Uptake of radiolabeled anti-CEA antibodies in human colorectal primary tumors as a function of tumor mass

Inhomogeneous activity distribution of 177Lu-DOTA0-Tyr3-octreotate and effects on somatostatin receptor expression in human carcinoid GOT1 tumors in nude mice

The aim of this study was to investigate the activity distribution in neouroendocrine tumors after diagnostic, or therapeutic, amounts of [(177)Lu-DOTA(0)-Tyr(3)]-octreotate and to investigate how the activity distribution influences the absorbed dose. Furthermore, the activity distribution of a second administration of radiolabeled octreotate was studied. Nude mice with subcutaneously grown human midgut carcinoid (GOT1) were injected intravenously with different amounts of (177)Lu-octreotate. At different time points thereafter (4 h to 13 days), a second injection of [(111)In-DOTA(0)-Tyr(3)]-octreotate was given to estimate the somatostatin receptor (sstr) expression. The activity distribution in the tumors was then determined. Monte Carlo simulations with PENELOPE were performed for dosimetry. Fifty-one out of 58 investigated tumors showed a lower activity concentration in the peripheral part than in the central part of the tumor. The amount of activity injected, or time after administration, did neither influence the relative activity nor the sstr distribution in the tumor. After an initial down-regulation (at 4-24 h), there was an up-regulation of sstr (1.5-2 times, at 7-14 days). Monte Carlo simulations demonstrated an inhomogeneous absorbed dose distribution in the tumor using (177)Lu, with twice as high absorbed dose centrally than peripherally. The high activity concentration centrally and the up-regulation of sstr demonstrated will facilitate fractionated therapy using radiolabeled somatostatin analogues if similar results will be obtained also in patients. The inhomogeneous activity distribution in the tumor has to be taken into account when the absorbed dose distribution in tumor is calculated.

How can we improve antibody-based cancer therapy?

Monoclonal antibodies (mAbs) as a class of novel oncology therapeutics are demonstrating clinical efficacy as measured by tumor response (shrinkage in tumor size), and prolongations in progression-free survival (PFS) and overall survival (OS). However, clinical benefits are often limited to when antibodies are used in combination with chemotherapy or radiation modalities, with tumor responses only seen in a fraction of patients, and improvements in PFS and OS are incremental.1 The potential of mAbs and mAb constructs has yet to be fully exploited for maximal clinical benefit. New approaches to further improve the effectiveness of these mAb therapies include (1) selection of patients who may derive the most benefit based on the molecular characteristics of their tumors; (2) improvements in biodistribution to maximize delivery of mAbs to susceptible tumor cells; and (3) optimization of antibody immune effector mechanisms such as antibody-dependent cellular cytotoxicity (ADCC).

Noninternalizing monoclonal antibodies are suitable candidates for 125I radioimmunotherapy of small-volume peritoneal carcinomatosis

We have previously shown that, in vitro, monoclonal antibodies (mAbs) labeled with the Auger electron emitter (125)I are more cytotoxic if they remain at the cell surface and do not internalize in the cytoplasm. Here, we assessed the in vivo biologic efficiency of internalizing and noninternalizing (125)I-labeled mAbs for the treatment of small solid tumors.

METHODS

Swiss nude mice bearing intraperitoneal tumor cell xenografts were injected with 37 MBq (370 MBq/mg) of internalizing (anti-HER1) (125)I-m225 or noninternalizing (anti-CEA) (125)I-35A7 mAbs at days 4 and 7 after tumor cell grafting. Nonspecific toxicity was assessed using the irrelevant (125)I-PX mAb, and untreated controls were injected with NaCl. Tumor growth was followed by bioluminescence imaging. Mice were sacrificed when the bioluminescence signal reached 4.5 x 10(7) photons/s. Biodistribution analysis was performed to determine the activity contained in healthy organs and tumor nodules, and total cumulative decays were calculated. These values were used to calculate the irradiation dose by the MIRD formalism.

