Introduction
Pancreatic neuroendocrine tumours (PanNET) are rare neoplasms with an annual incidence of 0.48/100,000 [
1]. Surgical removal is the only curative therapy. At presentation, 60%–80% of patients have unresectable disease due to local extension or metastases. Hence, only palliative interventions can be offered [
2]. Streptozocin-based chemotherapy (STZ CTx) has been an established first-line treatment since 1980. Initially, response rates (RR) exceeding 60% and a sustained median progression-free survival (PFS) of 36 months were reported [
3]. Subsequent series found very heterogeneous results with RR between 6% and 55% and PFS of 4–23 months [
4]. These conflicting results are attributed to heterogeneous patient cohorts and classification systems.
In 2010, a unified PanNET assessment was established and novel treatment options have been developed since then. The World Health Organisation (WHO) introduced a new classification system that abolished the distinction between well-differentiated endocrine tumours of benign behaviour and endocrine carcinomas, highlighting the malignant potential of all neuroendocrine tumours (NET) based on the proliferation marker, Ki-67 [
5]. The validity of this grading system was confirmed in several studies [
6].
Sunitinib, a multikinase inhibitor, approved by the European medicines agency (EMA) in 2010 and the food and drug administration (FDA) in 2011, was the first licensed therapeutic alternative to STZ CTx for progressive, well-differentiated PanNET. Approval of sunitinib was based on a double-blind, randomised study demonstrating an increase in PFS from 5.5 to 11.4 months [
7]. In the same year, everolimus, an oral mTOR inhibitor, was also approved by the EMA and FDA for the treatment of progressive, well-differentiated PanNET. Approval of everolimus was also based on double-blind, randomised data [
8]; PFS increased from 4.6 to 11.0 months in this study.
Since their approval, sunitinib and everolimus have competed with STZ CTx in the treatment of well-differentiated PanNET. However, the importance of the individual substances within the therapy algorithm has not been established because of the lack of comparative studies. Furthermore, peptide receptor radionuclide therapy (PRRT) is commonly used in PanNET, but whether prior PRRT influences the outcome of STZ-CTx is currently unknown [
9,
10]. In the current guidelines, STZ CTx is one of the standard therapies [
11‐
13]. The length of practical experience supports this approach. However, there are no studies in which the current therapeutic alternatives have been available. Tumour classification, therapeutic thresholds, and alternatives differ considerably in published studies [
4,
14‐
17]. In addition, grading based on Ki-67 has only been established as a mandatory part of NET baseline classification since 2010.
The aims of the present study were thus (1) to determine the outcome of STZ CTx in a well-defined patient population, treated according to current guidelines, in which the current therapeutic alternatives are available; (2) to assess the influence of previous targeted therapy and PRRT on objective response rate (ORR), PFS, and overall survival (OS), and (3) to detect factors influencing therapeutic outcomes.
Materials and methods
Patients
Patients with histologically confirmed, well-differentiated and locally advanced or metastatic PanNET, who received STZ CTx between January 2010 and January 2018, were identified from our prospective database at the European Neuroendocrine Tumour Society (ENETS) Centre of Excellence at the University Hospital of Essen. The follow-up period was extended until April 2020. Patients with hereditary tumours (multiple endocrine neoplasia type 1 or von Hippel–Lindau disease) were excluded. To ensure consistency, indication for therapies was determined according to ENETS guidelines by an experienced, multidisciplinary tumour board (MTB) [
11,
12]. All therapies were administered in-house at our centre.
Chemotherapy
The STZ CTx consisted of 500 mg/m² of streptozocin in 100 ml of 0.9% NaCl IV infusion given over 30 min, followed after 1 h by 400 mg/m² of fluorouracil (5-FU) infusion in 100 ml of 0.9% NaCl given over 30 min. Adequate peri-interventional hydration was ensured by administering at least 1000 ml of 0.9% NaCl IV infusion. A 5-hydroxytryptamine (5-HT3) antagonist was administered 30 min before the start of therapy. Dexamethasone (8 mg) was administered per os at the beginning of each chemotherapy day. The therapy was implemented over five consecutive days, with a cycle length of 42 days. In case of impaired performance status or toxicity, a delay of up to 2 weeks was provided. The first staging was performed using computed tomography (CT) after three cycles. Patients who did not show progression received the intended number of six cycles, unless unacceptable toxicity occurred.
Follow-up and evaluation of tumour response
A baseline CT scan was performed within 4 weeks before starting STZ CTx. After three completed treatment courses, the first evaluation of therapeutic response (history, physical examination, CT or MRI scan, and laboratory investigations) was scheduled. In case of stable disease (SD) or remission, STZ CTx was continued until the planned number of six cycles. Within 4 weeks after the last cycle and at 3-month intervals, follow-up examinations (CT or MRI) were performed. Hybrid imaging (68Ga DOTATOC positron emission tomography (PET)/CT) was included at the initial presentation and at least every 12 months within the surveillance schedule. After 1 year of SD, partial remission (PR) or complete remission (CR), follow-up intervals were extended from 3 to a maximum of 6 months, according to the MTB decision. Response to treatment was evaluated using the international criteria proposed by the Response evaluation criteria in solid tumours (RECIST) committee. At each scheduled time point, chromogranin A (CgA), hematologic, renal, hepatic, endocrine, and coagulation parameters were measured and clinical symptoms were recorded according to common terminology criteria for adverse events (CTCAE), version 3.0.
