Final results of a phase 2, open-label study of indisulam, idarubicin, and cytarabine in patients with relapsed or refractory acute myeloid leukemia and high-risk myelodysplastic syndrome
Eisai Pharmaceuticals and the National Cancer Institute had no role in study design, data collection, data analysis, data interpretation, or writing of the report.
Abstract
BACKGROUND
Indisulam possesses anticancer properties through down-regulation of various cell-cycle checkpoint molecules, thereby blocking the phosphorylation of retinoblastoma protein and inducing p53 and p21. Indisulam exhibits synergy with nucleoside analogs and topoisomerase inhibitors.
METHODS
The authors designed a phase 2 study of indisulam in combination with idarubicin and cytarabine in patients who had relapsed/refractory acute myeloid leukemia AML and high-risk myelodysplastic syndrome. In stage 1, patients received intravenous indisulam at 400 mg/m2 on days 1 and 8 of a 28-day cycle. If they had no response, then patients received same dose schedule of indisulam followed by intravenous idarubicin 8 mg/m2 daily for 3 days and cytarabine 1.0 g/m2 over 24 hours daily on days 9 through 12 (for those aged < 60 years) or days 9 through 11 (for those aged > 60 years) of a 28-day cycle. Primary endpoints included the overall response rate, and secondary objectives included overall survival.
RESULTS
Forty patients were enrolled. Of the 37 evaluable patients, 31 received indisulam with chemotherapy. Of these, 11 (35%) responded for a median duration of 5.3 months. The estimated 1-year overall survival rate was 51% for responders compared with 8 % for nonresponders (P < .001). The most common grade ≥3 nonhematologic toxicities were electrolyte abnormalities (50%) and febrile neutropenia (28%).
CONCLUSIONS
The combination of indisulam with idarubicin and cytarabine yielded a 35% response rate in heavily pretreated patients with AML. With emerging data identifying the expression of DCAF15 (DDB1 and CUL4-associated factor 15) as a potential biomarker for activity, the combination of indisulam with idarubicin and cytarabine should be studied in a biomarker-driven trial or in patients who have splicing factor mutations. Cancer 2018;124:2758-65. © 2018 American Cancer Society. Cancer 2018;124:2758-2765. © 2018 American Cancer Society
INTRODUCTION
The combination of an anthracycline (daunorubicin or idarubicin) and cytarabine remains a standard-of-care induction regimen for patients with newly diagnosed patients acute myeloid leukemia (AML) and leads to remission rates approaching 80%.1, 2 In general, relapsed or refractory AML portends a dire outcome,3 and therapeutic options are limited, with subsequent dismal response rates (RRs) for patients who have short first remissions or primary refractory disease.4 Investigational agents alone or in combination with standard chemotherapy in clinical trials should be the most appropriate approach for these patients.
Indisulam (also known as E7070) is a sulfonamide derivative that reportedly inhibits carbonic anhydrases and cytosolic malate dehydrogenase.5 This agent possesses antitumor properties because of its suppression of the expression of various cell-cycle checkpoint molecules, including cyclins A, B1, H, and cyclin-dependent kinase 2 (CDK2), with subsequent reduction in retinoblastoma (Rb) phosphorylation and induction of p53 and p21. In murine leukemia P388 cells, single-agent indisulam induced cell-cycle arrest and delay at G1/S and G2/M transitions in parallel with profound down-regulation of redox and energy metabolism genes, followed by induction of apoptosis at higher doses.6, 7 Despite substantial research efforts over 2 decades, until recently, the precise molecular target of indisulam was yet to be elucidated. Preclinical and clinical studies have established the synergy of indisulam with nucleoside analogs as well as topoisomerase inhibitors.6, 8, 9 In a phase 2 study combining indisulam with chemotherapy in colorectal cancer, indisulam was used at the recommended dose of 400 mg/m2 on days 1 and 8 of a 21-day cycle.10 The combination was tolerated with acceptable toxicities, including diarrhea, vomiting, and myelosuppression. The combination of nucleoside analogs (eg, cytarabine, fludarabine) with DNA-damaging agents like idarubicin is considered a standard salvage regimen for AML.
We hypothesized that, with its ability to down-regulate topoisomerase II, indisulam could potentiate the cytotoxic effect of an anthracycline and a nucleoside analog when these 3 agents are used in combination. Therefore, we designed the current phase 2 study to evaluate the safety and efficacy of indisulam in combination with cytarabine and idarubicin for patients with relapsed, refractory AML and high-risk myelodysplastic syndrome (MDS).
