Treatment of HER2-positive breast carcinomatous meningitis with intrathecal administration of alpha-particle-emitting (211)At-labeled trastuzumab

Brain metastasis: An insight into novel molecular targets for theranostic approaches

Brain metastases (BrM) are common malignant lesions in the central nervous system, and pose a significant threat in advanced-stage malignancies due to delayed diagnosis and limited therapeutic options. Their distinct genomic profiles underscore the need for molecular profiling to tailor effective treatments. Recent advances in cancer biology have uncovered molecular drivers underlying tumor initiation, progression, and metastasis. This, coupled with the advances in molecular imaging technology and radiotracer synthesis, has paved the way for the development of innovative radiopharmaceuticals with enhanced specificity and affinity for BrM specific targets. Despite the challenges posed by the blood-brain barrier to effective drug delivery, several radiolabeled compounds have shown promise in detecting and targeting BrM. This manuscript provides an overview of the recent advances in molecular biomarkers used in nuclear imaging and targeted radionuclide therapy in both clinical and preclinical settings. Additionally, it explores potential theranostic applications addressing the unique challenges posed by BrM.

Effective Treatment of Human Breast Carcinoma Xenografts with Single-Dose 211At-Labeled Anti-HER2 Single-Domain Antibody Fragment

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy, particularly with ²¹¹At, because of their rapid accumulation in tumor and clearance from normal tissues. Here, we evaluate the therapeutic potential of this strategy with 5F7 and VHH_1028-2 sdAbs that bind with high affinity to domain IV of human epidermal growth factor receptor type 2 (HER2). Methods: The HER2-specific sdAbs and HER2-irrelevant VHH_2001 were labeled using N-succinimidyl-3-²¹¹At-astato-5-guanidinomethyl benzoate (iso-²¹¹At-SAGMB). The cytotoxicity of iso- ²¹¹At-SAGMB-5F7 and iso- ²¹¹At-SAGMB-VHH_2001 were compared on HER2-expressing BT474 breast carcinoma cells. Three experiments in mice with subcutaneous BT474 xenografts were performed to evaluate the therapeutic effectiveness of single doses of iso- ²¹¹At-SAGMB-5F7 (0.7-3.0 MBq), iso- ²¹¹At-SAGMB-VHH_1028 (1.0-3.0 MBq), and iso- ²¹¹At-SAGMB-VHH_1028 and iso- ²¹¹At-SAGMB-VHH_2001 (∼1.0 MBq). Results: Clonogenic survival of BT474 cells was reduced after exposure to iso- ²¹¹At-SAGMB-5F7 (D₀ = 1.313 kBq/mL) whereas iso- ²¹¹At-SAGMB-VHH_2001 was ineffective. Dose-dependent tumor growth inhibition was observed with ²¹¹At-labeled HER2-specific 5F7 and VHH_1028 but not with HER2-irrelevant VHH_2001. At the 3.0-MBq dose, complete tumor regression was seen in 3 of 4 mice treated with iso- ²¹¹At-SAGMB-5F7 and 8 of 11 mice treated with iso- ²¹¹At-SAGMB-VHH_1028; prolongation in median survival was 495% and 414%, respectively. Conclusion: Combining rapidly internalizing, high-affinity HER2-targeted sdAbs with the iso- ²¹¹At-SAGMB residualizing prosthetic agent is a promising strategy for targeted α-particle therapy of HER2-expressing cancers.

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.

Theranostic Approach in Breast Cancer: A Treasured Tailor for Future Oncology

ABSTRACT

Breast cancer is the most frequent invasive malignancy and the second major cause of cancer death in female subjects mostly due to the considerable diagnostic delay and failure of therapeutic strategies. Thus, early diagnosis and possibility to monitor response to the treatment are of utmost importance. Identification of valid biomarkers, in particular new molecular therapeutic targets, that would allow screening, early patient identification, prediction of disease aggressiveness, and monitoring response to the therapeutic regimen has been in the focus of breast cancer research during recent decades. One of the intensively developing fields is nuclear medicine combining molecular diagnostic imaging and subsequent (radio)therapy in the light of theranostics. This review aimed to survey the current status of preclinical and clinical research using theranostic approach in breast cancer patients with potential to translate into conventional treatment strategies alone or in combination with other common treatments, especially in aggressive and resistant types of breast cancer. In addition, we present 5 patients with breast cancer who were refractory or relapsed after conventional therapy while presumably responded to the molecular radiotherapy with 177Lu-trastuzumab (Herceptin), 177Lu-DOTATATE, and 177Lu-FAPI-46.

Transportation of Single-Domain Antibodies through the Blood-Brain Barrier

Single-domain antibodies derive from the heavy-chain-only antibodies of Camelidae (camel, dromedary, llama, alpaca, vicuñas, and guananos; i.e., nanobodies) and cartilaginous fishes (i.e., VNARs). Their small size, antigen specificity, plasticity, and potential to recognize unique conformational epitopes represent a diagnostic and therapeutic opportunity for many central nervous system (CNS) pathologies. However, the blood–brain barrier (BBB) poses a challenge for their delivery into the brain parenchyma. Nevertheless, numerous neurological diseases and brain pathologies, including cancer, result in BBB leakiness favoring single-domain antibodies uptake into the CNS. Some single-domain antibodies have been reported to naturally cross the BBB. In addition, different strategies and methods to deliver both nanobodies and VNARs into the brain parenchyma can be exploited when the BBB is intact. These include device-based and physicochemical disruption of the BBB, receptor and adsorptive-mediated transcytosis, somatic gene transfer, and the use of carriers/shuttles such as cell-penetrating peptides, liposomes, extracellular vesicles, and nanoparticles. Approaches based on single-domain antibodies are reaching the clinic for other diseases. Several tailoring methods can be followed to favor the transport of nanobodies and VNARs to the CNS, avoiding the limitations imposed by the BBB to fulfill their therapeutic, diagnostic, and theragnostic promises for the benefit of patients suffering from CNS pathologies.

