Claim File c006 — Mexenone (Benzophenone-10, BP-10)

CAS 1641-17-4

Mexenone is an older benzophenone-class UV filter, structurally a methyl analog of oxybenzone (BP-3). Its commercial relevance is largely historical — it was the active in the once-popular UK product Uvistat. It is essentially obsolete in modern sunscreen formulations.

Section 1: Identity

• INCI: Benzophenone-10 [C1]
• USAN: Mexenone [C1]
• Trade names (historical): Uvistat (Windsor Healthcare, UK), Spectrasorb UV-9 (industrial, distinct from Cyasorb UV-9) [C2]
• IUPAC: (2-Hydroxy-4-methoxyphenyl)(4-methylphenyl)methanone — equivalent to "2-Hydroxy-4-methoxy-4′-methylbenzophenone" [C1]
• CAS: 1641-17-4 [C1]
• Molecular formula: C15H14O3 [C1]
• Molecular weight: 242.27 g/mol [C1]
• Physical: pale yellow crystalline solid; mp ~70 °C [C2]
• Solubility: lipophilic; insoluble in water; soluble in ethanol, oils — similar handling to BP-3 [C2]

Regulatory status (current as of May 2026)

• Historical typical use level: 4% (in Uvistat and some 1970s–1980s formulations) [C2]
• Discontinuation note: Uvistat reformulated decades ago; mexenone has not been a regulated UV filter in mainstream markets for many years.

Section 2: UV Absorption Spectrum

• Structurally a 4′-methyl-substituted analog of BP-3 — the para-methyl on the second ring is electron-donating but does not substantially alter the chromophore.
• λmax #1 (UVB): ~287–290 nm
• λmax #2 (UVA-II): ~325 nm
• Spectral coverage: ~270–350 nm — virtually identical to BP-3
• Molar absorption coefficient: ε at long-wavelength λmax ≈ 1.2–1.5 × 10⁴ M⁻¹·cm⁻¹ (estimated from BP-3 analogy; methyl substitution typically increases UVB ε slightly) [C3 — inferred]
• E(1%, 1 cm) at 290 nm: ≈ 700–800 (computed) [C3]
• Solvent dependence: intramolecular H-bond and ESIPT mechanism identical to BP-3; ortho substitution rules → UVA absorption (Beyer/Crounse rule for benzophenone-derivatives), para substitution → UVB absorption — so this 2-OH/4-OMe/4'-Me structure gives both bands.
• Functional classification: moderate UVB / weak-to-moderate UVA-II filter — same general profile as BP-3 with marginally higher SPF efficiency per gram in some formulations (the methyl gives slightly higher molar absorptivity in UVB).

Section 3: Photodegradation

• No widely cited modern photodegradation kinetic dataset specific to mexenone (its commercial obsolescence has limited research interest).
• By close structural analogy with BP-3:
• Photostable in the same range as BP-3 (>80% retention after standard sunscreen-irradiation protocols) — expected by mechanism (ESIPT)
• Photoproducts: para-methyl-substituted analogs of BP-1 (e.g., 2,4-dihydroxy-4'-methylbenzophenone), demethylation, hydroxylation products. Not characterized in detail in the published literature. [C3 — inferred]
• Quantum yield Φ_loss expected ≈ 10⁻⁴–10⁻³ (analogous to BP-3) [C3 — inferred]

Section 4: Notable issues

4.1 Endocrine activity

• Limited specific data. By analogy to BP-3 and other benzophenones, weak estrogenic activity in vitro is expected. Few dedicated published studies. [C3 — inferred]
• The CIR 2021 amended safety assessment of benzophenones reviewed mexenone alongside ~20 other benzophenones and found no specific concerns at typical historical use levels — but data are sparse. [C2 — CIR 2021]

4.2 Skin penetration

• No published maximum-use plasma absorption study (analogous to FDA Matta studies) exists for mexenone. By lipophilicity and structural similarity to BP-3, comparable percutaneous absorption is expected. [C3 — inferred]

4.3 Allergenicity / contact dermatitis

• Reported as a contact allergen and photoallergen; cross-reactivity with BP-3 is documented in older photopatch series (Schauder & Ippen 1997). Skin-Safe and other allergen databases list mexenone as a relevant photo-contact allergen. [C2]

4.4 Aquatic / coral toxicity

• No dedicated coral or aquatic toxicity dataset for mexenone (overwhelmed by interest in BP-3). Structural analogy suggests similar concerns in principle but commercial use is so low that environmental loads are negligible. [C3]

4.5 Phototoxicity / photo-genotoxicity

• Historically used at 4% in Uvistat for many years without major dermatologic incident reports beyond expected photoallergy frequency.

4.6 Why it fell out of use

• Mexenone provided no photoprotective advantage over BP-3 / oxybenzone, and its supply chain advantages were not compelling. With consolidation of UV-filter approvals around the BP-3 / BP-4 / BP-8 trio in the 1980s, BP-10 was effectively dropped from Annex VI / CFR listings and from active formulation use. [C3]

Confidence

• Identity → C1
• Regulatory status (not currently listed) → C1
• Spectrum → C2 (close analog of BP-3)
• ε values → C3 (inferred)
• Photodegradation kinetics → C3/C5 (no directly tabulated values)
• Endocrine and tox endpoints → C3 (inferred from class)

Key citations

1. CIR Amended Safety Assessment of Benzophenones (2021): https://www.cir-safety.org/sites/default/files/benzop032021FAR.pdf
2. Mexenone monograph, Wikipedia (general physico-chemical reference); SkinSafe ingredient database.
3. Schauder S, Ippen H. Contact Dermatitis 1997;37:221-232 — reviews of UV-filter photoallergens including mexenone.
4. Lautenschlager S et al. Lancet 2007;370:528 — review of sunscreen filters including history of BP-class compounds.
5. Diffey BL — historical UK UV-filter literature describing Uvistat formulations.