The main target of pharmacological therapy for growth hormone (GH)-secreting pituitary tumors (GH-PitNET) is the somatostatin receptor type 2 (SST2). However, approximately half of patients treated with octreotide, an SST2 agonist, show a low response rate or are octreotide-resistant. Here we present mechanistic data that shows co-treatment with simufilam, a novel oral therapeutic candidate, enhances sensitivity to octreotide. We previously showed that the cytoskeleton protein filamin A (FLNA) is recruited to bind SST2 upon agonist stimulation, and this interaction is required for SST2 signaling in GH-PitNET cells. However, when phosphorylated at Ser2152, FLNA no longer enables SST2 signaling and all SST2 anti-tumor effects are abolished. Simufilam is a FLNA-binding small molecule shown to modulate FLNAs conformation and its interactions with partner proteins in disease states. We postulated that simufilam may restore FLNAs linkage to SST2 and therefore FLNAs ability to enable SST2 signal transduction. To test this hypothesis, we assessed simufilams effects on FLNA phosphorylation, FLNA-SST2 complex formation, SST2 signal transduction, GH secretion and cell proliferation/apoptosis, in human primary cultured GH-PitNET cells and in the rat pituitary tumor cell line GH4C1. Simufilam treatment reduced FLNA phosphorylation on Ser2152 in GH4C1 cells (-28±13% after 10 min, P<0.01 vs basal) and in primary human GH-PitNET cells (-59%). Additionally, FLNA-SST2 complexes in GH4C1 cells fell below basal levels after 1h octreotide treatment (-29±6.8%, P<0.05 vs bas) but were still elevated after 1h co-incubation with octreotide+simufilam (135±19.7%, P<0.05 vs bas). Simufilam did not affect the ability of octreotide to inhibit GH secretion in GH4C1 or primary GH-PitNET cells that are in vitro responsive to octreotide; however, a combination of simufilam+octreotide reduced GH secretion in primary GH-PitNET cells that are in vitro resistant to octreotide (n=2) (-42±3.5%, P<0.001 vs bas). Simufilam slightly reduced cell proliferation (-15±10.1%, P<0.05 vs bas) and ERK phosphorylation (-21±18.8%, P<0.05 vs bas), while increasing cell apoptosis (+17.8±7.3%, P<0.05 vs bas) in the GH4C1 cell line. Interestingly, co-treatment with simufilam+octreotide in GH4C1 potentiated the pro-apoptotic effect of the single drugs (+13±5% octreotide, P<0.001 vs bas; +36.8±9.2% octreotide+simufilam, P<0.01 vs bas, P<0.05 vs octreotide or simufilam alone). In conclusion, simufilam reduced FLNA phosphorylation, enhanced and prolonged the octreotide-induced FLNA-SST2 interaction and promoted SST2 signal transduction in human primary cultured GH-PitNET cells. These data suggest that co-treatment with simufilam may enhance the efficacy of octreotide or other somatostatin analog drugs in the management of pituitary tumors.
13 May 2023 - 16 May 2023