Rapid optimization of the post-column fluorogenic ninhydrin reaction for the HPLC-based determination of bradykinin and related fragments

A flow injection analysis scheme is demonstrated for the rapid optimization of reagent concentrations, flow rates, delay time and temperature using the guanidino moiety specific fluorogenic ninhydrin reaction. Using the amino acid arginine, non-arginine containing peptides, and the arginine-containing peptides, bradykinin and related fragments, specificity is demonstrated. These results serve to extend previous descriptions of the post-column reaction by offering a time efficient approach for the optimization of newly assembled post-column reactors using this chemistry. The reactor is subsequently added to a gradient elution HPLC system with the separation of bradykinin and bradykinin fragments demonstrated. Detection sensitivity in the high femtomole-low picomole mass range was achieved for these substances.

Aspects of column fabrication for packed capillary electrochromatography

Various parameters have been evaluated to develop a process for optimization of column manufacture for packed capillary electrochromatography (CEC). Spherisorb ODS-1 was packed into 75 microm I.D. capillaries to establish a standard set of packing conditions to afford high-performance columns free of voids. Numerous silica-based packing materials including porous and non-porous reversed-phase and ion-exchange phases were employed to evaluate the applicability of the standard conditions. Success of column manufacture and performance demonstrate a relationship to the colligative properties of the packing materials under the applied conditions. Frequently encountered difficulties arising from inadequate column conditioning and void formation in the packed bed are identified and discussed.

Evaluation of 1.5 microM reversed phase nonporous silica in packed capillary electrochromatography and application in pharmaceutical analysis

Reversed-phase nonporous silica (RP-NPS) of 1.5 microm dp is employed to demonstrate rapid and efficient separations in packed capillary electrochromatography (CEC). Two methods for packing capillaries and two techniques to manufacture frits used to hold the packing in place are evaluated for their effect upon separation performance using polyaromatic hydrocarbons (PAHs) and polar neutral pharmaceutical compounds. Attention is given to conditioning of the packed capillaries for high efficiency separations without necessity for sodium dodecyl sulfate (SDS). Separation conditions for the nonporous materials were modified from those previously determined on porous reversed-phase silica. Feasibility for method development and validation of a parent pharmaceutical compound and related impurities in the range of 0.1-120% of a 5 mg/mL concentration was assessed and reported. An approach to improving detection sensitivity through use of large-bore capillaries is briefly discussed.

Application of sulfobutylether-beta-cyclodextrin with specific degrees of substitution for the enantioseparation of pharmaceutical mixtures by capillary electrophoresis

In this research the separation of the enantiomers of the basic drug bidisomide (SC-40230) from five closely related known process impurities was investigated using several neutral and anionic sulfobutylether beta-cyclodextrins (SBE-beta-CDs) as isomer selectors. Several novel sulfobutylether derivative mixtures and purified charge types having a specific degree of substitution were used to study the effect of selector charge on the efficiency and selectivity of both chiral and achiral separations. The effects of run buffer pH, selector type, and selector concentration on the chiral separation of bidisomide and the achiral separation of the related process impurities was also investigated. The related process impurity, SC-47500, displayed significant peak tailing with SBE-beta-CD mixtures which contained mono- to deca-substituted cyclodextrins. This problem was explored using isolated SBE-beta-CD charge types having degrees of substitution from one to seven. Peak tailing increased as the charge on the selector increased, suggesting that the distortion was due to electrodispersion and the large countercurrent mobility of the negatively charged complexes. Pure charge types having a lower degree of substitution provided adequate chiral and achiral selectivity, while eliminating the severe peak distortion caused by electrodispersion. The complete analysis of the bidisomide enantiomers and the related impurities was achieved with a pH 2.5 running buffer containing 5-10 mM of the isolated sulfobutylether charge types SBE[2]ds(1)sr-beta-CD or SBE[3]ds(1)sr-beta-CD. These conditions gave baseline resolution of bidisomide enantiomers and all five impurities, thus allowing both chiral and achiral purity to be determined in a single run.

Hydroquinone-based derivatization reagents for the quantitation of amines using electrochemical detection

Two new reagents, NDTE (2,5-dihydroxyphenylacetic acid, 2,5-bis-tetrahydropyranyl ether p-nitrophenyl ester) and HLTE (homogentisic gamma-lactone tetrahydropyranyl ether), are described for the chemical derivatization of primary and/or secondary amines to form an electrochemically active product. These reagents undergo reaction with the aforementioned analytes to form a product possessing the hydroquinone moiety, thus allowing for reversible electrochemical detection at mild oxidation potentials. The reactivity of each reagent was demonstrated by using N-ethylbenzylamine (EBzA) and the dipeptide isoleucine leucine methyl ester as model analytes. The investigation included the isolation and identification of the intermediates and final products from derivatization of EBzA. These isolated standards were subsequently characterized with respect to electrochemical properties by means of cyclic voltammetry. In LC-EC experiments, the concentration limit of detection (CLOD) of the purified EBzA product was determined to be 5 nM (100 fmol) at a detection potential of +200 mV vs Ag/AgCl ([Cl-] = 3 M). The CLOD values obtained by LC-EC after derivatization of aqueous solutions of EBzA and Ile-Leu-OMe with NDTE were 25 nM (250 fmol) and 250 nM (2.5 pmol), respectively.

