Systematic Evaluation of Osimertinib Population Pharmacokinetic Models in a Cohort of Dutch Adults with Non-Small Cell Lung Cancer.

BACKGROUND AND OBJECTIVE: Several population pharmacokinetic (popPK) studies have been reported that can guide the prediction of osimertinib plasma concentrations in individual patients. It is currently unclear which popPK model offers the best predictive performance and which popPK models are most suitable for nonadherence management and model-informed precision dosing. Therefore, the objective of this study was to externally validate all osimertinib popPK models available in the current literature. METHODS: Published popPK models for osimertinib were constructed using NONMEM version 7.4.4. The predictive quality of the identified models was assessed with goodness-of-fit (GoF) plots, conditional weighted residuals (CWRES) plots and a prediction-corrected visual predictive check (pcVPC) for osimertinib and its active metabolite AZ5104. A subset from the Dutch OSIBOOST trial, where 11 patients with low osimertinib exposure were included, was used as evaluation cohort. RESULTS: The population GoF plots for all four models poorly followed the line of identity. For the individual GoF plots, all models performed comparable and were closely distributed among the line of identity. CWRES of the four models were skewed. The pcVPCs of all four models showed a similar trend, where all observed concentrations fell in the simulated shaded areas, but in the lower region of the simulated areas. CONCLUSION: All four popPK models can be used to individually predict osimertinib concentrations in patients with low osimertinib exposure. For population predictions, all four popPK models performed poorly in patients with low osimertinib exposure. A novel popPK model with good predictive performance should be developed for patients with low osimertinib exposure. Ideally, the cause for the relatively low osimertinib exposure in our evaluation cohort should be known. CLINICAL TRIALS REGISTRATION: NCT03858491.

Lethal Capecitabine Toxicity in Patients With Complete Dihydropyrimidine Dehydrogenase Deficiency Due to Ultra-Rare DPYD Variants.

A DPD deficiency should be considered in case of severe toxicity even in the absence of common risk variants in DPYD.

Transcriptomics- and Genomics-Guided Drug Repurposing for the Treatment of Vesicular Hand Eczema.

Vesicular hand eczema (VHE), a clinical subtype of hand eczema (HE), showed limited responsiveness to alitretinoin, the only approved systemic treatment for severe chronic HE. This emphasizes the need for alternative treatment approaches. Therefore, our study aimed to identify drug repurposing opportunities for VHE using transcriptomics and genomics data. We constructed a gene network by combining 52 differentially expressed genes (DEGs) from a VHE transcriptomics study with 3 quantitative trait locus (QTL) genes associated with HE. Through network analysis, clustering, and functional enrichment analyses, we investigated the underlying biological mechanisms of this network. Next, we leveraged drug-gene interactions and retrieved pharmaco-transcriptomics data from the DrugBank database to identify drug repurposing opportunities for (V)HE. We developed a drug ranking system, primarily based on efficacy, safety, and practical and pricing factors, to select the most promising drug repurposing candidates. Our results revealed that the (V)HE network comprised 78 genes that yielded several biological pathways underlying the disease. The drug-gene interaction search together with pharmaco-transcriptomics lookups revealed 123 unique drug repurposing opportunities. Based on our drug ranking system, our study identified the most promising drug repurposing opportunities (e.g., vitamin D analogues, retinoids, and immunomodulating drugs) that might be effective in treating (V)HE.

Representation of older patients in the safety analysis of protein kinase inhibitor registration studies.

INTRODUCTION: Older patients (≥65 years old) make up the majority of the cancer population. Older patients seem to experience more adverse events (AEs) from protein kinase inhibitors (PKIs) in clinical practice. Yet they are underrepresented in clinical trials. We aimed to evaluate whether age-related safety differences were described at authorization of PKIs. Representation of older patients in registration studies was also evaluated. MATERIALS AND METHODS: European Public Assessment Reports (EPARs) of PKIs authorized between 2010 and 2015 were evaluated for the description of age-related safety- and pharmacokinetic differences. The International Council for Harmonization of Technical Requirement for Pharmaceuticals for Human Use (ICH) E7 guideline was applied to EPARs to assess the representation of older patients. Study results were presented descriptively. RESULTS: Eighteen PKIs with 19 EPARs were analyzed. Age-related safety differences were described in 14 out of 19 EPARs, and age-related pharmacokinetic differences in 1 out of 19 EPARs. More than 100 older patients were included in half of the studies. Older patients were not excluded solely by age, although other inclusion and exclusion criteria negatively influenced enrollment of older patients. None of the PKIs met all criteria from the ICH E7 guideline. DISCUSSION: Age-related safety differences are described for most PKIs. Older patients were underrepresented in PKI registration studies. Adequate representation of older patients in clinical trials for PKIs is vital, since they make up most of the cancer population.

