PEGylated antibodies and antibody fragments for improved therapy: a review

Abstract

The use of covalent attachment of poly(ethylene glycol) to various different proteins in order to modify their function has been reported over many years. One class of protein that this technology has more recently been applied to is antibodies and antibody fragments. PEG has been predominantly used to reduce the immunogenicity and increase the circulating half-lives of antibodies. It may also have a beneficial effect on the use of antibodies in certain clinical settings such as tumour targeting. This review describes previously reported experience with PEGylated antibodies and antibody fragments, and where these types of molecules may find clinical usefulness in the future.

Introduction

Over the last few years, there has been growing interest in the use of antibodies and antibody fragments as therapeutic entities. There are now more than 10 commercially available antibody products [1], and a recent survey recorded some 59 monoclonal antibody-based molecules in clinical development [2] making this the second largest biopharmaceutical product category after vaccines. The advent of genomic and in particular proteomic technologies is expected to lead to the identification of many more novel targets for antibody-based therapies. Coupled with the use of phage display, SLAM (Selected Lymphocyte Antibody Method) or alternative selection techniques, it is widely anticipated that there will be an increase in the number of antibodies entering clinical development in the next few years.

Over the past couple of decades, there have been significant advances in the field of antibody technology. For many years, the usefulness of antibodies therapeutically was limited by the immune response generated by the host to the administered antibodies (typically of murine origin), the therapeutic protein being recognised as foreign by the host immune system. In these circumstances, the known ability of PEG to reduce the immunogenicity of foreign proteins has been investigated. The need to do so however, has largely been removed by recent technological advances in enabling the production of engineered humanised antibodies, and latterly human antibodies [3]. These appear to have largely overcome the issue of triggering an immune response from the recipient such that the antibodies are no longer therapeutically beneficial, and has enabled the development of antibodies for the treatment of chronic conditions such as rheumatoid arthritis where repeat dosing is required.

The majority of current applications for antibodies are for acute diseases such as cancer. There are however, significant opportunities to develop these molecules for chronic conditions. These are likely to require large doses of protein over long periods of time, and for markets where the cost of goods is an important factor. The high cost of antibody production in mammalian cell culture, the traditional way of manufacturing antibodies and antibody fragments, has consequently discouraged the development of antibodies in these markets due to prohibitive cost of goods, difficulty in competing with small molecule drugs, and limited worldwide manufacturing capacity. One potential solution to this has been to evaluate the expression of antibodies in the milk of transgenic animals such as goats, but this has high start-up costs, long lead times, may have regulatory issues, and does carry a risk of a herd being lost with disease for example. An alternative approach has been to express fragments of antibodies such as Fab′ and scFv in microbial expression systems such as Escherichia coli. This is a much more economical manufacturing system and is expected to yield much higher amounts of protein as a result of larger fermenter volumes and much shorter fermentation times compared to mammalian cell fermentation. These fragments, however, have been found to have very short circulation times in vivo. This can be overcome by conjugation to PEG which has been found to confer an increased half-life to proteins to which it is attached. In this way, antibody fragments can be economically produced and enable the treatment of chronic diseases. This will be discussed in greater detail in this review.