Search

Supplementary Methods, Figures 1-5 from Preclinical Evaluation of AMG 900, a Novel Potent and Highly Selective Pan-Aurora Kinase Inhibitor with Activity in Taxane-Resistant Tumor Cell Lines

BackgroundInterleukin-12 (IL12) is a proinflammatory cytokine that induces differentiation of Th1 cells and increased cytotoxicity of T and NK cells. Stimulation by IL12 leads to production of IFNγ and an inflammatory tumor microenvironment critical for anti-tumor responses. Studies in mice revealed IL12 can dramatically shrink syngeneic tumors, however human clinical studies resulted in severe toxicity and a small therapeutic window, limiting response rates. Prior work at Xencor demonstrated that reduced-potency IL15/IL15Rα-Fc fusion proteins exhibited superior therapeutic index (TI) in non-human primates (NHP) by reducing receptor-mediated clearance. Applying similar principles to IL12, we created IL12 heterodimeric Fc-fusions (IL12-Fc) with reduced potency to improve TI.MethodsIL12 is a heterodimer of two subunits, so we engineered IL12-Fc fusions by fusing the IL12p35 subunit to one side of a heterodimeric (and inactive) Fc domain, and IL12p40 to the other side. These Fc-fusions were tuned for optimal activity by introducing amino acid substitutions at putative receptor-interface positions and screening for reductions of in vitro potency. In vitro activity was assessed on human PBMCs by measuring signaling in a STAT4 phosphorylation assay and IFNγ production in a mixed-lymphocyte reaction (MLR). In vivo anti-tumor activity of human IL12-Fc was assessed in huPBMC-NSG-DKO and huCD34+ MCF7 xenograft models. Surrogate mouse potency-reduced IL12-Fc were evaluated in syngeneic tumor models. Tolerability and pharmacodynamic activity were assessed in NHP.ResultsAn IL12-Fc potency series was created, and variants had up to a 10,000-fold reduction in STAT4 signaling and IFNγ production in an MLR assay compared to wild-type IL12-Fc. Anti-tumor activity was achieved with potency-reduced IL12-Fc as single-agents and in combination with anti-PD1, with weaker variants maintaining anti-tumor activity at higher dose levels. Analysis of peripheral lymphocytes indicated increased numbers of T and NK cells as well as activation of CD8+ T cells. Increased expression of immune checkpoints including PD1 was also observed. Analysis of serum indicated up to 200-fold increases in IFNγ levels. Surrogate potency-reduced IL12-Fc had improved tolerability and greater selectivity of IFNγ production in tumors compared to spleen and less production of IL10 compared to wild-type IL12-Fc. In NHP, potency-reduced IL12-Fc had superior exposure with slower, more sustained accumulation of IFNγ and IP10, and a more gradual dose-dependent peak response, as well as more sustained margination of T and NK cells compared to wild-type IL12-Fc.ConclusionsPotency-reduced IL12-Fc retain strong anti-tumor activity, while potentially overcoming safety and tolerability issues related to narrow TI associated with wild-type IL12 or IL12-Fc agents.

The voltage-gated sodium channel NaV1.7 has received much attention from the scientific community due to compelling human genetic data linking gain- and loss-of-function mutations to pain phenotypes. Despite this genetic validation of NaV1.7 as a target for pain, high quality pharmacological tools facilitate further understanding of target biology, establishment of target coverage requirements and subsequent progression into the clinic. Within the sulfonamide class of inhibitors, reduced potency on rat NaV1.7 versus human NaV1.7 was observed, rendering in vivo rat pharmacology studies challenging. Herein, we report the discovery and optimization of novel benzoxazine sulfonamide inhibitors of human, rat and mouse NaV1.7 which enabled pharmacological assessment in traditional behavioral rodent models of pain and in turn, established a connection between formalin-induced pain and histamine-induced pruritus in mice. The latter represents a simple and efficient means of measuring target engagement.

