Synthesis and evaluation of morpholinoalkyl ester prodrugs of indomethacin and naproxen

Spin-Labeled Diclofenac: Synthesis and Interaction with Lipid Membranes

Diclofenac is a non-steroidal anti-inflammatory drug (NSAID) from the group of phenylacetic acid derivatives, which has analgesic, anti-inflammatory and antipyretic properties. The interaction of non-steroidal anti-inflammatory drugs with cell membranes can affect their physicochemical properties, which, in turn, can cause a number of side effects in the use of these drugs. Electron paramagnetic resonance (EPR) spectroscopy could be used to study the interaction of diclofenac with a membrane, if its spin-labeled analogs existed. This paper describes the synthesis of spin-labeled diclofenac (diclofenac-SL), which consists of a simple sequence of transformations such as iodination, esterification, Sonogashira cross-coupling, oxidation and saponification. EPR spectra showed that diclofenac-SL binds to a lipid membrane composed of palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). 2H electron spin echo spectroscopy (ESEEM) was used to determine the position of the diclofenac-SL relative to the membrane surface. It was established that its average depth of immersion corresponds to the 5th position of the carbon atom in the lipid chain.

A supramolecular topical gel derived from a non-steroidal anti-inflammatory drug, fenoprofen, is capable of treating skin inflammation in mice

A new series of bioconjugates derived from a non-steroidal anti-inflammatory drug (NSAID), namely fenoprofen, has been synthesised by amidation with various biogenic molecules such as β-alanine, aminocaproic acid and tyramine with the aim of converting the NSAID into a supramolecular gelator for plausible biomedical applications. One such bioconjugate (2) showed gelation ability with methylsalicylate (MS) and 1% menthol in methyl salicylate (MMS) solvents. These gels were characterized by table top rheology, high resolution-transmission electron microscopy (HR-TEM) and dynamic rheology. Gelator 2 was found to be biostable both in proteolytic enzymes and in blood serum of BALB/c mouse under physiological conditions. It was also found to be biocompatible, as revealed by the methyl thiazolyldiphenyl tetrazolium bromide (MTT) assay in mouse macrophage RAW 264.7 and mouse myoblast C2C12 cells. The anti-inflammatory response (prostaglandin E2 assay, denoted PGE2 assay) of 2 was comparable to that of the parent drug fenoprofen calcium salt. Finally, a topical gel formulation of 2 displayed in vivo self-delivery application in treating imiquimod (IMQ) induced skin inflammation in BALB/c mice.

Potential of Piperazinylalkylester Prodrugs of 6-Methoxy-2-Naphthylacetic Acid (6-MNA) for Percutaneous Drug Delivery

Piperazinylalkyl ester prodrugs (4a-5d) of 6-methoxy-2-naphthylacetic acid (6-MNA) (1) were synthesized and evaluated in vitro for the purpose of percutaneous drug delivery. These ionizable prodrugs exhibited varying aqueous solubilities and lipophilicities depending on the pH of the medium. The prodrugs (4a-5c) showed higher aqueous solubility and similar lipophilicity at pH 5.0 and lower aqueous solubility and higher lipophilicity at pH 7.4 in comparison to 6-MNA. The chemical and enzymatic hydrolyses of the prodrugs was investigated in aqueous buffer solutions (pH 5.0 and 7.4) and in 80% human serum (pH 7.4) at 37°C. The prodrugs showed moderate chemical stability (t 1/2 = 6-60 h) but got readily hydrolyzed enzymatically to 6-MNA with half-life ranging from 10-60 min. In the in vitro permeation study using rat skin, the flux of 6-MNA and the prodrugs was determined in aqueous buffers of pH 5.0 and 7.4. The prodrug (5b) showed 7.9- and 11.2-fold enhancement in skin permeation compared to 6-MNA (1) at pH 5.0 and 7.4, respectively. It was concluded that the parent NSAIDs having favorable pharmacokinetic and pharmacodynamic properties coupled with increased skin permeability of their prodrugs could give better options for the treatment of rheumatic diseases.

