Radiosensitization by a new potent nucleoside analog: 1-(1',3',4'-trihydroxy-2'-butoxy)methyl-2-nitroimidazole(RP-343)

Design, Synthesis and Anticancer Evaluation of Nitroimidazole Radiosensitisers

The role of hypoxic tumour cells in resistance to radiotherapy, and in suppression of immune response, continues to endorse tumour hypoxia as a bona fide, yet largely untapped, drug target. Radiotherapy innovations such as stereotactic body radiotherapy herald new opportunities for classical oxygen-mimetic radiosensitisers. Only nimorazole is used clinically as a radiosensitiser, and there is a dearth of new radiosensitisers in development. In this report, we augment previous work to present new nitroimidazole alkylsulfonamides and we document their cytotoxicity and ability to radiosensitise anoxic tumour cells in vitro. We compare radiosensitisation with etanidazole and earlier nitroimidazole sulfonamide analogues and we identify 2-nitroimidazole and 5-nitroimidazole analogues with marked tumour radiosensitisation in ex vivo assays of surviving clonogens and with in vivo tumour growth inhibition.

Monitoring Mitochondrial Complex-I Activity Using Novel PET Probe 18F-BCPP-EF Allows Early Detection of Radiotherapy Effect in Murine Squamous Cell Carcinoma

Objectives

Aerobic glycolysis, the main pathway of energy production in tumors (Warburg effect) allows detection of tumors by positron emission tomography (PET) using ¹⁸F-fluoro-2-deoxy-D-glucose (¹⁸F-FDG). Since ionizing radiation (IR) is reported to switch aerobic glycolysis to mitochondrial oxidative phosphorylation, radiotherapeutic efficacy was monitored by the activity of mitochondrial complex I (MC-I), using a new PET probe ¹⁸F-BCPP-EF, ¹⁸F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoro-ethoxy)-ethoxy] -pyridine-3-ylmethoxy}-2H-pyridazin-3-one, compared with ¹⁸F-FDG uptake and the apoptosis index.

Methods

Tumor uptake of ¹⁸F-BCPP-EF or ¹⁸F-FDG was examined in C3H/HeN mice inoculated with murine squamous cell carcinoma SCCVII at various time points after a single dose of x-ray irradiation at 0, 6, 15, or 30 Gy. Apoptosis incidence was determined by TUNEL staining in excised tumor tissue.

Results

Tumor growth suppression was dose-dependent; tumor grew 10-fold (0 Gy), 5-fold (6 Gy), 2-fold (15 Gy), and reduced to half in its volume (30 Gy) 14 days after treatment. ¹⁸F-BCPP-EF uptake was significantly increased as early as 3 days after 15 Gy or 30 Gy, when tumor size and apoptosis index showed no difference among radiation doses. In contrast, ¹⁸F-FDG uptake was initially increased dose-dependently, remained elevated up to 7 days, and eventually decreased 10 days after 30 Gy and also 14 days after 15 Gy when tumor size was already reduced. Apoptosis index was increased after irradiation but failed to correlate with tumor response.

Conclusion

Tumor uptake of ¹⁸F-BCPP-EF was increased dose-dependently early after effective doses of IR when ¹⁸F-FDG uptake as well as apoptosis incidence were not indicative of tumor response. The results suggest that ¹⁸F-BCPP-EF is a promising “positive” MC-I imaging PET probe for early detection of efficacy of tumor radiotherapy.

Radiotherapy enhancement with gold nanoparticles

Gold is an excellent absorber of X-rays. If tumours could be loaded with gold, this would lead to a higher dose to the cancerous tissue compared with the dose received by normal tissue during a radiotherapy treatment. Calculations indicate that this dose enhancement can be significant, even 200% or greater. In this paper, the physical and biological parameters affecting this enhancement are discussed. Gold nanoparticles have shown therapeutic efficacy in animal trials and these results are reviewed. Some 86% long-term (>1 year) cures of EMT-6 mouse mammary subcutaneous tumours was achieved with an intravenous injection of gold nanoparticles before irradiation with 250-kVp photons, whereas only 20% were cured with radiation alone. The clinical potential of this approach is also discussed.

The radiosensitizing effect of doranidazole on human colorectal cancer cells exposed to high doses of irradiation

Background

This paper investigates the effects of a new radiosensitizer, doranidazole, and enhancing irradiation on colorectal cancer cells.

Methods

The radiosensitizing effect of doranidazole was determined using colony formation and propidium iodide (PI) assays to measure cell growth inhibition and the cell killing effect of human colorectal cancer cell lines exposed to high doses of γ-ray irradiation under hypoxic conditions in vitro. Fluorescence staining and cell migration assays were also used to assess the radiosensitizing effect.

Results

Cell proliferation evaluated by clonogenic survival curves was significantly inhibited by 5 mmol/L doranidazole, particularly at doses ranging from 10 to 30 Gy of irradiation. The radiosensitizing effect of doranidazole on colorectal cancer cells occurs in a time- and dose-dependent manner. Doranidazole also inhibited the mobility of cell invasion and migration.

Conclusion

Doranidazole can enhance the killing effect and the cell growth inhibition of colorectal cancer after high-dose irradiation in a time and dose-dependent manner.

Reevaluation of the radiosensitizing effects of sanazole and nimorazole in vitro and in vivo

Sanazole (AK-2123, 3-nitrotriazole derivative, N1-(3-methoxypropyl)-2-(3-nitro-1 H-1,2,4-triazol-1-yl)acetamide) and nimorazole (5-nitroimidazole derivative, 4-(2-(5-nitro-1H-1-imidazolyl)ethyl)morpholine) have been tested clinically as hypoxic cell radiosensitizers, mainly outside Japan. To determine if these sensitizers deserve clinical investigation in Japan, we reevaluated the radiosensitizing effects of these compounds in vitro and in vivo, in comparison with a fluorinated 2-nitroimidazole derivative KU-2285 (N1-(2-hydroxyethyl)-1,2-difluoro-3-(2-nitro-1 H-1-midazolyl)propanamide). KU-2285 is a known and established radiosensitizer, but is not suitable for clinical studies because of the high cost of synthesis. In vitro, the radiosensitizing effects of the three compounds on SCCVII (squamous cell carcinoma line in C3H mice) tumor cells were examined at 0.5 and 1 mM under aerobic or hypoxic conditions, using a colony assay. In vivo, SCCVII tumors grown subcutaneously in the hind legs of C3H/HeN mice were irradiated with or without prior intraperitoneal administration of 100, 200 or 400 mg/kg of the drugs. Thereafter, tumor growth delay was measured. In vitro, no sensitizing effect was observed under aerobic conditions at 1 mM. Under hypoxic conditions, the sensitizer enhancement ratio (SER) determined at 1% cell survival level for sanazole, nimorazole and KU-2285 was 1.55, 1.45 and 1.95, respectively, at 1 mM, and 1.40, 1.40 and 1.75, respectively, at 0.5 mM. In vivo, all three compounds had significant radiosensitizing effects; their effects appeared to decrease in the order of KU-2285, sanazole, and nimorazole. It was suggested that sanazole may be more suitable for clinical trials than nimorazole.

Effects of hypoxic cell radiosensitizer doranidazole (PR-350) on the radioresponse of murine and human tumor cells in vitro and in vivo

We have investigated the radiosensitizing effect of doranidazole, a hypoxic cells radiosensitizer, using SCCVII tumor cells of C3H mice and CFPAC-1 and MIA PaCa-2 human pancreatic tumor cells. The radiosensitivity of hypoxic SCCVII cells in vitro increased with 1 mM doranidazole by a factor of 1.34 and 1.68, when determined by clonogenic survival and micronucleus (MN) formation, respectively. The radiation-induced growth delay of SCCVII tumors was significantly enhanced and the TCD(50/120) was reduced by a factor of 1.33 when 200 mg/kg doranidazole was injected, i.v., 20 min prior to tumor irradiation. The in vivo-in vitro excision assay showed that radiosensitivity of SCCVII cells in vivo increased by a factor of 1.47 with 200 mg/kg doranidazole. The radiation-induced growth delay of CFPAC-1 xenografts in nude mice was significantly enhanced and the TCD(50/90) was reduced by a factor of 1.30 by 200 mg/kg doranidazole. On the other hand, 200 mg/kg of doranidazole exerted no influence on the radiation-induced growth delay in MIA PaCa-2 xenografts. The tumor oxygenation status, as determined with an oxygen sensitive needle probe and the immunohistological study using pimonidazole, indicated that MIA PaCa-2 tumors are better oxygenated than CFPAC-1 tumors. The relatively well-oxygenated status in MIA PaCa-2 tumor may account for the lack of radiosensitization by doranidazole. It is concluded that the magnitude of radiosensitization of tumors by doranidazole is dependent on the oxygenation status of the tumors and that doranidazole may be useful in increasing the response of hypoxic human pancreatic tumor to IORT.

