Design of BET Inhibitor Bottlebrush Prodrugs with Superior Efficacy and Devoid of Systemic Toxicities.

Prodrugs engineered for preferential activation in diseased versus normal tissues offer immense potential to improve the therapeutic indexes (TIs) of preclinical and clinical-stage active pharmaceutical ingredients that either cannot be developed otherwise or whose efficacy or tolerability it is highly desirable to improve. Such approaches, however, often suffer from trial-and-error design, precluding predictive synthesis and optimization. Here, using bromodomain and extra-terminal (BET) protein inhibitors (BETi)-a class of epigenetic regulators with proven anticancer potential but clinical development hindered in large part by narrow TIs-we introduce a macromolecular prodrug platform that overcomes these challenges. Through tuning of traceless linkers appended to a "bottlebrush prodrug" scaffold, we demonstrate correlation of in vitro prodrug activation kinetics with in vivo tumor pharmacokinetics, enabling the predictive design of novel BETi prodrugs with enhanced antitumor efficacies and devoid of dose-limiting toxicities in a syngeneic triple-negative breast cancer murine model. This work may have immediate clinical implications, introducing a platform for predictive prodrug design and potentially overcoming hurdles in drug development.

Pro-organic radical contrast agents ("pro-ORCAs") for real-time MRI of pro-drug activation in biological systems.

Nitroxide-based organic-radical contrast agents (ORCAs) are promising as safe, next-generation magnetic resonance imaging (MRI) tools. Nevertheless, stimuli-responsive ORCAs that enable MRI monitoring of prodrug activation have not been reported; such systems could open new avenues for prodrug validation and image-guided drug delivery. Here, we introduce a novel "pro-ORCA" concept that addresses this challenge. By covalent conjugation of nitroxides and drug molecules (doxorubicin, DOX) to the same brush-arm star polymer (BASP) through chemically identical cleavable linkers, we demonstrate that pro-ORCA and prodrug activation, i.e., ORCA and DOX release, leads to significant changes in MRI contrast that correlate with cytotoxicity. This approach is shown to be general for a range of commonly used linker cleavage mechanisms (e.g., photolysis and hydrolysis) and release rates. Pro-ORCAs could find applications as research tools or clinically viable "reporter theranostics" for in vitro and in vivo MRI-correlated prodrug activation.

An N-Heterocyclic-Carbene-Derived Distonic Radical Cation.

We present the discovery of a novel radical cation formed through one-electron oxidation of an N-heterocyclic carbene-carbodiimide (NHC-CDI) zwitterionic adduct. This compound possesses a distonic electronic structure (spatially separate spin and charge regions) and displays persistence under ambient conditions. We demonstrate its application in a redox-flow battery exhibiting minimal voltage hysteresis, a flat voltage plateau, high Coulombic efficiency, and no performance decay for at least 100 cycles. The chemical tunability of NHCs and CDIs suggests that this approach could provide a general entry to redox-active NHC-CDI adducts and their persistent radical ions for various applications.

Polyoxazoline-Based Bottlebrush and Brush-Arm Star Polymers via ROMP: Syntheses and Applications as Organic Radical Contrast Agents.

The synthesis of functional poly(2-alkyl-2-oxazoline) (PAOx) copolymers with complex nanoarchitectures using a graft-through ring-opening metathesis polymerization (ROMP) approach is described. First, well-defined norbornene-terminated poly(2-ethyl-2-oxazoline) (PEtOx) macromonomers (MM) were prepared by cationic ringopening polymerization. ROMP of these MMs produced bottlebrush copolymers with PEtOx side chains. In addition, PEtOx-based branched MMs bearing a terminal alkyne group were prepared and conjugated to an azide-containing bis-spirocyclohexyl nitroxide via Cu-catalyzed azide-alkyne cycloaddition (CuAAC). ROMP of this branched MM, followed by in situ cross-linking, provided PEtOx-based brush-arm star polymers (BASPs) with nitroxide radicals localized at the core-shell interface. These PEtOx-based nitroxide-containing BASPs displayed relaxivity values on par with state-of-the-art polyethylene glycol (PEG)-based nitroxide materials, making them promising as organic radical contrast agents for metal-free magnetic resonance imaging (MRI).

Thermally and Magnetically Robust Triplet Ground State Diradical.

High spin ( S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, Δ EST ≥ 1.7 kcal mol-1, leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at ∼160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 ( S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J'/ k = -14 K, which is associated with the N···N and N···O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains of 2 most isotropic, and therefore an excellent system for studies of low-dimensional magnetism. In polystyrene glass and in frozen benzene or dibutyl phthalate solution, both 1 and 2 are monomeric. Diradical 2 is thermally robust and is evaporated under ultrahigh vacuum to form thin films of intact diradicals on silicon substrate, as demonstrated by X-ray photoelectron spectroscopy. Based on C-K NEXAFS spectra and AFM images of the ∼1.5 nm thick films, the diradical molecules form islands on the substrate with molecules stacked approximately along the crystallographic a-axis. The films are stable under ultrahigh vacuum for at least 60 h but show signs of decomposition when exposed to ambient conditions for 7 h.