RESULTS

Median survival (MS) was 19 d in the NaCl-treated group. Similar values were obtained in mice treated with unlabeled PX (MS, 24 d) and 35A7 (MS, 24 d) or with (125)I-PX mAbs (MS, 17 d). Conversely, mice treated with unlabeled or labeled internalizing m225 mAb (MS, 76 and 77 d, respectively) and mice injected with (125)I-35A7 mAb (MS, 59 d) showed a significant increase in survival. Irradiation doses were comparable in all healthy organs, independently from the mAb used, whereas in tumors the irradiation dose was 7.4-fold higher with (125)I-labeled noninternalizing than with internalizing mAbs. This discrepancy might be due to iodotyrosine moiety release occurring during the catabolism of internalizing mAbs associated with high turnover rate.

CONCLUSION

This study indicates that (125)I-labeled noninternalizing mAbs could be suitable for radioimmunotherapy of small solid tumors and that the use of internalizing mAbs should not be considered as a requirement for the success of treatments with (125)I Auger electrons.

Antibody constructs in cancer therapy

Whereas over 85% of human cancers are solid tumors, of the 8 monoclonal antibodies (mAbs) currently approved for cancer therapy, 25% are directed at solid tumor surface antigens (Ags). This shortfall may be due to barriers to achieving adequate exposure in solid tumors. Advancements in tumor biology, protein engineering, and theoretical modeling of macromolecular transport are currently enabling identification of critical physical properties for antitumor Abs. It is now possible to structurally modify Abs or even replace full Abs with a plethora of Ab constructs. These constructs include Fab and Fab'(2) fragments, scFvs, multivalent scFvs (e.g., diabodies and tribodies), minibodies (e.g., scFv-CH3 dimers), bispecific Abs, and camel variable functional heavy chain domains. The purpose of the article is to provide investigators with a conceptual framework for exploiting the recent scientific advancements. The focus is on 2 properties that govern tumor exposure: 1) physical properties that enable penetration of and retention by tumors, and 2) favorable plasma pharmacokinetics. It is demonstrated that manipulating molecular size, charge, valence, and binding affinity can optimize these properties. These manipulations hold the key to promoting tumor exposure and to ultimately creating successful Ab therapies for solid tumors.

Modelling of metastatic cure after radionuclide therapy: Influence of tumor distribution, cross-irradiation, and variable activity concentration

The objective was to study the influence of tumor number and size, cross-irradiation from normal tissue, and of variable activity concentration on metastatic cure after radionuclide therapy. A model to calculate the metastatic cure probability (MCP) was developed, in which it was assumed that the tumor response was an exponential function of the absorbed dose. All calculations were performed for monoenergetic electron emitters with different energies (10-1000 keV). The influence of tumor size and number of tumors were investigated with different log uniform distributions; the basic tumor distribution consisted of tumors with 1, 10, ..., 10(11) cells. The influence of cross-irradiation was assessed by calculating MCP for various tumor-to-normal tissue activity concentration ratios (TNC). The influence of variable activity concentration between tumors was calculated by assuming that the activity concentration in tumors was an inverse power law function of tumor mass. The required activity concentration (C0.9) and absorbed dose (D0.9) to obtain MCP=0.9 was calculated in the different models. The C0.9 and D0.9 needed to obtain MCP were very high; more than 25 MBq/g and 80 Gy, respectively. The lowest C0.9 and D0.9 for equal activity concentration in the different tumor sizes were obtained for electron energies less than 80 keV. For higher energies the low absorbed energy fraction in small tumors will increase the required C0.9 and D0.9 markedly. Cross-irradiation from normal cells surrounding the tumor will cause sterilization of the smallest tumors and decrease the required C0.9 and D0.9 for higher electron energies. Assuming that the activity concentration decreased with increased tumor mass caused a marked increase in C0.9 and D0.9 in favor of higher electron energies. With the MCP model we demonstrated significant influence of the number of tumors, their size, TNC and variable activity concentration on MCP. The results are valuable when evaluating optimal choices for radionuclides for internal-emitter therapy.

Real-time assessment of intraperitoneal tumor growth in a rat model using CEA immunoscintigraphy

BACKGROUND

In most preclinical models, assessment of intraperitoneal tumor location and size require killing the animal. The dynamics of postoperative intraperitoneal tumor implantation and growth remain unclear. A noninvasive method allowing reliable in vivo, real-time assessment of tumor growth is desirable.

