Addition of hydrogen bond donating excipients to oil solution: effect on in vitro drug release rate and viscosity

In oily vehicles containing different hydrogen bond donating excipients rates of transfer of the weak electrolytes naproxen and lidocaine from the oil phase to the aqueous medium were measured by using the rotating dialysis cell. A logarithmic linear correlation was established between the apparent partition coefficient, P(app), and the first-order rate constant related to attainment of equilibrium between the two phases, k(obs), which fitted well with results from former publications. Further, release data for the non-electrolyte testosterone were found to fit into this linear correlation. Apparent partition coefficients were determined between oil vehicles containing various amounts of hydrogen bond donating excipients and phosphate buffer, pH 6.00, revealing a rise in log P(app) with increasing concentration of excipient. Viscosity was measured for castor oil containing vehicles showing a linear relationship between percentage (v/v) castor oil and log viscosity (mPas) of the mixed vehicle.

Assessment of rate of drug release from oil vehicle using a rotating dialysis cell

The rate constants for transfer of model compounds (naproxen and lidocaine) from oily vehicle (Viscoleo) to aqueous buffer phases were determined by use of the rotating dialysis cell. Release studies were done for the partly ionized compounds at several pH values. A correlation between the overall first-order rate constant related to attainment of equilibrium, k(obs), and the pH-dependent distribution coefficient, D, determined between oil vehicle and aqueous buffer was established according to the equation: logk(obs)=-0.71 logD-0.22 (k(obs) in h(-1)). Based on this correlation it was suggested that the rate constant of a weak electrolyte at a specified D value could be considered equal to the k(obs) value for a non-electrolyte possessing a partition coefficient, P(app), the magnitude of which was equal to D. Specific rate constants k(ow) and k(wo) were calculated from the overall rate constant and the pH-dependent distribution coefficient. The rate constant representing the transport from oily vehicle to aqueous phase, k(ow), was found to be significantly influenced by the magnitude of the partition coefficient P(app) according to: logk(ow)=-0.71 logP(app)-log(P(app)+1)-0.22 (k(ow) in h(-1)).

Modification of in vitro drug release rate from oily parenteral depots using a formulation approach

Rate constants for transfer of naproxen and lidocaine from different oils and oil mixtures to aqueous buffer, pH 6.00, were determined using the rotating dialysis cell. Significantly different first-order rate constants related to attainment of equilibrium, k(obs), were derived depending on the type of oil/oil mixtures used in the release experiments. For the drugs a linear correlation was found between logk(obs) and the logarithm of the partition coefficient P(app): logk(obs)=-0.68 logP(app)-0.25 (k(obs) in h(-1), n=26). A linear relationship was observed between the calculated and experimentally determined P(app) values for the oil mixtures investigated. The specific rate constants, k(ow) and k(wo), related to the partition process were derived from the determined k(obs) and P(app) values. The rate constant k(ow) representing the rate of transfer of the solute from the oil phase to the aqueous buffer was shown to be strongly dependent on the partition coefficient according to the relationship: logk(ow)=-0.68 logP(app)-log(P(app)+1)-0.25 (k(ow) in h(-1), n=26). In particular, diminished release rates were seen for oil mixtures containing castor oil most likely afforded by hydrogen bonding between the solute and the hydroxy groups of the latter vegetable oil. In this study it has been possible to alter P(app) for a specific compound up to a factor of 10 by variation of the composition of the oil vehicle. Such a span of P(app) values results in in vitro release rates differing a factor of 37. Thus, by proper design of the oil vehicle composition it should be possible to modify the release rate for a specific compound within certain limits.

Chemical and enzymatic stability as well as transport properties of a Leu-enkephalin analogue and ester prodrugs thereof

