The first in an occasional series on emerging imaging techniques.

CT, MRI, and ultrasound are not prognostic tools because they do not provide cellular target information; therefore, their use in the assessment of the effectiveness of cancer therapy is not optimal. When CT is nondiagnostic as a result of post-therapy anatomic alteration, positron emission tomography (PET) and single photon emission computed tomography (SPECT) agents can localize the disease. New agents, also known as microdosing imaging agents because there is no detectable pharmacologic effect, currently are being developed to integrate with PET and SPECT. These imaging agents provide important information concerning the characterization of varieties of tumors, including vascular angiogenesis, cellular signaling, and transcriptional activity.1 Uses for these agents include the determination of optimal therapeutic dosing, differential diagnosis between inflammation/infection and recurrence, sensitivity or resistance to treatment, whether tumors are low or high grade, and finally prediction of treatment response by selecting the patient who may respond to therapy. Technetium 99m and gallium 68 are the most attractive radioisotopes for SPECT and PET clinical applications due to their energy (140 and 511 keV) and half-life (360 minutes and 68 minutes), and could expand to chelator-based imaging techniques for the above applications. In addition to assessing molecular targets, they also may be useful in planning internal targeted radionuclide therapy with rhenium 188-labeled agents. 188Re has high ß energy (2.1 MeV), short physical half-life (16.9 hours), and 155 keV g-ray emission for radiotherapeutic dosimetric determination and imaging purposes. This unique chelator-based imaging technique provides a novel route in a kit formulation that may be used to discontinue ineffective treatment in an earlier phase and switch to a more efficient treatment that would be beneficial to patients early on in the course of treatment. In this report, a series of ethylenedicysteine (EC), diethylenetriamine pentaacetic acid (DTPA), glutamate peptide (GAP), and cyclam-agents in oncology target assessment are reviewed.

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To read a Q&A with the president of Cell>Point, read the “Oncology” section in the November issue of Medical Imaging.

Beyond FDG

Fluorine 18-labeled deoxyglucose (FDG), a gold standard for PET, is complementary to CT and MRI in diagnosing various tumors, and it allows detection of unsuspected distant metastases. However, FDG-PET also has its drawbacks. For example, a significant amount of FDG concentrates in cytosolic fraction,2,3 resulting in false-positive lesions at post-therapy between inflammation/infection and tumor recurrence.4,5 Additionally, FDG does not provide predictions of therapeutic response, and, due to high brain uptake, FDG radiation dosimetry is not suitable for internal radiotherapy. The use of FDG also is constrained by the availability of a local cyclotron and its high cost. On the other hand, radionuclide generator systems that produce radiopharmaceuticals in a well-controlled facility are permitted by current FDA policy, have a long history of successful clinical application, and are much more accessible and affordable.

Predicting Therapy Response

The agents that could provide therapeutic prediction are either labeled molecular biomarkers or radiolabeled drugs. For instance, radiolabeled annexin, a biomarker for apoptosis, is useful to evaluate the baseline level of apoptosis, predict the efficacy of therapy based on the detection of treatment-related apoptosis, and possibly predict disease progression and prognosis.6–9 Previously, our group reported that 18F-fluorotamoxifen provides useful information in selecting estrogen receptor-positive (ER+) patients for tamoxifen therapy.10–12 Others have used 18F-fluoroestradiol to image ER+ tumors.13,14 Though the clinical findings of these agents are encouraging, poor water solubility makes it difficult to dispense 18F-fluorotamoxifen and 18F-fluoroestradiol for routine clinical practice. Conjugation of estradiol (EDL) to GAP would not only allow cytosolic ERs targeting via a glutamate transporter but also improve EDL solubility. 99mTc- and 68Ga-GAP-EDL also have been able to image functional ER + diseases, such as breast cancer and endometriosis.15

Figure. In the CT (a), the red arrow identifies infection/inflammation, and the blue arrow indicates the tumor site. In the FDG-PET study (b), the SPECT 99m-Tc-EC-G study (c), and the postprocessed SPECT 99m-Tc-EC-G (d) study (Astonish software, Philips Medical Systems, Andover, Mass), the red arrow identifies infection/inflammation, and the blue and green arrows indicate the tumor site. Images are from a clinical Phase I trial conducted at the University of Texas MD Anderson Cancer Center, Houston, sponsored by Cell>Point, Centennial, Colo, and Philips Medical Systems, with the compound EC-Glucosamine (EC-G) imaging agent.

The Differential Diagnosis

Current Focus in Targeted Molecular Imaging

Myocardial Imaging
Celebrex (COX-2)
Metronidazole (hypoxia)
Glucosamine (glycolysis)
Annexin (apoptosis)

DNA Marker
Guanine
Glucosamine
Metronidazole
Adenosine

Endometrial Imaging
LHRH
Estrone
Tamoxifen

Transporters
Glucosamine (glucose)
Tyrosine (amino acid)
Alphamethyltyrosine
Guanine (nucleic acid)

Receptor Markers
LHRH
Estrone
Herceptin (EGFR)
TRAIL (death receptor)
Annexin
Glipizide (pancreas beta)

Neuroendocrine Imaging
Somatostatin
Octreotide
Tryptophan (serotonin)

Oncology Imaging
(Brain, Lung, Lymphoma)
Celebrex (COX-2)
Metronidazole (hypoxia)
Glucosamine (glycolysis)
Alphamethyltyrosine
Guanine (nucleic acid)
Annexin (apoptosis)

Angiogenesis Imaging
(Brain, Lung, Lymphoma)
VEGF
C-225
Celebrex
Endostatin
Angiostatin
Glycopeptide

