Financial Toxicity in Patients with Brain and Spine Metastases.

OBJECTIVE: Financial toxicity associated with cancer treatment has a deleterious impact on patient outcomes but has not been well characterized among patients with metastatic cancers. We characterize the extent of financial toxicity among this population and identify factors associated with financial toxicity. METHODS: We prospectively surveyed adult patients with brain and spine metastases who received radiosurgery at a large academic medical center between January 2018 and December 2019. Financial toxicity was measured with the Personal Financial Wellness (PFW) scale. RESULTS: In total, 93 patients were included, with a median survival of 17.7 months. Most patients had private insurance (47%) or Medicare with supplementary insurance (42%), whereas 11% of patients were uninsured or insured by Medicaid/Medicare/Veterans Affairs. Of patients, 60% were primary income earners, of whom 52% had dependents. The median PFW score was 7.0 (interquartile range, 5.1-9.1), with financial toxicity reported in 23 patients (25%). After adjusting for age and education level, private insurance (odds ratio [OR], 0.28; P = 0.080) was associated with a lower likelihood of financial toxicity. Having ≥1 emergency department visit (OR, 3.87; P = 0.024) and a cancer-related change in employment status (OR, 3.63; P = 0.036) were associated with greater likelihood of reporting financial toxicity. CONCLUSIONS: Most patients with cancer with brain and spine metastases with a poor prognosis treated at a tertiary center are primary income earners and experience financial toxicity. Further studies are warranted to assess the longitudinal impact of financial toxicity in patients with metastatic cancer, particularly those with ≥1 emergency department visit and a cancer-related change in employment status.

Predicting per-lesion local recurrence in locally advanced non-small cell lung cancer following definitive radiation therapy using pre- and mid-treatment metabolic tumor volume.

BACKGROUND: We evaluated whether pre- and mid-treatment metabolic tumor volume (MTV) predicts per lesion local recurrence (LR) in patients treated with definitive radiation therapy (RT, dose≥60 Gy) for locally advanced non-small cell lung cancer (NSCLC). METHODS: We retrospectively reviewed records of patients with stage III NSCLC treated from 2006 to 2018 with pre- and mid-RT PET-CT. We measured the MTV of treated lesions on the pre-RT (MTVpre) and mid-RT (MTVmid) PET-CT. LR was defined per lesion as recurrence within the planning target volume. Receiver operating characteristic (ROC) curves, cumulative incidence rates, and uni- and multivariable (MVA) competing risk regressions were used to evaluate the association between MTV and LR. RESULTS: We identified 111 patients with 387 lesions (112 lung tumors and 275 lymph nodes). Median age was 68 years, 69.4% were male, 46.8% had adenocarcinoma, 39.6% had squamous cell carcinoma, and 95.5% received concurrent chemotherapy. Median follow-up was 38.7 months. 3-year overall survival was 42.3%. 3-year cumulative incidence of LR was 26.8% per patient and 11.9% per lesion. Both MTVpre and MTVmid were predictive of LR by ROC (AUC = 0.71 and 0.76, respectively) and were significantly associated with LR on MVA (P = 0.004 and P = 7.1e-5, respectively). Among lesions at lower risk of LR based on MTVpre, higher MTVmid was associated with LR (P = 0.001). CONCLUSION: Per-lesion, larger MTVpre and MTVmid predicted for increased risk of LR. MTVmid was more highly predictive of LR than MTVpre and if validated may allow for further discrimination of high-risk lesions at mid-RT informing dose painting strategies.

Stage I-II nodular lymphocyte-predominant Hodgkin lymphoma: a multi-institutional study of adult patients by ILROG.

Nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) is an uncommon histologic variant, and the optimal treatment of stage I-II NLPHL is undefined. We conducted a multicenter retrospective study including patients ≥16 years of age with stage I-II NLPHL diagnosed from 1995 through 2018 who underwent all forms of management, including radiotherapy (RT), combined modality therapy (CMT; RT+chemotherapy [CT]), CT, observation after excision, rituximab and RT, and single-agent rituximab. End points were progression-free survival (PFS), freedom from transformation, and overall survival (OS) without statistical comparison between management groups. We identified 559 patients with median age of 39 years: 72.3% were men, and 54.9% had stage I disease. Median follow-up was 5.5 years (interquartile range, 3.1-10.1). Five-year PFS and OS in the entire cohort were 87.1% and 98.3%, respectively. Primary management was RT alone (n = 257; 46.0%), CMT (n = 184; 32.9%), CT alone (n = 47; 8.4%), observation (n = 37; 6.6%), rituximab and RT (n = 19; 3.4%), and rituximab alone (n = 15; 2.7%). The 5-year PFS rates were 91.1% after RT, 90.5% after CMT, 77.8% after CT, 73.5% after observation, 80.8% after rituximab and RT, and 38.5% after rituximab alone. In the RT cohort, but not the CMT cohort, variant immunoarchitectural pattern and number of sites >2 were associated with worse PFS (P < .05). Overall, 21 patients (3.8%) developed large-cell transformation, with a significantly higher transformation rate in those with variant immunoarchitectural pattern (P = .049) and number of involved sites >2 (P = .0006). OS for patients with stage I-II NLPHL was excellent after all treatments.

Use of Preoperative Radiation Therapy in Early-stage and Locally Advanced Breast Cancer.

Purpose There is growing interest in delivering radiation preoperatively (preopRT) rather than postoperatively (postopRT) for breast cancer. Using the National Cancer Database, we evaluated the use and outcomes of preopRT in breast cancer. Methods We identified adult females diagnosed with non-metastatic breast cancer treated with definitive surgery and radiation between 2004 and 2014. Logistic regression models evaluated factors associated with use of preopRT in early-stage (clinical T1-3/N0-1) and locally advanced (clinical T4/N2-3) disease. Rates of breast-conserving surgery, breast reconstruction, positive surgical margins, and 30-day surgical readmissions were compared between patients receiving preopRT and postopRT. Results Of 373,595 patients who met our inclusion criteria, 1,245 (0.3%) patients received preopRT. Patients receiving preopRT were more likely to be of lower socioeconomic status and have tumors with higher T stage. Younger age and N1 (vs N0) disease predicted for use of preopRT in early-stage disease, while older age and N0 disease predicted for use of preopRT in the locally advanced setting. PreopRT patients were less likely to undergo breast-conserving surgery and more likely to have positive surgical margins. Rates of unplanned readmissions within 30 days of surgery were similar among patients treated with preopRT and postopRT. Conclusions PreopRT is a new treatment strategy for patients with breast cancer with different clinical and sociodemographic drivers of its use in the early-stage and locally advanced settings. We await the results of clinical trials studying the efficacy of this approach.

Stereotactic Radiosurgery for Pediatric and Adult Intracranial and Spinal Ependymomas.

OBJECTIVE/BACKGROUND: We report efficacy and toxicity outcomes with stereotactic radiosurgery (SRS) for intracranial and spinal ependymoma. METHODS: We analyzed adult and pediatric patients with newly diagnosed or recurrent intracranial or spinal ependymoma lesions treated with SRS at our institution. Following SRS, local failure (LF) was defined as failure within or adjacent to the SRS target volume, while distant failure (DF) was defined as failure outside of the SRS target volume. Time to LF and DF was analyzed using competing risk analysis with death as a competing risk.Overall survival (OS) was calculated from the date of first SRS to the date of death or censored at the date of last follow-up using the Kaplan-Meier method. RESULTS: Twenty-one patients underwent SRS to 40 intracranial (n = 30) or spinal (n = 10) ependymoma lesions between 2007 and 2018, most commonly with 18 or 20 Gy in 1 fraction. Median follow-up for all patients after first SRS treatment was 54 months (range 2-157). The 1-year, 2-year, and 5-year rates of survival among patients with initial intracranial ependymoma were 86, 74, and 52%, respectively. The 2-year cumulative incidences of LF and DF after SRS among intracranial ependymoma patients were 25% (95% CI 11-43) and 42% (95% CI 22-60), respectively. No spinal ependymoma patient experienced LF, DF, or death within 2 years of SRS. Three patients had adverse radiation effects. CONCLUSIONS: SRS is a viable treatment option for intracranial and spinal ependymoma with excellent local control and acceptable toxicity.