The Leu-enkephalin analogue (Tyr-D-Ala-Gly-Phe-Leu-NH(2)) was synthesized together with three esters prodrugs hereof. The prodrugs synthesized were the O-acetyl, O-propionyl and O-pivaloyl99%) and in good yields (60-75%). The chemical and enzymatic stability of the prodrugs has been investigated in detail. The prodrugs studied are quite chemically stable and the degradation of the prodrugs follows the pattern previously shown for similar esters (U-shaped pH-profile; maximal stability at pH 4-5). The prodrugs are degraded quantitatively in plasma to the parent peptide with half-lives in the range 2.9 min-2.6 h. Type B esterases were shown to be involved in the degradation as the half-lives increased in the presence of paraoxon. No significant stabilization was seen in 10% porcine gut homogenate. Half-lives in the same order were seen for the analogue and the prodrugs in pure Leucine aminopeptidase solution. The analogue was stable in Carboxypeptidase A solution whereas a faster degradation of the prodrugs was seen in this media. Furthermore the transport properties of the compounds has been studied. A P(app) value of 0.284x10(-6) cm/s for the analogue was obtained for the transport across Caco-2 cell monolayers in the BL-AP direction. The P(app) values were increased by a factor of 2, 7 and 18 for the acetyl-, propionyl- and pivaloylprodrug. The increase could be explained by higher lipofilicities of the prodrugs compared to the analogue.

Stability and perfusion studies of Desmopressin (dDAVP) and prodrugs in the rat jejunum

Three aliphatic carboxylic acid esters of the tyrosine phenolic group in Desmopressin (dDAVP) were investigated in vitro for their stability and metabolism in rat gastrointestinal media. The degradation followed strictly first-order kinetics and the prodrugs were quantitatively converted to dDAVP. The n-hexanoyl (II) and n-octanoyl (III) esters were rapidly hydrolysed in 10% rat jejunal fluid showing half-lives of 1.1+/-0.2 min and 1.4+/-0.1 min, respectively. In 5 % rat jejunal homogenate the half-lives were 3.2+/-0.2 min and <30 sec, respectively. The sterically hindered pivalate ester (I) proved to be more stable. The half-lives were 10.3+/-0.3 min in 10% rat jejunal fluid and 1.5+/-0.1 min in 10% rat jejunal homogenate, respectively. The presence of paraoxon, an inhibitor of type B esterases significantly decreased the degradation rate of the pivalate ester (I) in rat jejunal fluid (t1/2 > 5 hrs) indicating that the prodrug is converted to dDAVP by rapid luminal breakdown of the ester bond. It was shown that approximately 13 % of prodrug I disappeared from the gut lumen during a single-pass perfusion experiment in rat jejunum. Our results indicate that the disappearance from the jejunal lumen was primarily caused by degradation of the prodrug to dDAVP by esterases rather than absorption. The better stability of the sterically hindered prodrug (I) indicate that even more sterically hindered prodrugs will be a better choice for a further optimization of stability and lipophilicity, and consequently a potentially improved intestinal absorption of dDAVP.

alpha-Chymotrypsin-catalyzed degradation of desmopressin (dDAVP): influence of pH, concentration and various cyclodextrins

Desmopressin [1-(mercaptopropanoic acid)-8-D-arginine vasopressin; dDAVP] is a vasopressin analogue with a selective antidiuretic effect. The oral bioavailability of desmopressin is limited due both to its high hydrophilicity leading to a low intestinal permeability and to low enzymatic stability. The degradation of desmopressin was investigated in aqueous buffer solutions (pH 6.00-9.00) containing the enzyme alpha-chymotrypsin at a concentration of 0.50 mg/ml at 37 degrees C. The degradation of desmopressin was also studied in solutions containing alpha-chymotrypsin in the concentration range 0.10-1.00 mg/ml (pH 7.40 and 37 degrees C). The rate of degradation was shown to be highly dependent on both enzyme concentration and pH. Maximal alpha-chymotrypsin activity was observed in the pH range 7.40-8.00. It was observed that phenylalanine was formed during the degradation of desmopressin. Phenylalanine was formed in the amount of 20% in 120 min. In the same time period 95% of desmopressin was degraded. The formation of phenylalanine can be explained from the substrate specificity of alpha-chymotrypsin. Cyclodextrins are known to stabilize drugs including peptides against both chemical and enzymatic degradation. In this study it was shown that hydroxypropyl cyclodextrins (alpha, beta and gamma) stabilized desmopressin against alpha-chymotrypsin-catalyzed degradation. The stabilization was by a factor of 3, 9 and 8 at the concentration 12.5% (w/v) for hydroxypropyl-alpha-cyclodextrin, hydroxylpropyl-beta-cyclodextrin and hydroxypropyl-gamma-cyclodextrin.