Radiation therapy for cT1-2 carcinoma of the palatine tonsil diagnosed via a simple tonsillectomy: Dosimetry and patterns of care in the IMRT era

OBJECTIVE

Small carcinomas of the palatine tonsil are often diagnosed via simple tonsillectomy, a maneuver with non-therapeutic intent. Herein, practice patterns for this unique situation are evaluated.

PATIENTS AND METHODS

A retrospective review was performed across 10 facilities to identify patients with cT1-2 squamous carcinomas of the tonsil diagnosed by simple tonsillectomy between 2010 and 2018. Patients who received curative-intent intensity modulated radiotherapy (IMRT) without additional surgery were included. Target volumes were reviewed, and cumulative incidences of local failure and severe late dysphagia were calculated.

RESULTS

From 638 oropharyngeal patients, 91 were diagnosed via simple tonsillectomy. Definitive IMRT with no additional surgery to the primary site was utilized in 57, and three with gross residual disease were excluded, leaving 54 for analysis. Margins were negative in 13%, close (<5 mm) in 13%, microscopically positive in 61%, and not reported in 13%. Doses typically delivered to gross disease (68-70.2 Gy in 33-35 fx or 66 Gy/30 fx) were prescribed to the tonsil bed in 37 (69%). Sixteen patients (29%) received doses from 60 to 66 Gy (≤2 Gy/fx) and one received 50 Gy (2 Gy/fx). No local failures were observed. One late oropharyngeal soft tissue ulcer occurred, treated conservatively (grade 2). At five years, the cumulative incidence of severe late dysphagia was 17.4% (95% CI 6.1-28.8%).

CONCLUSION

Small tonsil carcinomas diagnosed by simple tonsillectomy represent a niche subset with favorable oncologic outcomes. Regardless, radiation oncologists tend to deliver full-dose to the tonsil bed. The necessity of this routine could be questioned in the modern era.

Weekly vs. Bolus Cisplatin Concurrent with Definitive Radiotherapy for Squamous Carcinoma of the Head and Neck: A Systematic Review and Network Meta-Analysis

PURPOSE/OBJECTIVE(S)

The optimal schedule for cisplatin delivered concurrently with definitive radiation for squamous carcinoma of the head and neck remains controversial. Randomized data in the postoperative setting is mixed, and definitive studies are ongoing. Meanwhile, multiple trials have already compared cetuximab to cisplatin in the definitive setting. Across these trials, the cetuximab dosing was identical, but cisplatin dosing was variable and can be categorized as weekly (40 mg/m2 q1 week) or bolus (100 mg/m2 q3 weeks). We indirectly compared these two cisplatin schedules by performing a network meta-analysis of cetuximab trials.

MATERIALS/METHODS

We performed a PRISMA-concordant systematic review to identify randomized controlled trials comparing cisplatin to cetuximab for patients with non-metastatic squamous carcinoma of the head and neck treated with definitive radiation therapy. Trials of primary surgery, incorporating induction therapy, or mixing other therapeutics were excluded. The analysis was pre-registered with the Open Science Foundation. Individual patient survival data was extracted from the published overall survival curves using a digitizer, and outcomes were validated against published point-estimates and hazard ratios. A random effects Cox regression was used to perform the individual-patient analysis using a one-step approach under a frequentist framework. Random effects were applied to model heterogeneity in the baseline hazard function and treatment effect. Models were adjusted by HPV and smoking status, which were trial-level covariates. Alternative endpoints (DFS, LRF, DM, etc.) were analyzed qualitatively. IRB approval was not required.

RESULTS

Five randomized trials were identified, including 1,678 patients. Bolus cisplatin was delivered to 572 patients in 2 trials, and weekly to 271 in 3 trials. The risk of bias was low. Relative to cetuximab, both bolus and weekly cisplatin reduced the risk of death (adjusted HR 0.63, 95% CI 0.46-0.87, p = 0.004 & HR 0.56, 95% CI 0.37-0.86, p = 0.008 respectively). No interaction was identified between regimen and HPV or smoking status. Between-study heterogeneity (δ2) was 0.148 and treatment effect heterogeneity (τ2) was small (<0.0002). There was no statistical difference in OS between bolus vs. weekly regimens (weekly vs. bolus HR 0.90, 95% CI 0.53-1.52, p = 0.345). This Cox model therefore suggested that on average, the absolute difference in 5-year OS is <1.5% between the two regimens, which was not statistically significant. Secondary endpoints and toxicity were not obviously different by regimen, qualitatively.

CONCLUSION

Using cetuximab as a common reference, there was no significant difference in survival between weekly and bolus cisplatin schedules.

Prospective Phase II Trial of Primary Lung Tumor Stereotactic Body Radiation Therapy (SBRT) Followed By Concurrent Mediastinal Chemoradiation and Adjuvant Immunotherapy for Locally-Advanced Non-Small Cell Lung Cancer (LA NSCLC)

PURPOSE/OBJECTIVE(S)

To report the efficacy and toxicity outcomes of a prospective phase II trial of primary tumor SBRT followed by conventional chemoradiation to the lymph nodes and adjuvant immunotherapy in patients (pts) with unresectable LA NSCLC.

MATERIALS/METHODS

Eligible pts included stage II-III LA NSCLC with peripheral primary tumors ≤ 7cm or centrally based tumors that had at least 2 cm separation from involved nodal disease. Pts received SBRT to the primary tumor (50-54 Gy in 3-5 fractions) followed by standard radiation to 60 Gy in 30 fractions to the involved lymph nodes with concurrent platinum doublet chemotherapy. The trial was amended to allow pts without disease progression after chemoradiation to receive adjuvant durvalumab per the PACIFIC trial. The primary endpoint was 1 year progression free survival (PFS), evaluated as a binary variable. Frequencies and proportions were used for reporting this primary endpoint, in addition to adverse events and patterns of failure. Median PFS and OS were estimated using Kaplan Meier methods.

RESULTS

Safety and efficacy is reported on the first 50 pts enrolled in the trial with a median follow-up of 24 months (mos) (range, 1-54 mos). Pts were primarily stage IIIA (60%) or stage IIIB (34%), with 6% of pts stage IIB. Overall grade 3 or higher toxicity related to SBRT and/or mediastinal radiation was 8% with two pts (4%) developing grade 3 pneumonitis and one pt having a grade 5 lung infection possibly related to radiation. Overall grade 2 pneumonitis related to SBRT or mediastinal radiation was 20%. Only one pt (2%) developed grade 3 esophagitis. No late cardiac events have been observed. The one-year PFS for all pts was 62% with a median PFS of 26.3 mos and median overall survival of 40.8 mos. Of the 50 pts enrolled, 37 received at least one dose of adjuvant durvalumab. The one-year PFS for pts who received at least one dose of durvalumab was 70.3% with a median PFS not yet reached in this group (median follow-up 24 mos). Patterns of failure were mostly distant with 26% of pts experiencing distant failure, 6% regional, and 2% distant and regional. There was only one local failure (2%) after SBRT in all 50 pts.

