Pre-clinical evaluation of a methotrexate-albumin conjugate (MTX-HSA) in human tumor xenografts in vivo

Magnetic Platelets as a Platform for Drug Delivery and Cell Trapping

The possibility of using magnetically labeled blood cells as carriers is a novel approach in targeted drug-delivery systems, potentially allowing for improved bloodstream delivery strategies. Blood cells already meet the requirements of biocompatibility, safety from clotting and blockage of small vessels. It would solve the important problem of the patient's immune response to embedded foreign carriers. The high efficiency of platelet loading makes them promising research objects for the development of personalized drug-delivery systems. We are developing a new approach to use platelets decorated with magnetic nanoparticles as a targeted drug-delivery system, with a focus on bloodstream delivery. Platelets are non-nuclear blood cells and are of great importance in the pathogenesis of blood-clotting disorders. In addition, platelets are able to attach to circulating tumor cells. In this article, we studied the effect of platelets labeled with BSA-modified magnetic nanoparticles on healthy and cancer cells. This opens up broad prospects for future research based on the delivery of specific active substances by this method.

Deciphering albumin-directed drug delivery by imaging

Albumin is the most abundant plasma protein, exhibits extended circulating half-life, and its properties have long been exploited for diagnostics and therapies. Many drugs intrinsically bind albumin or have been designed to do so, yet questions remain about true rate limiting factors that govern albumin-based transport and their pharmacological impacts, particularly in advanced solid cancers. Imaging techniques have been central to quantifying - at a molecular and single-cell level - the impact of mechanisms such as phagocytic immune cell signaling, FcRn-mediated recycling, oncogene-driven macropinocytosis, and albumin-drug interactions on spatial albumin deposition and related pharmacology. Macroscopic imaging of albumin-binding probes quantifies vessel structure, permeability, and supports efficiently targeted molecular imaging. Albumin-based imaging in patients and animal disease models thus offers a strategy to understand mechanisms, guide drug development and personalize treatments.

Emerging Albumin-Binding Anticancer Drugs for Tumor-Targeted Drug Delivery: Current Understandings and Clinical Translation

Albumin has shown remarkable promise as a natural drug carrier by improving pharmacokinetic (PK) profiles of anticancer drugs for tumor-targeted delivery. The exogenous or endogenous albumin enhances the circulatory half-lives of anticancer drugs and passively target the tumors by the enhanced permeability and retention (EPR) effect. Thus, the albumin-based drug delivery leads to a potent antitumor efficacy in various preclinical models, and several candidates have been evaluated clinically. The most successful example is Abraxane, an exogenous human serum albumin (HSA)-bound paclitaxel formulation approved by the FDA and used to treat locally advanced or metastatic tumors. However, additional clinical translation of exogenous albumin formulations has not been approved to date because of their unexpectedly low delivery efficiency, which can increase the risk of systemic toxicity. To overcome these limitations, several prodrugs binding endogenous albumin covalently have been investigated owing to distinct advantages for a safe and more effective drug delivery. In this review, we give account of the different albumin-based drug delivery systems, from laboratory investigations to clinical applications, and their potential challenges, and the outlook for clinical translation is discussed. In addition, recent advances and progress of albumin-binding drugs to move more closely to the clinical settings are outlined.

Albumin Biomolecular Drug Designs Stabilized through Improved Thiol Conjugation and a Modular Locked Nucleic Acid Functionalized Assembly

Albumin-nucleic acid biomolecular drug designs offer modular multifunctionalization and extended circulatory half-life. However, stability issues associated with conventional DNA nucleotides and maleimide bioconjugation chemistries limit the clinical potential. This work aims to improve the stability of this thiol conjugation and nucleic acid assembly by employing a fast-hydrolyzing monobromomaleimide (MBM) linker and nuclease-resistant nucleotide analogues, respectively. The biomolecular constructs were formed by site-selective conjugation of a 12-mer oligonucleotide to cysteine 34 (Cys34) of recombinant human albumin (rHA), followed by annealing of functionalized complementary strands bearing either a fluorophore or the cytotoxic drug monomethyl auristatin E (MMAE). Formation of conjugates and assemblies was confirmed by gel shift analysis and mass spectrometry, followed by investigation of serum stability, neonatal Fc receptor (FcRn)-mediated cellular recycling, and cancer cell killing. The MBM linker afforded rapid conjugation to rHA and remained stable during hydrolysis. The albumin-nucleic acid biomolecular assembly composed of stabilized oligonucleotides exhibited high serum stability and retained FcRn engagement mediating FcRn-mediated cellular recycling. The MMAE-containing assembly exhibited cytotoxicity in the human MIA PaCa-2 pancreatic cancer cell line with an IC50 of 342 nM, triggered by drug release from breakdown of an acid-labile linker. In summary, this work presents rHA-nucleic acid module-based assemblies with improved stability and retained module functionality that further promotes the drug delivery potential of this biomolecular platform.

Recent Advancements in Serum Albumin-Based Nanovehicles Toward Potential Cancer Diagnosis and Therapy

Recently, drug delivery vehicles based on nanotechnology have significantly attracted the attention of researchers in the field of nanomedicine since they can achieve ideal drug release and biodistribution. Among the various organic or inorganic materials that used to prepare drug delivery vehicles for effective cancer treatment, serum albumin-based nanovehicles have been widely developed and investigated due to their prominent superiorities, including good biocompatibility, high stability, nontoxicity, non-immunogenicity, easy preparation, and functionalization, allowing them to be promising candidates for cancer diagnosis and therapy. This article reviews the recent advances on the applications of serum albumin-based nanovehicles in cancer diagnosis and therapy. We first introduce the essential information of bovine serum albumin (BSA) and human serum albumin (HSA), and discuss their drug loading strategies. We then discuss the different types of serum albumin-based nanovehicles including albumin nanoparticles, surface-functionalized albumin nanoparticles, and albumin nanocomplexes. Moreover, after briefly discussing the application of serum albumin-based nanovehicles used as the nanoprobes in cancer diagnosis, we also describe the serum albumin-based nanovehicle-assisted cancer theranostics, involving gas therapy, chemodynamic therapy (CDT), phototherapy (PTT/PDT), sonodynamic therapy (SDT), and other therapies as well as cancer imaging. Numerous studies cited in our review show that serum albumin-based nanovehicles possess a great potential in cancer diagnostic and therapeutic applications.

