The model system examined was a bsMAb composed of two
Fab' fragments, one from a humanized anti-
carcinoembryonic antigen antibody (hMN-14), and the other a murine antibody (679) against
histamine-succinyl-
glycine. These
Fab' fragments were chemically conjugated to form a F(ab')(2) that is joined by a stable
thioether bond. The
peptide used for these studies (IMP-241) contained two
histamine-succinyl-
glycine moieties for binding to the 679 portion of the bsMAb and a single 1,4,7,10-tetra-azacyclododecane N,N',N",N"'-tetraacetic
acid chelate for radiolabeling with (111)In.
RESULTS: The bsMAb cleared rapidly in nude mice bearing the GW-39 human
colonic cancer xenograft. Administration of a radiolabeled
peptide 1 day after the bsMAb, using a bsMAb/
peptide mole injection ratio of 10:1, allowed for higher
tumor accretion than if delayed by 2 days.
Tumor uptake measured 3 h after the
peptide injection given 1 day after the bsMAb was 11.3 +/- 2.2% percentage of injected dose/gram (%ID/g), with just 2.9 +/- 0.4% ID/g of the bsMAb in the
tumor at this time.
Tumor/blood ratios were 8.1 +/- 2.1.
Peptide uptake was highest in the kidneys, but even so, the
tumor/kidney ratio was 2.5 +/- 1.9 just 3 h after the
peptide injection. Although low bsMAb/
peptide mole injection ratios allow for greater concentrations of the
peptide in the
tumor, kidney uptake is increased at a proportionally higher amount than in the
tumor. Therefore, a bsMAb/
peptide injection ratio of 10:1 with a 24-h interval was preferred for pretargeting. Increasing the bsMAb dose, and thereby increasing the bsMAb/
peptide injection ratio, further enhanced the delivery of the radiolabeled
peptide to the
tumor, but the interval spacing between the bsMAb and
peptide had to be increased. Despite having a lower %ID/g of the bsMAb in the
tumor, with a bsMAb/
peptide injection ratio of 50:1 and a 48-h interval,
tumor uptake of the (111)In-peptide was nearly 30% ID/g, a 1.6-fold improvement over that seen with the 10:1/24-h interval pretargeting group, and
tumor/blood was 35:1, and
tumor/kidney ratio was 8:1. Two fractionation strategies were also examined. Giving two equal fractions of
peptide after a single injection of bsMAb loaded more moles of
peptide into the
tumor but would not permit higher radioactivity delivery than what could be achieved with a single injection. However, area under the curve analysis indicated that giving repeated cycles of the bsMAb followed by the
peptide would enable improvements in the amount of radioactivity delivered to the
tumor without increasing the amount delivered to normal tissues, but the timing of the bsMAb/
peptide cycles was important to optimize this process. Finally, it was noted that larger
tumors (e.g. those > 0.3 g) were more likely to have higher
peptide uptake in a pretargeting procedure than smaller
tumors (e.g., those of approximately 0.1 g), perhaps due to the greater mass of the bsMAb localized in the larger
tumors, but also possibly because of better blood supply in these
tumors.
CONCLUSIONS: