Novel insights into the biomarkers and therapies for primary central nervous system lymphoma

Circulating tumor DNAs

Circulating tumor DNAs (ctDNAs) are DNA fragments derived from apoptosis, necrosis, or secretion of tumor cells. Pretreatment ctDNA levels have been demonstrated to be independent prognostic predictors of aggressive lymphomas. ¹¹ In an investigation on six patients with CNSL (including five PCNSL), the digital polymerase chain reaction (PCR) analyses of cerebrospinal fluid (CSF)-derived ctDNAs proved effective for detecting somatic mutations in accordance with next generation sequencing (NGS) analysis of biopsy tissues. ¹² Another study evaluating the CSF and plasma from 6 patients with restricted CNSL, 1 with systemic and CNS lymphoma, and 12 with systemic lymphoma, indicated that CSF ctDNAs might detect CNS relapse earlier and the presence of residual disease better than conventional methods, such as flow cytometry (FC). ¹³ NGS analysis was performed on 11 newly diagnosed PCNSL patients who received ibrutinib-based therapy. CtDNAs in CSF were detected before and during treatment, and the results showed sensitive surveillance of tumor burden and assessment of treatment responses. ¹⁴

Although more than 70 aberrant genes have been reported in PCNSL, a half of which are related to the nuclear factor-κB (NF-κB) pathway, MYD88 and CD79B are the only two that have been reported to have the diagnostic abilities in liquid biopsy. ¹⁵ The potential application of MYD88 L265P mutation in noninvasive diagnosis of PCNSL has been implied in clinical cases. The successful detection of the MYD88 L265P mutation in CSF using droplet digital PCR (ddPCR) has been documented in secondary CNSL (SCNSL). ¹⁶ The detection rate of MYD88 L265P in the CSF of 14 patients with PCNSL was 86% in a retrospective study, and in three cases of which wherein no lymphoma cells were detected using cytology and FC, ddPCR could detect ctMYD88 L265P at the time of diagnosis. ¹⁷ In a recent retrospective study of 17 patients who were newly diagnosed with PCNSL, the concordance of MYD88 L265P mutation between CSF and tissue samples was found to be 82%. ¹⁸ In a phase Ib trial of an ibrutinib-based therapy for patients with r/r CNSL, high ctDNA mutations (60%) in B-cell antigen receptor (BCR) pathway genes, including MYD88 L265P and CD79B Y196, were detected in the CSF using MSK-HemPACT (an NGS-based tumor-sequencing assay), and the changes of these mutations were consistent with treatment responses. ¹⁹ In addition, concomitant assessment of CSF MYD88 L265P mutation and IL-10 could significantly improve the diagnostic sensitivity and specificity of distinguishing PCNSL from other CNS disorders by 94% and 98%, respectively. ¹⁸

Notably, a recent study that analyzed MYD88 and CD79B mutations in 27 patients with PCNSL at diagnosis showed significantly lower detection levels of these two hotspot mutations in blood-derived ctDNAs than in stereotactic biopsies, suggesting that MYD88 and CD79B mutations in blood-derived ctDNAs did not have reliable diagnostic value for PCNSL. ²⁰

MicroRNAs

MicroRNAs (miRNAs) are small non-coding RNAs, 20–24 nucleotide long, and some of them have been indicated to have diagnosis efficacy for lymphomas. Several miRNAs, such as miR-107, have shown diagnostic and prognostic indicator potential for DLBCL. ²¹ On one hand, a few members of CSF-derived miRNAs have demonstrated encouraging diagnostic accuracy for PCNSL from other brain diseases. A meta-analysis indicated that CSF-based miRNA assays seem to be more accurate and sensitive for the diagnosis of PCNSL than other widely used techniques, including neuroimaging MRI, cytopathologic, and immunophenotypic analyses of CSF cells. ²² Several studies have demonstrated that miR-19b, miR-21, and miR-92a quantified by qRT-PCR were significantly increased in CSF collected from patients with PCNSL, ²³ compared with those from the control group with other neurologic disorders. ²⁴ Combined analyses of these three miRNAs in CSF have shown diagnostic accuracy of PCNSL with a sensitivity of 95.7% and specificity of 96.7%. In addition, significantly lower miR-30c levels in CSF detected by miRNA arrays could distinguish patients with PCNSL from those with SCNSL with 90.9% sensitivity and 85.5% specificity. ²⁵

