Download Effects of Vincristine on Cell Survival, Cell Cycle

[CANCER RESEARCH 43, 3591-3597,
August 1983]
Effects of Vincristine on Cell Survival, Cell Cycle Progression, and Mitotic
Accumulation in Asynchronously Growing Sarcoma 180 Cells
Hamza Mujagic,1 Shan-Shan Chen, Richard Geist, Sandra J. Occhipinti, Bruce M. Conger, Charles A. Smith,
William H. Schuette, and Stanley E. Shackney
Section of Cell Kinetics, Clinical Pharmacology
S. E. S.], and Applied Clinical Engineering
Bethesda, Maryland 20205
Branch, Division of Cancer Treatment, National Cancer Institute [H. M., S-S. C., R. G., S. J. 0., B. M. C., C. A. S.,
Section, Biomédical Engineering and Instrumentation Branch, Division of Research Services [W. H. S.], NIH,
ABSTRACT
The effects of vincristine (VCR) on cell survival, cell cycle
progression, DMA synthesis, and metaphase accumulation were
studied in relation to drug concentration and drug exposure
duration in Sarcoma 180 cells in vitro. VCR was found to affect
cells in interphase, producing a transient G2 block at all drug
concentrations and drug exposure durations studied. VCR did
not affect DMA synthesis directly. Increases in the metaphase
index were delayed and always peaked at approximately 8 hr
after drug removal, regardless of the duration of drug exposure.
Increases in the metaphase index of sufficient magnitude to be
commensurate with VCR lethality were observed only with pro
longed drug exposure. VCR produced both nuclear fragmenta
tion and polyploidy. The proportion of cells undergoing polyploidy
increased progressively with increasing drug exposure duration.
Interference with cytokinesis during prolonged VCR exposure
may represent a lethal effect of VCR that is separate from its
short-term effects. This could serve as the basis for the clinical
study of the antitumor effects of prolonged VCR infusions.
indicate that metaphase arrest and cell necrosis are not the only
manifestations of VCR lethality. It has been suggested that
polyploidy might also be a dose-dependent phenomenon (3), but
this has not been clearly established.
We have undertaken a systematic study of the effects of VCR
on cell survival, cell cycle progression, DNA synthesis, and
metaphase accumulation in Sarcoma 180 in vitro in relation to
drug concentration and to drug exposure duration. The results
of studies carried out on asynchronously growing cells are
described in this paper. The results of VCR studies carried out
on recruited, partially synchronized cells are described in a
separate paper (14). The present studies demonstrated that VCR
exerted its lethal effects on cells before they entered mitosis.
VCR did not inhibit DNA synthesis. However, it did produce a
transient accumulation of cells in G2. With regard to the disposi
tion of lethally damaged cells, both nuclear fragmentation and
polyploidy were observed, and the latter was found to be highly
dependent on drug concentration and drug exposure duration.
MATERIALS AND METHODS
INTRODUCTION
VCR2 and vinblastine are cytotoxic
tubulin-binding
agents,
derived from the periwinkle (Vinca rosea), that have found their
place as effective drugs in combination chemotherapy regimens
for the treatment of human leukemias, lymphomas, and a variety
of solid tumors.
Early experimental studies of the mode of action of the Vinca
alkaloids focused on their disruptive effects on the mitotic spindle
(6,10,13), and the lethality of these drugs was generally attrib
uted to their effects on cells in mitosis (2, 6, 22). However, other
studies have suggested that VCR exerts its lethal effects on
interphase cells (4, 7, 8,11,12, 20, 23).
Studies of the disposition of cells affected by the Vinca alka
loids have produced conflicting results. In some studies, meta
phase arrest was found to be completely reversible (10, 13)
while, in other studies, it was reported that arrested metaphases
subsequently became necrotic (4,6,11,12).
Several authors
have emphasized that the reversibility of metaphase arrest was
dependent on drug concentration and drug exposure duration
(5, 9, 24). Alabaster and Cassidy (1) demonstrated by means of
flow cytometry that VCR produced polyploidy. These findings
1To whom requests for reprints should be addressed, at Building 10, Room
12C216, National Cancer Institute, Bethesda, Md. 20205.
2The abbreviations used are: VCR, vincristine; HBSS, Hanks' balanced salt
solution; dThd, thymidine.
Received October 22, 1982; accepted May 5, 1983.
