Secoisolariciresinol (SECO) analogues: oxidative metabolism, cytochrome P450 inhibition and implications for toxicity

Secoisolariciresinol (SECO) is the major lignan present in flaxseed, but unlike the structurally related lignan nordihydroguaiaretic acid, it is not associated with toxicity. The major phase I metabolite of SECO is lariciresinol, likely formed as a result of para-quinone methide (p-QM) formation followed by an intramolecular cyclization, thereby minimizing any toxicity associated with the p-QM. Four analogues of SECO were used to investigate substituent effects on lignan metabolism and formation of reactive quinones. HPLC methods were developed for analysis of SECO analogues and their metabolites. The stability of SECO analogues (1 mM) in a 50 mM Na2HPO4 buffer at pH 6.0 and 7.4 were quantified. Enzymatic oxidation experiments using mushroom tyrosinase and microsomes harvested from male Sprague-Dawley rats were performed with and without a GSH trapping system. Mass spectrometry and LC-MS were used to identify metabolites. Life Technologies was contracted to perform IC50 inhibition assays on SECO and the SECO analogues against CYP3A4, CYP3A5, CYP2C9 and CYP2C19 cytochrome P450 isoforms. All SECO analogues were stable at pH 6.0. SECO-2 was stable at pH 7.4 but SECO-1, -3 and -4 were unstable at pH 7.4. Autoxidation of SECO -1, -3 and -4 were 1st order reactions with t1/2 of 9.0 h, 1.7 h and 7.0 h respectively. Mushroom tyrosinase oxidations were performed to generate ortho-quinone standards. SECO-1 -3 and -4 were oxidized by mushroom tyrosinase but SECO-2 was not. Trapping with GSH produces aromatic ring conjugates for SECO-1, -3, -4. Results from microsomal oxidations for SECO-1, -3 and -4 are consistent with these standards. SECO-2 was metabolized by a microsomal system to produce a benzyl GSH adduct. Dealkylation products were also observed. All SECO analogues formed quinones but interestingly, GSH conjugation was competitive with intramolecular cyclization. All cytochrome P450 isoforms were inhibited by every analogue tested to varying degrees, a potential cause of toxicity concerns. Quinones are known to cause toxicity in vivo, including cytotoxicity, immunotoxicity, and carcinogenesis. Our results suggest that since the phenol and catechol lignans form GSH adducts in addition to intramolecular cyclization products, this class of lignans have the potential to cause toxicity