STEAP2 in Prostate Cancer: AstraZeneca’s Multi-Modality Bet on a Next-Wave Target
The American Association for Cancer Research (AACR) Annual Meeting has consistently served as the premier stage for unveiling novel preclinical research, therapeutic targets, and potential first-in-class (FIC) molecules. During the 2026 meeting, the "New Drugs on the Horizon" session, organized by the AACR Chemistry in Cancer Research (CICR) working group, highlighted the chemical and biological properties of several emerging oncology assets. Among these, AstraZeneca presented AZD8359, a next-generation CD8-biased T-cell engager (TCE) targeting STEAP2 [1].
A comparable saturation strategy was previously observed with Johnson amp; Johnsons (Jamp;J) advancement of multiple pipelines targeting KLK2, another prostate cancer antigen. Notably, early clinical data presented at ASCO GU for Jamp;Js KLK2 TCE, Pasritamig, demonstrated a remarkable zero-incidence rate of Cytokine Release Syndrome (CRS)—a safety profile that has sparked significant industry discussion [2]. This raises critical questions: Is there a fundamental scarcity of viable targets in prostate cancer, and what underlying biological attributes make STEAP2 a superior therapeutic anchor?
STEAP2 Biology and Expression: Rationale for a Therapeutic Target in Advanced Prostate Cancer
STEAP2 (Six-Transmembrane Epithelial Antigen of the Prostate 2) is a highly attractive therapeutic target due to its abundant overexpression across all stages of prostate adenocarcinoma and its fundamental role in driving aggressive cancer phenotypes [3]. As a metalloreductase, it catalyzes the reduction of Cu²⁺ to Cu⁺ and Fe³⁺ to Fe²⁺, facilitating critical cellular metabolism and proliferation [4]. Its overexpression correlates positively with Gleason scores, driving cell migration and invasion [4]. Crucially, STEAP2 exhibits highly restricted expression in normal tissues (with virtually zero expression in the brain and blood compartments), providing an exceptionally wide therapeutic window.
Structurally, STEAP2 is a Type II transmembrane protein belonging to the STEAP family (which includes the similarly prominent target STEAP1), featuring six transmembrane domains (TM1–TM6) [5]. Pathologically, it hyperactivates the PI3K/AKT/mTOR and MMP-7 signaling pathways to promote tumor invasion. From a pharmacological perspective, its extracellular domains are primarily restricted to ECL2 (conformation-dependent) and ECL3 [5-9]. Most importantly, the intracellular sequence contains a functional YXXØ internalization motif, facilitating highly efficient endosomal/lysosomal trafficking [5-9]. This structural feature provides a flawless foundation for endocytosis-dependent modalities such as ADCs.
Immunological Remodeling: AstraZeneca's CD8-Biased TCE Platforms
In the bispecific arena, AstraZeneca has deployed a dual-pipeline strategy: AZD8359 (TCR×CD8×STEAP2) and AZD6621 (CD3×CD8×STEAP2). Highlighted at AACR 2026, AZD8359 is a TCR-targeted, CD8-biased TCE that mediates STEAP2-dependent T-cell crosslinking, demonstrating an optimized therapeutic index in both in vitro and in vivo models [10]. AZD6621, a tri-specific TCE targeting STEAP2, CD3, and CD8, has recently entered the ACTIVATED-4-PC Phase I/II clinical trial for mCRPC [11-12].
The strategic logic behind these molecules is closely related to AstraZeneca’s TITAN platform, short for Target-Induced T-cell Activating Nanobodies. Unlike many conventional TCE strategies that primarily rely on CD3 engagement and may require careful tuning to reduce systemic immune activation, TITAN aims to use high-affinity CD8 binding to preferentially activate cytotoxic CD8+ T cells [13].
