Redox derivatives of tranylcypromine: syntheses, properties, and monoamine oxidase inhibitor activity of some chemical delivery systems

Several brain-targeting chemical delivery systems (CDS) based on a dihydropyridine----pyridinium salt type redox system were synthesized for the monoamine oxidase (MAO) inhibitor tranylcypromine (TCP). The dihydronicotinate moiety was chemically attached to the amino group of TCP by either an amide or substituted carbamate linkages. Physicochemical studies of the new derivatives, including chromatographic Rm determinations, were performed. Only the substituted carbamate-type derivatives manifested an increased lipophilicity relative to the parent compound. In vitro oxidation stability studies were also performed on selected derivatives using a ferricyanide-mediated method. Results of this assay showed that the dihydropyridine-type derivatives oxidized to the respective quaternary salt forms with stabilities which empirically correlated with other effective CDSs. Preliminary in vivo studies performed in rats indicated that some of the new derivatives exerted significant biological activity.     

Brain-specific chemical delivery systems for beta-lactam antibiotics. In vitro and in vivo studies of some dihydropyridine and dihydroisoquinoline derivatives of benzylpenicillin in rats

Four chemical delivery systems (CDS's) based on a dihydropyridine----quaternary pyridinium salt redox system were used for the brain delivery of benzylpenicillin (BP). CDS's 5 and 9 are diesters of C1 and C2 diols in which one hydroxyl group is esterified by the benzylpenicillin-3-carboxylic group and the other by dihydrotrigonelline. CDS's 13a and 17 are benzylpenicillin esters of amino alcohols in which the amine group is acylated by dihydro-trigonelline (13a) or by 1,2-dihydro-2-methyl-4-isoquinolinecarboxylic acid (17). In vitro relative stability studies showed that both CDS's and quaternary pyridinium salts were quite unstable in rat and rabbit blood or brain but much more stable in dog or human blood. Kinetic studies performed in rat brain homogenate demonstrated the facile enzymatic oxidation of the CDS's to the corresponding quaternary salts. Hydrolysis of the CDS's and the quaternary salts resulted in the release of benzylpenicillin. In biological media CDS 13a also yielded a water addition product, the 6-hydroxy-1,4,5,6-tetrahydropyridine derivative. In vivo distribution studies were carried out in rats. After iv administration of equimolar doses of BP and CDS's, brain benzylpenicillin levels were found to be substantially higher and more prolonged in case of 5 and 9 than of BP itself. However, administration of 13a and 17 resulted in lower brain benzylpenicillin levels due to the water addition reaction and a nonspecific brain delivery, respectively. The remarkable increase of BP levels as well as the prolonged effect after the administration of 5 and 9 is a result of an improved penetration through the blood-brain barrier of the lipophilic CDS's and a "lock-in" effect of the corresponding quaternary salts generated in situ.     

Improved anticonvulsant activity of phenytoin by a redox brain delivery system I: Synthesis and some properties of the dihydropyridine derivatives

Nine chemical delivery systems (CDSs) were synthesized for the efficient transport of phenytoin (DPH) across the blood-brain barrier. The CDSs were based on a dihydropyridine in equilibrium quaternary pyridinium ion redox system which relies on chemistry similar to the NADH in equilibrium NAD interconversion for activity. The chemical carriers, derivatives of trigonelline, 1-alkylcarboxynicotinamide, 3-pyridylacetic acid, and N-methylpicolinic acid, were esterified with 3-(hydroxymethyl)phenytoin. The CDSs proved to be more lipophilic (5-23 times) than DPH. The 1-alkylcarboxydihydronicotinamide CDSs, excluding the sterically hindered one (11e), were quite unstable in rat tissue homogenates and hydrolyzed to release DPH. In human blood, however, they were found to be much more stable (75 times) toward hydrolysis. All other CDSs were oxidized quantitatively to the corresponding pyridinium ion in rat brain homogenates. These compounds were found to possess the required physicochemical characteristics for delivering DPH into rat brain.     

