Final Report of the Cosmetic Ingredient Review Expert Panel on the Safety Assessment of Dicarboxylic Acids,Salts,and Esters

Malonic acid (C3)

Malonic acid, first prepared by malic acid oxidation, is commonly manufactured by more recent methods including the ozonolysis of cyclopentadiene or the air oxidation of 1,3-propanediol. ⁷

Succinic acid (C4)

Succinic acid is an intermediate of the citric acid cycle and is found in almost all plant and animal cells, although at very low concentrations. ⁸ Succinic acid is commonly produced synthetically by catalytic (eg, nickel or palladium catalyst) hydrogenation of maleic anhydride.

Glutaric (C5) and adipic(C6) acids

Although glutaric acid is often encountered in nature, adipic acid is not commonly encountered in nature. Glutaric and adipic acids were first synthesized by oxidation of castor oil with nitric acid. However, adipic acid is now more commonly manufactured by the oxidation of cyclohexane, cyclohexanol, or cyclohexanone, and glutaric acid may be manufactured by ozonolysis of cyclopentene. ⁹

Azelaic acid (C9)

Azelaic acid, first detected in rancid fats, was originally produced via nitric acid oxidation of oleic acid. ¹⁰ Azelaic acid is a naturally occurring dicarboxylic acid that can be found in dietary sources, such as whole grains. ¹¹ Azelaic acid is commonly manufactured by oxidative cleavage of oleic acid (obtained from grease or tallow) with chromic acid, nitric acid, or by ozonolysis.^10,7

Sebacic acid (C10)

Sebacic acid was originally isolated from distillation products of beef tallow. More recently, however, sebacic acid has been manufactured via alkali pyrolysis of castor oil, as mentioned above and drawn in Figure 1, or by alkali pyrolysis of ricinoleic acid.^12,7

Dodecanedioic acid (C12)

Dodecanedioic acid can be manufactured by fermentation of long-chain alkanes with a specific strain of Candida tropicalis. ¹³ Another method of manufacture involves the nitric acid oxidation of a mixture of cyclododecanone and cyclododecanol. ⁷

Physical and Chemical Properties

Table 2 lists the physical and chemical properties of the dicarboxylic acids and salts. Figure 2 presents the relationship between molecular weight of these ingredients and the octanol/water partition coefficient expressed as log K_ow.

OPEN IN VIEWER

Figure 2.

Dicarboxylic acids and their Salts; Log K_ow vs molecular weight

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Table 2.

Physical and Chemical Properties of the Alkyl Dicarboxylic Acid and Salt Ingredients^14–16

INCI Name	Malonic Acid	Succinic Acid	Sodium Succinate	Disodium Succinate	Glutaric Acid	Adipic Acid
Appearance	small crystals	colorless prisms	crystalline	crystalline	large monoclinic prisms	white, monoclinic prisms
Molecular Weight (g/mol)	104.06	118.09	140.07	162.05	132.11	146.14
Melting/Boiling Point (°C)	135 (dec.)/ 264 (est.)	185-187/ 235	206 (est.)/ 486 (est.)	156 (est.)/ 426 (est.)	97.5-98/ 302-304	152/265
Density (g/cm3)	1.63	1.56	--	--	1.429	1.360
Vapor pressure (mm Hg @ 25°C)	0.001 (est.)	0.0000002	7.3 E-10 (est.)	8.7 E-8 (est.)	0.000003	0.07
Solubility (g/L water @ 25°C)	1520	83	1000 (est.)	31 (est.)	639	30
Log Kow	−0.81	−0.59	−3.98 (est.)	−3.98 (est.)	−0.29	0.08

INCI Name	Azelaic Acid	Disodium Azelate	Dipotassium Azelate	Sebacic Acid	Disodium Sebacate	Dodecanedioic Acid
Appearance	monoclinic prismatic needles	crystalline	crystalline	Monoclinic prismatic tablets	crystalline	--
Molecular Weight (g/mol)	188.22	238.18	264.40	202.25	246.21	230.31
Melting/Boiling Point (°C)	106.5/ 286.5	186 (est.)/ 484 (est.)	186 (est.)/ 484 (est.)	134.5/ 294.5	194/496 (est.)	128/383 (est.)
Density (g/cm3)	1.0291	--	--	1.207	--	1.16
Vapor pressure (mm Hg @ 25°C)	0.00002 (est.)	1.4 E-9 (est.)	1.4 E-9 (est.)	0.000007 (est.)	5.9 E-10 (est.)	0.000002 (est.)
Solubility (g/L water @ 20°C)	2.4	1000 (est.)	1000 (est.)	1.0	1000 (est.)	0.040
Log Kow	1.57	−3.56 (est.)	−3.56 (est.)	2.19 (est.)	−3.01 (est.)	3.17 (est.)

The alkyl dicarboxylic acids vary considerably in their physical properties. The shorter chain (malonic, succinic, and glutaric) members are crystalline solids, very water soluble, and have limited solubility in organic solvents. As the chain length increases through adipic to dodecanedioic, water solubility decreases sharply (although still soluble in hot water). In other words, the water solubility of these acids is inversely proportional to their chain length. There is a marked alternation in melting point with changes in carbon number from even to odd. ⁷ Odd members (eg, malonic acid and glutaric acid) exhibit lower melting points and higher solubility than even carbon number alkyl dicarboxylic acids (eg, succinic acid and adipic acid). These alternating effects are believed to be the result of the inability of odd carbon number compounds to assume an in-plane orientation of both carboxyl groups with respect to the hydrocarbon chain.

Dicarboxylic acids react with Brønsted-Lowry bases (eg, sodium hydroxide) to form carboxylate salts (eg, sodium succinate or disodium succinate). Dicarboxylic acids also react with alcohols to give mono- and di-esters, such as those in this report.

Succinic acid

Methods used to analyze succinic acid include acidimetric titration for acidity; comparison with platinum–cobalt (Pt-Co) standard calibrated solutions for color; oxidation with potassium permanganate for detection of unsaturated compounds; atomic absorption or plasma spectroscopy for metals; and titration with silver nitrate or barium chloride for chloride or sulfate detection, respectively. ⁷ Small concentrations of succinic acid can be detected by common instrumentation such as gas–liquid chromatography (GLC) and polarography.

Adipic acid

Adipic acid can be extracted from a water sample and analyzed by gas chromatography/mass spectrometry. ¹⁷

Sebacic acid

Gas chromatography can be used to identify sebacic acid in air. ¹⁸

Diisopropyl adipate and diethylhexyl adipate

Diisopropyl adipate and diethylhexyl adipate can be identified through standard infrared (IR) spectroscopy. Gas–liquid chromatography, liquid–liquid extraction, mass spectrometry, and high-pressure liquid chromatography (HPLC) are also methods of analysis for the adipates. ²

Ultraviolet Absorption

The dicarboxylic acids and their salts included in this review would not be expected to have any meaningful ultraviolet (UV) absorption. Except for the acid functional group, these ingredients do not possess any conjugated π bonds or nonbonding electrons. The π bonds and nonbonding electrons in the acid functional group are not part of any conjugated systems. Accordingly, these ingredients are unlikely to absorb light within the UVA-UVB spectrum at a detectable molar absorptivity.

Malonic acid

Malonic acid is a useful intermediate in the manufacture of barbiturates. ¹⁴

Succinic acid

Succinic acid is listed by the FDA as a food additive that is generally recognized as safe (GRAS). ²⁶ Succinic acid is also utilized in detergents, pigments, toners, cement additives, soldering fluxes, and as an intermediate in the synthesis of a number of pharmaceutical products. ⁷

Adipic acid

Adipic acid is listed as a GRAS food additive by the FDA. ²⁷ Adipic acid has several industrial uses in the production of adhesives, plasticizers, gelatinizing agents, hydraulic fluids, lubricants, emollients, polyurethane foams, leather tanning, and urethane. ⁷ However, the bulk of the industrial production of adipic acid is driven by its usefulness in the manufacture of nylon-6,6 (in combination with 1,6-hexanediamine).