The development of human serum albumin-based drugs and relevant fusion proteins for cancer therapy

Human serum albumin (HSA)-based therapeutics have attracted tremendous attention in the development of anticancer agents. The versatile properties of HSA make HSA-based therapeutics possess improved pharmacokinetics, extended circulation half-life, enhanced efficacy, reduced toxicity, etc. Generally, the HSA-based therapeutics systems can be divided into four categories, i.e. HSA-drug nanoparticles, HSA-drug conjugates, HSA-binding prodrugs, and HSA-based recombinant fusion proteins: the latter mainly include antibody (domain)- and cytokine- fusion proteins. Advances in this area revealed the advantages of HSA-based systems in the development of tumor site-oriented therapeutics, partly referring to the enhanced penetration and retention (EPR) effect and the intensive macropinocytosis. Accordingly, a variety of technical platforms for the design and preparation of HSA-based therapeutics have been reported. Major strategies and directions for the drug development were discussed; those include (1) Tumor-site oriented drug delivery and enhanced drug retention, (2) Tumor-site prodrug release and activation, (3) Cancer cell bound intensive drug internalization, and (4) Tumor microenvironment (TME) directed immunomodulation. Notably, the multimodal HSA-based approach is promising for the development of tumor-oriented therapeutics for cancer therapy.

Theranostics in Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) has the potential to specifically destroy tumor cells without damaging the tissues infiltrated by the tumor. BNCT is a binary treatment method based on the combination of two agents that have no effect when applied individually: ¹⁰B and thermal neutrons. Exclusively, the combination of both produces an effect, whose extent depends on the amount of ¹⁰B in the tumor but also on the organs at risk. It is not yet possible to determine the ¹⁰B concentration in a specific tissue using non-invasive methods. At present, it is only possible to measure the ¹⁰B concentration in blood and to estimate the boron concentration in tissues based on the assumption that there is a fixed uptake of ¹⁰B from the blood into tissues. On this imprecise assumption, BNCT can hardly be developed further. A therapeutic approach, combining the boron carrier for therapeutic purposes with an imaging tool, might allow us to determine the ¹⁰B concentration in a specific tissue using a non-invasive method. This review provides an overview of the current clinical protocols and preclinical experiments and results on how innovative drug development for boron delivery systems can also incorporate concurrent imaging. The last section focuses on the importance of proteomics for further optimization of BNCT, a highly precise and personalized therapeutic approach.

The Uniqueness of Albumin as a Carrier in Nanodrug Delivery

Albumin is an appealing carrier in nanomedicine because of its unique features. First, it is the most abundant protein in plasma, endowing high biocompatibility, biodegradability, nonimmunogenicity, and safety for its clinical application. Second, albumin chemical structure and conformation allows interaction with many different drugs, potentially protecting them from elimination and metabolism in vivo, thus improving their pharmacokinetic properties. Finally, albumin can interact with receptors overexpressed in many diseased tissues and cells, providing a unique feature for active targeting of the disease site without the addition of specific ligands to the nanocarrier. For this reason, albumin, characterized by an extended serum half-life of around 19 days, has the potential of promoting half-life extension and targeted delivery of drugs. Therefore, this article focuses on the importance of albumin as a nanodrug delivery carrier for hydrophobic drugs, taking advantage of the passive as well as active targeting potential of this nanocarrier. Particular attention is paid to the breakthrough NAB-Technology, with emphasis on the advantages of Nab-Paclitaxel (Abraxane), compared to the solvent-based formulations of Paclitaxel, i.e., CrEL-paclitaxel (Taxol) in a clinical setting. Finally, the role of albumin in carrying anticancer compounds is depicted, with a particular focus on the albumin-based formulations that are currently undergoing clinical trials. The article sheds light on the power of an endogenous substance, such as albumin, as a drug delivery system, signifies the importance of the drug vehicle in drug performance in the biological systems, and highlights the possible future trends in the use of this drug delivery system.

Serum albumin: clinical significance of drug binding and development as drug delivery vehicle

Human serum albumin, the primary transport and reservoir protein in the human circulatory system, interacts with numerous endogenous and exogenous ligands of varying structural characteristics. The mode of binding of drugs to albumin is central to understanding their pharmacokinetic profiles and has a major influence on their in vivo efficacy. Altered drug binding to albumin due to drug-drug interactions or abnormal physiology may result in marked changes in the active drug concentration, thus affecting its pharmacokinetic and pharmacodynamic properties. The propensity of drug-drug interaction to be clinically significant as well as possible exploitation of such interactions for therapeutic purposes is reviewed. Being the major organs of albumin metabolism, any impairment in the liver and kidney functions frequently alter the level of serum albumin, which affects the pharmacokinetic profiles of drugs and may have serious clinical implications. The natural function of serum albumin as a drug carrier is facilitated by its interaction with various cellular receptors. These receptors not only promote the uptake of drugs into cells but are also responsible for the extraordinarily long circulatory half-life of albumin. This property in combination with the presence of multiple ligand binding pockets have led to the emergence of serum albumin as an attractive vehicle for novel drug delivery systems. Here, we provide an overview of various albumin-based drug delivery strategies, classified according to their methods of drug attachment, and highlight their experimental and clinical successes.

Albumin-based cancer therapeutics for intraperitoneal drug delivery: a review

Albumin is a remarkable carrier protein with multiple cellular receptor and ligand binding sites, which are able to bind and transport numerous endogenous and exogenous compounds. The development of albumin-bound drugs is gaining increased importance in the targeted delivery of cancer therapy. Intraperitoneal (IP) drug delivery represents an attractive strategy for the local treatment of peritoneal metastasis (PM). PM is characterized by the presence of widespread metastatic tumor nodules on the peritoneum, mostly originating from gastro-intestinal or gynaecological cancers. Albumin as a carrier for chemotherapy holds considerable promise for IP delivery in patients with PM. Data from recent (pre)clinical trials suggest that IP albumin-bound chemotherapy may result in superior efficacy in the treatment of PM compared to standard chemotherapy formulations. Here, we review the evidence on albumin-bound chemotherapy with a focus on IP administration and its efficacy in PM.