CONCLUSION

SBRT to the primary tumor followed by conventional chemoradiation to the involved lymph nodes and adjuvant immunotherapy was well tolerated and showed improved 1-year PFS compared to prior conventional chemoradiation trials for locally advanced NSCLC. The results of this trial will be further evaluated in a randomized phase III study, NRG LU-008. Pts will receive either conventional chemoradiation vs. SBRT to the primary tumor followed by chemoradiation to the involved lymph nodes followed by consolidative immunotherapy to evaluate the possibility of utilization of SBRT as a new standard of care for LA NSCLC.

Dosimetric Analysis of Intra-Fraction Motion Detected by Surface-Guided Radiation Therapy During Linac Stereotactic Radiosurgery

Purpose

Stereotactic radiosurgery (SRS) immobilization with an open face mask is more comfortable and less invasive than frame based, but concerns about intrafraction motion must be addressed. Surface-guided radiation therapy (SGRT) is an attractive option for intrafraction patient monitoring because it is continuous, has submillimeter accuracy, and uses no ionizing radiation. The purpose of this study was to investigate the dosimetric consequences of uncorrected intrafraction patient motion detected during frameless linac-based SRS.

Methods and Materials

Fifty-five SRS patients were monitored during treatment using SGRT between January 1, 2017, and September 30, 2020. If SGRT detected motion >1 mm, imaging was repeated and the necessary shifts were made before continuing treatment. For the 25 patients with intrafraction 3-dimensional vector shifts of ≥1 mm, we moved the isocenter in the planning system using the translational shifts from the repeat imaging and recalculated the plans to determine the dosimetric effect of the shifts. Planning target volume (PTV) coverage, minimum gross tumor volume (GTV) dose (relative and absolute), and normal brain V12 were evaluated. Wilcoxon signed rank tests were used to compare planned and simulated dosimetric parameters and median 2 sample tests were used to investigate these differences between cone and multileaf collimator (MLC) plans.

Results

For simulated plans, V12 increased by a median of 0.01 cc (P = .006) and relative GTV minimum dose and PTV coverage decreased by a median of 15.8% (P < .001) and 10.2 % (P < .001), respectively. Absolute minimum GTV dose was found to be significantly lower in the simulated plans (P < .001). PTV coverage decreased more for simulated cone plans than for simulated MLC plans (11.6% vs 4.7%, P = .011) but median V12 differences were found to be significantly larger for MLC plans (-0.34 cc vs -0.01 cc, P = .011). Differences in GTV minimum dose between cone and MLC plans were not statistically significant.

Conclusions

SGRT detected clinically meaningful intrafraction motion during frameless SRS, which could lead to large underdoses and increased normal brain dose if uncorrected.

Defining targets to improve care delivery for T4 larynx squamous cell carcinoma

Objective

United States oncology trends consistently demonstrate that nearly half of T4a larynx carcinoma patients are treated with larynx preservation, despite national guidelines favoring laryngectomy. This study identifies clinical decision-making drivers and defines patient subsets that should become targets for care improvement.

Methods

Retrospective analysis of patients with cT4 squamous cell carcinoma of the larynx from US National Cancer Database 2005-2016. Demographic data and survival rates between clinical pathways were compared. Survival was estimated by Kaplan-Meier method with statistical comparisons assessed by log-rank test.

Results

Of 11,556 patients with cT4 disease, laryngectomy (TL) was the initial treatment for 4627 (40%) patients. Larynx preservation via chemoradiation (CRT) occurred for 4307 patients. TL and CRT patients had similar Charlson-Deyo comorbidity indices and insurance status. TL patients had higher total tumor size, lower N3 rates and were more often seen at academic institutions (p < .0001). N0 surgery patients with adjuvant treatment demonstrated superior median survival (MS) compared to CRT (surgery + radiation MS: 69 months, surgery + chemoradiation MS: 66, CRT MS: 37.7), p < .0001. MS for N1/N2 disease patients was 56.5 months for surgery + radiation and 35.5 months for surgery + CRT, superior to CRT, MS 30.8 months, p < .0001. Tri-modality N3 patients with up front surgery had similar MS compared to CRT (surgery + chemoradiation 21.3 months vs. CRT 16.1), p = .95.

Conclusion

National quality improvement initiatives are needed to promote guideline adherence and improve survival in advanced larynx cancer. Targets for such initiatives should be patients with limited or no nodal disease burden, that meet clear T4a imaging criteria.

Level of Evidence

Level IV, non-randomized controlled cohort.

Oncologists' Perspective on Dental Care Around the Treatment of Head and Neck Cancer: A Pattern of Practice Survey

PURPOSE

Patients with head and neck cancer are at risk of long-term dental complications. Proper dental assessment pre- and post-treatment can improve outcomes but is logistically challenging. We surveyed oncologists to better understand their perspectives surrounding dental care in this unique population.

METHODS

We surveyed oncologists at institutions associated with an ongoing national study of oral health after treatment of head and neck cancer. Seventeen questions were used to assess provider characteristics, patterns of practice, patterns of referral, barriers to referral, and willingness to apply fluoride varnish in the oncology clinic.

RESULTS

Ninety-seven oncologists were invited from six institutions, of whom 40 (41%) responded. Surgeons represented 45% of the sample, followed by radiation oncologists (40%) and medical oncologists (15%). Both generalists and subspecialists were included. All practiced in a metropolitan area with an academic dental practice, and many felt that this improved access to care. Despite this, most oncologists thought that financial factors were a significant barrier to obtaining timely dental care. Most oncologists performed a dental assessment during visits. Oncologists felt qualified to identify the most significant complications of treatment, such as exposed bone, but felt underqualified to identify early changes in need of intervention. When asked if the oncology clinic could apply fluoride varnish during follow-ups, most stated that this seemed feasible but would require education and financial support.

CONCLUSION

Oncologists often perform limited dental evaluations during their routine visits. Given the challenges associated with access to proper dental care for this population, these oncology visits may provide a window for preventative intervention.

Use of surface-guided radiation therapy in combination with IGRT for setup and intrafraction motion monitoring during stereotactic body radiation therapy treatments of the lung and abdomen

Background and purpose

Multiple techniques can be used to assist with more accurate patient setup and monitoring during Stereotactic body radiation therapy (SBRT) treatment. This study analyzes the accuracy of 3D surface mapping with Surface‐guided radiation therapy (SGRT) in detecting interfraction setup error and intrafraction motion during SBRT treatments of the lung and abdomen.

Materials and Methods

Seventy‐one patients with 85 malignant thoracic or abdominal tumors treated with SBRT were analyzed. For initial patient setup, an alternating scheme of kV/kV imaging or SGRT was followed by cone beam computed tomography (CBCT) for more accurate tumor volumetric localization. The CBCT six degree shifts after initial setup with each method were recorded to assess interfraction setup error. Patients were then monitored continuously with SGRT during treatment. If an intrafractional shift in any direction >2 mm for longer than 2 sec was detected by SGRT, then CBCT was repeated and the recorded deltas were compared to those detected by SGRT.

Results

Interfractional shifts after SGRT setup and CBCT were small in all directions with mean values of <5 mm and < 0.5 degrees in all directions. Additionally, 25 patients had detected intrafraction motion by SGRT during a total of 34 fractions. This resulted in 25 (73.5%) additional shifts of at least 2 mm on subsequent CBCT. When comparing the average vector detected shift by SGRT to the resulting vector shift on subsequent CBCT, no significant difference was found between the two.

Conclusions

Surface‐guided radiation therapy provides initial setup within 5 mm for patients treated with SBRT and can be used in place of skin marks or planar kV imaging prior to CBCT. In addition, continuous monitoring with SGRT during treatment was valuable in detecting potentially clinically meaningful intrafraction motion and was comparable in magnitude to shifts from additional CBCT scans. PTV margin reduction may be feasible for SBRT in the lung and abdomen when using SGRT for continuous patient monitoring during treatment.

Preoperative stereotactic radiosurgery before planned resection of brain metastases: updated analysis of efficacy and toxicity of a novel treatment paradigm

OBJECTIVE

Preoperative stereotactic radiosurgery (SRS) is a feasible alternative to postoperative SRS and may lower the risk of radiation necrosis (RN) and leptomeningeal disease (LMD) recurrence. The study goal was to report the efficacy and toxicity of preoperative SRS in an expanded patient cohort with longer follow-up period relative to prior reports.

METHODS

The records for patients with brain metastases treated with preoperative SRS and planned resection were reviewed. Patients with classically radiosensitive tumors, planned adjuvant whole brain radiotherapy, or no cranial imaging at least 1 month after surgery were excluded. Preoperative SRS dose was based on lesion size and was reduced approximately 10-20% from standard dosing. Surgery generally followed within 48 hours.

RESULTS

The study cohort consisted of 117 patients with 125 lesions treated with single-fraction preoperative SRS and planned resection. Of the 117 patients, 24 patients were enrolled in an initial prospective trial; the remaining 93 cases were consecutively treated patients who were retrospectively reviewed. Most patients had a single brain metastasis (70.1%); 42.7% had non-small cell lung cancer, 18.8% had breast cancer, 15.4% had melanoma, and 11.1% had renal cell carcinoma. Gross total resection was performed in 95.2% of lesions. The median time from SRS to surgery was 2 days, the median SRS dose was 15 Gy, and the median gross tumor volume was 8.3 cm3. Event cumulative incidence at 2 years was as follows: cavity local recurrence (LR), 25.1%; distant brain failure, 60.2%; LMD, 4.3%; and symptomatic RN, 4.8%. The median overall survival (OS) and 2-year OS rate were 17.2 months and 36.7%, respectively. Subtotal resection (STR, n = 6) was significantly associated with increased risk of cavity LR (hazard ratio [HR] 6.67, p = 0.008) and worsened OS (HR 2.63, p = 0.05) in multivariable analyses.

CONCLUSIONS

This expanded and updated analysis confirms that single-fraction preoperative SRS confers excellent cavity local control with very low risk of RN or LMD. Preoperative SRS has several potential advantages compared to postoperative SRS, including reduced risk of RN due to smaller irradiated volume without need for cavity margin expansion and reduced risk of LMD due to sterilization of tumor cells prior to spillage at the time of surgery. Subtotal resection, though infrequent, is associated with significantly worse cavity LR and OS. Based on these results, a randomized trial of preoperative versus postoperative SRS is being designed.

Single-Fraction Stereotactic Radiosurgery (SRS) Alone Versus Surgical Resection and SRS for Large Brain Metastases: A Multi-institutional Analysis

PURPOSE

Stereotactic radiosurgery (SRS) dose is limited by brain metastasis (BM) size. The study goal was to retrospectively determine whether there is a benefit for intracranial outcomes and overall survival (OS) for gross total resection with single-fraction SRS versus SRS alone for patients with large BMs.

METHODS AND MATERIALS

A large BM was defined as ≥4 cm³ (2 cm in diameter) prior to the study. We reviewed the records of consecutive patients treated with single-fraction SRS alone or surgery with preoperative or postoperative SRS between 2005 and 2013 from 2 institutions.

RESULTS

Overall, 213 patients with 223 treated large BMs were included; 66 BMs (30%) were treated with SRS alone and 157 (70%) with surgery and SRS (63 preoperatively and 94 postoperatively). The groups (SRS vs surgery and SRS) were well balanced except regarding lesion volume (median, 5.9 cm³ vs 9.6 cm³; P<.001), median number of BMs (1.5 vs 1, P=.002), median SRS dose (18 Gy vs 15 Gy, P<.001), and prior whole-brain radiation therapy (33% vs 5%, P<.001). The local recurrence (LR) rate was significantly lower with surgery and SRS (1-year LR rate, 36.7% vs 20.5%; P=.007). There was no difference in radiation necrosis (RN) by resection status, but there was a significantly increased RN rate with postoperative SRS versus with preoperative SRS and with SRS alone (1-year RN rate, 22.6% vs 5% and 12.3%, respectively; P<.001). OS was significantly higher with surgery and SRS (2-year OS rate, 38.9% vs 19.8%; P=.01). Both multivariate adjusted analyses and propensity score-matched analyses demonstrated similar results.

CONCLUSIONS

In this retrospective study, gross total resection with SRS was associated with significantly reduced LR compared with SRS alone for patients with large BMs. Postoperative SRS was associated with the highest rate of RN. Surgical resection with SRS may improve outcomes in patients with a limited number of large BMs compared with SRS alone. Further studies are warranted.

Receptor tyrosine kinase EphA5 is a functional molecular target in human lung cancer

Lung cancer is often refractory to radiotherapy, but molecular mechanisms of tumor resistance remain poorly defined. Here we show that the receptor tyrosine kinase EphA5 is specifically overexpressed in lung cancer and is involved in regulating cellular responses to genotoxic insult. In the absence of EphA5, lung cancer cells displayed a defective G₁/S cell cycle checkpoint, were unable to resolve DNA damage, and became radiosensitive. Upon irradiation, EphA5 was transported into the nucleus where it interacted with activated ATM (ataxia-telangiectasia mutated) at sites of DNA repair. Finally, we demonstrate that a new monoclonal antibody against human EphA5 sensitized lung cancer cells and human lung cancer xenografts to radiotherapy and significantly prolonged survival, thus suggesting the likelihood of translational applications.

Palliative thoracic radiotherapy in lung cancer: An American Society for Radiation Oncology evidence-based clinical practice guideline

PURPOSE

To provide guidance to physicians and patients with regard to the use of external beam radiotherapy, endobronchial brachytherapy, and concurrent chemotherapy in the setting of palliative thoracic treatment for lung cancer, based on available evidence complemented by expert opinion.

METHODS AND MATERIALS

A Task Force authorized by the American Society for Radiation Oncology (ASTRO) Board of Directors synthesized and assessed evidence from 3 systematic reviews on the following topics: (1) dose fractionation in thoracic external beam radiotherapy (EBRT); (2) clinical utility of initial and salvage endobronchial brachytherapy (EBB); and (3) use of concurrent chemotherapy (CC) with palliative thoracic radiotherapy. Practice guideline recommendations were produced and are contained herein.

RESULTS

Studies suggest that higher dose/fractionation palliative EBRT regimens (eg, 30 Gy/10 fraction equivalent or greater) are associated with modest improvements in survival and total symptom score, particularly in patients with good performance status. As these improvements are associated with an increase in esophageal toxicity, various shorter EBRT dose/fractionation schedules (eg, 20 Gy in 5 fractions, 17 Gy in 2 weekly fractions, 10 Gy in 1 fraction), which provide good symptomatic relief with fewer side effects, can be used for patients requesting a shorter treatment course and/or in those with a poor performance status. No defined role for EBB in the routine initial palliative treatment of chest disease has been demonstrated; however, EBB can be a reasonable option for the palliation of endobronchial lesions causing obstructive symptomatology including lung collapse, or for hemoptysis after EBRT failure. The integration of concurrent chemotherapy with palliative intent/fractionated radiotherapy is not currently supported by the medical literature.

CONCLUSION

This Guideline is intended to serve as a guide for the use of EBRT, EBB, and CC in thoracic palliation of lung cancer outside the clinical trial setting. Further prospective clinical investigations with relevant palliative endpoints into the respective roles of EBB and CC/targeted therapy in the thoracic palliation of lung cancer are warranted, given the current state of the medical literature in these areas.

DNA repair biomarker profiling of head and neck cancer: Ku80 expression predicts locoregional failure and death following radiotherapy

PURPOSE

Radiotherapy plays an integral role in the treatment of head and neck squamous cell carcinoma (HNSCC). Although proteins involved in DNA repair may predict HNSCC response to radiotherapy, none has been validated in this context. We examined whether differential expression of double-strand DNA break (DSB) repair proteins in HNSCC, the chief mediators of DNA repair following irradiation, predict for treatment outcomes.

EXPERIMENTAL DESIGN

Archival HNSCC tumor specimens (n = 89) were assembled onto a tissue microarray and stained with antibodies raised against 38 biomarkers. The biomarker set was enriched for proteins involved in DSB repair, in addition to established mechanistic markers of radioresistance. Staining was correlated with treatment response and survival alongside established clinical and pathologic covariates. Results were validated in an independent intramural cohort (n = 34).

RESULTS

Ku80, a key mediator of DSB repair, correlated most closely with clinical outcomes. Ku80 was overexpressed in half of all tumors, and its expression was independent of all other covariates examined. Ku80 overexpression was an independent predictor for both locoregional failure and mortality following radiotherapy (P < 0.01). The predictive power of Ku80 overexpression was confined largely to HPV-negative HNSCC, where it conferred a nine-fold greater risk of death at two years.

CONCLUSIONS

Ku80 overexpression is a common feature of HNSCC, and is a candidate DNA repair-related biomarker for radiation treatment failure and death, particularly in patients with high-risk HPV-negative disease. It is a promising, mechanistically rational biomarker to select individual HPV-negative HNSCC patients for strategies to intensify treatment.

Discovery of DNA repair inhibitors by combinatorial library profiling

Small molecule inhibitors of DNA repair are emerging as potent and selective anticancer therapies, but the sheer magnitude of the protein networks involved in DNA repair processes poses obstacles to discovery of effective candidate drugs. To address this challenge, we used a subtractive combinatorial selection approach to identify a panel of peptide ligands that bind DNA repair complexes. Supporting the concept that these ligands have therapeutic potential, we show that one selected peptide specifically binds and noncompetitively inactivates DNA-PKcs, a protein kinase critical in double-strand DNA break repair. In doing so, this ligand sensitizes BRCA-deficient tumor cells to genotoxic therapy. Our findings establish a platform for large-scale parallel screening for ligand-directed DNA repair inhibitors, with immediate applicability to cancer therapy.

Leveraging molecular heterogeneity of the vascular endothelium for targeted drug delivery and imaging

Patterns of luminal vascular protein expression vary according to tissue of origin, and these so-called vascular zip codes play an important role in the maintenance of normal physiological processes in multiorgan species. These zip codes are also often modified in response to pathology and play a critical role in the response to and recovery from disease. These differentially expressed proteins offer opportunities for organ- and disease-specific ligand-targeted molecular delivery, offering clear pharmacologic advantages to traditional systemic drug delivery. However, this approach depends on the availability of frequently elusive targets. Combinatorial screening is a valuable tool for identifying optimal ligands for targeted delivery to the vasculature, especially when adapted for in vivo selection. Here we discuss molecular heterogeneity of the vascular endothelium and the use of phage display as a combinatorial screening method to exploit this heterogeneity for tissue or disease-specific vascular targeting. We also highlight applications of this approach for both drug delivery and molecular imaging.

Prospective imaging assessment of mortality risk after head-and-neck radiotherapy

PURPOSE

The optimal roles for imaging-based biomarkers in the management of head-and-neck cancer remain undefined. Unresolved questions include whether functional or anatomic imaging might improve mortality risk assessment for this disease. We addressed these issues in a prospective institutional trial.

METHODS AND MATERIALS

Ninety-eight patients with locally advanced pharyngolaryngeal squamous cell cancer were enrolled. Each underwent pre- and post-chemoradiotherapy contrast-enhanced computed tomography (CT) and (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/CT imaging. Imaging parameters were correlated with survival outcomes.

RESULTS

Low post-radiation primary tumor FDG avidity correlated with improved survival on multivariate analysis; so too did complete primary tumor response by CT alone. Although both imaging modalities lacked sensitivity, each had high specificity and negative predictive value for disease-specific mortality risk assessment. Kaplan-Meier estimates confirmed that both CT and FDG-PET/CT stratify patients into distinct high- and low-probability survivorship groups on the basis of primary tumor response to radiotherapy. Subset analyses demonstrated that the prognostic value for each imaging modality was primarily derived from patients at high risk for local treatment failure (human papillomavirus [HPV]-negative disease, nonoropharyngeal primary disease, or tobacco use).

CONCLUSIONS

CT alone and FDG-PET/CT are potentially useful tools in head-and-neck cancer-specific mortality risk assessment after radiotherapy, particularly for selective use in cases of high-risk HPV-unrelated disease. Focus should be placed on corroboration and refinement of patient selection for imaging-based biomarkers in future studies.

Antiangiogenic action of redox-modulating Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, MnTE-2-PyP(5+), via suppression of oxidative stress in a mouse model of breast tumor

MnTE-2-PyP(5+) is a potent catalytic scavenger of reactive oxygen and nitrogen species, primarily superoxide and peroxynitrite. It therefore not only attenuates primary oxidative damage, but was found to modulate redox-based signaling pathways (HIF-1alpha, NF-kappaB, SP-1, and AP-1) and thus, in turn, secondary oxidative injury also. Cancer has been widely considered an oxidative stress condition. The goal of this study was to prove if and why a catalytic SOD mimic/peroxynitrite scavenger would exert anti-cancer effects, i.e., to evaluate whether the attenuation of the oxidative stress by MnTE-2-PyP(5+) could suppress tumor growth in a 4T1 mouse breast tumor model. Tumor cells were implanted into Balb/C mouse flanks. Three groups of mice (n=25) were studied: control (PBS) and 2 and 15 mg/kg/day of MnTE-2-PyP(5+) given subcutaneously twice daily starting when the tumors averaged 200 mm(3) (until they reached approximately 5-fold the initial volume). Intratumoral hypoxia (pimonidazole, carbonic anhydrase), HIF-1alpha, VEGF, proliferating capillary index (CD105), microvessel density (CD31), protein nitration, DNA oxidation (8-OHdG), NADPH oxidase (Nox-4), apoptosis (CD31), macrophage infiltration (CD68), and tumor drug levels were assessed. With 2 mg/kg/day a trend toward tumor growth delay was observed, and a significant trend was observed with 15 mg/kg/day. The 7.5-fold increase in drug dose was accompanied by a similar (6-fold) increase in tumor drug levels. Oxidative stress was largely attenuated as observed through the decreased levels of DNA damage, protein 3-nitrotyrosine, macrophage infiltration, and NADPH oxidase. Further, hypoxia was significantly decreased as were the levels of HIF-1alpha and VEGF. Consequently, suppression of angiogenesis was observed; both the microvessel density and the endothelial cell proliferation were markedly decreased. Our study indicates for the first time that MnTE-2-PyP(5+) has anti-cancer activity in its own right. The anti-cancer activity via HIF/VEGF pathways probably arises from the impact of the drug on the oxidative stress. Therefore, the catalytic scavenging of ROS/RNS by antioxidants, which in turn suppresses cellular transcriptional activity, could be an appropriate strategy for anti-cancer therapy. Enhancement of the anti-cancer effects may be achieved by optimizing the dosing regime, utilizing more bioavailable Mn porphyrins (MnP), and combining MnP treatment with irradiation, hyperthermia, and chemotherapy. Mn porphyrins may be advantageous compared to other anti-cancer drugs, owing to their radioprotection of normal tissue and the ability to afford pain management in cancer patients via prevention of chronic morphine tolerance.

Targeting cancer-specific synthetic lethality in double-strand DNA break repair

There is interest in the use of DNA repair inhibitors as sensitizers of classic cytotoxic therapy against cancer. However, there is also risk -- theoretical, at least -- that such a strategy may increase the side effects of traditional therapies, including but not limited to treatment-related secondary malignancies. Before being brought to clinical application, therefore, important questions remain to be answered regarding how these therapies will be tailored to achieve benefit without concomitantly increasing harm. A potential solution may involve targeting so-called "synthetic lethalities" in tumor DNA repair pathways; taking advantage of defects acquired in DNA repair pathways during tumorigenesis by targeting alternative repair pathways on which the tumor critically depends. Conceivably, as repair pathways are functional in normal tissue, such targeted therapy should be relatively tumor-specific and non-toxic. We review here the rationale for this strategy, describe examples of its application, and outline potential strengths and weaknesses of this approach. For simplicity, a focus will be placed on the repair of double-strand breaks as a model system, but the conceptual framework is generally applicable to many other pathways of DNA repair.

Prospective risk-adjusted [18F]Fluorodeoxyglucose positron emission tomography and computed tomography assessment of radiation response in head and neck cancer

PURPOSE

[(18)F]Fluorodeoxyglucose positron emission tomography (FDG-PET)/computed tomography (CT) imaging may improve assessment of radiation response in patients with head and neck cancer, but it is not yet known for which patients this is most useful. We conducted a prospective trial to identify patient populations likely to benefit from the addition of functional imaging to the assessment of radiotherapy response.

PATIENTS AND METHODS

Ninety-eight patients with locally advanced cancer of the oropharynx, larynx, or hypopharynx were prospectively enrolled and treated with primary radiotherapy, with or without chemotherapy. Patients underwent FDG-PET/CT and contrast-enhanced CT imaging 8 weeks after completion of treatment. Functional and anatomic imaging response was correlated with clinical and pathologic response. Imaging accuracy was then compared between imaging modalities.

RESULTS

Although postradiation maximum standard uptake values were significantly higher in nonresponders compared with responders, the positive and negative predictive values of FDG-PET/CT scanning were similar to those for CT alone in the unselected study population. Subset analyses revealed that FDG-PET/CT outperformed CT alone in response assessment for patients at high risk for treatment failure (those with human papillomavirus [HPV] -negative disease, nonoropharyngeal primaries, or history of tobacco use). No benefit to FDG-PET/CT was seen for low-risk patients lacking these features.

CONCLUSION

These data do not support the broad application of FDG-PET/CT for radiation response assessment in unselected head and neck cancer patients. However, FDG-PET/CT may be the imaging modality of choice for patients with highest risk disease, particularly those with HPV-negative tumors. Optimal timing of FDG-PET/CT imaging after radiotherapy merits further investigation.

Spectral imaging facilitates visualization and measurements of unstable and abnormal microvascular oxygen transport in tumors

Abnormal microvasculature contributes to the pathophysiologic microenvironment of tumors. Understanding microvascular tumor oxygen transport is necessary to comprehend the factors that influence tumor biology, physiology, and therapy. Previously, we described an in vivo spectral imaging microscopy system for measurements of microvessel hemoglobin saturation (HbSat). We measure temporal fluctuations and spatial gradients in tumor microvessel oxygenation and identify instances of anastomoses between vessels with significantly different oxygenations. Slow periodic fluctuations in HbSat <0.2 cycles per minute were observed. These measurements are consistent with microelectrode measurements of fluctuating tumor oxygenation. Gradients in HbSat along individual tumor microvessels were measured that were larger in magnitude than normal tissue microvessels. Images were captured of anastomoses of tumor microvessels with diameters <or=100 microm and significantly different HbSat values (>20%). Shunting of inspired oxygen, presumably due to arteriovenous anastomoses, from tumor feeding arterioles to adjacent venules was imaged. This effect was confined to a region around the tumor and was not observed in nearby normal microvessels. Imaging measurements of tumor microvessel oxygen transport may offer insight to current questions regarding oxygen-related tumor biology and treatment responses, and spectral imaging may be a useful research tool in this regard.

Hypoxia and radiotherapy: opportunities for improved outcomes in cancer treatment

A large body of clinical evidence exists to suggest that tumor hypoxia negatively impacts radiotherapy. As a result, there has been longstanding active research into novel methods of improving tumor oxygenation, targeting hypoxic tumor cells, and otherwise modulating the effect hypoxia has on how tumors respond to radiation. Over time, as more has been learned about the many ways hypoxia affects tumors, our understanding of the mechanisms connecting hypoxia to radiosensitivity has become increasingly broad and complicated. This has opened up new potential avenues for interrupting hypoxia's negative effects on tumor radiosensitivity. Here, we will review what is currently known about the spectrum of influence hypoxia has over the way tumors respond to radiation. Particular focus will be placed on recent discoveries suggesting that hypoxia-inducible factor-1 (HIF-1), a transcription factor that upregulates its target genes under hypoxic conditions, plays a major role in determining tumor radiosensitivity. HIF-1 and/or its target genes may represent therapeutic targets which could be manipulated to influence hypoxia's impact on tumor radiosensitivity.

Radiation and a metalloporphyrin radioprotectant in a mouse prostate tumor model

BACKGROUND

Antioxidants have the potential to protect normal tissues against radiation-induced damage, but must not protect tumor cells during radiotherapy. The major objectives were to determine whether a metalloporphyrin antioxidant affects prostate tumor response to radiation and identify possible mechanisms of interaction.

MATERIALS AND METHODS

C57BL/6 mice with RM-9 tumor were treated with manganese (III) meso-tetrakis (1,3-diethylimidazolium-2-yl) porphyrin (MnTDE-2-ImP) and 10 gray (Gy) radiation. Tumor volume was quantified and a subset/group was evaluated for hypoxia-inducible factor-1alpha (HIF-1alpha), bone marrow-derived cell populations and cytokines.

RESULTS

The addition of MnTDE-2-ImP transiently increased tumor response compared to radiation alone. The group receiving drug plus radiation had reduced intratumoral HIF-1alpha and decreased capacity to secrete TNF-alpha, whereas production of IL-4 was increased. There were no toxicities associated with combination treatment.

CONCLUSION

The results demonstrate that MnTDE-2-ImP did not protect the RM-9 prostate tumor against radiation; instead, radiation effectiveness was modestly increased. Possible mechanisms include reduction of radiation-induced HIF-1alpha and an altered cytokine profile.

Epinephrine-induced activation of LW-mediated sickle cell adhesion and vaso-occlusion in vivo

Sickle red cell (SS RBC) adhesion is believed to contribute to the process of vaso-occlusion in sickle cell disease (SCD). We previously found that the LW RBC adhesion receptor can be activated by epinephrine to mediate SS RBC adhesion to endothelial alphavbeta3 integrin. To determine the contribution of LW activation to vaso-occlusive events in vivo, we investigated whether in vitro treatment of SS RBCs by epinephrine resulted in vaso-occlusion in intact microvasculature after RBC infusion into nude mice. Epinephrine enhanced human SS but not normal RBC adhesion to murine endothelial cells in vitro and to endothelium in vivo, promoting vaso-occlusion and RBC organ sequestration. Murine sickle RBCs also responded to epinephrine with increased adhesion to postcapillary endothelium in nude mice. Epinephrine-induced SS RBC adhesion, vaso-occlusion, and RBC organ trapping could be prevented by the beta-adrenergic receptor (beta-AR) antagonist, propranolol. Infusion of soluble recombinant LW also significantly reduced adhesion and vaso-occlusion. In addition, epinephrine-treated SS RBCs induced activation of murine leukocyte adhesion to endothelium as well. We conclude that LW activation by epinephrine via beta-AR stimulation can promote both SS RBC and leukocyte adhesion as well as vaso-occlusion, suggesting that both epinephrine and LW play potentially pathophysiological roles in SCD.

The type III TGF-beta receptor suppresses breast cancer progression

The TGF-beta signaling pathway has a complex role in regulating mammary carcinogenesis. Here we demonstrate that the type III TGF-beta receptor (TbetaRIII, or betaglycan), a ubiquitously expressed TGF-beta coreceptor, regulated breast cancer progression and metastasis. Most human breast cancers lost TbetaRIII expression, with loss of heterozygosity of the TGFBR3 gene locus correlating with decreased TbetaRIII expression. TbetaRIII expression decreased during breast cancer progression, and low TbetaRIII levels predicted decreased recurrence-free survival in breast cancer patients. Restoring TbetaRIII expression in breast cancer cells dramatically inhibited tumor invasiveness in vitro and tumor invasion, angiogenesis, and metastasis in vivo. TbetaRIII appeared to inhibit tumor invasion by undergoing ectodomain shedding and producing soluble TbetaRIII, which binds and sequesters TGF-beta to decrease TGF-beta signaling and reduce breast cancer cell invasion and tumor-induced angiogenesis. Our results indicate that loss of TbetaRIII through allelic imbalance is a frequent genetic event during human breast cancer development that increases metastatic potential.

HIF-1 and tumour radiosensitivity

Hypoxia-inducible factor-1 (HIF-1) plays important roles in regulating radiosensitivity, making it a potentially promising target for tumour radiosensitisation. Here, we discuss the rationale for, and the potential pitfalls of, combining HIF-1 blockade with radiotherapy. In doing so, we describe clinical scenarios in which HIF-1 inhibition might optimise tumour radiosensitivity.

The G12 family of heterotrimeric G proteins promotes breast cancer invasion and metastasis

Although the prognosis for patients with early-stage breast cancer has improved, the therapeutic options for patients with locally advanced and metastatic disease are limited. To improve the treatment of these patients, the molecular mechanisms underlying breast cancer invasion and metastasis must be understood. In this study, we report that signaling through the G12 family of heterotrimeric G proteins (Galpha12 and Galpha13) promotes breast cancer cell invasion. Moreover, we demonstrate that inhibition of G12 signaling reduces the metastatic dissemination of breast cancer cells in vivo. Finally, we demonstrate that the expression of Galpha12 is significantly up-regulated in the earliest stages of breast cancer, implying that amplification of G12 signaling may be an early event in breast cancer progression. Taken together, these observations identify the G12 family proteins as important regulators of breast cancer invasion and suggest that these proteins may be targeted to limit invasion- and metastasis-induced patient morbidity and mortality.

A manganese porphyrin superoxide dismutase mimetic enhances tumor radioresponsiveness

PURPOSE

To determine the effect of the superoxide dismutase mimetic Mn(III) tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)) on tumor radioresponsiveness.

METHODS AND MATERIALS

Various rodent tumor (4T1, R3230, B16) and endothelial (SVEC) cell lines were exposed to MnTE-2-PyP(5+) and assayed for viability and radiosensitivity in vitro. Next, tumors were treated with radiation and MnTE-2-PyP(5+)in vivo, and the effects on tumor growth and vascularity were monitored.

RESULTS

In vitro, MnTE-2-PyP(5+) was not significantly cytotoxic. However, at concentrations as low as 2 mumol/L it caused 100% inhibition of secretion by tumor cells of cytokines protective of irradiated endothelial cells. In vivo, combined treatment with radiation and MnTE-2-PyP(5+) achieved synergistic tumor devascularization, reducing vascular density by 78.7% within 72 h of radiotherapy (p < 0.05 vs. radiation or drug alone). Co-treatment of tumors also resulted in synergistic antitumor effects, extending tumor growth delay by 9 days (p < 0.01).

CONCLUSIONS

These studies support the conclusion that MnTE-2-PyP(5+), which has been shown to protect normal tissues from radiation injury, can also improve tumor control through augmenting radiation-induced damage to the tumor vasculature.

Cytokine profiling for prediction of symptomatic radiation-induced lung injury

PURPOSE

To analyze plasma cytokine profiles before the initiation of radiation therapy to define a cytokine phenotype that correlates with risk of developing symptomatic radiation-induced lung injury (SRILI).

METHODS AND MATERIALS

Symptomatic radiation-induced lung injury was evaluated in 55 patients (22 with SRILI and 33 without SRILI), according to modified National Cancer Institute common toxicity criteria. These plasma samples were analyzed by the multiplex suspension bead array system (Bio-Rad Laboratories; Hercules, CA), which included the following cytokines: interleukin (IL)-1beta, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12p70, IL-13, IL-17, granulocyte/macrophage colony-stimulating factor, interferon-gamma, monocyte chemotactic protein 1, macrophage inflammatory protein 1beta, tumor necrosis factor alpha, and granulocyte colony-stimulating factor.

RESULTS

Significant differences in the median values of IL-8 were observed between patients with and without SRILI. Patients who did not develop SRILI had approximately fourfold elevated levels of IL-8 as compared with patients who did subsequently develop SRILI. Significant correlations were not found for any other cytokine in this study, including transforming growth factor beta1.

CONCLUSIONS

Patients with lower levels of plasma IL-8 before radiation therapy might be at increased risk for developing SRILI. Further studies are necessary to determine whether IL-8 levels are predictive of SRILI in a prospective trial and whether this marker might be used to determine patient eligibility for dose escalation.

Pleiotropic effects of HIF-1 blockade on tumor radiosensitivity

We have previously shown that radiation increases HIF-1 activity in tumors, causing significant radioprotection of the tumor vasculature. The impact that HIF-1 activation has on overall tumor radiosensitivity, however, is unknown. We reveal here that HIF-1 plays an important role in determining tumor radioresponsiveness through regulating four distinct processes. By promoting ATP metabolism, proliferation, and p53 activation, HIF-1 has a radiosensitizing effect on tumors. Through stimulating endothelial cell survival, HIF-1 promotes tumor radioresistance. As a result, the net effect of HIF-1 blockade on tumor radioresponsiveness is highly dependent on treatment sequencing, with "radiation first" strategies being significantly more effective than the alternative. These data provide a strong rationale for pursuing sequence-specific combinations of HIF-1 blockade and conventional therapeutics.

Observation of Incipient Tumor Angiogenesis That Is Independent of Hypoxia and Hypoxia Inducible Factor-1 Activation

It is well established that hypoxia potently stimulates tumor angiogenesis by activating hypoxia inducible factor-1 (HIF-1)-induced proangiogenic factors, such as vascular endothelial growth factor. However, very little is known about the role of hypoxia in incipient angiogenesis in avascular tumors during their early stages of growth. To noninvasively investigate the functional significance of hypoxia and HIF-1 activation in incipient tumor angiogenesis, we genetically engineered HCT116 human colon carcinoma cells and 4T1 mouse mammary carcinoma cells with constitutively expressed red fluorescence protein as a tumor marker and green fluorescence protein (GFP) as a reporter for hypoxia and HIF-1 activation. The accuracy of GFP fluorescence in reporting hypoxia was confirmed by flow cytometry analysis and by immunohistochemical comparison with pimonidazole, a well-established hypoxia marker drug. Mouse dorsal skin-fold window chambers showed that incipient angiogenesis preceded a detectable level of hypoxia. The detectable levels of hypoxia were spatially and temporally related with more intensive secondary angiogenesis following the initial onset of new vessel formation. Selective killing of hypoxic cells by tirapazamine efficiently eliminated or delayed the detection of hypoxic cells, but it did not significantly delay the onset of incipient angiogenesis. These findings provide the first in vivo evidence that incipient tumor angiogenesis may not depend on hypoxia or HIF-1 activation. This is in contrast to the clear role of hypoxia in driving angiogenesis once initial tumor microvessel formation has occurred.

