Development, Characterization, and Evaluation of SLN-Loaded Thermoresponsive Hydrogel System of Topotecan as Biological Macromolecule for Colorectal Delivery.

BACKGROUND: Chemotherapeutic drugs cause severe toxicities if administered unprotected, without proper targeting, and controlled release. In this study, we developed topotecan- (TPT-) loaded solid lipid nanoparticles (SLNs) for their chemotherapeutic effect against colorectal cancer. The TPT-SLNs were further incorporated into a thermoresponsive hydrogel system (TRHS) (TPT-SLNs-TRHS) to ensure control release and reduce toxicity of the drug. Microemulsion technique and cold method were, respectively, used to develop TPT-SLNs and TPT-SLNs-TRHS. Particle size, polydispersive index (PDI), and incorporation efficiency (IE) of the TPT-SLNs were determined. Similarly, gelation time, gel strength, and bioadhesive force studies of the TPT-SLNs-TRHS were performed. Additionally, in vitro release and pharmacokinetic and antitumour evaluations of the formulation were done. RESULTS: TPT-SLNs have uniformly distributed particles with mean size in nanorange (174 nm) and IE of ~90%. TPT-SLNs-TRHS demonstrated suitable gelation properties upon administration into the rat's rectum. Moreover, drug release was exhibited in a control manner over an extended period of time for the incorporated TPT. Pharmacokinetic studies showed enhanced bioavailability of the TPT with improved plasma concentration and AUC. Further, it showed significantly enhanced antitumour effect in tumour-bearing mice as compared to the test formulations. CONCLUSION: It can be concluded that SLNs incorporated in TRHS could be a potential source of the antitumour drug delivery with better control of the drug release and no toxicity.

Improved Therapeutic Efficacy of Topotecan Against A549 Lung Cancer Cells with Folate-targeted Topotecan Liposomes.

BACKGROUND: Among all cancers, lung cancer has high mortality among patients in most of the countries in the world. Targeted delivery of anticancer drugs can significantly reduce the side effects and dramatically improve the effects of the treatment. Folate, a suitable ligand, can be modified to the surface of tumor-selective drug delivery systems because it can selectively bind to the folate receptor, which is highly expressed on the surface of lung tumor cells. OBJECTIVE: This study aimed to construct a kind of folate-targeted topotecan liposomes for investigating their efficacy and mechanism of action in the treatment of lung cancer in preclinical models. METHODS: We conjugated topotecan liposomes with folate, and the liposomes were characterized by particle size, entrapment efficiency, cytotoxicity to A549 cells and in vitro release profile. Technical evaluations were performed on lung cancer A549 cells and xenografted A549 cancer cells in female nude mice, and the pharmacokinetics of the drug were evaluated in female SD rats. RESULTS: The folate-targeted topotecan liposomes were proven to show effectiveness in targeting lung tumors. The anti-tumor effects of these liposomes were demonstrated by the decreased tumor volume and improved therapeutic efficacy. The folate-targeted topotecan liposomes also lengthened the topotecan blood circulation time. CONCLUSION: The folate-targeted topotecan liposomes are effective drug delivery systems and can be easily modified with folate, enabling the targeted liposomes to deliver topotecan to lung cancer cells and kill them, which could be used as potential carriers for lung chemotherapy.

Insights into accelerated liposomal release of topotecan in plasma monitored by a non-invasive fluorescence spectroscopic method.

A non-invasive fluorescence method was developed to monitor liposomal release kinetics of the anticancer agent topotecan (TPT) in physiological fluids and subsequently used to explore the cause of accelerated release in plasma. Analyses of fluorescence excitation spectra confirmed that unencapsulated TPT exhibits a red shift in its spectrum as pH is increased. This property was used to monitor TPT release from actively loaded liposomal formulations having a low intravesicular pH. Mathematical release models were developed to extract reliable rate constants for TPT release in aqueous solutions monitored by fluorescence and release kinetics obtained by HPLC. Using the fluorescence method, accelerated TPT release was observed in plasma as previously reported in the literature. Simulations to estimate the intravesicular pH were conducted to demonstrate that accelerated release correlated with alterations in the low intravesicular pH. This was attributed to the presence of ammonia in plasma samples rather than proteins and other plasma components generally believed to alter release kinetics in physiological samples. These findings shed light on the critical role that ammonia may play in contributing to the preclinical/clinical variability and performance seen with actively-loaded liposomal formulations of TPT and other weakly-basic anticancer agents.

