Immunotoxins and central nervous system neoplasia

Mechanisms of Action and Limitations of Monoclonal Antibodies and Single Chain Fragment Variable (scFv) in the Treatment of Cancer

Monoclonal antibodies are among the most effective tools for detecting tumor-associated antigens. The U.S. Food and Drug Administration (FDA) has approved more than 36 therapeutic antibodies for developing novel alternative therapies that have significant success rates in fighting cancer. However, some functional limitations have been described, such as their access to solid tumors and low interaction with the immune system. Single-chain variable fragments (scFv) are versatile and easy to produce, and being an attractive tool for use in immunotherapy models. The small size of scFv can be advantageous for treatment due to its short half-life and other characteristics related to the structural and functional aspects of the antibodies. Therefore, the main objective of this review was to describe the current situation regarding the mechanisms of action, applications, and limitations of monoclonal antibodies and scFv in the treatment of cancer.

A new scfv-based recombinant immunotoxin against EPHA2-overexpressing breast cancer cells; High in vitro anti-cancer potency

Immunotoxin therapy is one of the immunotherapy strategies providing a new, effective and high potency treatment against various cancers. Breast cancer is the most common cancer among women in many countries. The EPH receptors are a large part of tyrosine kinase receptors family and play an effective role in tumor development and angiogenesis. Among EPH receptors, EPHA2 is more commonly well-known and widely expressed in many cancers like breast cancer. In this study, we evaluated the specification of a designed immunotoxin formed by EPHA2-specific scfv linked with PE38KDEL on EPHA2-overexpressing breast cancer cell line. This new scfv-based recombinant immunotoxin was studied in terms of features such as binding potency, cytotoxicity effects, apoptosis induction ability, and internalization. The flow cytometry results showed that the immunotoxin can significantly (approximately 99%) bind to EPHA2-overexpressing breast cancer cell line (MDA-MB-231) in a low concentration (2.5 ng/ul) while cannot significantly bind to the normal cell line (HEK-293) or even EPHA2-very low expressing cell line (MCF-7). Using the MTT assay and Annexin V/Propidium iodide (PI) double staining method by flow cytometry, we observed significant killing and apoptosis induction of the MDA-MB-231 cells at different concentrations. Immunotoxin tracking by confocal microscopy at 2 h and 6 h revealed a massive presence of immunotoxin in the cytoplasm. Finally, given the in vitro results, it seems that this immunotoxin is competent enough to serve as a good candidate for in vivo studies to further explore the possibility of breast cancer treatment.

Leptomeningeal metastasis

Leptomeningeal metastasis (LM) results from dissemination of cancer cells to both the leptomeninges (pia and arachnoid) and cerebrospinal fluid (CSF) compartment. Breast cancer, lung cancer, and melanoma are the most common solid tumors that cause LM. Recent approval of more active anticancer therapies has resulted in improvement in survival that is partly responsible for an increased incidence of LM. Neurologic deficits, once manifest, are mostly irreversible, and often have a significant impact on patient quality of life. LM-directed therapy is based on symptom palliation, circumscribed use of neurosurgery, limited field radiotherapy, intra-CSF and systemic therapies. Novel methods of detecting LM include detection of CSF circulating tumor cells and tumor cell-free DNA. A recent international guideline for a standardization of response assessment in LM may improve cross-trial comparisons as well as within-trial evaluation of treatment. An increasing number of retrospective studies suggest that molecular-targeted therapy, such as EGFR and ALK inhibitors in lung cancer, trastuzumab in HER2+ breast cancer, and BRAF inhibitors in melanoma, may be effective as part of the multidisciplinary management of LM. Prospective randomized trials with standardized response assessment are needed to further validate these preliminary findings.

Advances in strategies to improve drug delivery to brain tumors

Brain tumors remain a significant health problem. Advances in the biology of the blood-brain barrier are improving the ability of researchers to target therapeutic peptides, small molecules and other drugs to brain tumors. Simple methods to improve blood-brain barrier penetration include chemical modification, glycosylation and pegylation. Drug-delivery vehicles, such as nanoparticles and liposomes, are also under study. Targeting vectors include natural ligands (e.g., epidermal growth factor) or monoclonal antibodies to receptors (e.g., transferrin or insulin). Other vector-mediated delivery approaches involve the conjugation of a therapeutic peptide or protein with a targeting molecule that can induce transcytosis across blood-brain barrier endothelial cells. The most commonly used vectors are peptidomimetic antibodies to endothelial receptors, such as the transferrin and insulin receptors.

Diphtheria toxin-based targeted toxin therapy for brain tumors

Targeted toxins (TT) are molecules that bind cell surface antigens or receptors such as the transferrin or interleukin-13 receptor that are overexpressed in cancer. After internalization, the toxin component kills the cell. These recombinant proteins consist of an antibody or carrier ligand coupled to a modified plant or bacterial toxin such as diphtheria toxin (DT). These fusion proteins are very effective against brain cancer cells that are resistant to radiation therapy and chemotherapy. TT have shown an acceptable profile for toxicity and safety in animal studies and early clinical trials have demonstrated a therapeutic response. This review summarizes the characteristics of DT-based TT, the animal studies in malignant brain tumors and early clinical trial results. Obstacles to the successful treatment of brain tumors include poor penetration into tumor, the immune response to DT and cancer heterogeneity.

Current status of local therapy in malignant gliomas--a clinical review of three selected approaches

Malignant gliomas are the most frequently occurring, devastating primary brain tumors, and are coupled with a poor survival rate. Despite the fact that complete neurosurgical resection of these tumors is impossible in consideration of their infiltrating nature, surgical resection followed by adjuvant therapeutics, including radiation therapy and chemotherapy, is still the current standard therapy. Systemic chemotherapy is restricted by the blood-brain barrier, while methods of local delivery, such as with drug-impregnated wafers, convection-enhanced drug delivery, or direct perilesional injections, present attractive ways to circumvent these barriers. These methods are promising ways for direct delivery of either standard chemotherapeutic or new anti-cancer agents. Several clinical trials showed controversial results relating to the influence of a local delivery of chemotherapy on the survival of patients with both recurrent and newly diagnosed malignant gliomas. Our article will review the development of the drug-impregnated release, as well as convection-enhanced delivery and the direct injection into brain tissue, which has been used predominantly in gene-therapy trials. Further, it will focus on the use of convection-enhanced delivery in the treatment of patients with malignant gliomas, placing special emphasis on potential shortcomings in past clinical trials. Although there is a strong need for new or additional therapeutic strategies in the treatment of malignant gliomas, and although local delivery of chemotherapy in those tumors might be a powerful tool, local therapy is used only sporadically nowadays. Thus, we have to learn from our mistakes in the past and we strongly encourage future developments in this field.

