Morpholine

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1983

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Objective of study:

distribution

metabolism

Qualifier:

no guideline available

Principles of method if other than guideline:

The in vivo and in vitro metabolism of Morpholine was assessed through the investigation of the subcellular binding interactions, the generation of nitrosamines during Morpholine metabolism, biologically active Morpholine metabolites in a UDS assay, the biochemical basis for species differences noted in Morpholine metabolism, and the differences in human, rat, hamster and guinea pig liver capacity for Morpholine metabolism.

GLP compliance:

not specified

Radiolabelling:

yes

Species:

other: Sprague Dawley rats, strain II guinea pigs, human liver biopsy samples

Strain:

not specified

Sex:

not specified

Details on test animals or test system and environmental conditions:

Please refer to "Any other information on materials and methods incl. tables"

Route of administration:

intraperitoneal

Vehicle:

not specified

Details on exposure:

Please refer to "Any other information on materials and methods incl. tables"

Duration and frequency of treatment / exposure:

Study 1: single injection
Study 2: single injection
Study 3: single administration
Study 4: not applicable (analytical study)
Study 5: single injection
Study 6: not applicable

Remarks:

Doses / Concentrations:
Study 1: 125 mg/kg bw (50 or 100 µCi/animal)
Study 2: 200 µCi/animal
Study 3: 50 or 200 µCi/animal
Study 4: not applicable (analytical study)
Study 5: no data
Study 6: not applicable

No. of animals per sex per dose / concentration:

no data

Control animals:

yes

Positive control reference chemical:

no

Details on study design:

No rationale for dose level selection was provided.

Details on dosing and sampling:

Please refer to "Any other information on materials and methods incl. tables"

Statistics:

no data

Details on distribution in tissues:

1. Uniform distribution of 14C in TCA-insoluble fractions indicated a non-specific binding and/or incorporation of Morpholine. No significant amounts of covalently bound 14C were detected in the subcellular fractions of the liver.

2. Very little, if any, 14C was bound to liver DNA.

In this report including in vivo and in vitro metabolism studies, the metabolism of Morpholine was assessed through the investigation of subcellular binding interactions, nitrosamines generation as part of Morpholine metabolism, biologically active Morpholine metabolites in a UDS assay, the biochemical basis for species differences noted in Morpholine metablism, and the differences in human, rat, hamster and guinea pig liver capacity for Morpholine metabolism.

A uniform distribution of 14C in TCA-insoluble fractions indicated a non-specific binding and/or incorporation of Morpholine. No significant amounts of covalently bound 14C were detected in the subcellular fractions of the liver. Very little, if any, radioactivity derived from 14C Morpholine was bound to liver DNA. Therefore, it was concluded that the TCA-insoluble macromolecules of the nuclear fraction probably reflected the 14C associated with proteins and/or RNA. The results did not provide evidence for the in vivo formation of N-nitrosomorpholine after administration of Morpholine under the conditions described. In vivo methylation of Morpholine (step 1 in the metabolism process) proceeded via the SAM pathway. Guinea pig liver microsomes showed the most metabolic activity in vitro. Ziegler's flavin monooxygenase was exlusively involved in the N-oxidation of N-methylmorpholine and at least partially so in the N-hydroxylation of Morpholine and was evidence against the involvement of cytochrome P450 in these reactions. Of the species examined, human liver most resembled that of the rat in its ability to metabolize Morpholine.Morpholine was not bound to serum proteins.

This metabolism study is classified as acceptable.

1. In the initial part of this study, the amount of 14C in the tissue macromolecules, expressed as µg eq. of Morpholine/g tissue, remained rather constant in all organs throughout a 24 hour period. In contrast, the total radioactivity determined in the whole tissue homogenates (representing bound plus unbound forms) declined rapidly with time. Since a considerable amount of 14C was found to be present in fractions from the liver and kidney homogenates, a second phase was conducted to determine whether there was preferential uptake in some particular subcellular fraction. While cytosol contained the highest level of 14C (expressed in terms of ng eq. Morpholine/µg protein) in both the liver and kidney, when 14C was determined in various subcellular fractions no signficant differences among these were detected.

2. The specific activity of the DNA was 31 and 29 dpm per mg DNA for 4 and 24 hours, respectively. The U.V. absorption monitored at 260 nm clearly demonstrated the presence of guanine and adenine, eluted at 6 and 12 mL and pyrimidine oligonucleotides eluting between 2 and 5 mL, respectively. However, no significant radioactivity associated with either peak or at any other elution volume were detected.

3. In no case was any radioactivity above background found to be associated with N-nitrosomorpholine peak in either HPLC system.