MATERIALS AND METHODS
Study Design and Participants
This open-label, phase 2 study of indisulam in combination with idarubicin and cytarabine was conducted at The University of Texas MD Anderson Cancer Center. Patients aged ≥ 18 years who had relapsed or refractory, de novo or secondary AML (excluding acute promyelocytic leukemia) according to World Health Organization criteria or high-risk MDS were enrolled.11 Primary refractory or resistant AML was defined by not achieving complete remission (CR) after 2 cycles of conventional dose cytarabine or 1 cycle of high-dose cytarabine.12 High-risk MDS was defined as intermediate-2 risk or high risk according to the International Prognostic Scoring System13 or having > 10% blasts in the bone marrow (BM). Other eligibility criteria included a performance status from 0 to 2, normal cardiac ejection fraction, and adequate renal and hepatic functions. Main exclusion criteria were a QTc interval >480 msec upon screening and recent (within 14 days) treatment for AML other than hydroxyurea, which was permitted up to 48 hours before enrollment. Pregnant or lactating women and patients who had any uncontrolled clinically significant illness were excluded. Infection prophylaxis was used according to institutional standards. The trial was approved by the Institutional Review Board. All patients provided written informed consent before enrollment according to the Declaration of Helsinki. The trial was registered in ClinicalTrials.gov (NCT01692197).
BM examination to assess response was done on day 28 (±3 days) of cycle 1 and, after that, as indicated at the discretion of the treating physician until documentation of response. The entire coding sequences of 28 genes known to be frequently mutated in myeloid hematologic malignancies (ABL proto-oncogene, nonreceptor tyrosine kinase [ABL1]; additional sex combs like 1, transcriptional regulator [ASXL1]; B-raf proto-oncogene, serine/threonine kinase [BRAF]; DNA methyltransferase 3 α [DNMT3A]; epidermal growth factor receptor [EGFR]; enhancer of zeste 2 polycomb repressive complex 2 subunit [EZH2]; Fms-related tyrosine kinase 3 [FLT3]; GATA binding protein 1 [GATA1]; GATA binding protein 2 [GATA2]; HRAS proto-oncogene, guanosine triphosphate [GTP]ase [HRAS]; isocitrate dehydrogenase 1 [nicotinamide adenine dinucleotide phosphate], cytosolic [IDH1]; isocitrate dehydrogenase 2 [nicotinamide adenine dinucleotide phosphate], mitochondrial [IDH2]; KIT proto-oncogene receptor tyrosine kinase [KIT]; KRAS proto-oncogene, GTPase [KRAS]; MDM2 proto-oncogene [MDM2]; IKAROS family zinc finger 2 [IKZF2]; Janus kinase 2 [JAK2]; mixed-lineage leukemia [MLL]; MPL proto-oncogene, thrombopoietin receptor [MPL]; myeloid differentiation primary response 88 [MYD88]; notch 1 [NOTCH1]; nucleophosmin 1 [NPM1]; NRAS proto-oncogene, GTPase [NRAS]; protein tyrosine phosphatase, nonreceptor type 11 [PTPN11]; runt-related transcription factor 1 [RUNX1]; Tet methylcytosine dioxygenase 2 [TET2]; tumor protein P53 [TP53]; and Wilms tumor 1 [WT1]) were sequenced using the Illumina MiSeq platform (Illumina Inc, San Diego, CA), as previously described. Testing for splicing mutations was not included in the panel at the time of this study.14 Safety electrocardiograms were obtained on days 2 (±1 day) and 8 (±2 days) of cycle 1 at least 1 hour after completion of indisulam infusion.
Treatment Plan
To assess single-agent response to indisulam, patients initially received 1 cycle of intravenous indisulam alone at 400 mg/m2 on days 1 and 8 of a 28-day cycle. Responding patients were to receive postremission therapy with indisulam at the same dosing schedule for up to 5 additional cycles as long as they continued to have clinical benefit. Nonresponders were to receive cycle 2 with indisulam (at the same dosing schedule as above) combined with intravenous idarubicin 8 mg/m2 daily for 3 days (on days 9-11) and intravenous cytarabine 1 g/m2 administered over 24 hours daily on days 9 through 12 (for those aged < 60 years) or days 9 through 11 (for those aged > 60 years). The administered doses of idarubicin and cytarabine, which were lower than the usual 12 mg/m2 and 1.5 g/m2 used at our institution, were chosen to avoid undue toxicity when combined with indisulam. Responding patients who attained CR, CR with incomplete platelet recovery (CRp), partial remission (PR), or BM clearance of blasts received up to 2 additional cycles of indisulam in combination with chemotherapy at the above doses. Each cycle was given every 4 weeks (±2 days). Dose modifications and delays were allowed for persistent cytopenias or nonhematologic toxicities. Because there was a lack of single-agent activity with indisulam, the protocol was modified after the first 20 patients to eliminate the cycle of single-agent indisulam and follow the schema illustrated in Figure 1.

