Radiochemical purity control of radiolabeled drugs

Stoichiometry and substrate affinity of the mannitol transporter, EnzymeIImtl, from Escherichia coli

Uptake and consecutive phosphorylation of mannitol in Escherichia coli is catalyzed by the mannitol permease EnzymeIImtl. The substrate is bound at an extracellular-oriented binding site, translocated to an inward-facing site, from where it is phosphorylated, and subsequently released into the cell. Previous studies have shown the presence of both a high- and a low-affinity binding site with K(D)-values in the nano- and micromolar range, respectively. However, reported K(D)-values in literature are highly variable, which casts doubts about the reliability of the measurements and data analysis. Using an optimized binding measurement system, we investigated the discrepancies reported in literature, regarding both the variability in K(D)-values and the binding stoichiometry. By comparing the binding capacity obtained with flow dialysis with different methods to determine the protein concentration (UV-protein absorption, Bradford protein detection, and a LDH-linked protein assay to quantify the number of phosphorylation sites), we proved the existence of only one mannitol binding site per dimeric species of unphosphorylated EnzymeIImtl. Furthermore, the affinity of EnzymeIImtl for mannitol appeared to be dependent on the protein concentration and seemed to reflect the presence of an endogenous ligand. The dependency could be simulated assuming that >50% of the binding sites were occupied with a ligand that shows an affinity for EnzymeIImtl in the same range as mannitol.

The metabolic fate of the dopamine agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin in rats after intravenous and oral administration. I. Disposition and metabolic profiling

1. The disposition and metabolic profiling of 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin(I), a dopamine agonist, were studied in anaesthetized rats after i.v. administration and in non-anaesthetized rats after i.v. and oral dosing. No major differences due to narcosis were observed. 2. Independent of dosing route or anaesthetic, clearance of I was rapid. Bile was the main route of excretion, accounting for 88% dose, compared with 9% in urine. 3. Drug metabolic profiling revealed that I is almost completely metabolized before elimination; less than 0.5% total radioactivity in bile and urine was due to parent compound. 4. The biliary metabolic profiles after i.v. and oral administration were similar. One major metabolite was detected, accounting for 50% (i.v.) or 65% (oral) dose. The major biliary metabolite was identified as the glucuronide of I. 5. Urinary metabolic profiles were quantitatively different from those of bile. After i.v. administration one major metabolite was detected in urine, but this was not the major biliary metabolite. After oral administration, the major urine metabolite was the same as the major biliary metabolite. These differences can be explained by first-pass gastro-intestinal metabolism.

Bioconversion of alpha-[14C]zearalenol and beta-[14C]zearalenol into [14C]zearalenone by Fusarium roseum 'Gibbosum'

Cultures of Fusarium roseium 'Gibbosum' on rice were treated with [14C]zearalenone, alpha[14C]zearalenol, or beta-[14C]zearalenol to determine whether a precursor-product relationship exists among these closely related fungal metabolites. Culture extracts were purified by silica gel column chromatography and fractionated by high-pressure liquid chromatography, and the level of radioactivity was determined. Within 7 days, the beta-[14C]zearalenol was converted to zearalenone, and no residual beta-[14C]zearalenol was detectable. Most of the alpha-[14C]zearalenol added was also converted into zearalenone with 14 days. In cultures treated with [14C]zearalenone, no radioactivity was noted in any other components.

Screening for impurities in butoprozine

The purity analysis of butoprozine is described. Both gas chromatography-mass spectrometry (GC-MS) and high pressure liquid chromatography (HPLC) with UV-vis detection (conventional and multichannel) were used. In the butoprozine example disadvantages for both techniques became apparent: incorrect conclusions with regard to the purity of the drug would have been drawn if only one of these chromatographic techniques had been used. GC-MS allowed the identification of an impurity not found by HPLC.