Chitosan-coated magnetic graphene oxide for targeted delivery of doxorubicin as a nanomedicine approach to treat glioblastoma

In this study, magnetic graphene oxide (mGO) was first prepared and modified with chitosan to prepare chitosan-coated mGO (mGOC). Gastrin-releasing peptide (GRP)-conjugated mGOC (mGOCG) was then prepared from mGOC. The chemo drug doxorubicin (DOX) was adsorbed to mGOCG surface for dual active/magnetic targeted drug delivery. The DOX loading to mGOCG is 1.71 mg/mg, and drug release is pH-sensitive to facilitate drug delivery in endosomes. In vitro studies confirmed enhanced mGOCG endocytosis by U87 glioblastoma cells, with which enhanced cytotoxicity towards cancer cells could be achieved. This could be revealed from the drastically reduced half-maximal inhibitory concentration of mGOCG/DOX compared with DOX and mGOC/DOX. Furthermore, mGOCG/DOX can be localized under the influence of a magnetic field (MF) to exert this cytotoxic effect. An orthotopic brain tumor model by implanting U87 cells in the intracranial area of BALB/c nude mice was used to study the in vivo anti-tumor efficacy by intravenous injection of different samples and followed with bioluminescence imaging. The tumor size in the mGOCG/DOX + MF group demonstrated the best potency to suppress tumor growth and prolong animal survival time compared with mGOCG/DOX, mGOC/DOX, or DOX groups, indicating this new dual-targeting delivery system for DOX can effectively treat glioblastoma.

Effect of Conventional and Ultrahigh Dose Rate FLASH Irradiations on Preclinical Tumor Models: A Systematic Analysis

PURPOSE

Compared with conventional dose rate irradiation (CONV), ultrahigh dose rate irradiation (UHDR) has shown superior normal tissue sparing. However, a clinically relevant widening of the therapeutic window by UHDR, termed "FLASH effect," also depends on the tumor toxicity obtained by UHDR. Based on a combined analysis of published literature, the current study examined the hypothesis of tumor isoefficacy for UHDR versus CONV and aimed to identify potential knowledge gaps to inspire future in vivo studies.

METHODS AND MATERIALS

A systematic literature search identified publications assessing in vivo tumor responses comparing UHDR and CONV. Qualitative and quantitative analyses were performed, including combined analyses of tumor growth and survival data.

RESULTS

We identified 66 data sets from 15 publications that compared UHDR and CONV for tumor efficacy. The median number of animals per group was 9 (range 3-15) and the median follow-up period was 30.5 days (range 11-230) after the first irradiation. Tumor growth assays were the predominant model used. Combined statistical analyses of tumor growth and survival data are consistent with UHDR isoefficacy compared with CONV. Only 1 study determined tumor-controlling dose (TCD50) and reported statistically nonsignificant differences.

CONCLUSIONS

The combined quantitative analyses of tumor responses support the assumption of UHDR isoefficacy compared with CONV. However, the comparisons are primarily based on heterogeneous tumor growth assays with limited numbers of animals and short follow-up, and most studies do not assess long-term tumor control probability. Therefore, the assays may be insensitive in resolving smaller response differences, such as responses of radioresistant tumor subclones. Hence, tumor cure experiments, including additional TCD50 experiments, are needed to confirm the assumption of isoeffectiveness in curative settings.

Treatment of Orthotopic U251 Human Glioblastoma Multiforme Tumors in NRG Mice by Convection-Enhanced Delivery of Gold Nanoparticles Labeled with the β-Particle-Emitting Radionuclide, 177Lu

