Enhanced efficacy in drug-resistant cancer cells through synergistic nanoparticle mediated delivery of cisplatin and decitabine.

There are several limitations with monodrug cancer therapy, including poor bioavailability, rapid clearance and drug resistance. Combination therapy addresses these by exploiting synergism between different drugs against cancer cells. In particular, the combination of epigenetic therapies with conventional chemotherapeutic agents can improve the initial tumour response and overcome acquired drug resistance. Co-encapsulation of multiple therapeutic agents into a single polymeric nanoparticle is one of the many approaches taken to enhance therapeutic effect and improve the pharmacokinetic profile. In this study, different types of poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs), matrix and core-shell (CS), were investigated for simultaneous encapsulation of a demethylating drug, decitabine, and a potent anticancer agent, cisplatin. It was shown that by altering the configuration of the CS structure, the release profile could be tuned. In order to investigate whether this could enhance the anticancer effect compared to cisplatin, human ovarian carcinoma cell line (A2780) and its cisplatin resistant variant (A2780cis) were exposed to free cisplatin and the CS-NPs. A better response was obtained in both cell lines (11% and 51% viability of A2780 and A2780cis, respectively) using CS-NPs than cisplatin alone (27%, 82% viability of A2780 and A2780cis, respectively) or in combination with decitabine (22%, 96% viability of A2780 and A2780cis, respectively) at equivalent doses (10 µM).

Texture analysis of (125)I-A5B7 anti-CEA antibody SPECT differentiates metastatic colorectal cancer model phenotypes and anti-vascular therapy response.

BACKGROUND: We aimed to test the ability of texture analysis to differentiate the spatial heterogeneity of (125)I-A5B7 anti-carcinoembryonic antigen antibody distribution by nano-single photon emission computed tomography (SPECT) in well-differentiated (SW1222) and poorly differentiated (LS174T) hepatic metastatic colorectal cancer models before and after combretastatin A1 di-phosphate anti-vascular therapy. METHODS: Nano-SPECT imaging was performed following tail vein injection of 20 MBq (125)I-A5B7 in control CD1 nude mice (LS174T, n=3 and SW1222, n=4), and CA1P-treated mice (LS174T, n=3; SW1222, n=4) with liver metastases. Grey-level co-occurrence matrix textural features (uniformity, homogeneity, entropy and contrast) were calculated in up to three liver metastases in 14 mice from control and treatment groups. RESULTS: Before treatment, the LS174T metastases (n=7) were more heterogeneous than SW1222 metastases (n=12) (uniformity, P=0.028; homogeneity, P=0.01; contrast, P=0.045). Following CA1P, LS174T metastases (n=8) showed less heterogeneity than untreated LS174T controls (uniformity, P=0.021; entropy, P=0.006). Combretastatin A1 di-phosphate-treated SW1222 metastases (n=11) showed no difference in texture features compared with controls (all P>0.05). CONCLUSIONS: Supporting the potential for novel imaging biomarkers, texture analysis of (125)I-A5B7 SPECT shows differences in spatial heterogeneity of antibody distribution between well-differentiated (SW1222) and poorly differentiated (LS174T) liver metastases before treatment. Following anti-vascular treatment, LS174T metastases, but not SW1222 metastases, were less heterogeneous.

Hepatic arterial spin labelling MRI: an initial evaluation in mice.

The development of strategies to combat hepatic disease and augment tissue regeneration has created a need for methods to assess regional liver function. Liver perfusion imaging has the potential to fulfil this need, across a range of hepatic diseases, alongside the assessment of therapeutic response. In this study, the feasibility of hepatic arterial spin labelling (HASL) was assessed for the first time in mice at 9.4 T, its variability and repeatability were evaluated, and it was applied to a model of colorectal liver metastasis. Data were acquired using flow-sensitive alternating inversion recovery-arterial spin labelling (FAIR-ASL) with a Look-Locker readout, and analysed using retrospective respiratory gating and a T1 -based quantification. This study shows that preclinical HASL is feasible and exhibits good repeatability and reproducibility. Mean estimated liver perfusion was 2.2 ± 0.8 mL/g/min (mean ± standard error, n = 10), which agrees well with previous measurements using invasive approaches. Estimates of the variation gave a within-session coefficient of variation (CVWS) of 7%, a between-session coefficient of variation (CVBS) of 9% and a between-animal coefficient of variation (CVA) of 15%. The within-session Bland-Altman repeatability coefficient (RCWS) was 18% and the between-session repeatability coefficient (RCBS) was 29%. Finally, the HASL method was applied to a mouse model of liver metastasis, in which significantly lower mean perfusion (1.1 ± 0.5 mL/g/min, n = 6) was measured within the tumours, as seen by fluorescence histology. These data indicate that precise and accurate liver perfusion estimates can be achieved using ASL techniques, and provide a platform for future studies investigating hepatic perfusion in mouse models of disease.

