Human Gene Therapy for Hemophilia 
 
 
Guidance for Industry 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Additional copies of this guidance are available from the Office of Communication, Outreach 
and Development (OCOD), 10903 New Hampshire Ave., Bldg. 71, Rm. 3128, Silver Spring, 
MD 20993-0002, or by calling 1-800-835-4709 or 240-402-8010, or email ocod@fda.hhs.gov, or 
from the Internet at https://www.fda.gov/vaccines-blood-biologics/guidance-compliance-
regulatory-information-biologics/biologics-guidances. 
 
For questions on the content of this guidance, contact OCOD at the phone numbers or email 
address listed above. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Biologics Evaluation and Research 
January 2020  Contains Nonbinding Recommendations 
 
Table of Contents 
 
 
I.  INTRODUCTION............................................................................................................. 1 
II.  BACKGROUND ............................................................................................................... 2 
III.  CONSIDERATIONS FOR CHEMISTRY, MANUFACTURING AND CONTROLS  
............................................................................................................................................. 2 
IV.  CONSIDERATIONS FOR FACTOR VIII/FACTOR IX ACTIVITY 
MEASUREMENTS ASSESSED BY DIFFERENT CLINICAL LABORATORY 
ASSAYS ............................................................................................................................. 3 
V.  CONSIDERATIONS FOR PRECLINICAL STUDIES................................................ 4 
VI.  CONSIDERATIONS FOR CLINICAL TRIALS .......................................................... 6 
A.  Efficacy Endpoints ................................................................................................ 6 
B.  Study Design .......................................................................................................... 8 
C.  Study Population ................................................................................................... 9 
D.  Statistical Considerations ................................................................................... 11 
E.  Study Monitoring ................................................................................................ 11 
F.  Patient Experience .............................................................................................. 13 
VII.  EXPEDITED PROGRAMS ........................................................................................... 13 
VIII.  COMMUNICATION WITH FDA ................................................................................ 13 
IX.  REFERENCES ................................................................................................................ 14 
i  Contains Nonbinding Recommendations 
 
Human Gene Therapy for Hemophilia 
 
 
Guidance for Industry 
 
 
This guidance represents the current thinking of the Food and Drug Administration (FDA or 
Agency) on this topic.  It does not establish any rights for any person and is not binding on FDA 
or the public.  You can use an alternative approach if it satisfies the requirements of the 
applicable statutes and regulations.  To discuss an alternative approach, contact the FDA staff 
responsible for this guidance as listed on the title page. 
 
 
I.  INTRODUCTION 
 
This guidance provides recommendations to sponsors developing human gene therapy 
(GT)1products for the treatment of hemophilia including clinical trial design and related 
development of coagulation factor VIII (hemophilia A) and IX (hemophilia B) activity assays, 
including how to address discrepancies in factor VIII and factor IX activity assays.  This 
guidance also includes recommendations regarding preclinical considerations to support 
development of GT products for the treatment of hemophilia.  Additional clinical and preclinical 
recommendations are available in other guidances (Refs. 1 and 2).  This guidance does not 
provide recommendations for products for the treatment of hemophilia C (factor XI deficiency) 
or for the treatment of any bleeding disorders other than hemophilia A and B, because of the 
unique nature of those bleeding disorders.  This guidance finalizes the draft guidance of the same 
title dated July 2018. 
 
FDAs guidance documents, including this guidance, do not establish legally enforceable 
responsibilities.  Instead, guidances describe the FDAs current thinking on a topic and should be 
viewed only as recommendations, unless specific regulatory or statutory requirements are cited.  
The use of the word should in FDAs guidances means that something is suggested or 
recommended, but not required. 
 
 
   
                                                 
1 Human gene therapy seeks to modify or manipulate the expression of a gene or to alter the biological properties of 
living cells for therapeutic use.  FDA generally considers human gene therapy products to include all products that 
mediate their effects by transcription or translation of transferred genetic material, or by specifically altering host 
(human) genetic sequences.  Some examples of gene therapy products include nucleic acids (e.g., plasmids, in vitro 
transcribed ribonucleic acid (RNA)), genetically modified microorganisms (e.g., viruses, bacteria, fungi), engineered 
site-specific nucleases used for human genome editing, and ex vivo genetically modified human cells.  Gene therapy 
products meet the definition of biological product in section 351(i) of the Public Health Service (PHS) Act (42 
U.S.C. 262(i)) when such products are applicable to the prevention, treatment, or cure of a disease or condition of 
human beings (see Federal Register Notice:  Application of Current Statutory Authorities to Human Somatic Cell 
Therapy Products and Gene Therapy Products (58 FR 53248, October 14, 1993), 
https://www.fda.gov/media/76647/download). 
1  Contains Nonbinding Recommendations 
 
