flavopiridol (l86-.8275): selective antitumor …...vol. 3. 273-279. fehruart /997 clinical cancer...

Vol. 3. 273-279. Fehruart /997 Clinical Cancer Research 273

Flavopiridol (L86-.8275): Selective Antitumor Activity in Vitro and

Activity in Vivo for Prostate Carcinoma Cells

Markus Drees, Wolfgang A. Dengler,

Thomas Roth, Heike Labonte, Joseph Mayo,

Louis Maispeis, Michael Grever,

Edward A. Sausville, and Heinz H. Fiebigt

Department of Internal Medicine, University of Freiburg. Hugstetter

Strasse 55, D-79l06 Freiburg, Germany IM. D., W. A. D., T. R..

H. L., H. H. F.], and Biological Testing Branch Ii. M.]. Laboratory ofPharmaceutical Chemistry IL. M.]. and Office of Associate Director.

Developmental Therapeutics Program [M. G.. E. A. S.]. National

Cancer Institute. Bethesda. Maryland 20892

ABSTRACT

We have selected a panel of human tumor xenografts

for in vitro and in vivo studies that allows an indication of

selectivity of action of novel chemotherapeutic agents. We

report here the antitumor activity of the flavone flavopiridol

(previously designated L86-8275), which has been selected

for further studies based in part on its behavior in the

anticancer drug screening system of the United States Na-

tional Cancer Institute. Eighteen human tumor and five cell

line-derived xenografts established by serial passage in nude

mice in our laboratory were used as tumor models for in

vitro investigations using a modified double-layer soft agar

assay. In vivo investigations were completed in nude mice

bearing advanced-stage s.c. growing prostate cancer xe-

nografts. Antitumor activity in vitro (test/control � 30%) of

flavopiridol was observed at the very low concentration of

0.1 ng/ml in three of four prostatic xenografts and in one

melanoma xenograft. Overall, in 14 of 23 (61 %) tumor

xenografts, drug treatment resulted in a IC70 of < 10 ng/ml,

demonstrating the high antiproliferative potential of fla-

vopiridol. Toxicity to in vitro bone marrow cultures was

evident only at 100 ng/ml, indicating potential high selectiv-

ity for susceptible tumor cells. Comparison of tumor cells

with bone marrow samples tested showed clear prostate

carcinoma and moderate melanoma selectivity. In vivo stud-

ies of flavopiridol confirmed antitumor activity in both pros-

tate cancer xenografts investigated. At the maximal tolerated

dose of 10 mg/kg/day administered p.o. on days 1-4 and 7-11,

flavopiridol effected tumor regression in PRXF1337 and tumor

stasis lasting for 4 weeks in PRXF1369. We conclude that

flavopiridol shows strong prostate- and moderate melanoma-

specific antitumor activity in vitro. The prostate antitumor

Received 4/5/96: revised 10/29/96; accepted I 1/12/96.

The costs of publication of this article were defrayed in part by the

payment of page charges. This article must thereftre be hereby marked

aclveltise,ne,It in accordance with 18 U.S.C. Section 1734 solely to

indicate this fact.

I To whom requests for reprints should be addressed. Phone: 49 761

51559 11: Fax: 49 761 51559 55.

activity is also reflected by the two in vivo models studied.

Initial clinical efforts with flavopiridol might consider early

evaluation in patients with prostate carcinoma.

INTRODUCTION

One of the main objectives in the development of novel

antineoplastic agents is the identification of compounds with

tumor-specific or tumor-selective antiproliferative activity. With

this goal in mind, the National Cancer Institute has developed a

primary screening system consisting of 60 human tumor cell

lines from eight tumor types ( 1-3). However, in vitro culture

conditions may result in the selection of poorly differentiated

cell lines, limiting the predictive value for the in viva situation.

In addition, in vitro screens may identify potent molecules that

have little in vito therapeutic index or that are metabolized

readily to inactive species. Thus, whether such a system can

detect agents that retain in vito or clinical activity is uncertain.

