Correlation between the expression of soluble BCMA and short-term/long-term curative effect and survival outcomes of anti-BCMA CAR-T cell therapy in relapsed/refractory multiple myeloma

Abstract

This study aimed to investigate whether soluble B-cell maturation antigen (sBCMA) levels could be predictive biomarker for short-term and long-term therapeutic efficacy and survival outcomes following anti-BCMA CAR-T cell therapy in relapsed/refractory multiple myeloma (R/R MM). We enrolled 29 R/R MM patients who received anti-BCMA CAR-T cell therapy. In short-term observation, proportion of MM cells, expression of B-cell maturation antigen (BCMA) and sBCMA in bone marrow (BM) were evaluated, along with adverse events, correlation between sBCMA levels and short-term efficacy or survival outcomes were evaluated. In long-term observation, expressions of sBCMA were observed up to 24 months after therapy or until disease progression again in patients who achieved an objective response (ORR). Progression-free survival (PFS), overall survival (OS), correlation between sBCMA levels, and long-term outcomes were analyzed. In short-term observation, high expressions of sBCMA in BM were associated with poor efficacy of CAR-T cell therapy, while the proportion of MM cells in BM and BCMA expression in MM cells were not associated with poor efficacy of therapy. After 2 months of infusion, sBCMA levels decreased significantly, especially in patients who obtained ORR. In long-term follow-up, for patients who achieved ORR, the sBCMA levels significantly increased again when their disease progressed once more. Notably, R/R MM patients with extramedullary disease (EMD) demonstrated a higher likelihood of disease progression again. In patients achieved ORR, peaks of CAR-T cells correlated with proportion of MM cells, not with BCMA and sBCMA expression. Additionally, sBCMA levels were independent of cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS) severity. We suggest that sBCMA levels in BM might serve as a predictive biomarker for anti-BCMA CAR-T cell therapy efficacy prior to treatment and for disease progression during long-term monitoring. The trail register name is China Clinical Trial Register. URL are https://www.chictr.org.cn/bin/project/edit?pid=28999 and https://www.chictr.org.cn/bin/project/edit?pid=53962. Registration numbers are ChiCTR1800017051 and ChiCTR2000033925.

Graphical Abstract

Keywords

anti-BCMA CAR-T multiple myeloma BCMA expression soluble BCMA expression efficacy

Introduction

Prognosis and survival for patients with relapsed/refractory (R/R) multiple myeloma (MM) is very poor^1,2. Anti-B-cell maturation antigen (anti-BCMA) chimeric antigen receptor (CAR)-T cell therapy is one of the most effective immunotherapies for patients with relapsed/refractory (R/R) hematologic malignancies^3–5. B-cell maturation antigen is specifically expressed in almost all MM patients⁶. As a transmembrane glycoprotein, it is part of the tumor necrosis factor receptor superfamily (TNFRSF17) and is found on normal plasma cells and malignant MM cells. It has been used as the main target of CAR-T cell therapy in R/R MM patients, and anti-BCMA CAR-T cell therapy has significantly extended the survival of R/R MM patients, with a response rate of 64% to 85%^7–11.

Before R/R MM patients receive anti-BCMA CAR-T cell therapy, are there indicators that could predict the efficacy and side effects? sBCMA is cleaved by γ-secretase (GS) complex from the extracellular portion of BCMA which reduces ligand density on tumor cells and releases a soluble BCMA (sBCMA) fragment. High expression of sBCMA could accumulate in bone marrow (BM) and inhibit the recognition of MM cells by T cells¹². GS inhibitors could decrease sBCMA concentrations of MM cells and improve recognition by CAR-T cells in vitro, and GS inhibitors combined with anti-BCMA CAR-T cell therapy showed good tolerance and depth of clinical response^13,14. Does the sBCMA levels in BM affect the efficacy of anti-BCMA CAR-T cell therapy? This is worthy of clinical exploration. Studies reported the use of whole genome sequencing and in vitro experimental methods to demonstrate that sBCMA could independently predict primary refractory to anti-BCMA therapies¹⁵. In BCMA-targeting immunotherapies, including bispecific antibodies and anti-BCMA CAR-T cell therapy, R/R MM patients who respond to treatment showed constantly decreasing sBCMA levels^16,17. Therefore, sBCMA levels could be potentially used as additional biomarker to monitor R/R MM responses following anti-BCMA CAR-T cell therapy.

