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8023 Background: MGTA-145 (GroβT), a CXCR2 agonist, has shown promising activity for HSC mobilization with plerixafor in pre-clinical models and healthy volunteers. Methods: This phase 2 single center study evaluates HSC mobilization with MGTA-145 + plerixafor and same day apheresis in patients with multiple myeloma. Patients received plerixafor 0.24 mg/kg (0.16 mg/kg if renal dysfunction) SQ, followed 2 hours later by MGTA-145 (0.03 mg/kg) IV over 3-10 minutes and apheresis within 30 minutes. Mobilization was repeated for a second day if day 1 yield was < 6 x 106 CD34+ cells/kg. This interim analysis reports on mobilization in 10 patients (of 25 planned), including safety cohort of first 6 patients completing transplant. Primary endpoint is collection of 2 x 106 CD34+ cells/kg. Results: Median age was 63 years (range: 46-68), 50% were female, 22% had ISS stage 3 & 50% had high-risk FISH. Induction therapy was VRD in 7 and daratumumab + VRD in 3 patients; median induction duration: 4 months (3-6) & median lenalidomide exposure: 6 cycles (4-6), with > VGPR in 70%. Median total stem cell yield (CD34+ cells/kg x 106) was 7.1 (3-16.2), day 1 yield was 5.4 (1.1-16.2) & yield per apheresis session was 4 (1.1-16.2). 100% of patients met the primary endpoint of collecting sufficient HSCs in < 2 days of mobilization + apheresis to proceed to transplant (2 x 106 CD34+ cells/kg). Secondary endpoints of 4 and 6 x 106 CD34+ cells/kg in < 2 days were met in 90% & 80% patients. 30% patients underwent 1 apheresis, while 70% underwent 2 sessions. MGTA-145 was well tolerated. At least 1 adverse event (AE) was seen in 90% of patients, 20% had grade 2 AEs (anemia, hypokalemia) and 20% had grade 3 AEs (worsening of baseline grade 3 anemia; hypocalcemia); all resolved. Acute & transient bone pain was seen in 40% of patients (back-2, hip-1, sternum-1), all grade 1, all on day 1, & resolved without intervention after 6 minutes (3-10). All 6 patients in the safety cohort have completed transplant with melphalan 200 mg/m2. Median of 4.1 (3.4-5.6) x 106 CD34+ cells/kg were infused. All patients have engrafted timely (DiPersio Blood 2009); median time to neutrophil engraftment: 12 days (11-13) & platelet engraftment: 17 days (16-19). Apheresis graft analysis is available in these 6 patients. Grafts with MGTA-145+plerixafor showed high enrichment for CD90+CD45RA- among CD34+ cells, a CD34 subset of long term engrafting HSCs (median: 31% of CD34+ cells, 27-52), higher than seen with G-CSF (6%, Goncalves TCT 2021). 67% of grafts were minimal residual disease negative with next generation flow cytometry. Conclusions: This is the first study to evaluate the novel regimen of MGTA-145 + plerixafor for same day stem cell mobilization & collection in myeloma/hematologic malignancies, with 100% efficacy in interim analysis and the first to demonstrate successful engraftment in patients with cells collected with this GCSF free regimen. Clinical trial information: NCT04552743.

e20557 Background: The ideal choice of maintenance therapy in patients with HRMM high-risk multiple myeloma remains unknown. We analyzed the outcomes of patients with HRMM undergoing transplant receiving different maintenance approaches. Methods: Patients with MM undergoing their first ASCT from 2012-19 within 1 year of diagnosis were identified from the prospectively maintained database of patients undergoing ASCT. HRMM was defined as having t(4;14), t(14;16), t(14;20), del17p13, or gain 1q detected on fluorescent in situ hybridization (FISH). Results: Of the 412 patients undergoing ASCT within 1 year of diagnosis, 333 had FISH data available and of these, 37% (124/333) patients had high-risk cytogenetics. Distribution of HR cytogenetics was as follows: deletion 17p: 37% (n = 46), t(4;14): 27% (n = 34), t(14;16) or t(14;20): 12% (n = 26), gain1q: 31% (n = 41). 9% (n = 12) had more than one HR abnormality. In patients with HRMM, median age at transplant was 59 years (range: 39 to 73), and 61% (n = 103) were males. 64% (n = 107) of high-risk patients received post-transplant maintenance therapy. Maintenance therapy in this group included a proteasome inhibitor (PI) in 34% (n = 29), immunomodulatory drug (IMiD) in 59% (n = 51), or both in 7% (n = 6). There was no difference in baseline characteristics of HRMM patients receiving PI vs. IMiD maintenance, except that patients with del17p were more likely to receive PI maintenance therapy (55% vs 28%, p = 0.01). (Table) After a median follow-up of 3.1 years from diagnosis, patients with HRMM had inferior PFS compared to patients with standard risk disease, with median PFS of 3 vs. 4.8 years, p < 0.001. Amongst the 86 HRMM patients receiving maintenance therapy, median PFS in patients receiving PI vs. IMiD vs. both PI + IMiD maintenance was 3 vs. 3.2 vs. 2.2 years, respectively, log-rank p = 0.7. In the sub-group of patients with 17p deletion, median PFS in the three groups was 3 vs. 2.9 vs. 2.2 years, respectively, log-rank p = 0.7. Conclusions: Patients with HRMM have inferior PFS compared to patients with standard risk disease. We observed similar outcomes in HRMM patients post-transplant regardless of the choice of maintenance therapy. [Table: see text]

