Targeting the sodium-dependent multivitamin transporter (SMVT) for improving the oral absorption properties of a retro-inverso Tat nonapeptide

Dual-Targeted Nanoparticle-in-Microparticle System for Ulcerative Colitis Therapy

Conventional oral therapy for ulcerative colitis (UC) is associated with premature release or degradation of drugs in the harsh gastrointestinal environment, resulting in reduced therapeutic effectiveness. Consequently, the present study aims to develop a dual-targeted delivery system with a nanoparticle-in-microparticle (nano-in-micro) structure. The prepared Asiatic Acid-loaded delivery system (AA/CDM-BT-ALG) has pH-sensitive properties. Cellular uptake evaluation confirms that nanoparticles exhibit targeted absorption by macrophages and Caco-2 cells through mannose (Man) receptor and biotin-mediated endocytosis, respectively. Therefore, this mechanism effectively enhances intracellular drug concentration. Additionally, the biodistribution study conducted on the gastrointestinal tract of mice indicates that the colon of the microspheres group shows higher fluorescence intensity with longer duration than the other groups. This finding indicates that the microspheres exhibit selective accumulation in areas of colon inflammation. In vivo experiments in colitis mice showed that AA/CDM-BT-ALG significantly alleviates the histopathological characteristics of the colon, reduced neutrophil, and macrophage infiltration, and decreases pro-inflammatory cytokine expression. Furthermore, the effect of AA/CDM-BT-ALG on colitis is validated to be closely related to the TLR4/MyD88/NF-κB signaling pathway. The present findings suggest that the development of a dual-targeted delivery system is accomplished effectively, with the potential to serve as a drug-controlled release system for treating UC.

Biotin conjugates in targeted drug delivery: is it mediated by a biotin transporter, a yet to be identified receptor, or (an)other unknown mechanism(s)?

Conjugation of drugs with biotin is a widely studied strategy for targeted drug delivery. The structure-activity relationship (SAR) studies through H3-biotin competition experiments conclude with the presence of a free carboxylic acid being essential for its uptake via the sodium-dependent multivitamin transporter (SMVT, the major biotin transporter). However, biotin conjugation with a payload requires modification of the carboxylic acid to an amide or ester group. Then, there is the question as to how/whether the uptake of biotin conjugates goes through the SMVT. If not, then what is the mechanism? Herein, we present known uptake mechanisms of biotin and its applications reported in the literature. We also critically analyse possible uptake mechanism(s) of biotin conjugates to address the disconnect between the results from SMVT-based SAR and "biotin-facilitated" targeted drug delivery. We believe understanding the uptake mechanism of biotin conjugates is critical for their future applications and further development.

Quantitative live-cell imaging of lipidated peptide transport through an epithelial cell layer

Oral drug delivery increases patient compliance and is thus the preferred administration route for most drugs. However, for biologics the intestinal barrier greatly limits the absorption and reduces their bioavailability. One strategy employed to improve on this is chemical modification of the biologic through the addition of lipid side chains. While it has been established that lipidation of peptides can increase transport, a mechanistic understanding of this effect remains largely unexplored. To pursue this mechanistic understanding, end-point detection of biopharmaceuticals transported through a monolayer of fully polarized epithelial cells is typically used. However, these methods are time-consuming and tedious. Furthermore, most established methods cannot be combined easily with high-resolution live-cell fluorescence imaging that could provide a mechanistic insight into cellular uptake and transport. Here we address this challenge by developing an axial PSF deconvolution scheme to quantify the transport of peptides through a monolayer of Caco-2 cells using single-cell analysis with live-cell confocal fluorescence microscopy. We then measure the known cross-barrier transport of several compounds in our model and compare the results with results obtained in an established microfluidic model finding similar transport phenotypes. This verifies that already after two days the Caco-2 cells in our model form a tight monolayer and constitute a functional barrier model. We then apply this assay to investigate the effects of side chain lipidation of the model peptide drug salmon calcitonin (sCT) modified with 4‑carbon and 8‑carbon-long fatty acid chains. Furthermore, we compare that with experiments performed at lower temperature and using inhibitors for some endocytotic pathways to pinpoint how lipidation length modifies the main avenues for the transport. We thus show that increasing the length of the lipid chain increases the transport of the drug significantly but also makes endocytosis the primary transport mechanism in a short-term cell culture model.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

The Impact of Breast Cancer Resistance Protein (BCRP/ABCG2) on Drug Transport Across Caco-2 Cell Monolayers

