Gene therapy enhances the antiproliferative effect of radiation in intimal hyperplasia

Background. Although ionizing radiation (IR) has been demonstrated to attenuate vessel wall restenosis and intimal hyperplasia (IH), dose-related mural injury and atrophy are possible deleterious side effects. We tested the hypothesis that a radiosensitizing strategy may improve IR-induced inhibition of in vivo vascular smooth muscle cells (VSMCs) without influencing apoptotic cell death. Methods. In 28 New Zealand White rabbits, the right common carotid artery (CCA) was injured and subjected to low-flow conditions to promote IH. The CCA was transfected with an adenoviral vector incorporating the cytosine deaminase (CD) gene (1 x 10⁹ PFU/ml). 5-Fluorocytosine (5-FC), a prodrug that is converted to the radiosensitizing agent 5-fluorouracil (5- FU) by CD, was thereafter administered intravenously. The CCA was exposed to 5 Gy IR at 24 h. Intimal/medial (I/M) area and thickness ratios were determined in the harvested CCAs at 14 days. VSMC proliferative and apoptotic indices were assessed with immunohistochemistry. Results. A 50% reduction in I/M area was found in rabbits treated with IR and IR + CD/5-FC (0.19 ± 0.03 and 0.18 ± 0.02) when compared with untreated controls (UC) (0.37 ± 0.06) (P = 0.005). This finding was substantiated by attenuation of I/M thickness in the IR groups [0.47 ± 0.13 (IR), 0.41 ± 0.11 (IR + CD/5-FC), 0.61 ± 0.17 (UC)] (P = 0.007). The number of proliferating VSMCs was notably smaller when IR was combined with CD/5-FC (4.17 ± 1.16 vs 2.97 ± 1.09 log transformed cells/mm², P < 0.07). Apoptosis was similar in all groups. Conclusions. Both IR alone and IR combined with a radiosensitizing agent are effective in attenuating experimental IH. However, combination therapy is synergistic and achieves greater inhibition of VSMC proliferation and may involve selective killing of radioresistant S-phase VSMCs. IR + CD/5-FC represents a novel therapeutic strategy that offers potential for long-term control of IH. (C) 2000 Academic Press.