All studies were carried out in Sarcoma 180 (Foley strain CCRF11 ;
supplied by American Tissue Type Culture, Rockville, Md.) grown in vitro
in Earle's Medium 199 (Flow Laboratories, Rockville, Md.), which was
supplemented with 10% fetal bovine serum, 2 rriM L-glutamine, 100 units
of penicillin per ml, and 100 ng of streptomycin per ml. Cultures were
grown in monolayer in 250-ml plastic tissue culture flasks (growth surface
area, 75 sq cm) (Costar, Cambridge, Mass.) containing 10 ml of medium.
Cells were plated at an initial concentration of 1 x 105 cells/ml. Medium
was changed on Days 2 and 4, and cells were subcultured on Day 5.
Cell Survival Studies. Two-day-old log-phase cell cultures were in
cubated at 37°with VCR at final concentrations ranging from 0.01 to 10
/¿M
for drug exposure durations ranging from 1 to 24 hr. At the end of
the drug exposure period, the medium containing drug was removed,
and the cells were rinsed 4 times with 5 ml of HBSS. Cells were harvested
by incubation (37°)with 0.25% trypsin for 8 min. Controls were obtained
at each time point. Total cell counts were determined using a Coulter
Counter (Coulter Electronics, Hialeah, Fla.). Cell suspensions were di
luted with Medium 199, and known numbers of cells were cloned in soft
agar. Nine-day-old colonies were fixed, stained with Giemsa stain, and
counted visually. Clonogenic cell yield, defined as the number of colonies
enumerated divided by the number of cells plated, was averaged for 5
replicate flasks at each concentration. The viable cell count per flask was
calculated as the total cell number per flask multiplied by the clonogenic
cell yield at a given time point. The surviving viable cell fraction following
drug exposure was calculated as the number of viable drug-treated cells
per flask divided by the number of viable untreated control cells per flask.
Each experiment was performed at least 3 times, and the reported
results represented the log means of replicate studies.
Metaphase Index Studies. Serial metaphase indices were obtained
at intervals during and after VCR exposure. Aliquots of cells were
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VÃ-ncrÃ-stine
Effects in Sarcoma 180
(Chart 2A), there was no significant decrease in cell number, and
there was no delay in cell population growth. Exposure to drug
for 4 or 12 hr (Chart 2, B ana C) produced complete inhibition of
cell population growth throughout the drug exposure period at
all drug concentrations studied and for 6 to 8 hr after drug
removal. Cells exposed to 0.1 ¿tM
VCR resumed growth within
18 to 20 hr of the initiation of drug exposure, whether the drug
was present or not (in Chart 2, compare C and D). Exposure to
0.5 or 2 ¡IMVCR produced not only an inhibition of cell growth,
but also a relative decrease in cell number of up to 10 to 25% in
some experiments (Chart 2, B to D).
Metaphase Index Studies. The effects of VCR on the metaphase index are shown in Chart 3. In control cells, the metaphase
index ranged from 0.01 to 0.02 throughout the period of obser
vation in all 3 studies. After exposure to 0.1 ¡MVCR for 1 hr,
the metaphase index rose modestly to 0.03 at 4 hr, remained in
this range through 8 hr, and fen to control levels at 12 hr (Chart
3A). After exposure to 0.5 ¿<M
VCR for 1 hr, the metaphase index
remained at the same level as that of controls at 2 hr, rose to
0.08 at 8 hr, and fell at 12 hr. After exposure to 2.0 MMVCR for
1 hr, the metaphase index remained at the same level as controls
through 4 hr, rose to 0.15 at 8 hr, and fell sharply at 12 hr.
The effects of exposure to 0.1 //M VCR for 4 hr were similar
in magnitudeto those observed after a 1-hr exposure (Chart
04812182404812182404812
2026048121824
Chart 2. Total eel number as a function of time during and after exposure to
VCR. A, VCR exposure duration. 1 hr. e. VCR exposure duration. 4 hr. C. VCR
exposure duration. 12 hr. D. VCR exposure duration. 24 hr. controls, •0.1 JIM
VCR. T: 0.5 UMVCR. A. 2 «MVCR. •Bars, SE
SB). There was a modest rise in the metaphase index during the
first 8 to 12 hr and a return to control values by 18 hr. During
the period of exposure to 0.5 and 2 ¿>M
VCR for 4 hr, there was
no rise m the metaphase index (Chart 35), and the metaphase
index remained at control levels for 2 hr after drug removal. The
subsequent behavior of the metaphase index after exposure to
0.5 and 2 fit»VCR for 4 hr was comparable to that observed
after drug exposure for 1 hr, except that the peaks in the
metaphase index occurred at 12 hr rather than at 8 hr.