Interfacing a Potential Purely Organic Molecular Quantum Bit with a Real-Life Surface.

By using a multidisciplinary and multitechnique approach, we have addressed the issue of attaching a molecular quantum bit to a real surface. First, we demonstrate that an organic derivative of the pyrene-Blatter radical is a potential molecular quantum bit. Our study of the interface of the pyrene-Blatter radical with a copper-based surface reveals that the spin of the interface layer is not canceled by the interaction with the surface and that the Blatter radical is resistant in presence of molecular water. Although the measured pyrene-Blatter derivative quantum coherence time is not the highest value known, this molecule is known as a "super stable" radical. Conversely, other potential qubits show poor thin film stability upon air exposure. Therefore, we discuss strategies to make molecular systems candidates as qubits competitive, bridging the gap between potential and real applications.

Triply Loaded Nitroxide Brush-Arm Star Polymers Enable Metal-Free Millimetric Tumor Detection by Magnetic Resonance Imaging.

Nitroxides occupy a privileged position among plausible metal-free magnetic resonance imaging (MRI) contrast agents (CAs) due to their inherently low-toxicity profiles; nevertheless, their translational development has been hindered by a lack of appropriate contrast sensitivity. Nanostructured materials with high nitroxide densities, where each individual nitroxide within a macromolecular construct contributes to the image contrast, could address this limitation, but the synthesis of such materials remains challenging. Here, we report a modular and scalable synthetic approach to nitroxide-based brush-arm star polymer (BASP) organic radical CAs (ORCAs) with high nitroxide loadings. The optimized ∼30 nm diameter "BASP-ORCA3" displays outstanding T2 sensitivity with a very high molecular transverse relaxivity ( r2 > 1000 mM-1 s-1). BASP-ORCA3 further exhibits excellent stability in vivo, no acute toxicity, and highly desirable pharmacokinetic and biodistribution profiles for longitudinal detection of tumors by MRI. When injected intravenously into mice bearing subcutaneous plasmacytomas, BASP-ORCA3 affords distinct in vivo visualization of tumors on translationally relevant time scales. Leveraging its high sensitivity, BASP-ORCA3 enables efficient mapping of tumor necrosis, which is an important biomarker to predict therapeutic outcomes. Moreover, BASP-ORCA3 allows for detection of millimetric tumor implants in a disseminated murine model of advanced-stage human ovarian cancer that possess genetic, histological, and vascular characteristics that are similar to those seen in patients. This work establishes BASP-ORCA3 as a promising metal-free spin contrast agent for MRI.

Scalable Synthesis of Multivalent Macromonomers for ROMP.

The polymerization of functional monomers provides direct access to functional polymers without need for postpolymerization modification; however, monomer synthesis can become a bottleneck of this approach. New methods that enable rapid installation of functionality into monomers for living polymerization are valuable. Here, we report the three-step convergent synthesis (two-step longest linear sequence) of a divalent exo-norbornene imide capable of efficient coupling with various nucleophiles and azides to produce diversely functionalized branched macromonomers optimized for ring-opening metathesis polymerization (ROMP). In addition, we describe an efficient iterative procedure for the synthesis of tri-and tetra-valent branched macromonomers. We demonstrate the use of these branched macromonomers for the synthesis of Janus bottlebrush block copolymers as well as for the generation of bottlebrush polymers with up to three conjugated small molecules per repeat unit. This work significantly expands the scalability and diversity of nanostructured macromolecules accessible via ROMP.

High-Spin Organic Diradical with Robust Stability.

Triplet ground-state organic molecules are interesting with respect to several emerging technologies but typically exhibit limited stability. We report two organic diradicals, one of which possesses a triplet ground state (2J/kB = 234 ± 36 K) and robust stability at elevated temperatures. We are able to sublime this high-spin diradical under high vacuum at 140 °C with no significant decomposition.

High-spin organic molecules.

Organic molecules with high-spin ground states, besides being fundamentally interesting, possess numerous potential applications in diverse fields such organic magnetism, MRI contrast agents, and spintronics. Such molecules, once thought to exist only as highly reactive intermediates, can be rationally designed to have adequate stability for organic synthesis and characterization. This Synopsis provides an overview of the factors that lead to high-spin ground states as well as recent progress in the design and synthesis of high-spin organic molecules.

Aromatic triazole foldamers induced by C-H···X (X = F, Cl) intramolecular hydrogen bonding.

Aryl-triazole oligomers based on isobutyl 4-fluorobenzoate and isobutyl 4-chlorobenzoate were designed and synthesized. Crystal structure and (1)H-(1)H NOESY experiments demonstrate that the oligomers adopt stable helical conformation, which are induced by C(5)-H···X-C (X = F, Cl) intramolecular hydrogen bonding between triazole protons and halogen atoms. The stabilities of the folded conformations are confirmed by DFT calculations, which show that each C(5)-H···F-C planar interaction lowers the energy by ~3 kcal mol(-1) on average, and by ~1 kcal mol(-1) when C(5)-H···Cl-C bridges are formed. The hydrogen-bonding networks are disrupted in competitive hydrogen-bonding media such as DMSO, generating the unfolded oligomers.