Application of enzymatically stable dipeptides for enhancement of intestinal permeability. Synthesis and in vitro evaluation of dipeptide-coupled compounds.

Transport across the intestinal barrier of compounds with low permeability may be facilitated by targeting the human oligopeptide transporter, hPepT1. A flexible synthetic pathway for attaching compounds to dipeptides through ester or amide bonds was developed. Furthermore, a synthetic approach to functionalize model drugs from one key intermediate was generated and applied to a glucose-6-phosphatase active model drug. The model drug was coupled to D-Glu-Ala through various linkers, and the G-6-Pase activity as well as the aqueous solubility and transport properties of these prodrugs, as compared to those of the parent drugs, were examined. None of the peptide-coupled compounds seemed to be transported by hPepT1, though one of the peptide-coupled compounds had affinity for hPepT1. Interestingly, in one case the parent drug was actively effluxed, while the corresponding peptide-coupled prodrug was not. The low aqueous solubility of the parent compounds was not increased after attachment to a dipeptide. This suggests that only compounds with a certain intrinsic aqueous solubility should be targeted to hPepT1 by attachment to a dipeptide. Important information about the design of peptide-coupled drugs targeted for hPepT1 is presented.

Model prodrugs designed for the intestinal peptide transporter. A synthetic approach for coupling of hydroxy-containing compounds to dipeptides.

The human peptide transporter, hPepT1, situated in the small intestine, may be exploited to increase absorption of drugs or model drugs by attaching them to a dipeptide, which is recognised by hPepT1. A synthetic protocol for this kind of model prodrugs was developed, in which model drugs containing a hydroxy group were attached to enzymatically stable dipeptides by hydrolysable ester linkages. Furthermore, a number of benzyl alcohols with various substituents in the 4-position of the phenyl ring were coupled to D-Asp-Ala and D-Glu-Ala. Ideally, a prodrug should be stable in the upper small intestine and be converted to the parent drug during or after transport into the blood circulation. Therefore, we investigated the influence of the electronegativity of the substituent in the 4-position of the phenyl ring on stability in aqueous solution at pH 6.0 and 7.4, corresponding to pH in jejunum and blood, respectively. In addition, the influence of the electronegativity of the substituent on stability upon storage was examined. Model prodrugs containing electron donating substituents in the 4-position of the phenyl ring decomposed upon storage, while model prodrugs containing no substituents or electron withdrawing substituents in the 4-position were stable. In aqueous solution (pH 6.0 and 7.4), electron withdrawing substituents in the 4-position decreased the half-life of the model prodrug. These data provide important information on stability of this kind of model prodrugs upon storage and under aqueous conditions. The results may be applied in the rational design of oligopeptide ester prodrugs to obtain prodrugs, which are stable upon storage and have an optimal release profile of the drug.

Stability and in vitro metabolism of dipeptide model prodrugs with affinity for the oligopeptide transporter.

One approach to increase drug stability and to facilitate oral absorption of low bioavailability drugs may be to design oligopeptide ester prodrugs which are stable in the gastrointestinal tract, are transported via the oligopeptide transporter, and finally release the parent drug molecule into the blood circulation and/or by its site of action. In these kinds of prodrugs the ester linkage may be broken by pH dependent and/or enzyme catalyzed hydrolysis. The objective of the present study was to investigate the degradation mechanism and rate of the model compounds Glu(OBzl)-Sar, D-Glu(OBzl)-Ala and Asp(OBzl)-Sar in aqueous solution and in relevant biological media and to compare these results with those of our previous study of D-Asp(OBzl)-Ala. Furthermore, the resulting aqueous stability and in vitro metabolism data are related to our previous affinity data to evaluate if Glu-Sar, D-Glu-Ala, and Asp-Sar have potential as pro-moieties in these kinds of prodrugs. The degradation rates follow first-order kinetics, show maximun stability at pH 4-5 with maximum half-lives for Asp(OBzl)-Sar, Glu(OBzl)-Sar, and D-Glu(OBzl)-Ala of 115 h, 30 days and 152 days, respectively. The stability was dependent on buffer concentration, temperature, pH, and ionic strength. In biological media such as 80% human plasma, human gastric juice and intestinal fluid, and 10% rat jejunal homogenate at 37 degrees C, the half-lives were greater than 1 h except for the hydrolysis of Glu(OBzl)-Sar in 10% rat jejunal homogenate, where the half-life was approximately 16 min. All the stabilized dipeptides may have potential as drug carriers targeting hPepT1.

[Studies on the combined effects of shift work and noise on permanent hearing loss]. / Untersuchungen zur Wirkung der Belastungskombination von Schichtarbeit und Lärm auf den permanenten Hörverlust.

Combined exposure to shiftwork and noise was investigated with respect to its aural damaging effect. The investigations were carried out on 347 male and female noise-exposed textile workers working in single-shift and three-shift systems. The average hearing loss for the PTS at 3 kHz, 4 kHz, the total hearing loss at 0.5, 1, 2, 4, 6, 8 kHz and percentage of hearing loss according to Fowler/Sabine served as indicators. To fulfil this task an epidemiological cross-sectional study was performed. The results of the present study show lower average hearing loss values in the case of three-shift workers as compared with single-shift workers.