Article

PET and Metallomic Imaging: New Frontier of Cancer Diagnosis?

Increasing accessibility of scanners and isotopes have allowed positron emission tomography (PET) to transform diagnostic nuclear medicine.1,2 Today the emerging area of PET metallomics has made nuclear medicine an indispensable tool for managing a range of diseases linked to trace metal disturbances because it connects intracellular trace metal homeostasis to whole body metabolism and movement.1,2

Growing availability of isotopes for key trace metals, including 64Cu, 63Zn, 52Fe, and 52Mn, serve as new tools for imaging and whole body tracking of trace metals that was not previously possible.1 Routes taken by trace metals in vivo to reach their sites of action are now understood to be complex trafficking systems expert at achieving and preserving homeostasis while at the same time preventing the accumulation of toxic levels that induce disease states.1

Expanding the application of molecular imaging means even more new PET probes are needed to meet the challenges of early diagnosis and treatment selection for an ever-widening range of diseases and patients.2,3 Given this, significant research already underway is resulting in substantial progress that is leading the way to improved diagnostics and treatment.2,3

PET metallomics illuminates trace metal trafficking

Trace metal equilibrium disturbances are linked to the pathogenesis of dementia, cancer, and inherited metabolic abnormalities, including diabetes, cystic fibrosis, prostate cancer, Alzheimer’s disease, and neuroblastomas, among numerous others.1,2

Increasingly, zinc, iron, and copper are being confirmed in the pathology of beta-amyloid (ß-amyloid) trace metal accumulation linked to Alzheimer's disease (AD).1 The specific role of these trace metals - consequence, cause, or red herring - is not well known but could be verified by imaging studies of variations in brain metal homeostasis.1 In particular, the 63Zn isotope is showing promise in AD where its biodistribution in mice has already been confirmed.1 A preliminary study of 63Zn in 12 human participants confirmed low levels of detectable accumulation in the brain and high absorption rates in the liver and pancreas similar to what has been described in animals.1 Outflow from brain regions was also different in healthy patients compared to those with AD confirming further research will provide additional valuable verification.1

Similarly, the role of copper isotope 64Cu-GTSM is being used to observe AD intracellular copper trafficking pathways, including redistribution and outflow.1 Its effectiveness is due to lipophilic properties that allow it to pass through the blood-brain barrier and passively enter cells where it is reduced to 64Cu+.1 In a mouse model of AD this isotope displayed substantially greater absorption in the brain of AD mice compared to the control group.1

New production methods are making it possible to use manganese (52Mn) as a radiotracer for PET imaging of pancreatic beta-cell functional mass in vivo for the diagnosis and treatment of diabetes mellitus.1,4 Its biodistribution in tissues was confirmed in rodents where the high absorption rate in the pancreas showed significant variation between healthy, obese, and diabetic mouse models.1 Researchers reported that 52Mn offers greater sensitivity, an improved dynamic range and clearance, as well as decreased toxicity.4

In Wilson's disease (WD), copper and iron (52Fe-citrate) radiotracers are being used to determine homeostasis disturbances in the brain.1 Neurological and psychiatric symptoms can be associated with disrupted cerebral trace metal levels.1 In mice with mutated ATP7B genes, investigators reported the use of radiocopper to assess whole-body copper metabolism may reduce invasive liver biopsies as well as complement and simplify interpretation of genetic testing for the more than 300 disease-causing mutations of the WD gene.1 Outcome monitoring of treatments, such as hepatocyte transplantation, gene therapy, and copper chelation, are additional ways radiocopper is expected to advance treatment.1

Disrupted copper homeostasis is also highly correlated in numerous pathways of human cancers, including metabolism, plasma levels, tumor tissue concentrations, and tumorigenesis.1 As a protein cofactor, copper affects modulation of metastatic progression, angiogenesis, and proliferation.1

