[In]-Labeled monovalent Fab fragment of chimeric monoclonal antibody cG250 directed against carbonic anhydrase IX

A common feature of most solid cancerous tumor types is the presence of hypoxic conditions (1) and the overexpression of carbonic anhydrase IX (CA IX), a transmembrane cell-surface enzyme that is known to regulate the pH and adhesion of tumor cells (2). Hypoxic tumors are often resistant to radio- and chemotherapy, have a high metastatic potential, and usually predict a poor outcome for the cancer patient (3). Although several methods (invasive and noninvasive) are available for the detection of hypoxia in tumors, including the use of radiolabeled small molecules, these methods are not completely reliable because they either yield variable diagnoses or have functional limitations due to incomplete penetration of tumors and fail to detect hypoxia in all tumor types (3, 4). Because CA IX is overexpressed in most solid tumors, it is considered to be a hypoxia biomarker, and targeting the CA IX for the detection of hypoxic tumors is of great interest to investigators (1, 3-5). A I-labeled murine monoclonal antibody (mAb) that targets the CA IX, designated G250, was developed and evaluated for the radiotherapy of metastatic renal cell carcinoma (RCC) patients, but no major responses were observed because the individuals developed immunity to the mAb (6). Subsequently, a I-labeled chimeric form of G250, [I]-cG250, was developed and evaluated as an immunotherapeutic agent for the treatment of RCC (7). cG250 has been labeled with other nuclides (such as Zr, Lu, Y, etc.) and has been used in preclinical studies in rats (8) and for the treatment of RCC (7). However, only minor responses were observed in the clinical investigations, and dose escalation studies are ongoing (7). Radiolabeled antibodies (Abs) have a limited ability to detect or treat cancer because these agents show only a peripheral penetration of solid tumors (due to a large size, 150 kDa) and leave many neoplastic cells in the lesion untreated (9). In addition, Abs have prolonged blood circulation and present a high radiation dose risk to the bone marrow (10). In comparison, the smaller monovalent Fab (50 kDa) and the divalent F(ab’) (~100 kDa) fragments derived from the parent Ab exhibit better tumor penetration and a shorter circulating half-life and are likely to yield better results if used to detect or treat solid malignant tumors (9). Between the two fragment types, the divalent F(ab’) fragments may be more useful for the detection or treatment of malignant tumors because they have a higher affinity for the antigen (11). With these observations in mind, a divalent F(ab’) fragment of cG250 was developed, labeled with I, and compared with the intact [I]-cG250 Ab for its pharmacokinetic behavior and its ability to target tumors in mice and RCC patients (10). However, from this study the investigators concluded that the intact Ab was superior to the divalent fragment for targeting the RCC tumors. A clinical trial to investigate the safety of a I-labeled version of cG250 in patients with renal masses has been reported (12). In addition, cG250 is also under evaluation in several other clinical trials. Recently, Zr-labeled F(ab’) fragments of cG250 were shown to be suitable for the visualization of hypoxic head and neck cancer xenograft tumors in mice (5). Brouwers et al. compared the use of [In]-isothiocynate-diethylenetriamine pentaacetic acid-cG250 and [I]-cG250 for the detection of RCC metastases in five patients and concluded that the former tracer was superior to the latter for visualization of the tumors (13). In another study involving three patients, it was shown that neither I-labeled cG250 nor In-labeled cG250 were suitable for the radioimmunotherapy of biliary cancer (14). In a recent study using 1,4,7,10-tetraazacyclododecane-,’,'',’’’-tetraacetic acid (DOTA) as a nuclide conjugating agent, In-labeled cG250 Ab ([In]-DOTA-cG250) and its Fab ([In]-DOTA-Fab-cG250) and F(ab’) ([In]-DOTA-F(ab’)-cG250) fragments were generated and compared for their biodistribution and detection of hypoxic HT-29 cell (of human colorectal adenocarcinoma origin) xenograft tumors in mice (1). This chapter details the studies performed with [In]-DOTA-Fab-cG250. Studies performed with [In]-DOTA-cG250 (15) and [In]-DOTA-F(ab’)-cG250 (16) are discussed in separate chapters of MICAD (www.micad.nih.gov).