An Automated Radiosynthesis of [68Ga]Ga-FAPI-46 for Routine Clinical Use.

[68Ga]Ga-FAPI-46 is a promising new tracer for the imaging of fibroblast activation protein (FAP) by positron emission tomography (PET). Labeled FAP inhibitors (FAPIs) have demonstrated uptake in various types of cancers, including breast, lung, prostate, pancreatic and colorectal cancer. FAPI-PET also possesses a practical advantage over FDG-PET as fasting and resting are not required. [68Ga]Ga-FAPI-46 exhibits enhanced pharmacokinetic properties, improved tumor retention, and higher contrast images than the earlier presented [68Ga]Ga-FAPI-02 and [68Ga]Ga-FAPI-04. Although a manual synthesis protocol for [68Ga]Ga-FAPI-46 was initially described, in recent years, automated methods using different commercial synthesizers have been reported. In this work, we describe the development of the automated synthesis of [68Ga]Ga-FAPI-46 using the iPHASE MultiSyn synthesizer for clinical applications. Initially, optimization of the reaction time and comparison of the performance of four different solid phase extraction (SPE) cartridges for final product purification were investigated. Then, the development and validation of the production of 0.6-1.7 GBq of [68Ga]Ga-FAPI-46 were conducted using these optimized parameters. The product was synthesized in 89.8 ± 4.8% decay corrected yield (n = 6) over 25 min. The final product met all recommended quality control specifications and was stable up to 3 h post synthesis.

Automated radiosynthesis of [89Zr]Zr-DFOSq-Durvalumab for imaging of PD-L1 expressing tumours in vivo.

OBJECTIVES: 89Zr-labelled proteins are gaining importance in clinical research in a variety of diseases. To date, no clinical study has been reported that utilizes an automated approach for radiosynthesis of 89Zr-labelled radiopharmaceuticals. We aim to develop an automated method for the clinical production of 89Zr-labelled proteins and apply this method to Durvalumab, a monoclonal antibody targeting PD-L1 immune-checkpoint protein. PD-L1 expression is poorly understood and can be up-regulated over the course of chemo- and radiotherapy treatment. The ImmunoPET multicentre study aims to examine the dynamics of PD-L1 expression via 89Zr-Durvalumab PET imaging before, during, and after chemoradiotherapy. The developed automated technique will enable reproducible clinical production of [89Zr]Zr-DFOSq-Durvalumab for this study at three different sites. METHODS: Conjugation of Durvalumab to H3DFOSqOEt was optimized for optimal chelator-to-antibody ratio. Automated radiolabelling of H3DFOSq-Durvalumab with zirconium-89 was optimized on the disposable cassette based iPHASE technologies MultiSyn radiosynthesizer using a modified cassette. Activity losses were tracked using a dose calibrator and minimized by optimizing fluid transfers, reaction buffer, antibody formulation additives and pH. The biological profile of the radiolabelled antibody was confirmed in vivo in PD-L1+ (HCC827) and PD-L1- (A549) murine xenografts. Clinical process validation and quality control were performed at three separate study sites to satisfy clinical release criteria. RESULTS: H3DFOSq-Durvalumab with an average CAR of 3.02 was obtained. Radiolabelling kinetics in succinate (20 mM, pH 6) were significantly faster when compared to HEPES (0.5 M, pH 7.2) with >90 % conversion observed after 15 min. Residual radioactivity in the 89Zr isotope vial was reduced from 24 % to 0.44 % ± 0.18 % (n = 7) and losses in the reactor vial were reduced from 36 % ± 6 % (n = 4) to 0.82 % ± 0.75 % (n = 4) by including a surfactant in the reaction and formulation buffers. Overall process yield was 75 % ± 6 % (n = 5) and process time was 40 min. Typically, 165 MBq of [89Zr]Zr-DFOSq-Durvalumab with an apparent specific activity of 315 MBq/mg ± 34 MBq/mg (EOS) was obtained in a volume of 3.0 mL. At end-of-synthesis (EOS), radiochemical purity and protein integrity were always >99 % and >96 %, respectively, and dropped to 98 % and 65 % after incubation in human serum for 7 days at 37 °C. Immunoreactive fraction in HEK293/PD-L1 cells was 83.3 ± 9.0 (EOS). Preclinical in vivo data at 144 h p.i. showed excellent SUVmax in PD-L1+ tumour (8.32 ± 0.59) with a tumour-background ratio of 17.17 ± 3.96. [89Zr]Zr-DFOSq-Durvalumab passed all clinical release criteria at each study site and was deemed suitable for administration in a multicentre imaging trial. CONCLUSION: Fully automated production of [89Zr]Zr-DFOSq-Durvalumab for clinical use was achieved with minimal exposure to the operator. The cassette-based approach allows for consecutive productions on the same day and offers an alternative to currently used manual protocols. The method should be broadly applicable to other proteins and has the potential for clinical impact considering the growing number of clinical trials investigating 89Zr-labelled antibodies.

