Histological verification of the treatment effect of tirabrutinib for relapsed/refractory primary central nervous system lymphoma

Tirabrutinib (ONO/GS-4059; Ono Pharmaceutical) is a newly developed drug that selectively and irreversibly inhibits Bruton’s tyrosine kinase (BTK) and has been approved in Japan for treating relapsed/refractory primary central nervous system lymphoma (PCNSL). However, its therapeutic effect is yet to be verified at the pathological level in human patients. A 64-year-old patient with recurrent PCNSL enrolled in the phase I/II clinical trial of tirabrutinib, a second-generation BTK inhibitor designed for treating relapsed/refractory PCNSL. The left cerebellum lesions on magnetic resonance imaging disappeared one month after tirabrutinib treatment. The patient died because of suspected pneumocystis pneumonia and acute exacerbation of interstitial pneumonia 43 days after starting tirabrutinib. An autopsy confirmed no viable tumor cells in the entire brain, including the left cerebellum lesion, confirming complete obliteration of tumor cells by tirabrutinib. This letter pathologically confirms the effect of tirabrutinib on relapsed/refractory PCNSL for the first time in humans.Trial registration: JapicCTI-173646. Registered 14 July 2017, https://www.clinicaltrials.jp/cti-user/trial/ShowDirect.jsp?japicId=JapicCTI-173646.