Rituximab

-01-引言治疗性单克隆抗体（mAb）不仅仅通过其抗原结合结构域（Fabs）结合靶标，而且要通过其Fc区与细胞表面受体FcγRs和FcRn结合，从而产生各种功能结果，包括抗体依赖性细胞毒性（ADCC）、抗体依赖性细胞吞噬作用（ADCP）、补体依赖性细胞毒作用（CDC）和循环半衰期的改变。对Fc区进行工程化改造以实现理想的药理学和药代动力学是药物开发中广泛采用的策略。-02-一、增强FcγR结合的FC工程化改造人类含有6种不同的FcγR: hFcγRI，hFcγRIIA，hFcγRIIB， hFcγRIIC，hFcγRIIIA 和 hFcγRIIIB，这些受体与Fc有不同的亲和力。其中hFcγRI细胞外结构域含有三个免疫球蛋白结构域并且其与Fc的结合力最强，而其它受体细胞外区域仅含有两个免疫球蛋白结构域，它们与Fc的亲和力从较低到中等。 从功能上讲，人FcγRs可分为激活受体和抑制受体，它们分别通过细胞内免疫受体酪氨酸基激活基序（ITAM）或免疫受体酪氨酸基础抑制基序（ITIM）传递信号，以影响与靶结合IgG抗体交叉结合后的细胞功能。FcγRs可以影响抗肿瘤抗体的有效性，特别是通过激活Fc效应功能（如ADCC和ADCP）的抗体。ADCC被认为主要是由NK细胞发挥的效应功能。IgG与FcγRIIIA可以激活NK细胞并刺激释放含有穿孔蛋白和颗粒酶等分子的裂解颗粒，最终导致目标靶标细胞的裂解。许多临床上相关的抗癌抗体，如利妥昔单抗、赫赛汀和西妥昔单抗已被证明可以在体外激起NK细胞介导的ADCC。ADCP是由吞噬细胞，如巨噬细胞、单核细胞和中性粒细胞介导的，该效应是由抗体与相应的激活性FcγRs相互作用诱发的。由Fc介导的另一个重要的相互作用是它与补体的第一个成分C1q的结合。一旦有足够的Fc分子与C1q的6个球状头结合，它就会启动血清中补体蛋白的蛋白分解级联，导致过敏毒素的释放，如C3a和C5a，并在目标细胞表面形成膜攻击复合物，产生补体依赖性细胞毒性。下表总结了与FcγR和补体结合并激活相关的一些Fc突变。此外，还有增强FcγRIIB结合的策略来抑制自身反应性B细胞。下表总结了与FcγRI1B结合激活相关的一些Fc突变，最常用的突变是“SELF”和“V12”。多种携带Fc变体且FcγRIIB结合增强的单克隆抗体正在临床试验中。-03- 二、降低FcγR结合的FC工程化改造上面主要介绍的是通过工程化方法增强抗体Fc的相关效应，如ADCC和CDC等。但是，当治疗目标是阻断免疫细胞的功能而不产生与效应细胞或补体激活相关的促炎作用时，需要通过各种方法减弱甚至消除Fc的相关效应。目前主要有三种方式减弱或者消除Fc的效应：1）氨基酸工程化改造Fc（与增强型改造相反，其主要减弱对相关受体的结合）；2）糖基化改造；3）IgG4型抗体改造。氨基酸突变降低效应功能，目前主要的一些氨基酸突变包括L234A/L235A（IgG1）, L234F/L235E/P331S(IgG1), G236R/L328R(IgG1), E233P/L234V/L235A/ G236del（IgG1/ IgG2）, L234A/L235A/P329G(IgG1)等。糖工程的氨基酸置换如N297A、N297Q和N297G是实现高水平沉默的一种非常简单的方法，因为它消除了聚糖的附着。这大大降低了与所有低亲和力FcγRs和C1q的结合，导致ADCC和CDC活性大大降低。在人类IgG的4个亚型中，IgG4是一个特殊的存在，其在血液中可以以半抗的形势存在，并且可以与其它IgG4抗体交换从而形成双抗。另外，其与FcRs 和C1q结合很弱，因此Fc相关效应功能很弱。在IgG4型抗体开发中，S228P突变可以有效的防止抗体以半抗形式存在，而且目前已经获批上市的抗体基本上都经过该位点的突变。下表总结了与FcγR结合减弱或沉默相关的一些Fc突变。-04-三、延长半衰期的Fc工程化改造在肿瘤治疗中，抗体半衰期的延长可以降低病人给药的频率，而且较长的半衰期可以使得抗体持续的在体内发挥抗肿瘤作用。但是另外一方面，抗体持续的暴露可能会导致相关副作用的产生，而且对于部分免疫调节型抗体，长的半衰期可能导致一些副作用无法控制，如细胞因子风暴等。因此，在药物开发中要充分考虑半衰期延长的危险与收益的平衡。延长半衰期的改造主要改变Fc与FcRn的相互作用。FcRn能够使被内化的IgG返回循环细胞表面并重新进入血液循环。这个过程中，FcRn以一种依赖于pH值的方式结合Fc的CH2-CH3界面。一旦FcRn：IgG复合物循环到细胞外环境，中性pH值允许IgG重新释放到血液中，从而使得抗体得到保护不被清除。M252Y/S254T/T256E的突变在细胞内酸性环境中使Fc与FcRn的结合能力提高了11倍，但几乎没有影响两者在细胞外中性环境中的结合。在猴子体内实验中M252Y/S254T/T256E突变抗体的半衰期延长了4倍。但是，其在延长半衰期的同时也降低了ADCC效应。一些其它的氨基酸突变，如M428L/N434S，T250Q/M428L，H433K/N434F等都是通过增强Fc与FcRn在细胞内酸性环境中的结合，而几乎不改变两者在细胞外的结合，从而达到增加半衰期的目的。下表总结了与FcRn增强结合延长半衰期相关的一些Fc突变。-05-结语目前，基于Fc与相应FC受体的结构，已经有多种技术如氨基酸突变和糖基化工程等可以用于改变抗体的效应功能。Fc工程化改造可以并且已经为患者提供了临床益处和价值，一些关键Fc修饰（如“LALA”，“LS”，“YTE”及其衍生物）得到广泛使用。但同时，这些突变也带来了一系列复杂的潜在安全风险的考量因素。我们需要深入了解这些FC改造背后的功能和生物学，在它们提供的优势和带来的风险之间保持平衡。毫无疑问，新的突变将继续推动进入临床，从而不断改进药物发现。参考资料：1.Impact of antibody Fc engineering on translational pharmacology, and safety: insights from industry case studies. MAbs.2025 Dec;17(1):2505092欢迎加入我的知识星球，可免费下载每次直播的原始文献和PPT，小药邀请你到知识星球一起学习！公众号已建立“小药说药专业交流群”微信行业交流群以及读者交流群，扫描下方小编二维码加入，入行业群请主动告知姓名、工作单位和职务。

THOUSAND OAKS, Calif.--(BUSINESS WIRE)--Atara Biotherapeutics, Inc. (Nasdaq: ATRA), a leader in T-cell immunotherapy, leveraging its novel allogeneic Epstein-Barr virus (EBV) T-cell platform to develop transformative therapies for patients with cancer and autoimmune diseases, today announced that it has resubmitted a Biologics License Application (BLA) to the U.S. Food and Drug Administration (FDA) for tabelecleucel (EBVALLO™ or tab-cel®) indicated as monotherapy for treatment of adult and pediatric patients two years of age and older with Epstein-Barr virus positive post-transplant lymphoproliferative disease (EBV+ PTLD) who have received at least one prior therapy. There are no FDA approved therapies in this treatment setting. “The BLA resubmission for tab-cel represents the collaborative efforts with our partner, Pierre Fabre Laboratories, to address the third-party manufacturing facility observations outlined in the January 2025 Complete Response Letter,” said Cokey Nguyen, President and Chief Executive Officer of Atara. “We look forward to continued engagement with the FDA throughout its review and with