Alan J. Green

MIRD pamphlet no. 20: The effect of model assumptions on kidney dosimetry and response - Implications for radionuclide therapy

Renal toxicity associated with small-molecule radionuclide therapy has been shown to be dose-limiting for many clinical studies. Strategies for maximizing dose to the target tissues while sparing normal critical organs based on absorbed dose and biologic response parameters are commonly used in external-beam therapy. However, radiopharmaceuticals passing though the kidneys result in a differential dose rate to suborgan elements, presenting a significant challenge in assessing an accurate dose–response relationship that is predictive of toxicity in future patients. We have modeled the multiregional internal dosimetry of the kidneys combined with the biologic response parameters based on experience with brachytherapy and external-beam radiation therapy to provide an approach for predicting radiation toxicity to the kidneys. Methods: The multiregion kidney dosimetry model of MIRD pamphlet no. 19 has been used to calculate absorbed dose to regional structures based on preclinical and clinical data. Using the linear quadratic model for radiobiologic response, we computed regionally based surviving fractions for the kidney cortex and medulla in terms of their concentration ratios for several examples of radiopharmaceutical uptake and clearance. We used past experience to illustrate the relationship between absorbed dose and calculated biologically effective dose (BED) with radionuclide-induced nephrotoxicity. Results: Parametric analysis for the examples showed that high dose rates associated with regions of high activity concentration resulted in the greatest decrease in tissue survival. Higher dose rates from short-lived radionuclides or increased localization of radiopharmaceuticals in radiosensitive kidney subregions can potentially lead to greater whole-organ toxicity. This finding is consistent with reports of kidney toxicity associated with early peptide receptor radionuclide therapy and 166Ho-phosphonate clinical investigations. Conclusion: Radionuclide therapy dose–response data, when expressed in terms of biologically effective dose, have been found to be consistent with external-beam experience for predicting kidney toxicity. Model predictions using both the multiregion kidney and linear quadratic models may serve to guide the investigator in planning and optimizing future clinical trials of radionuclide therapy.

A phase I study of single administration of antibody-directed enzyme prodrug therapy with the recombinant anti-carcinoembryonic antigen antibody-enzyme fusion protein MFECP1 and a bis-iodo phenol mustard prodrug

Purpose: Antibody-directed enzyme prodrug therapy is a two-stage treatment whereby a tumor-targeted antibody-enzyme complex localizes in tumor for selective conversion of prodrug. The purpose of this study was to establish optimal variables for single administration of MFECP1, a recombinant antibody-enzyme fusion protein of an anti–carcinoembryonic antigen single-chain Fv antibody and the bacterial enzyme carboxypeptidase G2 followed by a bis-iodo phenol mustard prodrug. MFECP1 is manufactured in mannosylated form to facilitate normal tissue elimination. Experimental Design: Pharmacokinetic, biodistribution, and tumor localization studies were used to test the hypothesis that MFECP1 localizes in tumor and clears from normal tissue via the liver. Firstly, safety of MFECP1 and a blood concentration of MFECP1 that would avoid systemic prodrug activation were tested. Secondly, dose escalation of prodrug was done. Thirdly, the dose of MFECP1 and timing of prodrug administration were optimized. Results: MFECP1 was safe and well tolerated, cleared rapidly via the liver, and was less immunogenic than previously used products. Eighty-fold dose escalation from the starting dose of prodrug was carried out before dose-limiting toxicity occurred. Confirmation of the presence of enzyme in tumor and DNA interstrand cross-links indicating prodrug activation were obtained for the optimal dose and time point. A total of 28 of 31 patients was evaluable for response, the best response being a 10% reduction of tumor diameter, and 11 of 28 patients had stable disease. Conclusions: Optimal conditions for effective therapy were established. A study testing repeat treatment is currently being undertaken.

A Phase I Trial of Radioimmunotherapy with 131I-A5B7 Anti-CEA Antibody in Combination with Combretastatin-A4-Phosphate in Advanced Gastrointestinal Carcinomas

Purpose: In preclinical models, radioimmunotherapy with 131I-A5B7 anti–carcinoembryonic antigen (CEA) antibody (131I-A5B7) combined with the vascular disruptive agent combretastatin-A4-phosphate (CA4P) produced cures unlike either agent alone. We conducted a phase I trial determining the dose-limiting toxicity (DLT), maximum tolerated dose, efficacy, and mechanism of this combination in patients with gastrointestinal adenocarcinomas. Experimental Design: Patients had CEA of 10 to 1,000 μg/L, QTc ≤450 ms, no cardiac arrhythmia/ischaemia, and adequate hematology/biochemistry. Tumor was suitable for blood flow analysis by dynamic contrast enhanced-magnetic resonance imaging (MRI). The starting dose was 1,800 MBq/m2 of 131I-A5B7 on day 1 and 45 mg/m2 CA4P given 48 and 72 hours post-131I-A5B7, then weekly for up to seven weeks. Results: Twelve patients were treated, with mean age of 63 years (range, 32-77). Two of six patients at the first dose level had DLTs (grade 4 neutropenia). The dose was reduced to 1,600 MBq/m2, and CA4P escalated to 54 mg/m2. Again, two of six patients had DLTs (neutropenia). Of ten assessable patients, three had stable disease and seven had progressive disease. Single-photon emission computed tomography confirmed tumor antibody uptake in all 10 patients. DCE-MRI confirmed falls in kinetic parameters (Ktrans/IAUGC60) in 9 of 12 patients. The change of both pharmacokinetic parameters reached a level expected to produce efficacy in one patient who had a minor response on computed tomography and a reduced serum tumor marker level. Conclusions: This is believed to be the first trial reporting the combination of radioimmunotherapy and vascular disruptive agent; each component was shown to function, and myelosuppression was dose-limiting. Optimal dose and timing of CA4P, and moderate improvements in the performance of radioimmunotherapy seem necessary for efficacy.