Breast cancer resistance protein (BCRP) is expressed on the apical membrane of small intestinal epithelial cells and functions as an efflux pump with broad substrate recognition. Therefore, quantitative evaluation of the contribution of BCRP to the intestinal permeability of new chemical entities is very important in drug research and development. In this study, we assessed the BCRP-mediated efflux of several model drugs in Caco-2 cells using WK-X-34 as a dual inhibitor of P-glycoprotein (P-gp) and BCRP and LY335979 as a selective inhibitor of P-gp. The permeability of daidzein was high with an apparent permeability coefficient for apical-to-basal transport (P _AB) of 20.3 × 10^-6 cm/s. In addition, its efflux ratio (ER) was 1.55, indicating that the contribution of BCRP to its transport is minimal. Estrone-3-sulfate and ciprofloxacin showed relatively higher ER values (>2.0), whereas their BCRP-related absorptive quotient (AQ _BCRP) was 0.21 and 0.3, respectively. These results indicate that BCRP does not play a major role in regulating the permeability of estrone-3-sulfate and ciprofloxacin in Caco-2 cells. Nitrofurantoin showed a P _AB of 1.8 × 10^-6 cm/s, and its ER was 7.6. However, the AQ _BCRP was 0.37, suggesting minimal contribution of BCRP to nitrofurantoin transport in Caco-2 cells. In contrast, topotecan, SN-38, and sulfasalazine had low P _AB values (0.81, 1.13, and 0.19 × 10^-6 cm/s, respectively), and each AQ _BCRP was above 0.6, indicating that BCRP significantly contributes to the transport of these compounds in Caco-2 cells. In conclusion, Caco-2 cells are useful to accurately estimate the contribution of BCRP to intestinal drug absorption. SIGNIFICANCE STATEMENT: We performed an in vitro assessment of the contribution of breast cancer resistance protein (BCRP) to the transport of BCRP and/or P-glycoprotein (P-gp) substrates across Caco-2 cell monolayers using absorptive quotient, which has been proposed to represent the contribution of drug efflux transporters to the net efflux. The present study demonstrates that the combined use of a BCRP/P-gp dual inhibitor and a P-gp selective inhibitor is useful to estimate the impact of BCRP and P-gp on the permeability of tested compounds in Caco-2 cells.

Transporter-Guided Delivery of Nanoparticles to Improve Drug Permeation across Cellular Barriers and Drug Exposure to Selective Cell Types

Targeted nano-drug delivery systems conjugated with specific ligands to target selective cell-surface receptors or transporters could enhance the efficacy of drug delivery and therapy. Transporters are expressed differentially on the cell-surface of different cell types, and also specific transporters are expressed at higher than normal levels in selective cell types under pathological conditions. They also play a key role in intestinal absorption, delivery via non-oral routes (e.g., pulmonary route and nasal route), and transfer across biological barriers (e.g., blood–brain barrier and blood–retinal barrier. As such, the cell-surface transporters represent ideal targets for nano-drug delivery systems to facilitate drug delivery to selective cell types under normal or pathological conditions and also to avoid off-target adverse side effects of the drugs. There is increasing evidence in recent years supporting the utility of cell-surface transporters in the field of nano-drug delivery to increase oral bioavailability, to improve transfer across the blood–brain barrier, and to enhance delivery of therapeutics in a cell-type selective manner in disease states. Here we provide a comprehensive review of recent advancements in this interesting and important area. We also highlight certain key aspects that need to be taken into account for optimal development of transporter-assisted nano-drug delivery systems.

Using peptides to increase transport across the intestinal barrier

The oral route is the preferred for the administration of drugs; however, it has some serious limitations. One of the main disadvantages is poor permeability across the intestinal barrier. Various approaches are currently being adopted to overcome this issue. In this review, we describe the alternatives that use peptides to enhance intestinal absorption. First, we define the various sources of peptide enhancers followed by the analysis of the absorption mechanism used. We then comment on the possible toxic effects derived from their use as permeation enhancers, as well as potential formulation strategies. Finally, the advantages and drawbacks of peptides as intestinal enhancers are examined.

Oral drug delivery systems using chemical conjugates or physical complexes

Oral delivery of therapeutics is extremely challenging. The digestive system is designed in a way that naturally allows the degradation of proteins or peptides into small molecules prior to absorption. For systemic absorption, the intact drug molecules must traverse the impending harsh gastrointestinal environment. Technologies, such as enteric coating, with oral dosage formulation strategies have successfully provided the protection of non-peptide based therapeutics against the harsh, acidic condition of the stomach. However, these technologies showed limited success on the protection of therapeutic proteins and peptides. Importantly, inherent permeability coefficient of the therapeutics is still a major problem that has remained unresolved for decades. Addressing this issue in the context, we summarize the strategies that are developed in enhancing the intestinal permeability of a drug molecule either by modifying the intestinal epithelium or by modifying the drug itself. These modifications have been pursued by using a group of molecules that can be conjugated to the drug molecule to alter the cell permeability of the drug or mixed with the drug molecule to alter the epithelial barrier function, in order to achieve the effective drug permeation. This article will address the current trends and future perspectives of the oral delivery strategies.

