Abstract:
Purpose. The use of fractionated radiotherapy has underpinned cancer therapy for the last 50 years and currently more than 50% of all cancer patients receive radiotherapy as part of their treatment. A new innovation in radiotherapy delivery, stereotactic body radiotherapy (SBRT), allows dose intensification to solid tumors while minimizing the effects to adjacent normal tissue. This technique uses hypo-fractionated (1–5 doses), high dose (25–60 Gy total dose) radiotherapy. The role of SBRT is expanding, now targeting an increasing array of primary tumors in the torso as well as inoperable metastases.
However, large radiation doses delivered in several fractions accentuate the role of hypoxia in radioresistance. This provides an opportunity to use oxygen-mimetic nitroimidazole radiosensitizers to eradicate any hypoxic tumor cells that limit the efficacy of radiation. Although various nitroimidazole radiosensitizers have been explored with conventional fractionated radiotherapy previously, these have limitations that undermine their development in the SBRT setting. We recently identified a class of nitroimidazole radiosensitizer that provides both novelty and chemical versatility and have continued our exploration of this class.
Methods. We developed synthetic routes to prepare all possible nitroimidazole regioisomers. We determined the cytotoxicity of the compounds against HCT116 tumor cells under both oxic and anoxic conditions using a SRB proliferation assay. Sensitizer enhancement ratios (SER: Ratio of radiation dose for 1% survival +/- drug) were determined for compounds at non-toxic drug concentrations in HCT116 cells. Maximum tolerated doses (MTD) or maximum achievable doses (MAD) were determined in non-tumor bearing mice. Plasma and tumor drug concentration-time profiles were determined for single i.v. doses at the MTD (or MAD) to determine achievable drug concentrations and the optimal time for irradiation. Compounds were compared with etanidazole for in vivo radiosensitization in HCT116 human tumor xenografts using clonogenic survival as an endpoint and sensitization ratios (SR: difference in log surviving fraction after radiation +/- drug) were determined. Antitumor efficacy was evaluated in HCT116 tumors in mice treated with a single dose of compound in combination with 12.5 Gy of radiation.
Results. We prepared 40 nitroimidazole sulfonamides covering a wide range of aqueous solubility, lipophilicity and electron affinity. Electron affinity dominated the SAR for hypoxic cytotoxicity and radiosensitization in vitro, while side chain substituents dominated lipophilicity and aqueous solubility. Some ten compounds were comparable to etanidazole as radiosensitizers in vitro and their MTD/MAD and pharmacokinetic profiles were determined. Comparison of tumor Cmax with in vitro sensitization data allowed prediction of in vivo sensitization ratios. Where aqueous solubility limited the MAD, a phosphate prodrug approach provided significantly increased aqueous solubility, along with corresponding increases in tumor Cmax and in vivo radiosensitization. 2-nitroimidazole (SN36660) and 5-nitroimidazole (SN36876) phosphate prodrugs had comparable SRs to etanidazole with SN36660 plus radiation providing a significant delay in HCT116 tumor growth compared to radiation alone.
Conclusions. We have identified several nitroimidazole sulfonamide radiosensitizers that demonstrate comparable radiosensitization to etanidazole in vivo. We continue to evaluate these leads and identify related analogues with improved pharmacokinetic profiles.