Theoretical estimation of absorbed dose to organs in radioimmunotherapy using radionuclides with multiple unstable daughters

The toxicity and clinical utility of long-lived alpha emitters such as Ac-225 and Ra-223 will depend upon the fate of alpha-particle emitting unstable intermediates generated after decay of the conjugated parent. For example, decay of Ac-225 to a stable element yields four alpha particles and seven radionuclides. Each of these progeny has its own free-state biodistribution and characteristic half-life. Therefore, their inclusion for a more accurate prediction of absorbed dose and potential toxicity requires a formalism that takes these factors into consideration as well. To facilitate the incorporation of such intermediates into the dose calculation, a previously developed methodology (model 1) has been extended. Two new models (models 2 and 3) for allocation of daughter products are introduced and are compared with the previously developed model. Model 1 restricts the transport to a function that yields either the place of origin or the place(s) of biodistribution depending on the half-life of the parent radionuclide. Model 2 includes the transient time within the bloodstream and model 3 incorporates additional binding at or within the tumor. This means that model 2 also allows for radionuclide decay and further daughter production while moving from one location to the next and that model 3 relaxes the constraint that the residence time within the tumor is solely based on the half-life of the parent. The models are used to estimate normal organ absorbed doses for the following parent radionuclides: Ac-225, Pb-212, At-211, Ra-223, and Bi-213. Model simulations are for a 0.1 g rapidly accessible tumor and a 10 g solid tumor. Additionally, the effects of varying radiolabled carrier molecule purity and amount of carrier molecules, as well as tumor cell antigen saturation are examined. The results indicate that there is a distinct advantage in using parent radionuclides such as Ac-225 or Ra-223, each having a half-life of more than 10 days and yielding four alpha particles per parent decay, in that lower doses to normal organs result for a given tumor dose in comparison to those radionuclides yielding fewer alpha particles. In model 2, which accounts for transit time through the blood, a dose of 20 Gy to a rapidly accessible 0.1 g tumor will result in a liver and kidney dose of 1.7 and 0.9 Gy, respectively from Ac-225. An equivalent dose to tumor from Ra-223 would yield a maximum normal organ dose of 0.4 and 0.3 Gy to bone and small intestines, respectively; the corresponding absorbed dose to small intestines from Pb-212 and Bi-213 is 2.2 and 3.0 Gy, respectively.

Cellular dose conversion factors for alpha-particle--emitting radionuclides of interest in radionuclide therapy

alpha-Particle--emitting radionuclides are of increasing interest in radionuclide therapy. The decay scheme of alpha-emitting radionuclides typically includes a chain of unstable progeny. It is generally assumed that alpha-particle emission by the parent radionuclide will break the chemical bond with its carrier molecule and that the resulting daughter atom will no longer be associated with the carrier molecule. If the daughter is very short lived, it will not have enough time to be carried any significant distance from the site of parent decay and a cellular, absorbed dose estimate must consider the energy deposited by the daughter as well as the parent. Depending on the site of parent decay and the expected removal rate of daughter atoms from this site, the contribution of emissions from longer-lived daughters may also be warranted. In this study, dose conversion factors (DCFs) for cellular dimensions that incorporate the fate of daughter radionuclides were derived for (225)Ac, (213)Bi, (211)At, and (223)Ra, the alpha-particle--emitting radionuclides of interest in radionuclide therapy.

METHODS

The dose contribution of daughter radionuclides at the site of parent decay was made dependent on a cutoff time parameter, which was used to estimate the fraction of daughter decays expected at the site of parent decay. Previously tabulated S values (cell-surface to nucleus and cell-surface to cell) for each daughter in the decay scheme were scaled by this fraction and a sum over all daughters was performed to yield a cutoff time--dependent set of corresponding DCF values for each radionuclide.

RESULTS

DCF values for the absorbed dose to the nuclear or cellular volume from cell-surface decays are presented as a function of the cutoff time for 4 different cellular and nuclear dimensions.

CONCLUSION

In contrast to the cellular S values that account only for parent decay, the DCF values provided in this study make it possible to easily include the contribution of daughter decays in cellular alpha-particle emitter dose calculations.

