Introduction

Small cell lung cancer (SCLC) is a highly metastatic and recalcitrant carcinoma, which accounts for approximately 10–13% of all lung cancers [1,2,3]. Platinum-based chemotherapy has been the backbone of SCLC treatment over the past decades. While in addition of extensive-stage SCLC, the programmed death-ligand 1 (PD-L1) inhibitors has demonstrated a sustained overall survival (OS) benefit and currently is considered as the standard first-line treatment [4, 5]. Unfortunately, most patients inevitably develop resistance to these therapies. There is an absence of effective therapies for recurrent and progressive diseases.

Angiogenesis plays an important role in tumor initiation and progression in SCLC, and the therapeutic activity of single-agent vascular endothelial growth factor receptor (VEGFR) tyrosine kinase inhibitors (TKI) by targeting VEGFRs has been shown a disheartening outcomes from previous phase 2 studies in second or later line treatment for SCLC [6,7,8]. Anlotinib, a potent novel oral multi-target anti-angiogenesis receptor tyrosine kinase (RTK) inhibitor, inhibits tumor angiogenesis by inhibiting VEGFRs and PDGFR. It also suppresses tumor cell proliferation by blocking c-Kit, PDGFR, Ret, c-FMS, Aurora-B, and discoidin domain receptor 1 (DDR1) [9]. Several clinical trials demonstrated that anlotinib had robust antitumor efficacy in many types tumor, therefore, it has been approved by the Chinese FDA to treat several types of cancer as a second or later-line option, such as non-small cell lung cancer [10]. advanced or metastatic medullary thyroid carcinoma [11], and refractory metastatic soft-tissue sarcom [12]. In a multinational, randomized phase II study (ALTER 1202), anlotinib monotherapy significantly improved overall response rate (ORR) ( 4.9% vs. 2.6%) compared with placebo in patients with progression SCLC who has been given at least two lines of chemotherapy, and median progression-free survival (PFS) was 4.1 and 0.7 months, respectively [13]. However, whether the combination of anlotinib and chemotherapy can improve outcome in second-line or later setting is unknown.

S-1, a new oral fluoropyrimidine cytotoxic anticancer drug, is a combination of three pharmacological compounds tegafur (prodrug of 5-fluorouracil), gimeracil, and oteracil potassium in a molar ratio of 1:0.4:1. S-1 was approved in Europe for the treatment of patients with non-small cell lung cancer (NSCLC) and gastrointestinal tumors. In a Japanese phase II trial enrolled 26 patients with relapsed SCLC who were treated with S-1 monotherapy, only one patient (3.8%) had an overall response, with a median PFS of 1.1 months [14].

Our previous basic research found that the treatment of combination of antitumor angiogenesis agent anlotinib with chemotherapy drug 5-FU may have synergistic cytotoxicity to SCLC in vitro and in vivo. This treatment modality reduced cell proliferation and migration via Src/AKT pathway [15]. We conducted this open-label, single arm, multicenter, Phase II Trial (SALTER TRIAL, NCT03823118) to assess the efficacy and safety of anlotinib in combination with oral fluoropyrimidine S-1 in patients with refractory or relapsed SCLC.

Study design and methods

Study design and eligibility criteria

SALTER TRIAL was an open-label, multicenter, single arm phase II study designed to evaluate the efficacy and safety of anlotinib in combination with oral fluoropyrimidine S-1 in patients with relapsed or refractory SCLC (ClinicalTrial.gov number, NCT03823118).

Patients eligible for enrollment were required to meet the following criteria: histologically or cytologically confirmed diagnosis of SCLC; patients with platinum-resistant (relapse < 90 days after or during chemotherapy) or platinum-sensitive (relapse ≥ 90 days after chemotherapy) whose disease progression after at least one previous platinum-based chemotherapy regimen, for platinum-sensitive population whose disease progression after at least two chemotherapy regimens. another line chemotherapy is required; aged 18 to75 years; an estimated life expectancy of at least three months; adequate organ function per protocol; at least one measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1 [16]; the Eastern Cooperative Oncology Group performance status (ECOG PS) of 0–2. Patients with limited-stage SCLC in our study conventionally received prophylactic brain irradiation.

