Radioimmunotherapy of micrometastases in lung with vascular targeted 213Bi

Targeted α-therapy in non-prostate malignancies

Progress in unraveling the complex biology of cancer, novel developments in radiochemistry, and availability of relevant α-emitters for targeted therapy have provided innovative approaches to precision cancer management. The approval of 223Ra dichloride for treatment of men with osseous metastatic castrate-resistant prostate cancer unleashed targeted α-therapy as a safe and effective cancer management strategy. While there is currently active research on new α-therapy regimens for prostate cancer based on the prostate-specific membrane antigen, there is emerging development of radiopharmaceutical therapy with a range of biological targets and α-emitting radioisotopes for malignancies other than the prostate cancer. This article provides a brief review of preclinical and first-in-human studies of targeted α-therapy in the cancers of brain, breast, lung, gastrointestinal, pancreas, ovary, and the urinary bladder. The data on leukemia, melanoma, myeloma, and neuroendocrine tumors will also be presented. It is anticipated that with further research the emerging role of targeted α-therapy in cancer management will be defined and validated.

Targeted α-Therapy in Cancer Management: Synopsis of Preclinical and Clinical Studies

The approval of ²²³Ra dichloride (²²³RaCl₂) in 2013 was a principal event in introducing targeted α-therapy as a form of safe and effective management strategy in cancer. There is an increasing interest in research and development of new targeted α-therapy agents spearheaded by advancements in cancer biology, radiochemistry, and availability of clinically relevant α particles. There are active clinical studies on sequencing or combining ²²³RaCl₂ with other drug regimens in the setting of metastatic prostate cancer and in other cancers such as osteosarcoma and bone-dominant breast cancer. Targeted α-therapy strategy is also being actively explored through many preclinical and few early clinical studies using ²²⁵Ac, ²¹³Bi, ²¹¹At, ²²⁷Th, and ²¹²Pb. Investigations incorporating ²²⁵Ac are more robust and active at this time with promising results. The author provide a brief synopsis of the preclinical and clinical studies in the rapidly evolving field of targeted α-therapy in cancer management.

Innovation, Impact, and Strategic Importance of Alpha-Emitting Radionuclides

This talk briefly reviews the earlier work in the field and highlights the contributions of colleagues whose clear vision paved the road for success of TAT. The talk primarily will focus on the development of the radioisotopes for application in TAT. The challenges regarding release of daughter radioisotopes will be briefly discussed, and a summary of the alternative approaches for production of ²²⁵Ac will be presented.

Cure of Human Ovarian Carcinoma Solid Xenografts by Fractionated α-Radioimmunotherapy with 211At-MX35-F(ab')2: Influence of Absorbed Tumor Dose and Effect on Long-Term Survival

The goal of this study was to investigate whether targeted α-therapy can be used to successfully treat macrotumors, in addition to its established role for treating micrometastatic and minimal disease. We used an intravenous fractionated regimen of α-radioimmunotherapy in a subcutaneous tumor model in mice. We aimed to evaluate the absorbed dose levels required for tumor eradication and growth monitoring, as well as to evaluate long-term survival after treatment. Methods: Mice bearing subcutaneous tumors (50 mm³, NIH:OVCAR-3) were injected repeatedly (1-3 intravenous injections 7-10 d apart, allowing bone marrow recovery) with ²¹¹At-MX35-F(ab')₂ at different activities (close to acute myelotoxicity). Mean absorbed doses to tumors and organs were estimated from biodistribution data and summed for the fractions. Tumor growth was monitored for 100 d and survival for 1 y after treatment. Toxicity analysis included body weight, white blood cell count, and hematocrit. Results: Effects on tumor growth after fractionated α-radioimmunotherapy with ²¹¹At-MX35-F(ab')₂ was strong and dose-dependent. Complete remission (tumor-free fraction, 100%) was found for tumor doses of 12.4 and 16.4 Gy. The administered activities were high, and long-term toxicity effects (≤60 wk) were clear. Above 1 MBq, the median survival decreased linearly with injected activity, from 44 to 11 wk. Toxicity was also seen by reduced body weight. White blood cell count analysis after α-radioimmunotherapy indicated bone marrow recovery for the low-activity groups, whereas for high-activity groups the reduction was close to acute myelotoxicity. A decrease in hematocrit was seen at a late interval (34-59 wk after therapy). The main external indication of poor health was dehydration. Conclusion: Having observed complete eradication of solid tumor xenografts, we conclude that targeted α-therapy regimens may stretch beyond the realm of micrometastatic disease and be eradicative also for macrotumors. Our observations indicate that at least 10 Gy are required. This agrees well with the calculated tumor control probability. Considering a relative biological effectiveness of 5, this dose level seems reasonable. However, complete remission was achieved first at activity levels close to lethal and was accompanied by biologic effects that reduced long-term survival.

Long-Term Toxicity of 213Bi-Labelled BSA in Mice

BACKGROUND

Short-term toxicological evaluations of alpha-radioimmunotherapy have been reported in preclinical assays, particularly using bismuth-213 (213Bi). Toxicity is greatly influenced not only by the pharmacokinetics and binding specificity of the vector but also by non-specific irradiation due to the circulating radiopharmaceutical in the blood. To assess this, an acute and chronic toxicity study was carried out in mice injected with 213Bi-labelled Bovine Serum Albumin (213Bi-BSA) as an example of a long-term circulating vector.

METHOD

Biodistribution of 213Bi-BSA and 125I-BSA were compared in order to evaluate 213Bi uptake by healthy organs. The doses to organs for injected 213Bi-BSA were calculated. Groups of nude mice were injected with 3.7, 7.4 and 11.1 MBq of 213Bi-BSA and monitored for 385 days. Plasma parameters, including alanine aminotransferase (ALT), aspartate aminotransferase (AST), blood urea nitrogen (BUN) and creatinine, were measured and blood cell counts (white blood cells, platelets and red blood cells) were performed. Mouse organs were examined histologically at different time points.

RESULTS

Haematological toxicity was transient and non-limiting for all evaluated injected activities. At the highest injected activity (11.1 MBq), mice died from liver and kidney failure (median survival of 189 days). This liver toxicity was identified by an increase in both ALT and AST and by histological examination. Mice injected with 7.4 MBq of 213Bi-BSA (median survival of 324 days) had an increase in plasma BUN and creatinine due to impaired kidney function, confirmed by histological examination. Injection of 3.7 MBq of 213Bi-BSA was safe, with no plasma enzyme modifications or histological abnormalities.

CONCLUSION

Haematological toxicity was not limiting in this study. Liver failure was observed at the highest injected activity (11.1 MBq), consistent with liver damage observed in human clinical trials. Intermediate injected activity (7.4 MBq) should be used with caution because of the risk of long-term toxicity to kidneys.

