Clinical application of monoclonal antibody-drug conjugates for immunotargeting chemotherapy of colorectal carcinoma

Challenges and Opportunities to Develop Enediyne Natural Products as Payloads for Antibody-Drug Conjugates

Calicheamicin, the payload of the antibody-drug-conjugates (ADCs) gemtuzumab ozogamicin (Mylotarg^®) and inotuzumab ozogamicin (Besponsa^®), belongs to the class of enediyne natural products. Since the isolation and structural determination of the neocarzinostatin chromophore in 1985, the enediynes have attracted considerable attention for their value as DNA damaging agents in cancer chemotherapy. Due to their non-discriminatory cytotoxicity towards both cancer and healthy cells, the clinical utilization of enediyne natural products relies on conjugation to an appropriate delivery system, such as an antibody. Here we review the current landscape of enediynes as payloads of first-generation and next-generation ADCs.

[Analysis of Target Molecules towards Anti-cancer Therapeutic Antibodies]

Target molecules of existing anti-cancer therapeutic monoclonal antibodies (mAbs) are divided into 1) receptor-type tyrosine kinases, such as human epidermal growth factor receptor (HER) family, 2) differentiation antigens, such as CD20 (Rituxan target), 3) angiogenesis-related molecules, and 4) immune checkpoint molecules (PD-1, etc.). We have recently reported a novel therapy targeting lymphangiogenesis, but not angiogenesis, using an anti-LYVE-1 (lymphatic vessel endothelial hyaluronan receptor 1) mAb. At present, many transporters are not considered to be target molecules for the cancer therapy; however, our study strongly suggested that the inhibition of cancer metabolism by mAbs against amino acid transporters will play a significant role in future cancer therapies. Most anti-cancer therapeutic mAbs bind cell-surface molecules on viable cancer cells: therefore, it is necessary to produce mAbs recognizing epitopes on the extracellular domains of native and non-denatured proteins. We concluded that viable cancer cells or cells transfected with cDNA encoding target proteins are suitable immunogens for the production of anti-cancer therapeutic mAbs. We introduce our efforts to develop seeds for therapeutic mAbs using whole cancer cells and transfectants as the immunogen. As many target candidates in the future are multi-pass membrane proteins, such as 12-pass amino acid transporter proteins belonging to the solute carrier (SLC) family, and their possible immunogenic extracellular regions are small, the production of specific mAbs is highly difficult. In this review, we summarize the successful preparation and characterization of mAbs recognizing the extracellular domain of oncoproteins, including transporters.

Targeting strategies for superparamagnetic iron oxide nanoparticles in cancer therapy

Among various nanoparticles, superparamagnetic iron oxide nanoparticles (SPIONs) have been increasingly studied for their excellent superparamagnetism, magnetic heating properties, and enhanced magnetic resonance imaging (MRI). The conjugation of SPIONs with drugs to obtain delivery nanosystems has several advantages including magnetic targeted functionalization, in vivo imaging, magnetic thermotherapy, and combined delivery of anticancer agents. To further increase the targeting efficiency of drugs through a delivery nanosystem based on SPIONs, additional targeting moieties including transferrin, antibodies, aptamers, hyaluronic acid, folate, and targeting peptides are coated onto the surface of SPIONs. Therefore, this review summarizes the latest progresses in the conjugation of targeting molecules and drug delivery nanosystems based on SPIONs, especially focusing on their performances to develop efficient targeted drug delivery systems for tumor therapy. STATEMENT OF SIGNIFICANCE: Some magnetic nanoparticle-based nanocarriers loaded with drugs were evaluated in patients and did not produce convincing results, leading to termination of clinical development in phase II/III. An alternative strategy for drug delivery systems based on SPIONs is the conjugation of these systems with targeting segments such as transferrin, antibodies, aptamers, hyaluronic acid, folate, and targeting peptides. These targeting moieties can be recognized by specific integrin/receptors that are overexpressed specifically on the tumor cell surface, resulting in minimizing dosage and reducing off-target effects. This review focuses on magnetic nanoparticle-based nonviral drug delivery systems with targeting moieties to deliver anticancer drugs, with an aim to provide suggestions on the development of SPIONs through discussion.

Monoclonal antibody conjugated to gadolinium as a contrast agent for magnetic resonance imaging of human rectal carcinoma

BACKGROUND AND OBJECTIVES

Local disease is the most frequent recurrence pattern of rectal carcinoma, and its prognosis is not good. One reason for the poor prognosis is the difficulty of making the diagnosis at an early stage. To detect local recurrence as early as possible, we produced the monoclonal antibody, A7-gadolinium (Mab A7-Gd), conjugate as a contrast agent for magnetic resonance imaging to distinguish between carcinoma and normal tissue.

METHODS

We examined the in vitro immunoreactivity of Mab A7 coupled to Gd by chelate, and stability of Mab A7-EDTA-Gd in human serum. Its in vivo distribution in nude mice with human colorectal carcinoma was also examined.

RESULTS

Mab A7-Gd retained binding activities that were nearly identical to intact Mab A7. Mab A7-Gd was stable in human serum. More radiolabeled Mab A7-Gd accumulated in the tumor than normal mouse IgG-Gd. Both Mab A7-Gd and normal mouse IgG-Gd disappeared from blood linearly over time. Accumulation levels in normal tissues decreased linearly over time but were lower than those in tumors.

CONCLUSIONS

Mab A7 conjugated to gadolinium selectively accumulated in the tumor. Our results suggest that it is potentially suitable as a contrast agent for MR imaging to detect local rectal carcinoma recurrence.

Targeted drug conjugates: principles and progress

Reports of targeting drugs using antibodies have appeared in the literature since 1958, but exciting clinical results in this field have only been reported in the last few years. Progress in this field has occurred largely through an understanding how drug-immunoconjugates work. The objective of this review is to draw together the fundamental principles on which this field of work is based, to examine the evidence supporting those principles, and the effectiveness and selectivity of targeted drug conjugates. The activity of many drug-immunoconjugates can now largely be accounted for by the underlying principles. Excellent development work, both with conventional anti-cancer agents and very potent drugs have led to a number of interesting clinical trials. In the best Phase I and II trials, good evidence of effectiveness have been reported, which suggest that drug-immunoconjugates may now be heralding a new era for chemotherapy.

