2.5. Benfotiamine metabolism and mechanism of action

Benfotiamine (Fig. 1) shows better bioavailability and absorption than thiamine, and results in at least five times higher plasma concentrations than an equivalent dose of thiamine [59]. Most human studies have shown that benfotiamine is safe and well tolerated, even in higher concentrations [24,25,[60], [61], [62]]. A study examining the pharmacokinetics of benfotiamine treatment in Alzheimer's patients showed that maximum thiamine concentrations in the blood were reached 1–2h after a single dose of benfotiamine, while ThMP and ThDP peaked at later time points, between 3,5 and 8h for ThMP and 8–24h for ThDP. Also, thiamine and ThDP were moderately accumulated after repeated treatments of benfotiamine [60].

Following oral administration, benfotiamine is dephosphorylated to S-benzoylthiamine by ecto-alkaline phosphatases in the small intestine. S-benzoylthiamine is lipophilic and passes easily through intestinal and endothelial cells and enters the bloodstream where erythrocytes convert it to free thiamine [5]. Part of S-benzoylthiamine is hydrolyzed in the liver to thiamine and benzoic acid by thioesterases [4]. Because dephosphorylation is necessary in order to obtain the lipophilic S-benzoylthiamine, oral route of administration is deemed to be the most effective [31]. Volvert et al. showed that in mice, oral treatment of benfotiamine resulted in maximum thiamine concentrations in the liver after 1 h, while maximum concentrations in the blood are reached after 2 h, which led to the conclusion that S-benzoylthiamine from the blood is taken up by the liver and converted to thiamine [4]. Higher concentrations of thiamine monophosphate (ThMP) and ThDP were also found in the blood and liver after benfotiamine ingestion [4]. This study did not find elevated thiamine levels in the brain neither after acute nor chronic (14 days) treatment with benfotiamine in mice. However, several other studies found higher thiamine levels in the brain after oral benfotiamine treatment in mice [20,21] and rats [22,63]. Specifically, after longer benfotiamine treatment (10 days) Pan et al. found higher levels of thiamine in the brain, but no difference in ThMP or ThDP concentrations. Moraes et al. had a model of chronic benfotiamine treatment (30 days) after which they found higher ThDP concentrations in the hippocampus and entorhinal cortex [22]. Also, many studies have found neuroprotective effects of benfotiamine, without measuring benfotiamine metabolites in the brain tissue. The situation is further complicated because analytical methods that are used register only the metabolites with an intact thiazolium ring [64]. Thus, to what extent thiamine and other benfotiamine metabolites enter the brain is still inconclusive, however its neuroprotective effects have been corroborated by a number of studies.

Study by Sambon et al. done on neuroblastoma cells reported that benfotiamine does not pass through the cell membrane in a considerable amount, which is in line with previous research and the fact that benfotiamine has a hydrophilic phosphate group. The authors propose that benfotiamine is dephosphorylated to S-benzoylthiamine by phosphatases present on the cell membrane or in the culture medium. S-benzoylthiamine then enters the cell and is converted to thiamine by thioesterases. A portion of thiamine is further phosphorylated by thiamine pyrophosphokinase to ThDP (Fig. 3) [64]. The authors of this study propose that protective and antioxidative effects of benfotiamine are mediated by thiamine or its metabolites, because benfotiamine, sulbutiamine and higher concentrations of thiamine all protected the cells from oxidative stress. However, when investigating the effect of benfotiamine and thiamine on Nrf2/ARE pathway, master regulator of cellular antioxidative response, Tapias et al. found that benfotiamine and its metabolites activate this pathway [21]. However, this is not true for thiamine, suggesting that antioxidative effects of benfotiamine are not exerted through thiamine, or that potentially there is another mechanism involved.

The metabolic pathways of benfotiamine. Benfotiamine is dephosphorylated to S-benzoylthiamine by phosphatases present on the cell membrane. S-benzoylthiamine then enters the cell and is converted to thiamine by thioesterases. A portion of thiamine is further phosphorylated by thiamine pyrophosphokinase to TPP that serves as coenzyme in glycolysis, Krebs cycle and pentose phosphate cycle. Thiamine and benfotiamine metabolites show anti-inflammatory, antioxidative and neuroprotective effects.