JNJ-88549968 Advanced Drug Monograph

Report on JNJ-88549968: A Novel CALRmut-Targeting T-Cell Redirecting Bispecific Antibody for Myeloproliferative Neoplasms

I. Executive Summary

JNJ-88549968 is an investigational, first-in-class, T-cell redirecting bispecific antibody developed by Janssen Research & Development, LLC, a Johnson & Johnson company.[1] This therapeutic agent is engineered to dually target the mutated form of the calreticulin protein (CALRmut), a neoantigen specifically expressed on the surface of myeloproliferative neoplasm (MPN) cells, and the CD3 antigen present on T-lymphocytes.[1] The core mechanism of JNJ-88549968 involves forming a bridge between these two cell types, thereby inducing T-cell activation and subsequent T-cell-mediated cytotoxic destruction of the CALRmut-expressing cancer cells.[1]

The primary therapeutic focus for JNJ-88549968 encompasses CALR-mutated MPNs, with essential thrombocythemia (ET) and myelofibrosis (MF) being lead indications.[1] Currently, JNJ-88549968 is advancing through Phase 1 clinical development, with the first-in-human trial, identified as NCT06150157, actively enrolling patients.[2]

Preclinical investigations have provided substantial evidence supporting the drug's mode of action. These studies have demonstrated the antibody's high selectivity for CALRmut-expressing cells and its potent anti-tumor activity in both in vitro and in vivo models.[1] The development of JNJ-88549968 is driven by the significant unmet medical need for curative or disease-modifying therapies for patients with CALR-mutated MPNs, as current treatments are often palliative and do not target the underlying genetic driver of the disease.[1] JNJ-88549968 represents a promising targeted immunotherapy that leverages the specificity of CALR mutations as neoantigens, offering the potential to overcome the limitations of existing MPN therapies by selectively eliminating the malignant clone.

II. Introduction: Myeloproliferative Neoplasms and the Role of CALR Mutations

Overview of Myeloproliferative Neoplasms (MPNs)

Myeloproliferative neoplasms (MPNs) constitute a group of chronic hematologic malignancies that originate from clonal abnormalities within hematopoietic stem cells (HSCs). These disorders are fundamentally characterized by the excessive production of one or more types of mature myeloid blood cells.[1] The classical Philadelphia-negative MPNs include essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF).

ET is primarily defined by a sustained elevation in platelet counts (thrombocytosis), which predisposes patients to an increased risk of thrombotic and hemorrhagic events. Over time, ET can progress to myelofibrosis or, less commonly, transform into acute myeloid leukemia (AML).[6] Myelofibrosis, which can arise de novo (as PMF) or evolve from pre-existing ET or PV (secondary MF), is characterized by progressive bone marrow fibrosis, ineffective hematopoiesis leading to cytopenias (particularly anemia), extramedullary hematopoiesis resulting in significant splenomegaly, and debilitating constitutional symptoms.[1] Patients with MF face a heightened risk of transformation to sAML, which is a frequent cause of mortality in this patient population.[1] The clinical course of MPNs can be indolent for many years but carries the persistent threat of severe complications and leukemic transformation.

The Significance and Molecular Biology of Calreticulin (CALR) Mutations in MPN Pathogenesis

The discovery of somatic driver mutations has revolutionized the understanding of MPN pathogenesis. Mutations in the calreticulin (CALR) gene are among the most significant genetic alterations identified in MPNs, occurring in approximately 20-35% of patients with ET and PMF.[1] These mutations are typically absent in PV and are generally mutually exclusive with mutations in JAK2 (Janus kinase 2) and MPL (myeloproliferative leukemia virus oncogene, encoding the thrombopoietin receptor).[10]

CALR mutations are predominantly frameshift mutations, specifically insertions or deletions within exon 9. These genetic alterations invariably lead to a +1 base pair frameshift, resulting in the generation of a novel C-terminal amino acid sequence in the CALR protein.[7] A critical consequence of this altered C-terminus is the loss of the KDEL (Lys-Asp-Glu-Leu) motif, which normally functions as an endoplasmic reticulum (ER) retention signal.[10] The absence of this KDEL sequence allows the mutant CALR protein (CALRmut) to escape the ER and traffic to the cell surface.

