Neomycin sulfate is an aminoglycoside antibiotic used for preventing or treating bacterial infections. In Vitro: Neomycin inhibits thrombin-stimulated release of inositol 1,4,5-trisphosphate (IP₃), by selectively binding PIP₂, but does not inhibit ³²P incorporation into PI or initiation of DNA synthesis^[1]. Neomycin (10 μM-1 mM) induces considerable release of [³H]arachidonic acid from phosphatidylinositol, phosphatidylcholine and phosphatidylethanolamine in saponin-permeabilized human platelets prelabeled with [³H]arachidonic acid. Moreover, neomycin enhances arachidonic acid release induced by thrombin. Addition of neomycin (100 μM) to ⁴⁵Ca²⁺-preloaded platelets elicits ⁴⁵Ca²⁺ mobilizatioin from intracellular stores^[2]. Neomycin (0-10 mM) inhibits guanosine 5'-[gamma-thio]triphosphate-stimulated PLD activity in digitonin-permeabilized NG108-15 cells in a concentration-dependent manner (50% inhibition at 100 μM). Neomycin similarly inhibits PLD activity present in rat brain membranes and assayed in vitro with [³H]phosphatidylcholine as substrate (50% inhibition at 65 μM)^[3]. Neomycin (5 mM) causes reversible reductions in the level of intracellular Ca²⁺, but PtdIns(4,5)P2 is not required for the channel activity^[4].