A review of immunotargeted therapy for Philadelphia chromosome positive acute lymphoblastic leukaemia: making progress in chemotherapy-free regimens

ABSTRACT

Philadelphia chromosome-positive acute lymphoblastic leukemia (PH + ALL) is the most common cytogenetic abnormality of B-ALL in adults and is associated with poor prognosis. Previously, the only curative treatment option in PH + ALL was allogeneic hematopoietic stem cell transplantation (Allo-HSCT). Since 2000, targeted therapy combined with chemotherapy, represented by the tyrosine kinase inhibitor Imatinib, has become the first-line treatment for PH + ALL. Currently, the remission rate and survival rate of Imatinib are superior to those of simple chemotherapy, and it can also improve the efficacy of transplantation. More recently, some innovative immune-targeted therapy greatly improved the prognosis of PH + ALL, such as Blinatumomab and Inotuzumab Ozogamicin. For patients with ABL1 mutations and those who have relapsed or are refractory to other treatments, targeted oral small molecule drugs, monoclonal antibodies, Bispecific T cell Engagers (BiTE), and chimeric antigen receptor (CAR) T cells immunotherapy are emerging as potential treatment options. These new therapeutic interventions are changing the treatment landscape for PH + ALL. In summary, this review discusses the current advancements in targeted therapeutic agents shift in the treatment strategy of PH + ALL towards using more tolerable chemotherapy-free induction and consolidation regimens confers better disease outcomes and might obviate the need for HSCT.

KEYWORDS:

• BCR-ABL1
• acute lymphoblastic leukemia
• Philadelphia chromosome
• tyrosine kinase inhibitors
• immunotherapy

1. Introduction

The Philadelphia chromosome (PH) is characterized by the t(9;22)(q34;q11) translocation, which leads to the formation of a fusion gene called BCR-ABL1. This cytogenetic abnormality is the most common in adult ALL, occurring in 15-25% of adults, and its frequency increases with age, accounting for approximately 50% of ALL patients over 60 years old [1,2]. Prior to the TKIs era, receiving HSCT was the only chance of long-term survival for patients with PH + ALL. With the emergence of TKIs such as Imatinib, the overall survival (OS) rate for PH + ALL patients has increased from 20% to 70% [3,4]. Following this, the second and third generations TKIs, including Dasatinib and Ponatinib, have demonstrated enhanced and more rapid molecular responses. Ponatinib, as the third-generation TKI have also shown effectiveness in overcoming mutations in the ABL1 domain, including T315I. With more potent inhibition of ABL kinase by next-generation TKIs, low dose of chemotherapy or chemotherapy-free regimen has becoming more effective and safer induction options, especially for elderly or frail patients. Alongside the advancement of immunotherapy and targeted therapy, the therapeutic landscape of PH + ALL is changing dramatically [5–7]. PH + ALL is characterized by abnormal proliferation and impaired differentiation of B lymphocytes in the pre-B stage, and abnormal expression of leukemia-specific surface antigens, including CD19, CD20, and CD22, making it an ideal target for immunotherapy. Blinatumomab, Inotuzumab Ozogamicin, and CAR-T have been developed to fill this gap [2,8]. In addition to immunotherapy, novel oral targeted drugs like Venetoclax and Histone deacetylase inhibitors (HDACi) are being evaluated in combination with TKIs for their clinical efficacy in treating refractory relapse(R/R) PH + ALL. This represents a novel direction of exploration aimed at achieving deeper remissions by inducing endogenous apoptosis of cells and restoring the expression of tumour suppressive genes. This review summarizes the latest advances in targeted therapies for PH + ALL, discusses the latest clinical advances in chemotherapy-free strategies with immunotherapies such as bispecific antibodies as well as antibody–drug conjugates(ADC) in combination with TKIs (Table 3), and reviews the available data and ongoing clinical trials (Table 1). The timeline of the development of treatment strategies for PH + ALL from initial chemotherapy to today's immunotherapy is shown in Figure 1.

Figure 1. Timeline of the development of PH + ALL targeted therapy. CML: chronic myeloid leukemia.

Table 1. Summary of ongoing or completed clinical trials for treating PH + ALL.

Clinical trials identifier	Trial phase	Status	Eligible ages	Intervention	Comments
NCT04329325	2	Recruiting	≥18 Years and older	Blinatumomab, Dasatinib, Dexamethasone, Methotrexate	Evaluating efficacy
NCT03147612	2	Recruiting	≥18 Years and older	Blinatumomab, Ponatinib, Chemotherapy	Evaluating efficacy
NCT03318770	Observ-ational Study	Recruiting	≥18 Years and older	Blinatumomab, Dasatinib	Evaluating safety and efficacy
NCT02997761	2	Recruiting	≥18 Years and older	Blinatumomab, Ibrutinib	Evaluating safety and efficacy
NCT06061094	2	Recruiting	18 - 65 Years	Blinatumomab, Dasatinib, Ponatinib	Evaluating safety and efficacy
NCT03984968	1/2	Recruiting	18 - 60 Years	CD19-CAR-T, Feeding T cells	Evaluating safety and efficacy
NCT03233854	1	Recruiting	≥18 Years and older	CD19/CD22-CAR-T, NKTR-255, Cyclophosphamide, Fludarabine Phosphate	Evaluating safety and efficacy
NCT04747912	2	Recruiting	≥18 Years and older	Inotuzumab Ozogamicin, Dasatinib, Ponatinib, Dexamethasone, Methotrexate, Vincristine	Evaluating efficacy
NCT02081378	1	Active	≥18 Years and older	ABL001, Nilotinib, Imatinib, Dasatinib	Evaluating safe and tolerated dose of ABL001
NCT05594784	2	Recruiting	≥14 Years and older	Venetoclax, Olverembatinib, Prednisone, Vincristine	Evaluating safety and efficacy
NCT02000427	2	Completed	≥18 Years and older	Blinatumomab	CR/CRh rate: 35.6% (n:16/45)
NCT00560794	2	Completed	≥18 Years and older	Blinatumomab	Incidence of MRD negativity within four cycles of treatment (n:3/5)
NCT01319981	2	Completed	≥18 Years and older	Rituximab, Imatinib, Chemotherapy	CR rate: H + C + R = 80% (12/15) H + C = 81.3% (13/16)
NCT00352677	1	Completed	≥18 Years and older	INNO-406	Evaluating safety and tolerability

