GB5121, a Brain-Penetrant, Irreversible BTK Inhibitor, Demonstrates Potent Anti-Tumor Activity in a Novel Mouse Model of Intracranial Lymphoma

Background: Inhibitors of Bruton's tyrosine kinase (BTKi) are approved treatments for several B cell lymphomas and more recently have shown promising results in CNS lymphomas. GB5121 was designed to be a brain-penetrant, potent, highly selective, irreversible small molecule BTKi with the potential to improve patient outcomes for hematologic malignancies with CNS involvement. Here, GB5121 was evaluated for brain penetration in Non-Human Primates (NHP) using a [¹¹C] PET tracer and for in vivo efficacy in pre-clinical lymphoma xenograft models.

Methods: GB5121 brain penetration was assessed in NHP using a [¹¹C] GB5121 PET ligand. Two rhesus monkeys were administered [¹¹C] GB5121 PET ligand intravenously and PET scans were collected at baseline and over 90 minutes. Anti-tumor activity of GB5121 was evaluated in the subcutaneous (SC) and orthotopic TMD8 cell-line derived xenograft (CDOX) mouse models (Tohda et al. Leuk Res. 2006; 30(11):1385-1390). For the SC model, mice (n=10) received daily oral doses of GB5121 (10 or 30 mg/kg) or ibrutinib (30 mg/kg) for 24 days. For the orthotopic model, TMD8 cells were implanted intracranially; 14-days post implantation animals (n=12) were dosed orally once daily with GB5121 (10 or 30 mg/kg) or ibrutinib (30 mg/kg) for 28 days. Tumor volume was measured by MRI on days 28 and 35 post-implant. Human IL-10 levels were measured in plasma, and percentage of human CD20-positive cells in brain was determined by immunohistochemistry (IHC) at study termination or at humane endpoints. In a cohort of mice (n=3), BTK target occupancy (TO) was measured in brain, and drug exposure was measured in plasma and brain on day 35 at 1-hour post-dose for all groups.

Results: PET imaging in NHP receiving [¹¹C] GB5121 confirms CNS penetration consistent with pharmacokinetic studies we previously conducted in NHP, demonstrating unimpeded access of free plasma GB5121 into the CNS. In the TMD8 SC tumor model, therapeutic dosing with GB5121 led to complete inhibition of tumor growth at both the 10 and 30 mg/kg doses. In the intracranial CDOX model, treatment with GB5121 resulted in prolonged survival when compared to vehicle treated mice, with 90% and 100% of the mice surviving till study termination at the 10 and 30 mg/kg doses, respectively. In comparison, ibrutinib-treated animals did not exhibit a significant survival benefit, with 58% of the mice surviving at study termination compared to 46% in vehicle group. GB5121 also demonstrated a decrease in tumor burden as measured by MRI and hCD20 staining by IHC. In addition, human IL-10 levels, which are used as a biomarker for tumor burden clinically, were significantly reduced in plasma of mice dosed with 30mg/kg of GB5121. Finally, GB5121 demonstrated higher brain exposure and BTK TO compared to ibrutinib in a cohort of tumor bearing mice.

Conclusions: GB5121 was designed to be a brain-penetrant BTKi; a PET imaging study confirms the results of previously conducted NHP PK studies demonstrating CNS penetration of GB5121. In a novel model of intracranial lymphoma, treatment with GB5121 resulted in better anti-tumor activity compared to ibrutinib, likely resulting from higher brain penetration and TO. This higher brain penetration and TO exhibited by GB5121 may result in more durable responses in malignancies with CNS involvement and supports the investigation of GB5121 in clinical trials for CNS lymphomas. Therefore, we are currently evaluating GB5121 in a phase 1b/2 open-label, international study in adult subjects with relapsed/refractory primary/secondary CNS lymphoma, or primary vitreoretinal lymphoma (STAR CNS; NCT05242146).

Yusuf:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Steinberg:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Gozo:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Clemons:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Hou:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Guimond:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Rose:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Taylor Meadows:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Carter:Gossamer Bio, Inc.: Current Employment, Current equity holder in publicly-traded company; Falk Trust: Honoraria, Membership on an entity's Board of Directors or advisory committees; AAPS: Honoraria; Asteroid Therapeutics: Consultancy, Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees.