The Committee evaluated a group of flavouring agents that includes cinnamyl alcohol (No. 647), cinnamaldehyde (No. 656), cinnamic acid (No. 657), and 52 structurally related substances (see Table 1). The evaluations were conducted according to the Procedure for the Safety Assessment of Flavouring Agents. One member of this group, allyl cinnamate (No. 19), had been evaluated previously by the Committee in a separate group of allyl ester flavouring agents examined by the Procedure (Annex 1, reference 122). Cinnamaldehyde (No. 656) was evaluated by the Committee at its eleventh meeting (Annex 1, reference 14), when it established a conditional ADI of 0–1.25 mg/kg bw. At its twenty-third meeting, the Committee converted the previous conditional ADI to a temporary ADI of 0–0.7 mg/kg bw (Annex 1, reference 50), which was extended at its twenty-fifth and twenty-eighth meetings (Annex 1, references 56 and 66). At its thirty-fifth meeting, the Committee did not extend the temporary ADI (Annex 1, reference 88) because the data that were required were not available. At its twenty-fifth meeting, the Committee concluded that cinnamyl anthranilate should not be used as a food additive (Annex 1, reference 56). This substance is structurally related to members of the group of cinnamyl alcohol and related flavouring agents but differs in that it is hydrolysed to cinnamyl alcohol and anthranilic acid only slowly, resulting in systemic intake of the intact ester. In contrast, the members of the group of flavouring agents undergo rapid hydrolysis.

Twenty-two of the 55 flavouring agents in this group are natural components of foods. Concentrations of cinnamaldehyde of up to 750 g/kg have been detected in oils from natural sources, such as cinnamon, cinnamomum, and cassia leaves; however, these agents are consumed predominantly as flavouring agents (Maarse et al., 1996).

The total annual volume of production of the 55 cinnamyl compounds considered here is approximately 60 t in Europe (International Organization of the Flavor Industry, 1995) and 480 t in the USA (National Academy of Sciences, 1987; Lucas et al., 1999; see Table 2). Approximately 30% of the total annual volume in Europe and over 93% of that in the USA is accounted for by cinnamaldehyde (No. 656), while 54% of the total annual volume in Europe is accounted for by the use of cinnamyl alcohol (No. 647) and methyl cinnamate (No. 658). The estimated intakes in Europe are 2.5 mg/day of cinnamaldehyde, 1.8 mg/day of cinnamyl alcohol, and 2.8 mg/day of methyl cinnamate. The estimated intakes in the USA are 59 mg/day of cinnamaldehyde, 1.9 mg/day of cinnamyl alcohol, and 0.83 mg/day of methyl cinnamate. The intakes of all the other flavouring agents in the group are in the range 0.003–690 µg/day, most of the values being at the low end of this range. Production volumes and intake of each substance are reported in Table 2.

Cinnamyl alcohol (No. 647), cinnamaldehyde (No. 656) and its para- and ortho-methoxy derivatives (Nos 687 and 688), cinnamic acid (No. 657) and its corres-ponding methyl ester (No. 658), and the saturated analogue (3-phenylpropionic acid, No. 646) have all been shown to be rapidly absorbed from the gut, metabolized, and excreted primarily in the urine and to a minor extent in the faeces.

Esters of cinnamic acid and structurally related aromatic esters have been shown to be hydrolysed rapidly to the component acid and alcohol. The aromatic primary alcohols and aldehydes in this group and those formed by the hydrolysis of esters and acetals are readily oxidized to cinnamic acid or one of its structurally related carboxylic acids. In animals, most carboxylic acids, such as cinnamic acid, are converted to acyl coenzyme A esters. Cinnamoyl coenzyme A undergoes either glycine conjugation catalysed by a glycine N-acyl transferase or beta-oxidation, eventually leading to the formation of benzoyl coenzyme A. This is in turn either conjugated with glycine, yielding hippuric acid, or hydrolysed to yield free benzoic acid, which is then excreted (Nutley et al., 1994).