CR/CRh: Complete remission/Complete remission with partial haematological recovery, Hyper-CVAD: hyper-fractionated Cyclophosphamide, Vincristine, Doxorubicin, and Dexamethasone, MRD: minimal residual disease

2. TKIs based treatment

Imatinib and Dasatinib have been approved by the FDA for the treatment of PH + ALL patients. At present, TKIs combined with cytotoxic drugs has become the first-line treatment, and researchs are underway for the next generation of TKIs combined with glucocorticoid and immunotherapy drugs. The following is a summary of the latest TKIs-based treatment strategies (Table 2).

Table 2. Summary of TKIs-based treatment for PH + ALL.

Basic information	Interventions	Condition	N	Clinical outcomes	Reference
UMIN000001226 (phase 2)	Imatinib + CT	ND	68	CR = 95.6% (65/68) 3-y EFS: 52% 3-y OS: 62%	Fujisawa, [9]
NCT00287105 (phase 2)	Imatinib + CT	NA	155	CR = 97% (151/155)	Biondi, [10]
NCT00327678 (Phase 3)	A: Imatinib + low-intensity CT B: Imatinib + hyper-CVAD	ND	145	CR/EFS/OS A = 98%/42%/48% B = 91%/32%/43%	Chalandon, [4]
Retrospective analysis	Imatinib + CT	ND	153	CR = 96% (148/153) 3-y EFS = 49.2% 3-y OS = 49.5%	Lou, [11]
NCT00038610 (Phase 2)	Imatinib + hyper-CVAD	ND	45	CR = 93% (42/45)	Daver, [12]
NCT00618501 (phase 2)	Imatinib + CT	ND	87	CR = 94% (82/87)	Lim, [13]
ChiCTR-IPR-14005706 (phase 3)	Dasatinib vs Imatinib	ND	189	Dasatinib Imatinib 4-y EFS: 71%, 48% 4-y OS: 88%, 69%	Shen, [14]
UMIN000012173 (phase 2)	Dasatinib + CT	ND	78	CR + CRi = 100% 3-y EFS: 66.2% 3-y OS: 80.5%	Sugiura, [15]
NCT00720109 (Phase 2,3)	Dasatinib vs CT	ND	60	CR = 98% (59/60)	Slayton, [16]
NCT01256398 (Phase 2)	Dasatinib + Dexamethasone	NA	65	CR = 98.5% (64/65)	Wieduwilt, [17]
2006-005694-21	Dasatinib + low-intensity CT	ND	71	CR = 96% (67/71)	Rousselot, [18]
NCT00792948 (Phase 2)	Dasatinib + CT	ND	94	CR = 88% (83/94)	Ravandi, [19]
ChiCTR-ONC-12002469 (Phase 2)	Nilotinib + CT	ND	30	HCR = 100% 4-y HRFS: 54% 4-y OS: 45%	Liu, [20]
NCT00844298	Nilotinib + CT	ND	90	HCR = 91% (82/90)	Kim, [21]
NCT02776605 (phase 2)	Ponatinib + CT	ND	30	CR = 100% 3-y EFS: 70% 3-y OS: 96%	Ribera, [22]
NCT01424982	Ponatinib + hyper-CVAD	ND	76	CMR = 83% (63/76)	Jabbour, [23]
Propensity score analysis	Ponatinib + hyper-CVAD vs Dasatinib + hyper-CVAD	ND	41 vs 41	Po Da CR: 100% 95% 3-y EFS: 69% 46% 3-y OS: 83% 56%	Sasaki, [24]
Retrospective study	Hyper-CVAD + Ponatinib	ND	13	3-month CMR: 76.9% 3-y OS/EFS: 92.3%	Othman, [25]
NCT01207440 (phase 2)	Ponatinib	R/R	32	McyR: 47% (15/32)	Cortes, [26]
NCT01641107 (phase 2)	Ponatinib + Steroid	ND	44	CHR: 86.4% (38/44)	Martinelli, [27]
NCT03576547 (phase 1/2)	Ponatinib + Venetoclax + Dexamethasone	R/R	9	CR/CRi: 56% (5/9) 1-y OS: 72%	Short, [28]

ND: newly diagnosed, R/R: refractory/relapse, CT: chemotherapy, CMR: complete molecular response, HCR: haematologic complete remission, HRFS: haematologic relapse-free survival, CRi: incomplete haematologic recovery, DFS: disease-free survival, CHR: complete haematologic response,

