Metabolism, uptake, and transepithelial transport of the stereoisomers of Val-Val-Val in the human intestinal cell line, Caco-2

Radiolabeled (R)-(-)-5-iodo-3'-O-[2-(ε-guanidinohexanoyl)-2-phenylacetyl]-2'-deoxyuridine: A new theranostic for neuroblastoma

Neuroblastoma, the most common extracranial solid tumor in children, accounts for nearly 8% of childhood cancers in the United States. It is a disease with pronounced clinical and biological heterogeneities. The amplification of MYCN, whose key tumorigenic functions include the promotion of proliferation, facilitation of the cell's entry into the S phase, and prevention of cells from leaving the cell cycle, correlates with poor prognosis. Patients with a high proliferation index disease have low survival rates. Neuroblastoma is one of the most radioresponsive of all human tumors. To exploit this radiosensitivity, radioactive guanidine (R)-(-)-5-[125 I]iodo-3'-O-[2-(ε-guanidinohexanoyl)-2-phenylacetyl]-2'-deoxyuridine (9, GPAID) was designed. This compound enters neuroblastoma cells much like metaiodobenzylguanidine (MIBG). Additionally, it cotargets DNA of proliferating cells, an attribute especially advantageous in the treatment of MYCN-amplified tumors. GPAID was synthesized from the trimethylstannyl precursor with an average yield of >90% at the no-carrier-added specific activities. The norepinephrine transporter-aided delivery of GPAID to neuroblastoma cells was established in the competitive uptake studies with nonradioactive MIBG. The intracellular processing and DNA targeting properties were confirmed in the subcellular distribution experiments. Studies in a mouse model of neuroblastoma demonstrated the therapeutic potential of GPAID. The tin precursor of GPAID can be used to prepare compounds radiolabeled with single-photon emission computed tomography (SPECT)- and positron-emission tomography (PET)-compatible radionuclides. Accordingly, these reagents can function as theranostics useful in the individualized and comprehensive treatment strategies comprising treatment planning and the assessment of tumor responses as well as the targeted molecular radiotherapy employing treatment doses derived from the imaging data.

Prodrug strategies in ocular drug delivery

Poor bioavailability of topically instilled drug is the major concern in the field of ocular drug delivery. Efflux transporters, static and dynamic ocular barriers often possess rate limiting factors for ocular drug therapy. Different formulation strategies like suspension, ointment, gels, nanoparticles, implants, dendrimers and liposomes have been employed in order to improve drug permeation and retention by evading rate limiting factors at the site of absorption. Chemical modification such as prodrug targeting various nutrient transporters (amino acids, peptide and vitamin) has evolved a great deal of interest to improve ocular drug delivery. In this review, we have discussed various prodrug strategies which have been widely applied for enhancing therapeutic efficacy of ophthalmic drugs. The purpose of this review is to provide an update on the utilization of prodrug concept in ocular drug delivery. In addition, this review will highlight ongoing academic and industrial research and development in terms of ocular prodrug design and delivery.

Angiotensin converting enzyme inhibitory peptides from a lactotripeptide-enriched milk beverage are absorbed intact into the circulation

Food products containing angiotensin converting enzyme (ACE) inhibitory peptides reportedly play a role in treatment of mild hypertension. The aim of this placebo-controlled crossover study was to assess the bioavailability of Ile-Pro-Pro and 7 other ACE-inhibiting peptides present in a lactotripeptide (LTP)-enriched yogurt beverage and whether meal intake affects Ile-Pro-Pro bioavailability. Six male and female subjects randomly consumed an LTP-enriched yogurt beverage or a placebo in the fasted state and an LTP-enriched yogurt beverage in the fed or fasted state. The area under the curve (AUC) of Ile-Pro-Pro after the LTP treatment in the fasted state was 2.1-fold of that after the placebo treatment (P < 0.001). The maximum peptide plasma concentration (C(max)) value was greater after consumption of the LTP-enriched beverage (897 +/- 157 pmol/L) than after the placebo treatment (555 +/- 0.09 pmol/L; P < 0.001) with a greater time after ingestion when reaching C(max) (T(max)) in the placebo treatment. Plasma concentrations of the peptides Leu-Trp, Phe-Tyr, Ile-Tyr, and Leu-Pro-Pro increased compared with baseline (P < 0.05) in the LTP-enriched and placebo treatment when consumed in the fasted state. However, DeltaC(max) values differed significantly between the placebo and LTP-enriched treatment only for Leu-Pro-Pro. Meal intake affected Ile-Pro-Pro concentrations. When the beverage was consumed after a meal, the AUC of Ile-Pro-Pro was 1.3-fold (P < 0.05) of the AUC derived from premeal intake. This was due to an increase in the plasma elimination half-life (P < 0.05); C(max) and T(max) were not affected by meal intake. In summary, this is the first demonstration, to our knowledge, that the tripeptide Ile-Pro-Pro selectively escapes from intestinal degradation and reaches the circulation undegraded.

