Objective:

Compare three BTK inhibitors in Phase 3 development in multiple sclerosis (MS) according to relative potencies and CNS exposure.

Background:

Modulating CNS neuroinflammation represents a paradigm shift for treating MS. Tolebrutinib is a covalent BTK inhibitor designed and selected to optimize both potency and exposure to modulate BTK signaling within the CNS. 

Design/Methods:

We used in vitro kinase assays, cellular activation assays, and pharmacokinetic (PK) sampling of cerebrospinal fluid (CSF) in the non-human primate (NHP) cynomolgus to compare tolebrutinib, evobrutinib, fenebrutinib under identical conditions. We then evaluated the CSF PK of tolebrutinib in healthy volunteers.

Results:

Kinase assays demonstrated that tolebrutinib reacted with BTK 65-times faster than evobrutinib (K_inact/K_i = 4.37 x 10^-3 and 6.82 x 10^-5 nM^-1*s^-1, respectively). Fenebrutinib, a reversible antagonist had a K_i value of 4.7 nM. Because of the slow off-rate (1.54 x 10^-5 s^-1), the forward rate was also very slow (3.28 x 10³ M^-1 * s^-1). Estimated cellular potency (using B cell activation) was consistent with kinase data (IC50 = 0.7 nM, 34.5 nM, and 2.9 nM, respectively). In NHP, we observed similar levels of exposure in the CSF after oral doses of 10 mg/kg for each candidate. However, tolebrutinib CSF exposure (4.8 ng/mL) (kp,uu CSF=0.40) exceeded the IC90, while evobrutinib (3.2 ng/mL) (kp,uu CSF=0.13) and fenebrutinib (12.9 ng/mL) (kp,uu CSF=0.15) failed to reach the IC50. In healthy volunteers, a single oral dose of 60 or 120 mg resulted in bioactive CSF levels between 2 and 4 hrs. (0.51 and 1.03 ng/mL, respectively).

Conclusions:

Next-generation drug candidates in MS must achieve pharmacologically-relevant concentrations in the CNS to address the existing treatment gap. In these NHP experiments, tolebrutinib was the only BTK inhibitor to show bioactive CSF levels, adding to the evidence that tolebrutinib has potential to slow disability accumulation via modulating neuroinflammation.