Ritalinic acid in urine: Impact of age and dose.

OBJECTIVES: The objective of this work was to study the results of urine drug testing for ritalinic acid (RA), the major urinary metabolite of methylphenidate (MP) (e.g., Ritalin®). The impact of age from 4 to 65 years old and older on median levels of RA was investigated as well as potential variations in pH, specific gravity and creatinine content of the patient urine samples. DESIGN AND METHODS: Samples from patients who were 1) prescribed MP and found to be positive for RA, 2) prescribed MP but found to be negative for RA and 3) not prescribed MP but tested positive for RA were examined by liquid chromatography - mass spectrometry/mass spectrometry (LC-MS/MS) for RA concentration. The levels of RA were examined for median and average levels and further normalized and transformed to reveal a near gaussian distribution. RESULTS: Over 20,000 samples from patients who were prescribed MP were examined for this work. Analysis of these data for a subset of patients prescribed MP and testing positive for RA revealed statistically different median values of RA for school age patients of 6 years old through 17 years old from adult patients 18 through 64 years old. Another 6751 samples were positive for RA without a prescription but were not included in the overall assessment of these data. CONCLUSIONS: While not clear as to the reason, these data indicate that school age children under the age of 18 have much higher levels of RA than adult patients. These results can be used to estimate "normal" levels of RA in these chronically dosed populations.

The effects of visual discomfort and chromaticity separation on neural processing during a visual task.

Visual stimuli that are uncomfortable to look at evoke a large neural response suggesting altered processing. While there is some evidence linking uncomfortable achromatic stimuli to impaired visual processing, the effect of uncomfortable chromatic patterns on visual cognition has yet to be explored. Large differences in chromaticity separation (e.g. red and blue) elicit visual discomfort, larger metabolic responses, larger visual evoked potentials, and greater alpha suppression compared to small chromaticity separations (e.g. pink and purple). We investigated the impact of stimuli that varied in their chromaticity separation (calculated in perceptual color space) on a visual task and their effect on neural responses across the cortex. Thirty participants completed a continuous pairs task (letters changed at 3 Hz) while grating patterns that differed in their chromaticity separation alternated with a grey screen at 5 Hz. The different temporal frequencies allowed for steady-state visual evoked potentials (SSVEPs) to the two stimulus-types to be measured simultaneously using electroencephalography (EEG). A subset of participants rated the gratings on a 9-point scale of discomfort. We observed greater ratings of discomfort and increased power at 5 Hz with the larger chromaticity separations. The increase in 5 Hz power with greater chromaticity separation was evident across the cortex. However, there was no significant effect of chromaticity separation on power at 3 Hz, or on reaction times, and no consistent effect on behavioral accuracy. Despite eliciting heightened neural responses across the cortex, short term exposure to uncomfortable chromatic stimuli does not adversely impact visual task performance.

A Rapid LC-MS-MS Method for the Quantitation of Antiepileptic Drugs in Urine.

Epilepsy is a common neurologic disease that requires treatment with one or more medications. Due to the polypharmaceutical treatments, potential side effects, and drug-drug interactions associated with these medications, therapeutic drug monitoring is important. Therapeutic drug monitoring is typically performed in blood due to established clinical ranges. While blood provides the benefit of determining clinical ranges, urine requires a less invasive collection method, which is attractive for medication monitoring. As urine does not typically have established clinical ranges, it has not become a preferred specimen for monitoring medication adherence. Thus, large urine clinical data sets are rarely published, making method development that addresses reasonable concentration ranges difficult. An initial method developed and validated in-house utilized a universal analytical range of 50-5,000 ng/mL for all antiepileptic drugs and metabolites of interest in this work, namely carbamazepine, carbamazepine-10,11-epoxide, eslicarbazepine, lamotrigine, levetiracetam, oxcarbazepine, phenytoin, 4-hydroxyphenytoin, and topiramate. This upper limit of the analytical range was too low leading to a repeat rate of 11.59% due to concentrations >5,000 ng/mL. Therefore, a new, fast liquid chromatography-tandem mass spectrometry (LC-MS-MS) method with a run time under 4 minutes was developed and validated for the simultaneous quantification of the previously mentioned nine antiepileptic drugs and their metabolites. Urine samples were prepared by solid-phase extraction and analyzed using a Phenomenex Phenyl-Hexyl column with an Agilent 6460 LC-MS-MS instrument system. During method development and validation, the analytical range was optimized for each drug to reduce repeat analysis due to concentrations above the linear range and for carryover. This reduced the average daily repeat rate for antiepileptic testing from 11.59% to 4.82%. After validation, this method was used to test and analyze patient specimens over the course of approximately one year. The resulting concentration data were curated to eliminate specimens that could indicate an individual was noncompliant with their therapy (i.e., positive for illicit drugs) and yielded between 20 and 1,700 concentration points from the patient specimens, depending on the analyte. The resulting raw quantitative urine data set is presented as preliminary reference ranges to assist with interpreting urine drug concentrations for the nine aforementioned antiepileptic medications and metabolites.

