Redo cardiac surgery in the era of percutaneous solutions: pattern of presentation and outcomes in a single tertiary care center

Introduction

The advent of new medical and interventional approaches has recently impacted on the referral for redo surgical operations. Little is known on current practice pattern and outcomes.

Purpose

We reviewed our institutional experience of 10 years (January 2011–December 2020) in a single university affiliated center to document frequency, outcome, and complications of the various types of redo procedures.

Methods

Retrospective analysis of a prospectively collected computed institutional database developed according to STS/EACTS recommendations.

Results

During study period 616 patients were referred for redo cardiac procedures, of which 129 patients underwent either medical or interventional procedures (75 thrombolysis, 22 valve-in-valve, 19 native mitral and or tricuspid percutaneous treatment, 13 paravalvular leakage devices), 459 patients were operated on, and 28 patients were denied any invasive treatment because of futility. Study group included these 459 surgical patients with a mean age of 62±12.7 years (octogenarians 5.6%, female sex 52.8%, diabetes 19.5%, chronic kidney disease 30%, urgent/emergent status 34.8%, third-time sternotomy 9.8%). The EuroSCORE II averaged 25.7±15.4%. Study group patients represented a nearly constant subgroup over the 6890 patients operated over this decade. Most frequent surgical procedures were valve operations, which were accomplished in 48.6% (223 pts), whereas isolated coronary bypass surgery was performed in 1.9% (9 pts) only. Valve thrombosis, mechanical prosthesis malfunction, paravalvular leakage, bioprosthetic failure and endocarditis (87% on prosthetic valve) were the most frequent indications. Combined procedures were performed in 15.2% (70 pts). Aortic root, ascending and arch replacement procedures were performed in 10.9% (50 pts, aortic dissection 7.8%). Cardiac transplantation in the setting of previous cardiac surgery was performed in 13.1% (60 pts including: previous conventional procedures [29 pts], mechanical circulatory devices [28 pts, 25 left ventricular assist device and 3 total artificial heart] and re-transplantation [3 pts]). Other procedures were performed in 10.2% (47 pts). Overall hospital mortality was 23,9%, rates of major complications were: surgical revision for bleeding 2,9%, Acute Kidney Injury (I/F) 17,4%, prolonged mechanical ventilation 12,9%, stroke 2,7%. Age, surgical priority, endocarditis, and heart failure represented the major independent predictors of morbidity and mortality.

Conclusions

Although percutaneous solutions are increasing their consensus, the rate of redo cardiac surgical procedures remained stable during the last 10 years. Outcomes are satisfactory despite increasingly complex patients features. Careful Heart Team evaluation is mandatory.

Funding Acknowledgement

Type of funding sources: None.

Activity of platinum and cetuximab in cutaneous squamous cell cancer not amenable to curative treatment

Background

Unresectable or metastatic cutaneous squamous cell cancers (cSCCs) are rare but potentially life-threatening diseases. In this setting, systemic therapy has a palliative intent with limited benefit, but there is no established consensus regarding the proper management of this tumour. This retrospective study aimed to review outcomes in patients with non-curable cSCC treated with platinum-based chemotherapy and cetuximab.

Methods

We considered 12 consecutive patients treated between June 2010 and March 2016. All patients had received previous treatment for the local disease.

Results

The overall response rate was 50%, and the disease control rate was 67%. Median progression-free survival and overall survival were 6.6 (95% confidence interval [CI]: 1.9–8.4) and 14.6 (95% CI: 9.4–20.1) months, respectively. The median duration of response was 4.8 months (95% CI: 1.2–5.9). The most frequent toxicities were skin reactions (58%; grade 3: 25%) and anaemia (10%). No grade 4 toxicities were observed.

Conclusions

Cetuximab and platinum-based chemotherapy were shown to be feasible and active in cSCC, with an acceptable toxicity profile, even if with a limited duration of response.

