Harnessing Albumin as a Carrier for Cancer Therapies

Harnessing Albumin as a Carrier for Cancer Therapies

1. Introduction
   1.            Albumin as a Carrier
   2.            Cancer Implications of Albumin
      1.      Passive Albumin Tumor Accumulation
      2.      Tumor Albumin Metabolism
2. General Albumin Binding Strategies

2.1 In Situ Binders

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2.1.1 Covalent Conjugation

2.1.2 Native Ligand Conjugates

2.1.3 Small Molecule Binders

2.1.4 Albumin Binding Domain and Nanobodies

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2.2 Exogenous Formulations

2.2.1 Covalent Conjugation

2.2.2 Nanoparticle

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2.2.3 Recombinant Albumin Fusion Proteins

3. Survey of Results

3.1 Chemotherapeutics

3.1.1 Nanoparticles

3.1.2 Covalent Conjugates

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3.2 Biologics

3.2.1 Oligonucleotides

3.2.2 Immunomodulatory Drugs

3.3 Theranostics

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3.4 Clinical Results

4. Conclusions and Future Directions

Acknowledgements

References

Abstract

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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 in to 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.

1. Introduction

Albumin, a long-circulating and highly-abundant protein in the blood, has unique promise as a carrier for cancer therapeutics based on several key characteristics: (1) it is a natural carrier of native ligands and other hydrophobic cargo (2) it is rescued from systemic clearance and degradation by natural mechanisms (3) it accumulates at sites of vascular leakiness and (4) it is more highly taken up and metabolized by rapidly growing, nutrient-starved cancer cells. Investigators have sought to leverage these characteristics for the delivery of several classes of approved and investigational anticancer agents, which will be reviewed herein.

1.1 Albumin as a Carrier

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Albumin is the most abundant protein in human blood with a concentration of about 40 mg/mL and a molecular weight of ~67 kDa [1]. Notably, it exhibits an extraordinarily long half-life of 19 days [2] [3]. Albumin is synthesized in the liver with approximately 13-14g of albumin entering the circulation every day [2]. When albumin extravasates into the tissue, it is returned to the vascular space via the lymphatic system through a nature recycling mechanism. The same approximate mass of 13-14g of albumin entering the intravascular space is also catabolized from it every day. Importantly, albumin is known to be a carrier of a wide variety of both endogenous and exogenous compounds [4]. This facilitates the colloidal solubilization and transport of normally hydrophobic molecules such as long chain fatty acids as well as a variety of other ligands such as bilirubin, metal ions such as zinc and copper, and therapeutic drugs such as warfarin and ibuprofen [5]. Figure 1shows the crystal structure of albumin and the sites where these ligands can bind. Some albumin-bound ligands may also benefit from albumin shielding the cargo from degradative serum enzymes.

Albumin naturally transcytoses across the vascular endothelium. This process is attributed to the receptor GP 60, also known as albondin. GP 60 is a receptor present in continuous vascular endothelium and alveolar epithelium. Albumin binds to GP60 which induces clustering of albumin-gp60 at cell surface and association with Cav-1, the main protein critical to caveolae formation. Cav-1 induces invagination of surface membrane surrounding the clustered GP 60-albumin receptors and the subsequent internalization of a vesicle composed of albumin and compounds bound to albumin. Cav-1 is transported to and fuses with basolateral membrane and completes transcytosis. [2] (Figure 2A) However, GP 60 only binds to native albumin. GP 18 and GP 30, by comparison, exhibit preferential binding to modified albumin to traffic it for lysosomal degradation and are widely distributed in the body [7]. These receptors may be part of a pathway to remove old, damaged, or potentially deleterious albumin [8]. Albumin-gold conjugates and maleic anhydride treated albumin, for example, are modified forms of this protein subject to this pathway, necessitating careful design consideration for the modification of albumin for drug delivery [7].

