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The therapeutic potential of radiolabeled antibodies is usually evaluated in experimental animal models bearing s.c. xenografts. We have established a micrometastatic model of the GW-39 human colonic carcinoma in the nude mouse lung (J. Natl. Cancer Inst., 83: 627-632, 1991) and presented preliminary findings on the efficacy of a 131I-anticarcinoembryonic antigen (CEA) antibody in this model. We now extend our observations on the use of radioiodinated labeled monoclonal antibodies (MAbs) to treat multiple small tumor nodules. Biodistribution and dosimetry analysis was performed for intact and F(ab')2 of NP-4 anti-CEA IgG, Mu-9 anti-colon-specific antigen IgG, isotype-matched irrelevant anti-AFP IgG, and intact MAb 34A anti-lung endothelial IgG antibody. Comparisons were made for rad dose delivered to small s.c. tumors, normal lung, lung with tumor nodules, and isolated tumor nodules. Survival curves were generated for tumor-bearing animals treated 1, 7, or 14 days after tumor cell implantation with these antibodies using the maximal tolerated dose for intact antibodies (275 microCi) and for F(ab')2 fragments (1.2 mCi). The studies established the following observations: (a) in contrast to previous results in a bulky tumor model in hamsters, intact antibodies are more therapeutic than MAb fragments for both NP-4 and Mu-9; (b) tumor nodule size, even on the microscopic level, affects therapeutic outcome; antibodies were more effective when administered 7 days postimplantation (mean nodule diameter, 150 microns) compared with treatment 14 days postimplantation (mean nodule diameter, 750 microns); (c) administration of radioiodinated Mu-9 was exquisitely effective on single avascular tumor cells that had seeded in lung; irrelevant antibody was minimally radiotoxic; (d) as in the bulky disease model, the anti-colon-specific antigen p antibody delivers a higher rad dose than the anti-CEA antibody and is significantly more therapeutic in the micrometastasis model; (e) a higher affinity anti-CEA antibody (MN-14) recognizing the same epitope on CEA as NP-4 was equally therapeutic; (f) the use of MAb directed against the lung endothelium was not as therapeutic as a tumor-associated antibody; and (g) all tumor-associated antibodies were more efficacious than administration of the maximal tolerated dose of 5-fluorouracil and leucovorin in this human tumor-xenograft model. These results provide further support for the use of radioimmunotherapy in the handling of minimal disease, probably as part of an adjuvant treatment regimen.

We evaluated the accretion of 131I-labeled NP-4 anticarcinoembryonic antigen (CEA) into 4 size-matched human colonic carcinomas grown s.c. in nude mice. Antibody uptake for LS174T and GW-39 tumors was relatively high (19 to 23% ID/g on day 3), whereas moderate uptake was seen in the Moser tumor (7.5% on day 3) and low uptake was detected in the GS-2 tumor (1.8% on day 3). Blood clearance of radioantibody was twice as fast in mice with GS-2 tumors than in mice with GW-39, LS174T or Moser tumors. Seven physiological parameters that might influence radioantibody accretion were evaluated in order to better understand the differences in observed tumor targeting: vascular volume, blood flow rate, vascular permeability, tumor antigen content, serum antigen content and complexation of radioantibody, intratumoral antigen distribution, and intracellular antigen distribution. Although marked variability in vascular physiology, antigen content and antibody complexation of the 4 tumors grown in the same host and site existed, it was insufficient to explain the differences in antibody uptake. However, intra-tumoral distribution of antigen, and sub-cellular accessibility of antigen for radioantibody were important considerations. GS-2 tumors are well differentiated and have polarized cells. CEA in GS-2 is largely inaccessible to radioantibody; most of the antigen is located in the lumen of the glands or on the apical surface of gland cells and most of the antibody distributes to the stromal region on the basolateral surface. The low antibody targeting in GS-2 could therefore be explained by restricted intra-tumor accessibility of antibody. Scatchard analysis of NP-4 binding to Moser cells under non-internalizing and internalizing conditions revealed that 90% of the antigen is found within the cell, unavailable to bind with the NP-4 antibody, which is slow to internalize. In contrast, CEA in LS174T cells was almost entirely accessible. The reduced antibody targeting to Moser xenografts might therefore, be explained by restricted antibody accessibility at the cellular level.