Patients with adenoid cystic carcinomas of the salivary glands treated with lenvatinib: Activity and quality of life

BACKGROUND

The treatment of patients with recurrent and/or metastatic (R/M) salivary gland adenoid cystic carcinoma (ACC) remains an unmet need.

METHODS

Patients with R/M disease with a history of clinical or symptomatic disease progression within 6 months and a maximum of 1 previous line of chemotherapy or a multiple kinase inhibitor received oral lenvatinib at a dose of 24 mg/day. The primary endpoint was the objective response rate; secondary endpoints included quality of life (QOL) (according to the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire-Core 30 Items [EORTC QLQ-C30] and the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire-Core Module Head and Neck Module [EORTC QLQ-H&N35]), progression-free survival and overall survival, duration of response, and toxicities.

RESULTS

Twenty-eight patients with R/M ACC were enrolled. Among 26 evaluable patients, 3 partial responses (11.5%) were reported. Target lesion reductions between 23% to 28% were observed in 4 of 20 patients with stable disease. Treatment-related adverse events were frequent (all grades, 96%; grade≥3 in 50% of cases according to version 4.03 of the National Cancer Institute Common Terminology Criteria for Adverse Events). The dose of lenvatinib was reduced in 24 patients, whereas in 21 patients the dose was reduced within the first 12 weeks and 4 patients maintained the full dose throughout treatment. The QOL deteriorated between baseline and 6 months with regard to Fatigue and Dry Mouth. There was no evidence of changes in Swallowing and Physical Functioning. At a median follow-up of 29 months, 2 patients remained on treatment, 10 patients were off protocol for disease progression and were alive with disease, and 14 patients had died of disease progression. The median overall survival, progression-free survival, and duration of response were 27 months, 9.1 months, and 3.1 months, respectively.

CONCLUSIONS

Lenvatinib appears to have modest activity in ACC. Toxicities are common but manageable and QOL was found to deteriorate in some domains.

PD-L1 Expression in Unresectable Locally Advanced or Metastatic Skin Squamous Cell Carcinoma Treated with Anti-Epidermal Growth Factor Receptor Agents

BACKGROUND

Recurrent or metastatic (R/M) skin squamous cell carcinoma (sSCC) not amenable of surgery or irradiation may benefit from systemic therapies. Epidermal growth factor receptor (EGFR) inhibitors and, more recently, immune checkpoint blockers (ICBs) showed activity in R/M sSCC. In this study, we aimed at exploring the possible role of PD-L1 expression in predicting response to anti-EGFR agents.

METHODS

Patients affected by R/M sSCC, treated with pan-HER inhibitor dacomitinib or with platinum-based chemotherapy with cetuximab (CT-cet) from 2010 to 2016, were considered. PD-L1 expression was assessed with immunohistochemistry on tumor cells (TCs) and on microenvironment (TC and tumor-infiltrating lymphocyte [IC] scores, respectively). Prognostic role of PD-L1 and the correlation with response to EGFR inhibitors and survival were analyzed.

RESULTS

Twenty-eight R/M sSCCs were analyzed (19 treated with dacomitinib, 9 with CT-cet). TC and IC were negative in 82 and 45% of cases, respectively; 15% sSCCs were both TC and IC positive. Progression-free survival (PFS) was longer in IC-positive cases (median 7.5 months vs. 2.1 in IC0, p = 0.02). No statistically significant differences were observed between PD-L1 expression and both overall survival and response rates.

CONCLUSION

PD-L1 expression in microenvironment predicted better PFS. The combination of EGFR inhibitors and ICB could help deepening the knowledge about the interrelations between the EGFR and PD-1/PD-L1 pathways.