Because of its strong genetic validation, NaV1.7 has attracted significant interest as a target for the treatment of pain. We have previously reported on a number of structurally distinct bicyclic heteroarylsulfonamides as NaV1.7 inhibitors that demonstrate high levels of selectivity over other NaV isoforms. Herein, we report the discovery and optimization of a series of atropisomeric quinolinone sulfonamide inhibitors [Bicyclic sulfonamide compounds as sodium channel inhibitors and their preparation. WO 2014201206, 2014] of NaV1.7, which demonstrate nanomolar inhibition of NaV1.7 and exhibit high levels of selectivity over other sodium channel isoforms. After optimization of metabolic and pharmacokinetic properties, including PXR activation, CYP2C9 inhibition, and CYP3A4 TDI, several compounds were advanced into in vivo target engagement and efficacy models. When tested in mice, compound 39 (AM-0466) demonstrated robust pharmacodynamic activity in a NaV1.7-dependent model of histamine-induced pruritus (i...

Several reports have recently emerged regarding the identification of heteroarylsulfonamides as NaV1.7 inhibitors that demonstrate high levels of selectivity over other NaV isoforms. The optimization of a series of internal NaV1.7 leads that address a number of metabolic liabilities including bioactivation, PXR activation, as well as CYP3A4 induction and inhibition led to the identification of potent and selective inhibitors that demonstrated favorable pharmacokinetic profiles and were devoid of the aforementioned liabilities. The key to achieving this within a series prone to transporter-mediated clearance was the identification of a small range of optimal cLogD values and the discovery of subtle PXR SAR that was not lipophilicity dependent. This enabled the identification of compound 20, which was advanced into a target engagement pharmacodynamic model where it exhibited robust reversal of histamine-induced scratching bouts in mice.

Efforts to improve upon the physical properties and metabolic stability of Aurora kinase inhibitor 14a revealed that potency against multidrug-resistant cell lines was compromised by increased polarity. Despite its high in vitro metabolic intrinsic clearance, 23r (AMG 900) showed acceptable pharmacokinetic properties and robust pharmacodynamic activity. Projecting from in vitro data to in vivo target coverage was not practical due to disjunctions between enzyme and cell data, complex and apparently contradictory indicators of binding kinetics, and unmeasurable free fraction in plasma. In contrast, it was straightforward to relate pharmacokinetics to pharmacodynamics and efficacy by following the time above a threshold concentration. On the basis of its oral route of administration, a selectivity profile that favors Aurora-driven pharmacology and its activity against multidrug-resistant cell lines, 23r was identified as a potential best-in-class Aurora kinase inhibitor. In phase 1 dose expansion studies with G-CSF support, 23r has shown promising single agent activity.

The voltage-gated sodium channel Na

Human genetic evidence has identified the voltage-gated sodium channel Na

Herein the discovery of a novel class of aminoheterocyclic Na(v)1.7 antagonists is reported. Hit compound 1 was potent but suffered from poor pharmacokinetics and selectivity. The compact structure of 1 offered a modular synthetic strategy towards a broad structure-activity relationship analysis. This analysis led to the identification of aminopyrazine 41, which had vastly improved hERG selectivity and pharmacokinetic properties.

In mammalian cells, the aurora kinases (aurora-A, -B, and -C) play essential roles in regulating cell division. The expression of aurora-A and -B is elevated in a variety of human cancers and is associated with high proliferation rates and poor prognosis, making them attractive targets for anticancer therapy. AMG 900 is an orally bioavailable, potent, and highly selective pan-aurora kinase inhibitor that is active in taxane-resistant tumor cell lines. In tumor cells, AMG 900 inhibited autophosphorylation of aurora-A and -B as well as phosphorylation of histone H3 on Ser10, a proximal substrate of aurora-B. The predominant cellular response of tumor cells to AMG 900 treatment was aborted cell division without a prolonged mitotic arrest, which ultimately resulted in cell death. AMG 900 inhibited the proliferation of 26 tumor cell lines, including cell lines resistant to the antimitotic drug paclitaxel and to other aurora kinase inhibitors (AZD1152, MK-0457, and PHA-739358), at low nanomolar concentrations. Furthermore, AMG 900 was active in an AZD1152-resistant HCT116 variant cell line that harbors an aurora-B mutation (W221L). Oral administration of AMG 900 blocked the phosphorylation of histone H3 in a dose-dependent manner and significantly inhibited the growth of HCT116 tumor xenografts. Importantly, AMG 900 was broadly active in multiple xenograft models, including 3 multidrug-resistant xenograft models, representing 5 tumor types. AMG 900 has entered clinical evaluation in adult patients with advanced cancers and has the potential to treat tumors refractory to anticancer drugs such as the taxanes. Cancer Res; 70(23); 9846–54. ©2010 AACR.

