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

Assessment of glyceride-structured oleogels as an injectable extended-release delivery system of bupivacaine

This manuscript systematically assesses three different glycerides (tripalmitin, glyceryl monostearate, and a blend of mono-, di- and triesters of palmitic and stearic acids (Geleol™)) as potential gelator structuring agents of medium-chain triglyceride oil to form an oleogel-based injectable long-acting local anesthetic formulation for postoperative pain management. Drug release testing, oil-binding capacity, injection forces, x-ray diffraction, differential scanning calorimetry, and rheological testing were serially performed to characterize the functional properties of each oleogel. After benchtop assessment, the superior bupivacaine-loaded oleogel formulation was compared to bupivacaine HCl, liposomal bupivacaine, and bupivacaine-loaded medium-chain triglyceride oil in a rat sciatic nerve block model to assess in vivo long-acting local anesthetic performance. In vitro drug release kinetics were similar for all formulations, indicating that drug release rate is primarily dependent on the drug's affinity to the base oil. Glyceryl monostearate-based formulations had superior shelf-life and thermal stability. The glyceryl monostearate oleogel formulation was selected for in vivo evaluation. It was found to have a significantly longer duration of anesthetic effect than liposomal bupivacaine and was able to provide anesthesia twice as long as the equipotent bupivacaine-loaded medium-chain triglyceride oil, indicating that the increased viscosity of the oleogel provided enhanced controlled release over the drug-loaded oil alone.

Evaluation of the Efficacy, Biocompatibility, and Permeation of Bupivacaine-Loaded Poly(epsilon-caprolactone) Nano-Capsules as an Anesthetic

In this study bupivacaine (BVC) was encapsulated in Nano-capsules of poly-ε-caprolactone (PCL) and its cytotoxicity in HaCaT (MTT) cells, its permeability in the oesophageal epithelium of pigs, as well as its anesthetic effect in the incision model of rat's hind paw (electronic von Frey anesthesiometer) were evaluated. BVC and epinephrine-associated bupivacaine (BVC-Epi) have been compared to BVC-Nano and it was demonstrated that BVC-Nano had high physicochemical properties and remained stable for 120 days; also, encapsulation of bupivacaine did not affect its toxicity to HaCaT cells, but epinephrine reduced its toxicity. Although both methods of combination with epinephrine and encapsulation in nanocapsules resulted in an extended time of anesthesia, the efficacy of epinephrine was more favorable. The permeation evaluation indicated that encapsulation increased both the permeability coefficient and the steady-state flux of bupivacaine across the esophageal epithelium. BVC permeation was enhanced by encapsulation into Nano-capsules, as a new novel therapeutic strategy, facilitating future research as a topical anesthetic.

A Portable Device for the Generation of Drug-Loaded Three-Compartmental Fibers Containing Metronidazole and Iodine for Topical Application

The use of combination therapies for the treatment of a range of conditions is now well established, with the component drugs usually being delivered either as distinct medicaments or combination products that contain physical mixes of the two active ingredients. There is, however, a compelling argument for the development of compartmentalised systems whereby the release, stability and incorporation environment of the different drugs may be tailored. Here we outline the development of polymeric fine fiber systems whereby two drugs used for the treatment of wounds may be separately incorporated. Fibers were delivered using a newly developed handheld electrospinning device that allows treatment at the site of need. Crucially, the delivery system is portable and may be used for the administration of drug-loaded fibers directly into the wound in situ, thereby potentially allowing domiciliary or site-of-trauma administration. The three-layered fiber developed in this study has polyethylene glycol as the outermost layer, serving as a structural support for the inner layers. The inner layers comprised iodine complexed with polyvinylpyrrolidone (PVP) and metronidazole dispersed in polycaprolactone (PCL) as a slow release core. The systems were characterized in terms of structure and architecture using scanning electron microscopy, transmission electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy and diffractometry. As antibacterial creams are still used for managing infected wounds, the performance of our trilayered fiber was studied in comparison with creams containing similar active drugs. Drug release was measured by UV analysis, while antimicrobial efficiency was measured using agar diffusion and suspension methods. It was found that the trilayered systems, averaging 3.16 µm in diameter, released more drug over the study period and were confirmed by the microbacterial studies to be more effective against P. aeruginosa, a bacterium commonly implicated in infected wounds. Overall, the portable system has been shown to be capable of not only incorporating the two drugs in distinct layers but also of delivering adequate amounts of drugs for a more effective antibacterial activity. The portability of the device and its ability to generate distinct layers of multiple active ingredients make it promising for further development for wound healing applications in terms of both practical applicability and antimicrobial efficacy.