Radiopharmaceutical Chemistry and Drug Development-What's Changed?

Radiation oncologists and nuclear medicine physicians have seen a resurgence in the clinical use of radiopharmaceuticals for the curative or palliative treatment of cancer. To enable the discovery and the development of new targeted radiopharmaceutical treatments, the United States National Cancer Institute has adapted its clinical trial enterprise to accommodate the requirements of a development program with investigational agents that have a radioactive isotope as part of the studied drug product. One change in perspective has been the consideration of investigational radiopharmaceuticals as drugs, with maximum tolerable doses determined by normal organ toxicity frequency like in drug clinical trials. Other changes include new clinical trial enterprise elements for biospecimen handling, adverse event reporting, regulatory conduct, writing services, drug master files, and reporting of patient outcomes. Arising from this enterprise, the study and clinical use of alpha-particle and beta-particle emitters have emerged as an important approach to cancer treatment. Resources allocated to this enterprise have brought forward biomarkers of molecular pathophysiology now used to select treatment or to evaluate clinical performance of radiopharmaceuticals. The clinical use of diagnostic and therapeutic radionuclide pairs is anticipated to accelerate radiopharmaceutical clinical development.

Preclinical Targeted α- and β--Radionuclide Therapy in HER2-Positive Brain Metastasis Using Camelid Single-Domain Antibodies

HER2-targeted therapies have drastically improved the outcome for breast cancer patients. However, when metastasis to the brain is involved, current strategies fail to hold up to the same promise. Camelid single-domain antibody-fragments (sdAbs) have been demonstrated to possess favorable properties for detecting and treating cancerous lesions in vivo using different radiolabeling methods. Here we evaluate the anti-HER2 sdAb 2Rs15d, coupled to diagnostic γ- and therapeutic α- and β⁻-emitting radionuclides for the detection and treatment of HER2^pos brain lesions in a preclinical setting. 2Rs15d was radiolabeled with ¹¹¹In, ²²⁵Ac and ¹³¹I using DTPA- and DOTA-based bifunctional chelators and Sn-precursor of SGMIB respectively and evaluated in orthotopic tumor-bearing athymic nude mice. Therapeutic efficacy as well as systemic toxicity were determined for ¹³¹I- and ²²⁵Ac-labeled sdAbs and compared to anti-HER2 monoclonal antibody (mAb) trastuzumab in two different HER2^pos tumor models. Radiolabeled 2Rs15d showed high and specific tumor uptake in both HER2^pos SK-OV-3-Luc-IP1 and HER2^pos MDA-MB-231Br brain lesions, whereas radiolabeled trastuzumab was unable to accumulate in intracranial SK-OV-3-Luc-IP1 tumors. Administration of [¹³¹I]-2Rs15d and [²²⁵Ac]-2Rs15d alone and in combination with trastuzumab showed a significant increase in median survival in 2 tumor models that remained largely unresponsive to trastuzumab treatment alone. Histopathological analysis revealed no significant early toxicity. Radiolabeled sdAbs prove to be promising vehicles for molecular imaging and targeted radionuclide therapy of metastatic lesions in the brain. These data demonstrate the potential of radiolabeled sdAbs as a valuable add-on treatment option for patients with difficult-to-treat HER2^pos metastatic cancer.

Observations on the Effects of Residualization and Dehalogenation on the Utility of N-Succinimidyl Ester Acylation Agents for Radioiodination of the Internalizing Antibody Trastuzumab

Trastuzumab is an antibody used for the treatment of human epidermal growth factor receptor 2 (HER2)-overexpressing breast cancers. Since trastuzumab is an internalizing antibody, two factors could play an important role in achieving high uptake and prolonged retention of radioactivity in HER2-positive tumors after radioiodination-residualizing capacity after receptor-mediated internalization and susceptibility to dehalogenation. To evaluate the contribution of these two factors, trastuzumab was radiolabeled using the residualizing reagent N-succinimidyl 4-guanidinomethyl-3-[*I]iodobenzoate ([*I]SGMIB) and the nonresidualizing reagent N-succinimidyl-3-[*I]iodobenzoate ([*I]SIB), both of which are highly dehalogenation-resistant. Paired-label uptake and intracellular retention of [¹²⁵I]SGMIB-trastuzumab and [¹³¹I]SIB-trastuzumab was compared on HER2-expressing BT474 human breast carcinoma cells. Tumor uptake and normal tissue distribution characteristics for the two labeled conjugates were assessed in mice bearing BT474M1 xenografts. The internalization and intracellular retention of initially-bound radioactivity in BT474 cells was similar for the two labeled conjugates up to 4 h, but were significantly higher for [¹²⁵I]SGMIB-trastuzumab at 6 and 24 h. Similarly, [*I]SGMIB labeling resulted in significantly higher uptake and retention of radioactivity in BT474M1 xenografts at all studied time points. Moreover, tumor-to-tissue ratios for [¹²⁵I]SGMIB-trastuzumab were consistently higher than those for [¹³¹I]SIB-trastuzumab starting at 12 h postinjection. Thus, optimal targeting of HER2-positive breast cancers with a radioiodinated trastuzumab conjugate requires an acylation agent that imparts residualizing capacity in addition to high stability towards dehalogenation in vivo.