The discovery of aurora kinases as essential regulators of cell division has led to intense interest in identifying small molecule aurora kinase inhibitors for the potential treatment of cancer. A high-throughput screening effort identified pyridinyl-pyrimidine 6a as a moderately potent dual inhibitor of aurora kinases -A and -B. Optimization of this hit resulted in an anthranilamide lead (6j) that possessed improved enzyme and cellular activity and exhibited a high level of kinase selectivity. However, this anthranilamide and subsequent analogues suffered from a lack of oral bioavailability. Converting the internally hydrogen-bonded six-membered pseudo-ring of the anthranilamide to a phthalazine (8a-b) led to a dramatic improvement in oral bioavailability (38-61%F) while maintaining the potency and selectivity characteristics of the anthranilamide series. In a COLO 205 tumor pharmacodynamic assay measuring phosphorylation of the aurora-B substrate histone H3 at serine 10 (p-histone H3), oral administration of 8b at 50 mg/kg demonstrated significant reduction in tumor p-histone H3 for at least 6 h.

Selective small molecule inhibitors of Tie-2 kinase are important tools for the validation of Tie-2 signaling in pathological angiogenesis. Reported herein is the optimization of a nonselective scaffold into a potent and highly selective inhibitor of Tie-2 kinase.

A novel method, which utilizes a key halogen dance step for the preparation of iodotetrahydronaphthyridine, iodoazaindoline, and iodotetrahydropyridoazepine ring-systems is described. A variety of transformations of the iodo-functional group are also reported to demonstrate the utility of this method.

We have established a set of technologies called Tunable Drug Conjugates (TDCs) that will differentiate from other DCs via a rapid payload release/rapid systemic clearance approach intended to quickly drive high toxic “payload” concentrations within tumor cells while minimizing toxicities to patients. TDCs use proprietary Silicon-based linker chemistries (SiLinkers) in novel Payload Cassettes (PCs) that enable the rapid and uniform release of multiple and/or mixed payloads both in the endosome after internalization and within the necrotic microenvironment of tumors. PCs can incorporate multiple copies of the same payload or optimal ratios of different therapeutic agents that work together synergistically to kill cancer cells. A second technology - a Dual variable Domain-Fab (DVD-Fab) targeting capability provides a proprietary means of specifically targeting a broad array of cell surface proteins. DVD-Fab containing TDCs should be cleared from the body within hours, enabling better management of side-effects. Our Silinker system is differentiated by the exquisite and tunable sensitivity of the linkers to cleavage under the modestly acidic conditions encountered in the endosomal and lysosomal compartments of a cell, and in the microenvironment of solid tumors. Good stability at physiological pH (and in human plasma) compares to rapid cleavage at relevant lower pHs as demonstrated using model systems. SiLinkers exhibit good stability at 37°C in pH7.4 buffer (and plasma) and show rapid hydrolysis at pH ~5.5 to 6.5 (t1/2 = 15 - 120 min). Confocal microscopy studies with imaging constructs demonstrate the translation of the in vitro data to cellular systems. We assembled SiLinker based prototype TDCs to target the folate receptor α (FAR) with folic acid as the targeting ligand and vinblastine analogs as the payload(s). We used single and PC drug conjugates to show cellular and in vivo proof-of-concept. We tested our TDCs in cell lines overexpressing FAR and showed that cytotoxicity was blocked by excess folic acid. Subsequent in vivo studies, using a nude mouse xenograft model, showed good antitumor activity for these prototype TDCs. BB-01 (a single payload construct) showed a clear dose response and had cures in 3 out of 5 mice after 24 days on a 3 μmol/kg TIW schedule. BB-03 (a triple PC construct) shows essentially complete tumor regression compared to modest inhibition by BB-01 at doses that deliver equivalent amounts of payload, demonstrating the synergistic potential of the PC concept to enhance the activity. We have also synthesized DVD-Fab TDCs. DVD-Fabs contain a reactive chemical group for seamless connection with our SiLinkers and PCs. We will describe conjugation chemistries and present cellular and in vivo Xenograft data for prototype DVD-Fab TDCs. Citation Format: Sara C. Hickey, Alex R. Nanna, Hanh N. Nguyen, Leslie Ofori, Christoph Rader, Michael K. Rood, Jutta Wanner, Doug S. Werner. Tunable Drug Conjugates: a differentiated drug conjugate (DC) platform [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5153. doi:10.1158/1538-7445.AM2017-5153

Treatment of aminophthalimides with organometallic reagents leads to 3-substituted 3-hydroxyisoindolinones.

