Robotic versus vaginal radical trachelectomy for reproductive-aged patients with early-stage cervical carcinoma: A multi-center cohort study.

BACKGROUND: A randomized non-inferiority trial showed worse survival in women with early-stage cervical cancer treated with radical hysterectomy by minimally invasive approach compared to laparotomy; the impact of surgical approach on survival following radical trachelectomy is unknown. OBJECTIVE: To examine oncologic outcomes in women with early-stage cervical cancer who underwent robotic or vaginal radical trachelectomy at Canadian cancer centers with the highest volumes of radical trachelectomy procedures. STUDY DESIGN: Retrospective multi-centre cohort analysis which includes patients who had surgery between 2006 and 2019. Women with International FIGO 2009 stage IA-IB cervical cancer who underwent radical trachelectomy and lymph node assessment were grouped by surgical approach (vaginal versus robotic surgery). RESULTS: A total of 197 patients were included from 4 regional referral centres. 56 women underwent robotic radical trachelectomy and 141 underwent vaginal radical trachelectomy. All patients had lymph node assessment by a minimally invasive technique. Median age was 32 years, median tumor size was 12 mm, and median depth of invasion was 5 mm. Recurrence-free survival was 97% in both groups at a median follow-up of 57 months. On multivariable analysis, after adjusting for previously chosen confounders (high risk pathologic criteria, tumor size, and LVSI) there was no statistically significant difference in PFS between the 2 groups (HR 2.1, 95%CI 0.3-7.1, p = 0.5). Tumor size larger than 2 cm (HR 9.4, 95%CI 2.8-26, p = 0.003) was the only variable predictive of recurrence. CONCLUSION: Survival outcomes were excellent in both cohorts of patients undergoing robotic vs. vaginal radical trachelectomy. The surgical approach was not significantly associated with risk of recurrence after adjusting for clinically important confounders.

Surgical margin status in relation to surgical approach in the management of early-stage cervical Cancer: A Canadian cervical Cancer collaborative (4C) study.

OBJECTIVE: Surgical margin status in women undergoing surgery for early-stage cervical cancer is an important prognostic factor. We sought to determine whether close (<3 mm) and positive surgical margins are associated with surgical approach and survival. METHODS: This is a national retrospective cohort study of cervical cancer patients treated with radical hysterectomy. Patients with stage IA1/LVSI-Ib2(FIGO 2018) with lesions up to 4 cm at 11 Canadian institutions from 2007 to 2019 were included. Surgical approach included robotic/laparoscopic (LRH), abdominal (ARH) or combined laparoscopic-assisted vaginal/vaginal (LVRH) radical hysterectomy. Recurrence free survival(RFS) and overall survival (OS) were estimated using Kaplan-Meier analysis. Chi-square and log-rank tests were used to compare groups. RESULTS: 956 patients met inclusion criteria. Surgical margins were as follows: negative (87.0%), positive (0.4%) or close <3 mm (6.8%), missing (5.8%). Most patients had squamous histology (46.9%); 34.6% had adenocarcinomas and 11.3% adenosquamous. Most were stage IB (75.1%) and 24.9% were IA. Mode of surgery included: LRH(51.8%), ARH (39.2%), LVRH (8.9%). Predictive factors for close/positive margins included stage, tumour diameter, vaginal involvement and parametrial extension. Surgical approach was not associated with margin status (p = 0.27). Close/positive margins were associated with a higher risk of death on univariate analysis (HR = non calculable for positive and HR = 1.83 for close margins, p = 0.017), but not significant for OS when adjusted for stage, histology, surgical approach and adjuvant treatment. There were 7 recurrences in patients with close margins (10.3%, p = 0.25). 71.5% with positive/close margins received adjuvant treatment. In addition, MIS was associated with a higher risk of death (OR = 2.39, p = 0.029). CONCLUSION: Surgical approach was not associated to close or positive margins. Close surgical margins were associated with a higher risk of death. MIS was associated with worse survival, suggesting that margin status may not be the driver of worse survival in these cases.

Comparison of outcomes between abdominal, minimally invasive and combined vaginal-laparoscopic hysterectomy in patients with stage IAI/IA2 cervical cancer: 4C (Canadian Cervical Cancer Collaborative) study.

