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In order to select a species for drug absorption studies of ester prodrugs and to identify a possible absorption window with low esterase activity and hence increased absorption of the ester prodrug, the esterase activity was investigated in homogenates from various intestinal segments of different species. p-Nitrophenyl acetate and tenofovir disoproxil [bis(POC)-PMPA] were used as substrates for esterases. p-Nitrophenyl acetate is a model substrate for esterase activity, while tenofovir disoproxil (fumarate salt) is an ester prodrug of the potent antiviral nucleoside phosphonate analogue tenofovir. As esterase-mediated degradation during transepithelial transport may be a limiting factor for its oral absorption, targeting the prodrug to a region of the intestine with lower esterase activity may lead to an increase in oral absorption of the prodrug. The results obtained with p-nitrophenyl acetate and tenofovir disoproxil showed both a site-specific (duodenum > or = jejunum > ileum > or = colon) and species-dependent (rat > man > pig) degradation in intestinal homogenates. Degradation of tenofovir disoproxil in homogenates from Caco-2 monolayers (0.016+/-0.003 nmol. s(-1). mg protein(-1)) was low compared to its degradation in homogenates from human ileum (0.177+/-0.052 nmol. s(-1). mg protein(-1)). Rat ileum appears to be a suitable model to evaluate the influence of esterase activity on the oral absorption of the ester prodrug, as the degradation rate for tenofovir disoproxil (0.245+/-0.054 nmol. s(-1). mg protein(-1)) in rat ileum was similar to degradation in human ileum. The results also suggest that colon targeting may be a useful strategy to reduce the esterase-mediated degradation of ester prodrugs, hence resulting in a possible increase in their oral absorption.

A series of straight chain, branched and cyclo-delta-aminolaevulinic acid (ALA) esters have been synthesized and their photosensitizing properties analysed using an in vitro system of rat pancreatoma cells. Structurally favourable ALA esters not only induced the formation of more of the endogenous photosensitizer, protoporphyrin IX (PpIX), but they did so at a faster rate than ALA itself. This action was reflected in a substantial increase in photocytotoxicity of some 270 times, using the more potent ALA esters. An important structural feature was identified in two of the ALA esters which greatly limited PpIX production, i.e. a branch point located next to the site of ester cleavage. Experiments on the transport of ALA and of ALA esters across the cell membrane showed that ALA, but not ALA esters, gain access to the cell via the di- and tripeptide transporter, PEPTI. Finally, these results show that the esterification of ALA can greatly increase its cellular uptake, so generating more intracellular PpIX, improved tumour cell photosensitization and enhanced photocytotoxicity.

While P-glycoprotein (Pgp) is the most studied protein involved in resistance to anti-cancer drugs, its mechanism of action is still under debate. Studies of Pgp have used cell lines selected with chemotherapeutics which may have developed many mechanisms of resistance. To eliminate the confounding effects of drug selection on understanding the action of Pgp, we studied cells transiently transfected with a Pgp-green fluorescent protein (GFP) fusion protein. This method generated a mixed population of unselected cells with a wide range of Pgp-GFP expression levels and allowed simultaneous measurements of Pgp level and drug accumulation in living cells. The results showed that Pgp-GFP expression was inversely related to the accumulation of chemotherapeutic drugs. The reduction in drug concentration was reversed by agents that block multiple drug resistance (MDR) and by the UIC2 anti-Pgp antibody. Quantitative analysis revealed an inverse linear relationship between the fluorescence of Pgp-GFP and MDR dyes. This suggests that Pgp levels alone limit drug accumulation by active efflux; cooperativity between enzyme, substrate, or inhibitor molecules is not required. Additionally, Pgp-GFP expression did not change cellular pH. Our study demonstrates the value of using GFP fusion proteins for quantitative biochemistry in living cells.

The hydrolysis of tertiary amidomethyl ester prodrugs of carboxylic acids by rat liver homogenates is reported. Amidomethyl esters are rapidly and quantitatively converted to the corresponding acid and secondary amide. Reactivity is inversely dependent upon the molar refractivity and lipophilicity of the ester, as well as with steric bulk in the carboxylic acid moiety. In contrast to chemical and plasma hydrolyses, no dependence upon the pK(a) of the carboxylate leaving group was observed, nor was there any dependence upon the amide N-substituent. The rate of decomposition was inhibited by the carboxylesterase inhibitor eserine but not by the cytochrome P450 inhibitor SKF-525A, indicating the involvement of esterases in the hydrolysis reaction. These results indicate that amidomethyl esters may be expected to be readily cleaved in vivo.

Trialkylammonium acetoxymethyl esters of dexanabinol were synthesized and evaluated as water-soluble prodrugs. Syntheses were performed by conventional methods; solubility in water and stability in buffers and human plasma were determined by HPLC, and in vivo tissue distribution studies were performed in a rat model. Most of the new derivatives were soluble in water (approximately 50 mg/mL). They were relatively stable in water, while rapidly hydrolyzed in human plasma. Distribution studies indicated that peak concentrations of drug both in blood (30 microg/mL) and brain (2 microg/mL) were rapidly (5 min) achieved after iv administration of a selected prodrug to rats. The blood concentration decreased faster than brain levels which were detectable even after 24 h. Some of the examined esters could be further developed as water soluble prodrugs of dexanabinol.

Scavenger receptor BI (SR-BI) mediates the selective uptake of high density lipoprotein (HDL) cholesteryl esters (CE) by cells, i.e., the uptake of CE without degradation of HDL protein. Mice with attenuated expression of SR-BI, because of targeted gene mutation (SR-BIatt mice), have increased plasma HDL levels as a result of decreased selective uptake in the liver. To further evaluate the role of SR-BI in lipoprotein metabolism, compound apolipoprotein E knock-out (apoE0)/SR-BIatt mice were bred. Hepatic SR-BI protein was increased (2.3-fold) in apoE0 mice compared with wild type (wt) and was reduced significantly in apoE0/SR-BIatt mice. However, the plasma lipoprotein profile of apoE0 and apoE0/SR-BIatt mice was identical. This was explained by HDL turnover studies that revealed that the selective clearance of HDL CE by the liver and adrenal was already profoundly impaired in apoE0 mice compared with wt (28% of wt in liver). A similar decrease in selective uptake was seen when apoE0 HDL was incubated with isolated apoE0 hepatocytes. The results suggest that apoE plays a major role in the selective clearance of HDL CE by the liver and adrenal gland, possibly by facilitating the presentation of HDL to SR-BI at the cell surface.

A new prodrug approach for intracellular delivery of phosphonates was developed via the synthesis of 3-phthalidyl esters of 1-naphthalenemethylphosphonate. This approach is advantageous over the traditional acyloxymethyl phosphonate prodrugs, because these prodrugs do not generate formaldehyde and have improved plasma half-lives.