Once on the cell surface, CALRmut aberrantly interacts with the extracellular domain of the thrombopoietin receptor, MPL.[10] This interaction is believed to induce ligand-independent dimerization or conformational changes in MPL, leading to the constitutive activation of its associated downstream signaling pathways, most notably the JAK2-STAT3/5 (Signal Transducer and Activator of Transcription 3/5) pathway.[10] This persistent, uncontrolled signaling drives the clonal proliferation and survival of hematopoietic cells, contributing directly to the MPN phenotype. The unique C-terminal peptide sequence generated by CALR frameshift mutations serves as a tumor-specific neoantigen, as it is not present in wild-type CALR or other normal cellular proteins.[11] JNJ-88549968 is designed to recognize epitopes within this common neoantigenic C-terminus, which is shared across the vast majority of pathogenic CALR mutations, including the most frequent Type 1 (a 52-bp deletion, often denoted L367fs*46 or del52) and Type 2 (a 5-bp insertion, K385fs*47 or ins5) mutations.[1] While different CALR mutation subtypes, such as Type 1 and Type 2, can arise from distinct frameshift events (e.g., CALRdel52 and CALRins5), they converge on producing this shared novel C-terminal peptide, which is the target for therapies like JNJ-88549968. Studies have explored whether these different mutation types might have differential effects on downstream pathways; for instance, Type 1 and Type 2 CALR mutations have been shown to differentially regulate mTORC2 activity.[17] However, the critical aspect for CALRmut-targeting antibodies is the presence of the common neoepitope on the cell surface.

Current Treatment Landscape for CALR-mutated MPNs and Existing Unmet Medical Needs

The current therapeutic landscape for MPNs, including those harboring CALR mutations, is primarily focused on managing symptoms, reducing the risk of thrombotic and hemorrhagic complications, and, in some cases, controlling disease proliferation. For ET, treatment strategies often involve observation for low-risk patients, or cytoreductive agents such as hydroxyurea, anagrelide, or pegylated interferon-alpha for higher-risk patients.[10] In MF, JAK inhibitors like ruxolitinib have become a standard of care for alleviating splenomegaly and constitutional symptoms, but their impact on bone marrow fibrosis or clonal evolution is limited.[1]

These existing treatments are generally not curative and do not specifically target the underlying CALRmut driver. They do not typically lead to the eradication of the malignant clone, and patients often continue to be at risk of disease progression or transformation.[1] Allogeneic stem cell transplantation (alloHSCT) is the only therapeutic modality with curative potential for MF, but its application is restricted by patient eligibility (age, comorbidities) and the significant risks of treatment-related morbidity and mortality.[10]

Consequently, a substantial unmet medical need persists for novel therapies in CALR-mutated MPNs. There is a pressing demand for treatments that can specifically target the CALR-mutant clone, achieve deep molecular remissions, alter the natural history of the disease, and potentially offer a cure.[1] The development of therapies like JNJ-88549968, which are designed to selectively eliminate cells expressing the CALRmut neoantigen, directly addresses this critical unmet need. The unique biology of CALR mutations, resulting in a tumor-specific surface protein, offers a rational target for such innovative immunotherapeutic strategies.

III. JNJ-88549968: Drug Profile and Mechanism of Action

Classification and Developer

JNJ-88549968 is classified as a novel, first-in-class, T-cell redirecting bispecific antibody.[1] Some sources also refer to this class of molecules as Bispecific T-cell Engagers (BiTEs).[4] The drug is under development by Janssen Research & Development, LLC, which is a pharmaceutical company of Johnson & Johnson.[2]

Alternative Names/Synonyms

The investigational drug JNJ-88549968 is also referred to by several synonyms and developmental codes in the literature and clinical trial registries. These include:

• anti-CALRmut/anti-CD3 bispecific antibody JNJ-88549968 [7]
• anti-mutant calreticulin/anti-CD3 bispecific antibody JNJ-88549968 [7]
• CALRmutxCD3 T-cell redirecting antibody JNJ-88549968 [1]
• JNJ 88549968 [7]
• JNJ88549968 [7]

Detailed Mechanism of Action

The therapeutic strategy of JNJ-88549968 is predicated on its bispecific nature, enabling it to concurrently engage two distinct cellular targets, thereby orchestrating a targeted immune response against CALR-mutated cancer cells.

