Challenges and interventions of chimeric antigen receptor-T cell therapy in solid tumors

Abstract

Adoptive cellular therapy is rapidly improving immunotherapy in hematologic malignancies and several solid tumors. Remarkable clinical success has been achieved in chimeric antigen receptor (CAR)-T cell therapy which represents a paradigm-shifting strategy for the treatment of hematological malignancies. However, many challenges such as resistance, antigen heterogeneity, poor immune cell infiltration, immunosuppressive microenvironment, metabolic obstructive microenvironment, and T cell exhaustion remain as barriers to broader application especially in solid tumors. Encouragingly, the development of new approaches such as multidimensional omics and biomaterials technologies was aided to overcome these barriers. Here, in this perspective, we focus on the most recent clinical advancements, challenges, and strategies of immune cellular therapy in solid tumor treatment represented by CAR-T cell therapy, to provide new ideas to further overcome the bottleneck of immune cell therapy and anticipate future clinical advances.

Current clinical landscape of chimeric antigen receptor (CAR)-T cell therapy in solid tumors

Eight CAR-T cell products have been approved by the US Food and Drug Administration (FDA) and National Medical Products Administration (NMPA) in China for 12 indications of hematologic malignancies (Figure 1) (1). The tremendous success of CAR-T cell therapy in the clinical treatment of hematologic tumors has led to the translation of CAR-T cell therapy into challenging solid tumors. CAR-T cell therapy of targeting GPC3, Claudin18.2, B7H3, EphA2, etc. in solid tumors already have been evaluated in preclinical or clinical research (2). Although the majority of clinical trials constructed the efficacy and safety of CAR-T cell in solid tumors exhibited disappointing results, some studies showed promising outcomes. In a recent clinical trial, the efficacy of GD2 CAR-T cells, which incorporated both CD28 and 4-1BB costimulatory domains, was evaluated in young high-risk neuroblastoma patients. The results indicated that 9 out of 27 patients achieved complete response (CR), and the three-year overall survival (OR) rate was 60% (3). Another clinical trial of CAR-T cells targeting CLDN18.2 in 37 gastrointestinal cancer patients also reports promising results. The rate of overall response (OR) and disease control (DC) were 48.6% and 73.0%, respectively (NCT03874897) (4). Furthermore, CAR-T cells have demonstrated feasibility and safety in clinical trials for solid tumors, including pancreatic, colorectal, glioblastoma and mesothelioma, as evidenced by studies registered under the identifiers NCT01935843, NCT02349724, NCT02208362, and NCT03545815 (5).

Figure 1.

FDA and NMPA approved CAR-T cell therapies. FDA, Food and Drug Administration; NMPA, National Medical Products Administration; CAR-T, chimeric antigen receptor-T; ALL, acute lymphoblastic leukemia; LBCL, large B-cell lymphoma; FL, follicular lymphoma; MCL, mantle cell lymphoma; MM, multiple myeloma.

Current limitations and strategies of CAR-T cell therapy in solid tumors

Although immune cellular therapy has made significant advancements and demonstrated clinical efficacy in patients with hematologic malignancies and some solid tumors, there are still many challenges in the field of solid tumor therapy (Figure 2) (by www.home-for-researchers.com). These challenges include: 1) antigen heterogeneity and immune escape; 2) inadequate immune cell infiltration; 3) tumor immune suppressive microenvironment; and 4) impaired metabolism of tumor microenvironment (TME) and CAR-T cells (6,7).

Figure 2.

Challenges and strategies of CAR-T cell therapy for solid tumors. (A) Improving CAR-T cell antigen recognition. Multiple CAR-T, Tan CAR, and BiTEs are designed to improve CAR-T cell antigen recognition for preventing antigen immune escape; (B) Improving CAR-T cells trafficking into the tumor site. Overexpression of chemokine receptors, such as CXCR1, CXCR2 and CCR4 can promote the infiltration of CAR-T cells into the tumor site. FAP or VEGFR-targeting CAR-T cells or with the anti-VEGF treatment can break through physical barriers to increase CAR-T cell infiltration; (C) Overcoming the suppressive TME and CAR-T cell dysfunction. Arming CAR-T cells with cytokines overexpression such as IL-7, IL-12, IL-15 and IL-18 to improve CAR-T cell persistence and function in TME. Blocking the immune checkpoint through utilizing DNRs without transmembrane or intracellular signaling domains block negative signaling mediated by PD-1 or TGFBR can overcome suppression of TME; (D) Rewiring metabolism to sustain function of CAR-T cells. Overexpression of PGC1-α can strengthen the mitochondrial fitness in CAR-T cells. CAR-T, chimeric antigen receptor-T; Tan CAR, Tandem CAR; FAP, fibroblast activation protein; VEGFR, vascular endothelial growth factor receptor; VEGF, vascular endothelial growth factor; TME, tumor microenvironment; DNR, dominant-negative receptor; TGFBR, transforming growth factor beta receptor.

