放射免疫治疗中的α核素微剂量研究

微剂量研究在临床α核素放免治疗的计划设计和疗效评价中起重要作用。设计一整套既能准确描述α粒子在细胞、亚细胞水平上剂量的不均匀分布,又适于临床使用的微剂量估算模型,一直是该领域的重要研究内容。本文介绍了α核素微剂量计算的几种常用方法以及最新发现的显著影响其结果精确度的因素。     

Cyclotron production of Ac-225 for targeted alpha therapy.

The feasibility of producing Ac-225 by proton irradiation of Ra-226 in a cyclotron through the reaction Ra-226(p,2n)Ac-225 has been experimentally demonstrated for the first time. Proton energies were varied from 8.8 to 24.8 MeV and cross-sections were determined by radiochemical analysis of reaction yields. Maximum yields were reached at incident proton energies of 16.8 MeV. Radiochemical separation of Ac-225 from the irradiated target yielded a product suitable for targeted alpha therapy of cancer.     

Bismuth-213 and actinium-225 -- generator performance and evolving therapeutic applications of two generator-derived alpha-emitting radioisotopes

The alpha emitters (225)Ac and (213)Bi are promising therapeutic radionuclides for application in targeted alpha therapy of cancer and infectious diseases. Both alpha emitters are available with high specific activity from established radionuclide generators. Their favourable chemical and physical properties have led to the conduction of a large number of preclinical studies and several clinical trials, demonstrating the feasibility, safety and therapeutic efficacy of targeted alpha therapy with (225)Ac and (213)Bi. This review describes methods for the production of (225)Ac and (213)Bi and gives an overview of (225)Ac/(213)Bi radionuclide generator systems. Selected preclinical studies are highlighted and the current clinical experience with (225)Ac and (213)Bi is summarized.     

Production of Ac-225 for cancer therapy by photon-induced transmutation of Ra-226.

The increasing application of Ac-225 for cancer therapy indicates the potential need for its increased production and availability. The production of Ac-225 has been achieved using bremsstrahlung photons from an 18 MV medical linear accelerator (linac) to bombard a Ra-226 target. A linac dose of 2800 Gy produced about 64 microCi of Ra-225, which decays to Ac-225. This result, while consistent with the theoretical calculations, is far too low to be of practical use. A more powerful linac is required that runs at a higher current, longer pulse length and higher frequency for practical production. This process could also lead to the reduction of the nuclear waste product Ra-226.     

Production and characterization of 188Re-C595 antibody for radioimmunotherapy of transitional cell bladder cancer.

Bladder cancer was responsible for >12,000 deaths in the United States in 1999. The high-molecular-weight glycoprotein MUC1 mucin is overexpressed on bladder tumors and represents a useful target for radioimmunoscintigraphy and radioimmunotherapy. We report on the production and initial tracer studies of a 188Re-antibody complex directed against this target and intended for intravesical radioimmunotherapy of superficial bladder cancer. METHODS: 188Re perrhenate was eluted from a 188W/188Re generator. C595 antibody was reduced with 2-mercaptoethanol and was labeled in the presence of stannous tartrate. The final reaction mixture contained high-molecular-weight contamination, which was removed from the complex using an affinity separation technique. The specificity and integrity of the antibody complex were tested by radioimmunoassay and size exclusion chromatography. Tumor localization was investigated using an ex vivo model in human cystectomy specimens. Tracer amounts of the complex were also administered intravesically to three patients with bladder cancer, who were then imaged by gamma scintigraphy. RESULTS: The complex was immunoreactive (70% +/- 17%) and specific for MUC1 antigens. A peak corresponding to a protein of 150 kDa was observed on size exclusion chromatography, showing that the complex was homogeneous. Binding to bladder tumors was observed in an ex vivo model in which tumors were successfully imaged in four specimens. The mean tumor-to-normal tissue ratio in ex vivo bladders was 7:1. Tumor uptake after intravesical administration was confirmed in three patients with bladder cancer (mean tumor-to-normal tissue ratio, 4:1). CONCLUSION: The C595 antibody was labeled with 188Re, providing a radioimmunoconjugate with high immunoreactivity and specificity. Its ability to localize in tumors both in an ex vivo model and after intravesical administration to patients has been shown. This approach will now be extended for the therapy of superficial bladder cancer.     

Implications of nonuniform tumor doses for radioimmunotherapy.