2.1. Imatinib + chemotherapy

The classic combination regimen of Imatinib and chemotherapy has shown impressive results in adult patients, with a CR rate exceeding 95% and 5-year OS rate of approximately 60%−70% [11,29]. In primary pediatric patients with PH + ALL, several Phase 2 studies have explored the addition of Imatinib to first-line chemotherapy. These studies reported a 97% achievement of first CR after induction, with 5-year event-free survival (EFS) and OS rates of 57% and 71.3%, respectively. The estimated cumulative rate of death over 5 years while in remission was 16.1%, and the relapse rate was 26.9% [10,30]. In a retrospective study of adolescent and young adult (AYA)patients, adding Imatinib early in the induction phase of a pediatric chemotherapy regimen resulted in a greater OS benefit [31]. Furthermore, in patients who were intolerant to chemotherapy, the combination of low-dose chemotherapy with Imatinib reduced early mortality and achieved a higher CR rate of 98.5% compared to hyper-CVAD [4]. This reduced toxicity early in treatment led to 77% of patients undergoing HSCT and achieving longer EFS (2.5 vs. 1.8 years) and OS (4.1 vs. 3.3 years). Nonetheless, disease relapse and the development of drug resistance represent significant challenges for the prognosis of PH + ALL patients. There have been documented instances of patients developing resistance to Imatinib [32]. The constrained effectiveness of imatinib against ABL1 domain mutations and its limited ability to penetrate the central nervous system (CNS) have been pivotal in driving the advancement of TKIs such as Dasatinib [33–35] (Table 3).

Table 3. Summary of Immune-targeted therapy for PH + ALL.

Basic information	Interventions	Condition	N	Clinical outcomes	Reference
Case report	Venetoclax + Ponatinib or Flumatinib	R/R	5	CMR/CR = 80%	Yin, [76]
NCT03576547 (phase 1/2)	Venetoclax + Pobatinib	R/R	9	CRi/CR = 56% 1-year OS = 72%	Farhad, [28]
Retrospective study	Venetoclax + Pobatinib or Dasatinib +Dexamethasone	R/R	19	CRi/CR = 89.5%	Zhu, [77]
NCT02744768 (Phase 2)	Dasatinib + Blinatumomab	ND	63	2-cycles CMR: 60% OS: 95% DFS: 88%	Robin, [62]
NCT02458014 (phase 2)	Blinatumomab + Ponatinib or Dasatinib	MRD+	18	CMR = 61%	Elias J, [78]
NCT02000427 (phase 2)	Blinatumomab	R/R	45	CR/CRh = 35.6%	Giovanni, [79]
Retrospective study	Blinatumomab	R/R	34	CR/CRh/CRi = 41%	Nicolas, [80]
NCT03263572 (phase 2)	Blinatumomab + Ponatinib	ND R/R	60	CR = 87% 1-year OS = 95%	Prof, [81]
NCT02143414 (Phase 2)	Dasatinib + Prednisone+ Blinatumomab	ND	24	CR: 92% 3-y OS: 87% 3-y DFS: 77%	Advani, [40]
Case report	anti-CD19 CAR-T	R/R	7	CR = 6/7 CRq = 1/7	Qiu, [82]
Retrospective study	anti-CD19 CAR-T	R/R	18	CR = 100% 2-year OS = 88% 2-year RFS = 88%	Allison, [83]
NCT03825718 (Phase 1)	anti-CD19 CAR-T	R/R	24	CR = 2/25 CRi = 23/25	Yang, [84]
NCT03173417 (phase 1/2)	anti-CD19 CAR-T	R/R	14	CR = 100%	Zhang, [85]
NCT01319981 (Phase 2)	Rituximab + Imatinib or Dasatinib	ND	20	CMR = 70% MMR = 95%	Elias, [86]
NCT01085617 (Phase 3)	Rituximab + Imatinib + SOC	ND	86	CR = 97% 3-year OS = 59% 3-year EFS = 51%	Prof, [87]
NCT02311998 (phase 1/2)	Inotuzumab ozogamicin + Bosutinib	R/R	18	CR/CRi = 83%	Nitin, [88]
Phase 2 Study	Inotuzumab ozogamicin + Pobatinib or Dasatinib	MRD+	17	MRD- = 59% CR1 = 88%	Jayastu, [89]
NCT01363297 (phase 1/2) NCT01564784 (Phase 3)	Inotuzumab ozogamicin	R/R	22	CR/CRi = 73% MRD- = 81%	Wendy, [90]

CRp: complete remission with incomplete recovery of platelets

2.2. The second generation TKIs

Dasatinib, in particular, exhibits a broader inhibition of tyrosine kinases, including SRC family kinases and c-KIT, and is effective against ABL kinase domain mutations (except T315I) [36,37]. Research by X. Gong et al. suggests that higher doses of Dasatinib (140 mg/day) may increase systemic drug exposure and enhance its penetration into the cerebrospinal fluid [38]. Multiple phase 2 and 3 clinical trials have shown that Dasatinib combined with chemotherapy or low-intensity chemotherapy can lead to rapid and profound remission in patients with PH + ALL [14,16–18,39], increasing CR rates to 96% to 98.5% and 5-year OS rates to 45% to 68%. A phase 2 clinical trial involving 78 PH + ALL patients utilized two-stage Dasatinib induction therapy [15]. Dasatinib and Prednisolone were used as induction therapy to achieve CR, followed by follow-up chemotherapy. All patients achieved CR and 52.6% were minimal residual disease (MRD) negative. 58 patients received HSCT in first complete remission (CR₁). The 3-year EFS and OS rates were 66.2% and 80.5%, respectively. The 3-year relapse and non-relapse mortality (NRM) rates were 26.1% and 7.8%, respectively. Due to the limited tolerance of chemotherapy in elderly patients, Anjali S. et al conducted a study using a chemotherapy-free regimen of Dasatinib combined Prednisone [40]. The study included 24 patients with a median age of 73 years. The induction therapy with Dasatinib and Prednisone achieved a CR in 92% of patients. The median follow-up was 2.7 years, and the 3-year OS and disease-free survival (DFS) rates were 87% and 77%, respectively. However, pulmonary and cardiac adverse events were significant reasons for adjusting the dose of Dasatinib.