Copper trafficking research in cancer has shown particular promise in those areas of the body where radiocopper (64CuCl2) absorption is decreased, such as the brain and pelvic region.1 For example, one study found no correlation of radiocopper absorption with high-affinity copper transporter 1 (hCTR1) expression, while other studies confirmed hCTR1 knock-down in prostate cancer xenografts decreased absorption.1 Increased expression of hCTR1 in a breast cancer model more than doubled the absorption of radiocopper.1

Additionally, intravenous administration of radiocopper targeting hCTR1 in athymic mice models with hepatoma, prostate, hepatocellular carcinoma, melanoma, head and neck, glioblastoma, and breast cancer xenografts confirmed that while hCTR1 is important it is not acting alone in copper absorption by tumors warranting further research.1

Visualizing trafficking pathways and mechanisms noninvasively

Until recently, radioisotopes could not be used to identify trafficking pathways in vivo for trace metals in humans due in part to tissue sampling requirements.1 Today, PET scanner advanced technology combined with increased accessibility is providing a noninvasive imaging method for acquiring whole body dynamic in vivo measurements.1

Assessing whole body copper variability in the context of diseases that correlate with disrupted copper homeostasis using 64CuCl2 is allowing clinicians to visualize in vivo pathways and mechanisms not previously possible.1 Radiocopper is being used to measure tissue absorption as well as outflow and excretion.1 For example, in 18/19 glioblastoma multiforme (GBM) patients, 64CuCl2 tumor imaging successfully detected primary and secondary lesions.1

Low urinary excretion of 64CuCl2 in a preliminary study of seven prostate cancer patients resulted in the delineation of local tumors using PET.1 Tumor recurrence and lymph node metastases were detected with greater sensitivity using 64CuCl2 in a larger study of 50 prostate cancer patients, and particularly for those patients with decreased PSA levels.1

Preclinical achievements using 64Cu+2 ions as a PET probe have resulted in its move to the clinical setting as a noninvasive imaging and staging tool for prostate cancer in human patients.3 Additionally, absorption of 64Cu+2 ions by tumors from a wide range of cancers in animal studies, including hepatoma, colorectal cancer, prostate cancer, lung cancer, breast cancer, head and neck cancer, fibrosarcoma, melanoma, glioblastoma, and ovarian cancer, have produced a large volume of useful imaging data that continues to advance research initiatives and improve treatment selection.3

PET imaging protocol for metallomics monitoring

A novel PET imaging protocol using [64Cu]CuCl2 was developed to monitor patient response to the anti-cancer activity of a copper-targeting drug in a study of preclinical human neuroblastoma xenograft models in mice.5 The PET method was designed to detect copper trafficking and any changes to the pathways and mechanisms caused by the drug.5

Results from study imaging data confirmed a link between the anti-cancer action of the copper-targeting drug and a significant reduction of intracellular copper in the tumor tissues.5 In vivo imaging analysis found increased radiocopper retention in the tumor mass before treatment compared with a significant reduction in tracer absorption and tumor growth measured after therapy.5

Evidence acquired via PET imaging of the anti-cancer compound's copper-targeted activity in vivo is essential to evaluate its applicability in other copper-dependent cancer types, tumor models, and clinical studies that in the future could provide clinicians a method for predicting the efficacy of similar anti-cancer treatments.5

Using the [64Cu]CuCl2 PET imaging method may also help physicians identify patients with greater chances for therapeutic response to copper targeting drugs that improve treatment personalization.5 Additionally, this study marked the first use of [64Cu]CuCl2 PET to assess the molecular mechanism of copper-targeting drugs for cancer treatment.5

PET metallomics reveals greater relapse sensitivity

In a prospective study to detect relapse in 50 prostate cancer patients after first-line surgery or radiation therapy, radiocopper agent copper-64-chloride (64CuCl2) was compared with the more commonly used nonmetal radiotracer fluorine-18-choline (18F-choline).6,7 Results demonstrated 64CuCl2 was able to identify prostate cancer relapse with statistically greater sensitivity (82 percent versus 74 percent) for lesion detection, including metastasis in lymph nodes and bones, than the standard tracer.5,6 Patients with PSA levels below one nanogram per milliliter also demonstrated higher detection rates.6,7