Beyond the Barrier: Targeted Radionuclide Therapy in Brain Tumors and Metastases

Brain tumors are notoriously difficult to treat. The blood-brain barrier provides a sanctuary site where residual and metastatic cancer cells can evade most therapeutic modalities. The delicate nature of the brain further complicates the decision of eliminating as much tumorous tissue as possible while protecting healthy tissue. Despite recent advances in immunotherapy, radiotherapy and systemic treatments, prognosis of newly diagnosed patients remains dismal, and recurrence is still a universal problem. Several strategies are now under preclinical and clinical investigation to optimize delivery and maximize the cytotoxic potential of pharmaceuticals with regards to brain tumors. This review provides an overview of targeted radionuclide therapy approaches for the treatment of primary brain tumors and brain metastases, with an emphasis on biological targeting moieties that specifically target key biomarkers involved in cancer development.

Trastuzumab-Modified Gold Nanoparticles Labeled with 211At as a Prospective Tool for Local Treatment of HER2-Positive Breast Cancer

Highly localized radiotherapy with radionuclides is a commonly used treatment modality for patients with unresectable solid tumors. Herein, we propose a novel α-nanobrachytherapy approach for selective therapy of human epidermal growth factor receptor 2 (HER2)-positive breast cancer. This uses local intratumoral injection of 5-nm-diameter gold nanoparticles (AuNPs) labeled with an α-emitter (²¹¹At), modified with polyethylene glycol (PEG) chains and attached to HER2-specific monoclonal antibody (trastuzumab). The size, shape, morphology, and zeta potential of the 5 nm synthesized AuNPs were characterized by TEM (Transmission Electron Microscopy) and DLS (Dynamic Light Scattering) techniques. The gold nanoparticle surface was modified by PEG and subsequently used for antibody immobilization. Utilizing the high affinity of gold for heavy halogens, the bioconjugate was labelled with ²¹¹At obtained by α irradiation of the bismuth target. The labeling yield of ²¹¹At was greater than 99%. ²¹¹At bioconjugates were stable in human serum. Additionally, in vitro biological studies indicated that ²¹¹At-AuNP-PEG-trastuzumab exhibited higher affinity and cytotoxicity towards the HER2-overexpressing human ovarian SKOV-3 cell line than unmodified nanoparticles. Confocal and dark field microscopy studies revealed that ²¹¹At-AuNP-PEG-trastuzumab was effectively internalized and deposited near the nucleus. These findings show promising potential for the ²¹¹At-AuNP-PEG-trastuzumab radiobioconjugate as a perspective therapeutic agent in the treatment of unresectable solid cancers expressing HER2 receptors.

A model of breast cancer meningeal metastases: characterization with in vivo molecular imaging

Meningeal metastasis is a fatal complication of breast cancer which affects 8-15% of patients who experience severe neurological complications of cranial nerves, cerebrum, and spinal cord. Survival once diagnosed is less than 4 months. Currently there is no cure. Aggressive multimodal radiation, intra-CSF, or systemic chemotherapy is palliative. Investigation of urgently needed new treatment modalities is hindered by the lack of suitable animal models to effectively study tumor growth kinetics. We present a model of meningeal metastases where tumor growth and associated neurological symptoms have been characterized over 3 weeks by sequential molecular imaging, tumor growth kinetics, and histopathology. Meningeal metastases were induced by stereotaxic injection of human breast cancer cells (MDA-MB-231-Rluc) into the lateral ventricle. Tumor identified by Gd-MRI and Rluc-bioluminescence depict growth in 3 phases, namely lag, exponential, and plateau phase. Invasive tumor growth was highlighted by changes in contrast distribution in the meninges, ventricle and brain compartments over time where moderate contrast uptake in the early growth phase gave rise to a heavy tumor burden in the base of the brain in the latter phases. Tumor growth was accompanied with debilitating neurological symptoms and change in body mass. Tumor was confirmed by ex vivo histology. The reliability of the model to study novel therapeutics was confirmed by oncolytic virus delivered into the lateral ventricle showed potential for treatment. This effective and reliable model resembles human disease progression and is ideally suited to investigate novel treatments.

Phase I feasibility study for intrathecal administration of trastuzumab in patients with HER2 positive breast carcinomatous meningitis

PURPOSE

Leptomeningeal carcinomatosis (MC) is commonly associated with HER2-positive breast cancer (HER2-BC), with a poor prognosis and no standardised treatment. We conducted a phase I dose-escalation study of intrathecal (IT) administration of trastuzumab in HER2-BC patients with MC to determine the maximum tolerated dose (MTD), which was based on both the achievement of a trastuzumab intra-cerebrospinal fluid concentration close to a conventional therapeutic plasma concentration (30 mg/L) and/or dose-limiting toxicity (DLT).

