Page 969 - Hematology_ Basic Principles and Practice ( PDFDrive )

852 Part VII Hematologic Malignancies

represents the percentage of cycling cells relative to the total cell significantly less sensitive to the actions of antimetabolites, including
population. The generation time represents the time required for a methotrexate.
cell to proceed through a single cell cycle (generally 24–36 hours for In vivo the growth of tumors is limited by various factors such as
hematopoietic tissues). Surprisingly, in the case of acute myeloid vascular supply, nutritional requirements, and possibly physical
leukemia (AML), the generation time of leukemic blasts is not shorter restraints. Consequently the rate of tumor growth declines as the
than that of normal hematopoietic progenitors and may be longer. number of cells increases. To the extent that tumor-doubling times
The proliferative advantage of malignant hematopoietic cells (and of are inversely correlated with drug responsiveness, large, late-stage
many nonhematopoietic tumors) stems, at least in part, from the fact tumors are less susceptible to cytotoxic drugs than early-stage tumors,
that a higher percentage of cells are in cycle at any one point in time with higher growth fractions. Most chemotherapeutic drugs kill by
(i.e., the growth fraction is higher). The doubling time represents the first-order kinetics. The implication of this phenomenon is that it
period required for a tumor to double in mass and is, in general, requires the same drug dose to reduce the number of tumor cells from
1
4
10
inversely related to the tumor’s growth fraction. Tumor-doubling 10 to 10 cells as it does to reduce the tumor burden from 10 to
7
times range from longer than 120 days in the case of some solid 10 cells.
tumors (e.g., lung and colon) to less than 2 weeks (in some leukemias Hematopoietic malignancy-initiating or stem cells, such as leuke-
and lymphomas). Tumors with high growth fractions and short mia stem cells, appear to explain resistance and treatment failure.
doubling times tend to be more sensitive to chemotherapy than These cells express high levels of hematopoietic stem cell proteins and
slowly growing neoplasms with low growth fractions and long dou- markers, and are resistant to cell cycle–specific agents because of an
bling times. increase in quiescent cell populations. They overlap with normal
Cell cycle progression is governed by a complex network of pro- hematopoiesis stem cells and there appears to be a set of HSCs that
teins consisting of cyclins, cyclin-dependent kinases (CDKs), and contain preleukemic-promoting mutations that both predispose and
CDK inhibitors. Progression through S phase is regulated primarily may be sufficient for conversion to leukemic stem cells. The proteins
by CDK2 in association with cyclins A and E; progression through expressed by these cells have become targets of therapy, including
G 2 M is regulated by CDK1 (p34 cdc2 ) and cyclins A and B; and pro- alterations in DNA damage, quiescent cell cycle factors such as the
gression through G 1 involves CDKs 4–6 in conjunction with cyclin thrombopoietin receptor MPL, stem cell proliferation signals such as
D. CDK inhibitors fall into two major categories: the low-molecular- NOTCH and WNT protein families, and niche occupancy proteins
weight inhibitors (pINK14, -15, -16, -17, and -18), which primarily such as KIT.
inhibit cyclin D (and to some extent, CDK2) complexes, and the Cytotoxic agents may be divided into several categories with
higher molecular-weight inhibitors, p21, p27, and p57, which are respect to their effects on the cell cycle or the cell cycle specificity of
more universal in their actions and inhibit most or all CDKs. Signals their actions, or both.
for the progression of cells through G 1 S are essential for maintenance
of the neoplastic phenotype. In the commonly accepted model of 1. Noncycle-active drugs kill both cycling and noncycling cells in all
G 1 S progression, inactivation of the retinoblastoma protein (pRb) is phases of the cell cycle. Examples include steroids and antitumor
required. In quiescent cells, pRb is in an active dephosphorylated antibiotics (except bleomycin).