Method development in pharmaceutical analysis employing capillary electrochromatography

Capillary electrochromatography (CEC) has been employed to explore method development for a series of structurally related polar neutral compounds of pharmaceutical relevance. Capillaries with dimensions of 75 microm ID x 25 cm length (34.5 cm total) were packed with Spherisorb ODS-1, Hypersil phenyl, and Hypersil MOS (all 3 microm particles) and were compared in the reversed-phase mode in order to determine which phase provided the best initial performance and thus serve as the phase of choice for additional method development experiments. The various separation parameters examined for their effect on efficiency, k', resolution, and linear velocity included percent and type of organic modifier, buffer concentration, voltage, and temperature. All separations were conducted with an acidic mobile phase (aqueous mobile phase component, pH 3.0). The separation efficiencies obtained were on the order of 200,000-260,000 plates/m, which equates to reduced plate heights of 1.22 for columns packed with Spherisorb ODS-1. Repeatable column-to-column separation performance was demonstrated.

Plasma analysis of alpha-difluoromethylornithine using pre-column derivatization with naphthalene-2,3-dicarboxaldehyde/CN and multidimensional chromatography

A procedure for the plasma analysis of alpha-difluoromethylornithine (DFMO) has been developed that utilizes pre-column derivatization with naphthalene-2,3-dicarboxaldehyde/cyanide (NDA/CN) in pH 9.2 borate buffer. Selective derivatization of delta-amine of DFMO followed by quenching of the reaction results in the formation of a cyanobenz [f] isoindole (CBI) derivative that is stable for 24 h. Plasma was prepared for derivatization by a single step procedure which resulted in an ultrafiltrate compatible with derivatization and analysis. The DFMO derivative (CBI-DFMO) was separated from plasma interferences by multidimensional chromatography with an analysis time of 28 min. The response for DFMO in plasma was linear over the range of 2.1 x 10(-8) 2.1 x 10(-6) M after derivatization. This procedure encompasses a useful linear range and offers the advantages of minimal sample preparation and production of a stable fluorophor.

Chiral separations of enantiomeric pharmaceuticals by capillary electrophoresis using sulphobutyl ether beta-cyclodextrin as isomer selector

Capillary electrophoresis has developed into an extremely useful technique for the separation of optical isomers. High efficiencies and the availability of many types of isomer selectors allowing rapid and inexpensive methods development make capillary electrophoresis (CE) an attractive alternative to gas chromatography (GC) and high-pressure liquid chromatography (HPLC) for the determination of chiral purity. In this research the separation of the enantiomers of some chiral pharmaceuticals was investigated using anionic sulphobutyl ether-beta-cyclodextrins as isomer selectors. These chiral selectors have a large countercurrent mobility, making them inherently advantageous as selectors as compared to neutral cyclodextrins. The effects of pH, buffer composition and selector concentration on the chiral separation of these compounds was investigated. All of the compounds studied were successfully resolved by the sulphobutyl ether beta-cyclodextrins (SBE-beta-CDs) typically with run times of less than 20 min using low concentrations of the SBE selector (1-5 mM).

Resolution of acylated dipeptide stereoisomers by capillary electrophoresis using sulfobutylether derivatized beta-cyclodextrin

The separation of enantiomerically and diastereomerically related stereoisomers of acylated Asp-Phe dipeptides was explored using capillary electrophoresis (CE). This series of dipeptides included the alpha-L,L parent compound and the three other potential Asp containing stereoisomers (alpha-D,D, alpha-L,D, and alpha-D,L), as well the four possible isoAsp containing stereoisomers (beta-L,L, beta-D,D, beta-L,D and beta-D,L). The separation of these substances was explored using both neutral and charged cyclodextrins as the stereoisomer selector added to the running electrolyte. The major experimental parameters investigated included pH, the cyclodextrin type, and the cyclodextrin concentration. Due to differences in the pKa values of the carboxylic acid groups, adjustment of the separation buffer to between pH 3.0 and 4.0 provided for sufficient electrophoretic mobility differences to result in excellent separations of the diastereomerically related peptides in this pH region. The resolution of the enantiomerically related peptide stereoisomers was accomplished using low concentrations (1 mM) of the anionic cyclodextrin derivative, sulfobutylether-beta-cyclodextrin (SBE-beta-CD). This negatively charged cyclodextrin was found to be superior for the resolution of the enantiomerically related peptides as compared to native beta-cyclodextrin or the neutral derivatives, dimethyl beta-cyclodextrin and hydroxypropyl beta-cyclodextrin. An alternative approach using anionic or neutral surfactants in conjunction with the SBE-beta-CDs was also explored and found to be successful but problematic.

Selective fluorogenic derivatization of a peptide nucleic acid trimer with naphthalene-2,3-dicarboxaldehyde

The reversed-phase high-performance liquid chromatography of a Peptide Nucleic Acid (PNA) trimer has been studied after its preseparation fluorogenic derivatization with naphthalene-2,3-dicarboxaldehyde in the presence of cyanide (NDA/CN). Trace levels of the PNA trimer were determined in cell homogenate samples containing the PNA trimer at prederivatization concentrations as low as 48.9 ng ml-1. The sample pretreatment operations included a deproteination step, achieved by ultra-filtration, followed by fluorogenic derivatization (NDA/CN). Subsequently, to achieve adequate selectivity, the fluorescently labeled PNA was subjected to high performance anion exchange chromatography prior to quantitation via fluorescence detection. The various problems encountered during sample pretreatment and separation of derivatized PNA trimer in biological samples are presented and discussed.

