AS2863619 is a potent, orally active cyclin-dependent kinase 8 (CDK8) and CDK19 inhibitor with IC₅₀s of  0.61 nM and 4.28 nM, respectively.

Firstly, in vitro, AS2863619 is a potent Foxp3 inducer in T_conv cells. AS2863619 can generate Foxp3⁺ T cells from naïve Foxp3⁻CD4⁺ T_conv cells in a dose-dependent fashion. Similarly, AS2863619 treatment induces Foxp3 in CD8⁺ T_conv cells as well. Furthermore, although TCR stimulation per se elicits the expression at a low-level, AS2863619 substantially enhances FOXP3 expression in human CD4⁺ and CD8⁺ T_conv cells in the peripheral blood.

This compound induces Foxp3 expression in vitro without antigen-presenting cells (APCs). The induction is TGF-β independent and IL-2 dependent and requires TCR stimulation. It enables the conversion of antigen-specific Tconv cells into Foxp3⁺ suppressive T cells without TGF-β.

Moreover, in mouse CD4⁺ T cells, AS2863619 (1 μM; 22 hours) suppresses serine phosphorylation of the PSP motif of STAT5 to ~40%, enhancing tyrosine phosphorylation in the C-terminal domain to ~160% of control-treated samples.

Nextly, in vivo, To determine whether AS is able to induce Foxp3 in vivo in Tconv cells in an antigen-specific manner.

In DO11.10 TCR transgenic mice on the RAG2-deficient background,  AS2863619 (orally administration; 30mg/kg)  attains a serum concentration equivalent to an in vitro Foxp3-inducing dose, in addition, it does not cause discernible toxicity in mouse. In contrast, in RAG2-sufficient DO11.10 mice, AS2863619 specifically generates KJ 1.26⁺ Foxp3⁺ T cells, it inhibits the activation of KJ 1.26⁺ rather than KJ 1.26^– Foxp3^– T_conv cells.

After sensitization with 2,4-dinitrofluorobenzene (DNFB), AS2863619 (30 mg/kg; oral administration; daily; for 2 weeks; mice) dampen the degree of the secondary response, with milder infiltration of inflammatory cells into the skin and decreases ratios of interferon-γ⁺ (IFN-γ⁺) cells in a skin contact hypersensitivity model, when compared with vehicle-treated control mice. T_reg depletion before the elicitation of the secondary response abolishes AS2863619-induced suppression.AS2863619 specifically increases KLRG1⁺ Foxp3⁺ T cells in DNFB-sensitized mice.

In conclusion, AS2863619, as a potent CDK8 and CDK19 inhibitor, has the potential for the treatment of various immunological diseases.

Akamatsu M, et al. Sci Immunol. 2019 Oct 25;4(40). pii: eaaw2707.