OBJECTIVE: Although minimally invasive hysterectomy (MIS-H) has been associated with worse survival compared to abdominal hysterectomy (AH) for cervical cancer, only 8% of patients in the LACC trial had microinvasive disease (Stage IA1/IA2). We sought to determine differences in outcome among patients undergoing MIS-H, AH or combined vaginal-laparoscopic hysterectomy (CVLH) for microinvasive cervical cancer. METHODS: A retrospective cohort study of all patients undergoing hysterectomy (radical and non radical) for FIGO 2018, microinvasive cervical cancer across 10 Canadian centers between 2007 and 2019 was performed. Recurrence free survival (RFS) was estimated using Kaplan Meier Survival analysis. Chi-square and log-rank tests were used to compare outcomes. RESULTS: 423 patients with microinvasive cervical cancer were included; 259 (61.2%) Stage IA1 (22/8.5% with LVSI) and 164(38.8%) IA2. The median age was 44 years (range 24-81). The most frequent histology was squamous (59.4%). Surgical approach was: 50.1% MIS-H (robotic or laparoscopic), 35.0% AH and 14.9% CVLH. Overall, 70.9% underwent radical hysterectomy and 76.5% had pelvic lymph node assessment. There were 16 recurrences (MIS-H:4, AH:9, CVLH: 3). No significant difference in 5-year RFS was found (96.7% MIS-H, 93.7% AH, 90.0% CVLH, p = 0.34). In a sub-analysis of patients with IA1 LVSI+/IA2(n = 186), survival results were similar. Further, there was no significant difference in peri-operative complications (p = 0.19). Patients undergoing MIS-H had a shorter median length of stay(0 days vs 3 (AH) vs. 1.5 (CVLH), p < 0.001), but had more ER visits (16.0% vs 3.6% (AH), 3.5% (CVLH), p = 0.036). CONCLUSION: In this cohort, including only patients with microinvasive cervical cancer, no difference in recurrence was found by surgical approach. This may be due to the low rate of recurrence making differences hard to detect or due to a true lack of difference. Hence, this patient population may benefit from MIS without compromising oncologic outcomes.

Monocyte- and Neutrophil-Derived CXCL10 Impairs Efficient Control of Blood-Stage Malaria Infection and Promotes Severe Disease.

CXCL10, or IFN-γ-inducible protein 10, is a biomarker associated with increased risk for Plasmodium falciparum-mediated cerebral malaria (CM). Consistent with this, we have previously shown that CXCL10 neutralization or genetic deletion alleviates brain intravascular inflammation and protects Plasmodium berghei ANKA-infected mice from CM. In addition to organ-specific effects, the absence of CXCL10 during infection was also found to reduce parasite biomass. To identify the cellular sources of CXCL10 responsible for these processes, we irradiated and reconstituted wild-type (WT) and CXCL10(-/-) mice with bone marrow from either WT or CXCL10(-/-) mice. Similar to CXCL10(-/-) mice, chimeras unable to express CXCL10 in hematopoietic-derived cells controlled infection more efficiently than WT controls. In contrast, expression of CXCL10 in knockout mice reconstituted with WT bone marrow resulted in high parasite biomass levels, higher brain parasite and leukocyte sequestration rates, and increased susceptibility to CM. Neutrophils and inflammatory monocytes were identified as the main cellular sources of CXCL10 responsible for the induction of these processes. The improved control of parasitemia observed in the absence of CXCL10-mediated trafficking was associated with a preferential accumulation of CXCR3(+)CD4(+) T follicular helper cells in the spleen and enhanced Ab responses to infection. These results are consistent with the notion that some inflammatory responses elicited in response to malaria infection contribute to the development of high parasite densities involved in the induction of severe disease in target organs.

Immunological processes underlying the slow acquisition of humoral immunity to malaria.

Malaria is one of the most serious infectious diseases with ~250 million clinical cases annually. Most cases of severe disease are caused by Plasmodium falciparum. The blood stage of Plasmodium parasite is entirely responsible for malaria-associated pathology. Disease syndromes range from fever to more severe complications, including respiratory distress, metabolic acidosis, renal failure, pulmonary oedema and cerebral malaria. The most susceptible population to severe malaria is children under the age of 5, with low levels of immunity. It is only after many years of repeated exposure, that individuals living in endemic areas develop clinical immunity. This form of protection does not result in sterilizing immunity but prevents clinical episodes by substantially reducing parasite burden. Naturally acquired immunity predominantly targets blood-stage parasites and it is known to require antibody responses. A large body of epidemiological evidence suggests that antibodies to Plasmodium antigens are inefficiently generated and rapidly lost in the absence of ongoing exposure, which suggests a defect in the development of B cell immunological memory. This review summarizes the main findings to date contributing to our understanding on cellular processes underlying the slow acquisition of humoral immunity to malaria. Some of the key outstanding questions in the field are discussed.