Overview of Dual-Acting Drug Methotrexate in Different Neurological Diseases, Autoimmune Pathologies and Cancers

Methotrexate, a structural analogue of folic acid, is one of the most effective and extensively used drugs for treating many kinds of cancer or severe and resistant forms of autoimmune diseases. In this paper, we take an overview of the present state of knowledge with regards to complex mechanisms of methotrexate action and its applications as immunosuppressive drug or chemotherapeutic agent in oncological combination therapy. In addition, the issue of the potential benefits of methotrexate in the development of neurological disorders in Alzheimer’s disease or myasthenia gravis will be discussed.

Self-Assembling Peptides and Their Application in the Treatment of Diseases

Self-assembling peptides are biomedical materials with unique structures that are formed in response to various environmental conditions. Governed by their physicochemical characteristics, the peptides can form a variety of structures with greater reactivity than conventional non-biological materials. The structural divergence of self-assembling peptides allows for various functional possibilities; when assembled, they can be used as scaffolds for cell and tissue regeneration, and vehicles for drug delivery, conferring controlled release, stability, and targeting, and avoiding side effects of drugs. These peptides can also be used as drugs themselves. In this review, we describe the basic structure and characteristics of self-assembling peptides and the various factors that affect the formation of peptide-based structures. We also summarize the applications of self-assembling peptides in the treatment of various diseases, including cancer. Furthermore, the in-cell self-assembly of peptides, termed reverse self-assembly, is discussed as a novel paradigm for self-assembling peptide-based nanovehicles and nanomedicines.

Smart Targeting To Improve Cancer Therapeutics

Cancer is the second largest cause of death worldwide with the number of new cancer cases predicted to grow significantly in the next decades. Biotechnology and medicine can and should work hand-in-hand to improve cancer diagnosis and treatment efficacy. However, success has been frequently limited, in particular when treating late-stage solid tumors. There still is the need to develop smart and synergistic therapeutic approaches to achieve the synthesis of strong and effective drugs and delivery systems. Much interest has been paid to the development of smart drug delivery systems (drug-loaded particles) that utilize passive targeting, active targeting, and/or stimulus responsiveness strategies. This review will summarize some main ideas about the effect of each strategy and how the combination of some or all of them has shown to be effective. After a brief introduction of current cancer therapies and their limitations, we describe the biological barriers that nanoparticles need to overcome, followed by presenting different types of drug delivery systems to improve drug accumulation in tumors. Then, we describe cancer cell membrane targets that increase cellular drug uptake through active targeting mechanisms. Stimulus-responsive targeting is also discussed by looking at the intra- and extracellular conditions for specific drug release. We include a significant amount of information summarized in tables and figures on nanoparticle-based therapeutics, PEGylated drugs, different ligands for the design of active-targeted systems, and targeting of different organs. We also discuss some still prevailing fundamental limitations of these approaches, eg, by occlusion of targeting ligands.

Current multifunctional albumin-based nanoplatforms for cancer multi-mode therapy

Albumin has been widely applied for rational design of drug delivery complexes as natural carriers in cancer therapy due to its distinct advantages of biocompatibility, abundance, low toxicity and versatile property. Hence, various types of multifunctional albumin-based nanoplatforms (MAlb-NPs) that adopt multiple imaging and therapeutic techniques have been developed for cancer diagnosis and treatment. Stimuli-responsive release, including reduction-sensitive, pH-responsive, concentration-dependent and photodynamic-triggered, is important to achieve low-toxicity cancer therapy. Several types of imaging techniques can synergistically improve the effectiveness of cancer therapy. Therefore, combinational theranostic is considered to be a prospective strategy to improve treatment efficiency, minimize side effects and reduce drug resistance, which has received tremendous attentions in recent years. In this review, we highlight several stimuli-responsive albumin nanoplatforms for combinational theranostic.

Combined antitumoral effects of pretubulysin and methotrexate

Pretubulysin (PT), a potent tubulin-binding antitumoral drug, and the well-established antimetabolite methotrexate (MTX) were tested separately or in combination (PT+MTX) for antitumoral activity in L1210 leukemia cells or KB cervix carcinoma cells in vitro and in vivo in NMRI-nu/nu tumor mouse models. In cultured L1210 cells, treatment with PT or MTX displays strong antitumoral effects in vitro, and the combination PT+MTX exceeds the effect of single drugs. PT also potently kills the MTX resistant KB cell line, without significant MTX combination effect. Cell cycle analysis reveals the expected arrest in G1/S by MTX and in G2/M by PT. In both cell lines, the PT+MTX combination induces a G2/M arrest which is stronger than the PT-triggered G2/M arrest. PT+MTX does not change rates of apoptotic L1210 or KB cells as compared to single drug applications. Confocal laser scanning microscopy images show the microtubule disruption and nuclear fragmentation induced by PT treatment of L1210 and KB cells. MTX changes the architecture of the F-actin skeleton. PT+MTX combines the toxic effects of both drugs. In the in vivo setting, the antitumoral activity of drugs differs from their in vitro cytotoxicity, but their combination effects are more pronounced. MTX on its own does not display significant antitumoral activity, whereas PT reduces tumor growth in both L1210 and KB in vivo models. Consistent with the cell cycle effects, MTX combined at moderate dose boosts the antitumoral effect of PT in both in vivo tumor models. Therefore, the PT+MTX combination may present a promising therapeutic approach for different types of cancer.

Harnessing albumin as a carrier for cancer therapies

Serum albumin, a natural ligand carrier that is highly concentrated and long-circulating in the blood, has shown remarkable promise as a carrier for anti-cancer agents. Albumin is able to prolong the circulation half-life of otherwise rapidly cleared drugs and, importantly, promote their accumulation within tumors. The applications for using albumin as a cancer drug carrier are broad and include both traditional cancer chemotherapeutics and new classes of biologics. Strategies for leveraging albumin for drug delivery can be classified broadly into exogenous and in situ binding formulations that utilize covalent attachment, non-covalent association, or encapsulation in albumin-based nanoparticles. These methods have shown remarkable preclinical and clinical successes that are examined in this review.

Recent advancements in erythrocytes, platelets, and albumin as delivery systems

In the past few years, nanomaterial-based drug delivery systems have been applied to enhance the efficacy of therapeutics and to alleviate negative effects through the controlled delivery of targeting and releasing agents. However, few drug carriers can achieve high targeting efficacy, even when targeting modalities and surface markers are introduced. Immunological problems have also limited their wide applications. Biological drug delivery systems, such as erythrocytes, platelets, and albumin, have been extensively investigated because of their unique properties. In this review, erythrocytes, platelets, and albumin are described as efficient drug delivery systems. Their properties, applications, advantages, and limitations in disease treatment are explained. This review confirms that these systems can be used to facilitate a specific, biocompatible, and smart drug delivery.

