4,4'-isopropylidenediphenol

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2014 and 2017

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Data source

Reference

Reference Type:

publication

Title:

Pharmacokinetics of bisphenol A in humans following dermal administration

Author:

Alan F. Sassoa, Ralph Pirowb, Syam S. Andrac, Rebecca Churchd, Rebecca M. Nachmana, Susanne Linkeb, Dustin F. Kaprauna, Shepherd H. Schurmand, Manish Arorac, Kristina A. Thayera, John R. Buchere, Linda S. Birnbaumf

Year:

2020

Bibliographic source:

Environment International 144 (2020) 106031

Report date:

2020

Materials and methods

Objective of study:

other: The objective was to examine the absorption, distribution, metabolism and excretion of BPA in humans following dermal admistration.

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

Deuterated BPA (d6-BPA) was dermally administered to 10 subjects (6 men and 4 women) at a dose of 100 µg/kg over a 12-hour period and blood and urine analysis were conducted from the beginning of dosing through a three- or six-day period. Time-course serum and urine concentrations of total and unconjugated(free) d6-BPA were determined and was used to calculate the terminal half-life and the area under the curve. The protocol (protocol number 12-E-0089; clinicaltrials.gov identifier: NCT01573429), blood, and urine collection method was approved by the NIEHS Institutional Review Board (IRB).

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

4,4'-isopropylidenediphenol

EC Number:

201-245-8

EC Name:

4,4'-isopropylidenediphenol

Cas Number:

80-05-7

Molecular formula:

C15H16O2

IUPAC Name:

4-[2-(4-hydroxyphenyl)propan-2-yl]phenol

Test material form:

other: aqueous or ethanolic solution

Details on test material:

Purity: 98.6% isotopic purity and > 99.7% chemical purity
Supplier: CDN Isotopes, Pointe-Claire, Quebec
Concentration of test solution: 25 mg/mL in USP grade 95% ethanol or in an aqueous suspension containing 0.3% CMC

Specific details on test material used for the study:

Deuterated d6-BPA (dimethyl‐d6; 98% isotopic purity and 99% chemical purity; product # 588806-1G) was purchased from Sigma Aldrich (St. Louis, MO) and the corresponding labeled internal standard 13C12-BPA (99%) was obtained from Cambridge Isotope Laboratories, Inc. (Andover, MA).

Radiolabelling:

no

Test animals

Species:

other: humans

Strain:

other: not applicable

Sex:

male/female

Details on test animals or test system and environmental conditions:

Men and non-pregnant women were recruited by the National Institute of Environmental Health Sciences Clinical Research Unit (NIEHS CRU) in 2014 and 2017 from the Raleigh Durham region of North Carolina using standard flyer advertisements. Criteria included age (25–45), ability to fast overnight, and agreement to avoid conceiving a child and not donate eggs or sperm for six months following their participation.

The average age of male (n = 6) and female (n = 4) volunteers was 33 years (range 25–42) and 31 years (range 26–35), respectively. Half of the subjects were non-Hispanic whites (n = 5;), while the remainder were black or African-American (n = 4) and mixed-race (n = 1). The average BMI of male and female volunteers was 27.2 (22.4–34.2) and 28.7 (21.6–34), respectively. The average body weight of male and female volunteers was 91.2 kg (67.7–124.0) and 80.0 kg (70.0–95.4), respectively. Three of the 10 subjects (two males and one female) participated in both experimental protocols (2014 and 2017) and received a different dosing vehicle (CMC or ethanol) at each visit.

Criteria for exclusion included specified pre-existing health conditions, pregnancy, and use of medications that may affect glucuronidation of BPA. Blood samples used to determine eligibility status was collected at the time of the initial screening visit.