The half-life of albumin is one of its remarkable properties that make it an attractive carrier for improving the pharmacokinetic properties of anticancer agents. This long half-life is attributed to neonatal Fc receptor (FcRn), an intracellular receptor that is responsible for rescuing albumin from degradation. This receptor is widely distributed in the body and is known to extend the half-life of both serum albumin and immunoglobulin G. The FcRn functions by binding these proteins in the acidic endosome and diverting them from the highly-degradative lysosomal pathway. Both molecules are exocytosed into the extracellular compartment where, at physiological pH, they are released from the FcRn.  Albumin can then re-enter the circulation through the lymphatics, thereby prolonging its half-life. [2] This receptor interaction is pH dependent with strong affinity binding occurring at the low pH of the endosome (Figure 2B). Additionally, albumin can avoid renal clearance by reabsorption through the megalin/cubilin receptors. Cubilin is a 460 kDa glycoprotein which binds to transmembrane endocytic receptor megalin and has been demonstrated to be important for binding albumin and tubular albumin reabsorption [9]. Albumin binds cubilin with high affinity (Kd~0.6µM), and studies have shown that megalin facilitates the endocytosis and intracellular trafficking of cubilin [10]. Both megalin and cubilin are highly expressed in the renal proximal tubule brush-border and endocytic apparatus, and it is thought that megalin may be directly involved in reabsorption as an albumin receptor, and/or indirectly by affecting the expression and/or endocytic function of cubilin [10]. Moreover, as a result of the size selective properties of the glomerular filter, the primary urine contains proteins of low molecular weight (<60 kDa), whereas larger proteins are excluded [11]. Using albumin as a carrier can leverage this natural pathway for extending drug circulation half-life. This approach circumvents the usage of synthetic systems which may involve complicated synthesis and confer some toxicity or immunogenicity. For instance, modifying drugs with poly(ethylene glycol) is a common method for improving hydrophilicity and circulation of a drug by making it larger the renal size cutoff.  However, humans can mount antibody responses to PEG, resulting in potentially dangerous allergic reactions and potentially limiting the utility of repeated administration of PEG-containing formulations [12]. The biocompatibility and physiological transport pathways used by albumin make it an exciting platform for anticancer drug development.

2. Cancer Implications of Albumin

1.2.1 Passive Albumin Tumor Accumulation

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The internal vascular network of growing tumors often becomes insufficient for supplying the oxygen and nutrients necessary to support their aberrant, hyper-proliferative local environments. Consequent hypoxic conditions and cell death are associated with the release of angiogenic factors that cause rapid formation of new blood vessels at the tumor site. These new vessels are characterized by irregularities such as larger than normal fenestrations in the endothelium [13]. Tumors also have disruption of the lymphatic system, which in healthy tissues, continuously drains the extracellular fluid, facilitating reentry of macromolecules into the circulation. In tumors, there is either poor or heterogeneous lymphatic drainage due to the compression and eventual collapse of lymphatic vessels by rapidly growing cancer cells [13][14]. The tumor vasculature leakiness, paired with poorly formed lymphatic drainage, is thought to be responsible for the preferential tumor accumulation of nanostructures and macromolecules in the phenomenon known as the enhanced permeability and retention effect (EPR)  (Figure 2C) [15].

Albumin is especially adept at accumulating in regions of proliferating tumor cells as a consequence of the EPR effect. This is because albumin is the most concentrated protein in the blood at the aforementioned concentration of 40 mg/mL compared to its interstitial concentration of about 14 mg/mL, driving its diffusional transport [16].  Furthermore, the molecular weight of albumin (67 kDa) is near the reported optimum size of ~50 nm nanomedicine for deep tissue penetration and high retention in tumors [17]. Additionally, the reliance of albumin on the lymphatic system to return to the circulation from the extracellular space makes it susceptible to accumulation in tumors with their poor lymphatic drainage[15] [18]. Indeed, the first observations of macromolecule accumulation in tumor interstitium were based on the preferential distribution and retention of radiolabeled albumin and other serum proteins [15]. Importantly, Evans blue dye, which naturally complexes with albumin, demonstrates prolonged retention in tumors compared to normal tissue, from which it is rapidly cleared [15] . Albumin accumulation has since been observed in a variety of solid tumors animal models including sarcoma, ovarian carcinoma, and Novikof hepatoma [19].

