Small Peptide-Based Probes Successfully Image HER2-Positive Tumors Using PET Scans in Mice

What the researchers found

All three Cu-64-labeled affibody variants demonstrated excellent HER2 targeting with similar tumor uptake at 24 hours (4.0-4.3 %ID/g). The binding affinities were in the low nanomolar range: [64Cu]DOTA-Cys-ZHER2:342 (KD = 25.2 ± 9.2 nM) showed the strongest binding, followed by the C-terminal variant (32.6 ± 14.7 nM) and the internal variant (77.6 ± 22.2 nM).

The N-terminal labeled probe ([64Cu]DOTA-Cys-ZHER2:342) also demonstrated the highest in vivo stability. Specificity was confirmed by co-injecting unlabeled affibody, which reduced tumor uptake. All probes showed fast tumor targeting, good tumor accumulation, and good tumor-to-normal tissue contrast on PET imaging.

Why it matters

HER2 status guides treatment decisions in breast and ovarian cancer, but current detection methods rely on biopsies. Non-invasive PET imaging with peptide-based probes could allow doctors to determine HER2 status across all tumor sites in the body simultaneously, track changes over time, and guide therapy selection — all without surgery. Small affibody peptides offer advantages over full antibody-based imaging agents, including faster blood clearance and earlier imaging timepoints.

How the study worked

Three affibody variants were synthesized using solid-phase peptide synthesis, each with the radioactive label attached at a different position (N-terminal, C-terminal, or internal). They were conjugated with a DOTA chelator and radiolabeled with Cu-64. Cell uptake, binding affinity, and stability were tested in vitro using HER2-positive SKOV3 ovarian cancer cells. In vivo performance was evaluated in nude mice bearing SKOV3 tumors using PET imaging, biodistribution analysis, and metabolic stability studies.

What this study cannot tell us

This is a preclinical mouse study using human tumor cells implanted under the skin, which doesn't fully represent how cancers grow in humans. The xenograft model lacks a functioning immune system. The study did not test the probes in larger animals or humans. Kidney uptake of small peptides can be high and was not extensively discussed. Long-term toxicity of the probes was not assessed.

How to read the evidence

This is a preclinical study in mouse xenograft models. While the results are thorough and well-quantified with multiple probe comparisons, the findings require validation in larger animals and clinical trials before human use.

When this study was published

Published in 2019, this study is relatively recent and represents ongoing progress in peptide-based cancer imaging probe development.

The bigger picture

This study contributes to the growing field of peptide-based molecular imaging for cancer. Affibody molecules are much smaller than traditional antibodies, allowing faster imaging with better contrast. Developing reliable, non-invasive ways to image HER2 could improve how cancers are diagnosed, monitored, and treated — particularly as targeted therapies like trastuzumab and newer antibody-drug conjugates require knowing HER2 status.

Questions still open

• Would the lead probe [64Cu]DOTA-Cys-ZHER2:342 perform as well in human patients with naturally occurring HER2-positive tumors?
• How does the imaging quality of these affibody probes compare to antibody-based HER2 imaging agents in clinical use?
• Can this approach detect small metastatic lesions or changes in HER2 status during treatment?

Common questions

Read the original research

PET Imaging of HER2-Positive Tumors with Cu-64-Labeled Affibody Molecules.

Molecular imaging and biology, 21(5), 907-916

Citation

Qi, Shibo; Hoppmann, Susan; Xu, Yingding; Cheng, Zhen. (2019). PET Imaging of HER2-Positive Tumors with Cu-64-Labeled Affibody Molecules.. Molecular imaging and biology, 21(5), 907-916. https://doi.org/10.1007/s11307-018-01310-5