ALBERT

Description: CD123 (IL-3 receptor alpha) is highly expressed on acute myeloid leukemia stem cells and blasts, and represents a promising target of antibody therapies for AML. Anti-CD123 antibodies such as CSL-362 and KHK2823 are currently in clinical development; however, a limitation of these molecules is that they are unable to stimulate T cell-mediated killing of CD123+ AML cells. To exploit the potent activity inherent to T cell immunotherapy while maintaining the favorable dosing regimen of a therapeutic antibody, we have designed a novel bispecific antibody that recruits T cells to attack CD123+ AML stem and blast cells. Such antibodies act via a mechanism known as "redirected T cell-cytotoxicity" (RTCC), because they stimulate targeted T cell-mediated killing regardless of T cell antigen specificity. Unlike other bispecific formats, these antibodies possess a full Fc domain and spontaneously form stable heterodimers that are readily manufactured. Their Fc domain was also engineered to abolish binding to Fcγ receptors (to reduce the potential for nonselective T cell activation), yet preserve binding to human FcRn (to maintain long serum half-life). We first generated a library of humanized and affinity-optimized anti-CD123 × anti-CD3 bispecific antibodies and assessed their potency using RTCC assays, in which bispecifics stimulated killing by human T cells of the CD123+ AML cell lines KG-1a and TF-1. From this cell-based screen, we selected the bispecific antibody XmAb14045 for testing in animal models. This antibody has 0.1 nM affinity for human CD123, and a T cell-engaging domain with 8 nM affinity for human CD3. XmAb14045 stimulated T cell-mediated killing of KG-1a and TF-1 cells with an EC50 〈 1 ng/ml (8 pM). In contrast, XmAb14045 had no cytotoxic activity against the CD123− Raji B cell line, demonstrating target specificity of the T cells. XmAb14045 had a prolonged serum half-life in mice of 6.2 days, in marked contrast to non-Fc domain-containing bispecific formats. Because this antibody was optimized for human CD123 and CD3 targets and does not crossreact with mouse antigens, we evaluated efficacy in cynomolgus monkeys. We treated 3 monkeys per group with a single dose of XmAb14045 at 0.01, 0.1, or 1 mg/kg. We quantified CD123+ cell numbers, including basophils and plasmacytoid dendritic cells (pDC) as CD123+ surrogate populations for AML stem and blast cells. Within 4 hours of dosing, XmAb14045 strongly activated T cells and stimulated depletion of over 99% of circulating CD123+ cells within 1 hr, particularly at the 0.1 and 1 mg/kg doses. Basophil and pDC counts fell to baseline within 4 hr and remained low for several weeks. Circulating CD4+ and CD8+ T cells were activated immediately after dosing and this was sustained for 48 hr, as measured by markedly increased levels of the activation markers CD25 and CD69. Notably, XmAb14045 induced rapid margination of CD4+ and CD8+ T cells from the circulation, with blood T cell populations returning to baseline within several days. Bone marrow CD123+ cells were depleted by over 95% at all doses, and these cell populations had not recovered by 8 days after treatment. Our results demonstrate that bispecific antibodies can recruit and activate T cells to efficiently kill CD123+ cells not only in the circulation but also in the bone marrow. Results in monkeys also suggest that changes in basophil and/or plasmacytoid dendritic cell numbers are readily quantifiable in peripheral blood, and thus these populations may serve as biomarkers for clinical efficacy. Our preclinical data provide a rationale for clinical assessment of anti-CD123 × anti-CD3 bispecific antibodies in patients with acute myeloid leukemia. Disclosures Chu: Xencor: Employment, Equity Ownership. Pong:Xencor, Inc.: Employment, Equity Ownership. Chen:Xencor, Inc.: Employment, Equity Ownership. Phung:Xencor, Inc.: Employment, Equity Ownership. Chan:Xencor, Inc.: Employment, Equity Ownership. Endo:Xencor, Inc.: Employment, Equity Ownership. Rashid:Xencor, Inc.: Employment, Equity Ownership. Bonzon:Xencor, Inc.: Employment, Equity Ownership. Leung:Xencor, Inc.: Employment, Equity Ownership. Muchhal:Xencor, Inc.: Employment, Equity Ownership. Moore:Xencor, Inc.: Employment, Equity Ownership. Bernett:Xencor, Inc.: Employment, Equity Ownership. Szymkowski:Xencor, Inc.: Employment, Equity Ownership. Desjarlais:Xencor, Inc.: Employment, Equity Ownership.