A Dilute and Shoot LC-MS/MS Method for Antipsychotics in Urine.

Adherence to prescribed antipsychotics is an ongoing problem. Traditionally, estimates of adherence have been made from patient interviews, pill counting and blood testing. A number of methods for the analysis of antipsychotics in blood have been reported for both therapeutic drug monitoring and postmortem testing for toxicity. This report details a dilute and shoot method for the analysis of 19 different antipsychotics and metabolites. The method takes advantage of earlier reports demonstrating unique, prevalent urine metabolites for aripiprazole, brexpiprazole, haloperidol and lurasidone to enhance sensitivity for these analytes. With a fast analysis time and minimal sample preparation, this method can be used for quantitation of antipsychotics in urine. Finally, this method has been used to test samples for over a year with the results summarized in this report. While further improvements are certainly possible, this method is selective and sensitive for this group of important compounds.

Development and Validation of a Novel All-Inclusive LC-MS-MS Designer Drug Method.

Designer drugs including synthetic cannabinoids and synthetic cathinones are an increasing problem due to the ease of access to these compounds. They present analytical challenges inasmuch as the compound structures are numerous and growing within each class. Typically each class of designer compounds is analyzed separately due to differences in chemistry, desired cut-offs or other reasons. Physicians treating "high-risk" patients typically order tests for all "illicit" substances which can span several test classes. Despite that multiple classes of designer drugs are ordered together, there has not been a comprehensive confirmatory test developed to date. Presented here is a novel comprehensive designer drug LC-MS-MS method that combines synthetic cannabinoids and synthetic cathinones, etizolam, a designer benzodiazepine and mitragynine (kratom), a natural product analgesic. This method improves laboratory throughput with a cycle time of ~4.5 min which affords resolution of crucial isomers, such as ethylone and butylone. Development of this method also provided an opportunity to update the list of compounds within the method. Analytes with fewer than five positive specimens in a year of testing with previous separate methods were removed as old and not current. New analytes were added based on reports from NMS Laboratories and the US Drug Enforcement Administration testing and drug seizures, which included etizolam, its major metabolite α-hydroxyetizolam as well as newer synthetic cannabinoids (5-fluoro ADB metabolite 7, AB-FUBINACA metabolite 3, AB-FUBINACA metabolite 4 and MDMB-FUBINACA metabolite M1) and synthetic cathinones (N-ethyl pentylone). Finally, the impact of the new analytes and cut-off changes are discussed in context with patient results from the first 4 months of testing after implementation of the method in the lab.

Nicotine and cotinine in oral fluid: Passive exposure vs active smoking.

Scheidweiler and colleagues have clinically tested and identified a reporting cutoff (10 ng/mL) of nicotine and cotinine in oral fluid that could reliably determine active smoking in patients. The results from that study were reevaluated using a large data set of oral fluid nicotine and cotinine results available from pain medication monitoring. Additionally, test results from patients using a nicotine transdermal patch delivery device are compared with those from smokers. Finally, oral fluid test results collected over a 2-year period were normalized and transformed to yield a near Gaussian distribution for nicotine. The normalized and transformed data reveal the presence of two independent populations: a larger population consistent with active smokers and a smaller population consistent with those passively exposed to smoke. Furthermore, application of this model to patients prescribed transdermal nicotine reveals oral fluid levels consistent with those of active smokers. The clinical delineation of smokers from non-smokers reported earlier is supported by the oral fluid nicotine data modelling presented herein. These data indicate that oral fluid is an acceptable sample matrix for determining the smoking status of patients. Further, these data indicate that oral fluid test results are indistinguishable between patients prescribed transdermal patches and active smokers; however, oral fluid testing can determine absence of patches or smoking.

