Synthesis and Evaluation of Ginkgolic Acid Derivatives as SUMOylation Inhibitors

Abstract

SUMOylation has emerged as an important post-translational modification that has been shown to modulate protein activity associated with various signaling pathways, and consequently, it has emerged as an important therapeutic target. While several natural products have been shown to inhibit enzymes involved in the SUMOylation process, there has been little progress toward the development of more selective and potent SUMOylation inhibitors. Ginkgolic acid was one of the first natural products discovered to inhibit the SUMO E1 enzyme. Despite its use to mechanistically investigate the SUMOylation process, ginkgolic acid also modulates other pathways as well. In this Letter, preliminary structure-activity relationships for ginkgolic acid as a SUMOylation inhibitor are presented.

Figure 1

SUMOylation catalytic cycle: (1) maturation, (2) activation, (3) conjugation, (4) ligation, and (5) hydrolysis.

Figure 2

(A) Ginkgolic acid (1) and anacardic acid (2). (B) Compound numbering scheme. The first letter represents position of the alkyl tail, and the second letter represents alkyl tail length. Example compound 8bc shown.

Scheme 1. Synthesis of Ginkgolic/Anacardic Acid Analogues

(a) SOCl₂, DMAP, acetone, 1,2-DME, 24 h (77%); (b) trifluoroacetic anhydride, trifluoroacetic acid, acetone, 24 h (27–53%); (c) Tf₂O, pyridine, CH₂Cl_2, 3 h (35–89%); (d) alkene, Pd(dppf)Cl₂, K₂CO₃, DMF, 75 °C, 12 h (38–86%); (e) KOH, DMSO, 80 °C, 2 h (42-94%); (f) H₂, EtOAc, Pd/C, 16 h (64–98%).

Figure 3

Western blot results from initial library screen. All compounds were tested at 50 μM.

Figure 4

C8 and C6 ginkgolic acid derivatives.

Figure 5

(A) Effects of lead compounds on global SUMOylation with SUMO1, apoptosis, and autophagy related factors on PC3 cells after 24 h treatment. (B) Densitometric quantification of the cleaved PARP data from A using ImageJ. (C) Densitometric quantification of the LC3II data from A using ImageJ. All experiments were performed at least three times, and the data are presented as the mean ± SEM.

Figure 6

(A) The 48 h viability studies of four active and one inactive compound at 10 or 20 μM against PC3 cells. (B) The 48 h viability studies of four active and one inactive compound at 10 or 20 μM against MDA-MB-231 cells. (C) Time-dependent viability studies with two concentrations against PC3 cells (*P ≤ 0.05 significant versus nontreated cells). All experiments were performed at least three times and the data are presented as the mean ± SEM.