Azelaic acid

FDA has approved azelaic acid for use in treating acne and rosacea. A skin cream containing 20% (w/w) azelaic acid is indicated for the topical treatment of mild-to-moderate inflammatory acne vulgaris, ²⁸ and a gel containing 15% azelaic acid is approved for treating rosacea. ²⁹ These drugs are available by prescription only. (As a reference point, azelaic acid is reported to be used in cosmetics at 0.3% in leave-on and 10% in rinse-off formulations that have dermal exposure. ²³ )

Azelaic acid is used in the manufacture of plasticizers, lubricants, and greases. Azelaic acid was identified as a molecule that accumulated at elevated levels in some parts of plants and was shown to be able to enhance the resistance of plants to infections. ³⁰

Sebacic acid

Before 1973, sebacic acid was widely used in the United States, as an aromatic in food. ³¹

Sebacic acid is used in resorbable polymer systems that deliver chemotherapeutic agents (eg, cisplatin, carboplatin) that are implanted at the site of tumors to provide for sustained release of the drugs. ³² Sebacic acid and its derivatives have a variety of industrial uses as plasticizers, lubricants, diffusion pump oils, candles, and as intermediates in the synthesis of polyamides and various alkyd resins. ⁷

Dodecanedioic acid

Dodecanedioic acid is used in the production of nylon (nylon-6,12), polyamides, coatings, adhesives, greases, polyesters, dyestuffs, detergents, flame retardants, and fragrances. ³³

Diethyl malonate

Diethyl malonate finds great utility as the starting material in malonic ester synthesis, a classic organic chemistry reaction wherein a very wide variety of esters can be synthesized. ²⁵

Diisobutyl adipate

Diisobutyl adipate is considered by FDA to be a Prior-Sanctioned Food Ingredients, Plasticizer (21 CFR § 181.27).

Diethylhexyl adipate

Diethylhexyl adipate is used as a plasticizer for polyvinyl chloride (PVC) plastics. ³⁴

Diethyl sebacate

Before 1973, diethyl sebacate was widely used in the United States, as an aromatic in food. ³¹

Dibutyl sebacate

Dibutyl sebacate is a component of PVC. ³⁵

Azelaic acid

The in vitro percutaneous absorption of a 15% azelaic acid gel through human skin, prior to or after the application of 3 different moisturizer formulations, was determined. ⁵² All doses were applied as 5 µL/cm². The second dose was applied 15 minutes after the first. [ ¹⁴ C]Azelaic acid had a finite dose absorption profile, with a rise to peak penetration followed by a slow but steady decline. In vitro, 70% of the azelaic acid diffused into the reservoir solution over 48 hours. The application of a moisturizer, and whether it was applied prior to or following azelaic acid administration, did not have a statistically significant effect on the penetration of azelaic acid. However, there was a trend toward greater percutaneous penetration and mass distribution with the application of a moisturizer lotion prior to the azelaic acid gel.

The percutaneous absorption of azelaic acid was determined using 6 male participants. A total of 5 g of a cream containing 20% azelaic acid was applied to the face (1 g), chest (2 g), and upper back (2 g) of each participant, giving an area dose of approximate 5 mg cream/cm² skin. The test areas were covered 1 hour after dosing with cotton tissues and washed 24 hours after dosing. After 1 week, 100 mL of an aqueous microcrystalline suspension containing 1 g azelaic acid was given orally to each participant. Urinary excretion of unchanged azelaic acid was measured after each dose. Following dermal application, 1.29% of the dose was recovered unchanged in the urine in 24 hours, and a total of 2.2% was recovered by day 3. Following oral administration, 61.2% of the dose was recovered within 4 hours; excretion was complete at this point. Assuming similar rates and pathways in biotransformation following both routes of exposure, percutaneous absorption of azelaic acid was determined to be 3.6% of the dermally applied dose. ⁵³

Cellular effects

Dicarboxylic acids have a cytotoxic effect on the abnormally hyperactive and malignant epidermal melanocytes. Dicarboxylic acids, C8 to C13, have been shown to inhibit mitochondrial oxidoreductases, ⁶² and they have been shown to reversibly inhibit microsomal NADPH and cytochrome P450 reductase. ⁶³ Medium chain length dicarboxylic acids are also competitive inhibitors of tyrosinase in vitro.

Adipic acid

The effect of adipic acid on primary keratinocyte cultures was evaluated using epidermal cells from neonatal NMRI mice. ⁶⁴ Concentrations of ≤30 mmol/L did not inhibit ³ H-thymidine incorporation or affect DNA synthesis, while 40 and 50 mmol/L inhibited both of these parameters. No effect on labeling indices was observed with 1 to 30 mmol/L adipic acid.

Azelaic acid

Azelaic acid, a naturally occurring competitive inhibitor of tyrosinase, has a cytotoxic effect on malignant melanocytes. ⁶⁵ Azelaic acid is also a competitive inhibitor of a number of oxidoreductive enzymes, enzymes involved in DNA synthesis, and of oxidoreductases of the respiratory chain. ⁶⁶ It has been reported that, in vitro, azelaic acid has time- and dose-dependent, reversible, and antiproliferative and cytotoxic effects on a number of tumoral cell lines. Azelaic acid had no effect on normal cell lines.

Disodium azelate

Disodium azelate inhibited cell proliferation and affected viability of Cloudman and Harding-Passey murine melanomata at concentrations ≥10⁻² mol/L when incubated over a 3-day period. ⁶² The mitochondria were the prime target of action.

The effect of disodium azelate on primary keratinocyte cultures was evaluated using epidermal cells from neonatal NMRI mice. ⁶⁴ A dose-dependent inhibition of ³ H-thymidine incorporation into DNA, ranging from 50% inhibition with 20 mmol/L to 90% inhibition with 50 mmol/L disodium azelate, was observed following a 12-hour incubation period. Concentrations of 1 and 10 mmol/L did not affect DNA synthesis, but a marked reduction was seen with 20 to 50 mmol/L. The effects on DNA synthesis were time dependent, with the maximum inhibitory effect observed at 4 hours; this effect was reversible. RNA and protein synthesis were also inhibited during the first 4 hours of incubation with 50 mmol/L disodium azelate. Cellular structure was altered upon incubation with disodium azelate, primarily affecting mitochondria and the rough endoplasmic reticulum. These effects were also reversible.

Dodecanedioic acid

The disodium salt of dodecanedioic acid inhibited cell proliferation and affected viability of Cloudman and Harding-Passey murine melanomata at concentrations ≥10⁻² mol/L, when incubated over a 3-day period. ⁶² The mitochondria were the prime target of action.

Animal studies

Most of the available animal studies were oral exposures, although limited inhalation studies were available for adipic acid.

Adipic acid Oral

Groups of 6 male Sprague-Dawley rats were dosed orally (method not specified) with 3600 to 5600 mg/kg bw adipic acid as an 18.6% to 24.9% solution in saline for 14 days. ⁴⁰ Three animals of the 3600 mg/kg bw group, 5 of the 4000 mg/kg bw group, and all of the 4500 to 5600 mg/kg bw groups died prior to study termination. Signs of toxicity included depressed activity, labored respiration, ataxia, and convulsions. No gross findings were noted at necropsy at study termination.