During a 12-hr exposure to 0.1 ¿IM
VCR, a modest but pro
gressive rise in the metaphase index was observed from 4 to 14
hr. Between 14 and 16 hr, the metaphase index doubled from
0.05 to 0.1; it then feugradually to control values by 26 hr (Chart
3C). In the presence of 0.5 and 2 /^MVCR, the metaphase index
did not increase above control values until 8 hr and continued to
rise modestly through 14 hr to values of 0.08 to 0.12. The
metaphase index then rose sharply to values of 0.45 to 0.5 at
20 hr, and then fell rapidly to control values by 26 hr.
In summary, for all 3 drug schedules. VCR produced modest
increases in the metaphase index during the period of drug
exposure and for 2 to 4 hr thereafter. At the higher drug concen
trations, the metaphase index rose sharply at 4 to 8 hr after the
termination of drug exposure, regardless of the duration of drug
exposure.
Tubulin-bindingagentscan producea rise in ine mitolicindex
that may be due in large part to selective loss of eels in
interphaserather than a true increasein mrtoticcete (11. 12,
19). Wien the data in Charts 2 and 3 are consideredtogether,
ittsdear that eventhe modest3-foldincreasesinthe metaphase
index that were observedduringand after exposureto 0.1 KM
VCR couid not be attributedto the selectivetossof interphase
cells and reflected true increasesin the absolute numbers as
weMas the relativenumbersof mitoticceHs.
In general, the peak in the metaphase index varied directly
with drug concentrationand with drug exposure duration,but
these were not simple, linear relationships.Peak metaphase
indicesdidnotchangeas drugexposuredurationincreasedfrom
1 to 4 hr for all 3 drug concentrationsstudied(Chart 3, A and
0.5
Charta. Effects of VCR on the metaphase
index. A, VCR exposure duration, 1 hr; B, VCfl
exposure duration. 4 hr; C. VCR exposure du
ration, 12 hr: controls. •0.1 *M VCR, T; 05
«cu
VCR, A; 2 0*1 VCR. •.Ban, SJE.
TIME, HR
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1983
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H. Mujagic et al.
8); when drug exposure duration was extended to 12 hr, peak
metaphase indices increased 4- to 5-fold (Chart 3C). The peak
metaphase index of cells exposed to 0.5 MMVCR was one-half
that of cells exposed to 2 MMVCR for drug exposures of 1 or 4
hr. After 12 hr of drug exposure, the peak metaphase index of
cells exposed to 0.5 MMVCR rose to 90% of that of cells exposed
to 2 MM VCR. Thus, while both drug concentration and drug
exposure duration are important, it would appear that prolonged
duration of drug exposure is the dominant factor in producing
metaphase arrest.
Parallel changes in the peak metaphase index and in clonogenic cell killing as a function of drug concentration and drug
exposure duration might suggest a simple causal relation be
tween the two. However, when Charts 1 and 3 are considered
together, it is clear that this is not the case. Following exposure
to 2 MM VCR for 12 hr, 90% of the clonogenic cells were killed
(Chart 1). Allowing that 70% of all of the cells in the population
were clonogenic, then the overall fraction of the cell population
that was lethally damaged by the drug was 0.63 (0.9 x 0.7). The
peak metaphase index of 0.5 after exposure to 2 UM VCR for 12
hr (Chart 3C) is in reasonably good agreement with this value.
By comparison, following exposure to 2 MM VCR for only 4 hr,
over 80% of the clonogenic cells were killed (Chart 1). Again,
allowing for the fact that 70% of all the cells were clonogenic,
one can estimate that the overall fraction of lethally damaged
cells was 0.56 (0.8 x 0.7). However, the peak metaphase index
of 0.16 (Chart 38) was too low to account for all of these lethally
damaged cells. Thus, while delayed arrest in metaphase might
be taken as a prominent feature of lethally damaged cells follow
ing continuous exposure to high concentrations of VCR for 12
hr, it would appear that most clonogenic cells that were killed
after only 4 hr of drug exposure did not exhibit this feature.
Flow Cytometry Studies. Flow cytometry studies were carried
out in parallel with the metaphase index studies on separate
aliquots of cells from the same flasks. Representative druginduced changes in the DMA histogram are illustrated by the
data obtained after a 12-hr exposure to 0.5 tiM VCR (Chart 4).
