A new painkiller nanomedicine to bypass the blood-brain barrier and the use of morphine

The clinical use of endogenous neuropeptides has historically been limited due to pharmacokinetic issues, including plasma stability and blood-brain barrier permeability. In this study, we show that the rapidly metabolized Leu-enkephalin (LENK) neuropeptide may become pharmacologically efficient owing to a simple conjugation with the lipid squalene (SQ). The corresponding LENK-SQ bioconjugates were synthesized using different chemical linkers in order to modulate the LENK release after their formulation into nanoparticles. This new SQ-based nanoformulation prevented rapid plasma degradation of LENK and conferred on the released neuropeptide a notable antihyperalgesic effect that lasted longer than after treatment with morphine in a rat model of inflammation (Hargreaves test). The biodistribution study as well as the use of brain-permeant and -impermeant opioid receptor antagonists indicated that LENK-SQ NPs act through peripherally located opioid receptors. This study represents a novel nanomedicine approach, allowing the specific delivery of LENK neuropeptide into inflamed tissues for pain control.

Reversible lipidization for the oral delivery of leu-enkephalin

The endogenous opioid peptide leu-enkephalin (ENK) was chemically modified by a method known as reversible aqueous lipidization (REAL) with a novel amine-reacting lipophilic dimethylmaleic anhydride analog, 3,4-bis(decylthiomethyl)-2,5-furandione. The binding affinity of the product, REAL-ENK, to opioid receptors was greatly reduced. This prodrug was stable in neutral and basic phosphate buffers but underwent rapid hydrolysis under acidic conditions in the presence of 50% acetonitrile. It also showed increased stability toward enzymatic degradations in various tissue preparations. The half-lives of REAL-ENK in mouse small intestinal mucosal homogenate and liver homogenate were 12 and 80 min, representing a 12- and 32-fold increase over those of ENK itself. In contrast to ENK (t(1/2) 6.7 min), REAL-ENK was stable in mouse plasma. More importantly, REAL-ENK produced significant and sustained antinociception mediated by peripheral opioid receptors in a rodent inflammatory pain model. Pharmacokinetic studies employing a radioimmunoassay (RIA) demonstrated that significantly higher and sustained plasma peptide levels were detected up to 24 h following the oral administration of REAL-ENK in normal mice. The peak concentration and area under the curve of oral REAL-ENK were 4.4 and 21 times higher than that of oral ENK. Our results indicate that like its disulfide-based counterpart, amine-based REAL may be an enabling technology which can be applied to enhance metabolic stability, increase oral absorption, and preserve and possibly prolong the pharmacological activity of peptide drugs.

Investigation of the route of absorption of lipid and sugar modified leu-enkephalin analogues and their enzymatic stability using the caco-2 cell monolayer system

It has been demonstrated that conjugation of lipoamino acids or glucose units to the endogenous opioid peptide, Leu-enkephalin can significantly improve the peptide's metabolic stability and absorption across biological barriers. The purpose of this study was to investigate the possible involvement of specific carrier proteins in the absorption of these peptide conjugates. A series of lipo- glycol- and liposaccharide peptide conjugates were synthesised and examined using the Caco-2 monolayer assay for evidence of interaction with the human H(+)-coupled oligopeptide transporter (hPepT1), glucose transporters and the multidrug resistance efflux pump, p-glycoprotein. The investigation involved determining the apparent permeability of each compound in the absence of any inhibitors and comparing this to the apparent permeabilities of each compound in the presence of glycylsarcosine, glucose or vinblastine, respective inhibitors of the above mentioned transporters. None of the peptide conjugates were found to be substrates for p-glycoprotein. Of the six peptide conjugates examined, only the C-terminus glucose conjugate of Leu-enkephalin (Enk-glu) showed evidence of transport by both glucose transporters and hPepT1. In contrast, N-terminus conjugation of both lipids and sugars appeared to provide the greatest protection against enzymatic degradation.

In vitro evaluation of the potential of thiomers for the nasal administration of Leu-enkephalin

It was the aim of this study to evaluate the potential of thiolated polycarbophil for the nasal administration of Leucine-enkephalin (Leu-enkephalin). The enzymatic degradation of Leu-enkephalin on freshly excised bovine nasal mucosa was analysed qualitatively via thin layer chromatography and quantitatively via high performance liquid chromatography (HPLC). The potential of thiolated polycarbophil gels to provide a sustained release for the therapeutic peptide was investigated via diffusion studies. Permeation studies were performed in Ussing-type diffusion chambers with freshly excised bovine nasal mucosa. Results demonstrated that Leu-enkephalin is mainly degraded by the cleavage of tyrosine from the N-terminus of the peptide. Within one hour more than 63.5 +/- 2% of this therapeutic peptide are degraded on the nasal mucosa. In the presence of 0.25% thiolated polycarbophil, this degradation process, however, could be significantly lowered. Diffusion studies demonstrated that Leu-enkephalin being incorporated in a 0.5% thiolated polycarbophil gel is sustained released out of it. The appearent permeability coefficient (P(app)) for Leu-enkephalin on the nasal mucosa was determined to be 1.9 +/- 1.2 x 10(-7) cm/sec. Furthermore, in the presence of 0.5% thiolated polycarbophil and 1% glutathione, which is used as permeation mediator for the thiomer, the uptake of Leu-enkephalin from the nasal mucosa was even 82-fold improved. According to these results thiolated polycarbophil might be a promising excipient for nasal administration of Leu-enkephalin.

Determination of transport in the Caco-2 cell assay of compounds varying in lipophilicity using LC-MS: enhanced transport of Leu-enkephalin analogues

PURPOSE

To synthesize a number of analogues of Leu-enkephalin with different lipophilicities and to develop an LC-MS method for determining the Caco-2 cell permeability values of these compounds.

METHODS

A number of sugar and sugar plus lipoamino acid analogues of Leu-enkephalin were synthesized by solid-phase and solution methods. An LC-MS method was developed for analyzing the Caco-2 cell assay samples and validated against the traditional method using radiolabelled compounds.