LnPO4 nanoparticles doped with Ac-225 and sequestered daughters for targeted alpha therapy

For targeted alpha therapy (TAT) with 225Ac, daughter radioisotopes from the parent emissions should be controlled. Here, we report on a second-generation layered nanoparticle (NP) with improved daughter retention that can mediate TAT of lung tumor colonies. NPs of La3+, Gd3+, and 225Ac3+ ions were coated with additional layers of GdPO4 and then coated with gold via citrate reduction of NaAuCl4. MAb 201b, targeting thrombomodulin in lung endothelium, was added to a polyethylene glycol (dPEG)-COOH linker. The NPs:mAb ratio was quantified by labeling the mAb with 125I. NPs showed 30% injected dose/organ antibody-mediated uptake in the lung, which increased to 47% in mice pretreated with clodronate liposomes to reduce phagocytosis. Retention of daughter 213Bi in lung tissue was more than 70% at one hour and about 90% at 24 hours postinjection. Treatment of mice with lung-targeted 225Ac NP reduced EMT-6 lung colonies relative to cold antibody competition for targeting or phosphate-buffered saline injected controls. We conclude that LnPO4 NPs represent a viable solution to deliver the 225Ac as an in vivo α generator. The NPs successfully retain a large percentage of the daughter products without compromising the tumoricidal properties of the α-radiation.

Microdosimetry for targeted alpha therapy of cancer

Targeted alpha therapy (TAT) has the advantage of delivering therapeutic doses to individual cancer cells while reducing the dose to normal tissues. TAT applications relate to hematologic malignancies and now extend to solid tumors. Results from several clinical trials have shown efficacy with limited toxicity. However, the dosimetry for the labeled alpha particle is challenging because of the heterogeneous antigen expression among cancer cells and the nature of short-range, high-LET alpha radiation. This paper demonstrates that it is inappropriate to investigate the therapeutic efficacy of TAT by macrodosimetry. The objective of this work is to review the microdosimetry of TAT as a function of the cell geometry, source-target configuration, cell sensitivity, and biological factors. A detailed knowledge of each of these parameters is required for accurate microdosimetric calculations.

Targeted alpha therapy approach to the management of pancreatic cancer

Evidence for the efficacy of targeted alpha therapy for the control of pancreatic cancer in preclinical models is reviewed. Results are given for in vitro pancreatic cancer cells and clusters and micro-metastatic cancer lesions in vivo. Two complementary targeting vectors are examined. These are the C595 monoclonal antibody that targets the MUC1 antigen and the PAI2 ligand that targets the uPA receptor. The expression of the tumor-associated antigen MUC-1 and the uPA receptor on three pancreatic cancer cell lines is reported for cell clusters, human mouse xenografts and lymph node metastases, as well as for human pancreatic cancer tissues, using immuno-histochemistry, confocal microscopy and flow cytometry. The targeting vectors C595 and PAI2 were labeled with the alpha emitting radioisotope 213Bi using the chelators cDTPA and CHX-A″ to form the alpha-conjugates (AC). Cell clusters were incubated with the AC and examined at 48 hours. Apoptosis was documented using the TUNEL assay. In vivo, the anti-proliferative effect for tumors was tested at two days post-subcutaneous cell inoculation. Mice were injected with different concentrations of AC by local or systemic administration. Changes in tumor progression were assessed by tumor size. MUC-1 and uPA are strongly expressed on CFPAC-1, PANC-1 and moderate expression was found CAPAN-1 cell clusters and tumor xenografts. The ACs can target pancreatic cells and regress cell clusters (~100 µm diameter), causing apoptosis in some 70-90 % of cells. At two days post-cell inoculation in mice, a single local injection of 74 MBq/kg of AC causes complete inhibition of tumor growth. Systemic injections of 111, 222 and 333 MBq/kg of alpha-conjugate caused significant tumor growth delay in a dose dependent manner after 16 weeks, compared with the non-specific control at 333 MBq/kg. Cytotoxicity was assessed by the MTS and TUNEL assays. The C595 and PAI2-alpha conjugates are indicated for the treatment of micro-metastatic pancreatic cancer with over-expression of MUC1 and uPA receptors in post-surgical patients with minimal residual disease. The observation of tumor regression in a Phase I clinical trial of targeted alpha therapy for metastatic melanoma indicates that alpha therapy can regress tumors by a process called tumor anti-vascular alpha therapy (TAVAT). As a consequence, this therapy could be indicated for the management of non-surgical pancreatic cancer tumors.

Solid-tumor radionuclide therapy dosimetry: new paradigms in view of tumor microenvironment and angiogenesis

PURPOSE

The objective of this study is to evaluate requirements for radionuclide-based solid tumor therapy by assessing the radial dose distribution of beta-particle-emitting and alpha-particle-emitting molecules localized either solely within endothelial cells of tumor vasculature or diffusing from the vasculature throughout the adjacent viable tumor cells.

METHODS

Tumor blood vessels were modeled as a group of microcylindrical layers comprising endothelial cells (one-cell thick, 10 microm diameter), viable tumor cells (25-cell thick, 250 microm radius), and necrotic tumor region (> 250 microm from any blood vessel). Sources of radioactivity were assumed to distribute uniformly in either endothelial cells or in concentric cylindrical 10 microm shells within the viable tumor-cell region. The EGSnrc Monte Carlo simulation code system was used for beta particle dosimetry and a dose-point kernel method for alpha particle dosimetry. The radioactive decays required to deposit cytocidal doses (> or = 100 Gy) in the vascular endothelial cells (endothelial cell mean dose) or, alternatively, at the tumor edge [tumor-edge mean dose (TEMD)] of adjacent viable tumor cells were then determined for six beta (32P, 33P, 67Cu, 90Y 131I, and 1188Re) and two alpha (211At and 213Bi) particle emitters.

RESULTS

Contrary to previous modeling in targeted radionuclide therapy dosimetry of solid tumors, the present work restricts the region of tumor viability to 250 microm around tumor blood vessels for consistency with biological observations. For delivering > or = 100 Gy at the viable tumor edge (TEMD) rather than throughout a solid tumor, energetic beta emitters 90Y, 32P, and 188Re can be effective even when the radionuclide is confined to the blood vessel (i.e., no diffusion into the tumor). Furthermore, the increase in tumor-edge dose consequent to beta emitter diffusion is dependent on the energy of the emitted beta particles, being much greater for lower-energy emitters 131I, 67Cu, and 33P relative to higher-energy emitters 90Y, 32P, and 188Re. Compared to alpha particle emitters, a approximately 150-400 times higher number of beta-particle-emitting radioactive atoms is required to deposit the same dose in tumor neovasculature. However, for the alpha particle emitters 211At and 213Bi to be effective in irradiating viable tumor-cell regions in addition to the vasculature the carrier molecules must diffuse substantially from the vasculature into the viable tumor.

CONCLUSION

The presented data enable comparison of radionuclides used for antiangiogenic therapy on the basis of their radioactive decay properties, tumor neovasculature geometry, and tumor-cell viability. For alpha particle emitters or low-energy beta particle emitters, the targeting carrier molecule should be chosen to permit the radiopharmaceutical to diffuse from the endothelial wall of the blood vessel, while for long-range energetic beta particle emitters that target neovasculature, a radiopharmaceutical that binds to newly formed endothelial cells and does not diffuse is preferable. The work is a first approximation to modeling of tumor neovasculature that ignores factors such as pharmacokinetics and targeting capability of carrier molecules. The calculations quantify the interplay between irradiation of neovasculature, the surrounding viable tumor cells, and the physical properties of commonly used radionuclides and can be used to assist estimation of radioactivity to be administered for neovasculature-targeted tumor therapy.

