Enteral absorption of octreotide: absorption enhancement by polyoxyethylene-24-cholesterol ether

The role of neuropeptide somatostatin in the brain and its application in treating neurological disorders

Somatostatin (SST) is a well-known neuropeptide that is expressed throughout the brain. In the cortex, SST is expressed in a subset of GABAergic neurons and is known as a protein marker of inhibitory interneurons. Recent studies have identified the key functions of SST in modulating cortical circuits in the brain and cognitive function. Furthermore, reduced expression of SST is a hallmark of various neurological disorders, including Alzheimer's disease and depression. In this review, we summarize the current knowledge on SST expression and function in the brain. In particular, we describe the physiological roles of SST-positive interneurons in the cortex. We further describe the causal relationship between pathophysiological changes in SST function and various neurological disorders, such as Alzheimer's disease. Finally, we discuss potential treatments and possibility of novel drug developments for neurological disorders based on the current knowledge on the function of SST and SST analogs in the brain derived from experimental and clinical studies.

Overexpression of P-glycoprotein, MRP2, and CYP3A4 impairs intestinal absorption of octreotide in rats with portal hypertension

Background

Portal hypertension (PH) is the main cause of complications and death in liver cirrhosis. The effect of oral administration of octreotide (OCT), a drug that reduces PH by the constriction of mesenteric arteries, is limited by a remarkable intestinal first-pass elimination.

Methods

The bile duct ligation (BDL) was used in rats to induce liver cirrhosis with PH to examine the kinetics and molecular factors such as P-glycoprotein (P-gp), multidrug resistance-associated protein 2 (MRP2) and cytochrome P450 3A4 (CYP3A4) influencing the intestinal OCT absorption via in situ and in vitro experiments on jejunal segments, transportation experiments on Caco-2 cells and experiments using intestinal microsomes and recombinant human CYP3A4. Moreover, RT-PCR, western blot, and immunohistochemistry were performed.

Results

Both in situ and in vitro experiments in jejunal segments showed that intestinal OCT absorption in both control and PH rats was largely controlled by P-gp and, to a lesser extent, by MRP2. OCT transport mediated by P-gp and MRP2 was demonstrated on Caco-2 cells. The results of RT-PCR, western blot, and immunohistochemistry suggested that impaired OCT absorption in PH was in part due to the jejunal upregulation of these two transporters. The use of intestinal microsomes and recombinant human CYP3A4 revealed that CYP3A4 metabolized OCT, and its upregulation in PH likely contributed to impaired drug absorption.

Conclusions

Inhibition of P-gp, MRP2, and CYP3A4 might represent a valid option for decreasing intestinal first-pass effects on orally administered OCT, thereby increasing its bioavailability to alleviate PH in patients with cirrhosis.

Progress in the formulation and delivery of somatostatin analogs for acromegaly

A 14 amino acid cystin bridge containing neuropeptide was discovered in 1973 and designated as growth hormone-inhibiting hormone, in other words, somatostatin. Its discovery led to the synthesis of three analogs which were licensed for the treatment of acromegaly: octreotide, lanreotide and pasireotide. Somatostatin analogs are currently approved only as either subcutaneous or intramuscular long-acting injections. We examine the challenges that must be overcome to create oral formulations of somatostatin analogs and examine selected clinical trial data. While octreotide has low intestinal permeability, similar to almost all other peptides, it has an advantage of being more stable against intestinal peptidases. The development of new oral formulation strategies may eventually allow for the successful oral administration of potent somatostatin analogs with high therapeutic indices.

Both PepT1 and GLUT Intestinal Transporters Are Utilized by a Novel Glycopeptide Pro-Hyp-CONH-GlcN

Pro-Hyp (PO) accounts for many beneficial biological effects of collagen hydrolysates for skin and bone health. The objective of this study was to conjugate PO with glucosamine (GlcN) to create a novel glycopeptide Pro-Hyp-CONH-GlcN (POGlcN) and then to investigate the potential involvement of multiple transepithelial transport pathways for this glycopeptide. Nuclear magnetic resonance results revealed the amide nature of this glycopeptide with α and β configurations derived from GlcN. This glycopeptide was very resistant to simulated gastrointestinal digestion. Also, it showed a rate of transepithelial transport [permeability coefficient (P_app) of (2.82 ± 0.15) × 10^-6 cm/s] across the Caco-2 cell monolayer superior to those of parental dipeptide PO and GlcN [P_app values of (1.45 ± 0.17) × 10^-6 and (1.87 ± 0.15) × 10^-6 cm/s, respectively]. A transport mechanism experiment indicated that the improved transport efficiency of POGlcN is attributed to the introduction of glucose transporters.