Cinnamyl derivatives containing alpha-methyl substituents, such as alpha-methylcin-namaldehyde (No. 683), are extensively metabolized by beta-oxidation and cleavage to yield mainly the corresponding hippuric acid derivative. Because ortho-oxygenated ring substituents (e.g. ortho-methoxycinnamaldehyde, No. 688) selectively inhibit oxidation of coenzyme A esters of beta-hydroxyacids via the beta-oxidation pathway, these hydroxyacid derivatives are excreted as glycine conjugates. In contrast, para-oxygenated ring substituents (e.g. para-methoxycinnamaldehyde, No. 687) are oxidized via the beta-oxidation pathway, eventually yielding hippuric acid derivatives.

Table 1 summarizes the evaluation of cinnamyl alcohol and 54 related substances used as flavouring agents.

In the unlikely event that all foods containing the 50 substances in structural class I were consumed concurrently on a daily basis, the estimated combined intake would exceed the threshold for human intake for class I. In the unlikely event that all foods containing the five substances in structural class II were consumed concurrently on a daily basis, the estimated combined intake would exceed the threshold for human intake for class II. However, all 55 substances in the group are expected to be efficiently metabolized and would not saturate the metabolic pathways. Overall evaluation of the data indicates that combined intake would not raise concern about safety.

The Committee concluded that the safety of the flavouring agents in this group would not raise concern at the current estimated levels of intake. Other data on the toxicity of cinnamyl derivatives were consistent with the results of the safety evaluation.

This monograph summarizes data relevant to an evaluation of the safety of cinnamyl alcohol (No. 647), cinnamaldehyde (No. 656), cinnamic acid (i.e., trans-3-phenylpropenoic acid, No. 657), and 52 structurally related substances. All members of this group are primary alcohols, aldehydes, or carboxylic acids or their corresponding esters and acetals. The primary oxygenated functional group is located on a three-carbon chain containing an aromatic ring at position 3 (i.e. a 3-phenylpropyl group). Structural variations among these substances include the presence of unsaturation in the propyl side-chain and/or alkyl, alkoxy, or hydroxy substituents on the aromatic ring. The parent saturated alcohol is 3-phenyl-1-propanol, and the parent unsaturated alcohol is cinnamyl alcohol (3-phenyl-2-propen-1-ol, No. 647).

The group includes 3-phenyl-1-propanol, (No. 636), eight related esters (Nos. 637-644), and the corresponding aldehyde (No. 645) and carboxylic acid (No. 646). It also includes cinnamyl alcohol (No. 647), eight related cinnamyl esters (Nos 649–655 and cinnamyl benzoate), cinnamaldehyde (No. 656), an acetal of cinnamaldehyde (No. 648), cinnamic acid (No. 657), and 16 cinnamic acid esters (Nos 658–673). Thirteen cinnamyl derivatives contain additional ring side-chain alkyl substituents (Nos 674–686), and three contain alkoxy-ring substituents (Nos 687–689).

The available data on cinnamyl anthranilate, which is no longer used as a flavouring agent (voluntarily discontinued in 1986), are not presented in this review. A review of the metabolism and the proposed mechanism of toxicity of this agent has been published (Newberne et al., 2000).

Cinnamyl compounds are fundamental to plant biochemistry. trans-Cinnamic acid is ubiquitous in the plant kingdom and is required for lignin formation in plants. It is derived from the action of L-phenylalanine ammonia lyase on L-phenylalanine, with the formation of ammonia and cinnamic acid (No. 657). Cinnamic acid is also converted to para-hydroxy cinnamic acid (para-coumaric acid) by plants. para-Coumaric acid is one of the more important precursors of lignins as it can be converted to polyphenolic alcohols, which readily polymerize to form lignin (Goodwin & Mercer, 1972).

Twenty-two of the 55 flavouring substances in this group have been detected as natural components of traditional foods (Maarse et al., 1996; see Table 2). A considerable amount (38 642 kg) of cinnamaldehyde (No. 656) occurs naturally in foods. This agent has been detected in oils derived from natural sources such as cinnamon, cinnamomum, and cassia leaf, at concentrations up to 750 000 mg/kg (Maarse et al., 1996). Quantitative data on natural occurrence have also been reported for 3-phenylpropyl acetate (No. 638), ethyl 3-phenylpropionate (No. 644), cinnamyl alcohol (No. 647), cinnamic acid (No. 657), methyl cinnamate (No. 658), and ethyl cinnamate (No. 659). Generally, the consumption ratios indicate that intake occurs predominately from use of these substances as flavours (i.e. the consumption ratio is < 1) (Stofberg & Kirschman, 1985; Stofberg & Grunschober, 1987).

The cinnamyl derivatives used as flavouring substances are simple aromatic compounds with a propyl side-chain containing a primary oxygenated functional group, and they participate in common routes of absorption, distribution, and metabolism. The members of this group may be hydrolysed to yield the component alcohol, aldehyde, or acid. If the product is an alcohol or aldehyde, it is oxidized to yield the corresponding 3-phenylpropenoic acid or a 3-phenylpropanoic acid derivative which undergoes further side-chain beta-oxidation and cleavage to yield mainly the corresponding benzoic acid derivatives (Figure 2; Williams, 1959). The benzoic acid derivatives are conjugated with glycine and, to a lessor extent, glucuronic acid and excreted primarily in the urine (Snapper et al., 1940). ortho-Alkyl- and ortho-alkoxy-substituted cinnamaldehyde derivatives undergo beta-oxidation to a minor extent, to yield beta-hydroxy-3-phenylpropanoic acid metabolites that are excreted as the glucuronic acid conjugates (Solheim & Scheline, 1973, 1976; Samuelsen et al., 1986).

2.3.1 Absorption, distribution, and excretion

Cinnamyl alcohol (No. 647), cinnamaldehyde (No. 656) and its ortho- and para-methoxy derivatives (Nos 687 and 688), cinnamic acid (No. 657) and its corres-ponding methyl ester (No. 658), and the saturated analogue (3-phenylpropanoic acid, No. 646) have all been shown to be rapidly absorbed from the gut, metabolized, and excreted primarily in the urine and, to a minor extent, in the faeces. Results of studies conducted as long ago as 1909 indicate that cinnamyl derivatives are absorbed, metabolized, and excreted as polar metabolites within 24 h (see Table 3).

In groups of male Fischer 344 rats, 83% of an oral dose of 2.5 mmol/kg bw of [3-14C-d5]-cinnamyl alcohol (335 mg/kg bw), 77% of a dose of [3-14C-d5]-cinnamaldehyde (330 mg/kg bw), and 79% of a dose of [3-14C-d5]-cinnamic acid (370 mg/kg bw) were excreted mainly in the urine within 24 h. Excretion in the faeces accounted for only minor amounts of the administered alcohol (6.1%), aldehyde (16%), and acid (0.9%). More than 90% of the administered dose of any of the three substances was recovered in the urine and faeces within 72 h. Administration of the same doses of the parent alcohol, aldehyde, or acid to groups of CD-1 mice by intraperitoneal injection resulted in a similar pattern of excretion in the urine and faeces at 24 h (75%, 80%, and 93%, respectively) and 72 h (> 93%) (Nutley, 1990).

The tissue distribution and excretion of cinnamaldehyde (No. 656) were studied in groups of eight male Fischer 344 rats pretreated with single daily oral doses of 5, 50, or 500 mg/kg bw of cinnamaldehyde by gavage for 7 days and the same single oral dose of [3-14C]cinnamaldehyde 24 h later. Further groups received no pretreatment but the same single doses. The radiolabel was distributed primarily to the gastrointestinal tract, kidneys, and liver in all groups. After 24 h, > 80% of the radiolabel was recovered in the urine and < 7% in the faeces from all rats, regardless of dose. In all groups, a small amount of the dose was distributed to fat. Radiolabel was still present in animals killed 3 days after receiving 50 or 500 mg/kg bw. In animals pretreated with the two lower doses, the main urinary metabolite was hippuric acid, with small amounts of cinnamic and benzoic acid. In those pretreated with the high dose, benzoic acid was the major metabolite, suggesting that saturation of the glycine conjugation pathway occurs with repeated high doses of cinnamaldehyde (Sapienza et al., 1993).

The effect of dose and sex on the disposition of [3-14C]cinnamaldehyde was studied in Fischer 344 rats and CD-1 mice. More than 85% of doses of 2.0 and 250 mg/kg bw administered to groups of four male and four female rats and six male and six female mice by intraperitoneal injection was recovered in the urine and faeces within 24 h, and > 90% was recovered by 72 h. Of a dose of 250 mg/kg bw of [3-14C]cinnamaldehyde administered orally to Fischer 344 rats, 98% was recovered from the urine (91%) and faeces (7%) within 24 h (Peters & Caldwell, 1994). The effect of dose on the disposition of [3-14C-d5]-cinnamic acid was also studied in Fischer 344 rats and CD-1 mice. Five doses of cinnamic acid in the range 0.0005–2.5 mmol/kg bw were given orally to groups of four rats or by intraperitoneal injection to groups of four mice. After 24 h, 73–88% of the radiolabel was recovered in the urine of rats and 78–93% in the urine of mice; after 72 h, 85–100% of the radiolabel was recovered from rats and 89–100% from mice, mainly in the urine (Caldwell & Nutley, 1986). Only trace amounts of radiolabel were present in the carcass, indicating that cinnamic acid was readily and quantitatively excreted at all doses. The parent alcohol, aldehyde, and acid therefore appear to undergo rapid absorption, metabolism, and excretion, independently of dose up to 250 mg/kg bw, species, sex, and mode of administration (Nutley et al., 1994).

Eleven persons each received a single intravenous dose of cinnamic acid (No. 657) equivalent to 5 mg/kg bw. Analysis of blood showed that 100% of the dose was present within 2.5 min and none after 20 min (Quarto di Palo & Bertolini, 1961).

An oral dose of 1.5 mmol/kg bw (240 mg/kg bw) methyl cinnamate (No. 658) was rapidly and almost completely (95%) absorbed from the gut in rats. The agent was partially hydrolysed to cinnamic acid in the stomach (9%) and gut (40%), and the rate of absorption of cinnamic acid and methyl cinnamate from the gut was similar. No ester was detected in the peripheral blood of dosed rabbits or rats. Only traces were detected in portal and heart blood taken from the rats, indicating that almost complete hydrolysis of methyl cinnamate had occurred during intestinal absorption (Fahelbum & James, 1977).

After administration of a single oral dose of 57 mg of ring-deuterated 3-phenyl-propionic acid to one person, deuterobenzoic acid corresponding to 110% of the dose was isolated from alkaline hydrolysed urine within 100 min (Pollitt, 1974).

These studies indicate that cinnamyl derivatives can be anticipated to be rapidly absorbed, metabolized, and excreted, mainly in the urine, within 24 h.

2.3.1.1 Hydrolysis of esters and acetals

In general, esters containing an aromatic ring system are expected to be hydrolysed in vivo. The hydrolysis is catalysed by classes of enzymes recognized as carboxylesterases or esterases (Heymann, 1980), the most important of which are the A-esterases. In mammals, A-esterases occur in most tissues of the body (Anders, 1989; Heymann, 1980) but predominate in hepatocytes (Heymann, 1980). Acetals are rapidly hydrolysed in acidic media (Morgareidge, 1962).

Esters of cinnamic acid (No. 657) and structurally related aromatic esters have been shown to hydrolyse rapidly to the component acid and alcohol. Oral administration of a single dose of 50 mg/kg bw of methyl cinnamate (No. 658) resulted in the urinary excretion, after 24 h, of hippuric acid (66%) and benzoylglucuronide (5%). This distribution of metabolites, which was nearly identical to that for cinnamic acid, indicates that rapid hydrolysis of the ester in vivo precedes metabolism of the acid (Fahelbum & James, 1977). Ethyl cinnamate (No. 659) administered subcutaneously to a cat also produced only cinnamic acid metabolites in the urine (Dakin, 1909). Incubation of benzyl cinnamate (No. 670) or benzyl acetate with simulated intestinal fluid (pH 7.5; pancreatin) at 37 °C for 2 h resulted in 80% and 50% hydrolysis, respectively (Grundschober, 1977). Incubation of the structurally related aromatic acetal, 2-phenylpropanal dimethyl acetal (1 mmol/L) with simulated gastric juice at 37 °C resulted in 97% hydrolysis within 1 h. Under the same experimental conditions, benzaldehyde propylene glycol acetal (1 mmol/L) was 97% hydrolysed within 5 h (Morgareidge, 1962).

