Indium-111 labeling of low density lipoproteins with the DTPA-bis(stearylamide): evaluation as a potential radiopharmaceutical for tumor localization

Low-density lipoprotein: a versatile nanoscale platform for targeted delivery

Low-density lipoprotein (LDL) is a small lipoprotein that plays a vital role in controlling lipid metabolism. LDL has a delicate nanostructure with unique physicochemical properties: superior payload capacity, long residence time in circulation, excellent biocompatibility, smaller size, and natural targeting. In recent decades, the superiority and feasibility of LDL particles as targeted delivery carriers have attracted much attention. In this review, we introduce the structure, composition, advantages, defects, and reconstruction of LDL delivery systems, summarize their research status and progress in targeted diagnosis and therapy, and finally look forward to the clinical application of LDL as an effective delivery vehicle.

Radiolabeled liposomes and lipoproteins as lipidic nanoparticles for imaging and therapy

Radiolabeled lipidic nanoparticles, particularly liposomes and lipoproteins, are of great interest as agents for imaging and therapy, due not only to their peculiar physicochemical and biological properties, but also to their great versatility and the ability to manipulate them to obtain the desired properties. This review provides an overview of radionuclide labeling strategies for preparing diagnostic and therapeutic nanoparticles based on liposomes and lipoproteins that have been developed to date, as well as the main quality control methods and in vivo applications.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Biomimetics: reconstitution of low-density lipoprotein for targeted drug delivery and related theranostic applications

Low-density lipoprotein (LDL), one of the four major groups of lipoproteins for lipid transport in vivo, is emerging as an attractive carrier for the targeted delivery of theranostic agents. In contrast to the synthetic systems, LDL particles are intrinsically biocompatible and biodegradable, together with reduced immunogenicity and natural capabilities to target cancerous cells and to escape from the recognition and elimination by the reticuloendothelial system. Enticed by these attributes, a number of strategies have been developed for reconstituting LDL particles, including conjugation to the apolipoprotein, insertion into the phospholipid layer, and loading into the core. Here we present a tutorial review on the development of reconstituted LDL (rLDL) particles for theranostic applications. We start with a brief introduction to LDL and LDL receptor, as well as the advantages of using rLDL particles as a natural and versatile platform for the targeted delivery of theranostic agents. After a discussion of commonly used strategies for the reconstitution of LDL, we highlight the applications of rLDL particles in the staging of disease progression, treatment of lesioned tissues, and delivery of photosensitizers for photodynamic cancer therapy. We finish this review with a perspective on the remaining challenges and future directions.

Lipoprotein-Related and Apolipoprotein-Mediated Delivery Systems for Drug Targeting and Imaging

The integration of lipoprotein-related or apolipoprotein-targeted nanoparticles as pharmaceutical carriers opens new therapeutic and diagnostic avenues in nanomedicine. The concept is to exploit the intrinsic characteristics of lipoprotein particles as being the natural transporter of apolar lipids and fat in human circulation. Discrete lipoprotein assemblies and lipoprotein-based biomimetics offer a versatile nanoparticle platform that can be manipulated and tuned for specific medical applications. This article reviews the possibilities for constructing drug loaded, reconstituted or artificial lipoprotein particles. The advantages and limitations of lipoproteinbased delivery systems are critically evaluated and potential future challenges, especially concerning targeting specificity, concepts for lipoprotein rerouting and design of innovative lipoprotein mimetic particles using apolipoprotein sequences as targeting moieties are discussed. Finally, the review highlights potential medical applications for lipoprotein-based nanoparticle systems in the fields of cardiovascular research, cancer therapy, gene delivery and brain targeting focusing on representative examples from literature.

Passively Targeted Curcumin-Loaded PEGylated PLGA Nanocapsules for Colon Cancer Therapy In Vivo

