Preclinical evaluation of somatostatin receptor agonist versus antagonist radioligands for breast cancer imaging

Objectives Somatostatin receptor (SSTR) expression has been reported in a high proportion of human breast cancers, yet published results using ¹¹¹In-pentetreotide have been disappointing. Antagonists have been reported to be superior to agonists for the visualization of SSTR positive tumors. This work aims to evaluate whether antagonists can improve the visualization of SSTR positive breast cancers by PET imaging. We compared two routinely used agonists, ⁶⁸Ga-DOTA-tyr³-octreotide (⁶⁸Ga-DOTATOC) and ⁶⁸Ga-DOTA-tyr³-octeotate (⁶⁸Ga-DOTATATE) with a recently reported potent antagonist ⁶⁸Ga-NODAGA-Cpa-c[D-Cys-Aph(Hor)- D-Aph(Cbm)-Lys-Thr-Cys]-D-Tyr-NH₂ (⁶⁸Ga-NODAGA-JR11), in a human breast cancer model.

Methods All three peptides were prepared by solid-phase peptide synthesis and conjugated with their respective chelator at the N-terminus. Peptides were purified by HPLC and verified by mass-spectroscopy. ^natGa-DOTATOC, ^natGa-DOTATATE and ^natGa-NODAGA-JR11 were assessed for binding affinity to SSTR 2a (SSTR2a) using a membrane-based competition binding assay. ⁶⁸Ga labelling was performed in HEPES buffer (pH 5.0) under microwave heating for 1 min. NOD scid gamma female mice bearing the SSTR-positive breast cancer tumor ZR-75-1 were injected intravenously with the labeled tracers and used for imaging and biodistribution at 1h post-injection.

Results All three tracers had favourable binding affinity to SSTR2a, with the exception of the antagonist, which had a higher Ki value (28.9 ± 6.92 compared to 2.45 ± 0.09 for ^natGa-DOTATOC and 2.02 ± 0.55 for ^natGa-DOTATATE). The tracers were obtained in good radiochemical yield (>58%), purity (>95%) and specific activity (>2.8 μCi/mol); see Table 1 for summary of radiochemical data. The tumor uptake was highest for ⁶⁸Ga-DOTATOC (28.2 ± 4.39 %ID/g) followed by ⁶⁸Ga-NODAGA-JR11 (17.8 ± 2.32 %ID/g) and lastly ⁶⁸Ga-DOTATE (15.1 ± 2.18 %ID/g). Tumor-to-blood and tumor-to-muscle ratios were more favourable for the agonists (41.1 ± 5.68 and 171 ± 55.3 respectively for ⁶⁸Ga-DOTATOC, 44.6 ± 13.1 and 163 ± 60.7 for ⁶⁸Ga-DOTATATE) and significantly lower for the antagonist ⁶⁸Ga-NODAGA-JR11 (15.9 ± 2.01 and 47.2 ± 9.03).

Conclusions In the human ZR-75-1 breast cancer xenograft, agonists were superior to the antagonist in terms of tumor-to-blood and tumor-to-muscle ratios. ⁶⁸Ga-DOTATOC had the highest tumor uptake among the three tested peptides. While it is known that SSTR2 is highly expressed in this cell line, it is possible that other SSTR subtypes are also expressed. The investigation of the SSTR subtype expression profile in ZR-75-1 cells is currently under way.

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