Specific Base Catalysis Research Articles

Macromolecular prodrugs IV. Kinetics of hydrolysis of metronidazole monosuccinate dextran ester conjugates in aqueous solution and in plasma — sequential release of metronidazole from the conjugates at physiological pH

The kinetics of hydrolysis of metronidazole monosuccinate dextran ester conjugates in aqueous solution over the pH range 6.61–7.80 (37°C) has been investigated. As demonstrated by HPLC the degradation of the dextran conjugates proceeds through parallel formation of metronidazole and metronidazole monosuccinate, respectively. The regeneration rates of the latter compounds followed first-order kinetics. The pH dependence of the various rate constants showed almost parallel straight line portions with slopes varying from 0.93 to 0.95, indicating that the hydrolysis reactions were subject to specific base catalysis. A 5-fold increase in the hydrolysis rate of the metronidazole-spacer arm ester bond compared to the stability of metronidazole monosuccinate, per se, was observed. Similarly, the carbohydrate succinic acid ester linkage showed greater susceptibility to undergo degradation at neutral pH in proportion to corresponding aliphatic ester derivatives, suggesting participation of intramolecular catalysis in the hydrolysis of the conjugate ester bonds by the neighbouring carbohydrate hydroxy groups. Almost identical stability of the conjugates was observed after incubation in 0.05 M phosphate buffer pH 7.40 and in 80% human plasma ( t 1 2 ~ 32 h at 37° C) revealing that the hydrolysis in plasma proceeds without enzymatic catalysis.

Non-steroidal anti-psoriatic prodrugs. Hydrolysis and aminolysis of naphthyl esters in aqueous solution

The anti-psoriatic compound, 6-chloro-2,3-dimethoxy-1,4-naphthalene diol diacetate (lonapalene) ( 1), undergoes ester hydrolysis and oxidation to give 2,3-dimethoxy-6-chloro-1,4-naphthoquinone (3). The hydrolysis reaction of 1 obeys the equation k obs = k H +[H +] + k 0 + k HO −[HO −] where k H + , k 0 and k HO − at 25° C are 1.42 × 10 −5 M −1 · s −1, 3.68 × 10 −8 s −1 and 1.78 M −1 · s −1, respectively. Both general acid and general base catalysis of 1 were observed, as well as an aminolysis reaction with primary amines. It is concluded that naphthyl esters undergo specific base catalysis by rate-determining hydroxide ion attack on the ester group to form a tetrahedral intermediate, whereas nucleophilic catalysis by amines proceeds by rate-determining proton transfer in the tetrahedral intermediate to base. It was also demonstrated that the hydrolysis rates of 1-naphthyl esters do not change greatly by altering the substituent in the 6- and 7-positions, and that the maximum shelf-life of 1 in aqueous formulations is approximately 1 month at room temperature and pH 4.

Liquid chromatographic determination of the pH-dependent degradation of eseroline — hydrolysis product of physostigmine

The stability of eseroline, the hydrolysis product of physostigmine under aerobic conditions, was studied by liquid chromatography. A reversed-phase, ion-pair technique was used to separate eseroline from its degradation products. The degradation of eseroline in phosphate buffer solutions of pH 6.91, 7.40, 7.98, 8.41 and 8.94 appears to follow first-order kinetics; the rate constant increased with an increase in pH. The degradation appears to follow specific base catalysis.

Pyridoxal and pyridoxal 5′-phosphate catalyzed β-deuteration of α-aminobutyric acid and homoserine. I. Metal-free systems

As the initial step in the investigation of metal ion- and vitamin B6-catalyzed, β, γ-elimination in α-amino acids, the pyridoxal-and pyridoxal 5′-phosphate-catalyzed β-deuteration of α-aminobutyric acid and homoserine in D2O in metal-free systems has been studied by nuclear magnetic resonance. The reaction kinetics of α-H, β-H, and 4′-CH exchange rates were measured by following the rates of deuteration of these positions. The kinetic data obtained for these reactions lead to a proposed mechanism for β-deuteration. The relative rates of the deuteration reactions observed, and the order in which deuteration products were obtained are: α-H exchange > transamination (4′-CH exchange) > β-H exchange. The observed rates of β-proton exchange exhibit specific base catalysis and are dependent on the activation of the α-hydrogen of the amino acid moiety in the aldimine, and on the activation of the β-hydrogens of the amino acids in the ketimine.

Poly(dA-dT).poly(dA-dT) two-pathway proton exchange mechanism. Effect of general and specific base catalysis on deuteration rates.

