Characterization of prednisolone in controlled porosity osmotic pump pellets using solid-state NMR spectroscopy.

The overall objective of this study was to demonstrate the influence of formulation and processing variables on the physical state of prednisolone (PDL) in formulations consisting of PDL, microcrystalline cellulose (MCC), and sulfobutylether-beta-cyclodextrin (CD). PDL was used as a model drug in controlled porosity osmotic pump pellet (CP-OPP) formulations, and was characterized using solid-state NMR spectroscopy and other complimentary analytical techniques. Dosage forms and the solid-state properties of drugs and excipients in a formulation may be influenced by the processing conditions used. Several processing parameters, such as amount of water used in wet granulation and subsequent drying conditions, were found to affect the solid-state transformation of PDL. In addition, the presence of excipients in the CP-OPP was observed to decrease the degree of PDL crystallinity, presumably by creating an inclusion complex with the CD. A hydrated form of PDL was created when PDL was ground with water alone; however, this form was not observed in formulated products. Solid-state NMR spectroscopy was shown to be a powerful technique for the analysis of drug formulations and investigations of the effects of processing conditions.

Evaluation of various properties of alternative salt forms of sulfobutylether-beta-cyclodextrin, (SBE)7M-beta-CD.

The goal of this study was to evaluate alternative salt forms of (SBE)7M-beta-CD (currently the sodium salt). The potential salt form would ideally decrease the rate of (SBE)7M-beta-CD release from osmotic pump formulations and result in an increase in the rate and extent of drug release in osmotic pump tablet and pellet dosage forms. Several (SBE)7M-beta-CD salt forms (potassium, calcium, and two ethylene diamine salt forms) were prepared by either titration or ultrafiltration and characterized by elemental analysis and capillary electrophoresis, CE. The physical properties (water uptake behavior, osmolality, complexation characteristics, etc.) were then compared to the sodium salt form. Although the water isotherm and the binding characteristics using various model drugs were similar among all the salt forms, the calcium salt form appeared to be the best alternative candidate due to its lower osmolality and slower intrinsic dissolution rate.

Kinetics and mechanism of degradation of epothilone-D: an experimental anticancer agent.

The objective of this study was to investigate the stability and the degradation pathway of epothilone-D (Epo-D), an experimental anticancer agent. In pH range 4-9, Epo-D displayed pH-independent stability and the highest stability was observed at pH 1.5-2 where its thiazole group is protonated. Increasing the pH >9 or <1.5 resulted in an increase in the degradation rate. Epo-D contains an ester group that can be hydrolyzed. The formation of the hydrolytic product was confirmed by the nuclear magnetic resonance (NMR), fast atom bombardment mass spectroscopy and liquid chromatography/mass spectroscopy/mass spectroscopy techniques. The largely sigmoidal pH-rate profile is not consistent with the normal pH dependency of ester hydrolysis involving an addition/elimination mechanism. Hence, a hydrolysis mechanism through a carbonium ion was suggested. At pH 4 and 7.4, no buffer catalysis was observed (0.01, 0.02, and 0.05 M buffers) and no significant deuterium kinetic solvent isotope effect was noted. The degradation was very sensitive to changes in the dielectric constant of the solvents as significant enhancement in the stability was observed in buffer-acetonitrile and 0.1 M (SBE)7m-beta-cyclodextrin solutions compared with just buffer, suggesting that the rate-determining step in the degradation pathway involved formation of a polar transition state. Mass spectral analysis of the reaction run in 18O water was consistent with incorporation of the 18O in the alcohol hydroxyl rather than the carboxylate group. These observations strongly support the carbonium ion mechanism for the hydrolysis of Epo-D in the pH range 4-9. A pKa value of 2.86 for Epo-D was estimated from the fit of the pH-rate profile. This number was confirmed independently by the changes in ultraviolet absorbance of Epo-D as a function of pH (pKa 3.1) determined at 25 degrees C and the same ionic strength.

Degradation of NSC-281612 (4-[bis[2-[(methylsulfonyl)oxy]ethyl]amino]-2-methyl-benzaldehyde), an experimental antineoplastic agent: effects of pH, solvent composition, (SBE)7m-beta-CD, and HP-beta-CD on stability.

