Discovery of 4,7-Diamino-5-(4-phenoxyphenyl)-6-methylene-pyrimido[5,4-b]pyrrolizines as Novel Bruton’s Tyrosine Kinase Inhibitors

An alternative medicinal chemistry approach was conducted on Bruton's tyrosine kinase (BTK) inhibitor 1 (ibrutinib) by merging the pyrazolo[3,4- d]pyrimidine component into a tricyclic skeleton. Two types of compounds were prepared, and their biochemical activities on BTK as well as stereochemistry effects were determined. Structural optimization focusing on the reactive binding group to BTK Cys481 and on the metabolic site guided by metabolic study were conducted. 7S was identified as the most potent showing an IC₅₀ value of 0.4 nM against BTK and 16 nM against BTK-dependent TMD8 cells. Compared to 1, 7S was slightly more selective with strong inhibition on the B-cell receptor signaling pathway. In a TMD8 cell-derived animal xenograft model, 7S showed a relative tumor volume of 5.3 at 15 mg/kg QD dosage that was more efficacious than 1 (RTV 6.6) at a higher dose of 25 mg/kg QD. All these results suggest 7S as a new BTK inhibitor worthy of further profiling.