Molecular imaging of cyclooxygenase-2 (COX-2) and autotaxin (ATX) in cancer using positron emission tomography (PET)

• Author / Creator

  Litchfield, Marcus

• Positron emission tomography (PET) relies on the use of β+-emitting radionuclide-bearing molecules congregating and decaying at a specific site of interest. Use of these molecules, called radiotracers, which target common tumor characteristics has become a pillar in the diagnosis of cancer and the monitoring of treatment response. Within the last two decades, a significant appreciation for the tumorigenic and metastasis-promoting role of inflammation has emerged and biomarkers of cancer-associated inflammation have been identified. Two key players in this relationship are cyclooxygenase-2 (COX-2) and autotaxin (ATX), and their main lipid mediator products, prostaglandin E2 (PGE2) and lysophosphatidic acid (LPA), respectively. Having emerged as key inflammatory regulators, COX-2 and ATX have become therapeutic targets in the treatment of inflammatory diseases. Recently, our lab developed a small molecule inhibitor of COX-2, called triacoxib, using in situ click chemistry, a target-guided synthesis method which used the COX-2 active site to select and form its own ligand from a diverse group of building blocks. Although a relatively new effort, several inhibitors of ATX have been developed, and one has even been tested as an ATX PET imaging probe: [18F]PRIMATX. With the recognition of the role inflammatory mediators play in tumor progression, novel methods of biomarker detection and measurement are required. The focus of this work was to develop a radiochemical synthesis method for and validate novel radiotracers which target COX-2 or ATX as a step toward the clinical detection and monitoring of inflammatory biomarkers in vivo.
  Using both nucleophilic and late-stage radiofluorination chemistry, we converted three small molecule inhibitors of COX-2 and ATX into the PET molecular imaging probes [18F]triacoxib, [18F]PRIMATX, and [18F]F-GLPG1690, derived from an ATX inhibitor nearing FDA approval.
  For the [18F]triacoxib project we present the radiosynthesis as well as the in vitro and in vivo validation of this radiotracer in COX-2-expressing HCA-7 human colorectal cancer cells. Radiosynthesis and purification of [18F]triacoxib afforded the radiotracer in yields of 72% ± 6.5% (n = 7) at molar activities exceeding 90 GBq/µmol. Baseline cellular uptake levels of [18F]triacoxib could be reduced by 63% with pre-treatment using 0.1 mM of the COX-2 inhibitor celecoxib. The radiotracer showed high metabolic stability, remaining 90% intact after 60 min. PET imaging of [18F]triacoxib revealed a favorable baseline uptake of SUV60min = 0.76 ± 0.02 (n = 4) that could be blocked by 20% with pre-treatment using celecoxib. Autoradiographic and immunohistochemical experiments further confirmed the COX-2-mediated uptake of [18F]triacoxib in vivo. Despite a substantial amount of non-specific uptake in vivo, [18F]triacoxib represents a suitable radiotracer for PET imaging of COX-2.
  For the [18F]PRIMATX project we present an improved radiosynthetic method and a further in vivo validation of the radiotracer in ATX-expressing 8305C human thyroid tumors and 4T1 murine breast tumors, where ATX is expressed in the adipose tissues rather than the tumor cells. Radiosynthesis and purification of [18F]PRIMATX afforded the radiotracer in yields of 25% ± 6.7% (n = 9) at molar activities exceeding 55 GBq/µmol. The radiotracer showed moderate metabolic stability, remaining ~65% intact after 60 min. PET imaging of [18F]PRIMATX in both models revealed favorable baseline uptakes of SUV60min4T1 = 0.81 ± 0.05 (n = 4) and SUV60min8305C = 0.61 ± 0.02 (n = 4). Uptake into 8305C tumors could be blocked by 12 and 8% after 30 and 60 min, respectively. Notably, brain uptake was high and could be blocked by 34%. Despite some additional work needed [18F]PRIMATX appears to be a valid radiotracer for the PET imaging of ATX in multiple cells lines.
  For the [18F]F-GLPG1690 project we present the radiosynthesis and in vivo PET imaging of the radiotracer in the 8305C human thyroid tumor model. Radiosynthesis and purification of [18F]F-GLPG1690 afforded the radiotracer in yields of 30% ± 5% (n = 2). PET imaging of [18F]F-GLPG1690 in the 8305C human thyroid tumor model resulted in very low baseline uptake of SUV60min8305C = 0.31 ± 0.04 (n = 3).
  The results of these projects demonstrate the feasibility of using PET imaging for the in vivo detection of inflammatory biomarkers. Further development and implementation of these systems should allow for increased personalization of therapeutic strategies for patients with inflammatory diseases such as cancer.

• Subjects / Keywords

  • ATX
  • PRIMATX
  • COX-2
  • Cancer
  • Positron emission tomograph
  • PET
  • Imaging
  • triacoxib
  • Autotaxin
  • molecular imaging
  • Inflammation
  • Cyclooxygenase-2

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-bwy7-7a26

• License

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