Recent clinical trends in Toll-like receptor targeting therapeutics

Abstract

Toll-like receptors (TLRs) are germline-encoded receptors that are central to innate and adaptive immune responses. Owing to their vital role in inflammation, TLRs are rational targets in clinics; thus, many ligands and biologics have been reported to overcome the progression of various inflammatory and malignant conditions and support the immune system. For each TLR, at least one, and often many, drug formulations are being evaluated. Ligands reported as stand-alone drugs may also be reported based on their use in combinatorial therapeutics as adjuvants. Despite their profound efficacy in TLR-modulation in preclinical studies, multiple drugs have been terminated at different stages of clinical trials. Here, TLR modulating drugs that have been evaluated in clinical trials are discussed, along with their mode of action, suggestive failure reasons, and ways to improve the clinical outcomes. This review presents recent advances in TLR-targeting drugs and provides directions for more successful immune system manipulation.

Keywords: Toll-like receptor; adjuvant; clinical trial; drug; innate immunity.

Figure 1

Generalized structure and signaling mechanisms of Toll‐like receptors. A, A typical TLR is composed of three distinct domains, an ECD, a TM domain, and a TIR domain. B, Conventionally, TLRs are divided into two categories; cell surface functional, and endosomally functional TLRs. Endosomal TLRs are mainly activated through nucleic acids, while cell surface–expressed TLRs are activated by a variety of ligands, including proteins and lipoproteins. Upon sensing PAMPs or DAMPs, TLRs dimerize and reorient their TIR domains, allowing the docking of intracellularly localized TIR‐containing proteins, including MAL, MyD88, TRIF, and TRAM. The majority of TLRs convey downstream signals through MyD88; however, TLR3 can signal only through TRIF. Exceptionally, TLR4 can transmit signals through both the MyD88 and TRIF adapter proteins. Therapeutics targeting immune‐related diseases mediated by TLRs are reported to modulate these signaling mechanisms. There are many internal mechanisms that come into action to regulate TLR‐mediated inflammation. These act at all levels starting from cell surface interaction to dent the TLR dimerization, and cytoplasmic interactions to block adapter molecules, alter the posttranslational modification state, and finally in the nucleus to counter overexpression of various interleukins and cytokines. There are many microRNAs that reduce the mRNA stability of different cytokines. All these mechanisms ensure a balanced response toward the invading pathogen or DAMP that unbalance the homeostasis. ABIN3, A20 binding and inhibitor of NFκB‐3; AhR, aryl hydrocarbon receptor; AP1, activated protein 1; ATF3, activating transcription factor 3; Bcl‐3, B‐cell lymphoma 3‐encoded protein; CYLD, cylindromatosis; DOK, downstream of tyrosine kinases; DUSP, dual specificity phosphatases; ECD, extracellular domain; ERK, extracellular‐regulated kinase; IFN, interferon; IkB, inhibitor of κB; IKK, inhibitor of κB kinase; IL, interleukin; IRF, interferon response factor; IRAK‐M, interleukin receptor‐associated kinase M; JNK, c‐Jun N‐terminal kinase; MAL, MyD88 adapter like; MD2, myeloid differentiation factor 2; MIR, microRNA; MKK, mitogen‐activated protein kinase kinase; mRNA, messenger RNA; MyD88, myeloid differentiation primary response 88; MyD88s, myeloid differentiation primary response 88 short; NEMO, NF‐κB essential modulator; NFKBID, NF‐κB inhibitor δ; NF‐κB, nuclear factor κB; Nurr1, nuclear receptor related 1 protein; p38, protein 38; PDLIM2, PDZ and LIM domain 2; PTP1B, protein tyrosine phosphatase‐1B; Reg‐1, regnase‐1; RIPK‐1, receptor interacting protein kinase 1; RP105, radioprotective 105 kDa protein; SARM, sterile α and armadillo‐motif containing protein; SHP‐1, Src homology region 2 domain‐containing phosphatase‐1; SIGGR, single immunoglobulin IL1R‐related molecule; SOCS, suppressor of cytokine signaling; ST2, suppression of tumorigenity 2; ST2L, membrane bound ST2; STAT, signal transducers and activators of transcription; sTLRs, soluble Toll‐like receptor; TAB, TAK‐1‐binding protein; TAK1, transforming growth factor β‐activated kinase 1; TANK, TRAF‐associated NF‐κB activator; TBK1, TANK‐binding kinase 1; TIR, Toll/interleukin‐1 receptor; TLR, Toll‐like receptor; TM, transmembrane domain; TNF‐α, tumor necrosis factor α; TRAF, tumor necrosis factor receptor (TNF‐R)‐associated factor; TRAM, TRIF‐related adapter molecule; TRIF, TIR‐domain‐containing adapter‐inducing interferon‐β; TRIM38, tripartite motif 38; TTP, tristetraprolin; USP4, ubiquitin‐specific protease 4 [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

TLRs targeting ligands with respect to their relative clinical trials and disease conditions. A, The total number of clinical trials, activators (including agonists) and inhibitors (including antagonists), and the diversity of ligands are presented. The majority of ligands have been extensively pursued in different diseases, making it difficult to determine their exact numbers. The data indicate that total number of clinical trials exceeds the number of active drugs, suggesting the use of single drugs in multiple clinical trials. B, Clinical trial data showing the current status of drugs targeting TLRs from the disease perspectives. TLR ligands have been evaluated in multiple diseases including cancers, immune disorders, and viral and bacterial diseases. The largest proportion of clinical trials focuses on cancers, followed by immune disorders. “Mixed” indicates those cases where cancer and immune disease have been targeted simultaneously. The category “general” covers vaccination, clinical trials involving healthy volunteers, and those that are not covered by prior instances. This data was gathered from the clinical trials website (clinicaltrials.gov) using various keywords (cancers, immune disorders, TLR, TLRs, TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, and adjuvant) from June 2017 to Jan 2018. TLR, Toll‐like receptor

Figure 3

TLRs with bound ligands. The ligand binding mechanism of the extracellular TLRs (left, TLR1, 2, 4, 5, 6) and endosomal TLRs (right, TLR3, 7, 8) has been presented. Each monomer has been labeled; however, for the homodimers, the other monomer has been labeled with asterisk (*). In the case of TLR5, flagellin‐bound single monomer has been given. The respective protein databank (PDB) ID has also been given at the bottom of each structure. TLRs recognize several molecules, including protein, lipopeptide, small molecules and nucleic acids, and the bound ligand with individual TLR has been shown in 2D interaction diagram. The color code for the 2D interaction is given at the bottom of the figure. Black arrows indicate the bound TLR‐ligand. dsRNA, double‐stranded RNA; TLR, Toll‐like receptor; 2D, two‐dimensional [Color figure can be viewed at wileyonlinelibrary.com]