In vitro and preclinical studies of targeted alpha therapy (TAT) for colorectal cancer

INTRODUCTION

Effective targeted cancer therapy requires high selectivity and cytotoxicity of the labelled product. We report the preparation and testing of anticolorectal cancer monoclonal antibody c30.6 radioimmunoconjugates (RIC) labelled with alpha-emitting Bismuth-213 and positron emitting Terbium-152 using two chelators, viz. Cyclic dianhydride of diethylenetriaminepentacetic acid (DTPA) and CHX-A" (a DTPA derivative).

METHODS

Selectivity and stability of the RIC were tested in vitro (flow cytometry) and in vivo (biodistribution, organ/tumour uptake and retention). Cytotoxicity assays were carried out using tritiated thymidine uptake (inhibition of DNA synthesis) and MTS assay.

RESULTS

High labelling efficiency (ranging between 89 and 91%) and stability over 2-5 half-lives of the isotopes were seen. Kidney retention was not seen in contrast to high uptake and retention of both conjugates in tumours. Flow cytometry studies showed high specificity of the antibody before and after labelling and this unchanged targeting behaviour was reflected in cytotoxicity assays. These assays showed that only alpha-labelled antibody could selectively kill the cancer cells for activities as low as 2-3 microCi. The study also revealed that free isotopes or isotopes bound to nonspecific antibodies did not kill cancer cells.

CONCLUSION

The stability of the RICs and outstanding cytotoxicity of the alpha emitter, together with no kidney retention and high tumour uptake and retention of the radiolabel, offers a new approach for the potential control of colorectal cancer.

Active drug targeting with immunoconjugates to choroidal neovascularization

PURPOSE

Active drug targeting with monoclonal antibody to neovascular vessels may be a potential treatment for choroidal neovascularization (CNV) in age-related macular degeneration (AMD). Endoglin (CD105) is a proliferating endothelial cell marker with excellent potential for targeting. The goals of this study were to investigate the expression of CD105 in CNV membranes surgically excised from patients with AMD and CNV lesions induced by intense laser photocoagulation in a cynomolgus monkey and to evaluate the in vitro effect of immunoconjugates on endothelial cells.

METHODS

CNV membranes were surgically excised from 10 patients with AMD. Experimental CNV was induced by intense laser photocoagulation in a cynomolgus monkey. Immunolocalization of CD105 on frozen sections of CNV lesions was studied by immunohistochemical evaluation. Anti-von Willebrand's factor antibody was used as an endothelial cell marker. The cytotoxic effect of immunoconjugates of anti-CD105 monoclonal antibody and dextran binding mitomycin C on human umbilical vein endothelial cells (HUVECs) was evaluated in vitro.

RESULTS

Endothelial cells demonstrated strong immunoreactivity of CD105 in all surgically excised CNV membranes. In the monkey eye, CD105-positive cells were detected only in CNV lesions but not in normal chorioretinal tissues. Immunoconjugates with anti-CD105 monoclonal antibody showed a specific inhibitory effect on proliferating HU-VECs.

CONCLUSIONS

These results suggest that anti-CD105 monoclonal antibody-mediated drug targeting has a potential to treat CNV in AMD.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

Iodine-131 human-mouse chimeric Fab monoclonal antibody A7 guided surgery for colorectal cancer patients: a pilot study

This study was conducted to determine the clinical usefulness of radioimmunoguided surgery (RIGS) using human-mouse chimeric Fab monoclonal antibody A7 (ch-Fab-A7) for colorectal cancer patients. Ten colorectal cancer patients were given iodine-131-labeled ch-Fab-A7 intravenously (i.v.) 2 to 7 days prior to RIGS. The RIGS was carried out using a portable gamma detecting probe (GDP). Tumor localization was identified by GDP intraoperatively in 4 of the ten patients, while liver metastasis and lymph node metastasis were identified in 2 patients and 1 patient, respectively. The GDP revealed tumor/surrounding tissue radio(gamma)count ratios of 1.5 or greater in 8 of the ten resected tumors. Although RIGS using ch-Fab-A7 is a promising tool for intraoperatively identifying the tumor localization of colorectal cancer, 125I, rather than 131I, should be used as a tracer for RIGS to enhance the accuracy of ch-Fab-A7.

Rhenium-186-mercaptoacetyltriglycine-labeled monoclonal antibody for radioimmunotherapy: in vitro assessment, in vivo kinetics and dosimetry in tumor-bearing nude mice

Stability and immunoreactivity of 186Re-labeled monoclonal antibody were examined, and its in vivo kinetics was investigated in tumor-bearing Balb/c nu/nu female mice to assess the feasibility of using it in radioimmunotherapy (RIT). A murine IgG1, A7, against a 45 kD glycoprotein in human colon cancer was radiolabeled with 186Re by using a chelating method with a mercaptoacetyltriglycine (MAG3). 186Re-MAG3 complex was conjugated to A7 after esterification of 186Re-MAG3 with tetrafluorophenol (TFP). The efficiency of 186Re-MAG3-TFP production and the labeling efficiency of A7 were 51-59% and 57-60%, respectively. Immunoreactivity of purified 186Re-MAG3-A7 was 68.2% at infinite antigen excess. In 0.9% NaCl at 4 degrees C, the radioactivity (12.7 MBq/mg, 3.55 MBq/ml) dissociated with time from 186Re-MAG3-A7 as a small molecular weight moiety because of autoradiolysis. The addition of ascorbic acid, 5 mg/ml, as a radioprotectant or storage at -80 degrees C could effectively prevent the radiolysis of 186Re-MAG3-A7 for 7 days. Immunoreactivity of 186Re-MAG3-A7, 6.70 MBq/mg (6.66 MBq/ml), stored in the presence of ascorbic acid was well retained up to 8 days after the preparation. In colon cancer xenografted mice, 31.0% of the injected dose/g of 186Re-MAG3-A7 had accumulated in the tumors at 24 h postinjection. Estimated radiation dose to tumors was 14.9 cGy/37 kBq up to 8 days postinjection which was 12-fold greater than the whole-body radiation dose. These in vivo characteristics were superior to those of A7 labeled with radioiodine, affording greater therapeutic ratios than 131I-A7. Because of the better image quality of 186Re-MAG3-A7 as well as more favorable dosimetry, 186Re-MAG3-A7 would be a better choice for RIT of colon cancer than 131I-A7. These results indicated the feasibility of RIT with 186Re-MAG3-A7, though the prevention of radiolysis of the labeled antibody should be considered.