Groups of 5 rats were dosed with 0 or 3000 mg/kg bw of a neutralized 20% adipic acid solution orally, by gavage, for 4 weeks. ⁴⁹ A nonsignificant decrease in bw gain was observed. In a 4-week study in which a group of 3 rats was dosed orally, by gavage, with 2400 mg/kg bw adipic acid, no significant toxicological effects were noted.

In a 4-week dietary study in which groups of 17 to 20 female rats were fed 0 to 40 mg/d (0-435 mg/kg bw per d) adipic acid, no effects were reported. ⁴⁹ The no-observable adverse effect level (NOAEL) was >435 mg/kg bw per d. In a 5-week dietary study in which groups of 15 to 18 male rats were fed 0 to 800 mg/d (0-13 333 mg/kg bw/d) decreased bw gains, an unkempt appearance, and diarrhea were observed for the animals fed 800 mg/d the first 3 weeks. In another 5-week dietary study in which groups of 4 rats, gender not specified, were fed 100 or 200 mg/d (310-922 mg/kg bw/d) of a 20% adipic acid solution in ethanol, 5 days/week, no signs of toxicity were observed.

Ten rats were dosed orally, method not specified, with 199 mg/d (638-1332 mg/kg bw/d) sodium adipate, 5 days/week for 9 weeks. ⁴⁹ No toxicological effects were observed.

A group of 5 guinea pigs, gender not specified, were dosed orally using capsules with 400 mg/d (682-942 mg/kg bw/d) adipic acid for 5 days, followed by dosing with 600 mg/d (1032-1739 mg/kg bw/d), 5 days/week for 5 weeks. ⁴⁹ No signs of toxicity were observed.

No toxicity was observed in a study in which pigs were fed 1% adipic acid in the diet for 7 days. ⁴⁹

Groups of 8 to 10 male rats were given 0, 420, 840, 1700, or 3400 mg/kg bw/d sodium adipate for 19 weeks in a protein deficient diet. ⁵⁷ Animals were killed after either 7 weeks or at study termination. For unexplained reasons, only 5 to 7 animals/group survived until study termination. Rats of the 3400 mg/kg bw/d group had decreased bw gains and decreased bws. (Statistical significance not stated.) Slight effects were seen in the liver, and the NOAEL was 3333 mg/kg bw.

Groups of 13 to 15 male and female rats were fed a diet containing 0, 1600, or 3200 mg/kg bw/d adipic acid for 33 weeks. ⁴⁰ Rats were killed at various intervals throughout the study. Ten of 14 rats fed 3200 mg/kg bw/d died during weeks 0 to 4; surviving rats had decreased weight gains during this time. However, at study termination, bws were for surviving animals of this group were similar to controls. Slight effects were seen in the liver. (Statistical significance not stated.)

In a 2-year study, groups of 20 male rats were fed a diet containing 0%, 0.1%, 1%, 3%, and 5% adipic acid (equiv. to 0, 75, 750, 2250, and 3750 mg/kg bw/d), and groups of 10 and 19 females were fed 0% and 1% adipic acid, respectively. ⁴⁹ Weight gains of male rats fed 3% and 5% adipic acid were significantly less than controls. There were no significant toxicological findings upon gross or microscopic observation. The NOAEL was 1% adipic acid for male and female rats.

The effect of adipic acid on hepatic peroxisome proliferation was evaluated in an in vivo study in which 4 male F344 rats were fed chow containing 2% adipic acid dissolved in alcohol. ⁶⁷ After 3 weeks of dosing, the animals were killed. Adipic acid did not induce peroxisome proliferation and did not affect relative liver to bws.

Sodium succinate

The oral toxicity of sodium succinate was evaluated using F344 rats. ⁵⁸ Groups of 10 males and 10 females were given 0, 0.3, 0.6, 1.25, 2.5, 5 or 10% sodium succinate in the drinking water for 13 weeks. All animals were killed at the termination of dosing. Body weight gains of animals of the 10% group were significantly decreased, and all animals of this group died by week 4. These animals were extremely emaciated; however, no compound-related microscopic lesions were found. Body weight gains were decreased in animals given ≥2.5% sodium succinate, as compared to controls. No toxicological treatment-related effects were observed.

Glutaric acid

Groups of 15 male and 15 female Sprague Dawley rats were fed a diet containing 0% to 2% glutaric acid for 90 days. ⁵⁵ Body weight gains were decreased for males and statistically significantly decreased for females of the 2% group. No differences were noted between test and control animals in hematology, clinical chemistry, or urinalysis. There were no microscopic findings or organ weight changes attributable to the test substance. There was no treatment-related mortality. The NOAEL was ≥1%, and the LOAEL was 2% glutaric acid.

Four male and 4 female Beagle dogs were fed a diet containing 0% to 5% glutaric acid for 90 days. ⁶⁸ Decreased bws, accompanied by reduced feed consumption, were observed for the males and females of the 5% group and females of the 3% group. No other treatment-related effects were observed. The NOAEL was ≥2% and the LOAEL was 3%.

Adipic/glutaric/succinic acid mixture

Groups of 15 male and 15 female rats were dosed orally, by gavage, for 90 days with 0% to 30% of a mixture that contained 4% adipic, 16% glutaric, and 5% succinic acid. ⁵⁵ The vehicle was deionized water, and the dosing volume was 10 mL/kg. Two males and 1 female of the 30% group died, and the deaths were considered dose-related. Also in this group, bws were reduced for males and females, and feed consumption was statistically significantly reduced in males. An increased incidence of labored breathing and rales was noted. The urine pH was statistically significantly reduced in both males and females dosed with 30% of the mixture. In the 10% group, bw gains were slightly, but not statistically significantly, reduced in females and feed consumption was statistically significantly reduced in males. The NOAEL was 3% and the LOAEL was 10%.

Azelaic acid

Groups of 15 male and 15 female Wistar rats were fed a diet containing 140 or 280 mg/kg bw azelaic acid for 180 days, and a control group of 10 males and 10 females was given untreated feed. ⁵⁹ No significant toxicological effects were observed. Growth was similar between test and control groups, as were the microscopic examinations and clinical chemistry parameters. The researchers found similar, negative, results when groups of 10 male and 10 female New Zealand rabbits were fed diets containing 0, 200, or 400 mg/kg bw azelaic acid for 180 days.

Disodium sebacate

Groups of 10 male and 10 female Wistar rats were fed a diet containing 0, 500, or 1000 mg/kg bw disodium sebacate for 6 mos, after which time they were killed and necropsied. ⁵⁴ Growth was similar between test and control groups, as were the microscopic examinations and clinical chemistry parameters. The researchers found similar, negative, results when groups of 10 male and 10 female New Zealand rabbits were fed diets containing 0, 750, or 1000 mg/kg bw disodium sebacate for 6 mos.

Succinic acid

The ocular irritation potential of succinic acid was evaluated using albino rabbits. ⁵⁵ Undiluted test material, 0.005 mL, was applied to the center of the cornea. The eyes were not rinsed. Succinic acid was a severe eye irritant, with necrosis visible upon staining. The score for ocular irritation, on a scale of 1 to 10, was 8.

Glutaric acid

A Draize ocular irritation study was performed in which 100 mg of glutaric acid was instilled in the eyes of 3 rabbits and the eyes were rinsed 24 hours after application. ⁵⁵ Glutaric acid was irritating to rabbit eyes, with a primary irritation index (PII) of 35.2/110. Mild erythema, slight edema, and slight dullness were still present after 7 days.

