A Review of Approaches to Potentiate the Activity of Temozolomide against Glioblastoma to Overcome Resistance.

A glioblastoma (GBM) is one of the most aggressive, infiltrative, and treatment-resistant malignancies of the central nervous system (CNS). The current standard of care for GBMs include maximally safe tumor resection, followed by concurrent adjuvant radiation treatment and chemotherapy with the DNA alkylating agent temozolomide (TMZ), which was approved by the FDA in 2005 based on a marginal increase (~2 months) in overall survival (OS) levels. This treatment approach, while initially successful in containing and treating GBM, almost invariably fails to prevent tumor recurrence. In addition to the limited therapeutic benefit, TMZ also causes debilitating adverse events (AEs) that significantly impact the quality of life of GBM patients. Some of the most common AEs include hematologic (e.g., thrombocytopenia, neutropenia, anemia) and non-hematologic (e.g., nausea, vomiting, constipation, dizziness) toxicities. Recurrent GBMs are often resistant to TMZ and other DNA-damaging agents. Thus, there is an urgent need to devise strategies to potentiate TMZ activity, to overcome drug resistance, and to reduce dose-dependent AEs. Here, we analyze major mechanisms of the TMZ resistance-mediated intracellular signaling activation of DNA repair pathways and the overexpression of drug transporters. We review some of the approaches investigated to counteract these mechanisms of resistance to TMZ, including the use of chemosensitizers and drug delivery strategies to enhance tumoral drug exposure.

A Phase 0/I Pharmacokinetic and Pharmacodynamics and Safety and Tolerability Study of Letrozole in Combination with Standard Therapy in Recurrent High-Grade Gliomas.

PURPOSE: High-grade gliomas (HGG) carry a poor prognosis, with glioblastoma accounting for almost 50% of primary brain malignancies in the elderly. Unfortunately, despite the use of multiple treatment modalities, the prognosis remains poor in this population. Our preclinical studies suggest that the presence of aromatase expression, encoded by CYP19A1, is significantly upregulated in HGGs. Remarkably, we find that letrozole (LTZ), an FDA-approved aromatase inhibitor, has marked activity against HGGs. PATIENTS AND METHODS: We conducted a phase 0/I single-center clinical trial (NCT03122197) to assess the tumoral availability, pharmacokinetics (PK), safety, and tolerability of LTZ in recurrent patients with HGG. Planned dose cohorts included 2.5, 5, 10, 12.5, 15, 17.5, and 20 mg of LTZ administered daily pre- and postsurgery or biopsy. Tumor samples were assayed for LTZ content and relevant biomarkers. The recommended phase 2 dose (R2PD) was determined as the dose that resulted in predicted steady-state tumoral extracellular fluid (ECF; Css,ecf) >2 µmol/L and did not result in ≥33% dose-limiting adverse events (AE) assessed using CTCAE v5.0. RESULTS: Twenty-one patients were enrolled. Common LTZ-related AEs included fatigue, nausea, musculoskeletal, anxiety, and dysphoric mood. No DLTs were observed. The 15 mg dose achieved a Css,ecf of 3.6 ± 0.59 µmol/L. LTZ caused dose-dependent inhibition of estradiol synthesis and modulated DNA damage pathways in tumor tissues as evident using RNA-sequencing analysis. CONCLUSIONS: On the basis of safety, brain tumoral PK, and mechanistic data, 15 mg daily is identified as the RP2D for future trials.

Solubilization of flurbiprofen into aptamer-modified PEG-PLA micelles for targeted delivery to brain-derived endothelial cells in vitro.

Novel aptamer-functionalized polyethylene glycol-polylactic acid (PEG-PLA) (APP) micelles were developed with the objective to target the transferrin receptor on brain endothelial cells. Flurbiprofen, a potential drug for therapeutic management of Alzheimer's disease (AD), was loaded into the APP micelles using the co-solvent evaporation method. Results indicated that 9.03% (w/w) of flurbiprofen was entrapped in APP with good retention capacity in vitro. Targeting potential of APPs was investigated using the transferring receptor-expressing murine brain endothelial bEND5 cell line. APPs significantly enhanced surface association of micelles to bEND5 cells as quantified by fluorescence spectroscopy. Most importantly, APPs significantly enhanced intracellular flurbiprofen delivery when compared to unmodified micelles. These results suggest that APP micelles may offer an effective strategy to deliver therapeutically effective flurbiprofen concentrations into the brain for AD patients.

