Background

Desmoplastic Small Round Cell Tumors (DSRCT) are soft tissue sarcomas that grow in the abdomen/pelvic area. DSRCT could be treated with bispecific antibodies (BsAbs), which redirect T cells to kill the tumor. These BsAbs typically targeting only one tumor-associated antigen. However, this could lead to toxicity against healthy tissues with basal antigen expression. 

Therefore, using a heterodimeric BsAb to simultaneously target two antigens may reduce toxicity since DSRCT is the only type of tissue with a high expression of both proteins. However, we also need to assess whether this BsAb can retain equal efficacy as homodimeric BsAbs targeting only one antigen.

Aim

Compare the efficacy of the heterodimeric T cell-engaging BsAb against homodimeric BsAbs when performing immunotherapy against DSRCT in vitro and in vivo. 

Hypothesis

The heterodimeric BsAb will retain the same anti-DSRCT efficacy as homodimeric BsAbs.

Methods

Chromium release assays were performed on JN-DSRCT-1 and BOD DSRCT cell lines. Next, 6 groups of 7 mice bearing DSRCT xenografts were each treated with human T cells and one BsAb. On Day 4, tumors were harvested from 2 mice from each group to perform immunohistochemistry for T cell localization. The remaining 5 mice were treated with BsAbs 2x/week as well as T cells and IL-15 1x/week. Their tumors were measured 2x/week and the mice were weighed 1x/week.

Chromium Release Assays

All BsAbs led to a sigmoidal killing curve except for the negative control, with greater cytotoxicity at greater BsAb concentrations for both cell lines. 

IHC

BsAbs induce T cell trafficking to BER xenografts. 

In Vivo Efficacy

All BsAbs exhibited a dramatic decrease in tumor volume except for the negative control. Even the monovalent controls showed a strong response, with efficacy between the groups indistinguishable at this dose. 

Remarkably, none of the mice exhibited recurrences over the 6-month monitoring period, highlighting the potency of T cell-engaging BsAbs against DSRCT. 

Weight 

The mice retained the same weight, meaning there is no obvious toxicity. This is expected since the T cells are only cytotoxic towards human cells, while leaving the rest of the mouse alone. 

Conclusion

The heterodimeric BsAb retains the same anti-DSRCT efficacy as homodimeric BsAbs.

Future Directions

The next step of our study is evaluating whether the heterodimeric BsAb reduces toxicity compared to homodimeric BsAbs. This would be done by testing the same panel of BsAbs against single-target knockdown DSRCT cell lines (e.g., DSRCT with only one antigen expressed), which are surrogates for normal human tissues. In theory, the heterodimeric BsAb should be less effective against these knockdown cell lines than homodimeric BsAbs.