Artesunate Activation by Heme in an Aqueous Medium.

The reaction between the antimalarial drug artesunate (ATS) and ferriprotoporphyrin_(IX) (FPIX) in the presence of glutathione (GSH) has been monitored by nuclear magnetic resonance (NMR) spectroscopy. By following the disappearance of resonances of protons near the endoperoxide group in ATS, the rate at which the drug is activated can be directly measured. In an aqueous medium, the rate of ATS activation is limited by the rate of reduction of the FPIX Fe(III) center by GSH. The reaction is observed to slow dramatically in the presence of other heme binding antimalarial drugs. These findings explain the long observed antagonism between artemisinin derivatives and quinoline-based drugs. This discovery suggests that combination therapy that involves artemisinin or any of its derivatives and a quinoline-based drug may be compromised.

Relative to quinine and quinidine, their 9-epimers exhibit decreased cytostatic activity and altered heme binding but similar cytocidal activity versus Plasmodium falciparum.

The 9-epimers of quinine (QN) and quinidine (QD) are known to exhibit poor cytostatic potency against P. falciparum (Karle JM, Karle IL, Gerena L, Milhous WK, Antimicrob. Agents Chemother. 36:1538-1544, 1992). We synthesized 9-epi-QN (eQN) and 9-epi-QD (eQD) via Mitsunobu esterification-saponification and evaluated both cytostatic and cytocidal antimalarial activities. Relative to the cytostatic activity of QN and QD, we observed a large decrease in cytostatic activity (higher 50% inhibitory concentration [IC(50)s]) against QN-sensitive strain HB3, QN-resistant strain Dd2, and QN-hypersensitive strain K76I, consistent with previous work. However, we observed relatively small changes in cytocidal activity (the 50% lethal dose), similar to observations with chloroquine (CQ) analogues with a wide range of IC(50)s (see the accompanying paper [A. P. Gorka, J. N. Alumasa, K. S. Sherlach, L. M. Jacobs, K. B. Nickley, J. P. Brower, A. C. de Dios, and P. D. Roepe, Antimicrob. Agents Chemother. 57:356-364, 2013]). Compared to QN and QD, the 9-epimers had significantly reduced hemozoin inhibition efficiency and did not affect pH-dependent aggregation of ferriprotoporphyrin IX (FPIX) heme. Magnetic susceptibility measurements showed that the 9-epimers perturb FPIX monomer-dimer equilibrium in favor of monomer, and UV-visible (VIS) titrations showed that eQN and eQD bind monomer with similar affinity relative to QN and QD. However, unique ring proton shifts in the presence of zinc(II) protoporphyrin IX (ZnPIX) indicate that binding of the 9-epimers to monomeric heme is via a distinct geometry. We isolated eQN- and eQD-FPIX complexes formed under aqueous conditions and analyzed them by mass, fluorescence, and UV-VIS spectroscopies. The 9-epimers produced low-fluorescent adducts with a 2:1 stoichiometry (drug to FPIX) which did not survive electrospray ionization, in contrast to QN and QD complexes. The data offer important insight into the relevance of heme interactions as a drug target for cytostatic versus cytocidal dosages of quinoline antimalarial drugs and further elucidate a surprising structural diversity of quinoline antimalarial drug-heme complexes.

Cytostatic versus cytocidal activities of chloroquine analogues and inhibition of hemozoin crystal growth.

We report an improved, nonhazardous, high-throughput assay for in vitro quantification of antimalarial drug inhibition of ß-hematin (hemozoin) crystallization performed under conditions that are more physiological relative to previous assays. The assay uses the differential detergent solubility of crystalline and noncrystalline forms of heme and is optimized via the use of lipid catalyst. Using this assay, we quantify the effect of pH on the crystal growth-inhibitory activities of current quinoline antimalarials, evaluate the catalytic efficiencies of different lipids, and test for a possible correlation between hemozoin inhibition by drugs versus their antiplasmodial activity. Consistent with several previous reports, we found a good correlation between hemozoin inhibition potency versus cytostatic antiplasmodial potency (50% inhibitory concentration) for a series of chloroquine (CQ) analogues. However, we found no correlation between hemozoin inhibition potency and cytocidal antiplasmodial potency (50% lethal dose) for the same drugs, suggesting that cellular targets for these two layers of 4-aminoquinoline drug activity differ. This important concept is also explored further for QN and its stereoisomers in the accompanying paper (A. P. Gorka, K. S. Sherlach, A. C. de Dios, and P. D. Roepe, Antimicrob. Agents Chemother. 57:365-374, 2013).

