A Phase I Dose Finding and Proof-of-concept Study of the Histone Deacetylase Inhibitor Panobinostat (LBH589) in Combination With Standard Dose Cytarabine and Daunorubicin for Older Patients With Untreated Acute Myeloid Leukemia or Advanced Myelodysplastic Syndrome

Trial Information

A Phase I Dose Finding and Proof-of-concept Study of the Histone Deacetylase Inhibitor Panobinostat (LBH589) in Combination With Standard Dose Cytarabine and Daunorubicin for Older Patients With Untreated Acute Myeloid Leukemia or Advanced Myelodysplastic Syndrome

In the United States, the incidence of acute myeloid leukemia (AML) is approximately 3.5
cases per 100,000 persons per year. Approximately 13,000 people were diagnosed with AML in
2009 and 9,000 died of the disease, making AML the 6th leading cause of cancer death. Over
the past three decades, AML survival has improved for younger patients with 5-year survival
rates of greater than 60% for adults under the age of 45 years likely owing to improvements
in induction and consolidation chemotherapy, allogeneic hematopoietic stem cell transplant
(HSCT) and supportive care. Post-remission therapy with high-dose cytarabine-based regimens
after cytarabine and anthracycline based induction has improved disease free and overall
survival at the expense of increased treatment related mortality limiting its use in many
older patients and those with significant comorbidities. Although allogeneic HSCT remains
the standard of care for patients with poor risk AML or relapsed disease, advanced age,
comorbidities and donor availability preclude this option for a large number of patients
making improvement in the tolerability and efficacy of induction therapy an important goal.

Over half of newly diagnosed AML patients are over 65 years of age with a third over the age
of 75 years. Unlike younger patients, the prognosis of elderly patients with AML is still
dismal with five-year survival rates of less than 10% for patients over the age of 65 years.
For the last thirty years, induction therapy with standard dose cytarabine with an
anthracycline has remained the standard of care for elderly patients with AML. In the
elderly, complete response rates to induction chemotherapy are lower than younger patients
at 40 to 60% with median survival approaching 12 months. New strategies using novel agents
to increase the sensitivity of malignant myeloid precursors to standard induction
chemotherapy may improve complete response and relapse rates without increasing treatment
related mortality.

Myelodysplastic syndromes (MDS)

The myelodysplastic syndromes are neoplasms of hematopoietic progenitor cells characterized
by ineffective hematopoiesis and increased risk of transformation to AML. Clinically,
patients develop symptoms related to with cytopenias, typically progressive anemia with or
without thrombocytopenia or neutropenia that is unrelated to a defined reversible cause such
as nutritional deficiency. Histologically, MDS is suggested by the presence of dysplasia in
>10% of cells in one or more myeloid lineage on bone marrow evaluation. Characteristic
cytogenetic abnormalities also aid in making the diagnosis of MDS.

The incidence of MDS in the U.S. has been estimated at 3.4 cases per 100,000 people per year
with the incidence increasing 10 fold in people over the age of 70. Risk factors for the
development of MDS include advanced age, male sex, and prior exposure to DNA-damaging
chemotherapy or radiation therapy, typically for treatment of other malignancies. As a
group, patients with advanced MDS and those with MDS progressing to AML have treatment
resistant disease with low response rates and short durations of response after induction
therapy.

The International Prognostic Scoring System (IPSS) for primary MDS assigns four MDS risk
categories (Low, INT-1, INT-2, High) based on bone marrow myeloblast percentage, specific
cytogenetic abnormalities, and number of cytopenias to estimate survival and risk of
transformation to AML. Low and INT-1 risk MDS patients have a median survival of 5.7 and
3.3 years, respectively, in the absence of therapy. Advanced MDS patients in IPSS risk
groups INT-2 and High fare much less well with median survival of 1.1 and 0.4 years,
respectively. INT-2 and High-risk MDS is also associated with a higher risk of
transformation to AML. In addition, hematopoietic precursors from patients with advanced
MDS more frequently express the multi-drug resistance (MDR1) gene product P-glycoprotein,
possibly explaining the low response rates and short duration of responses in this group
after conventional induction therapies. A Phase III trial of the P-glycoprotein inhibitor
valspodar in combination with mitoxantrone, etoposide and cytarabine in relapsed or
refractory AML and high-risk MDS failed to show improved outcomes with P-glycoprotein
inhibition. As such, novel therapeutic strategies to overcome the intrinsic resistance to
chemotherapy seen in advanced MDS are needed to improve induction chemotherapy as primary
therapy and as a bridge to allogeneic hematopoietic cell transplant.