Guidelines for the welfare and use of animals in cancer research

Animal experiments remain essential to understand the fundamental mechanisms underpinning malignancy and to discover improved methods to prevent, diagnose and treat cancer. Excellent standards of animal care are fully consistent with the conduct of high quality cancer research. Here we provide updated guidelines on the welfare and use of animals in cancer research. All experiments should incorporate the 3Rs: replacement, reduction and refinement. Focusing on animal welfare, we present recommendations on all aspects of cancer research, including: study design, statistics and pilot studies; choice of tumour models (e.g., genetically engineered, orthotopic and metastatic); therapy (including drugs and radiation); imaging (covering techniques, anaesthesia and restraint); humane endpoints (including tumour burden and site); and publication of best practice.

Treatment of solid tumors by interstitial release of recoiling short-lived alpha emitters

A new method utilizing alpha particles to treat solid tumors is presented. Tumors are treated with interstitial radioactive sources which continually release short-lived alpha emitting atoms from their surface. The atoms disperse inside the tumor, delivering a high dose through their alpha decays. We implement this scheme using thin wire sources impregnated with (224)Ra, which release by recoil (220)Rn, (216)Po and (212)Pb atoms. This work aims to demonstrate the feasibility of our method by measuring the activity patterns of the released radionuclides in experimental tumors. Sources carrying (224)Ra activities in the range 10-130 kBq were used in experiments on murine squamous cell carcinoma tumors. These included gamma spectroscopy of the dissected tumors and major organs, Fuji-plate autoradiography of histological tumor sections and tissue damage detection by Hematoxylin-Eosin staining. The measurements focused on (212)Pb and (212)Bi. The (220)Rn/(216)Po distribution was treated theoretically using a simple diffusion model. A simplified scheme was used to convert measured (212)Pb activities to absorbed dose estimates. Both physical and histological measurements confirmed the formation of a 5-7 mm diameter necrotic region receiving a therapeutic alpha-particle dose around the source. The necrotic regions shape closely corresponded to the measured activity patterns. (212)Pb was found to leave the tumor through the blood at a rate which decreased with tumor mass. Our results suggest that the proposed method, termed DART (diffusing alpha-emitters radiation therapy), may potentially be useful for the treatment of human patients.

Biodistribution of 225Ra citrate in mice: retention of daughter radioisotopes in bone

Alpha-particle-emitting radionuclides have potential for therapy of localized disease due to their high linear energy transformation and short pathlengths. Radiometals that home naturally to bone can be exploited for this purpose, and 223Ra (t(1/2)=11.4 days) recently has been studied for therapy of bone tumors in mice and rats. Actinium-225 (t(1/2)=10 days) is also an attractive radioisotope for endoradiotherapy. In a single decay of a 225Ac nucleus and its subsequent decay daughters, over 27 MeV ( approximately 90% of total energy) is released by sequential emission of four alpha particles, ranging in energy from 5.7 to 8.4 MeV. Although Ac3+ does not home naturally to bone, its parent radioisotope 225Ra (beta(-), t(1/2)=15 days) can be used as an in vivo source for 225Ac. Thus, injection of 225Ra takes advantage of the bone-homing properties of radium coupled with the significant amount of energy released from the 225Ac decay chain. Our data confirm that a large fraction of radium citrate injected intravenously into mice localizes rapidly in bone. Injected doses per gram (ID/g) for 225Ra range from 25% in skull to about 10% in sternum. Once deposited, the 225Ra remains in the bone with a biological half life of >40 days. Furthermore, >95% of the daughter radioisotope, 225Ac, is retained in the bone. However, a significant fraction of one of the daughter radioisotopes, 213Bi, is found in kidney. The biodistribution data indicate that 225Ra injection should be a powerful agent for killing cells associated with bone; however, the toxicity of this radioisotope which is similar to that of other alpha emitters limits the dose that can be tolerated.

Identification of an antilaminin-1 scFv that preferentially homes to vascular solid tumors

The tumor vasculature and extracellular matrix make attractive targets for distinguishing solid tumors from normal cells. In solid tumors, the processes of angiogenesis and metastasis potentially give rise to unique epitopes not usually accessible in homeostatic organs. Specific targeting of solid tumors for radioimmunotherapy requires that the targeting agent accumulate rapidly and at high levels at the tumor site. This study involved the selection of scFvs that recognize laminin-1 in vitro from the Tomlinson I and J phage display libraries. Selected, purified scFvs were radioiodinated and injected in tumor-bearing mice. One of these, scFv 15-9, exhibited preferential accumulation at subcutaneous tumors when compared to other antilaminin scFvs or to a control scFv. Autoradiographic analysis indicated that scFv15- 9 also displayed a higher vessel:parenchyma ratio than did two other antilaminin scFvs, scFv 15-6 and scFv 15-1, indicating a preferential accumulation of scFv 15-9 around vessel structures. Immunohistochemistry confirmed that scFv 15-9 accumulated at sites of endothelial cells lining vessel structures where significant levels of laminin were present. These data demonstrate that scFv 15-9 binds to a specific epitope on laminin and has potential for tumor endoradiotherapy in subcutaneous tumors.

Mechanisms of cell sensitization to alpha radioimmunotherapy by doxorubicin or paclitaxel in multiple myeloma cell lines

PURPOSE

The purpose of this study was to analyze different mechanisms (cell cycle synchronization, DNA damage, and apoptosis) that might underlie potential synergy between chemotherapy (paclitaxel or doxorubicin) and radioimmunotherapy with alpha radionuclides.

EXPERIMENTAL DESIGN

Three multiple myeloma cell lines (LP1, RMI 8226, and U266) were treated with 213Bi-radiolabeled B-B4, a monoclonal antibody that recognizes syndecan-1 (CD138) 24 hours after paclitaxel (1 nmol/L) or doxorubicin (10 nmol/L) treatment. Cell survival was assessed using a clonogenic survival assay. Cell cycle modifications were assessed by propidium iodide staining and DNA strand breaks by the comet assay. Level of apoptosis was determined by Apo 2.7 staining.

RESULTS

Radiation enhancement ratio showed that paclitaxel and doxorubicin were synergistic with alpha radioimmunotherapy. After a 24-hour incubation, paclitaxel and doxorubicin arrested all cell lines in the G2-M phase of the cell cycle. Doxorubicin combined with alpha radioimmunotherapy increased tail DNA in the RPMI 8226 cell line but not the LP1 or U266 cell lines compared with doxorubicin alone or alpha radioimmunotherapy alone. Neither doxorubicin nor paclitaxel combined with alpha radioimmunotherapy increased the level of apoptosis induced by either drug alone or alpha radioimmunotherapy alone.

CONCLUSION

Both cell cycle arrest in the G2-M phase and an increase in DNA double-strand breaks could lead to radiosensitization of cells by doxorubicin or paclitaxel, but apoptosis would not be involved in radiosensitization mechanisms.

Pre-clinical study of 213Bi labeled PAI2 for the control of micrometastatic pancreatic cancer

PURPOSE

The urokinase plasminogen activator (uPA) and its receptor (uPAR) are expressed by pancreatic cancer cells and can be targeted by the plasminogen activator inhibitor type 2 (PAI2). We have labeled PAI2 with (213)Bi to form the alpha conjugate (AC), and have studied its in vitro cytotoxicity and in vivo efficacy.

METHODS AND MATERIALS

The expression of uPA/uPAR on pancreatic cell lines, human pancreatic cancer tissues, lymph node metastases, and mouse xenografts were detected by immunohistochemistry, confocal microscopy, and flow cytometry. Cytotoxicity was assessed by the MTS and TUNEL assay. At 2 days post-cancer cell subcutaneous inoculation, mice were injected with AC by local or systemic injection.