METHODS

The protocol planned IT administration of trastuzumab (30 mg, 60 mg, 100 mg or 150 mg dose levels) once a week, over the course of at least 4 weeks. Sixteen patients with MC from HER2-BC received IT trastuzumab. Intra-cerebrospinal fluid samples were obtained before each injection for pharmacokinetics.

RESULTS

We did not observe DLT of IT trastuzumab. Eleven patients had no toxicity attributed to IT trastuzumab. For 60 mg or higher dose levels, minor toxicities attributed to IT trastuzumab included headache (2 patients), nausea (2 patients), vomiting (1 patient), cervical pain (1 patient) and peripheral neuropathy (1 patient). Two patients experienced immediate toxicity including headache or vomiting. The mean residual intra-cerebrospinal fluid concentration of trastuzumab was 27.9 mg/L for the 150 mg dose level. Three patients achieved a clinical response, seven patients had stable disease and four patients had progressive disease.

CONCLUSIONS

The MTD and recommended phase II weekly dose of IT trastuzumab in patients with HER2-BC and MC is 150 mg. A phase II trial using this dose regimen in MC from HER2-BC is ongoing.

REGISTRATION IDENTIFICATION

ClinicalTrials.gov Identifier: NCT01373710 (https://clinicaltrials.gov/ct2/show/NCT01373710?term=trastuzumab+intrathecal&rank=1).

212Pb-Labeled Antibody 225.28 Targeted to Chondroitin Sulfate Proteoglycan 4 for Triple-Negative Breast Cancer Therapy in Mouse Models

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer with a poor prognosis. There is a clinical need for effective, targeted therapy strategies that destroy both differentiated TNBC cells and TNBC cancer initiating cells (CICs), as the latter are implicated in the metastasis and recurrence of TNBC. Chondroitin sulfate proteoglycan 4 (CSPG4) is overexpressed on differentiated tumor cells and CICs obtained from TNBC patient specimens, suggesting that CSPG4 may be a clinically relevant target for the imaging and therapy of TNBC. The purpose of this study was to determine whether α-particle radioimmunotherapy (RIT) targeting TNBC cells using the CSPG4-specific monoclonal antibody (mAb) 225.28 as a carrier was effective at eliminating TNBC tumors in preclinical models. To this end, mAb 225.28 labeled with ²¹²Pb (²¹²Pb-225.28) as a source of α-particles for RIT was used for in vitro Scatchard assays and clonogenic survival assays with human TNBC cells (SUM159 and 2LMP) grown as adherent cells or non-adherent CIC-enriched mammospheres. Immune-deficient mice bearing orthotopic SUM159 or 2LMP xenografts were injected i.v. with the targeted (225.28) or irrelevant isotype-matched control (F3-C25) mAbs, labeled with ^99mTc, ¹²⁵I, or ²¹²Pb for in vivo imaging, biodistribution, or tumor growth inhibition studies. ²¹²Pb-225.28 bound to adherent SUM159 and 2LMP cells and to CICs from SUM159 and 2LMP mammospheres with a mean affinity of 0.5 nM. Nearly ten times more binding sites per cell were present on SUM159 cells and CICs compared with 2LMP cells. ²¹²Pb-225.28 was six to seven times more effective than ²¹²Pb-F3-C25 at inhibiting SUM159 cell and CIC clonogenic survival (p < 0.05). Radiolabeled mAb 225.28 showed significantly higher uptake than radiolabeled mAb F3-C25 in SUM159 and 2LMP xenografts (p < 0.05), and the uptake of ²¹²Pb-225.28 in TNBC xenografts was correlated with target epitope expression. ²¹²Pb-225.28 caused dose-dependent growth inhibition of SUM159 xenografts; 0.30 MBq ²¹²Pb-225.28 was significantly more effective than 0.33 MBq ²¹²Pb-F3-C25 at inhibiting tumor growth (p < 0.01). These results suggest that CSPG4-specific ²¹²Pb-225.28 is a useful reagent for RIT of CSPG4-expressing tumors, including metastatic TNBC.

Progress in Targeted Alpha-Particle Therapy. What We Learned about Recoils Release from In Vivo Generators

This review summarizes recent progress and developments as well as the most important pitfalls in targeted alpha-particle therapy, covering single alpha-particle emitters as well as in vivo alpha-particle generators. It discusses the production of radionuclides like ²¹¹At, ²²³Ra, ²²⁵Ac/²¹³Bi, labelling and delivery employing various targeting vectors (small molecules, chelators for alpha-emitting nuclides and their biomolecular targets as well as nanocarriers), general radiopharmaceutical issues, preclinical studies, and clinical trials including the possibilities of therapy prognosis and follow-up imaging. Special attention is given to the nuclear recoil effect and its impacts on the possible use of alpha emitters for cancer treatment, proper dose estimation, and labelling chemistry. The most recent and important achievements in the development of alpha emitters carrying vectors for preclinical and clinical use are highlighted along with an outlook for future developments.