A novel retro-inverso peptide inhibitor reduces amyloid deposition, oxidation and inflammation and stimulates neurogenesis in the APPswe/PS1ΔE9 mouse model of Alzheimer's disease

Previously, we have developed a retro-inverso peptide inhibitor (RI-OR2, rGffvlkGr) that blocks the in vitro formation and toxicity of the Aβ oligomers which are thought to be a cause of neurodegeneration and memory loss in Alzheimer’s disease. We have now attached a retro-inverted version of the HIV protein transduction domain ‘TAT’ to RI-OR2 to target this new inhibitor (RI-OR2-TAT, Ac-rGffvlkGrrrrqrrkkrGy-NH₂) into the brain. Following its peripheral injection, a fluorescein-labelled version of RI-OR2-TAT was found to cross the blood brain barrier and bind to the amyloid plaques and activated microglial cells present in the cerebral cortex of 17-months-old APPswe/PS1ΔE9 transgenic mice. Daily intraperitoneal injection of RI-OR2-TAT (at 100 nmol/kg) for 21 days into 10-months-old APPswe/PS1ΔE9 mice resulted in a 25% reduction (p<0.01) in the cerebral cortex of Aβ oligomer levels, a 32% reduction (p<0.0001) of β-amyloid plaque count, a 44% reduction (p<0.0001) in the numbers of activated microglial cells, and a 25% reduction (p<0.0001) in oxidative damage, while the number of young neurons in the dentate gyrus was increased by 210% (p<0.0001), all compared to control APPswe/PS1ΔE9 mice injected with vehicle (saline) alone. Our data suggest that oxidative damage, inflammation, and inhibition of neurogenesis are all a downstream consequence of Aβ aggregation, and identify a novel brain-penetrant retro-inverso peptide inhibitor of Aβ oligomer formation for further testing in humans as a potential disease-modifying treatment for Alzheimer’s disease.

Oral delivery of glucagon-like peptide-1 and analogs: alternatives for diabetes control?

Type 2 diabetes mellitus (T2DM) is one of the most prevalent diseases worldwide. Current treatments are often associated with off-target effects and do not significantly impact disease progression. New therapies are therefore urgently needed to overcome this social burden. Glucagon-like peptide-1 (GLP-1), an incretin hormone, has been used to control T2DM symptomatology. However, the administration of peptide or proteins drugs is still a huge challenge in the pharmaceutical field, requiring administration by parenteral routes. This article reviews the main hurdles in oral administration of GLP-1 and focuses on the strategies utilized to overcome them.

Targeted lipid based drug conjugates: a novel strategy for drug delivery

A majority of studies involving prodrugs are directed to overcome low bioavailability of the parent drug. The aim of this study is to increase the bioavailability of acyclovir (ACV) by designing a novel prodrug delivery system which is more lipophilic, and at the same time site specific. In this study, a lipid raft has been conjugated to the parent drug molecule to impart lipophilicity. Simultaneously a targeting moiety that can be recognized by a specific transporter/receptor in the cell membrane has also been tethered to the other terminal of lipid raft. Targeted lipid prodrugs i.e., biotin-ricinoleicacid-acyclovir (B-R-ACV) and biotin-12hydroxystearicacid-acyclovir (B-12HS-ACV) were synthesized with ricinoleicacid and 12hydroxystearicacid as the lipophilic rafts and biotin as the targeting moiety. Biotin-ACV (B-ACV), ricinoleicacid-ACV (R-ACV) and 12hydroxystearicacid-ACV (12HS-ACV) were also synthesized to delineate the individual effects of the targeting and the lipid moieties. Cellular accumulation studies were performed in confluent MDCK-MDR1 and Caco-2 cells. The targeted lipid prodrugs B-R-ACV and B-12HS-ACV exhibited much higher cellular accumulation than B-ACV, R-ACV and 12HS-ACV in both cell lines. This result indicates that both the targeting and the lipid moiety act synergistically toward cellular uptake. The biotin conjugated prodrugs caused a decrease in the uptake of [(3)H] biotin suggesting the role of sodium dependent multivitamin transporter (SMVT) in uptake. The affinity of these targeted lipid prodrugs toward SMVT was studied in MDCK-MDR1 cells. Both the targeted lipid prodrugs B-R-ACV (20.25 ± 1.74 μM) and B-12HS-ACV (23.99 ± 3.20 μM) demonstrated higher affinity towards SMVT than B-ACV (30.90 ± 4.19 μM). Further, dose dependent studies revealed a concentration dependent inhibitory effect on [(3)H] biotin uptake in the presence of biotinylated prodrugs. Transepithelial transport studies showed lowering of [(3)H] biotin permeability in the presence of biotin and biotinylated prodrugs, further indicating a carrier mediated translocation by SMVT. Overall, results from these studies clearly suggest that these biotinylated lipid prodrugs of ACV possess enhanced affinity towards SMVT. These prodrugs appear to be potential candidates for the treatment of oral and ocular herpes virus infections, because of higher expression of SMVT on intestinal and corneal epithelial cells. In conclusion we hypothesize that our novel prodrug design strategy may help in higher absorption of hydrophilic parent drug. Moreover, this novel prodrug design can result in higher cell permeability of hydrophilic therapeutics such as genes, siRNA, antisense RNA, DNA, oligonucleotides, peptides and proteins.