Inhibition of P-glycoprotein, multidrug resistance-associated protein 2 and cytochrome P450 3A4 improves the oral absorption of octreotide in rats with portal hypertension

The aim of the present study was to increase the intestinal transport of octreotide (OCT) by targeting the first-pass impact to identify a potential method for decreasing portal vein pressure (PVP) using oral OCT. Thus, the bioavailability of intestinally absorbed OCT was evaluated in normal rats and rats with portal hypertension (PH) that had been administered P-glycoprotein/multidrug resistance-associated protein 2/cytochrome P450 3A4 (P-gp/MRP2/CYP3A4) inhibitors. The mRNA and protein expression levels of P-gp, MRP2 and CYP3A4 were evaluated in normal and PH rats with or without OCT and the inhibitors using RT-PCR, western blot and immunohistochemical analyses. The potential effects of the inhibitor administration on PVP were also examined. The results suggest that P-gp, MRP2 and CYP3A4 play important roles in prohibiting the enteral absorption of OCT, particularly under a PH environment. Moreover, inhibitors of P-gp, MRP2 and CYP3A4 decrease the first-pass effects of OCT and effectively reduce PVP under PH conditions. Therefore, the present results suggest P-gp, MRP2 and CYP3A4 are key factors in the intestinal absorption of OCT. The inhibition of P-gp, MRP2 and CYP3A4 can markedly decrease the first-pass effects of OCT, and their use may facilitate the use of orally administered OCT.

Intestinal permeation enhancers for oral peptide delivery

Intestinal permeation enhancers (PEs) are one of the most widely tested strategies to improve oral delivery of therapeutic peptides. This article assesses the intestinal permeation enhancement action of over 250 PEs that have been tested in intestinal delivery models. In depth analysis of pre-clinical data is presented for PEs as components of proprietary delivery systems that have progressed to clinical trials. Given the importance of co-presentation of sufficiently high concentrations of PE and peptide at the small intestinal epithelium, there is an emphasis on studies where PEs have been formulated with poorly permeable molecules in solid dosage forms and lipoidal dispersions.

Update on the use of oral octreotide therapy for acromegaly

Somatostatin analogs are most commonly used in pharmacological treatment of acromegaly. Pegvisomant and dopamine agonists are alternatives, which are used to a lesser extent. Dopamine agonists are the only orally applicable medication but are less effective than the other options. For a large number of patients, life-long pharmacotherapy has to be applied and frequent injections represent a reduction of quality of life for many of them. Areas covered: Recently published evidence for the use of oral octreotide therapy for acromegaly. Expert commentary: Oral octreotide is a novel and effective treatment for acromegaly and the side effects have been shown to be comparable to the injectable SSAs. The combination with a transient permeability enhancer allows intestinal permeation but also enables molecules with a size <70 kDa to pass transiently. This does not seem to have an acute or subacute consequence, but the long-term effect is still elusive. Therefore, more long-term trials are desirable.

Macrocycles: lessons from the distant past, recent developments, and future directions

A noticeable increase in molecular complexity of drug targets has created an unmet need in the therapeutic agents that are larger than traditional small molecules. Macrocycles, which are cyclic compounds comprising 12 atoms or more, are now recognized as molecules that "are up to the task" to interrogate extended protein interfaces. However, because macrocycles (particularly the ones based on peptides) are equipped with large polar surface areas, achieving cellular permeability and bioavailability is anything but straightforward. While one might consider this to be the Achilles' heel of this class of compounds, the synthetic community continues to develop creative approaches toward the synthesis of macrocycles and their site-selective modification. This perspective provides an overview of both mechanistic and structural issues that bear on macrocycles as a unique class of molecules. The reader is offered a historical foray into some of the classic studies that have resulted in the current renaissance of macrocycles. In addition, an attempt is made to overview the more recent developments that give hope that macrocycles might indeed turn into a useful therapeutic modality.

A novel suspension formulation enhances intestinal absorption of macromolecules via transient and reversible transport mechanisms

PURPOSE

Medium chain fatty acid salts promote absorption by increasing paracellular permeability of the intestinal epithelium. Novel oily suspension (OS) formulation disperses a powder containing sodium caprylate and macromolecules such as octreotide or fluorescent dextran (FD). Formulation safety, macromolecule absorption and pharmacokinetic (PK)/pharmacodynamic (PD) were evaluated.

METHODS

Octreotide/OS toxicity was evaluated in monkeys following 9 months of daily oral enteric-coated capsule administration. The OS permeation effect was also assessed in rats, using FD/OS and octreotide/OS preparations. Octreotide/OS effects on circulating growth hormone (GH) levels were also measured.

