A phase 2 randomized multicenter study of 2 extended dosing schedules of oral ezatiostat in low to intermediate-1 risk myelodysplastic syndrome

Authors


Abstract

BACKGROUND:

Ezatiostat is a glutathione analog prodrug glutathione S-transferase P1-1 (GSTP1-1) inhibitor. This study evaluated 2 extended dose schedules of oral ezatiostat in 89 heavily pretreated patients with low to intermediate-1 risk myelodysplastic syndrome (MDS).

METHODS:

Patients were randomized by 1 stratification factor—baseline cytopenia (anemia only vs anemia with additional cytopenias)—to 1 of 2 extended dosing schedules. Multilineage hematologic improvement (HI) responses were assessed by International Working Group 2006 criteria.

RESULTS:

Overall, 11 of 38 (29%) red blood cell (RBC) transfusion-dependent patients had HI-Erythroid (HI-E) response. The median duration of HI-E response was 34 weeks. Multilineage responses were observed. There was 1 cytogenetic complete response in a del (5q) MDS patient. An important trend was the effect of prior therapy on response. A 40% HI-E rate (6 of 15 patients) was observed in patients who had prior lenalidomide and no prior hypomethylating agents (HMAs), with 5 of 11 (45%) patients achieving significant RBC transfusion reduction and 3 of 11 (27%) achieving transfusion independence. A 28% HI-E rate (5 of 18 patients) was observed in patients who were both lenalidomide and HMA naive, with 4 of 8 (50%) patients achieving clinically significant RBC transfusion reductions. Most common ezatiostat-related adverse events were grade 1 and 2 gastrointestinal including: nausea (45%, 17%), diarrhea (26%, 7%), and vomiting (30%, 12%).

CONCLUSIONS:

Ezatiostat is the first GSTP1-1 inhibitor shown to cause clinically significant and sustained reduction in RBC transfusions, transfusion independence, and multilineage responses in MDS patients. The tolerability and activity profile of ezatiostat may offer a new treatment option for patients with MDS. Cancer 2012. © 2011 American Cancer Society.

INTRODUCTION

Myelodysplastic syndromes (MDS) are clonal hematopoietic stem cell disorders characterized by dysplasia of 1 or more cell lineages, ineffective hematopoiesis, and cytopenias. MDS defects in cell growth, differentiation, and apoptosis are associated with a 30% incidence of transformation to acute myelogenous leukemia (AML).1 The estimated incidence rate is 3.4 per 100,000 per year, rising to ≥30 per 100,000 per year for people over 70 years of age.2 Although the natural history of distinct subtypes of myelodysplasia ranges from indolent chronic anemia to rapid progression to AML, most patients eventually die from complications of bone marrow failure. Two-thirds of MDS patients at diagnosis have low or intermediate-1 risk disease by International Prognostic Scoring System (IPSS),3 with the primary goal of therapy being improved hematopoiesis. Management of cytopenias ranges from supportive care measures to the use of disease-modifying agents. Of the 3 US Food and Drug Administration (FDA) approved drugs available for the treatment of MDS, the lenalidomide label is specific for lower risk red blood cell (RBC) transfusion-dependent patients with the deletion 5q (del 5q) MDS cytogenetic abnormality, although 26% of such patients without this chromosome abnormality also achieve transfusion independence.4 The 2 hypomethylating agents (HMAs) are beneficial for patients with higher risk MDS. Therapies are urgently needed for the lower risk MDS patients, particularly those failing other treatment approaches.

Ezatiostat hydrochloride (Telintra®, TLK199), a tripeptide glutathione analog reversible inhibitor of the enzyme glutathione S-transferase P1-1 (GSTP1-1), is being developed for the treatment of cytopenias associated with lower risk MDS. Ezatiostat facilitates dissociation of GSTP1-1 from jun-N-terminal kinase (JNK), leading to activation of JNK and the subsequent promotion of growth and maturation of multilineage hematopoietic progenitors, while promoting apoptosis in human leukemia blasts.5 The role of apoptosis in MDS remains complicated. Some studies show a defect in apoptosis, and clearly the progression to acute leukemia may argue for a role for these, and other, defects in apoptotic pathways in certain premalignant clones. The ability of ezatiostat to activate the caspase-dependent pathway may help eliminate or inhibit the emergence of malignant clones. Alternatively, ezatiostat's ability to increase reactive oxygen species (ROS) in dysplastic cells may also contribute to apoptotic death and elimination, because MDS cells may lack the reducing ability to eliminate these oxidative stresses.6 On the basis of promising multilineage responses and good tolerability profile in low to intermediate-2 MDS patients in a phase 1 study of oral ezatiostat,7 as well as the results of a phase 1 to 2a proof-of-concept study with the intravenous formulation,8 this randomized multicenter phase 2 study of oral ezatiostat on extended dose schedules was conducted.