In our previous study, we showed that sBCMA expression in BM is related to the short-term curative effect and survival of anti-BCMA CAR-T cell therapy in patients with R/R MM, but is not significantly correlated with the severity of adverse events¹⁸. Here, we further explore the correlation between sBCMA expression biomarkers and the long-term curative effect and survival of this therapy. We aimed to identify the indicator that could predict the efficacy before anti-BCMA CAR-T cell therapy and predict the disease progresses again after anti-BCMA CAR-T cell therapy.

Patients and methods

Clinical data of the patients before anti-BCMA CAR-T cell therapy

In this retrospective non-randomized controlled observational study, 25 R/R MM patients were admitted to our hospital to receive anti-BCMA CAR-T cell therapy between January 2019 and December 2021. All patients had MM cells in BM that were detectable by flow cytometry (FCM). Follow-up was conducted from the date of anti-BCMA CAR-T cell infusion to the cutoff date or the date of death. The cutoff date was October 31, 2024.

Anti-BCMA CAR-T cell therapy

All 29 patients with R/R MM were enrolled in a clinical trial of anti-BCMA CAR-T cell therapy (ChiCTR1800017051 and ChiCTR2000033925) at our hospital. Autologous peripheral blood mononuclear cells (PBMCs) were collected from all R/R MM patients. From day −4 to day −2, the patients received fludarabine (30 mg/m²) and cyclophosphamide (400 mg/m²) as lymphodepleting chemotherapy. The target dose of 2 × 10⁶ cells/kg autologous anti-BCMA CAR-T cells was infused on day 0.

Evaluation criteria for diagnosis and therapeutic efficacy

After the last patient received anti-BCMA CAR-T cell therapy in December 2021, the curative effect and survival after anti-BCMA CAR-T cell therapy were assessed according to the International Myeloma Working Group (IMWG) Uniform Response Criteria for Multiple Myeloma 2 months after CAR-T cell infusion¹⁹. Clinical responses included stringent complete response (sCR), complete response (CR), very good partial response (VGPR), partial response (PR), minimal response (MR), stable disease (SD), and progression of disease (PD). The overall response rate (ORR), progression-free survival (PFS) and overall survival (OS) were observed. ORR was defined as the proportion of patients who achieved sCR, CR, VGPR, or PR.

Detection of BCMA expression in MM cells and sBCMA expression in BM

B-cell maturation antigen (CD269) expression in MM cells in BM, pleural effusion, and cerebrospinal fluid was detected by FCM (MACS Miltenyi Biotec, Inc., 5200206336). Serum from BM was extracted and sBCMA expression was detected by FCM (QuantoBio Biotec, Inc., B113377).

The sBCMA expressions in short-term and long-term observation

The short-term observation is to observe the expression of sBCMA during anti-BCMA CAR-T cell therapy. Before and 2 months after the anti-BCMA CAR-T cell infusion, BM serum was extracted and frozen at −80°C. The expression of sBCMA was detected by FCM.

The long-term observation is to observe the expression of sBCMA within 2 years after anti-BCMA CAR-T cell therapy or during disease re-progression. The expressions of sBCMA were observed every 2 months. Six months after infusion, it was observed every 6 months until 24 months after CAR-T cell therapy or disease progress again in all patients who obtained ORR in this therapy. We divided the patients into Sustained remission group and Progress again after ORR group.

The proportions of anti-BCMA CAR-T cell in peripheral blood

In our study, the proportions of anti-BCMA CAR-T cells in CD3+ T cells in peripheral blood were observed on days 0, 7, 14, 28, and 60 postinfusion by FCM. Proportions of anti-BCMA CAR-T cells in pleural effusion, in cerebrospinal fluid were also observed by FCM.

Adverse events (AEs) observation in therapy

Interleukin-6 (IL-6) was measured on days 0, 7, 14, 28, and 60 using enzyme-linked immunosorbent assay (ELISA). AEs were monitored during anti-BCMA CAR-T cell therapy. The cytokine release syndrome (CRS) grade was determined according to the National Cancer Institute Common Terminology Criteria for AEs v4.03²⁰. Neurotoxicity syndrome was assessed according to the Immune Effector Cell-Associated Neurotoxicity Syndrome (ICANS)²⁰.

