In vitro testing of the leukaemia monoclonal antibody WM-53 labeled with alpha and beta emitting radioisotopes

DNA Damage by Radiopharmaceuticals and Mechanisms of Cellular Repair

DNA is an organic molecule that is highly vulnerable to chemical alterations and breaks caused by both internal and external factors. Cells possess complex and advanced mechanisms, including DNA repair, damage tolerance, cell cycle checkpoints, and cell death pathways, which together minimize the potentially harmful effects of DNA damage. However, in cancer cells, the normal DNA damage tolerance and response processes are disrupted or deregulated. This results in increased mutagenesis and genomic instability within the cancer cells, a known driver of cancer progression and therapeutic resistance. On the other hand, the inherent instability of the genome in rapidly dividing cancer cells can be exploited as a tool to kill by imposing DNA damage with radiopharmaceuticals. As the field of targeted radiopharmaceutical therapy (RPT) is rapidly growing in oncology, it is crucial to have a deep understanding of the impact of systemic radiation delivery by radiopharmaceuticals on the DNA of tumors and healthy tissues. The distribution and activation of DNA damage and repair pathways caused by RPT can be different based on the characteristics of the radioisotope and molecular target. Here we provide a comprehensive discussion of the biological effects of RPTs, with the main focus on the role of varying radioisotopes in inducing direct and indirect DNA damage and activating DNA repair pathways.

Antibody Therapies for Acute Myeloid Leukemia: Unconjugated, Toxin-Conjugated, Radio-Conjugated and Multivalent Formats

In recent decades, therapy for acute myeloid leukemia (AML) has remained relatively unchanged, with chemotherapy regimens primarily consisting of an induction regimen based on a daunorubicin and cytarabine backbone, followed by consolidation chemotherapy. Patients who are relapsed or refractory can be treated with allogeneic hematopoietic stem-cell transplantation with modest benefits to event-free and overall survival. Other modalities of immunotherapy include antibody therapies, which hold considerable promise and can be categorized into unconjugated classical antibodies, multivalent recombinant antibodies (bi-, tri- and quad-specific), toxin-conjugated antibodies and radio-conjugated antibodies. While unconjugated antibodies can facilitate Natural Killer (NK) cell antibody-dependent cell-mediated cytotoxicity (ADCC), bi- and tri-specific antibodies can engage either NK cells or T-cells to redirect cytotoxicity against AML targets in a highly efficient manner, similarly to classic ADCC. Finally, toxin-conjugated and radio-conjugated antibodies can increase the potency of antibody therapies. Several AML tumour-associated antigens are at the forefront of targeted therapy development, which include CD33, CD123, CD13, CLL-1 and CD38 and which may be present on both AML blasts and leukemic stem cells. This review focused on antibody therapies for AML, including pre-clinical studies of these agents and those that are either entering or have been tested in early phase clinical trials. Antibodies for checkpoint inhibition and microenvironment targeting in AML were excluded from this review.

Global comparison of targeted alpha vs targeted beta therapy for cancer: In vitro, in vivo and clinical trials

Targeted therapy for cancer is a rapidly expanding and successful approach to the management of many intractable cancers. However, many immunotherapies fail in the longer term and there continues to be a need for improved targeted cancer cell toxicity, which can be achieved by radiolabelling the targeting vector with a radioisotope. Such constructs are successful in using a gamma ray emitter for imaging. However, traditionally, a beta emitter is used for therapeutic applications. The new approach is to use the short range and highly cytotoxic alpha radiation from alpha emitters to achieve improved efficacy and therapeutic gain. This paper sets out to review all experimental and theoretical comparisons of efficacy and therapeutic gain for alpha and beta emitters labelling the same targeting vector. The overall conclusion is that targeted alpha therapy is superior to targeted beta therapy, such that the use of alpha therapy in clinical settings should be expanded.