MATERIALS AND METHODS

Patients

This study was conducted in accordance with the International Conference on Harmonization and Good Clinical Practice standards. Institutional review board approval was obtained from all participating institutions. All patients provided written informed consent before study participation.

Eligible patients were age ≥18 years, with histologically confirmed diagnosis of MDS, with an Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1. Patients were required to have adequate hepatic and renal function. Inclusion criteria included cytopenia per the International Working Group 2006 (IWG 2006) criteria9 for >2 months and ineligibility for stem cell bone marrow transplantation. Patients were excluded for prior allogeneic bone marrow transplantation, higher risk MDS or AML, cytogenetic abnormalities consistent with de novo AML,10, 11 proliferative chronic myelomonocytic leukemia, chronic myelogenous leukemia blast crisis, use of oral corticosteroids (>10 mg), history of hepatitis B/C, human immunodeficiency virus, or an active infection requiring intravenous antibiotics. Patients were not allowed to receive growth factors while on study, and a 4-week washout period for growth factors was required before study enrollment.

Study Design

This was a randomized, multicenter, phase 2 study evaluating 2 extended dose schedules of oral ezatiostat in patients with low to intermediate-1 IPSS risk MDS.12

The hematologic improvement (HI) rates for erythroid (E), neutrophils (N), and platelets (P) by IWG 2006 criteria, safety, and tolerability on each treatment dose schedule were evaluated to select the optimal extended dose and dose schedule for future studies. After randomization of the initial 14 patients at the 4500-mg dose on 2 dose schedules, subsequent patients were randomized to 2 selected lower doses/dose schedules to improve tolerability and patient convenience.

Patients were randomized by 1 stratification factor-baseline cytopenia (anemia only vs anemia with additional cytopenias) to either dose schedule 1 (DS1) or dose schedule 2 (DS2). In DS1, patients received 1500 mg orally (p.o.) twice daily (b.i.d.) for 2 weeks followed by a 1-week rest period in a 3-week treatment cycle, and in DS2, patients received 1000 mg p.o. b.i.d. for 3 weeks followed by a 1-week rest period on a 4-week treatment cycle. Divided daily doses were administered approximately every 12 hours, 1 tablet at a time with adequate fluids. The randomized treatment was planned for a maximum of 6 months; however, upon completion of 6 months of therapy, patients with ongoing clinical benefit and good tolerability were allowed to continue for an additional 6 months of continuous daily dosing.

Patients were treated until MDS disease progression, unacceptable toxicity, or withdrawal from the study. Red blood cell (RBC) transfusion guidelines were provided in the protocol and RBC transfusions were to be given for a hemoglobin (Hgb) level ≤9.0 g/dL. All treated patients were monitored for safety and efficacy with physical and laboratory examinations, and hematologic response assessments (IWG 2006) were evaluated every 2 cycles.9 Adverse events (AEs) were graded in accordance with the National Cancer Institute-Common Toxicity Criteria for Adverse Events, Version 3.0 (NCI-CTCAE, v3.0; Bethesda, MD).13

Drug Formulation

Ezatiostat is formulated as 500 mg tablets. Each tablet contains ezatiostat hydrochloride with the following excipients: mannitol, croscarmellose sodium, hypromellose, magnesium stearate, and Opadry Clear. (Opradry Clear is a mixture of hypromellose and polyethylene glycol 400.)

Assessments

On day 1 of each treatment cycle, a physical examination and laboratory assessments (complete blood count [CBC] with differential, platelet count, and chemistry profile) were obtained, use of concomitant medication(s) was documented, AEs were assessed, and RBC transfusion requirements were documented. CBC with differential and platelet count was obtained weekly.

Dose Modifications

Patients who experienced a treatment-related nonhematologic AE grade 3 or higher had treatment delayed for up to 3 weeks or until recovery to grade 1 or baseline and subsequent treatment resumed at a dose reduced by the amount of 500 mg (1 tablet) per day. If recovery did not occur after a delay of 21 days, treatment was discontinued and patients were followed until resolution of the AE.

Statistical Analysis

With 1-sided 5% significance level, 38 patients in each dose schedule ensured approximately 90% power to test for a 20% HI-E response rate in each treatment arm versus a 5% HI-E response rate in the null hypothesis. The primary endpoint of the study was the hematologic improvement response rate in the erythroid cell line (HI-E) by the IWG MDS criteria (2006) in the efficacy evaluable (EE) population. The HI-E response rate was also calculated in the intent-to-treat (ITT) population. All randomized patients were included in the ITT population. All randomized patients who received any amount of ezatiostat treatment were included in the safety population. Patients who received at least 2 cycles of ezatiostat treatment and for whom an HI response assessment was completed were included in the EE population. All responses were reviewed and confirmed. Efficacy analyses were performed in the ITT and the EE populations overall and by the 2 selected dose schedules. Exact binomial 95% confidence intervals (CIs) were provided for response rates. Rate comparisons were performed using Cochran-Mantel-Haenszel test stratified by significant confounders or Fisher's exact tests when applicable. Multiple comparisons were not addressed in this phase 2 study.