Novel flurbiprofen derivatives with improved brain delivery: synthesis, in vitro and in vivo evaluations

Tarenflurbil (R-flurbiprofen) was acknowledged as a promising candidate in Alzheimer's disease (AD) therapy. However, the Phase III study of tarenflurbil was extremely restricted by its poor delivery efficiency to the brain. To tackle this problem, the novel carriers for tarenflurbil, racemic flurbiprofen (FLU) derivatives (FLU-D1 and FLU-D2) modified by N,N-dimethylethanolamine-related structures were synthesized and characterized. These derivatives showed good safety level in vitro and they possessed much higher cellular uptake efficiency in brain endothelial cells than FLU did. More importantly, the uptake experiments suggested that they were internalized via active transport mechanisms. Biodistribution studies in rats also illustrated a remarkably enhanced accumulation of these derivatives in the brain. FLU-D2, the ester linkage form of these derivatives, achieved a higher brain-targeting efficiency. Its C_max and AUC_0-t were enhanced by 12.09-fold and 4.61-fold, respectively compared with those of FLU. Additionally, it could be hydrolyzed by esterase in the brain to release the parent FLU, which might facilitate its therapeutic effect. These in vitro and in vivo results highlighted the improvement of the brain-targeted delivery of FLU by making use of N,N-dimethylethanolamine ligand, with which an active transport mechanism was involved.

β-Amino acid and amino-alcohol conjugation of a nonsteroidal anti-inflammatory drug (NSAID) imparts hydrogelation displaying remarkable biostability, biocompatibility, and anti-inflammatory properties

A well-known nonsteroidal anti-inflammatory drug (NSAID), namely, naproxen (Np), was conjugated with β-alanine and various combinations of amino alcohols and l-alanine. Quite a few bioconjugates, thus synthesized, were capable of gelling pure water, NaCl solution (0.9 wt %), and phosphate-buffered saline (PBS) (pH 7.4). The hydrogels were characterized by rheology and electron microscopy. Hydrogelation was probed by FT-IR and temperature-variable (1)H NMR studies. Single-crystal X-ray diffraction (SXRD) of a nonhydrogelator and a hydrogelator in the series established a useful structure-property (gelation) correlation. MTT assay of the hydrogelators in the mouse macrophage RAW 264.7 cell line showed excellent biocompatibility. The prostaglandin E2 (PGE2) assay of the hydrogelators revealed their anti-inflammatory response, which was comparable to that of the parent NSAID naproxen sodium (Ns).

Reactive oxygen species-activated nanoprodrug of Ibuprofen for targeting traumatic brain injury in mice

Traumatic brain injury (TBI) is an enormous public health problem, with 1.7 million new cases of TBI recorded annually by the Centers for Disease Control. However, TBI has proven to be an extremely challenging condition to treat. Here, we apply a nanoprodrug strategy in a mouse model of TBI. The novel nanoprodrug contains a derivative of the nonsteroidal anti-inflammatory drug (NSAID) ibuprofen in an emulsion with the antioxidant α-tocopherol. The ibuprofen derivative, Ibu2TEG, contains a tetra ethylene glycol (TEG) spacer consisting of biodegradable ester bonds. The biodegradable ester bonds ensure that the prodrug molecules break down hydrolytically or enzymatically. The drug is labeled with the fluorescent reporter Cy5.5 using nonbiodegradable bonds to 1-octadecanethiol, allowing us to reliably track its accumulation in the brain after TBI. We delivered a moderate injury using a highly reproducible mouse model of closed-skull controlled cortical impact to the parietal region of the cortex, followed by an injection of the nanoprodrug at a dose of 0.2 mg per mouse. The blood brain barrier is known to exhibit increased permeability at the site of injury. We tested for accumulation of the fluorescent drug particles at the site of injury using confocal and bioluminescence imaging of whole brains and brain slices 36 hours after administration. We demonstrated that the drug does accumulate preferentially in the region of injured tissue, likely due to an enhanced permeability and retention (EPR) phenomenon. The use of a nanoprodrug approach to deliver therapeutics in TBI represents a promising potential therapeutic modality.

The hydrolysis kinetics of monobasic and dibasic aminoalkyl esters of ketorolac

Six aminoethyl and aminobutyl esters of ketorolac containing 1-methylpiperazine (MPE and MPB), N-acetylpiperazine (APE and APB) or morpholine (ME and MB), were synthesized and their hydrolysis kinetics were studied. The hydrolysis was studied at pH 1 to 9 (for MPE, APE and ME) and pH 1 to 8 (for MPB, APB and MB) in aqueous phosphate buffer (0.16 M) with ionic strength (0.5 M) at 37°C. Calculation of k(obs), construction of the pH-rate profiles and determination of the rate equations were performed using KaleidaGraph® 4.1. The hydrolysis displays pseudo-first order kinetics and the pH-rate profiles shows that the aminobutyl esters, MPE, APB and MB, are the most stable. The hydrolysis of the ethyl esters MPE, APE and ME, depending on the pH, is either fast and catalyzed by the hydroxide anion or slow and uncatalyzed for the diprotonated, monoprotonated and nonprotonated forms. The hydrolysis of the butyl esters showed a similar profile, albeit it was also catalyzed by hydronium cation. In addition, the hydroxide anion is 105 more effective in catalyzing the hydrolysis than the hydronium cation. The hydrolysis pattern of the aminoethyl esters is affected by the number and pKa of its basic nitrogen atoms. The monobasic APE and ME, show a similar hydrolysis pattern that is different than the dibasic MPE. The length of the side chain and the pKa of the basic nitrogen atoms in the aminoethyl moiety affect the mechanism of hydrolysis as the extent of protonation at a given pH is directly related to the pKa.

Nanoprodrugs of NSAIDs: Preparation and Characterization of Flufenamic Acid Nanoprodrugs

We demonstrated that hydrophobic derivatives of the nonsteroidal anti-inflammatory drug (NSAID)flufenamic acid (FA), can be formed into stable nanometer-sized prodrugs (nanoprodrugs) that inhibit the growth of glioma cells, suggesting their potential application as anticancer agent. We synthesized highly hydrophobic monomeric and dimeric prodrugs of FA via esterification and prepared nanoprodrugs using spontaneous emulsification mechanism. The nanoprodrugs were in the size range of 120 to 140 nm and physicochemically stable upon long-term storage as aqueous suspension, which is attributed to the strong hydrophobic interaction between prodrug molecules. Importantly, despite the highly hydrophobic nature and water insolubility, nanoprodrugs could be readily activated into the parent drug by porcine liver esterase, presenting a potential new strategy for novel NSAID prodrug design. The nanoprodrug inhibited the growth of U87-MG glioma cells with IC₅₀ of 20 μM, whereas FA showed IC₅₀ of 100 μM, suggesting that more efficient drug delivery was achieved with nanoprodrugs.

Thiazolidin-4-one and hydrazone derivatives of capric acid as possible anti-inflammatory, analgesic and hydrogen peroxide-scavenging agents

Starting from capric acid, hydrazone and thiazolidin-4-one derivatives have been synthesized in the present investigation. Decanoic acid hydrazide was reacted appropriately to yield hydrazones, which were then cyclized to yield the corresponding thiazolidin-4-ones. The structures of the newly synthesized compounds were confirmed by analytical and spectral methods. Anti-inflammatory, analgesic, and hydrogen peroxide-scavenging activity of the title compounds were evaluated. Among synthesized compounds, 2-hydroxyphenyl thiazolidinone with 44.90% inhibition of inflammation was the most potent anti-inflammatory agent. Similarly, 4-methoxybenzylidine hydrazide with 64.90% inhibition of writhing was observed to be the most potent analgesic agent of the synthesized compounds. All the synthesized compounds exhibited potent hydrogen peroxide-scavenging activity.

Clinical pharmacokinetics of the cyclooxygenase inhibiting nitric oxide donator (CINOD) AZD3582

The clinical pharmacokinetics of the COX-inhibiting nitric oxide donator (CINOD) AZD3582 and its metabolites, including naproxen, nitric oxide and nitrate, are summarized. AZD3582 has low aqueous solubility, moderate and passive intestinal permeability and is degraded by intestinal esterases. Its oral bioavailability (F) appears to be maximally a few per cent, and increases by several-fold after food intake. Ninety-four per cent or more of an AZD3582 dose is absorbed, of which at least 9–20% appears to be taken up as intact substance. AZD3582 has a predicted plasma protein binding degree of ∼ 0.1%, a half-life (t½) of 3 to 10 h and does not accumulate after repeated once- and twice-daily dosing. In patients AZD3582 does not provide a significantly better gastrointestinal (GI) side-effect profile than the highly permeable and locally irritating naproxen. Possible reasons for this include considerable GI uptake as naproxen, limited duration and extent of nitric oxide donation in the GI mucosa and the circulation, tolerance development (involving auto-inhibition of nitric oxide catalysing enzymes) and mucosal damage caused by nitric oxide. Blood pressure data suggest that nitric oxide is mainly donated within 3h. The uptake of naproxen is slightly slower and lower (≥ 94% relative GI uptake and 80–85% relative F) after AZD3582 administration compared with naproxen dosing. The naproxen t½ and trough steady-state concentrations after AZD3582 and naproxen dosing are similar. The average systemic nitrate exposure is approximately doubled after dosing of 375 to 750 mg AZD3582 twice daily.

1-Toluene-sulfonyl-3-[(3'-hydroxy-5'-substituted)-gamma-butyrolactone]-indoles: synthesis, COX-2 inhibition and anti-cancer activities

Indoles carrying a cyclic ester (gamma-butyrolactone) at C-3 position have been synthesized by the allylation of 3-indoleglyoxylate followed by iodocyclisation and the nucleophilic replacement of the iodo-group. Screening of these molecules for COX-2 inhibition and anti-cancer activities has identified compounds 10 and 11 as highly potent and selective for COX-2 as well as showing remarkable anti-cancer activities (better than that of indomethacin).

Aminocarbonyloxymethyl Ester Prodrugs of Flufenamic Acid and Diclofenac: Suppressing the Rearrangement Pathway in Aqueous Media

Aminocarbonyloxymethyl ester prodrugs are known to undergo rearrangement in aqueous solutions to form the corresponding N-acylamine side product via an O-->N intramolecular acyl transfer from the carbamate conjugate base. Novel aminocarbonyloxymethyl esters of diclofenac and flufenamic acid containing amino acid amide carriers were synthesized and evaluated as potential prodrugs displaying less ability to undergo rearrangement. These compounds were prepared in reasonable yield by a four-step synthetic method that uses the appropriate N-Boc-protected amino acid N-hydroxysuccinimide ester and secondary amine and chloromethyl chloroformate as key reactants. Their reactivity in pH 7.4 buffer and 80% human plasma at 37 degrees C was assessed by RP-HPLC. The aminocarbonyloxymethyl esters containing a secondary carbamate group derived from amino acids such as glycine or phenylalanine were hydrolyzed quantitatively to the parent drug both in non-enzymatic and enzymatic conditions, with no rearrangement product being detected. The oral bioavailability in rats was determined for selected diclofenac derivatives. These derivatives displayed a bioavailability of 25 to 68% relative to that of diclofenac, probably due to their poor aqueous solubility and lipophilicity. These results suggest that further optimization of aminocarbonyloxymethyl esters as potential prodrugs for non-steroidal anti-inflammatory drugs require the use of amino acid carriers with ionizable groups to improve aqueous solubility.