Cell-penetrating conjugates of pentaglutamylated methotrexate as potential anticancer drugs against resistant tumor cells

The emerging resistance of tumor cells against methotrexate (MTX) is one of the major limitations of the MTX treatment of tumorous diseases. The disturbance in the polyglutamation which is a main step in the mechanism of methotrexate action is often the reason of the resistance. Delivery of polyglutamylated MTX into cells may evade the mechanisms that are responsible for drug resistance. In this study conjugates of methotrexate and its pentaglutamylated derivatives with cell-penetrating peptides - penetratin and octaarginine - were investigated. The cellular-uptake and in vitro cytostatic activity of conjugates were examined on breast cancer cell cultures (MDA-MB-231 as resistant and MCF-7 as sensitive cell culture). These cell cultures showed very different behaviour towards the conjugates. Although the presence of pentaglutamyl moiety significantly decreased the internalisation of conjugates, some of them were significantly active in vitro. All of the conjugates were able to penetrate in some extent into both cell types, but only the conjugates of penetratin showed in vitro cytostatic activity. The most effective conjugates were the MTX-Glu5-Penetratin(desMet) and MTX-Glu5-GFLG-Penetratin(desMet). The latter was effective on both cell cultures while the former was active only on the resistant tumor cells. Our results suggest that the translocation of polyglutamylated MTX may be a new way to treat sensitive and more importantly resistant tumors. While both penetratin and octaarginine peptides were successfully used to deliver several kinds of cargos earlier in our case the activity of penetratin conjugates was more pronounced.

Kinetics and efficiency of a methyl-carboxylated 5-Fluorouracil-bovine serum albumin adduct for targeted delivery

5-Fluorouracil (5-FU) is a clinically well-established anti-cancer drug effectively applied in chemotherapy, mainly for the treatment of breast and colorectal cancer. Substantial disadvantages are adverse effects, arising from serious damage of healthy tissues, and shortcoming pharmacokinetics due to its low molecular weight. A promising approach for improvement of such drugs is their coupling to suitable carriers. Here, a 5-FU adduct, 5-fluorouracil acetate (FUAc) is synthesized and covalently coupled to bovine serum albumin (BSA) as model carrier molecule. On average, 12 molecules FUAc are bound to one BSA. Circular dichriosm (CD)-spectra of BSA and FUAc-BSA are identical, suggesting no significant conformational differences. FUAc-BSA is tested on T-47D and MDA-MB-231 breast cancer cells. Proliferation inhibition of membrane albumin-binding protein (mABP)-expressing T-47D cells by FUAc-BSA is similar to that of 5-FU and only moderate for MDA-MB-231 cells that lack such expression. Therefore, a crucial role of mABP expression in effective cell growth inhibition by FUAc-BSA is assumed.

Development and characterization of small bispecific albumin-binding domains with high affinity for ErbB3

Affinity proteins based on small scaffolds are currently emerging as alternatives to antibodies for therapy. Similarly to antibodies, they can be engineered to have high affinity for specific proteins. A potential problem with small proteins and peptides is their short in vivo circulation time, which might limit the therapeutic efficacy. To circumvent this issue, we have engineered bispecificity into an albumin-binding domain (ABD) derived from streptococcal Protein G. The inherent albumin binding was preserved while the opposite side of the molecule was randomized for selection of high-affinity binders. Here we present novel ABD variants with the ability to bind to the epidermal growth factor receptor 3 (ErbB3). Isolated candidates were shown to have an extraordinary thermal stability and affinity for ErbB3 in the nanomolar range. Importantly, they were also shown to retain their affinity to albumin, hence demonstrating that the intended strategy to engineer bispecific single-domain proteins against a tumor-associated receptor was successful. Moreover, competition assays revealed that the new binders could block the natural ligand Neuregulin-1 from binding to ErbB3, indicating a potential anti-proliferative effect. These new binders thus represent promising candidates for further development into ErbB3-signaling inhibitors, where the albumin interaction could result in prolonged in vivo half-life.

Pharmacokinetic considerations for targeted drug delivery

Drug delivery systems involve technology designed to maximize therapeutic efficacy of drugs by controlling their biodistribution profile. In order to optimize a function of the delivery systems, their biodistribution characteristics should be systematically understood. Pharmacokinetic analysis based on the clearance concepts provides quantitative information of the biodistribution, which can be related to physicochemical properties of the delivery system. Various delivery systems including macromolecular drug conjugates, chemically or genetically modified proteins, and particulate drug carriers have been designed and developed so far. In this article, we review physiological and pharmacokinetic implications of the delivery systems.

Pharmaceutical aspects of the recombinant human serum albumin dimer: structural characteristics, biological properties, and medical applications

Human serum albumin is the most abundant protein in the blood. It is clinically used in the treatment of severe hypoalbuminemia and as a plasma expander. The use of albumins as a carrier for drugs is currently being developed, and some are now in the preclinical and clinical trial stages. The main technologies for utilizing an albumin as a drug carrier are protein fusion, polymerization and surface modification, and so on. Among these technologies, albumin dimerization has wide clinical applications as a plasma expander as well as a drug carrier. Despite the fact that many reports have appeared on drugs using an albumin dimer as a carrier, our knowledge of the characteristics of the albumin dimer itself is incomplete. In this review, we summarize the structural characteristics of recombinant albumin dimers produced by two methods, namely, chemical linkage with 1,6-bis(maleimido)hexane and genetically linked with an amino acid linker, and the physicochemical characteristics and biological properties of these preparations. Finally, the potential for pharmaceutical applications of albumin dimers in clinical situations is discussed.

