Originally published as JCO Early Release 10.1200/JCO.2007.15.7404 on March 17 2008

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The "Other" Signaling of Trastuzumab: Antibodies Are Immunocompetent Drugs

Division of Medical Oncology, Clinical Pharmacology and New Drug Development, Fondazione Istituto Nazionale dei Tumori, Milano, Italy

The outstanding pace of the clinical success of trastuzumab was always intimately connected to its ability to target and block the erbB2 receptor and to obviate to the malignant consequences of the activation of the intricate network of erbB2-associated intracellular signals.¹ The wealth of data derived from probing the erbB2-associated signals has helped to clarify very relevant features of the mechanisms of sensitivity and resistance to trastuzumab. At the same time, such successful research somewhat obscured the fact that trastuzumab is an immunoglobulin G (IgG) 1 antibody endowed with all of the powerful armamentarium of immunologic effects characteristic of antibodies.

In this issue of the Journal of Clinical Oncology, a report from Musolino et al² shows that the likelihood of response to trastuzumab is different in carriers of different variants of the IgG fragment C receptor (FcR) present on immune effectors, such as neutrophils, macrophages, and natural-killer cells. The FcR engagement by antibodies linked to cancer cells triggers antibody-dependent cellular cytotoxicity (ADCC) in a critically relevant way in model systems.³ The study by Musolino et al supports the concept that ADCC is a key mechanism of antitumor activity of trastuzumab in humans and opens scenarios relevant to the current and future use of the monoclonal antibody in the clinic.

In the 54 consecutive patients with erbB2/neu-amplified metastatic breast cancer studied by Musolino et al,² the FcRIIIa-158 valine/valine (V/V) genotype was significantly correlated with response rate and progression-free survival, and borderline significance was observed for the association between therapeutic effects and FcRIIa-131 histidine/histidine (H/H) genotype. This finding could be predicted based on a number of prior reports in the literature. In a seminal work, Clynes et al showed in a variety of relevant syngeneic and xenograft murine models that engagement of Fc receptors on effector cells was a dominant component of the antitumor activity of trastuzumab, and that knocking out the FcR left a significantly lower residual antitumor effect of the antibody, presumably linked to the blockade of erbB2 signaling in the tumor.⁴ Interestingly, in that report, similar conclusions were reached for rituximab that targets the B-cell marker CD20 and is identical to trastuzumab except for the epitope-specific portion of the variable region of the antibody.⁴ That preclinical observation has already been matched by evidence in humans demonstrating the link between the same FcR polymorphisms indicated by Musolino et al² for trastuzumab and the therapeutic benefit of the anti-CD20 antibody.^5,6 More recently, the association between the same polymorphisms and therapeutic effects was also documented for the chimeric antibody directed to the epidermal growth factor receptor cetuximab when used as single agent in patients with colorectal cancer.⁷ Finally, a pilot investigation of preoperative trastuzumab conducted in women with operable breast cancer overexpressing erbB2 already provided preliminary evidence that the antibody's therapeutic effects could preferentially be driven by ADCC.⁸

No matter how well and easily predictable, the results of Musolino et al² represent the first conclusive evidence of the crucial role that ADCC modulation plays for trastuzumab's antitumor activity, and lead to many further considerations. The first and obvious one is the opportunity to test germline DNA of individual patients for FcR polymorphisms, using the results for deciding whether to treat with trastuzumab. The data presented indicate a huge differential benefit between carriers of V/V or H/H genotype and other variants.² A very recent independent report also showed that the differential propensity for triggering ADCC as measured from the quantity and lytic efficiency of CD16-positive effector cells was strongly associated with different type of response after a short course of primary trastuzumab before surgery.⁹ The relative comparison of outcome of Musolino et al's² analysis is insufficient to conclude that carriers of other FcR genotypes than V/V or H/H should be excluded from trastuzumab therapy, but together with the work of Varchetta et al⁹ the data strongly support that the predictive hypothesis should be tested. Today's methodology allows for the extraction and dependable analysis of germline DNA from formalin-fixed paraffin embedded material so that probing of FcR polymorphisms can be done. In the trials of adjuvant trastuzumab that were recently completed, tumor blocks were collected and proved of outstanding relevance for association studies between different markers, such as c-MYC¹⁰ and topo-II¹¹, and likelihood of benefit. Analysis of nontumoral formalin-fixed paraffin embedded material in the same studies could be of outstanding value to define the role of FcR polymorphisms in dictating the sensitivity to trastuzumab. Such studies would also provide an invaluable and unique source of information with reference to the indication of trastuzumab in women with ebB2-positive breast tumors and unfavorable FcR because no future study will enroll patients in arms without erbB2-directed therapy. As a consequence, none of the ongoing or future trials in patients eligible for trastuzumab will ever clarify whether some benefit is present irrespective of FcR genotype.