This design may be particularly relevant in prostate cancer. Conventional TCEs can be limited by cytokine release syndrome, T-cell exhaustion, and insufficient activity in immunologically cold tumors. A CD8-biased approach is intended to focus immune activation on the cytotoxic T-cell compartment while reducing activation of CD4+ helper T cells, which are major contributors to inflammatory cytokine release [13–15]. For older patients with prostate cancer, who may have limited tolerance for severe systemic inflammation, this type of immune-selective engineering could be clinically meaningful.
AstraZeneca has also advanced other TITAN-based TCE programs, including CD20 and GPC3 programs, providing broader platform-level validation for this design concept [14,15]. Beyond AstraZeneca, other companies are also exploring CD8-biased or CD8-selective T-cell engager formats, reflecting growing industry interest in immune redirection strategies that may improve safety while preserving antitumor potency [16].
Antibody-Drug Conjugates (ADC): AZD0516 and Bystander Effect in STEAP2-Positive Tumors
STEAP2’s internalization biology provides a strong rationale for antibody–drug conjugate development. AZD0516 is AstraZeneca’s STEAP2-directed ADC and has been described as a potential first-in-class ADC for prostate cancer [17].
AZD0516 uses an antibody component conjugated through interchain cysteines to a β-glucuronidase-cleavable linker, carrying a topoisomerase I inhibitor payload derived from exatecan, with a reported drug-to-antibody ratio of 8 [17]. In preclinical models, AZD0516 demonstrated low-nanomolar potency, DNA damage induction, apoptotic pathway activation, and durable tumor regression in both CDX and PDX models [17].
One of the most important features of ADCs in advanced prostate cancer is the ability to address antigen heterogeneity. Late-stage prostate tumors often contain spatially heterogeneous tumor cell populations, meaning that not all malignant cells express a target antigen at the same level. A TOP1 inhibitor payload with bystander activity may help eliminate neighboring tumor cells with lower or absent STEAP2 expression, potentially overcoming a resistance mechanism that limits more strictly antigen-dependent approaches.
Radiopharmaceutical Targeting: AZD2284 and STEAP2-Directed RDC Development
Radioconjugate therapy has already reshaped the therapeutic landscape of prostate cancer through PSMA-targeted approaches. AstraZeneca’s AZD2284 extends this concept to STEAP2. AZD2284 is a STEAP2-directed radioconjugate carrying actinium-225, an alpha-emitting radionuclide [18,19].
Alpha emitters such as ²²⁵Ac deliver high linear energy transfer over a very short tissue range, enabling potent DNA double-strand break induction while limiting radiation exposure to surrounding healthy tissue. This property is attractive for prostate cancer, particularly when tumor cells express a target antigen in a disease setting where radiopharmaceutical delivery has already shown clinical and commercial feasibility.
AstraZeneca’s acquisition of Fusion Pharmaceuticals further strengthened its radioconjugate capabilities, particularly in actinium-based radiopharmaceutical development [20]. In preclinical evaluation of AZD2284, Fusion-developed lutetium-177-labeled analogs were used for pharmacological assessment, supporting the translational path toward clinical development [18–20]. AZD2284 has entered Phase I clinical evaluation in metastatic castration-resistant prostate cancer [21].
Armored CAR-T Cell Therapy: AZD0754 and STEAP2 CAR-T Development
Solid tumor CAR-T development remains challenging because of antigen heterogeneity, limited T-cell trafficking, poor persistence, and immunosuppressive tumor microenvironments. Prostate cancer is particularly difficult due to its cold tumor phenotype and suppressive cytokine milieu.
AstraZeneca’s AZD0754 is a STEAP2-targeted CAR-T cell therapy designed for advanced prostate cancer. Its CAR construct includes a STEAP2-targeting single-chain variable fragment and a second-generation signaling framework containing 4-1BB and CD3ζ domains. Importantly, AZD0754 is armored with a dominant-negative TGFβ receptor II, or dnTGFβRII, to resist TGFβ-mediated immunosuppression in the prostate tumor microenvironment [22].
