In Situ Vaccination with a TLR9 Agonist and Local Low-Dose Radiation Induces Systemic Responses in Untreated Indolent Lymphoma

This multicenter phase I/II clinical trial evaluated intratumoral SD-101, a TLR9 agonist, and low-dose radiation in patients with untreated indolent lymphoma. Twenty-nine enrolled patients received 4 Gy of radiation followed by 5 weekly intratumoral injections of SD-101 at a single tumor site. No treatment-related grade 4 or serious adverse events occurred. Nearly all patients had tumor reduction at their treated site. More importantly, 24 patients had tumor reduction at their nontreated sites, with 5 patients achieving a partial response and one achieving a complete response. Treatment-related increases of CD8⁺ and CD4⁺ effector T cells and decreases of T follicular helper and T regulatory cells (Treg) were observed in the tumor microenvironment. Low pretreatment levels of CD4⁺ Tregs, proliferating CD8⁺ T cells, and Granzyme B⁺ CD8⁺ T cells were associated with favorable outcomes. Intratumoral SD-101 in combination with low-dose radiation is well tolerated and results in regression of both treated and untreated sites of disease.Significance: In situ vaccination with the TLR9 agonist SD-101, along with low-dose radiation, was safe and induced systemic responses in patients with indolent lymphoma. Low levels of CD4⁺ Tregs, proliferating CD8⁺ T cells, and Granzyme B⁺ CD8⁺ T cells in the tumor microenvironment predicted favorable response to treatment. Cancer Discov; 8(10); 1258-69.