Radiobiologic studies of radioimmunotherapy and external beam radiotherapy in Vitro and in Vivo in human renal cell carcinoma xenografts

BACKGROUND Previous studies suggest that the radiobiologic characteristics of in vitro survival curves are important determinants of the response of tumors to both conventional radiotherapy and radioimmunotherapy (RIT). The purpose of this study was to elucidate the relationship between in vitro radiation survival curve parameters and the relative sensitivity of tumor to RIT, exponentially decreasing low dose rate (ED LDR) irradiation and conventional high dose rate (HDR) fractionated external beam radiotherapy. METHODS Two human renal cell carcinoma cell lines, Caki‐1 and A498, were used in vitro and nude mouse xenograft studies. HDR external beam gamma irradiation (dose rate, 430 centigray [cGy]/minute) and ED LDR irradiation (initial dose rate, 22‐25 cGy/hour) were performed with a cesium‐137 ( 137 Cs) gamma irradiator. RIT was carried out with yttrium‐90 ( 90 Y)‐labeled monoclonal antibody NR‐LU‐10, and the absorbed radiation doses were calculated by medical internal radiation dose methodology. A clonogenic assay was used to generate radiation survival curves, and a computer FIT program was used to calculate the radiobiologic parameters. The antitumor efficacy of the different treatments was compared in vivo using a tumor regrowth delay assay in these two tumor xenograft models. RESULTS The radiation survival curves showed that the Caki‐1 cell line was more sensitive to both HDR and ED LDR irradiation than A498 in vitro. The Caki‐1 cell line, compared with A498, had a larger α (0.39 vs. 0.15 Gy following HDR and 0.32 vs. 0.21 Gy following ED LDR) and α‐to‐β ratio (6.92 vs. 2.60 Gy for HDR and 40.0 vs. 19.2 Gy for ED LDR), a smaller n number (5.13 vs. 23 for HDR and 1.16 vs. 3.53 for ED LDR), a lower quasi‐threshold dose (Dq) (1.60 vs. 3.15 Gy for HDR and 0.35 vs. 1.76 Gy for ED LDR), and a lower surviving fraction at 2 Gy (SF 2 ) (0.37 vs. 0.60 for HDR and 0.51 vs. 0.61 for ED LDR), suggesting that Caki‐1, compared with A498, had a steep initial slope and a small shoulder. The final slope represented by the β value and D o dose (the dose (Gy) required to reduce the fraction of surviving cells of 37% of its previous value in the exponential region of the survival curves) did not vary significantly between these two cell lines at either HDR or ED LDR irradiation. Tumor volume doubling times were 4.0 ± 1.5 days for Caki‐1 and 4.2 ± 1.8 days for A498 tumor xenografts. One hundred μCi/50 μg of 90 Y‐labeled, isotype‐matched irrelevant monoclonal antibody CCOO16‐3 produced a tumor growth delay time (TGD) of 2.1 days in Caki‐1 tumors but had no effect on A498 tumors ( P < 0.05). RIT with 100 μCi of 90 Y‐NR‐LU‐10 resulted in a TGD of 4.8 days for Caki‐1 tumors, whereas 100 μCi and 150 μCi of 90 Y‐NR‐LU‐10 produced a TGD of 1.9 and 2.7 days for A498 tumors, respectively. Estimated absorbed doses were 21.9 Gy in Caki‐1 tumors treated with 100 μCi of 90 Y‐NR‐LU‐10 and 14.5 Gy and 21.8 Gy in A498 tumors treated with 100 μCi and 150 μCi of 90 Y‐NR‐LU‐10, respectively. The weighted normal tissue absorbed doses were 7.4 Gy for Caki‐1 tumor‐bearing miceand 9.0 Gy for A498 tumor‐bearing mice ( P > 0.05). To compare the responses of Caki‐1 and A498 xenografts to RIT with external beam ED LDR and HDR irradiation, tumor‐bearing mice were treated with equivalent doses (20‐22 Gy) of 1) RIT with 90 Y‐NR‐LU‐10 (100 μCi for Caki‐1 and 150 μCi for A498), 2) continuous ED LDR 137 Cs irradiation with a initial dose rate of 22 cGy/hour, or 3) HDR X‐irradiation (2 Gy × 10 fractions in 2 weeks). The TGDs produced by RIT, ED LDR, and HDR were 5.3, 9.7, and 8.3 days for Caki‐1 and 2.7, 5.1, and 5.8 days for A498. The relative efficacy of RIT in these xenograft models correlated well with the radiobiologic parameters (i.e., the size of the initial slope and shoulder) of in vitro survival curves following HDR and ED LDR irradiation in these cell lines. CONCLUSIONS The results demonstrated that Caki‐1 tumors were more sensitive than A498 tumors to RIT, HDR, and ED LDR irradiation in vitro and in vivo in nude mice. The results suggest that the radiosensitivity of human tumor cells to low dose rate RIT correlated with the size of the initial slope and the shoulder of in vitro survival curves. Tumors with radiation survival curves characterized by a steep initial slope (i.e., a large α and α/β ratio) and/or a small shoulder (i.e., a small n number and a low D q dose) tend to be relatively more sensitive to RIT than tumors with a flat initial slope and/or a large shoulder. The SF2 also correlated well with the radiosensitivity of these tumor cell lines. The final slope of the survival curves represented by the β value and D 0 was not a useful indicator of the relative radiosensitivity of these two cell lines. These studies suggest that some of the radiobiologic parameters (the size of the initial slope and the shoulder) of in vitro radiation survival curves may help to predict the sensitivity of tumors to RIT.19‐28. Cancer 1997; 80:2519‐28. © 1997 American Cancer Society.