Cyclosporin H (CsH) is a potent and selective FPR antagonist. In human neutrophils, cyclosporin H inhibited FMLP-induced superoxide anion (O2-) formation. CsH inhibited the binding of FMLP (3 nM) to HL-60 membrane with IC50 and Ki values of 0.7 μM and 0.10 μM, respectively. CsH inhibited GTP hydrolysis stimulated by FMLP (100 nM) with IC50 and Ki values of 1.3 μM and 0.79 μM, respectively. CsH also inhibited the increase of [Ca2+Ii, O2- formation and β-glucuronidase release induced by FMLP with IC50 values of 0.23, 0.6 and 2.2 μM, and with Ki values of 0.08, 0.24, and 0.45 μM, respectively. In peripheral blood basophils, CsH (8-800 nM) competitively and concentration-dependently inhibited leukotriene C4 and histamine release activated by FMLP. In eosinophils isolated from bronchoalveolar lavage fluid of mice challenged with inhaled allergen, cyclosporin H increased apoptosis in the presence of 10 U/ml interleukin 5 in a concentration-dependent way. Cyclosporin H also increased apoptosis in the presence of 10 U/ml GM-CSF, but to a lesser extent .
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