Loss of immunoreactivity of I-131 labeled monoclonal antibody with storage is related to radiation damage

In order to use a single preparation of I-131 monoclonal antibody on more than one day, it is important to determine the shelf life of these compounds. Fifteen I-131 (.128 to 15 mCi/ml) preparations of IgG or Fab fragments of antimelanoma antibody (96.5 or 48.7) were stored at 0-40C in pharmaceutical vials. Daily aliquots were tested for total immunoreactivity (IR) (JNM 24:123,1983), TCA precipitability and fractions using size exclusion HPLC. Loss of IR ranges from 0-54% during the first 24 hours to 0-92% over 6 days. Loss of IR occurred with both Fab and IgG. The rate of loss of IR correlated with the initial specific concentration (r = .8,p < .01) and specific activity (r = .78,p < .01) but dilution of the concentrated solution by 1000x or more stopped the deterioration process (p < .01) suggesting specific concentration to be the important variable. When mM cysteamine or cystamine were added to the concentrated solutions the loss of IR with time was inhibited (X/sup 2/,p < .001). TCA precipitation and HPLC analysis showed loss of antibody associated radioactivity but not to the extent of the change in IR. There can be significant loss of IR of I-131 monoclonal antibody during storage at 0-40C. and high concentration solutions (>5 mCi/ml.) must be used within 24 hours of labeling to assure an active preparation. The inhibition of IR loss by dilution or by addition of radioprotectors suggests that the process is due to radiation effects on the antibody