Adipic acid

The ocular irritation of adipic acid was evaluated using groups of 2 albino rabbits. ⁵⁵ Ten or 57.1 mg of adipic acid was placed in the eye of each rabbit, and the eye of 1 animal in each group was rinsed. With 10 mg followed by rinsing, mild conjunctival irritation was observed; and the eye was normal within 3 days. In the unrinsed eye, mild conjunctival irritation and a minimal iritic effect were observed; minimal conjunctival irritation was still observed after 7 days and the eye was normal after 14 days. With instillation of 57.1 mg adipic acid followed by rinsing, moderate to mild conjunctival irritation and transient mild opacity were observed; the eye was normal in 3 days. In the unrinsed eye, moderate to mild conjunctival irritation, mild opacity of the cornea, and a minimal iritic effect were observed; the eye was normal at day 7. However, other studies have reported that adipic acid produced severe irritation in rabbit eyes, and the signs of irritation were still present after 8 days. ⁴⁰

Adipic/glutaric/succinic acid mixture

The ocular irritation potential of a mixture of adipic, glutaric, and succinic acid, percentages not specified, was evaluated using 2 male albino rabbits. ⁵⁵ One-tenth milliliter of the test substance was instilled in the conjunctival sac of each animal, and the eye of 1 animal, but not the other, was rinsed. The contralateral eye served as the negative control. Mild to severe conjunctivitis was observed in on both the rinsed and unrinsed rabbit eyes. Both eyes were normal within 21 days.

Dodecanedioic acid

In studies using rabbits that evaluated the ocular irritation of dodecanedioic acid, slight irritation was reported in 1 study, with a PII of 11.96/110, and small areas of corneal opacity and mild conjunctival irritation were seen in the other study. ⁵⁶ Details were not provided.

Succinic acid

Succinic acid was a slight irritant to rabbit skin. ⁵⁵ Details were not provided.

Glutaric acid

The dermal irritation potential of glutaric acid was determined using 2 male and 4 female New Zealand white rabbits. ⁵³ A 0.5 g aliquot of glutaric acid was applied to the clipped skin on the back of the rabbits. The test site was scored for irritation after 3 minutes, and the site was then washed. The test material was then applied to 2 other test sites, which were covered with a rubber wrap. The sites were examined at 1 hour and 4 hours, and the site was washed after both examinations. The sites were then evaluated at 24 and 48 hours after application. Slight erythema was seen in 1 rabbit throughout the study. Irritation was not observed in the other rabbits.

Adipic Acid Nonhuman

A dermal irritation study was performed in which 500 mg of 50% aqueous adipic acid was applied under an occlusive patch to a 5 cm x 5 cm area of intact and abraded skin of 6 rabbits for 24 hours. ³⁸ With intact skin, an erythema score of 2 to 3/4 was reported, with clearing by day 3. With abraded skin, mild to severe erythema and edema were reported, which cleared by day 7.

Adipic acid, undiluted or as an 80% aqueous paste, was applied occlusively to the backs of ears of rabbits for 24 hours. Two rabbits were used per group. No irritation was observed on the backs of animals. Erythema was observed on the ear, with clearing by 72 hours. In another study in which adipic acid was applied occlusively for 24 hours, irritation was not observed. Details were not provided.

A semi-occlusive application of 500 mg of a paste of 50% adipic acid in propylene glycol to 6 rabbits produced slight to mild irritation in 3 of the rabbits. A semi-occlusive application of undiluted adipic acid was not corrosive. Adipic acid, 50% in propylene glycol, was not irritating to a group of 10 guinea pigs.

The sensitization potential of adipic acid was evaluated using groups of 10 guinea pigs. For induction, 0.1 mL of 1% aqueous adipic acid was given as a sacral intradermal injection, once a week for 4 weeks. After a 2-week nontreatment period, the dermal challenge was performed with 0.05 mL of 50 and 25% adipic acid in propylene glycol. Adipic acid produced very mild or no irritation and it was not a sensitizer.

Human

In 2 case reports involving occupational exposure to adipic acid, positive sensitization reactions were reported with follow-up testing. ³⁸

Adipic/glutaric/succinic acid mixture

A mixture of adipic, glutaric, and succinic acid (percentages not specified) was evaluated for irritation and for sensitization using groups of 10 male guinea pigs. ⁵³ The primary irritation potential was evaluated by applying 0.05 mL of an 8 or 80% suspension in dimethyl phthalate to the shaved, intact skin on the shoulder of the animals. The sensitization potential was also evaluated, using 4 sacral intradermal injections of 0.1 mL of a 1% suspension for induction. After a 13-day nontreatment period, a dermal challenge was performed with 0.05 mL of an 8% and 80% suspension of the mixture. Ten previously untreated guinea pigs were exposed to the same challenge applications as the test animals. In the test for primary irritation, the 8% suspension produced no irritation, and no to mild irritation was observed 24 hours after exposure to the 80% suspension. No sensitization was observed at either dose.

Azelaic acid

The cumulative irritation potential of a 15% azelaic acid gel (prescription formulation; vehicle not identified) was determined in a study using 31 female and 2 male participants. ⁶⁴ (During the study, 1 participant withdrew for personal reasons.) White petrolatum was used as a negative control. Azelaic acid and petrolatum, 0.2 g of each, were applied under occlusion to 2 cm x 2 cm sites on the back of each participant 3 times per week for 3 weeks. Weekday patches were removed after 24 hours, while the patches applied on Fridays were removed after 72 hours. The test sites were evaluated 15 to 30 minutes after removal of the patch, and then a new patch was applied. Application was discontinued if severe irritation, which was designated by a maximum erythema score of 3, was observed. A 15% azelaic acid gel was statistically significantly more irritating than the negative control, with a mean cumulative irritancy index of 1.05/3. Individual reaction scores for the test article ranged from 0 to 3, and 5 participants discontinued patching with azelaic acid due to an irritation score ≥3. Cumulative irritancy increased with successive patching. The researchers noted that since the vehicle used for azelaic acid was not tested, there was uncertainty as to whether the vehicle components affected the irritation scores.

Twice daily application of a cream containing 20% azelaic acid has been reported to cause erythema, irritation, pruritus, dryness, scaling, and burning. ⁶⁵

Dodecanedioic acid

Dodecanedioic acid was not an irritant to rabbit skin in a 4-hour exposure study or upon application of 0.5 g. ⁵⁴ In a maximization study using female guinea pigs, 0.5% dodecanedioic acid was injected intracutaneously at induction and 25 and 50% was used for the dermal challenge. Dodecanedioic acid was not a sensitizer.

Succinic acid

Succinic acid has been considered to be an exacerbating factor in ulcerative colitis, therefore its influence on rat colonic mucosa in terms of mucosal blood flow and superoxide generation was investigated. ⁶⁶ The left side of the colon of 5 male and 5 female rats was exposed, and 0.9% to 5% succinic acid in physiological saline was instilled into the colonic lumen. A segment of the colon was then ligated as to not include the mesenteric blood vessel. Mucosal blood flow decreased with all dose levels. Microscopically, the higher the concentration of succinic acid, the greater was the erosion formation in the colonic mucosa. Significant polymorphonuclear cell infiltration superoxide generation from colon tissue was observed with 0.01% succinic acid, as compared to higher or lower concentrations. Succinic acid, at fecal concentrations found in active stage ulcerative colitis, appears to be implicated in mucosal injury, mediated by a decrease in colonic mucosal blood flow and infiltration of superoxide-generating polymorphonuclear cells into the mucosa.