The usefulness of human tumor xenografts for studies of

tumor biology and cancer therapy. however, has been demonstrated

by various groups (4, 5). We have developed a testing procedure

that uses human tumors grown by serial passage in athymic mice as

the source of cells for in vitro colony assays, and have also selected

a panel of human tumor xenografts for primary in vitro studies

(5-7). This test system has shown an excellent correlation of drug

response In %‘itr() and in vito (7). We report here the activity of the

flavone flavopiridol [( - )cis-5,7-dihydroxy-2-(2-chlorophenyl)-8-

[4-(3-hydroxy- 1 -methyl)-piperidinyl]-4H- 1 -benzopyran-4-one,

Fig. I ; Refs. 8 -1 1] in 23 human tumor xenografts in the colony-

forming assays, as well as in human bone marrow in vitro. We then

tested the agent in prostate xenografts in vito.

Flavonoids such as quercetin and genistein have been

shown to inhibit tumor cell growth in vitro (12-16). Quereetin

inhibits cellular signal transduction (Refs. 15 and 16: e.g.,

senine/threonine or tyrosine phosphorylation). and this may en-

hance the potency of “conventional” agents ( I 7). Genistein is a

more specific inhibitor of tyrosine kinase activities, such as

epidermal growth factor, pp6O’�, and ppl l0�’#{176}�CS ( 13, 18).

Other flavonoids are less potent tyrosine kinase inhibitors. The

new flavone flavopinidol (formerly designated L86-8275: Fig. I)

is derived by synthesis from a parent structure obtained from

Dvsovv!umn binectarijerum (8. 9). a plant native to India. Fla-

vopinidol has recently been recognized as a potent inhibitor

(KATP = 45 nM) of CDK1,2 the mediator of cell cycle pro-

gression from G, to M phase (19, 20). Subsequent studies (21,

22) have also demonstrated it to be a potent inhibitor of CDK2,

a CDK whose activity appears at the G,-S boundary (reviewed

in Ref. 23). Therefore. flavopinidol is a structure with potent

activity against members of the CDK family of cell cycle

2 The abbreviations used are: CDK. cyclin-dependent kinase; T/C. test!

control; MTD. maximal tolerated dose.

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274 Flavopiridol Preclinical Prostate Cancer Activity

3 E. Sausville, unpublished results.

Fig. 1 Structure of flavopinidol (L86-8275; National Service Center

649890).

regulatory kinases. It is considerably less potent against every other

kinase family thus far studied, including the epidermal growth

factor receptor kinase, cyclic AMP-dependent protein kinase, pro-

tein kinase C, and mitogen-aetivated protein kinase (1 l).� Ha-

vopiridol has potent antiproliferative effects in vitro, with IC50s

ranging from 25 to 150 nM in a series of breast and lung carcinoma

cell lines (24). The mechanism of this effect is not clear, although

the capacity of the compound to inhibit cell cycle progression in G1

or G2 depending on the cell-synchronization protocol used (24) and

to alter selectively the phosphorylation state of CDKI in living

cells (19) is concordant with a mechanism directed at the CDK

family of cell cycle regulatory kinases. In our disease-oriented

panel of human tumor xenograft cells tested in vitro in a soft agar

colony-forming assay, we were able to demonstrate a clear prostate

carcinoma and a moderate melanoma-selective antiproliferative

activity on the part of flavopinidol. The prostate antitumor activity

was also demonstrated in vivo.

MATERIALS AND METHODS

Drug. Flavopinidol (Behringwerke L86-8275; National

Service Center 649890) was originally supplied by Hoechst

India and Behring AG, Marburg, Germany, to the National

Cancer Institute. Other chemicals were obtained from commer-

cia] sources.

Nude Mice and Tumors. Eighteen human tumors estab-

lished in serial passage in nude mice (NMRI genetic back-

ground) in our laboratory and five cell line-derived xenografts

were used as tumor material (25, 26). The animals were housed

in macrolon cages set in laminar flow rackets. Tumor designa-

tion and tumor types are shown in Fig. 2. Tumor models were

selected out of more than 300 regularly growing xenografts of

different histology that we established in serial passage (26).

Characterization of these models included histology, immuno-

histochemistry (eareinoembryonie antigen, human ehorionie go-

nadotropin, epithelial membrane antigen, �32-mieroglobulin, cy-

tokeratin, and others; Refs. 27 and 28), and growth behavior, as

well as chemosensitivity to standard anticancer drugs in vitro

and in vivo (29, 30), isoenzyme phenotype analysis (31), hor-

mone receptor analysis, and DNA histogram as generated by

flow cytometry (32).

