Venetoclax + hypomethylating agents or low dose cytarabine
Early studies using venetoclax as monotherapy in AML demonstrated only modest efficacy in high-risk relapsed/refractory (R/R) AML patients with an overall response rate (ORR) of 38% and complete remission/complete remission with incomplete hematologic recovery (CR/CRi) of 19%. The responses were short lived, with overall survival (OS) of only 4.7 months [
4]. Based on promising results from two large Phase 1b/II trials using combination of a hypomethylating agent (HMA) or low-dose cytarabine (LDAC) with venetoclax in untreated older AML patients [
5,
6], FDA granted accelerated approval to venetoclax in combination with azacitidine (AZA) or decitabine (DEC) or LDAC for the treatment of newly-diagnosed (ND) AML in adults who are age 75 years or older, or who have comorbidities that preclude use of intensive induction chemotherapies in 2018.
Recently published Phase III randomized studies confirmed the results from these early single arm trials, and demonstrated a significant survival benefit from adding venetoclax to azacitidine and to LDAC [
7,
8]. The major findings from the VIALE-A and VIALE-C trials are summarized in Table
1. In summary, the VIALE-A trial included 431 patients without history of exposure to azacitidine. At a median follow-up of 20.5 months, the median OS was 14.7 months in the azacitidine-venetoclax group and 9.6 months in the control group. The incidence of CR and composite complete remission rate (cCR) (CR + CRi) were significantly higher with azacitidine-venetoclax than with the control regimen. However, there were higher rates in key adverse events in the azacitidine-venetoclax group than those in the control group, but they were manageable [
7]. The VIALE-C study assigned 211 patients to either venetoclax (
n = 143) or placebo (
n = 68) in 28-day cycles, plus LDAC on days 1 to 10. In contrast to VIALE-A trial, 20% enrolled patients had received prior HMA treatments. The planned primary analysis showed a 25% reduction in risk of death with venetoclax plus LDAC
vs LDAC alone, although this was not statistically significant. Median OS was 7.2
vs 4.1 months, respectively. Unplanned analyses with an additional 6-months follow-up demonstrated median OS of 8.4 months for the venetoclax arm (HR, 0.70; 95% CI, 0.50–0.98;
P = 0.04). CR/CRi rates were 48% and 13% for venetoclax plus LDAC and LDAC alone, respectively. Thus, venetoclax plus LDAC demonstrated clinically meaningful improvement in remission rate and OS
vs LDAC alone, with a manageable safety profile [
8]. Based on these confirmatory data, FDA granted full approval to these venetoclax combinations for treating newly diagnosed AML patients. Both trials established new standard of care for unfit newly diagnosed AML patients. Since VIALE-A trial excluded patients with previous exposure to azacitidine, and 20% patients enrolled on the VIALE-C trial had exposure to HMA, venetoclax plus LDAC might be a preferred consideration for patients who received HMAs in the past.
Table 1
Comparison of randomized prospective studies on venetoclax-based combinations in AML: AZA + venetoclax vs LDAC + venetoclax
Phase | III VIALE-A trial | III VIALE-C trial |
Population | Age > 75 years or unfit for chemotherapy |
Control arm | AZA | LDAC |
h/o HMA | No | Yes, allowed (20%) |
Patient number | 431 (286 in AZA + venetoclax) | 211 (143 in LDAC + venetoclax) |
Median age (range), years | 76 (49–91) | 76 (36–93) |
30-day mortality, % | 7% | 13% |
cCR (CR) rate, % | 66.4% (36.7%) | 48% (27%) |
MRD negativity, % | N/A | 6% |
Time to CR (response) | 1.3 months (0.6–9.9) | N/A most response at the end of cycle 2 |
Median DOR, months | 17.5 (13.6 to NR) | NA |
Median OS, months | 14.7 (11.9–18.7) | 8.4 (5.9–10.1) |
Reference | | |
Both trials also identified that patients with NPM1 and IDH1/2 mutations had high CR rates of 91%, and 71%, respectively with HMA + venetoclax [
5] and high CR/CRi rates (89% and 72%), respectively, when treated with venetoclax + LDAC [
6]. Patients with FLT3 mutations (Internal tandem duplication (ITD) and/or tyrosine kinase domain (TKD) also demonstrated high CR rate of 72% [
5]. On the other hand, inhibitors to these mutations have been developed and will be discussed in the following sections. It would be continued debate on how to choose the first line treatment for AML with these mutations: hypomethylating agents with IDH1/2 inhibitors
vs venetoclax-based combination; how to sequence the treatment options: venetoclax-based combinations first followed by IDH1/2 inhibitors at disease relapse/ progression or the other way around; or use three drugs combination with HMA + venetoclax + IDH1/2 inhibitor to get deeper remission. Only randomized clinical trials could eventually answer these important clinical questions.
