No abstract

The aim of this study was to evaluate the intestinal disposition of abiraterone acetate, an ester prodrug of the anticancer agent abiraterone. Stability of the prodrug and solubility and dissolution characteristics of both abiraterone and abiraterone acetate were monitored in vitro. Moreover, the in vivo intraluminal concentrations of abiraterone and abiraterone acetate upon intake of one tablet of 250 mg abiraterone acetate were assessed in healthy volunteers. The intestinal absorption resulting from the intraluminal behavior of the ester prodrug was determined using the rat in situ intestinal perfusion technique with mesenteric blood sampling. Simulated and aspirated human intestinal fluids of the fasted state were used as solvent systems. Upon incubation of abiraterone acetate in human intestinal fluids in vitro, rapid hydrolysis of the prodrug was observed, generating abiraterone concentrations largely exceeding the apparent solubility of abiraterone, suggesting the existence of intestinal supersaturation. These findings were confirmed in vivo, by intraluminal sampling of duodenal fluids upon oral intake of an abiraterone acetate tablet by healthy volunteers. Rat in situ intestinal perfusion experiments performed with suspensions of abiraterone and abiraterone acetate in human intestinal fluids of the fasted state revealed significantly higher flux values upon perfusion with the prodrug than with abiraterone. Moreover, rat in situ intestinal perfusion with abiraterone acetate suspensions in simulated fluids of the fasted state in presence or absence of esterases demonstrated that increased hydrolytic activity of the perfusion medium was beneficial to the intestinal absorption of abiraterone. In conclusion, the rapid hydrolysis of abiraterone acetate in the intraluminal environment appears to result in fast and extensive generation of abiraterone supersaturation, creating a strong driving force for abiraterone absorption.

Microcin C (McC), a natural antibacterial compound consisting of a heptapeptide attached to a modified adenosine, is actively taken up by the YejABEF transporter, after which it is processed by cellular aminopeptidases, releasing the nonhydrolyzable aminoacyl adenylate, an inhibitor of aspartyl-tRNA synthetase. McC analogues with variable length of the peptide moiety were synthesized and evaluated in order to characterize the substrate preferences of the YejABEF transporter. It was shown that a minimal peptide chain length of 6 amino acids and the presence of an N-terminal formyl-methionyl-arginyl sequence are required for transport.In the current ongoing quest for new antibiotics, aminoacyltRNA synthetases (aaRSs) have been regarded as promising targets (5,11,14). The natural antibiotic microcin C (McC) (Fig. 1, compound 1a) targets an aaRS and has been envisaged as a lead compound for further development as an antibacterial agent (17). McC consists of a heptapeptide that is covalently linked through a phosphoramidate bond to adenosine, with, in addition, an aminopropyl moiety esterified to the phosphoramidate linker (4). Once inside a sensitive cell, McC is processed by peptide deformylase and several peptidases that remove the N-terminal formyl group and the peptide part, respectively (7). As a result of intracellular processing, the active compound (compound 2), a modified nonhydrolysable aspartyl-adenylate, is released. Processed McC is a potent inhibitor of aspartyl-tRNA synthetase (AspRS) (8).McC penetrates the outer membrane of the Escherichia coli cell mostly through the OmpF porin, but also through other, yet-unidentified transport systems (M. Novikova, A. Metlitskaya, and K. Severinov, unpublished data), and is subsequently transported through the inner membrane by the YejABEF transporter (10). YejABEF is the only complex responsible for McC transport, since yej mutants are highly resistant to McC and its maturation intermediates and chemical analogues. While intact McC inhibits the growth of sensitive E. coli cells at low micromolar concentrations, processed McC does not affect cell growth, even at millimolar concentrations. Thus, the peptide chain enables McC to function through a Trojan-horse mechanism by promoting active uptake via the YejABEF transporter. The recently improved synthetic approach for the production of McC analogues has led us to investigate the uptake properties of the Yej transporter in more detail. The results obtained could be important for further drug development, where peptides function as carrier moieties for drugs that otherwise would not be able to penetrate the bacterial membranes. Here, we used a number of McC analogues truncated either from their C-or N-terminal sides, or otherwise modified, to determine the minimal peptide chain length sufficient for facilitated transport by Yej. MATERIALS AND METHODSChemistry. Reagents and solvents were purchased from commercial suppliers (Acros, Sigma-Aldrich, Bachem, and Novabiochem) and used as provided unless otherwise indicated....

To relate the reported positive effect of food on the oral bioavailability of abiraterone to the intraluminal behavior of abiraterone acetate, an in vivo experiment was performed, in which duodenal fluids and plasma samples were collected from healthy volunteers after the administration of abiraterone acetate in fasted and postprandial conditions. The plasma concentration-time profiles confirmed the positive food effect. Nevertheless, intraduodenal concentrations of abiraterone acetate and abiraterone did not fully reflect this observation. This apparent discrepancy was explored by performing several in vitro experiments including solubility, dissolution, and transfer studies. Gastrointestinal transfer studies illustrated a positive impact of gastric processing of the abiraterone acetate formulation on the duodenal concentrations in the fasted state, which could not be observed in the postprandial condition. As the influence of gastric dissolution on the intraluminal concentrations in the small intestine declines aborally, it is most likely the superior solubility of abiraterone acetate and abiraterone in intestinal fluids of the fed state that dictates the food effect. Furthermore, N-oxide abiraterone sulfate and abiraterone sulfate appeared in the duodenum at significantly later time points than abiraterone, suggesting biliary excretion of these abiraterone metabolites; this was confirmed by in situ biliary excretion experiments in rats.