In Vitro Studies. A modification of the double-layer soft

agar assay as described by Hamburger and Salmon (33) was

used as described earlier (5). The target cell population is the

stem cell population, which is responsible for the unlimited

growth of a tumor. An excellent correlation of drug response in

the patient and in the elonogenie assay has been published by

various groups (34, 35).

Preparation of a Single-Cell Suspension. Solid human

tumor xenografts were mechanically disaggregated and subse-

quently incubated with a disaggregating solution consisting of

0.05% collagenase, 0.07% DNase, and 0.1% hyaluronidase in

RPMI 1640 at 37#{176}Cfor 30 mm. Cells were washed twice with

PBS and passed through sieves of 200- and 50-p.m mesh size.

The percentage of viable cells was determined in a hemoeytom-

eter using trypan blue exclusion. The freshly prepared tumor cell

suspension was used for colonogenie assay.

Human Bone Marrow. Human bone marrow cells were

aspirated from the iliac crest of healthy volunteers into preser-

vative-free heparinized syringes. Mononuclear cells with a den-

sity of less than 1 .007 g/ml were separated by density centrif-

ugation in Ficoll-Paque, washed, and plated in 24-multiwell

plates as described below. Granulocyte maerophage colony-

stimulating factor (1000 U/mI) was added to stimulate the

growth of granulopoietie colonies (colony forming unit, granu-

locyte macrophage).

Culture Methods. The tumor cell suspension was plated

into 24-multiwell plates over a bottom layer consisting of 0.2 ml of

Iseove’s modified Dulbeeco’s medium with 20% FCS and 0.7%

agar. A total of 20,000 to 200,000 cells was added to 0.2 ml of the

same culture medium and 0.4% agar and plated onto the base layer.

Flavopiridol was applied by continuous exposure (drug overlay) in

0.2 ml of medium. In each assay, six control plates received the

vehicle only, and drug-treated groups were plated in triplicate in six

concentrations ranging from 0.1 ng/ml to 10 p.g/ml.

Cultures were incubated at 37#{176}Cand 5% CO, in a humid-

ified atmosphere for 6-18 days and monitored closely for col-

ony growth using an inverted microscope. Within this period, in

vitro tumor growth led to the formation of colonies with a

diameter of �50 p.m. At the time of maximum colony forma-

tion, counts were performed with an automatic image analysis

system (Bausch & Lomb OMNICON FAS IV). Twenty-four h

before evaluation, vital colonies were stained using a tetrazo-

hum chloride dye.

Drug effects were expressed in terms of the percentage of

survival, obtained by comparison of the mean number of cob-

nies in the treated plates with the mean colony count of the

untreated controls (colony count T/C X 100). A compound was

considered active if it reduced colony formation to less than

30% of the control group value (T/C � 30%). Furthermore.

effective IC�t), IC70, and IC90 were calculated corresponding to

T/C values of 50, 30, and 10%, respectively.

Using these evaluation parameters, previous studies have

demonstrated that the majority of clinically established antican-

cer agents were active at drug concentrations of < I p.g/ml.

Some drugs. such as nitrosoureas or 5-(3,3-dimethyl-l-triaze-

nyl)-1H-imidazole-4-carboxamide, required dose levels of 6-30

p�g/ml, respectively. An assay was considered fully evaluable if



0.1* 8

1C50 lC7O

10 100 ng/ml ng/mI

36

5841

55

78

2340

394097

98

115

206

85

58155

38

60165

154

58

44

3827

I

.

_

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2 177

0.3 0.8

2 53 7

3 5

1 2

<0.1 <0.1

< 0.1< 0.1

3

< 0.1

< 0.1

< 0.1

10

< 0.1

U

Clinical Cancer Research 275

Cell Colony No.