e20509 Background: We evaluated outcomes of patients with multiple myeloma (MM) ≥ 70 years who were seen for Autologous Stem Cell Transplant (ASCT) consult, based on whether they underwent transplant vs. non-transplant treatment. Methods: 138 patients with MM ≥ 70 years (median 71, range 70-78) were evaluated in the BMT clinic from 1/2010 to 11/2019 for a transplant consult. Results: Of the 138 patients, 53 proceeded to ASCT. ASCT was not pursued in 85 patients despite most (79%) being eligible for transplant. Reasons for deferring ASCT in eligible patients included patient preference (48%, n = 32) and physician preference (52%, n = 35). 68 patients were seen during first-line therapy, of which 29 underwent upfront transplant. The remainder were seen at second-line or beyond. There was no difference in baseline characteristics among the 113 patients with available follow-up (ASCT = 53, non-ASCT = 60), including median age (71 vs. 72 years, p = 0.4), high-risk cytogenetics (41% vs. 31%, p = 0.4), high-risk HCT-CI (32% vs. 20% p = 0.2), and ISS stage III (34% vs. 27%, p = 0.6). In the 53 patients who underwent transplant, conditioning melphalan dose was 200 mg/m2 (75%, n = 40) and 140 mg/m2 (25%, n = 13). Day 100 transplant related mortality was 0% (n = 0). Progression-free survival (PFS) and overall survival (OS) were compared in patients who were seen for consultation within 1 year of diagnosis (n = 80). With a median follow-up of 27 months, median PFS amongst patients ≥ 70 years undergoing ASCT (n = 39) was 47 months compared to 34 months in the non-ASCT (n = 41) group, p = 0.006. Median OS was not reached in either group. Estimated 5-years OS was 76% in the ASCT group and 82% in the non-ASCT group (p = 0.6). There was no difference in PFS of patients ≥ 70 undergoing ASCT compared to a cotemporaneous cohort of patients < 70 (n = 639) from our institution (47 vs. 57 months, p = 0.3). Conclusions: Selected patients ≥ 70 years with MM undergoing ASCT have better PFS compared to patients with similar characteristics who do not undergo ASCT. ASCT is safe in this older population and outcomes were similar compared to younger patients. [Table: see text]

Purpose In contrast to Hodgkin lymphoma and systemic anaplastic large-cell lymphoma, CD30 expression of malignant lymphocytes in mycosis fungoides (MF) and Sézary syndrome (SS) is quite variable. Clinical activity and safety of brentuximab vedotin, a CD30 targeting antibody-drug conjugate, was evaluated in MF and SS. Tissue and blood biomarkers of clinical response were explored. Patients and Methods In this phase II study, patients with MF or SS with negligible to 100% CD30 expression levels were treated with brentuximab vedotin (1.8 mg/kg) every 3 weeks for a maximum of sixteen doses. The primary end point was overall global response rate. Secondary end points included correlation of tissue CD30 expression level with clinical response, time to response, duration of response, progression-free and event-free survivals, and safety. Results Of the 32 patients enrolled and treated, 30 patients had available efficacy evaluations. Objective global response was observed in 21 (70%) of 30 patients (90% CI, 53% to 83%). CD30 expression assessed by immunohistochemistry was highly variable, with a median CD30max of 13% (range, 0% to 100%). Those with Conclusion Brentuximab vedotin demonstrated significant clinical activity in treatment-refractory or advanced MF or SS with a wide range of CD30 expression levels. Additional biomarker studies may help optimize rational design of combination therapies with brentuximab vedotin.