Key exclusion criteria included tumour histology that was predominantly NSCLC, mixed small cell, or a combination of both histologies; previous treatment with anti-angiogenesis (VEGF/VEGFR) inhibitor; active symptomatic brain or meningeal metastases (a brain magnetic resonance imaging scan was mandatory); history of malignancy within the last 5 years except for in basal cell carcinoma or situ cervical carcinoma.

This study was approved by the Institutional Review Boards at each participating institution, and was carried out in accordance with Good Clinical Practices and the Declaration of Helsinki [17], and local ethical and legal requirements. All patients provided written informed consent before their participation in the study.

Treatment

Patients were treated with oral anlotinib (12 mg, QD) plus oral fluoropyrimidine S-1 (60 mg, BID). Each cycle was defined as consecutive 2 weeks on-treatment follow by 1 week off-treatment [8]. After six cycles of combined treatment, S-1 single-agent maintenance treatment was continually followed (3-week as a cycle, 2 week on-treatment follow by 1 week off-treatment) unless the occurrence of disease progression, unacceptable toxicity, withdrawal of consent, or study completion.

Endpoints and assessments

The primary endpoint of this study was investigator-assessed ORR. Key secondary endpoints were PFS, OS and treatment-related adverse events (TRAEs). Treatment response was evaluated using a combination of conventional CT scans and MRI, selected to provide a comprehensive assessment, particularly for the detection of brain and liver metastases. Tumor response was assessed by at least two study investigators according to RECIST criteria v1.1 [16].

Investigator-assessed ORR was defined as the proportion of patients achieving a complete response (CR) or partial response (PR). PFS was defined at the time from initiation of therapy to the date of first documented tumor progression according to RECIST criteria v1.1 [16], or death due to any cause. OS was defined as the time between initiation of therapy and the date of death or censoring on the day of the last follow-up visit. Disease control rate (DCR) was defined as the proportion of patients who achieved a best ORRs of CR, PR, or stable disease (SD). TRAEs were assessed and graded according to National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE), version 5.0 [18, 19].

Sample size and statistical analysis

We planned to enroll 37 patients to accept the hypothesis that the true ORR was 50% of with 80% power and to reject the hypothesis that the ORR was < 20%, with a two-sided alpha level of 5%. Considering a 20% drop-off rate, the total number of patients was estimated to be 45.

Descriptive statistics were used for baseline characteristics of included patients, tumor-response data analysis, and summary statistics for TRAEs. PFS and OS were calculated using the Kaplan–Meier method. Statistical analyses were performed using GraphPad Prism software (Version 9, GraphPad Software) and R software (version 4.1.3, R Foundation for Statistical Computing). Two-sided values of p ≤ 0.05 were considered statistically significant.

Results

Patients and baseline characteristics

Between March 2019 to June 2020, a total of 71 patients with relapsed or refractory SCLC were reviewed for eligibility. Among those eligible, 19 patients were excluded (12 patients did not meet the eligibility criteria, 5 patients declined to participate and 2 patients with others), and 4 patients were excluded from the final analysis because of inadequate data (two patients received only one course combination treatment, two patients were not received response assessment). Therefore, a total of 48 patients were included for final analysis in the study, including 44 men (91.70%) and 4 women (8.30%) (Fig. 1). Their median age was 65 (range 37–75) years. Among those 48 patients, 47 (95.8%) were extensive-stage SCLC, and 27 (56.30%) were platinum-resistant (progression during or 90 days after platinum). A total of 29 patients (60.4%) in our study received 4–6 cycles of chemotherapy as part of their first-line treatment. including 21 platinum-sensitive and 8 platinum-resistant patients. The baseline characteristics are shown in Table 1.

