Contrasting effects of interleukin-2 secretion by rat glioma cells contingent upon anatomical location: accelerated tumorigenesis in the central nervous system and complete rejection in the periphery

Radiolabeled cyclic RGD peptides as radiotracers for tumor imaging

The integrin family comprises 24 transmembrane receptors, each a heterodimeric combination of one of 18α and one of 8β subunits. Their main function is to integrate the cell adhesion and interaction with the extracellular microenvironment with the intracellular signaling and cytoskeletal rearrangement through transmitting signals across the cell membrane upon ligand binding. Integrin α_vβ₃ is a receptor for the extracellular matrix proteins containing arginine–glycine–aspartic (RGD) tripeptide sequence. The α_vβ₃ is generally expressed in low levels on the epithelial cells and mature endothelial cells, but it is highly expressed in many solid tumors. The α_vβ₃ levels correlate well with the potential for tumor metastasis and aggressiveness, which make it an important biological target for development of antiangiogenic drugs, and molecular imaging probes for early tumor diagnosis. Over the last decade, many radiolabeled cyclic RGD peptides have been evaluated as radiotracers for imaging tumors by SPECT or PET. Even though they are called “α_vβ₃-targeted” radiotracers, the radiolabeled cyclic RGD peptides are also able to bind α_vβ₅, α₅β₁, α₆β₄, α₄β₁, and α_vβ₆ integrins, which may help enhance their tumor uptake due to the “increased receptor population.” This article will use the multimeric cyclic RGD peptides as examples to illustrate basic principles for development of integrin-targeted radiotracers and focus on different approaches to maximize their tumor uptake and T/B ratios. It will also discuss important assays for pre-clinical evaluations of the integrin-targeted radiotracers, and their potential applications as molecular imaging tools for noninvasive monitoring of tumor metastasis and early detection of the tumor response to antiangiogenic therapy.

Combination of temozolomide with immunocytokine F16-IL2 for the treatment of glioblastoma

Background:

Glioblastoma patients are still not cured by the treatments available at the moment. We investigated the therapeutic properties of temozolomide in combination with F16–IL2, a clinical-stage immunocytokine consisting of human interleukin (IL)-2 fused to the human antibody F16, specific to the A1 domain of tenascin-C.

Methods:

We conducted three preclinical therapy studies, using subcutaneous and intracranial U87MG glioblastoma tumours xenografted in BALB/c nude mice. The same therapeutic schedule was used, consisting of five total administrations every third day, of 0.525 mg temozolomide, 20 μg F16–IL2, the combination, or the control solution.

Results:

Immunohistochemical analysis of U87MG xenografts and of human glioblastoma specimens showed selective tumour staining of F16. A quantitative biodistribution confirmed the preferential tumour accumulation of radiolabelled F16–IL2. In the study with subcutaneous xenografts, the combination of F16–IL2 with temozolomide induced complete remission of the animals, which remained tumour free for over 160 days. The same treatment led to a consistent size reduction of intracranial xenografts and to a longer survival of animals. The immunocytokine promoted the recruitment of leukocytes into tumours of both models.

Conclusion:

The combined use of temozolomide with F16–IL2 deserves clinical investigations, which will be facilitated by the excellent safety profile in cynomolgus monkeys, and by the fact that F16–IL2 is in clinical trials in patients with cancer.

Tumor-infiltrating, myeloid-derived suppressor cells inhibit T cell activity by nitric oxide production in an intracranial rat glioma + vaccination model

In rats bearing an intracranial T9 glioma, immunization with tumor antigens induces myeloid suppressor cells, which express neutrophil (His48) and monocyte (CD11bc) markers, to infiltrate the tumors. The His48(+)/CD11bc(+) cells were not derived from CNS microglia but were hematogenous; suppressed multiple T cell effector functions; and are myeloid-derived suppressor cells (MDSC). The glioma-infiltrating MDSC expressed arginase I, iNOS, indoleamine 2,3-dioxygenase and TGF-beta; however, inhibitor/blocking studies demonstrated that NO production was the primary mechanism of suppression which induced T cell apoptosis. These findings suggest that neuro-immunomodulation by MDSC in rat gliomas maybe mediated by a pathway requiring NO production.