Dendrimers as Innovative Radiopharmaceuticals in Cancer Radionanotherapy

Radiotherapy is one of the most commonly used cancer treatments, with an estimate of 40% success that could be improved further if more efficient targeting and retention of radiation at the tumor site were achieved. This review focuses on the use of dendrimers in radionanotherapy, an emerging technology aimed to improve the efficiency of radiotherapy by implementing nanovectorization, an already established praxis in drug delivery and diagnosis. The labeling of dendrimers with radionuclides also aims to reduce the dose of radiolabeled materials and, hence, their toxicity and tumor resistance. Examples of radiolabeled dendrimers with alpha, beta, and Auger electron emitters are commented, along with the use of dendrimers in boron neutron capture therapy (BNCT). The conjugation of radiolabeled dendrimers to monoclonal antibodies for a more efficient targeting and the application of dendrimers in gene delivery radiotherapy are also covered.

Preclinical in vivo application of (152)Tb-DOTANOC: a radiolanthanide for PET imaging

BACKGROUND

Terbium has attracted the attention of researchers and physicians due to the existence of four medically interesting radionuclides, potentially useful for SPECT and PET imaging, as well as for α- and β(-)-radionuclide therapy. The aim of this study was to produce (152)Tb (T 1/2 = 17.5 h, Eβ+av = 1140 keV) and evaluate it in a preclinical setting in order to demonstrate its potential for PET imaging. For this purpose, DOTANOC was used for targeting the somatostatin receptor in AR42J tumor-bearing mice.

METHODS

(152)Tb was produced by proton-induced spallation of tantalum targets, followed by an online isotope separation process at ISOLDE/CERN. After separation of (152)Tb using cation exchange chromatography, it was directly employed for radiolabeling of DOTANOC. PET/CT scans were performed with AR42J tumor-bearing mice at different time points after injection of (152)Tb-DOTANOC which was applied at variable molar peptide amounts. (177)Lu-DOTANOC was prepared and used in biodistribution and SPECT/CT imaging studies for comparison with the PET results.

RESULTS

After purification, (152)Tb was obtained at activities up to ~600 MBq. Radiolabeling of DOTANOC was achieved at a specific activity of 10 MBq/nmol with a radiochemical purity >98 %. The PET/CT scans of mice allowed visualization of AR42J tumor xenografts and the kidneys, in which the radiopeptide was accumulated. After injection of large peptide amounts, the tumor uptake was reduced as compared to the result after injection of small peptide amounts. PET images of mice, which received (152)Tb-DOTANOC at small peptide amounts, revealed the best tumor-to-kidney ratios. The data obtained with (177)Lu-DOTANOC in biodistribution and SPECT/CT imaging studies confirmed the (152)Tb-based PET results.

CONCLUSIONS

Production of 30-fold higher quantities of (152)Tb as compared to the previously performed pilot study was feasible. This allowed, for the first time, labeling of a peptide at a reasonable specific activity and subsequent application for in vivo PET imaging. As a β(+)-particle-emitting radiolanthanide, (152)Tb would be of distinct value for clinical application, as it may allow exact prediction of the tissue distribution of therapeutic radiolanthanides.

Targeted alpha anticancer therapies: update and future prospects

Targeted alpha therapy (TAT) is an emerging option for local and systemic cancer treatment. Preclinical research and clinical trials show that alpha-emitting radionuclides can kill targeted cancer cells while sparing normal cells, thus reducing toxicity. (223)RaCl2 (Xofigo(®)) is the first alpha emitting radioisotope to gain registration in the US for palliative therapy of prostate cancer bone metastases by indirect physiological targeting. The alpha emitting radioisotopes (211)At, (213)Bi, (225)Ac and (227)Th are being used to label targeting vectors such as monoclonal antibodies for specific cancer therapy indications. In this review, safety and tolerance aspects are considered with respect to microdosimetry, specific energy, Monte Carlo model calculations, biodosimetry, equivalent dose and mutagenesis. The clinical efficacy of TAT for solid tumors may also be enhanced by its capacity for tumor anti-vascular (TAVAT) effects. This review emphasizes key aspects of TAT research with respect to the PAI2-uPAR complex and the monoclonal antibodies bevacizumab, C595 and J591. Clinical trial outcomes are reviewed for neuroendocrine tumors, leukemia, glioma, melanoma, non-Hodgkins lymphoma, and prostate bone metastases. Recommendations and future directions are proposed.

