Pharmacokinetics of intratumoral RK-28, a new hypoxic radiosensitizer

A phase I/II study of a hypoxic cell radiosensitizer KU-2285 in combination with intraoperative radiotherapy

A fluorinated 2-nitroimidazole radiosensitizer KU-2285 was given before intraoperative radiotherapy (IORT) to 30 patients with unresectable, unresected or macroscopic residual tumours. Twenty-three patients had pancreatic cancer and five had osteosarcoma. The IORT dose was 30 Gy for unresectable pancreatic cancer and 60 Gy for osteosarcoma. The dose of KU-2285 administered ranged from 1 to 9 g m-2. Four patients received a dose of 9 g m-2, and ten received 6.8-7 g m-2. All patients tolerated KU-2285 well, and no drug-related toxicity was observed. The average tumour concentration of KU-2285 immediately after IORT was 166 microg g-1 at dose of 6.8-7 g m-2 and 333 microg g-1 at 9 g m-2. The average tumour-plasma ratio was > or = 0.82. Eleven patients with unresectable but localized pancreatic cancer treated with KU-2285 plus IORT and external beam radiotherapy had a median survival time of 11 months and 1-year local control rate of 50%, which compares favourably with those of 8 months (P = 0.26) and 28% (P = 0.10) for 22 matched historical control patients. The five patients with osteosarcoma attained local control. The results of this first study on KU-2285 and IORT appear encouraging, and further studies of this compound seem to be warranted.

Optical isomers of a new 2-nitroimidazole nucleoside analog (PR-350 series): radiosensitization efficiency and toxicity

PURPOSE

A new 2-nitroimidazole nucleoside radiosensitizer, PR-350 (1-[1',3',4'-trihydroxy-2'-butoxy]-methyl-2-nitroimidazole), has been reported to be as efficient as and less toxic than etanidazole. This compound is racemic, and it was recently optically resolved into two isomers, PR-68 (2'R,3'S type) and PR-69 (2'S,3'R type). The other two isomers, PR-28 (2'S,3'S type) and PR-44 (2'R,3'R type), were asymmetrically synthesized. In the present study, we investigated the properties, sensitizing activity, and toxicity of PR-350 and the four optical isomers in comparison with those of other 2-nitroimidazole hypoxic cell radiosensitizers, etanidazole, KU-2285, KIN-804, and RP-170. Because PR-350 and PR-28 can be industrially synthesized, we evaluated whether either of these two drugs are suitable for further investigation.

METHODS AND MATERIALS

In an in vitro study, EMT-6 cells were irradiated at a dose of 1-3 Gy under hypoxic conditions in the presence of the drugs at a concentration of 1 mM. A combined cytokinesis-block micronucleus and chromosomal aberration assay was performed. To assess the in vivo effects, colony assay and growth delay assay were performing using SCCVII tumor-bearing C3H mice. The mice received 16-24 GY 10-40 min after administration of 50-200 mg/kg of the drugs. Toxicity and pharmacokinetics in mice were also investigated.

RESULTS

The sensitizer enhancement ratio (SER) in the in vitro cytokinesis-block micronucleus assay increased in the following order: PR-69 (1.27) approximately equal to PR-28 (1.31) approximately equal to PR-44 (1.38) approximately equal to PR-350 (1.41) approximately equal to PR-68 (1.47) < etanidazole (1.79) < KIN-804 (2.03) approximately equal to KU-2285 (2.30). The SER at a dose of 200 mg/kg and at an interval of 20 min (optimal interval) in the in vivo-in vitro colony assay increased as follows: PR-44 (1.26) approximately equal to PR-28 (1.29) < PR-69 (1.34) approximately equal to etanidazole (1.35) approximately equal to PR-350 (1.36) < RP-170 (1.41) approximately equal to PR-68 (1.41) < KU-2285 (1.49). The growth delay assay also showed that PR-350 was less efficient than KU-2285 and more efficient than PR-28. PR-350 and the four isomers had similar reduction potentials, but PR-28 and PR-44 were more hydrophilic than PR-68 and PR-69. The LD50 in mice were 5.8 g/kg for PR-350, approximately 7 g/kg for PR-28, 4 g/kg for PR-68, and 6 g/kg for PR-44 and PR-69. The concentration of PR-28 in the murine sciatic nerve was lower than that of PR-350.

CONCLUSION

In vivo radiosensitizing activity differed among the four optical isomers, which appeared to be due, at least in part, to differences in lipophilicity. Although PR-28 was the least toxic, its low sensitization efficiency does not warrant clinical trials. Among the PR compounds, PR-68 appears to be most efficient, but optical resolution of PR-68 from PR-350 is expensive, and asymmetrical synthesis of PR-68 is not established. Therefore, PR-350 seems to be most suitable for further investigation among the PR-350 series compounds, considering its higher efficiency compared with PR-28 and PR-44, and established synthesis.