Structure–Activity Relationships of (+)-Naltrexone-Inspired Toll-like Receptor 4 (TLR4) Antagonists

Abstract

Activation of Toll-like receptors has been linked to neuropathic pain and opioid dependence. (+)-Naltrexone acts as a Toll-like receptor 4 (TLR4) antagonist and has been shown to reverse neuropathic pain in rat studies. We designed and synthesized compounds based on (+)-naltrexone and (+)-noroxymorphone and evaluated their TLR4 antagonist activities by their effects on inhibiting lipopolysaccharide (LPS) induced TLR4 downstream nitric oxide (NO) production in microglia BV-2 cells. Alteration of the N-substituent in (+)-noroxymorphone gave us a potent TLR4 antagonist. The most promising analog, (+)-N-phenethylnoroxymorphone ((4S,4aR,7aS,12bR)-4a,9-dihydroxy-3-phenethyl-2,3,4,4a,5,6-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one, 1j) showed ~ 75 times better TLR-4 antagonist activity than (+)-naltrexone, and the ratio of its cell viability IC50, a measure of its toxicity, to TLR-4 antagonist activity (140 μM /1.4 μM) was among the best of the new analogs. The 1j was active in vivo; it significantly increased and prolonged morphine analgesia.

Therapeutic Developments Targeting Toll‐like Receptor‐4‐Mediated Neuroinflammation

“…The lead opioid in this study was also tested in vivo in a Hargreaves assay, and was able to demonstrate efficacy as compared to a morphine/vehicle [30] .…”

Stereochemistry and innate immune recognition: (+)‐norbinaltorphimine targets myeloid differentiation protein 2 and inhibits toll‐like receptor 4 signaling

“…As shown in Fig. 2A, (+)-1 inhibited LPS-induced TLR4 signaling of NO in a concentration-dependent manner, with an IC 50 of 4.7 6 1.8 mM, which possesses ;25 times better TLR4 antagonist activity than (+)-naltrexone (105.5 6 10.1 mM) (19,21). In addition to NO inhibition, (+)-1 also inhibited the LPS-induced TLR4 signaling of proinflammatory factors TNF-a (IC 50 = 12.1 6 2.0 mM, Fig.…”

“…However, the TLR4 antagonist activity of (+)-naltrexone was moderate [half maximal inhibitory concentration (IC 50 ) = 105.5 6 10.1 mM], and it is not long acting (19), which severely prevents its wide in vivo application. Structure-activity relationship studies of (+)-naltrexone showed that the substituted group at N-17 affected the binding with MD-2 and TLR4 antagonistic activity, increasing the hydrophobicity of the substituent at N-17 and improving its TLR4 antagonistic activity (19,20). However, it should be noted that increasing the hydrophobicity of substituted groups meanwhile compromised the drug likeness of (+)-naltrexone-inspired TLR4 antagonists.…”

Low-Dose Naltrexone (LDN)—Review of Therapeutic Utilization

“…In discrete ‘low-doses’ ranging from 1 to 5 mg, naltrexone acts as a glial modulator [14,15]. It specifically binds to Toll-like receptor 4, where it acts as an antagonist [8,16,17]. Toll-like receptor 4 downstream cellular signaling includes myeloid differentiation primary response 88 (MyD88) and toll-interleukin receptor (TIR)-domain-containing adapter-inducing interferon-β (TRIF) pathways, both ultimately leading to inflammatory end-products such as interleukin (IL)-1, tumor necrosis factor (TNF)-α, interferon-β, and nitric oxide [18].…”

“…Low-dose naltrexone disrupts the TRIF portion of the signaling cascade which reduces TNF-α and interferon-β synthesis [8]. Consequently, activated microglial cells expressing Toll-like receptor 4, otherwise a non-constitutive receptor, exert an attenuated pro-inflammatory profile [16]. The span and importance of neuronal Toll-like receptor 4 signaling is still under debate with ex vivo [19] and in vitro [20] investigations emphasizing its role in neuroinflammation, a role traditionally reserved for glia in the central nervous system (CNS) [21].…”