© 2004 American Thoracic Society DOI: 10.1165/rcmb.2003-0315TR
Molecular Mechanisms of Pulmonary Peptidomimetic Drug and Peptide TransportDepartments of Medicine/Pediatric Pneumology and Immunology, Charité School of Medicine, Humboldt University, Berlin, Germany; Department of Thoracic Medicine, National Heart and Lung Institute, Imperial College of Science, Technology and Medicine, London, United Kingdom; and Molecular Nutrition Unit, Technical Unviersity of Munich, Freising-Weihenstephan, Germany Address correrspondence to: David A. Groneberg, M.D., Deptartment of Pediatric Pneumology and Immunology/Medicine, Charité School of Medicine, Humboldt-University; CVK OR-1 R.3.0073, Augustenburger Platz 1, D-13353 Berlin, Germany. E-mail: david.groneberg{at}charite.de
The aerosolic administration of peptidomimetic drugs could play a major role in the future treatment of various pulmonary and systemic diseases, because rational drug design offers the potential to specifically generate compounds that are transported efficiently into the epithelium by distinct carrier proteins such as the peptide transporters. From the two presently known peptide transporters, PEPT1 and PEPT2, which have been cloned from human tissues, the high-affinity transporter PEPT2 is expressed in the respiratory tract epithelium. The transporter is an integral membrane protein with 12 membrane-spanning domains and mediates electrogenic uphill peptide and peptidomimetic drug transport by coupling of substrate translocation to a transmembrane electrochemical proton gradient serving as driving force. In human airways, PEPT2 is localized to bronchial epithelium and alveolar type II pneumocytes, and transport studies revealed that both peptides and peptidomimetic drugs such as antibiotic, antiviral, and antineoplastic drugs are carried by the system. PEPT2 is also responsible for the transport of delta-aminolevulinic acid, which is used for photodynamic therapy and the diagnostics of pulmonary neoplasms. Based on the recent progress in understanding the structural requirements for substrate binding and transport, PEPT2 becomes a target for a rational drug design that may lead to a new generation of respiratory drugs and prodrugs that can be delivered to the airways via the peptide transporter.
Abbreviations: cystic fibrosis, CF • delta-aminolevulinic acid, d-ALA • photodynamic therapy, PDT • transmembrane domains, TMDs
Besides regulating the airway tone and producing the respiratory lining fluid, the airway epithelium displays the largest barrier between the human organism and its environment. This barrier can be used for drug treatment, but the effectiveness in delivery of topically-adminstered drugs is dependent on various factors, including the chemical nature of the drug, the characteristics of the delivery system and aerosol administration, and the deposition to pulmonary clearance mechanisms (reviewed elsewhere [1]).
In view of the large respiratory surface area of
Inhalation of pharmacologically active substances for the treatment of respiratory diseases actually dates back at least 4,000 years when Atropa belladonna leaves were smoked to suppress cough. Later, the inhalation of sea mists, aerosols, and hot vapors was commonly used to ease airway obstruction in the times of Hippocrates (4, 5). In times of the industrial revolution, "asthma cigarettes" were used containing stramonium from the plant Datura stramonium. In the late nineteenth century, a variety of new nebulizers driven either by compressed air or ultrasonic vibrations were invented. The history of modern aerosolic drug delivery dates back to the first half of the last century (6). After the demonstration of the great potential to deliver drugs topically due to the airways' unique anatomic and physiologic features (7–9), various possibilities of topical and systemic aerosol therapy were propagated (10). It soon became clear that the most successful candidates for aerosolically-administered compounds were low-molecular-weight substances that were able to diffuse rapidly either to the pulmonary veins to reach the systemic circulation or to reach the target cells in the respiratory tract. Consequently, anaesthesiology and emergency medicine were the first disciplines using the delivery of medicines through the aerosolic route to reach the systemic circulation. Here, a large number of compounds was developed starting from anesthetic