In this study, we investigated convection-enhanced delivery (CED) of 23 ± 3 nm gold nanoparticles (AuNPs) labeled with the β-particle-emitting radionuclide ¹⁷⁷Lu (¹⁷⁷Lu-AuNPs) for treatment of orthotopic U251-Luc human glioblastoma multiforme (GBM) tumors in NRG mice. The cytotoxicity in vitro of ¹⁷⁷Lu-AuNPs (0.0-2.0 MBq, 4 × 10¹¹ AuNPs) on U251-Luc cells was also studied by a clonogenic survival assay, and DNA double-strand breaks (DSBs) caused by β-particle emissions of ¹⁷⁷Lu were measured by confocal immunofluorescence microscopy for γH2AX. NRG mice with U251-Luc tumors in the right cerebral hemisphere of the brain were treated by CED of 1.1 ± 0.2 MBq of ¹⁷⁷Lu-AuNPs (4 × 10¹¹ AuNPs). Control mice received unlabeled AuNPs or normal saline. Tumor retention of ¹⁷⁷Lu-AuNPs was assessed by single-photon emission computed tomography/computed tomography (SPECT/CT) imaging and biodistribution studies. Radiation doses were estimated for the tumor, brain, and other organs. The effectiveness for treating GBM tumors was determined by bioluminescence imaging (BLI) and T2-weighted magnetic resonance imaging (MRI) and by Kaplan-Meier median survival. Normal tissue toxicity was assessed by monitoring body weight and hematology and blood biochemistry analyses at 14 d post-treatment. ¹⁷⁷Lu-AuNPs (2.0 MBq, 4 × 10¹¹ AuNPs) decreased the clonogenic survival of U251-Luc cells to 0.005 ± 0.002 and increased DNA DSBs by 14.3-fold compared to cells treated with unlabeled AuNPs or normal saline. A high proportion of ¹⁷⁷Lu-AuNPs was retained in the U251-Luc tumor in NRG mice up to 21 d with minimal re-distribution to the brain or other organs. The radiation dose in the tumor was high (599 Gy). The dose in the normal right cerebral hemisphere of the brain excluding the tumor was 93-fold lower (6.4 Gy), and 2000-3000-fold lower doses were calculated for the contralateral left cerebral hemisphere (0.3 Gy) or cerebellum (0.2 Gy). The doses in peripheral organs were <0.1 Gy. BLI revealed almost complete tumor growth arrest in mice treated with ¹⁷⁷Lu-AuNPs, while tumors grew rapidly in control mice. MRI at 28 d post-treatment and histological staining showed no visible tumor in mice treated with ¹⁷⁷Lu-AuNPs but large GBM tumors in control mice. All control mice reached a humane endpoint requiring sacrifice within 39 d (normal saline) or 45 d post-treatment (unlabeled AuNPs), while 5/8 mice treated with ¹⁷⁷Lu-AuNPs survived up to 150 d. No normal tissue toxicity was observed in mice treated with ¹⁷⁷Lu-AuNPs. We conclude that CED of ¹⁷⁷Lu-AuNPs was highly effective for treating U251-Luc human GBM tumors in the brain in NRG mice at amounts that were non-toxic to normal tissues. These ¹⁷⁷Lu-AuNPs administered by CED hold promise for treating patients with GBM to prevent recurrence and improve long-term outcome.

Targeted delivery of irinotecan and SLP2 shRNA with GRP-conjugated magnetic graphene oxide for glioblastoma treatment

Magnetic nanoparticles (MNPs) are useful for magnetic targeted drug delivery while ligand-mediated active targeting is another common delivery strategy for cancer therapy. In this work, we intend to prepare magnetic graphene oxide (mGO) by chemical co-precipitation of MNPs on the GO surface, followed by conjugation of the gastrin releasing peptide (GRP) as a targeting ligand, for dual targeted drug/gene delivery in invasive brain glioma treatment. mGO was grafted with chitosan, complexed with shRNA plasmid DNA for stomatin-like protein 2 (SLP2) gene silencing, modified with urocanic acid for plasmid DNA endosomal escape, PEGylated for GRP conjugation, and loaded with the chemotherapeutic drug irinotecan (CPT-11) by π-π interaction for pH-responsive drug release (mGOCUG/CPT-11/shRNA). In addition to the in depth characterization of the physico-chemical and biological properties during each preparation step, we also study the loading/pH-responsive release behavior of CPT-11 and the shRNA plasmid loading/cell transfection efficiency. The targeting and antitumor efficacies of the nanocomposite were studied with U87 human glioblastoma cells in vitro. An in vivo study revealed that intravenous administration followed by magnetic guidance results in the efficient targeted delivery of mGOCUG/CPT-11/shRNA to orthotopic brain tumors in nude mice, and it exhibits excellent antitumor efficacy with a reduced tumor growth rate and prolonged animal survival time. Our work thus highlights a multifunctional mGO-based drug/gene delivery platform for effective combination cancer therapy in glioblastoma treatment.