Retrospective analysis of baseline clinical factors associated to CDDP-nephrotoxicity in locally advanced head and neck cancer (LAHNC) patients

e17537

Background: CDDP is the standard of care for HNC. We performed a retrospective analysis related to baseline clinical characteristics and CDDP-nephrotoxicity in LAHNC patients (pts) treated in a curative setting with CDDP and concomitant RT (CRT) or with CDDP as part of induction chemotherapy. Methods: We collected data of LAHNC pts treated between 2008 and 2018 at the National Cancer Institute of Milan. CDDP was administered 100 mg/sm every 3 weeks or 50 mg/sm once a week during CRT, or 75 mg/sm during induction. Nephrotoxicity was assessed using creatinine increase (CreaIncr) recorded at day 6-20 post CDDP, graded according to CTCAE v4.0 and analyzed as a dichotomous variable (Grade>0 vs Grade = 0). We used univariable logistic regression to investigate associations between CreaIncr and clinical variables. Continuous variables were modeled using 3-knots restricted cubic splines. Results: Overall, 204 pts were considered. Male: 74%; median age: 56 yrs [IQR 51-63]. CRT was performed in 179 pts (88%). Three-weekly schedule was performed in 75% of pts. The median CDDP dose was 250 mg [IQR 200-300]. One fourth had history of hypertension and 9% were on diuretics. At baseline, median [IQR] creatinine was 0.80 mg/dL [0.69-0.89], creatinine clearance 109 mL/min [98-129] and uricaemia 4.90 mg/dL [4.11-5.74]. In total, 44% pts suffered from CreaIncr (G1: 46 pts; G2: 39 pts; G3-G4: 5 pts) all of them occurred within the first 2 cycles. CreaIncr was observed in 92% of pts treated with CRT, 84% received 3-weekly CDDP. Baseline creatinine clearance, azotaemia and hypertension were not significantly associated with CreaIncr. Loop diuretics alone appeared a potential relevant factor [OR 4.72;95%CI 0.96-23.32;p = 0.057]. Concomitant drugs (ACE inhibitors, Sartans, Ca-blockers) [OR 1.48;95%CI 1.00-2.18;p = 0.050] and uricaemia [OR 1.95;95%CI 1.27-3.0;p = 0.006] were statistically significantly associated to CreaIncr. Conclusions: In our selected population we found that concomitant medications and uricaemia were significantly related to CreaIncr. A better understanding of clinical baseline factors may improve decision making in a setting where CDDP has a curative significance.

Clinical prognostic factors in patients with recurrent or metastatic carcinoma of the head and neck treated with immune checkpoints therapies

e17530

Background: Immunotherapy (IT) provides advantages in a small proportion of recurrent / metastatic head and neck squamous cell carcinoma (RM HNSCC) patients (pts). Predictive biomarkers are still an unmet need. We aimed to determine if known prognostic clinical factors in RM HNSCC as well as new variables hold prognostic value in the IT era. Methods: We conducted a retrospective analysis of a cohort of 106 RM HNSCC pts enrolled into IT trials at National Cancer Institute of Milan-Italy between Oct 2014 - Jul 2018. Using Log Rank test and Cox proportional hazard model we computed univariate and multivariate analysis (UVA; MVA) of demographics, clinical and biological data to assess their prognostic value for progression free survival (PFS) and overall survival (OS). Results: Ninety-three pts (M 75; F: 18) were included in the final analysis with a median follow up of 20.8 months. The majority (96%) were treated in platinum-refractory setting. Approximately half of pts received anti-PD-1 or anti-PDL-1 monotherapy while the remaining 52% received them in combination with other IT agents. Median OS and PFS were 6.5 (CI 95%: 4.6-10.4) and 2.1 months (CI 95%: 1.8-2.2), respectively. Statistically significant factors at UVA for PFS were smoking history - both pos vs neg and < vs ≥ 30 PY, disease subsite, weight loss > 5%, Karnosfky performance status (PSK), occurrence of any and G3-G4 drug related (DR) toxicities, neutrophil to lymphocytes ratio (NLR). Without stratification for disease subsite, MVA showed independent predictive value of prolonged PFS for positive smoking history (HR 0.5, p 0.02) and any DR toxicities (HR 0.43, p < 0.01). Several factors reached significance for OS in UVA (smoking history and PY, weight loss > 5%, ECOG PS, PSK, occurrence of any toxicities, CRP, baseline C-reactive protein, antibiotic use, steroid use in the 30 days before IT). Of these, PSK (HR 0.95, p 0.01) and any DR toxicities (HR 0.3, p < 0.01) resulted to be predictive in the MVA. Conclusions: Clinical parameters, especially pretreatment PSK, smoking history and occurrence of any DR toxicities, appear to be strong predictors of outcome in RM HNSCC pts treated with IT. These results should be validated in an independent cohort.

