Background: Janus kinase inhibitors (JAKis) are used to treat chronic inflammatory diseases, e.g., rheumatoid arthritis (RA), with a high efficacy. However, tofacitinib, a JAKi, has been associated with increased cardiovascular (CV) side effects, which have led to a class warning by the U.S. Food and Drug Administration (FDA) and restriction of use, excluding a large patient group from this efficient treatment. The mechanisms behind these side effects remain unknown. Thromboxane A ₂ (TXA ₂ ) and prostaglandin E ₂ (PGE ₂ ) are lipid mediators derived from arachidonic acid (AA). TXA ₂ is a major product generated in platelets and plays a key role in thrombosis, while PGE ₂ is crucial in inflammation and cancer.[1]

Objectives: This study aimed to investigate the impact of JAKis on the biosynthesis of TXA ₂ from platelets in response to thrombin, and prostanoids PGE ₂ and TXA ₂ from leukocytes in response to the inflammatory stimulus bacterial lipopolysaccharides (LPS) in human whole blood assays (WBAs), which are in vitro models that reflect AA metabolism in vivo .

Methods: Four JAKis, approved for clinical use, i.e ., tofacitinib, baricitinib, filgotinib, and upadacitinib, were investigated for their impact on the biosynthesis of platelet TXB ₂ (the stable hydrolysis product of TXA ₂ ). Whole blood from healthy subjects (HC, n=12) were allowed to clot for 1 hour, at 37°C and serum TXB ₂ was detected. In this assay, TXB ₂ is generated from platelet cyclooxygenase (COX)-1 in response to endogenous thrombin. The JAKis were also studied in LPS-stimulated heparinized whole blood, collected from HC (n=17) and patients with systemic lupus erythematosus (SLE, n=12) or treatment-naïve axial spondyloarthritis (axSpA, n=12), incubated for 24 hour at 37°C. PGE ₂ and TXB ₂ were assessed by liquid chromatography-mass spectrometry (LC-MS/MS).

Results: The four JAKis, investigated at 0.04-20 μM, enhanced serum TXB ₂ generation. At 4 μM of tofacitinib, baricitinib, and upadacitinib, % serum TXB ₂ vs. vehicle was 135.70±53.65, 139.00±65.25, and 131.20±45.88%, respectively (mean±SD, n=12). At 0.04 μM of filgotinib, this was 144.40±48.91%. Addition of exogenous AA did not further increase the TXB ₂ production, suggesting that JAKis would rather be involved in the enhanced release of platelet AA, rather than downstream regulators. The antiplatelet agent aspirin (acetylsalicylic acid, ASA, 100µM) almost completely inhibited serum TXB ₂ synthesis (96.2±1.8%), and this effect was not altered by the presence of JAKis. In the LPS-stimulated WBA, tofacitinib (1 µM) increased both PGE ₂ (48% ± 36%, mean±SD, P<0.01, n=17) and TXB ₂ (42% ± 33%, P<0.01, n=17) in HC. This increase was also seen in patients with SLE (PGE ₂ : 89% ± 127%, P<0.01, n=12; TXB ₂ : 57% ± 39%, P<0.01, n=12) and axSpA (PGE ₂ : 50% ± 56%, P<0.01, n=12; TXB ₂ : 29% ± 25%, P<0.01, n=12).

Conclusion: In conclusion, JAKis lead to an increased production of prostanoids in platelets and leukocytes. The increased production of PGE ₂ can reduce the anti-inflammatory benefits of these drugs and possibly promote tumorigenesis. Moreover, the increased production of TXA ₂ from platelets and leukocytes can increase the risk of thrombotic vascular events supporting a class effect for JAKis. Our findings suggest that co-administering low-dose aspirin might help reduce the cardiovascular side effects of JAKis. To confirm this hypothesis, it is necessary to conduct randomized clinical trials on specific patients.

REFERENCES: [1] Steinmetz-Späh, J. & Jakobsson, P.-J. The anti-inflammatory and vasoprotective properties of mPGES-1 inhibition offer promising therapeutic potential. Expert Opinion on Therapeutic Targets 27 , 1115-1123 (2023).

Acknowledgements: We want to acknowledge patients, participants, medical doctors, research nurses and nurses involved in this study at the Karolinska University Hospital at Solna/Huddinge and at the Center for Rheumatology (Sweden) and the nurse and students of “G. d’Annunzio” University (Chieti, Italy).

Disclosure of Interests: None declared.