Time to response and duration of response were estimated by Kaplan-Meier method. Hematologic improvement-erythroid is defined as: 1) RBC transfusion-dependent patients who experience a transfusion reduction by ≥4 units from baseline over 8 weeks after the initiation of study treatment (given for Hgb ≤9.0 g/dL); or 2) Patients with symptomatic anemia with Hgb <10 g/dL at baseline who experienced an Hgb increase by ≥1.5 g/dL sustained for a period of 8 weeks.

RBC transfusion independence is defined as no RBC transfusions required over an 8-week period. Time to HI-E response is calculated from the day of the initiation of study treatment to the end day of the 8-week period when an HI-E is declared. Time to RBC transfusion independence is calculated from the day of the initiation of study treatment to the end day of an 8-week period free of transfusions. Duration of HI-E response is calculated from the end day of the 8-week period when an HI-E is declared to the first day when an HI-E criterion is no longer met. Duration of RBC transfusion independence is calculated from the last day after an 8-week period free of transfusions to the date a RBC transfusion is required. These duration-of-response definitions pertain only when transfusion reduction or independence has been sustained beyond 8 weeks, per IWG 2006 criteria.

A multivariate stepwise logistic regression analysis was conducted to identify significant MDS disease factors associated with HI-E rates, including prior MDS treatment, age, gender, IPSS, ECOG PS, years from MDS diagnosis, MDS World Health Organization (WHO) subtypes, anemia only versus anemia plus other cytopenias, dose schedule, and starting dose.

RESULTS

Patient Population Disposition

Eighty-nine patients were randomized to 2 dose schedules, with 14 patients treated at 4500 mg daily in the initial dose-ranging stage of the study. These 14 patients were randomized to either ezatiostat at 4500 mg total dose daily in divided doses p.o. b.i.d. days 1-14 followed by a 1-week rest per 3-week cycle (DS1) or 4500 mg total dose daily in divided doses p.o. b.i.d. days 1-21 followed by a 1-week rest in a 4-week cycle (DS2). Upon review of the initial 14-patient data, 2 lower doses were selected for randomized treatment of the subsequent 75 patients. Seventy-three patients were treated on the 2 selected ezatiostat dose schedules, with 37 patients at 3000 mg daily in divided doses (1500 mg p.o. b.i.d.) on DS1 (3-week cycle) and 36 patients at 2000 mg daily in divided doses (1000 mg p.o. b.i.d.) on DS2 (4-week cycle). The patient population disposition schema is shown in Figure 1.

Figure 1.

Patient population disposition schema. Abbreviations: E, erythroid (anemia); EN, erythroid-neutrophil (anemia + neutropenia); EP, erythroid-platelet (anemia + thrombocytopenia); NP, neutrophil-platelet (neutropenia + thrombocytopenia); ENP, erythroid-neutrophil-platelet (anemia + neutropenia + thrombocytopenia).

Patient Demographics Characteristics

Seventy-three patients, 51 (70%) males and 22 (30%) females, median age 73 years, (range, 48-89), with the majority of patients having an ECOG PS of 1 (46 patients, 63%), were treated on the 2 selected doses and dose schedules at 22 centers in the United States between October 8, 2008 and August 24, 2009 (Table 1). As a result of the randomization, the MDS disease characteristics were well balanced between the 2 treatment groups. Twenty-eight (38%) patients had abnormal karyotypes, with 23 (82%) of 28 patients being of complex (poor prognostic) types. Fifty (69%) patients had an intermediate-1 risk IPSS; 23 (32%) were low risk. The WHO subtypes included 9 (12%) patients with refractory anemia (RA), 11 (15%) RA with ringed sideroblasts (RARS), 4 (6%) RA with excess blasts (RAEB-1), 24 (33%) refractory cytopenias with multilineage dysplasia (RCMD), 14 (19%) refractory cytopenias with multilineage dysplasia with ringed sideroblasts (RCMD-RS), 4 (6%) MDS-unclassified (U), 2 (3%) MDS/myeloproliferative disorder-U, 4 (6%) MDS del 5q, and 1 (1%) unknown.