RESULTS

A sensitive and specific LC-MS assay was developed. Standard curves were linear in the range of 0.025-5 microM. Apparent permeability values determined by LC-MS and liquid scintillation counter were identical, for both a hydrophilic drug, cephalexin and a lipophilic Leu-enkaphalin analogue. Caco-2 permeability values for the analogues of Leu-enkephalin were determined and it was found that attachment of sugar or sugar and lipoamino acid to the Leu-enkephalin peptide resulted in an increase in the apparent permeability values compared to the native peptide, which was not transported across the Caco-2 cell monolayers.

CONCLUSIONS

A rapid, generic LC-MS method for analyzing a range of compounds was developed. Attachment of a sugar or sugar and lipoamino acid to Leu-enkephalin improves the apparent permeability across Caco-2 cell monolayers.

Peptidase activity on the surface of the porcine buccal mucosa

Peptide drugs in buccal bioadhesive delivery systems are exposed to the surface of the buccal mucosa at high concentrations over long periods of time. The peptidase activity on the surface of the buccal mucosa has not been evaluated as a barrier to peptide buccal delivery. The in vitro stability of various synthetic substrates on the surface of intact porcine buccal mucosa was determined. No carboxypeptidase or dipeptidyl peptidase IV activity was detected on the buccal mucosa, while aminopeptidase N activity was detected using Leu-p-nitroanilide. No endopeptidase activity was observed towards the peptide substrates. Insulin and insulin B-chain were intact at the 2 h time point at 37 degrees C, while the percent of parent Leu-enkephalin remaining was 18+/-9 (mean+/-S.D., n=9). In the presence of aminopeptidase inhibitors, amastatin, sodium deoxycholate and EDTA, the degradation of Leu-enkephalin was dramatically reduced. This work suggests that the buccal route maybe advantageous for the delivery of peptides that are susceptible to such activities. The inclusion of aminopeptidase inhibitors in buccal bioadhesive delivery systems could improve buccal bioavailability of Leu-enkephalin. We suggest that compared with the existing in vitro metabolism methods, the analysis of peptide or protein metabolism on intact buccal mucosa could better predict the degradation of the drug as it crosses the tissue.

Improved blood-brain barrier penetration and enhanced analgesia of an opioid peptide by glycosylation

Neuropeptide pharmaceuticals have potential for the treatment of neurological disorders, but the blood-brain barrier (BBB) limits entry of peptides to the brain. Several strategies to improve brain delivery are currently under investigation, including glycosylation. In this study we investigated the effect of O-linked glycosylation on Ser(6) of a linear opioid peptide amide Tyr-D-Thr-Gly-Phe-Leu-Ser-NH(2) on metabolic stability, BBB transport, and analgesia. Peptide stability was studied in brain and serum from both rat and mouse by high-performance liquid chromatography. BBB transport properties were investigated by rat in situ perfusion. Tail-flick analgesia studies were performed on male ICR mice, injected i.v. with 100 microg of peptide ligand. Glycosylation of Ser(6) of the peptide led to a significant increase in enzymatic stability in both serum and brain. Glycosylation significantly increased the BBB permeability of the peptide from a value of 1.0 +/- 0.2 microl x min(-1) x g(-1) to 2.2 +/- 0.2 microl x min(-1) x g(-1) (p < 0.05), without significantly altering the initial volume of distribution. Analgesia studies showed that the glycosylated peptide gave a significantly improved analgesia after i.v. administration compared with nonglycosylated peptide. The improved analgesia profile shown by the glycosylated peptide is due in part to an improvement in bioavailability to the central nervous system. The bioavailability is increased by improving stability and transport into the brain.

Development of novel lipophilic derivatives of DADLE (leucine enkephalin analogue): intestinal permeability characateristics of DADLE derivatives in rats

PURPOSE

The objective of this study is to examine the intestinal permeability of novel lipophilic derivatives of DADLE (Tyr-D-Ala-Gly-Phe-D-Leu), an enkephalin analogue, using isolated rat intestinal membranes.

METHODS

The novel lipophilic derivatives of DADLE were synthesized by chemical modification with various fatty acids at the C terminus. The pharmacological activities of these DADLE derivatives were assessed by a hot plate test. The intestinal permeability of these derivatives was estimated by the in vitro Ussing chamber method.

RESULTS

We obtained four different DADLE derivatives including acetyl-DADLE (DADLE-C2), butyryl-DADLE (DADLE-C4), caproyl-DADLE (DADLE-C6), and caprylyl-DADLE (DADLE-C8). All the derivatives of DADLE had at least 75% of the activity of native DADLE, suggesting that chemical modification of DADLE at the C terminus did not markedly affect its pharmacological activity. These DADLE derivatives were more stable than native DADLE in jejunal and colonic homogenates. A "bell-shaped" profile was observed between the apparent permeability coefficients (Papp) of DADLE derivatives and lipophilicity. In particular, DADLE-C4 had the greatest permeability characteristics across the intestinal membrane of the acyl derivatives studied in this experiment. The permeability of DADLE-C4 across the jejunal membrane was further improved in the presence of puromycin, amastatin, and sodium glycocholate (NaGC), all at a concentration of 0.5 mM.

CONCLUSIONS

We suggest that the combination of chemical modification with butyric acid and the application of a protease inhibitor are effective for improving the absorption of DADLE across the intestinal membrane.

