Multiple Therapeutic Applications of RBM-007, an Anti-FGF2 Aptamer
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cells Review Multiple Therapeutic Applications of RBM-007, an Anti-FGF2 Aptamer Yoshikazu Nakamura 1,2 1 Division of RNA Medical Science, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan; [email protected] 2 RIBOMIC Inc., Tokyo 108-0071, Japan Abstract: Vascular endothelial growth factor (VEGF) plays a pivotal role in angiogenesis, but is not the only player with an angiogenic function. Fibroblast growth factor-2 (FGF2), which was discovered before VEGF, is also an angiogenic growth factor. It has been shown that FGF2 plays positive pathophysiological roles in tissue remodeling, bone health, and regeneration, such as the repair of neuronal damage, skin wound healing, joint protection, and the control of hypertension. Targeting FGF2 as a therapeutic tool in disease treatment through clinically useful inhibitors has not been developed until recently. An isolated inhibitory RNA aptamer against FGF2, named RBM- 007, has followed an extensive preclinical study, with two clinical trials in phase 2 and phase 1, respectively, underway to assess the therapeutic impact in age-related macular degeneration (wet AMD) and achondroplasia (ACH), respectively. Moreover, showing broad therapeutic potential, preclinical evidence supports the use of RBM-007 in the treatment of lung cancer and cancer pain. Keywords: fibroblast growth factor 2; RNA aptamer; age-related macular degeneration; achondropla- sia; lung cancer; cancer pain Citation: Nakamura, Y. Multiple Therapeutic Applications of RBM-007, 1. Introduction an Anti-FGF2 Aptamer. Cells 2021, 10, In mammals, fibroblast growth factors (FGF) have 22 known members that exert 1617. https://doi.org/10.3390/ important functions in regulating cell proliferation, differentiation, and migration [1,2]. cells10071617 Upon binding to tyrosine kinase FGF receptors FGFR1–FGFR4, FGFs activate essential signaling pathways, such as the mitogen-activated protein kinase (MAPK)/ERK and JNK Academic Editor: Zhixiang Wang pathways, that are centrally involved in angiogenesis, tissue remodeling, and regeneration, including the repair of neuronal damage, skin wound healing, joint protection, and the Received: 1 June 2021 control of hypertension. Among the FGFs, human FGF2 is an 18-kDa non-glycosylated Accepted: 23 June 2021 Published: 28 June 2021 polypeptide composed of 146 amino acids [3]. FGF2 generally plays a positive role in bone health; however, several in vitro studies Publisher’s Note: MDPI stays neutral have demonstrated the adverse role of FGF2 in the progression of bone disease [4–7]. Recent with regard to jurisdictional claims in advances in understanding the role of FGF2 in bone formation alternatively posit that the published maps and institutional affil- pharmaceutical manipulation of FGF2 signaling may be a promising approach for bone iations. disease therapy. In the literature, a number of anti-FGF2 neutralizing monoclonal antibodies (mAbs) have been reported [8–12], while no anti-FGF2 mAbs have been investigated for the clinical potency. Focusing on the therapeutic potential in inhibiting FGF2, we developed the inhibitory RNA aptamer, RBM-007 [13]. In the literature, a few aptamers (DNA & RNA-based) Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. against FGF2 (or bFGF) were described in the 1990s, but there appeared no subsequent This article is an open access article studies on these aptamers. Aptamers are single-stranded short oligonucleotides selected distributed under the terms and in vitro from a large random sequence library, and are applicable to therapies because of conditions of the Creative Commons several pharmaceutical advantages, such as a medium size between small molecules and Attribution (CC BY) license (https:// antibodies, chemical synthesis, production cost, and low antigenicity [14]. Aptamers are creativecommons.org/licenses/by/ built on the foundation of targeted molecules using the systematic evolution of ligands 4.0/). by exponential enrichment (SELEX) [15,16]. The concept relies on the potential of short Cells 2021, 10, 1617. https://doi.org/10.3390/cells10071617 https://www.mdpi.com/journal/cells Cells 2021, 10, 1617 2 of 10 oligonucleotides to fold, in the presence of a target, unique tertiary structures that bind with high specificity and affinity. Our studies highlight the broad therapeutic potential of RBM-007 and the multifunctionality of FGF2 (Figure1) in the treatments of wet AMD, achondroplasia, cancer pain and lung cancer, respectively, while providing perspectives on these therapeutic applicabilities. Figure 1. Diverse therapeutic applications of RBM-007 (anti-FGF2 aptamer). 