The revolution in rheumatoid arthritis (RA) therapeutics has been transformative for many patient outcomes. Yet most patients continue to experience life disabling pain. Strikingly, even those who achieve full disease remission with state-of-the-art anti-tumour necrosis factor (TNF) treatments report substantially higher levels of pain when compared to the general population. Such disconnect presents one of the greatest contemporary challenges to the care of patients with RA.~Considering the ongoing excess burden of pain in this patient population, trials of Janus kinase inhibitors (JAKinibs) present welcome data. JAKinibs deliver superior pain improvements in comparison to those receiving anti-TNF therapy. Of note, the majority of this effect has not been fully explained by markers of peripheral inflammation and remains to be understood. Moreover, JAKinibs appear to offer rapid analgesic benefit. Traditional DMARDS and modern biologics commonly take several weeks to bring relief whereas JAKinibs, such as filgotinib, begin to improve pain as early as 2 weeks, even before the observed attenuation of peripheral clinical inflammation.~In light of these clinical observations, the investigators believe that RA is a mixed pain state i.e., pain pathways exist in addition to established peripheral inflammatory nociceptive mechanisms. In particular, the central nervous system (CNS) may have an important role in determining RA pain. Recently our group were the first to delineate distinct neurobiological pain signatures in the brains of RA patients by employing functional connectivity magnetic resonance imaging (fcMRI) - a recent adaptation of functional MRI data that examines the synchrony of neural activity which modulates the efficiency and extent of neuronal transmission between brain regions. Specifically, the investigators identified and replicated two distinct pain signatures:~enhanced functional connectivity between the Default Mode Network (DMN) and insula, which was unrelated to levels of peripheral inflammation but, intriguingly, is an established neurobiological marker of fibromyalgia (the prototypical CNS pain sensitization disorder, and~enhanced functional connectivity between the Dorsal Attention Network (DAN) and the left inferior parietal lobule (LIPL) which was related to levels of peripheral inflammation.~Pre-clinical experiments have not only implicated the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway with peripheral immune system functioning but also the brain. In the CNS, this pathway promotes gene expression associated with inflammation which in turn generates pro-nociceptive cytokines. However, there is now also emerging evidence to support the pathway's direct role in synaptic transmission and neurotransmitter receptor modulation. Specifically, the JAK-STAT pathway appears important in N-methyl-d-aspartate (NMDA) related synaptic plasticity - a ubiquitous glutamate receptor of the human brain. Their induction is selectively blocked by JAK inhibitors. Increases in glutamate and subsequent binding to NMDA receptors cause chaotic and incoherent neuronal functional activity. Human studies of fibromyalgia have consistently evidenced both elevated glutamate levels within the insula and dysfunctional neural connectivity. Moreover, fibromyalgia pharmacotherapy (pregablin), considered to reduce neural glutamate, rectifies both insular glutamate and brain functional connectivity (DMN-insula). JAK inhibition (JAKi) may facilitate the reduction of glutamate-NMDA binding and ultimately pain alleviation by normalising the functional activity of these same neural connections.
This is an experimental medicine, single-centre, observational test-retest study to evaluate Filgotinib's mechanism of analgesic action in RA patients.~The investigators hypothesize that Filgotinib's mechanism of analgesic action is determined by at least two factors. The first is related to those CNS sensitization pathways seen in fibromyalgia, specifically DMN-insula brain functional connectivity and insular glutamate.~The second is related to peripheral inflammation, specifically joint synovitis, blood cytokines/chemokines and DAN-LIPL functional brain connectivity. The CNS sensitization pain pathways related to fibromyalgia are more quickly modified compared to those related to peripheral inflammation and help explain Filgotinib's rapid onset of effect.