gases and going later to other drugs such as opioids. Although the oral and intravenous administration of drugs still dominates the pharmacologic treatment of systemic diseases, the interest in new pulmonary delivery pathways grows, last but not least driven by the patients' desire for more convenient treatment alternatives (11). In view of the recent advances in drug formulation design and aerosol delivery technology (12), systemic pulmonary therapy with proteins and peptidomimetic drugs will enable us to treat systemic diseases, such as diabetes mellitus, noninvasively (13). In contrast to the area of aerosolic drugs for the treatment of systemic diseases, in respiratory medicine, aerosol therapy with small molecules has become the predominant mode of managing lung diseases such as asthma. A variety of substrate classes such as steroids (14–16), ß2-adrenergic agonists (17), and anticholinergic drugs (18, 19) are commonly administered via inhalation. Also, aerosolized drugs are used for the treatment of other respiratory diseases: nebulized vasodilators such as prostaglandins have been shown to be effective in the treatment of pulmonary hypertension (20) and antibiotic agents are commonly used in the treatment of recurrent infections in cystic fibrosis (CF) (12, 21, 22). A new therapeutic avenue for the delivery of peptidomimetic drugs to the airways has evolved during the past decade by the identification of distinct drug transporters in the airway epithelium. In this respect, the molecular identification of a high-affinity peptide and drug transporter PEPT2 in 1996 (23) and the demonstration of its functional expression in the airways (2, 3) revealed a promising new target protein for future strategies in airway drug delivery.
Expression and Function of PEPT2 in the Respiratory Tract Extending these observations, experiments that allowed the visualization of cell types that possess functional peptide transport activity were developed. A fluorophore-conjugated dipeptide D-Ala-Lys-AMCA was used for the visualization of pulmonary peptide uptake processes. The incubation of murine lungs with the reporter molecule revealed an uptake and intracellular accumulation of the labeled PEPT2-substrate along the repiratory tract and in type II cells as well as into bronchial and tracheal epithelial cells, confirming thereby the morphologic data of immunohistochemistry and in situ hybridization (2). This simultaneous demonstration of the PEPT2-mRNA and protein together with the protein function unequivocally demonstrated that the peptidomimetic was transported by PEPT2 (2). Finally, the human airway peptide transport system was identified using immunohistochemistry, and the pattern of PEPT2 protein expression was found to be similar to that observed in murine and rat airways (32). Recurrent pulmonary infections in patients with CF represent the primary indication for an aerosolic treatment with antibiotics. In view of the potential of the PEPT2-transporter to be used as a drug delivery system in the treatment of pulmonary diseases based on its capability to transport antibiotics, it was then examined whether PEPT2 is also expressed in human CF airway tissues with unaltered expresssion levels (3). When uptake experiments were performed to assess the activity of the transport system to carry drugs after topical administration, it was found that the transporter exhibited a regular function (3). However, the accumulation of labeled substrate in the airway epithelial cells was significantly reduced due to the large quantities of airway obstructing mucus (33), which was later shown to consist of mucus-forming mucin proteins from both epithelial goblet cells and mucus gland cells (34, 35).
PEPT2 belongs to the familiy of proton-dependent oligopeptide transporters (POT), which currently encompass  70 cloned transporters from a large variety of organisms. Together with the intestinal transporter PEPT1, PEPT2 was the first mammalian transporter identified that uses the electrochemical proton gradient as a driving force (Figure 2) (36).
The mechanisms of substrate recognition, uptake, and transfer by PEPT2 are strongly dependent on the membrane potential and the extracellular pH, as suggested by the term POT. PEPT2-mediated transport of drugs and peptides is electrogenic and the substrate translocation process has a pH optimum of 4.5–6.5, which depends on the substrate's net charge (37).
As the microenvironment of the respiratory epithelial lining fluid is relatively stable and slightly acidic (pH The rate of transport increases as the membrane potential hyperpolarizes and a lower pH with a surplus of extracellular protons may be found under pathophysiologic conditions such as bronchial asthma. Therefore, the functional capacity of PEPT2 to transport selected drugs may be particularly facilitated in certain pathologic states. Due to coupling the substrate translocation to the electrochemical proton gradient, the PEPT2 system can be regarded as a cellular acid loader in the airway epithelium, which is strongly dependent on secondary transport systems such as Na+–H+ exchangers, which in turn increase the airway cell uptake of Na+ to allow intracellular pH homeostasis. For the alimentary tract, it has been shown that the Na+–H+ exchanger NHE3 is involved as a secondary active transport system (38). This exchanger is not expressed in airway epithelial cells (39). Therefore, other transporter systems such as NHE1 (40) may account for the Na+ exchange in the airway epithelium. After uptake into the airway epithelial cell, di- and tripeptides can be rapidly hydrolyzed by airway cell cytosolic peptidases. The generated amino acids may then be used by the cells for specific synthesis or nutritional needs or are released into circulation via multiple basolateral amino acid transporters. By contrast, hydrolysis-resistant substrates of PEPT2 such as peptidomimetic drugs accumulate in the airway cells, as shown by uptake studies, and are then released to the extracellular space via transport systems not yet molecularly identified. Transporters in the basolateral membrane of renal and intestinal epithelial cells with similar but clearly not identical features as PEPT1 or PEPT2 have been described that seem to mediate the efflux of oligopeptides and peptidomimetic drugs (41–43). Whether similar carrier proteins exist in the basloteral membranes of airway epithelia is not known. Whereas PEPT2 is a high-affinity, low-capacity transporter with substrate-dependent Km values ranging between 5 and 500 µM, PEPT1 is a low-affinity, high-capacity transporter with substrate-dependent Km values ranging from 200 µM to 10 mM for the same substrates (44). Besides these functional differences, the transporters are also differentially expressed. Whereas PEPT2 is expressed in the airways (2, 3), the kidneys (45, 46), peripheral and central nervous system (47, 48), and mammary gland (49), PEPT1 expression has been demonstrated for the intestinal tract (50, 51), kidneys (52), and bile duct epithelium (53).
The human PEPT2 (hPEPT2) transporter was identified by homology screening (54) and mapped to the human chromosome 3q13.3-q21 (55). It is part of the POT superfamily which is also termed peptide transporter family. These transporter proteins vary in size from 450 to over 700 amino acids, with prokaryotic variants being considerably smaller than eukaryotic variants such as hPEPT2. The cDNA consists of a 2,190-bp-long open reading frame with a predicted protein of 729 amino acids (Figure 3). The molecular size of hPEPT2 has not been identified, but the highly homologous rabbit PEPT2 has been reported to be glycosylated with a molecular mass of 107 kD for the mature protein and of 83 kD for the nonglycosylated transporter protein. Hydropathy analysis of the amino acid sequence indicates the presence of twelve putative transmembrane domains in parallel to the structure of PEPT1. Both the N- and the C-terminal sequences are facing the cytosol (Figure 3). The primary structure of the PEPT2 protein exhibits 50% identity and 70% similarity to the human intestinal H+/peptide cotransporter PEPT1 (54). The lowest identity between both transporter proteins is found in the large extracellular loop connecting the transmembrane domains (TMDs) 9 and 10. So far, PEPT2 has been cloned from human (54), rabbit (23), rat (56), and murine (57) tissues. Parallel to the human form, the murine Pept2 gene was localized to a syntenic region with human chromosome 3q13.3-q21, on central mouse chromosome 16 close to D16Mit4 and D16Mit59 (57).