2. Anti-FGF2 Aptamer RBM-007 is an anti-FGF2 aptamer composed of 37 nucleotides, whose ribose 20 po- sitions are modified to resist ribonucleases, in addition to being 50-PEGylated and 30- conjugated with an inverted dT to confer an advantageous pharmacokinetic profile [13]. RBM-007 binds strongly and specifically to FGF2 and does not cross-react with other FGF family proteins or heparin-binding proteins, blocking the interaction between human FGF2 and its receptors FGFR1 through FGFR4 [16]. The dissociation constant (KD) of the non-PEGylated form of RBM-007 to human FGF2 protein is 2 pM, compared to 5, 7, and 27 pM in rat, mouse, and rabbit protein, respectively, showing the high affinity of RBM-007 for different FGF2s regardless of the species difference. 3. Therapeutic Application in Age-Related Macular Degeneration Age-related macular degeneration (AMD) is a leading cause of blindness in the United States and Europe, and the annual incidence of neovascular AMD (referred to as “wet” AMD) in white Americans is 3.5% per 1000 people aged ≥50 years [17]. Wet AMD is caused by choroidal neovascularization, which results in disabled central visual activity due to vascular leakage and exudation, along with subretinal hemorrhage, which are often associated with subretinal fibrosis (Figure1). Vascular endothelial growth factor (VEGF) plays a pivotal role in the pathogenesis of wet AMD. When secreted by the retinal pigment epithelial (RPE) cells from the basal side, VEGF stimulates choroidal blood vessels and promotes the expression of VEGF receptors [18,19]. Ultimately, these signaling networks lead to formation of new blood vessels that originate from the choroid, break through Bruch’s membrane, and infiltrate the macula [20]. The current major therapies approved by the FDA for wet AMD are pegaptanib (Macugen®, Eyetech Pharmaceuticals, New York, NY, USA), ranibizumab (Lucentis®, Roche/Genentech, San Francisco, CA, USA), aflibercept (Eylea®, Regeneron Pharmaceuti- cals, New York, NY, USA), and bevacizumab (Avastin®, Roche/Genentech, San Francisco, CA, USA) [19,21]. All of them target the same molecule, VEGF. Frequent intravitreal injec- tions of anti-VEGF drugs have been shown to be associated with great visual benefits in patients with AMD [22–24]. Nevertheless, studies into the limitations of these therapies Cells 2021, 10, 1617 3 of 10 are well documented. In clinical trials, one-third of all treatments for naïve (i.e., untreated) patients did not respond to anti-VEGF treatment [22–24]. Additionally, a large number of patients successfully treated with monthly intravitreal injections of ranibizumab relapsed with worsening of vision even after two years of therapy. Poor vision and persistent exu- dation are often associated with macular atrophy and submacular fibrotic scar formation regardless of different anti-VEGF treatments [25]. The reasons for scarring after anti-VEGF therapy are unknown. Clinical trials have also shown submacular fibrosis developing regardless of anti-VEGF treatment, causing further health complications in wet AMD patients [26]. Finally, patients’ intravitreal injections are an additional major barrier to treatment compliance. There remains an obvious need for novel therapies with alternative mechanisms of action compared to anti-VEGF treatment for wet AMD. Four decades ago, FGF2 was discovered as an angiogenic factor prior to VEGF [27] and has been implicated in the pathophysiology of both angiogenesis and fibrosis (Figure 2)[28,29]. It has been demonstrated that FGF2 stimulates the growth of vascular endothelial cells and tubular structure formation [30] in addition to promoting VEGF expression [31,32]. The angiogenic activity of FGF2 was reportedly stronger than that of VEGF in a mouse corneal micropocket assay [33,34]. However, the role of FGF2 in the progression of wet AMD has never been established. Figure 2. Schematic model of the dual role of FGF2 in angiogenesis and fibrotic scar formation in the retina. The presence of TGFb2 stimulates retinal pigment endothelial (RPE) cells to undergo epithelial- mesenchymal transformation (EMT) to fibroblasts, leading to scar formation. In parallel, FGF2 and VEGF act as vascular endothelial cell mitogens in the initiation and maturation of angiogenic vessels. Similarly, despite the possible engagement of FGF2 in fibroblast proliferation, the involvement of FGF2 in fibrotic scar formation in the retina has long been unknown. It is known that FGF2 induces epithelial-mesenchymal transformation (EMT) in the presence of transforming growth factor (TGF)-β isomers for organ remodeling during fibrogenesis, wherein the joint action of FGF2 and TGFβ1 induced EMT of tubular epithelial cells, leading to renal tubulointerstitial scarring [35]. It has also been reported that TGFβ2, not TGFβ1, is engaged in scarring associated with eye pathologies, including proliferative vitreoretinopathy [36–39]. However,