The rate of hydrolysis of polyethylene glycol esters of hydrocortisone 21-succinate (H-PEGs) rate by a commercial esterase and by enzymes from ovine cornea was studied. Both the commercial esterase and the corneal enzymes rapidly hydrolysed the H-PEGs. Both lipophilicity and the PEG chain length had an influence on the rate of enzymatic hydrolysis, a significant decrease in enzymatic hydrolysis rate being associated with an increase in the polymer chain length. The in vitro penetration of H-PEGs across ovine cornea and sclera was assessed. In corneal penetration studies no prodrug was detected in the receiver phase while in the sclera penetration study both drug and prodrug were detected. Results from in vitro corneal and scleral absorption studies showed that the use of H-PEG400 increased the rate at which hydrocortisone diffused through excised ovine cornea compared to the use of hydrocortisone itself, particularly when the donor phase was removed after 15 min in order to mimic the situation in vivo. In addition, H-PEG200, H-PEG400, H-PEG600, H-PEG900 and H-PEG2000 gave a significant increase in the rate of diffusion of hydrocortisone+H-PEG through the sclera compared to hydrocortisone itself. Our results on ovine eye show that the rate of diffusion of hydrocortisone across the sclera is about six times higher than that in cornea and for the H-PEGs the scleral diffusion was 10-100 times higher than that observed in the cornea., (Copyright.)

The percutaneous delivery of PGE1 and its alkyl esters in alcoholic saline solution through hairless mouse skin was compared. The quantification of alkyl esters was based on the same principle as that for PGE1, which was converted to PGB1 to enhance the sensitivity and minimize the interference. Results showed that it was PGE1 that appeared in the receiver compartment for all alkyl esters examined. The flux of all alkyl esters of PGE1 in the same concentration was higher than PGE1 itself at most of saline vehicle with various fractions of alcohol. The maximal flux for a fixed concentration of each alkyl ester appeared at different fractions of alcohol. When the fractions of alcohol was kept constant, the alkyl ester that showed the maximal flux at this concentration appeared to have a longer chain length with increasing the fraction of alcohol. But isopropyl ester deviated from this order. It was concluded that the alkyl ester derivatives promoted the penetration of PGE1 mainly as a result of enhancing the drug partitioning into the stratum corneum. The alcohol fraction that needed to achieve the maximal flux at the same concentration increased with the increase of alkyl chain length, which resulted in the decrease of solubility parameter. It is necessary to optimize the fraction of alcohol in the saline solution in order to achieve the maximal flux at a fixed concentration for these alkyl esters with different alkyl chain length.

An inhibition study showed that the stereoselective hydrolysis of butyryl propranolol (butyryl PL) in rat liver microsomes and plasma involves carboxylesterase. The hydrolysis of (S)-butyryl PL in plasma was specifically inhibited by eserine and bis-nitrophenyl phosphate (BNPP), compared to the (R)-isomer, despite the non-stereoselective hydrolysis of butyryl PL in plasma. In addition, inhibition of hydroloysis by eserine and BNPP showed little stereoselectivity for butyryl PL in liver, although liver microsomes showed an (S)-preferential hydrolysis for butyryl PL (R/S ratio of Vmax/Km: 2.1 +/- 0.2). The hydrolysis of butyryl PL was not inhibited by a polyclonal antibody against a high affinity carboxylesterase (hydrolase A, RH1). Moreover, the high Km value and the high IC50 for phenylmethylsulfonyl fluoride (PMSF) against the hydrolysis of butyryl PL in rat liver microsomes suggest that a low affinity carboxylesterase (perhaps hydrolase B) might be involved in this hydrolysis in rat liver.

A series of acyloxyalkyl esters of ketoprofen and naproxen were synthesized and investigated as topical prodrugs with the aim of improving the dermal delivery of the drugs. In addition, some hydroxyalkyl esters of ketoprofen and naproxen were synthesized as possible intermediates of acyloxyalkyl prodrugs. All of the prodrugs were more lipophilic than their parent molecules, as evaluated by drug partitioning between 1-octanol and phosphate buffer at pH 7.4 (log Papp). However, their solubilities in aqueous solutions decreased markedly compared with the parent molecules. The prodrugs were stable toward chemical hydrolysis in aqueous solutions (pH 7.4), but were hydrolyzed to the parent drug both in 80% human serum and in human skin homogenate, with half-lives ranging from 4 to 137 min and from 13 to 403 min, respectively. The abilities of the selected naproxen acyloxyalkyl prodrugs to deliver naproxen through excised human skin were evaluated. Generally, the prodrugs showed similar dermal delivery as the parent drug through cadaver skin. In the present series of lipophilic prodrugs of naproxen, the prodrug with the highest aqueous solubility was the most effective prodrug to deliver naproxen through the skin.

Purpose: This study characterized the cellular uptake mechanism and hydrolysis of the amino acid ester prodrugs of nucleoside antiviral drugs in the transiently transfected Caco-2 cells overexpressing a human intestinal peptide transporter, hPEPT1 (Caco-2/hPEPT1 cells)., Methods: Amino acid ester prodrugs of acyclovir and AZT were synthesized and their apical membrane permeability and hydrolysis were evaluated in Caco-2/hPEPT1 cells. The cellular uptake mechanism of prodrugs was investigated through the competitive inhibition study in Caco-2/hPEPT1 cells., Results: L-Valyl ester of acyclovir (L-Val-ACV) was approximately ten fold more permeable across the apical membrane than acyclovir and four times more permeable than D-valyl ester of acyclovir (D-Val-ACV). Correspondingly, L-valyl ester of AZT (L- Val-AZT) exhibited three fold higher cellular uptake than AZT. Therefore, amino acid ester prodrugs significantly increased the cellular uptake of the parent drugs and exhibited the D,L-stereoselectivity. Furthermore, prodrugs were rapidly hydrolyzed to the parent drugs by the intracellular hydrolysis, following the apical membrane transport. In the inhibition studies, cephalexin and small dipeptides strongly inhibited the cellular uptake of L-Val-ACV while L-valine had no effect, indicating that the peptide transporter is primarily responsible for the apical membrane transport of L-Val-ACV. In addition, the cellular uptake of L-Val-ACV was five times higher in Caco-2/hPEPT1 cells than the uptake in the untransfected Caco-2 cells, implying the cellular uptake of L-Val-ACV was related to the enhancement of the peptide transport activity in Caco-2/hPEPT1 cells., Conclusions: Caco-2/hPEPT1 system is an efficient in vitro model for the uptake study of peptidyl derivatives. Amino acid ester prodrugs significantly improved the cellular uptake of the parent drugs via peptide transport mechanism and were rapidly converted to the active parent drugs by the intracellular hydrolysis.

