Esterase-sensitive cyclic prodrugs of peptides: evaluation of an acyloxyalkoxy promoiety in a model hexapeptide

Improvement on Permeability of Cyclic Peptide/Peptidomimetic: Backbone N-Methylation as A Useful Tool

Peptides have a three-dimensional configuration that can adopt particular conformations for binding to proteins, which are well suited to interact with larger contact surface areas on target proteins. However, low cell permeability is a major challenge in the development of peptide-related drugs. In recent years, backbone N-methylation has been a useful tool for manipulating the permeability of cyclic peptides/peptidomimetics. Backbone N-methylation permits the adjustment of molecule’s conformational space. Several pathways are involved in the drug absorption pathway; the relative importance of each N-methylation to total permeation is likely to differ with intrinsic properties of cyclic peptide/peptidomimetic. Recent studies on the permeability of cyclic peptides/peptidomimetics using the backbone N-methylation strategy and synthetic methodologies will be presented in this review.

Permeation-Enhancing Nanoparticle Formulation to Enable Oral Absorption of Enoxaparin

This study tests the hypothesis that association complexes formed between enoxaparin and cetyltrimethylammonium bromide (CTAB) augment permeation across the gastrointestinal mucosa due to improved encapsulation of this hydrophilic macromolecule within biocompatible poly (lactide-co-glycolide, PLGA RG 503) nanoparticles. When compared with free enoxaparin, association with CTAB increased drug encapsulation efficiency within PLGA nanoparticles from 40.3 ± 3.4 to 99.1 ± 1.0%. Drug release from enoxaparin/CTAB PLGA nanoparticles was assessed in HBSS, pH 7.4 and FASSIFV2, pH 6.5, suggesting effective protection of PLGA-encapsulated enoxaparin from unfavorable intestinal conditions. The stability of the enoxaparin/CTAB ion pair complex was pH-dependent, resulting in more rapid dissociation under simulated plasma conditions (i.e., pH 7.4) than in the presence of a mild acidic gastrointestinal environment (i.e., pH 6.5). The intestinal flux of enoxaparin complexes across in vitro Caco-2 cell monolayers was greater when encapsulated within PLGA nanoparticles. Limited changes in transepithelial transport of PLGA-encapsulated enoxaparin complexes in the presence of increasing CTAB concentrations suggest a significant contribution of size-dependent passive diffusion as the predominant transport mechanism facilitating intestinal absorption. Graphical abstract.

Bio-accessible milk casein derived tripeptide (LLY) mediates overlapping anti- inflammatory and anti-oxidative effects under cellular (Caco-2) and in vivo milieu

Inflammation and oxidative stress are closely linked patho-physiological processes which occur concurrently in many diseased conditions. Recently, interdependence between these two processes explains the antioxidant paradox associated with failure to select appropriate agents required for prevention of diseases known to be induced by oxidative stress. Present study established the overlapping anti-inflammatory and anti-oxidative potential along with bio-accessibility of milk casein derived tripeptide (LLY). Tripeptide exhibited anti-inflammatory response under ex vivo conditions by suppressing (P<.01) mice splenocytes proliferation and modulating their cytokines (IFN-γ, IL-10 and TGF-β) with improved phagocytosis of peritoneal macrophages. Conversely, tripeptide displayed extraordinary radical scavenging ability and cellular anti-oxidative potential using chemical assays and H₂O₂ induced oxidative stress model on Caco-2 cells. Under cellular assessment, on one hand tripeptide inhibited (P<.01) intracellular ROS generation and reduced MDA and protein carbonyls but on the other also increased (P<.01) the activity of anti-oxidative enzyme, catalase without much effect on SOD and GPx. This anti-oxidative potential was further established by studying relative expression of genes (Nrf-2 and Keap1) and Nrf-2 nuclear translocation associated with anti-oxidative signaling in Caco-2 cells. Bio-accessibility of tripeptide and its intact transport across Caco-2 cell monolayer was also found to be 1.72±0.22% through PepT1 mediated transport mechanism. Besides, tripeptide displayed strong anti-oxidative and anti-inflammatory potential under in vivo conditions in mice against ethanol induced oxidative stress by elevating (P<.01) liver GSH content and by decreasing (P<.01) the activities of anti-oxidative enzymes, MDA along with reduced expression of CYP2E1, PPAR-α, TNF-α and COX-2 genes than ethanol control.

The protein kinase A pathway contributes to Hg2+-induced alterations in phosphorylation and subcellular distribution of occludin associated with increased tight junction permeability of salivary epithelial cell monolayers

Hg(2+) is commonly used as an inhibitor of many aquaporins during measurements of transcellular water transport. To investigate whether it could also act on the paracellular water transport pathway, we asked whether addition of Hg(2+) affected transport of radiolabeled probes through tight junctions of a salivary epithelial cell monolayer. Inclusion of 1 mM Hg(2+) decreased transepithelial electrical resistance by 8-fold and augmented mannitol and raffinose flux by 13-fold, which translated into an estimated 44% increase in pore radius at the tight junction. These Hg(2+)-induced effects could be partially blocked by the protein kinase A (PKA) inhibitor N-[2-((p-bromocinnamyl) amino) ethyl]-5-isoquinolinesulfonamide, 2HCl (H89), suggesting that both-PKA dependent and PKA-independent mechanisms contribute to tight junction regulation. Western blot analyses showed a 2-fold decrease in tight junction-associated occludin after Hg(2+) treatment and the presence of a novel hyperphosphorylated form of occludin in the cytoplasmic fraction. These findings were corroborated by confocal imaging. The results from this study reveal a novel contribution of the PKA pathway in Hg(2+)-induced regulation of tight junction permeability in the salivary epithelial barrier. Therapeutically, this could be explored for pharmacological intervention in the treatment of dry mouth, Sjögren's syndrome, and possibly other disorders of fluid transport.

Simultaneous determination of a selective adenosine 2A agonist, BMS-068645, and its acid metabolite in human plasma by liquid chromatography-tandem mass spectrometry—Evaluation of the esterase inhibitor, diisopropyl fluorophosphate, in the stabilization of a labile ester-containing drug

BMS-068645 is a selective adenosine 2A agonist that contains a methyl ester group which undergoes esterase hydrolysis to its acid metabolite. To permit accurate determinations of circulating BMS-068645 and its acid metabolite, blood samples must be rapidly stabilized at the time of collection. A sensitive, rapid and specific liquid chromatography-tandem mass spectrometry (LC/MS/MS) method for the simultaneous quantitation of BMS-068645 and its acid metabolite in human plasma has been developed and validated using diisopropyl fluorophosphate (DFP) as the esterase inhibitor to prevent BMS-068645 from converting to its acid metabolite. The D(5)-stable isotope labeled analogs of BMS-068645 and its metabolite were used as the internal standards (IS). Analytes and IS in plasma containing 20 mM DFP were acidified and extracted into methyl tert-butyl ether. The liquid-liquid extraction effectively eliminated the strong matrix effect caused by the esterase inhibitor. The chromatographic separation was achieved on a Waters Atlantis C18 column with a run time of 4 min. Detection was performed on a Sciex API 4000 with positive ion electrospray mode (ESI/MS/MS), monitoring the ion transitions m/z 487>314 and 473>300 for BMS-068645 and its acid metabolite, respectively. The method was validated over the range from 0.020 to 10.0 ng/mL for BMS-068645 and 0.050 to 10.0 ng/mL for its acid metabolite. Inter- and intra-run precision for the quality control samples during validation were less than 8.7% and 4.0%, respectively, for the two analytes. The assay accuracy was within +/-5.4% of the nominal values. The esterase inhibitor effectively stabilized BMS-068645 during blood collection and storage. Blood collection tubes containing DFP were easily prepared and used at the clinical sites and could be stored at -30 degrees C for 3 months. This method demonstrated adequate sensitivity, specificity, accuracy, precision, stability and ruggedness to support the analysis of human plasma samples in pharmacokinetic studies.

