Circular dichroic investigations of the binding of salicylate and related compounds to human serum albumin

Distribution of parabens and 4-HB in human blood

Human blood has been commonly and routinely analyzed to determine internal human exposure to parabens. However, data on the occurrence of parabens and their common metabolite, p-hydroxybenzoic acid (4-HB), in different human blood matrixes is still limited. In this study, 139 pairs of serum and whole blood samples were collected from Chinese adults, and then analyzed them for 5 parabens and 4-HB. Methylparaben (MeP) and propylparaben (PrP) were consistently the predominant parabens in human serum (mean 2.3 and 2.1 ng/mL, respectively) and whole blood (1.9 and 1.3 ng/mL, respectively). Mean concentrations of 4-HB in human serum and whole blood were 7.7 and 12 ng/mL, respectively. Concentrations of parabens, except benzylparaben (BzP), and 4-HB in human serum were significantly (p < 0.01) correlated with that in whole blood. Distribution pattern of parabens and 4-HB in human blood was evaluated, for the first time, based on their partitioning between human serum and whole blood (Kp). Mean Kp values of parabens, except BzP, increased with the alkyl chain length from 0.83 to 1.6. BzP (mean 1.4) had a comparable mean Kp value to PrP (mean 1.4). Among target analytes, 4-HB had the lowest mean Kp value (0.75). These data are important to select appropriate blood matrixes for conducting human exposure assessment and epidemiological studies on parabens.

3 Final Report on the Safety Assessment of Methylparaben, Ethylparaben, Propylparaben, and Butylparaben

The Parabens are esters of p-hydroxybenzoic acid (PHBA) and are the most commonly used as preservatives in cosmetic formulations. Data obtained from chronic administration studies indicate that Parabens are rapidly absorbed, metabolized, and excreted.

Acute chronic and subchronic toxicity studies in animals indicate that Parabens are practically nontoxic by various routes of administration. Methylparaben and Ethylparaben at 100 percent concentration were slightly irritating when instilled into the eyes of rabbits.

Numerous in vitro mutagenicity studies indicate that the Parabens are non-mutagenic. Methylparaben was noncarcinogenic when injected in rodents or when administered intravaginally in rats. Cocarcinogenesis studies on Propyl- and Methylparaben were negative. Teratogenic studies on Methyl- and Ethylparaben were also negative.

Parabens are practically nonirritating and nonsensitizing in the human population with normal skin. Paraben sensitization has been reported when Paraben-containing medicaments have been applied to damaged or broken skin. Photo-contact sensitization and phototoxicity tests on product formations of Methyl-, Propyl-, and/or Butylparaben gave no evidence of significant photoreactivity.

It is concluded that Methylparaben, Ethylparaben, Propylparaben, and Butylparaben are safe as cosmetic ingredients in the present practices of use.

Final Amended Report on the Safety Assessment of Methylparaben, Ethylparaben, Propylparaben, Isopropylparaben, Butylparaben, Isobutylparaben, and Benzylparaben as used in Cosmetic Products

