Ionizing Radiation Gamma Radiation

Several studies have been reported in which ionizing radiation with 60Co was utilized to reduce the toxicity of bacterial endotoxin. Bertok and Szeberenyi (47) describe the use of a 60Co-irradiated endotoxin preparation, TOLERIN that significantly decreased the endotoxin's lethal and hypotensive effects in a dose-related manner. Endotoxin's ability to activate the complement system was also affected, and immunoadjuvant properties and ability to stimulate nonspecific resistance were retained. Csako et al. (48) investigated the physical and biological properties of 60Co ionizing radiation. Physical and biological changes were reported to be dose dependent. A gradual loss of the polysaccharide components (O side chain and R core) was observed, and activity tests suggested that destruction of lipid A was dose related. Both pyrogenicity and LAL reactivity of the endotoxin were destroyed by increasing the doses of radiation. However, because it increases the possibility of unknown chemical changes to drugs and parenteral solutions, the use of the ionizing radiation in depyrogenating these materials is unlikely. Ionizing radiation would be of far greater use in producing endotoxin that has lost its harmful pyrogenic and toxic properties but retained beneficial properties, such as adjuvanicity, that increase the body's natural defense.

Gamma irradiation has proven capable of reducing endotoxin loads and is currently used in sterilization and is notable for its ability to penetrate medical devices (49), raw materials, excipients, new drug delivery systems includes liposomes and monoclonal antibodies (50). The ease of validation has been described by Reid as being dependent upon a single variable: time:

Gamma radiation processing with its single variable, time, makes validation very easy. Time is the only variable as Co60 (cobalt 60) decays at a fixed rate. Once the total number of curies installed and the required dose is known, only the time, which control the dwell time of the carrier/tote at each position around the source rack, need to be set. This is in direct contrast to gas and steam sterilization where a vacuum system, gas/steam mixture and the uniformity of the heat/gas within the sterilization chamber must be monitored. Similarly for electron beam, the power variables (i.e., current, voltage) and the rate of movement of the transport system must be strictly controlled and monitored throughout the radiation process.

Csako et al. also studied the effects of gamma radiation (Co60) on standard endotoxin preparations and found that a dose of 1 Mrad brought about a reduction of about 200X in the pyrogenicity of LPS, however, only an "eight-fold decrease in the titer of LAL activity was found." Their study demonstrated a dose dependent, exponential destruction of LPS via irradiation. In summary they state:

At higher doses of radiation, a direct relation was observed between the degradation of the molecular and supramolecular structure and the loss of biologic function. At lower doses of radiation, however, there was variability among the functional assays in their rate of change with progressive irradiation of the RSE. The results suggest that the carbohydrate moiety plays an important role both in determining the supramolecular structure and in modulating certain biologic activities of bacterial endotoxins (51).

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