Stability of oxymethyl-modified coumarinic acid cyclic prodrugs of diastereomeric opioid peptides in biological media from various animal species including human

In vitro stability studies of oxymethyl-modified coumarinic acid (OMCA) cyclic prodrugs of the diastereomeric opioid peptides DADLE ([D-Ala2,D-Leu5]-Enk, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), [Ala2,D-Leu5]-Enk (H-Tyr-Ala-Gly-Phe-D-Leu-OH), [D-Ala2,Leu5]-Enk (H-Tyr-D-Ala-Gly-Phe-Leu-OH), and [Ala2,Leu5]-Enk (H-Tyr-Ala-Gly-Phe-Leu-OH) were conducted to evaluate how the chirality of specific amino acid residues (Ala2 and Leu5) in the peptide portion affects their bioconversion by esterases. The stability studies were conducted at 37 degrees C in plasma and tissue homogenates (liver and brain) from five animal species (rat, mouse, canine, guinea pig, and hamster) and human in an attempt to identify an animal species that had a "prodrug bioconversion profile" comparable to that of humans. Initially, the total esterase activity in these biological media was measured using p-nitrophenyl butyrate (PNPB) as a substrate. By repeating this activity assay in the presence of paraoxon, a potent esterase B inhibitor, it was possible to estimate the relative amounts of esterases B and esterases A/C in a biological sample. Stability studies of the cyclic prodrugs were carried out under identical conditions, that is, in the presence and absence of paraoxon. Significant differences in the rates of hydrolysis of the cyclic prodrugs were observed, particularly between cyclic prodrugs with differences in the chirality of the amino acid on the C-terminus of the peptide portion, for example, L-amino acids at the C-terminus hydrolyzed more rapidly than D-amino acids. This stereoselective hydrolysis was independent of the animal species but tended to be more pronounced in brain and liver homogenates compared to plasma. Increased esterase specific activity, as measured by PNPB, in the biological media did not necessarily correlate with increased bioconversion rates of the cyclic prodrugs. The enzymatic stability profiles of the cyclic prodrugs in biological media from canine and guinea pig most closely resembled the profiles from human biological media. Therefore, canine and guinea pig appear to be the most relevant animal models for conducting pharmacokinetic studies on these cyclic prodrugs of opioid peptides.

Methyl esters of N-(dicyclohexyl)acetyl-piperidine-4-(benzylidene-4-carboxylic acids) as drugs and prodrugs: a new strategy for dual inhibition of 5 alpha-reductase type 1 and type 2

Steroid 5alpha-reductase (5alphaR) inhibitory potency of three N-(dicyclohexyl)acetyl-piperidine-4-(benzylidene-4-carboxylic acids) and their corresponding methyl esters was monitored for type 2 isoenzyme in a benign prostatic hyperplasia cell free preparation and for type 1 isoenzyme in DU145 cells and in a cell free assay. The hydrolytic stability of the esters and their bioconversion to the corresponding acids was assessed in aqueous buffered solution (pH 7.4) and in selected biological media having measurable esterase activities. The carboxylic acids 1, 2, and 3 with high type 2 inhibitory potencies displayed only little type 1 inhibition. The esters 1a, 2a, and 3a, originally designed as prodrugs to enhance cell permeation, proved to be potent type 1 inhibitors and are therefore acting as drugs themselves. They are stable in buffered salt solution (pH 7.4), Caco-2 cells, and human plasma, whereas all esters are cleaved into the corresponding acids in benign prostatic hyperplasia tissue homogenate. Methyl esters, applied as hydrolytically stable precursor drugs to facilitate cell permeation, will yield the corresponding carboxylic acids as type 2 inhibitors after hydrolysis in the target organ. The esters themselves--stable in human plasma and Caco-2 cells--are acting as potent drugs toward 5alphaR type 1. Thus, dual inhibition of 5alphaR type 1 and type 2 can be achieved by applying a single parent compound.

Improved alkylation and product stability in phosphotriester formation through quinone methide reactions with dialkyl phosphates

Investigating reactions of functionalized p-quinone methides continues to advance our design of a reagent being developed for controlled, in situ modification of DNA via phosphodiester alkylation. Previously reported investigations of p-quinone methides derived from catechols allowed for trapping of isolable trialkyl phosphates for characterization and mechanistic information. However, lactone formation with these derivatives required long reaction times, resulting in an unfavorable mixture of trialkyl phosphate and hydrolysis products. To enhance the rate and efficacy of trialkyl phosphate formation and trapping, a phenol derived p-quinone methide has been designed to enforce a conformation favoring lactonization of the dialkyl phosphate alkylated intermediate. The relative rates of phosphodiester alkylation and subsequent trapping of the phosphotriester adduct have been examined by UV and (1)H NMR analysis for p-quinone methide precursor 1 and the corresponding control, 1'. The incorporation of a methyl group at the meta-position of 1 (relative to 1') significantly improves the rate of lactionization to provide a much higher yield of the desired product, lactonized phosphotriester 5. The control reaction with 1' afforded only a minor amount of the corresponding lactonized trialkyl phosphate 5'.

Amino acid ester prodrugs of 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole enhance metabolic stability in vitro and in vivo

2-Bromo-5,6-dichloro-1-(beta-d-ribofuranosyl)benzimidazole (BDCRB) is a potent and selective inhibitor of human cytomegalovirus (HCMV), but it lacks clinical utility due to rapid in vivo metabolism. We hypothesized that amino acid ester prodrugs of BDCRB may enhance both in vitro potency and systemic exposure of BDCRB through evasion of BDCRB-metabolizing enzymes. To this end, eight different amino acid prodrugs of BDCRB were tested for N-glycosidic bond stability, ester bond stability, Caco-2 cell uptake, antiviral activity, and cytotoxicity. The prodrugs were resistant to metabolism by BDCRB-metabolizing enzymes, and ester bond cleavage was rate-limiting in metabolite formation from prodrug. Thus, BDCRB metabolism could be controlled by the selection of promoiety. In HCMV plaque-formation assays, l-Asp-BDCRB exhibited 3-fold greater selectivity than BDCRB for inhibition of HCMV replication. This potent and selective antiviral activity in addition to favorable stability profile made l-Asp-BDCRB an excellent candidate for in vivo assessment and pharmacokinetic comparison with BDCRB. In addition to rapid absorption and sufficient prodrug activation after oral administration to mice, l-Asp-BDCRB exhibited a 5-fold greater half-life than BDCRB. Furthermore, the sum of area under the concentration-time profile (AUC)(BDCRB) and AUC(prodrug) after l-Asp-BDCRB administration was roughly 3-fold greater than AUC(BDCRB) after BDCRB administration, suggesting that a reservoir of prodrug was delivered in addition to parent drug. Overall, these findings demonstrate that amino acid prodrugs of BDCRB exhibit evasion of metabolizing enzymes (i.e., bioevasion) in vitro and provide a modular approach for translating this in vitro stability into enhanced in vivo delivery of BDCRB.

