Halothane-anesthetized rabbit: a new experimental model to test the effects of besipirdine and duloxetine on lower urinary tract function

INTRODUCTION

The effects of besipirdine and its main metabolite, HP-748, as well as duloxetine and tomoxetine in the lower urinary tract (LUT) were studied using in vitro and in vivo techniques.

MATERIALS AND METHODS

For in vivo studies, besipirdine or duloxetine effects on cystometric parameters and striated sphincter electromyographic (SS-EMG) activity were investigated. On the isolated urethra, norepinephrine (NE) concentration-response curves (CRC) were performed in the presence of besipirdine, duloxetine or tomoxetine. Moreover, CRC to HP-748 were constructed in the absence or presence of prazosin. Potency (pEC(50)) and maximal responses (E(max)) were determined.

RESULTS

Besipirdine at 1, 3 and 5 mg/kg intravenously (i.v.) induced a significant increase in SS-EMG activity (250, 273 and 241%, respectively), bladder capacity (172, 197, and 235%, respectively), intercontraction interval (ICI; 208, 242, and 400%, respectively), and residual volume (181, 191, and 236%, respectively). Duloxetine at 2 mg/kg i.v. increased significantly SS-EMG activity (219%), micturition volume (222%), and ICI (205%). In the isolated urethra, besipirdine, tomoxetine and duloxetine significantly displaced to the left the NE CRC. In addition, HP-748 induced contraction of the isolated urethra with a pEC(50) of 5.89 and an E(max) of 37%.

CONCLUSIONS

These data support the potential of besipirdine as a new drug for LUT dysfunctions such as stress and mixed urinary incontinence.

Characterization of two pharmacophores on the multidrug transporter P-glycoprotein

The multidrug transporter P-glycoprotein is a plasma membrane protein involved in cell and tissue detoxification and the multidrug resistance (MDR) phenotype. It actively expels from cells a number of cytotoxic molecules, all amphiphilic but chemically unrelated. We investigated the molecular characteristics involved in the binding selectivity of P-glycoprotein by means of a molecular modeling approach using various substrates combined with an enzymological study using these substrates and native membrane vesicles prepared from MDR cells. We determined affinities and mutual relationships from the changes in P-glycoprotein ATPase activity induced by a series of cyclic peptides and peptide-like compounds, used alone or in combination. Modeling of the intramolecular distribution of the hydrophobic and polar surfaces of this series of molecules made it possible to superimpose some of these surface elements. These molecular alignments were correlated with the observed mutual exclusions for binding on P-glycoprotein. This led to the characterization of two different, but partially overlapping, pharmacophores. On each of these pharmacophores, the ligands compete with each other. The typical MDR-associated molecules, verapamil, cyclosporin A, and actinomycin D, bound to pharmacophore 1, whereas vinblastine bound to pharmacophore 2. Thus, the multispecific binding pocket of P-glycoprotein can be seen as sites, located near one another, that bind ligands according to the distribution of their hydrophobic and polar elements rather than their chemical motifs. The existence of two pharmacophores increases the possibilities for multiple chemical structure recognition. The size of the ligands affects their ability to compete with other ligands for binding to P-glycoprotein.

Analysis of the tangled relationships between P-glycoprotein-mediated multidrug resistance and the lipid phase of the cell membrane

P-glycoprotein (Pgp), the so-called multidrug transporter, is a plasma membrane glycoprotein often involved in the resistance of cancer cells towards multiple anticancer agents in the multidrug-resistant (MDR) phenotype. It has long been recognized that the lipid phase of the plasma membrane plays an important role with respect to multidrug resistance and Pgp because: the compounds involved in the MDR phenotype are hydrophobic and diffuse passively through the membrane; Pgp domains involved in drug binding are located within the putative transmembrane segments; Pgp activity is highly sensitive to its lipid environment; and Pgp may be involved in lipid trafficking and metabolism. Unraveling the different roles played by the membrane lipid phase in MDR is relevant, not only to the evaluation of the precise role of Pgp, but also to the understanding of the mechanism of action and function of Pgp. With this aim, I review the data from different fields (cancer research, medicinal chemistry, membrane biophysics, pharmaceutical research) concerning drug-membrane, as well as Pgp-membrane, interactions. It is emphasized that the lipid phase of the membrane cannot be overlooked while investigating the MDR phenotype. Taking into account these aspects should be useful in the search of ways to obviate MDR and could also be relevant to the study of other multidrug transporters.

Flavonoid-related modulators of multidrug resistance: synthesis, pharmacological activity, and structure-activity relationships

A series of 28 flavonoid derivatives containing a N-benzylpiperazine chain have been synthesized and tested for their ability to modulate multidrug resistance (MDR) in vitro. At 5 microM, most compounds potentiated doxorubicin cytotoxicity on resistant K562/DOX cells. They were also able to increase the intracellular accumulation of JC-1, a fluorescent molecule recently described as a probe of P-glycoprotein-mediated MDR. This suggests that these compounds act, at least in part, by inhibiting P-glycoprotein activity. As in other studies, lipophilicity was shown to influence MDR-modulating activity but was not the only determinant. Diverse di- and trimethoxy substitutions on N-benzyl were examined and found to affect the activity differently. The most active compounds had a 2,3, 4-trimethoxybenzylpiperazine chain attached to either a flavone or a flavanone moiety (13, 19, 33, and 37) and were found to be more potent than verapamil.

Reversion of multidrug resistance with polyalkylcyanoacrylate nanoparticles: towards a mechanism of action

Polyalkylcyanoacrylate (PACA) nanoparticles loaded with doxorubicin allowed multidrug resistance to be overcome in vitro. However, increased cytotoxicity is not always correlated with an increased level of intracellular drug. Although we have previously shown that PACA nanoparticles are not endocytosed by tumour cells, we report here that a direct interaction between nanoparticles and cells is a necessary requirement for overcoming resistance. In addition, the results showed that the degradation products of PACA (mainly polycyanoacrylic acid) in the presence of doxorubicin are able to increase both accumulation and cytotoxicity, thus suggesting the formation of a doxorubicin-polycyanoacrylic acid ion pair. It is therefore concluded that resistance is overcome as a result of both the adsorption of nanoparticles to the cell surface and increased doxorubicin diffusion by the accumulation of an ion pair at the plasma membrane.