Redox targeting of LY231617, an antioxidant with potential use in the treatment of brain damage

Several brain-targeting chemical delivery systems (CDS) based on a dihydropyridine ⇌ pyridinium salt-type targetor were synthesized and evaluated for LY231617 (1), a di-tert-butylated phenolic amine antioxidant with potential use in the treatment of brain injuries. The dihydropyridine moiety was chemically attached to the amine (by either amide or various substituted carbamate linkages) or to the phenolic hydroxyl (by carboxylic ester linkage) functionalities of LY231617. In vitro stability and in vivo tissue distribution studies (in the rat) were performed with the novel derivatives. The results indicated that a simple amide-type CDS demonstrated efficient delivery of LY231617-targetor conjugate to the CNS. This derivative which contains the intact pharmacophore might possess intrinsic pharmacological antioxidant activity. Favorable in vitro properties suggested that a substituted carbamate-type CDS might be a better delivery modality for LY231617.     

Metabolism in rats of NC100692, an RGD-peptide for imaging of angiogenesis

The 99mTc complex of NC100692 is being evaluated as a diagnostic agent for imaging of angiogenesis. We here report in vivo studies performed with NC100692 and 14C-labelled NC100692 using liquid chromatography coupled to either an ion-trap mass spectrometer or a radiochemical detector. Following injection of 14C-labelled NC100692, only the parent compound and no metabolites was observed in rat blood, whereas no parent compound and only 1 metabolite was observed in urine. Analysis of rat urine samples with liquid chromatography with mass spectrometric detection following administration of NC100692 verified the absence of the parent compound and showed the presence of 2 metabolites. The structures of the 2 metabolites were identified based on mass spectra and accurate mass determinations. The major metabolite was identified as the molecule obtained following hydrolytic cleavage at the end of the C-terminal amino acid of NC100692. The minor metabolite was identified as that obtained following removal of phenylalanine within the cyclic structure of the major metabolite.     

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.     

Oral delivery of 1,3-dicyclohexylurea nanosuspension enhances exposure and lowers blood pressure in hypertensive rats

Cytochrome P450-derived epoxyeicosatrienoic acids (EET) are biologically active metabolites of arachidonic acid that have potent effects on renal vascular reactivity and tubular ion transport and have been implicated in the control of blood pressure. EETs are hydrolyzed to their less active diols, dihydroxyeicosatrienoic acids (DHET), by the enzyme soluble epoxide hydrolase (sEH). 1,3-dicyclohexylurea (DCU), a potent sEH inhibitor, lowers systemic blood pressure in spontaneously hypertensive rats when dosed intraperitoneally. However, DCU has poor aqueous solubility, posing a challenge for in vivo oral delivery. To overcome this limitation, we formulated DCU in a nanosuspension using wet milling. Milling reduced particle size, increasing the total surface area by approximately 40-fold. In rats chronically infused with angiotensin II, the DCU nanosuspension administered orally twice daily for 4 days produced plasma exposures an order of magnitude greater than unmilled DCU and lowered blood pressure by nearly 30 mmHg. Consistent with the mechanism of sEH inhibition, DCU increased plasma 14,15-EET and decreased plasma 14,15-DHET levels. These data confirm the antihypertensive effect of sEH inhibition and demonstrate that greatly enhanced exposure of a low-solubility compound is achievable by oral delivery using a nanoparticle drug delivery system.     

Photoinactivation of the mu opioid receptor using a novel synthetic morphiceptin analog

The benzophenone chromophore has been incorporated into a synthetic amino acid (p-benzoyl-L-phenylalanine; L-Bpa) to produce a chemically stable photoaffinity probe. L-Bpa was found to retain the photochemical reactivity of benzophenone. To test the utility of this synthetic amino acid as a photo-reactive probe for receptors, a tetrapeptide analog of morphiceptin was made as a model peptide in which the C-terminal prolinamide was replaced by L-Bpa amide. The affinity of the mu opioid receptor for this peptide is comparable to that for the parent compound, morphiceptin. Irradiation of the peptide-receptor complex reduced the subsequent binding of [3H]naloxone and virtually eliminated that of [3H]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAGO). Binding studies with [3H]naloxone indicated that both the affinity and the capacity were reduced. Competition studies with [3H]D-Ala2-D-Leu5-enkephalin (DADLE) and naloxone indicated selective inactivation of a mu type opioid receptor.     

