Calcium cyanamide

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:

not reported

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Objective of study:

toxicokinetics

Qualifier:

no guideline followed

Principles of method if other than guideline:

Different methods used for characterisation of the pharmacokinetic behaviour of calcium cyanamide. No guidelines were indicated.

GLP compliance:

not specified

Species:

rat

Strain:

not specified

Sex:

not specified

Details on test animals or test system and environmental conditions:

No details on test animals

Route of administration:

other: oral and intraperitoneal

Vehicle:

water

Details on exposure:

Pharmacokinetics of calcium cyanamide were studied after intraperitoneal and oral administration.

Duration and frequency of treatment / exposure:

no details given

Remarks:

Doses / Concentrations:
In one of the tests 7.0 mg/kg bw was administered orally.

No. of animals per sex per dose / concentration:

no details given

Control animals:

not specified

Positive control reference chemical:

no data

Details on study design:

Distribution, metabolism and excretion were studied measuring cyanamide and metabolite concentration in different tissues and in the urine.
The time-course of plasma cyanamide concentration in the rat was determined following oral administration of 7.0 mg/kg calcium cyanamide.

Details on dosing and sampling:

no details given

Statistics:

no details given

Preliminary studies:

It has been demonstrated that calcium cyanamide is quantitatively hydrolysed to cyanamide in one hour under simulated gastric conditions. This hydrolysis is required for absorption, as calcium cyanamide is insoluble in aqueous solutions.

Details on distribution in tissues:

Intraperitoneal:
The disposition of cyanamide in the rat has been studied after the intraperitoneal administration of 14C-cyanamide. Radioactivity was found only in the liver at the 1-h interval after drug administration. Radioactivity was excreated primarily in the urine and to a small extent in the breath as carbon dioxide.

Details on excretion:

Intraperitoneal:
At 6 h following intraperitoneal administration of 14C-cyanamide, 94% and 1.5% of the radioactivity were excreted in the urine and breath respectively.
Oral:
Peak plasma cyanamide concentration was achieved at 80 min after calcium cyanamide administration. The plasma cyanamide concentration decreased monoexponentially, which indicated first-order kinetics of elimination, and the apparent plasma elimination half-life was 92.4 min. Throughout this 6h study, no cyanamide was detected in the liver.

Key result

Toxicokinetic parameters:

half-life 1st: 92.4 min

Metabolites identified:

no

Details on metabolites:

Intraperitoneal:
The radioactivity in the urine was not associated with cyanamide, its dimer cyanoguanidine, or urea. A metabolite containing a cyano group was isolated from the urine and appeared to be an acid with a pKa of 3.9.

Conclusions:

Interpretation of results: no bioaccumulation potential based on study results
In the rat, peak plasma cyanamide concentration was achieved 60 minutes after oral administration of calcium cyanamide and plasma elimination half-life was 92.4 minutes.

Executive summary:

In the review of Brien and Loomis (1983), data on the absorption, distribution, biotransformation, excretion and pharmacokinetics of calcium cyanamide in experimental animals and man after oral and i.p. administration are summarised. It has been mentioned that calcium cyanamide is quantitatively hydrolysed to cyanamide in one hour under simulated gastric conditions (see toxicokinetics study entry of Loomis and Brien, 1981). This hydrolysis is required for absorption as calcium cyanamide is insoluble in aqueous solutions.

Also, an in vivo study on the disposition of cyanamide using [¹⁴C]-cyanamide revealed that radioactivity was found only in liver and excretion was primarily via urine. The time course of plasma concentration was determined following oral administration of 7 mg calcium cyanamide/kg bw to rats. Peak plasma cyanamide concentration was achieved 60 minutes after administration and plasma elimination half-life was 92.4 minutes. Calcium cyanamide inhibits aldehyde dehydrogenase in vitro and is used as a drug in the treatment of alcoholism. No indication for bioaccumulation or sex-difference in toxicokinetic were found.

Reference 2

Endpoint:

basic toxicokinetics in vitro / ex 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:

metabolism

toxicokinetics

Qualifier:

no guideline followed

Principles of method if other than guideline:

The hydrolysis of calcium cyanamide (also known as calcium carbimide) in various forms under simulated gastic conditions was quantified in this in vitro study.