Because indisulam (E7070) lacked single-agent activity, the protocol was modified after the first 20 patients to eliminate the cycle of single-agent indisulam and follow this schema for responders and nonresponders.
Outcomes
The primary endpoints were the overall RR (ORR) and the safety and tolerability of indisulam combined with idarubicin and cytarabine. Responses were defined based on criteria from the International Working Group for AML15: CR was defined as a BM sample that had <5% blasts with normal maturation of all cell lines and no dysplasia, a peripheral blood absolute neutrophil count ≥1 × 109/L, hemoglobin ≥ 10 g/dL, and platelet count ≥100 × 109/L; PR was defined as counts as above and ≥ 50% decrease in the percentage of BM aspirate blasts to 5% to 25% or BM blasts < 5% with persistent Auer rods. CRp and CRi indicate CR with <5% BM myeloblasts, normal maturation of all cell lines, and no dysplasia but with incomplete platelet recovery (CRp) or incomplete blood count recovery (CRi). A morphologic leukemia-free state was defined as <5% BM blasts, irrespective of cytopenias. Secondary endpoints included response duration and overall survival (OS).
Statistical Considerations
Summary statistics were used to describe continuous variables in the study population. Categorical endpoints were summarized using frequencies and percentages. The Fisher exact and the Wilcoxon rank test were used in univariate analyses of categorical and continuous variables, respectively. The primary objective of this study was to assess the efficacy and safety of indisulam plus chemotherapy in patients with leukemia. Efficacy was measured by using the ORR (CR + CRp + PR + BM clearance of blasts). Patients were considered evaluable for efficacy if they received indisulam alone and achieved a response or if they received at least 1 treatment with indisulam plus chemotherapy. Efficacy was assessed based on the Simon 2-stage min-max design. In particular, 27 patients were to be enrolled at the first stage. If ≤ 2 patients had a response, then the trial was to be terminated; otherwise, another 13 patients were to receive treatment for a total of 40 patients. If, among 40 patients, ≤ 6 had a response, then it was to be concluded that the treatment was ineffective. A maximum sample size of 40 patients on the study was chosen to differentiate between RRs of 10% and 25%, with 90% power at a significance level (P) of .1. We used a 1-sample exact binomial test with a 2-sided significance level of .02 to evaluate the trial's observed RR in salvage 1 and 2 against the expected RRs previously recorded in historic cohorts. The parameters were chosen based on our historic expectation of a 30% to 40% CR rate in patients receiving first salvage16 and a 13% CR rate among patients receiving second salvage therapy for AML.17 A Bayesian sequential monitoring method was used to monitor the toxicity, and the trial was to stop if clinically significant nonhematologic toxicity was >33% at any point.
OS was defined as the time between the date of starting treatment and the date of death or last follow-up. Patients who remained alive at their last follow-up date were censored for OS. OS was estimated using the Kaplan-Meier method. Statistical analysis was performed using Stata/SE version 14.1 (Stata Corporation LP, College Station, TX). Adverse events (AEs) were classified according to the Common Terminology Criteria for Adverse Events version 4.0 and were captured from the time of enrollment until 30 days after the last dose of therapy.
RESULTS
Between February 2013 and June 2014, 40 patients were enrolled, including 39 with AML and 1 with high-risk MDS. Baseline characteristics are summarized in Table 1. The median patient age was 63 years (range, 25-75 years), and the median number of prior therapies was 2 (range, 1-6 prior therapies). Seventeen patients (43%) patients had a diploid karyotype, and 16 (40%) had poor risk cytogenetics. Twenty-eight patients (70%) and 23 patients (58%) had received prior intermediate-dose to high-dose cytarabine-based regimens and hypomethylating agents, respectively, whereas 9 patients (23%) patients progressed after undergoing stem cell transplantation (SCT).