Given the relatively long survival and low rate of progression to AML seen in patients with
IPSS Low and INT-1 risk disease, allogeneic transplant is typically reserved for those who
fail conservative management with erythropoiesis stimulating agents, G-CSF, hypomethylating
agents such as azacitidine or decitabine, lenalidomide, or immune suppression therapy. For
patients 60 years of age and younger with advanced MDS (INT-2, High), allogeneic
hematopoietic cell transplant is the most appropriate therapy as it prolongs life
expectancy. Due to advanced age and significant co-morbidities, allogeneic hematopoietic
transplant is not an appropriate treatment modality for a large number of patients with
advanced MDS. In addition, stem cell donors are not available for all patients. For
patients with advanced MDS, hypomethylating agents or enrollment on clinical trials are both
appropriate treatment options given the poor outcomes in this patient population. A phase
III open-label, randomized trial of azacitidine versus conventional care regimens in
advanced MDS showed superior overall survival for patients treated with azacitidine (24.5
versus 15.0 months, HR 0.58; 95% CI 0.43-0.77). Although decitabine has activity in MDS, it
has not been shown to prolong survival in advanced MDS to date. In a phase III study
comparing decitabine to supportive care, decitabine showed a superior response rate and
delayed the time to the development of AML.

Anthracyclines in AML therapy

Anthracycline chemotherapy agents (daunorubicin, idarubicin, mitoxantrone) are highly active
in AML and are an essential part initial induction therapy in those fit for intensive
chemotherapy. The optimal dose of anthracycline to maximize response and survival while
preserving safety is still being determined. For daunorubicin, a Phase III randomized study
of younger patients ages 17 to 60 years with AML demonstrated that standard dose cytarabine
100 mg/m2 daily for 7 days in combination with daunorubicin 90 mg/m2 daily for 3 days was
superior to daunorubicin 45 mg/m2 daily for 3 days with improved complete remission rates
(70.6% vs. 57.3%, p<0.001) and median overall survival (23.7 vs. 15.7 months, p=0.003).
Toxicity was not significantly different between the two groups. A similar Phase III study
in AML patients 60 years of age or older compared daunorubicin 45 mg/m2 to 90 mg/m2 for 3
days in combination with cytarabine 200 mg/m2 daily for 7 days. Although the complete
remission rate was higher in patients receiving the 90 mg/m2 daunorubicin dose (64% vs 54%,
p=0.002), no difference was seen in survival. Notably, patients aged 60-65 years receiving
higher daunorubicin dose had superior complete remission rates, event-free survival and
overall survival. To date, no head-to-head comparisons of daunorubicin at 60 mg/m2 versus
90 mg/m2 have been published.

Histone deacetylases and their inhibitors in AML and MDS

HDAC inhibitors have shown activity in Phase I monotherapy trials for AML and advanced MDS.
A Phase I trial of panobinostat as monotherapy in primarily AML yielded transient
hematologic responses with reduction in peripheral blood blast counts in 8 of 11 patients
consistent with the documented in vitro activity of the drug. Major toxicities included
nausea, diarrhea, hypokalemia, anorexia, thrombocytopenia and reversible QTcF prolongation.
Similar responses with rare complete responses have also been seen with the HDAC inhibitors
romidepsin and MGCD0103 in AML and MDS.

Synergy between HDAC inhibitors and anthracyclines

As monotherapy, HDAC inhibitors are unlikely to impact the treatment of AML and advanced MDS
although there is a strong biologic rationale for use of these agents in combination
therapies. By inhibiting deacetylation of histones, HDAC inhibitors generate a more open
chromatin structure more susceptible to the DNA damaging effects of anthracycline
chemotherapeutic agents, in some instances when administered 48 hours after the HDAC
inhibitor. In vitro, HDAC inhibitors potentiate the cytotoxic effects of anthracyclines in
leukemia cell lines. Panobinostat, specifically, acts synergistically with the
anthracycline doxorubicin to induce DNA damage, increase histone acetylation and activate
programmed cell death in AML cell lines and primary AML cells. The administration of the
anthracycline daunorubicin with panobinostat is predicted to be synergistic in vivo and may
improve complete response and relapse rates for AML.

Proper sequencing of HDAC inhibitors with anthracyclines will likely be important to the
success of these combinations. Pretreatment with HDAC inhibitors prior to anthracycline
exposure may provide synergistic effects as well by increasing nuclear DNA exposure to
anthracycline. In cultured MCF-7 breast cancer cells, treatment with the HDAC inhibitor
vorinostat leads to chromatin decondensation which is maximal after 48 hours of HDAC
inhibitor treatment. In this system, co-administration of vorinostat and epirubicin did not
lead to increased apoptosis whereas 48 hour pre-incubation with vorinostat led to
synergistic increases in apoptosis associated with increased nuclear accumulation of
epirubicin and increased DNA damage. In AML, maximal epigenetic effects appear to occur at
about 48 hours after HDAC inhibitor exposure as well.

Anticancer activity of DAC inhibitors

Alterations in chromosome structure play critical roles in the control of gene
transcription. These epigenetic alterations include modification of histones and others
proteins by acetylation and/or phosphorylation. Normally, these modifications are balanced
finely and are highly reversible in normal tissues, but they may be imbalanced and heritable
in tumor cells. DAC inhibitors increase histone acetylation, thereby modulating the
expression of a subset of genes in a coordinated fashion. Several tumors suppressor genes
associated with the malignant phenotype are repressed by epigenetic mechanisms in sporadic
cancers. Thus therapy with DAC inhibitors may alter tumor phenotype and inhibit growth in
such tumors.

