Contributions
Abstract: PB1987
Type: Publication Only
Background
Beta thalassemia is a genetic blood disorder characterized by reduced red blood cell (RBC) lifespan, low blood hemoglobin and the need for lifelong transfusion. CTX001 is a gene-edited cellular therapy for the treatment of beta thalassemia, in which autologous hematopoietic stem cells (HSCs) have been edited to disrupt the erythroid enhancer region (EER) of BCL11a on chromosome 2. Treatment with CTX001 is intended to increase expression of fetal hemoglobin (HbF) in RBCs. CTX001 is being co-developed by CRISPR Therapeutics and Vertex Pharmaceuticals.
Aims
A mathematical model was developed to describe the relationship between a dose of genetically modified HSCs and resulting blood Hb following engraftment.
Methods
Literature and original research data were used to develop a model comprised of distinct biological processes characterizing the effect of gene editing on HbF in the RBC, the proportion of modified HSC administered, bone-marrow engraftment of HSCs, surviving RBC in circulation, and, finally, an estimate of blood Hb levels. Individual components included the ratio of gamma/alpha globin mRNA, RBC survival post ablation, lifespan of RBCs as a function of globin protein ratios (alpha-partner/alpha globin) and extrapolation of RBC lifespan to Hb. The relationship between dosing and Hb was evaluated across a range of input variables.
Data were manually extracted from literature sources with individual-level data reported on endpoints relevant to the models (Ferrari, 2017; Vigi et al., 1969; Clegg et al., 1979). The input data were independently quality controlled. R with the rstanarm library was used to generate mathematical models using Bayesian inference and minimally-informative priors. Models were selected to represent the data available and were a combination of linear fixed effect, linear mixed effect, and nonlinear fixed effect models. The best model was determined by statistical properties and visual checks of the analysis results.
Results
RBC lifespan, as measured by 51Cr pulse, appears directly and linearly related to alpha-partner/alpha globin ratio (Vigi et al 1969). Original research data indicated that CRISPR modification of the BCL11a EER increased the gamma/alpha globin mRNA ratio. Efficiency of transfection (% cells transfected) was modeled as a variable process without covariates. A linear mixed effects model estimated that the percent of leukocytes in blood emanating from transfused HSCs is 77.3 % [50.1, 105, 95% credible interval] x the efficiency of gene modification in the HSCs. When the alpha-partner/alpha globin ratio is 1, the RBC half-life is estimated to be 29.4 [27.3, 31.6] days consistent with the normal 51Cr pulsed RBC half-life in subjects without beta thalassemia. When no alpha partner is present, the estimated lifespan is 0.522 [-0.935, 1.80] days, aligning with expectations that alpha partners are required for RBC survival. Using these models and internal data on the distribution of alpha-partner/alpha mRNA and protein ratios in reticulocyte-like cells, the model estimate for the alpha-partner/alpha ratio is 0.746 following bi-allelic genetic modification of HSC cells (CTX001). The estimated ratio would suggest an average 51Cr pulsed RBC lifespan of 21 -34 days, depending on assumptions for maximum RBC lifespan constraints.
Conclusion
Simulations across the model suggest that the proportion of biallelically modified HSCs must be 20 to 50% for notable improvement in blood Hb levels (target 10 g/dL) following treatment with CTX001.
Session topic: 25. Gene therapy, cellular immunotherapy and vaccination – Biology & Translational Research
Keyword(s): Autologous hematopoietic stem cell transplantation, Gene therapy, Hemoglobin variants, Thalassemia
Abstract: PB1987
Type: Publication Only
Background
Beta thalassemia is a genetic blood disorder characterized by reduced red blood cell (RBC) lifespan, low blood hemoglobin and the need for lifelong transfusion. CTX001 is a gene-edited cellular therapy for the treatment of beta thalassemia, in which autologous hematopoietic stem cells (HSCs) have been edited to disrupt the erythroid enhancer region (EER) of BCL11a on chromosome 2. Treatment with CTX001 is intended to increase expression of fetal hemoglobin (HbF) in RBCs. CTX001 is being co-developed by CRISPR Therapeutics and Vertex Pharmaceuticals.
Aims
A mathematical model was developed to describe the relationship between a dose of genetically modified HSCs and resulting blood Hb following engraftment.
Methods
Literature and original research data were used to develop a model comprised of distinct biological processes characterizing the effect of gene editing on HbF in the RBC, the proportion of modified HSC administered, bone-marrow engraftment of HSCs, surviving RBC in circulation, and, finally, an estimate of blood Hb levels. Individual components included the ratio of gamma/alpha globin mRNA, RBC survival post ablation, lifespan of RBCs as a function of globin protein ratios (alpha-partner/alpha globin) and extrapolation of RBC lifespan to Hb. The relationship between dosing and Hb was evaluated across a range of input variables.
Data were manually extracted from literature sources with individual-level data reported on endpoints relevant to the models (Ferrari, 2017; Vigi et al., 1969; Clegg et al., 1979). The input data were independently quality controlled. R with the rstanarm library was used to generate mathematical models using Bayesian inference and minimally-informative priors. Models were selected to represent the data available and were a combination of linear fixed effect, linear mixed effect, and nonlinear fixed effect models. The best model was determined by statistical properties and visual checks of the analysis results.
Results
RBC lifespan, as measured by 51Cr pulse, appears directly and linearly related to alpha-partner/alpha globin ratio (Vigi et al 1969). Original research data indicated that CRISPR modification of the BCL11a EER increased the gamma/alpha globin mRNA ratio. Efficiency of transfection (% cells transfected) was modeled as a variable process without covariates. A linear mixed effects model estimated that the percent of leukocytes in blood emanating from transfused HSCs is 77.3 % [50.1, 105, 95% credible interval] x the efficiency of gene modification in the HSCs. When the alpha-partner/alpha globin ratio is 1, the RBC half-life is estimated to be 29.4 [27.3, 31.6] days consistent with the normal 51Cr pulsed RBC half-life in subjects without beta thalassemia. When no alpha partner is present, the estimated lifespan is 0.522 [-0.935, 1.80] days, aligning with expectations that alpha partners are required for RBC survival. Using these models and internal data on the distribution of alpha-partner/alpha mRNA and protein ratios in reticulocyte-like cells, the model estimate for the alpha-partner/alpha ratio is 0.746 following bi-allelic genetic modification of HSC cells (CTX001). The estimated ratio would suggest an average 51Cr pulsed RBC lifespan of 21 -34 days, depending on assumptions for maximum RBC lifespan constraints.
Conclusion
Simulations across the model suggest that the proportion of biallelically modified HSCs must be 20 to 50% for notable improvement in blood Hb levels (target 10 g/dL) following treatment with CTX001.
Session topic: 25. Gene therapy, cellular immunotherapy and vaccination – Biology & Translational Research
Keyword(s): Autologous hematopoietic stem cell transplantation, Gene therapy, Hemoglobin variants, Thalassemia