Evaluation of the utility of capillary electrophoresis for the analysis of sulfobutyl ether beta-cyclodextrin mixtures

A capillary electrophoresis (CE) method for the analysis of a sulfobutyl ether beta-cyclodextrin (SBE-beta-CD) mixture is described. The SBE-beta-CD has been prepared as a parenterally safe solubilizing agent and has historically been characterized by elemental analysis and nuclear magnetic resonance spectroscopy. While these methods provide gross values for the degree of substitution, the CE method described resolves the mixture of positional and regional isomers based on the degree of SBE substitution. The method uses benzoic acid in the running buffer and detects the CD by a decrease in background absorbance of the benzoic acid due to complexation. The necessity of a defined column was sequence between injections was evaluated. The reproducibility of migration times and peak areas/heights for 10 components of the mixture was determined. The modular CE system gave a relative standard deviation of 2.5% (n = 3) for six of the 10 peaks. Further refinements (pH buffer effects) were explored to improve the reproducibility with remaining components. The method was used to evaluate the reproducibility of the synthesis (21 different lots) and the effect of reaction variables (time, temperature and base) on the composition pattern of the modified CD.

Determination of alpha-difluoromethylornithine in blood by microdialysis sampling and capillary electrophoresis with UV detection

A procedure is described for the analysis of alpha-difluoromethylornithine (DFMO), an anti-cancer agent, in plasma microdialysis (MD) samples. DFMO has been shown to be effective alone or in combination with other agents in the treatment of several cancers. Precolumn derivatization of DFMO with naphthalene-2,3-dicarboxaldehyde-cyanide (NDA-CN) in pH 10.0 borate buffer results in the rapid formation of a stable mono-derivatized product (N-substituted 1-cyanobenz[f]isoindole, CBI), which is UV active. An analytical method has been developed to separate CBI-DFMO from NDA-CN derivatization products of 20 standard amino acids using capillary electrophoresis (CE). This method is then employed for the determination of DFMO in plasma microdialysis samples. Separation of DFMO from other components in the dialysate was achieved within 20 min. The response for DFMO in Ringer's solution was linear over the range of 1.2 x 10(-6) to 1.6 x 10(-4) M after derivatization. The detection limit of DFMO in the plasma dialysate is 5 microM using UV detection at 254 nm. This method has been proven to have adequate sensitivity for quantitation of DFMO in i.v. microdialysate samples and has been successfully applied to monitoring the pharmacokinetics of DFMO by CE-UV.

Continuous in vivo monitoring of amino acid neurotransmitters by microdialysis sampling with on-line derivatization and capillary electrophoresis separation

A separation-based biosensor has been developed that is capable of near-real-time analysis of aspartate and glutamate with a temporal resolution of less than 2 min in anesthetized or awake, freely moving animals. The instrument consists of a microdialysis sampling system, an on-line reactor, an injection interface, and a CE-LIF system. Primary amine analytes are derivatized with NDA/CN following microdialysis sampling using an on-line reactor to produce fluorescent CBI derivatives. The reaction takes approximately 1 min. The derivatized sample then travels to a microinjection valve which alternately sends CE running buffer and reacted microdialysis sample to the CE column via an injection interface. The interface allows a controllable volume of 10-20 nL to be injected onto the CE separation capillary. Separation of aspartate and glutamate from the other amino acids present in the microdialysis sample was achieved within 70 s. Detection limits for glutamate and aspartate using laser-induced fluorescence detection were 0.1 microM. The linear dynamic range was acceptable for the determination of aspartate and glutamate in dialysate samples where the levels are between 1 and 10 microM. Full automation of the system was achieved by computer control of the valve, the interface, and the data collection system. The performance of this system was demonstrated in an anesthetized rat by monitoring ECF levels of aspartate and glutamate released in brain after stimulation with high concentrations of K+.

Determination of intracellular levels of 6-mercaptopurine metabolites in erythrocytes utilizing capillary electrophoresis with laser-induced fluorescence detection

Capillary electrophoresis proved to be a useful technique for the analysis of intracellular levels of 6-thioguanosine mono-, di-, and triphosphate with analysis times of 20 min. Conditions required for baseline separation of the thioguanine nucleotides consisted of a 25 mM KH2PO4 (pH 8.0) buffer and a separation voltage of +28 kV. Laser-induced fluorescence detection (lambda ex = 325 nm, lambda em = 410 nm) of the thioguanine nucleotide metabolites of 6-mercaptopurine (6-MP) was possible following oxidation of the thiol functionality. Tedious extraction procedures involving mercury cellulose resins or phenyl mercury adduct formation, which had been required previously for the selective extraction of thiopurines from erythrocytes, were unnecessary due to the overall specificity of the approach. However, the inclusion of 50 mM EDTA in the sample preparation was required to inhibit the anabolic/catabolic enzymatic activity, which was responsible for the degradation of the analytes. The method demonstrated linearity from 5 to 1700 pmol/100 microliters red blood cells for the three analytes (RSDs < or = 8%). The feasibility of the method was demonstrated for the quantitation of 6-thioguanine nucleotides in patients receiving either oral or intravenous 6-MP therapy.