Enhanced Survival of High-Risk Medulloblastoma-Bearing Mice after Multimodal Treatment with Radiotherapy, Decitabine, and Abacavir

Children with high-risk SHH/TP53-mut and Group 3 medulloblastoma (MB) have a 5-year overall survival of only 40%. Innovative approaches to enhance survival while preventing adverse effects are urgently needed. We investigated an innovative therapy approach combining irradiation (RT), decitabine (DEC), and abacavir (ABC) in a patient-derived orthotopic SHH/TP53-mut and Group 3 MB mouse model. MB-bearing mice were treated with DEC, ABC and RT. Mouse survival, tumor growth (BLI, MRT) tumor histology (H/E), proliferation (Ki-67), and endothelial (CD31) staining were analyzed. Gene expression was examined by microarray and RT-PCR (Ki-67, VEGF, CD31, CD15, CD133, nestin, CD68, IBA). The RT/DEC/ABC therapy inhibited tumor growth and enhanced mouse survival. Ki-67 decreased in SHH/TP53-mut MBs after RT, DEC, RT/ABC, and RT/DEC/ABC therapy. CD31 was higher in SHH/TP53-mut compared to Group 3 MBs and decreased after RT/DEC/ABC. Microarray analyses showed a therapy-induced downregulation of cell cycle genes. By RT-PCR, no therapy-induced effect on stem cell fraction or immune cell invasion/activation could be shown. We showed for the first time that RT/DEC/ABC therapy improves survival of orthotopic SHH/TP53-mut and Group 3 MB-bearing mice without inducing adverse effects suggesting the potential for an adjuvant application of this multimodal therapy approach in the human clinic.

Validation of a combined ultrasound and bioluminescence imaging system with magnetic resonance imaging in orthotopic pancreatic murine tumors

Preclinical mouse solid tumor models are widely used to evaluate efficacy of novel cancer therapeutics. Recent reports have highlighted the need for utilizing orthotopic implantation to represent clinical disease more accurately, however the deep tissue location of these tumors makes longitudinal assessment challenging without the use of imaging techniques. The purpose of this study was to evaluate the performance of a new multi-modality high-throughput in vivo imaging system that combines bioluminescence imaging (BLI) with robotic, hands-free ultrasound (US) for evaluating orthotopic mouse models. Long utilized in cancer research as independent modalities, we hypothesized that the combination of BLI and US would offer complementary advantages of detection sensitivity and quantification accuracy, while mitigating individual technological weaknesses. Bioluminescent pancreatic tumor cells were injected into the pancreas tail of C57BL/6 mice and imaged weekly with the combination system and magnetic resonance imaging (MRI) to serve as a gold standard. BLI photon flux was quantified to assess tumor activity and distribution, and US and MRI datasets were manually segmented for gross tumor volume. Robotic US and MRI demonstrated a strong agreement (R2 = 0.94) for tumor volume measurement. BLI showed a weak overall agreement with MRI (R2 = 0.21), however, it offered the greatest sensitivity to detecting the presence of tumors. We conclude that combining BLI with robotic US offers an efficient screening tool for orthotopic tumor models.

Assessing Human Embryonic Stem Cell-Derived Dopaminergic Neuron Progenitor Transplants Using Non-invasive Imaging Techniques

PURPOSE

Human pluripotent stem cell (hPSC)-derived dopaminergic neuron progenitor cells (DAPCs) are a potential therapy for Parkinson's disease (PD). However, their intracranial administration raises safety concerns including uncontrolled proliferation, migration and inflammation. Here, we apply a bimodal imaging approach to investigate the fate of DAPC transplants in the rat striatum.

PROCEDURES

DAPCs co-expressing luciferase and ZsGreen or labelled with micron-sized particles of iron oxide (MPIOs) were transplanted in the striatum of RNU rats (n = 6 per group). DAPCs were tracked in vivo using bioluminescence and magnetic resonance (MR) imaging modalities.

RESULTS

Transgene silencing in differentiating DAPCs accompanied with signal attenuation due to animal growth rendered the bioluminescence undetectable by week 2 post intrastriatal transplantation. However, MR imaging of MPIO-labelled DAPCs showed that transplanted cells remained at the site of injection for over 120 days. Post-mortem histological analysis of DAPC transplants demonstrated that labelling with either luciferase/ZsGreen or MPIOs did not affect the ability of cells to differentiate into mature dopaminergic neurons. Importantly, labelled cells did not elicit increased glial reactivity compared to non-labelled cells.