Table 1. Patient Demographics and MDS Disease Characteristics
 DS1 3000 mg/Day 2 Wk On/1 Wk Off (N = 37)DS2 2000 mg/Day 3 Wk On/1 Wk Off (N = 36)Total (N = 73)
  1. Abbreviations: Del5q, deletion 5q; DS1, dose schedule 1; DS2, dose schedule 2; ECOG, Eastern Cooperative Oncology Group; IPSS, International Prognostic Scoring System; MDS, myelodysplastic syndrome; MDS-U, MDS-unknown; MPD-U, myeloproliferative disorder-unknown; RA, refractory anemia; RARS, refractory anemia with ringed sideroblasts; RAEB-1, refractory anemia with excess blasts; RBC, red blood cell.; RCMB, refractory cytopenias with multilineage dysplasia; RCMD-RS, refractory cytopenias with multilineage dysplasia with ringed sideroblasts; WHO, World Health Organization; wk, week(s).

Age (y)   
 Median (range)74 (49-86)71 (48-89)73 (48-89)
Gender   
 Male292251
 Female81422
ECOG performance status, n (%)   
 013 (35)14 (39)27 (37)
 124 (65)22 (61)46 (63)
Baseline cytogenetics, n (%)   
 Normal22 (60)17 (47)39 (53)
 Abnormal13 (35)15 (42)28 (38)
  Del5q + 1 other abnormal1 (3)0 (0)1 (1)
  Del5q + ≥3 other complex3 (8)4 (11)7 (10)
  1 Other abnormal3 (8)1 (3)4 (6)
  ≥3 Other complex6 (16)10 (28)16 (22)
 Unknown2 (5)4 (11)6 (8)
IPSS classification, n (%)   
 Low risk13 (35)10 (28)23 (32)
 Intermediate-1 risk24 (65)26 (72)50 (69)
WHO classification, n (%)   
 RA4 (11)5 (14)9 (12)
 RARS4 (11)7 (19)11 (15)
 RAEB-13 (8)1 (3)4 (6)
 RCMD12 (32)12 (33)24 (33)
 RCMD-RS8 (22)6 (17)14 (19)
 MDS-U2 (5)2 (6)4 (6)
 MDS/MPD-U0 (0)2 (6)2 (3)
 MDS-del5q3 (8)1 (3)4 (6)
 Unknown (Other)1 (3)0 (0)1 (1)
Transfusion dependency   
 RBC transfusion dependent, n (%)24 (65)20 (56)44 (60)
 Median requirements (unit/8 wk) (range)6 (4-19)6 (4-12)6 (4-19)
Cell lineage cytopenia, n (%)   
 Anemia34 (92)36 (100)70 (96)
 Neutropenia12 (32)13 (36)25 (34)
 Thrombocytopenia18 (49)16 (44)34 (47)
 Bilineage cytopenia20 (54)22 (61)42 (58)
  Anemia + neutropenia10 (27)13 (36)23 (32)
  Anemia + thrombocytopenia16 (43)16 (44)32 (44)
  Neutropenia + thrombocytopenia8 (22)7 (19)15 (21)
 Trilineage cytopenia7 (19)7 (19)14 (19)

Forty-four (60%) patients were RBC transfusion dependent at baseline, with a median requirement of 6 units/8 weeks (range, 4-19). There were 70 (96%) patients with anemia, 25 (34%) with neutropenia, and 34 (47%) with thrombocytopenia.

Patients had received a median of 3 prior MDS therapies (range, 0-8) (Table 2) including recombinant erythropoietin in 56 (77%) patients (with the remaining 23% not being erythropoietin resistant but having high baseline erythropoietin levels), granulocyte colony stimulating factor in 32 (44%), lenalidomide in 28 (38%), other immunotherapy in 8 (11%), HMAs (eg, azacitidine and decitabine) in 34 (47%), other chemotherapy in 4 (6%), corticosteroids in 4 (6%), vitamins in 5 (7%), and investigational agents in 9 (12%).

Table 2. Prior MDS-Related Therapies
 3000 mg/Day 2 Wk on/1 Wk Off n (%)2000 mg/Day 3 Wk On/1 Wk Off n (%)Total n (%)
  1. Abbreviations: MDS, myelodysplastic syndrome; wk, week(s).