RESULTS

Safety assessment of octreotide/OS in monkeys after 9 months showed minor drug-related findings, comparable to the injectable octreotide. Octreotide exposure levels were similar across the treatment periods. In rats, OS facilitated FD permeation up to 70 kDa in a reversible, spatial and dose-dependent manner, independent of the intestinal dosing site. Following OS administration, the staining pattern of the tight-junction protein, ZO-1, changed transiently, and a paracellular penetration marker, LC-biotin, permeated between adjacent epithelial cells. Enteral octreotide/OS absorption was dose-dependent and suppressed rat GH levels.

CONCLUSIONS

Oral octreotide/OS dosing was shown to be safe in monkeys. OS enhances intestinal absorption of active octreotide, likely by transient alteration of the tight junction protein complex.

Opportunities and challenges for oral delivery of hydrophobic versus hydrophilic peptide and protein-like drugs using lipid-based technologies

Peptide and protein-like drugs are macromolecules currently produced in increasing numbers by the pharmaceutical biotechnology industry. The physicochemical properties of these molecules pose barriers to oral administration. Lipid-based drug-delivery systems have the potential to overcome these barriers and may be utilized to formulate safe, stable and efficacious oral medicines. This review outlines the design of such lipid-based technologies. The mechanisms whereby these formulations enhance the absorption of lipophilic versus hydrophilic peptide and protein-like drugs are discussed. In the case of lipophilic compounds, the advantages of lipid-based drug-delivery systems including increased solubilization, decreased intestinal efflux, decreased intracellular metabolism and possible lymphatic transport are well established as is evident from the success of Neoral® and other drug products on the market. In contrast, with respect to hydrophilic compounds, the situation is more complex and, while promising formulation approaches have been studied, issues including reproducibility of response, intersubject variability and duration of response require further optimization before commercially viable products are possible.

Effect of structural and conformation modifications, including backbone cyclization, of hydrophilic hexapeptides on their intestinal permeability and enzymatic stability

A library of 18 hexapeptide analogs was synthesized, including sub-libraries of N- or C-methylation of the parent hexapeptide Phe-Gly-Gly-Gly-Gly-Phe, as well as backbone cyclized analogs of each linear analog with various ring sizes. N- or C-methylation as well as cyclization (but not backbone cyclization) have been suggested to improve intestinal permeability and metabolic stability of peptides in general. Here we aimed to assess their applicability to hydrophilic peptides. The intestinal permeability (Papp) of the 18-peptide library was in the range of 0.2-6.8 x 10-6 cm/sec. Based on several tests, we concluded that the absorption mechanism of all tested analogs is paracellular, regardless of the structural or conformational modifications. In all cases, backbone cyclization increased Papp (5-fold) in comparison to the linear analogs due to the smaller 3D size and also dramatically decreased peptide proteolysis by brush border enzymes. N- or C-methylation did not enhance the permeability of the linear analogs in this series.

Correlation of in vitro and in vivo models for the oral absorption of peptide drugs

The aim of this study was to evaluate two in vitro models, Caco-2 monolayer and rat intestinal mucosa, regarding their linear correlation with in vivo bioavailability data of therapeutic peptide drugs after oral administration in rat and human. Furthermore the impact of molecular mass (Mm) of the according peptides on their permeability was evaluated. Transport experiments with commercially available water soluble peptide drugs were conducted using Caco-2 cell monolayer grown on transwell filter membranes and with freshly excised rat intestinal mucosa mounted in Using type chambers. Apparent permeability coefficients (P (app)) were calculated and compared with in vivo data derived from the literature. It was shown that, besides a few exceptions, the Mm of peptides linearly correlates with permeability across rat intestinal mucosa (R (2) = 0.86; y = -196.22x + 1354.24), with rat oral bioavailability (R (2) = 0.64; y = -401.90x + 1268.86) as well as with human oral bioavailability (R (2) = 0.91; y = -359.43x + 1103.83). Furthermore it was shown that P (app) values of investigated hydrophilic peptides across Caco-2 monolayer displayed lower permeability than across rat intestinal mucosa. A correlation between P (app) values across rat intestinal mucosa and in vivo oral bioavailability in human (R (2) = 0.98; y = 2.11x + 0.34) attests the rat in vitro model to be a very useful prediction model for human oral bioavailability of hydrophilic peptide drugs. Presented correlations encourage the use of the rat in vitro model for the prediction of human oral bioavailabilities of hydrophilic peptide drugs.

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.