GLP compliance:

no

Specific details on test material used for the study:

- Name used in the publication: Temposil

SOURCE OF TEST MATERIAL
- Source and lot/batch No.of test material: Lederle (Montreal, Canada)

OTHER SPECIFICS: Three forms of calcium cyanamide were tested in vitro: [1] pure calcium cyanamide, [2] the Temposil tablet containing 50 mg of calcium cyanamide and 100 mg of citric acid and [3] the pulverised Temposil tablet containing 50 mg of calcium cyanamide in a gelatinous capsule.

Radiolabelling:

no

Species:

other: human

Sex:

not specified

Route of administration:

not specified

Details on study design:

- Dose selection rationale: 50 mg was reported to be the clinically recommended dose in man.

Details on dosing and sampling:

METABOLITE CHARACTERISATION STUDIES
- Fluids sampled (delete / add / specify): Simulated gastric conditions (50 mL of 0.1 M hydrochloric acid; pH 1.14-1.16)
- Time and frequency of sampling: 1 h after incubation
- Method type for identification: Gas-liquid chromatography with electron-capture detection (GLC-ECD)
- Other: 1 mL of the acidic incubate was extracted for the measurement.

Statistics:

Not specified

Type:

metabolism

Results:

92% of the 50 mg calcium cyanamide incubated under simulated gastric conditions for 1 hour converted to cyanamide.

Type:

metabolism

Results:

57% of the 50 mg calcium cyanamide in tablet form (Temposil) containing 100 mg citric acid incubated under simulated gastric conditions for 1 hour converted to cyanamide.

Type:

metabolism

Results:

100% of the pulverised Temposil tablet in a gelatinous capsule containing 50 mg calcium cyanamide incubated under simulated gastric conditions for 1 hour converted to cyanamide.

Metabolites identified:

yes

Details on metabolites:

Only hydrogen cyanamide was quantified.

Conclusions:

This in vitro study quantified the percent of conversion of test substance calcium cyanamide after 1 h incubation under simulated gastric conditions. It was reported that 92% of pure calcium cyanamide (50 mg) hydrolysed to cyanamide. However, when calcium cyanamide (also 50 mg) was moulded in a form of a tablet (Temposil®), which contained also 100 mg citric acid, it resulted in a slower conversion to cyanamide (57% under the same in vitro conditions) than pure calcium cyanamide alone. After 10 hours, the hydrolysis of calcium cyanamide in tablet form was complete (i.e. 100%). On the other hand, the pulverised tablet in a gelatinous capsule resulted in 100% conversion of calcium cyanamide to cyanamide. The study points out the kinetic difference in hydrolysis of calcium cyanamide to cyanamide when tested in different forms.

Executive summary:

This study quantified the percent of conversion of test substance calcium cyanamide (also known as calcium carbimide) under simulated gastric conditions (50 mL of 0.1 M hydrochloric acid, pH 1.14 -1.16), which mimic the volume and pH of the stomach contents in man. The hydrolysis of calcium cyanamide (50 mg) was tested three different forms: [1] pure calcium cyanamide, [2] the Temposil tablet containing 50 mg of calcium cyanamide and 100 mg of citric acid and [3] the pulverised Temposil tablet in a gelatinous capsule. The substance was incubated in simulated gastric conditions for 1 h; afterwards, 0.1 mL acidic incubate was extracted and the cyanamide concentration was quantified using gas-liquid chromatography with electron capture detection.

It was reported that 92% of pure calcium cyanamide (50 mg) hydrolysed to cyanamide (or carbimide) within 1 h under simulated gastric conditions. However, when calcium cyanamide (also 50 mg) was moulded in a form of a tablet (Temposil^®), which contained also 100 mg citric acid, it resulted in a slower conversion to cyanamide (57% within 1 h under the same in vitro gastric conditions) than pure calcium cyanamide alone. After 10 hours, the hydrolysis of calcium cyanamide in tablet form was complete (i.e. 100%). On the other hand, the pulverised tablet in a gelatinous capsule resulted in 100% conversion of calcium cyanamide to cyanamide within 1 h under simulated gastric conditions. The study points out the kinetic difference in hydrolysis of calcium cyanamide to cyanamide when tested in different forms.

This study also includes an in vivo assessment of the conversion of calcium cyanamide to cyanamide in rats, which is described in details in the study of Brien and Loomis (1983).