Herein we describe the discovery, optimization, and structure-activity relationships of novel potent pyrrolopyrimidine Na(v)1.7 antagonists. Hit-to-lead SAR studies of the pyrrolopyrimidine core, head, and tail groups of the molecule led to the identification of pyrrolopyrimidine 48 as exceptionally potent Na(v)1.7 blocker with good selectivity over hERG and improved microsomal stability relative to our hit molecule and pyrazolopyrimidine 8 as a promising starting point for future optimization efforts.

Clinical genetic data have shown that the product of the SCN9A gene, voltage-gated sodium ion channel Nav1.7, is a key control point for pain perception and a possible target for a next generation of analgesics. Sodium channels, however, historically have been difficult drug targets, and many of the existing structure-activity relationships (SAR) have been defined on pharmacologically modified channels with indirect reporter assays. Herein we describe the discovery, optimization, and SAR of potent aminopyrimidinone Nav1.7 antagonists using electrophysiology-based assays that measure the ligand-receptor interaction directly. Within this series, rapid functionalization at the polysubstituted aminopyrimidinone head group enabled exploration of SAR and of pharmacokinetic properties. Lead optimized N-Me-aminopyrimidinone 9 exhibited improved Nav1.7 potency, minimal off-target hERG liability, and improved rat PK properties.

Clinical human genetic studies have recently identified the tetrodotoxin (TTX) sensitive neuronal voltage gated sodium channel Nav1.7 (SCN9A) as a critical mediator of pain sensitization. Herein, we report structure-activity relationships for a novel series of 2,4-diaminotriazines that inhibit hNav1.7. Optimization efforts culminated in compound 52, which demonstrated pharmacokinetic properties appropriate for in vivo testing in rats. The binding site of compound 52 on Nav1.7 was determined to be distinct from that of local anesthetics. Compound 52 inhibited tetrodotoxin-sensitive sodium channels recorded from rat sensory neurons and exhibited modest selectivity against the hERG potassium channel and against cloned and native tetrodotoxin-resistant sodium channels. Upon oral administration to rats, compound 52 produced dose- and exposure-dependent efficacy in the formalin model of pain.

[36982-84-0] C15H23N3O2S (MW 309.48) InChI = 1S/C15H23N3O2S/c1-9(2)12-7-13(10(3)4)15(14(8-12)11(5)6)21(19,20)18-17-16/h7-11H,1-6H3 InChIKey = AEMWUHCKKDPRSK-UHFFFAOYSA-N (agent for diazo2 and azide3 transfer to enolates) Alternate Name: trisyl azide. Physical Data: mp 41–43 °C. Solubility: freely sol most organic solvents. Preparative Methods: readily prepared by treatment of the commercially available triisopropylbenzenesulfonyl chloride (trisyl chloride) with sodium azide.4

[769-15-3] C9H11N2+ (MW 147.2) InChI = 1S/C9H11N2/c1-10-7-11(2)9-6-4-3-5-8(9)10/h3-7H,1-2H3/q+1 InChIKey = DSPUBTKMQXMJCI-UHFFFAOYSA-N [7181-87-5] C9H11IN2 (MW 274.1) InChI = 1S/C9H11N2.HI/c1-10-7-11(2)9-6-4-3-5-8(9)10;/h3-7H,1-2H3;1H/q+1;/p-1 InChIKey = RQGURHMTNSNBQX-UHFFFAOYSA-M (reagent used in Umpolung reactions to convert an electrophilic aldehyde to a nucleophile in the benzoin condensation, Stetter-type reaction, and SNAr reaction; the generation of homoenolate species from α,β-unsaturated aldehydes; formylation; and in coupling reactions) Physical Data: mp 200–201 °C,1 201–202 °C (methanol),2 200–201 °C (ethanol);3 1H NMR (60 MHz, DMSO-d6) δ 9.65 (s, 1H), 7.50–8.10 (m, 4H), 4.09 (s, 6H).2, 4 1H NMR (500 MHz, D2O, 50 mM acetate buffer pH 5.6) δ 9.17 (br s, 1H), 7.86 (m, AA′XX′, 2H), 7.70 (m, AA′XX′, 2H), 4.10 (d, J = 0.6 Hz, 6H).5 13C NMR (60 MHz, DMSO-d6) δ 9.65 142.9 (d), 131.5 (s), 126.2 (d), 113.2 (d), 33.3 (q).4 IR (KBr, cm−1)λ 3095, 1597, 1572.3 Solubility: sol THF, MeCN, acetone, H2O, DMSO, and methanol. Known Forms: the iodide salt is the most widely used form. Other counteranions such as chloride, bromide, perchlorate, hydroxide, tetrafluoroborate, and trifluoroacetate are also known. Most of the benzimidazolium salts are not commercially available. The iodide salt can be synthesized from commercially available 1-methylbenzimidazole. Preparation and Purification: 1,3-dimethyl-1H-benzimidazolium iodide was prepared by methylation of 1-methylbenzimidazole with an excess amount of iodomethane2, 4, 5 To an oven-dried pressure tube equipped with a stir bar was added 1-methylbenzimidazole (1.0 equiv) and iodomethane (5.5 equiv). The tube was sealed and heated to 100 °C for 1 h. The reaction was cooled to room temperature and concentrated under vacuo. The product was recrystallized from acetone, methanol, or a mixture of MeCN-toluene and dried under vacuum in the presence of P2O5. The iodide salt was obtained as light yellow crystals. Handling, Storage, and Precautions: the iodide salt can be stored in the dessicator and handled quickly in air.6

