Ibrutinib in Refractory or Relapsing Primary Central Nervous System Lymphoma: A Systematic Review

Primary Central Nervous System Lymphoma (PCNSL) is a rare variant of Non-Hodgkin Lymphoma (NHL) representing 1–2% of all NHL cases. PCNSL is defined as a lymphoma that occurs in the brain, spinal cord, leptomeninges, or eyes. Efforts to treat PCNSL by traditional chemotherapy and radiotherapy have generally been unsuccessful as a significant proportion of patients have frequent relapses or are refractory to treatment. The prognosis of patients with Refractory or Relapsed (R/R) PCNSL is abysmal. The optimal treatment for R/R PCNSL is poorly defined as there are only a limited number of studies in this setting. Several studies have recently shown that ibrutinib, a Bruton tyrosine kinase (BTK) inhibitor, has promising results in the treatment of R/R PCNSL. However, these are preliminary studies with a limited sample size. In this systematic review, we explored and critically appraised the evidence about the efficacy of the novel agent ibrutinib in treating R/R PCNSL.

Role of Tyrosine Kinase Syk in Thrombus Stabilisation at High Shear

Understanding the pathways involved in the formation and stability of the core and shell regions of a platelet-rich arterial thrombus may result in new ways to treat arterial thrombosis. The distinguishing feature between these two regions is the absence of fibrin in the shell which indicates that in vitro flow-based assays over thrombogenic surfaces, in the absence of coagulation, can be used to resemble this region. In this study, we have investigated the contribution of Syk tyrosine kinase in the stability of platelet aggregates (or thrombi) formed on collagen or atherosclerotic plaque homogenate at arterial shear (1000 s−1). We show that post-perfusion of the Syk inhibitor PRT-060318 over preformed thrombi on both surfaces enhances thrombus breakdown and platelet detachment. The resulting loss of thrombus stability led to a reduction in thrombus contractile score which could be detected as early as 3 min after perfusion of the Syk inhibitor. A similar loss of thrombus stability was observed with ticagrelor and indomethacin, inhibitors of platelet adenosine diphosphate (ADP) receptor and thromboxane A2 (TxA2), respectively, and in the presence of the Src inhibitor, dasatinib. In contrast, the Btk inhibitor, ibrutinib, causes only a minor decrease in thrombus contractile score. Weak thrombus breakdown is also seen with the blocking GPVI nanobody, Nb21, which indicates, at best, a minor contribution of collagen to the stability of the platelet aggregate. These results show that Syk regulates thrombus stability in the absence of fibrin in human platelets under flow and provide evidence that this involves pathways additional to activation of GPVI by collagen.

Spontaneous Platelet Aggregation in Blood Is Mediated by FcγRIIA Stimulation of Bruton’s Tyrosine Kinase

High platelet reactivity leading to spontaneous platelet aggregation (SPA) is a hallmark of cardiovascular diseases; however, the mechanism underlying SPA remains obscure. Platelet aggregation in stirred hirudin-anticoagulated blood was measured by multiple electrode aggregometry (MEA) for 10 min. SPA started after a delay of 2–3 min. In our cohort of healthy blood donors (n = 118), nine donors (8%) with high SPA (>250 AU*min) were detected. Pre-incubation of blood with two different antibodies against the platelet Fc-receptor (anti-FcγRIIA, CD32a) significantly reduced high SPA by 86%. High but not normal SPA was dose-dependently and significantly reduced by blocking Fc of human IgG with a specific antibody. SPA was completely abrogated by blood pre-incubation with the reversible Btk-inhibitor (BTKi) fenebrutinib (50 nM), and 3 h after intake of the irreversible BTKi ibrutinib (280 mg) by healthy volunteers. Increased SPA was associated with higher platelet GPVI reactivity. Anti-platelet factor 4 (PF4)/polyanion IgG complexes were excluded as activators of the platelet Fc-receptor. Our results indicate that high SPA in blood is due to platelet FcγRIIA stimulation by unidentified IgG complexes and mediated by Btk activation. The relevance of our findings for SPA as possible risk factor of cardiovascular diseases and pathogenic factor contributing to certain autoimmune diseases is discussed.

