Overview of NTP Proposal

							                CTGTAC #41

      National Toxicology Program
        Proposed Study Design:
   Model for Retroviral-Mediated
     Insertional Mutagenesis

          Carolyn A. Wilson, Ph.D.
Division of Cellular and Gene Therapies, Office of
       Cellular, Tissue, and Gene Therapies
            carolyn.wilson@fda.hhs.gov
Overview of FDA Presentation
• Why consider performing the proposed
  study?
• What are the study goals?
• What are the goals for today’s meeting?
  Gammaretroviral-Mediated
 Endogenous Gene Activation
             Psi     (Internal Promotor) Transgene
      LTR
      LTR                                            LTR

 U3
U3     R    U5



                   Read-through
                   Transcription
                                                Dysregulated
Distal Gene                                     Gene Expression
 Activation


                                               Tumorigenesis
Retroviral Vectors and Tumorigenesis

• Wildtype Retroviruses
   – Used as models to study tumorigenesis
   – Used as models to induce genetic
     aberrations
• Acknowledged risk in use of retroviral
  vector-mediated gene therapy
   – In absence of replication, finite number
     of sites for genomic integration
     (reduced risk)
            X-SCID Gene Therapy*
      Increased
                                                 THERAPEUTIC
  Transduction Rates                               BENEFIT


                  +                                        +
    In vivo Selective                               INCREASED
                                                       RISK
        Advantage
*Cavazzana-Calvo, M., et al. 2000. Science 288:669-72.
 Hacein-Bey-Abina, et al. 2002. New England Journal of Medicine 346:1185-1193.
 Hacein-Bey-Abina, S., et al. 2003.. N Engl J Med 348:255-6.
            CRITICAL PATH:
Success of retroviral vectors will be limited
      by potential for tumorigenesis




 A valid preclinical model is needed to assess
      the relative risks of vectors modified
   to reduce the likelihood of tumorigenesis
   Previous CTGTAC Advice
• Investigators should be encouraged to
  explore alternative retroviral vector
  structures to reduce the risk of
  tumorigenesis, for example:
  – Deletion of U3
  – Insulator elements
  – Suicide genes
• Alternative vectors should be adequately
  tested in animal models
         Considerations in choice of
            preclinical models
• In vitro genomics that map vector insertion sites
  do not show biological effect
• Large animal studies are limited by
  – study size that is feasible
  – cost of long-term observation
• Rodent Studies
  – Allow analysis for biological effects of vector insertion
  – Allow use of large study size
• Model should mimic human clinical studies
  – Target cells used for retroviral vector transduction
  – Measurable rate of tumorigenesis in animals
     Leukemias Observed in Mouse Model
      of Retroviral Vector Hematopoietic
           Stem Cell Transduction
                                           Leukemic Clone



                                                 Evi-1




                                                  Control
    AML-like disease
                                    Li, et al, Science, 2002
Additional relevant preclinical studies will be presented by Chris Baum
Goals of Proposed NTP Study
• Develop and assess the sensitivity of a
  preclinical model for assessing the risk of
  retroviral vector-mediated insertional
  tumorigenesis
   – 50 mice per group of primary recipients
   – 2 secondary recipients/asymptomatic primary
• Assess the effect of vector dose on tumor
  frequency
• Assess the effect of deleting U3 from the LTR

  Study design will be presented by Rick Irwin
        Goal for Today:
  Discussion of FDA Questions
1. Please comment on the general
   scientific approach proposed to
   evaluate a mouse bone marrow
   transplantation model for its feasibility
   to assess pre-clinical safety of
   retroviral vectors.
                      FDA Question #2
     2. The FDA/NTP partnership may have
        opportunities to explore other models in the
        future. Please comment on future studies
        that may be useful to assess retroviral
        vector safety.
            a. Specifically, please comment on whether the
               use of an in utero gene transfer model, such as
               that used by Themis, et al*, should be examined
               through the NTP program for its potential as a
               toxicology model for assessing lentivirus vector
               tumorigenicity.
*Themis, M., et al., Oncogenesis following delivery of a nonprimate
lentiviral gene therapy vector to fetal and neonatal mice.
Mol Ther, 2005. 12(4): p. 763-71.
         FDA Question #3
3. If time permits, we would welcome
   your comments on the following:
  a. Possible toxicology models of other
     cellular or gene therapies that would be
     useful to study through NTP.
  b. The use of NTP as a resource for
     development of toxicological testing
     models for novel therapies.