The EPR effect has recently come under scrutiny based on the disparity observed between nanocarrier tumor biodistribution and therapeutic efficacy in preclinical tumor models versus human clinical trials. Tumor vasculature formation occurs at a faster rate in commonly used flank tumor mouse models compared to most human disease, and this physiologic difference may exaggerate the EPR effect in some animal models. Additionally, there is growing appreciation that the EPR effect may be more relevant for certain tumor or patient subsets among the wildly heterogeneous spectrum of human cancers. The development of companion diagnostic nanoparticles is a promising approach for predicting patient responses to nanomedicine, which may be key to best leveraging the EPR effect for clinical delivery [20]. Interestingly, in observations among a variety of tumor models including syngeneic breast tumors, mouse mammary intraepithelial neoplasia outgrowths, and epithelial-mesenchymal transition tumors, permeability to albumin is ~4 fold greater than 100 nm liposomes [21]. Further, the prolonged circulation time associated with albumin may further enhance tumor accumulation by EPR. These finding suggest promise for the improved penetration of albumin therapeutics.

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1.2.2 Albumin Tumor Metabolism

In addition to the desirable passive tumor tissue accumulation of albumin, it also demonstrates preferential uptake by tumor cells which can be leveraged for intracellular delivery of therapeutic cargo. In 2013, Commisso et al observed a mechanism by which cancer cells can support their increased metabolic and growth needs by active uptake of extracellular proteins through micropinocytosis. They observed that cancer cells expressing oncogenic Ras, an inner plasma membrane protein whose aberrant activation is associated with virtually all aspects of the malignant cancer phenotype, more highly utilize extracellular proteins as a source of amino acids to drive cellular growth [22][23]. Indeed, pancreatic ductal adenocarcinoma cells have since been shown to grow indefinitely in media lacking essential amino acids in the presence of physiologic albumin [24]. These findings provide important insight into the ability of albumin to be internalized by cancer cells in a more targeted manner. It has been additionally observed that hypoalbuminemia is a characteristic feature of patients with advanced solid tumors [25]. Decreased serum albumin in these patients may be indicative of the increased catabolism of albumin by the proliferating tumor as an abnormal source of amino acids utilized to meet high metabolic demands [26].

Some cancer cells also preferentially use receptor-mediated albumin uptake pathways in addition to upregulated use of nonspecific macropinocytic mechanisms.  This has become particularly evident based on the discovery that a correlation exists between expression of albumin-related receptors and relative efficacy of albumin-based therapies among different cancer types. Nab-paclitaxel (nab-P), also known as Abraxane, is an FDA approved albumin-bound paclitaxel particle. Recently, Chatterjee et al attempted to elucidate why certain pancreatic cancer patient populations responded better to treatment with nab-P than others [27]. It had previously been posited that SPARC (secreted protein acidic and rich in cysteine) was a critical albumin-binding protein that facilitates the efficacy of nab-P in metastatic pancreatic cancer. This idea was based on small scale retrospective studies (n=16) on nab-P treatment of head and neck cancer where the relationship was examined between SPARC tumor expression and patient outcomes [28]. This hypothesis was centered around the notion that the presence of SPARC in the tumor environment would concentrate nab-P and thus possibly enhance its therapeutic effect. However, further exploration in a phase III clinical trial on nab-P in metastatic pancreatic cancer patients demonstrated no association between SPARC level and treatment efficacy [29]. Indeed, additional preclinical studies in a mouse model of pancreatic cancer treated with nab-paclitaxel plus gemcitabine also demonstrated a lack of association with SPARC knockout and tumor progression [29]. Chatterjee et al instead examined the role that caveolae, omega-shaped invaginations of the plasma membrane, play in albumin uptake in cancer cells [30]. Caveolae have previously been implicated in a wide variety of cellular processes including endocytosis, transcytosis, and signal transmission [31][32]. The primary protein necessary for caveolae formation, Caveolin-1 (Cav-1), is upregulated in a wide variety of cancer types including pancreatic cancer, prostate cancer, and breast cancer and is associated with cancer progression [27], [33]–[35]. It has been shown in Cav-1 knockdown mice that caveolae are critical in albumin uptake and transport [36]. Interestingly, Chatterjee et al demonstrated that Cav-1 expression is critical to the entry and tumor response of nab-P in pancreatic cancer models. This new development in the mechanistic understanding of albumin-bound chemotherapeutics may aid in better identification of patients who are likely to respond to albumin-based therapies based on prescreening of tumor biopsies. By exploiting the natural affinity of albumin for tumor biodistribution and preferential tumor cell uptake, albumin provides great promise as a carrier for increasing the therapeutic efficacy of cancer drugs.