Description: CD20 is highly expressed on normal and malignant B cells, and is a well-established target of antibody therapeutics for B cell leukemias and lymphomas. However, a limitation of approved anti-CD20 antibodies such as rituximab, ofatumumab, and obinutuzumab is that they are unable to stimulate T cell-mediated killing of CD20+ B cells. To exploit the potent activity intrinsic to T cell immunotherapy while maintaining the favorable dosing regimen of a therapeutic antibody, we have designed novel humanized bispecific antibodies that bind to both CD20+ B cells and CD3+ T cells. Such antibodies act via a mechanism known as "redirected T cell-cytotoxicity" (RTCC), because they stimulate targeted T cell-mediated killing regardless of T cell receptor antigen specificity. Unlike other bispecific formats, these antibodies possess a full Fc domain and spontaneously form stable heterodimers that are readily manufactured. Their Fc domain was also engineered to abolish binding to Fcγ receptors (to reduce the potential for nonselective T cell activation), yet preserve binding to human FcRn (to maintain long serum half-life). We first generated a series of affinity-optimized anti-CD20 × anti-CD3 bispecific antibodies and screened these using RTCC assays in which bispecifics stimulated killing of the CD20+ Ramos B cell line by purified human T cells. From this cell-based screen, we selected two candidates for further study in animal models. The bispecific antibodies XmAb13676 and XmAb13677 have identical T cell-engaging domains with 8 nM affinity for human CD3. XmAb13676 stimulated T cell killing of Ramos cells with an EC50 of ~53 ng/ml (~420 pM), while XmAb13677 (with higher affinity for CD20) had an EC50 of ~2 ng/ml (~16 pM). To assess in vivo half-life, we next dosed mice with 2 mg/kg of XmAb13676 or XmAb13677. In marked contrast to non-Fc domain-containing bispecific antibody formats, XmAb13676 and XmAb13677 had an extended serum half-life in mice of 6.7 and 6.6 days, respectively. Because these bispecifics were optimized for binding to human CD20 and CD3 targets and do not crossreact with mouse antigens, we evaluated efficacy in cynomolgus monkeys. We treated 3 monkeys per group with a single dose of XmAb13676 or XmAb13677 at 0.03, 0.3, or 3 mg/kg. Within 4 hr after dosing, T cells were strongly activated and stimulated depletion of over 97% of circulating CD40+ B cells, with the two 3 mg/kg groups showing greatest depletion. B cells continued to decrease for 24 to 48 hr after dosing, with the high-dose groups remaining at baseline levels for the duration of the study (29 days). CD4+ and CD8+ T cells in the circulation were activated immediately after treatment with XmAb13676 and XmAb13677, and this state was sustained for over 48 hr, as measured by greatly increased levels of the activation markers CD25 and CD69. Bispecific antibodies also induced rapid margination of CD4+ and CD8+ T cells from the circulation, with blood T cell populations returning to baseline from 2 to 7 days after dosing. Notably, CD40+ cells in lymph nodes and in bone marrow were depleted by over 90% at all doses, and at the higher dose levels, these B cell populations had not recovered by 29 days after treatment. Our results demonstrate that bispecific antibodies can recruit and activate T cells to efficiently kill CD20+ B cells not only in the circulation but also in the more resistant reservoir of lymphoid organs. These preclinical data in cynomolgus monkeys provide a rationale for clinical assessment of anti-CD20 × anti-CD3 bispecific antibodies in patients with CD20+ B cell leukemias and lymphomas. Disclosures Chu: Xencor: Employment, Equity Ownership. Lee:Xencor, Inc.: Employment, Equity Ownership. Rashid:Xencor, Inc.: Employment, Equity Ownership. Chen:Xencor, Inc.: Employment, Equity Ownership. Chan:Xencor, Inc.: Employment, Equity Ownership. Phung:Xencor, Inc.: Employment, Equity Ownership. Pong:Xencor, Inc.: Employment, Equity Ownership. Endo:Xencor, Inc.: Employment, Equity Ownership. Miranda:Xencor, Inc.: Employment, Equity Ownership. Bonzon:Xencor, Inc.: Employment, Equity Ownership. Leung:Xencor, Inc.: Employment, Equity Ownership. Muchhal:Xencor, Inc.: Employment, Equity Ownership. Moore:Xencor, Inc.: Employment, Equity Ownership. Bernett:Xencor, Inc.: Employment, Equity Ownership. Szymkowski:Xencor, Inc.: Employment, Equity Ownership. Desjarlais:Xencor, Inc.: Employment, Equity Ownership.