Strategies for non-invasive delivery of biologics

Macromolecular therapeutics, in particular, many biologics, is the most advancing category of drugs over conventional chemical drugs. The potency and specificity of the biologics for curing certain disease made them to be a leading compound in the pharmaceutical industry. However, due to their intrinsic nature, including high molecular weight, hydrophilicity and instability, they are difficult to be administered via non-invasive route. This is a major quest especially in biologics, as they are frequently used clinically for chronic disorders, which requires long-term administration. Therefore, many efforts have been made to develop formulation for non-invasive administration, in attempt to improve patient compliance and convenience. In this review, strategies for non-invasive delivery, in particular, oral, pulmonary and nasal delivery, that are recently adopted for delivery of biologics are discussed. Insulin, calcitonin and heparin were mainly focused for the discussion as they could represent protein, polypeptide and polysaccharide drugs, respectively. Many recent attempts for non-invasive delivery of biologics are compared to provide an insight of developing successful delivery system.

Prodrugs for improving tumor targetability and efficiency

As the mainstay in the treatment of various cancers for several decades, chemotherapy is successful but still faces challenges including non-selectivity and high toxicity. Improving the selectivity is therefore a critical step to improve the therapeutic efficacy of chemotherapy. Prodrug is one of the most promising approaches to increase the selectivity and efficacy of a chemotherapy drug. The classical prodrug approach is to improve the pharmaceutical properties (solubility, stability, permeability, irritation, distribution, etc.) via a simple chemical modification. This review will focus on various targeted prodrug designs that have been developed to increase the selectivity of chemotherapy drugs. Various tumor-targeting ligands, transporter-associated ligands, and polymers can be incorporated in a prodrug to enhance the tumor uptake. Prodrugs can also be activated by enzymes that are specifically expressed at a higher level in tumors, leading to a selective anti-tumor effect. This can be achieved by conjugating the enzyme to a tumor-specific antibody, or delivering a vector expressing the enzyme into tumor cells.

Transporters, Trojan horses and therapeutics: suitability of bile acid and peptide transporters for drug delivery

Membrane transporters are major determinants for the pharmacokinetic, safety and efficacy behavior of drugs. Available technologies to study function and structure of transport proteins has strongly stimulated research in transporter biology and uncovered their importance for the drug discovery and development process, especially for drug absorption and disposition. Physiological transport systems are investigated as potential ferries to improve drug absorption and membrane permeation and to achieve organ-specific drug action. In particular, the bile acid transport systems in the liver and the small intestine and the oligopeptide transporters are of significant importance for molecular drug delivery.

Vitreal pharmacokinetics of biotinylated ganciclovir: role of sodium-dependent multivitamin transporter expressed on retina

PURPOSE

The objective of this study was to investigate the role of sodium-dependent multiple vitamin transporter (SMVT) on Biotin-Ganciclovir (biotin-GCV) uptake on both human retinal pigmented epithelium cell line (ARPE-19) and rabbit retina. Study also aims to delineate the vitreal pharmacokinetics of biotin-GCV.

METHOD

ARPE-19 was employed to study the in vitro uptake experiments. New Zealand white albino rabbits were used to study in vivo retinal uptake and vitreal pharmacokinetics following intravitreal administration of biotin-GCV. In vitro uptake kinetics of [3H] biotin was determined at various initial concentrations. Competitive inhibition studies were conducted in the presence of unlabelled biotin, desthiobiotin, pantothenic acid, and lipoic acid. Various other uptake studies were performed to functionally characterize the transporter. To provide the molecular evidence of this transporter, Reverse Transcription-Polymerase Chain Reaction (RT-PCR) studies were also conducted. In vivo retinal/choroidal uptake studies were carried out with New Zealand albino rabbits. Unconscious animal ocular microdialysis studies were performed in order to evaluate intravitreal pharmacokinetics of GCV and Biotin-GCV.

RESULTS

Uptake of [3H] biotin into ARPE-19 was linear over 7 min, and found to be saturable with K(m) of 138.25 muM and Vmax of 38.85 pmol/min/mg protein. Both pantothenic acid and lipoic acid decreased significantly in uptake of biotin in the concentration-dependent manner. Uptake of biotin into ARPE-19 was found to be temperature, energy, and Na+ dependent but Cl(-)independent. Further, RT-PCR studies identified a band exhibiting presence of hSMVT on ARPE-19. Biotin-GCV is recognized by SMVT system present on the ARPE-19 and rabbit retina. Vitreal Pharmacokinetics profile reveals that most of the parameters were not significantly different for GCV and Biotin-GCV. However, use of Biotin-GCV may result in sustain levels of regenerated GCV in vitreous.