Current strategies used to enhance the paracellular transport of therapeutic polypeptides across the intestinal epithelium

The intent of this paper is to update the reader on various strategies which have been utilized to increase the paracellular permeability of protein and polypeptide drugs across the intestinal epithelium. Structural features of protein and polypeptide drugs, together with the natural anatomical and physiological features of the gastrointestinal (GI) tract, have made oral delivery of this class of compounds extremely challenging. Interest in the paracellular route for the transport of therapeutic proteins and polypeptides following oral administration has recently intensified and continues to be explored. The assumption that molecules with a large molecular weight are not able to diffuse through the tight junctions of the intestinal membrane has been challenged by current research, along with an increased understanding of tight junction physiology.

Solid-phase total synthesis of scytalidamide A

The first total synthesis of the natural cyclic heptapeptide scytalidamide A was achieved on solid phase using two different resins, a phenylalanine silane resin and a 4-methoxybenzaldehyde backbone linker resin. The synthetic product confirms the structure of the natural product reported in the preceding paper in this issue (Tan, L. T.; Cheng, X. C.; Jensen, P. R.; Fenical, W. J. Org. Chem. 2003, 68, 8767).

Syntheses of cyclic prodrugs of RGD peptidomimetics with various macrocyclic ring sizes: evaluation of physicochemical, transport and antithrombic properties

The objective of this work was to synthesize cyclic prodrugs 1a-d of RGD peptidomimetics 2a-d with various ring sizes (n[CH2] = 1, 3, 5 and 7) and to evaluate the effect of ring size on their transport, physicochemical, enzymatic stability, and antithrombic properties. The syntheses of cyclic prodrugs 1a-d were achieved by converging two key intermediates, Boc-Phe-O-CH2-OCO-OpNP (5) and H2N-(CH2)n-CO-Asp(OBzl)-OTce (8a-d), to give linear precursors Boc-Phe-O-CH2-OCO-HN-(CH2)n-CO-Asp(OBzl)-OTce (9a-d). The N- and C-terminus protecting groups were removed from 9a-d to give 10a-d. Linear precursors 10a-d were cyclized, and the remaining Bzl-protecting group was removed to produce cyclic prodrugs 1a-d in around 20% overall yield. The linear RGD peptidomimetics (2a-d) were synthesized using standard Boc-amino acid chemistry by solution-phase method. Increasing the ring size by adding methylene groups also increases the hydrophobicity of the cyclic prodrugs and parent RGD peptidomimetics. The transport properties of cyclic prodrugs 1c and 1d were 2.6- and 4.4-fold better than those of parent compounds 2c and 2d, respectively. These results suggest that increasing the hydrophobicity of the cyclic prodrugs and parent RGD peptidomimetics enhanced their transport properties. The hydrodynamic radii of the cyclic prodrugs were also smaller than those of their respective parent compounds, suggesting that the change in size may contribute to their transport properties. The chemical stability of the cyclic prodrugs was affected by the ring size, and the cyclic prodrug with the larger ring size (i.e. 1d) was more stable than the smaller one (i.e. 1a). All the cyclic prodrugs were more stable at pH 4 than at pH 7 and 10. Prodrug-to-drug conversion could be induced by isolated esterase as well as esterase found in human plasma. An increase in the length of methylene group (n[CH2] = 1, 3, 5, 7) enhanced the antithrombic activity of the prodrugs and the parent compounds. In summary, the ring size of cyclic prodrugs affected their transport, physicochemical, and antithrombic properties.

Synthesis and stability study of a modified phenylpropionic acid linker-based esterase-sensitive prodrug

An esterase-sensitive amide prodrug 1 with a modified phenylpropionic acid linker was synthesized. The prodrug can be converted to the drug using isolated porcine esterase and human plasma. Paraoxon, an esterase inhibitor, can inhibit prodrug-to-drug conversion. The conversion of prodrug 1 was via phenol intermediate 9 followed by a lactonization reaction to give lactone 2 and the drug.

Quantitative analysis of a model opioid peptide and its cyclic prodrugs in rat plasma using high-performance liquid chromatography with fluorescence and tandem mass spectrometric detection

Two analytical methods were developed for quantitative determination of DADLE (H(2)N-Tyr-D-Ala-Gly-Phe-D-Leu-COOH) and its two cyclic prodrugs in rat plasma. For high-performance liquid chromatography with fluorescence detection (LC-FLU), precolumn derivatization of DADLE was accomplished by labeling the N-terminal amino group with the reagent naphthalene-2,3-dicarboxaldehyde in the presence of cyanide (NDA/CN) to form a highly fluorescent 1-cyanobenz[f]isoindole (CBI) derivative. A multi-dimensional LC system was employed to improve selectivity, and solid-phase extraction (SPE) was used for plasma sample preparation. The cyclic prodrugs were converted to DADLE prior to their derivatization. With fluorescence detection after derivatization, the limit of quantitation (LOQ) was 6 ng ml(-1) for the analysis of DADLE, and good linearity was observed up to 6000 ng ml(-1) in rat plasma. Quantitative analysis of DADLE and its cyclic prodrugs was also performed using liquid chromatography interfaced to electrospray ionization tandem mass spectrometry (LC-ESI-MS-MS). Chromatographic separation was achieved on a C(18) column using gradient elution in a water-acetonitrile system containing 0.1% (v/v) formic acid. The tandem mass spectrometric analysis was performed in the multiple reaction monitoring mode using internal standardization to improve assay precision and accuracy. For plasma sample pretreatment, acetonitrile was added first to precipitate proteins and SPE was used to minimize matrix effects. Using LC-ESI-MS-MS, the LOQ was 0.5 ng ml(-1) for DADLE and 2 to 5 ng ml(-1) for its prodrugs. Good linearity was observed from the LOQ up to 1000 ng ml(-1) for all compounds. For the analysis of DADLE, both analytical methods showed good precision, accuracy and stability. However, for prodrug analysis, LC-FLU showed some sensitivity and accuracy problems, while the LC-ESI-MS-MS method provided consistent and satisfactory results. In conclusion, LC-ESI-MS-MS is the method of choice for the analysis of DADLE and its cyclic prodrugs in rat plasma samples due to its good selectivity, high sensitivity, and fast analysis. Its application was demonstrated through biodisposition and bioconversion studies of the coumarinic acid-based prodrug after intravenous administration in rats.