PEPT2 gene-deleted mouse strains have been established recently (58, 59), but have not been examined for the contribution of PEPT2 to airway physiology and pathophysiology so far. Future studies may therefore target specific questions such as the association of PEPT2 to water–ion exchange or the involvement in the formation or resolution of pulmonary edema. To assess the precise role of protein domains for the functional properties of PEPT2 and PEPT1, site-directed mutagenesis studies were performed and chimeric peptide transporters consisting of distinct regions of PEPT1 and PEPT2 were generated (Figure 4). These studies demonstrated that conserved histidine residues in the second and fourth TMDs of PEPT2 and PEPT1 are essential for transport activity (60–62). The studies using chimeric peptide transporters also revealed that the first six TMDs and loops of the PEPT2 protein determine most of its phenotypical characteristics (63, 64) and that the TMDs 7 to 9 are important for its affinity to zwitterionic substrates. The first six TMDs were shown to form a major part of the transmembrane basis of the substrate-binding pocket (Figure 3) (65).
Other PEPT1–PEPT2 chimeras were constructed to identify smaller segments within the N-terminal region of the transporter proteins that contribute to the kinetic properties (66). In particular, the first 59 or 91 amino-acid residues of PEPT1 were used to replace the corresponding region in PEPT2 (Figure 4). Using Xenopus laevis oocytes as expression system, it was shown that substrate affinities of both chimeras for the zwitterionic substrate D-Phe-Ala ranged between those that are characteristic for either PEPT2 or PEPT1. However, when charged dipeptide substrates were employed, both chimeras possessed PEPT1-like affinities. The chimera carrying the N-terminal 59-amino-acid residues of PEPT1 exhibited a PEPT2-like phenotype with respect to pHout-dependency as well as to the current–voltage relationship of inward currents. In the chimera possessing the 91 amino-terminal residues of PEPT1, a pronounced alteration in the pHout dependence was observed, with highest transport rates occurring at pH values as low as pH 4.0 (Figure 4). It was therefore assumed that the two identified aminoterminal regions play the prominent role in determining the substrate affinity (66).
PEPT2 was originally identified as a transport system for oligopeptides (di- and tripeptides). In view of the 20 proteinogenic L-  -amino acids, 400 different di- and up to 8,000 different tripeptides that cover a range of molecular weights from 96.2 D (di-glycine) to 522.6 D (tri-Tryptophan) may be transported by PEPT2. As oligopeptides with a D-enantiomeric amino acid in N-terminal position have also been demonstrated to exhibit high affinities and transport rates, the number of possible PEPT2 substrates is even larger. Next to these peptides, PEPT2 accepts also a large number of pharmacologically active substrates (Figure 5) and therefore the transporter has become an attractive target for drug delivery in the airways. Together with endogenous oligopeptides, the drugs and prodrugs carried by PEPT2 display a broad range of molecular structures with a multitude of different physical and chemical characteristics.
To perform rational drug design for PEPT2-carried airway drugs and prodrugs, the essential molecular features of substrates that determine affinity and transport by PEPT2 needs to be assessed first. Therefore, a series of amino acid derivatives as well as modified dipeptides was synthesized recently (67) and kinetic constants for the interaction of test compounds with PEPT2 were obtained in competition assays using a heterologous PEPT2 expression system and the substrates' electrogenic transport properties were examined using the two-electrode voltage-clamp technique in PEPT2-expressing Xenopus laevis oocytes. In these experiments, it was demonstrated that in contrast to omega-amino fatty acids that displayed no affinity for PEPT2, the introduction of a single carbonyl group into the backbone increased both affinity and transport currents more than 30-fold. Omega-amino fatty acids, at their amino or carboxyl group coupled to an alanine residue allowed to determine the importance of the spatial position of the functional head groups within the molecule and it was found that both affinity and transport function declined by elongating the fatty acid chain when located in the N-terminal position. In contrast, the elongation in the carboxyl terminal with an N-terminal alanine caused less pronounced effects (67). These results demonstrated that a free amino-terminus, a correctly positioned backbone carbonyl group, and a carboxylic group positioned in a suitable distance from the intramolecular carbonyl function and the amino terminal head group are the major features for substrate recognition and transport by PEPT2. With the substrate-binding site of PEPT2 allowing a large number of substrates to be recognized and transported, and the recently reported molecular and structural restrictions for transporter substrates, new prodrugs and drugs may be designed and tested for their transport via PEPT2 in heterologous expression systems (68). Following this line, some of the known drug substrates of PEPT2 and their potential use in respiratory pharmacology are discussed (Figure 5).