Accurate quantification of 131I distribution by gamma camera imaging.

The development of targeted therapy requires that the concentration of the therapeutic agent can be estimated in target and normal tissues. Single photon emission tomography (SPET), with and without scatter correction, and planar imaging using131I have been compared to develop a method for investigation of targeted therapy. Compton scatter was investigated using line spread functions in air and water, these data were used to set a second peak, adjacent to the photopeak, for scatter correction. The system was calibrated with an eliptical phantom containing sources in background activity of various intensities. Scatter corrected reconstructions gave accurate estimates of activity in the sources regardless of background activity. For planar scanning and SPET without scatter correction there was an overestimate of activity in the source of 290% and 40% respectively. The validity of this method was confirmed in patients by comparing activity in the cardiac ventricles measured by SPET with scatter correction with that in a simultaneous blood sample. A coefficient of correlation of 0.955 was achieved with 25 data points. SPET with scatter correction was compared with planar imaging in measuring activity in the liver and spleen of patients receiving 75 mCi131I-antibody to CEA intravenously. Planar imaging gave significantly higher values than SPET for the spleen (t=5.4,P<0.001 by the pairedt-test) but no significant difference for the liver. SPET with scatter correction forms a basis for an improved technique of quantifying the targeting efficiency.

Engineering Antibodies for Clinical Applications in Cancer

The ‘magic bullet’ concept predicted over a century ago that antibodies would be used to target cancer therapy. Since then initial problems that were related to specificity, purity and immungenicity of antibody-based reagents have slowly been overcome due to developments in technology and increased knowledge. As a result, antibodies are in use for many clinical applications and now comprise the second largest category of medicines in clinical development after vaccines. For antibody-based cancer therapeutics the last 20 years have met with an explosion of knowledge about the biology of the disease and potential targets as well as new technology which allows cloning and manipulation of multifunctional antibody-based molecules. However, the focus still remains on developing therapeutics that will have potential for treating cancer in people and this is efficiently assessed in mechanistic clinical trials that feed back to the laboratory for further development. This review illustrates the mechanistic approach to making new molecules for antibody imaging and therapy of cancer. It is illustrated by examples of radioimmunotherapy and antibody-directed enzyme prodrug therapy developed by the authors.

An overview of imaging techniques and physical aspects of treatment planning in radioimmunotherapy

Planar and tomographic imaging techniques and methods of treatment planning in clinical radioimmunotherapy are reviewed. In clinical trials, the data needed for dosimetry and treatment planning are, in most cases, obtained from noninvasive imaging procedures. The required data include tumor and normal organ volumes, the activity of radiolabeled antibodies taken up in these volumes, and the pharmacokinetics of the administered activity of radiolabeled antibodies. Therefore, the topics addressed in this review include: (1) Volume determination of tumors and normal organs from x-ray-computed tomography and magnetic resonance imaging, (2) quantitation of the activity of radiolabeled antibodies in tumors and normal organs from planar gamma camera views, (3) quantitative single-photon emission computed tomography and positron emission tomography, (4) correlative image analysis, and (5) treatment planning in clinical radioimmunotherapy.

MIRD Pamphlet No. 24: Guidelines for Quantitative 131I SPECT in Dosimetry Applications

The reliability of radiation dose estimates in internal radionuclide therapy is directly related to the accuracy of activity estimates obtained at each imaging time point. The recently published MIRD pamphlet no. 23 provided a general overview of quantitative SPECT imaging for dosimetry. The present document is the first in a series of isotope-specific guidelines that will follow MIRD 23 and focuses on one of the most commonly used therapeutic radionuclides, 131I. The purpose of this document is to provide guidance on the development of protocols for quantitative 131I SPECT in radionuclide therapy applications that require regional (normal organs, lesions) and 3-dimensional dosimetry.

Tumour targeting of humanised cross-linked divalent-Fab' antibody fragments: a clinical phase I/II study.