PK/TD modeling for prediction of the effects of 8C2, an anti-topotecan mAb, on topotecan-induced toxicity in mice.

To facilitate the development of an inverse targeting strategy, where anti-topotecan antibodies are administered to prevent systemic toxicity following intraperitoneal topotecan, a pharmacokinetic/toxicodynamic (PK/TD) model was developed and evaluated. The pharmacokinetics of 8C2, a monoclonal anti-topotecan antibody, were assessed following IV and SC administration, and the data were characterized using a two compartmental model with nonlinear absorption and elimination. A hybrid PK model was constructed by combining a PBPK model for topotecan with the two-compartment model for 8C2, and the model was employed to predict the disposition of topotecan, 8C2, and the topotecan-8C2 complex. The model was linked to a toxicodynamic model for topotecan-induced weight-loss, and simulations were conducted to predict the effects of 8C2 on the toxicity of topotecan in mice. Increasing the molar dose ratio of 8C2 to topotecan resulted in a dose-dependent decrease in the unbound (i.e., not bound to 8C2) topotecan exposure in plasma (AUCf) and a decrease in the extent of topotecan-induced weight-loss. Consistent with model predictions, toxicodynamic experiments showed substantial reduction in the percent nadir weight loss observed with 30 mg/kg IP topotecan after co-administration of 8C2 (20 ± 8% vs. 10 ± 8%). The investigation supports the use of anti-topotecan mAb to reduce the systemic toxicity of IP topotecan chemotherapy.

Predicting the effects of 8C2, a monoclonal anti-topotecan antibody, on plasma and tissue disposition of topotecan.

We are investigating an inverse targeting strategy to reduce the dose limiting systemic toxicities resultant from intraperitoneal administration of topotecan, a model chemotherapeutic drug. This approach utilizes systemic co-administration of anti-topotecan antibodies to alter the plasma and tissue disposition kinetics of topotecan. To better predict the effects of 8C2, a high affinity anti-topotecan monoclonal antibody, on the pharmacokinetics of topotecan, two mathematical models have been developed and evaluated. Model 1 is a hybrid physiologically based pharmacokinetic (PBPK) model that was created by merging a PBPK model for topotecan with a simple two compartment model of 8C2 pharmacokinetics. Model 2 is a comprehensive PBPK model developed by merging a PBPK model for IgG with a PBPK model for topotecan. To help validate the simulation results from both the models, a tissue distribution experiment was conducted, in which topotecan and 8C2 were co-administered in mice. Experimental and simulated data were compared by calculating the median percent prediction error (%PE) for all tissues. For both models, the median %PE values for all the tissues were less than 100 %, indicating that the predicted values were, on average, less than twofold the observed plasma and tissue topotecan concentrations values. In general model 2 was found to be more predictive of the data set than model 1, as the overall median %PE value for model 2 (%PE = 63) was less than model 1 (%PE = 73).

Development and validation of the HPLC method for simultaneous estimation of Paclitaxel and topotecan.

A simple, rapid, accurate and precise high performance liquid chromatography (HPLC) method for simultaneous analysis of Paclitaxel and Topotecan was developed. Different analytical parameters, such as linearity, accuracy, precision, specificity with intentional degradation, limit of detection and limit of quantification (LOQ), were determined according to the ICH guidelines. Acetonitrile-water (70:30, 0.1% trifluoroacetic acid) was run on a Phenomenex Luna C-18(2) column in isocratic mode at a flow rate of 1.2 mL/min for simultaneous analysis of the two drugs using a UV detector set at 227 nm. The proposed method showed a retention time (Rt) of 14.56 min for Topotecan and 23.81 min for Paclitaxel with a continuous run up to 30 min. The linearity of the calibration curves for each analyte in the desired concentration range was found to be good (r(2) > 0.9995). The recovery ranged from 97.9 to 101% for each drug with a relative standard deviation (%RSD) of <2%. Peaks corresponding to each of the drugs exhibited  positive values for the minimum peak purity index over the entire range of integrated chromatographic peak indicating high purity of the peaks. Stability analysis revealed that the drugs remained stable for sufficient time. Thus, the developed method was found to be robust and it can be employed to quantify Paclitaxel and Topotecan in commercial sample and rat blood/serum.

Oral topotecan: Bioavailability, pharmacokinetics and impact of ABCG2 genotyping in Chinese patients with advanced cancers.

