Received for publication July 30, 2004.
Revised September 16, 2004.
Accepted for publication October 7, 2004.

Identification of Quenchers of Photoexcited States as Novel Agents For Skin Photoprotection

Abstract

Photooxidative stress is a key mechanism in UVA-induced skin photodamage. Photoexcited states of endogenous UVA-chromophores such as porphyrins, melanin precursors, and crosslink-fluorophores of skin collagen exert skin photodamage by direct reaction with substrate molecules (type I photosensitization) or molecular oxygen (type II) leading to formation of reactive oxygen species. Based on our previous research on the role of photoexcited states of endogenous skin chromophores as sensitizers of photooxidative stress, we describe here the identification of a novel class of chemopreventive agents for topical skin photoprotection: quenchers of photoexcited states (QPES). QPES compounds antagonize the harmful excited state chemistry of endogenous sensitizers by physical quenching, facilitating the harmless return of the sensitizer excited state to the electronic ground state by energy dissipation. To identify QPES compounds suitable for development, we designed a primary screening assay based on QPES-suppression of photosensitized plasmid cleavage using conditions that exclude antioxidants. This screen is followed with a screen to test for non-sacrificial quenching of dye-sensitized singlet oxygen (¹O₂) formation by electron paramagnetic resonance detection of 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO), a stable free radical indicative of ¹O₂ formation. These initial screens identified a pyrrolidine-pharmacophore with pronounced QPES activity and L-proline and other non-cytotoxic proline-derivatives containing this pharmacophore were then screened for efficacy in cellular models of sensitized photodamage. These compounds showed QPES-protection against dye-sensitized and psoralen-UVA (PUVA) induced apoptosis and suppression of proliferation in cultured human skin keratinocytes and fibroblasts. Further, QPES-photoprotection of reconstructed full thickness human skin exposed to solar simulated light has been demonstrated.

Key words: antioxidants, free radical, photocarcinogenesis, quencher, singlet oxygen, skin photoprotection