A novel method, which utilizes a key halogen dance step for the preparation of iodotetrahydronaphthyridine, iodoazaindoline, and iodotetrahydropyridoazepine ring-systems is described. A variety of transformations of the iodo-functional group are also reported to demonstrate the utility of this method.

The first general method for the palladium-catalyzed Suzuki-Miyaura and carbonyl enolate coupling of unactivated aryl arenesulfonates was developed utilizing XPhos, 1, and Pd(OAc)2. This is of significant interest because aryl tosylates and aryl benzenesulfonates are more easily handled and considerably less expensive than aryl triflates. This catalyst system effects the coupling of a variety of aryl, heteroaryl, and extremely hindered arylboronic acids with different aryl tosylates, under mild conditions. The same catalyst was employed in the first carbonyl enolate coupling of aryl arensulfonates.

Copper-mediated regioselective allylation and propargylation of 2-(n-butylthio)oxazole at the C5-position provided 2,5-disubstituted oxazoles in moderate to good yields. Subsequent removal of the n-butylthio group with deactivated W2-Raney nickel gave 5-substituted oxazoles.

Bulky trialkylsilyl-protected alkynes such as triethylsilyl (TES), tert-butyldimethylsilyl (TBS), and triisopropylsilyl (TIPS) acetylenes underwent the Cadiot-Chodkiewicz cross-coupling reaction with different bromoalkynes to form a variety of synthetically useful unsymmetrical diynes in good yields. The diyne alcohol 10 was transformed regio- and stereoselectively into enynes by hydrotelluration, carbometalation, and reduction reactions.

A novel electrotelluration process is described in which a Michael addition of an alkyl or aryl tellurolate anion occurs onto an activated alkyne with subsequent trapping of a vinyl anion with electrophiles (aldehydes and ketones) other than a proton. This process provides an efficient regio- and stereospecific route to tri- and tetrasubstituted alkenes. Methodologically significant examples of this chemistry were studied in which aryl and alkyl tellurolate anions were added to omega-keto alkynyl esters in a Michael reaction, and the incipient vinyl anions were trapped intramolecularly by the internal aldehydes. The reactive centers were tethered by different lengths of alkyl chains to form highly functionalized five-, six-, seven-, and eight-membered rings in modest to good yields.