Development of a high-performance liquid chromatography method for rapid radiochemical purity measurement of [18 F]PSMA-1007, a PET radiopharmaceutical for detection of prostate cancer.

Since first becoming commercially available in 2018, the PET radiopharmaceutical [18 F]PSMA-1007 has been used widely for the diagnosis and staging of prostate cancer. A pharmacopoeia monograph first became available in 2021, prescribing a radiochemical purity specification of >91%, based on analytical results from both TLC (for [18 F]fluoride impurity alone) and HPLC (for all other 18 F-impurities). Though this monograph has provided clarity for the quality control testing of [18 F]PSMA-1007, it prescribes a HPLC method using phosphate buffer mobile phase that may present a risk of precipitation of phosphate salts in the HPLC system. The method also requires specialised hardware not immediately available to all laboratories. This work describes the development of a simple, rapid reversed-phase HPLC method utilising 0.1 M ammonium formate mobile phase for the accurate assessment of both [18 F]fluoride impurity and overall radiochemical purity in a single test. This method is especially useful for assessment of product stability over time. A more accurate TLC method for [18 F]fluoride impurity is also described.

Synthesis of human amyloid restricted to liver results in an Alzheimer disease-like neurodegenerative phenotype.

Several lines of study suggest that peripheral metabolism of amyloid beta (Aß) is associated with risk for Alzheimer disease (AD). In blood, greater than 90% of Aß is complexed as an apolipoprotein, raising the possibility of a lipoprotein-mediated axis for AD risk. In this study, we report that genetic modification of C57BL/6J mice engineered to synthesise human Aß only in liver (hepatocyte-specific human amyloid (HSHA) strain) has marked neurodegeneration concomitant with capillary dysfunction, parenchymal extravasation of lipoprotein-Aß, and neurovascular inflammation. Moreover, the HSHA mice showed impaired performance in the passive avoidance test, suggesting impairment in hippocampal-dependent learning. Transmission electron microscopy shows marked neurovascular disruption in HSHA mice. This study provides causal evidence of a lipoprotein-Aß /capillary axis for onset and progression of a neurodegenerative process.

Direct in vivo comparison of [18F]PSMA-1007 with [68Ga]Ga-PSMA-11 and [18F]AlF-PSMA-11 in mice bearing PSMA-expressing xenografts.

The aim of this preclinical study was to directly compare [18F]PSMA-1007 with both [68Ga]Ga-PSMA-11 and [18F]AlF-PSMA-11 in mice bearing PSMA-positive tumor xenografts. Uptake was assessed by PET/CT at 1, 2 and 4 h post-injection, and by ex vivo measurement after 4 h. [18F]PSMA-1007 demonstrated the highest tumor uptake of the three tracers. The high uptake in bone for mice injected with [18F]AlF-PSMA-11 suggested rapid in vivo decomposition. This was confirmed by an in vitro plasma stability study.

Comparison between gallium-68 citrate positron emission tomography-computed tomography and gallium-67 citrate scintigraphy for infection imaging.