Tuning molecular dynamics by hydration and confinement: antiplasticizing effect of water in hydrated prilocaine nanoclusters

In glass-forming substances, the addition of water tends to produce the effect of lowering the glass transition temperature, Tg. In a previous work by some of us (Ruiz et al., Sci. Rep., 2017, 7, 7470) we reported on a rare anti-plasticizing effect of water on the molecular dynamics of a simple molecular system, the pharmaceutically active prilocaine molecule, for which the addition of water leads to an increase of Tg. In the present work, we study pure and hydrated prilocaine confined in 0.5 nm and 1 nm pore size molecular sieves, and carry out a comparison with the bulk compounds in order to gain a better understanding of the microscopic mechanisms that result in this rare effect. We find that the Tg of the drug under nanometric confinement can be lower than the bulk value by as much as 17 K. Through the concurrent use of differential scanning calorimetry and broadband dielectric spectroscopy we are able to observe the antiplasticizing effect of water in prilocaine also under nanometric confinement, finding an increase of Tg of up to almost 6 K upon hydration. The extension of our analysis to nanoconfined systems provides a plausible explanation for the very uncommon antiplasticizing effect, based on the formation of water-prilocaine molecular complexes. Moreover, this study deepens the understanding of the behavior of drugs under confinement, which is of relevance not only from a fundamental point of view, but also for practical applications such as drug delivery.

Citrem–phosphatidylcholine nano-self-assemblies: solubilization of bupivacaine and its role in triggering a colloidal transition from vesicles to cubosomes and hexosomes

In concentration- and lipid composition-dependent manners, bupivacaine triggers lamellar–nonlamellar phase transitions in citrem/soy phosphatidylcholine nanodispersions.

Polymeric nanocapsules as drug carriers for sustained anticancer activity of calcitriol in breast cancer cells

Clinical use of calcitriol (1,25-dihydroxyvitamin D₃) as an anticancer agent is currently limited by the requirement of supraphysiological doses and associated hypercalcemia. Nanoencapsulation of calcitriol is a strategy to overcome these drawbacks, allowing reduced administrated doses and/or frequency, while retaining the therapeutic activity towards cancer cells. For this purpose, we investigated the impact of calcitriol encapsulation on its antiproliferative activity and optimized formulation parameters with that respect. Calcitriol-loaded polymeric nanoparticles with different polymer:oil ratios were prepared by the nanoprecipitation method. Nanoparticles had similar mean size (200 nm) and EE (85%) whereas their release profile strongly depended on formulation parameters. Antiproliferative and cytotoxic activities of formulated calcitriol were evaluated in vitro using human breast adenocarcinoma cells (MCF-7) and showed that calcitriol-induced cell growth inhibition was closely related to its release kinetics. For the most suitable formulation, a sustained cell growth inhibition was observed over 10 days compared to free form. Advantages of calcitriol encapsulation and the role of formulation parameters on its biological activity in vitro were demonstrated. Selected nanoparticle formulation is a promising calcitriol delivery system ensuring a prolonged anticancer activity that could improve its therapeutic efficiency.