Clinical applications of curcumin for the treatment of cancer and other chronic diseases have been mainly hindered by its short biological half-life and poor water solubility. Nanotechnology-based drug delivery systems have the potential to enhance the efficacy of poorly soluble drugs for systemic delivery. This study proposes the use of poly(lactic-co-glycolic acid) (PLGA)-based polymeric oil-cored nanocapsules (NCs) for curcumin loading and delivery to colon cancer in mice after systemic injection. Formulations of different oil compositions are prepared and characterized for their curcumin loading, physico-chemical properties, and shelf-life stability. The results indicate that castor oil-cored PLGA-based NC achieves high drug loading efficiency (≈18% w(drug)/w(polymer)%) compared to previously reported NCs. Curcumin-loaded NCs internalize more efficiently in CT26 cells than the free drug, and exert therapeutic activity in vitro, leading to apoptosis and blocking the cell cycle. In addition, the formulated NC exhibits an extended blood circulation profile compared to the non-PEGylated NC, and accumulates in the subcutaneous CT26-tumors in mice, after systemic administration. The results are confirmed by optical and single photon emission computed tomography/computed tomography (SPECT/CT) imaging. In vivo growth delay studies are performed, and significantly smaller tumor volumes are achieved compared to empty NC injected animals. This study shows the great potential of the formulated NC for treating colon cancer.

Functionalized low-density lipoprotein nanoparticles for in vivo enhancement of atherosclerosis on magnetic resonance images

To allow visualization of macrophage-rich and miniature-sized atheromas by magnetic resonance (MR) imaging, we have converted low-density lipoprotein (LDL) into MR-active nanoparticles via the intercalation of a 1,4,7,10-tetraazacyclodecane-1,4,7-triacetic acid (DO3A) derivative and the subsequent coordination reaction with Gd(3+). After careful removal of nonchelated Gd(3+), an MR-active LDL (Gd(3+)-LDL) with a remarkably high payload of Gd(3+) (in excess of 200 Gd(3+) atoms per particle) and a high relaxivity (r(1) = 20.1 s(-1) mM(-1) per Gd(3+) or 4040 s(-1) mM(-1) per LDL) was obtained. Dynamic light-scattering photon correlation spectroscopy (DLS) and cryo transmission electron microscope (cryoTEM) images showed that Gd(3+)-LDL particles did not aggregate and remained of a similar size (25-30 nm) to native LDL. Intravenous injection of Gd(3+)-LDL into an atherosclerotic mouse model (ApoE(-/-)) resulted in an extremely high enhancement of the atheroma-bearing aortic walls at 48 h after injection. Free Gd(3+) dissociation from Gd(3+)-LDL was not detected over the imaging time window (96 h). Because autologous LDL can be isolated, modified, and returned to the same patient, our results suggest that MR-active LDL can potentially be used as a noninfectious and nonimmunogenic imaging probe for the enhancement of atheroplaques presumably via the uptake into macrophages inside the plaque.

Degree of chemical functionalization of carbon nanotubes determines tissue distribution and excretion profile

Getting rid of the tubes: An assessment of the retention of functionalized multi-walled carbon nanotubes (MWNTs) in the organs of mice was carried out using single photon emission computed tomography and quantitative scintigraphy (see scheme). Increasing the degree of functionalization on MWNTs enhanced renal clearance, while lower functionalization promoted reticuloendethelial system accumulation.

Low-density lipoprotein-bound aluminum sulfophthalocyanine: targeting tumor cells for photodynamic therapy

The aim of this study was to evaluate the role of low-density lipoproteins (LDLs) as a vehicle for aluminum sulfophthalocyanine ( AlPcS ) to target tumor cells for photodynamic therapy (PDT). LDLs are biological particles containing an apolipoprotein which recognizes with high affinity specific receptors on many cell types, including cancer cells. LDL was loaded with AlPcS in two different manners. In the first procedure, the tetrasulfonated AlPcS4was covalently bound to the protein part of the LDL via amide bonds to 6-carboxypentylaminosulfonyl spacer chains attached to two of the four sulfonate groups, i.e. AlPcS4A2. In the second procedure, the AlPcS4was substituted with a linear dodecylaminosulfonyl chain, i.e. AlPcS4( C12) and non-covalently inserted into the phospholipid monolayer of the LDL. Both preparations contained over 50 moles AlPcS /mole LDL. They were tested in vitro for their phototoxic properties against the EMT-6 mouse mammary tumor cell line and the A-549 human lung adenocarcinoma cell line. Cell survival was assessed using the MTT colorimetric assay. Association of the free AlPcS4( C12) with LDL increased the in vitro phototoxicity of the dye substantially against both cell lines while the covalently loaded AlPcS4A2-LDL preparation showed little cytotoxicity, even at a 10-fold-higher light or drug doses. It was postulated that the covalent labeling of the protein moiety with the Pc greatly reduced LDL receptor recognition, rendering this derivative photo-inactive. Photodynamic therapy of EMT-6 tumor-bearing mice showed that the free and LDL-associated AlPcS4( C12) exhibited similar activities, with 100% cure one week post-PDT at 0.2 μmol kg−1. We conclude that the attached aliphatic chain of the AlPcS4( C12) greatly enhances the phototoxicity of the parent AlPcS4but that pre-association of the AlPcS4( C12) with LDL does not further augment its in vivo photodynamic efficacy.