Hypomethylating agents   
 Azacitidine15 (41)15 (42)30 (41)
 Decitabine6 (16)5 (14)'11 (15)
Other chemotherapy1 (3)3 (8)4 (6)
Lenalidomide14 (38)14 (39)28 (38)
Other immunotherapy4 (11)4 (11)8 (11)
Erythropoietin30 (81)26 (72)56 (77)
Granulocyte colony growth dactor16 (43)16 (44)32 (44)
Investigation agent/drug4 (11)5 (14)9 (12)
Steroid1 (3)3 (8)4 (6)
Vitamin2 (5)3 (8)5 (7)
Other1 (3)3 (8)4 (6)
Median No. of prior MDS therapies (range)3 (0-7)3 (0-8)3 (0-8)

Ezatiostat Tablets Study Treatment Administration

Eighty-seven of 89 patients were treated (Table 3). A total of 447 cycles were administered, with the median number of 4 cycles per patient. Dose reductions were infrequent, with only 3% of all cycles requiring dose reduction and 9% of all cycles requiring a dose delay. Dose reductions were due to nausea (9 cycles), diarrhea (3 cycles), vomiting (3 cycles), fatigue (2 cycles), and abdominal distension, back pain, fatigue, dizziness, principal investigator discretion, platelet count increased (1 cycle each). Six patients discontinued treatment with ezatiostat due to ezatiostat-related AEs.

Table 3. Ezatiostat Treatment Administration
Safety PopulationDS1 4500 mg/ Day 2 Wk on/ 1 Wk OffDS2 4500 mg/ Day 3 Wk On/ 1 Wk OffDS1 3000 mg/ Day 2 Wk On/ 1 Wk OffDS2 2000 mg/ Day 3 Wk On/ 1 Wk OffTotal
  1. Abbreviations: DS1, dose schedule 1; DS2, dose schedule 2; wk, week(s).

No. of Patients68373687
Total no. of cycles administered2829201189447
Median no. of cycles per patient (range)5 (2-8)4 (1-6)4 (1-18)4 (1-17)4 (1-18)
No. of cycles with dose reduction (% of total cycles)3 (11)3 (10)3 (2)6 (3)15 (3)
No. of cycles with dose delay (% of total cycles)4 (14)2 (7)9 (5)23 (12)38 (9)

Safety

Ezatiostat was generally well tolerated, and the most common treatment-related AEs were nonhematologic, grade 1 and 2 gastrointestinal (GI) including nausea (45%, 17%), diarrhea (26%, 7%), and vomiting (30%, 12%). Grade 3 events were nausea (1%), diarrhea (3%), and vomiting (2%) (Table 4). Ezatiostat-related AEs in general appeared to be related to dose, with the 4500 mg/day starting dose being associated with the highest incidence of AEs and the 2000 mg/day starting dose associated with the lowest incidence of AEs. However, prior HMA treatment appeared to be associated with an increased incidence of ezatiostat-related AEs. Patients with prior HMA use had GI AEs (grades 1, 2, and 3, respectively): nausea (51%, 20%, 2%), vomiting (34%, 7%, 2%), diarrhea (22%, 10%, 5%). HMA-naive patients had fewer GI AEs (grades 1, 2, 3): nausea (39%, 15%, 0%), vomiting (26%, 15%, 2%), diarrhea (30%, 4%, 2%). This observation was also true of all treatment-emergent GI AEs regardless of ezatiostat treatment relatedness.

Table 4. Hematologic (All Events) and Nonhematologic (≥5% of Events) Adverse Events “Possibly Related,” “Probably Related,” or “Related” to Ezatiostat by Grade for All Dose Levels Combined
For Both Dose Schedules Combined NCI-CTCAE v3.0 Maximum Toxicity Grade (N = 87)
Adverse Event (Preferred Term)Grade 1 n (%)Grade 2 n (%)Grade 3 n (%)Grade 4 n (%)Total n (%)
Hematologic
 Anemia0 (0)0 (0)1 (1)0 (0)1 (1)
 Eosinophilia0 (0)1 (1)0 (0)0 (0)1 (1)
 Febrile neutropenia0 (0)0 (0)1 (1)0 (0)1 (1)
 Thrombocytopenia0 (0)0 (0)0 (0)1 (1)1 (1)
Nonhematologic
 Nausea39 (45)15 (17)1 (1)0 (0)55 (63)
 Vomiting26 (30)10 (12)2 (2)0 (0)38 (44)
 Diarrhea23 (26)6 (7)3 (3)0 (0)32 (37)
 Fatigue3 (3)10 (12)4 (5)0 (0)17 (20)
 Skin odor abnormal9 (10)8 (9)0 (0)0 (0)17 (20)
 Abdominal distention6 (7)0 (0)0 (0)0 (0)6 (7)
 Abdominal pain2 (2)2 (2)2 (2)0 (0)6 (7)
 Abdominal pain upper5 (6)1 (1)0 (0)0 (0)6 (7)
 Abnormal feces5 (6)0 (0)1 (1)0 (0)6 (7)
 Constipation3 (3)2 (2)1 (1)0 (0)6 (7)
 Dysgeusia5 (6)1 (1)0 (0)0 (0)6 (7)
 Pyrexia1 (1)4 (5)1 (1)0 (0)6 (7)
 Urine odor abnormal5 (6)1 (1)0 (0)0 (0)6 (7)
 Dyspepsia2 (2)2 (2)1 (1)0 (0)5 (6)
 Dysphagia1 (1)2 (2)2 (2)0 (0)5 (6)
 Headache3 (3)2 (2)0 (0)0 (0)5 (6)