CONCLUSIONS

In summary, our findings support the transplantation of hPSC-derived DAPCs as a safe treatment for PD.

Therapeutic targeting of KSP in preclinical models of high-risk neuroblastoma

Neuroblastoma is a childhood malignancy with often dismal prognosis; relapse is common despite intense treatment. Here, we used human tumor organoids representing multiple MYCN-amplified high-risk neuroblastomas to perform a high-throughput drug screen with approved or emerging oncology drugs. Tumor-selective effects were calculated using drug sensitivity scores. Several drugs with previously unreported anti-neuroblastoma effects were identified by stringent selection criteria. ARRY-520, an inhibitor of kinesin spindle protein (KSP), was among those causing reduced viability. High expression of the KSP-encoding gene KIF11 was associated with poor outcome in neuroblastoma. Genome-scale loss-of-function screens in hundreds of human cancer cell lines across 22 tumor types revealed that KIF11 is particularly important for neuroblastoma cell viability. KSP inhibition in neuroblastoma patient-derived xenograft (PDX) cells resulted in the formation of abnormal monoastral spindles, mitotic arrest, up-regulation of mitosis-associated genes, and apoptosis. In vivo, KSP inhibition caused regression of MYCN-amplified neuroblastoma PDX tumors. Furthermore, treatment of mice harboring orthotopic neuroblastoma PDX tumors resulted in increased survival. Our results suggested that KSP inhibition could be a promising treatment strategy in children with high-risk neuroblastoma.

Model-based evaluation of image-guided fractionated whole-brain radiation therapy in pediatric diffuse intrinsic pontine glioma xenografts

Radiation therapy (RT) is currently the standard treatment for diffuse intrinsic pontine glioma (DIPG), the most common cause of death in children with brain cancer. A pharmacodynamic model was developed to describe the radiation-induced tumor shrinkage and overall survival in mice bearing DIPG. CD1-nude mice were implanted in the brain cortex with luciferase-labeled patient-derived orthotopic xenografts of DIPG (SJDIPGx7 H3F3AWT / K27 M and SJDIPGx37 H3F3AK27M / K27M ). Mice were treated with image-guided whole-brain RT at 1 or 2 Gy/fraction 5-days-on 2-days-off for a cumulative dose of 20 or 54 Gy. Tumor progression was monitored with bioluminescent imaging (BLI). A mathematical model describing BLI and overall survival was developed with data from mice receiving 2 Gy/fraction and validated using data from mice receiving 1 Gy/fraction. BLI data were adequately fitted with a logistic tumor growth function and a signal distribution model with linear radiation-induced killing effect. A higher tumor growth rate in SJDIPGx37 versus SJDIPGx7 xenografts and a killing effect decreasing with higher tumor baseline (p < 0.0001) were identified. Cumulative radiation dose was suggested to inhibit the tumor growth rate according to a Hill function. Survival distribution was best described with a Weibull hazard function in which the hazard baseline was a continuous function of tumor BLI. Significant differences were further identified between DIPG cell lines and untreated versus treated mice. The model was adequately validated with mice receiving 1 Gy/fraction and will be useful in guiding future preclinical trials incorporating radiation and to support systemic combination therapies with RT.

Supramolecularly enabled pH- triggered drug action at tumor microenvironment potentiates nanomedicine efficacy against glioblastoma

The crucial balance of stability in blood-circulation and tumor-specific delivery has been suggested as one of the challenges for effective bench-to-bedside translation of nanomedicines (NMs). Herein, we developed a supramolecularly enabled tumor-extracellular (T^ex) pH-triggered NM that can maintain the micellar structure with the entrapped-drug during systemic circulation and progressively release drug in the tumor by rightly sensing heterogeneous tumor-pH. Desacetylvinblastine hydrazide (DAVBNH), a derivative of potent anticancer drug vinblastine, was conjugated to an aliphatic ketone-functionalized poly(ethylene glycol)-b-poly(amino acid) copolymer and the hydrolytic stability of the derived hydrazone bond was efficiently tailored by exploiting the compartmentalized structure of polymer micelle. We confirmed an effective and safe therapeutic application of T^ex pH-sensitive DAVBNH-loaded micelle (T^ex-micelle) in orthotopic glioblastoma (GBM) models, extending median survival to 1.4 times in GBM xenograft and 2.6 times in GBM syngeneic model, compared to that of the free DAVBNH. The work presented here offers novel chemical insights into the molecular design of smart NMs correctly sensing T^ex-pH via programmed functionalities. The practical engineering strategy based on a clinically relevant NM platform, and the encouraging therapeutic application of T^ex-micelle in GBM, one of the most lethal human cancers, thus suggests the potential clinical translation of this system against other types of common cancers, including GBM.

