Small molecules possessing the properties of biologics hold promise for a wide range of therapeutic applications. Antibody-recruiting molecules (ARMs) – ‘bifunctional’ (two-headed) agents that can hijack antibody proteins – represent one member of this group and have the potential to herald a new era in pharmaceutical design.
The revolution in biopharmaceuticals is currently underway. Sales of protein- and cell-based therapeutic agents have yielded a market value of US$99 billion in 2009 alone Citation[1], and sales of biologics are expected to continue showing robust growth in 2013 Citation[2]. Therapeutic monoclonal antibodies (mAbs) have experienced particularly rapid growth; the number of marketed mAbs has more than doubled since 2006 Citation[3], and there are almost 350 antibody-based agents undergoing clinical trials, including antibody–drug conjugates, bispecific antibodies and glycoengineered antibodies or antibody fragments Citation[4]. Given the many advantages of mAbs, their surge in popularity is not surprising. They can be developed simply and rapidly, possess unparalleled levels of selectivity for molecular targets, possess consistent, predictable pharmacokinetic profiles and can function both by antagonizing/agonizing biomolecular targets and by engaging immune effector mechanisms such as antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. In short, mAbs have rendered an impressive array of conventionally refractory drug targets now ‘druggable’, including cytokines, growth factor receptors and others Citation[5].
Despite their many enabling features, mAbs and other biologics possess certain notable drawbacks, primarily because they are large proteins Citation[6]. For example, immunologic recognition of mAb-derived peptide sequences can lead to severe anaphylactic reactions or the development of immunity to the therapeutic itself Citation[7,8]. Indeed, even fully humanized agents can elicit such responses Citation[9]. Furthermore, because of their large size and high polar surface area, protein-based therapeutics lack oral bioavailability Citation[10], and they have difficulties penetrating into tumor tissues owing to endothelial and interstitial barriers Citation[11]. Finally, due in part to high production costs, they are extremely expensive. For example, a year’s supply of Herceptin® (Genentech, CA, USA), a therapeutic antibody used in the treatment of metastatic breast cancer, costs approximately US$60,000 per patient Citation[12]. Such high costs would render the truly widespread application of antibody therapeutics economically impractical.
With the goal of mitigating these drawbacks, researchers have developed various alternatives to full-length therapeutic mAbs. These ‘next-generation’ agents have included (among others): bispecific antibodies, which are heterodimeric antibody constructs capable of recognizing two different epitopes through Fab regions as well as Fc receptors on effector cells; BiTE constructs, which are bifunctional proteins that possess dual specificity for disease-associated epitopes and the activating receptor CD3, found on T-cell surfaces; and nanobodies, which are derived from low-molecular-weight camelid immunoglobulins, and can achieve molecular weights as low as 11 kDa Citation[13]. Although these agents show great promise, since they are peptide based, they remain limited by poor oral bioavailability, low thermal stability and immunogenicity.
Low-molecular-weight organic molecules (i.e., small molecules), capable of harnessing the powerful properties of antibodies, have the potential to address all of these problems without compromising the advantages of biologics. For example antibody-recruiting molecules are bifunctional (i.e., ‘two-headed’) agents designed such that one terminus interacts with disease-relevant biomolecular targets, while the other interacts with the Fab domain of antibody proteins. ARMs have been designed for delivery as preformed complexes with antibodies, or to hijack antibodies found innately in the human bloodstream Citation[14]. Both such ARM subclasses represent exciting biomedical advances with significant therapeutic potential.
The advantages of ARMs are multifold. Compared with biologics, small molecules are generally inexpensive to produce on a large scale, readily optimized for oral bioavailability Citation[15], often stable to storage at elevated temperatures and unlikely to cause unwanted allergic or immunogenic responses Citation[16]. Moreover, because of their small size compared with antibodies and other protein-based therapeutics, ARMs are expected to penetrate more efficiently into disease-affected tissues such as solid tumors. Unlike many conventional small molecules, however, ARM constructs will neither require cell permeability nor the ability to interfere with protein–ligand interactions, to function effectively as cytotoxic agents. In order to trigger immune-mediated clearance, ARMs simply need to enhance antibody opsonization of disease-relevant target surfaces. Moreover, most ARMs are entirely nonpeptidic, and are therefore unlikely to induce unwanted immune responses, yet they are also expected to enhance trafficking of disease-specific antigens through antigen-presenting cells, thus augmenting long-lasting immunity. In this sense, ARMs have the potential to serve simultaneously as treatments and vaccines, similar to therapeutic antibodies.
Like many therapeutic mAbs, ARMs function through mechanisms that afford multiple levels of regulation; they only induce cell killing upon binding disease-causing cells in sufficiently large numbers to induce immune-cell activation. To the extent that disease-associated biomolecular targets are displayed at unusually high levels, ARMs are expected to exhibit exquisite selectivity for pathogenic cells or virus particles. This behavior stands in contrast to traditional chemotherapeutics or toxin conjugates, which can kill ‘off-target’ cells, either due to nonspecific uptake of lethal toxins or low levels of target antigen expression Citation[17]. In addition, association of small molecules with antibody proteins greatly enhances their plasma half-life, thus enabling even highly metabolically labile chemical functionality to serve effectively as targeting motifs. For example, small-molecule–antibody conjugates using a peptide-based motif targeting apo-2 are currently being evaluated in Phase II clinical trials in patients with advanced renal cell carcinoma Citation[18]. Moreover, because antibodies are inherently multivalent, their association with targeting motifs enhances binding avidity compared with simple monovalent molecules.