Photochemically generated elemental selenium forms conjugates with serum proteins that are preferentially cytotoxic to leukemia and selected solid tumor cells

The objective of this study was to determine if and how photoproducts contribute to the antitumor effect of merocyanine-mediated PDT. A panel of barbituric, thiobarbituric and selenobarbituric acid analogues of Merocyanine 540 was photobleached, and the resulting photoproducts were characterized by absorption, fluorescence emission, mass, energy dispersive X-ray, and X-ray photoelectron spectroscopy and tested for cytotoxic activity against tumor cell lines and freshly explanted bone marrow cells. While all dyes were readily photobleached, only photoproducts of selone dyes showed cytotoxic activity. One-hour incubations with micromolar concentrations of selone-derived photoproducts were sufficient to reduce leukemia/lymphoma cells ≥10 000 fold, whereas preserving virtually all normal CD34-positive bone marrow cells. Of six multidrug-resistant tumor cell lines tested, five were as sensitive or more sensitive to photoproducts than the corresponding wild-type lines. Physicochemical characterizations of the cytotoxic activity indicated that it consisted of conjugates of subnano particles of elemental selenium and (lipo)proteins. The discovery of cytotoxic Se-protein conjugates provides a rare example of photoproducts contributing substantially to the antitumor effect of PDT and challenges the long-held view that Se in oxidation state zero is biologically inert. Agents modeled after our Se-protein conjugates may prove useful for the treatment of leukemia.

Magnitude of enhanced permeability and retention effect in tumors with different phenotypes: 89Zr-albumin as a model system

Targeted nanoparticle-based technologies show increasing prevalence in radiotracer design. As a consequence, quantitative contribution of nonspecific accumulation in the target tissue, mainly governed by the enhanced permeability and retention (EPR) effect, becomes highly relevant for evaluating the specificity of these new agents. This study investigated the influence of different tumor phenotypes on the EPR effect, hypothesizing that a baseline level of uptake must be exceeded to visualize high and specific uptake of a targeted macromolecular radiotracer.

METHODS

These preliminary studies use (89)Zr-labeled mouse serum albumin ((89)Zr-desferrioxamine-mAlb) as a model radiotracer to assess uptake and retention in 3 xenograft models of human prostate cancer (CWR22rv1, DU-145, and PC-3). Experiments include PET and contrast-enhanced ultrasound imaging to assess morphology, vascularization, and radiotracer uptake; temporal ex vivo biodistribution studies to quantify radiotracer uptake over time; and histologic and autoradiographic studies to evaluate the intra- and intertumoral distribution of (89)Zr-desferrioxamine-mAlb.

RESULTS

Early uptake profiles show statistically significant but overall small differences in radiotracer uptake between different tumor phenotypes. By 20 h, nonspecific radiotracer uptake was found to be independent of tumor size and phenotype, reaching at least 5.0 percentage injected dose per gram in all 3 tumor models.

CONCLUSION

These studies suggest that minimal differences in tumor uptake exist at early time points, dependent on the tumor type. However, these differences equalize over time, reaching around 5.0 percentage injected dose per gram at 20 h after injection. These data provide strong support for the introduction of mandatory experimental controls of future macromolecular or nanoparticle-based drugs, particularly regarding the development of targeted radiotracers.

Tuning the serum persistence of human serum albumin domain III:diabody fusion proteins

The long circulation persistence of human serum albumin (HSA) is enabled by its domain III (DIII) interaction with the neonatal Fc receptor (FcRn). A protein scaffold based on HSA DIII was designed. To modify the serum half life of the scaffold, residues H535, H510, and H464 were individually mutated to alanine. HSA DIII wild type (WT) and variants were fused to the anti-carcinoembryonic antigen (CEA) T84.66 diabody (Db), radiolabeled with (124)I and injected into xenografted athymic mice for serial PET/CT imaging. All proteins targeted the CEA-positive tumor. The mean residence times (MRT) of the proteins, calculated by quantifying blood activity from the PET images, were: Db-DIII WT (56.7 h), H535A (25 h), H510A (20 h), H464A (17 h), compared with Db (2.9 h). Biodistribution confirmed the order of blood clearance from slow to fast: Db-DIII WT > H535A > H510A > H464A > Db with 4.0, 2.0, 1.8, 1.6 and 0.08 %ID/g of remaining blood activity at 51 h, respectively. This study demonstrates that attenuating the DIII-FcRn interaction provides a way of controlling the pharmacokinetics of the entire Db-DIII fusion protein without compromising tumor targeting. H464 appears to be most crucial for FcRn binding (greatest reduction in MRT), followed by H510 and H535. By mutating the DIII scaffold, we can dial serum kinetics for imaging or therapy applications.

In vitro and in vivo evaluation of microparticulate drug delivery systems composed of macromolecular prodrugs

Macromolecular prodrugs are very useful systems for achieving controlled drug release and drug targeting. In particular, various macromolecule-antitumor drug conjugates enhance the effectiveness and improve the toxic side effects. Also, polymeric micro- and nanoparticles have been actively examined and their in vivo behaviors elucidated, and it has been realized that their particle characteristics are very useful to control drug behavior. Recently, researches based on the combination of the concepts of macromolecular prodrugs and micro- or nanoparticles have been reported, although they are limited. Macromolecular prodrugs enable drugs to be released at a certain controlled release rate based on the features of the macromolecule-drug linkage. Micro- and nanoparticles can control in vivo behavior based on their size, surface charge and surface structure. These merits are expected for systems produced by the combination of each concept. In this review, several micro- or nanoparticles composed of macromolecule-drug conjugates are described for their preparation, in vitro properties and/or in vivo behavior.

Imaging tumors with an albumin-binding Fab, a novel tumor-targeting agent

Association with albumin as a means to improve biodistribution and tumor deposition of a Fab was investigated using AB.Fab4D5, a bifunctional molecule derived from trastuzumab (HERCEPTIN) capable of binding albumin and tumor antigen HER2 (erbB2) simultaneously. AB.Fab4D5 was compared with trastuzumab and a trastuzumab-derived Fab (Fab4D5) for the ability to target tumors overexpressing HER2 in mouse mammary tumor virus/HER2 allograft models. Biodistribution was monitored using intravital microscopy, histology, and integrated single-photon emission computed tomography/computed tomography analysis. Fab4D5 tumor deposition was characterized by rapid but transient appearance in tumor at 2 h with little retention, followed by rapid accumulation in kidney by 6 h. Trastuzumab was slow to accumulate in tumors and slow to clear from normal tissues, although significant tumor deposition was achieved by 24 h. In contrast, AB.Fab4D5 was observed at 2 h in tumor and its presence was sustained beyond 24 h similar to trastuzumab. Intravital microscopy revealed that at peak tumor accumulation, tumor cell staining by AB.Fab4D5 was more uniform than for Fab4D5 or trastuzumab. Similar tumor deposition was achieved for both AB.Fab4D5 and trastuzumab at 48 h (35.9 +/- 1.8% and 38.2 +/- 3.1% injected dose/g); however, AB.Fab4D5 targeted tumors more rapidly and quickly cleared from blood, leading to a lower overall normal tissue exposure. Importantly, unlike Fab4D5, AB.Fab4D5 did not accumulate in kidney, suggesting that association with albumin leads to an altered route of clearance and metabolism. Rapid targeting, excellent tumor deposition and retention, coupled with high tumor to blood ratios may make AB.Fab an exceptional molecule for imaging and cancer therapy.