After 12 hr of drug exposure, many cells had progressed from
the Gìand early S regions to the late S and G2-M regions of the
DMA histogram (Chart 46). At 16 and 20 hr, the majority of cells
progressed to the G2-M region of the histogram and accumulated
there (Chart 4, C and D); by 20 hr, some cells had gone on to
divide, resulting in a slight increase in the height of the d peak
(Chart 40). By 26 hr, the height of the G2-M peak decreased
AHRPRE«,
CONTROL
B
12 HR
C
16
POSTUMi
0, S G.M
;\I
i
20HR
: '•
ñM
1
ji\.i
26 HR
32 HR
>.
DNA CONTENT
Chart 4. Series of DNA histograms obtained at various times after exposure to
0.5 UM VCR for 12 hr. Histogram is divided into PRE-G,, G,, S, G2-M, and POSTGi-M regions, as shown in A; G, and G2-M regions, shaded panels.
3594
considerably (Chart 4E). The fate of cells that had accumulated
earlier in the G2-M region could be traced through one of 3 paths.
Many of these cells appeared as nuclear fragments in the preGi region. The presence of micronuclei was confirmed histologically in Feulgen-stained smears of these cells. Some of the cells
that had been blocked in the G2-M region went on to divide,
appearing in the d and early-S regions of the histogram (com
pare G, regions of Chart 4, D and £).Cells that persisted in the
G2-M region of the histogram at 26 hr went on to endoreduplicate
and were found to proceed in a broad wave through the postG2-M region at 32 hr (Chart 4F). Feulgen-stained smears of cells
obtained at this time point contained many large cells with
multilobed nuclei or multiple micronuclei.
Systematic changes in the fractions of cells in different regions
of the DNA histogram in relation to VCR concentration and drug
exposure duration are described quantitatively in Chart 5.
The most striking change that was observed with all drug
schedules was the accumulation of cells in the G2-M region
during and after exposure to VCR (Chart 5, A4 to D4). In general,
the accumulation of cells was greater and more prolonged with
increasing drug concentration and increasing drug exposure
duration. However, the patterns were quite different from those
of the metaphase index data (Chart 3). Let us first consider the
G2-M accumulation during and after exposure to 2 UM VCR
(Chart 5, A4 to D4, uppermost curves). The earliest drug effect
was observed within 2 hr of the onset of drug exposure. Peak
accumulation in the G2-M region always occurred at 8 hr after
the onset of drug exposure, regardless of the duration of drug
exposure, and involved 70 to 90% of the cells with drug exposure
durations of 4 hr or longer. The peak G2-M accumulation was
not sustained, even in the continued presence of the drug (Chart
5, C4 and D4). When the relative numbers of cells and the times
of peak accumulations in the flow cytometry data (Chart 5, At to
C4) are compared with those of the metaphase index data (Chart
3, A to C), it is clear that the peaks in the flow cytometry data
consisted predominantly of premitotic cells. For example, during
a 12-hr exposure to 2 MMVCR, 75% of the cells accumulated in
the G2-M region of the histogram at 8 hr (Chart 5 C4); however,
at 8 hr, the metaphase index was only 0.05 (Chart 3C). Thus,
approximately 70% of the cells in the G2-M region of the DNA
histogram were in G2 and not in M at this time. The same can
be said for peak accumulations of the cells in the G2-M region at
8 hr during exposure to 0.5 MM VCR (compare Chart 5C4 with
Chart 3C). The fractions of cells in the G2-M region following
exposure to 0.5 and 2 MM VCR fell at 20 hr to 0.4 and 0.5,
respectively (Chart 5C4); these values corresponded to the peak
values of the metaphase index at 20 hr at the 2 respective drug
concentrations (Chart 3C).
Of the cells that accumulated in G2 and were killed by the
drug, some may have been permanently arrested in G2 or M.
However, it is apparent from Chart 5 that the cells that accu
mulated transiently in G2 during and after drug exposure under
went one of several subsequent fates:
1. Decreases in the fractions of cells in the G2-M region of the
DNA histogram were accompanied by increases of 10 to 20% in
the fractions of cells in the pre-d region of the histogram. The
magnitude of these increases was modest and only weakly
dependent on drug concentration and drug exposure duration
(Chart 5, A, to Di).
