Marianne Johansen

Macromolecular prodrugs. XVI. Colon-targeted delivery--comparison of the rate of release of naproxen from dextran ester prodrugs in homogenates of various segments of the pig gastrointestinal (GI) tract.

We have determined initial rates of naproxen formation from dextran-naproxen ester prodrugs incubated in homogenates of various segments of the pig GI tract. Drug liberation proceeded 15–17 times faster in cecum and colon homogenates than in aqueous pH 7.4 buffer or homogenates of the small intestine. The degree of conjugate substitution did not affect the liberation rates, whereas enhanced drug activation was observed with decreasing molecular size of the carrier dextran. During incubation in colon homogenates the average molecular weight of the dextran prodrugs decreased. The mechanism of drug activation from the prodrugs may therefore involve an initial depolymerization step of the dextran chains by dextranases secreted from bacteria in the pig colon. The generated small fragments then serve as substrates for esterases and other hydrolases.

Macromolecular Prodrugs. XV. Colon-Targeted Delivery—Bioavailability of Naproxen from Orally Administered Dextran–Naproxen Ester Prodrugs Varying in Molecular Size in the Pig

The bioavailability of naproxen after oral administration of aqueous solutions of various dextran–naproxen ester prodrugs in pigs was determined. The dextran prodrugs employed ranged in molecular weight from 10,000 to 500,000. As calculated relative to an equivalent oral dose of parent naproxen, the absorption fractions of all the derivatives were close to 100%. Only small interindividual variation of naproxen bioavailability was observed. The naproxen plasma profiles for all the administered prodrugs exhibited a characteristic lag time of naproxen appearance in the blood (2–3 hr). Compared to administration of the prodrugs alone, coadministration of excess of the parent dextran further delayed the absorption of naproxen from the GI tract. The results of the present study demonstrate the potential of dextran prodrugs for colon site-specific delivery of drugs containing a carboxylic acid functional group.

Pro-drugs as drug delivery systems VIII. Bioreversible derivatization of hydantoins by N-hydroxymethylation

Abstract The kinetics and mechanism of decomposition of the N 3 -hydroxymethyl derivatives of various hydantoins (phenytoin, nitrofurantoin and 5,5-dimethylhydantoin) in aqueous solution at 37°C was studied to assess their potential utility as pro-drugs for the parent substances. The derivatives were found to undergo an apparent hydroxide ion-catalyzed decomposition in the pH range 3.3–6.1, the specific rate constants being 7.1 × 10 7 , 6.2 × 10 8 and 1.0 × 10 7 M −1 min −1 for the phenytoin, nitrofurantoin and dimethylhydantoin derivative, respectively. This difference m decomposition rates was correlated with the difference in the acidity of the parent hydantoins and the rate data for the hydantoins were shown to fit well with a rate—acidity relationship previously derived for a number of N-hydroxymethylated amides and imides. The N-hydroxymethyl hydantoins are very rapidly cleaved to formaldehyde and the parent compounds at pH 7.4 and 37°C (half-lives calculated to range from 0.1 to 6.9 s). The derivatives were shown to possess higher water solubilities than the parent compounds and it is suggested that N-hydroxymethylation may be a potentially useful means of obtaining pro-drug forms of hydantoins. In addition, it was demonstrated by determination of the hydrolysis kinetics of the acetate ester of 3-(hydroxymethyl)phenytoin that the N-hydroxymethylated hydantoins are amenable to further bioreversible derivatization.

Macromolecular prodrugs. XIV. Absorption characteristics of naproxen after oral administration of a dextran T-70-naproxen ester prodrug in pigs

Abstract The bioavailability of naproxen after oral administration of aqueous solutions of a dextran T-70-naproxen ester prodrug in pigs was assessed. Compared to the administration of an oral solution of an equivalent dose of naproxen the average absorption fraction for the conjugate amounted to 91%. It was established that several features of the prodrug indicated that naproxen was released from the prodrug prior to systemic absorption and that drug activation involved the action of one or more enzyme systems located in the gastrointestinal tract. As was observed in rabbits, the plasma concentration-time curves for the conjugate were characterized by an initial lag time of about 2–3 h, whereas naproxen was detected in plasma immediately after p.o. administration of the drug compound, per se. The distribution of the prodrug along the GI tract at various times after conjugate administration was assessed qualitatively by HPLC analysis of the content of conjugated and free naproxen in various segments of the GI tract. From these experiments it was suggested that drug regeneration was effective in the bowel below the ileum. No visible allergic reactions appeared after oral administration of the dextran derivative to already i.v. sensitized pigs, indicating that the intact conjugate did not reach the systemic circulation.

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

Abstract 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.