Hyperspectral imaging of hemoglobin saturation in tumor microvasculature and tumor hypoxia development

Tumor hypoxia has been shown to have prognostic value in clinical trials involving radiation, chemotherapy, and surgery. Tumor oxygenation studies at microvascular levels can provide understanding of oxygen transport on scales at which oxygen transfer to tissue occurs. To fully grasp the significance of blood oxygen delivery and hypoxia at microvascular levels during tumor growth and angiogenesis, the spatial and temporal relationship of the data must be preserved and mapped. Using tumors grown in window chamber models, hyperspectral imaging can provide serial spatial maps of blood oxygenation in terms of hemoglobin saturation at the microvascular level. We describe our application of hyperspectral imaging for in vivo microvascular tumor oxygen transport studies using red fluorescent protein (RFP) to identify all tumor cells, and hypoxia-driven green fluorescent protein (GFP) to identify the hypoxic fraction. 4T1 mouse mammary carcinoma cells, stably transfected with both reporter genes, are grown in dorsal skin-fold window chambers. Hyperspectral imaging is used to create image maps of hemoglobin saturation, and classify image pixels where RFP alone is present (tumor cells), or both RFP and GFP are present (hypoxic tumor cells). In this work, in vivo calibration of the imaging system is described and in vivo results are shown.

The relationship between hypoxia and angiogenesis

Recent studies have generated a large amount of data supporting the hypothesis that hypoxia drives tumor angiogenesis. The relationship between the two is often considered a matter of supply and demand: ineffectively-vascularized tumor tissue becomes hypoxic, stimulating neoangiogenesis to improve the influx of oxygen, thereby diminishing the angiogenic drive. Although this paradigm is logically pleasing, much of what is known about tumor biology argues against such a straightforward relationship. In fact, some preclinical data convincingly shows that tumor hypoxia and angiogenesis do not always go hand in hand. It is important to begin to explore means of reconciling these discrepancies. Although poor oxygenation is a strong stimulus for tumor angiogenesis, (1) the pathogenesis of tumor hypoxia is much more complicated than the supply-demand paradigm lets on and (2) hypoxia is not necessarily sufficient or necessary for neovascularization to occur. These subtleties may help to explain why so much data disagrees with the current hypoxia-angiogenesis model and may begin to build a better understanding of the role hypoxia plays in tumor vascularization. This article will review what is known about hypoxia and angiogenesis in nononcological processes and will apply these lessons to tumor biology to more deeply describe their relationship.

Raising the bar: how HIF-1 helps determine tumor radiosensitivity

Through a poorly understood mechanism, tumors respond to radiation by secreting cytokines which inhibit endothelial cell apoptosis, thereby limiting treatment response by minimizing vessel damage. We have recently discovered that this pathway is governed by a major angiogenesis regulator, hypoxia-inducible factor-1 (HIF-1). We uncovered dual mechanisms initiated by radiation that both simultaneously lead to HIF-1 activation: (1) reoxygenation-induced stabilization of the HIF-1 dimer through free radical intermediates, and (2) reoxygenation-mediated depolymerization of hypoxia-induced translational suppressors known as stress granules. These findings have implications both for understanding the basic science of hypoxic signaling in tumors, and for discovering novel methods of enhancing conventional anti-tumor therapeutics in the clinic. In this article, we will highlight the apparent importance of free radical species in protecting tumor vasculature, stress granules in regulating hypoxic gene expression, and HIF-1 in regulating tumor sensitivity to ionizing radiation. The potential therapeutic utility of these findings will also be explored, with emphasis placed on putative targets in these pathways which may enhance tumor radiotherapy.

Radiation activates HIF-1 to regulate vascular radiosensitivity in tumors: role of reoxygenation, free radicals, and stress granules

Through a poorly understood mechanism, tumors respond to radiation by secreting cytokines capable of inhibiting apoptosis in endothelial cells, thereby diminishing treatment response by minimizing vascular damage. We reveal here that this pathway is governed by a major angiogenesis regulator, HIF-1. Following radiotherapy, tumor reoxygenation leads to: (1) nuclear accumulation of HIF-1 in response to reactive oxygen, and (2) enhanced translation of HIF-1-regulated transcripts secondary to stress granule depolymerization. The resulting increase in HIF-1-regulated cytokines enhances endothelial cell radioresistance. Inhibiting postradiation HIF-1 activation significantly increases tumor radiosensitivity as a result of enhanced vascular destruction. These data describe novel pathways contributing significantly to our understanding of HIF-1 regulation which may be major determinants of tumor radiosensitivity, potentially having high clinical relevance.

Mutagenicity of 5-aza-2'-deoxycytidine is mediated by the mammalian DNA methyltransferase

The cytosine analog 5-aza-2'-deoxycytidine has been used clinically to reactivate genes silenced by DNA methylation. In particular, patients with beta-thalassemia show fetal globin expression after administration of this hypomethylating drug. In addition, silencing of tumor suppressor gene expression by aberrant DNA methylation in tumor cells may potentially be reversed by a similar regimen. Consistent with its function in maintaining tumor suppressor gene expression, 5-aza-2'-deoxycytidine significantly reduces intestinal tumor multiplicity in the predisposed Min mouse strain. Despite its utility as an anti-cancer agent, the drug is highly mutagenic by an unknown mechanism. To gain insight into how 5-aza-2'-deoxycytidine induces mutations in vivo, we examined the mutational spectrum in an Escherichia coli lac I transgene in colonic DNA from 5-aza-2'-deoxycytidine-treated mice. Mutations induced by 5-aza-2'-deoxycytidine were predominantly at CpG dinucleotides, which implicates DNA methyltransferase in the mutagenic mechanism. C:G-->G:C transversions were the predominant class of mutations observed. We suggest a model for how the mammalian DNA methyltransferase may be involved in facilitating these mutations. The observation that 5-aza-2'-deoxycytidine-induced mutations are mediated by the enzyme suggests that novel inhibitors of DNA methyltransferase, which can inactivate the enzyme before its interaction with DNA, are needed for chemoprevention or long term therapy.