In vitro skin penetration and degradation of peptides and their analysis using a kinetic model

The main purpose of this study was to estimate the net percutaneous absorption of physiologically active peptides in vitro. The degradation of two peptides, Leu-enkephalin (Enk) and Tyr-Pro-Leu-Gly amide (TPLG), during skin penetration and on the dermal side following penetration, and the prevention of degradation by some protease inhibitors, were investigated using rat skin in vitro. In addition, these permeation and degradation data were analyzed using a kinetic model. These peptides were rapidly degraded in the receptor fluid of a Franz diffusion cell (rate constant: 0.977 h(-1) for Enk and 0.250 h(-1) for TPLG). The addition of phenylmethylsulfonyl fluoride (PMSF) and phenanthroline and the pretreatment of skin with these inhibitors prevented almost completely any degradation in the receptor fluid and skin, respectively. The pretreatment of skin with PMSF and phenanthroline had no effect on the penetration of dextran (1000 Da). The degradation rate constant during skin penetration, calculated from the difference in the penetration rate constants via pretreated and untreated skins, was also high (0.037 h(-1) for Enk and 0.050 h(-1) for TPLG). A kinetic model including an input rate (zero-order), the permeation rate across the viable skin (first-order) and the degradation rate in skin (first-order) was sufficient to describe the apparent steady-state flux of the peptides through skin. We have, thus, established a method for measuring the true flux of peptides across skin in vitro and a kinetic model which simply describes the skin penetration of peptides.

Mutation of tyrosine 318 (Y318F) in the delta-opioid receptor attenuates tyrosine phosphorylation, agonist-dependent receptor internalization, and mitogen-activated protein kinase activation

Opioid receptors are known for their ability to activate diverse second messenger systems. Previously, we showed that selective delta-opioid agonists were able to induce the rapid tyrosine phosphorylation of delta-opioid receptors (delta-ORs) through Src. Src-dependent tyrosine phosphorylation of delta-ORs appears to be important for activation of the mitogen-activated protein kinase cascade and for receptor sequestration into clathrin-coated endosomes, as the Src antagonist, PP1, inhibited both. In an attempt to clarify the role of tyrosine phosphorylation in delta-OR signalling and regulation, we constructed a mutant receptor in which the tyrosine located in the conserved NPXXY motif of the C-terminus was replaced by a phenylalanine (Y318F-delta-OR). Mutation of Y318 resulted in a receptor that was comparable to the wild type in its expression level in HEK-293 cells and in its affinity for opioid ligands. Both receptors showed effective coupling to G proteins and were capable of inhibiting forskolin-stimulated cAMP accumulation with similar potencies. However, the mutant receptor was able to stimulate (35)S-GTPgammaS binding with a lower EC(50) than the wild type receptor. The stimulation of tyrosine phosphorylation in delta-ORs by [D-Thr(2)]-Leu-enkephalin-Thr (DTLET) was significantly less in cells expressing the Y318F-delta-OR than in cells expressing the wild type. In addition, both rapid receptor internalization and down-regulation were markedly attenuated in the mutant. Finally, the mutant receptor was unable to induce a robust activation of the MAPK pathway, suggesting that tyrosine phosphorylation of the delta-OR protein is important for this signalling pathway. These findings implicate tyrosine phosphorylation of Y318 in receptor signalling and agonist-mediated regulation.

TR146 cells grown on filters as a model of human buccal epithelium: V. Enzyme activity of the TR146 cell culture model, human buccal epithelium and porcine buccal epithelium, and permeability of leu-enkephalin

The objective of the present study was to characterise the TR146 cell culture model as an in vitro model of human buccal mucosa with respect to the enzyme activity in the tissues. For this purpose, the contents of aminopeptidase, carboxypeptidase and esterase in homogenate supernatants of the TR146 cell culture model, and human and porcine buccal epithelium were compared. The esterase activity in the intact cell culture model and in the porcine buccal mucosa was compared. Further, the TR146 cell culture model was used to study the permeability rate and metabolism of leu-enkephalin. The activity of the three enzymes in the TR146 homogenate supernatants was in the same range as the activity in homogenate supernatants of human buccal epithelium. In the TR146 cell culture model, the activity of aminopeptidase (13.70+/-2.10 nmol/min per mg protein) was approx. four times the activity of carboxypeptidase (3.73+/-0.53 nmol/min per mg protein), whereas the level of esterase activity was significantly higher (223.39+/-69.82 nmol/min per mg protein). In the TR146 cell culture model, the apical esterase activity was found significantly higher than the basal activity, and found comparable to the porcine buccal mucosa. However, the esterase activity on the serosal side of the porcine buccal mucosa was higher than in the TR146 cell culture model. Approx. 1.5% of leu-enkephalin permeated the TR146 cell layers within 5 h (P(app) 7.38+/-0.83x10(-7) cm/s) and approx. 77% of intact peptide was still present in the donor phase after 5 h. The present study suggests that the TR146 cell culture model is a valuable in vitro model for permeability and metabolism studies with enzymatically labile drugs, such as leu-enkephalin, intended for buccal drug delivery.

Degradation kinetics of leucine5-enkephalin by plasma samples from healthy controls and various patient populations: in vitro drug effects

Incubation of [3H]tyrosine leucine5-enkephalin with platelet-poor human plasma (final concentration 1 x 10(-8) M; 1:9 ratio to Trizma base buffer, pH 7.4) resulted in rapid and complete peptide degradation in each of the subjects studied, with more than 95% of the initial labeled tyrosine consistently recovered as the free amino acid (< or =30 minutes). Essentially, and irrespective of the incubation time (1-180 minutes), tyrosine was the only Leu metabolite formed; we were unable to identify significant amounts (> or =3%) of any other possible labeled or nonlabeled Leu degradation fragments. Neither gender (64 men and 20 women; samples tested individually), age (men, 23-70; women, 25-65 years), nor the subjects' medical condition appeared to make a significant difference in either the t1/2 of Leu elimination, the initial velocity of this reaction (x +/- SD, median, minimum and maximum of 12.0 +/- 0.9, 12.0, and 10.6-13.7 minutes; 1.2 +/- 0.3, 1.1, and 0.6-2.0 pg/min, respectively), or in the Km and Vmax values for aminopeptidase Leu degradation (x +/- SD; 0.81 +/- 0.01 mM and 14.30 +/- 1.17 micromol/L/min, respectively). Subjects were diagnosed as chronic schizophrenics (n = 15), polydrug abusers including alcohol (n = 9) and polydrug abusers excluding alcohol (n = 8), chronic alcoholics (n = 12), and migraineurs (n = 10) during or outside an acute migraine episode; for comparison we used a group of gender-matched (20 men and 10 women), age-comparable, drug-free, healthy volunteers. Differences in plasma storage time or repeated sample freezing and thawing failed to alter significantly any of these kinetic parameters of Leu metabolism or to change the identity and/or relative ratio of the products formed. The Leu degradation rate was pH and temperature dependent (optimum, 7.4 and 37 degrees C, respectively). Leu degradation was strongly and similarly inhibited by puromycin, bacitracin, and bestatin (IC50 [+/- SD] of 1.4 +/- 0.2 micromol/L) and to a lesser extent by various L-tyrosine-containing Leu fragments. The kinetics of this reaction was not significantly affected by either thiorphan, N-carboxyphenylmethyl leucine, or any other of a number of monoamine neurotransmitters, substances of abuse, nonsteroidal anti-inflammatory agents, and miscellaneous compounds tested (concentration up to 10(-4) mol/L).