The incidence of overall grade 1 and grade 2 ezatiostat-related abnormal skin odor was 9 of 87 (10%) and 8 of 87 (9%), respectively. Prior HMA treated patients had significantly increased incidence and severity of abnormal skin odor (P = .0386). Patients with prior HMA treatment had grade 1 and grade 2 abnormal skin order, 4 of 41 (10%) and 7 of 41 (17%), respectively, compared with HMA-naive patients, 5 of 46 (11%) and 1 of 46 (2%), respectively.

There were a total of 11 ezatiostat possibly treatment-related serious adverse events. Of these 11 possibly treatment-related events, there were 2 events of gastritis/gastroenteritis, 2 events of fever, 2 events of diarrhea, and 1 event each with abdominal pain, pneumonia, vomiting, dizziness, and pulmonary embolism. These events resolved without sequelae.

Efficacy

Seventy-five patients were in the ITT population on the 2 selected dose and dose schedules and 61 patients were in the EE population. The HI-E response rate was similar for the EE (22%) and ITT (19%) populations. The HI-E response rate was also similar for the evaluable patients on the 2 dose schedules (P = .76); therefore, the data were pooled (Table 5). The overall HI-E rate was 22% (13 of 60 patients; 95% CI, 12-34) as protocol-targeted for this heavily prior treated population. Eleven (29%) of 38 RBC transfusion-dependent patients had transfusion reductions (≥4 units/8 weeks), with 4 (11%) patients achieving transfusion independence. The maximum Hgb during the HI-E responding period was a median of 10 mg/dL (range, 8-15) with a median of 8 mg/dL (range, 7-10) at baseline. The IWG 2006 criteria time-to-response and duration-of-response definitions pertain to transfusion reduction or independence after it had been sustained beyond 8 weeks; this is the more stringent criteria, because it does not include the first 8 weeks of response in the calculation. The median duration of response was 34 weeks. However, exposure time was observed to be important, as the median duration of response was 18 weeks on DS1 and 46 weeks on DS2, respectively. The lower dose, longer exposure time dose schedule was observed to have a longer duration of response. The median time to HI-E response was 8 weeks (range, 8-11). A 19% HI-N response rate was observed in patients with neutropenia (4 of 21; 95% CI, 5-42) and a bilineage (HI-E and HI-N) response rate of 20% (4 of 20; 95% CI, 6-44) in patients with anemia and neutropenia. A 4% HI-P response rate (1 of 27; 95% CI, 0-19) was observed in thrombocytopenic patients and a trilineage (HI-E, HI-N, HI-P) response rate of 9% (1 of 11; 95% CI, 0-41) in patients with trilineage cytopenia. There was 1 cytogenetic complete response in a patient with del 5q.

Table 5. Efficacy
ITT Population3000 mg/Day 2 Wk On/1 Wk Off (n = 38)2000 mg/Day 3 Wk On/1 Wk Off (n = 37)Total (n = 75)
HI-E (%) (95% CI)7 (21) (9-38)6 (17) (6-33)13 (19) (10-30)
HI-N (%) (95% CI)1 (8) (0-39)3 (23) (5-54)4 (16) (5-36)
HI-P (%) (95% CI)1 (6) (0-27)0 (0) (0-21)1 (3) (0-15)
Bilineage (HI-E and HI-N) (%) (95% CI)1 (10) (0-45)3 (23) (5-54)4 (17) (5-39)
Trilineage (HI-E, HI-N, and HI-P) (%) (95% CI)1 (14) (0-58)0 (0) (0-41)1 (7) (0-34)
Median time (wk) to HI-E response (range)8 (8-10)8 (8-11)8 (8-11)
Median duration (wk) of HI-E response (range)18 (2-51)46 (2-63)34 (2-63)
# of RBC Transfusion-dependent patients (%)24 (71)20 (56)44 (63)
RBC transfusions reduced by 4 units/8wk (%) (95% CI)6 (25) (10-47)5 (25) (8-49)11 (25) (13-40)
RBC transfusion independence (%) (95% CI)1 (4) (0-21)3 (15) (3-38)4 (9) (3-22)
EE Population3000 mg/Day 2 Wk On/1 Wk Off (n = 30)2000 mg/Day 3 Wk On/1 Wk Off (n = 31)Total (n = 61)
  1. CI, confidence interval; EE, efficacy evaluable; HI-E, hematologic improvement-erythroid; HI-N, hematologic improvement-neutrophil; HI-P, hematologic improvement-platelet; ITT, intent-to-treat; RBC, red blood cell; Wk, week(s).

