Real-world Outcomes With Cumulative Bortezomib Dose and Efficacy in the Treatment of Transplant-ineligible Multiple Myeloma With Cyclophosphamide, Bortezomib, and Dexamethasone

BACKGROUND

Higher cumulative dose of bortezomib, a key component of Multiple Myeloma (MM) treatment regimens, has been shown to improve outcomes in MM patients, but must be balanced with toxicities including peripheral neuropathy. In this study, we studied the effect of cumulative bortezomib dose on survival, depth of response, and discontinuation rate in transplant ineligible MM patients.

PATIENTS AND METHODS

Data from 70 patients treated with Cyclophsophamide, Bortezomib, and Dexamethasone (CyBorD) in a single Canadian center were grouped according to above vs below median cumulative bortezomib dose and analyzed for progression-free survival (PFS), overall survival (OS), depth of response, and discontinuation rate.

RESULTS

There was a trend for lower discontinuation rate (45.7% vs. 68.6%, P = .052) and significantly lower rate of neuropathy-related discontinuation (5.7% vs. 22.9%, P = .035) in patients who received higher than 43.1 mg/m² of bortezomib. The higher-dose group showed a trend for higher rate of complete response (14.3% vs. 5.7%, P = .225) and significantly higher rate of very good partial response or better (77.1% vs. 51.4%, P = .024). There was significantly longer PFS (24.3 vs. 9.1 months, P = .012) and a trend for longer OS (22.4 vs. 61.3 months, P = .061) in the higher-dose group. In landmark analysis after 180 days, PFS (23.5 vs. 24.3 months, P = .941) and OS were similar in both groups.

CONCLUSION

Higher cumulative bortezomib dose showed a lower rate of discontinuation, longer survival, and deeper response. Determining risk of treatment intolerance remains important for treatment.

Amyloidosis and COVID-19: experience from an amyloid program in Canada

Severe acute respiratory syndrome coronavirus (SARS-CoV2) and associated COVID-19 infection continue to impact patients globally. Patients with underlying health conditions are at heightened risk of adverse outcomes from COVID-19; however, research involving patients with rare health conditions remains scarce. The amyloidoses are a rare grouping of protein deposition diseases. Light-chain and transthyretin amyloidosis are the most common disease forms, often present with systemic involvement of vital organs including the heart, nerves, kidneys, and GI tracts of affected individuals. The Amyloidosis Program of Calgary examined 152 ATTR patients and 103 AL patients analyzing rates of vaccination, COVID-19 testing, infection outcomes, influence referrals, and excess deaths. Results showed 15 total PCR-confirmed COVID-19 infections in the tested population of amyloid patients, with a higher frequency of infections among patient with AL compared to the ATTR cohort (26.2% vs 5.1%). Four patients (26.6%) required hospital admission for COVID-19 infection, 2 ATTR, and 2 AL patients. Of the confirmed cases, 1 (0.07%) unvaccinated ATTR patient died of a COVID-19 infection. An excess of deaths was found in both the ATTR and AL cohorts when comparing pre-pandemic years 2018 and 2019 to the pandemic years of 2020 and 2021. The finding suggests that amyloidosis patients are likely at a high risk for severe COVID-19 infection and mortality, especially those of advanced age, those on an active treatment with chemotherapy, and those with concomitant B-cell or plasma cell disorder. The impact of virtual healthcare visits and pandemic measures on the excess of deaths observed requires further research.

Cyclophosphamide, Bortezomib and Methylprednisolone (CyBorMe) for the Treatment of AL Amyloidosis: Initial Experience From a Single Center

The use of cyclophosphamide, bortezomib and dexamethasone (CyBorD) is widely accepted in the treatment of AL amyloidosis (AL). Recently, the substitution of dexamethasone by methylprednisolone (CyBorMe) appeared to improve response rates and survival outcomes. All consecutive newly diagnosed AL amyloidosis treated with CyBorMe from 01/19 to 08/20 were evaluated. A historic cohort of patients treated with CyBorD was used for comparison (01/13-08/20). Methylprednisolone was given IV at 500 mg weekly for 4 weeks in the CyBorMe group. 43 patients were treated with CyBorD and 14 with CyBorMe. After a median of 4 cycles of CyBorD and 3 cycles of CyBorMe, Hematological Response was seen in 90.6% and 92.8% of cases, including CR in 28.5% and 35.7%, VGPR in 33.3% and 35.7% and PR in 30.9% and 21.4% for CyBorD and CyBorMe, respectively. Time to first response was faster in the CyBorMe group (4 vs. 6 weeks) and cardiac response was observed in 44% and 31% of patients treated with CyBorMe and CyBorD, respectively. CyBorMe appeared to be efficacious and well tolerated in patients with AL amyloidosis. Prospective studies with CyBorMe in the stage III/IV group are warranted aiming to minimize toxicity.