Akaluc bioluminescence offers superior sensitivity to track in vivo glioma expansion

Background

Longitudinal tracking of tumor growth using noninvasive bioluminescence imaging (BLI) is a key approach for studies of in vivo cancer models, with particular relevance for investigations of malignant gliomas in rodent intracranial transplant paradigms. Akaluciferase (Akaluc) is a new BLI system with higher signal strength than standard firefly luciferase (Fluc). Here, we establish Akaluc BLI as a sensitive method for in vivo tracking of glioma expansion.

Methods

We engineered a lentiviral vector for expression of Akaluc in high-grade glioma cell lines, including patient-derived glioma stem cell (GSC) lines. Akaluc-expressing glioma cells were compared to matching cells expressing Fluc in both in vitro and in vivo BLI assays. We also conducted proof-of-principle BLI studies with intracranial transplant cohorts receiving chemoradiation therapy.

Results

Akaluc-expressing glioma cells produced more than 10 times higher BLI signals than Fluc-expressing counterparts when examined in vitro, and more than 100-fold higher signals when compared to Fluc-expressing counterparts in intracranial transplant models in vivo. The high sensitivity of Akaluc permitted detection of intracranial glioma transplants starting as early as 4 h after implantation and with as little as 5000 transplanted cells. The sensitivity of the system allowed us to follow engraftment and expansion of intracranial transplants of GSC lines. Akaluc was also robust for sensitive detection of in vivo tumor regression after therapy and subsequent relapse.

Conclusion

Akaluc BLI offers superior sensitivity for in vivo tracking of glioma in the intracranial transplant paradigm, facilitating sensitive approaches for the study of glioma growth and response to therapy.

Injectable Thermo-Sensitive Chitosan Hydrogel Containing CPT-11-Loaded EGFR-Targeted Graphene Oxide and SLP2 shRNA for Localized Drug/Gene Delivery in Glioblastoma Therapy

In this study, we aimed to develop a multifunctional drug/gene delivery system for the treatment of glioblastoma multiforme by combining the ligand-mediated active targeting and the pH-triggered drug release features of graphene oxide (GO). Toward this end, we load irinotecan (CPT-11) to cetuximab (CET)-conjugated GO (GO-CET/CPT11) for pH-responsive drug release after endocytosis by epidermal growth factor receptor (EGFR) over-expressed U87 human glioblastoma cells. The ultimate injectable drug/gene delivery system was designed by co-entrapping stomatin-like protein 2 (SLP2) short hairpin RNA (shRNA) and GO-CET/CPT11 in thermosensitive chitosan-g-poly(N-isopropylacrylamide) (CPN) polymer solution, which offers a hydrogel depot for localized, sustained delivery of the therapeutics after the in situ formation of CPN@GO-CET/CPT11@shRNA hydrogel. An optimal drug formulation was achieved by considering both the loading efficiency and loading content of CPT-11 on GO-CET. A sustained and controlled release behavior was found for CPT-11 and shRNA from CPN hydrogel. Confocal microscopy analysis confirmed the intracellular trafficking for the targeted delivery of CPT-11 through interactions of CET with EGFR on the U87 cell surface. The efficient transfection of U87 using SLP2 shRNA was achieved using CPN as a delivery milieu, possibly by the formation of shRNA/CPN polyplex after hydrogel degradation. In vitro cell culture experiments confirmed cell apoptosis induced by CPT-11 released from acid organelles in the cytoplasm by flow cytometry, as well as reduced SLP2 protein expression and inhibited cell migration due to gene silencing. Finally, in vivo therapeutic efficacy was demonstrated using the xenograft of U87 tumor-bearing nude mice through non-invasive intratumoral delivery of CPN@GO-CET/CPT11@shRNA by injection. Overall, we have demonstrated the novelty of this thermosensitive hydrogel to be an excellent depot for the co-delivery of anticancer drugs and siRNA. The in situ forming hydrogel will not only provide extended drug release but also combine the advantages offered by the chitosan-based copolymer structure for siRNA delivery to broaden treatment modalities in cancer therapy.

