Molecular imaging of chemokine receptor CXCR4

Straightforward synthesis of bis-tetraazacycloalkanes: towards new potential CXCR4 antagonists?

We report herein an efficient and general method for the synthesis of new bismacrocyclic compounds, structural analogues of biscyclam AMD3100, in which the two macrocycles are linked together through carbon atoms of the cycles.

Al[18F]NOTA-T140 Peptide for Noninvasive Visualization of CXCR4 Expression

PURPOSE

Chemokine receptor CXCR4 plays an important role in tumor aggressiveness, invasiveness, and metastasis formation. Quantification of CXCR4 expression by tumors may have an impact on prediction and evaluation of tumor response to therapies. In this study, we developed a robust and straightforward F-18 labeling route of T140, a CXCR4 peptide-based antagonist.

PROCEDURES

T140 derivative was conjugated to 1,4,7-triazacyclononane-triacetic acid (NOTA) and labeled with Al[(18)F]. Al[(18)F]NOTA-T140 was evaluated in vitro in cell-based assay and stability in mouse serum and in vivo using CXCR4 positive and negative tumor xenograft models.

RESULTS

Labeling of Al[(18)F]NOTA-T140 was completed within 30 min with a radiochemical yield of 58 ± 5.3 % at the end of synthesis, based on fluoride-18 activity. Al[(18)F]NOTA-T140 accumulated in CHO-CXCR4 positive but not negative tumors. Al[(18)F]NOTA-T140 uptake in the tumors correlated with CXCR4 protein expression. Moreover, Al[(18)F]NOTA-T140 had high accumulation in CXCR4-positive metastatic tumors.

CONCLUSIONS

The simplicity of Al[(18)F]NOTA-T140 labeling along with its properties to specifically image CXCR4 expression by tumors warrant further clinical application for the diagnosis of CXCR4 clinically.

Aspartate-Based CXCR4 Chemokine Receptor Binding of Cross-Bridged Tetraazamacrocyclic Copper(II) and Zinc(II) Complexes

The CXCR4 chemokine receptor is implicated in a number of diseases including HIV infection and cancer development and metastasis. Previous studies have demonstrated that configurationally restricted bis-tetraazamacrocyclic metal complexes are high-affinity CXCR4 antagonists. Here, we present the synthesis of Cu(2+) and Zn(2+) acetate complexes of six cross-bridged tetraazamacrocycles to mimic their coordination interaction with the aspartate side chains known to bind them to CXCR4. X-ray crystal structures for three new Cu(2+) acetate complexes and two new Zn(2+) acetate complexes demonstrate metal-ion-dependent differences in the mode of binding the acetate ligand concomitantly with the requisite cis-V-configured cross-bridged tetraazamacrocyle. Concurrent density functional theory molecular modelling studies produced an energetic rationale for the unexpected [Zn(OAc)(H2 O)](+) coordination motif present in all of the Zn(2+) cross-bridged tetraazamacrocycle crystal structures, which differs from the chelating acetate [Zn(OAc)](+) structures of known unbridged and side-bridged tetraazamacrocyclic Zn(2+) -containing CXCR4 antagonists.

Gold Nanoclusters Doped with (64)Cu for CXCR4 Positron Emission Tomography Imaging of Breast Cancer and Metastasis

As an emerging class of nanomaterial, nanoclusters hold great potential for biomedical applications due to their unique sizes and related properties. Herein, we prepared a (64)Cu doped gold nanocluster ((64)CuAuNC, hydrodynamic size: 4.2 ± 0.5 nm) functionalized with AMD3100 (or Plerixafor) for targeted positron emission tomography (PET) imaging of CXCR4, an up-regulated receptor on primary tumor and lung metastasis in a mouse 4T1 orthotopic breast cancer model. The preparation of targeted (64)CuAuNCs-AMD3100 (4.5 ± 0.4 nm) was done via one-step reaction with controlled conjugation of AMD3100 and specific activity, as well as improved colloid stability. In vivo pharmacokinetic evaluation showed favorable organ distribution and significant renal and fecal clearance within 48 h post injection. The expression of CXCR4 in tumors and metastasis was characterized by immunohistochemistry, Western blot, and reverse transcription polymerase chain reaction analysis. PET imaging with (64)CuAuNCs-AMD3100 demonstrated sensitive and accurate detection of CXCR4 in engineered tumors expressing various levels of the receptor, while competitive receptor blocking studies confirmed targeting specificity of the nanoclusters. In contrast to nontargeted (64)CuAuNCs and (64)Cu-AMD3100 alone, the targeted (64)CuAuNCs-AMD3100 detected up-regulated CXCR4 in early stage tumors and premetastatic niche of lung earlier and with greater sensitivity. Taken together, we believe that (64)CuAuNCs-AMD3100 could serve as a useful platform for early and accurate detection of breast cancer and metastasis providing an essential tool to guide the treatment.

Molecular imaging to guide systemic cancer therapy: Illustrative examples of PET imaging cancer biomarkers

Molecular imaging agents have the ability to non-invasively visualize, characterize, and quantify the molecular biology of disease. Recent advances in nuclear probe development, particularly in PET radiotracers, have generated many new imaging agents with precise molecular targets. With such specificity, PET probes may be utilized as biomarkers to objectively interrogate and evaluate pathology. Whereas the current indications for PET imaging are predominately confined to staging and restaging of malignancy, the utility of PET greatly expands when utilized as a biomarker, the topic of this review. As an imaging biomarker, PET may be used to (1) measure target expression to select subsets of patients who would most benefit from targeted therapy; (2) measure early treatment response to predict therapeutic efficacy; and (3) relate tumor response to survival. This review will discuss the application of radiotracers to targeted cancer therapy. Particular attention is given to new radiotracers evaluated in recently completed clinical trials and those with current or potential clinical utility. The diverse roles of PET in clinical trails for drug development are also examined.

The influence of different metal-chelate conjugates of pentixafor on the CXCR4 affinity

Background

The overexpression of the chemokine receptor 4 (CXCR4) in different epithelial, mesenchymal, and hematopoietic cancers makes CXCR4 an attractive diagnostic and therapeutic target. However, targeting the CXCR4 receptor with small cyclic pentapeptide-based radiopharmaceuticals remains challenging because minor structural modifications within the ligand-linker-chelate structure often significantly affect the receptor affinity. Based on the excellent in vivo properties of CXCR4-directed pentapeptide [⁶⁸Ga]pentixafor (cyclo(-d-Tyr-N-Me-d-Orn(AMB-DOTA)-l-Arg-l-2-Nal-Gly-)), this study aims to broaden the spectrum of applicable (radio)metal-labeled pentixafor analogs.

Methods

Cyclic pentapeptides, based on the pentixafor scaffold, were synthesized by a combined solid- and solution-phase peptide synthesis. The CXCR4 receptor affinities of the cold reference compounds were determined in competitive binding assays using CXCR4-expressing Jurkat T - cell leukemia cells and [¹²⁵I]FC131 as the radioligand.

Results

Metalated pentixafor derivatives with cyclic and acyclic chelators were synthesized by solid-phase peptide synthesis and evaluated in vitro. The resulting CXCR4 affinities (IC₅₀) were highly dependent on the chelator and metal used. Two pentapeptides, Ga-NOTA and Bi-DOTA conjugates, offer an improved affinity compared to [⁶⁸Ga]pentixafor.

Conclusions

Based on the pentapeptide [⁶⁸Ga]pentixafor, a broad range of metal-labeled analogs were investigated. The affinities of the new compounds were found to be strongly dependent on both the chelator and the metal used. Bi-labeled pentixafor showed high receptor affinity and seems to be a promising ligand for further preclinical evaluation and future α-emitter-based endoradiotherapy.

Differential expression and prognostic value of the chemokine receptor CXCR4 in bronchopulmonary neuroendocrine neoplasms

Introduction

For many tumors, the overexpression of the chemokine receptor CXCR4 is associated with increased malignancy and poor patient outcomes. However, comprehensive data for neuroendocrine neoplasms of the lung are still lacking.

