Efficacy and safety of fruquintinib combined with immunotherapy in later‑line treatment of metastatic colorectal cancer

Introduction

Colorectal cancer (CRC) is one of the most common malignancies. According to the 2022 cancer report in China, CRC remains the second most prevalent cancer and ranks fourth in cancer-associated mortalities (1). Despite advances in diagnostic technologies and the ongoing development of therapeutic strategies, including chemotherapy and targeted therapy, the prognosis for metastatic CRC (mCRC) remains poor with a 5-year survival rate of <15% (2). Due to the limited options for later-line treatment and the deterioration of the physical condition of patients following multiple treatment lines, identifying effective, low-toxicity therapy for subsequent lines remains a major focus of current research. At present, the standard later-line treatments recommended by the Chinese Society of Clinical Oncology Diagnosis and Treatment Guidelines include fruquintinib, regorafenib and TAS-102 (3).

Immune checkpoint inhibitors (ICIs), such as programmed death-1 (PD-1) inhibitors, have recently transformed the landscape of clinical oncology. The KEYNOTE-016 study demonstrated that pembrolizumab provides durable antitumor activity and fewer treatment-related adverse events (AEs) supporting it as an efficacious first-line therapy in patients with microsatellite instability-high (MSI-H) or mismatch repair-deficient mCRC (4). However, ~95% of patients with mCRC are classified as microsatellite-stable (MSS)/mismatch repair proficient (pMMR), characterized by a tumor microenvironment with limited immune cell infiltration, which results in suboptimal efficacy of immunotherapy monotherapy (5–8). Therefore, improving survival outcomes in the MSS/pMMR population remains a notable challenge in the management of mCRC.

At present, the primary challenge in treatment is how to modify therapeutic strategies to augment the sensitivity of patients with MSS-type mCRC to immunotherapy, thereby improving prognosis. Emerging evidence demonstrates that the combination of vascular endothelial growth factor receptor (VEGFR) inhibitors with ICIs has a synergistic antitumor effect (9). Fruquintinib, a highly selective tyrosine kinase inhibitor (TKI) developed independently in China, specifically targets VEGFR-1, VEGFR-2 and VEGFR-3, effectively suppressing tumor angiogenesis (10). The global, multicenter FRESCO-2 trial demonstrated that fruquintinib significantly improved outcomes compared with the placebo, with a median progression-free survival (PFS) of 3.7 months and a median overall survival (OS) of 7.4 months (11). Based on these findings, fruquintinib was approved by the US. Food and Drug Administration (FDA) for the treatment of recurrent mCRC on November 8, 2023 (12).

Moreover, the combination of fruquintinib and ICIs has shown promising therapeutic potential for CRC in both preclinical and clinical studies. Li et al (13) demonstrated that the combination of fruquintinib and sintilimab more effectively inhibited the growth of colorectal tumors and significantly extended survival compared with monotherapy with either agent in a mouse xenograft model. Mechanistically, this combination therapy targeted tumor angiogenesis and modulated the tumor immune microenvironment, enhancing T-cell infiltration and thereby suppressing tumor progression. The findings of a study reported by Li et al (13) suggest a synergistic effect between TKI and PD-1 inhibitors, providing a solid theoretical foundation for the treatment of CRC. Clinically, Guo et al (14) reported a median OS of 20.0 months and a median PFS of 6.9 months in patients with MSS/pMMR mCRC receiving fruquintinib plus sintilimab, significantly extending OS. Further evidence from a propensity score-matched retrospective study presented at the 2024 American Society of Clinical Oncology Gastrointestinal Cancer Symposium (15) confirmed that fruquintinib plus a PD-1 inhibitor significantly improved PFS in patients with MSS/pMMR mCRC. Similarly, several small-scale retrospective studies have demonstrated that combining fruquintinib with other ICIs yields superior efficacy compared with fruquintinib monotherapy in patients with MSS/pMMR mCRC, with manageable AEs, indicating promising clinical potential (16,17).

