Background The rising prevalence of rheumatic diseases in our society has caused a dramatic impact on the welfare of the population as well as becoming an economic burden to the health system. Osteoarthritis (OA), the most common rheumatic disease, is defined by joint-space narrowing due to progressive cartilage degradation. Despite the growing knowledge in OA pathophysiology, no treatment has yet proved to be efficient enough. The activation of innate immune receptors, such Toll-like receptor 4 (TLR4), by damage-associated molecular patterns (DAMPS) has been involved in chondrocyte-mediated inflammatory responses. There are currently no available drugs aimed to block TLR4-mediated inflammatory responses. Nonetheless, there are already known drugs being employed in other indications that could have this activity; namely amitriptyline, naloxone, and thalidomide.
Objectives Determine the ability of amitriptyline, naloxone and thalidomide to block TLR4-mediated innate immune responses in chondrocytes.
Methods The effect of amitriptyline, naloxone and thalidomide on TLR4-mediated inflammatory responses was determined in mouse chondrogenic cell line (ATDC5) and in human primary OA chondrocytes. The mRNA expression of key inflammatory factors lipocalin-2 (LCN2), interleukin-6 (IL-6), and monocyte chemoattractant protein-1 (MCP-1) was studied by RT-PCR. Cell viability was tested using the methyl-thiazolyl-tetrazolium (MTT) reagent and nitrite accumulation (nitric oxide production) in cell culture media was assessed by Griess reaction and validated by determining nitric oxide synthase 2 (NOS-2) mRNA expression.
Results The co-stimulation of human OA chondrocytes with the TLR4 agonist LPS [100 ng/ml] and amitriptyline [1 µM], reduced the mRNA expression of LCN2 (90%), IL-6 (95%) and MCP-1 (87%). The pre-stimulation of naloxone [100 µM] with LPS also reduced the mRNA expression of LCN2 (53%), IL-6 (78%) and MCP-1 (79%). Similar results LCN2 (63%), IL-6 (74%), and MCP-1 (78%) were obtained upon the pre-stimulation of these cells with thalidomide [500 µM]. The anti-inflammatory effect of these drugs on these pro-inflammatory factors was also observed but lowered in ATDC5 cells. Consistent with these results in ATDC5 cells these drugs also reduced the expression of mRNA NOS-2 gene expression as well as nitrite accumulation in the cell culture medium. At the studied concentrations, amitriptyline, naloxone and thalidomide did not affect to chondrocytes viability.
Conclusions The data presented here have shown that amitriptyline, naloxone and thalidomide block TLR4 innate immune responses in human OA chondrocytes. These drugs have passed many toxicology and safety tests for their clinical use and could be ready for its repurposing in the management of TLR4-mediated OA cartilage inflammation.
Acknowledgements This research is supported by Fondo de Investigación Sanitaria funded by the Instituto de Salud Carlos III and FEDER (PI16/01870, CP15/00007). R.G. is funded by the Instituto de Salud Carlos III through a Miguel Servet programme. R.G. is a member of the RETICS Programme, RD12/0009/0008 Instituto de Salud Carlos III (ISCIII).
Disclosure of Interest None declared
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