Fundamental studies in reversed-phase liquid-solid extraction of basic drugs; I: Ionic interactions

Quantitation of Canadian organic ultraviolet filters using polarity switching and ultra-high performance liquid chromatography-tandem mass spectrometry

Ultraviolet filters (UVFs) absorb UV light and are comprised of numerous classes of compounds including inorganic and organic. They have been used for decades in protecting humans from skin damage and cancer. Recent studies have shown that UVFs are found in many phases of abiotic and biotic systems with their physical-chemical characteristics determining environmental fate and potential biological impacts such as bioaccumulation. This study developed a unified method to quantify eight UVFs (avobenzone, dioxybenzone, homosalate, octinoxate, octisalate, octocrylene, oxybenzone, and sulisobenzone) by solid phase extraction and ultra-high performance liquid chromatography-tandem mass spectrometry using polarity switching. The validated method resulted in accuracies ranging from 75 to 112%, MLD/MLQs of 0.00015/ 0.00049 to 0.0020/ 0.0067 ng mL^-1, and precisions of 1.8 to 22.6% (intraday) and 1.3 to 17.2% (interday). The method was applied to chlorinated outdoor pool waters in the City of Winnipeg, Manitoba, Canada. This method could be adapted for a variety of chlorinated and unchlorinated waters such as drinking water, wastewater, and surface waters.

Drug permeation in biomembranes

In the past decades, it has become increasingly apparent that in addition to therapeutic effect, drugs need to exhibit favourable absorption, distribution, metabolism and excretion (ADME) characteristics to produce a desirable response in vivo. As the recent progress in drug discovery technology enables rapid synthesis of vast numbers of potential drug candidates, robust methods are required for the effective screening of compounds synthesized within such programs, so that compounds with poor pharmacokinetic properties can be rejected at an early stage of drug development. Furthermore, a viable in silico method would save resources by enabling virtual screening of drug candidates already prior to synthesis. This review gives a general overview of the approaches aimed at predicting biological permeation, one of the cornerstones behind the ADME behaviour of drugs. The most important experimental and computational models are reviewed. Physicochemical factors underlying the permeation process are discussed.

Contributions of theory to method development in solid-phase extraction

The kinetic and retention properties of solid-phase extraction devices are reviewed from the perspective of method development strategies. Models based on frontal analysis are used to correct retention properties of solid-phase extraction devices to account for the fact that too few theoretical plates are provided for retention to be independent of kinetic factors. The available pressure drop for the sampling device largely dictates the choice of useful particle sizes and maximum bed length. The use of octanol--water partition coefficients and extrapolated values of the retention factor obtained by liquid chromatography are poor empirical models for the estimation of breakthrough volumes with water as the sample solvent. The solvation parameter model provides an adequate description of sorbent retention for the estimation of breakthrough volumes, rinse solvent volume and composition, and elution solvent volume and composition. Combining the frontal analysis and solvation parameter models offers a comprehensive approach to computer-aided method development in solid-phase extraction. This is the first step in the development of a structure-driven approach to method development in solid-phase extraction that should be more reliable and less tedious than traditional trial and error approaches.

Theory and methodology of antibiotic extraction from biomatrices

A short review on the basic theory and practices of the extraction and clean-up of agricultural antibiotics from biomatrices is presented. For the analysis of residues of ionophores, beta-lactams, macrolides, chloramphenicol, aminoglycosides, tetracyclines and peptide antibiotics, the use of solid-phase extraction has become nearly ubiquitous as part of the basic extraction methodology. The majority of the methodologies for these compounds report recoveries greater than 70%, with relative standard deviations usually less than 15%. Each of the antibiotic classes, as well as antibiotics within each class, have unique chemistries that must be taken into account when developing a viable extraction method.

Determination of a HIV protease inhibitor (DMP 450) in animal and human plasma by solid-phase extraction and high-performance liquid chromatography

Extraction of DMP 450 from plasma was performed with C2 solid-phase extraction columns, using 0.1 M ammonium acetate in 90% methanol to elute DMP 450. The extraction recovery over the range of 10 to 10 000 ng/ml averaged 81.0, 96.2, 77.4, 95.2 and 68.0% from rat, dog, monkey, chimpanzee (25-10 000 ng/ml) and human plasma, respectively. HPLC analysis was carried out with a C18 column and a mobile phase of acetonitrile, methanol and 30 mM potassium phosphate (pH 3), the composition dependent on the type of plasma being analyzed, and monitored at a wavelength of 229 nm. Intra-day and inter-day coefficients of variation were less than 9.9 and 12.9%, respectively. Absolute differences were less than 11.5%.

Application of reversed-phase liquid chromatography and prepacked C18 cartridges for the analysis of oxytetracycline and related compounds

The reversed-phase (RP) chromatographic separation of oxytetracycline (OTC) 4-epioxytetracycline (4-epiOTC), alpha-apooxytetracycline (alpha-apoOTC), and beta-apooxytetracycline (beta-apoOTC) has been accomplished on an Inertsil C8 column at ambient temperature. Using the simplex method of solvent optimization, a 0.1 M ammonium acetate buffer (pH 3.0)-acetonitrile-tetrahydrofuran (72.5:12.5:15, v/v/v) mobile phase was found to give excellent separation of the compounds. OTC, 4-epiOTC, alpha-apoOTC and beta-apoOTC were resolved in 35 min with calculated detection limits of 40, 20, 50 and 140 ng/ml, respectively. Solid-phase extraction (using RP C18 cartridges) of OTC and OTC degradation compounds from distilled water and porcine muscle was tested at four concentration levels ranging from 200 to 2000 ng/ml (g); overall mean recovery of OTC from distilled water and porcine tissue was greater than 90% and 70%, respectively.

