RGD-Binding Integrins Revisited: How Recently Discovered Functions and Novel Synthetic Ligands (Re-)Shape an Ever-Evolving Field

Integrins have been extensively investigated as therapeutic targets over the last decades, which has been inspired by their multiple functions in cancer progression, metastasis, and angiogenesis as well as a continuously expanding number of other diseases, e.g., sepsis, fibrosis, and viral infections, possibly also Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2). Although integrin-targeted (cancer) therapy trials did not meet the high expectations yet, integrins are still valid and promising targets due to their elevated expression and surface accessibility on diseased cells. Thus, for the future successful clinical translation of integrin-targeted compounds, revisited and innovative treatment strategies have to be explored based on accumulated knowledge of integrin biology. For this, refined approaches are demanded aiming at alternative and improved preclinical models, optimized selectivity and pharmacological properties of integrin ligands, as well as more sophisticated treatment protocols considering dose fine-tuning of compounds. Moreover, integrin ligands exert high accuracy in disease monitoring as diagnostic molecular imaging tools, enabling patient selection for individualized integrin-targeted therapy. The present review comprehensively analyzes the state-of-the-art knowledge on the roles of RGD-binding integrin subtypes in cancer and non-cancerous diseases and outlines the latest achievements in the design and development of synthetic ligands and their application in biomedical, translational, and molecular imaging approaches. Indeed, substantial progress has already been made, including advanced ligand designs, numerous elaborated pre-clinical and first-in-human studies, while the discovery of novel applications for integrin ligands remains to be explored.

A biocompatible phosphorescence probe based on iridium(III) solvent complex for monitoring integrin expression in living cells

Visualizing and monitoring of integrin expression on or within living cells is essential for cancer diagnosis and treatment as they correlate well with tumor invasion and metastasis. Here, a phosphorescence probe, named as Ir-[HRGDH], was designed and chemically synthesized for selectively monitoring integrin expression in living cells by simply functionalizing histidine-arginine-glycine-aspartate-histidine (HRGDH) motifs with an iridium(III) solvent complex ([(pbz)2Ir(DMSO)Cl], pbz = 3-(2-pyridyl)benzoic acid, Ir-DMSO) via coordination reaction. The synthesized Ir-[HRGDH] shows strong photoluminescence (PL) emission with maximum wavelength at 483 nm and 506 nm. Absolute fluorescence quantum yield was determined to be 6.5 % and lifetime was determined to be 453 ns (67 ns, 1 %; 569 ns, 12 %; 442 ns, 87 %) for Ir-[HRGDH]. The synthesized Ir-[HRGDH], confirmed by mass spectrometry and optical technique, displayed high specificity to RGD-binding integrins and excellent biocompatibility. Good cell permeability enabled Ir-[HRGDH] to be distributed throughout the entire cell region. Cellular uptake pathways of Ir-[HRGDH] were further investigated for A549 cells and an energy-dependent endocytosis was revealed, in which the PL intensity of Ir-[HRGDH] within the cells decreased significantly by incubation at 4 °C, or uptake pathway inhibitor pretreatments. Importantly, time-dependent integrin expression within living cells stimulated by phorbol myristate acetate was successfully monitored by Ir-[HRGDH]. Particularly, Ir-[HRGDH] staining allowed for distinguishing cancer cells from normal cells based on their different intracellular levels of RGD-binding integrins. This work demonstrates that the synthesized Ir-[HRGDH] is a promising PL probe for targeting RGD-binding integrins within living cells, which can be further used for cancer diagnosis and treatment.

Structural insights into the molecular recognition of integrin αVβ3 by RGD-containing ligands: The role of the specificity-determining loop (SDL)

Integrin αVβ3 is a prominent member of the "RGD-recognizing" integrin family of cell surface receptors. αVβ3 binds to various extracellular matrix (ECM) proteins and oxysterols such as 25-hydroxycholesterol, is implicated in several diseases, including cancer metastasis, lung fibrosis, inflammation, and autoimmune diseases, and is pursued as a valuable therapeutic target. Despite enormous efforts to seek a pure antagonist, to date, no single drug candidate has successfully reached clinics due to associated partial agonism and toxicity issues. Developing effective and safe inhibitors require a thorough understanding of the molecular interactions and structural changes related to the receptor's activation and inhibition mechanisms. This study offers a comprehensive residue-residue contact and network analyses of the ligand-binding β-propeller βI domains (headpiece) based on all available experimental structures of integrin αVβ3 in unliganded, agonist-, antagonist-, and antibody-bound states. The analyses reveal many critical interactions that were not reported before and show that specific orientation and interactions of residues from the specificity-determining loop (SDL) are critical in molecular recognition and regulation. Also, the network analysis reveals that residues from the nearby allosteric site (site II) connect to the primary RGD-binding site via SDL, which likely acts as an interface between the two sites. Our results provide valuable insights into molecular interactions, structural changes, distinct features of the active and inactive headpiece conformations, the role of SDL in ligand recognition, and SDL-mediated allostery. Thus, the insights from this study may facilitate the designing of pure antagonists or site II-mediated allosteric modulators to integrin αVβ3 to treat various diseases.