Monday, May 10, 2010
Baby's breath for cancer?
Baby's Breath flower can boost anti-leukaemia drugs by up to a million times
So read the headline in the Daily Mail.
The article refers to work done in Southampton by David and Bee Flavell with antibodies tagged to Saponin. This couple have been beavering away for more than 20 years ever since they lost their child, Simon, to acute lymphoblastic leukemia.
Here is an abstract of their work from a presentation at the American Association for Cancer Research last month.
Saponins are a highly diverse group of glycosides of plant origin containing either a steroidal or triterpenoid aglycone to which one or more sugar chains are attached. They have been variously described as enhancing the cytotoxic activity of ribosome-inactivating proteins (RIPs) and ligand-directing conjugates constructed with these for eukaryotic cells. Saponins are thought to facilitate the entry of RIPs into the cytosol across the membranes of intracellular vesicular structures. This characteristic of saponins therefore makes them of interest as molecules that could be useful in improving the therapeutic index of immunotoxins (IT) in a clinical setting. In the current study we used a protein synthesis inhibition assay based on leucine incorporation together with cell proliferation studies in culture to demonstrate that saponins from Gypsophila paniculata L. potentiate the cytotoxicity of the anti-CD19 and CD38 saporin-based immunotoxins (BU12-SAPORIN and OKT10-SAPORIN, respectively) by several thousand-fold for the human lymphoma cell line Daudi. Various experiments revealed that the enhancing effect of saponins on immunotoxin activity did not appear to be dependent on extra cellular pH in the assay system we used. Kinetic studies further revealed that saponin at a predetermined optimal concentration of 2 μg/ml reduced the t10 (time taken to reduce protein synthesis to 10% of the level seen in control Daudi cells) for OKT10-SAPORIN at 0.1μg/ml from 50 hours without saponin to 12.5 hours with saponin. In blocking experiments using OKT10-SAPORIN IT at the IC50 concentration in combination with saponin together with a gross excess of native OKT10 antibody we established that the immunospecific cytotoxic activity of OKT10-SAPORIN was fully retained when leucine incorporation was used as the surrogate measure of cytotoxicity but only partially so when thymidine incorporation was used. This observation of a partial uncoupling of protein and DNA synthesis as a measure of cytotoxicity was further supported by cell proliferation experiments in culture which were undertaken in parallel. If saponins are to be of any clinical value as candidate molecules for improving the therapeutic index of immunotoxins it is imperative that the immunospecificity of IT activity is fully retained. Whilst this study clearly demonstrates that saponins from G. paniculata L. do indeed significantly potentiate IT activity it seems that this effect may only be partially immunospecific. Nevertheless, the dramatic improvements that saponins confer on IT activity argue in favor of in vivo experiments in transgenic animal models of human lymphoma combined with additional studies to explore other strategies that might be employed to improve the immunospecific effect achievable.
I have written about these 'immunotoxins' at length in the past and we published a small series back in the 1990s:
Lancet. 1995 Jul 22;346(8969):223-4.
Treatment of B-cell lymphomas with combination of bispecific antibodies and saporin.
French RR, Hamblin TJ, Bell AJ, Tutt AL, Glennie MJ.
Tenovus Research Laboratory, Southampton General Hospital, UK.
We report the use of a bispecific F(ab')2 antibody to target the ribosome-inactivating protein saporin to the surface antigen CD22 in the treatment of low-grade, end-stage, B-cell lymphoma. Four patients were treated. Toxic effects were minimal (grade I), with mild fever, weakness, and myalgia for 1-2 days after treatment. One patient showed an antibody response to mouse Fab' and saporin. All patients showed rapid and beneficial responses to treatment with improvements in most disease sites and in peripheral blood cytopenia. The responses were short-lived (less than 28 days) but further study of this targeting system is warranted.
We need to distinguish between saporin and saponin. Saporin is a ribosome-inactivating protein similar to ricin, that needs an antibody to hitch a lift on to get into the cell. Saponins are a group of plant compounds which seem to enhance the activity of immunotoxins.
It is important to recognize that these experiments, while promising, are still at the stage of killing cells in a test tube. Many substances kill cells in test tubes, the acid test comes when you put them into patients. We found that the OKT10-saporin conjugate caused transient blindness when given to patients with myeloma.