Estimates of dose to intraperitoneal micrometastases from alpha and beta emitters in radioimmunotherapy

Dosimetry in targeted alpha therapy. A systematic review: current findings and what is needed

Objective. A systematic review of dosimetry in Targeted Alpha Therapy (TAT) has been performed, identifying the common issues. Approach. The systematic review was performed in accordance with the PRISMA guidelines, and the literature was searched using the Scopus and PubMed databases. Main results. From the systematic review, three key points should be considered when performing dosimetry in TAT. (1) Biodistribution/Biokinetics: the accuracy of the biodistribution data is a limit to accurate dosimetry in TAT. The biodistribution of alpha-emitting radionuclides throughout the body is difficult to image directly, with surrogate radionuclide imaging, blood/faecal sampling, and animal studies able to provide information. (2) Daughter radionuclides: the decay energy of the alpha-emissions is sufficient to break the bond to the targeting vector, resulting in a release of free daughter radionuclides in the body. Accounting for daughter radionuclide migration is essential. (3) Small-scale dosimetry and microdosimetry: due to the short path length and heterogeneous distribution of alpha-emitters at the target site, small-scale/microdosimetry are important to account for the non-uniform dose distribution in a target region, organ or cell and for assessing the biological effect of alpha-particle radiation. Significance. TAT is a form of cancer treatment capable of delivering a highly localised dose to the tumour environment while sparing the surrounding healthy tissue. Dosimetry is an important part of treatment planning and follow up. Being able to accurately predict the radiation dose to the target region and healthy organs could guide the optimal prescribed activity. Detailed dosimetry models accounting for the three points mentioned above will help give confidence in and guide the clinical application of alpha-emitting radionuclides in targeted cancer therapy.

GD2-targeted immunotherapy and radioimmunotherapy

Ganglioside GD2 is a tumor-associated surface antigen found in a broad spectrum of human cancers and stem cells. They include pediatric embryonal tumors (neuroblastoma, retinoblastoma, brain tumors, osteosarcoma, Ewing sarcoma, rhabdomyosarcoma), as well as adult cancers (small cell lung cancer, melanoma, soft tissue sarcomas). Because of its restricted normal tissue distribution, GD2 has been proven safe for antibody targeting. Anti-GD2 antibody is now incorporated into the standard of care for the treatment of high-risk metastatic neuroblastoma. Building on this experience, novel combinations of antibodies, cytokines, cells, and genetically engineered products all directed at GD2 are rapidly moving into the clinic. In this review, past and present immunotherapy trials directed at GD2 will be summarized, highlighting the lessons learned and the future directions.

Cancer radioimmunotherapy with alpha-emitting nuclides

In lymphoid malignancies and in certain solid cancers such as medullary thyroid carcinoma, somewhat mixed success has been achieved when applying radioimmunotherapy (RIT) with beta-emitters for the treatment of refractory cases. The development of novel RIT with alpha-emitters has created new opportunities and theoretical advantages due to the high linear energy transfer (LET) and the short path length in biological tissue of alpha-particles. These physical properties offer the prospect of achieving selective tumoural cell killing. Thus, RIT with alpha-emitters appears particularly suited for the elimination of circulating single cells or cell clusters or for the treatment of micrometastases at an early stage. However, to avoid non-specific irradiation of healthy tissues, it is necessary to identify accessible tumoural targets easily and rapidly. For this purpose, a small number of alpha-emitters have been investigated, among which only a few have been used for in vivo preclinical studies. Another problem is the availability and cost of these radionuclides; for instance, the low cost and the development of a reliable actinium-225/bismuth-213 generator were probably determining elements in the choice of bismuth-213 in the only human trial of RIT with an alpha-emitter. This article reviews the literature concerning monoclonal antibodies radiolabelled with alpha-emitters that have been developed for possible RIT in cancer patients. The principal radio-immunoconjugates are considered, starting with physical and chemical properties of alpha-emitters, their mode of production, the possibilities and difficulties of labelling, in vitro studies and finally, when available, in vivo preclinical and clinical studies.

Monte Carlo investigation of single cell beta dosimetry for intraperitoneal radionuclide therapy

Single event spectra for five beta-emitting radionuclides (Lu-177, Cu-67, Re-186, Re-188, Y-90) were calculated for single cells from two source geometries. The first was a surface-bound isotropically emitting point source and the second was a bath of free radioactivity in which the cell was submerged. Together these represent a targeted intraperitoneal radionuclide therapy. Monoenergetic single event spectra were calculated over an energy range of 11 keV to 2500 keV using the EGSnrc Monte Carlo system. Radionuclide single event spectra were constructed by weighting monoenergetic single event spectra according to radionuclide spectra appropriate for each source geometry. In the case of surface-bound radioactivity, these were radionuclide beta decay spectra. For the free radioactivity, a continuous slowing down approximation spectrum was used that was calculated based on the radionuclide decay spectra. The frequency mean specific energy per event increased as the energy of the beta emitter decreased. This is because, at these energies, the stopping power of the electrons decreases with increasing energy. The free radioactivity produced a higher frequency mean specific energy per event than the corresponding surface-bound value. This was primarily due to the longer mean path length through the target for this geometry. This information differentiates the radionuclides in terms of the physical process of energy deposition and could be of use in the radionuclide selection procedure for this type of therapy.