II.  BACKGROUND 
 
Hemophilia therapy in the United States has progressed from replacement therapies for 
on-demand treatment of bleeding to prophylaxis to reduce the frequency of bleeding.  Current 
replacement therapies utilize plasma-derived coagulation factor or recombinant factor 
concentrates.  Prophylaxis has been shown to prevent joint damage in children and allows lower 
factor usage compared to on-demand therapy.  Routine prophylaxis is currently the optimal 
treatment strategy for hemophilia.  Compliance with dosing is critical for effective prophylaxis, 
and dosing intervals aim to maintain trough levels >1% between doses.  Patients on prophylaxis 
may experience breakthrough bleeding episodes that require treatment with replacement 
therapies for control of bleeding.  The main adverse event associated with factor replacement 
therapy is the development of inhibitors (neutralizing antibodies) to the administered factor (VIII 
or IX), which then requires use of alternative therapies to overcome the effect of the inhibitor. 
The marketing approvals of factor VIII and factor IX replacement products currently indicated 
for prophylaxis were based on clinical trials with reduction of the annualized bleeding rate as the 
primary efficacy endpoint.  
 
GT products for the treatment of hemophilia are being developed as single-dose treatments that 
may provide long-term expression of the deficient coagulation factor at steady levels to reduce or 
eliminate the need for exogenous factor replacement.  GT products currently in clinical 
development may use a vector to deliver the coagulation factor gene to the liver.  The 
coagulation factor that is expressed may be different from the wild type (normal) form.  For 
example, the coagulation factor may be a truncated variant, such as B domain-deleted factor 
VIII, or a hyper-functional natural variant (such as the Padua variant of factor IX). 
 
 
III.  CONSIDERATIONS FOR CHEMISTRY, MANUFACTURING AND CONTROLS  
 
The general chemistry, manufacturing and controls (CMC) considerations for product 
manufacturing, testing and release of GT products for the treatment of hemophilia are the same 
as those described for other GT products (Ref. 3).  For early-phase clinical trials, a sponsor 
should be able to evaluate the identity, purity, quality, dose, and safety of a GT product.  A 
potency assay to assess the biological activity of the final GT product, with relevant lot release 
specifications, should be established prior to the initiation of clinical trials intended to provide 
substantial evidence of effectiveness for a marketing application (Ref. 4).  To support licensure 
of a GT product, manufacturing processes and all testing methods for product release must be 
validated (21 CFR 211.165(e)).  Sponsors developing GT products for hemophilia are strongly 
encouraged to contact the Office of Tissues and Advanced Therapies (OTAT) in the Center for 
Biologics Evaluation and Research (CBER) early in product development to discuss product-
specific issues. 
 
 
   
2  Contains Nonbinding Recommendations 
 
IV.  CONSIDERATIONS FOR FACTOR VIII/FACTOR IX ACTIVITY 
MEASUREMENTS ASSESSED BY DIFFERENT CLINICAL LABORATORY 
ASSAYS 
 
One stage clotting (OC) assays and chromogenic substrate (CS) assays have been used to 
measure factor activity; however, discrepancies in factor activity measurements between the OC 
and CS methods have been observed (Refs. 5-11).  For example, in patients with hemophilia A 
treated with recombinant B-domain-deleted factor VIII products, CS assays indicate higher 
factor activity than OC assays.  In contrast, for patients with hemophilia A who receive GT 
products that express a B-domain-deleted factor VIII transgene, OC assays indicate higher factor 
activity than CS assays.  These contrasting results pose challenges in generalizing our previous 
experience with recombinant factor VIII products to clinical benefits related to factor VIII levels 
produced by recipients of GT products.  Similarly, for hemophilia B patients who receive GT 
products that express the Padua variant of factor IX, discrepancies between results of the OC and 
CS assays have been observed across products. 
 
Factor activity assay discrepancies are not limited to differences between OC and CS assays, but 
are also observed between OC assays using different OC reagents.  These discrepancies indicate 
structural and functional differences between the transgene proteins and normal factor proteins 
used as an assay standard.  The discrepancies hinder reliable interpretation of factor activity 
measurements and present a challenge when factor activity levels are proposed as surrogate 
endpoints for hemostatic efficacy.  Even if factor activity is not used as a surrogate endpoint to 
support accelerated approval, safe clinical management of patients in GT trials depends on an 
understanding of any assay discrepancies.  
 
To better interpret factor activity, we, FDA, recommend that sponsors also consider:  
 
  Performing in vitro studies using samples containing the transgene product from animal 
plasma or ex vivo-transduced cells to compare the performance of OC and CS assays.  
Both assays should be calibrated in International Units (IU) of factor activity and should 
use a reference standard analogous to the expressed transgene, if available.  
 
