SAFit1 is a FK506 binding protein 51 (FKBP51)-specific inhibitor with a Ki value of 4 ± 0.3nM[1]. The selective inhibition of FKBP51 using SAFfit1 has emerged as a possible treatment for chronic pain, obesity-induced diabetes, or depression[2][3].
In vitro, SAFit1 (1-1000nM) effectively stimulated neurite growth in two neuronal cell lines, specifically the mouse N2a line and the human SH-SY5Y line, as well as in primary hippocampal neurons. The active concentration range of SAFit1 is very wide[1]. The NanoBRET assay shows that SAFit1 (1-100000nM) treated HEK293T cells transiently expressing FKBP51-NLuc for 2h dose-dependently competed with a fluorescent NanoBRET tracer, which indicated that SAFit1 was able to engage FKBP51 inside cells. SAFit1 (0.4, 0.2, 1μM) incubated murine microglia SIM-A9 cells for 16h could engage FKBP51 in cells and inhibited IKKα phosphorylation in a dose-dependent manner[4]. A375 melanoma incubated in the presence of 20nM SAFit1 for 5h decreased transcript and protein levels of the receptor type I of TGF-β (TβRI) and reduced the protein expression of phosphorylated (p)-Smad2,3. SAFit1 finally inhibited FKBP51-dependent TGF-β pro-tumoral function in vitro[5].
In vivo, mice received SAFit1 (30mg/kg) and SAFit2 (30mg/kg) as a cassette dose orally in a pharmacokinetic study. The experimental results showed that the maximum concentration (Cmax) of SAFit1 in mice was 1467ng/mL, and the time to reach Cmax was 0.5 hours, indicating that SAFIT1 was absorbed quickly in the gastrointestinal tract[6].
References:
[1] Gaali S, Kirschner A, Cuboni S, et al.Selective inhibitors of the FK506-binding protein 51 by induced fit. Nat Chem Biol. 2015 Jan;11(1):33-7.
[2] Sidibeh C O, Pereira M J, Abalo X M,et al.FKBP5 expression in human adipose tissue: potential role in glucose and lipid metabolism, adipogenesis and type 2 diabetes.Endocrine. 2018 Oct;62(1):116-128.
[3] Knaup F H, Meyners C, Sugiarto W O, et al.Structure-Based Discovery of a New Selectivity-Enabling Motif for the FK506-Binding Protein 51. J Med Chem. 2023 Apr 27;66(8):5965-5980.
[4] Voll A M, Meyners C, Taubert M C, et al. Macrocyclic FKBP51 Ligands Define a Transient Binding Mode with Enhanced Selectivity. Angew Chem Int Ed Engl. 2021 Jun 7; 60(24): 13257-13263.
[5] Marrone L, Giacomo V D, Malasomma C, et al. Exploring the potential of selective FKBP51 inhibitors on melanoma: an investigation of their in vitro and in vivo effects. Cell Death Discov. 2025 Apr 3;11(1):138.
[6] Gabani B B, Sulochana S P, Siddesh A H,et al. Validated LC-MS/MS Method for Simultaneous Quantitation of SAFit-1 and SAFit-2 in Mice Plasma: Application to a Pharmacokinetic Study. Drug Res (Stuttg). 2020 Jul;70(7):325-332.