In vitro evaluation of dermal prodrug delivery — transport and bioconversion of a series of aliphatic esters of metronidazole

Abstract The ability of a series of aliphatic ester prodrugs to improve cutaneous delivery of metronidazole has been investigated. In permeation studies using full thickness human skin in vitro the derivatives lead only to a 1.5-fold enhancement of permeation compared to the parent drug. The influence of non-specific cutaneous hydrolytic enzymes on the rate of hydrolysis of the ester derivatives was determined in homogenates prepared from human skin. It was observed that the susceptibility of the esters to undergo enzyme-catalyzed cleavage was strongly affected by the length of the acyi side chain making the valerate and caproate esters the best substrates. The experimental data demonstrate that no single ester derivative can be selected with optimal properties regarding both skin permeation and enzymatic regeneration to metronidazole. Due to the relatively small variation in permeation rate of the compounds, however, the butyrate ester seems to fulfill the requirements of good permeation characteristics as well as a quick conversion to metronidazole (t 1 2 in skin homogenate is 1.0 h at 37°C).

Macromolecular prodrugs I. Kinetics and mechanism of hydrolysis of O-benzoyl dextran conjugates in aqueous buffer and in human plasma

Abstract Benzoate esters of dextran (Mw = 65,600) with varying degrees of substitution have been synthesized. The kinetics and mechanism of hydrolytic cleavage of the ester bond in aqueous solution over the pH range 3.0–9.5 (60°C) has been investigated. The degradation reactions followed strict first-order kinetics and a rate expression encompassng hydrogen ion-, hydroxide ion- and water-catalyzed hydrolysis of the conjugates is derived. General acid-base catalysis of release of benzoic acid from the conjugates was negligible. No influence of the degree of substitution on the reaction rates was observed. Nearly identical rates of release of benzoic acid from the conjugates at 37°C in 80% human plasma and in 0.05 M phosphate buffer pH 7.40 have been found (t 1 2 ∼ 190 h), revealing that hydrolysis in plasma proceeds without enzymatic catalysis. The lack of susceptibility to undergo enzyme-mediated hydrolysis is most likely due to steric hindrance by the dextran backbone. Various chemical methods to attach drug compounds covalently to dextrans and the potential, therapeutic utility of the conjugates are discussed.

Spectrophotometric determination of the rates of hydrolysis of aldehyde-releasing pro-drugs in aqueous solution and plasma

Abstract A previously described spectrophotometric method for the determination of formaldehyde or other aliphatic aldehydes has been modified and demonstrated to be a highly useful and convenient means for assessing the rate of hydrolysis of various pro-drugs which upon reconversion to their parent drugs release an aldehyde. The pro-drugs tested include a number of acyloxyalkyl esters of carboxylic acids (pivampicillin, bacampicillin, pivmecillinam and esters of isoguvacine), N-acyloxymethyl esters of 5-fluorouracil and theophylline, N-Mannich bases and N-hydroxymethyl derivatives. Rate data are given for the hydrolysis of these pro-drugs in buffer solutions as well as in solutions containing human plasma and are compared with data obtained by various other methods.

Stability and kinetics of hydrolysis of metronidazole monosuccinate in aqueous solution and in plasma

Abstract The kinetics of hydrolysis of metronidazole monosuccinate in aqueous solution at pH 1.5–10 and 60°C has been investigated. The decomposition was monitored by a high-performance liquid chromatographic method capable of determining the monosuccinate ester and the parent metronidazole simultaneously. At any given pH the reactions displayed strict first-order kinetics and were subject to specific acid-base catalysis; no general acid-base catalytic effect by various buffer substances was observed. Maximal stability was observed at pH 3–6, where the degradation rate was independent of pH indicating that intramolecular catalysis by the terminal carboxylate ion on the ester group is not involved in the hydrolysis reactions in this pH range. The lack of intramolecular catalysis is suggested to be due to the poor leaving ability of the strongly basic metronidazole alkoxide ion. The rates of conversion of the ester to metronidazole at 37°C in 80% human plasma and in 0.05 M phosphate buffer pH 7.40, respectively, were found to be nearly identical, t 1 2 ~ 580 h, revealing that the degradation in plasma proceeds in the absence of enzymatic catalysis.

Pro-drugs as drug delivery systems XIII. Kinetics of decomposition of N-Mannich bases of salicylamide and assessment of their suitability as possible pro-drugs for amines☆

Abstract The kinetics of decomposition of various N-Mannich bases of salicylamide in aqueous solution at 37°C was studied to assess their suitability as pro-drugs for amino compounds. The decomposition, yielding salicylamide, amine and formaldehyde in stoichiometric amounts, showed bell-shaped pH-rate profiles which could be accounted for by assuming spontaneous decomposition of both neutral and protonated Mannich base and unreactivity of the derivatives in the anionic form. For the Mannich bases with the amines piperidine, α-alanine, methylamine and morpholine, the half-lives of decomposition at pH 7.40 and 37°C were 14, 17, 28 and 41 min, respectively, suggesting that salicylamide N-Mannich bases are possible candidates as pro-drugs for compounds containing a primary or secondary amino group. N-Amidomethylation of the amines with salicylamide resulted in a pronounced lowering of their basicity corresponding to 3–4 pK a units which may be of potential utility for the application of N-Mannich bases as pro-drug forms for amines.