PEGylated dextran as long-circulating pharmaceutical carrier

Dextran-polyethylene glycol (PEG) conjugates were synthesized by activating dextran hydroxy groups with carbonyldiimidazole, introducing amino groups by attaching ethylenediamine, and reacting amino groups with a succinimidyl-activated derivative of PEG. Conjugates with an average of 12 and 21 PEG (5 kDa) residues per single dextran (73 kDa) molecule were prepared. These conjugates have circulation half-lives of 5.3 h and 7.0 h, respectively, compared to 4.0 h for non-PEGylated dextran. The modification of dextran with PEG inhibits the uptake of polymer by the major organ of the reticuloendothelial system, the liver. Dextran-PEG conjugates may represent a convenient platform for long-circulating pharmaceutical preparations.

Lipoprotein-inspired nanoparticles for cancer theranostics

Over hundreds of millions of years, animals have evolved endogenous lipoprotein nanoparticles for shuttling hydrophobic molecules to different parts of the body. In the last 70 years, scientists have developed an understanding of lipoprotein function, often in relationship to lipid transport and heart disease. Such biocompatible, lipid–protein complexes are also ideal for loading and delivering cancer therapeutic and diagnostic agents, which means that lipoprotein and lipoprotein-inspired nanoparticles also offer opportunities for cancer theranostics. By mimicking the endogenous shape and structure of lipoproteins, the nanocarrier can remain in circulation for an extended period of time, while largely evading the reticuloendothelial cells in the body’s defenses. The small size (less than 30 nm) of the low-density (LDL) and high-density (HDL) classes of lipoproteins allows them to maneuver deeply into tumors. Furthermore, lipoproteins can be targeted to their endogenous receptors, when those are implicated in cancer, or to other cancer receptors.

In this Account, we review the field of lipoprotein-inspired nanoparticles related to the delivery of cancer imaging and therapy agents. LDL has innate cancer targeting potential and has been used to incorporate diverse hydrophobic molecules and deliver them to tumors. Nature’s method of rerouting LDL in atherosclerosis provides a strategy to extend the cancer targeting potential of lipoproteins beyond its narrow purview. Although LDL has shown promise as a drug nanocarrier for cancer imaging and therapy, increasing evidence indicates that HDL, the smallest lipoprotein, may also be of use for drug targeting and uptake into cancer cells. We also discuss how synthetic HDL-like nanoparticles, which do not include human or recombinant proteins, can deliver molecules directly to the cytoplasm of certain cancer cells, effectively bypassing the endosomal compartment. This strategy could allow HDL-like nanoparticles to be used to deliver drugs that have increased activity in the cytoplasm. Lipoprotein nanoparticles have evolved to be ideal delivery vehicles, and because of that specialized function, they have the potential to improve cancer theranostics.

Gd(III)-EPTPAC16, a new self-assembling potential liver MRI contrast agent: in vitro characterization and in vivo animal imaging studies

The recently reported amphiphilic chelate, [Gd(EPTPAC16)(H2O)]2-, forms supramolecular aggregates in aqueous solution by self-assembly of the monomers with a relaxometrically determined critical micellar concentration (CMC) of 0.34 mM. The effect of sonication on the aggregate size was characterized by dynamic light scattering and relaxometry, indicating the presence of premicellar aggregates and an overall decrease in aggregate size and polydispersity upon sonication, slightly below the CMC. [[153Sm](EPTPAC16)(H2O)]2- radiotracer was evaluated in vivo from gamma scintigraphy and biodistribution in Wistar rats. It was found to depend strongly on the sample concentration, below or above the CMC, and its sonication, in a way that correlates with the effect of the same factors on the size of the aggregates formed in solution. Below CMC, the very large aggregates of the [153Sm]3+ -labeled chelate were persistently and mainly taken up by the lungs, and also by the macrophage-rich liver and spleen. Sonication of this solution led to loss of the lung uptake. Above CMC, the metal chelate was mainly taken up by the liver, with very little uptake by the spleen and lungs. In vivo, dynamic contrast-enhanced (DCE)-MRI evaluation of the micellar [Gd(EPTPAC16)(H2O)]2- compound in Wistar rats showed a persistent hepatic positive-contrast effect in T1-weighted images, qualitatively similar to the clinically established Gd(III)-based hepatobiliary-selective agents. No enhancement effect was observed in the lungs because of the scarcity of mobile protons in this organ, despite the scintigraphic evidence of significant lung retention of the [153Sm]3+ -labeled chelate at concentrations below the CMC.

