Background A transformative approach to medicine involves expressing complex and therapeutically relevant biologics utilizing patients’ own body and tissues via mRNA administration. Here, we engineered complex molecules targeting CD47, as multivalent SIRPa-Fc fusion proteins or bi-specifics targeting both CD47 and CCR4 molecules. CD47 is a widely expressed transmembrane glycoprotein that sends ‘do not eat me’ signals to macrophages by binding to signal regulatory protein alpha (SIRPa). With more than 50 ongoing clinical trials, anemia is the most common adverse effect due to the expression of CD47 on the Red Blood Cells (RBCs).1 We show that in-vitro activity, selectivity, and in-vivo efficacy could be significantly improved either by increasing the valency or by targeting dual antigens via mRNA produced proteins.
Methods The SIRPa N-terminal IgV domain was genetically fused to IgG1 Fc to generate bivalent, tetravalent, hexavalent, and octavalent molecules as well as bispecific molecules by combining with anti-CCR4 binding domain (figure 1). We produced, characterized, and compared these molecules with the bivalent SIRPa Wild Type-Fc and SIRPa High Affinity (HA)-Fc2 proteins that are similar to the molecules currently in clinical trials. Specifically developed western and mass spectrometry methods were employed to assess the purity of the in-vivo expressed proteins following IV administration of mRNA in Nutshell® nanoparticles. Antibody-dependent cellular cytotoxicity (ADCC) assay as well as Raji xenograft mouse model was utilized to demonstrate the activity and efficacy.
Results We observed that the activity of octavalent SIRPa WT was similar to that of the affinity improved (>50000-fold) SIRPa HA, however, it showed low to no binding to RBCs in-vitro potentially mitigating anemia risk (figure 2). Further mechanistic studies demonstrated a direct correlation between valency and target density, suggesting a role for the avidity effect. The observed activity could be further improved with addition of aCCR4 Fab, leading to >1000-fold improvement in EC50. Administering mice with formulated mRNAs encoding the tetravalent, octavalent, and bispecific molecules resulted in robust protein expression (~10–100ug/ml) with high purity/homogeneity similar to or better than the DNA expressed proteins. Further, the engineered molecules fully eradicated established subcutaneous tumors in the Raji xenograft mouse model (figure 3).
Conclusions We demonstrate that significant enhancement in therapeutic window and efficacy could be achieved by engineering complex multivalent and bispecific molecules. Further, systemic delivery of mRNA encoding these complex molecules avoids the need to produce and retain stability using the traditional cell line development paradigm which is often challenging.
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