Cobicistat as a Potential Booster of Ponatinib and Dasatinib Exposure in a CML Patient: A Case Study.

ABSTRACT: The authors present a case of a 57-year-old patient with chronic myeloid leukemia who was treated with ponatinib and subsequently treated with dasatinib. The patient showed a major molecular response; however, the BCR-ABL1 signal increased with low ponatinib and dasatinib trough concentrations. Cobicistat was used as a pharmacokinetic booster to increase ponatinib and dasatinib exposure, as opposed to increasing the dose. However, ponatinib exposure was not sufficiently increased by cobicistat. The peak dasatinib concentration was successfully increased with cobicistat treatment. Dasatinib and cobicistat cotreatment induced a response in BCR-ABL1 PCR signal, was well tolerated, and led to a substantial reduction in drug costs.

A Systematic Evaluation of Cost-Saving Dosing Regimens for Therapeutic Antibodies and Antibody-Drug Conjugates for the Treatment of Lung Cancer.

BACKGROUND: Expensive novel anticancer drugs put a serious strain on healthcare budgets, and the associated drug expenses limit access to life-saving treatments worldwide. OBJECTIVE: We aimed to develop alternative dosing regimens to reduce drug expenses. METHODS: We developed alternative dosing regimens for the following monoclonal antibodies used for the treatment of lung cancer: amivantamab, atezolizumab, bevacizumab, durvalumab, ipilimumab, nivolumab, pembrolizumab, and ramucirumab; and for the antibody-drug conjugate trastuzumab deruxtecan. The alternative dosing regimens were developed by means of modeling and simulation based on the population pharmacokinetic models developed by the license holders. They were based on weight bands and the administration of complete vials to limit drug wastage. The resulting dosing regimens were developed to comply with criteria used by regulatory authorities for in silico dose development. RESULTS: We found that alternative dosing regimens could result in cost savings that range from 11 to 28%, and lead to equivalent pharmacokinetic exposure with no relevant increases in variability in exposure. CONCLUSIONS: Dosing regimens based on weight bands and the use of complete vials to reduce drug wastage result in less expenses while maintaining equivalent exposure. The level of evidence of our proposal is the same as accepted by regulatory authorities for the approval of alternative dosing regimens of other monoclonal antibodies in oncology. The proposed alternative dosing regimens can, therefore, be directly implemented in clinical practice.

Pharmacokinetic Boosting of Kinase Inhibitors.

(1) Introduction: Pharmacokinetic boosting of kinase inhibitors can be a strategy to enhance drug exposure and to reduce dose and associated treatment costs. Most kinase inhibitors are predominantly metabolized by CYP3A4, enabling boosting using CYP3A4 inhibition. Kinase inhibitors with food enhanced absorption can be boosted using food optimized intake schedules. The aim of this narrative review is to provide answers to the following questions: Which different boosting strategies can be useful in boosting kinase inhibitors? Which kinase inhibitors are potential candidates for either CYP3A4 or food boosting? Which clinical studies on CYP3A4 or food boosting have been published or are ongoing? (2) Methods: PubMed was searched for boosting studies of kinase inhibitors. (3) Results/Discussion: This review describes 13 studies on exposure boosting of kinase inhibitors. Boosting strategies included cobicistat, ritonavir, itraconazole, ketoconazole, posaconazole, grapefruit juice and food. Clinical trial design for conducting pharmacokinetic boosting trials and risk management is discussed. (4) Conclusion: Pharmacokinetic boosting of kinase inhibitors is a promising, rapidly evolving and already partly proven strategy to increase drug exposure and to potentially reduce treatment costs. Therapeutic drug monitoring can be of added value in guiding boosted regimens.

Optimising pharmacotherapy in older cancer patients with polypharmacy.