  Using various clinical factor activity laboratory assays in preclinical animal studies and, 
where feasible, assays intended for human use. 
 
We also recommend that sponsors perform analytical studies to clarify the biochemical root-
causes for any discrepancies observed, addressing: 
 
  Methodology (OC vs. CS) 
 
  Reagents (phospholipids, activators, chromogenic substrates) 
 
  Conditions (incubation times, temperature) 
 
  Choice of reference standards  
 
3  Contains Nonbinding Recommendations 
 
  Vendors/kits/lab being used 
 
  Correlations between factor activity and antigen levels (by immunoassay) 
 
In early-phase clinical studies, interpretation of factor activity can be particularly challenging 
because the magnitude of discrepancies using different assays is not yet established for patient 
samples.  Initially, data from in vitro factor assay studies using samples containing the transgene 
product from animal plasma or ex vivo-transduced cells may predict assay discrepancy in patient 
plasma.  Patient samples collected during early-phase studies can clarify the actual range of 
discrepancies between factor activity values in GT patients.  This assay discrepancy information 
should be obtained as early as possible if the assay is proposed to: 
 
  Measure factor activity intended as a surrogate endpoint to support accelerated approval; 
and  
 
  Guide exogenous replacement therapy for the treatment of bleeding. 
 
During clinical trials, we recommend that sponsors: 
 
  Perform a comparative field study2 with patient plasma samples using assays routinely 
performed in clinical laboratories to evaluate the range of discrepancies 
 
  Perform bridging studies on patient samples if changes to the assay(s) are initiated after a 
clinical trial is underway. 
 
  For GT products for hemophilia that express a variant protein with specific activity that 
deviates from normal, consider developing dedicated activity and antigen assays for the 
variant protein.  Calibrate the factor activity and antigen assay standards for the variant 
proteins with the coagulation factor International Standards, as well as with the plasma 
samples obtained from normal healthy donors (Ref. 12).3  
 
 
V.  CONSIDERATIONS FOR PRECLINICAL STUDIES  
 
A preclinical program that is tailored to the investigational product and planned early-phase 
clinical trial contributes to characterization of the products benefit/risk profile for the intended 
patient population.  The overall objectives of a preclinical program for a GT product include:  1) 
identification of a biologically active dose range; 2) recommendations for an initial clinical dose 
level, dose-escalation schedule, and dosing regimen; 3) establishment of feasibility and  
   
                                                 
2 For additional information on comparative field studies, please see references 5-11. 
3 The specific activity (i.e., ratio of factor activity to factor antigen of the variant proteins) may differ from that of 
normal coagulation factor proteins.  The activity assay may be used to measure factor activity as a surrogate 
endpoint, and the factor antigen assay may be used to measure the level of protein expression.  
4  Contains Nonbinding Recommendations 
 
reasonable safety of the proposed clinical route of administration (ROA); 4) support of patient 
eligibility criteria; and 5) identification of potential toxicities and physiologic parameters that 
help guide clinical monitoring for a particular investigational product.  
  
Further details for general considerations in preclinical studies are available in a separate 
guidance document (Ref. 2).  The following elements are recommended in the development of a 
preclinical program for an investigational GT product for treatment of hemophilia. 4  
 
  Preclinical in vitro and in vivo proof-of-concept (POC) studies are recommended to 
establish feasibility and support the scientific rationale for administration of the 
investigational GT product in a clinical trial.  Data derived from preclinical POC studies 
may guide the design of both the preclinical toxicology studies, as well as the early-phase 
clinical trials.  Several animal models of hemophilia (Ref. 13) can be used to demonstrate 
biological activity of an investigational GT product.  
 
  Biodistribution studies should be conducted to assess the distribution, persistence, and 
clearance of the vector and possibly the expressed transgene product in vivo, from the 
site of administration to target and non-target tissues, including applicable biofluids (e.g., 
blood, lymph node fluid, cerebrospinal fluid (CSF)), as feasible.  These data can 
determine extent of tissue transduction and transgene expression, evaluate whether 
expression is transient or persistent, and guide the design of the preclinical toxicology 
studies as well as the early-phase clinical trials (Refs. 2, 14, and 15). 
  Toxicology studies for an investigational GT product should incorporate elements of the 
planned clinical trial (e.g., dose range, ROA, dosing schedule, evaluation endpoints, etc.), 
to the extent feasible.  Study designs should be sufficiently comprehensive to permit 
identification, characterization, and quantification of potential local and systemic 
toxicities, their onset (i.e., acute or delayed) and potential resolution, and the effect of 
dose level on these findings.   
 
  To support translation of effective and safe dose levels determined in preclinical studies 
to clinical trials, the assay for vector titer determination of the preclinical lots should use 
the same methodology as that used for clinical lots.  The assays for measuring factor 
activity in animals administered the GT product should be consistent to the assays used in 
humans.  The factor activity assays are discussed in detail under section IV of this 
document.   
 