Pathology of the tumours
The presence of PanNET was confirmed morphologically and immunohistochemically in all patients. The Ki-67 index was indicated using the MIB-1 antibody, taking into consideration the area of highest activity. Tumour grading was performed according to the WHO/ENETS criteria [
18,
19]. Low-grade (G1) PanNET were defined as tumours having a Ki-67 index of ≤2% and intermediate-grade (G2) PanNET was defined as tumours having a Ki-67 index between 3% and 20%. Three patients had well-differentiated, high proliferative PanNET with a Ki-67 index of >20%. The analyses were performed by one pathologist with expertise in endocrine and pancreatic tumours. The pathologist was blinded to the patients’ history.
Statistical methods
Response and tumour characteristics were compared using Fisher’s exact test. PFS was recorded as the time between the start of treatment and radiological progression (based on RECIST 1.1) or death. Survival rates were calculated using the Kaplan-Meier method. OS from diagnosis was defined as the time between PanNET diagnosis and death or the last follow-up. OS from the start of chemotherapy was defined as the time between the start of treatment and death or the last follow-up. Univariate and multivariate Cox regression analyses were performed to identify prognostic factors. Statistical differences in PFS and OS between patient groups were estimated using the log-rank test. A p-value of <0.05 was considered significant. All statistical calculations were performed using IBM SPSS Statistics for Windows, Version 25.0 (IBM Corp., Armonk, NY).
Discussion
In this study, the efficacy of STZ CTx in patients selected according to current guidelines was investigated [
13,
20,
21]. Since the introduction of STZ CTx in the 1970s, the therapy threshold has risen from the mere detection of residual disease to the clinical or radiological progression of an incurable condition [
3,
13,
22]. In 2010, competing targeted therapies with sunitinib and everolimus were approved and PRRT became widely available [
7,
8,
11]. Since then, STZ CTx has been used competitively with these new therapies, preferably in short-term progressive PanNET with multiple-organ manifestations. This study indicates a persistent ORR of 38% with a lower PFS at 12 months and lower OS of 38 months compared with previous studies (Table
6).
Table 6
Patient characteristics and outcome of STZ CTx in PanNET
| STZ/5-FU 1981–2014 | 133 | 100 (75.2%) | 100% pancreas | 36% | 88% | 63.2% | n.ra | 32% | 28% | 92% | 23 | 51.9 | CgA decrease > 50% | Grading Stage IV | Grading Previous surgery |
| STZ/5-FU 1998–2014 | 96 | 93 (96.8%) | 100% pancreas | 12% | 93.8% | 56.3% | 11.8 | 74% | 42.7% | 83.3% | 19.4 | 54.8 | CgA decrease >30% | Ki-67 > 15% | Ki-67 > 15% Metastatic sites ≥2 |
| STZ/Dox/5-FU 1995–2013 | 77 | 64 (83.1%) | 84.4% pancreas (+bronchial, duodenal, CUP-NEN) | 12% | 90.9% | 19.5% | 33 | n.ra | 34% | 72% | 16 | 28 | CgA decrease >30%, positive Octreo-Scan | Ki-67 > 10% | —b |
| STZ/5-FU 2002–2018 | 30 | 28 (93.3%) | 100% pancreas | n.ra | 92.9% | 61% | n.ra | n.ra | 36% | 86% | 21 | 69 | —b | Previous surgery | —b |
Lahner et al. | STZ/5-FU 2010–2018 | 50 | 50 (100%) | 100% pancreas | 6% | 96% | 54% | 6.5 | 100% | 38% | 76% | 12 | 38 | CgA decrease >30% | —b | Bone metastases Therapy line |
Baseline characteristics of our cohort confirmed a shift toward a more aggressive clinical course compared with published data [
14‐
17]. The median duration of illness from the first diagnosis to the beginning of STZ CTx was considerably shorter in our investigation (6.5 months) than in recent studies from Berlin and Marburg (11.8 and 33.0 months, respectively) [
15,
16]. At the same time, 50% of our patients had a tumour spread to two or more distant organ systems; in a previous study, tumours spread in only 33% of patients [
15]. The proportion of patients with a low proliferative G1 PanNET, corresponding to a Ki-67 index of ≤2%, was significantly smaller in our cohort (6%) compared with that in previous studies, with G1 PanNET of 12%–36% [
14‐
16]. Finally, when commencing STZ CTx, all of our patients were morphologically and/or clinically progressive. Taken together, our cohort illustrates the influence of the guidelines, treating preferably short-term progressive PanNET G2 or G3 with multiple-organ manifestations.
