Retinyl palmitate

Endpoint summary

Additional information

Basic toxicokinetics:

Vitamin A is a micronutrient essential for most mammalian species and is one of three fat soluble Vitamins. The term “Vitamin A” is used as a generic descriptor for compounds that exhibit the biological properties of retinol or other closely related naturally occurring derivatives. Vitamin A occurs in nature primarily as retinyl ester and retinol. It is an essential factor for the growth and maintenance of higher organisms. It is required for visual function, epithelial cell differentiation and reproduction.

Dietary Vitamin A, which is mostly in the form of retinyl esters, is absorbed in the upper part of the small intestine by mechanisms similar to those for lipid absorption. The esters undergo hydrolysis to release retinol, which is incorporated into mixed micelles and absorbed by enterocytes, where it is bound to an intra-cellular protein called CRBPII (cellular retinol binding protein II). It is then re-esterified to form retinyl esters. The esters are incorporated into chylomicrons and are hydrolyzed in the general circulation. Chylomicron remnants are taken up by tissues, particularly the liver. Remnants are degraded within the hepatocytes, and the released retinol is transferred to stellate cells for storage after re-esterification.

In the present study on basic toxicokinetics, the exposure to retinyl palmitate and its main metabolites in plasma was examined within the framework of prenatal developmental toxicity studies in cynomolgus monkeys (Hendrickx 2000). Retinyl palmitate was administered daily to female monkeys by nasogastric intubation at dose levels of approx. 4.1, 11, 22, 44 mg/kg (7500, 20000, 40000, 80000 IU/kg) during early pregnancy (i. e. between gestational days 16 and 27). Control monkeys were administered the vehicle (physiological saline or distilled water). Retinol, retinyl esters and the metabolites were determined using a reversed-phase HPLC assay.

Administration of retinyl palmitate resulted in the formation of the following metabolites:

retinol

retinyl esters

all-trans-retinoic acid

13-cis-retinoic acid

all-trans-4-oxo-retinoic acid

13-cis-4-oxo-retinoic acid

A dose-related increase in AUC_0-8on gd 27 was observed for the four metabolically generated retinoic acids after administration of retinyl palmitate (20000 IU/kg and 40000 IU/kg). Moreover, after application of 80000 IU/kg, the AUC_0-8 for retinol and the four retinoic acid metabolites were higher on gd 16 than on gd 27, whereas the AUC_0-8remained unchanged for retinyl esters. After application of 7500 or 20000 IU/kg, the AUC_0-24of retinol in the treatment groups was found to be slightly higher than in controls, whereas the AUC_0-24 of retinyl esters was found to be 30 times higher compared to controls. The AUC_0-24values indicate that the exposure in plasma to the retinoic acids on GD16 increased with rising doses. There were no differences between GD16 and GD27 in AUC_0-24of retinol, retinyl esters and retinoic acids at dose level 7500 IU/kg (4.1 mg/kg). However, at 20000 IU/kg (11 mg/kg), the AUC_0-24of all retinoic acids were consistently lower on GD27 than on GD16, whereas the AUC_0-24of retinylesters and retinol remained essentially unchanged. 

Single oral dose of Retinyl palmitate (RP) at 10000 IU or 30000 IU to humans resulted in dose-related and sustained increases (up to 15 fold) in Cmax and AUC (0-24h) values of plasma RP, RO (all trans retinyl oleate), RS (all trans retinyl stereate),13-cis- and 13-cis-4-oxo-RAs, as well as a transient increase in AT-RA (Nohyek et al, 2006). In contrast, 

cosmetic creams containing retinyl palmitate at the same dose levels induced no detectable changes in constitutive plasma levels of these metabolites when applied topically under enhanced experimental conditions (Nohyek et al, 2006).

 

Dermal absorption:

For the assessment of dermal absorption of retinoids, retinol (0.3%) containing 3H-retinol was applied on viable fuzzy rat or human skin in flow-through diffusion cells for in vitro absorption studies under relevant exposure conditions, i.e. cosmetic formulations (hydroalcoholic gel and oil-in-water emulsion) (Yourick 2008). Furthermore, in vivo absorption studies using fuzzy rats were performed in glass metabolism cages for collection of urine, feces, and body content. In vitro application on human skin for 24 hours showed a vehicle dependent penetration rate of 2.4%/ 1.5% (gel) or 4.3%/5.1% (oil in water) excluding the levels found in the stratum corneum 24/72 hours after test substance application, respectively. Higher retinol levels were found in the stratum corneum (3.5%/ 2.8% (gel) or 5.9%/4.8% (oil in water) after a 24/72 hour application).

The same setup using rat skin in vitro resulted in higher dermal penetration, i.e. 24.9% (gel) or 28.7% (oil in water) of the applied dose exluding the stratum corneum 24 hours after test substance application. In line, a dermal penetration of 16.3% (gel) or 20.6% (oil in water) of the test substance was found 24 hours after administration in vivo in rats (excluding the stratum corneum). The setup in vivo demonstrated, that based on the stable retinol levels in urine, feces and the carcass directly after removal of the test substance and 48 hours after removal, little additional absorption of the retinol stored in the stratum corneum occurred.

 

In an in vitro study using domestic pig skin, the application of radioactive retinyl palmitate (0.0087% in toluol) resulted in a total penetration rate of 21 -22 % of the applied dose after 6 or 16 hour application time including the stratum corneum (DSM B-116'727). However, considering viable skin and receptor fluid, four to five fold lower levels of the test substance were found (4.7% and 6.2% of the applied dose, respectively). The use of naked rat skin in the same experimental setup yielded significantly higher penetration rates (61.5% of the applied dose including the stratum corneum).

 

Comparable results were obtained after application of a radioactive retinyl ester with similar structure, i.e. retinyl acetate. Retinyl acetate (5% in ethanol) on domestic pig skin in vitro resulted in a total penetration rate of 12 % or 30% of the applied dose after 6 or 16 hour application time, respectively, when levels in the stratum corneum were included (DSM B-105'520). Three to four fold lower levels (3.8% and 7.4% of the applied dose) remained in viable skin layers and the receptor fluid after a 6 and 16 hour application time. Application of retinyl acetate for 6 hours on porcine skin in vitro using alternative vehicles resulted in remarcable lower penetration rates, i.e. 2.2% (in TWEEN-20) and 1.8 %(in NEOBEE-M-5) of the applied dose including the stratum corneum (DSM B-105'700).

 

Overall, species and vehicle dependent differences in the dermal absorption of retinoids have been observed. In the study of Yourick et al. (Yourick 2008), the results in vivo in rat warrant not to consider the total retinol levels in skin as bioavaliable material, since no further systemic absorption (urine, feces, carcass) from the stratum corneum depot after test substance removal was observed. For further risk assessment purposes, the dermal absorption rates found in setups using human and porcine skin are included, and test substance levels found in the viable skin and receptor fluid (excluding the stratum corneum) were considered systemically available.

In a weight of evidence, the dermal absorption rate of retinol and the respective esters in humans is considered to be approx. 5% of the applied dose. Considering the comparative assessment of oral versus dermal administration observed by Nohynek et al. (Nohynek 2006), the oral absorption is expected to be extensively higher (up to 15 fold increases), which substantiates the assumption of a 100% oral absorption of retinyl palmitate.