Automated analytical systems for drug development studies. I--A system for the determination of drug stability

An automated system consisting of a pH-stat, microdialysis sampling and a liquid chromatograph was assembled to measure the rate of rapid chemical reactions. 2',3',5'-Triacetyl-6-azauridine was used as a model compound to validate the performance of the automated system. Buffer catalysis was minimized by using a non-catalytic concentration of borate buffer along with a pH-stat to maintain the pH during the kinetic run. The microdialysis sampling technique permitted sample quenching and buffering of the solutions to a pH compatible with the LC column materials. The combination of microdialysis sampling and rapid LC analysis allowed reactions with a half-life of approximately 1 min to be sampled every 30 s. The rates of hydrolysis of the drug, measured at different conditions of temperature (37-70 degrees C) and pH (9.0-10.5) using the automated system, compared well with the previously determined values.

On-line coupling of in vivo microdialysis sampling with capillary electrophoresis

Microdialysis sampling has become an important means of continuously monitoring reactions in vivo. This sampling technique places a constraint on the analysis method because of the very small sample volume provided. On the other hand, microdialysis provides the advantage of clean samples that do not require cleanup prior to analysis. An on-line coupling of microdialysis sampling to capillary electrophoretic (CE) analysis is described that uses the advantages of microcolumn separations to overcome the small volume limitation. An interface was designed which converts the continuous microdialysis sample stream into discrete 60-nL sample plugs and then injects a portion of this plug into the CE system. The on-line interface provided precision of 2.6% with minimal band broadening or peak height loss relative to off-line sampling. Using a high-speed micellar electrokinetic chromatography (MEKC) separation, resolution of the investigational antineoplastic SR 4233 from its main metabolite SR 4317 was achieved in less than 60 s. This allowed the on-line system to achieve a 90-s temporal resolution for determining the pharmacokinetics of SR 4233 in vivo.

Improvements in detection sensitivity for the determination of ivermectin in plasma using chromatographic techniques and laser-induced fluorescence detection with automated derivatization

Improvement in detection sensitivity for the analysis of ivermectin was observed through utilization of laser-induced fluorescence detection and by manipulation of chromatographic conditions. Gradient elution used in combination with narrow-bore chromatography and conventional fluorescence detection resulted in a limit of quantitation for the major homologue of ivermectin of 0.01 ng/ml in dog plasma. Laser-induced fluorescence detection with isocratic chromatographic conditions also resulted in a limit of quantitation of 0.01 ng/ml in dog plasma, which is a six-fold improvement over previously reported methods. Introduction of an automated procedure for the derivatization and injection of samples reduced the amount of sample handling, eliminated the potential for analyte/internal standard degradation and contributed to the overall ease of analysis.

Applications of capillary electrophoresis in pharmaceutical analysis

The role of capillary electrophoresis (CE) in the analysis of peptide/proteins, chiral pharmaceuticals, and other small-molecule drugs has been reviewed. Potential uses of CE range from purity and structural confirmation to a micropreparative technique. Strategies for the prevention of protein wall adsorption include the use of extreme pH values, surface-modified capillaries, and high ionic strengths employing salts of alkali metals or by the addition of zwitterionic surfactants to the background electrolyte. Chiral separations of amino acids and other racemic pharmaceuticals have been achieved by micellar electrokinetic chromatography or by the introduction of cyclodextrins/modified cyclodextrins or other reagents to the running buffer. Applications of capillary electrophoresis to the analysis of small-molecule pharmaceuticals include determinations of drugs and/or excipients in various pharmaceutical preparations and the analysis of miscellaneous pharmaceuticals in standard solutions and biological fluids. The complementary nature of capillary electrophoresis and HPLC, in addition to future expectations of CE in pharmaceutical analysis, is discussed.

Reversed-phase liquid chromatography of the opioid peptides. 3. Development of a microanalytical system for opioid peptides involving microbore liquid chromatography, post-column derivatization and laser-induced fluorescence detection

A microanalytical system has been developed for the determination of peptides in small samples. Naphthalene-2,3-dicarboxaldehyde-beta-mercaptoethanol (NDA-BME) was used as the labelling reagent system as an alternative to NDA-cyanide (NDA-CN) because of the faster labelling when CN was replaced by a thiol. The fluorescence characteristics of the NDA-thiol adducts, N-substituted 1-alkylthiobenz[f]isoindoles (TBIs), were found to be different from the previously described cyanobenz[f]isoindole (CBIs) adducts formed by the reaction of primary amines with NDA-CN. The excitation maximum of the TBI adducts was at 460 nm, which was closer to the 457.9 nm argon-ion laser line, than the 440-nm maximum of the CBI adduct. The limit of detection for leucine enkephalin was 36 fmol (S/N = 3) and linearity was proven for greater than 2 orders of magnitude, from 45 fmol to 9 pmol for an injection volume of 60 nl. The detectability was limited by the high background noise produced by the post-column derivatization system. The utility of the system was demonstrated for the analysis of methionine enkephalin and its potential oxidation products, using leucine enkephalin as a suitable internal standard.