The TKI Era in Chronic Leukemias

Tyrosine kinases are proteins involved in physiological cell functions including proliferation, differentiation, and survival. However, the dysregulation of tyrosine kinase pathways occurs in malignancy, including hematological leukemias such as chronic myeloid leukemia (CML) and chronic lymphocytic leukemia (CLL). Particularly, the fusion oncoprotein BCR-ABL1 in CML and the B-cell receptor (BCR) signaling pathway in CLL are critical for leukemogenesis. Therapeutic management of these two hematological conditions was fundamentally changed in recent years, making the role of conventional chemotherapy nearly obsolete. The first, second, and third generation inhibitors (imatinib, dasatinib, nilotinib, bosutinib, and ponatinib) of BCR-ABL1 and the allosteric inhibitor asciminib showed deep genetic and molecular remission rates in CML, leading to the evaluation of treatment discontinuation in prospective trials. The irreversible BTK inhibitors (ibrutinib, acalabrutinib, zanubrutinib, tirabrutinib, and spebrutinib) covalently bind to the C481 amino acid of BTK. The reversible BTK inhibitor pirtobrutinib has a different binding site, overcoming resistance associated with mutations at C481. The PI3K inhibitors (idelalisib and duvelisib) are also effective in CLL but are currently less used because of their toxicity profiles. These tyrosine kinase inhibitors are well-tolerated, do have some associated in-class side effects that are manageable, and have remarkably improved outcomes for patients with hematologic malignancies.

Zanubrutinib (Brukinsa)

CADTH recommends that Brukinsa be reimbursed by public drug plans for the treatment of adult patients with relapsed or refractory Waldenström macroglobulinemia (WM), if certain conditions are met. Brukinsa should only be covered to treat patients with relapsed or refractory WM who have received at least 1 prior line of therapy, meet at least 1 criterion for treatment according to International Workshop on WM (IWWM) consensus panel criteria, and have good performance status. Patients eligible for reimbursement of Brukinsa should not have disease transformation, which is WM that has transformed into another type of cancer, or received prior treatment with a drug of the same class (i.e., a Bruton tyrosine kinase [BTK] inhibitor) unless such therapy was stopped because the drug was not tolerated and the disease had not progressed. Brukinsa should only be reimbursed if prescribed by a clinician with expertise and experience in the treatment of WM and monitoring of therapy and if it does not cost more than other treatments for WM.

ML-11 Tirabrutinib treatment for recurrent or refractory PCNSL

Since 2020, tirabrutinib which is a Bruton’s tyrosine kinase (BTK) inhibitor has been available for recurrent or refractory PCNSL cases. The number of studies reporting efficiency and adverse effect of tirabrutinib treatment for recurrent or refractory PCNSL has been limited yet. In this study, we investigated clinical course of eight refractory or recurrent PCNSL cases treated with tirabrutinib in our institute. Eight PCNSL cases treated with tirabrutinib included four recurrent cases and four refractory cases. Five cases obtained CR or PR after 26.8 days administration of tirabrutinib and other two cases also exhibited obvious improvement of clinical symptoms after 23.5 days administration of tirabrutinib. Among three cases exhibiting intraocular lesions, two cases revealed improvement of visual dysfunction and the other case obtained SD status of intraocular lesion. The most frequently found adverse effect was the skin rash. CTCAE grade 2 (n=2) or 3 (n=2) rash was found after mean 16 days or 94 days of tirabrutinib administration, respectively. Two cases with grade 3 rash could start taking the low-dose tirabrutinib after improvement of rash. Althouth one case experienced shingles, no other case experienced serious adverse effects. Although adverse effect of rash was frequently found, we could obtain high response rate of tirabrutinib treatment for recurrent or refractory PCNSL cases. We need to establish quantitative assessment method for analysis of treatment response of tirabrutinib for intraocular lesions.

