Development of a HER3 PET probe for breast cancer imaging

Objectives HER3 is a receptor tyrosine kinase that forms heterodimers with HER2 and EGFR and plays an important role in the pro-oncogenic signaling pathways of HER2+ breast cancer. We developed an antibody-based PET probe specific for HER3, characterized it in vitro in a panel of HER3 expressing breast cancer cell lines, and performed in vivo PET-CT imaging in mouse xenografts.

Methods 64Cu-DOTA-HER3 F(ab’)2 was prepared from whole HER3 monoclonal antibody with F(ab)’2 fragmentation and chelator conjugation, and its radiochemical purity confirmed using size-exclusion chromatography. Probe affinity was assessed using 67Ga labeling and saturation binding studies with HER3 expressing MDA-MB-468 tumors. In vitro uptake studies were conducted with 67Ga-DOTA-HER3 F(ab’)2 incubation with the HER3 expressing cell lines MDA-MB-468, HCC-70, and MCF-7. Results of cell uptake studies were correlated with ratio of HER3/beta-actin staining by Western blot. In vivo PET-CT imaging with 64Cu-DOTA-HER3 F(ab’)2 was conducted using mouse xenografts of MDA-MB-468 and HCC-70 tumors (n=3 for both groups).

Results The HER3 PET probe was generated with >95% radiochemical purity, and binds with a Kd of 6.8 nM to HER3. Cell uptake studies demonstrated counts/minute/cell of 0.28, 0.45, 0.82 for MCF-7, HCC-70, and MDA-MB-468 cells, respectively. CPM/cell correlates with Western-blot HER3/beta-actin ratio with an R2 of 0.79. In vivo imaging with the HER3 PET Probe of MDA-MB-468 and HCC-70 tumor xenografts demonstrate SUVs of 0.35 and 0.59, with TBRs of 6.0 and 11.4 respectively.

Conclusions We demonstrate that HER3 expressing breast tumors can be imaged using a HER3 specific PET probe and that SUV correlates with HER3 expression level. HER3 PET imaging may enable the clinical development and use of HER3 targeted therapies, which are currently in clinical development.