The hydroxyl functionality and a rigid proximal N are required for forming a novel non-covalent quinine-heme complex.

Quinoline antimalarial drugs bind both monomeric and dimeric forms of free heme, with distinct preferences depending on the chemical environment. Under biological conditions, chloroquine (CQ) appears to prefer to bind to µ-oxo dimeric heme, while quinine (QN) preferentially binds monomer. To further explore this important distinction, we study three newly synthesized and several commercially available QN analogues lacking various functional groups. We find that removal of the QN hydroxyl lowers heme affinity, hemozoin (Hz) inhibition efficiency, and antiplasmodial activity. Elimination of the rigid quinuclidyl ring has similar effects, but elimination of either the vinyl or methoxy group does not. Replacing the quinuclidyl N with a less rigid tertiary aliphatic N only partially restores activity. To further study these trends, we probe drug-heme interactions via NMR studies with both Fe and Zn protoporphyrin IX (FPIX, ZnPIX) for QN, dehydroxyQN (DHQN), dequinuclidylQN (DQQN), and deamino-dequinuclidylQN (DADQQN). Magnetic susceptibility measurements in the presence of FPIX demonstrate that these compounds differentially perturb FPIX monomer-dimer equilibrium. We also isolate the QN-FPIX complex formed under mild aqueous conditions and analyze it by mass spectrometry, as well as fluorescence, vibrational, and solid-state NMR spectroscopies. The data elucidate key features of QN pharmacology and allow us to propose a refined model for the preferred binding of QN to monomeric FPIX under biologically relevant conditions. With this model in hand, we also propose how QN, CQ, and amodiaquine (AQ) differ in their ability to inhibit Hz formation.

Synthesis and antimalarial activity of new chloroquine analogues carrying a multifunctional linear side chain.

We report the synthesis and in vitro antimalarial activity of several new 4-amino- and 4-alkoxy-7-chloroquinolines carrying a linear dibasic side chain. Many of these chloroquine analogues have submicromolar antimalarial activity versus HB3 (chloroquine sensitive) and Dd2 (chloroquine resistant strain of Plasmodium falciparum) and low resistance indices were obtained in most cases. Importantly, compounds 11-15 and 24 proved to be more potent against Dd2 than chloroquine. Branching of the side chain structure proved detrimental to the activity against the CQR strain.

Antimalarial drugs and heme in detergent micelles: An NMR study.

Proton nuclear magnetic resonance relaxation times were measured for the protons of micelles formed by the detergents sodium dodecyl sulfate, dodecyltrimethyl ammonium bromide, and polyethylene glycol sorbitan monolaureate in the presence of ferriprotoporphyrin IX and the antimalarial drugs chloroquine, 7-chloro-4-quinolyl 4-N,N-diethylaminobutyl sulfide, and primaquine. Diffusion coefficients were extracted from pulsed gradient NMR experiments to evaluate the degree of association of these drugs with the detergent micelles. Results indicate that at low or neutral pH when the quinolyl N is protonated, chloroquine does not associate with neutral or cationic detergent micelles. For this reason, chloroquine's interaction with heme perturbs the partitioning of heme between the aqueous medium and detergent micelles.

Synthesis and antimalarial activity of new 4-amino-7-chloroquinolyl amides, sulfonamides, ureas and thioureas.

We report the synthesis and in vitro antimalarial activities of more than 50 7-chloro-4-aminoquinolyl-derived sulfonamides 3-8 and 11-26, ureas 19-22, thioureas 23-26, and amides 27-54. Many of the CQ analogues prepared for this study showed submicromolar antimalarial activity versus HB3 (chloroquine sensitive) and Dd2 (chloroquine resistant strains of Plasmodium falciparum) and low resistance indices were obtained in most cases. Systematic variation of the side chain length and introduction of fluorinated aliphatic and aromatic termini revealed promising leads that overcome CQ resistance. In particular, sulfonamide 3 exhibiting a short side chain with a terminal dansyl moiety combined high antiplasmodial potency with a low resistance index and showed IC(50)s of 17.5 and 22.7 nM against HB3 and Dd2 parasites.

Quinine and chloroquine differentially perturb heme monomer-dimer equilibrium.

