Diquat dibromide

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

18 Jan 2011 to 16 Jul 2011

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

EPA OPPTS 835.2120 (Hydrolysis of Parent and Degradates as a Function of pH at 25°C)

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Version / remarks:

adopted 13th April 2004

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Radiolabelling:

yes

Analytical monitoring:

yes

Details on sampling:

The definitive sampling regime is summarised in Table 1 in 'Any other information on materials and methods incl tables'.

Buffers:

BUFFER SOLUTIONS
Hydrolysis was performed at pH 4, 7 and 9. The buffer solutions used can be found in Table 2 in 'Any other information on materials and methods incl. tables'.

ANALYSIS OF BUFFER SAMPLES
After incubation, each unit was removed from the water bath and cooled to room temperature. The cap was removed and the contents were transferred to a pre-weighed vial. HPLC water (1 mL) was added to the incubation unit to wash the unit and this was combined with the buffer sample. Aliquots (50 µL) were removed for analysis by LSC. An aliquot of each sample was analysed by HPLC and selected samples were also analysed by TLC.

Details on test conditions:

TEST SUBSTANCE STOCK SOLUTIONS
A [14C]-substance dibromide stock solution in acetonitrile (35%) and methanol (65%) was available at a concentration of 3.778 mg/mL (Stock solution 1). The concentration of the test substance cation in stock solution 1 was 2.02 mg/mL. The non-radiolabelled treatment solution (application solution 2 in sterilised water) was prepared by weighing 1.75 mg of the test substance into a vial and adding water (0.73 mL) to give a concentration of 2.4 mg/mL (the test substance ion concentration was 1.29 mg/mL).

STANDARD REFERENCE COMPOUNDS
The substance dibromide was supplied by the Sponsor on 2 November 2009 for the identification by thin layer chromatography (TLC) and high performance liquid chromatography (HPLC). Standard reference compounds were used for the verification of the presence of M1, M2, M3, M4 by HPLC and TLC. Compounds were supplied by the Sponsor (along with a structural chemistry interpretation report). After receipt at Covance, they were stored at 2 – 8°C in the dark.

SOLVENTS AND REAGENTS
Reagents and solvents were of analytical grade (or a suitable alternative). The buffer solutions were prepared using commercially available bottled water of known chemical characteristics.

STUDY DESIGN
The experimental design is summarised in Table 3 in 'Any other information on materials and methods incl. tables'.

PREPARATION OF TEST SYSTEMS
The pH of the bulk buffer solutions was adjusted at 50 °C with a calibrated pH meter (precision at least 0.1 pH units) to pH values of 4.0 ± 0.1, 7.0 ± 0.1, and 9.0 ± 0.1. The buffers were deoxygenated by sonication, followed by bubbling nitrogen through for ca 5 mins. Buffer (3 mL) was transferred to glass vials. The vials were sealed with PTFE-lined crimp caps and sterilised by autoclaving. The pH before incubation was determined using two control units at each pH and was within ± 0.1 pH units. The pH after incubation (5 DAT) was within ± 0.2 pH units, for details refer to Table 4 in 'Any other information on materials and methods incl. tables'. The pH values were measured at 50 °C. The sterility of the buffer solutions was determined by the analysis of two solutions at each pH value at the end of the incubation period. At the end of the test, samples were confirmed to be sterile.

RADIOCHEMICAL PURITY
The radiochemical purity of the application solution was determined before and after application by HPLC.

PREPARATION OF APPLICATION SOLUTIONS
Application solutions of [14C]-the substance dibromide (main test samples) and of non-radiolabelled the substance dibromide (sterility and post incubation pH samples) were prepared as described below. An aliquot of the [14C]-the substance stock solution (Stock Solution 1, 720 µL) was added to a vial and the solvent was removed under a gentle stream of sterilised air. HPLC water (1.12 mL) was then added to the vial. This treatment solution (application solution 1) had a concentration of 2.27 mg/mL (6.627 MBq/mL) by LSC. The concentration of the substance ion in application solution 1 was 1.22 mg/mL. An application solution of non-radiolabelled the substance was prepared by dissolving the solid the substance dibromide (1.75 mg) in sterilised HPLC water (0.73 mL) to give a concentration of 2.4 mg/mL. The solution was mixed by sonication. The concentration of the substance ion in application solution 2 was 1.29 mg/mL. Preparation of both solutions was performed under sterilised conditions.