Multiple hallmarks of cancer are regulated by acetylation/deacetylation:

- DAC inhibition targets both histone and nonhistone proteins. Targeting the acetylation
status of nonhistone, tumor-associated proteins that mediate proliferation may be the
underlying antitumor mechanism of DAC inhibitors.

- Nonhistone proteins regulated by acetylation include α-tubulin, p53, HIF-1α, and HSP90.
These proteins are substrates of DACs.

- The ability of a single agent to target multiple molecular features of tumor cells may
result in good efficacy against a range of different tumor types.

- HSP90 is involved in protein stability and degradation; the inhibition of HSP90 affects
protein turnover in diseases such as multiple myeloma and B-cell malignancies.

- Acetylated HIF-1α is degraded and can no longer act as a tumor growth factor. Class II
DAC inhibitors target histone deacetylase (HDAC or DAC) 6, resulting in increased
acetylation of HIF-1α and decreased vascular endothelial growth factor (VEGF), thereby
inhibiting angiogenesis.

- Both acetylation and ubiquitylation often occur on the same lysine residue, but these
processes cannot occur simultaneously. Acetylation allows for increased stability, and
ubiquitylation leads to protein degradation. Therefore, DACs decrease the half-life of
a protein by exposing the lysine residue for ubiquitylation.

Panobinostat (LBH589)

Panobinostat (LBH589) is a deacetylase inhibitor (DACi) belonging to a structurally novel
cinnamic hydroxamic acid class of compounds. It is a potent class I/II pan-DAC inhibitor
(pan-DACi) that has shown anti-tumor activity in pre-clinical models and cancer patients.
Deacetylases (DAC) target lysine groups on chromatin and transcription factors and various
non-histone proteins such as p53, tubulin, HSP90 and Rb. Panobinostat is formulated as an
oral capsule and a solution for intravenous (i.v.) injection. Both the oral and i.v.
formulations are currently being investigated in ongoing Phase I and Phase II studies in
advanced solid tumors and hematological malignancies.

Inhibition of DAC provides a novel approach for cancer treatment. Histones are part of the
core proteins of nucleosomes, and acetylation and deacetylation of these proteins play a
role in the regulation of gene expression. Highly charged deacetylated histones bind
tightly to the phosphate backbone of DNA, inhibiting transcription, presumably, by limiting
access of transcription factors and RNA polymerases to DNA. Acetylation neutralizes the
charge of histones and generates a more open DNA conformation. This conformation allows
transcription factors and associated transcription apparatus access to the DNA, promoting
expression of the corresponding genes. The opposing activities of two groups of enzymes,
histone acetyltransferase (HAT) and DAC control the amount of acetylation. In normal cells
a balance exists between HAT and DAC activity that leads to cell specific patterns of gene
expression. Perturbation of the balance produces changes in gene expression.

Several lines of evidence suggest that aberrant recruitment of DAC and the resulting
modification of chromatin structure may play a role in changing the gene expression seen in
transformed cells. For example, silencing of tumor suppressor genes at the level of
chromatin is common in human tumors and DAC complexes have been shown to be crucial to the
activity of the AML-specific fusion proteins PLZF-RAR-α, PML-RAR-α, and AML1/ETO. DAC
inhibitors (DACi) have been shown to induce differentiation, cell cycle arrest or apoptosis
in cultured tumor cells, and to inhibit the growth of tumors in animal models. In addition,
DACi have been shown to induce expression of p21, a key mediator of cell cycle arrest in G1
phase and cellular differentiation.

Tumor growth inhibition and apoptosis in response to DACi treatment may also be mediated
through changes in acetylation of non-histone proteins (e.g., HSP90, p53, HIF-1α,
α-tubulin). For example, the chaperone protein HSP90 has been shown to be acetylated in
cells treated with DACi. Acetylation of HSP90 inhibits its ability to bind newly
synthesized client proteins, thus preventing proper client protein folding and function. In
the absence of HSP90 function, misfolded proteins are targeted for degradation in the
proteasome. Many proteins that require HSP90 association are critical to cancer cell
growth, including ErbB1, ErbB2, AKT, Raf, KDR, and BCR-ABL. Acetylation of HSP90 in cells
treated with DACi inhibits the chaperone function of HSP90, leading to degradation of the
client proteins and eventual cell death.

The potential clinical utility of the use of DACi in cancer therapy was first suggested by
the activity of the DACi, sodium phenylbutyrate, against acute promyelocytic leukemia (APL).
An adolescent female patient with relapsed APL, who no longer responded to all
trans-retinoic acid (ATRA) alone, achieved a complete clinical remission after treatment
with a combination of ATRA and the DACi sodium phenylbutyrate.