Protracted exposure radiosensitization of experimental human malignant glioma

Potential treatment of inflammatory and proliferative diseases by ultra-low doses of ionizing radiations

Ultra-low doses and dose- rates of ionizing radiation are effective in preventing disease which suggests that they also may be effective in treating disease. Limited experimental and anecdotal evidence indicates that low radiation doses from radon in mines and spas, thorium-bearing monazite sands and enhanced radioactive uranium ore obtained from a natural geological reactor may be useful in treating many inflammatory conditions and proliferative disorders, including cancer. Optimal therapeutic applications were identified via a literature survey as dose-rates ranging from 7 to 11μGy/hr or 28 to 44 times world average background rates. Rocks from an abandoned uranium mine in Utah were considered for therapeutic application and were examined by γ-ray and laser-induced breakdown fluorescence spectroscopy. The rocks showed the presence of transuranics and fission products with a γ-ray energy profile similar to aged spent uranium nuclear fuel (93% dose due to β particles and 7% due to γ rays). Mud packs of pulverized uranium ore rock dust in sealed plastic bags delivering bag surface β,γ dose-rates of 10-450 μGy/h were used with apparent success to treat several inflammatory and proliferative conditions in humans.

Overview of radiosensitivity of human tumor cells to low-dose-rate irradiation

PURPOSE

We compared clonogenic survival in 27 human tumor cell lines that vary in genotype after low-dose-rate (LDR) or high-dose rate (HDR) irradiation. We measured susceptibility to LDR-induced redistribution in the cell cycle in eight of these cell lines.

METHODS AND MATERIALS

We measured clonogenic survival after up to 96 hours of LDR (0.25 Gy/h) irradiation. We compared these with clonogenic survival after HDR irradiation (50 Gy/h). Using flow cytometry, we measured LDR-induced redistribution as a function of time during LDR irradiation in eight of these cell lines.

RESULTS

Coefficients that describe clonogenic survival after both LDR and HDR irradiation segregate into four radiosensitivity groups that associate with cell genotype: mutant (mut)ATM, wild-type TP53, mutTP53, and an unidentified gene in radioresistant glioma cells. The LDR and HDR radiosensitivity correlates at lower doses ( approximately 2 Gy HDR, approximately 6 Gy LDR), but not at higher doses (HDR > 4 Gy; LDR > 6 Gy). The rate of LDR-induced loss of clonogenic survival changes at approximately 24 hours; wild-type TP53 cells become more resistant and mutTP53 cells become more sensitive. Redistribution induced by LDR irradiation also changes at approximately 24 hours.

CONCLUSIONS

Radiosensitivity of human tumor cells to both LDR and HDR irradiation is genotype dependent. Analysis of coefficients that describe cellular radiosensitivity segregates 27 cell lines into four statistically distinct groups, each associating with specific genotypes. Changes in cellular radiosensitivity and redistribution in the cell cycle are strongly time dependent. Our data establish a genotype-dependent time-dependent model that predicts clonogenic survival, explains the inverse dose-rate effect, and suggests possible clinical applications.

Systemic targeted radionuclide therapy: potential new areas

Radiation oncology is entering an exciting new era with therapies being delivered in a targeted fashion through an increasing number of novel approaches. External beam radiotherapy now integrates functional and anatomic tumor imaging to guide delivery of conformal radiation to the tumor target. Systemic targeted radionuclide therapy (STaRT) adds an important new dimension by making available to the radiation oncologist biologically targeted radiation therapy. Impressive clinical results with antibody-targeted radiotherapy, leading to the Food and Drug Administration's approval of two anti-CD20 radiolabeled antibodies, highlight the potential of STaRT. Optimization strategies will further improve the efficacy of STaRT by improving delivery systems, modifying the tumor microenvironment to increase targeted dose, and maximizing dose effect. Ultimately, the greatest potential for STaRT will not be as monotherapy, but as therapy integrated into established multimodality regimens and used as adjuvant or consolidative therapy in patients with minimal or micrometastatic disease.

Treatment of malignant gliomas: radiotherapy, chemotherapy and integration of new targeted agents

Progress in the biological and molecular characterization of gliomas and studies of factors associated with tumor growth and progression have led to translational research projects and the development of rational new approaches regarding prognostic models, better prediction of response to treatment and innovative therapeutic strategies. This review summarizes the available data on established and emerging prognostic factors and prognostic scores, and discusses their limitations as well as their potential influence on future therapeutic efforts. Recent developments in standard treatment options (i.e., surgery, radiotherapy and chemotherapy) are reviewed. Experimental data indicate that inhibition of several signaling pathways (e.g., epidermal growth factor, transforming growth factor-beta and phosphatidylinositol 3 kinase) may represent a promising therapeutic strategy. Some inhibitory agents (i.e., drugs, antibodies and antisense oligonucleotides) have now entered clinical trials, mainly for recurrent gliomas and a small number are being tested in combination with radiotherapy. Early results of such approaches are presented.