2. With the decrease in the peak fraction of cells in the G2-M
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Vincristine Effects in Sarcoma 180
VCR 1 HR
4HR
12 HR
24 HR
PRE-G
0 4 8 12
18
24 0 4 8 12 18
24 0 4 8 12
20 26 0 4 8 12
18 24
TIME,HR
Charts. Changes in the fractions of cells in different regions of the DMA
histograms in relation to VCR concentration and drug exposure duration. Data for
1-hr, 4-hr, and 12-hr exposure durations, means of 3 experiments; Hata for 24-hr
exposure duration are taken from one experiment. Drug exposure duration periods,
shaded panels. Controls, •;0.1 t¡u VCR, T; 0.5 UNI VCR, A; 2 >»M
VCR, •.
Changes are shown in pre-Gi, Gì,S, Gz-M, and post-G2-M regions. Bars, S.E.
region of the histogram, there also occurred a concomitant
increase in the fraction of cells in the Gìregion of the histogram
that was greatest after a 1-hr exposure to 0.1 /¿M
VCR (Chart
5A2) and became less pronounced at higher drug concentrations
and longer drug exposure durations (Chart 5, A2 to D2). Whenever
a wave of cells was observed to enter and move through the Gì
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H. Mujagic et al.
8
12
16
20
24
Time, hr
Chart 6. ['HJdThd tf'H/dTft) incorporation during continuous exposure to VCR.
Controls, • •;0.1 «MVCR, •
•;0.5 /*M VCR, •—•;
2 MMVCR,
• ».Bars.S.E.
region of the histogram (Chart 5, A2 to C2), the wave was seen
to be propagated through the S region of the histogram 6 to 8
hr later (Chart 5, A3 to C3).
3. Another fate of G^blocked cells was the development of
polyploidy. White this effect was drug concentration-dependent,
it was most striking with prolonged drug exposure durations
(Chart 5, As to D5). In these respects, polyploidy is similar in its
behavior to that of the metaphase index, and the fractions of
polyploid cells at 26 hr after the onset of a 12-hr drug exposure
(Chart 5C5) were comparable to corresponding peak values of
the metaphase index at 20 hr (Chart 3C).
[3H]dThd Incorporation during Continuous Exposure to
VCR. The patterns of [3H]dThd incorporation during continuous
exposure to VCR concentrations of 0.1,0.5, and 2 pM are shown
in Chart 6. There was a gradual, progressive decline in [3H]dThd
incorporation for up to 12 hr. There followed an increase in [3H]dThd incorporation, with a return toward control values in the
presence of 0.1 UM VCR, and a 2.5- to 3-fdd overshoot above
controls in the presence of 0.5 and 2 //M VCR. The overshoot
can be attributed both to the fact that there is twice as much
DMA being synthesized in polyploid cells, and to the partial
synchronization of the polyploid cells following transient meta
phase accumulation (Chart 4F).
DISCUSSION
The present studies indicate that VCR affects cells in interphase and that metaphase arrest is a late consequence of VCRinduced cell damage. The earliest effect of VCR was manifest
within 1 to 2 hr of drug administration as a progressive accu
mulation of cells in G2 that was observed at all drug concentra
tions and drug exposure durations studied (Chart 5, At to D4).
The accumulation of cells in G2 was transient; the fraction of
cells in the G2 region of the histogram peaked at 4 to 8 hr after
the onset of drug administration and decreased thereafter, even
in the continued presence of the drug (Chart 5, A4 to D4). A block
in G2 following VCR administration has also been reported by
others (24). Although the cause of this G2 block could not be
3596
determined directly from our studies, it would seem reasonable
to suppose that it might be due to the disruption or inhibition of
formation of microtubular structures other than the mitotic spin
dle that might be required for the initiation of mitosis.
We have examined the susceptibility of interphase cells to the
lethal effects of VCR in greater detail in a separate study in
synchronized Sarcoma 180 cells (14). In brief, cells were found
to become more susceptible to the lethal effects of VCR as they
progressed through late S and G2, suggesting an underlying
mechanism of action that was independent of DMA synthesis
but involved a cell constituent, presumably tubulin, whose syn
thesis overlapped with that of DMA.
It is clear from the present study that VCR did not inhibit DNA
synthesis directly. Cells that were in the d and S regions of the
DNA histogram at the onset of drug administration succeeded in
traversing the S region during continuous exposure to drug for
24 hr and accumulated transiently in the G2-M region (Chart 5D4).
Thus, the gradual decrease in [3H]dThd incorporation that was
observed during the first 12 hr of VCR exposure (Chart 6)
reflected the failure of cells to divide and replenish the d and S
regions of the DNA histogram (Chart 5, D2 and D3) but did not
represent a direct inhibitory effect on DNA synthesis itself. This
was confirmed by the overshoot in [3H]dThd incorporation in the
presence of VCR (Chart 6), corresponding to the wave of poly
ploid cells traversing S (Chart 5D5).
