BACKGROUND

The radiation-induced abscopal effect (RIAE) is a desirable phenomenon involving radiation-induced activation of the immune system and regression of metastatic disease after local radiotherapy. However, the majority of patients undergoing radiotherapy do not experience abscopal responses. One potential barrier to the RIAE is tumor-associated macrophages (TAMs), which can be recruited to the tumor after radiotherapy and have an immunosuppressive effect on the tumor microenvironment (TME).

PURPOSE

We aim to evaluate the dual capabilities of the FDA-approved iron nanoparticle ferumoxytol for (1) enhancing the RIAE and (2) measuring TAMs by magnetic resonance imaging (MRI). We hypothesized that (1) the immunomodulating effect of ferumoxytol could enhance the RIAE by repolarizing the M2 TAMs to M1 TAMs, and (2) the TAMs could be non-invasively imaged by ferumoxytol-MRI.

METHODS

Twenty-eight BALB/c mice were subcutaneously implanted with 4T1 primary orthotopic tumor (mammary fat pad) and flank tumor (abscopal tumor). At 14 days post-implantation, mice were separated into four groups: control (Ctrl), radiotherapy (RT) only (8-Gy×3), ferumoxytol only (FMX; 40 mg/kg) and combined (Comb) group (a single dose of 40 mg/kg FMX 24 h prior to 8-Gy×3) (n = 7 mice per group; 56 tumors). At 23- and 24-day post-implantation the pre- and post-FMX injection MRI was performed for mice in FMX and Comb group. The percent change in transverse relaxation time (%T2*) from pre to post ferumoxytol injection was calculated from MR images for both tumors and lymph nodes (LNs). At 25 days post-implantation, both tumors were harvested, and the TAMs were analyzed by flow cytometry.

RESULTS

At 25 days post-implantation, the primary tumor volume in the RT and Comb groups was significantly lower than the Ctrl and FMX groups (p < 0.05). No significant size difference of abscopal tumors was observed among all groups. In addition, there was no significant difference in lung metastasis nodules. A significant decrease in %T2* values of tumors and LNs in the FMX and Comb group 24 h post-ferumoxytol injection was observed, suggesting ferumoxytol uptake in TAMs. The flow cytometry result showed that the CD80 CD206 M1 macrophage population was similar among all tumors and groups. The CD80 CD206 M2 macrophage population was also similar in all tumors and groups, with the exception of the FMX group, where the M2 tumor macrophage levels were significantly higher when compared to the Ctrl group (p < 0.05). Tumors in the FMX group had a significant negative Pearson correlation between tumor %T2* change and M1 tumor macrophage levels (r = -0.76, p < 0.05) but this correlation was not significant in any other treatment group.

CONCLUSIONS

Radiotherapy combined with ferumoxytol led to significant growth delays of irradiated tumors, but no abscopal effects were observed in non-irradiated tumors. Additionally, our hypothesis that the immunomodulating effect of ferumoxytol could enhance the RIAE by repolarizing the M2 TAMs to M1 was not supported by our findings. However, our second hypothesis that the TAMs could be non-invasively imaged by ferumoxytol-MRI was supported by our findings. This includes observation of a significant negative correlation between M1 TAMs and %T2* change in tumors in the ferumoxytol treatment group.