Carcinomatous meningitis: Leptomeningeal metastases in solid tumors

Leptomeningeal metastasis (LM) results from metastatic spread of cancer to the leptomeninges, giving rise to central nervous system dysfunction. Breast cancer, lung cancer, and melanoma are the most frequent causes of LM among solid tumors in adults. An early diagnosis of LM, before fixed neurologic deficits are manifest, permits earlier and potentially more effective treatment, thus leading to a better quality of life in patients so affected. Apart from a clinical suspicion of LM, diagnosis is dependent upon demonstration of cancer in cerebrospinal fluid (CSF) or radiographic manifestations as revealed by neuraxis imaging. Potentially of use, though not commonly employed, today are use of biomarkers and protein profiling in the CSF. Symptomatic treatment is directed at pain including headache, nausea, and vomiting, whereas more specific LM-directed therapies include intra-CSF chemotherapy, systemic chemotherapy, and site-specific radiotherapy. A special emphasis in the review discusses novel agents including targeted therapies, that may be promising in the future management of LM. These new therapies include anti-epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors erlotinib and gefitinib in nonsmall cell lung cancer, anti-HER2 monoclonal antibody trastuzumab in breast cancer, anti-CTLA4 ipilimumab and anti-BRAF tyrosine kinase inhibitors such as vermurafenib in melanoma, and the antivascular endothelial growth factor monoclonal antibody bevacizumab are currently under investigation in patients with LM. Challenges of managing patients with LM are manifold and include determining the appropriate patients for treatment as well as the optimal route of administration of intra-CSF drug therapy.

Radioimmunotargeting of malignant glioma by monoclonal antibody D2C7 reactive against both wild-type and variant III mutant epidermal growth factor receptors

INTRODUCTION

Malignant glioma remains a significant therapeutic challenge, and immunotherapeutics might be a beneficial approach for these patients. A monoclonal antibody (MAb) specific for multiple molecular targets could expand the treatable patient population and the fraction of tumor cells targeted, with potentially increased efficacy. This motivated the generation of MAb D2C7, which recognizes both wild-type epidermal growth factor receptor (EGFRwt) and a tumor-specific mutant, EGFRvIII.

METHODS

D2C7 binding affinity was determined by surface plasmon resonance and its specificity characterized through comparison to EGFRwt-specific EGFR.1 and EGFRvIII-specific L8A4 MAbs by flow cytometry and immunohistochemical analysis. The three MAbs were labeled with (125)I or (131)I using Iodogen, and paired-label internalization assays and biodistribution experiments in athymic mice with human tumor xenografts were performed.

RESULTS

The affinity of D2C7 for EGFRwt and EGFRvIII was 5.2×10(9) M(-1) and 3.6×10(9) M(-1), and cell-surface reactivity with both receptors was documented by flow cytometry. Immunohistochemical analyses revealed D2C7 reactivity with malignant glioma tissue from 90 of 101 patients. Internalization assays performed on EGFRwt-expressing WTT cells and EGFRvIII-expressing NR6M cells indicated a threefold lower degradation of (125)I-labeled D2C7 compared with (131)I-labeled EGFR.1. Uptake of (125)I-labeled D2C7 in NR6M xenografts (52.45±13.97 %ID g(-1) on Day 3) was more than twice that of (131)I-labeled L8A4; a threefold to fivefold tumor delivery advantage was seen when compared to (131)I-labeled EGFR.1 in mice with WTT xenografts.

CONCLUSIONS

These results suggest that D2C7 warrants further evaluation for the development of MAb-based therapeutics against cancers expressing EGFRwt and EGFRvIII.

Molecular targeting of malignant glioma cells with an EphA2-specific immunotoxin delivered by human bone marrow-derived mesenchymal stem cells

Immunotoxins have shown great promise as an alternative treatment for brain malignancies such as gliomas, but their failure to penetrate into the tumor mass remains a major problem. Mesenchymal stem cells exhibit tropism to tumor tissue and may serve as a cellular vehicle for the delivery and local production of antitumor agents. In this study, we used human bone marrow-derived mesenchymal stem cells (hMSCs) as a vehicle for the targeted delivery of EphrinA1-PE38, a very specific immunotoxin against the EphA2 receptor that is overexpressed in gliomas. hMSCs were transduced with adenovirus to express secretable EphrinA1-PE38. Our invitro assays confirmed the expression, release and selective killing effect of the immunotoxin produced by hMSCs. Furthermore, the intratumoral injection of engineered hMSCs was effective at inhibiting tumor growth in a malignant glioma tumor model. These results indicate that gene therapy utilizing EphrinA1-PE38-secreting hMSCs may provide a novel approach for the local treatment of malignant gliomas.