Oral topotecan (Hycamtin(®)) has been recently approved for the treatment of relapsed small cell lung cancer (SCLC) in 2007, however, the bioavailability and pharmacokinetic data of topotecan for Chinese patients is still limited. Xinze(®) is a new and the only capsule formulation of topotecan used in China that is similar to Hycamtin(®). The current study aimed to investigate the absolute bioavailability and pharmacokinetics of Xinze(®) in Chinese patients with advanced cancers. On day 1, an IV dose of 1.5 mg/m(2)/d as a 30 min continuous infusion was administered. Patients took the oral topotecan at one of two dose levels: 1.5 mg/m(2)/d (six patients) or 1.9 mg/m(2)/d (seven patients) on day 2. Plasma pharmacokinetics of total topotecan and topotecan in the lactone form were performed on both days using ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Single-nucleotide polymorphisms (SNPs) identified in exon 5 (421C>A) and in exon 2 (34G>A) in ATP-binding cassette sub-family G member 2 (ABCG2) were analyzed by direct sequencing. Safety assessments were performed throughout the study. The maximum plasma concentration (Cmax) reached at 1-2 h and the elimination half-life time (T1/2) was approximately 4.2 h after oral administration. The absolute bioavailability of total topotecan in the 1.5 mg/m(2)/d and 1.9 mg/m(2)/d groups averaged 41.23 ± 11.8% and 36.00 ± 14.8%, respectively. The patients with heterozygous SNPs had essentially the same bioavailability and pharmacokinetics. The bioavailability of topotecan after oral administration illustrates good systemic exposure at dosages of 1.5 mg/m(2)/d and 1.9 mg/m(2)/d over a five-day schedule in Chinese patients. On a dose-normalized basis, the values of Cmax and AUC0-t for total topotecan in Chinese patients were higher than in Caucasians following oral and intravenous administration, while the T1/2 was consistent.

Effect of venlafaxine and desvenlafaxine on drug efflux protein expression and biodistribution in vivo.

Venlafaxine, and to a lesser extent desvenlafaxine, has previously been shown to induce the expression of the drug efflux transporters P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) in whole cells and alter the cellular permeability of a known drug efflux probe (rhodamine 123). To validate these in vitro findings, wild-type mice were treated for 4 days with 10 mg/kg venlafaxine or desvenlafaxine, and drug efflux transporter expression was examined in the brain, liver, and intestine. P-gp and BCRP expression was significantly upregulated in the intestine, following a treatment with venlafaxine (2.6- and 6.7-fold, respectively) or desvenlafaxine (2.3- and 4.8-fold, respectively). In addition, venlafaxine increased the BCRP expression in the brain (40%) and liver (60%), whereas desvenlafaxine had no effect on drug efflux transporter levels in these tissues. Using the same treatment paradigm, we observed a minimal impact of either drug on the brain disposition of the known drug efflux probe, topotecan. However, in the periphery, venlafaxine treatment significantly reduced the topotecan oral bioavailability by nearly 40%, whereas the impact of desvenlafaxine on topotecan plasma levels was more modest (23%). These studies demonstrate an effect of venlafaxine on the drug efflux transport activity and the potential for clinical drug-drug interactions.

Development and validation of a liquid chromatography-tandem mass spectrometry method for topotecan determination in beagle dog plasma and its application in a bioequivalence study.

Topotecan (TPT) is an important anti-cancer drug that inhibits topoisomerase I. A sensitive and robust liquid chromatography-tandem mass spectrometry (LC-MS/MS) method that potentially determines TPT in beagle dog plasma is needed for a bioequivalence study of TPT formulations. We developed and validated LC-MS/MS to evaluate TPT in beagle dog plasma in terms of specificity, linearity, precision, accuracy, stability, extraction recovery and matrix effect. Plasma samples were treated with an Ostro(TM) sorbent plate (a robust and effective tool) to eliminate phospholipids and proteins before analysis. TPT and camptothecin (internal standard) were separated on an Acquity UPLC BEH C18 column (1.7 µm, 2.1 × 50 mm) with 0.1% formic acid and methanol as the mobile phase at a flow rate of 0.25 mL/min. TPT was analyzed using positive ion electrospray ionization in multiple-reaction monitoring mode. The obtained lower limit of quantitation was 1 ng/mL (signal-to-noise ratio > 10). The standard calibration curve for TPT was linear (correlation coefficient > 0.99) at the concentration range of 1-400 ng/mL. The intra-day and inter-day precision, accuracy, stability, extraction recovery and matrix effect of TPT were within the acceptable limits. The validated method was successfully applied in a bioequivalence study of TPT in healthy beagle dogs.

