Immunotherapies
Mark Klein. Outside of prostate cancer, there has been a lot of research trying to determine how to improve PD-L1 expression. Where are immunotherapy trials moving? How radiation might play a role in conjunction with immunotherapy.
Julie Graff. Two phase 3 studies did not show statistically improved survival or statistically significant survival improvement on ipilimumab, an immunotherapy agent that targets CTLA4. Some early studies of the PD-1 drugs nivolumab and pembrolizumab did not show much response with monotherapy. Despite the negative phase 3 studies for ipilimumab, we periodically see exceptional responses.
In prostate cancer, enzalutamide is FDA approved. And there’s currently a phase 3 study of the PD-L1 inhibitor atezolizumab plus enzalutamide in patients who have progressed on abiraterone. That trial is fully accrued, but the results are not yet known. Soon a study will compare pembrolizumab plus enzalutamide vs enzalutamide alone. So the combinations are getting more interesting.
I just received a Prostate Cancer Foundation Challenge Award to open a VA-only study looking at fecal microbiota transplant from responders to nonresponders to see how manipulating host factors can increase potential responses to PD-1 inhibition.
Abhishek Solanki. The classic mechanism by which radiation therapy works is direct DNA damage and indirect DNA damage through hydroxyl radicals that leads to cytotoxicity. But preclinical and clinical data suggest that radiation therapy can augment the local and systemic immunotherapy response. The radiation oncologist’s dream is what is called the abscopal effect, which is the idea that when you treat one site of disease with radiation, it can induce a response at other sites that didn’t get radiation therapy through reactivation of the immune system. I like to think of the abscopal effect like bigfoot—it’s elusive. However, it seems that the setting it is most likely to happen in is in combination with immunotherapy.
One of the ways that radiation fails locally is that it can upregulate PD-1 expression, and as a result, you can have progression of the tumor because of local immune suppression. We know that T cells are important for the activity of radiation therapy. If you combine checkpoint inhibition with radiation therapy, you can not only have better local control in the area of the tumor, but perhaps you can release tumor antigens that will then induce a systemic response.
The other potential mechanism by which radiation may work synergistically with immunotherapy is as a debulking agent. There are some data that suggest that the ratio of T-cell reinvigoration to bulk of disease, or the volume of tumor burden, is important. That is, having T-cell reinvigoration may not be sufficient to have a response to immunotherapy in patients with a large burden of disease. By using radiation to debulk disease, perhaps you could help make checkpoint inhibition more effective. Ultimately, in the setting of prostate cancer, there are not a lot of data yet showing meaningful benefits with the combination of immunotherapy and radiotherapy, but there are trials that are ongoing that will educate on potential synergy.