1, 2-dihydroisoquinoline-N-acetic acid derivatives as new carriers for specific brain delivery I: synthesis and estimation of oxidation kinetics using multivariate calibration method

In order to overcome the slow oxidation of 1, 2-dihydro-N-alkylisoquinoline, 1, 2-dihydroisoquinoline-N-acetic acid derivatives (3b-d) were designed and synthesized as new chemical delivery systems (CDS) for the brain. Molecular orbital calculations for the suggested derivatives revealed that these carriers are stable against oxidation. However, hydrolysis to their corresponding anions will accelerate the rate of oxidation, and accordingly increase the efficiency of brain specific delivery. A multivariate calibration method for in vitro determination of the oxidation rate for the suggested brain specific chemical delivery system is described. The method is based on measurement of individual rates of oxidation of prepared 1, 2-dihydroisoquinolines, using silver ions to provide the corresponding quaternary forms. Components of binary mixtures formed through the oxidation step (composed of the dihydro-compound and its quaternary form), showed a considerable degree of spectral overlapping - more than 90% in all cases. Resolution of these binary mixtures under investigation has been accomplished mainly using classical least squares analysis. Kinetic oxidation data of the tested compounds revealed that these new CDSs have a reasonable oxidation rate for efficient brain delivery.     

A Dihydroisoquinoline Targetor-Based Acid Resistant Chemical Delivery System of Azidothymidine (AZT)

A 1,2-dihydroisoquinoline targetor-based brain-specific chemical delivery system (CDS) (6) was prepared for 3'-deoxyazidothymidine (AZT). The synthesis involved acylation of the 5'-OH group of the AZT by the anhydride of isoquinoline-4-carboxylic acid, followed by N-methylation with methyl iodide of the resulting ester and subsequent sodium borohydride reduction of the quaternary salt. Lipophilicity and stability studies in buffers and biological tissues were performed for 6 in comparison with a similar 1,4-dihydropyridine targetor-based CDS of AZT (7). The dihydroisoquinoline derivative 6 proved to be more lipophilic than 7 and quite stable to acidic conditions, including in pH 1.2 simulated gastric fluid, while the dihydropyridine analog rapidly degraded under these conditions as a result of an irreversible water addition reaction. The oxidation of the isoquinoline CDS in brain tissue was some-what slow compared to the dihydropyridine derivative. The study indicated that the isoquinoline-based CDS, 6, generally possesses the properties required for oral administration.     

Study of the ameliorating effects of an enteral nutrient for liver failure on hepatic encephalopathy: effects of SF-1008C on plasma and brain free amino acids, intracerebral amine concentrations and electroencephalogram in portacaval shunted rats with ammonia loading

