Teprotumumab for the Treatment of Active Thyroid Eye Disease.

BACKGROUND: Thyroid eye disease is a debilitating, disfiguring, and potentially blinding periocular condition for which no Food and Drug Administration-approved medical therapy is available. Strong evidence has implicated the insulin-like growth factor I receptor (IGF-IR) in the pathogenesis of this disease. METHODS: In a randomized, double-masked, placebo-controlled, phase 3 multicenter trial, we assigned patients with active thyroid eye disease in a 1:1 ratio to receive intravenous infusions of the IGF-IR inhibitor teprotumumab (10 mg per kilogram of body weight for the first infusion and 20 mg per kilogram for subsequent infusions) or placebo once every 3 weeks for 21 weeks; the last trial visit for this analysis was at week 24. The primary outcome was a proptosis response (a reduction in proptosis of ≥2 mm) at week 24. Prespecified secondary outcomes at week 24 were an overall response (a reduction of ≥2 points in the Clinical Activity Score plus a reduction in proptosis of ≥2 mm), a Clinical Activity Score of 0 or 1 (indicating no or minimal inflammation), the mean change in proptosis across trial visits (from baseline through week 24), a diplopia response (a reduction in diplopia of ≥1 grade), and the mean change in overall score on the Graves' ophthalmopathy-specific quality-of-life (GO-QOL) questionnaire across trial visits (from baseline through week 24; a mean change of ≥6 points is considered clinically meaningful). RESULTS: A total of 41 patients were assigned to the teprotumumab group and 42 to the placebo group. At week 24, the percentage of patients with a proptosis response was higher with teprotumumab than with placebo (83% [34 patients] vs. 10% [4 patients], P<0.001), with a number needed to treat of 1.36. All secondary outcomes were significantly better with teprotumumab than with placebo, including overall response (78% of patients [32] vs. 7% [3]), Clinical Activity Score of 0 or 1 (59% [24] vs. 21% [9]), the mean change in proptosis (-2.82 mm vs. -0.54 mm), diplopia response (68% [19 of 28] vs. 29% [8 of 28]), and the mean change in GO-QOL overall score (13.79 points vs. 4.43 points) (P≤0.001 for all). Reductions in extraocular muscle, orbital fat volume, or both were observed in 6 patients in the teprotumumab group who underwent orbital imaging. Most adverse events were mild or moderate in severity; two serious events occurred in the teprotumumab group, of which one (an infusion reaction) led to treatment discontinuation. CONCLUSIONS: Among patients with active thyroid eye disease, teprotumumab resulted in better outcomes with respect to proptosis, Clinical Activity Score, diplopia, and quality of life than placebo; serious adverse events were uncommon. (Funded by Horizon Therapeutics; OPTIC ClinicalTrials.gov number, NCT03298867, and EudraCT number, 2017-002763-18.).

Risankizumab compared with adalimumab in patients with moderate-to-severe plaque psoriasis (IMMvent): a randomised, double-blind, active-comparator-controlled phase 3 trial.