This article describes a method of assessing the biologic consequences of nonuniform dose distributions produced in tumors by biologically targeted radionuclide therapy. The analysis is based on a simple mathematical model that assumes all tumor cells are uniformly radiosensitive. METHODS: Using the linear-quadratic radiobiologic model, it is possible to represent an absorbed dose distribution by a biologically effective dose (BED) volume histogram (BVH). The Laplace transform of the BVH yields an equivalent uniform biologically effective dose. This is a one-number value that fully describes the biologic effect of a nonuniform absorbed dose distribution. In this article, for the purposes of exposition, nonuniform BED distributions are represented by normal distributions. RESULTS: Nonuniform absorbed dose distributions are inefficient in sterilizing tumors and become proportionately less effective as the mean dose increases. The loss in effectiveness is most severe for radiosensitive tumors. CONCLUSION: Several approaches may alleviate the consequences of dosimetric nonuniformity. These include the use of smaller targeting molecules, radionuclides with longer emission ranges, fractionated administration of biologically targeted radionuclide therapy and combined modality treatments.     

Rapid preparation of short-lived alpha particle emitting radioimmunopharmaceuticals.

Alpha emitting radionuclides are of considerable interest for targeted radioimmunotherapy. Generator supplied 213Bi emitting 8.5 MeV alpha particles with a 45.6 min half-life has been conjugated to a monoclonal antibody (HuM195-CHX-A-DTPA) for targeted therapy of leukemia in a clinical trial. The clinical dose preparation of pharmaceutical formulation by a pair of skilled radiochemists took 25 min, which corresponds, to an overall decay loss of 30% of the initial 213Bi activity eluted from the generator. In order to allow more widespread and practical clinical use of targeted 213Bi alpha particle therapy, we developed a new procedure that is simpler, more rapid and adaptable to a hospital pharmacy. The new 10 min process includes a tandem elution and labeling, and an anion exchange column purification method that can be reproducibly used.     

Radiolabeled bivalent haptens for tumor immunodetection and radioimmunotherapy.

The pretargeting technique referred to as the Affinity Enhancement System (AES) uses bispecific antibodies and radiolabeled bivalent haptens that bind cooperatively to target cells in vivo. Experimental and clinical data demonstrate that DTPA bivalent haptens can deliver large radiation doses to tumor cells with high tumor to normal tissue contrast ratios and long activity residence time in tumors. Preliminary clinical results of radioimmunotherapy of medullary thyroid carcinomas and lung cancers look promising. Very encouraging results in biodistribution and radioimmunotherapy experiments in animals have been obtained with new haptens bearing two histamine-hemisuccinate suitable for 131I, 99mTc and 188Re labeling. Targeting isotopes to double antigen positive tumor cells provides a binding enhancement that increases specificity for tumor cells as compared to single antigen targeting on normal cells. This approach may be beneficial for targeting isotopes to B type acute lymphoblastic leukemia and Burkitt lymphoma, as well as others tumors co-expressing two markers of low specificity, and might increase tumor irradiation with minimal irradiation of normal cells.     

Importance of receptor density in alpha radioimmunotherapy in B cell malignancies: an in-vitro study

BACKGROUND: External beam radiotherapy and beta radioimmunotherapy (RIT) are effective treatments for lymphoid malignancies. The development of RIT with alpha emitters is attractive because of the high linear energy transfer (LET) and short path length, allowing higher tumour cell kill and lower toxicity to healthy tissues. AIM: To assess the binding of rituximab to samples of B cell chronic lymphocytic leukaemia (B-CLL) and splenic lymphoma with villous lymphocytes (SLVL), and to evaluate the induction of apoptosis by conventional therapies as well as with Bi conjugated to rituximab. METHOD: 213Bi was eluted from a 225Ac generator and conjugated to CD20 antibody (rituximab) with CHX-A'-DTPA as chelator. Binding assays with 213Bi-rituximab were correlated to antibody binding capacity obtained by flow cytometry. Apoptosis was scored by flow cytometric analyses of the cells stained with annexin V-FITC and 7-amino-actinomycin D. RESULTS: Binding of 213Bi-rituximab was significantly lower for B-CLL compared to SLVL samples (12+/-3 and 42+/-10 213Bi atoms per cell, respectively, at 370 kBq.ml(-1)). The induction of apoptosis did not differ significantly between the two groups (B-CLL and SLVL) after external gamma irradiation or treatment with methylprednisolone and fludarabine (17+/-12% and 18+/-11%; 23+/-14% and 21+/-12%; 9+/-9% and 11+/-8%, respectively; all results expressed as percentages of all cells). Rituximab conjugated or not to 213Bi induced significantly more apoptosis in SLVL (42+/-19% and 42+/-17%) compared to B-CLL samples (27+/-12% and 6+/-8%). CONCLUSION: Binding assays confirm that SLVL samples present more CD20 antigens compared to B-CLL samples. Conventional therapies such as fludarabine, methylprednisolone or external gamma irradiation induce similar responses in the two populations but SLVL samples present higher sensitivity towards 213Bi-rituximab. These data are in favour of alpha-RIT in SLVL patients.     