In the present studies, the metaphase index peaked at ap
proximately 4 to 8 hr after drug removal, regardless of the
duration of the preceding drug exposure period (Chart 3). This
delay was attributable at least in part to the prior transient block
in G2 (compare Chart 5, /44 to C4, with Chart 3). In any event, it
is clear that the accumulation of cells in metaphase was the
result of drug-induced damage that was sustained earlier in the
cell cycle. It is also apparent from a comparison of Chart 5, A4
to C4, with Chart 3 that, following brief exposure to high concen
trations of VCR, not all cells that were blocked transiently in G2
subsequently accumulated in metaphase. Furthermore, the de
gree of metaphase accumulation following exposure to high
concentrations of VCR for 1 to 4 hr was not sufficient to account
for the magnitude of the cell kill that was observed (compare
Chart 3 with Chart 1). Metaphase accumulation was pronounced
only with prolonged drug exposure (Chart 3C), and only then did
the peak metaphase index correspond with drug-induced cell
kill. Thus, it would appear that VCR may have multiple effects
on cells and that some of these effects may become more
prominent with prolonged drug exposure duration.
The fate of arrested metaphases has been studied by a
number of investigators (4-6, 8, 10-13, 24). In some early
studies, metaphase arrest was considered to be reversible (10,
13). However, the reversibility of metaphase arrest was found to
be dependent on drug concentration and drug exposure duration
(5,10, 24). Following exposure to cytotoxic drug concentrations,
arrested metaphases were found commonly to undergo necrosis,
multipolar divisions, and/or dissolution (4-6, 8,11,12, 24). Our
findings are consistent with the fragmentation of some mitotic
cells, particularly after relatively brief exposure to high VCR
concentrations. The evidence for this was the appearance of cell
fragments in the pre-d region of the histogram (Chart 5, A, to
Ci), usually peaking at 4 to 6 hr after peak metaphase accumu
lation (compare with Chart 3). However, most of the metaphases
that accumulated after 12 hr of exposure to 0.5 and 2 ^M VCR
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Vincristine Effects in Sarcoma 180
could not be accounted for either by fragmentation (compare
Chart 3C with Chart 5C,) or by dissolution (compare Chart 3C
with Chart 2C). On the other hand, both the magnitude and
timing of the rise in the polyploid cell fraction are consistent with
the progression of most of the arrested metaphases to polyploidy
(compare Chart 3C with Chart 5C5).
The development of polyploidy is an effect of VCR that became
increasingly more pronounced with the prolongation of drug
exposure duration. Indeed, as drug exposure duration was in
creased, this effect was demonstrated at progressively lower
VCR concentrations (Chart 5, /45 to D5). The development of
polyploidy following VCR administration in vivo has been re
ported by Alabaster and Cassidy (1). Camplejohn (3) suggested
that this finding may have been due to the very high doses of
VCR that were used in that study. However, the maintenance of
effective drug levels for prolonged periods after administration
of high doses in vivo may be an alternative explanation.
The mechanism underlying the development of polyploidy
following prolonged drug exposure is of special interest. The
lethal effects of the Vinca alkaloids have been attributed to their
binding to tubulin subunits and the inhibition of polymerization of
tubulin subunits into microtubules (15-17). However, microtubules participate in a wide variety of cellular processes, and the
mitotic sequence itself involves several different classes of mi
crotubules. Early studies focused on the inhibition or disruption
of the mitotic spindle by tubulin-binding agents and the resultant
prevention of centriole separation during mitosis (6, 21). The
development of polyploidy calls attention to another microtubuledependent mitotic function that may be inhibited by VCR, namely,
cytokinesis. The Vinca alkaloids are known to inhibit the devel
opment of the cleavage furrow of sea urchin eggs (18) and
interfere with the normal separation of mammalian cells in telophase (10, 11, 24). Thus, interference with cytokinesis during
prolonged exposure to VCR would represent a lethal effect of
the drug that may be distinct from its short-term effects, and
which could account for the development of polyploidy during
and after prolonged drug exposure. This could serve as the basis
for clinical studies of the antitumor effects of prolonged VCR
infusions.
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3597
Effects of Vincristine on Cell Survival, Cell Cycle Progression,
and Mitotic Accumulation in Asynchronously Growing Sarcoma
180 Cells
Hamza Mujagic, Shan-Shan Chen, Richard Geist, et al.
Cancer Res 1983;43:3591-3597.
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