2.3.1.2 Oxidation and conjugation reactions of cinnamyl alcohol (No. 647) and cinnamaldehyde derivatives

The aromatic primary alcohols and aldehydes used as flavouring substances or formed by the hydrolysis of esters and acetals are readily oxidized to a cinnamic acid derivative (see Figure 2). Human NAD⁺-dependent alcohol dehydrogenase catalyses oxidation of primary alcohols to aldehydes (Pietruszko et al., 1973), and isoenzyme mixtures of NAD⁺-dependent aldehyde dehydrogenase (Weiner, 1980) catalyse oxidation of aldehydes to carboxylic acids. Aromatic alcohols and aldehydes have been reported to be excellent substrates for alcohol dehydrogenase (Sund & Theorell, 1963) and aldehyde dehydrogenase (Feldman & Wiener, 1972), respec-tively. The urinary metabolites of cinnamyl alcohol (No. 647) and cinnamaldehyde (No. 656) are mainly those derived from metabolism of cinnamic acid (see Figure 2).

In four rats given cinnamyl alcohol (No. 647) orally at a dose of 335 mg/kg bw, 52% was recovered in the urine within 24 h as the glycine conjugate of benzoic acid (hippuric acid). Ten minor metabolites cumulatively accounted for about 10% of the dose. When cinnamyl alcohol (No. 647) was administered to mice by intraperitoneal injection, hippuric acid was the major urinary metabolite (Nutley, 1990).

trans-[3-14C]Cinnamaldehyde was given at doses of 2 and 250 mg/kg bw by intraperitoneal injection to male and female Fischer 344 rats and CD-1 mice, and doses of 250 mg/kg bw were administered by oral gavage to male rats and mice only. In both species, the major urinary metabolites were formed from oxidation of cinnamaldehyde (No. 656) to cinnamic acid (No. 657), which was subsequently oxidized in the -oxidation pathway. The major urinary metabolite was hippuric acid (71–75% in mice and 73–87% in rats), and this was accompanied by small amounts of metabolites, including 3-hydroxy-3-phenylpropionic acid (0.4–4%), benzoic acid (0.4–3%), and benzoyl glucuronide (0.8–7.0%). The glycine conjugate of cinnamic acid was formed to a considerable extent only in the mice (4–13%). Glutathione conjugation of cinnamaldehyde competes to a small extent with the oxidation pathway. Approximately 6–9% of either dose was excreted within 24 h as glutathione conjugates of cinnamaldehyde. The authors concluded that the excretion pattern and metabolic profile of cinnamaldehyde in rats and mice are not systematically affected by sex, dose size, or route of administration (Peters & Caldwell, 1994).

The toxicokinetics of cinnamaldehyde (No. 656) has been investigated in male Fischer 344 rats. Cinnamaldehyde (limit of detection, < 0.1 µg/ml) and cinnamic acid (No. 657; < 1 µg/ml) were not detectable in the plasma of groups of three to six rats given a single oral dose of 50 mg/kg bw by gavage in corn oil. At doses of 250 and 500 mg/kg bw, the plasma concentrations of cinnamaldehyde and cinnamic acid were approximately 1 and > 10 µg/ml, respectively. The bioavailability of cinnamaldehyde was calculated to be < 20% at both doses. A dose-dependent increase in the amount of hippuric acid, the major urinary metabolite, occurred 6 h after gavage and continued over the next 18 h. Only small amounts of cinnamic acid were excreted in the urine either free or as the glucuronic acid conjugate. The hippuric acid recovered in the urine over 50 h accounted for 72–81% over doses ranging from 50 to 500 mg/kg bw (Yuan et al., 1992).

Approximately 15% of an oral dose of 250 mg/kg bw of cinnamaldehyde (No. 656) administered to rats by gavage was excreted in the urine as two mercapturic acid derivatives, N-acetyl-S-(1-phenyl-3-hydroxypropyl)cysteine and N-acetyl-S-(1-phenyl-2-carboxyethyl)cysteine, in a ratio of 4:1. Approximately 9% of an oral dose of 125 mg/kg bw of cinnamyl alcohol (No. 647) was excreted in the urine as N-acetyl-S-(1-phenyl-3-hydroxypropyl)cysteine (Delbressine et al., 1981).