Nilotinib binds to the inactive conformation of the ABL kinase structural domain and has increased specificity for most ABL mutant forms [41,42] In a phase 2 trial of Nilotinib in combination with chemotherapy conducted in 30 patients with PH + ALL [20], all patients achieved haematologic complete remission (HCR) after 4 weeks of induction therapy. The cumulative molecular complete response (MCR) rate was 83.3%. The median haematological relapse-free survival (HRFS) was 18 months, and the median OS was 47.5 months. The 4-year HRFS and OS rates were 54% and 45%, respectively. Intestinal obstruction was a significant cause of treatment interruption with Nilotinib.

2.3. Ponatinib

The T315I mutation leading to resistance occurs in 37% of PH + ALL patients treated with first and second generation TKIs [43]. Ponatinib, a third-generation TKI, has been shown to inhibit BCR-ABL1 clones that include the T315I mutation, however, Ponatinib treatment is associated with considerable dose-dependent cardiovascular toxicity, most commonly thrombosis and hepatotoxicity [44]. An initial dose of 45 mg was approved for use through extensive efficacy and safety trials [45]. The current study demonstrated that low-dose Ponatinib maintains stable blood levels and has improved antileukaemic activity in patients with the T315I mutation and relapsed CNS leukaemia PH + ALL [46,47]. A phase II study in 30 patients reduced Ponatinib to 30 mg in combination with standard chemotherapy to reduce cardiovascular side effects, at the end of consolidation therapy [22], the complete molecular response (CMR) rate was 71%, and 26 patients received HSCT, 3-year EFS and OS rates were 70% and 96%, respectively, only 1 patient experienced a cardiovascular event. The phase II clinical trial enrolled 76 patients with PH + ALL treated with Ponatinib in combination with hyper-CVAD [23]. Ponatinib dose could be reduced to 30 or 15 mg daily. major molecular remission rate (MMR) was 95%, CMR was 78%, and 2-year EFS and OS rates were 81% and 80%, respectively. At long-term follow-up, the MRD negative response rate was 99% and the CMR rate was 83%. At the longest follow-up of 77 months, 72% of patients sustained CR, with 3-year EFS and OS rates of 70% and 76%, respectively.

Ponatinib was evaluated in combination with Steroids in elderly patients [27]. 44 patients with PH + ALL were enrolled in the phase 2 trial, with a complete haematologic response (CHR) rate of 86% and 77% achieving a complete cytogenetic response rate (CCyR) with a median EFS of 14.3 months. Several studies have used Ponatinib for prophylactic treatment after HSCT in patients carrying the T315I mutation to reduce recurrence rates [48,49]. Patients who received Ponatinib for a short period of time again achieved CMR with long-term OS rates around 60-90%. Correct treatment regimen (prophylactic, pre-emptive) or dose adjustment of Ponatinib is beneficial to improve the outcome of patients with relapse or T315I mutation.

3. Allogeneic haematopoietic stem cell(allo-hSCT)

3.1. Maintenance of TKIs after HSCT

Numerous studies have reported that the administration of TKIs post-transplantation yields favourable outcomes in OS and cumulative incidence of relapse (CIR) rates. Furthermore, there appears to be no correlation with the occurrence of graft-versus-host disease (GVHD)[50–52]. It is worth discussing that the study by Zhang et al. shows that PH + ALL patients who underwent transplantation had a 63.6% relapse rate at the 3-year mark when they received imatinib post-transplantation [53], compared to a 24.2% relapse rate for patients who did not receive imatinib post-transplantation. Dasatinib seems to offer enhanced survival prospects, particularly for patients who are MRD-positive following transplantation. Nonetheless, determining the use of Dasatinib may necessitate a more personalized approach, taking into account post-transplantation concerns related to toxicity, notably GVHD and cytopenia [15,54,55]. Among patients harbouring the T1315I mutation [49], the utilization of Ponatinib as a maintenance therapy following HSCT resulted in a 5-year DFS rate of 81.5%, an OS rate of 91.7%, and a cumulative relapse rate of 18.5%. Moreover, given the scarcity of follow-up records regarding the cessation of TKIs after HSCT, there remains a necessity for further studies to delve into more potent maintenance dosages and determine the appropriate duration for maintenance therapy.

3.2. Treatment of co-mutations

PH + ALL patients are often complicated with other gene mutations, these co-occurring mutations can impact the disease progression and therapeutic responsiveness [56]. The IKZF1 deletion, which is the primary co-mutation with BCR-ABL1, is evident in around 50–70% of cases [16,57]. IKZF1 deletion leads to a reduced number of early molecular responders and has been linked to a significant decline in OS and DFS, with an increased likelihood of recurrence [58]. Due to the lack of targeted drugs specifically targeting IKZF1 deletion, TKI-based consolidation chemotherapy followed by HSCT is still the standard treatment for CR₁ [59–61]. Robin F et al. conducted a single-group trial of Blinatumomab plus Dasatinib in 63 patients with newly diagnosed PH + ALL (median age, 54 years) [62]. Of these, 54% were IKZF1 deletion and 24% were IKZF1plus. At a median follow-up of 18 months, OS rates was 93% in patients with IKZF1 deletion and 82% in those with IKZF1plus, suggesting that the additional KIZF1 abnormalities in this study had less impact on overall survival and that treatment with Blinatumomab enhancement may reduce the negative prognostic impact of this genotype. In a recent multicenter phase 2 study [63], Dasatinib plus Prednisone was evaluated asa chemotherapy-free regimen in 41 patients with PH + ALL, of whom 30% (9/30) had IKZF1 deletion. The overall CR rate was 95%. IKZF1 gene deletion was associated with relapse, and IKZF1plus was not an ideal response. Bridging to HSCT in CR₁ provided a clear survival advantage, as 2-year progression-free survival (PFS) and OS were 100% in patients undergoing HSCT.