Changes in expression of the antigen recognized by monoclonal antibody A7 in human pancreatic carcinoma cells following exposure to anticancer agents

Techniques which can increase the expression level of tumor-associated antigens may improve immunotargeting therapy. We studied the reactivity of MAb A7 toward an antigen expressed on the surface of the human pancreatic cancer cell line HPC-YS after treatment with various antitumoral agents. When we applied 1 microg/ml mitomycin C (MMC) or 0.1 microg/ml neocarzinostatin (NCS) for 1 h, A7 recognizing antigen expression was enhanced until 24 h after the treatments. At a dose that completely suppressed cell growth, increased antigen expression was maintained for 96 h. Therefore, this study suggests that the combined application of an anticancer drug and MAb A7 may be useful for immunotargeting chemotherapy.

Diagnostic and therapeutic applications of monoclonal antibodies in colorectal cancer

PURPOSE

The study contained herein was undertaken to review and summarize the current literature on diagnostic and therapeutic applications of monoclonal antibodies in colorectal cancer.

RESULTS

Limitations of traditional imaging techniques have encouraged development of targeted imaging strategies using radiolabeled monoclonal antibodies. Diagnostic immunoscintigraphy can detect lesions not identified by conventional imaging modalities, although it has not proven useful in the management of primary colorectal cancers and in hepatic metastases. Immunoscintigraphy shows promise in cases of local recurrence and rising carcinoembryonic antigen values; however, the impact of immunoscintigraphy on clinical outcomes and cost-effectiveness remains unproven. Radioimmunoguided surgery has been advocated as a method of more accurately detecting tumor extension and accomplishing radical resection. The technique remains controversial, and its use is not widespread. With respect to therapeutic applications, immunotherapy has most often been investigated in the setting of advanced stage disease. Results in this setting have been poor. In contrast, adjuvant immunotherapy after resection of Dukes C carcinoma has achieved convincing results, with improvements in survival comparable with that of adjuvant chemotherapy. Adjuvant trials are now under way to examine the effectiveness of monoclonal antibodies in the postoperative treatment of early-stage (II) tumors and the combination of monoclonal antibodies and chemotherapy in advanced-stage (III) tumors. Bispecific antibodies, or immunoconjugates with cytokines or toxins, represent additional areas of interest and future investigations.

CONCLUSIONS

At present, immunoscintigraphy is not sufficient to determine, by itself, resectability of colorectal tumor and has limited usefulness in select cases of recurrent cancer and possibly in cases of rising carcinoembryonic antigen values. Immunotherapy with monoclonal antibodies as a postoperative adjuvant treatment shows promise and is currently being investigated in national trials.

Targeted chemotherapy in mice with peritoneally disseminated gastric cancer using monoclonal antibody-drug conjugate

The murine monoclonal antibody A7 (MAb A7) is reactive against most human gastric cancer cell lines. Using a nude mouse peritoneal dissemination model of human gastric cancer, we investigated targeted chemotherapy using a conjugate of neocarzinostatin (NCS) with MAb A7 (A7-NCS). After demonstrating cytotoxicity of the complex against the human gastric cancer cell line MKN45 in vitro, we intraperitoneally injected A7-NCS, NCS or saline into nude mice bearing peritoneally disseminated human gastric cancer. A7-NCS inhibited peritoneal dissemination significantly more effectively than NCS. MAb A7 may prove to be an effective carrier for antineoplastic drugs in patients with peritoneal dissemination of gastric cancer.

Decreased renal accumulation of biotinylated chimeric monoclonal antibody-neocarzinostatin conjugate after administration of avidin

Murine monoclonal antibodies (mAbs) such as A7 administered to humans induce a human anti-mouse antibody response. Moreover, because Fab fragments of mAbs are able to penetrate target tumors easily, they may be more suitable than intact mAb to be carriers of anticancer agents such as neocarzinostatin (NCS), which are rapidly inactivated in the blood. To address these problems, chimeric A7 Fab fragment-NCS conjugate (chA7Fab-NCS) was produced. However, large amounts of 125I-labeled chA7Fab-NCS accumulate in the kidney and can lead to renal dysfunction. To decrease renal accumulation of chA7Fab-NCS, chA7Fab was biotinylated and administered with a subsequent injection of avidin. Human pancreatic carcinoma-bearing nude mice were injected with 125I-labeled biotinylated chA7Fab-NCS with or without subsequent administration of avidin. The accumulation of 125I-labeled biotinylated chA7Fab-NCS in tissue samples was measured at appropriate time intervals. 125I-labeled biotinylated chA7Fab-NCS was cleared more rapidly from the blood and the kidney with the administration of avidin than without it. There was no difference between tumor accumulation in these groups. The tumor/blood ratio of radioactivity of 125I-labeled biotinylated chA7Fab-NCS was significantly higher with subsequent administration of avidin than without avidin. The administration of biotinylated chA7Fab-NCS followed by avidin may enhance safety and permit the administration of larger doses of NCS without the subsequent development of renal failure. A larger amount of 125I-labeled biotinylated chA7Fab-NCS was retained in the liver and spleen with the subsequent administration of avidin than without avidin.

