Evidence that tricyclic small molecules may possess toll-like receptor and myeloid differentiation protein 2 activity

Abstract

Opioids have been discovered to have Toll-like receptor (TLR) activity, beyond actions at classical opioid receptors. This raises the question whether other pharmacotherapies for pain control may also possess TLR activity, contributing to or opposing their clinical effects. We document that tricyclics can alter TLR4 and TLR2 signaling. In silico simulations revealed that several tricyclics docked to the same binding pocket on the TLR accessory protein, MD-2, as do opioids. Eight tricyclics were tested for effects on TLR4 signaling in HEK293 cells over-expressing human TLR4. Six exhibited mild (desipramine), moderate (mianserin, cyclobenzaprine, imiprimine, ketotifen) or strong (amitriptyline) TLR4 inhibition, and no TLR4 activation. In contrast, carbamazepine and oxcarbazepine exhibited mild and strong TLR4 activation, respectively, and no TLR4 inhibition. Amitriptyline but not carbamazepine also significantly inhibited TLR2 signaling in a comparable cell line. Live imaging of TLR4 activation in RAW264.7 cells and TLR4-dependent interleukin-1 release from BV-2 microglia revealed that amitriptyline blocked TLR4 signaling. Lastly, tricyclics with no (carbamazepine), moderate (cyclobenzeprine), and strong (amitriptyline) TLR4 inhibition were tested intrathecally (rats) and amitriptyline tested systemically in wildtype and knockout mice (TLR4 or MyD88). While tricyclics had no effect on basal pain responsivity, they potentiated morphine analgesia in rank-order with their potency as TLR4 inhibitors. This occurred in a TLR4/MyD88-dependent manner as no potentiation of morphine analgesia by amitriptyline occurred in these knockout mice. This suggests that TLR2 and TLR4 inhibition, possibly by interactions with MD2, contributes to effects of tricyclics in vivo. These studies provide converging lines of evidence that several tricyclics or their active metabolites may exert their biological actions, in part, via modulation of TLR4 and TLR2 signaling and suggest that inhibition of TLR4 and TLR2 signaling may potentially contribute to the efficacy of tricyclics in treating chronic pain and enhancing the analgesic efficacy of opioids.

Identification of Hepatitis C Virus Inhibitors Targeting Different Aspects of Infection Using a Cell-Based Assay

“…NSC 5159 (c2) also has anti-HCV potential as it has been shown to be an inhibitor of N-linked glycosylation, a process that is critical for HCV E1 and E2 processing (28,32), and has in fact been proposed as a potential antiviral target (14,18). Also notable is the tricyclic compound NSC 78206 (c3), as tricyclics or their active metabolites are believed to exert their biological actions, in part, via modulation of Toll-like receptor 4 (TLR4) and TLR2 signaling (36). Likewise, HCV infection correlates with TLR expression and signaling (33,50,76).…”

Mechanisms of Pain in Sickle Cell Disease

“…TLR4 has been shown to be associated with several modalities of pain including inlammatory pain [46,77] whereas knocking out TLR4 afected cisplatin-induced mechanical allodynia in both male and female mice [91]. No adverse of-target efects of targeting of TLR4 in other disease conditions have been observed so far [92,93]. Therefore, the contribution of TLR4 in sickle pain needs to be evaluated.…”

Medication-overuse headache and opioid-induced hyperalgesia: A review of mechanisms, a neuroimmune hypothesis and a novel approach to treatment

“…107 These findings raise the possibility that attenuation of glial activation via TLR-4 blockade could contribute to the efficacy of this medication in tension-type headache.…”

“…[98][99][100][101][102][103] It is now clear that glia-to-neuron signaling via toll-like receptor 4 (TLR-4) can play a causal role in the initiation and maintenance of pathological pain. [104][105][106][107] The toll-like receptors (TLRs) are a family of innate immune pattern recognition receptors, which respond to a wide variety of pathogen-derived and tissue damage-related ligands. 108 The TLR-4 receptor, which is primarily expressed upon microglia, 109 is an important contributor to activation of these cells, although expression on astrocytes, endothelial cells and neurons has also been reported.…”

“…107 Pre-clinically opioid-induced glial activation is known to oppose opioid analgesia and enhance opioid adverse effects including tolerance, dependence, reward and respiratory depression. 115,116 Interestingly, opioid-induced glial activation is mediated through activation of TLR-4, exposing the potential to separate the beneficial actions of opioids from their unwanted adverse effects.…”