In vitro stability and in vivo pharmacokinetic studies of a model opioid peptide, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (DADLE), and its cyclic prodrugs

In vitro stability and in vivo pharmacokinetic studies of a model opioid peptide, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH (DADLE), and its cyclic prodrugs (acyloxyalkoxy-based cyclic prodrug of DADLE, coumarinic acid-based cyclic prodrug of DADLE, and oxymethyl-modified coumarinic acid-based cyclic prodrug of DADLE) were conducted. The enzymatic stability of DADLE and its prodrugs in various biological media was determined at 37 degrees C in the presence and absence of paraoxon, a known esterase inhibitor. The prodrugs exhibited metabolic stability to exo- and endopeptidases, and esterase-catalyzed bioconversion of the prodrugs to DADLE was observed. For pharmacokinetic studies in rats, various biological samples (blood, bile, urine, and brain) were collected after i.v. administration of DADLE and its prodrugs. The samples were analyzed by high-performance liquid chromatography with tandem mass spectrometric detection, and the conversion from the prodrugs to intermediates to DADLE was monitored. The prodrugs exhibited similar pharmacokinetic properties and showed improved stability compared with DADLE in rat blood. This increased stability led to higher plasma concentrations of DADLE after i.v. administration of the prodrugs compared with i.v. administration of DADLE alone. In terms of elimination pathways, metabolism by endopeptidases was the major route for DADLE elimination, whereas rapid biliary excretion was the major route of elimination for the prodrugs. The rapid elimination of the prodrugs by the liver and the formation of stable intermediates after esterase hydrolysis limited the bioconversion efficiencies of the prodrugs to DADLE after i.v. administration. The substrate activity of the prodrugs for efflux transporters (e.g., P-glycoprotein) in the blood-brain barrier significantly restricted their access to the brain.

Synthesis of a novel cyclic prodrug of RGD peptidomimetic to improve its cell membrane permeation

The objective of this work was to synthesize cyclic prodrug 2 derived from the parent RGD peptidomimetic 1 and to evaluate its chemical and enzymatic stabilities and antithrombic activity. Cyclic prodrug 2 was formed to improve the cell membrane permeation of RGD peptidomimetic 1 by transiently masking the unfavorable physicochemical properties of compound 1. Cyclic prodrug 2 was synthesized by linking the amino and carboxylic acid groups of parent 1 via the (acyloxy)alkoxy promoiety. The prodrug-to-drug conversion of cyclic prodrug 2 was evaluated in isolated esterase and human plasma in the absence and presence of the esterase inhibitor paraoxon. The rate of hydrolysis of cyclic prodrug 2 was significantly faster in plasma (t(1/2)=33.5+/-0.6 min) than in PBS (t(1/2)=314+/-11 min). Cyclic prodrug 2 was converted by esterase to the parent compound 1 and this conversion was inhibited by an esterase inhibitor, paraoxon. The IC50 (4 micro M) of cyclic prodrug 2 was higher than the IC50 (1.9 micro M) of parent drug 1. The antithrombic activity of cyclic prodrug 2 depends on the incubation time in platelet-rich plasma; the activity increases with incubation time, suggesting that the prodrug-to-drug conversion is time-dependent and mediated by esterase. Cyclic prodrug 2 was more stable under acidic and neutral conditions than under basic conditions, suggesting that handling and formulation of this prodrug should be undertaken under acidic conditions.

Characterization of acetylcholinesterase in Caco-2 cells

Acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) was solubilized from cultured Caco-2 cells. It was established that this enzyme activity is acetylcholinesterase by substrate specificity (acetylthiocholine, acetyl-beta-methylthiocholine>propionylthiocholine>butyrylthiocholine), substrate inhibition, and specificity of inhibitors (BW284c51>iso-OMPA). The acetylcholinesterase activity increased proportional to the degree of differentiation of the cells. Most of the enzyme was membrane bound, requiring detergent for solubilization, and the active site faced the external fluid. Only one peak of activity, which corresponded to a monomeric form, could be detected on linear sucrose density gradients. The sedimentation of this form of the enzyme was shifted depending on whether Triton X-100 or Brij 96 detergent was used. These results indicate that the epithelial-derived Caco-2 cells produce predominantly an amphiphilic, monomeric form of acetylcholinesterase that is bound to the plasma membrane and whose catalytic center faces the extracellular fluid.

Characterization of the efflux transporter(s) responsible for restricting intestinal mucosa permeation of an acyloxyalkoxy-based cyclic prodrug of the opioid peptide DADLE

PURPOSE

To elucidate the efflux transporter(s) responsible for restricting the permeation of an acyloxyalkoxy-based cyclic prodrug of the opioid peptide DADLE (AD) through Caco-2 cell monolayers.

METHODS

The cellular permeation characteristics of AD were investigated using Caco-2 cells, Madin-Darby canine kidney wild-type II cells (MDCK-WT), MDCK cells transfected with the human MDR1 gene (MDCK-MDR1), and MDCK cells transfected with the human MRP2 gene (MDCK-MRP2). These cells were grown as monolayers onto microporous membranes. The disappearance of AD from the donor side and its appearance on the receiver side were monitored by high-performance liquid chromatography. The substrate activity of AD for P-glycoprotein (P-gp) was determined using GF120918, a known P-gp specific inhibitor. The substrate activity of AD for MRP2 was determined by using cyclosporin A, a known MRP2 and P-gp inhibitor.

RESULTS

In Caco-2 cells, the ratio of the apparent permeability coefficients (Papp) of AD flux measured in the basolateral (BL) to apical (AP) direction vs. the flux in the AP-to-BL direction (Papp BL-to-AP/ Papp AP-to-BL) was 99. In the presence of 2 microM GF120918 or 25 microM cyclosporin A. the Papp BL-to-AP/Papp AP-to-BL ratio was decreased to 11. In MDCK-WT, MDCK-MDR1, and MDCK-MRP2 cells, the Papp BL-to-AP/Papp AP-to-BL ratios of AD were 4.7, 10, and 5.8, respectively. A mixture of GF120918 (2 microM) and cyclosporin A (25 microM) decreased the Papp BL-to-AP/Papp AP-to-BL ratios of AD in MDCK-WT, MDCK-MDR1, and MDCK-MRP2 cells to 1.2,1.8, and 2.3, respectively.

CONCLUSIONS

These data suggest that AD is a much better substrate for P-gp than MRP2 and that the restricted permeation of this cyclic prodrug in Caco-2 cells and in the intestinal mucosa probably is due primarily to its substrate activity for P-gp.

A modified coumarinic acid-based cyclic prodrug of an opioid peptide: its enzymatic and chemical stability and cell permeation characteristics

PURPOSE

To evaluate the chemical/enzymatic stability and the cell permeation characteristics of the modified coumarinic acid-based cyclic prodrug 2 of DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), which has an aldehyde equivalent (oxymethyl) inserted between the phenolic group of the promoiety and the carboxylic acid group of the peptide.

METHODS

The rates of the chemical/enzymatic conversion of the oxymethyl-modified prodrug 2 to DADLE were measured by HPLC. The cellular permeation characteristics of DADLE and its oxymethyl-modified prodrug 2 were measured by HPLC using Caco-2 cells, wild type Madin-Darby Canine Kidney cells (MDCK-WT), MDCK cells transfected with human MDR1 gene (MDCK-MDR1), and MDCK cells transfected with human MRP2 gene (MDCK-MRP2) grown onto microporous membranes.