RESULTS

uPA/uPAR is strongly expressed on pancreatic cancer cell lines and cancer tissues. The AC can target and kill cancer cells in vitro in a concentration-dependent fashion. Some 90% of TUNEL positive cells were found after incubation with 1.2 MBq/ml of AC. A single local injection of approximately 222 MBq/kg 2 days post-cell inoculation can completely inhibit tumor growth over 12 weeks, and an intraperitoneal injection of 111 MBq/kg causes significant tumor growth delay.

CONCLUSIONS

(213)Bi-PAI2 can specifically target pancreatic cancer cells in vitro and inhibit tumor growth in vivo. (213)Bi-PAI2 may be a useful agent for the treatment of post-surgical pancreatic cancer patients with minimum residual disease.

Targeted delivery of oligodeoxynucleotides to mouse lung endothelial cells in vitro and in vivo

Pulmonary endothelium plays an important role in the maintenance of normal pulmonary physiology and its dysfunction is involved in a number of pulmonary diseases. Correction of endothelial dysfunction via antisense oligodeoxynucleotides (ODN) is dependent on the development of a delivery vehicle that can efficiently deliver the ODN to pulmonary endothelium with minimal toxicity. To this end, we have developed a novel lipidic vector that is highly efficient in targeted delivery of ODN to pulmonary endothelium. This is based on a method that utilizes an ionizable aminolipid (1,2-dioleoyl-3-dimethylammonium propane) and an ethanol-containing buffer system for encapsulating large quantities of polyanionic ODN in lipid vesicles. An endothelium-specific antibody (273-34A) is incorporated into the lipid vesicles via a distearoylphosphatidylethanolamine-poly(ethylene glycol) spacer. The 273-34A antibody efficiently mediated delivery of ODN to mouse lung endothelial cells in vitro and in vivo. Furthermore, systemic administration of this formulation is associated with minimal hematological toxicities and induces little acute change in systemic and pulmonary hemodynamics. These results provide a basis for lipid-mediated delivery of ODN for the treatment of pulmonary diseases. They also suggest the utility of this approach as a research tool to characterize the function of genes in the pulmonary endothelium.

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.

Phage display selection of scFv to murine endothelial cell membranes

The diversity of endothelial cells is becoming more apparent and more important in defining vessel systems that supply blood to normal organs and to tumors. Reagents that identify expression of cell surface determinants on these cells are crucial for differentiating among different vessel types. As a first step in this process we have selected a panel of 25 scFvs from a phage display library that bind to the endothelial cell line LEII. The scFvs are of high affinity and bind to some tumor cells as well as to the target endothelial cell. The scFvs can be divided into 8 epitope groups by use of competition binding studies. DNA sequencing of the members of these groups generally support the classification. This work shows that phage display is a rapid and efficient method for identification of reagents for cell surface molecules.

MUC1 expression in primary and metastatic pancreatic cancer cells for in vitro treatment by (213)Bi-C595 radioimmunoconjugate

Control of micrometastatic pancreatic cancer remains a major objective in pancreatic cancer treatment. The overexpression of MUC1 mucin plays an important role in cancer metastasis. The aim of this study was to detect the expression of MUC1 in human primary tumour tissues and three pancreatic cancer cell lines (CAPAN-1, CFPAC-1 and PANC-1), and target MUC1-positive cancer cells in vitro using ²¹³Bi-C595 alpha-immunoconjugate (AIC). The expression of MUC1 on pancreatic tumour tissues and cancer cell lines was performed by immunohistochemistry and further confirmed by confocal microscope and flow cytometry analysis on the cell surface. Cytotoxicity of ²¹³Bi-C595 was tested by MTS assay. Apoptosis was documented using TUNEL assay. Overexpression of MUC1 was found in ∼90% of tested tumour samples and the three pancreatic cancer cell lines. ²¹³Bi-C595 is specifically cytotoxic to pancreatic cancer cells in a concentration-dependent fashion. These results suggest that overexpression of MUC1 in pancreatic cancer is a useful target, and that the novel ²¹³Bi-C595 AIC selectively targets pancreatic cancer cells in vitro. ²¹³Bi-C595 may be a useful agent for the treatment of micrometastases or minimal residual disease (MRD) in pancreatic cancer patients with overexpression of MUC1 antigen.

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.

Production of actinium-225 for alpha particle mediated radioimmunotherapy

The initial clinical trials for treatment of acute myeloid leukemia have demonstrated the effectiveness of the alpha emitter (213)Bi in killing cancer cells. Bismuth-213 is obtained from a radionuclide generator system from decay of 10-days (225)Ac parent. Recent pre-clinical studies have also shown the potential application of both (213)Bi, and the (225)Ac parent radionuclide in a variety of cancer systems and targeted radiotherapy. This paper describes our five years of experience in production of (225)Ac in partial support of the on-going clinical trials. A four-step chemical process, consisting of both anion and cation exchange chromatography, is utilized for routine separation of carrier-free (225)Ac from a mixture of (228)Th, (229)Th and (232)Th. The separation of Ra and Ac from Th is achieved using the marcoporous anion exchange resin MP1 in 8M HNO(3) media. Two sequential MP1/NO(3) columns provide a separation factor of approximately 10(6) for Ra and Ac from Th. The separation of Ac from Ra is accomplished on a low cross-linking cation exchange resin AG50-X4 using 1.2M HNO(3) as eluant. Two sequential AG50/NO(3) columns provide a separation factor of approximately 10(2) for Ac from Ra. A 60-day processing schedule has been adopted in order to reduce the processing cost and to provide the highest levels of (225)Ac possible. Over an 8-week campaign, a total of approximately 100 mCi of (225)Ac (approximately 80% of the theoretical yield) is shipped in 5-6 batches, with the first batch typically consisting of approximately 50 mCi. After the initial separation and purification of Ac, the Ra pool is re-processed on a bi-weekly schedule or as needed to provide smaller batches of (225)Ac. The averaged radioisotopic purity of the (225)Ac was 99.6 +/- 0.7% with a (225)Ra content of < or =0.6%, and an average (229)Th content of (4(-4)(+5)) x 10(-5)%.

Pretargeted alpha emitting radioimmunotherapy using (213)Bi 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid-biotin

PURPOSE

The use of an alpha emitter for radioimmunotherapy has potential advantages compared with beta emitters. When administered systemically optimal targeting of intact antibodies requires >24 h, therefore limiting the use of short-lived alpha emitters. This study investigated the biodistribution of bismuth-labeled biotin in A431 tumor-bearing mice pretargeted with antibody B3-streptavidin (B3-SA) and examined the therapeutic efficacy of the alpha emitter, (213)Bi-labeled biotin.

EXPERIMENTAL DESIGN

Biotinidase-resistant 7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA)-biotin was radiolabeled with (205,206)Bi or (213)Bi. Treatment of tumor-bearing mice began by administration of B3-SA (400 micro g) to target the tumor sites for 24 h. Then, an agent containing biotin and galactose groups was used to clear the conjugate from the circulation. Four h later, bismuth-radiolabeled DOTA-biotin was given, and biodistribution or therapy was evaluated. Dose escalation treatment from 3.7-74 MBq was performed, and the effects on tumors of different sizes were investigated. Tumor growth, complete blood cell counts, toxicity, and survival were monitored.