Development and characterization of interleukin-18-loaded biodegradable microspheres

Immunostimulation represents a promising approach designed to specifically eradicate malignant cells. Since glioma tumour cells hole up in the central nervous system (CNS) in a particularly inauspicious milieu to antitumour immune reactions we here propose a new strategy to revert the properties of this microenvironment by administering an antitumour cytokine into the CNS tumour itself. Thus, biodegradable poly(D,L-lactide-co-glycolide) (PLGA) sustained-release microspheres for stereotaxic implantation loaded with interleukin-18 (IL-18), that is known to exert antitumour activity and trigger immune cell-mediated cytotoxicity, were developed. Different tests for assessing IL-18 bioactivity were set-up and evaluated. A specific bioassay was considered as the most reliable test. The stability and integrity of IL-18 was then verified during the encapsulation process. Consequently, two procedures of IL-18 encapsulation in PLGA microparticles (W/O/W and S/O/W) were investigated. As determined by radiolabelling studies using 125I-IL-18 and a continuous flow system, the in vitro release profile of IL-18 was optimum with S/O/W method with a moderate burst effect and a subsequent progressive discharge of 16.5+/-8.4 ng/day during the next 21 days against 6.1+/-4.2 ng/day with the W/O/W method. Considering analytical testing of IL-18 together with its preserved biological activity after release from microspheres, amounts of the active cytokine obtained with S/O/W method were relevant to plan in vivo evaluation to validate the therapeutic strategy.

Immunotherapy for malignant glioma: current approaches and future directions

Traditional therapies for the treatment of malignant glioma have failed to make appreciable gains regarding patient outcome in the last decade. Therefore, immunotherapeutic approaches have become increasingly popular in the treatment of this cancer. This article reviews general immunology of the central nervous system and the immunobiology of malignant glioma to provide a foundation for understanding the rationale behind current glioma immunotherapies. A review of currently implemented immunological treatments is then provided with special attention paid to the use of vaccines, gene therapy, cytokines, dendritic cells and viruses. Insights into future and developing avenues of glioma immunotherapy, such as novel delivery systems, are also discussed.

Tumor infiltration by myeloid suppressor cells in response to T cell activation in rat gliomas

We have recently reported that activation of tumor-specific T cells by subcutaneous vaccination with irradiated T9 glioma cells of syngeneic rats with a pre-existing, intracranial (i.c.) T9 glioma (T9+vaccination) promotes the mobilization of myeloid suppressor cells (MSC) which inhibit T cell function resulting in unregulated tumor progression. The current study investigated if this immunological paradigm could be recapitulated in T cell deficient rats, in other rat glioma models or using a dendritic cell (DC) vaccine. When nude rats were used in the T9+vaccination model, the level of MSC tumor infiltration remained low in vaccinated and control groups and there was no significant difference in tumor size between the groups. Increased tumor infiltration by MSC after vaccination with respective irradiated tumor cells was observed in the 9L, F98 and D74 gliomas. RT-2 tumors were markedly infiltrated with MSC regardless of vaccination. Enhanced tumor progression in response to immunization and T cell activation was observed in rats bearing F98 and D74 gliomas, although less pronounced than in the T9 model, and there was a trend for increased tumor size in the 9L glioma model. Increased MSC infiltrate and augmented T9 glioma growth were observed when DC pulsed with T9 cell lysate was used as a vaccine. These results suggest that MSC infiltration and unregulated tumor growth in response to vaccination is T cell-dependent; is not unique to the T9 glioma; and can be recapitulated with an alternate immunization approach.