CONCLUSIONS

SMVT was identified and functionally characterized on ARPE-19 cells. Further, Biotin-GCV shares this transport system. Vitreal pharmacokinetics of the conjugate was determined in unconscious rabbit model.

Preparation and characterization of salmon calcitonin-biotin conjugates

This study was performed to prepare and characterize the biotinylated Salmon calcitonin (sCT) for oral delivery and evaluate the hypocalcemic effect of biotinylated-sCTs in rats. Biotinylated sCTs was characterized by using high performance liquid chromatography (HPLC) and MALDITOF-MS. The effect of biotinylation on permeability across Caco-2 cell monolayers was examined. Their hypocalcemic effect was determined in rats. Mono- and di-bio-sCTs were separated by reverse phase HPLC. The molecular weights of mono-bio-sCT and di-bio-sCT were determined to be 3,660.5 and 3,900.2 Da, respectively. The permeability of biotinylated-sCTs across Caco-2 cell monolayers was observed with a significant enhancement compared with sCT. Intrajejunal (ij) administration of mono-bio-sCT and di-bio-sCT resulted in sustained reduction in serum calcium levels, with a maximum reduction (% max(d)) of 21.6% and 30% after 4 h and 6 h of application, respectively. The biotin conjugation of sCT may be a promising strategy for increasing the oral bioavailability of sCT and achieving sustained calcium-lowering effects.

A new orally available glucagon-like peptide-1 receptor agonist, biotinylated exendin-4, displays improved hypoglycemic effects in db/db mice

An orally active glucagon-like peptide-1 (GLP-1) formulation would have great advantages over conventional injectable therapies for the treatment of diabetic patients. Because GLP-1 absorption in the intestine is restricted by its natural physiological characteristics, biotinylated exendin-4 analogues might useful as orally active GLP-1 receptor agonists. Three different biotinylated exendin-4 analogues, Lys(27)-Biotin-Exendin-4 (MB1-Ex-4), Lys(12)-Biotin-Exendin-4 (MB2-Ex-4), and Lys(12, 27)-Biotin-Exendin-4 (DB-Ex-4) were prepared, and their biological activities and enzymatic stabilities were studied in vitro. The hypoglycemic effects and pharmacokinetics of these analogues after oral administration were evaluated in db/db mice and Sprague-Dawley rats, respectively. These biotinylated exendin-4 analogues preserved GLP-1 receptor binding affinity and stimulated insulin secretion in RIN-m5F murine insulinoma cells and in isolated rat islets, respectively, and were as potent as exendin-4. In particular, DB-Ex-4 showed 9.0-fold better stability against rat intestinal fluid than exendin-4. When 0.1, 1, and 10 microg/mouse of DB-Ex-4 were orally administered, mean total hypoglycemic degrees (HGD) were increased by 36.8+/-1.2, 46.9+/-1.8, and 54.3+/-4.5%, respectively, whereas 1 microg/mouse of native exendin-4 showed an increase of 8.8+/-7.3%. This study demonstrates that biotinylated exendin-4 analogues are absorbed in the intestine and that they have biological efficacies of exendin-4. Furthermore, it indicates that biotinylated exendin-4 analogues could be used as potential oral antidiabetic agent for the treatment of type 2 diabetes.

Biotin uptake by rabbit corneal epithelial cells: role of sodium-dependent multivitamin transporter (SMVT)

PURPOSE

The objective of this research was to investigate the presence of sodium-dependent multivitamin transporter (SMVT) on rabbit corneal epithelial cells.

METHODS

Primary cultured rabbit corneal epithelial cells (rPCECs)and freshly excised rabbit corneas were used for characterization of biotin uptake and transport, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) was performed to confirm the molecular identity of SMVT. Liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis was performed to examine the presence of biotin in rabbit tears.

RESULTS

Uptake of biotin by rPCECs was found to be time and concentration dependent with Km of 32.52 microM and Vmax of 10.43 pmol min- 1 mg protein- 1. Biotin was significantly inhibited in the presence of pantothenic acid and lipoic acid. Biotin uptake was found to be energy and Na+ dependent but H+ and Cl- independent. The uptake was inhibited by valeric acid in a concentration-dependent manner but not much affected in the presence of biotin methyl ester and biocytin with no free carboxyl group. Modulators of both PKC- and PKA-mediated pathways had no effect on biotin uptake, but calcium-calmodulin inhibitor significantly inhibited its uptake. Sodium-dependent multivitamin transporter was identified by RT-PCR in rPCECs. Transport experiments across the rabbit corneas revealed the functional localization of SMVT on the apical side of the cornea, and thereby corroborating with in vitro results with cultured corneal cells. Finally, LC-MS/MS analysis showed the presence of biotin in rabbit tears.