• Dual Targeting Specificity: JNJ-88549968 possesses two distinct antigen-binding domains. One domain is engineered to specifically recognize and bind to the mutated form of calreticulin (CALRmut). This mutant protein, characterized by its novel C-terminal neoantigen, is aberrantly expressed on the cell surface of CALR-mutated MPN cells, including the critical CD34-positive hematopoietic stem and progenitor cells that drive the disease.[1] The other antigen-binding domain of JNJ-88549968 is directed against the CD3 epsilon (CD3ε) subunit, a component of the T-cell receptor (TCR) complex that is ubiquitously expressed on the surface of virtually all T-lymphocytes.[1]
• Immune Synapse Formation and T-Cell Activation: By simultaneously binding to CALRmut on the MPN cell and CD3 on the T-cell, JNJ-88549968 functions as a molecular bridge. This cross-linkage physically draws the T-cell into close proximity with the cancer cell, facilitating the formation of a cytotoxic immunological synapse.[1] The engagement of the CD3 complex by JNJ-88549968, when co-ligated with the tumor cell, mimics the natural TCR activation signal, leading to potent, polyclonal activation of the T-cell. This activation occurs independently of the T-cell's intrinsic antigen specificity and does not require traditional MHC (Major Histocompatibility Complex)-restricted antigen presentation.[1]
• Redirected T-Cell Cytotoxicity: Once activated, the T-cells unleash their cytotoxic effector mechanisms directly against the adjoined CALRmut-expressing MPN cell. These mechanisms include the release of cytotoxic granules containing perforin and granzymes, which induce apoptosis in the target cell, and potentially the engagement of death receptor pathways such as Fas/FasL.[1] This redirected lysis results in the selective elimination of the CALR-mutated cells.
• Broad Recognition of CALRmut Variants: A key aspect of JNJ-88549968's design is its ability to recognize the common neoantigenic C-terminus generated by the various frameshift mutations in exon 9 of the CALR gene. This includes the prevalent Type 1 (del52) and Type 2 (ins5) mutations, as well as other less common pathogenic variants that result in the same altered C-terminal peptide sequence.[1] This broad recognition ensures that the therapeutic potential of JNJ-88549968 extends to the majority of patients with CALR-mutated MPNs, irrespective of the precise underlying nucleotide change, as long as it results in the targetable neoepitope. The specificity for the mutant form over wild-type CALR is critical for minimizing toxicity to healthy cells.

The bispecific T-cell engaging approach aims to harness the potent killing capacity of a patient's own T-cells and redirect it with high precision against the malignant clone defined by the CALR mutation. This strategy offers the potential for an "off-the-shelf" immunotherapy, distinct from more complex cell-based therapies like CAR-T cells, which require ex vivo genetic modification of patient T-cells. The success of JNJ-88549968 relies on achieving a therapeutic window where potent on-target cell killing is maximized while systemic T-cell activation and associated toxicities are manageable. The specific antibody format and any Fc-engineering of JNJ-88549968, which could influence its half-life and non-T-cell mediated effector functions or side effects, are not detailed in the provided information but are important considerations for bispecific antibody design. Janssen's access to technologies like the DuoBody platform for bispecific antibody generation suggests sophisticated engineering may be involved.[18]

IV. Preclinical Evidence for JNJ-88549968

The advancement of JNJ-88549968 into clinical trials is supported by a body of preclinical data demonstrating its specificity, mechanism of action, and anti-tumor efficacy.

In Vitro Studies

• Binding Selectivity and Affinity: In vitro assays have confirmed that JNJ-88549968 selectively binds to cell lines engineered to express various forms of mutant CALR (CALRmut) and, importantly, to primary CD34+ hematopoietic stem and progenitor cells isolated from MPN patients harboring CALR mutations. A critical finding is the lack of measurable binding to cells expressing only the wild-type (WT) CALR protein, highlighting the antibody's specificity for the tumor-associated neoantigen.[1] This selectivity is fundamental to its proposed therapeutic window, aiming to spare healthy cells.
• T-Cell Activation: Co-culture experiments involving JNJ-88549968, CALRmut-expressing target cells, and T-lymphocytes (from both healthy donors and CALRmut MPN patients) have demonstrated potent, CALRmut-dependent T-cell activation. This activation is evidenced by the upregulation of T-cell surface markers such as CD25 and CD69, and the secretion of effector cytokines.[1]
• Cytotoxicity Against MPN Cells:
• JNJ-88549968 has shown the ability to mediate concentration-dependent cytotoxic killing of CALRmut-engineered target cell lines. This killing is T-cell dependent, confirming the T-cell redirecting mechanism.[1]
• Of particular significance is the demonstration of robust T-cell mediated cytotoxicity against primary CD34+ cells derived from CALRmut MPN patients in an autologous setting (i.e., using T-cells from the same patient). This activity was observed across various CALR mutation subtypes, including the common CALRdel52 (Type 1) and CALRins5 (Type 2) mutations, as well as less common variants like CALRdel34 and CALRdel4, while CALR WT cells were spared.[1] This broad activity across different mutation types is a key attribute.
• Functional assessments confirmed that T-cells from CALRmut MPN patients, despite potential alterations in their immune phenotype compared to healthy controls, remain competent in mediating cytotoxicity when engaged by JNJ-88549968.[15]