ML-10 Tirabrutinib, a second-generation BTK inhibitor in relapsed and refractory primary CNS lymphoma: A single institute study

BACKGROUND: The prognosis of relapsed and refractory (r/r) primary CNS lymphoma (PCNSL) is poor, and the development of new therapeutic agents is desirable. Comprehensive genetic analysis of PCNSL has shown that MYD88 and CD79B are frequently mutated and are oncogenic drivers, suggesting that Bruton’s tyrosine kinase (BTK), which is located downstream of MYD88 and CD79B, may be a reasonable therapeutic target. Tirabrutinib is a second-generation oral BTK inhibitor recently approved in Japan for the treatment of r/r PCNSL. In this study, we evaluated the efficacy and safety of tiraburtinib treatment of r/r PCNSL at Saitama Medical University. MATERIAL AND METHODS: Eighteen patients with r/r PCNSL to HD-MTX-based regimens were treated with 480 mg tiraburtinib daily under fasting conditions until disease progression. RESULTS: The median age was 63.5 years, and the median KPS was 70. Nine patients (50%) achieved a CR, 2 (11%) had a partial response, 3 (17%) had stable disease, and 4 (22%) had progressive disease. After a median follow-up of 17.3 months, the median progression-free survival was 7.9 months, and the median overall survival was 23.6 months. There were four cases of long-term treatment lasting more than one year. Grade 3 or higher adverse events were observed in 1 case of maculopapular rash, 1 case of cardiac failure, 1 case of neutropenia, and 1 case of lymphopenia. CONCLUSION: Tiraburtinib can be administered relatively safely to patients with relapsed or refractory PCNSL, and a certain degree of efficacy can be expected. Which patients can be treated with tiraburtinib over the long term, when can stop tirabrutinib treatment for patients with long-term CR, and the mechanism of tiraburtinib resistance needs to be determined.

ML-12 Clinical impact and management of skin-related disorders during treatment of relapsed PCNSL by tirabrutinib

BACKGROUNDS: Tirabrutinib is a second-generation Bruton’s tyrosine kinase (BTK) inhibitor, approved by the Japanese Pharmaceutical and Medical Devices Agency (PMDA) for relapsed and refractory PCNSL in March 2020. Skin-related disorder (SRD)s are the most prevalent adverse events in tirabrutinib, which accounted for 54.5% in a phase I/II trial. While the use of tirabrutinib is increasingly considered in clinical practice, the prevalence and clinical impact of tirabrutinib-related SRDs in real-world practice remains unclear. METHODS: Relapsed PCNSL patients treated with tirabrutinib at the author’s institution were identified, and divided into those with SRDs (SRD group), and without SRDs (non-SRD group). Response rate and progression-free survival (PFS) were retrospectively analyzed and compared between the two groups. RESULTS: Eleven patients were identified (median age: 73 [range: 50–83], median KPS: 70 [range: 40–90]), which included six (54.5%) from the SRD group and five (45.5%) from the non-SRD group. Response rate was 100% in the SRD group and 60% in the non-SRD group. Median PFS was 2.8 months in the SRD group and 36.3 months in the non-SRD group, which yielded no significant difference (p=0.446). While antihistamine prophylaxis using fexofenadine was performed in seven patients, among them SRDs were observed in three (27.3%). SRDs lead to tirabrutinib interruption (for seven days or more) in two (18.2%), dose reduction in three (27.3%), and discontinuation in two (18.2%) patients. Four patients in whom tirabrutinib was interrupted or discontinued due to SRDs had shorter PFS, compared with the two patients from the SRD group in whom tirabrutinib was continued (median PFS: 2.3 and 29.6 months, respectively) (p=0.049). CONCLUSIONS: SRDs substantially lead to tirabrutinib interruption or discontinuation, which could result in early PD. Since fexofenadine prophylaxis seems ineffective for preventing SRDs, other antihistamines should be considered. Establishment of the optimal management of tirabrutinib-related SRDs is warranted.

