PB0710 - Evaluation of the feasibility and safety of the long-term expression of hemopexin using a viral (AAV8) vector in an animal model of sickle cell anemia

Background: Extracellular heme mediates tissue damage and coagulation activation in sickle cell anemia (SCA). Hemopexin (HPX) is responsible for removing excess heme from the circulation. However, previous studies have shown that HPX levels are depleted in SCA.

Aims: To explore the feasibility and safety of using a viral (AAV8) vector to obtain the long-term expression of HPX in a mouse model of SCA.

Methods: Two viral vectors based on AAV8 expressing human (hHPX) or murine HPX (mHPX). Sustained hHPX expression using the first vector was reported previously. C57Bl/6j or Townes mice were intravenously injected with one of these vectors, or with a control (Empty). HPX expression was evaluated by qPCR, western blot or Elisa. Safety was evaluated by clinical and hematological parameters.

Results: hHPX expression was confirmed in Townes mice in liver samples after 28 weeks (figures 1a-b). However, circulating hHPX could be detected in only 1/10 transduced mice (HPX level: 0.033 mg/mL). To optimize expression we moved to a vector expressing mHPX, which increased mHPX levels up to 3-fold, in a dose-dependent fashion (figure 1c). Townes mice were then treated with the higher vector dose and observed for 28 weeks. No differences in mortality were observed. Gained weight and hematological parameters did not differ treated and control mice. Despite a trend towards higher circulating mHPX in transduced mice after 8 weeks (figure 1d), levels of circulating mHPX remained low, consistent with hemolysis-mediated depletion in both groups.

Conclusion(s): While long-term gene transfer of HPX is feasible in C57BL/6 mice, achievement of therapeutic HPX levels in an animal model of SCA is challenging, possibly due to HPX consumption. Additional experiments are required to explore whether transgenic HPX expression can result in clinical and laboratory parameters in Townes mice, despite the lower circulating levels.