Enhancement of colorectal tumor targeting using a novel biparatopic monoclonal antibody against carcinoembryonic antigen in experimental radioimmunoguided surgery

Biparatopic antibodies: therapeutic applications and prospects

Biparatopic antibodies (bpAbs) bind distinct, non-overlapping epitopes on an antigen. This unique binding mode enables new mechanisms of action beyond monospecific and bispecific antibodies (bsAbs) that can make bpAbs effective therapeutics for various indications, including oncology and infectious diseases. Biparatopic binding can lead to superior affinity and specificity, promote antagonism, lock target conformation, and result in higher-order target clustering. Such antibody-target complexes can elicit strong agonism, increase immune effector function, or result in rapid target downregulation and lysosomal trafficking. These are not only attractive properties for therapeutic antibodies but are increasingly being explored for other modalities such as antibody-drug conjugates, T-cell engagers and chimeric antigen receptors. Recent advances in bpAb engineering have enabled the construction of ever more sophisticated formats that are starting to show promise in the clinic.

Recent trends in antibody-based oncologic imaging

Antibodies, with their unmatched ability for selective binding to any target, are considered as potentially the most specific probes for imaging. Their clinical utility, however, has been limited chiefly due to their slow clearance from the circulation, longer retention in non-targeted tissues and the extensive optimization required for each antibody-tracer. The development of newer contrast agents, combined with improved conjugation strategies and novel engineered forms of antibodies (diabodies, minibodies, single chain variable fragments, and nanobodies), have triggered a new wave of antibody-based imaging approaches. Apart from their conventional use with nuclear imaging probes, antibodies and their modified forms are increasingly being employed with non-radioisotopic contrast agents (MRI and ultrasound) as well as newer imaging modalities, such as quantum dots, near infra red (NIR) probes, nanoshells and surface enhanced Raman spectroscopy (SERS). The review article discusses new developments in the usage of antibodies and their modified forms in conjunction with probes of various imaging modalities such as nuclear imaging, optical imaging, ultrasound, MRI, SERS and nanoshells in preclinical and clinical studies on the diagnosis, prognosis and therapeutic responses of cancer.

Selecting Potential Targetable Biomarkers for Imaging Purposes in Colorectal Cancer Using TArget Selection Criteria (TASC): A Novel Target Identification Tool

Peritoneal carcinomatosis (PC) of colorectal origin is associated with a poor prognosis. However, cytoreductive surgery combined with hyperthermic intraperitoneal chemotherapy is available for a selected group of PC patients, which significantly increases overall survival rates up to 30%. As a consequence, there is substantial room for improvement. Tumor targeting is expected to improve the treatment efficacy of colorectal cancer (CRC) further through 1) more sensitive preoperative tumor detection, thus reducing overtreatment; 2) better intraoperative detection and surgical elimination of residual disease using tumor-specific intraoperative imaging; and 3) tumor-specific targeted therapeutics. This review focuses, in particular, on the development of tumor-targeted imaging agents. A large number of biomarkers are known to be upregulated in CRC. However, to date, no validated criteria have been described for the selection of the most promising biomarkers for tumor targeting. Such a scoring system might improve the selection of the correct biomarker for imaging purposes. In this review, we present the TArget Selection Criteria (TASC) scoring system for selection of potential biomarkers for tumor-targeted imaging. By applying TASC to biomarkers for CRC, we identified seven biomarkers (carcinoembryonic antigen, CXC chemokine receptor 4, epidermal growth factor receptor, epithelial cell adhesion molecule, matrix metalloproteinases, mucin 1, and vascular endothelial growth factor A) that seem most suitable for tumor-targeted imaging applications in colorectal cancer. Further cross-validation studies in CRC and other tumor types are necessary to establish its definitive value.