The deuteration rates of the poly(dA-dT).poly(dA-dT) amino and imino protons have been measured with stopped-flow spectrophotometry as a function of general and specific base catalyst concentration. Two proton exchange classes are found with time constants differing by a factor of 10 (4 and 0.4 s-1). The slower class represents the exchange of the adenine amino protons whereas the proton of the faster class has been assigned to the thymine imino proton. The exchange rates of these two classes of protons are independent of general and specific base catalyst concentration. This very characteristic behavior demonstrates that in our experimental conditions the exchange rates of the imino and amino protons in poly(dA-dT).poly(dA-dT) are limited by two different conformational fluctuations. We present a three-state exchange mechanism accounting for our experimental results.

Study of formation of the spiro-Meisenheimer adduct of NN′-dimethyl-N-(2,4,6-trinitrophenyl)glycinamide and its rearrangement to 2-methylamino-N-methyl-N-(2,4,6-trinitrophenyl)acetamide

NN′-Dimethyl-N-(2,4,6-trinitrophenyl)glycinamide (5) is cyclized in methanol under specific base catalysis and gives the spiro-adduct (6). The spiro-adduct is opened by the action of methanolic hydrogen chloride to give the Z- and E-isomers of 2-methylamino-N-methyl-N-(2,4,6-trinitrophenyl)-acetamide hydrochloride (7). In aniline-anilinium chloride buffers the E-isomer of (7) is cyclized to the spiro-adduct (6) with a half-life <1 s, the rate-limiting step of the cyclization of Z-(7) isomer being its isomerization to E-(7). In methanolic acetate buffers the rate-limiting step is gradually shifted to the isomerization of the neutral (Z)-2-methylamino-N-methyl-N-(2,4,6-trinitrophenyl)acetamide Z-(8) to the E-(8) isomer.

Base catalyzed cyclization of substituted esters of hydantoic and thiohydantoic acids

Base catalyzed cyclization rates have been measured of 22 derivatives of hydantoic and thiohydantoic acid esters in water and methanol. The cyclization of methyl and ethyl esters of hydantoic and 5-methylhydantoic acids is accompanied by hydrolysis of the ester group, whereas with the other derivatives the hydrolysis does not take place. Hydrolysis of the cyclization products (hydantoin and thiohydantoin derivatives) is not significant under the kinetic conditions. The cyclization of methyl ester of 5-phenylhydantoic acid in methanol is reversible; the equilibrium mixture contains 30% of the starting ester. In all the cases the cyclization is subject to specific base catalysis; exceptions are esters of 5-phenylthiohydantoic and 5-phenyl-2-methylthiohydantoic acids whose cyclizations are subject to general base catalysis. Substituents always accelerate the cyclization. The 3-substituents have the greatest effects, the cyclization rate being considerably increased with bulk of the substituents; similarly large effect of 5-phenyl group consists mainly in its polar effects on the pre-equilibrium. The cyclization are slower in methanol at the same concentration of the lyate ion: the greatest difference (up to 3 orders of magnitude) is observed with the 5-phenyl derivatives.

Allopurinol prodrugs. II. Synthesis, hydrolysis kinetics and physicochemical properties of various N-acyloxymethyl allopurinol derivatives

Fourteen novel N-acyloxymethyl derivatives of allopurinol were synthesized and evaluated as potential prodrugs with the aim of enhancing the rectal delivery characteristics of the parent drug. The hydrolysis of the compounds (1- or 2- acyloxymethyl and 1,5- or 2,5-bis(acyloxymethyl) derivatives) was subject to specific base catalysis as well as enzymatic catalysis by plasma enzymes. In 80% human plasma solutions all the compounds were converted quantitatively to allopurinol, passing through an unstable N-(hydroxymethyl)allopurinol intermediate. The derivatives were more lipophilic than allopurinol as expressed by octanol-water partition coefficients and reversed-phase liquid Chromatographic capacity factors but the water-solubility was only slightly reduced or, for some derivatives, even greater than that of allopurinol. This behaviour was attributed to differences in the crystal lattice energy and a relationship between melting points, partition coefficients and water-solubilities for these and four previously studied N 1-acyl allopurinol prodrug derivatives was established. The results suggested, that N-acyloxymethylation may be a promising means of obtaining prodrug forms of allopurinol with improved physicochemical properties with regard to drug delivery.