Despite this selection, we observed an ORR in 38% of the patients, which is in line with the results of 34%, 36%, and 42.7% in previous studies [
15‐
17]. In contrast, 24% of our patients progressed during therapy, corresponding to a DCR of 76%, which is lower than previous reports [
14‐
17]. Interestingly, the highest DCR of STZ CTx (92%) was found in a study that had a uniquely high proportion of low proliferative G1 PanNET patients (36%), possibly reflecting the natural course of the disease rather than anti-tumour activity [
14]. With a decreasing proportion of G1 differentiated PanNET, the DCR also decreased [
15,
17]. Even progression status at baseline may play a relevant role. In previous reports focused on STZ CTx, progression status was rarely reported. However, in two prospective, placebo-controlled PanNET studies with sunitinib and everolimus in progressive patients, the DCRs of 72% and 73% match the 76% DCR of our STZ CTx study [
7,
8]. Compared with targeted therapy, our results show a superior ORR with a similar DCR. As shown before, the biochemical response (CgA decrease >30%) was associated with significantly higher ORR. However, while there was a trend toward better response of STZ–CTx without preceding therapy with targeted therapy or PRRT, this difference was not statistically significant as the sample size might lack the power to reliably detect a difference.
The median PFS of 12 months and median OS of 38 months were considerably shorter in our cohort than in recent analyses. Clewemar et al. reported a much higher PFS of 23 months with an OS of 51.9 months in patients treated between 1981 and 2014 [
14]. Dilz et al. showed a time to progression (TTP) of 19.4 months with an OS of 54.8 months in patients treated between 1998 and 2014 [
15]. Krug et al. and Schrader et al. reported similar results; the OS was exceptionally short in the first study, most likely because of the inclusion of bronchial and CUP-NET [
16,
17] (Table
6). PFS and OS in our study reflected the performance of STZ CTx in progressive PanNET patients with multiple-organ manifestations.
Previous lines of therapy, including surgery, had no measurable impact on PFS. However, data on targeted therapy were limited to seven patients and on PRRT to 13 patients. The impact of prior treatments, therefore, needs to be further elucidated in a larger cohort.
Interestingly, we could not find any influence of the Ki-67 marker on PFS, as described in other studies. However, only three of our patients (6% of the cohort) had a WHO G1 tumour; thus, a much smaller proportion of our patients had G1 tumours than that in the other studies (Table
6). Again, this demonstrates the influence of patient selection according to the current guidelines from 2010 onwards. The G1 PanNET patients whose biologically slow course ultimately shows the influence of the Ki-67 marker are only rarely treated with STZ CTx nowadays.
STZ CTx as the first-line therapy was associated with a significantly longer median OS in this study. Thus, beginning the therapy sequence with STZ CTx may result in a more favourable outcome for dynamically growing PanNET. On the other hand, available therapy alternatives may have an impact. Everolimus, sunitinib, PRRT, or temozolomide/capecitabine may be administered in subsequent lines of therapy to our patients. First-line therapy may be associated with the longest survival in this setting. In contrast, no reliable therapy alternatives were available for patients in past decades.
Another key finding in this study is the prognostic importance of bone metastases. In this study, 38% of our patients had bone metastases, which was significantly higher than would have been expected. An analysis of 14,685 GI-NEN patients in the Surveillance, Epidemiology, and End Results database (SEER-9 registry) from 1973 to 2015 showed a bone metastases rate of 5.7% in stage-IV patients [
23]. In contrast, the rate of bone metastases in an analysis by a tertiary referral centre in Germany was 26% in stage-IV patients [
24]. Notably, a significant increase in bone metastases was detected after the introduction of 68Ga-DOTATOC PET/CT. In our study, all patients had received hybrid imaging before beginning STZ CTx and at least once a year thereafter, so the increased incidence of bone metastases in our study may have been due to improved detection corresponding to true incidence. In addition, the selection of patients with a more aggressive course may have also impacted the bone metastasis rate. Overall, the significant influence of bone metastases on the median OS is remarkable; the OS was reduced to nearly half in patients with bone metastases compared with that in patients without bone metastases (24 vs. 46 months) (Fig.
4).
In contrast to previous studies, we detected no effects of primary tumour resection, the Ki-67 index, or the number of metastatic sites on median OS [
14,
15]. Surgical removal of the primary tumour, as a prognostic factor, is prone to selection bias, because patients with a smaller tumour burden, lower grading and better performance status may be more likely to undergo surgery. A higher Ki-67 index and more affected organs, which were previously mentioned as prognostic parameters, are statistically included in our collective as an initial finding and, therefore, are no longer detectable. Interestingly, neither preceding targeted therapy nor PRRT showed a significant correlation with median PFS or OS. It should be noted that the number of patients with targeted therapy prior to STZ CTx was small in our analysis. The ongoing phase-3 SEQTOR study (NCT 02246127), which compares the STZ CTx followed by everolimus upon progression with the reverse sequence, will further elucidate the optimal therapy sequencing.
The safety profile of STZ CTx in our study was consistent with the previous experience in advanced PanNET. The most frequent toxic reactions were of grade 1 or 2 severity and included renal insufficiency, anaemia, fatigue, and nausea; the frequencies were similar to those reported previously [
14‐
16].
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