Gene Therapy With Lentivirus Is Acceptable For Clinical Trials, ICH Panel Says

Lentiviral vectors are acceptable for use in human clinical trials but the safety studies required will depend on the patient population and disease indication under study, an International Conference on Harmonization workgroup said Sept. 9 during a gene therapy workshop in Vienna, Va.

The types of studies that could be required include non-clinical toxicity studies, replication-competent lentivirus (RCL) assays, and mobilization.

FDA's main safety concern with use of lentiviral vectors is the potential for recombination events that may generate RCL, especially when the vector is based on a known human pathogen like the human immunodeficiency virus.

The agency "is clearly concerned about the replication competent viruses," Center for Biologics Evaluation & Research Director Kathryn Zoon, PhD, said during the workshop. "This is important and since we know that they are present in each dose of the vector given...the safety concerns for these are clear."

"Some of the assays that have been developed by the FDA to monitor and measure [replication competent viruses] have been very important, and I think that FDA will continue to take a very proactive role in this area," Zoon predicted.

FDA's Biological Response Modifiers Advisory Committee recommended in October 2001 that lentiviral vector products should undergo RCL assays (¹ (Also see "VirxSys Lentivirus IND For HIV Will Be Filed By First Quarter of 2002" - Pink Sheet, 19 Nov, 2001.), p. 18).

The advisory committee also recommended that Gaithersburg, Md.-based biotech firm VirxSys collect more in vitro data before proceeding with clinical trials of its HIV-1 lentiviral vector. VirxSys has since completed a long-term in vitro assessment and hopes to initiate clinical trials by the end of 2002.

Participants at the ICH meeting acknowledged the challenge of trying to convince patients to use a viral vector prepared from the HIV virus. Lentiviral vector systems under development include HIV, simian immunodeficiency virus, feline immunodeficiency virus and equine infectious anemia virus.

Other safety concerns related to use of lentiviral vectors include the level and persistence of gene transfer and expression, as well as insertional mutagenesis and inappropriate expression of the gene product by non-target cells and tissues, CBER Office of Therapeutics Research & Review Toxicologist Anne Pilaro, PhD, noted.

The agency's latest guidance addressing lentiviral vectors, "Supplemental Guidance on Testing for Replication Competent Retrovirus in Retroviral Vector Based Gene Therapy Products and During Follow-up of Patients in Clinical Trials Using Retroviral Vectors," was released in October 2000 and says that lentiviral vectors are considered new molecular entities and require in vivo safety studies.

ICH panel members generally agreed that safety issues with lentiviral vectors are similar to those identified with murine onco-retroviruses, with three added concerns: generation of RCL during manufacturing, mobilization of integrated lentiviral vector or endogenous retroviral sequences, and the potential for insertional mutagenesis and germline transmission.

The group also agreed that there were no specific safety issues identified for non-primate lentiviral vectors that would necessitate their safety and toxicity testing in the species of viral origin.

The panel suggested that while mobilization of integrated lentivirus should be assayed, there should be also be efforts to develop appropriate animal models to study mobilization in vivo.

The panel also agreed that nonclinical studies to evaluate the potential for germline dissemination and transmission should be developed.

Zoon said future issues to consider include "additional discussions and workshops on germline transmission issues, or inadvertent transmissions of genetic materials to future generations. Perhaps even some of the societal and ethical issues, versus the therapeutic effects...may be important to address collectively."

The development of viral standards should also be continued, Zoon said. "I think we just entered our first toe in the water...and I think [viral standards] will be important to pursue to the level that these are appropriate."

The ICH Steering Committee is expected to issue recommendations from the workshops for FDA's consideration.

During the workshop, CBER Office of Therapeutics Research and Review Branch Chief Stephanie Simek, PhD, discussed the Aug. 26 release of adenovirus reference material.

The reference material is intended to allow the comparison of data from different studies using different adenoviral vectors, and was prompted by the 1999 death of a subject in a gene therapy trial at the University of Pennsylvania.

The adenovirus reference material includes a wild-type adenovirus preparation that can be used as a reference material to define infectious titer and virus particle concentration for adenoviral gene therapy vectors, Simek said. Each new virus must be validated using different dilutions in an assay.

However, the adenovirus reference material does not represent standardization of specific assay methods, she explained.

The agency continues to recommend that adenoviral products contain a level of less than 1 RCA per 3 x 10¹⁰ viral particles for all lots regardless of clinical use (² (Also see "Adenovirus Vector Standards Should Be Loosened For Healthier Patients" - Pink Sheet, 23 Jul, 2001.), p. 30).

"The release of the adenovirus reference material is...a small step, I think, but a positive step," Zoon said. "It's not a panacea, and I don't think that anybody's making a claim that it is, but I think it does give us an opportunity to make progress."

"Standards help provide an opportunity to help us to move the field forward to make comparisons that are scientifically valid where appropriate," she added. "I do think that is, in many ways, a focus that we're trying to put forward here at this meeting, and will continue to do so."

"We're also looking at data regarding potential toxicity related to use of...adenoviral vectors, where we have observed dose limiting toxicity," Zoon said. "My sense is that we will continue to be vigilant, looking at parameters that can help evaluate this better."

Whether "we're looking at particle count dosing or infectious titer, my sense is that this is going to be key and I think the emphasis has been put on particle counts and not on the infectivity is because of some of the surprises that we have seen."

FDA has 196 active gene therapy INDs, including 90 for ex vivo gene transfer and 106 for in vivo gene transfer.

"Based on the distribution of most of these, ex vivos are using retroviruses, and as we look at other types of vectors, whether it be adeno, plasma, [adeno-associated virus] and herpes virus, we're looking more closely at the in vivo administration of these types of vectors," Zoon said. "So we still have relatively few INDs with [adeno-associated virus]...compared to some of the other systems."

Zoon endorsed "sharing information on the viral vector systems, both the pros and the cons and some of the challenges behind there."

The ICH workshop included a presentation from Austin, Texas-based Introgen, which has the adenoviral p53 product Advexin in Phase III trials for head and neck cancer. Studies for the indication are expected to finish at the end of 2003, and BLA filing is slated for 2004 (³ (Also see "Aventis Spins Off Gene Therapy, Returns p53 Commercial Rights To Introgen" - Pink Sheet, 9 Apr, 2001.), p. 25).

Introgen Associate VP-Clinical Research Deborah Wilson, PhD, told the meeting that the company's biodistribution analysis showed that Advexin is systematically distributed following intratumoral administration.

While the vector appears transiently in patient body fluids, no vector DNA was detected in semen, and anti-adenoviral antibody titers increase in response to Advexin but have little impact on pharmacokinetics, Wilson said.

Wilson also noted that that no vector-derived replication-competent adenovirus was found in body fluids of subjects treated with Advexin, and that there is no clear and direct evidence of transmission to household members under normal living conditions.

Genzyme Senior VP-Gene Therapy Richard Gregory, PhD, observed that independent gene therapy companies are becoming "an endangered species" and that because big pharma investment has dropped, there are few products in the late stages of development.