Evaluation of Tc99m-glucoheptonate for SPECT functional imaging of medulloblastoma

CNS penetration of methotrexate and its metabolite 7-hydroxymethotrexate in mice bearing orthotopic Group 3 medulloblastoma tumors and model-based simulations for children

The brain penetration of methotrexate (MTX) and its metabolite 7-hydroxymethotrexate (7OHMTX) was characterized in non-tumor bearing mice and mice bearing orthotopic Group 3 medulloblastoma. Plasma pharmacokinetic studies and cerebral and ventricular microdialysis studies were performed in animals dosed with 200 or 1000 mg/kg MTX by IV bolus. Plasma, brain/tumor extracellular fluid (ECF) and lateral ventricle cerebrospinal fluid (CSF) MTX and 7OHMTX concentration-time data were analyzed by validated LC-MS/MS methods and modeled using a population-based pharmacokinetic approach and a hybrid physiologically-based model structure for the brain compartments. Brain penetration was similar for MTX and 7OHMTX and was not significantly different between non-tumor and tumor bearing mice. Overall, mean (±SD) model-derived unbound plasma to ECF partition coefficient K_p,uu were 0.17 (0.09) and 0.17 (0.12) for MTX and 7OHMTX, respectively. Unbound plasma to CSF K_p,uu were 0.11 (0.06) and 0.18 (0.09) for MTX and 7OHMTX, respectively. The plasma and brain model were scaled to children using allometric principles and pediatric physiological parameters. Model-based simulations were adequately overlaid with digitized plasma and CSF lumbar data collected in children receiving different MTX systemic infusions. This model can be used to further explore and optimize methotrexate dosing regimens in children with brain tumors.

CNS penetration and pharmacodynamics of the CHK1 inhibitor prexasertib in a mouse Group 3 medulloblastoma model

Prexasertib (LY2606368) is a potent and selective small molecule inhibitor of cell-cycle checkpoint CHK1 and CHK2 protein kinases and is currently under clinical evaluation for treatment of pediatric malignancies. As a candidate therapy for pediatric Group 3 medulloblastoma (G3MB), prexasertib CNS penetration was evaluated in mice using cerebral microdialysis and pharmacokinetic modeling. A plasma pharmacokinetic study with a population-based design was performed in CD1 nude mice bearing G3MB orthotopically implanted in the brain and receiving a single dose of prexasertib (10 mg/kg, subcutaneously) to characterize prexasertib disposition and to establish a limited plasma sampling model for the microdialysis studies. The microdialysis studies were performed in both non-tumor bearing mice and in mice bearing G3MB receiving 10 mg/kg prexasertib subcutaneously, for up to 24 h post-dose. Plasma and extracellular fluid (ECF) concentrations were quantified using validated LC MS/MS methods, and analyzed using a population pharmacokinetic model. Model-derived prexasertib tumor/ECF to plasma partition coefficient K_p,uu (ratio of tumor/brain ECF to unbound plasma AUC_0-24 h) was significantly greater in G3MB tumor-bearing mice (0.17 ± 0.08) compared to non-tumor bearing mice (0.09 ± 0.04, p = 0.04). A pharmacodynamic study was then performed in mice bearing G3MB (20 mg/kg, IV) to evaluate prexasertib-induced target engagement after a single dose. Phosphorylated CHK1 serine 345 (pCHK1 S345), phosphorylated Histone 2A variant (γ-H2AX), and cleaved caspase-3 were quantified in mouse G3MB tumor tissues by immunohistochemistry at different time points up to 24 h post-dose. The induction of pCHK1 S345 and γ-H2AX peaked at 2 h after the dose and was elevated above baseline for at least 6 h, reflecting relevant CHK1 inhibition and DNA damage. Cleaved caspase-3 levels increased at 24 h suggesting initiation of cell apoptosis. Adequate unbound prexasertib exposure reached the brain tumor site relative to target engagement in G3MB tumor bearing mice at a clinically relevant dosage. These results support further preclinical and clinical development of prexasertib to treat children with medulloblastoma.

Evaluation of Technetium-99m glucoheptonate single photon emission computed tomography for brain tumor grading

BACKGROUND

This study is designed to appraise the diagnostic value of technetium-99m glucoheptonate (Tc-99m GHA) single photon emission computed tomography (SPECT) in brain tumor grading.

SUBJECTS AND METHODS

The study was performed on 30 patients referred from the Department of Neurosurgery, who were from both urban and rural areas. Data were collected through interview, history taking, and clinical examination followed by recording the desired parameters and finally imaging. The study subjects were divided into five groups: Controls (n = 4), low-grade tumors (n = 8), high-grade tumors (n = 8), metastases (n = 5), and nonneoplastic lesions (n = 5). This division was based on the World Health Organization (WHO) classification postclinico-histological diagnosis. Each of the subjects underwent contrast-enhanced computed tomography/contrast-enhanced magnetic resonance and Tc-99m GHA SPECT preoperatively. All were followed up postoperatively, and histopathological reports were regarded as the gold standard for tumor grading wherever available.

RESULTS

It was found that high-grade tumors (Grades III/IV and IV/IV according to the WHO classification) showed significantly higher tumor to normal (T/N) ratios as well as Tmax/N ratios when compared with low-grade tumors (Grades I/IV and II/IV), metastases or nonneoplastic lesions.