Permeability of porcine blood brain barrier to somatostatin analogues

1. Transport of a fluorescent somatostatin analogue (NBD-octreotide) across freshly isolated functionally intact capillaries from porcine brain was visualized by confocal microscopy and quantitated by image analysis. 2. Luminal accumulation of NBD-octreotide showed all characteristics of specific and energy-dependent transport. Steady-state luminal fluorescence averaged 2 - 3 times cellular fluorescence and was reduced to cellular levels when metabolism was inhibited by NaCN. 3. The accumulation of NBD-octreotide in capillary lumens was inhibited in a concentration-dependent manner by unlabelled octreotide, by verapamil, PSC-833 and cyclosporin A, potent inhibitors of p-glycoprotein, and by leucotriene C(4), a strong modulator of Mrp2. Conversely, unlabelled octreotide reduced luminal accumulation of fluorescent BODIPY-verapamil on p-glycoprotein and of fluorescein-methotrexate, on Mrp2. None of the inhibitors used significantly reduced cellular accumulation of the fluorescent substrates. 4. Together, the data are consistent with octreotide being transported across the luminal membrane of porcine brain capillaries by both P-gp and Mrp2, providing further evidence that both transporters contribute substantially to the active barrier function of this endothelium.

Intestinal absorption of octreotide using trimethyl chitosan chloride: studies in pigs

PURPOSE

To investigate the enhancing effect of trimethyl chitosan chloride (TMC) on the enteral absorption of octreotide and to delineate the required doses of both TMC and peptide in vivo in juvenile pigs.

METHODS

Six female pigs (body weight, 25 kg) were operated to induce a stoma at the beginning of their jejunum and to insert an in-dwelling fistula for intrajejunal (IJ) administration of the formulations. A silicone cannula was inserted at the jugular vein for blood sampling. One week after surgery the pigs received IJ octreotide solution administrations with or without TMC at pH 7.4 or chitosan HCl at pH 5.5. For determining bioavailability (F) values, the pigs also received an octreotide solution intravenously (IV). Blood samples were taken from the cannulated jugular vein and subsequently analyzed by radioimmunoassay.

RESULTS

Intrajejunal administration of 10 mg octreotide without any polymer (control solution) resulted in F values of 1.7 +/- 1.1% (mean +/- SE). Chitosan HCl 1.5% (w/v) at pH 5.5 led to a 3-fold increase in F compared to the control (non-polymer containing) formulations. Co-administration of octreotide with 5 and 10% (w/v) TMC at pH 7.4 resulted in 7.7- and 14.5-fold increase of octreotide absorption, respectively (F of 13.9 +/- 1.3% and 24.8 +/- 1.8%). IJ administration of 5 mg octreotide solutions resulted in low F values of 0.5 +/- 0.6%, whereas co-administration with 5% (w/v) TMC increased the intestinal octreotide bioavailability to 8.2 +/- 1.5%.

CONCLUSIONS

Cationic polymers of the chitosan type are able to enhance the intestinal absorption of the peptide drug octreotide in pigs. In this respect, TMC at neutral pH values of 7.4 appears to be more potent than chitosan HCl at a weak acidic pH of 5.5.

Intestinal absorption of octreotide: N-trimethyl chitosan chloride (TMC) ameliorates the permeability and absorption properties of the somatostatin analogue in vitro and in vivo

Octreotide acetate is a somatostatin analogue used for the control of endocrine tumors of the gastrointestinal (GI) tract and the treatment of acromegaly. The oral absorption of octreotide is limited because of the limited permeation across the intestinal epithelium. Both chitosan hydrochloride and N-trimethyl chitosan chloride (TMC), a quaternized chitosan derivative, are nonabsorbable and nontoxic polymers that have been proven to effectively increase the permeation of hydrophilic macromolecules across mucosal epithelia by opening the tight junctions. This study investigates the intestinal absorption of octreotide when it is coadministered with the polycationic absorption enhancer TMC. Caco-2 cell monolayers were used as an in vitro intestinal epithelium model, and male Wistar rats were used for in vivo studies. Octreotide with or without polymers (TMC; chitosan hydrochloride) was administered intrajejunally in rats, and serum peptide levels were measured by radioimmunoassay. All applications and administrations were performed at neutral pH values (i.e., pH = 7.4). In vitro transport studies with Caco-2 cells revealed an increased permeation of octreotide in the presence of TMC. Enhancement ratios ranged from 34 to 121 with increasing concentrations of the polymer (0.25-1.5%, w/v). In rats, 1.0% (w/v) TMC solution significantly increased the absorption of the peptide analogue, resulting in a 5-fold increase of octreotide bioavailability compared with the controls (octreotide alone). Coadministration of 1.0% (w/v) chitosan hydrochloride did not enhance octreotide bioavailability. These results in combination with the nontoxic character of TMC suggest that this polymer is a promising excipient in the development of solid dosage forms for the peroral delivery and intestinal absorption of octreotide.