Three-dimensional analysis of tumour vascular corrosion casts using stereoimaging and micro-computed tomography.

OBJECTIVE: In order to perform effective translational research for cancer therapy, we need to employ pre-clinical models which reflect the clinical situation. The purpose of this study was to quantitatively compare the vascular architecture of human colorectal cancer and experimental tumour models to determine the suitability of animal models for vascular studies and antivascular therapy. METHODS: In this study we investigated the three-dimensional properties of colonic tumour vasculature in both human clinical tissues (normal mucosa control [n=20], carcinoma [n=20] and adenoma [n=6]) and murine colorectal xenografts (LS147T [n=6] and SW1222 [n=6]). Scanning Electron Microscope Stereoimaging (SEM) and X-ray Micro-Computed Tomography (Micro-CT) methods were employed for 3D analyses of the vascular corrosion casts from these tissues. RESULTS: Morphological measurements showed that there were significant differences in the underlying morphology in the different tissues. Of the studied xenografts, LS147T is more consistently similar to the vascular architecture of the human carcinoma than SW1222. The only reversal of this is for the inter-vessel distance. CONCLUSION: While SEM stereoimaging provided better surface detailed resolution of the corrosion casts, it was complimented by the fully 3D micro-CT method. Comparison made between the xenografts and clinical tumours showed that the LS147T xenografts shared many similarities with the clinical tumour vasculature. This study provides insight into how to select the most suitable pre-clinical models for translational studies of clinical cancer therapy.

Human monoclonal antibodies targeting carbonic anhydrase IX for the molecular imaging of hypoxic regions in solid tumours.

BACKGROUND: Hypoxia, which is commonly observed in areas of primary tumours and of metastases, influences response to treatment. However, its characterisation has so far mainly been restricted to the ex vivo analysis of tumour sections using monoclonal antibodies specific to carbonic anhydrase IX (CA IX) or by pimonidazole staining, after the intravenous administration of this 2-nitroimidazole compound in experimental animal models. METHODS: In this study, we describe the generation of high-affinity human monoclonal antibodies (A3 and CC7) specific to human CA IX, using phage technology. RESULTS: These antibodies were able to stain CA IX ex vivo and to target the cognate antigen in vivo. In one of the two animal models of colorectal cancer studied (LS174T), CA IX imaging closely matched pimonidazole staining, with a preferential staining of tumour areas characterised by little vascularity and low perfusion. In contrast, in a second animal model (SW1222), distinct staining patterns were observed for pimonidazole and CA IX targeting. We observed a complementary pattern of tumour regions targeted in vivo by the clinical-stage vascular-targeting antibody L19 and the anti-CA IX antibody A3, indicating that a homogenous pattern of in vivo tumour targeting could be achieved by a combination of the two antibodies. CONCLUSION: The new human anti-CA IX antibodies are expected to be non-immunogenic in patients with cancer and may serve as broadly applicable reagents for the non-invasive imaging of hypoxia and for pharmacodelivery applications.

Data standards for minimum information collection for antibody therapy experiments.

Research groups developing antibody therapies generate diverse data sets; the value of these sets would be compounded when shared or amalgamated. A complete amalgamation of diverse data sets requires data standards for information collection during experiments. We propose to define elements of the data standards in the form of common data elements (CDEs) in order to clarify each experiment's targets and data values. We have created a set of core information elements which we suggest should be collected from antibody therapy experiments. We propose these as a basis for community consultation with a view to defining a set of data standards which can be developed under the auspices of the Antibody Society.

In vivo high-resolution 3D photoacoustic imaging of superficial vascular anatomy.