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

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

Optimizing oral absorption of peptides using prodrug strategies

Prodrug strategies applied to peptides have tended to focus on modification of a single functional group (e.g., N-terminal end). Recently, our laboratory introduced the concept of making cyclic prodrugs of peptides as a way to modify their physicochemical properties sufficiently to allow them to permeate biological barriers (i.e., intestinal mucosa). This cyclization strategy required the development of new 'chemical linkers,' including an acyloxyalkoxy linker, a phenylpropionic acid linker, and a coumarinic acid linker. All three chemical linkers were designed to be susceptible to esterase metabolism (slow step), leading to a cascade of chemical reactions (fast steps) that result in release of the peptide. These cyclic prodrug strategies have been applied to opioid peptides in an attempt to stabilize them to metabolism and/or improve their intestinal mucosal permeation. Specifically, we prepared acyloxyalkoxy-, phenylpropionic acid- and coumarinic acid-based cyclic prodrugs of [Leu(5)]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and its metabolically stable analog DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH) and determined their metabolic and biopharmaceutical properties. The cyclic prodrugs of these opioid peptides were shown to have: (i) favorable physicochemical properties (e.g., increased lipophilicity) for membrane permeation; (ii) unique solution structures (e.g., beta-turns) that reduce their hydrogen bonding potential; and (iii) metabolic stability to exo- and endopeptidases. The cell membrane permeation characteristics of [Leu(5)]-enkephalin, DADLE and the cyclic peptide prodrugs were evaluated using Caco-2 cell monolayers, a cell culture model of the intestinal mucosa. The phenylpropionic acid- and coumarinic acid-based cyclic prodrugs of [Leu(5)]-enkephalin and DADLE were shown to have significantly better cell permeation characteristics than the parent opioid peptides. Furthermore, these cyclic prodrugs were shown to be transcellular permeants (in contrast to the opioid peptides, which are paracellular permeants), and were not substrates for polarized efflux systems. Surprisingly, the acyloxyalkoxy-based prodrugs of [Leu(5)]-enkephalin and DADLE were shown to exhibit very low permeation through Caco-2 cell monolayers, which could be attributed to their substrate activity for efflux systems.

Transport kinetics of leucine enkephalin across Caco-2 monolayers: quantitative analysis for contribution of enzymatic and transport barrier

In this study, we determined the activities of four aminopeptidases such as aminopeptidase B (APB), M (APM), N (APN) and dipeptidylpeptidase IV (DPP IV) in Caco-2 cells and compared with those in the rat intestinal mucosae. The activities of APB, APM and APN appeared to be highest in rat small intestinal mucosa, while DPP IV activity was much higher in Caco-2 cells than that in the rat intestinal mucosa. Next the inhibitory effects of various protease inhibitors were examined in Caco-2 homogenate. Three tested inhibitors, bacitracin, amastatin and puromycin, effectively inhibited the activities of APM, APN and DPP IV except for APB. Further, we quantitatively evaluated the permeation and degradation properties of leucine enkephalin (Leu-Enk) in the presence or absence of inhibitors in Caco-2 monolayer system. Leu-Enk had a high degradation clearance (CLd) and a low permeation clearance (CLp) in Caco-2 monolayers. This finding indicates that the very rapid degradation of Leu-Enk on the apical side of Caco-2 monolayers was due to aminopeptidases. However, these protease inhibitors besides sodium glycocholate were able to reduce the CLd values markedly, thereby increasing the permeation amount of Leu-Enk across Caco-2 monolayers. In particular, amastatin significantly decreased the CLd value and increased the CLp value. This enhanced CLp value was further increased by the coadministration with an absorption enhancer, EDTA or laurylmaltoside. These findings are relevant to the oral administration of peptide drugs and to developing an efficient oral delivery system.

Intestinal absorption and analgesic activity of aminopeptidase-resistant cellobiose-coupled leucine enkephalinamide

Leucine enkephalinamide (LEamide), aminopeptidase-degradable opioid peptide, was coupled with cellobiose (cellobiose-coupled LEamide, CcpLEamide). CcpLEamide was absorbed from the rat small intestine in vitro, whereas LEamide was not. CcpLEamide on the mucosal side was more stable than aminopeptidase-resistant cellobiose-coupled leucine enkephalin (CcpLE) in the presence of inhibitors of enkephalinase and angiotensin converting enzyme, and much more stable than LEamide. The absorption rate (clearance) of CcpLEamide was comparable with that of CcpLE in the presence of these peptidase inhibitors. Analgesic activities of these enkephalins were tested by the acetic acid writhing assay and hot-plate assay after subcutaneous administration to mice. Both assays indicated CcpLEamide-induced analgesia. On the other hand, the analgesic activity of LEamide was not observed, but pretreatment with amastatin (a peptidase inhibitor) produced LEamide-induced analgesia. These results indicate that CcpLEamide is stable in the body and has analgesic effects without pretreatment with peptidase inhibitors, and was stable enough to be absorbed from the small intestine. We propose CcpLEamide as an orally active analgesic peptide candidate.