Improvement of intestinal peptide absorption by a synthetic bile acid derivative, cholylsarcosine

The potential of the nontoxic bile salt derivative, cholylsarcosine, to enhance the intestinal absorption of peptides was investigated in vitro and in situ. The permeation of the two model peptides octreotide and vasopressin-[arg(8)CT>/=CS, whereas ursodeoxycholic acid exhibited no absorption enhancement. Determination of the cytotoxic potential of the bile salts revealed the same rank order. In rats, octreotide and desmopressin were absorbed from the gastrointestinal-tract with moderate absorption efficiency. Coadministration of bile salts resulted in an increased absorption efficiency. The effect of CS was similar to that of CT. In conclusion, CS shows absorption enhancement properties and a relatively low cytotoxicity. It offers an alternative as absorption enhancer as compared to conventional bile acids which may have a potential cocarcinogenic risk.

P-glycoprotein- and mrp2-mediated octreotide transport in renal proximal tubule

1. Transepithelial transport of a fluorescent derivative of octreotide (NBD-octreotide) was studied in freshly isolated, functionally intact renal proximal tubules from killifish (Fundulus heteroclitus). 2. Drug accumulation in the tubular lumen was visualized by means of confocal microscopy and was measured by image analysis. Secretion of NBD-octreotide into the tubular lumen was demonstrated and exhibited the all characteristics of specific and energy-dependent transport. Steady state luminal fluorescence averaged about five times cellular fluorescence and was reduced to cellular levels when metabolism was inhibited by NaCN. 3. NBD-octreotide secretion was inhibited in a concentration-dependent manner by unlabelled octreotide, verapamil and leukotriene C(4) (LTC(4)). Conversely, unlabelled octreotide reduced in a concentration dependent manner the p-glycoprotein (Pgp)-mediated secretion of a fluorescent cyclosporin A derivative (NBDL-CS) and the mrp2-mediated secretion of fluorescein methotrexate (FL-MTX). 4. This inhibition was not due to impaired metabolism or toxicity since octreotide had no influence on the active transport of fluorescein (FL), a substrate for the classical renal organic anion transport system. 5. The data are consistent with octreotide being transported across the brush border membrane of proximal kidney tubules by both Pgp and mrp2.

Direct solid-phase synthesis of octreotide conjugates: precursors for use as tumor-targeted radiopharmaceuticals

Somatostatin analogues, such as octreotide, are useful for the visualization and treatment of tumors. Unfortunately, these compounds were produced synthetically using complex and inefficient procedures. Here, we describe a novel approach for the synthesis of octreotide and its analogues using p-carboxybenzaldehyde to anchor Fmoc-threoninol to solid phase resins. The reaction of the two hydroxyl groups of Fmoc-threoninol with p-carboxybenzaldehyde was catalyzed with p-toluenesulphonic acid in chloroform using a Dean-Stark apparatus to form Fmoc-threoninol p-carboxybenzacetal in 91% yield. The Fmoc-threoninol p-carboxybenzacetal acted as an Fmoc-amino acid derivative and the carboxyl group of Fmoc-threoninol p-carboxybenzacetal was coupled to an amine-resin via a DCC coupling reaction. The synthesis of protected octreotide and its conjugates were carried out in their entirety using a conventional Fmoc protocol and an autosynthesizer. The acetal was stable during the stepwise elongation of each Fmoc-amino acid as shown by the averaged coupling yield (> 95%). Octreotide (74 to 78% yield) and five conjugated derivatives were synthesized with high yields using this procedure, including three radiotherapy octreotides (62 to 75% yield) and two cellular markers (72 to 76% yield). This novel approach provides a strategy for the rapid and efficient large-scale synthesis of octreotide and its analogues for radiopharmaceutical and tagged conjugates.

The effect of a drug-delivery system consisting of soybean phosphatidyl choline and medium-chain monoacylglycerol on the intestinal permeability of hexarelin in the rat

The aim of this study was to investigate if the effective in-situ permeability (Peff) of a new growth hormone-releasing peptide, hexarelin, along rat intestine was enhanced by a lipid matrix drug-delivery system comprising a mixture of soybean phosphatidyl choline and medium-chain monoacylglycerol (PC-MG). The study was performed with and without a protease inhibitor, Pefabloc SC. To enable better understanding of the mechanism of action of this delivery system we also studied the uptake of a small hydrophilic molecule, atenolol. PC-MG at a concentration of 15 mmol L(-1) increased the jejunal Peff of hexarelin approximately 20-fold, both in the presence and absence of Pefabloc SC, whereas Peff was not increased in the ileum and colon. PC-MG had no effect on the jejunal, ileal and colonic Peff of atenolol. Complete recovery of the non-absorbable molecule PEG 4000 showed that functional intestinal viability was maintained in all experiments. Although the results obtained in this study are promising, pharmacokinetic and toxicological studies are required to investigate if this delivery system is a suitable and safe candidate for improving the oral bioavailability of hexarelin.

