Effect of drug lipophilicity on in vitro release rate from oil vehicles using nicotinic acid esters as model prodrug derivatives

Development and evaluation of intramuscularly administered nano/microcrystal suspension

INTRODUCTION

Formulation of nanocrystals is one of the most important drug delivery systems for poorly soluble drug molecules. Nanocrystals are produced by techniques like precipitation, media milling, high-pressure homogenization, and so on. In order to achieve sustained release and higher absorption of nanosuspensions, intramuscularly administered nanosuspensions have been developed. As well, intramuscularly administered nanosuspensions have been implemented in order to improve the bioavailability of drug nanocrystals which have both a low oral bioavailability and cannot be administered by intravenous injection routes.

AREAS COVERED

This review summarizes studies that have focused on the production, classification, in vitro release and in vivo pharmacokinetics of intramuscularly administered nanosuspensions. In order to avoid common drawbacks of intramuscularly administered nanosuspensions, such as tissue residues and some local tissue damage, nanosuspensions with a reduced administration volume of high drug loading and extended therapeutic effects are developed.

EXPERT OPINION

Intramuscularly administered nano/micro crystal suspensions have been developed for the treatment of various diseases such as schizophrenia, hormone disordered diseases, HIV and more. Additionally, intramuscularly administered nanosuspensions are also a good route for the development of traditional chinese medicines which have lower oral bioavailability and are not suitable for intravenous injection.

Characterization of bupivacaine-loaded formulations based on liquid crystalline phases and microemulsions: the effect of lipid composition

This report details the structural characterization and the in vitro drug-release properties of different local anesthetic bupivacaine (BUP)-loaded inverted-type liquid crystalline phases and microemulsions. The effects of variations in the lipid composition and/or BUP concentration on the self-assembled nanostructures were investigated in the presence of the commercial distilled glycerol monooleate Myverol 18-99K (GMO) and medium-chain triglycerides (MCT). Synchrotron small-angle X-ray scattering (SAXS) and rotating dialysis cell model were used to characterize the BUP formulations and to investigate the in vitro BUP release profiles, respectively. The evaluation of SAXS data for the BUP-loaded GMO/MCT formulations indicates the structural transition of inverted-type bicontinuous cubic phase of the symmetry Pn3m → inverted-type hexagonal (H(2)) phase → inverted-type microemulsion (L(2)) with increasing MCT content (0-40 wt %). In the absence of MCT, the solubilization of BUP induces the transition of Pn3m → H(2) at pH 7.4; whereas a transition of Pn3m → (Pn3m + H(2)) is detected as the hydration is achieved at pH 6.0. To mimic the drug release and transport from in situ formed self-assembled systems after subcutaneous administration, the release experiments were performed by injecting low viscous stimulus-responsive precursors to a buffer in the dialysis cell leaving the surface area between the self-assembled system and the release medium variable. Our results suggest that the pH-dependent variations in the lipidic partition coefficient, K(l/w), between the liquid crystalline nanostructures and the surrounding buffer solution are significantly affecting BUP release rates. Thus, a first step toward understanding of the drug-release mechanism of this drug-delivery class has been undertaken tackling the influence of drug ionization as well as the type of the self-assembled nanostructure and its release kinetics under pharmaceutically relevant conditions.

Role of in vitro release models in formulation development and quality control of parenteral depots

This review article provides an assessment of advantages/limitations of the use of current in vitro release models to predict in vivo performance of parenteral sustained release products (injectable depots). As highlighted, key characteristics influencing the in vivo drug fate may vary with the route of administration and the type of sustained release formulation. To this end, an account is given on three representative injection sites (intramuscular, subcutaneous and intra-articular) as well as on in vitro release mechanism(s) of drugs from five commonly investigated depot principles (suspensions, microspheres, hydrogels, lipophilic solutions, and liposomes/other nano-size formulations). Current in vitro release models are, to a different extent, able to mimic the rate, transport and equilibrium processes that the drug substance may experience in the environment of the administration site. Their utility for the purpose of quality control including in vitro-in vivo correlations and formulation design is discussed.

Critical factors influencing the in vivo performance of long-acting lipophilic solutions--impact on in vitro release method design

Parenteral long-acting lipophilic solutions have been used for decades and might in the future be used in the design of depots with tailored delivery characteristics. The present review highlights major factors influencing the in vivo performance of lipophilic solutions. Furthermore, an account is given of the characteristics of employed in vitro release methods with a focus on the "state" of sink condition, the stirring conditions, and the oil-water interfacial area. Finally, the capability of in vitro release data to predict the in vivo performance of drug substances administrated in the form of lipophilic solutions is discussed. It is suggested that as long as the major rate-limiting in vivo release mechanism is governed by the drug partitioning between the oil vehicle and the tissue fluid, the use of in vitro release testing in quality control appears to be realistic. With increasing lipophilicity of the drug substances and longer duration of action, the in vivo drug release process may become more complex. As discussed, practical analytical problems together with the inability of release methods to mimic two or more concomitant in vivo events may constitute severe impediments for establishment of in vitro in vivo correlations.