Next-generation therapy for residual prostate cancer

Prostate cancer claimed an estimated 136,500 lives globally in 2011. Ironically, the best existing treatment strategies provide survival benefits and missed opportunities to further improve outcomes. Prostatectomy provides the greatest survival benefit, albeit the risk of systemic recurrence increases dramatically with extracapsular involvement. To date, further systemic treatment is not generally prescribed for these 'high-risk' patients until such time as advanced disease is diagnosed based on persistent high PSA levels and/or when larger tumors are confirmed by imaging. This recurrent form of the disease is most often terminal. Androgen deprivation therapy (ADT) provides outstanding early control for these patients, which is rather tragic as the early benefits of ADT are lost within 2 years for most men, as the cancer again progresses to an incurable 'late-stage' castration-resistant form of the disease with a median survival of approximately 18 months. We review the potential of targeted α-therapy as an adjuvant with minimal side effects for early-stage high-risk patients to be administered immediately following prostatectomy and/or during ADT.

Targeted alpha-therapy: past, present, future?

Monoclonal antibodies have become a viable strategy for the delivery of therapeutic, particle emitting radionuclides specifically to tumor cells to either augment anti-tumor action of the native antibodies or to solely take advantage of their action as targeting vectors. Proper and rational selection of radionuclide and antibody combinations is critical to making radioimmunotherapy (RIT) a standard therapeutic modality due to the fundamental and significant differences in the emission of either alpha- and beta-particles. The alpha-particle has a short path length (50-80 microm) that is characterized by high linear energy transfer (100 keV microm(-1)). Actively targeted alpha-therapy potentially offers a more specific tumor cell killing action with less collateral damage to the surrounding normal tissues than beta-emitters. These properties make targeted alpha-therapy an appropriate therapy to eliminate minimal residual or micrometastatic disease. RIT using alpha-emitters such as (213)Bi, (211)At, (225)Ac, and others has demonstrated significant activity in both in vitro and in vivo model systems. Limited numbers of clinical trials have progressed to demonstrate safety, feasibility, and therapeutic activity of targeted alpha-therapy, despite having to traverse complex obstacles. Further advances may require more potent isotopes, additional sources and more efficient means of isotope production. Refinements in chelation and/or radiolabeling chemistry combined with rational improvements of isotope delivery, targeting vectors, molecular targets, and identification of appropriate clinical applications remain as active areas of research. Ultimately, randomized trials comparing targeted alpha-therapy combined with integration into existing standards of care treatment regimens will determine the clinical utility of this modality.

[Alpha-radioimmunotherapy: a review of recent developments]

The use of heavy particles in the treatment of cancer is increasing remarkably, whether with external radiation or using a vector such as an antibody in radioimmunotherapy. Recent pre-clinical and clinical developments of alpha-radioimmunotherapy have provided more interesting information in parallel of the use of high Linear Energy Transfer (LET) external irradiation. This review aims at presenting recent advances of this therapeutic approach, and at detailing the biological specificities of this kind of radiation.

Internal high linear energy transfer (LET) targeted radiotherapy for cancer

High linear energy transfer (LET) radiation for internal targeted therapy has been a long time coming on to the medical therapy scene. While fundamental principles were established many decades ago, the clinical implementation has been slow. Localized neutron capture therapy, and more recently systemic targeted alpha therapy, are at the clinical trial stage. What are the attributes of these therapies that have led a band of scientists and clinicians to dedicate so much of their careers? High LET means high energy density, causing double strand breaks in DNA, and short-range radiation, sparing adjacent normal tissues. This targeted approach complements conventional radiotherapy and chemotherapy. Such therapies fail on several fronts. Foremost is the complete lack of progress for the control of primary GBM, the holy grail for cancer therapies. Next is the inability to regress metastatic cancer on a systemic basis. This has been the task of chemotherapy, but palliation is the major application. Finally, there is the inability to inhibit the development of lethal metastatic cancer after successful treatment of the primary cancer. This review charts, from an Australian perspective, the developing role of local and systemic high LET, internal radiation therapy.