Targeted toxins in brain tumor therapy

Targeted toxins, also known as immunotoxins or cytotoxins, are recombinant molecules that specifically bind to cell surface receptors that are overexpressed in cancer and the toxin component kills the cell. These recombinant proteins consist of a specific antibody or ligand coupled to a protein toxin. The targeted toxins bind to a surface antigen or receptor overexpressed in tumors, such as the epidermal growth factor receptor or interleukin-13 receptor. The toxin part of the molecule in all clinically used toxins is modified from bacterial or plant toxins, fused to an antibody or carrier ligand. Targeted toxins are very effective against cancer cells resistant to radiation and chemotherapy. They are far more potent than any known chemotherapy drug. Targeted toxins have shown an acceptable profile of toxicity and safety in early clinical studies and have demonstrated evidence of a tumor response. Currently, clinical trials with some targeted toxins are complete and the final results are pending. This review summarizes the characteristics of targeted toxins and the key findings of the important clinical studies with targeted toxins in malignant brain tumor patients. Obstacles to successful treatment of malignant brain tumors include poor penetration into tumor masses, the immune response to the toxin component and cancer heterogeneity. Strategies to overcome these limitations are being pursued in the current generation of targeted toxins.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

The PEPvIII-KLH (CDX-110) vaccine in glioblastoma multiforme patients

Conventional therapies for glioblastoma multiforme (GBM) fail to target tumor cells exclusively, resulting in non-specific toxicity. Immune targeting of tumor-specific mutations may allow for more precise eradication of neoplastic cells. EGFR variant III (EGFRvIII) is a tumor-specific mutation that is widely expressed in GBM and other neoplasms and its expression enhances tumorigenicity. This in-frame deletion mutation splits a codon, resulting in a novel glycine at the fusion junction producing a tumor-specific epitope target for cellular or humoral immunotherapy. We have previously shown that vaccination with a peptide that spans the EGFRvIII fusion junction (PEPvIII-KLH/CDX-110) is an efficacious immunotherapy in syngeneic murine models. In this review, we summarize our results in GBM patients targeting this mutation in multiple, multi-institutional Phase II immunotherapy trials. These trials demonstrated that a selected population of GBM patients who received vaccines targeting EGFRvIII had an unexpectedly long survival time. Further therapeutic strategies and potential pitfalls of using this approach are discussed.

Convection-enhanced drug delivery of interleukin-4 Pseudomonas exotoxin (PRX321): increased distribution and magnetic resonance monitoring

Convection-enhanced drug delivery (CED) enables achieving a drug concentration within brain tissue and brain tumors that is orders of magnitude higher than by systemic administration. Previous phase I/II clinical trials using intratumoral convection of interleukin-4 Pseudomonas exotoxin (PRX321) have demonstrated an acceptable safety and toxicity profile with promising signs of therapeutic activity. The present study was designed to assess the distribution efficiency and toxicity of this PRX321 using magnetic resonance imaging (MRI) and to test whether reformulation with increased viscosity could enhance drug distribution. Convection of low- [0.02% human serum albumin (HSA)] and high-viscosity (3% HSA) infusates mixed with gadolinium-diethylenetriamine pentaacetic acid and PRX321 were compared with low- and high-viscosity infusates without the drug, in normal rat brains. MRI was used for assessment of drug distribution and detection of early and late toxicity. Representative brain samples were subjected to histological examination. Distribution volumes calculated from the magnetic resonance images showed that the average distribution of 0.02% HSA was larger than that of 0.02% HSA with PRX321 by a factor of 1.98 (p < 0.02). CED of 3.0% HSA, with or without PRX321, tripled the volume of distribution compared with 0.02% HSA with PRX321 (p < 0.015). No drug-related toxicity was detected. These results suggest that the impeded convection of the PRX321 infusate used in previous clinical trials can be reversed by increasing infusate viscosity and lead to tripling of the volume of distribution. This effect was not associated with any detectable toxicity. A similar capability to reverse impeded convection was also demonstrated in a CED model using acetic acid. These results will be implemented in an upcoming phase IIb PRX321 CED trial with a high-viscosity infusate.

Toward effective immunotherapy for the treatment of malignant brain tumors

The immunologic treatment of cancer has long been heralded as a targeted molecular therapeutic with the promise of eradicating tumor cells with minimal damage to surrounding normal tissues. However, a demonstrative example of the efficacy of immunotherapy in modulating cancer progression is still lacking for most human cancers. Recent breakthroughs in our understanding of the mechanisms leading to full T-cell activation, and recognition of the importance of overcoming tumor-induced immunosuppressive mechanisms, have shed new light on how to generate effective anti-tumor immune responses in humans, and sparked a renewed and enthusiastic effort to realize the full potential of cancer immunotherapy. The immunologic treatment of invasive malignant brain tumors has not escaped this re-invigorated endeavor, and promising therapies are currently under active investigation in dozens of clinical trials at several institutions worldwide. This review will focus on some of the most important breakthroughs in our understanding of how to generate potent anti-tumor immune responses, and some of the clear challenges that lie ahead in achieving effective immunotherapy for the majority of patients with malignant brain tumors. A review of immunotherapeutic strategies currently under clinical evaluation, as well as an outline of promising novel approaches on the horizon, is included to provide perspective on the active and stalwart progress toward effective immunotherapy for the treatment of malignant brain tumors.

Immunotherapeutic approaches for glioma

The development of effective immunotherapy strategies for glioma requires adequate understanding of the unique immunological microenvironment in the central nervous system (CNS) and CNS tumors. Although the CNS is often considered to be an immunologically privileged site and poses unique challenges for the delivery of effector cells and molecules, recent advances in technology and discoveries in CNS immunology suggest novel mechanisms that may significantly improve the efficacy of immunotherapy against gliomas. In this review, we first summarize recent advances in the CNS and CNS tumor immunology. We address factors that may promote immune escape of gliomas. We also review advances in passive and active immunotherapy strategies for glioma, with an emphasis on lessons learned from recent early-phase clinical trials. We also discuss novel immunotherapy strategies that have been recently tested in non-CNS tumors and show great potential for application to gliomas. Finally, we discuss how each of these promising strategies can be combined to achieve clinical benefit for patients with gliomas.