Zolpidem and Zolpidem Carboxylic Acid Results from Medication Monitoring.

Zolpidem (Ambien®) is one of the "Z" drugs often used to improve sleep in older patients and those suffering from insomnia. Schwope, D.M., DePriest, A., Black, D.L., Caplan, Y.H., Cone, E.J., Heltsley, R. (2014) Determing zolpidem compliance: urinary metabolite detection and prevalence in chronic pain patients . Journal of Analytical Toxicology, 38, 513-518 reported that zolpidem in urine is not very prevalent being present <23% of the time in patient urine while the major metabolite, zolpidem 4-phenyl carboxylic acid (ZCA), is much more prevalent in urine with positive rates as high as 50% of the patient samples reviewed. Results from patient testing over a year's time are in agreement with the reported zolpidem results. However, the data observed herein for ZCA are not consistent with the earlier report. These data suggest that monitoring ZCA may result in even higher levels of positivity. Further, while the Food and Drug Administration has pointed out that female dosing should be half that given to males, results of this population testing indicate that the majority of patients (83% male and 73% female) receive 10 mg/day or 12.5 mg/day for Ambien CR® with females demonstrating statistically significantly higher levels of ZCA albeit zolpidem levels are not statistically significantly different between men and women. Estimates of patient positivity are dependent upon the value of the limit of quantification (LOQ) as demonstrated by the zolpidem results herein (LOQ = 50 ng/mL vs. 4 ng/mL). However, even with a much higher LOQ of 50 ng/mL for ZCA in this work, the positivity from ZCA results is significantly higher (e.g., 64.8%) than reported earlier (50.3%). Nevertheless, these data support the addition of ZCA for monitoring zolpidem in urine.

Impact of ß-Glucuronidase Mediated Hydrolysis on Haldol® Urinalysis.

Reports have suggested that patients with mental health disorders including major depressive disorder and schizophrenia have dramatically low adherence levels to prescribed medications. Patients on haloperidol (Haldol®) therapy, regardless of their disease, were found to have higher adherence levels-though still strikingly low. This work shows that high levels of the glucuronidated form of haloperidol are present in patient urine samples. Time-of-Flight (TOF) mass spectrometry experiments are consistent with both the presence of haloperidol glucuronide and that hydrolysis of haloperidol patient urine samples leads to significantly increased concentrations of free haloperidol. Urine samples collected from patients prescribed haloperidol were tested with and without hydrolysis revealing a significant increase in the number of patients testing positive when the samples were hydrolyzed before analysis. These data demonstrate that hydrolysis greatly improves the sensitivity and consistency of results for patients on haloperidol therapy resulting in positivity data that strongly correlates with the dosage form administered.

Analytical Considerations When Developing an LC-MS/MS Method for More than 30 Analytes.

BACKGROUND: While validation of analytical (LC-MS/MS) methods has been documented in any number of articles and reference texts, the optimal design and subsequent validation of a method for over 30 analytes presents special challenges. Conventional approaches to calibration curves, controls, and run time are not tenable in such methods. This report details the practical aspects of designing and implementing such a method in accordance with College of American Pathologists validation criteria. METHODS: Conventional criteria were followed in the design and validation of a method for 34 analytes and 15 internal standards by LC-MS/MS. These criteria are laid out in a standard operating procedure, which is followed without exception and is consistent with College of American Pathologists criteria. RESULTS: The method presented herein provides quality results and accurate medication monitoring. The method was optimized to negate interferences (both from within the method and from potential concomitant compounds), increase throughput, and provide reproducible quality quantification over relevant analyte concentrations ranges. CONCLUSIONS: The method was designed primarily with quality and accurate medication monitoring in mind. The method achieves these goals by use of novel approaches to calibration curves and controls that both improve performance and minimize risk (financial and operational). As automation and LC-MS/MS equipment continue to improve, it is expected that more methods like this one will be developed.

Patterns of Marijuana Use in a 6-Month Pain Management Sample in the United States.

