Evaluation of 177Lu and 47Sc Picaga-Linked, Prostate-Specific Membrane Antigen-Targeting Constructs for Their Radiotherapeutic Efficacy and Dosimetry

Novel [68Ga/177Lu]Ga/Lu-AZ-093 as PSMA-Targeting Agent for Diagnosis and Radiotherapy

Prostate-specific membrane antigen (PSMA) overexpressed in prostate cancer cells can serve as a target for imaging and radioligand therapy (RLT). Previously, [68Ga]Ga-P16-093, containing a Ga(III) chelator, N,N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid (HBED-CC), displayed excellent PSMA-targeting properties and showed a high tumor uptake and retention useful for diagnosis in prostate cancer patients. Recently, [177Lu]Lu-PSMA-617 has been approved by the U.S. food and drug administration (FDA) for the treatment of prostate cancer patients. Derivatives of PSMA-093 using AAZTA (6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid), as the chelator, were designed as alternative agents forming complexes with both diagnostic and therapeutic radiometals, such as gallium-68 (log K = 22.18) or lutetium-177 (log K = 21.85). The aim of this study is to evaluate AAZTA-Gly-O-(methylcarboxy)-Tyr-Phe-Lys-NH-CO-NH-Glu (designated as AZ-093, 1) leading to a gallium-68/lutetium-177 theranostic pair as potential PSMA targeting agents. Synthesis of the desired precursor, AZ-093, 1, was effectively accomplished. Labeling with either [68Ga]GaCl3 or [177Lu]LuCl3 in a sodium acetate buffer solution (pH 4-5) at 50 °C in 5 to 15 min produced either [68Ga]Ga-1 or [177Lu]Lu-1 with high yields and excellent radiochemical purities. Results of in vitro binding studies, cell uptake, and retention (using PSMA-positive prostate carcinoma cells line, 22Rv1-FOLH1-oe) were comparable to that of [68Ga]Ga-P16-093 and [177Lu]Lu-PSMA-617, respectively. Specific cellular uptake was determined with or without the competitive blocking agent (2 μM of "cold" PSMA-11). Cellular binding and internalization showed a time-dependent increase over 2 h at 37 °C in the PSMA-positive cells. The cell uptakes were completely blocked by the "cold" PSMA-11 suggesting that they are competing for the same PSMA binding sites. In the mouse model with implanted PSMA-positive tumor cells, both [68Ga]Ga-1 and [177Lu]Lu-1 displayed excellent uptake and retention in the tumor. Results indicate that [68Ga]Ga/[177Lu]Lu-1 (68Ga]Ga/[177Lu]Lu-AZ-093) is potentially useful as PSMA-targeting agent for both diagnosis and radiotherapy of prostate cancer.

Towards Clinical Development of Scandium Radioisotope Complexes for Use in Nuclear Medicine: Encouraging Prospects with the Chelator 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic Acid (DOTA) and Its Analogues

Scandium (Sc) isotopes have recently attracted significant attention in the search for new radionuclides with potential uses in personalized medicine, especially in the treatment of specific cancer patient categories. In particular, Sc-43 and Sc-44, as positron emitters with a satisfactory half-life (3.9 and 4.0 h, respectively), are ideal for cancer diagnosis via Positron Emission Tomography (PET). On the other hand, Sc-47, as an emitter of beta particles and low gamma radiation, may be used as a therapeutic radionuclide, which also allows Single-Photon Emission Computed Tomography (SPECT) imaging. As these scandium isotopes follow the same biological pathway and chemical reactivity, they appear to fit perfectly into the "theranostic pair" concept. A step-by-step description, initiating from the moment of scandium isotope production and leading up to their preclinical and clinical trial applications, is presented. Recent developments related to the nuclear reactions selected and employed to produce the radionuclides Sc-43, Sc-44, and Sc-47, the chemical processing of these isotopes and the main target recovery methods are also included. Furthermore, the radiolabeling of the leading chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and its structural analogues with scandium is also discussed and the advantages and disadvantages of scandium complexation are evaluated. Finally, a review of the preclinical studies and clinical trials involving scandium, as well as future challenges for its clinical uses and applications, are presented.