Microsatellite Instability and Adjuvant Chemotherapy in Stage II Colon Cancer.

BACKGROUND: Randomized control trials and population-based studies do not demonstrate a definitive benefit for adjuvant chemotherapy (ACT) in stage II colon cancer (CC). Tumor sidedness and microsatellite instability (MSI) status may predict response to ACT, but previous studies have limited microsatellite data. We assessed the efficacy of ACT and possible interaction with MSI status and tumor sidedness in patients with resected stage II CC diagnosed between 2010 and 2013 using the National Cancer Database. MATERIALS AND METHODS: Overall survival was evaluated with the Kaplan-Meier method and multivariate and propensity score matched Cox proportional hazards models. The interaction between receipt of ACT, MSI status, and tumor sidedness was evaluated. The efficacy of ACT was assessed in patient subgroups by MSI status and tumor sidedness. RESULTS: Among 6964 stage II CC patients with known MSI status, 1497 (21.5%) received ACT, 843 had MSI tumors, and 6121 had microsatellite stable (MSS) tumors. In multivariate and propensity score matched analyses, ACT was associated with improved survival after adjusting for factors including high-risk features, MSI status, and tumor sidedness (multivariate hazard ratio, 0.52; P<0.001). There was no interaction between receipt of ACT and MSI status (P=0.25). Patients with MSS tumors benefitted from ACT (multivariate hazard ratio, 0.47; P<0.001), even without other high-risk features. Patients with MSI tumors did not (P=0.671). ACT was associated with improved survival regardless of tumor sidedness. CONCLUSIONS: MSS alone may warrant ACT in stage II CC while patients with MSI tumors may not derive significant benefit from ACT.

Dynamic control of hippocampal spatial coding resolution by local visual cues.

The ability to flexibly navigate an environment relies on a hippocampal-dependent cognitive map. External space can be internally mapped at different spatial resolutions. However, whether hippocampal spatial coding resolution can rapidly adapt to local features of an environment remains unclear. To explore this possibility, we recorded the firing of hippocampal neurons in mice navigating virtual reality environments, embedding or not local visual cues (virtual 3D objects) in specific locations. Virtual objects enhanced spatial coding resolution in their vicinity with a higher proportion of place cells, smaller place fields, increased spatial selectivity and stability. This effect was highly dynamic upon objects manipulations. Objects also improved temporal coding resolution through improved theta phase precession and theta timescale spike coordination. We propose that the fast adaptation of hippocampal spatial coding resolution to local features of an environment could be relevant for large-scale navigation.

Adverse Radiation Effect and Disease Control in Patients Undergoing Stereotactic Radiosurgery and Immune Checkpoint Inhibitor Therapy for Brain Metastases.