4. Not applicable.

5. When N-methylmorpholine-N-oxide was isolated from the urine (in vivo), 10.5 % of the radioactivity of the L-[methyl-14C]-methionine administered was found to be incorporated in the metabolite. When tested in vitro, the initial rate of formation of N-methylmorpholine by guinea pig liver cytosol was approximately 0.13 nmol/min/mg protein, while the rate using hamster liver cytosol was less than 1/10 of this and was almost undetectable using rat liver cytosol. When evaluating the metabolic rates of oxidation of N-methylmorpholine to N-methylmorpholine-N-oxide or of Morpholine to N-hydroxymorpholine, the N-hydroxylase activity was almost undetectable using rat liver microsomes; however, hamster liver microsomes contained significant levels of this enzyme and guinea pig liver microsomes were even more active. When the liver microsome catalyzed N-oxidation of N-methylmorpholine was evaluated, liver microsomes from the guinea pig were more active than those from the hamster, which, in turn, were more active than rat liver microsomes. While the species differences in the rates of N-oxidation were not as marked as those observed in the case of N-methylation or N-hydroxylation of Morpholine, a similar pattern was observed, with liver microsomes from the guinea pig being more active than those from the hamster, which, in turn, were more active than rat liver microsomes. When in vitro incubations were conducted in the presence of methimazole, SKF 525 -A and liver microsomes, SKF 525 -A failed to inhibit N-hydroxylation of Morpholine and the N-oxidation of N-methylmorpholine; however, methimazole inhibited the N-hydroxylation by approximately 60 % and almost completely blocked the N-oxidation of methylmorpholine.

6. Using human liver cytosols, no detectable Morpholine N-methylase activity was found. When the formation of N-methylmorpholine-N-oxide from N-methylmorpholine was monitored using microsomes, human livers were found to be active in carrying out this reaction, though the activity was lower than that observed with rat liver. In addition, studies using enzyme modifiers showed that the reaction was inhibited 70% by methimazole, indicating that the flavin-containing monooxygenase was also involved in the human liver in carrying out this reaction. Determination of Morpholine hydroxylase indicated that the human liver microsomes were able to N-hydroxylate Morpholine, with an activity of approximately 0.008 µmol/mg protein/30 minutes.

Reference 2

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1981

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Objective of study:

distribution

excretion

Qualifier:

no guideline followed

Principles of method if other than guideline:

Following intraperitoneal administration of 125 mg/kg bw (approx. 50 µCi), male Sprague Dawley rats were individually placed in Delmar-Roth glass metabolism cages. The expired air was passed through gas washers and the contents of the gas washers was sampled at appropriate intervals. Urine was collected and selected organs were removed at appropriate intervals for analysis. In a further experiment, the tissue distribution of 14C Morpholine was determined following inhalation exposure to vapour at 75 or 150 ppm for appropriate exposure periods. Analyses were performed by means of chromatography and mass spectrometry.

GLP compliance:

not specified

Radiolabelling:

yes

Species:

rat

Strain:

Sprague-Dawley

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Camm Research, Wayne, New Jersey, USA
- Age at study initiation: young adult (based on body weight)
- Weight at study initiation: 250 to 290 g
- Individual metabolism cages: yes
- Diet: standard NIH-07 diet, ad libitum
- Water: ad libitum

Route of administration:

other: A. intraperitoneal administration for assessment of pulmonary elimination. B. vapour inhalation for assessment of tissue distribution

Vehicle:

other: A. isotonic saline. B. nitrogen (N2)

Details on exposure:

A. Rats dosed with 14C Morpholine (i.p., 125 mg/kg bw, approx. 50 µCi) were individually placed in Delmar-Roth glass metabolism cages. Air, dried and freed of carbon dioxide, was pulled through the cages at 250-300 mL/min by means of peristaltic pumps. The expired air was passed through three gas washers in series, containing respectively, 150 mL of 2M NaOH (two washers) and 0.5 M H2S04 (one washer). The contents of the gas washers were sampled at appropriate intervals and changed at 6, 24, 30 and 48 hrs. Urine was collected in receptacles thermoelectrically cooled at 0°C. Animals were killed at appropriate intervals after administration and selected organs were removed and homogenized for analysis.

B. Animals were placed in Delmar-Roth (glass) metabolism chambers and exposed to 75 ppm or 150 ppm 14C Morpholine. The vapour was generated by passing N2 through 14C-Morpholine at a constant temperature (25°C). The Morpholine vapour was mixed with air at a ratio of approximately 1:30 such that the total air plus N2-Morpholine flowing through the cage was 250 mL/min. The concentration of Morpholine was monitored by gas chromatography. Radioactivity of the vapor was also assayed.

Duration and frequency of treatment / exposure:

A. single injection
B. single exposure for 1 hr, 2 hrs, or 4 hrs

Dose / conc.:

125 mg/kg bw (total dose)

Remarks:

i.p.

Dose / conc.:

75 ppm

Remarks:

vapour

Dose / conc.:

150 ppm

Remarks:

vapour

No. of animals per sex per dose / concentration:

No data

Control animals:

no

Positive control reference chemical:

No data

Details on study design:

No rationale for dose level selection was provided.

Details on dosing and sampling:

After appropriate periods of exposure, the rats were removed from the inhalation chamber and blood samples were obtained from the orbital sinus. The animals were sacrificed, and selected organs (e.g. kidney, spleen, liver, lung) were excised and homogenized. Please refer to "Details on exposure".

Statistics:

Mean values and standard deviations were calculated where appropriate.

Details on distribution in tissues:

The 14C content in various tissues appeared to be concentration-dependent as well as time-dependent for the inhalation exposure . At the concentration of both 75 and 150 ppm, the 14C levels in all tissues increased continuously during 1, 2 and 4 hour(s) exposure, without reaching a plateau, indicating that the organs were not yet saturated with 14C Morpholine.

