(Arg)9 (Nona-L-arginine;Peptide R9) is a cell-penetrating peptide; exhibits neuroprotective activity with an IC₅₀ of 0.78 μM in the glutamic acid model. IC50 & Target: IC50: 0.78 μM (neuroprotection)^[1] In Vitro: Poly-arginine (e.g. (Arg)9) and arginine-rich peptides (e.g. TAT, penetratin), which belong to a class of peptides with cell-penetrating properties are neuroprotective. (Arg)9 provides significant neuroprotection in a dose–response manner following glutamic acid exposure (IC₅₀=0.78 μM). Following kainic acid exposure, (Arg)9 is neuroprotective, but less effective than in the glutamic acid model (IC₅₀=0.81 μM). (Arg)9 also shows neuroprotection following in vitro ischemia (IC₅₀=6 μM)^[1]. In Vivo: (Arg)9) (D-isoform) is neuroprotective in rat stroke models. (Arg)9) is highly neuroprotective, with efficacy increasing with increasing arginine content, has the capacity to reduce glutamic acid-induced neuronal calcium influx and requires heparan sulfate preotoglycan-mediated endocytosis to induce a neuroprotective effect^[2]. (Arg)9) (D-isoform) administered intravenously at a dose of 1000 nmol/kg 30 min after permanent middle cerebral artery occlusion (MCAO) reduces infarct volume^[3].