Conformational restrictions in ligand binding to the human intestinal di-/tripeptide transporter: implications for design of hPEPT1 targeted prodrugs

The aim of the present study was to develop a computational method aiding the design of dipeptidomimetic pro-moieties targeting the human intestinal di-/tripeptide transporter hPEPT1. First, the conformation in which substrates bind to hPEPT1 (the bioactive conformation) was identified by conformational analysis and 2D dihedral driving analysis of 15 hPEPT1 substrates, which suggested that psi(1) approximately 165 degrees , omega(1) approximately 180 degrees , and phi(2) approximately 280 degrees were descriptive of the bioactive conformation. Subsequently, the conformational energy required to change the peptide backbone conformation (DeltaE(bbone)) from the global energy minimum conformation to the identified bioactive conformation was calculated for 20 hPEPT1 targeted model prodrugs with known K(i) values. Quantitatively, an inverse linear relationship (r(2)=0.81, q(2)=0.80) was obtained between DeltaE(bbone) and log1/K(i), showing that DeltaE(bbone) contributes significantly to the experimentally observed affinity for hPEPT1 ligands. Qualitatively, the results revealed that compounds classified as high affinity ligands (K(i)<0.5 mM) all have a calculated DeltaE(bbone)<1 kcal/mol, whereas medium and low-affinity compounds (0.5 mM<K(i)<15 mM) have DeltaE(bbone) values in the range 1-3 kcal/mol. The findings also shed new light on the basis for the experimentally observed stereoselectivity of hPEPT1.

Amino acid ester prodrugs of floxuridine: synthesis and effects of structure, stereochemistry, and site of esterification on the rate of hydrolysis

PURPOSE

To synthesize amino acid ester prodrugs of floxuridine (FUdR) and to investigate the effects of structure, stereochemistry, and site of esterification of promoiety on the rates of hydrolysis of these prodrugs in Caco-2 cell homogenates.

METHODS

Amino acid ester prodrugs of FUdR were synthesized using established procedures. The kinetics of hydrolysis of prodrugs was evaluated in human adenocarcinoma cell line (Caco-2) homogenates and pH 7.4 phosphate buffer.

RESULTS

3'-Monoester, 5'-monoester, and 3',5'-diester prodrugs of FUdR utilizing proline, L-valine, D-valine, L-phenylalanine, and D-phenylalanine as promoieties were synthesized and characterized. In Caco-2 cell homogenates, the L-amino acid ester prodrugs hydrolyzed 10 to 75 times faster than the corresponding D-amino acid ester prodrugs. Pro and Phe ester prodrugs hydrolyzed much faster (3- to 30-fold) than the corresponding Val ester prodrugs. Further, the 5'-monoester prodrugs hydrolyzed significantly faster (3-fold) than the 3',5'-diester prodrugs.

CONCLUSIONS

Novel amino acid ester prodrugs of FUdR were successfully synthesized. The results presented here clearly demonstrate that the rate of FUdR prodrug activation in Caco-2 cell homogenates is affected by the structure, stereochemistry, and site of esterification of the promoiety. Finally, the 5'-Val and 5'-Phe monoesters exhibited desirable characteristics such as good solution stability and relatively fast enzymatic conversion rates.

Relationship between structure and permeability of dipeptide derivatives containing tryptophan and related compounds across human intestinal epithelial (Caco-2) cells

The permeability of dipeptide derivatives containing tryptophans and indole derivatives through Caco-2 cells was used as an in vitro intestinal absorption model in order to clarify structural factors which influence their intestinal epithelial permeation and metabolism. Most peptide derivatives were hydrolysed not only by the cytosolic enzymes in Caco-2 cells during permeation but also by enzymes released to the apical solution before cell permeation. The N-terminal blocked dipeptides were more resistant to hydrolases expressed in the Caco-2 cells and indole derivatives were not entirely degraded. Based on compound concentration dependency and comparison of permeability coefficients in apical-to-basolateral and basolateral-to-apical directions, the main absorption mechanism of compounds were determined. Compounds were then classified into three groups; (1) passively transported compounds, (2) actively transported compounds and (3) compounds excreted by P-glycoprotein.