Malonic acid

Malonic acid, 3333 µg/plate, was not mutagenic in a National Toxicology Program (NTP) preincubation assay, with or without metabolic activation. ⁷⁰

Succinic acid

The genotoxic potential of succinic acid was evaluated in an Ames test and in a chromosomal aberration study using a Chinese hamster fibroblast cell line. ⁷¹ Succinic acid, at a concentration of ≤5.0 mg/plate in phosphate buffer, was not mutagenic in the Ames test. (Whether metabolic activation was used is not stated.) Concentrations of ≤1.0 mg/mL in saline were not genotoxic in the chromosomal aberration assay.

Disodium succinate

The genotoxic potential of disodium succinate was evaluated in an Ames test and in a chromosomal aberration study using a Chinese hamster fibroblast cell line. ⁷¹ In the Ames test, disodium succinate was not mutagenic at concentration up to 5.0 mg/plate in phosphate buffer. (Whether metabolic activation was used is not stated.) Equivocal genotoxic results were obtained in the chromosome aberration assay of ≤15.0 mg/mL disodium succinate in saline using Chinese hamster fibroblast cells.

Disodium succinate, ≤10 mg/plate, was negative in another Ames test, with and without metabolic activation. ⁷⁷

Glutaric acid

Glutaric acid was evaluated in vitro in a standard Ames assay, the L5178Y/TK ± mouse lymphoma assay with and without metabolic activation, and the mammalian in vitro Balb/c-3T3 cell transformation assay with and without metabolic activation. ⁷⁸ The Ames tests were negative. However, the cell transformation assay was positive both in the presence and absence of metabolic activation and the results in the mouse lymphoma assay were dependent upon pH of the culture medium. The researchers stated that the variable response in the mouse lymphoma assay and the positive effect in the cell transformation assay may have been an indirect effect of other factors (such as the pH or osmolarity of the media in which the cells were exposed), rather than a direct effect of glutaric acid.

Adipic acid

Adipic acid was evaluated in a number of Ames assays using Salmonella typhimurium and Escherichia coli; results were negative, with or without metabolic activation, at concentrations as high as 10 000 mg/plate. ^40,74,75 Negative results were also obtained in an Ames test with 0 to 200 mg/L adipic acid using S typhimurium TA1530 and G-46 without metabolic activation. ⁴⁰ Results were negative in a yeast gene mutation assay using Saccharomyces cerevisiae without metabolic activation at concentrations ≤200 mg/L. A mouse lymphoma assay using L5178Y/TK ± cells was negative with and without metabolic activation at concentrations of ≤2000 µg/plate, ⁷⁵ as was a cytogenetic assay using human embryonic lung fibroblast cells with ≤200 mg/L adipic acid. ⁴⁰ In a viral enhanced cell transformation assay using Syrian hamster embryo cells at doses of 62 to 1000 µg/mL adipic acid, results were negative.

Adipic/glutaric/succinic acid mixture

A mixture of adipic, glutaric, and succinic acid, percentages not specified, tested as a 50% aqueous solution, was not mutagenic in an Ames assay using S typhimurium, with or without metabolic activation, at concentrations of ≤300 µg/plate. ⁵⁵ Negative results were also obtained in an unscheduled DNA synthesis assay at concentrations of ≤5000 µg/plate using rat hepatocytes and in an HGPRT assay at concentrations of ≤2500 µg/plate, without, and of ≤3500 µg/plate, with, metabolic activation. In an invitro transformation assay using Chinese hamster ovary (CHO) cells at concentrations of ≤1500 µg/mL without and ≤2500 µg/mL with metabolic activation, positive results were obtained with, but not without, metabolic activation at 2000 µg/plate.

Azelaic acid

Azelaic acid, 20%, was not mutagenic or genotoxic in an Ames assay, HGPRT test in CHO cells, or human lymphocyte test. ⁴⁴ Details were not provided.

Dodecanedioic acid

Dodecanedioic acid was not mutagenic in an Ames assay at concentrations of ≤5000 µg/plate, with and without metabolic activation. ⁵⁶ Toxicity occurred at ≥500 µg/plate.

Glutaric acid

Glutaric acid was evaluated in a mammalian micronucleus cytogenetic assay in mice. ⁷⁸ Glutaric acid was not genotoxic in this assay. (Details not specified.)

Adipic acid

Adipic acid was not genotoxic in in vivo cytogenetic assays using chromosomes from rats dosed orally, by gavage, with a single dose of 5000 mg/kg bw or daily for 5 days with 2500 mg/kg bw. ⁴⁰ Adipic acid was also not genotoxic in dominant lethal studies with doses up to 5000 mg/kg bw.

Adipic/glutaric/succinic acid mixture

A mixture of adipic, glutaric, and succinic acid, percentages to specified, was not genotoxic in vivo using male and female Sprague Dawley rats dosed orally by gavage with 2750 and 1375 mg/kg of the mixture, respectively. ⁵⁵

Azelaic acid

Azelaic acid was not genotoxic in a dominant lethal assay in mice. ⁴⁴ (Details not specified.)

Dodecanedioic acid

Dodecanedioic acid, ≤5000 mg/kg bw, was not mutagenic in a micronucleus assay using mice. ⁵⁶

Succinic acid, sodium succinate, disodium succinate

The promotion of urinary bladder carcinogenesis by sodium succinate was evaluated using male F344 rats. ⁷⁹ Groups of 16 male F344 rats were given 5% succinic acid, sodium succinate, or disodium succinate with 0.05% N-butyl-N- (4-hydroxybutyl)nitrosamine (BBN) in the drinking water for 4 weeks, followed by dietary administration of 5% of the respective test article without BBN for 32 weeks. Negative controls were given water with BBN only and untreated feed. Groups of 8 male F344 rats followed the same protocol without the addition of BBN to the drinking water, as did a group of non-BBN-treated negative controls. The animals were killed at week 37.

In the BBN-pretreated groups, many rats given sodium or disodium succinate developed hematuria towards the end of the study. There were no statistically significant differences in body or organ weights between the control and test groups. (Information on organ and bws was not provided for the non-BBN groups.) Large tumors were found on the urinary bladders of the BBN-pretreated animals given sodium and disodium succinate; tiny lesions were found in the control or succinic acid BBN-pretreated animals. The incidence and number of urinary bladder carcinomas and papillomas and of papillary or nodular hyperplasia (preneoplastic lesions) were statistically significantly increased in the sodium and disodium succinate BBN-pretreated groups as compared to the succinic acid and control BBN-pretreated groups. The incidence and numbers observed in the sodium and disodium succinate groups were not statistically significantly different from each other. An association between tumor area and sodium intake was noted. Urinary bladder lesions were not observed in any of the animals that were not pretreated with BBN. Urinary pH and electrolyte concentrations were affected by dosing with sodium or disodium succinate with BBN, as compared to the control and succinic acid groups, and statistically significant differences between these 2 groups were observed as well.

The researchers also evaluated cell proliferation and DNA synthesis in the urinary bladder epithelium. Groups of 20 male F344 rats were given 5% succinic acid, sodium succinate, or disodium succinate in the feed, without BBN pretreatment for 8 weeks. Negative controls were given basal diet. Five rats per group were given an ip injection of 50 mg/kg bw 5-bromo-2’-deoxyuridine (BrdU) 1 hour prior to being killed. Compared to control values, BrdU uptake was statistically significantly increased by increased disodium succinate and was increased, but not in a statistically significant manner, by sodium succinate. Succinic acid did not have any effect on DNA synthesis. Microscopically, simple hyperplasia was observed in the urinary bladders of animals given sodium and disodium succinate. The appearance of the urinary bladder epithelial surface was altered by sodium and disodium succinate. Spermidine/spermine N ¹-acetyltransferase activity in the urinary bladder epithelium was increased for disodium succinate, but not sodium succinate, when compared to controls. Urinary pH and electrolyte concentrations were affected as described previously.