In addition, CDKN2A/2B Deletions account for approximately 40% of concurrent mutations in PH + ALL and are considered poor prognostic markers for long-term outcomes in some studies [64,65]. A study by XU et al. reported that CDKN2A/2B gene deletion was associated with drug resistance [6]. The group with wild-type CDKN2 gene had a higher CMR rate before HSCT than the group with CDKN2 deletion, while no difference in CMR rate was observed after HSCT. This suggests that HSCT has some survival advantage. Other concurrent mutations such as PAX5, TP53, etc., need more clinical trials to evaluate the impact on prognosis, so as to carry out finer prognostic stratification and more personalized management of PH + ALL patients [66].

3.3. Role of HSCT in TKIs management

Prior to the era of TKIs, HSCT at the time of CR₁ in PH + ALL patients had been the standard strategy for achieving long-term DFS and OS, improving long-term survival from 20% to 40% compared to chemotherapy regimens alone [59,67]. Thereafter, TKIs combined with chemotherapy followed by HSCT continued to benefit long-term survival and was independently associated with favourable OS and EFS [68,69]. However, with the advent of TKIs, some studies have shown that the benefit of HSCT is gradually decreasing, especially in pediatric population, the survival benefit of TKI combined with chemotherapy is not inferior to HSCT [16,70]. The study by Mixue X et al showed that among patients who obtained CMR [59], survival in the nontransplant subgroup was comparable to that in the transplant subgroup, with estimated 5-year OS of 64% versus 58% and 5-year DFS of 58% versus 51%, respectively. In a randomized trial of PH + ALL patients treated with Imatinib plus chemotherapy [9], 3-year OS and EFS did not differ significantly between patients who underwent HSCT and those who did not undergo HSCT. Patients younger than 55 years old may benefit from HSCT, with a 3-year OS and EFS of 81% and 77%. In a clinical study focused on adults with PH + ALL [16], the participants who were administered Dasatinib in conjunction with chemotherapy exhibited 5-year OS and EFS rates that were equivalent with those who underwent HSCT. Several retrospective studies have validated this by contrasting the outcomes of PH + ALL patients who either underwent or did not undergo HSCT after receiving a combination of chemotherapy and TKIs(primarily Dasatinib) [71,72]. The 3-year OS rate reached an impressive 90% among PH + ALL patients who received the initial treatment regimen of Ponatinib combined with hyper-CVAD [23,73], while those who underwent HSCT had a slightly lower OS rate of 68%. Both Ponatinib and Blinatumomab produce high molecular response rates in PH + ALL, and the combination regimen of the two drugs may produce a durable response. In a Phase II clinical trial conducted at MD Anderson [74], 44 PH + ALL patients were included. For the initial induction phase of treatment, a combination of Ponatinib and Blinatumomab was administered as the first-line regimen. Only one patient received HSCT at first response, with an overall CMR rate of 85% and an estimated 3-year incidence of both PFS and OS of 95%. The same results were obtained ina retrospective study of 26 patients with R/R PH + ALL treated with Blinatumomab in combination with Ponatinib [75]. 96% of these patients achieved CR and 32% underwent HSCT. However, there was no significant difference in median OS and EFS between the transplant and non-transplant groups. These findings imply that the introduction of a novel generation of immunotargeted drugs, combined with second and third-generation TKIs, could potentially decrease the reliance on transplantation.

4. Venetoclax

The pro-survival effect of BCR-ABL1-mediated transformation is attributed to the upregulation of Bcl-2 family proteins. Several authors have reported that BCL-2 inhibitor Venetoclax induces endogenous apoptosis by targeting the pro-survival Bcl-2 signalling pathway and its downstream pathways, such as Lck/Yes novel tyrosine kinase and CDK6, leading to the downregulation of Bcl-2 and Mcl-1 when combined with TKIs [91,92]. Clinical evaluations have demonstrated the efficacy of Venetoclax in combination with TKIs for treating patients who have developed resistance [93]. A case study reported 5 patients with R/R PH + ALL who received Venetoclax and low dose Dexamethasone in combination with TKIs (Ponatinib in 2 patients with T315I mutation and Flumatinib in 3 patients) [76]. The dose of Venetoclax ranged from 100 to 400 mg. CMR/ CR were achieved by 3 patients and 1 patient, respectively, indicating good activity in clearing MRD. The main adverse effect was myelosuppression. In a phase 2 clinical study of 9 patients with R/R PH + ALL [28], the combined application of Ponatinib and Venetoclax was evaluated. Patient was administered daily dosages of either 400 mg or 800 mg of Venetoclax, with no observed dose-limiting toxicities. As a result, 56% of patients achieved CR or incomplete haematologic recovery (CRi), and specifically among those receiving 800 mg daily of Venetoclax, the CR/CRi rate was as high as 83%. Furthermore, no relapses were reported. The median follow-up duration was 13.2 months, reporting a 1-year OS rate of 72%. In a retrospective study [77], 19 PH + ALL patients with T315I mutations were treated with VPD programme (Venetoclax, Ponatinib, Dexamethasone). After one cycle, 89.5% achieved CR/CRi and 82.4% achieved MRD-negative, 64.7% achieved MMR. The most common grade 3,4 adverse events were neutropenia and thrombocytopenia. The results indicated that Venetoclax combined with TKIs was effective and safe for R/R PH + ALL patients as a new chemotherapy-free regimen.