Characterization of sulphoalkyl ether derivatives of beta-cyclodextrin by capillary electrophoresis with indirect UV detection

A capillary electrophoresis method which characterizes the degrees of substitution of heterogeneous sulphoalkyl ether beta-cyclodextrin derivatives is described. The separation is based on the different electrophoretic mobilities observed from changes in the overall charge of the molecule as a result of substitution. Individual peaks of the electropherogram then provide a measure of each degree of substitution of the present beta-cyclodextrin. Detection of these beta-cyclodextrin derivatives is performed by indirect UV detection.

Reversed-phase liquid chromatography of the opioid peptides--2. Quantitative structure-retention relationships and isocratic retention prediction

The ability of Snyder's theory of linear gradient elution to predict the starting isocratic reversed-phase LC conditions (k' = 4-10) for the opioid peptides was investigated. The errors in predicting the concentration of acetonitrile (phi) required to elute the peptides with a k' value of 4 were high, ranging from 13.5 to 38.1%. At k' = 10 the errors were generally reduced to less than 20%. This analysis was repeated with the same peptides after conversion to their fluorescent 1-cyanobenz[f]isoindoles (CBIs) by reaction with naphthalene-2,3-dicarboxaldehyde/cyanide. For the CBI derivatives, the errors in predicting the required concentration of acetonitrile for isocratic elution were markedly reduced and ranged from 0 to 14.3 for k' = 4 and 0 to 11.9% for k' = 10. The errors in the model in predicting the required isocratic mobile phase accurately were attributed to a mixed mechanism of retention involving solvophobic and silanophilic interactions and leading to non-linear relationships between log k' and phi. Even when the errors in predicting the required value of phi were relatively high, the Snyder approach was found to be very useful in predicting the initial starting conditions for the reversed-phase LC of the native opioid peptides as well as their fluorescence CBI derivatives.

Reversed-phase ion-pair liquid chromatography of the angiotensins

The isocratic reversed-phase liquid chromatography of the angiotensins and a number of their synthetic analogues is described. Complete separation of 10 out of 12 peptides was achieved through a solvent optimization strategy with a total analysis time of about 20 min. The retention behavior of the angiotensins studied was described in terms of the hydrophobic contribution of their amino acid residues; there was good correlation between predicted and experimental retention for those peptides that were retained by a common mechanism. However, because ion-pair chromatography was required for good peak symmetry, retention was substantially modulated by the presence of acidic and basic residues. The limit of detection of these peptides was 3-5 pmol by UV absorbance at 214 nm. For those peptides containing a primary amino group the detection limit was improved by two orders of magnitude by fluorogenic derivatization with naphthalene-2,3-dicarboxaldehyde/cyanide to the corresponding N-substituted 1-cyanobenz[f]isoindole (CBI) derivatives. The contribution of the CBI ring system to retention was also investigated.

Stereoselective pharmacokinetics and metabolism of the enantiomers of cyclophosphamide. Preliminary results in humans and rabbits

[R(+),S(-)]-Cyclophosphamide [(R,S)-CP] is an anticancer drug, containing a chiral phosphorous atom, which is prepared and used clinically as the racemic mixture. A new high-performance liquid chromatographic assay suitable for pharmacokinetic studies of CP enantiomers in plasma has been reported recently by this laboratory (Reid et al., Anal Chem 61: 441-446, 1989). Briefly, the assay involves ethyl acetate extraction of CP enantiomers from plasma followed by derivatization to diastereomers in a two-step process utilizing chloral and (+)-naproxen acid chloride. Chromatographic analysis was performed on a reversed phase (ODS) column with detection at 232 nm. In the present study, preliminary results on the applicability of this assay to pharmacokinetic studies are presented. Several rabbits were used to compare the influence of i.p., i.v., and oral routes of administration on the stereoselective disposition of (R,S)-CP. Following i.p. administration, S-CP was cleared faster than R-CP. Following oral administration, only R-CP was detectable in plasma, while i.v. administration resulted in minor or no stereoselective disposition. These results indicated that there was a marked stereoselective metabolism of the S-CP enantiomer, with the i.p. and oral routes producing the greatest differences due to first-pass metabolism. Incubation of rabbit-liver microsomes with (R,S)-CP demonstrated that the monooxygenase system can exhibit marked stereoselectivity in its metabolism of CP. The ratio of R-CP to S-CP in the incubation medium increased during the incubation period from 1:1 initially to 4.5:1 after 60 min. The results from the experiments with rabbits indicate that the first-pass metabolism of this drug is highly stereoselective; in contrast, cancer patients who had received (R,S)-CP as an i.v. infusion showed no stereoselectivity in the elimination of the enantiomers. Pharmacokinetic studies with cancer patients, receiving (R,S)-CP as an oral dose, are in progress in order to determine if stereoselective first-pass metabolism of this drug also occurs in humans.

Multidimensional liquid chromatography of opioid peptides: fluorogenic labelling, retention prediction and separation optimization

The ultra-trace analysis of opioid peptides in biological samples can be achieved by multidimensional liquid chromatography with pre-column fluorogenic derivatization with naphthalene-2,3-dicarboxaldehyde in the presence of cyanide ion. However, in order to take full advantage of the high sensitivity possible with detectors based on laser-induced fluorescence or chemiluminescence, each component of the analytical method must be carefully optimized. In this study, strategies are presented for the prediction of retention time and the optimization of separations of derivatized opioid peptides in multidimensional LC systems.