BACKGROUND: Preliminary studies have reported promising results for the utility of gallium-68 (Ga-68) citrate positron emission tomography-computed tomography (PET-CT) for infection imaging. This technique offers reduced radiation dose to patients, shorter time between injection and imaging and reduced time for image acquisition compared to the 'gold standard' nuclear imaging technique: gallium-67 (Ga-67) citrate scintigraphy. AIMS: To compare the two imaging modalities to ascertain whether Ga-68 citrate PET-CT is of equivalent diagnostic efficacy for bone and joint infection or pyrexia of unknown origin (PUO) and to assess image quality and reporter confidence. METHODS: Patients with PUO and suspected bone or joint infection underwent Ga-67 citrate scintigraphy and Ga-68 citrate PET-CT. Participants were followed up for 3 months to record subsequent treatment, investigations and outcome. RESULTS: 60 patients were recruited to this multicentre prospective study: 32 for bone and joint infection, 28 for PUO. The results show a sensitivity of 81% for Ga-67 citrate scintigraphy and 69% for Ga-68 citrate PET-CT, a specificity of 79% for Ga-67 citrate and 67% for Ga-68 citrate and were concordant for 76% of the participants. The reporting physician confidence was significantly lower for Ga-68 citrate (P < 0.05), frequently due to prominent physiologic blood pool activity adjacent to the site of infection. CONCLUSION: The sensitivity and specificity of Ga-68 citrate PET-CT were found to be consistently lower than Ga-67 citrate scintigraphy. Additionally, due to the insufficient level of confidence of the reporting physicians for the Ga-68 citrate PET-CT, this modality could not currently be recommended to replace Ga-67 citrate scintigraphy for routine clinical use.

68 Ga-somatostatin analogue PET-CT: Analysis of costs and benefits in a public hospital setting.

INTRODUCTION: Between 2009 and 2012, 68 Ga-somatostatin analogue PET-CT progressively replaced 111 In-octreotide scintigraphy for imaging neuroendocrine tumours in WA public hospitals due to published literature demonstrating improved diagnostic accuracy and increased availability. Despite significantly improved sensitivity and specificity, 68 Ga-somatostatin analogue PET is currently unfunded in Australia. This study sought to undertake cost analysis of the two modalities in a public hospital setting and to compare them with regard to patient factors such as imaging time and radiation dose. METHODS: This analysis was based on retrospective clinical data from 95 111 In-octreotide scintigraphies performed in 2007 and 2008 at Sir Charles Gairdner (SCGH) and Royal Perth (RPH) hospitals and 219 68 Ga-somatostatin analogue PET-CT studies performed in 2013 at SCGH. Whole body effective radiation dose was derived from the radiopharmaceutical and low-dose CT scan. The cost analysis included radiopharmaceutical and imaging costs. RESULTS: The median imaging time for an 111 In-octreotide scintigraphy was 152 min at SCGH, 100 min at RPH and 20 min for a 68 Ga-somatostatin analogue PET-CT scan. The mean effective radiation dose for 111 In-octreotide scintigraphy was 18.1 mSv at SCGH and 13.8 mSv at RPH. The effective dose for 68 Ga-somatostatin analogue PET-CT was 8.7-10.8 mSv. The average cost of 68 Ga-somatostatin analogue PET-CT was four times less than 111 In-octreotide scintigraphy. CONCLUSION: 68 Ga-somatostatin analogue PET-CT is not only more accurate than 111 In-octreotide scintigraphy, this study has also shown that it is significantly less expensive, delivers a lower radiation dose to patients and requires less imaging time for patients and staff. 68 Ga-somatostatin PET-CT provides an important combination of both reduced cost and improved clinical care for patients.

A pilot study of the utility of choline PET-CT in prostate cancer biochemical relapse following radical prostatectomy.