However, existing studies have notable limitations. Most trials focus on the combined effects of a single PD-1 inhibitor and fruquintinib. There is a lack of direct comparison between fruquintinib combined with different ICIs and an introduction to the side effects of the combination regimens. The present study systematically analyzed the efficacy and safety of different ICIs combined with fruquintinib in patients with MSS/pMMR mCRC. In contrast to the traditional fixed-dose model, the present study reviewed the drug dose intensity, administration time and sequence in the real world, which provided specific guidance for the optimization of the future administration regimen of immunotherapy combined with fruquintinib. At the same time, the present study also explored a more efficient and low-toxicity combination therapy model and further precisely selected the beneficiary population. The present study supports important clinical needs and scientific value.

Materials and methods

Study design and patients

The present retrospective study analyzed the medical data of patients with MSS/pMMR mCRC between January 2022 and December 2023 at the Jiangsu Cancer Hospital (Nanjing, China). Patients who were aged 18–75 years with histopathologically or cytologically confirmed CRC and MSS/pMMR were eligible for the study. Patients were required to have failed at least two prior lines of treatment (all patients had received standard first- and second-line chemotherapy) and to have received fruquintinib plus PD-1 inhibitors (PD-1 inhibitor combination group) or programmed death-ligand 1 (PD-L1) inhibitors (PD-L1 inhibitor combination group). Patients also had to have radiologically confirmed metastases with at least one radiological-target lesion, a life expectancy of ≥3 months and an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 (18). Patients were excluded if they had received a live vaccine within 28 days before treatment, were allergic to fruquintinib or ICIs, had active hepatitis B or C, HIV infection, active tuberculosis, had a history of active autoimmune disease, had severe liver or kidney dysfunction or other serious underlying conditions, and had a history of or concurrent untreated malignancy.

The present study was conducted in compliance with Good Clinical Practice guidelines and the Declaration of Helsinki (2013 version) and approved by the ethics committee of the Jiangsu Cancer Hospital (grant no. KY-2024-071). The requirement for informed consent was waived because of the retrospective nature of the present study.

Treatments

Regarding the initial dose of fruquintinib, the fruquintinib dose in the FRESCO-2 study (11) was used, which is 5 mg daily q28 with a 1-week break from days 22–28. However, most patients cannot tolerate a 5 mg daily dose. Therefore, all patients received oral fruquintinib once daily, with initial doses of 3, 4 or 5 mg. Treatment continued for 2 weeks, followed by a 1-week break, with each cycle lasting 21 days. The actual initial dose was mainly selected by clinicians after a full assessment in combination with the physical condition, body surface area, previous chemotherapy response, and liver and kidney functions of the patient. Regarding the issue of drug administration time, a Ib/II study conducted by Guo et al (14) was referenced; compared with the treatment group taking 3 mg fruquintinib daily, the group taking 5 mg/day fruquintinib for 2 weeks and then having a 1-week break showed a marked improvement in efficacy. The present treatment continued for 2 weeks, followed by a 1-week break, with each cycle lasting 21 days. If the patient experienced intolerance, the dose could be adjusted and treatment discontinued if intolerance persisted.

In the PD-1 inhibitor combination group, the PD-1 inhibitors camrelizumab (200 mg), sintilimab (200 mg), tislelizumab (200 mg), toripalimab (240 mg) or serplulimab (300 mg) were chosen by the physician based on the condition of the patient, with intravenous infusion on day 1 of each 21-day cycle. In the PD-L1 inhibitor combination group, envafolimab (200 mg) was administered by subcutaneous injection on days 1 and 15 of each 28-day cycle. All ICIs were continued until disease progression or severe intolerance.

Outcomes and assessment

Tumor responses were evaluated every 2 cycles through computed tomography (CT) in accordance with the Response Evaluation Criteria in Solid Tumors guidelines (version 1.1) (19). The tumor responses monitored included complete response (CR), partial response (PR), stable disease (SD) and progressive disease (PD). Efficacy outcomes included objective response rate (ORR), disease control rate (DCR) and PFS. The ORR was defined as the proportion of patients with CR or PR. The DCR was the proportion of patients who achieved CR, PR or SD. The DCR time node in the present study was 3 months. The PFS was calculated as the time from treatment initiation to first disease progression or death from any cause. All AEs occurring during the treatment in both groups were collected and evaluated according to the Common Terminology Criteria for Adverse Events version 5.0 (20).