APPLICATION OF [14C]-SUBSTANCE IN MAIN TEST
For each incubation condition, an aliquot of treatment solution was added by syringe into each of six sterilised glass vials containing buffer. A further four vials per incubation condition were treated with an equivalent concentration of non-radiolabelled test substance. [14C]-Substance dibromide (application solution 1) was added drop wise (26 µL) to units A1-A6 (pH 4), B1-B6 (pH 7) and C1-C6 (pH 9). On each occasion a steret was used to swab the needle and septum in order to maintain sterile conditions. The application solution (application solution 2, 25 µL) was added in the same manner as the radiolabelled treatment solution, to units, A9-A12, B9-B12 and C9-C12. Units were maintained at 50 ± 0.5°C in a water bath. A7-8, B7-8 and C7-8 were not treated but analysed for pH prior to treatment of main test samples. Triplicate aliquots (26 µL) of the treatment solution were transferred to 10 mL volumetric flasks pre- and post-application (6 samples in total). The volume in each flask was made up to the mark with HPLC water. Triplicate aliquots (50 µL) of each solution were quantified by LSC to check the homogeneity of the treatment solution and to determine the exact application rate.

INCUBATION, MONITORING OF TEMPERATURE AND PH MEASUREMENT
Samples were placed in a water bath maintained at 50 ± 0.5 ºC, in the dark. The temperature was monitored and values periodically recorded throughout the incubation period. Buffer pH was checked after autoclaving, buffer pH and sterility were further checked after incubation using the four units treated with non-radiolabelled test substance.

Duration:

5 d

pH:

4

Temp.:

50 °C

Initial conc. measured:

20 mg/L

Duration:

5 d

pH:

7

Temp.:

50 °C

Initial conc. measured:

20 mg/L

Duration:

5 d

pH:

9

Temp.:

50 °C

Initial conc. measured:

20 mg/L

Number of replicates:

2

Positive controls:

yes

Remarks:

Positive controls comprised broth cultures of Escherichia coli and Staphylococcus aureus at concentrations of 1E-05 and 1E-06 cells per mL.

Negative controls:

yes

Remarks:

Phosphate buffered saline

Preliminary study:

The test substance was considered to be hydrolytically stable at pH 4, 7 and 9 as < 10% of parent degraded when compared to the study initiation. The details are described below.

Transformation products:

yes

No.:

#1

No.:

#2

No.:

#3

No.:

#4

Details on hydrolysis and appearance of transformation product(s):

An overview of the results is provided in Table 2 - Table 4 in 'Any other information on results incl. tables'.

RADIOACTIVE RESIDUES IN AQUEOUS SAMPLES
The substance represented ≥ 95% of the sample radioactivity for pH 4 and 7 buffer samples during the incubation period. In the case of the pH 9 buffer, the difference between the levels of the substance at study initiation (95.7%) and at 5 DAT (88.3%) was 7.4%. As this was less than 10%, the substance was considered to be hydrolytically stable at this pH. Nevertheless, further attempts were made to identify the degradation products. Two degradation products, #1 and Unknown A were present at up to 1.7 and 6.7 % of the sample radioactivity in pH 9 buffer at 5 DAT, respectively. Unknown A was analysed further by LC-MS and was found to be a mixture of two components with mass ions of m/z 171 and 183. Ion m/z 171 was confirmed to be transformation product #3 and ion m/z 183 had an elemental composition consistent with C12H10N2. Transformation product #2 was also confirmed to be present by LC-MS but this was only present at less than the HPLC limit of detection (< 0.3%). HPLC chromatograms and confirmatory TLC radioluminograms showed a single zone of radioactivity corresponding to the substance, thereby confirming its identity. A degradation product corresponding to Unknown A detected by HPLC was also observed in the pH 9 buffer at 5 DAT.