In Vivo Studies

• Xenograft Models: The anti-tumor efficacy of JNJ-88549968 has been evaluated in at least two distinct CALRmut-positive xenograft murine leukemia models. While specific details of these models (e.g., cell lines used, engraftment methods) are not fully elaborated in the available abstracts [1], such models typically involve implanting human CALRmut-expressing cells into immunodeficient mice, often co-administered with human T-cells.
• Survival Benefit: In an established disseminated leukemia model, systemic administration of JNJ-88549968 resulted in a statistically significant increase in the lifespan (Improved Lifespan, ILS) of treated mice compared to vehicle-treated control animals.[1] This finding provides crucial in vivo proof-of-concept for the drug's anti-leukemic activity and its potential to confer a survival benefit.

The preclinical data package, therefore, robustly supports the therapeutic hypothesis for JNJ-88549968. The consistent demonstration of selective binding to CALRmut, leading to specific T-cell activation and potent lysis of malignant cells across various in vitro systems (including patient-derived material) and culminating in in vivo efficacy, provides a strong rationale for its clinical investigation in patients with CALR-mutated MPNs. The activity across multiple CALR mutation types further strengthens its potential clinical utility.

Table 1: Summary of Key Preclinical Findings for JNJ-88549968

Study Type	Model System	Key Parameter Measured	Key Result/Observation	CALRmut Specificity Demonstrated	Source Snippet(s)
In Vitro	CALRmut-engineered cell lines (ELF-153 MPL CALRdel52+, CALRins5+), CALR WT cells	Binding affinity/selectivity	Selective binding to CALRmut cells; no measurable binding to CALR WT cells.	Yes	1
In Vitro	Co-culture: CALRmut+ cells + T-cells (healthy donor or patient)	T-cell activation (e.g., CD25, CD69 expression)	CALRmut-dependent T-cell activation.	Yes	1
In Vitro	Co-culture: CALRmut-engineered cells + T-cells	% Specific Lysis / Cytotoxicity	Concentration-dependent T-cell mediated cytotoxicity of CALRmut+ cells.	Yes	1
In Vitro	Co-culture: Patient-derived CD34+ CALRmut cells + autologous T-cells	% Specific Lysis / Cytotoxicity	Potent cytotoxicity against patient CALRmut CD34+ cells (del52, ins5, del34, del4); CALR WT cells spared.	Yes	1
In Vivo	CALRmut+ xenograft murine leukemia models (at least 2 independent models)	Tumor burden reduction / Survival	Robust in vivo efficacy; significant increase in lifespan (ILS) in a disseminated model vs. vehicle.	Implied by CALRmut model use	1

V. Clinical Development: The NCT06150157 First-in-Human Study

The preclinical validation of JNJ-88549968 has paved the way for its evaluation in human subjects. The cornerstone of its current clinical development is the first-in-human (FIH) Phase 1 trial, NCT06150157.