Hyperprogressive disease (HPD) in head and neck squamous cell carcinoma (HNSCC) patients treated with immune checkpoint inhibitors (ICI)

6029

Background: HPD was described in 9% of cancer patients (pts) treated in phase I trials, in 13.8% of advanced non-small cell lung cancer and 29% of 34 HNSCC pts upon ICI. A better definition of the hallmarks and survival outcomes of HPD pts in a larger cohort of HNSCC is still lacking. Methods: We retrospectively analyzed all advanced HNSCC pts treated with ICI at our Institution between October 2014 and December 2018. Three scans, performed before ICI, at baseline and at first evaluation during ICI, were assessed according to RECIST 1.1. Tumor Growth Kinetics (TGK) pre- (TGKpre) and post-baseline (TGKpost) were measured as previously reported (Saâda-Bouzid E, Ann Oncol 2017). Pts were defined HPD if they had progression at first radiological evaluation and TGKpost/TGKpre ≥2. Correlation between HPD and clinical characteristics was performed by Fisher or t-student test. Median overall survival (mOS) and progression free survival (mPFS) were estimated using the Kaplan-Meier method and compared between HPD and non-HPD using the log-rank test. Results: Ninety pts were eligible: 18% were female, 4% had ECOG PS ≥ 2, 73% smoking history, 37% oropharyngeal cancer (61% HPV+), 65% locoregional disease (89% previously irradiated), 54% received combined immunotherapy, 75% in ≥ 2nd line. Two out of 90 pts had TGKpre = 0 and were not evaluable for TGK ratio. HPD was observed in 7.9% (7/88) of pts. HPD pts were significantly younger compared to non-HPD pts (median age 53 ± 3.7 vs 63.3 ± 0.9 years, p = 0.002) and had a significantly higher median neutrophil-lymphocyte ratio (NLR) (11.5 ± 3.5 vs 6.4 ± 0.4, p = 0.004). Overall, mOS and mPFS were 7.5 (95% CI: 4.2-10.8) and 2.2 months (95% CI: 0.9-3.4), respectively. At a median follow-up of 20.9 months (95% CI: 19-22.8), HPD pts had a significantly worse mPFS compared to non-HPD pts [1.8 (95% CI: 1.5-2.2) vs 3.5 (95% CI: 2.2-4.8) months; p = 0.001]. HPD correlated with a not significant trend in lower mOS compared to non-HPD group [3.7 (95% CI: 2.4-5.1) vs 8.3 (95% CI: 4.1-12.5) months; p = 0.348]. Three (43%) out of 7 HPD pts early switched to chemotherapy after PD to ICI having a mOS of 8.1 months (range 3.7-25.3). Excluding these 3 pts, HPD correlated with a significantly worse mOS compared to non-HPD [2.6 (95% CI: 1.9-3.3) vs 8.3 (95% CI: 4.1-12.5) months; p = 0.006]. Conclusions: HPD was identified in 7.9% of HNSCC and correlated with younger age and higher NLR. HPD pts who did not receive a subsequent treatment had poorer mPFS and mOS. The assessment of HPD in a control cohort of advanced HNSCC upon standard chemotherapy is ongoing.

