PABA esters

PABA is acidic, which means it can sting if it gets in the eyes. Some people are sensitive to PABA when it is applied to their skin. PABA also darkens and can stain clothing. For these reasons, modified forms of PABA have become popular as sunscreens. PABA can be reacted with long-chain alcohols to form PABA esters, such as polyethylene glycol 25 PABA ... 

This PABA molecule eliminates the irritation and staining problems. Other PABA esters are glycerol PABA, padimate A, and padimate O. 

PABA, 1,6-7, 11, 70. See also para-amino benzoic acid PABA esters, 6-7 padimate A, 7 padimate O, 7 pain relievers, 158,170-189 paints, 44, 70, 112, 129 acrylic, 234 latex, 232, 236, 237 oil, 94... 

Figure 12.9.2 The molecular structure of dioxybenzone. and sulisobenzone (Fig. 12.9.3), 250-380 nm. PABA and PABA esters have...

There are numerous UVB filters, including (1) PABA esters (e.g., padimate O), (2) cinnamates (odi-noxate),- (3) octocrylene (2-ethylhexyl-2-cyano-3,3 diphenylacrylate) and (4) salicylates focti-salate). 

Bruze M, Gruvberger B, Thulin I (1990) PABA, benzocaine, and other PABA esters in sunscreens and after-sun products. Photodermatol Photoimmunol Photomed 7 106-108... 

Denatonium benzoate is the bitterest-tasting substance known. It gets its name from denatured alcohol—alcohol that has been rendered unfit for drinking—and is often used in the denaturing process. Specially denatured alcohol 40, or SD-40, is ethanol that has been denatured by a tiny amount of denatonium benzoate. Denatonium benzoate is an ester of PABA, and is related to lidocaine, benzocaine, novocaine, and cocaine. 

Benzocaine is an ester, a compound made from the organic acid PABA (para-amino benzoic acid) and the alcohol ethanol. 

Other esters of PABA have similar anesthetic properties. 

The metabolic degradation of local anesthetics depends on whether the compound has an ester or an amide linkage. Esters are extensively and rapidly metabolized in plasma by pseudochoUnesterase, whereas the amide linkage is resistant to hydrolysis. The rate of local anesthetic hydrolysis is important, since slow biotransformation may lead to drug accumulation and toxicity. In patients with atypical plasma cholinesterase, the use of ester-linked compounds, such as chloroprocaine, procaine and tetracaine, has an increased potential for toxicity. The hydrolysis of all ester-linked local anesthetics leads to the formation of paraaminobenzoic acid (PABA), which is known to be allergenic. Therefore, some people have allergic reactions to the ester class of local anesthetics. 

Benzocaine is a PABA derivative used primarily for topical application to skin and mucous membranes. Its low aqueous solubility allows it to stay at the site of application for long periods. Its minimal rate of absorption after topical administration is associated with a low incidence of systemic toxicity. Benzocaine is contraindicated in patients with known sensitivity to ester-linked anesthetics or PABA-containing compounds. 

Tetracaine hydrochloride (Pontocaine) is an ester of PABA that is an effective topical local anesthetic agent... 

Contraindications Hypersensitivityto ester local anesthetics, sulfites, PABA, patients on anticoagulant therapy, and in patients with coagulopathy, infection, thrombocytopenia, Should not be given via intra-arterial, intrathecal, or intravenous routes. 

Para-aminobenzoic acid (PABA) and its esters, benzophenones IX Acne preparations... 

These are the agents which protect the skin from harmful effects of exposure to sunlight by absorbing ultraviolet light. The most commonly used compounds are paraamino benzoic acid (PABA) and its esters and ben-zophenones include oxybenzone, dioxybenzone and sulisobenzone. These... 

Topical medications useful in protecting against sunlight contain either chemical compounds that absorb ultraviolet light, called sunscreens, or opaque materials such as titanium dioxide that reflect light, called sunshades. The three classes of chemical compounds most commonly used in sunscreens are p-aminobenzoic acid (PABA) and its esters, the benzophenones, and the dibenzoylmethanes. 

