Preclinical Efficacy of a PSMA-Targeted Actinium-225 Conjugate (225Ac-Macropa-Pelgifatamab): A Targeted Alpha Therapy for Prostate Cancer.

PURPOSE: Initially, prostate cancer responds to hormone therapy, but eventually resistance develops. Beta emitter-based prostate-specific membrane antigen (PSMA)-targeted radionuclide therapy is approved for the treatment of metastatic castration-resistant prostate cancer. Here we introduce a targeted alpha therapy (TAT) consisting of the PSMA antibody pelgifatamab covalently linked to a macropa chelator and labeled with actinium-225 and compare its efficacy and tolerability with other TATs. EXPERIMENTAL DESIGN: The in vitro characteristics and in vivo biodistribution, antitumor efficacy, and tolerability of 225Ac-macropa-pelgifatamab (225Ac-pelgi) and other TATs were investigated in cell line- and patient-derived prostate cancer xenograft models. The antitumor efficacy of 225Ac-pelgi was also investigated in combination with the androgen receptor inhibitor darolutamide. RESULTS: Actinium-225-labeling of 225Ac-pelgi was efficient already at room temperature. Potent in vitro cytotoxicity was seen in PSMA-expressing (LNCaP, MDA-PCa-2b, and C4-2) but not in PSMA-negative (PC-3 and DU-145) cell lines. High tumor accumulation was seen for both 225Ac-pelgi and 225Ac-DOTA-pelgi in the MDA-PCa-2b xenograft model. In the C4-2 xenograft model, 225Ac-pelgi showed enhanced antitumor efficacy with a T/Cvolume (treatment/control) ratio of 0.10 compared with 225Ac-DOTA-pelgi, 225Ac-DOTA-J591, and 227Th-HOPO-pelgifatamab (227Th-pelgi; all at 300 kBq/kg) with T/Cvolume ratios of 0.37, 0.39, and 0.33, respectively. 225Ac-pelgi was less myelosuppressive than 227Th-pelgi. 225Ac-pelgi showed dose-dependent treatment efficacy in the patient-derived KuCaP-1 model and strong combination potential with darolutamide in both cell line- (22Rv1) and patient-derived (ST1273) xenograft models. CONCLUSIONS: These results provide a strong rationale to investigate 225Ac-pelgi in patients with prostate cancer. A clinical phase I study has been initiated (NCT06052306).

Targeted thorium-227 conjugates as treatment options in oncology.

Targeted alpha therapy (TAT) is a promising approach for addressing unmet needs in oncology. Inherent properties make α-emitting radionuclides well suited to cancer therapy, including high linear energy transfer (LET), penetration range of 2-10 cell layers, induction of complex double-stranded DNA breaks, and immune-stimulatory effects. Several alpha radionuclides, including radium-223 (223Ra), actinium-225 (225Ac), and thorium-227 (227Th), have been investigated. Conjugation of tumor targeting modalities, such as antibodies and small molecules, with a chelator moiety and subsequent radiolabeling with α-emitters enables specific delivery of cytotoxic payloads to different tumor types. 223Ra dichloride, approved for the treatment of patients with metastatic castration-resistant prostate cancer (mCRPC) with bone-metastatic disease and no visceral metastasis, is the only approved and commercialized alpha therapy. However, 223Ra dichloride cannot currently be complexed to targeting moieties. In contrast to 223Ra, 227Th may be readily chelated, which allows radiolabeling of tumor targeting moieties to produce targeted thorium conjugates (TTCs), facilitating delivery to a broad range of tumors. TTCs have shown promise in pre-clinical studies across a range of tumor-cell expressing antigens. A clinical study in hematological malignancy targeting CD22 has demonstrated early signs of activity. Furthermore, pre-clinical studies show additive or synergistic effects when TTCs are combined with established anti-cancer therapies, for example androgen receptor inhibitors (ARI), DNA damage response inhibitors such as poly (ADP)-ribose polymerase inhibitors or ataxia telangiectasia and Rad3-related kinase inhibitors, as well as immune checkpoint inhibitors.

Immunostimulatory effects of targeted thorium-227 conjugates as single agent and in combination with anti-PD-L1 therapy.