Astatine-211 labeled anti-HER2 5F7 single domain antibody fragment conjugates: radiolabeling and preliminary evaluation

INTRODUCTION

Derived from heavy chain only camelid antibodies, ~15-kDa single-domain antibody fragments (sdAbs) are an attractive platform for developing molecularly specific imaging probes and targeted radiotherapeutics. The rapid tumor accumulation and normal tissue clearance of sdAbs might be ideal for use with 211At, a 7.2-h half-life α-emitter, if appropriate labeling chemistry can be devised to trap 211At in cancer cells after sdAb binding. This study evaluated two reagents, [211At]SAGMB and iso-[211At]SAGMB, for this purpose.

METHODS

[211At]SAGMB and iso-[211At]SAGMB, and their radioiodinated analogues [131I]SGMIB and iso-[131I]SGMIB, were synthesized by halodestannylation and reacted with the anti-HER2 sdAb 5F7. Radiochemical purity, immunoreactivity and binding affinity were determined. Paired-label internalization assays on HER2-expressing BT474M1 breast carcinoma cells directly compared [131I]SGMIB-5F7/[211At]SAGMB-5F7 and iso-[131I]SGMIB-5F7/iso-[211At]SAGMB-5F7 tandems. The biodistribution of the two tandems was evaluated in SCID mice with subcutaneous BT474M1 xenografts.

RESULTS

Radiochemical yields for Boc2-iso-[211At]SAGMB and Boc2-[211At]SAGMB synthesis, and efficiencies for coupling of iso-[211At]SAGMB and [211At]SAGMB to 5F7 were similar, with radiochemical purities of [211At]SAGMB-5F7 and iso-[211At]SAGMB-5F7 >98%. iso-[211At]SAGMB-5F7 and [211At]SAGMB-5F7 had immunoreactive fractions >80% and HER2 binding affinities of less than 5 nM. Internalization assays demonstrated high intracellular trapping of radioactivity, with little difference observed between corresponding 211At- and 131I-labeled 5F7 conjugates. Higher BT474M1 intracellular retention was observed from 1-6 h for the iso-conjugates (iso-[211At]SAGMB-5F7, 74.3 ± 2.8%, vs. [211At]SAGMB-5F7, 63.7 ± 0.4% at 2 h) with the opposite behavior observed at 24 h. Peak tumor uptake for iso-[211At]SAGMB-5F7 was 23.4 ± 2.2% ID/g at 4 h, slightly lower than its radioiodinated counterpart, but significantly higher than observed with [211At]SAGMB-5F7. Except in kidneys and lungs, tumor-to-normal organ ratios for iso-[211At]SAGMB-5F7 were greater than 10:1 by 2 h, and significantly higher than those for [211At]SAGMB-5F7.

CONCLUSION

These 211At-labeled sdAb conjugates, particularly iso-[211At]SAGMB-5F7, warrant further evaluation for targeted α-particle radiotherapy of HER2-expressing cancers.

Comparative studies on the therapeutic benefit of targeted α-particle radiation therapy for the treatment of disseminated intraperitoneal disease

Identification of the appropriate combination of radionuclide, target and targeting vehicle is critical for successful radioimmunotherapy. For the treatment of disseminated peritoneal diseases such as pancreatic or ovarian cancer, α-emitting radionuclides have been proposed for targeted radiation therapy. This laboratory has taken a systematic approach investigating targeted α-radiation therapy, allowing comparisons to now be made between 211At, 227Th, 213Bi and 212Pb. Herein, trastuzumab radiolabeled with 211At and 227Th was evaluated for therapeutic efficacy in the LS-174T i.p. tumor model. A dose escalation study was conducted with each radioimmunoconjugate (RIC). Therapeutic benefit was realized with 211At-trastuzumab with doses of 20, 30 and 40 μCi. At doses >40 μCi, toxicity was observed with greater weight loss and 2-fold higher decrease in the platelet counts. Following a second study comparing the effect of 20, 30 and 40 μCi of 211At-trastuzumab, 30 μCi was selected as the dose for future studies. A parallel study was performed evaluating 0.25, 0.5, 1.0, 2.0 and 5.0 μCi of 227Th-trastuzumab. The 0.5 and 1.0 μCi injected dose resulted in a therapeutic response; a lower degree of weight loss was experienced by the mice in the 0.5 μCi cohort. When the data is normalized for comparing 211At, 227Th, 213Bi and 212Pb, the choice of radionuclide for RIT is perhaps not entirely based on simple therapeutic efficacy, other factors may play a role in choosing the "right" radionuclide.

Radioimmunotherapy with α-particle-emitting radionuclides

α-particle-emitting radionuclides are highly cytotoxic and are thus promising candidates for use in targeted radioimmunotherapy of cancer. Due to their high linear energy transfer (LET) combined with a short path length in tissue, α-particles cause severe DNA double-strand breaks that are repaired inaccurately and finally trigger cell death. For radioimmunotherapy, α-emitters such as 225Ac, 211At, 212Bi/212Pb, 213Bi and 227Th are coupled to antibodies via appropriate chelating agents. The α-emitter immunoconjugates preferably target proteins that are overexpressed or exclusively expressed on cancer cells. Application of α-emitter immunoconjugates seems particularly promising in treatment of disseminated cancer cells and small tumor cell clusters that are released during the resection of a primary tumor. α-emitter immunoconjugates have been successfully administered in numerous experimental studies for therapy of ovarian, colon, gastric, blood, breast and bladder cancer. Initial clinical trials evaluating α-emitter immunoconjugates in terms of toxicity and therapeutic efficacy have also shown positive results in patients with melanoma, ovarian cancer, acute myeloid lymphoma and glioma. The present problems in terms of availability of therapeutically effiective α-emitters will presumably be solved by use of alternative production routes and installation of additional production facilities in the near future. Therefore, clinical establishment of targeted α-emitter radioimmunotherapy as one part of a multimodal concept for therapy of cancer is a promising, middle-term concept.

