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Background Tilsotolimod, a synthetic toll-like receptor 9 (TLR9) agonist, modulates the tumor immune microenvironment and has antitumor activity as monotherapy in preclinical models. In the ILLUMINATE-204 phase I/II study of tilsotolimod in combination with ipilimumab in patients with refractory melanoma, increased antitumor immune activity was observed in injected and uninjected tumors at 24 hours following tilsotolimod treatment. The ILLUMINATE-101 phase Ib study explored the safety, efficacy, and immune effects of tilsotolimod monotherapy in multiple solid tumors. Methods Adults with histologically or cytologically confirmed diagnosis of cancer not amenable to curative therapies received intratumoral tilsotolimod in doses escalating from 8 mg to 32 mg into a single lesion at weeks 1, 2, 3, 5, 8, and 11. Additional patients with advanced melanoma were enrolled into an expansion cohort at the 8-mg dose, the recommended phase II dose established for melanoma. Objectives included characterizing safety and efficacy, and immunological assessment. Blood samples and tumor biopsies of injected and distal lesions were obtained at baseline and 24 hours post treatment for immune analyses. Results As of May 6 2019, 54 patients have been enrolled, including 38 patients into the dose evaluation portion and 16 patients into the melanoma expansion cohort. No dose-limiting toxicities were observed. The most common treatment-related adverse events were fever, fatigue, and chills. Within 24 hours, fresh tumor biopsies showed increased gene expression for multiple immune checkpoint pathways, increased IFN gamma levels, activation of the type 1 IFN pathway, and upregulation of MHC class I and II, compared to pretreatment biopsies. Of 43 evaluable patients, 15 (35%) had a RECIST v1.1 disease assessment of stable disease (duration 1.2 to 11.1 months, 1 patient ongoing). Conclusions Tilsotolimod was well tolerated and induced robust alterations in the tumor microenvironment, including immune checkpoint upregulation, activation of dendritic cells, and induction of Type 1 IFN signaling. A phase II study of tilsotolimod in combination with nivolumab and ipilimumab has been initiated for the treatment of multiple solid tumors (ILLUMINATE-206; NCT03865082). Clinical trial identification NCT03052205. Editorial acknowledgement Ted Everson, Idera Pharmaceuticals, Inc. Legal entity responsible for the study Idera Pharmaceuticals, Inc. Funding Idera Pharmaceuticals, Inc. Disclosure H. Babiker: Advisory / Consultancy: Endocyte; Advisory / Consultancy: Celgene. V. Subbiah: Advisory / Consultancy: Idera Pharmaceuticals; Advisory / Consultancy: Novartis; Advisory / Consultancy: Bayer; Advisory / Consultancy: Blueprint Medicines; Advisory / Consultancy: Loxo Oncology. S. Rahimian: Shareholder / Stockholder / Stock options, Full / Part-time employment: Idera Pharmaceuticals. C. Haymaker: Research grant / Funding (institution): Idera Pharmaceuticals. C. Bernatchez: Research grant / Funding (institution): Idera Pharmaceuticals. G. Bindra: Shareholder / Stockholder / Stock options, Full / Part-time employment: Idera Pharmaceuticals. I. Iverson: Shareholder / Stockholder / Stock options, Full / Part-time employment: Idera Pharmaceuticals. S. Chunduru: Shareholder / Stockholder / Stock options, Full / Part-time employment: Idera Pharmaceuticals. A. Diab: Advisory / Consultancy: Array Biopharma; Advisory / Consultancy: Nektar Therapeutics; Honoraria (self): Bristol-Myers Squibb; Advisory / Consultancy: Jounce Therapeutics; Honoraria (self): Novartis; Advisory / Consultancy: Idera. All other authors have declared no conflicts of interest.