Description: Recombinant adenovirus-engineered dendritic cells (Ad.DCs) are potent immunologic adjuvants of antiviral and anticancer vaccines. The effectiveness of Ad.DC-based vaccines may depend on the ability of Ad.DCs to crosstalk with natural killer (NK) cells and to activate, polarize, and bridge innate and adaptive immunity. We investigated, for the first time, whether and how human Ad.DCs activate NK cells, and compared the Ad.DC function with that of immature DCs and matured DCs (mDCs). We found that adenovirus transduction and lipopolysaccharide/interferon-γ-induced maturation increased expression of transmembrane tumor necrosis factor (TNF) and trans-presented (trans) interleukin-15 (IL-15) on DCs, leading to enhanced NK cell activation without enhancing DC susceptibility to NK cell-mediated killing. This crosstalk enhanced NK cell CD69 expression, interferon-γ secretion, proliferation, and antitumor activities, with Ad.DCs being significantly more effective than immature DCs, but less effective than mDCs. The Ad.DC and mDC crosstalk with NK cells was largely prevented by physical separation of DCs and NK cells, and neutralization of total TNF and IL-15, but not by selective sequestration of soluble TNF. These findings demonstrate that both Ad.DCs and mDCs can efficiently promote innate immune functions by activation of NK cells through the cooperative activities of tmTNF and trans-IL-15 mediated by cell-to-cell contact.

Description: CD38 is highly expressed on plasma cells and is an attractive target for multiple myeloma (MM) therapies. Several anti-CD38 antibodies including daratumumab and SAR650984 show promising results in clinical development, though such antibodies are not able to stimulate T cell-mediated killing of myeloma cells. To exploit a T cell immunotherapy mechanism while retaining the favorable drug properties of therapeutic antibodies, we designed bispecific antibodies that recruit T cells to CD38+ MM cells. Such bispecifics act via redirected T cell-cytotoxicity (RTCC) to stimulate T cell-mediated target cell killing regardless of T cell receptor antigen specificity. These anti-CD38 × anti-CD3 antibodies possess a full Fc domain and spontaneously form stable heterodimers that are readily manufactured. Their Fc domain lacks binding to Fcγ receptors to minimize nonselective T cell activation, yet retains binding to FcRn to maintain long serum half-life. We have previously reported that XmAb13551, a humanized and affinity-optimized anti-CD38 × anti-CD3 antibody, stimulates killing of the CD38+ MM cell line RPMI8226 by human T cells and suppresses human Ig levels in SCID mice engrafted with human PBMCs, showing much greater efficacy than daratumumab in these models (Blood 2014 124:4727). We also investigated efficacy of XmAb13551 in monkeys given a single dose of 2, 5, and 20 μg/kg. Within 1 hr after dosing, CD25 and CD69 activation markers were upregulated on T cells and within 8 hr, circulating CD38+ cells were depleted by 〉 95% at the 20 μg/kg dose. However, depletion of peripheral CD38+ cells was not sustained, suggesting that a large antigen sink was limiting drug exposure. Although higher dosing might overcome an antigen sink, higher doses of XmAb13551 (0.2 mg/kg or higher) resulted in a T cell-mediated cytokine release syndrome (CRS) in