Methods

CXCR4 expression was evaluated in a panel of bronchopulmonary neuroendocrine neoplasms (BP-NEN) comprising typical carcinoids (n = 26), atypical carcinoids (n = 30), and small cell lung cancers (SCLC, n = 34). Samples were analyzed by immunohistochemistry using the novel monoclonal rabbit anti-human CXCR4 antibody UMB-2 and by qRT-PCR. The expression was correlated with clinical data and overall patient survival.

Results

CXCR4 was predominantly localized at the plasma membrane of the tumor cells. CXCR4 was expressed with a high intensity in almost all of the 30 SCLC samples. In contrast, it was detected infrequently and with low intensity in the typical carcinoid and atypical carcinoid samples. There was a significant correlation between the immunohistochemistry and qRT-PCR data. Additionally, there was a significant negative relationship between CXCR4 expression and overall survival.

Conclusions

With increasing malignancy, BP-NEN clearly differ in the extent of CXCR4 expression. As in other tumor entities, CXCR4 overexpression significantly correlates with negative patient outcome. Due to its particular high expression rate in SCLC, CXCR4 may serve as a promising new target for diagnostic and pharmacological intervention as well as for peptide receptor-based radionuclide therapy.

AMD3100: A Versatile Platform for CXCR4 Targeting (68)Ga-Based Radiopharmaceuticals

CXCR4 is a G protein-coupled receptor (GPCR), which is overexpressed in numerous diseases, particularly in multiple cancers. Therefore, this receptor represents a valuable target for imaging and therapeutic purposes. Among the different approaches, which were developed for CXCR4 imaging, a CXCR4 antagonist biscyclam system (AMD3100, also called Mozobil), currently used in the clinic for the mobilization of hematopoietic stem cells, was radiolabeled with different radiometals such as (62)Zn, (64)Cu, (67)Ga, or (99m)Tc. However, cyclam is not an ideal chelator for most of these radiometals, and could lead to the release of the radionuclide in vivo. In the current study, a new family of CXCR4 imaging agents is presented, in which AMD3100 is used as a carrier for specific delivery of an imaging reporter, i.e., a (68)Ga complex for PET imaging. AMD3100 was functionalized on the phenyl moiety with different linkers, either ethylenediamine or diamino-polyethylene glycol 3 (PEG3). The resulting AMD3100 analogues were further coupled with two different chelators, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) or 1,4,7-triazacyclononane-1-glutaric acid-4,7-acetic acid (NODAGA). Five potential CXCR4 targeting agents were obtained. The derived AMD3100-based ligands were labeled with (68)Ga, highlighting the influence of the spacer nature on the (68)Ga-labeling yield. The lipophilic character of the different systems was also investigated, as well as their affinity for the CXCR4 receptor. The most promising compound was further evaluated in vivo in H69 tumor xenografts by biodistribution and PET imaging studies, validating the proof of principle of our concept.

First Experience with Chemokine Receptor CXCR4-Targeted PET Imaging of Patients with Solid Cancers

CXCR4 is a chemokine receptor that is overexpressed in various human cancers and is involved in tumor metastasis. The aim of this proof-of-concept study was to evaluate a novel CXCR4-targeted PET probe in patients with solid cancers with reported in vitro evidence of CXCR4 overexpression and to estimate its potential diagnostic value.

METHODS

Twenty-one patients with histologically proven pancreatic cancer, laryngeal cancer, non-small cell lung cancer, prostate cancer, melanoma, breast cancer, hepatocellular carcinoma, glioblastoma, sarcoma, or cancer of unknown primary underwent PET imaging using the novel CXCR4 nuclear probe (68)Ga-pentixafor. The SUVmax of the liver, spleen, and bone marrow was measured to determine physiologic tracer distribution. For evaluation of tracer accumulation in solid cancers, SUVmax and tumor-to-background (T/B) ratios were determined in a total of 43 malignant lesions, including 8 primary tumors, 3 locally recurrent tumors, and 32 metastases. When available, the SUVmax of malignant lesions was compared with the corresponding SUVmax measured in routine (18)F-FDG PET.

