Bruton’s tyrosine kinase (BTK) inhibitors for the treatment of primary central nervous system lymphoma (PCNSL): current progress and latest advances

Abstract The incidence of primary central nervous system lymphoma (PCNSL) has steadily increased, particularly in elderly patients. Although highly responsive to first-line chemotherapy and radiotherapy, approximately 50% of patients relapse or become refractory within 1 year. Prognosis following relapse is dismal and no standard salvage therapy exists. Bruton’s tyrosine kinase (BTK), a key regulator of the B-cell receptor (BCR) pathway, has emerged as a promising therapeutic target. The first BTK inhibitor ibrutinib has been evaluated in the relapsed/refractory PCNSL setting, with overall response rates of 51.9%–89.0% and median progression-free survival of 4.6–4.8 months. However, ibrutinib inhibits several kinases in addition to BTK, leading to off-target effects. Second-generation BTK inhibitors have since been developed, which afford greater selectivity for BTK and fewer off-target effects. We review current practices in the diagnosis and evaluation of PCNSL, as well as clinical trials of BTK inhibitors in PCNSL and future developments in PCNSL treatment.


Introduction
Primary central nervous system lymphoma (PcNSl) is a highly aggressive non-Hodgkin lymphoma (NHl) restricted to the brain, meninges, spinal cord, and eyes and is histopathologically predominantly classified as diffuse large B-cell lymphoma (DlBcl) (90%). in immunocompetent patients, advancing age is a primary risk factor for PcNSl, with a median age at diagnosis of 65 years, and approximately one-third of patients aged ≥70 years [1].PcNSl is considered a relatively rare tumor, accounting for 4%-6% of all extranodal lymphomas in the USA [2,3].However, epidemiological data shows that the incidence of PcNSl has been steadily rising in recent decades in parallel with the burgeoning aging population. in an analysis of data from the central Brain tumor Registry of the USA and the Surveillance, epidemiology, and end Results registry, the incidence of PcNSl rose from 0.1 patients per 100,000/years in the 1970s to 0.4 patients per 100,000/years in 2013 [4].Of these, elderly patients were disproportionately affected, with an incidence rate of 4.32 per 100,000/years in patients aged 70-79 years in the USA [4].
High-dose methotrexate (HD-MtX)-based regimens are considered the standard of care for newly diagnosed PcNSl [5].Although PcNSl is highly responsive to chemotherapy and radiation therapy in the first-line setting [5], median time to relapse is 10-18 months and most relapses occur within 2 years [3]. the prognosis for patients with relapsed/refractory (r/r) PcNSl is dismal, with a median overall survival (OS) of 8.4 months despite salvage therapy [6].Furthermore, many elderly patients are not able to tolerate intensive treatments due to poor performance status, impaired renal function (ccr < 50 ml/min), and other comorbidities, and are particularly susceptible to treatment-associated toxicity [7].whole-brain radiotherapy (wBRt) has traditionally been used as consolidation following HD-MtX-based chemotherapy but is associated with a significant risk of permanent and irreversible neurotoxicity, particularly in patients aged ≥60 years [8].
Bruton's tyrosine kinase (BtK) is a key regulator of the B-cell receptor (BcR) pathway and a promising target in the treatment of lymphomas with constitutive activation of the nuclear factor kappa β (NF-κB) pathway.Due to the central role of the BcR signaling pathway in B-cell survival and proliferation, BtK represents an attractive molecular target for the treatment of B-cell NHl, and PcNSl in particular.characterized by DlBcl, PcNSl exhibits diverse molecular features, including alterations in key genes associated with B-cell lymphomas.MYD88 (primarily l265P) and cD79B (primarily Y196) are the most frequently mutated genes in DlBcl [9], which lead to constitutive activation of the BcR pathway, including NF-κβ and downstream signals responsible for the activation, proliferation, and survival of B-cell lymphomas (Figure 1) [9,10].Over 75% of patients with PcNSl possess mutations in one or both of these genes [11]. in recent genomic analyses, PcNSl also frequently exhibits 9p24.1/PD-l1(programmed cell death-ligand 1)/PD-l2 (programmed cell death-ligand 2) copy number alterations and translocations, which are likely genetic bases of immune evasion. in a small case series of 21 patients with PcNSl, 67% had 9p24.1/PD-l1/PD-l2copy gain and copy number-associated increased expression of the 2 ligands [12].Another study reported that only 4% of PcNSl tissue specimens had PD-l1-positive tumor cells; however, 52% of the samples had PD-l1-positive cells in the tumor microenvironment [13].Mutations in tP53, PiM1, and iRF4 are also frequently reported in PcNSl and may play a role in disease development and progression [14].However, it should be noted that no known molecular signature to predict therapy failure/resistance. in this review, we summarize recent progress and the latest advances in the development of BtK inhibitors for the treatment of PcNSl.