Trial Identification and Basic Information:

• NCT Number: NCT06150157.[4]
• Official Title: "A First-in-Human Study of the Safety, Pharmacokinetics, and Pharmacodynamics of JNJ-88549968, a T-cell Redirecting Bispecific Antibody for CALR-mutated Myeloproliferative Neoplasms".[6]
• Sponsor: Janssen Research & Development, LLC.[3]
• Phase: Phase 1.[4]
• Status: Currently recruiting participants (as of May 2025, based on available information).[3]
• Secondary Identifiers: IRAS 1008190, CPMS 57124 (UK specific); Janssen study ID 88549968MPN1001.[21]

Study Design and Objectives:

The trial employs a standard open-label, multicenter design typical for FIH oncology studies, structured in two main parts:

• Part 1 (Dose Escalation): The primary goal of this part is to determine the safety and tolerability profile of JNJ-88549968 across ascending dose levels. This phase aims to identify any Dose-Limiting Toxicities (DLTs) and establish the Recommended Phase 2 Dose(s) (RP2D[s]) and the optimal dosing schedule(s) for subsequent investigation.[6]
• Part 2 (Cohort Expansion): Once the RP2D(s) are determined, this part will enroll additional patients into specific cohorts to further characterize the safety of JNJ-88549968 at these dose levels and to gather more extensive data on its activity.[6]

Secondary Objectives and Endpoints:

Beyond the primary safety and dose-finding objectives, the study will assess several secondary and exploratory endpoints:

• Pharmacokinetics (PK): To characterize the absorption, distribution, metabolism, and excretion (ADME) profile of JNJ-88549968 in humans. This includes measuring serum concentrations of the antibody over time to determine parameters such as maximum concentration (Cmax​), time to maximum concentration (Tmax​), area under the concentration-time curve (AUC), elimination half-life (t1/2​), clearance (CL/F), and volume of distribution (Vz​/F).[6]
• Pharmacodynamics (PD): To evaluate the biological effects of JNJ-88549968 on the immune system and on tumor cells. This may include assessments of T-cell activation markers, cytokine release profiles, and changes in CALRmut allele burden in peripheral blood or bone marrow.[6]
• Immunogenicity: To monitor for the development of anti-drug antibodies (ADAs) against JNJ-88549968, which could potentially impact its efficacy or safety.[21]
• Preliminary Anti-Tumor Activity: To obtain initial signals of clinical efficacy, including Overall Response Rate (ORR), Complete Response (CR) rate, Time to Response (TTR), and Duration of Response (DoR). Responses will be assessed according to modified International Working Group-Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria and the modified European LeukemiaNet (ELN) consensus report. Other efficacy measures may include spleen volume reduction and changes in symptom scores.[21]

Target Population and Key Eligibility Criteria:

The study is designed for adult patients with specific CALR-mutated MPNs who have limited treatment options:

• Inclusion Criteria:
• Age ≥ 18 years (or the legal age of majority).[3]
• Confirmed diagnosis of ET or MF with a documented CALR driver mutation.[3]
• Specific prior therapy requirements:
• For ET patients: Must have received at least two prior lines of cytoreductive therapy, with at least one of these being hydroxyurea.[21]
• For MF patients: Must have received at least one prior JAK inhibitor (JAKi) therapy, unless deemed ineligible for JAKi treatment.[21]
• Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2, indicating reasonable functional status.[3]
• Exclusion Criteria:
• Known allergies, hypersensitivity, or intolerance to JNJ-88549968 or its excipients.[3]
• Certain concurrent or recently diagnosed/treated other malignancies (with exceptions for cured, low-risk cancers).[3]
• Prior solid organ transplantation.[3]
• Allogeneic stem cell transplantation within 6 months prior to the first dose of JNJ-88549968, or evidence of active graft-versus-host disease (GVHD) requiring immunosuppressive therapy.[3]
• History of clinically significant cardiovascular disease within 6 months prior to study entry.[3]

Intervention:

JNJ-88549968 is administered via subcutaneous (SC) injection.[22] The dosing regimen (dose level and frequency) will be determined during the dose escalation phase (Part 1) of the study.

Current Status, Enrollment, and Study Locations:

• Status: The trial is actively recruiting participants.[3]
• Enrollment: The target enrollment number is not explicitly stated in the provided snippets.
• Study Locations: This is a multicenter international trial with sites in the United States (including MD Anderson Cancer Center with Dr. Naveen Pemmaraju as a physician contact, and Memorial Sloan Kettering Cancer Center with Dr. Raajit Rampal as an investigator), the United Kingdom, Spain, Germany, Canada, and France.[3]

Timelines:

• Recruitment Start Date: Ethics approval in the UK was granted in December 2023, with the study officially starting around October/November 2023 according to different registry entries.[3]
• Estimated Primary Completion Date: Anticipated around April 2025 to October 2025.[3]
• Estimated Study Completion Date: October 2025.[6]