Fluorogenic derivatization of peptides with naphthalene-2,3-dicarboxaldehyde/cyanide: optimization of yield and application in the determination of leucine-enkephalin spiked human plasma samples

Initial attempts to derivatize the alpha-amino site of several tripeptides with naphthalene-2,3-dicarboxaldehyde/cyanide (NDA/CN) resulted in poor yields of the expected N-substituted 1-cyanobenz[f]isoindole (CBI) products. Examination of the CBI-formation mechanism, in conjunction with knowledge of the general structure-reactivity properties of the tripeptides, led to the recognition of a competing non-productive reaction pathway. Through the use of model reactions and the isolation and structural elucidation of a predicted side-product the viability of the competing pathway was confirmed. From an understanding of the key features of both the productive and non-productive reaction pathways, a rational approach for the optimization of CBI-derivative yield was proposed and confirmed experimentally. This information led, in turn, to the development of HPLC methodology suitable for the determination of leu-enkephalin spiked into human plasma; fluorescence detection was used in conjunction with leu-enkephalin amide as the internal standard. The method enabled leu-enkephalin to be determined at a concentration of 0.31 nmol ml-1 with an error of less than 4% for 25 pmol injected.

Determination of the anticancer drug, 15-deoxyspergualin, in plasma ultrafiltrate by liquid chromatography and precolumn derivatization with naphthalene-2,3-dicarboxaldehyde/cyanide

An alternative analytical method for the determination of 15-deoxyspergualin in plasma is described. The drug was initially separated from the plasma matrix by ultrafiltration and a precolumn derivatization step was performed with naphthalene-2,3-dicarboxaldehyde in the presence of sodium cyanide to yield the fluorescent N-substituted 1-cyanobenz[f]isoindole (CBI) derivative. The CBI derivative was separated and quantitated by reversed-phase chromatography using an ODS Hypersil column and mobile phase of KH2PO4 (0.1 M)-H3PO4-acetonitrile (48:0.8:52, v/v/v) containing dodecyl sodium sulphate (18 mM). The excitation and emission wavelengths for the fluorescence detector were 420 and 490 nm, respectively. The peak height was linearly related to drug concentration over the range from 5 ng ml-1 (10 nM) to 10 micrograms ml-1 (20 microM) in phosphate buffer (0.1 M, pH 7.0), spiked plasma ultrafiltrate and ultrafiltrate obtained from spiked plasma. Measurements could be made with a relative standard deviation of 4.5% or less in phosphate buffer (0.1 M, pH 7.0), 6.1% or less in spiked plasma ultrafiltrate and 12% or less in ultrafiltrate obtained from spiked plasma.

Chemiluminescence: a sensitive and versatile method for analyte detection

The generation of light from the oxidation of oxalic esters with hydrogen peroxide has been applied to the detection of luminescent materials. In order to improve the efficiency of this method, which is less than 0.1%, and to enhance the selectivity for target analytes, an in-depth investigation of the oxalate ester-hydrogen peroxide reaction has been conducted. A kinetic model has been developed based on the effects of catalysts, reagents and reaction conditions for maximum light production. Application of the model to liquid chromatography through the "time-dependent emission window" concept affords a predictable maximum sensitivity for selected analytes. Application to the detection and quantitation of met- and leu-enkephalins which have been labelled with naphthalene-2,3-dicarboxyaldehyde/cyanide provides support for this methodology. Other bioanalytical applications are presented.

Multi-dimensional high-performance liquid chromatography of opioid peptides following pre-column derivatization with naphthalene-2,3-dicarboxaldehyde in the presence of cyanide ion. Preliminary results on the determination of leucine- and methionine-enkephalin-like fluorescence in the striatum region of the rat brain

The reversed-phase high-performance liquid chromatography of three synthetic opioid peptides, 5leucine-enkephalin, 5methionine-enkephalin and [D-2alanine]-5methionine enkephalin, has been studied after their pre-column fluorogenic derivatization with naphthalene-2,3-dicarboxaldehyde in the presence of cyanide to the corresponding 1-cyanobenz[f]isoindole (CBI) derivatives. The chromatographic properties of the three synthetic CBI-peptides were characterized using three different stationary phases, ODS Hypersil, CPS Hypersil and Spherisorb Phenyl, eluted with mobile phases containing various concentrations of methanol, tetrahydrofuran or acetonitrile in 26 mM trifluoroacetic acid, adjusted to pH 3.5. The data obtained using single chromatographic columns were used to design a multi-dimensional system in which the three synthetic CBI-peptides of interest were transferred as a single fraction from one column to a second. The first column served to separate the peptides from the majority of the material in the samples, and the second column was used to separate the three CBI-peptides from each other. The best separation was achieved in which the first column was Spherisorb Phenyl and the second column was ODS Hypersil. Both columns were eluted with a mobile phase of 45% acetonitrile (v/v) in 26 mM trifluoroacetic acid (pH 3.5) at a flow-rate of 1.0 ml/min. The method has been applied to the determination of leucine- and methionine-enkephalin-like fluorescence in the striatum of the rat brain.

Analysis of 5-fluorouracil in plasma by precolumn derivatization with 4-bromomethyl-7-methoxycoumarin, followed by multi-dimensional high-performance liquid chromatography

An assay for 5-fluorouracil (5-FU) has been developed that utilizes a double extraction with ethyl acetate, followed by precolumn derivatization with 4-bromo-methyl-7-methoxycoumarin. The reaction mixture was quenched with 5% acetic acid, extracted with hexane, and analyzed by multi-dimensional high-performance liquid chromatography. Derivatized 5-FU was injected into a cyanopropyl column and a heart cut containing the analyte was then switched to an octadecyl column and quantitated by fluorescence detection. The assay had a limit of detection of 0.5 ng 5-FU/ml plasma and was linear to 20 micrograms/ml. It was shown to be free of interferences from the other anticancer agents commonly used in combination with 5-FU. This assay should have the sensitivity needed to measure the low levels that occur after low-dose, continuous infusion of 5-FU.

Liquid chromatographic determination of cyclophosphamide enantiomers in plasma by precolumn chiral derivatization

On the basis of reactions described in the synthetic literature, a two-step chiral derivatization sequence was developed for the anticancer agent cyclophosphamide (CP). The sequence involves amidoalkylation of CP with anhydrous chloral containing 1% dimethylformamide followed by acylation of the resulting secondary alcohol with a chiral carboxylic acid chloride, (+)-6-methoxy-alpha-methyl-2-naphthaleneacetyl chloride, to form a diastereomeric pair. Derivatized (-)-CP and (+)-CP exhibited retention times of 17.2 and 20.7 min, respectively, when chromatographed on Hypersil ODS with acetonitrile/phosphate buffer (pH 6.8) as the mobile phase. Preparation of the individual diastereomers from enantiomerically pure CP enabled correlation of the chromatographically observed peaks with a particular enantiomer. Various aspects of the overall assay methodology have been systematically investigated (derivatization solvents, temperatures, reaction time, and work-up procedure) and optimized on the scale required for trace analysis in biological fluids. Calibration curves were established for each enantiomer in spiked human plasma over the therapeutically relevant concentration range of 0.99-49.94 micrograms/mL.

Suitability of DTAF as a fluorescent labelling reagent for direct analysis of primary and secondary amines--spectral and chemical reactivity considerations

DTAF has been used successfully to prepare fluorescent labelled reagents for fluorescence polarization immunoassays. Its applicability as a derivation reagent for direct fluorescence analysis of primary and secondary amines was evaluated. DTAF was shown to have spectral properties that closely resemble those of fluorescein and that are apparently insensitive to the presence of the triazine nucleus. Spectrally determined pKa values also closely resemble those of fluorescein and other amino aryl-s-triazines. DTAF is prone to hydrolytic degradation with the rate of reaction increasing with increasing pH, until a pH value is reached at which ionization of the amine bridging the two aromatic nuclei occurs; at this pH the rate reaches a plateau value. Both primary and secondary amines react efficiently with DTAF, and the reactivity increases with the increasing basicity of the amine reactants. The reaction is pH-dependent, proceeding most efficiently at pH values at which both the "bridging amine" of DTAF and the amine substrate are unionized. Methyl substituted secondary amines were consistently more reactive than the corresponding primary amine, but further imposition of steric bulk about the amine nitrogen significantly reduced the reactivity of amines toward DTAF. In cases where such steric bulk is minimal. DTAF appears to be a suitable fluorescent labelling reagent for direct analytical applications.

High-performance liquid chromatography of 5-fluorouracil after derivatization with 4-bromomethyl-7-methoxycoumarin. Characterization of the derivative and the use of column switching for the improvement of resolution and the enhancement of sensitivity

The derivatization of 5-fluorouracil with 4-bromomethyl-7-methoxycoumarin has been reported previously; however, the structure of the derivative was not confirmed. The synthesis and purification of the 5-FU derivative is described along with the spectroscopic (MS and NMR) determination that it is labelled at both heterocyclic nitrogens as expected. A column switching HPLC system is also presented which consists of primary separation on a cyanopropyl column followed by a final separation on an ODS column with fluorescence detection. This system removes all interferences from the derivatization system and has a limit of detection for the pure derivative of less than 50 fmol (injection volume = 100 microliters).

High-performance liquid chromatography of bis[1,2-bis-(diphenylphosphino)ethane]gold(I) chloride, a potential antineoplastic agent

The chromatography of [Au(dppe)2]+ (I), a potential antineoplastic drug, was studied on a variety of stationary phases (ODS Hypersil, PLRP-S, Partisil SAX and Partisil SCX) using aqueous mobile phases containing 60% acetonitrile, 15% tetrahydrofuran and various electrolytes. The effects of both the concentration (0-20 mM) and the nature of the electrolytes, added to the mobile phase, on the chromatography of I were investigated. A wide variety of electrolytes were investigated in which the hydrophobicity of both the anion and the cation were varied. The analyte of interest was found to be unretained by the like-charged Partisil SAX column. On the other hand, I was retained on the Partisil SCX by an ion-exchange mechanism and retention could be controlled by manipulating the electrolyte composition of the mobile phase. I was retained on the two reversed-phase materials by a mixture of solvophobic and electrostatic interactions but, under the conditions studied, the latter mechanism was the dominant one. The retention of I on the two reversed-phase materials was influenced much more by the nature and concentration of the cation added to the mobile phase than it was by the nature and concentration of the anion. Therefore, manipulation of the nature and concentration of the cationic species in the mobile phase appears to afford the most useful means of manipulating the retention of I, and presumably its analogues, on reversed-phase columns.