Production of [(211)At]-astatinated radiopharmaceuticals and applications in targeted α-particle therapy

(211)At is a promising radionuclide for α-particle therapy of cancers. Its physical characteristics make this radionuclide particularly interesting to consider when bound to cancer-targeting biomolecules for the treatment of microscopic tumors. (211)At is produced by cyclotron irradiation of (209)Bi with α-particles accelerated at ~28 MeV and can be obtained in high radionuclidic purity after isolation from the target. Its chemistry resembles iodine, but there is also a tendency to behave as a metalloid. However, the chemical behavior of astatine has not yet been clearly established, primarily due to the lack of any stable isotopes of this element, which precludes the use of conventional analytical techniques for its characterization. There are also only a limited number of research centers that have been able to produce this element in sufficient amounts to carry out extensive investigations. Despite these difficulties, chemical reactions typically used with iodine can be performed, and a number of biomolecules of interest have been labeled with (211)At. However, most of these compounds exhibit unacceptable instability in vivo due to the weakness of the astatine-biomolecule bond. Nonetheless, several compounds have shown high potential for the treatment of cancers in vitro and in several animal models, thus providing a promising basis that has allowed initiation of the first two clinical studies.

Response of meningeal carcinomatosis from breast cancer to capecitabine monotherapy: a case report

A 62-year-old woman with breast cancer received neoadjuvant chemotherapy followed by breast-conserving surgery and sentinel node biopsy. During adjuvant endocrine therapy with aromatase inhibitor, she developed multiple bone metastases. Thereafter, she received tamoxifen and zoledronate therapy. In May 2011, she developed a tongue deviation and was diagnosed as having meningeal carcinomatosis. The tongue deviation disappeared 3 weeks after taking capecitabine (2,400 mg/day). Magnetic resonance imaging of the brain showed regression of meningeal carcinomatosis. Levels of tumor markers CEA and CA15-3 changed from 96.0 IU/ml and 3.5 ng/ml to 47.0 IU/ml and 1.5 ng/ml, respectively. Progression-free survival with capecitabine monotherapy was 5 months.

Nanovectorized radiotherapy: a new strategy to induce anti-tumor immunity

Recent experimental findings show that activation of the host immune system is required for the success of chemo- and radiotherapy. However, clinically apparent tumors have already developed multiple mechanisms to escape anti-tumor immunity. The fact that tumors are able to induce a state of tolerance and immunosuppression is a major obstacle in immunotherapy. Hence, there is an overwhelming need to develop new strategies that overcome this state of immune tolerance and induce an anti-tumor immune response both at primary and metastatic sites. Nanovectorized radiotherapy that combines ionizing radiation and nanodevices, is one strategy that could boost the quality and magnitude of an immune response in a predictable and designable fashion. The potential benefits of this emerging treatment may be based on the unique combination of immunostimulatory properties of nanoparticles with the ability of ionizing radiation to induce immunogenic tumor cell death. In this review, we will discuss available data and propose that the nanovectorized radiotherapy could be a powerful new strategy to induce anti-tumor immunity required for positive patient outcome.

An overview of targeted alpha therapy

The effectiveness of targeted α-therapy (TAT) can be explained by the properties of α-particles. Alpha particles are helium nuclei and are ~8,000 times larger than β(-)-particles (electrons). When emitted from radionuclides that decay via an α-decay pathway, they release enormous amounts of energy over a very short distance. Typically, the range of α-particles in tissue is 50-100 μm and they have high linear energy transfer (LET) with a mean energy deposition of 100 keV/μm, providing a more specific tumor cell killing ability without damage to the surrounding normal tissues than β(-)-emitters. Due to these properties, the majority of pre-clinical and clinical trials have demonstrated that α-emitters such as (225)Ac, (211)At, (212)Bi, (213)Bi, (212)Pb, (223)Ra, and (227)Th are ideal for the treatment of smaller tumor burdens, micrometastatic disease, and disseminated disease. Even though these α-emitters have favorable properties, the development of TAT has been limited by high costs, unresolved chemistry, and limited availability of the radionuclides. To overcome these limitations, more potent isotopes, additional sources, and more efficient isotope production methods should be addressed. Furthermore, better chelation and labeling methods with the improvements of isotope delivery, targeting vehicles, molecular targets, and identification of appropriate clinical applications are still required.

Applications of 211At and 223Ra in targeted alpha-particle radiotherapy

Targeted radiotherapy using agents tagged with α-emitting radionuclides is gaining traction with several clinical trials already undertaken or ongoing, and others in the advanced planning stage. The most commonly used α-emitting radionuclides are 213Bi, 211At, 223Ra and 225Ac. While each one of these has pros and cons, it can be argued that 211At probably is the most versatile based on its half life, decay scheme and chemistry. On the other hand, for targeting bone metastases, 223Ra is the ideal radionuclide because simple cationic radium can be used for this purpose. In this review, we will discuss the recent developments taken place in the application of 211At-labeled radiopharmaceuticals and give an overview of the current status of 223Ra for targeted α-particle radiotherapy.