Liquid crystalline drug delivery vehicles for oral and IV/subcutaneous administration of poorly soluble (and soluble) drugs

Poorly soluble drug molecules often have low bioavailability issues and absorption problems in the clinical setting. As the number of poorly soluble drugs increases from discovery, developing technologies to enhance their solubility, while also controlling their release is one of the many challenges facing the pharmaceutical industry today. Liquid crystalline systems, nanoparticulate or macro-matrix, self-assemble in the presence of an aqueous environment and can provide a solubility enhancement, while also controlling the drug release rate. This review examines the fundamentals of liquid crystalline systems through the representative literature, concluding with examples of liquid crystalline systems in clinical trials development. The review focus is on the potential of utilizing liquid crystalline systems for poorly soluble drugs, in the areas of oral delivery and IV/subcutaneous, followed by water soluble molecules. Key considerations in utilizing liquid crystalline systems advantages while also discussing potential areas of key research that may be needed will be highlighted.

Novel strategies for the formulation and processing of poorly water-soluble drugs

Low aqueous solubility of active pharmaceutical ingredients presents a serious challenge in the development process of new drug products. This article provides an overview on some of the current approaches for the formulation of poorly water-soluble drugs with a special focus on strategies pursued at the Center of Pharmaceutical Engineering of the TU Braunschweig. These comprise formulation in lipid-based colloidal drug delivery systems and experimental as well as computational approaches towards the efficient identification of the most suitable carrier systems. For less lipophilic substances the preparation of drug nanoparticles by milling and precipitation is investigated for instance by means of microsystem-based manufacturing techniques and with special regard to the preparation of individualized dosage forms. Another option to overcome issues with poor drug solubility is the incorporation into nanospun fibers.

On the structure of prilocaine in aqueous and amphiphilic solutions

The solvation of prilocaine has been investigated in pure water and in amphiphilic solutions using a combination of neutron diffraction and simulations.

Computational prediction of drug solubility in lipid based formulation excipients

PURPOSE

To investigate if drug solubility in pharmaceutical excipients used in lipid based formulations (LBFs) can be predicted from physicochemical properties.

METHODS

Solubility was measured for 30 structurally diverse drug molecules in soybean oil (SBO, long-chain triglyceride; TGLC), Captex355 (medium-chain triglyceride; TGMC), polysorbate 80 (PS80; surfactant) and PEG400 co-solvent and used as responses during PLS model development. Melting point and calculated molecular descriptors were used as variables and the PLS models were validated with test sets and permutation tests.

RESULTS

Solvation capacity of SBO and Captex355 was equal on a mol per mol scale (R (2) = 0.98). A strong correlation was also found between PS80 and PEG400 (R (2) = 0.85), identifying the significant contribution of the ethoxylation for the solvation capacity of PS80. In silico models based on calculated descriptors were successfully developed for drug solubility in SBO (R (2) = 0.81, Q (2) = 0.76) and Captex355 (R (2) = 0.84, Q (2) = 0.80). However, solubility in PS80 and PEG400 were not possible to quantitatively predict from molecular structure.

CONCLUSION

Solubility measured in one excipient can be used to predict solubility in another, herein exemplified with TGMC versus TGLC, and PS80 versus PEG400. We also show, for the first time, that solubility in TGMC and TGLC can be predicted from rapidly calculated molecular descriptors.

A novel class of small-molecule caspase-3 inhibitors prepared by multicomponent reactions

A series of tetra- and pentasubstituted polyfunctional dihydropyrroles 5 and 6 were synthesized via practical multicomponent reactions (MCRs) for research on their structure-activity relationship as caspase-3 inhibitors. Among 39 compounds evaluated, 14 of them exhibited inhibition against caspase-3 with IC(50) ranging from 5 to 20 μM. The inhibitory activities of 5 and 6 depend on the nature of substituents on different positions. 5 and 6 possess a different scaffold from those previously reported and are the first caspase-3 inhibitors prepared via MCRs. The most active compounds 5k (IC(50) = 5.27 μM) could therefore be used as a lead for the development of highly potent caspase-3 inhibitors as drug candidates for therapeutic agents by taking advantage of MCRs.

Lipid nanoparticles for the delivery of poorly water-soluble drugs

OBJECTIVES

This review discusses important aspects of lipid nanoparticles such as colloidal lipid emulsions and, in particular, solid lipid nanoparticles as carrier systems for poorly water-soluble drugs, with a main focus on the parenteral and peroral use of these carriers.

