Development of a new quantitative approach for the isobolographic assessment of the convulsant interaction between pefloxacin and theophylline in rats.

PURPOSE: A new mathematical approach was developed to quantify convulsant interaction between pefloxacin and theophylline in rats. METHODS: Animals received each compound separately or in different combination ratios. Infusion was stopped at the onset of maximal seizures. Cerebrospinal fluid (CSF) and plasma samples were collected for HPLC drug determination. The nature and intensity of the pharmacodynamic (PD) interaction between drugs was assessed with a new modeling approach which includes (a) data transformation to create an essentially error-free X-variable and (b) estimation of an interaction parameter a by fitting a nonlinear hyperbolic model to the combination data with unweighted nonlinear regression. RESULTS: Drug disposition to the biophase was linear within the range of administered doses. The estimates of a suggested a Loewe antagonistic interaction between pefloxacin and theophylline at the induction of maximal seizures in rats. Similar intensity of PD interaction was observed at the dose and biophase level (alpha was -0.415 +/- 0.069 and -0.567 +/- 0.079, respectively). CONCLUSIONS: The suitability of the proposed model was assessed by Monte Carlo simulation. This new mathematical approach enabled the characterization of the Loewe antagonistic nature of the PD (convulsant) interaction between pefloxacin and theophylline, whereas previously used methodologies failed to do so.

A model allowing the design of modified nucleosides as HIV-RT inhibitors.

A chemical, structural, molecular electrostatic potential (MEP) analysis of modified nucleosides allows the understanding of how nucleosides interact with different receptors. The interaction with kinases is sensitive to base modifications, while the interaction with the reverse transcriptase receptor HIV active site is more affected by ribose modifications. The model herein indicates a geometrical lower limit in the width of the modified sugar that corresponds to the 3' erythro position. This characteristic allows one to predict a potential activity of the 3' substituted compounds. The 4'-hydroxymethyl group position with respect to the nucleic base is also important for antiviral activity. The model gives the geometric parameters of this position (related to kinetic effects) that corresponds to an increase in the activation energy required to fit the active site of the kinases and the RT. Our model is compatible with the 3D structure of the HIV RT active site. It allows the design of potent new active compounds where the sugar can be substituted by any group answering to the defined parameters.

Synthesis and antiviral activity of new carbonylphosphonate 2',3'-dideoxy-3'-thiacytidine conjugates.

The synthesis of new potential PFA-BCH-189 conjugate analogues is described and their molecular structure clearly identified through NMR and mass spectra techniques. The anti-HIV-1 activity was determined according to the inhibition of syncytium formation in MT-4 cells, while the anti-HBV activity was determined in infected duck hepatocytes. Both antiviral activities of the PFA-BCH-189 conjugates were much lower than those of the parent BCH-189 (2',3'-dideoxy-3'-thiacytidine) (1). Whereas a prodrug effect, following cleavage and release of the free BCH-189 and PFA, cannot be ruled out, poor cellular permeation of the drug seems to be the most likely reason for the reduced activities against HIV and DHBV. The presence of the PFA moiety appears to be detrimental for both the anti-HIV and anti-DHBV activity of PFA-BCH-189 cases.

Inhibition of human immunodeficiency virus type 1 replication by phosphonoformate- and phosphonoacetate-2',3'-dideoxy-3'-thiacytidine conjugates.

The synthesis of potential "combined prodrugs" where phosphonoformic acid (PFA) or phosphonoacetic acid (PAA) was attached to the 5'-O or N4 position of 2',3'-dideoxy-3'-thiacytidine (BCH-189) is described. The anti-HIV-1 activity of 11 analogues which included carboxylic ester or phosphoric ester linkages of PFA or PAA to BCH-189 was determined in MT-4 cells. Of these compounds, the IC50 of analogues 3, 4, 6, and 7 ranged from 0.2 to 100 microM, while IC50 for BCH-189 in this system was 0.1 microM. In vitro hydrolysis of the various esters or amides in human plasma indicated that these agents were relatively stable in the presence of plasma esterases with t1/2 values of up to 120 min. Moreover, lipophilicity of these compounds (partition coefficient) was determined in order to establish correlation between lipophilicity and diffusion of BCH-189 analogues into the cells. The active compounds may exert their effects by extracellular or intracellular hydrolysis to the corresponding antiviral agent BCH-189, but intrinsic anti-HIV-1 activity of some of PAA and PFA adducts, themselves, may also be involved.

Tetrazole isosteres of biologically active acids and their effects on enzymes.

A number of tetrazole analogs of carboxylic substrates and inhibitors have been tested. Lactic and pyruvic tetrazoles were found to be competitive inhibitors of rabbit muscle L-lactate dehydrogenase in both the pyruvate reduction and the lactate oxidation reactions (Ki's of 0.04 M and 0.08 M D,L-lactic tetrazole and 0.02 M and 0.035 M pyruvic tetrazole, respectively). Lactic tetrazole is a non-competitive inhibitor of yeast L-lactate dehydrogenase (Ki = 0.10 M D,L-lactic tetrazole) while pyruvic tetrazole is predominantly competitive (Ki = 0.15 M). Alanine tetrazole is a poorer substrate than alanine for D-amino acid oxidase. It also acts as weak inhibitor. Benzoic tetrazole is a substrate-competitive inhibitor of D-amino acid oxidase (Ki = 0.7 mM) and is also a stronger ethanol-competitive inhibitor than benzoic acid (Ki = 0.03 M) of liver alcohol dehydrogenase. In all the substrates and inhibitors tested, substitution of a tetrazole ring for a carboxylic group has resulted in decreased binding, presumably due to a dilution of the negative charge density and the larger size of the tetrazoyl anion.