Mimicking nature’s nanocarrier: synthetic low-density lipoprotein-like nanoparticles for cancer-drug delivery

Evaluation of: Nikanjam M, Blakely EA, Bjornstad KA, Shu X, Budinger TF, Forte TK: Synthetic nano-low density lipoprotein as targeted drug delivery vehicle for glioblastoma multiforme. Int. J. Pharm. 3287, 86–94 (2007) [1] . Low-density lipoproteins have long been recognized as a viable nanocarrier for targeted delivery of drug and imaging agents. Many groups have published promising initial findings; however, progress in this field has been impeded by the need to isolate low-density lipoproteins from fresh donor plasma. In a recent paper by Nikanjam and colleagues, synthetic low-density lipoprotein-like nanoparticles were prepared from commercial lipids and a bifunctional synthetic peptide containing the low-density lipoprotein receptor-binding domain and the lipid-binding motif. These particles were shown to behave similarly to native low-density lipoproteins and also to bind to the low-density lipoprotein receptor on cancer cells. Herein, we evaluate the utility of this novel delivery vehicle and discuss what role this technology may have in nanomedicine.

Low-density lipoprotein nanoparticles as magnetic resonance imaging contrast agents

Low-density lipoproteins (LDLs) are a naturally occurring endogenous nanoplatform in mammalian systems. These nanoparticles (22 nm) specifically transport cholesterol to cells expressing the LDL receptor (LDLR). Several tumors overexpress LDLRs presumably to provide cholesterol to sustain a high rate of membrane synthesis. Amphiphilic gadolinium (Gd)-diethylenetriaminepentaacetic acid chelates have been incorporated into the LDL to produce a novel LDLR-targeted magnetic resonance imaging (MRI) contrast agent. The number of Gd chelates per LDL particle ranged between 150 and 496 Gd(III). In vitro studies demonstrated that Gd-labeled LDL retained a similar diameter and surface charge as the native LDL particle. In addition, Gd-labeled LDL retained selective cellular binding and uptake through LDLR-mediated endocytosis. Finally, Gd-labeled LDLs exhibited significant contrast enhancement 24 hours after administration in nude mice with human hepatoblastoma G2 xenografts. Thus, Gd-labeled LDL demonstrates potential use as a targeted MRI contrast agent for in vivo tumor detection.

Tissue biodistribution and blood clearance rates of intravenously administered carbon nanotube radiotracers

Carbon nanotubes (CNT) are intensively being developed for biomedical applications including drug and gene delivery. Although all possible clinical applications will require compatibility of CNT with the biological milieu, their in vivo capabilities and limitations have not yet been explored. In this work, water-soluble, single-walled CNT (SWNT) have been functionalized with the chelating molecule diethylentriaminepentaacetic (DTPA) and labeled with indium ((111)In) for imaging purposes. Intravenous (i.v.) administration of these functionalized SWNT (f-SWNT) followed by radioactivity tracing using gamma scintigraphy indicated that f-SWNT are not retained in any of the reticuloendothelial system organs (liver or spleen) and are rapidly cleared from systemic blood circulation through the renal excretion route. The observed rapid blood clearance and half-life (3 h) of f-SWNT has major implications for all potential clinical uses of CNT. Moreover, urine excretion studies using both f-SWNT and functionalized multiwalled CNT followed by electron microscopy analysis of urine samples revealed that both types of nanotubes were excreted as intact nanotubes. This work describes the pharmacokinetic parameters of i.v. administered functionalized CNT relevant for various therapeutic and diagnostic applications.

Synthesis and Physicochemical Characterization of Gd−DTPA−B(sLex)A, a New MRI Contrast Agent Targeted to Inflammation

A new magnetic resonance imaging (MRI) contrast agent designed to mimic sialyl Lewis X (sLeX) and to target inflammation has been synthesized and characterized. The evolution of its proton longitudinal relaxivity as a function of the magnetic field (NMRD) and temperature has been studied. The exchange rate of the water coordinated to the metal has been assessed by oxygen-17 relaxometry. The transmetalation by zinc(II) ions and the noncovalent binding to human serum albumin have been evaluated. The results show no limitation by the residence time of the coordinated water molecule above room temperature, a higher stability of the complex versus transmetalation by zinc(II) ions than a parent complex, the clinically used Gd-DTPA-BMA, and negligible interaction with human serum albumin.

De novo synthesis of a new diethylenetriaminepentaacetic acid (DTPA) bifunctional chelating agent

Diethylene triamine pentaacetic acid (DTPA) has been in extensive use as a metal chelator in the development of radiopharmaceuticals and contrast agents. The former application uses DTPA mostly as a bifunctional chelating agent (BCA) conjugated to tumor-targeting vehicles (TTVs) such as monoclonal antibodies (MAbs) and receptor-directed peptides. A new bifunctional DTPA derivative was synthesized by a fully organic scheme. This compound, N(4),N(alpha),N(alpha),N(epsilon),N(epsilon)-[pentakis(carboxymethyl)]-N(4)-(carboxymethyl)-2,6-diamino-4-azahexanoic hydrazide (20) was prepared by a convergent synthesis strategy using N(alpha)-benzyloxycarbonyl-2,3-diaminopropionic acid as the starting compound. This commercially available material was used to build a functionalized triamine which served as the molecular core template for assembling the target molecule. To evaluate the conjugation and radiolabeling capabilities of this new molecule, it was covalently attached to the anti-TAG-72 MAb, Delta CH2HuCC49, and the conjugate was radiolabeled in near-quantitative yields with yttrium-90 ((90)Y) and lutetium-177 ((177)Lu). Biodistribution of the (177)Lu-labeled DTPA-Delta CH2HuCC49 in tumor-bearing nude mice demonstrated preservation of the immunoreactivity of the MAb as indicated by high tumor uptake. In addition to the introduction of a new bifunctional DTPA, this work reports on a novel synthetic approach for preparation of this useful metal chelator and introduces a new conjugation protocol.

Noninvasive imaging of peripherally injected Alzheimer's disease type synthetic A beta amyloid in vivo

The pathological hallmark of Alzheimer's disease (AD) is accumulation in the brain of amyloid composed of the 40-mer peptide A beta. Many fundamental questions about the biology of (AD) remain unanswered because there is currently no method of quantifying A beta amyloid in vivo. A noninvasive method of detecting and quantifying A beta amyloid in vivo would have wide application for the premortem diagnosis of AD and the efficient evaluation of candidate therapeutics aimed at inhibiting the formation and growth of A beta amyloid. Taking advantage of the extraordinarily high affinity of A beta for itself, we have synthesized an N'-terminal diethylenetriaminepentaacetic acid (DTPA) derivative of A beta possessing the kinetic activity and specificity for A beta amyloid desired of a probe to be used for noninvasive imaging. DTPA-A beta(3-40) is readily labeled with (111)InOAc(3) to yield a stable probe with exquisite specificity for naturally occurring and synthetic A beta amyloid in vitro. Moreover, (111)In-DTPA-A beta(3-40), administered intravascularly can specifically deposit onto and label previously injected synthetic A beta amyloid and be imaged in vivo with a gamma camera. The present results demonstrate the design, synthesis, and use of an A beta amyloid-specific probe and methods for its use as a noninvasive imaging agent. In vivo imaging of A beta amyloid represents an important step toward the development of biochemically based objective tools for the assessment of progression of AD and efficacy of potential therapeutics.

Synthesis of the first diethylenetriaminepentahydroxamic acid (DTPH) bifunctional chelating agent