99mTc-ethylene dicysteine diethyl ester (99mTc-L,L-ECD) is a neutral lipophilic tracer agent that crosses the blood-brain barrier and is retained in the brain of primates following enzymatic hydrolysis of one of the ester functions to the ionized mono-ester, mono-acid metabolite. Up to now, it is not clear whether the second ethylcarboxylate group is essential for brain uptake and retention. Therefore, we have synthesized and studied two derivatives of 99mTc-L,L-ECD that contain only one ethylcarboxylate function, namely 99mTc-labelled L- and D-ethylene cysteamine cysteine ethyl ester (99mTc-ECCE). Direct labelling of L- or D-ECCE at neutral pH and room temperature resulted for each of them in the formation of two probably diastereomeric 99mTc-complexes in a 1:1 ratio. This means that four different isomers could be isolated. The 99mTc-labelled complexes formed after labelling ECCE (A and B, in order of elution during HPLC) are slightly less lipophilic than 99mTc-L,L-ECD. In mice, all four isomers show a low brain activity at 10 min post injection (p.i.), approximately 0.1% to 0.3% of the injected dose versus 0.9% for 99mTc-L,L-ECD. The clearance from the blood is comparable with (isomers LA and LB) or slower (isomers DA and DB) than that of 99mTc-L,L-ECD. Isomers LA and DA show high liver uptake and rapid excretion to the intestines. Both 99mTc-ECCE-LB and 99mTc-ECCE-DB, the most lipophilic isomers, are cleared from the blood mainly by the kidneys and excreted more efficiently to the urine. 99mTc-ECCE-LB is characterized by a surprisingly high heart uptake (about 1.5% of i.d.) at 10 min p.i. versus 1.0% for 99mTc-methoxyisobutylisonitrile (99mTc-MIBI) and 0.2-0.3% for the other isomers, but also lung uptake is relatively high. In the baboon, brain and heart uptake of isomers LA and LB of 99mTc-ECCE were negligible. Activity concentrated mostly in the hepatobiliary system for isomer LA and in the renal system for isomer LB. The results indicate a clear difference in biological behaviour between 99mTc-L,L-ECD and the four isomers of 99mTc-ECCE. This shows that the presence of both ester functions in 99mTc-L,L-ECD is an essential structural requirement for its brain uptake and retention in primates.

beta-Blockers including timolol and propranolol are administered in eye-drops for the treatment of glaucoma. Due to high incidence of cardiovascular and respiratory side-effects, their therapeutic value is limited. As a result of poor ocular bioavailability, many ocular drugs are applied in high concentrations, which give rise to both ocular and systemic side-effects. Therefore, some methods have been employed to increase ocular bioavailability such as (a) the development of drug delivery devices designed to release drugs at controlled rates, (b) the use of various vehicles that retard precorneal drug loss, and (c) the conversion of drugs to biologically reversible derivatives (prodrugs) with increased corneal penetration properties, from which the active drugs are released by enzymatic hydrolysis. A series of structurally related oxprenolol esters were synthesized and investigated as potential prodrugs for improved ocular use. The stability of each ester was studied in phosphate buffer (pH 7.4), also in the presence of (a) 30% human plasma, (b) aqueous humor, and (c) corneal extract at pH 7. 4 and at 37 degreesC. An account is given of how the stability of a homologous series of oxprenolol esters in the presence of biological enzymes is affected by an increase in the carbon chain length of the ester moiety.

Effects of oleic acid or triolein on lymphatic recovery of docosahexaenoic acid (DHA) given as an ethyl ester were examined in rats with cannulated thoracic ducts. Lymphatic recovery of ethyl DHA given with oleic acid or triolein was significantly higher than in rats given ethyl DHA alone. DHA distributed almost exclusively at the 1- and 3-position of triglyceride in lymph collected at 0-3 h after the administration, when it was given with oleic acid or triolein. A small part of DHA distributed at the 2-position when ethyl DHA was the sole fatty acid given. Oleic acid given as free acid or triolein with ethyl DHA was a major fatty acid at the 2-position. Intramolecular distribution of DHA and oleic acid in lymph triglyceride was similar when ethyl DHA was given with oleic acid or triolein.

Four derivatives of 4-isobutylphenyl-2-propionic acid (Ibuprofen), in which the drug was bound by ester linkages to poly(ethylene glycols) (PEG 2000-I), monomethoxy poly(ethylene glycols) (PEG 1900-I), poly(N-vinyl pyrrolidinone) (PVP-I) and poly(N-acryloyl morpholine) (PACM-I), all having approximatively the same number average molecular weight (Mn congruent equal to 2000), were prepared and tested for their pharmacokinetic properties after oral administration. It was found that the two end-hydroxylated amphiphilic oligomers of polyvinylic structure, PACM and PVP, whose physico-chemical properties are comparable to those of PEGs especially as regards solvent affinity, have in principle a similar potential as promoieties for preparing oligomeric prodrugs.

The tissue concentration of retinol and its metabolites was determined in a group of vitamin A deficient rats after a single dose of 53 micrograms of [11,12-3H]retinol. The sensitive technique of high performance liquid chromatography was used to analyse the metabolites of retinol. The analysis of the metabolites in tissues at different days after the administration of radioactive retinol showed a rapid decrease in the amount of retinol and retinyl esters in the liver tissue, accompanied by an increase in the retinol and retinyl ester values in the kidney. In addition, the content of retinoic acid was higher in liver and kidney compared with intestine, testis, and blood. It reached maximum at 4 and 11 days, respectively. After 17 days the retinoid(s) concentrations decreased markedly in all tissues studied; yet the kidney showed higher concentrations of retinoic acid and retinyl esters. These studies indicate that the kidney retains more vitamin A as vitamin A becomes depleted in the body, probably as a reserve for the production of the active metabolite retinoic acid, needed for the growth and differentiation.

Purpose: The purpose of this study was to evaluate the stereoselective permeation and concurrent cutaneous hydrolysis of a series of ester prodrugs of propranolol (PL)., Methods: In vitro studies were performed across full-thickness, stripped and diisopropylfluorophosphate (DFP) treated skins of hairless mouse with flow-through diffusion cells at 37 degrees C., Results: The permeability coefficients (Kp), which were dependent on partition coefficients (PC), of all the prodrugs were markedly increased compared to the parent drug. In full-thickness skin, the (R) caproyl-PL (CR-PL) showed the highest Kp, which was about 52-fold greater than that of PL. Most of the more lipophilic prodrugs showed stereoselectivity in Kp (R > S). All the prodrugs underwent stereoselective hydrolysis (R > S) during penetration. The prodrugs which showed stereoselectivity in permeation were comparatively lipophilic and showed great differences in hydrolysis percentages between the enantiomers. Permeation studies with stripped skin revealed that prodrugs were more permeable across stratum corneum compared to PL, whereas reverse was happened across viable skin. Although CR-PL showed high stereoselectivity in permeation across full-thickness skin and underwent higher percent of concurrent stereoselective cutaneous hydrolysis, the prodrug showed no stereoselectivity in permeation across DFP, an esterase inhibitor, treated skin and the concurrent cutaneous hydrolysis was also stopped., Conclusions: Lipophilic prodrugs may readily pass the stratum corneum but may not be able to penetrate so easily through the deeper tissues. Unlike the (S) isomers, the (R) isomers of lipophilic prodrugs almost completely converted to propranolol in epidermis and can easily pass through the dermis layer, resulting in stereoselective penetration.

