The advent of proteins as therapeutic agents and the promise and expectations of large revenues has initiated several lines of new and rapid developments. Not that proteins had not been used as drugs, but they had been purified from blood or tissues and were not available in large scale with the exception of perhaps pig insulin. Only after the wave of discoveries in molecular biology as gene cloning and recombinant technology, in antibody technology, identification of hematopoietic and other growth factors and last but not the least, the begining of some understanding of the complex interactions in the hematopoietic system enabled industry to tackle the preparation of a broad range of proteins for therapeutic purposes. Still, considering the vast amount of public, corporate and private money spent for the search of target molecules and the selection of specific binders in the antibody construct field, the discovery of more and more growth factors and their receptors, the yield in registered therapeutic agents is small.
In this report i want to focus on molecules for administration to patients with solid cancers.
In the "old" days, this was from the late seventies into the early nineties of last century, bio-therapeutics in general were considered to better target aberrant cells and, in contrast to the available chemotherapy, leave normal cells unharmed. However, before recombinant molecules were designed to beware the patient from undesired immune reactions against animal proteins and protect animal species derived molecules against neutralization and elimination by host responses, one was poorly blessed by therapeutic success.
For the production of recombinant proteins animal cell lines were selected to host the recombinant construct without generating unfavorable host cell derived neo-antigens. Highly sensitive quality control tests had to be established to warrant the purity and homogeneity of the protein preparation. i.e. proving the absences of contamination from down-stream processing. Finally, formulations had to be invented to prevent degradation and aggregation of the final vial.
Let's talk first about antibody preparations. Although mixtures of antibodies had been administered as anti-toxins for a long time, only the discovery and creation of monoclonal antibodies opened the field to the wide application of antibodies we see now, particularly as research tools. Set aside their use in routine laboratory practice, a special design was needed to make them suitable for the use in humans.
Somehow, human B-cells in most cases proved resistant to genetic manipulation. As a consequence, mice and their B-cells have been subjected to various vaccination procedures in order to gain antibody specificities thought to have therapeutic value.
At the same time experiments were performed to alter the mouse antibodies into molecules which would be tolerated by the patient's immune system. Different strategies have been pursued to create non-immunogenic antibodies. The aim was to preserve the antibody specificity but free the molecule of the species sequences. In the chimeric antibody version, the molecule consists of the original variable chains which contain the binding capacity and the so called constant parts of human origin. Since the constant part harbors other functional sites. a type with the desirable property can be chosen. Genetic recombination of the gene parts coding for the antigen binding sequences and constant parts allows after transfection of host cells for the production of the gene product in large scale cell cultures.
Another strategy called "reshaping" uses only the complementarity determining regions (CDR) of the original antibody and integrates them in the framework of a human antibody species. Such construct in its properties comes closest to a human antibody molecule. Instead of using CDRs one can also use randomly created sequences from phage display systems selected for their binding capacity and again integrate them in an antibody framework.
Obviously, the nature of a construct is not without significance for its effect on the patient's disease. Size and immunogenicity will greatly determine the half-life and thus the duration of the possible efficacy, the functions in the constant part might add to the blocking of the receptor binding for tumor cell destruction.
It seems there are no limits to the inventiveness and construction of antibodies or antibody constructs. Yet, finally all have to prove that they are functional and safe and valuable in the defeat of the disease if meant to do so.
There are major obstacles we are facing in the discovery of new specificities for the therapy of cancer. One is the failure to define tumor specific target molecules. For the treatment of solid tumors we are making use of the over-expression of growth factor receptors either on tumor cells or surrounding tissue like blood vessels. Another problem is the fact, that the biology of tumors is not well understood and a marker for difference to normal has not been detected.
The general concepts is to block the binding of the natural ligand to the growth factor receptor on the tumor cell and thus interfere with the positive signaling of the receptor into the cell. In addition, antibodies naturally carry binding sites for phagocytic cells add factors of the so called complement system which in the activated form can lead to cell destruction.
However, tumor cell populations are heterogeneous. This means that only part of the tumor cell population will bind the antibody and is being attacked while others continue to grow unaffected. The escaping cells might belong to the more recent lineage of tumor stem cells, which in particular posses the capability for proliferation and survival.
Much attention is recently paid to the tumor environment. It is also described as stroma, what collectively describes all cells surrounding the tumor cells. They might be cells forming blood vessels or others releasing growth factors for their own growth, which are supporting the tumor cell growth as well.
Another important field is the research on cytokines. Compared to antibodies they are small molecules consisting of one amino acid chain. This made cloning and expression in bacterial, yeast or animal systems much easier. Amongst the first and most important to colony stimulating factors were produced and entered into clinical trials. The outstanding performance of one of them, G-CSF, in reconstituting leukocyte populations made it to one of the biggest productions. However, only the clinical trials taught us, that the others were less effective or induced adverse effects.
Similarly, from their discovery until ongoing, interleukins raised hope as benefactors in the defeat of cancer cells. Being small proteins they did not pose much difficulty in their development. So far, all of them failed to grow into the small high revenue market.
There are several reasons, why the number of registered recombinant drugs is relatively small. First of all would say their mode of action is misunderstood. The expectation was and still is, that one drug can do the job of tumor eradication by itself. On top, it should act in a large indication, where chemotherapy and radiation failed. This assumption can be fatal. Naturally immune molecules such as antibodies, cytokines and interleukins interact on short distance with the immune cells at low molar concentrations. Bacterial, viral infections or parasitic invasion are efficiently suppressed. In the case of cancer and in anticipation of the fact, that the manifest cancer cells already escaped immune surveillance, we ask the isolated molecule with possibly reduced function to do the impossible. The failure then should not come as a surprise.
Another reason comes from the reluctance to recognize that only combination and cooperation of several factors including chemotherapy and radiation will lead to the goal. This is largely dominated by marketing aspects, because sales might be less prominent than for the single drug and development costs for the cocktail would be higher.
There is certainty about the further way of bio-therapeutics: Less costly and faster development of a number of key molecules and their application in cancer indications which are likely to respond well to the treatment will lead us and teach us how to tackle the more difficult cases. This requires some patience.
About the author.
Prof. Gerd-Albrecht Luckenbach is Professor of Immunology,
Universitat fur Bodenkultur Wien, Austria.
[Ref by: THE BIO MEDIC Issue No. 3, Jan-Mar-2008]
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