The application of a photoacoustic imaging instrument based upon a Fabry-Perot polymer film ultrasound sensor to imaging the superficial vasculature is described. This approach provides a backward mode-sensing configuration that has the potential to overcome the limitations of current piezoelectric based detection systems used in superficial photoacoustic imaging. The system has been evaluated by obtaining non-invasive images of the vasculature in human and mouse skin as well as mouse models of human colorectal tumours. These studies showed that the system can provide high-resolution 3D images of vascular structures to depths of up to 5 mm. It is considered that this type of instrument may find a role in the clinical assessment of conditions characterized by changes in the vasculature such as skin tumours and superficial soft tissue damage due to burns, wounds or ulceration. It may also find application in the characterization of small animal cancer models where it is important to follow the tumour vasculature over time in order to study its development and/or response to therapy.

Fractionated 131I anti-CEA radioimmunotherapy: effects on xenograft tumour growth and haematological toxicity in mice.

Dose fractionation has been proposed as a method to improve the therapeutic ratio of radioimmunotherapy (RIT). This study compared a single administration of 7.4 MBq 131I-anti-CEA antibody given on day 1 with the same total activity given as fractionated treatment: 3.7 MBq (days 1 and 3), 2.4 MBq (days 1, 3, and 5) or 1.8 MBq (days 1, 3, 5, and 8). Studies in nude mice, bearing the human colorectal xenograft LS174T, showed that increasing the fractionation significantly reduced the efficacy of therapy. Fractionation was associated with a decrease in systemic toxicity as assessed by weight, but did not lead to any significant decrease in acute haematological toxicity. Similarly, no significant decrease in marrow toxicity, as assessed by colony-forming unit assays for granulocytes and macrophages (CFUgm), was seen. However, there was a significant depression of CFUgm counts when all treated animals were compared with untreated controls, suggesting that treatment did suppress marrow function. In conclusion, in this tumour model system, fractionated RIT causes less systemic toxicity, but is also less effective at treating tumours.

Relationship between human tumour angiogenic profile and combretastatin-induced vascular shutdown: an exploratory study.

Combretastatin-A4-phosphate (CA4P) acts most effectively against immature tumour vasculature. We investigated whether histological angiogenic profile can explain the differential sensitivity of human tumours to CA4P, by correlating the kinetic changes demonstrated by dynamic MRI (DCE-MRI) in response to CA4P, with tumour immunohistochemical angiogenic markers. Tissue was received from 24 patients (mean age 59, range 32-73, 18 women, 6 men). An angiogenic profile was performed using standard immunohistochemical techniques. Dynamic MRI data were obtained for the same patients before and 4 h after CA4P. Three patients showed a statistically significant fall in K(trans) following CA4P, and one a statistically significant fall in IAUGC(60). No statistically significant correlations were seen between the continuous or categorical variables and the DCE-MRI kinetic parameters other than between ang-2 and K(trans) (P=0.044). In conclusion, we found no strong relationships between changes in DCE-MRI kinetic variables following CA4P and the immunohistochemical angiogenic profile.

From laboratory to Phase I/II cancer trials with recombinant biotherapeutics.

Many promising recombinant cancer medicines are generated by academic research and increasing the number of these products that are translated into the clinic will increase the pipeline of new therapies. Recombinant proteins for use in Phase I/II cancer trials must be produced to standards of Good Manufacturing Practice (GMP) in compliance with EU law. This can be a major obstacle for translating experimental products to clinical reality especially when there is no established process or prior experience with GMP. Here, we illustrate the principals of GMP with a step-by-step guide and we show that GMP can be achieved on a relatively small scale in the researchers own institution. The process is exemplified with an antibody-based therapeutic expressed in the yeast Pichia pastoris. The purified product has been used safely in patients and the principles are applicable to any recombinant protein required for Phase I/II cancer trials.

Technological advances in radioimmunotherapy.

Radioimmunotherapy (RIT) is a method of selectively delivering radionuclides with toxic emissions to cancer cells, while reducing the dose to normal tissues. Although primary tumours can often be treated successfully with external beam radiotherapy or surgery, metastases often escape detection and treatment, leading to therapy failure, and these can be treated with systemic targeted therapies such as RIT. This review describes more recent developments in the field, including both technological developments from the laboratory and increasingly encouraging findings from clinical studies.

Characterisation and radioimmunotherapy of L19-SIP, an anti-angiogenic antibody against the extra domain B of fibronectin, in colorectal tumour models.