Statistical analysis

Statistical analyses were performed using SPSS 18.0 software and GraphPad Prism 8. Data were expressed as mean ± SE. Overall survival was defined as the time from the start of treatment to the date of death from any cause or the last follow-up day. Progression-free survival was defined as the time from treatment to disease progression or death from any cause. The probabilities of PFS and OS were analyzed using the Kaplan-Meier method. Data comparisons between groups were conducted using Fisher’s exact test. Correlations between variables with a normal distribution were assessed using Pearson’s correlation test, while those with abnormal distributions were assessed using Spearman’s rho test. All reported P-values are two-sided, and a P-value < 0.05 was considered statistically significant.

Results

Patient characteristics

Table 1 shows the characteristics of all twenty-nine patients with R/R MM enrolled in our clinical trial.

Table 1.

Baseline characteristics of the R/R multiple myeloma patients.

Pt	Sex	Age	KPS	Subtype	ISS stage	MM cells in BM, %	BCMA expression(%)	sBCMA expression (pg/ml)	High-riskcytogenetics	Extramedullary disease	Numbers of prior therapy	BCMA CAR-T cell therapy before(murine CAR-T)	Auto-HSCT before
1	Female	70	100	IgG-κ	III	32.6	90.8	29,415	Del(17p), t(4;14)	Yes	10	Yes	No
2	Male	72	100	Nonsecretory	III	28.9	88.2	16,007	None	None	4	No	No
3	Female	77	90	IgD-κ	III	66.8	1.5	48,945	None	None	13	No	No
4	Male	57	70	k light chain	III	86.0	55.8	40,801	t(4;14), t(14;16), Del(17p)	None	14	No	Yes
5	Male	63	80	IgG-κ	III	35.6	92.9	11,751	t(14,16)	None	5	No	Yes
6	Female	52	100	IgA-λ	III	20.7	62.6	33,816	None	None	9	No	No
7	Male	66	70	IgA-κ	II	19.2	49.1	33,659	t(14;16), Del(17p)	None	16	No	No
8	Male	44	100	IgG-κ	II	22.1	96.5	28,790	t(14;16)	None	5	No	No
9	Female	58	90	IgG-λ	III	88.1	1.6	49,623	None	Yes	30	No	No
10	Male	59	100	k light chain	III	41.8	77.3	21,276	t(14,16)	Yes	7	Yes	No
11	Female	54	90	IgG-κ	III	17.5	16.8	11,245	None	None	8	No	No
12	Male	56	70	IgG-κ	II	32.3	78.1	79,448	None	Yes	10	No	No
13	Female	38	90	k light chain	II	20.4	69.7	41,187	None	Yes	17	No	Yes
14	Male	73	90	IgD-λ	II	23.0	69.8	37,856	None	Yes	12	No	No
15	Male	45	90	IgG-κ	II	19.4	92.5	32,045	None	None	7	No	No
16	Male	46	100	IgG-κ	III	22.2	71.1	18,742	None	None	9	No	No
17	Male	65	90	k light chain	III	17.0	29.1	26,425	None	Yes	6	No	Yes
18	Female	79	60	λ light chain	III	79.4	12.9	37,973	Del(17p)	Yes	28	No	No
19	Male	63	80	IgG-κ	III	32.4	30.8	26,425	t(14,16)	Yes	25	No	No
20	Female	58	90	IgG-κ	II	16.1	87.3	27,427	None	None	8	No	No
21	Female	64	80	IgA-κ	III	64.4	76.2	44,437	Del(17p)	None	14	No	No
22	Female	65	60	k light chain	III	89.6	92.1	33,816	t(14;16), Del(17p)	None	22	No	No
23	Male	55	100	IgG-κ	II	22.1	82.4	17,378	None	Yes	15	No	No
24	Male	68	60	IgD-λ	III	40.1	26.3	49,826	t(4;14)	Yes	21	No	Yes
25	Female	55	90	IgG-λ	III	44.2	80.2	30,731	t(14,16)	None	12	No	Yes
26	Female	59	70	λ light chain	III	60.8	0.6	47,046	Del(17p)	None	27	No	No
27	Female	47	80	IgG-λ	III	14.8	73.1	7,496	t(14;16)	None	9	No	Yes
28	Female	58	80	λ light chain	III	31.6	0.2	16,634	None	None	21	No	No
29	Female	65	80	IgG-κ	III	29.6	83.2	19,441	None	None	9	No	No

BCMA expression in MM cells and sBCMA expression in BM before therapy

Before anti-BCMA CAR-T cell therapy, BCMA expression in MM cells and sBCMA expression in BM, as measured by FCM, are listed in Table 1. The median BCMA expression was 58.2% (IQR 29.1–83.2), and the median sBCMA expression was 31,712 pg/ml (IQR 19,441–40,801). There was no significant correlation between BCMA expression in MM cells and sBCMA expression in BM (R² =0.0506, P = 0.2405).