monkeys. We reasoned that an anti-CD38 × anti-CD3 antibody with reduced CD3 affinity would stimulate sufficient RTCC to deplete MM cells, yet would attenuate the acute T cell activation (and associated CRS) induced by high-affinity coengagement of T cells with CD38+ target cells. Using XmAb13551 as a starting point, we engineered a series of bispecifics retaining the same high-affinity (0.2 nM) binding to CD38, but with reduced affinity to CD3. We selected two antibodies - XmAb15426 and XmAb14702 - that have significantly reduced CD3 affinity. As expected, these molecules showed reduced potency in RTCC assays using T cells to kill RPMI8226 cells, with potency correlating with CD3 affinity (XmAb13551 〉 XmAb15426 〉〉 XmAb14702). We next tested XmAb15426 and XmAb14702 at single doses of 0.5 mg/kg and 3 mg/kg, respectively, in cynomolgus monkeys. Both antibodies were well-tolerated at these higher doses, consistent with the moderate levels of IL6 observed in serum from the treated monkeys. Moreover, XmAb15426, with intermediate CD3 affinity, more effectively depletes CD38+ cells at 0.5 mg/kg compared to the original high-affinity XmAb13551 dosed at 2, 5 or 20 µg/kg. Depletion by XmAb15426 was more sustained compared to the highest dose of XmAb13551 in the previous study (7 vs. 2 days, respectively). Notably, although target cell depletion was greater for XmAb15426, T cell activation (CD69, CD25 and PD1 induction) was much lower in monkeys treated with XmAb15426 even dosed 25-fold higher than the 20 µg/kg XmAb13551 group. XmAb14702, with very low CD3 affinity, had little effect on CD38+ cells and T cell activation. Our results demonstrate that modulating T cell activation by attenuating CD3 affinity is a promising method to improve the therapeutic window of T cell-engaging bispecific antibodies. This strategy has potential to expand the set of antigens amenable to targeted T cell immunotherapy by improving tolerability and enabling higher dosing to overcome antigen sink clearance with targets such as CD38. We have shown that by reducing affinity for CD3, XmAb15426 effectively depletes CD38+ cells while minimizing the CRS effects seen with comparable doses of its high-affinity counterpart XmAb13551. Our preclinical data for XmAb15426 provide a rationale for clinical testing of this bispecific antibody in patients with multiple myeloma and other CD38+ malignancies. Disclosures Moore: Xencor, Inc.: Employment, Equity Ownership. Lee:Xencor, Inc.: Employment, Equity Ownership. Schubbert:Xencor, Inc.: Employment, Equity Ownership. Miranda:Xencor, Inc.: Employment, Equity Ownership. Rashid:Xencor, Inc.: Employment, Equity Ownership. Pong:Xencor, Inc.: Employment, Equity Ownership. Phung:Xencor, Inc.: Employment, Equity Ownership. Chan:Xencor, Inc.: Employment, Equity Ownership. Chen:Xencor, Inc.: Employment, Equity Ownership. Endo:Xencor, Inc.: Employment, Equity Ownership. Ardila:Xencor, Inc.: Employment, Equity Ownership. Bernett:Xencor, Inc.: Employment, Equity Ownership. Chu:Xencor, Inc.: Employment, Equity Ownership. Leung:Xencor, Inc.: Employment, Equity Ownership. Muchhal:Xencor, Inc.: Employment, Equity Ownership. Bonzon:Xencor, Inc.: Employment, Equity Ownership. Szymkowski:Xencor, Inc.: Employment, Equity Ownership. Desjarlais:Xencor, Inc.: Employment, Equity Ownership.