Bacterial infections play a crucial role in the progression of chronic lung diseases such as CF and lead via a progressive destruction of lung tissue to respiratory failure (69). Infections of the lower respiratory tract invariably occur in patients with CF, and despite major advantages in antibiotic drug therapy, pneumonia remains a major cause of morbidity and mortality (70). Because the local concentration of antimicrobial agents in the lung is the most important factor for a successful eradication of bacteria, the alveolar and bronchial epithelium is a site with a major opportunity for drug delivery and therapy (71). Direct delivery of antibiotics to the airways by aerosol administration and PEPT2-mediated high-affinity uptake has potential advantages: Deposition at the site of the infection allows high local concentration (72) and therefore inhaled drugs can reduce the occurence of serious systemic adverse effects by dose reduction. Among the different pulmonary compartments, the airway epithelium and macrophages are considered to be of major importance for host defense (73–76). Therefore, high concentrations of antimicrobial agents in these compartments are most crucial, and a large number of studies have assessed the drug concentration in these cell types in relation to the effectiveness of antibiotics (77–81). In view of the central role of antibiotic treatment and the multitude of clinical studies concerning systemic and topical antibiotic therapy (82–84), the recent identification of PEPT2 as a major respiratory pathway for antibiotic transport into the airway epithelium, in which high concentrations of antibiotic agents are considered to be important to defend intracellular and extracellular airway pathogens, leads to new possibilities in respiratory drug treatment. Among the different antibiotics classes, the betalactam family, including penicillins and cephalosporins, represents the most commonly used antibacterial drugs with high effectiveness and safety profiles. Most of the orally active betalactams are transported by PEPT2, as their basic structure resembles that of tripeptides. Betalactams have the C-terminal peptide bond incorporated into the betalactam ring. Furthermore, when the C–N bond of the betalactam ring is rotated by 180°, the D-enantiomeric stereochemistry of the cephalosporins matches the L-enantiomeric stereochemistry of tripeptides, explaining the high affinity for binding and transport by PEPT2. The aminocephalosporin cefadroxil which is a substrate of PEPT2 has been used for the demonstration of functional expression of PEPT2 in the human respiratory tract (3) with a transport affinity (Km) of 50 µM (85). As with endogenous oligopeptides, there is a difference in substrate recognition and transport between PEPT2 and the intestinal transporter form PEPT1. In this respect, PEPT2 has a higher affinity for all zwitterionic betalactam antibiotics, whereas anionic betalactams such as ceftibuten and cefixime display a higher affinity for PEPT1 (86, 87). With regard to the decrease of the airway lining pH as found in airway diseases such as allergic asthma (88), it is important to add that a more acidic external pH is required for an efficient transport of anionic or dianionic compounds such as cefixime. It may therefore be assumed that under pathologic airway conditions, the transporter function may be even increased due to a more pronounced acidic airway milieu. Despite such a proposed enhanced function of PEPT2 in the airways at lower pH values, larger amounts of mucus proteins such as MUC5AC and MUC5B demonstrated in airway obstructing mucus plugging (89, 90) may hamper antibiotic delivery to PEPT2. Besides the airway epithelial cells which accumulate antibiotics via PEPT2, the epithelial lining fluid and macrophages have also been regarded as critical sites for antibiotic deposition. By efficient uptake into epithelial cells, PEPT2 could reduce the antibiotic concentration in the lining fluid and thereby reduce antibiotic efficacy in this compartment. However, as with the intestinal transporter PEPT1 that transports drugs across the absorptive enterocyte to enter the portal circulation, drugs transported by PEPT2 also have to leave the epithelium on the basolateral pole to reach subepithelial areas, in which a high antibiotic drug concentration can also be crucial, depending on the bacterial species and the grade of infiltration. The recent development (58, 59) of PEPT2 gene-depleted mouse strains should enable to characterize the precise role of PEPT2 as an antibiotic drug transporter in the airways and show whether topically administered antibiotics display an altered efficacy in gene-depleted mouse strains. Therefore, before a rational drug design for PEPT2-carried substances, further information on the expression and function of the transporter in forms of bacterial pneumonia using wild-type and gene-depleted mouse strains is needed.
