Background: Photoacoustic tomography (PAT) provides a direct readout of tumor haemoglobin (Hb) concentration and oxygenation. This readout could be used to provide an early indication of response to anti-angiogenic drugs, thus optimizing the management of these therapies.

Experimental Procedures: Two cohorts of nude Balb/c were inoculated with either estrogen-dependent MCF-7 (n=7) or estrogen-independent MDA-MB-231 (n=19) cells. Mice were randomly split into Control (Ctrl) and Bevacizumab (Bev,10 mg/Kg, IP, weekly) groups. PAT was performed weekly, starting just before enrolment. Oxy- and deoxy-Hb were quantified in the tumour and reference areas. Total Hb (THb) and O2 saturation (SO2) were calculated. Blood samples and tissues were collected after imaging. Hypoxia (Hypoxyprobe and CA-IX) and vascular density/maturity (CD31 and ASMA) were analyzed by immunohistochemistry. Circulating levels of human and mouse vascular endothelial growth factor (hVEGF and mVEGF) and Hb were measured.

Summary of New Data: Reflecting clinical observations, the MCF-7 Bev group and most of the mice from the MDA-MB-231 Bev group (8/11) showed no improvement in survival with Bev treatment. All resistant tumours (Bev MCF-7 and Bev MDA-MB-231 non-responders (Bev-NR)) showed an increase in hVEGF production (Ctrl vs Bev; 73.0513.06 vs. 497103.9 for MCF-7; 247.984.81 vs. 330.4 85.97 for Bev NR). MDA-MB-231 responders (Bev-R) showed a significant decrease in hVEGF (247.984.81 vs. 80.53 26.13). These results indicate that VEGF was only successfully sequestered in a small subset of animals (3/11) and that VEGF overload might be a resistance mechanism. At the final time point, PAT showed no difference between Ctrl and Bev-NR in THb for either group (THbMDA-MB-231 7.2±1.3 vs. 5.5±1.2). THb was found significantly increased in the Bev-R group by PAT (13.9±3.3 p-valuevs.Crtl=0.0339; p-valuevs.Bev=0.0072) and biochemical measurements. SO2 showed a slight but significant decrease between Ctrl and Bev-NR (0.58±0.03 vs. 0.48±0.01). A dramatic decrease was seen in the Bev-R group (0.31±0.07) and significantly different from Ctrl and Bev-NR. SO2 was sensitive to early changes, SO2 values increased in Ctrl and Bev-NR 48h after enrolment (p-values paired t-test pCtrl= 0.0134; pNR=0.0268) while Bev-R shows a trend (p=0.0649). Analysis of SO2MSOT over time shows a significant (p<0.0001) change in slope at 3 weeks after enrolment in the Bev-R group compared to either of the Ctrl or Bev-NR groups. Our results indicate that PAT SO2 can differentiate responders at very early stages of the treatment. Histologically, we observed fewer blood vessels but better structured in Bev-NR than in Bev-R, which showed higher vascular density and also higher levels of hypoxia makers.

Statement of the Conclusions: Our results postulates PAT as a low-cost, label-free and non-invasive candidate to monitor response to Bevacizumab over time. Our results also indicate that tumours with functional vasculature may be resistant to Bevacizumab treatment.

Citation Format: Isabel Quiros-Gonzalez, Michal R. Tomaszewski, Monika A. Golinska, Emma Brown, Laura Ansel-Bollepalli, Lina Hacker, Dominique-Laurent Couturier, Rosa M. Sainz, Sarah E. Bohndiek. Bevacizumab leaves a photoacoustic fingerprint 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 LB057.