Pitfalls of the application of microdialysis in clinical oncology: controversial findings with docetaxel

A rapid screening method to select microdialysis carriers for hydrophobic compounds

Microdialysis is a minimally invasive sampling technique which is widely applied in many fields including clinical studies. This technique usually has limitation on sampling hydrophobic compounds as aqueous solutions are commonly used as the perfusates. The relative recovery of hydrophobic compounds is often low and irreproducible because of the non-specific binding to microdialysis membranes or catheter tubing. Carriers such as cyclodextrins have been used to improve the recovery and consistency, however the identification of an optimal carrier can only be achieved after time-consuming and costly microdialysis experiments. We therefore developed a rapid, convenient, and low-cost method to identify the optimal carriers for sampling hydrophobic compounds with the use of centrifugal ultrafiltration. Doxorubicin was used as the model compound and its relative recoveries obtained from centrifugal ultrafiltration and from microdialysis were compared. The results show that the relative recoveries are highly correlated (correlation coefficient ≥ 0.9) between centrifugal ultrafiltration and microdialysis when different types or different concentrations of cyclodextrins were used as the carriers. In addition to doxorubicin, this method was further confirmed on three other drugs with different hydrophobicity. This method may facilitate and broaden the use of microdialysis perfusion on sampling or delivering hydrophobic substances in various applications.

Nose-to-Brain Delivery of Diazepam from an Intranasal Aqua-Triggered In-Situ (ATIS) Gelling Microemulsion: Monitoring Brain Uptake by Microdialysis

BACKGROUND AND OBJECTIVES

An innovative intranasal aqua-triggered in-situ (ATIS) gel is a polymer-free in-situ gelling microemulsion which gels instantaneously on contact with minute quantities of water to form a mucoadhesive gel. The objective of the study was to develop ATIS diazepam (ATIS-diazepam) as an alternative to the injection for epileptic emergencies and evaluate its brain uptake and nose-to-brain targeting efficiency in rats.

METHODS

ATIS-diazepam (1 mg/100 µL) was prepared and characterized for in vitro formulation characteristics. An LC-MS/MS method was developed and validated for the bioanalysis of diazepam. In vivo studies for pharmacokinetics, brain uptake and nasal irritation of intranasal ATIS-diazepam were conducted in rats. Brain uptake was investigated with brain microdialysis, a highly sensitive technique enabling quantification of free drug, which correlates to efficacy.

RESULTS

ATIS-diazepam exhibited globule size < 200 nm, low viscosity, negative zeta potential and good stability. A significant increase in mucoadhesion was exhibited by ATIS-diazepam following the addition of a small quantity of water. ATIS-diazepam showed burst release in pH 6.4 with 50% diazepam release in ~ 10 min, which was sustained over 1 h. The absolute bioavailability was ~ 50% with both intranasal free-diazepam and ATIS-diazepam. Intranasal administration of ATIS-diazepam revealed immediate absorption with rapid and high brain extracellular fluid concentration compared to intravenous free-diazepam solution. The estimated direct transport potential and drug targeting efficiency of intranasal ATIS-diazepam was significantly higher (2-fold) than intranasal free-diazepam solution, which was attributed to the mucoadhesive and microemulsion properties of ATIS-diazepam. The nasal irritation study revealed the safety of ATIS-diazepam compared to free-diazepam solution.

CONCLUSION

Intranasal ATIS-diazepam showed promise of higher direct nose-to-brain targeting, better safety and hence has an immense implication in the treatment of epileptic emergencies.

