Klaus-Jürgen Schaper

Tertiary Structure of Bacterial Murein: the Scaffold Model

Although the chemical structure and physical properties of peptidoglycan have been elucidated for some time, the precise three-dimensional organization of murein has remained elusive. Earlier published computer simulations of the bacterial murein architecture modeled peptidoglycan strands in either a regular (D. Pink, J. Moeller, B. Quinn, M. Jericho, and T. Beveridge, J. Bacteriol. 182: 5925-5930, 2000) or an irregular (A. Koch, J. Theor. Biol. 204: 533-541, 2000) parallel orientation with respect to the plasma membrane. However, after integrating published experimental data on glycan chain length distribution and the degree of peptide side chain cross-linking into this computer simulation, we now report that the proposed planar network of murein appears largely dysfunctional. In contrast, a scaffold model of murein architecture, which assumes that glycan strands extend perpendicularly to the plasma membrane, was found to accommodate published experimental evidence and yield a viable stress-bearing matrix. Moreover, this model is in accordance with the well-established principle of murein assembly in vivo, i.e., sequential attachment of strands to the preexisting structure. For the first time, the phenomenon of division plane alternation in dividing bacteria can be reconciled with a computer model of the molecular architecture of murein.

Free-Wilson-Type Analysis of Non-Additive Substituent Effects on THPB Dopamine Receptor Affinity Using Artificial Neural Networks

Recently published brain dopamine D2 receptor affinity data of 15 tetrahydroprotoberberine (THPB) derivatives acting as dopamine receptor antagonists have been analyzed by two different QSAR techniques. The following main results were obtained by this analysis: In contrast to an unsuccessful Free-Wilson/Fujita-Ban analysis the investigated receptor binding data could be described by a neural network approach using only binary substructural indicator variables. The artificial/computational neural network was able to recognize that the affinity depends significantly on the simultaneous presence or absence of two or more substituents. A 4-D plot demonstrates the non-additivity/-variability of substituent effects on D2 receptor affinity.

A combined Hansch/Free-Wilson approach as predictive tool in QSAR studies on propafenone-type modulators of multidrug resistance

A series of 48 propafenone‐type modulators of multidrug resistance was synthesized and their P‐glycoprotein inhibitory activity was measured using the daunomycin efflux assay. Both a Free‐Wilson and a combined Hansch/Free‐Wilson analysis were performed using log P, partial log P and molar refraction values as Hansch descriptors. The results of the Free‐Wilson analysis show that modifications on the central aromatic ring generally influence pharmacological activity, whereby in almost all cases a decrease in MDR‐modulating potency is observed (Q2cv = 0.66). The combined approach results in equations with remarkably higher predictive power (Q2cv = 0.83), specifically molar refractivity shows high significance in all equations derived. This indicates that polar interactions also contribute to protein binding.

Substituted Xanthones as Antimycobacterial Agents

Quantitative Structure Activity‐Relationships between the antituberculous activity of a series of 61 substituted xanthones and their 13C NMR chemical shifts, lipophilicity, and molar refractivities of the substituents were investigated. In addition to these structural parameters, the test concentrations of the compounds were considered because of the varying solubility. While the multiple linear regression‐based Adaptive Least Squares analysis revealed only weak correlations between the antituberculous activity classes of the compounds and their physicochemical parameters, significantly better results were obtained by the Artificial Neural Network technique, which describes nonlinear relationships between the activity as dependent and the physicochemical parameters as independent variables.