This design reflects a broader principle in next-generation cellular therapy: antigen recognition alone is not enough for solid tumors. The cell therapy must also survive, expand, and remain functional within a suppressive microenvironment. By combining STEAP2 targeting with TGFβ resistance, AZD0754 attempts to solve both the targeting and persistence problems simultaneously [22].
AstraZeneca’s collaboration with AbelZeta further adds strategic depth to this area. AbelZeta has experience in armored CAR-T design and has advanced its own STEAP2 CAR-T program, A-CAR032, into clinical development, with AstraZeneca listed as a partner [23]. While public information does not fully clarify the relationship between A-CAR032 and AZD0754, the collaboration suggests shared technical foundations in STEAP2-directed armored cell therapy.
Cyagen Perspective: Why Translational Models Matter for STEAP2 Programs
For STEAP2 and similar next-generation oncology targets, target expression is only the starting point. The real translational challenge is to select preclinical systems that can evaluate modality-specific biology with sufficient relevance.
A STEAP2 ADC program requires models that preserve antigen density, internalization kinetics, payload sensitivity, and tumor heterogeneity. A STEAP2 TCE program requires systems that can interrogate immune-cell engagement, cytokine release risk, and tumor-cell killing across antigen-expression gradients. A STEAP2 CAR-T program requires models capable of assessing cell expansion, persistence, and resistance to suppressive factors such as TGFβ. A radioconjugate program requires careful evaluation of target expression, biodistribution, tumor retention, and normal-tissue safety.
Cyagen supports translational oncology research through integrated preclinical capabilities, including genetically engineered and humanized rodent model generation, target-specific model development, antibody discovery platforms, viral vector and cell engineering services, and in vivo pharmacology workflows. For research teams exploring STEAP2 or other emerging tumor targets, a scientifically rigorous model strategy is essential for bridging target biology, therapeutic design, and in vivo proof-of-concept.
Conclusion
STEAP2 is emerging as more than another prostate cancer-associated antigen. Its restricted normal tissue expression, cell-surface accessibility, internalization potential, and association with aggressive tumor biology make it a compelling target for multiple therapeutic modalities.
AstraZeneca’s multi-modality STEAP2 strategy—spanning CD8-biased T-cell engagers, ADCs, radioconjugates, and armored CAR-T cells—illustrates how major pharmaceutical companies may increasingly develop high-value targets across the full modality spectrum. Rather than treating each modality as an isolated program, this approach uses different therapeutic formats to address different disease stages, tumor burdens, antigen-expression patterns, and microenvironmental barriers.
For the broader drug discovery community, STEAP2 offers a valuable case study in modern target development: when a target combines tumor specificity, functional relevance, internalization capacity, and platform compatibility, it can become the foundation for an entire therapeutic ecosystem.
Reference
[1] Friedman LS, Hamann LG. (2026) Four novel molecules disclosed at the first New Drugs on the Horizon session. AACR Meeting News. Available at: https://www.aacrmeetingnews.org/four-novel-molecules-disclosed-at-the-first-new-drugs-on-the-horizon-session/.
[2] Patel MR et al. (2026) Safety and efficacy of pasritamig (PAS) + docetaxel (DOCE) in participants with metastatic castration-resistant prostate cancer (mCRPC): Initial results of a phase 1b study. J Clin Oncol. 44(7_suppl):171.
[3] AstraZeneca. (2026) STEAP2 T-cell engager: AZD6621. Available at: https://www.az-oncology-pipeline.com/home/scientific-pillars/steap2-t-cell-engager.html.
[4] AstraZeneca. (2026) STEAP2 ADC: AZD0516. Available at: https://www.az-oncology-pipeline.com/home/scientific-pillars/steap2-adc.html.
[5] Xu M et al. (2022) STEAP1-4 (Six-Transmembrane Epithelial Antigen of the Prostate 1-4) and their clinical implications for prostate cancer. Cancers (Basel). 14(16):4034.