Prodrugs of scutellarin: ethyl, benzyl and N,N-diethylglycolamide ester synthesis, physicochemical properties, intestinal metabolism and oral bioavailability in the rats

In an effort to enhance the oral bioavailability of scutellarin, ethyl, benzyl and N,N-diethylglycolamide ester of scutellarin were synthesized. The hydrolysis of the prodrugs follows first-order kinetics in aqueous solution, and produced a V-shaped pH profile. The N,N-diethylglycolamide ester is highly susceptible to enzymatic hydrolysis in human plasma (t(1/2) approximately 7 min) with a high stability in aqueous solution (t(1/2) approximately 16 day, pH 4.2). Compared with the solubility of scutellarin, the solubility of glycolamide ester was about ten times in pH 4.0 buffer, and about thirty five times in water. Its apparent partition coefficient increased significantly from -2.56 to 1.48. Glycolamide ester of scutellarin was chosen to investigate the intestinal metabolism and in vivo bioavailability. Degradation studies in the intestinal tract content and homogenates indicated intestinal metabolism before absorption was a crucial obstacle for the prodrug. N,N-Diethylglycolamide ester can be protected from the degradation in the intestinal lumen by an emulsion. A significant increase in the plasma AUC and C(max) of the prodrug emulsion was observed in rats, compared with that of the scutellarin-cyclodextrin complex (P<0.01). The emulsion of N,N-diethylglycolamide ester produces a 1.58-fold enhancement in apparent bioavailability and 1.4-fold increase in the absolute bioavailability compared to the scutallarin-cyclodextrin complex.

Chemical stability of esters of nicotinic acid intended for pulmonary administration by liquid ventilation

PURPOSE

It has been suggested that fluorocarbon liquid may be a unique vehicle for the delivery of drugs directly to the acutely injured lung. A prodrug approach was used as a means of enhancing the solubility of a model drug (nicotinic acid) in the fluorocarbon. The solubility, the chemical stability of the putative prodrugs, and the sensitivity to enzymatic hydrolysis was investigated.

METHODS

The solubility of each nicotinic acid ester was determined in buffer as a function of pH and in perflubron. The octanol/buffer partition coefficient was determined at pH 7.4. The chemical stability of the putative prodrugs was determined as a function of pH, temperature, buffer content, and ionic strength. In addition, sensitivity of the esters to enzymatic degradation was evaluated.

RESULTS

Compared with nicotinic acid, the solubility in perflubron of the esters was significantly enhanced. In aqueous buffers, the esters exhibited pseudo-first order degradation kinetics, with both acid and base catalyzed loss. Studies of the fluorobutyl ester indicate quantitative loss of the putative prodrug and release of the parent nicotinic acid. Porcine esterase accelerated the loss of fluorobutyl ester by a factor of over 200 compared with chemical hydrolysis at pH 7.4.

CONCLUSIONS

The properties of the fluorinated esters suggest that they may be suitable candidates for further testing as possible prodrugs of nicotinic acid based upon higher solubility in perflubron, rapid release of the parent drug after simple hydrolysis, and sensitivity to the presence of a model esterase enzyme.

Synthesis, stability and in vitro dermal evaluation of aminocarbonyloxymethyl esters as prodrugs of carboxylic acid agents

Aminocarbonyloxymethyl esters based on (S)-amino acid carriers were synthesised and evaluated as potential prodrugs of carboxylic acid agents. In addition, the compounds were evaluated as topical prodrugs with the aim of improving the dermal delivery of two non-steroidal anti-inflammatory agents: naproxen and flufenamic acid. The lipophilicities of these compounds were determined and their hydrolyses in aqueous solutions and in human plasma were examined. Compounds containing a secondary carbamate group were hydrolysed at pH 7.4 by two different routes: (i) direct nucleophilic attack at the ester carbonyl carbon leading to the release of the parent carboxylic acid and (ii) intramolecular rearrangement involving an O-->N acyl migration, leading to the formation of the corresponding amide. The rearrangement pathway is highly dependent on the size of the carboxylic acid and amino acid substituents, being eliminated when the amino acid is valine or leucine. In contrast, compounds decomposed in plasma exclusively through ester hydrolysis, most releasing the parent carboxylic acid quantitatively with half-lives shorter than 5 min. The permeation of selected prodrugs across excised postmortem human skin was studied in vitro. All prodrugs evaluated exhibited a lower flux than the corresponding parent carboxylic acid. The poor skin permeation observed for compounds is most probably due to their low aqueous solubility and high partition coefficient.

Glycosyl derivatives of 2-bromosugar of selected non-steroidal anti-inflammatory drugs. Synthesis and QSAR data

On the basis of molecular modeling and calculation of physicochemical SAR data we obtained new derivatives of 2-bromosugars 5a-d and 6 for widely used non-steroidal anti-inflammatory drugs (NSAIDs). Compounds 5a-d and 6 were synthesized as stable, crystalline compounds from the reaction of phosphoroates 1-3 with salicylic acid (4a), aspirin (4b), diclofenac (4c) and indomethacin (4d) in an aprotic solvent. The reaction took place in the presence of silver carbonate as the activator of leaving groups with high stereoselectivity and good yields. The structures of the new derivatives of 5a-d and 6 for NSAIDs were established by spectroscopic methods 1H and 13C NMR, and elemental analyses.

Piperazinylalkyl prodrugs of naproxen improve in vitro skin permeation

Novel morpholinyl (4a) and piperazinylalkyl (4b-e) esters were synthesized and evaluated in vitro for their properties as bioreversible topically administered dermal prodrugs of naproxen. These ionizable prodrugs exhibited various aqueous solubilities and lipophilicities, depending on the pH of medium. As indicated by octanol-buffer partition coefficients (logP(app)) at pH 7.4, all of the prodrugs were significantly more lipophilic (logP(app)=0.7-3.9) than naproxen (logP(app)=0.3). Furthermore, the most aqueous of the soluble prodrugs (4b-d) were only 2-3-fold less soluble in an aqueous buffer of pH 7.4 ( approximately 30-50 mM) than was naproxen ( approximately 100 mM). At a pH of 5.0, prodrugs showed a generally higher aqueous solubility and similar logP(app) values, compared to naproxen. The chemical and enzymatic hydrolysis of prodrugs at 37 degrees C was investigated in aqueous buffer solutions (pH 5.0 and 7.4) and in 80% human serum (pH 7.4), respectively. The prodrugs showed moderate chemical stability (t(1/2)=15-150 days at pH 5.0), and they were hydrolyzed enzymatically to naproxen, with half-lives ranging from 0.4 to 77 min. In permeation studies using post-mortem human skin in vitro, the flux of naproxen was 6.5 and 1.6 nmol/cm(2). h in a saturated aqueous buffer vehicle of pH 7.4 and 5.0, respectively. Among the prodrugs, two piperazinyl derivatives (4c and 4d) resulted in a 9- and 4-fold enhancement of permeation, respectively, when compared to naproxen itself at pH 7.4. 4c also resulted in a significantly (4-fold) better permeation than naproxen at pH 5.0. In conclusion, piperazinyl esters improved skin permeation of naproxen and are promising prodrugs of naproxen for topical drug delivery.

Synthesis and in vitro evaluation of novel morpholinyl- and methylpiperazinylacyloxyalkyl prodrugs of 2-(6-methoxy-2-naphthyl)propionic acid (Naproxen) for topical drug delivery

Various novel morpholinyl- (3a,b) and methylpiperazinylacyloxyalkyl (3c-f) esters of 2-(6-methoxy-2-naphthyl)propionic acid were synthesized and evaluated in vitro for topical drug delivery as potential prodrugs of naproxen (1). Compounds 3a-f were prepared by coupling the corresponding naproxen hydroxyalkyl ester with the morpholinyl- or (4-methyl-1-piperazinyl)acyl acid in the presence of dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP) and quantitatively hydrolyzed (t(1/2) = 1-26 min) to naproxen in human serum. Compounds 3c-f showed higher aqueous solubility and similar lipophilicity, determined by their octanol-buffer partition coefficients (log P(app)), at pH 5.0 when compared to naproxen. At pH 7.4 they were significantly more lipophilic than naproxen. The best prodrug 3c led to a 4- and 1.5-fold enhancement of skin permeation when compared to naproxen at pH 7.4 and 5.0, respectively. The present study indicates using a methylpiperazinyl group yields prodrugs that are partially un-ionized under neutral and slightly acidic conditions, and thus, a desirable combination is achieved in terms of aqueous solubility and lipophilicity. Moreover, the resulting combination of biphasic solubility and fast enzymatic hydrolysis of the methylpiperazinylacyloxyalkyl derivatives gave improved topical delivery of naproxen.

Cyclic amide derivatives as potential prodrugs II: N-hydroxymethylsuccinimide- / isatin esters of some NSAIDs as prodrugs with an improved therapeutic index

Ester prodrugs of aspirin 1a, ibuprofen 1b, naproxen 1c and indomethacin 1d were synthesized using N-Hydroxymethylsuccinimide (HMSI) 3 and N-hydroxymethylisatin (HMIS) 4 as promoieties to reduce their gastrointestinal toxicity and improve bioavailability. Additionally, the kinetics of hydrolysis of the synthesized prodrugs 5a-d and 6a-d were studied at 37 degrees C in non-enzymatic simulated gastric fluid (SGF; hydrochloric acid buffer pH = 1.2); 0.02 M phosphate buffer (pH = 7.4); 80% human plasma and 10% rat liver homogenate. The results indicate higher chemical stability of the ester prodrugs in non-enzymatic SGF (t(1/2) congruent with 6.5-18.6 h) and rapid conversion to the parent drugs in 80% human plasma (t(1/2) congruent with 11.4-235 min) as well as in 10% rat liver homogenates (t(1/2) congruent with 12.0-90.0 min). As a general pattern, the HMSI esters 5a-d revealed higher chemical stability than the corresponding HMIS analogues 6a-d. The pH-rate profile of 5c and 6a indicated maximum stability of the former at pH = 1.2-8.0 and of the latter at pH = 1.2-4.0. The distribution coefficient (D(7.4)) values of the prodrugs 5a-d, 6a-d and the parent drugs 1a-d in an n-octanol/phosphate buffer (pH = 7.4) system indicated enhanced lipophilic properties of the prodrugs. Furthermore, the HMIS ester prodrugs 6a-d are more lipophilic than the corresponding HMSI derivatives 5a-d. In vivo ulcerogenicity studies using scanning electron microscopy on stomach specimens of rats treated with an oral dose for 4 d revealed that the synthesized ester prodrugs are significantly less irritating to gastric mucosa than the parent drugs. These results suggested HMSI and/or HMIS esters possess good potential as prodrugs with an improved therapeutic index for oral delivery of NSAIDs.

Surfactant effect on enhancing (S)-naproxen prodrug production from racemic naproxen by lipase

In the enantioselective esterification of racemic naproxen with 4-(2-hydroxyethyl) morpholine by Lipase MY in organic solvents, a productivity improvement of the desired (S)-naproxen ester from 0.42 to 0.72 mM at the reaction time of 130 h was observed, when the surfactant bis (2-ethylhexyl) sodium sulfosuccinate (AOT) was added in the reaction mixture. The presence of a small amount of exogenously added water dramatically activated the enzyme in AOT/cyclohexane-reversed micelles. Desorption of the surfactant molecule from the enzyme mass and solubilization of the enzyme into reversed micelles were used to elucidate an existing maximum of the initial rate of (S)-naproxen synthesis with the water content. Moreover, the effects of alcohol and surfactant concentration on the enzyme activity are reported.

Comparative evaluation of the severity of gastric ulceration by solid dispersions and coprecipitates of indomethacin

The ulcerogenic activity of indomethacin was studied in rats following single and chronic doses of indomethacin in the form of pure drug, solid dispersions and coprecipitates. Each formulation was administrated as a suspension in a 2% methylcellulose solution. Gastrointestinal ulceration was assessed, four hours after a single dose and 24 hours following the last dose of a chronic four day dosing regimen, by counting the number of lesions and ulcers present. A rating scale was employed to evaluate the severity index. The coprecipitate formulation produced less severe ulceration than the solid dispersion and pure drug. This suggests that the severity of ulceration than the solid dispersion and pure drug. This suggests that the severity of ulceration may be related to the preparation methodology and drug release kinetics.

Stereoselective disposition of flurbiprofen from a mutual prodrug with a histamine H2-antagonist to reduce gastrointestinal lesions in the rat

The in vitro and in vivo stereoselective hydrolysis characteristics of the mutual prodrug FP-PPA, which is a conjugate of flurbiprofen (FP) with the histamine H2-antagonist PPA, to reduce gastrointestinal lesions induced by FP were investigated and compared with those of FP methyl ester (rac-FP-Me) and FP ethyleneglycol ester (rac-FP-EG). The rac-FP derivatives were hydrolyzed preferentially to the (+)-S-isomer in plasma and to the (-)-R-isomer in liver and small intestinal mucosa. Interestingly, in the gastric mucosa, the stereoselectivity of hydrolysis of (-)-R-FP-PPA was opposite from that of rac-FP-Me and rac-FP-EG, which suggested that the stereoselective hydrolysis of FP-PPA was helpful in reducing gastric damage induced by (+)-S-FP. However, hydrolysis of all rac-FP derivatives was found to be catalyzed by carboxylesterases in the gastric mucosa. The stereoselective disposition of FP enantiomers early after intravenous administration of rac-FP-PPA could be explained by the stereoselective formation of (-)-R-FP from rac-FP-PPA in the liver. (-)-R-FP-PPA was completely hydrolyzed to form (-)-R-FP in vivo, while 78% of (+)-S-FP-PPA was hydrolyzed to (+)-S-FP, with a corresponding decrease in the area under the curve. Twenty-five percent of (+)-S-FP-PPA might be eliminated as the intact prodrug or its metabolites other than FP. The most important bioconversion of FP-PPA occurred in plasma, and additional hydrolysis of the R-enantiomer in liver resulted in the stereoselectivity observed following both i.v. and p.o. administration.

Pharmacokinetic analysis of diethylcarbonate prodrugs of ibuprofen and naproxen

The pharmacokinetics of ibuprofen diethylcarbonate (ibudice) and naproxen diethythylcarbonate (napdice), two new diethylcarbonate prodrugs of ibuprofen and naproxen, was investigated in dogs. The rationale for the development of ibudice and napdice was that esterification of the carboxylic moiety of the parent compounds would suppress gastrotoxicity without adversely affecting their anti-inflammatory activity. In addition the biotransformation of the prodrugs to the parent compounds may be utilized to achieve rate and time controlled drug delivery of the active entities. Following oral administration the diethylcarbonate esters were rapidly biotransformed to the parent compounds and no ibudice or napdice was detected in the plasma. The relative bioavailability of ibuprofen and naproxen, following oral administration of ibudice and napdice, was 96% and 74%, respectively, and the rate of absorption was not significantly different from that obtained following oral dosing of the parent compound. Stability studies in gastric and intestinal juice showed that, unlike napdice, ibudice was stable in gastric juice, with both prodrugs undergoing rapid biotransformation to their parent compounds in intestinal juice. This rapid conversion led to the lack of sustained release performance following oral administration of ibudice or napidice.