Hypoxia imaging and theranostic potential of [64Cu][Cu(ATSM)] and ionic Cu(II) salts: a review of current evidence and discussion of the retention mechanisms

Abstract

Background: Tumor hypoxia (low tissue oxygenation) is an adverse condition of the solid tumor environment, associated with malignant progression, radiotherapy resistance, and poor prognosis. One method to detect tumor hypoxia is by positron emission tomography (PET) with the tracer [⁶⁴Cu][Cu-diacetyl-bis(N(4)-methylthiosemicarbazone)] ([⁶⁴Cu][Cu(ATSM)]), as demonstrated in both preclinical and clinical studies. In addition, emerging studies suggest using [⁶⁴Cu][Cu(ATSM)] for molecular radiotherapy, mainly due to the release of therapeutic Auger electrons from copper-64, making [⁶⁴Cu][Cu(ATSM)] a "theranostic" agent. However, the radiocopper retention based on a metal-ligand dissociation mechanism under hypoxia has long been controversial. Recent studies using ionic Cu(II) salts as tracers have raised further questions on the original mechanism and proposed a potential role of copper itself in the tracer uptake. We have reviewed the evidence of using the copper radiopharmaceuticals [^60/61/62/64Cu][Cu(ATSM)]/ionic copper salts for PET imaging of tumor hypoxia, their possible therapeutic applications, issues related to the metal-ligand dissociation mechanism, and possible explanations of copper trapping based on studies of the copper metabolism under hypoxia.

Results: We found that hypoxia selectivity of [⁶⁴Cu][Cu(ATSM)] has been clearly demonstrated in both preclinical and clinical studies. Preclinical therapeutic studies in mice have also demonstrated promising results, recently reporting significant tumor volume reductions and improved survival in a dose-dependent manner. Cu(II)-[Cu(ATSM)] appears to be accumulated in regions with substantially higher CD133⁺ expression, a marker for cancer stem cells. This, combined with the reported requirement of copper for activation of the hypoxia inducible factor 1 (HIF-1), provides a possible explanation for the therapeutic effects of [⁶⁴Cu][Cu(ATSM)]. Comparisons between [⁶⁴Cu][Cu(ATSM)] and ionic Cu(II) salts have showed similar results in both imaging and therapeutic studies, supporting the argument for the central role of copper itself in the retention mechanism.

Conclusions: We found promising evidence of using copper-64 radiopharmaceuticals for both PET imaging and treatment of hypoxic tumors. The Cu(II)-[Cu(ATSM)] retention mechanism remains controversial and future mechanistic studies should be focused on understanding the role of copper itself in the hypoxic tumor metabolism.

Keywords: Copper-64; Metabolism; Positron emission tomography; Radiation; Tumor hypoxia.

Fig. 1

Simplified illustration of copper transport mechanism. Cu(II) is carried by albumin or transcuprein in blood, and is believed to be reduced to Cu(I) by reductases near the surface of cells. Copper transporter 1 (CTR-1) is the major high-affinity copper transporter, which transports Cu(I) into the cells and deliver to the copper chaperone Cox17, the antioxidant protein 1 (Atox1), or the copper chaperone for superoxide dismutase (CCS). Atox1 further deliver copper to either the copper transporting adenosine triphosphatase (ATPase) ATP7A in neuron/astrocyte cells, or the ATPase ATP7B in the liver, which either pass the copper to target cuproenzymes, or to excretion

Fig. 2

Possible role of copper in tumor hypoxia. a Cu(II) is reduced to Cu(I) by metalloreductases. b Hypoxic tumor activates the hypoxia-inducible factor 1 (HIF-1). c Activation of HIF-1 promotes cancer stem cells (CSCs) via survival advantages, which d enhances the self-renewal ability and inhibitsdifferentiation of CSCs. e In effect this is in favor of tumor proliferation, which f generates oxidative stress, and g further worsen tumor hypoxia. On the other hand, h tumor hypoxia stimulates copper transporter 1 (CTR-1) expression, increases the production of CTR-1 to i transport incoming Cu(I), j upregulating the HIF-1 expression, where Cu(I) is required