Clinical responses of anti-BCMA CAR-T cell therapy

Patients received an infusion dose of (2.09 ± 0.16) × 10^⁶ cells/kg autologous anti-BCMA CAR-T cells on day 0 after lymphodepleting chemotherapy. The median time for patients to obtain the best effect was 2.07 ± 0.94 months. Twenty patients (68.97%) had an ORR, with 10 achieving sCR/CR, four achieving VGPR, and six achieving PR. The other nine patients achieved MR and SD only (Fig. 1). One patient (Pt 15) who achieved VGPR received auto-HSCT 3 months after anti-BCMA CAR-T cell therapy. Pt 22 died of CRS and infection, Pt 18 died of an unexpected cerebral hemorrhage 4 months after CAR-T cell therapy, and Pt 5 and Pt 28 died of infection 18 and 6 months after CAR-T cell therapy (Fig. 1).

Figure 1.

Clinical responses and survival observation of anti-BCMA CAR-T cell therapy.

sBCMA expressions in short-term and long-term observation

In short-term observation, there was no significant difference in the proportion of MM cells between the ORR group and the MR + SD group (P = 0.8433), nor in BCMA expression in MM cells between these groups (P = 0.2948). However, sBCMA levels in BM were significantly higher in the MR + SD group than in the ORR group (P = 0.0048) (Fig. 2a–c). The sBCMA levels in BM decreased significantly 2 months after infusion (P < 0.001) in all the 25 patients (Fig. 2d). This indicator was significantly lower in ORR group than in that of in MR + SD group (P < 0.001) 2 months after infusion (Fig. 2e).

Figure 2.

The sBCMA expressions in short-term and long-term observation. (a) No difference in the proportion of MM cells in ORR group and MR + SD group. (b) No difference in the BCMA expressions in MM cells in two groups. (c) The sBCMA expressions in BM was higher in MR + SD group than in ORR group. (d) The sBCMA expressions in BM decreased significantly 2 months after infusion. (e) The sBCMA was lower in ORR group than that of in MR + SD group 2 months after therapy. (f) The expressions of sBCMA in Sustained remission group. (g) The expressions of sBCMA in Progress again after ORR group.

In our study, the survival time of R/R MM patients who obtained ORR ranged from 12 to 68 months. In long-term observation, the expressions of sBCMA were observed for 11 to 24 months (Fig. 2f and g). In all R/R MM patients who obtained ORR, the expressions of sBCMA before therapy were no significant difference in Sustained remission group and in Progress again after ORR group (P = 0.090). Two months after therapy, it was significantly higher in Progress again after ORR group than that of in Sustained remission group (P = 0.024). The expressions of sBCMA were significantly higher in Progress again after ORR group when their disease progresses again than that of in Sustained remission group when their disease remained stable for 24 months (P = 0.008).

Patients with extramedullary lesions were also observed and compared in long-term observation. Among the patients who relapsed again in ORR group, seven of the nine patients had extramedullary disease prior to CAR-T therapy. In contrast, in Sustained remission group, only two out of 11 patients had extramedullary lesions prior to CAR-T therapy (Fig. 2f and g).

Expressions of anti-BCMA CAR-T cells and serum levels of cytokines

During therapy, proportions of CAR-T cells were measured on day 0, 4, 7, 14, 28, and 60 after infusion (Fig. 3a). The median peaks of CAR-T cells in CD3+ T cells in peripheral blood was 27.3% (IQR 3.2–43.7). The average time to reach the expansion peak was 11.1 ± 3.4 days. The median peaks of anti-BCMA CAR-T cells in CD3+ T cells in the ORR group was 33.2% (IQR 21.8–43.7), significantly higher than in the MR + SD group (14.3% [IQR 3.2–21.7]) (P = 0.0150) (Fig. 3b).

Figure 3.

