Page 990 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 57 Pharmacology and Molecular Mechanisms of Antineoplastic Agents for Hematologic Malignancies 873

clinical trials in humans. Flavopiridol binds to the ATP-binding site SNS032
of CDKs, resulting in reversible, competitive enzyme inhibition at
concentrations of less than 100 nM. As noted earlier, flavopiridol is SNS032 is a small-molecule CDK inhibitor discovered through high-
a relatively nonspecific CDK inhibitor and inhibits all CDKs, throughput screening that is primarily active against CDK2. In
although it is less effective against CDK7. Flavopiridol induces G 1S preclinical studies, SNS032 has shown potent antiproliferative activ-
or G 2M arrest, presumably a consequence of inhibition of CDK1 and ity against ovarian and breast cancer cells in vitro. As is the case with
CDK2. Flavopiridol may also act to block cell cycle progression by other CDK inhibitors, SNS032 induces cell cycle arrest, pRb dephos-
downregulating cyclin D 1 levels, inhibiting the CDK-activating phorylation, and, under some circumstances, apoptosis in CLL and
complex (CDK7), or both. myeloma cell lines. At least in some tumor cell lines, however, CDK2
In addition to blocking cell cycle progression, flavopiridol has may be dispensable for cell cycle progression. In CLL cells, SNS-032
been shown to be a potent inducer of apoptosis in malignant hema- was cytotoxic in vitro in untreated and refractory cell lines. RNA
topoietic cells (e.g., acute and chronic leukemia) at low concentrations synthesis was suppressed after treatment, with evidence of CDK 2
(e.g., <100 nM). Moreover, flavopiridol has shown activity against and 7 inhibition. 14
MM cells in vitro. Several phase I trials of SNS032 have been initiated. SNS032 has
The proapoptotic actions of flavopiridol have been attributed to been administered as either a 24-hour or 1-hour infusion every 3
2
its capacity to inhibit the positive transcription elongation factor-β weeks. Dose levels of 4–59 mg/m have proven to be tolerable; the
(PTEF-β), cyclin T/CDK9 complex by inhibiting phosphorylation maximum tolerated dose for either of these schedules has not been
of the C-terminal domain of RNA polymerase II. This leads to reached. Toxicities have been mild and include rash, nausea and
downregulation of several antiapoptotic proteins, including BIRC4, vomiting, diarrhea, and fatigue. In a clinical trial in patients with
CIP1
P 21 , and, in the case of MM cells, Mcl-1. Flavopiridol has also CLL (19 patients) and myeloma (18 patients), CDK7 and -9 inhibi-
recently been shown to block the antiapoptotic actions of the IAP tion was observed, but responses as a single agent were modest.
family member survivin.
In clinical studies, flavopiridol was initially administered as a
72-hour continuous infusion every 2 weeks, with a maximally toler- Hypomethylating Agents
2
ated dose of 40 mg/m . Steady-state plasma levels in excess of those
necessary to inhibit CDKs and induce apoptosis in leukemia cells Promoter methylation within CpG islands regulates gene expression in
(e.g., 350 nM) were achieved. Dose-limiting toxicities were fatigue, all cells. Disruption of normal gene expression profiles accompanies
diarrhea, nausea, and myelosuppression. However, the occurrence malignant transformation, giving rise to complex patterns of gene
of thromboembolic phenomena and the general lack of single-agent expression. As a consequence, promoters of many genes have an altered
activity have limited enthusiasm for administering flavopiridol by pattern of methylation, resulting in either gene activation or gene
this schedule. Flavopiridol has also been administered as a daily IV repression. The link between these concepts and the interest in agents
bolus for 1, 3, or 5 days every 3 weeks with manageable toxicity, that alter promoter methylation began with the realization that azaciti-
and other schedules are being examined, including a hybrid schedule dine, an agent used sparingly for treating myeloid leukemias, had
with half the dose administered as an IV bolus and the other half efficacy when given at low doses either IV or SC for extended periods
as a more prolonged infusion. When administered as a daily bolus of time, and that in these cases, altered gene expression accompanied
infusion for 3 days every 3 weeks, flavopiridol exhibited modest responses. Both azacitidine and 5-aza-2′-deoxycytidine (decitabine) act