Targeting immunotherapy using the avidin-biotin system for a human colon adenocarcinoma in vitro

A new method of targeting immunotherapy using the avidin-biotin system in vitro was investigated. Both an anti-carcinoembryonic antigen monoclonal antibody (anti-CEA MAb) and an anti-cancer drug, neocarzinostatin (NCS), were biotinylated. A human colon adenocarcinoma cell line (LoVo) was immunized with biotinylated anti-CEA MAb; avidin was added, and the cell line was incubated with various concentrations of biotinylated NCS for either 72 h or 7 min. In the incubation for 72 h, the IC50 was similar (approximately 0.45 microgram/ml) for biotinylated NCS for LoVo cells immunized with biotinylated anti-CEA MAb and those without immunization. In the incubation for 7 min, the IC50 (concentration producing 50% cytotoxicity) of biotinylated NCS for LoVo cells immunized with biotinylated anti-CEA MAb (0.35 microgram/ml) was five times less than that of non-immunized LoVo cells (1.8 micrograms/ml). Thus the present system has the potential to reduce the dosage of anti-cancer drugs needed, and this strategy seems likely to be a valuable clinical tool in targeting immunotherapy.

Sensitivity of monoclonal antibodies to carcinoembryonic antigen, tissue polypeptide antigen, alpha-fetoprotein, carbohydrate antigen 50, and carbohydrate antigen 19-9 in the diagnosis of colorectal adenocarcinoma

PURPOSE

This study was designed to establish the sensitivity of monoclonal antibodies to carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), tissue polypeptide antigen (TPA), carbohydrate antigen 50 (CA 50), and carbohydrate antigen 19-9 (CA 19-9) and the efficacy of the joint determination of several tumor markers, as well as the dynamics of postoperative normalization of each marker in the absence of recurrence.

MATERIALS AND METHODS

A prospective study was carried out in 100 patients subjected to surgical resection of colon adenocarcinoma. Serum concentrations of these markers were determined the day before surgery and seven days, two months, and six months after surgery.

RESULTS

The results demonstrate that sensitivity increased as the disease spread and that CA 19-9 was the most sensitive tumor marker. The rate of false negatives was 40 percent for Dukes Stage A lesions, 19 percent for Dukes Stage B, 7 percent for Dukes Stage C, and 0 percent for Dukes Stage D. Determination of two markers (CA 19-9 and CEA) provided the greatest sensitivity in Stages A and D tumors (60 percent and 100 percent, respectively); the incidence did not change when measurements of other antigens were associated. For Stages B and C, determination of at least three markers was necessary, the association of CEA, TPA, and CA 19-9 being that which showed the greatest sensitivity, 78 percent and 91 percent, respectively.

CONCLUSIONS

It would be advisable to include monoclonal antibody determination of CEA, TPA, and CA 19-9 in the diagnosis of adenocarcinoma, despite the fact that ultimate sensitivity will depend on the degree of tumor extension or on the presence of metastasis.

The effect of intravenous and intra-tumoural chemotherapy using a monoclonal antibody-drug conjugate in a xenograft model of pancreatic cancer

In order to investigate the efficacy of the intra-tumoural administration of an anticancer drug-monoclonal antibody conjugate in athymic nude mice bearing xenografts of a human pancreatic carcinoma, we examined the clearance of the murine monoclonal antibody A7 from the xenografts after intravenous or intra-tumoural administration and measured the antitumour effect of neocarzinostatin conjugated to MAb A7 following intravenous or intra-tumoural injection. Compared with 125I-labelled normal mouse IgG, a larger amount of 125I-labelled A7 remained in the tumour after both intravenous and intra-tumoural injection, and a significantly larger amount of 125I-labelled A7 remained in the tumour after intra-tumoural injection than that after intravenous injection. Moreover, a larger amount of 125I-labelled A7-NCS localized in the tumour after intra-tumoural injection than that after intravenous injection. Neocarzinostatin conjugated to MAb A7 showed greater activity against human pancreatic cancer than neocarzinostatin alone after both intravenous and intra-tumoural administration. Tumour growth was suppressed completely by the intra-tumoural administration of A7-NCS at a dose that did not suppress tumour growth via the intravenous route. These observations suggest that the intra-tumoural injection of neocarzinostatin conjugated to MAb A7 offers promise in treating pancreatic carcinoma.

In vitro reactivity and in vivo biodistribution of the monoclonal antibody A7 using human gastric carcinoma cell lines

The monoclonal antibody (MAb) A7 has been used to treat patients with colorectal or pancreatic carcinoma with encouraging results. We therefore determined if MAb A7 would also react with gastric carcinoma cell lines. MAb A7 reacted with seven of eight gastric carcinoma cell lines tested. The intensity of the reaction, measured by flow cytometry, was equal to that of WiDr (colon) and HPC-YS (pancreas) cell lines. In nude mice bearing xenografts of the MAb A7-reactive gastric cancer line MKN45, the percentage injected dose of MAb A7 per g of tumour tissue on day 7 was 9.79; this value was 77% of that on day 1. The in vivo tumour-to-blood ratio of MAb A7 was 2.77 on day 7. Therefore, MAb A7 has long-term retention at binding sites as well as a high probability, high intensity and high specificity of reactivity against gastric cancer, which make it an ideal drug carrier for immunotargeted chemotherapy and immunodiagnosis.

Biodistribution of neocarzinostatin conjugated to chimeric Fab fragments of the monoclonal antibody A7 in nude mice bearing human pancreatic cancer xenografts

In this study, we conjugated chimeric Fab fragments of the monoclonal antibody (MAb) A7, which reacts with pancreatic cancers, to the antitumor drug neocarzinostatin (chA7Fab-NCS) and intravenously injected 125I-labeled chA7Fab-NCS into nude mice bearing a human pancreatic cancer xenograft. We compared the tumor localization of 125I-labeled chA7Fab-NCS with that of conventional 125I-labeled A7-NCS, which was produced by conjugation of MAb A7 and NCS. 125I-Labeled chA7Fab-NCS accumulated in the tumor earlier than 125I-labeled A7-NCS, and significantly larger amounts of 125I-labeled chA7Fab-NCS had accumulated in the tumor 1 hour after injection. The results suggest that chA7Fab may be a suitable carrier for NCS in immunotargeting therapy against pancreatic cancer.