Diethyl malonate

Malonic acid esters can be produced either by cobalt-catalyzed alkoxycarbonylation of chloroacetates with carbon monoxide in the presence of the appropriate alcohol, or by hydrolysis of cyanoacetic acid followed by esterification with the respective alcohol. ³⁹ Diethyl malonate is prepared from chloroacetic acid and sodium cyanide followed by esterification with ethanol and sulfuric acid. ⁸⁰

Diisopropyl adipate

Diisopropyl adipate is produced by esterification of adipic acid with an excess of isopropanol. The excess alcohol is removed by vacuum stripping and the ester is then alkali-refined and filtered. ²

Dibutyl adipate

Adipic acid is esterified with butyl alcohol by a continuous distillation process. ⁸¹

Diethylhexyl adipate

Diethylhexyl adipate can be prepared by the reaction of adipic acid and 2-ethylhexanol in the presence of an esterification catalyst such as sulfuric acid or para-toluenesulfonic acid (Figure 2). ¹⁷ Purification of the reaction product includes removal of the catalyst, alkali refining, and stripping. ²

Alkyl succinates

Succinic anhydride reacts readily with alcohols to give monoesters of succinic acid (eg, decyl succinate from decanol), which are readily further esterified to the diesters by Fischer methods. ⁷ Dimethyl succinate can be produced from methanol and succinic anhydride or succinic acid, or by hydrogenation of dimethyl maleate. Diethyl succinate can be prepared by the same methods (from ethanol or diethyl maleate).

Diethyl malonate

Diethyl malonate is a colorless organic liquid with an ester like odor. ³⁹ The purity is typically > 99 %. Impurities from the production process include ethanol (ca. 0.1 % w/w), ethyl acetate (ca. 0.05 % w/w), and ethyl methyl malonate (ca. 0.05 % w/w).

Dibutyl adipate

Impurities are generally not found due to the manufacturing process, but available data demonstrate that arsenic levels are below a detection limit of 1 ppm, heavy metals (as lead) are below a detection limit of 10 ppm, and sulfated ash is below a detection limit of 0.1%. ⁸¹

Diisopropyl adipate and diethylhexyl adipate

Diisopropyl adipate and diethylhexyl adipate are considered stable; however, hydrolysis of the ester groupings may occur in the presence of aqueous acids or bases. No known impurities occur in either diisopropyl adipate or diethylhexyl adipate, although the acid values imply the presence of adipic acid or of the monoester in both. ²

Diethylhexyl adipate is commercially available with the following specifications: purity—99% to 99.9%; acidity—0.25 µg/100g max; moisture—0.05% to 0.10% max. ¹⁷

Diisopropyl sebacate

A supplier reported that the expected impurities in diisopropyl sebacate are the starting material sebacic acid, <0.3%, and isopropyl alcohol, <0.2%. ⁸²

Ditridecyl adipate

The percutaneous absorption of [ ¹⁴ C]ditridecyl adipate was determined using groups of 10 male and 10 female Sprague-Dawley rats that were untreated or that had previously been exposed to unoccluded dermal applications of 0 or 2000 mg/kg bw ditridecyl adipate, 5 days/week for 13 weeks. ⁹² (This study is described in the section on ‘Subchronic Dermal Toxicity’). A single 58 µl dose of 2000 mg/kg bw [ ¹⁴ C]ditridecyl adipate was applied topically (size of test site not specified), and urine and feces were collected for 4 days. In the previously untreated rats, a total of 11.6 and 10.6% of the [ ¹⁴ C] solution was absorbed by male and female rats, respectively, over 4 days. Approximately 63 and 52% of the absorbed dose (7.4 and 5.5% of the applied dose, respectively) was found in the tissues of males and females, respectively. A total of 3.5% to 4.7% of the applied dose was recovered in the urine and 0.4% to 0.7% in the feces of previously untreated rats. The values for the animals previously dosed with 2000 mg/kg bw ditridecyl adipate were not statistically significantly different from the controls. In the previously dosed animals, a total of 10.8 and 9.1% of the dose was absorbed by males and females, respectively, over the 4 days, with approximately 87 and 81% of the absorbed dose (9.4 and 7.4% of the applied dose, respectively) found in the tissues of the male and female rats, respectively. A total of 0.7% to 1.3% of the [ ¹⁴ C] was recovered in the urine and 0.4% to 0.6% in the feces. Based on the radioactivity recovered in the urine, the bioavailability of ditridecyl adipate was 2% to 6%, and previous dosing did not significantly affect absorption.

Diethylhexyl Adipate In Vitro

The in vitro hydrolysis of diethylhexyl adipate (and mono-(2-ethylheyxl) adipate [MEHA]) using tissue preparations from the liver, pancreas, and small intestine of 2 rats was examined, as were the effects of diethylhexyl adipate on serum and hepatic enzymatic activities in vitro. ⁹³ Diethylhexyl adipate was readily hydrolyzed to MEHA or adipic acid by each tissue preparation. The formation of adipic acid was rapid and approximately the same for all 3 tissues, while the formation of MEHA was rapid only in pancreatic tissue and was negligible in the intestine. The rate of hydrolysis from MEHA to adipic acid was greater that than from diethylhexyl adipate and the highest activity was found in intestinal tissue. In examining the effects on serum and hepatic enzymes, only N-demethylase activity was considerably inhibited by diethylhexyl adipate.

Diethylhexyl sebacate

Diethylhexyl sebacate is not readily absorbed through the skin of guinea pigs (no further details were provided). ¹ It was noted that the metabolism of diethylhexyl sebacate in rodents and humans may follow partially common pathways.

Diethylhexyl adipate

The potential of diethylhexyl adipate to bind to liver DNA of female NMRI mice was evaluated by administering a solution of 119 mg diethylhexyl adipate/mL with 3.85 mCi/mL of [ ¹⁴ C]diethylhexyl adipate (labelled at C1 of the alcohol moiety) and 27.7 mCi/mL of [ ³ H]diethylhexyl adipate (tritiated at positions 2 and 3 of the alcohol moiety) in olive oil. ¹⁰³ The animals were dosed by gavage, and the livers were excised 16 hours after dosing. Some animals were pretreated with 10 g/kg of unlabeled dietary diethylhexyl adipate for 4 weeks. Diethylhexyl adipate did not covalently bind to hepatic DNA in mice. Pretreatment with diethylhexyl adipate caused an increase in liver weight, but no increase in DNA binding. The researchers stated that tumorigenicity of diethylhexyl adipate must be due to an activity other than DNA binding.

The ability of diethylhexyl adipate to stimulate liver DNA synthesis in male F344 rats was investigated using radiolabeled thymidine. ¹⁰⁴ Contrary to expected results, diethylhexyl adipate did stimulate DNA synthesis. The stimulation factor, which is indicated by the ratio of the thymidine incorporation in treated animals compared to controls, was 10.5 and the doubling dose, which is the dose that produced a doubling of the control level DNA synthesis, was 0.7 mmol/kg.

The effect of dosing with diethylhexyl adipate on 8-hydroxydeoxyguanosine (8-OH-dG) in liver and kidney DNA of rats was examined. ¹⁰⁵ Groups of 10 male F344 rats were fed a diet containing 0 or 2.5 diethylhexyl adipate. Five animals per group were killed after 1 week, and the other 5 after 2 weeks of dosing. Relative liver to bws were statistically significantly increased after 1 and 2 weeks of dosing, and the relative kidney to bws were statistically significantly increased only after 2 weeks. A statistically significant increase in 8-OH-dG was observed in the liver DNA, but not the kidney DNA, at week 1 and 2.