Description of key information

Two kinetics studies on calcium cyanamide were considered for the basic toxicokinetics. The in vitro study of Loomis and Brien (1981) demonstrates that pure calcium cyanamide rapidly hydrolyses to cyanamide under simulated gastric conditions of low pH. However, when calcium cyanamide is mixed with other components such as citric acid and moulded into a tablet, the rate of hydrolysis is significantly lower compared to calcium cyanamide alone (57% in tablet form vs. 92% in powder form). This demonstrates that physical state and other constituents or impurities found in calcium cyanamide might affect the rate of conversion to cyanamide.

Calcium cyanamide technical grade is a black solid with a characteristic odour and comprises up to 72% calcium cyanamide and the remaining percentage of other constituents such as calcium oxide and graphite (see Chapter 1.2 Composition). This is different from pure cyanamide, which is a colourless, odourless solid. Thus, it is probable that the conversion of calcium cyanamide to cyanamide occurs at a slower rate than pure calcium cyanamide, and this subsequently might affect the overall toxicokinetics of calcium cyanamide technical grade. For example, the slower dissolution kinetics of calcium cyanamide technical grade (i.e. conversion to cyanamide) might result in lower systemic concentrations of cyanamide present in the body over a period of time in contrast to exposure to much higher concentrations of cyanamide via a single bolus dose. Furthermore, lower systemic concentrations may allow opportunities for the body to cope with the exposure, e.g. enabling metabolic capacities of protective pathways or repair mechanisms, which allows earlier detoxification and/or elimination of cyanamide.

Overall, it is challenging to predict the dissolution kinetics of calcium cyanamide (in its technical grade form) to cyanamide, but it is relevant to consider the influence of other components found in the technical grade form on the dissolution kinetics to cyanamide. With this said, there are 4 toxicokinetics studies available on cyanamide (e.g. Cyanamide L500/F1000) provided in this dossier, but they are not considered suitable for the toxicokinetics evaluation of calcium cyanamide technical grade for the aforementioned reasons. For further information see IUCLID chapter 13.2 under the title “Scientific Rationale for not using Cyanamide as Read-Across Substance for Calcium cyanamide on Toxicological Endpoints”.

Please see the endpoint summary "Dermal absorption" for the key information for this specific endpoint.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - dermal (%):

10

Additional information

For the basic toxicokinetics of calcium cyanamide technical grade, two key studies are evaluated.

An in vitro study of Loomis and Brien (1981) quantified the percent of conversion of test substance calcium cyanamide after 1 h incubation under simulated gastric conditions. This hydrolysis is required for absorption as calcium cyanamide is insoluble in aqueous solutions. It was reported that 92% of pure calcium cyanamide (50 mg) hydrolysed to cyanamide. However, when calcium cyanamide (also 50 mg) was moulded in a form of a tablet (Temposil®), which contained also 100 mg citric acid, it resulted in a slower conversion to cyanamide (57% under the same in vitro conditions) than pure calcium cyanamide alone. After 10 hours, the hydrolysis of calcium cyanamide in tablet form was complete (i.e. 100%). On the other hand, the pulverised tablet in a gelatinous capsule resulted in 100% conversion of calcium cyanamide to cyanamide. The study points out the kinetic difference in hydrolysis of calcium cyanamide to cyanamide when tested in different forms.

In the review of Brien and Loomis (1982), data on the absorption, distribution, biotransformation, excretion and pharmacokinetics of calcium cyanamide in experimental animals and man after oral and i.p. administration are summarised. An in vivo study on the disposition of cyanamide using [¹⁴C]-cyanamide revealed that radioactivity was found only in liver and excretion was primarily via urine. The time course of plasma concentration was determined following oral administration of 7 mg calcium cyanamide/kg bw to rats. Peak plasma cyanamide concentration was achieved 60 minutes after administration and plasma elimination half-life was 92.4 minutes. Calcium cyanamide inhibits aldehyde dehydrogenase in vitro and is used as a drug in the treatment of alcoholism. No indication for bioaccumulation or sex-difference in toxicokinetic were found.

For additional information on dermal absorption of calcium cyanamide, please see the endpoint summary "Dermal absorption" for more details.

On the other hand, 4 toxicokinetics studies on cyanamide (with oral or intravenous exposure) are available for evaluation, but they are considered not suitable for the basic toxicokinetics of calcium cyanamide due to the differences in physical properties and composition between the two substances. These differences could result in different ADME kinetics. However, they do provide further information and indication of the subsequent metabolism of cyanamide to acetylcyanamide, the main urinary metabolite of cyanamide. The toxicokinetic studies on cyanamide are briefly summarised in this section below.