Details on excretion:

In 72 hours, approximately 0.5 % of the 14C Morpholine administered by a single i.p. injection appeared in the exhaled air as 14C carbon dioxide. This indicated a very low rate of complete metabolism of the chemical in the rat. Negligibly low amounts of radioactivity were found in the H2SO4 absorption towers indicating that virtually none of the Morpholine administered was exhaled.

Metabolites identified:

no

In this toxicokinetic study by Sohn et al. (1981), male Sprague Dawley rats were exposed to 14C Morpholine vapour at 75 or 150 ppm for 1, 2, or 4 hours. Animals were sacrificed and the 14C-content in various tissues was examined. In a further experiment, the amount of exhaled radioactivity was assessed following intraperitoneal administration of 14C Morpholine at 125 mg/kg bw. Results indicated that only about 0.5 % of the dose of labeled Morpholine was exhaled as 14C carbon dioxide, and about 1.5% was excreted in the faeces. The route of administration, intraperitoneal injection or inhalation, did not influence the tissue distribution of Morpholine and in both cases Morpholine levels were highest in the kidneys. This inhalation study dealing in rats is considered as acceptable.

Reference 3

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1982

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Objective of study:

excretion

metabolism

Qualifier:

no guideline followed

Principles of method if other than guideline:

Urinary and plasma clearance, and urinary metabolites were determined in three rodent species following intraperitoneal injection of 14C Morpholine at 125 mg/kg bw (50 µCi/animal). Sample analyses were perfomed by means of chromatography and mass spectrometry.

GLP compliance:

not specified

Radiolabelling:

yes

Species:

other: rats, hamsters, guinea-pigs

Strain:

other: Sprague Dawley rat, Syrian golden hamster, strain II guinea pig

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS

1. male Sprague-Dawley rats
- Source: Camm Research, Wayne, New Jersey, USA
- Age at study initiation: young adult (based on body weight)
- Weight at study initiation: 250 to 290 g
- Individual metabolism cages: yes
- Diet: standard NIH-07 diet, ad libitum
- Water: ad libitum

2. male Syrian golden hamsters
- Source: Sprague Dawley, Indianapolis, Indiana, USA
- Age at study initiation: young adult (based on body weight)
- Weight at study initiation: 95 to 135 g
- Individual metabolism cages: yes
- Diet: standard NIH-07 diet, ad libitum
- Water: ad libitum

3. male strain II guinea pigs
- Source: Charles River, Wilmington, Massachussetts, USA
- Age at study initiation: young adult (based on body weight)
- Weight at study initiation: 300 to 360 g
- Individual metabolism cages: yes
- Diet: rabbit chow, ad libitum
- Water: 0.1 % ascorbic acid in water, ad libitum

Route of administration:

intraperitoneal

Vehicle:

physiological saline

Details on exposure:

The animals received a total dose volume of approximately 1 mL.

Duration and frequency of treatment / exposure:

single treatment

Dose / conc.:

125 mg/kg bw (total dose)

Remarks:

of 14C Morpholine (50 µCi/animal)

No. of animals per sex per dose / concentration:

3 male animals of each species

Control animals:

no

Positive control reference chemical:

no

Details on study design:

No dose selection rationale was provided.

Details on dosing and sampling:

Animals were injected i.p. with 14C Morpholine at 125 mg/kg bw (50 µCi/animal) in 0.9 % NaCl (total volume was approximately 1 mL). The animals
were placed in individual stainless-steel metabolism cages. Blood samples were taken from the orbital sinus with a heparinized capillary while the animals were under light ether anaesthesia. Urine was collected in cooled receptacles. Samples were collected for up to 72 hours following treatment.

Statistics:

Mean values and standard deviations were calculated where appropriate.

Details on excretion:

In all three rodent species, approximately 80 % of the radioactivity was excreted in the urine in 24 hours.

Metabolites identified:

yes

Details on metabolites:

While non-metabolized 14C Morpholine constituted up to 99 % of the urinary radioactivity in the rat and hamster, a significant portion of the dose (approximately 20 %) appeared in guinea pig urine as N-methylmorpholine-N-oxide.

In this toxicokinetic study by Sohn et al. (1982), the blood plasma levels and urinary metabolites of Morpholine were examined in three rodent species: the Sprague-Dawley rat, the Syrian golden hamster and the strain II guinea pig. Marked differences were found between the guinea pig and the other two species with respect to plasma levels and metabolism of Morpholine. After i.p. administration of 125 mg/kg bw 14C Morpholine (50 µCi/animal; 3 males of each species), the blood plasma halflives in the rat, hamster and guinea pig were 115, 120 and 300 min, respectively. In all three species, approximately 80 % of the radioactivity was excreted in the urine in 24 hours. However, while non-metabolized 14C Morpholine constituted up to 99 % of the urinary radioactivity in the rat and hamster, a significant portion of the dose (approximately 20 %) appeared in guinea pig urine as N-methylmorpholine-N-oxide.

This toxicokinetic study in three rodent species is classified as acceptable.