RESULTS

The oxymethyl-modified coumarinic acid-based cyclic prodrug 2 degraded chemically to DADLE in a pH-dependent manner, i.e., rates of conversion increased with increasing pH. The prodrug 2 degraded rapidly in rat plasma (t1/2 = 39 min) and rat liver homogenate (t1/2 = 59.2 min), but much slower in Caco-2 cell homogenate (t1/2 = 678.7 min) and human plasma (t1/2 = 264.3 min). In all four cell lines used for transport studies, the flux rates of the oxymethyl prodrug 2 in the basolateral (BL)-to-apical (AP) direction (Papp BL-to-AP) were significantly greater than the flux rates in the AP-to-BL direction (Papp AP-to-BL). The Papp BB-to-AP/Papp AP-to-BL ratios were >116, 35.1, 21.2, and 12.6 in Caco-2, MDCK-MDR1, MDCK-MRP2, and MDCK-WT cells, respectively. The efflux of the modified prodrug could be inhibited by GF120918 (an inhibitor for P-gp) and cyclosporin A (an inhibitor for P-gp and MRP2).

CONCLUSIONS

The oxymethyl-modified coumarinic acid-based cyclic prodrug 2 of DADLE could be converted to DADLE in both chemical and enzymatic media. However, the prodrug was a good substrate for both P-gp and MRP2 suggesting that its permeation across intestinal mucosa and blood-brain barrier would be significantly restricted.

Synthesis of cyclic prodrugs of Aggrastat and its analogue with a modified phenylpropionic acid linker

[structure: see text] The objective of this work was to synthesize cyclic prodrugs 1a and 1b from Aggrastat 2a and its analogue 2b, respectively, to improve their membrane permeation. Cyclic prodrugs 1a and 1b were formed using an ester bond between the -COOH group of Aggrastat or its analogue and the phenylpropionic acid linker 3 and an amide bond between the piperidinylamine and the -COOH group of the linker 3, respectively, as outlined in Scheme 4.

Prodrugs in nasal drug delivery

Prodrugs have been used to overcome poor solubility, insufficient stability, incomplete absorption across biological membranes and premature metabolism to active species. This review examines the importance of various physicochemical factors affecting nasal absorption of drugs. Novel trends in nasal prodrug development in the areas of targeted delivery to the CNS and selective targeting of the nutrient transporter system of the nasal mucosa have received considerable attention.

A coumarin-based prodrug strategy to improve the oral absorption of RGD peptidomimetics

In recent years, major progress has been made in the design and synthesis of fibrinogen antagonists, which are peptidomimetic Arg-Gly-Asp (RGD) analogs. These RGD analogs are very promising antiplatelet agents. However, the clinical development of orally active RGD analogs has been hindered by the low oral bioavailability of many such RGD analogs. Aimed at enhancing their oral bioavailability, we have synthesized several coumarin-based cyclic prodrugs of RGD analogs, which have the two most polar functional groups, a carboxyl and an amino group, masked as an ester and an amide, respectively. As expected, these cyclic prodrugs have higher membrane interaction potentials as estimated by determining their partitioning between aqueous buffer and an immobilized artificial membrane than the corresponding RGD analogs. Consequently, these cyclic prodrugs are 5-6-fold more able to permeate monolayers of Caco-2 cells, an in vitro cell culture model of the intestinal mucosa barrier. Preliminary studies using dog also indicate the promising potential of using this coumarin-based prodrug strategy to improve the oral bioavailability of such RGD analogs.

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.

Synthesis and evaluation of the physicochemical properties of esterase-sensitive cyclic prodrugs of opioid peptides using coumarinic acid and phenylpropionic acid linkers

In an attempt to improve the membrane permeabilities of opioid peptides, we have synthesized cyclic prodrugs of [Leu5]-enkephalin and DADLE using a coumarinic acid or a phenylpropionic acid linker. The synthesis of the coumarinic acid- and phenylpropionic acid-based cyclic prodrugs followed similar strategies. Key intermediates were the compounds with the C-terminal amino acids of opioid peptides (L-Leu, [Leu5]-enkephalin; D-Leu, DADLE) attached to the phenol hydroxyl group and the remaining amino acids of the peptide linked via the N-terminal amino acid (L-Tyr) attached to the carboxylic acid groups of the prodrug moieties (coumarinic acid or propionic acid). Cyclization of these linear precursors gave the cyclic prodrugs in 30-50% yields. These cyclic prodrugs exhibited excellent transcellular permeation characteristics across Caco-2 cell monolayers, an in vitro model of the intestinal mucosa. To correlate the cellular permeabilities of these cyclic prodrugs with their physicochemical properties, we calculated their Stokes-Einstein molecular radii from their diffusion coefficients which were determined by NMR and we determined their membrane interaction potentials using immobilized artificial membrane (IAM) column chromatography. The cyclic prodrugs exhibited molecular radii similar to those of the parent compounds, [Leu5]-enkephalin and DADLE. However, these cyclic prodrugs were shown to have much higher membrane interaction potentials than their corresponding opioid peptides. Therefore, the enhanced cellular permeation of the cyclic prodrugs is apparently due to the alteration of their lipophilicity and hydrogen bonding potential, but not their molecular sizes.

The effect of conformation on the membrane permeation of coumarinic acid- and phenylpropionic acid-based cyclic prodrugs of opioid peptides

In an earlier study using Caco-2 cells, an in vitro cell culture model of the intestinal mucosa, we have shown that the coumarinic-based (3 and 4) and the phenylpropionic acid-based (5 and 6) cyclic prodrugs were more able to permeate the cell monolayers than were the corresponding opioid peptides, [Leu5]-enkephalin (1, H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (2, H-Tyr-D-Ala-Gly-Phe-D-Leu-OH). In an attempt to explain the increased permeation of the cyclic prodrugs, we have determined the possible conformations of these cyclic prodrugs in solution, using spectroscopic techniques (2D-NMR, CD) and molecular dynamics simulations. Spectroscopic as well as molecular dynamic studies indicate that cyclic prodrug 4 exhibits two major conformers (A and B) in solution. Conformer A exhibited a type I beta-turn at Tyr1-D-Ala2-Gly3-Phe4. The presence of a turn was supported by ROE cross-peaks between the NH of D-Ala2 and the NH of Gly3 and between the NH of Gly3 and the NH of Phe4. Conformer B of cyclic prodrug 4 consisted of type II beta-turns at the same positions. The type II turn was stabilized by hydrogen bonding, thus forming a more compact structure, whereas the type I turn did not exhibit similar intramolecular hydrogen bonding. Spectroscopic data for compounds 3, 5 and 6 are consistent with the conclusion that these cyclic prodrugs have solution structures similar to those observed with cyclic prodrug 4. The increased lipophilicity and well-defined secondary structures in cyclic prodrugs 3-6, but not in the linear peptides 1 and 2, could both contribute to the enhanced ability of these prodrugs to permeate membranes.

Acyloxyalkoxy-based cyclic prodrugs of opioid peptides: evaluation of the chemical and enzymatic stability as well as their transport properties across Caco-2 cell monolayers

PURPOSE

To evaluate the chemical and enzymatic stability, as well as the cellular permeation characteristics, of the acyloxyalkoxy-based cyclic prodrugs 1 and 2 of the opioid peptides [Leu5]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively.

METHODS

The rates of conversion of 1 and 2 to [Leu5]-enkephalin and DADLE, respectively, were measured by HPLC in HBSS, pH = 7.4, and in various biological media (e.g., human plasma and Caco-2 cell and rat liver homogenates) having measurable esterase activity. The cellular permeation and metabolism characteristics of [Leu5]-enkephalin, DADLE and the cyclic prodrugs 1 and 2 were measured using Caco-2 cell monolayers grown onto microporous membranes and monitored by HPLC.

RESULTS

Cyclic prodrugs 1 and 2 degraded slowly but stoichiometrically to [Leu5]-enkephalin and DADLE, respectively, in HBSS, pH = 7.4. In homogenates of Caco-2 cells and rat liver, as well as 90% human plasma, the rates of disappearance of the cyclic prodrugs were significantly faster than in HBSS. The stabilities of the cyclic prodrugs 1 and 2 were increased significantly in 90% human plasma and Caco-2 cell homogenates when paraoxon, a potent inhibitor of serine-dependent esterases, was included in the incubation mixtures. A similar stabilizing effect of paraoxon was not observed in 50% rat liver homogenates, but was observed in 10% homogenates of rat liver. When applied to the AP side of a Caco-2 cell monolayer, DADLE and cyclic prodrugs 1 and 2 exhibited significantly greater stability than [Leu5]-enkephalin. Based on their physicochemical properties (i.e., lipophilicity), cyclic prodrugs 1 and 2 should have exhibited high permeation across Caco-2 cell monolayers. Surprisingly, the AP-to-BL apparent permeability coefficients (P(App)) for cyclic prodrugs 1 and 2 across Caco-2 cell monolayers were significantly lower than the P(App) value determined for the metabolically stable opioid peptide DADLE. When the P(App) values for cyclic prodrugs 1 and 2 crossing Caco-2 cell monolayers in the BL-to-AP direction were determined, they were shown to be 36 and 52 times greater, respectively, than the AP-to-BL values.

CONCLUSIONS

Cyclic prodrugs 1 and 2, prepared with an acyloxyalkoxy promoiety, were shown to degrade in biological media (e.g., 90% human plasma) via an esterase-catalyzed pathway. The degradation of cyclic prodrug 1, which contained an ester formed with an L-amino acid, degraded more rapidly in esterase-containing media than did prodrug 2, which contained an ester formed with a D-amino acid. Cyclic prodrugs 1 and 2 showed very low AP-to-BL Caco-2 cell permeability, which did not correlate with their lipophilicities. These low AP-to-BL permeabilities result because of their substrate activity for apically polarized efflux systems.

Coumarinic acid-based cyclic prodrugs of opioid peptides that exhibit metabolic stability to peptidases and excellent cellular permeability

To evaluate the cellular permeation characteristics and the chemical and enzymatic stability of coumarinic acid-based cyclic prodrugs 1 and 2 of the opioid peptides [Leu5]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively.

METHODS

The rates of conversion of the cyclic prodrugs 1 and 2 to [Leu5]-enkephalin and DADLE, respectively, in HBSS, pH 7.4 (Caco-2 cell transport buffer) and in various biological media having measurable esterase activity were determined by HPLC. The cell permeation characteristics of [Leu5]-enkephalin, DADLE and cyclic prodrugs 1 and 2 were measured using Caco-2 cell monolayers grown onto microporus membranes and monitored by HPLC.

RESULTS

In HBSS, pH 7.4, cyclic prodrugs 1 and 2 degraded chemically to intermediates that further degraded to [Leu5]-enkephalin and DADLE, respectively, in stoichiometric amounts. In 90% human plasma and rat liver homogenate, the disappearance of cyclic prodrugs 1 and 2 was significantly faster than in HBSS, pH 7.4. The half-lives in 90% human plasma and in rat liver homogenate were substantially longer after pretreatment with paraoxon, a known inhibitor of serine-dependent esterases. When applied to the AP side of a Caco-2 cell monolayer, cyclic prodrug 1 exhibited significantly greater stability against peptidase metabolism than did [Leu5]-enkephalin. Cyclic prodrug 2 and DADLE exhibited similar stability when applied to the AP side of the Caco-2 cell monolayer. Prodrug 1 was 665-fold more able to permeate the Caco-2 cell monolayers than was [Leu5]-enkephalin, in part because of its increased enzymatic stability. Prodrug 2 was shown to be approximately 31 fold more able to permeate a Caco-2 cell monolayer than was DADLE.

CONCLUSIONS

Cyclic prodrugs 1 and 2, prepared with the coumarinic acid promoiety, were substantially more able to permeate Caco-2 cell monolayers than were the corresponding opioid peptides. Prodrug 1 exhibited increased stability to peptidase metabolism compared to [Leu5]-enkephalin. In various biological media, the opioid peptides were released from the prodrugs by an esterase-catalyzed reaction, which is sensitive to paraoxon inhibition.

Phenylpropionic acid-based cyclic prodrugs of opioid peptides that exhibit metabolic stability to peptidases and excellent cellular permeation

PURPOSE

To evaluate the cellular permeation characteristics and the chemical and enzymatic stability of phenylpropionic acid-based cyclic prodrugs 1 and 2 of opioid peptides [Leu5]-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and DADLE (H-Tyr-D-Ala-Gly-Phe-D-Leu-OH), respectively.

METHODS

The rates of conversion of cyclic prodrugs 1 and 2 to [Leu5]-enkephalin and DADLE, respectively, in HBSS, pH 7.4 (Caco-2 cell transport buffer) and in various biological media having measurable esterase activity were determined by HPLC. The cell permeation characteristics of [Leu5]-enkephalin, DADLE, and cyclic prodrugs 1 and 2 were measured using Caco-2 cell monolayers grown onto microporus membranes and monitored by HPLC.

RESULTS

In HBSS, pH 7.4, cyclic prodrugs 1 and 2 degraded to [Leu5]-enkephalin and DADLE, respectively, in stoichiometric amounts. In 90% human plasma, the rates of disappearance of cyclic prodrugs 1 and 2 were slightly faster than in HBSS, pH 7.4. These accelerated rates of disappearance in 90% human plasma could be reduced to the rates observed in HBSS, pH 7.4, by pretreatment of the plasma with paraoxon, a known inhibitor of serine-dependent esterases. In homogenates of Caco-2 cells and rat liver, accelerated rates of disappearance of cyclic prodrugs 1 and 2 were not observed. When applied to the AP side of a Caco-2 cell monolayer, cyclic prodrug 1 exhibited significantly greater stability against peptidase metabolism than did [Leu5]-enkephalin. Cyclic prodrug 2 and DADLE exhibited stability similar to prodrug 1 when applied to the AP side of the Caco-2 cell monolayers. Prodrug 1 was 1680 fold more able to permeate the Caco-2 cell monolayers than was [Leu5]-enkephalin, in part because of its increased enzymatic stability. Prodrug 2 was shown to be approximately 77 fold more able to permeate a Caco-2 cell monolayer than was DADLE.

CONCLUSIONS

Cyclic prodrugs 1 and 2, prepared with the phenylpropionic acid promoiety, were substantially more able to permeate Caco-2 cell monolayers than were the corresponding opioid peptides. Prodrug 1 exhibited increased stability to peptidase metabolism compared to [Leu5]-enkephalin. In 90% human plasma but not in Caco-2 cell and rat liver homogenates, the opioid peptides were released from the cyclic prodrugs by an esterase-catalyzed reaction that is sensitive to paraoxon inhibition. However, the rate of this bioconversion appears to be extremely slow.