The proportions of anti-BCMA CAR-T cells and levels of cytokines. (a) The proportions of anti-BCMA CAR-T cells in peripheral blood in all patients. (b) The median expansion peak of the anti-BCMA CAR-T cells in ORR group was higher than that of the in MR + SD group. (c–e) No correlation between the peaks of CAR-T cells and the proportion of MM cells, the BCMA expressions or sBCMA expressions. (f–h) In all patients obtained ORR, the peaks of CAR-T cells had correlation with the proportion of MM cells only. (i–k) Serum levels of IL-6 were correlated with proportion of MM cells in BM and sBCMA expressions in BM.

There was no significant correlation between the peaks of CAR-T cells and the proportion of MM cells (P = 0.1050), BCMA expression in MM cells (P = 0.3400), or sBCMA levels in BM (P = 0.7100) (Fig. 3c–e). However, in patients who achieved ORR, the peaks of CAR-T cells correlated with the proportion of MM cells (P = 0.0045). There was no significant correlation between the peak of CAR-T cells and BCMA expression (P = 0.1067) or sBCMA levels (P = 0.1956) (Fig. 3f–h). In patients who achieved ORR, IL-6 levels were significantly correlated with proportion of MM cells in BM (P = 0.0016) and with sBCMA levels in BM (P = 0.0265), but not with BCMA expression in MM cells (P = 0.0921) (Fig. 3i–k).

The AEs in anti-BCMA CAR-T cell therapy

In anti-BCMA CAR-T cell therapy, except for Pt 9 and Pt 22 patients who were diagnosed as grade 4 CRS, Pt 13 and Pt 18 who were diagnosed as grade 3 CRS, while the remaining patients were all diagnosed with grade 0–2 CRS. In this study, except for Pt 18 who was diagnosed with grade 3 ICANS, Pt 22 who was diagnosed with grade 2 ICANS, the other patients were all diagnosed with grade 0–1 ICANS.

There was no significant difference in the proportion of MM cells in BM, BCMA expression in MM cells, or sBCMA levels in BM between the grade 0–1 CRS group and the grade 2–4 CRS group (Fig. 4a–c). Similar results were found between the grade 0 ICANS group and the grade 1–3 ICANS group (Fig. 4d–f).

Figure 4.

The CRS and ICANS in anti-BCMA-CAR T cell therapy. (a–f) In all patients, the proportion of MM cells, BCMA expressions and sBCMA expressions showed no difference between grade 0–1 CRS group and grade 2–4 CRS group. The same results were found between grade 0 ICANS group and grade 1–3 ICANS group. (g–i) In patients obtained ORR, the proportion of MM cells, BCMA expressions and sBCMA expressions showed no difference between grade 0–1 CRS group and grade 2–4 CRS group. (j–k) The proportion of MM cells and BCMA expressions were lower in grade 0 ICANS group than that of in grade 1–3 ICANS group. (l) No difference in sBCMA expressions between grade 0 ICANS group and grade 1–3 ICANS group. (m–n) No difference in CRS levels or ICANS levels between patients with and without extramedullary disease. (o) The CRS levels were higher in patients with extramedullary disease than that of without extramedullary disease. (p) No difference in ICANS levels between patients with and without extramedullary disease.

In all R/R MM patients who achieved ORR, there was no significant difference in the proportion of MM cells in BM, BCMA expression in MM cells, or sBCMA levels in BM between the grade 0–1 CRS group and the grade 2–4 CRS group (Fig. 4g–i). The proportion of MM cells in BM and BCMA expression in MM cells were significantly lower in the grade 0 ICANS group than in the grade 1–3 ICANS group (Fig. 4j and k). However, there was no significant difference in sBCMA levels in BM between the grade 0 ICANS group and grade 1–3 ICANS group, although levels were higher in grade 1–3 ICANS group (Fig. 4l).

There was no significant difference in CRS levels or ICANS levels between patients with and without extramedullary disease (Fig. 4m and n). In all patients who achieved ORR, CRS levels were significantly higher in patients with extramedullary disease compared to those without it (Fig. 4o), but there was no significant difference of ICANS grades between patients with or without extramedullary disease (Fig. 4p).

During therapy, patients developed symptoms such as fever with or without chills, fatigue, headache, nausea, reduced appetite, hypotension, and edema from 3 to 7 days after infusion. These symptoms resolved 14 to 60 days postinfusion. Hematological toxicity occurred 5 to 8 days after infusion, ranging from grade 1–4, and resolved 18 to 60 days after infusion. After treatment of tocilizumab, methylprednisolone, antipyretic analgesics, and supportive therapy, all symptoms were alleviated (Table 2).