NSC-281612 (4-[bis[2-[(methylsulfonyl)oxy]ethyl]amino]-2-methyl-benzaldehyde, 1), is a chemically unstable, poorly water soluble, experimental antineoplastic agent. The saturated solubility in water at 25 degrees C was determined as approximately 30 microg/mL. In the pH range 2-11, 1 displayed pH-independent stability (t(50) was around 24 hr). However, an increase in the degradation rate was observed at pH 12. The hydrolysis of the methane sulfonate groups to the corresponding hydroxyl groups was the major degradation pathway in water in the absence of buffers and added halide ions. In phosphate buffer solutions without sodium chloride, phosphate degradants appear to be formed in addition to the mono- and dihydroxy degradants. Additional degradants, the mono- and dichloro degradation products, were formed when the ionic strength of the solution was adjusted with sodium chloride. When bromide and iodide ions were added, the corresponding mono- and dihalides were formed. The chloro compounds subsequently underwent further degradation to the hydroxy products. A deuterium kinetic solvent isotope effect study showed that water was minimally involved in the rate-determining step. The addition of either (SBE)(7m)-beta-cyclodextrin (CD) or HP-beta-CD resulted in a significant enhancement in drug solubility and stability. The apparent binding constants for HP-beta-CD and (SBE)(7m)-beta-CD were 1,486 and 2,740 M(-1), respectively. The stability of 1 in the presence of 0.1 M HP-beta-CD and (SBE)(7m)-beta-CD was enhanced 9- and 15-fold, respectively. Thus, (SBE)(7m)-beta-CD displayed better solubilization and stabilization efficacy than HP-beta-CD.

Photostability of 2-hydroxymethyl-4,8-dibenzo[1,2-b:5,4-b']dithiophene-4,8-dione (NSC 656240), a potential anticancer drug.

Stability studies of 2-hydroxymethyl-4,8-dibenzo[1,2-B:5,4-b']dithiophene-4,8-dione (NSC 656240, dithiophene), a poorly water-soluble (approximately 5 microg/ml) potential anticancer drug are reported. Dithiophene stability turned out to be very sensitive to laboratory fluorescent lighting. The rate of photodegradation of dithiophene was studied in aqueous solutions at room temperature (approximately 25 degrees C) at various pH values, in MeOH, CH(3)CN, DMF, DMA, and in mixed nonbuffered aqueous/organic solutions. The aqueous pH-rate profile indicated no sensitivity to changing pH values. 1H NMR and LC/MS methods were used to characterize the degradation products. Dithiophene photodegradation in the presence of air followed an apparent autoxidation pathway with dithiophene-2-aldehyde and dithiophene-2-carboxylic acid as the major degradants. The structures were confirmed against authentic samples. Dithiophene photodegradation under anaerobic conditions followed an apparent disproportionation pathway with only one identified major product, dithiophene-2-aldehyde.

Osmotic properties of sulfobutylether and hydroxypropyl cyclodextrins.

PURPOSE: The purpose of this study was to determine the osmolality of sulfobutylether (SBE) and hydroxypropyl (HP) derivatives of cyclodextrins (CDs) via vapor pressure osmometry (VPO) and freezing point depression (FPD). (SBE) and HP-CDs are efficient excipients capable of solubilizing and stabilizing poorly water-soluble drugs in parenteral formulations. (SBE)-CDs have also been used as solubility enhancers and osmotic agents for the sustained release of poorly water-soluble drugs from osmotic pump tablets. The knowledge of the CD's osmolality in solution or inside such tablets would allow one to further characterize the release mechanisms. METHODS: Experiments were conducted at 37 degrees C with eight types of HP and (SBE)-CDs. The aqueous solutions ranged from 0.005-0.350 mol(-1). Methods were developed to allow the measurement of high osmolalities using a vapor pressure osmometer or a differential scanning calorimeter. RESULTS: The osmolality calculations from the VPO and FPD measurements correlated well. The osmolality of (SBE)-CDs was significantly higher than the osmolality of HP-CDs and increased with the total degree of substitution (TDS). All CDs showed deviations from ideality at high concentrations. CONCLUSIONS: Empirical correlations of osmolality with concentration and TDS allowed the prediction of osmolality over a wide concentration range. This study also gave some useful insights into the behavior of CD derivatives in solution.

