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Introduction Central nervous system lymphoma (CNSL) is a rare neoplasia and arises as primary (PCNSL) or secondary CNSL (SCNSL) and most commonly occurs as diffuse large B-cell lymphoma. Although prognosis of this disease has significantly improved due to advantages in diagnostic procedures and intensification of treatment over the last decade there remain major challenges in the clinical management. Magnetic resonance imaging (MRI) - as the standard imaging modality - has difficulties to discriminate CNSL from other brain-derived tumors or metastasis. Moreover, prognostic scores in CNSL lack the ability to reliable define patients with high relapse risk 1. Therefore, novel strategies to facilitate diagnosis and to select patients that profit from intense treatment protocols are urgently needed. The C-X-C chemokine receptor 4 (CXCR4) is a transmembrane chemokine receptor with pivotal roles in cell homing and is often overexpressed in hematologic malignancies. CXCR4-directed positron emission tomography (PET) imaging with the tracer [68Ga]Pentixafor has been proven to be a suitable in vivo imaging modality for CXCR4 expression in lymphoid malignancies 2. To evaluate the feasibility of CXCR4-directed PET and the prognostic value of this imaging modality this retrospective proof-of-concept study evaluated [68Ga]Pentixafor PET imaging in patients with CNSL. Methods 11 patients with lymphoma of the CNS (n=8 PCNSL, n=3 SCNSL) were imaged with the CXCR4-directed PET tracer [68Ga]Pentixafor in this retrospective proof-of-concept study after signing inform consent. Lymphoma tissue was assessed for CXCR4 expression ex vivo by immunohistochemistry. The prognostic value of CXCR4-directed PET imaging was evaluated in a computed analysis in 7 patients with follow-up MRI. Treatment response calculated as treatment efficiency η by sequential MRI was correlated with [68Ga]Pentixafor PET derived parameters at diagnosis such as volume of PET positive lymphoma lesion V(PET), maximal PET uptake value within the lesion max(PET) or integrated uptake values over the lymphoma lesion ∫(PET). Analysis was performed in a lesion- and patient-based manner. Results [68Ga]Pentixafor PET imaging was positive in all patients with active disease (10/11 patients) with excellent contrast characteristics to the surrounding brain parenchyma. PET positive lesions correlated well with lymphoma lesions in MRI-T1c sequences. The surrounding edema depicted in MRI-FLAIR sequences was evaluated as PET negative (Figure 1). Semi-quantitative analysis revealed maximum standard uptake values of [68Ga]Pentixafor-derived PET from 4.2 to 23.3 within the lesions with a high tumor-to-background-ratio ranging from 13.2 to 83.0. The computed analysis revealed that CXCR4-directed PET parameters at diagnosis given by max(PET) and ∫(PET) significantly correlated with treatment response in the lesion-based as well as in the patient-based analysis. Specifically, ∫(PET) was the most significant prognostic factor in the present study (Figure 2 C, D, F). On the other hand tumor volume at diagnosis measured either by MRI or by [68Ga]Pentixafor PET did not denote a predictive marker for treatment response (Figure 2 A, B, E). Conclusion/Outlook CXCR4-directed PET imaging represents a novel imaging modality for CNSL. Due to its excellent contrast properties it might help to facilitate diagnosis and refine response assessment. Moreover, owing the predictive value for treatment response, CXCR4-directed PET could serve as a biomarker for the selection of patients profiting from intense treatment protocols. Furthermore, the feasibility of endoradiotherapeutic approaches targeting CXCR4 with Pentixather - the therapeutic twin of the imaging peptide Pentixafor - has been shown in hematologic malignancies and could be incorporated in the treatment of CNSL. References 1. Han CH, Batchelor TT. Diagnosis and management of primary central nervous system lymphoma. Cancer. 2017;123(22):4314-4324. 2.Kircher M, Herhaus P, Schottelius M, et al. CXCR4-directed theranostics in oncology and inflammation. Ann Nucl Med. 2018;32(8):503-511. Disclosures Wester: Scintomics: Other: Spouse CEO of Company; CXCR4-targeted radiopharmaceuticals: Other: Inventor; Scintomics GmbH, Germany: Other: Shareholder. Bassermann:Celgene: Consultancy, Research Funding.