This study is a 6-month retrospective analysis of urine drug testing (UDT) data from a pain management population among specimens with clinician-ordered marijuana testing (N = 194 809). Descriptive statistics about the specimen positivity of clinician-ordered marijuana UDT are provided as well as other drug positivity. Specimens from men and adults aged 18 to 34 years had the highest prevalence rates of marijuana positivity. The prevalence of past-month marijuana use among a comparative national population was lower than the prevalence of positive marijuana tests in the UDT specimens by all characteristics. Among the specimens tested for illicit drugs and marijuana, 4.0% were positive for amphetamine, 2.8% were positive for cocaine, and 0.9% were positive for heroin. The most common prescription drugs listed were opioids (64.7%), benzodiazepines (20.5%), and antidepressants (19.9%). In sum, the findings reflect previous research showing high rates of marijuana use, illicit drug use, and prescription drug use in a pain management population.

Quetiapine Carboxylic Acid and Quetiapine Sulfoxide Prevalence in Patient Urine.

Treatment adherence is often an issue with mental health patients. For those prescribed quetiapine (Seroquel®), the low levels of parent drug and plasma metabolite(s) (e.g., 7-hydroxyquetiapine) typically used in urine drug monitoring can result in false negatives with concomitant unfavorable impacts on patient care. Literature review coupled with liquid chromatography/time-of-flight mass spectrometry analysis of patient positive urine samples indicated the presence of quetiapine carboxylic acid and quetiapine sulfoxide as significant urinary metabolites of quetiapine. Analysis of these two metabolites determined that they are abundant in the urine of quetiapine patients and can result in apparent adherence rates that are improved relative to those determined using only quetiapine and 7-hydroxyquetiapine. For example, analysis of a random set of 114 patients who were prescribed quetiapine exhibited an apparent adherence rate of 47% using the quetiapine carboxylic acid and quetiapine sulfoxide metabolites. Traditional metabolite testing with quetiapine and 7-hydroxyquetiapine yielded apparent adherence rates of ~31% while all four analytes resulted in apparent adherence of 48%. The prevalence of these metabolites suggests that quetiapine urine drug testing would be more consistent with prescriptions when they are included in the analysis.

Normalizing Oral Fluid Hydrocodone Data Using Calculated Blood Volume.

Oral fluid testing to assist in the assessment of treatment adherence for chronic pain patients is attractive for a number of reasons. However, efforts focused on interpreting patient results have been modest when compared to urine drug testing. This work details a retrospective approach developed to transform and normalize oral fluid testing results to provide a historical picture of patient values in this important test fluid. Using this approach, a model was developed using data from 6,800 independent patients who were both prescribed hydrocodone and tested positive (with limitations: reporting cutoff < X < upper limit of quantitation) by liquid chromatography-mass spectrometry. Patient demographic data were used to calculate the relevant parameters (e.g., calculated blood volume (CBV)) used in the transformation and normalization of the oral fluid data. The crucial normalizing factor in oral fluids was found to be the CBV which parallels the use of creatinine to normalize drug concentration levels in urine and is consistent with the view that oral fluid samples reflect plasma concentrations of the respective drugs. The resulting near Gaussian distribution is dose independent and as such should be of value to physicians in quickly assessing whether their patient is consistent with this historical population in the broad terms of this model. While this comparison alone is not definitive for adherence with a treatment regimen, together with patient interviews, prescription history and other clinical criteria, it can add an idea of expected patient values from oral fluid testing.

Fentanyl-Norfentanyl Concentrations During Transdermal Patch Application: LC-MS-MS Urine Analysis.

Poklis and Backer published a survey of the concentrations of fentanyl and norfentanyl that could be expected in urine from patients using Duragesic®, a transdermal fentanyl patch. That study employed a relatively small number of patient data points and analysis by Gas Chromatography/Mass Spectrometry. This work examines a larger population of patient positives for fentanyl and norfentanyl to determine whether more than a decade later the original report remains accurate in predicting the range and median levels of fentanyl and norfentanyl concentrations physicians can expect to see from their patients. Additionally, these data were transformed to develop a model that results in a near Gaussian distribution of urine drug test results. This retrospective approach was developed to transform and normalize urine drug testing results to provide a historical picture of expected patient values for this important analgesic. The resulting near Gaussian distribution is dose independent and as such should be of value to physicians in quickly assessing whether their patient is consistent with this historical population in the broad terms of this model. While this comparison alone is not definitive for adherence with a treatment regimen, together with patient interviews, prescription history and other clinical criteria, it can add an idea of expected patient values from urine drug testing.

Improved Chiral Separation of Methamphetamine Enantiomers Using CSP-LC-MS-MS.