Preclinical application of radioimmunoguided surgery using anti-carcinoembryonic antigen biparatopic antibody in the colon cancer

Radioimmunoguided surgery (RIGS) has been known as a sophisticated tool to detect micrometastasis intraoperatively. A preclinical model of RIGS was designed to test the possible clinical applicability of the biparatopic antibody in detecting colorectal cancer. The biparatopic antibody was constructed using two anti-carcinoembryonic antigen (CEA)-specific antibodies, T84.66 and PR1A3, reacting against two different epitopes. (125)I-labeled biparatopic antibody was introduced via the principal colonic arteries at the end of operation in 10 operable patients with colon cancer. After 24 h, the radioactivities of the tumors and lymph nodes were counted using the gamma-detecting probe. The radioactivity count was performed ex vivo. The accurate detection in the primary tumors and metastatic lymph nodes were 100 and 88.7% respectively. False-positive detections occurred in 24 of 256 lymph nodes (9.4%), whereas false-negative detections occurred in 5 of them (2%). The most frequent cause of false-positive detection was dissociated radionuclides trapped in the lymphatic tissues. False-negative detections occurred mainly from weak targeting by radiolabeled antibody, probably due to weak expression of tumor CEA. Conclusively, as most detection errors appear to be reduced within 3 days in vivo, the biparatopic antibody can efficiently be applied to the clinical RIGS, thereby facilitating accurate detection and removal of occult cancer foci in colorectal cancer.

Experimental radioimmunoguided surgery for peritoneal metastases of gastric cancer using anticarcinoembryonic antigen-specific T84.66 F(ab')2

PURPOSE

In gastric cancer, peritoneal dissemination is the most frequent cause of the noncurative resection and recurrence after curative resection. We therefore evaluated the feasibility of radioimmunoguided surgery (RIGS) in the treatment of peritoneal metastases of gastric cancer and the use of anti-CEA-specific T84.66 F(ab')2 as an efficient immune agent.

METHODS

Two human gastric cancer cell lines, MKN45 and RF48, were intraperitoneally xenografted into nude mice, which were later injected with 125I-labeled T84.66 F(ab')2. Peritoneal tumors were localized by RIGS 5 days after antibody injection. The minimum number of cells detected by a gamma probe was assayed by in vitro tumor cell localization.

RESULTS

We observed 37 peritoneal metastases: 8 invisible (long diameter, <1 mm), 6 small (1- < 5 mm), and 23 large (> or =5 mm) tumors. The accuracy, sensitivity and specificity of RIGS in detecting peritoneal metastasis were 82% (69/84), 76% (28/37), and 87% (41/47), respectively. RIGS accuracy did not differ with respect to tumor diameter. Mean labeling indices over minimal and maximal normal counts were 6.1+/-1.2 (mean +/- SEM) and 4.7+/-1, respectively. Mean scores of CEA immunostaining and silver grains in tumors were significantly higher than those in the nontumor-bearing peritoneum (P < 0.001). There was a close correlation among radioactivity, immunostaining and microautoradiography (P < 0.001-0.005). We observed six false-positive and nine false-negatives which may have been due to high blood background and negative radioimmune reactivity, respectively.

CONCLUSIONS

125I-labeled T84.66 F(ab')2 efficiently targeted peritoneally disseminated gastric cancer cells, suggesting that RIGS using this immune agent may accurately detect occult peritoneal metastases in patients with gastric cancer.

Efficient Cancer Therapy with a Nanobody-Based Conjugate

Nanobodies are the smallest fragments of naturally occurring single-domain antibodies that have evolved to be fully functional in the absence of a light chain. Nanobodies are strictly monomeric, very stable, and highly soluble entities. We identified a nanobody with subnanomolar affinity for the human tumor-associated carcinoembryonic antigen. This nanobody was conjugated to Enterobacter cloacae beta-lactamase, and its site-selective anticancer prodrug activation capacity was evaluated. The conjugate was readily purified in high yields without aggregation or loss of functionality of the constituents. In vitro experiments showed that the nanobody-enzyme conjugate effectively activated the release of phenylenediamine mustard from the cephalosporin nitrogen mustard prodrug 7-(4-carboxybutanamido) cephalosporin mustard at the surface of carcinoembryonic antigen-expressing LS174T cancer cells. In vivo studies demonstrated that the conjugate had an excellent biodistribution profile and induced regressions and cures of established tumor xenografts. The easy generation and manufacturing yield of nanobody-based conjugates together with their potent antitumor activity make nanobodies promising vehicles for new generation cancer therapeutics.

Tumor markers and colorectal cancer: utility in management

Colorectal cancer (CRC) is a leading cause of morbidity and mortality. Although genetic testing can screen for rare hereditary CRC syndromes, there is no ideal means of screening for sporadic forms of CRC. This review will focus on markers that are currently used in the management of sporadic CRC and their limitations, as well as possible future clinical applications.