Effect of platelet-derived growth factor receptor-beta inhibition with STI571 on radioimmunotherapy

Abstract

Whereas radioimmunotherapy of hematologic malignancies has evolved into a viable treatment option, the responses of solid tumors to radioimmunotherapy are discouraging. The likely cause of this problem is the interstitial hypertension inherent to all solid tumors. Remarkable improvements in tumor responses to radioimmunotherapy were discovered after the inclusion of STI571 in the therapy regimen. A combination of the tumor stroma-reactive STI571, a potent platelet-derived growth factor receptor-beta (PDGFr-beta) antagonist, and the tumor-seeking radiolabeled antibody B72.3 yielded long-lasting growth arrest of the human colorectal adenocarcinoma LS174T grown as s.c. xenografts in athymic mice. The interaction of STI571 with the stromal PDGFr-beta reduced tumor interstitial fluid pressure (P(IF)) by >50% and in so doing improved the uptake of B72.3. The attenuation of P(IF) also had a positive effect on the homogeneity of antibody distribution. These effects were dose-dependent and under optimized dosing conditions allowed for a 2.45 times increase in the tumor uptake of B72.3 as determined in the biodistribution studies. Single-photon emission computed tomography imaging studies substantiated these results and indicated that the homogeneity of the radioisotope distribution was also much improved when compared with the control mice. The increased uptake of radioimmunotherapy into the tumor resulted in >400% increase in the tumor absorbed radiation doses in STI571 + radioimmunotherapy-treated mice compared with PBS + radioimmunotherapy-treated mice. The improved antibody uptake in response to the attenuation of tumor P(IF) was identified as the primary reason for the growth arrest of the STI571 + radioimmunotherapy-treated tumors. Two related causes were also identified: (a) the improved homogeneity of monoclonal antibody distribution in tumor and (b) the increased tumor radiosensitivity resulting from the improved tumor oxygenation.

Figure 1

Characterization of PDGFr-β interactions with STI571 in LS174T cells in vitro and in vivo. A. Immunoblot of human adenocarcinoma LS174T cells and porcine aortic endothelial cells expressing the PDGFr-α and -β (αβ -PAE) (control). Cells were left untreated or were stimulated with PDGF-BB and following sequential immunoprecipitation (IP) of PDGF receptors, immunoblotting to detect activated PDGFr was performed using anti-phosphotyrosine antibodies. B. Survival of LS174T cells in the presence of low concentrations of STI571. Cells were treated for 24 h or 48 h with STI571 and subsequently the metabolic/proliferative activities measured. The surviving fraction of the untreated control cells is one. C. Survival of LS174T cells grown in the presence of various STI571 concentrations and irradiated with 1 Gy and 6 Gy. D. Immunohistochemical staining using antibodies against PDGFr-β or nonspecific rabbit IgG as a control in 5-μ sections of formalin-fixed LS174T tumors.

Figure 2

Response of LS174T tumors to various treatments with STI571. A. Changes in the tumor interstitial fluid pressure P_IF. Tumor-bearing mice received vehicle (n=6) or STI571 (n=5) for four consecutive days before the measurement of tumor P_IF. B. Impact of STI571 on the production of phospho-PDGFr-β in LS174T tumors grown as SQ xenografts in athymic mice treated with either PBS (control) or STI571 (n = 15 to 18). C. Arrest of LS174T tumor growth in response to the combination radioimmunotherapy and STI571 treatment. Mice were treated as outlined in Table 1. Data is plotted as a relative tumor growth normalized to the tumor size on day -3 when the first dose of STI571 was given. D. Kaplan-Meier analysis of the response of LS174T xenografts to the external beam radiotherapy in mice treated with PO doses of either PBS or STI571. E. Single photon emission computed tomography images (SPECT) acquired 72 h after administration of ¹²⁵I-B72.3 in LS174T-bearing mice treated with PBS (control) or STI571 as shown in Table 1. The PBS-treated tumor had an early onset of ulceration typical of LS174T tumors of this size. The pooling of the blood in the area of the ulcer is clearly noticeable in images shown in Fig.2E and in panels 16, 19, and 22 of Fig. 4A (NT).

Figure 3

SPECT images of athymic mice bearing SQ LS174T xenografts acquired with the LumaGEM^™ scintillation camera. Mice treated with STI571 as indicated in Table 1 and their images acquired 24, 48, and 72 h after the administration of ¹²⁵I-B72.3.

Figure 4

The effect of STI571 on the tumor uptake of the radioactive tracers. A. Comparison of ¹²⁵I-B72.3 uptake in tumors of STI571- treated (upper panels) and PBS-treated (lower panels) mice 72 h after the administration of radioactivity. Images were acquired using a radius of rotation of 3.29 cm and a pixel size of 0.78 mm. Volume images have been reconstructed and the Butterworth bandpass postfiltering was applied. B. Differences in the tumor uptake of ¹²⁵I-B72.3 in the center of the tumor. The size of the region of interest was 6 × 6 pixels size (4.68 mm × 4.68 mm) and was located in the core of the tumor; the diameter of tumor was 8.7 mm (PBS) and 8.9 mm (STI). C. Changes in the tumor hypoxia in response to STI571 treatment as determined by the tumor uptake of 1-[ethyl-(3′-[¹²⁵I]iodobenzamide)]-2-nitroimidazole. Data are expressed as the decrease in tumor-associated radioactivity relative to the radiotracer uptake after two doses of STI571.