Role of Forkhead Box P3 in IFNγ-Mediated PD-L1 Expression and Bladder Cancer Epithelial-to-Mesenchymal Transition.

Antagonism of the PD-1/PD-L1 axis is a critical therapeutic strategy for patients with advanced bladder cancer. IFNγ functions as a key regulator of PD-L1 in both immune as well as cancer cells. Forkhead box P3 (FOXP3) is a transcription factor synonymous in T regulatory cell function but with increasingly described functions in cancer cells. Here, we investigated the relationship between FOXP3 and PD-L1 in bladder cancer. We showed that FOXP3 is critical in the ability for IFNγ to activate PD-L1 in bladder cancer cells. FOXP3 can bind to the PD-L1 promoter and induces a gene program that leads to regulation of multiple immune-related genes and genes involved in epithelial-to-mesenchymal transition (EMT). Using in vitro and in vivo human and murine models, we showed that FOXP3 can influence bladder cancer EMT as well as promote cancer metastases. Furthermore, FOXP3 may be a convergent factor for multiple activators of PD-L1, including the chemotherapeutic drug cisplatin. SIGNIFICANCE: Historically a key transcription factor driving T regulatory cell function, FOXP3 has an increasingly recognized role in cancer cells. In bladder cancer, we defined a novel mechanism whereby FOXP3 mediates the activation of the immune checkpoint PD-L1 by the cytokine IFNγ. We also showed that FOXP3 induces other immune checkpoints as well as genes involved in EMT, promoting immune resistance and cancer metastases.

Global distribution and career outcomes of international fellows trained in Canadian gynecologic oncology programs.

Objective: We assessed the global distribution and academic, administrative and research outcomes of international fellows (IFs) trained in Canadian gynecologic oncology (GO) programs. Methods: A web-based survey was sent to IFs who completed GO training in Canada. Using the Web of science database, we identified the publication list, citation record and H-index of IFs and classified them according to their region of practice: high-income countries (HIC), middle income countries (MIC), and low-income countries (LIC). Results: From 1996 to 2020, 81 IFs from 23 countries were trained in English-speaking (62,9%) and French-speaking Canadian universities (37,1%). Most IFs came from HIC (87,6%) and none from LIC. Only 12 IFs (14,8%) are now practicing in Canada. Of the 55 IFs who completed the survey (response rate: 67,9%), the majority (58,2%) reported working in an academic hospital and 29,1% were holding an executive position in a national scholar organization. IFs participated in mentoring residents (96.4 %) and medical students (83,6%) and 36,3% initiated a GO fellowship program in their home country. 67,3% of IFs were involved in international research collaboration and 52,7% participated in international clinical trials. The mean number of publications (22,36 vs 7,75, p = 0.007), citations (369,15 vs 45,12 p = 0.0006) and H-Index (6,88 vs 2,37 p = 0.0001) were significantly higher among IFs working in HIC compared to those in MIC. Most IFs (98,2%) recommended their Canadian GO fellowship program to a colleague from their home country. Conclusion: Most IFs trained in Canadian GO fellowship programs returned to their home countries and achieved important milestones in terms of academic, clinical and research accomplishments.

Safety of vaginal surgery for early-stage cervical cancer: A retrospective multicenter cohort study.

OBJECTIVE: Abdominal Radical hysterectomy (ARH) with pelvic lymph node assessment is considered the standard treatment for early-stage cervical cancer. Accepted routes have previously included laparoscopic or robotic approaches (LRH). Laparoscopy-assisted vaginal or vaginal radical hysterectomy (LVRH) are performed in some centers. The objective of this study is to compare surgical and oncological outcomes of LVRH, to laparoscopic and abdominal approaches. DESIGN PATIENTS SETTING: A retrospective multicenter analysis of consecutive cervical cancer cases who underwent a radical hysterectomy between 2007 and 2017 in eleven regional cancer centers across Canada. MEASUREMENTS: A comparison of patients stratified by surgical technique was undertaken. T-test, Wilcoxon rank-sum and chi-square were used to compare patient characteristics. Log-rank tests and Cox proportional hazards models were employed to compare recurrence and survival across surgical groups. MAIN RESULTS: A total of 1071 patients with cervical cancer stage IA1 with lymphovascular invasion to stage IIIC (FIGO 2018) <4 cm were identified. Postoperative complication rate was lowest for women undergoing LVRH (9.1 %, vs 18.3 % and 22.1 % for minimally invasive and open respectively). During follow up, 114 women recurred, and 70 women died. 5-year recurrence-free survival was 85.4 % for LRH, 89.4 % for ARH and 92.2 % for LVRH. LVRH was not found to be associated with a higher risk of recurrence or death than ARH on multivariable analysis (aHR for recurrence 0.62, CI 0.21-1.77; aHR for death 0.63, CI 0.14-2.77) CONCLUSION: In this retrospective study, vaginal or laparoscopy-assisted vaginal radical hysterectomy for cervical cancer was associated with favorable perioperative and oncological outcomes.

Oncologic Outcomes of Surgically Treated Cervical Cancer with No Residual Disease on Hysterectomy Specimen: A 4C (Canadian Cervical Cancer Collaborative) Working Group Study.

Minimally invasive surgery for the treatment of macroscopic cervical cancer leads to worse oncologic outcomes than with open surgery. Preoperative conization may mitigate the risk of surgical approach. Our objective was to describe the oncologic outcomes in cases of cervical cancer initially treated with conization, and subsequently found to have no residual cervical cancer after hysterectomy performed via open and minimally invasive approaches. This was a retrospective cohort study of surgically treated cervical cancer at 11 Canadian institutions from 2007 to 2017. Cases initially treated with cervical conization and subsequent hysterectomy, with no residual disease on hysterectomy specimen were included. They were subdivided according to minimally invasive (laparoscopic/robotic (MIS) or laparoscopically assisted vaginal/vaginal hysterectomy (LVH)), or abdominal (AH). Recurrence free survival (RFS) and overall survival (OS) were estimated using Kaplan-Meier analysis. Chi-square and log-rank tests were used to compare between cohorts. Within the total cohort, 238/1696 (14%) had no residual disease on hysterectomy specimen (122 MIS, 103 AH, and 13 VLH). The majority of cases in the cohort were FIGO 2018 stage IB1 (43.7%) and underwent a radical hysterectomy (81.9%). There was no statistical difference between stage, histology, and radical vs simple hysterectomy between the abdominal and minimally invasive groups. There were no significant differences in RFS (5-year: MIS/LVH 97.7%, AH 95.8%, p = 0.23) or OS (5-year: MIS/VLH 98.9%, AH 97.4%, p = 0.10), although event-rates were low. There were only two recurrences. In this large study including only patients with no residual cervical cancer on hysterectomy specimen, no significant differences in survival were seen by surgical approach. This may be due to the small number of events or due to no actual difference between the groups. Further studies are warranted.

Estimation risk of lymph nodal invasion in patients with early-stage cervical cancer: Cervical cancer application.

Introduction: Lymph node status is a major prognostic factor in early-stage cervical cancer. Predicting the risk of lymph node metastasis is essential for optimal therapeutic management. The aim of the study was to develop a web-based application to predict the risk of lymph node metastasis in patients with early-stage (IA1 with positive lymph vascular space invasion, IA2 and IB1) cervical cancer. Materials and methods: We performed a secondary analysis of data from two prospective multicenter trials, Senticol 1 and 2 pooled together in the training dataset. The histological risk factors were included in a multivariate logistic regression model in order to determine the most suitable prediction model. An internal validation of the chosen prediction model was then carried out by a cross validation of the 'leave one out cross validation' type. The prediction model was implemented in an interactive online application of the 'Shinyapp' type. Finally, an external validation was performed with a retrospective cohort from L'Hôtel-Dieu de Québec in Canada. Results: Three hundred twenty-one patients participating in Senticol 1 and 2 were included in our training analysis. Among these patients, 280 did not present lymph node invasion (87.2%), 13 presented isolated tumor cells (4%), 11 presented micrometastases (3.4%) and 17 macrometastases (5.3%). Tumor size, presence of lymph-vascular space invasion and stromal invasion were included in the prediction model. The Receiver Operating Characteristic (ROC) Curve from this model had an area under the curve (AUC) of 0.79 (95% CI [0.69- 0.90]). The AUC from the cross validation was 0.65. The external validation on the Canadian cohort confirmed a good discrimination of the model with an AUC of 0.83. Discussion: This is the first study of a prediction score for lymph node involvement in early-stage cervical cancer that includes internal and external validation. The web application is a simple, practical, and modern method of using this prediction score to assist in clinical management.

High-dimensional mass cytometry identifies T cell and B cell signatures predicting reduced risk of Plasmodium vivax malaria.

IFN-γ-driven responses to malaria have been shown to modulate the development and function of T follicular helper (TFH) cells and memory B cells (MBCs), with conflicting evidence of their involvement in the induction of antibody responses required to achieve clinical immunity and their association with disease outcomes. Using high-dimensional single-cell mass cytometry, we identified distinct populations of TH1-polarized CD4+ T cells and MBCs expressing the TH1-defining transcription factor T-bet, associated with either increased or reduced risk of Plasmodium vivax (P. vivax) malaria, demonstrating that inflammatory responses to malaria are not universally detrimental for infection. Furthermore, we found that, whereas class-switched but not IgM+ MBCs were associated with a reduced risk of symptomatic malaria, populations of TH1 cells with a stem central memory phenotype, TH17 cells, and T regulatory cells were associated with protection from asymptomatic infection, suggesting that activation of cell-mediated immunity might also be required to control persistent P. vivax infection with low parasite burden.

Development of B Cell Memory in Malaria.

A single exposure to many viral and bacterial pathogens typically induces life-long immunity, however, the development of the protective immunity to Plasmodium parasites is strikingly less efficient and achieves only partial protection, with adults residing in endemic areas often experiencing asymptomatic infections. Although naturally acquired immunity to malaria requires both cell-mediated and humoral immune responses, antibodies govern the control of malarial disease caused by the blood-stage form of the parasites. A large body of epidemiological evidence described that antibodies to Plasmodium antigens are inefficiently generated and rapidly lost without continued parasite exposure, suggesting that malaria is accompanied by defects in the development of immunological B cell memory. This topic has been of focus of recent studies of malaria infection in humans and mice. This review examines the main findings to date on the processes that modulate the acquisition of memory B cell responses to malaria, and highlights the importance of closing outstanding gaps of knowledge in the field for the rational design of next generation therapeutics against malaria.

Transcription Factor T-bet in B Cells Modulates Germinal Center Polarization and Antibody Affinity Maturation in Response to Malaria.

Despite the key role that antibodies play in protection, the cellular processes mediating the acquisition of humoral immunity against malaria are not fully understood. Using an infection model of severe malaria, we find that germinal center (GC) B cells upregulate the transcription factor T-bet during infection. Molecular and cellular analyses reveal that T-bet in B cells is required not only for IgG2c switching but also favors commitment of B cells to the dark zone of the GC. T-bet was found to regulate the expression of Rgs13 and CXCR3, both of which contribute to the impaired GC polarization observed in the absence of T-bet, resulting in reduced IghV gene mutations and lower antibody avidity. These results demonstrate that T-bet modulates GC dynamics, thereby promoting the differentiation of B cells with increased affinity for antigen.

A fully automated conversational agent for promoting mental well-being: A pilot RCT using mixed methods.

Fully automated self-help interventions can serve as highly cost-effective mental health promotion tools for massive amounts of people. However, these interventions are often characterised by poor adherence. One way to address this problem is to mimic therapy support by a conversational agent. The objectives of this study were to assess the effectiveness and adherence of a smartphone app, delivering strategies used in positive psychology and CBT interventions via an automated chatbot (Shim) for a non-clinical population - as well as to explore participants' views and experiences of interacting with this chatbot. A total of 28 participants were randomized to either receive the chatbot intervention (n = 14) or to a wait-list control group (n = 14). Findings revealed that participants who adhered to the intervention (n = 13) showed significant interaction effects of group and time on psychological well-being (FS) and perceived stress (PSS-10) compared to the wait-list control group, with small to large between effect sizes (Cohen's d range 0.14-1.06). Also, the participants showed high engagement during the 2-week long intervention, with an average open app ratio of 17.71 times for the whole period. This is higher compared to other studies on fully automated interventions claiming to be highly engaging, such as Woebot and the Panoply app. The qualitative data revealed sub-themes which, to our knowledge, have not been found previously, such as the moderating format of the chatbot. The results of this study, in particular the good adherence rate, validated the usefulness of replicating this study in the future with a larger sample size and an active control group. This is important, as the search for fully automated, yet highly engaging and effective digital self-help interventions for promoting mental health is crucial for the public health.

