[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 [ 64/67 Cu]-FAP2287 in combination with immunotherapy (IMT) to treat triple-negative breast cancer (TNBC). Methods 4T1 tumor-bearing BALB/c mice (N = 20) were injected and imaged with 100µCi [ 64 Cu]-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 [ 67 Cu]-FAP2287 or 4)[ 67 Cu]-FAP2287 + IMT and 1600µCi [ 67 Cu]-FAP2287, or [ 67 Cu]-FAP2287 + IMT (N = 5-10/group) and imaged with [ 64 Cu]-FAP2287 PET followed by treatment. All [ 67 Cu]-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. Results At baseline, there was a strong positive correlation between [ 64 Cu]-FAP2287 PET SUV Mean and autoradiography (R 2  = 0.7806,p < 0.001). No changes in [ 67 Cu]-FAP2287 retention over time was observed following the IMT. [ 64 Cu]-FAP2287 SUV Mean proved to be a reliable predictor of survival with 800µCi (R 2  = 0.84,p = 0.03) and 1600µCi (R 2  = 0.62,p = 0.01). FAP-TRT increased survival compared to controls and IMT. 1600µCi [ 67 Cu]-FAP2287 + IMT significantly decreased tumor volume (p = 0.0003) and increased survival (p = 0.0005) compared to [ 67 Cu]-FAP2287. Conclusion We can noninvasively target and predict response to FAP radiolabeled with elementally paired theranostic probe, [ 64/67 Cu]-FAP2287 in TNBC. IMT synergistically increased response to FAP-TRT and can guide personalized approaches to improve treatment efficacy.