Type Control 0.01*

HBM

CXF

60910861103

LXFL

529

LXFS387605

MAXF4011322

MCF7XMCF7XAD

MEXF

514

622

1341

OVXF89910231353

PRXF1369

PC3MXDU145XLNCAPX

RXF631

1183

1332

Test/Control (%) at Drug Concentration (ng/rnI)

log scaled axis

12 34

0.8 > 10000

6 68

10 33

2 6

3 9

8 69

4 8

6 22

> 25 > 10000

2 5

0.2 0.510 41

Mean n=23 8ng/ml 1 8

Fig. 2 Selectivity of flavopiridol in the colony-forming assay. Variations of individual lC70s (drug concentration needed to reduce colony formation

to 30% of control value) from mean value are expressed as bars in the logarithmically scaled axis. Bars to the left demonstrate IC70s lower than the

mean value; bars to the right demonstrate higher values. The mean potency of the compound was 8 ng/ml. HBM. human bone marrow; CXF. colorectal

xenograft; LXF. lung xenograft: L. large cell; S. small cell; MAXF, mammary xenograft: MEXF. melanoma xenograft: OVXF, ovarian xenograft:

PRXF. prostate xenograft; RXF, renal xenograft.

the following quality control criteria were fulfilled (36): (a)

Mean number of colonies in the control group wells were �20

colonies, with a colony diameter of �50 pm. (b) Initial plate

counts on day 0 or day 2 were �20% of the final control group

count. (�) The positive reference compound 5-fluorouracil (at

the toxic dose of 1000 p.g/ml) had to reduce colony survival to

�30% of the controls. (d) Coefficient of variation in the control

group was �50’7e.

Prostate Cancer Xenografts. Two human prostate can-

cer xenografts, PRXFI337 and PRXFI369, grown s.c. in nude

mice were used. Histological examination showed a moderately

differentiated cnibniform adenoearcinoma (PRXFI337) and a

poorly differentiated adenocareinoma with solid components

(PRXFI369), respectively. Both xenografts are androgen de-

pendent, with growth in nude mice occurring only with andro-

gen supplementation. These models have a moderate tumor

growth rate, with tumor volume doubling times between 10 and

17 days.

Nude Mouse Tests. Male nude mice 6-8 weeks old of

NMRI genetic background were used for all experiments. Tu-



276 Flavopinidol Preclinical Prostate Cancer Activity

Table I In vitro colony inhibition of flavopiridol in human tumor xenografts and human bone marrow

Cell type

Active”/total at drug concentration (ng/ml)

0.1 1 10 100 1000 10,000

Human bone marrow 0/1 0/1 0/1 1/1 1/1 1/1

Colon cancer 0/3 0/3 0/3 2/3 2/3 2/3Lung cancer 0/3 0/3 2/3 2/3 2/3 3/3Mammary cancer 0/4 1/4 3/4 3/4 4/4 4/4

Melanoma cancer 1/3 1/3 3/3 3/3 3/3 3/3Ovarian cancer 0/3 0/3 1/3 2/3 2/3 2/3Prostate cancer 3/4 3/4 3/4 4/4 4/4 4/4Renal cancer 0/3 1/3 2/3 3/3 3/3 3/3Active”/total 4/23 6/23 14/23 19/23 20/23 2 1/23

Xenografts only (%) 17 26 61 83 87 91

(1 Active: colony number in test group is less than 30% of the control group (T/C < 30%).

mors were implanted s.c. in both flanks of athymie nude mice.

Treatment was started as soon as the tumors reached a median

tumor volume of 500 mm3 for PRXF1337 and 300 mm3 for

PRXF1369, depending on the individual tumor doubling time

between days 10 and 17 (5). Mice were randomly assigned to

treatment groups and control group (8-10 tumors per group).

Flavopiridol was dissolved in water and administered p.o. by

gavage on days 1-4 and days 7-1 1 . Tumor size was measured

weekly by two-dimensional measurement with calipers. Tumor

volumes were calculated according to the formula V (a X

b2)/2, where a is the larger diameter and b the smaller. Data

evaluation was performed using specifically designed software

by plotting relative tumor volumes against time. Relative tumor

volume values were calculated for each single tumor by dividing

the tumor volume day X by the tumor volume day 0 at the time

of randomization. Tumor doubling time of test and control

groups was defined as the period required to double the initial

tumor volume. Growth curves were analyzed in terms of max-

imal tumor inhibition (treated/control, T/C) and growth delay

(the difference in days to double the initial tumor volume of the

test minus the control group). Toxicity was assessed by lethality

of tumor-bearing nude mice. At the maximal tolerable dose, the

mice were allowed approximately a LD10 (or a body weight loss

with recovery within 2 weeks after the last injection).