CONFIRMATION OF THE SUBSTANCE AND IDENTIFICATION OF HYDROLYSIS PRODUCTS
- The substance was identified at each pH by HPLC and TLC co-chromatography.
- Transformation product #1was identified at pH 9 by HPLC and TLC co-chromatography. Structural confirmation of #1 was obtained by LC-MS.
- Transformation product #2 was present as a very minor metabolite, less than the limit of detection of initial HPLC. Structural confirmation of #2 was obtained by LC-MS at pH 9.
- Unknown A was detected at pH 9 by HPLC. This peak was found to have two mass ions of m/z 171 and 183. Ion m/z 171 was confirmed to be transformation product #3 and ion m/z 183 had an elemental composition consistent with C12H10N2 (#4).

% Recovery:

94.2

pH:

4

Temp.:

50 °C

Duration:

0 d

Remarks on result:

hydrolytically stable based on preliminary test

% Recovery:

96.4

pH:

4

Temp.:

50 °C

Duration:

5 d

Remarks on result:

hydrolytically stable based on preliminary test

% Recovery:

98.8

pH:

7

Temp.:

50 °C

Duration:

0 d

Remarks on result:

hydrolytically stable based on preliminary test

% Recovery:

99.3

pH:

7

Temp.:

50 °C

Duration:

5 d

Remarks on result:

hydrolytically stable based on preliminary test

% Recovery:

99

pH:

9

Temp.:

50 °C

Duration:

0 d

Remarks on result:

hydrolytically stable based on preliminary test

% Recovery:

99.5

pH:

9

Temp.:

50 °C

Duration:

5 d

Remarks on result:

hydrolytically stable based on preliminary test

Key result

Remarks on result:

hydrolytically stable based on preliminary test

Remarks:

at pH 4, 7 and 9

Details on results:

MASS BALANCE
The results are shown in Table 1 in 'Any other information on results incl. tables'.

RADIOCHEMICAL PURITY
The radiochemical purity value for [14C]-substance dibromide in the treatment solution, determined by HPLC, pre- and post-application ranged from 95.3% to 96.5%.

TREATMENT RATE
The application rate for all hydrolyses ranged from 10.5 to 10.7 µg/mL (substance ion) and 19.7 µg/mL to 20.0 µg/mL (test substance). The treatment solution was shown to be homogenous during application (coefficient of variation = 1.28%)

TEMPERATURE THROUGHOUT MEASUREMENTS
Throughout the study the measured temperatures were within ±0.5 ºC of 50ºC. pH checks carried out immediately before application and following the incubation period confirmed the pH of the solutions as 4, 7 and 9 (± 0.2). See Table 4 in “Any other information on materials and methods incl tables”.

STERLITY
No colonies were observed on any of the nutrient agar plates, therefore, the sterility of the buffer solutions was confirmed. Colonies were observed on the positive control plates, confirming the validity of the testing method.

ADSORPTION TO VESSELS
The majority of the applied radioactivity results (> 94% AR; see Table 1 in 'Any other information on results incl tables'), was recovered in the aqueous solution. This indicated that adsorption of the test compound to the vessel wall was negligible. Based on these results the application method was considered to be acceptable.