Parabens is the name given to a group of p-hydroxybenzoic acid (PHBA) esters used in over 22,000 cosmetics as preservatives at concentrations up to 0.8% (mixtures of parabens) or up to 0.4% (single paraben). The group includes Methylparaben, Ethylparaben, Propylparaben, Isopropylparaben, Butylparaben, Isobutylparaben, and Benzylparaben. Industry estimates of the daily use of cosmetic products that may contain parabens were 17.76 g for adults and 378 mg for infants. Parabens in cosmetic formulations applied to skin penetrate the stratum corneum in inverse relation to the ester chain length. Carboxylesterases hydrolyze parabens in the skin. Parabens do not accumulate in the body. Serum concentrations of parabens, even after intravenous administration, quickly decline and remain low. Acute toxicity studies in animals indicate that parabens are not significantly toxic by various routes of administration. Subchronic and chronic oral studies indicate that parabens are practically nontoxic. Numerous genotoxicity studies, including Ames testing, dominant lethal assay, hostmediated assay, and cytogenic assays, indicate that the Parabens are generally nonmutagenic, although Ethylparaben and Methylparaben did increase chromosomal aberrations in a Chinese Hamster ovary cell assay. Ethylparaben, Propylparaben, and Butylparaben in the diet produced cell proliferation in the forestomach of rats, with the activity directly related to chain length of the alkyl chain, but Isobutylparaben and Butylparaben were noncarcinogenic in a mouse chronic feeding study. Methylparaben was noncarcinogenic when injected subcutaneously in mice or rats, or when administered intravaginally in rats, and was not cocarcinogenic when injected subcutaneously in mice. Propylparaben was noncarcinogenic in a study of transplacental carcinogenesis. Methylparaben was nonteratogenic in rabbits, rats, mice, and hamsters, and Ethylparaben was nonteratogenic in rats. Parabens, even at levels that produce maternal toxicity, do not produce fetal anomalies in animal studies. Parabens have been extensively studied to evaluate male reproductive toxicity. In one in vitro study, sperm were not viabile at concentrations as low as 6 mg/ml Methylparaben, 8 mg/ml Ethylparaben, 3 mg/ml Propylparaben, or 1 mg/ml Butylparaben, but an in vivo study of 0.1% or 1.0% Methylparaben or Ethylparaben in the diet of mice reported no spermatotoxic effects. Propylparaben did affect sperm counts at all levels from 0.01% to 1.0%. Epididymis and seminal vesicle weight decreases were reported in rats given a 1% oral Butylparaben dose; and decreased sperm number and motile activity in F1 offspring of rats maternally exposed to 100 mg/kg day–1 were reported. Decreased sperm numbers and activity were reported in F1 offspring of female rats given Butylparaben (in DMSO) by subcutaneous injection at 100 or 200 mg/kg day–1, but there were no abnormalities in the reproductive organs. Methylparaben was studied using rats at levels in the diet up to an estimated mean dose of 1141.1 mg/kg day–1 with no adverse testicular effects. Butylparaben was studied using rats at levels in the diet up to an estimated mean dose of 1087.6 mg/kg day–1 in a repeat of the study noted above, but using a larger number of animals and a staging analysis of testicular effects—no adverse reproductive effects were found. Butylparaben does bind to estrogen receptors in isolated rat uteri, but with an affinity orders of magnitude less than natural estradiol. Relative binding (diethylstilbesterol binding affinity set at 100) to the human estrogen receptors α and β increases as a function of chain length from not detectable for Methylparaben to 0.267 ± 0.027 for human estrogen receptor α and 0.340 ± 0.031 for human estrogen receptor β for Isobutylparaben. In a study of androgen receptor binding, Propylparaben exhibited weak competitive binding, but Methylparaben had no binding effect at all. PHBA at 5 mg/kg day–1 subcutaneously (s.c.) was reported to produce an estrogenic response in one uterotrophic assay using mice, but there was no response in another study using rats (s.c. up to 5 mg/kg day–1) and mice (s.c. up to 100 mg/kg day–1) and in a study using rats (s.c. up to 100 mg/kg day–1). Methylparaben failed to produce any effect in uterotrophic assays in two laboratories, but did produce an effect in other studies from another laboratory. The potency of Methylparaben was at least 1000 × less when compared to natural estradiol. The same pattern was reported for Ethylparaben, Propylparaben, and Butylparaben when potency was compared to natural estradiol. In two studies, Isobutylparaben did produce an estrogenic response in the uterotrophic assay, but the potency was at least 240,000 × less than estradiol. In one study, Benzylparaben produced an estrogenic response in the uterotrophic assay, but the potency was at least 330,000 × less than estradiol. Estrogenic activity of parabens and PHBA was increased in human breast cancer cells in vitro, but the increases were around 4 orders of magnitude less than that produced by estradiol. Parabens are practically nonirritating and nonsensitizing in the population with normal skin. Paraben sensitization has occurred and continues to be reported in the case literature, but principally when exposure involves damaged or broken skin. Even when patients with chronic dermatitis are patch-tested to a parabens mix, parabens generally induce sensitization in less than 4% of such individuals. Many patients sensitized to paraben-containing medications can wear cosmetics containing these ingredients with no adverse effects. Clinical patch testing data available over the past 20 years demonstrate no significant change in the overall portion of dermatitis patients that test positive for parabens. As reviewed by the Cosmetic Ingredient Review (CIR) Expert Panel, the available acute, subchronic, and chronic toxicity tests, using a range of exposure routes, demonstrate a low order of parabens' toxicity at concentrations that would be used in cosmetics. Parabens are rarely irritating or sensitizing to normal human skin at concentrations used in cosmetics. Although parabens do penetrate the stratum corneum, metabolism of parabens takes place within viable skin, which is likely to result in only 1% unmetabolized parabens available for absorption into the body. The Expert Panel did consider data in the category of endocrine disruption, including male reproductive toxicity and various estrogenic activity studies. The CIR Expert Panel compared exposures to parabens resulting from use of cosmetic products to a no observed adverse effect level (NOAEL) of 1000 mg/kg day–1 based on the most statistically powerful and well-conducted study of the effects of Butylparabens on the male reproductive system. The CIR Expert Panel considered exposures to cosmetic products containing a single parabens preservative (use level of 0.4%) separately from products containing multiple parabens (use level of 0.8%) and infant exposures separately from adult exposures in determining margins of safety (MOS). The MOS for infants ranged from ~6000 for single paraben products to ~3000 for multiple paraben products. The MOS for adults ranged from 1690 for single paraben products to 840 for multiple paraben products. The Expert Panel considers that these MOS determinations are conservative and likely represent an overestimate of the possibility of an adverse effect (e.g., use concentrations may be lower, penetration may be less) and support the safety of cosmetic products in which parabens preservatives are used.

Safety assessment of esters of p-hydroxybenzoic acid (parabens)

Parabens are widely used as preservatives in food, cosmetic and pharmaceutical products. Acute, subchronic, and chronic studies in rodents indicate that parabens are practically non-toxic. Parabens are rapidly absorbed, metabolized, and excreted. In individuals with normal skin, parabens are, for the most part, non-irritating and non-sensitizing. However, application of compounds containing parabens to damaged or broken skin has resulted in sensitization. Genotoxicity testing of parabens in a variety of in vitro and in vivo studies primarily gave negative results. The paraben structure is not indicative of carcinogenic potential, and experimental studies support these observations. Some animal studies have reported adverse reproductive effects of parabens. In an uterotrophic assay, methyl and butyl paraben administered orally to immature rats were inactive, while subcutaneous administration of butyl paraben produced a weak positive response. The ability of parabens to transactivate the estrogen receptor in vitro increases with alkyl group size. The detection of parabens in a small number of breast tumor tissue samples and adverse reproductive effects of parabens in animals has provoked controversy over the continued use of these substances. However, the possible estrogenic hazard of parabens on the basis of the available studies is equivocal, and fails to consider the metabolism and elimination rates of parabens, which are dose, route, and species dependent. In light of the recent controversy over the estrogenic potential of parabens, conduct of a reproductive toxicity study may be warranted.

High-resolution and high-throughput protocols for measuring drug/human serum albumin interactions using BIACORE

Characterizing how chemical compounds bind to human serum albumin (HSA) is essential in evaluating drug candidates. Using warfarin as a test system, we validate the application of BIACORE SPR biosensors to reliably determine binding constants for drug/HSA interactions. The binding responses for warfarin over HSA surfaces were extremely reproducible even though warfarin is small compared to the size of the immobilized protein. At high concentrations, warfarin bound at more than one site on HSA, which is consistent with its known binding properties. The affinity we determined for the high-affinity site (K(25 degrees C)(d) = 3.7 +/- 1.2 microM), as well as the dissociation rate constant (k(25 degrees C)(d) = 1.2 s(-1)), are also consistent with binding constants determined previously. These results validate the biosensor technology and illustrate how BIACORE can be used to study drug/HSA interactions in a high-resolution mode. Using a set of 10 test compounds, we present a protocol for determining equilibrium dissociation constants for HSA in a high-throughput mode. Our method involves working at low compound concentrations and fitting the equilibrium data for all compounds simultaneously. We show that the % bound values determined by SPR correlate with the values determined by solution-based methods. The ability to examine directly the binding of small molecules (130-800 Da), coupled with minimal sample requirements and automated instrumentation, makes BIACORE technology applicable for evaluating drug/HSA interactions.

