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Basic toxicokinetics

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basic toxicokinetics in vitro / ex vivo
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Study period:
no data
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well-documented publication which meets basic scientific principles, please refer to IUCLID section 13 for read across justification.

Data source

Reference Type:
Kinetics of permeation and metabolism of alpha-tocopherol and alpha-tocopheryl acetate in micro-Yucatan pig skin
Rangarjan M and Zatz JL
Bibliographic source:
J. Cosmetc. Sci. 52, 35-50

Materials and methods

Objective of study:
Test guideline
no guideline required
GLP compliance:
not specified

Test material

Constituent 1
Reference substance name:
D-alpha Tocopherol
D-alpha Tocopherol
Constituent 2
Reference substance name:
D-alpha Tocopherly acetate
D-alpha Tocopherly acetate

Results and discussion

Any other information on results incl. tables


HPLC chromatograms of viable skin samples showed the appearance of two peaks at retention times of about 13.4 and 16.0 minutes. Spiking studies were carried out to identify the nature of the metabolite. When this mixture was spiked (1:1) with known amounts of a-TAc Standard, and the volume of injection remained the same, the peak at 16 minutes increased in area whereas the peak at 13 minutes was halved in area due to the 1:1 dilution of the mix. Correspondingly, when the original mixture was spiked (1: 1) with Standard a-T, the peak at 13 minutes increased in area whereas the peak at 16 minutes was halved in area, showing that the peak at 13 minutes was indeed the metabolite, a-T, and the peak at 16 minutes was the prodrug peak (a-TAc). Spiking studies were also carried out with the viable skin sample obtained from the a-T permeation experiment. Only one peak at 13 minutes was obtained in the original eluent. When spiked with known amounts of a-TAc, a new peak was obtained at 16 minutes, accounting for the added prodrug. With Standard a-T spiking, a existing parent peak increased in area. No peak was found at 16 minutes. This confirmed unequivocally that the metabolite obtained from a-TAc bioconversion was indeed a-T. In the case of a-TAc formulations no more than two peaks (corresponding to a-TAc and a-T) were observed, and for the a-T formulation only one peak (corresponding to a-T itself) was observed. No further metabolites of a-T were detectable in viable pig skin during the 24-hour time period of our study in any of the samples.

The total amount of a-TAc permeated is the sum of both a-T and a-TAc in the SC and viable skin. Sufficient quantity of the prodrug was detectable even at the end of two hours in the SC. lnterestingly, a-TAc levels in the viable skin were below the limits of detection at the end of two hours. This implied essentially complete metabolism of a-TAc into a-T during this time period. Bioconversion was restricted to the viable skin, and no a-T was detected in the stratum corneum for both the a-TAc formulations. No significant difference was obtained between the two a-TAc formulations with respect to the total amount of a-TAc permeated at the end of two hours. A lower amount of a-TAc was detectable in the stratum corneum at six hours than at two hours, but more was detected in the viable skin. The emulsion formulation showed higher amounts of total drug permeated than the IPM solution at six hours, and this difference was statistically significantly different. The amount of a-TAc that permeated in the SC of the IPM solution at 24 hours was statistically significantly higher than at 2, 6 and 12 hours. The amount of prodrug permeated or metabolite formed in viable skin or the total amount of a-TAc permeated at both 12 and 24 hours were not statistically significantly different between the two formulations. In terms of the total amount permeated for the IPM solution, the amount at 24 hours was significantly higher than that at two and six hours. The total amount permeated in this case probably reaches a plateau at six hours and does not significantly increase after that. Total a-TAc and a-T permeated in viable skin was significantly higher in the a-TAc emulsion at 24 hours and six hours than at two hours. The emulsion had higher amounts of metabolite at 12 hours, although at the end of the study period (24 hours), there was no significant difference between the two formulations in terms of the amount of metabolite a-T formed in viable skin.

The extent of metabolism was calculated in the viable skin and total skin. Almost complete metabolism of the prodrug occurred in the viable skin at two hours. At six hours in cases of both the IPM solution and emulsion, the extent of metabolism was significantly lower in the viable skin compared to that at two hours. At the end of 24 hours the IPM solution had a significantly higher extent of metabolism in viable skin than the emulsion. When calculated for total skin, the highest extent of metabolism occurred at six hours for the IPM solution and 12 hours for the emulsion.


Viable skin samples for a-T permeation and metabolism studies showed the presence of only the a-T peak, and no metabolite peaks were detected by HPLC. a-T was detected in the SC and viable skin from an a-T IPM solution as early as two hours by HPLC. The level of active in both tissues remained constant throughout the 24-hour study period.

The results clearly show that although detectable quantities of a-T are obtained in the viable skin when a-TAc is bioconverted to a-T as early as two hours, a formulation containing pure drug a-T is a more efficient delivery system. In terms of micrograms of drug, a somewhat larger amount of a-T was obtained in the viable skin at 2, 6, and 24 hours as a metabolite of a-TAc than from an a-T solution, because the a-TAc concentration used in the original formulation was 5 % , whereas a 1% formulation of a-T was used.

Applicant's summary and conclusion

This study demonstrated the metabolism of alpha-tocopheryl acetate (a-TAc) to alpha-tocopherol (a-T) in viable pig skin. Topically applied a-TAc was bioconverted to the active molecule and free radical scavenger a-T within the skin tissue. No metabolism was detectable in the stratum corneum. This study has also elucidated the kinetics of metabolism of a-TAc. The extent of metabolism was highest at 6-12 hours after application. Longer time periods failed to produce a higher extent of metabolism, probably due to the saturation of the hydrolytic pathway.
Executive summary:

The objective of this research was to investigate the permeation and metabolism of alpha-tocopheryl acetate (a-TAc) and alpha-tocopherol (a-T) from solution and emulsion formulations and to delineate the kinetics of such a metabolism. Simple formulations containing a-TAc and a-T were applied to fresh, viable micro-Yucatan skin dermatomed to a thickness of 250 -300 µm as a finite dose in a flow-through diffusion system.The experiments were stopped at time intervals of 2, 6,12, and 24hours. At the end of each time interval, the amounts removed by washing, retained in the stratum corneum (SC), and penetrated into the viable skin and receptor were determined by a validated HPLC method. Receptor concentrations were below the limit of detection. a-TAc underwent metabolis m in pig skin to the active antioxidant a-T. The metabolite appeared as early as two hours after application. The extent of metabolism was highest at 6 -12 hours after application. No metabolism was detected in the Stratum corneum. Delivery of a-T from isopropyl myristate (IPM) solution was more efficient than utilization of a-TAc from the same solution. Approximately 1.5%of a-T yielded the same viable skin concentration as 5% a-TAc.