Potentiation of temozolomide activity against glioblastoma cells by aromatase inhibitor letrozole.

PURPOSE: The DNA alkylating agent temozolomide (TMZ), is the first-line therapeutic for the treatment of glioblastoma (GBM). However, its use is confounded by the occurrence of drug resistance and debilitating adverse effects. Previously, we observed that letrozole (LTZ), an aromatase inhibitor, has potent activity against GBM in pre-clinical models. Here, we evaluated the effect of LTZ on TMZ activity against patient-derived GBM cells. METHODS: Employing patient-derived G76 (TMZ-sensitive), BT142 (TMZ-intermediately sensitive) and G43 and G75 (TMZ-resistant) GBM lines we assessed the influence of LTZ and TMZ on cell viability and neurosphere growth. Combination Index (CI) analysis was performed to gain quantitative insights of this interaction. We then assessed DNA damaging effects by conducting flow-cytometric analysis of Ë H2A.X formation and induction of apoptotic signaling pathways (caspase3/7 activity). The effects of adding estradiol on LTZ-induced cytotoxicity and DNA damage were also evaluated. RESULTS: Co-treatment with LTZ at a non-cytotoxic concentration (40 nM) reduced TMZ IC50 by 8, 37, 240 and 640 folds in G76, BT-142, G43 and G75 cells, respectively. The interaction was deemed to be synergistic based on CI analysis. LTZ co-treatment also significantly increased DNA damaging effects of TMZ. Addition of estradiol abrogated these LTZ effects. CONCLUSIONS: LTZ increases DNA damage and synergistically enhances TMZ activity in TMZ sensitive and TMZ-resistant GBM lines. These effects are abrogated by the addition of exogenous estradiol underscoring that the observed effects of LTZ may be mediated by estrogen deprivation. Our study provides a strong rationale for investigating the clinical potential of combining LTZ and TMZ for GBM therapy.

Letrozole targets the human ether-a-go-go-related gene potassium current in glioblastoma.

Aberrant expression of human ether-a-go-go-related gene (hERG) potassium channels has been implicated in the pathophysiology of glioblastoma (GBM). Letrozole has demonstrated efficacy in pre-clinical GBM models. The objective of this research was to assess the potential for hERG inhibition by letrozole to mediate efficacy in GBM. hERG currents were assessed using patch-clamp electrophysiology in an overexpression system during treatment with letrozole, exemestane or vehicle (dimethyl sulphoxide). Relative to vehicle, peak hERG tail current density was reduced when treated with 300 nmol/L and 1 µmol/L letrozole but not when treated with exemestane (up to 1 µmol/L). Cell proliferation was assessed in cultured glioblastoma cell lines (U87 and U373) treated with letrozole, exemestane, doxazosin (hERG blocker) or vehicle. Letrozole, but not exemestane, reduced cell proliferation relative to vehicle in U87 and U373 cells. The associations between expression of hERG (KCNH2), aromatase (CYP19A1) and the oestrogen receptors (ESR1 and ESR2) and time to all-cause mortality were assessed in GBM patients within The Cancer Genome Atlas (TCGA) database. hERG expression was associated with reduced overall survival in the TCGA GBM cohort. Future work is warranted to investigate hERG expression as a potential biomarker to predict the therapeutic potential of hERG inhibitors in GBM.

Venetoclax Increases Intratumoral Effector T Cells and Antitumor Efficacy in Combination with Immune Checkpoint Blockade.

The antiapoptotic protein BCL2 plays critical roles in regulating lymphocyte development and immune responses, and has also been implicated in tumorigenesis and tumor survival. However, it is unknown whether BCL2 is critical for antitumor immune responses. We evaluated whether venetoclax, a selective small-molecule inhibitor of BCL2, would influence the antitumor activity of immune checkpoint inhibitors (ICI). We demonstrate in mouse syngeneic tumor models that venetoclax can augment the antitumor efficacy of ICIs accompanied by the increase of PD-1+ T effector memory cells. Venetoclax did not impair human T-cell function in response to antigen stimuli in vitro and did not antagonize T-cell activation induced by anti-PD-1. Furthermore, we demonstrate that the antiapoptotic family member BCL-XL provides a survival advantage in effector T cells following inhibition of BCL2. Taken together, these data provide evidence that venetoclax should be further explored in combination with ICIs for cancer therapy. SIGNIFICANCE: The antiapoptotic oncoprotein BCL2 plays critical roles in tumorigenesis, tumor survival, lymphocyte development, and immune system regulation. Here we demonstrate that venetoclax, the first FDA/European Medicines Agency-approved BCL2 inhibitor, unexpectedly can be combined preclinically with immune checkpoint inhibitors to enhance anticancer immunotherapy, warranting clinical evaluation of these combinations.This article is highlighted in the In This Issue feature, p. 1.