My mentors.

This paper traces the academic genealogy of Professor Takeru Higuchi, who is considered "The Father of Physical Pharmacy" because of his contributions to the science of drug formulation and delivery and his impact in mentoring a whole generation of post-World War II pharmaceutical scientists. As one who was mentored by Professor Higuchi (Tak), I was challenged to ask the question, "If Tak was my mentor, who was his mentor, and who were those who formed his academic lineage?" This question led me to attempt to follow Professor Higuchi's academic genealogy. I was quite surprised at what I found and I thought it would be of interest to many readers who will correctly claim direct or indirect connection to Professor Higuchi. A greater purpose of my paper is to encourage all readers to think about their mentors and to take the time for a note of gratitude (if possible) to these people who have so deeply affected their life.

Controlled and complete release of a model poorly water-soluble drug, prednisolone, from hydroxypropyl methylcellulose matrix tablets using (SBE)(7m)-beta-cyclodextrin as a solubilizing agent.

Sustained-release formulations such as hydroxypropyl methylcellulose (HPMC)-based hydrophilic matrix tablets of poorly water-soluble drugs often result in incomplete release because of the poor solubility and dissolution rate of the drug in the hydrophilic matrix. Sulfobutylether-beta-cyclodextrins ((SBE)(7M)-beta-CDs) have been known to improve the solubility of such drugs by forming inclusion complexes. The present paper deals with the modification of drug release from an HPMC-based matrix tablet of a sparingly water-soluble drug, prednisolone (PDL), using (SBE)(7M)-beta-CD as a solubilizing agent. Tablets were prepared by direct compression of a physically mixed PDL, (SBE)(7M)-beta-CD, and polymer. On exposure to water, an in situ PDL:(SBE)(7M)-beta-CD complex was formed in the gel layer, and enhanced drug release relative to a control formulation was observed (lactose used as the excipient instead of (SBE)(7M)-beta-CD ). Other possible changes due to the incorporation of (SBE)(7M)-beta-CD in the formulation were also probed. Incorporation of (SBE)(7M)-beta-CD lead to a higher water uptake relative to the control (lactose) formulation. For a fixed total tablet weight, polymer type, and loading, the drug release rate appeared to depend on the molar ratio of (SBE)(7M)-beta-CD to PDL and not the absolute amount of (SBE)(7M)-beta-CD present in the matrix tablet. This work shows that incorporation of (SBE)(7M)-beta-CD into the matrix tablets could be considered in designing a sustained-release tablet of poorly water-soluble drugs.

Effect of cyclodextrin charge on complexation of neutral and charged substrates: comparison of (SBE)7M-beta-CD to HP-beta-CD.

PURPOSE: To understand the role of charge in substrate/cyclodextrin complexation by comparing the binding of neutral and charged substrates to a neutral cyclodextrin, such as hydroxypropyl beta-CD (HP-beta-CD) with 3.5 degrees of substitution, and an anionically charged cyclodextrin, such as sulfobutyl ether beta-CD ((SBE)7M-beta-CD) with 6.8 degrees of substitution. METHOD: HP-beta-CD and (SBE)7M-beta-CD were evaluated in their ability to form inclusion complexes with neutral compounds, as well as to cationic and anionic substrates in their charged and uncharged forms. The complexation constants (Kc) were determined via a UV spectrophotometric technique, by monitoring the change in substrate absorbance upon incremental addition of a concentrated cyclodextrin solution. The role of electrostatic interaction was probed by observing Kc as a function of solution ionic strength. RESULTS: Neutral molecules displayed a stronger interaction with (SBE)7M-beta-CD compared to HP-beta-CD. In those cases where the guest possessed a charge (positive or negative), HP-beta-CD/substrate complexes exhibited a decrease in complexation strength (2 to 31 times lower) compared to the neutral forms of the same substrate. The same was true (but to a larger extent, 41 times lower) for negatively charged molecules binding to (SBE)7M-beta-CD due to charge-charge repulsion. However, positively charged molecules interacting with the negatively charged (SBE)7M-beta-CD displayed a similar binding capability as their neutral counterpart, due to charge-charge attraction. Further evaluation through manipulation of solution ionic strength revealed strong electrostatic interactions between substrate and cyclodextrin charges. In addition, the studies suggested that on average two sulfonates out of seven may be involved in forming ionic attraction or repulsion effects with the positive charges on prazosin and papaverine, or negative charges of ionized naproxen and warfarin. CONCLUSIONS: Presence of charge on the cyclodextrin structure provides an additional site of interaction compared to neutral cyclodextrins, which may be modified using solution ionic strength.