INTRODUCTION: To evaluate the detection rate of positive choline PET-CT and its clinical role in assisting with management decisions and the correlation between positive choline PET-CT and clinical/pathological parameters in prostate cancer patients with biochemical relapse following radical prostatectomy. METHODS: This was a longitudinal observational pilot study of 34 patients who received choline PET-CT scans with biochemical relapse after radical prostatectomy. Variables including peak PSA, PSA doubling time (DT), Gleason score, age, initial PSA at diagnosis, use of ADT prior to PET and initial clinical staging were statistically analysed to assess for independent predictive factors for positive PET findings. RESULTS: Choline PET-CT was positive in 38.2% of patients (13/34). The only statistically significant predictor for positive PET-CT was the use of ADT prior to PET-CT, with OR 18.7 (95% CI, 2.87-122.45), P < 0.01. Mean peak PSA for patients with positive PET-CT was 5.5 ± 4.8 ng/mL. Patients with positive PET-CT had a mean PSA DT of 5.1 ± 3.8 months and mean total Gleason of 7.6 ± 0.8. Although these variables were not statistically significant, they showed a tendency towards significance. At Receiver Operator Characteristics (ROC) analysis, a peak PSA value of 1.65 ng/mL and PSA DT of 4.4 months were determined to be the optimal cut-off values predicting positive PET-CT. CONCLUSION: Choline PET-CT has its potential as a diagnostic modality enabling the detection of occult prostate cancer recurrence and to differentiate localised disease from systemic disease thus guiding management. Use of ADT prior to PET-CT is a significant predictor of positive PET-CT. Patients with a short PSA DT, high-peak PSA and high Gleason score should also be considered for choline PET-CT.

Characterization of hypoxia in malignant pleural mesothelioma with FMISO PET-CT.

OBJECTIVES: Malignant pleural mesothelioma (MPM) is a chemotherapy resistant tumor with a poor prognosis. Hypoxia is increasingly recognized as an important factor in tumor aggressiveness and cellular resistance to chemotherapy and radiation treatment. This prospective pilot study was performed with [F-18] fluoromisonidazole (FMISO) PET-CT to characterize hypoxia in patients with MPM. MATERIALS AND METHODS: Twenty prospectively recruited patients with histologically or cytologically confirmed MPM not currently receiving systemic or local treatment underwent both FMISO and fluorodeoxyglucose (FDG) PET-CT scans within 2 weeks. FMISO and FDG PET-CT scans were independently analyzed visually and semi-quantitatively using SUVmax and tumor to background ratio (TBR) in order to assess tumor hypoxia and metabolic activity. Lesion by lesion analysis was performed in sites of measurable pleural masses. RESULTS: Visual analysis demonstrated tumor FMISO activity in 17 of 20 patients, and tumor FDG activity in 19 of 20 patients. Focal areas of bulky tumor were most likely to demonstrate hypoxia. In 19 patients suitable for semi-quantitative analysis the median FDG SUVmax was 6.4 (range 1.9-19.1), median FMISO SUVmax was 2.5 (range 1.4-3.7) and median FMISO TBR was 1.8 (1.1-2.5). There was a positive correlation between intensity of metabolic activity and hypoxia (r=0.72, p=0.001). Lesion by lesion analysis demonstrated a positive correlation between tumor thickness and FMISO activity (r=0.77, p<0.001). CONCLUSION: This pilot study confirms that MPM is a tumor with significant areas of hypoxia, particularly in dominant tumor masses. The relationship of tumor hypoxia to effectiveness of chemotherapy and/or radiation therapy warrants prospective assessment.

Excitation functions of (nat)Zn(p,x) nuclear reactions with proton beam energy below 18 MeV.

We measured the excitation functions of (nat)Zn (p,x) reactions up to 17.6MeV, using the stacked-foils activation technique. High-purity natural zinc (and copper) foils were irradiated with proton beams generated by an 18MeV isochronous cyclotron. Activated foils were measured using high-purity Ge gamma spectroscopy to quantify the radionuclides (61)Cu, (66)Ga, (67)Ga, and (65)Zn produced from the reactions. Thick-target integral yields were also deduced from the measured excitation functions of the produced radioisotopes. These results were compared with the published literature and were found to be in good agreement with most reports, particularly those most recently compiled.