To address the heterogeneity of solid tumors, designing multitarget CAR-T cells with different structures such as Tandem CAR (Tan CAR), bispecific T cell engagers (BiTEs), and “logical gating”, is the effective strategy (6,8). The CAR-T cells with dual targets, split co-stimulatory signaling, and shared CD3ζ chain provided potent and sustained anti-tumor activity under stressful conditions in a mouse model of neuroblastoma, which effectively prevented tumor escape (9). Another investigation utilizing bispecific CAR-T cells co-expressing IL13Rα2 and HER2 CAR molecules demonstrated the ability to eradicate solid tumor cells in glioblastoma with minimal antigen escape (10).

Normalization of vessels, optimizing CAR-T cells expressing appropriate chemokine receptors, and disrupting physical barriers could promote CAR-T cells infiltration into TME (11-13). The results of phase I clinical trial of overexpressed IL-7 and CCL19 CAR-T cell therapy (NCT03198546) showed that a patient with advanced pancreatic cancer had almost complete tumor disappearance after 240 d of intravenous infusion of CAR-T cells (14). The ability of CAR-T cells to migrate into tumor tissues, however, may not just be exclusively regulated by chemokine signals, and it may be simultaneously controlled by the cell-intrinsic program that T cells acquired during the ex vivo manufacturing process (15). In addition, the problem of inadequate infiltration of TME can be directly circumvented by modifying immune cell transfusion, such as intratumoral injection, topical administration, or delivery of specialized biomaterials (16,17).

The activation, differentiation, proliferation and function of T cells are all significantly influenced by mitochondria. Key characteristics of dysfunctional CAR-T cells include impaired mitochondrial function and dynamics (18). The destiny, functionality, persistence, and longevity of CAR-T cells after infusion, as well as the clinical outcome, were all linked with mitochondrial biogenesis (19). The previous report showed that CD19 CAR-T cells from chronic lymphocytic leukemia (CLL) patients who had achieved a CR exhibited enhanced mitochondrial biogenesis compared with non-responders (NR) (20). The characteristic of larger mitochondrial size, increased level of proliferator-activated receptor-γ coactivator 1α (PGC-1α), and GLUT1 of CAR-T cells were examined in patients who achieved CR accompanied with an early memory T-cell phenotype than NR patients. Exhaustion can be prevented by genetic modification by altering the mitochondrial metabolism of CAR-T cells. Mitochondrial reprogramming caused by PGC-1α-engineered CAR-T cells demonstrated resistance to post-translational regulation, giving T cells more effector-like programs and a longer-lasting memory state, which improves therapeutic efficacy against solid tumors (21). The production of cancer metabolism including _D-2-hydroxyglutarate (_D-2HG) (22), reactive oxygen species (ROS), prostaglandin E2 (PEG2), adenosine, _L-arginine, tryptophan, lactic acid in the TME has been confirmed their suppressive function on T cells (23,24). Strategies modifying metabolic pathways to strengthen CAR-T cell function or reverse effector function inhibited by immunosuppressive metabolites in acidic, nutrient-deprived and hypoxic TME have shown potential improvement in multiple preclinical investigations (25). Moreover, the development of single-cell RNA sequencing (26,27), metabolomics, and spatial omics technology provides a novel perspective for looking for key metabolic interventions which determine the efficiency of CAR-T cell infusion. Thus, rewiring metabolism could be a promising approach for boosting the efficacy of CAR-T cells in solid tumor treatment.

Conclusions and perspectives

With the development of precision medicine, CAR-T cell therapy will also develop towards individualized treatment. Genetic testing and tumor characterization allow for the selection of the most appropriate CAR-T cell treatment strategy for patients. Further investigation needs to be explored by combining clinical trial results with advanced technologies such as genomic, proteomic, metabolomic, single-cell sequencing, and CRISPR/Cas9 technology to explain the main reasons why patients with solid tumors resist CAR-T cell therapy at the level of gene, protein, and epigenetic modifications. At the same time, multidisciplinary cross-fertilization has opened up new ideas to break the dilemma of CAR-T cell therapy for solid tumors. The combination of biomaterials such as nano-encapsulation, implantable scaffolds, and injectable biomaterials may allow for more effective and precise delivery of CAR-T cells into the tumor sites and reduce off-target effects (28). In addition, strategies to control safety when treating solid tumors with immune cells, such as dose reduction, addition of suicide genes, and finding more suitable antigenic targets for solid tumors, need to be considered. Finally, improving production processes and reducing costs are also important to improve the accessibility and sustainability of CAR-T cell therapy which could benefit more oncology patients. With the advancement of technology and accumulation of clinical experience in solid tumors (Table 1), CAR-T cell therapy is expected to be gradually improved and optimized, which can benefit more patients with solid tumors and become one of the important modalities in the field of cancer treatment in the future.

Table 1. Targets of CAR-T cell therapy in solid tumors with clinical evidence of efficacy.

Target	Disease	Response rate	Survival	Comments	Ref.
CAR-T, chimeric antigen receptor-T; GPC3, glypican-3; MSLN, mesothelin; PSMA, prostate-specific membrane antigen; TAG-72, tumor-associated glycoprotein-72; HER2, human epidermal growth factor receptor 2; EGFR, epidermal growth factor receptor; GC, gastrointestinal cancers; HCC, hepatocellular carcinoma; PC, pancreatic carcinoma; OC, ovarian carcinoma; mCRPC, metastatic castration-resistant prostate cancer; CRC, metastatic colorectal cancer; NSCLC, non-small cell lung cancer; MPM, malignant pleural mesothelioma; BC, breast cancer; GBM, glioblastoma; CR, complete response; ORR, objective response rate; DCR, disease control rate; PR, partial response; SD, stable disease; PD, progressive disease; OS, overall survival; PFS, progression-free survival; CRS, cytokine release syndrome; 95% CI, 95% confidence interval.
GD2	High-risk neuroblastoma	CR: 27% of patients with active disease	Overall response: 63% Median OS: 31 months	Among patients who received the recommended dose, the 3-year OS and event-free survival were 60% and 36%, respectively.	(3)
Claudin-18.2	GC	ORR: 48.6% DCR: 73.0%	Median PFS: 3.7 months OS: 81% at 6 months	Eighty-three percent of patients showed tumor regression; 11% showed reversible grade 3/4 gastrointestinal toxicities	(4)
GPC3 or MSLN	HCC or PC or OC	CR: 1/6 PR: 1/6 SD: 2/6	Not available	One PC patient (1/6, 16.7%) achieved CR; one HCC patient (1/6, 16.7%) achieved PR; and 2 HCC (2/6, 33.3%) achieved SD	(14)
PSMA	mCRPC	Five of 13 patients achieved a PSMA reduction of ≥30%	Median PFS: 4.4 months Median OS: 15.9 months	Five of 13 patients achieved a PSA reduction of ≥30%, 5 out of 13 patients developed high-grade CRS	(29)
TAG-72	CRC	Seven of 8 evaluable patients in trial C-9701 and 2 evaluable patients in trial C-9702 showed an increase in TAG-72 in post-infusion sera following heat treatment	Not available	Detectable, but mostly short-term (≤14 weeks), persistence of CART72 cells was observed in blood; one patient had CART72 cells detectable at 48 weeks	(30)
HER2	Sarcomas	SD: 4/17	Median OS: 13 months	Of 17 evaluable patients, 4 had SD for 12−14 months. Three of these patients had their tumor removed, with one showing ≥90% necrosis.	(31)
4-1BB and PD1	CRC or OC or NSCLC	DCR: 65.6%	Not available	DuoBody-PD-L1×4-1BB (GEN1046) is a first-in-class bispecific immunotherapy with a manageable safety profile and encouraging preclinical and early clinical activity	(32)
Mesothelin	MPM or BC	PR: 2/16 SD: 9/16 PD: 5/16	Median OS: 23 months One-year OS: 83%	/	(33)
IL-13Rα2	GBM	CR: 1/1	Not available	This first-in-human experience establishes a foundation for future adoptive therapy studies using off-the-shelf, zinc-finger modified, and/or glucocorticoid resistant CAR-T cells.	(34)
EGFR	NSCLC	PR: 1/9 SD: 6/9 PD: 2/9	Median OS: 15.63 months PFS: 7.13 months	PFS of these nine patients was 7.13 (95% CI: 2.71−17.10 ) months, while the median OS was 15.63 (95% CI: 8.82−22.03) months.	(35)