Available Preliminary Clinical Data:

• EHA 2024 Presentation: While not direct results from treated patients in NCT06150157, Janssen presented in vitro data at the European Hematology Association (EHA) 2024 congress. This presentation focused on T-cell phenotyping and functional assessment using samples from CALRmut MPN patients and healthy donors. The findings indicated that T-cells from CALRmut MPN patients, despite some phenotypic alterations, are functionally competent and can be effectively engaged by JNJ-88549968 to mediate cytotoxicity against CALRmut-expressing target cells (including engineered lines expressing CALRdel52 or CALRins5, and patient-derived CD34+ cells with various CALR mutations). These data further support the rationale for the ongoing clinical trial.[11]

The rigorous design of this FIH study, with its careful dose escalation and comprehensive safety monitoring, is critical for a novel immunotherapy like JNJ-88549968. The inclusion of patients who have exhausted standard therapies underscores the unmet need this agent aims to address. The subcutaneous route of administration offers a potential convenience advantage over intravenous therapies. Early pharmacodynamic markers of T-cell engagement and CALRmut clone reduction will be keenly anticipated as indicators of biological activity in patients.

Table 2: Overview of NCT06150157 Clinical Trial Design

Trial Aspect	Details from Snippets	Source Snippet(s)
Official Title	A First-in-Human Study of the Safety, Pharmacokinetics, and Pharmacodynamics of JNJ-88549968, a T-cell Redirecting Bispecific Antibody for CALR-mutated Myeloproliferative Neoplasms	6
Phase	Phase 1	4
NCT ID	NCT06150157	4
Sponsor	Janssen Research & Development, LLC	3
Status	Recruiting	3
Primary Objectives (Part 1: Dose Escalation)	Characterize safety and tolerability; identify DLTs; determine RP2D(s) and optimal dosing schedule(s).	6
Primary Objectives (Part 2: Cohort Expansion)	Characterize safety of JNJ-88549968 at RP2D(s).	6
Key Secondary Objectives	Evaluate PK, PD (immune response), immunogenicity (ADAs), preliminary anti-tumor activity (ORR, CR, TTR, DoR, spleen volume, symptoms, CALRmut allele burden).	6
Intervention & Route	JNJ-88549968; Subcutaneous (SC) injection.	22
Target Population	Adults (≥18 years) with CALR-mutated Essential Thrombocythemia (ET) or Myelofibrosis (MF).	3
Key Inclusion Criteria	CALR driver mutation; ET: ≥2 prior cytoreductive therapies (incl. HU); MF: ≥1 prior JAKi (unless ineligible); ECOG PS ≤2.	3
Key Exclusion Criteria	Known allergies to excipients; certain other/recent malignancies; prior solid organ transplant; recent allo-HSCT (≤6 mo) or active GVHD requiring immunosuppressants; significant cardiovascular disease (within 6 mo).	3
Key Endpoints	Safety (AEs, DLTs), PK parameters, PD markers (T-cell activation, cytokines), ADA incidence, ORR, CR, DoR, TTR, spleen/symptom changes, CALRmut allele burden.	6
Estimated Timelines	Start: Oct 2023; Ethics Approval (UK): Dec 2023; Est. Primary Completion: Apr-Oct 2025; Est. Study Completion: Oct 2025.	3

VI. Therapeutic Potential, Comparative Landscape, and Future Directions

Therapeutic Potential of JNJ-88549968

The development of JNJ-88549968 holds significant therapeutic potential for patients with CALR-mutated MPNs, primarily by addressing critical unmet needs in the current treatment paradigm.

• Targeting the Driver Mutation for Clonal Eradication: Unlike existing therapies for MPNs, which largely focus on symptom control or general cytoreduction, JNJ-88549968 is designed to specifically target cells bearing the CALR mutation.[1] By mediating the elimination of these malignant clones, it offers the prospect of achieving deep molecular remissions and potentially altering the natural history of the disease, which current treatments like JAK inhibitors typically do not achieve.[11] The ability to eliminate the MPN clone could be a "game changer" in this field.[11]
• Specificity and Safety: The high specificity of JNJ-88549968 for the CALRmut neoantigen, with no measurable binding to wild-type CALR observed in preclinical studies, suggests a potential for a favorable therapeutic index.[1] This specificity is intended to minimize damage to healthy hematopoietic cells and reduce off-target toxicities, a common concern with broader-acting cancer therapies.
• Broad Applicability within CALR-mutated MPNs: The antibody's design to recognize epitopes common to all known pathogenic CALR mutation types (including Type 1/del52 and Type 2/ins5) means it could benefit a wide spectrum of CALR-mutated ET and MF patients.[1]

Comparative Landscape (Other CALR-Targeted Immunotherapies)

JNJ-88549968 is part of an emerging landscape of immunotherapies targeting CALR mutations. Understanding its position relative to other approaches is key:

• Monoclonal Blocking Antibodies (e.g., INCA033989 by Incyte): INCA033989 is a monoclonal antibody that also selectively targets the mutant CALR C-terminus. However, its mechanism involves direct inhibition of the CALRmut-MPL interaction, leading to receptor internalization and blockade of downstream oncogenic signaling, rather than direct T-cell mediated killing.[13] INCA033989 is also in Phase 1 clinical trials (NCT06034002, NCT05936359).[23] The T-cell redirecting mechanism of JNJ-88549968 is hypothesized to be potentially more effective in directly eliminating CALR-mutant cells compared to an antibody that solely blocks signaling.[1] The comparative clinical outcomes of these distinct antibody-based strategies will be highly informative.
• Vaccine Approaches: Therapeutic cancer vaccines aiming to elicit an endogenous T-cell response against CALRmut neoantigens have been explored. However, early clinical results from some CALRmut vaccine studies have been disappointing, with reports of limited efficacy (e.g., a "big goose egg" response mentioned by experts).[11] Challenges in vaccine development include overcoming immune tolerance and generating a sufficiently robust and durable T-cell response. Research into optimized peptide vaccines, such as heteroclitic peptides designed to enhance binding to patient-specific MHC-I alleles, is ongoing to address these limitations.[16] JNJ-88549968 bypasses the need for de novo T-cell priming by directly engaging and activating existing T-cells.
• Chimeric Antigen Receptor (CAR)-T Cells: The development of CAR-T cells engineered to recognize CALRmut is in the preclinical stage.[12] CAR-T cell therapy has shown remarkable efficacy in some hematologic malignancies, but it involves complex ex vivo cell manufacturing, patient lymphodepletion, and carries risks of significant toxicities such as CRS and neurotoxicity. An "off-the-shelf" bispecific antibody like JNJ-88549968, if proven effective and safe, could offer a more readily accessible and potentially safer immunotherapeutic option.

Potential Challenges and Considerations for JNJ-88549968

Despite its promise, the clinical development of JNJ-88549968 faces several potential challenges inherent to T-cell engaging bispecific antibodies:

• Immune-Related Adverse Events (irAEs): The potent T-cell activation induced by JNJ-88549968 can lead to systemic inflammatory responses. Cytokine Release Syndrome (CRS) and Immune Effector Cell-Associated Neurotoxicity Syndrome (ICANS) are well-recognized class-specific toxicities for T-cell engagers and are listed as expected risks for JNJ-88549968 in trial information.[6] Effective monitoring, mitigation, and management strategies for these irAEs will be critical for the drug's success.
• On-Target, Off-Tumor Toxicity: While the CALRmut arm of JNJ-88549968 is designed for tumor specificity, the CD3-binding arm targets all T-cells. Widespread, non-specific T-cell activation, even if primarily localized to the tumor microenvironment by CALRmut binding, could lead to unintended immune-mediated damage to healthy tissues. The degree of this risk will be a key aspect of safety evaluations.
• Immunogenicity: As with all biologic therapies, there is a potential for patients to develop anti-drug antibodies (ADAs) against JNJ-88549968. ADAs can neutralize the drug's activity, alter its pharmacokinetic profile, or, in some cases, lead to hypersensitivity reactions. The NCT06150157 trial includes immunogenicity assessment as a secondary endpoint.[21]
• Impact of Soluble CALRmut: Some evidence suggests that mutant CALR protein may be secreted by MPN cells. If present in significant amounts in the circulation, soluble CALRmut could potentially bind to JNJ-88549968, acting as a "decoy" target and reducing the concentration of the antibody available to engage cell-surface CALRmut on MPN cells.[12] The EHA 2024 presentation on JNJ-88549968 noted the presence of soluble CALRmut in the assay conditions for in vitro functional tests [15], suggesting this is a recognized factor. The clinical relevance of this phenomenon and its potential impact on dosing or efficacy will need to be assessed.
• Disease Heterogeneity and Resistance Mechanisms: MPNs are clonal disorders that can evolve and acquire additional genetic mutations over time, particularly in advanced stages or after transformation.[14] The efficacy of JNJ-88549968 in patients with more complex genomic profiles or in advanced disease phases, and the potential for tumor cells to develop mechanisms of resistance to T-cell mediated killing (e.g., downregulation of CALRmut expression, upregulation of immune checkpoint ligands), will be important long-term considerations.

