PRECLINICAL EVALUATION OF 5-IODO-2-PYRIMIDINONE-2'-DEOXYRIBOSE AS A PRODRUG FOR 5-IODO-2'-DEOXYURIDINE-MEDIATED RADIOSENSITIZATION IN MOUSEAND HUMAN TISSUES

We reported previously that p.o. administered 5-iodo-2-pyrimidinone-2' -deoxyribose (IPdR) was efficiently converted to 5-iodo-2'-deoxyuridin e (IUdR) in athymic mice (T. J. Kinsella et al., Cancer Res., 54: 2695 -2700, 1994). Here, we further evaluate IPdR metabolism, systemic toxi city, and percentage DNA incorporation in athymic mouse normal tissues and a human colon cancer xenograft (HT29) using higher p.o. doses of IPdR. These data are compared to results using a continuous infusion o f IUdR at the maximum tolerable dose, We also evaluate IPdR metabolism in cytosolic extracts from normal human liver, normal human intestine , and human colorectal cancer specimens, Athymic mice tolerated a dail y p.o. bolus of up to 2 g/kg IPdR for 6 days with minimal host toxicit y (less than or equal to 10% body weight loss), There was rapid conver sion of IPdR to IUdR, with peak plasma levels of IUdR of 40-75 mu M at 10 min following a p.o. IPdR bolus of 250-1500 mg/kg. The percentage IUdR-DNA in the HT29 s.c. human tumor xenografts increased 1.5 times ( 2.3-3.6%) with IPdR doses above 1 g/kg/day for 6 days, whereas the per centage IUdR-DNA incorporation in two proliferating normal tissues (4- 4.5% in intestine; 1.6-2.2% in bone marrow) and a quiescent normal tis sue (less than or equal to 1% in liver) showed <1.5-fold increases wit h the IPdR dose escalation between 1-2 g/kg/day for 6 days, In contras t, using a continuous infusion of IUdR at 100 mg/kg/day, significant s ystemic toxicity (>20% body weight loss) was found by day 6 of the inf usion, Steady-state plasma IUdR levels were 1.0-1.2 mu M during the 6- day infusion, and percentage IUdR-DNA incorporations of 2.3, 8, 6, and 1% were measured in s.c. tumors, normal intestine, normal bone marrow , and normal liver, respectively, following the 6-day infusion, Thus, the p.o. IPdR schedule has an improved therapeutic index, based on per centage IUdR-DNA incorporation in normal and tumor tissues, compared t o continuous infusion IUdR at the maximum tolerable dose in athymic mi ce with this human tumor xenograft, Additionally, a tumor regrowth ass ay to assess the radiation response of HT29 s.c. xenografts showed a 1 .5-fold enhancement (time to regrow to 300% initial tumor volume) with IPdR (1000 mg/kg/day for 6 days) plus fractionated irradiation (XRT; 2 Gy/day for 4 days), compared to XRT (2 Gy/day for 4 days) alone. No enhancement in the radiation response of HT29 s.c. xenografts was foun d with continuous infusion IUdR (100 mg/kg/day for 6 days) plus XRT (2 Gy/day for 4 days), compared to XRT alone. Using cytosolic extracts f rom normal human liver specimens, we found a rapid (15-min) conversion of IPdR to IUdR. Coincubation of liver cytosol with IPdR and allopuri nol, an inhibitor of xanthine oxidase, had no inhibitory effect on IPd R metabolism, whereas coincubation with IPdR and isovanillin or menadi one, analogue substrates for aldehyde oxidase, effectively reduced the amount of IPdR oxidized to IUdR, Significantly less metabolism of IPd R to IUdR was seen in cytosolic extracts from normal human intestine s pecimens, and no metabolism of IPdR was found in cytosolic extracts fr om colorectal liver metastases in two patients and from the HT29 human colon cancer xenografts in athymic mice, These additional data indica te that IPdR has the potential for clinical use as a p.o. prodrug for IUdR-mediated radiosensitization of resistant human cancers.