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Metastatic Castration-Resistant Prostate CancerPhase 1/2NCT07850921

REGN21353 Alone or in Combination With Nezastomig for Metastatic Castration-Resistant Prostate Cancer in Adult Participants

A Phase 1/2 study testing REGN21353, Nezastomig and Sarilumab Prophylaxis (Contingency) in people with Metastatic Castration-Resistant Prostate Cancer and mCRPC, run by Regeneron Pharmaceuticals.

Not yet recruiting

Opening soon. The study is approved but enrolment has not started yet. Sites may already take enquiries.

Sites and contacts
Official record
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Trial summary

This study will test an experimental Regeneron drug called REGN21353 ("study drug") to see if it can help treat previously treated prostate cancer that has spread (also called metastatic castration-resistant prostate cancer or mCRPC). Read more about this study

Who is it for?
Ages 18 years and older, men only
What is being tested?
REGN21353, Nezastomig and Sarilumab Prophylaxis (Contingency)
Could I receive a placebo?
No, everyone receives an active treatment
How many people
192 participants
Where
No sites listed yet
Study dates
28 Jun 2027to 28 Jul 2031
Phase
Phase 1/2
Are there results?
Not yet: the study is under way

Key requirements

Who can take part, as listed by the sponsor. Only the study team can confirm whether you are eligible, so take this list to your doctor.

  • Ages 18 years and older
  • Men only
  • No healthy volunteers
  • Adult, Older adult

Who may be able to join

  • Is male with histologically or cytologically confirmed adenocarcinoma of the prostate without pure small cell carcinoma
  • Has metastatic castration-resistant prostate cancer with prostate-specific antigen value as described in the protocol and that has progressed within 6 months prior to screening
  • Has received ≥2 lines of prior systemic therapy approved in the metastatic and/or castration-resistant setting (in addition to androgen deprivation therapy) including at least one androgen receptor pathway inhibitor (eg, abiraterone, enzalutamide, apalutamide, or darolutamide)
  • Has had either orchiectomy or be on luteinizing hormone-releasing hormone (LHRH) agonist or antagonist therapy with serum testosterone levels as described in the protocol and agree to stay on LHRH agonist or antagonist therapy during the trial
  • Is able and willing to provide tumor tissue, either archival or newly obtained
  • Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
  • Has adequate organ, bone marrow, renal and hepatic function

Who cannot join

  • Has not recovered (ie, grade ≤1 or baseline) from immune-mediated adverse event within 3 months prior to initiation of trial intervention therapy except for endocrinopathies adequately managed with hormone replacement
  • Has permanently discontinued anti-cancer immune modulating therapies due to immune-mediated adverse events
  • Has another malignancy that is progressing or requires active treatment, except non-melanoma skin cancer that has undergone potentially curative therapy or any tumor that has been deemed to be effectively treated with definitive local control
  • Has ongoing or recent (within 5 years) evidence of significant autoimmune disease (including any history of systemic vasculitis) that required treatment with systemic immunosuppressive treatments
  • Has active liver metastases
  • Has current evidence of or history of central nervous system metastases as described in the protocol
Show all 11Show fewer
  • Has encephalitis, meningitis, neurodegenerative disease (with the exception of mild dementia that does not interfere with activities of daily living), or uncontrolled seizures in the year prior to first dose of trial intervention
  • Has a known history of, or any evidence of, interstitial lung disease or active, noninfectious pneumonitis (past 5 years). A history of radiation pneumonitis in the radiation field is permitted
  • Has active or recent infection requiring hospitalization or treatment with intravenous anti-infectives within 2 weeks of first dose of trial intervention
  • Has significant cardiovascular disease
  • Note: Other protocol defined Inclusion/ Exclusion criteria apply

Trial locations and contacts

No sites are listed yet. Contact the study team or a site near you to ask about taking part.

Clinical Trials Administrator contact

844-734-6643

clinicaltrials@regeneron.com

About this study

Official title: Phase 1/2 Trial of REGN21353 (a STEAP1xCD3 Bispecific Antibody) Administered Alone or in Combination With Nezastomig (a PSMAxCD28 Bispecific Antibody) in Participants With Metastatic Castration-Resistant Prostate Cancer (mCRPC)

This study will test an experimental Regeneron drug called REGN21353 ("study drug") to see if it can help treat previously treated prostate cancer that has spread (also called metastatic castration-resistant prostate cancer or mCRPC). Some participants may get additional experimental drugs in combination with REGN21353. These additional drugs are nezastomig and sarilumab. "Experimental" means that the drugs are being tested and have not been approved for use in prostate cancer by a country's national health agency that gives approval for new medicines to be prescribed.

The study is looking at:

  • What is the best dose of REGN21353 alone and together with nezastomig for treating prostate cancer
  • What side effects REGN21353 alone and together with nezastomig might cause
  • How well REGN21353 alone and together with nezastomig works
  • How much REGN21353 alone and together with nezastomig is in the blood at different times
  • If the body reacts to REGN21353 alone or together with nezastomig, in a way that may cause them to not work as well
  • How REGN21353 alone and together with nezastomig could affect prostate cancer, which can help researchers understand why REGN21353 alone or together with nezastomig work better in some than others
Being tested
REGN21353, Nezastomig, Sarilumab Prophylaxis (Contingency)
Keywords
Prostate cancer, Metastatic prostate cancer, Castration resistant prostate cancer, Advanced prostate cancer, Stage 4 prostate cancer, Prostate cancer that has spread, Hormone refractory prostate cancer, Androgen-resistant prostate cancer, Androgen-independent prostate cancer, PSA progression, Androgen receptor pathway inhibitor (ARPI), Luteinizing hormone-releasing hormone (LHRH), Androgen deprivation therapy (ADT)
Medical subject headings
Prostatic Neoplasms

What participants receive

What participants receive. In a randomised study each person is assigned to one group by chance.