CONCLUSIONS

Results obtained from both in vitro and exvivo studies suggest the possible role of SMVT expressed on corneal epithelial cells for the uptake of biotin, which co-transports pantothenic acid and lipoic acid. Further, the presence of biotin in tears suggests the physiological significance of this transporter in rabbit corneal epithelium.

Prion protein 90-231 contains a streptavidin-binding motif

The biological function of prion protein (PrP) and the physiological relevance of its truncated subtypes and glycoforms is still enigmatic. In this paper, we adduce evidence that recombinant murine PrP fragment 90-231 (mPrP90-231) contains a biotin-mimicking sequence motif that causes binding of the bacterial protein streptavidin to mPrP90-231. As indicated by epitope mapping and proven by analysis of a deletion mutant (mPrP101-231), streptavidin binding is primarily mediated by the amino-terminus of mPrP90-231 with the core-binding sequence represented by residues 94-100. Competition with biotin significantly reduces the interaction pointing to an involvement of streptavidin's biotin-binding site (BBS). Since the BBS of streptavidin shares similarities with the active sites of proteins involved in biotin metabolism we speculate that biotin mimicry by truncated PrP-species may have an impact in vivo.

Functional characterization of sodium-dependent multivitamin transporter in MDCK-MDR1 cells and its utilization as a target for drug delivery

The objective of this research is to characterize a sodium-dependent multivitamin transporter (SMVT) in MDCK-MDR1 cells (Madin-Darby canine kidney cells transfected with the human MDR1 gene) and to investigate the feasibility of utilizing the MDCK-MDR1 cell line as an in vitro model to study the permeability of biotin-conjugated prodrugs of anti-HIV protease inhibitors. Mechanism of [3H]biotin uptake and transport was delineated. Transepithelial permeability of the biotin-conjugated prodrug, i.e., biotin-saquinavir, was also studied. Reverse transcription polymerase chain reaction (RT-PCR) was carried out to confirm the existence of SMVT in MDCK-MDR1 cells. Biotin uptake was Na+, pH, and temperature dependent, but energy independent. Uptake of biotin was found to be saturable with a Km of 13.0 microM, Vmax 21.5 of pmol min-1 (mg of protein)-1, and Kd of 0.12 microL min-1 (mg of protein)-1. Both apical and basal uptake and transepithelial transport of [3H]biotin showed that SMVT localized predominantly on the apical membrane of MDCK-MDR1 cells. [3H]Biotin uptake was inhibited by excess unlabeled biotin and its structural analogues, i.e., desthiolbiotin and valeric acid, and other vitamins such as lipoic acid and pantothenic acid, but not by acetic acid, benzoic acid, biotin methyl ester, and biocytin. Biotin-saquinavir caused lowering of [3H]biotin uptake, which indicates that it is recognized by SMVT. Apical to basal transport of [3H]biotin was also significantly inhibited in the presence of excess biotin or biotin-saquinavir. Transepithelial transport studies of biotin-saquinavir in MDCK-MDR1, wild type MDCK, and Caco-2 cells revealed that permeability of biotin-saquinavir was similar in all three cell lines. A band of SMVT mRNA at 862 bp was identified by RT-PCR. A sodium-dependent multivitamin transporter, SMVT, responsible for biotin uptake and transport, was identified and functionally characterized in MDCK-MDR1 cells. Therefore, the MDCK-MDR1 cell line may be utilized as an in vitro model to study the permeability of biotin-conjugated prodrugs such as HIV protease inhibitors.

Biotin uptake and cellular translocation in human derived retinoblastoma cell line (Y-79): A role of hSMVT system

The objective of this research was to investigate the presence of a specialized carrier-mediated system for biotin and delineate uptake mechanism and intracellular trafficking of biotin in the human derived retinoblastoma cell line (Y-79). Human derived retinoblastoma cell line, Y-79, was used for uptake studies. Uptake of [3H]Biotin was determined at various concentrations, pH, temperatures, in the absence of sodium and in the presence of other vitamins and metabolic inhibitors to delineate the mechanism of uptake. Uptake was determined in the presence of various intracellular regulatory pathways (protein kinase A & C, protein tyrosine kinase and calcium-calmodulin) modulators. Reverse transcription polymerase chain reaction (RT-PCR) was performed to confirm the molecular identity of human sodium-dependent multivitamin transporter (hSMVT). Uptake of [3H]Biotin in Y-79 cells were found to be saturable at micromolar concentration range, with apparent Km of 8.53 microM and Vmax of 14.12 pmol/min/mg protein, but linear at nanomolar concentration range. Uptake was sodium, pH, temperature and energy-dependent, but chloride independent; inhibited by the structural analogue desthiobiotin, pantothenic acid and lipoic acid at milimolar concentrations and not at nanomolar concentrations. Uptake of [3H]Biotin was trans-stimulated by the intracellular biotin. Ca2+/calmodulin pathways appeared to play important roles in the regulation of riboflavin uptake in Y-79 cells via significant reduction in Vmax (66%) and Km (28%) of the uptake process. A human sodium-dependant multivitamin transporter, hSMVT, was identified by RT-PCR in Y-79. These studies demonstrated for the first time the existence of a human sodium dependant multivitamin transporter (hSMVT), a specialized carrier-mediated system for biotin uptake, in human derived retinoblastoma cells.