2532 Background: EBV is implicated in a variety of diseases. Tab-cel is an investigational off-the-shelf, allogeneic T-cell immunotherapy utilizing endogenous T cell receptors targeting EBV antigens. We hypothesized the clinical activity of tab-cel is mediated by expansion and persistence of EBV-specific T cells. Therefore, we quantified circulating EBV- CTLp after tab-cel administration and examined the correlation between expansion and clinical response. Methods: Samples from 10 patients with EBV+ post-transplant lymphoproliferative disease (PTLD) and other EBV-associated diseases enrolled in an multicenter expanded access protocol (EAP) study ( NCT02822495 ) were analyzed. To evaluate CTLp frequencies, limited dilution analysis was performed on samples taken at baseline and day 34 post first tab-cel dose (end cycle 1). The day 34 persistence of circulating EBV-CTLp from best overall response to initial tab-cel product was tested using the two-tailed Mann-Whitney test. Changes in inflammatory cytokines were also measured. Results: Responders represented in this sampling (n=6; 2 PR and 4 CR) showed a median 5.8-fold increase in circulating CTLp between baseline and day 34 (range: 0.8 to 133-fold). Five of 6 responders showed an increase in EBV-CTLp at day 34 of ≥ 3.8-fold while 1 pt showed no change in CTLp (0.8-fold change). In contrast, the 4 non-responders (3 SD; 1 PD) showed a median 0.3-fold decrease in EBV-CTLp from baseline (range: 1.2 to 0.02-fold; ns). Cumulative analyses revealed a statistically significant correlation between the fold-change of circulating CTLp at day 34 and clinical response (p=0.038) which did not appear to correlate with the type of the EBV-associated disease. Inflammatory cytokines showed no meaningful change from baseline. The safety profile remains consistent with previously reported data. Conclusions: These data support the correlation of clinical activity of tab-cel with the expansion and persistence of EBV-specific T-cells at day 34 post-treatment, as well as the use of circulating CTLp as a biomarker for response in clinical studies. Clinical trial information: NCT02822495.

Purpose: Although rituximab is now routinely used in the treatment of B-cell non-Hodgkin’s lymphoma, the mechanism of its antitumor effect is not clear. One potential mechanism of action involves antibody-dependent cellular cytotoxicity (ADCC). Two aspects of ADCC influence the effectiveness of this process: the susceptibility of tumor cells and the activation of effector cells via their immunoglobulin G fragment C receptors (FcγRs). Several FcγR polymorphisms have been identified that may affect the killing function of natural killer cells and macrophages.Patients and Methods: The pretreatment tumor cells from 43 patients with follicular lymphoma were tested for their intrinsic susceptibility to rituximab-mediated ADCC. In addition, the FcγRIIIa (CD16) and FcγRIIa (CD32) polymorphisms were determined in an expanded group of 87 patients. The results were then correlated with clinical outcome of these patients.Results: No difference was found between the susceptibility of tumors from patients who clinically responded to rituximab versus those who did not respond. Conversely, both the FcγRIIIa 158 valine/valine and the FcγRIIa 131 histidine/histidine genotypes were found to be independently associated with the response rate and freedom from progression.Conclusion: These data support the hypothesis that ADCC plays an important role in the clinical effect of rituximab at the level of the effector cell. It will be important to include information on Fc receptor polymorphisms in future trials of rituximab therapy.

8026Background: Autologous hematopoietic cell transplantation (AHCT) is an important component of multiple myeloma treatment. Single-agent melphalan is the most commonly used conditioning regimen. ...

7046Background: The HCT-CI has been extensively studied in Myeloablative (MA) and reduced intensity conditioning (RIC) regimens. However there is paucity of data about the validity of HCT-CI in NMA...

8565 Background: We have pioneered a method to sequence the diversity of the TCR CDR3 rearrangements that exploits the capacity of high-throughput sequencing (HTS) to document the diverse repertoir...