Purpose: Homologous 3-alkyl-ester prodrugs (C2 to C4) of buprenorphine with decreased crystallinity have been synthesized and evaluated for transdermal delivery commensurate with opioid dependence treatment., Methods: To assess the influence of derivatization on delivery, the permeation of the prodrugs through human skin was determined in vitro. Prodrug metabolism was measured in human blood and skin supernatant in vitro along with chemical hydrolysis controls. The prodrugs octanol/water partition coefficients were measured., Results: Without exception, the prodrugs were completely hydrolyzed on passing through the skin and appeared as buprenorphine in the receptor compartment. However, using saturation conditions, in no instance did the buprenorphine flux through skin from a prodrug solution exceed the flux of buprenorphine base itself in vitro. Moreover, the flux of the acetyl ester, the least hydrophobic of the prodrugs, was not significantly elevated upon stripping the skin. Whether in blood or the skin supernatant, the prodrugs hydrolyzed in an apparent first-order fashion and rate constants and half-lives were calculated., Conclusions: We conclude from the results that the prodrugs' very high octanol/water partition coefficients (hydrophobicity) placed them in viable tissue layer controlled diffusion. Consequently, one does not derive the potential flux-increasing benefit of reducing crystallinity that was expected.

In order to develop a potential prodrug of indomethacin (IM) which causes less irritation to the gastrointestinal mucosa, the ester prodrugs [butyl ester (IM-BE) and octyl ester (IM-OE)] of IM were synthesized and evaluated for their ulcerogenic activity and hepatic injury after oral administration in rats. Additionally, the kinetics of hydrolysis of the prodrugs were examined to characterize the tissues or organs capable of hydrolyzing the ester bonds. The plasma levels of IM after the oral administration of IM-OE and IM-BE were comparatively low compared with those after IM, with a small bioavailability (2.1 and 15.0%, respectively). Ulcerogenic activity and hepatic injury, expressed by decreased hepatic microsomal enzyme activities, were hardly seen after repeated oral administration of the prodrugs, in contrast with the severely irritating effects of IM alone. Hydrolysis of the prodrugs was adequately described by first-order kinetics. IM-BE was relatively rapidly hydrolyzed in plasma, skin and whole blood, but the hydrolysis in the intestinal mucosa and liver was very slow. The hydrolytic rates for IM-OE were exceedingly small or negligible. These results indicate that the main part of IM-BE and IM-OE administered orally might not be hydrolyzed to IM in the gastrointestinal tract, and that the ester prodrugs themselves were absorbed through the mucosa; also, that the hydrolysis of ester bonds would be carried out mainly in the circulatory system. Consequently, IM-BE seems to be an ideal prodrug of IM.

Nasal absorption of 2',3'-didehydro-3'-deoxythymidine (D4T) and its esters (5'-acetyl D4T: C2-D4T and 5'-hemisuccinyl D4T: Suc-D4T) was investigated in rats. Bioavailability of D4T following intranasal (i.n.) administration was 98.0%, and the elimination from plasma was as rapid as that following intravenous administration of D4T. There seemed to be complete and rapid absorption of D4T from the nasal cavity. The plasma concentration-time profile of D4T following i.n. administration of C2-D4T was almost the same as that after administration of D4T itself. This suggests that C2-D4T was rapidly absorbed from the nasal cavity, and that some amount of dosing C2-D4T was hydrolyzed to D4T before entering the systemic circulation. In contrast, Suc-D4T showed slower absorption in the i.n. administration, and the plasma D4T level was maintained for a long period.

The kinetics of vitamin A and its major metabolites were investigated in humans. Eleven healthy male subjects ingested 105 mumol (100,000 IU) of [8,9,19-13C]retinyl palmitate in an oily solution. Twenty-seven blood samples were collected during the 1-week study. Plasma samples were analyzed for retinyl esters and for [12C]- and [8,9,19-13C]retinol. Retinol isotopes were quantified using a newly developed GC-MS method. Total retinyl esters peaked at about 4.45 mumol/L from 3.5 to 12 h after dosing. As a result of the perturbation of the tracee system, the plasma concentration of [12C]retinol increased and then decreased as the concentration of [8,9,19-13C]retinol increased, indicating rapid distribution kinetics. A broad single peak (1.16 +/- 0.32 mumol/L) was observed for [8,9,19-13C]retinol at about 10 to 24 h postdose; this likely reflects hepatic secretion of [8,9,19-13C]retinol associated with retinol-binding protein. Then, declining levels of the tracer and increasing levels of the tracee were observed. At its peak, the ingested [8,9,19-13C]retinol reached about 51% of the observed total plasma retinol concentration. This percentage dropped to 13.4% on day 7 indicating slow final elimination from plasma. Our data support the concept that the liver follows the principle "last in/first out' in maintaining vitamin A homeostasis.

The objectives of the present study were (i) to develop a mechanistic algorithm for predicting blood:air partition coefficients (PCs) of volatile organic chemicals (VOCs), and (ii) to apply this algorithm to predict the rat blood:air PCs of several VOCs. The approach consisted initially of developing an algorithm to predict the blood:air PCs of VOCs solely based on the solubility phenomenon and then of extending the algorithm to include protein binding. The algorithm based on solubility phenomenon predicted blood:air PCs by dividing the estimated solubility of chemicals in blood by their saturable vapor concentrations at 37 degrees C. The rat blood:air PCs predicted using this algorithm were in close agreement with the experimental values for relatively hydrophilic VOCs such as ketones, alcohols, acetate esters, and diethyl ether (with an average ratio of 0.80 between predicted and experimental values), whereas there was a marked discrepancy in the case of relatively lipophilic VOCs such as alkanes, haloalkanes, and aromatic hydrocarbons (with an average ratio of 0.21 between predicted and experimental values). This discrepancy was hypothesized to be due to the occurrence of reversible binding of these substances in rat hemoglobin based on literature evidence of the existence of hydrophobic holes (or "xenon-binding" pockets). The association constants (Ka) for the presumed reversible hemoglobin binding of several alkanes, haloalkanes, and aromatic hydrocarbons were estimated from the difference between chemical concentration in rat erythrocytes predicted by the solubility-based algorithm and that deduced from the previously published experimental blood:air PCs for these chemicals (which presumably included contribution of hemoglobin binding in addition to "true" solubility). The Ka values estimated in this manner ranged from 504 to 4725 M-1 for the chemicals investigated in the present study. The a priori predictions of the percentage of several VOCs (diethyl ether, methyl isobutyl ketone, n-hexane, toluene, and chloroform) in rat erythrocytes obtained with the algorithm using these Ka estimates corresponded well with previously published experimental data. The mechanistic algorithm developed in the present study should be useful for predicting the "apparent" blood:air PCs of VOCs regardless of exposure concentrations, by accounting for the relative contributions of both the true chemical solubility and reversible hemoglobin binding.