  As the clinical development program for an investigational GT product progresses to late-
phase clinical trials and possible marketing approval, additional nonclinical studies may 
need to be considered to address:  1) the potential for reproductive/developmental 
                                                 
4 The preclinical program for any investigational product should be individualized with respect to scope, complexity, 
and overall design.  We support the principles of the 3Rs, to reduce, refine, and replace animal use in testing when 
feasible.  Proposals, with justification for any potential alternative approaches (e.g. in vitro or in silico testing), 
should be submitted during early communication meetings with FDA (see section VIII of this document).  We will 
consider if such an alternative method could be used in place of an animal test method. 
 
5  Contains Nonbinding Recommendations 
 
toxicity5 and 2) any significant changes in the product manufacturing process or 
formulation changes for which product comparability may be an issue.  
 
 
VI.  CONSIDERATIONS FOR CLINICAL TRIALS  
 
The fundamental considerations for clinical development programs of GT products for 
hemophilia are similar to those for other biologic products.  Early-phase trials of GT products 
should not only evaluate safety and feasibility, but also gauge bioactivity and preliminary 
efficacy.  Sponsors should evaluate the discrepancies between OC and CS assays early in the 
course of clinical development, prior to considering whether to pursue accelerated approval 
using factor activity levels as a surrogate endpoint.  Later-phase trials should be designed as 
adequate and well-controlled studies that can provide substantial evidence of effectiveness to 
support a BLA.  For further details of general considerations for GT clinical trials, please refer to 
relevant FDA guidance documents (Refs. 14 and 16).  
 
With respect to late-phase clinical trials that are intended to form the primary basis of an 
effectiveness claim for hemophilia GT products, we have the following recommendations:   
 
A.  Efficacy Endpoints 
 
Approval of GT products could be based on factor activity levels, if scientifically 
justified.  Our ability to use factor activity levels as validated surrogate endpoints that 
could support traditional approval, however, is currently limited by several scientific 
considerations.  One of these considerations is that discrepant results may be obtained 
when using conventional factor assay methodology to assess the plasma factor levels in 
patients treated with GT products in comparison to the assessment of levels following the 
administration of their recombinant or plasma-derived counterparts.  Another 
consideration is the lack of molecular characterization of the protein translated in vivo 
(e.g., factor VIII produced by hepatocytes), in contrast to recombinant factor concentrate 
products produced in vitro.  Additional uncertainty exists regarding the extent of clinical 
benefit introduced by genetically engineered modifications that increase the specific 
activity of coagulation factor protein (e.g., factor IX Padua constructs).  Thus, we 
recommend that sponsors seeking accelerated approval based on factor activity levels 
provide evidence, specific to their GT product, that correlates the factor levels with 
relevant clinical outcomes (e.g., through long term observation of patients exposed to the 
GT product in early stage clinical trials).  We may consider revising these 
recommendations following the development of reliable evidence on correlates between 
factor activity and clinical benefit, when such evidence can be generalized to multiple GT 
products. 
 
                                                 
5 For additional information on developmental and reproductive toxicity, please see section III.B. of the Preclinical 
Assessment of Investigational Cellular and Gene Therapy Products; Guidance for Industry, November 2013, 
https://www.fda.gov/regulatory-information/search-fda-guidance-documents/preclinical-assessment-investigational-
cellular-and-gene-therapy-products. 
6  Contains Nonbinding Recommendations 
 
Sponsors may consider using the following efficacy endpoints as primary endpoints in 
clinical trials of GT products for hemophilia: 
 
1.  Traditional Approval  
 
Historically, applicants of products approved for treatment of hemophilia, (e.g., 
plasma-derived, recombinant replacement factors; bi-specific antibodies) have relied 
on annualized bleeding rate (ABR) to demonstrate clinical benefit.  Although ABR is 
a direct assessment of clinical benefit, ABR has limitations in that it is a relatively 
infrequent event in patients on prophylactic factor regimens and the decision by a 
patient to treat a possible bleeding episode is usually somewhat subjective.  For these 
reasons, more objective endpoints, such as factor activity levels, are desirable.  
 