BACKGROUND: Psoriasis is an autoimmune disease that affects approximately 100 million people worldwide, and is a disease that can be ameliorated by anti-cytokine treatment. We aimed to compare the efficacy and safety of risankizumab with adalimumab in patients with moderate-to-severe plaque psoriasis. METHODS: IMMvent was a phase 3, randomised, double-blind, active-comparator-controlled trial completed at 66 clinics in 11 countries. Eligible patients were aged 18 years or older with moderate-to-severe chronic plaque psoriasis. Patients were randomly assigned 1:1 using interactive response technology to receive 150 mg risankizumab subcutaneously at weeks 0 and 4 or 80 mg adalimumab subcutaneously at randomisation, then 40 mg at weeks 1, 3, 5, and every other week thereafter during a 16-week double-blind treatment period (part A). For weeks 16-44 (part B), adalimumab intermediate responders were re-randomised 1:1 to continue 40 mg adalimumab or switch to 150 mg risankizumab. In part A, participants and investigators were masked to study treatment. Randomisation was stratified by weight and previous tumour necrosis factor inhibitor exposure. Co-primary endpoints in part A were a 90% improvement from baseline (PASI 90) and a static Physician's Global Assessment (sPGA) score of 0 or 1 at week 16, and for part B was PASI 90 at week 44 (non-responder imputation). Efficacy analyses were done in the intention-to-treat population and safety analyses were done in the safety population (all patients who received at least one dose of study drug or placebo). This study is registered with ClinicalTrials.gov, number NCT02694523. FINDINGS: Between March 31, 2016, and Aug 24, 2017, 605 patients were randomly assigned to receive either risankizumab (n=301, 50%) or adalimumab (n=304, 50%). 294 (98%) of patients in the risankizumab group and 291 (96%) in the adalimumab group completed part A, and 51 (96%) of 53 patients re-randomised to risankizumab and 51 (91%) of 56 patients re-randomised to continue adalimumab completed part B. At week 16, PASI 90 was achieved in 218 (72%) of 301 patients given risankizumab and 144 (47%) of 304 patients given adalimumab (adjusted absolute difference 24·9% [95% CI 17·5-32·4]; p<0·0001), and sPGA scores of 0 or 1 were achieved in 252 (84%) patients given risankizumab and 252 (60%) patients given adalimumab (adjusted absolute difference 23·3% [16·6-30·1]; p<0·0001). In part B, among adalimumab intermediate responders, PASI 90 was achieved by 35 (66%) of 53 patients switched to risankizumab and 12 (21%) of 56 patients continuing adalimumab (adjusted absolute difference 45·0% [28·9-61·1]; p<0·0001) at week 44. Adverse events were reported in 168 (56%) of 301 patients given risankizumab and 179 (57%) of 304 patients given adalimumab in part A, and among adalimumab intermediate responders, adverse events were reported in 40 (75%) of 53 patients who switched to risankizumab and 37 (66%) of 56 patients who continued adalimumab in part B. INTERPRETATION: Risankizumab showed significantly greater efficacy than adalimumab in providing skin clearance in patients with moderate-to-severe plaque psoriasis. No additional safety concerns were identified for patients who switched from adalimumab to risankizumab. Treatment with risankizumab provides flexibility in the long-term treatment of psoriasis. FUNDING: AbbVie and Boehringer Ingelheim.

Efficacy and safety of risankizumab in moderate-to-severe plaque psoriasis (UltIMMa-1 and UltIMMa-2): results from two double-blind, randomised, placebo-controlled and ustekinumab-controlled phase 3 trials.