BTK inhibitors for the treatment of PCNSL
to date, five BtK inhibitors -ibrutinib, acalabrutinib, zanubrutinib, tirabrutinib, and orelabrutinib -have been approved for the treatment of hematological malignancies, all of which are covalent and irreversible inhibitors of BtK activity (table 1).BtK inhibitors act by binding to the active site of BtK, cysteine-481 (cys-481) to inhibit the overexpression and phosphorylation of tyrosine-223 and aberrant activation of the NF-κβ pathway.

BTK kinase selectivity and safety profile of existing BTK inhibitors
each BtK inhibitor possesses its own unique safety profile, with differences in kinase selectivity believed to be largely responsible for the variations observed in adverse event (Ae) profiles [15]. in addition to BtK, ibrutinib has been shown to irreversibly inhibit the activity of several important kinases, including epidermal growth factor receptor (eGFR), which is associated with severe skin toxicities [16], il-2-inducible t-cell kinase (itK), an important regulator of natural killer cell-mediated cytotoxicity [17], and other members in the tyrosine kinase expressed in hepatocellular carcinoma (tec) family of kinases (such as bone marrow tyrosine kinase gene on chromosome X [BMX]), which are involved in diverse biological processes related to cell survival and differentiation [18]. in contrast, in vitro kinase assays demonstrate that the second-generation BtK inhibitors, tirabrutinib, acalabrutinib, and zanubrutinib, possess more selective kinase binding compared with ibrutinib and thus fewer off-target effects (Figure 2) [19]. in a study evaluating half-maximal inhibitory concentrations (ic 50 ) of BtK inhibitors against BtK and 13 kinases, the ic 50 of the second-generation inhibitors was considerably higher than ibrutinib against all kinases tested, and there were 5 kinases that ibrutinib inhibited to a similar degree as BtK (ic 50 within 10-fold of BtK) compared with only 2-3 kinases with the second-generation BtK inhibitors (table 2) [19].
compared with second-generation BtK inhibitors, ibrutinib is a more potent inhibitor of human epidermal growth factor 2 (HeR2) and 4 (HeR4) [15], and inhibition of HeR2/HeR4 expression in cardiomyocytes by ibrutinib is widely considered to be the mechanism responsible for the increased incidence of cardiovascular events observed [15]. in a review of studies evaluating kinase activity of BtK inhibitors, ibrutinib was found to inhibit all kinases implicated in causing cardiovascular Aes, namely HeR2, HeR4, and tec [15].conversely, acalabrutinib was found to inhibit HeR4, and only slightly tec, but not HeR2, zanubrutinib inhibited tec and HeR4, but not HeR2, and tirabrutinib inhibited tec but not HeR2 or HeR4.More recently, c-terminal Src kinase (cSK) has emerged as another potential molecular target underlying AF after it was identified on a short list of candidate kinases, and cardiac symptoms were reproduced in cardiac-specific cSK knockout mice [27].consistent with the low incidence of cardiovascular events, zanubrutinib demonstrates less inhibition of cSK compared with ibrutinib and acalabrutinib does not inhibit Src family kinases [28,29].the cumulative incidence of new or worsened hypertension in patients treated with ibrutinib at 1.8 months was also reported to be 50%, and the cardiovascular effects of long-term use should be considered [22].BTk: Bruton's tyrosine kinase; CLL: chronic lymphocytic leukemia; SLL: small lymphocytic leukemia; Wm: Waldenstrom macroglobulinemia; mCL: mantle cell lymphoma; pCNSL: primary central nervous system lymphoma; mZL: marginal zone lymphoma; r/r: relapsed/refractory; NCCN: National Comprehensive Cancer Network; QD: daily; GVhD: Graft Versus host Disease; BiD: twice daily.a ibrutinib (560 mg) was approved for the treatment of r/r mCL.b NCCN guideline recommended for the treatment of r/r pCNSL.c orelabrutinib was approved for the treatment of r/r mCL and r/r CLL in China.
d Tirabrutinib was approved for the treatment of r/r pCNSL in Japan, korea, and Taiwan.e Tirabrutinib was approved for the treatment of Wm in Japan.
Although considered to be less immunosuppressive than conventional immunochemotherapy, several cases of invasive fungal infections have been reported in clinical trials of BtK inhibitors in PcNSl [32,41].Ghez et al. reported on 33 patients who developed invasive fungal infections during ibrutinib treatment, the majority of which were invasive aspergillosis [42].However, the majority of patients also had at least one additional factor that increased their risk for fungal infections, including chemotherapy within the last 6 months, neutropenia, or corticosteroid use.Second-generation BtK inhibitors such as acalabrutinib and zanubrutinib possess greater binding selectivity and fewer off-target effects compared to ibrutinib but disseminated fungal infections have still been reported, albeit less frequently [32,41]. in a Phase 1/2 study of tirabrutinib in R/R PcNSl, grade ≥3 bronchopulmonary aspergillosis, pneumonia, and Pneumocystis jirovecii pneumonia occurred in 1 of 44 patients (2.3%) each [32]. in a phase 2 study of orelabrutinib in combination with an anti-PD-1 monoclonal antibody in R/R PcNSl (n = 13), 1 patient experienced a grade 3 Ae of interstitial pneumonitis-related Pneumocystis jirovecii infection [41].these findings indicate that susceptibility to invasive fungal infection and pneumocystis pneumonia is not merely a consequence of off-target effects on t cell response and suggests a greater role of B-cell pathways in antifungal immunity, although the exact mechanism/s by which BtK inhibitors raise the risk of these events are still under investigation.Given that corticosteroid use is common in the treatment of PcNSl and the risk of invasive fungal infections and pneumocystis pneumonia appears higher in this cohort compared with other non-central nervous system (cNS) lymphomas and chronic lymphocytic leukemia (cll), caution is warranted in those patients.