Cancer radioimmunotherapy

Targeting of radionuclides with antibodies, or radioimmunotherapy, has been an active field of research spanning nearly 50 years, evolving with advancing technologies in molecular biology and chemistry, and with many important preclinical and clinical studies illustrating the benefits, but also the challenges, which all forms of targeted therapies face. There are currently two radiolabeled antibodies approved for the treatment of non-Hodgkin lymphoma, but radioimmunotherapy of solid tumors remains a challenge. Novel antibody constructs, focusing on treatment of localized and minimal disease, and pretargeting are all promising new approaches that are currently under investigation.

Experimental α-particle radioimmunotherapy of breast cancer using 227Th-labeled p-benzyl-DOTA-trastuzumab

Background

The aim of the present study was to explore the biodistribution, normal tissue toxicity, and therapeutic efficacy of the internalizing low-dose rate alpha-particle-emitting radioimmunoconjugate ²²⁷Th-trastuzumab in mice with HER2-expressing breast cancer xenografts.

Methods

Biodistribution of ²²⁷Th-trastuzumab and ²²⁷Th-rituximab in nude mice bearing SKBR-3 xenografts were determined at different time points after injection. Tumor growth was measured after administration of ²²⁷Th-trastuzumab, ²²⁷Th-rituximab, cold trastuzumab, and saline. The toxicity of ²²⁷Th-trastuzumab was evaluated by measurements of body weight, blood cell, and clinical chemistry parameters, as well as histological examination of tissue specimens.

Results

The tumor uptake reached peak levels of 34% ID/g (4.6 kBq/g) 3 days after injection of 400 kBq/kg of ²²⁷Th-trastuzumab. The absorbed radiation dose to tumor was 2.9 Gy, while it was 2.4 Gy to femur due to uptake of the daughter nuclide ²²³Ra in bone; the latter already explored in clinical phases I and II trials without serious toxicity. A significant dose-dependent antitumor effect was observed for dosages of 200, 400, and 600 kBq/kg of ²²⁷Th-trastuzumab but no effect of 400 and 600 kBq/kg ²²⁷Th-rituximab (non-tumor binding). No serious delayed bone marrow or normal organ toxicity was observed, but there was a statistical significant reduction in blood cell parameters for the highest-dose group of ²²⁷Th-trastuzumab treatment.

Conclusion

Internalizing ²²⁷Th-trastuzumab therapy was well tolerated and resulted in a dose-dependent inhibition of breast cancer xenograft growth. These results warrant further preclinical studies aiming at a clinical trial in breast cancer patients with metastases to bone.

Reagents for astatination of biomolecules. 5. Evaluation of hydrazone linkers in (211)At- and (125)I-labeled closo-decaborate(2-) conjugates of Fab' as a means of decreasing kidney retention

Evaluation of monoclonal antibody (mAb) fragments (e.g., Fab', Fab, or engineered fragments) as cancer-targeting reagents for therapy with the α-particle emitting radionuclide astatine-211 ((211)At) has been hampered by low in vivo stability of the label and a propensity of these proteins localize to kidneys. Fortunately, our group has shown that the low stability of the (211)At label, generally a meta- or para-[(211)At]astatobenzoyl conjugate, on mAb Fab' fragments can be dramatically improved by the use of closo-decaborate(2-) conjugates. However, the higher stability of radiolabeled mAb Fab' conjugates appears to result in retention of radioactivity in the kidneys. This investigation was conducted to evaluate whether the retention of radioactivity in kidney might be decreased by the use of an acid-cleavable hydrazone between the Fab' and the radiolabeled closo-decaborate(2-) moiety. Five conjugation reagents containing sulfhydryl-reactive maleimide groups, a hydrazone functionality, and a closo-decaborate(2-) moiety were prepared. In four of the five conjugation reagents, a discrete poly(ethylene glycol) (PEG) linker was used, and one substituent adjacent to the hydrazone was varied (phenyl, benzoate, anisole, or methyl) to provide varying acid sensitivity. In the initial studies, the five maleimido-closo-decaborate(2-) conjugation reagents were radioiodinated ((125)I or (131)I), then conjugated with an anti-PSMA Fab' (107-1A4 Fab'). Biodistributions of the five radioiodinated Fab' conjugates were obtained in nude mice at 1, 4, and 24 h post injection (pi). In contrast to closo-decaborate(2-) conjugated to 107-1A4 Fab' through a noncleavable linker, two conjugates containing either a benzoate or a methyl substituent on the hydrazone functionality displayed clearance rates from kidney, liver, and spleen that were similar to those obtained with directly radioiodinated Fab' (i.e., no conjugate). The maleimido-closo-decaborate(2-) conjugation reagent containing a benzoate substituent on the hydrazone was chosen for study with (211)At. That reagent was conjugated with 107-1A4 Fab', then labeled (separately) with (125)I and (211)At. The radiolabeled Fab' conjugates were coinjected into nude mice bearing LNCaP human tumor xenografts, and biodistribution data were obtained at 1, 4, and 24 h pi. Tumor targeting was achieved with both (125)I- and (211)At-labeled Fab', but the (211)At-labeled Fab' reached a higher concentration (25.56 ± 11.20 vs 11.97 ± 1.31%ID/g). Surprisingly, while the (125)I-labeled Fab' was cleared from kidney similar to earlier studies, the (211)At-labeled Fab'was not (i.e., kidney conc. for (125)I vs (211)At; 4 h, 13.14 ± 2.03 ID/g vs 42.28 ± 16.38%D/g; 24 h, 4.23 ± 1.57 ID/g vs 39.52 ± 15.87%ID/g). Since the Fab' conjugate is identical in both cases except for the radionuclide, it seems likely that the difference in tissue clearance seen is due to an effect that (211)At has on either the hydrazone cleavage or on the retention of a metabolite. Results from other studies in our laboratory suggest that the latter case is most likely. The hydrazone linkers tested do not provide the tissue clearance sought for (211)At, so additional hydrazones linkers will be evaluated. However, the results support the use of hydrazone linkers when Fab' conjugated with closo-decaborate(2-) reagents are radioiodinated.

