Investigation of microdialysis sampling calibration approaches for lipophilic analytes: doxorubicin

Microdialysis as a sampling tool for the chemotherapeutic agent Doxorubicin

Doxorubicin is a chemotherapeutic agent used for more than fifty years to treat a great variety of cancers in both children and adults. Despite hereof, pharmacokinetic knowledge is almost solely based on systemic plasma concentrations. Microdialysis is a catheter-based pharmacokinetic sampling tool enabling simultaneous target site sampling of unbound molecules of interest. The aim of this study was to thoroughly evaluate the feasibility of applying microdialysis for sampling of Doxorubicin in both in vitro experiments and an in vivo setting. Doxorubicin relative recovery by gain and by loss was tested for different catheter types, perfusion fluids, concentrations and collection vials. Adsorption tests revealed polystyrene/santoprene vials to be the biggest contributor of unwanted adsorption between Doxorubicin and the microdialysis equipment, and confirmed LoBind Eppendorf tubes to be a suitable alternative. The methodological combination of polyamide membranes, saline as perfusion fluid and LoBind Eppendorf sampling tubes demonstrated no statistically significant differences for relative recovery by gain and by loss, and the relative recovery was also found to be concentration independent. We conclude, that a proper microdialysis set-up can be used to collect samples containing concentrations of the chemotherapeutic drug Doxorubicin in vitro and in vivo, which encourage future pharmacokinetic studies to evaluate current treatment regimens to find the most effective and least toxic anti-neoplastic treatment for the patients.

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.

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.

Pharmacokinetics of doxorubicin in glioblastoma multiforme following ultrasound-Induced blood-brain barrier disruption as determined by microdialysis

The goal of this study was to investigate the in vivo extracellular kinetics of doxorubicin (Dox) in glioblastoma multiforme (GBM)-bearing mice following focused ultrasound (FUS)-induced blood-brain barrier (BBB) disruption using microdialysis. An intracranial brain tumor model in NOD-scid mice using human brain GBM 8401 cells was used in this study. Prior to each sonication, simultaneous intravenous administration of Dox and microbubbles, and the Dox concentration in the brains was quantified by high performance liquid chromatography (HPLC). Drug administration with sonication elevated the tumor-to-normal brain Dox ratio of the target tumors by about 2.35-fold compared with the control tumors. The mean peak concentration of Dox in the sonicated GBM dialysate was 10 times greater than without sonication, and the area under the concentration-time curve was 3.3 times greater. This study demonstrates that intracerebral microdialysis is an effective means of evaluating real-time target BBB transport profiles and offers the possibility of investigating the pharmacokinetics of drug delivery in the sonicated brain.

Cerebral open flow microperfusion (cOFM) an innovative interface to brain tissue

Cerebral open flow microperfusion (cOFM) is a new in-vivo technique for continuous sampling of the interstitial fluid in brain tissue. cOFM can be used to monitor substance transport across the blood-brain barrier (pharmacokinetics) and to investigate metabolic changes in brain tissue after drug application (pharmacodynamics). The possibility of long-term implantation into the brain makes cOFM an outstanding tool in the development of brain relevant pharmaceutics.

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.

Pharmacokinetics, brain delivery, and efficacy in brain tumor-bearing mice of glutathione pegylated liposomal doxorubicin (2B3-101)

Brain cancer is a devastating disease affecting many people worldwide. Effective treatment with chemotherapeutics is limited due to the presence of the blood-brain barrier (BBB) that tightly regulates the diffusion of endogenous molecules but also xenobiotics. Glutathione pegylated liposomal doxorubicin (2B3-101) is being developed as a new treatment option for patients with brain cancer. It is based on already marketed pegylated liposomal doxorubicin (Doxil®/Caelyx®), with an additional glutathione coating that safely enhances drug delivery across the BBB. Uptake of 2B3-101 by human brain capillary endothelial cells in vitro was time-, concentration- and temperature-dependent, while pegylated liposomal doxorubicin mainly remained bound to the cells. In vivo, 2B3-101 and pegylated liposomal doxorubicin had a comparable plasma exposure in mice, yet brain retention 4 days after administration was higher for 2B3-101. 2B3-101 was overall well tolerated by athymic FVB mice with experimental human glioblastoma (luciferase transfected U87MG). In 2 independent experiments a strong inhibition of brain tumor growth was observed for 2B3-101 as measured by bioluminescence intensity. The effect of weekly administration of 5 mg/kg 2B3-101 was more pronounced compared to pegylated liposomal doxorubicin (p<0.05) and saline (p<0.01). Two out of 9 animals receiving 2B3-101 showed a complete tumor regression. Twice-weekly injections of 5 mg/kg 2B3-101 again had a significant effect in inhibiting brain tumor growth (p<0.001) compared to pegylated liposomal doxorubicin and saline, and a complete regression was observed in 1 animal treated with 2B3-101. In addition, twice-weekly dosing of 2B3-101 significantly increased the median survival time by 38.5% (p<0.001) and 16.1% (p<0.05) compared to saline and pegylated liposomal doxorubicin, respectively. Overall, these data demonstrate that glutathione pegylated liposomal doxorubicin enhances the effective delivery of doxorubicin to brain tumors and could become a promising new therapeutic option for the treatment of brain malignancies.