Exploring Aqueous Coordination Chemistry of Highly Lewis Acidic Metals with Emerging Isotopes for Nuclear Medicine

Nuclear medicine harnesses radioisotopes for the diagnosis and treatment of disease. While the isotopes 99mTc and 111In have enabled the clinical diagnosis of millions of patients over the past 3 decades, more recent clinical translation of numerous 68Ga/177Lu-based radiopharmaceuticals for diagnostic imaging and therapy underscores the clinical utility of metal-based radiopharmaceuticals in mainstream cancer treatment. In addition to such established radionuclides, advancements in radioisotope production have enabled the production of radionuclides with a broad range of half-lives and emission properties of interest for nuclear medicine. Chemical means to form kinetically inert, in vivo-compatible species that can be modified with disease-targeting vectors is imperative. This presents a challenge for radiosiotopes of elements where the aqueous chemistry is still underdeveloped and poorly understood. Here, we discuss our efforts to date in exploring the aqueous, radioactive coordination chemistry of highly Lewis acidic metal ions and how our discoveries apply to the diagnosis and treatment of cancer in preclinical models of disease. The scope of this Account includes approaches to aqueous coordination of to-date understudied highly Lewis acidic metal ions with radioisotopes of emerging interest and the modulation of well-understood coordination environments of radio-coordination complexes to induce metal-catalyzed reactivity for separation and pro-drug applications.First, we discuss the development of seven-coordinate, small-cavity macrocyclic chelator platform mpatcn/picaga as an exemplary case study, which forms robust complexes with 44Sc/47Sc isotopes. Due to the high chemical hardness and pronounced Lewis acidity of the Sc3+ ion, the displacement of ternary ligand H2O by 18/natF- can be achieved to form an inert Sc-18/natF bond. Corresponding coordination complex natSc-18F is in vivo compatible and forms a theranostic tetrad with corresponding 44Sc/47Sc, 177Lu complexes all exhibiting homologous biodistribution profiles. Another exceptionally hard, highly Lewis acidic ion with underdeveloped aqueous chemistry and emerging interest in nuclear medicine is 45Ti4+. To develop de novo approaches to the mononuclear chelation of this ion under aqueous conditions, we employed a fragment-based bidentate ligand screening approach which identified two leads. The screen successfully predicted the formation of [45Ti][Ti(TREN-CAM)], a Ti-triscatechol complex that exhibits remarkable in vivo stability. Furthermore, the fragment-based screen also identified approaches that enabled solid-phase separation of Ti4+ and Sc3+ of interest in streamlining the isotope production of 45Ti and accessing new ways to separate 44Ti/44Sc for the development of a long-lived generator system. In addition to establishing the inert chelation of Ti4+ and Sc3+, we introduce controlled, metal-induced reactivity of corresponding coordination complexes on macroscopic and radiotracer scales. Metal-mediated autolytic amide bond cleavage (MMAAC) enables the temperature-dependent release of high-molar-activity, ready-to-inject radiopharmaceuticals; cleavage is selectively triggered by coordinated trivalent Lewis acid nat/68Ga3+ or Sc3+. Following the scope of reactivity and mechanistic studies, we validated MMAAC for the synthesis of high-molar-activity radiopharmaceuticals to image molecular targets with low expression and metal-mediated prodrug hydrolysis in vivo.This Account summarizes how developing the aqueous coordination chemistry and tuning the chemical reactivity of metal ions with high Lewis acidity at the macroscopic and tracer scales directly apply to the radiopharmaceutical synthesis with clinical potential.

