A Phase 2 interventional study of PF-06838435 (formerly SPK-9001) in Hemophilia B, sponsored by Pfizer. Terminated at 25 sites in 4 countries. Open to male participants aged 18 Years to 65 Years. Per ClinicalTrials.gov, last updated 2026-10-05.
Sponsored by Pfizer · Phase 2, Interventional, and Treatment
The study was terminated early due to Sponsor and participants may transition into a long-term follow-up study. The decision to terminate the trial was not based on any safety and/or efficacy concerns.
Among 33 studies in Hemophilia B by Pfizer — 5 open to participants now, 6 under way, 5 of them phase 3. Hemophilia B · Pfizer.
Among the 33 studies in Hemophilia B by Pfizer, this one is the 14th largest by enrollment of the 19 interventional ones that give a count, is one of 2 that started in 2017, is one of 2 in phase 2, and was the 23rd registered.
292 studies on the registry are indexed under Hemophilia B; 51 are open to participants now.
Browse Hemophilia B studies →5 of the 33 studies in Hemophilia B by Pfizer are open now, in 12 countries. Recruiting first, the newest first:
Pfizer · Sites in United States
Ages 12–74
Pfizer · Sites in United States, Australia, Canada and 10 more countries
Ages 18+ · Men only
Pfizer · Sites in United States, Argentina, Australia and 20 more countries
Ages 1–17 · Men only
Pfizer · Sites in United States, Argentina, Australia and 21 more countries
Ages 1–74 · Men only
Pfizer
Ages 6+
By country, each count listing its studies:
Of the 30 Hemophilia B studies recruiting now, 17 (57%) set no upper age limit and 11 (37%) stop at 75 or younger; 16 take children under 18; 3 take healthy volunteers.
This study: 18 Years to 65 Years · male · no healthy volunteers.
The full table on the hub →This tab as a report to print or save as PDF → With an account.
33 studies; the count open now beside each. This study's row is marked.
First in 2001; this study’s year is marked. 2 started in 2017.
This study's enrollment of 21 is below the median of 26 across 171 interventional studies indexed under Hemophilia B.
Median enrollment 35 across the 19 interventional studies, 80 across the 13 observational. Of the 26 that completed or were terminated, 17 (65%) have results posted.
This tab as a report to print or save as PDF → With an account.
Pfizer is the lead sponsor of 3,337 studies on the registry; 150 are open to participants now.
Of its 601 completed or terminated interventional studies of FDA-regulated products, 392 (65%) have results posted.
None of the studies under way reaches primary completion within a year.
Among the 33 studies in Hemophilia B by Pfizer, from their registry updates:
This tab as a report to print or save as PDF → With an account.
Long-term safety and efficacy follow-up for participants with Hemophilia B who were previously treated in the C0371005 (formerly SPK-9001-101) study, and a dose-escalation sub-study evaluating safety, tolerability, and kinetics of a higher dose with long-term safety and efficacy follow-up. Participants in the substudy do not need to have participated in C0371005.
Evaluation of the long-term level of persistence and potential late or delayed adverse events associated with PF-06838435 (formerly SPK-9001), assessment of the durability of the transgene expression, and determination of the effects of PF-06838435 on clinical outcomes in individuals who have previously received a single administration of PF-06838435 in the C0371005 study. Amendment 2 of this study incorporates a dose-escalation substudy to evaluate the safety, tolerability, and kinetics of a single IV infusion of PF-06838435 at a higher dose than that used in the C0371005 study. The dose-escalation participants will also be followed for long-term safety and efficacy.
Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
This study is currently only enrolling into the dose-escalation substudy with subsequent long-term follow-up. The Eligibility Criteria for entry into the dose-escalation substudy is presented below:
Inclusion Criteria:
Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Ways of preventing pregnancy. “Highly effective” methods fail less than 1% of the time a year when used correctly — for example an IUD or an implant.
Criteria usually say which methods count and how long to keep using them after the last dose, sometimes for male participants too. Some studies accept a partner’s vasectomy if it was confirmed successful and that person is the participant’s only sexual partner.
Hepatitis B is a virus that infects the liver. These are separate tests: HBsAg positive usually means a current infection (a recent vaccination can briefly cause it too); total anti-HBc (HBcAb) positive means an infection now or in the past; HBV DNA measures how much virus is in the blood.
Some studies accept a past infection if HBV DNA is undetectable, sometimes with antiviral medicine during the study. Check what this study allows.
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
The number of CD4 cells, a type of white blood cell, in a given amount of blood. It is used to follow how well the immune system is doing in HIV.