The IARC remarked that the weight of evidence for diethylhexyl adipate, and other rodent peroxisome proliferators in general, demonstrated that rodent peroxisome proliferators do not act as direct DNA-damaging agents. ¹⁷

Diethylhexyl adipate

Dietary administration of diethylhexyl adipate affects hepatic lipid metabolism. ¹⁷ Hepatic fatty acid-binding protein and microsomal stearoyl-CoA desaturation were increased in Wistar rats fed 2% diethylhexyl adipate for 7 days. ^106,107 When fed to rats for 14 days, an increase in hepatic phospholipid levels and a decrease in phosphatidylcholine:phosphatidylethanolamine ratio was reported. ¹⁰⁸ In male NZB mice fed 2% diethylhexyl adipate for 5 days, an induction of fatty acid translocase, fatty acid transporter protein, and fatty acid binding protein in the liver was reported. ¹⁰⁹

Dibutyl adipate

Dibutyl adipate was tested for cytotoxicity in the metabolic inhibition test. A dilution series of dibutyl adipate was suspended in HeLa cells. Dibutyl adipate had no acute toxicity to the cells, which was attributed to its insolubility in water. ⁵

Dibutyl Adipate

Oral

Male and female Crj:CD(SD) rats, number per group not specified, were dosed orally, by gavage, with 0, 20, 140, or 1000 mg/kg bw dibutyl adipate in olive oil daily for 28 days. ¹¹⁰ No clinical, hematological, or microscopic test-article related changes were observed.

Dermal

Groups of 10 rabbits were dosed dermally with 0.5 or 1.0 mL/kg/d of a 20% dispersion of dibutyl adipate, 5x/week for 6 weeks. A significant reduction in bw gain was seen for animals of the 1.0 mL/kg/d group, and renal lesions were seen in 1 animal of each group. ⁵

No adverse effects were reported in a study using 4 dogs in which entire-body applications of an emulsion con×tain×ing 6.25% dibutyl adipate were made 2x/week for 3 mos. ⁵

Diethylhexyl Adipate

Oral

In a 14-day dietary study, groups of 5 male rats and mice were given ≤50 000 ppm and groups of 5 female rats and mice were given ≤100,000 ppm diethylhexyl adipate. Male rats and mice fed 50 000 ppm and female rats and mice fed ≥25,000 ppm had decreased weight gains or weight loss. (It is not specified whether the results were statistically significant.) One female rat and all female mice of the 100 000 ppm group died. ²

In a 14-day study in which 5 male and 5 female Wistar and F344 rats and Swiss and B6C3F₁ mice were dosed with 0 to 2.5 g/kg diethylhexyl adipate in corn oil for 14 days, diethylhexyl adipate was toxic to female B6C3F₁ mice, causing mortality, at a dose level of 2.5 g/kg. ⁹⁷ The toxicity of 2 metabolites of diethylhexyl adipate, 2-ethylhexanol and 2-ethylhexanoic acid, was also examined using Wistar rats and Swiss mice. 2-Ethylhexanol was toxic to male and female rats, with mortality reported at doses >1.05 g/kg in male and female rats. 2-Ethylhexanoic acid was toxic to female rats, with mortality reported at doses ≥1.9 g/kg; mortality was not reported for male rats. These effects were not reported in mice.

In a 1 and 4-week dietary study in which groups of 5 to 8 rats and mice were fed diets containing 0% to 4.0% and 0% to 2.5% diethylhexyl adipate, respectively, feed consumption by rats was decreased in the 4.0% group at 1 week and in the 2.5 and 4.0% dose groups at 4 weeks. ¹⁰¹ Body weights were significantly decreased in these groups. Feed consumption by mice was not affected, but a significant decrease in bws was seen in the 1.2 and 2.5% dose groups at 4 weeks.

Toxicity was evaluated in a study in which groups of 10 female Crl:CD(SD) rats were dosed, by gavage, with 5 mL/kg of 0, 200, 1000, or 2000 mg/kg bw diethylhexyl adipate in corn oil for 2 or 4 weeks. ¹²⁵ All animals survived until study termination. In the 2-week study, no statistically significant findings were observed for the animals dosed with 200 mg/kg bw, and the only statistically significant finding in the 1000 mg/kg bw dose group was an increase in relative liver to bw. In the 2000 mg/kg bw dose group, there was staining around the perineum, statistically significant increases in relative liver and kidney to bws, and a statistically significant decrease in the relative weight of the left ovary. Microscopically, abnormal findings were reported for both the ovary and kidney. In the ovary, an increase in atresia of the large follicle and a decrease in currently formed corpora lutea were seen in animals dosed with 1000 and 2000 mg/kg bw, and in the 2000 mg/kg bw group, an increase in follicular cysts was observed. In the kidney, an eosinophilic change of the proximal tubule was observed for the 2000 mg/kg bw dose group. The NOAEL was 200 mg/kg bw.

In the rats dosed for 4 weeks, similar observations were made. There was staining around the perineum of animals dosed with 1000 and 2000 mg/kg bw diethylhexyl adipate, and final bws of animals dosed with 2000 mg/kg bw were statistically significantly decreased. The relative kidney to bws were statistically significantly increased in animals at all dose levels, and liver weights were statistically significantly increased in animals of the 1000 and 2000 mg/kg bw dose groups. The mean estrous cycle length was statistically significantly decreased in the 200 mg/kg bw dose group, but this was not considered treatment-related since a dose-response was not seen. The same microscopic abnormalities reported in the 2 weeks study were seen in the ovaries and kidneys of the animals dosed with 1000 and 2000 mg/kg bw in the 4-week study. As in the 2-week study, the NOAEL for ovarian toxicity was 200 mg/kg bw.

In a 13-week dietary study, groups of 10 rats and 10 mice were fed ≤25 000 ppm diethylhexyl adipate. With the exception of decreased weight gain for some of the groups, no compound-related toxicologic effects were observed. ²

In a 90-day dietary study, groups of 10 rats per group were fed 0 to 4740 mg/kg bw diethylhexyl adipate for 90 days. ¹¹³ Mortality occurred in the 4740 mg/kg bw group, but the number of deaths was not specified. Decreased growth and feed consumption was reported for animals fed 2920 mg/kg bw. Changes in kidney and liver weights were noted, but no details were given. The NOEL was 610 mg/kg bw, and the LOEL was 2920 mg/kg bw diethylhexyl adipate.

Groups of 15 male and 15 female Sprague Dawley rats were fed 0 or 2.5% diethylhexyl adipate for 90 days. ⁹² At study termination, all animals were killed for necropsy. Body weight gains were statistically significantly decreased for treated males and females, and relative kidney and liver to bws were statistically significantly increased for treated females, when compared to controls.

In a 13-week dietary study in which groups of 5 to 8 rats and mice were fed diets containing 0% to 4.0% and 0% to 2.5% diethylhexyl adipate, feed consumption by rats was decreased in the 2.5 and 4.0% dose groups, and bws were significantly reduced in these groups. ¹⁰¹ Feed consumption by mice was not affected, but a significant decrease in bws was seen in the 1.2 and 2.5% dose groups.

Intragastric doses of ≤2.0 g/kg diethylhexyl adipate to rats (number not stated) for 6 mos produced no enzymatic changes, but levels of sulphydryl compounds in the blood were increased. Hepatic detoxification appeared depressed at the onset of the study, but it was accelerated after 6 mos. Administration of 0.1 g/kg for 10 mos decreased central nervous system excitability. ²

Diisopropyl adipate

An immersion study was performed using guinea pigs in which a product containing 20.75% diisopropyl adipate was diluted, giving an actual adipate concentration of 0.10%. The animals were immersed 4 h/d for 3 days. There were no signs of systemic toxicity, and the degree of dermal irritation was considered minimal. ²

Diethylhexyl sebacate

No deaths occurred when 4 rats, 2 guinea pigs, 2 rabbits and 1 cat were exposed to 400 mg diethylhexyl sebacate/m³, 7 h/d, for 10 days. ¹ Details were not provided.