Table 1. Percentage Recovery of Radioactivity in Buffer Solutions Incubated at 50°C

	pH 4 buffer				pH 7 buffer				pH 9 buffer
	0 DAT		5 DAT		0 DAT		5 DAT		0 DAT		5 DAT
	A1	A2	A3	A4	B1	B2	B3	B4	C1	C2	C3	C4
Ext 1 (buffer)	95.0	93.4	96.7	96.1	98.8	98.8	99.1	99.5	99.1	98.8	99.3	99.7
	94.2		96.4		98.8		99.3		99.0		99.5

Mean values are in bold font

 

Table 2. Recovery and Quantification of Radioactive Components in Buffer Solutions Incubated at 50 °C

	pH 4 buffer				pH 7 buffer				pH 9 buffer
	0 DAT		5 DAT		0 DAT		5 DAT		0 DAT		5 DAT
	A1	A2	A3	A4	B1	B2	B3	B4	C1	C2	C3	C4
Substance (parent)	94.5	95.4	98.1	97.4	95.5	95.3	98.7	98.5	96.0	95.5	87.6	89.0
	94.9		97.7		95.4		98.6		95.7		88.3(1)
M4	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	2.1	1.2
	ND		ND		ND		ND		ND		1.7
Unknown A	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	6.4	7.0
	ND		ND		ND		ND		ND		6.7
Total minor unknowns	4.9	4.0	1.7	2.3	3.9	3.8	0.9	1.0	3.0	3.7	3.1	1.9
	4.5		2.0		3.8		1.0		3.4		2.5
Unresolved background	0.6	0.6	0.2	0.3	0.7	0.9	0.5	0.5	1.0	0.8	0.9	0.9
	0.6		0.3		0.8		0.5		0.9		0.9
Total	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0
	100.0		100.0		100.0		100.0		100.0		100.0
Largest other unknown	1.7	1.7	1.2	1.5	2.1	2.2	0.8	0.8	0.8	0.8	1.4	0.8
	NA		NA		NA		NA		NA		NA

ND = Not Detected NA = Not Applicable

All values are as % of total HPLC Chromatogram

(1) At pH 9, the percentage of parent substance degraded was 95.7% – 88.3% = 7.4% and the degradation components comprised of a total of 8.4% (1.7% + 6.7%), the remainder are minor unknowns and unresolved background totalling 3.4%. Unknown A was composed of at least two components.

 

Table 3. Concentration (mg/L) in Terms of the substance cation of components in Buffer solution incubated at 50 ºC

	pH 4 buffer				pH 7 buffer				pH 9 buffer
	0 DAT		5 DAT		0 DAT		5 DAT		0 DAT		5 DAT
	A1	A2	A3	A4	B1	B2	B3	B4	C1	C2	C3	C4
Substance (parent)	9.971	10.063	10.348	10.271	10.071	10.058	10.408	10.387	10.128	10.074	9.242	9.393
	10.017		10.309		10.065		10.398		10.101		9.317
M4	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	0.217	0.131
	ND		ND		ND		ND		ND		0.174
Unknown A	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	0.673	0.739
	ND		ND		ND		ND		ND		0.706
Total minor unknowns	0.520	0.423	0.183	0.244	0.410	0.396	0.090	0.110	0.319	0.394	0.323	0.198
	0.471		0.213		0.403		0.100		0.357		0.260
Unresolved background	0.060	0.064	0.018	0.035	0.069	0.095	0.052	0.052	0.102	0.081	0.094	0.091
	0.062		0.027		0.082		0.052		0.092		0.092
Total	10.551	10.549	10.549	10.550	10.550	10.549	10.550	10.549	10.549	10.549	10.549	10.551
	10.550		10.550		10.550		10.550		10.549		10.550
Largest other unknown	0.176	0.184	0.128	0.160	0.222	0.236	0.083	0.087	0.081	0.084	0.152	0.085
	NA		NA		NA		NA		NA		NA

ND = Not Detected NA = Not Applicable mg/L ≡ ppm

 

Table 4. Comparison of HPLC and TLC analysis of selected samples

pH	Unit code	Timepoint (DAT)	Sample radioactivity (%) as substance cation
			By HPLC	By TLC
4	A3	5	98.3	99.5
7	B3	5	98.8	99.4
9	C3	5	88.2	90.1

Validity criteria fulfilled:

yes

Conclusions:

The substance was found to be hydrolytically stable in buffer solutions at pH 4, 7 and 9 at 50 ± 0.5 °C based on the fact that less than 10% of the applied substance was hydrolysed in line with OECD guideline 111. Hydrolysis of the substance is therefore, not expected to be a significant process under environmental conditions.

Executive summary:

The hydrolysis of [14C]-substance cation at 11 mg/L (20 mg/L registered test substance) was studied in the dark in sterile, aqueous buffered solutions at pH 4 [0.05M potassium phthalate], pH 7 [0.05M potassium dihydrogen orthophosphate] and pH 9 [0.05M sodium tetra-borate] at 50 °C for 5 days. Duplicate samples from each pH were analysed at study initiation and after 5 days incubation at 50 °C. The aqueous solutions were analysed directly by liquid scintillation counting and high performance liquid chromatography with radio-detection. Selected samples were also analysed by thin layer chromatography (TLC) to confirm the identity of the substance and degradation products.The mean radioactivity balance was 98.4% (range 96.1% to 99.7%) for the 5 DAT test samples. The major component in all three buffers (pH 4, 7 and 9) was determined to be the parent, which represented 87.6 to 98.7% of the sample radioactivity. The substance was stable at pH 4 and 7 with some degradation occurring at pH 9. The substance was considered to be hydrolytically stable at pH 4, 7 and 9 as < 10% of parent degraded when compared to study initiation. Two degradation products, #1 and Unknown A (identified by further LC-MS analysis as a mixture of compound #3 and another substance #4 of elemental composition C12H10N2) were present at up to 1.7 and 6.7 % of the sample radioactivity in pH 9 buffer at 5 DAT, respectively. Transformation product #2 was also confirmed to be present by LC-MS but this was only present at less than the HPLC limit of detection (< 0.3%). The temperatures remained constant throughout the incubation period (50 ± 0.5 °C) and there was no significant variation in the pH values of the buffered solutions. The samples also remained sterile throughout the study. The substance was shown to be hydrolytically stable under all the conditions tested in this study.

Description of key information

pH 4 at 50°C less than 10% degradation

pH 7 at 50°C less than 10% degradation

pH 9 at 50°C less than 10% degradation (after 5 days)

Thus, the test substance is hydrolytically stable under all tested conditions; OECD TG 111; Dixon 2012

Additional information

The hydrolysis of [14C]-substance cation at 11 mg/L (20 mg/L registered test substance) was studied in the dark in sterile, aqueous buffered solutions at pH 4 [0.05M potassium phthalate], pH 7 [0.05M potassium dihydrogen orthophosphate] and pH 9 [0.05M sodium tetra-borate] at 50 °C for 5 days. Duplicate samples from each pH were analysed at study initiation and after 5 days incubation at 50 °C. The aqueous solutions were analysed directly by liquid scintillation counting and high performance liquid chromatography with radio-detection. Selected samples were also analysed by thin layer chromatography (TLC) to confirm the identity of the substance and degradation products.The mean radioactivity balance was 98.4% (range 96.1% to 99.7%) for the 5 DAT test samples. The major component in all three buffers (pH 4, 7 and 9) was determined to be the parent, which represented 87.6 to 98.7% of the sample radioactivity. The substance was stable at pH 4 and 7 with some degradation occurring at pH 9. The substance was considered to be hydrolytically stable at pH 4, 7 and 9 as < 10% of parent degraded when compared to study initiation. Two degradation products, #1 and Unknown A (identified by further LC-MS analysis as a mixture of compound #3 and another substance #4 of elemental composition C12H10N2) were present at up to 1.7 and 6.7 % of the sample radioactivity in pH 9 buffer at 5 DAT, respectively. Transformation product #2 was also confirmed to be present by LC-MS but this was only present at less than the HPLC limit of detection (< 0.3%). The temperatures remained constant throughout the incubation period (50 ± 0.5 °C) and there was no significant variation in the pH values of the buffered solutions. The samples also remained sterile throughout the study. The substance was shown to be hydrolytically stable under all the conditions tested in this study.