In the drug development process, validated surrogate endpoints may be used to obtain 
traditional approval for products.  However, as noted above, at the present time factor 
activity levels have limitations that make it difficult for them to be used as a validated 
surrogate endpoint.  To consider factor activity level as a validated surrogate 
endpoint, clinical data to assess and quantitate the relationship between steady-state 
factor activity levels and bleeding outcomes in patients with severe hemophilia are 
critical.  One proposed approach to quantify such a relationship is to generalize factor 
activity levels observed in patients with mild phenotypes to predict factor activity 
levels that improve bleeding outcomes in patients with more severe phenotypes.  
However, factor activity assay discrepancies (section IV), transgene products with 
enhanced factor activity, and uncertainties regarding the applicability of extrapolation 
of factor activity levels in milder phenotypes to more severe phenotypes hinder such 
an approach.  As data from ongoing clinical studies to assess and quantitate the 
relationship between steady-state factor activity levels and bleeding outcomes 
become available, along with reliable calibration methods for factor activity assays, 
FDA is optimistic that factor activity levels could potentially serve as a validated 
surrogate endpoint.  Pending the availability of such data and reliable calibration 
methods, we recommend ABR as a primary endpoint to demonstrate clinical benefit.   
 
2.  Accelerated Approval 
  Factor activity may be considered as a surrogate endpoint6 for primary 
efficacy assessment under the accelerated approval pathway7 (Ref. 17).  
 
To support the use of this surrogate endpoint, we recommend that you:  
                                                 
6 According to section 507(e)(9) of the FD&C Act [21 USC 357(e)(9] [t]he term surrogate endpoint means a 
marker, such as a laboratory measurement, radiographic image, physical sign, or other measure, that is not itself a 
direct measurement of clinical benefit, and 
(A) is known to predict clinical benefit and could be used to support traditional approval of a drug or biological 
product; or 
(B) is reasonably likely to predict clinical benefit and could be used to support the accelerated approval of a drug 
or biological product in accordance with section 506(c). 
7 Section 506(c) of the FD&C Act; 21 CFR Part 314, Subpart H  Accelerated Approval of New Drugs for Serious 
and Life Threatening Illnesses; 21 CFR Part 601, Subpart E. 
7  Contains Nonbinding Recommendations 
 
 
  Conduct root-cause analyses to evaluate and explain discrepancies in 
o
factor assay results obtained from the various assay methods.  If 
discrepancies are unresolved after addressing issues identified in the 
root-cause analyses, propose and justify a specific assay and target 
factor activity level as a surrogate endpoint for Agency consideration. 
  Determine a target factor activity level for your product that predicts 
o
clinically meaningful hemostatic outcomes.  Functional and structural 
differences between the transgene and normal factor protein may 
present a challenge in understanding the relationship between 
numerically identical factor activity levels or levels that are within the 
normal range8 for the transgene protein and normal factor protein.  In 
general, we expect that clinical study data will be necessary to support 
that any proposed target factor activity level is likely to predict a 
clinically meaningful hemostatic outcome.  We intend to consider the 
determination of whether a specific hemostatic outcome is clinically 
meaningful in the context of available therapies.9 
 
  For products that receive accelerated approval, we recommend that the post-
approval study to verify the products predicted effect on clinical benefit use 
ABR as the primary endpoint.   
 
B.  Study Design 
 
While designing the clinical study, sponsors should consider the following pre-and post-
administration recommendations:   
 
1.  Pre-administration Considerations 
 
We recommend:  
  Enrolling patients who have not required dose adjustments to their 
prophylactic replacement therapy to reduce bleeding events in the prior 6 
to 12 months.  This baseline dose stability can facilitate efficacy 
assessments following administration of the GT product. 
  Observing subjects for 6 months (lead-in period) in-study to collect ABR 
data.  ABRs based on retrospective data collection from medical records 
may be subject to recall bias and missing information.  Collecting:   
                                                 
8 The normal ranges of factor assays in each laboratory depend on the local population, sample preparation, assay 
methodology and a variety of additional circumstances 
9 Guidelines to determine the severity of hemophilia and correlation of severity of bleeding to factor activity are 
available.  However, at present, evidence based guidelines with recommendations for a minimum target level of 
steady-state therapeutic factor activity for patients with severe hemophilia and pre-existing joint damage are 
unavailable. 
8  Contains Nonbinding Recommendations 
 
  ABR on an optimized prophylactic regimen to allow for within-
o
subject (paired) comparison, increasing the statistical power 
relative to a design with parallel control.  
  Data for supportive endpoints (e.g., utilization of exogeneous 
o
replacement therapy or trough levels of factor activity).  
  Enrolling patients who use on-demand therapy prior to study entry in a 
separate cohort.  Analysis of efficacy in this cohort may provide evidence 
to support the primary endpoint results. 
 