Table 2.

Adverse events in anti-BCMA CAR-T cell therapy.

	Any	Grade 1	Grade 2	Grade 3	Grade 4
Any	29 (100%)	0 (0%)	16 (55.17%)	7 (24.14%)	5 (17.24%)
Pyrexia	23 (79.31%)	4 (13.79%)	11 (3.45%)	7 (24.14%)	1 (3.45%)
Chills	16 (55.17%)	2 (6.9%)	7 (24.14%)	5 (17.24%)	2 (6.9%)
Fatigue	22 (75.86%)	9 (31.03%)	5 (17.24%)	6 (20.69%)	2 (6.9%)
Hematological toxicity (Leukopenia)	21 (72.41%)	9 (31.03%)	7 (24.14%)	4 (13.79%)	1 (3.45%)
Hematological toxicity (Anemia)	18 (62.07%)	8 (27.58%)	5 (17.24%)	3 (10.34%)	2 (6.9%)
Hematological toxicity (Thrombocytopenia)	20 (68.97%)	11 (37.93%)	6 (20.69%)	2 (6.9%)	1 (3.45%)
Hypocalcaemia	7 (24.14%)	3 (10.34%)	3 (10.34%)	1 (3.45%)	0 (0%)
Hyponatraemia	6 (20.69%)	4 (13.79%)	1 (3.45%)	1 (3.45%)	0 (0%)
Hypokalaemia	10 (34.48%)	5 (17.24%)	2 (6.9%)	3 (10.34%)	0 (0%)
Nausea	10 (34.48%)	5 (17.24%)	3 (10.34%)	1 (3.45%)	1 (3.45%)
Vomiting	6 (20.69%)	4 (13.79%)	1 (3.45%)	1 (3.45%)	0 (0%)
Decreased appetite	23 (79.31%)	11 (37.93%)	7 (24.14%)	3 (10.34%)	2 (6.9%)
Constipation	5 (17.24%)	4 (13.79%)	1 (3.45%)	0 (0%)	0 (0%)
Diarrhea	10 (34.48%)	6 (20.69%)	1 (3.45%)	2 (6.9%)	1 (3.45%)
Increased aminotransferase	16 (55.17%)	8 (27.58%)	4 (13.79%)	2 (6.9%)	2 (6.9%)
Hypoxemia	6 (20.69%)	4 (13.79%)	1 (3.45%)	1 (3.45%)	0 (0%)
Dyspnoea	4 (13.79%)	2 (6.9%)	1 (3.45%)	0 (0%)	1 (3.45%)
Cough	7 (24.14%)	4 (13.79%)	2 (6.9%)	1 (3.45%)	0 (0%)
Pleural effusion	11 (37.93%)	4 (13.79%)	2 (6.9%)	3 (10.34%)	2 (6.9%)
Creatinine increased	11 (37.93%)	5 (17.24%)	1 (3.45%)	2 (6.9%)	2 (6.9%)
Hyperkalaemia	4 (13.79%)	3 (10.34%)	0 (0%)	1 (3.45%)	0 (0%)
Peripheral oedema	11 (37.93%)	7 (24.14%)	2 (6.9%)	1 (3.45%)	1 (3.45%)
Tachycardia	4 (13.79%)	2 (6.9%)	1 (3.45%)	1 (3.45%)	0 (0%)
Sinus tachycardia	15 (51.72%)	9 (31.03%)	4 (13.79%)	1 (3.45%)	1 (3.45%)
Hypertension	7 (24.14%)	5 (17.24%)	1 (3.45%)	1 (3.45%)	0 (0%)
Hypotension	4 (13.79%)	3 (10.34%)	1 (3.45%)	0 (0%)	0 (0%)
Headache	12 (41.38%)	8 (27.58%)	3 (10.34%)	1(3.45%)	0 (0%)
Dizziness	8 (27.58%)	5 (17.24%)	3 (10.34%)	0 (0%)	0 (0%)
Confusion	6 (20.69%)	4 (13.79%)	2 (6.9%)	0 (0%)	0 (0%)
Tremor	4 (13.79%)	3 (10.34%)	1 (3.45%)	0 (0%)	0 (0%)
Epilepsy Coagulopathy	4 (13.79%) 8 (27.58%)	2 (6.9%) 5 (17.24%)	1 (3.45%) 2 (6.9%)	1 (3.45%) 0 (0%)	0 (0%) 1 (3.45%)

AEs were according to the National Cancer Institute’s Common Terminology Criteria for Adverse Events (version 5.0).