Response of LNCaP spheroids after treatment with an alpha-particle emitter (213Bi)-labeled anti-prostate-specific membrane antigen antibody (J591)

A theoretical drawback to alpha-particle therapy with 213Bi is the short range of the particle track coupled with the short half-life of the radionuclide, thereby potentially limiting effective cytotoxicity to rapidly accessible, disseminated individual tumor cells (e.g., as in leukemia). In this work, a prostate carcinoma spheroid model was used to evaluate the feasibility of targeting micrometastatic clusters of tumor cells using 213Bi-labeled anti-prostate-specific membrane antigen (PSMA) antibody, J591. In prostate cancer, vascular dissemination of tumor cells or tumor cell clusters to the marrow constitutes an important step in the progression of this disease to widespread skeletal involvement, an incurable state. Such prevascularized clusters are ideal targets for radiolabeled antibodies because the barriers to antibody penetration that are associated with the capillary basal lamina have not yet formed. Beta- and gamma-emitting radionuclides such as 131I, which are widely used in radioimmunotherapy, are not expected to be effective when targeting single cells or small cell clusters. This is because the range of the emissions is one to two orders of magnitude greater than the target size, and the energy deposited per traversal is insufficient to produce any significant radiobiological effect. Spheroids of the prostate cancer cell line, LNCaP-LN3, were used as a model of prevascularized micrometastases; their response to an anti-PSMA antibody, J591, radiolabeled with the alpha-particle emitter 213Bi (T(1/2), 45.6 min.) has been measured. The time course of spheroid volume reductions was found to be sensitive to the initial spheroid volume. J591 labeled with 0.9 MBq/ml 213Bi resulted in a 3-log reduction in spheroid volume on day 33, relative to control, for spheroids with an initial diameter of 130 microm; 1.8 MBq/ml were required to achieve a similar response for spheroids with an initial diameter of 180 microm. Equivalent spheroid responses were observed after 12 Gy of acute external beam photon irradiation. Monte Carlo-based microdosimetric analyses of the 213Bi decay distribution in individual spheroids of 130-microm diameter yielded an average alpha-particle dose of 3.7 Gy to the spheroids, resulting in a relative biological effectiveness factor of 3.2 over photon irradiation. The activity concentrations used in the experiments were clinically relevant, and this work supports the possibility of using 213Bi-labeled antibodies not only for disseminated single tumor cells, as found in patients with leukemia, but also for micrometastatic tumor deposits up to 180 microm in diameter (1200 cells).

Parametric images of antibody pharmacokinetics in Bi213-HuM195 therapy of leukemia

Kinetic analysis of gamma camera patient images can provide time-dependent information about antibody behavior. Current region-of-interest-based techniques for the kinetic analysis of these images rely on user selection and drawing of regions to be analyzed. Such analyses do not reveal unexpected kinetic activity outside of the selected regions of interest and do not provide a whole-image assessment regarding the pharmacokinetics of an agent. At Memorial Sloan-Kettering Cancer Center, a method for generating images in which the pixel value represents a kinetic parameter has been developed. This work extends the method into a new application in which whole-body parametric images are used to examine the kinetics of Bi213-HuM195 in patients with leukemia.

METHODS

Bi213-HuM195 is typically administered in multiple injections over 2-4 d, yielding a progressive increase in the amount of antibody administered. Patients are injected with individual doses while positioned in a gamma camera, and imaging is initiated at the start of the injection. The acquisition is performed in dynamic mode with images collected at several time intervals over 1 h. Using software developed in-house, images are corrected for patient movement through iterative alignments, decay corrected, and summed to yield a series of images over regular time intervals. Parametric rate images are obtained by fitting a linear expression to the counts in each pixel. In this study, rate images from a patient's first injection were compared with rate images from the last injection.

RESULTS

The conventional planar images of antibody distribution showed significant uptake in liver, spleen, and marrow, whereas the generated rate images displayed different patterns, sometimes with negative values in liver and spleen and positive values in marrow, reflecting clearance and uptake rates rather than total accumulation. The impact of the progressive increase in antibody administration was observed by comparing the first with the last rate images. Interpatient comparisons were also made and showed that rate image patterns varied depending on patient-specific conditions such as the amount of disease and previous therapies undergone by the patient.

CONCLUSION

Rate images make it possible to succinctly display kinetic information about an agent's behavior over the entire acquired image.

A schema for estimating absorbed dose to organs following the administration of radionuclides with multiple unstable daughters: a matrix approach

The dosimetry of alpha particle emitters requires that all decays, including those of unstable intermediates be included in the calculation. Such calculations are complicated by the potential differential biologic distribution of each of the intermediates. In this work we present a formalism which will account for the known biodistribution factors of the daughters and the resulting effective biodistribution which will depend upon the site at which the parent radionuclide decays. The number of decays or cumulated activity of a daughter radionuclide present in a particular tissue is estimated using a probability matrix which describes the likelihood of daughter decay in a particular tissue as a function of the decay site of the parent. An example of three initial compartments is provided to illustrate the use of this formalism. Such modeling may be used to evaluate the feasibility of using radionuclides whose decay includes alpha-emitting intermediates. Model validation and refinement will require an assessment of the fate of free, alpha-emitting intermediates in various biological milieus.