D.S. Fairweather

Computer Analysis of Antibody Scans

Abstract A variety of tumours can be detected with radiolabelled antibodies and a gamma camera. Unfortunately, Low count rates and poor contrast combine to produce images that are often difficult to interpret. Subtraction, using a second isotope, may induce further artifacts. We have used automatic techniques to avoid observer bias, estimate statistical significance, and enhance resolution. Three separate techniques have been used: Firstly, the variance in the subtraction image was calculated and two standard deviations removed for each pixel and only remaining counts were regarded as significant. Secondly, noise was removed by processing with a Wiener filter and the result expressed in contour plots, each stepped in units of standard deviation. Thirdly, a non-linear deconvolution technique (maximum entropy) was used to remove both noise and camera blurring. Subtraction can then be effected using a local normalising factor, and lesions assessed by their statistical significance and shape. These techniques allow objective assessment of scans as well as improving resolution and sensitivity.

Characteristics of First and Second Antibodies for Tumour Imaging

Abstract Radiolabelled anti-tumour antibodies (1-Ab) can be cleared from non-tumour areas using a 2nd antibody (2-Ab) to give enhanced tumour:normal tissue ratios and improved tumour detection. Rabbit, mouse and pig IgGs showed higher affinities for human Fc receptors, and were considered potential 2nd antibodies, since immune complexes containing them are rapidly catabolised. In contrast, sheep and chicken antibodies showed no Fc receptor uptake and were considered useful locating antibodies, but of no value as second antibodies. 5 patients were given radiolabelled anti CEA and pig anti sheep IgG 24h later, resulting in a two-fold fall in blood counts, without a large fall in tumour counts. In contrast, patients given mouse IgM and sheep 2-Ab showed no clearance, as predicted by Fc receptor studies.

Scanning for tumours with radiolabelled antibodies: a review.

The demonstration that certain antigenic substances occur in tumours in much higher concentrations than in normal tissues inevitably resulted in two substantial fields of enquiry: serological immunodiagnosis and radioimmunodetection (RID). Of these the former has been of moderate clinical return, whilst the latter has not been fully assessed and in certain areas does appear to be clinically valuable.

Preparation of 111-Indium Labelled Antibodies

Abstract DTPA was covalently coupled to sheep anti CEA using a mixed anhydride. This was formed by mixing DTPA and a trace amount of 14-C-DTPA with triethylamine to form the triethylammonium salt, then isobutyl chloroformate to form the anhydride. The anti CEA was then covalently linked to the anhydride in 0.1M bicarbonate buffer, pH 9.5, and non-covalently bound DTPA removed by gel filtration on Sephadex G25 in 0.01M acetate buffer, pH 6. Polyvalent metal ions were removed from all buffers by extraction with dithizone. 74MBq of 111-In chloride was neutralised with acetate (pH 6.5), mixed with 0.8 mg of chelated antibody and left at 4°C overnight. Unbound 111-In was removed by gel filtration. Labelling efficiency was 50-75%, giving a specific activity of up to 74 MBq/mg protein. The chelated antibody formed stable complexes with indium. Ill-indium labelled antibodies have proved superior to iodinated antibodies for in vivo tumour imaging purposes.

111-Indium-Labelled Antibodies for Tumour Detection

Abstract 131-iodine is frequently used as the radiolabel for tumour radioimmunodetection but produces high energy gamma and beta emissions which are disadvantageous. 111-indium, by contrast, has a much more suitable photon emission and is known to be a stable cell label. This study describes our experience with Ill-Indium labelled antibodies to CEA used for in-vivo tumour localisation. The results indicate that despite similar dosimetry 111-In is superior to 131-I. This is due to higher count rates and prolonged intracellular stability of indium giving improved picture contrast.

Indium Chelates for the Radioimmunodetection of Tumours

The localising properties of 111-Indium labelled anti CEA has been assessed in 11 patients with tumours producing CEA. Of 31 potential tumour areas 28 were positive on 111-In scans and 25 positive by a combination of clinical and other techniques. Five of the patients had 131-I antibody scans for comparison. Of 15 potential areas 13 were detected with 111-In and 8 with 131-I. Also, the 111-In scans were positive for longer and count rates were higher per MBq of isotope injected. The biological distribution of the 111In was as follows: 20% accumulated in the liver; 10% was passed in the urine on the first day and 3% per day thereafter. This retention of 111-In meant the absorbed dose (whole body) of the two isotopes were similar per MBq. The main advantage of 111-In is that it can be detected more efficiently than 131-I by present gamma cameras. If both 111-In photo peaks are utilised, this gives an eight fold advantage over 131-I in terms of counts. Also, spatial resolution with 111-In is better using a medium energy collimator and it may acheive higher tumour to normal tissue ratios.

Radioimmunodetection of Endocrine Tumours

Abstract Radiolabelled antibodies have been used to locate three different types of endocrine tumour: a/differentiated thyroid tumours using anti-thyroglobulin, b/medullary thyroid carcinomas (MTC) using anti-carcinoembryonic antigen (CEA) and c/insulinomas using anti-insulin. In 11 patients with papillary and follicular tumours the antibody located 27 malignant areas. This compared with 9 areas detected with one mCi of 131-Iodide alone and 24 areas by conventional clinical detection techniques. In 5 patients with MTC and elevated serum calcitonin, only one lesion in the neck was detected with anti-CEA and a subsequent scan after surgical excision was negative. In 5 patients with insulinomas, 3 of the tumours were correctly located in the pancreas; one was a technical failure and the other gave a false positive scan when the tumour was only of microscopic size. In only one of the 5 patients was the lesion detected by pancreatic angiography. The sensitivity of the method is at present about 2 cm.

Localisation of a Monoclonal Antibody Labelled with Astatine-211 to a Human Heterograft Tumour in Nude Mice

Abstract Antibodies labelled with Astatine-211 are potent cytotoxic agents in an in-vitro system. In an in-vivo tumour model however, the high level of blood radioactivity precludes its direct use as a specific, antitumour agent. A method of simply reducing the level of blood radioactivity is presented.