Penetration of BTK inhibitors across the bloodbrain barrier
Given that PcNSl is restricted to the brain, eyes, meninges, and other structures of the cNS, drugs that are capable of crossing the blood-brain barrier (BBB) to deliver sufficient cytotoxic doses to malignant cells and tissues in the cNS are necessary to achieve durable responses.However, the vast majority of chemotherapeutic agents that are effective in other systemic lymphomas have limited penetration of the BBB.For example, standard doses of MtX do not cross the BBB, necessitating administration of high doses and/or rapid infusion, which increase the risk of systemic and neurological toxicity.

Drug-drug interaction with methotrexate
A lack of high-quality data exists regarding a potential interaction between MtX and BtK inhibitors in the literature.However, when BtK inhibitors are used in combination with high-dose MtX-based therapy, pharmacokinetic interactions via cYP3A4 are a concern and should be considered with caution [45,46]. in a phase 1b study evaluating ibrutinib in combination with HD-MtX and rituximab (RtX) in patients with r/r cNS lymphoma, the ibrutinib dose was reduced due to a drug interaction with the cYP3A inhibitor, amlodipine, which was initiated to control AF [47].However, no dose-limiting toxicities (Dlts), treatment-related deaths, or cases of aspergillosis were observed and all grade 4 Aes were seen in patients treated with RtX, HD-MtX, and ibrutinib (840 mg, n = 2; 560 mg, n = 2).Similarly, most (62%) grade 3 Aes were seen in patients treated with this combination.Using a rat model of collagen-induced arthritis, potential drug-drug interactions between HM71224, a BtK inhibitor, and MtX were explored.HM71224 in combination with MtX was found to decrease the arthritis score, bone erosion, synovitis, and cartilage degradation without apparent interaction [48].