The diagnostic efficiency and prognostic value of blood-derived miRNAs in PCNSL have also been investigated. A study based on available blood samples from patients entering the G-PCNSL-SG1 trial compared the peripheral blood miRNA levels in a short-term survival group (median OS of 3 months) of 20 patients with PCNSL with those in a long-term survival group (median OS of 55 months) using NGS. According to the results, a total of 12 miRNAs were significantly deregulated between these two groups. Among them, miR-151a-5p and miR-151b differed prominently and exhibited the largest differential expression. ²⁶ While considering the limited number of available patients, verifications in larger population cohorts are required. Higher serum miR-21 levels were also associated with poorer prognosis in another survival analysis that included 56 patients with PCNSL. Multivariate Cox proportional hazard analysis demonstrated that serum miR-21 expression level could be an independent prognostic biomarker for PCNSL. ²⁷

For formalin-fixed paraffin-embedded (FFPE) biopsy tissues from PCNSL, there was also a distinction in the miRNA expression panel between PCNSL and nodal DLBCL, with 13 significantly upregulated and 5 significantly downregulated miRNAs. ²⁸ Among them, the upregulated miRNAs in PCNSL included miR-17-5p, miR-20a, miR-9 (related to the Myc pathway), miR-30b/c, and miR-155. The downregulated miRNAs consisted of miR-199a, miR-214, miR-193b, and miR-145, which are putative tumor-suppressor miRNAs. Furthermore, PCNSL showed a significantly increased expression level of miR-17-5p and a significantly decreased level of miR-127 compared to those in primary testicular lymphoma (PTL). ²³ Nevertheless, a recent study indicated that the miRNA levels in CSF and FFPE biopsy tissues from PCNSL patients did not match with non-overlapped miRNAs differentiated compared with tissues from nonmalignant brain lesions. ²⁹ One explanation for such discrepancies is that CSF miRNAs may be derived not only from lymphoma cells but also from other cells, including those from the subarachnoid space, ventricular system, ventricular choroid plexus, and spinal cord. ³⁰ Differentially expressed miRNAs between PCNSL and other diseases are shown in Table 1.

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Table 1.

MiRNAs differentially expressed between PCNSL and other diseases at diagnosis.

Samples	Comparation	Methods	Upregulated	Downregulated
FFPE biopsy samples	Nodal DLBCL	qRT-PCR	miR-9 miR-20b miR-155 miR-340 miR-17-5p miR-148a miR-30b miR-27b miR-26b miR-146b miR-20a miR-30c miR-let-7g	miR-199a miR-214 miR-432 miR-193b miR-145
	PTL	qRT-PCR	miR-17-5p	miR-127
	n-ML a	qRT-PCR	miR-155miR-196b	miR-9miR-125b,miR-let-7b
Blood samples	LTS: STS b	NGS	miR-151a-5pmiR-151bmiR-106a-5pmiR-17-5pmiR-183-3pmiR-30d-5pmiR-96-5pmiR-194-5pmiR-503-5p	miR-219-5pmiR-6130miR-181c-3p
	Glioblastomas, CNS inflammation, metastases, healthy controls, respectively	qRT-PCR	miR-21
CSF samples	Various neurologic disorders c	qRT-PCR	miR-21miR-19bmiR-92a
	SCNSL	microRNA arrays	miR-222	miR-16miR-30bmiR-30cmiR-191miR-204
	n-ML a	qRT-PCR	miR-21miR-19bmiR-92a

CSF, cerebrospinal fluid; FFPE, formalin-fixed paraffin-embedded; MiRNA, MicroRNAs; NGS, next generation sequencing; OS, overall survival; PCNSL, primary central nervous system lymphoma; PCR, polymerase chain reaction; PTL, primary testicular lymphoma; qRT-PCR, quantitative reverse transcription-PCR; SCNSL, secondary central nervous system lymphoma.

a

n-ML, nonmalignant brain lesions (including gliosis, cavernous malformation, cyst, abscess, hematoma, ischemic necrosis, lymphocytic inflammation, toxoplasmosis, focal cortical dysplasia).

b

LTS: STS, long-term survival group (median OS of 55 months) with those in short-term survival group (median OS of 3 months) of PCNSL patients.

c

Various neurologic disorders (including CNS inflammation, neurocardiogenic syncope, tension headache, brain infarction, polyneuropathy, migraine, epilepsy, and transient ischemic attack).