Nuclear magnetic resonance (NMR) measurements of magnetic susceptibility have been utilized to study the equilibrium between two forms (high-spin monomer vs the antiferromagnetically coupled mu-oxo dimer) of ferriprotoporphyrin(IX) as a function of pH. The pH dependence of this equilibrium is significantly altered by the addition of either chloroquine or quinine. Chloroquine promotes the mu-oxo dimer whereas quinine promotes the monomer.

4-N-, 4-S-, and 4-O-chloroquine analogues: influence of side chain length and quinolyl nitrogen pKa on activity vs chloroquine resistant malaria.

Using predictions from heme-quinoline antimalarial complex structures, previous modifications of chloroquine (CQ), and hypotheses for chloroquine resistance (CQR), we synthesize and assay CQ analogues that test structure-function principles. We vary side chain length for both monoethyl and diethyl 4-N CQ derivatives. We alter the pKa of the quinolyl N by introducing alkylthio or alkoxy substituents into the 4 position and vary side chain length for these analogues. We introduce an additional titratable amino group to the side chain of 4-O analogues with promising CQR strain selectivity and increase activity while retaining selectivity. We solve atomic resolution structures for complexes formed between representative 4-N, 4-S, and 4-O derivatives vs mu-oxo dimeric heme, measure binding constants for monomeric vs dimeric heme, and quantify hemozoin (Hz) formation inhibition in vitro. The data provide additional insight for the design of CQ analogues with improved activity vs CQR malaria.

Overcoming drug resistance to heme-targeted antimalarials by systematic side chain variation of 7-chloro-4-aminoquinolines.

Systematic variation of the branching and basicity of the side chain of chloroquine yielded a series of new 7-chloro-4-aminoquinoline derivatives exhibiting high in vitro activity against four different strains of P. falciparum. Many of the compounds tested showed excellent potency against chloroquine sensitive and resistant strains. In particular 4b, 5a, 5b, 5d, 17a, and 17b were found to be significantly more potent than chloroquine against the resistant strains Dd2 and FCB.

Ab initio calculations of NMR chemical shifts.

The nuclear magnetic resonance chemical shift is one of the most powerful properties available for structure determination at the molecular level. A review of advances made in the ab initio calculation of chemical shielding during the past five years is presented. Specifically, progress in the areas including the effects of an unpaired electron, electron correlation, and relativistic effects into ab initio chemical shielding calculations, the tensor nature of the chemical shift, and intramolecular and intermolecular effects on the chemical shift will be covered.

Progress in 13C and 1H solid-state nuclear magnetic resonance for paramagnetic systems under very fast magic angle spinning.

High-resolution solid-state NMR (SSNMR) of paramagnetic systems has been largely unexplored because of various technical difficulties due to large hyperfine shifts, which have limited the success of previous studies through depressed sensitivity/resolution and lack of suitable assignment methods. Our group recently introduced an approach using "very fast" magic angle spinning (VFMAS) for SSNMR of paramagnetic systems, which opened an avenue toward routine analyses of small paramagnetic systems by (13)C and (1)H SSNMR [Y. Ishii et al., J. Am. Chem. Soc. 125, 3438 (2003); N. P. Wickramasinghe et al., ibid. 127, 5796 (2005)]. In this review, we discuss our recent progress in establishing this approach, which offers solutions to a series of problems associated with large hyperfine shifts. First, we demonstrate that MAS at a spinning speed of 20 kHz or higher greatly improves sensitivity and resolution in both (1)H and (13)C SSNMR for paramagnetic systems such as Cu(II)(DL-alanine)(2)H(2)O (Cu(DL-Ala)(2)) and Mn(acac)(3), for which the spectral dispersions due to (1)H hyperfine shifts reach 200 and 700 ppm, respectively. Then, we introduce polarization transfer methods from (1)H spins to (13)C spins with high-power cross polarization and dipolar insensitive nuclei enhanced by polarization transfer (INEPT) in order to attain further sensitivity enhancement and to correlate (1)H and (13)C spins in two-dimensional (2D) SSNMR for the paramagnetic systems. Comparison of (13)C VFMAS SSNMR spectra with (13)C solution NMR spectra revealed superior sensitivity in SSNMR for Cu(DL-Ala)(2), Cu(Gly)(2), and V(acac)(3). We discuss signal assignment methods using one-dimensional (1D) (13)C SSNMR (13)C-(1)H rotational echo double resonance (REDOR) and dipolar INEPT methods and 2D (13)C(1)H correlation SSNMR under VFMAS, which yield reliable assignments of (1)H and (13)C resonances for Cu(Ala-Thr). Based on the excellent sensitivity/resolution and signal assignments attained in the VFMAS approach, we discuss methods of elucidating multiple distance constraints in unlabeled paramagnetic systems by combing simple measurements of (13)C T(1) values and anisotropic hyperfine shifts. Comparison of experimental (13)C hyperfine shifts and ab initio calculated shifts for alpha- and beta-forms of Cu(8-quinolinol)(2) demonstrates that (13)C hyperfine shifts are parameters exceptionally sensitive to small structural difference between the two polymorphs. Finally, we discuss sensitivity enhancement with paramagnetic ion doping in (13)C SSNMR of nonparamagnetic proteins in microcrystals. Fast recycling with exceptionally short recycle delays matched to short (1)H T(1) of approximately 60 ms in the presence of Cu(II) doping accelerated 1D (13)C SSNMR for ubiquitin and lysozyme by a factor of 7.3-8.4 under fast MAS at a spinning speed of 40 kHz. It is likely that the VFMAS approach and use of paramagnetic interactions are applicable to a variety of paramagnetic systems and nonparamagnetic biomolecules.