Phase III trial of radiosensitizer PR-350 combined with intraoperative radiotherapy for the treatment of locally advanced pancreatic cancer

A randomized, controlled trial was conducted to clarify the effect of novel radiosensitizer, PR-350, accompanied by intraoperative radiology (IOR) on locally advanced pancreatic cancer. Between July 1999 and March 2002, 48 patients were enrolled in this clinical trial and received either PR-350 or placebo. Any differences between the PR-350 group (n = 22) and control group (n = 25) were not statically significant. All patients were evaluated, and none of them showed toxicity, with the exception of 1 patient from the control group, and the PR-350 compound was considered to be safe. The efficacy of IOR with PR-350 was evaluated using CT examination. The committee responsible for evaluating efficacy reported that 47.4% of the PR-350 group showed the effective response, compared with 21.7% of the control group (P = 0.1067, Fisher analysis). At 6 months following treatment, the tumor mass reduction rate in the PR-350 group was significantly improved (P = 0.0274). By the time of the last follow-up in July 2003, 17 PR-350 patients and 24 control patients group had died of the disease. The median survival period of the PR-350 group was thus 318.5 days and that of the control group is 303.0 days. One-year survival rates of the PR-350 group and control group were 36.4% and 32.0%, respectively. Although the PR-350 group did not demonstrate significantly better survival than the control group, 4 of 22 PR-350 patients were still living more than 2 years after the end of the trial, compared with only 1 of 25 patients from the control group. The mechanism of this increased therapeutic response to radiotherapy using PR-350 must be clarified to establish more effective strategy for pancreatic cancer treatment.

Effect of a hypoxic cell sensitizer doranidazole on the radiation-induced apoptosis of mouse L5178Y lymphoma cells

We investigated the sensitizing effect of the 2-nitroimidazole analogue doranidazole, a new hypoxic radiosensitizer, on radiation-induced apoptosis in L5178Y cells. Apoptosis was assessed by checking DNA ladder formation, the presence of sub-G1 peaks in flow cytometry, and chromatin condensation. A radiosensitizing effect of doranidazole was also confirmed by a soft-agar colony assay of surviving cells. In the assay of DNA ladder formation, DNA fragmentation was observed following irradiation under an aerobic or hypoxic condition with or without doranidazole. The proportions of the cells at the sub-G1 peak in a flow cytometric measurement was not very different among the irradiations at 5 Gy under the aerobic condition, 15 Gy under hypoxia, and 10 Gy with 1 mM doranidazole under hypoxia. The fraction of cells with chromatin condensation was found to be significantly increased with doranidazole up to 3 mM when applied under hypoxic irradiation, but did not increase even at 10 mM. The sensitizer enhancement ratio was estimated to be about 1.7 with a concentration of 1 mM. This enhancement ratio was not different from that observed by assaying cell survivals. On the other hand, doranidazole showed no radiosensitizing effect under aerobic conditions with 1 mM. In conclusion, the radiation-induced apoptosis of L5178Y cells was enhanced by doranidazole under hypoxia.

A nitroimidazole derivative, PR-350, enhances the killing of pancreatic cancer cells exposed to high-dose irradiation under hypoxia

The radiosensitizing effects of PR-350, a nitroimidazole derivative, were examined concerning the cell killing of human pancreatic cancer cell lines exposed to high doses of gamma-ray irradiation in vitro. The percentages of dead cells were analyzed with a multiwell plate reader to measure the fluorescence intensity of propidium iodide before and after a digitonin treatment. The sensitizing effect of PR-350 on cell killing by high-dose irradiation was confirmed by time-course, dose-dependency, and microscopic observations. In five of seven pancreatic cancer cell lines in which the number of dead cells was determined 5 days after 30 Gy irradiation in the presence of PR-350, the number was significantly increased under hypoxic conditions, but not under aerobic conditions. The selective radiosensitive effect of PR-350 on hypoxic cells was also confirmed by flow cytometry. The results indicate that PR-350 can enhance the killing of pancreatic cancer cells by high-dose irradiation under hypoxia, which supports its clinical radiosensitizing effects when administered during intraoperative irradiation to pancreatic cancer.

Evaluation of the radiosensitizing effects of RK28 intravenous, intraarterial, and intratumoral injections on the rabbit VX2 tumor system

PURPOSE

To evaluate the differences in the radiosensitizing effects of intravenous (i.v.) injection, intraarterial (i.a.) injection, and intratumoral (i.t.) injections of the hypoxic cell radiosensitizer RK28 ([1-(4'-hydroxy-2'-butenoxy)methyl-2-nitroimidazole], a 2-nitroimidazole with an acyclic sugar analogue substituted at the N-1 position of the imidazole ring) using an animal experimental system.

METHODS AND MATERIALS

Rabbit VX2 tumors, which were implanted in the muscle of left hind legs and grown to 3 cm in diameter, were treated with RK28 (80 mg/kg x b.wt.) before 15 Gy of local x-ray irradiation. The auricular vein and the left saphenous artery were used for systemic injection and regional injection, respectively. For i.t. injection, a 21-gauge needle with three lateral holes was positioned in the central area of the tumor. Tumor regression was precisely evaluated by computed tomograpy (CT), and survival time was also studied. Using high-performance liquid chromatography (HPLC), pharmacokinetic studies for RK28 and its seven major metabolites were performed in tumor and serum at 0, 10, 20, 30, and 60 min after drug injection was completed.

RESULTS

Radiosensitizing effects of RK28 were considered present after i.a. injection (p < 0.05) and i.t. injection (p < 0.05) after analyzing tumor volumes on day 21 after treatment. Increased survival was not observed in any group with RK28 injection compared with survival in the group treated by x-ray irradiation alone. Pharmacokinetic studies showed the average concentration of RK28 in the tumor during x-ray irradiation was 1.3 times higher after i.a. injection and 3.5 times higher after i.t. injection than that after i.v. injection. The time modifying factor50 (TMF50: ratio of time for tumor to decrease by 50%, radiation alone vs. radiation plus drug) was calculated to be 1.5 after i.v. injection, 1.7 after i.a. injection, and 2.3 after i.t. injection. The values of TMF50 correlated to the average concentrations of RK28 in the tumor. As to metabolites of RK28, beta-glucuronated compound and cysteine conjugate were highly detected. The concentrations of cysteine conjugate were higher in the tumor than in serum via i.v. injection.

CONCLUSIONS

Radiosensitizing effects of RK28 were observed on the rabbit VX-2 tumor system after i.a. or i.t. injection. Pharmacokinetic studies proved that radiosensitizing effects depended on the concentration in the tumor, though the administration routes were different. Combined forms with nonprotein thiols were detected. However, survival benefits were not obtained by RK28. For clinical applications of RK28, i.a. or i.t. injection could facilitate better local control of cancer.

Detection of substances with alcoholic or phenolic hydroxyl groups by generation of hydrogen peroxide with imidazole and peroxyoxalate chemiluminescence

On-line detection of substances with an alcoholic or phenolic hydroxyl group using imidazole and peroxyoxalate chemiluminescence was investigated qualitatively using a flow-injection method. The substances tested included six polyphenols, five monophenols and six sugars. After incubation at 80 degrees C with an imidazole buffer (pH 9.5) the substances were detected by peroxyoxalate chemiluminescence. The polyphenols tested (e.g., pyrogallol, purpurogallin, and dopamine) showed the strongest light emission. The sugars with hydroxyl groups (e.g., fructose and lactose) and the monophenols (e.g., phenol, serotonin, and beta-estradiol) produced only a weak light emission. Imidazole served two roles, it catalysed the reaction with the hydroxyl compound and initiated peroxyoxalate chemiluminescence on-line. A novel reactor formed by packing glass beads into a flow cell (Teflon) of a chemiluminometer improved the sensitivity of light detection.

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.