OBJECTIVE: Polypharmacy is frequent among older cancer patients and increases the risk of potential drug-related problems (DRPs). DRPs are associated with adverse drug events, drug-drug interactions and hospitalisations. Since no standardised polypharmacy assessment methods for oncology patients exist, we aimed to develop one that can be integrated into routine care. METHODS: Based on the Systematic Tool to Reduce Inappropriate Prescribing (STRIP), we developed OncoSTRIP, which includes a polypharmacy anamnesis, a concise geriatric assessment, a polypharmacy analysis taking life expectancy into account and an optimised treatment plan. Patients ≥65 years with ≥5 chronic drugs visiting our outpatient oncology clinic were eligible for the polypharmacy assessment. RESULTS: OncoSTRIP was integrated into routine care of our older cancer patients. In 47 of 60 patients (78%), potential DRPs (n = 101) were found. In total, 85 optimisations were recommended, with an acceptance rate of 41%. It was possible to reduce the number of potential DRPs by 41% and the number of patients with at least one potential DRP by 30%. Mean time spent per patient was 71 min. CONCLUSIONS: Polypharmacy assessment of older cancer patients identifies many pharmacotherapeutic optimisations. With OncoSTRIP, polypharmacy assessments can be integrated into routine care.

Reversible Impaired Methotrexate Clearance After Platinum-Based Chemotherapy for Osteosarcoma.

The authors present a case of an 18-year-old man with metastasized osteosarcoma, admitted for methotrexate (MTX) treatment combined with cisplatin and doxorubicin. During the first cycle, severe MTX toxicity was observed with increased MTX serum levels and delayed MTX clearance requiring rescue treatment with intensified leucovorin. In the following cycles, cisplatin and doxorubicin were discontinued, and MTX dose was reduced. The elimination half-life slowly improved over the following cycles suggesting a reversible cause responsible for reduced MTX clearance and toxicity during the first cycle. Cisplatin is well-known for its nephrotoxic effects and can induce reversible tubular injury. Previous treatment with cisplatin may well have been responsible for decreased MTX clearance, and combination treatment should be used with adequate monitoring of MTX levels. Other factors that may have contributed, such as urine alkalization, gene polymorphisms, and other drug-drug interactions are discussed.

Amiodarone Rifampicin Drug-Drug Interaction Management With Therapeutic Drug Monitoring.

The authors present a case of a 69-year-old man with arrhythmogenic right ventricular cardiomyopathy controlled with amiodarone and an infected orthopedic prosthesis requiring treatment with rifampicin. This combination involves a pharmacokinetic drug-drug interaction leading to subtherapeutic drug concentrations of amiodarone and its active metabolite. The long half-life of amiodarone and its active metabolite in combination with the late onset and offset of cytochrome P4503A (CYP3A4) induction by rifampicin makes this a challenging drug-drug interaction to cope with in clinical practice. Before, during, and after rifampicin treatment, the serum concentrations of amiodarone and its active metabolite were measured and the amiodarone dose was adjusted accordingly. The amiodarone dose required to maintain effective concentrations was 450% of the initial dose. The drug-drug interaction between amiodarone and rifampicin is relevant, both clinically and pharmacokinetically, and can be managed by dose adjustments of amiodarone based on serum concentrations.

TGF-ß Antibody Uptake in Recurrent High-Grade Glioma Imaged with 89Zr-Fresolimumab PET.

UNLABELLED: Transforming growth factor-ß (TGF-ß) signaling is involved in glioma development. The monoclonal antibody fresolimumab (GC1008) can neutralize all mammalian isoforms of TGF-ß, and tumor uptake can be visualized and quantified with (89)Zr-fresolimumab PET in mice. The aim of this study was to investigate the fresolimumab uptake in recurrent high-grade gliomas using (89)Zr-fresolimumab PET and to assess treatment outcome in patients with recurrent high-grade glioma treated with fresolimumab. METHODS: Patients with recurrent glioma were eligible. After intravenous administration of 37 MBq (5 mg) of (89)Zr-fresolimumab, PET scans were acquired on day 2 or day 4 after tracer injection. Thereafter, patients were treated with 5 mg of fresolimumab per kilogram intravenously every 3 wk. (89)Zr-fresolimumab tumor uptake was quantified as maximum standardized uptake value (SUVmax). MR imaging for response evaluation was performed after 3 infusions or as clinically indicated. RESULTS: Twelve patients with recurrent high-grade glioma were included: 10 glioblastomas, 1 anaplastic oligodendroglioma, and 1 anaplastic astrocytoma. All patients underwent (89)Zr-fresolimumab PET 4 d after injection. In 4 patients, an additional PET scan was obtained on day 2 after injection. SUVmax on day 4 in tumor lesions was 4.6 (range, 1.5-13.9) versus a median SUVmean of 0.3 (range, 0.2-0.5) in normal brain tissue. All patients showed clinical or radiologic progression after 1-3 infusions of fresolimumab. Median progression-free survival was 61 d (range, 25-80 d), and median overall survival was 106 d (range, 37-417 d). CONCLUSION: (89)Zr-fresolimumab penetrated recurrent high-grade gliomas very well but did not result in clinical benefit.

