Radioiodinated Fab

When the peptide bond between glycine and lysine is cleaved, meta-iodohippuric acid is released from the conjugate. For comparison, two additional radioiodinated Fab fragments were prepared. Meto-iodohippuric acid was conjugated to Fab fragments though an amide bond to prepare MPH-Fab by procedures similar to those employed for HML-Fab. 

Figure 4. Chemical structures of radioiodinated MPH-Fab and directly radioiodinated Fab fragments.

No significant differences were observed in the radioactivity levels in the blood between the two. However, significant differences were observed in the kidney. MPH-Fab showed high radioactivity levels in the kidney and reached its peak at 1 h postinjection, whereas HML-Fab demonstrated significantly lower levels of the radioactivity in the kidney as early as 10 minutes postinjection. The radioactivity levels in the blood and the kidney after concomitant administration of [ I]HML-Fab and directly radioiodinated Fab ( I-Fab)are also illustrated in Figure 5. 

To further understand the mechanisms responsible for the rapid and selective release of meto-iodohippuric acid from [ I]HML-Fab fragment, subcellular localization of the radioactivity was investigated at 10 and 30 minutes postinjection of the three radioiodinated Fab fragments. Both [ I]MIH-Fab and l-Fab showed the major radioactivity at membrane fractions at 10 minutes postinjection. At 30 minutes postinjection, the majority of the radioactivity of the two Fab fragments migrated to the lysosomal fractions. This suggested that most of the administered Fab fragments would be transported to and metabolized in the lysosomal... 

The heart is imaged whole and as 1-cm bread-loaf slices for 201T1, inIn, and radioiodine activities. When 123I-labeled AM-Fab is used in the study, a photopeak of 159 keV with a 20% window is employed. However, when 125I-labeled AM-Fab is used, the X-ray peak of 125I was used to image the iodine distribution. 

Radioiodine was also attached directly to the Fab fragment, as shown in Figure 4. 

Fiffire 3. Conjugation of radioiodinated HML to Fab fragments using 2-iminothiolane. 

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