Reference 4

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1978

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Objective of study:

toxicokinetics

Qualifier:

no guideline followed

Principles of method if other than guideline:

Elimination, distribution and metabolism of Morpholine salts in rats were investigated by means of chemical analysis and/or radioassay. Gas-liquid chromatography was used for chemical analysis of Morpholine in the rat urine and faeces. The analytical results of the excreta accorded with those made by the tracer technique.

GLP compliance:

no

Radiolabelling:

yes

Species:

rat

Strain:

Wistar

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Age at study initiation: young adult (based on body weight)
- Weight at study initiation: 200 to 350 g
- Metabolism cages: yes
- Diet: standard diet (CE-2, Japan CLEA, Tokyo, Japan), ad libitum (fasted overnight in the case of peroral administration)
- Water: ad libitum

Route of administration:

other: oral and intravenous

Vehicle:

not specified

Duration and frequency of treatment / exposure:

single treatment

Dose / conc.:

200 mg/kg bw (total dose)

Remarks:

oral (14C-Morpholine), radioassay

Dose / conc.:

400 mg/kg bw (total dose)

Remarks:

oral (14C-Morpholine palmitate), radioassay

Dose / conc.:

150 mg/kg bw (total dose)

Remarks:

intravenous injection (14C-Morpholine), radioassay

Dose / conc.:

500 mg/kg bw (total dose)

Remarks:

oral (Morpholine-HCI), chemical assay

Dose / conc.:

250 mg/kg bw (total dose)

Remarks:

intravenous injection (Morpholine-HCI), chemical assay

No. of animals per sex per dose / concentration:

3 animals

Control animals:

not specified

Positive control reference chemical:

Not indicated

Details on study design:

A rationale for dose selection was not provided.

Details on dosing and sampling:

Elimination study:
Rats were given an oral dose of 200 mg/kg bw of 14C-Morpholine or were treated intravenously at 150 mg/kg bw. 14C-Morpholine palmitate was administered by gavage at 400 mg/kg bw. Morpholine-HCl was dosed as a 5% aqueous solution, and palmitate as a dimethyl sulfoxide solution. Urine and faeces were collected every 24 hours. A water wash of the cages at the end of the experiment was combined with the final day urine. Dose levels of Morpholine-HCI were 500 mg/kg bw for oral administration and 250 mg/kg bw for intravenous injection.

Distribution study:
After dosing wih 14C-Morpholine HCl, the animals were killed at 2, 6 and 12 hour intervals. Organs and tissues were removed, dried in air, powdered and weighed. Portions of the samples were oxidized to carbon dioxide for measuring radioactivity in a scintillator. Only the intestine was solubilized in a 0.5 N sodium hydroxide solution under gentle reflux and an aliquot of the solution was mixed with a dioxane scintillator for counting. The organ-affinity was compared with regard to the level of radioactivity content in terms of RSA (relative specific activity).

Metabolic studies:
The collected urine was extracted with ether or isopropyl ether, and an aliquot of the concentrate was spotted onto thin-layer films, and subsequently developed with several solvents to detect urinary metabolites.

Statistics:

Mean values and standard deviations were calculated.

Details on distribution in tissues:

The distribution of 14C-labeled Morpholine following oral administration (200 mg/kg bw) or i.v. injection (150 mg/kg bw) to 3 animals per group was investigated. The highest level was found in the muscles and intestines.

Details on excretion:

The analytical method by GLC was sufficient to determine free Morpholine in biological fluids. Morpholine salts were rapidly excreted by rats after peroral or intravenous administration. The urinary excretion accounted for most of the dose and a small fraction was found in the faeces. The elimination patterns were essentially similar for Morpholine-HCl and –palmitate. Morpholine was excreted almost unchanged in the rat urine. Ninety percent of the original dose was found in the urine after 3 days and 0.08 to 0.14 % was found in the faeces.

Metabolites identified:

no

Details on metabolites:

Thin-layer chromatograpy of concentrated urine revealed only the existence of 14C-Morpholine which indicated the same Rf values as those of the authentic sample in some solvents. It became clear that Morpholine was mainly excreted unchanged in the urine.

In this toxicokinetic study provided by Tanaka et al. (1977), elimination, distribution and metabolism of Morpholine salts in rats were investigated by means of chemical analysis and/or radioassay. Gas-liquid chromatography was used for chemical analysis of Morpholine in the rat urine and faeces. The distribution of 14C-labeled Morpholine via oral administration (200 mg/kg bw) or i.v. injection (150 mg/kg bw) to 3 male animals per group was investigated. The analytical results of the excreta accorded with those made by the tracer technique. When rats were given Morpholine-HCl or -palmitate, about 90% of the dose was excreted in the urine over a period of 3 days and the remaining in the faeces (0.08 to 0.14 %). Morpholine was largely excreted unchanged in the urine: Ninety percent of the original dose was found in the urine after 3 days. The lowest affinity was found for adipose regardless of routes of administration. The elimination of Morpholine from organs, tissues and blood was generally rapid and specific organ-affinity was not observed in other organs except the intestine. The highest level was found in the muscle and intestine.This toxicokinetic study in the rat is as classified acceptable. Morpholine exhibited a very low bioaccumulation potential. Morpholine appeared to be an inert substance in the body.