Emerging concepts in T follicular helper cell responses to malaria.

Antibody responses to malaria and candidate malaria vaccines are short-lived in children, leaving them susceptible to repeated malaria episodes. Because T follicular helper (TFH) cells provide critical help to B cells to generate long-lived antibody responses, they have become the focus of recent studies of Plasmodium-infected mice and humans. The emerging data converge on common themes, namely, that malaria-induced TH1 cytokines are associated with the activation of (i) T-like memory TFH cells with impaired B cell helper function, and (ii) pre-TFH cells that acquire Th1-like features (T-bet expression, IFN-γ production), which impede their differentiation into fully functional TFH cells, thus resulting in germinal center dysfunction and suboptimal antibody responses. Deeper knowledge of TFH cells in malaria could illuminate strategies to improve vaccines through modulating TFH cell responses. This review summarizes emerging concepts in TFH cell responses to malaria.

Severe Malaria Infections Impair Germinal Center Responses by Inhibiting T Follicular Helper Cell Differentiation.

Naturally acquired immunity to malaria develops only after years of repeated exposure to Plasmodium parasites. Despite the key role antibodies play in protection, the cellular processes underlying the slow acquisition of immunity remain unknown. Using mouse models, we show that severe malaria infection inhibits the establishment of germinal centers (GCs) in the spleen. We demonstrate that infection induces high frequencies of T follicular helper (Tfh) cell precursors but results in impaired Tfh cell differentiation. Despite high expression of Bcl-6 and IL-21, precursor Tfh cells induced during infection displayed low levels of PD-1 and CXCR5 and co-expressed Th1-associated molecules such as T-bet and CXCR3. Blockade of the inflammatory cytokines TNF and IFN-γ or T-bet deletion restored Tfh cell differentiation and GC responses to infection. Thus, this study demonstrates that the same pro-inflammatory mediators that drive severe malaria pathology have detrimental effects on the induction of protective B cell responses.

The contribution of natural killer complex loci to the development of experimental cerebral malaria.

BACKGROUND: The Natural Killer Complex (NKC) is a genetic region of highly linked genes encoding several receptors involved in the control of NK cell function. The NKC is highly polymorphic and allelic variability of various NKC loci has been demonstrated in inbred mice, providing evidence for NKC haplotypes. Using BALB.B6-Cmv1r congenic mice, in which NKC genes from C57BL/6 mice were introduced into the BALB/c background, we have previously shown that the NKC is a genetic determinant of malarial pathogenesis. C57BL/6 alleles are associated with increased disease-susceptibility as BALB.B6-Cmv1r congenic mice had increased cerebral pathology and death rates during P. berghei ANKA infection than cerebral malaria-resistant BALB/c controls. METHODS: To investigate which regions of the NKC are involved in susceptibility to experimental cerebral malaria (ECM), intra-NKC congenic mice generated by backcrossing recombinant F2 progeny from a (BALB/c x BALB.B6-Cmv1r) F1 intercross to BALB/c mice were infected with P. berghei ANKA. RESULTS: Our results revealed that C57BL/6 alleles at two locations in the NKC contribute to the development of ECM. The increased severity to severe disease in intra-NKC congenic mice was not associated with higher parasite burdens but correlated with a significantly enhanced systemic IFN-γ response to infection and an increased recruitment of CD8+ T cells to the brain of infected animals. CONCLUSIONS: Polymorphisms within the NKC modulate malarial pathogenesis and acquired immune responses to infection.

Repair of guanyl radicals in plasmid DNA by electron transfer is coupled to proton transfer.

By using gamma-irradiation in the presence of thiocyanate ions, we have generated guanyl radicals in plasmid DNA. These can be detected by using an Escherichia coli base excision repair endonuclease to convert their stable end products to strand breaks. The yield of enzyme-sensitive sites is strongly attenuated by the presence of micromolar concentrations of one of a series of singly substituted phenols, and it is possible to derive bimolecular rate constants for the reduction of DNA guanyl radicals by these phenols. More strongly reducing phenols were found to react more rapidly. This electron-transfer reaction also involves a proton transfer. By comparing the expected energetics of the reaction with the observed rate constants, the electron transfer is found to be mechanistically coupled with the proton transfer.