Rationales, evidence, and design considerations for fractionated radioimmunotherapy

Although fractionation can be used in a discrete radiobiologic sense, herein it is generally used in the broader context of administration of multiple, rather than single, doses of radionuclide for radioimmunotherapy (RIT) or other targeted radionuclide therapies. Fractionation is a strategy for overcoming heterogeneity of monoclonal antibody (MAb) distribution in the tumor and the consequent nonuniformity of tumor radiation doses. Additional advantages of fractionated RIT are the ability to 1) provide patient-specific radionuclide and radiation dosing, 2) control toxicity by titration of the individual patient, 3) reduce toxicity, 4) increase the maximum tolerated dose (MTD) for many patients, 5) increase tumor radiation dose and efficacy, and 6) prolong tumor response by permitting treatment over time. However, fractionated RIT has logistic and economic implications. Preclinical and clinical data substantiate the advantages of fractionated RIT, although the radiobiology for conventional external beam radiotherapy does not provide a straightforward rationale for RIT unless fractionation leads to more uniform distribution of radiation dose throughout the tumor. Preclinical data have shown that toxicity and mortality can be reduced while efficacy is increased, thereby providing inferential evidence of greater uniformity of radiation dose. Direct evidence of superior dosimetry and tumor activity distribution has also been found. Clinical data have shown that toxicity can be better controlled and reduced and the MTD extended for many patients. It is clear that fractionated RIT can only fulfill its potential if the effects of critical issues, such as the number and amount of radionuclide doses, the radionuclide physical and effective half-life, and the dose interval, are better characterized.

IUdR polymers for combined continuous low-dose rate and high-dose rate sensitization of experimental human malignant gliomas

Local polymeric delivery enhances IUdR radiosensitization of human malignant gliomas (MG). The combined low-dose rate (LDR) (0.03 Gy/h) and fractionated high-dose rate (HDR) treatments result in cures of experimental MGs. To enhance efficacy, we combined polymeric IUdR delivery, LDR, and HDR for treatments of both subcutaneous and intracranial MGs. In vitro: Cells (U251 MG) were trypsinized and replated in triplicate 1 day prior to LDR irradiation in media either without (control) or with 10 microM IUdR. After 72 hr, LDR irradiation cells were acutely irradiated (1.1 Gy/min) with increasing (0, 1.25, 2.5, 5.0, or 10 Gy) single doses. Implantable IUdR polymers [(poly(bis(p-carboxyphenoxy)-propane) (PCPP): sebaic acid (PCPP:SA), 20:80] (50% loading; 10 mg) were synthesized. In vivo: For flank vs. intracranial tumors, mice had 6 x 10(6) subcutaneous vs. 2 x 10(5) intracranial cells. For intracranial or subcutaneous MGs, mice had intratumoral blank (empty) vs. IUdR polymer treatments. One day after implantation, mice had immediate external LDR (3 cGy/h x 3 days total body irradiation) or HDR (2 Gy BID x 4 days to tumor site) or concurrent treatments. For the in vitro IUdR treatments, LDR resulted in a striking increase in cell-killing when combined with HDR. For the in vivo LDR treatments of flank tumors, the growth delay was greater for the IUdR vs. blank polymer treatments. For the combined LDR and HDR, the IUdR treatments resulted in a dramatic decrease in tumor volumes. On day 60 the log V/V0 were -1.7 +/- 0.22 for combined LDR + HDR + IUdR polymer (P < 0.05 vs. combined LDR + HDR + blank polymer). Survival for the intracranial controls was 22.9 +/- 1.2 days. For the blank polymer + LDR vs. blank polymer + LDR + HDR treatments, survival was 25.3 +/- 1.7 (P = NS) vs. 48.1 +/- 3.5 days (P < 0.05). For IUdR polymer + LDR treatment survival was 27.3 +/- 2.3 days (P = NS). The most striking improvement in survival followed the IUdR polymer + LDR + HDR treatment: 66.0 + 6.4 days (P < 0.05 vs. blank polymer + LDR + HDR). The polymeric IUdR delivery plus combined continuous LDR and HDR treatments results in growth delay and improved survival in animals bearing the MG xenografts. This treatment may hold promise for the treatment of human MGs.