Carbonate ester prodrugs of salicylic acid. Synthesis, solubility characteristics, in vitro enzymatic hydrolysis rates, and blood levels of total salicylate following oral administration to dogs

Synthesis and Biological Evaluation of Amino Acid Based Mutual Amide Prodrugs of Phenytoin as Anticonvulsant Agents

BACKGROUND

Phenytoin (5,5-diphenyl hydantoin) has poor water solubility, which results in incomplete oral availability. Other problems associated with the oral and intramuscular administration of phenytoin are gastric irritation and inflammation at the site of injection.

OBJECTIVE

The purpose of this study was to synthesize mutual amide prodrugs of phenytoin by using amino acids like glycine, L-tryptophan, L-lysine and taurine.

METHODS

These prodrugs were synthesized and characterized by Fourier Transform Infrared (FTIR), Proton nuclear magnetic resonance (1H NMR) and Mass Spectra. Physical and spectral characterization was performed by determination of solubility, maximum wavelength, partition coefficient (log P), ionization constant (pKa), specific (α) and molar rotation (μ), refractive index (n), specific refraction (RS) and molar refraction (RM).

RESULTS

The results obtained from solubility and log P values determination indicated that phenytoin prodrugs can be administered by oral as well as a parenteral route by minimizing the limitations associated with phenytoin. Anticonvulsant activity of prodrugs (4a-4d) was evaluated by using maximal electroshock (MES) and strychnine induced seizure test on albino mice of either sex weighing 25-30 g in which 4b and 4d were found to have significant anticonvulsant activity for MES and strychnine induced seizure test. In vitro enzymatic hydrolysis study of 4b and 4d was performed on liver, intestinal mucosa and plasma sample of male Sprague Dawley rats weighing 280-300 g in which phenytoin was eluted at 10.13 to 10.68 minutes at 220 nm.

CONCLUSION

The results obtained from the present work showed that amino acid-based mutual prodrug strategy can be a promising method to increase the solubility and anticonvulsant activity of phenytoin for the development of anticonvulsant agents.

Bioreversible derivatives of phenol. 2. Reactivity of carbonate esters with fatty acid-like structures towards hydrolysis in aqueous solutions

A series of model phenol carbonate ester prodrugs encompassing derivatives with fatty acid-like structures were synthesized and their stability as a function of pH (range 0.4 - 12.5) at 37 degrees C in aqueous buffer solutions investigated. The hydrolysis rates in aqueous solutions differed widely, depending on the selected pro-moieties (alkyl and aryl substituents). The observed reactivity differences could be rationalized by the inductive and steric properties of the substituent groups when taking into account that the mechanism of hydrolysis may change when the type of pro-moiety is altered, e.g. n-alkyl vs. t-butyl. Hydrolysis of the phenolic carbonate ester 2-(phenoxycarbonyloxy)-acetic acid was increased due to intramolecular catalysis, as compared to the derivatives synthesized from omega-hydroxy carboxylic acids with longer alkyl chains. The carbonate esters appear to be less reactive towards specific acid and base catalyzed hydrolysis than phenyl acetate. The results underline that it is unrealistic to expect that phenolic carbonate ester prodrugs can be utilized in ready to use aqueous formulations. The stability of the carbonate ester derivatives with fatty acid-like structures, expected to interact with the plasma protein human serum albumin, proved sufficient for further in vitro and in vivo evaluation of the potential of utilizing HSA binding in combination with the prodrug approach for optimization of drug pharmacokinetics.

Nonsteroidal anti-inflammatory drugs in cats: a review

OBJECTIVE

To review the evidence regarding the use of nonsteroidal anti-inflammatory drugs (NSAIDs) in cats.

DATABASES USED

PubMed, CAB abstracts.

CONCLUSIONS

Nonsteroidal anti-inflammatory drugs should be used with caution in cats because of their low capacity for hepatic glucuronidation, which is the major mechanism of metabolism and excretion for this category of drugs. However, the evidence presented supports the short-term use of carprofen, flunixin, ketoprofen, meloxicam and tolfenamic acid as analgesics in cats. There were no data to support the safe chronic use of NSAIDs in cats.