Participant Inclusion Criteria:
• Male or female 25-45 years of age at the time of enrollment
• Able to fast overnight
• Able to understand and provide written informed consent in English
• Able to travel to the NIEHS Clinical Research Unit (CRU) for all required study visits
• Male and females of reproductive age agree to use contraception to avoid conceiving a child and agree not to donate eggs or sperm for six months following their participation in the study

Participant Exclusion Criteria:
• Uncontrolled diabetes:
- Hemoglobin A1C of >6.5% or a fasting blood glucose of >126 mg/dL
• Known liver dysfunction or disease:
-ALT - higher than the normative value or determined abnormal by the PI
- AST – higher than the normative value or determined abnormal by the PI
- ALP – higher than the normative value or determined abnormal by the PI
• Known kidney dysfunction or disease:
- Estimated Glomerular Filtration Rate (eGFR)- <60 ml/min per the MDRD equation
•Clinically relevant anemia as defined as hemoglobin concentration <13 g/dL for males and hemoglobin concentration <11 g/dL for females
• Pregnancy: Positive serum quantitative hCG pregnancy test
• Current lactation
• BMI ≤19 and ≥35
•Medication use: Given the widespread use of medications, it may not be practical to instruct subjects to avoid all medication prior to and during the study. Thus, participant exclusion will be based on use of medications within 48 hours of the exposure and for the 6 days following the exposure that affect glucuronidation of the d-BPA dosage: Salicylic acid, acetaminophen, ibuprofen, naproxen, mefenamic acid, diclofenac, gliclazide carbamazepine, valproic acid, cimetidine, sulfasalazine, amoxicillin and erythromycin (Verner et al. 2010).
• Recent blood donation within the past 8 weeks of the BPA exposure visit (so as not to exceed donation of 10.5 mL/kg or 550 mL over an 8 week period)

BPA Exposure Questionnaire:
Each participant was asked to complete a short questionnaire, to collect demographic information including age, ethnicity (required for NIH human participants reporting) and gender. Information about BPA exposure status at home and at work was collected, including the following:
• Information on activities and practices that may impact oral exposure to BPA (i.e., use of polycarbonate bottles, consumption of canned foods).
• Occupational information (e.g., works in the manufacture of BPA or BPA-containing products such as polycarbonate bottles or epoxy resins, handles cash register receipts, etc.).

Administration / exposure

Route of administration:

dermal

Vehicle:

other: 0.3% carboxymethylcellulose (6 participants) suspension and 95% ethanol (7 participants)

Details on exposure:

TEST SITE
- Area of exposure: Volar surface of the forearm - 4.91 cm2.
- % coverage: Generally, one application site was used per subject. For the 2014-CMC study arm, two or more application sites were used (with simultaneous application) depending on the subjects weight. This was done because a practice session indicated that larger individuals with correspondingly larger dose formulation volumes would have the solution run off their arms, so the formulation was divided to prevent this runoff.
- Type of wrap if used: The application area(s) were encircled with petroleum jelly and each area was then covered with the Hill Top Chamber (HTC) without a pad covering the area. The chamber was secured with tape to prevent the spreading of the dried d6-BPA layer/film on the skin.

REMOVAL OF TEST SUBSTANCE
- Washing (if done): The HTC was removed and skin was rinsed and/or wiped with rinsates and wipe materials saved for analysis.
- Time after start of exposure:12 h

TEST MATERIAL
- Concentration (if solution): The solution for dermal application (25 mg/mL) was prepared by dissolving a weighed portion of d6-BPA (dimethyl-d6; 98.6% isotopic purity and > 99.7% chemical purity, CDN Isotopes, Pointe-Claire, Quebec) (Twaddle et al., 2010) in USP grade 95% ethanol (Decon Laboratories, King of Prussia, PA) or an aqueous suspension containing 0.3% CMC. The d6-BPA solutions were stored as 0.5 mL individual aliquots in 2-mL tubes and stored frozen at -20 °C until the day before the participant’s dosing visit when they were thawed in the refrigerator. For most participants, the nominal d6-BPA concentration (25 mg/mL) was used to calculate the appropriate amount X (mL) of the dosing solution/suspension from the participant body weight Y (kg) measured on the day of the study and the target dose (100 µg/kg BW) (see below). The d6-BPA dosing concentration was validated for four selected participants using HPLC-MS/MS, and this analytical value was used in the calculation instead of the nominal concentration (e.g., a 70 kg subject will receive an application of 0.28 mL of the dosing solution): X mL (100 g/kg BW) Y kg BW/(25, 000 g/mL dosing solution)

