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Toxicological information

Basic toxicokinetics

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Administrative data

basic toxicokinetics in vivo
Type of information:
experimental study
Adequacy of study:
key study
Study period:
13 April 2017 – 30 April 2017
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 Type:
study report
Report date:

Materials and methods

Objective of study:
Test guideline
no guideline followed
Principles of method if other than guideline:
This study was not designed to fulfil any guidelines.
GLP compliance:
yes (incl. QA statement)

Test material

Constituent 1
Chemical structure
Reference substance name:
EC Number:
EC Name:
Cas Number:
Molecular formula:
Specific details on test material used for the study:
Test Material Name: Dodecamethylcyclohexasiloxane
Chemical Name: Dodecamethylcyclohexasiloxane
Synonyms: D6
Molecular Formula: C12H36O6Si6
Molecular Weight: 444.9
CAS Number: 540-97-6

Radiochemical Test Material Name: D6, [14C]-
Chemical Name: 2,2,4,4,6,6,8,8,10,10,12,12-D6, [14C]-
Synonyms: 14C-D6
Molecular Formula (unlabelled): C12H36O6Si6
Molecular Weight (unlabelled): 444.9
Specific Activity (mCi/g): 144.736
CAS Number (unlabelled): 540-97-6

Test animals

New Zealand White
Details on species / strain selection:
NZW Rabbits were selected because of they are a suitable species for the analysis of metabolism of chemicals in vivo. NZW Rabbit are also suitable due to the availability of historical background data, and the reliability of the commercial supplier.
Details on test animals or test system and environmental conditions:
Species and Sex: Female time-mated rabbits with Vascular access port (VAP)
Supplier and Location: Covance Research Products, Inc. (CRP), (Denver, Pennsylvania), USA
Age at Study Start: Sexually mature adult at GD 12 upon arrival
Health Status and Acclimation: Upon arrival all rabbits were acclimated to the laboratory for approximately 7 days prior to the start of the study. Upon arrival and once during the acclimation period, each animal was evaluated by a veterinarian.
Housing: Upon arrival until administration of test material, animals were housed one per cage in stainless steel/plastic cages. Cages had perforated plastic floors and were suspended above catch pans with absorbent non-contact bedding. Cages contained a J-type feeder and a water bottle.
Following administration of test material, the rabbits were single housed in stainless steel metabolism cages, which were designed for the separation and collection of urine and feces.

The following environmental conditions were targeted in the animal room from the day of arrival until necropsy:
Temperature: 20°C with a range of 18.7°C-22.7°C
Humidity: 52% with a range of 44-95%
Air Changes: 10-15 changes/hour
Photoperiod: 12-hour light/dark (on at 6:00 a.m. and off at 6:00 p.m.)
Rabbits were then transferred to room 162; 53.3% humidity with a range of 44-61% (hand-recorded humidity remained in normal range of 43-55% as well) was recorded.
Enrichment: Enrichment upon arrival until test material administration included an elevated resting platform, a variety of stainless steel objects attached inside the cage, and a cardboard tray for manipulation.
Enrichment after test material administration included stainless steel metal spoons attached inside the cage.
Randomization and Identification: The rabbits were stratified by body weight and/or patency/viability of the VAP and then randomly assigned to treatment groups using a computer program. Animals were received from the vendor with an alphanumeric identification tattoo.
Feed and Water: Upon receipt, rabbits received approximately 62.5 g of LabDiet 5325 (PMI Nutrition International, St. Louis, Missouri, USA) in pelleted form. The amount of feed was increased up to the full daily amount of approximately 125 g the following day. The rabbits were not fasted prior to dosing.
Analyses of the feed were performed by PMI Nutrition International to confirm the diet provided adequate nutrition and to quantify the levels of selected contaminants.
Municipal water was supplied to all study animals ad libitum throughout the study.