  • REGN21353Drug

    Administered per the protocol

  • NezastomigDrug

    Also called REGN5678

    Administered per the protocol

  • Sarilumab Prophylaxis (Contingency)Drug

    Also called REGN88, SAR153191, KEVZARA®

    May be administered as pretreatment, per the protocol

Study groups in NCT07850921
Study groupRoleWhat they receive
Module 1: monotherapyExperimentalNot described
Receives: Drug: REGN21353; Drug: Sarilumab Prophylaxis (Contingency)
Module 2: combinationExperimentalNot described
Receives: Drug: REGN21353; Drug: Nezastomig; Drug: Sarilumab Prophylaxis (Contingency)

What is being measured

The primary outcome is the main question the study is designed to answer.

  1. Occurrence of Treatment Emergent Adverse Events (TEAEs)

    Dose escalation

    Measured: Up to 5 years

  2. Occurrence of Dose-Limiting Toxicities (DLTs)

    Dose escalation

    Measured: Up to 42 days

  3. Occurrence of Serious Adverse Events (SAEs)

    Dose escalation

    Measured: Up to 5 years

  4. Occurrence of Adverse Events of Special Interests (AESIs)

    Dose escalation

    Measured: Up to 5 years

  5. Composite Response Rate (CRR) of ≥50% decline of prostate specific antigen (PSA) and/or confirmed radiographic response of complete response (CR) or partial response (PR)

    Dose expansion

    Measured: Up to 5 years

Secondary outcomes (12)

  1. (CRR) of ≥50% decline of prostate specific antigen (PSA) and/or confirmed radiographic response of complete response (CR) or partial response (PR)

    Dose escalation

    Measured: Up to 5 years

  2. Occurrence of TEAEs

    Dose expansion

    Measured: Up to 5 years

  3. Occurrence of SAEs

    Dose expansion

    Measured: Up to 5 years

  4. Occurrence of AESIs

    Dose expansion

    Measured: Up to 5 years

  5. Percentage of participants with ≥50% reduction in PSA confirmed by a second PSA test ≥3 weeks later

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

  6. Percentage of participants with ≥90% reduction in PSA confirmed by a second PSA test ≥3 weeks later

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

  7. REGN21353 serum concentrations

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

  8. Nezastomig serum concentrations

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

  9. Occurrence of Anti-Drug Antibodies (ADA) to REGN21353

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

  10. Occurrence of ADA to nezastomig

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

  11. Magnitude of ADA to REGN21353

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

  12. Magnitude of ADA to nezastomig

    Dose Escalation and Dose Expansion

    Measured: Up to 5 years

Timeline

From the start of the study to its completion; the mark is today. Estimated dates are the sponsor’s plan.

  1. First posted30 Sep 2026
  2. Study start28 Jun 2027 (estimated)
  3. Primary completion28 Jul 2031 (estimated)
  4. Study completion28 Jul 2031 (estimated)

Resources

More details

For doctors and researchers: how the study is designed, who runs it, and its registry IDs.

Study design
Study type
Interventional
Phase
Phase 1/2
Primary purpose
Treatment
Allocation
Non-randomised
Intervention model
Sequential
Masking
None (open label)
Enrolment
192 participants (estimated)
IDs, dates and publications
ClinicalTrials.gov ID
NCT07850921
Sponsor's protocol ID
R21353-ONC-25105
Status last verified
Sep 2026
First submitted
23 Sep 2026
Last updated
30 Sep 2026
Results posted
No
Data monitoring committee
No
FDA-regulated drug
Yes
FDA-regulated device
No
Individual data sharing
Yes: All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing

Questions about this trial

What is this clinical trial studying?

A Phase 1/2 study testing REGN21353, Nezastomig and Sarilumab Prophylaxis (Contingency) in people with Metastatic Castration-Resistant Prostate Cancer and mCRPC, run by Regeneron Pharmaceuticals.

The full registered description is in the About this study section and the official record.

Who may be eligible to take part?

The sponsor lists 7 inclusion and 11 exclusion criteria. Eligibility is decided by the study team after screening, so read the full criteria and contact a listed site.

Is this clinical trial currently recruiting?

The record says: opening soon, as of the sponsor's last update on 30 Sep 2026. Status can change between updates, so confirm with the study contact before making plans.

What are the possible benefits and risks?

They depend on the study and the treatment, and no study can promise a benefit. The study team explains the known risks during informed consent, before anyone joins; the record itself does not list them.

What does this combined phase mean?

Phase 1 studies test a treatment in a small group of people for the first time or in a new way, mainly to check safety and find the right dose.

Phase 2 studies give the treatment to a larger group to see whether it has the intended effect and to keep checking safety.

Not every study follows the same path, and some studies (observational studies, device studies) have no phase.

Are results available for this study?

This study is still under way. Results are not available in the source record yet.

How can a doctor refer a patient or contact the research team?

Contact Clinical Trials Administrator (844-734-6643, clinicaltrials@regeneron.com), or a site near the patient. AzTrials does not enrol participants or take referrals.

What does the primary outcome measure?

Occurrence of Treatment Emergent Adverse Events (TEAEs) (time frame: Up to 5 years). The study lists 5 primary outcomes in all. It is what the study was designed to measure, not a finding.

Where can I read the original study information?

On the ClinicalTrials.gov record for NCT07850921.

Does a listing mean the treatment is approved or proven to work?

No. A listing means the study is registered. It does not show that the treatment is safe, works, or is approved by a regulator.

How often is this information updated?

AzTrials copies changed records from ClinicalTrials.gov every night and reloads the whole registry every week. The sponsor last updated this record on 30 Sep 2026.

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