activity in patients with MCL. A novel pharmacologically directed by irreversible inhibition of the DNA methyltransferases responsible
schedule of flavopiridol has been developed in which half of the for methylation of the cytidine in CpG islands and are thus S-phase–
2
14a
dose (e.g., 30 mg/m ) is administered as a 30-minute bolus-loading specific agents. Wijermans et al first described that a related com-
2
infusion and 30 mg/m as a 4-hour infusion. In a phase I study, pound, 5-aza-2′-deoxycytidine, was effective when given as a continuous
objective response rates of 45% were obtained with this schedule infusion to elderly patients with high-risk MDS, with a 54% response
in patients with progressive CLL, including some with high-risk rate. The mechanism of action included both a direct change in pro-
13
disease. Because of the rapidity of response, particularly in patients moter methylation and independent changes, with up to 70% genome-
with high white blood cell counts, aggressive measures designed to wide demethylation, suggesting either that genes with altered expression
avoid TLS (e.g., hydration, alkalinization of the urine, administration either up- or downregulated a second set of genes through altered
of rasburicase) are advisable. Efforts are now underway to use this pathways such as induction of WAFp21, p15, and p16, or in a more
novel flavopiridol schedule in combination with other agents and in direct fashion through altered transcription factor expression. More
other hematologic malignancies. recent studies have identified activity of these agents in both AML and
Because of limited single-agent activity, combination regimens CML. Most often, PRs or short-lived CRs are seen in AML, with CRs
involving flavopiridol are being explored in hematologic malignan- more common in MDS. Recent studies also describe a number of
14b
cies. On the basis of preclinical evidence of synergism with the extended therapy regimens. Kantarjian et al reported a comparison
2
2
antimetabolite ara-C, a regimen combining flavopiridol on a daily of 20 mg/m decitabine IV for 5 days with 20 mg/m SC daily for 5
2
IV bolus schedule followed by high-dose ara-C has been initiated in days and 10 mg/m IV for 10 days. The 5-day IV dose schedule was
patients with AML and has shown some activity. In a phase II study, judged superior on the basis of clinical outcome with 39% CR, a higher
flavopiridol was administered as a 1-hour bolus infusion of 50 mg/ proportion of reactivation of p15, and achievement of a hypomethyl-
2
m daily for 3 days before administration of high-dose ara-C (day 6) ation state as well as clinical tolerance. Azacitidine has been given after
and mitoxantrone (day 9). CR rates of 67% were obtained in patients allogeneic stem cell transplantation, at lower doses, as a “maintenance”
14c
with high-risk AML and more than half were durable. More recently, regimen, to prevent relapse. De Lima et al established the MTD of
2
evidence of synergism between flavopiridol and other signal transduc- azacitidine after transplant as 32 mg/m for 5 days. Posttransplant
tion modulators has become the focus of considerable attention. For azacitidine may also augment graft-versus-leukemia effect and attenu-
example, the observation that flavopiridol interacts synergistically with ate GVHD after transplant through expansion of T-regulatory cells.
imatinib mesylate against CML cells, including some that are imatinib An oral formulation of azacitidine is currently in clinical development
mesylate resistant, has prompted the initiation of a phase I trial of which would permit convenient and prolonged exposure to the drug,
flavopiridol and imatinib mesylate in patients with progressive BCR- which may translate into better outcomes. It was recently shown that
+
ABL hematologic malignancies. Four out of 21 patients responded. pretreatment with azacitidine, followed by R-CHOP chemotherapy,
Evidence that flavopiridol interacts synergistically with HDAC and can restore chemosensitivity in DLBCL cells in vitro and in clinical
proteasome inhibitors in human leukemia cells has appeared, and samples from patients with DLBCL. The combination between
clinical trials combining flavopiridol with the HDI vorinostat or the azacitidine and R-CHOP was feasible, safe, and associated with a
proteasome inhibitor bortezomib in patients with refractory AML/ higher ORR compared with historic controls treated with R-CHOP
MDS and MM/indolent NHL are currently underway. alone. Azacitidine “priming” decreased SMAD1 methylation, and