Prodrugs of 5-fluorouracil. IV. Hydrolysis kinetics, bioactivation and physicochemical properties of various N-acyloxymethyl derivatives of 5-fluorouracil

The kinetics and mechanism of hydrolysis of various 1-, 3- and 1,3-acyloxymethyl derivatives of 5-fluorouracil were studied to assess their potential as prodrugs with the aim of enhancing the delivery characteristics of the parent drug. All the derivatives hydrolyzed to yield 5-fluorouracil in quantitative amounts, passing through an unstable N-hydroxymethyl-5-fluorouracil intermediate. The pH-rate profiles obtained revealed the occurrence of specific acid and base catalysis as well as of a water-catalyzed reaction. The rates of hydrolysis were accelerated markedly in the presence of human plasma or rat liver homogenate, suggesting the utility of the derivatives as prodrugs. The derivatives were all more lipophilic than 5-fluoro-uracil as determined by partition experiments in octanol-aqueous buffer systems but the aqueous solubility was only slightly reduced or, for some derivatives, even greater than that of 5-fluorouracil. This behaviour was attributed to differences in the crystal lattice energy, and relationships between melting points, partition coefficients and water-solubilities for these and 11 other prodrug derivatives of 5-fluoro-uracil were established.

Kinetics and mechanism of rearrangement and methanolysis of acylphenylthioureas

S-Acyl-1-phenylthioureas and their 3-methyl derivatives are rearranged to 1-acyl derivatives of thiourea in methanolic solution. The rearrangement of the 1-acyl-1-phenyl derivative to the thermodynamically more stable 3-acyl derivative is subject to specific base catalysis. The rearrangement of acetyl group is about 2 orders of magnitude slower than that of benzoyl group. 1-Acetyl-l-phenylthiourea undergoes base-catalyzed methanolysis (giving phenylthiourea and methyl acetate) instead of the rearrangement. The methanolysis rates of l-acyl-3-phenylthioureas and their N-methyl derivatives have been measured. The acetylthioureas react at most 3x faster than the benzoyl derivatives. The methyl group at the nitrogen adjacent to acyl group accelerates the solvolysis by almost 2 orders of magnitude; the methyl group at the other nitrogen atom retards the solvolysis by almost 1 order of magnitude. Replacement of hydrogen atom by methyl group at the phenyl-substituted nitrogen increases acidity of the phenylacetylthiourea by 2 orders of magnitude. The same replacement at the benzoyl-substituted nitrogen increases the acidity by 3 orders of magnitude, the increase in the case of the acetyl derivative being as large as 4 orders of magnitude.

Hydrolysis of N-(α-hydroxybenzyl)benzamide and other N-(α-hydroxyalkyl) amide derivatives: implications for the design of N-acyloxyalkyl-type prodrugs

N-Acyloxyalkylation has become a commonly used approach to obtain prodrug forms of various NH-acidic drug substances. The regeneration of the parent drug occurs via a two-step reaction, enzymatic cleavage of the ester grouping followed by spontaneous decomposition of the N-hydroxyalkyl intermediate. The usefulness of this approach depends on the rate of decomposition of the latter intermediate and for several compounds such as amides their N-hydroxymethyl derivatives are too stable to make the approach useful. In this work the kinetics of decomposition of various N-α-hydroxyalkyl compounds derived from benzamide, thiobenzamide and various aldehydes (benzaldehyde, acetaldehyde and chloral) in aqueous solution at 37° C was determined and compared with that for the corresponding N-hydroxymethyl compounds derived from formaldehyde. All compounds showed apparent specific base catalysis and for N-(α-hydroxybenzyl)benzamide, specific acid catalysis was also observed. The derivatives were much more unstable at physiological conditions of pH and temperature than the N-hydroxymethyl derivatives; thus, whereas the half-life of decomposition of N-(hydroxymethyl)benzamide is 183 h at pH 7.4 and 37 ° C the half-life for N-(α-hydroxybenzyl)benzamide is only 6.5 min under the same conditions. The latter compound appears to be the first reported N-alkylol amide derived from an aromatic aldehyde and its synthesis is described. It is concluded that N-acyloxyalkylation may be a useful means of obtaining prodrug forms of also weakly NH-acidic compounds if the (acyloxy)alkyl α-halide normally used for the N-acyloxyalkylation is prepared from other aldehydes than formaldehyde e.g. benzaldehyde.