Population pharmacokinetic modeling of the unbound levofloxacin concentrations in rat plasma and prostate tissue measured by microdialysis

Levofloxacin is a broad-spectrum fluoroquinolone used in the treatment of both acute and chronic bacterial prostatitis. Currently, the treatment of bacterial prostatitis is still difficult, especially due to the poor distribution of many antimicrobials into the prostate, thus preventing the drug to reach effective interstitial concentrations at the infection site. Newer fluoroquinolones show a greater penetration into the prostate. In the present study, we compared the unbound levofloxacin prostate concentrations measured by microdialysis to those in plasma after a 7-mg/kg intravenous bolus dose to Wistar rats. Plasma and dialysate samples were analyzed using a validated high-pressure liquid chromatography-fluorescence method. Both noncompartmental analysis (NCA) and population-based compartmental modeling (NONMEM 6) were performed. Unbound prostate tissue concentrations represented 78% of unbound plasma levels over a period of 12 h by comparing the extent of exposure (unbound AUC0-∞) of 6.4 and 4.8 h·μg/ml in plasma and tissue, respectively. A three-compartment model with simultaneous passive diffusion and saturable distribution kinetics from the prostate to the central compartment gave the best results in terms of curve fitting, precision of parameter estimates, and model stability. The following parameter values were estimated by the population model: V1 (0.38 liter; where V1 represents the volume of the central compartment), CL (0.22 liter/h), k12 (2.27 h(-1)), k21 (1.44 h(-1)), k13 (0.69 h(-1)), Vmax (7.19 μg/h), kM (0.35 μg/ml), V3/fuprostate (0.05 liter; where fuprostate represents the fraction unbound in the prostate), and k31 (3.67 h(-1)). The interindividual variability values for V1, CL, Vmax, and kM were 21, 37, 42, and 76%, respectively. Our results suggest that levofloxacin is likely to be substrate for efflux transporters in the prostate.

Enhanced human tissue microdialysis using hydroxypropyl-ß-cyclodextrin as molecular carrier

Microdialysis sampling of lipophilic molecules in human tissues is challenging because protein binding and adhesion to the membrane limit recovery. Hydroxypropyl-ß-cyclodextrin (HP-ß-CD) forms complexes with hydrophobic molecules thereby improving microdialysis recovery of lipophilic molecules in vitro and in rodents. We tested the approach in human subjects. First, we determined HP-ß-CD influences on metabolite stability, delivery, and recovery in vitro. Then, we evaluated HP-ß-CD as microdialysis perfusion fluid supplement in 20 healthy volunteers. We placed 20 kDa microdialysis catheters in subcutaneous abdominal adipose tissue and in the vastus lateralis muscle. We perfused catheters with lactate free Ringer solution with or without 10% HP-ß-CD at flow rates of 0.3–2.0 µl/min. We assessed tissue metabolites, ultrafiltration effects, and blood flow. In both tissues, metabolite concentrations with Ringer+HP-ß-CD perfusate were equal or higher compared to Ringer alone. Addition of HP-ß-CD increased dialysate volume by 10%. Adverse local or systemic reactions to HP-ß-CD did not occur and analytical methods were not disturbed. HP-ß-CD addition allowed to measure interstitial anandamide concentrations, a highly lipophilic endogenous molecule. Our findings suggest that HP-ß-CD is a suitable supplement in clinical microdialysis to enhance recovery of lipophilic molecules from human interstitial fluid.

Analytical and biological methods for probing the blood-brain barrier

The blood-brain barrier (BBB) is an important interface between the peripheral and central nervous systems. It protects the brain against the infiltration of harmful substances and regulates the permeation of beneficial endogenous substances from the blood into the extracellular fluid of the brain. It can also present a major obstacle in the development of drugs that are targeted for the central nervous system. Several methods have been developed to investigate the transport and metabolism of drugs, peptides, and endogenous compounds at the BBB. In vivo methods include intravenous injection, brain perfusion, positron emission tomography, and microdialysis sampling. Researchers have also developed in vitro cell-culture models that can be employed to investigate transport and metabolism at the BBB without the complication of systemic involvement. All these methods require sensitive and selective analytical methods to monitor the transport and metabolism of the compounds of interest at the BBB.