# Presentation: 825225

## 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

- Gene Disruption

## 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)

**Notes:**
Low transduction rates have resulted in good safety record, but poor or no therapeutic benefit.

## X-SCID Gene Therapy* 

- Increased
- Transduction Rates

- *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

*Success of retroviral vectors will  be limited by potential for tumorigenesis*

**Actual Risk**

**Therapeutic**
** ****Benefit**

**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

- Evi-1

- Leukemic Clone

- AML-like disease

- Li, et al, Science, 2002

*Additional relevant preclinical studies will be presented by Chris Baum*

- Control

## 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

- 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

- 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

- 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.