Myelodisplastic Syndromes (MDS) are a heterogeneous group of diseases characterized by ineffective hematopoiesis (as a result of increased apoptosis of precursor cells), progressive peripheral cytopenia, with a tendency to evolve to acute leukemia.
The term "syndrome" represents the wide clinical spectrum of this group of diseases, ranging from mild and stable cytopenia, with a low risk of leukemic conversion and a life expectancy of several years, to true pre-leukemia.
The International MDS Risk Analysis Workshop recently developed a consensus risk-based International Prognostic Score System (IPSS) for primary MDS, which has markedly improved prognostic stratification of MDS patients. Following IPSS,it is now possible to identify patients (i.e. Hig-Risk and Intermediate-2-Risk patients) with a bad prognosis (i.e. a life-expectancy < 1 year) due to a high risk of leukemic evolution.
Currently, allogeneic stem cell transplantation represents the only curative therapy for this subgroup of high-risk patients. However, this therapeutic option is often precluded for several reasons (old age, comorbidity, lack of suitable donor).
Among the experimental treatments hitherto tested, 5-Azacitidine (5-Aza) has recently shown promising results. Moreover, the biological experimental data suggest that the association of 5-Aza with histone deacetylase inhibitors, such as Valproic Acid, and with differentiating agents, such as retinoic acid, might be synergistic.
Azacitidine (Aza C), a pyrimidine nucleoside analog, was developed as an antitumor agent. In addition to cytotoxic effects, it induces differentiation of malignant cells in vitro. Aza C inhibits DNA methyltransferase, the enzyme in mammalian cells responsible for methylating newly synthesized DNA, resulting in synthesis of hypomethylated DNA and changes in gene transcription and expression. The mechanism by which Aza C produces its effects is most likely multifactorial. Aza C can produce significant myelosuppression, particularly at higher doses. Aza C could also be acting as a biologic response modifier. The response of hematopoietic progenitors to cytokines is impaired in patients with MDS. This may be attributable in part to abnormalities of the signal transduction pathway downstream from the cytokine receptors. In vitro data suggest that Aza-C can modulate the cytokine signal transduction pathway, rendering sensitive unresponsive cells to the effects of cytokines, partially restoring normal hematopoietic regulation. Incorporation of Aza-C into DNA inhibits DNA methyltransferase and induces DNA hypomethylation. 5-Azacytidine and 5-Aza-2_-deoxycytidine (decitabine), 2 potent inhibitors of cytosine methylation, have shown strong antileukemic activity in acute myeloid leukemia (AML). In addition, both induce trilineage responses in myelodysplastic syndromes (MDS) at dose levels allowing for outpatient management, with moderate myelotoxicity and no significant nonhematologic toxicity.
Azacytidine is the only drug approved by FDA for the treatment of MDS. The median survival for patients with MDS and a Intermediate/high IPSS score, treated with conventional care regimens, is assumed to be 11 months. Patients treated with Azacytidine in the CALGB 9221 trial had a median survival of 18 months. Responses occurred in 60% of patients on the Aza C arm (7% complete response, 16% partial response, 37% improved) compared with 5% (improved) receiving supportive care (P <.001). Median time to leukemic transformation or death was 21 months for Aza C versus 13 months for supportive care (P =.007). Transformation to acute myelogenous leukemia occurred as the first event in 15% of patients on the Aza C arm and in 38% receiving supportive care (P =.001). Furthermore, Aza-C improved quality of life, compared to supportive care.
The opposing actions of histone acetyltransferases (HATs) and histone deacetylases (HDACs) allow gene expression to be exquisitely regulated through chromatin remodelling. Aberrant transcription due to altered expression or mutation of genes that encode HATs, HDACs or their binding partners, is a key event in the onset and progression of cancer. HDAC inhibitors can reactivate gene expression and inhibit the growth and survival of tumour cells. The remarkable tumour specificity of these compounds, and their potency in vitro and in vivo, underscore the potential of HDAC inhibitors as exciting new agents for the treatment of cancer. Histone deacetylase (HDAC) inhibitors have been shown to be potent inducers of growth arrest, differentiation, and/or apoptotic cell death of transformed cells in vitro and in vivo. One class of HDAC inhibitors, hydroxamic acid-based hybrid polar compounds (HPCs), induce differentiation at micromolar or lower concentrations. Studies (x-ray crystallographic) showed that the catalytic site of HDAC has a tubular structure with a zinc atom at its base and that these HDAC inhibitors, such as suberoylanilide hydroxamic acid (SAHA) and trichostatin A, fit into this structure with the hydroxamic moiety of the inhibitor binding to the zinc. HDAC inhibitors cause acetylated histones to accumulate in both tumor and normal tissues, and this accumulation can be used as a marker of the biologic activity of the HDAC inhibitors. Hydroxamic acid-based HPCs act selectively to inhibit tumor cell growth at levels that have little or no toxicity for normal cells. These compounds also act selectively on gene expression, altering the expression of only about 2% of the genes expressed in cultured tumor cells. In general, chromatin fractions enriched in actively transcribed genes are also enriched in highly acetylated core histones, whereas silent genes are associated with nucleosomes with a low level of acetylation. However, HDACs can also acetylate proteins other than histones in nucleosomes. The role that these other targets play in the induction of cell growth arrest, differentiation, and/or apoptotic cell death has not been determined. Our working hypothesis is that inhibition of HDAC activity leads to the modulation of expression of a specific set of genes, which control has been disrupted by ectopic expression of EVI1 or similar oncogene, that, in turn, result in growth arrest, differentiation, and/or apoptotic cell death. The hydroxamic acid-based HPCs are potentially effective agents for cancer therapy and, possibly, cancer chemoprevention.
