Enzymic hydrolysis of nicotinate esters: comparison between plasma and liver catalysis

Activation of benzoate model prodrugs by mycobacteria. Comparison with mammalian plasma and liver hydrolysis

Due to difficulties in drug penetration in M. tuberculosis, a prodrug approach based on mycobacterial activation appears as a promising strategy to increase the delivery of antitubercular drugs to the target microorganisms. Esters have been successful used by us and others to deliver drugs to mycobacteria, however because very little is known about the metabolic hydrolysis of esters by mycobacteria in connection with prodrug activation, we decided to study the process further. For that we selected a series of 13 benzoates with different chain lengths and ramifications in the alkoxy side as model prodrugs and examined their hydrolysis by a mycobacterial homogenate, comparing the results with those obtained parallelly in human plasma and in total rat liver homogenate. In all biological media, the benzoates with a linear alkyl group showed a parabolic dependence between log(k) and logP (or the number of carbons of the linear alkyl chain) that reached a maximal value for the n-butyl chain. Considering linear correlations for the total number of compounds between log(k) and chosen descriptors, for mycobacterial esterases, pKa of the leaving alcohol (pKa_LG) seem to be the most important descriptor. Plasma esterases seem to be quite sensitive to the Taft polarity parameter σ* and also to pKa_LG and less sensitive to steric effects. Liver esterases seem to be more sensitive to the Taft steric descriptor E_S^c. Lipophilicity correlates weakly with log(k) in all the 3 media, however, is more important when one looks for mycobacterial activation selectivity in relation to plasma metabolism or in relation to liver homogenate metabolism. The importance of lipophilicity increases further when biparametric expressions are considered. We showed that it is easy to activate a wide variety of benzoate esters using a mycobacterial homogenate. The data also suggest that with careful design is possible to obtain tuberculostatic prodrug esters sensitive to mycobacterial hydrolases while reasonably resistant to plasma and liver hydrolysis. One important observation is that mycobacterial hydrolysis is less affected by bulky substituents than liver homogenate or plasma hydrolysis. tert-Butyl is probably the substituent in the alkoxy side that seems more adequate to resist simultaneously plasma and liver metabolism, while allowing activation by mycobacterial esterases. Hexyl is also a good option for the medicinal chemist if a linear alkoxy chain is needed.

Synthesis of novel flexible tamoxifen analogues to overcome CYP2D6 polymorphism and their biological evaluation on MCF-7 cell line

Tamoxifen (TAM) is currently the endocrine treatment of choice for all stages of breast cancer; it has proven success in ER positive and ER negative patients. TAM is activated by endogenous CYP450 enzymes to the more biologically active metabolites 4-hydroxytamoxifen and endoxifen mainly via CYP2D6 and CYP3A4/5. CYP2D6 has been investigated for polymorphism; there is a large interindividual variation in the enzyme activity, this drastically effects clinical outcomes of tamoxifen treatment. Here in we report the design and synthesis of 10 novel compounds bearing a modified tamoxifen skeleton, ring C is substituted with different ester groups to bypass the CYP2D6 enzyme metabolism and employ esterase enzymes for activation. All compounds endorse flexibility on ring A. Compounds (II-X) showed MCF-7% growth inhibition >50% at a screening dose of 10 μM. These results were validated by yeast estrogen screen (YES) and E-Screen assay combined with XTT assay. Compound II (E/Z 4-[1-4-(3-Dimethylamino-propoxy)-phenyl)-3-(4-methoxy-phenyl)-2-methyl-propenyl]-phenol) showed nanomolar antiestrogenic activity (IC₅₀ = 510 nM in YES assay) and was five times more potent in inhibiting the growth of MCF-7 BUS (IC₅₀ = 96 nM) compared to TAM (IC₅₀ = 503 nM). Esterified analogues VI, VII were three times more active than TAM on MCF-7 BUS (IC₅₀ = 167 nM). Novel analogues are prodrugs that can ensure equal clinical outcomes to all breast cancer patients.

Reversible inhibitory effects of saturated and unsaturated alkyl esters on the carboxylesterases activity in rat intestine

This study was conducted to investigate the relationship between the carbon chain length/double bonds of alkyl esters and their inhibitory potency/mechanism on carboxylesterases (CESs). CESs activity was evaluated by inhibition of adefovir dipivoxil (ADV) metabolism in rat intestinal homogenates. Furthermore, the inhibitory effect of BNPP and ethyl (E)-hex-2-enoate (C8:1) on drug absorption was evaluated in situ intestinal perfusion model. The results showed that the rank order of the inhibitory potency on CESs was C10:0 > C8:0 > C6:0 > C4:0 > C12:0, C8:1 > C8:0, C6:1 > C6:0, while the esters (C14:0, C13:1, C16:0, C18:0, C17:1, C20:0) were found to have no inhibitory effect at investigated concentrations. However, the unsaturated esters (C20:1, C20:2, C20:3) displayed the inhibitory effect on CESs. Moreover, the double reciprocal plots indicated that alky esters inhibited the CESs in competitive and mixed competitive ways which were reversible. In addition, the result of most effective CESs inhibitor C8:1 from in situ experiment showed that C8:1 can inhibit the CESs-mediated intestinal metabolism and improve the drug absorption. And the inhibition had no time-dependent effect, compared with that of BNPP groups. The study suggested that alkyl esters can be served as effective and reversible CESs inhibitors, besides that their inhibitory potency/mechanism can be affected by their carbon chain length/double bonds.