The establishment of the immunocompetence of trastuzumab as a leading mechanism of therapeutic effect in humans triggers some consideration about combination studies of the monoclonal antibody. Musolino et al's study was conducted in patients undergoing treatment with trastuzumab combined with either paclitaxel or docetaxel.² The effect of such drugs on the immune system—and more specifically on ADCC—has rarely been tested. The common wisdom is that taxanes have an immunosuppressive effect. This is probably true in view of the generous doses of corticosteroids that are an integral part of the premedication regimens of either taxane. However, in the few reports referenced in the literature paclitaxel and docetaxel seem to enhance natural killer activity in vivo^12,13 and to lead to persistently increased natural killer cell cytotoxicity after the conclusion of taxane-containing chemotherapies.¹⁴ The observation provides a clue to clarify why the concomitant treatment with chemotherapy did not obscure the association of trastuzumab benefit with FcR genoypes. When assessing the relevance of ADCC modulation and trastuzumab activity, it should be taken into consideration that the antibody is most often administered with taxanes and has a reported synergistic activity in their combination that might be in part dependent on immune effects.¹³ In addition, the demonstration that ADCC might be crucial for the antitumor activity of trastuzumab lends further weight to the rationale of combining the antibody with erbB2-directed tyrosine-kinase inhibitors, such as lapatinib in the ongoing international Adjuvant Lapatinib and/or Trastuzumab Treatment Optimization trial, because the same target would be hit through two completely different and independent mechanisms of action. Also, the recently reported synergistic activity between trastuzumab and pertuzumab,¹⁵ a monoclonal that shares the same IgG1 frame of trastuzumab but targets a different epitope of the erbB2 extracellular domain¹⁶ could at least be due in part to the fact that no steric hindrance affects the binding of the two monoclonals to the receptor and their concomitant administration would result in a higher density of FcR binding sites on erbB2-overexpressing breast cancer cells and eventually to enhanced ADCC. These examples underscore the consideration that future studies of trastuzumab combinations cannot be conducted without investigating FcR polymorphisms and ADCC.

A final and very relevant point prompted by the report of Musolino et al² still deserves discussion. If ADCC and Fc receptors play the suspected pivotal role for the clinical activity of trastuzumab and are involved in defining patterns of sensitivity and relative resistance, the enhancement of ADCC should be a primary goal and research investment for future drugs or future combinations of trastuzumab. A number of strategies can been used to increase the therapeutic impact of antibodies.^17-20 Among them, genetic engineering of monoclonal antibodies to improve binding to Fc stimulatory receptors while decreasing or abolishing binding to inhibitory Fc receptors has been successful and point mutations in the Fc region yielded a twofold enhancement of ADCC whose corresponding increment of antitumor activity in vivo was not explored.²¹ Research to increase the benefit of immunotherapy with antibodies should be revitalized.

In brief, the report by Musolino et al² shows in a solid and well-conceived study that trastuzumab signals to the FcR of effector cells as IgGs do and that antibody-dependent cancer cell cytotoxicity should be triggered. Modulation of this dominant mechanism of therapeutic activity is possible and might be crucial for optimizing therapy with trastuzumab as well as with other therapeutic antibodies used in cancer therapy. The evidence of this immune mechanism and its implications should no longer be overlooked nor neglected.

AUTHOR'S DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST

Although all authors completed the disclosure declaration, the following author(s) indicated a financial or other interest that is relevant to the subject matter under consideration in this article. Certain relationships marked with a "U" are those for which no compensation was received; those relationships marked with a "C" were compensated. For a detailed description of the disclosure categories, or for more information about ASCO's conflict of interest policy, please refer to the Author Disclosure Declaration and the Disclosures of Potential Conflicts of Interest section in Information for Contributors.

Employment or Leadership Position: None Consultant or Advisory Role: Luca Gianni, Genentech (C), Roche (C) Stock Ownership: None Honoraria: Luca Gianni, Roche, GlaxoSmithKline Research Funding: None Expert Testimony: None Other Remuneration: None

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