Discovery of a Novel Pharmacological and Structural Class of Gamma Secretase Modulators Derived from the Extract of Actaea racemosa

Abstract

A screen of a library of synthetic drugs and natural product extracts identified a botanical extract that modulates the processing of amyloid precursor protein (APP) in cultured cells to produce a lowered ratio of amyloid-beta peptide (1-42) (Aβ42) relative to Aβ40. This profile is of interest as a potential treatment for Alzheimer's disease. The extract, from the black cohosh plant (Actaea racemosa), was subjected to bioassay guided fractionation to isolate active components. Using a combination of normal-phase and reverse-phase chromatography, a novel triterpene monoglycoside, 1, was isolated. This compound was found to have an IC(50) of 100 nM for selectively reducing the production of amyloidogenic Aβ42 while having a much smaller effect on the production of Aβ40 (IC(50) 6.3 μM) in cultured cells overexpressing APP. Using IP-MS methods, this compound was found to modulate the pool of total Aβ produced by reducing the proportion of Aβ42 while increasing the relative amounts of shorter and less amyloidogenic Aβ37 and Aβ39. Concentrations of 1 sufficient to lower levels of Aβ42 substantially (up to 10 μM) did not significantly affect the processing of Notch or other aspects of APP processing. When 1 (10 μg) was administered to CD-1 normal mice intracerebroventricularly, the level of Aβ42 in brain was reduced. Assays for off-target pharmacology and the absence of overt signs of toxicity in mice dosed with compound 1 suggest a comparatively selective pharmacology for this triterpenoid. Compound 1 represents a new lead for the development of potential treatments for Alzheimer's disease via modulation of gamma-secretase.

Gamma Secretase Modulators: New Alzheimer’s Drugs on the Horizon?

“…Bioassay guided fractionation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 48 identified compound 84 as the active component from a black cohash extract. 134 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Researchers at the University of Florida have evaluated the ability of steroids to reduce Aβ42 by screening 170 natural and synthetic steroids at 10 µM in CHO cells. 139 Many non-acidic steroids were found to increase Aβ42 while acidic steroids were found to lower Aβ42.…”

Initial Optimization of a New Series of γ-Secretase Modulators Derived from a Triterpene Glycoside

“…33 This initial lead (1, Scheme 1) was isolated from extracts of the black cohosh plant (Actaea racemosa) and represents the first entry in a unique class of GSMs that features a complex molecular architecture characterized by a plant sterol core structure. Most importantly, 1 exhibits a unique and compelling profile toward γ-secretase modulation, selectively reducing levels of Aβ42 and Aβ38 in vitro, while raising levels of Aβ37 and Aβ39.…”

Optimization of a Kilogram-Scale Synthesis of a Potent Cycloartenol Triterpenoid-Derived γ-Secretase Modulator

“…The crude residue was then purified via silica gel flash column chromatography (gradient elution, 25− 100% EtOAc/hexane) to provide the C15 reduction product 4 (21.960 g, 38% yield over three steps, 61% corrected yield based on the purity of 2). 1 was treated with imidazole (4.26 g, 62.5 mmol) followed by chlorotriethylsilane (5.2 mL, 31.3 mmol), and the reaction mixture was stirred at RT while the reaction was followed by TLC. After 1 h, some starting material was still observed by TLC, so additional imidazole (0.85 g, 12.5 mmol) and chlorotriethylsilane (1.05 mL, 6.24 mmol) were added.…”

“…The crude residue was carried on without further purification, assuming quantitative yield. 1 (6). Diol 5 (17.145 g, 20.29 mmol) was first concentrated from toluene to ensure dryness and then was dissolved in DMF (200 mL), and the solution was cooled to 0°C.…”

“…1 A recent series of publications documents our early work on this series of molecules, which is derived from a complex plant triterpene found in black cohosh (Actaea racemosa). 2−4 Semisynthetic derivatives featuring a C3 morpholine acetal and a C24 ether on the modified triterpene scaffold emerged as particularly potent GSMs.…”