BACKGROUND: Risankizumab is a humanised IgG1 monoclonal antibody that binds to the p19 subunit of interleukin-23, inhibiting this key cytokine and its role in psoriatic inflammation. We aimed to assess the efficacy and safety of risankizumab compared with placebo or ustekinumab in patients with moderate-to-severe chronic plaque psoriasis. METHODS: UltIMMa-1 and UltIMMa-2 were replicate phase 3, randomised, double-blind, placebo-controlled and active comparator-controlled trials done at 139 sites in Australia, Austria, Belgium, Canada, Czech Republic, France, Germany, Japan, Mexico, Poland, Portugal, South Korea, Spain, and the USA. Eligible patients were 18 years or older, with moderate-to-severe chronic plaque psoriasis. In each study, patients were stratified by weight and previous exposure to tumour necrosis factor inhibitor and randomly assigned (3:1:1) by use of interactive response technology to receive 150 mg risankizumab, 45 mg or 90 mg ustekinumab (weight-based per label), or placebo. Following the 16-week double-blind treatment period (part A), patients initially assigned to placebo switched to 150 mg risankizumab at week 16; other patients continued their originally randomised treatment (part B, double-blind, weeks 16-52). Study drug was administered subcutaneously at weeks 0 and 4 during part A and at weeks 16, 28, and 40 during part B. Co-primary endpoints were proportions of patients achieving a 90% improvement in the Psoriasis Area Severity Index (PASI 90) and a static Physician's Global Assessment (sPGA) score of 0 or 1 at week 16 (non-responder imputation). All efficacy analyses were done in the intention-to-treat population. These trials are registered with ClinicalTrials.gov, numbers NCT02684370 (UltIMMa-1) and NCT02684357 (UltIMMa-2), and have been completed. FINDINGS: Between Feb 24, 2016, and Aug 31, 2016, 506 patients in UltIMMa-1 were randomly assigned to receive 150 mg risankizumab (n=304), 45 mg or 90 mg ustekinumab (n=100), or placebo (n=102). Between March 1, 2016, and Aug 30, 2016, 491 patients in UltIMMa-2 were randomly assigned to receive 150 mg risankizumab (n=294), 45 mg or 90 mg ustekinumab (n=99), or placebo (n=98). Co-primary endpoints were met for both studies. At week 16 of UltIMMa-1, PASI 90 was achieved by 229 (75·3%) patients receiving risankizumab versus five (4·9%) receiving placebo (placebo-adjusted difference 70·3% [95% CI 64·0-76·7]) and 42 (42·0%) receiving ustekinumab (ustekinumab-adjusted difference 33·5% [22·7-44·3]; p<0·0001 vs placebo and ustekinumab). At week 16 of UltIMMa-2, PASI 90 was achieved by 220 (74·8%) patients receiving risankizumab versus two (2·0%) receiving placebo (placebo-adjusted difference 72·5% [95% CI 66·8-78·2]) and 47 (47·5%) receiving ustekinumab (ustekinumab-adjusted difference 27·6% [16·7-38·5]; p<0·0001 vs placebo and ustekinumab). In UltIMMa-1, sPGA 0 or 1 at week 16 was achieved by 267 (87·8%) patients receiving risankizumab versus eight (7·8%) receiving placebo (placebo-adjusted difference 79·9% [95% CI 73·5-86·3]) and 63 (63·0%) receiving ustekinumab (ustekinumab-adjusted difference 25·1% [15·2-35·0]; p<0·0001 vs placebo and ustekinumab). In UltIMMa-2, 246 (83·7%) patients receiving risankizumab versus five (5·1%) receiving placebo (placebo-adjusted difference 78·5% [95% CI 72·4-84·5]) and 61 (61·6%) receiving ustekinumab achieved sPGA 0 or 1 at week 16 (ustekinumab-adjusted difference 22·3% [12·0-32·5]; p<0·0001 vs placebo and ustekinumab). The frequency of treatment-emergent adverse events in UltIMMa-1 and UltIMMa-2 was similar across risankizumab (part A: 151 [49·7%] of 304 and 134 [45·6%] of 294; part B: 182 [61·3%] of 297 and 162 [55·7%] of 291), placebo (part A: 52 [51·0%] of 102 and 45 [45·9%] of 98), ustekinumab (part A: 50 [50·0%] of 100 and 53 [53·5%] of 99; part B: 66 [66·7%] of 99 and 70 [74·5%] of 94), and placebo to risankizumab (part B: 65 [67·0%] of 97 and 61 [64·9%] of 94) treatment groups throughout the study duration. INTERPRETATION: Risankizumab showed superior efficacy to both placebo and ustekinumab in the treatment of moderate-to-severe plaque psoriasis. Treatment-emergent adverse event profiles were similar across treatment groups and there were no unexpected safety findings. FUNDING: AbbVie and Boehringer Ingelheim.

Brodalumab, an anti-interleukin-17-receptor antibody for psoriasis.

BACKGROUND: In this phase 2, randomized, double-blind, placebo-controlled, dose-ranging study, we assessed the efficacy and safety of brodalumab (AMG 827), a human anti-interleukin-17-receptor monoclonal antibody, for the treatment of moderate-to-severe plaque psoriasis. METHODS: We randomly assigned patients with a score of 12 or higher on the psoriasis area-and-severity index (PASI, on which scores range from 0 to 72, with higher scores indicating more severe disease) and with 10% or more of their body-surface area affected by psoriasis to receive brodalumab (70 mg, 140 mg, or 210 mg at day 1 and weeks 1, 2, 4, 6, 8, and 10 or 280 mg monthly) or placebo. The primary end point was the percentage improvement from baseline in the PASI score at week 12. Secondary end points included improvement of at least 75% and at least 90% in the PASI score and the score on the static physician's global assessment at week 12. RESULTS: A total of 198 patients underwent randomization. At week 12, the mean percentage improvements in the PASI score were 45.0% among patients receiving 70 mg of brodalumab, 85.9% among those receiving 140 mg, 86.3% among those receiving 210 mg, 76.0% among those receiving 280 mg, and 16.0% among those receiving placebo (P<0.001 for all comparisons with placebo). An improvement of at least 75% and at least 90% in the PASI score at week 12 was seen in 77% and 72%, respectively, of the patients in the 140-mg brodalumab group and in 82% and 75%, respectively, of the patients in the 210-mg group, as compared with 0% in the placebo group (P<0.001 for all comparisons). The percentage of patients with a static physician's global assessment of clear or minimal disease was 26%, 85%, 80%, and 69% with the 70-mg, 140-mg, 210-mg, and 280-mg doses, respectively, of brodalumab, as compared with 3% with placebo (P<0.01 for all comparisons with placebo). Two cases of grade 3 neutropenia were reported in the 210-mg brodalumab group. The most commonly reported adverse events in the combined brodalumab groups were nasopharyngitis (8%), upper respiratory tract infection (8%), and injection-site erythema (6%). CONCLUSIONS: Brodalumab significantly improved plaque psoriasis in this 12-week, phase 2 study. (Funded by Amgen; ClinicalTrials.gov number, NCT00975637.).

