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Stimulator of interferon genes (STING) is a promising target for potentiating antitumor immunity, but multiple pharmacological barriers limit the clinical utility, efficacy, and/or safety of STING agonists. Here we describe a modular platform for systemic administration of STING agonists based on nanobodies engineered for in situ hitchhiking of agonist cargo on serum albumin. Using site-selective bioconjugation chemistries to produce molecularly defined products, we found that covalent conjugation of a STING agonist to anti-albumin nanobodies improved pharmacokinetics and increased cargo accumulation in tumor tissue, stimulating innate immune programs that increased the infiltration of activated natural killer cells and T cells, which potently inhibited tumor growth in multiple mouse tumor models. We also demonstrated the programmability of the platform through the recombinant integration of a second nanobody domain that targeted programmed cell death ligand-1 (PD-L1), which further increased cargo delivery to tumor sites while also blocking immunosuppressive PD-1/PD-L1 interactions. This bivalent nanobody carrier for covalently conjugated STING agonists stimulated robust antigen-specific T cell responses and long-lasting immunological memory, conferred enhanced therapeutic efficacy, and was effective as a neoadjuvant treatment for improving responses to adoptive T cell transfer therapy. Albumin-hitchhiking nanobodies thus offer an enabling, multimodal, and programmable platform for systemic delivery of STING agonists with potential to augment responses to multiple immunotherapeutic modalities., Competing Interests: Competing Interests J.T.W., K.A., and B.R.K. are inventors on U.S. Provisional Application No. 63/472,528 “NANOBODY-DRUG CONJUGATES AND METHODS OF PREPARING THEREOF” which describes nanobody conjugation and delivery technologies. W.K.R. is an employee of Vanderbilt University Medical Center and appointed to the Vanderbilt University School of Medicine. She has received research support from Incyte Corp. within the past three years, and serves on the Board of Scientific Advisors for the National Cancer Institute. J.C.R. is an employee of Vanderbilt University Medical Center and appointed to the Vanderbilt University School of Medicine. He is a founder, scientific advisory board member, and stockholder of Sitryx Therapeutics, a scientific advisory board member and stockholder of Caribou Biosciences and holds stock options for Nirogy Therapeutics. He has consulted and received speaker fees from Merck, Pfizer, and Abbie. He has received research support from Incyte Corp. within the past three years. J.M.B. receives research support from Genentech/Roche and Incyte Corporation, has received advisory board payments from AstraZeneca and Mallinckrodt and is an inventor on patents regarding immunotherapy targets and biomarkers in cancer.