2. General Albumin Binding Strategies

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Multiple classes of albumin-based formulations have been tested for cancer-targeting therapies (Figure 3). Herein we will review two general categories- preformed albumin therapeutics and in situ binders. In situ binders can dock on to endogenous albumin after delivery into the body. Exogenous formulations, by comparison, rely on drug loading in or attachment to recombinantly produced albumin, bovine serum albumin, or human serum albumin isolated from donors prior to administration to patients. This discussion will focus specifically surveying albumin-based cancer therapeutics, but other types of clinically-mature therapeutics will be highlighted in each class as applicable.

2.1 In Situ Binders

2.1.1 Covalent Conjugation

Covalent bonds between native albuminand therapeutics can be formed in situ. The primary method for in situ covalent attachment to albumin leverages the cysteine-34 amino acid of albumin. Importantly, albumin cysteine-34 represents the most abundant free thiol in the blood, and competitive side reactions with other free thiols are not a significant concern because cysteines are typically found in nonreactive disulfide bridges. Kratz and colleagues pioneered this method for binding endogenous albumin [37]. Their approach used a maleimide carboxylic hydrazone derivative of doxorubicin to form a covalent thioether bond in situ with the cysteine-34 position of albumin. This approach is made possible by the unique properties of this amino acid residue, with approximately 70% of circulating albumin possessing the Cys-34 amino acid in its accessible form. Indeed, the authors posit that the lack of a free thiol on the majority of other circulating serum proteins makes this a relatively specific reaction. The preferential reaction with albumin is further bolstered by the low cysteine 34 pKa of 7, making it the most reactive thiol group in human plasma. The incorporation of an acid-sensitive hydrazone linker was utilized by the authors to create triggered release of the doxorubicin cargo within the highly acidic environment of endosomes and lysosomes. A variety of groups have used this approach to improve the pharmacokinetic properties of their cancer therapeutics ranging from small molecule drugs [38]–[42] to biologics [43]. Additionally, rather than employing the maleimide to sulfhydryl reaction, a disulfide bond with this free thiol can be formed to make a more readily reducible link between therapeutic cargo and native albumin. One such example involves the addition of a cysteine residue to a tumor penetrating peptide to form a disulfide bond with albumin in situ [44]. Finally, both ruthenium-based anticancer complexes and copper pro-drugs have been synthesized to bind endogenously to the large hydrophobic cavity  at the IIA subdomain of albumin, followed by subsequent exchange with the N-donor residues of Lys 199 and His 242 to form a stable albumin complex [45], [46].

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Several strategies have been employed to facilitate the liberation of the therapeutic cargo from albumin after it reaches the site of interest. In addition to the acid-labile hydrazone and reducible disulfide linkages mentioned above, another example albumin-binding prodrug incorporated a caspase cleavable peptide spacer [47] to promote drug release from albumin at sites of tumor radiotherapy.  In this design, the DEVD peptide, a well-known substrate of caspase- 3, was attached on one end to albumin cysteine-34 through a maleimide functional group and on the other end to doxorubicin by a self-immolative linker. Because the enzyme caspase-3 is upregulated during apoptotic cell death that occurs in an irradiated tumor, this design allows for targeted release of doxorubicin chemotherapy at the site of tumor radiotherapy.

An alternative liberation approach from in situ covalently bound albumin involves cathepsin cleavable linkers. Cathepsins B and D are lysosomal enzymes known to be overexpressed in a variety of malignant tumors [48]. In their prodrug, Schmid and colleagues incorporated a pentapetide linker, Ala-Leu-Ala-Leu-Ala, which is a known cathepsin cleavable sequence previously employed in targeted drug release [48]. The group conjugated ε-maleimidocaproic acid to camptothecin (a small molecule inducer of apoptosis) and doxorubicin using this linker flanked by two arginines which were incorporated to promote water solubility. A final method for liberating cancer therapeutics covalently bound to endogenous albumin leverages MMP-2 linkers. MMP-2, a matrix metalloproteinase, has been shown to be overexpressed in melanoma and plays a significant role in tumor proliferation, angiogenesis, and metastasis [49]. Mansour and colleagues introduced an octapeptide, Gly-Pro-Leu-Gly-Ile-Ala-Gly-Gln, that has been shown to be effectively cleaved by MMPs 2 and 9 when used to link doxorubicin and an albumin-binding maleimide group [49]. These covalent binding and targeted release strategies could be generalized for the delivery of a wide range of cargo including other small molecule drugs and biologics.

2.1.2 Native Ligand Conjugates

Another approach to promote in situ interaction with albumin to alter drug pharmacokinetics involves the conjugation of therapeutic agents to ligands that naturally bind albumin non-covalently in the body. For instance, fatty acids are naturally ferried by albumin in the bloodstream due to their insolubility in plasma. Indeed, albumin is known to have seven fatty binding sites distributed asymmetrically on the protein and to bind around ~0.1 mol of fatty acid per mol human serum albumin under normal physiological conditions [19]. A common approach to promoting in situ docking of therapeutics through the fatty acid binding pockets of albumin   involves direct conjugation of fatty acids to therapeutic payloads. Notably, commercially available Semaglutide (Figure 3) uses an 18-carbon fatty diacid linked to a glucagon-like peptide-1 analog via a glutamyl ethylene glycol spacer for the treatment of diabetes albumin binding K_d~0.38 nM and half-life of 46.1 hr in mini-pigs) [50]. This was a remarkable improvement compared to its precursor, Liraglutide, (16-carbon monoacid with γGlu spacer) which demonstrated an i.v. half-life of 8-10 hours. This drastic enhancement underscores the impact of both linker and binding chemistry in the use of albumin as a drug carrier. Similar noteworthy approaches have used a diacyl lipid linked by polytheylene glycol to deliver antigen/adjuvant cargo [51], palmitoyl modifications to the 2’ position of antisense oligonucleotides [52], and DNA nanocages modified with branching 12 carbon chains for encapsulation of cargo[53]. Interestingly, increasing the valency of lipid-modified drugs appears to result in superior albumin-binding and pharmacokinetics, but may hinder potency depending on modification site [54].

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In the context of cancer treatment, Sarrett et al used lipid-mediated in situ albumin binding to deliver therapeutic short interfering RNA (siRNA) to solid breast tumors. Short interfering RNA is a powerful gene silencing platform that involves the degradation of a specific mRNA, which can be used to directly target expression and phenotypes associated with a wide spectrum of cancers. However, siRNA pharmacokinetics are typically poor because it is subject to rapid renal clearance when introduced in the body. Further, unmodified siRNA has no mechanism for targeting cancer cells or reaching their cytosolic site of action. This approach involved using simple and specific “click” chemistry to conjugate modified siRNA to a PEGylated diacyl lipid, and resulted in a 5.7-fold increase in half-life [55]. Similarly, cholesterol is another native ligand that is non covalently bound to albumin in the body and has been used as a means to improve the pharmacokinetic properties of biologics [56] [57], [58]. In a study on a variety of lipophilic siRNA conjugates including cholesterol, oleyl alcohol, lithocholic acid, and oleylamide of lithocholic acid modifications, it was found that a 6 to 10 carbon aliphatic linker conferred the optimal uptake and gene silencing properties; shortening of the linker reduced efficiency of cellular uptake, whereas lengthening increased uptake but decreasing silencing efficiency in KB-8-5, HepG2, and HEK 293 cells [59]. These lipophilic moieties allow for non-covalent association of cargo which may circumvent hindered pharmacokinetic properties and allow for easier offloading into sites of interest due to reversibility.

2.1.3 Small Molecule Binders

In contrast to leveraging native ligands, synthetically produced small molecules can also be used for binding to endogenous albumin. A notable example of these small molecule binders is the dye Evans Blue (EB). Remarkably, each albumin protein can bind up to 14 Evans Blue molecules [60]. Truncated versions of this molecule have been derived to retain its albumin-binding properties but allow for modifications such as conjugation to other drugs or imaging agents[61] [62][63]. For instance, truncated EB conjugated to the anti-diabetic drug Exendin-3 resulted in markedly improved half-life (5 to 32 hours) and yielded improvement in hypoglycemic effects [64] [65]. A recent and interesting use of this platform was the development of an albumin/vaccine nanocomplex that conjugated molecular vaccines to EB derivatives for use in cancer immunity (K_d for mouse serum albumin = 1.0 µM )[66].