KEY FINDINGS

A short historical background of the development of colloidal lipid emulsions and solid lipid nanoparticles is provided and their similarities and differences are highlighted. With regard to drug incorporation, parameters such as the chemical nature of the particle matrix and the physicochemical nature of the drug, effects of drug partition and the role of the particle interface are discussed. Since, because of the crystalline nature of their lipid core, solid lipid nanoparticles display some additional important features compared to emulsions, their specificities are introduced in more detail. This mainly includes their solid state behaviour (crystallinity, polymorphism and thermal behaviour) and the consequences of their usually non-spherical particle shape. Since lipid nanoemulsions and -suspensions are also considered as potential means to alter the pharmacokinetics of incorporated drug substances, some underlying basic considerations, in particular concerning the drug-release behaviour of such lipid nanodispersions on dilution, are addressed as well.

CONCLUSIONS

Colloidal lipid emulsions and solid lipid nanoparticles are interesting options for the delivery of poorly water-soluble drug substances. Their specific physicochemical properties need, however, to be carefully considered to provide a rational basis for their development into effective carrier systems for a given delivery task.

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.

Interactions between local anesthetics and lipid dispersions studied with liposome electrokinetic capillary chromatography

In the case of local anesthetic intoxication, intravenous administration of lipid-based Intralipid dispersion (Fresenius Kabi) can be used for the entrapment of hydrophobic drugs. Our long-term aim is to develop a sensitive, efficient, and non-harmful lipid-based formulation to specifically trap harmful substances. In this study liposome electrokinetic capillary chromatography (LEKC) was used to study the interactions between local anesthetics and Intralipid or liposome dispersions. Intralipid dispersion and extruded liposomes with different concentrations of 1-palmitoyl-2-oleyl-sn-glycerophosphatidylcholine (POPC), phosphatidylglycerol, cardiolipin, cholesterol, oleic acid, and linoleic acid were used as a pseudostationary phase in LEKC and their interactions with lidocaine, prilocaine, and bupivacaine were studied. POPC liposomes containing 1mol% of palmitoyl-2-[12-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine as a fluorescent marker were used for the first time in LEKC connected with laser-induced fluorescent detection in order to calculate the retention factor for anesthetics.

Intra-articular depot formulation principles: role in the management of postoperative pain and arthritic disorders

The joint cavity constitutes a discrete anatomical compartment that allows for local drug action after intra-articular injection. Drug delivery systems providing local prolonged drug action are warranted in the management of postoperative pain and not least arthritic disorders such as osteoarthritis. The present review surveys various themes related to the accomplishment of the correct timing of the events leading to optimal drug action in the joint space over a desired time period. This includes a brief account on (patho)physiological conditions and novel potential drug targets (and their location within the synovial space). Particular emphasis is paid to (i) the potential feasibility of various depot formulation principles for the intra-articular route of administration including their manufacture, drug release characteristics and in vivo fate, and (ii) how release, mass transfer and equilibrium processes may affect the intra-articular residence time and concentration of the active species at the ultimate receptor site.

What determines drug solubility in lipid vehicles: is it predictable?

Lipid-based drug delivery systems are of increasing interest to the pharmaceutical scientist because of their potential to solubilize drug molecules that may be otherwise difficult to develop. The ability to predict lipid solubility is an important step in being able to identify the right excipients to solubilize and formulate drugs in lipid formulations. However, predicting lipid solubility is complicated by the fact that interfacial effects may play a dominant role in these mixtures and the solubility may be affected by the microstructure (microemulsions, emulsions, oily solutions, etc.), as well as by the physicochemical properties of the oil, surfactant, co-solvent, and the drug. This review illustrates the fundamental factors that govern solubility in lipid mixtures and discusses models built at varying levels of sophistication to estimate the solubility. Examples from the literature are presented that demonstrate the application of these models, how their choice is related to the drug/lipid employed, and the challenges involved in solubility prediction. New data on the role water plays in altering lipid solubility, not only through its interaction with the solute, but also by changing the structure of lipids by promoting lipid organization are highlighted. The available data demonstrate that a rational understanding of solubilization in lipids is a worthwhile pursuit and models to predict at least the relative solubility from chemical structure have potential. Prediction of absolute solubility is more difficult as it requires knowledge of the drug's escaping tendency from the crystalline state. In recent years, it has become amply clear that for polar solutes, specific interactions are a critical factor governing solubility. Methods that can better take into account the specific as well as non-specific interactions between the solute and solvent, and the lipid microstructure, hold considerable promise for the future.