Methotrexate enhances the antianabolic and antiproliferative effects of 5-aminoimidazole-4-carboxamide riboside

Because of its ability to mimic a low energy status of the cell, the cell-permeable nucleoside 5-aminoimidazole-4-carboxamide (AICA) riboside was proposed as an antineoplastic agent switching off major energy-consuming processes associated with the malignant phenotype (lipid production, DNA synthesis, cell proliferation, cell migration, etc.). Key to the antineoplastic action of AICA riboside is its conversion to ZMP, an AMP mimetic that at high concentrations activates the AMP-activated protein kinase (AMPK). Here, in an attempt to increase the efficacy of AICA riboside, we pretreated cancer cells with methotrexate, an antimetabolite blocking the metabolism of ZMP. Methotrexate enhanced the AICA riboside-induced accumulation of ZMP and led to a decrease in the levels of ATP, which functions as an intrasteric inhibitor of AMPK. Consequently, methotrexate markedly sensitized AMPK for activation by AICA riboside and potentiated the inhibitory effects of AICA riboside on tumor-associated processes. As cotreatment elicited antiproliferative effects already at concentrations of compounds that were only marginally effective when used alone, our findings on the cooperation between methotrexate and AICA riboside provide new opportunities both for the application of classic antimetabolic chemotherapeutics, such as methotrexate, and for the exploitation of the energy-sensing machinery as a target for cancer intervention.

Methotrexate-Albumin and Aminopterin-Albumin Effectively Prevent Experimental Acute Graft-versus-Host Disease

BACKGROUND

During the last several years, major progress has been made in developing targeted chemotherapy using cytotoxic drugs covalently bound to human serum albumin (HSA). We explored the activity of two antifolates methotrexate (MTX) and aminopterin (AMPT) bound to HSA in prophylaxis and treatment of experimental acute graft-versus-host disease (aGVHD).

METHODS

In all, 113 female F1 hybrid BN/Lew-rats were irradiated with 8.2 Gy (n=103) or 9.9 Gy (n=10). One day after irradiation each rat received 4 x 10 bone marrow cells and 1.5 x 10 spleen T-cells from female Lew-rats. GVHD prophylaxis consisted of MTX-HSA 2 mg/kg (n=25), MTX-HSA 0.5 mg/kg (n=8), AMPT-HSA 0.65 mg/kg (n=8), MTX 0.5 mg/kg (n=17), or native HSA (n=39) given intraperitoneally (IP) on days 0, 4, 8, and 12. Treatment of aGVHD consisted of MTX-HSA 2 mg/kg (n=8) or AMPT-HSA 0.5 mg/kg (n=8) given intraperitoneally at the time of onset of aGVHD and subsequently every fourth day (a total of four doses).

RESULTS

All animals receiving native HSA developed lethal aGVHD. Prophylactic treatment with MTX-HSA 2 mg/kg prevented aGVHD in 18 of 25 animals and in 6 of 8 receiving AMPT-HSA. In contrast, five out of nine rats receiving free MTX died due to aGVHD. The survival rates in the prophylactic MTX-HSA 2 mg/kg and AMPT-HSA groups were significantly higher compared to the MTX and control groups (P<0.05), while non hematologic toxicity of MTX-HSA was not detectable. AMPT-HSA at a dose of 0.65 mg/kg as well as MTX at a dose of 0.5 mg/kg were associated with temporary weight loss and lethargy.

CONCLUSIONS

The albumin conjugates MTX-HSA and AMPT-HSA effectively prevent experimental aGVHD.

Phase II study of MTX-HSA in combination with Cisplatin as first line treatment in patients with advanced or metastatic transitional cell carcinoma

PURPOSE

To assess the efficacy, tolerability and safety of MTX-HSA (methotrexate (MTX) covalently linked to human serum albumin (HSA)) combined with cisplatin as first line therapy for advanced bladder cancer.

METHODS

Patients (pat) were treated with a loading dose of 110 mg/m(2) of MTX-HSA followed by a weekly dose of 40 mg/m(2) starting on day 8. Cisplatin was given on day 2 of each 28 day cycle at a dose of 75 mg/m(2).

RESULTS

Tumor response evaluation was possible in 7 patients. Complete response (CR) and partial response (PR) was observed in 1 patient each (overall response rate: 29%). Key toxicities included CTC Grade (G) 3/4 stomatitis in 6 patients, vomiting G3 in 1 patient, fatigue G3 in 1 patient and thrombocytopenia G3 in 3 patients.

CONCLUSION

The combination of MTX-HSA with cisplatin is feasible and shows antitumor activity against urothelial carcinomas combined with an acceptable toxicity profile.

Polymer Therapeutics: Concepts and Applications

Polymer therapeutics encompass polymer-protein conjugates, drug-polymer conjugates, and supramolecular drug-delivery systems. Numerous polymer-protein conjugates with improved stability and pharmacokinetic properties have been developed, for example, by anchoring enzymes or biologically relevant proteins to polyethylene glycol components (PEGylation). Several polymer-protein conjugates have received market approval, for example the PEGylated form of adenosine deaminase. Coupling low-molecular-weight anticancer drugs to high-molecular-weight polymers through a cleavable linker is an effective method for improving the therapeutic index of clinically established agents, and the first candidates have been evaluated in clinical trials, including, N-(2-hydroxypropyl)methacrylamide conjugates of doxorubicin, camptothecin, paclitaxel, and platinum(II) complexes. Another class of polymer therapeutics are drug-delivery systems based on well-defined multivalent and dendritic polymers. These include polyanionic polymers for the inhibition of virus attachment, polycationic complexes with DNA or RNA (polyplexes), and dendritic core-shell architectures for the encapsulation of drugs. In this Review an overview of polymer therapeutics is presented with a focus on concepts and examples that characterize the salient features of the drug-delivery systems.