Chemical derivatization as a strategy to enhance delectability of agents used in cancer management

The bioanalysis of drugs used in the management of cancer is often complicated by the lack of selectivity and sensitivity. Chemical derivatization of these drugs prior to their chromatographic analysis represents a viable strategy to improve chromatographic resolution and to enhance detectability. This review provides examples of how this approach can meet these objectives. Derivatization of racemic cyclophosphamide with a chiral acylating agent, following hydroxyalkylation to introduce a reactive centre into the molecule, provides the basis for its stereospecific analysis. The analysis of dianhydrogalactitol is described, in which diethyldithiocarbamate is used as a nucleophilic derivatizing agent that improves chromatographic behaviour and analytical sensitivity. The final example that is described is the design and preparation of improved fluorogenic reagents (o-phthalaldehyde analogues) for the derivatization of peptides and application of these reagents to the trace analysis of leu-enkephalin in plasma.

Aspects of the stability of isoindoles derived from the reaction of o-phthalaldehyde—ethanethiol with primary amino compounds

The stability of a series of fluorescent isoindoles formed under analytical conditions following the reaction of o-phthalaldehyde (OPA) and ethanethiol (ET) with a series of primary amines is reported. Increasing the bulk and degree of substitution of the isoindole N-substituent resulted in substantial increases in isoindole stability. The effects of excess reagents on isoindole stability is examined and OPA is observed to accelerate isoindole degradation whilst ET provides a stabilizing effect. Comparison with previously reported data involving the use of 2-mercaptoethanol revealed that ET clearly forms the more stable isoindole derivatives, i.e. a minimum of five-fold improved stability based on the time for 10% degradation to occur. Identification of the major degradation product together with kinetic data suggests that degradation proceeds via autoxidation.

Rational design and evaluation of improved o-phthalaldehyde-like fluorogenic reagents

Evidence was presented suggesting that the fluorescent isoindole produced by reaction of o-phthalaldehyde (OPA), ethanethiol, and primary amine was formed by initial imine formation followed by conversion to an alpha-alkylaminobenzylsulfide and subsequent ring closure to form the isoindole nucleus. This mechanism suggested that the minimum structural requirement for condensation to an isoindole was an o-diacyl benzene in which one of the carbonyl groups was aldehydic. A major drawback of OPA as an analytical reagent is the limited stability of the fluorescent 1,2-disubstituted isoindole. Since isoindole instability is related to autoxidation at C-3, the use of o-(formyl) arylketones as alternatives to OPA is attractive in increasing the lifetime of the fluorescent species in that such reagents would form 1,2,3-trisubstituted isoindoles. Two compounds, o-acetylbenzaldehyde (OAB) and o-benzoylbenzaldehyde (OBB), were synthesized and evaluated as potential fluorogenic reagents. Both formed fluorescent products. The rate of formation of isoindole from the latter was too slow to make it of practical analytical value; however, OAB formed isoindoles with t1/2 less than 10 s and offered markedly improved stability over that observed with OPA.

Factors affecting the stability of fluorescent isoindoles derived from reaction of o-phthalaldehyde and hydroxyalkylthiols with primary amines

The stability of a series of fluorescent isoindole derivatives formed in situ under analytical conditions following the reaction of o-phthalaldehyde (OPA) and 2-mercaptoethanol (2-ME) with a series of primary amines are reported. Increasing the bulk and degree of substitution at C-10 of the resulting isoindole resulted in substantial increases in product stability. The effects of excess OPA and 2-ME on isoindole stability were examined and OPA was observed to catalyze isoindole degradation while 2-ME had no effect. Previously proposed degradation mechanisms were reexamined in light of the present data and an alternate degradation pathway is proposed. 3-Mercapto-1-propanol (3-MP) was found to be a superior thiol for use in the fluorogenic OPA reaction. The OPA/3-MP reagent combination was utilized to derive several amino acids and offered detection limits (S/N = 2) of less than 200 fmol.

Synthesis and biological evaluation of gem-dichlorocyclopropyl and cyclopropyl analogs of stilbene congeners as potential antiestrogens

A series of gem-cichlorocyclopropyl and cyclopropyl analogs of stilbene congeners was synthesized and examined for estrogenic and antiestrogenic activity using the uterotropic assay in the immature mouse. The relative receptor affinity in vitro was determined by measuring [3H]estradiol displacement from the rat uterine cytosol receptor. The 11 test compounds synthesized in this study did not produce estrogenic or antiestrogenic activity at the dosage levels used (1-25 micrograms), but did produce a significant displacement of [3H]estradiol in the rat uterine receptor binding assay with analog XVIII possessing the greatest binding affinity and compound XI the lowest affinity. Structure-affinity relationships of this series were established from the receptor binding assay and comparisons between these analogs and a previously reported series are summarized.