Characteristic | No. of Patients/Total No. (%) or Median [Range] |
---|---|
Age: Mean [range], y | 63 [25-75] |
Men | 21 (53) |
Diagnosis | |
AML | 39 (98) |
Secondary AML | 12 (31) |
HRMDS | 1 (2) |
No. of prior regimens | 2 [1-6] |
Intermediate/high-dose cytarabine-based | 28 (70) |
HMA | 23 (58) |
Clofarabine-based | 11 (28) |
Salvage | |
1 | 14 (35) |
2 | 14 (35) |
≥3 | 12 (30) |
Duration of first remission when achieved, mo | 11 [1-52] |
Duration of remission on therapy received closest to clinical trial, mo | 4.3 [1-21] |
Prior SCT | 9 (23) |
WBC, × 106/L | 1.8 [0.4-56] |
Hemoglobin, g/dL | 9.5 [7.2-14.7] |
Platelets, × 109/L | 27 [2-231] |
Bone marrow blasts, % | 34 [3-95] |
Peripheral blood blasts, % | 11 [0-90] |
Cytogenetics | |
Diploid | 17 (43) |
Complex | 16 (40) |
−5/5q− and/or −7/7q− | 13 (33) |
Miscellaneous | 7 (17) |
IDH1/IDH2 mutations | 7/37 (19) |
RUNX1 mutation | 6/37 (16) |
NRAS mutation | 5/39 (13) |
TP53 mutation | 4/37 (11) |
FLT3 mutation | 3/39 (8) |
NPM1 mutation | 3/38 (8) |
TET2 mutation | 2/37 (5) |
ASXL1 mutation | 2/37 (5) |
- Abbreviations: AML, acute myeloid leukemia; ASXL1, additional sex combs like 1, transcriptional regulator; FLT3, Fms-related tyrosine kinase 3; HRMDS, high-risk myelodysplastic syndrome; HMA, hypomethylating agents; IDH1, isocitrate dehydrogenase 1 (nicotinamide adenine dinucleotide phosphate), cytosolic; IDH2, isocitrate dehydrogenase 2 (nicotinamide adenine dinucleotide phosphate), mitochondrial; NRAS, NRAS proto-oncogene, guanosine triphosphate (GTP)ase; RUNX1, runt-related transcription factor 1; SCT, stem cell transplantation; TET2, Tet methylcytosine dioxygenase 2; TP53, tumor protein 53; WBC: white blood cell count.
Response to Therapy
The median treatment duration was 7 weeks (range, 1-21 weeks) for all patients. Three patients (8%) were not evaluable for response, including 1 who was taken off study after 2 days because the patient had a targetable FLT3 mutation identified, 1 who was taken off because of chest pain and a grade 1 increase in cardiac troponin level while receiving indisulam alone, and a third who was taken off because of rapid disease progression on day 2 of therapy.
Indisulam had no single-agent activity. Of the 37 evaluable patients, 31 received indisulam plus idarubicin and cytarabine, and 6 received indisulam alone without proceeding to receive the combination (within the first 20 patients who received indisulam alone in cycle 1 before protocol amendment); of these 31 patients, 8 (26%) achieved CR, including 1 patient who underwent prior SCT, whereas 2 (6%) achieved Cri, and 1 (3%) achieved CRp. Responders completed a median of 3 cycles of therapy (range, 1-4 cycles), with a median time to respond of 8 weeks (range, 3-12 weeks) and a median response duration of 5.3 months (range, 0.4-13 months). Six responders later proceeded to SCT. Nine of the 11 responders had prior exposure to cytarabine, or clofarabine, or both. The median number of regimens received by the responders before enrolling in this trial was 2 (range, 1-6 regimens). A comparison of characteristics revealed that nonresponders had lower median platelets count (25 vs 34 × 109/L; P = .03), higher median BM blasts (37.5% vs 16%; P = .009), and higher peripheral blast percentages (23% vs 0%; P = .01) (Table 2) compared with responders. In addition, none of the 11 responders had poor cytogenetics (0% vs 12%; P = .006) compared with nonresponders.