state and bound to the transcription factor E2F. Phosphorylation of 2. Cycle-active, nonphase-specific drugs are more active against cycling
pRb by CDK4, CDK6 and CDK2 leads to release of E2F, which is cells and can kill cells in each phase of the cell cycle. However,
then free to activate diverse genes essential for S-phase progression, such drugs may preferentially kill cells in a particular phase of the
such as MYC (also known as c-Myc), TYMS (thymidylate synthetase), cell cycle. Examples include alkylating agents, cisplatin, and
and DHFR (dihydrofolate reductase). Conversely, induction of CDK 5-fluorouracil (5-FU).
inhibitors (e.g., by transforming growth factor-β [TGF-β] or 3. Cycle-active, phase-specific drugs primarily kill cells in a specific
differentiation-inducing agents) results in inactivation of CDK4, phase of the cell cycle. Examples include most antimetabolites,
CDK6 and CDK2, dephosphorylation of pRb, inactivation of E2F, which are active against cells engaged in DNA synthesis (S-phase
and inhibition of the progression through S phase. Aberrant expres- cells), and microtubule-active drugs (e.g., vinca alkaloids, taxanes),
sion of cyclins and CDK inhibitors is commonly encountered in which kill cells in G 2 M.
hematopoietic malignancies.
In addition to growth control, cell cycle proteins are intimately An example of a cytokinetically rational approach to chemotherapy
involved in the regulation of programmed cell death (apoptosis) involves the combination of a noncycle-active agent (e.g., daunorubi-
and checkpoint control mechanisms. Consequently, cell cycle cin) with a cycle- and a phase-specific agent (e.g., ara-C, fludarabine,
regulatory proteins can exert a major influence on the response of decitabine, gemcitabine, clofarabine and nelarabine). From a theoreti-
neoplastic cells to cytotoxic agents. For example, when cells undergo cal standpoint, administration of a noncycle-active agent may reduce
DNA damage, they may arrest in G 2 M or G 1 , during which repair tumor mass, leading in turn to an increase in the growth fraction
occurs, or if the damage is too severe, the cells undergo apoptosis. caused by recruitment of cells into cycle. Such cells would then be
In particular, the tumor suppressor gene TP53 and its downstream more susceptible to a cycle- and phase-specific agent, particularly one
inducible target p21 have been implicated in the G 1 arrest process administered over a prolonged interval. In the case of hematopoietic
after genotoxic insult. Dysregulation of various cell cycle regulatory malignancies, attempts have been made to recruit neoplastic cells
proteins can have a major impact on the sensitivity of neoplastic into the more susceptible S phase of the cell cycle through the
cells to chemotherapeutic agents. Loss of the TP53 gene renders use of hematopoietic growth factors. The success of such a strategy
cells resistant to diverse chemotherapeutic agents, presumably by has been limited because of several factors, including the inability
preventing cells from undergoing repair in G 1 and thereby inhibiting of growth factors to increase the S-phase fraction significantly, the
the cell death processes and allowing DNA damage to accumulate, lack of selectivity of this strategy, and the theoretical possibility that
culminating in cellular transformation. Conversely, transfection of growth factors may protect neoplastic cells from apoptosis.
P53-negative cells with wild-type P53 restores responsiveness to most Unfortunately, cytokinetic differences between normal and neo-
drugs. Dysregulation of the CDK inhibitors p21 (a downstream plastic tissues have been difficult to exploit. Both normal hematopoi-
target of P53) and p27 increases the sensitivity of neoplastic cells etic stem cells and hematologic malignant stem cells have a low
to various cytotoxic agents, possibly by uncoupling S-phase pro- proportion of cells in G 1 . However, prolonged dosage schedules can
gression and mitosis. After DNA damage, checkpoints block the provoke these malignant cells into cell cycle and may explain their
cell cycle, but loss of the CDK inhibitors p21 or p27 prevents efficacy. Consequently, rapidly dividing normal tissues such as gastro-
cells from arresting in G 1 and cells die during G 2M. Mutations intestinal epithelium and normal hematopoietic progenitors tend to
in the E2F protein have been shown to lengthen S phase and be very sensitive to most chemotherapeutic agents. As a result,
increase the sensitivity of malignant cells to S-phase–specific agents. mucositis and myelosuppression represent frequent dose-limiting
Furthermore, cells lacking functional pRb have been shown to be toxicities for many cytotoxic drugs.