Effects of various protease inhibitors on the stability and permeability of [D-Ala2,D-Leu5]enkephalin in the rat intestine: comparison with leucine enkephalin

The effects of various protease inhibitors on the stability of leucine enkephalin (Leu-Enk) and [D-Ala2,D-Leu5] enkephalin (DADLE) were investigated, and the permeability of these peptides was also examined in an in vitro Ussing chamber. Captopril, thiorphan, bacitracin, bestatin, puromycin, amastatin, and sodium glycocholate (Na-GC) were chosen as protease inhibitors. Regional differences in the stability of Leu-Enk and DADLE were observed, and the rank order of the stability of these peptides was colon > duodenum > ileum > jejunum. Na-GC, amastatin, and puromycin were effective protease inhibitors for improving the stability of these peptides, although captopril and thiorphan did not improve the stability of Leu-Enk. In the transport studies, Leu-Enk did not cross the intestinal membrane in the absence of protease inhibitors, but its transport was improved in the presence of Na-GC. In addition, Na-GC, amastatin, and puromycin improved the permeability of DADLE in both jejunum and colon, while the permeability of DADLE was not improved by the addition of captopril, thiorphan, and bestatin. Furthermore, the permeability of 6-carboxyfluorescein, a poorly absorbable and stable compound, was also improved in the presence of Na-GC and bacitracin at a concentration of 10 mM. These findings indicated that amastatin, puromycin, and Na-GC at a concentration of 0.5 mM might increase the permeability of DADLE due to the improved stability of DADLE in the donor site. However, Na-GC and bacitracin at a concentration of 10 mM had absorption-enhancing activities which might be also related to the enhanced permeability of DADLE across the intestinal membrane.

Quantitative analysis of two opioid peptides in plasma by liquid chromatography-electrospray ionization tandem mass spectrometry

Quantitative analysis of two opioid peptides, DSLET [(D-Ser2)Leu-enkephalin-Thr6] and Met-enkephalin-Arg-Gly-Leu, was performed using microbore liquid chromatography interfaced to electrospray ionization tandem mass spectrometry. Validation of the methodology was demonstrated for each peptide in plasma. Quantitative analyses were performed through the use of a deuterium labelled peptide analog as an internal standard. Linearity was observed for the analysis of DSLET (5-1000 ng/ml) and Met-enkephalin-Arg-Gly-Leu (1-1000 ng/ml) in plasma with a limit of detection of 0.25 ng/ml for Met-enkephalin-Arg-Gly-Leu and 1.0 ng/ml for DSLET. In general, the observed concentrations showed good reproducibility with coefficients of variation of within 15%. In the concentration range studied, only 0.5 ml of plasma was required for optimal detection of Met-enkephalin-Arg-Gly-Leu and 0.25 ml for DSLET. Application of this method was demonstrated by studying the disposition of DSLET in a rat. DSLET administered to a rat exhibited a short half-life and a high clearance value.

Hepatic inactivation of Leu-enkephalin

Leu-enkephalin radiolabelled at the N-terminal tyrosine by two different methods was presented to isolated perfused rat livers. Approximately 10% of a pulse of tritiated Leu-enkephalin was taken up first-pass; this was increased to 62% when the peptide was iodinated with Bolton and Hunter reagent. Uptake of both forms of radiolabelled Leu-enkephalin was inhibited by taurocholate in a concentration-dependent manner. The proportion of internalised radioactivity secreted into bile also differed but in both cases showed a very rapid time-course similar to that of [24-(14)C]taurocholate and suggestive of non-endocytic transfer via membrane transport proteins. Pre-perfusion with the aminopeptidase inhibitor bestatin increased uptake of 3H-labelled Leu-enkephalin from 10% to 23%; no further increase occurred when the endopeptidase 24.11 inhibitor thiorphan was also present. On infusion of the native peptide into rat livers, 80% of Leu-enkephalin immunoreactivity was lost between the pre- and post-hepatic perfusate; this was reduced to 65% in the presence of 10(-5) M bestatin. The almost total release of the N-terminal tyrosine from 3H-labelled Leu-enkephalin which escaped first-pass uptake confirmed that substantial sinusoidal metabolism had occurred. Low levels of aminopeptidase N were visualised in the sinusoidal membrane using a specific monoclonal antibody coupled to peroxidase staining. Thus, hepatic inactivation of Leu-enkephalin is primarily via hydrolysis mediated by cell surface peptidase (including aminopeptidases) whilst uptake of the intact peptide, probably by a bile salt transport protein, is quantitatively minor unless the N-terminus is blocked by Bolton and Hunter reagent or peptidase inhibitors are present.

Improvement of intestinal absorption of leucine enkephalin by sugar coupling and peptidase inhibitors

Peptidase-degradable leucine enkephalin (LE) was coupled with cellobiose or gentiobiose. In the absorption experiments, cellobiose-coupled LE (CcpLE) was more stable than LE itself on the mucosal side, and CcpLE appeared on the serosal side. Destyrosyl LE coupled with cellobiose was not formed, indicating that sugar coupling provided LE with aminopeptidase resistance. In the presence of inhibitors of angiotensin-converting enzyme and enkephalinase, the stability of CcpLE on the mucosal side was increased, and as a result more was absorbed. Furthermore, the absorption clearance was much higher than the value expected from the mucosal concentration of CcpLE. Similar results were observed in the absorption of gentiobiose-coupled LE. In the LE absorption experiment, however, LE was not detected on the serosal side even in the presence of these peptidase inhibitors. Improvement of intestinal absorption by sugar coupling and peptidase inhibitors was evaluated kinetically, indicating the exclusive contribution of metabolic degradation of LE through intestinal tissues to the absorption process.

Transdermal iontophoretic delivery of enkephalin formulated in liposomes

Transdermal iontophoretic transport of a liposomal formulation of [Leu5]enkephalin, across human cadaver skin, was investigated. Franz (vertical) cells were supplied with 0.5 mA/cm2 current density via silver/silver chloride electrodes from a Scepter power supply. Enkephalin spiked with [3H]enkephalin was transported across skin from anode or cathode, depending on the charge on the molecule. Liposomes or their constituents were shown to penetrate into the skin. Enkephalin, when delivered iontophoretically at its isoelectric point, from liposomes carrying positive or negative charge on their surface, resulted in permeation of radioactivity which was same or less than that of the controls when analyzed by liquid scintillation counting. When analyzed by radiochromatography detector on HPLC, degradation of enkephalin during transport was observed, with several degradation peaks in the chromatogram. The degradation was less in liposome formulations, as compared to controls. This is the first report of the combined use of liposomes and iontophoresis for transdermal delivery.