Statistical analysis

All statistical analyses were conducted using SPSS version 22.0 (IBM Corp.) Baseline categorical variables were compared using the χ2 test. Survival curves were generated using the Kaplan-Meier method, with group differences assessed by the log-rank test. Univariate and multivariate analyses were performed using the Cox proportional hazards model to identify independent prognostic factors. P<0.05 was considered to indicate a statistically significant difference.

Results

Patient characteristics

A total of 78 patients with mCRC were included, with 47 (60.3%) males and 31 (39.7%) females and a median age of 57 years. Subsequently, 63 patients received fruquintinib plus a PD-1 inhibitor (20 sintilimab, 13 tislelizumab, 11 camrelizumab, 9 toripalimab or 10 serplulimab) and 15 received fruquintinib plus a PD-L1 inhibitor (envafolimab). In the PD-1 inhibitor combination group, 10, 23 and 30 patients received initial doses of fruquintinib at 3, 4 and 5 mg, respectively, while in the PD-L1 inhibitor combination group, two, 9 and four patients received initial doses of 3, 4 and 5 mg, respectively. There were no significant differences in the baseline characteristics between the two groups (P>0.05; Table I).

Table I. Baseline characteristics of the included patients.

Table I.

Baseline characteristics of the included patients.

Characteristic	PD-1 inhibitor combination group (n=63)	PD-L1 inhibitor combination group (n=15)	χ2	P-value
Sex			1.43	0.231
Male	40 (63.5)	7 (46.7)
Female	23 (36.5)	8 (53.3)
Age			2.03	0.154
<60	38 (60.3)	6 (40.0)
≥60	25 (39.7)	9 (60.0)
ECOG performance status			0.37	0.542
0	24 (38.1)	7 (46.7)
1	39 (61.9)	8 (53.3)
Degree of differentiation			4.44	0.109
High	2 (3.2)	3 (20.0)
Middle	44 (69.8)	9 (60.0)
Low	17 (27.0)	3 (20.0)
Tumor location			0.32	0.571
Left	49 (77.8)	10 (66.7)
Right	14 (22.2)	5 (33.3)
Number of metastases			0.03	0.872
<3	50 (79.4)	11 (73.3)
≥3	13 (20.6)	4 (26.7)
Presence of liver metastases	37 (58.7)	8 (53.3)	0.15	0.704
Presence of lung metastases	31 (50.8)	5 (66.7)	1.23	0.268
Number of treatment lines			0.99	0.319
>3	19 (30.2)	2 (13.3)
3	44 (69.8)	13 (86.7)
Baseline carcinoembryonic antigen			0.56	0.452
<5	32 (50.8)	6 (40.0)
≥5	31 (49.2)	9 (60.0)
BRAF status			0.53	0.872
Wild-type	43 (68.3)	10 (66.7)
Mutated	3 (4.8)	0 (0.0)
Unknown	17 (27.0)	5 (33.3)
KRAS status			2.64	0.267
Wild-type	21 (33.3)	2 (13.3)
Mutated	25 (39.7)	8 (53.3)
Unknown	17 (27.0)	5 (33.3)
Prior antitumor treatment
TKI therapy	13 (20.6)	3 (20.0)	0.003	0.956
Radiotherapy	20 (31.7)	8 (53.3)	2.45	0.117
Surgery	51 (81.0)	14 (93.3)	0.59	0.441
Initial dose of fruquintinib			2.90	0.234
3 mg	10 (15.9)	2 (13.3)
4 mg	23 (36.5)	9 (60.0)
5 mg	30 (47.6)	4 (26.7)

[i] Data are expressed as n (%). ECOG, Eastern Cooperative Oncology Group; PD-1, programmed death-1 inhibitors; PD-L1, programmed death-ligand 1 inhibitors; TKI, tyrosine kinase inhibitors; BRAF, B-Raf proto-oncogene, serine/threonine kinase; KRAS, KRAS proto-oncogene, GTPase.

Efficacy

In the PD-1 inhibitor combination group, 5 (7.9%) patients had a PR and 42 (66.7%) patients had an SD, achieving an ORR of 7.9% and a DCR of 74.6% (Table II). In comparison, the PD-L1 inhibitor combination group included 3 patients (20.0%) with PR and 6 patients (40.0%) with SD, yielding an ORR of 20.0% and a DCR of 60.0% (Table II). No statistically significant differences were observed between the PD-1 and PD-L1 inhibitor combination group in ORR (7.9 vs. 20.0%; P=0.363) or DCR (74.6 vs. 60%; P=0.418). Moreover, there was no significant difference in median PFS between the two groups (7.0 vs. 8.3 months; P=0.14; Fig. 1).

