Non-clinical modeling the therapeutic window of HX044, a novel CTLA4xCD47 BsAb

MOAs of the CTLA4-mAb ( e.g. Ipilimumab or Ipi) for anti-cancer efficacy include: 1) ADCC/the associated TIL-T reg -depletion, 2) CTLA4-ligand (CD80/CD86) blockade/the associated T-cell activation in tdLN (tumor draning lymphonoda) (priming), and 3) Fc-mediated effector activities/the associated TME remodeling enhancing immunogenicity. Ligand-blockade is also blamed for the associated immunotoxicity (irAE). We previously described a bispecific CTLA4-antibody/SIRPalpha-fusion protein, HX044, where it is considered to be a next generation CTLA4-therapy with enhanced efficacy via increased ADCC on CTLA4 +++ CD47 +++ TIL-T reg and lowered irAE via reduced ligand-blockade over Ipi. Its design concept was based on the assuption that Til-T reg have higher CTLA4/CD47 expression over those in peripheral in both levels and frequency. The present study assayed HX044 in various in vitro/in vivo experimental systems to approximately simulate its therapeutic window by modeling a variety of possible relevant parameters of pharmacokinetic exposures, irAE and efficacy. First, we surveyed available cancer patient T reg scRNAseq-datasets, which indeed confirmed both higher frequencies and levels of CTLA4/CD47 expressions in TIL-T reg over peripheral T reg , a benefiting factor in term of less irAE, although their thresholds for irAE remained to be determined. Second, HX044 was confirmed to have significantly enhanced ADCC over Ipi in double-high cells, confirming greater potential in TIL-T reg depletion, which is also confirmed along with associated enhanced antitumor efficacy in humanized syngeneic tumor models. Third, HX044's ligand-blockade in double-positive cells was determined to be slightly reduced (~50%) as compared to Ipi, while little ligand-blockade in CTLA4-single-positive cells, contrasting to potent blockade by Ipi, suggesting that HX044 could have reduced irAE if the dual receptor expression in peripheral are below the assumed thresholds. As a reference, blocking assay using recombinant CTLA4 protein revealed ~300x fold reduction of CD80-blocking for HX044 in comparison to Ipi. Fourth, NHP study of HX044 demonstrated excellent PK profile with long half-life and dose-proportional exposure as well as dose-dependent irAE, mostly seen in the highest dose level at 40mg/kg rather than lower dose levels at 1mg/kg and 6mg/kg, respectively. The data could not necessarily confirm significant better safety over Ipi in the 40mg/kg dose level, thus higher tolerable dose for irAE in the presence of sufficient double-positive T reg cells in peripheral. Nevertheless, HX044 has significantly broadened therapeutic window over Ipi, via lower efficacious dose levels attributed to significantly enhanced ADCC activities.