Exclusion of unsuitable CNS drug candidates based on their physicochemical properties and unbound fractions in biomatrices for brain microdialysis investigations

Microdialysis has been the only direct method of continuously measuring the unbound drug concentrations in extracellular fluid at a specific brain region with respect to time in the same animal. However, not every compound is suitable for microdialysis system as demonstrated by their inconsistent "by gain" and "by loss" in-vitro microdialysis probe recoveries leading to over- or under- estimated in-vivo concentrations. Therefore, our current study was proposed aiming to develop simple exclusion criteria for drug candidates that are not suitable for microdialysis system investigation. Through literature research, the properties ((LogP, pKa, water solubility and unbound fraction in plasma and brain) of drugs that have been reported for microdialysis studies were summarized. The exclusion criteria were developed by evaluating the impact of such properties on the consistency of in-vitro "by gain" and "by loss" recoveries of microdialysis probe. As a result, forty-five compounds were identified from literatures, among which doxorubicin, docetaxel, omeprazole, donepezil and phenytoin were found to have inconsistent in-vitro "by gain" and "by loss" microdialysis probe recoveries and subsequently selected for the exclusion criteria analysis. It was found that compounds with limited water solubility (less than 1 g/L) and unbound fraction in plasma (f_u,plasma less than 30%) and brain homogenate (f_u,brain less than 10%) were more likely to have inconsistent "by gain" and "by loss" microdialysis probe recoveries. Our proposed exclusion criteria were further validated using carbamazepine (limited water solubility only), DB213 (limited f_u,brain only) and piperine (both limited water solubility and limited f_u,plasma, f_u,brain). Our current proposed exclusion criteria will help excluding the CNS drug candidates that are highly unlikely suitable for brain microdialysis approach leading to a better success rate in brain microdialysis approach development.

Challenges for the in vivo quantification of brain neuropeptides using microdialysis sampling and LC-MS

In recent years, neuropeptides and their receptors have received an increased interest in neuropharmacological research. Although these molecules are considered relatively small compared with proteins, their in vivo quantification using microdialysis is more challenging than for small molecules. Low microdialysis recoveries, aspecific adsorption and the presence of various multiply charged precursor ions during ESI-MS/MS detection hampers the in vivo quantification of these low abundant biomolecules. Every step in the workflow, from sampling until analysis, has to be optimized to enable the sensitive analysis of these compounds in microdialysates.

Combined Microdialysis-Tumor Homogenate Method for the Study of the Steady State Compartmental Distribution of a Hydrophobic Anticancer Drug in Patient-Derived Xenografts

PURPOSE

To develop a reproducible microdialysis-tumor homogenate method for the study of the intratumor distribution of a highly hydrophobic anticancer drug (SN-38; 7-ethyl-10-hydroxycamptothecin) in neuroblastoma patient-derived xenografts.

METHODS

We studied the nonspecific binding of SN-38 to the microdialysis tubing in the presence of 2-hydroxypropyl-beta-cyclodextrin (HPBCD) in the perfusate. We calibrated the microdialysis probes by the zero flow rate (ZFR) method and calculated the enhancement factor (f = extrapolated SN-38 concentration at the ZFR / SN-38 concentration in the dialysed solution) of HPBCD. We characterized the extravasation of HPBCD to tumors engrafted in mice. In vivo microdialysis and terminal homogenate data at the steady state (subcutaneous pump infusions) were used to calculate the volume of distribution of unbound SN-38 (Vu,tumor) in neuroblastoma.

RESULTS

HPBCD (10% w/v) in the perfusate prevented the nonspecific binding of SN-38 to the microdialysis probe and enhanced SN-38 recovery (f = 1.86). The extravasation of HPBCD in the tumor during microdialysis was lower than 1%. Vu,tumor values were above 3 mL/g tumor for both neuroblastoma models and suggested efficient cellular penetration of SN-38.

CONCLUSIONS

The method contributes to overcome the limitations of the microdialysis technique in hydrophobic drugs and provides a powerful tool to characterize compartmental anticancer drug distribution in xenografts.

Toward prediction of efficacy of chemotherapy: a proof of concept study in lung cancer patients using [¹¹C]docetaxel and positron emission tomography

PURPOSE

Pharmacokinetics of docetaxel can be measured in vivo using positron emission tomography (PET) and a microdose of radiolabeled docetaxel ([(11)C]docetaxel). The objective of this study was to investigate whether a [(11)C]docetaxel PET microdosing study could predict tumor uptake of therapeutic doses of docetaxel.