Thermodynamic and structural aspects of sulfonamide crystals and solutions

The crystal structures of three sulfonamides with the general structure 4-NH(2)-C(6)H(4)-SO(2)NH-C(6)H(4/3)-R (R = 4-Et; 4-OMe; 5-Cl-2-Me) have been determined by X-ray diffraction. On the basis of our previous data and the results obtained a comparative analysis of crystal properties was performed: molecular conformational states, packing architecture, and hydrogen bond networks using graph set notations. The thermodynamic aspects of the sulfonamide sublimation process have been studied by investigating the temperature dependence of vapor pressure using the transpiration method. A regression equation was derived describing the correlation between sublimation entropy terms and crystal density data calculated from X-ray diffraction results. Also correlations between sublimation Gibbs energies and melting points, on the one hand, and between sublimation enthalpies and fusion enthalpies at 298 K, on the other hand, were found. These dependencies give the opportunity to predict sublimation thermodynamic parameters by simple thermo-physical experiments (fusion characteristics). Solubility processes of the compounds in water, n-hexane, and n-octanol (as phases modeling various drug delivery pathways and different types of membranes) were investigated and corresponding thermodynamic functions were calculated as well. Thermodynamic characteristics of sulfonamide solvation were evaluated. For compounds with similar structures processes of transfer from one solvent to another one were studied by a diagram method combined with analysis of enthalpic and entropic terms. Distinguishing between enthalpy and entropy, as is possible through the present approach, leads to the insight that the contribution of these terms is different for different molecules (entropy- or enthalpy-determined). Thus, in contrast to interpretation of only the Gibbs energy of transfer, being extensively used for pharmaceuticals in the form of the partition coefficient (log P), the analysis of thermodynamic functions of the transfer process provides additional mechanistic information. This may be important for further evaluation of the physiological distribution of drug molecules and may provide a better understanding of biopharmaceutical properties of drugs.

New N1-Hetarylmethylene-Substituted Amidrazones with Potential Antimycobacterial Activity

N1‐Hetarylmethylene (e.g. pyridyl, 2‐benzoxazolyl, 2‐benzofuryl) substituted picolinic acid amidrazones and pyrazine‐2‐carbox‐amidrazones were investigated. The compounds were accessible by reaction of the unsubstituted amidrazones with aldehydes. Pyrazine‐2‐carbox‐N1‐(2‐benzoxazolylmethylene)amidrazones were obtained by a new multistep reaction. The investigated amidrazones were tested for their antibacterial activity against four strains of mycobacteria (M. tuberculosis, M. avium, M. intracellulare, M. lufu) and were shown to have rather low activity.

Estimation of the chemosensitizing activity of modulators of multi-drug resistance via combined simultaneous analysis of sigmoidal dose-response curves

The potency of modulators which re‐establish sensitivity of resistant tumour cells to cytotoxic drugs is not usually described by ED50 values, even though such values are needed for comparison of modulator activity.

Development of effective drug combinations for the inhibition of multiply resistant Mycobacteria, especially of the Mycobacterium avium complex

Rationally designed combinations of rifampicin (RAMP) and thiacetazone plus isonicotinic acid hydrazide and/or ethambutol are highly effective in the treatment of patients (including HIV-positive) infected with multiply resistant mycobacteria of the Mycobacterium avium complex (MAC). Clinical results are very promising. The high efficacy of these combinations is due to the synergistic potentiation of single-drug activities. As soon as rifabutin is marketed, it should replace RAMP in the combination treatment of patients with highly RAMP-resistant MAC bacteria.

Thermodynamic aspects of solubility process of some sulfonamides

The thermodynamic aspects of solubility process of sulfonamides with the general structures C(6)H(5)-SO(2)NH-C(6)H(4)-R (R=4-NO(2); 4-Cl) and 4-NH(2)-C(6)H(4)-SO(2)NH-C(6)H(4)-R (R=4-NO(2); 4-CN; 4-Cl; 4-OMe; 4-C(2)H(5)) in water, phosphate buffer with pH 7.4 and n-octanol (as phases modeling various drug delivery pathways) were studied using the isothermal saturated method.

Experimental drugs and combination therapy.

The worldwide increase in tuberculosis, the additional problem of increasing multiple drug resistance (MDR) and the primary resistance of Mycobacterium avium requires new strategies in drug development and in therapy. The reason for development of MDR is manifold. One important factor is the change in cell wall construction which limits the penetration of the drug to the target receptor. This is supported by the observation that within a class of tuberculostatic drugs (identical mode of action) the more lipophilic derivative is more effective. In addition, it has been shown that mycobacteria within macrophages are able to synthesize additional multilamellar cell wall components. Several possibilities exist to overcome MDR. Besides improving the permeation properties of drugs, the development of synergistic drug combinations based on their special mode of action is a promising approach. This is illustrated with the highly synergistic combination of newly developed hydrazones and thiacetazone respectively with rifampicin. Chance combinations which may even lead to antagonism have to be avoided. Examples of antagonistic behavior of the combinations clofazimine-dapsone and ofloxacin-rifampicin are discussed. An optimization procedure has been developed based on the determination of the specific resistance of patient-derived mycobacteria against single drugs and their combinations. With its use, an individual optimal treatment becomes feasible. Preliminary clinical experience is encouraging.