Figure 1. Kaplan-Meier survival curves of progression-free survival for the PD-1 and PD-L1 inhibitor combination groups. PFS, progression-free survival; PD-1, programmed death-1; PD-L1, programmed death-ligand 1.

Table II. Efficacy between the PD-1 and PD-L1 inhibitor combination groups.

Table II.

Efficacy between the PD-1 and PD-L1 inhibitor combination groups.

Response	PD-1 inhibitor combination group (n=63)	PD-L1 inhibitor combination group (n=15)	χ2	P-value
Objective response
CR	0 (0.0)	0 (0.0)	Not applicable	>0.999
PR	5 (7.9)	3 (20.0)	0.83	0.363
SD	42 (66.7)	6 (40.0)	3.64	0.056
PD	16 (25.4)	6 (40.0)	1.28	0.259
ORR	5 (7.9)	3 (20.0)	0.83	0.363
DCR	47 (74.6)	9 (60.0)	0.66	0.418

[i] Data are expressed as n (%). CR, complete response; PR, partial response; SD, stable disease; PD, progressive disease; ORR, objective response rate; DCR, disease control rate; PD-1, programmed death-1 inhibitors; PD-L1, programmed death-ligand 1 inhibitors.

In the PD-1 inhibitor combination group, five PD-1 inhibitors were administered. Subgroup analysis showed no statistically significant differences in PFS between patients treated with sintilimab, tislelizumab, camrelizumab, toripalimab or serplulimab (P=0.37; Fig. 2). Univariate analysis showed that age, ECOG performance status and liver metastases were associated with PFS (Table III). Further multivariate analysis using a COX regression model revealed that an ECOG performance status of 1 was an independent factor of poor prognosis for PFS in patients with mCRC treated with fruquintinib plus ICIs (Table III).

Figure 2. Kaplan-Meier survival curves of progression-free survival for fruquintinib plus different PD-1 inhibitors; PD-1, programmed death-1 inhibitors; PFS, progression-free survival.

Table III. Univariate and multivariate analysis for progression-free survival.

Table III.

Univariate and multivariate analysis for progression-free survival.

	Univariate analysis			Multivariate analysis
Characteristic	HR	95% CI	P-value	HR	95% CI	P-value
Sex (male vs. female)	0.97	0.58–1.63	0.912
Age (<60 vs. ≥60)	0.54	0.32–0.92	0.022	0.69	0.40–1.19	0.180
ECOG performance status (0 vs. 1)	0.25	0.14–0.46	<0.001	0.28	0.15–0.52	<0.001
Degree of differentiation
Low vs. high	1.36	0.77–2.39	0.293
Low vs. middle	1.21	0.58–3.20	0.703
High vs. middle	1.28	0.48–3.38	0.620
Tumor location (right vs. left)	1.14	0.64–2.04	0.652
Number of metastases (<3 vs. ≥3)	1.18	0.63–2.23	0.603
Liver metastases (no vs. yes)	0.55	0.33–0.93	0.027	0.72	0.42–1.23	0.226
Lung metastases (no vs. yes)	1.28	0.77–2.13	0.349
Number of treatment lines (3 vs. >3)	1.45	0.81–2.58	0.207
Baseline carcinoembryonic antigen (<5 vs. ≥5)	0.73	0.43–1.22	0.227
BRAF status
Unknown vs. mutated	1.00	0.55–1.80	0.989
Unknown vs. wild-type	1.43	0.34–5.97	0.622
Mutated vs. wild-type	1.46	0.35–6.09	0.608
KRAS status
Unknown vs. mutated	0.86	0.43–1.71	0.670
Unknown vs. wild-type	0.81	0.44–1.48	0.491
Mutated vs. wild-type	0.44	0.43–1.44	0.788
Prior TKI drugs therapy (no vs. yes)	0.74	0.41–1.33	0.309
Prior radiotherapy (no vs. yes)	1.55	0.91–2.65	0.109
Prior surgery (no vs. yes)	1.64	0.86–3.13	0.133
Initial dose of fruquintinib
3 mg vs. 4 mg	0.73	0.33–1.61	0.430
3 mg vs. 5 mg	1.50	0.87–2.59	0.145
4 mg vs. 5 mg	1.53	0.89–2.63	0.127
ICIs (PD-1 inhibitors vs. PD-L1 inhibitors)	1.71	0.82–3.54	0.151

