Gene therapy product developers are seeking more clarity from the US Food and Drug Administration on when viral vectors from the same class will be considered sufficiently different for purposes of awarding orphan drug designation or exclusivity.
In addition, stakeholders want the FDA to better define, and provide examples of, the types of minor differences and additional features that would impact the agency’s “sameness” determination under the orphan drug regulations, and they seek assurance that the agency will share its lessons learned as it gains more experience in making such determinations for gene therapy products.
In a much-anticipated draft guidance issued in January, the agency said that if two gene therapy products are for the same use or indication, it will consider certain key features, such as transgenes and vectors, to be the “principal molecular structural features” in determining sameness for orphan designation and exclusivity purposes. (Also see “Orphan Exclusivity For Gene Therapies Hinges On Two Big Factors” – Pink Sheet, 28 Jan, 2020.)
If two gene therapy products express different transgenes and/or use vectors from different viral classes, the agency generally intends to consider them to be different drugs. However, it will decide whether two vectors from the same viral class (e.g., adeno-associated virus 2 (AAV2) vs. adeno-associated virus 5 (AAV5)) are the same or different on a case-by-case basis.
In an effort to guard against rewarding trivial changes between products, the agency said it will not consider two gene therapies to be different drugs based solely on “minor differences” between their transgenes and/or vectors. (Also see “Gene Therapies: ‘Trivial’ Changes Will Not Be Rewarded With Orphan Drug Designation And Exclusivity” – Pink Sheet, 14 Apr, 2020.)
If two gene therapy products express the same transgene and use the same vector, determining whether they are the “same” for orphan designation purposes may depend upon “additional features” of the final product that can contribute to the therapeutic effect, such as regulatory elements, the guidance states.
VIRAL VECTORS FROM THE SAME CLASS
In comments on the draft guidance, product sponsors raised concerns about the vagueness of the agency’s proposed case-by-case approach to determining sameness when viral vectors come from the same class.
“We appreciate FDA’s clarification that FDA intends to make the determination of whether two vectors from the same viral class (e.g., AAV2 vs. AAV5) are the same or different on a case-by-case basis,” BioMarin Pharmaceutical Inc.’s comments state. “Considering that there is tremendous development of gene therapy products using AAV-based vectors, it would be helpful to further clarify FDA’s thinking regarding factors that FDA may consider in making the determination that vectors within the same viral class are the same or different.”
Pfizer Inc. proposed that two serotypes from the same viral class, such as AAV2 and AAV5, be considered different. “Such differentiation is supported by the science,” the company said. “Evolutionary differences are captured by the parvovirus phylogenetic classification and are known to impact biological properties such as immunogenicity, tropism and infectivity.”
Similarly, Regeneron Pharmaceuticals, Inc. requested the agency define two vectors from the same viral class, but with different serotypes, as a characteristic sufficient to classify two gene therapies as different products. Different AAV serotypes are likely to exhibit differences in tissue tropism, transgene expression and immunogenicity, which are important characteristics that can significantly impact a product’s safety and efficacy, the company’s comments state.
“Therefore, we recommend that gene therapy products from different isotype classes should be classified as different products,” Regeneron said. “An approach that makes this determination on a case-by-case basis will lead to uncertainty, debate and potential disputes that will impede innovation in the development of certain types of gene therapy products.”
“MINOR DIFFERENCES” AND “ADDITIONAL FEATURES”
Numerous commenters urged the FDA to clarify what may constitute “minor differences” in transgenes and vectors, and to provide specific examples.
Likewise, industry comments urged the FDA to clarify, and provide examples of, what is meant by “additional features” of gene therapy products, such as what “regulatory elements” may be relevant and considered for determination of sameness.
CSL Behring asked the agency to
