Experimental radioimmunotherapy

B7-H3-targeted Radioimmunotherapy of Human Cancer

BACKGROUND

Targeted Radioimmunotherapy (RIT) is an attractive approach to selectively localize therapeutic radionuclides to malignant cells within primary and metastatic tumors while sparing normal tissues from the effects of radiation. Many human malignancies express B7-H3 on the tumor cell surface, while expression on the majority of normal tissues is limited, presenting B7-H3 as a candidate target for RIT. This review provides an overview of the general principles of targeted RIT and discusses publications that have used radiolabeled B7-H3-targeted antibodies for RIT of cancer in preclinical or clinical studies.

METHODS

Databases including PubMed, Scopus, and Google Scholar were searched for publications through June 2018 using a combination of terms including "B7-H3", "radioimmunotherapy", "targeted", "radiotherapy", and "cancer". After screening search results for relevancy, ten publications were included for discussion.

RESULTS

B7-H3-targeted RIT studies to date range from antibody development and assessment of novel Radioimmunoconjugates (RICs) in animal models of human cancer to phase II/III trials in humans. The majority of clinical studies have used B7-H3-targeted RICs for intra- compartment RIT of central nervous system malignancies. The results of these studies have indicated high tolerability and favorable efficacy outcomes, supporting further assessment of B7-H3-targeted RIT in larger trials. Preclinical B7-H3-targeted RIT studies have also shown encouraging therapeutic outcomes in a variety of solid malignancies.

CONCLUSION

B7-H3-targeted RIT studies over the last 15 years have demonstrated feasibility for clinical development and support future assessment in a broader array of human malignancies. Future directions worthy of exploration include strategies that combine B7-H3- targeted RIT with chemotherapy or immunotherapy.

Methotrexate, paclitaxel, and doxorubicin radiosensitize HER2-amplified human breast cancer cells to the Auger electron-emitting radiotherapeutic agent (111)In-NLS-trastuzumab

Our goal in this study was to elucidate the mechanisms by which methotrexate radiosensitizes HER2-positive human breast cancer cells to the Auger electron emitter (111)In-trastuzumab modified with nuclear-localization sequence peptides ((111)In-NLS-trastuzumab) and to compare these mechanisms with the potential sensitizing effects of paclitaxel and doxorubicin when combined with this radiopharmaceutical.

METHODS

Experiments were performed in MDA-MB-231 human breast cancer cells, their HER2-transfected subclones (231-H2N), and 2 trastuzumab-resistant variants (trastuzumab-resistant-1 and -2 [TrR1 and TrR2]). Effects of coexposure of these cells to (111)In-NLS-trastuzumab and low-dose, radiosensitizing methotrexate, paclitaxel, or doxorubicin were assessed by clonogenic cell-survival assay. Quantification of residual DNA damage was measured by the gammaH2AX-immunofluorescence assay, and cell cycle distribution was measured by fluorescence-activated cell sorting analysis. The radiation-enhancement ratio was calculated as the ratio of the surviving fraction (SF) of cells treated with (111)In-NLS-trastuzumab alone to that of cells treated concurrently with (111)In-NLS-trastuzumab and methotrexate, paclitaxel, or doxorubicin.

RESULTS

A reduction in the SF in HER2-positive 231-H2N (55.7% +/- 1.3%) and TrR1 (62.6% +/- 6.5%) cells was demonstrated after exposure to (111)In-NLS-trastuzumab (approximately 0.2 MBq/microg, 100 nmol/L) but not in MDA-MB-231 or TrR2 cells expressing low levels of HER2 (SF > 90%, P > 0.05). Coadministration of methotrexate, paclitaxel, or doxorubicin enhanced the cytotoxicity of (111)In-NLS-trastuzumab toward 231-H2N and TrR1 cells but not toward MDA-MB-231 or TrR2 cells. The radiation-enhancement ratios for methotrexate, paclitaxel, and doxorubicin for 231-H2N or TrR1 cells were 2.0-2.2, 1.6-1.8, and 2.7-2.8, respectively. Methotrexate or doxorubicin combined with (111)In-NLS-trastuzumab, compared to treatment with (111)In-NLS-trastuzumab alone, significantly increased residual gammaH2AX foci in 231-H2N and TrR1 cells but not in MDA-MB-231 or TrR2 cells or in any cell line treated concurrently with paclitaxel and (111)In-NLS-trastuzumab. Cells exposed to low-dose methotrexate accumulated in the G(1)/S phase of the cell cycle, whereas low-dose paclitaxel or doxorubicin caused cells to arrest in the G(2)/M phase.