Long acting local anesthetic–polymer formulation to prolong the effect of analgesia

Prolonged postoperative analgesia cannot be achieved using single injections of local anesthetic solutions. The study objective was to evaluate the efficacy and toxicity of a new formulation of bupivacaine loaded in an injectable fatty acid based biodegradable polymer poly(sebacic-co-ricinoleic acid) for producing motor and sensory block when injected near the sciatic nerve. Bupivacaine was dissolved in poly(fatty ester-anhydride) paste and tested for drug release in vitro and in vivo after injection in mice. The efficacy and toxicity of the polymer-drug combination was determined by injecting the polymer formulation near the sciatic nerve of mice and measure the sensory and motor nerve blockade for 48 h, while monitoring the animal general health and the injection site. Seventy percent of the incorporated drug was released during 1 week in vitro. Single injection of 10% bupivacaine in the polymer caused motor and sensory block that lasted 30 h. Microscopic examination of the injection sites revealed only mild infiltration in three of eight examined tissues with no pathological findings for internal organs were found. In conclusion the polymer poly(sebacic-co-ricinoleic acid) is a safe carrier for prolonged activity of bupivacaine.

In vitro assessment of drug release rates from oil depot formulations intended for intra-articular administration

In vitro drug release rates from oil depot formulations intended for intra-articular injection have been investigated by using the rotating dialysis cell. The rate of drug appearance in the acceptor phase after instillation of sesame oil solutions of naproxen and lidocaine into the small aqueous donor compartment applied to first-order kinetics. In the present three-compartment model oil-aqueous phase distribution equilibrium was maintained at all times in the donor phase and thus drug efflux from the donor compartment was dictated by the distribution coefficient. A mathematical description of the rate of drug release into the acceptor phase and the interdependence of the observed apparent first-order rate constants and the drug oil-water distribution coefficients is provided. The in vitro model may constitute a valuable tool in formulation design and development allowing comparison of drug release rates originating from alteration of the oil vehicle composition, the drug compound or the composition of the release media to be performed.

The "inverted cup" -- a novel in vitro release technique for drugs in lipid formulations

The aim of this study was to develop a membrane-free in vitro release method for drugs in lipid formulations. It was intended to be applicable to as wide a range as possible of preparations, independently of their polarity and viscosity. The principle of the novel technique is to keep the sample suspended in the release medium in an inverted glass cup, allowing a possible phase transition or swelling. Thirteen formulations containing bupivacaine, lidocaine and/or prilocaine in lipid vehicles with different physical properties were prepared and examined. When possible, in vitro release profiles obtained by the new method were compared to profiles obtained by earlier techniques. For three formulations of either bupivacaine or lidocaine in polar lipid formulations, in vitro release profiles were evaluated in relation to in vivo data, from nerve block and pharmacokinetic studies in rats. Preparations that could be investigated both by the "inverted cup" and by the earlier published "single drop" technique generally showed good agreement between the two release profiles. In the case of the polar lipid formulations, arterial blood concentration curves in rats could reasonably be predicted from the in vitro release profiles. In conclusion, the "inverted cup" technique should potentially be applicable to a wide range of lipid formulations of drugs, both for physico-chemical characterisation and for obtaining 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.