Octanol-water partition of nonzwitterionic peptides: predictive power of a molecular size-based model

A remarkably simple, molecular size-based model developed to predict octanol-water partition coefficients for organic compounds is tested on a set of 188 neutral peptides with available experimental partition data. Despite using only two parameters, it gives a promising correlation (r2 = 0.914; sigma = 0.455, F = 1978.0), and predictions are in a realistic range even for larger peptides (cyclosporin, melanotan, sandostatin) where common, overparametrized fragment methods become quite unreliable. Ion-pair partitioning and the extraction constant formalism is briefly reviewed to describe the sigmoidal lipophilicity profile of ionizable, nonzwitterionic peptides. It seems possible to extend the present model to estimate apparent partition coefficients measured around neutral pH and physiological conditions for monoionic peptides; however, as no standard conditions are yet defined and only relatively small number of experimental data are available, the situation here is more complex.

Cholesterol derivative of poly(ethylene glycol) inhibits clathrin-independent, but not clathrin-dependent endocytosis

The effect of poly(ethylene glycol) cholesteryl ethers (PEG(n)-Chols) with two different numbers of units (n = 50 and 200) in the hydrophilic PEG moiety on cellular endocytic activity was studied on HT-1080 cells. The amphipathic molecules were soluble in aqueous solution. When fluorescein derivatives of PEG-Chols (one fluorescein at the distal end of PEG) were incubated with the cells in culture, the cellular fluorescence was localized at the plasma membrane level and in intracellular vesicles. Fluorescence quantification indicated that for the same external concentration, twice more FPEG(50)-Chol than FPEG(200)-Chol was associated with the cells under the same conditions. Regardless of the length of PEG moiety, PEG-Chols' interaction with cells reduced the endocytic internalization of a fluid phase marker, horseradish peroxidase (HRP) depending on the cell-associated amount. In contrast, internalization of 125I-labeled epidermal growth factor (EGF) through receptor-mediated endocytosis did not change upon incubation with PEG(50)-Chol. The effect of PEG(200)-Chol was also small, since EGF internalization showed a reduction of 10-20%, while at the same concentration as much as 80% of HRP uptake was inhibited. PEG(50)-Chol did not influence the internalization of a larger ligand, 125I-transferrin (Tfn). However, in the presence of PEG(200)-Chol, the uptake of 125I-Tfn decreased remarkably, and yet, PEG(200)-Chol has no influence on the binding and internalization of a monoclonal antibody directed toward the ectodomain of the Tfn-receptor. These results suggested that incorporation of PEG-Chols in the outer monolayer of the plasma membrane specifically inhibited clathrin-independent, but not clathrin-dependent endocytosis.

The effect of monomers and of micellar and vesicular forms of non-ionic surfactants (Solulan C24 and Solulan 16) on Caco-2 cell monolayers

Measurements of transepithelial electrical resistance (TEER), the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) test and monitoring of poly(ethylene glycol) (PEG) transport have been used to study the effects of the non-ionic surfactants Solulan C24 and Solulan 16, either free in solution or as an integral part of niosome bi-layers, on intestinal epithelial cells from man (Caco-2 cell monolayers). The effects on epithelial integrity and on the transport of the hydrophilic drug metformin depend on the concentration of the surfactants. At concentrations above 1% the effect on TEER of the surfactant in niosomal form and free in solution were equivalent whereas cell viability was preserved to a higher concentration of Solulans when the Solulans were present in the niosomal form. It was concluded that the toxic effect of niosomes arises from free surfactant present in the niosome suspension.

Effect of restricted conformational flexibility on the permeation of model hexapeptides across Caco-2 cell monolayers

PURPOSE

To determine how restricted conformational flexibility of hexapeptides influences their cellular permeation characteristics.

METHODS

Linear (Ac-Trp-Ala-Gly-Gly-X-Ala-NH2; X = Asp, Asn, Lys) and cyclic (cyclo[Trp-Ala-Gly-Gly-X-Ala]; X = Asp, Asn, Lys) hexapeptides were synthesized, and their transport characteristics were assessed using the Caco-2 cell culture model. The lipophilicities of the hexapeptides were determined using an immobilized artificial membrane. Diffusion coefficients used to calculate molecular radii were determined by NMR. Two-dimensional NMR spectroscopy, circular dichroism, and molecular dynamic simulations were used to elucidate the most favorable solution structure of the cyclic Asp-containing peptide.