EXPERIMENTAL DESIGN

Docetaxel-naïve lung cancer patients underwent 2 [(11)C]docetaxel PET scans; one after bolus injection of [(11)C]docetaxel and another during combined infusion of [(11)C]docetaxel and a therapeutic dose of docetaxel (75 mg·m(-2)). Compartmental and spectral analyses were used to quantify [(11)C]docetaxel tumor kinetics. [(11)C]docetaxel PET measurements were used to estimate the area under the curve (AUC) of docetaxel in tumors. Tumor response was evaluated using computed tomography scans.

RESULTS

Net rates of influx (Ki) of [(11)C]docetaxel in tumors were comparable during microdosing and therapeutic scans. [(11)C]docetaxel AUCTumor during the therapeutic scan could be predicted reliably using an impulse response function derived from the microdosing scan together with the plasma curve of [(11)C]docetaxel during the therapeutic scan. At 90 minutes, the accumulated amount of docetaxel in tumors was less than 1% of the total infused dose of docetaxel. [(11)C]docetaxel Ki derived from the microdosing scan correlated with AUCTumor of docetaxel (Spearman ρ = 0.715; P = 0.004) during the therapeutic scan and with tumor response to docetaxel therapy (Spearman ρ = -0.800; P = 0.010).

CONCLUSIONS

Microdosing data of [(11)C]docetaxel PET can be used to predict tumor uptake of docetaxel during chemotherapy. The present study provides a framework for investigating the PET microdosing concept for radiolabeled anticancer drugs in patients.

Quantification of cabazitaxel in human plasma by liquid chromatography/triple-quadrupole mass spectrometry: a practical solution for non-specific binding

A rapid and sensitive liquid chromatography/tandem mass spectrometry (LC-MS/MS) method has been developed and validated for the quantitative determination of cabazitaxel, a novel tubulin-binding taxane, in 100 μl aliquots of human lithium heparinized plasma with deuterated cabazitaxel as internal standard. The sample extraction and cleaning-up involved a simple liquid-liquid extraction with 20 μl aliquots of 4% ammonium hydroxide, 100 μl aliquots of acetonitrile and 1 ml aliquots of n-butylchloride. Chromatographic separations were achieved on a reversed phase C₁₈ column eluted at a flow-rate of 0.20 ml/min on a gradient of acetonitrile. The overall cycle time of the method was 5 min, with cabazitaxel eluting at 3.0 min. The multiple reaction monitoring transitions were set at 836>555 (m/z), and 842>561 (m/z) for cabazitaxel and the internal standard, respectively. The calibration curves were linear over the range of 1.00-100 ng/ml with the lower limit of quantitation validated at 1.00 ng/ml. The within-run and between-run precisions, also at the level of the LLQ, were within 8.75%, while the accuracy ranged from 88.5 to 94.1%. As dilution of samples prior to extraction resulted in a loss of cabazitaxel of approximately 6.5% per dilution step, a second calibration curve ranging from 40.0 to 4000 ng/ml was validated and was also linear. The within-run and between-run precisions in this range were within 4.99%, while the accuracy ranged from 95.8 to 100.3%. The method was successfully applied to samples derived from a clinical study.

Role of ATP-binding cassette and solute carrier transporters in erlotinib CNS penetration and intracellular accumulation

PURPOSE

To study the role of drug transporters in central nervous system (CNS) penetration and cellular accumulation of erlotinib and its metabolite, OSI-420.

EXPERIMENTAL DESIGN

After oral erlotinib administration to wild-type and ATP-binding cassette (ABC) transporter-knockout mice (Mdr1a/b(-/-), Abcg2(-/-), Mdr1a/b(-/-)Abcg2(-/-), and Abcc4(-/-)), plasma was collected and brain extracellular fluid (ECF) was sampled using intracerebral microdialysis. A pharmacokinetic model was fit to erlotinib and OSI-420 concentration-time data, and brain penetration (P(Brain)) was estimated by the ratio of ECF-to-unbound plasma area under concentration-time curves. Intracellular accumulation of erlotinib was assessed in cells overexpressing human ABC transporters or SLC22A solute carriers.