Impact of COVID-19 on the Diagnosis and Management of Multiple Myeloma: Experience from a Canadian Center

BACKGROUND

The impact of coronavirus disease-19 on the management of multiple myeloma (MM) has been recognized. However, the real effect on clinical outcomes remains poorly understood.

OBJECTIVE

We describe a local experience of the management of MM patients and report their outcomes during the current pandemic.

METHODS

All consecutive symptomatic MM patients seen at our center since 03/20 were evaluated.

RESULTS

A cohort of 156 patients diagnosed from 01/19 to 12/20 was analyzed to interrogate differences in presentation patterns. A total of 553 MM patients were seen and/or treated at Tom Baker Cancer Center in the year of 2020. From those, 47.1% (n = 261) were tested for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Sixteen patients tested positive and data are presented. In addition, a decrease of 21.7% in the rate of new smoldering MM/MM diagnosis was observed in 2020 as compared to 2019. Further, an increase in deaths was also observed in 2020.

CONCLUSIONS

Our study confirms an increase lethality for MM patients infected with SARS-CoV-2. A balance between safety and need for cancer control should be emphasized.

Treatment response measurements and survival outcomes in a cohort of newly diagnosed AL amyloidosis

INTRODUCTION

The assessment of AL amyloidosis response is based on serum free light chains (sFLC) levels, and serum and urine monoclonal protein investigations. Recently, difference between involved and uninvolved free light chains (dFLC), involved free light chain (iFLC) and complete response (CR) has been reported as independent predictor of survival and a refinement of the hematological response criteria has been proposed by several groups.

METHODS

In the current study, all consecutive newly diagnosed symptomatic AL amyloidosis patients were evaluated. The primary objective of the study was to assess hematological and organ response after first line of treatment.

RESULTS

A cohort of 76 cases with upfront treatment was used for this analysis. After a median of 3 months post-therapy, hematological response was seen in 88% of cases including CR in 26.3%, VGPR in 38.2% and PR in 23.7%. Median OS was longer in patients with dFLC < 10 mg/L at 3 months, iFLC <20 mg/L at 1 and 3 months, and those achieving CR. Multivariate analysis showed presence of CR as the most important independent prognostic factors for survival.

CONCLUSIONS

Our study suggests that maximal sFLC response and CR are potential endpoints to define clinical outcomes. Large collaborative studies are required to validate and optimize response criteria.

N-Terminal pro-brain natriuretic peptide (NTproBNP) in patients with symptomatic multiple myeloma: report from a single institution

Elevated levels of serum cardiovascular markers including natriuretic peptides (NPs) such as amino terminal pro-brain natriuretic peptide (NTproBNP) have been associated with disease severity and survival in cancer patients and more recently in multiple myeloma (MM). In the present study, we retrospectively reviewed 87 consecutive symptomatic TEMM (transplant-eligible) and 126 TIMM (transplant-ineligible) patients treated at our institution that did undergo NTproBNP testing from 2017 to 2020. Median age at diagnosis was 59.3 years and 75.4 years for the TEMM and TIMM groups, respectively (p = 0.0001). NTproBNP ≥ 300 ng/L was used to assess survival outcomes in the group of symptomatic MM. Patients with AL amyloidosis and symptomatic MM were excluded from the study. Median OS for patients with NTproBNP ≥ 300 ng/L was shorter (45.9 months) as compared to those with NTproBNP of < 300 ng/L (non-reached) (p = 0.0001). In addition, OS was shorter for those with CCI > 2, ISS2-3, and high-risk cytogenetics by FISH and ≥ 70 years of age. Multivariate analysis showed that HR cytogenetics and ISS2-3 were independent predictors for OS in the entire cohort of MM patients. When restricted to TIMM, age ≥ 80 years and NTproBNP ≥ 800 ng/L were predictors for OS in univariate and multivariate analyses. In conclusion, NTproBNP appears to be an independent predictor factor for OS in symptomatic TIMM patients. The use of NTproBNP as a frailty marker remains to be elucidated. However, NTproBNP could potentially be used to guide treatment decisions aimed to minimize cardiovascular and renal toxicity for myeloma therapies that potentially do have cardio-renal implications.