The role of IL-6 and STAT3 in inflammation and cancer

The defense of the host from foreign pathogens is the commonly accepted function of the vertebrate immune system. A complex system consisting of many differing cells and structures communicating by both soluble and cell bound ligands, serves to protect the host from infection, and plays a role in preventing the development of certain types of tumours. Numerous signalling pathways are involved in the coordination of the immune system, serving both to activate and attenuate its responses to attack. The ability of the immune system, specifically those cells involved in acute inflammatory responses, to mediate the directed (and sometimes indirect) killing of cells and pathogens, make it a potential threat to host survival. Furthermore, the production and release of various survival factors such as the pleiotropic cytokine IL-6, a major mediator of inflammation and activator of signal transducer and activator of transcription 3, serves to block apoptosis in cells during the inflammatory process, keeping them alive in very toxic environments. Unfortunately, these same pathways serve also to maintain cells progressing towards neoplastic growth, protecting them from cellular apoptotic deletion and chemotherapeutic drugs. Here, we discuss the relationships between cancer and inflammation, and some of the molecular mechanisms involved in mediating the unintended consequences of host defense and tumour survival.

The partial retro-inverso modification: a road traveled together

In the mid-1970s, Dr. Murray Goodman was interested in a reversed peptide bond as a surrogate to understand the functional role of the amide bond in aspartame, a dipeptide sweetener. Very soon, realizing the breath and potential of this modification, Murray expanded this activity into a full program and I was fortunate to be part of it. Together we formulated new concepts such as the partially modified retro-inverso and end-group modified retro-inverso transformations, tested hypotheses, generated novel nomenclature, developed synthetic routes, characterized the preferred conformations of the unique building blocks employed in this modification, the gem-diaminoalkyl and the C2-substituted malonyl residues, and studied the biological activity of retro-inverso isomers of bioactive peptides. In the early 1980s several laboratories initiated extensive research targeted at the retro-inverso modification. The revival of this field led to new applications, new methods of synthesis, and new insights on the conformational and topological properties of the retro-inverso modification. Among the fields that embraced the retro-inverso concept were immunology as pertains to subjects such as synthetic vaccines, immunomodulators, and diagnostic tools, and drug delivery field as pertains to targeted and nontargeted cell permeation vectors loaded with bioactive cargo. Doctor Murray Goodman's sudden death leaves behind not only family, friends, and colleagues, but also an impressive record of scientific achievements among which is the revival of the modern era of the retro-inverso transformation. Murray's numerous contributions, excellent leadership, enthusiastic promotion, and outstanding teachings in this field will carry and illuminate his memory far into the future.

Characterization of acetylcholinesterase in Caco-2 cells

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

Multivalent interactions between biotin-polyrotaxane conjugates and streptavidin as a model of new targeting for transporters

Kinetic analysis of interactions between biotin-polyrotaxane or biotin-alpha-cyclodextrin (biotin-alpha-CD) conjugates and streptavidin was carried out as a model of new targeting to transporters using the surface plasmon resonance (SPR) technique. The biotin-polyrotaxane conjugates, in which biotin-introduced alpha-CDs are threaded onto a poly(ethylene oxide) chain capped with bulky end-groups, are expected to increase the valency of biotin from monovalent to multivalent binding. The number of biotins conjugated with one polyrotaxane molecule varied from 11 to 78, and apparently increased the association equilibrium constant (K(a)), assuming pseudo-first-order kinetics. A detailed dissociation kinetics was analyzed and the re-binding of the biotin-polyrotaxane conjugates was observed on the streptavidin-deposited SPR surface. The magnitude of the re-binding is likely to become larger with increasing the number of biotins, suggesting multivalent interaction on the SPR surface. To quantify the effect of valency, competitive inhibition assay was performed in terms of the supramolecular structure of the polyrotaxane. The inhibitory potency of the biotin-polyrotaxane conjugate was found to be 4-5 times greater than that of the biotin-alpha-CD conjugate. Therefore, the biotin-polyrotaxane conjugates by supramolecular formation of the biotin-alpha-CD conjugate significantly switches from monovalent to multivalent bindings to the model binding protein, streptavidin.