RESULTS

P(Brain) in wild-type mice was 0.27 ± 0.11 and 0.07 ± 0.02 (mean ± SD) for erlotinib and OSI-420, respectively. Erlotinib and OSI-420 P(Brain) in Abcg2(-/-) and Mdr1a/b(-/-)Abcg2(-/-) mice were significantly higher than in wild-type mice. Mdr1a/b(-/-) mice showed similar brain ECF penetration as wild-type mice (0.49 ± 0.37 and 0.04 ± 0.02 for erlotinib and OSI-420, respectively). In vitro, erlotinib and OSI-420 accumulation was significantly lower in cells overexpressing breast cancer resistance protein (BCRP) than in control cells. Only OSI-420, not erlotinib, showed lower accumulation in cells overexpressing P-glycoprotein (P-gp) than in control cells. The P-gp/BCRP inhibitor elacridar increased erlotinib and OSI-420 accumulation in BCRP-overexpressing cells. Erlotinib uptake was higher in OAT3- and OCT2-transfected cells than in empty vector control cells.

CONCLUSION

Abcg2 is the main efflux transporter preventing erlotinib and OSI-420 penetration in mouse brain. Erlotinib and OSI-420 are substrates for SLC22A family members OAT3 and OCT2. Our findings provide a mechanistic basis for erlotinib CNS penetration, cellular uptake, and efflux mechanisms.

Microdialysis for assessing intratumoral drug disposition in brain cancers: a tool for rational drug development

IMPORTANCE OF THE FIELD

many promising targeted agents and combination therapies are being investigated for brain cancer. However, the results from recent clinical trials have been disappointing. A better understanding of the disposition of drug in the brain early in drug development would facilitate appropriate channeling of new drugs into brain cancer clinical trials.

AREAS COVERED IN THIS REVIEW

barriers to successful drug activity against brain cancer and issues affecting intratumoral drug concentrations are reviewed. The use of the microdialysis technique for extracellular fluid (ECF) sampling and its application to drug distribution studies in brain are reviewed using published literature from 1995 to the present. The benefits and limitations of microdialysis for performing neuorpharmacokinetic (nPK) and neuropharmacodynamic (nPD) studies are discussed.

WHAT THE READER WILL GAIN

the reader will gain an appreciation of the challenges involved in identifying agents likely to have efficacy in brain cancer, an understanding of the general principles of microdialysis, and the power and limitations of using this technique in early drug development for brain cancer therapies.

TAKE HOME MESSAGE

a major factor preventing efficacy of anti-brain cancer drugs is limited access to tumor. Intracerebral microdialysis allows sampling of drug in the brain ECF. The resulting nPK/nPD data can aid in the rational selection of drugs for investigation in brain tumor clinical trials.

Application of prolonged microdialysis sampling in carboplatin-treated cancer patients

Purpose

To better understand the mechanisms underlying (in)sensitivity of tumors to anticancer drugs, assessing intra-tumor drug pharmacokinetics (PKs) could be important. We explored the feasibility of microdialysis in tumor tissue for multiple days in a clinical setting, using carboplatin as model drug.

Methods

Plasma and microdialysate samples from tumor and adipose normal tissues were collected up to 47 h after dosing in eight carboplatin-treated patients with an accessible (sub)cutaneous tumor.

Results

Pharmacokinetics were evaluable in tumor tissue in 6/8 patients and in adipose normal tissue in 3/8 patients. Concentration–time curves of unbound platinum in both the tissues followed the pattern of the curves in plasma, with exposure ratios of tissue versus plasma ranging from 0.64 to 1.46.

Conclusions

Microdialysis can be successfully employed in ambulant patients for multiple days, which enables one to study tissue PK of anticancer drugs in normal and malignant tissues in more detail.