The ameliorating effects of an enteral nutrient for liver failure (SF-1008C), which is enriched with branched-chain amino acids (BCAA) and includes few aromatic amino acids (AAA), were investigated. The blood ammonia, plasma and brain free amino acids, intracerebral amine concentrations and electroencephalogram were measured in portacaval shunted rats with 10% ammonium acetate (3 ml/kg, i.p.) (PCS) as a model of hepatic encephalopathy. The blood ammonia and plasma free amino acid concentrations in PCS rats were significantly increased in comparison to sham-operated (Sham) rats. Thus, the plasma BCAA/AAA ratio in PCS rats was appreciably reduced. Concomitant with the abnormal plasma amino acid concentrations, the brain free amino acid concentrations in PCS rats were markedly increased in comparison to the Sham rats. Moreover, the intracerebral tryptophan (Trp) and 5-hydroxyindol acetic acid (5-HIAA) concentrations were significantly increased, and the intracerebral dopamine (DA) concentration was significantly decreased in the PCS rats. The intracerebral serotonin (5-HT) and norepinephrine (NE) concentrations were, however, hardly changed. A smaller voltage for the electroencephalogram was used in the PCS rats than in the Sham rats. Abnormal plasma and brain free amino acid concentrations in PCS rats were normalized by oral administration of SF-1008C, and the low voltage electroencephalograms in the PCS rats were suppressed. On the other hand, abnormal plasma and brain free amino acid concentrations in the PCS rats were hardly normalized by oral administration of ED-AC, an elemental diet based on an amino acid composition of egg protein. These results suggest that SF-1008C affects brain free amino acids, intracerebral amine concentrations and electroencephalogram by ameliorating abnormal plasma free amino acid concentrations. Moreover, there is a highly significant correlation between the plasma BCAA/AAA ratio and the brain BCAA/AAA ratio, and this finding suggests that the plasma free amino acid patterns reflect the brain free amino acid patterns.     

Enhancement of Transport of D-Melphalan Analogue by Conjugation with L-Glutamate across Bovine Brain Microvessel Endothelial Cell Monolayers

Abstract

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°C to 4°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 2-hydroxypropyl-beta-cyclodextrin on the solubility, stability, and pharmacological activity of the chemical delivery system of TRH analogs

To improve the aqueous solubility and stability of the chemical delivery system (CDS) of the thyrotropin-releasing hormone (TRH) analogs, 2-hydroxypropyl-beta-cyclodextrin (HPBCD) has been attempted. TRH analogs were [Leu2]-TRH, [Nva2]-TRH and [Nva2, Pip3]-TRH. Excess amount of CDS was added in various HPBCD in water solutions (0%-50%, pH 6.5). The mixture was saturated by ultra-sonication for 1 h at 15 degrees C and filtered. The concentration of CDS in the filtrate (solubility) was determined with UV detector, and subsequently the stability was investigated. By HPBCD complexation, the aqueous solubility and stability (half-life) of CDS were significantly improved from undetectable levels to about 15 mg/ml and 30 h, respectively. In pH 6.5 and 7.4 HPBCD solution, the degradation of CDS was mainly via acid catalyzed water addition reaction, thus, e.g. [Leu2]-TRH-CDS was more stable in pH 7.4 than in pH 6.5 aqueous solutions. After lyophilizing the saturated CDSs in 50% HPBCD complex solutions, the amount of CDS in the complex was determined as 26.22, 26.79, and 30.34 mg/g for [Leu2]-TRH, [Nva2]-TRH and [Nva2, Pip3]-TRH, respectively. The half-life of [Leu2]-TRH-CDS/HPBCD solid complex at 25 degrees C, 4 degrees C and -15 degrees C was about 100 days, 440 days and no detectable change in three months, respectively. Argon protected condition did not improve the stability of lyophilized [Leu2]-TRH-CDS/HPBCD complex. Dimethyl sulfoxide although increased the solubility of [Leu2]-TRH-CDS in the 50% HPBCD solution by 1.3 times, significantly decreased its stability by 6.6 times. After intravenous administration of CDS (in 30% HPBCD) at a dose of 10 mumole/kg in mice, compared to the vehicle control or the same dose of [Leu2]-TRH (in 30% HPBCD), a significant increase in pharmacological effect (decrease in barbiturate-induced sleeping time) was observed. These results demonstrate the usefulness of cyclodextrin in the formulation of the CDSs of TRH analogs.     