Antibody engineering has made it possible to design antibodies with optimal characteristics for delivery of radionuclides for tumour imaging and therapy. A humanised divalent-Fab′ cross-linked with a bis-maleimide linker referred to as humanised divalent-Fab′ maleimide was produced as a result of this design process. It is a humanised divalent antibody with no Fc, which can be produced in bacteria and has enhanced stability compared with F(ab′)2. Here we describe a clinical study in patients with colorectal cancer using humanised divalent-Fab′ maleimide generated from the anti-carcinoembryonic antigen antibody A5B7 radiolabelled with iodine-131. Ten patients received an i.v. injection of iodine-131-labelled A5B7 humanised divalent-Fab′ maleimide, and positive tumour images were obtained by gamma camera imaging in eight patients with known lesions, and one previously undetected lesion was identified. True negative results were obtained in two patients without tumour. Area under the curve analysis of serial blood gamma counting and gamma camera images showed a higher tumour to blood ratio compared to A5B7 mF(ab′)2 used previously in the clinic, implying this new molecule may be superior for radioimmunotherapy. MIRD dose calculations showed a relatively high radiation dose to the kidney, which may limit the amount of activity that could be administered in radioimmunotherapy. However the reduction in immunogenicity was also a major advantage for A5B7 humanised divalent-Fab′ maleimide over murine versions of this antibody suggesting that humanised divalent-Fab′ maleimide should be a useful vehicle for repeated therapies.

Assessment of tumour response with 18 F-fluorodeoxyglucose positron emission tomography using three-dimensional measures compared to SUVmax—a phantom study

SUVmax is currently the most common semi-quantitative method of response assessment on FDG PET. By defining the tumour volume of interest (VOI), a measure of total glycolytic volume (TGV) may be obtained. We aimed to comprehensively examine, in a phantom setting, the accuracy of TGV in reflecting actual lesion activity and to compare TGV with SUVmax for response assessment. The algorithms for VOI generation from which TGV was derived included fixed threshold techniques at 50% of maximum (MAX50), 70% of maximum (MAX70), an adaptive threshold of 50% of (maximum + background)/2 (BM50) and a semi-automated iterative region-growing algorithm, GRAB. Comparison with both actual lesion activity and response scenarios was performed. SUVmax correlated poorly with actual lesion activity (r = 0.651) and change in lesion activity (r = 0.605). In a response matrix scenario SUVmax performed poorly when all scenarios were considered, but performed well when only clinically likely scenarios were included. The TGV derived using MAX50 and MAX70 algorithms performed poorly in evaluation of lesion change. The TGV derived from BM50 and GRAB algorithms however performed extremely well in correlation with actual lesion activity (r = 0.993 and r = 0.982, respectively), change in lesion activity (r = 0.972 and r = 0.963, respectively) and in the response scenario matrix. TGV(GRAB) demonstrated narrow confidence bands when modelled with actual lesion activity. Measures of TGV generated by iterative algorithms such as GRAB show potential for increased sensitivity of metabolic response monitoring compared to SUVmax, which may have important implications for improved patient care.

The Nonuniformity of Antibody Distribution in the Kidney and its Influence on Dosimetry

Abstract Flynn, A. A., Pedley, R. B., Green, A. J., Dearling, J. L., El-Emir, E., Boxer, G. M., Boden, R. and Begent, R. H. J. The Nonuniformity of Antibody Distribution in the Kidney and its Influence on Dosimetry. Radiat. Res. 159, 182–189 (2003). The therapeutic efficacy of radiolabeled antibody fragments can be limited by nephrotoxicity, particularly when the kidney is the major route of extraction from the circulation. Conventional dose estimates in kidney assume uniform dose deposition, but we have shown increased antibody localization in the cortex after glomerular filtration. The purpose of this study was to measure the radioactivity in cortex relative to medulla for a range of antibodies and to assess the validity of the assumption of uniformity of dose deposition in the whole kidney and in the cortex for these antibodies with a range of radionuclides. Storage phosphor plate technology (radioluminography) was used to acquire images of the distributions of a range of antibodies of various sizes, labeled with 125I, in kidney sections. This allowed the calculation of the antibody concentration in the cortex relative to the medulla. Beta-particle point dose kernels were then used to generate the dose-rate distributions from 14C, 131I, 186Re, 32P and 90Y. The correlation between the actual dose-rate distribution and the corresponding distribution calculated assuming uniform antibody distribution throughout the kidney was used to test the validity of estimating dose by assuming uniformity in the kidney and in the cortex. There was a strong inverse relationship between the ratio of the radioactivity in the cortex relative to that in the medulla and the antibody size. The nonuniformity of dose deposition was greatest with the smallest antibody fragments but became more uniform as the range of the emissions from the radionuclide increased. Furthermore, there was a strong correlation between the actual dose-rate distribution and the distribution when assuming a uniform source in the kidney for intact antibodies along with medium- to long-range radionuclides, but there was no correlation for small antibody fragments with any radioisotope or for short-range radionuclides with any antibody. However, when the cortex was separated from the whole kidney, the correlation between the actual dose-rate distribution and the assumed dose-rate distribution, if the source was uniform, increased significantly. During radioimmunotherapy, the extent of nonuniformity of dose deposition in the kidney depends on the properties of the antibody and radionuclide. For dosimetry estimates, the cortex should be taken as a separate source region when the radiopharmaceutical is small enough to be filtered by the glomerulus.