Fig. 1
figure 1

CONSORT flow diagram of study enrollment

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Table 1 Patient demographics and baseline characteristics
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Treatment and efficacy

Two-hundred one S-1 and anlotinib combination treatment courses in total were administered, with a median of three courses per patient, and 12 patients received S-1 single-agent maintenance treatment.

At data cut-off April 17, 2021, median follow-up time was 25.1 months. According to the investigator assessment of all treated patients, the ORR was 43.8% (Fig. 2), all responses were PR. The DCR was 89.3%, including the 21 (43.8%) patients had PR, and 22 (45.5%) patients had SD. Antitumor activity by investigator assessment are summarized in Table 2.

Fig. 2
figure 2

Waterfall plot showing the best change in tumor burden from baseline (n = 47). A waterfall plot depicting the best response to the study treatment in patients with refractory or relapsed small-cell lung cancer is presented. The Y-axis represents the percentage of maximum tumor reduction, assessed according to RECIST version 1.1. Out of the 48 study participants, 47 were eligible for inclusion in this analysis, as one patient (ID14) experienced rapid progression during the first-course combination treatment and did not undergo response assessment. The upper and lower dotted lines on the plot represent 20% tumor growth and 30% tumor reduction, respectively, which define progressive disease and partial response. Objective partial responses are denoted in blue (n = 21), stable disease is shown in gray (n = 22), and progression disease is shown in red (n = 4)

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Table 2 Summary of tumor response
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The Kaplan–Meier estimates for PFS and OS are shown in Fig. 3. Investigator-assessed median PFS was 4.5 months (95% CI, 3.5–5.5 months), median OS was 6.3 months (95% CI, 4.6–7.3 months).

Fig. 3
figure 3

Efficacy of S-1 in combination with anlotinib. A Kaplan–Meier estimates progression-free survival. B Kaplan–Meier estimates overall survival

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We performed a subgroup analysis to assess the impact of response to first-line therapy, brain metastasis, liver metastasis, and smoking status on treatment outcomes, focusing on key clinical endpoints including ORR, median PFS, and median OS, as summarized in Tables 3 and 4. Notably, in the liver metastasis subgroup, patients without liver metastases demonstrated a significantly higher ORR compared to those with liver metastases (56.7% vs. 22.2%, p = 0.037). However, despite this difference in ORR, no significant differences in PFS and OS were observed between these two groups. Furthermore, subgroup analyses based on response to first-line therapy, brain metastasis status, and smoking status did not reveal any significant differences in ORR, PFS, or OS.

Table 3 Clinical outcomes by subgroup analysis
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Table 4 Treatment-related adverse events occurring in ≥ 10% of patients
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Additionally, we conducted an analysis to examine whether there is a correlation between dose reduction and treatment response. Our results showed that in the dose-adjustment group, 3 out of 6 patients (50.0%) achieved PR, while 18 out of 42 patients (42.9%) in the full-dose group achieved PR. This analysis indicated that there was no significant difference in response rates between the dose-reduction group and the full-dose group.

Safety

Three patients had a dose reduction of 10 mg per day, one patient had a dose reduction to 8 mg in anlotinib treatment, and one patient had S-1 dose reduction to 40 mg per day due to anemia. Both anlotinib and S-1 dose administration was reduced in 1 (2.0%) patient because of TRAEs.

A safety summary of TRAEs was presented in Table 3 (grade 1–2 TRAEs that reported ≥ 10% of patients and all grade 3–4 TRAEs). At least one TRAE was reported in all 48 patients (100%), the most frequently reported TRAEs of any grade were hematological AEs (62.5%), hypertension (47.9%), skin hyperpigmentation (37.5%), fatigue (35.4%), appetite decrease (35.4%), weight loss (31.2%) and hyperbilirubinaemia (20.8%).