RESULTS

Radiolabeled biotin cleared rapidly. Rapid tumor uptake resulted in much higher tumor:nontumor targeting ratios than achieved with the directly labeled monoclonal antibody. Dose escalation revealed that 74 MBq caused acute death of mice, whereas 0.37-37 MBq doses inhibited tumor growth and prolonged survival significantly. Evidence of mild hematological toxicity was noted. At therapeutically effective doses renal toxicity was observed.

CONCLUSIONS

(213)Bi-DOTA-biotin, directed by the Pretarget method to tumor-targeted B3-SA, showed a therapeutic effect, although the therapeutic index was low. The source of the toxicity was most likely related to the renal toxicity.

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.

In vitro and preclinical targeted alpha therapy of human prostate cancer with Bi-213 labeled J591 antibody against the prostate specific membrane antigen

Limited options for the treatment of prostate cancer have spurred the search for new therapies. One innovative approach is the use of targeted alpha therapy (TAT) to inhibit cancer growth, using an alpha particle emitting radioisotope such as (213)Bi. Because of its short range and high linear energy transfer (LET), alpha-particles may be particularly effective in the treatment of cancer, especially in inhibiting the development of metastatic tumors from micro-metastases. Prostate-specific membrane antigen (PSMA) is expressed in prostate cancer cells and the neovasculature of a wide variety of malignant neoplasms including lung, colon, breast and others, but not in normal vascular endothelium. The expression is further increased in higher-grade cancers, metastatic disease and hormone-refractory prostate cancer (PCA). J591 is one of several monoclonal antibodies (mabs) to the extracellular domain of PSMA. Chelation of J591 mab with (213)Bi forms the alpha-radioimmunoconjugate (AIC). The objective of this preclinical study was to design an injectable AIC to treat human prostate tumors growing subcutaneously in mice. The anti-proliferative effects of AIC against prostate cancer were tested in vitro using the MTS assay and in vivo with the nude mice model. Apoptosis was documented using terminal deoxynucleotidyl transferase [TdT]-mediated deoxyuridinetriphosphate [dUTP] nick end-labeling (TUNEL) assay, while proliferative index was assessed using the Ki-67 marker. We show that a very high density of PSMA is expressed in an androgen-dependent human PCA cell line (LNCaP-LN3) and in tumor xenografts from nude mice. We also demonstrate that the AIC extensively inhibits the growth of LN3 cells in vitro in a concentration-dependent fashion, causing the cells to undergo apoptosis. Our in vivo studies showed that a local AIC injection of 50 microCi at 2 days post-cell inoculation gave complete inhibition of tumor growth, whereas results for a non-specific AIC were similar to those for untreated mice. Further, after 1 and 3 weeks post-tumor appearance, a single (100 microCi/100 microl) intra-lesional injection of AIC can inhibit the growth of LN3 tumor xenografts (volume<100 mm(3)) in nude mice. Tumors treated with AIC decreased in volume from a mean 46+/-14 mm(3) in the first week or 71+/-15 mm(3) in the third week to non-palpable, while in control mice treated with a non-specific AIC using the same dose, tumor volume increased from 42 to 590 mm(3). There were no observed side effects of the treatment. Because of its in vitro cytotoxicity and these anti-proliferative properties in vivo, the (213)Bi-J591 conjugate has considerable potential as a new therapeutic agent for the treatment of prostate cancer.

Cytotoxicity of human prostate cancer cell lines in vitro and induction of apoptosis using 213Bi-Herceptin α-conjugate

HER-2 has been implicated in the oncogenesis of human prostate cancer (CaP) and is the target of a new treatment for metastatic breast cancer using the humanised monoclonal antibody (MAb) trastuzumab (Herceptin). In this study, a novel alpha-particle emitting [213Bi]Herceptin construct, which targets the HER-2 extracellular domain on CaP cells, was prepared and evaluated in vitro. We used immunocytochemistry, flow cytometry and Western blot analysis to examine the expression of HER-2 in a panel of established human CaP cell lines, used the MTS assay to evaluate the cytotoxicity of 213Bi-Herceptin on these cell lines and the TUNEL assay to document apoptosis. The results indicate that LNCaP-LN3 cells express high levels of HER-2 protein, in contrast, DU 145 cells express low to medium levels, and PC-3 cells express an undetectable level of HER-2 protein. 213Bi-Herceptin was specifically cytotoxic to LNCaP-LN3 cells in a concentration-dependent fashion, cause the cells to undergo apoptosis, whereas DU 145 showed an HER-2 level-dependent response to 213Bi-Herceptin cytotoxicity. In contrast, PC-3 cells were resistant to 213Bi-Herceptin-induced cytotoxicity. The 213Bi-Herceptin induced apoptosis in LNCaP-LN3 cells could be inhibited by incubation with unlabeled Herceptin. Our results suggest that 213Bi-Herceptin alpha-conjugate might be a promising new agent for the treatment of preangiogenic cancer cell clusters or micro-metastases with high levels of HER-2 expression.

The therapeutic potential of novel antiangiogenic therapies

Promising new antiangiogenic strategies are emerging for the treatment of cancer. Numerous candidate drugs that target vascular endothelial growth factor, vascular endothelial growth factor receptors, integrins, matrix metalloproteinases and other blood vessel targets are being developed and tested in clinical trials. This review highlights the numerous drugs in clinical trials and expands on potential new approaches to inhibiting angiogenesis. These approaches include gene therapy, vaccine strategies and antiangiogenic radioligands. New insight has been gained from completed Phase III trials with antiangiogenic drugs and some of the major obstacles include design of trials, dosing, toxicities and resistance. This review will discuss these barriers and methods by which they can be overcome.

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.

Mechanisms and future directions for angiogenesis-based cancer therapies

Targeting angiogenesis represents a new strategy for the development of anticancer therapies. New targets derived from proliferating endothelial cells may be useful in developing anticancer drugs that prolong or stabilize the progression of tumors with minimal systemic toxicities. These drugs may also be used as novel imaging and radiommunotherapeutic agents in cancer therapy. In this review, the mechanisms and control of angiogenesis are discussed. Genetic and proteomic approaches to defining new potential targets on tumor vasculature are then summarized, followed by discussion of possible antiangiogenic treatments that may be derived from these targets and current clinical trials. Such strategies involve the use of endogenous antiangiogenic agents, chemotherapy, gene therapy, antiangiogenic radioligands, immunotherapy, and endothelial cell-based therapies. The potential biologic end points, toxicities, and resistance mechanisms to antiangiogenic agents must be considered as these therapies enter clinical trials.