2.  Post-administration Considerations 
 
We recommend: 
  Including a plan for exogenous factor replacement therapy based on most 
recent factor activity levels for the management of interventional 
procedures and bleeding episodes.  
  Using the same exogenous replacement therapy as in the lead-in phase to 
prevent (or treat) bleeding during the interval from post-GT product 
administration to steady state factor levels.  
  Including a washout period following exogenous factor replacement 
therapy to measure factor activity.  
  Including a pre-specified target factor activity level or duration from 
treatment that specifies the timing to discontinue exogeneous factor 
prophylaxis. 
  Specifying when assessment of ABR rates and durability of response is to 
begin (e.g., 3 weeks after steady state levels of factor activity is reached, 
and exogenous factor prophylaxis is discontinued).  
  Collecting data for analyses of supportive endpoints as related to the pre-
treatment phase. 
  Including a plan for initiation, dosing and tapering of corticosteroids for 
management (treatment or prophylaxis) of immune-mediated liver 
dysfunction. 
  Including an assessment plan to correlate factor activity and bleeding 
rates.  
C.  Study Population 
 
Sponsors should consider the following recommendations when identifying the target 
population:  
 
  Pre-existing antibodies to the GT product may block delivery of the coagulation 
factor gene to its target (e.g., liver cells), limiting its therapeutic potential.  
Therefore, sponsors may choose to exclude patients with pre-existing antibodies 
9  Contains Nonbinding Recommendations 
 
to the GT product.  In such cases, the sponsor should strongly consider 
contemporaneous development of a companion diagnostic to detect antibodies to 
the GT product. (Ref. 18)  If an in vitro companion diagnostic is needed to 
appropriately select patients for study (and later, once the GT product is approved, 
for treatment), then submission of the marketing application for the companion 
diagnostic and submission of the BLA for the GT product should be coordinated 
to support contemporaneous marketing authorizations.  In addition, the clinical 
development plan should include studies to assess the effect of such pre-existing 
antibodies on the safety and efficacy of the product. 
  As hemophilia affects both children and adults, pediatric studies are a critical part 
of drug development.  It is important that clinical investigations in pediatric 
patients address ethical considerations for conducting investigations in vulnerable 
populations.  FDA regulations at 21 CFR Part 50, Subpart D contain additional 
safeguards for children in clinical investigations.  Clinical investigations 
involving no greater than minimal risk may involve children in accordance with 
21 CFR 50.51.  Clinical investigations involving greater than minimal risk but 
presenting the prospect of direct benefit to individual subjects may involve 
children as set forth in 21 CFR 50.52.  An investigation involving greater than 
minimal risk and no prospect of direct benefit to individual subjects, but which is 
likely to yield generalizable knowledge about the disorder or condition, may 
involve children as set forth in 21 CFR 50.53, which includes, for example, a 
finding by the IRB that the risk represents a minor increase over minimal risk.  
FDAs regulation at 21 CFR 50.54 also addresses clinical investigations not 
otherwise approvable and describes a process to follow to determine whether the 
investigation may involve children.  In addition to the determinations required 
under applicable provisions of subpart D, adequate provisions must be made to 
obtain the permission of the parents and the assent of the child as described in 21 
CFR 50.55.   
  Plans to enroll pediatric subjects should be discussed with the Agency in the 
context of the anticipated benefit-risk profile of the product.  Benefit-risk 
calculations for GT trials in pediatric patients with hemophilia should consider 
durability of expression, potential insertional mutagenesis, episomal dilution 
during liver growth, and the effect of transgene expression.  To justify enrolling 
pediatric subjects, you should: 
  Submit clinical data from studies in adults to demonstrate sustained and 
o
robust steady state factor levels and hemostatic outcomes. 
  Select an appropriate pediatric population that represents a population 
o
with an unmet medical need in the context of available therapies.  
  Consider conducting studies in appropriate animal models to support 
o
sustained factor expression levels following GT product administration.  
  Consider characterizing the risks of your GT product (e.g., vector 
o
integration, insertional mutagenesis) based on available preclinical and 
clinical data for your product.  
10  Contains Nonbinding Recommendations 
 
  Plans to address inclusion of clinically relevant populations with regard to age, 
gender, race, and ethnicity should be discussed with the Agency.  
 
D.  Statistical Considerations 
 
To support a BLA for traditional approval, we recommend a non-inferiority (NI) clinical 
trial design with ABR as the primary efficacy endpoint, using within-subject comparison.  
Specifically, 
  We recommend that you develop an NI margin (M) for comparing ABR of the 
investigational GT product to that of current prophylaxis therapies in the within-
subject comparison trial. 
 
  The hypothesis test and confidence interval should accommodate the paired 
nature of the ABRs before and after GT for the same subject.  
 
  Descriptive statistics, including graphical displays such as histograms and scatter 
plots of ABRs, can add valuable information to understand the treatment effect, 
and, therefore, should also be considered. 
 
The within-subject comparison design provides an added advantage in evaluating the 
treatment effect of the investigational product by controlling for factors that may also 
influence the bleeding outcomes.  Additional information on general statistical and 
clinical considerations for non-inferiority trials is described in FDA guidance (Ref. 19). 
 