BACKGROUND: Targeted thorium-227 conjugates (TTCs) are an emerging class of targeted alpha therapies (TATs). Their unique mode of action (MoA) is the induction of difficult-to-repair clustered DNA double-strand breaks. However, thus far, their effects on the immune system are largely unknown. Here, we investigated the immunostimulatory effects of the mesothelin-targeted thorium-227 conjugate (MSLN-TTC) in vitro and in vivo in monotherapy and in combination with an inhibitor of the immune checkpoint programmed death receptor ligand 1 (PD-L1) in immunocompetent mice. METHODS: The murine cell line MC38 was transfected with the human gene encoding for MSLN (hMSLN) to enable binding of the non-cross-reactive MSLN-TTC. The immunostimulatory effects of MSLN-TTC were studied in vitro on human cancer cell lines and MC38-hMSLN cells. The efficacy and MoA of MSLN-TTC were studied in vivo as monotherapy or in combination with anti-PD-L1 in MC38-hMSLN tumor-bearing immunocompetent C57BL/6 mice. Experiments were supported by RNA sequencing, flow cytometry, immunohistochemistry, mesoscale, and TaqMan PCR analyses to study the underlying immunostimulatory effects. In vivo depletion of CD8+ T cells and studies with Rag2/Il2Rg double knockout C57BL/6 mice were conducted to investigate the importance of immune cells to the efficacy of MSLN-TTC. RESULTS: MSLN-TTC treatment induced upregulation of DNA sensing pathway transcripts (IL-6, CCL20, CXCL10, and stimulator of interferon genes (STING)-related genes) in vitro as determined by RNASeq analysis. The results, including phospho-STING activation, were confirmed on the protein level. Danger-associated molecular pattern molecules were upregulated in parallel, leading to dendritic cell (DC) activation in vitro. MSLN-TTC showed strong antitumor activity (T:C 0.38, p<0.05) as a single agent in human MSLN-expressing MC38 tumor-bearing immunocompetent mice. Combining MSLN-TTC with anti-PD-L1 further enhanced the efficacy (T:C 0.08, p<0.001) as evidenced by the increased number of tumor-free surviving animals. MSLN-TTC monotherapy caused migration of CD103+ cDC1 DCs and infiltration of CD8+ T cells into tumors, which was enhanced on combination with anti-PD-L1. Intriguingly, CD8+ T-cell depletion decreased antitumor efficacy. CONCLUSIONS: These in vitro and in vivo data on MSLN-TTC demonstrate that the MoA of TTCs involves activation of the immune system. The findings are of relevance for other targeted radiotherapies and may guide clinical combination strategies.

Darolutamide Potentiates the Antitumor Efficacy of a PSMA-targeted Thorium-227 Conjugate by a Dual Mode of Action in Prostate Cancer Models.

PURPOSE: Androgen receptor (AR) inhibitors are well established in the treatment of castration-resistant prostate cancer and have recently shown efficacy also in castration-sensitive prostate cancer. Although most patients respond well to initial therapy, resistance eventually develops, and thus, more effective therapeutic approaches are needed. Prostate-specific membrane antigen (PSMA) is highly expressed in prostate cancer and presents an attractive target for radionuclide therapy. Here, we evaluated the efficacy and explored the mode of action of the PSMA-targeted thorium-227 conjugate (PSMA-TTC) BAY 2315497, an antibody-based targeted alpha-therapy, in combination with the AR inhibitor darolutamide. EXPERIMENTAL DESIGN: The in vitro and in vivo antitumor efficacy and mode of action of the combination treatment were investigated in preclinical cell line-derived and patient-derived prostate cancer xenograft models with different levels of PSMA expression. RESULTS: Darolutamide induced the expression of PSMA in androgen-sensitive VCaP and LNCaP cells in vitro, and the efficacy of darolutamide in combination with PSMA-TTC was synergistic in these cells. In vivo, the combination treatment showed synergistic antitumor efficacy in the low PSMA-expressing VCaP and in the high PSMA-expressing ST1273 prostate cancer models, and enhanced efficacy in the enzalutamide-resistant KUCaP-1 model. The treatments were well tolerated. Mode-of-action studies revealed that darolutamide induced PSMA expression, resulting in higher tumor uptake of PSMA-TTC, and consequently, higher antitumor efficacy, and impaired PSMA-TTC-mediated induction of DNA damage repair genes, potentially contributing to increased DNA damage. CONCLUSIONS: These results provide a strong rationale to investigate PSMA-TTC in combination with AR inhibitors in patients with prostate cancer.

