Small molecule- and peptide-drug conjugates addressing integrins: A story of targeted cancer treatment

Targeted cancer treatment should avoid side effects and damage to healthy cells commonly encountered during traditional chemotherapy. By combining small molecule or peptidic ligands as homing devices with cytotoxic drugs connected by a cleavable or non-cleavable linker in peptide-drug conjugates (PDCs) or small molecule-drug conjugates (SMDCs), cancer cells and tumours can be selectively targeted. The development of highly affine, selective peptides and small molecules in recent years has allowed PDCs and SMDCs to increasingly compete with antibody-drug conjugates (ADCs). Integrins represent an excellent target for conjugates because they are overexpressed by most cancer cells and because of the broad knowledge about native binding partners as well as the multitude of small-molecule and peptidic ligands that have been developed over the last 30 years. In particular, integrin αVβ3 has been addressed using a variety of different PDCs and SMDCs over the last two decades, following various strategies. This review summarises and describes integrin-addressing PDCs and SMDCs while highlighting points of great interest.

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

Homomultimer strategy for improvement of radiolabeled peptides and antibody fragments in tumor targeting

A homomultimeric radioligand is composed of multiple identical ligands connected to the linker and radionuclide to detect a variety of overexpressed receptors on cancer cells. Multimer strategy holds great potential for introducing new radiotracers based on peptide and monoclonal antibody (mAb) derivatives in molecular imaging and therapy. It offers a reliable procedure for the preparation of biological-based targeting with diverse affinities and pharmacokinetics. In this context, we provide a useful summary and interpretation of the main results by a comprehensive look at multimeric radiopharmaceuticals in nuclear oncology. Therefore, there will be explanations for the strategy mechanisms and the main variables affecting the biodistribution results. The discussion is followed by highlights of recent work in the targeting of various types of receptors. The consequences are expressed based on comparing some parameters between monomer and multimer counterparts in each relevant section.

Development of a hydroxamamide-based bifunctional chelating agent to prepare technetium-99m-labeled bivalent ligand probes

Hydroxamamide (Ham) is a thiol-free chelating agent that forms technetium-99m (^99mTc)-complexes with a metal-to-ligand ratio of 1:2 under moderate reaction conditions. Therefore, Ham-based chelating agents will produce ^99mTc-labeled compounds with a bivalent targeting scaffold. For their universal usage, we developed a novel Ham-based bifunctional chelating agent, “Ham-Mal”, with a maleimide group that can easily conjugate with a thiol group, for to preparing ^99mTc-labeled bivalent ligand probes. Ham-Mal was synthesized by a four-step reaction, and then reacted with cysteine or c(RGDfC) to produce Ham-Cys or Ham-RGD. These precursors were reacted with ^99mTcO₄^- for 10 min under room temperature to obtain ^99mTc-(Ham-Cys)₂ and ^99mTc -(Ham-RGD)₂. The cellular uptake level of ^99mTc-(Ham-RGD)₂ by U87MG (high Integrin ɑ_vβ₃ expression) cells was significantly higher than that by PC3 (low Integrin ɑ_vβ₃ expression) cells at 60 min after the incubation, and the uptake was significantly suppressed by pre-treatment for 15 min with excess c(RGDfK) peptide. In the in vivo study with U87MG/PC3 dual xenografted BALB/c-nu mice, the radioactivity of U87MG tumor tissue was significantly higher than that of PC3 tumor tissue at 360 min after the administration of ^99mTc-(Ham-RGD)₂. These results suggest Ham-Mal may have potential as a bifunctional chelating agent for ^99mTc-labeled bivalent ligand probes.