METHODS

An intraperitoneal tumor homograft using cultured CC531 colorectal cells was created by laparotomy in 24 Wistar Albino Glaxo rats. Eight additional rats were used as controls. Then, 10 MBq technetium 99m-labeled anticarcinoembryonic antigen (anti-CEA) monoclonal antibodies were administrated intravenously and radioactivity uptake was measured by using extracorporeal gamma counting at various time points. Subsequently, the animals were killed for tumor weighting. In 2 more groups of 8 animals, real-time, repeated measures were performed.

RESULTS

Correlation between gamma counting and tumor weight was highly significant (P <.001). The regression equation obtained by using the least squares method was: tumor weight (g) = 2.422 + 0.267 x counts. It was possible to obtain real-time tumor growth curves when repeated measurements of radioactivity were performed. At day 25, the predicted tumor weight was 8.49 +/- 0.76 g, the measured weight was 8.16 +/- 0.99 g.

CONCLUSIONS

Immunoscintigraphic measurements with technetium 99m anti-CEA antibodies are highly correlated with tumor weight in this model. As opposed to other tumor graft models based on autopsy findings, real-time monitoring is possible. This will allow dynamic studies of intraperitoneal tumor implantation and growth and will reduce the number of animals used in further studies.

Evaluation of an anti-carcinoembryonic monoclonal antibody suitable for immunoscintigraphy

An anti-carcinoembryonic antigen (CEA) monoclonal antibody (mAb 6D1. 1) was evaluated in vitro and in vivo to determine its suitability as a tracer for immunoscintigraphy of colorectal carcinomas. Determination of mAb affinity for CEA showed a constant of association of 0.63 +/- 0.11 x 10(9) M-1. Binding of technetium-99m (99mTc)-6D1.1, labeled by a direct method, to human cultured lineages was highly specific. Binding to only CEA-positive LS-174T cells resulted in a saturable curve inhibited by pre-incubation with unlabeled mAb. No binding at all was observed for the human lineages MeWo (melanoma) or ZR75-30 (breast carcinoma), neither of them expressing CEA cells. Intravenous injection of 99mTc-6D1.1 into nude mice xenografted with human LS-174T tumors resulted in planar images of excellent quality. Localization of an irrelevant mAb labeled with either 99mTc or iodine-125 was never observed in tumor masses. Biodistribution studies on excised tumoral tissue showed retention of 28.48% of the injected dose per gram of LS-174T tumor. The tumor-to-blood ratio was 3.46. The same analysis performed on the other three human xenografted tumors studied demonstrated that only the CEA-producing HT-29 (colorectal adenocarcinoma) retained 99mTc-6D1.1 while the other two (ZR75-30 and MeWo) did not. These data demonstrate that this mAb is an adequate tool for targeting CEA-expressing tumors in experimental models.

Tumor uptake of intravenously administered radiolabeled antibodies in ovarian carcinoma patients in relation to antigen expression and other tumor characteristics

To study factors that possibly influence the heterogeneous tumor uptake of radiolabeled antibodies, tissues from 34 ovarian-carcinoma patients were obtained 2 to 8 days after i.v. injection with radiolabeled murine OV-TL3 or chimeric MOv18 (cMOv18). The tumor uptake and the ratio of tumor to normal tissue (T/NT) were studied in relation to the histopathological classification, prior treatment, site of tumor, time interval, antigen expression, volume percentage of (malignant) epithelium in the tumor tissue, and the size of the tumor. The results of immunoscintigraphy were also included. In addition, autoradiography using storage phosphor technology was performed on tissue sections from patients injected with iodine-labeled cMOv18. Tumor uptake varied largely, not only between patients, but also between tumor deposits within the same patient. Uptake of OV-TL3 F(ab')2 was higher than of cMOv18 F(ab')2, but the T/NT ratios were similar. The antibody uptake was positively correlated with the pattern of antigen expression and inversely correlated with the time interval between injection and surgery. No correlation was observed with any of the other factors studied. The visibility with immunoscintigraphy was related to the size of the detected lesion, but not to the other factors studied. Autoradiography showed that antibodies preferentially localized in areas with cancer cells, which were immunohistochemically positive for MOv18. In areas with weak antigen expression, autoradiography revealed less activity. The antigen expression by the tumor is an important factor for estimation of the tumor uptake of radiolabeled antibodies.

