In vitro activity: U73122 significantly inhibits aggregation of human platelets induced by a variety of agonists, including collagen, thrombin, ADP, arachidonic acid, with IC50 of 1-5 μM. U-73122 (10 μM) inhibits the production of IP3 and the subsequent rapid increase in cytosolic Ca2+ induced by either thrombin or U-46619 through inhibiting hydrolysis of phosphatidyl[3H]inositol and phosphatldyl[3H]inosito1 4,5-bisphosphate catalyzed by a soluble fraction from platelets (Ki=9 and 40 μM, respectively). U-73122 inhibits thromboxane B production induced by collagen through inhibiting receptor-coupled mobilization of arachidonic acid. U73122 inhibits also FMLP-induced aggregation of human polymorphonuclear neutrophils and the associated production of IP3 and diacylglycerol. U-73122 causes a concentration-dependent inhibition of C5a, FMLP, PAF and LTB4-induced MPO and B12-BP release from cyto-chalasin B-treated PMNs. The IC50 values are 60 (FMLP), 110 (LTB4), 115 (C5a) and 120 (PAF) nM for MPO release; and 105 (FMLP), 110 (LTB4), 120 (C5a) and 140 (PAY) nM for B12-BP release. U-73122 is also a potent inhibitor of superoxide anion production by cytochalasin B-treated PMNs activated with either C5a or FMLP with IC50 of 160 and 300 nM, respectively. U-73122 causes suppression of the rise in [Ca2+]i, IP3 production and DAG production in FMLP-stimulated PMNs with IC50 of 500 nM, 2 μM, and 2 μM, respectively. 3 μM U-73122 causes 100% inhibition of FMLP-induced GTPase activity. U-73122 causes a concentration-dependent inhibition of the FMLP-evoked association of PKC with the extractable particulate fraction of PMNs, but not a soluble preparation of PMN PKC. U73122 significantly inhibits recombinant human PLC-β2, with an IC50 of ~6 μM. U73122 has little effect on PLC-β1, PLC-β3, or PLC-β4. U73122 reduces interleukin-8 and leukotriene B4-induced Ca2+ flux and chemotaxis in human neutrophils with IC50 of ~6 μM and ~5 μM, respectively.

Kinase Assay: U-73122 is a phospholipase C (PLC) and 5-LO (5-lipoxygenase) inhibitor with an IC50 of 1-2.1 μM for PLC.

Cell Assay: Agonist-induced production of IP3 in PMN is measured by use of the competitive radiobinding assy. PMN (2 x 106-107) in 0.2 mL of phosphate-buffered saline, pH 7.4 [NaC1 (138 mM), Na2HPO4 (8.1 mM), KH2PO4 (1.5 mM), KCI (2.7 mM), CaCl2 (1.0 mM), MgC12 (1.0 mM) and glucose (0.1%, w/v)] are incubated in conical polypropylene tubes at 37°C in a shaking water bath. U-73122 or U-73343 is added (in 1 μL of DMSO) 3 min before the addition of agonist, FMLP (0.1 μM) plus cytochalasin B (5 μg/mL). FMLP and cytochalasin B are added in 1 μL each of DMSO and ethanol, respectively. Appropriate vehicle controls are included in each experiment. PMN incubation mixtures are quenched with the addition of 0.07 mL of ice-cold TCA (20%, w/v) and a portion (0.2 mL) of the TCA extract is processed for the measurement of IP3 by competitive radiobinding as described above for platelets.