Distribution of the anticancer drugs doxorubicin, mitoxantrone and topotecan in tumors and normal tissues.

PURPOSE: Pharmacokinetic analyses estimate the mean concentration of drug within a given tissue as a function of time, but do not give information about the spatial distribution of drugs within that tissue. Here, we compare the time-dependent spatial distribution of three anticancer drugs within tumors, heart, kidney, liver and brain. METHODS: Mice bearing various xenografts were treated with doxorubicin, mitoxantrone or topotecan. At various times after injection, tumors and samples of heart, kidney, liver and brain were excised. RESULTS: Within solid tumors, the distribution of doxorubicin, mitoxantrone and topotecan was limited to perivascular regions at 10 min after administration and the distance from blood vessels at which drug intensity fell to half was ~25-75 µm. Although drug distribution improved after 3 and 24 h, there remained a significant decrease in drug fluorescence with increasing distance from tumor blood vessels. Drug distribution was relatively uniform in the heart, kidney and liver with substantially greater perivascular drug uptake than in tumors. There was significantly higher total drug fluorescence in the liver than in tumors after 10 min, 3 and 24 h. Little to no drug fluorescence was observed in the brain. CONCLUSIONS: There are marked differences in the spatial distributions of three anticancer drugs within tumor tissue and normal tissues over time, with greater exposure to most normal tissues and limited drug distribution to many cells in tumors. Studies of the spatial distribution of drugs are required to complement pharmacokinetic data in order to better understand and predict drug effects and toxicities.

Abcc4 together with abcb1 and abcg2 form a robust cooperative drug efflux system that restricts the brain entry of camptothecin analogues.

PURPOSE: Multidrug resistance-associated protein 4 (ABCC4) shares many features with P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2), including broad substrate affinity and expression at the blood-brain barrier (BBB). However, the pharmacologic relevance of ABCC4 at the BBB is difficult to evaluate, as most drugs are also substrates of ABCB1 and/or ABCG2. EXPERIMENTAL DESIGN: We have created a mouse strain in which all these alleles are inactivated to assess their impact on brain delivery of camptothecin analogues, an important class of antineoplastic agents and substrates of these transporters. Wild-type (WT), Abcg2(-/-), Abcb1a/b(-/-), Abcc4(-/-), Abcb1a/b;Abcg2(-/-), Abcg2;Abcc4(-/-), and Abcb1a/b;Abcg2;Abcc4(-/-) mice received i.v. topotecan, irinotecan, SN-38, or gimatecan alone or with concomitant oral elacridar. Drug levels were analyzed by high-performance liquid chromatography (HPLC). RESULTS: We found that additional deficiency of Abcc4 in Abcb1a/b;Abcg2(-/-) mice significantly increased the brain concentration of all camptothecin analogues by 1.2-fold (gimatecan) to 5.8-fold (SN-38). The presence of Abcb1a/b or Abcc4 alone was sufficient to reduce the brain concentration of SN-38 to the level in WT mice. Strikingly, the brain distribution of gimatecan in brain of WT mice was more than 220- and 40-fold higher than that of SN-38 and topotecan, respectively. CONCLUSION: Abcc4 limits the brain penetration of camptothecin analogues and teams up with Abcb1a/b and Abcg2 to form a robust cooperative drug efflux system. This concerted action limits the usefulness of selective ABC transport inhibitors to enhance drug entry for treatment of intracranial diseases. Our results also suggest that gimatecan might be a better candidate than irinotecan for clinical evaluation against intracranial tumors.

Sildenafil is not a useful modulator of ABCB1 and ABCG2 mediated drug resistance in vivo.

