Abstract
PURPOSE: Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors in combination with endocrine-therapy have emerged as an important regimen of care for estrogen receptor (ER)-positive metastatic breast cancer, although identifying predictive biomarkers remains a challenge. We assessed the ability of two PET-proliferation tracers, [(18)F]FLT and [(18)F]ISO-1, for evaluating response to CDK4/6-inhibitor (palbociclib) and ER-antagonist (fulvestrant). EXPERIMENTAL DESIGN: To determine the effect of CDK4/6 inhibition combined with estrogen-blockade, we assessed cell proliferation in six breast cancer cell lines after 1, 3, and 6 days of treatment with palbociclib and/or fulvestrant. These data were correlated to in vitro radiotracer assays and results were verified by longitudinal [(18)F]FLT and [(18)F]ISO-1 micro-PET imaging performed in MCF7 tumor-bearing mice. RESULTS: All palbociclib-sensitive cell lines showed decreased [(18)F]FLT accumulation and S-phase depletion after treatment, with both measures augmented by combination therapy. In contrast, these cells showed changes in [(18)F]ISO-1 analogue-binding and G(0) arrest only after prolonged treatment. MicroPET imaging of MCF7 xenografts showed a significant decrease in [(18)F]FLT but no changes in [(18)F]ISO-1 uptake in all treated mice on day 3. On day 14, however, mice treated with combination therapy showed a significant decrease in [(18)F]ISO-1, corresponding to G(0) arrest, while maintaining reduced [(18)F]FLT uptake, which corresponded to S-phase depletion. CONCLUSIONS: Our data suggest complementary roles of [(18)F]FLT and [(18)F]ISO-1 PET in evaluating tumor-proliferation after combined CDK4/6 inhibitor and endocrine therapy in breast cancer. [(18)F]FLT is more sensitive to immediate changes in S-phase, whereas [(18)F]ISO-1 can assess more delayed changes related to cell-cycle arrest and transition to G(0) quiescence from combination therapy. These data suggest a potential role for early prediction of long-term response using these imaging biomarkers.