Relationship between NMR shielding and heme binding strength for a series of 7-substituted quinolines.

Chemical shielding tensors are calculated for the carbons in a series of 4-aminoquinolines with different substituents at the 7-position. The sigma(11) component is used as a measure of the relative pi-electron density at each carbon. By comparing the pi-electron density at each carbon with the log K of binding to heme (Kaschula et al. J. Med. Chem. 2002, 45, 3531), the drug-heme association is found to increase with increasing pi-electron density at the carbons meta to the substituent and with decreasing pi-electron density at the carbons ortho and para to the substituent. The greatest change in pi-electron density is at the ortho carbons, and log K increases with a decrease in pi-electron density on the ring containing the substituent, which corresponds to an increase in the pi-dipole between the two rings. An examination of the solution structures of the pi-pi complexes formed by amodiaquine and quinine with heme (Leed et al. Biochemistry 2002, 41, 10245. de Dios et al. Inorg. Chem. 2004, 43, 8078) shows that the pi-dipoles in each drug and in the porphyrin ring of heme may be paired. The chloro-substituted compound has an association constant that is an order of magnitude higher than the other compounds in the series, but the pi-electron density at the ring containing the substituent is not correspondingly low. This lack of correlation indicates that the Cl-substituted compound may be binding to heme in a manner that differs from the other compounds in the series.

Interactions between pairs of antimalarial drugs studied by experimental and ab initio (13)C NMR chemical shifts.

The interactions between the antimalarial drugs chloroquine (CQ) and amodiaquine (AQ), chloroquine and quinine (QN), and amodiaquine and quinine are studied by (13)C NMR. Experimental changes in chemical shift are compared to nucleus-independent chemical shifts to determine the best structure of the complex formed by each drug pair in solution. Structures of the CQ-AQ and CQ-QN complexes are found to be similar to those found previously for the drug dimers. On the other hand, the best solution structure for the AQ-QN complex suggests that the quinoline rings of the two drugs are at an angle with respect to each other.

Carbon chemical shift tensor components in quinolines and quinoline N-oxides.

Chemical shift calculations are carried out for the quinoline carbons in 1,8-bis(2-isopropyl-4-quinolyl)naphthalene, 2-isopropylquinoline, amodiaquine, chloroquine, and quinine and the N-oxide of each compound. Ab initio calculations of the isotropic shielding values are in agreement with experimental chemical shifts. The calculations indicate that changes to the principal components of the shielding tensor upon N-oxidation are similar for each compound. Carbons 2, 4, 8, and 10 are largely shielded in each case as the nitrogen is oxidized. For C2, C4, and C10, this shielding is due to a large change in sigma11 and/or sigma22, indicating a change in pi-electron density. For C8, the large shielding change is due mainly to a change in sigma33, indicating a change in sigma-electron density. Upon examination and comparison of the calculated 13C shielding tensor components in the antimalarial drugs versus those in unsubstituted quinolines, it is found that amodiaquine and chloroquine have increased pi-electron density in the ring containing the amino side chain and quinine has increased pi-electron density in the opposite ring, containing the methoxy substituent.

Correlations between 31P chemical shift anisotropy and molecular structure in polycrystalline O,O'-dialkyldithiophosphate zinc(II) and nickel(II) complexes: 31P CP/MAS NMR and ab initio quantum mechanical calculation studies.