Venetoclax + intensive chemotherapy
Not surprisingly, venetoclax has been studied in combinations with intensive chemotherapy as well (summarized in Table
2). A retrospective report of 13 patients treated with FLAVIDA salvage therapy (fludarabine, cytarabine, and idarubicin in combination with venetoclax 100 mg daily for 7 days; dose reduced due to concurrent azole administration) compared to a control cohort received FLA-Ida (fludarabine, cytarabine, and idarubicin) reported a higher but not statistically significant CR/CRi rate of 69% compared to 47% in the control cohort [
9]. A phase 1b/II trial of medically fit patients with R/R AML receiving FLAG-Ida induction and consolidation in combination with a 14 days course of venetoclax was conducted at MD Anderson. Early results were promising with CRc of 74% in all the patients and an impressive CRc of 91% in newly diagnosed (ND) patients. Consistent with known venetoclax resistance mechanisms, high levels of MCL-1 expression were found in patients who relapsed following FLAG-Ida + venetoclax [
10]. The updated data of 62 patients (27 with ND AML and 35 with R/R AML) from the trial was recently presented. The ORR was 84%, with 89% of ND AML and 66% of R/R AML patients achieving a CRc. 83% of patients achieved minimal residual disease (MRD) negative (MRD-) status assessed by flow cytometry. After a median follow up of 11 months, median OS was not reached. The addition of venetoclax to FLAG-ida demonstrated robust efficacy with acceptable safety profile [
11].
Table 2
Summary of venetoclax-based combinations in AML
FLA-Ida | Retrospective | R/R AML | 13 | 69% | |
FLAG-ida | Ib/II | ND AML R/R AML | 27 35 | 89% in ND AML 66% in R/R AML | |
CAVEAT (5 + 2) | Ib | ND AML | 51 | 72% in all 97% in de novo AML 43% secondary AML | |
DEC10 | II | ND AML R/R AML | 70 55 | 86% in ND AML 42% in R/R AML | |
CLIA | II | ND AML | 18 | 88% | |
CLAD/LDAC, alternating with AZA | II | ND AML | 48 | 94% | |
CPX-351 | II | R/R AML ND AML | 17 1 | 37% | |
CPX-351 LIT | Ib | ND AML | 44 planned | NA | |
GO | Ib | R/R AML | 24 planned | NA | |
The CAVEAT study reported data on 51 newly diagnosed patients with AML, either de novo or secondary, who were treated in five venetoclax dose-escalation cohorts (50–600 mg; venetoclax was given over 14 days, day -6 to 7 with induction chemotherapy (cytarabine 100 mg/m
2 days 1–5 and idarubicin 12 mg/m
2 intravenously days 2–3)). The same venetoclax dose and schedule was given for four cycles of consolidation (cytarabine, days 1–2, and idarubicin, day 1), and as maintenance (up to seven 28-day cycles). The overall CR/CRi rate was 72%, but was 97% in the 28 patients with de novo AML and only 43% in secondary AML. [
12]. In our center, we have used HiDAC + mitoxantrone + venetoclax for several heavily pretreated patients with R/R acute leukemia to control the disease prior to allogeneic stem cell transplantation (allo-SCT) (personal experience). This combination warrants further study in both newly diagnosed and R/R AML setting.