Future Directions

The successful progression of JNJ-88549968 through clinical development could open several future avenues:

• Expansion to Later-Phase Trials: Positive safety and preliminary efficacy data from the ongoing Phase 1 trial (NCT06150157) would support advancement to Phase 2 and Phase 3 studies designed to definitively establish its clinical benefit in specific CALR-mutated MPN populations (ET and MF).
• Combination Therapies: While not explicitly detailed for JNJ-88549968 in the provided snippets, there is a general trend in oncology and hematology towards exploring combination therapies to enhance efficacy, overcome resistance, or reduce toxicity. Future studies could investigate JNJ-88549968 in combination with other agents used in MPNs, such as JAK inhibitors, hypomethylating agents, or other immunomodulatory drugs. Janssen's broader R&D strategy emphasizes combination therapies and bispecific antibodies.[18]
• Use in Earlier Lines of Therapy: If JNJ-88549968 demonstrates a compelling efficacy and safety profile, particularly if it shows potential for deep and durable remissions or clonal eradication, its use could be explored in earlier lines of treatment for CALR-mutated MPNs, potentially delaying or preventing disease progression and complications.
• Biomarker Development: Further refinement of biomarkers to predict response or monitor treatment efficacy (beyond CALR mutation status itself) could help optimize patient selection and treatment strategies.

The development of JNJ-88549968 is aligned with Janssen's strategic focus on precision medicine and innovative immunotherapies in oncology.[18] The successful clinical translation of this agent would represent a significant step forward in treating CALR-mutated MPNs and would further validate the approach of targeting mutation-derived neoantigens with T-cell redirecting bispecific antibodies. The outcomes of NCT06150157 and subsequent trials will be critical in determining the ultimate role of JNJ-88549968 in the management of these challenging hematologic malignancies.

VII. Conclusion

JNJ-88549968 emerges from preclinical and early clinical insights as a highly innovative and potentially transformative therapeutic agent for patients with CALR-mutated myeloproliferative neoplasms. As a first-in-class CALRmutxCD3 T-cell redirecting bispecific antibody, its mechanism of action is designed to harness the patient's own immune system to specifically target and eliminate malignant cells harboring the CALR mutation, a key driver in ET and MF.[1]

The scientific rationale for JNJ-88549968 is robust, founded on the unique biology of CALR mutations which create a novel, tumor-specific neoantigen on the surface of MPN cells.[10] Preclinical studies have compellingly demonstrated the antibody's ability to selectively bind to CALRmut-expressing cells (across diverse mutation subtypes including del52 and ins5) and to mediate potent T-cell activation and cytotoxicity, leading to significant anti-tumor effects in vitro and in vivo.[1]

The ongoing Phase 1 first-in-human clinical trial (NCT06150157) represents a critical milestone in the development of JNJ-88549968. This study will provide essential data on its safety, tolerability, pharmacokinetics, pharmacodynamics, and preliminary efficacy in patients with CALR-mutated ET and MF.[6] Early in vitro functional data using patient T-cells, presented at EHA 2024, further support the drug's potential by showing that patient T-cells can be effectively engaged by JNJ-88549968 to kill CALR-mutated cells.[15]

JNJ-88549968 stands at the forefront of a new wave of targeted immunotherapies for MPNs. If its promising preclinical profile translates into meaningful clinical benefit with a manageable safety profile, it could significantly alter the treatment landscape for patients with CALR-mutated MPNs, addressing a major unmet medical need by offering a therapy aimed at clonal eradication and potentially achieving durable, disease-modifying responses.[1] The continued clinical investigation of JNJ-88549968 is therefore of considerable importance to the hematology-oncology community and to patients awaiting more effective and targeted treatment options.

VIII. References

[1]

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