One advantage of using 64CuCl2 for detecting relapse in this study is that its excretion occurs through the liver instead of in the urine like with standard imaging agents.6,7 This means the radiocopper tracer does not interfere in the pelvis and prostate region which are located in the assessment area.6,7

Researchers concluded based on study findings that 64CuCl2 is better suited as a tracer to evaluate the pelvic areas surrounding the prostate as well as the prostate itself.6,7 They found 64CuCl2 exhibited more powerful detection of relapse over the standard fluorine tracer and is reported to be the first time it was compared to the standard tracer in a greater number of prostate cancer patients with biochemical relapse.6,7

Monoclonal antibody labeled PET tracer increases specificity

Combining the high sensitivity and resolution of PET imaging with the specificity of monoclonal antibodies created an immuno-PET radiotracer used to study biodistribution in multiple myeloma preclinical mice models.8 Radioimmunoconjugate copper-64 labeled mCD138-specific 9E7.4 antibody (64Cu-TE2A-9E7.4) imaging showed higher sensitivity and specificity for detecting subcutaneous and bone marrow lesions compared to more commonly known and studied 18F-FDG and 64CuCl2.8 The antibody-labeled tracer was designed to target biomarkers on multiple myeloma cancer cells.8 Results support 64Cu-anti-CD138 antibody as a promising imaging tool for selecting patients before antibody-based therapy.8 Radiolabeled anti-CD20 monoclonal antibody was previously studied and demonstrated a stronger response in the imaging of lymphoma tissue.8

Opportunities to use trace metal PET probes in vivo to illuminate their role in disease pathogenesis, observe whole body variations, improve diagnostics, and monitor therapy outcomes longitudinally are increasing. Studies are already revealing the possibilities for new treatment strategies along with methods to evaluate efficacy as well as the improved selection of treatment for those patients most likely to respond. Early studies have opened new possibilities for launching novel research in the fast-emerging field of PET metallomics.

REFERENCES:

  1. Insights into Trace Metal Metabolism in Health and Disease from PET: “PET Metallomics.” Mettallomics. http://jnm.snmjournals.org/content/59/9/1355  Accessed 2/22/2019
  2. Across the spectrum: integrating multidimensional metal analytics for in situ metallomic imaging. Metallomics https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6350628/ Accessed 3/1/2019
  3. 64Cu2+ Ions as PET Probe: An Emerging Paradigm in Molecular Imaging of Cancer. American Chemical Society  https://pubs.acs.org/doi/abs/10.1021/acs.molpharmaceut.6b00582 Accessed 3/1/2019
  4. [GE PETrace cyclotron used] Radio-manganese PET imaging of pancreatic beta cells. Journal of Nuclear Medicine http://jnm.snmjournals.org/content/57/supplement_2/5.short Accessed 3/1/2019
  5. In vivo [64Cu]CuCl2 PET imaging reveals activity of Dextran-Catechin on tumor copper homeostasis. Theranostics http://www.thno.org/v08p5645.htm Accessed 3/1/2019
  6. Tracing Copper Accumulation Is Better at Detecting Prostate Cancer Recurrence, Study Finds. Prostate Cancer News Today https://prostatecancernewstoday.com/2018/03/09/tracing-copper-accumulation-better-finding-prostate-cancer-relapse/ Accessed 3/1/2019
  7. 64CuCl2 PET/CT in Prostate Cancer Relapse. Journal of Nuclear Medicine http://jnm.snmjournals.org/content/59/3/444 Accessed 3/1/2019
  8. Comparison of Immuno-PET of CD138 and PET imaging with 64CuCl2 and 18F-FDG in a preclinical syngeneic model of multiple myeloma. Oncotarget http://www.oncotarget.com/index.php?journal=oncotarget&page=article&op=view&path[]=23886&path[]=75134 Accessed 3/1/2019