Imaging of colorectal carcinoma with technetium-99m radiolabeled Fab' fragments

The Use of Monoclonal Antibodies for Radioscintigraphic Detection of Cancer

Beginning with Ehrlich's original “magic bullet” concept of 1904, the pioneering human trials in the late 1970s of Goldenberg and Mach using polyclonal antibodies, and the Nobel Prize-winning work of Kohler and Milstein in 1975 for developing monoclonal antibody (MAb) technology, there has been much interest in the use of antibodies for detecting and treating cancer. Although not the revolutionary breakthrough that was initially hoped for, marked progress has been made. The Food and Drug Administration (FDA) has recently approved the intact murine IgG, 111-indium CYT-103 (satumomoab, Oncoscint™ Cytogen, Princeton, NJ) for clinical use in detecting colorectal and ovarian cancer. However, the agent has been approved for only a single, one-time use, because patients developed an immune response (human anti-mouse antibody, or HAMA) that alters MAb biodistribution and may limit the clinical effectiveness of this agent when repeat studies are performed. Other MAbs reacting with a variety of antigens and targeting numerous tumors, including breast, lung, prostate, and melanomas, are currently undergoing large-scale clinical trials. To reduce induction of immune responses, many of the agents use immunoglobulin fragments [Fab, or F(ab)2] labeled with the short-lived isotope 99m-technetium used for most routine nuclear medicine diagnostic testing. Future developments will use even smaller fragments such as single chain antibodies or custom synthesized molecular recognition units (small peptides containing only the specific antigen combining site). Presented herein is an overview of the past results and an assessment of the current status of radioimmunoscintigraphy for various neoplasms.

Pretargeted molecular imaging and radioimmunotherapy

Pretargeting is a multi-step process that first has an unlabeled bispecific antibody (bsMAb) localize within a tumor by virtue of its anti-tumor binding site(s) before administering a small, fast-clearing radiolabeled compound that then attaches to the other portion of the bsMAb. The compound's rapid clearance significantly reduces radiation exposure outside of the tumor and its small size permits speedy delivery to the tumor, creating excellent tumor/nontumor ratios in less than 1 hour. Haptens that bind to an anti-hapten antibody, biotin that binds to streptavidin, or an oligonucleotide binding to a complementary oligonucleotide sequence have all been radiolabeled for use by pretargeting. This review will focus on a highly flexible anti-hapten bsMAb platform that has been used to target a variety of radionuclides to image (SPECT and PET) as well as treat tumors.

Advances in Radioimmunotherapy in the Age of Molecular Engineering and Pretargeting

Now that radioimmunotherapy is an approved method for the treatment of certain types of non-Hodgkin's lymphoma, investigators are turning to new approaches to further improve radionuclide targeting in hopes of expanding the use of this technology. A number of innovative recombinant proteins have been developed with more favorable pharmacokinetic and targeting properties than standard whole IgG, which conceivably could improve the therapeutic index for cancer treatment. Pretargeting methods also are coming of age, with preclinical and early clinical studies in a variety of cancers illustrating how this alternative approach can enhance the therapeutic window several-fold of what has been possible with directly radiolabeled IgG. This review will discuss some of these promising new developments.

Application of 99mTc labeled chimeric Fab fragments of monoclonal antibody A7 for immunoscintigraphy of pancreatic carcinoma

BACKGROUND

One of the reasons for the poor prognosis of pancreatic carcinoma is the difficulty of obtaining an early diagnosis of pancreatic carcinoma. One possibility is the application of radioimmunoscintigraphy using radiolabeled monoclonal antibodies (Mabs).

METHODS

We labeled chimeric (human/mouse) Fab fragments of Mab A7 (chA7Fab) with (99m)Tc and examined the distribution of (99m)Tc-labeled chA7Fab in nude mice bearing human pancreatic carcinoma.

RESULTS

The tumor accumulation of (99m)Tc-labeled chA7Fab was larger than that of (99m)Tc-labeled A7 from 2 to 6 hr after injection. (99m)Tc-labeled chA7Fab disappeared from blood more rapidly than (99m)Tc-labeled A7. For all resected normal tissues except kidney, the accumulation of (99m)Tc-labeled chA7Fab was low and similar to that of (99m)Tc-labeled A7.

CONCLUSIONS

Because the half-life of (99m)Tc is short (6 hr), chA7Fab, which accumulates rapidly in tumors, may be a better carrier of (99m)Tc than intact Mab A7.

Comparison of recombinant derivatives of chimeric TNT-3 antibody for the radioimaging of solid tumors

Although intact monoclonal antibodies (MAbs) are well suited as therapeutic reagents, their relatively slow clearance rates render them less useful for imaging applications. Over the last several years, our laboratory has developed a unique targeting approach to solid tumors that utilizes MAbs directed against DNA and its components to bind to degenerating cells and necrotic regions of tumors in a specific manner. Because these MAbs have considerable potential for the early diagnosis of cancer and for the monitoring of cytoreductive therapies, the availability of an effective imaging agent is highly desirable. To accomplish this goal, a series of genetically engineered derivatives of MAb chTNT-3 including the single-chain Fv, diabody, triabody, Fab, and F(ab')(2) were generated and expressed in NS0 myeloma cells using the Glutamine Synthetase Amplification System. Initial in vitro studies demonstrated that each of the antibody derivatives maintained its antigen binding in a stable manner. In vivo analyses after radiolabeling were then performed to evaluate their pharmacokinetic, biodistribution, and tumor-imaging properties in solid tumor-bearing mice. The results of these studies showed that compared with intact parental chTNT-3, which has a half-life of 134.2 h, the smaller derivatives were eliminated more rapidly (4.9-8.1 h). Importantly, the smaller derivatives were found to have significantly higher tumor-to-organ ratios, but lower overall uptake levels compared with parental (125)I-chTNT-3 in two different tumor models. A comparison of the five derivatives showed that the F(ab')(2) reagent consistently gave the best results in imaging and biodistribution studies. Based upon these results, further studies are warranted to demonstrate the potential of this reagent for the diagnosis and monitoring of solid tumors using noninvasive imaging techniques such as immunoscintigraphy and positron emission tomography (PET).

Radioimmunoscintigraphy in the differential diagnosis of hepatic mass lesion

A patient with suspected recurrent cancer of the colon underwent a variety of imaging procedures for the differential diagnosis of a hepatic mass lesion. Computed tomography (CT) showed a low-density lesion in the left hepatic lobe, and the initial CT-guided biopsy of the liver mass was reported to demonstrate a benign lesion. Ultrasonography (US) showed a hypoechoic lesion, and technetium-99m red blood cell (RBC) scan failed to suggest a hemangioma. However, radioimmunoscintigraphy (RIS) using 99mTc- labeled anti-carcinoembryonic antigen (CEA) monoclonal antibody clearly demonstrated increase uptake of antibody in the liver lesion. Scheduled hepatic angiography was canceled and subsequent exploratory laparotomy confirmed liver metastasis. RIS appears most helpful in the diagnosis of hepatic metastasis in patients with colorectal cancer and a rising CEA level. CT, US, and 99mTc-RBC studies for the investigation of hepatic masses are briefly discussed.