Targeted α particle immunotherapy for myeloid leukemia

Unlike β particle–emitting isotopes, α emitters can selectively kill individual cancer cells with a single atomic decay. HuM195, a humanized anti-CD33 monoclonal antibody, specifically targets myeloid leukemia cells and has activity against minimal disease. When labeled with the β-emitters 131I and 90Y, HuM195 can eliminate large leukemic burdens in patients, but it produces prolonged myelosuppression requiring hematopoietic stem cell transplantation at high doses. To enhance the potency of native HuM195 yet avoid the nonspecific cytotoxicity of β-emitting constructs, the α-emitting isotope 213Bi was conjugated to HuM195. Eighteen patients with relapsed and refractory acute myelogenous leukemia or chronic myelomonocytic leukemia were treated with 10.36 to 37.0 MBq/kg 213Bi-HuM195. No significant extramedullary toxicity was seen. All 17 evaluable patients developed myelosuppression, with a median time to recovery of 22 days. Nearly all the 213Bi-HuM195 rapidly localized to and was retained in areas of leukemic involvement, including the bone marrow, liver, and spleen. Absorbed dose ratios between these sites and the whole body were 1000-fold greater than those seen with β-emitting constructs in this antigen system and patient population. Fourteen (93%) of 15 evaluable patients had reductions in circulating blasts, and 14 (78%) of 18 patients had reductions in the percentage of bone marrow blasts. This study demonstrates the safety, feasibility, and antileukemic effects of 213Bi-HuM195, and it is the first proof-of-concept for systemic targeted α particle immunotherapy in humans.

Therapy of rat tracheal carcinoma IC-12 in SCID mice: vascular targeting with [213Bi]-MAb TES-23

In previous work, we have demonstrated that vascular targeting of [213Bi], an alpha-emitter, to lung blood vessels could efficiently destroy tumour colonies growing in the lung. In order to expand this approach to treatment of tumours growing in other sites, we employed the monoclonal antibody (MAb) TES-23, which reacts with CD44H, preferentially expressed on new blood vessels in tumours. Biodistribution studies of N-succinimidyl [125I] 3-iodobenzoate (SIB)-radiolabelled MAb TES-23 in ICR-severe combined immunodeficient (SCID) mice bearing subcutaneous (s.c.) and intramuscular (i.m.) IC-12 tumours, demonstrated efficient tumour uptake. At 24 h, accumulation in small tumours was 45%ID/g for s.c. tumours, and 58%ID/g for i.m. tumours and in large tumours it was 25%ID/g for s.c. tumours and 17%ID/g for i.m. tumours. Micro-autoradiography data confirmed that radiolabel accumulated in or near tumour blood vessels. Normal tissues had very low levels of radioactivity. Treatment of mice bearing small IC-12 tumours with [213Bi] MAb TES-23 retarded tumour growth relative to animals treated with cold MAb TES-23. Biodistribution and therapy experiments were also performed in BALB/c mice bearing both s.c. and i.m. syngeneic, lung carcinoma (line 498) tumours. [I(125)] SIB MAb TES-23 accumulated efficiently in both s.c. and i.m. tumours (14%ID/g and 15%ID/g, respectively, at 4 h); however, no therapeutic effect of [213Bi] MAb TES-23 treatment could be demonstrated in this model system. The data demonstrate that the timing of vascularisation of the tumours and the delivery kinetics of MAb relative to the half-life of the therapeutic radionuclide are critical for effective therapy.

Vascular-targeted radioimmunotherapy with the alpha-particle emitter 211At

Astatine-211, an alpha-particle emitter, was employed in a model system for vascular-targeted radioimmunotherapy of small tumors in mouse lung to compare its performance relative to other radioisotopes in the same system. Astatine-211 was coupled to the lung blood vessel-targeting monoclonal antibody 201B with N-succinimidyl N-(4-[211At]astatophenethyl) succinamate linker. Biodistribution data showed that the conjugate delivered 211At to the lung (260-418% ID/g), where it remained with a biological half-time of about 30 h. BALB/c mice bearing about 100 lung tumor colonies of EMT-6 cells, each about 2000 cells in size, were treated with 211At-labeled monoclonal antibody 201B. The administered activity of 185 kBq per animal extended the life span of treated mice over untreated controls. Injections of 370 kBq, corresponding to an absorbed dose of 25-40 Gy, were necessary to eradicate all of the lung tumors. Mice receiving 740 kBq of 211At-labeled monoclonal antibody 201B developed pulmonary fibrosis 3-4 months after treatment, as did mice treated with 3700 kBq of the alpha-particle emitter 213Bi-labeled monoclonal antibody 201B in previous work. Animals that were injected with 211At bound to untargeted IgG or to glycine, as control agents, also demonstrated therapeutic effects relative to untreated controls. Control groups that received untargeted 211At required about twice as much administered activity for effective therapy as did groups with lung-targeted radioisotope. These results were not consistent with radioisotope biodistribution and dosimetry calculations that indicated that lung-targeted 211At should be at least 10-fold more efficient for lung colony therapy than 211At bound to nontargeting controls. The data showed that 211At is useful for vascular-targeted radioimmunotherapy because lung tumor colonies were eradicated in the mice. Work in this model system demonstrates that vascular targeting of alpha-particle emitters is an efficient therapy for small perivascular tumors and may be applicable to human disease when specific targeting agents are identified.

213Bi-PAI2 conjugate selectively induces apoptosis in PC3 metastatic prostate cancer cell line and shows anti-cancer activity in a xenograft animal model

A novel alpha-particle emitting ((213)Bi) plasminogen activator inhibitor type 2 construct, which targets the membrane-bound urokinase plasminogen activator on prostate cancer cells, was prepared and evaluated in vitro and in a xenograft animal model. The PC3 prostate cancer cell line expresses urokinase plasminogen activator which binds to its receptor on the cell membrane; plasminogen activator inhibitor type 2 is bound to urokinase plasminogen activator/urokinase plasminogen activator receptor to form stable complexes. In vitro, the cytotoxicity of (213)Bi-plasminogen activator inhibitor type 2 against prostate cancer cells was tested using the MTS assay and apoptosis was documented using terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labelling (TUNEL) assay. In vivo, antiproliferative effects for tumours and prostate cancer lymph node metastasis were carried out in an athymic nude mouse model with a subcutaneous xenograft of PC3 cells. (213)Bi-plasminogen activator inhibitor type 2 was specifically cytotoxic to PC3 cells in a concentration-dependent fashion, causing the cells to undergo apoptosis. A single local or i.p. injection of (213)Bi-plasminogen activator inhibitor type 2 was able to completely regress the growth of tumours and lymph node metastases 2 days post subcutaneous inoculation, and obvious tumour regression was achieved in the therapy groups compared with control groups with (213)Bi-plasminogen activator inhibitor type 2 when the tumours measured 30-40 mm(3) and 85-100 mm(3). All control animals and one of five (20%) mice treated with 3 mCi kg(-1) (213)Bi-plasminogen activator inhibitor type 2 developed metastases in the lymph nodes while no lymphatic spread of cancer was found in the 6 mCi kg(-1) treated groups at 2 days and 2 weeks post-cell inoculation. These results demonstrate that this novel (213)Bi-plasminogen activator inhibitor type 2 conjugate selectively targets prostate cancer in vitro and in vivo, and could be considered for further development for the therapy of prostate cancer, especially for the control of micro-metastases or in minimal residual disease.