89Zr-bevacizumab PET visualizes heterogeneous tracer accumulation in tumor lesions of renal cell carcinoma patients and differential effects of antiangiogenic treatment.

UNLABELLED: No validated predictive biomarkers for antiangiogenic treatment of metastatic renal cell carcinoma (mRCC) exist. Tumor vascular endothelial growth factor A (VEGF-A) level may be useful. We determined tumor uptake of (89)Zr-bevacizumab, a VEGF-A-binding PET tracer, in mRCC patients before and during antiangiogenic treatment in a pilot study. METHODS: Patients underwent (89)Zr-bevacizumab PET scans at baseline and 2 and 6 wk after initiating either bevacizumab (10 mg/kg every 2 wk) with interferon-α (3-9 million IU 3 times/wk) (n = 11) or sunitinib (50 mg daily, 4 of every 6 wk) (n = 11). Standardized uptake values were compared with plasma VEGF-A and time to disease progression. RESULTS: (89)Zr-bevacizumab PET scans visualized 125 evaluable tumor lesions in 22 patients, with a median SUV(max) (maximum standardized uptake value) of 6.9 (range, 2.3-46.9). Bevacizumab/interferon-α induced a mean change in tumor SUV(max) of -47.0% (range, -84.7 to +20.0%; P < 0.0001) at 2 wk and an additional -9.7% (range, -44.8 to +38.9%; P = 0.015) at 6 wk. In the sunitinib group, the mean change in tumor SUV(max) was -14.3% at 2 wk (range, -80.4 to +269.9; P = 0.006), but at 6 wk the mean change in tumor SUV(max) was +72.6% (range, -46.4 to +236%; P < 0.0001) above baseline. SUV(max) was not related to plasma VEGF-A at all scan moments. A baseline mean tumor SUV(max) greater than 10.0 in the 3 most intense lesions corresponded with longer time to disease progression (89.7 vs. 23.0 wk; hazard ratio, 0.22; 95% confidence interval, 0.05-1.00). CONCLUSION: Tumor uptake of (89)Zr-bevacizumab is high in mRCC, with remarkable interpatient and intrapatient heterogeneity. Bevacizumab/interferon-α strongly decreases tumor uptake whereas sunitinib results in a modest reduction with an overshoot after 2 drug-free weeks. High baseline tumor SUV(max) was associated with longer time to progression.

89Zr-trastuzumab and 89Zr-bevacizumab PET to evaluate the effect of the HSP90 inhibitor NVP-AUY922 in metastatic breast cancer patients.

PURPOSE: HSP90 chaperones have key client proteins that are involved in all hallmarks of breast cancer growth and progression. The primary aim of this clinical trial was to evaluate the feasibility of using (89)Zr-trastuzumab PET (for HER2-positive breast cancer) or (89)Zr-bevacizumab PET [for estrogen receptor (ER)-positive breast cancer] to determine in vivo degradation of client proteins caused by the novel HSP90 inhibitor NVP-AUY922. EXPERIMENTAL DESIGN: Of note, 70 mg/m(2) NVP-AUY922 was administered intravenously in a weekly schedule to patients with advanced HER2 or ER-positive breast cancer. Biomarker analysis consisted of serial PET imaging with 2[18F]fluoro-2-deoxy-D-glucose (FDG), (89)Zr-trastuzumab, or (89)Zr-bevacizumab. Response evaluation was performed according to RECIST1.0. FDG, (89)Zr-trastuzumab, and (89)Zr-bevacizumab distributions were scored visually and quantitatively by calculating the maximum standardized uptake values (SUVmax). In blood samples, serial HSP70 levels, extracellular form of HER2 (HER2-ECD), and pharmacokinetic and pharmacodynamic parameters were measured. RESULTS: Sixteen patients (ten HER2-positive and six ER-positive tumors) were included. One partial response was observed; seven patients showed stable disease. SUVmax change in individual tumor lesions on baseline versus 3 weeks (89)Zr-trastuzumab PET was heterogeneous and related to size change on CT after 8 weeks treatment (r(2) = 0.69; P = 0.006). Tumor response on (89)Zr-bevacizumab PET and FDG-PET was not correlated with CT response. CONCLUSIONS: NVP-AUY922 showed proof-of-concept clinical response in HER2-amplified metastatic breast cancer. Early change on (89)Zr-trastuzumab PET was positively associated with change in size of individual lesions assessed by CT.