Transport of peptidomimetic drugs by the intestinal Di/tri-peptide transporter, PepT1

The apical membrane of small intestinal enterocytes possess an uptake system for di- and tripeptides. The physiological function of the system is to transport small peptides resulting from digestion of dietary protein. Moreover, due to the broad substrate specificity of the system, it is also capable of transporting a number of orally administered peptidomimetic drugs. Absorbed peptides may be hydrolysed in the cells due to the high peptidase activity present in the cytosol. Peptidomimetic drugs may, if resistant to the cellular enzyme activity, pass the basolateral membrane via a basolateral peptide transport mechanism and enter the systemic circulation. As the number of new peptide and peptidomimetic drugs are rapidly increasing, the peptide transport system has gained increasing attention as a possible drug delivery system for small peptides and peptide-like compounds. In this paper we give an updated introduction to the transport system and discuss the substrate characteristics of the di/tri-peptide transporter system with special emphasis on chemically modified substrates and prodrugs.

Transport characteristics of peptides and peptidomimetics: II. Hydroxyethylamine bioisostere-containing peptidomimetics as substrates for the oligopeptide transporter and P-glycoprotein in the intestinal mucosa

Peptide bond bioisosteres, such as hydroxyethylamine (Hea), have frequently been used to stabilize metabolically labile peptide bonds in peptidomimetic drug design in an effort to increase the oral bioavailability of drug candidates. However, the impact of the peptide bond bioisosteres on the cell permeation characteristics of peptidomimetics is not well understood, particularly with respect to the effects on the substrate activity for proteins that can restrict (e.g. P-glycoprotein, P-gp) or facilitate (e.g. the oligopeptide transporter, OPT) intestinal mucosal permeation of peptidomimetics. In this study, terminally free and terminally modified (N-acetylated and C-amidated) peptidomimetics of H-Ala-Phe-OH and H-Ala-Phe-Ala-OH with the Ala-Phe peptide bonds replaced by Hea bioisosteres were synthesized. Transport characteristics of these peptidomimetics were investigated using Caco-2 cell monolayers as an in vitro model of the intestinal mucosa. The study showed that the Hea bioisostere stabilized the peptidomimetics to protease metabolism in Caco-2 cells. All terminally free peptidomimetics showed significant affinity and substrate activity for OPT. The affinity and substrate activity for OPT were stereoselective for peptidomimetics containing an S,S-configuration for the two adjacent chiral centers related to the Hea bioisostere. Three of the four terminally modified peptidomimetics showed significant substrate activity for P-gp and, interestingly, the substrate activity for P-gp was also stereoselective; however, it was in favor of an R,R-configuration for the two adjacent chiral centers related to the Hea bioisostere.

Model prodrugs for the intestinal oligopeptide transporter: model drug release in aqueous solution and in various biological media

The human intestinal di/tri-peptide carrier, hPepT1, has been suggested as a target for increasing intestinal transport of low permeability compounds by creating prodrugs designed for the transporter. Model ester prodrugs using the stabilized dipeptides D-Glu-Ala and D-Asp-Ala as pro-moieties for benzyl alcohol have been shown to have affinity for hPepT1. Furthermore, in aqueous solution at pH 5.5 to 10, the release of the model drug seems to be controlled by a specific base-catalyzed hydrolysis, indicating that the compounds may remain relatively stable in the upper small intestinal lumen with a pH of approximately 6.0, but still release the model drug at the intercellular and blood pH of approximately 7.4. Even though benzyl alcohol is not a low molecular weight drug molecule, these results indicate that the dipeptide prodrug principle is a promising drug delivery concept. However, the physico-chemical properties such as electronegativity, solubility, and log P of the drug molecule may also have an influence on the potential of these kinds of prodrugs. The purpose of the present study is to investigate whether the model drug electronegativity, estimated as Taft substitution parameter (sigma*) may influence the acid, water or base catalyzed model drug release rates, when released from series of D-Glu-Ala and D-Asp-Ala pro-moieties. Release rates were investigated in both aqueous solutions with varying pH, ionic strength, and buffer concentrations as well as in in vitro biological media. The release rates of all the investigated model drug molecules followed first-order kinetics and were dependent on buffer concentration, pH, ionic strength, and model drug electronegativity. The electronegativity of the model drug influenced acid, water and base catalyzed release from D-Asp-Ala and D-Glu-Ala pro-moieties. The model drug was generally released faster from D-Asp-Ala- than from the D-Glu-Ala pro-moieties. In biological media the release rate was also dependent on the electronegativity of the model drug. These results demonstrate that the model drug electronegativity, estimated as Taft (sigma*) values, has a significant influence on the release rate of the model drug.