Dosimetric Analysis of a Phase I Study of PSMA-Targeting Radiopharmaceutical Therapy With [177Lu]Ludotadipep in Patients With Metastatic Castration-Resistant Prostate Cancer

OBJECTIVE

177Lutetium [Lu] Ludotadipep is a novel prostate-specific membrane antigen targeting therapeutic agent with an albumin motif added to increase uptake in the tumors. We assessed the biodistribution and dosimetry of [177Lu]Ludotadipep in patients with metastatic castration-resistant prostate cancer (mCRPC).

MATERIALS AND METHODS

Data from 25 patients (median age, 73 years; range, 60-90) with mCRPC from a phase I study with activity escalation design of single administration of [177Lu]Ludotadipep (1.85, 2.78, 3.70, 4.63, and 5.55 GBq) were assessed. Activity in the salivary glands, lungs, liver, kidneys, and spleen was estimated from whole-body scan and abdominal SPECT/CT images acquired at 2, 24, 48, 72, and 168 h after administration of [177Lu]Ludotadipep. Red marrow activity was calculated from blood samples obtained at 3, 10, 30, 60, and 180 min, and at 24, 48, and 72 h after administration. Organ- and tumor-based absorbed dose calculations were performed using IDAC-Dose 2.1.

RESULTS

Absorbed dose coefficient (mean ± standard deviation) of normal organs was 1.17 ± 0.81 Gy/GBq for salivary glands, 0.05 ± 0.02 Gy/GBq for lungs, 0.14 ± 0.06 Gy/GBq for liver, 0.77 ± 0.28 Gy/GBq for kidneys, 0.12 ± 0.06 Gy/GBq for spleen, and 0.07 ± 0.02 Gy/GBq for red marrow. The absorbed dose coefficient of the tumors was 10.43 ± 7.77 Gy/GBq.

CONCLUSION

[177Lu]Ludotadipep is expected to be safe at the dose of 3.7 GBq times 6 cycles planned for a phase II clinical trial with kidneys and bone marrow being the critical organs, and shows a high tumor absorbed dose.

H3 Derivatives Possessing Picolyl and Picolinate Pendants for Ga3+ Coordination and 67Ga3+ Radiolabeling

We synthesized, thanks to the regiospecific N-functionalization using an orthoamide intermediate, two 1,4,7-triazacyclononane derivatives containing an acetate arm and either a methylpyridine or a picolinic acid group, respectively, Hnoapy and H2noapa, as new Ga3+ chelators for potential use in nuclear medicine. The corresponding Ga3+ complexes were synthesized and structurally characterized in solution by 1H and 13C NMR. The [Ga(noapy)]2+ complex appears to exist in solution as two diasteroisomeric pairs of enantiomers, as confirmed by density functional theory (DFT) calculations, while for [Ga(noapa)]+, a single species is present in solution. Solid-state investigations were possible for the [Ga(noapa)]+ complex, which crystallized from water as a pair of enantiomers. The average length of the N-Ga bonds of 2.090 Å is identical with that found for the [Ga(nota)] complex, showing that the presence of the picolinate arm does not hinder the coordination of the ligand to the metal ion. Protonation constants of noapy- and noapa2- were determined by potentiometric titrations, providing an overall basicity ∑log KiH (i = 1-4) that increases in the order noapy- < noapa2- < nota3- with increases in the negative charge of the ligand. Stability constants determined by pH-potentiometric titrations supplemented with 71Ga NMR data show that the stabilities of [Ga(noapy)]2+ and [Ga(noapa)]+ are lower compared to that of [Ga(nota)] but higher than those of other standards such as [Ga(aazta)]-. 67Ga radiolabeling studies were performed in order to demonstrate the potential of these chelators for 67/68Ga-based radiopharmaceuticals. The labelings of Hnoapy and H2noapa were nearly identical, outperforming H3nota. Stability studies were conducted in phosphate-buffered saline and in the presence of human serum transferrin, revealing no significant decomplexation of [67Ga][Ga(noapy)]2+ and [67Ga][Ga(noapa)]+ compared to [67Ga][Ga(nota)]. Finally, all complexes were found to be highly hydrophilic, with calculated log D7.4 values of -3.42 ± 0.05, -3.34 ± 0.04, and -3.00 ± 0.23 for Hnoapy, H2noapa, and H3nota, respectively, correlating with the charge of each complex and the electrostatic potentials obtained with DFT.