Alterations in delta opioid receptor levels in discrete areas of the neocortex and in the globus pallidus of the aging guinea pig: a quantitative autoradiographic study

The effect of aging on delta opioid receptors was examined in the brains of guinea pigs aged 1, 6, 24 and 36 months. Quantitative autoradiography was used to monitor the concentrations of delta receptors in various anatomical regions at five rostro-caudal levels. delta opioid receptor populations were found to be remarkably stable throughout the life span of this species. We have, however, discovered anatomical areas which offer striking exceptions. In the globus pallidus, progressive age-related losses of delta receptors reached 50% in the senescent animal. In contrast, laminae I, II of the lateral agranular frontal cortex and laminae I, II and III, IV of the primary somatosensory cortex demonstrated age-related increases in the concentrations of delta receptors ranging from 30 to 45%. These changes are discussed with the view to their being functionally related components of motor circuitry involving pyramidal and extrapyramidal elements.

Enhancement of systemic delivery of met-enkephalin and leu-enkephalin eyedrops with permeation enhancers

It was found that methionine enkephalin (Met-Enk) and leucine-enkephalin (Leu-Enk) can be delivered efficiently into the systemic circulation through the outer route in rabbits. When 50 mcl of 1% Met-Enk eyedrops without absorption enhancers were instilled into eyes, the rabbits' blood concentration rose from 92 pg/ml to 153 pg/ml in 10 min. When either of two permeation enhancers (BL-9 or Brij-78) was added at a 0.5% concentration to the ophthalmic solution, the systemic absorption of 1% Met-Enk was markedly improved. Its blood level was increased to approximately 3.4 times that without the enhancer. When the eyedrops of 0.3% Met-Enk plus 0.5% absorption enhancer were administered, the blood concentration reached higher than that reached by 1% Met-Enk without permeation enhancers. Similar results were obtained with Leu-Enk except the systemic absorption was enhanced by BL-9 and Brij-78 even further to 10-fold and 8.3-fold, respectively. These results indicate that the systemic delivery of Met-Enk and Leu-Enk through eyes, especially with the addition of permeation enhancers, is a feasible alternative route to parenteral injection.

Oral absorption of peptides: influence of pH and inhibitors on the intestinal hydrolysis of leu-enkephalin and analogues

Leu-enkephalin (YGGFL) and several analogues were chosen as model peptides for the study of peptide absorption and hydrolysis in the rat jejunum. An HPLC assay was adapted to detect YGGFL or the analogues and metabolites. Peptide hydrolysis was studied in the rat jejunum using a single-pass perfusion method. Extensive hydrolysis of YGGFL was observed in the rat jejunum and approaches to reduce its metabolism were studied. The brush border enzymes are a major site of enkephalin hydrolysis. Lumenal peptidases were secondary to the brush border enzymes in hydrolyzing the enkephalins in this system. In the in situ perfusion system, YGGFL is hydrolyzed primarily to Tyr and GGFL by the brush border aminopeptidase and to YGG and FL by brush border endopeptidase. Lowering the jejunal pH below 5.0 significantly reduces aminopeptidase activity and, to a lesser extent, endopeptidase activity. An aminopeptidase inhibitor, amastatin, produced more pronounced inhibitory effects at higher pH and the endopeptidase inhibitors, tripeptides YGG and GGF, are effective even below pH 5.0. Coperfusion of YGGFL with a combination of aminopeptidase and endopeptidase inhibitors, e.g., amastatin and YGG, is more effective in inhibiting hydrolysis since both metabolic pathways are inhibited. Leu-D(Ala)2-enkephalin, while showing enhanced stability against aminopeptidase hydrolysis, is hydrolyzed at the Gly-Phe bond by the endopeptidase. Its hydrolysis is not affected by pH changes or amastatin but is decreased by YGG. The YGGFL wall permeability was estimated and is not a limiting factor for oral absorption.

Transdermal delivery of bioactive peptides: the effect of n-decylmethyl sulfoxide, pH, and inhibitors on enkephalin metabolism and transport

We investigated the effects of the nonionic surfactant, n-decylmethyl sulfoxide (NDMS), pH, and inhibitors on the metabolism and the permeation of amino acids, dipeptides, and the pentapeptide enkephalin, through hairless mouse skin. An HPLC gradient method was developed to identify the possible peptide and amino acid metabolites of leucine-enkephalin. NDMS increased the permeability of all amino acids and peptides tested. At neural pH, the enzyme activity within the skin was such that no flux of leucine-enkephalin (YGGFL) was observed and the donor cell concentration of YGGFL decreased rapidly. The major cleavage occurred at the Tyr-Gly bond. At pH 5.0 the metabolic activity was reduced significantly and a substantial flux of YGGFL was observed. Enzymatically stable YGGFL analogues, Tyr-D-Ala-Gly-Phe-Leu (YDAGFL) and its amide, exhibited significant fluxes even at neutral pH in the presence of NDMS, but with substantial metabolism. YDAGFL amide was more stable to metabolism than YDAGFL. The rates of metabolism of the peptides in the skin homogenates were in the order: FL much greater than YGGFL greater than GFL greater than GGFL much greater than YG, YGG much greater than YDAGFL amide. In the skin homogenates puromycin and amastatin showed the highest inhibitory effects, while FL and GFL were only slightly active. However, in the skin diffusion experiments, FL allowed the highest amount of intact parent compound to permeate, making it the most potent inhibitor. These results show that the complex proteolytic enzyme activities occurring during skin permeation are different from those in skin homogenates and that a combination of enhancer, pH adjustment, and inhibitors can increase the transdermal delivery of peptides.

Mechanism of nasal absorption of drugs. IV: Plasma levels of radioactivity following intranasal administration of [3H]leucine enkephalin

To study the factors influencing nasal absorption of a model pentapeptide, plasma levels of total radioactivity were determined following the administration of [3H]tyr-leucine enkephalin to rats intravenously, intranasally alone, and intranasally in the presence of puromycin. The major pathway for transport of radioactivity into the blood from the nasal cavity appeared to be hydrolysis of [3H]tyr-leucine enkephalin to [3H]L-tyrosine, followed by absorption of [3H]L-tyrosine. When puromycin was added to the nasal solution in concentrations at which the in vitro hydrolysis of leucine enkephalin was completely inhibited, the appearance of radioactivity in the plasma was slowed, but plasma concentrations of radioactivity eventually reached levels comparable to those observed in the absence of puromycin. In view of the inhibitory effect of puromycin on the hydrolysis of leucine enkephalin, it was assumed that a significant fraction of the [3H]tyr-leucine enkephalin was absorbed intact in the presence of this substance. However, an assay method for intact leucine enkephalin in plasma is needed to confirm these preliminary observations.

Mechanism of nasal absorption of drugs. III: Nasal absorption of leucine enkephalin

The nasal absorption of a model peptide, leucine enkephalin (LE), was studied in rats using an in situ technique in which 4 mL of perfusion solution was circulated. Leucine enkephalin (LE) was found to undergo hydrolysis to its major metabolite des-tyrosine leucine enkephalin (DTLE). The addition of 1% sodium glycocholate (SGC) to the perfusion solution resulted in an increase in the overall rate of disappearance of LE and a decrease in the rate of formation of DTLE. When LE was added to nasal washings (i.e., Ringer's buffer that was precirculated through the nasal cavity to extract enzymes), LE was found to form DTLE. When SGC or puromycin was added to the nasal washings prior to the addition of LE, the rate of conversion of LE to DTLE was significantly reduced, suggesting that these two agents can inhibit peptidase enzyme activity in the nasal cavity. Since the volume of the solution has been shown to influence the kinetics of absorption of drugs administered nasally, a new experimental technique, the in vivo-in situ technique, which utilizes small volumes of solution and simulates realistic use of nose drops, was employed to further examine the mechanism of absorption and hydrolysis of LE in rats. Leucine enkephalin (LE) dissolved in 100 microL of Ringer's buffer was placed in the isolated nasal cavities of rats. The disappearance of LE and the appearance of DTLE were followed by rinsing the nasal cavity with fresh buffer. Disappearance of LE was always accompanied by appearance of DTLE, and the fraction of LE converted to DTLE decreased as the concentration of LE increased, suggesting a saturable enzymatic process.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of leucine enkephalin metabolism in rat blood, plasma and tissues in vitro by an aminoboronic acid derivative

Boroleucine, an aminoboronic acid derivative, is a potent inhibitor of aminopeptidases. Its effects on leucine enkephalin metabolism in rat whole blood, plasma and skeletal muscle and brain homogenates were determined in vitro. In the absence of the inhibitor, leucine enkephalin disappeared very rapidly from each medium. The rank order of metabolism rates was brain greater than muscle greater than whole blood approximately plasma. Des-tyrosyl-leucine enkephalin rapidly appeared as a result of metabolism by aminopeptidases. In whole blood and in the muscle and brain homogenates the aminopeptidase metabolite could account for all of the lost leucine enkephalin. In plasma, however, it appeared that another metabolic route contributed. Boroleucine prolonged the leucine enkephalin degradation half-life 1.6-1.9-fold in each medium at 0.1 microM concentrations, 1/1000th the concentration of leucine enkephalin. At 1.0 microM boroleucine concentrations the half-lives were prolonged 4.0-6.4-fold. Boroleucine also inhibited the degradation of des-tyrosyl-leucine enkephalin added to whole blood, but did not inhibit its degradation in muscle or brain homogenates. Boroleucine and other aminoboronic acid derivatives may be useful tools for studying peptide metabolism, and as pharmaceutical adjuvants to inhibit the degradation of peptide drugs metabolized by aminopeptidases.

Attempts to target antitumor drugs toward opioid receptor-rich mouse tumor cells with enkephalin-ellipticinium conjugates

Human colorectal and pulmonary carcinomas have been shown to contain high levels of opioid peptides and their corresponding membrane-bound receptors. Therefore possible targeted drugs, consisting of modified enkephalins linked to cytotoxic drugs, were designed. Such conjugates were expected to be specifically internalized within opioid receptor-bearing cells. As a model to this approach, we have synthesized enkephalin-ellipticinium conjugates in which the D-Ala2-D-Leu5-enkephalin (DADLE) was coupled to the 2-nitrogen of either ellipticine or 9-hydroxyellipticine, two drugs acting through different mechanisms of cytotoxicity. These conjugates, DADLE-ellipticinium (NME) and DA-DLE-9-hydroxyellipticinium (NMHE), respectively, were previously shown to retain in vitro both opioid receptors and DNA affinities close to those of the parent compounds. In this paper, we first show that each individual moiety in the complexes remains capable of recognizing its cellular targets. Thus, pretreatment of NG108-15 cells containing delta-opioid receptors by the DADLE-ellipticinium conjugates induced a loss of opioid receptor (down-regulation), while the smaller peptide conjugates, tyrosinyl-D-alanylglycine-ellipticinium, prepared as control, do not. On the other hand, peptide-NMHE conjugates were able to induce DNA topoisomerase II-associated DNA strand breaks suggesting that they have a mode of action similar to that of their parent molecule, NMHE. We then examined whether or not these molecules could exert a specific toxicity on opioid receptor-bearing cells. However, when tested on NG108-15 tumor cells and L-fibroblasts as control, the enkephalin-ellipticinium conjugates (DADLE-NME and DADLE-NMHE) proved to be similarly more cytotoxic on both cell lines than their ellipticinium (NME and NMHE) precursors. In order to understand this apparent lack of specificity we examined the cellular accumulation and distribution of DADLE-NME by fluorescence techniques. These experiments revealed that an important intracellular overconcentration caused by a nonspecific process is probably masking the specific targeted effect of the conjugates. Hence, the project of linking DADLE to highly cytotoxic molecules which cannot cross the plasma membrane without site-directed targeting is discussed.

