Xencor to Present Data from the Phase 1 Study of Plamotamab in B-Cell Malignancies at the American Society of Hematology Annual Meeting

Xencor, Inc. (NASDAQ: XNCR), a clinical-stage biopharmaceutical company developing engineered monoclonal antibodies and cytokines for the treatment of cancer and autoimmune diseases, today announced that updated clinical data from its Phase 1 dose-escalation study of plamotamab, a CD20 x CD3 bispecific antibody, in patients with B-cell malignancies will be presented in a poster session during the 63^rd American Society of Hematology (ASH) Annual Meeting in Atlanta, Georgia on Sunday, December 12, 2021.

“Plamotamab is generally well tolerated and demonstrates encouraging clinical activity at our recommended intravenous Phase 2 dose, 50 mg flat dosing every two weeks after step-up dosing. We believe the best outcomes for patients require our focus on studying unique combinations of plamotamab with chemotherapy-free partners, and we are initiating the first of these studies in patients with relapsed or refractory diffuse large B cell lymphoma in late 2021 or early 2022,” said Allen Yang, M.D., Ph.D., senior vice president and chief medical officer at Xencor. “Additionally, our recently announced collaboration for advancing plamotamab development will close soon, and this will expand our strategy to develop multiple highly active chemotherapy-free regimens across B-cell cancers, importantly with tumor-selective, co-stimulatory CD28 bispecific antibodies.”

Key Highlights from the Abstract

The accepted abstract with data from the study is available through the ASH website. Updated results will be shared at the ASH Annual Meeting.

At data cut off on July 1, 2021, 80 patients with relapsed or refractory non-Hodgkin’s lymphoma (NHL) had received doses of plamotamab. Patients had a median age of 62 years, a median of 4 prior therapies and had been diagnosed a median of 28 months prior to treatment. The study was originally designed in two parts: Part A to establish an initial priming dose with fixed, weight-based dosing regimens and Part B to escalate dosing on administrations subsequent to the priming dose. A third part, Part C, was added to establish a step-up dosing regimen with higher, flat and less frequent, every other week, dosing.

The most common treatment-related adverse event was cytokine release syndrome (CRS), which occurred in 62.5% (50/80) of patients, with 5.0% (4/80) experiencing Grade 3 or 4 events. CRS was generally manageable with premedication. No related neurotoxicity Grade 2 or higher was observed.

The efficacy analysis included 53 evaluable patients who were treated at doses between 80 and 360 mcg/kg (n=45) or at flat doses of 50 mg (n=8). The overall response rate (ORR) was 38.2% (13/34) in patients with diffuse large B-cell lymphoma. The ORR was 80% (8/10) for patients with follicular lymphoma. The median duration of response was 57 days.

After the implementation of higher doses in the flat-dosing regimen, the ORR among all patients with NHL had improved to 50% (4/8) from 42.2% (19/45) in prior weight-based dosing cohorts. At data cut off, the median duration of response was 19.5+ days, with three of four patients continuing to respond to plamotamab monotherapy.

The ORR for patients with prior CAR-T therapy was 25% (4/16).

Presentation Details

• Abstract 2494, “Safety and Anti-Tumor Activity of Plamotamab (XmAb13676), an Anti-CD20 x Anti-CD3 Bispecific Antibody, in Subjects with Relapsed/Refractory Non-Hodgkin’s Lymphoma”
• Session: 626. Aggressive Lymphomas: Prospective Therapeutic Trials: Poster II
• Date & Time: Sunday, December 12, 2021. 6:00 - 8:00 p.m. EST
• Location: Georgia World Congress Center, Hall B5

About Plamotamab

Plamotamab is an investigational tumor-targeted XmAb® bispecific antibody that contains both a CD20 binding domain and a cytotoxic T-cell binding domain (CD3). CD20 is highly expressed across a range of B-cell tumors, including non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL). Engagement of CD3 by plamotamab activates T cells for highly potent and targeted killing of CD20-expressing tumor cells.

Plamotamab is currently being evaluated in a Phase 1 clinical study for the treatment of patients with CD20-expressing hematologic malignancies, including NHL and CLL. Preliminary safety and anti-tumor activity from the Phase 1 study indicated that plamotamab was generally well tolerated and demonstrated encouraging clinical activity as a monotherapy.

About Xencor, Inc.

Xencor is a clinical-stage biopharmaceutical company developing engineered monoclonal antibodies and cytokines for the treatment of cancer and autoimmune diseases. Currently, 22 candidates engineered with Xencor's XmAb^® technology are in clinical development internally and with partners. Xencor's XmAb antibody engineering technology enables small changes to the structure of proteins resulting in new mechanisms of therapeutic action. For more information, please visit www.xencor.com.

Forward Looking Statements

Certain statements contained in this press release may constitute forward-looking statements within the meaning of applicable securities laws. Forward-looking statements include statements that are not purely statements of historical fact, and can generally be identified by our use of words such as “potential,” “can,” “will,” “plan,” “may,” “could,” “would,” “expect,” “anticipate,” “seek,” “look forward,” “believe,” “committed,” “investigational,” and similar terms, or by express or implied discussions relating to the Phase 1 dose-escalation study of plamotamab, the quotations from Xencor’s senior vice president and chief medical officer; and other statements that are not purely statements of historical fact. Such statements are made on the basis of the current beliefs, expectations, and assumptions of the management of Xencor and are subject to significant known and unknown risks, uncertainties and other factors, including the ability of publicly disclosed preliminary clinical trial data to support continued clinical development and regulatory approval for specific treatments, that may cause actual results, performance or achievements and the timing of events to be materially different from those implied by such statements, and therefore these statements should not be read as guarantees of future performance or results. Such risks include, without limitation, the risks associated with the process of discovering, developing, manufacturing and commercializing drugs that are safe and effective for use as human therapeutics and other risks described in Xencor's public securities filings. For a discussion of these and other factors, please refer to Xencor's annual report on Form 10-K for the year ended December 31, 2020 as well as Xencor's subsequent filings with the Securities and Exchange Commission. You are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof. This caution is made under the safe harbor provisions of the Private Securities Litigation Reform Act of 1995, as amended to date. All forward-looking statements are qualified in their entirety by this cautionary statement and Xencor undertakes no obligation to revise or update this press release to reflect events or circumstances after the date hereof, except as required by law.

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