The aurora family of serine/threonine kinases (Aurora-A, -B, -C) regulate cell-cycle progression in mammalian cells. Whereas aurora kinase C function appears restricted to meiosis in males, aurora kinases A and B are essential for proper chromosome congression, segregation, and cytokinesis during mitosis. Aurora kinases A and B have been implicated in tumorigenesis, with overexpression levels correlating to clinical staging of cancers and poor prognosis. Thus, these mitotic kinases have become the subject of much interest as targets for anticancer therapy. N-(4-((3-(2-amino-4-pyrimidinyl)-2-pyridinyl)oxy)phenyl)-4-phenyl-1-phthalazinamine was a key aurora kinase inhibitor lead, possessing oral bioavailability in rats that was lacking in the anthranilamide compounds from which it was derived. This phthalazine compound possessed a key feature that was deemed important to maintain in a clinical candidate: potency against a model multidrug resistant (MDR) cell line (MES-SA Dx5) commensurate with its activity against a cell line that does not overexpress P-gp (HeLa). Improved in vivo potency was desired, as measured by suppression of the phosphorylation of the aurora kinase B substrate Histone H3 on Ser10 six hours after dosing. SAR from targeting this improvement in in vivo activity uncovered a delicate balance between protein binding, pharmacokinetic parameters, and cell potency in MES-SA Dx5 cells. AMG 900 was identified as a suitable candidate for clinical development based on its low single digit nanomolar potency against MDR cell lines, robust PD response (with complete suppression of Histone H3 phosphorylation at six hours), and high selectivity against other kinases. Oral administration of AMG 900 at a well-tolerated dose of 4 mg/kg BID inhibited tumor growth (83% TGI; p < 0.0001) in an HCT116 xenograft model. AMG 900 is currently undergoing phase 1 clinical evaluation in patients with advanced solid tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5776.

Abstract 609 HM1.24/CD317 or BST2, a cell surface protein highly expressed on malignant plasma cells, represents a potential target of immunotherapy for multiple myeloma (MM). Here we characterized XmAb®5592, a novel Fc-engineered and humanized anti-HM1.24 antibody (Ab), and studied mechanisms of its anti-MM activity. XmAb®5592, with double amino acid substitution in Fc region of the wild type IgG1, has approximately 40-fold and 10-fold increases in affinity for Fc gamma receptor III (FcγRIIIa) and (FcγRIIa), respectively, expressed on effector cells including NK cells. The Fv region of XmAb®5592 was humanized and engineered to achieve high affinity and specificity of binding to HM1.24-expressing target cells. XmAb®5592 reacts against a panel of MM cell lines (n=19) which are both sensitive or resistant to current anti-MM conventional and novel therapies. Importantly, it triggers 10-100-fold higher antibody-dependent cell-mediated cytotoxicity (ADCC) against these MM cell lines than a native/non Fc-engineered version (anti-HM1.24 IgG1) of the Ab. Specifically, the maximum specific lysis of MM1S, MM1R, and RPMI8226 target cells induced by XmAb®5592 is at a concentration of 0.001-0.01 μg/ml, whereas the IgG1 analog did not induce maximum cell lysis until 0.1 μg/ml. The maximum 100% specific lysis of INA-6 target cells occurred at 0.1 μg/ml of XmAb5592, in contrast to 60% maximum lysis induced by10 μg/ml of the IgG1 analog. Since the bone marrow (BM) microenvironment induces resistance in MM cells to conventional therapies, we next asked whether XmAb®5592 induced ADCC against MM cells even in the presence of BM stromal cells (BMSCs). Importantly, XmAb®5592 triggered significant ADCC against MM1S, MM1R, and INA-6 MM cells in the context of BMSCs. XmAb5592 also reacts against patient MM cells, and triggers robust ADCC against CD138-purified patient MM cells in assays using NK effector cells from normal donors. Furthermore, cross-linked XmAb5592 inhibited RMPI 8226 cell growth in the absence of effector cells. The in vivo efficacy of XmAb®5592 was next evaluated in murine subcutaneous (sc) xenograft murine models using RPMI 8226 cells. Administration of XmAb5592 (9mg/kg, ip, 2x/week for 4 weeks) led to a significant reduction in growth of established tumors in vivo compared to a non-engineered IgG1 anti-HM1.24 analog. At termination of the study. 7/15 mice were tumor free in the XmAb- treated group versus only 1/15 tumor free mice in the IgG1 analog treated group. An anti-HM1.24 antibody with Fc region engineered to completely ablate binding to FcγRs (knock-out) behaved equivalent to the PBS vehicle control in these studies, again underlining the significance of interaction with FcγR for anti-tumor efficacy. These results therefore suggest that XmAb5592, an anti-HM1.24 antibody engineered for improved effector function and antitumor potency in vitro and in vivo, is a promising next-generation immunotherapeutic for MM. Disclosures: Muchhal: Xencor: Employment. Horton:Xencor: Employment. Nguyen:Xencor: Employment. Karki:Xencor: Employment. Desjarlais:Xencor: Employment. Munshi:Millennium Pharmaceuticals: Honoraria, Speakers Bureau. Richardson:Keryx Biopharmaceuticals: Honoraria. Anderson:Millennium Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau.