A study was conducted to evaluate the bioavailability of α, γ and δ tocotrienols administered via oral, intravenous, intramuscular and intraperitoneal routes in rats. Three separate experiments, each conducted according to a two-way crossover design, were carried out to compare intravenous and oral, intramuscular and oral, and intraperitoneal and oral administration. Oral absorption of all three tocotrienols was found to be incomplete. Of the three tocotrienols, α-tocotrienol had the highest oral bioavailability, at about 27.7± 9.2%, compared with γ- and δ-tocotrienols, which had values of 9.1± 2.4% and 8.5±3.5%, respectively. Such biodiscrimination was also observed in their total clearance rates (estimated from the intravenous data). -Tocotrienol showed the lowest clearance rate at about 0.16 L kg 1 h 1, whereas that of δ- and γ-tocotrienols was quite similar, with values of 0.24 and 0.23 L kg 1 h 1, respectively. Interestingly, all three tocotrienols were found to be negligibly absorbed when administered intraperitoneally and intramuscularly. Thus, these two routes of administration should be avoided when evaluating the biological activities of the tocotrienols in whole animal experiments.
Tocopherols and tocotrienols are metabolized by side chain degradation via initial omega-oxidation and subsequent beta-oxidation. omega-Oxidation is performed by cytochrome P450 (CYP) enzymes which are often regulated by their substrates themselves. Results presented here show that all forms of Vitamin E are able to activate gene expression via the pregnane X receptor (PXR), a nuclear receptor regulating a variety of drug metabolizing enzymes. In HepG2 cells transfected with the human PXR and the chloramphenicol acetyl transferase (CAT) gene linked to two PXR responsive elements, CAT activity was most strongly induced by alpha- and gamma-tocotrienol followed by rifampicin, delta-, alpha- and gamma-tocopherol. The inductive efficacy was concentration-dependent; its specificity was underscored by a lower response when cotransfection with PXR was omitted. Up-regulation of endogenous CYP3A4 and CYP3A5 mRNA was obtained by gamma-tocotrienol, the most potent activator of PXR, with the same efficacy as with rifampicin. This points to a potential interference of individual forms of Vitamin E with the metabolism and efficacy of drugs.
Benzoyl peroxide (BPO) is a commonly used drug in the treatment of acne vulgaris, but it induces unwanted side effects related to stratum corneum (SC) function. Since it has been recently shown to oxidize SC antioxidants, it was hypothesized that antioxidant supplementation may mitigate the BPO-induced SC changes. To test this, 11 subjects were selected to be topically supplemented with alpha-tocotrienol (5% w/vol) for 7 d on defined regions of the upper back, while the contralateral region was used for vehicle-only controls. Starting on day 8, all test sites were also treated with BPO (10%) for 7 d; the alpha-tocotrienol supplementation was continued throughout the study. A single dose of BPO depleted 93.2% of the total vitamin E. While continuing the BPO exposure for 7 d further depleted vitamin E in both vehicle-only and alpha-tocotrienol-treated sites, significantly more vitamin E remained in the alpha-tocotrienol-treated areas. Seven BPO applications increased lipid peroxidation. Alpha-tocotrienol supplementation significantly mitigated the BPO-induced lipid peroxidation. The transepidermal water loss was increased 1.9-fold by seven BPO applications, while there was no difference between alpha-tocotrienol treatment and controls. The data suggest that alpha-tocotrienol supplementation counteracts the lipid peroxidation but not the barrier perturbation in the SC induced by 10% BPO.
Alpha-tocopherol and its derivatives have been shown to be effective in reducing cerebral ischemia-induced brain damage. However, the effects of other vitamin E isoforms have not been characterized. In the present study, we investigated the effects of six different isoforms of vitamin E on the ischemic brain damage in the mice middle cerebral artery (MCA) occlusion model. All vitamin E isoforms were injected i.v., twice, immediately before and 3 h after the occlusion. Alpha-tocopherol (2 mM), alpha-tocotrienol (0.2 and 2 mM) and gamma-tocopherol (0.2 and 2 mM) significantly decreased the size of the cerebral infarcts 1 day after the MCA occlusion, while gamma-tocotrienol, delta-tocopherol and delta-tocotrienol showed no effect on the cerebral infarcts. These results suggest that alpha-tocotrienol and gamma-tocopherol are potent and effective agents for preventing cerebral infarction induced by MCA occlusion.