Diastereoselective metabolism of homomenthyl salicylate (homosalate): Identification of relevant human exposure biomarkers

Food additive salicylates inhibit human and rat placental 3β-hydroxysteroid dehydrogenase: 3D-QSAR and in silico analysis

The use of salicylates as flavoring agents in food and beverages is common, but their potential to disrupt the endocrine system remains unclear. Human placental 3β-hydroxysteroid dehydrogenase 1 (h3β-HSD1) plays a role in progesterone synthesis and is the potential target. This study evaluated the inhibition of 13 salicylates on h3β-HSD1, structure-activity relationship (SAR) and compared with rat placental homolog r3β-HSD4. Salicylates inhibited h3β-HSD1, depending on carbon chain number in the alcohol moiety and the IC50 values for hexyl, ethylhexyl, homomenthyl, and menthyl salicylates were 53.27, 15.78, 2.35, and 2.31 μM, as mixed inhibitors, respectively, while methyl to benzyl salicylates were ineffective at 100 μM. Interestingly, only hexyl salicylate inhibited r3β-HSD4 with IC50 of 31.05 μM. Bivariate analysis revealed a negative correlation between IC50 and hydrophobicity (LogP), molecular weight, heavy atoms, and carbon number in the alcohol moiety against h3β-HSD1. Docking analysis demonstrated that these salicylates bind to cofactor binding sites or between the steroid and cofactor binding sites. Additionally, 3D-QSAR showed distinct binding via hydrogen bond donors and hydrophobic regions. In conclusion, the inhibition of h3β-HSD1 by salicylates appears to be dependent on factors such as LogP, molecular weight, heavy atoms, and carbon-chain length and there is species-dependent inhibition sensitivity.

Toxicokinetics of homosalate in humans after dermal application: applicability of oral-route data for exposure assessment by human biomonitoring

Homosalate (HMS) is a UV filter used in sunscreens and personal care products as a mixture of cis- and trans-isomers. Systemic absorption after sunscreen use has been demonstrated in humans, and concerns have been raised about possible endocrine activity of HMS, making a general population exposure assessment desirable. In a previous study, it was shown that the oral bioavailability of cis-HMS (cHMS) is lower than that of trans-HMS (tHMS) by a factor of 10, calling for a separate evaluation of both isomers in exposure and risk assessment. The aim of the current study is the investigation of HMS toxicokinetics after dermal exposure. Four volunteers applied a commercial sunscreen containing 10% HMS to their whole body under regular-use conditions (18-40 mg HMS (kg bw)-1). Parent HMS isomers and hydroxylated and carboxylic acid metabolites were quantified using authentic standards and isotope dilution analysis. Further metabolites were investigated semi-quantitatively. Elimination was delayed and slower compared to the oral route, and terminal elimination half-times were around 24 h. After dermal exposure, the bioavailability of cHMS was a factor of 2 lower than that of tHMS. However, metabolite ratios in relation to the respective parent isomer were very similar to the oral route, supporting the applicability of the oral-route urinary excretion fractions for dermal-route exposure assessments. Exemplary calculations of intake doses showed margins of safety between 11 and 92 (depending on the approach) after single whole-body sunscreen application. Human biomonitoring can reliably quantify oral and dermal HMS exposures and support the monitoring of exposure reduction measures.

Identification, Organic Synthesis, and Sensitive Analysis of a cis-Homosalate-Specific Exposure Biomarker

Homosalate (HMS) is an organic UV filter used in sunscreens and personal care products. Despite its widespread use and detection in environmental matrices, little is known regarding its exposure in humans. HMS is used as a mixture of cis- and trans-isomers, and we recently revealed major differences in human toxicokinetics, indicating the need to consider these isomers separately in exposure and risk assessments. In the course of these previous investigations of human HMS toxicokinetics, we identified two trans-HMS-specific and one cis-HMS-specific biomarker candidates. However, the latter lacks sensitivity due to only low amounts excreted in urine, prompting the search for another cis-HMS-specific biomarker. Our toxicokinetic investigations revealed a total of five isomers of HMS carboxylic acid metabolites (HMS-CA). Of these, only one was specifically formed from cis-HMS (HMS-CA 5), but its full identity in terms of constitution and configuration had, so far, not been elucidated. Here, we describe the synthesis of three HMS-CA isomers, of which the isomer (1R,3S,5S)/(1S,3R,5R)-3-((2-hydroxybenzoyl)oxy)-1,5-dimethylcyclohexane-1-carboxylic acid turned out to be HMS-CA 5. Taken together with two previously synthesized HMS-CA isomers, we were able to identify the constitution and configuration of all five HMS-CA isomers observed in human metabolism. We integrated the newly identified cis-HMS-specific metabolite HMS-CA 5 into our previously published human biomonitoring LC-MS/MS method. Intra- and interday precisions had coefficients of variation below 2% and 5%, respectively, and the mean relative recovery was 96%. The limit of quantification in urine was 0.02 μg L-1, enabling the quantification of HMS-CA 5 in urine samples for at least 96 h after sunscreen application. The extended method thus enables the sensitive and separate monitoring of cis- and trans-HMS in future human biomonitoring studies for exposure and risk assessment.

