Challenges in the bioanalysis of tetracyclines: Epimerisation and chelation with metals.

Tetracyclines (TCs) are important broad spectrum antibiotics which are active against gram-positive and gram-negative bacteria. TCs readily form epimers, especially under weakly acidic conditions. The epimers are reported to have different antibacterial and toxicological properties and pose a significant challenge for selective bioanalysis, being isobaric with the parent drug and possessing very similar physicochemical properties. During the development, validation and use of bioanalytical methods for minocycline in plasma, urine and renal dialysate there were two unexpected findings. The first was that the analyte and the internal standard, tetracycline, were found to be unexpectedly stable and resistant towards epimerisation in the presence of the deproteinising agent, trichloroacetic acid (TCA). The second was that keeping minocycline spiked dialysate in a freezer led to significant losses which were worse for low concentrations at lower storage temperatures. Investigations into the stability of tetracycline, minocycline, omadacycline and tigecycline in aqueous acidic solutions, under typical analytical conditions, revealed that TCA acts as a stabiliser with respect to both epimerisation and other degradation pathways for these TCs. This gives the rarely used TCA a significant advantage over the commonly used deproteinising agents such as acetonitrile when analysing TCs. Studies of the recoveries of tetracycline and tigecycline from frozen renal dialysis buffer demonstrated similar losses to those for minocycline. These were assigned to deposition of insoluble Mg2+ or Ca2+ complexes on freezing, as pre-storage treatment of the samples by incubation in EDTA coated tubes at room temperature prevented the losses. Minocycline was stable in renal dialysis buffer samples when frozen, for up to ca. 3 months, when this treatment was employed. The TCs were analysed using LC-MS/MS based methods developed in-house using the assay that was originally developed for minocycline in plasma, urine and dialysate as a template.

Development, validation and application of a novel HPLC-MS/MS method for the measurement of minocycline in human plasma and urine.

New treatments are urgently required to treat infections caused by multi-drug resistant Acinetobacter baumanni,. To address this need, a new formulation of Minocin®, (minocycline for injection) has been developed that allows for higher doses of minocycline to be administered. Phase 1 clinical trials were conducted in healthy volunteers to assess the safety and pharmacokinetics (PK) of this new formulation at higher doses. In order to generate PK data, novel, selective and simple HPLC-MS/MS based assays were developed and validated for the determination of minocycline (MC) in human plasma and urine. The respective working ranges were 0.05 to 30 mg/L and 0.1 to 30 mg/L. Removal of endogenous proteins with trichloroacetic acid was used as a simple means of extracting MC from the samples. An analogue, tetracycline was used as the internal standard (IS). Chromatographic separation, including that of MC from its 4-epimer (4-EMC), was achieved on a Waters XBridge BEH C18 column (50 x 4.6 mm ID, 5 µm) with gradient elution. The mobile phases comprised water containing 5 mM ammonium formate at a pH of 2.5, and methanol containing 5 mM ammonium formate. The internal standard (IS) was tetracycline, a structural analogue of minocycline. The methods were fully validated and met regulatory acceptance criteria for intra-run and inter-run accuracy and precision, carryover, dilution integrity and matrix effects. Mean extraction recoveries ranged between 64.3% and 84.6% for MC and 64.3% for the IS. There was no significant ion suppression or enhancement for MC or the IS. The validated assays were successfully applied to 1423 plasma and 689 urine samples from a Phase 1 clinical study. There was no evidence of instability, or significant interconversion between MC and 4-EMC, in stored clinical samples, spiked plasma and urine samples, or their extracts, under various test conditions.

Utilization of DBS within drug discovery: development of a serial microsampling pharmacokinetic study in mice.

BACKGROUND: Dried blood spots (DBS) are rapidly gaining a foothold in the pharmaceutical industry. However, applications in exploratory drug discovery are limited mainly owing to method development time. The development of a generic DBS assay is presented with its application in serial microsampling from mice. RESULTS: A generic 'fit-for-purpose' assay was developed on FTA(®) Elute, which allowed 90% of compounds tested to reach sensitivity levels of 1 ng/ml. A ten-time point serial mouse pharmacokinetic study was conducted using 20-µl microsamples and DBS. Application of generic 'fit-for-purpose' approach did not compromise data delivery or quality. CONCLUSIONS: Serial microsampling and DBS in exploratory mouse pharmacokinetic has been shown to provide superior data quality when compared with traditional plasma-based composite studies.

Determination of a potent urokinase-type plasminogen activator, UK-356,202, in plasma at pg/mL levels using column-switching HPLC and fluorescence detection.

A rapid, sensitive and selective method using column-switching HPLC with fluorescence detection has been developed for the determination of UK-356,202, a potent urokinase-type plasminogen activator, in human plasma. A structural isomer of UK-356,202 is used as an internal standard. The lower limit of quantification is 20 pg/mL and the method is linear over a 100-fold concentration range. UK-356,202 is extracted from plasma simply through the removal of proteins by precipitation with acetonitrile. The HPLC system comprises three columns and the cycle time is 9.5 min per sample. The eluate from the extraction column is heart-cut onto a trace enrichment cartridge which is then back-flushed onto a narrow-bore Supelco ABZ+ Plus analytical column. The method has been used to analyze many thousands of samples from clinical and toxicological studies support. Its ruggedness is demonstrated by the use of a single extraction column for the analysis of over 1200 clinical samples.