Philanthotoxin 74 dihydrochloride (PhTx 74) is an AMPAR antagonist; inhibits GluR3 and GluR1 with IC50s of 263 and 296 nM, respectively.

Philanthotoxin 74 in the micromolar concentration range displays selective inhibition between the two major subtypes of GluA2R-containing AMPARs, GluA1/A2R and GluA2R/A3, when these are coexpressed with γ-2 in oocytes. Philanthotoxin 74 is reported to fully inhibit GluA1/A2R receptors when applied at a concentration of 500 μM while producing 10% inhibition at GluA2R/A3 receptors. Philanthotoxin 74, when tested at concentrations of 100 and 500 μM, displays pronounced channel block (more than 80%) of GluA1/A2R but minimal block (less than 10%) of GluA2R/A3. Oocytes expressing GluA2R alone that homomeric GluA2R is virtually inert to philanthotoxin 74 in the 0.1-300 μM concentration range, displaying less than 5% inhibition at the maximum tested concentration of 300 μM. Philanthotoxin 74 inhibits these receptors nonselectively, with IC50 values of about 30 μM, in both the presence and absence of γ-2[1].

References:
[1]. Poulsen MH, et al. Evaluation of PhTX-74 as subtype-selective inhibitor of GluA2-containing AMPA receptors. Mol Pharmacol. 2014 Feb;85(2):261-8.
[2]. Kromann H, et al. Solid-phase synthesis of polyamine toxin analogues: potent and selective antagonists of Ca2+-permeable AMPA receptors. J Med Chem. 2002 Dec 19;45(26):5745-54.