CONCLUSIONS

In summary, the results of this study suggest that in situations where a preoperative grading of tumor is required Tc-99m GHA can be used in tumor grading and its use should be encouraged. Semi-quantitative analysis using both T/N as well as Tmax/N can be used in differentiating high-grade tumors from low-grade ones.

ABCG2 Transporter Expression Impacts Group 3 Medulloblastoma Response to Chemotherapy

While a small number of plasma membrane ABC transporters can export chemotherapeutic drugs and confer drug resistance, it is unknown whether these transporters are expressed or functional in less therapeutically tractable cancers such as Group 3 (G3) medulloblastoma. Herein we show that among this class of drug transporters, only ABCG2 was expressed at highly increased levels in human G3 medulloblastoma and a mouse model of this disease. In the mouse model, Abcg2 protein was expressed at the plasma membrane where it functioned as expected on the basis of export of prototypical substrates. By screening ABC substrates against mouse G3 medulloblastoma tumorspheres in vitro, we found that Abcg2 inhibition could potentiate responses to the clinically used drug topotecan, producing a more than 9-fold suppression of cell proliferation. Extended studies in vivo in this model confirmed that Abcg2 inhibition was sufficient to enhance antiproliferative responses to topotecan, producing a significant survival advantage compared with subjects treated with topotecan alone. Our findings offer a preclinical proof of concept for blockade of ABCG2 transporter activity as a strategy to empower chemotherapeutic responses in G3 medulloblastoma.

Silencing of the miR-17~92 cluster family inhibits medulloblastoma progression

Medulloblastoma, originating in the cerebellum, is the most common malignant brain tumor in children. Medulloblastoma consists of four major groups where constitutive activation of the Sonic Hedgehog (SHH) signaling pathway is a hallmark of one group. Mouse and human SHH medulloblastomas exhibit increased expression of microRNAs encoded by the miR-17~92 and miR-106b~25 clusters compared with granule progenitors and postmitotic granule neurons. Here, we assessed the therapeutic potential of 8-mer seed-targeting locked nucleic acid (LNA)-modified anti-miR oligonucleotides, termed tiny LNAs, that inhibit microRNA seed families expressed by miR-17~92 and miR-106b~25 in two mouse models of SHH medulloblastomas. We found that tumor cells (medulloblastoma cells) passively took up 8-mer LNA-anti-miRs and specifically inhibited targeted microRNA seed-sharing family members. Inhibition of miR-17 and miR-19a seed families by anti-miR-17 and anti-miR-19, respectively, resulted in diminished tumor cell proliferation in vitro. Treatment of mice with systemic delivery of anti-miR-17 and anti-miR-19 reduced tumor growth in flank and brain allografts in vivo and prolonged the survival of mice with intracranial transplants, suggesting that inhibition of the miR-17~92 cluster family by 8-mer LNA-anti-miRs might be considered for the treatment of SHH medulloblastomas.

In vitro evaluation of apoptosis with 99mTc-glucoheptonate

Radiopharmaceuticals are useful to evaluate effectiveness of cancer treatments as well as diagnosis of diseases. (99m)Tc-Glucoheptonate has high sensitivity for imaging lung cancer tissues. In this study, the potential use of (99m)Tc-glucoheptonate for monitoring apoptosis related to chemotherapeutic agents is investigated in vitro using A549 lung cancer cell line. A decrease in (99m)Tc-glucoheptonate uptake ratio was observed depending on the level of apoptosis. (99m)Tc-glucoheptonate is found to be useful for the detection of apoptosis following treatment in A549 lung tumor cells.

Clinical applications of PET and PET/CT in pediatric malignancies

The common childhood cancers are leukemia, CNS tumors, lymphomas, soft-tissue tumors (such as rhabdomyosarcoma and fibrosarcoma), neuroblastoma, malignant bone tumors, germ cell tumors with neoplasms of gonads and hepatic tumors. Usually the conventional imaging modalities, such as x-ray, ultrasound, computed tomography (CT) and MRI, are being routinely used for the management of these pediatric malignancies. However, most of these modalities provide structural information and are lacking in functional/metabolic status of these malignancies. Recently, PET and PET/CT have emerged as a functional diagnostic imaging modality for the management of various cancers in adult population. Up to now most of the data published in the literature are on PET alone. PET used in conjunction with CT is useful as it provides an enhanced view of the anatomical details and the malignant focus then can be located with highest accuracy. PET and PET/CT has been found to be useful in, for example, CNS tumors, lymphomas, soft-tissue tumors, neuroblastoma, malignant bone tumors and germ cell tumors. PET/CT has a limited role in early diagnosis, however, it plays an important role in initial staging, treatment response evaluation and detection of metastatic disease in these cancers. Despite the fact that PET/CT has better diagnostic value when compared with conventional imaging, such as CT and MRI, in the management of many pediatric cancers, there are certain limitations. PET/CT has a limited role in detection of lesions smaller than 5 mm, well-differentiated tumors and tumors with low metabolic rate. Many infections and inflammation can lead to false-positive PET/CT results. In the present review we will discuss the various clinical indications of PET and PET/CT in pediatric cancers.