Details on exposure:
Dermal administration started in the morning after a fasting period beginning at midnight on the day of the visit. Locations on the skin of the volar forearm were identified and a circle drawn using a Hill Top® Chamber (Hill Top Research, Inc., Cincinnati, OH) as a template. A pipette with a polypropylene pipet tip was used to apply the d6-BPA solution (ethanol) or suspension (CMC) to the unshaven volar forearm of the participant, choosing an area with the least amount of hair and with no visible scratches or open wounds. For both the CMC and ethanol experiments, the solution was allowed to dry for approximately five minutes, using a gentle stream of air from a hairdryer if needed.
Drying time was noted in the participant’s record. The beginning of application of the d6-BPA solution or suspension was considered to be time zero for data-recording purposes. After the solution dried, the application area(s) was encircled with petroleum jelly and each area was then covered with the Hill Top Chamber (HTC) without a pad covering the area. The chamber provided a viewing window for study staff to monitor for possible skin irritation. The area under the HTC was 4.91 cm2. The chamber was secured with tape to prevent the spreading of the dried d6-BPA layer/film on the skin. At 12 h after the start of administration, the HTC(s) were removed and skin was rinsed and/or wiped with rinsates and wipe materials saved for analysis.

Recovery of unabsorbed d6-BPA after dermal administration:
Following dermal administration and removal of the HTC at 12 hrs, the application area(s) were rinsed with a polypropylene squeeze bottle filled with 95% ethanol to recover unabsorbed d6-BPA for quantification.
A volume of 1–2 mL rinsing solution was squirted on the exposed skin area, and a sponge or gauze was used to wipe the exposed skin with the rinsing solution. For the 2014 protocol using CMC, no wiping material was used. The site rinsate was collected in a specimen cup. The rinsing step was typically repeated several times, and the total volume of site rinsate was recorded. Rinsing of the HTC with ethanol and recording the total volume of HTC rinsate was done similarly. Polypropylene vials with 1–2 mL aliquots of the rinsates were sent for d6-BPA analysis. The HTC and the sponge/gauze were also sent to the analytical lab to recover and determine any residual d6-BPA adsorbed to these materials.

Duration and frequency of treatment / exposure:

12 hour/single dose

Doses / concentrations

Dose / conc.:

100 other: µg/kg bw

No. of animals per sex per dose / concentration:

10 subjects (6 men and 4 women)

Control animals:

no

Details on study design:

2014 Protocol versus 2017 protocol
The target dermal dose of 100 µg/kg was selected in order to match the administered dose used in an oral arm of this study (Thayer et al., 2015). A dermal study was initially conducted in 2014 to evaluate the suitability of the oral study sample collection protocol to evaluate pharmacokinetics following dermal exposure (Fig. 1 – included below in attached background information), and to help select a vehicle to best model the nature of human exposure (i.e., handling thermal paper). More specifically, the 2014 protocol was conducted to verify that detectable levels of d6-BPA could be measured using either a 0.3% aqueous carboxymethylcellulose (CMC) suspension vehicle or a 95% ethanol solution vehicle, and whether follow-up for 3 days after dermal administration was sufficiently long to observe complete (or nearly complete) urinary elimination.
Preliminary results of the 2014 study on four subjects using 0.3% CMC suspension vehicle indicated that a longer follow-up period post-treatment was warranted. The protocol was subsequently amended to allow for extending the follow-up period to six days for subjects participating in the 2017 studies.
The preliminary analysis of the 2014 CMC data also included estimates of free d6-BPA serum concentrations. For the current study, urine and serum samples from the 2014 preliminary analysis were re-analyzed using the method described below to better compare across the 2014 and 2017 experiments.
Analysis of free d6-BPA in serum was limited to samples from the ethanol study arms. This was done to complement the 2014 CMC analysis by expanding the database to include free d6-BPA measurements for the ethanol vehicle, and to focus post-2014 analyses on the vehicle believed to facilitate higher absorption.
The study was designed to evaluate the pharmacokinetics of dermal uptake, which required prolonged exposure at a high surface concentration. Ethanol and CMC vehicles on the forearm are not representative of typical exposures (such as thermal receipts on the hands), and the use of a Hill Top chamber caused occlusive conditions, which likely promoted sweat layer formation on the covered skin. This administration regimen might have enhanced dermal absorption relative to what might be expected following skin contact to thermal receipt paper. The aim was to ensure that dermal uptake would indeed occur so that kinetic parameters can be evaluated and compared against other controlled exposure scenarios.