A proliferation inducing ligand, B-cell activating factor of the tumor necrosis factor family, and their receptors

Two significantly homologous members of the tumor necrosis factor (TNF) superfamily, B-cell activating factor of the TNF family (BAFF) and a proliferation inducing ligand (APRIL), play important roles in the pathogenesis of B-cell malignancies. Serum BAFF levels have been observed to be higher in patients with aggressive NHL and are related to treatment response and OS. ⁴¹ Immunohistochemical staining has shown that BAFF and APRIL are both expressed in PCNSL tissue specimens. ⁴² APRIL and BAFF have the potential to be reliable diagnostic biomarkers for PCNSL. A prospective analysis of the CSF collected from 116 patients with focal brain lesions containing 53 CNSLs (30 of which were PCNSL) demonstrated that CSF levels of APRIL and BAFF in patients with CNSL were significantly higher than those in patients with any other focal lesions, such as primary brain tumors (PBTs), MBT, and autoimmune-inflammatory diseases. ⁴³ The elevated APRIL levels in CSF revealed promising potential application as diagnostic marker for CNSL, with a specificity of 93.7% and sensitivity of 62.3%. The integration of BAFF levels with APRIL levels further improved the diagnostic accuracy to a specificity of 96.1% and a sensitivity of 77.3%. In the subgroup analysis, patients with PCNSL who achieved complete response (CR) showed remarkable decrease in both APRIL and BAFF CSF levels, compared with those in untreated patients.

APRIL and BAFF, as well as their two receptors, B-cell maturation antigen (BCMA) and transmembrane activator and CAML interactor (TACI), could serve as potential diagnostic biomarkers for PCNSL. Soluble TACI and BCMA were found to be elevated in the CSF of patients with PCNSL, compared with those in patients with other neurological diseases. ⁴⁴ The TACI level in the CSF could differentiate PCNSL with high specificity (88.3%) and sensitivity (87.9%). The diagnostic specificity increased to 96.7% when CSF BCMA was combined with TACI levels. Notably, serum TACI and BCMA levels were not elevated in patients with PCNSL and do not exhibit diagnostic value. In a small sample study of nine patients with PCNSL, the combination of elevated levels of CSF BAFF and its receptor TACI revealed 100% sensitivity and 100% specificity for the diagnosis of PCNSL. ⁴⁵

Currently, no single diagnostic modality is either sensitive enough or specific enough for clinical application. Integration of multivariable diagnostic modalities is necessary for the early diagnosis and treatment selection in PCNSL. When a patient presents with initial clinical symptoms of a brain tumor, MRI is commonly the first important diagnostic test. Functional imaging such as PET can be a more specific complement to this step. If PCNSL is suspected based on imaging findings, tissue-based brain pathology biopsy, which is still considered as the gold standard, is necessary for the definite diagnosis of PCNSL, unless fluid-based analysis is diagnostic. Unfortunately, the clinical use of tissue-based biopsy is limited mainly due to the risk of postoperative complications, such as intracranial hemorrhage and functional damage. In some cases, the tumor location is difficult or unsuitable to access for tissue-based biopsy, so fluid-based diagnostic techniques could be less invasive and more helpful. Besides, the use of corticosteroids hampers histopathological diagnosis, as they can reduce diagnostic sensitivity. ¹⁵ Fluid-based analysis of cytomorphology, such as FC should be performed. Compared with traditional liquid analysis, novel biomarkers including ctDNAs, miRNAs, and metabolomic markers might have advantages in terms of sensitivity and specificity. These fluid-based diagnostic markers facilitate more accurate stratification, prognosis evaluation, and choice of targeted treatment at an early stage, which could be optional choices. An overview of diagnostic value of novel biomarkers is summarized in Table 2. Liquid biopsy can overcome tumor heterogeneity to detect minimal residual disease (MRD) during early relapse. However, in current clinical practice, only a subset of patients can be diagnosed definitively based on typical liquid biopsy. Also, concerns regarding the waiting time for fluid-based diagnostic results and prioritization of different techniques remain to be resolved. Therefore, tissue-based brain biopsy should be carried out in a timely manner if there are no contraindications. In addition, to distinguish PCNSL from SCNSL, whole-body PET-CT and bone marrow biopsy should be performed. Furthermore, ocular examinations, such as vitrectomy, are required for the detection of ocular involvement.

