Rheumatoid arthritis (RA) is a chronic, systemic, autoimmune inflammatory disorder that primarily targets the synovial joints and periarticular soft tissues. Its pathological hallmarks encompass persistent synovitis, hyperplastic pannus formation, progressive cartilage and bone destruction [1,2]. RA can lead to severe disability, increased morbidity, and premature mortality [3,4]. Epidemiological data indicate that RA affects up to 1% of the adult population globally, imposing substantial health burdens and socioeconomic challenges [5]. Therefore, the development of effective and well-tolerated therapeutic agents for RA assumes a position of paramount importance. Currently, RA treatment options are largely categorized into several drug classes, including non-steroidal anti-inflammatory drugs (NSAIDs, e.g., celecoxib, naproxen) [6], biologic disease-modifying antirheumatic drugs (e.g., tocilizumab, adalimumab) [7], glucocorticoids (e.g., prednisone, dexamethasone) [8,9], and conventional synthetic disease-modifying antirheumatic drugs (DMARDs) such as methotrexate (MTX) [10]. However, these therapies are also associated with a spectrum of adverse effects, including gastrointestinal ulcers, immunosuppression, increased infection risk, pulmonary fibrosis, and pneumonia [11,12]. Hence, the development of therapeutics that are both more effective and have lower adverse effects for the treatment of rheumatoid arthritis represents an urgent need.

Janus kinases (JAKs), a family of non-receptor tyrosine kinases, comprise four members: janus kinases 1 (JAK1), janus kinases 2 (JAK2), janus kinases 3 (JAK3), and tyrosine kinase 2 (TYK2) [13,14]. The JAK/STAT (signal transducer and activator of transcription) signaling cascade represents a pivotal pathway governing immune responses, development, and various pathological conditions [15,16]. Notably, multiple pro-inflammatory cytokines, such as interleukin-6 (IL-6), interferon-gamma (IFN-γ), and granulocyte-macrophage colony-stimulating factor (GM-CSF), activate the JAK/STAT pathway, thereby promoting synovial inflammation and joint destruction in RA [17,18]. The development of small-molecule agents targeting the JAK-STAT pathway is expected to advance safer and more effective therapies for autoimmune diseases such as RA [19]. JAK inhibitors represent a novel class of orally bioavailable disease-modifying agents for RA. Clinical trials reveal that JAK inhibitors induce rapid abatement of rheumatoid arthritis symptomatology, with particularly pronounced amelioration of articular pain, and exhibit significantly superior therapeutic efficacy compared with the conventional anchor drug methotrexate [20]. Presently, several JAK inhibitors have been approved for RA treatment including Tofacitinib, Upadacitinib, Filgotinib and Baricitinib (Fig. 1). Tofacitinib was the first in-class and a broad-spectrum JAK inhibitor. However, the pan-JAK inhibition profile is associated with notable safety concerns, including an increased risk of herpes zoster infections, anemia [21], and cardiovascular adverse events [22]. To mitigate these risks, the development of more selective JAK inhibitors has gained momentum, particularly those targeting JAK1, e.g., Upadacitinib and Filgotinib, as well as JAK1/2 inhibitors such as Baricitinib. Compared to pan-JAK inhibitors, JAK-selective inhibitors significantly reduce hematologic side effects such as anemia and thrombocytopenia [23].

In rheumatoid arthritis (RA), multiple inflammatory pathways involve JAK/STAT signaling, and JAK2/STAT3 has been implicated in inflammatory and immune responses that contribute to RA pathogenesis [24]. Therefore, a dual JAK1/2 inhibition profile may provide broader pathway coverage and robust anti-inflammatory activity. Developing dual JAK1/2 inhibitors is expected to enhance the therapeutic efficacy in rheumatoid arthritis (RA). However, JAK2 inhibition is associated with potential adverse risks such as thrombocytopenia [25]. Therefore, achieving a favorable balance between JAK1 and JAK2 inhibition is particularly important to improve efficacy while minimizing JAK2-mediated side effects as much as possible. Intriguingly, the marketed JAK1/2 inhibitor Baricitinib demonstrated superior therapeutic efficacy compared to methotrexate in RA clinical trials [26]. Baricitinib not only exhibited significant clinical efficacy but also had a favorable safety profile [27,28]. A study indicated that Baricitinib maintained stable long-term safety over a drug exposure period of up to 9.3 years [29]. Thus, JAK1/2 inhibitors constitute a promising strategy for investigation in RA therapeutics.

In this work, we analyzed the target characteristics of the JAK1/2 proteins and designed a novel class of compounds based on [1,2,4]triazolo[1,5-a]pyridine and imidazo[1,2-a]pyridine pharmacophore cores. Their inhibitory effects on JAK1/2 kinases and Ba/F3 cells were further evaluated. The optimal compound H018 (RAI-20) exhibited high selectivity for JAK1/2, superior Ba/F3 inhibitory capacity compared to Filgotinib, and enhanced therapeutic efficacy in a rat model of RA. Importantly, compound RAI-20 displayed favorable properties of druggability, pharmacokinetic characteristics, and safety profiles. Currently, this compound has advanced to Phase II clinical trials and holds promise as a therapeutic agent with both high efficacy and safety for the treatment of RA.