The impact of COVID-19 in the management of AL amyloidosis and Immunoglobulin Deposition Disease: A single-center experience

Introduction

Patients with AL amyloidosis and immunoglobulin deposition diseases (IDD) are vulnerable during the COVID‐19 pandemic due to the immune compromise from the plasma cell disorder and therapy‐related immune defects. We describe a local experience in providing care for patients with AL amyloidosis and IDD.

Method

Patient treatment and disease status since the beginning of the pandemic on March 11, 2020, as declared by WHO, were collected and analyzed.

Results

Ninety‐six patients with AL amyloidosis and IDD were included. Four patients with IDD and 22 patients with systemic AL amyloidosis were receiving treatment during the pandemic. Since the pandemic, patients’ treatments were discontinued if they achieved VGPR or better postinduction. Seven patients discontinued all treatment after achieving VGPR, and others required treatment modifications. 28 patients have been tested for COVID‐19, and all tests have been negative. Three patients died since the pandemic, two from organ complications of systemic AL amyloidosis and one from an unrelated cause.

Conclusion

The management of AL amyloidosis and IDD must be individualized on the clinical characteristics, centers’ access to care under the pandemic restrictions, and the epidemiological aspects of the outbreak.

A81 POST COLONOSCOPY COLORECTAL CANCERS IN MANITOBA: A POPULATION-BASED ANALYSIS

Background

Recent consensus guidelines from the World Endoscopy Organization (WEO) recommend all jurisdictions report unadjusted rates of post colonoscopy (PC) colorectal cancers (CRC). Until recently, prior reports have mostly focused on PC-CRC in the CRC screening age groups.

Aims

We evaluated the rate and predictors of PC-CRC in the adult population for the province of Manitoba from 1990 to 2016.

Methods

Individuals 18+ years at CRC diagnosis were identified from the Manitoba Cancer Registry. Colonoscopies in the 3 years preceding CRC diagnosis were identified via linkage to Manitoba Health (MH) physicians billing claims. CRCs were classified, based on WEO recommendations, as: (1) detected CRC (colonoscopy up to 6 months before CRC diagnosis) and (2) PC-CRC-3y (colonoscopy 6–36 months before CRC diagnosis). Generalized linear models with generalized estimating equations (to adjust for clustering within endoscopy physicians) were used to test for differences in rates over 3-time intervals (1990/91 – 1999/00; 2000/01 - 2009/10; 2010/11 – Dec 31 2016), provincial region of performance of colonoscopy and identify other associations from the MH data.

Results

Overall, 10.5% of the 16,639 CRCs diagnosed in the study period and with colonoscopy in the preceding 3 years were PC-CRC-3y. CRCs diagnosed between April 2000 and March 2010 were more likely to be PC-CRC-3y than those diagnosed between April 2010 and December 2016 (odds ratio [OR] 1.18; 95% confidence interval [CI]: 1.03–1.37). Female sex (OR for male: 0.86; 95% CI: 0.77–0.94), IBD diagnosis (OR 3.04; 95% CI: 2.56–4.52), prior CRC (OR 5.41; 95% CI: 4.61–6.34), prior colonoscopy (OR 2.10; 95% CI 1.88–2.36), diverticulosis (OR 2.39; 95% CI: 2.16–2.6), colonoscopy by GP (OR: 1.62; 95% CI 1.16–2.26 vs. surgeons) were associated with increased odds of PC-CRC-3y. There were no regional differences, and no effect of colonoscopy volume or age greater than 75 (or lower than 50).

Conclusions

In Manitoba, the PC-CRC-3y rate decreased slightly in recent years. The study results of large number of PC-CRC-3y along with only a slight decrease in rates over the years, support calls for root cause analysis to evaluate individual cases of PC-CRC. An initial focus could be the groups with increased risk of PC-CRC.

Funding Agencies

Manitoba Health

A24 RATE OF GASTROINTESTINAL ENDOSCOPY USE AND CONCOMITTANT ANESHESIOLOGY ASSISTANCE AND PREDICTORS OF ANESTHESIOLOGY ASSISTANCE: A POPULATION-BASED ANALYSIS

Background

There are limited Canadian data on time trends of gastrointestinal endoscopy (GIE) use and concomitant anesthesiology assistance.