Ibrutinib
to date, two prospective clinical studies have been conducted evaluating the efficacy and safety of ibrutinib monotherapy for the treatment of r/r PcNSl (table 3). in an open-label, Phase 1 dose escalation study evaluating 560 mg and 840 mg of ibrutinib, 77% (10/13) achieved a clinical response (5 each with a complete response [cR] and partial response [PR]), and the median progression-free survival (PFS) was 4.6 months [9]. in a phase 2 study of 52 patients receiving ibrutinib 560 mg once daily for r/r PcNSl and primary vitreoretinal lymphoma, a cR rate of 19% (10/52 patients) and a PR rate of 33% (17/52 patients) was achieved, and the median PFS was 4.8 months [49].Grommes et al. reported that while PcNSl patients with MYD88/cD79B co-mutations that activate BcR signaling were expected to respond better to ibrutinib than wild-type patients, there was no apparent response differentiation between patients with or without the co-mutations [9]. in the phase 1 study, hyperglycemia (95%), thrombocytopenia (70%), anemia (70%), and hypertriglyceridemia (70%) were the most common Aes, however, in the above two studies three patients experienced pulmonary aspergillosis, with a favorable (n = 1) and fatal (n = 2) outcome [49].Based on the results of the aforementioned studies, ibrutinib monotherapy is listed in the NccN guidelines as a treatment option for r/r PcNSl [50].
More recently, ibrutinib has been investigated in combination with other therapies for the treatment of r/r PcNSl (table 3). in a phase 1 study of ibrutinib in combination with HD-MtX and RtX in patients with r/r cNS lymphoma, an overall response rate (ORR) of 89% (8/9 patients) was achieved and the median PFS was not reached in the PcNSl cohort [47].the most common Aes were anemia, thrombocytopenia, alanine aminotransferase/aspartate aminotransferase elevation, and lymphopenia.No Dlts, treatment-related death, or cases of aspergillosis were observed.A phase 1b study has also been conducted of ibrutinib monotherapy followed by ibrutinib plus chemotherapy (DA-teDDi-R) in patients with PcNSl and cD79B and/or MYD88 mutations, in which 18 patients were treated with ibrutinib monotherapy, 16 patients started teDDi-R treatment and 14 were evaluated [11].A total of 94% of patients showed tumor reductions with ibrutinib alone, and 12 (86%) achieved a complete remission with DA-teDDi-R.However, the combination of ibrutinib with chemotherapeutic agents was associated with invasive aspergillosis in 7/18 patients (39%), three of which were related, one was probably related, and three were possibly related.

Tirabrutinib
A phase 1/2 study (ONO-4059-02) has been conducted of tirabrutinib in 44 Japanese patients with r/r PcNSl using three dosing regimens (320 mg, 480 mg, and 480 mg in the fasting state) (table 3) [32].During the 9-month follow-up period, the ORR was 63.6% ( and the most common grade ≥3 Aes were neutropenia (9.1%), lymphopenia, leukopenia, and erythema multiforme (6.8% each).if administration of tirabrutinib was interrupted in response to a skin disorder, prophylactic use of antiallergic drugs or corticosteroids at the resumption of administration was considered [32].Skin disorders resolved after treatment with an antihistamine or corticosteroid, which facilitated continued tirabrutinib administration.there were no cardiovascular-related Aes of any grade, including AF or hypotension.the final 3-year follow-up data have now been published for the entire 44 patients [52].the ORR was 63.6%, including 36.4% of patients who achieved a cR, the median duration response was 9.2 months, and the median PFS was 2.9 months.No new grade 5 Aes nor new safety profiles were observed since the last data cutoff.A prospective, noninterventional, multicenter, observational post-marketing all-case surveillance study (JRct iD: jRct2011210002) has since been conducted to evaluate adverse drug reactions (ADRs) and the ORR of tirabrutinib in r/r PcNSl in Japan [53].During an interim analysis (data cutoff date: 31 October 2020), 140 patients who received tirabrutinib treatment were analyzed for safety, and data from 127 of those patients were analyzed for effectiveness.the most common ADRs were skin and subcutaneous tissue disorders (30.7%), and no new safety concerns were identified compared with those reported in the phase 1/2 study.No instances of cardiovascular ADRs or grade 5 ADRs were observed.Furthermore, the ORR was 63.0% and the rate of cR or cRu response (37% each) was similar to that achieved in the 480 mg fasted group in the phase 1/2 ONO-4059-02 study [32].By the end of the 52 weeks, 73 patients (52.1%) were still alive, 46 (32.9%) had died, and the status of 21 (15.0%) was unknown.