Reference 5

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1981

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Objective of study:

distribution

excretion

Qualifier:

no guideline followed

Principles of method if other than guideline:

In each anaesthetised male New Zealand White rabbit, polyethylene catheters were secured in a jugular vein, urinary bladder, and common bile duct for the collection of samples. Approximately 5 mmol/kg bw of Morpholine was injected intravenously, and samples of urine and bile were collected at 20-min intervals for 3 to 4 hours. Blood was collected in the middle of each sampling period. Other rabbits were given similar injections and blood was collected at 1, 2, 3 or 4 hours later by cardiac puncture without anesthesia. Urine was collected from the bladder at 4 hours. Animals used for the clearance studies were prepared in a manner similar to that of the preceding experiments except that the Morpholine was given by continuous intravenous infusion. GFR was measured by the clearance of 3H inulin. Expired air was passed through activated charcoal to extract Morpholine and NaOH to extract carbon dioxide. The binding of Morpholine to serum proteins was evaluated by equilibrium dialysis.

GLP compliance:

no

Radiolabelling:

yes

Species:

rabbit

Strain:

New Zealand White

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Age at study initiation: young adult (based on body weight)
- Weight at study initiation: 2200 to 2500 g

Route of administration:

intravenous

Vehicle:

physiological saline

Details on exposure:

Morpholine was neutralized with HCl, diluted with 0.9% NaCl solution and labeled with 14C Morpholine

Duration and frequency of treatment / exposure:

Single iv injection (bolus) / 30 min, or
Single iv injection (bolus) / 3 to 4 hours

Dose / conc.:

435 mg/kg bw (total dose)

Remarks:

5 mmol/kg bw

No. of animals per sex per dose / concentration:

7 male animals

Control animals:

yes, concurrent vehicle

Positive control reference chemical:

none

Details on study design:

A rationale for dose level selection was not provided.

Details on dosing and sampling:

5 mmol/kg bw of Morpholine was neutralized with HCl, diluted with 0.9% NaCl solution, labeled with 14C Morpholine, and injected intravenously in a volume of 0.5 mL. Blood was taken by cardiac puncture after 30 min, the rabbits were promptly killed with pentobarbital, and tissues were taken for weighing and the determination of the content of radioactivity.
Other rabbits were given similar injections and blood was collected at 1, 2, 3 or 4 hours later by cardiac puncture without anaesthesia. Urine was collected from the bladder at 4 hours. Animals used for the clearance studies were prepared in a manner similar to that of the preceding except that Morpholine was given by continuous intravenous infusion. GFR was measured by the clearance of labeled inulin.

Statistics:

Mean values and standard errors of the mean were calculated.

Details on distribution in tissues:

Morpholine was preferentially distributed to the kidneys. The ratios of the concentration in lung and liver to that of blood were approximately 2, but were greater than 6 for the kidneys, with further preferential uptake by the renal medulla. 60 minutes after intravenous injection of boluses of 5 mmol/kg bw of 14C Morpholine, the concentrations were 5.1 ± 1.0 mmol/kg bw (444 mg/kg bw) in lung tissue, 4.7 ± 0.9 mmol/kg bw (410 mg/kg bw) in the liver, 2.3 ± 0.5 mmol/L (200 mg/L) in the blood, 15.4 ± 2.9 mmol/kg bw (252 mg/kg bw) in the renal cortex and 36.0 ± 5.5 mmol/kg (3136 mg/kg bw) in the renal medulla.

Details on excretion:

The distribution alpha phase was complete by the end of the first hour postinjection. Extrapolation of the alpha phase to time zero yielded a serum concentration of approximately 15 mM Morpholine. The volume of distribution of 12 mmol of Morpholine in a rabbit weighing 2.5 kg was approximately 800 mL. Volumes of distribution in the range of minor fractions of total body volume are characteristic of chemicals of low lipid solubility. The volume of distribution of Morpholine in the rabbit is in the range of the volumes reported for the extracellular space in the rabbit. The t1/2 for the elimination of Morpholine from the serum averaged 3 hours between 2 and 4 hours after injection, and 11 hours from 4 to 28 hours after injection.
An average of 0.6 % of the dose that was administered was recovered as radioactivity that was derived from Morpholine in the bile during the first 3 hours after injection. An average of 43 % of the administered dose was recovered from the urine during the same period. 16 to 21 % of the administered dose was recovered from the control rabbits during the first 4 hours, and 43 to 44 % was recovered from the rabbits whose urine had been acidified by the prior administration of ammonium chloride in the drinking water.
During the course of the infusion the average serum concentration rose from 0.5 to 1.5 mmol/mL. Both the mean renal clearances of Morpholine and of inulin (GFR) decreased during the infusion period. The average ratio of the renal clearance of Morpholine to the GFR remained steady from 1.8 to 2.2 throughout the infusion period. No significant binding of 14C Morpholine to serum proteins was detected by equilibrium dialysis.

Metabolites identified:

no

Details on metabolites:

The urine and bile from 2 rabbits injected with 14C Morpholine were examined for the presence of acid-labile metabolites and were compared with control urine and bile to which 14C Morpholine had been added. About 0.2 % of the radioactivity was extractable into n-heptane prior to hydrolysis as compared to 0.4 % after hydrolysis, the same as for the controls both before and after hydrolysis. Urine was then precipitated with basic lead acetate and no significant amount of radioactivity was found associated with the glucuronides. No radioactivity was recovered from the charcoal through which the expired air was drawn and 0.0008 % of the administered dose of radioactivity was recovered as 14C carbon dioxide. The results indicated that
Morpholine was not significantly metabolized to carbon dioxide through oxidative pathways or conjugated with glucuronic acid.

In this toxicokinetic study by Van Stee et al. (1981), seven male New Zealand rabbits received an intravenous injection of 14C Morpholine at a dose level of 5 mmol/kg bw (435 mg/kg bw). After 30 minutes, the distribution of radioactivity in these animals was determined. Other rabbits were given similar injections and blood was collected at 1, 2, 3 or 4 hours later. Urine was collected from the bladder of these animals at 4 hours. Animals used for the assessment of the renal clearance received a continuous infusion. The glomerular filtration rate was measured by the clearance of labelled inulin. The radioactivity was distributed preferentially to the kidneys, indicating that the kindneys were a significant organ for Morpholine elimination. 63% of the dose was excreted in the bile and 43 % in the urine during the first 3 hours in another set of experiments. Most of the radioactivity excreted was in the form of unchanged Morpholine, which is consistent with the observation in other species that Morpholine is stable in vivo. Morpholine was considered as neither significantly metabolized nor being bound to plasma proteins. The Morpholine clearance exceeded the inulin clearance by a factor of 2, pointing towards the involvement of active tubular transport in renal elimination of Morpholine. Only 0.0008 % of the total dose was eliminated as carbon dioxide by exhalation. This toxicokinetic study in the rabbit is classified as acceptable. There is a low bioaccumulation potential based on study results.

Description of key information

Oral absorption and distribution through-out the body is expected for Morpholine. This is supported by the results of single and repeated-dose toxicity studies in vivo were systemic effects are reported and target organs are identified. Furthermore, available toxicokinetic studies in rat demonstrate that Morpholine is rapidly and quantitatively absorbed after oral administration. Based on the recovery of Morpholine or Morpholine salts in urine following inhalation or oral exposure, absorption is expected to be at least 55 or 90%, respectively. Since Morpholine is corrosive it induces damage to skin and mucous membranes which will enhance dermal penetration and absorption via the respiratory epithelium. Based on a predictive algorithm it was concluded that Morpholine has the potential to be absorbed through the skin and to become available systemically following dermal exposure (NIOSH, 2017). Morpholine is preferentially distributed to the rabbit kidney (Van Stee et al., 1981) and the rat kidney and intestine (Tanaka, 1978). Morpholine has been reported to be resistant to metabolism in the rat (Tanaka et al., 1978; Sohn et al., 1982) and rabbit (Van Stee et al., 1981). In contrast, Sohn et al. (1982) reported that Morpholine is metabolized by N-methylation and N-oxidation in the guinea pig. Morpholine is metabolized to a much smaller extent by N-methylation in the rat and hamster (Sohn et al., 1982). Bioaccumulation of the substance is not expected after continuous exposure. The test substance is primarily excreted unchanged in urine.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Morpholine is a colorless liquid with a molecular weight of 87.12 g/mol. The test item is completely miscible with water. The log Pow is measured to be -0.84 at 25°C (pH 10.3) and -2.55 at 25°C (pH 7). The vapor pressure is 9.8 hPa at 20.3°C. The substance has a measured melting point of -4.9°C and a boiling point of 128.3°C at 1013 hPa.

 

Absorption

Oral

Absorption is a property of a substance to diffuse across biological membranes.Generally, oral absorption is favored for molecular weights below 500 g/mol andlog Pow values between -1 and 4. In the GI tract absorption of small water-soluble molecules (molecular weight up to around 200 g/mol) occurs through aqueous pores or carriage of such molecules across membranes with the bulk passage of water. Based on the physico-chemical properties of the substance, it can be considered as likely that Morpholine becomes bioavailable following the oral route. Oral uptake is confirmed by results of toxikokinetic studies. Following both oral and i.v. administration of Morpholine or Morpholine salts, approximately 90% of the administered dose was absorbed (OECD SIDS, 2013, Tanaka et al. 1978).

Inhalative

Absorption via the respiratory route also depends on physico-chemical properties like vapor pressure, log Pow and water solubility. In general, highly volatile substances are those with a vapor pressure greater than 25 kPa or boiling point below 50°C. Substances with log Pow values between -1 and 4 and moderate water solubility are favored for absorption directly across the respiratory tract epithelium by passive diffusion. Due to the low vapor pressure of 9.8 hPa and boiling point of 128.3 °C only minor availability of vapors for inhalative absorption is anticipated for Morpholine. In a toxikokinetic study on distribution and excretion performed by Tombropoulos (1979), female New Zealand White rabbits were exposed for 5 hours to 905 mg/m³ Morpholine vapor by nose-only exposure. Following the exposure, 50-58% of the total dose was recovered in the urine, indicating that at least 50% of the Morpholine had been absorbed. Rats exposed by inhalation to Morpholine at concentrations of 0, 36, 181 or 543 mg/m³, 6 hours/day, 5 days/week for 104 weeks showed normal growth, survival and hematology and clinical chemistry parameters (Huntsman, 1983). A systemic intoxication was not observed. Findings in this study were limited to local effects of irritation noted during clinical observation and ophthalmoscopic examination, and were confirmed histopathologically.

