Cyclic dinucleotides are second messengers in the cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) pathway, which plays an important role in recognizing tumor cells and viral or bacterial infections. They bind to the STING adaptor protein and trigger expression of cytokines via TANK binding kinase 1 (TBK1)/interferon regulatory factor 3 (IRF3) and inhibitor of nuclear factor-κB (IκB) kinase (IKK)/nuclear factor-κB (NFκB) signaling cascades. In this work, we describe an enzymatic preparation of 2′–5′,3′–5′-cyclic dinucleotides (2′3′CDNs) with use of cyclic GMP–AMP synthases (cGAS) from human, mouse, and chicken. We profile substrate specificity of these enzymes by employing a small library of nucleotide-5′-triphosphate (NTP) analogues and use them to prepare 33 2′3′CDNs. We also determine affinity of these CDNs to five different STING haplotypes in cell-based and biochemical assays and describe properties needed for their optimal activity toward all STING haplotypes. Next, we study their effect on cytokine and chemokine induction by human peripheral blood mononuclear cells (PBMCs) and evaluate their cytotoxic effect on monocytes. Additionally, we report X-ray crystal structures of two new CDNs bound to STING protein and discuss structure–activity relationship by using quantum and molecular mechanical (QM/MM) computational modeling.

The most important experimental quantity describing the thermodynamics of metal-ion binding with various (in)organic ligands, or biomolecules, is the stability constant of the complex (β). In principle, it can be calculated as the free-energy change associated with the metal-ion complexation, i.e., its uptake from the solution under standard conditions. Because this process is associated with the interactions of charged species, large values of interaction and solvation energies are in general involved. Using the standard thermodynamic cycle (in vacuo complexation and solvation/desolvation of the reference state and of the resulting complexes), one usually subtracts values of several hundreds of kilocalories per mole to obtain final results on the order of units or tens of kilocalories per mole. In this work, we use density functional theory and Møller-Plesset second-order perturbation theory calculations together with the conductor-like screening model for realistic solvation to calculate the stability constants of selected complexes--[M(NH3)4](2+), [M(NH3)4(H2O)2](2+), [M(Imi)(H2O)5](2+), [M(H2O)3(His)](+), [M(H2O)4(Cys)], [M(H2O)3(Cys)], [M(CH3COO)(H2O)3](+), [M(CH3COO)(H2O)5](+), [M(SCH2COO)2](2-)--with eight divalent metal ions (Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Hg(2+)). Using the currently available computational protocols, we show that it is possible to achieve a relative accuracy of 2-4 kcal·mol(-1) (1-3 orders of magnitude in β). However, because most of the computed values are affected by metal- and ligand-dependent systematic shifts, the accuracy of the "absolute" (uncorrected) values is generally lower. For metal-dependent systematic shifts, we propose the specific values to be used for the given metal ion and current protocol. At the same time, we argue that ligand-dependent shifts (which cannot be easily removed) do not influence the metal-ion selectivity of the particular site, and therefore it can be computed to within 2 kcal·mol(-1) average accuracy. Finally, a critical discussion is presented that aims at potential caveats that one may encounter in theoretical predictions of the stability constants and highlights the perspective that theoretical calculations may become both competitive and complementary tools to experimental measurements.

A carefully selected set of acyclic and cyclic model peptides and several other macrocycles, comprising 13 compounds in total, has been used to calibrate the accuracy of the DFT(-D3) method for conformational energies, employing BP86, PBE0, PBE, B3LYP, BLYP, TPSS, TPSSh, M06-2X, B97-D, OLYP, revPBE, M06-L, SCAN, revTPSS, BH-LYP, and ωB97X-D3 functionals. Both high- and low-energy conformers, 15 or 16 for each compound adding to 196 in total, denoted as the MPCONF196 data set, were included, and the reference values were obtained by the composite protocol, yielding the CCSD(T)/CBS extrapolated energies or their DLPNO-CCSD(T)/CBS equivalents in the case of larger systems. The latter was shown to be in near-quantitative (∼0.10-0.15 kcal·mol) agreement with the canonical CCSD(T), provided the TightPNO setting is used, and, therefore, can be used as the reference for larger systems (likely up to 150-200 atoms) for the problem studied here. At the same time, it was found that many D3-corrected DFT functionals provide results of ∼1 kcal·mol accuracy, which we consider as quite encouraging. This result implies that DFT-D3 methods can be, for example, safely used in efficient conformational sampling algorithms. Specifically, the DFT-D3/DZVP-DFT level of calculation seems to be the best trade-off between computational cost and accuracy. Based on the calculated data, we have not found any cheaper variant for the treatment of conformational energies, since the semiempirical methods (including DFTB) provide results of inferior accuracy (errors of 3-5 kcal·mol).