The results of ten-days of decitabine (DEC10) with venetoclax (DEC10-VEN) in AML and high-risk MDS were reported. DEC10-VEN is safe and highly effective in newly diagnosed AML and can serve as an effective bridge to SCT. Median OS in treatment naïve AML patients who subsequently underwent SCT was not reached (1 year OS of 100%). For previously treated AML patients, OS was 22.1 months [
13]. In addition, propensity score matched analysis (PSMA) was employed to compare outcomes of 54 younger adult patients with R/R AML treated on the prospective phase 2 trial of 10-day decitabine and venetoclax (DEC10-VEN) with a historical cohort of patients treated with intensive chemotherapy. The analysis demonstrated that DEC10-VEN provided comparable response of CR/CRi, OS, and rate of patient to proceed SCT to non-venetoclax based intensive chemotherapy. Thus, DEC10-VEN represents an appropriate salvage therapy, and provides an appropriate backbone for adding novel therapies in R/R AML patients [
19].
The addition of venetoclax to cladribine, idarubicin, and Ara C (CLIA) was safe and effective in ND patients with AML. The combination was not associated with early mortality or prolonged myelosuppression, but did result in high rates of durable MRD negative remissions (NCT02115295) [
14]. Addition of venetoclax to a low-intensity backbone of cladribine + LDAC (CLAD/LDAC) alternating with HMA for older patients with newly diagnosed AML provided a CR/CRi rate of 94%; and among the subset of patients who had CR with complete count recovery, the MRD negative rate was 92%. The regimen was well tolerated, with 4-week mortality rates of 0%. With a median follow-up of more than 11 months, the median OS has not been reached (NR), with 12-month OS rates of 70% [
15]. Full dose CPX-351 plus 7 days of VEN (300 mg on D2-8) was demonstrated to be tolerable with acceptable toxicities in patients with R/R AML with an ORR of 44%; and ORR was high at 60% in patient without prior VEN exposure, compared to just 17% among those who had prior VEN. 86% of responding patients proceeded to SCT. The median OS overall was 6.4 months; and the median OS was not reached among the responders [
16].
Other ongoing trials include open-label, multicenter, 2-part, phase 1b study (NCT04038437) to determine the maximum tolerated dose and evaluate the safety, efficacy, and pharmacokinetics of CPX-351 lower-intensity therapy (LIT) plus venetoclax [
17]. Another single arm, open-label, multi-center, dose-escalation phase Ib study is evaluating the combination of venetoclax and gemtuzumab ozogamicin in R/R CD33 + AML patients (NCT04070768) [
18].
Venetoclax + experimental drugs or targeted inhibitors
Given the proven synergies of BCL-2 inhibition, multiple combinations with targeted agents, and venetoclax are under investigation. There are many ongoing combinations of therapies targeting BCL-2 and other pathways, including FLT3 inhibitors (gilteritinib) and IDH1 and 2 inhibitors (Ivosidenib and enasidenib) (will be discussed in the later sections), MCL-1 inhibitors (VU661013, A-1210477); MEK1/2 inhibitor (cobimetinib), and MDM2 inhibitor (idasanutlin) (reviewed in [
20]), combination with TKI in Ph + acute leukemia [
21] and other emerging pre-clinical combinations including small-molecule inhibitors of CDK9 (the orally active A-1592668 and the related analog A-1467729) leading to down-expression of MCL-1 [
22]; the Exportin inhibitor, Selinexor, [
23]; BET inhibitors, ABBV-075, [
24]; SRC family kinases (SFK) and Bruton's tyrosine kinase (BTK) inhibitor, ArQule 531 (ARQ 531), [
25]; and it is expecting much more novel combinations to come.