Although ARMs combine many of the advantages of antibody- and small-molecule-based therapeutics, certain obstacles to their widespread application still remain. For example, general methods are not yet available for identifying small-molecule-targeting ligands with affinities and/or selectivities comparable with those of antibodies. Research in the areas of high-throughput screening and rational ligand design is advancing rapidly, and is well positioned to help address this challenge in the years to come Citation[19]. Another obstacle is that the extent of variability in the levels, isotype distributions and recognition properties of endogenous antibodies is not comprehensively understood. Although available data are encouraging, and indicate that antihapten antibodies (e.g., antidinitrophenyl, anti-Gal and others) are highly prevalent among humans, future studies will be necessary to illuminate this issue Citation[14,20]. Such data, considered in light of similarities in vaccination patterns, and/or dietary and environmental antigen exposures, are likely to provide useful insights and starting points for the development of broadly applicable antibody-binding motifs.
In short, I believe that leveraging the combined efforts of immunologists and medicinal chemists will afford a straightforward path for the development of ARMs to treat a wide range of disease states. Indeed, published examples of ARMs targeting viral, bacterial and neoplastic diseases are quite encouraging, and ARMs might prove useful either as adjuncts to available treatment protocols or as single agents. One might imagine that the addition of ARMs to existing therapeutic regimens could mitigate the high costs and the untoward side-effect profiles of available therapies, thus imparting significant benefits to patients. By bridging cutting-edge developments in synthetic organic chemistry with an emerging molecular understanding of immunobiology, ARMs are well positioned to help catalyze biomedical advances both at the bench and at the bedside.
Financial & competing interests disclosure
DA Spiegel has served as a consultant for Novartis Institute for Biomedical Research, and the Spiegel laboratory has received research funding from Novartis Institute for Biomedical Research and Bristol–Myers Squibb. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
References
- Walsh G. Biopharmaceutical benchmarks 2010. Nat. Biotechnol. 28(9), 917–924 (2010).
- Goodman M. Market watch: sales of biologics to show robust growth through to 2013. Nat. Rev. Drug Discov. 8(11), 837 (2009).
- Reichert JM. Marketed therapeutic antibodies compendium. MAbs 4(3), 413–415 (2012).
- Reichert JM. Which are the antibodies to watch in 2013? MAbs 5(1), 1–4 (2013).
- Murad JP, Lin OA, Espinosa EV, Khasawneh FT. Current and experimental antibody-based therapeutics: insights, breakthroughs, setbacks and future directions. Curr. Mol. Med. 13(1), 165–178 (2013).
- Allen TM. Ligand-targeted therapeutics in anticancer therapy. Nat. Rev. Cancer 2(10), 750–763 (2002).
- Leon JA, Goldstein NI, Fisher PB. New approaches for the development and application of monoclonal antibodies for the diagnosis and therapy of human cancer. Pharmacol. Ther. 61(1–2), 237–278 (1994).
- Presta LG. Engineering of therapeutic antibodies to minimize immunogenicity and optimize function. Adv. Drug Deliv. Rev. 58(5–6), 640–656 (2006).
- Imai K, Takaoka A. Comparing antibody and small-molecule therapies for cancer. Nat. Rev. Cancer 6(9), 714–727 (2006).
- Veber DF, Johnson SR, Cheng HY, Smith BR, Ward KW, Kopple KD. Molecular properties that influence the oral bioavailability of drug candidates. J. Med. Chem. 45(12), 2615–2623 (2002).
- Christiansen J, Rajasekaran AK. Biological impediments to monoclonal antibody-based cancer immunotherapy. Mol. Cancer Ther. 3(11), 1493–1501 (2004).
- Waltz E. GlaxoSmithKline cancer drug threatens Herceptin market. Nat. Biotechnol. 23(12), 1453–1454 (2005).
- Wurch T, Pierré A, Depil S. Novel protein scaffolds as emerging therapeutic proteins: from discovery to clinical proof-of-concept. Trends Biotechnol. 30(11), 575–582 (2012).
- McEnaney PJ, Parker CG, Zhang AX, Spiegel DA. Antibody-recruiting molecules: an emerging paradigm for engaging immune function in treating human disease. ACS Chem. Biol. 7(7), 1139–1151 (2012).
- Wu AM, Senter PD. Arming antibodies: prospects and challenges for immunoconjugates. Nat. Biotechnol. 23(9), 1137–1146 (2005).
- Imai K, Takaoka A. Comparing antibody and small-molecule therapies for cancer. Nat. Rev. Cancer 6(9), 714–727 (2006).
- Carlson CB, Mowery P, Owen RM, Dykhuizen EC, Kiessling LL. Selective tumor cell targeting using low-affinity, multivalent interactions. ACS Chem. Biol. 2(2), 119–127 (2007).
- Huang H, Lai JY, Do J et al. Specifically targeting angiopoietin-2 inhibits angiogenesis, Tie2-expressing monocyte infiltration, and tumor growth. Clin. Cancer Res. 17(5), 1001–1011 (2011).
- Kodadek T. Protein binding: antibody surrogates click into place. Nat. Chem. 1(3), 183–185 (2009).
- Oyelaran O, McShane LM, Dodd L, Gildersleeve JC. Profiling human serum antibodies with a carbohydrate antigen microarray. J. Proteome Res. 8(9), 4301–4310 (2009).