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Table 2.

Overview of diagnostic value of novel biomarkers for PCNSL.

Biomarker	Number of patients	Sample	Methods	Sensitivity	Specificity	Reference
Combination of MYD88 and IL-10	36 PCNSL and 106 neurological controls	CSF	TaqMan-based PCR assay for MYD88	94%	98%	Ferreri et al. 18
Combination of miR-21, miR-19b, and miR-92a	23 PCNSL and 30 controls	CSF	qRT-PCR	95.7%	96.7%	Baraniskin et al. 24
	30 PCNSL and 23 n-ML	CSF	qRT-PCR	63.33%	80.77%	Zajdel et al. 29
MiR-30c	55 PCNSL and 11 SCNSL	CSF	qRT-PCR	90.9%	85.5%	Baraniskin et al. 25
Combination of miR-155 and miR-let-7b	35 PCNSL and 23 n-ML	FFPET	qRT-PCR	96%	98%	Zajdel et al. 29
MiR-21 and RNU2-1f	72 PCNSL and 47 controls	CSF	qRT-PCR	91.7%	95.7%	Baraniskin et al. 46
APRIL	53 CNSL (30 PCNSL) and 63 controls	CSF	ELISA	62.3%	93.7%	Mulazzani et al. 43
BAFF		CSF	ELISA	47.1%	93.7%
Elevation of APRIL and/or BAFF				77.3%	96.1%
TACI	33 PCNSL and 143 controls	CSF	ELISA	87.9%	88.3%	Thaler et al. 44
BCMA			ELISA	72.7%	71.8%
Combination of TACI and BCMA				63.9%	96.7%
Combination of TACI and BAFF	9 PCNSL and 73 controls	CSF	ELISA	100%	100%	Mizutani et al. 45

APRIL, a proliferation inducing ligand; BAFF, B-cell activating factor of the tumor necrosis factor family; BCMA, B-cell maturation antigen; CSF, cerebrospinal fluid; ELISA, enzyme linked immunosorbent assay; FFPET, formalin-fixed paraffin-embedded tissue; n-ML, nonmalignant brain lesions; PCNSL, primary central nervous system lymphoma; PCR, polymerase chain reaction; qRT-PCR, quantitative reverse transcription-PCR; RNU2-1f, circulating U2 small nuclear RNA fragments; SCNSL, secondary central nervous system lymphoma; TACI, transmembrane activator and CAML interactor.

Ibrutinib

Bruton tyrosine kinase (BTK) links BCR and Toll-like receptor (TLR) signaling, thus preventing the activation of NF-κB pathway. ⁴⁷ Ibrutinib is a primary inhibitor of BTK, and has single-drug activity in chronic lymphocytic leukemia (CLL) ⁴⁸ and other B-cell malignant tumors, ⁴⁹ including mantle cell lymphoma (MCL), Waldendström macroglobulinemia (WM), marginal zone lymphoma, and follicular lymphoma (FL). Systematic sequencing of PCNSL revealed that CD79B of the BCR signaling pathway accompanied with MYD88 is more frequently mutated in PCNSL than in systemic DLBCL, ⁵⁰ suggesting promising antitumor efficacy of ibrutinib.