[i] ECOG, Eastern Cooperative Oncology Group; ICIs, Immune checkpoint inhibitors; HR, hazard ratio; PD-1, programmed death-1 inhibitors; PD-L1, programmed death-ligand 1 inhibitors; TKI, tyrosine kinase inhibitors; CI, confidence interval; BRAF, B-Raf proto-oncogene, serine/threonine kinase; KRAS, KRAS proto-oncogene, GTPase.

Safety

The AEs during treatment in both groups were predominantly grade 1–2 according to the Common Terminology Criteria for Adverse Events version 5.0 (20), with the most common being hand-foot syndrome (PD-1 inhibitor combination group vs. PD-L1 inhibitor combination group, 20.6 vs. 33.3%), proteinuria (23.8 vs. 33.3%) and hypothyroidism (31.7 vs. 26.7%). Notably, grade 3 or worse AEs were infrequent. Throughout the treatment, no patients in either group died due to serious AEs, such as life-threatening, permanent or severe disability or loss of function.

The treatment was discontinued in 2 patients in the PD-1 inhibitor combination group due to immune-related diabetes and immune-related hepatic injury, respectively. In the PD-L1 inhibitor combination group, no patients discontinued treatment for AEs. Additionally, the incidence of AEs did not differ significantly between the PD-1 and PD-L1 inhibitor combination group (P>0.05, Table IV) and the incidence of AEs did not differ significantly between PD-1 inhibitor combination groups (P>0.05, Table V).

Table IV. Adverse events.

Table IV.

Adverse events.

	Any grade				Grade ≥3
Adverse event	PD-1 inhibitor combination group (n=63)	PD-L1 inhibitor combination group (n=15)	χ2	P-value	PD-1 inhibitor combination group (n=63)	PD-L1 inhibitor combination group (n=15)	χ2	P-value
Hypertension	10 (15.9)	3 (20.0)	NA	>0.999
Hand-foot syndrome	13 (20.6)	5 (33.3)	0.50	0.479	4 (6.3)	3 (20.0)	1.35	0.246
Diarrhea	4 (6.3)	0 (0.0)	0.12	0.726
Fatigue	7 (11.1)	4 (26.7)	1.31	0.253
Decreased appetite	3 (4.8)	3 (20.0)	2.11	0.147
Nausea/vomiting	1 (1.6)	1 (6.7)	0.04	0.834
Leukopenia	7 (11.1)	2 (13.3)	NA	>0.999
Thrombocytopenia	5 (7.9)	1 (6.7)	NA	>0.999	2 (3.2)		NA	>0.999
Hepatic injury	1 (1.6)	1 (6.7)	0.04	0.834	1 (1.6)		NA	>0.999
Proteinuria	15 (23.8)	5 (33.3)	0.19	0.667	1 (1.6)	1 (6.7)	0.04	0.834
Hyperthyroidism	3 (4.8)	0 (0.0)	0.01	0.909
Hypothyroidism	20 (31.7)	4 (26.7)	0.01	0.943
Mucositis oral	3 (4.8)	0 (0.0)	0.01	0.909
Rash	3 (4.8)	1 (6.7)	NA	>0.999
Hoarseness	1 (1.6)	0 (0.0)	NA	>0.999
Muscle tenderness	2 (3.2)	0 (0.0)	NA	>0.999
Hyperglycemia	2 (3.2)	0 (0.0)	NA	>0.999	1 (1.6)		0.43	0.512
Infection	1 (1.6)	0 (0.0)	NA	>0.999

[i] Data are expressed as n (%). PD-1, programmed death-1 inhibitors; PD-L1, programmed death-ligand 1; NA, not applicable.

Table V. PD-1 inhibitor combination group adverse events (any grade).

Table V.

PD-1 inhibitor combination group adverse events (any grade).