Aims

To test the time trends and variations in concomitant anesthesiology assistance during GIE in the central Canadian province of Manitoba.

Methods

Physicians claims reimbursement data from the provincial health ministry were used to ascertain upper & lower GIE procedures and anesthesiology assistance annually from 1984 to 2016. The annual provincial population was determined from health insurance registration files. Generalized linear models with generalized estimating equations were used to test the linear trend over time and differences amongst age groups, health regions and income quintiles. Logistic regression analysis with generalized estimating equations was used to assess predictors of anesthesiology use.

Results

410, 685 individuals had at least one procedure in the observation period. Approximately half of the procedures were performed outside the recommended CRC screening age group of 50–74 years, with 38% among those less than 50 years. The average annual rate of increase was 4.6% (95% CI 4.4–4.8%). The rate of increase stabilized among the &gt; 75 years group over the last 5 years of the study. There were marked regional variation in GIE procedures, with the smallest increase in Winnipeg region (the largest urban region):2.9% (95%CI: 2.5–3.3) vs. 5.2% (95%CI: 4.8–5.6) in Southern Health; RR: 2.2(figure 1 A). There were no differences in GIE rates by income quintile. Similar patterns were seen in analysis stratified by upper and lower GIE. Concomitant anesthesiology use ranged from 1% in Winnipeg region to 80% in the southern rural region (figure 1B). In the logistic regression analysis, independent predictors of concomitant anesthesiology use in rural regions included younger age, female sex, lower income quintile, and higher comorbidities of patients, GP endoscopist, region of physician practice, and lower volume endoscopy physician.

Conclusions

Marked regional variations in GIE use and concomitant anesthesiology use were observed in a universal health care system. Efforts to standardize care are needed to reduce variations.

Funding Agencies

Manitoba Health

Greater occipital nerve blocks in chronic cluster headache: a prospective open-label study

BACKGROUND AND PURPOSE

Greater occipital nerve blockade (GONB) has been shown to be effective in episodic cluster headache. However, its use in chronic cluster headache (CCH) is less certain. The study aims to prospectively assess the efficacy and consistency of response to GONB in a large series of CCH patients.

METHODS

CCH patients who had a unilateral GONB and were referred to the National Hospital for Neurology and Neurosurgery were studied prospectively. Headache characteristics were collected using headache diaries. Responders were considered to be patients with a complete or partial response lasting at least 7 days. In a subgroup of responders the outcomes of serial GONB performed at 3-monthly intervals were also analysed.

RESULTS

Eighty-three CCH patients were studied. After the first GONB, a positive response was observed in 47 (57%) patients: 35 (42%) were rendered pain free, 12 (15%) had a partial benefit and one patient obtained <50% improvement. The duration of a positive response lasted a median of 21 days (range 7-504 days). There was a transient worsening of condition in 6% of patients. The overall rate and average duration of response remained consistent after the second [n = 37; 31 responders (84%); median duration 21 days], third [n = 28; 20 responders (71%); median duration 25 days] and fourth [n = 14; 10 responders (71%); median duration 23 days] injections.

CONCLUSION

GONB seems to be an efficacious treatment with reproducible effects in CCH patients. Performed three monthly, GONB may have a useful role in the management of CCH.

Eicosanoid and muscarinic receptor blockade abolishes hyperventilation-induced bronchoconstriction

This study was designed to test the hypothesis that hyperventilation-induced bronchoconstriction (HIB) results from the combined effects of prostanoid and leukotriene metabolism. A bronchoscope was used in anesthetized dogs to record peripheral airway resistance and HIB before and after combined treatment with inhibitors of cyclooxygenase (indomethacin) and 5-lipoxygenase (MK-0591). Bronchoalveolar lavage fluid (BALF) cells and mediators from hyperventilated and control airways were also measured. Pretreatment with MK-0591 and indomethacin significantly attenuated, but did not abolish, HIB. However, addition of atropine nearly eliminated the residual response. Blockade of eicosanoid metabolism markedly reduced the concentrations of eicosanoids recovered in BALF after hyperventilation. Positive correlations between posthyperventilation BALF prostanoid and epithelial cell concentrations are suggestive of mucosal injury-induced mediator production and release. We conclude that HIB is prevented in the presence of eicosanoid and muscarinic-receptor blockade and that both classes of eicosanoids contribute similarly to the development of HIB.