2.3.1.3 Metabolism of cinnamic acid (No. 657)

In animals, aromatic carboxylic acids such as cinnamic acid (No. 657) that enter the cell are converted to acyl coenzyme A (CoA) esters. Cinnamoyl CoA either conjugates with glycine, a reaction catalysed by N-acyl transferase, or undergoes -oxidation eventually leading to the formation of benzoyl CoA. The reactions which form benzoic acid from cinnamic acid are reversible but the equilibrium favours formation of the benzoic acid CoA ester. Benzoyl CoA is in turn conjugated with glycine, yielding hippuric acid, or the CoA thioester is hydrolysed to yield free benzoic acid, which is then excreted (Nutley et al., 1994). CoA thioesters of carboxylic acids are obligatory intermediates in amino acid conjugation reactions (Hutt & Caldwell, 1990). The reactions in this sequence are of historical significance in biochemistry, since it was studies on cinnamic acid and fatty acids that revealed the beta-oxidation pathway of fatty acid catabolism (Nutley et al., 1994). Regardless of dose or species, the beta-oxidation pathway is the predominant pathway of metabolic detoxication of cinnamic acid in animals (see Table 3).

Six doses in the range 0.0005–2.5 mmol/kg (0.08–400 mg/kg bw) of [14C]- or [14C/5H2]-cinnamic acid (No. 657) were administered orally to male Fischer 344 rats or by intraperitoneal injection to male CD-1 mice. In both species, 84–101% was recovered within 72 h, most (73–93%) being recovered from the urine within 24 h. The metabolites identified at all doses included hippuric acid, benzoyl glucuronide, 3-hydroxy-3-phenyl-propionic acid, benzoic acid, and unchanged cinnamic acid. The major metabolite at all doses was hippuric acid (44–77%). At the highest dose tested (2.5 mmol/kg bw), the percentage of hippuric acid decreased while the percentages of benzoyl glucuronide and benzoic acid increased. The formation of larger amounts of benzoyl glucuronide (0.5–5%) and free benzoic acid (0.4–2%) at doses > 0.5 mmol/kg bw provides evidence that saturation of the glycine conjugation pathway occurs at these doses. The fact that the excretion of 3-hydroxy-3-phenyl-propionic acid differed little over the dose range (0.2–0.9%) supports the conclusion that the capacity of the beta-oxidation pathway is not limited at doses of cinnamic acid up to 2.5 mmol/kg bw in male rats (Nutley et al., 1994). The increasing role of glucuronic acid conjugation relative to glycine conjugation as the dose increases is a general trend observed in the metabolism of carboxylic acids (Caldwell et al., 1980).

In mice, glycine conjugation of cinnamic acid (No. 657) competes with the beta-oxidation pathway, but only at low doses. As the dose was increased from 0.0005 to 2.5 mmol/kg bw, the urinary concentration of hippuric acid increased from 44 to 67%, while that of cinnamoylglycine decreased from 29 to 2.4%. These results suggest that gylcine N-acetyl transferase has a stronger affinity but a lower capacity for cinnamic acid than for benzoic acid. At the highest dose, 2.5 mmol/kg bw, the amount of free benzoic acid excreted was increased from 0.8 to 8.6%, suggesting that the capacity of glycine conjugation of benzoyl CoA is also limited in mice. At all doses, mice excreted only a small proportion of benzoyl glucuronide, indicating that this conjugation reaction is of minimal importance in this species (Nutley et al., 1994).

Eleven volunteers received a single intravenous dose of cinnamic acid (No. 657) equivalent to 5 mg/kg bw. In blood plasma, 100% of the dose was found within 2.5 min and 0% after 20 min. Ninety minutes after dosing, hippuric acid, cinnamoyl-glucuronide, and benzoylglucuronide were found in a ratio of 74:24:1.5 (Quarto di Palo & Bertolini, 1961).

2.3.1.4 Effects of ring and chain substituents on metabolism of cinnamyl derivatives

The position and size of the substituent play a role in the metabolism of cinnamyl derivatives. Those containing alpha-methyl substituents (e.g. alpha-methylcinnamaldehyde, No. 683) are extensively metabolized via beta-oxidation and cleavage to yield mainly the corresponding hippuric acid derivative. A benzoic acid metabolite was isolated from the urine of dogs given either alpha-methylcinnamic acid or alpha-methylphenylpropionic acid (Kay & Raper, 1924). Larger substituents located at the alpha- or beta-position inhibited beta-oxidation to some extent (Deuel, 1957; Kassahun et al., 1991), in which case there may be direct conjugation of the carboxylic acid with glucuronic acid, followed by excretion. While alpha-methylcinnamic acid undergoes oxidation to benzoic acid, alpha-ethyl- and alpha-propylcinnamic acids are excreted unchanged (Carter, 1941). alpha-Ethylcinnamic alcohol and alpha-ethylcinnamaldehyde administered orally to rabbits resulted in urinary excretion of alpha-ethylcinnamic acid and of small amounts of benzoic acid (Fischer & Bielig, 1940). These observations suggest that alpha-methylcinnam-aldehyde undergoes oxidation to benzoic acid, while higher homologues are excreted primarily unchanged or as the conjugated form of the cinnamic acid derivative.