Detection of hypoxia with 18F-fluoromisonidazole (18F-FMISO) PET/CT in suspected or proven pancreatic cancer.

PURPOSE OF THE REPORT: Pancreatic carcinoma is known to demonstrate molecular features of hypoxia. The aim of this prospective pilot study is to analyze the hypoxia agent fluoromisonidazole (FMISO) using PET/CT in pancreatic carcinoma and to compare FMISO activity with glucose metabolism reflected by FDG. PATIENTS AND METHODS: Ten patients with pancreatic carcinoma underwent FMISO and FDG PET scans. FMISO and FDG PET/CT scans were analyzed by 2 PET physicians. Regions of interest drawn on the FDG images were transposed to the FMISO images after study coregistration. The FDG SUVmax was used to quantify metabolic activity and FMISO SUVmax and tumor-to-background (muscle) ratio to quantify hypoxia. RESULTS: Seven patients were diagnosed with pancreatic adenocarcinoma. The remaining patients had a neuroendocrine tumor, poorly differentiated/sarcomatoid carcinoma, and mucinous neoplasm. Visual analysis demonstrated increased FMISO activity in 2 pancreatic adenocarcinomas. All patients, however, had increased FDG activity at the tumor site. Mean FDG SUVmax was 6 (range: 3.8 to 9.5) compared to 2.3 for FMISO (range: 1 to 3.4). The 2 positive studies on visual analysis of FMISO did not correspond to the largest tumors, the studies with the highest FMISO or FDG SUVmax. There was no significant correlation between FMISO and FDG SUVmax values. CONCLUSIONS: The hypoxia imaging agent, FMISO, demonstrates minimal activity in pancreatic tumors. If FMISO PET/CT is to be included in clinical trial protocols of hypoxia in pancreatic cancer, it would require correlation with other imaging modalities to localize the tumor and allow semiquantitative analysis.

Novel molecular imaging in lung and pleural diseases.

Molecular imaging provides an opportunity to study biological processes in vivo. Specific molecular 'probes' are labelled with radioactive tracers, and imaging is carried out using either PET or gamma-cameras. The imaging is quantitative, and therefore the activity of a specific biological process (e.g. metabolism or proliferation) can be numerically assessed, which may be important for prognosis or therapy monitoring. The use of molecular imaging may lead to the development of a 'molecular profile' of a disease, therefore facilitating individualization of therapy and rational treatment approaches. This review article summarizes the most commonly used molecular imaging agents and their role in lung and pleural diseases. This is a rapidly developing field as new targets and imaging probes are being developed and as their clinical roles are being established.

¹8F-Fluoromethylcholine (FCH) PET imaging in patients with castration-resistant prostate cancer: prospective comparison with standard imaging.

PURPOSE: The aim of the study was to assess the utility of (18)F-fluorocholine (FCH), compared to standard imaging of bone scan (BS) and contrast-enhanced abdominopelvic computed tomography (CT), in patients with castration-resistant prostate carcinoma. METHODS: FCH has shown promise as a metabolic imaging agent for prostate carcinoma. Twenty-six patients with castration-resistant prostate carcinoma had FCH, BS and CT imaging within a 2-month period. Individual FCH-positive lesions in bone were compared to the BS and soft tissue lesions were compared to CT. The lesions were then classified as concordant or discordant for the presence or absence of prostate cancer metastases. Discordant bone or soft tissue lesions were followed up with BS or CT, respectively, at 6-month intervals for up to 2 years or until a definitive diagnosis of the discordant lesion could be made. RESULTS: In 13 (50%) of the patients, all lesions identified were concordant; this included 5 patients in whom no lesions could be identified with any imaging modality. In 21 patients, 183 lesions were observed with 149 being concordant and 34 (19%) being discordant (13 patients). Based on follow-up, FCH correctly identified the presence or absence of disease in 27 of 34 lesions, and in 14 cases FCH-positive lesions, not identified on initial imaging, were confirmed as disease on follow-up. The sensitivity, specificity, accuracy, positive predictive and negative predictive values for lesion detection by FCH are 96% (92-98%), 96% (81-99%), 96% (93-97%), 99% (96-100%) and 81% (64-88%), respectively, with 95% confidence intervals shown in parentheses. CONCLUSION: In this patient cohort, FCH shows good initial concordance (81%) with BS and CT in the detection of metastatic prostate carcinoma. Follow-up of the cases where FCH was initially discordant with subsequent BS or CT shows that FCH was accurate in determining the presence or absence of prostate metastasis in 79% of lesions. While FCH imaging as compared to BS and CT in this patient group has a good sensitivity and specificity for the detection of lesions representing prostate metastasis, further prospective studies are needed to determine its role.