CONCLUSION

Low-dose methotrexate, paclitaxel, or doxorubicin potently sensitized HER2-overexpressing human breast cancer cells, with and without acquired trastuzumab-resistance, to the Auger electron emissions from (111)In-NLS-trastuzumab through cell cycle distribution changes and in part through the inhibitory effects of these agents on DNA damage repair.

Tumor radioimmunolocalization in nude mice by mono- and divalent- single-chain Fv antiplacental alkaline phosphatase antibodies

One single-chain Fv antibody fragment (scFv) and a new recombinant covalently linked dimeric scFv antibody (sc(Fv)(2)) against placental alkaline phosphatase (PLAP) were investigated for selective tumor targeting. The biological behavior of these new antibodies was compared to that of the original native antibody, H7 MAb. The sc(Fv)(2)) antibody displayed convincing tumor localization properties with a rapid excretion pattern comparable to the scFv, but with a longer retention time in the tumor, and higher tumor-to-nontumor ratio (27:1), compared to the scFv (15:1), at 48 hours. For the sc(Fv)(2) antibody, more than 50% of the remaining activity in the mouse was present in the tumor between 24 and 48 hours after the injection. With this antibody, scintigraphic visualization of the tumor was also possible 1 week after the injection. It is concluded that this sc(Fv)(2) antibody fragment, with two binding sites, displays properties suitable for in vivo targeting of PLAP expressing tumors.

Targeted radionuclide therapy for solid tumors: an overview

Although radioimmunotherapy (RIT) has been effective in non-Hodgkin's lymphoma (NHL) as a single agent, solid tumors have shown less clinically significant therapeutic response to RIT alone. The clinical impact of RIT or other forms of targeted radionuclide therapy for solid tumors depends on the development of a high therapeutic index (TI) for the tumor vs. normal tissue effect, and the implementation of RIT as part of synergistic combined modality therapy (CMRIT). Preclinical and clinical studies have provided a wealth of information, and new prototypes or paradigms have shed light on future possibilities in many instances. Evidence suggests that combination and sequencing of RIT in CMRIT appropriately can provide effective treatment for many solid tumors. Vascular targets provide RIT enhancement opportunities and nanoparticles may prove to be effective carriers for RIT combined with intracellular drug delivery or alternating magnetic frequency (AMF) induced thermal tumor necrosis. The sequence and timing of combined modality treatments will be of critical importance to achieve synergy for therapy while minimizing toxicity. Fortunately, the radionuclide used for RIT also provides a signal useful for nondestructive quantitation of the influence of sequence and timing of CMRIT on events in animals and patients. This can be readily accomplished clinically using quantitative high-resolution imaging (e.g., positron emission tomography [PET]).

Antibodies as delivery vehicles for radioimmunotherapy of infectious diseases

The field of infectious diseases is in crisis and there is a need for strategies that can facilitate the rapid development of new antimicrobial agents. Radioimmunotherapy (RIT), a therapeutic modality originally developed for cancer treatment, has recently been suggested as a novel therapy for the treatment of a variety of infectious diseases. Because specific antibodies are used in RIT as delivery vehicles of cytocidal radiation, their molecular weight influences the nonspecific accumulation in infectious foci and blood clearance, and their affinity-specific accumulation of antibodies in infectious foci. Like the problems encountered in oncology, relevant variables in the development of RIT of infectious diseases include target antigen-shedding; delivering radionuclides to infectious foci in organs, abscesses, granulomas, heart and brain, and potential safety concerns. Dadachova and Casadevall anticipate that RIT can be developed for many types of infectious diseases, including microbes resistant to conventional antimicrobial therapy and agents of biological warfare.

