Cytotoxic and Noncytotoxic Steroidal Constituents of Cryptolepis dubia

Abstract

(-)-Cryptanoside A (1) was identified previously as a major cytotoxic component of the stems of Cryptolepis dubia collected in Laos, which mediates its activity by targeting Na⁺/K⁺-ATPase (NKA), with hydrogen bonds formed between its 11- and 4'-hydroxy groups and NKA being observed in its docking profile. In a continuing investigation, 1 and its 17-epimer, (-)-17-epi-cryptanoside A (2), and the new (+)-2-hydroxyandrosta-4,6-diene-3-one-17-carboxylic acid (3) and the known (+)-2,21-dihydroxypregna-4,6-diene-3,20-dione or 2-hydroxy-6,7-didehydrocortexone (4) pregnane-type steroids were isolated from C. dubia. In addition, (-)-11,4'-di-O-acetylcryptanoside A (1a) has been synthesized from the acetylation of 1. The structures of these compounds were determined by analysis of their spectroscopic data, with their cytotoxic and NKA inhibitory activities being evaluated. In contrast to 1 that exhibited potent activities, the other compounds were largely inactive. Molecular docking profiles indicated that 1-3 and 1a bind to NKA, but some subtle differences were observed in their interactions with NKA, which may contribute to their differential cytotoxic and NKA inhibitory potency.

Figure 1.

COSY (, ¹H→¹H) (C-7/8 changed to normal bond, 010625) and key HMBC (, ¹H→¹³C) and selected NOESY (, ¹H→¹H) correlations of and 2 and 3. The conformations shown for the NOESY correlations were generated based on the crystal structures of (−)-cryptanoside A (1) for 2 and (12R,16R,17S)-16,17-epoxy-12-hydroxypregna-4,6-diene-3,20-dione for 3.

Figure 2.

ECD (left column) and UV (right column) spectra of compounds 1 (green), 1a (blue), 2 (orange), 3 (red), and 4 (purple). The ECD data were obtained in HPLC grade MeOH as the average of three scans corrected by subtracting a spectrum of the appropriate solution in the absence of the samples recorded under identical conditions. Each scan in the range 200–400 nm was obtained by taking points every 0.1 nm for 1⁹ and every 1 nm for other compounds, with a 50 nm/min scanning speed and a 1 nm band width.

Figure 3.

Inhibition of Na⁺/K⁺-ATPase by 1a, 2, and 3 and digitoxin.

Figure 4.

Docking profile for 1a (left), 2 (middle), and 3 (right) in Na⁺/K⁺-ATPase (NKA).