Synthesis of a new pentahydroxamic acid bifunctional chelating agent (BCA), constructed on the aminoazaalkyl core of diethylenetriaminepentaacetic acid (DTPA), is reported. Rational modifications in the structure of DTPA, which could result in an enhancement of its chelation properties, add to the collection of diagnostic and therapeutic metals bound by this chelator, and might implement significant improvements in the in vivo behavior of this compound, are described. Further improvements in the stability of the ligand-metal complexes of DTPA may improve both diagnostic and therapeutic outcomes such as tumor-to-normal tissue ratios and target-delivered radioactivity. A combination of hydroxamate functions with the azaalkyl backbone of DTPA might be a suitable approach to generate such higher stabilities. This rationale may be justified by the well-known affinity of hydroxamates against different transition metals and favorable properties of DTPA as a versatile chelator. Thus, the N(4),N(alpha),N(alpha),N(epsilon),N(epsilon)-pentakis[[((N-hydroxy-N-methyl]carbonyl)methyl]-2, 6-diamino-4-azahexanoic hydrazide (5, DTPH) was designed and synthesized through a convergent synthesis and in 40.7% overall yield. Conjugation of this compound to the monoclonal antibody (MAb) Delta Ch2HuCC49, used as a model protein, was carried out to evaluate the efficiency of this molecule as a BCA. Radiolabeling of the DTPH-Delta CH2HuCC49 conjugate with lutetium-177 ((177)Lu) and biodistribution of the labeled conjugate in athymic nude mice, bearing LS174T human colon carcinoma xenografts, are reported.

Preparation of indium-115-labeled diethylenetriaminetetraacetic acid monoacetamide peptides purified by 8-hydroxyquinoline

In this communication we describe a novel procedure for the preparation and purification of diethylenetriaminepentaacetic (DTPA)-acylated and (115)In(3+)-labeled oligopeptides using 8-hydroxyquinoline for the removal and quantification of nonbound indium ions. First the N(alpha)- or N(alpha),N(epsilon)-DTPA oligopeptides containing C-terminal KDEL signal motif were produced by solid-phase synthesis. For this the free carboxyl group of DTPA dianhydride was activated in situ for a short period of time yielding a major product. Reversed-phase HPLC-purified DTPA oligopeptides were labeled with (115)In(3+) in aqueous buffer solution at pH 3.8. For the removal as well as for the detection of uncoordinated (115)In(3+) ions we have utilized the (115)In(3+) complex-forming ability of 8-hydroxyquinoline in chloroform. Following an optimized extraction procedure the free indium ion content was measured by spectrophotometry in the organic phase. Data obtained by this method and verified by total-reflection X-ray fluorescence spectroscopy and thin-layer chromatography demonstrated that free (115)In(3+) could be efficiently removed and sensitively detected in the presence of DTPA oligopeptide chelator. No release of (115)In(3+) from its DTPA complex was observed. This method could be useful for the preparation of indium complexes of peptides and perhaps proteins containing a DTPA moiety and nonradioactive isotope ligand.

Low density lipoprotein and liposome mediated uptake and cytotoxic effect of N4-octadecyl-1-beta-D-arabinofuranosylcytosine in Daudi lymphoma cells

Low density lipoprotein (LDL) receptor-mediated uptake and cytotoxic effects of N4-octadecyl-1-beta-D-arabinofuranosylcytosine (NOAC) were studied in Daudi lymphoma cells. NOAC was either incorporated into LDL or liposomes to compare specific and unspecific uptake mechanisms. Binding of LDL to Daudi cells was not altered after NOAC incorporation (K(D) 60 nM). Binding of liposomal NOAC was not saturable with increasing concentrations. Specific binding of NOAC-LDL to Daudi cells was five times higher than to human lymphocytes. LDL receptor binding could be blocked and up- or down-regulated. Co-incubation with colchicine reduced NOAC-LDL uptake by 36%. These results suggested that NOAC-LDL is taken up via the LDL receptor pathway. In an in vitro cytotoxicity test, the IC50 of NOAC-LDL was about 160 microM, whereas with liposomal NOAC the IC50 was 40 microM. Blocking the LDL receptors with empty LDL protected 50% of the cells from NOAC cytotoxicity. The cellular distribution of NOAC-LDL or NOAC-liposomes differed only in the membrane and nuclei fraction with 13% and 6% respectively. Although it is more convenient to prepare NOAC-liposomes as compared to the loading of LDL particles with the drug, the receptor-mediated uptake of NOAC-LDL provides an interesting rationale for the specific delivery of the drug to tumours that express elevated numbers of LDL receptors.