5. HDAC inhibitor

Histone deacetylases (HDACs) are one of the key enzyme classes that maintain histone acetylation homeostasis in nucleosomes. BCR-ABL1 through its downstream signalling transduction such as c-Myc can interact with HDAC1, leading to a reduction in the expression of tumour-suppressor genes, which in turn drives the development of leukaemia [94]. It has been shown that histone deacetylase inhibitor (HDACi) leads to loss of chaperone function through increased acetylation of heat shock protein 90 (HSP90) and exposes its client protein BCR-ABL1 to polyubiquitination and degradation via the proteasome pathway [95]. Linyu Y et al. reported that Purinostat Mesylate treatment increased acetylation of Ac-H3 and Ac-H4 in cells and decreased BCR-ABL1, HSP90, HCK, and p-SRC levels, reversing the progression of leukaemia in the BCR-ABL1 (T315I) PDX mouse model with a good pharmacokinetic and low toxicity profile [96]. S. T. Promod et al. showed that HDAC1,2 inhibitor treatment in PH + ALL cell lines resulted in a 1.5-fold increase in chromatin-bound levels of cell cycle regulatory and apoptosis-related proteins (p53, granzyme C and granzyme B) [97]. In addition, the HDAC1,2 inhibitor combined with adriamycin impaired the Mre11-Rad51-DNA ligase 1 axis, a central hub of DNA repair, and blocked BCR-ABL1-mediated double-strand break (DSB) repair signalling. This overcame the survival advantage of PH + ALL cell lines and progenitor cells in a cardiotoxicity-free PDX mouse model, reducing leukaemic load in vivo. Seiichi O. et al. showed that the aurora kinase inhibitor (MK-0457) in combination with histone deacetylase inhibitor (Vorinostat) was effective in PH + ALL cell lines carrying the T315I mutation [98], which was observed to be activated by caspase 3 and poly (ADP-ribose) polymerase (PARP) in primary cell lines. Down-regulation of Crk-L and Lyn phosphorylation was observed, leading to a dose-dependent decrease in HSP90 and HSP70 expression and an increase in apoptosis. Several preclinical studies have applied TKIs in combination with HDACi regimens to the treatment of CML and have yielded notable advances. These findings provide strong evidence for further clinical application of PH + ALL combination regimens [99,100].

6. CD19: blinatumomab and CAR-T

CD19, a surface antigen found on B lymphocytes, is present from the pre-B cell stage all the way to maturation [101]. Over 90% of patients with PH + ALL show high levels of CD19 expression, making it an optimal target for monoclonal antibody and CAR-T treatments. However, the variability in CD19 antigen expression can potentially influence the effectiveness of these targeted therapies [102,103].

6.1. Blinatumomab

Blinatumomab, a traditional BiTE, has two arms which respectively bind to CD19 and resting T cells. This dual binding elicits direct cytotoxicity in CD19-positive cells and boosts the production of inflammatory cytokines [104]. Unlike the mechanism of action of TKIs, Blinatumomab exerts its killing effect by binding directly to T cells and navigating to leukaemia cells, activating them and prompting them to release perforin and granzyme [105]. Several studies have been conducted to evaluate the efficacy of Blinatumomab in patients with MRD-positive and T315I mutations, with CR/Complete remission with partial haematological (CRh) rates ranging from 35% to 41%, and MRD responses in 67% to 88% of patients after receiving approximately two cycles of treatment, this provides a bridge for patients to undergo HSCT, and effectively improves haematologic relapse-free survival (HRFS) and OS time [79,80,106]. The combination of Blinatumomab and TKIs for treating PH + ALL patients represents a promising chemotherapy-free regimen that offers substantial benefits, particularly in terms of achieving high rates of MRD negativity [78]. A clinical study evaluated the efficacy of Blinatumomab in combination with Dasatinib in PH + ALL patients [62,107]. The patients received Blinatumomab as consolidation therapy and complete remission was observed in 98%. At a median follow-up of 18 months, OS rate was 95%, and DFS rate was 88%. A decrease in Tregs cells and an increase in CD8 + cells were observed after Blinatumomab treatment, suggesting a positive effect on the activation of the host immune system. The safety and efficacy of combining Ponatinib with Blinatumomab were evaluated by MD Anderson Cancer Center in a study involving patients with newly diagnosed and R/R PH + ALL [81]. After receiving five cycles of Blinatumomab treatment, the CMR rates were 87% for newly diagnosed patients and 92% for R/R patients. Additionally, the 1-year OS rate was 95%. Overall, the efficacy of the chemotherapy-free regimen combining TKIs and Blinatumomab was encouraging.