E.  Study Monitoring  
 
The goal of the follow-up is to monitor the safety and durability of response.  Sponsors 
may consider the following recommendations for short-term and long-term monitoring:   
 
1.  Short-Term Monitoring (first 2 years following GT product administration) 
 
We recommend:  
 
  Monitoring factor activity levels and liver function once or twice weekly 
in the interval between administration of the GT product and until steady 
state factor levels are reached.  
 
  Decreasing the frequency of monitoring of factor activity once steady state 
levels are achieved (for instance, monthly).  
 
  Periodic monitoring for levels of vector-related antibodies and assessing 
interferon- secretion from peripheral blood mononuclear cells by 
ELISPOT assays for the detection of anti-vector and anti-factor T cell 
reactivity (more frequent monitoring may be appropriate if immune-
mediated hepatic dysfunction is suspected).  
 
11  Contains Nonbinding Recommendations 
 
  Monitoring for inhibitor antibodies to factor VIII or factor IX. 
 
  Assessing for viral shedding for products where a viral vector is used for 
gene transfer (Ref. 20). 
 
2.  Long-Term Monitoring (2 years following GT product administration):  Please 
refer to the FDA guidance on Long Term Follow-Up After Administration of Human 
Gene Therapy Products for recommendations on duration and design of long-term 
follow up (Ref. 14).  Please be aware that, as additional relevant scientific data 
becomes available, FDA may revise its recommendations for duration of follow-up 
for adverse events. 
 
We recommend:  
  Monitoring for adverse events should consider:  1) several product 
characteristics (e.g., integrating vector, nonintegrating vector, genome 
editing) that may be specific to the product under study and 2) the findings 
from preclinical studies (Ref. 14). 
 
  Monitoring for adverse events to include: eliciting history of and non-
invasive screening for hepatic malignancies; physical examination; and 
laboratory testing for hepatic function. 
 
  Monitoring for inhibitor antibodies to factor VIII or factor IX. 
 
  Monitoring for the emergence of new clinical conditions, including new 
malignancies and new incidence or exacerbation of pre-existing 
neurologic, rheumatologic, or autoimmune disorders. 
 
  Monitoring factor activity at least once every 6 months for 5 years.  
 
  Clinical evaluations and imaging to assess the impact of steady state factor 
activity levels on long-term functional and structural outcomes of 
hemophilic arthropathy.  
 
 
   
12  Contains Nonbinding Recommendations 
 
F.  Patient Experience 
 
Patient experience data10 may provide important additional information about the clinical 
benefit of a GT product.  FDA encourages sponsors to collect patient experience data 
during product development, and to submit such data in a BLA.  
 
The treatment landscape for hemophilia is evolving.  As is the case for other products we 
review, we intend to evaluate the benefit-risk profile of the investigational product in the 
context of the treatment landscape at the time of our review of a marketing application.  
 
 
VII.  EXPEDITED PROGRAMS 
 
There are several programs that may be available to sponsors of GTs intended to address unmet 
medical needs in the treatment of serious or life-threatening conditions.  These programs, 
including regenerative medicine advanced therapy designation, breakthrough therapy 
designation, fast track designation, accelerated approval, and priority review, are intended to 
facilitate and expedite development and review of these therapies.  For example, regenerative 
medicine advanced therapy designation and breakthrough therapy designation call for increased 
FDA attention to these potentially promising therapies, offering sponsors more frequent 
interactions with FDA on efficient trial design and overall drug development.  Further 
information on these expedited programs is available in separate guidance documents (Refs. 17 
and 21). 
 
 
VIII.  COMMUNICATION WITH FDA 
 
FDA recommends communication with OTAT early in product development, before submission 
of an investigational new drug application (IND).  There are different meeting types that can be 
used for such discussions, depending on the stage of product development and the issues to be 
considered.  These include pre-IND meetings prior to submission of the IND (Ref. 22), and 
INitial Targeted Engagement for Regulatory Advice on CBER producTs (INTERACT) 
meetings, which can be used earlier in development to discuss issues such as preclinical 
development or manufacturing, so that the sponsor can obtain non-binding regulatory advice.11  
 
 
                                                 
10 As defined in section 569(c) of the FD&C Act, the term patient experience data includes data that are: 
  Collected by any persons (including patients, family members and caregivers of patients, patient advocacy 
organizations, disease research foundations, researchers, and drug manufacturers); and  
  Intended to provide information about patients experiences with a disease or condition, including the impact 
(including physical and psychosocial impacts) of such disease or condition, or a related therapy or clinical 
investigation, on patients lives; and patient preferences with respect to treatment of such disease or condition. 
Additional information on Patient-Focused Drug Development can be found on this website:  
https://www.fda.gov/drugs/development-approval-process-drugs/cder-patient-focused-drug-development. 
11 Going forward, INTERACT meetings will serve in place of pre-pre-IND meetings.  For additional information 
about INTERACT meetings, please see https://www.fda.gov/vaccines-blood-biologics/industry-biologics/interact-
meetings-initial-targeted-engagement-regulatory-advice-cber-products.  
13  Contains Nonbinding Recommendations 
 