Efficacy and safety of brodalumab in subpopulations of patients with difficult-to-treat moderate-to-severe plaque psoriasis.

BACKGROUND: Novel therapies are needed for difficult-to-treat populations of patients with psoriasis. OBJECTIVE: We sought to assess the efficacy and safety of the interleukin-17 Receptor A inhibitor brodalumab in patients with psoriasis with or without a self-reported history of psoriatic arthritis (PsA) and with or without a history of biologic use. METHODS: Subset analyses of a phase II, randomized, double-blind, placebo-controlled study of brodalumab in patients with moderate-to-severe plaque psoriasis were performed. Improvement from baseline in Psoriasis Area and Severity Index score of 75%, 90%, and 100% at week 12; static Physician Global Assessment (0/1) score; Dermatology Life Quality Index response; and Psoriasis Symptom Inventory response were evaluated within subgroups. RESULTS: Efficacy and quality-of-life measures were generally similar between subgroups of patients with or without a history of PsA and with or without a history of biologic use across brodalumab doses and were significantly higher among patients who received brodalumab 140 mg every 2 weeks or 210 mg every 2 weeks versus placebo. LIMITATIONS: Differences between subgroups were not compared statistically, PsA was self-reported, only skin involvement/symptoms were reported, and reasons for discontinuation of prior biologic were not captured. CONCLUSION: Brodalumab is efficacious in patients with psoriasis with or without a history of PsA or biologic use.

Safety and efficacy of brodalumab for psoriasis after 120 weeks of treatment.

BACKGROUND: Brodalumab (anti-interleukin-17-receptor antibody) was effective in treating moderate to severe psoriasis in a 12-week, dose-ranging, placebo-controlled trial. OBJECTIVE: We sought to evaluate efficacy and safety of long-term brodalumab treatment. METHODS: In this interim analysis at week 120 of an open-label extension study, patients received brodalumab 210 mg every 2 weeks. Protocol amendments reduced the dose (140 mg) in patients weighing 100 kg or less and subsequently increased the dose (210 mg) in patients with inadequate responses. Efficacy was measured by static physician global assessment and 75% or greater, 90% or greater, or 100% improvement in Psoriasis Area and Severity Index score (PASI-75, PASI-90, and PASI-100, respectively). RESULTS: Of 181 patients, 144 completed week 120. Static physician global assessment scores of clear/almost clear and clear were achieved by 90% and 63% of patients, respectively, at week 12 and by 72% and 51% at week 120. The PASI-75, PASI-90, and PASI-100 response rates at week 12 (95%/85%/63%) were sustained through week 120 (86%/70%/51%). Most commonly reported adverse events were nasopharyngitis (26.5%), upper respiratory tract infection (19.9%), arthralgia (16.0%), and back pain (11.0%). Four patients had grade-2 absolute neutrophil count. LIMITATIONS: There was no control group in this open-label extension. CONCLUSION: Brodalumab demonstrated sustained clinical response and an acceptable safety profile through 120 weeks in patients with moderate to severe psoriasis.

A randomized study to evaluate the efficacy and safety of adding topical therapy to etanercept in patients with moderate to severe plaque psoriasis.

BACKGROUND: Few clinical trials have evaluated the combination of topical corticosteroids plus systemic therapies for psoriasis. OBJECTIVE: We sought to evaluate efficacy and safety of etanercept plus topical clobetasol propionate (CP) foam versus etanercept monotherapy for treatment of moderate to severe plaque psoriasis. METHODS: Adults with Psoriasis Area and Severity Index (PASI) score greater than or equal to 10 and psoriasis-affected body surface area greater than or equal to 10% were randomized to etanercept with CP as needed to clear (2 up-to-2-week courses, weeks 11-12 and 23-24) or etanercept alone (each arm at 50 mg twice weekly for 12 weeks followed by 50 mg once weekly for 12 weeks). RESULTS: A total of 592 patients enrolled (295 etanercept + CP arm; 297 etanercept arm). At week 12, significant differences were observed for response of 75% improvement in PASI score (primary end point, 65.2% vs 48.3% in the etanercept + CP vs etanercept arms, respectively; P < .001), response of 90% improvement in PASI score (29.7% vs 19.4%; P = .009), percentage PASI score improvement (76.5% vs 68.2%; P < .001), static physician global assessment of clear/almost clear (63.1% vs 47.3%; P < .001), and patient satisfaction with treatment (P = .006). Response of 75% improvement in PASI score and static physician global assessment of clear/almost clear were not significantly different between arms at week 24. Patient satisfaction with treatment (P = .001) and percentage improvement in PASI score (P = .031) were also greater in the etanercept + CP arm compared with etanercept only at week 24. Comparable numbers of adverse events occurred in each arm. LIMITATIONS: No placebo for CP foam was provided in the etanercept arm. CONCLUSIONS: Addition of CP to etanercept yielded increased efficacy compared with etanercept alone at week 12 without an increase in treatment-related adverse events.

Effect of Risankizumab on Patient-Reported Outcomes in Moderate to Severe Psoriasis: The UltIMMa-1 and UltIMMa-2 Randomized Clinical Trials.

Importance: Demonstrating the value of therapies from a patient's perspective is increasingly important for patient-centered care. Objective: To compare patient-reported outcomes (PROs) with risankizumab vs ustekinumab and placebo in psoriasis symptoms, health-related quality of life (HRQL), and mental health among patients with moderate to severe psoriasis. Design, Setting, and Participants: The UltIMMa-1 and UltIMMa-2 studies were replicate 52-week phase 3, randomized, multisite, double-blind, placebo-controlled and active comparator-controlled trials conducted in 139 sites (including hospitals, academic medical centers, clinical research units, and private practices) globally in Asia-Pacific, Japan, Europe, and North America. Adults (≥18 years) with moderate to severe chronic plaque psoriasis with body surface area (BSA) involvement of 10% or more, Psoriasis Area Severity Index (PASI) scores of 12 or higher, and static Physician's Global Assessment (sPGA) scores of 3 or higher were included. Interventions: In each trial, patients were randomly assigned (3:1:1) to 150 mg of risankizumab, 45 mg or 90 mg of ustekinumab (weight-based per label) for 52 weeks, or matching placebo for 16 weeks followed by risankizumab. Main Outcomes and Measures: Integrated data from 2 trials were used to compare Psoriasis Symptom Scale (PSS) (total score and item scores for pain, redness, itchiness, and burning), Dermatology Life Quality Index (DLQI), 5-level EuroQoL-5D (EQ-5D-5L), and Hospital Anxiety and Depression Scale (HADS), at baseline, week 16, and week 52. Results: A total of 997 patients with moderate to severe chronic plaque psoriasis were analyzed. Across all arms, the mean age was 47.2 to 47.8 years and 68.3% (136/199 for ustekinumab) to 73.0% (146/200 for placebo) were men. Patients' characteristics and PROs were comparable across all treatment arms at baseline (n = 598, 199, 200 for risankizumab, ustekinumab, and placebo, respectively). At week 16, a significantly greater proportion of patients treated with risankizumab than those treated with ustekinumab or placebo achieved PSS = 0, indicating no psoriasis symptoms (30.3% [181/598], 15.1% [30/199], 1.0% [2/200], both P < .001), and DLQI = 0 or 1 indicating no impact on skin-related HRQL (66.2%, 44.7%, 6.0%, P < .001). Significantly greater proportions of patients treated with risankizumab achieved minimally clinically important difference (MCID) than ustekinumab or placebo for DLQI (94.5% [516/546], 85.1% [149/175], 35.6% [64/180]; both P < .001), EQ-5D-5L (41.7% [249/597] vs 31.5% [62/197], P = .01; vs 19.0% [38/200], P < .001), and HADS (anxiety: 69.1% [381/551] vs 57.1% [104/182], P = .004; vs 35.9% [66/184], P < .001; depression: 71.1% [354/598] vs 60.4% [96/159], P = .01; vs 37.1% [59/159], P < .001). At week 52, improvements in patients treated with risankizumab compared with those treated with ustekinumab were sustained for PSS, DLQI, and EQ-5D-5L. Conclusions and Relevance: Risankizumab significantly improved symptoms of moderate to severe psoriasis, improved HRQL, and reduced psychological distress compared with ustekinumab or placebo. Trial Registration: ClinicalTrials.gov Identifiers: NCT02684370 (UltIMMa-1) and NCT02684357 (UltIMMa-2).