Cyclam derivatives bearing one geminal bis(phosphinic acid), -CH2PO2HCH2PO2H2 (H2L(1)), or phosphinic-phosphonic acid, -CH2PO2HCH2PO3H2 (H3L(2)), pendant arm were synthesized and studied as potential copper(II) chelators for nuclear medical applications. The ligands showed good selectivity for copper(II) over zinc(II) and nickel(II) ions (log KCuL = 25.8 and 27.7 for H2L(1) and H3L(2), respectively). Kinetic study revealed an unusual three-step complex formation mechanism. The initial equilibrium step leads to out-of-cage complexes with Cu(2+) bound by the phosphorus-containing pendant arm. These species quickly rearrange to an in-cage complex with cyclam conformation II, which isomerizes to another in-cage complex with cyclam conformation I. The first in-cage complex is quantitatively formed in seconds (pH ≈5, 25 °C, Cu:L = 1:1, cM ≈ 1 mM). At pH >12, I isomers undergo nitrogen atom inversion, leading to III isomers; the structure of the III-[Cu(HL(2))] complex in the solid state was confirmed by X-ray diffraction analysis. In an alkaline solution, interconversion of the I and III isomers is mutual, leading to the same equilibrium isomeric mixture; such behavior has been observed here for the first time for copper(II) complexes of cyclam derivatives. Quantum-chemical calculations showed small energetic differences between the isomeric complexes of H3L(2) compared with analogous data for isomeric complexes of cyclam derivatives with one or two methylphosphonic acid pendant arm(s). Acid-assisted dissociation proved the kinetic inertness of the complexes. Preliminary radiolabeling of H2L(1) and H3L(2) with (64)Cu was fast and efficient, even at room temperature, giving specific activities of around 70 GBq of (64)Cu per 1 μmol of the ligand (pH 6.2, 10 min, ca. 90 equiv of the ligand). These specific activities were much higher than those of H3nota and H4dota complexes prepared under identical conditions. The rare combination of simple ligand synthesis, very fast copper(II) complex formation, high thermodynamic stability, kinetic inertness, efficient radiolabeling, and expected low bone tissue affinity makes such ligands suitably predisposed to serve as chelators of copper radioisotopes in nuclear medicine.

Cyclic dinucleotides (CDNs) are second messengers that bind to the stimulator of interferon genes (STING) and trigger the expression of type I interferons and proinflammatory cytokines. Here we evaluate the activity of 3′,3′-c-di(2′F,2′dAMP) and its phosphorothioate analogues against five STING allelic forms in reporter-cell-based assays and rationalize our findings with X-ray crystallography and quantum mechanics/molecular mechanics calculations. We show that the presence of fluorine in the 2′ position of 3′,3′-c-di(2′F,2′dAMP) improves its activity not only against the wild type (WT) but also against REF and Q STING. Additionally, we describe the synthesis of the acyloxymethyl and isopropyloxycarbonyl phosphoester prodrugs of CDNs. Masking the negative charges of the CDNs results in an up to a 1000-fold improvement of the activities of the prodrugs relative to those of their parent CDNs. Finally, the uptake and intracellular cleavage of pivaloyloxymethyl prodrugs to the parent CDN is rapid, reaching a peak intracellular concentration within 2 h.