[CANCER RESEARCH 61, 2008 –2014, March 1, 2001] Response of LNCaP Spheroids after Treatment with an -Particle Emitter ( 213 Bi)- labeled Anti-Prostate-specific Membrane Antigen Antibody (J591) 1 Åse M. Ballangrud, Wei-Hong Yang, David E. Charlton, Michael R. McDevitt, Klaus A. Hamacher, Katherine S. Panageas, Dangshe Ma, Neil H. Bander, 2 David A. Scheinberg, and George Sgouros 3 Departments of Medical Physics [Å. M. B., W-H. Y., K. A. H., G. S.], Medicine [M. R. M., D. M., D. A. S.], and Epidemiology and Biostatistics [K. S. P.], Memorial Sloan- Kettering Cancer Center, New York, New York 10021; Weill Medical College of Cornell University, New York, New York 10021 [N. H. B.]; and Physics Department, Concordia University, Montreal, Quebec, Canada [D. E. C.] ABSTRACT A theoretical drawback to -particle therapy with 213 Bi is the short range of the particle track coupled with the short half-life of the radio- nuclide, thereby potentially limiting effective cytotoxicity to rapidly ac- cessible, 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 213 Bi- 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 be- cause the barriers to antibody penetration that are associated with the capillary basal lamina have not yet formed. - and -emitting radionu- clides such as 131 I, 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 magni- tude 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 anti- body, J591, radiolabeled with the -particle emitter 213 Bi (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 213 Bi resulted in a 3-log reduction in spheroid volume on day 33, relative to control, for spheroids with an initial diameter of 130 m; 1.8 MBq/ml were required to achieve a similar response for spheroids with an initial diameter of 180 m. Equivalent spheroid responses were observed after 12 Gy of acute external beam photon irradiation. Monte Carlo-based microdosimetric analyses of the 213 Bi decay distribution in individual spheroids of 130-m diameter yielded an average -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 ex- periments were clinically relevant, and this work supports the possibility of using 213 Bi-labeled antibodies not only for disseminated single tumor cells, as found in patients with leukemia, but also for micrometastatic tumor deposits up to 180 m in diameter (1200 cells). INTRODUCTION Radiolabeled antibody therapy has already demonstrated efficacy in the treatment of non-Hodgkin’s lymphoma (1–3). Results have been largely disappointing, however, in the targeting of bulky disease. To target bulky disease, i.v. administered antibody must extravasate, diffuse across an interstitial fluid space, and then distribute throughout antigen-positive cells. Each of these steps is associated with a barrier to delivery (4 –9). By targeting hematologically distributed, single tumor cells or tumor cell clusters, the barriers to antibody delivery are diminished. -Particle-emitting radionuclides such as 131 I and 90 Y have been used in most applications of radioimmunotherapy. These radionu- clides are suboptimal for sterilizing single tumor cells or small tumor cell clusters because the concentration of radioactivity required to provide the thousands to tens of thousands of -particle traversals through the cell nucleus needed to achieve cytotoxicity would also yield prohibitive normal organ toxicity. -Particles are much more cytotoxic than -particles and would, therefore, be ideal candidates in targeting individual tumor cells or small clusters. The effectiveness of -particles arises because the amount of energy deposited per unit distance traveled (linear energy transfer) can be several orders of magnitude greater than that of -particles. Cell survival studies have shown that -particle-induced killing is independent of oxygenation state or cell cycle during irradiation (10, 11). Spheroids have been used by a number of investigators as models of tumor cell micrometastases (12–17). These multicellular clusters provide the experimental flexibility of monolayer cultures while pre- serving the three-dimensional structure that is important for the cell- to-cell interaction that exists in vivo. The spheroid model is particu- larly important in establishing a relationship between antibody binding kinetics, antigen density, internalization, and external anti- body concentration, as well as to assess kill probability under different antibody concentrations and specific activities. It is an ideal model to optimize treatment parameters. Such optimization is essential in the treatment of micrometastases because objective measures of response will not be generally available in vivo. Previous spheroid studies with large (800-m-diameter) spheroids using antibodies labeled with the -particle emitter, 212 Bi, concluded that this -particle emitter would be ineffective because the short, 50 –90-m range of the -particles, coupled with the short, 1-h half-life, restricted tumor cell targeting (18). The requirement of adequate oxygen and nutrient supply, however, limits prevascularized micrometastases to maximum diameters of 150 –200 m. The emis- sion properties of 213 Bi (Fig. 1) used in the experiments reported here are similar to those of 212 Bi, with the exception that 213 Bi does not emit the highly energetic and penetrating photon emissions found in 212 Bi. Recently, Kennel et al. (19) compared surviving fraction of cells in monolayers and in spheroids irradiated by surface-bound 213 Bi. These studies were carried out using spheroids derived from the murine EMT6 cell line and the 13A antibody against murine CD44. Tumor cells in spheroids were efficiently killed for spheroids up to 20 –30 cells in diameter. Animal studies have also been performed and show that -particle emitters yield superior tumor control relative to  or Auger electron emitters (10, 20 –23). Human use of -particle emitters has also been reported (24 –26). The first implementation was with 213 Bi conjugated to the anti-CD33 antibody, HuM195, targeting myeloid leukemia. This trial demonstrated feasibility and anticancer activity with minimal toxicity (24). The anti-tenascin antibody, 81C6, Received 8/8/00; accepted 1/3/01. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 Supported, in part, by NIH Grants PO1 CA-33049, R01 CA-55349, and R01 CA-72683 and also by the CapCure Foundation. D. A. S. is the recipient of the Doris Duke Distinguished Clinical Scientist Award. 2 N. H. B. is a consultant to BZL Biologics, Inc. His association with BZL is managed in accordance with the conflict of interest policies of Cornell University. 3 To whom requests for reprints should be addressed, at Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021. E-mail: sgourosg@mskcc.org. 2008 on May 1, 2016. © 2001 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from