Conjugation with L-Glutamate for in vivo brain drug delivery.

In vitro studies have shown that conjugation of a model compound [p-di(hydroxyethyl)-amino-D-phenylalanine (D-MOD)] with L-Glu can improve D-MOD permeation through the bovine brain microvessel endothelial cell monolayers (Sakaeda et al., 2000). The transport of this D-MOD-L-Glu conjugate is facilitated by the L-Glu transport system. In this paper, we evaluate the in vivo brain delivery of model compounds (i.e. D-MOD, p-nitro-D-phenylalanine (p-nitro-D-Phe), 5,7-dichlorokynurenic acid (DCKA) and D-kyotorphin) and their L-Glu conjugates. DCKA was also conjugated with L-Asp and L-Gln amino acids. The analgesic activities of D-kyotorphin and its L-Glu conjugate were also evaluated. The results showed that the brain-to-plasma concentration ratio of D-MOD-L-Glu was higher than the D-MOD alone; however, the plasma concentration of both compounds were the same. The plasma concentration of p-nitro-D-Phe-L-Glu conjugate was higher than the parent p-nitro-D-Phe; however, the brain-to-plasma concentration ratio of p-nitro-D-Phe was higher than its conjugate. On the other hand, both DCKA and DCKA conjugates have a low brain-to-plasma concentration ratio due to their inability to cross the blood-brain barrier (BBB). The L-Asp and L-Glu conjugates of DCKA have elevated plasma concentrations relative to DCKA; however, the DCKA-L-Gln conjugate has the same plasma concentration as DCKA. For D-kyotorphin, both the parent and the L-Glu conjugate showed similar analgesic activity. In conclusion, conjugation of a non-permeable drug with L-Glu may improve the drug's brain delivery; however, this improvement may depend on the physicochemical and receptor binding properties of the conjugate.

Thermodynamics of binding of neutral molecules to sulfobutyl ether beta-cyclodextrins (SBE-beta-CDs): the effect of total degree of substitution.

PURPOSE: To understand the role of degree of substitution on binding of molecules to beta-Cyclodextrins (beta-CDs) with varying degrees of sulfobutyl ether (SBE) substitution. METHODS: Using UV spectroscopy, complexation constants of molecules to SBE-beta-CDs were estimated as a function of temperature, allowing for calculation of thermodynamic parameters, including the enthalpy and entropy of binding. RESULTS: Binding constants of various molecules to SBE-beta-CDs did not show a uniform trend to total degree of SBE substitution. However, a distinct pattern was observed with the enthalpy and entropy of complexation. The results showed the complexation of substrates to SBE-beta-CDs to be more entropy-favored as the number of SBE groups increased. This favorable entropy of interaction was compensated by a less favorable enthalpy of interaction. CONCLUSIONS: Enthalpy and entropy of complexation provided additional insight into the role that the alkylsulfonate groups may play in the complexation of molecules with SBE-beta-CDs.

Enhancement of transport of D-melphalan analogue by conjugation with L-glutamate across bovine brain microvessel endothelial cell monolayers.

