The amazing in vivo multistep rearrangement of bilobalide to furopyranons and cyclopentadifurans

Bilobalide 1, the principal terpenlactone of the Ginkgo special extract EGb 761®, is a trilactone substituted with a very rare tertiary butyl alcohol group. The compound has several modes of action, which may act together in neuroprotection [1]. The absolute oral bioavailability is about 60 – 80% in rats and humans [2,3]. In rats, 31% of bilobalide is excreted unchanged in urine [3]. Therefore, we were interested in the yet unknown 69% metabolites of bilobalide.

In order to evaluate the metabolism of bilobalide, rats were treated orally either with pure 1 (10 mg/kg) or with EGb 761® (300 mg/kg). Urine was collected during 0 – 4h and 4 – 8h after administration and the samples were analyzed after work-up by solid phase extraction with an ion trap LC-MS². In agreement with former results no phase II metabolites were observed beside some very minor oxidation products. In contrast, several hydrolysis products characterized in the mass spectrum by added 18 Da to the parent mass of 1 were detected.

Since urine samples of treated animals contain only low concentrations of metabolites, the hydrolysis of bilobalide was mimicked in vitro by incubation of 1 under weak basic condition. Indeed, this reaction produced a large variety of compounds that were in agreement with the in vivo products. Preparative isolation and subsequent detailed 2D-NMR studies established an amazing multistep rearrangement of 1. The product formation is probably initiated by a retro-aldol reaction in combination with hydrolysis and condensation steps (partially presented in Fig. 1) which results for example in furopyranons 2 and cyclopentadifurans 3.

Fig. 1

[1] Defeudis FV. Pharmacol Res 2002; 46: 565 – 568.

[2] Dew TP, Wang G, Williamson G. Biofactors 2014; 40: 268 – 274.

[3] Biber A. Pharmacopsychiatry 2003; 36 Suppl 1: S32-S37.