Bioreversible quaternary N-acyloxymethyl derivatives of the tertiary amines bupivacaine and lidocaine—synthesis, aqueous solubility and stability in buffer, human plasma and simulated intestinal fluid

Design of water-soluble prodrugs may constitute a means to improve the oral bioavailability of drugs suffering from dissolution rate-limited absorption. The model drug bupivacaine containing a tertiary amine function has been converted into bioreversible quaternary N-acyloxymethyl derivatives. The pH-independent solubility of the N-butanoyloxymethyl derivate exceeded 1000 mg ml-1 corresponding approximately to a 10,000-fold increase in water solubility compared to that of bupivacaine base. The kinetics of hydrolysis of the prodrugs was studied in the pH range 0.1-9.8 (37 degrees C). Decomposition was found to follow first-order kinetics and U-shaped pH-rate profiles were constructed. The observed differences between the hydrolytic lability of the derivatives might most likely be ascribed to steric effects. In most cases, the prodrugs were quantitatively converted into bupivacaine. However, for the hydrolysis of the N-butanoyloxymethyl derivative at neutral to slightly alkaline pH parallel formation of bupivacaine (approximately 80%) and an unknown compound X (approximately 20%) was observed. LC-MS analysis of the latter compound suggests that an aromatic imide structure has been formed from an intramolecular acyl transfer reaction involving a nucleophilic attack of the amide nitrogen atom on the ester carbonyl carbon atom. Whereas the derivatives were poor substrates for plasma enzymes; they were hydrolyzed rapidly to parent bupivacaine in the presence of pancreatic enzymes (simulated intestinal fluid) at 37 degrees C. The data indicate that such prodrugs possess sufficient stability in the acidic environment of the stomach to reach the small intestine in intact form where they can be cleaved efficiently by action of pancreatic enzymes prior to drug absorption. Thus, the N-acyloxymethyl approach might be of potential utility to enhance oral bioavailability of tertiary amines exhibiting pKa values below approximately 6 and intrinsic solubilities in the low microM range.

Kinetics of degradation and oil solubility of ester prodrugs of a model dipeptide (Gly-Phe)

Oil-based depot formulations may constitute a future delivery method for small peptides. Thus, a requirement is attainment of sufficient oil solubility for such active compounds. A model dipeptide (Gly-Phe) has been converted into lipophilic prodrugs by esterification at the C-terminal carboxylic acid group. The decomposition kinetics of octyl ester of Gly-Phe (IV) has been investigated at pH 7.4 (37 degrees C) and IV was shown to degrade by first-order kinetics via two parallel pathways (1) intramolecular aminolysis resulting in formation of a 2,5-diketopiperazine and (2) hydrolysis of the ester bond producing the dipeptide. The cyclisation reaction was dominating in the decomposition of methyl (II) butyl (III) octyl (IV) decyl (V) and dodecyl (VI) esters of Gly-Phe at pH 7.4. However, this degradation pathway was almost negligible for pH below 6. During degradation of the dipeptide esters in 80% human plasma pH 7.4 (37 degrees C) a minimal amount of cyclo(-Gly-Phe) was formed. A faster degradation of the esters in 80% human plasma pH 7.4 compared to those in aqueous solution pH 7.4 was suggested to be due to fast cleavage of the peptide bond. Low oil solubilities for Gly-Phe and the hydrochlorides of the dipeptide esters III and VI were observed. Although the solubility of Gly-Phe in oil solutions was enhanced by hydrophobic ion pairing with sodium decyl sulfonate the oil solubility was still less than 1 mg Gly-Phe/ml. By addition of a solubiliser, 10% N,N-dimethylacetamide (DMA), to Viscoleo the solubility of the HIP complexes increased significantly. The present study indicates that sufficient oil solubility might only be obtained for relatively small peptides by using the prodrug approach in combination with solubility enhancing organic solvents like DMA.

