Diethylstilbestrol-scaffold-based pregnane X receptor modulators.

Due to its function as a regulator of drug-metabolizing enzymes and transporters, pregnane X receptor (PXR) represents an important factor involved in drug metabolism. In this work, we describe the discovery of diethylstilbestrol-based PXR modulators, which were designed from marine sulfated steroids with PXR agonistic activity, solomonsterols A and B, and our recently reported bazedoxifene scaffold-derived PXR antagonists. The methylated diethylstilbestrol derivative 1 displayed potent PXR agonistic activity with an EC50 value of 10.5 µM, whereas compounds 3, 4 and 6 (IC50 for 6 = 27.4 µM) and diethylstilbestrol (2) itself (IC50 = 14.6 µM) exhibited PXR antagonistic effects in HepG2 cells. The PXR modulatory effects of the synthesized diethylstilbestrol derivatives were further confirmed by the induction of PXR-regulated CYP3A4 expression with compound 1, as well as by the inhibition of the rifaximin-promoted up-regulation of CYP3A4 expression with 2 and its derivative 6.

Allosteric modulation of the RNA polymerase catalytic reaction is an essential component of transcription control by rifamycins.

Rifamycins, the clinically important antibiotics, target bacterial RNA polymerase (RNAP). A proposed mechanism in which rifamycins sterically block the extension of nascent RNA beyond three nucleotides does not alone explain why certain RNAP mutations confer resistance to some but not other rifamycins. Here we show that unlike rifampicin and rifapentin, and contradictory to the steric model, rifabutin inhibits formation of the first and second phosphodiester bonds. We report 2.5 A resolution structures of rifabutin and rifapentin complexed with the Thermus thermophilus RNAP holoenzyme. The structures reveal functionally important distinct interactions of antibiotics with the initiation sigma factor. Strikingly, both complexes lack the catalytic Mg2+ ion observed in the apo-holoenzyme, whereas an increase in Mg2+ concentration confers resistance to rifamycins. We propose that a rifamycin-induced signal is transmitted over approximately 19 A to the RNAP active site to slow down catalysis. Based on structural predictions, we designed enzyme substitutions that apparently interrupt this allosteric signal.

Highly efficient uptake of a rifamycin derivative via the FhuA-TonB-dependent uptake route in Escherichia coli.

Rifamycin CGP 4832 is a semisynthetic rifamycin derivative. It is at least 200 times more active than rifampicin against Escherichia coli and related bacteria. This increased activity is shown here to be due to the efficient uptake of CGP 4832 across the E. coli outer membrane via the ferrichrome transport system comprising the outer membrane FhuA (TonA) protein, the ferrichrome receptor, and the inner membrane TonB protein. CGP 4832 competed with ferrichrome and other iron siderophore complexes, and with bacteriophage T5 and colicin M for binding sites on the FhuA protein. Mutations in fhuA or tonB genes reduce CGP 4832 sensitivity to a level comparable to that to rifampicin. There is no evidence that CGP 4832 or rifampicin utilize the inner membrane ferrichrome transport system to gain entry into the cytoplasm.

[Mechanisms of detecting organic cholephilic anions in man: comparative studies on plasma depuration of rifamycin-SV. Interference of nicotinic acid and phenobarbital]. / Meccanismi di captazione di anioni organici colefili nell'uomo: osservazioni comparative sulla depurazione plasmatica di rifamicina-SV. Interferenza di acido nicotinico e fenobarbital.

We have studied plasmatic half-life of R-SV administered alone and in association with nicotinic acid, before and after treatment with phenobarbital, in 10 normobilirubinaemic subjects and in 10 patients Gilbert's syndrome, used like controls. Ouer results confirm the existence of some alterations of drug-metabolism produced by associated administration of other drugs, in both healthy and hyperbilirubinaemic subjects, and in these one even more.

Detergent effects on a reverse transcriptase activity and on inhibition by rifamycin derivatives.

A reverse transcriptase activity, extracted from virus-transformed cells, is activated by very low concentrations of nonionic detergents. These same detergents also significantly reduce the effectiveness of certain rifamycin derivatives as inhibitors of the polymerase activity when the detergents are present at micelle-forming concentrations.