Analysis of urine for drugs of abuse using mixed-mode solid-phase extraction and gas chromatography-mass spectrometry

A method for the simultaneous analysis of urine for the major drugs of abuse is described. The analytical procedure uses solid-phase extraction (SPE), gas chromatography-mass spectrometry (GC-MS) and a semi-automated identification process. It allows simultaneous extraction, derivatization and analysis of acidic, neutral and basic drugs from urine. Urine samples were subjected to enzymatic hydrolysis followed by SPE using Bakerbond narc-2 columns. The eluant was selectively derivatized with N-methyl-bis-trifluoroacetamide (MBTFA) and N-methyl-N-trimethylsilyltrifluoroacetamide + 1% trimethylchlorosilane. Analysis was performed using a GC-MS system operating in full scan mode. A simple macro programme was written to enhance the mass spectra identification capabilities of the MS software by producing extracted ion chromatograms (EIC) for the drugs of interest. Once a suspect compound was indicated by EIC, the mass spectrum of the compound was searched manually against reference libraries for positive identification and the retention time checked against that of the standard. This procedure has increased both the amount and the reliability of information given to clinicians without increasing the cost per sample. The system has been in routine operation for 24 months, processing up to 40 urine samples per day, with a usual turn-around time of 48 h.

Electrical burn of urethra as cause of stricture after transurethral resection

BACKGROUND AND PURPOSE

One of the most common complications of transurethral resection is urethral stricture. The exact etiology is still controversial. Postulated pathophysiology ranges from mechanical trauma to the urethra during the resection to inflammatory reaction secondary to local anesthesia. We propose electrical burn to the urethra as a contributory factor in some cases.

MATERIALS AND METHODS

Electrical events during transurethral resection were simulated in vitro. The distribution of current was measured at the loop/rollerball and in the outer metal sheath. Both an intact loop/rollerball and loop/rollerball with faulty insulation were tested. Various power outputs simulating coagulation, cutting, and vaporization were used. Data were recorded both in settings where the outer metal sheath was and was not smeared with nonconductive lubricating gel.

RESULTS

Approximately 20% of the total current output was shunted to the metal sheath when an intact loop/rollerball was used. One hundred percent of the current was short-circuited to the sheath when a loop/rollerball with faulty insulation was retracted into the sheath. Little current was detectable in the nonconductive lubricating gel. We then calculated the length of time required, in various settings, to reach tissue temperature of 45 degrees C, when protein denaturation starts. The higher the power output and the smaller the contacting surface area, the more likely it is for urethral burns to occur.

CONCLUSION

Our in vitro experiment showed that urethral burns secondary to stray current in the resectoscope sheath are unlikely when an intact loop is used. However, urethral burns can occur when a loop/rollerball with faulty insulation is used, especially in combination with nonconductive gel that is dispersed unevenly within the urethra.

Genital tract electrical burns during hysteroscopic endometrial ablation: report of 13 cases in the United States and Canada

We investigated 13 alleged thermal injuries to the genital tract of women undergoing hysteroscopic endometrial ablation. Possible mechanisms proposed to explain these injuries are hot-weighted speculum, povidone-iodine scrub solution, inadequate rinsing of Cidex sterilizing solution, and electrical burns. The history, nature, and distribution, as well as experimental evidence strongly support the hypothesis that these injuries are electrical due to capacitive coupled currents induced onto the sheath of the resectoscope, and/or stray currents generated by arcing or direct coupling from defective electrode insulation to the telescope, electrifying the entire resectoscope.

Intended and stray radiofrequency electrical currents during resectoscopic surgery

STUDY OBJECTIVE

To test the hypothesis that electrical burns of the genital tract and urethral strictures after hysteroscopic endometrial ablation and transurethral prostatectomy, respectively, are related to capacitive coupling and/or stray currents induced by intact and defective electrodes and/or resectoscopes.

DESIGN

Basic in vitro measurements.

SETTING

Laboratory.

MATERIALS

Porcine muscle and liver, resectoscope, electrosurgical unit (ESU), and ESU analyzer.

INTERVENTION

We measured electrical coagulation and cutting currents of rollerball and loop electrodes and the external sheath of the resectoscope from 80 to 200 W through a resistance load of 200 and 250 ohms, using intact electrodes and conditions simulating potential insulation defects along the shaft of the electrodes.