Triethylenetetramine-N,N,N',N",N"',N"'-hexaacetic Acid (TTHA) and TTHA-Bis(butanamide) as Chelating Agents Relevant to Radiopharmaceutical Applications

The N,N'-bis(butanamide) derivative of TTHA (TTHA = triethylenetetramine-N,N,N',N",N"',N"'-hexaacetic acid), and its Ga(3+) and In(3+) complexes were synthesized and characterized. The crystal X-ray diffraction structure of [Ga(2)(OH)(2)(TTHA)][Na(2)(H(2)O)(6)].2H(2)O was determined. The complex crystallizes in the monoclinic space group P2(1)/n with a = 7.179(2) Å, b = 20.334(3) Å, c = 10.902(5) Å, beta = 101.90(2) degrees, and Z = 2. Each gallium atom is bonded to six donor atoms (N(2)O(4)) in a slightly distorted octahedral geometry. The values of the protonation constants and the protonation sequence were determined by potentiometry and NMR. The stability constants of the Al(3+), Ga(3+), Fe(3+), and In(3+) complexes of TTHA-(BuA)(2) and of the Ga(3+) complex of TTHA were determined by potentiometry. The structures, in solution, of the Al(3+), Ga(3+), and In(3+) complexes of TTHA-(BuA)(2) and TTHA were analyzed by (1)H, (13)C, (27)Al, (71)Ga, and (115)In NMR techniques. Derivatization of two terminal carboxylates by butanamide substituents leads to a significant decrease of the total ligand basicity (5.77 log units) and to a change of the solubility of the resulting complexes. The stability constant of the ML complexes of TTHA-(BuA)(2) with Fe(3+) exhibits the highest value of the series (10(23.92)). The In(3+) complex is more stable than that of Ga(3+) and almost as stable as that of the Fe(3+). However, the decrease in indium and iron complex stability is less drastic going from TTHA to TTHA-(BuA)(2) (about 3 log units) than for Al(3+) or Ga(3+) (about 6 log units). pM values calculated under physiological conditions for DTPA, TTHA, and the bis(butanamide) derivatives have shown that while DTPA remains a ligand of choice to chelate Fe(3+) and In(3+) ions in vivo compared to transferrin as competitor ligand, TTHA, surprisingly, appears to be the best of these four ligands (pM = 22.71) to chelate Ga(3+).

Internalization of indium-labeled LDL through a lipid chelating anchor in human pancreatic-cancer cells as a potential radiopharmaceutical for tumor localization

Low-density lipoproteins (LDL) labeled with indium via a lipid-chelating agent, the bis(stearylamide) of diethylenetri-aminepentaacetic acid (L), were evaluated as a potential radiopharmaceutical (111In-L-LDL) for tumor localization by studying their internalization in human pancreatic cancer cells (Capan-1). Using Dil-LDL (1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate-LDL), this cell line was shown to bind human LDL with a high-affinity saturable component and a low-affinity non-saturable (40%) component. The single saturable high-affinity binding site had a KD of 27.5 +/- 2.1 micrograms/ml and a maximal binding of 610 +/- 7.5 ng/ml protein. Electron-microscopic examination of the In-L-LDL particles revealed the peripheral distribution of the electron-dense indium atoms at the outer surface of LDL. The modified LDL were then shown to be internalized by the cells. After conjugation of In-L-LDL to colloidal gold to follow the different stages of internalization, electron-microscopic examination showed that the In-L-LDL gold conjugates were stuck to the external sheet of the plasma apical and microvilli membrane, into earlier and later endosomes and into multivesicular bodies, suggesting the penetration of the In-L-LDL particles into lysosomal vacuoles. The observation of In-L-LDL-gold conjugates in deep-seated cytoplasm suggests that LDL could be employed as a drug-transport vehicle for targeting cytotoxics or radionuclides close to the cell nucleus.

Synthesis, biodistribution, and estrogen receptor scintigraphy of indium-111-diethylenetriaminepentaacetic acid-tamoxifen analogue

This study was aimed at developing a hydrophilic diethylenetriaminepentaacetic acid-tamoxifen (DTPA-Tam) analogue for use in imaging estrogen receptor positive (ER+) lesions. In rat uterine cytosol, the IC50 of DTPA-Tam conjugate was 1 microM and of tamoxifen, 2 microM. Biodistribution, autoradiography, and radionuclide imaging of 111In-DTPA-Tam in breast-tumor-bearing rats showed that tumor-to-tissue ratios increased steadily between 30 min and 48 h. The in vivo response of MCF-7 breast cancer xenografts to tamoxifen and DTPA-Tam in nude mice demonstrated that DTPA-Tam could reduce tumor growth rate. These results indicate that DTPA-Tam, a new hydrophilic ER+ ligand, might be useful in diagnosing ER+ lesions.