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
Informed consent
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Contraception
Ways of preventing pregnancy. “Highly effective” methods fail less than 1% of the time a year when used correctly — for example an IUD or an implant.
Criteria usually say which methods count and how long to keep using them after the last dose, sometimes for male participants too. Some studies accept a partner’s vasectomy if it was confirmed successful and that person is the participant’s only sexual partner.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Hepatitis B tests
Hepatitis B is a virus that infects the liver. These are separate tests: HBsAg positive usually means a current infection (a recent vaccination can briefly cause it too); total anti-HBc (HBcAb) positive means an infection now or in the past; HBV DNA measures how much virus is in the blood.
Some studies accept a past infection if HBV DNA is undetectable, sometimes with antiviral medicine during the study. Check what this study allows.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
HIV
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
CD4 count
The number of CD4 cells, a type of white blood cell, in a given amount of blood. It is used to follow how well the immune system is doing in HIV.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Clinically significant
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Single intravaneous infusion of PF-06838435. After 2 participants receive initial dose, data will be evaluated and a decision will be made to escalate or reduce the dose being evaluated, increase the number of participants receiving the dose, or stop dosing. Multiple iterations may be undertaken.
Biological: PF-06838435 (formerly SPK-9001)
Gene Therapy: A novel, bioengineered adeno-associated viral vector carrying human factor IX variant
Incidence of PF-06838435 related adverse events
Time frame: Baseline up to Year 6
Incidence of clinically significant changes from baseline
Clinically significant changes in physical examination, vital signs, laboratory values. (to be reported as AEs, regardless of causality)
Time frame: Baseline up to 52 weeks
Incidence of protocol-defined medically important events
Clinical thrombotic events, FIX inhibitor development as assessed by Nijmegen Bethesda assay, Hypersensitivity reaction (eg, bronchospasm and anaphylaxis), Hepatic malignancy, Study intervention-related elevated hepatic transaminases that fail to improve or resolve, Malignancy assessed as having reasonable possibility of being related to study intervention (to be reported as SAEs).
Time frame: Baseline up to 52 weeks
Immune response against AAV capsid protein and hFIX transgene
Positive immune response based on peripheral blood mononuclear cell (PBMC) results by interferon gamma enzyme-linked immunospot assay (ELISPOT).
Time frame: Baseline up to 52 weeks
Coagulation Clotting Assay for FIX activity levels
Coagulation Clotting assays to assess FIX activity levels (percent of normal)
Time frame: Baseline up to Year 6
Mean and standard deviation of vector-derived FIX Activity levels
Mean and standard deviation of peak and steady-state FIX Activity
Time frame: Baseline up to 52 weeks
Mean and standard deviation of FIX Antigen levels
Mean and standard deviation of FIX Antigen levels
Time frame: Baseline up to 52 weeks
Annualized bleeding rate (ABR)
ABR (not including those for surgery)
Time frame: Baseline up to Year 6
Annualized (factor FIX) infusion rate
AIR (not including those for surgery)
Time frame: Baseline up to Year 6
Total factor consumption (IU)
total quantity of factor infused annually (not including those for surgery) as recorded on the infusion log
Time frame: Baseline up to Year 6
Total number of bleeding events
spontaneous and traumatic
Time frame: Baseline up to Year 6
Haem-A-QoL
Quality-of-life (QoL) assessment
Time frame: Baseline up to Year 6
EQ-5D-5L
Quality-of-life (QoL) assessment
Time frame: Baseline up to Year 6
Brief Pain Inventory
Quality-of-life (QoL) assessment
Time frame: Year 2 up to Year 6
McGill Pain Questionnaire
Quality-of-life (QoL) assessment
Time frame: Baseline up to 52 weeks
Plan to share: Yes — Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical_trials/trial_data_and_results/data_requests.
Data sharing details ↗ · www.pfizer.com
Ran from Jun 2017 to primary completion in Jul 2026, 109 months.
How these figures are worked out, and which studies are left out of them.
The study was terminated early due to Sponsor and participants may transition into a long-term follow-up study. The decision to terminate the trial was not based on any safety and/or efficacy concerns.
The study was terminated early due to Sponsor and participants may transition into a long-term follow-up study. The decision to terminate the trial was not based on any safety and/or efficacy concerns.
From the registry record's own update history. This site started tracking changes on Sep 25, 2026; for anything earlier, see the record history on ClinicalTrials.gov ↗
This study is terminated, as verified in Sep 2026. You cannot join it, but the record below documents what was studied.
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Studies of the drug tested here: Fidanacogene elaparvovec