Tailoring antibodies for radionuclide delivery

Therapeutic antibodies are well established as an important class of drugs in modern medicine. The exquisite specificity and affinity for a specific target offered by antibodies has also encouraged their development as delivery vehicles for agents such as radionuclides to target tissues, for radioimmunoimaging and radioimmunotherapy. Specifically, in nuclear medicine, radionuclide-conjugated antibody molecules make it possible to image diseased loci with greater sensitivity than other imaging modalities such as magnetic resonance imaging. Furthermore, two radionuclide-conjugated antibodies have recently been approved for the therapy of non-Hodgkin's lymphoma. However, optimal implementation of antibodies has been limited by the extended circulation persistence that is characteristic of native antibodies, which is responsible for increased background activity in radioimmunoimaging applications and dose-related normal organ toxicities in radioimmunotherapy. In this article the current status of radiolabelled intact antibodies is reviewed, focusing on strategies to improve their pharmacokinetic properties to suit a desired application. Examples from the literature that represent different approaches to accomplishing this task in terms of their successes as well as limitations, and perspectives for the future are discussed.

Hapten-directed targeting to single-chain antibody receptors

Artificial recombinant receptors may be useful for selectively targeting imaging and therapeutic agents to sites of gene expression. To evaluate this approach, we developed transgenes to express highly on cells a single-chain antibody (scFv) against the hapten 4-ethoxymethylene-2-phenyl-2-oxazoline-5-one (phOx). A phOx enzyme conjugate was created by covalently attaching phOx molecules to polyethylene glycol (PEG)-modified beta-glucuronidase. Cells expressing phOx scFv but not control scFv receptors were selectively killed after exposure to ss-glucuronidase derivatized with phOx and PEG (phOx-beta G-PEG) and a glucuronide prodrug (p-hydroxy aniline mustard beta-D-glucuronide, HAMG) of p-hydroxyaniline mustard. Targeted activation of HAMG produced bystander killing of receptor-negative cells in mixed populations containing as few as 10% phOx-receptor-positive cells. Functional phOx scFv receptors were stably expressed on B16-F1 melanoma tumors in vivo. Treatment of mice bearing established phOx-receptor-positive tumors with phOx-beta G-PEG and HAMG significantly (P< or =.0005) suppressed tumor growth as compared with treatment with beta G-PEG and HAMG or prodrug alone. phOx was unstable in the serum, suggesting alternative haptens may be more suitable for in vivo applications. Our results show that therapeutic agents can be targeted to artificial hapten receptors in vitro and in vivo. The expression of artificial receptors on target cells may allow preferential delivery of therapeutic or imaging molecules to sites of transgene expression.

Ionizing radiation delivered by specific antibody is therapeutic against a fungal infection

There is an urgent need for new antimicrobial therapies to combat drug resistance, new pathogens, and the relative inefficacy of current therapy in compromised hosts. Ionizing radiation can kill microorganisms quickly and efficiently, but this modality has not been exploited as a therapeutic antimicrobial strategy. We have developed methods to target ionizing radiation to a fungal cell by labeling a specific mAb with the therapeutic radioisotopes Rhenium-188 and Bismuth-213. Radiolabeled antibody killed cells of human pathogenic fungus Cryptococcus neoformans in vitro, thus converting an antibody with no inherent antifungal activity into a microbicidal molecule. Administration of radiolabeled antibody to mice with C. neoformans infection delivered 213Bi and 188Re to the sites of infection, reduced their organ fungal burden, and significantly prolonged their survival without apparent toxicity. This study establishes the principle that targeted radiation can be used for the therapy of an infectious disease, and suggests that it may have wide applicability as an antimicrobial strategy.