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.

Intestinal absorption enhancement of the ester prodrug tenofovir disoproxil fumarate through modulation of the biochemical barrier by defined ester mixtures

The effect of discrete esters and ester mixtures on the intestinal stability and absorption of tenofovir disoproxil fumarate (tenofovir DF, an esterase-sensitive prodrug of the antiviral tenofovir) was compared with the effect of strawberry extract, which has been shown to enhance the absorption of the prodrug across Caco-2 monolayers and in rat ileum. In addition, the mechanism of absorption enhancement was investigated. In rat intestinal homogenates, complete inhibition of the conversion of tenofovir DF (as obtained by strawberry extract) could only be obtained at relatively high concentrations of the discrete esters or by using mixtures of esters (e.g., propyl p-hydroxybenzoate 0.02%, octyl acetate 0.02%, ethyl caprylate 0.01%). Coincubation of tenofovir DF with this mixture also resulted in an enhancement of its absorption in the in vitro Caco-2 system as well as in rat ileum. As tenofovir DF is a substrate for P-glycoprotein (P-gp)-related efflux carriers in the Caco-2 model, the modulatory effect of the ester mixtures was studied on the functionality of P-gp using cyclosporin A (CsA) as a model substrate. Strawberry extract as well as the mixture of three esters interfered with the absorptive transport of CsA across Caco-2 monolayers, illustrating that both mixtures interfere with both esterase-activity and P-gp functionality. This concerted barrier was not observed in rat ileum, suggesting differential functional activities of the biochemical barrier toward tenofovir DF in different absorption systems. Overall, our results illustrate that modulation of the biochemical barrier (metabolism and efflux) of tenofovir DF by ester mixtures can be used to increase the intestinal absorption of tenofovir DF in an in vitro and an in situ absorption model; the mechanism of action appears to be a complex interplay of different systems; the differential expression of carriers and enzymes in different systems illustrates the difficulty of extrapolating observations between different systems/species.

Quantitative structure-metabolism relationships: steric and nonsteric effects in the enzymatic hydrolysis of noncongener carboxylic esters

An attempt to quantitatively describe human blood in vitro hydrolysis data for more than 80 compounds belonging to seven different noncongener series of ester-containing drugs is presented. A parameter not yet explored in pharmaceutical studies, the inaccessible solid angle Omega(h), calculated around different atoms was used as a measure of steric hindrance, and the steric hindrance around the carbonyl sp(2) oxygen (Omega(h)(O=)) proved the most relevant parameter. The obtained final equation, log t(1/2) = -3.805 + 0.172Omega(h)(O=) - 10.146q(C=) + 0.112QLogP, also includes the AM1-calculated charge on the carbonyl carbon (q(C=)) and a calculated log octanol-water partition coefficient (QLogP) as parameters and accounts for 80% of the variability in the log half-lives of 67 compounds. A number of structures are still mispredicted, but the equation agrees very well with a recently proposed mechanism for hydrolysis by carboxylesterases. The model, with a predictive power tested here on three unrelated structures, should be useful in estimating approximate rates of hydrolysis for prodrug or soft drug candidates ahead of their synthesis.

A continuous fluorimetric method to monitor the enzymatic hydrolysis of medicinal esters

This paper describes a new spectrofluorimetric assay for continuously monitoring the enzymatic hydrolysis of medicinal esters. The procedure is based on the stoichiometric quantity of protons generated by the hydrolysis of the substrate, which produces changes in the fluorescence of a pH-sensitive dye. The pH indicator, 2', 7'-bis(car-boxyethyl)-5(6)-carboxyfluorescein, was selected due to its favourable pKa for studies under physiological conditions. Moreover, the presence of a domain in the spectra (< 442 nm) where fluorescence intensities are independent of pH allows measurements of wavelength ratios that cancel artifacts and lower sample-to-sample variability. The indicator did not affect the catalytic activity of purified hog liver carboxylesterase or human serum albumin. This assay is easy to perform and appears to be especially useful for studying enzymatic reactions with half-lives of the order of minutes or hours.

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.

Nicotinate esters: their binding to and hydrolysis by human serum albumin

Nicotinate esters were studied for their binding to, and hydrolysis by, human serum albumin. Some esters (ethyl, isopropyl, t-butyl, cyclohexyl, benzyl) were bound but not hydrolysed, while others (2-chloroethyl, 2-butoxyethyl) displayed the opposite behaviour; 1-carbamoylethyl ester was neither bound nor readily hydrolysed. Only p-methoxyphenyl nicotinate was both a ligand and a substrate, and its rate constants of binding and hydrolysis were calculated in a stepwise procedure using a kinetic model.