The objectives of the present study were (i) to develop an algorithm for predicting the tissue:air partition coefficients (PCs) of volatile organic chemicals (VOCs) and (ii) to apply this algorithm to predict the rat tissue:air PCs of 45 VOCs. The approach consisted of estimating the tissue:air PCs by dividing the tissue solubility of chemicals by their saturable vapor concentrations. The tissue solubility of chemicals was calculated as the sum total of their solubility in neutral lipid, phospholipid, and water fractions of tissues. The rat liver:air, muscle:air, and adipose tissue:air PCs predicted using this algorithm compared well with literature data available for several ketones, alcohols, acetate esters, alkanes, haloalkanes, aromatic hydrocarbons, and diethyl ether. The average ratios between the predicted and experimental values of the tissue:air PC values were 0.94 (liver), 0.93 (muscle), and 1.10 (adipose tissue). The mechanistic algorithm developed in the present study should be useful for predicting tissue:air PCs of VOCs and for verifying the current default assumption of considering tissue:air PCs to be species-invariant.

The preparation of 99mTcO-, 99mTcN- and 99mTc(CO)-complexes of cysteamine (CYA), cysteine (CYS) and the methyl (CME) and ethyl (CEE) esters of cysteine is described. All complexes were stable and their biological behaviour in mice was studied. Most complexes were found to clear predominantly via the renal pathway with TcN- and Tc(CO)-complexes exhibiting greater urinary clearance than the corresponding TcO-complex. 99mTcN-CME, 99mTc(CO)-CME and 99mTc(CO)-CYS were found to be rapidly cleared via the kidneys and the effect of probenicid on renal clearance was studied to determine whether tubular excretion was involved. The results suggest that tubular secretion is involved in the renal clearance of 99mTcN-CME and Tc(CO)-CYS.

The efficacy and toxicity of vitamin K depends on the pathway and the extent of enzymatic reductive activation to vitamin K hydroquinone, which is an essential cofactor for the synthesis of clotting factors. Parenteral use of vitamin K is impaired by its water insolubility. With the aim to improve delivery problems associated with menahydroquinone-4 (MKH, 2), an active form of menaquinone-4, N,N-dimethylglycine esters of 2 (1-mono, 4-mono, and 1,4-bis) were synthesized and assessed as potential water-soluble prodrugs for parenteral use. The esters can deliver the hydroquinone to its active site without a quinone reductive activation step. The hydrochloride salts of the esters were found to be quite soluble in water. The hydrolysis of the esters in 20% rat liver homogenate 9000 x g supernatant, rat plasma and phosphate buffer, pH 7.4, at 37 degrees C was kinetically studied in the presence and absence of an esterase inhibitor. The hydrolysis was catalyzed by esterases located in the rat liver and rat plasma and quantitatively yielded 2. These results suggest that esterification of 2 with N,N-dimethylglycine is a promising way for obtaining water-soluble prodrug forms of 2. Based on the high susceptibility to liver esterase, the esters are potential prodrugs for achieving the site-specific delivery of 2.

Five indomethacin oligoethylene ester derivatives (3-7) were synthesized and evaluated for their anti-inflammatory, analgesic, and ulcerogenic activity after oral administration. The molecular weight of the oligoethylene glycols used for synthesizing esters 3-7 ranged from 106 to 282. The chemical and enzymatic stabilities of esters 3-7 were evaluated in pH 7.4 and 2.0 buffers and in human plasma, respectively. All the prodrugs showed a good stability both in pH 7.4 phosphate buffer and in pH 2.0 buffer, and they were readily hydrolyzed by human plasma. Esters 3-7 showed an anti-inflammatory activity, determined as the percent inhibition of carrageenan-induced edema, similar to that of indomethacin, although at higher doses. From writhing test results, we observed that all the prodrugs exhibited better or similar analgesic activity compared to indomethacin. Esters 3-7 were significantly less irritating to the gastric mucosa than indomethacin, after oral administration, and esters 3-5 did not show any ulcerogenic activity, although they were administered at higher doses than indomethacin.

Morpholinoalkyl esters (HCl salts) of diclofenac (1) were synthesized and evaluated in vitro and in vivo for their potential use as prodrugs for oral delivery. Prodrugs were freely soluble in simulated gastric fluid (SGF) and pH 7.4 phosphate buffer and showed a minimum of a 2000-fold increase in solubility over the parent drug. All prodrugs were more lipophilic than 1 as indicated by n-octanol/pH 7.4 buffer partition coefficients, but less lipophilic than 1 in terms of n-octanol/SGF partition coefficients. Potentiometrically determined ionization constants (pKas) were in the range of 7.52 to 8.40 at 25 degrees C. The chemical and enzymatic hydrolyses of prodrugs were evaluated in SGF/pH 7.4 phosphate buffer and rat plasma, respectively, at 37 degrees C. All prodrugs were quantitatively hydrolyzed to 1 by either chemical and/or enzymatic means. An increase in carbon chain length rendered the prodrugs more stable at pH 7.4, but less stable in SGF. In general, the esters were hydrolyzed rapidly in rat plasma at 37 degrees C, the half-lives of hydrolysis being in the range of 4.85 to 23.49 min. Based on in vitro results, prodrug 2 was chosen to evaluate solid-state stability, bioavailability, and in vivo ulcerogenicity. At elevated temperatures, the solid-state decomposition of 2 followed biphasic kinetics, with rapid decomposition occurring initially. The extent, but not the rate, of absorption was significantly greater in rats for prodrug 2 than 1 following single dose oral administration. Prodrug 2 was significantly less irritating to gastric mucosa than 1 following single and chronic oral administration in rats.

In this study we characterized liposomes containing the synthetic glycolipids, lactose mono-fatty acid esters (LFEs), whose structures were the most simple ones compared to the other synthetic glycolipids. Gel filtration chromatography was used to confirm that 78 per cent of lactose monostearic acid ester (LSE) or 85 per cent of lactose monoarachidic acid ester (LAE) existed in the liposomal fraction. The size distribution of liposomes containing LFEs was similar to that of the control liposomes which did not contain LFEs. Liposomes containing LFEs were aggregated by Ricinus communis agglutinin from caster bean (RCA120) while the control liposomes were not, with the agglutinability of the liposomes containing LAE being greater than that of those containing LSE. These findings therefore suggested that the LFEs were incorporated into the liposomal membrane with the galactose residues exposed to the outer aqueous phase. Next, the tissue distribution of liposomes containing LAE at 30 min after intravenous administration was compared to that of the control liposomes in rats. Hepatic uptake was 2-fold higher than that of the control liposomes. These results thus demonstrate that liposomes containing LFEs represent potential drug carriers to the liver.

