Design, synthesis and preclinical evaluation of a tumor extracellular nucleotidase CD73 targeted theranostic radiotracer

High levels of adenosine are often associated with immunosuppression in malignant tumors, acting through adenosine receptors to inhibit the functions of T cells and NK cells. The ectonucleotidase CD73, as a key enzyme that catalyzes adenosine monophosphate to adenosine, plays an important role in the tumor immune microenvironment and promotes resistance to immune checkpoint blockade, and thus was regarded as an important biomarker for tumor prognosis. The inhibition of overexpressed CD73 was also believed as a promising strategy to restore tumor immune function in anti-tumor therapies. However, the in vivo evaluation of CD73 expression in patients is still challenging. In this investigation, a series of novel non-nucleotide small-molecule targeting CD73 was developed, and the most potent compounds (HX-6 and HX-8) were radiolabeled with ⁶⁸Ga for in vivo PET imaging of CD73 expression. The highest tumor uptake of [⁶⁸Ga]Ga-HX-6 and [⁶⁸Ga]Ga-HX-8 in CD73-positive LS174T xenograft mice was 6.98 ± 0.74 %ID/g and 6.31 ± 0.99 %ID/g, respectively, both significantly higher than that observed in the blocked and CD73-negative tumors. Considering its relatively lower IC₅₀ value, HX-6 was selected for labeling with ¹⁷⁷Lu to conduct a preliminary radio-ligand anti-tumor therapy. 37 MBq of [¹⁷⁷Lu]Lu-HX-6 for 4 cycles displayed favorable anti-tumor efficacy when used alone or combined with anti-PD-1 agent. Thus, [⁶⁸Ga]Ga-HX-6 and [¹⁷⁷Lu]Lu-HX-6 may be used as a theranostic pair for non-invasive CD73 imaging and anti-tumor therapies.

(177)Lu; (68)Ga; CD73; Positron emission tomography; Radiopharmaceutical.