Pretargeting for Cancer Radioimmunotherapy with Bispecific Antibodies:  Role of the Bispecific Antibody's Valency for the Tumor Target Antigen

Abstract

The use of a divalent effector molecule improves bispecific antibody (bsMAb) pretargeting by enabling the cross-linking of monovalently bound bsMAb on the cell surface, thereby increasing the functional affinity of a bsMAb. In this work, it was determined if a bsMAb with divalency for the primary target antigen would improve bsMAb pretargeting of a divalent hapten. The pretargeting of a (99m)Tc-labeled divalent DTPA-peptide, IMP-192, using a bsMAb prepared by chemically coupling two Fab' fragments, one with monovalent specificity to the primary target antigen, carcinoembryonic antigen (CEA), and to indium-loaded DTPA [DTPA(In)], was compared to two other bsMAbs, both with divalency to CEA. One conjugate used the whole anti-CEA IgG, while the other used the anti-CEA F(ab')(2) fragment to make bsMAbs that had divalency to CEA, but with different molecular weights to affect their pharmacokinetic behavior. The rate of bsMAb blood clearance was a function of molecular weight (IgG x Fab' < F(ab')(2) x Fab' < Fab' x Fab' conjugate). The IgG x Fab' bsMAb conjugate had the highest uptake and longest retention in the tumor. However, when used for pretargeting, the F(ab')(2) x Fab' conjugate allowed for superior tumor accretion of the (99m)Tc-IMP-192 peptide, because its more rapid clearance from the blood enabled early intervention with the radiolabeled peptide when tumor uptake of the bsMAb was at its peak. Excellent peptide targeting was also seen with the Fab' x Fab' conjugate, albeit tumor uptake was lower than with the F(ab')(2) x Fab' conjugate. Because the IgG x Fab' bsMAb cleared from the blood so slowly, when the peptide was given at the time of its maximum tumor accretion, the peptide was captured predominantly by the bsMAb in the blood. Several strategies were explored to reduce the IgG x Fab' bsMAb remaining in the blood to take advantage of its 3-4-fold higher tumor accretion than the other bsMAb conjugates. A number of agents were tested, including those that could clear the bsMAb from the blood (e.g., galactosylated or nongalactosylated anti-id antibody) and those that could block the anti-DTPA(In) binding arm [e.g., DTPA(In), divalent-DTPA(In) peptide, and DTPA coupled to bovine serum albumin (BSA) or IgG]. When clearing agents were given 65 h after the IgG x Fab' conjugate (time of maximum tumor accretion for this bsMAb), (99m)Tc-IMP-192 levels in the blood were significantly reduced, but a majority of the peptide localized in the liver. Increasing the interval between the clearing agent and the time the peptide was given to allow for further processing of the bsMAb-clearing agent complex did not improve targeting. At the dose and level of substitution tested, galacosylated BSA-DTPA(In) was cleared too quickly to be an effective blocking agent, but BSA- and IgG-DTPA(In) conjugates were able to reduce the uptake of the (99m)Tc-IMP-192 in the blood and liver. Tumor/nontumor ratios compared favorably for the radiolabeled peptide using the IgG x Fab'/blocking agent combination and the F(ab')(2) x Fab' (no clea...

An experimental and theoretical evaluation of the influence of pretargeting antibody on the tumor accumulation of effector

“…Reports on a large number of these pretargeting studies have appeared in recent years (33 -36). Furthermore, if the dosage of effector is above that required to saturate the MORF -antibody in tumor, both the absolute and the percent tumor accumulation of the effector will depend on the antibody, because antibodies differ in tumor accumulation (28), affinity for their antigen or the effector (37), the disappearance rate by either internalization or shedding of their antigen complexes, and the accessibility in tumor for the effector (38,39).…”

Stably tethered multifunctional structures of defined composition made by the dock and lock method for use in cancer targeting

“…We have previously demonstrated the advantage of using a chemically prepared bispecific antibody with bivalent, instead of monovalent, binding to the tumor antigen for enhancing the amount and retention of radiotracer bound to the tumor (15). A recombinant bispecific trivalent construct, referred to as hBS14, with bivalent carcinoembryonic antigen (CEA) binding was produced in myeloma cell culture and performed very well as a pretargeting agent (16,17).…”

Improved Targeting of Pancreatic Cancer

“…The stability of the bsMAbs in fresh sterile-filtered mouse serum was analyzed over a period of 5 days, with no aggregation and/or degradation products evident by size-exclusion chromatography. In vitro and in vivo characteristics of the peptides, IMP-156 [Ac-Phe-Lys(DTPA)-Tyr-Lys(DTPA)-NH 2 ], for use with 111 In, and IMP-192 [Ac-Lys(DTPA)-Tyr-Lys(DTPA)-Lys(thiosemicarbazonyl-glyoxyl-cysteinyl-)-NH 2 ], for use with 99m Tc, have been reported (61,65). Each of the peptide haptens was divalent with respect to reactivity with MAb-m734 (antiindium-DTPA).…”