AIM: Recently, sildenafil was reported to be an inhibitor of P-glycoprotein (P-gp/ABCB1) and breast cancer resistance protein (BCRP/ABCG2) in vitro. We have now investigated the in vivo potency of sildenafil. METHODS: By using wild-type and Abcb1; Abcg2 knockout mice we have investigated the effect of sildenafil on the brain penetration of two substrate drugs (docetaxel and topotecan). Next we have investigated if sildenafil was able to improve the efficacy of doxorubicin against P-glycoprotein expressing CT26 colon cancer cells in syngeneic Balb/c mice. RESULTS: Sildenafil administered orally at a dose of 50mg/kg did not improve the brain penetration of docetaxel and topotecan, although the plasma level of sildenafil was already much higher than can be achieved in humans. On the other hand, sildenafil increased the plasma levels of the cytotoxic drugs, but not by inhibition of Abcb1 or Abcg2, since this effect was also seen in Abcb1;Abcg2 knockout mice. The brain penetration of sildenafil was more than 20-fold higher in Abcb1;Abcg2 mice versus wild-type mice, indicating that sildenafil is a good substrate of the two transporters. Sildenafil was also not able to improve the efficacy of doxorubicin against subcutaneous CT26 tumours. The doxorubicin level in tumour tissue did increase, but so did the concentration of doxorubicin in plasma and heart. CONCLUSION: These results demonstrate that the potency and specificity of sildenafil as an inhibitor of ABCB1 and ABCG2 is not sufficient to warrant further clinical testing of this agent in combination with anticancer drugs.

Prolongation of time interval between doses could eliminate accelerated blood clearance phenomenon induced by pegylated liposomal topotecan.

Repeated injection of pegylated liposomes could elicit the disappearance of long-circulating characteristic, referred to as "accelerated blood clearance phenomenon." ABC phenomenon typically occurs when entrapped drugs are not cytotoxic, but recently it was reported that multiple doses of pegylated liposomal topotecan, a cytotoxic drug, could also induce the phenomenon in rats. To reveal whether the phenomenon could be induced in dogs and the effect of time interval between doses on the magnitude of ABC, pLT was repeatedly injected into beagle dogs with a time interval of 7, 21 and 28 days. The anti-PEG Ig M levels were detected using ELISA. It was found that ABC phenomenon could be induced in dogs by pLT. Inter-individual difference in anti-PEG antibody production could be observed, and antibody levels were directly correlated with the magnitude of ABC. Furthermore, time interval between doses had marked effect on the magnitude of ABC phenomenon. When the time interval was prolonged from 1 week to 4 weeks, ABC phenomenon could be eliminated. By comparing the pharmacokinetic profiles of lipid vesicles and entrapped topotecan, it was found that "empty pegylated vesicles" be formed in circulation, which might be responsible for the occurrence of ABC phenomenon.

Accelerated blood clearance of pegylated liposomal topotecan: influence of polyethylene glycol grafting density and animal species.

Upon repeated administration, empty pegylated liposomes lose long-circulating characteristics, referred to as accelerated blood clearance (ABC) phenomenon. However, pegylated liposomal cytotoxic drug formulations could not elicit the phenomenon. In the study, it was found that repeated injection of pegylated liposomal topotecan could induce ABC phenomenon in Wistar rats, beagle dogs, and mice, which might be associated with the formation of empty liposomes in circulation because of the rapid drug release rate. In rats, the 9% polyethylene glycol (PEG) formulation induced more severe ABC phenomenon than 3% PEG formulation despite the similar anti-PEG immunoglobulin M (IgM) levels following the first dose. Antibody neutralization experiments revealed that high PEG formulation was easily neutralized by IgM. Repeated administration of 3% PEG formulation in dogs could result in more severe ABC phenomenon. It seems that slow infusion was liable to cause ABC phenomenon. In all animal species, considerable intraindividual variability of IgM levels could be observed. Our observations may have important implications for the development, evaluation, and therapeutic use of pegylated liposomal cytotoxic drug formulations because using the current drug loading technology, most of the cytotoxic drugs could not be stably loaded in liposomes and rapid drug leakage from liposomes might occur in circulation.

Topotecan vitreous and plasma levels and retinal toxicity after transcorneal intravitreal delivery in healthy albino rabbits: alternative retinoblastoma treatment.

AIM: To determine intravitreal and plasma concentrations and retinal toxicity after transcorneal intravitreal injection of 1 µg and 2 µg of topotecan (Hycamtin). METHOD: Twelve healthy albino rabbits were included in this in vivo experiment. Six anesthetized albino rabbits received a single transcorneal intravitreal injection of 1 µg (group A) or 2 µg (group B) of topotecan. Vitreous and blood samples were collected until 168 h. Left eyes were treated with the same volume of saline. Plasma and vitreous levels of topotecan were determined by high-performance liquid chromatography. Area under the plasma concentration time curve (AUC) was calculated using trapezoidal rule. Clinical evidence of toxicity was classified into four grades according to anatomical structures. Electroretinograms (ERGs) were recorded. RESULTS: Time to maximum concentration was observed up to 2 h after drug injection in group A whereas up to 1 h in group B. Low levels of topotecan were detected in plasma in both groups and in the vitreous humor of the contralateral eye in group B. Topotecan levels (mean vitreous AUC in group A 2.55 µg/mL.h and in group B 5.338 µg/mL.h) were detectable up to 6 h in both groups. We observed following structural changes in rabbit eyes: corneal vascularization, cataract, hemophthalmus, choroidal edema and focal retinal atrophy. Abnormal ERGs were obtained. CONCLUSION: Our findings proved that transcorneal intravitreal administration of 1 µg and 2 µg of topotecan results in potentially cytotoxic intraocular concentrations. More studies are needed to establish the safety of topotecan for retinoblastoma in children.

Topotecan exposure estimation in pediatric acute myeloid leukemia supported by LC-MS-based drug monitoring and pharmacokinetic analysis.

Individualization of the topotecan dosing can reduce inter-patient variability, toxicity, and at the same time increases chemotherapy efficacy. Topotecan dosing based on simultaneous drug monitoring and pharmacokinetic analysis can yield more accurate and precise estimation of the topotecan systemic exposure than that attainable with the fixed dosing approach. Therefore, a combined approach could provide a tool assisting the clinicians in individualization of the topotecan dosing. The aim of the study was to estimate the topotecan exposure in pediatric patients with acute myeloid leukemia (AML) based on the plasma concentration-time data and using the pharmacokinetic analysis. The primary goal was achieve the correct estimation of the target plasma area against the topotecan concentration-time curve (AUC) in a 5 day course of cladribine followed by monitored topotecan in pediatric patients with recurrent/refractory AML. A sensitive and selective reversed-phase liquid chromatographic-mass spectrometry (LC-MS) assay was developed to quantify total topotecan in the human plasma samples. This method, with its lower quantification limit of 1 ng/ml, was validated over a linear range of 1-150 ng/ml. Under the proposed approach, the topotecan dosing was selected so as to achieve the final AUC value of 140±20 ng/ml h. The presented analytical and pharmacokinetic data demonstrate that the proposed approach can be a practical, useful, efficient, and accurate tool for individualizing the topotecan dosing in children with AML.

Repeated injections of PEGylated liposomal topotecan induces accelerated blood clearance phenomenon in rats.

The "accelerated blood clearance (ABC) phenomenon" of PEGylated liposomes following multiple injections has been reported recently. This immunogenicity poses a problem for research into liposomes and hinders their clinical application. However, since doxorubicin liposomes and mitoxantrone liposomes have been reported to fail to induce the ABC phenomenon, some people believe that cytotoxic drugs loaded liposomes will not produce this ABC phenomenon under multiple-dosing regimens. Nevertheless, in the present study, we report that a first injection of the PEGylated liposomal topotecan (a cell cycle-specific drug for the S phase) still produced a strong ABC phenomenon. Likewise, when the first dose of "empty" PEGylated liposomes or topotecan liposomes was increased, the ABC phenomenon of the subsequent dose was accordingly attenuated. Unlike doxorubicin and mitoxantrone, the blood clearance rate of topotecan was dramatically rapid, and the hepatic and splenic accumulations of topotecan liposomes were anomalous because of the ABC phenomenon. These findings may present new challenges to the clinical application of formulations of cytotoxic drugs loaded liposomes that require repeated administrations.

Physiologically based pharmacokinetic model for topotecan in mice.

Topotecan is a chemotherapeutic agent of choice for the second-line treatment of recurrent ovarian cancer. In this article, we have developed a physiologically based pharmacokinetic model to characterize and predict topotecan concentrations in mouse plasma and tissues. Single intravenous (IV) doses (5, 10 and 30 mg/kg) of topotecan were administered to male Swiss Webster mice, with plasma and tissue samples collected over 24 h, and with sample analysis by high performance liquid chromatography. Topotecan disposition in the lungs, heart, muscle, skin, spleen, gut, liver, brain and adipose was described by perfusion rate-limited compartments, whereas the testes and intraperitoneal (IP) fluid were described with permeability rate-limited compartments. The kidneys were modeled as a permeability rate-limited compartment with nonlinear efflux. The model included enterohepatic recycling of topotecan, with re-absorption of drug secreted in the bile and nonlinear bioavailability. Topotecan demonstrated dose-dependent, nonlinear pharmacokinetics and its elimination was described by nonlinear clearance from the liver and a parallel nonlinear and linear clearance from the kidneys. Mean tissue-to-plasma partition coefficients ranged from 0.123 (brain) to 55.3 (kidney). The model adequately characterized topotecan pharmacokinetics in plasma and tissue for all three doses. Additionally, the model provided good prediction of topotecan pharmacokinetics from several external data sets, including prediction of topotecan tissue pharmacokinetics following administration of 1 or 20 mg/kg IV, and prediction of plasma pharmacokinetics following doses of 1, 1.25, 15, 20 and 80 mg/kg IV and 20 mg/kg IP.