Pierre Fabre Laboratories as they actively prepare for the potential launch of this innovative therapy in the U.S.” Tab-cel is an allogeneic, EBV-specific T-cell immunotherapy designed to target and eliminate EBV-infected cells. The BLA is supported by pivotal and supportive data covering more than 430 patients treated with tab-cel across multiple life-threatening diseases, including the latest pivotal ALLELE study data that demonstrated a statistically significant 48.8% Objective Response Rate (ORR) (p<0.0001) and favorable safety profile consistent with previous analyses. Tab-cel has been granted Breakthrough Therapy Designation for the treatment of rituximab-refractory EBV-associated lymphoproliferative disease by the U.S. FDA and has orphan drug designation for the treatment of Epstein-Barr virus-positive post-transplant lymphoproliferative disorders. Corporate Updates Tab-cel Transition Activities: The company is finalizing the transfer of the following clinical studies associated with tab-cel: NCT03394365: Tabelecleucel for Solid Organ or Allogeneic Hematopoietic Cell Transplant Participants with Epstein-Barr Virus-Associated Post-Transplant Lymphoproliferative Disease (EBV+ PTLD) After Failure of Rituximab or Rituximab and Chemotherapy (ALLELE) NCT04554914: A Study to Evaluate Tabelecleucel in Participants with Epstein-Barr Virus-associated Diseases. This study is a multicenter, multicohort, open-label, single-arm, Phase 2 study investigating the efficacy and safety of tabelecleucel for the treatment of EBV-associated diseases. Upon completion substantially all operational activities and associated costs related to tab-cel will transfer to Pierre Fabre Laboratories. The sponsorship of the BLA continues to be maintained by Atara. Cash Runway and Future Tabelecleucel (Tab-cel®) Milestone and Royalty Income: Atara projects that its cash, cash equivalents and short-term investments of approximately $22M as of June 30, 2025, combined with the cost reduction initiatives implemented in the first half of 2025, will enable funding of all currently planned operations, including one-time restructuring costs, into the first quarter of 2026, that Atara believes will be sufficient to fund the ongoing activities required to achieve potential BLA approval. Additionally, under its commercialization agreement with Pierre Fabre Laboratories, Atara is eligible to receive a $40 million milestone payment upon FDA approval of the tab-cel BLA, as well as significant double-digit tiered royalties as a percentage of net sales, and milestones related to commercial sales of EBVALLO. The estimate of our cash, cash equivalents and short-term investments as of June 30, 2025, is preliminary, has not been audited and it is subject to change upon completion of our financial statement closure procedures. Our independent registered public accounting firm has not audited or completed any procedures with respect to this estimate. Such estimates are based on assumptions and plans that are subject to change and such changes could materially impact our expected cash runway. These assumptions include the completion of specific development and regulatory activities by us and actions taken by third parties, and are, therefore, uncertain at this time. Any delay in these activities will create additional expenses and cash needs for us. We have not yet completed our quarter-end financial close process for the quarter ended June 30, 2025. This estimate of our cash, cash equivalents and short-term investments as of June 30, 2025, is preliminary and is subject to change upon completion of our financial statement closing procedures. Additional information and disclosure would be required for a more complete understanding of our financial position and results of operations as of and for the quarter ended June 