In view of a potential use of peptidomimetics and prodrugs in the treatment of pulmonary neoplasms, PEPT2 may be a new tool for target-specific drug delivery, as it has been shown that peptide transporter expression and activity can be upregulated in epithelial and nonepithelial tumor cells (53, 91, 92). The peptidomimetic compound bestatin (ubenimex) was proven to be a substrate of PEPT2 and PEPT1 and is considered to act as an antineoplastic agent. It competitively inhibits leucine aminopeptidase and aminopeptidase B, and serves thereby as an indirectly acting antineoplastic compound and biological response modifier that also may potentiate host immune responses (93, 94). When peptide transporter expressing HeLa cells were implanted in Balb/c nu/nu mice to demonstrate the contribution of peptide transporters to the tumor-selective drug delivery, it was shown that administration of bestatin for one month significantly decreased the viability of the implanted tumor cells (93). In view of the potential therapeutic relevance of peptide transporter–mediated drug delivery to tumors, further studies on the expression of PEPT2 in pulmonary carcinomas need to be conducted. Peptide transporter expression or activity in tumor cells has been described for a bile epithelium tumor cell line (53), pancreatic carcinoma cell lines (91), and fibrosarcoma cell line (92).
Based on the demonstration of a functionally active PEPT2-mediated transport in the human respiratory tract (3, 32), the identification of delta-aminolevulinic acid (d-ALA) as a substrate of PEPT2 (95) added an important perspective to the field of photodynamic therapy (PDT), with a possible involvement of lung peptide transport processes in PDT. The PDT is a new technique for treatment of cancers that uses the accumulation of different photosensitizers by the tumor cells (96). When activated by light exposure with a distinct wavelength, the sensitizing agents generate reactive oxygen species, which induce apoptosis and necrosis in the neoplastic cells (97). The use of porphyrins, which are generated endogenously after administration of a precursor, displays an attractive alternative to photosensitizer injection for photodynamic therapy and is currently a rapidly evolving area of active research. One of the most promising methods is the intracellular accumulation of porphyrins by providing orally, parenterally, or topically the precursor d-ALA for protoporphyrin IX synthesis (98). The porphyrin accumulation may also be accomplished by the use of enzyme inhibitors of the heme biosynthetic pathway (98). As peptide transporter gene expression and activity has been shown to be induced in several tumor cells (53, 91, 92), PEPT2 may be a target transporter for photosensitizers when it is overexpressed in epithelial tumor cells. There are only a few reports on uptake mechanisms of photosensitizers in mammalian tissues (99). As the carrier-mediated transport of d-ALA by PEPT2 was demonstrated (95), the pulmonary expression profile of PEPT2 provides a basis for a more rational understanding of pulmonary d-ALA uptake mechanisms. Future studies of PEPT2 expression levels in different respiratory neoplasms need to be performed to assess the use of a PEPT2-mediated, tumor-cell delivery of PDT compounds. In this respect, PEPT2 gene expression and activity may be induced in pulmonary epithelial tumors, as previously demonstrated for other organs (53, 91, 92). As respiratory neoplastic cells require large quantities of amino nitrogen for metabolism and growth, PEPT2 might also be upregulated in neoplastic cells due to increased nutritional needs and could provide an optimal molecular therapeutic tool to target d-ALA or other antineoplastic drugs/prodrugs to the cells. The uptake of d-ALA and accumulation of porphyrins is also used diagnostically to identify early stage tumors in the respiratory tract. Here, numerous reports have focused on the development of guidelines for the use of this method in the early detection of neoplasms (100–102). The identification of PEPT2 in the respiratory tract gives a first insight into the molecular mechanisms underlying the d-ALA uptake.