Design of configuration-restricted triazolylated β-d-ribofuranosides: a unique family of crescent-shaped RNase A inhibitors

Seven new carbohydrate-bistriazole hybrid molecules were designed taking into consideration the crescent shaped active site of ribonuclease A (RNase A). In this case, the β-d-ribofuranose structure was used as the basic building unit; both the C1 and C4 arms protruding out towards the β-face of the tetrahydrofuran moiety of the ribose sugar provided an overall "U" shape to the basic building block. Several combinations of bistriazole moieties were constructed on the two arms of this basic building block. These mono- and/or bis-substituted 1,2,3-triazole units were linked to acidic functional groups because of the overall basic nature of the hydrolytic site of RNase A. All these compounds were efficient competitive inhibitors of RNase A with inhibition constants (Ki) in the micromolar range. In contrast to the carboxylic acid-modified hybrid molecules, molecules carrying sulfonic acids were found to be more potent because of the stronger interactions with the positively charged active site. The most efficient inhibitor of the series was the disulfonic acid-functionalized carbohydrate-bis-triazole hybrid molecule. Docking studies disclosed that the molecule, because of its well defined "U" shape with flexible arms, fits effectively in the active site; moreover, in all cases, besides the acid groups, the triazole and sugar rings also actively participated in creating the hydrogen bonding network in the cavity of the enzyme active site.

Binding of αvβ3 Integrin-Specific Radiotracers Is Modulated by Both Integrin Expression Level and Activation Status

PURPOSE

Molecular imaging of α_vβ₃ integrin has exhibited real potential to guide the appropriate use of anti-angiogenic therapies. However, an incomplete understanding of the factors that influence binding of α_vβ₃ integrin-specific radiotracers currently limits their use for assessing response to therapy in cancer patients. This study identifies two fundamental factors that modulate uptake of these radiotracers. Procedures Experiments were performed in prostate cancer (PC3) and glioblastoma (U87MG) cells, which differentially express α_vβ₃ integrin. α_vβ₃ integrin-specific radiotracers were used to investigate the effect of manipulating α_vβ₃ integrin expression or activation in cellular binding assays. β₃ integrin and α_vβ₃ integrin expression were measured by western blotting and flow cytometry, respectively. The effect of select pharmacological inhibitors on α_vβ₃ integrin activation and expression was also determined.

RESULTS

Radiotracer binding was proportional to α_vβ₃ integrin expression when it was decreased (β₃ knock-down cells) or increased, either using pharmacological inhibitors of cell signalling or by culturing cells for different times. Studies with both small molecule and arginine-glycine-aspartic acid (RGD)-based radiotracers revealed increased radiotracer binding after activation of α_vβ₃ integrin with Mn²⁺ or talin head domain. Moreover, inhibition of fundamental signalling pathways (mitogen-activated protein kinase kinase (MEK), Src and VEGFR2) decreased radiotracer binding, reflecting reduced α_vβ₃ integrin activity.

CONCLUSION

Binding of small molecule ligands and radiolabelled RGD peptides is modulated by expression and activation status of α_vβ₃ integrin. α_vβ₃ integrin-specific radiotracers can provide otherwise inaccessible information of the effect of signalling pathways on α_vβ₃ integrin. This has significant implications for assessing response to anti-angiogenic therapies in clinical studies.

Timely Visualization of the Collaterals Formed during Acute Ischemic Stroke with Fe3 O4 Nanoparticle-based MR Imaging Probe

Ischemic stroke is one of the major leading causes for long-term disability and mortality. Collateral vessels provide an alternative pathway to protect the brain against ischemic injury after arterial occlusion. Aiming at visualizing the collaterals occurring during acute ischemic stroke, an integrin α_v β₃ -specific Fe₃ O₄ -Arg-Gly-Asp (RGD) nanoprobe is prepared for magnetic resonance imaging (MRI) of the collaterals. Rat models are constructed by occluding the middle cerebral artery for imaging studies of cerebral ischemia and ischemia-reperfusion on 7.0 Tesla MRI using susceptibility-weighted imaging sequence. To show the binding specificity to the collaterals, the imaging results acquired with the Fe₃ O₄ -RGD nanoprobe and the Fe₃ O₄ mother nanoparticles, respectively, are carefully compared. In addition, an RGD blocking experiment is also carried out to support the excellent binding specificity of the Fe₃ O₄ -RGD nanoprobe. Following the above experiments, cerebral ischemia-reperfusion studies show the collateral dynamics upon reperfusion, which is very important for the prognosis of various revascularization therapies in the clinic. The current study has, for the first time, enabled the direct observation of collaterals in a quasi-real time fashion and further disclosed that the antegrade flow upon reperfusion dominates the blood supply of primary ischemic tissue during the early stage of infarction, which is significantly meaningful for clinical treatment of stroke.