Formulation development for a zidovudine chemical delivery system 2. Towards oral and non-parenteral dosage forms

Steps toward the development of an oral dosage form for a dihydronicotinate chemical delivery system for zidovudine (AZT-CDS) were examined. Administration of the AZT-CDS by gavage to rats indicated poor bioavailability consistent with the acid lability of the CDS. Furthermore, administration of the AZT-CDS in dimethyl sulfoxide (DMSO) intraintestinally did not result in therapeutically relevant brain or blood levels of the AZT-CDS or its metabolites. Use of a liposome formulation, however, did provide for significant uptake with administration to the jejunum more effective than AZT-CDS administration to the ileum or colo-caecum. Invasive administration of AZT-CDS complexed with various chemically modified cyclodextrins to the intestine also resulted in good bioavailability. Perfusion of a section of jejunum with a solution of AZT-CDS in 2-hydroxypropyl-β-cyclodextrin (HPβCD) resulted in demonstrable AZT-CDS uptake and pre-liver/post-liver blood concentration ratio of approx. 0.5. These results suggest that an enterically coated AZT-CDS tablet may provide for pharmacologically useful oral bioavailability. A second route of administration considered was rectal dosing. AZT was significantly bioavailable from prototype suppositories in the rat and although AZT-CDS could be detected after AZT-CDS treatment, the absolute bioavailability for AZT after such treatment was low.     

Importance of tryptophan pyrrolase and aromatic amino acid decarboxylase in the catabolism of tryptophan.

Benserazide, an inhibitor of aromatic amino acid decarboxylase, potentiates the rise of plasma tryptophan in rats given a tryptophan load. It also inhibits ¹⁴CO₂ release from animals given carboxyl-labeled tryptophan. These results are explained by the ability of benserazide to inhibit tryptophan pyrrolase, the most important enzyme catabolizing tryptophan. Direct decarboxylation is not a quantitatively important pathway of tryptophan catabolism and carboxyl-labeled tryptophan is metabolized to ¹⁴CO₂ primarily by the pyrrolase pathway. These data have implications for the clinical use of tryptophan as an antidepressant. Pyridoxine, which is often given with tryptophan in clinical use, can activate aromatic amino acid decarboxylase. However, pyridoxine does not inhibit the rise of plasma tryptophan in rats given a tryptophan load and is unlikely to antagonize the therapeutic effect of tryptophan. It may be possible to potentiate the therapeutic effect of tryptophan by administering it with benserazide to inhibit its peripheral catabolism through the pyrrolase pathway.     

Tryptophan-free diet: a new means for rapidly decreasing brain tryptophan content and serotonin synthesis.

Changes in the synthesis rate of brain serotonin are positively correlated with changes in the concentration of brain tryptophan, indicating that the concentration of tryptophan in the whole brain reflects that at sites of serotonin synthesis. In turn, the concentration of brain tryptophan is positively correlated with that of free serum tryptophan (tryptophan is the only amino acid bound to serum proteins) and negatively to that of other amino acids competing with tryptophan for the same transport from blood to brain. Consistently, experiments in rats have shown that treatments which increase free tryptophan in serum (in respect to competing amino acids) also increase brain tryptophan and serotonin turnover. Conversely, the ingestion of diets containing all amino acids except tryptophan cause a dramatic fall in free serum tryptophan and a parallel decline in brain tryptophan and serotonin synthesis. In man the administration of an amino acid mixture lacking trytophan produces a marked depletion in serum tryptophan concentration.     

Intestinal absorption and biodistribution of cosalane and its amino acid conjugates: novel anti-HIV agents.

Cosalane and its amino acid conjugates are potent inhibitors of HIV replication. The purpose of this study was to investigate: (1) the pharmacokinetic disposition of the diglycine (GC) and the diaspartic acid (ASPC) conjugates of cosalane in male Sprague-Dawley rats; (2) intestinal absorption of cosalane and its amino acid conjugates using in vitro (small intestinal segments), in situ (closed loop); and (3) biodistribution of GC and its absolute oral bioavailability in rat. Cosalane and its conjugates exhibited biexponential disposition with very long half-lives upon intravenous dosing. However, these compounds failed to permeate the small intestine unless sodium desoxycholate (5-20 mM) was used as an intestinal permeation enhancer. A rank order correlation in terms of permeation enhancement in a descending order is as follows: GC>Cosalane>ASPC. In situ studies revealed that although the bile salt enhanced the permeation of cosalane across the enterocyte, its hepatic uptake was extensive. However, 66% of the absorbed dose of GC escaped uptake by the reticuloendothelial system (RES) and its biodistribution studies showed that the uptake by the RES was significantly lower compared to the parent compound. GC had an absolute oral bioavailability of 5.10+/-1.51%. Therefore, GC appears to be a favorable candidate for further development.     