Ibrutinib monotherapy has been observed to reveal an encouraging overall response rate (ORR) of 50% in 14 r/r PCNSL patients. Among them, two patients achieved long-lasting CR of 8 months without severe toxicities in a multicenter retrospective case series. ⁵¹ The median duration of response and progression-free survival (PFS) in all patients were 4 and 6 months, respectively. The final outcomes of a phase II clinical trial for r/r PCNSL and primary vitreoretinal lymphoma (PVRL) conducted by LYSA and LOC also demonstrated that ibrutinib monotherapy (560 mg/day) was highly clinically effective (70% of evaluable patients achieved disease control) and tolerated (only one patient experienced fatal pulmonary aspergillosis), with a median PFS of 4.8 months in all. ⁵² In addition, another single-center study (NCT02315326) including 13 r/r PCNSL and 7 r/r SCNSL patients also showed good single-agent activity of ibrutinib. In total, 10 (77%) patients with PCNSL [5 CR and 5 partial response (PR)] and 5 with SCNSL (including 4 CR) responded. However, the duration of response was short, with the median PFS of 4.6 months (95% CI = [2.4, 7.5] months) in PCNSL and 7.43 months in SCNSL. Genomic analysis revealed that the mutation within the coiled-coil domain of CARD11 seemed to be associated with ibrutinib resistance. ⁵³ The latest study by Lewis also reported similar results. ⁵⁴ Even with an encouraging ORR of 58% and CR rate of 55% for ibrutinib across the entire cohort of 33 patients with CNSL (9 PCNSL and 24 SCNSL), the median PFS was 10.2 months for SCNSL and short, at 3.1 months, for PCNSL. Thus, considering the relatively short duration of responses in PCNSL, as several studies have reported, combination therapies of ibrutinib for the treatment of PCNSL are needed. A phase Ib study of 18 patients with PCNSL (five untreated, 13 r/r) investigating the potential of ibrutinib monotherapy and together with chemotherapy (DA-TEDDi-R), showed significant response improvement with ibrutinib achieving significant CNS penetration and high free-drug concentrations. ⁵⁵ During ibrutinib monotherapy (maximum dose of 840 mg/day) treatment, 94% of patients (17/18) showed tumor reductions and 83% (15/18) achieved PR. Out of the 14 evaluable patients who carried on with DA-TEDDi-R, 86% achieved CR, suggesting ibrutinib could strengthen the efficacy of chemotherapy.

Considering the high incidence of aspergillosis (39%) in this trial, two patients developed aspergillosis during ibrutinib monotherapy and five patients developed aspergillosis during DA-TEDDi-R treatment, and other clinical trials of ibrutinib-based combination therapies are needed to reduce the risk of adverse events (AEs). Another phase Ib clinical trial (NCT02315326) explored ibrutinib (560 or 840 mg/d) /HD-MTX/rituximab combination in 15 patients with CNSL (9 PCNSL and 6 SCNSL). ¹⁹ This three-drug combination was well tolerated without dose-limiting toxicity (DLT) and a high response rate (89% in PCNSL and 67% in SCNSL) with a median duration of response of 12.8 months. A phase Ib clinical trial (NCT03703167) of ibrutinib combined with lenalidomide and rituximab for r/r CNSL is ongoing. In addition, ibrutinib as a maintenance treatment for elderly patients with primary CNS DLBCL is being investigated in a phase II clinical trial (NCT02623010). Patients who achieved MRI-documented CR or PR after rituximab and HD-MTX induction treatment would receive maintenance treatment including ibrutinib 560 mg/day until disease progression or occurrence of limiting toxicities.

Lenalidomide

Lenalidomide is an oral second-generation immunomodulatory drug (IMiD) that has exhibited direct effects against B-NHL cells ⁶² and indirect antitumor effects mediated by various immune cells in the tumor microenvironment (TME), including T, natural killer (NK), and dendritic cells in B-NHL. ⁶³

Lenalidomide monotherapy as a salvage therapy has shown promising efficiency and good tolerance for the treatment of PCNSL in several clinical studies. A retrospective case series reported that two out of six patients with recurrent PCNSL who received lenalidomide monotherapy as salvage treatment achieved CR, including one patient with a remission duration of 24 months. ⁶⁴ One MTX-resistant acquired immune deficiency syndrome (AIDS)-related PCNSL patient was also reported to maintain remission for up to 4 years with lenalidomide monotherapy (10 mg/day) plus anti-retroviral therapy. ⁶⁵ A phase I study (NCT01542918) including 14 relapsed CNSL patients (six with PCNSL and eight with SCNSL) demonstrated the tolerability, antitumor efficiency, and CSF penetration of lenalidomide as the ORR was 64% (9/14), and the response duration to this monotherapy in four patients prolonged for 18 months with all manageable toxicities. Considering the effectiveness of lenalidomide through the BBB and the absence of DLT, this study recommended an initial dose of lenalidomide monotherapy at 15 mg/day to treat relapsed CNSL. ³⁴