Angiogenesis is a characteristic feature of tumours and other disorders. The human monoclonal antibody L19- SIP targets the extra domain B of fibronectin, a marker of angiogenesis expressed in a range of tumours. The aim of this study was to investigate whole body distribution, tumour localisation and the potential of radioimmunotherapy with the L19-small immunoprotein (SIP) in colorectal tumours. Two colorectal tumour models with highly different morphologies, the SW1222 and LS174T xenografts, were used in this study. Localisation and retention of the L19-SIP antibody at tumour vessels was demonstrated using immunohistochemistry and Cy3-labelled L19-SIP. Whole body biodistribution studies in both tumour models were carried out with (125)I-labelled L19-SIP. Finally, (131)I-labelled antibody was used to investigate the potential of radioimmunotherapy in SW1222 tumours. Using immunohistochemistry, we confirmed extra domain B expression in the tumour vasculature. Immunofluorescence demonstrated localisation and retention of injected Cy3-labelled L19-SIP at the abluminal side of tumour vessels. Biodistribution studies using a (125)I-labelled antibody showed selective tumour uptake in both models. Higher recorded values for localisation were found in the SW1222 tumours than in the LS174T (7.9 vs 6.6 %ID g(-1)), with comparable blood clearance for both models. Based on these results, a radioimmunotherapy study was performed in the SW1222 xenograft using (131)I-Labelled L19-SIP (55.5 MBq), which showed selective tumour uptake, tumour growth inhibition and improved survival. Radio- and fluorescence-labelled L19-SIP showed selective localisation and retention at vessels of two colorectal xenografts. Furthermore, (131)I-L19-SIP shows potential as a novel treatment of colorectal tumours, and provides the foundation to investigate combined therapies in the same tumour models.

Radiolabelling of glycosylated MFE-23::CPG2 fusion protein (MFECP1) with 99mTc for quantitation of tumour antibody-enzyme localisation in antibody-directed enzyme pro-drug therapy (ADEPT).

MFECP1 is a glycosylated recombinant fusion protein composed of MFE-23, a high-affinity anti-carcinoembryonic antigen (CEA) single chain Fv (scFv), fused to the enzyme carboxypeptidase G2 (CPG2), and has been constructed for use in antibody-directed enzyme pro-drug therapy (ADEPT). Radiolabelling of glycosylated MFECP1 with technetium-99m was developed for the purpose of determining tumour localisation of MFECP1 in a phase I ADEPT clinical study. The method used was 99mTc-carbonyl [99mTc(H2O)3(CO)3]+ (abbreviated to TcCO) mediated labelling of 99mTc to the hexahistidine (His) tag of MFECP1. MFECP1 fusion protein was labelled with TcCO under a variety of conditions, and this was shown to be a relatively simple and robust method. Tissue biodistribution was assessed in a CEA-expressing LS174T (human colon carcinoma) nude mouse xenograft model. Tissues were taken at 1, 4 and 6 h for assessment of distribution of radioactivity and for measurement of CPG2 enzyme levels. The amount of radioactivity retained by the tumour proved to be an accurate estimation of actual measured enzyme activity, indicating that this radiolabelling method does not appear to damage the antibody-antigen binding or the enzyme activity of MFECP1. However, correlation between CPG2 enzyme activity and measured radioactivity in liver, spleen and kidney was poor, indicating retention of radioactivity in non-tumour sites but loss of enzyme activity. The high retention of technetium radioisotope in normal tissues may limit the clinical applicability of this radiolabelling method for MFECP1; however, these results suggest that this technique does have applicability for measuring the biodistribution of His-tagged recombinant proteins.

Higher dose and dose-rate in smaller tumors result in improved tumor control.

Small tumors are more sensitive to radioimmunotherapy (RIT) than larger ones. A greater proportion of viable radiosensitive areas in small tumors, higher antibody uptake, and radiation dose may be responsible. Six groups of mice with small (median tumor size 0.06 cm3) or large LoVo xenografts (median tumor size 0.38 cm3) received either RIT using a 131I-labeled anti-CEA antibody A5B7, 5-fluorouracil (5-FU) modulated with folinic acid (FA), or no treatment. The % injected activity/gram, antibody distribution in viable and necrotic areas, and dose distribution were determined. High-power microscopy images of the original section were reconstructed to estimate the proportion of viable areas. Mice with small and large tumors grew significantly less rapidly when treated with RIT compared to the control group (p < 0.0004 and p < 0.003, respectively), while 5-FU was ineffective. Small tumors treated with RIT grew less than large tumors (p < 0.02). A higher amount of % injected activity/gram of tumor (median 26.6% vs. 8.1%, p = 0.0007) and a higher dose-rate were found in small tumors at 24 hours post injection (viable areas: 56.2 +/- 23.7 vs. 13.3 +/- 7 cGy/h, necrosis 19.2 +/- 16.3 vs. 4.9 +/- 4.7 cGy/h, p = 0.0007). It appears that as viable tumor masses grow the access to them decreases and this has a fourfold effect on dose delivered for RIT in this example. These data support the consideration of use of RIT for adjuvant treatment in colon cancer.

