VX-166 does not inhibit the activation of human T-cells at concentrations below 100 μM. VX-166 and the calcineurin-inhibitor FK506 were tested in an IL-2 assay using anti-CD28/anti-CD3 stimulated human peripheral blood mononuclear cells. The compounds were added directly after stimulation and incubated for 24 hours. IL-2 secretion into the supernatant was determined by ELISA. FK506 inhibited the release of IL-2 with a 50% inhibitory concentration (IC50) value of 0.2 nM while VX-166 partly inhibited the release of IL-2 by ca. 25% at 100 μM.