Immunotherapy of malignant brain tumors

Despite aggressive multi-modality therapy including surgery, radiation, and chemotherapy, the prognosis for patients with malignant primary brain tumors remains very poor. Moreover, the non-specific nature of conventional therapy for brain tumors often results in incapacitating damage to surrounding normal brain and systemic tissues. Thus, there is an urgent need for the development of therapeutic strategies that precisely target tumor cells while minimizing collateral damage to neighboring eloquent cerebral cortex. The rationale for using the immune system to target brain tumors is based on the premise that the inherent specificity of immunologic reactivity could meet the clear need for more specific and precise therapy. The success of this modality is dependent on our ability to understand the mechanisms of immune regulation within the central nervous system (CNS), as well as counter the broad defects in host cell-mediated immunity that malignant gliomas are known to elicit. Recent advances in our understanding of tumor-induced and host-mediated immunosuppressive mechanisms, the development of effective strategies to combat these suppressive effects, and a better understanding of how to deliver immunologic effector molecules more efficiently to CNS tumors have all facilitated significant progress toward the realization of true clinical benefit from immunotherapeutic treatment of malignant gliomas.

EGFRvIII-targeted immunotoxin induces antitumor immunity that is inhibited in the absence of CD4+ and CD8+ T cells

PURPOSE

Immunotoxins as anti-cancer therapeutics have several potential advantages over conventional agents including a high specificity, extraordinary potency, and a lack of an identified mechanism for resistance. It has been clearly demonstrated that Pseudomonas-based immunotoxins have a direct cytotoxic effect. However, delayed and often dramatic antitumor responses seen in human studies with targeted toxins led us to hypothesize that immunologic responses may be a secondary mechanism that enhances the therapeutic efficacy of these novel drugs.

EXPERIMENTAL DESIGN

This hypothesis was tested in a murine system using an immunotoxin, MR1-1 [MR1-1(dsFv)-PE38KDEL], that targets a syngeneic murine homologue of the tumor-specific human epidermal growth factor mutation, EGFRvIII, expressed on a murine cell line.

RESULTS

Intratumoral treatment with MR1-1 eliminated EGFRvIII-expressing tumors (P < 0.0001). The antitumor activity of MR1-1 was dependent on the expression of EGFRvIII on some, but not all tumors cells, and was significantly inhibited in the absence of CD4+ (P = 0.0193) and CD8+ (P = 0.0193) T cells. MR1-1 induced EGFRvIII-specific immunity (P < 0.0005) and produced long lasting immunity against tumors expressing EGFRvIII as well as EGFRvIII-negative tumors.

CONCLUSIONS

These data suggest that immunotoxins may not be strictly dependent on direct cytotoxicity for their efficacy, but may also be potent inducers of antitumor immunity active even against cells that do not express the targeted antigen.

Recent developments in the use of adenoviruses and immunotoxins in cancer gene therapy

Despite setbacks in the past and apparent hurdles ahead, gene therapy is advancing toward reality. The past several years have witnessed this new field of biomedicine developing rapidly both in breadth and depth, especially for the treatment of cancer, thanks largely to the better understanding of molecular and genetic basis of oncogenesis and the development of new and improved vectors and technologies for gene delivery and targeting. This article is intended to provide a brief review of recent advances in cancer gene therapy using adenoviruses, both as vectors and as oncolytic agents, and some of the recent progress in the development of immunotoxins for use in cancer gene therapy.

Photodynamic therapy of high grade glioma - long term survival

Haemetaporphyrin derivative (HpD) mediated photodynamic therapy (PDT) has been investigated as an adjuvant treatment for cerebral glioma. This study records the survival of patients at the Royal Melbourne Hospital with residences in the State of Victoria, utilizing the Victorian Cancer Registry database for patients treated with adjuvant PDT following surgical resection of the tumour. For primary (newly diagnosed) tumours, median survival from initial diagnosis was 76.5 months for anaplastic astrocytoma (AA) and 14.3 months for glioblastoma multiforme (GBM). Seventy-three percent of patients with AA and 25% with GBM survived longer than 36 months. For recurrent tumour, median survival from the time of surgery was 66.6 months for AA and 13.5 months for GBM. Fifty-seven percent of patients with recurrent AA and 41% of patients with recurrent GBM survived longer than 36 months. Older age at the time of diagnosis was associated with poorer prognosis. Laser light doses above the sample median of 230 J/cm2 were associated with better prognosis in the 136 patients studied (primary tumour patients - (HR=0.50[0.27,0.95],p=0.033); recurrent tumour patients (HR=0.75[0.42,1.31],p=0.312). There was no mortality directly associated with the therapy, three patients had increased cerebral oedema thought to be related to photodynamic therapy that was controlled with conventional therapies.

Leptomeningeal metastases from solid malignancy: a review

Leptomeningeal metastases (LMM) consist of diffuse involvement of the leptomeninges by infiltrating cancer cells. In solid tumors, the most frequent primary sites are lung and breast cancers, two tumors where the incidence of LMM is apparently increasing. Careful neurological examination is required to demonstrate multifocal involvement of the central nervous system (CNS), cranial nerves, and spinal roots, which constitute the clinical hallmark of the disease. Cerebro-spinal fluid (CSF) analysis is almost always abnormal but only a positive cytology or demonstration of intrathecal synthesis of tumor markers is diagnostic. T1-weighted gadolinium-enhanced sequence of the entire neuraxis (brain and spine) plays an important role in supporting the diagnosis, demonstrating the involved sites and guiding treatment. Radionuclide CSF flow studies detect CSF compartmentalization and are useful for treatment planning. Standard therapy relies mainly on focal irradiation and intrathecal or systemic chemotherapy. Studies using other therapeutic approaches such as new biological or cytotoxic compounds are ongoing. The overall prognosis remains grim and quality of life should remain the priority when deciding which treatment option to apply. However, a sub-group of patients, tentatively defined here, may benefit from an aggressive treatment.