Another small synthetic small molecule binder was developed by Dumelin and colleagues as a “portable” albumin binder using a DNA-encoded chemical library [67]. The resulting binders were characterized by a common butanoyl moiety and require hydrophobic groups in the para position of the phenyl ring for good retention. The most successful candidate was a 4-(p-iodophenyl)butyric acid derivative that demonstrated affinity for albumin with K_d=3.2µM. Conjugation with this low molecular weight albumin binding entity has prolonged half-life of antibody fragments (~30 mins to 1000 mins) [68] and reduced renal clearance of a chelator-folate conjugate (∼70 %ID/g vs 28 %ID/g at 4 h after injection) [69]. Other groups have found that a wide variety of synthetic and naturally occurring aromatic compounds can be used for non-covalent association with albumin, which may be attributed to their hydrophobic nature. For instance, the FDA-approved MRI agent gadofosveset trisodium utilizes a 4-diphenylccyclohexyl group to reversibly bind serum albumin, resulting in extension of elimination half-life from approximately 90 to 1000 minutes [70] [71]. Modifications with these simple albumin-binding moieties are a synthetically appealing approach for improving drug pharmacokinetic properties.

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2.1.4 Albumin-Binding Domains and Nanobodies

Albumin-binding peptides have also been leveraged to non-covalently associate with albumin to improve the pharmacokinetics of cancer therapeutics. These albumin-binding domains (ABDs) are often genetically fused to recombinantly-produced therapeutic proteins. The heptapeptide WQRPSSW is an albumin-binding domain that was identified using phage display by Su and colleagues [72].  This group genetically fused the chimeric peptide BB28, a fusion of tumor homing bombesin and mitochondria disrupting peptide, to this albumin binding domain in order to extend its half-life from several minutes to 2 hours [72]. Li et al, by comparison, used a 46-amino acid albumin-binding domain derived from streptococcal protein G [73] [74]. Streptococcal protein G is a cell-surface exposed protein produced by gram-positive bacteria that is capable of binding to serum proteins, namely albumin [75]. This group sought to extend the half-life of human recombinant tumor necrosis factor-related apoptosis-inducing ligand (hTRAIL), a tumor specific inducer of apoptosis whose lack of efficacy in clinical trials has been attributed to its poor pharmacokinetic properties. Their study identified fusion of the N terminus of this 46-amino acid ABD to hTRAIL as a promising technique for improving its antitumor efficacy. The resulting conjugate bound human serum albumin with high affinity (K_d=0.4nM) and extended the half-life of hTRAIL from 0.32h to about 14.1h. Other successful examples of this technique include ABD genetic fusion to human soluble complement receptor type 1 [76], insulin-like growth factor II [77], and respiratory syncytial virus subgroup protein [78].

Monoclonal antibodies are a common means for high affinity binding to biologic targets and have had marked success in clinical translation. However, intact IgG antibodies are large (~150 kDa), and thus are considered to have limited tumor penetration capability [79][80]. This challenge has prompted investigation into monoclonal antibody fragments to achieve high affinity target binding with a molecule that is smaller in size. However, these antibody fragments can have diminished target affinity and can be too small in size to avoid renal clearance.  To overcome these obstacles, Tijink et al investigated the class known as nanobodies for binding to albumin [79]. Nanobodies are derived from the unique antibody format present in dromedaries that is comprised only of the heavy chain. These “heavy chain only” antibodies are approximately 15 kDa, but, upon the desired binding to albumin would be able to avoid renal clearance. Tijink and colleagues constructed a 50 kDa albumin-binding nanobody for the purpose of improving upon existing epidermal growth factor receptor (EGFR) antibody treatments. Their bivalent nanobody comprised two EGFR binding units and one albumin binding unit [79]. The group details that, by themselves, nanobodies are quickly excreted, making intermediate association with albumin an additionally appealing approach.  The EGFR-EGFR-Alb nanobody showed faster and deeper tumor penetration than the unmodified EGFR-EGFR binder in A431 xenograft-bearing mice with a tumor to blood ratio greater than 80 achieved after 6 hours.