Different potassium salts and zinc(II) and nickel(II) O,O'-dialkyldithiophosphate complexes were studied by solid-state 31P CP/MAS and static NMR and ab initio quantum mechanical calculations. Spectra were obtained at different spinning frequencies, and the intensities of the spinning sidebands were used to estimate the chemical shift anisotropy parameters. Useful correlations between the shapes of the 31P chemical shift tensor and the type of ligand were found: terminal ligands have negative values of the skew kappa, while bridging and ionic ligands have positive values for this parameter. The experimental results were compared with known X-ray diffraction structures for some of these complexes as well as with ab initio quantum mechanical calculations, and a useful correlation between the delta22 component of the 31P chemical shift tensor and the S-P-S bond angle in the O,O'-dialkyldithiophoshate zinc(II) and nickel(II) complexes was found: delta22 increases more than 50 ppm with the increase of S-P-S bond angle from ca. 100 degrees to 120 degrees , while the other two principal values of the tensor, delta11 and delta33, are almost conserved. This eventually leads to the change in sign for kappa in the bridging type of ligand, which generally has a larger S-P-S bond angle than the terminally bound O,O'-dialkyldithiophosphate group forming chelating four-membered P(ss)Me heterocycles.

Structure of the amodiaquine-FPIX mu oxo dimer solution complex at atomic resolution.

Using NMR inversion recovery experiments and XPLOR distance restraint calculations, we recently deduced the structure of ferriprotoporphyrin IX (FPIX) heme mu oxo dimer-antimalarial drug complexes for chloroquine (CQ), quinine (QN), and quinidine (QD) at atomic resolution [A. Leed et al., Biochemistry 2002, 41, 10245-55]. Using similar methods, we now report an unexpected structure for the complex formed between FPIX and the related drug amodiaquine (AQ). The deduced structure is further supported by comparing AQ chemical-shift data to restricted Hartree-Fock calculations. The structure further highlights the critical nature of quinoline drug side-chain composition in stabilizing noncovalent association to FPIX. Heme Fe-AQ proton distances are longer, relative to those of the CQ complex, and the AQ aromatic side chain seems to have a significant role in stabilizing the complex. Relative to the FPIX-CQ complex, a similar 2:1 stoichiometry was determined for the AQ complex, in contrast to a 4:1 stoichiometry previously suggested from calorimetry data. These solution structures add to our rapidly growing understanding of the mechanism of quinoline antimalarial drug action and will help elucidate the mechanism(s) of quinoline antimalarial drug resistance phenomena.

NMR studies of peptide T, an inhibitor of HIV infectivity, in an aqueous environment.

The synthetic octapeptide peptide T (ASTTTNYT) has been shown to interfere with binding of the HIV-1 envelope glycoprotein gp120 to the chemokine receptor R5, thus preventing viral infection. This study investigated the degree of conformational order of two analogs of peptide T, one biologically active (D-Ala peptide T amide) and one inactive (D-Ala, D-Tyr peptide T amide) using nuclear magnetic resonance (NMR) spectroscopy in an aqueous environment, both in solution and in the frozen solid state. Standard solution NMR techniques such as DQFCOSY, HMQC, ROESY and inversion recovery measurements have been utilized to characterize these peptides. Solid state NMR experiments were likewise employed to study the peptides in a frozen glycerol:water mixture. The NMR results indicate that the monomeric form of both peptide T analogs have considerable conformational heterogeneity. Solid state NMR studies indicate aggregation of D-Ala peptide T, possibly into a beta-sheet structure, at concentrations higher than 10 mM.

13C NMR spectroscopy of 13C1-labeled octanethiol-protected Au nanoparticles: shifts, relaxations, and particle-size effect.

We report here an investigation of metal-ligand interactions in nanoparticles with 13C NMR, using a labeled 13C1-octanethiol, a protecting ligand for self-assembled monolayer (SAM) systems, in which close proximity of the 13C1 to the metal surface serves as an effective probe for the changing electronic environment. Several remarkable results have been obtained: as the metal core size increases from 1.6 to 4.0 nm, the 13C1 spectrum is downshifted from 40.5 to 53 ppm, and the spin-spin relaxation rate, T2-1, increases while the spin-lattice relaxation ratio decreases. Although the spin-lattice relaxation may be due to particle tumbling and ligand motion in the liquid state, the main source of the spin-spin relaxation, and NMR shift, is most possibly due to the changing electronic properties of the metal core.