Actinium-225 conjugates of MAb CC49 and humanized delta CH2CC49

Radioisotopes with moderate half-lives are essential for conventional radioimmunotherapy using tumor-selective MAbs which require days for localization. Actinium-225, with a half-life of 10 days and a yield of 4 alpha particles in its decay chain, may be an ideal choice for tumor-targeted radioimmunotherapy. Release of daughter radioisotopes from the primary chelator after the first decay has been a complication with the use of 225Ac. It has been reported that the domain-deleted product of MAb CC49, Hu-delta CH2 CC49, is able to extravasate and penetrate more deeply into tumors than the parent IgG molecule. We reasoned that once the 225Ac-chelate-MAb had penetrated into the tumor, the daughter radioisotopes would remain trapped even if they had been released from the primary chelator. Actinium-225 HEHA MAb CC49 conjugates were tested for distribution, micro-distribution and therapy in immunocompromised mice which had LS174T tumors growing at subcutaneous or intramuscular sites. Both 125I and 225Ac CC49 and Hu-delta CH2 CC49 were efficient in delivery of the radioisotopes to tumor sites. Tissue micro-autoradiography for the two antibody forms did not demonstrate any differences in micro-distribution of either 125I or 225Ac in the tumor. Furthermore, there was no detectable difference for the two carriers in the tumor retention of daughter radioisotopes from 225Ac. Therapy experiments with 225Ac were complicated by radiotoxicity of the conjugates. The lethal dose was about 0.5 microCi in two strains of mice regardless of the carrier. At injected doses of 0.5 and 0.25 microCi, CC49 was slightly active in tumor stasis, whereas no consistent significant effect of 225Ac-Hu-delta CH2 CC49 on growth of tumors was observed. The potential of 225Ac in radioimmunotherapy is limited by the radiotoxicity of its daughter radioisotopes. Its potential will only be realized if stable conjugates, capable of daughter radioisotope retention, can be devised.

The development of immunotherapies for non-small cell lung cancer

Standard of care for non-small cell lung cancer (NSCLC) (surgery, chemotherapy and radiation) may enhance patient survival but the enhancement is typically transient and quite uncommon with advanced disease. Researchers and medical professionals are using new approaches to improve patient mortality and morbidity. One of these approaches, immunotherapy, seeks to stimulate antitumour immunity above a threshold level needed for tumour regression or to induce stability in the face of progression. Among the most established approaches are vaccines involving monoclonal antibodies (mAbs) or immune effector cells. These approaches stimulate the humoral and cell-mediated arms of the immune system, respectively. As the development of humanised or fully human antibodies has spurred exploration of radioimmunoconjugates and immunotoxins, mAbs have enjoyed a revival of sorts. Cell-based therapies using the tumour cell itself as a vaccine component has resulted in disease stabilisation or regression. In addition, immune cells (e.g., T-lymphocytes and dendritic cells [DCs]) are the focal point of numerous patient trials in which meaningful clinical impact was achieved. In general, there are many tactics under development for the treatment of NSCLC. This review primarily concerns immunotherapeutic cancer treatments that are either already in clinical trial or well progressed into preclinical studies.

Radioimmunotherapy for acute leukemia

BACKGROUND

The use of monoclonal antibodies to deliver radioactive isotopes directly to tumor cells has become a promising strategy to enhance the antitumor effects of native monoclonal antibodies. In this article, we summarize the role of radioimmunotherapy in the treatment of leukemia.

METHODS

The authors reviewed the published clinical trials of radioimmunotherapy in acute leukemia.

RESULTS

Radioimmunoconjugates that emit beta-particles, such as 131I-anti-CD33, 90Y-anti-CD33, 131I-anti-CD45, and 188Re-anti-CD66c, deliver significant doses of radiation to the bone marrow and may be particularly effective when used as part of a conditioning regimen for hematopoietic stem cell transplantation. Radioimmunoconjugates that emit short-ranged alpha-particles, such as 213Bi-anti-CD33, are better suited for the treatment of low-volume or residual disease.

CONCLUSIONS

Radiolabeled antibodies can be administered safely to patients with advanced leukemias and have significant antileukemic activity. Radiolabeled antibodies can potentially intensify the antileukemic effects of conditioning regimens when used in conjunction with hematopoietic stem cell transplantation. Whether or not radiolabeled antibodies improve the outcome of patients with leukemia remains to be demonstrated by randomized studies.

In vitro cytotoxicity of bismuth-213 (213Bi)-labeled-plasminogen activator inhibitor type 2 (alpha-PAI-2) on human breast cancer cells

Metastasis is the principal cause of death in breast cancer patients. New and improved treatments for eradicating micrometastases are needed. To this end, a novel alpha-emitting protein construct, 213Bi-labelled plasminogen activator inhibitor type-2 (PAI-2) (alpha-PAI-2), was evaluated in vitro. This construct exploits: (a) the overexpression of the cell-surface receptor bound urokinase plasminogen activator (uPA) in the metastatic spread of breast cancer cells; (b) the binding and inhibition of receptor-bound uPA by PAI-2; and (c) the high cytotoxicity of alpha radiation. High labeling efficiencies and stability of 213Bi bound to human recombinant PAI-2 conjugated with cyclic diethylenetriaminepentaacetic acid anhydride were achieved (greater than 90%). The uPA inhibitory activity of the chelated PAI-2 was maintained as determined by complex formation with uPA and by inhibition of uPA activity. Furthermore, the reactivity of alpha-PAI-2 was confirmed in a cell assay as this construct was highly cytotoxic to breast cancer cell lines that express active, receptor bound uPA. The specificity of alpha-PAI-2 targeting was shown using several controls. Firstly, an active uPA blocking agent that limits PAI-2 binding significantly improved cell survival by a factor greater than three. Secondly, a non-specific alpha-BSA construct had minimal cytotoxic effect. Moreover, alpha-PAI-2 was not cytotoxic to freshly isolated normal human leukocytes, confirming that cells which do not contain active, receptor bound uPA cannot be targeted by alpha-PAI-2. In conclusion, we have validated, in vitro, the potential of alpha-PAI-2 as a novel therapeutic agent for breast cancer.

Breakthrough of 225Ac and its radionuclide daughters from an 225Ac/213Bi generator: development of new methods, quantitative characterization, and implications for clinical use

Bisumth-213, a short-lived alpha particle emitting radionuclide, is generated from the decay of 225Ac, which has a half-life of 10 days. The development of a clinical 225Ac/213Bi generator and the preparation of a 213Bi radiolabeled antibody for radioimmunotherapy of leukemia have been reported. The 225Ac decay scheme is complex; therefore a thorough understanding of the impact of both the parent 225Ac and its daughters on radiolabeling, purification, and quantification is necessary for optimal use of the generator system. This paper reports: (i) unique new methods to measure 221Fr, 213Bi, and 209Pb, the prominent daughters of 225Ac; and (ii) a quantitative evaluation of 225Ac/213Bi generator breakthrough and the radionuclidic purity of 213Bi labeled radiopharmaceutical dose formulations. A quantitative multi-dimensional proportional scanning method was employed to distinguish and measure specific daughter radionuclides. This method combines thin layer chromatography in two perpendicular directions with attenuated collimation as a function of time for data collection and analysis. Francium-221 and 213Bi eluted differentially from the generator, and 221Fr contributed minimally to unchelated 213Bi in the reaction and final products. Lead-209 was present in the reaction solution, but not strongly bound by the chelating moiety either (i) under the 213Bi labeling reaction conditions or (ii) following chelated 213Bi decay. As a consequence of incorporating several new procedures to the operation of the generator, 225Ac breakthrough in the final product was further reduced and represented a trivial contaminant in the final drug formulations.

