Vaccination With Flt3L, Radiation, and Poly-ICLC

Phase 1 and Phase 2 will occur sequentially:

Phase 1 is the preliminary safety assessment portion of the study, which will follow a modified 3 + 3 design to assess toxicity. The trial would be held for review should there be more than 1 dose-limiting toxicity (DLT) observed within the first 3-6 patients. The toxicity of the in situ vaccine without pembrolizumab (ongoing and previous trials) has been minimal, limited to transient febrile response to the intratumoral Toll-like receptor (TLR) ligand poly-ICLC, typically resolving with oral acetaminophen. As such, the primary objective of Phase 1 is to assess the safety of the addition of pembrolizumab to this in situ vaccine protocol. In Phase 1, patients will only be enrolled at the lead institution (Mount Sinai). After enrollment of the initial 3 patients, enrollment will pause until all three patients have completed cycle 1 of pembrolizumab (DLT evaluation window is 63 days from initiation of in situ vaccine). During the pause, new patients may be consented and screened, but not initiated on therapy. If 1 or no DLTs are observed in the first 3 patients, an additional 3 patients will be enrolled. The regimen will be considered safe (in order to progress to Phase 2) if no more than 1 out of 6 patients experience a DLT. If two or more DLTs are observed at any point in Phase 1 then accrual will be stopped for protocol review by Data Safety Monitoring Board, PIs and sponsor for possibility of protocol amendment.

Phase 2 will follow a Simon's two-stage design. This Phase will proceed only if toxicity is acceptable as determined in Phase 1.

Phase 1/2 design: Patients will be enrolled concurrently into three independent cohorts, with patients from Phase 1 evaluated within their subsequent disease-specific cohorts. iNHL, MBC and HNSCC often have peripherally accessible tumors amenable to intratumoral injection, and have all had relatively modest responses to single-agent checkpoint blockade, so are good candidates for this novel combination. Patients will be enrolled regardless of the PD-L1 staining of their tumor. We will also enroll patients previously exposed to PD-1/PD-L1 targeted therapies either on a clinical trial or as standard of care (as for HNSCC).

Cohort A: iNHL including but not limited to chronic lymphocytic leukemia (CLL/SLL), follicular lymphoma (FL), and marginal zone lymphoma (MZL). This cohort excludes patients with diffuse large B cell lymphoma (DLBCL) and any other lymphoma determined to be progressing rapidly by investigator (PI or site PI).

Cohort B: MBC with peripherally palpable disease amenable to intratumoral injection.

Cohort C: HNSCC with peripherally palpable disease amenable to intratumoral injection.

Stage 1: Seven patients will be evaluated in Stage 1 of Phase 2 in each disease-specific cohort, inclusive of patients enrolled in Phase 1. If no patient in a cohort achieves a response, the study in that cohort will close due to a lack of efficacy. The first seven patients in each histology will only be enrolled at Mount Sinai.

Stage 2: If one or more responses are seen in Stage 1, enrollment of an additional 12 patients for Cohorts A and B or 11 patients for Cohort C will proceed. These cohorts may be also be enrolled at collaborator sites (to be determined).

• Cohorts A or B: If at least three out of the 19 patients achieve a response, the intervention will be considered promising and worthy of further investigation.
• Cohort C: If at least four out of the 18 patients achieve a response, the intervention will be considered promising and worthy of further investigation.