Resistance mechanisms
HMA + venetoclax or LDAC + venetoclax have clearly advanced the treatment of AML for older or unfit AML patients. Unfortunately, these regimens are unlikely to provide cure as most patients have relapsed at the median of 7 cycles of treatment. A retrospective study demonstrated that the outcome of 41 patients who failed to respond to HMA + venetoclax was very poor with the median OS of only 2.4 months despite salvage therapy [
26]. To understand the resistance mechanisms, DiNardo CD et al. analyzed 81 patients receiving these venetoclax-based combinations to identify molecular correlates of durable remission, initial response followed by relapse (adaptive resistance), or refractory disease (primary resistance). Acquisition or enrichment of clones with activation of the signaling pathways such as FLT3 or RAS or bi-allelic mutations perturbing TP53 were most commonly identified among primary and adaptive resistance to venetoclax-based combinations. Single-cell studies identified heterogeneous and sometimes divergent interval changes in leukemic clones within a single cycle of therapy, highlighting the dynamic and rapid occurrence of therapeutic selection in AML. In functional studies, gain of FLT3-ITD mutation or loss of TP53 conferred cross-resistance to both venetoclax and cytotoxic-based therapies [
27]. These data confirmed the previous findings that TP53 apoptotic network is the primary mediator of resistance to BCL-2 inhibition in AML cells [
28]. Interestingly, recent study demonstrated that monocytic AML is intrinsically resistant to venetoclax + AZA due to loss of expression of the venetoclax target of BCL-2, but instead preferentially reliant on MCL-1 for the survival. Thus, venetoclax + AZA treatment selects monocytic disease at disease relapse, which is derived from pre-existing monocytic subclones [
29]. AML patients with monocytic disease or TP53 mutation might have high risk to be resistant to venetoclax-based combinations, and clinical trials targeting TP53 mutation or trials specifically targeting monocytic AML might be considered over venetoclax-based combinations.
Future clinical research will focus on deepening the responses provided by HMA + venetoclax with additional targeted agents, like ivosidenib in IDH1 mutated AML (to be discussed in next section), FLT3 inhibitors, and novel pathways inhibitors to eventually cure a greater fraction of newly diagnosed AML, and to explore new strategies to deal with relapses after venetoclax-based therapies.
IDH1/2 inhibitors
IDH1 and IDH2 are critical enzymes for the oxidative carboxylation of isocitrate. A mutation in one of these genes results in increased concentration of 2-hydroxyglutarate (2-HG). 2-HG causes DNA and histone hypermethylation, leading to blocked cellular differentiation and tumorigenesis. Mutations in IDH1 or IDH2 are present in 5% to 15% and 10% to 15% of patients with newly diagnosed AML, respectively [
30]. Oral, small-molecule inhibitors have been developed for both mutant IDH1 (ivosidenib) and IDH2 (enasidenib). In R/R AML, ivosidenib and enasidenib as single agent produced promising responses for the corresponding mutations with ORR of 41.6% (CR: 21.6%) with median OS of 8.8 months [
31] and ORR of 40.3% (CR 20.6%) with median OS of 9.3 months [
32] respectively. FDA approved ivosidenib and enasidenib for patients with relapsed or refractory IDH1 and IDH2 mutated AML, respectively, in 2018. In the front line setting, both inhibitors have also demonstrated clinical effectiveness [
33,
34], leading to FDA approval of ivosidenib for patients with newly diagnosed IDH1 mutated AML based on an ORR of 42% (CR: 30%) with median OS of 12.6 months in older patients not eligible for intensive therapy [
34].
The Phase 3 IDHENTIFY study evaluating enasidenib plus best supportive care (BSC) versus conventional care regimens, which included BSC only, azacitidine plus BSC, low-dose cytarabine plus BSC, or intermediate-dose cytarabine plus BSC, did not meet the primary endpoint of OS in patients with R/R AML with an IDH2 mutation. The safety profile of enasidenib was consistent with previously reported findings. IDH inhibitors alone are unlikely to provide cure or durable remission for R/R AML, but they might provide excellent disease control with low toxicity and a bridge to allo-SCT.
IDH inhibitors work in part through induction of differentiation of malignant cells, leading to differentiation syndrome in 10% to 20% of patients. Clinical features are similar to those seen in patients with acute promyelocytic leukemia (APL) treated with ATRA-based regimens [
35,
36]. Early studies established a firm association between IDH mutations and serum 2-HG concentration in AML, and confirmed that serum oncometabolite measurements provide useful diagnostic and prognostic information that can improve patient selection for IDH-targeted therapies [
37]. However, 2-HG level reduction and clearance of IDH mutation by next generation sequencing (NGS) assay does not correlate with the clinical response. These inhibitors are unlikely to provide cure of the AML due to primary resistance from co-mutations in other pathways especially the NRAS/KRAS, and MAPK pathway effectors PTPN11, NF1, FLT3 and others [
38] and secondary resistance from development of second-site IDH2 missense mutations or isoform switching [
39,
40].