Local corrosive properties of Morpholine are considered to be more critical after inhalation exposure since damage on mucous membranes will occur which may enhances absorption via the respiratory epithelium.

Dermal

In general, dermal absorption is favored by small molecular weights less than 100 and high water solubility of the substance. For substances with a log Pow value <0 poor lipophilicity will limit penetration into the stratum corneum and hence dermal absorption. Since Morpholine is corrosive, damage to skin will enhance penetration. Shea (1939) assessed the dermal toxicity and skin absorption of Morpholine in one investigation using rabbits. Unneutralized, diluted Morpholine (1 part Morpholine, 2 parts water) was applied at a daily dose of 900 mg/kg bw to the clipped skin. All rabbits (7/7) died before the eleventh dose. Necrosis of the treated skin, and inflammation and congestion of the underlying organs were evident upon gross examination. Microscopic lesions of the liver and effects on kidneys and spleen were observed. This indicated that dermal absorption has occurred. The potential of Morpholine to pose a skin absorption hazard was evaluated by using a predictive algorithm for estimating and evaluating the health hazards of dermal exposure to substances. On the basis of this algorithm, a ratio of the skin dose to the inhalation dose (SI ratio) of 1.76 was calculated for Morpholine. An SI ratio of ≥0.1 indicates that skin absorption may significantly contribute to the overall body burden of a substance; therefore, Morpholine has the potential to be absorbed through the skin and to become available systemically following dermal exposure (NIOSH, 2017).

 

Distribution

In general, the smaller the molecule the wider is its distribution. Small water-soluble molecules will diffuse through aqueous channels and pores in the membranes.After being absorbed into the body, Morpholine is expected to distribute throughout the body water. In repeated dose toxicity studies target tissues like kidneys have been identified which indicates distribution of the substance.

 

Tanaka et al. (1978) determined the distribution of [14C]-labeled Morpholine in male Wistar rats (3 animals/group) after oral (200 mg/kg bw) and intravenous administration (150 mg/kg bw). The radioactivity was determined in the dried, powdered organs. Large amounts of [14C]-labeled Morpholine were only found in muscle and intestine regardless of route of administration. In rats sacrificed 2 hours after oral administration of Morpholine HCl in the same study, 29% of the radioactivity was found in the intestine and 26% in muscle tissue. Similarly, 2 hours after intravenous injections, 19 and 27 % of the dose was found in the intestine and muscle tissue, respectively. Elimination of radioactivity from other organs, tissues and blood was very rapid in cases of both oral and i.v. administration.

Female New Zealand White rabbits were exposed to Morpholine (905 mg/m³) for 5 hours by nose-only inhalation (Tombropoulos, 1979). At the end of the exposure, the animals were sacrificed and the tissue and body fluids analyzed. Concentrations of Morpholine were highest in urine (324 µg/mL) and kidney (118 µg/g), the other tissues having concentrations below 50 µg/g.

Van Stee et al. (1981) injected six male New Zealand White rabbits intravenously with 5 mmol [14C]-labeled Morpholine/kg bw (435 mg/kg bw). The distribution of radioactivity after 30 min showed the highest concentrations in the renal medulla (36 mmol/kg bw) and cortex (15.4 mmol/kg bw), followed by lung (5.1 mmol/kg bw), liver (4.7 mmol/kg bw) and blood (2.3 mmol/L). Morpholine was not bound to serum proteins. Furthermore, the subcellular binding interactions of Morpholine were investigated (Naylor Dana Institute, 1983). Uniform distribution of [14C] indicated a non-specific binding and/or incorporation of Morpholine. No significant amounts of covalently bound [14C] were detected in the subcellular fractions of the liver.

Plasma concentration-time curves of [14C] after intraperitoneal injections of [14C] Morpholine (125 mg/kg bw in 0.9% NaCl) in Sprague-Dawley rats, Syrian golden hamsters, and strain II guinea pigs declined bi-exponentially. Marked differences were noted between the guinea pig and the other two species with respect to plasma levels (as well as the metabolism of Morpholine). Whereas rates of the first phase of elimination from plasma in rats and hamsters were similar (half-lives of 115 and 120 min, respectively), the half-life in guinea-pigs was significantly longer (300 min) (Sohn et al., 1982).

 

In summary, Morpholine is well distributed following is well distributed following all routes of exposure, with distribution primarily to the kidney, intestine, and muscle. The highest concentration is expected to be in the kidney.

 

Based on the toxikokinetic data described below in the section “Excretion” a bioaccumulation potential can be excluded. Furthermore, due to the low log Pow Morpholine is unlikely to bioaccumulate in tissue,and there are no other physicochemical properties indicating bioaccumulating properties.