Spatial accuracy of 3D reconstructed radioluminographs of serial tissue sections and resultant absorbed dose estimates.

Many agents using tumour-associated characteristics are deposited heterogeneously within tumour tissue. Consequently, tumour heterogeneity should be addressed when obtaining information on tumour biology or relating absorbed radiation dose to biological effect. We present a technique that enables radioluminographs of serial tumour sections to be reconstructed using automated computerized techniques, resulting in a three-dimensional map of the dose-rate distribution of a radiolabelled antibody. The purpose of this study is to assess the reconstruction accuracy. Furthermore, we estimate the potential error resulting from registration misalignment, for a range of beta-emitting radionuclides. We compare the actual dose-rate distribution with that obtained from the same activity distribution but with manually defined translational and rotational shifts. As expected, the error produced with the short-range 14C is much larger than that for the longer range 90Y; similarly values for the medium range 131I are between the two. Thus, the impact of registration inaccuracies is greater for short-range sources.

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.

Antibody and radionuclide characteristics and the enhancement of the effectiveness of radioimmunotherapy by selective dose delivery to radiosensitive areas of tumour.

PURPOSE: Estimating the absorbed dose to tumour relative to normal tissues has often been used in the assessment of the therapeutic efficacy of radiolabelled antibodies for radioimmunotherapy. Typically, the calculations assume a uniform dose deposition and response throughout the tumour. However, the heterogeneity of the dose delivery and response within tumours can lead to a radiobiological effect inconsistent with dose estimates. The aim was to assess the influence of antibody and radionuclide characteristics on the heterogeneity of dose deposition. MATERIALS AND METHODS: Quantitative images of the temporal and spatial heterogeneity of a range of antibodies in tumour were acquired using radioluminography. Subsequent registration with images of tumour morphology then allowed the delineation of viable and necrotic areas of tumour and the measurement of the antibody concentration in each area. A tumour dosimetry model then estimated the absorbed dose from 131I and 90Y in each area. RESULTS: Tumour-specific antibodies initially localized in the viable radiosensitive areas of tumour and then penetrated further into tumour with continued tumour accretion. Multivalent antibodies were retained longer and at higher concentrations in viable areas, while monovalent antibodies had greater mobility. In contrast, non-specific antibodies penetrated into necrotic regions regardless of their size. As a result, multivalent, specific antibodies delivered a significantly larger dose to viable cells compared with monovalent antibodies, while non-specific antibodies deposited most of the dose in necrotic areas. There was a significant difference in dose estimates when assuming a uniform dose deposition and accounting for heterogeneity. The dose to the viable and necrotic areas also depended on the properties of the radionuclide where antibodies labelled with 131I generally delivered a higher dose throughout the tumour even though the instantaneous dose-rate distribution for 90Y was more uniform. CONCLUSIONS: The extent of heterogeneity of dose deposition in tumour is highly dependent on the antibody characteristics and radionuclide properties, and can enhance therapeutic efficacy through the selective dose delivery to the radiosensitive areas of tumour.

Relationship between tumour morphology, antigen and antibody distribution measured by fusion of digital phosphor and photographic images.

Antibody-directed cancer therapy has achieved encouraging responses despite poor localisation in tumour. This discrepancy may be attributed to heterogeneity of antibody delivery within tumours: preferential localisation in the better perfused and more radio- and chemosensitive areas provides a therapeutic advantage. Antibody distribution depends upon the interactions of many complex mechanisms. We have started to investigate this by studying the single and combined influence of two tumour-associated parameters, morphology and antigen, on antibody distribution. Tumours were taken from mice at 24 and 48 h after 125I-labeled anti-CEA antibody injection. Images of antibody distribution, antigen distribution and tumour morphology were acquired by radioluminography, radioimmunoluminography and digitisation of morphology, respectively. Image registration allowed correlation of pixel values of antibody distribution with corresponding values of antigen distribution and morphology. At 24 h there was little correlation between antibody and antigen distribution, but strong positive correlation between antibody distribution and morphology, with preferential localisation in viable tumour areas. Correlation between antibody distribution and morphology fell significantly between 24 and 48 h, while that between antibody and antigen distribution remained low. However, the combination of morphology and antigen distribution showed the largest influence on antibody distribution. This novel technique demonstrates potential for combining multi-factor information in order to provide a greater understanding of antibody distribution in tumours, facilitating the optimisation of clinical treatments.