Structural-functional relationships of TNF-alpha antagonists: next steps.

The balance between effective tumor necrosis factor (TNF) blockade to control aggressive autoimmune disease states and adequate remaining TNF activity to confer immunoprotection against infections such as tuberculosis is an important and complex issue. An increased scientific understanding of how each of the TNF antagonist agents affects the complex interactions of the inflammation cascade and apoptosis, and whether the effects are modulatory or destructive, is needed. The data presented in this supplement highlight the need for further research into these key areas, and illustrate our current understanding of the mode of action of TNF blockers as only the tip of the iceberg.

3'-5' exonuclease of Klenow fragment: role of amino acid residues within the single-stranded DNA binding region in exonucleolysis and duplex DNA melting.

The mechanism of the 3'-5' exonuclease activity of the Klenow fragment of DNA polymerase I has been investigated with a combination of biochemical and spectroscopic techniques. Site-directed mutagenesis was used to make alanine substitutions of side chains that interact with the DNA substrate on the 5' side of the scissile phosphodiester bond. Kinetic parameters for 3'-5' exonuclease cleavage of single- and double-stranded DNA substrates were determined for each mutant protein in order to probe the role of the selected side chains in the exonuclease reaction. The results indicate that side chains that interact with the penultimate nucleotide (Q419, N420, and Y423) are important for anchoring the DNA substrate at the active site or ensuring proper geometry of the scissile phosphate. In contrast, side chains that interact with the third nucleotide from the DNA terminus (K422 and R455) do not participate directly in exonuclease cleavage of single-stranded DNA. Alanine substitutions of Q419, Y423, and R455 have markedly different effects on the cleavage of single- and double-stranded DNA, causing a much greater loss of activity in the case of a duplex substrate. Time-resolved fluorescence anisotropy decay measurements with a dansyl-labeled primer/template indicate that the Q419A, Y423A, and R455A mutations disrupted the ability of the Klenow fragment to melt duplex DNA and bind the frayed terminus at the exonuclease site. In contrast, the N420A mutation stabilized binding of a duplex terminus to the exonuclease site, suggesting that the N420 side chain facilitates the 3'-5' exonuclease reaction by introducing strain into the bound DNA substrate. Together, these results demonstrate that protein side chains that interact with the second or third nucleotides from the terminus can participate in both the chemical step of the exonuclease reaction, by anchoring the substrate in the active site or by ensuring proper geometry of the scissile phosphate, and in the prechemical steps of double-stranded DNA hydrolysis, by facilitating duplex melting.

Determinants of DNA mismatch recognition within the polymerase domain of the Klenow fragment.

The Klenow fragment of Escherichia coli DNA polymerase I catalyzes template-directed synthesis of DNA and uses a separate 3'-5' exonuclease activity to edit misincorporated bases. The polymerase and exonuclease activities are contained in separate structural domains. In this study, nine Klenow fragment derivatives containing mutations within the polymerase domain were examined for their interaction with model primer-template duplexes. The partitioning of the DNA primer terminus between the polymerase and 3'-5' exonuclease active sites of the mutant proteins was assessed by time-resolved fluorescence anisotropy, utilizing a dansyl fluorophore attached to the DNA. Mutation of N845 or R668 disrupted favorable interactions between the Klenow fragment and a duplex containing a matched terminal base pair but had little effect when the terminus was mismatched. Thus, N845 and R668 are required for recognition of correct terminal base pairs in the DNA substrate. Mutation of N675, R835, R836, or R841 resulted in tighter polymerase site binding of DNA, suggesting that the side chains of these residues induce strain in the DNA and/or protein backbone. A double mutant (N675A/R841A) showed an even greater polymerase site partitioning than was displayed by either single mutation, indicating that such strain is additive. In both groups of mutant proteins, the ability to discriminate between duplexes containing matched or mismatched base pairs was impaired. In contrast, mutation of K758 or Q849 had no effect on partitioning relative to wild type, regardless of DNA mismatch character. These results demonstrate that DNA mismatch recognition is dependent on specific amino acid residues within the polymerase domain and is not governed solely by thermodynamic differences between correct and mismatched base pairs. Moreover, this study suggests a mechanism whereby the Klenow fragment is able to recognize polymerase errors following a misincorporation event, leading to their eventual removal by the 3'-5' exonuclease activity.