Column liquid chromatographic determination of bupivacaine in human serum using solid-phase extraction

A convenient and economical procedure for the extraction of bupivacaine and its major metabolites, desbutylbupivacaine and 4'-hydroxybupivacaine, from serum is described. A 0.25-ml aliquot of 4 micrograms/ml solution of N-pentyl-2,6-pipecoloxylidide used as the internal standard and an equal volume of the serum sample are applied to a 1-ml BondElut C18 silica extraction column which has been conditioned by subsequent washing once with 1 M HCl, twice with methanol and once with water. After passing the sample at a slow rate, the column is washed twice with water and once with acetonitrile. The desired compounds are then eluted with a 0.25-ml aliquot of a mixture of 35% perchloric acid-methanol (1:40, v/v). A 5-microliters aliquot of the eluate is injected onto a 150 x 4.6 mm I.D. column packed with 5-microns C8 silica particles, which is eluted at ambient temperature with a mobile phase of acetonitrile-10 mM KH2PO4 (25:80, v/v) at a flow-rate of 1.5 ml/min. The peaks are detected at 205 nm. The extraction recovery of all the compounds is in the range of 90%. The chromatogram is clean and the desired peaks are well separated from each other and from the extraneous peaks.

An investigation of the effects of carbon loading and endcapping on the solid-phase extraction of beta-blockers onto C18 bonded silica gel

The effects of carbon loading and endcapping on the solid-phase extraction onto C18 bonded silica gel of a range of beta-blockers from aqueous buffer and from dog plasma has been investigated. The highest extraction efficiencies were obtained for those phases with carbon loadings of between 5 and 16% for phases without endcapping or 10.5-14% for endcapped material. With carbon loadings of 18 and 22% (plus endcapping) poor extractions from the matrix were obtained combined with further losses at the wash steps. Matrix effects were observed with dog plasma which accentuated the effects seen with buffer. These results are best explained by assuming that a cationic interaction of the secondary amino group present in the analytes with residual silanols on the silica surface is primarily responsible for the extraction of these analytes.

Combined reversed phase and anion exchange solid phase extraction for the assay of the neuroprotectant NBQX in human plasma and urine

A combination of reversed phase and anion exchange solid phase extraction (SPE) was investigated for the liquid chromatographic assay of the neuroprotectant agent NBQX in human plasma and urine. Reversed phase SPE alone using a variety of phases failed to yield sufficiently pure extracts for the assay of plasma in the low ng/mL concentration range using UV detection. However, using Bond Elut Certify II SPE columns containing a mixture of reversed phase and anion exchange functionalities markedly improved the purity of plasma extracts. Furthermore, a change of detector wavelength from UV (294 nm) to the visible region (380 nm) removed some minor interfering peaks originating from plasma. Using optimized SPE conditions with an extraction recovery of 92.3% and a high performance liquid chromatographic procedure with Lichrospher C18 as the stationary phase, the lower limit of quantitation in human plasma was 2 ng/mL (corresponding to 0.75 ng injected on-column). Intra-assay coefficients of variation ranged from 9.6% at 2 ng/mL to 1.3% at 10 ng/mL. A similar SPE procedure was applied to human urine with acceptable recovery (88.3%), but an analytical column of different selectivity (Chromspher B C18) was necessary in order to avoid interference from the urine. The limit of quantitation for the urine assay was 25 ng/mL and the intra-assay precision ranged from 4.6% at 25 ng/mL to 2.2% at 500 ng/mL.

Fundamental studies in reversed-phase liquid-solid extraction of basic drugs; III: Sample matrix effects

Using a set of 13 basic solutes (including a quarternary ammonium compound) with controlled pKa, log P and plasma protein binding, the factors which influence the extraction of these compounds from plasma by reversed-phase liquid-solid extraction have been studied. Direct analysis of plasma, avoiding deproteination, etc., is possible providing the sample is applied to the cartridge under the appropriate conditions. These conditions are dictated by the degree of plasma protein binding and are controlled and predicted from the lipophilicity of the compound. Plasma samples containing lipophilic compounds (log D(pH 7.4) > 2.03) require acidification and samples containing more polar compounds (log D(pH 7.4) < 0.9) should be applied untreated. Failure to use the appropriate conditions can result in significant losses from the cartridge on application or washing. In the intermediate lipophilicity range the optimal application conditions cannot be predicted. The effect of various aqueous-organic wash solvents have been investigated, and it was found that the cationic but not the proteinaceous components of plasma also affect the extraction process. This phenomenon which is probably due to an attenuation of the secondary cation exchange mechanism, results in more facile elution than when compounds are applied in water.

Fundamental studies in reversed-phase liquid-solid extraction of basic drugs; II: Hydrogen bonding effects

Anomalies in the elution order of the beta-blockers atenolol and propranolol on reversed-phase liquid-solid extraction cartridges have been studied. The hydrogen bond acceptor properties of the polar ring substituent in atenolol was found to result in greater retention than would be expected from the reversed-phase or cation-exchange mechanisms alone. This hydrogen bonding interaction could be attenuated by inclusion of a strong hydrogen bond acceptor in the eluent or more successfully by increasing the eluent water content. The conclusions from this work have been used to explain previously reported anomalies in the solid-phase extraction of polar basic drugs.