ortho-ring substituents (e.g. ortho-methoxycinnamaldehyde, No. 688) selectively inhibit oxidation of CoA esters of beta-hydroxyacids within the beta-oxidation pathway, and the hydroxyacid derivative is excreted as a glycine conjugate. The metabolism of ortho-methoxycinnamaldehyde ceases after formation of the beta-hydroxy derivative (Samuelsen et al., 1986).

The glycine conjugates of ortho-methoxycinnamic and ortho-methoxyphenyl-propionic acids are the principal urinary metabolites of ortho-methoxycinnamaldehyde in rats. Relatively large amounts of the beta-hydroxylated phenylpropionic acid derivatives were also detected, but only traces of benzoic and hippuric acid derivatives (products of further beta-oxidation) were excreted. The detection of relatively large amounts of a beta-hydroxylated derivative suggests that this metabolite is not readily oxidized, perhaps because of steric hindrance of the ortho substituent (Solheim & Scheline, 1973).

In contrast, para-ring substituents (e.g. 3-(para-isopropyl-phenyl)propional-dehyde, No. 680, and para-methylcinnamaldehyde, No. 682) may not affect metabolism via beta-oxidation significantly. In male albino rats, para-methoxycinnamic acid was metabolized primarily to para-methoxybenzoic acid and its corresponding glycine conjugate (Solheim & Scheline, 1973). Similar results were reported with 3,4-dimethoxycinnamic acid (which is meta and para substituted) (Solheim & Scheline, 1976). The structurally related substance para-tolualdehyde has been reported to be metabolized to para-methylbenzoic acid with no apparent oxidation of the methyl group (Williams, 1959). These observations indicate that the presence of side-chain alkyl substituents with more than one carbon atom and of ortho-ring substituents inhibits the beta-oxidation pathway. In these cases, the parent acid (cinnamic acid derivative) or an intermediary beta-oxidation metabolite (e.g. beta-hydroxy-3-phenylpropanoic acid derivative) is excreted as the glycine or glucuronic acid conjugate.

2.3.2 Toxicological studies

2.3.2.1 Acute toxicity

LD₅₀ values after oral administration have been reported for 39 of the 55 substances in this group. In rats, the values were in the range 1500 to > 5000 mg/kg bw (Jenner et al., 1964; Moreno, 1971; Weir & Wong, 1971; Keating, 1972; Levenstein & Wolven, 1972; Moreno, 1972; Denine & Palanker, 1973; Moreno, 1973; Russell, 1973; Moreno, 1974; Opdyke, 1974; Wohl, 1974; Zaitsev & Rakhmanina, 1974; Levenstein, 1975; Moreno, 1975; Levenstein, 1976; Moreno, 1976, 1977, 1981, 1982; Schafer et al., 1983), demonstrating that the acute toxicity of these substances after oral administration is low. In rats, the values were 900 to > 5000 mg/kg bw (Draize et al., 1948; Harada & Ozaki, 1972; Zaitsev & Rakhmanina, 1974; Levenstein, 1975; Schafer & Bowles, 1985), and in guinea-pigs they were 3100 to > 5000 mg/kg bw (Draize et al., 1948; Zaitsev & Rakhmanina, 1974).

2.3.2.2 Short-term and long-term studies of toxicity

Toxicological studies have been reported for 11 substances in the group. The results of those with the parent alcohol cinnamyl alcohol (No. 647), the corresponding aldehyde, two cinnamate esters, two alpha-alkyl substituted cinnamaldehyde derivatives, two alkoxy-substituted cinnamaldehyde derivatives, and a mixture of five cinnamyl derivatives are described below and summarized in Table 4.