Safety assessment of propyl paraben: a review of the published literature

Propyl paraben (CAS no. 94-13-3) is a stable, non-volatile compound used as an antimicrobial preservative in foods, drugs and cosmetics for over 50 years. It is an ester of p-hydroxybenzoate. Propyl paraben is readily absorbed via the gastrointestinal tract and dermis. It is hydrolyzed to p-hydroxybenzoic acid, conjugated and the conjugates are rapidly excreted in the urine. There is no evidence of accumulation. Acute toxicity studies in animals indicate that propyl paraben is relatively non-toxic by both oral and parenteral routes, although it is mildly irritating to the skin. Following chronic administration, no-observed-effect levels (NOEL) as high as 1200-4000 mg/kg have been reported and a no-observed-adverse-effect level (NOAEL) in the rat of 5500 mg/kg is posited. Propyl paraben is not carcinogenic, mutagenic or clastogenic. It is not cytogenic in vitro in the absence of carboxyesterase inhibitors. The mechanism of propyl paraben may be linked to mitochondrial failure dependent on induction of membrane permeability transition accompanied by the mitochondrial depolarization and depletion of cellular ATP through uncoupling of oxidative phosphorylation. Sensitization has occurred when medications containing parabens have been applied to damaged or broken skin. Parabens have been implicated in numerous cases of contact sensitivity associated with cutaneous exposure, but high concentrations of 5-15% in patch testing are needed to elicit reaction in susceptible individuals. Allergic reactions to ingested parabens have been reported, although rigorous evidence of the allergenicity of ingested paraben is lacking.

A quantitative circular dichroic investigation of the binding of the enantiomers of ibuprofen and naproxen to human serum albumin

The binding constants for racemic, R and S naproxen and ibuprofen to human serum albumin have been determined by a circular dichroic technique. The ibuprofens and naproxens show no measurable extrinsic optical activity on interaction with the protein, and so the extrinsic Cotton effect shown following the diazepam-albumin interaction is used as a probe. The presence of the drugs reduce the amount of diazepam bound as shown by the interaction is used as a probe. The presence of the drugs reduce the amount of diazepam bound as shown by the reduced size of the induced ellipticity. The calculated primary binding constants show that the S form of both drugs bind to the albumin more tightly than the R form and that the racemic forms bind less tightly than either enantiomer.

Effect of different parameters on the binding of two anti-inflammatory drugs to human serum albumin

The binding to human serum albumin of two anti-inflammatory drugs, indomethacin and indoprofen, has been studied by chromatographic and spectroscopic techniques. The results shown indicate that the binding of both drugs--but more notably of indoprofen--is very sensitive to variations of the environmental conditions. The binding is also dependent upon limited modifications in the tertiary structure of the protein. The evidences shown tend to indicate that these two phenomena are related, and that the binding is permitted under conditions of a relatively open structure of the protein molecule.

Sensitive high-performance liquid chromatographic determination of indomethacin in human plasma. Pharmacokinetic studies after single oral dose

A sensitive high-performance liquid chromatographic assay for the specific determination of indomethacin at concentrations down to 20 ng/ml in human plasma is described. This method has been applied to investigate the disappearance of indomethacin from plasma of ten subjects following the intake of two formulations (Indocid and generic form). An initial half-life of 1.32 +/- 0.44 h-1 was found which is in good agreement with other findings, but the terminal phase was much longer (13.6 +/- 6.9 h-1) than previously reported. There is no difference between the two galenic forms (p less than 0.001).

Rapid determination of indomethacin and salicylic acid in serum by means of reversed-phase liquid chromatography

A method for the quantitative analysis of indomethacin and salicylic acid in blood serum and urine by high-performance liquid chromatography is described. A C18-bonded silica was used as the stationary phase and mixtures of ethanol, n-butanol and aqueous buffer as the mobile phase. Before injection the serum is deproteinized and extracted in one step. The recovery of the extraction was found to be 88% and 77% for indomethacin and salicylic acid, respectively. The relative standard deviations of the analysis for 0.5 micrograms indomethacin and 5 micrograms salicyclic acid per ml serum were 3.6% and 3.2%, respectively. The detection limits for indomethacin and salicylic acid were 2 ng. This corresponds for both substances to 0.1 micrograms/ml serum for an injection volume of 100 microliters. The method enables simultaneous determination of possibly formed metabolites. A number of concurrently administered drugs do not interfere with the analysis. The interactive effects of co-medication of indomethacin and salicylic acid on the serum concentration of indomethacin is demonstrated by measuring the pharmacokinetic curves.

Mutual displacement interactions in the binding of two drugs to human serum albumin by frontal affinity chromatography

A continuous frontal analysis chromatographic method was developed for studying the simultaneous binding of two drugs or ligands with an immobilized macromolecule. The usefulness of this method was demonstrated in the interactions of sulphamethizole and salicylic acid with human serum albumin (HSA). The mutual inhibitory effect on the binding of one drug of the presence of the other was directly shown to be due to displacement of the bound drug from HSA by the other. On the basis of a double-reciprocal plot analysis, these two drugs are interpreted as competing for the same primary binding sites.