Valproic acid Valproic acid (VPA, 2-propylpentanoic acid) is an established drug in the long-term therapy of epilepsy. During the past years, it has become evident that VPA is also associated with anti-cancer activity. VPA not only suppresses tumor growth and metastasis, but also induces tumor differentiation in vitro and in vivo. Several modes of action might be relevant for the biological activity of VPA: (1) VPA increases the DNA binding of activating protein-1 (AP-1) transcription factor, and the expression of genes regulated by the extracellular-regulated kinase (ERK)-AP-1 pathway; (2) VPA downregulates protein kinase C (PKC) activity; (3) VPA inhibits glycogen synthase kinase-3beta (GSK-3beta), a negative regulator of the Wnt signaling pathway; (4) VPA activates the peroxisome proliferator-activated receptors PPARgamma and dagger (Rif. 25); (5) VPA blocks HDAC (histone deacetylase), causing hyperacetylation. The findings elucidate an important role of VPA for cancer therapy. VPA might also be useful as low toxicity agent given over long time periods for chemo prevention and/or for control of residual minimal disease. Authors show that the well-tolerated antiepileptic drug valproic acid is a powerful HDAC inhibitor. Valproic acid relieves HDAC-dependent transcriptional repression and causes hyperacetylation of histones in cultured cells and in vivo. Valproic acid inhibits HDAC activity in vitro, most probably by binding to the catalytic center of HDACs. Most importantly, valproic acid induces differentiation of carcinoma cells, transformed hematopoietic progenitor cells and leukemic blasts from acute myeloid leukemia patients. More over, tumor growth and metastasis formation are significantly reduced in animal experiments. Therefore, valproic acid might serve as an effective drug for cancer therapy. These findings constitute a "proof of concept" to use valproic acid for the therapy of acute leukemia and myelodisplastic syndrome.
ATRA
Retinoids have been shown to have a major role in the treatment of APL. There is also in vitro evidence that primary blast cells from FAB groups other than M3 can demonstrate phenotypic evidence of maturation when exposed to retinoid with or without other agents. Additional relevant information on AML/MDS comes from a number of sources:
(I) The addition of retinoid increases the sensitivity of AML blasts to Ara C in vitro.
(II) BCL 2 protein is over-expressed in a proportion of AML cases. There is some evidence that this correlates with treatment response and survival and, since chemotherapy results in apoptotic cell death, this may constitute a mechanism of chemo-resistance. Retinoids down regulate BCL 2 expression AML blasts in vitro. The increased sensitivity of AML blasts to chemotherapy mentioned above, mediated by retinoid, is due to shortening of the BCL 2 half-life.
(III) In a pilot study by Tallman et al, 39 high risk patients with a median age of 65 years, most of whom (n=25) had failed to respond to chemotherapy, were treated with ATRA (150 mg/m2) and LD Ara C. Forty-two percent of patients achieved a CR (25%) or PR (17%) and a further 17% had stable disease.
(IV) In a recent study from M.D. Anderson Cancer Center, Houston, 170 patients with a median age of 66 years with AML or high risk MDS were randomized to receive FAI (fludarabine, Ara C and idarubicin) alone, or FAI combined with either G CSF or ATRA or both. The overall 6 month survival was 49%. In a comparison of ATRA-treated versus ATRA-non-treated patients, there was a more favourable outcome in recipients of retinoids, with CR rates of 56% vs 42% respectively (p=0.06) and superior 6 month survival and event free survival (both p=0.02) but no long-term benefit was demonstrable. In this study the exposure to retinoid was relatively short, namely from day -2 to day +8 of chemotherapy (Rif. 30).
(V) In a recent study from Dusseldorf, 18 patients with MDS and AML secondary to MDS were treated with Valproic Acid (VPA) alone, and 5 patients received a combination of VPA and ATRA. A favourable response was observed in 44% of the patients treated with VPA alone, with a median response duration of 4 (3-9) months. Four of the five patients who subsequently relapsed were treated with VPA + ATRA, two of them responding again. The Authors hypotize that pre-treatment with VPA might be necessary for a synergistic effect of both drugs.
The above evidence suggests that retinoids may be a useful biological response modifier (when combined with chemotherapy and/or VPA) with little or no additional toxicity. Their role will therefore be examined in this trial, in association with 5-Azacytidine and Valproic Acid.