Amyloid imaging results from the Australian Imaging, Biomarkers and Lifestyle (AIBL) study of aging.

The Australian Imaging, Biomarkers and Lifestyle (AIBL) study of aging, a participant of the worldwide Alzheimer's Disease Neuroimaging Initiative (ADNI), performed (11)C-Pittsburgh Compound B (PiB) scans in 177 healthy controls (HC), 57 mild cognitive impairment (MCI) subjects, and 53 mild Alzheimer's disease (AD) patients. High PiB binding was present in 33% of HC (49% in ApoE-epsilon4 carriers vs 21% in noncarriers) and increased with age, most strongly in epsilon4 carriers. 18% of HC aged 60-69 had high PiB binding rising to 65% in those over 80 years. Subjective memory complaint was only associated with elevated PiB binding in epsilon4 carriers. There was no correlation with cognition in HC or MCI. PiB binding in AD was unrelated to age, hippocampal volume or memory. Beta-amyloid (Abeta) deposition seems almost inevitable with advanced age, amyloid burden is similar at all ages in AD, and secondary factors or downstream events appear to play a more direct role than total beta amyloid burden in hippocampal atrophy and cognitive decline.

Melanoma affinity in mice and immunosuppressed sheep of [(125)I]N-(4-dipropylaminobutyl)-4-iodobenzamide, a new targeting agent.

The increasing incidence of melanoma and the lack of effective therapy have prompted the development of new vectors, more specific to the pigmented tumor, for early detection and treatment. Targeted agents have to exhibit a rapid, high tumor uptake, long tumor retention and rapid clearance from nontarget organs. This joint work presents results obtained with a new melanoma targeting agent, [(125)I]-N-(4-dipropylaminobutyl)-4-iodobenzamide or [(125)I]BZ18. After labeling with a high specific activity, the biodistribution of the compound was investigated in two animal models, the mouse and the sheep. Melanotic tumor retention was observed lasting several days. We visualized the internalization of the agent inside the melanosomes by secondary ion mass spectroscopy imaging, we measured the affinity constants of [(125)I]BZ18 on a synthetic melanin model and we demonstrated a radiotoxic effect of this labeled agent on B16F0 melanoma cell culture due to its cellular internalization. From this work, [(125)I]BZ18 appeared a promising melanoma targeting agent in the nuclear medicine field.

High-activity radio-iodine labeling of conventional and stealth liposomes.

A new method to label preformed liposomes with high activities of radiohalogenated compounds has been developed. It uses activated esters of simple synthetic molecules that may be readily halogenated, such as Bolton-Hunter reagent (BH), and arginine-containing liposomes. BH, in the form of an activated ester, crosses the liposome membrane to react with arginine inside the liposomes, as demonstrated by thin-layer chromatography and by the fact that saline-containing liposomes, or hydrolyzed BH or the water soluble sulfo-BH afforded only marginal encapsulation yields. Under optimized conditions, between 37 and 55 degrees C, 62 +/- 4% (mean +/- SD) of radiolabeled BH were consistently encapsulated in the liposomes within 30 min. In molar amounts, this corresponds to a mean of 56 nmol of BH per micromol of lipids. Based on achievable specific activity, up to 2.8 GBq of iodine-131 could be entrapped per micromol of lipids. Leakage of radioactivity was very low, with less than 5% of the encapsulated activity released within 6 days at 4 degrees C in phosphate-buffered saline and less than 50% within 24 h in human serum at 37 degrees C. The labeling stability, and the fact that both conventional and PEGylated liposomes can be readily labeled with high doses of radioactivity, will make this technique useful for in vivo targeting applications, such as tumor detection (using iodine-123 or iodine-124) or therapy (with iodine-131 or astatine-211).

