[(18)F]-FLT-PET to evaluate how the sequencing of chemotherapies impacts the efficacy of combination treatment in mouse models of triple-negative breast cancer.

INTRODUCTION: Triple-negative breast cancer (TNBC) lacks targeted therapies due to an absence of biomarkers, making chemotherapy the primary treatment option for early-stage cancer. This study evaluates whether the order and sequence of combination chemotherapy-doxorubicin (DRB) and paclitaxel (PTX)-affects treatment efficacy in TNBC. METHODS: In vitro and in vivo models (MDA-MB-231 human and 4T1 syngeneic mouse TNBC) were used to assess treatment efficacy across three groups: saline control, DRB→PTX, and PTX→DRB. [(18)F]fluorothymidine (FLT) Positron emission tomography (PET) imaging was performed at baseline, day 3, and day 6 to monitor changes in tumor proliferation, and flow cytometry on day 6 examined immune profile differences in endpoint cohorts. Statistical significance was evaluated using the ANOVA and Kolmogorov-Smirnov test. RESULTS: In vitro experiments showed PTX→DRB treatment significantly reduced S/G2/M cell cycles and cancer cell viability. The MDA-MB-231 tumor model showed that PTX→DRB treatment significantly decreased cell proliferation and tumor heterogeneity comparing day 6 to baseline. In 4T1 models, DRB→PTX suppressed tumor growth and enhanced B cell and macrophage recruitment in immunocompetent but not immunocompromised mice. In both models, [(18)F]-FLT-PET plays a crucial role in directing the sequencing of chemotherapy in TNBC. CONCLUSIONS: Our study highlights the immune system's critical role in enhancing chemotherapy's efficacy. It provides compelling evidence that imaging can guide the sequencing of therapies by tracking changes in cellular proliferation and the heterogeneity of tumor response. This approach underscores the potential to refine treatment strategies for improved therapeutic outcomes.