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Radionuclides Linked to a CD74 Antibody as Therapeutic Agents for B-Cell Lymphoma: Comparison of Auger Electron Emitters with ß-Particle Emitters

Immunomedics, Inc., Morris Plains; Garden State Cancer Center, Center for Molecular Medicine and Immunology, Belleville, New Jersey; National Institutes of Health, Bethesda, Maryland; and Memorial Sloan-Kettering Cancer Center, New York, New York

View larger version (21K): [in this window] [in a new window]   FIGURE 1. (A) Toxicity of 90Y-LL1 for Raji cells (filled symbols) compared with nonreactive control Ab MN-14 labeled in same way (open symbols). Cells were incubated for 2 d with radiolabeled Ab at starting concentration of 0.74 MBq/mL (20 µCi/mL) (circles), 0.37 MBq/mL (10 µCi/mL) (squares), 0.185 MBq/mL (5 µCi/mL) (triangles), or 0.095 MBq/mL (2.5 µCi/mL) (inverted triangles). Growth rate of control, untreated cells is also shown (dotted line without symbols). Data shown are cell counts obtained at various times and are representative of 2 experiments, each done in duplicate. Cells treated with highest concentration of LL1 were 100% killed, because no viable cells were detected after day 6, and growth of a single viable cell would be readily detected in 22 d. (B) Fraction surviving was calculated from growth curves, as described in Materials and Methods section, and plotted versus the initial µCi/mL for LL1 (filled circle) or the nonreactive control Ab (filled square). One hundred percent killing cannot be shown on exponential y-axis, but next-higher concentration of 90Y-LL1, 20 µCi/mL, produced 100% killing. Results shown are representative of 2 experiments, each done in duplicate.

View larger version (28K): [in this window] [in a new window]   FIGURE 2. Toxicity of Ab LL1 conjugated to 4 radionuclides for Raji B-lymphoma cells (filled circles). Results are also shown for a nonreactive control Ab (filled squares). Cells were incubated for 2 d with indicated starting concentration of Ab, then diluted and counted at various times. Fraction surviving was calculated from growth curves as described in Materials and Methods section. One hundred percent killing cannot be shown on the exponential y-axis, but all the LL1 conjugates used produced 100% killing at next-higher concentration, which was 2 or 3 times higher than highest concentration shown. Results shown are representative of at least 2 experiments performed with each radiolabel and Ab, each done in duplicate. Note different scales on x-axis. Similar experiments with 90Y label are shown in Figure 1.

View larger version (29K): [in this window] [in a new window]   FIGURE 3. Nonspecific toxicity of 5 radionuclides conjugated to control nonreactive Ab. Experimental protocol was as described in Figure 1, except that higher concentrations of radioactivity were used. Results are shown for 90Y (open squares), 131I (open circles), 125I (filled circles), 111In (filled triangles), and 67Ga (filled squares). Note that ß-particle emitters have open symbols, whereas Auger electron emitters have filled symbols. Separate curves indicate different experiments with same radiolabel. Iodine labels used a chloramine-T label, rather than IMP-R2 label used with LL1, because there was virtually no nonspecific uptake of radionuclide by cells (1), and therefore there was no need to use residualizing label. (A) x-axis shows initial µCi/mL; (B) x-axis is total disintegrations per well (total volume, 1.5 mL) in 2 d. Only substantial difference between parts A and B is small rightward shift of 125I curves.

View larger version (28K): [in this window] [in a new window]   FIGURE 4. Relationship between fraction surviving and total disintegrations per cell. Raji cells were incubated with radiolabeled LL1 for 2 d, and fraction surviving was determined by clonogenic assay. Cell-bound CPM was determined at days 1, 2, 3, and 6, or days 1, 2, and 5, and cumulative disintegrations per initial cell number were calculated. Results are shown for 90Y (open squares), 131I (open circles), 125I (filled circles), 111In (filled triangles), and 67Ga (filled squares). Note that ß-particle emitters have open symbols, whereas Auger electron emitters have filled symbols. Separate curves indicate different experiments with same radiolabel.

View larger version (24K): [in this window] [in a new window]   FIGURE 5. Relationship between fraction surviving and calculated cGy dose. Using data presented in Figure 4, cGy dose per cell was calculated from disintegrations per cell. Separate curves indicate different experiments with same radiolabel. Dotted line shows toxicity caused by irradiation from 137Cs, which was determined previously (1).

View larger version (25K): [in this window] [in a new window]   FIGURE 6. Energy spectrum of electrons emitted by 125I, 111In, and 67Ga. Electrons are grouped by energy in increments of 5 keV. Note break in the scale of y-axis and different scales used.