Indications
In the United States and Europe, baricitinib is prescribed for adult patients with moderately to severely active rheumatoid arthritis who have not responded adequately to one or more tumor necrosis factor (TNF) blockers. It may be administered either as monotherapy or in combination with methotrexate or other disease-modifying antirheumatic drugs (DMARDs). Additionally, in Europe, baricitinib is approved for treating moderate to severe atopic dermatitis in adults who are candidates for systemic therapy. In the United States, it is also authorized for the treatment of COVID-19 in hospitalized adults who require supplemental oxygen, non-invasive or invasive mechanical ventilation, or extracorporeal membrane oxygenation. Recently, baricitinib has also been approved for the treatment of severe alopecia areata in adults.
Pharmacodynamics
Baricitinib functions as a DMARD aimed at alleviating symptoms and impeding the progression of rheumatoid arthritis. Studies conducted on animal models of inflammatory arthritis have demonstrated significant anti-inflammatory properties, along with preservation of cartilage and bone. Importantly, these studies revealed no measurable suppression of humoral immunity or adverse hematologic effects. In clinical settings, baricitinib has been shown to reduce levels of immunoglobulins and serum C-reactive protein in patients with rheumatoid arthritis.
Absorption
Baricitinib exhibits an absolute bioavailability of approximately 80%. The maximum concentration (Cmax) is achieved approximately one hour following oral administration. Consuming a high-fat meal can reduce the mean area under the curve (AUC) and Cmax of baricitinib by around 11% and 18%, respectively, and delay the time to reach maximum concentration (Tmax) by 0.5 hours.
Metabolism
Baricitinib is metabolized primarily through the cytochrome P450 enzyme CYP3A4. Approximately 6% of the orally administered dose is identified as metabolites in urine and feces, although no quantifiable metabolites of baricitinib are found in plasma.
Mechanism of Action
Baricitinib functions as a selective and reversible inhibitor, primarily targeting Janus Kinases JAK1 and JAK2, with reduced affinity for JAK3 and TYK2. These kinases, members of the tyrosine kinase family, are crucial for transmitting signals from cytokines and growth factor receptors, thereby influencing processes such as hematopoiesis, inflammation, and immune cell activities. Following the binding of extracellular cytokines and growth factors, JAKs phosphorylate and activate Signal Transducers and Activators of Transcription (STATs). Activated STATs regulate intracellular functions, including gene transcription of inflammatory mediators like IL-2, IL-6, IL-12, IL-15, IL-23, IFN-γ, GM-CSF, and various interferons, which can trigger autoimmune responses. The involvement of the JAK-STAT pathway in rheumatoid arthritis is well-documented, as it correlates with an excessive production of inflammatory mediators. Among the four known JAK proteins-JAK1, JAK2, JAK3, and TYK2-JAKs form different pairings, either as homodimers or heterodimers, to transmit cytokine signals through distinct cell receptors. Although it remains unclear how the inhibition of specific JAK enzymes directly contributes to therapeutic outcomes, Baricitinib effectively disrupts the activity of these JAK proteins, modulating signaling pathways associated with various interleukins, interferons, and growth factors. Additionally, Baricitinib has demonstrated the capacity to reduce the proliferation of cells expressing JAK1/JAK2 mutations and to promote apoptosis in these cells.