Production, binding and cytotoxicity of human/mouse chimeric monoclonal antibody-neocarzinostatin conjugate

A human/mouse chimeric Fab monoclonal antibody A7 (chFabA7) was covalently coupled to neocarzinostatin (NCS) by the SPDP method at various chFabA7:NCS substitution ratios. The antigen-binding activity of the conjugate, examined by ELISA using fixed antigen-positive colon cancer cells, was identical to that of the parent chFabA7 when one mole of NCS was conjugated, but was reduced with 2 or 3 moles of conjugated NCS. By means of a colony-forming assay, the cytocidal effect of the conjugate on antigen-positive cancer cells was found to be stronger than that of free NCS, whereas in antigen-negative cancer cells it was similar to that of free NCS. This effect was attenuated by adding an excess amount of monoclonal antibody A7. These findings indicate that the conjugate has an antigen-specific cytocidal action, and thus chFabA7-NCS is a promising tool for targeting cancer chemotherapy.

Follow-up study of patients treated with monoclonal antibody-drug conjugate: report of 77 cases with colorectal cancer

A total of 77 patients with advanced colorectal cancer, including postoperative patients with liver, lung and peritoneal metastases, were treated with single or multiple injections of monoclonal antibody A7-neocarzinostatin (A7-NCS). A follow-up study of the patients treated with A7-NCS was done and the clinical outcome was compared with that of patients given other chemotherapies. In the postoperative patients with liver metastasis, the A7-NCS treatment prolonged survival time when compared with systemic administration of anticancer drugs, while it showed a similar survival time to chemoembolization using multiple anticancer agents suspended in a lipid contrast medium. Among the patients who underwent surgical resection of primary cancer, with or without liver metastasis, there was no difference in overall 5-year survival rate between the group treated with A7-NCS and the group treated with the other chemotherapies. However, the survival time of the patients treated with A7-NCS was longer than that of the patients treated with the other chemotherapies. In addition, the patients given a higher dose of A7-NCS had a longer survival time than the patients given a lower dose of A7-NCS. Human anti-mouse antibody was detected in all the A7-NCS-treated patients examined. There were no serious side effects in any of the patients given A7-NCS. Thus, this study indicates that the A7-NCS treatment is safe and useful for colorectal cancer patients, though some problems remain, such as optimization of injection dose, route, interval, etc., and overcoming human anti-mouse antibody development.

Enhanced tumor localization of radiolabeled Fab fragments of monoclonal antibody A7 in nude mice bearing human pancreatic carcinoma xenografts

Much recent research has been directed toward the use of monoclonal antibodies (MAb) for the immunodetection of solid tumors. In pancreatic cancer, the results of conventional immunoscintigraphy using intact MAb remain disappointing. Clear immunoscintigraphy with radiolabeled MAb requires a high tumor tissue/blood ratio of radioactivity and a low normal tissue/blood ratio of radioactivity. In this study, 125I-labeled Fab fragments produced by papain digestion of MAb A7 were injected intravenously into nude mice bearing a human pancreatic cancer (HPC-YS) xenograft previously shown to react specifically with MAb A7. The radioactivity of tumors and normal organs was subsequently measured. The tumor tissue/blood ratio of 125I-labeled Fab fragments of MAb A7 was 1.00 +/- 0.24 and 9.68 +/- 2.54 at 2 and 24 h after injection, respectively. The tumor tissue/blood ratio of radioactivity was significantly higher than those of normal organs at 24 h after injection. Moreover, the tumor tissue/blood ratio of 125I-labeled Fab fragments of MAb A7 was greater than that of intact MAb A7, although the 125I-labeled Fab accumulation level was much less than that of 125I-labeled intact MAb A7 in the tumor. When mice bearing tumors which did not react with MAb A7 were studied, 125I-labeled Fab fragments did not specifically localize to the tumors. These results suggest that Fab fragments of MAb A7 may be suitable carriers of radionuclides for the immunodetection of human pancreatic cancer.

Intratumoral administration of neocarzinostatin conjugated to monoclonal antibody A7 in a model of pancreatic cancer

We investigated the following in athymic nude mice with xenografts of a human pancreatic carcinoma: 1) clearance of the murine monoclonal antibody A7 from the carcinoma; and 2) the antitumor effect of neocarzinostatin conjugated to MAb A7 (A7-NCS) on the carcinoma following intratumoral injection. Compared with 125I-labeled normal mouse IgG, a significantly larger amount of 125I-labeled A7 remained in the tumor after intratumoral injection. Neocarzinostatin conjugated to MAb A7 showed a greater antitumor activity against human pancreatic cancer than neocarzinostatin alone after intratumoral administration. The conjugate completely suppressed tumor growth macroscopically during the experiment. Tumor tissue in mice became necrotic 32 days after injection with A7-NCS. These observations suggest that the intratumoral injection of A7-NCS offers promise in treating pancreatic carcinoma.

Increased tumor localization by monoclonal antibody A7 after F(ab')2 fragmentation in athymic nude mice bearing human pancreatic carcinomas

Much recent research has been directed toward the use of monoclonal antibodies (MoAbs) for the immunodetection of solid tumors. In pancreatic cancer, conventional immunoscintigraphy using intact MoAbs remains disappointing. In this study, 125I-labeled F(ab')2 fragments produced by pepsin digestion of MoAb A7 were injected intravenously into nude mice bearing human pancreatic cancer, HPC-YS, xenografts that have previously been shown to react specifically with MoAb A7. The tumor tissue/blood ratio of 125I-labeled F(ab')2 fragments of MoAb A7 increased with time and was much higher than those for normal tissues. Moreover, the tumor tissue/blood ratio of 125I-labeled F(ab')2 fragments was greater than that of intact MoAb A7, although the F(ab')2 accumulation was less than that of intact MoAb A7 in the tumor. These results suggest that F(ab')2 fragments of MoAb A7 may be suitable carriers of radionuclides for immunodetection of human pancreatic cancer.