RESULTS

Moderate tracer accumulation was detectable in the liver, bone marrow, and spleen, with a mean SUVmax of 3.1, 3.7, and 5.6, respectively. By visual interpretation criteria, 9 of 11 primary and locally recurrent tumors were detectable, exhibiting a mean SUVmax of 4.7 (range, 2.1-10.9) and a mean T/B ratio of 2.9. Twenty of 32 evaluated metastases were visually detectable (mean SUVmax, 4.5 [range, 3.2-13.8]; mean T/B ratio, 2.8). The highest signal was detected in a patient with non-small cell lung cancer (SUVmax, 10.9; T/B ratio, 8.4) and a patient with cancer of unknown primary (SUVmax, 13.8; T/B ratio, 8.1). Compared with (18)F-FDG PET, which was additionally performed in 10 patients, (68)Ga-pentixafor PET had a lower SUVmax in all measured malignant lesions.

CONCLUSION

On the basis of these first observations in a small and heterogeneous patient cohort, the in vitro CXCR4 expression profile of solid cancers and metastases described in the previous literature does not seem to sufficiently depict the in vivo distribution revealed by CXCR4-targeted PET. Moreover, the detectability of solid cancers seems to be generally lower for (68)Ga-pentixafor than for (18)F-FDG PET.

In vivo molecular imaging of chemokine receptor CXCR4 expression in patients with advanced multiple myeloma

CXCR4 is a G-protein-coupled receptor that mediates recruitment of blood cells toward its ligand SDF-1. In cancer, high CXCR4 expression is frequently associated with tumor dissemination and poor prognosis. We evaluated the novel CXCR4 probe [(68)Ga]Pentixafor for in vivo mapping of CXCR4 expression density in mice xenografted with human CXCR4-positive MM cell lines and patients with advanced MM by means of positron emission tomography (PET). [(68)Ga]Pentixafor PET provided images with excellent specificity and contrast. In 10 of 14 patients with advanced MM [(68)Ga]Pentixafor PET/CT scans revealed MM manifestations, whereas only nine of 14 standard [(18)F]fluorodeoxyglucose PET/CT scans were rated visually positive. Assessment of blood counts and standard CD34(+) flow cytometry did not reveal significant blood count changes associated with tracer application. Based on these highly encouraging data on clinical PET imaging of CXCR4 expression in a cohort of MM patients, we conclude that [(68)Ga]Pentixafor PET opens a broad field for clinical investigations on CXCR4 expression and for CXCR4-directed therapeutic approaches in MM and other diseases.

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.

CXCR4-targeted near-infrared imaging allows detection of orthotopic and metastatic human osteosarcoma in a mouse model

CXCR4 is expressed at primary and metastatic sites of osteosarcoma. We developed a novel CXCR4-targeted near-infrared (NIR) fluorescent imaging agent (referred to as CXCR4-IR-783). The binding to representative osteosarcoma cells (F5M2 and F4 for high- and low- CXCR4 expression) was examined. CXCR4-IR-783 fluorescence was also examined in a mouse xenograft model of human osteosarcoma using NIR fluorescence microscopy and a Kodak in-vivo multispectral system. Pulmonary metastases in mice bearing osteosarcoma xenografts were detected by micro CT, ¹⁸F-PET scan and NIR imaging scan. Briefly, the binding of CXCR4-IR-783 was significantly higher in F5M2 than in F4 cells. Intense NIR fluorescence signals were detected in osteosarcoma xenografts, with signal/background ratio at 4.87 in mice bearing the F5M2 cell. At 4 weeks after F5M2 cell inoculation, metastatic lesions in the lungs were detectable using CXCR4-IR-783 and micro-CT scan, but not with ¹⁸F-FDG PET scan. In conclusion, CXCR4-IR-783 is a promising tool for detection of high CXCR4-expressing osteosarcoma, and particularly for its metastatic lesions.

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.

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.

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.