Advancing role of radiolabeled antibodies in the therapy of cancer

This review focuses on the use of radiolabeled antibodies in the therapy of cancer, termed radioimmunotherapy (RAIT). Basic problems concerned with the choice of antibody, radionuclide, and physiology of the tumor and host are discussed, followed by a review of the pertinent clinical publications of various radioantibody constructs in the treatment of hematopoietic and solid tumors of diverse histopathology, grade, and stage, and in different clinical settings. Factors such as dose rate delivered, tumor size, and radiosensitivity play a major role in determining therapeutic response, while target-to-nontarget ratios and, particularly, circulating radioactivity to the bone marrow determine the principal dose-limiting toxicities. RAIT appears to be gaining a place in the therapy of hematopoietic neoplasms, such as non-Hodgkin's lymphoma: several agents are advancing in clinical trials toward registration, and one has recently been approved by the FDA. Although RAIT of solid tumors has shown less progress, use of pretargeting strategies, such as an affinity-enhancement system consisting of bispecific antibodies separating targeting from delivery of the radiotherapeutic, appears to enhance tumor-to-nontumor ratios, and may increase radiation doses to tumors more selectively than directly labeled antibodies.

Tumour therapy with radionuclides: assessment of progress and problems

Radionuclide therapy is a promising modality for treatment of tumours of haematopoietic origin while the success for treatment of solid tumours so far has been limited. The authors consider radionuclide therapy mainly as a method to eradicate disseminated tumour cells and small metastases while bulky tumours and large metastases have to be treated surgically or by external radiation therapy. The promising therapeutic results for haematological tumours give hope that radionuclide therapy will have a breakthrough also for treatment of disseminated cells from solid tumours. New knowledge related to this is continuously emerging since new molecular target structures are being characterised and the knowledge on pharmacokinetics and cellular processing of different types of targeting agents increases. There is also improved understanding of the factors of importance for the choice of appropriate radionuclides with respect to their decay properties and the therapeutic applications. Furthermore, new methods to modify the uptake of radionuclides in tumour cells and normal tissues are emerging. However, we still need improvements regarding dosimetry and treatment planning as well as an increased knowledge about the tolerance doses for normal tissues and the radiobiological effects on tumour cells. This is especially important in targeted radionuclide therapy where the dose rates often are lower than 1Gy/h.

Combined modality radioimmunotherapy. Promise and peril

BACKGROUND

Single-agent radioimmunotherapy (RIT), although potentially useful for slowing solid tumor growth, has not been effective in curing aggressive tumors, such as breast cancer. These cancers typically have p53 mutations and are less susceptible to apoptosis, the apparent mechanism of cell death from low dose-rate radiation. Thus, synergistic or combined modality radioimmunotherapy (CMRIT) agents are needed to increase radiosensitivity for therapeutic enhancement without additive toxicity.

METHODS

To assess synergy in CMRIT in a breast cancer xenograft model, we evaluated RGD peptide EMD 121974, an inhibitor of alpha(v)beta(3) integrin; paclitaxel, an antimicrotubule; IMC-C225, a monoclonal antibody to epidermal growth factor receptor (EGFR); and bcl-2 antisense oligonucleotide G3139. Groups of mice received (90)Y-DOTA-ChL6 in combination with each agent. Tumor size, survival, and blood counts were monitored for efficacy and toxicity. Immunopathologic evaluation of apoptosis was performed at selected time points after RIT and RIT + RGD CMRIT.

RESULTS

CMRIT with RGD peptide increased apoptosis and resulted in 57% cures, compared with 0% cures with RIT alone. CMRIT with paclitaxel after RIT increased cures to 88%, compared with 25% cures with RIT before paclitaxel administration. CMRIT with IMC-C225 resulted in up to 20% cures if given before RIT. A time-dependent increase in toxicity was observed with IMC-C225 after RIT. CMRIT with bcl-2 antisense G3139 resulted in no cures and an increased rate of regrowth compared with RIT alone.

CONCLUSIONS

Some combined modality therapies resulted in higher numbers of cures, while others decreased cures and responses and increased toxicity compared with RIT alone. These results support the potential for CMRIT but illustrate the complexity of predicting the efficacy and toxicity and the importance of the relationship between dose and sequence of administration.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.