MEASUREMENTS AND MAIN RESULTS

Approximately 20% to 25% of current was induced by capacitive coupling to the resectoscope sheath. Touching porcine muscle or liver with small areas of the sheath while the generator was activated resulted in superficial tissue burn. Surrounding large segments of the sheath with tissue did not result in visible burns, indicating that under normal conditions the sheath acts as a dispersive electrode. Defective insulation of distal segments of the electrodes resulted in 100% transfer of current to the sheath and caused extensive electrical burns of tissue in contact with the sheath.

CONCLUSIONS

Capacitive coupled currents induced by intact resectoscopes and electrodes may cause thermal injury to surrounding tissue during prolonged resectoscopic surgery. Stray currents from defective insulation of the electrodes result in direct coupling of current to the telescope and sheath and cause extensive burns of surrounding tissues in contact with the sheath.

Mucosal injury and eicosanoid kinetics during hyperventilation-induced bronchoconstriction

Bronchoalveolar lavage (BAL) of canine peripheral airways was performed at various times after hyperventilation, and BAL fluid (BALF) cell and mediator data were used to evaluate two hypotheses: 1) hyperventilation-induced mucosal injury stimulates mediator production, and 2) mucosal damage is correlated with the magnitude of hyperventilation-induced bronchoconstriction. We found that epithelial cells increased in BALF immediately after a 2- and a 5-min dry air challenge (DAC). Prostaglandins D(2) and F(2alpha) and thromboxane B(2) were unchanged immediately after a 2-min DAC but were significantly increased after a 5-min DAC. Leukotriene C(4), D(4), and E(4) did not increase until 5 min after DAC. Hyperventilation with warm moist air did not alter BALF cells or mediators and caused less airway obstruction that occurred earlier than DAC. BALF epithelial cells were correlated with mediator release, and mediator release and epithelial cells were correlated with hyperventilation-induced bronchoconstriction. These observations are consistent with the hypothesis that hyperventilation-induced mucosal damage initiates peripheral airway constriction via the release of biochemical mediators.

Venous allografts prepared from stripped long saphenous vein. Is there a need for antibiotic sterilisation?

AIM

Can useful lengths of vein be retrieved from varicose vein stripping procedures; is it necessary to sterilise this tissue prior to use as vein allografts?

METHOD

Stripped long saphenous vein was retrieved at operation. Vein samples were cultured using direct plate inoculation and enrichment culture. Further samples were immersed in two low concentration antibiotic solutions and recultured. Smooth muscle viability was assessed after antibiotic immersion and storage by cryopreservation.

RESULTS

High quality vein could be retrieved by vein stripping. Vein segments grew skin commensals on enrichment culture despite negative cultures with standard media plate inoculation (Chi-squared = 53.34 1 d.f. p < 0.001). Low concentration antibiotic solutions sterilised processed vein. Smooth muscle cell viability was reduced by cryopreservation, Mann-Whitney p = 0.008 (control 98% S.E. 0.93 vs. cryopreserved 64% S.E. 6.58), but prior exposure to antibiotics did not compound this effect.

CONCLUSION

Useful lengths of vein grafts can be retrieved from varicose vein stripping procedures. Venous segments are frequently contaminated by skin commensals. Enrichment culture is required to detect contamination. Low concentration antibiotics sterilise venous-tissue without affecting smooth muscle cell viability.

ATP-dependent interaction of human mismatch repair proteins and dual role of PCNA in mismatch repair

DNA mismatch repair ensures genomic stability by correcting biosynthetic errors and by blocking homologous recombination. MutS-like and MutL-like proteins play important roles in these processes. In Escherichia coli and yeast these two types of proteins form a repair initiation complex that binds to mismatched DNA. However, whether human MutS and MutL homologs interact to form a complex has not been elucidated. Using immunoprecipitation and Western blot analysis we show here that human MSH2, MLH1, PMS2 and proliferating cell nuclear antigen (PCNA) can be co-immunoprecipitated, suggesting formation of a repair initiation complex among these proteins. Formation of the initiation complex is dependent on ATP hydrolysis and at least functional MSH2 and MLH1 proteins, because the complex could not be detected in tumor cells that produce truncated MLH1 or MSH2 protein. We also demonstrate that PCNA is required in human mismatch repair not only at the step of repair initiation, but also at the step of repair DNA re-synthesis.