Renal uptake of bismuth-213 and its contribution to kidney radiation dose following administration of actinium-225-labeled antibody

Clinical therapeutic studies using (225)Ac-labeled antibodies have begun. Of major concern is renal toxicity that may result from the three alpha-emitting progeny generated following the decay of (225)Ac. The purpose of this study was to determine the amount of (225)Ac and non-equilibrium progeny in the mouse kidney after the injection of (225)Ac-huM195 antibody and examine the dosimetric consequences. Groups of mice were sacrificed at 24, 96 and 144 h after injection with (225)Ac-huM195 antibody and kidneys excised. One kidney was used for gamma ray spectroscopic measurements by a high-purity germanium (HPGe) detector. The second kidney was used to generate frozen tissue sections which were examined by digital autoradiography (DAR). Two measurements were performed on each kidney specimen: (1) immediately post-resection and (2) after sufficient time for any non-equilibrium excess (213)Bi to decay completely. Comparison of these measurements enabled estimation of the amount of excess (213)Bi reaching the kidney (γ-ray spectroscopy) and its sub-regional distribution (DAR). The average absorbed dose to whole kidney, determined by spectroscopy, was 0.77 (SD 0.21) Gy kBq(-1), of which 0.46 (SD 0.16) Gy kBq(-1) (i.e. 60%) was due to non-equilibrium excess (213)Bi. The relative contributions to renal cortex and medulla were determined by DAR. The estimated dose to the cortex from non-equilibrium excess (213)Bi (0.31 (SD 0.11) Gy kBq(-1)) represented ∼46% of the total. For the medulla the dose contribution from excess (213)Bi (0.81 (SD 0.28) Gy kBq(-1)) was ∼80% of the total. Based on these estimates, for human patients we project a kidney-absorbed dose of 0.28 Gy MBq(-1) following administration of (225)Ac-huM195 with non-equilibrium excess (213)Bi responsible for approximately 60% of the total. Methods to reduce renal accumulation of radioactive progeny appear to be necessary for the success of (225)Ac radioimmunotherapy.

Radioimmunotherapy of solid tumors: searching for the right target

Radioimmunotherapy of solid tumors remains a challenge despite the tremendous success of ⁹⁰Y ibritumomab tiuxetan (Zevalin) and ¹³¹I Tositumomab (Bexxar) in treating non-Hodgkin's lymphoma. For a variety of reasons, clinical trials of radiolabeled antibodies against solid tumors have not led to responses equivalent to those seen against lymphoma. In contrast, promising responses have been observed with unlabeled antibodies that target solid tumor receptors associated with cellular signaling pathways. These observations suggest that anti-tumor efficacy of the carrier antibody might be critical to achieving clinical responses. Here, we review and compare tumor antigens targeted by radiolabeled antibodies and unlabeled antibodies used in immunotherapy. The review shows that the trend for radiolabeled antibodies under pre-clinical development is to also target antigens associated with signaling pathways that are essential for the growth and survival of the tumor.

Unconventional nuclides for radiopharmaceuticals

Rapid and widespread growth in the use of nuclear medicine for both diagnosis and therapy of disease has been the driving force behind burgeoning research interests in the design of novel radiopharmaceuticals. Until recently, the majority of clinical and basic science research has focused on the development of 11C-, 13N-, 15O-, and 18F-radiopharmaceuticals for use with positron emission tomography (PET) and 99mTc-labeled agents for use with single-photon emission computed tomography (SPECT). With the increased availability of small, low-energy cyclotrons and improvements in both cyclotron targetry and purification chemistries, the use of "nonstandard" radionuclides is becoming more prevalent. This brief review describes the physical characteristics of 60 radionuclides, including beta+, beta-, gamma-ray, and alpha-particle emitters, which have the potential for use in the design and synthesis of the next generation of diagnostic and/or radiotherapeutic drugs. As the decay processes of many of the radionuclides described herein involve emission of high-energy gamma-rays, relevant shielding and radiation safety issues are also considered. In particular, the properties and safety considerations associated with the increasingly prevalent PET nuclides 64Cu, 68Ga, 86Y, 89Zr, and 124I are discussed.