Preclinical Development in Radiopharmaceutical Therapy for Prostate Cancer

Prostate cancer is a leading cause of cancer death in men worldwide. Among the various treatment options, radiopharmaceutical therapy has shown notable success in metastatic, castration-resistant disease. Radiopharmaceutical therapy is a systemic approach that delivers cytotoxic radiation doses precisely to the malignant tumors and/or tumor microenvironment. Therapeutic radiopharmaceuticals are composed of a therapeutic radionuclide and a high-affinity, tumor-targeting carrier molecule. Therapeutic radionuclides used in preclinical prostate cancer studies are primarily α-, β--, or Auger-electron-emitting radiometals or radiohalogens. Monoclonal antibodies, antibody-derived fragments, peptides, and small molecules are frequently used as tumor-targeting molecules. Over the years, several important membrane-associated proteases and receptors have been identified, validated, and subsequently used for preclinical radiotherapeutic development for prostate cancer. Prostate-specific membrane antigen (PSMA) is the most well-studied prostate cancer-associated protease in preclinical literature. PSMA-targeting radiotherapeutic agents are being investigated using high-affinity antibody- and small-molecule-based agents for safety and efficacy. Early generations of such agents were developed simply by replacing radionuclides of the imaging agents with therapeutic ones. Later, extensive structure-activity relationship studies were conducted to address the safety and efficacy issues obtained from initial patient data. Recent regulatory approval of the 177Lu-labeled low-molecular-weight agent, 177Lu-PSMA-617, is a significant accomplishment. Current preclinical experiments are focused on the structural modification of 177Lu-PSMA-617 and relevant investigational agents to increase tumor targeting and reduce off-target binding and toxicity in healthy organs. While lutetium-177 (177Lu) remains the most widely used radionuclide, radiolabeled analogs with iodine-131 (128I), yttrium-90 (89Y), copper-67 (67Cu), and terbium-161 (161Tb) have been evaluated as potential alternatives in recent years. In addition, agents carrying the α-particle-emitting radiohalogen, astatine-211 (211At), or radiometals, actinium-225 (225Ac), lead-212 (212Pb), radium-223 (223Ra), and thorium-227 (227Th), have been increasingly investigated in preclinical research. Besides PSMA-based radiotherapeutics, other prominent prostate cancer-related proteases, for example, human kallikrein peptidases (HK2 and HK3), have been explored using monoclonal-antibody-(mAb)-based targeting platforms. Several promising mAbs targeting receptors overexpressed on the different stages of prostate cancer have also been developed for radiopharmaceutical therapy, for example, Delta-like ligand 3 (DLL-3), CD46, and CUB domain-containing protein 1 (CDCP1). Progress is also being made using peptide-based targeting platforms for the gastrin-releasing peptide receptor (GRPR), a well-established membrane-associated receptor expressed in localized and metastatic prostate cancers. Furthermore, mechanism-driven combination therapies appear to be a burgeoning area in the context of preclinical prostate cancer radiotherapeutics. Here, we review the current developments related to the preclinical radiopharmaceutical therapy of prostate cancer. These are summarized in two major topics: (1) therapeutic radionuclides and (2) tumor-targeting approaches using monoclonal antibodies, small molecules, and peptides.