Combined Effects of Polystyrene Nanosphere and Homosolate Exposures on Estrogenic End Points in MCF-7 Cells and Zebrafish

BACKGROUND

Micro- and nanoplastics (MNPs) and homosalate (HMS) are ubiquitous emerging environmental contaminants detected in human samples. Despite the well-established endocrine-disrupting effects (EDEs) of HMS, the interaction between MNPs and HMS and its impact on HMS-induced EDEs remain unclear.

OBJECTIVES

This study aimed to investigate the influence of MNPs on HMS-induced estrogenic effects and elucidate the underlying mechanisms in vitro and in vivo.

METHODS

We assessed the impact of polystyrene nanospheres (PNSs; 50  nm , 1.0 mg / L ) on HMS-induced MCF-7 cell proliferation (HMS: 0.01 - 1 μ M , equivalent to 2.62 - 262 μ g / L ) using the E-SCREEN assay and explored potential mechanisms through transcriptomics. Adult zebrafish were exposed to HMS (0.0262 - 262 μ g / L ) with or without PNSs (50  nm , 1.0 mg / L ) for 21 d. EDEs were evaluated through gonadal histopathology, fertility tests, steroid hormone synthesis, and gene expression changes in the hypothalamus-pituitary-gonad-liver (HPGL) axis.

RESULTS

Coexposure of HMS and PNSs resulted in higher expression of estrogen receptor α (ESR1) and the mRNAs of target genes (pS2, AREG, and PGR), a greater estrogen-responsive element transactivation activity, and synergistic stimulation on MCF-7 cell proliferation. Knockdown of serum and glucocorticoid-regulated kinase 1 (SGK1) rescued the MCF-7 cell proliferation induced by PNSs alone or in combination with HMS. In zebrafish, coexposure showed higher expression of SGK1 and promoted ovary development but inhibited spermatogenesis. In addition, coexposure led to lower egg hatchability, higher embryonic mortality, and greater larval malformation. Coexposure also modulated steroid hormone synthesis genes (cyp17a2, hsd17[Formula: see text]1, esr2b, vtg1, and vtg2), and resulted in higher 17 β -estradiol (E 2 ) release in females. Conversely, males showed lower testosterone, E 2 , and gene expressions of cyp11a1, cyp11a2, cyp17a1, cyp17a2, and hsd17[Formula: see text]1.

DISCUSSION

PNS exposure exacerbated HMS-induced estrogenic effects via SGK1 up-regulation in MCF-7 cells and disrupting the HPGL axis in zebrafish, with gender-specific patterns. This offers new mechanistic insights and health implications of MNP and contaminant coexposure. https://doi.org/10.1289/EHP13696.

A time trend of urinary 4-methylbenzylidene camphor metabolites in young adults from Germany

4-methylbenzylidene camphor (4-MBC) is used as a UV-B filter in cosmetics. Two oxidized metabolites of 4-MBC - 3-(4-carboxybenzylidene)camphor (cx-MBC) and 3-(4-carboxybenzylidene)-6-hydroxycamphor (cx-MBC-OH) - were analyzed in 250 24-h urine samples from young adults in Germany. The samples were from the German Environmental Specimen Bank (ESB) and represented exposure in the years 1995, 2005, 2010, 2015 and 2019. A UHPLC-MS/MS method enabled the sensitive determination of both metabolites, with limits of quantification at 0.15 μg L^-1 (cx-MBC) and 0.30 μg L^-1 (cx-MBC-OH), respectively. A temporal trend of the internal exposure to 4-MBC was clearly noticeable. The metabolite cx-MBC was frequently quantifiable at the beginning of the period: in 70% of the samples in 1995, and 56% in 2005. After 2005, urinary concentrations and detection rates of cx-MBC dropped to reach very low levels. In 2015 and 2019, the detection rate was only 2% and 0%, respectively. A similar trend was observed for cx-MBC-OH, though overall, this metabolite was detected less often and at lower concentration levels than cx-MBC. Nowadays, measurable levels of urinary 4-MBC metabolites are an extremely rare occurrence in Germany. These trends are consistent with the history of 4-MBC use by the cosmetic industry. The highest measured individual concentration of 16.20 μg L^-1 (in a sample of the year 2005) was still more than 30 times below the health-based guidance value (HBM-I). An investigation of the ratios between both metabolites uncovered several features of the 4-MBC metabolism which have been essentially overlooked until now. In particular, stereochemical aspects should be explored in future studies. As urine was collected in autumn/winter in Northwestern Germany, the 4-MBC metabolites measured probably do not arise from sunscreen products in a narrow sense. They rather may reveal the use of other skin care products containing 4-MBC for UV protection as an added feature.