RESULTS

The cyclic hexapeptides used in this study were 2-3 times more able to permeate (e.g., Papp = 9.3 +/- 0.3 x 10(-8) cm/sec, X = Asp) the Caco-2 cell monolayer than were their linear analogs (e.g., Papp = 3.2 +/- 0.3 x 10(-8) cm/sec, X = Asp). In contrast to the linear hexapeptides, the flux of the cyclic hexapeptides was independent of charge. The cyclic hexapeptides were shown to be more lipophilic than the linear hexapeptides as determined by their retention times on an immobilized phospholipid column. Determination of molecular radii by two different techniques suggests little or no difference in size between the linear and cyclic hexapeptides. Spectroscopic data indicate that the Asp-containing linear hexapeptide exists in a dynamic equilibrium between random coil and beta-turn structures while the cyclic Asp-containing hexapeptide exists in a well-defined compact amphophilic structure containing two beta-turns.

CONCLUSIONS

Cyclization of the linear hexapeptides increased their lipophilicities. The increased permeation characteristics of the cyclic hexapeptides as compared to their linear analogs appears to be due to an increase in their flux via the transcellular route because of these increased lipophilicities. Structural analyses of the cyclic Asp-containing hexapeptide suggest that its well-defined solution structure and, specifically the existence of two beta-turns, explain its greater lipophilicity.

Intestinal absorption of sugar-coupled somatostatin analogs

The intestinal absorption of glycosylated somatostatin analogs was compared in rat enterocyte brush border membranes as an in vitro test system and rats as an in situ absorption model. Derivatives of the cyclic octapeptide octreotide with mono-, di-, and trisaccharide residues were used. The uptake of octreotide by the vesicles was inhibited by the glycosylated analogs. The uptake was not inhibited by the bicyclic octapeptide alpha-amanitin, which exhibits structural similarity but is not absorbed in rats. The inhibition of octreotide permeation into the vesicles decreased in the presence of derivatives with an increasing length of the carbohydrate residues. To evaluate, whether the vesicle system is predictable for the in situ situation, the extent of absorption of the peptides was determined after intrajejunal administration. A linear relationship between inhibitory capacity of the octreotide derivatives in the vesicle system and their in situ absorption efficiency was found when blood was taken from a mesenteric vein. However, after sampling from a peripheral vein, deviations from the predicted values were noted. These differences reflected changes in pharmacokinetics (e.g., hepatic elimination) rather than in absorption. In summary, the data indicate that the vesicle system is a useful tool to predict the absorption efficiency of glycosylated somatostatin analogs in situ.

Effects of permeation enhancers on the transport of a peptidomimetic thrombin inhibitor (CRC 220) in a human intestinal cell line (Caco-2)

PURPOSE

The effects of five different permeation enhancer systems on the transport properties of a peptidomimetic thrombin inhibitor. CRC 220, were investigated in monolayers of a human intestinal cell line (Caco-2).

METHODS

The transepithelial transport rates and additionally the cytotoxic properties of these enhancers were characterized using the following tests: measurement of the transepithelial electrical resistance (TEER), the MTT-transformation, the protein content and the release of cytosolic lactate dehydrogenase (LDH), as well as FITC-phalloidin and propidium iodide staining.

RESULTS

All permeation enhancer systems showed concentration-dependent effects on cell permeability and toxicity. The most prominent effects on peptide transport were seen at the highest concentration (40 mM), yielding the rank order, NaTC > NaTC/Cholesterol > Solulan C24 > NaTC/Oleic acid > NaTC/PC18. Using the TEER after 120 min exposure as the most sensitive parameter describing cytotoxicity, the following order was obtained: Solulan C24 > NaTC > NaTC/PC18 = NaTC/Cholesterol > NaTC/Oleic acid > NaTC/PC. Generally, efficient enhancement of peptide transport was associated with a noticeable influence on cell viability under in-vitro conditions.

CONCLUSIONS

Taking into account permeation and cytotoxicity as a function of concentration, both NaTC at 15 mM and the mixed micellar system NaTC/oleic acid at 0.75 mM offer interesting enhancement properties, showing an 18-fold increase in CRC 220 transport rates. The effects on cell viability and cytotoxicity were comparatively low and of reversible nature.

Current concepts in intestinal peptide absorption

Today there is considerable interest in oral peptide delivery. However, oral administration of peptides is limited by a low bioavailability and a high variability in plasma levels. A review is given of the literature describing the major barriers in peptide absorption, the basic mechanisms of intestinal peptide transport, the experimental models and the pharmaceutical approaches currently used in the investigation of peptide and protein absorption processes.