(111)In-trastuzumab scintigraphy in HER2-positive metastatic breast cancer patients remains feasible during trastuzumab treatment.

Human epidermal growth factor receptor (HER)2 imaging with radiolabeled trastuzumab might support HER2-targeted therapy. It is, however, frequently questioned whether HER2 imaging is also possible during trastuzumab treatment as the receptor might be saturated. We studied the effect of trastuzumab treatment on 111In-trastuzumab uptake. Patients received trastuzumab weekly and paclitaxel once every 3 weeks. 111In-trastuzumab was injected on day 1 of cycle 1 and day 15 of cycle 4. Whole-body planar scintigraphy was acquired at different time points postinjection. Tumor uptake and organ distribution between the first and repeated scan series were calculated via residence times. Twenty-five tumor lesions in 12 patients were visualized on both scintigraphy series. Tumor uptake decreased (19.6%; p  =  .03). The residence times of normal organs remained similar except for the cardiac blood pool (+ 16.3%; p  =  .014). Trastuzumab treatment decreases tumor 111In-trastuzumab uptake around 20%. HER2 imaging is feasible during trastuzumab treatment.

Everolimus Reduces (89)Zr-Bevacizumab Tumor Uptake in Patients with Neuroendocrine Tumors.

UNLABELLED: Everolimus increases progression-free survival in patients with advanced neuroendocrine tumors (NETs). Currently, no biomarkers are available for early selection of patients who will benefit from everolimus. Everolimus can reduce vascular endothelial growth factor A (VEGF-A) production by tumor cells. Therefore, we aimed to investigate the effect of everolimus on tumor uptake of the radioactive-labeled VEGF-A antibody bevacizumab with PET in NET patients. METHODS: Patients with advanced progressive well-differentiated NETs underwent (89)Zr-bevacizumab PET scans before and at 2 and 12 wk during everolimus treatment. (89)Zr-bevacizumab uptake was quantified by the maximum standardized uptake value (SUVmax). Tumor response and the percentage change in the sum of target lesion diameters were determined according to Response Evaluation Criteria in Solid Tumors 1.1 on CT (3 monthly). RESULTS: In 4 of the 14 patients entered, no tumor lesions were visualized with (89)Zr-bevacizumab PET. In the remaining patients, 19% of tumor lesions 1 cm or greater known by CT were visualized. Tumor SUVmax decreased during everolimus treatment, with a median of -7% at 2 wk (P = 0.09) and a median of -35% at 12 wk (P < 0.001). The difference in SUVmax at 2 and 12 wk with respect to SUVmax at baseline correlated with percentage change on CT at 6 mo (r(2) = 0.51, P < 0.05, and r(2) = 0.61, P < 0.01, respectively). CONCLUSION: This study demonstrates variable (89)Zr-bevacizumab PET tumor uptake in NET patients. (89)Zr-bevacizumab tumor uptake diminished during everolimus treatment. Serial (89)Zr-bevacizumab PET might be useful as an early predictive biomarker of anti-VEGF-directed treatment in NET patients.

Placental growth factor (PlGF)-specific uptake in tumor microenvironment of 89Zr-labeled PlGF antibody RO5323441.

