Abstract 2482: Evaluating immune checkpoint blockade treatment efficacy via [89Zr]-CD4 and [89Zr]-CD8 PET imaging in breast cancer mouse models
Authors: Lu Yun , Houson Hailey , Mascioni Alessandro , Jia Fang , Song Patrick N. , Napier Tiara , Mansur Amer M. , Gallegos Carlos A. , Larimer Benjamin M. , Lapi Suzanne E. , Sorace Anna G. 
Published: June 2022

Objective: Immune checkpoint blockades have shown a great promise in cancer therapy. However, as the overall response rate varies, there is a profound unmet need to monitor the treatment efficacy and predict therapeutic responders.

Methods: In vivo blocking and biodistribution studies were performed to validate the specificity of [89Zr]-mouse-CD4 and [89Zr]-mouse-CD8. 4T1 and MMTV-HER2 mouse models of breast cancer (N=80 per model) were imaged on Days 0, 2, and 6 during treatment (saline, anti-PD1, anti-CTLA4, or combinational therapies) and evaluated for long-term changes in tumor response. Therapeutic responders were determined through the thresholding of tumor mass at the end of study. Intratumoral and splenic CD4+ and CD8+ cells were characterized in vivo via [89Zr]-mouse-CD4 and [89Zr]-mouse-CD8 positron emission tomography (PET) imaging during immunotherapy treatment. An additional of mice (N=16) were euthanized on day 7 post treatment for biological validation studies. Autoradiography and immunofluorescence staining (CD8 and CD4) were performed to validate the biological accuracy of [89Zr]-mouse-CD4 and [89Zr]-mouse-CD8 PET imaging.

Results: Splenic CD4 and CD8 signal showed three-fold and two-fold decreased in the blocking group compared to non-blocking group, respectively (p&lt;0.01). In 4T1 model, the changes of intratumoral CD4 SUVmean (the mean value of standard uptake value) from day 0 to day 6 significantly increased in responder group with anti-PD1 treatment (p&lt;0.01). The baseline intratumoral CD8 SUVmean showed no significant difference between responders and non-responders; inversely, the changes from day 0 to day 6 increased in MMTV-HER2 (p=0.02) and 4T1 model (p=0.07) with combinational therapies. Spleen masses of responders decreased with anti-CTLA4 treatment (p&lt;0.05) and for all treatments (p=0.06), in 4T1 model only. Terminal tumor mass and spleen mass were positively correlated in 4T1 model (r=0.26, p=0.02). In 4T1 model, splenic CD4 SUVmean on day 6 was negatively correlated with terminal spleen mass (r=-0.36, p=0.02) and tumor mass (r=0.33, p=0.04). PET, autoradiography, and immunofluorescence staining showed strong correlations when comparing in vivo imaging to ex vivo validations (r&gt;0.75, p&lt;0.01).

Conclusion: [89Zr]-CD4 and [89Zr]-CD8 PET imaging can accurately evaluate CD4+ and CD8+ cell populations in vivo. Biomarkers for predicting immunotherapy response varies with different targeted drugs and tumor models. Characterizing immune cell population during immune checkpoint blockade treatment via advanced PET imaging can provide understanding of immunological alterations, monitor immunotherapy response and may potentially help in clinical decision making.

Citation Format: Yun Lu, Hailey Houson, Alessandro Mascioni, Fang Jia, Patrick N. Song, Tiara Napier, Amer M. Mansur, Carlos A. Gallegos, Benjamin M. Larimer, Suzanne E. Lapi, Anna G. Sorace. Evaluating immune checkpoint blockade treatment efficacy via [89Zr]-CD4 and [89Zr]-CD8 PET imaging in breast cancer mouse models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2482.