Conjugates bearing multiple formyl-methionyl peptides display enhanced binding to but not activation of phagocytic cells

N-Formyl-methionyl peptides can specifically bind to surface receptors on phagocytic cells. A single copy of N-formyl-methionine-leucine-phenylalanine (fMLF) covalently linked to a poly(ethylene glycol)-based polymer displayed reduced binding avidity (K(d) = 190 nM) for differentiated HL-60 cells relative to free fMLF (K(d) = 28 nM). Increasing the number of fMLF residues (up to eight) attached to a single polymer results in enhanced avidity for these cells (K(d) = 0.18 nM), which appears to be independent of whether the polymer backbone is linear or branched. However, no conjugate showed enhanced ability to activate phagocytic cells, relative to the free peptide (EC(50) = 5 nM), as measured by transient stimulation of release of calcium ions from intracellular stores into the cytoplasm. A polymer bearing four fMLF and four digoxigenin residues showed specific enhancement in binding to differentiated HL-60 cells and mouse peritoneal macrophages in situ relative to a polymer lacking fMLF; no such enhancement was seen in binding to receptor-negative lymphocytic Jurkat cells. These results suggest that multiple fMLF residues linked to a drug-delivery polymer can be used to target appended drugs to phagocytic cells with relatively little toxicity due to cellular activation.

Targeted PEG-based bioconjugates enhance the cellular uptake and transport of a HIV-1 TAT nonapeptide

We previously described the enhanced cell uptake and transport of R.I-K(biotin)-Tat9, a large ( approximately 1500 Da) peptidic inhibitor of HIV-1 Tat protein, via SMVT, the intestinal biotin transporter. The aim of the present study was to investigate the feasibility of targeting biotinylated PEG-based conjugates to SMVT in order to enhance cell uptake and transport of Tat9. The 29 kDa peptide-loaded bioconjugate (PEG:(R.I-Cys-K(biotin)-Tat9)8) used in these studies contained eight copies of R.I-K(biotin)-Tat9 appended to PEG by means of a cysteine linkage. The absorptive transport of biotin-PEG-3400 (0.6-100 microM) and the bioconjugate (0.1-30 microM) was studied using Caco-2 cell monolayers. Inhibition of biotin-PEG-3400 by positive controls (biotin, biocytin, and desthiobiotin) was also determined. Uptake of these two compounds was also determined in CHO cells transfected with human SMVT (CHO/hSMVT) and control cells (CHO/pSPORT) over the concentration ranges of 0.05-12.5 microM and 0.003-30 microM, respectively. Nonbiotinylated forms of these two compounds, PEG-3350 and PEG:(R.I-Cys-K-Tat9)8, were used in the control studies. Biotin-PEG-3400 transport was found to be concentration-dependent and saturable in Caco-2 cells (K(m)=6.61 microM) and CHO/hSMVT cells (K(m)=1.26 microM). Transport/uptake was significantly inhibited by positive control substrates of SMVT. PEG:(R.I-Cys-K(biotin)Tat9)8 also showed saturable transport kinetics in Caco-2 cells (K(m)=6.13 microM) and CHO/hSMVT cells (K(m)=8.19 microM). Maximal uptake in molar equivalents of R.I-Cys-K(biotin)Tat9 was 5.7 times greater using the conjugate versus the biotinylated peptide alone. Transport of the nonbiotinylated forms was significantly lower (P<0.001) in all cases. The present results demonstrate that biotin-PEG-3400 and PEG:(R.I-Cys-K(biotin)Tat9)8 interact with human SMVT to enhance the cellular uptake and transport of these larger molecules and that targeted bioconjugates may have potential for enhancing the cellular uptake and transport of small peptide therapeutic agents.

Pharmacogenomics of drug transporters: the next drug delivery challenge

Scientifically, the third millennium begins with a major triumph--the publishing of the human genomic map, which is destined to have a momentous impact on the quality of life in our time. Disease prevention, individualized medicine, and genotyped-based medicine will soon become a reality. Pharmacogenetics, the forerunner of pharmacogenomics, began in the 1950s with a series of observations relating drug response to various genetic factors. It took almost two more decades for scientists to discover that cytochrome p450 2D6 was responsible for the metabolism of many drugs. This landmark discovery helped focus attention on how gene expression could impact the response to drugs. The stage was set for a revolution in therapeutics some 30 years later as the Human Genome Project crossed the finishing line triumphantly. A parallel development in drug delivery that may also benefit from the fruits of the Human Genome Project is the growing acceptance/awareness of drug transporters as a gateway to epithelial drug transport. This presentation addresses an area in need of attention: the possible impact of genetic polymorphism of drug transporters in pharmacokinetics and the challenge it poses in drug delivery.