Radiochemical and Biological Evaluation of 3p-C-NETA-ePSMA-16, a Promising PSMA-Targeting Agent for Radiotheranostics

Bifunctional chelators (BFCs) are a key element in the design of radiopharmaceuticals. By selecting a BFC that efficiently complexes diagnostic and therapeutic radionuclides, a theranostic pair possessing almost similar biodistribution and pharmacokinetic properties can be developed. We have previously reported 3p-C-NETA as a promising theranostic BFC, and the encouraging preclinical outcomes obtained with [¹⁸F]AlF-3p-C-NETA-TATE led us to conjugate this chelator to a PSMA-targeting vector for imaging and treatment of prostate cancer. In this study, we synthesized 3p-C-NETA-ePSMA-16 and radiolabeled it with different diagnostic (¹¹¹In, ¹⁸F) and therapeutic (¹⁷⁷Lu, ²¹³Bi) radionuclides. 3p-C-NETA-ePSMA-16 showed high affinity to PSMA (IC₅₀ = 4.61 ± 1.33 nM), and [¹¹¹In]In-3p-C-NETA-ePSMA-16 showed specific cell uptake (1.41 ± 0.20% ID/10⁶ cells) in PSMA expressing LS174T cells. Specific tumor uptake of [¹¹¹In]In-3p-C-NETA-ePSMA-16 was observed up to 4 h p.i. (1.62 ± 0.55% ID/g at 1 h p.i.; 0.89 ± 0.58% ID/g at 4 h p.i.) in LS174T tumor-bearing mice. Only a faint signal could be seen at 1 h p.i. in the SPECT/CT scans, whereas dynamic PET/CT scans performed after administration of [¹⁸F]AlF-3p-C-NETA-ePSMA-16 in PC3-Pip tumor xenografted mice resulted in a better tumor visualization and imaging contrast. Therapy studies with short-lived radionuclides such as ²¹³Bi could further elucidate the therapeutic potential of 3p-C-NETA-ePSMA-16 as a radiotheranostic.

Radiolabeling and in vivo evaluation of lanmodulin with biomedically relevant lanthanide isotopes

Short-lived, radioactive lanthanides comprise an emerging class of radioisotopes attractive for biomedical imaging and therapy applications. To deliver such isotopes to target tissues, they must be appended to entities that target antigens overexpressed on the target cell's surface. However, the thermally sensitive nature of biomolecule-derived targeting vectors requires the incorporation of these isotopes without the use of denaturing temperatures or extreme pH conditions; chelating systems that can capture large radioisotopes under mild conditions are therefore highly desirable. Herein, we demonstrate the successful radiolabeling of the lanthanide-binding protein, lanmodulin (LanM), with medicinally relevant radioisotopes: ¹⁷⁷Lu, ^132/135La and ⁸⁹Zr. Radiolabeling of the endogenous metal-binding sites of LanM, as well exogenous labeling of a protein-appended chelator, was successfully conducted at 25 °C and pH 7 with radiochemical yields ranging from 20-82%. The corresponding radiolabeled constructs possess good formulation stability in pH 7 MOPS buffer over 24 hours (>98%) in the presence of 2 equivalents of ^natLa carrier. In vivo experiments with [¹⁷⁷Lu]-LanM, [^132/135La]-LanM, and a prostate cancer targeting-vector linked conjugate, [^132/135La]-LanM-PSMA, reveal that endogenously labeled constructs produce bone uptake in vivo. Exogenous, chelator-tag mediated radiolabeling to produce [⁸⁹Zr]-DFO-LanM enables further study of the protein's in vivo behavior, demonstrating low bone and liver uptake, and renal clearance of the protein itself. While these results indicate that additional stabilization of LanM is required, this study establishes precedence for the radiochemical labeling of LanM with medically relevant lanthanide radioisotopes.