Optical and Optoacoustic Imaging

The spatiotemporal determination of molecular events and cells is important for understanding disease processes, especially in oncology, and thus for the development of novel treatments. Equally important is the knowledge of the biodistribution, localization, and targeted accumulation of novel therapies as well as monitoring of tumor growth and therapeutic response. Optical imaging provides an ideal versatile platform for imaging of all these problems and questions.

Magnetic Resonance Imaging and Bioluminescence Imaging for Evaluating Tumor Burden in Orthotopic Colon Cancer

Quantifying tumor burden is important for following the natural history of orthotopic colon cancer and therapeutic efficacy. Bioluminescence imaging (BLI) is commonly used for such assessment and has both advantages and limitations. We compared BLI and magnetic resonance imaging (MRI) for quantifying orthotopic tumors in a mouse model of colon cancer. Among sequences tested, T2-based MRI imaging ranked best overall for colon cancer border delineation, contrast, and conspicuity. Longitudinal MRI detected tumor outside the colon, indistinguished by BLI. Colon tumor weights calculated from MRI in vivo correlated highly with tumor weights measured ex vivo whereas the BLI signal intensities correlated relatively poorly and this difference in correlations was highly significant. This suggests that MRI may more accurately assess tumor burden in longitudinal monitoring of orthotopic colon cancer in this model as well as in other models.

Cousins at work: How combining medical with optical imaging enhances in vivo cell tracking

Microscopy and medical imaging are related in their exploitation of electromagnetic waves, but were developed to satisfy differing needs, namely to observe small objects or to look inside subjects/objects, respectively. Together, these techniques can help elucidate complex biological processes and better understand health and disease. A current major challenge is to delineate mechanisms governing cell migration and tissue invasion in organismal development, the immune system and in human diseases such as cancer where the spatiotemporal tracking of small cell numbers in live animal models is extremely challenging. Multi-modal multi-scale in vivo cell tracking integrates medical and optical imaging. Fuelled by basic research in cancer biology and cell-based therapeutics, it has been enabled by technological advances providing enhanced resolution, sensitivity and multiplexing capabilities. Here, we review which imaging modalities have been successfully used for in vivo cell tracking and how this challenging task has benefitted from combining macroscopic with microscopic techniques.

A multimodality imaging model to track viable breast cancer cells from single arrest to metastasis in the mouse brain

Cellular MRI involves sensitive visualization of iron-labeled cells in vivo but cannot differentiate between dead and viable cells. Bioluminescence imaging (BLI) measures cellular viability, and thus we explored combining these tools to provide a more holistic view of metastatic cancer cell fate in mice. Human breast carcinoma cells stably expressing Firefly luciferase were loaded with iron particles, injected into the left ventricle, and BLI and MRI were performed on days 0, 8, 21 and 28. The number of brain MR signal voids (i.e., iron-loaded cells) on day 0 significantly correlated with BLI signal. Both BLI and MRI signals decreased from day 0 to day 8, indicating a loss of viable cells rather than a loss of iron label. Total brain MR tumour volume on day 28 also correlated with BLI signal. Overall, BLI complemented our sensitive cellular MRI technologies well, allowing us for the first time to screen animals for successful injections, and, in addition to MR measures of cell arrest and tumor burden, provided longitudinal measures of cancer cell viability in individual animals. We predict this novel multimodality molecular imaging framework will be useful for evaluating the efficacy of emerging anti-cancer drugs at different stages of the metastatic cascade.