To determine the true enantiomeric composition of methamphetamine urine drug testing results, chiral separation of dextro (D) and levo (L) enantiomers is necessary. While enantiomeric separation of methamphetamine has traditionally been accomplished using gas chromatography-mass spectrometry (GC-MS), chiral separation of D- and L-methamphetamine by chiral stationary phase (CSP) liquid chromatography-mass spectrometry/mass spectrometry (LC-MS-MS) has proved more reliable. Chirally selective detection of methamphetamine by GC-MS is often performed using L-N-trifluoroacetyl-prolyl chloride (TPC). L-TPC, a chiral compound, is known to have impurities that can affect the chiral composition percentages of the methamphetamine sample, potentially leading to inaccurate patient results. The comparative analysis of the samples run by GC and LC methods showed preferential bias of the GC method for producing error rates, consistent with previous research, of 8-19%. The CSP-LC-MS-MS method produces percent deviation errors of <2%. Additionally, the GC method failed to produce results that were 100% D- or L-isomer even for enantiomerically pure standards. A higher rate of D- and L-methamphetamine isomer racemization is seen in samples when analyzed by GC-MS using L-TPC-derivatizing agent. This racemization is not seen when these samples are tested with CSP-LC-MS-MS. Thus, a more accurate method of enantiomeric analysis is provided by CSP-LC-MS-MS.

A Dilute-and-Shoot LC-MS Method for Quantitating Opioids in Oral Fluid.

Opioid testing represents a dominant share of the market in pain management clinical testing facilities. Testing of this drug class in oral fluid (OF) has begun to rise in popularity. OF analysis has traditionally required extensive clean-up protocols and sample concentration, which can be avoided. This work highlights the use of a fast, 'dilute-and-shoot' method that performs no considerable sample manipulation. A quantitative method for the determination of eight common opioids and associated metabolites (codeine, morphine, hydrocodone, hydromorphone, norhydrocodone, oxycodone, noroxycodone and oxymorphone) in OF is described herein. OF sample is diluted 10-fold in methanol/water and then analyzed using an Agilent chromatographic stack coupled with an AB SCIEX 4500. The method has a 2.2-min LC gradient and a cycle time of 2.9 min. In contrast to most published methods of this particular type, this method uses no sample clean-up or concentration and has a considerably faster LC gradient, making it ideal for very high-throughput laboratories. Importantly, the method requires only 100 µL of sample and is diluted 10-fold prior to injection to help with instrument viability. Baseline separation of all isobaric opioids listed above was achieved on a phenyl-hexyl column. The validated calibration range for this method is 2.5-1,000 ng/mL. This 'dilute-and-shoot' method removes the unnecessary, costly and time-consuming extraction steps found in traditional methods and still surpasses all analytical requirements.

Capillary electrophoresis-mass spectrometry determination of morphine and its isobaric glucuronide metabolites.

The determination of morphine and its isobaric metabolites morphine-3-beta-d-glucuronide (M3G) and morphine-6-beta-d-glucuronide (M6G) is useful for therapeutic drug monitoring and forensic identification of drug use. In particular, capillary electrophoresis with mass spectrometry (CE-MS) represents an attractive tool for opioid analysis. Whereas volatile background electrolytes in CE often improve electrospray ionization for coupled MS detection, such electrolytes may reduce CE separation efficiency and resolution. To better understand the effects of background electrolyte (BGE) composition on separation efficiency and detection sensitivity, this work compares and contrasts method development for both volatile (ammonium formate and acetate) and nonvolatile (ammonium phosphate and borate) buffers. Peak efficiencies and migration times for morphine and morphine metabolites were optimal with a 25mM ammonium borate buffer (pH=9.5) although greater sensitivities were achieved in the ammonium formate buffer. Optimized CE methods allowed for the resolution of the isobaric morphine metabolites prior to high mass accuracy, electrospray ionization quadrupole time-of-flight (ESI-QTOF) MS detection applicable to the analysis of urine samples in under seven minutes. Urine sample preparation required only a 10-fold dilution with BGE prior to analysis. Limits of detection (LOD) in normal human urine were found to be 1.0µg/mL for morphine and 2.5µg/mL for each of M3G and M6G by CE-ESI-QTOF-MS. These LODs were comparable to those for CE-UV analysis of opioid standards in buffer, whereas CE-ESI-QTOF-MS analysis of opioid standards in buffer yielded LODs an order of magnitude lower. Patient urine samples (N=12) were analyzed by this new CE-ESI-QTOF-MS method and no significant difference in total morphine content relative to prior liquid chromatography-mass spectrometry (LC-MS) results was found as per a paired-t test at the 99% confidence level. Whereas the LC-MS method applied to these samples determined only total morphine content, this new CE-ESI-QTOF-MS method allowed for species differentiation in addition to total morphine determination. By this method, it was found that M3G and M6G metabolites were present in a 5:1 concentration ratio, on average, in patient samples. Therefore, the CE-ESI-QTOF-MS method not only allows for total morphine concentration determination comparable to established LC-MS methods, but also allows for differentiation between morphine and its trace glucuronides, yielding additional biochemical information about drug metabolism.