No. of Patients (%) or Mean [Range] | |||
---|---|---|---|
Characteristic | Responders, N = 11 | Nonresponders, N = 26 | P |
Age, y | 65 [50-75] | 63 [25-75] | .47 |
Men | 6 (55) | 13 (50) | > .99 |
Secondary AML | 2 (18) | 7 (27) | .69 |
AML status | .27 | ||
Primary refractory | 2 (18) | 10 (38) | |
Relapsed | 9 (82) | 16 (62) | |
Prior regimens | 2 [1-3] | 2 [1-6] | .11 |
Intermediate/high-dose cytarabine-based | 7 (64) | 19 (73) | .69 |
HMA | 5 (45) | 16 (62) | .47 |
Clofarabine-based | 3 (27) | 7 (27) | > .99 |
Prior SCT | 1 (9) | 8 (31) | .22 |
WBC, × 106/L | 1.5 [1-5.6] | 1.8 [0.4-7.7] | .76 |
Hemoglobin, g/dL | 9.7 [7.4-13.6] | 9.5 [7.7-14.6] | .85 |
Platelets, × 109/L | 34 [6-231] | 25 [2-148] | .03 |
Bone marrow blasts, % | 16 [7-68] | 37.5 [3-95] | .009 |
Peripheral blasts, % | 0 [0-41] | 23 [0-84] | .01 |
Cytogenetics | |||
Diploid | 10 (91) | 8 (31) | .001 |
Complex | 0 (0) | 12 (46) | .006 |
−5/5q− and/or −7/7q− | 0 (0) | 12 (46) | .006 |
- Abbreviations: AML, acute myeloid leukemia; HMA, hypomethylating agents; SCT, stem cell transplantation; WBC, white blood cell count.
With regard to salvage status, 5 of 13 evaluable patients (39%) in salvage 1 responded. On the basis of an expected RR of 30% to 40%,16 the observed RR was not significantly different in our trial (P = .69 and P = 1.0). Of the 13 evaluable patients in salvage 2, 5 (39%) also responded, and the observed RR was significantly higher (P = .039) than the expected 13% RR observed in a historic cohort that received second salvage therapy at The University of Texas MD Anderson Cancer Center.17
Survival Endpoints
At a median follow-up of 6 months (range, 1-40 months), 5 patients remained alive. The median OS was 5.2 months (range, 0.6-40 months; 95% confidence interval, 4.96-10.21 months) (Fig. 2). The median OS for responders (n = 11) was 17.4 months, compared with 4.3 months for nonresponders (P = .004; HR, 0.32; 95% confidence interval, 0.16-0.65) (Fig. 3). Six of the responding patients were successfully bridged to SCT.

Overall survival (OS) of patients treated on the trial is plotted using the Kaplan-Meier method.

Overall survival (OS) of patients who achieved a clinical response (complete response or partial response), compared with the OS of those who had no response, is plotted using the Kaplan-Meier method.
Toxicities
All 40 patients who received the study therapy were eligible for toxicity evaluation (Table 3). The most common grade 1 and 2 toxicities were electrolyte imbalances (n = 30; 75%), nausea (n = 26; 65%), transaminitis (n = 25; 63%), and hyperbilirubinemia (n = 21; 53%). These were generally transient and manageable with supportive measures. The most common grade 3 and 4 nonhematologic toxicities, irrespective of attribution, were transient electrolyte abnormalities (n = 20; 50%). Febrile neutropenia without a clear source of infection was encountered in 11 patients (28%). Pneumonia and soft tissue infections requiring hospital admission (grade ≥ 3) occurred in 7 patients (18%) each, and colitis and/or diarrhea (grade ≥ 3) in 4 patients (10%). Four patients (10%) died within the first 4 weeks from progressive disease and associated complications.