Since IDH1/2 mutations lead to DNA and histone hypermethylation, HMAs might have synergistic effects in combination of IDH inhibitors. Combination of HMAs with IDH inhibitors has been studied. The combination of ivosidenib and azacitidine was studied in 23 patients with IDH1 mutated AML as front line treatment. The ORR was 78% with CR/CRh rate of 70%, and median time to response of 1.8 months; median response duration was not yet reached. The ivosidenib and azacitidine combination was well tolerated with a safety profile consistent with ivosidenib or AZA monotherapy and with 17% incidence of IDH differentiation syndrome. Clearance of mutated IDH1 was seen in 63% patients with CR/CRh. CR and ORR rates exceeded those expected from AZA alone [
41]; 83% CR/CRh patients achieved MRD negativity by flow cytometry [
42]. AGILE, a global, double-blind, randomized, placebo-controlled, phase III trial for patients with previously untreated IDH1 mutated AML who are not candidates for intensive therapy (NCT03173248) is actively enrolling patients from 172 study centers across the world [
43]. Patients are randomly assigned to AZA + ivosidenib or AZA + placebo.
As for the IDH2 inhibitor of enasidenib, the phase II portion of an open-label, randomized phase I/II study of enasidenib (E) + AZA (“E + A”)
vs AZA monotherapy (“A”) in patients with mutated IDH2 (mIDH2) ND AML (NCT02677922) was recently reported [
44]. 101 patients with intermediate- or poor-risk cytogenetics were randomized 2:1 to E + A or A in 28-day cycles. ORR (71%
vs 42%) and CR (53%
vs 12%) rates were significantly improved with E + A with greater clearance of mIDH2 allele frequency. Time to first response was about 2 months in each arm and the time to CR was 5.5 months (range, 0.7–19.5). There was no difference in median PFS and OS so far [
44].
As discussed in the section of Azacitidine and venetoclax, this combination is very effective in patients with IDH1/2 mutation. In a pooled retrospective study, 79 patients with IDH1/2 mutation were identified and treated with VEN + AZA on either the Phase Ib or the randomized Phase III (VIALE-A) trials. CR/CRh was 72% (95% CI: 61%-82%) in the whole population. In patients with IDH1, CR/CRh was 59%, median time to first CR/CRh response was 2.3 months, and median duration of response (DOR) and OS were 21.9 (7.8–29.5) months and NR. In patients with IDH2, CR/CRh rates were 80%, median time to first CR/CRh response was 1.0 month. Median DOR and median OS (mOS) were NR. Thus, VEN + AZA provided high response rates, long DOR, and mOS among treatment-naïve patients with IDH1/2 mutation ineligible for intensive chemotherapy with acceptable safety profile [
45]. As mentioned previously, it will be a continued debate to optimize the front line treatment for unfit AML patients with IDH1/2 mutations.
There is also a rationale for combining IDH inhibitors with BCL-2 inhibitors, since the accumulation of 2-HG caused by IDH mutations could decrease the mitochondrial threshold for induction of apoptosis induced by BCL-2 inhibition with venetoclax [
46]. The combination therapy of ivosidenib (IVO) plus venetoclax (VEN) with or without azacitidine was found to be effective against AML harboring an IDH1 mutation in a phase Ib/II trial [
47]. Patients with AML or high-risk MDS were assigned to one of three cohorts, either receiving IVO + VEN 400 mg, IVO + VEN 800 mg, or IVO + VEN 400 mg + AZA. The median time to best response was 2 months. In 18 evaluable patients, cCR rate was 78% overall (treatment naive: 100%; R/R: 75%), and 67%, 100%, and 67% by cohort with median time to best response of 2 months. IVO + VEN + AZA therapy was well tolerated and highly effective for patients with IDH1 mutated AML [
47]. It is reasonable to expect that combination of the IDH2 inhibitor enasidenib with venetoclax and azacitidine might also provide better outcomes than enasidenib alone or enasidenib with azacitidine, since enasidenib plus venetoclax demonstrated superior anti-leukemic activity against IDH2 mutated AML in patient-derived xenograft models [
48]. Table
3 summarizes the trials of combinational therapies for newly diagnosed unfit AML patients with IDH1/2 mutation.
