Discussion
Due to potential long-term safety risks, FIH study of most GT products
is a dose-finding study conducted in a dozen or so patients with a rare
disease of genetic disorders. Each patient usually receives a single
dose of AAV vector only because the immune responses caused by repeated
dosing may render subsequent administrations
ineffective9. A patient receiving a low and
ineffective starting dose may be ineligible to receive an escalated dose
due to activated immunity against this AAV vector. Thus, an accurate
prediction of FIH dose is critical for clinical development of GT
products. In addition to the direct vg/kg conversion approach widely
used in clinical development of AAV GT16 and recently
reported allometric scaling approach,5 this study
demonstrated that interspecies dose-response normalization could be used
as an additional approach for FIH dose prediction for AAV-mediated GT.
This new approach bypasses pharmacokinetic data and extrapolates
dose-response relationship from preclinical species to hemophilia
patients. This approach is especially useful at the early stage of
clinical development when clinical data are insufficient to support the
development and validation of complex quantitative systems pharmacology
models. However, the performance of the interspecies dose-response
normalization approach can be compromised by poor quality of preclinical
data, uncertainties in human immune responses to vector/transgene
product, and substantial interspecies differences in vector transduction
efficiency and transgene expression efficiency.
The quality of preclinical data is critical for successfully
extrapolating dose response from animals to humans. Preclinical
dose-finding studies for AAV-mediated GT products are usually conducted
in dogs or monkeys with a small sample size (N = 2 – 6 per dose level)
and within a limited dose range. Due to the large inter-individual
variability in transgene product levels, a preclinical dose-finding
study with appropriate sample size is recommended. As shown in our
regression analysis, the total vector dose range often did not overlap
between dogs/monkeys and humans, which might compromise the quality of
interspecies dose-response extrapolation. This issue can be addressed by
(1) including large animal models with a body weight comparable to human
(i.e., swine and cattle) into does-finding studies, and (2) testing GT
products in the large animal models with a wide dose range. Since
intracellular synthesis rate of transgene product is reversely
correlated with W0.25, large animal models are
anticipated to be more representative of humans in term of transgene
product expression efficiency. The quality of both allometric scaling
and interspecies dose-response normalization are expected to be improved
by including dose response data of large animal
models.17
Immune responses to vectors, transgene products, and AAV capsid antigens
expressed on the surface of transduced cells in animals and humans are a
big challenge in extrapolating dose-response from animals to
humans.8 AAV capsid-specific T cell responses were
detected in hemophilia B patients receiving moderate- or high-dose
rAAV2-hAAT-FIX or scAAV2/8-LP1-hFIXco vectors. The presence of T-cell
responses might eliminate vector-transduced human hepatocytes, decrease
transgene product expression, and thus lead to substantial
underprediction of FIH dose.8 Several steps may be
taken to reduce the risk of AAV capsid-specific T cell responses: (1)
develop appropriate in vitro tests (i.e., enzyme-linked immunospot assay
and cytotoxic T lymphocyte assay) to identify and eliminate vector
candidates with a high risk of T cell responses16; (2)
exclude patients with preexisting immunity from FIH studies; and (3)
maximize vector potency through capsid engineering and use high-activity
transgene product variant and thus reduce FIH dose because AAV
capsid-specific T cell responses are dose-dependent.8,
18
With the advance in capsid engineering and availability of new potent
promoters/enhancers, some bioengineered AAV vectors showed a much higher
transduction efficiency and a similar or even higher transgene
expression efficiency in human hepatocytes than that in animal
hepatocytes,15 which might cause an overprediction of
FIH dose using allometric scaling or interspecies dose-response
normalization approaches. The large interspecies differences in
transduction efficiency and transgene expression efficiency can be
detected using a humanized mouse liver model.15 These
interspecies differences should be considered when dose response is
extrapolated from animals to humans.
In this study, the proposed interspecies dose-response normalization was
assessed with AAV vectors for hemophilia therapy only. Further work is
needed to assess if this approach can be applied to (1) AAV-encoding
transgene products that function in systemic circulation for other
diseases; (2) other intravenously administered viral (e.g., adenoviral
and lentiviral vectors) and non-viral GT modalities (e.g., lipid
nanoparticles); and (3) locally administered GT products (e.g.,
intrathecal or intravitreal administration).