Bone metastasis imaging with SPECT/CT/MRI: a preclinical toolbox for therapy studies

Bone is one of the most common metastatic target sites in breast cancer, with more than 200 thousand new cases of invasive cancer diagnosed in the US alone in 2011. We set out to establish a multimodality imaging platform for bone metastases in small animals as a tool to non-invasively quantify metastasis growth, imaging the ensuing bone lesions and possibly the response to treatment. To this end, a mouse model of osteolytic metastatic bone tumors was characterized with SPECT/CT and MRI over time. A cell line capable of forming bone metastases, MDA-MB-231, was genetically modified to stably express the reporter gene herpes simplex virus-1 thymidine kinase (hsv-1 tk). The intracellular accumulation of the radiolabeled tracer [(123)I]FIAU promoted by HSV-1 TK specifically pinpoints the location of tumor cells which can be imaged in vivo by SPECT. First, a study using tumors implanted subcutaneously was performed. The SPECT/MRI overlays and the ex vivo γ-counting showed a linear correlation in terms of %ID/cm(3) (R(2)=0.93) and %ID/g (R(2)=0.77), respectively. Then, bone metastasis growth was imaged weekly by SPECT/CT and T2-weighted MRI over a maximum of 40 days post-intracardiac injection of tumor cells. The first activity spots detectable with SPECT, around day 20 post-cell injection, were smaller than 2mm(3) and not yet visible by MRI and increased in volume and in %ID over the weeks. Osteolytic bone lesions were visible by CT (in vivo) and μCT (ex vivo). The SPECT/MRI overlays also showed a linear correlation in terms of %ID/cm(3) (R(2)=0.86). In conclusion, a new multimodality imaging platform has been established that non-invasively combines images of active tumor areas (SPECT), tumor volume (MRI) and the corresponding bone lesions (CT and μCT). To our knowledge this is the first report where the combination of soft tissue information from MRI, bone lesions by CT, and reporter gene imaging by SPECT is used to non-invasively follow metastatic bone lesions.

Using [¹⁸F]FBAU for imaging brain tumor progression in an F98/tk-luc glioma-bearing rat model

1-(2-Deoxy-2-[18F]fluoro-β-D-arabinofuranosyl)-5-bromouracil ([18F]FBAU), a substitute for thymine, has been reported as an effective reporter probe by which to trace cellular metabolism with its positron emission. In the present study, a rat xenograft model bearing F98 glioma transfected with dual reporter genes, herpes simplex virus type 1 thymidine kinase (HSV1-tk) and firefly luciferase (luc) was used for monitoring tumor progression by multimodalities of molecular imaging using [18F]FBAU and D-luciferase as probes. Rat F98 glioma cells were transfected with the pC1-tk-IRES-luc vectors. The selected stable clone was renamed as the F98/tk-luc cell line. Fischer 344 male rats bearing orthotropic F98/tk-luc gliomas in the left brain were used. On day 13 post tumor inoculation, biodistribution, positron emission tomography (PET), magnetic resonance imaging (MRI) and ex vivo autoradiography were performed. The surviving fraction of F98/tk-luc cells treated with 15 µM ganciclovir (GCV) was 15.9%, and the uptake of [131I]FIAU in these cells was significantly enhanced when compared with F98 cells. The correlation coefficient of tumor volume vs. the bioluminescence in the F98/tk-luc glioma-bearing rats was 0.90. The biodistribution showed that the accumulation ratios of [18F]FBAU for glioma-to-normal brain were 9.16, 14.24, 5.7 and 13.7 at 30, 60, 90 and 120 min post i.v. injection, respectively. Consistent tumor enhancement of [18F]FBAU/PET imaging was also noted from 30-90 min post injection. Ex vivo autoradiography also confirmed significant [18F]FBAU uptake in tumors. In conclusion, [18F]FBAU may be used as a PET probe for monitoring glioma progression in animal models and may have potential for clinical use as well.

18F-AFETP, 18F-FET, and 18F-FDG imaging of mouse DBT gliomas

The goal of this study was to evaluate the (18)F-labeled nonnatural amino acid (S)-2-amino-3-[1-(2-(18)F-fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid ((18)F-AFETP) as a PET imaging agent for brain tumors and to compare its effectiveness with the more-established tracers O-(2-(18)F-fluoroethyl)-l-tyrosine ((18)F-FET) and (18)F-FDG in a murine model of glioblastoma. The tracer (18)F-AFETP is a structural analog of histidine and is a lead compound for imaging cationic amino acid transport, a relatively unexplored target for oncologic imaging.