Zanubrutinib
Zanubrutinib in combination with lenalidomide, temozolomide (tMZ), and RtX, with or without methotrexate (RlZt ± MtX) has been shown to be an effective and safe treatment option in the first-line setting for patients with PcNSl (table 3). in a prospective, open-label study, the efficacy and safety of RlZt + MtX were compared against RlZt alone in newly diagnosed patients with PcNSl [51].At data cutoff (30 June 2022), 24 patients were enrolled in the study, including 12 patients each in the RlZt + MtX and RlZt groups, respectively.the ORR in the overall population was 79.2%, including 9 (37.5%)patients with cR and 10 (41.7%) patients with PR; 2 (8.3%) patients had SD, and 3 (12.5%)patients had progressive disease (PD).the ORR was 66.7% (cR: 16.7%; PR: 50%) in the RlZt + MtX group and 91.7% (cR: 58.3%; PR: 33.3%) in the RlZt group.Median PFS and OS were not achieved in the overall population, and the estimated 18-month OS and PFS were 95.8% and 78.2%, respectively.Overall, 50% of patients experienced hematologic toxicity.the incidence of non-hematologic toxicity was 50% and included gastrointestinal reactions, rash, and liver and kidney damage.Only one patient experienced grade ≥3 rash.However, pneumonia was reported in 16.7% (4/24) of patients.

Orelabrutinib
Preliminary results have recently been published from an ongoing prospective, multicenter, single-arm phase 2 study evaluating orelabrutinib with the programmed cell death protein 1 (PD-1) inhibitor, sintilimab, for r/r PcNSl [41].thirteen patients were enrolled from March 2021 to January 2022, with a median follow-up of 7.0 (1.5-10.5)months.At data cutoff, the ORR was 61.5%; 4 (30.7%)achieved cR, 1 (7.7%) achieved cRu, and 3 (23.1%)achieved PR, and the estimated 1-year PFS rate was 67.7%.ten patients completed 4 cycles of the combination regimen, while 3 patients discontinued treatment in the first 2 cycles due to PD.Aside from one grade 3 Ae of interstitial pneumonitis-related Pneumocystis jirovecii infection, no other grade 3-4 hematological or non-hematological Aes were reported, and toxicities were mild.
Despite these promising results, it should be noted that cohorts of PcNSl patients are extremely challenging to collect given the relative rarity of the disease, and many of the clinical trials included ≤20 patients, thus making new interventions incredibly difficult to assess.

Development status of BTK inhibitors for PCNSL
currently, there are several BtK inhibitors in development for the treatment of PcNSl, and their use in combination with chemotherapy, molecular targeted drugs, and immune checkpoint inhibitors is also being explored (table 4).Newer BtK inhibitors such as pirtobrutinib and vecabrutinib are also under investigation in hemato-oncology, with the possibility for expansion into PcNSl in the future (table 5).

Secondary CNS lymphoma
Secondary cNS lymphoma (ScNSl) refers to lymphoma that has metastasized to the cNS during or after systemic therapy.Relapse outside of the cNS, including at extranodal sites, is observed in approximately 16% of patients [55].ScNSl, which is classified into DlBcl, is a highly malignant tumor like PcNSl and early diagnosis is therefore necessary to achieve optimal therapeutic effects and improved prognosis.there is no standard treatment for ScNSl and prognosis is extremely dismal (median OS of 1.5 months) [56].conventional PcNSl treatment with MtX or cytarabine chemotherapy is the treatment strategy of choice for ScNSl but is often inactive against primary chemorefractory disease [57].As is the case with PcNSl, BtK inhibitors are also expected to demonstrate a therapeutic effect in ScNSl. in a phase 2 study in 44 patients with r/r cNS lymphoma (PcNSl; n = 29, ScNSl; n = 15), treatment with ibrutinib was associated with an ORR of 69% in patients with ScNSl [58] which is in contrast to the less impressive efficacy in non-cNS lymphomas, where only 37% responded [59].that being said, the proportion of MYD88 mutations is lower in ScNSl than in PcNSl [60], and further investigation is therefore required to evaluate whether a similar level of efficacy observed with PcNSl may also be achieved with ScNSl.combination regimens utilizing BtK inhibitors and other novel agents currently being investigated for ScNSl are presented in table 4.

Next-generation BTK inhibitors
First-and second-generation BtK-targeting agents have shown impressive clinical responses in PcNSl; however, reports detailing the development of acquired resistance to ibrutinib have been published and are of concern [61].Mutations in the active site of BtK have been identified as the main mechanism of acquired resistance to ibrutinib in cll and mantle cell lymphoma (Mcl), with the most common mutation sites being c481S/F/t/R, as well as less frequent variants at t474i/S/M, R490H, Q516Km, l528w, and v537i [62].However, the mechanism of resistance in PcNSl is not currently clear and remains speculative.the rise of mutations that are refractory to current therapies has presented a need for new therapeutic approaches in PcNSl.to address the problem of acquired resistance, next-generation BtK inhibitors, which are potent inhibitors of BtK as well as different variants of the BtK c481 mutation, are being developed for use in patients with cll and Mcl refractory to first-or second-generation BtK inhibitors (table 5) [63].Pirtobrutinib, a highly selective reversible inhibitor that acts on both wild-type and c481 mutant BtK [64], is currently being developed, as are dual BtK-based inhibitors designed to overcome acquired resistance.Development of a novel BtK inhibitor with efficacy in patients with PcNSl despite acquired

Table 1 .
Summary of approved BTk inhibitors.

Table 2 .
Selectivity of first-and second-generation BTk inhibitors targeting 14 kinases.

Table 3 .
Clinical trials of BTk inhibitors for r/r pCNSL.

Table 4 .
Development status of BTk inhibitors for pCNSL.

Table 5 .
[65]-generation BTk inhibitors.BTk: Bruton's tyrosine kinase; CLL: chronic lymphocytic leukemia; SLL: small lymphocytic leukemia; mZL: marginal zone lymphoma; mCL: mantle cell lymphoma; FL: follicular lymphoma; Wm: Waldenstrom macroglobulinemia; NhL: non-hodgkin lymphoma; pCNSL: primary central nervous system lymphoma; DLBCL: diffuse large B-cell lymphoma; mS: multiple sclerosis; RRmS, relapsing-remitting multiple sclerosis.resistancetofirst-andsecond-generationBtKinhibitors is eagerly anticipated[65].Recently, protein degraders have been developed that target and degrade the BtK protein for BtK inhibition, with several molecules in Phase 1 clinical trials for various B-cell malignancies (Beigene [Nct05006716]; Nurix [Nct04830137, Nct05131022]; Haisco [Nct04861779]) and promising preliminary efficacy observed.ConclusionAlthough PcNSl is highly responsive to chemotherapy and radiation therapy in the first-line setting, the recurrence rate remains unacceptably high and prognosis is extremely poor.New treatment strategies with novel mechanisms of action distinct from conventional therapies are therefore of interest.PcNSl is associated with constitutive activation of BcR signaling, and BtK plays an important role in oncogenic signal transduction downstream of BcR, making it an attractive molecular target.Following the efficacy of the first-generation BtK inhibitor ibrutinib for the treatment of PcNSl, second-generation BtK inhibitors tirabrutinib, zanubrutinib, and orelabrutinib, have subsequently been developed, which possess greater selectivity for BtK and thus fewer off-target effects.these agents have delivered impressive treatment responses in the relapsed/refractory setting with acceptable safety.the expansion of these BtK inhibitors to newly diagnosed PcNSl, and potentially ScNSl, and the development of next-generation BtK inhibitors are expected to revolutionize the treatment landscape for PcNSl.