Grade 3 or 4 TRAES were observed in 19 (39.6%) patients, the most common grade 3 or 4 TRAEs were hematologcial (18.8%), hypertension (10.4%), fatigue (8.3%), hand-foot syndrome (4.2%), anorexia (6.3) and blurred vision (4.2%). All TRAEs during trial were controlled after dose modification or symptomatic treatment and no patients had discontinued treatment because of TRAEs. No treatment-related deaths occurred.

Discussion

The SALTER Trial meets its primary endpoint of improved ORR versus histological report standard treatment as second-line or later treatment in patients with SCLC whose disease progression after at least one platinum-based chemotherapy [18, 19]. Our findings showed that the combination of S-1 and anlotinib exhibited robust antitumor activity in ≥ 2L SCLC patients.

This study confirmed that the combination therapy of anlotinib and S-1 demonstrated greater clinical efficacy than either anlotinib or S-1 monotherapy in relapsed or refractory SCLC. Previous clinical trials reported objective response rates of only 3.8% with S-1 monotherapy and 4.9% with anlotinib monotherapy, whereas our study observed a notably higher ORR of 43.8% with combination therapy. This enhanced antitumor activity may be attributed to the synergistic effects of anlotinib and S-1, as combination therapies often produce better outcomes by targeting multiple pathways involved in tumor growth and progression. Our previous research supports this, showing that the combination of anlotinib and 5-FU (the active component of S-1) increases cytotoxicity against SCLC both in vitro and in vivo [15]. Additionally, differences in patient populations between studies may have contributed to the varying outcomes. Factors such as differences in patient characteristics, treatment history, and disease burden could all influence treatment response. The small sample size in our study might also amplify these differences, leading to the promising results observed. In addition, the ORR in our study was notably higher than immune-based treatment strategies and traditional cytotoxic chemotherapy for relapsed SCLC [20,21,22]. For Instance, FDA-approved topotecan, commonly used as second-line therapy, typically yields ORRs below 25% [18, 23, 24], while immune-based treatment strategies, such as nivolumab in the Checkmate 331 trial, showed response rates of just 13.6% [19]. Similarly, lurbinectedin monotherapy reported an ORR of 45% [25]and its combination with irinotecan reached an ORR of 62% [26], single agent amrubicin showed 31% ORRs [23]. PARP inhibitors, such as Olaparib combined with temozolomide, achieved a 41.7% ORR [27], while temozolomide with another PARP inhibitor veliparib showed a 39% ORR [28]. Meanwhile, in a phase 2 trial, PD-1inhibitor camrelizumab combined with apatinib demonstrated an ORR of 34.0% [29]. The most recent phase 2 DeLLphi-301 trial (NCT05060016), Ahn et al. evaluated the antitumor activity and safety of tarlatamab, a bispecific T-cell engager targeting delta-like ligand 3 (DLL3) and CD3, in patients with advanced SCLC who had previously received two or more lines of therapy. The study reported ORR,of 40% in the 10-mg cohort and 32% in the 100-mg cohort [30]. Furthermore, the ORR of the present trial was also higher than reported for other anti-VEGFR-TIKs monotherapy such as sorafenib [6], pazopanib [7], apatinib [8] in relapsed or refractory SCLC. Table 5 summarized the outcomes of anlotinib for relapsed SCLC from selected studies.

Table 5 Outcomes of anlotinib for relapsed SCLC from selected studies
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However, while the combination of S-1 and anlotinib significantly improved ORR, this did not translate into significant PFS or OS benefits in the overall population. Previous studies have shown that OS outcomes in relapsed SCLC are strongly influenced by the time from initial therapy to relapse and the availability of subsequent treatment options [40]. Several factors may explain the lack of significant survival benefits in this study. First, approximately 56.3% of patients had a treatment-free interval of fewer than 90 days at study entry. Second, most patients (95.8%) were diagnosed with extensive-stage disease at baseline. Third, 56.7% of patients had already received third-line therapy. Lastly, there was a high incidence of brain metastasis (27.1%) and liver metastasis (37.5%), both of which are associated with poorer prognoses in SCLC.