Tumor imaging with monoclonal antibodies

Immunoscintigraphy offers the possibility of specifically targeting human tumors, but the complexity of the human immune system, as well as tumor-related phenomena, prevent monoclonal antibodies from reaching a large number of tumor cells in which they can interact with the antigen. Possible ways to overcome these problems are the use of small fragments, in particular those of genetically engineered humanized antibodies including single immunoglobulin-variable domains, as well as techniques to label the antibody in vivo after a sufficient amount has been taken up by the tumor and the remainder has been eliminated. Despite the low absolute tumor uptake, results of European studies, presently available radiolabeled monoclonal antibodies in gastrointestinal and ovarian cancers yield an average sensitivity of more than 70% with an average specificity of more than 80%, even in otherwise occult tumors. Because of possible tracer uptake in normal liver, the detection rate of liver metastases varies from less than 10% to more than 90%. For the detection of local recurrence in the pelvis, immunoscintigraphy has been found to be more accurate than methods that are based on the imaging of structural changes. Fusion of morphological and functional images might improve the early detection of recurrent and metastatic disease. In melanoma, another tumor that has been extensively studied in Europe, similar results have been obtained, whereas only few data are presently available for other tumors (especially lung and breast cancer).

Differences in biodistribution of the anti-(carcinoembryonic antigen) murine monoclonal antibody CE-25, its F(ab')2 fragment and its intact mainly human chimeric form CE 4-8-13. Dependence on tumour size and amount of antibody injected

The effect of the size of the tumour and the amount of antibody injected on the biodistribution of a family of radioiodinated antibodies was studied. The intact mouse anti-(carcinoembryonic antigen) (anti-CEA) monoclonal antibody CE-25, its F(ab')2 fragment and the intact human-mouse chimeric from CE 4-8-13 were evaluated in a model system using the human CEA-producing colon xenograft T 380 grown in nude mice. The relative retention (the percentage of the injected dose per gram of tissue), of mouse mAb and F(ab')2 in tumour and most normal tissues 1 day after injection was independent of the antibody dose; after 4 days the mAb values increased with increasing antibody dose. The relative retention of chimeric mAb increased with increasing antibody dose 1 day after injection and also slightly after 4 days. The relative retention in tumour tissue was lower in bigger xenografts for all antibodies. The relative retention of mouse mAb in small tumours increased from day 1 to day 4; for chimeric mAb this value decreased. In normal tissues the relative retention of mouse mAb decreased from day 1 to day 4, but the relative retention of chimeric mAb in normal tissue dropped rapidly and changed little afterwards. Thus the biokinetics of antibodies is "species"-dependent: foreign, mainly human, chimeric antibody clears faster from normal mouse tissue than mouse antibody and reaches lower concentrations.

Radio-immunotherapy dosimetry with special emphasis on SPECT quantification and extracorporeal immuno-adsorption

Results from therapeutic trials with radiolabelled monoclonal antibodies are difficult to compare, because of lack of accurate macroscopic and microscopic dosimetry for both tumours and normal tissues. Requirements for such a dosimetry are covered in the paper. Accurate in vivo dosimetric measurement techniques for verification of calculated absorbed doses are also needed to verify treatment planning. In the review, important topics related to dosimetry in therapeutic trials in RIT are covered, such as, absorbed-dose calculations and activity-quantification techniques for planar imaging and SPECT. The latter is particularly discussed, including a summary of different correction techniques. Absorbed-dose calculations and treatment-planning techniques are also discussed. Possible ways of enhancing the therapeutic ratio are reviewed, especially the novel technique with extracorporeal immuno-adsorption. The review could form the basis of the development of future treatment-planning protocols and for dosimetry calculations in radio-immunotherapy, considering some of the most important parameters for approaching an accurate in vivo dosimetry.