Table 3
Summary of combination of targeted-therapy trials in IDH1/2-mutant newly diagnosed AML
HMA + venetoclax | Ib | 35 | 71 | 2.1 | 24.4 | |
AZA + venetoclax | III | 46 | 75.4 | N/A | N/A | |
AZA + venetoclax | Pooled data from two trials | 79 | 72 | 1.0 | 24.5 | |
LDAC + venetoclax (HMA naïve) | Ib/II III | 18 | 72 | 1.4 | 19.4 | |
AZA + ivosidenib | Ib | 23 | 69.6 | 3.7 | N/A | |
AZA + enasidenib | II | 68 | 68 | 5 | 22.0 | |
Venetoclax + ivosidenib | Ib/II | 12 | 83 | NA | NA | |
AZA + venetoclax + ivosidenib | Ib/II | 6 | 67 | NA | NA | |
Targeting FLT3 mutations
FLT3 Mutations occurs in approximately 30% patients with newly diagnosed AML (20% to 25% with FLT3-ITD mutation, 5% to 10% with FLT3-TKD), and associates with more proliferative disease, increased risk of relapse, and inferior survival. Randomized phase III RATIFY study led to approval of 7 + 3 + midostaurin as front line for young fit patients [
51], and randomized phase III ADMIRAL study with single-agent gilteritinib established the approval of gilteritinib for the treatment of R/R FLT3-mutated AML [
52]. The phase III randomized trial using quizartinib
vs investigator choice salvage chemotherapy in patients with R/R FLT3-ITD mutated AML met the primary objective of OS improvement [
53], leading its approval in Japan, but not in the US. Many studies to combine these FLT3 inhibitors are under investigation.
An open-label, phase 1 study (NCT02236013) assessed the safety/tolerability and anti-leukemic effects of gilteritinib plus 7 + 3 induction, consolidation, and maintenance therapy in fit adults with newly diagnosed FLT3-mutated AML. 80 patients with median age of 59 years were allocated to treatment. The maximum tolerated dose of gilteritinib was 120 mg daily. CRc was achieved by 81.8% of patients across all dose groups with mutational clearance (FLT3 ITD signal ratio of ≤ 10
–4 after induction or consolidation) was achieved by 70% of patients with FLT-ITD mutation receiving a gilteritinib dose of ≥ 120 mg [
54]. Two large randomized clinical trials of induction and consolidation chemotherapy plus gilteritinib
vs midostaurin in FLT3 mutated AML patients are ongoing in the US (PrECOG trial) (NCT03836209)) and in Europe (HOVON 156 AML / AMLSG 28–18 trial (NCT04027309)).
The LACEWING study is a phase 3 trial to randomize FLT3 mutated ND AML patients ineligible for intensive induction chemotherapy to get gilteritinib plus azacitidine
vs azacitidine alone. The safety cohort enrolled 15 patients and established dose of gilteritinib of 120 mg to be used in combination with azacitidine. Overall, a CRc of 67% was observed with median duration of remission of 10.4 months for the CRc responders. The combination treatment was well tolerated with no unexpected adverse effect [
55]. While the data provides a promising option of gilteritinib plus azacitidine for newly diagnosed FLT3-mutated unfit AML patients, the company announced that Phase 3 LACEWING trial failed to meet primary end point of OS at a planned interim analysis and the study was terminated for futility in December 2020. Many lessons have been learned in the AML field that high response rate in AML will not necessary transform into survival benefit.
In the R/R setting, a phase 1b study tested the safety and efficacy of combining venetoclax at 400 mg with gilteritinib at 120 mg daily. 39 patients were enrolled, and among them 64% had previous history of FLT3 TKI exposure. 37 patients were evaluable for response, 31 (84%) achieved CRc. This data compares favorably to the CRc of 54% with single agent gilteritinib in the ADMIRAL study; suggesting gilteritinib plus venetoclax might be better option for R/R FLT3-mutated AML, while longer follow-up with OS data is awaited [
56]. A Phase Ib/II trial explored the combination of quizartinib (Quiz) with decitabine (10 days) ± venetoclax mostly in patients with R/R AML. CRc of 90% was achieved in the DEC10 + VEN + Quiz cohort, and CRc rate of 40% was achieved in DEC10 + quiz cohort. In addition, CyTOF (single-cell mass cytometry) analysis could be used to select patients with the best response based on pre- and on-therapy apoptotic and signaling pathway profiles [
57].