Aqueous Conversion Kinetics and Mechanisms of Ancitabine, a Prodrug of the Antileukemic Agent Cytarabine

The kinetics of conversion of the prodrug ancitabine to the anticancer drug cytarabine have been studied in aqueous solutions in the pH range of 1.5–10.7, temperature range of 19.5–80.0°C, ionic strength range of 10−4 to 1.5, and in the presence of several general-base catalysts. Under all conditions ancitabine was quantitatively converted to cytarabine. The pH-rate profiles were linear with slope = 1 in alkaline pH, becoming pH independent in the region of maximum stability at pH ± 4, where buffer catalysis was found to be insignificant and kobs ± (1.12 × 1011 h−1)-exp {-10121 deg/T. At 30°C, pH ± 4, it is calculated that an aqueous ancitabine solution will maintain 90% of its initial concentration for 12 d. A novel method for measuring general-base catalysis in competition with predominating specific-base catalysis and in the presence of secondary salt effects at constant ionic strength was developed. Three mechanisms of hydrolytic prodrug conversion are proposed: nucleophilic hydroxide addition, general base-assisted nucleophilic water attack, and spontaneous water attack.

Theoretical Basis for the Detection of General-Base Catalysis in the Presence of Predominating Hydroxide Catalysis

The detection of general-base catalysis in the presence of predominating specific-base catalysis in aqueous buffer solutions is examined for various relationships between k0catand k0OH, the bimolecular rate constants for general-base and hydroxide-ion attack. The three experimental variables that affect the detection of buffer-base catalysis are the type of buffer, conjugate-acid concentration, and ionic strength. Various buffers used in pharmaceutical kinetic studies are considered, and it is concluded that buffers with high Ka values favor detection. Additionally, high conjugate-acid concentrations and ionic strengths appear to optimize the detection of general-base catalysis.

Stabilization of ampicillin analogs in aqueous solution. III. Kinetics of degradation of ampicillin with furfural in aqueous solution and physical properties of ampicillin-aldehyde adducts.

The degradation of ampicillin was found to be inhibited due to the formation of a 1 : 1 molar adduct between ampicillin and furfural in alkaline solution. Further, specific base catalysis for the ampicillin-furfural adduct was smaller, about one hundredth, than that for ampicillin-benzaldehyde adduct. Since the adduct formation was not observed at pH<6.00 and the formation constant of the adduct increased with increase of pH, the α-amino group of ampicillin was concluded to participate in the adduct formation. Thus, to estimate the structure of the adduct, various physical properties of samples prepared by freeze-drying of alkaline solution containing ampicillin and an aldehyde (furfural or benzaldehyde) were studied and compared. Nuclear magnetic resonance (NMR) spectra (1H and 13C), infrared (IR) spectra and the chemical properties of the adducts suggested that they have a Schiff base structure formed from the aldehyde moiety and the free amino group of ampicillin.

Acid and base catalysis of the mutarotation of D-arabinose oxime

Tha mutarotation of Z-D-arabinose oxime is subject to general acid and to specific base catalysis and is proposed to proceed by an addition–elimination pathway involving cyclic N-arabinosylhydroxylamine intermediates. It is suggested that general acid catalysis (Brönsted α 0.54) proceeds via a prior equilibrium proton transfer followed by a rate-determining reaction with conjugate base.

Studies on the stability of corticosteroids VI. Kinetics of the rearrangement of betamethasone-17-valerate to the 21-valerate ester in aqueous solution

The kinetics of the degradation of betamethasone-17-valerate in aqueous solutions of pH 0.5–8 have been investigated at 60°C using a reversed-phase HPLC procedure for determining remaining steroid and the products of its degradation, betamethasone-21-valerate and betamethasone. The overall degradation was shown to proceed entirely through a rearrangement of the 17-valerate ester to the 21-valerate ester followed by hydrolysis of the latter to yield betamethasone. The acyl group migration from C 17 to C 21 was subject to both specific acid and base catalysis as well as to catalysis by water. The pH-rate profile for the rearrangement showed a minimum at pH 3.5.

Kinetics and mechanism of the oxidation of 3,5-di-t-butyl-o-benzoquinone with hydrogen peroxide in aqueous methanol solution

The kinetics of the reaction between 3,5-di-t-butyl-o-benzoquinone (DTBQ)(1) and hydrogen peroxide have been investigated in 50% aqueous methanol in the pH range 8–10 and an ionic strength of 0.1M using spectrophotometric techniques at temperatures between 19.5 and 36.2 °C. The rate law –d [DTBQ]/dt=k[DTBQ][O2H–], with k(25 °C) 22.0 ± 0.9 dm3 mol–1 s–1 describes the kinetic data. The activation parameters at 25 °C are ΔH‡ 76 ± 4 kJ mol–1 and ΔS‡ 34 ± 1 J K–1 mol–1. The reaction is subject to specific base catalysis and the rate-determining step is presumably the nucleophilic attack of O2H– on the C(1) carbonyl group of DTBQ. The hydroperoxide intermediate so formed breaks down in two ways: (i) to 3,5-di-t-butylmuconic acid anhydride (4), which was detected by g.l.c.–m.s., and (ii) to 6-hydroxy-3,5-di-t-butyl-o-benzoquinone (8) which in the reaction with further hydrogen peroxide leads to 5-hydroxy-3,5-di-t-butylmuconic acid anhydride (9). All the products formed in the reaction and identified by g.l.c.–m.s. can be derived from the muconic acid anhydride derivatives (4) and (9).

Hydrolysis kinetics and mechanism of acyl-1,3-diphenyltriazenes

The hydrolysis rate constants of 3-(N-methylcarbamoyl)-1,3-diphenyltriazene (I), 3-(N-phenylcarbamoyl)-1,3-diphenyltriazene (II) and 3-acetyl-1,3-diphenyltriazene (III) have been determined in aqueous media within pH 0 to 14, Ho to 3 and H- to 15. The hydrolysis at pH &lt; 2 and pH &gt; 11 show specific acid and base catalysis, respectively, whereas within pH 2 to 11 non-catalyzed hydrolysis takes place. Activation parameters of the non-catalyzed and catalyzed hydrolysis have been determined. Quantum-chemical calculation of 3-carbamoyltriazene (IV) has been carried out by the MINDO/2 method. Hydrolysis mechanism of the studied compounds is suggested for the non-catalyzed, specific acid catalyzed and specific base catalyzed hydrolysis.

Degradation of Carmustine in Aqueous Media

The degradation rate of carmustine was investigated in buffered aqueous media at several pH values. The buffering agents studied were those with potential use in parenteral formulations of this drug: acetate, citrate, and phosphate. The apparent first-order degradation rate constants were calculated using a linear regression procedure. A pH range over which minimum degradation occurred was ascertained. General acid and specific base catalysis was demonstrated for the degradation of carmustine. From the data at 5, 22, and 37°, the apparent activation energies for carmustine degradation in buffered aqueous media were computed and were strongly pH dependent.

N-hydroxy-compounds as acyl transfer agents. Part 2. Kinetics and mechanism of hydrolysis and aminolysis of 1-hydroxypyrazole and 1-hydroxyimidazole esters

Benzoate esters of 3-(alkyl or aryl)-1-hydroxy-4,5-dimethylpyrazole (1) and 1-hydroxy-4,5-dimethyl-2-phenylimidazole (2) react with hydroxide ion, water (via acid catalysis), and primary amines in aqueous dioxan at 25°, but at a much slower rate than the corresponding esters of 1-hydroxybenzotriazole, pydrolysis of the pyrazole esters (1 a and b) shows specific acid and base catalysis, giving Hammett ρ values of 1.1 and 1.5 respectively. For the acid-catalysed process, protonation of the heterocyclic ring rather than the carbonyl oxygen is invoked. Acidcatalysed hydrolysis of the imidazole ester (2a) occurs at about the same rate, but the hydroxide ion-catalysed process is much slower (kOH– 8.32 l mol–1 s–1). The rate of hydroxide attack on (1a and b) and (2a) has a lower sensitivity to the pKa of the leaving group (β1˙g˙–0.2), than phenyl acetates (β1˙g˙–0.33). Only one pKa(7.22) could be measured for (le), in contrast to two pKa values reported previously (6.1 and 5.5). The reaction of (1 a) with primary amines gives βnuc0.89, and general base catalysis was observed with ethylenediamine monocation and glycine ethyl ester (kgb= 61.6 × 10–3 l2 mol–2 s–1 and 5.3 × 10–3 l2 mol–3 s–1). With amines the imidazole ester (2a) shows βnuc 0.80 and no general base-catalysed process was observed. Above pH 9, 2-phenyloxazolin-5-one (12) was observed as an intermediate in the hydrolysis and aminolysis of the hippurate ester (1c). The ester (1d) reacts with nucleophiles by a BAC2 mechanism at all pH. Compounds (1e and 1f) and (2b) and 1hydroxybenzotriazole catalyse the decomposition of p-nitrophenyl acetate, but the latter two compounds show a 50-fold negative deviation from structure–reactivity correlations (log knversus pKa).