Phase I trial of intravenous IL-4 pseudomonas exotoxin protein (NBI-3001) in patients with advanced solid tumors that express the IL-4 receptor

NBI-3001 is a novel immunotoxin of attenuated Pseudomonas exotoxin fused to circularly permutated IL-4, which has shown some antitumor effects in glioblastoma multiforme with intratumoral administration. The authors evaluated the safety and tolerability of NBI-3001 administered intravenously in a dose-escalation design to patients with renal cell and non-small cell lung carcinoma whose tumors showed at least 10% IL-4 receptor expression. Cohorts of three to six patients were treated at dose levels of 0.008, 0.016, and 0.027 mg/m2 daily x 5 days every 28 days. Neutralizing antibody (NAB) titers, plasma levels of NBI-3001, and patient tolerability were monitored sequentially. 14 patients received a total of 36 cycles of NBI-3001 (range 1-6). No dose-limiting toxicities were noted at dose levels 0.008 and 0.016 mg/m2. At 0.027 mg/m2, two patients developed self-limiting, grade 3 or 4 transaminase elevation during cycle 1. NAB titers of more than 1:100 were detected in five of the seven patients treated with at least two cycles; the median titer after cycle 1 and the median maximum titer in subsequent cycles were 1:50 and approximately 1:1,710, respectively. No objective tumor responses were noted. Eight of 12 evaluable patients with renal cell carcinoma had stable disease; four patients had disease progression. High NAB titers resulted in four patients being withdrawn from the study. The dose-limiting toxicity for intravenous NBI-3001 was transaminase elevation at 0.027 mg/m2. NBI-3001 at 0.016 mg/m2 was well tolerated. Low circulating levels of NBI-3001, coupled with rising NAB titers, may have contributed to the lack of response in tumors that express IL-4R.

Current treatment of leptomeningeal metastases: systemic chemotherapy, intrathecal chemotherapy and symptom management

Treatment of leptomeningeal metastases is multifaceted and includes symptomatic therapy, intrathecal and systemic chemotherapy, and radiotherapy. As the majority of patients have widespread incurable systemic tumor, treatment is predominantly palliative; however, some patients with leukemia, lymphoma or breast cancer may have prolonged remissions and the possibility of cure.

Treatment of solid tumors with immunotoxins

Immunotoxins are hybrid molecules that generally consist of a toxin coupled with a tumor-specific antibody or antibody fragment that is intended to target and concentrate the toxin within the tumor tissue. The biodistribution, specificity, immunogenicity, and cytotoxic activity of the immunotoxin are but a few of the factors that govern the effectiveness of these compounds in the treatment of patients with cancer. Improvements in design, synthesis, and delivery of these molecules may permit them to become significant components in the arsenal of targeted therapies for cancer.

Immunotoxin treatment targeted to the high-molecular-weight melanoma-associated antigen prolonging the survival of immunodeficient rats with invasive intracranial human glioblastoma multiforme

OBJECT

The aim of this study was to target immunotoxin treatment to the high-molecular-weight melanoma-associated antigen (HMW-MAA) and thereby examine any changes in the survival of immunodeficient rats with human glioblastoma multiforme (GBM).

METHODS

To target treatment specifically to human glioma cells, Pseudomonas exotoxin A (PE) was conjugated to the 9.2.27 antibody, which recognizes the HMW-MAA. Treatment of the antigen-positive glioma cell line U87MG with the resulting 9.2.27-PE caused cytotoxicity with a median inhibitory concentration of 1 ng/ml. Intratumoral 9.2.27-PE treatment of intracranial U87MG tumors in nude rats prolonged the survival of these animals by 43% compared with controls. In additional studies on the use of this targeted treatment, the authors precultured freshly dissected glioblastoma multiforme (GBM) biopsy tissue for 1 to 2 weeks. Inoculation of this tissue into the rat brain resulted in diffuse infiltrative gliomas. The markers glial fibrillary acidic protein and S100 protein were found to be expressed in the original biopsy specimens, as well as in the glioma xenografts in nude rat brains. Intratumoral immunotoxin treatment of such established tumors with 9.2.27-PE was effective and prolonged survival time from 30% to as high as 90% in animals with tumors originating from four different GBM specimens.

CONCLUSIONS

Targeted treatment of highly invasive GBMs proved effective, and these results emphasize the clinical relevance of this antigen as a target molecule for immunotoxin treatment of human GBMs.

Progress report of a Phase I study of the intracerebral microinfusion of a recombinant chimeric protein composed of transforming growth factor (TGF)-alpha and a mutated form of the Pseudomonas exotoxin termed PE-38 (TP-38) for the treatment of malignant brain tumors

TP-38 is a recombinant chimeric targeted toxin composed of the EGFR binding ligand TGF-alpha and a genetically engineered form of the Pseudomonas exotoxin, PE-38. After in vitro and in vivo animal studies that showed specific activity and defined the maximum tolerated dose (MTD), we investigated this agent in a Phase I trial. The primary objective of this study was to define the MTD and dose limiting toxicity of TP-38 delivered by convection-enhanced delivery in patients with recurrent malignant brain tumors. Twenty patients were enrolled in the study and doses were escalated from 25 ng/mL to 100 with a 40 mL infusion volume delivered by two catheters. One patient developed Grade IV fatigue at the 100 ng/mL dose, but the MTD has not been established. The overall median survival after TP-38 for all patients was 23 weeks whereas for those without radiographic evidence of residual disease at the time of therapy, the median survival was 31.9 weeks. Overall, 3 of 15 patients, with residual disease at the time of therapy, have demonstrated radiographic responses and one patient with a complete response and has survived greater than 83 weeks.

Targeting the over-expressed urokinase-type plasminogen activator receptor on glioblastoma multiforme

A recombinant fusion protein targeting the urokinase-type plasminogen activator receptor (uPAR) and delivering a potent catalytic toxin has the advantage of simultaneously targeting both over-expressed uPAR on glioblastoma cells and on the tumor neovasculature. Such a hybrid protein was synthesized consisting of the noninternalizing amino-terminal fragment (ATF) of urokinase-type plasminogen activator (uPA) for binding, and the catalytic portion of diphtheria toxin (DT) for killing, and the translocation enhancing region (TER) of DT for internalization. The protein was highly selective for human glioblastoma in vitro and in vivo. In vivo, this DT/ATF hybrid called DTAT caused the regression of small subcutaneous uPAR-expressing tumors with minimal toxicity to critical organs. In vitro, DTAT killed only uPAR-positive glioblastoma cell lines and human endothelial cells in the form of the HUVEC cell line. Killing was selective and blockable with specific antibody. DTAT was highly effective against tumor cells cultured from glioblastoma multiforme patients and in vitro mixing experiments combining DTAT with DTIL13 another highly effective anti-glioblastoma agent showed that the mixture was as toxic as the most potent immunotoxin. In this article, we review our progress to date with DTAT.