A functional assay for quantitation of the apparent affinities of ligands of P-glycoprotein in Caco-2 cells

PURPOSE

To develop a facile functional assay for quantitative determination of the apparent affinities of compounds that interact with the taxol binding site of P-glcoprotein (P-gp) in Caco-2 cell monolayers.

METHODS

A transport inhibition approach was taken to determine the inhibitory effects of compounds on the active transport of [3H]-taxol, a known substrate of P-gp. The apparent affinities (K(I) values) of the compounds were quantitatively determined based on the inhibitory effects of the compounds on the active transport of [3H]-taxol. Intact Caco-2 cell monolayers were utilized for transport inhibition studies. Samples were analyzed by liquid scintillation counting.

RESULTS

[3H]-Taxol (0.04 microM) showed polarized transport with the basolateral (BL) to apical (AP) flux rate being about 10-20 times faster than the flux rate in the AP-to-BL direction. This difference in [3H]-taxol flux could be totally abolished by inclusion of (+/-)-verapamil (0.2 mM), a known inhibitor of P-gp, in the incubation medium. However, inclusion of probenecid (1.0 mM), a known inhibitor for the multidrug resistance associated protein (MRP), did not significantly affect the transport of [3H]-taxol under the same conditions. These results suggest that P-gp, not MRP, was involved in taxol transport. Quinidine, daunorubicin, verapamil, taxol, doxorubicin, vinblastine, etoposide, and celiprolol were examined as inhibitors of the BL-to-AP transport of [3H]-taxol with resulting K(I) values of 1.5+/-0.8, 2.5+/-1.0, 3.0+/-0.3, 7.3+/-0.7, 8.5+/-2.8, 36.5+/-1.5, 276+/-69, and 313+/-112 microM, respectively. With the exception of that of quinidine, these K(I) values were comparable with literature values.

CONCLUSIONS

This assay allows a facile quantitation of the apparent affinities of compounds to the taxol-binding site in P-gp, however, this assay does not permit the differentiation of substrates and inhibitors. The potential of drug-drug interactions involving the taxol binding site of P-gp can be conveniently estimated using the protocol described in this paper.

Structure-activity relationship of carbacephalosporins and cephalosporins: antibacterial activity and interaction with the intestinal proton-dependent dipeptide transport carrier of Caco-2 cells

An intestinal proton-dependent peptide transporter located on the lumenal surface of the enterocyte is responsible for the uptake of many orally absorbed beta-lactam antibiotics. Both cephalexin and loracarbef are transported by this mechanism into the human intestinal Caco-2 cell line. Forty-seven analogs of the carbacephalosporin loracarbef and the cephalosporin cephalexin were prepared to evaluate the structural features necessary for uptake by this transport carrier. Compounds were evaluated for their antibacterial activities and for their ability to inhibit 1 mM cephalexin uptake and, subsequently, uptake into Caco-2 cells. Three clinically evaluated orally absorbed carbacephems were taken up by Caco-2 cells, consistent with their excellent bioavailability in humans. Although the carrier preferred the L stereoisomer, these compounds lacked antibacterial activity and were hydrolyzed intracellularly in Caco-2 cells. Compounds modified at the 3 position of cephalexin and loracarbef with a cyclopropyl or a trifluoromethyl group inhibited cephalexin uptake. Analogs with lipophilic groups on the primary amine of the side chain inhibited cephalexin uptake, retained activity against gram-positive bacteria but lost activity against gram-negative bacteria. Substitution of the phenylglycl side chain with phenylacetyl side chains gave similar results. Compounds which lacked an aromatic ring in the side chain inhibited cephalexin uptake but lost all antibacterial activity. Thus, the phenylglycl side chain is not absolutely required for uptake. Different structural features are required for antibacterial activity and for being a substrate of the transporter. Competition studies with cephalexin indicate that human intestinal Caco-2 cells may be a useful model system for initially guiding structure-activity relationships for the rational design of new oral agents.