In this paper, the L-glutamate (L-Glu) transport system was targeted to improve the delivery of a model compound, p-di(hydroxyethyl)-amino-D-phenylalanine (D-MOD), through the blood-brain barrier (BBB) in vitro cell culture model. D-MOD is an analogue of an antitumor agent D-melphalan. To target the L-Glu transport system, D-MOD was conjugated to L-Glu to give D-MOD-L-Glu conjugate. D-MOD and D-MOD-L-Glu transport properties were evaluated using the bovine brain microvessel endothelial cell (BBMEC) monolayers. The results suggest that D-MOD-L-Glu conjugate permeates through the BBMEC monolayers more readily than the parent D-MOD. The improvement of transport may be due to the recognition of D-MOD-L-Glu by the L-Glu transport system. The transport mechanism was evaluated using several different experiments including: (a) concentration-dependent studies; (b) temperature-dependent studies; (c) substrate inhibition studies; and (d) metabolic inhibitor studies. The D-MOD-L-Glu transport was inhibited by the change of temperature from 37 degrees C to 4 degrees C. At higher concentrations, the transport of D-MOD-L-Glu reached plateau due to saturation. Furthermore, some amino acids (i.e., L-Glu, L-Asp, D-Asp, and L-Gln) inhibited the transport of D-MOD-L-Glu; presumably the conjugate was competing with these amino acids for the same transport system. Metabolic inhibitors (i.e., 2,4-dinitrophenol and sodium azide) suppressed the transport of the conjugate. However, the conjugate was not transported by monocarboxylic acid, dipeptide and neutral amino acid transporters. In conclusion, the L-Glu transport system can be utilized to facilitate a non-permeable drug across the BBB by conjugating the drug with L-Glu amino acid.

Effect of conformation on the rate of deamidation of vancomycin in aqueous solutions.

The instability of vancomycin, a glycopeptide antibiotic, limits its shelf-life because the deamidation of its asparagine residue results in the formation of a zwitterion with limited aqueous solubility. Analysis of the pH-rate profile for vancomycin indicates that the deamidation reaction is notably sensitive to the ionic state of the molecule. This observation results in a hypothesis in which the ionic state of vancomycin may influence the conformation of the molecule and therefore affect its reactivity. Two-dimensional nuclear magnetic resonance (NMR), homonuclear Hartmann-Hahn (HOHAHA) and rotating frame Overhauser enhancement spectroscopy (ROESY) information combined with molecular dynamic simulations were used to estimate the apparent conformation of vancomycin in aqueous solution at pH 4 and pH 9 where the molecule exists primarily as a monocation and monoanion, respectively. The apparent conformation for vancomycin at pH 4 is compact, and the proximity of the backbone amide nitrogen to the side chain carbonyl carbon of asparagine is favorable for the rapid formation of the cyclic imide intermediate, thus increasing its reactivity. The apparent conformation for vancomycin at pH 9, however, is expanded in comparison with the conformation at pH 4, and the increase in distance between the reacting atoms leads to slower cyclic imide formation and thus decreased intrinsic reactivity. That cyclic imide formation was rate limiting at both pH values was confirmed by cyclic imide isolation and stability estimation. It becomes apparent from the analysis of the pH-rate and conformational profiles of vancomycin that the deamidation rate of vancomycin is largely influenced by the ionization state of the N-methyl leucine nitrogen.

Degradation pathways of a peptide boronic acid derivative, 2-Pyz-(CO)-Phe-Leu-B(OH)(2).

The peptide boronic acid derivative 2-Pyz-(CO)-Phe-Leu-B(OH)(2) is a potent inhibitor of 20S proteasome and a proposed anticancer agent. During preformulation studies, the compound presented erratic stability behavior. Efforts were made to isolate and identify the degradation products, thereby helping to identify possible mechanisms for the degradation. The reaction of 2-Pyz-(CO)-Phe-Leu-B(OH)(2) with hydrogen peroxide not only provided a convenient way to isolate the initial degradation products seen from hydrolysis in aqueous buffers but also showed that the major, initial degradation pathway was probably oxidative in nature. The isolated degradation products were characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and optical rotation dispersion. In the presence of hydrogen peroxide, the boronic acid group was cleaved from 2-Pyz-(CO)-Phe-Leu-B(OH)(2) to give an alcohol with an apparent retention of the original stereochemistry. Subsequent isomerization and further hydrolysis were then seen. Surprisingly, added ascorbate and EDTA accelerated rather than inhibited degradation. Degradation of 2-Pyz-(CO)-Phe-Leu-B(OH)(2) under acidic and basic conditions seemed to be mediated by an initial oxidative degradation pathway similar to that seen with the peroxide.

Cyclodextrin-catalyzed deacetylation of spironolactone is pH and cyclodextrin dependent.

The complexation of spironolactone (SP) with cyclodextrins (CDs) and the effect of pH on the CD catalyzed deacetylation of SP was studied in the presence of beta-cyclodextrin (beta-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD), sulfobutylether-beta-cyclodextrin ([SBE](7m)-beta-CD), gamma-cyclodextrin (gamma-CD), and sulfobutylether gamma-cyclodextrin (SBE-gamma-CD). The complexation of SP with beta-CD and the mechanism of deacetylation was confirmed using NMR. The complexation of SP with CDs was determined by means of the phase-solubility method at pH 2, in which chemical degradation was minimal. The phase-solubility diagrams were classified as A(L)-type and the apparent stability constants (K(1:1)) for 1 : 1 inclusion complex were calculated to be 9939 M(-1), 10,976 M(-1), 15,816 M(-1), 4792 M(-1) and 4118 M(-1) for beta-CD, HP-beta-CD, (SBE)(7m)-beta-CD, gamma-CD, and SBE-gamma-CD, respectively. The effect of pH on the degradation rate of SP was studied in the presence and absence of 4.4 mM CD solutions at pH 4, 5, 6, 7, and 8 (25 degrees C). The stability studies showed that CD-catalyzed degradation of SP can be decreased by lowering the pH. The pH-rate profiles of SP degradation with different CDs gave slopes of 1.0. Because no buffer catalysis was observed, the reaction appears to be specific-base catalyzed. The catalytic activity of CDs was as follows: SBE-gamma-CD < (SBE)(7m)-beta-CD < HP-beta-CD approximately gamma-CD < beta-CD. NMR studies confirmed that SP forms an inclusion complex with beta-CD and complexation occurs by means of the secondary face. The NMR studies also showed that during the deacetylation of SP, the secondary hydroxyl groups of beta-CD at the 2- and 3-position were acetylated. The decrease of catalytic activity of CDs at low pH values and the CDs differing ability to catalyze the degradation of SP correlated qualitatively with the ionization state of the CD hydroxyl groups, which were lower in SBE-CDs. The site of binding differences and the number of hydroxyl groups present probably also contribute to the differences.

Comparative effects of (SBE)7m-beta-CD and HP-beta-CD on the stability of two anti-neoplastic agents, melphalan and carmustine.

The purpose of this study was to evaluate and compare the potential use of two parenterally safe beta-cyclodextrins derivatives, (SBE)7m-beta-CD and HP-beta-CD, as solubilizers and stabilizers for melphalan and carmustine, two very unstable antineoplastic agents. Phase solubility and chemical stability of the compounds in the presence of the cyclodextrins were studied. UV, fluorescence, and several NMR techniques were used to probe the potential causes for the differences observed. The phase solubility method was found to provide only qualitative data on the binding of melphalan to the cyclodextrins since rapid degradation and the presence of products of degradation complicated the interpretation of the results. Qualitatively, however, the solubilizing potential was similar for the two cyclodextrins. The chemical stability studies indicate that both of the drugs had similar binding constants for both cyclodextrins; however, the intrinsic reactivities in the complexes were significantly lower with (SBE)7m-beta-CD than for HP-beta-CD. The main cause for this distinct difference appeared to correlate with differences in the site of binding and the polarity of the binding site.

New injectable melphalan formulations utilizing (SBE)(7m)-beta-CD or HP-beta-CD.