Apart from adenovirus infections, viral respiratory tract infections are also common in immunocompromised patients (103). In these patients, the three herpes viruses—herpes simplex, varicella-zoster, and cytomegalovirus—are the most common causes of viral pneumonia. As the diagnosis of, e.g., herpes virus infections is still very difficult and the treatment is not optimized, advances in therapeutic strategies may help to address this problem. Viruses such as herpes viruses or the infectious agent causing severe acute respiratory distress syndrome replicate in pulmonary epithelial cells (104–106) and similar to bacterial pneumonia high local drug concentrations are most crucial for the effectiveness of antiviral agents in this compartment. In this respect, the pulmonary expression of PEPT2 offers the potential to topically deliver antiviral compounds to the airway site of infections. Valacyclovir, the valyl ester of acyclovir, has been shown to be a substrate of PEPT2 (107). Also, valganciclovir, the valyl ester of ganciclovir, which is used for the treatment of cytomegalovirus infection, has been shown to be transported by PEPT2 (108). Next to these compounds, also the valyl ester of zidovudine, which is used in the treatment of HIV, has been reported to be a substrate of peptide transporters (109). It therefore appears that the esterification of a drug with an amino acid residue to create a prodrug, as in the case of the described antiviral nucleosides, may be a very promising approach to model PEPT2-transported drugs.
Transvascular drug transport is an important feature for the delivery of drugs either from the lungs to the circulation or from the circulation to the lungs. The primary expression site of PEPT2 in the respiratory tract seems to be the airway epithelium. This localization of PEPT2 mimics the localization of the intestinal peptide transporter PEPT1. PEPT1 is expressed in the enterocytes and mediates the transport of nutrients and drugs into the enterocytes (51), from which they reach the portal vein in high concentrations via yet unidentified basolateral mechanisms. Likewise, PEPT2 is expressed in the airway epithelium and may function there as a carrier for drugs into the circulation. Whereas its apical localization and function has been demonstrated (2), the basolateral exit mechanisms by which the compounds leave the epithelial cells have not thus far been characterized. Next to the function of epithelial cells for the pulmonary route of drugs to reach the circulation, endothelial cells are the key cell type of transvascular drug delivery. For the endothelial compartment, PEPT2 expression has been shown in the cells of smaller pulmonary vessels. As PEPT2 expression here was not correlated to transport activity, the transporter seems not to play a major role for drug transport in this cell type (2).
Pulmonary administration of drugs plays an important role in the treatment of various respiratory and systemic diseases and appears as an attractive area of future drug development. Recently, PEPT2 was identified as the respiratory carrier for uptake of peptidomimietic drugs. Using this transport protein, the aerosolic administration of known drugs or of new prodrugs designed to become a PEPT2 substrate, could play a major role in the treatment of various pulmonary and systemic diseases. Before rational drug design and delivery via PEPT2, future studies have to address the expression and function of the transporter in major airway diseases such as asthma by applying modern techniques of molecular biology such as gene arrays (110), nonisotopic in situ hybridization (111, 112), or laser-assisted airway cell harvesting (113), morphology analysis (114–116), and pharmacology (117).
The authors thank C. Peiser and F. Döring for helpful discussions. This study was supported by the German Academic Exchange Council (DAAD, D/00/10559) and by Deutsche Atemwegsliga. Received in original form August 24, 2003 Received in final form November 3, 2003
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Abstract
Introduction