Model-Informed Dosing of Venetoclax in Healthy Subjects: An Exposure-Response Analysis.

Venetoclax is an approved drug for the treatment of some hematological malignancies. Venetoclax can cause reduction in B-lymphocyte counts as an on-target effect. The purpose of this analysis is to quantify the relationship between venetoclax exposure and B-lymphocyte levels to inform dosing of venetoclax in healthy subjects. Data were pooled from 10 studies in healthy subjects with venetoclax doses ranging from 10 mg to 400 mg and food ranging from fasting to high-fat meals. Venetoclax pharmacokinetics (PK) was characterized in 203 subjects using a population approach, as implemented in NONMEM version 7.3 (Icon Development Solutions, Ellicott City, MD, USA). A semimechanistic pharmacodynamic (PD) model with a linear drug effect was fit to the B-lymphocyte data to determine the exposure-response relationship. The population PK and PD model described the observed data adequately. The 200 and 400 mg doses were shown to reduce the B-lymphocyte levels by 24% (15-35%) and 38% (25-54%), respectively. B-lymphocytes recovered to normal levels within an average of 48 (21-64) days and 59 (30-66) days, respectively, with 200 and 400 mg doses. Venetoclax can be safely administered in healthy subjects. The PK-PD model characterized the relationship between venetoclax exposure and reduction in B-lymphocytes and will help design future venetoclax studies in healthy subjects.

Pharmacokinetics of the BCL-2 Inhibitor Venetoclax in Subjects with Hepatic Impairment.

INTRODUCTION: Venetoclax is a selective B cell lymphoma-2 inhibitor. It is approved for treatment of chronic lymphocytic leukemia and is being investigated for other hematological malignancies. Venetoclax is predominantly eliminated by the liver; therefore, there is a need to investigate the effect of hepatic insufficiency on venetoclax pharmacokinetics. METHODS: A phase I study was carried out in 24 women with normal hepatic function or mild, moderate, or severe hepatic impairment (based on Child-Pugh scores), who received a single 50 mg dose of venetoclax with a low-fat meal. Blood samples were collected up to 120 h after venetoclax administration. Pharmacokinetic parameters were estimated using non-compartmental methods. RESULTS: Venetoclax maximum observed plasma concentration (Cmax) and area under the plasma concentration-time curve (AUC) in subjects with mild or moderate hepatic impairment were similar to subjects with normal hepatic function. Mean venetoclax AUC in subjects with severe hepatic impairment was 2.3- to 2.7-fold higher than in subjects with normal hepatic function. The half-life of venetoclax in subjects with severe hepatic impairment was approximately two-fold longer than in subjects with normal hepatic function and subjects with mild or moderate hepatic impairment. Unbound fractions of venetoclax in subjects with mild, moderate, and severe hepatic impairment were similar to the subjects with normal hepatic function. No significant adverse safety events were reported. CONCLUSIONS: No venetoclax dosage adjustment is required in subjects with mild or moderate hepatic impairment. In subjects with severe hepatic impairment, a 50% dose reduction of venetoclax is recommended to account for higher exposures and the longer half-life.

CYP2B6 Genotype-Dependent Inhibition of CYP1A2 and Induction of CYP2A6 by the Antiretroviral Drug Efavirenz in Healthy Volunteers.

We investigated the effect of efavirenz on the activities of cytochrome P450 (CYP)1A2, CYP2A6, xanthine oxidase (XO), and N-acetyltransferase 2 (NAT2), using caffeine as a probe. A single 150 mg oral dose of caffeine was administered to healthy volunteers (n = 58) on two separate occasions; with a single 600 mg oral dose of efavirenz and after treatment with 600 mg/day efavirenz for 17 days. Caffeine and its metabolites in plasma and urine were quantified using liquid chromatography/tandem-mass spectrometry. DNA was genotyped for CYP2B6*4 (785A>G), CYP2B6*9 (516G>T), and CYP2B6*18 (983T>C) alleles using TaqMan assays. Relative to single-dose efavirenz treatment, multiple doses of efavirenz decreased CYP1A2 (by 38%) and increased CYP2A6 (by 85%) activities (P < 0.05); XO and NAT2 activities were unaffected. CYP2B6*6*6 genotype was associated with lower CYP1A2 activity following both single and multiple doses of efavirenz. No similar association was noted for CYP2A6 activity. This is the first report showing that efavirenz reduces hepatic CYP1A2 and suggesting chronic efavirenz exposure likely enhances the elimination of CYP2A6 substrates. This is also the first to report the extent of efavirenz-CYP1A2 interaction may be efavirenz exposure-dependent and CYP2B6 genotype-dependent.

