[^64/67Cu]-FAP theranostics in combination with immunotherapy in a preclinical model of TNBC

Fibroblast activation protein (FAP) is a prominent transmembrane protein in the tumor microenvironment (TME) that promotes tumorigenesis. FAP has been a recent target for radionuclide-conjugated peptides for PET imaging and targeted radionuclide therapy (TRT). The objective of this study is to develop a theranostic approach using [^64/67Cu]-FAP2287 in combination with immunotherapy (IMT) to treat triple-negative breast cancer (TNBC).

4T1 tumor-bearing BALB/c mice (N = 20) were injected and imaged with 100μCi [⁶⁴Cu]-FAP2287 PET at 1, 4, and 24h to quantify baseline FAP expression followed by autoradiography. 4T1-tumor bearing BALB/c mice (N = 42) were divided into Control, IMT, 800μCi [⁶⁷Cu]-FAP2287 or 4)[⁶⁷Cu]-FAP2287 + IMT and 1600μCi [⁶⁷Cu]-FAP2287, or [⁶⁷Cu]-FAP2287 + IMT (N = 5-10/group) and imaged with [⁶⁴Cu]-FAP2287 PET followed by treatment. All [⁶⁷Cu]-FAP2287 groups were imaged with SPECT at 4, 24, and 48h. Tumor growth and survival rates were monitored. Standard uptake values (SUV) quantified, statistical analysis and probability of survival were done with significance at p < 0.05.

At baseline, there was a strong positive correlation between [⁶⁴Cu]-FAP2287 PET SUV_Mean and autoradiography (R² = 0.7806,p < 0.001). No changes in [⁶⁷Cu]-FAP2287 retention over time was observed following the IMT. [⁶⁴Cu]-FAP2287 SUV_Mean proved to be a reliable predictor of survival with 800μCi (R² = 0.84,p = 0.03) and 1600μCi (R² = 0.62,p = 0.01). FAP-TRT increased survival compared to controls and IMT. 1600μCi [⁶⁷Cu]-FAP2287 + IMT significantly decreased tumor volume (p = 0.0003) and increased survival (p = 0.0005) compared to [⁶⁷Cu]-FAP2287.

We can noninvasively target and predict response to FAP radiolabeled with elementally paired theranostic probe, [^64/67Cu]-FAP2287 in TNBC. IMT synergistically increased response to FAP-TRT and can guide personalized approaches to improve treatment efficacy.