Pharmacokinetic analysis:
Analyses and data visualizations were performed using the R programming language (R Core Team, 2019). Plots of serum concentrations of total and free d6-BPA at each time point were generated at the individual-level. The terminal half-life in serum (t1/2) was determined for each dataset by fitting an exponential curve, beginning at the peak concentration (typically the 12-hour timepoint). A single terminal phase was observed, and all data from the peak concentration through the end of data collection (3–6 days, depending on the protocol) were fitted to a single exponential curve by least-squares regression of log-transformed concentration vs time. The observed t1/2 represents both serum elimination kinetics and diffusion kinetics of absorption from the dermal compartment (which may be a rate-limiting step). If the absorption process would indeed be the rate-limiting step that governs the disappearance from the body, this kinetics would then be described as a flip-flop situation (Derendorf and Rowland, 2020; Gabrielsson and Weiner, 2016). To estimate the area-under-the-curve from time zero to infinity (AUC0–∞) for serum concentrations, the log-linear trapezoidal rule was applied using t1/2 and the serum concentration at the last time point.

Details on dosing and sampling:

TOXICOKINETIC / PHARMACOKINETIC STUDY (Absorption, distribution, excretion)
- Tissues and body fluids sampled: Urine and blood
- Time and frequency of sampling: Blood: after the start of administration, blood was collected for 12 h via an indwelling IV cannula/saline lock at: 0, 5, 10, 15, 20, 25, 30, 40, 45, 50, 60, 80 and 100 (± 5 min); 120, 150, 180, 210, 240, 270 and 300 (± 10 min); 330, 360, 390, 420, 480, 540 min (± 15 min); and 720 min (± 20 min). Additional blood samples were collected by peripheral blood draw at 24/48/72 h for all participants remaining in the study, and at 144 h for 2017 participants.
Urine: a fasting urine specimen was collected prior to dosing. Following the start of administration of d6-BPA, participants collected all unscheduled urine voids in separate containers from 0 to 2, 2 to 4, 4 to 8, and 8 to 12 h. Participants of the 2014 protocol performed a 24-hour collection for 3 days post-dose, while 2017 participants performed these collections for 6 days post-dose.


Analytical methods:
Sample preparation and quantification of total and free d6-BPA in urine and serum were based on a previously described method (Thayer et al., 2015; Twaddle et al., 2010) with some modifications. In brief, 13C12-BPA was added to an individual urine or serum sample (0.2 mL) for isotope-dilution high performance liquid chromatography-tandem mass spectrometry (HPLC–MS/MS) analysis. Further details can be found below under attached background material - Analytical method_d6 BPA measurements.pdf.

Results and discussion

Main ADME results

Main ADME results 1

Type:

absorption

Results:

Total d6-BPA: Cmax (serum) = 3.26 ± 2.31 nM. Free d6-BPA: Cmax (serum) = 0.272 ± 0.141 nM. Combined (2017 and 2014 protocol).

Main ADME results 2

Type:

absorption

Results:

Total d6-BPA: AUC0-∞ (serum) = 95.6 ± 54.4 nM x h. Free d6-BPA:AUC0-∞ (serum) = 7.51 ± 2.69 nM x h. Combined (2017 and 2014 protocol)

Main ADME results 3

Type:

absorption

Results:

Total d6-BPA: % free AUC (serum) = 8.81 ± 1.65 nM. Combined (2017 and 2014 protocol).

Main ADME results 4

Type:

absorption

Results:

Total d6-BPA: t1/2 (serum) = 21.4 ± 9.81 nM. Free d6-BPA: t1/2 (serum) = 17.6 ± 7.69 nM. Combined (2017 and 2014 protocol).

Main ADME results 5

Type:

excretion

Results:

Total d6-BPA: cumulative excreted (urine) = 0.998 µg/kg BW. Combined (2017 and 2014 protocol).

Toxicokinetic / pharmacokinetic studies

Details on absorption:

A summary of estimated pharmacokinetic parameters is presented in Table 2.