HI-E (%) (95% CI)7 (24) (10-44)6 (19) (8-38)13 (22) (12-34)
HI-N (%) (95% CI)1 (10) (0-45)3 (27) (6-61)4 (19) (5-42)
HI-P (%) (95% CI)1 (7) (0-34)0 (0) (0-25)1 (4) (0-19)
Bilineage (HI-E and HI-N) (%) (95% CI)1 (11) (0-48)3 (27) (6-61)4 (20) (6-44)
Trilineage (HI-E, HI-N, and HI-P) (%) (95% CI)1 (17) (0-64)0 (0) (0-52)1 (9) (0-41)
Median time (wk) to HI-E response (range)8 (8-10)8 (8-11)8 (8-11)
Median duration (wk) of HI-E response (range)18 (2-51)46 (2-63)34 (2-63)
No. of RBC transfusion-dependent patients (%)20 (69)18 (58)38 (63)
RBC transfusions reduced by 4 units/8 wk (%) (95% CI)6 (30) (12-54)5 (28) (10-54)11 (29) (15-46)
RBC transfusion independence (%) (95% CI)1 (5) (0-25)3 (17) (4-41)4 (11) (3-25)

There was an important trend in effect of prior therapy. A 40% HI-E rate (6 of 15 patients) (95% CI, 16-68) was observed in patients who had prior lenalidomide, but were HMA naive, with 5 of 11 (45%) patients achieving significant RBC transfusion reduction and 3 of 11 (27%) patients achieving transfusion independence. These 3 ongoing patients are still RBC transfusion independent at the data cutoff for this manuscript. They have been on ezatiostat treatment for 15, 17, and 18 months, respectively.

A 28% HI-E rate (5 of 18 patients; 95% CI, 10-54) was observed in patients who were both lenalidomide and HMA naive, with 4 of 8 (50%) patients achieving clinically significant RBC transfusion reduction. In the 16 patients who had received prior HMA but were lenalidomide naive, a 0% HI-E rate (95% CI, 0-21) was observed.

Prior HMA treatment was associated with a 6-fold decrease in the odds (odds ratio = 0.16; 95% CI, 0.03-0.81) for HI-E response to subsequent ezatiostat (P = .027). A recent study reported poor outcomes for MDS patients who have failed azacitidine therapy.14 This observation that prior HMA treatment is associated with less effective response to subsequent therapies is not unique to ezatiostat.

Comparison of ezatiostat efficacy in patients with prior HMA treatment versus being HMA naive showed an HI-E rate of 7% (2 of 27; 95% CI, 1-24) versus 33% (11 of 33; 95% CI, 18-52) respectively (P = .018). The transfusion reduction rate was 11% (2 of 19; 95% CI, 1-33) in patients with prior HMA treatment versus 47% (9 of 19; 95% CI, 24-71) in patients that were HMA naive (P = .023). The median duration of response in HMA treatment-naive patients was 46 weeks (range, 2-63) compared with 34 weeks (range, 2-34) in patients with a history of prior HMA treatment. The HI-N response was observed in 3 of 9 HMA-naive patients (33%; 95% CI, 8-70) compared with 1 in 12 patients (8%; 95% CI, 0-39) who had received prior HMA treatment.

Comparison of ezatiostat efficacy in patients with a history of prior lenalidomide treatment versus lenalidomide naive state showed an HI-E rate of 31% (8 of 26; 95% CI, 14-52) versus 15% (5 of 34; 95% CI, 5-31), respectively. Similarly, the clinically significant RBC transfusion reduction rate was 39% (7 of 18; 95% CI, 17-64) in patients who had received prior lenalidomide versus 20% (4 of 20; 95% CI, 6-44) in patients who were lenalidomide naive. Clinically and statistically significant findings were shown on the transfusion-independent rate of patients (22%) with prior lenalidomide treatment (4 of 18; 95% CI, 6-48) versus 0% (0%; 95% CI, 0-17) in patients with no prior lenalidomide therapy (P = .042).