6.2. CAR-T

CAR-T cells use gene modification techniques to make T cells express chimeric antigen receptors (CARs), which recognisze specific antigens on the surface of cancer cells and attack tumour cells. Very significant results have been achieved in B-ALL and AML [108]. Cytokine release syndrome (CRS) caused by T-cell activation and immune effector cells after CAR-T cell infusion is a serious clinical complication, and in some cases symptoms can progress to fever and hypotension [109]. Combination therapy with the anti-IL6 receptor antibody Tocilizumab controls and improves CRS, and early use of corticosteroids to prevent CRS can be effective [110,111]. CAR-T cell therapy or combination TKIs achieved sustained remission in TKIS-resistant and karyotypically complex R/R PH + ALL patients in a short period of time [82,112]. In the study of Danqing K et al. [83], the PH + ALL patients in the high-risk cytogenetic layer who received CTL019/tisagenlecleucel treatment had a CR rate of 100%, the BCR-ABL1 transcript was lower than the detectable level, and the 2-year OS and RFS was 88%. In another study by Peihua Lu et al. [84], a novel type of CAR-T cell called FasT CAR-T was developed, which exhibited a significant 2–10-fold expansion in vitro and in vivo upon stimulation with CD19 antigen. The FasT CAR-T cells demonstrated superior killing ability compared to conventional CAR-T cells, and BCR-ABL1-positive patients achieved MRD negativity and CR/CRi within two weeks of CAR-T cell infusion. A phase 1/2 study (NCT03173417) enrolled 14 PH + ALL patients [85]. The CAR-T cells were administered at doses ranging from ≥1 × 10⁵ cells per kg to <3 × 10⁵ cells per kg. The results showed that all 14 PH + ALL patients achieved CR, with 86% attaining MRD negativity. Notably, 1-year OS and leukaemia-free survival (LFS) rates were about 70-80% in patients with PH + ALL, and survival analyses showed no significant differences in OS and LFS rates between the PH + and PH-groups. It is worth noting that immune escape, leading to the survival of CD19-negative tumour cells after treatment with Blinatumomab and antibody–drug conjugates (ADC), is a significant factor contributing to relapse after CAR-T cell therapy [113]. To tackle this challenge, future studies should concentrate on optimizing the structure of CAR-T cells to effectively target a broader range of driver mutation-specific antigens.

7. CD20: rituximab

CD20, a B-cell marker, is expressed from late pre-B cells and diminishes as cells differentiate into terminal or plasma cells, it can be phosphorylated in response to external signals, initiating intracellular signalling pathways that regulate cell proliferation and differentiation by controlling calcium influx [114]. CD20 expression is prevalent in most patients with PH + ALL and is associated with poor disease characteristics [102,115]. Na Xu et al. discovered a positive correlation between CD20 expression and CDKN2 deficiency in PH + ALL patients [6], and those with CD20-positive CDKN2 deficiency exhibited lower OS and DFS rates compared to CD20-negative patients. Rituximab, a monoclonal antibody therapy, specifically binds to CD20 and primarily exerts its effects through complement-dependent cytotoxicity (CDC), by recruiting the C1 complex, it triggers the classical complement cascade, leading to the insertion of the membrane attack complex (MAC) and subsequent cell lysis [116]. Several clinical studies have investigated the impact of adding Rituximab to CD20-positive PH + ALL patients treated with hyper-CMAD(Liposome vincristine replaces regular vincristine) or hyper-CVAD and TKIs regimens [86,117]. These studies have shown that this combination may enhance the molecular response rate. Additionally, achieving early MRD negativity has been found to significantly improve the prognosis of patients. A phase 3 multicenter study recruited 172 PH + ALL patients who were randomly assigned to receive either standard of care (SOC: n = 86) or SOC plus Rituximab (n = 86) [87]. Rituximab was administered at a dose of 375 mg/m2. PH + ALLpatients treated with Rituximab demonstrated enhanced effectiveness, with a CR rate of 96.5% versus 97.7% after two cycles of induction therapy in the SOC and SOC plus Rituximab groups, respectively. The 3-year EFS and relapse rates were 45% versus 51.9% and 30.6% versus 19.9%, respectively. Considering these promising results, further optimization of the combination of CD20 monoclonal antibodies with other treatments is warranted for PH + ALL patients.

8. CD22: inotuzumab ozogamicin

CD22 belongs to the Siglec protein family and is primarily expressed in the bone marrow and spleen during the early stages of B-cell development, with minimal expression outside the haematopoietic system [118]. It has been observed that CD22 is expressed in over 90% of leukaemia cells in PH + ALL patients and has been investigated as a potential target for therapies based on the BCR-ABL1 [119]. Inotuzumab Ozogamicin is an ADC composed of the G544 IgG4 alloantigen linked to kallikrein, upon binding to CD22, the complex is internalized, releasing calicheamicin derivatives that induce DNA breaks and apoptosis, leading to cytotoxic effects [120]. In certain case reports [121–123], Inotuzumab Ozogamicin has shown potential clinical benefits in patients with relapsed PH + ALL carrying the T315I mutation after transplantation. However, achieving long-lasting remission rates with a single agent is challenging. Combining Inotuzumab Ozogamicin with Ponatinib has been considered an effective strategy for achieving prolonged molecular remissions. A phase 1/2 study investigated the safety and effectiveness of combining Inotuzumab Ozogamicin with second generation TKI Bosutinib in the treatment of 18 PH  + ALL patients (16 with PH + ALL and 2 with lymphoid blast phase-CML) with a median age of 62 years [88]. The patients received a total of 6 treatment cycles. The CR/CRi rate was 83%, and the CCyR was 81%. Additionally, 61% of patients achieved MRD negativity. 6 patients underwent HSCT after achieving a response. Over a median follow-up of 44 months, the median EFS was 7.7 months, and the OS was 13.5 months. In a recent phase II clinical trial, 17 PH + ALL patients who did not achieve MRD negativity or experienced relapse for at least 3 months after initial treatment were enrolled [89]. The trial aimed to assess the efficacy of combining Ponatinib or Dasatinib with Inotuzumab Ozogamicin in achieving MRD response. Among the patients, 59% achieved MRD negativity, and 88% achieved CR₁. The survival outcomes in PH + ALL patients were comparable to those of PH-ALL patients. In an open-label, randomized Phase 3 trial, CD22+ R/R PH + ALL patients were randomly assigned to Inotuzumab Ozogamicin group or SOC group [90]. Patients in the Inotuzumab Ozogamicin group received up to 6 cycles of treatment. The CR/CRi rate in Inotuzumab Ozogamicin group (73%) was higher than that in SOC group (56%). The rate of MRD negativity among patients who achieved CR/CRi was higher in the Inotuzumab Ozogamicin group (81%) versus SOC group (33%), which means that more patients received HSCT. The median PFS and OS rate was 3.9 vs 3.1 months and 8.7 vs 8.4 months for Inotuzumab Ozogamicin versus SOC group. These findings provide compelling evidence supporting the further clinical use of Inotuzumab Ozogamicin in the treatment of PH + ALL.