Meanwhile, lenalidomide monotherapy has been revealed to be an optional maintenance therapy following the response to genotoxic therapy for relapsed PCNSL, as an alternative to WBRT or HDC. The same group conducted a retrospective analysis of 10 patients with relapsed PCNSL with a median age of 61.5 years. These patients received salvage therapies of either HD-MTX or focal irradiation to demonstrate the outcomes of low-dose (5–10 mg/day) lenalidomide monotherapy as maintenance treatment, with an encouraging median OS of 45 months, and only one patient progressed to receive WBRT. ³⁴ Among them, the response duration of patients in CR 2 through 5 after salvage treatment followed by lenalidomide maintenance was more than 6 times longer than that of patients who relapsed in CR1 after receiving standard induction therapies of MT-R (HD-MTX, temozolomide, and rituximab). As the poorer outcomes in elderly patients with PCNSL remain a significant problem, ¹⁰ low-dose lenalidomide was again used as maintenance in 13 patients, all of whom were ⩾ 70 years old and achieved PR or CR after MT-R with or without EA (etoposide, cytarabine) induction therapy. ⁶⁶ Lenalidomide maintenance (median time of 18.9 months on lenalidomide) also revealed good tolerance in these elderly patients and median PFS was not reached, with only one death occurring due to complications from myocardial infarction in a median follow up of 31.6 months.

In addition to lenalidomide monotherapy, due to a synergistic effect between lenalidomide and rituximab in a mouse lymphoma model, ⁶⁷ investigations of the combination treatment are underway. LOC and LYSA conducted a phase II study of lenalidomide combined with intravenous rituximab (R²) for r/r PCNSL and PVRL. The ORR of 45 assessable patients (34 PCNSL and 11 PVRL) after a median number of 7 R² treatment cycles was 35.6%, including 13 CR/unconfirmed CR (uCR; 29%) and 3 PR (7%) cases. In total, 18 patients continued to the lenalidomide maintenance phase, and 5 completed 12 cycles. Without unexpected toxicities, the median PFS and OS reached 7.8 and 17.7 months at a median follow up of 19.2 months. The peripheral baseline CD4/CD8 ratio ⩾ 1.6 was significantly related to a longer PFS of 9.5 months. ⁶⁸

Pomalidomide

A novel third-generation IMiD, pomalidomide, has shown significant CNS penetration and effective antitumor effect against CNSL in murine models. ⁶⁹ A phase I clinical trial (NCT 01722305) investigated pomalidomide (5 mg/day orally on Days 1 to 4 of each 28-day cycle) and weekly dexamethasone for r/r PCNSL and PVRL. ⁷⁰ This combination showed a tolerable toxicity profile and significant curative activity, with an overall ORR of 48% (12 of 25), including 6 CR, 2 uCR, and 4 PR, as well as a median PFS of 9 months for responders.

Nivolumab

Programmed cell death protein 1 (PD-1) is an important immune checkpoint receptor expressed on activated T cells. The expression of PD-1 and its ligands (PD-L1 and PD-L2) have been extensively investigated for the treatment of lymphoma. High expression of transcript variants of PD-1 showed an ability to predict poor prognosis in PCNSL. ⁷¹ A recent study revealed that based on tumor mutational burden (TMB), high PD-L1 expression was observed in 37.5% of the 48 PCNSL subjects and more than 90% expressed intermediate-to-high TMB, indicating that immune checkpoint inhibitors may be beneficial to PCNSL. ⁷² Furthermore, larger clinical studies on PCNSL are warranted to investigate the checkpoint inhibitor therapies. A retrospective study of TME using high-plex immunohistochemistry in 36 CNSLs, 22 of which were PCNSLs, reported that PD-L1 expression level in lymphoma cells of all cases was 28%, which also supported immune checkpoint inhibitor treatment in PCNSL. ⁷³