Cancer radioimmunotherapy with alpha-emitting nuclides

In lymphoid malignancies and in certain solid cancers such as medullary thyroid carcinoma, somewhat mixed success has been achieved when applying radioimmunotherapy (RIT) with beta-emitters for the treatment of refractory cases. The development of novel RIT with alpha-emitters has created new opportunities and theoretical advantages due to the high linear energy transfer (LET) and the short path length in biological tissue of alpha-particles. These physical properties offer the prospect of achieving selective tumoural cell killing. Thus, RIT with alpha-emitters appears particularly suited for the elimination of circulating single cells or cell clusters or for the treatment of micrometastases at an early stage. However, to avoid non-specific irradiation of healthy tissues, it is necessary to identify accessible tumoural targets easily and rapidly. For this purpose, a small number of alpha-emitters have been investigated, among which only a few have been used for in vivo preclinical studies. Another problem is the availability and cost of these radionuclides; for instance, the low cost and the development of a reliable actinium-225/bismuth-213 generator were probably determining elements in the choice of bismuth-213 in the only human trial of RIT with an alpha-emitter. This article reviews the literature concerning monoclonal antibodies radiolabelled with alpha-emitters that have been developed for possible RIT in cancer patients. The principal radio-immunoconjugates are considered, starting with physical and chemical properties of alpha-emitters, their mode of production, the possibilities and difficulties of labelling, in vitro studies and finally, when available, in vivo preclinical and clinical studies.

Targeted alpha therapy for cancer

Targeted alpha therapy (TAT) offers the potential to inhibit the growth of micrometastases by selectively killing isolated and preangiogenic clusters of cancer cells. The practicality and efficacy of TAT is tested by in vitro and in vivo studies in melanoma, leukaemia, colorectal, breast and prostate cancers, and by a phase 1 trial of intralesional TAT for melanoma. The alpha-emitting radioisotope used is Bi-213, which is eluted from the Ac-225 generator and chelated to a cancer specific monoclonal antibody (mab) or protein (e.g. plasminogen activator inhibitor-2 PAI2) to form the alpha-conjugate (AC). Stable alpha-ACs have been produced which have been tested for specificity and cytotoxicity in vitro against melanoma (9.2.27 mab), leukaemia (WM60), colorectal (C30.6), breast (PAI2, herceptin), ovarian (PAI2, herceptin, C595), prostate (PAI2, J591) and pancreatic (PAI2, C595) cancers. Subcutaneous inoculation of 1-1.5 million human cancer cells into the flanks of nude mice causes tumours to grow in all mice. Tumour growth is compared for untreated controls, nonspecific AC and specific AC, for local (subcutaneous) and systemic (tail vein or intraperitoneal) injection models. The 213Bi-9.2.27 AC is injected into secondary skin melanomas in stage 4 patients in a dose escalation study to determine the effective tolerance dose, and to measure kinematics to obtain the equivalent dose to organs. In vitro studies show that TAT is one to two orders of magnitude more cytotoxic to targeted cells than non-specific ACs, specific beta emitting conjugates or free isotopes. In vivo local TAT at 2 days post-inoculation completely prevents tumour formation for all cancers tested so far. Intra-lesional TAT can completely regress advanced sc melanoma but is less successful for breast and prostate cancers. Systemic TAT inhibits the growth of sc melanoma xenografts and gives almost complete control of breast and prostate cancer tumour growth. Intralesional doses up to 450 microCi in human patients are effective in regressing melanomas, with no concomitant complications. These results point to the application of local and systemic TAT in the management of secondary cancer. Results of the phase 1 clinical trial of TAT of subcutaneous, secondary melanoma indicate proof of the principle that TAT can make tumours in patients regress.