Targeting TP53 mutation
The
TP53 gene, located on chromosome 17p13.1, is commonly mutated in tumors making it one of the most widely mutated genes in human malignancies.
TP53 mutations are detected in 5% to 20% of patients with newly diagnosed AML and MDS, with higher incidences in older patients and in those with secondary AML or therapy-related myeloid neoplasms.
TP53 mutation is enriched in patients with complex karyotype and monosomal karyotypes and also in patients with relapse or refractory disease.
TP53 mutation has been associated with a poor prognosis in both AML and MDS [
58,
59].
A recent study analyzed 3,324 patients with MDS for
TP53 mutations and allelic imbalances, and delineated two subsets of patients with distinct phenotypes and outcomes. One-third of
TP53-mutated patients had monoallelic mutations whereas two-thirds had multiple hits consistent with biallelic targeting. Established associations with complex karyotype, few co-occurring mutations, high-risk presentation and poor outcomes were specific to multi-hit patients only. The
TP53 multi-hit state predicted a high risk of death and leukemic transformation independently of the revised international prognostic scoring system (IPSS-R). Importantly, monoallelic patients did not differ from
TP53 wild-type patients in outcomes and response to therapy. This study demonstrates that consideration of
TP53 allelic state is critical for diagnostic and prognostic precision in MDS as well as for future correlative studies of treatment response [
60]. It is uncertain if a similar finding will be identified in AML patients.
TP53 mutation used to be considered as “undruggable”, but there has been an abundant effort recently to explore different mechanisms to overcome the negative impact of the mutant TP53 protein. Although one study using 10-days of decitabine reported a marrow remission rate of 100% in TP53-mutated patients with AML or MDS [
61], the results have not been confirmed in subsequent studies, including a randomized study of 5-day versus 10-day schedules of decitabine as first line therapy for older patients with AML [
62].
APR-246 (Eprenetapopt) is a novel, first-in-class small molecule that selectively induces apoptosis in TP53 mutated cancer cells via thermodynamic stabilization of the TP53 protein and shifting the equilibrium toward the wild-type conformation with restoration of the transcriptional activity of unfolded wild-type or mutant
TP53 [
63]. Updated results from the multicenter Phase 1b/2 trial demonstrated that APR-246 + AZA is a well-tolerated combination with high response rates in HMA-treatment naïve TP53 mutated higher risk MDS, MDS/MPN, and oligoblastic AML (20%-30% blasts) patients (NCT03072043). Patients enrolled on the Phase II portion received APR-246 4500 mg IV (days 1–4) + AZA 75 mg/m2 SC/IV × 7 days (days 4–10 or 4–5 and 8–12) in 28 day cycles. 55 patients were enrolled, and ORR by IWG criteria was 87% with CR of 53%. Median time to response was 2.1 months, and median duration of response of 6.5 months. CR rate was 50% for AML. An isolated TP53 mutation was predictive for a higher CR rate (69%
vs 25%;
P = 0.006) with a trend for higher ORR. By intention-to-treat analysis, median OS was 11.6 months (95% CI 9.2–14) with significantly longer OS in responding patients (12.8
vs 3.9 months;
P < 0001). The study concluded that APR-246 + AZA is a well-tolerated combination with high response rates in TP53 mutated MDS/AML. Response durations were promising and were accompanied by a high fraction of cytogenetic and deep molecular remissions leading to encouraging outcomes post-SCT. These data support the ongoing, randomized phase 3 study of APR-246 + AZA versus AZA alone in TP53 mutated MDS (NCT03745716) [
64]. A similar phase 2 study conducted by the Groupe Francophone Des Myelodysplasies (GFM) in a high-risk elderly population of TP53 mutated MDS and AML patients reported response rate of 76%, including 53% CR/CRi [
65].
Beyond
TP53 mutation or loss, MDM2 forms a complex with wild type TP53, leading to decreased
TP53 transcriptional activity, increased nuclear export, and degradation of TP53 through the proteasome. Inactivation of wild-type TP53 protein frequently occurs in the cancer cells through overexpression of its negative regulator MDM2. Thus, MDM2 antagonists have been explored to re-establish the function of wild TP53, and various compounds have been developed to disrupt this MDM2–TP53 interaction [
66]. These MDM2 inhibitors could synergistically activate the TP53 pathways with various chemotherapy to kill leukemia cells, but this class of drugs will be largely ineffective in
TP53-mutated disease [
67].
Several MDM2 inhibitors are being evaluated in patients with AML/MDS [
68]. Nutlins were the first small molecule inhibitors developed that bind to MDM2 and target its interaction with TP53 [
69]. Second-generation nutlin, such as idasanutlin, have improved potency and better toxicity profile. Data from early phase trials demonstrated clinical response with monotherapy with idasanutlin (RG7388) or in combination with other agents [
68]. In general, monotherapy with MDM2 inhibitors revealed very modest anti-leukemia effect in R/R AML, including RG7112 [
70], RO6839921 (an inactive pegylated prodrug of idasanutlin) [
71], and AMG-232 [
72]. Other MDM2 inhibitors under investigation pre-clinically or in early phase clinical trials were reviewed [
73]. Currently, efforts are focusing on combination strategies. In a multicenter Phase 1/1b study, idasanutlin in combination with cytarabine resulted in cCR of 29% with CR rate of 25% in the dose escalation, dose expansion, and bridging cohorts. The median duration of response was about 6.4 months (1.1–11.9 months) and some patients remained in CR at 1 year follow-up. Higher MDM2 expression in leukemic blasts and stem cells, and not TP53 mutational status was associated with CR, suggesting MDM2 expression in leukemic cells might serve as a predictive biomarker for response [
74]. Unfortunately, even with the promising results from the early phase, MIRROS trial (NCT02545283) [
75], a randomized Phase III trial evaluating idasanutlin + cytarabine versus placebo + cytarabine in R/R AML was terminated due to failure to meet its primary goal of prolonging the survival, further demonstrating the challenges to replicate the early promising data at the later large phase trial in AML patients.
Earlier studies demonstrated the cross-talk between TP53 pathway and apoptosis-related molecules, initially with BCL-2 and BAX [
76‐
81], and later with MCL-1 [
82]. There was synergistic apoptosis-induction effects in leukemia cells with TP53 activation following suppression of pro-apoptotic molecules, like BCL-2, BCL-XL and XIAP [
82,
83]. More recent studies clearly demonstrate that TP53 apoptotic network is a primary mediator of resistance to BCL-2 inhibition in AML cells [
28], and increased activities of TP53 through MDM2 inhibition negatively regulates the RAS/RAF/MEK/ERK pathway and activates GSK3 to modulate MCL-1 phosphorylation and promote its degradation, thus overcoming AML resistance to BCL-2 inhibition by venetoclax [
84]. The combination of venetoclax with the MDM2 inhibitor, idasanutlin, has been tested in a phase Ib study in older patients with relapsed or refractory AML [
85]. The response rates were promising with ORR of 37% across all dose cohorts, and an ORR of 50% in the dose cohorts being considered for recommended Phase II dose (RP2D). MRD negativity (< 0.1% by Flow) was achieved in 43% of patients with cCR. The median time to response was 1.8 months (range, 0.8–2.7), with median response duration of 8.1 months (range, 0.3–9.7). Median overall survival in the RP2D cohorts was 5.3 months (range, 0.2–17.6). The response rate was very high at 86% in patients with IDH2 mutation and 57% in patients with a RUNX1 mutation, but only 20% in patients with TP53 mutation [
85].
MDM2 antagonists have been combined with other agents in different cancer types, such as with PI3K, MEK, or FLT3-ITD pathway inhibition in AML, with CD20 antibody in lymphoma, with CDK4/6 inhibitor in locally advanced or metastatic liposarcoma, with PD-L1/PD-1 antibodies in patients with metastatic solid tumors, and a number of others [
73].