Statistical Analysis. The statistical data analysis of the in

vivo investigations was performed by the Wileoxon test. Data of

relative tumor volumes of each treatment group were compared

with control group.

RESULTS

Screening Studies. Initial screening studies of flavopiri-

dol in the National Cancer Institute in vitro cell line screen (2)

revealed evidence of potent growth-inhibiting effect, with an

average IC50 of 25 ng/ml evaluated in 60 cell lines. Against the

PC3 and DU145 cell lines, IC50s of 32 and 30 ng/ml, respec-

tively, were observed in a 2-day growth inhibition assay (data

not shown). Therefore, it became of interest to investigate in an

independent panel of cell lines evidence for a potent and selee-

tive antiproliferative effect.

Detailed in Vitro Studies. Table 1 and Fig. 2 show the

cytotoxie activity of flavopindol using the elonogenie assay in 23

human tumor cell models. Cytotoxicity (T/C < 30%) begins to be

apparent at concentrations as low as 0. 1 ng/ml. In three prostate

cancer xenograft (PRXFI369, PC3MX, and LNCaPX)- and one

melanoma xenograft (MEXFI341)-derived ebonogenic cell types,

we observed potent colony inhibition, even at the lowest eoneen-

tration tested (0. 1 ng/ml). Human bone marrow colony formation

was significantly growth inhibited only between 10 and 100 ng/ml.

Growth in 14 of the 23 tumor cell types was more potently affected

than bone marrow. MCF7XAD, resistant to doxorubiein, was as

potently inhibited as its parental cell type, suggesting indirectly that

the multidrug resistance phenotype was not likely a major deter-

minant of drug efficacy. Cytotoxicity in most of the other xe-

nografts was observed by 10 ng/ml.

Fig. 2 summarizes the distribution of IC70s (drug concen-

tration needed to reduce colony formation to 30% of control

value) for each tumor cell type related to the mean IC70 for all

cell lines. Considering all cell types, flavopinidol had a mean IC70

of 8 ng/ml, indicating high cytotoxic potency of the compound.

These data also clearly indicate the selectivity of flavopiridol for

prostate cancer xenograft-derived cells (PRXF), exhibiting IC70s

almost two log-steps lower than the mean value. Melanoma-de-

rived tumor cells also showed a higher sensitivity to flavopinidol

treatment than the other tumor types tested, but to a smaller extent

than the prostate tumor cells. Lung and breast cancer xenograft-

derived cells demonstrated intermediate responsiveness to fla-

vopiridol, whereas the colon carcinoma cells studied here seem to

be more resistant to flavopiridol. We conclude that in this panel of

tumor cell types using a colony-forming assay, flavopiridol dis-

plays clear evidence of selective toxicity for prostate carcinoma and

melanoma cells. Ovarian and colon carcinoma cells appear to be

relatively resistant.

In Vivo Studies. Dose-finding studies (data not shown)

with flavopinidol identified a dose of 10 mg/kg/day administered

P.O. on days 1-4 and days 7-1 1 as the MTD. In tumor-bearing

nude mice, we observed 12.5% drug-related deaths at the MTD.

Table 2 shows the in vi�o antitumor activity of flavopinidol

against two prostate cancer xenografts. In PRXFI369, we ob-

served at the MTD of 10 mg/kg/day an optimal T/C of 33% and

a growth delay of 30 days. After an initial tumor regression to

85% relative tumor volume, tumor volume increased slightly to

1 15% at day 28 (Fig. 3A). Flavopiridol administered at the dose

level of 15 mg/kg/day was toxic, whereas the dose of 5 mglkg/

day was not active. In PRXFI337, we observed antitumor



Table 2 Activity of flavopiridol against prostate cancer xenografts

“ Given p.o. at days 1-4 and days 7-I 1.I, Optimal T/C during therapy.

( Significantly different from control (P < 0. 1, Wilcoxon test).“Significantly different from control (P < 0.()Ol. Wilcoxon test).