A comparison of the bioconversion rates and the Caco-2 cell permeation characteristics of coumarin-based cyclic prodrugs and methylester-based linear prodrugs of RGD peptidomimetics

PURPOSE

To compare the bioconversion rates in various biological media and the Caco-2 cell permeation characteristics of coumarin based cyclic prodrugs (3a, 3b) and methylester-based linear prodrugs (1b, 2b) of two RGD peptidomimetics (1a, 2a).

METHODS

Bioconversion rates of the prodrugs to the RGD peptidomimetics were determined in Hank balances salt solution (HBSS), pH 7,4, at 37 degrees C and in various biological media (human blood plasma, rat liver homogenate, Caco-2 cell homogenate) known to have esterase activity. Transport rates of the prodrugs and the RGD peptidomimetics were determined using Caco-2 cell monolayers, an in vitro cell culture model of the intestinal mucosa. RESULTS. In HBSS, pH 7,4, the coumarin-based cyclic prodrugs 3a and 3b degraded slowly and quantitatively to the RGD peptidomimetics 1a and 2a, respectively (3a, t1/2 = 630+2-14 min; 3b, t1/2 = 301 +/-12 min). The methylester-based linear prodrugs 1b and 2b were more stable to chemical hydrolysis (1b and 2b, t1/2 > 2000 min). Both the coumarin-based cyclic prodrugs and the methylester-based linear prodrugs degraded more rapidly in biological media containing esterase activity (e.g., 90% human blood plasma: 1b, t1/2 < 5 min; 2b, t1/2 < 5 min; 3a, t1/2 < 91+/-1 min; 3b, 1/2 < 57+/-2 min). When the apical (AP)-to-basolateral (BL) permeation characteristics were determined using Caco-2 cell monolayers, it was found that the methylester prodrugs 1b and 2b underwent esterase bioconversion (>80%) to the RGD peptidomimetics 1a and 2a, respectively, In contrast, the cyclic prodrugs 3a and 3b permeated the cell monolayers intact. Considering the appearance of both the prodrug and the RGD peptidomimetic on the BL side, the methylester prodrugs 1b and 2b were approximately 12-fold more able to permeate than were the RGD peptidomimetics 1a and 2a. When similar analysis of the transport data for the coumarin prodrugs 3a and 3b was performed, they were shown to be approximately 6-fold and 5-fold more able to permeate than were the RGD peptidomimetics 1a and 12a, respectively.

CONCLUSION

The coumarin-based cyclic prodrugs 3a and 3b were chemically less stable, but metabolically more stable, then the methylester based linear prodrugs. The esterase stability of the cyclic prodrugs 3a and 3b means that they are transported intact across the Caco-2 cell monolayer in contrast to the methylester prodrugs 1b and 2b, which undergo facile bioconversion during their transport to the RGD peptidomimetics. However, both prodrug systems successfully delivered more (5-12-fold) of the RGD peptidomimetic and/or the precursor (prodrug) than did the RGD peptidomimetics themselves.

Transport characteristics of peptidomimetics. Effect of the pyrrolinone bioisostere on transport across Caco-2 cell monolayers

PURPOSE

To compare the permeation characteristics of amide bond-containing HIV-1 protease inhibitors and their pyrrolinone-containing counterparts across Caco-2 cell monolayers, a model of the intestinal mucosa.

METHODS

Transepithelial transport and cellular uptake of three pairs of amide bond-containing and pyrrolinone-based peptidomimetics were assessed in the presence and absence of cyclosporin A using the Caco-2 cell culture model. The potential of the peptidomimetics to interact with biological membranes was estimated by IAM chromatography.

RESULTS

In the absence of cyclosporin A, apical (AP) to basolateral (BL) flux of all compounds studied was less than the flux determined in the opposite direction (i.e., BL-to-AP). The ratio of the apparent permeability coefficients (Papp) calculated for the BL-to-AP and AP-to-BL transport (P(BL-->AP)/P(AP-->BL)) varied between 1.7 and 36.2. When individual pairs were ompared, P(BL-->AP)/P(AP-BL) ratios of the pyrrolinone-containing compounds were 1.5 to 11.5 times greater than those determined for the amide bond-containing analogs. Addition of 25 microM cyclosporin A to the transport buffer reduced the P(BL-->AP)/P(AP-->BL) ratios for all protease inhibitors to a value close to unity. Under these conditions, the amide bond-containing peptidomimetics were at least 1.6 to 2.8 times more able to permeate Caco-2 cell monolayers than were the pyrrolinone-containing compounds. The intrinsic uptake characteristics into Caco-2 cells determined in the presence of 25 microM cyclosporin A were slightly greater for the amide bond-containing protease inhibitors than for the pyrrolinone-containing analogs. These uptake results are consistent with the transepithelial transport results determined across this in vitro model of the intestinal mucosa.

CONCLUSIONS

The amide bond-containing and pyrrolinone-based peptidomimetics are substrates for apically polarized efflux systems present in Caco-2 cell monolayers. The intrinsic permeabilities of the amide bond-containing protease inhibitors are slightly greater than the intrinsic permeabilities of the pyrrolinone-based analogs through Caco-2 cell monolayers.

Prodrugs for nasal drug delivery

Recently, the delivery of xenobiotics via the nasal route has received increasing attention as this offers several advantages, i.e. high systemic availability, rapid onset of action. Both charged and uncharged forms of drugs can be transported across the nasal epithelium. This mucosa is rich in various metabolizing enzymes such as aldehyde dehydrogenase, glutathione transferases, epoxide hydrolases, cyt-P450-dependent monooxygenases. The presence of these enzymes may make it possible for pharmaceutical scientists to design prodrugs for better absorption and high systemic availability. Recent advances in peptide nasal delivery through prodrug modification has been thoroughly discussed in this paper. Finally, nasally delivered therapeutic agents targeted to various disease states have been examined.

The effect of beta-turn structure on the passive diffusion of peptides across Caco-2 cell monolayers

PURPOSE

To investigate the relationships between the beta-turn structure of a peptide and its passive diffusion across Caco-2 cell monolayers, an in vitro model of the intestinal mucosa.

METHODS

Linear hydrophilic peptides (Ac-TyrProXaaZaaVal-NH2; Xaa = Gly, Ile and Zaa = Asp, Asn) and hydrophobic (Ac-YaaPro-XaaIle Val-NH2; Yaa = Tyr, Phe and Xaa = Gly, Ile: and Ac-PhePro-XaaIle-NH2; Xaa = Gly, Ile) peptides were synthesized and their effective permeability coefficients (Peff) were determined across Caco-2 cell monolayers. The lipophilicities of the peptides were estimated by measuring their partition coefficients (Po/w) between 1-octanol and HBSS. Two-dimensional NMR (2D-NMR) spectroscopy and circular dichroism (CD) spectroscopy was used to determine the solution structures of these model peptides.

