Antibody–cytokine fusion proteins for the therapy of cancer

Abstract

Advances in genetic engineering and expression systems have led to rapid progress in the development of antibodies fused to other proteins. These ‘antibody fusion proteins’ have novel properties and include antibodies with specificity for tumor associated antigens fused to cytokines such as interleukin-2 (IL2), granulocyte/macrophage colony-stimulating factor (GM-CSF), and interleukin-12 (IL12). The goal of this approach to cancer therapy is to concentrate the cytokine in the tumor microenvironment and in so doing directly enhance the tumoricidal effect of the antibody and/or enhance the host immune response (T-cell, B-cell or NK) against the tumor. In the past decade, multiple antibody–cytokine fusion proteins have been developed with different specificities targeting a broad variety of tumors. These novel molecules retain both antibody and cytokine associated functions. In addition, in animals bearing tumors, antibody–cytokine fusion proteins are able to target the tumor and to elicit a significant anti-tumor response that in some cases results in a complete elimination of the tumor. These results suggest that antibody–cytokine fusion proteins have potential for use in the treatment of human cancer. In the present review, we describe strategies for construction of antibody–cytokine fusion proteins and discuss the properties of several antibody–cytokine fusion proteins with IgG genetically fused to the cytokines IL2, GM-CSF or IL12.

Introduction

The management of residual disease is a central problem in the treatment of cancer. Despite improvements in treatment protocols, relapse remains a critical and generally fatal problem in high risk cancer patients. Chemotherapeutic strategies are necessarily limited by various toxicities and of limited efficacy. Therefore, additional modalities are needed to achieve disease containment or elimination. Systemic treatment with cytokines such as IL2, IL12 and GM-CSF can render some non-immunogenic tumors immunogenic, activating a protective immune response (Rosenberg et al., 1998, Ruef and Coleman, 1990, Tsung et al., 1997). However, when cytokines are given systemically there are frequently problems with severe toxic side effects that make it impossible to achieve an effective dose at the site of the tumor (Cohen, 1995, Maas et al., 1993, Ruef and Coleman, 1990, Siegel and Puri, 1991).

More effective treatment with cytokines could be achieved if methods were developed to provide effective concentrations in the tumor while limiting generalized toxicity. Direct injection into the site of the tumor has been one approach to this problem (Cortesina et al., 1988, Forni et al., 1987, Maas et al., 1989, Maas et al., 1991, Pizza et al., 1984, Rutten et al., 1989). However, this requires that the tumor be localized and accessible and direct injection into micrometastases is not possible. Another approach has been cytokine gene therapy whereby tumor cells are removed from the patients, transduced or transfected with the cytokine of interest, and reintroduced into the patient with the expectation that a systemic immune response will be elicited against the tumors (Dranoff and Mulligan, 1995, Hurford et al., 1995, Maass et al., 1995, Schmidt et al., 1995, Soiffer et al., 1998, Su et al., 1994, Zatloukal et al., 1995). Although the results suggest that this immunization strategy has potential application in the treatment of minimal residual disease, the ex vivo genetic modification and reintroduction of cells into patients is limited by its patient specific nature. Additionally, it is technically difficult, time consuming and expensive to expand primary autologous human tumor cells to the numbers required for vaccination (Hrouda et al., 1999, Simons et al., 1997, Simons et al., 1999, Soiffer et al., 1998). While in vivo gene delivery using viral vectors has been considered, the in vivo low transfer efficiency of viral vectors and their immunogenicity and safety limit their use (Hrouda et al., 1999). In addition, surface glycoproteins of many viral vectors bind to receptors prevalent on a variety of cells, such non-specific interaction directly decreases the transfection efficiency in vivo (Smith and Wu, 1999). Moreover, a significant fraction of the human population carries preexisting antibodies to viral vectors and such unfavorable immune responses decrease the half-life of vectors (Piedra et al., 1998). Thus the challenge is to develop an alternative approach for achieving effective local concentrations of cytokines.

Tumor specific Abs genetically fused to cytokines provide an alternative approach for concentrating in the region of the tumors quantities of cytokine sufficient to elicit a significant anti-tumor activity without accompanying systemic toxicity. In fact, in the past decade, we and others have developed several Ab-cytokine fusion proteins specific for different TAAs. In preclinical trials using murine model such Ab-cytokine fusion proteins have been shown to be very effective anti-cancer agents suggesting that they may be useful in the treatment of human cancer. As the number and diversity of Ab-cytokine fusion proteins has dramatically increased in the last years the present review cannot include all of them. Instead we will focus this review on a subset of Ab-cytokine fusion proteins consisting of IgG genetically fused to the cytokines IL2, GM-CSF or IL12.