Hydrolysis of carbonates, thiocarbonates, carbamates, and carboxylic esters of alpha-naphthol, beta-naphthol, and p-nitrophenol by human, rat, and mouse liver carboxylesterases

Thirty carbonates, thiocarbonates, carbamates, and carboxylic esters of alpha-naphthol, beta-naphthol, and p-nitrophenol were synthesized and tested as substrates for liver carboxylesterases from the crude microsomal fractions of human and mouse, and purified isozymes, hydrolases A and B, from rat liver microsomes. The carbonates, thiocarbonates, and carboxylic esters of alpha-naphthol were cleaved more rapidly than the corresponding beta-naphthol isomers by the mammalian liver esterases. alpha-Naphthyl esters of acetic, propionic, and butyric acids were among the best substrates tested for these enzymes. The majority of the substrates was consistently hydrolyzed at higher rates by hydrolase B compared with hydrolase A, although the Michaelis-Menten constant (Km) values of selected substrates differed widely with these two isozymes. Malathion was a 15-fold better substrate for hydrolase B than for hydrolase A. Compared with the corresponding carboxylates, the carbonate moiety of alpha- and beta-naphthol and p-nitrophenol lowered the specific activities of the enzymes by about fivefold but improved stability under basic conditions. The optimum pH of mouse liver esterase with the acetate, methyl-carbonate, and ethylthiocarbonate of alpha-naphthol was between pH 7.0 and pH 7.6. Human and mouse liver microsomal esterase activities were about five orders of magnitude lower than the esterase activities of purified rat liver hydrolase B. A relationship between the catalytic activity of the enzymes and the lipophilicity of the naphthyl substrates indicated that (i) in the alpha- and beta-naphthyl carbonate series, an inverse relationship between enzyme activity and lipophilicity of the substrates was observed, whereas (ii) in the alpha-naphthyl carboxylate series, an increase in enzyme activity with increasing lipophilicity of the substrates up to a logP value of about 4.0 was observed, after which the enzyme activity decreased.

Discovery of diflunisal

1 Diflunisal (MK-647; 5-(2,4-difluorophenyl)-salicylic acid) is a new analgesic anti-inflammatory agent discovered after an extensive chemical and pharmacological study from 1962-71.

2 In the search for a superior salicylate our objectives were higher potency, better tolerance, and a longer duration of action.

3 An evaluation of many available and newly synthesized salicylates, in the granuloma and carrageenan foot oedema assays, revealed the activity-enhancing trend of a hydrophobic group—for example, phenyl, at the carbon-5 position of salicylic acid.

4 The attachment of a 5-(4-fluorophenyl) group, previously found to enhance the potency of anti-inflammatory (3,2-c)-pyrazole steroids and phenyl-α-propionic acids to acetyl salicylic acid yielded a clinical candidate flufenisal. As an analgesic, flufenisal is two times more potent than aspirin in man, but with a longer action; no distinct advantage in gastrointestinal tolerance has, however, been observed.

5 Further investigation of 5-heteroaryl salicylic acids, flufenisal congeners and their non-acylating carbonate esters identified diflunisal and 5-(1-pyrryl)-salicylic acid for subacute safety assessment. The O-acetyl group, commonly present in aspirin, benorylate and flufenisal, was purposely avoided in these two compounds for safety considerations.

6 Without an O-acetyl group, diflunisal cannot acetylate proteins and macro-molecules in vivo as aspirin does. In the prostaglandin synthetase assay in vitro, salicylic acid is much less active than aspirin. In contrast, the non-acetylated diflunisal and desacetyl flufenisal are both more active than flufenisal in vitro. A significant difference between aspirin and diflunisal in their biochemical mechanisms was noted.

7 On the basis of overall efficacy and tolerance data, diflunisal was finally chosen as a superior analgesic anti-inflammatory salicylate for clinical evaluation.