[6] Chen K et al. (2023) Mechanism of stepwise electron transfer in six-transmembrane epithelial antigen of the prostate (STEAP) 1 and 2. Elife. 12:RP88299.
[7] Chen WJ et al. (2021) Regulatory roles of six-transmembrane epithelial antigen of the prostate family members in the occurrence and development of malignant tumors. Front Cell Dev Biol. 9:752426.
[8] Rocha SM et al. (2021) The usefulness of STEAP proteins in prostate cancer clinical practice. In: Bott SRJ, Ng KL, eds. Prostate Cancer [Internet]. Exon Publications. Available at: https://www.ncbi.nlm.nih.gov/books/NBK571332/.
[9] Jones L A et al. (2023) Investigating STEAP2 as a potential therapeutic target for the treatment of aggressive prostate cancer. Cellular and Molecular Biology, 69(4), 179-187.
[10] Sitnikova SI et al. (2026) AZD8359: A novel CD8-biased T cell engager designed to deliver a wider therapeutic window in prostate cancer. Presented at: AACR Annual Meeting 2026; Abstract ND02.
[11] Sitnikova S et al. (2025) AZD6621: Improving the therapeutic index for prostate cancer T cell engagers with a next-generation CD8-guided format. Cancer Res. 85(8_Supplement_1):3514.
[12] AstraZeneca. (2026) A study to learn how safe AZD6621 is, how well it works, and how it moves throughout the body over time, in adult male participants with metastatic prostate cancer (ACTIVATED-4-PC). ClinicalTrials.gov. NCT07192614. Available at: https://clinicaltrials.gov/study/NCT07192614.
[13] Caytte C et al. (2024) Development of TITAN: A new CD8-guided T cell engager platform for hematological and solid tumor applications. J Immunother Cancer. 12(Suppl):1093.
[14] AstraZeneca. (2026) CD20 inhibition: AZD5492. Available at: https://www.az-oncology-pipeline.com/home/scientific-pillars/cd20-inhibition.html.
[15] AstraZeneca. (2026) GPC3 TITAN T-cell engager: AZD9793. Available at: https://www.az-oncology-pipeline.com/home/scientific-pillars/gpc3-titan-t-cell-engager.html#tabs-8e7cbf5fd8-item-080a36fa2f-tab.
[16] Yang Z et al. (2024) Development and characterization of a tri-specific selective CD8 T cell engager (CD8xCD3xCD19) for treatment of B cell lymphoma. Cancer Res. 84(6_Supplement):6704.
[17] Monlish D et al. (2025) Preclinical characterization of AZD0516, a novel STEAP2 antibody-drug conjugate (ADC) for the treatment of prostate cancer. Cancer Res. 85(8_Supplement_1):1158.
[18] AstraZeneca. (2026) STEAP2 radioconjugate: AZD2284. Available at: https://www.az-oncology-pipeline.com/home/scientific-pillars/steap2-radioconjugate.html/.
[19] Monlish D et al. (2025) AZD2284: A novel, alpha-particle emitting radioconjugate targeting STEAP2 in metastatic castration-resistant prostate cancer. Cancer Res. 85(8_Supplement_1):4303.
[20] AstraZeneca. (2024) Acquisition of Fusion completed. AstraZeneca Press Release. Available at: https://www.astrazeneca.com/media-centre/press-releases/2024/acquisition-of-fusion-completed.html.
[21] AstraZeneca. (2026) A phase I study of [225Ac]-AZD2284 in patients with metastatic castration-resistant prostate cancer. ClinicalTrials.gov. NCT06879041. Available at: https://clinicaltrials.gov/study/NCT06879041.
[22] Zanvit P et al. (2023) Antitumor activity of AZD0754, a dnTGFβRII-armored, STEAP2-targeted CAR-T cell therapy, in prostate cancer. J Clin Invest. 133(22):e169655.
[23] AbelZeta. (2026) Pipeline. Available at: https://www.abelzeta.com/pipeline/.