Targeted delivery of methotrexate to epidermal growth factor receptor–positive brain tumors by means of cetuximab (IMC-C225) dendrimer bioconjugates

We have constructed a drug delivery vehicle that targets the epidermal growth factor receptor (EGFR) and its mutant isoform EGFRvIII. The monoclonal antibody, cetuximab, previously known as C225, which binds to both EGFR and EGFRvIII, was covalently linked via its Fc region to a fifth-generation (G5) polyamidoamine dendrimer containing the cytotoxic drug methotrexate. As measured by mass spectrometry and UV/vis spectroscopy, the resulting bioconjugate, designated C225-G5-MTX, contained 12.6 molecules of methotrexate per unit of dendrimer. Specific binding and cytotoxicity of the bioconjugate was evaluated against the EGFR-expressing rat glioma cell line F98(EGFR). Using a competitive binding assay, it was shown that the bioconjugate retained its affinity for F98(EGFR) cells, with a 0.8 log unit reduction in its EC(50). Only cetuximab completely inhibited binding of the bioconjugate, which was unaffected by methotrexate or dendrimer. Cetuximab alone was not cytotoxic to F98(EGFR) cells at the concentration tested, whereas the IC(50) of the bioconjugate was 220 nmol/L, which was a 2.7 log unit decrease in toxicity over that of free methotrexate. The biodistribution of C225-G5-MTX in rats bearing i.c. implants of either F98(EGFR) or F98(WT) gliomas was determined 24 hours following convection enhanced delivery of (125)I-labeled bioconjugate. At this time, 62.9 +/- 14.7% ID/g tumor was localized in rats bearing F98(EGFR) gliomas versus 11.3 +/- 3.6% ID/g tumor in animals bearing F98(WT) gliomas, thereby showing specific molecular targeting of the tumor. The corresponding radioactivity of normal brain from the F98(EGFR) tumor-bearing right and non-tumor-bearing left cerebral hemisphere were 5.8 +/- 3.4% and 0.8 +/- 0.6% ID/g, respectively. Based on these results, therapy studies were initiated in F98(EGFR) glioma-bearing rats. Animals that received C225-G5-MTX, cetuximab, or free methotrexate had median survival times of 15, 17, and 19.5 days, respectively, which were not statistically different from each other or untreated control animals. Our results, which are both positive and negative, show that specific molecular targeting is but one of several requirements that must be fulfilled if an antibody-drug bioconjugate will be therapeutically useful.

Antitumor efficacy of a novel polymer-peptide-drug conjugate in human tumor xenograft models

We have designed a new dextran-peptide-methotrexate conjugate to achieve tumor-targeted delivery of chemotherapeutics. The dextran carrier was selected to allow passive targeting and enhanced permeation and retention (EPR). The peptide linker has also been optimized to allow drug release in the presence of matrix-metalloproteinase-2 (MMP-2) and matrix-metalloproteinase-9 (MMP-9), 2 important tumor-associated enzymes. The new conjugate was assessed for its in vivo antitumor efficacy and systemic side effects. It was compared with free methotrexate (MTX) and a similar conjugate, differing by an MMP-insensitive linker, at equivalent intraperitoneal dosages. The MMP-sensitive conjugate demonstrated tolerable in vivo side effects and effective inhibition of in vivo tumor growth by 83% in each of the 2 separate tumor models that overexpress MMP (HT-1080 and U-87). The antiproliferative effect of the drug contributed to the inhibition of tumor growth. In contrast, free MTX resulted in no significant tumor reduction in the same models. Neither free MTX nor the conjugate caused any tumor inhibition in the mice bearing RT-112, a slower growing model that does not overexpress MMP. MMP-insensitive conjugates, though able to inhibit tumor growth, caused toxicity in the small intestine and bone marrow.

In vivo antitumor efficacy of 17-DMAG (17-dimethylaminoethylamino-17-demethoxygeldanamycin hydrochloride), a water-soluble geldanamycin derivative

PURPOSE

To describe the preclinical basis for further development of 17-dimethyl aminoethylamino-17-demethoxygeldanamycin hydrochloride (17-DMAG, NSC 707545).

METHODS

In vitro proliferation assays, and in vivo model studies in metastatic pancreatic carcinoma and subcutaneous xenograft melanoma and small-cell lung carcinoma models.

RESULTS

17-DMAG emerged from screening studies as a potent geldanamycin analog, with the average concentration inhibiting the growth of the NCI anticancer cell line drug screen by 50% being 0.053 microM. "Head to head" comparison with 17-allylamino-17-demethoxygeldanamycin (17-AAG, NSC 330507) revealed 17-DMAG to possess potent activity against certain cell types, e.g., MDA-MB-231 breast carcinoma and HL60-TB leukemia which were relatively insensitive to 17-AAG. Evidence of oral bioavailability of 17-DMAG in a saline-based formulation prompted more detailed examination of its antitumor efficacy in vivo. 17-DMAG inhibited the growth of the AsPC-1 pancreatic carcinoma xenografts growing as intrahepatic metastases at doses of 6.7-10 mg/kg twice daily for 5 days administered orally under conditions where 17-AAG was without activity. 17-DMAG in an aqueous vehicle at 7.5-15 mg/kg per day for 3 days on days 1-3, 8-10 and 13-17, or 1-5 and 8-12 showed evidence of antitumor activity by the parenteral and oral routes in the MEXF 276 and MEXF 989 melanomas and by the parenteral route in the LXFA 629 and LXFS 650 adenocarcinoma and small-cell carcinoma models. The latter activity was comparable to the historical activity of 17-AAG.

CONCLUSIONS

Taken together, the in vivo activity of 17-DMAG supports the further development of this water-soluble and potentially orally administrable geldanamycin congener.