Detectable serum levels of total d6-BPA were observed at 1.4 h after the start of dosing. A maximum serum concentration (Cmax) of 3.26 nM was observed. Free d6-BPA was detectable in serum 2.8 h after start of dermal administration, with Cmax of 0.272 nM.

Based on data collected under the 2017 protocol, detectable serum levels of total d6-BPA were observed at approximately 1.4 h (ranging from 0.6 to 3.7 h) following the start of dermal administration. Detectable free serum d6-BPA appeared approximately 1.8 h (1.5–2.2 h) after the appearance of total d6-BPA at the individual-level, or approximately 2.8 h (2.5–3.1 h) after the start of dermal administration. Serum concentrations of free and total d6-BPA increased rapidly for seven hours. Free d6-BPA was a significantly greater percentage of the total serum BPA, with 10.9% (6.6–17%) at Cmax compared to the 0.39% observed in the oral arm (Thayer et al., 2015). Beginning at approximately seven hours and continuing to 12 h (which corresponds to the cessation of exposure), the concentration of free and total serum d6- BPA plateaued. After cessation of dermal exposure, elimination from the serum was slow, with t1/2 values for free and total BPA estimated to be approximately 15–20 h.

Details on excretion:

Total d6-BPA was detected in the urine at first urinary void for all individuals (within 2 h). Approximately 1% of the target dose was collected in urine by day three post-dose. The concentration of total urinary d6-BPA increased rapidly during 8–12 h and then decreased continuously, beginning around 24 h post-dose. For all individuals, there remained detectable levels of total urinary d6-BPA at the end of the experiment (three days post-dose for the 2014 study participants, and six days post-dose for the 2017 study participants). However, the daily urinary excretion of total d6-BPA was a negligible percent of the dose after about three days. There was high interindividual variability in the cumulative excreted dose (which did not plateau at three days post-dose).

The plateau of serum unconjugated and total d6-BPA indicates that a steady-state between absorption and elimination processes had been reached. Given the serum elimination half-life of ~ 6 h for unconjugated d6-BPA (observed in Thayer et al., 2015), steady-state was achieved faster than expected. If the dermal absorption rate of d6-BPA into blood was constant (as in the case of intravenous infusion), it would take approximately five elimination half-lives (30 h) to reach a quasi-steady state serum concentration of unconjugated d6-BPA (97% of the final level).

Metabolite characterisation studies

Metabolites identified:

not measured

Any other information on results incl. tables

Table 2. Pharmacokinetics parameters for total and free d6-BPA in serum and urine of human subjects administered d6-BPA via 12-hour dermal application.

 

	Serum					Urine
	Cmax (nM)	% free Cmax	AUC0-∞ (nM × h)	% free AUC	t ½ (h)	Cumulative excreted (µg/kg BW)
2017 protocol
Total d6-BPA	2.63 ± 1.69 (5)	9.66 ± 3.39 (3)	72.4 ± 45.7 (4)	8.41 ± 1.67 (2)	20.0 ± 6.84 (4)	1.16 ± 0.572 (4)
Free d6-BPA	0.282 ± 0.0583 (3)	-	8.68 ± 1.35 (2)	-	12.2 ± 5.16 (2)	-
2014 protocol
Total d6-BPA	3.66 ± 2.65 (8)	11.9 ± 4.15 (4)	107 ± 57.4 (8)	9.02 ± 1.85 (4)	22.1 ± 11.4 (8)	0.919 ± 0.554 (8)
Free d6-BPA	0.282 ± 0.0583 (3)	-	8.68 ± 1.35 (2)	-	12.2 ± 5.16 (2)	-
Combined protocol
Total d6-BPA	3.26 ± 2.31 (13)	10.9 ± 3.73 (7)	95.6 ± 54.4 (12)	8.81 ± 1.65 (6)	21.4 ± 9.81 (12)	0.998 ± 0.546 (12)
Free d6-BPA	0.272 ± 0.141 (7)	-	7.51 ± 2.69 (6)	-	17.6 ± 7.69 (6)	-

 

All values shown as mean ± SD (n), where n is the number of datasets used to estimate the parameter. AUC_0-∞ estimated by log-linear trapezoidal rule from time zero to infinity (using t_1/2 and the final serum concentration). C_max was directly obtained from the data for all datasets. t_1/2 estimated for individuals I and G of the 2014 protocol omitted data for t = 12 h, since concentrations increased to t = 24 h. AUC, t_1/2, and cumulative amount excreted could not be estimated for individual D due to early withdrawal from the study. Time to C_max (i.e., t_max) is not applicable because serum concentration increases to C_max during exposure, and predictably declines when exposure is stopped at 720 min. There was no discernable difference between vehicles (CMC or ethanol) given the interindividual variability, and data for free d6-BPA were only analyzed for the ethanol results. A pilot analysis was performed on the 2014 CMC data, which included an estimate of free d6-BPA. These preliminary results are not incorporated in this analysis due to inter-laboratory differences.

Table 3: Plausibility check of the fraction dermally absorbed (Fabs), as obtained by three different approaches, in terms of serum clearance of unconjugated BPA.

Approach	Fabs (%)	Dose D (nmol)	AUC (Nm x h)	CL (L/h)
Recovery of total BPA in urine	1	34,141	7.51	45
Serum AUCs of total BPA after oral and dermal administration	2.2			95.5
Mass-balance consideration based on recovery and unabsorbed BPA	12-29			545-1318

The absolute dose (D, nmol) derives from the per-body-weight dose of 100 µg/kg BW multiplied by a body weight of 80 kg and divided by the molecular mass of 234.32 g/mol for d6-BPA. The assumed body weight is represented for the six subjects from which the estimate for the mean serum AUC of 7.51 nM x h for unconjugated BPA was obtained. The serum clearance for unconjugated BPA (CL) was calculated as: CL = Fabs/100% x D/AUC.

Recovery of unabsorbed d6-BPA after dermal administration

A substantial amount of the applied dose was recovered from the application site at 12 h following dermal administration of d6-BPA in ethanol. For the ethanol experiments under both the 2014 and 2017 protocols, 71–99% of the applied dermal dose remained unabsorbed on the skin or the HTC. This is in good agreement with the experiments using CMC conducted in 2017, which found an unabsorbed dose of 75–79%. For the 2014 CMC data, a lower percentage (19–21%) of the applied dose was recovered. For this dataset, no sponge or gauze was used to wipe the exposed skin of the rinsing solution. This likely had a negative impact on the effectiveness of surface recovery. In all experiments conducted in 2014, the largest proportion of the unabsorbed dose was found on the skin, whereas only a minor portion was recovered from the HTC. The opposite is true for the experiments conducted in 2017, where the largest proportion of the unabsorbed dose was recovered from the Hill Top Chamber (HTC )and a minor portion from the skin rinsate. Multiple differences between the 2014 and 2017 experiments were documented, and these could have contributed to the recovery variations. As noted earlier, there was a lack of skin wiping for the 2014 CMC experiment. In addition, the 2014 CMC study used 2–4 application sites rather than a single application site. Distributing the dermal dose (100 µg/kg BW) across a larger skin surface area would enable the applied BPA to diffuse into a larger portion of skin surface to build up a skin depot. Different nurses implemented the surface and HTC rinsing protocol between 2014 and 2017, which could have also impacted surface recovery. Irrespective of these differences, there was good agreement in the unabsorbed dose (71–88%) across the subjects when excluding the implausible value of 99% from one participant (where 1.4% of the dose was still collected in urine) and excluding the low values from the 2014 study using CMC where no skin wiping was performed.

Estimates of absorbed d6-BPA after dermal administration:

The dermal and HTC recovery mass balance implies that up to 12–29% of the applied dermal dose penetrated the skin over a 12-hour period. This is likely an over-estimate due to the experimental variation in rinsate recovery as well as unaccounted amounts lost to the environment.

The percentage of total d6-BPA recovered in urine was significantly lower (approximately 1% of the target dose). This is likely an under-estimate of total absorbed dose because detectable levels of d6-BPA were measured in urine and serum at the end of the experiment for all individuals. This indicates that not all d6-BPA had been cleared from the subjects at the time the last observations were made. Comparing the average serum AUC for total d6-BPA between Thayer et al. (2015) (where the target oral 100 µg/kg of d6-BPA dose was almost completely absorbed) and the current work, the dermal AUC for a target 100 µg/kg dose was 2.2% of the oral AUC. This may be one estimate for the average percent dose absorbed into systemic circulation following 12-hour dermal application, but there is some uncertainty due to interindividual variability, and oral/dermal toxicokinetic differences. Given the high approximate dermal absorption of up to 12–29% as implied from surface recovery, and the fact that detectable levels were measured in serum at the end of the experiment, it is possible that a portion of the administered dose that penetrated the skin remained in the dermis and epidermis throughout the post-dose period and beyond six days. This depot of d6-BPA in the skin would have continued to diffuse into the blood until depleted or lost by other removal processes (contact with other surfaces, skin washing, and desquamation).

To further reduce the uncertainty surrounding the dermal estimates for the fraction dermally absorbed (Fabs) were used to calculate the serum clearance (CL) of unconjugated BPA for a human subject with a body weight (BW) of 80 kg (Table 3). Apart from Fabs, the absolute dermal dose (D) and the AUC for unconjugated BPA entered into the calculation (Table 3). The calculated serum clearances were then compared with the clearance predicted by allometric scaling based on animal data with intravenous administration of BPA.

The Fabs values derived from the three approaches (1%, 2.2%, and12–29%) translate into serum clearances of 45 L/h, 95.5 L/h, and 545–1318 L/h (Table 3). Allometric scaling approaches predict serum clearances of 140 L/h (=5.264 × BW0.749; Cho et al. (2002)) and 122 L/h (=2.34 × BW0.9014; Collet et al. (2015)), respectively, for an 80-kg human. The Fabs value of 1%, which was obtained by considering the recovery of total BPA in urine, translates into a serum clearance of half that predicted by allometric scaling. The Fabs value of 2.2%, which was calculated from the serum AUCs of total BPA after oral and dermal administration, translates into a serum clearance which is physiologically plausible as it is consistent with allometric scaling estimates. This conclusion holds true even assuming a small portion of the dermally absorbed BPA is metabolized in the skin before reaching systemic circulation.

The Fabs value of 12–29%, which derives from mass-balance considerations based on the recovery of unabsorbed BPA, yields a serum clearance of 545–1318 L/h which is physiologically implausible as it grossly exceeds not only the values predicted by allometric scaling but also the hepatic blood flow (Collet et al., 2015). This strongly suggests that the 12–29%, being the complement of the portion recovered from the application site, was mostly lost to the environment rather than absorbed.

 

 

Applicant's summary and conclusion

Executive summary:

Deuterated BPA (d6-BPA) was dermally administered to 10 subjects (6 men and 4 women) at a dose of 100 µg/kg over a 12-hour period and blood and urine analysis were conducted from the beginning of dosing through a three- or six-day period. Detectable serum levels of total d6-BPA were observed at 1.4 h after the start of dosing, and a maximum serum concentration (Cmax) of 3.26 nM was observed. Free d6-BPA was detectable in serum 2.8 h after the start of dermal administration, with Cmax of 0.272 nM. Beginning at approximately seven hours and continuing to 12 h (which corresponds to the cessation of exposure), the concentration of free and total serum d6-BPA plateaued. The terminal half-lives of total d6-BPA and free d6-BPA in the body were 21.4± 9.81 h and 17.6± 7.69 h, respectively. Elimination from the body was rate-limited by kinetics in the dermal compartment. Free d6-BPA was a greater percentage of the area under the curve of total serum BPA (8.81%) compared to the 0.56% observed in our previously published oral study. Recovery of total d6-BPA in urine was < 2% of the applied dose after six days. Analysis of the area under the curve for dermal and oral administration revealed that 2.2% of the dermal dose became systemically available. These data are in line with previous studies indicating how pharmacokinetics of BPA differ following oral and dermal exposures. Dermal exposure resulted in a longer apparent half-life and higher free:total d6-BPA ratio compared to oral. The fraction dermally absorbed (Fabs) value of 12–29%, which was derived from mass-balance considerations based on the recovery of unabsorbed BPA, yields a serum clearance of 545–1318 L/h which is physiologically implausible as it grossly exceeds not only the values predicted by allometric scaling but also the hepatic blood flow. This strongly suggests that the 12–29%, being the complement of the portion recovered from the application site, was mostly lost to the environment rather than absorbed.