Efficacy and safety evaluations of topical proretinal nanoparticles
Authors:   , Limcharoen Benchaphorn 
Published: September 2023

Retinal (retinaldehyde) is one of natural vitamin A metabolites. It is widely used in clinical dermatology such as acne, acne scar, photoaging, wrinkles, psoriasis, skin neoplasia and seborrheic dermatitis. Retinal is a pivotal key to regulate keratinocyte proliferation and differentiation in the epidermis resulting in epidermal thickening and inhibit collagen destruction in the dermis. However, topical application of retinal is still irritative to the skin and chemically and photochemically unstable.  Topically applied retinal can induce a retinoid dermatitis. This inflammation is induced by an overload of non-physiological amounts of exogenous retinoic acid in the skin. The adverse effects and physicochemical stability of retinal limit its topical therapeutic effects in long-term treatment. To eliminate this dose-related side effect and the instability of the chemical, the developingment of topical nanoparticulate controlled-release drug delivery system as proretinal nanoparticles (PRN) which releases retinal continuously to prevent an excessive amount of retinal on the skin immediately after application has been proposed. The aims of this study were to investigate the safety, efficacy and biological activities of topical application of proretinal nanoparticles in term of follicular and intercellular penetration as the major pathways of topical nanoparticles in skin and to assess the possibility of the intradermal derlivery og PRN-loaded microneedle by non-invasive imaging techniques.

In this study, proretinal nanoparticles had barely cytotoxic and apoptotic effects comparing with conventional retinal on exposure HaCaT cells, spontaneous immortalized keratinocytes, for 24 h. The biological activity of retinoid in PRN in HaCaT cells was investigated after 24 h exposure to PRN. CRABP-2, transporting protein for members of the vitamin A family to the cellular nucleus, was inducible and higher when compared with conventional at the same concentration of retinoids. To evaluate the safety and efficat aspects of PRN in vivo, daily topical application of PRN to rats for 28 consecutive days produced neither irritation nor inflammation but significantly increased of epidermal proliferation, epidermal thickness, CRABP-2 expression, and up-regulation of various differentiation markers of epidermis including keratin (K) 5, K10, K14, CRABP-2 ,and PCNA except IL-6, proinflammatory cytokine compared with topical applicaton of conventional retinal solution. Through the use of confocal laser scanning microscopy, we observed the in vivo follicular penetration of PRN with the depth of penetration independent of post-application time. As nanoparticles preferably penetrating the hair follicles, the follicular penetration depths of PRN at different time points were investigated further in ex vivo porcine skin. The release capacity of the nanoparticulate system was studied using fluorescein as a model drug. Additionally, the concentration of retinal in the stratum corneum and in the hair follicles was quantified after application in particulate and non-particulate form. The results showed that the nanocarriers reached the infundibular area of the hair follicles, irrespective of the incubation time. The nanoparticles were able to release their model drug within the hair follicle. The retinal concentration delivered to the stratum corneum and the hair follicles was significantly higher when retinal was applied in the particulate form. To assist the intradermal delivery of PRN for the dermal therapeutic aspect, the use of two combinations transdermal drug delivery strategies were developed as PRN and microneedle and then investigated by non-invasive imaging techniques as dermoscopy, optical coherence tomography and multiphoton microscopy. The localization and skin deposition of PRN localization in dermis has been studied and visualized successfully over the time following the application.

The present study showed that topical application proretinal nanoparticles are safe, non-irritative, successfully penetrate into hair follicles and possess retinoid biological activities to skin as the ability to induct and regulate epidermal proliferation and differentiation. Finally, it could be beneficial in some retinoid-responsive skin conditions in the future. Although further investigations are necessary to clarify the required doses and dose intervals in clinical settings, the suggested system may help to overcome the main problems of topical retinoid therapy, which are skin irritation, chemical ,and photochemical instability and low bioavailability.