Beta dose-rate distributions in microscopic spherical tumors for intraperitoneal radioimmunotherapy

PURPOSE

This work was designed to calculate the radial beta dose-rate profiles through microscopic spherical tumors. Its application is in the treatment of micrometastases in the peritoneal cavity by the intraperitoneal administration of radiolabeled immunoliposomes.

METHODS AND MATERIALS

Using previously published data for the dose-rate as a function of distance from a point source of activity, dose-rate profiles through five sizes of tumors (radii: 10 microm, 50 microm, 100 microm, 500 microm, 1 mm) for six different radionuclides ((188)Re, (186)Re, (32)P, (90)Y, (67)Cu, (131)I) were calculated. Dose-rate profiles were calculated for two source geometries: (1) a large bath of radioactivity in which the tumor is submerged, and (2) surface-bound radioactivity that results from tumor targeting.

RESULTS

The bath geometry produced profiles that were uniform for sufficiently small tumors. For high-energy emitters (i.e., (90)Y and (188)Re), uniformity was maintained up to a tumor radius of 100 microm. For lower energy emitters (i.e., (67)Cu and (131)I) deviations from uniformity start to appear at a tumor radius of 50 microm. Surface-bound radioactivity produced a much greater range of dose-rates within tumors of all sizes. Lower energy emitters bound to the surface of tumors produce higher dose-rates for very small micrometastases compared with high-energy emitters. Upon consideration of the simultaneous contributions from both source geometries, we believe that liposome-mediated radioimmunotherapy would benefit from the inclusion of a high-energy beta emitter, possibly as a component of a cocktail of radionuclides.

CONCLUSIONS

The calculated dose-rate profiles provide a tool for making tumor control probability estimations for micrometastases and for assessing the potential benefit offered by a targeted approach over a nontargeted approach. These calculations also suggest that the inclusion of a high-energy beta emitter is appropriate for this treatment modality.

Status of radioimmunotherapy in the new millennium

This synopsis attempts to summarize progress made in radioimmunotherapy (RIT) by the end of the 20th century addressing the problems, possible solutions, and recent developments. The reduction of minimal residual disease in an adjuvant setting appears to be a feasible goal for RIT utilizing short-range alpha-emitters. RIT has been more successful in the radiosensitive hematologic malignancies, for example lymphomas and leukemias as compared with small solid tumors. Several radiopharmaceuticals seem near approval for RIT in patients with non-Hodgkin's lymphoma (NHL) as therapeutic responses, including complete responses, are common. Obstacles to successful RIT have been recognized and strategies to overcome these hurdles and to improve efficacy are continuously being developed resulting in encouraging outcome particularly with locoregional routes of administration in solid tumors. Systemic RIT for solid tumors will need manipulating the tumor-host to improve the tumor uptake and retention of radioimmunoconjugates (RICs). The utilization of radiometals, stable chelators, biodegradable linkers and bone marrow transplantation should be able to deliver the radiation dose required for successful treatment. In conjunction with additional synergistic agents, RIT is likely to have a great impact on the treatment of solid tumors. The ability to generate new constructs, such as bivalent antibodies or fusion proteins incorporating two different functional proteins opens exciting opportunities for new therapeutic modalities. These developments will hopefully offset the impediments to the successful use of RIT.

Recent developments in radioimmunotherapy

With the advent of monoclonal antibody techniques, there has been renewed interest in RIT as a treatment modality in patients with a variety of tumour types. There has been a considerable research effort to increase understanding of the scientific basis of such therapy at all levels. Antibody, chelator and radioisotope factors are all the subject of research aimed at producing a potent effector system capable of maximal target cell kill with acceptable normal tissue toxicity. Improved knowledge of the host and tumour factors which limit access to the target cell offers the possibility of optimizing targeting and increasing the therapeutic index. Target cell factors that influence response to low dose rate RIT have been elucidated and provide an opportunity to integrate the treatment modality into radical therapy regimens. A number of Phase I and II trials have now been performed in various tumour types. The results have been promising but, as yet, the prospect of radical RIT remains a research goal. Before it can be achieved it will be necessary to improve specific tumour cell targeting and to increase both the initial dose rates and the total dose delivered to tumour deposits. Until such time, it is likely that RIT will be incorporated into multimodality protocols to deliver a moderate (10-20 Gy) tumour boost, or in an adjuvant setting in patients with minimal residual disease.