No. (%) | ||
---|---|---|
Adverse Event | Grade 1-2 | Grade 3-4 |
Nausea | 26 (65) | 0 (0) |
Vomiting | 12 (30) | 0 (0) |
Diarrhea | 14 (35) | 2 (5) |
Constipation | 16 (40) | 0 (0) |
Fatigue | 16 (40) | 0 (0) |
Anorexia | 11 (28) | 0 (0) |
Abdominal pain | 6 (15) | 2 (5) |
Musculoskeletal pain | 12 (30) | 2 (5) |
Chest pain | 4 (10) | 1 (3) |
Electrolyte Imbalance | 30 (75) | 20 (50) |
Hyperglycemia | 20 (50) | 9 (23) |
Hypoglycemia | 3 (8) | 2 (5) |
Hyperbilirubinemia | 21 (53) | 5 (13) |
Transaminitis | 25 (63) | 1 (3) |
Mucositis | 2 (5) | — |
Enterocolitis | 0 (0) | 2 (5) |
Acute kidney injury | 13 (33) | 1 (3) |
Hypertension | 2 (5) | 4 (10) |
Atrial fibrillation | 3 (8) | 2 (5) |
Capillary leak syndrome | 0 (0) | 1 (3) |
Edema | 17 (43) | 0 (0) |
Rash | 1 (3) | 1 (3) |
Febrile neutropenia | 5 (13) | 11 (28) |
Pneumonia | 1 (3) | 7 (18) |
Skin/soft tissue infections | 5 (13) | 7 (18) |
DISCUSSION
In this phase 2 study, we evaluated the efficacy and safety of idarubicin and cytarabine with indisulam, a sulfonamide drug, in patients with relapsed and refractory AML. In such a heavily pretreated population that failed a median of 2 prior treatments (range, 1-6 prior treatments), including cytarabine (70%), hypomethylating agents (58%), and SCT (23%), the combination in the current study led to a composite CR rate (CR + CRi + CRp) of 35%. It is important to note that the median duration of responses was 5.3 months (range, 0.4-13 months), which allowed 6 responders to eventually undergo allogeneic SCT. In our cohort of patients, the median duration of their first CR was 11 months (range, 1-52 months), and the median duration of remission immediately before enrollment in the clinical trial was 4.3 months (range, 1-21 months). To put this response in perspective, the RR achieved in this trial appears to be mostly significant in those patients who received second salvage treatment compared with a historic analysis of our institutional data, in which an RR of 13% was reported, with a median OS of 1.5 months and a median CR duration of 7 months.17 The cohort size precludes generalization of this observation.
For the combination of indisulam plus chemotherapy, responses were observed in patients who had diploid cytogenetics, and none of the patients who had adverse cytogenetics responded (P = .001 and P = .006, respectively). Responses appear to be clinically relevant, because responders had a significantly longer survival (Fig. 3).
Overall, the combination had a clinically manageable safety profile. Because cytopenia was a dose-limiting toxicity of indisulam in phase 1 studies of solid tumors,7, 18 there was a concern about extended hematologic toxicities from its combination with chemotherapy. The time interval between cycles was between 4 and 5 weeks, reflecting the absence of profound myelosuppression and prolonged hematologic recovery, except in 1 patient who had delayed myelosuppression beyond 42 days after the second course of treatment. With regard to nonhematologic toxicity, there were high rates of electrolyte abnormalities and hypoglycemia, likely related to the intrinsic pharmacologic activity of indisulam as a sulfonamide diuretic and insulin-releasing agent.19 Other nonhematologic AEs included fatigue, nausea, hyperbilirubinemia, and transaminitis. All these AEs were mostly of grade 1 and 2, transient, and manageable with supportive care. Mucositis was observed infrequently on this trial, with 4 grade 2 and 3 events, unlike in phase 1 trials7 and as expected from the cell cycle-inhibitory action of indisulam and the combination with idarubicin and cytarabine. Grade 3 and 4 events were mostly reversible electrolyte abnormalities. Hand-foot syndrome and stomatitis were reported in a phase I trial that combined indisulam and capecitabine,8 but these AEs were not encountered in our trial. The likely explanation for this better toxicity profile is that the doses of idarubicin and cytarabine received in the current study were lower than the usual 12 mg/m2 and 1.5 g/m2 doses, respectively, used as induction regimen at our institution. In addition, indisulam was given at a dose of 400 mg/m2 on days 1 and 8 every 4 weeks to allow for the addition of chemotherapy with limited scope of cumulative AEs. We note that 2 patients died of disease progression within the first 30 days on therapy, leading to an early mortality rate of 5% and indicating that the aforementioned AEs did not result in increased treatment-related mortality.
