Toll-like receptors (TLRs), which serve as a bridge between innate and adaptive immunity, may be viable treatment targets. TLRs are the first line of defense against microbes and activate signaling cascades that induce immune and inflammatory responses. Patients with "hot" versus "cold" tumors may respond more favorably to immune checkpoint inhibition, and through their downstream effects, TLR agonists have the potential to convert "cold tumors" into "hot tumors" making TLRs in combination with immune checkpoint inhibitors, potential targets for cancer therapies. Imiquimod is a topical TLR7 agonist, approved by the FDA for antiviral and skin cancer treatments. Other TLR adjuvants are used in several vaccines including Nu Thrax, Heplisav, T-VEC, and Cervarix. Many TLR agonists are currently in development as both monotherapy and in combination with immune checkpoint inhibitors. In this review, we describe the TLR agonists that are being evaluated clinically as new therapies for solid tumors.
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