Another interesting implementation of albumin-binding domains involves its fusion to an affibody molecule. Affibodies are a new class of small (~6.5 kDa) proteins with high target specificity derived from staphylococcal protein A [81].Multiple groups have sought to fuse an albumin binding domain to affibody targeting human epidermal growth factor receptors, which are overexpressed in a variety of cancers and are known to drive tumor cell proliferation [82] [83]. Interestingly, Orlova and colleagues were able to incorporate a radiolabel into these affibody molecules and suggest the potential of labeling with therapeutic radionuclides for radioimmunotherapy of solid tumors [82]. The elimination half-life of the parent affibody after fusion to the ABD was increased 80-fold from 0.5 to 41h. Similarly, a 46-amino acid ABD derived from streptococcal protein G was used to create a bispecific single-chain diabody for the retargeting of cyototoxic T cells to carcinoembryonic antigen (CEA)-expressing tumor cells by genetic fusion. This diabody was directed against the CEA and T cell receptor complex protein CD, and the resulting complex resulted in a 6 fold circulation time extension [84].

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2.2 Exogenous Formulations

2.2.1 Covalent Conjugation

Cancer drug formulation with exogenous albumin prior to delivery into the body has also been pursued in several formats, including direct, covalent conjugation of drug to albumin.  A common strategy is to conjugate therapeutic cargo to a primary amine available within a free lysine residue on albumin. This strategy has been used for the conjugation of small molecule drugs such as methotrexate [85], curcumin [86], and doxorubicin [87]. These albumin-drug conjugates used the formation of amide bonds and reductive amination for covalent attachment. The benefits to this covalent method include avoiding use of albumin’s free thiol which is not always available, and the presence of multiple lysine groups available during exogenous conjugation. However, Kuhlmann et al suggest that, although this method allows for the conjugation to multiple lysines, the absence of selectivity may compromise FcRn engagement and consequent albumin pharmacokinetics. Additionally, it may be more difficult to control the number of modifications per albumin as well as site specificity. The aforementioned binding method for conjugating drugs to the cysteine-34 of albumin can also be used for covalent binding to exogenous human serum albumin. Indeed, this method was used for the conjugation of oligodeoxynucleotides to this position to allow for the subsequent annealing of various complementary strands such as aptamers [88]. However, these exogenous albumin binding methods requires either donors or recombinant production, both of which are associated with their own challenges including possible transmission of disease and high cost.

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2.2.2 Recombinant Albumin Fusion Proteins

Direct genetic fusion of therapeutic proteins to whole recombinant albumin is another alternative. For instance, this approach was used to link the N-terminus of proaerolysin, a potent toxin, to recombinant albumin [89]. This group has termed the resulting therapeutic a “pro-toxin”. The goal is for these “pro-toxins” to only be cleaved by a defined protease that is present in the metastatic prostate cancer tumor microenvironment. The authors specifically engineered a peptide linker into their recombinant protein that was specific for the protease prostate specific antigen. Another recombinant HSA fusion protein, Albuleukin, combines recombinant interleukin-2 (rIL-2) and human serum albumin. This strategy aimed to maintain the excellent pharmacokinetic properties of albumin while conferring the immunomodulatory and anti-tumor properties of rIL-2 [90]. Other examples of this strategy include an interferon-β [91], anticarcinoembryonic antigen single-chain antibody [92], and barbourin [93]. This method is elegant in the sense that it does not require any complicated conjugation chemistry, merely the isolation of precisely-defined, recombinant protein.

2.2.3 Nanoparticle Formulations

One of the more widely explored methods that utilize albumin as a carrier for cancer therapeutics involves drug encapsulation into an exogenous albumin-based nanoparticle. The appeal of this method lies in the ability to leverage the native albumin mechanisms that facilitate its extraordinarily long half-life and cancer homing properties, while shielding therapeutic cargo until the particle is broken down at the therapeutic site of interest. The methods for synthesizing albumin nanoparticles can be generally categorized into the techniques of desolvation, emulsification, thermal gelation, and more recently, nano spray drying, and self-assembly [94] (Table 1). The most notable of these albumin nanoparticles is the aforementioned FDA approved nab-paclitaxel or Abraxane. Nab-paclitaxel synthesis involves passing a lipophilic drug and human serum albumin in an aqueous solvent through a jet under high pressure to form nab-paclitaxel nanoparticles with a mean particle size of 130 nm [95][96]. However, various albumin nanoparticle strategies have been employed for a wide variety of treatment agents and a myriad of chemical modifications. Herein, we will survey notable examples of the large pool of clinical and preclinical formulations that utilize the various aforementioned synthesis methods.