Description: CD38, being highly expressed on malignant plasma cells, is an attractive target of new therapies for multiple myeloma (MM). Several anti-CD38 antibodies including daratumumab are in clinical development; however, a limitation of such monospecific antibodies is their inability to stimulate cytotoxic T cell killing of myeloma cells. To exploit the potent mechanism of T cell immunotherapy yet preserve the favorable drug and dosing properties of therapeutic antibodies, we designed bispecific antibodies that recruit T cells to CD38+ MM cells. Such bispecifics act via a "redirected T cell-cytotoxicity" (RTCC) mechanism because they stimulate targeted T cell-mediated killing regardless of T cell receptor antigen specificity. Unlike other bispecific formats, these antibodies possess a full Fc domain and spontaneously form stable heterodimers that are readily manufactured. Their Fc domain was also engineered to abolish binding to Fcγ receptors (to reduce the potential for nonselective T cell activation), yet preserve binding to human FcRn (to maintain long serum half-life). We first generated a library of humanized and affinity-optimized anti-CD38 × anti-CD3 antibodies and measured their potency using RTCC assays in which antibodies stimulated killing of the human MM cell line RPMI8226 by human T cells. From this screen, we selected two candidates for further assessment. XmAb13243 and XmAb13551 have 21 and 0.2 nM affinities, respectively, for human CD38, and have identical T cell-engaging domains with 8 nM affinity for human CD3. XmAb13243 stimulated RTCC with an EC50 of 2.5 ng/ml (20 pM) after 24 hr, while XmAb13551 had an EC50 of ~100 pg/ml (~1 pM). In contrast to bispecific formats lacking an Fc domain, XmAb13243 and XmAb13551 had long half-lives in mice of ~7.6 and 8.3 days, respectively. Because these bispecifics were optimized for human CD38 and CD3 binding and do not crossreact with mouse antigens, we next evaluated efficacy in immunodeficient SCID mice engrafted with human PBMCs. In this model, engrafted human B cells differentiate into CD38+ plasma cells, which produce high levels of human Ig. Bispecific antibodies dosed at 0.2, 1, and 5 mg/kg, 7 and 15 days after engraftment, suppressed human IgG2, IgM, and IgE to below detectable levels by Day 14 (〉 50-fold for IgG2, 〉 1,000-fold for IgM, and 〉 80-fold for IgE). Daratumumab at 5 mg/kg was markedly less potent than bispecifics, reducing IgG2 by 2-fold, IgM by 6-fold, and IgE by 3-fold. The control bispecific anti-RSV × anti-CD3 (which binds to T cells but not to CD38+ cells) had no effect on IgG2, IgM, or IgE levels. To investigate activity against an immune response requiring production of new human plasma cells, mice were vaccinated with tetanus toxoid 8 days after engraftment. Anti-CD38 × anti-CD3 bispecifics suppressed human anti-tetanus antibody titers to baseline (〉 100-fold), while daratumumab suppressed titers by only 2-fold. We next assessed efficacy in cynomolgus monkeys. Unlike daratumumab, which does not crossreact with monkey CD38, XmAb13243 and XmAb13551 bind to both CD38 and CD3 in monkeys (23 and 0.3 nM, respectively, to CD38, and 6 nM to CD3 for both). We treated monkeys with a single dose of XmAb13243 or XmAb13551 at 2, 5, and 20 μg/kg. T cells were activated within 1 hr, as measured by dramatic increases in CD25 and CD69 activation markers. Within 8 hr, T cells depleted circulating CD38+ cells by 〉 95% at the 20 μg/kg dose. Our results demonstrate that XmAb13243 and XmAb13551 effectively recruit T cells to kill CD38+ cells in vivo. Our preclinical data in monkeys and humanized mice provide a rationale for clinical testing of anti-CD38 × anti-CD3 bispecific antibodies in patients with multiple myeloma and other CD38+ malignancies. Disclosures Chu: Xencor: Employment, Equity Ownership. Miranda:Xencor, Inc.: Employment, Equity Ownership. Phung:Xencor, Inc.: Employment, Equity Ownership. Chen:Xencor, Inc.: Employment, Equity Ownership. Rashid:Xencor, Inc.: Employment, Equity Ownership. Endo:Xencor, Inc.: Employment, Equity Ownership. Chan:Xencor, Inc.: Employment, Equity Ownership. Pong:Xencor, Inc.: Employment, Equity Ownership. Bonzon:Xencor, Inc.: Employment, Equity Ownership. Muchhal:Xencor, Inc.: Employment, Equity Ownership. Leung:Xencor, Inc.: Employment, Equity Ownership. Bernett:Xencor, Inc.: Employment, Equity Ownership. Moore:Xencor, Inc.: Employment, Equity Ownership. Szymkowski:Xencor, Inc.: Employment, Equity Ownership. Desjarlais:Xencor, Inc.: Employment, Equity Ownership.