Permeation enhancement of octreotide by specific bile salts in rats and human subjects: in vitro, in vivo correlations

1. The potential of bile salts to improve the enteral absorption of octreotide, an orally active somatostatin analogue, was investigated by a combination of in vitro, in situ and in vivo experiments. 2. Incorporation of octreotide into lipid monolayers (as measured by area increase of the monolayer at constant surface pressure using a Langmuir-Blodgett trough set-up) depended on the type of bile salt used for monolayer pre-treatment. Addition of 20 microM octreotide to the subphase containing 20 microM of the dihydroxylated bile salt ursodeoxycholate (UDCA) causes a 9% increase in area, whereas addition of octreotide to the subphase containing the 7 alpha-enantiomer of UDCA, chenodeoxycholate (CDCA), resulted in an area increase of the lipid monolayer of 20%. Area increase by octreotide alone was not significantly different from the increase of octreotide and UDCA in combination. 3. CDCA and UDCA in combination with octreotide increased the permeability of liposomal membranes for rubidium ions, whereas octreotide alone did not significantly change the permeability. This indicates membrane distortion as a possible cause for the enhanced absorption of octreotide by bile salts. 4. In polarized Caco-2 cell monolayers octreotide exhibited a permeation coefficient of 0.008 +/- 0.004 cm h-1. Addition of 0.2-1% of UDCA to the apical incubation medium had no significant effect upon the permeation coefficient. In contrast, 0.2-1% CDCA in the incubation medium resulted in a significant increase (P < 0.05) of the monolayer permeability of octreotide (0.015-0.037 cm h-1). 5. Octreotide was absorbed as the intact peptide from the gastrointestinal tract in rats with an absorption efficiency of 0.26%. Coadministration of bile salt resulted in a dose-dependent increase in absorption efficiency of the peptide up to 20.2%. The observed effect was more pronounced for CDCA than for UDCA. 6. The effect of CDCA and UDCA on octreotide absorption in vivo was assessed in a pharmacokinetic study with healthy volunteers. After oral administration of 4 mg octreotide in the presence of 100 mg bile salt, an average bioavailability of the peptide of 1.26% was achieved in the presence of CDCA, whereas in the presence of UDCA a bioavailability of only 0.13% was reached. This difference was statistically significant (P < 0.01). 7. In conclusion, the co-administration of CDCA is able to enhance the enteral absorption of octreotide. The in vitro and in situ experiments were predictive for the observed effect in human subjects.

In vitro tolerability of human nasal mucosa: histopathological and scanning electron-microscopic evaluation of nasal forms containing Sandostatin

An in vitro human nasal model was developed as a tool to study the local tolerability of nasal powder forms using excised nasal mucosa in a diffusion chamber. The suitability of this model was tested using Sandostatin (SMS) an octapeptide analog of somatostatin, as a reference drug enhanced by Avicel (microcrystalline cellulose) or lactose (100 mesh). The standard nasal spray vehicle was taken as a harmless control and 1% chenodeoxycholate (CDC) as a harmful control in terms of local tolerability. The extent of peptide permeation was determined by measuring SMS concentration in the receiving chamber. The labeling of SMS was detected by immunoperoxidase staining on cross sections. The local tolerability for all tested forms was assessed by histopathological examination and scanning electron microscopy. The apparent permeation coefficient allowed us to rank the absorption of the tested drug forms as Avicel > spray = lactose > 1%CDC. For all formulations, SMS was detected in the epithelium. No changes of the nasal mucosa could be observed with Avicel, lactose or nasal spray vehicle in the presence or absence of SMS. 1%CDC with or without drug showed an immediate destruction of the nasal epithelium. The validation of this in vitro model using human nasal mucosa will be further discussed as a tool for assessing the local tolerability of intranasally applied test substances.

Preformulation studies with melanotan-II: a potential skin cancer chemopreventive peptide

Melanotan-II (1) is a cyclic heptapeptide analogue of alpha-melanocyte-stimulating hormone (alpha-MSH) which tans the skin and is currently being evaluated for the prevention of sunlight-induced skin cancers. The dissociation constants of 1 were determined using potentiometric titration and ultraviolet spectrophotometry. The pKa1 (histidine) and pKa2 (arginine) were estimated to be 6.54 and 11.72, respectively. The apparent partition coefficient (PC) was measured at three pH values using both n-octanol and isooctane as the nonpolar phase. The PC(octanol) and delta log PC at pH 7.35 were 2.82 and 1.05, respectively. These data, together with the observance of a bioavailability of 4.6% in the rat, indicate that 1 may be a suitable candidate for oral delivery. The data presented here are useful in developing an appropriate dosage form for 1.