Despite the absence of survival benefits in the overall population, S-1 maintenance therapy appeared promising in a subgroup of patients. Among the 12 patients who received maintenance treatment, 3 achieved long-term disease control, with 2 remaining alive for more than 24 months. The median OS in the maintenance therapy group was nearly double that of the non-maintenance group (10.1 months vs. 5.1 months, HR = 0.390, p = 0.003). These results suggest that S-1 maintenance therapy may provide durable benefits for a subset of patients, highlighting the need for further investigation in future studies.

In the subgroup analysis, ORR was similar between platinum-sensitive and platinum-resistant groups (47.6% vs. 40.7%), which contrasts with previous studies reporting inferior ORR in platinum-resistant patients. A systematic analysis included 21 clinical studies reported ORRs of 27.7% in the platinum-sensitive group and 14.8% in the platinum-resistant group [40]. Treatment with the multi-kinase inhibitor sorafenib resulted in an ORR of 11% in the platinum-sensitive and 2% in the platinum-refractory stratum [6]. In our study, the median PFS (4.7 m vs. 4.3 m, HR = 1.09, p > 0.05) and OS (6.6 m vs. 5.9 m, HR = 0.50. p > 0.05) were comparable between the platinum-sensitive and platinum-resistant groups in the current study. The sensitive disease derived more clinical benefit from subsequent system treatment than resistant/refractory disease both in terms of PFS and OS. A system analysis showed the median OS was 7.73 months for sensitive SCLC and 5.45 months for refractory disease [40]. Specifically, in the present study, the median number of treatment lines was 2 for platinum-resistant patients and 3 for platinum-sensitive patients. Furthermore, our study revealed a significantly higher ORR in patients without liver metastases compared to those without liver metastases (56.7% vs. 22.2%, p = 0.037). This finding is consistent with previously, where the presence of liver metastases has been associated with poorer outcomes in SCLC. Liver metastases are often considered a marker of more aggressive disease and are linked to a higher tumor burden, which may contribute to the reduced efficacy of systemic therapies.

The toxicity profile was consistent with previously reported findings for monotherapies or combination therapies involving the component agents [7, 14, 41]. In our study, any grade TRAEs were observed in 100% of patients, with grade 3 or 4 TRAEs occurring in 39.6%. These rates appear slightly higher than those previously reported for either S-1 or anlotinib monotherapy. A Japanese phase II study of S-1 monotherapy for relapsed SCLC reported grade 3 or 4 AEs in 7.7% of patients [14], while another trial of anlotinib monotherapy reported grade 3 or 4 TRAEs in 35.8% of patients [7]. Furthermore, in our study, the combination of anlotinib and S-1 resulted in higher frequency of grade 3 or 4 TRAEs compared to prior reports in EGFR-mutant NSCLC populations treated with this combination [42]. Nonetheless, the toxicity profile observed in this combination treatment was manageable and acceptable within the context of the therapeutic benefit, supporting the overall safety of the treatment regimen for further clinical investigation.

There are several limitations to our study. First, the single-arm design lacked a control group for comparison, and the enrolled patient population was heterogeneous, which may have impacted the consistency of the findings. Additionally, the open-label nature of the trial introduces potential bias in both outcome reporting and assessment, as both investigators and patients were aware of the treatment being administered. Moreover, fresh tumor specimens were not collected before treatment, preventing biomarker analyses that could have offered deeper insights into the mechanisms underlying treatment response and resistance. Future studies should aim to incorporate a randomized control group and conduct biomarker analyses to better define the therapeutic potential of this regimen.

Conclusions

The combination of anlotinib and oral fluoropyrimidine S-1 showed robust antitumor activity in terms of ORR, with a manageable and acceptable safety profile as a second- or later-line treatment for relapsed or refractory SCLC. These results suggest that this regimen could be a viable therapeutic option for this patient population in second-line or beyond treatments.