Nivolumab is a humanized anti-PD-1 antibody approved by the FDA as second-line therapy for r/r classical Hodgkin lymphoma (cHL). ⁷⁴ Recent evidence of frequent increases in the copy numbers of 9p24.1/PD-L1 and PD-L2, accompanied by increased expression of PD-L1 and PD-L2 in PCNSL, ⁷⁵ has provided a rationale for immune checkpoint inhibitors treatment regimens. A case series of five patients (four with r/r PCNSL) demonstrated that nivolumab was effective and well tolerated, with all achieving objective radiographic responses (four CR and one PR) and symptomatic responses to nivolumab. ⁷⁶ Three out of all involved patients remained progression-free up to 13–17 months after the initiation of nivolumab. Another case report showed that a PCNSL patient with a very poor performance status (ECOG 4) achieved MRI almost CR and regained consciousness after six cycles of 3 mg/kg single-agent nivolumab administration. ⁷⁷ Nivolumab combined with lenalidomide exhibited good tolerance and antitumor activity as salvage therapy in a patient with sanctuary site CNS DLBCL, ⁷⁸ whose MRI showed near CR after 2 months of the combination treatment. A multicenter phase II trial of nivolumab for the treatment of r/r PCNSL and PTL has recently been completed (NCT02857426). Regrettably, with only one of 47 enrolled patients with PCNSL completing nivolumab treatment for 2 years, ORR was 6.4%, and PFS and OS were 1.41 and 8.64 months, respectively. Clinical trials in larger populations to investigate the effect of nivolumab are urgently required.

Nivolumab was also investigated as maintenance therapy after the patient achieved CR with MTX-based therapy followed by HDC/ASCT. In another case report of a patient with PCNSL and with two late relapses, a prolonged CR of more than 24 months without nivolumab-related toxicities indicated that nivolumab might be the drug of choice for the maintenance of remission. ⁷⁹ In addition to being used as salvage and maintenance therapy, nivolumab monotherapy in the front-line treatment of PCNSL has shown promising effects in one PCNSL patient with poor tolerance for induction chemotherapy. ⁷⁷

Pembrolizumab

Pembrolizumab, another humanized anti-PD-1 antibody that has different epitope binding sites compared with nivolumab, has been reported to have a promising antitumor effect in r/r cHL and was granted FDA approval in June 2018 for r/r primary mediastinal B-cell lymphoma (PMBCL). ⁸⁰ Pembrolizumab has also demonstrated antitumor responses for CNSL in a case series. ⁸¹ A patient with r/r PCNSL after treatment including lenalidomide, achieved MRI CR following two doses of 200 mg pembrolizumab with 1 mg dexamethasone. The CR was maintained for more than 22 months and to 24 months after discontinuing pembrolizumab. Registered phase II trials evaluating the role of pembrolizumab in the treatment of r/r PCNSL are now recruiting (NCT02779101, NCT03255018, NCT04421560).

Temsirolimus

The PI3K/mTOR pathway is a critical pro-survival pathway that adjusts the synthesis of proteins to modulate cell growth and proliferation. ⁸² Temsirolimus, an inhibitor of mTOR, has shown encouraging efficiency in the treatment of r/r MCL and DLBCL. A phase II trial of 37 r/r PCNSL patients with r/r PCNSL demonstrated that temsirolimus monotherapy at a dose of 75 mg/week had a positive effect with 54% ORR, including 13.5% CR. ⁸³ However, median PFS and OS were short at 2.1 and 3.7 months, respectively, with 13.5% treatment-associated mortality mainly from severe infections. In addition, 14 blood–CSF pairs were collected from nine patients, whereas only one patient showed a 2 ng/mL concentration of temsirolimus in the CSF. No temsirolimus or its main metabolite, sirolimus, was found in any other patient. The lack of a detectable concentration of temsirolimus and its main metabolite in CSF was inconsistent with its activity.

Other PI3K/mTOR inhibitors

In CD79B-mutant PCNSL, the PI3K/mTOR signaling axis is highly activated to promote tumor survival. ⁵³ The PI3K inhibitor BKM120, buparlisib, could result in cell death in slice cultures from xenograft CD79B-mutant PCNSL. ⁸⁴ This preclinical study demonstrated that combining ibrutinib with inhibitors of p110α/p110δ PI3K (BAY80-6946) or mTOR (INK128) could deepen and extend its responses in CD79B-mutant PCNSL. However, this needs to be investigated in advanced clinical trials. A phase Ib/II clinical trial (NCT03581942) on BAY80-6946 (copanlisib) for r/r PCNSL is ongoing. The recently completed phase II clinical trial (NCT02301364) of buparlisib in four patients with r/r CNSL showed three patients with PD and one PR. Another phase II trial (NCT02669511) investigating the pan PI3K/mTOR inhibitor, PQR309, in 21 relapsed PCNSL patients has been completed; however, no results had been posted at the time of writing this review.