Tetracaine, an ester of para-aminobenzoic acid (PABA), has been widely used for topical anesthesia of the eye. It is currently available in a 0.5% solution. Its onset, intensity, and duration of anesthesia are comparable with those of proparacaine and benoxinate (Figure 6-2). Onset of anesthesia sufficient to permit tonometry or other minor procedures involving the superficial cornea and conjimc-tiva is 10 to 20 seconds, and duration of anesthesia is 10 to 20 minutes. It has been reported, however, that the 1% solution produces anesthesia lasting nearly an hour. Tetracaine 1% has also been used successfully to provide anesthesia during phacoemulsification cataract surgery and intraocular lens implantation. 

Practitioners should record the event in the patient s chart and avoid using the same anesthetic on subsequent patient visits. Because there is apparently little cross-sensitivity between classes of local anesthetics, practitioners can usually change from proparacaine to an ester of PABA, or vice versa, with little risk of local allergy. Unfortunately, no topical anesthetics approved for ocular use have an amide linkage. Such anesthetics, because of their extremely low allergenic potential, would serve as ideal topical ocular anesthetics. 

The four-step synthesis begins with the hydrogenation of PABA using Rh/AliOs to furnish a 92% yield of 77 as an 80 20 cisitrans mixture. Subsequent heating at 250°C in Dowtherm A to effect epimerization-cyclization afforded the bicyclic lactam (78) in 84% yield. Red-Al reduction furnished 2-azabicyclo[2.2.2]octane which was subsequently protected as the tosylate salt (79) in 85% yield. Employment of more concentrated Dowtherm mixtures and a decreased amount of Red-Al increased the yield for the cyclization step to 89% and the tosylate salt to 91% (overall yield 70%) [85]. Earlier, Werner [86] demonstrated that using the carboxylate methyl ester instead of the carboxylic acid allows the cyclization to occur at lower temperatures (190°C). 

In the body, the ester link of procaine is hydrolyzed, yielding p-aminobenzoic acid (PABA) and N,h/-diethyl-aminoethanol (DEAE), an analog of DMAE. It is PABA (Figure 3.11) that is responsible for the large majority of allergic reactions to procaine it is excreted rapidly by the kidneys. PABA is most often used as a sunscreen, but is sometimes called vitamin B-x , although it is not essential for humans and the body cannot synthesize folate from PABA. According to some authors, the Aslan method relies solely on the combined action of PABA and DEAE/DMAE. It is assumed that the action of PABA is due to its anti-free-radical properties. 

Procaine, on the other hand, is an ester-type local anesthetic when it is injected into the organism, it is transformed into N,N-diethylaminoethanol and para-amino-benzoic acid (PABA) when the ester link that binds these two components of procaine is broken. PABA is a well-known allergen. Methyl para-hydroxybenzoate, whose structure is similar to that of PABA, is also called PAB or methylparaben. It is a preservative that binds, like a hapten, to the immunoglobulin E (IgE) on the surface of mast cells and basophils and maybe the cause of the few cases of anaphylactic shock that have been described. Other patients may be allergic to metabisulfite conservative, found in preparations containing adrenaline (epinephrine). 

Because of the structural limitations of ABA, several sunscreening chemicals based on ABA structure were developed with attempts to capitalize on the strengths and eliminate the drawbacks. There are four compounds that successfully addressed the problems of ABA These are iV,iV-dimethyl PABA ethyl ester, AT,JV-dimethyl PABA butyl ester, N,N-di-methyl PABA amyl ester (padimate A), and iV,iV -dimethyl PABA octyl ester (padimate O). Padimate 0 (24) has the most desirable prop-... 

Unlike esters, which are rapidly hydrolyzed in the body, benzophenones maybe potentially toxic if absorbed through the skin statistically, more frequent allergic reactions have been reported with oxybenzone than with PABA (299). Three benzophenones are... 

Hypersensitivity reactions occur with the ester-type of drugs. These drugs are metabolized to paraaminobenzoic acid (PABA) derivatives. Individuals sensitive to PABA may experience allergic reactions to these drugs. 

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