Improved Electrophile Design for Exquisite Covalent Molecule Selectivity

Covalent inhibitors continue to show therapeutic promise. However, off-target reactivity challenges the field. Extensive efforts have been exerted to solve this issue by varying the reactivity attributes of electrophilic warheads, with features such as reversibility or metabolic vulnerability. Here we report the development of a new approach to increase the selectivity of covalent probes and small molecule inhibitors that is independent of warhead reactivity features and can be used in concert with already-existing methods. Using the Bruton’s Tyrosine Kinase (BTK) inhibitor Ibrutinib scaffold for our proof-of-concept, we reasoned that increasing the steric bulk of fumarate-based electrophiles on Ibrutinib should improve selectivity via the steric exclusion of off-targets but ideally retain rates of cysteine reactivity comparable to that of an acrylamide. Using chemical proteomic techniques, we demonstrate that elaboration of the electrophile to a tert-Butyl (t-Bu) fumarate ester significantly decreases time-dependent off-target reactivity and abolishes time-independent off-target reactivity but retains BTK target engagement. While an alkyne-bearing probe analog of Ibrutinib has 247 protein targets, our t-Bu fumarate Ibrutinib probe analog has only 7 protein targets. Of these 7 targets, BTK is the only time-independent target. This 2-order-of-magnitude increase in selectivity is also conferred to the t-Bu inhibitor itself. By shotgun proteomics, we investigated the consequences of treatment with Ibrutinib and our t-Bu analog and discovered that only 8 proteins are downregulated in response to treatment with the t-Bu analog compared to 107 with Ibrutinib. Of these 8 proteins, 7 are also downregulated by Ibrutinib and a majority of these targets are associated with BTK biology. Taken together, these findings reveal a previously-unappreciated opportunity to increase cysteine-reactive covalent inhibitor selectivity through electrophilic structure optimization.

Improved Electrophile Design for Exquisite Covalent Molecule Selectivity

Covalent inhibitors continue to show therapeutic promise. However, off-target reactivity challenges the field. Extensive efforts have been exerted to solve this issue by varying the reactivity attributes of electrophilic warheads, with features such as reversibility or metabolic vulnerability. Here we report the development of a new approach to increase the selectivity of covalent probes and small molecule inhibitors that is independent of warhead reactivity features and can be used in concert with already-existing methods. Using the Bruton’s Tyrosine Kinase (BTK) inhibitor Ibrutinib scaffold for our proof-of-concept, we reasoned that increasing the steric bulk of fumarate-based electrophiles on Ibrutinib should improve selectivity via the steric exclusion of off-targets but ideally retain rates of cysteine reactivity comparable to that of an acrylamide. Using chemical proteomic techniques, we demonstrate that elaboration of the electrophile to a tert-Butyl (t-Bu) fumarate ester significantly decreases time-dependent off-target reactivity and abolishes time-independent off-target reactivity but retains BTK target engagement. While an alkyne-bearing probe analog of Ibrutinib has 247 protein targets, our t-Bu fumarate Ibrutinib probe analog has only 7 protein targets. Of these 7 targets, BTK is the only time-independent target. This 2-order-of-magnitude increase in selectivity is also conferred to the t-Bu inhibitor itself. By shotgun proteomics, we investigated the consequences of treatment with Ibrutinib and our t-Bu analog and discovered that only 8 proteins are downregulated in response to treatment with the t-Bu analog compared to 107 with Ibrutinib. Of these 8 proteins, 7 are also downregulated by Ibrutinib and a majority of these targets are associated with BTK biology. Taken together, these findings reveal a previously-unappreciated opportunity to increase cysteine-reactive covalent inhibitor selectivity through electrophilic structure optimization.