Chemical delivery systems for some penicillinase-resistant semisynthetic penicillins

Chemical delivery systems (CDS's) based on a dihydropyridine----quaternary pyridinium ion redox system analogous to the naturally occurring NADH----NAD+ system were synthesized for a group of staphylococcal penicillinase resistant penicillins, including methicillin, oxacillin, cloxacillin, and dicloxacillin, in order to improve their penetration of the central nervous system (CNS). The CDS's are penicillin monoesters of gem-diols in which the other hydroxyl group is esterified by the dihydrotrigonelline carrier. The CDS's were found to be much more lipopholic than the parent drugs by comparing their log k' values used as lipophilicity indexes. A study of the chemical oxidation of the CDS's performed by a UV spectrophotometric method showed relatively slow reaction. Stability studies were performed in buffers and different animal tissues for both the CDS's and the quaternary salt type derivatives. These studies showed that the CDS's were oxidized to the quaternary salt forms at neutral and basic pH and added water at lower pH. The quaternary salts released the parent drugs both in buffers and in vitro. A preliminary in vivo distribution study in the rat and rabbit demonstrated blood-brain barrier (BBB) penetration by the CDS, whereas no drug was detected by administering the drug itself.     

Brain-specific delivery of naproxen using different carrier systems

Naproxen is one of the most potent NSAIDs and plays an important role in the treatment of neurodegenerative diseases. Poor brain delivery of naproxen at therapeutic doses, in addition to its serious gastrointestinal side effects, has prompted research into the development of a specific carrier system that is capable of delivering naproxen to the brain at smaller doses. The purpose of this study was to evaluate two brain-specific carrier systems of naproxen. The first was the dihydropyridine/pyridinium redox system that utilized a lipophilic chemical delivery system coupled to the carboxylic acid group of naproxen through an ethanolamine linker. Secondly, an ascorbic acid system, which has reducing properties and acts as a biological carrier through sodium-dependent vitamin-C transporter, was used for brain-specific delivery of naproxen. The prepared prodrugs were stable in aqueous buffers (pH 1.2 and 7.4) and rapidly hydrolyzed in biological fluids. Bioavailability studies revealed that both prodrugs 10 and 17 were rapidly cleared from blood with half lives of about 1?h, which will likely decrease systemic adverse effects. The rapid clearance from the blood was accompanied by an increase in the prodrug concentration in the brain, which occurred as a result of the prodrug being more locked in compared to the parent drug naproxen.     

Basal level insulin delivery: in vitro release, stability, biocompatibility, and in vivo absorption from thermosensitive triblock copolymers

The major goal of this study was to develop the biodegradable and biocompatible thermosensitive polylactic acid-polyethylene glycol-polylactic acid triblock copolymer-based delivery systems for controlled release of basal level insulin for a longer duration after single subcutaneous injection. Insulin was dispersed into aqueous copolymer solutions to prepare the delivery system. The in vitro release profile of insulin from delivery systems was studied at 37°C in phosphate-buffered saline. Stability of released insulin was investigated using circular dichroism, differential scanning calorimetry, and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and skin histology were used to determine the in vitro and in vivo biocompatibility of the delivery systems, respectively. Streptozotocin-induced diabetic rat model was used to study the in vivo absorption and bioactivity of insulin. In vitro release studies indicated that the delivery systems released insulin over 3 months in structurally stable form. The delivery systems were biocompatible in vitro and in vivo. In vivo absorption and bioactivity studies demonstrated elevated insulin level and corresponding decreased blood glucose level in diabetic rats. Thus, the delivery systems released insulin at a controlled rate in vitro in conformationally and chemically stable form and in vivo in biologically active form up to 3 months.