Thalidomide (TL) has been used in the treatment of multiple myeloma and due to its anti-angiogenic effects has been postulated to be a potential radiosensitizer of tumors in vivo. We first investigated the effect of TL on growth of IL-3 dependent murine hematopoietic 32D cl 3 cells in semisolid methylcellulose containing medium also supplemented with TL ranging from 0 to 200 μM. The cells were incubated at 37°C for 7 days at which time colonies of greater than 50 cells were counted. TL increased cell colony formation 2-fold over the dose range of 10 to 100 μM. In serially decreasing concentrations of IL-3, TL (50 μM) was growth stimulatory of 32D cl 3 colony formation in 15% IL-3 and 10% IL-3 conditioned medium (146.2 ± 7.2 and 89.3 ± 2.4 colonies/500 cells plated in TL vs. 110.8 ± 11.4 and 58.0 ± 2.1 without TL, p = 0.0304 or 0.0006). To determine whether thalidomide was a radiosensitizer, cells were placed in: 0, 50, or 150 μM TL in three protocols:

  1. 1 hour before irradiation;

  2. 1 hour before irradiation plus also in methylcellulose following irradiation; or

  3. were placed in TL containing methylcellulose only following irradiation.

Cells were irradiated to doses ranging from 0 to 800 cGy and were plated in methylcellulose containing medium. Seven days later, colonies of greater than 50 cells were counted and analyzed by single hit, multi-target or linear quadratic models. Cells grown in 50 μM TL which stimulated cell growth resulted in an increased radiation resistance compared to control irradiated cells (D0 = 1.87 ± 0.02 vs. 1.30 ± 0.09 Gy, respectively, p = 0.0366). However, 150 μM TL (which did not stimulate cell growth) increased radiation sensitivity compared to control irradiated cells (D0 = 1.22 ± 0.5 vs. 1.64 ± 0.11 Gy, respectively, p = 0.0245). To determine whether thalidomide radiosensitized tumor cells in the therapeutic 150 μM range, SSC-VII murine squamous cell carcinoma cells were grown in TL at concentrations ranging from 0 to 200 μM and were assayed for colony formation. In contrast, SSC-VII cells showed no detectable stimulation or inhibition of cell growth for unirradiated cells in any concentration of TL. However, at 150 μM TL, SSC-VII cells were radiosensitive when incubated in TL before irradiation compared to control irradiated cells (D0 = 2.02 vs. 2.55 Gy, respectively). These results demonstrate that thalidomide may be a clinically valuable radiosensitizer.

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