Accelerating Drug Development in Pediatric Oncology With the Clinical Pharmacology Storehouse.

Pediatric drug development is a challenging process due to the rarity of the population, the need to meet regulatory requirements across the globe, the associated uncertainty in extrapolating data from adults, the paucity of validated biomarkers, and the lack of systematic testing of drugs in pediatric patients. In oncology, pediatric drug development has additional challenges that have historically delayed availability of safe and effective medicines for children. In particular, the traditional approach to pediatric oncology drug development involves conducting phase 1 studies in children once the drug has been characterized and in some cases approved for use in adults. The objective of this article is to describe clinical pharmacology factors that influence pediatric oncology trial design and execution and to highlight efficient approaches for designing and expediting oncology drug development in children. The topics highlighted in this article include (1) study design considerations, (2) updated dosing approaches, (3) ways to overcome the significant biopharmaceutical challenges unique to the oncology pediatric population, and (4) use of data analysis strategies for extrapolating data from adults, with case studies. Finally, suggestions for ways to use clinical pharmacology approaches to accelerate pediatric oncology drug development are provided.

Model-based meta-analysis of progression-free survival in non-Hodgkin lymphoma patients.

BACKGROUND: Non-Hodgkin lymphoma (NHL) is a group of lymphoproliferative malignancies with varying treatment responses and progression-free survival (PFS) times. The objective of this study was to quantify the effect of treatment and patient-population characteristics on PFS in patients with NHL. METHODS: A database was developed from 513 NHL clinical trials reported from 1993 to 2015. Summary-level PFS was obtained from 112 of these trials, which included 155 cohorts and 11,824 patients. Characteristics evaluated for their impact on PFS included cohort treatment, percentage of patients with each NHL subtype, percentage of patients with different numbers of prior treatments, percentage of subjects previously administered rituximab, performance status, disease stage, median age, and sex distribution. RESULTS: Rituximab, bendamustine, CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone combination)/CHOP-like, and other nonchemotherapy drugs, aside from bortezomib, prolonged median PFS time 2 to 4-fold. Follicular lymphoma patients had 60% longer median PFS time than mantle cell lymphoma (MCL) patients, while diffuse large B-cell lymphoma patients had a median PFS time that was 25% of MCL patients. Patients who received ≤1 prior treatment had median PFS times > 10-fold longer than patients who received ≥2 prior treatments. The final model predicted the hazard ratio in 75% of the studies within 25% of the observed value and the observed median PFS time of 92% of the studies fell within the predicted 90% confidence intervals. CONCLUSIONS: The developed PFS model predicts the median PFS time and hazard ratio for specific populations and treatment combinations quantitatively and can potentially be extended to link short-term and long-term clinical outcomes.

Preclinical pharmacological evaluation of letrozole as a novel treatment for gliomas.

We present data that letrozole, an extensively used aromatase inhibitor in the treatment of estrogen receptor-positive breast tumors in postmenopausal women, may be potentially used in the treatment of glioblastomas. First, we measured the in vitro cytotoxicity of letrozole and aromatase (CYP19A1) expression and activity in human LN229, T98G, U373MG, U251MG, and U87MG, and rat C6 glioma cell lines. Estrogen receptor (ER)-positive MCF-7 and ER-negative MDA-MB-231 cells served as controls. Cytotoxicity was determined employing the MTT assay, and aromatase activity using an immunoassay that measures the conversion of testosterone to estrogen. Second, in vivo activity of letrozole was assessed in Sprague-Dawley rats orthotopically implanted with C6 gliomas. The changes in tumor volume with letrozole treatment (4 mg/kg/day) were assessed employing µPET/CT imaging, employing [(18)F]-fluorodeoxyglucose (F18-FDG) as the radiotracer. Brain tissues were collected for histologic evaluations. All glioma cell lines included here expressed CYP19A1 and letrozole exerted considerable cytotoxicity and decrease in aromatase activity against these cells (IC50, 0.1-3.5 µmol/L). Imaging analysis employing F18-FDG µPET/CT demonstrated a marked reduction of active tumor volume (>75%) after 8 days of letrozole treatment. Immunohistochemical analysis revealed marked reduction in aromatase expression in tumoral regions of the brain after letrozole treatment. Thus, employing multifaceted tools, we demonstrate that aromatase may be a novel target for the treatment of gliomas and that letrozole, an FDA-approved drug with an outstanding record of safety may be repurposed for the treatment of such primary brain tumors, which currently have few therapeutic options.

The pharmacokinetics of letrozole in brain and brain tumor in rats with orthotopically implanted C6 glioma, assessed using intracerebral microdialysis.

PURPOSE: Emerging evidence suggests that primary and metastatic brain tumors may be sensitive to hormonal manipulations. However, the pharmacokinetics of compounds against such targets in the brain and, more importantly, in the brain tumor are not well characterized. Here, we investigated the pharmacokinetics of letrozole, a third-generation aromatase inhibitor, in the normal brain and in orthotopically implanted C6 glioma in Sprague-Dawley rats. METHODS: Intracerebral microdialysis was employed to determine the concentrations of unbound letrozole in the brain extracellular fluid (ECF) while simultaneously collecting blood samples (via jugular vein) to assess plasma levels of letrozole. Letrozole was administered intravenously at doses of 4, 6, 8 and 12 mg/kg, and ECF and blood samples were collected over 8 h. For assessing normal versus tumoral brain pharmacokinetics, letrozole (4 or 8 mg/Kg; i.v.) was administered 10 days after implantation of C6 glioma in the brain. Dual-probe intracerebral microdialysis was employed for assessing ECF samples from tumor-free and tumor-bearing regions of the brain. RESULTS: Normal brain ECF and plasma C max and AUC0-8h increased linearly with letrozole doses up to 8 mg/kg dose, but at 12 mg/kg, the pharmacokinetics were nonlinear. The relative brain distribution coefficients, AUCECF/AUCplasma (ub), were 0.3-0.98. The tumoral uptake of letrozole was 1.5- to 2-fold higher relative to tumor-free region. CONCLUSIONS: Thus, letrozole permeability across the blood brain barrier is high, and the exposure to the brain is dose dependent. Furthermore, the brain tumoral letrozole levels are markedly higher than those in the tumor-free regions, which underscore potential selectivity of its activity against tumor cells.

The pharmacokinetics of acyl, des-acyl, and total ghrelin in healthy human subjects.

BACKGROUND: Ghrelin stimulates GH secretion and regulates energy and glucose metabolism. The two circulating isoforms, acyl (AG) and des-acyl (DAG) ghrelin, have distinct metabolic effects and are under active investigation for their therapeutic potentials. However, there is only limited data on the pharmacokinetics of AG and DAG. OBJECTIVES: To evaluate key pharmacokinetic parameters of AG, DAG, and total ghrelin in healthy men and women. METHODS: In study 1, AG (1, 3, and 5 µg/kg per h) was infused over 65 min in 12 healthy (8 F/4 M) subjects in randomized order. In study 2, AG (1 µg/kg per h), DAG (4 µg/kg per h), or both were infused over 210 min in ten healthy individuals (5 F/5 M). Plasma AG and DAG were measured using specific two-site ELISAs (study 1 and 2), and total ghrelin with a commercial RIA (study 1). Pharmacokinetic parameters were estimated by non-compartmental analysis. RESULTS: After the 1, 3, and 5 µg/kg per h doses of AG, there was a dose-dependent increase in the maximum concentration (C(max)) and area under the curve (AUC(0-last)) of AG and total ghrelin. Among the different AG doses, there was no difference in the elimination half-life, systemic clearance (CL), and volume of distribution. DAG had decreased CL relative to AG. The plasma DAG:AG ratio was ~2:1 during steady-state infusion of AG. Infusion of AG caused an increase in DAG, but DAG administration did not change plasma AG. Ghrelin administration did not affect plasma acylase activity. CONCLUSIONS: The pharmacokinetics of AG and total ghrelin appears to be linear and proportional in the dose range tested. AG and DAG have very distinct metabolic fates in the circulation. There is deacylation of AG in the plasma but no evidence of acylation.