METHODS

(18)F-AFETP was prepared using the click reaction. BALB/c mice with intracranially implanted delayed brain tumor (DBT) gliomas (n = 4) underwent biodistribution and dynamic small-animal PET imaging for 60 min after intravenous injection of (18)F-AFETP. Tumor and brain uptake of (18)F-AFETP were compared with those of (18)F-FDG and (18)F-FET through small-animal PET analyses.

RESULTS

(18)F-AFETP demonstrated focally increased uptake in tumors with good visualization. Peak tumor uptake occurred within 10 min of injection, with stable or gradual decrease over time. All 3 tracers demonstrated relatively high uptake in the DBTs throughout the study. At late time points (47.5-57.5 min after injection), the average standardized uptake value with (18)F-FDG (1.9 ± 0.1) was significantly greater than with (18)F-FET (1.1 ± 0.1) and (18)F-AFETP (0.7 ± 0.2). The uptake also differed substantially in normal brain, with significant differences in the standardized uptake values at late times among (18)F-FDG (1.5 ± 0.2), (18)F-FET (0.5 ± 0.05), and (18)F-AFETP (0.1 ± 0.04). The resulting average tumor-to-brain ratio at the late time points was significantly higher for (18)F-AFETP (7.5 ± 0.1) than for (18)F-FDG (1.3 ± 0.1) and (18)F-FET (2.0 ± 0.3).

CONCLUSION

(18)F-AFETP is a promising brain tumor imaging agent, providing rapid and persistent tumor visualization, with good tumor-to-normal-brain ratios in the DBT glioma model. High tumor-to-brain, tumor-to-muscle, and tumor-to-blood ratios were observed at 30 and 60 min after injection, with higher tumor-to-brain ratios than obtained with (18)F-FET or (18)F-FDG. These results support further development and evaluation of (18)F-AFETP and its derivatives for tumor imaging.

Radiosynthesis and biological evaluation of alpha-[F-18]fluoromethyl phenylalanine for brain tumor imaging

OBJECTIVES

Radiolabeled amino acids have proven utility for imaging brain tumors in humans, particularly those that target system L amino acid transport. We have prepared the novel phenylalanine analogue, (FMePhe, 9), as part of an effort to develop new system L tracers that can be prepared in high radiochemical yield through nucleophilic [(18)F]fluorination. The tumor imaging properties of both enantiomers of this new tracer were evaluated through cell uptake, biodistribution and microPET studies in the mouse DBT model of high grade glioma.

METHODS

The non-radioactive form of 9 and the cyclic sulfamidate labeling precursor were prepared from commercially available racemic α-benzylserine. Racemic [(18)F]9 was prepared from the labeling precursor in two steps using standard[(18)F]fluoride nucleophilic reaction conditions followed by acidic deprotection. The individual enantiomers [(18)F]9a and [(18)F]9b were isolated using preparative chiral HPLC. In vitro uptake inhibition assays were performed with each enantiomer using DBT cells. Biodistribution and microPET/CT studies were performed with each enantiomer in male BALB/c mice at approximately 2 weeks after implantation of DBT tumor cells.

RESULTS

Radiolabeling of the cyclic sulfamidate precursor 5 provides racemic [(18)F]9 in high radiochemical yield (60%-70%, n=4) and high radiochemical purity (>96%, n=4). In vitro uptake assays demonstrate that both [(18)F]9a and [(18)F]9b undergo tumor cell uptake through system L transport. The biodistribution studies using the single enantiomers [(18)F]9a and [(18)F]9b demonstrated good tumor uptake with lower uptake in most normal tissues, and [(18)F]9a had higher tumor uptake than [(18)F]9b. MicroPET imaging demonstrated good tumor visualization within 10 min of injection, rapid uptake of radioactivity, and tumor to brain ratios of approximately 6:1 at 60 min postinjection.

CONCLUSIONS

The novel PET tracer, [(18)F]FMePhe, is readily synthesized in good yield from a cyclic sulfamidate precursor. Biodistribution and microPET studies in the DBT model demonstrate good tumor to tissue ratios and tumor visualization, with enantiomer [(18)F]9a having higher tumor uptake. However, the brain availability of both enantiomers was lower than expected for system L substrates, suggesting the [(18)F]fluorine group in the β-position affects uptake of these compounds by system L transporters.