance to in vivo transchelation of the metal ion. Understanding specific and nonspecific molecular mechanisms involved in oncogenesis is one major key to develop new molecular imaging tools. The present review focuses on peptide signaling involved in different oncogenic pathways. This peptide signalization might be common for tumoral and non-tumoral processes or could be specific of an oncological process. This review describes gallium chemistry and different (68)Ga-radiolabeled peptides already in use or under development aiming at developing molecular PET imaging of different oncological processes.

Molecular imaging of chemokine receptor CXCR4

CXCR4 was found to be expressed by many different types of human cancers and its expression has been correlated with tumor aggressiveness, poor prognosis and resistance to chemotherapy. CXCR4 was also shown to contribute to metastatic seeding of organs that express its ligand CXCL12 and support the survival of these cells. These findings suggest that CXCR4 is a potentially attractive therapeutic target, and several antagonists and antibodies for this receptor were developed and are under clinical evaluation. Quantifying CXCR4 expression non-invasively might aid in prognostication as a mean for personalized therapy and post treatment monitoring. Multiple attempts were done over the recent years to develop imaging agents for CXCR4 using different technologies including PET, SPECT, fluorescent and bioluminescence, and will be reviewed in this paper.

Molecular Imaging using PET/CT Applying 68Ga-Labeled Tracers and Targeted Radionuclide Therapy: Theranostics on the Way to Personalized Medicine

Theranostics is an acronym, which exemplifies the togetherness of diagnostics and therapeutics in the individualized management of disease. The key to personalized medicine in cancer is to determine the molecular phenotypes of neoplasms, so that specific probes can be selected to target the tumor and its microenvironment. Molecular imaging and radionuclide therapy using a particular probe is based on this premise. Neuroendocrine neoplasms express somatostatin receptors, enabling the use of somatostatin analogs for molecular imaging, when labeled with the positron-emitter 68Ga for receptor positron emission tomography/computed tomography (PET/CT), and targeted radionuclide therapy, when labeled with beta-emitters 90Y and 177Lu.

How to cite this article

Kulkarni HR, Baum RP. Molecular Imaging using PET/CT Applying 68Ga-Labeled Tracers and Targeted Radionuclide Therapy: Theranostics on the Way to Personalized Medicine. J Postgrad Med Edu Res 2013; 47(1):47-53.

An Increasing Role for 68Ga PET Imaging: A Perspective on the Availability of Parent 68Ge Material for Generator Manufacturing in an Expanding Market

The use of gallium-68 for molecular imaging is gaining momentum world-wide. While our understanding of 68Ga chemistry, generators, and associated synthesis modules appear to have advanced to a clinically-reliable stage, uncertainty in the supply of radiopharmaceutically-suitable parent is of significant concern. In this work, we examine the current supply of 68Ge in an effort to better understand the potential for expansion of manufacturing to meet an increasing demand for 68Ga. Although specific information on sales and demand of 68Ge is highly business sensitive and thus guarded, our examination finds no shortage in the current supply of 68Ge. On the other hand, increases in the use of 68Ge generators for clinical applications in the United States point to the need for continued support for production at DOE laboratories in the United States to ensure a reliable supply and suggests that new commercial facilities may be needed to meet the increasing demand.

How to cite this article

Schultz MK, Donahue P, Musgrave NI, Zhernosekov K, Naidoo C, Razbash A, Tworovska I, Dick DW, Watkins GL, Graham MM, Runde W, Clanton JA, Sunderland JJ. An Increasing Role for 68Ga PET Imaging: A Perspective on the Availability of Parent 68Ge Material for Generator Manufacturing in an Expanding Market. J Postgrad Med Edu Res 2013;47(1):26-30.

MR/PET: a new challenge in hybrid imaging

OBJECTIVE

This article provides a short overview of hybrid imaging and the potential advantages of combined MR/PET. We will address some of the challenges that had to be met before MR/PET could become clinically available as well as further scientific work that has to be done to increase the potential benefit of this emerging hybrid modality.

CONCLUSION

Hybrid imaging, the combination of two imaging modalities into one, promises the compensation of specific deficits of the modalities involved. PET/CT has gained wide acceptance for oncologic imaging in recent years; however, MRI has certain advantages that could make combined MR/PET more tempting in various clinical applications. The development of new clinical whole-body MR/PET systems offers new insights in metabolic and functional processes in oncology as well as cardiovascular and neurologic diseases.

In vivo imaging of therapy-induced anti-cancer immune responses in humans

Immunotherapy aims to re-engage and revitalize the immune system in the fight against cancer. Research over the past decades has shown that the relationship between the immune system and human cancer is complex, highly dynamic, and variable between individuals. Considering the complexity, enormous effort and costs involved in optimizing immunotherapeutic approaches, clinically applicable tools to monitor therapy-induced immune responses in vivo are most warranted. However, the development of such tools is complicated by the fact that a developing immune response encompasses several body compartments, e.g., peripheral tissues, lymph nodes, lymphatic and vascular systems, as well as the tumor site itself. Moreover, the cells that comprise the immune system are not static but constantly circulate through the vascular and lymphatic system. Molecular imaging is considered the favorite candidate to fulfill this task. The progress in imaging technologies and modalities has provided a versatile toolbox to address these issues. This review focuses on the detection of therapy-induced anticancer immune responses in vivo and provides a comprehensive overview of clinically available imaging techniques as well as perspectives on future developments. In the discussion, we will focus on issues that specifically relate to imaging of the immune system and we will discuss the strengths and limitations of the current clinical imaging techniques. The last section provides future directions that we envision to be crucial for further development.

Imaging agents for the chemokine receptor 4 (CXCR4)

The interaction between the chemokine receptor 4 (CXCR4) and stromal cell-derived factor-1 (SDF-1, also known as CXCL12) is a natural regulatory process in the human body. However, CXCR4 over-expression is also found in diseases such as cancer, where it plays a role in, among others, the metastatic spread. For this reason it is an interesting biomarker for the field of diagnostic oncology, and therefore, it is gaining increasing interest for applications in molecular imaging. Especially "small-molecule" imaging agents based on T140, FC131 and AMD3100 have been extensively studied. SDF-1, antibodies, pepducins and bioluminescence have also been used to visualize CXCR4. In this critical review reported CXCR4 targeting imaging agents are described based on their affinity, specificity and biodistribution. The level wherein CXCR4 is up-regulated in cancer patients and its relation to the different cell lines and animal models used to evaluate the efficacy of the imaging agents is also discussed (221 references).

Stroma targeting nuclear imaging and radiopharmaceuticals

Malignant transformation of tumor accompanies profound changes in the normal neighboring tissue, called tumor stroma. The tumor stroma provides an environment favoring local tumor growth, invasion, and metastatic spreading. Nuclear imaging (PET/SPECT) measures biochemical and physiologic functions in the human body. In oncology, PET/SPECT is particularly useful for differentiating tumors from postsurgical changes or radiation necrosis, distinguishing benign from malignant lesions, identifying the optimal site for biopsy, staging cancers, and monitoring the response to therapy. Indeed, PET/SPECT is a powerful, proven diagnostic imaging modality that displays information unobtainable through other anatomical imaging, such as CT or MRI. When combined with coregistered CT data, [(18)F]fluorodeoxyglucose ([(18)F]FDG)-PET is particularly useful. However, [(18)F]FDG is not a target-specific PET tracer. This paper will review the tumor microenvironment targeting oncologic imaging such as angiogenesis, invasion, hypoxia, growth, and homing, and also therapeutic radiopharmaceuticals to provide a roadmap for additional applications of tumor imaging and therapy.