Influence of trans-1,2-diaminocyclohexane structure and mixed carboxylic/phosphonic group combinations on samarium-153 chelation capacity and stability.

A simple procedure was developed to compare chelating agents for (153)Sm complexes as a preliminary step to synthesise bifunctional analogues. Several variables affecting the efficiency of complex stability were investigated, such as the pre-organisation concept, cavity size, and the nature of coordination sites. Four semi-rigid agents incorporating carboxylic and/or phosphonic groupings fixed at trans-1,2-diaminocyclohexane were evaluated for their (153)Sm chelation properties, and competition studies were performed. Data on the stability of the best chelating agent compound 3: trans-cyclohexane-1,2-bis(aminomethylphosphonic)-N,N'-bis(ethyl-2-iminodiacetic acid) in human serum are presented.

Comparison of the biologic effects of MA5 and B-B4 monoclonal antibody labeled with iodine-131 and bismuth-213 on multiple myeloma.

BACKGROUND: Using a specific monoclonal antibody (MAb), B-B4, coupled to bismuth-213 ((213)Bi) by a chelating agent (CITC-DTPA), the feasibility of alpha-radioimmunotherapy (RIT) for multiple myeloma (MM) has been demonstrated previously. METHODS: In this study, the two MAbs tested, MA5 and B-B4, target the epithelial antigens Muc-1 and syndecan-1, respectively, which are both expressed by MM cell lines. Antibody characterization was evaluated by flow cytometric analysis of normal and tumoral hematopoeitic cells of MM patients as well as immunohistochemical tests of normal, nonhematopoetic tissues. Radiobiologic effects were evaluated for (213)Bi- and iodine-131 ((131)I)--labeled antibodies. We assessed in vitro mortality (thymidine incorporation, MTT, and clonogenic assays) and cell cycle modifications with propidium iodide staining. These tests were performed on MM cell lines until 120 hours postirradiation at several time points, using radiolabeled antibody concentrations ranging from 0.5 to 20 nM and specific activities ranging from 240 to 1200 MBq/mg of MAb. RESULTS: MA5 stained all MM cells in only 50% of patients, whereas B-B4 recognized all MM cells in all patients. B-B4 principally showed hepatic, pulmonary, and duodenal staining, whereas MA5 marked renal and pulmonary tissues. RIT with (213)Bi-B-B4 induced specific mortality and G(2)/M phase cell cycle arrest, which depended on the concentrations and specific activity. For (213)Bi-MA5, this arrest appeared at concentrations above 10 nM, an amount fivefold higher than that required with B-B4. This difference was also found in thymidine incorporation assays. Furthermore, with (213)Bi-B-B4, the arrest at the G(2)/M phase appeared quickly, within 24 hours after irradiation, and affected up to 60% of the cells (for 20 nM of (213)Bi-B-B4 at 1,200 MBq/mg). Conversly, (131)I-B-B4 had a very limited effect on cell mortality and did not induce any cell cycle arrest. CONCLUSIONS: The results of this study show that B-B4 might be the more effective therapeutic antibody and suggest that alpha-RIT might be more suitable than beta-RIT for treating single-cell tumor models. Thus, these findings set the stage for the beginning of clinical trials using alpha-emitter--radiolabeled B-B4, with special attention paid to hepatic, pulmonary, and intestinal side effects.