227Th-Labeled Anti-CD22 Antibody (BAY 1862864) in Relapsed/Refractory CD22-Positive Non-Hodgkin Lymphoma: A First-in-Human, Phase I Study.

Background: BAY 1862864 is an α-particle emitting 227Th-labeled CD22-targeting antibody. This first-in-human dose-escalation phase I study evaluated BAY 1862864 in patients with CD22-positive relapsed/refractory B cell non-Hodgkin lymphoma (R/R-NHL). Materials and Methods: BAY 1862864 intravenous injections were administered at the starting 227Th radioactivity dose of 1.5 MBq (2 or 10 mg antibody), and the radioactivity dose escalated in ∼1.5 MBq increments (10 mg antibody) until the maximum tolerated dose (MTD) was reported. The primary objective was to determine the safety, tolerability, and MTD. Results: Twenty-one patients received BAY 1862864. Two dose-limiting toxicities (grade 3 febrile neutropenia and grade 4 thrombocytopenia) were reported in one patient in the 4.6 MBq (10 mg antibody) cohort. The MTD was not reached. Ten (48%) patients reported grade ≥3 treatment-emergent adverse events, with the most common being neutropenia, thrombocytopenia, and leukopenia, each occurring in 3 (14%) patients. Pharmacokinetics demonstrated the dose proportionality and stability of BAY 1862864 in the blood. The objective response rate (ORR) was 25% (5/21 patients) according to the LUGANO 2014 criteria, including 1 complete and 4 partial responses. The ORR was 11% (1/9) and 30% (3/10) in patients with relapsed high- and low-grade lymphomas, respectively. Conclusions: BAY 1862864 was safe and tolerated in patients with R/R-NHL. Clinical Trial Registration numbers: NCT02581878 and EudraCT 2014-004140-36.

Advances in Precision Oncology: Targeted Thorium-227 Conjugates As a New Modality in Targeted Alpha Therapy.

Targeted α therapy (TAT) offers the potential for the targeted delivery of potent α-particle-emitting radionuclides that emit high linear energy transfer radiation. This leads to a densely ionizing radiation track over a short path. Localized radiation induces cytotoxic, difficult-to-repair, clustered DNA double-strand breaks (DSBs). To date, radium-223 (223Ra) is the only TAT approved for the treatment of patients with metastatic castration-resistant prostate cancer. Thorium-227 (227Th), the progenitor nuclide of 223Ra, offers promise as a wider-ranging alternative due to the availability of efficient chelators, such as octadentate 3,2-hydroxypyridinone (3,2-HOPO). The 3,2-HOPO chelator can be readily conjugated to a range of targeting moieties, enabling the generation of new targeted thorium-227 conjugates (TTCs). This review provides a comprehensive overview of the advances in the preclinical development of TTCs for hematological cancers, including CD22-positive B cell cancers and CD33-positive leukemia, as well as for solid tumors overexpressing renal cell cancer antigen CD70, membrane-anchored glycoprotein mesothelin in mesothelioma, prostate-specific membrane antigen in prostate cancer, and fibroblast growth factor receptor 2. As the mechanism of action for TTCs is linked to the formation of DSBs, the authors also report data supporting combinations of TTCs with inhibitors of the DNA damage response pathways, including those of the ataxia telangiectasia and Rad3-related protein, and poly-ADP ribose polymerase. Finally, emerging evidence suggests that TTCs induce immunogenic cell death through the release of danger-associated molecular patterns. Based on encouraging preclinical data, clinical studies have been initiated to investigate the safety and tolerability of TTCs in patients with various cancers.