30, 2025. About Atara Biotherapeutics, Inc. Atara is harnessing the natural power of the immune system to develop off-the-shelf cell therapies for difficult-to-treat cancers and autoimmune conditions that can be rapidly delivered to patients from inventory. With cutting-edge science and differentiated approach, Atara is the first company in the world to receive regulatory approval of an allogeneic T-cell immunotherapy. Our advanced and versatile T-cell platform does not require T-cell receptor or HLA gene editing and forms the basis of a diverse portfolio of investigational therapies that target EBV, the root cause of certain diseases. Atara is headquartered in Southern California. For more information, visit atarabio.com and follow @Atarabio on X and LinkedIn. Forward-Looking Statements This press release contains or may imply "forward-looking statements" within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. For example, forward-looking statements include statements regarding: (1) the development, timing and progress of tab-cel, including the resubmission of the BLA and potential indications, the timing for FDA review of any resubmission of the BLA, the potential characteristics and benefits of tab-cel, and the results of, and prospects for, the global partnership with Pierre Fabre Laboratories involving tab-cel, and the potential financial benefits to Atara as a result of the global partnership with Pierre Fabre Laboratories, including the receipt, timing and amount of any payments to be received by Atara thereunder; (2) Atara’s estimate of its cash, cash equivalents, and short-term investments as of June 30, 2025, as well as Atara’s cash runway, receipt of potential milestone payments, and operating expenses, including Atara’s ability to fund its planned operations into the first quarter of 2026; and (3) Atara’s planned transition of substantially all remaining activities relating to tab-cel to Pierre Fabre Laboratories and the timing thereof. Because such statements deal with future events and are based on Atara’s current expectations, they are subject to various risks and uncertainties and actual results, performance or achievements of Atara could differ materially from those described in or implied by the statements in this press release. These forward-looking statements are subject to risks and uncertainties, including, without limitation, risks related with the timing of the transfer of substantially all operational activities related to tab-cel to Pierre Fabre Laboratories, with any potential delay creating additional expenses and cash needs for Atara; risks and uncertainties associated with the costly and time-consuming pharmaceutical product development process and the uncertainty of clinical success; risks related to FDA feedback and the ability of Atara, Pierre Fabre Laboratories and Pierre Fabre Laboratories third-party manufacturer to address issues identified in the Complete Response Letter (CRL); our ability to access capital, and the sufficiency of Atara’s cash resources and access to additional capital on favorable terms or at all; risks and uncertainties related to Atara’s financial close and audit procedures; and other risks and uncertainties affecting Atara, including those discussed in Atara’s filings with the Securities and Exchange Commission, including in the “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” sections of the Company’s most recently filed periodic reports on Form 10-K and Form 10-Q and subsequent filings and in the documents incorporated by reference therein. Except as otherwise required by law, Atara disclaims any intention or obligation to update or revise any forward-looking statements, which speak only as of the date hereof, whether as a result of new information, future events or circumstances or otherwise.