A series of aliphatic 2'-esters of acyclovir ([9-(2-hydroxyethoxymethyl)guanine]) were synthesized by direct acylation in a pyridine-N,N-dimethylformamide solution. The prodrugs were characterized as to their aqueous solubility in phosphate buffer (pH 7.4), partition coefficients in 1-octanol/phosphate buffer (pH 7.4), bioreversion kinetics by the soluble ocular esterases, and in vitro effectiveness against Herpes group viruses. The compounds exhibit an expected decrease in aqueous solubility upon esterification with a corresponding increase in the 1-octanol/water partition coefficient. The butyrate ester shows good aqueous stability in the neutral pH ranges. The apparent first order rate constants of bioreversion varied with the steric nature and polarity of the acyl substituent. The butyrate and pivalate esters were evaluated for their anti-herpesvirus activity and cellular toxicity. The butyrate ester possesses similar anti-herpesvirus activity to acyclovir and has a very high selectivity index. The pivalate ester shows poor anti-herpes simplex virus activity; however, it is unique in its effectiveness against the Epstein-Barr virus.

In vitro permeability studies were conducted on isolated rabbit corneal membranes using aliphatic acyl esters of acyclovir to determine the effect of lipophilicity on the transcorneal diffusion. Corneal membrane permeability coefficients increased with increasing lipophilicity of the straight chain aliphatic esters. The branch chain ester, acyclovir isobutyrate, displayed an anomalously low corneal permeability when compared to acyclovir esters having similar molecular size and 1-octanol/water partition coefficient. In vivo corneal penetration studies were conducted on unanesthetized rabbits. The aqueous humor concentrations of acyclovir and the ester prodrugs were measured at twenty five minutes after the topical instillation of an aqueous solution of the appropriate compound. The concentration of acyclovir in the aqueous humor increased with increasing 1-octanol/water partition coefficient. The lipophilic modification was shown to have a greater effect on increasing productive corneal absorption than the precorneal loss pathways. The effectiveness of acyclovir butyrate as a treatment for primary herpetic keratitis was evaluated in the McKrae strain infected rabbit model. The compound did not lose activity due to the esterification of the 2' hydroxyl group.

Six ester prodrugs of 2',3'-didehydro-3'-deoxythymidine (D4T) were synthesized, and their physicochemical properties were evaluated. Marked differences were observed. All of the prodrugs were chemically stable within the pH range 2-7. Hydrolysis of these esters was observed in all cases for four rat enzyme systems (plasma, liver, duodenum, and kidney), with D4T being regenerated. D4T or the prodrug was administered orally to rats, and the plasma concentrations of D4T and a corresponding prodrug were measured. The half-life of D4T after intravenous administration was 35.9 min. The half-life calculated from the terminal phase and the maximum concentration in plasma following oral administration of D4T were 35.9 min and 48.4 microM, respectively. After oral prodrug administration (with water or olive oil as a solvent), though none of the prodrugs was detected in plasma except for 5'-hemisuccinyl D4T and 5'-hemiglutaryl D4T with olive oil as a solvent, retention time of plasma D4T concentration was extended and the elevated D4T concentration in plasma decreased.

Enteral absorption by healthy male volunteers of cis-5,8,11,14,17-eicosapentaenoic acid (20:5(n-3), EPA) and cis-4,7,10,13,16,19-docosahexaenoic acid (22:6(n-3), DHA) from an ethyl ester enriched in EPA and DHA (K85) and from natural fish oil (TG30) was found to be similar after intake of equivalent doses. Thus, after administration twice daily for 14 days, the amounts of EPA and DHA in total serum lipids and in serum phospholipids were essentially identical for the two ester forms of the n-3 fatty acids. A dose-dependent relationship between intake and total phospholipid serum levels was observed for EPA, which increased 6.5-12-fold and 4.8-9-fold, respectively, but DHA reached a plateau at 2-2.5-fold over the control after supplementation. Arachidonic acid (AA) did not change significantly in total serum lipids, but fell 10-20% in serum phospholipids. The supplementation resulted in a 6.3-11-fold increase in the EPA/AA ratio. A significant and dose-dependent 25-45% reduction in linoleic acid (18:2(n-6), LA) was found in serum phospholipids, while a smaller decrease was observed in total serum lipids. 20:3(n-6), 20:3(n-9) and 22:5(n-6) also decreased significantly in serum phospholipids. Saturated fatty acids remained essentially unchanged. Some 35-47% decrease in serum triacylglycerols and approx. 10% decrease in cholesterol levels were also observed. In spite of the similar serum levels of EPA and DHA obtained in vivo, in vitro hydrolysis by porcine pancreatic lipase of K85 was 3-fold slower than hydrolysis of a glyceryl ester (TG85) similarly enriched in EPA and DHA, and 15-fold slower than the hydrolysis of the 3-fold less enriched TG30 with EPA and DHA predominantly in the 2-position. Under similar conditions release of AA from glyceryl ester and ethyl ester was essentially similar and approx. 1.5-fold faster than release of EPA and DHA from ethyl esters. In vitro hydrolysis of olive oil was 1.8-fold faster than hydrolysis of a natural fish oil (TG30) under similar conditions. Thus, the order of the rates of hydrolyses was olive oil > TG30 > TG85 > triarachidonin > LA ethyl ester > AA ethyl ester > ethyl ester K85. Our results show that in spite of differences in the rate of hydrolysis by lipase in vitro, the enteral absorption of EPA and DHA is as least as good from a synthetic ethyl ester highly enriched in EPA and DHA as it is from a natural triacylglycerol containing equivalent amounts of these fatty acids.