Assessment of the combined approach of N-alkylation and salt formation to enhance aqueous solubility of tertiary amines using bupivacaine as a model drug

Quaternary prodrug types of poorly water-soluble tertiary amines have been shown to exhibit significantly enhanced solubilities as compared to the parent amine. In the present study the combined effect of N-alkylation and salt formation to enhance aqueous solubility of tertiary amines have been investigated using bupivacaine as a model compound. X-ray structure analyses of selected salts were included to investigate the potential existence of correlations between salt solubility and crystal packing modes. Alkyl groups were methyl, ethyl, propyl, and butyl and the derivatives were isolated as their iodide salts. Chloride, mesylate, formate, acetate, glycolate, and tosylate salts were obtained by anion exchange of the N-methyl-bupivacaine derivative. N-Alkylation and salt formation afforded quaternary ammonium salts possessing pH-independent aqueous solubilities far exceeding that of the parent tertiary amine (up to a factor of 3200 at pH 8). A moderate reduction in solubility with increasing length of the alkyl chain was observed for the iodide salts of the N-alkylated bupivacaine derivatives. In case of the N-methyl-bupivacaine derivative variation of the counterion had a significant impact on the solubility with the iodide salt being 200 times less soluble than the chloride salt. X-ray analysis revealed that both the alkyl substituent and the anionic counterion influenced salt packing modes, however, in an unpredictable manner making establishment of quantitative correlations between crystal packing and solubility difficult even for a series of closely related derivatives.

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.

Transdermal penetration of vasoconstrictors--present understanding and assessment of the human epidermal flux and retention of free bases and ion-pairs

PURPOSE

As reductions in dermal clearance increase the residence time of solutes in the skin and underlying tissues we compared the topical penetration of potentially useful vasoconstrictors (VCs) through human epidermis as both free bases and ion-pairs with salicylic acid (SA).

METHODS

We determined the in vitro epidermal flux of ephedrine, naphazoline, oxymetazoline, phenylephrine, and xylometazoline applied as saturated solutions in propylene glycol:water (1:1) and of ephedrine, naphazoline and tetrahydrozoline as 10% solutions of 1:1 molar ratio ion-pairs with SA in liquid paraffin.

RESULTS

As free bases, ephedrine had the highest maximal flux, Jmax = 77.4 +/- 11.7 microg/cm2/h, being 4-fold higher than tetrahydrozoline and xylometazoline, 6-fold higher than phenylephrine, 10-fold higher than naphazoline and 100-fold higher than oxymetazoline. Stepwise regression of solute physicochemical properties identified melting point as the most significant predictor of flux. As ion-pairs with SA, ephedrine and naphazoline had similar fluxes (11.5 +/- 2.3 and 12.0 +/- 1.6 microg/cm2/h respectively), whereas tetrahydrozoline was approximately 3-fold slower. Corresponding fluxes of SA from the ion-pairs were 18.6 +/- 0.6, 7.8+/- 0.8 and 1.1 +/- 0.1 respectively. Transdermal transport of VC's is discussed.

CONCLUSIONS

Epidermal retention of VCs and SA did not correspond to their molar ratio on application and confirmed that following partitioning into the stratum corneum, ion-pairs separate and further penetration is governed by individual solute characteristics.

In vivo release of bupivacaine from subcutaneously administered oily solution. Comparison with in vitro release

A non-randomized cross-over study was performed with bupivacaine HCl (5 mg x ml(-1)) aqueous solution and bupivacaine free base (4.44 mg x ml(-1)) in Viscoleo/castor oil 2:1 (v/v) administered s.c. to male Wistar rats. Plasma levels were analyzed by LC-MS. Plasma profiles obtained after administration of oily solution showed a prolonged bupivacaine release with lower peak plasma levels as compared to administration of an aqueous formulation applied in the same compartment. t(1/2), t(max), C(max) and AUC(0-infinity) for the aqueous solution were 63+/-8 min, 19+/-16 min, 194+/-46 ng x ml(-1) and 25,000+/-3000 ng min x ml(-1), respectively, while the corresponding data for the oil solution were 368+/-89 min, 334+/-186 min, 36+/-25 ng x ml(-1) and 25,000+/-6000 ng x min x ml(-1). The present data indicate the potential of designing an oil formulation of bupivacaine with a prolonged local analgetic effect exhibiting a minimum of systemic toxicity. In vivo release of bupivacaine from the oil solution was evaluated by a numerical deconvolution method. In vivo release kinetics was found to be first-order and corresponded well with in vitro release kinetics found using a rotating dialysis cell. This led to establishment of an in vitro/in vivo correlation for this particular formulation.