90Y-DOTA-hLL2: an agent for radioimmunotherapy of non-Hodgkin's lymphoma.

The goal of this work was to determine an optimal radioimmunotherapy agent for further development against non-Hodgkin's lymphoma. We sought to establish the stability profile of (90)Y-labeled humanized LL2 (hLL2) monoclonal antibody (mAb) when prepared with different chelating agents and, from these data, to estimate the dosimetric improvement to be expected from use of the most stable (90)Y-chelate-hLL2 complex. METHODS: The complementarity-determining region-grafted (humanized) anti-CD22 mAb, hLL2 (epratuzumab), was conjugated to 3 different chelating agents, 2 of which were derivatives of diethylenetriaminepentaacetic acid (DTPA) and 1 of which was the macrocyclic chelate 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA). The 3 hLL2 conjugates were radiolabeled with (90)Y and tested for stability in vitro against a 10,000-fold molar excess of free DTPA over 9 d. They were also tested against normal human serum at 37 degrees C over 12 d. Each conjugate was radiolabeled with the gamma-emitting radionuclide, (88)Y, and compared for biodistribution in normal and lymphoma xenograft-bearing athymic mice. In vivo data were analyzed for statistical differences in the uptake of yttrium in bone and washed bone when either the DOTA or the Mx-DTPA chelates were used, and dosimetry calculations were made for each complex. RESULTS: (90)Y-DOTA complex of the hLL2 mAb was completely stable to either DTPA or serum challenge for the duration of either experiment (equivalent to 3.3-4.5 half-lives of (90)Y radionuclide or >90% of possible (90)Y decays from any initial starting activity). Complexes of hLL2 that had been prepared using the DTPA-type chelates lost 3%-4% of initially bound (90)Y over the first few days and about 10%-15% over the duration of the challenges. In vivo, these stability differences manifested as significantly lower yttrium uptake in bone and cortical bone over a 10-d period when DOTA was used as the yttrium chelating agent. Absorbed doses per 37 MBq (1 mCi) of (90)Y-mAb were 3,555 and 5,405 cGy for bone and 2,664 and 4,524 cGy for washed bone for (90)Y-DOTA-hLL2 and (90)Y-MxDTPA-hLL2, respectively, amounting to 52.0% and 69.8% increases in absorbed radiation doses for bone and washed bone, respectively, when a DOTA chelate was switched to a Mx-DTPA chelate. CONCLUSION: (90)Y-hLL2 prepared with the DOTA chelate represents an improved agent for radioimmunotherapy of non-Hodgkin's lymphoma, with an in vivo model demonstrating a large reduction in bone-deposited yttrium, compared with (90)Y-hLL2 agents prepared with open-chain DTPA-type chelating agents. Dosimetry suggests that this benefit will result in a substantial toxicologic advantage for a DOTA-based hLL2 conjugate.     

Simulation of alpha particle spectra from aerosol samples

A Monte Carlo code is developed for simulating energy spectra of alpha particles from aerosol samples. Geometrical detection efficiency and energy loss of the alpha particles in the source itself and in the material between the source and detector are simulated. Different characteristics of the aerosol particles and medium material are taken into account in the computation. An excellent agreement with earlier results and measurements is found. The code can be applied for example in designing optimal aerosol sampling methods in direct alpha spectrometry.     

Evaluation of 225Ac decay data.