The lipophilic derivatives of the anticancer alkylating agent chlorambucil, chlorambucil-methyl, -isopropyl and -tertiary butyl esters, were synthesized and administered i.v. to anesthetized rats. Plasma and brain concentrations of these agents and of their active metabolites, chlorambucil and phenylacetic mustard, then were determined by high-performance liquid chromatography between 5 and 60 min. Whereas large amounts of chlorambucil-tertiary butyl ester entered and were maintained in brain, lower amounts of chlorambucil-isopropyl ester and no chlorambucil-methyl ester were found in brain. The comparative brain/plasma concentration-time integral ratios of the total active agents generated from chlorambucil-tertiary butyl, -isopropyl and -methyl esters were 0.85, 0.12 and 0.06, respectively, compared to a ratio of 0.02 for chlorambucil. In vitro alkylating activity of each ester was compared to that of equimolar chlorambucil, by reaction with 4-(p-nitrobenzyl)pyridine. Each ester possessed high intrinsic alkylating activity, equal to 38.4, 57.0 and 69.9% of chlorambucil activity, for the -tertiary butyl, -isopropyl and -methyl esters, respectively. Therefore each is an active antineoplastic agent irrespective of whether or not chlorambucil is regenerated. The rates of ester hydrolysis of these derivatives to chlorambucil were measured in fresh rat blood and in liver and brain homogenates at 37 degrees C. Chlorambucil-methyl and -isopropyl esters were hydrolysed quickly within 30 s in blood and liver, whereas chlorambucil-tertiary butyl ester was more stable with half-lives of approximately 7 h and 2 h, respectively. All proved to be relatively stable in brain homogenate. Steric hindrance around the ester linkage of chlorambucil-tertiary butyl ester reduces its affinity to and rate of hydrolysis by plasma and liver esterases, and allows it to accumulate within the brain. Chlorambucil-tertiary butyl ester maintains high levels in brain despite rapidly declining plasma concentrations and, due to these favorable pharmacokinetics and to its intrinsic anticancer activity, it possess promising characteristics for the treatment of malignant brain tumors.

Lymphatic transport of docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids given in the forms of triglyceride, ethyl ester of free acid and their effect on cholesterol transport was compared in lymph-cannulated rats. Lymphatic recovery of DHA and EPA given by stomach tube in the form of triglyceride in which they were mainly located at the 2-position was significantly higher than that of the ethyl ester or free acid during the first 6 hr after the administration and the tendency continued until 9 hr. In contrast, the 9 to 24 hr recovery of DHA and EPA in the forms of ethyl ester and free acid was considerably higher than that of triglyceride. Consequently, cumulative 24 hr recovery of EPA was comparable among the three forms. However, the 24 hr recovery of DHA was highest in free acid, lowest in ethyl ester and intermediate in triglyceride. Recovery of the free acid between 9 and 24 hr after administration was significantly higher than that given in the forms of triglyceride or ethyl ester. Cholesterol recovery in lymph of rats given with ethyl ester or free acid was lower than that given with triglyceride at an early stage after the administration in both EPA and DHA. Cumulative 24 hr recovery of cholesterol in rats given these fatty acids as ethyl ester was significantly lower than in those given as the other two forms.

The pharmacokinetics of five monoester prodrugs of valproic acid (VPA) were investigated: propyl valproate (P-VPA), butyl valproate (B-VPA), isobutyl valproate (IB-VPA), isoamyl valproate (IA-VPA), and hexyl valproate (H-VPA). In addition, the anticonvulsant activity of these compounds was evaluated and compared with that of VPA and valpromide (VPD). The pharmacokinetics of VPA and its five ester derivatives were determined after intravenous administration of equivalent doses (400 mg of VPA) to six dogs. The five ester prodrugs of VPA were biotransformed to VPA; the biotransformation was complete for P-VPA, B-VPA, and H-VPA but was only partial for IB-VPA and IA-VPA. Because of the rapid conversion of the prodrugs to the parent drug, levels of VPA in plasma after administration of the prodrugs peaked at 6-26 min after dosing and did not yield an in vivo sustained-release dosage profile. Of the five ester prodrugs of VPA, only P-VPA demonstrated anticonvulsant activity. P-VPA also was less neurotoxic than VPA and VPD; therefore, it has a better protective index.

The concentration of AZT in mice plasma and brain was measured using HPLC after an ingestion of 20 mg/kg of AZT or the molar equivalent of hexadecyl 2-(alpha-D-mannopyranosidyl)ethyl 3'-azido-3'-deoxy-5'-thymidinyl phosphate 3. The results demonstrated the promising qualities of the prodrug 3 which gave AZT-5'-phosphate as the main metabolite: the total concentration of AZT derivatives detected in brain presented a peak of 156 nmol/g (5 nmol/g for AZT) at 1 h; the half-life was about 24 h (1 h for AZT) with an AUC of 4366 nmol h/g as compared to 4 nmol h/g for AZT. The lipophilic properties of 3 were confirmed by its in vitro transport of inside synaptosomes. The derivative 2-(alpha-D-mannopyranosidyl)ethyl 3'-azido-3'-deoxy-5'-thymidinyl phosphate (2) provided also a good delivery of AZT to the central nervous system, with values intermediate between those of AZT and 3.

2-(Alkyloxycarbonyl)-2-alkylideneethyl esters of various aminothiazole-oxyimino cephalosporins have been synthesized and studied. They are useful alternatives to the currently existing orally active esters. Among the new esters synthesized, the 3'-azidomethyl cephem ester Ro 41-3399 (7k) presented an oral bioavailability superior to the corresponding pivaloyloxymethyl ester (9) in a rat model and was selected as a candidate for further evaluation.

Five ester prodrugs of 2'3'-dideoxyinosine (DDI) were synthesized for the purpose of improving oral bioavailability. The prodrugs, acetate (C2-DDI), octanoate (C8-DDI), stearate (C18-DDI), benzoate (Bz-DDI), and hemisuccinate (Suc-DDI) were proved to quantitatively regenerate their parent drug by enzymatic hydrolysis. Though the chemical stability of the prodrugs under acidic conditions was not improved, their solubility in water was significantly decreased by esterification, except for Suc-DDI. Bioavailability was evaluated by oral administration to rats. Two hydrophobic prodrugs (C8-DDI and Bz-DDI) showed higher absolute bioavailability (23.5% and 31.0%, respectively) than did DDI (15.2%), though that of C2-DDI (11.5%) and Suc-DDI (4.5%) was poor.

O,O'-(1,4-Xylylene) bispilocarpic acid esters are pilocarpine prodrugs containing two pilocarpic acid monoesters linked with one pro-moiety. Each mole of prodrug forms two pilocarpine moles in the presence of esterases. Corneal uptake and permeability of various bispilocarpic acid diesters were investigated in vitro using isolated albino rabbit corneas. The permeability coefficient of pilocarpine was 2.8 x 10(-6) cm/sec, whereas for bispilocarpic acid diesters, despite their large molecular weights (between 638 and 722), permeability coefficients were 6.5-20.2 x 10(-6) cm/sec. Only pilocarpine, and no intact prodrug, was observed at the endothelial side. Corneal uptake was increased with increasing lipophilicity, but a parabolic relationship between the logarithm of the apparent partition coefficient (1-octanol-pH 7.4 phosphate buffer) (log PC) and the corneal permeability was noticed. Corneal permeability and the rate of enzymatic hydrolysis of the compounds correlated well. The corneal permeability of pilocarpine given as lipophilic bispilocarpic acid diester (log PC greater than or equal to 3) prodrugs seems to be controlled by the formation of pilocarpine in the corneal epithelium rather than by the absorption of prodrugs into the epithelium or their epithelium-stroma transport rate.