Groups of 12 F344 rats, gender not specified, were exposed 4 hours/d, 5 days/week, to 25 or 250 mg/m³ diethylhexyl sebacate for ≤13 weeks. ⁹² No adverse systemic or lung effects were observed.

Diethyl malonate

Groups of 10 to 16 male and female CD rats were fed diets containing either 0, 36 (males) or 41 mg/kg bw/d (females) diethyl malonate for 90 days. ³⁹ No treatment related effects were observed.

Di-C7-9 branched and linear alkyl esters of adipic acid

Groups of 15 male and 15 female Sprague-Dawley rats were fed a diet containing 0, 0.1, 0.5, or 2.5% di-C7-9 branched and linear alkyl esters of adipic acid for 90 days, corresponding to approximately 1500 mg/kg bw/d for high-dose males and 1900 mg/kg bw/d for high-dose females. ⁹² All rats were killed for necropsy at study termination. No systemic toxicity was reported. Small, but significant, increases in absolute and relative kidney to bws reported for females of the 2.5% dose group were not considered treatment-related. The NOAELs for male and female rats were 1500 and 1950 mg/kg bw/d, respectively.

Diisononyl adipate

Groups of 10 male and 10 female rats were fed 0, 50, 150, or 500 mg/kg bw diisononyl adipate for 13 weeks. ⁹² A statistically significant increase in relative kidney to bws was reported for males and females given 500 mg/kg bw, but absolute kidney weights were not affected and no significant microscopic effects were seen. Microscopic changes in any of the organs, including the testes and epididymis of males and ovaries of females, were not observed. There were no significant toxicological findings, and the NOAEL was 500 mg/kg bw/d.

In another 13-week study, groups of 4 male and 4 female Beagle dogs were fed 0, 0.3, 1.0, or 3.0% diisononyl adipate; the high dose was increased to 6% during weeks 9 to 13. ⁹² No significant findings were reported for the 0.3 or 1.0% groups. In the high-dose group, decreased bws, testes weight, and feed consumption, increased liver weights, elevated enzyme levels, liver and kidney discoloration, and microscopic changes in the liver, testes, spleen, and kidneys were reported. The dietary NOAEL for diisononyl adipate was 1.0%.

Ditridecyl adipate

Ditridecyl adipate, 0, 800, or 2000 mg/kg bw, was applied to the backs of groups of 10 male and 10 female Sprague-Dawley rats, 5 days/week for 13 weeks. ⁹² The test sites were not occluded, but the animals wore Elizabethan collars. Slight erythema and flaking of the skin was observed in the treated groups, with hyperplasia of the sebaceous glands in the dermis, but otherwise no significant differences were observed between test and control animals. Differences in relative organ to bws were not statistically significant, and ditridecyl adipate did not appear to cause systemic toxicity.

Dibutyl sebacate

Groups of 5 male and 5 female Sprague-Dawley rats were fed a diet containing 0, 0.01, 0.05, 0.25, or 1.25% dibutyl sebacate for 1 year. ¹²² Necropsies were performed whenever rats exhibited significant weight losses or other evidences of severe concurrent infection. Dibutyl sebacate had no effect on growth or well-being.

The researches then fed groups of 16 male Sprague Dawley rats a diet containing 0.01, 0.05, 0.25, 1.25, or 6.25% dibutyl sebacate for 2 years. ¹²² Two control groups were given untreated feed. Necropsies were performed on 3 rats from each group after 1yr, and the experiment was terminated at the end of the 2-year feeding period. Interim, animals were killed whenever they became moribund. In such instances the rats usually had incapacitating tumors or severe intercurrent infections. Dibutyl sebacate did not adversely affect growth or survival, and it did not produce significant hematological changes in peripheral blood. As the rats increased in age, slight changes in distribution of leukocytes were found, but these trends occurred in both the control and treatment groups.

Diethyl malonate

The ocular irritation potential of diethyl malonate was evaluated using rabbits, number and gender not specified. ³⁹ A volume of 0.1 mL was instilled into the conjunctival sac of 1 eye, which was not rinsed, and the contralateral eye was untreated and served as the negative control. Diethyl malonate produced slight to moderate irritation.

In a similar study, undiluted dimethyl malonate produced slight to moderate irritation in rabbit eyes. ³⁹ All signs of irritation were cleared by day 8.

Dibutyl Adipate Nonhuman

Undiluted dibutyl adipate was minimally irritating to the eyes of rabbits, and 0.1% in olive oil was nonirritating. ⁵

Human

Dibutyl adipate, 0.1% in paraffin oil, was not an ocular irritant in 2 participants. ⁵

Diisopropyl Adipate

The ocular irritation potential of 2 lots of undiluted diisopropyl adipate was evaluated using rabbits. One caused negligible irritation, while the other was nonirritating. A formulation containing 0.7% diisopropyl adipate produced some corneal stippling in rabbit eyes, while a formulation containing 5.0% and 1 containing 20.75% were nonirritating to rabbit eyes. ²

The ocular irritation of undiluted diisopropyl adipate was evaluated using 3 albino rabbits. ¹¹² A volume of 0.1 mL was instilled into the conjunctival sac of 1 eye, which was not rinsed. The contralateral eye was untreated and served as the negative control. Diisopropyl adipate was not irritating.

Diethylhexyl adipate

Undiluted diethylhexyl adipate was nonirritating to rabbit eyes and a formulation containing 0.0175% was, at most, a mild transient irritant. ²

Diisopropyl Sebacate

A primary ocular irritation study was performed using 6 New Zealand white rabbits to determine the ocular irritation potential of diisopropyl sebacate. ¹²⁶ A volume of 0.1 mL was instilled into 1 eye of each animal, which was not rinsed, and the contralateral eye of each animal served as the control. The average Draize scores were 2.0 at 24 and 48 hours, 0.3 at 72 hours, and 0.0 at 4 days. Diisopropyl sebacate was a minimal ocular irritant.

Diethylhexyl sebacate

The ocular irritation of a cream containing 1.2% diethylhexyl sebacate was evaluated using the in vitro EpiOcular MTT viability assay. ¹²⁷ The tissue samples were exposed to undiluted test material for 64 minutes, 256 minutes, or 1200 minutes. Following treatment, the viability of those tissues were calculated. The ET₅₀ (time for tissue viability to be reduced by 50%) was 484.9 minutes, and diethylhexyl sebacate was considered to be nonirritating.

Dioctyldodecyl dodecanedioate

The primary eye irritation of dioctyldodecyl dodecanedioate was evaluated using 6 albino rabbits. ¹¹⁹ A volume of 0.1 mL was instilled into 1 eye of each animal, which was not rinsed. and the contralateral eye served as a negative control. They eyes were evaluated at 24, 48, and 72 hours. At 24 hours, the maximum mean total score (MMTS) was 0.00, and dioctyldodecyl dodecanedioate was considered not irritating.

Diisocetyl dodecanedioate

The primary eye irritation of diisocetyl dodecanedioate was evaluated using the procedure described above. ¹²⁸ The MMTS was 0.00, and diisocetyl dodecanedioate was considered not irritating to the eyes of rabbits.

Dibutyl adipate

Dibutyl adipate, 10% to 100% (vehicle not stated), was not comedogenic in clinical testing. ⁵