Rapid enzymatic hydrolysis using a novel recombinant ß-glucuronidase in benzodiazepine urinalysis.

Only trace amounts of parent benzodiazepines are present in urine following extensive metabolism and conjugation. Thus, hydrolysis of glucuronides is necessary for improved detection. Enzyme hydrolysis is preferred to retain identification specificity, but can be costly and time-consuming. The assessment of a novel recombinant ß-glucuronidase for rapid hydrolysis in benzodiazepine urinalysis is presented. Glucuronide controls for oxazepam, lorazepam and temazepam were treated with IMCSzyme™ recombinant ß-glucuronidase. Hydrolysis efficiency was assessed at 55°C and at room temperature (RT) using the recommended optimum pH. Hydrolysis efficiency for four other benzodiazepines was evaluated solely with positive patient samples. Maximum hydrolysis of glucuronide controls at 5 min at RT (mean analyte recovery ≥ 94% for oxazepam and lorazepam and ≥ 80% for temazepam) was observed. This was considerably faster than the optimized 30 min incubation time for the abalone ß-glucuronidase at 65°C. Mean analyte recovery increased at longer incubation times at 55°C for temazepam only. Total analyte in patient samples compared well to targets from abalone hydrolysis after recombinant ß-glucuronidase hydrolysis at RT with no incubation. Some matrix effect, differential reactivity, conjugation variability and transformation impacting total analyte recovery were indicated. The unique potential of the IMCSzyme™ recombinant ß-glucuronidase was demonstrated with fast benzodiazepine hydrolysis at RT leading to decreased processing time without the need for heat activation.

Disposition of docosahexaenoic acid-paclitaxel, a novel taxane, in blood: in vitro and clinical pharmacokinetic studies.

PURPOSE: Docosahexaenoic acid-paclitaxel is as an inert prodrug composed of the natural fatty acid DHA covalently linked to the C2'-position of paclitaxel (M. O. Bradley et al., Clin. Cancer Res., 7: 3229-3238, 2001). Here, we examined the role of protein binding as a determinant of the pharmacokinetic behavior of DHA-paclitaxel. EXPERIMENTAL DESIGN: The blood distribution of DHA-paclitaxel was studied in vitro using equilibrium dialysis and in 23 cancer patients receiving the drug as a 2-h i.v. infusion (dose, 200-1100 mg/m(2)). RESULTS: In vitro, DHA-paclitaxel was found to bind extensively to human plasma (99.6 +/- 0.057%). The binding was concentration independent (P = 0.63), indicating a nonspecific, nonsaturable process. The fraction of unbound paclitaxel increased from 0.052 +/- 0.0018 to 0.055 +/- 0.0036 (relative increase, 6.25%; P = 0.011) with an increase in DHA-paclitaxel concentration (0-1000 microg/ml), suggesting weakly competitive drug displacement from protein-binding sites. The mean (+/- SD) area under the curve of unbound paclitaxel increased nonlinearly with dose from 0.089 +/- 0.029 microg.h/ml (at 660 mg/m(2)) to 0.624 +/- 0.216 microg.h/ml (at 1100 mg/m(2)), and was associated with the dose-limiting neutropenia in a maximum-effect model (R(2) = 0.624). A comparative analysis indicates that exposure to Cremophor EL and unbound paclitaxel after DHA-paclitaxel (at 1100 mg/m(2)) is similar to that achieved with paclitaxel on clinically relevant dose schedules. CONCLUSIONS: Extensive binding to plasma proteins may explain, in part, the unique pharmacokinetic profile of DHA-paclitaxel described previously with a small volume of distribution ( approximately 4 liters) and slow systemic clearance ( approximately 0.11 liters/h).