Preclinical Efficacy of a PSMA-Targeted Thorium-227 Conjugate (PSMA-TTC), a Targeted Alpha Therapy for Prostate Cancer.

PURPOSE: Prostate-specific membrane antigen (PSMA) is an attractive target for radionuclide therapy of metastatic castration-resistant prostate cancer (mCRPC). PSMA-targeted alpha therapy (TAT) has shown early signs of activity in patients with prostate cancer refractory to beta radiation. We describe a novel, antibody-based TAT, the PSMA-targeted thorium-227 conjugate PSMA-TTC (BAY 2315497) consisting of the alpha-particle emitter thorium-227 complexed by a 3,2-HOPO chelator covalently linked to a fully human PSMA-targeting antibody. EXPERIMENTAL DESIGN: PSMA-TTC was characterized for affinity, mode of action, and cytotoxic activity in vitro. Biodistribution, pharmacokinetics, and antitumor efficacy were investigated in vivo using cell line and patient-derived xenograft (PDX) models of prostate cancer. RESULTS: PSMA-TTC was selectively internalized into PSMA-positive cells and potently induced DNA damage, cell-cycle arrest, and apoptosis in vitro. Decrease in cell viability was observed dependent on the cellular PSMA expression levels. In vivo, PSMA-TTC showed strong antitumor efficacy with T/C values of 0.01 to 0.31 after a single injection at 300 to 500 kBq/kg in subcutaneous cell line and PDX models, including models resistant to standard-of-care drugs such as enzalutamide. Furthermore, inhibition of both cancer and cancer-induced abnormal bone growth was observed in a model mimicking prostate cancer metastasized to bone. Specific tumor uptake and efficacy were demonstrated using various PSMA-TTC doses and dosing schedules. Induction of DNA double-strand breaks was identified as a key mode of action for PSMA-TTC both in vitro and in vivo. CONCLUSIONS: The strong preclinical antitumor activity of PSMA-TTC supports its clinical evaluation, and a phase I trial is ongoing in mCRPC patients (NCT03724747).

Mesothelin-Targeted Thorium-227 Conjugate (MSLN-TTC): Preclinical Evaluation of a New Targeted Alpha Therapy for Mesothelin-Positive Cancers.

PURPOSE: Targeted thorium-227 conjugates (TTC) represent a new class of molecules for targeted alpha therapy (TAT). Covalent attachment of a 3,2-HOPO chelator to an antibody enables specific complexation and delivery of the alpha particle emitter thorium-227 to tumor cells. Because of the high energy and short penetration range, TAT efficiently induces double-strand DNA breaks (DSB) preferentially in the tumor cell with limited damage to the surrounding tissue. We present herein the preclinical evaluation of a mesothelin (MSLN)-targeted thorium-227 conjugate, BAY 2287411. MSLN is a GPI-anchored membrane glycoprotein overexpressed in mesothelioma, ovarian, pancreatic, lung, and breast cancers with limited expression in healthy tissue. EXPERIMENTAL DESIGN: The binding activity and radiostability of BAY 2287411 were confirmed bioanalytically. The mode-of-action and antitumor potency of BAY 2287411 were investigated in vitro and in vivo in cell line and patient-derived xenograft models of breast, colorectal, lung, ovarian, and pancreatic cancer. RESULTS: BAY 2287411 induced DSBs, apoptotic markers, and oxidative stress, leading to reduced cellular viability. Furthermore, upregulation of immunogenic cell death markers was observed. BAY 2287411 was well-tolerated and demonstrated significant antitumor efficacy when administered via single or multiple dosing regimens in vivo. In addition, significant survival benefit was observed in a disseminated lung cancer model. Biodistribution studies showed specific uptake and retention of BAY 2287411 in tumors and enabled the development of a mechanistic pharmacokinetic/pharmacodynamic model to describe the preclinical data. CONCLUSIONS: These promising preclinical results supported the transition of BAY 2287411 into a clinical phase I program in mesothelioma and ovarian cancer patients (NCT03507452).

Meeting report from the Prostate Cancer Foundation PSMA-directed radionuclide scientific working group.

INTRODUCTION: The Prostate Cancer Foundation (PCF) convened a PSMA-Directed Radionuclide Scientific Working Group on November 14, 2017, at Weill Cornell Medicine, New York, NY. METHODS: The meeting was attended by 35 global investigators with expertise in prostate cancer biology, radionuclide therapy, molecular imaging, prostate-specific membrane antigen (PSMA)-targeted agents, drug development, and prostate cancer clinical trials. The goal of this meeting was to discuss the potential for using PSMA-targeted radionuclide agents for the treatment of advanced prostate cancer and to define the studies and clinical trials necessary for validating and optimizing the use of these agents. RESULTS: Several major topic areas were discussed including the overview of PSMA biology, lessons and applications of PSMA-targeted PET imaging, the nuances of designing PSMA-targeted radionuclide agents, clinical experiences with PSMA-targeted radionuclides, PCF-funded projects to accelerate PSMA-targeted radionuclide therapy, and barriers to the use of radionuclide treatments in widespread clinical practice. DISCUSSION: This article reviews the major topics discussed at the meeting with the goal of promoting research that will validate and optimize the use of PSMA-targeted radionuclide therapies for the treatment of advanced prostate cancer.

Dimethylfumarate induces immunosuppression via glutathione depletion and subsequent induction of heme oxygenase 1.

A mixture of different fumaric acid esters (FAE) is established for systemic therapy of psoriasis, a frequent inflammatory skin disease. The main active compound of FAE, however, has not been identified so far, and the mechanisms of activity are only partially understood. We analyzed the impact of FAE on in vitro immune function and aimed to gain knowledge about the mode of action. Dimethylfumarate (DMF) and diethylfumarate (DEF), but not fumaric acid, methylhydrogenfumarate and ethylhydrogenfumarate, exhibited potent depression of inflammatory cytokine secretion (e.g., tumor necrosis factoralpha, IL-12, and IFNgamma) in activated human peripheral blood mononuclear cells. Moreover, solely DMF and DEF inhibited alloreactive T-cell proliferation in mixed leukocyte reaction. Interestingly, these immunosuppressive effects were accompanied by the strong induction of the anti-inflammatory stress protein heme oxygenase 1 (HO-1). Supplementation with exogenous glutathione (GSH), which is known to bind DMF, prevented both HO-1 induction as well as the anti-inflammatory effects of DMF. Moreover, inhibition of HO-1 activity restored the diminished IL-12 and IFNgamma production after FAE treatment. These results suggest that DMF acts as active compound within the FAE mixture and at least partially mediates its immunomodulatory activity by the induction of the anti-inflammatory stress protein HO-1 ascribed to the functional depletion of reduced GSH.

Dissociation of transactivation from transrepression by a selective glucocorticoid receptor agonist leads to separation of therapeutic effects from side effects.

Glucocorticoids (GCs) are the most commonly used antiinflammatory and immunosuppressive drugs. Their outstanding therapeutic effects, however, are often accompanied by severe and sometimes irreversible side effects. For this reason, one goal of research in the GC field is the development of new drugs, which show a reduced side-effect profile while maintaining the antiinflammatory and immunosuppressive properties of classical GCs. GCs affect gene expression by both transactivation and transrepression mechanisms. The antiinflammatory effects are mediated to a major extent via transrepression, while many side effects are due to transactivation. Our aim has been to identify ligands of the GC receptor (GR), which preferentially induce transrepression with little or no transactivating activity. Here we describe a nonsteroidal selective GR-agonist, ZK 216348, which shows a significant dissociation between transrepression and transactivation both in vitro and in vivo. In a murine model of skin inflammation, ZK 216348 showed antiinflammatory activity comparable to prednisolone for both systemic and topical application. A markedly superior side-effect profile was found with regard to increases in blood glucose, spleen involution, and, to a lesser extent, skin atrophy; however, adrenocorticotropic hormone suppression was similar for both compounds. Based on these findings, ZK 216348 should have a lower risk, e.g., for induction of diabetes mellitus. The selective GR agonists therefore represent a promising previously undescribed class of drug candidates with an improved therapeutic index compared to classical GCs. Moreover, they are useful tool compounds for further investigating the mechanisms of GR-mediated effects.