Alginate-based in situ gelling suspensions and emulsions comprising N4-alkyloxycarbonyl derivatives of cytosine: zero-order release and effect of physicochemical properties

As an alternative to incorporation of various excipients, N(4)-alkyloxycarbonyl-cytosine derivatives possessing various physicochemical properties and cytosine regeneration rates have been examined to modify release rate and kinetics from in situ gelling alginate formulations, e.g., liquid formulations that gel in acidic gastric juice and release the entrapped derivative or parent cytosine. Linear relationships were obtained between the release rate constants and the square root of the solubility for suspension formulations. Calculated diffusion coefficients were observed to be similar for suspension and solution formulations; however, for in situ gelling emulsion formulations, diffusivity correlated linearly to log P. Zero-order release of parent cytosine was observed from in situ gelling suspensions of the poorly soluble acid-labile N(4)-adamantyloxycarbonyl-cytosine prodrug.

Sustained release veterinary parenteral products

Controlled release parenteral dosage forms have application in veterinary medicine. Systems that minimize the need for repeated injections while achieving therapeutic effects for extended periods offer benefits that make commercial development of these products desirable. While some products have already found commercial success, others will result from application of new controlled release technologies. This review highlights the formulation and technology challenges in developing some of these controlled release technologies into products. Further, examples of application of controlled release technologies in the veterinary field are discussed.

Kinetics of degradation of 4-imidazolidinone prodrug types obtained from reacting prilocaine with formaldehyde and acetaldehyde

The kinetics of decomposition of 4-imidazolidinone prodrug types obtained by reacting prilocaine (I) with formaldehyde and acetaldehyde has been studied in aqueous solution in the pH range 1-7.4 at 60 and 37 degrees C, respectively. At pH<5 the hydrolysis of the derivative derived from formaldehyde (II) to yield I obeyed apparent first-order kinetics. At higher pH, the decomposition reactions proceeded to an equilibrium and the reactions could be described by first- and second-order reversible kinetics. A plot of the logarithm of the apparent first-order rate constants for hydrolysis of II against pH resulted in a sigmoidal-shaped pH-rate profile characteristic for the hydrolysis of many N-Mannich bases. A half-life at pH 7.4 (60 degrees C) of 6.9h for compound II was calculated. Compared to II the 4-imidazolidinone derived from acetaldehyde (III) exhibited enhanced instability in aqueous buffer solutions. The decomposition was followed at 37 degrees C monitoring the decrease in concentration of intact (III). At acidic pH the reactions displayed strict first-order kinetics and the disappearance of III was accompanied by a concomitant formation of I. At pH 7.4, the rate data also applied reasonably well to first-order kinetics despite the observation that small amounts of III was formed at pH 7.4 from a solution containing equimolar concentrations of acetaldehyde and prilocaine (10(-4)M). In case of III, a bell-shaped pH-rate profile was obtained by plotting the logarithm of the pseudo-first-order rate constants against pH indicating the involvement of a kinetically significant intermediate in the reaction pathway and a change of the rate-limiting step in the overall reaction with pH. For the stability studies performed at pH 6.9 and 7.4 product analysis revealed that parallel to formation of (I) an unknown compound (X) emerged. Compared to III, compound X is hydrolysed to give I at a slower rate (t(50%)=30 h at 37 degrees C). Based on LC-MS data it is suggested that (X) is an isomeric form of III, which may exist in four diastereomeric forms. Thus, at physiological pH an initial relatively fast regeneration of I from III is to be expected followed by a slower drug activation resulting from hydrolysis of the isomeric form of III.

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.

Characteristics of drug substances in oily solutions. Drug release rate, partitioning and solubility

In vitro rate of drug release from oil solutions was investigated in a rotating dialysis cell. A log linear correlation was established between the rate constant (k(obs)) for attainment of equilibrium and apparent partition coefficient (P(app)) between oil vehicle and release media using various weak acids and bases and non-electrolytes. Collander like linear free energy relationships were observed allowing various oil-aqueous buffer partition coefficients to be calculated from known octanol-aqueous buffer partition coefficients. Solubility of the various drug substances in oil vehicles were investigated. A linear correlation was observed between log molar solubility and melting point of the solutes. Release profiles obtained for release of two local anaesthetics dissolved in the same oil vehicle exhibited an unexpected behavior involving an initial delayed release of the most lipophilic local anaesthetic.

Determination of the disappearance rate of iodine-125 labelled oils from the injection site after intramuscular and subcutaneous administration to pigs

The rate of disappearance of clinically used vegetable oils, Viscoleo, sesame oil, castor oil and isopropyl myristate, from the injection site after intramuscular (i.m.) or subcutaneous (s.c.) administration to pigs were determined by using a non-invasive gamma-scintigraphy method. All the oil vehicles were spiked with 2.5% (v/v) (125)I-triolein and six injections of 1.9 ml were given to each of 12 pigs. No significant difference (ANOVA) in disappearance rate of each individual oil vehicle from the different injection sites was observed after administration of the oils: i.m. in the lower back, s.c. in the neck and s.c. in the mid-back. Likewise, no inter-individual difference between the pigs was observed. The half-life of 14 days for Viscoleo was significantly smaller than those of the other oil vehicles (P<0.0001), i.e. 23,20,20 days for sesame oil, castor oil and isopropyl myristate, respectively. Due to the spreading effect of the oils and reflux of the oils through the injection canal, the half-lives were calculated omitting the data for the first sampling day.