Measurement of the critical DNA lesions produced by antibody-directed enzyme prodrug therapy (ADEPT) in vitro, in vivo and in clinical material.

An antibody-directed enzyme prodrug therapy (ADEPT) system against CEA-positive tumours is currently in phase I clinical trials. It consists of a prodrug, 4-[N,N-bis(2-iodoethyl) amino] phenoxycarbonyl L -glutamic acid (ZD2767P) and a conjugate of the F(ab')(2) anti-CEA antibody A5B7 and the bacterial enzyme carboxypeptidase G2 (CPG2). ZD2767P is converted by antibody-targeted CPG2 into an active bifunctional alkylating drug (ZD2767) at the tumour site. The IC(50) value of the prodrug against the human colorectal tumour LS174T cell line was 55 +/- 9 microM following a 1 h exposure. In contrast, co-incubation of ZD2767P with CPG2 resulted in 229-fold increase in activity. Using a modified comet assay, DNA interstrand cross links (ISC) were detected within 1 h of ZD2767P + CPG2 treatment and were repaired by 24 h. A clear dose-response was seen between the level of ISC, growth inhibition and ZD2767 concentration. Administration of a therapeutic dose of ZD2767P 72 h after the F(ab')(2) A5B7 conjugate to mice bearing LS147T xenografts resulted in extensive ISC in the tumour after 1 h; repair was seen at 24 h. Tumour biopsies and peripheral lymphocytes were studied in 5 patients on the ADEPT phase I clinical trial. In 4 patients no ISC were detected. These patients also demonstrated poor localization of conjugate and no tumour response was seen. However a significant level of ISC was detected in one tumour biopsy, which also showed evidence of conjugate localization and clinical response. These studies demonstrate the application of the comet assay in the measurement of ISC in vitro and in clinical material and confirm that activation of ZD2767P results in the formation of DNA crosslinks.

A mouse model for calculating the absorbed beta-particle dose from (131)I- and (90)Y-labeled immunoconjugates, including a method for dealing with heterogeneity in kidney and tumor.

Flynn, A. A., Green, A. J., Pedley, R. B., Boxer, G. M., Boden, R. and Begent, R. H. J. A Mouse Model for Calculating the Absorbed Beta-Particle Dose from (131)I- and (90)Y-Labeled Immunoconjugates, Including a Method for Dealing with Heterogeneity in Kidney and Tumor. Radiat. Res. 156, 28-35 (2001). Conventional internal radiation dosimetry methods assume that the beta-particle energy is absorbed uniformly and completely in the source organ and that the radioactivity is distributed uniformly in the source. However, in mice, a considerable proportion of the beta-particle energy can escape the source organ, resulting in large cross-organ doses. Furthermore, the distribution of radioactivity is generally heterogeneous in kidney and tumor. Therefore, a model was developed to account for cross-organ doses and for the effects of heterogeneity in kidney and tumor in mice for two of the most important radionuclides used in therapy, (131)I and (90)Y. Most mouse organs were modeled as single-compartment ellipsoids or cylinders, while heterogeneity in kidney and in tumor was addressed by using two compartments to represent the cortex and the medulla and viable and necrotic cells, respectively. The dimensions of these models were taken from previous studies, with the exception of kidney and tumor, which were defined using radioluminography and mosaics of high-power microscopy images. The absorbed fractions in each compartment were calculated using beta-particle point dose kernels. The self-organ dose was significantly higher for (131)I compared to (90)Y in all compartments, but a considerable amount of beta-particle energy was shown to escape the source organ for both radionuclides, with as much as 85% and 36% escaping the marrow for (90)Y and (131)I, respectively. The cortex was found to occupy a greater proportion of the total kidney volume than the medulla, and consequently the self-dose was higher in the cortex. In addition, the thickness of the viable shell in the tumor increased with tumor size, as did the self-dose fractions in both necrotic and viable areas. This dosimetry model improves dose estimates in mice and gives a conceptual basis for considering dosimetry in humans.