9. CD33: gemtuzumab ozogamicin

CD33 is a tumour-associated target antigen that is highly expressed in progenitor cells in most patients with AML, but not in mature granulocytes or other tissues [124]. Studies have shown that CD33 is expressed in 34% of PH + ALL patients, and its expression is associated with P190BCR-ABL1 transcript levels, based on these findings, the CD33 monoclonal antibody gemtuzumab ozogamicin (GO), which is used in the treatment of AML, may exhibit potent activity against PH + ALL [125,126]. In an in vivo xenograft model developed by Josee G et al. [127], GO demonstrated cytotoxic activity against CD33-positive ALL cells in both in vitro and in vivo assays, leading to a significant prolongation of survival in mice. Patrice C et al. reported the feasibility of using GO in CD33-positive PH + ALL patients who were resistant to conventional chemotherapy. Their study demonstrated that GO synergistically works with standard chemotherapy, inducing potentially CR in patients without excessive liver toxicity [128]. Additionally, in rare cases of BCR-ABL1-positive AML (M5), the addition of GO treatment after chemotherapy selectively eliminates malignant cells while preserving normal stem cells, resulting in cytogenetic complete responses [129]. The use of myeloid antigens that are expressed aberrantly as effective therapeutic targets in PH + ALL patients remains controversial due to limited clinical data. Therefore, further basic and clinical studies are needed to elucidate the underlying mechanisms and develop more effective drug combinations.

10. Conclusion

Philadelphia chromosome-positive acute lymphoblastic leukaemia (PH + ALL) has historically been associated with a poor prognosis. It is often resistant to chemotherapy, has a high relapse rate. However, with the introduction of TKIs, the use of low-/high-intensity conventional chemotherapy or hyper-CVAD combinations has increased CR rates to 90% in PH + ALL patients, leading to improved estimated 5-year survival rates of 50%, compared to the previous range of 10-20%. Historically, HSCT has been the only curative treatment for PH + ALL patients. As the emergence of next-generation TKIs, especially in the pediatric population, HSCT may not have the advantage of chemotherapy combined with TKIs. According to the latest data, long-term survival was better with Dasatinib or Ponatinib combined with multiple chemotherapy or corticosteroids, with little difference compared to the transplant group. Given the advancements in new immunotherapies like Blinatumomab, it is imperative to conduct future investigations to ascertain whether AYA and adult patients can attain favourable survival outcomes by adopting a better-tolerated, chemotherapy-free regimen that reducing the dependence on HSCT. After HSCT, patients should be preferentially treated with TKIs maintenance for more than one year in the absence of drug resistance and ABL1 domain mutations. Enhanced MRD monitoring is warranted because MRD clearance before HSCT leads to a better prognosis compared to MRD positivity. If MRD continues to be positive after HSCT, it is possible to achieve long-term remission by second or third generation TKIs combined with multi-drug chemotherapy, Blinatumomab, and CAR-T. For elderly patients who cannot tolerate chemotherapy, low-toxicity chemotherapy-free regimens are a preferable option. Further research is needed to determine whether TKIs in combination with oral agents can achieve better efficacy than chemotherapy regimens as a first-line treatment.

In the future development trend of PH + ALL treatment mode, there is a growing focus on targeted agents such as BCL-2 inhibitors and HDAC inhibitors, which provide additional treatment options for relapsed/refractory (R/R) patients and newly diagnosed elderly patients who are not suitable for intensive chemotherapy. Immunotherapies targeting tumour-specific antigens expressed in PH + ALL patients, such as CD20 monoclonal antibodies (mAb), bispecific T-cell engagers (BiTEs), Inotuzumab ozogamici, and CAR-T cell therapies, have shown promising results and are now included in first-line treatments for R/R patients. Studies have demonstrated that combining bispecific antibodies with second- and third-generation TKIs leads to higher complete response rates and deeper clearance of MRD at an early stage, providing a bridge for R/R patients to undergo HSCT. Prior to treatment, careful consideration of the patient's immunophenotype is necessary, requiring precise disease monitoring and risk stratification to better target and combine these drug regimens to overcome resistance and minimize toxicity. The mechanisms of action of multiple drugs targeting PH + ALL are shown in Figure 2. The treatment landscape for PH + ALL is constantly evolving, with a current trend towards moving away from low-toxicity drugs and adopting chemotherapy-free regimens, which holds the potential for further improvements in prognosis in the future.

Figure 2. Mechanism of action of multiple targeted therapeutics in PH + ALL. CDC: complement-dependent cytotoxicity. ADCC: antibody-dependent cell-mediated cytotoxicity.

Author contributions

H-H.Z and S-Z.Z conceived the idea for this paper. Z-Y.X and Y-J.S contributed equally to data collection and wrote the manuscript. All authors contributed to the article and approved the submitted version.

Disclosure statement

No potential conflict of interest was reported by the author(s).