Compartment-specific roles of ATP-binding cassette transporters define differential topotecan distribution in brain parenchyma and cerebrospinal fluid.

Topotecan is a substrate of the ATP-binding cassette transporters P-glycoprotein (P-gp/MDR1) and breast cancer resistance protein (BCRP). To define the role of these transporters in topotecan penetration into the ventricular cerebrospinal fluid (vCSF) and brain parenchymal extracellular fluid (ECF) compartments, we performed intracerebral microdialysis on transporter-deficient mice after an intravenous dose of topotecan (4 mg/kg). vCSF penetration of unbound topotecan lactone was measured as the ratio of vCSF-to-plasma area under the concentration-time curves. The mean +/- SD ratios for wild-type, Mdr1a/b(-/-), Bcrp1(-/-), and Mdr1a/b(-/-)Bcrp1(-/-) mice were 3.07 +/- 0.09, 2.57 +/- 0.17, 1.63 +/- 0.12, and 0.86 +/- 0.05, respectively. In contrast, the ECF-to-plasma ratios for wild-type, Bcrp1(-/-), and Mdr1a/b(-/-)Bcrp1(-/-) mice were 0.36 +/- 0.06, 0.42 +/- 0.06, and 0.88 +/- 0.07. Topotecan lactone was below detectable limits in the ECF of Mdr1a/b(-/-) mice. When gefitinib (200 mg/kg) was preadministered to inhibit Bcrp1 and P-gp, the vCSF-to-plasma ratio decreased to 1.29 +/- 0.09 in wild-type mice and increased to 1.13 +/- 0.13 in Mdr1a/b(-/-)Bcrp1(-/-) mice, whereas the ECF-to-plasma ratio increased to 0.74 +/- 0.14 in wild-type and 1.07 +/- 0.03 in Mdr1a/b(-/-)Bcrp1(-/-) mice. Preferential active transport of topotecan lactone over topotecan carboxylate was shown in vivo by vCSF lactone-to-carboxylate area under the curve ratios for wild-type, Mdr1a/b(-/-), Bcrp1(-/-), and Mdr1a/b(-/-)Bcrp1(-/-) mice of 5.69 +/- 0.83, 3.85 +/- 0.64, 3.61 +/- 0.46, and 0.78 +/- 0.19, respectively. Our results suggest that Bcrp1 and P-gp transport topotecan into vCSF and out of brain parenchyma through the blood-brain barrier. These findings may help to improve pharmacologic strategies to treat brain tumors.

Application of a highly specific and sensitive fluorescent HPLC method for topotecan lactone in whole blood.

Individualization of topotecan dosing reduces inter-patient variability in topotecan exposure, presumably reducing toxicity and increasing efficacy. However, logistical limitations (e.g. requirement for plasma, intensive bedside plasma processing) have prevented widespread application of this approach to dosing topotecan. Thus, the objectives of the present study were to develop and validate an HPLC with fluorescence detection method to measure topotecan lactone in whole blood samples and to evaluate its application to individualizing topotecan dosing. Plasma samples (200 microL) were prepared using methanolic precipitation, a filtration step and then injection of 100 microL of the methanolic extract onto a Novapak C(18), 4 microm, 3.9 x 150 mm column with an isocratic mobile phase. Analytes were detected with a Shimadzu Fluorescence RF-10AXL detector with an excitation and emission wavelength of 370 and 520 nm, respectively. This method had a lower limit of quantification of 1 ng/mL (S/N >or= 5; RSD 4.9%) and was validated over a linear range of 1-100 ng/mL. Results from a 5-day validation study demonstrated good within-day and between-day precision and accuracy. Data are presented to demonstrate that the present method can be used with whole blood samples to individualize topotecan dosing in children with cancer.