Changes in expression of transferrin, insulin-like growth factor 1, and interleukin 4 receptors after irradiation of cells of primary malignant brain tumor cell lines

Various immunotoxins have been developed for the treatment of cancer. The toxin is internalized by target cells through cell-surface receptors, and it is essential for these receptors to be expressed for the immunotoxin to have specific anti-tumor activity. Radiation therapy is one of the main treatment modalities for primary malignant brain tumors. The purpose of this study was to determine whether radiation influences the expression of cell-surface receptors. Cells of one human medulloblastoma (Daoy) and two glioblastoma (U373-MG and T98-G) cell lines were tested by exposing the cells to a single dose of 5 Gy gamma rays. Expression of transferrin receptors, type-1 insulin-like growth factor receptors (IGF1R), and interleukin 4 receptors (IL4R) was measured by flow cytometry analysis on unirradiated cells and on cells 3 to 120 h after irradiation. In Daoy cells, the absolute expression index of transferrin receptors increased during the 24 h after irradiation with the greatest change of 26% above control at 9 h. The absolute expression index of IGF1R increased 26.5% above control at 12 h. The absolute expression index of IL4R decreased 9 h after irradiation. In U373-MG cells the absolute expression index of transferrin receptors increased during the 24 h after irradiation, and the greatest increase was 45% above control at 9 h. The absolute expression index of IGF1R increased during the 12 h after irradiation with a maximum increase of 33% above control at 6 h. The absolute expression index of IL4R decreased with time after irradiation. In T98-G cells, the absolute expression index of both transferrin receptors and IL4R decreased after irradiation. The results suggest that the expression of growth factor receptors on brain tumor cells may be influenced by radiation. The effect of ionizing radiation on receptor expression should be considered when administration of targeted toxin is combined with radiation. Similar studies with other growth factor receptors used in targeted toxin therapy are recommended.

Immunotoxin therapy for CNS tumor

Surgery, chemotherapy, and radiation therapy have become standard of practice in treating malignant brain tumors. Unfortunately, the prognosis of these malignant tumors still remains poor. Immunotoxins are a relatively new adjuvant treatment for brain tumors. Within the last few years an increased amount of clinically-oriented research involving immunotoxins has been published. This has led to numerous clinical trials which although encouraging have not yet born out the "magic bullet" concept envisioned for immunotoxins. In this review article the history, design, toxicity, and pharmokinetics of immunotoxins will be discussed in detail.

Convection-enhanced delivery in clinical trials

The poor prognosis associated with the current management of malignant gliomas has led investigators to develop alternative treatments such as targeted toxin therapy. The optimal method for administering these agents is under development but appears to be convection-enhanced delivery (CED).

The direct intratumoral infusion of targeted toxins was first performed in nude mouse flank tumor models of human malignant glioma. After the demonstration of in vivo efficacy, these potent cytotoxic compounds were tested in Phase I and Phase II clinical trials.

Using a high-flow microinfusion technique, volumes of up to 180 ml were infused by CED through catheters placed directly into brain tumors. Minor systemic toxicity was seen in the form of hepatic enzyme elevation. Neural toxicity manifested as seizure activity and hemiparesis resulted from peritumoral edema that followed the completion of the infusion. Peritumoral toxicity was believed to be more related to the concentration of the infused immunotoxin than to the infusion volume. In approximately half of patients treated with CED a stable disease course, a partial response, or a complete response was demonstrated in some clinical trials.

Targeted toxin therapy has clinical efficacy in patients with malignant gliomas. Convection-enhanced delivery appears to represent an effective method for administering these agents in patients with malignant brain tumors.

Systemic chemotherapy, intrathecal chemotherapy, and symptom management in the treatment of leptomeningeal metastasis

Metastasis to the leptomeninges occurs in many common cancers, including leukemia; lung, breast, and gastrointestinal cancers; and tumors of the brain. By way of the flow of cerebrospinal fluid, leptomeningeal metastasis spreads throughout the neuraxis. Consequently, therapy for leptomeningeal metastasis must be directed to the entire central nervous system (CNS). Treatment often consists of involved-field radiotherapy, systemic chemotherapy, and intrathecal chemotherapy. However, because meningeal spread occurs most often in advanced disease, treatment is mainly palliative, except in childhood leukemia, where durable remission has been reported. This article outlines the role of systemic and intrathecal chemotherapy in patients with leptomeningeal metastases. Strategies for symptom management in these patients are also described.

Clinical immunotherapy for brain tumors

As an immunization platform for brain tumors, dendritic cells supply an impressive host of advantages. On the simplest level, they provide the safety and tumor-specificity so wanted by current therapeutic options. Yet, in addition, as the fundamental antigen-presenting cell, they circumvent many of the immunologic challenges that gliomas and the CNS proffer and that other immunotherapeutic modes fail to overcome. Directions to take now include the identification of new tumor-specific and tumor-associated antigens; the determination of the optimal dendritic cell subtype, generation, loading method, maturation state, dose, and route of delivery for immunizations; the further characterization of dendritic cells and their activities; and, potentially, the discovery of ways to pulse dendritic cells efficiently in vivo. Preclinical studies continue to play an important role in refining this form of active immunotherapy.

Transferrin toxin but not transferrin receptor immunotoxin is influenced by free transferrin and iron saturation

BACKGROUND

Cytotoxic agents can be targeted successfully to cancer cells. The efficacy of such novel and potent anticancer strategies may be influenced by variables of iron metabolism.

METHODS

The in vitro cytotoxicity against glioma cells of transferrin (Tf)-based targeted toxins was compared with that of alpha-transferrin receptor (TfR)-immunotoxin.

RESULTS

Of four Tf-based targeted toxins, Tf-gelonin, Tf-pokeweed antiviral protein, Tf-momordin and Tf-saporin, inhibitory concentration 50% values against glioma-derived cell lines HS683 and U251, ranged from [4.8 +/- 1.5] x 10(-10) m for Tf-saporin to [26.9 +/- 15.3] x 10(-10) m for Tf-gelonin in [(3)H]-leucine incorporation assays. Tf-saporin and alpha-TfR-saporin-immunotoxin had similar efficacy, even in the more quantitative clonogenic assay (4-5 log kill with 1 x 10(-9) m) using the myeloma cell line RPMI 8226 and glioma cell line U251. However, on RPMI 8226, the efficacy of Tf-saporin 1 x 10(-9) m was reduced by 90% in the presence of 150 microg mL(-1)(=20% of normal plasma value) competing diferric transferrin, whereas the efficacy of the corresponding immunotoxin was affected only marginally. In addition, the efficacy of Tf-based conjugates will depend on their iron saturation state. Iron desaturation of Tf-saporin was demonstrated by [(59)Fe]-labelling, subsequent CM-Sepharose chromatography and SDS-PAGE. Desaturation led to virtually complete loss of affinity for the transferrin receptor, as determined by flow cytometry, which could be largely restored upon resaturation.

CONCLUSION

Transferrin-based toxin conjugates are strongly influenced by the presence of free transferrin and the iron saturation state. The corresponding alpha-transferrin receptor-immunotoxin does not show these disadvantages, has similar efficacy and should be preferred for further experiments.

In vivo documentation of photochemical internalization, a novel approach to site specific cancer therapy

Photochemical internalization (PCI) is a unique procedure for site-specific delivery of several types of membrane-impermeable molecules to the cytosol of target cells. The technology is based on photochemical-induced release of endocytosed macromolecules from endosomes and lysosomes into the cytosol. The purpose of this study was to evaluate the therapeutic potential of PCI of the type I ribosomal-inactivating protein gelonin in an animal model. The photosensitizer aluminum phthalocyanine disulfonate (AlPcS(2a)) was injected intraperitoneally (10 mg/kg) into athymic female BALB/c (nu/nu) nude mice (8-9 mice per group) with subcutaneously growing human adenocarcinoma (WiDr) tumors 48 hr before exposure to 135 J/cm(2) of red light focused on the tumor. Six hours before light exposure a single dose of 50 microg gelonin was administrated intratumorally. Tumor growth was measured at least twice a week. After immunomagnetic separation of in vivo growing tumor cells the subcellular localization of the photosensitizer was evaluated by fluorescence microscopy. The photosensitizer localized in endocytic vesicles in in vivo growing WiDr cells. Furthermore, it was found that in vitro gelonin treatment of WiDr cells isolated from photosensitizer-treated mice potentiated a light-induced decrease of clonal survival. Complete remission in 6 of 9 (67%) of the treated mice were induced. Our findings indicate that photochemical treatment with the photosensitizer AlPcS(2a) activates the cytotoxic potential of gelonin in vivo. These results demonstrate that the synergistic effect of combining photoactivation of photosensitizer located in endocytic vesicles and gelonin is indeed a result of PCI of gelonin.

Systemic immunotoxin treatment inhibits formation of human breast cancer metastasis and tumor growth in nude rats

Adjuvant chemotherapy in breast cancer patients has had limited success, which is possibly because of lack of effect on non-proliferating cells accompanied by the emergence of drug-resistant cell clones. Since immunotoxins (ITs) are known to exert proliferation-independent cytotoxicity, we investigated the efficacy of systemically administered anti-carcinoma ITs in nude rat models, simulating micrometastatic disease. The monoclonal antibodies MOC31, BM7 and 425.3, which recognize epithelial glycoprotein 2, MUC-1 mucin and the epidermal growth factor receptor, chemically conjugated to Pseudomonas exotoxin A (PE), inhibited protein synthesis of the 2 breast cancer cell lines at concentrations of 0.3-0.4 ng/ml, except for BM7-PE, which was less efficacious (65 ng/ml). In the MA-11 model in nude rats, a single i. v. dose of 20 microg MOC31-PE prevented development of metastasis in the spinal cord in 11/19 (58%) of the animals. Similarly, 425.3-PE treatment gave 6/9 (66%) long-term survivors. In rats injected intracardially or intratibially with MT-1 cells, treatment with 425. 3-PE prevented metastasis in 4/10 (40%) and intratibial tumor growth in 17/18 (94%) of the rats. Importantly, an equimolar dose of free 425.3 (antibody) was ineffective, whereas PE alone was toxic. With BM7-PE, 5/17 (29%) cures were obtained in the intratibial model. The results demonstrate that systemic short-term treatment with non-toxic doses of the 3 ITs tested can effectively inhibit the development of experimental breast cancer metastasis and/or local tumor growth in bone. The results support the development of the ITs towards clinical evaluation for possible use as short-term adjuvant therapy in patients at high risk of early relapse.

In vitro efficacy of recombinant diphtheria toxin–murine interleukin-4 immunoconjugate on mouse glioblastoma and neuroblastoma cell lines and the additive effect of radiation

Object

The prognosis for patients with primary malignant brain tumors is poor despite aggressive treatment, and tumor recurrence is common regardless of the chosen therapy. Although multimodal treatment does not provide a cure, it is necessary to determine which treatment modalities have the greatest cytotoxic effect and can potentially prolong survival. Immunotoxin therapy is a novel approach for the treatment of tumors, and it has been successfully used in the central nervous system. Because the interleukin (IL)–4 receptor is commonly expressed on brain tumor cells, the purpose of this study was to evaluate the cytotoxic effect of using a modified diphtheria toxin–murine IL-4 (DT390-mIL4) immunoconjugate for the treatment of murine brain tumor cell lines and to determine whether the addition of radiation therapy could potentiate the effect of this agent.

Methods

Spontaneous murine glioblastoma (SMA-560) and two neuroblastoma (Neuro-2a and NB41A3) cell lines were treated using DT390-mIL4 at different concentrations, and the anti–mouse IL-4 monoclonal antibody (11B11) was used for blocking its cytotoxicity. Other SMA-560 and Neuro-2a cell lines were treated using 500 cGy of radiation 3 hours before DT390-mIL4 treatment. Cytotoxity was evaluated using a trypan blue viability assay.

The immunoconjugate exhibited a dose-dependent cytotoxic effect with 50% inhibitory concentration values of 0.56 × 10−9 M in SMA-560, 1.28 × 10−9 M in Neuro-2a, and 0.95 × 10−10 M in NB41A3 cell lines. The cytotoxicity of DT390-mIL4 was specifically blocked by an excess of 11B11. Cytotoxicity was additive when the DT390-mIL4 at 10−9 M immunoconjugate administration was followed by radiation therapy.

Conclusions

These results indicate that the IL-4 receptor can be a target for diphtheria toxin fusion proteins and that radiation can potentiate the effects of DT390-mIL4. The development of multimodal approaches to treat malignant brain tumors with agents that have different mechanisms of action may be beneficial.

Brain tumors in the older person

BACKGROUND

The incidence of brain tumors is increasing rapidly, particularly in the older population. Advances in molecular biology help to explain differences in biologic behavior and response to therapy of brain tumors in the elderly compared with younger patients. The number of elderly patients who desire and receive therapy for brain tumors and are included in clinical trials is increasing.

METHODS

This article reviews the literature on the epidemiology, clinical aspects, and therapy of brain tumors, with emphasis on the older patient population.

RESULTS

The increased incidence of brain tumors in the elderly is principally due to the increasing number of people who comprise the older population. Age and performance status are important independent prognostic indicators, together with tumor histology. Surgery, radiation therapy, and chemotherapy can benefit elderly patients with brain tumors with favorable histologies, tumor location, and good performance status. The response rates to available therapies are less favorable than in younger patients, and only a small number of elderly patients are enrolled in clinical studies addressing new treatment modalities.

CONCLUSIONS

Brain tumors in the elderly have specific characteristics that determine their biologic behavior and response to therapy. There is a need for clinical studies designed for treatment of brain tumors in older patients, and requirements for rehabilitation and support systems for the elderly need to be addressed.

Interstitial quinacrine for elimination of abnormal tissue; therapy of experimental glioma

BACKGROUND

When quinacrine is injected interstitially, an intense migration of leukocytes and accumulation of various lymphokines is obtained locally, and the reaction is followed by cicatricial fibrosis. This property has been used in humans to induce tubal fibrosis in women and pleurodesis in patients with pleural effusion.

METHODS

In a controlled study, a single dose of 150 mg of quinacrine was injected interstitially into a C6 glioma implanted in the subcutaneous tissue of Wistar rats. Changes in size, histologic variations, and microscopic characteristics of leukocyte subpopulations infiltrating the tumor were studied by immunohistochemistry. Tumor necrosis factor and interleukin-1 beta were measured at different times in tumor homogenates.

RESULTS

The day after the injection of quinacrine, infiltration of leukocytes and macrophages was observed, accompanied by an accumulation of proinflammatory endogenous cytokines. Tumoral necrosis soon ensued; complete tumor disappearance was obtained in 72% of the animals. Cicatrization proceeded without injury of perilesional structures. In all controls injected with the vehicle, a large tumor developed (P <.0001).

CONCLUSIONS

Quinacrine, when administered interstitially in a single dose, elicits an intense local recruitment and proliferation of activated immune cells that, at the dose used in this study, induces tissue necrosis within a radius of 1 cm around the site of quinacrine injection, leaving the surrounding tissue unharmed.

Targeted toxin therapy for malignant astrocytoma

The poor prognosis associated with malignant astrocytoma has led investigators to seek new, innovative methods of treatment. Targeted toxins represent a unique form of therapy that has two components, a carrier molecule with high specificity for tumor-associated antigens and a potent protein toxin. These compounds are extremely cytotoxic to malignant astrocytoma cell lines in vitro. Animal studies have shown prolongation of survival and complete tumor regression when targeted toxins were administered by a variety of routes. The promising results seen in vivo have formed the basis for proceeding with clinical trials in humans with leptomeningeal neoplasia and malignant brain tumors, in which these agents are administered intrathecally or directly into tumor, respectively. To date, in these clinical trials, targeted toxins have been delivered safely without significant neurological toxicity, and cytological analysis of cerebrospinal fluid and radiological findings have shown evidence of a therapeutic response. These studies have confirmed the existence of a therapeutic window between normal brain tissue and malignant cells that can be exploited with targeted therapy directed against the transferrin receptor. The successful delivery of targeted toxins directly into malignant brain tumors has established this route of administration as practical and feasible. Identification of other receptors that are preferentially expressed on brain tumors, such as the interleukin-4 receptor, has resulted in the creation of a fusion protein against this receptor that contains a modified toxin from the bacteria Pseudomonas aeruginosa. This chimeric fusion toxin is currently under investigation in a Phase I clinical trial with patients with recurrent malignant astrocytoma, and other targeted toxins are under development for the treatment of these uniformly fatal tumors. Owing to these recent advances in targeted toxin therapy for malignant primary brain tumors, a review of the development of these agents for practicing neurosurgeons seems timely.

Antisense oligonucleotides for central nervous system tumors

The poor prognosis associated with malignant primary brain tumors has led investigators to seek and develop new, innovative treatment modalities. Current adjuvant therapies lack tumor specificity, which can lead to toxic central nervous system side effects. Advances in molecular biology now allow specific gene sequences to be inserted or targeted in the malignant cell genome. Antisense oligodeoxynucleotides represent complementary nucleic acid sequences that can recognize and bind to target genes, resulting in the arrest of deoxyribonucleic acid transcription or the translation of messenger ribonucleic acid. Although the use of antisense oligodeoxynucleotides is still in the experimental stages, these molecules enter cells in tissue culture by simple diffusion or active endocytosis and temporarily inhibit cell proliferation in a time- and dose-dependent fashion. The ability of antisense oligodeoxynucleotides to recognize specific gene sequences and to down-regulate gene expression make them ideal agents for use in targeting oncogenes, such as c-myb, that are expressed in central nervous system neoplasms.