BACKGROUND: Immune checkpoint inhibitors (ICIs) and stereotactic radiosurgery (SRS) are increasingly used together to manage brain metastases (BMs). We assessed adverse radiation effect, disease control, and overall survival in patients with BMs who received SRS with anticytotoxic T-lymphocyte-associated protein 4 and/or anti- programmed cell death protein receptor/ligand therapies. METHODS: We retrospectively reviewed the records of patients with intact or resected BMs treated with SRS and ICIs within 5 months of SRS between 2010 and 2018. Patients were defined as receiving concurrent SRS and ICI if a dose of ICI was given within 4 weeks of SRS. Local failure, distant intracranial failure, extracranial failure, and adverse radiation effect were assessed using cumulative incidence rates and competing risk regressions with death as a competing risk. Overall survival was assessed using the Kaplan-Meier method and Cox proportional hazards models. RESULTS: A total of 97 patients with 580 BMs were included in our analysis. Competing risk analyses showed that concurrent SRS-ICI therapy is associated with higher rates of adverse radiation effect (6.4% vs. 2.0% at 1 year; multivariable hazard ratio [HR], 4.47; 95% confidence interval [CI], 1.57-12.73; P = 0.005), lower rates of extracranial failure (69.7% vs. 80.8% at 1 year; multivariable HR, 0.60; 95% CI, 0.42-0.87; P = 0.007), and better overall survival (48.6% vs. 25.4% at 1 year; multivariable HR, 0.57; 95% CI, 0.33-0.99; P = 0.044) compared with nonconcurrent therapy. SRS-ICI timing was not associated with local failure or distant intracranial failure. CONCLUSIONS: Concurrent SRS-ICI therapy has a tolerable adverse event profile and may improve extracranial disease control and overall survival, supporting concurrent use in the management of BMs.

Less Than Whole Uterus Irradiation for Locally Advanced Cervical Cancer Maintains Locoregional Control and Decreases Radiation Dose to Bowel.

OBJECTIVE: This study aimed to evaluate whether our institutional standard of less-than-whole-uterus irradiation affects locoregional control in patients with locally advanced cervical cancer. METHODS AND MATERIALS: We retrospectively reviewed 53 patients with stage IB to IVB cervical carcinoma who were treated with image guided intensity modulated radiation therapy and brachytherapy. The entire uterus was not included in the clinical target volume, as per our institutional standard. Dosimetric parameters were obtained, including positron emission tomography gross tumor volume (GTV), uterus volume excluding GTV, proportion of uterus included in the planning target volume (PTV; percentage), volume of overlap between uterus and prescription dose (cm3), minimum and mean dose to the uterus, and bowel V40 and D200cc. Local, regional, and distant failure and death were recorded. RESULTS: The median proportion of the uterus included in the PTV was 66%. With a median follow-up of 44 months, no patient experienced isolated local recurrence, and 2-year locoregional failure was 10.9%. Positron emission tomography GTV correlated significantly with increased chance of any failure (P = .049; 95% confidence interval, 1.000-1.018). Compared with patients who had ≥90% of the uterus included in the PTV (n = 12), patients who had <90% (n = 41) of the uterus included in the PTV had significantly lower bowel V40 (P = .049) and D200cc (P = .006). CONCLUSIONS: Less-than-whole-uterus irradiation for locally advanced cervical cancer does not compromise locoregional control and reduces bowel V40 and D200cc. Further investigation is required to evaluate whether this reduction in bowel dose translates to a clinically significant reduction in bowel toxicity and whether modifications should be made to the recommended definitive cervix intensity modulated radiation therapy volumes.

Outcomes and Characteristics of Patients Treated with Emergent Palliative Radiation Therapy.

PURPOSE: Emergent palliative radiation therapy (PRT) of symptomatic metastases can significantly increase the quality of life of patients with cancer. In some contexts, this treatment may be underused, but in others PRT may represent an excessively aggressive intervention. The characterization of the current use of emergent PRT is warranted for optimized value and patient-centered care. METHODS AND MATERIALS: This study is a cross-sectional retrospective analysis of all emergent PRT courses at a single academic tertiary institution across 1 year. RESULTS: A total of 214 patients received a total of 238 treatment courses. The most common indications were bone (39%) and brain (14%) metastases. Compared with outpatients, inpatients had lower mean survival rates (2 months vs 6 months; P < .001), higher rates of stopping treatment early (19.1% vs 9.0%; P = .034), and greater involvement of palliative care (44.8% vs 24.1%; P < .001), but the same mean planned fractions (9.10 vs 9.40 fractions; P = .669). In a multiple predictor survival analysis, palliative care involvement (P = .025), male sex (P = .001), ending treatment early (P = .011), and having 1 of 3 serious indications (airway compromise, leptomeningeal disease, and superior/inferior vena cava involvement; P = .007) were significantly associated with worse overall survival. CONCLUSIONS: Survival is particularly poor in patients who receive emergent PRT, and patient characteristics such as functional status and indication should be considered when determining fractionation schedule and dosing. A multi-institutional study of practice patterns and outcomes is warranted.

Renal ultrasound abnormalities in children with syndromic and non-syndromic microtia.

OBJECTIVE: Renal abnormalities are commonly considered in the work up of pediatric patients with external ear malformations. However, there is little consensus regarding an appropriate renal screening protocol for patients with microtia. We sought to characterize renal abnormalities detected on ultrasonography in pediatric patients with microtia. METHODS: We conducted a retrospective cohort study of pediatric patients diagnosed with microtia who underwent renal ultrasound from 1991 to 2014 at a single tertiary academic institution. Renal ultrasound reports and medical records were reviewed to assess for renal abnormalities and to determine whether patients required specialist follow-up or interventions. Audiograms and otolaryngology notes were used to determine patterns of hearing loss. The following additional information was recorded from the electronic medical records: patient sex, microtia grade (I-IV), microtia laterality, and known associated syndromes. Characteristics were compared between those who did and did not have renal ultrasound findings using Fisher's exact test. Univariate logistic regression analysis was performed to determine factors associated with renal ultrasound findings. RESULTS: The majority of patients in this cohort were syndromic (n = 51, 64%) with grade III microtia (n = 46, 58%) and conductive hearing loss (n = 58, 72%). Syndromic children with microtia demonstrated a higher crude rate of renal ultrasound abnormalities (22%) than children with isolated microtia (7%). Of these patients, 69% required specialist follow-up. Univariate logistic regression analysis did not identify predictors that were significantly associated with renal ultrasound findings. CONCLUSION: Fairly high rates of abnormalities in syndromic and non-syndromic patients may warrant screening renal ultrasound in all patients with microtia, especially given the high percentage of findings requiring renal follow-up. A prospective study to formally evaluate screening efficacy is needed.

Impact of Insurance Coverage on Outcomes in Primary Breast Sarcoma.

Private insurance is associated with better outcomes in multiple common cancers. We hypothesized that insurance status would significantly impact outcomes in primary breast sarcoma (PBS) due to the additional challenges of diagnosing and coordinating specialized care for a rare cancer. Using the National Cancer Database, we identified adult females diagnosed with PBS between 2004 and 2013. The influence of insurance status on overall survival (OS) was evaluated using the Kaplan-Meier estimator with log-rank tests and Cox proportional hazard models. Among a cohort of 607 patients, 67 (11.0%) had Medicaid, 217 (35.7%) had Medicare, and 323 (53.2%) had private insurance. Compared to privately insured patients, Medicaid patients were more likely to present with larger tumors and have their first surgical procedure further after diagnosis. Treatment was similar between patients with comparable disease stage. In multivariate analysis, Medicaid (hazard ratio (HR), 2.47; 95% confidence interval (CI), 1.62-3.77; p < 0.001) and Medicare (HR, 1.68; 95% CI, 1.10-2.57; p=0.017) were independently associated with worse OS. Medicaid insurance coverage negatively impacted survival compared to private insurance more in breast sarcoma than in breast carcinoma (interaction p < 0.001). In conclusion, patients with Medicaid insurance present with later stage disease and have worse overall survival than privately insured patients with PBS. Worse outcomes for Medicaid patients are exacerbated in this rare cancer.

Superenhancer Analysis Defines Novel Epigenomic Subtypes of Non-APL AML, Including an RARα Dependency Targetable by SY-1425, a Potent and Selective RARα Agonist.

We characterized the enhancer landscape of 66 patients with acute myeloid leukemia (AML), identifying 6 novel subgroups and their associated regulatory loci. These subgroups are defined by their superenhancer (SE) maps, orthogonal to somatic mutations, and are associated with distinct leukemic cell states. Examination of transcriptional drivers for these epigenomic subtypes uncovers a subset of patients with a particularly strong SE at the retinoic acid receptor alpha (RARA) gene locus. The presence of a RARA SE and concomitant high levels of RARA mRNA predisposes cell lines and ex vivo models to exquisite sensitivity to a selective agonist of RARα, SY-1425 (tamibarotene). Furthermore, only AML patient-derived xenograft (PDX) models with high RARA mRNA were found to respond to SY-1425. Mechanistically, we show that the response to SY-1425 in RARA-high AML cells is similar to that of acute promyelocytic leukemia treated with retinoids, characterized by the induction of known retinoic acid response genes, increased differentiation, and loss of proliferation.Significance: We use the SE landscape of primary human AML to elucidate transcriptional circuitry and identify novel cancer vulnerabilities. A subset of patients were found to have an SE at RARA, which is predictive for response to SY-1425, a potent and selective RARα agonist, in preclinical models, forming the rationale for its clinical investigation in biomarker-selected patients. Cancer Discov; 7(10); 1136-53. ©2017 AACR.See related commentary by Wang and Aifantis, p. 1065.This article is highlighted in the In This Issue feature, p. 1047.

Lineage-specific and single-cell chromatin accessibility charts human hematopoiesis and leukemia evolution.

We define the chromatin accessibility and transcriptional landscapes in 13 human primary blood cell types that span the hematopoietic hierarchy. Exploiting the finding that the enhancer landscape better reflects cell identity than mRNA levels, we enable 'enhancer cytometry' for enumeration of pure cell types from complex populations. We identify regulators governing hematopoietic differentiation and further show the lineage ontogeny of genetic elements linked to diverse human diseases. In acute myeloid leukemia (AML), chromatin accessibility uncovers unique regulatory evolution in cancer cells with a progressively increasing mutation burden. Single AML cells exhibit distinctive mixed regulome profiles corresponding to disparate developmental stages. A method to account for this regulatory heterogeneity identified cancer-specific deviations and implicated HOX factors as key regulators of preleukemic hematopoietic stem cell characteristics. Thus, regulome dynamics can provide diverse insights into hematopoietic development and disease.

Leukemia-Associated Cohesin Mutants Dominantly Enforce Stem Cell Programs and Impair Human Hematopoietic Progenitor Differentiation.

Recurrent mutations in cohesin complex proteins have been identified in pre-leukemic hematopoietic stem cells and during the early development of acute myeloid leukemia and other myeloid malignancies. Although cohesins are involved in chromosome separation and DNA damage repair, cohesin complex functions during hematopoiesis and leukemic development are unclear. Here, we show that mutant cohesin proteins block differentiation of human hematopoietic stem and progenitor cells (HSPCs) in vitro and in vivo and enforce stem cell programs. These effects are restricted to immature HSPC populations, where cohesin mutants show increased chromatin accessibility and likelihood of transcription factor binding site occupancy by HSPC regulators including ERG, GATA2, and RUNX1, as measured by ATAC-seq and ChIP-seq. Epistasis experiments show that silencing these transcription factors rescues the differentiation block caused by cohesin mutants. Together, these results show that mutant cohesins impair HSPC differentiation by controlling chromatin accessibility and transcription factor activity, possibly contributing to leukemic disease.

New and distinct hippocampal place codes are generated in a new environment during septal inactivation.

The hippocampus generates distinct neural codes to disambiguate similar experiences, a process thought to underlie episodic memory function. Entorhinal grid cells provide a prominent spatial signal to hippocampus, and changes in their firing pattern could thus generate a distinct spatial code in each context. We examined whether we would preclude the emergence of new spatial representations in a novel environment during muscimol inactivation of the medial septal area, a manipulation known to disrupt theta oscillations and grid cell firing. We found that new, highly distinct configurations of place fields emerged immediately and remained stable during the septal inactivation. The new place code persisted when theta oscillations had recovered. Theta rhythmicity and feedforward input from grid cell networks were thus not required to generate new spatial representations in the hippocampus.

Parallel and convergent processing in grid cell, head-direction cell, boundary cell, and place cell networks.

The brain is able to construct internal representations that correspond to external spatial coordinates. Such brain maps of the external spatial topography may support a number of cognitive functions, including navigation and memory. The neuronal building block of brain maps are place cells, which are found throughout the hippocampus of rodents and, in a lower proportion, primates. Place cells typically fire in one or few restricted areas of space, and each area where a cell fires can range, along the dorsoventral axis of the hippocampus, from 30 cm to at least several meters. The sensory processing streams that give rise to hippocampal place cells are not fully understood, but substantial progress has been made in characterizing the entorhinal cortex, which is the gateway between neocortical areas and the hippocampus. Entorhinal neurons have diverse spatial firing characteristics, and the different entorhinal cell types converge in the hippocampus to give rise to a single, spatially modulated cell type-the place cell. We therefore suggest that parallel information processing in different classes of cells-as is typically observed at lower levels of sensory processing-continues up into higher level association cortices, including those that provide the inputs to hippocampus. WIREs Cogn Sci 2014, 5:207-219. doi: 10.1002/wcs.1272 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

Assessment of the genotoxic potential of azidothymidine in the comet, micronucleus, and Pig-a assay.

The genotoxic potential of azidothymidine (Zidovudine, AZT), chosen as a model compound for nucleotide analogs, was comprehensively assessed in vivo for gene mutation, clastogenicity, and DNA breakage endpoints. Male Wistar rats were treated by oral gavage over 7 days with AZT at dose levels of 2×0 (control), 2×250, 2×500, and 2×1000mg/kg/day with a final single dose given on day 8. DNA damage was then evaluated with the comet assay in liver, stomach, and peripheral blood and with the micronucleus test in bone marrow and peripheral blood (by flow cytometry) in the same animals. After a treatment-free period of upto 42 days, the Pig-a gene mutation assay was performed in peripheral blood of the high-dose animals. In the comet assay as well as the micronucleus test, AZT caused a considerable dose-dependent increase in DNA damage in all tissues evaluated and was highly cytotoxic to bone marrow and peripheral blood cells. These data are well in line with published results. Surprisingly, AZT did not significantly increase the number of Pig-a mutant cells. We speculate that two factors likely contributed to this negative result: a predominance of large deletions caused by AZT, and the relatively low statistical power of the first-generation scoring method used for this study.

Genotoxicity profile of azidothymidine in vitro.

Azidothymidine (Zidovudine, AZT) is part of the standard care of treatment for acquired immunodeficiency syndrome since many years. A great number of studies on the genotoxic potential of AZT have been published, but no comprehensive hypothesis yet explains all observations. We investigated a multitude of genotoxic endpoints, both in vitro and in vivo, with the goal to complete the picture. The mutagenic potential of AZT in bacteria was found to be restricted to strains with an "ochre" target sequence and could be abrogated both by thymidine supplementation and rat liver S9 mix. Single-strand breaks in mammalian cells were detected in the comet assay after short-term treatment (3h) with AZT, which did not induce micronuclei. The latter were mainly seen after prolonged exposure (24 and 48h) and are probably not directly related to AZT incorporation into DNA. Our data demonstrate that short-term exposure to low AZT concentrations does not induce biologically relevant micronucleation. Only treatment with high concentrations of AZT for prolonged time periods manifests in substantial micronucleus induction. Furthermore, we found that high concentrations of thymidine have no effect in the comet assay but increase micronucleus frequency in a manner very similar to AZT. These results lead us to the following hypothesis: AZT is triphosphorylated and then incorporated into DNA strands, leading to mutations and cytotoxicity. Cellular attempts to repair these DNA lesions as well as stalled replication forks due to chain termination are detectable with the comet assay. Increased micronucleus frequency is likely related to nucleotide pool imbalance.