Chemokine (C-X-C motif) receptor 4 RNA interference inhibits bone metastasis in breast cancer

Chemokine (C-X-C motif) receptor 4 (CXCR4) has been found to closely correlate with the incidence, development, treatment and prognosis of breast cancer. The aim of the present study was to investigate the effects of CXCR4 on bone metastasis in breast cancer and to explore the mechanisms of this process. CXCR4 small interfering RNA was transfected into the breast cancer cell line, MDA-MB-231BA-rfp, and the cell proliferation and invasion abilities of the cells were measured using cell counting kit-8 cell proliferation and Transwell assays. A mouse model of breast cancer with bone metastasis was prepared and the bone metastasis was confirmed using micro-positron emission tomography. The associated proteins were detected by western blot analysis and the results showed that CXCR4 RNAi inhibited the cell proliferation and invasion ability of the MDA-MB-231BA-rfp cells. In addition, CXCR4 RNAi inhibited the duration and extent of bone metastasis in the MDA-MB-231BA-rfp cells in the mouse model, while the inhibition of CXCR4 RNAi blocked the phosphatidylinositide 3-kinase (PI3K)/protein kinase B (AKT)/matrix metalloproteinase (MMP)-9 pathway. In conclusion, the present study demonstrated that CXCR4 RNAi inhibits bone metastasis and the cell proliferation and invasion abilities of breast cancer cells. Furthermore, the CXCR4/PI3K/AKT/MMP-9 pathway may be important in the bone metastasis of breast cancer.

Molecular imaging agents for SPECT (and SPECT/CT)

The development of hybrid single photon emission computed tomography/computed tomography (SPECT/CT) cameras has increased the diagnostic value of many existing single photon radiopharmaceuticals. Precise anatomical localization of lesions greatly increases diagnostic confidence in bone imaging of the extremities, infection imaging, sentinel lymph node localization, and imaging in other areas. Accurate anatomical localization is particularly important prior to surgery, especially involving the parathyroid glands and sentinel lymph node procedures. SPECT/CT plays a role in characterization of lesions, particularly in bone scintigraphy and radioiodine imaging of metastatic thyroid cancer. In the development of novel tracers, SPECT/CT is particularly important in monitoring response to therapies that do not result in an early change in lesion size. Preclinical SPECT/CT devices, which actually have spatial resolution superior to PET/CT devices, have become essential in characterization of the biodistribution and tissue kinetics of novel tracers, allowing coregistration of serial studies within the same animals, which serves both to reduce biological variability and reduce the number of animals required. In conclusion, SPECT/CT increases the utility of existing radiopharmaceuticals and plays a pivotal role in the evaluation of novel tracers.

Interrogating tumor metabolism and tumor microenvironments using molecular positron emission tomography imaging. Theranostic approaches to improve therapeutics

Positron emission tomography (PET) is a noninvasive molecular imaging technology that is becoming increasingly important for the measurement of physiologic, biochemical, and pharmacological functions at cellular and molecular levels in patients with cancer. Formation, development, and aggressiveness of tumor involve a number of molecular pathways, including intrinsic tumor cell mutations and extrinsic interaction between tumor cells and the microenvironment. Currently, evaluation of these processes is mainly through biopsy, which is invasive and limited to the site of biopsy. Ongoing research on specific target molecules of the tumor and its microenvironment for PET imaging is showing great potential. To date, the use of PET for diagnosing local recurrence and metastatic sites of various cancers and evaluation of treatment response is mainly based on [(18)F]fluorodeoxyglucose ([(18)F]FDG), which measures glucose metabolism. However, [(18)F]FDG is not a target-specific PET tracer and does not give enough insight into tumor biology and/or its vulnerability to potential treatments. Hence, there is an increasing need for the development of selective biologic radiotracers that will yield specific biochemical information and allow for noninvasive molecular imaging. The possibility of cancer-associated targets for imaging will provide the opportunity to use PET for diagnosis and therapy response monitoring (theranostics) and thus personalized medicine. This article will focus on the review of non-[(18)F]FDG PET tracers for specific tumor biology processes and their preclinical and clinical applications.