CONCLUSIONS

Ezatiostat, a reversible oral inhibitor of the enzyme GSTP1-1, in this phase 2 study, showed promising hematopoietic promoting activity in heavily pretreated patients with low to intermediate-1 risk MDS. These results confirm the previously reported proof of concept study using intravenous administration,8 as well as the phase 1 study of the current oral formulation using an abbreviated dosing schedule.7

The population in this study was heavily prior treated: 77% resistant to erythropoietin, 38% resistant to lenalidomide, and 47% resistant to HMAs. In addition, the majority of these patients (69%) were of an intermediate-1 risk IPSS group, with 94% being in the non-del 5q subtypes by the WHO classification. Efficacy results showed a clinically meaningful and sustained reduction in responding patients' transfusion requirements including, in some responders, the achievement of transfusion independence. Among the transfusion-dependent patients, the HI-E rate was 29%, with 11% achieving complete transfusion independence. The overall median duration of HI-E response was 34 weeks, whereas on the 4-week cycle dose schedule the median duration of response was 46 weeks. The trends of longer duration of HI-E response on DS2, better tolerability of the lower daily dose utilized on DS2, combined with more convenience for patients of dosing of 2 tablets twice a day, led to our decision to select DS2 for future ezatiostat studies. Beneficial effects were also observed in a subset of those with thrombocytopenia and neutropenia, confirming the multilineage efficacy of ezatiostat. As previously reported, some patients also achieved cytogenetic responses, including 1 patient with del 5q cytogenetic abnormality.

Currently, the recommended approach for this group of MDS patients is to start with erythroid stimulating agents for those who are not heavily transfusion dependent and have a low baseline serum erythropoietin level. Next, lenalidomide is recommended for the del 5q group, and for the rest, the choice ranges from lenalidomide (although not FDA approved for this indication) to 1 of the hypomethylating drugs (azacitidine or decitabine). All 3 of these drugs, however, are associated with considerable myelosuppression.

Ezatiostat had a very favorable tolerability profile, with gastrointestinal side effects being the predominant adverse event; however, these were mainly restricted to grades 1 and 2, confirming the results of the prior studies. Importantly, a significant increase in frequency and severity of adverse events and decrease in efficacy were noted in patients who had received prior treatment with hypomethylating drugs. The tolerability and efficacy profile described above supports a dual role for ezatiostat in the management of low to intermediate-1 risk MDS patients, as a disease modifier and as a potential cytoprotective agent. This drug improves hematopoiesis in a subset of these MDS patients without causing myelosuppression. Because the safety and efficacy were significantly worse in patients treated with prior HMAs, ezatiostat would best be used in HMA-naive patients. Ezatiostat has shown efficacy both in patients who are lenalidomide naive and in patients who have had prior lenalidomide treatment. These observations will help guide patient selection in future trials. We have previously reported on a patient from this trial who was a del 5q lenalidomide failure relapsing with a new t(2;3) clonal abnormality who experienced a prolonged trilineage response lasting over a year after a brief 6-week exposure to ezatiostat.15 This suggests a potential treatment application for ezatiostat in a group of del 5q lenalidomide failures in whom the therapeutic efficacy of HMAs has proved to be limited. Given the absence of myelosuppression, it is reasonable to postulate a cytoprotective effect of ezatiostat. This profile has provided the basis for a current ongoing phase 1 study testing the combination of lenalidomide plus ezatiostat in low to intermediate-1 risk non-del 5q MDS. In this heavily pretreated population, ezatiostat is a very tolerable drug compared with the other disease-modifying agents available. In addition, ezatiostat was observed to be active in patients who had failed prior treatment with lenalidomide. This patient population represents a group with a particular unmet medical need and is of interest for further study.

In addition to the clinical benefit of reducing transfusion requirments in MDS patients, there is also a potential for an oral pharmaceutical small-molecule therapy to have a pharmacoeconomic benefit. A recent study by Frytak et al16 reported that transfusion dependence in adults with MDS was associated with an incremental cost of $31,255 per patient per year. This study demonstrated that an important consequence of transfusion dependence for MDS patients was markedly greater use of and consequently higher costs associated with inpatient and outpatient services. They concluded that pharmaceutical therapies may help more patients achieve transfusion independence, thereby reducing the financial burden of MDS. The mechanism by which a reversible inhibitor of the enzyme GSTP1-1 can cause significant improvement in the MDS patient is of great interest and may shed additional insight into the molecular defects associated with the development and evolution of MDS. Recent reports have shown that GSTP1-1 is a significant mediator of signaling in hematopoietic cells, most likely through negative regulation of JNK activity. Consequently, inhibition of GSTP1-1 would be expected to stimulate growth and differentiation of normal progenitor cells as well as apoptosis of dysplastic cells.17 Inhibition of GSTP1-1 may also elevate levels of reactive oxygen species that would contribute to apoptosis of abnormal cells. In addition, studies are also in progress to identify genomic markers that predict enhanced response to ezatiostat.

In conclusion, this study supports the further development of ezatiostat in low to intermediate-1 risk MDS.

FUNDING SUPPORT

This work was supported by Telik, Inc., Palo Alto, Calif, USA.

CONFLICT OF INTEREST DISCLOSURES

Dr. Brown is an employee of Telik, Inc, is fully compensated, and holds stock in the company. Dr. Sekeres serves on an Advisory Board for Celgene Corporation. All other authors declare no conflicts of interest.

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