Monoclonal antibodies in cancer detection and therapy

Anticancer antibodies have had a long history in the management of cancer, with major applications having been shown in the immunohistochemistry and immunoassay of tumor-associated antigen markers. With the advent of hybridoma-derived monoclonal antibodies, attempts to use these more reproducible reagents in vivo for cancer detection and therapy have intensified. Radiolabeled monoclonal antibodies appear to be gaining a role in the management of cancer by means of imaging methods to detect sites of increased radioactivity, and several products have been developed and tested clinically. In the area of radioimmunotherapy, a number of problems still need to be solved, including low tumor uptake of the radioimmunoconjugate, dose-limiting myelotoxicity, and the induction of an immune response to repeated doses of murine (foreign) immunoglobulins. Similar problems exist for toxin and drug immunoconjugates, but these also fail to benefit from the "bystander" effect of the ionizing radiation delivered with radioimmunoconjugates, and plant and bacterial toxin molecules appear to have additional immunogenicity that restricts repeated injections. Despite these limitations, recombinant engineering and other chemical approaches are making progress in developing second-generation immunoconjugates that may be more efficacious and less immunogenic as cancer-selective therapeutics. Although nonconjugated, "naked", murine monoclonal antibodies have shown limited success in the therapy of human neoplasms, human and "humanized" forms may be more effective, particularly in lymphatic tumors. Some evidence also suggests that anti-idiotype antibodies (antiantibodies) may serve as surrogate antigens in cancer vaccines. Thus, a number of promising immunologic approaches for cancer diagnosis, detection, and therapy have made important progress in recent years.

Anti-tumor activity of T-2 toxin-conjugated A7 monoclonal antibody (T-2-A7 MoAb) against human colon carcinoma

Monoclonal antibody A7 (A7 MoAb), from splenocytes of a mouse immunized against human colorectal carcinoma, was used as a T-2 toxin (T-2) carrier targeting colon cancer. T-2 was converted to T-2 hemiglutarate by glutaric anhydride treatment, and T-2-A7 MoAb conjugates containing up to 20 T-2 per antibody molecule were obtained from the antibody and T-2 hemiglutarate activated with N-hydroxysuccinimide. The in vitro cytotoxicity against human colon cancer (LS174T) cells indicated that the conjugates were markedly less toxic than the toxin itself. The immunoreactivity was evaluated from the in vitro binding activity of A7 MoAb with LS174T cells, and from the in vivo localization in LS174T-bearing nude mice; it remained essentially intact after conjugation with T-2. The efficacy of the T-2-A7 MoAb conjugate was tested against LS174T-bearing nude mice. The conjugate significantly suppressed the growth of the tumor in comparison with both phosphate-buffered saline and free T-2. These results suggest that the conjugate of T-2 with A7 MoAb might be useful as a selective immunotoxin for cancer immunotherapy, with less serious side effects than T-2.

Antitumour activity of an immunoconjugate composed of anti-human astrocytoma monoclonal antibody and neocarzinostatin

Neocarzinostatin (NCS) linked to the thiol group on the hinge region of the Fab' fragment of GA-17, a murine monoclonal antibody reacting with tyrosine-specific phosphorylated antigens, which are exclusively expressed on the cell surface of human astrocytomas, was evaluated for in vivo activity. GA-17-NCS immunoconjugates significantly suppressed the growth of human malignant glioma cell line U87-MG subcutaneous xenografts in nude mice until day 50 when administered intravenously into the tail vein. Disulphide- and thioether-linked GA-17-NCS were nearly equipotent immunoconjugates, but thioether-linked GA-17-NCS was more effective than disulphide-linked conjugates with 250 U/kg NCS content on day 50 (P < 0.05). Thioether-linked GA-17-NCS was significantly more effective on day 50 than free NCS with 500 U/kg or 250 U/kg NCS content (P < 0.05, P < 0.01, respectively). These results suggest that GA-17-NCS may prove useful in the treatment of human malignant gliomas.

Monoclonal antibodies for the treatment of metastases. Evaluation of strategies using a syngeneic rat model

To investigate critical factors influencing the localization and antitumor effects of monoclonal antibodies (MAb) or toxic conjugates, we have adapted a single rat sarcoma, HSN, for preferential growth in the lungs, liver, and lymph nodes (the major sites of metastasis in humans) and have raised a panel of syngeneic rat MAbs to a stably-expressed cell surface antigen. Using this model we have shown that localization in tumors is significantly influenced by their anatomical location and vascularization, and the degree of MAb interaction with host cells. Uptake in small hepatic tumors was excellent, but access to lung tumors was limited by the poor permeability of pulmonary vessels. HSN cells transfected with th human IL-2 gene and coinjected in low numbers with parental tumors secreted sufficient cytokine to enhance the local permeability of vessels and doubled MAb localization in tumors without any systemic toxicity, suggesting that regional delivery of IL-2 may be used to enhance MAb localization in this situation. In order to extent the applicability of the model to studies of MAbs raised against human tumor targets, we have transfected the human c-erb B-2 gene (homolog of the rat neu) into the highly metastatic HSN.LV subline. MAbs raised against the external domain of the p185 product can now be screened for their ability to localize in metastases, and for various conjugates to inhibit tumor growth either independently of, or in association with, a fully functional immune system.

Immunotherapy of colorectal cancer

Specific and nonspecific stimulation of the host immune system to reject cancer is an attractive concept that is just beginning to mature. Results with crude extracts and nonspecific immune stimulation have been variable. However, the recent observations of improved survival after administration of levamisole plus 5-fluorouracil in the adjuvant setting have made an impact on the treatment of colorectal cancer. Animal studies consistently show that immune therapies are most effective for disease that is not advanced. Thus, the small benefit seen with levamisole, a low toxicity immunomodulator, suggests that much more impressive results can be anticipated with more potent and specific agents. Postsurgical autologous tumor cell vaccine has been effective in some prospective randomized trials; in others, no benefit was found. The identification and purification of allogeneic tumor-associated antigens has lead to enhanced antigen-specific host cell-mediated immunity; this may result in more consistent antitumor effects. The current development of chemically defined immune adjuvants of low toxicity allows tumor-specific immune stimulation to be tested in high-risk apparently healthy patients after resection of colorectal cancer (Stages II and III). The influx of information regarding immune cell populations, cell-surface markers, and cytokines has fostered extensive exploration of lymphocyte stimulation, in vitro cell growth and expansion, and in vivo evaluation in patients with advanced cancer. Modest tumor response rates have been documented with adoptive transfer of lymphokine-activated killer cells and interleukin-2. Improved results are anticipated with the more potent tumor-infiltrating lymphocytes and specific in vitro sensitization of draining lymph node cells to autologous and allogeneic tumor antigens. Murine monoclonal antibodies specific for cell-surface markers, such as carcinoembryonic antigen, have been tested for their value in the diagnosis and therapy of colorectal cancer. A small response rate has been seen with single and multiple injections of C017-1A, a monoclonal antibody specific for colonic and pancreatic cancer. The development of antiidiotypic antibodies in these patients may have been important in those that responded to this type of therapy. However, laboratory evidence suggests that monoclonal antibody conjugated to a cytotoxic agent (i.e., radionuclide, drug, or toxin) should be much more effective. Radioimmunotherapy trials in the nude mouse model bearing human colon cancer xenografts showed good tumor incorporation of the radionuclide (yttrium 90 or iodine 131), inhibition of tumor growth, and long-term survival.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo and in vitro antitumor activity of mitomycin C conjugates at 7-N position through a linker containing thiocarbamate bond with CD10 monoclonal antibody

Through a linker containing thiocarbomate bound to the 7-N position of mitomycin C (MMC), conjugates with a monoclonal antibody to CD10 (NL-1) were prepared, and their antitumor activities were examined. All five conjugates, except one, showed in vitro cytotoxicity to two CD10+ lymphoid cell lines superior to MMC. The conjugate displaying the highest cytotoxicity was selected and further tested against three CD10+ and two CD10- lymphoid cell lines in vitro. The conjugate with NL-1 antibody demonstrated higher cytotoxic activity against CD10+ tumor cells than the control conjugate with normal immunoglobulin, while there was no significant difference, when tested against CD10- tumors. The cytotoxic activity of the NL-1 conjugate to CD10+ tumors was significantly blocked by NL-1 antibody. In vivo antitumor activity of the NL-1 conjugate was then tested against a CD10+ tumor transplanted to nude mice, and side effects were recorded. The NL-1 conjugate (4 mg/kg) showed an in vivo antitumor effect similar to MMC (2 mg/kg), which is at nearly maximal tolerable dose; the latter induced decreases in numbers of leukocytes and platelets, while the former did not, suggesting less side effect by the NL-1 conjugate. Since MMC demonstrates a broad spectrum of antitumor activity, the conjugate, as such, may be applicable for the treatment of cancer patients.

Physiological factors influencing radioantibody uptake: a study of four human colonic carcinomas

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.

Biodistribution of monoclonal antibody A7 and its F(ab')2 fragment in athymic nude mice bearing human pancreatic carcinoma

Xenografts of a human pancreatic carcinoma cell line, HPC-YS, which reacted with the monoclonal antibody (MAb) A7, were used to investigate the in vivo localization of radioiodinated MAb A7 after intraperitoneal injection. MAb A7 localized to the tumor 4 days and 8 days after injection with a tissue/blood ratio of 1.45 +/- 0.18 and 2.04 +/- 0.20, respectively. The accumulation of MAb A7 in the tumor was 5%/g and 3.3%/g of the injected dose on day 4 and on day 8, respectively. In contrast, the F(ab')2 fragment of MAb A7 localized to the tumor 4 days after intravenous injection with a tissue/blood ratio of 14.2. The accumulation of the F(ab')2 fragment in the tumor was 1.2%/g of the injected dose. These results suggested that MAb A7 might be a suitable carrier of anticancer drugs for immunotargeting chemotherapy and that the F(ab')2 fragment might be potentially useful for the immunodetection of human pancreatic carcinomas.

Efficacy and specificity of a monoclonal antibody-drug conjugate in chemotherapy by intratumoral injection

The murine monoclonal antibody (Mab) A7 conjugated to neocarzinostatin (A7-NCS) was injected intratumorally (IT) into tumor bearing nude mice. Its pharmacokinetics and tumoricidal effects were compared in the high, moderate and low antigen expressing xenograft for SW1116, WiDr and KB tumor-bearing nude mice, respectively. When injected IT into nude mice, [125I]A7-NCS was retained in the tumors according to the degree of antigen expression; it was also disseminated into the blood inverse proportion to the antigen expression. Addition of an excess amount of Mab A7 reduced [125I]-A7-NCS accumulation in SW1116 xenograft and elevated the [125I]A7-NCS concentration in the circulation. Complete tumor reduction was found in all 5 mice with SW1116 tumor, and 2 of 5 mice with WiDr tumor. However, only incomplete tumor suppression was observed in mice with the KB tumor. The significant tumor reduction in SW1116 bearing nude mice was attenuated when excess of Mab A7 was simultaneously administered with A7-NCS. These findings indicate that A7-NCS was localized in the target tumors and exerted its tumoricidal effects depending on the degree of antigen-antibody interaction when administered IT. Thus, A7-NCS can be used successfully in vivo for local therapy, auguring new and promising applications for local cancer therapy.

Enhanced tumor localization of monoclonal antibody by treatment with kininase II inhibitor and angiotensin II

The effect of kininase II inhibitor, enalapril, on the delivery of monoclonal antibody A7 to the targeted tumor was investigated using athymic mice bearing human colon cancer, SW1116. Enalapril alone, which enhances tumor vascular permeability through the kinin-generating cascade, did not increase the uptake of 125I-labeled A7 (125I-A7) in SW1116 due to the systemic hypotension induced by its inhibitory effect on angiotensin converting enzyme. However, with combined angiotensin II (AT-II) and enalapril treatment, a 2-fold increase in the accumulation of 125I-A7 was seen when compared to A7 alone. This marked increase was presumably due to increased tumor vascular permeability induced by enalapril combined with the absence of hypotension due to the actions of AT-II. This approach might be useful in radioimmunodetection and immunotargeting chemotherapy using monoclonal antibody.

Radioimmunodetection of human colon cancer using 99mTc-MDP-MoAb-A7 in mice

Radioimmunoscintigraphy (RIS) using 99mTc-labeled A7, a monoclonal antibody (MoAb) against a human colorectal cancer, was performed in nude mice bearing a human colon cancer, HCT-15. MoAb-A7 was labeled with 99mTc in the presence of methylene diphosphate (MDP). At 24 h post-injection of 99mTc-MDP-A7, colon cancer was clearly visualized, and the tumor-to-tissue ratio of 99mTc-MDP-A7 was higher than the ratio of 111In-DTPA-A7 which was described previously. These results suggest the possibility of the clinical application of 99mTc-MDP-A7 for RIS of a human colon cancer.

Expression of the cell surface antigen detected by the monoclonal antibody A7 in pancreatic carcinoma cell lines

In a previous study, we used a murine monoclonal antibody, A7, against human colon carcinoma as a drug-carrier to treat colorectal cancer. In the present study, we found that MAb A7 also reacted immunohistochemically with 73% of human pancreatic carcinoma cell lines, with the A7 antigen mainly being detected on the cell surface. However, the A7 antigen was found in only 9% of the spent media of these human pancreatic carcinoma cell lines by ELISA. On the other hand, the positive incidence of CA19-9, POA, ferritin, CEA, DU-PAN-2 and SLX in those spent media was 100%, 64%, 64%, 55%, 55% and 36%, respectively. These results suggest that the A7 antigen may only rarely be shed into the sera of pancreatic cancer patients, in which case MAb A7 could be a suitable drug-carrier in targeting chemotherapy for pancreatic cancer patients.

Comparative pharmacokinetic properties of murine monoclonal antibody A7 modified with neocarzinostatin, dextran and polyethylene glycol

The murine monoclonal antibody A7 (Mab A7) was chemically modified with several macromolecules: dextran, polyethylene glycol and the anti-cancer polypeptide neocarzinostatin. The pharmacokinetic properties of the combinations were subsequently examined. Radioimmunoassay revealed that all preparations retained their antigen-binding activities. The Mab A7-neocarzinostatin conjugate was cleared from the blood circulation with a kinetic pattern almost identical to that of the parent Mab A7. Of the three preparations, Mab A7-dextran (A7-Dx) was removed the most rapidly from the circulation. Mab A7-polyethylene glycol (A7-PEG) exhibited the slowest blood clearance curve, with twice the half life of the parent Mab A7 in the circulation. In normal organ distributions, A7-Dx exhibited the highest liver, spleen and kidney uptake, and A7-PEG showed the lowest uptake, when expressed as tissue:blood ratio. Although A7-Dx exhibited lower tumor uptake, there was no significant difference among the three conjugates in tumor-bearing nude mice. A7-PEG seems to be a good candidate for targeted cancer therapy using antibody due to its high blood retention but low normal organ uptake.

Tumor localization and in vivo antitumor activity of the immunoconjugate composed of anti-human colon cancer monoclonal antibody and mitomycin C-dextran conjugate

The tissue distribution and in vivo antitumor activity of a novel monoclonal antibody-mitomycin C conjugate (A7-MMCD) composed of anti-human MAb A7 and MMC-dextran conjugate were investigated using tumor-bearing mice. A7-MMCD was prepared via an anionic dextran intermediate for the purpose of keeping the non-specific uptake by the reticuloendothelial system to a minimum. 111In-labeled A7-MMCD showed about a 5-times-greater accumulation in SW1116 (targeted tumor) than in S180 (non-targeted tumor) 48 h after injection, and produced a tumor-to-blood ratio which was 3 times higher in SW1116-bearing mice than in S180-bearing mice 96 h after injection. Accumulations in the liver, spleen, and kidney were also observed to some extent. Pharmacokinetic analysis revealed that A7-MMCD had nearly the same properties in the body as MMCDan (MMCD with an anionic charge), i.e., those of a negatively charged macromolecule. Both A7-MMCD and MMCDan had relatively similar tissue uptake rate indices for the liver and spleen. The tumor uptake rate index for SW1116 was about 2.5 times greater than that for S180, and the total amount of 111In-A7-MMCD accumulated in SW1116 was calculated to be approximately 5 times greater than the amount in S180. These results indicated that A7-MMCD could achieve site-specific targeting in the body. Furthermore, in the therapeutic experiment using SW1116 implanted subcutaneously, A7-MMCD suppressed tumor growth significantly, compared to free MMC and MMCDan. These results suggest that in designing an monoclonal antibody-drug conjugate via an intermediary, the physicochemical properties of intermediate macromolecules must also be taken into consideration to obtain a high degree of efficacy in vivo.

Identification and characterization of a colon-cancer-associated antigen expressed on leukemia-lymphoma cell lines

A total of 72 human leukemia-lymphoma cell lines were studied for reactivity with the monoclonal antibody (MAb) A7, an anti-human colon-cancer-cell-associated antigen reagent, by indirect membrane immunofluorescence. Nine of the 72 cell lines expressed the antigen recognized by A7 MAb. Five of the 34 T-cell lines, 2 of the 21 B-cell lines, and 2 of the 3 non-lymphoid-non-myeloid cell lines were reactive with A7 MAb. By means of SDS-PAGE and immunoblotting, the antigens isolated from both colon cancer cell lines (WiDr, SW1116 and LoVo) and leukemia cell lines (A3/KAWAKAMI, H9, RPMI 8226 and SPI-801) showed an identical MW of 42-43 kDa. The non-glycosylated antigen recognized by A7 MAb, which was expressed on both the colon cancer line (SW1116) and the leukemia line (H9) in the presence of tunicamycin, also showed an identical MW of 36 kDa. However, the quantity of the antigen in the leukemia cells was significantly lower than in the colon cancer cells. Although expression of this colon-cancer-associated antigen in the non-colon cancer cells is real, the significant expression of this antigen in colon-cancer cells makes it useful for clinical monitoring of colon cancer patients.