Adverse event	Camrelizumab (n=11)	Sintilimab (n=10)	Tislelizumab (n=9)	Toripalimab (n=13)	Serplulimab (n=20)	χ2	P-value
Hypertension	4 (36.4)	2 (20)	2 (22.2)	1 (7.7)	1 (5)	6.177	0.186
Hand-foot syndrome	4 (36.4)	3 (30)	1 (11.1)	0 (0.0)	5 (25)	8.734	0.068
Diarrhea	0 (0.0)	1 (10)	0 (0.0)	2 (15.4)	1 (5)	4.191	0.381
Fatigue	1 (9.1)	1 (10)	0 (0.0)	1 (7.7)	4 (20)	3.681	0.451
Decreased appetite	0 (0.0)	0 (0)	1 (11.1)	1 (7.7)	1 (5)	2.851	0.583
Nausea/vomiting	0 (0.0)	0 (0)	0 (0.0)	1 (7.7)	0 (0)	3.991	0.407
Leukopenia	0 (0.0)	2 (20)	2 (22.2)	2 (15.4)	1 (5)	5.307	0.257
Thrombocytopenia	0 (0.0)	0 (0)	1 (11.1)	3 (23.1)	1 (5)	6.664	0.155
Hepatic injury	0 (0.0)	0 (0)	1 (11.1)	0 (0.0)	0 (0)	3.991	0.407
Proteinuria	5 (45.5)	3 (30)	0 (0.0)	2 (15.4)	5 (25)	8.190	0.085
Hyperthyroidism	0 (0.0)	1 (10)	0 (0.0)	0 (0.0)	2 (10)	4.617	0.329
Hypothyroidism	2 (18.2)	4 (40)	2 (22.2)	2 (15.4)	10 (50)	6.428	0.169
Mucositis oral	0 (0.0)	1 (10)	1 (11.1)	0 (0.0)	1 (5)	3.401	0.493
Rash	1 (9.1)	0 (0)	1 (11.1)	1 (7.7)	0 (0)	4.090	0.394
Hoarseness	0 (0.0)	0 (0)	0 (0.0)	1 (7.7)	0 (0)	3.219	0.522
Muscle tenderness	0 (0.0)	1 (10)	0 (0.0)	0 (0.0)	1 (5)	3.294	0.510
Hyperglycemia	0 (0.0)	0 (0)	0 (0.0)	1 (7.7)	1 (5)	2.744	0.601
Infection	0 (0.0)	0 (0)	0 (0.0)	1 (7.7)	0 (0)	3.219	0.522

[i] Data are expressed as n (%). PD-1, programmed death-1.

The present findings are preliminary and recommend conducting well-designed prospective cohort studies or multi-center collaborations in the future, including a larger sample size, to validate the present findings and draw more conclusive conclusions.

Discussion

Immunotherapy has become a research hotspot in the treatment of mCRC, with the combination of ICIs and TKIs being widely endorsed and established as the mainstream treatment strategy in later-line therapies. The REGONIVO trial reported an ORR of >30% in patients with MSS-type mCRC who received regorafenib plus nivolumab therapy, demonstrating favorable efficacy along with favorable safety (21). Although studies have shown that the combination of TKI and ICIs is more effective than TKI monotherapy, to the best of our knowledge, no studies have yet compared the efficacy and safety of TKIs plus different ICIs. The present study retrospectively assessed the efficacy and safety of fruquintinib plus different ICIs in MSS/pMMR mCRC. The present study aimed to identify the optimal combination, understand the factors influencing outcomes and determine the patient populations most likely to benefit from this therapy. The results showed no statistically significant differences in ORR (7.9 vs. 20.0%; P=0.363), DCR (74.6 vs. 60.0%; P=0.418) and median PFS (7.0 vs. 8.3 months; P=0.14) between the PD-1 inhibitor and PD-L1 inhibitor combination groups. When comparing the ORR and DCR between the two groups, the DCR was found to be more promising in the PD-1 group, while the ORR was improved in the PD-L1 group, although neither difference was statistically significant.