Introduction: IELSG32 is an international randomized phase II trial with 2 key clinical questions in pts with PCNSL. The first question focused on optimal induction therapy, and results of the 1st randomization have demonstrated that MATRix combination (methotrexate (MTX), cytarabine (ARAC), thiotepa, rituximab (R)) is associated with significantly better outcome [Ferreri AJ, et al. Lancet Haematol 2016]. The second question addresses the efficacy and neurotolerability of ASCT, as an alternative to WBRT as consolidation. Herein, we report the results of the 2nd randomization (NCT01011920). Methods: HIV-neg pts 18-70 ys and ECOG PS ≤3 with new histology-proven PCNSL and measurable disease were randomly assigned to receive 4 courses of MTX-ARAC (arm A); or R-MTX-ARAC (arm B); or MATRix (arm C). ASC were collected after 2nd c. Pts with responsive or stable disease were further randomized between WBRT 36 ± 9 Gy (arm D) and BCNU-thiotepa conditioned/ASCT (arm E). Stratification parameters were induction arm and response . Primary endpoint of the 2ndrandomization was 2-year PFS. To demonstrate a 2-yr PFS improvement from 65% (P0) to 85% (P1), 52 pts/arm (one-sided; α 5%; β 95%) were required. Consolidation arm would be considered effective if ≥40 pts were progression-free survivors at 2 ys. Analyses were performed on an intention-to-treat (ITT) basis. Effects of consolidation treatments on neurocognitive functions and quality of life (QoL) were assessed with the IPCG tests panel and EORTC-QLQ at baseline, after treatment and every 6 months afterwards. Results: 227 pts were enrolled in 52 centers of 5 countries; 219 assessable pts were referred to 1st randomization (arm A 75; B 69; C 75). 167 pts had responsive or stable disease after induction: 49 pts were excluded from 2nd randomization (poor mobilization, poor condition or patients' refusal); 118 pts were thus randomized (59 pts/arm) and constitute the study population (median age 58 ys; range 18-70). 15 (13%) pts had high IELSG risk, 3 pts had ocular disease and 20 pts had meningeal disease; with the exception of a higher male prevalence in arm D, there were no differences in clinical presentation between two arms; There were 6 protocol violations: 4 pts randomly allocated to arm D were treated with ASCT and 2 pts randomly allocated to arm E were treated with WBRT; 5 pts (D 2; E 3) refused consolidation. Therefore, per-protocol (PP) groups consisted of 55 pts treated with WBRT and 58 with ASCT. Both consolidation therapies were well tolerated. Grade 4 toxicity was uncommon (≤5% of pts); as expected, hematological toxicity was more common in arm E (neutropenia 5% vs. 71%; p=0.00001 - thrombocytopenia 2% vs. 72%; p=0.00001). There were 2 toxic deaths (infections), both in arm E. Neuropsychological tests showed a significant impairment of attention/executive functions and a non-significant trend to impaired memory among pts treated with WBRT, whereas pts treated with ASCT exhibited improved functions. Both consolidation therapies were associated with significant improvement in language and QoL. Both WBRT and ASCT were active and resulted in significant improvement in CR rate: from 54% after induction to 95% (95%CI: 90-100) after consolidation in arm D, and from 53% to 93% (95%CI: 87-99) in arm E. After a median follow-up of 40 months (range 24-76), there were 20 events (16 relapses, 2 PDs, 2 off-therapy deaths) in arm D, and 25 events (18 relapses, 2 PDs, 2 toxic deaths, 3 off-therapy deaths) in arm E. Importantly, WBRT and ASCT were both effective, with a number of progression-free survivors at 2 ys equal to the pre-determined efficacy threshold: 40 among both the first 52 arm-D and 52 arm-E pts. There were no significant differences in PFS between WBRT and ASCT; on ITT basis, the 2-yr PFS was 80 ± 5% after WBRT and 70 ± 6% after ASCT; per protocol, the 2-yr PFS was 76 ± 6% and 75 ± 6%, respectively. In multivariate analysis, IELSG risk group, number of lesions and induction arm were independently associated with PFS; gender, CSF cytology status and consolidation arm were not. Forty-two pts randomized to arm D and 37 randomized to arm E are alive, with 2-year OS of 85 ± 5% and 71 ± 6% (p=0.12), respectively. Conclusions: WBRT and ASCT are both feasible, active and effective as consolidation therapies after high-dose-MTX-based chemoimmunotherapy in pts ≤70 ys with PCNSL. Potential impairment of specific neurocognitive functions after WBRT should be considered at the time of therapeutic decision. Disclosures Ferreri: Sandoz: Membership on an entity's Board of Directors or advisory committees; Mundipharma: Research Funding, Speakers Bureau; Italfarmaco: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Adienne: Speakers Bureau; Gilead: Membership on an entity's Board of Directors or advisory committees. Fox:Adienne: Honoraria, Research Funding; Takeda: Honoraria, Other: travel funding; Gilead: Honoraria, Other: travel funding; Janssen: Honoraria, Other: travel funding; AbbVie: Consultancy. Röth:Alexion Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Honoraria. Johnson:Celldex Therapeutics: Research Funding. Linton:Takeda: Research Funding. Hess:Roche: Honoraria; Novartis: Honoraria; Janssen: Honoraria; Roche, CTI, Pfizer, Celgene: Research Funding; Celgene: Honoraria; Pfizer: Honoraria. Stilgenbauer:Janssen: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses, Research Funding, Speakers Bureau; Genentech: Consultancy, Honoraria, Research Funding; Sanofi: Consultancy, Honoraria, Research Funding; Gilead: Consultancy, Honoraria, Research Funding; Boehringer-Ingelheim: Consultancy, Honoraria, Research Funding; AbbVie: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses, Research Funding, Speakers Bureau; Hoffman La-Roche: Consultancy, Honoraria, Research Funding; Mundipharma: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; GlaxoSmithKline: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Genzyme: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria, Research Funding; Pharmacyclics LLC, an AbbVie Company: Consultancy, Honoraria, Other: Travel, Accommodations, Expenses, Research Funding, Speakers Bureau. Rummel:Roche Pharma AG: Other: Personal fees, Research Funding; Mundipharma GmbH: Other: Personal fees, Research Funding. Illerhaus:Riemser: Honoraria; Amgen: Honoraria.

ETV6 is the only known transcriptional repressor which plays an important role in the malignant transformation of haematopoietic cells in different leukemias. In most cases of ETV6-AML1 + leukemias, the second ETV6 allele is knocked out by deletions. Other mechanisms of knocking out the second ETV6 allele in acute leukemias carrying ETV6 fusion genes, have not been described as of yet. Recently, we have published that the MT-ALL cell line with multilineage differentiation potential carries an ETV6-ARG fusion gene. The second ETV6 allele carries a point mutation in the highly conserved ETS-domain (G to A, bp 1220), leading to the replacement of arginine, normally directly binding to DNA, by histidine (R399H). Therefore, we investigated whether the R399H point mutation leads to a functional double knock-out and will reduce the DNA binding affinity of ETV6. For the ETV6 binding studies the single point mutated and wild type ETS-domains were amplified and the proteins were expressed. Electro mobility shift assays (EMSA) were performed with the EBS (ETS binding site) carrying the specific GGAA core motif. The wild type ETS domain (ETSwt) incubated with the EBS showed a clear shift of the protein DNA complex loaded on a gel, but not the point mutated ETS domain (ETSmt). In order to quantify the binding of the ETSwt and mt-domains to EBS we established a transient transfection reporter gene assay. For this assay we constructed vectors which contain the EBS fused to the TK promoter, firefly luciferase serves as reporter gene. After transient cotransfections of HeLa cells with plasmids which expressed the ETSwt- or ETSmt-domain together with the vector containing the EBS(wt or mt), the activities of luciferase were measured in a tube luminometer. As expected, cotransfection of ETSwt and EBSwt reduced luciferase activity up to 75% compared with control vector and EBSwt cotransfection. Cotransfection of ETSmt with EBSwt did not lead to a reduction of luciferase activity. The specificity of the reporter gene analysis was controlled by using EBSmt. These results show that the transcriptional repressor function of ETV6 is dependent on binding of ETV6-ETS domain to EBS and that the point mutation completely alters the physiological function of ETV6. Further studies would explain whether the multilineage differentiation potential of MT-ALL is dependent on ETV6 double-knock-out.