IX.  REFERENCES 
 
1.  Considerations for the Design of Early-Phase Clinical Trials of Cellular and Gene 
Therapy Products; Guidance for Industry, June 2015. 
https://www.fda.gov/media/106369/download 
2.  Guidance for Industry:  Preclinical Assessment of Investigational Cellular and Gene 
Therapy Products, November 2013.  https://www.fda.gov/media/87564/download 
3.  Chemistry, Manufacturing, and Control (CMC) Information for Human Gene Therapy 
Investigational New Drug Applications (IND); Guidance for Industry, January 2020 
https://www.fda.gov/media/113760/download  
4.  Guidance for Industry:  Potency Tests for Cellular and Gene Therapy Products:  
Guidance for Industry, January 2011.  https://www.fda.gov/media/79856/download 
5.  Kitchen S, et al., BAY 81-8973, a full-length recombinant factor VIII: results from an 
International comparative laboratory field study, Haemophilia, 2016; 22(3):192-199. 
6.  Kitchen S, et al., Factor VIII assay variability in postinfusion samples containing full 
length and B-domain deleted FVIII, Haemophilia, 2016; 22(5):806-812. 
7.  Pickering W et al., Factor VIII chromogenic assays can be used for potency labeling and 
post administration monitoring of N8-GP, J Thromb Haemost, 2016; 14(8):1579-1587. 
8.  Sommer JM et al., Comparative field study evaluating the activity of recombinant factor 
VIII Fc fusion protein in plasma samples at clinical haemostasis laboratories. 
Haemophilia, 2014; 20(2):294-300. 
9.  Sommer JM et al., Comparative field study: impact of laboratory assay variability on the 
assessment of recombinant factor IX Fc fusion protein (rFIXFc) activity, Thromb 
Haemost, 2014; 112(5):932-940. 
10.  St. Ledger K et al., International comparative field study evaluating the assay 
performance of AFSTYLA in plasma samples at clinical hemostasis laboratories, J 
Thromb Haemost, 2018; 16(3):555-564. 
11.  Turecek PL et al., A world-wide survey and field study in clinical haemostasis 
laboratories to evaluate FVIII:C activity assay variability of ADYNOVATE and 
OBIZUR in comparison with ADVATE, Haemophilia, 2016; 22(6):957-965. 
12.  Raut S and AR Hubbard, International reference standards in coagulation, Biologicals,  
2010; 38(4):423-429. 
13.  Lozier JN et al., Animal models of hemophilia and related bleeding disorders, Semin 
Hematol, 2013; 50(2):175-184. 
14.  Long Term Follow-Up After Administration of Human Gene Therapy Products:  
Guidance for Industry, January 2020.  https://www.fda.gov/media/113768/download  
15.  Expectations for Biodistribution (BD) Assessments for Gene Therapy (GT) Products.  
International Pharmaceutical Regulators Programme (IPRP) Reflection Paper, 2018.  
http://development.iprp.backend.dev6.penceo.com/sites/default/files/2018-
09/IPRP_GTWG_ReflectionPaper_BD_Final_2018_0713.pdf 
14  Contains Nonbinding Recommendations 
 
16.  Guidance for Industry:  Providing Clinical Evidence of Effectiveness for Human Drug 
and Biological Products, May 1998.  https://www.fda.gov/media/71655/download 
17.  Guidance for Industry:  Expedited Programs for Serious Conditions  Drugs and 
Biologics: Guidance for Industry, May 2014.  
https://www.fda.gov/media/86377/download 
18.  In Vitro Companion Diagnostic Devices:  Guidance for Industry and Food and Drug 
Administration Staff, August 2014.  https://www.fda.gov/media/81309/download 
19.  Non-Inferiority Clinical Trials to Establish Effectiveness; Guidance for Industry, 
November 2016.  https://www.fda.gov/media/78504/download 
20.  Design and Analysis of Shedding Studies for Virus or Bacteria-Based GT and Oncolytic 
Products:  Guidance for Industry, August 2015.  
https://www.fda.gov/media/89036/download 
21.  Expedited Programs for Regenerative Medicine Therapies for Serious Conditions; 
Guidance for Industry, February 2019.  https://www.fda.gov/media/120267/download 
22.  Guidance for Industry:  Formal Meetings Between the FDA and Sponsors or Applicants, 
May 2009.  https://www.fda.gov/downloads/drugs/guidances/ucm079744.pdf 
 
 
 
 
 
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