The objective of this work was to evaluate the potential of using (SBE)(7m)-beta-CD and HP-beta-CD as enabling excipients to improve on the current melphalan injectable formulation. Melphalan is an anti-neoplastic agent formulated for parenteral use as a sterile, non-pyrogenic, freeze-dried powder. It is marketed by Glaxo-Wellcome as ALKERAN((R)) for Injection (Alkeran). A major concern with melphalan therapy, other than its intrinsic cytotoxicity and biocompatibility, arises from its marginal aqueous solubility and chemical stability; thus, co-solvents are used in the current two-vial formulation. Because of the two-vial system, the product is also inconvenient to use. Two approaches to improve melphalan's formulation utilizing cyclodextrins, including the use of aqueous (SBE)(7m)-beta-CD or HP-beta-CD solutions as the reconstitution diluents, and/or the use of (SBE)(7m)-beta-CD as a freeze-drying excipient in a melphalan formulation, are presented. Results showed that, when the cyclodextrins were used as diluents, the use of organic co-solvents can be eliminated and the shelf-life of the reconstituted melphalan greatly enhanced. When the freeze-dried melphalan/(SBE)(7m)-beta-CD formulation was prepared, the formulation was found to be stable; and a simplified one-vial delivery system was achieved. In conclusion, the parenterally safe beta-cyclodextrins derivatives can provide promising alternatives and improved formulations for melphalan injectable and perhaps similar problematic drugs.

Variation in the diffusion and release of ribonuclease through and from esterified hyaluronic acid membranes: effect of changes in matrix characteristics.

50 h) were detected for the transport and release of a model protein (ribonuclease A) compared with that for the translucent region which showed no lag time. The results highlight the importance of carefully controlling matrix formation to ensure reproducible transport and release characteristics from polymer matrices.

A novel prodrug approach for tertiary amines. 2. Physicochemical and in vitro enzymatic evaluation of selected N-phosphonooxymethyl prodrugs.

Quaternary amine prodrugs resulting from N-phosphonooxymethyl derivatization of the tertiary amine functionality of drugs represents a novel approach for improving their water solubility. Separate reports have demonstrated the synthetic feasibility and rapid and quantitative prodrug to parent drug conversion in rats and dogs. This work is a preliminary evaluation of the physicochemical and in vitro enzymatic reversion properties of selected prodrugs. The loxapine prodrug had over a 15 000-fold increase in aqueous solubility relative to loxapine free base at pH 7.4. The loxapine prodrug was also shown to be quite stable at neutral pH values. The time for degradation product (parent drug) precipitation from an aqueous prodrug formulation would be expected to dictate the shelf life. Using this assumption, together with solubility and elevated temperature chemical stability studies, the shelf life of a parenteral formulation of the loxapine prodrug was projected to be close to 2 years at pH 7.4 and 25 degrees C. In addition, the prodrugs of cinnarizine and loxapine have been shown to be substrates for alkaline phosphatase, an enzyme found throughout the human body, and revert to the parent compound in its presence. The results from these evaluations demonstrate that the derivatives examined have many of the ideal properties required for potential clinical application.

A novel prodrug approach for tertiary amines. 3. In vivo evaluation of two N-phosphonooxymethyl prodrugs in rats and dogs.

N-phosphonooxymethyl derivatives of tertiary amine containing drugs have been identified as a novel prodrug approach for improving aqueous solubility. The in vivo reversion of two prodrugs to the corresponding parent compounds following iv and im administration to rats and dogs was investigated. Equimolar doses of parent drugs (loxapine or cinnarizine) and the corresponding prodrugs were each administered via a rapid iv infusion to rats and dogs. Equimolar doses of loxapine and its prodrug were each administered im to rats only. Blood samples were collected over 12 h, and plasma was assayed for both parent drug and intact prodrug by HPLC. Comparison of the plasma AUC for the parent drugs following administration of the parent drugs and prodrugs allowed estimation of the apparent bioavailability of parent drug from prodrug dosing. Plasma levels of the prodrugs fell below the limit of detection 5 min after iv infusion with an approximate half-life of 1 min. The mean AUCs following iv and im dosing of parent drugs were not statistically different from the parent drug AUCs obtained after prodrug dosing. The results are consistent with rapid and quantitative prodrug to parent drug reversion following administration of the phosphonooxymethyl prodrugs to the rats and dogs. This information, together with previous studies on the synthesis and physicochemical evaluation of the prodrugs, suggests that this novel prodrug strategy is a very promising approach for overcoming solubility limitations seen with many tertiary amine containing drugs at physiological pH values.