RESULTS

Using 2D-NMR spectroscopy and CD spectroscopy, the hydrophilic Gly-containing peptides (Ac-TyrProGlyZaaVal-NH2; Zaa = Asp, Asn) were shown to exhibit a higher degree of beta-turn structure in solution than the Ile-containing peptides (Ac-TyrProIleZaaVal-NH2; Zaa = Asp, Asn). CD spectroscopy was used to show that the Gly-containing hydrophobic peptides (Ac-YaaProGlyIleVal-NH2; Yaa = Tyr, Phe: and Ac-PheProGlyIle-NH2) exhibited a higher degree of beta-turn structure in solution than the Ile-containing hydrophobic peptides. The Peff values of all four hydrophilic peptides across unperturbed Caco-2 cell monolayers were very low and no statistically significant differences were observed between the Gly- and Ile-containing pentapeptides within either the Asp or Asn series. The Peff values for the hydrophobic Gly-containing peptides were significantly greater than the Peff values determined for their Ile-containing counterparts. The Gly-containing penta- and tetrapeptides in the Phe series, which exhibited high permeation, were shown to be metabolically unstable. In contrast, the Gly- and Ile-containing pentapeptides in the Tyr series and the Ile-containing penta- and tetrapeptides in the Phe series, which exhibited low permeation, were metabolically stable.

CONCLUSIONS

Hydrophobic peptides that exhibit significant beta-turn structure in solution are more lipophilic as measured by log Po/w and more readily permeate Caco-2 cell monolayers via the transcellular route than hydrophobic peptides that lack this type of solution structure. The ability of these peptides to permeate Caco-2 cell monolayers via the transcellular route also exposed them to metabolism, presumably by cytosolic endopeptidase. Similar secondary structural features in hydrophilic peptides do not appear to sufficiently alter the physicochemical properties of the peptides so as to alter their paracellular flux through unperturbed Caco-2 cell monolayers.

The effect of beta-turn structure on the permeation of peptides across monolayers of bovine brain microvessel endothelial cells

PURPOSE

To investigate the effects of the beta-turn structure of a peptide on its permeation via the paracellular and transcellular routes across cultured bovine brain microvessel endothelial cell (BBMEC) monolayers, an in vitro model of the blood-brain barrier (BBB).

METHODS

The effective permeability coefficients (Peff) of the model peptides were determined across BBMEC monolayers. The dimensions of the aqueous pores in the tight junctions (TJs) of the BBMEC monolayers were determined using a series of hydrophilic permeants. This value and the molecular radius of each peptide were used to calculate the theoretical paracellular (PP*) and transcellular (PT*) permeability coefficients for each peptide.

RESULTS

A comparison of the theoretical PP* values with the observed Peff values was made for a series of model peptides. For the most hydrophobic peptides (Ac-PheProXaaIle-NH2 and Ac-PheProXaaIleVal-NH2; Xaa = Gly, Ile), it was concluded that the Gly-containing peptide of each pair more readily permeates BBMEC monolayers via the transcellular pathway than the Ile-containing analog. In addition, the Gly-containing peptides, which exhibit more beta-turn structure, were shown to be more lipophilic than the Ile-containing peptides as estimated by the log of their 1-octanol:HBSS partition coefficients (log Po/w). However, the three hydrophilic peptide pairs (Ac-TyrProXaaAspVal-NH2, Ac-TyrProXaaAsnVal-NH2, and Ac-TyrProXaaIleVal-NH2; Xaa = Gly, Ile) were found to permeate BBMEC monolayers predominantly via the paracellular pathway. No differences were observed in the Peff values of the hydrophilic peptides having higher beta-turn structures as compared to the peptides lacking these structural features. In addition, the Ile-containing peptides exhibited significantly higher log Po/w values than the Gly-containing hydrophilic peptides.

CONCLUSIONS

Hydrophobic peptides that exhibit significant beta-turn structure in solution are more lipophilic as measured by log Po/w, and more readily permeate BBMEC monolayers via the transcellular route than hydrophobic peptides that lack this type of solution structure. Similar secondary structural features in hydrophilic peptides do not appear to sufficiently alter the physicochemical properties of the peptides so as to alter their paracellular flux through BBMEC monolayers.

Effect of stereochemistry on the transport of Aca-linked beta-turn peptidomimetics across a human intestinal cell line

Transcellular transport is one of the most important barriers facing the development of new therapeutic agents. However, little is known about the specific effects of structure and particularly stereochemistry on cell permeability. An attractive in vitro model has been developed for the direct assessment of cell transport, using the immortalized human epithelial cell line, Caco-2. The present study assesses the effects of stereochemistry on transport in a commonly used beta-turn model system. Thus, L,L- and L,D-Ala-Ala were cyclized with aminocaproic acid, resulting in macrocycles in which the dipeptides correspond to the i + 1 and i + 2 positions of a beta-turn. The transport of these dipeptides across a Caco-2 cell monolayer was determined, along with corresponding acyclic models (L,L- and L,D-CH3CH2C(O)-Ala-Ala-n-Pr). The transport studies were carried out in the presence and absence of verapamil, a known inhibitor of the apically polarized efflux system present in Caco-2 cells. Both apical-->basolateral and basolateral-->apical transport were measured. Measurements made in the presence of verapamil showed that the cyclic peptides experienced a ca. 4-5-fold difference in intrinsic flux depending on stereochemistry, with the L,D isomer being transported at a higher rate. These differences disappeared in the acyclic cases examined (permeability coefficient ratios of the L,D/L,L isomers were 1.04-1.13). These observations are discussed in terms of the conformations and hydrogen-bonding characteristics of the compounds as determined by NMR spectroscopy.

Effect of size and charge on the passive diffusion of peptides across Caco-2 cell monolayers via the paracellular pathway

PURPOSE

To evaluate the effect of size and charge on the permeation characteristics of peptides across the intestinal mucosa.

METHODS

The lipophilicities of neutral, positively and negatively charged capped amino acids (Asn, Lys, Asp), tripeptides (Ac-Gly-X-Ala-NH2; X = Asn, Lys, Asp) and hexapeptides (Ac-Trp-Ala-Gly-Gly-X-Ala-NH2; X = Asn, Lys, Asp) were estimated using an immobilized artificial membrane. The diffusion coefficients used to calculate the molecular radii were measured by NMR. The transport characteristics of the model peptides were determined across Caco-2 cell monolayers.

RESULTS

When model compounds having the same charge were compared, permeation was highly size-dependent (capped amino acids > tripeptides > hexapeptides), suggesting transport predominantly via the paracellular route. For example, the flux of the negatively charged Asp amino acid (Papp = 10.04 +/- 0.43 x 10(-8) cm/s) was 3 times greater than that observed for the Asp-containing hexapeptide (Papp = 3.19 +/- 0.27 x 10(-8) cm/s). When model compounds of the same size were compared, permeation across the cell monolayer was charge-dependent (negative < positive < or = neutral). For example, the neutral, Asn-containing tripeptide (Papp = 25.79 +/- 4.86 x 10(-8) cm/s) was substantially more able to permeate the Caco-2 cell monolayer than the negatively charged Asp-containing tripeptide (Papp = 7.95 +/- 1.03 x 10(-8) cm/s) and the positively charged Lys-containing tripeptide (Papp = 9.86 +/- 0.18 x 10(-8) cm/s). The permeability of the cell monolayer to peptides became less sensitive to net charge as the size of the peptides increased.

CONCLUSIONS

A positive net charge of hydrophilic peptides enhances their permeation across the intestinal mucosa via the paracellular pathway. With increasing molecular size, molecular sieving of the epithelial barrier dominates the transport of peptides, and the effect of the net charge becomes less significant.