Three patients who diagnosed with grade 3–4 hematological toxicity and grade 1–3 CRS developed gram-negative bacterial infections during therapy (Table 3). One patient (Pt 5), who maintained PR after therapy, died of invasive fungal disease and cytomegalovirus infection 18 months postinfusion. Another patient (Pt 28) who maintained VGPR after therapy, died of bacterial infection 6 months postinfusion. Four patients developed renal insufficiency or failure during therapy and were treated with hemofiltration or hemodialysis. One patient (Pt 22) who diagnosed with grade 4 CRS and grade 2 ICANS, died of traumatic cerebral hemorrhage 62 days postinfusion. The other three patients recovered from hemodialysis or hemofiltration about 30 days postinfusion (Table 3).

Table 3.

Secondary infection and renal insufficiency in anti-BCMA CAR-T cell therapy.

Infection	Effect evaluation	Days after infusion	CRS/ICANS	Leukopenia (×10⁹/l)	Pathogen/therapy	Outcomes	ICU admission
Pt 1	CR	16	2/0	0.12	Escherichia coli sepsis/Meropenem	Healing	No
Pt 9	PR	8	4/2	0.02	E. coli sepsis/Meropenem	Healing	No
Pt 26	MR	18	1/0	0.04	Staphylococcus aureus sepsis/Vancomycin + Meropenem	Healing	No
Renal insufficiency	Effect evaluation	Days after infusion	CRS/ICANS	Peak of creatinine(μmol/ml)	Therapy	Outcomes	ICU admission
Pt 3	VGPR	12	2/1	553.2	Hemodialysis	Healing	No
Pt 4	SD	9	1/0	415.0	Bedside hemofiltration	Healing	Yes
Pt 18	CR	5	2/0	611.6	Bedside hemofiltration	Healing	No
Pt 22	PR	7	4/2	707.2	Bedside hemofiltration	Death from cerebral hemorrhage	No

Survival observation

As of the cutoff date, Kaplan-Meier analysis showed that PFS was 69.0% at 6 months, 58.6% at 12 months, and 37.9% at 60 months. Overall survival was 75.9% at 6 months, 65.5% at 12 months and 37.9% at 60 months (Fig. 5).

Figure 5.

Survival observation.

Discussion

Advances in treatment over the past two decades have significantly improved the survival of MM patients. However, there is still a need for more effective treatments, especially for R/R MM patients resistant to existing therapies. Anti-BCMA CAR-T cell therapy has emerged as a promising treatment with potential for long-term disease control in these patients and represents a breakthrough in therapy for R/R MM^21–23. Targeting BCMA in CAR T cell therapy and antibody-based approaches is currently being explored in many clinical trials^24,25. However, incomplete tumor eradication and subsequent disease progression remain challenges to be addressed in CAR T cell therapy^26,27. In previous studies, one factor contributing to incomplete tumor eradication and recurrent disease in anti-BCMA CAR-T cell therapy is the failure to completely eliminate MM cells, as BCMA is often expressed at low levels in these cells^28,29.

B-cell maturation antigen is a highly selective target for CAR-T cell therapy in R/R MM³⁰. However, the heterogeneity of BCMA expression on MM cells poses a challenge to the efficacy of these therapies³¹. In our study, BCMA was significantly differentially expressed among R/R MM patients (median expression: 58.2%, IQR 29.1%–83.2%). Another factor that might be associated with clinical status among R/R MM patients is sBCMA expression³². Other studies have reported that the half-life of sBCMA and changes in sBCMA levels in clinical status are much more rapid than M-protein in MM patients^33,34.

In our previous study, we showed that sBCMA expression is related to the short-term curative effect and survival of BCMA CAR-T cell therapy in R/R MM. And there was no significant correlation between the short-term efficacy of anti-BCMA CAR-T cell therapy and MM cells or BCMA expression¹⁸. A previous study also reported similar response rates to BCMA CAR-T cell therapy in patients with high or low expression of BCMA⁸. The response rates were 100% for those with sBCMA expression less than 50 and 91% for those with sBCMA expression of 50% or more^32,35,36. This result is consistent with our findings before that the sBCMA could be a biomarker correlated with short-term curative effect and survival of BCMA CAR-T cell therapy in R/R MM patients¹⁸.