Rapid preparation of short-lived alpha particle emitting radioimmunopharmaceuticals

Alpha emitting radionuclides are of considerable interest for targeted radioimmunotherapy. Generator supplied 213Bi emitting 8.5 MeV alpha particles with a 45.6 min half-life has been conjugated to a monoclonal antibody (HuM195-CHX-A-DTPA) for targeted therapy of leukemia in a clinical trial. The clinical dose preparation of pharmaceutical formulation by a pair of skilled radiochemists took 25 min, which corresponds, to an overall decay loss of 30% of the initial 213Bi activity eluted from the generator. In order to allow more widespread and practical clinical use of targeted 213Bi alpha particle therapy, we developed a new procedure that is simpler, more rapid and adaptable to a hospital pharmacy. The new 10 min process includes a tandem elution and labeling, and an anion exchange column purification method that can be reproducibly used.

Labeling and distribution of linear peptides identified using in vivo phage display selection for tumors

To develop targeting molecules to be used for vascular targeting of short half-lived alpha-emitters for radioimmunotherapy, linear peptide phage display libraries were selected in vivo for binding to IC-12 rat tracheal tumors growing in severe combined immune deficient mice. After three rounds of selection, 15 phage clones were analyzed for DNA sequence, and the deduced translation products of cDNA inserts were compared. Three consensus sequences were chosen from three separate experimental selection series and peptides of these sequences with added -gly-gly-tyr were obtained. Peptides were radiolabeled on tyrosine with (125)I and the biodistribution in tumor-bearing mice was determined. The radioiodinated peptides were stable in vitro and when injected in tumor-bearing mice approximately 3.0 %ID/g accumulated in the tumor; however, much of the (125)I was found in the gastrointestinal tract and thyroid, indicative of dehalogenation of the labeled peptide. Radiolabeling peptide 2 with N-succinimidyl-3-(125)I-iodobenzoate resulted in faster excretion, which in turn resulted in lower levels in tumor and other organs, especially thyroid and gastrointestinal tract. Peptide 2 was derivatized with the bifunctional isothiocyanates of cyclohexyl-B diethylenetriaminepentaacetic acid (DTPA) or CHX-A" DTPA by direct conjugation or with a hydroxylamine derivative of 1B4M-DTPA (2-(p-[O-(carboxamylmethyl)hydroxylamine]benzyl)-6-methyl-diethylenetriamine-N,N,N',N",N"-pentaacetic acid ) coupled at the N-terminus. The primary molecular species in the conjugated products were shown by mass spectrometry to have one DTPA per peptide. Peptide chelate conjugates were radiolabeled with (213)Bi and the products tested for biodistribution in tumor-bearing mice. The data show that chelation of (213)Bi to peptides was accomplished by both the direct method of DTPA attachment and by the method using the linker at the N-terminus. Only small amounts of peptide accumulated at tumor sites. We conclude that phage display is a powerful tool to select peptides with restricted binding specificity; however, the peptides isolated to date do not bind with high retention to tumor sites in vivo.

Evaluation of 225Ac for vascular targeted radioimmunotherapy of lung tumors

Several alpha particle emitting radioisotopes have been studied for use in radioimmunotherapy. Ac-225 has the potential advantages of a relatively long half life of 10 days, and a yield of 4 alpha emissions in its decay chain with a total energy release of approximately 28 MeV. A new, 12 coordination site chelating ligand, HEHA, has been chemically modified for coupling to targeting proteins without loss of chelating ability. HEHA was coupled with MAb 201B which binds to thrombomodulin and accumulates efficiently in murine lung. Ac-225 was bound to the HEHA-MAb 201B conjugate and injected into BALB/c mice bearing lung tumor colonies of EMT-6 mammary carcinoma. Biodistribution data at 1 and 4 h postinjection indicated that, as expected, 225Ac was delivered to lung efficiently (> 300% ID/g). The 225Ac was slowly released from the lung with an initial t1/2 = 49 h, and the released 225Ac accumulated in the liver. Injection of free HEHA was only partially successful in scavenging free 225Ac. In addition to the slow release of 225Ac from the chelate, data indicated that decay daughters of 225Ac were also released from the lung. Immediately after organ harvest, the level of 213Bi, the third alpha-decay daughter, was found to be deficient in the lungs and to be in excess in the kidney, relative to equilibrium values. Injected doses of 225Ac MAb 201B of 1.0 microCi, delivering a minimum calculated absorbed dose of about 6 Gy to the lungs, was effective in killing lung tumors, but also proved acutely radiotoxic. Animals treated with 1.0 microCi or more of the 225Ac radioconjugate died of a wasting syndrome within days with a dose dependent relationship. We conclude that the potential for 225Ac as a radioimmunotherapeutic agent is compromised not only by the slow release of 225Ac from the HEHA chelator, but most importantly by the radiotoxicity associated with decay daughter radioisotopes released from the target organ.

High resolution computed tomography and MRI for monitoring lung tumor growth in mice undergoing radioimmunotherapy: correlation with histology

A model lung tumor system has been developed in mice for the evaluation of vascular targeted radioimmunotherapy. In this model, EMT-6 mammary carcinoma tumors growing in the lung are treated with 213Bi, an alpha particle emitter, which is targeted to lung blood vessels using a monoclonal antibody. Smaller tumors (< 100 microm in diameter) are cured, but larger tumors undergo a period of regression and then regrow and ultimately prove lethal. The goal of this work was to determine if external imaging with MRI or CT could be used routinely to monitor the growth/ regression of lung tumors in live mice. To attempt to evaluate individual tumors in vivo, animals were initially imaged with magnetic resonance imaging (MRI). High resolution MRI images could be obtained only after sacrifice when lungs were not moving. In contrast, high resolution computed tomography (CT) produced evaluable images from anesthetized animals. Serial CT images (up to 5/animal) were collected over a 17 day period of tumor growth and treatment. When tumored animals became moribund, animals were sacrificed and lungs were inflated with fixative, embedded in paraffin, and then sectioned serially to compare the detection of tumors by high resolution CT with detection by histology. CT proved most useful in detecting lung tumors located in the hilar area and least useful in detecting serosal surface and anterior lobe tumor foci. Overall, CT images of live animals revealed tumors in approximately 2/3 of cases detected in histologic serial sections when relatively few tumors were present per lung. Detection of lesions and their resolution post therapy were complicated due to residual hemorrhagic, regressing tumor nodules and the development of lung edema both of which appeared as high density areas in the CT scans. We conclude that the microCT method used could identify some lung tumors as small as 100 microm in diameter; however, no concrete evaluation of therapy induced regression of the tumors could be made with CT analyses alone.

Experimental radioimmunotherapy

Experimental radioimmunotherapy (RIT) studies in animal models have contributed significantly to the design of clinical RIT protocols, although the results have not always been directly translated. Reviewed in this article are current areas of active research in experimental RIT to increase the therapeutic ratio that are likely to have a significant impact on the design of future clinical studies. Approaches for increasing the therapeutic efficacy of RIT include the development of new targeting molecules (genetically engineered monoclonal antibodies, antibody fragments, single-chain antibodies, diabodies and minibodies, fusion toxins, or peptides); improved labeling chemistry; novel radionuclide use and fractionation; locoregional administration; pretargeting; use of biological response modifiers or gene transfer techniques to increase target receptor expression; bone marrow transplantation; and combined modality therapy with external-beam radiation therapy, chemotherapy, or gene therapy. Further research with these new experimental approaches in preclinical animal models is necessary to contribute to advances in the treatment of cancer patients using radiolabeled antibodies and peptides.