Efficacy and safety of single-agent pan-human epidermal growth factor receptor (HER) inhibitor dacomitinib in locally advanced unresectable or metastatic skin squamous cell cancer

BACKGROUND

In recurrent or metastatic (R/M) skin squamous cell cancer (sSCC) not amenable to radiotherapy (RT) or surgery, chemotherapy (CT) has a palliative intent and limited clinical responses. The role of oral pan-HER inhibitor dacomitinib in this setting was investigated within a clinical trial.

METHODS

Patients with diagnosis of R/M sSCC were treated. Dacomitinib was started at a dose of 30 mg daily (QD) for 15 d, followed by 45 mg QD. Primary end-point was response rate (RR). Tumour samples were analysed through next-generation sequencing using a custom panel targeting 36 genes associated with sSCC.

RESULTS

Forty-two patients (33 men; median age 77 years) were treated. Most (86%) received previous treatments consisting in surgery (86%), RT (50%) and CT (14%). RR was 28% (2% complete response; 26% partial response), disease control rate was 86%. Median progression-free survival and overall survival were 6 and 11 months, respectively. Most patients (93%) experienced at least one adverse event (AE): diarrhoea, skin rash (71% each), fatigue (36%) and mucositis (31%); AEs grade 3-4 occurred in 36% of pts. In 16% of cases, treatment was discontinued because of drug-related toxicity. TP53, NOTCH1/2, KMT2C/D, FAT1 and HER4 were the most frequently mutated genes. BRAF, NRAS and HRAS mutations were more frequent in non-responders, and KMT2C and CASP8 mutations were restricted to this subgroup.

CONCLUSIONS

In sSCC, dacomitinib showed activity similar to what was observed with anti-epidermal growth factor receptor agents, and durable clinical benefit was observed. Safety profile was comparable to previous experiences in other cancers. Molecular pt selection could improve therapeutic ratio.

Prognostic role of PIK3CA and TP53 in human papillomavirus-negative oropharyngeal cancers

BACKGROUND

Human papilloma virus (HPV)-negative oropharyngeal squamous cell carcinomas (OPCs) have a poorer prognosis and best management is an unmet need. We studied the prognostic role of epidermal growth factor receptor (EGFR) and PIK3CA amplifications and TP53 functional status.

METHODS

Between 1992 and 2000, 90 consecutive patients with OPCs were treated with surgery, followed by radiotherapy in case of high-risk pathologic features. Of those, 73 cases were HPV-negative and therefore were selected for molecular analysis ( PIK3CA and EGFR fluorescent in situ hybridization [FISH] analysis and TP53 mutation analysis).

RESULTS

FISH analyses of EGFR and PIK3CA were successfully conducted on 69 and 63 of 73 tumor samples, respectively. EGFR alterations were detected in 43% of patients but just 7% showed amplification. Seven cases (11%) carried PIK3CA amplification and 18 (29%) gene gain or high polysomy. TP53 was detected as nonfunctional in 24 of 67 (36%) successfully analyzed cases. Both univariable and multivariable analysis showed statistically significantly worse disease-free survival (DFS) for patients with PIK3CA disomy compared to those with gene gain or high polysomy. No differences in overall survival or DFS for EGFR and TP53 alteration were evident. The combined evaluation of PIK3CA and TP53 showed that PIK3CA gene copy number gain separated a population with better outcome, defining an overall worse prognosis population (disomy) now clearly further divided according to TP53 functional status.

CONCLUSION

PIK3CA gene copy number increase is associated with a favorable clinical outcome in HPV-negative OPCs treated with surgery ± postoperative radiotherapy. In patients without PIK3CA alteration, TP53 nonfunctional mutations are associated with poor prognosis.

RANK expression in EBV positive nasopharyngeal carcinoma metastasis: a ready-to-treat target?

Epstein Barr Virus (EBV) related Nasopharyngeal Carcinoma (NPC), is an highly chemo- and radiosensitive endemic malignancy in southeast Asia. More than one third of locally advanced cases relapse after curative treatment, especially because of bone, liver and lung metastases. Lymphocyte sub-populations favour EBV-associated carcinogenesis and tumour progression and several strategies aim to reverse this phenomenon. Receptor activator of NF-kB (RANK) and its Ligand (RANKL), key regulator of bone metabolisms, are expressed in several malignancies and tumor-infiltrating Tregs. We collected 17 paired FFPE specimen of primary and metachronous metastatic or regionally relapsed EBV related NPC and evaluated RANK expression by immunohistochemistry. All primary tumour specimens resulted not evaluable whereas all metastatic specimens, regardless of sites, showed high RANK IHC expression in the tumor with no staining in normal surrounding tissues. This observation deserves further clarifications and could open the way to trials testing the hypotesis that targeting the RANK/RANKL pathway with denosumab, an already available, clinically approved monoclonal antibody for metastatic bone lesions, might restore proper anti-tumor immune response in NPC metastatic patients.

Management of tyrosine kinase inhibitors (TKI) side effects in differentiated and medullary thyroid cancer patients

Four tyrosine kinase inhibitors (TKIs) have been recently licensed in thyroid cancer (TC), sorafenib and lenvatinib for differentiated TC, vandetanib and cabozantinib for medullary TC. Others TKIs such as axitinib, pazopanib, sunitinib, have been tested within phase II trials. The toxicity burden associated to TKIs is not negligible. Drug reductions and interruptions are common, definitive drug withdrawals have also been reported as well as toxic deaths in more rare cases. In this context, the prevention of toxicities is mandatory to allow patients to stay on treatment as long as possible without dose and schedule modifications. Both physicians and patients should be educated to recognize drug-related toxicities in order to manage them in an early phase. Tools (e.g. toxicities summary booklet) for physicians and patients could be considered to improve the knowledge on side effects management. Guidelines, whenever available, should be followed.

Efficacy and safety of single agent pan-HER inhibitor dacomitinib in locally advanced unresectable or metastatic skin squamous cell cancer (sSCC)

9543

Background: In recurrent/metastatic skin squamous cell cancer (sSCC) not amenable to radiotherapy (RT) or surgery, chemotherapy (CT) has a palliative intent and limited clinical responses. We investigated the role of pan-HER inhibitor dacomitinib within a phase II trial in this setting. Methods: Patients (pts) with diagnosis of sSCC not amenable to curative approaches were included. Oral dacomitinib was started at a dose of 30 mg daily for 15 days, followed by 45 mg daily. Primary endpoint was response rate (RR). Tumor samples were analyzed through Next Generation Sequencing methods (pgm, Ion torrent) using a custom panel targeting 36 genes associated with sSCC. Results: Forty-two pts (33 M, 9 F; median age 77 years, range 45-92) were treated. ECOG PS was 0 in 58%, 1 in 40% and 2 in 2%. One fifth of the pts had distant metastasis. Most pts (86%) received previous treatments consisting in surgery (86%), RT (50%) and CT (14%). Among evaluable pts, overall RR was 28% (complete response CR 2%, partial response PR 26%), disease control rate 86%. Median duration of response was 11 months (range 1-26+). Median treatment duration was 4 months (range 1-26+). Reason for discontinuation were disease progression in 69%, adverse events (AEs) in 19%, non drug-related death in 5%; 3 pts are still on treatment. Median PFS and OS were 6 and 12 months, respectively. Most pts (93%) had at least one AE, mainly consisting in diarrhea and skin rash (71% each), fatigue (36%) and stomatitis (31%). G3/G4 AEs occurred in 36% of pts (diarrhea and skin rash 17% each). Tumor material was available from 7 responding (R: 6 PR, 1 CR) and 15 non-responding (NR: 13SD, 2PD) pts. Frequent TP53 (60%), NOTCH1/2 (60%) and FAT1 (40%) mutations were observed. NR pts showed a higher occurrence of HRAS/BRAF/NRAS mutations (40%) than R ones (28%). Moreover, HER3 (27%) CASP8 (27%), KMT2C (33%) and DCLK1 (27%) mutations were restricted to NR pts. Conclusions: In sSCC dacomitinib showed activity, similar to what observed with anti-EGFR monoclonal antibody cetuximab and panitumumab (RR 28% and 31%); safety profile was comparable to previous experiences in other diseases. Molecular pt selection could improve therapeutic ratio. Clinical trial information: NCT02268747.

Obesity increases the incidence of distant metastases in oestrogen receptor-negative human epidermal growth factor receptor 2-positive breast cancer patients

BACKGROUND

Obesity is a major negative determinant of breast cancer outcome. However, there are contrasting data on the differential impact of obesity on specific breast cancer subtypes. In particular, very little is known on human epidermal growth factor receptor 2-positive (HER2+) tumours.

PATIENTS AND METHODS

We assessed the prognostic role of increased body mass index (BMI) on a consecutive series of non-metastatic HER2+ patients treated at our institution before the introduction of adjuvant Trastuzumab. We separately analysed oestrogen receptor-positive (ER+) and -negative (ER-) HER2+ cases.

RESULTS

In ER-/HER2+ tumours we observed a significantly worse overall survival (Hazard ratio (HR) 1.79, p-value 0.041) and cumulative incidence of distant metastases (HR 2.03, p-value 0.019) in obese (BMI>30) versus normal/underweight (BMI<25) patients. Local relapses appeared to be non-significantly reduced in obese patients, masking the overall effect on disease-free survival. Outcome in ER+ tumours, instead, was not significantly different between BMI groups.

CONCLUSIONS

Obesity significantly correlates with worse overall survival and cumulative incidence of distant metastases in ER-/HER2 positive breast cancer. Differences in the biology of breast tumours may determine individual susceptibility to obesity. The biology of the underlying tumour should be taken into account in the design of dietary intervention trials in breast cancer.

Limited proteolysis of a disulfide-linked apoA-I dimer in reconstituted HDL

The apolipoprotein A-I(Milano) (apoA-I(M)) is a molecular variant of apoA-I characterized by the Arg(173)-->Cys substitution, leading to the formation of homodimers A-I(M)/A-I(M). Upon interaction with palmitoyloleoylphosphatidylcholine, A-I(M)/A-I(M) forms only two species of reconstituted HDL (rHDL) particles, with diameters of 7.8 and 12.5 nm. We used limited proteolysis to analyze the conformation of A-I(M)/A-I(M) in the two rHDL particles, in comparison with that of apoA-I in rHDL of similar size. ApoA-I in the small, 7.8-nm rHDL is degraded to a greater extent (50% after 6 h) than in the large rHDL (<10% degraded after 6 h). The protease susceptibility of A-I(M)/A-I(M) in small and large rHDL is instead remarkably the same, with A-I(M)/A-I(M) being much more sensitive to proteolytic digestion (50% degraded after 10 min) than apoA-I. The identification of the proteolytic fragments by immunoblotting, N-terminal sequencing, and molecular mass determination, shows that the N-terminus of both proteins is resistant to proteolysis, with six cleavage sites located in the central and carboxy-terminal portions of the molecules. Cleavage in the middle of apoA-I occurs at distinct sites in 7.8-nm (Lys(118)) and 12.7-nm (Arg(123)) rHDL, indicating a different conformation in small and large rHDL particles. The A-I(M)/A-I(M) instead adopts a unique and identical conformation in small and large rHDL, with the carboxy-terminal portion of the molecule being remarkably more accessible to the proteases than in apoA-I. This suggests the presence of a novel carboxy-terminal domain in A-I(M)/A-I(M), not organized in a compact structure and not shared by wild-type apoA-I, which may account for the unique functional properties of A-I(M)/A-I(M).