The inconsistency between the DCR and ORR is a common and well-elucidated phenomenon (22–24). The most direct source of the inconsistency between the values is the patients who only achieved SD. These patients are included in the DCR but not in the ORR. In terms of the mechanism of action, it takes time to activate the immune system. Thus, the ORR may not be high at early evaluation, but the proportion of patients achieving SD may be relatively high (with a higher DCR). In addition, tumors exhibit heterogeneity and have drug resistance. At present, the drug-resistance mechanisms of targeted immunotherapy combinations are not fully understood. From the perspective of the patient population, patients with a large tumor burden may find it more difficult to achieve a deep response (CR/PR). The resistance to treatment may increase with increased previous treatment lines and the drugs are more likely to control the tumor rather than significantly shrink it (high proportion of SD). The difference in the number of cases between the PD-1 and PD-L1 inhibitor combination group) may also lead to the inconsistency of ORR and DCR, which is also the limitation of this study.. In the future, the number of the population will be further expanded, the follow-up time will be extended and the data will be optimized to verify and obtain more accurate results.

The findings of the present study were consistent with another retrospective study reported by Yang et al (16), which also analyzed the efficacy and safety of fruquintinib + PD-1 inhibitors in mCRC, reporting an ORR of 11.4%, a DCR of 84.3% and a median PFS of 5.5 months. The PD-1 inhibitors in the present retrospective study included sintilimab, tislelizumab, toripalimab and pembrolizumab. However, due to significant differences in the sample sizes between groups, subgroup analyses were not performed. A study by Bai et al (25) evaluated the fruquintinib + geptanolimab for the treatment of CRC. The results indicated that, in all evaluable patients with mCRC, the overall ORR was 26.7%, with an ORR of 33.3% in the recommended phase II dose (RP2D) group, a DCR of 80.0% and a median PFS of 7.3 months. For patients with MSS-type mCRC, the ORR was 25.0%, the DCR was 75% and the median PFS was 5.5 months. The findings of the present study suggested that fruquintinib plus ICI exhibited manageable safety and promising antitumor activity in patients with mCRC. In another retrospective study reported by Gou et al (17), fruquintinib plus PD-1 inhibitors in 45 patients with MSS-type mCRC resulted in a DCR of 62.2%, ORR of 11.1% and median PFS of 3.8 months. The limited efficacy observed might be attributed to the lack of strict ECOG performance status criteria for inclusion criteria. Additionally, considering patient tolerance, the dosing of fruquintinib might be lower in real-world settings, which could also impact efficacy. The present study evaluated the PFS of patients treated with fruquintinib plus different PD-1 inhibitors and observed no statistically significant differences between the different PD-1 inhibitors. As a result, the most effective ICIs could not be identified.

The CheckMate-142 (26) and CheckMate 8HW (27) studies both confirmed the favorable efficacy of PD-1 inhibitors plus cytotoxic T-lymphocyte protein 4 (CTLA-4) inhibitors in MSI-H CRC. Similarly, studies are presently exploring the efficacy of dual ICI therapy in MSS-type mCRC. In a phase I clinical trial presented at the 2022 ESMO-GI conference (28), the combination of the botensilimab (a CTLA-4 inhibitor) and balstilimab (a PD-1 inhibitor) showed strong clinical activity and sustained efficacy in patients with MSS mCRC who had previously failed multiple lines of treatment. Importantly, the treatment was well-tolerated with no serious AEs, indicating the broad potential of dual immunotherapy in the management of mCRC. However, the present study did not include patients receiving the combination of fruquintinib and dual immunotherapy due to economic limitations, as the high cost of dual immunotherapy might have been unaffordable for some patients. By contrast, PD-1 or PD-L1 inhibitors are more cost-effective and therefore more accessible to patients, while still providing significant therapeutic benefits.

In the present study, univariate analysis indicated that age, ECOG performance status and liver metastasis were associated with PFS. Multivariate Cox regression analysis identified an ECOG performance status of 1 as an independent risk factor for PFS. Immunotherapy exerts its effect by activating immune cells that specifically recognize and eliminate tumor cells, requiring a sufficient immune cell reserve. Patients with an ECOG performance status of 0 generally have improved physical conditions and a stronger immune response. In the CheckMate 153 trial (29), patients with an ECOG performance status of 0–1 derived significantly more benefit from immunotherapy compared with those with a score of 2. Similarly, the KEYNOTE-177 trial (30) observed that PD-1 inhibitors did not significantly improve PFS in patients with an ECOG performance status of 1. Additionally, Fakih et al (31) found that liver metastasis is a poor prognostic factor. However, other studies have shown no association between KRAS mutations, lung metastasis, liver metastasis and PFS or OS, warranting further validation in larger clinical trials (32,33).

The most common AEs previously reported with fruquintinib plus ICIs were hypertension, hand-foot syndrome and hypothyroidism. In the present study, the most common AEs in the PD-1 inhibitor combination group were hypothyroidism (31.7%), proteinuria (23.8%) and hand-foot syndrome (20.6%). In the PD-L1 inhibitor combination group, the most common AEs were proteinuria (33.3%), hand-foot syndrome (33.3%) and hypothyroidism (26.7%). No deaths due to serious AEs were reported in either group.

The present study has several limitations. Firstly, it is a single-center, retrospective analysis, which might introduce selection bias. Secondly, the administration of five different PD-1 inhibitors and three initial doses of fruquintinib in the PD-1 inhibitor combination group might have affected treatment consistency. Next, the sample size was relatively small and the follow-up period was relatively short. The reason why the sample size, especially the PD-L1 inhibitor combination group, is small is that fruquintinib is only reimbursed by Chinese medical insurance for third-line colorectal cancer treatment, with no coverage beyond this line. Immunotherapy drugs are not covered for colorectal cancer at all. Consequently, few patients can afford the combination of immunotherapy and fruquintinib. Affordability is even lower for combinations involving PD-L1 inhibitors due to their higher cost. Additionally, the present study has limitations in safety assessment, as the small sample size limits the ability to detect rare toxicities and the relatively short follow-up time affects the assessment of long-term toxicities. In the future, large-scale real-world research should be carried out to confirm safety, explore biomarkers for toxicity prediction and optimize adverse reaction management strategies.

Although there was no statistically significant difference in efficacy between the groups of fruquintinib combined with PD-1 inhibitor and fruquintinib combined with PD-L1 inhibitor, it should be noted that the statistical power of this result was low and it should be regarded as a preliminary finding that needs to be verified in future larger-scale studies. Similarly, for the present results where no significant difference was found, the possibility of clinically relevant differences should not be ruled out as the lack of significance may be due to insufficient sample size. Finally, one of the other major limitations of the present study is the lack of mature OS data. This is mainly because the follow-up time at the data cut-off was relatively short and some patients were lost to follow-up. Therefore, it is currently impossible to evaluate the long-term survival benefits of the present study based on OS. Against the backdrop of a high risk of loss to follow-up, the present study considered earlier-occurring endpoints with more reliable data acquisition (such as PFS or ORR) as the primary endpoints. The data collection was relatively complete (loss to follow-up occurred after the PFS event), so the analysis results were reliable and clinically meaningful. Future studies should pre-set a more rigorous analysis plan for OS in the case of a high loss-to-follow-up rate in the protocol. In future studies, we will be committed to improving the survival follow-up strategy and investing more resources in designing and implementing a more powerful survival follow-up strategy.

In conclusion, although no statistically significant differences in efficacy were observed between the PD-1 inhibitor combination group and the PD-L1 inhibitor combination group, fruquintinib plus ICIs improved survival compared with the previously reported efficacy of fruquintinib monotherapy in mCRC. Furthermore, this combination therapy did not increase serious AEs, indicating an acceptable safety profile. Future clinical trials with larger sample sizes are needed to confirm the findings of the present study and explore the potential of immunotherapy plus targeted therapy to establish new treatment strategies for mCRC in clinical practice.

Acknowledgements

Not applicable.

Funding

The present study was funded by the Jiangsu Provincial Cancer Hospital 2023 Hospital Science and Technology Development Fund (grant no. ZL202308).

Availability of data and materials

The data generated in the present study may be requested from the corresponding author.

Author's contributions

YP was responsible for the curation of data, formal analysis, investigation, validation, visualization, writing the original draft and reviewing and editing of the manuscript. SL analyzed data and edited the manuscript. LZ was responsible for the conceptualization and supervision of the present study. All authors read and approved the final version of the manuscript. YP, SL and LZ confirm the authenticity of all the raw data.

Ethics approval and consent to participate

The study was conducted in compliance with Good Clinical Practice guidelines and the Declaration of Helsinki (2013 version) and was approved by the ethics committee of the Jiangsu Cancer Hospital (approval no. KY-2024-071). The requirement for informed consent was waived because of the retrospective nature of the study.

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

References