3015 Background: Cetuximab therapy results in beneficial, yet limited, clinical improvement for patients with KRAS wildtype (WT) colorectal (CRC) and head and neck (HN) cancer. The efficacy of cetuximab, an IgG1 monoclonal antibody against EGFR, is due in part to antibody-dependent cell-mediated cytotoxicity (ADCC) by natural killer (NK) cells. CD137 is a costimulatory molecule expressed following activation on NK and memory, antigen-specific, CD8 T cells. Methods: We investigated the hypothesis that the combination of cetuximab with anti-CD137 mAb will enhance innate and adaptive immunity, thereby improving cetuximab’s anti-tumor efficacy in preclinical models and a prospective trial, NCT01114256. Results: NK cells increased their expression of CD137 by a factor of 30-40 when exposed to cetuximab-coated, EGFR-expressing HN and CRC cell lines. An agonistic anti-CD137 mAb enhanced NK cell degranulation and cytotoxicity 2-fold (~45 to 90% tumor lysis assayed by chromium release). The combination of cetuximab and anti-CD137 mAbs was synergistic in a syngeneic, human-EGFR-transfected murine tumor leading to complete tumor resolution and prolonged survival. NK cell depletion, significantly, and CD8 T cell depletion, partly, abrogated the anti-tumor efficacy of this combination. A series of HN and both KRAS WT and mutant CRC xenotransplant models demonstrated synergy with cetuximab and anti-CD137 mAbs. In our clinical trial, 54 patients with HN cancer receiving cetuximab therapy, circulating and intratumoral NK cells upregulated CD137 with amplitude influenced by duration post-cetuximab and host FcyRIIIa polymorphism. Interestingly, in 10 HLA-A2+ patients, following cetuximab, an increase in EGFR-specific, CD137-expressing, CD8 T cells directly correlated with the percent increase in CD137-expressing NK cells. Conclusions: Our results demonstrate the synergy of combining an agonistic mAb, anti-CD137, augmenting ADCC and T cell memory following a tumor-targeting mAb, cetuximab, in HN and KRAS mutant and WT CRC cancer. These results support a novel, sequential antibody approach by targeting first the tumor and then the host innate and adaptive immune system. Clinical trial information: NCT01114256.

2514 Background: Antibody-dependent cell-mediated cytotoxicity (ADCC), mediated by natural killer (NK) cells, plays an important role in the efficacy of monoclonal antibodies (mAb)s. CD137 is a costimulatory molecule expressed on immune cells following activation, including NK cells. We hypothesize that as the antitumor efficacy of mAbs is due to ADCC, their activity can be enhanced by stimulation of NK cells with an anti-CD137 agonistic mAb. Methods: Upregulation of CD137 on NK cells was assessed using CD20+lymphoma, HER2+breast, and EGFR+head and neck cell lines and primary patient samples. NK cell degranulation, cytokine release and cytotoxicity were assessed by CD107a mobilization, IFN-γ secretion, and chromium release. Mechanism of synergy was explored by cell depletion in an immune competent mouse model. Xenotransplanted models were used to demonstrate anti-tumor activity and sufficiency of an innate immune response. Results: NK cells in human primary patient samples do not express CD137 at baseline, however CD137 is highly upregulated when encountering mAb-coated tumor cells. MAb-induced NK cell degranulation and cytotoxicity are enhanced by anti-CD137 agonistic mAb. In a murine lymphoma model, anti-CD137 mAb significantly enhances anti-tumor activity of anti-CD20 mAb leading to complete tumor resolution and prolonged survival. NK cell depletion completely abrogates the therapeutic effect. In seven xenotransplant models, sequential administration of rituximab, trastuzumab or cetuximab plus anti-CD137 mAb provided superior reduction in tumor burden and prolonged overall survival. In a phase 0 biomarker study, level of CD137 expression on circulating and intratumoral NK cells was influenced by disease burden, prior treatment, FcγRIII polymorphism, and time since mAb therapy. Conclusions: Our results demonstrate the synergy of anti-CD137 mAb and a tumor-targeting mAb by stimulation of mAb-activated NK cells with anti-CD137 mAb to enhance ADCC. These results support a novel, sequential antibody approach against CD20+B cell, HER2+breast, and EGFR+head and neck malignancies by targeting first the tumor and then the host immune system.

2509 Background: Mantle cell lymphoma (MCL) has a poor long-term prognosis. Though autologous transplant prolongs survival, novel therapies are needed to target the residual, myeloablation-resistan...

6503 Background: The Ig idiotype (Id) expressed by B cell lymphoma is a target for immunotherapy. Id vaccination (Vac) induces humoral and/or cellular anti-Id immune response (IR). However, the cli...