Evaluation of the complete decay scheme and data for (225)Ac including new measurements are presented in this report; literature data available up to March 2006 are included. The half-life is determined to be 10.0+/-0.1 days. All known measured gamma-ray relative intensities have been examined; the gamma-ray emission probability of the reference gamma-ray line of 150.04 keV is recommended to be 0.693+/-0.012%. The calculated internal conversion coefficients and their uncertainties have been used to obtain the complete decay intensity balance. The other decay characteristics are calculated using the ENSDF analysis program. Finally, the new decay scheme for (225)Ac is presented.     

Single-dose versus fractionated radioimmunotherapy: model comparisons for uniform tumor dosimetry.

Targeting molecules with reduced immunogenicity will enable repetitive administrations of radioimmunotherapy. In this work a mathematical model was used to compare 2 different treatment strategies: large single administrations (LSAs) and rapid fractionation (RF) of small individual administrations separated by short time intervals. METHODS: An integrated compartmental model of treatment pharmacokinetics and tumor response was used to compare alternative treatments that delivered identical absorbed doses to red marrow. RESULTS: Based on the key assumption of uniform dose distributions, the LSA approach consistently produced smaller nadir values of tumor cell survival and tumor size. The predicted duration of remission was similar for both treatment structures. These findings held for both macroscopic and microscopic tumors and were independent of tumor cell radiosensitivity, proliferation rate, rate of tumor shrinkage, and uptake characteristics of radiolabeled material in tumor. CONCLUSION: Clinical situations for which each treatment is most appropriate may be tentatively identified. An LSA using a short-range-emitting radionuclide would be most appropriate for therapy of microscopic disease, if uptake is relatively homogeneous. RF using a longer range emitter would be most appropriate for macroscopic disease, if uptake is heterogeneous and varies from one administration to another. There is a rationale for combining LSA and RF treatments in clinical situations in which slowly growing macroscopic disease and rapidly growing microscopic disease exist simultaneously.     

Labeling of monoclonal antibodies with 153Sm for potential use in radioimmunotherapy.

The labeling of a monoclonal (anti-CEA) and a polyclonal (IgG) antibody with 153Sm has been investigated, using the bicyclic anhydride of DTPA (cDTPAa) as the chelating agent. The radiochemical study was performed using a combination of radioanalytical techniques (gel filtration, HPLC, ITLC-SG and SDS-PAGE). Optimization of factors affecting labeling (pH, Ab, Ab-DTPA concentration, etc.) leads to a labeling yield higher than 90%. Biodistribution studies in normal mice showed slow blood clearance and high uptake into the liver, kidney and lungs.     

Impact of nodal regression on radiation dose for lymphoma patients after radioimmunotherapy.

Radioimmunotherapy for non-Hodgkin's lymphoma often results in surprisingly high response rates compared with those expected from estimated absorbed radiation doses. Several factors, including radiobiologic response, selective targeting, and heterogeneous absorbed radiation within the lymphoma, are likely to contribute to the lack of a dose-response relationship. This article investigates the impact of nodal regression on absorbed radiation dose and applies a correction factor to account for its effect. METHODS: The radioactivity in and regression of 37 superficial lymph nodes were measured in 7 non-Hodgkin's lymphoma patients treated with 775-3,450 MBq/m(2) of (131)I-Lym-1 monoclonal antibody. Nodal dimensions were measured with calipers and radioactivity was quantitated using gamma-camera imaging on multiple days after (131)I-Lym-1 injection. Both nodal regression and radioactivity were fit with monoexponential functions. Formulas were developed to account for simultaneous change in nodal mass and radioactivity. All lymph nodes with size and radioactivity measurements, and exponential-fit coefficients of determination of >0.8, were included in the analysis. RESULTS: A 3 orders-of-magnitude node-to-node variation in initial radiopharmaceutical concentration (MBq/g) was observed, with the highest concentrations in the smallest nodes. Reduction in radioactivity as a function of time (biologic half-life) varied by about a factor of 2. In contrast, the rate of nodal regression varied by orders of magnitude, from a 14-h half-time to no regression at all. Five nodes regressed with a half-time that was shorter than their observed effective radiopharmaceutical half-life. Accounting for the effect of nodal regression resulted in dose corrections ranging from 1 (no correction) to a factor of >10, with 70% of nodes requiring a correction factor of at least 20% and >50% of nodes requiring a correction factor of >2. Corrected for nodal regression, 46% of nodes analyzed had absorbed radiation doses of >10 Gy and 32% had doses of >20 Gy. CONCLUSION: These results highlight the importance of accounting for change in mass, particularly tumor regression, when assessing absorbed radiation dose for tissues whose mass changes during the time the radiation dose is being absorbed. The increase in calculated absorbed dose when this change is considered provides better insight into the high nodal response rates observed in non-Hodgkin's lymphoma patients.     

Evaluation of a remote radioiodination system for radioimmunotherapy

This report describes a remote radioiodination system which is inexpensive, easy to assemble, disposable, and capable of radioiodinating curie levels of activity safely. In addition to the safety afforded by this system, an immobilized oxidant and anion exchange resin are used to generate electrophilic iodine and remove free iodine, respectively. Reducing agents are not used and, therefore, when radioiodinating F(ab')2 fragments, degradation does not occur. In contrast, chloramine-T, sodium metabisulfite (CT/SMB) iodinations of F(ab')2 fragments resulted in products with up to 40% Fab' fragments. Radiolabeling yields (65.8% +/- 8.1%) and antibody immunoreactivity (68.8% +/- 8.0%) were not statistically different (p less than 0.001) from those obtained in remote CT/SMB iodinations. The system is currently being used to radioiodinate both IgG and F(ab')2 monoclonal antibodies with up to 450 mCi 131I for clinical radioimmunotherapy trials.     

Radiochemical purification of no-carrier-added scandium-47 for radioimmunotherapy

A procedure, adaptable to large-scale remote operation, was developed to purify no-carrier-added (NCA) ⁴⁷Sc from irradiated Ti targets. Methods based on extraction chromatography and cation-exchange chromatography were compared. Results of this comparison led to the development of an optimized procedure based on cation-exchange with Dowex AG 50W-X4 and ⁴⁷Sc elution with HCl/HF. This method gave 90–97% overall ⁴⁷Sc recovery, with a Ti separation factor greater than 2.4×10⁻⁵, and specific activities ≥0.9 GBq μg⁻¹. Use of the ⁴⁷Sc product, for labeling monoclonal antibodies, resulted in consistent labeling yields of ≥90%.     

Determination of biological vector characteristics and nanoparticle dimensions for radioimmunotherapy with radioactive nanoparticles.

Radioimmunotherapy with biological vector labeled with radioactive nanoparticles is investigated from a dosimetric point of view. Beta (32P, 90Y) and low-energy X-ray radionuclides (103Pd) are considered. Dose distributions inside solid tumors have been calculated using MCNPX 2.5.0. Nanoparticle dimensions and biological vector characteristics are also determined in order to reach the 50 Gy prescribed dose inside the entire tumor volume. The worst case of an avascular tumor is considered. Results show that for beta-emitting nanoparticles, a set of data (covering fraction, biological half-life, and nanoparticle radius) can be found within acceptable ranges (those of classical radioimmunotherapy). These sources (with Emax approximately few MeV) can be used for the treatment of tumors with a maximum diameter of about 1 cm. Low-energy X-rays (E<25 keV) can be used to extend the range of tumor diameter to 4-5 cm but require very tight biological vector characteristics.     

Pretargeted radioimmunotherapy of cancer: progress step by step.

To enhance the therapeutic efficacy of radioimmunotherapy of cancer, several pretargeting strategies have been developed. In pretargeted radioimmunotherapy, the tumor is pretargeted with an antibody construct that has affinity for the tumor-associated antigen on the one hand and for a radiolabeled hapten on the other. The radiolabeled hapten is administered in a later phase, preferably after the antibody construct has cleared from the circulation. In pretargeted radioimmunotherapy, 2 main approaches can be distinguished: pretargeting strategies based on the avid interaction between streptavidin (SA) or avidin and biotin, and pretargeting strategies based on the use of bispecific antibodies. In pretargeting strategies based on biotin and SA or avidin, the use of a clearing agent that could remove the pretargeting construct from the circulation markedly improved the targeting of the radiolabeled biotin to the tumor. Thus, multistep injection schemes in which 3-5 different agents are subsequently injected were developed. In bispecific antibody-based pretargeting strategies, the use of bivalent haptens improved the efficacy of the tumor targeting, and a 2-step pretargeted radioimmunotherapy strategy is now being tested in cancer patients. Preclinical studies as well as studies on cancer patients have shown that these pretargeting strategies can result in higher radiation doses to the tumor than can directly radiolabeled antitumor antibodies. Here, the development and state of the art of the most effective approaches for pretargeted radioimmunotherapy are reviewed.