Basic toxicokinetics studies on (hydrogen) cyanamide

Unlabeled and non-radiolabeled hydrogen cyanamide was applied to 5 male and 5 female rats per dose group orally and intravenously (Sturble, 1993). The doses applied were the following: an oral high dose of 20 mg/kg bw, an oral low dose of 1 mg/kg bw, an intravenous dose of 1 mg/kg bw, and an oral dose of 1 mg/kg bw of unlabeled test substance for 14 days followed by a single oral dose of 1 mg/kg bw of radiolabeled material. [¹⁴C]-hydrogen cyanamide was rapidly excreted regardless of the route of administration, and radioactivity of [¹⁴C]-hydrogen cyanamide was apparently completed absorbed after oral administration. The percent excreted in the urine post-oral administration ranged from approx. 79.0% to 97.7% after 168 hours. There was evidence of biliary excretion of radioactivity after the intravenous route of exposure but not for oral high dose. Repeated doses resulted in a decrease in the amount of radioactivity as CO₂ and in the concentration of radioactivity in the liver 168 hours post-dose, suggesting that the disposition of radioactivity was influenced by repeated doses of the test substance. The metabolism of cyanamide after oral and intravenous dosing can be described as follows: Starting with the unchanged parent compound, the major metabolic reaction was acetylation of the nitrogen, forming N-acetylcyanamide. N-acetylcyanamide is the major metabolite in both urine and faeces. Other radioactive metabolites were present in the urine and faeces but they represented less than 10% of the administered dose in each case. It can be concluded that results indicated that cyanamide has no potential for bioaccumulation as it is rapidly absorbed and excreted after oral and i.v. administration. According to the amounts excreted with the urine, cyanamide can be regarded to be nearly quantitatively bioavailable both after single low, single high and repeated low dose administration via the oral route.

A three-part study (Gloxhuber, 1989) was conducted to examine the absorption, metabolism, distribution, and excretion of the hydrogen cyanamide applied orally to rats (20 mg/kg bw by gavage) and orally (20 mg) and dermally (10 mg per forearm; 20 mg per person) to human volunteers. Altogether results of all three parts of the study (rat: gavage, human: oral, dermal) confirmed that N-acetylcyanamide is the main urinary metabolite of cyanamide in rats and humans. Both species showed an efficient excretion of orally or dermally administered cyanamide. Cyanamide is absorbed through human skin, however, the dermal absorption is considerably delayed compared to the oral one. The degree of dermal absorption of a 1% aqueous cyanamide solution for a period of 6 hours exposure was found to be between 0.87 and 3.53%. Based on the data described in a review article, urine samples were analysed for acetylcyanamide and thiocyanate content, and cyanide levels were determined in blood after administration of cyanamide in rats (10 mg/kg bw; gavage) and male human volunteers (0.25 mg/kg bw; oral and dermal application). The results of the urinary excretion of acetylcyanamide after oral application of cyanamide show that acetylcyanamide is one major metabolite of cyanamide in rats and humans. The main part of the applied cyanamide was found to be excreted in the form of its metabolite during the first 12 and 22 hours after application in humans and rats, respectively. The concentration of thiocyanate in the persons urine showed no significant differences before and up to 48 hours after cyanamide application and cyanide concentration in the blood did not show significant differences from the controls before application of the test substance, indicating that cyanide is no degradation product or metabolite of cyanamide in vivo.

In one investigation the bioavailability of cyanamide in fasted and not fasted rats was studied after intravenous and oral administration of 35 mg cyanamide/kg bw (Obach et al., 1986). Results revealed a half-life of cyanamide of about one hour and a distribution throughout the whole body water. Plasma clearance was 0.02 L/kg/min and the relative bioavailability was 93.3% in fasted and 85.5% in unfasted rats. It was shown that food does not modify the absolute bioavailability of cyanamide, but appears to delay drug absorption. Comparison with results from a study where 7 mg/kg of calcium cyanamide were applied to rats showed that oral absorption of cyanamide is faster and the half-life is shorter than from calcium cyanamide. This might be explained by the different forms of cyanamide applied; as calcium cyanamide has to be hydrolysed, thus producing cyanamide, prior to absorption.