Metabolism

Morpholine is eliminated mainly in a non-metabolized form in the urine of the rat, mouse, hamster and rabbit (Griffiths, 1968; Tanaka et al., 1978; Van Stee et al., 1981; Sohn et al., 1982). Sohn et al. (1982) reported that Morpholine is metabolized by N-methylation followed by N-oxidation in the guinea-pig. After an intraperitoneal injection of 125 mg/kg bw [14C]-labelled Morpholine in guinea-pigs, 20 % of the radioactivity was found in the urine as N-methylmorpholine- N-oxide. The Morpholine ring can be cleaved in mammalian systems: in several studies on the metabolism of Morpholine derivatives in the rat, ring cleavage products have been reported (Tatsumi et al., 1975; Hecht & Young, 1981; Kamimura et al., 1987).

 

In the presence of nitrite, Morpholine can be nitrosated to the carcinogenic N-nitrosomorpholine (NMOR). For instance, NMOR was found in the stomach of rodents whose feed contained nitrite and Morpholine (Sander et al., 1968; Inui et al., 1978; Kitano et al., 1979). Hecht & Morrison (1984) developed a method to monitor the in vivo formation of NMOR by measuring N-nitroso(2-hydroxyethyl)glycine, its major urinary metabolite. The formation of NMOR was measured in F-344 rats over wide range of doses of Morpholine (38.3 - 0.92 µmol) and sodium nitrite (191 - 4.8 µmol). According to estimates by the authors, 0.5 to 12 % of the administered Morpholine, depending on the dose, was nitrosated. Furthermore,in vitroexperiments have demonstrated that nitrosation of Morpholine is also possible in human saliva and in gastric juices (Ziebarth, 1997; Ziebarth, 1973; Boyland et al., 1971).

Excretion

Expired air

Following intraperitoneal injection, the elimination of [14C] from labeled Morpholine through expired air is minimal. In rats, only about 0.5 % of the dose of radioactively labeled Morpholine was exhaled as [14C] carbon dioxide (Sohn et al., 1981). In rabbits, only 0.0008 % of the administered Morpholine dose was [14C] carbon dioxide (Van Stee et al., 1981).

Urine:

Elimination studies on male Wistar rats (200-350 g) were carried out by administering Morpholine HCl (500 mg/kg bw) or [14C]-labeled Morpholine HCl (200 mg/kg bw) orally and Morpholine-HCl (250 mg/kg bw) intravenously. In all cases, over 85 % of the total dose was excreted in urine within 24 hours. A further portion, up to 5 %, was excreted during the next three days. The [14C]-labeled Morpholine palmitate was eliminated slightly slower, but the urinary excretion within 3 days following oral administration amounted to 90 % of the dose (Tanaka et al., 1978). Of the radioactive Morpholine administered to rats, up to 73.5 % was excreted in the urine after 24 hours (Griffiths, 1968).

The time-course of urinary excretion of [14C] by Sprague-Dawley rats, Syrian golden hamsters, and strain II guinea pigs treated with [14C]-labeled Morpholine was compared by Sohn et al. (1982). In all three species over 80 % of the dose was excreted in 3 days, while the rate of urinary excretion within the first 6 hours was greatest in the hamster and least in the guinea pig. Van Stee et al. (1981) infused rabbits intravenously with [14C]-labeled Morpholine (5 mmol/kg bw) which had been neutralized with HCl. After 4 hours, 18.5 % of the dose was excreted in the urine. When the pH of the urine was lowered from 7.8 - 7.9 to 7.1 - 7.2 by administration of ammonium chloride (10 g/L) in drinking-water prior to the Morpholine injection, the urinary excretion more than doubled (to 43 %). These data suggest that the urinary excretion of Morpholine is enhanced by its neutralization with acid.

Feces:

Rats dosed orally or intravenously with Morpholine HCl excreted not more than 1.7 % of the dose in the feces (Griffiths, 1968; Tanaka et al., 1978). When dosed orally with Morpholine palmitate (Tanaka et al., 1978), up to 7% was excreted in feces.

Summary and conclusion

Oral absorption and distribution through-out the body is expected for Morpholine. This is supported by the results of single and repeated-dose toxicity studies in vivo were systemic effects are reported and target organs are identified. Furthermore, available toxicokinetic studies in rat demonstrate that Morpholine is rapidly and quantitatively absorbed after oral administration. Based on the recovery of Morpholine or Morpholine salts in urine following inhalation or oral exposure, absorption is expected to be at least 55 or 90%, respectively Since Morpholine is corrosive it induces damage to skin and mucous membranes which will enhance dermal penetration and absorption via the respiratory epithelium. Based on a predictive algorithm it was concluded that Morpholine has the potential to be absorbed through the skin and to become available systemically following dermal exposure (NIOSH, 2017). Morpholine is preferentially distributed to the rabbit kidney (Van Stee et al., 1981) and the rat kidney and intestine (Tanaka, 1978). Morpholine has been reported to be resistant to metabolism in the rat (Tanaka et al., 1978; Sohn et al., 1982) and rabbit (Van Stee et al., 1981). In contrast, Sohn et al. (1982) reported that Morpholine is metabolized by N-methylation and N-oxidation in the guinea pig. Morpholine is metabolized to a much smaller extent by N-methylation in the rat and hamster (Sohn et al., 1982). Bioaccumulation of the substance is not expected after continuous exposure. The test substance is primarily excreted unchanged in urine.

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