In recent years, several attempts at optimizing salvage regimens in patients with AML have failed to improve upon remission rates, survival, and overall outcomes, especially in heavily pretreated patients who received prior standard treatments, including allogeneic SCT. Mutational studies identified new targets in AML, with these mostly representing driver mutations. Nevertheless, considering the complex clonal and molecular dynamics of this disease, there is increasing understanding that rational combinatorial approaches are needed to address the redundant survival pathways in leukemic cells. CDKs are crucial components of the cell-cycle apparatus that regulate proper transition between different phases of this cycle.20 These enzymes are sequentially controlled by phosphorylation and activated by cyclins.20 Deregulated CDK function with subsequent overexpression of cyclins or low or absent levels of CDK inhibitors is a universal occurrence in human cancers that is facilitated through various genetic and epigenetic abnormalities.21 These events induce Rb phosphorylation and malignant cell-cycle progression, thereby granting neoplastic cells a selective proliferation advantage.21
Indisulam is a synthetic aryl sulfonamide that halts the progression of G1/S-phase.22 Compared with other CDK inhibitors, indisulam does not competitively inhibit the adenosine triphosphate-binding site of the CDK enzymes,19 thus making it an attractive agent to combine with cell cycle-active agents and potentially with adenosine triphosphate-competitive CDK inhibitors. The cell-cycle events modulated by indisulam include depletion of cyclin E and transcriptional repression of cyclin H, with subsequent reduced phosphorylation of the Rb protein, a CDK2 substrate, during the G1/S-phase transition.6 In addition, indisulam may up-regulate p53 and p21, leading to a further decrease in Rb phosphorylation and enhanced apoptosis.23 Therefore, the anticipated effects of indisulam on leukemic proliferation would be both cytostatic and cytotoxic. In our study, however, we did not observe any single-agent activity of indisulam with the dose and schedule used.
Cytosolic malate dehydrogenase, a metabolic enzyme essential for gluconeogenesis and glycolysis, was identified as an indisulam-specific binding protein.23, 24 However, a cytosolic malate dehydrogenase-indisulam binding property has never been demonstrated as conditional for the antineoplastic activity of the drug. Very recent studies have revealed that indisulam stimulates the recruitment and bridging of the splicing factor RNA binding motif protein 39 (RBM39), also designated as CAPERα, to the CUL4-DCAF15 E3 ubiquitin ligase, leading to indisulam-induced RBM39 polyubiquitination and proteasomal degradation.25-27 This results in aberrant pre-messenger RNA splicing, along with transcriptional down-regulation of redox and energy metabolism genes as well as cell-cycle checkpoint genes. This process is somewhat analogous to the means by which immunomodulatory drugs like lenalidomide bridge cereblon to degrade IKZF1, IKZF3, and CK1.28 The same studies have revealed that the sensitivity of hematopoietic cancer cell lines to indisulam correlates with the level of DCAF15 (DDB1 CUL4-associated factor 15) expression, whereas mutations in RBM39 that prevent CUL4-DCAF15 recruitment increase RBM39 stability and confer resistance to indisulam.26, 27 Nevertheless, the process by which indisulam binds these proteins remains to be established. Given the occurrence of spliceosome mutations in myeloid malignancies, the sulfonamide class of antineoplastic agents may prove to be effective in certain patient subgroups.26, 27
In conclusion, the combination of indisulam with idarubicin and cytarabine in patients with relapsed and refractory AML is effective and largely well tolerated. With the current knowledge of dependence of indisulam's antineoplastic activity on DCAF15 and RBM39, we propose that the combination should be studied in a more homogeneous group of patients with AML or high-risk MDS whose leukemic cells express mutant splicing factors (eg, U2 small nuclear RNA auxiliary factor 1 [U2AF1], splicing factor 3B subunit 1 [SF3B1], serine and arginine-rick splicing factor 2 [SRSF2], and zinc finger CCH-type RNA binding motif and serine/arginine-Rich 2 [ZRSR2]) or express high levels of DCAF15. Preclinical studies to establish rationale are underway.
FUNDING SUPPORT
This work was supported by Eisai Pharmaceuticals, the University of Texas MD Anderson Cancer Center Support Grant (CA016672), and the University of Texas MD Anderson Cancer Center Leukemia Specialized Programs of Research Excellence grant (SPORE CA100632) from the National Cancer Institute.
CONFLICT OF INTEREST DISCLOSURES
The authors made no disclosures.
AUTHOR CONTRIBUTIONS
Gautam Borthakur, Hagop M. Kantarjian, and Rita Assi contributed to the design, conception, and writing of the article; provided study materials or patients; contributed to the collection and assembly of data, data analysis, and interpretation; reviewed and revised the content; and approved the final version. Tapan M. Kadia, Naveen Pemmaraju, Elias Jabbour, Nitin Jain, Naval Daver, Zeev Estrov, Taisuke Uehara, Takashi Owa, and Jorge E. Cortes provided study materials or patients; contributed to the collection and assembly of data, data analysis, and interpretation; reviewed and revised the content; and approved the final version.