UNLABELLED: Placental growth factor (PlGF) is a member of the proangiogenic vascular endothelial growth factor family, which is upregulated in many tumors. RO5323441, a humanized monoclonal antibody against PlGF, showed antitumor activity in human tumor xenografts. We therefore aimed to radiolabel RO5323441 and preclinically validate this tracer to study drug tumor uptake and organ distribution by PET imaging. (89)Zr-RO5323441 was tested for stability and immunoreactivity in vitro. METHODS: The tumor uptake and organ distribution for 10, 50, and 500 µg of (89)Zr-RO5323441 was assessed in mice bearing human PlGF-expressing hepatocellular cancer (Huh7) xenografts or human renal cell carcinoma (ACHN) xenografts without detectable human PlGF expression. The effect of pretreatment with RO5323441 (20 mg/kg) on (89)Zr-RO5323441 tumor uptake was analyzed in Huh7 xenografts. (111)In-IgG served as a control for nonspecific tumor uptake and organ distribution. Cy5-RO5323441 was injected to study the intratumor distribution of RO5323441 with fluorescence microscopy. RESULTS: (89)Zr-RO5323441 showed a time- and dose-dependent tumor accumulation. Uptake in Huh7 xenografts at 10 µg of (89)Zr-RO5323441 was 8.2% ± 1.7% injected dose (ID)/cm(3) at 144 h after injection, and in ACHN xenografts it was 5.5 ± 0.3 %ID/cm(3) (P = 0.03). RO5323441 pretreatment of Huh7 xenograft-bearing mice reduced (89)Zr-RO5323441 tumor uptake to the level of nonspecific (111)In-IgG uptake. Cy5-RO5323441 was present in the tumors mainly in the microenvironment. CONCLUSION: The findings show that RO5323441 tumor uptake is PlGF-specific and time- and dose-dependent.

Bevacizumab-induced normalization of blood vessels in tumors hampers antibody uptake.

In solid tumors, angiogenesis occurs in the setting of a defective vasculature and impaired lymphatic drainage that is associated with increased vascular permeability and enhanced tumor permeability. These universal aspects of the tumor microenvironment can have a marked influence on intratumoral drug delivery that may often be underappreciated. In this study, we investigated the effect of blood vessel normalization in tumors by the antiangiogenic drug bevacizumab on antibody uptake by tumors. In mouse xenograft models of human ovarian and esophageal cancer (SKOV-3 and OE19), we evaluated antibody uptake in tumors by positron emission tomographic imaging 24 and 144 hours after injection of (89)Zr-trastuzumab (SKOV-3 and OE19), (89)Zr-bevacizumab (SKOV-3), or (89)Zr-IgG (SKOV-3) before or after treatment with bevacizumab. Intratumor distribution was assessed by fluorescence microscopy along with mean vessel density (MVD) and vessel normalization. Notably, bevacizumab treatment decreased tumor uptake and intratumoral accumulation compared with baseline in the tumor models relative to controls. Bevacizumab treatment also reduced MVD in tumors and increased vessel pericyte coverage. These findings are clinically important, suggesting caution in designing combinatorial trials with therapeutic antibodies due to a possible reduction in tumoral accumulation that may be caused by bevacizumab cotreatment.

CXCR4 inhibition with AMD3100 sensitizes prostate cancer to docetaxel chemotherapy.

Several in vitro and in vivo models have revealed the key role of CXCR4/CXCL12 axis in tumor-stroma interactions. Stromal cells present in the tumor microenvironment express high levels of CXCL12 protein, directly stimulating proliferation and migration of CXCR4-expressing cancer cells. This specific prosurvival influence of stromal cells on tumor cells is thought to protect them from cytotoxic chemotherapy and is postulated as a possible explanation for the minimal residual disease in hematological and solid cancers. Therefore, CXCR4/CXCL12 signaling is an attractive therapeutic target in cancer, as proven in preclinical leukemia mouse models, where CXCR4 inhibition sensitized cancer cells to conventional chemotherapy. This study investigates whether inhibition of CXCR4 with the specific inhibitor AMD3100 sensitizes human prostate cancer cells to docetaxel. We showed that both mouse and human stromal cell lines have a protective effect on PC3-luc cells by promoting their survival after chemotherapy. Furthermore, we demonstrated that AMD3100 sensitizes PC3-luc cells to docetaxel. In a subcutaneous xenograft mouse model of human prostate carcinoma, we showed that a combination of docetaxel and AMD3100 exerts increased antitumor effect compared with docetaxel alone. We concluded that CXCR4 inhibition chemosensitizes prostate cancer cells, both in vitro and in vivo. To explore the relevance of these findings, we analyzed CXCR4 expression levels in human prostate cancer samples. We found that cancer cells present in bone metastatic lesions express higher CXCR4 levels relative to the cells present in primary tumors and lymph node metastatic lesions. These findings underscore the potential of CXCR4 inhibitors as chemosensitizing agents.