[Bioavailability of dalargin and its metabolism during intranasal administration to rats]

Bioavailability and metabolism of a peptide drug Dalargin with a chemical structure Tyr-D-Ala-Gly-Phe-Leu-Arg have been examined. Dalargin is applied for the treatment of gastric and duodenal ulcers. Bioavailability was estimated following intramuscular (i/m) and intranasal (i/n) routes of administration of 3H-dalargin in anesthetized dogs. The highest dalargin concentration was achieved about 10 min after i/m and i/n administration. Absolute dalargin bioavailability was 15% and 8%, while its elimination half-life was 23.2 min and 21.3 min, respectively. Tyrosine, N-terminated tetra- and pentapeptides were the main metabolites detected in the blood. The intranasal route of dalargin administration is concluded to be possible in the clinical practice.

Effect of centrally administered encephalinamides on regional cerebral blood flow in the dog

(D-ala2)-met5-encephalinamide (AM encephalinamide) and (D-ala2)-leu5-encephalinamide (AL encephalinamide) were administered into the cisterna magna in anesthetized dogs to determine whether these opiates effected the neurohypophyseal circulation differently than the circulation of other brain areas. At the beginning of the experimental protocol, animals were given either mock cerebral spinal fluid (CSF) or 5 or 25 mg of AM encephalinamide or 5 mg of AL encephalinamide in equal volumes of mock CSF into the cisterna magna. By 60 min after intracisternal injection, radiolabeled AM encephalinamide distributed throughout the brain with the highest concentration being in the area of the brainstem. Sixty minutes after intracisternal injection, heart rate was decreased 29.0 +/- 5.1%, 41.3 +/- 4.4%, and 36.0 +/- 3.6%, and MABP was decreased 25.2 +/- 8.0%, 26.4 +/- 2.4%, and 32.3 +/- 2.6% in animals treated with AL encephalinamide (5 mg), AM encephalinamide (5 mg), and AM encephalinamide (25 mg), respectively. Neither AL encephalinamide or AM encephalinamide altered CBF or CMRO2 when compared with animals treated with mock CSF, whereas both AL encephalinamide and AM encephalinamide reduced neurohypophyseal blood flow by 30 min (43 +/- 11%, AL encephalinamide; 35 +/- 7%, AM encephalinamide, 5 mg; 46 +/- 8%, AM encephalinamide, 25 mg); the reduction was sustained throughout the 60-min protocol (34 +/- 10%, AL encephalinamide; 37 +/- 3%, AM encephalinamide, 5 mg; 38 +/- 4% AM encephalinamide, 25 mg). Plasma arginine vasopressin was transiently elevated 15 (326 +/- 75%, AL encephalinamide; 323 +/- 109%, AM encephalinamide, 25 mg) and 30 min (271 +/- 68%, AL encephalinamide; 368 +/- 136%, AM encephalinamide, 25 mg) in animals treated with AL encephalinamide or AM encephalinamide (25 mg). Intravenous naloxone administered at the end of the 60-min encephalinamide protocol was associated with a rise toward control values in heart rate and MABP in the AL encephalinamide group and in heart rate, MABP, and neurohypophyseal blood flow in both the AM encephalinamide 5 mg and 25 mg groups. These data suggest that encephalinamides may play a role in the regulation of neurohypophyseal blood flow through their actions on opiate receptors.

[Pharmacokinetics of dalargin]

Pharmacokinetics of a new anti-ulcer drug, synthetic opioid hexapeptide dalargin (Tyr-D-Ala-Gly-Phe-Leu-Arg), which is an analogue of Leu-enkephalin, was studied in rats. Distribution and metabolism of 3H-Tyr-dalargin were examined after intravenous administration at a dose of 150 mg/kg. Main Tyr-containing metabolites of dalargin were detected and dynamics of their concentrations was evaluated in blood within 1 hr after administration. Kinetic curve of dalargin was described as three-exponential function.

LY164929: a highly selective ligand for the lower affinity [3H]D-Ala2-D-Leu-5-enkephalin binding site

Most radiolabeled ligands used to label opiate receptors bind to multiple binding sites. Subtype-selective ligands make possible the labeling of single sites by virtue of their ability to "block" binding of the radiolabeled ligand to selected subtypes. This study compares the selectivity of several ligands for the higher and lower affinity [3H]D-Ala2-D-Leu-5-enkephalin binding sites. The results demonstrated that while morphine and D-ala2-MePhe4,Gly-ol5-enkephalin were 80- and 256-fold selective for the lower affinity [3H]D-Ala2-D-Leu-5-enkephalin binding site, LY164929 was 1,986-fold selective. Additional experiments indicated that whereas morphine was a noncompetitive inhibitor at the lower affinity [3H]D-Ala2-D-Leu-5-enkephalin binding site, LY164929 was a competitive inhibitor, suggesting that this peptide might exhibit different properties in vivo than other mu-like ligands.

Synthesis, receptor binding activity and fluorescence property of fluorescent enkephalin analogs containing L-1-pyrenylalanine

The novel fluorescent amino acid, L-1-pyrenylalanine (L-Pya), was prepared by the asymmetric hydrogenation of cyclic dehydrodipeptide. Fluorescent enkephalins containing one or two Pya residues at position 1,4 or 5 of [D-Ala2, Leu5]enkephalin were synthesized by the solution method. Mono-Pya-enkephalins showed strong fluorescence intensities and potent binding affinities with specificity and selectivity for opiate receptors. However, di-Pya-enkephalins showed markedly decreased receptor binding affinities. These results indicate that the incorporation of two Pya residues into enkephalin makes the peptide unable to interact with the opiate receptors, although introduction of one Pya residue is effective to elicit a specific receptor interaction. Di-Pya-enkephalins showed intramolecular excimer spectra, indicating that the peptides are able to take possible folded conformations.