Effect of eribulin on remodeling of tumor vascular confirmed by FMISO PET imaging-The small animal PET study in a human breast cancer cell xenograft

Objectives: Hypoxia is an important resistance factor in radio- and chemotherapies. Eribulin is an inhibitor of microtubule dynamics and has an antivascular (antiangiogenetic or vascular-disrupting) activity that can target abnormal tumor vessels. Thus, eribulin can induce remodeling of tumor vasculature through its novel antivascular activity and improve tumor hypoxic condition. However, the elimination of tumor hypoxic condition by eribulin treatment has yet to be clarified. The remodeling of tumor vasculature may enhance the delivery of chemotherapeutic drugs or increase the sensitivity of tumor to radiotherapy. ¹⁸F-fluoromisonidazole (FMISO) is the most widely used probe for imaging tumor hypoxia. Tumor hypoxic condition may be measured by FMISO PET to assess early responses to a therapy. Thus, in this study, to clarify the elimination of tumor hypoxic condition by eribulin treatment on the basis of the antivascular activity of eribulin and the remodeling of tumor vasculature by eribulin treatment, we determined the tumor hypoxic condition using small-animal PET with FMISO in a human breast cancer cell xenograft.

Methods: A human breast cancer cell xenograft (MDA-MB435s) was established in female nude mice. When tumor volumes reached 200‐400 mm³, mice were assigned to the ex vivo study group (n=11) and PET imaging group (n=4). In the ex vivo study group, mice were further assigned to the control (n=5) and eribulin-treated (n=6) groups. Mice in the eribulin-treated group were administered a single dose (0.3 mg/kg, i.p.). The day of administration was designated as Day 1. Three days after eribulin treatment (Day 4), the mice were injected with FMISO and pimonidazole (a hypoxia marker) 4 and 2 h before sacrifice, respectively. Radioactivity in tissues was expressed as the percentage of injected dose per gram of tissue (%ID/g). Immunohistochemistry of pimonidazole and CD31 (a vascular marker) was also performed. Tumor size was measured on Days 1 and 4. In the PET imaging study, mice were imaged by small-animal PET/CT 4 h after FMISO injection, and then treated with single-dose eribulin (0.3 mg/kg, i.p.) on Day 1. Seven days (Day 8) after eribulin treatment, the mice were injected with FMISO and imaged again by PET/CT 4 h after FMISO injection. The ¹⁸F-FMISO accumulation level in tumors was quantified by calculating the mean standardized uptake value (SUV_mean). Tumor size was measured on Days 1, 4 and 8.

Results: In the ex vivo study, the FMISO accumulation level in the tumor significantly decreased to 37% of the control value after 0.3 mg eribulin treatment [0.71 ± 0.23 and 0.27 ± 0.12[asterisk] (%ID/g) for control and 0.3 mg treatment; [asterisk]p < 0.001 vs control]. As compared with the control, the pimonidazole-positive area also significantly decreased to 28% of the control value after 0.3 mg eribulin treatment [8.7 ± 3.3 and 2.5 ± 2.1[asterisk] (%positive area) for control and 0.3 mg treatment; [asterisk]p < 0.001 vs control]. The number of microvessels in tumors increased after eribulin treatment. In the PET imaging study, intratumoral FMISO SUV_mean also significantly decreased to 41% of the pre-treatment value after the treatment [0.21 ± 0.11 and 0.12 ± 0.08[asterisk] for pre- and post-treatment; [asterisk]p < 0.05 vs pre-treatment]. No significant changes in tumor size were observed between the pre- and post-treatments with eribulin in all groups.

Conclusion: In the human breast cancer cell xenograft, theFMISO accumulation level in the tumor decreased after eribulin treatment, which was consistent with the decrease in pimonidazole-positive area. The FMISO PET study further supported the ex vivo results. Thus, by using small-animal PET imaging and FMISO, we for the first time demonstrated the elimination of tumor hypoxic conditions by eribulin treatment, indicating the antivascular activity of eribulin and the remodeling of tumor vasculature. Research Support: Figure: FMISO hypoxia images at pre- and post-treatments with eribulin in breast cancer cell xenograft. $$graphic_E7873F5A-BB18-4E9E-B456-06AF1DC5FA91$$