Absorption, Distribution and Excretion
The toxicokinetics of para-aminobenzoic acid (PABA) are as follows: rapid oral absorption, with the primary biotransformation pathway being acetylation and glycine conjugation, and secondary pathways being hepatic and renal glucuronidation. It is almost completely excreted in the urine within 24 hours. PABA is extensively acetylated during transdermal absorption in humans. Studies have shown that PABA can rapidly cross the placenta. Furthermore, one study indicated that the human placenta has a significant PABA acetylation capacity. In three male volunteers, 82% of 1 gram of PABA was excreted in the urine within 4 hours after oral administration; para-aminohippuric acid and acetylated para-aminohippuric acid were the main metabolites. Concomitant administration of sodium benzoate completely inhibited the excretion of these glycine conjugates. This study determined the transdermal absorption and metabolism of three structurally related compounds—benzoic acid, para-aminobenzoic acid (PABA), and ethyl aminobenzoate (benzocaine)—in vitro using hairless guinea pig skin. Benzocaine was also studied in human skin. Benzocaine absorption was rapid and similar on both living and dead skin. The two acidic compounds—benzoic acid and PABA—were absorbed more readily on dead skin. A small amount (6.9%) of absorbed benzoic acid conjugated with glycine to form hippuric acid. Although N-acetylbenzocaine was not observed as a metabolite of benzocaine in studies using other routes of administration, both PABA and benzocaine were extensively N-acetylated during transdermal absorption. Therefore, the metabolism of these compounds should be considered when accurately assessing absorption after topical application. In six male volunteers who received three different formulations of para-aminobenzoic acid (PABA), urinary tests revealed that the amount of PABA absorbed through the skin ranged from 1.6% to 9.6% of the topical PABA dose. No significant differences were observed among the three formulations. For more complete data on the absorption, distribution, and excretion of 10 4-aminobenzoic acids, please visit the HSDB records page.

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Metabolism/Metabolites
In a variety of animals (guinea pigs, rabbits, and rats, but excluding dogs), the two main metabolic pathways are aminoacetylation and the conjugation of carboxyl groups with glycine or glucuronic acid. Acetylation occurs in the liver, heart, lungs, blood, and kidneys of rats, and in the gastrointestinal mucosa of cattle. In the presence of para-aminobenzoic acid, N-acetyltransferase activity is similar in the liver and lung tissues of rabbits. Both para-aminobenzoic acid (30-40%) and para-aminohippuric acid (70%) are acetylated in the kidneys of rabbits; acetylation of para-aminohippuric acid also occurs in the kidneys of guinea pigs. Acetylation is dose-dependent. In rats, 75% of the metabolites are acetylated at doses up to 5 mg/kg body weight; as the dose increases, the degree of acetylation decreases to 40%. There is an inverse relationship between acetylation and glycine conjugation: glycine conjugation increases as acetylation decreases. This decrease is observed in pantothenic acid-deficient rats. Male rats excreted higher levels of acetylated conjugates in their urine than female rats. After ingestion of 1.0 gram in humans, the main metabolites are para-aminohippuric acid (PABA) and acetylated para-aminohippuric acid (PAHA). PABA is primarily metabolized through acetylation and glycine conjugation to produce PAABA, PAHA, and PAAHA. In rapidly acetylated rabbits, the half-life of PABA was 7.01 ± 0.32 minutes; in slowly acetylated rabbits, the half-life was 7.08 ± 0.78 minutes. Significant differences were observed in the formation of PAABA and PAHA derived from PABA between the two acetylated phenotypes. For more complete metabolite/metabolite data on 4-aminobenzoic acid (9 metabolites in total), please visit the HSDB record page.

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Biological half-life
……In rapidly acetylated rabbits, the half-life of PABA was 7.01 ± 0.32 minutes, and in slowly acetylated rabbits it was 7.08 ± 0.78 minutes.

Interactions
Sulfonamides exert their effects by competitively inhibiting para-aminobenzoic acid (PABA) in microorganisms. Therefore, adequate amounts of PABA can antagonize the antibacterial activity of sulfonamides. PABA appears to block the conversion of salicylic acid to salicyluric acid, leading to elevated blood salicylate levels. Aminosalicylic acid appears to act on Mycobacterium tuberculosis in a similar manner to sulfonamides on other microorganisms (by competing with PABA). Therefore, administration of PABA may inhibit the antibacterial activity of aminosalicylic acid. 0.007% PABA and pluduan-induced interferon titers were compatible after conjunctival injection but incompatible after anterior chamber injection. For more complete data on interactions of 4-aminobenzoic acids (14 in total), please visit the HSDB records page.

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Non-human toxicity values
Rabbit intravenous LD50: 2000 mg/kg
Rabbit oral LD50: 1830 mg/kg
Dog oral LD50: 1000 mg/kg
Mouse oral LD50: 2850 mg/kg
Rats oral LD50: 6000 mg/kg body weight

Therapeutic Uses
Sunscreen
Oral use is used to treat conditions such as scleroderma, dermatomyositis, and Peroni's disease. Topical use is as a sunscreen and protectant.
Residents living in areas with high solar radiation and who frequently work or engage in outdoor recreational activities are advised to use a high SPF (greater than 15) sunscreen daily on normally exposed skin. Daily use of sunscreen can reduce the cumulative sun exposure that can lead to actinic keratosis and squamous cell carcinoma.
Because para-aminobenzoic acid (PABA) is rapidly and almost completely excreted in urine, PABA has long been recognized as an objective marker for verifying the integrity of 24-hour urine samples. For this reason, PABA has long been used clinically as an indicator for pancreatic and liver function tests.
Sunscreen should be applied evenly and in sufficient quantity to all exposed skin surfaces, including the lips, before exposure to UVB radiation. For optimal protection, it may be necessary to apply sunscreen twice. Sunscreens containing para-aminobenzoic acid (PABA) are most effective when applied 1-2 hours before sun exposure. Non-waterproof sunscreens should be reapplied after swimming, towel drying, or heavy sweating. Since most sunscreens wash off easily, they typically need to be reapplied every 1-2 hours or according to the manufacturer's instructions to provide adequate UVB protection. /Sunscreen/

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Drug Warnings
In vitro studies have shown that PABA can displace methotrexate from its plasma protein binding sites, thereby increasing the concentration of free methotrexate.
PABA derivatives have been reported to have low sensitizing properties, but the incidence of allergic and photosensitive contact dermatitis associated with their use is increasing. There have been reports that PABA derivatives may be contaminated with benzocaine, which can cause allergic reactions. In patients allergic to compounds structurally similar to para-aminobenzoic acid (PABA) (e.g., ester anesthetics, aniline dyes, thiazides, sulfonylureas, and p-phenylenediamine drugs), cross-sensitivity to PABA derivatives has been occasionally reported; therefore, sunscreens containing PABA derivatives may be contraindicated in patients with a history of allergy to such chemicals. Sunscreen manufacturers using propellants warn that inhaling the fumes produced by these formulations can be harmful or even fatal. /Propellant/ Because the skin absorption characteristics of infants under 6 months of age may differ from those of adults, and their metabolic and excretory pathways are not yet mature, potentially limiting their ability to clear transdermal sunscreens, sunscreen products should only be used on infants under 6 months of age under the guidance of a clinician. The skin characteristics of older adults may differ from those of younger adults, but these characteristics and the specific precautions required when using sunscreen products in this age group are not yet fully understood. /Sunscreen/ For more complete data on drug warnings for 4-aminobenzoic acid (13 in total), please visit the HSDB record page.

C7H7NO2

137.1360

137.047

150-13-0

25136-77-0

978

Off-white to light brown solid powder

1.3±0.1 g/cm3

339.9±25.0 °C at 760 mmHg

187-189 °C(lit.)

159.4±23.2 °C

0.0±0.8 mmHg at 25°C

1.637

0.83

2

3

1

10

128

0

ALYNCZNDIQEVRV-UHFFFAOYSA-N

InChI=1S/C7H7NO2/c8-6-3-1-5(2-4-6)7(9)10/h1-4H,8H2,(H,9,10)

4-aminobenzoic acid

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ~75 mg/mL (~546.89 mM)
H2O : ~4.55 mg/mL (~33.18 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (18.23 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (18.23 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (18.23 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 4.17 mg/mL (30.41 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication (<60°C).

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 (Please use freshly prepared in vivo formulations for optimal results.)