Initial velocities of ketoprofen formation from ketoprofen-dextran ester prodrugs incubated in homogenates of various segments of the pig GI-tract were determined. Enzyme-mediated drug release was found in caecum and colon homogenates with their contents, whereas release rates in the stomach, duodenum, jejunum and ileum homogenates were comparable to those determined in pure buffer solutions of identical pH. In colon homogenates adjusted to various pH values between 6.0 and 7.9, little variation in release rates was observed. However, the contribution of enzyme-catalyzed drug regeneration to the overall initial velocity of ketoprofen formation increased significantly as a function of decreasing pH. The presence of several antibiotics and betamethasone in colon homogenates did not affect the drug activation process, whereas the addition of various enzyme inhibitors slowed down the ketoprofen release rates. During incubation in colon homogenates the average molecular weight of the dextran esters decreased. The drug release may therefore involve an initial fragmentation of the drug-liganded dextran chains carried out by dextranases secreted from the microflora which reside in the pig's large bowel.

We have recently shown the kinetic behavior of liver retinyl esters in rats with adequate vitamin A levels receiving oral vitamin supplementation. In the present work we have studied the effects of intramuscular administration of a vitamin A preparation on the metabolism of vitamin A in the rat. Retinol administered intramuscularly to rats in the form of an emulsion brought about a significant increase in the serum and liver concentration of vitamin A; this increase was slightly less than in orally treated rats. In each group, retinyl palmitate constituted 80-85% of the total retinyl esters, followed by stearate (9-13%), laurate, palmitoleate, myristate, linoleate and pentadecanoate making up 3-10%. The subcellular localization of all retinyl esters is similar and dependent on age but not on the route of administration. These results indicate that although the best hepatic storage is achieved with an orally administered vitamin A emulsion, the intramuscular administration of a physiological dose might provide an effective supplementation method if oral vitamin A is contraindicated.

In an attempt to improve the skin delivery characteristics of Zidovudine (AZT, azidothymidine), five aliphatic esters (acetate, butyrate, hexanoate, octanoate, and decanoate) of AZT were synthesized and assessed as prodrugs of AZT. While the water solubility of the esters is lower than that of AZT, the solubilities in isopropylmyristate (IPM) and the partition coefficients (n-octanol:buffer) are higher. Susceptibility to enzymatic hydrolysis in the rat skin homogenate increases as the acyl chain of the ester is lengthened. Among the esters, acetate (C2-AZT) and hexanoate (C6-AZT) showed 2.4- and 4.8-fold enhanced permeation in human skin from an apolar vehicle (IPM) relative to application of AZT itself, respectively.

Comparative Bioavailability of Eicosapentaenoic Acid and Docosahexaenoic Acid from Triglycerides, Free Fatty Acids and Ethyl Esters in Volunteers. The bioavailability of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from triglycerides, free fatty acids and ethyl esters was investigated in 8 female volunteers in a randomized triple cross-over trial with baseline control. EPA/DHA was administered in capsules in form of triglycerides (1.68/0.72 g), free fatty acids (1.35/1.065 g) and ethyl esters (1.86/1.27 g). The resulting EPA/DHA plasma levels were determined and evaluated. The mean relative bioavailability of EPA/DHA compared to triglycerides was 186/136% from free fatty acids and 40/48% from ethyl esters. Maximal plasma levels were about 50% higher with free fatty acids and about 50% lower with ethyl esters as compared to triglycerides. The tolerability of the free fatty acids was much worse than that of triglycerides and ethyl esters. The main side effect was eructation.

An increasing number of chemicals have been identified as being toxic to the nasal mucosa of rats. While many chemicals exert their effects only after inhalation exposure, others are toxic following systemic administration, suggesting that factors other than direct deposition on the nasal mucosa may be important in mechanisms of nasal toxicity. The mucosal lining of the nasal cavity consists of a heterogeneous population of ciliated and nonciliated cells, secretory cells, sensory cells, and glandular and other cell types. For chemicals that are metabolized in the nasal mucosa, the balance between metabolic activation and detoxication within a cell type may be a key factor in determining whether that cell type will be a target for toxicity. Recent research in the area of xenobiotic metabolism in nasal mucosa has demonstrated the presence of many enzymes previously described in other tissues. In particular, carboxylesterase, aldehyde dehydrogenase, cytochromes P-450, epoxide hydrolase, and glutathione S-transferases have been localized by histochemical techniques. The distribution of these enzymes appears to be cell-type-specific and the presence of the enzyme may predispose particular cell types to enhanced susceptibility or resistance to chemical-induced injury. This paper reviews the distribution of these enzymes within the nasal mucosa in the context of their contribution to xenobiotic metabolism. The localization of the enzymes by histochemical techniques has provided important information on the potential mechanism of action of esters, aldehydes, and cytochrome P-450 substrates known to injure the nasal mucosa.

In an effort to find a replacement for the iv antiarrhythmic drug lidocaine having reduced systemic and central nervous system effects, activity against supraventricular as well as ventricular arrhythmias, and a biological half-life of less than 15 min, derivatives of the orally active class Ic clinical agent 2,6-bis(1-pyrrolidinylmethyl)-4-benzamidophenol, 1 (ACC-9358), were synthesized and tested. Compounds with ester groups attached to the phenyl ring were either weakly active or toxic. Replacement of the formanilide function with alkyl esters afforded compounds with antiarrhythmic activity in the range of 1. When the ester carboxyl was separated from the bis(aminomethyl)phenol by methylene units, very short half-lives were observed in human blood. In general, these compounds also had low lipophilic character.

Radioiodinated benzoyl esters and amides of epimeric 20-hydroxy- and 20-aminopregn-5-en-3 beta-ols were synthesized in an effort to find an agent that would be rapidly and selectively taken up by adrenal cortical tissue. Achievement of such a goal would provide a basis for the development of adrenal imaging agents superior to those currently available for clinical use. The iodobenzoyl derivatives were obtained by treating the appropriate epimer with 2-iodobenzoic acid in the presence of dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine. The resulting esters and amides were readily labeled with radioiodine by isotope exchange with sodium iodide-125 in pivalic acid. Tissue distribution studies in female rats revealed that only the esters displayed appreciable adrenal specificity, and the ester having the same configuration at C-20 as cholesterol was significantly better than the corresponding C-20 epimer.

Naloxone was shown to induce a slight increase of the blood supply to the brain, an elevation of blood pressure and an enhancement of the bioelectrical activity in the sympathetic nerves. Unlike opioid peptides, naloxone was found to exert a marked alleviating effect on the nervous control of the cerebral circulation. The drug increases neurogenic spasms of the cerebral vessels that is followed by an enhancement of vasomotor and somatosympathetic reflexes. The data obtained indicate the role of the opioid system in the central regulation of the cerebral circulation.