Hypoxic biomarker. It has been well established that hypoxic tumors are known to be resistant to traditional radiotherapy and chemotherapy, which results in higher local recurrence rates. The ability to quantify tissue hypoxia allows physicians to select patients for additional or alternative treatment regimens that would circumvent the ominous impact of hypoxia.16 Metronidazole (MN), a 5-nitroimidazole analogue, was shown to sensitize only anoxic cells in a dose-dependent manner. However, clinical studies showed that 99mTc-EC-MN, a hypoxic biomarker, was able to differentiate hypoxic regions in stroke patients.17,18

Metabolic imaging biomarker. Glucosamine is a glucose analogue similar to FDG, and its cellular uptake is via a glucose transporter process.19,20 However, its regulatory products of glucosamine-6-phosphate mediate insulin activation, downstream signalling, and translocation, which upregulate mRNA expression and tumor growth.2,3,21 For instance, Sp-1 is one of the known transcription factors whose activity may be post-translationally targeted by glucosamine.21 Sp-1 binding sites are possible to be present in the promoters of potent angiogenic growth factors such as vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8) and regulate oncogenic factors with cell cycle factors at basal level of transcription or repression. Because of this phenomenon, studies have shown that 99mTc-EC-glucosamine was able to differentiate inflammation/infection and tumor growth in animal models and human subjects.9,22

Cell proliferation biomarker. Synthesis and biological activity of labeled thymidine or uridine, which were incorporated into DNA/RNA, have been reported.23,24 For instance, 3′-deoxy-3′-18F- fluorothymidine (18F-FLT) is a new tracer that images cellular proliferation by entering the salvage pathway of DNA synthesis. However, the DNA incorporation rate of FLT is low, and the chemistry is complex.25 To continue to explore other purine-based analogues using chelation radiochemistry, we then synthesized 99mTc-EC-guanine (EC-Guan) for evaluation of cell proliferation.26 68Ga-EC-Guan was able to differentiate inflammation versus tumors.9 In vitro cell confluence, cell cycle analysis, cellular uptake, and in vivo imaging studies suggest that 99mTc- and 68Ga-EC-Guan may be useful as tumor proliferation imaging agents.

Radiopharmaceuticals and Treatment Response

Amino acid transporter. The epidermal growth factor receptor (EGFR) is expressed in a variety of human solid tumors. When EGFR is triggered, tyrosine kinase (TK) is phosphorylated and leads to initiate the receptor-mediated signal transduction, cell proliferation, survival, angiogenesis, and metastasis. TK inhibitors (Iressa, Tarceva, Gleevec) have shown to be effective against tumor growth. Radiolabeled tyrosine, an amino acid transporter, is involved in protein synthesis, which is suitable to assess the end point of TK activity. PET and planar scintigraphy in animal models has demonstrated that the tumors can be clearly visualized by 68Ga- and 99mTc-amino acid.27,28

Receptor-mediated imaging. In the pancreas, the beta cell comprised 60% of all cell types. Assessment of beta cell activity would provide early diagnosis of pancreas function and monitor drug treatment response on pancreatic beta cells. For example, animal studies showed that 99mTc-DTPA-nateglinide uptake occurs in the pancreas via a receptor-mediated process. Planar images confirmed that the pancreas could be visualized with 99mTc-DTPA-nateglinide within 5 to 50 min. 99mTc-DTPA-nateglinide may be helpful in monitoring and selecting the patients who may respond to beta cell therapy in the pancreas.29

Current Status in Imaging Technology in Drug Development

To demonstrate the specific target assessment from an imaging agent, biologic correlations are needed. The biologic correlations are provided by biologists, immunologists, or pharmacologists. In oncology, numerous molecular targets have been identified; thus, useful imaging technologies must be developed. In January 2006, the FDA issued a guidance for industry, investigators, and reviewers in exploratory investigational new drug (eIND) studies. In eIND, limited acute toxicity in a single mammalian species is required for acute toxicity studies instead of the traditional IND requirement of two species or multiple administration routes. Novel radiopharmaceuticals for clinical trials could undergo this new eIND application process, which may accelerate the drug development.

David J. Yang, PhD, is associate professor in the Division of Diagnostic Imaging at The University of Texas MD Anderson Cancer Center, Houston; Jerry L. Bryant, MS, is chief technology officer of Cell>Point, Centennial, Colo; and E. Edmund Kim, MD, is professor in the Department of Nuclear Medicine’s Division of Diagnostic Imaging at The University of Texas MD Anderson Cancer Center.

Glossary

Annexin V—A protein binding with high affinity to an apoptosis marker (membrane-bound phosphatidylserine); radiolabeled annexin V can be used for imaging of apoptosis in mice and humans

Apoptosis—Programmed cell death involving a regulated intracellular proteolytic enzyme cascade

Biomarker—Any molecule whose presence, absence, or abnormal concentration suggests an abnormal physiological status associated with injury or disease

Chelator—A cross-linker that binds metals; chelates are used to detoxify metals (isotopes, paramagnetic lanthanides) and to attach such metals to targeting molecules

Interleukins—A group of molecules involved in signaling between cells of the immune system

Signal transduction—A sequence of intermolecular reactions involved in the forwarding and amplification of extracellular signals within the cell

Transcription—The production of RNA from DNA template by the enzyme RNA polymerase

Vascular endothelial growth factor (VEGF)—A protein that binds to receptors on epithelial cells and promotes their growth, stimulates angiogenesis, and increases permeability of the endothelium

Definitions selected from: Wagenaar DJ, Weissleder R, Hengerer A. Glossary of molecular imaging terminology. JACR. 2004;1(suppl):24–32.


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