Evaluation of a Radio-IMmunoStimulant (RIMS) in a Syngeneic Model of Murine Prostate Cancer and ImmunoPET Analysis of T-cell Distribution

An immunosuppressive tumor microenvironment and tumor heterogeneity have led to the resilience of metastatic castrate resistant prostate cancer (mCRPC) to current treatments. To address these challenges, we developed and evaluated a new drug paradigm, Radio-IMmunostimulant (RIMS), in a syngeneic model of murine prostate cancer. RIMS-1 was generated using a convergent synthesis employing solid phase peptide and solution chemistries. The prostate-specific membrane antigen (PSMA) inhibitory constant for ^natLu-RIMS-1 was determined, and radiolabeling with ¹⁷⁷Lu generated ¹⁷⁷Lu-RIMS-1. The TLR 7/8 agonist payload release from ^natLu-RIMS-1 was determined using a cathepsin B assay. The biodistribution of ¹⁷⁷Lu-RIMS-1 was evaluated in a bilateral xenograft model in NCru nude mice bearing PSMA(+) (PC3-PiP) and PSMA(-) (PC3-Flu) tumors at 2, 24, and 72 h. The therapeutic effect of ¹⁷⁷Lu-RIMS-1 was evaluated in C57BL/6J mice bearing RM1-PGLS (PSMA-positive, green fluorescent protein-positive, and luciferase-positive) tumors and compared to that of ¹⁷⁷Lu-PSMA-617 at the same total administered radioactivity of 57 MBq and molar activity of 5.18 MBq/nmol. ^natLu-RIMS-1 and vehicle were evaluated as the controls. Immuno-positron emission tomography (PET) using ⁸⁹Zr-DFO-anti-CD3 was used to visualize T-cell distribution during treatment. ¹⁷⁷Lu-RIMS-1 was quantitatively radiolabeled at >99% radiochemical purity and maintained a high affinity toward PSMA (K_i = 3.77 ± 0.5 nM). Cathepsin B efficiently released the entire immunostimulant payload in 17.6 h. ¹⁷⁷Lu-RIMS-1 displayed a sustained uptake in PSMA(+) tumor tissue up to 72 h (2.65 ± 1.03% ID/g) and was not statistically different (P = 0.1936) compared to ¹⁷⁷Lu-PSMA-617 (3.65 ± 0.59% ID/g). All animals treated with ¹⁷⁷Lu-RIMS-1 displayed tumor growth suppression and provided a median survival of 30 days (P = 0.0007) while ¹⁷⁷Lu-PSMA-617 provided a median survival of 15 days, which was not statistically significant (P = 0.3548) compared to the vehicle group (14 days). ImmunoPET analysis revealed 2-fold more tumor infiltrating T-cells in ¹⁷⁷Lu-RIMS-1-treated animals compared to ¹⁷⁷Lu-PSMA-617-treated animals; ¹⁷⁷Lu-RIMS-1 improves therapeutic outcomes in a syngeneic model of mouse prostate cancer and elicits greater T-cell infiltration to the tumor compared to ¹⁷⁷Lu-PSMA-617. These results support further investigation of the RIMS paradigm as the first example of a single molecular entity combining radiotherapy and immunostimulation.

Advances in PSMA theranostics

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PSMA is an appealing target for theranostic because it is a transmembrane protein with a known substrate that is overexpessed on prostate cancer cells and internalizes upon ligand binding.

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There are a number of PSMA theranostic ligands in clinical evaluation, clinical trial, or clinically approved.

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PSMA theranostic ligands increase progression-free survival, overall survival, and pain in patients with metastatic castration resistant prostate cancer.

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A major obstacle to PSMA-targeted radioligand therapy is off-target toxicity in salivary glands.

The validation of prostate specific membrane antigen (PSMA) as a molecular target in metastatic castration-resistant prostate cancer has stimulated the development of multiple classes of theranostic ligands that specifically target PSMA. Theranostic ligands are used to image disease or selectively deliver cytotoxic radioactivity to cells expressing PSMA according to the radioisotope conjugated to the ligand. PSMA theranostics is a rapidly advancing field that is now integrating into clinical management of prostate cancer patients. In this review we summarize published research describing the biological role(s) and activity of PSMA, highlight the most clinically advanced PSMA targeting molecules and biomacromolecules, and identify next generation PSMA ligands that aim to further improve treatment efficacy. The goal of this review is to provide a comprehensive assessment of the current state-of-play and a roadmap to achieving further advances in PSMA theranostics.