17-(Allylamino)-17-demethoxygeldanamycin activity in human melanoma models

17-(Allylamino)-17-demethoxygeldanamycin (17-AAG) is a semisynthetic antitumor agent, which has entered phase I/II clinical trials. Melanoma cell lines in the NCI in vitro screen (mean GI50 = 84 nM) were relatively sensitive to the agent, which was therefore tested in vivo in four s.c. growing human melanoma xenografts (MEXF 276, 989, 462 and 514) in athymic mice. 17-AAG markedly inhibited tumor growth at doses of 80 (maximum tolerated dose) and 60 mg/kg/day in a qd x 5 (h: 0, 6; i.p.) schedule in two of four xenograft models. Cell lines derived from the 17-AAG-sensitive MEXF 276 and -resistant MEXF 514 melanomas, MEXF 276L and 514L, were chosen to study the effects of 17-AAG on its target Hsp90 as well as the Hsp90 'client' protein c-Raf-1 in vitro. Cells were exposed to drug concentrations which just cause total growth inhibition (total growth inhibition = 375 nM in MEXF 276L and 10 microM in MEXF 514L). Pharmacokinetic determinations confirmed that 17-AAG concentrations producing growth inhibition invitro are readily achievable in vivo at the MTD (AUC0- infinity 1068 microM x min). Whilst 17-AAG treatment did not affect Hsp90 expression in the relatively resistant MEXF 514L cells, it caused a rapid transient decline in the markedly sensitive MEXF 276L cell line. In contrast, Hsp72 expression increased. Following Hsp90 depletion at 2-8 h in MEXF 276L cells, down-regulation of c-Raf-1 was seen starting at 16 h after drug addition. In MEXF 276 xenograft tissues treated with effective dose levels, loss of Hsp90 was seen and was associated with occurrence of apoptotic figures. The apoptotic index rose from 9% after 48 h, greater than 12% at 72 h to 45% at 10 days. These data support the hypothesis that in some melanoma models, a very good response (e.g. with tumor regressions) to 17-AAG may be associated with modulation of Hsp90 expression. The expression of this target should be followed in clinical studies with 17-AAG.

Models of breast cancer: is merging human and animal models the future?

Survival rates of patients with early breast cancer in the United Kingdom and in the United States have improved steadily over the past 15 years. The only way to continue or even accelerate this progress, however, is the discovery and development of new preventative and therapeutic strategies. With the massive explosion in potential therapeutic strategies becoming available, in the postgenomic era, better and more representative breast cancer models are urgently required for preclinical trials. Development of better in vivo models of human breast cancer are thus of crucial importance in the development of new cancer therapeutics.

Albumin-based drug delivery as novel therapeutic approach for rheumatoid arthritis

We reported recently that albumin is a suitable drug carrier for targeted delivery of methotrexate (MTX) to tumors. Due to pathophysiological conditions in neoplastic tissue, high amounts of albumin accumulate in tumors and are metabolized by malignant cells. MTX, covalently coupled to human serum albumin (MTX-HSA) for cancer treatment, is currently being evaluated in phase II clinical trials. Because synovium of patients with rheumatoid arthritis (RA) shares various features observed also in tumors, albumin-based drug targeting of inflamed joints might be an attractive therapeutic approach. Therefore, the pharmacokinetics of albumin and MTX in a mouse model of arthritis was examined. Additionally, uptake of albumin by synovial fibroblasts of RA patients and the efficacy of MTX and MTX-HSA in arthritic mice were studied. The results show that when compared with MTX, significantly higher amounts of albumin accumulate in inflamed paws, and significantly lower amounts of albumin are found in the liver and the kidneys. The protein is metabolized by human synovial fibroblasts in vitro and in vivo. MTX-HSA was significantly more effective in suppression of the onset of arthritis in mice than was MTX. In conclusion, albumin appears to be a suitable drug carrier in RA, most likely due to effects on synovial fibroblasts, which might increase therapeutic efficacy and reduce side effects of MTX.

Efficacy and tolerability of an aminopterin-albumin conjugate in tumor-bearing rats

The antifolate aminopterin (AMPT) was developed before methotrexate (MTX), but was not clinically established or generally used due its increased toxicity compared to MTX. Recently, we reported on the increased metabolism of albumin conjugates such as methotrexate-albumin (MTX-SA) in malignant tumors and the feasibility of using albumin as a carrier for drug targeting. Consequently, AMPT was covalently bound to serum albumin (AMPT-SA) at a 1:1 molar ratio. Biodistribution, tolerability and efficacy of this novel conjugate were studied in Walker-256 (W-256) carcinoma-bearing rats. As compared to native albumin, the same biodistribution and plasma clearance were found for AMPT-SA, which achieved 20.1% tumor uptake (estimated uptake per g tumor 6.4%) within 24 h after i.v. administration in rats. In a randomized study, AMPT-SA, repeatedly i.v. injected, was compared with low-molecular-weight AMPT. Depending on the molar concentration, the maximum tolerated dose (MTD) of AMPT covalently bound to SA was twice that of unbound AMPT (three repeated injections of 1.0 mg AMPT-SA/kg body weight versus three repeated injections of 0.5 mg AMPT/kg body weight; p=0.0006). Efficacy was studied at the level of the MTD and MTD/2, and demonstrated that AMPT-SA was significantly more active. At the MTD/2 in W-256 carcinoma-bearing rats, AMPT-SA achieved a 100% volume reduction and an optimal volume reduction during treatment/control (T/C) of 8.3% compared to a 53% volume reduction of AMPT and a T/C of 16.5% (p=0.032). Tumor relapses were reduced and occurred later in the AMPT-SA group (two tumor recurrences for AMPT-SA versus seven for AMPT; p=0.05). In this comparative study, the AMPT-SA conjugate showed high antitumor activity in vivo and a favorable toxicity compared to low-molecular-weight AMPT. These effects are attributed to the albumin carrier which seems to be an effective tool for selective tumor drug targeting.

Magnetic resonance imaging of nude mice with heterotransplanted high-grade squamous cell carcinomas: use of a low-loaded, covalently bound Gd-Hsa conjugate as contrast agent with high tumor affinity

RATIONALE

Malignant tumors often show an increased uptake and metabolism of plasma proteins, especially albumin.

OBJECTIVES

Determine whether the accumulation of low loaded Gd-albumin improves visualization of malignant tumors by MRI.

METHODS

Twelve nude mice with heterotransplanted squamous cell carcinomas were studied. The signal intensity of tumor, blood, liver, kidney and muscle tissue was studied in MR images after application of Gd-albumin during a period of 144 hours. MRI results were histologically correlated after simultaneously injection of Gd- and fluorescein-labeled albumins in 9 nude mice.

RESULTS

Although liver and kidney had a maximum increase in signal intensity within 30 minutes, tumors showed a delayed 51% increase in the 24 hours after application. Histologic and fluorescence evaluation demonstrated albumin localization in tumors predominantly in stroma and necroses.

CONCLUSIONS

Gd-albumin is efficiently accumulated in SCC transplants. MRI with low loaded Gd-albumin may offer relevant opportunities for recognizing tumors sensitive to a therapy with cyostic drug-labeled albumins.