Lysine-Derived Charge-Altering Releasable Transporters: Targeted Delivery of mRNA and siRNA to the Lungs

Targeted delivery of nucleic acid therapeutics to the lungs could transform treatment options for pulmonary disease. We have previously developed oligomeric charge-altering releasable transporters (CARTs) for in vivo mRNA transfection and demonstrated their efficacy for use in mRNA-based cancer vaccination and local immunomodulatory therapies against murine tumors. While our previously reported glycine-based CART-mRNA complexes (G-CARTs/mRNA) show selective protein expression in the spleen (mouse, >99%), here, we report a new lysine-derived CART-mRNA complex (K-CART/mRNA) that, without additives or targeting ligands, shows selective protein expression in the lungs (mouse, >90%) following systemic IV administration. We further show that by delivering siRNA using the K-CART, we can significantly decrease expression of a lung-localized reporter protein. Blood chemistry and organ pathology studies demonstrate that K-CARTs are safe and well-tolerated. We report on the new step economical, organocatalytic synthesis (two steps) of functionalized polyesters and oligo-carbonate-co-α-aminoester K-CARTs from simple amino acid and lipid-based monomers. The ability to direct protein expression selectively in the spleen or lungs by simple, modular changes to the CART structure opens fundamentally new opportunities in research and gene therapy.

Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols

The palladium complex [(L₁)Pd(μ-OAc)]₂[OTf]₂ (L₁ = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2'-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L₂)Pd(μ-OAc)]₂[OTf]₂ (L₂ = 2,2'-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L₂ were a consequence of precipitation of an insoluble trinuclear palladium species─(L₂Pd)₃(μ-O)₂²⁺─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L₃ = 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear (LPd)₃(μ-O)₂²⁺ species. This design strategy was validated as catalytic aerobic oxidation with [(L₃)Pd(μ-OAc)]₂[OTf]₂ is both robust and rapid, marrying the kinetics of the parent L₁-supported system with the high aerobic turnover numbers of the L₂-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.

Fingolimod-Conjugated Charge-Altering Releasable Transporters Efficiently and Specifically Deliver mRNA to Lymphocytes In Vivo and In Vitro

Charge-altering releasable transporters (CARTs) are a class of oligonucleotide delivery vehicles shown to be effective for delivery of messenger RNA (mRNA) both in vitro and in vivo. Here, we exploited the chemical versatility of the CART synthesis to generate CARTs containing the small-molecule drug fingolimod (FTY720) as a strategy to increase mRNA delivery and expression in lymphocytes through a specific ligand-receptor interaction. Fingolimod is an FDA-approved small-molecule drug that, upon in vivo phosphorylation, binds to the sphingosine-1-phosphate receptor 1 (S1P1), which is highly expressed on lymphocytes. Compared to its non-fingolimod-conjugated analogue, the fingolimod-conjugated CART achieved superior transfection of activated human and murine T and B lymphocytes in vitro. The higher transfection of the fingolimod-conjugated CARTs was lost when cells were exposed to a free fingolimod before transfection. In vivo, the fingolimod-conjugated CART showed increased mRNA delivery to marginal zone B cells and NK cells in the spleen, relative to CARTs lacking fingolimod. Moreover, fingolimod-CART-mediated mRNA delivery induces peripheral blood T-cell depletion similar to free fingolimod. Thus, we show that functionalization of CARTs with a pharmacologically validated small molecule can increase transfection of a cellular population of interest while conferring some of the targeting properties of the conjugated small molecule to the CARTs.

Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring p Ka Effects and Demonstrating Electrocatalysis

Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N2, CO2, and O2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P3BFe+, for catalyzing the nitrogen reduction reaction (N2RR, N2-to-NH3 conversion) versus HER changes as a function of acid p Ka. We find that there is a strong correlation between p Ka and N2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N2 reduction (e.g., Fe(NNH) or Fe(NNH2)) in the presence of the metallocene reductant Cp*2Co. This suggests that the interaction of acid and reductant is playing a critical role in N-H bond-forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N-H bonds critical for N2RR. Furthermore, DFT studies also suggest that the observed p Ka effect on N2RR efficiency is attributable to the rate and thermodynamics of Cp*2Co protonation by the different anilinium acids. Inclusion of Cp*2Co+ as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH3. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv of NH3 per Fe at -2.1 V vs Fc+/0).

Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

We have recently reported on several Fe catalysts for N₂-to-NH₃ conversion that operate at low temperature (-78 °C) and atmospheric pressure while relying on a very strong reductant (KC₈) and acid ([H(OEt₂)₂][BAr^F₄]). Here we show that our original catalyst system, P₃^BFe, achieves both significantly improved efficiency for NH₃ formation (up to 72% for e^- delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH₃ per Fe site), when employing a significantly weaker combination of reductant (Cp*₂Co) and acid ([Ph₂NH₂][OTf] or [PhNH₃][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido-hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system.

A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench (57)Fe Mössbauer Data, and a Hydride Resting State

The mechanisms of the few known molecular nitrogen-fixing systems, including nitrogenase enzymes, are of much interest but are not fully understood. We recently reported that Fe-N2 complexes of tetradentate P3(E) ligands (E = B, C) generate catalytic yields of NH3 under an atmosphere of N2 with acid and reductant at low temperatures. Here we show that these Fe catalysts are unexpectedly robust and retain activity after multiple reloadings. Nearly an order of magnitude improvement in yield of NH3 for each Fe catalyst has been realized (up to 64 equiv of NH3 produced per Fe for P3(B) and up to 47 equiv for P3(C)) by increasing acid/reductant loading with highly purified acid. Cyclic voltammetry shows the apparent onset of catalysis at the P3(B)Fe-N2/P3(B)Fe-N2(-) couple and controlled-potential electrolysis of P3(B)Fe(+) at -45 °C demonstrates that electrolytic N2 reduction to NH3 is feasible. Kinetic studies reveal first-order rate dependence on Fe catalyst concentration (P3(B)), consistent with a single-site catalyst model. An isostructural system (P3(Si)) is shown to be appreciably more selective for hydrogen evolution. In situ freeze-quench Mössbauer spectroscopy during turnover reveals an iron-borohydrido-hydride complex as a likely resting state of the P3(B)Fe catalyst system. We postulate that hydrogen-evolving reaction activity may prevent iron hydride formation from poisoning the P3(B)Fe system. This idea may be important to consider in the design of synthetic nitrogenases and may also have broader significance given that intermediate metal hydrides and hydrogen evolution may play a key role in biological nitrogen fixation.

Evaluating molecular cobalt complexes for the conversion of N2 to NH3

Well-defined molecular catalysts for the reduction of N2 to NH3 with protons and electrons remain very rare despite decades of interest and are currently limited to systems featuring molybdenum or iron. This report details the synthesis of a molecular cobalt complex that generates superstoichiometric yields of NH3 (>200% NH3 per Co-N2 precursor) via the direct reduction of N2 with protons and electrons. While the NH3 yields reported herein are modest by comparison to those of previously described iron and molybdenum systems, they intimate that other metals are likely to be viable as molecular N2 reduction catalysts. Additionally, a comparison of the featured tris(phosphine)borane Co-N2 complex with structurally related Co-N2 and Fe-N2 species shows how remarkably sensitive the N2 reduction performance of potential precatalysts is. These studies enable consideration of the structural and electronic effects that are likely relevant to N2 conversion activity, including the π basicity, charge state, and geometric flexibility.

Inorganic click (iClick) synthesis of heterotrinuclear Pt(II)/Au(I)2 complexes

Metal-azide-metal-acetylide cycloaddition (iClick) reactions to synthesize heterotrimetallics and an unexpected novel tetranuclear gold(I) complex, are described. In addition, a discussion regarding the connection between traditional azide-alkyne cycloaddition reactions and iClick is presented focusing on applications towards linking multiple metal ions.