PSMA-reactive NB7 single domain antibody fragment: A potential scaffold for developing prostate cancer theranostics.

Introduction: Single domain antibody fragments (sdAbs) are an appealing scaffold for radiopharmaceutical development due to their small size (~15 kDa), high solubility, high stability, and excellent tumor penetration. Previously, we developed NB7 sdAb, which has very high affinity for an epitope on PSMA that is different from those targeted by small molecule PSMA inhibitors. Herein, we evaluated NB7 after radioiodination using [*I]SGMIB (1,3,4-isomer) and iso-[*I]SGMIB (1,3,5-isomer), as well as their At-labeled analogues.

A third generation PSMA-targeted agent [At]YF2: Synthesis and in vivo evaluation.

Introduction: Targeted α-particle therapy agents have shown promising responses in patients who have developed resistance to β-particle emitting radionuclides, albeit off-target toxicity remains a concern. Astatine-211 emits only one α-particle per decay and may alleviate the toxicity from α-emitting daughter radionuclides. Previously, we developed the low-molecular-weight PSMA-targeted agent [At]L3-Lu that showed suitable therapeutic efficacy and was well tolerated in mice.

DNA Repair Inhibitors: Potential Targets and Partners for Targeted Radionuclide Therapy.

The present review aims to explore the potential targets/partners for future targeted radionuclide therapy (TRT) strategies, wherein cancer cells often are not killed effectively, despite receiving a high average tumor radiation dose. Here, we shall discuss the key factors in the cancer genome, especially those related to DNA damage response/repair and maintenance systems for escaping cell death in cancer cells. To overcome the current limitations of TRT effectiveness due to radiation/drug-tolerant cells and tumor heterogeneity, and to make TRT more effective, we propose that a promising strategy would be to target the DNA maintenance factors that are crucial for cancer survival.

Site-Specific Radiohalogenation of a HER2-Targeted Single-Domain Antibody Fragment Using a Novel Residualizing Prosthetic Agent.

Because of their rapid tumor accumulation and normal tissue clearance, single-domain antibody fragments (sdAbs) are an attractive vehicle for developing radiotherapeutics labeled with the α-emitter At. Herein, we have evaluated -[At]AGMB-PODS, a prosthetic agent that combines a functionality for residualizing radiohalogens with a phenyloxadiazolyl methylsulfone (PODS) moiety for site-specific sdAb conjugation. -[At]AGMB-PODS and its radioiodinated analogue were evaluated for thiol-selective conjugation to anti-HER2 5F7 sdAb bearing a C-terminus GGC tail.

Brachytherapy via a depot of biopolymer-bound I synergizes with nanoparticle paclitaxel in therapy-resistant pancreatic tumours.

Locally advanced pancreatic tumours are highly resistant to conventional radiochemotherapy. Here we show that such resistance can be surmounted by an injectable depot of thermally responsive elastin-like polypeptide (ELP) conjugated with iodine-131 radionuclides (I-ELP) when combined with systemically delivered nanoparticle albumin-bound paclitaxel. This combination therapy induced complete tumour regressions in diverse subcutaneous and orthotopic mouse models of locoregional pancreatic tumours.

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

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy, particularly with At, 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). The HER2-specific sdAbs and HER2-irrelevant VHH_2001 were labeled using -succinimidyl-3-At-astato-5-guanidinomethyl benzoate (-At-SAGMB).

Stapled peptides as scaffolds for developing radiotracers for intracellular targets: Preliminary evaluation of a radioiodinated MDM2-binding stapled peptide in the SJSA-1 osteosarcoma model.

Stapled peptides are promising scaffolds for inhibiting protein-protein interactions in cells, including between the intracellular oncoprotein MDM2 and p53. Herein, we have investigated the potential utility of a stapled peptide, VIP116, for developing radiolabeled agents targeting MDM2. VIP116 was radioiodinated using the prosthetic agent N-succinimidyl-3-[*I]iodobenzoate ([*I]SIB).

Evaluation of an I-labeled HER2-specific single domain antibody fragment for the radiopharmaceutical therapy of HER2-expressing cancers.

Radiopharmaceutical therapy (RPT) is an attractive strategy for treatment of disseminated cancers including those overexpressing the HER2 receptor including breast, ovarian and gastroesophageal carcinomas. Single-domain antibody fragments (sdAbs) exemplified by the HER2-targeted VHH_1028 evaluated herein are attractive for RPT because they rapidly accumulate in tumor and clear faster from normal tissues than intact antibodies. In this study, VHH_1028 was labeled using the residualizing prosthetic agent N-succinimidyl 3-guanidinomethyl 5-[I]iodobenzoate (iso-[I]SGMIB) and its tissue distribution evaluated in the HER2-expressing SKOV-3 ovarian and BT474 breast carcinoma xenograft models.

Gold Nanostars: A Novel Platform for Developing At-Labeled Agents for Targeted Alpha-Particle Therapy.

Aim: To develop an innovative At nanoplatform with high radiolabeling efficiency and low in vivo deastatination for future targeted alpha-particle therapy (TAT) to treat cancer.

Methods: Star-shaped gold nanoparticles, gold nanostars (GNS), were used as the platform for At radiolabeling. Radiolabeling efficiency under different reaction conditions was tested.

Fluorine-18 Labeling of the MDM2 Inhibitor RG7388 for PET Imaging: Chemistry and Preliminary Evaluation.

RG7388 (Idasanutlin) is a potent inhibitor of oncoprotein murine double minute 2 (MDM2). Herein we investigated the feasibility of developing F-labeled RG7388 as a radiotracer for imaging MDM2 expression in tumors with positron emission tomography (PET). Two fluorinated analogues of RG7388, and , were synthesized by attaching a fluoronicotinyl moiety to RG7388 via a polyethylene glycol (PEG) or a propyl linker.

Labeling single domain antibody fragments with F using a novel residualizing prosthetic agent - N-succinimidyl 3-(1-(2-(2-(2-(2-[F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate.

Introduction: Labeling single domain antibody fragments (sdAbs) with F is an attractive strategy for immunoPET. Earlier, we developed a residualizing label, N-succinimidyl 3-((4-(4-fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([F]RL-I), synthesized via a click reaction for labeling sdAbs with F, that has attractive features but suffered from modest radiochemical yields and suboptimal hydrophobicity. Herein, we have evaluated the potential utility of an analogous agent, N-succinimidyl 3-(1-(2-(2-(2-(2-[F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate ([F]SFETGMB; [F]RL-III) designed to address these limitations.

Production, purification and availability of At: Near term steps towards global access.

The promising characteristics of the 7.2-h radiohalogen At have long been recognized; including having chemical properties suitable for labeling targeting vectors ranging from small organic molecules to proteins, and the emission of only one α-particle per decay, providing greater control over off-target effects. Unfortunately, the impact of At within the targeted α-particle therapy domain has been constrained by its limited availability.

An Improved At-Labeled Agent for PSMA-Targeted α-Therapy.

α-Particle emitters targeting the prostate-specific membrane antigen (PSMA) proved effective in treating patients with prostate cancer who were unresponsive to the corresponding β-particle therapy. At is an α-emitter that may engender less toxicity than other α-emitting agents. We synthesized a new At-labeled radiotracer targeting PSMA that resulted from the search for a pharmacokinetically optimized agent.

Feasibility of Developing Radiotracers for MDM2: Synthesis and Preliminary Evaluation of an F-Labeled Analogue of the MDM2 Inhibitor SP-141.

Murine double minute 2 (MDM2), a negative regulator of the p53 tumor suppressor protein, is overexpressed in several human cancers. Herein we investigate the feasibility of developing F-labeled compounds based on the small molecule inhibitor SP-141 for imaging tumor MDM2 expression levels with positron emission tomography (PET). Three nonradioactive fluorinated SP-141 analogues, -, were synthesized, and their binding to the MDM2 protein was analyzed by surface plasmon resonance (SPR).

Site-Specific and Residualizing Linker for F Labeling with Enhanced Renal Clearance: Application to an Anti-HER2 Single-Domain Antibody Fragment.

Single-domain antibody fragments (sdAbs) are promising vectors for immuno-PET; however, better methods for labeling sdAbs with F are needed. Herein, we evaluate a site-specific strategy using an F residualizing motif and the anti-epidermal growth factor receptor 2 (HER2) sdAb 5F7 bearing an engineered C-terminal GGC tail (5F7GGC). 5F7GGC was site-specifically attached with a tetrazine-bearing agent via thiol-maleimide reaction.

Synthesis and preliminary evaluation of At-labeled inhibitors of prostate-specific membrane antigen for targeted alpha particle therapy of prostate cancer.

Introduction: The high potency and short tissue range of α-particles are attractive features for targeted radionuclide therapy, particularly for cancers with micro-metastases. In the current study, we describe the synthesis of a series of At-labeled prostate-specific membrane antigen (PSMA) inhibitors and their preliminary evaluation as potential agents for metastatic prostate cancer treatment.

Methods: Four novel Glu-urea based PSMA ligands containing a trialkyl stannyl group were synthesized and labeled with At, and for comparative purposes, I, via halodestannylation reactions with N-chlorosuccinimide as the oxidant.

Labeling a TCO-functionalized single domain antibody fragment with F via inverse electron demand Diels Alder cycloaddition using a fluoronicotinyl moiety-bearing tetrazine derivative.

Single domain antibody fragments (sdAbs) exhibit a rapid tumor uptake and fast blood clearance amenable for labeling with F (t = 110 min) but suffer from high kidney accumulation. Previously, we developed a method for F-labeling of sdAbs via trans-cyclooctene (TCO)-tetrazine (Tz) inverse electron demand Diel's Alder cycloaddition reaction (IEDDAR) that incorporated a renal brush border enzyme (RBBE)-cleavable linker. Although >15 fold reduction in kidney activity levels was achieved, tumor uptake was compromised.

Heterogeneity and vascular permeability of breast cancer brain metastases.

Improvements in the diagnosis and treatment of systemic breast cancer have led to a prolongation in patient survival. Unfortunately, these advances are also associated with an increased incidence of brain metastases (BM), with the result that many patients succumb due to BM treatment failure. Intracranial delivery of many chemotherapeutic agents and other therapeutics is hindered by the presence of an impermeable blood-brain barrier (BBB) designed to protect the brain from harmful substances.

Site-specific radioiodination of an anti-HER2 single domain antibody fragment with a residualizing prosthetic agent.

Introduction: As a consequence of their small size, high stability and high affinity, single domain antibody fragments (sdAbs) are appealing targeting vectors for radiopharmaceutical development. With sdAbs binding to internalizing receptors like HER2, residualizing prosthetic agents can enhance tumor retention of radioiodine, which until now has been done with random labeling approaches. Herein we evaluate a site-specific strategy utilizing a radioiodinated, residualizing maleimido moiety and the anti-HER2 sdAb 5F7 bearing a GGC tail for conjugation.

Poly(ADP-Ribose)Polymerase (PARP) Inhibitors and Radiation Therapy.

Poly(ADP-ribose)polymerase-1 (PARP1) is a DNA repair enzyme highly expressed in the nuclei of mammalian cells, with a structure and function that have attracted interest since its discovery. PARP inhibitors, moreover, can be used to induce synthetic lethality in cells where the homologous recombination (HR) pathway is deficient. Several small molecule PARP inhibitors have been approved by the FDA for multiple cancers bearing this deficiency These PARP inhibitors also act as radiosensitizing agents by delaying single strand break (SSB) repair and causing subsequent double strand break (DSB) generation, a concept that has been leveraged in various preclinical models of combination therapy with PARP inhibitors and ionizing radiation.

Labeling Monoclonal Antibody with α-emitting At at High Activity Levels via a Tin Precursor.

In a previous clinical study, the authors evaluated the potential of antitenascin C monoclonal antibody (mAb) 81C6 labeled with At via the prosthetic agent -succinimidyl 3-[At]astatobenzoate (SAB) for the treatment of primary brain tumors. Although encouraging results were obtained, labeling chemistry failed while attempting to escalate the dose to 370 MBq. The goal of the current study was to develop a revised procedure less susceptible to radiolysis-mediated effects on At labeling that would be suitable for use at higher activity levels of this α-emitter.

Delivery systems exploiting natural cell transport processes of macromolecules for intracellular targeting of Auger electron emitters.

The presence of Auger electrons (AE) among the decay products of a number of radionuclides makes these radionuclides an attractive means for treating cancer because these short-range electrons can cause significant damage in the immediate vicinity of the decomposition site. Moreover, the extreme locality of the effect provides a potential for selective eradication of cancer cells with minimal damage to adjacent normal cells provided that the delivery of the AE emitter to the most vulnerable parts of the cell can be achieved. Few cellular compartments have been regarded as the desired target site for AE emitters, with the cell nucleus generally recognized as the preferred site for AE decay due to the extreme sensitivity of nuclear DNA to direct damage by radiation of high linear energy transfer.

Observations on the Effects of Residualization and Dehalogenation on the Utility of -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 -succinimidyl 4-guanidinomethyl-3-[*I]iodobenzoate ([*I]SGMIB) and the nonresidualizing reagent -succinimidyl-3-[*I]iodobenzoate ([*I]SIB), both of which are highly dehalogenation-resistant.

Pharmacodynamic study of radium-223 in men with bone metastatic castration resistant prostate cancer.

Background: Radium-223 is a targeted alpha-particle therapy that improves survival in men with metastatic castration resistant prostate cancer (mCRPC), particularly in men with elevated serum levels of bone alkaline phosphatase (B-ALP). We hypothesized that osteomimicry, a form of epithelial plasticity leading to an osteoblastic phenotype, may contribute to intralesional deposition of radium-223 and subsequent irradiation of the tumor microenvironment.

Methods: We conducted a pharmacodynamic study (NCT02204943) of radium-223 in men with bone mCRPC.