Cytotoxicity of breast cancer cells overexpressing HER2/neu by 213Bi-Herceptin radioimmunoconjugate

HER2 is the target of a new treatment for metastatic breast cancer using the humanized monoclonal antibody (MAb) trastuzumb (Herceptin). A novel alpha-particle emitting (213)Bi-Herceptin construct, targeting the HER2 extracellular domain on breast cancer cells, was produced by chelation and characterized in vitro in this study. We used Western blot and flow cytometry analysis to examine the expression of HER2 in a panel of established human metastatic breast cancer cell lines (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt) MTS assay to evaluate the cytotoxicity and the TUNEL assay to analyze cellular apoptosis. Our results demonstrate that the human breast cancer cell lines BT-474 and SK-BR-3 express high levels of HER2 protein while MDA-231 expresses low levels of HER2. (213)Bi-Herceptin alpha conjugate (AC) was specifically cytotoxic to these cell lines in a HER2 level-dependent fashion, resulting in the cellular death through apoptosis. These results suggest that (231)Bi-Herceptin AC could be a novel agent for the treatment of breast cancer cell clusters or micro-metastases with high levels of HER2 expression.

Antigenic expression of human metastatic prostate cancer cell lines for in vitro multiple-targeted α-therapy with 213Bi-conjugates

PURPOSE

Control of metastatic prostate cancer (CaP) is an elusive objective. Some 30% of patients with clinically localized CaP will develop micrometastatic disease. Defining the expression of tumor-associated antigens on CaP will enable appropriate selection of therapeutic targets.

METHODS AND MATERIALS

The expression of tumor-associated antigens on CaP cell lines (PC-3, DU 145, and LNCaP-LN3) was detected by immunohistochemistry and flow cytometry. Test and control alpha-conjugates were prepared using monoclonal antibodies, an inhibitor, plasminogen activator inhibitor type 2, that binds to the cell-membrane-bound protease, urokinase plasminogen activator, and a control protein labeled with (213)Bi using standard methods. These were used singly or together against three different CaP cell lines in vitro. The cytotoxicity of the alpha-conjugates was assessed using the [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt] (MTS) assay.

RESULTS

The PC-3 and DU 145 cancer cell lines expressed antigens that bind monoclonal antibodies BLCA-38 and #394 (mouse anti-human urokinase plasminogen activator B-chain) but not J591. The LNCaP-LN3 cells bound J591 but not #394 or BLCA-38. For the PC-3, DU 145, and LNCaP-LN3 cell lines, multiple-targeted alpha-therapy combining four alpha-conjugates (one-quarter doses of each) gave D(0) (37% cell survival) values of 15, 17, and 27 microCi/mL compared with those of the controls of 272, 289, and 281 microCi/mL, respectively.

CONCLUSION

Metastatic prostate cancer-associated antigens recognized by multiple monoclonal antibodies are potential targets for alpha-therapy. Multiple-targeted alpha-therapy produced cytotoxicity specific to three CaP cell lines and may form the basis of treatment for micrometastatic CaP, overcoming the heterogeneity of expression of the targeted antigens.

Preclinical studies of targeted alpha therapy for breast cancer using 213Bi-labelled-plasminogen activator inhibitor type 2

The control of micrometastatic breast cancer remains problematic. To this end, we are developing a new adjuvant therapy based on (213)Bi-PAI2, in which an alpha-emitting nuclide ((213)Bi) is chelated to the plasminogen activator inhibitor-2 (PAI2). PAI2 targets the cell-surface receptor bound urokinase plasminogen activator (uPA), which is involved with the metastatic spread of cancer cells. We have successfully labelled and tested recombinant human PAI2 with the alpha radioisotope (213)Bi to produce (213)Bi-PAI2, which is highly cytotoxic towards breast cancer cell lines. In this study, the 2-day postinoculation model, using MDA-MB-231 breast cancer cells, was shown to be representative of micrometastatic disease. Our in vivo efficacy experiments show that a single local injection of (213)Bi-PAI2 can completely inhibit the growth of tumour at 2 days postcell inoculation, and a single systemic (i.p.) administration at 2 days causes tumour growth inhibition in a dose-dependent manner. The specific role of uPA as the target for (213)Bi-PAI2 therapy was determined by PAI2 pretreatment blocking studies. In vivo toxicity studies in nude mice indicate that up to 100 microCi of (213)Bi-PAI2 is well tolerated. Thus, (213)Bi-PAI2 is successful in targeting isolated breast cancer cells and preangiogenic cell clusters. These results indicate the promising potential of (213)Bi-PAI2 as a novel therapeutic agent for micrometastatic breast cancer.