In long-term observation of our study, nine patients experienced disease progression again after CAR-T cell therapy. Interestingly, there was no significant correlation between the expressions of sBCMA before therapy and disease progression again after CAR-T cell therapy, but at the time 2 months after therapy, it was significantly higher in Progress again after ORR group than that of in the Sustained remission group. We further explore the correlation between sBCMA expression and the long-term curative effect and survival of BCMA CAR-T cell therapy. Could sBCMA further predict the persistence of efficacy and the disease progression again? After our long-term follow-up of the patients who obtained ORR, we found that the expressions of sBCMA were significantly higher in Progress again after ORR group when their disease progresses again than that of in Sustained remission group when their disease remained stable for 24 months. These results suggest that the expressions of sBCMA might be a marker for the disease progresses again in R/R MM patients who obtained ORR in CAR-T cell therapy.

In our previous study^37,38, we found no difference in ORR between R/R MM patients with or without extramedullary disease, but patients with extramedullary disease were more likely to have disease progresses again. Similar results were obtained in our current long-term observation. In progression again after ORR group, seven of the nine patients were patients with extramedullary disease.

Anti-BCMA CAR-T cell therapy might lead to life-threatening toxicities, such as CRS and neurotoxicity^39,40, Close monitoring and attention are essential when administering anti-BCMA CAR-T cell therapy^41–43. In our study, the peak of CAR-T cells was not related to the proportion of MM cells, BCMA expression in MM cells, or sBCMA expression in BM. Only in R/R MM patients who achieved ORR anti-BCMA CAR-T cell therapy, the peak of CAR-T cells was significantly associated with the proportion of MM cells, while there was still no significant correlation with BCMA expression in MM cells or sBCMA expression in BM.

To explore the factors related to the curative effect and survival of anti-BCMA CAR-T cell therapy in R/R MM, we summarized predictive results of different indicators for its efficacy. We found that the expression of sBCMA in BM could be a predictor of the short-term of efficacy, while the proportions of MM cells in BM and BCMA expression in MM cells did not. Moreover, the expression of sBCMA could be a predictor of the disease progresses again in long-term follow-up. We suggest that sBCMA levels in BM could be a predictor of the efficacy before CAR-T cell therapy and the disease progress again in long-term observation. Further clinical verification is needed to confirm it.

Conclusions

In conclusion, we suggest that sBCMA levels in BM could be a predictor of the efficacy before CAR-T cell therapy and the disease progress again in long-term observation.

Footnotes

Acknowledgements

We sincerely thank all our patients for their participation in our clinical trials. We also thank Shanghai Genbase Biotechnology Co., Ltd. for providing the anti-BCMA CAR T cells.

ORCID iD

Siyan Niu

Ethical considerations

This study was approved by the Medical Ethics Committee of the Department of Hematology, Tianjin First Center Hospital (Tianjin, China) (approval numbers: 2015002X and 2020N028KY).

Consent for publication

All patients, participants and their guardians have provided written consent for the use of patient data involved in this study for publication.

Author contributions

Mingwei Fu: Writing—original draft, Writing—review & editing, Methodology, Formal analysis. Siyan Niu: Writing—original draft, Writing—review & editing. Can Liu: Investigation. Juan Mu: Investigation. Shuquan Gao: Investigation. Gang An: Formal analysis. Rui Cui: Investigation. Qi Deng: Conceptualization, Writing—review & editing, Supervision, Project administration.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by Tianjin Health Research Projects (TJWJ2023ZD003), and the Chinese Society of Clinical Oncology Beijing Xisike Clinical Oncology Research Foundation (Y-NCJH202201-0027 and Y-2022YMJN/MS-0001).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability statement

The datasets used and/or analyzed in the current study are available from the corresponding authors on reasonable request.

Clinical trial registration

ChiCTR1800017051,

ChiCTR2000033925,

Statement of human and animal rights

This article does not contain any studies with animal subjects.

Statement of informed consent

The patients gave their written informed consent in accordance with the Declaration of Helsinki. The patients agreed to have their data used in our study.

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