Laboratory / Animal / Preclinical Studies
The third study investigated the ability of amygdalin and beta-glucosidase to indirectly sensitize the hypoxic (oxygen-starved) cells at the center of a tumor to the lethal effects of gamma irradiation. Cells at the periphery (outer edge) of a tumor are more sensitive to gamma irradiation because they are not oxygen-deprived. Radiation kills cells, in part, by splitting molecules, including oxygen molecules, to form free radicals, which are highly reactive chemicals that can damage DNA and other important cellular components. It has been proposed that, by inhibiting oxygen uptake by peripheral tumor cells, more oxygen will diffuse to the hypoxic cells, thereby increasing their sensitivity to radiation. In this study, beta-glucosidase was used to break down amygdalin to release cyanide, with the cyanide inhibiting oxygen uptake by peripheral tumor cells. Presumably, cyanide uptake by interior tumor cells is less than that of cells located at a tumor's periphery. Spheres of tumor cells created in the laboratory and tumor slices were used in the study. The investigators found that amygdalin and beta-glucosidase could act as indirect radiation sensitizers of hypoxic tumor cells. It should be noted, however, that independent confirmation of this positive finding has not been published in a peer-reviewed scientific journal. A major hurdle in the application of this technique to animals and humans is the development of a method for delivering a sufficient amount of cyanide to tumors without causing substantial systemic or regional toxicity.
In the fourth study, cultured human bladder cancer cells were treated with amygdalin alone or a combination of amygdalin and an antibody that was coupled (chemically) to beta-glucosidase. The target for this antibody was the glycoprotein (a protein with sugar molecules attached) MUC1. Aberrant forms of MUC1 are produced and displayed at high levels on the outside of several types of cancer cells, including bladder cancer cells. In this study, amygdalin alone was not very effective in killing the bladder cancer cells, but its cell-killing ability was 36 times greater in the presence of the antibody-enzyme complex. There are two possible explanations for this increase in cell-killing ability. The first is that antibody-enzyme complexes bound via MUC1 produce high rates of amygdalin breakdown at the cell surface. This breakdown leads to high local production of cyanide, which is quickly taken up by the cells and kills them. The second explanation is that antibody-enzyme complexes bound to the cells are internalized, thereby increasing the intracellular concentration of beta-glucosidase. Increased beta-glucosidase activity inside a cell would result in increased breakdown of amygdalin taken up by it, and increased cyanide production and cell death. These two potential mechanisms are not mutually exclusive. In another experiment, the researchers cultured bladder cancer cells in the presence of human brain tumor cells, which do not express MUC1. When this coculture was treated with amygdalin and the antibody-enzyme complex, the bladder cancer cells were killed selectively. In view of the mechanisms proposed above, this result is not surprising, since the bladder cancer cells and the brain tumor cells in this coculture formed homogeneous colonies (colonies that contained exclusively bladder cancer cells or brain tumor cells). Conceivably, selective killing of some types of human cancer cells might be achievable through application of this method; however, these positive results must be confirmed independently, and the effectiveness of this approach in animal models must be demonstrated before its use in humans can be considered.