Administration / exposure

Route of administration:
oral: gavage
unchanged (no vehicle)
Details on exposure:
Oral Dose Administration: The dose was administered via a ball-tipped gavage needle attached to a syringe. Animals from Group A (on GD 19) and Groups 1-2 (on GD 26), received a single oral dose. The volume of 14C-radiolabelled dose or 12C-non-radiolabelled test material administered to each animal was calculated based on the body weight taken on the day of dose administration. The actual amount of dose administered to each animal was determined by weighing the dose syringe before and after dose administration.
Dose Levels and Justification: A dose of 450 mg/kg bw was used in this study which was expected to be adequate to provide sufficient levels in plasma that were easily detected by chemical analysis.
Dose Preparation: Appropriate amounts of 14C-labeled and/or non-radiolabeled D6 were used to obtain the target dose of 450 mg D6/kg body weight and the target radioactivity of ~400 µCi/kg bw. Radioactivity in the dose was quantified by LSS. Due to the limited amount of radioactive test material available, the dose was only able to be made at 78.2% of target radioactivity.
Duration and frequency of treatment / exposure:
Group 1 received a single neat oral dose of a mixture of 14C-D6 (radiolabelled) and D6 (non-radiolabelled). Group A and Group 2 received a single oral dose of non-radiolabelled D6.
Doses / concentrationsopen allclose all
Dose / conc.:
450 mg/kg bw/day
Group A. 12C-Time-course: 1 rabbit
Dose / conc.:
450 mg/kg bw/day
Group 1. 14C-Time-course: 2 rabbits
Dose / conc.:
450 mg/kg bw/day
Group 2. 12C-Time-course: 1 rabbit
No. of animals per sex per dose / concentration:
Group A: 1 rabbit
Group 1: 2 rabbits
Group 2: 1 rabbit
Details on study design:
This study consisted of three groups of rabbits orally gavaged with either 12C-D6 (non-radiolabelled) or 14C-D6 (radiolabelled). A single rabbit administered 12C-D6 (Group A) was used only for analytical method development. Group 1, which had one pregnant and one non-pregnant rabbit, consisted of a blood, plasma and red blood cell (RBC) 14C concentration-time course which determined pharmacokinetics. In addition, the absorption, metabolism and elimination of 14C-D6 were also determined. The last rabbit (Group 2) was administered 12C-D6 and plasma samples were obtained, but these contingency samples were not analyzed.

Homogeneity of Dose Preparation: LSS analysis of aliquots of the 14C-labeled dose preparation taken from various locations (top, middle and bottom) was used to confirm the concentration of radioactivity and the homogeneity. The respective dose preparation for Group 1 had radiochemical homogeneity determined (via LSS) prior to administration.
Stability: Stability was not conducted as it was administered neat.
Solubility: Solubility was not applicable to this study due to utilizing neat test material
Details on dosing and sampling:
The rabbit of Group A (n=1) was given a single oral dose of 12C-D6 and blood collected via the Vascular access port (VAP) from the same time-points as Group 1-2, centrifuged and the plasma placed in glass vials and shipped for parent analysis.
The rabbits of Group 1 (n=2) were given a single oral dose of 14C-D6 and 12C-D6 and a blood, plasma and RBC 14C-activity concentration-time course was determined. Approximately 1-mL blood was individually collected via the VAP at chosen times (0.08, 0.17, 0.25, 0.5, 1, 2, 3, 6, 12, 24, and 48 hours post-dosing). From an aliquot of blood, RBC and plasma were prepared by centrifugation. Another aliquot of blood and RBC was oxidized and the blood, plasma and RBC analyzed for radioactivity via liquid scintillation spectrometry (LSS). Moreover, as a larger blood volume was obtained from each time point interval, blood that was not immediately analyzed for radioactivity was centrifuged to obtain plasma, placed in a glass vial and shipped for parent analysis.
Finally, one rabbit (Group 2) was given a single oral dose of 12C-D6 and blood collected via the VAP from the same time-points as Group 1, was centrifuged and the plasma placed in glass vials and stored at -80ºC, but were not shipped or analyzed.
Descriptive statistics were used, i.e., mean ± standard deviation, when applicable. All calculations in the database were conducted using Microsoft Excel (Microsoft Corporation, Redmond, Washington) spreadsheets and databases in full precision mode (15 digits of accuracy). Certain pharmacokinetic parameters were calculated for blood data, including AUC (area-under-the-curve), Cmax, ½Cmax, and elimination rate constants, using a pharmacokinetic computer modeling program, PK Solutions (v.2.0.6., Summit Research Services, Montrose, Colorado).

Results and discussion

Preliminary studies:
There were no signs of toxicity observed in any animals following oral administration of 450 mg 14C-D6/kg bw or 12C-D6/kg bw.
Main ADME resultsopen allclose all
D6 derived-radioactivity was metabolized (based on the AUC data between the radioactivity in plasma and parent D6, it is estimated that 46% and 22% of the radioactivity is due to metabolites, in the pregnant and non-pregnant rabbits, respectively).
other: Elimination
D6 derived-radioactivity was eliminated in the faeces with 56% of the administered dose recovered within 48 hours post-dosing.

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Absorption rates could not be calculated due to low levels of radioactivity in plasma samples.
Details on distribution in tissues:
Carcass and tissues were not analyzed for radioactivity.
Details on excretion:
2% of the administered dose was absorbed (and excreted in the urine). Rabbits excreted 56% of the radiolabelled dose in the feces and since rabbits are known to ingest their night feces, there is the possibility that D6 blood levels reflected the re-ingestion of D6.
Toxicokinetic parametersopen allclose all
Key result
Toxicokinetic parameters:
Cmax: 48 hours (last blood collection),
slowly absorbed
Key result
Toxicokinetic parameters:
AUC: based on comparison of 14C-activity and parent plasma AUCs and 2% of the dose in urine
Key result
Toxicokinetic parameters:
other: eliminated in the feces with 56% of the administered dose recovered within 48 hours post-dosing

Metabolite characterisation studies

Metabolites identified:
Details on metabolites:
Identification of metabolites was not possible due to low levels of radioactivity in plasma samples.

Any other information on results incl. tables

 At GD 19 (Group A rabbit), the Cmaxfor parent D6 was determined to be 6 hours post-dosing and levels dropped off at 12-48 hours post-dosing. Based on Group A rabbit results, it appeared that steady-state would be reached in the time frame of the next study (i.e. developmental range finder with estimation of KMD).

Whole blood, RBC and plasma 14C-activity time-courses were determined for Group 1 animals dosed on GD26. Excretion in urine and feces was followed up to 48 hours. The 14C-activity concentrations (µg-eq/g) in blood, plasma and RBC for the two rabbits dosed with 14C-D6 were still increasing at 48 hours (GD 28) post-dosing. The plasma time course of parent D6 concentrations followed the time course of radioactivity concentrations on GD 26-28.

Orally administered 14C-D6 was slowly absorbed. The maximum blood, plasma or RBC concentration (Cmax) was achieved at 24 or 48-hours post-dosing. All three sample matrices plateaued or were increasing, thus absorption and elimination rates could not be calculated. Concentrations of time-course radioactivity in whole blood were similar to that detected in plasma and RBC. The AUC of the radioactivity in the non-pregnant rabbit was lower in whole blood than in plasma, but similar in the pregnant rabbit. The14C-activity AUC was ~2 to 2.5-fold less in the pregnant rabbit vs. the non-pregnant rabbit in both the plasma and blood, but equivalent in the RBC.

Based only on urine/rinse data, 2% of the administered dose was absorbed (and eliminated in the urine). A mass balance was not conducted in this study since carcass and tissues were not analysed for radioactivity; therefore the total percent absorbed in the study cannot be determined. Feces contained the majority of the administered dose at ~56%. Since rabbits are known to ingest their night feces, there is the possibility that D6 blood levels reflected the re-ingestion of D6. The % administered dose recovered in the urine and feces were similar between the pregnant and non-pregnant rabbits in Group 1.

Parent D6 was detected in Group 1 plasma samples with levels above the limit of quantitation (LOQ) beginning at 2 hours until sacrifice at 48 hours post-dosing. In the Group 1 rabbit plasma samples, parent D6 follows the same pattern as radioactivity, where the highest concentration was at 48 hours.  The radioactivity in the collected Cmax plasma samples was low and profiling and metabolite identification was not conducted. Based on the Group 1 rabbit D6 blood concentration time course, steady-state would not be reached in 20 days of a repeated dose study.

Comparison of the plasma time-course for14C-activity and parent D6 gives insight that metabolism is occurring. Plasma parent D6 AUCs were calculated and compared to14C-activity AUCs and the difference in these AUCs for each animal suggest metabolism. With only two animals in Group 1, with one being pregnant and the other non-pregnant, it is difficult to draw conclusions regarding whether metabolism differs with pregnancy status. 


Dose (Concentration, Administration and Recovery of the Administered Dose:

The concentration of total test material in each dose preparation was within 102-110% of the target concentration. The concentration of radioactivity in the dose preparations for Group 1 was 75.3% of the target radioactivity.


At the time of test material administration, the body weights of the female rabbits averaged 3.2 kg.  The administered dose of 12C-D6 with or without 14C-D6 administered to various groups ranged from 460-494 mg/kg bw for the targeted 450 mg/kg bw dose.  A mean range of 338-340 µCi/kg bw (or ~1097-1143 µCi per rabbit) was administered to Group 1 rabbits of the 450 mg/kg bw oral dose groups.  The actual radioactivity administered to rabbits through oral dosing was slightly less than the targeted dose range of ~400 µCi/kg bw.  The difference between the target and actual doses administered had no effects on the results of this study.

Applicant's summary and conclusion

In conclusion, D6 derived-radioactivity was: 1) slowly absorbed and Cmax was at 48 hours (last blood collection), 2) metabolized (based on comparison of 14C-activity and parent plasma AUCs and 2% of the dose in urine) and 3) eliminated in the feces with 56% of the administered dose recovered within 48 hours post-dosing. Absorption and elimination rates could not be calculated and plasma profiling of metabolites was not possible due to low levels of radioactivity in plasma samples.