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TumorDose

(mg/kg/day)”

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Tumor inhibitionT/C (%)“

Growth delay(days)

PRXFI369 1510

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90

Toxic30

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PRXF1337 10S

27”55”

1712

activity with a minor remission on day 14 (Fig. 3B) at the MTD

of 10 mg/kg/day. Tumor recurrence occurred when the therapy

was stopped. The optimal T/C was 27% (day 14), and the

growth delay was 17 days. The dose level of S mg/kg/day

ef’feeted a moderate tumor volume inhibition with an optimal

T/C of 55%.

DISCUSSION

The flavone flavopiridol has been evaluated here for

antitumor activity in vitro and in viva. in vitro evaluation was

performed in a disease-oriented human tumor xenograft-

derived panel using a clonogenic assay. In 23 human tumor

models, flavopiridol was identified as a highly potent cyto-

toxic compound with a mean IC70 of 8 ng/ml. Other flavone

derivatives such as quereetin or genistein also have shown

cytotoxic activity in vitro (13-16). However. they are less

potent (24) and, compared with flavopinidol, require higher

concentrations for cytotoxie activity. Previous results from

Kaur et a!. (24) also described flavopinidol as a compound

with potent antiproliferative activity in breast and lung cancer

cell lines using a formazan colonimetnie assay. Our results

show clear prostate-selective and moderate melanoma-selec-

tive antiproliferative activity of flavopinidol. Three of four

prostate cancer xenograft-derived cell types showed an mdi-

vidual IC70 almost two log-steps lower than the mean. With

melanoma cells, we observed notable potency as well,

whereas breast, lung, renal, and ovarian cancer showed in-

termediate sensitivity. Flavopinidol was considerably less

potent when directed against human bone marrow, an impor-

tant result favoring its further clinical development.

However, the ultimate mechanism of in vitro activity is still

unclear. Several flavones are known to inhibit tyrosine kinases.

Quereetin also inhibits various senine-threonine protein kinase

activities, whereas genistein is a more specific tyrosine kinase

inhibitor. The tyrosine kinase inhibition capacity of flavonoids

correlates closely with the number of hydroxy groups on the

flavone ring ( 16), and the replacement of hydroxy groups with

methoxy groups has resulted in a substantial loss of activity

(37). Two new types of substituents are present in flavopinidol:

a chloro substituent in the 2’ position of the aryl ring and a

N-hydroxy methyl piperidinyl substituent at the 8 position of the

flavone ring system. Flavopinidol has recently been described as

a potent inhibitor of the CDK family of cell cycle regulatory

molecules CDK1 and CDK2 (20-23). Of great interest will be

Days

Fig. 3 Growth delay by flavopiridol in prostate carcinoma athymicmouse xenografts. A, PRXFI369/12 treated at 0 (O), 5 (0), 10 (s), and15 (A) mg/kg/dose. B. PRXF1337/19 treated at 0 (0), 5 (LI). and 10 (A)

mg/kg/dose. Both experimental groups were treated p.o. on days 1-4

and days 7-Il.

an examination of the CDK complex of cell cycle regulatory

molecules in the prostate carcinoma cells examined here in an

effort to understand the basis of the potent activity of flavopini-

dol. Endogenous activating phosphorylations, as well as novel

endogenous intracellular CDK inhibitors, have recently been

described (38. 39). Variations in different tumor cell types of the

mass of CDK catalytic subunits, the amount or type of regula-

tory cyclins (40), or the mass of potential enogenous CDK

inhibitors could all be a basis for differential sensitivity to the

agent. In addition, differential metabolism of flavopinidol in the

different cell types obviously could also be a factor in deter-

mining susceptibility.

Because flavopinidol demonstrated prostate cancer spec-

ificity in vitro. it was studied in viva in the prostate cancer

xenografts PRXF1337 and PRXFI369. PRXF1369 was

highly sensitive ii? vitro, whereas PRXFI337 could not be

investigated due to insufficient growth in the clonogenic

assay. Flavopiridol was administered p.o. As shown by Czech

et a!. (41), this application form is as effective as the iv.

application. Also, a sufficient bioavailability of flavopinidol

is indicated by similar MTDs for both routes (41 ). Flavopiri-

dol proved to be active in both prostate cancer xenografts.

However, the therapeutic window is small. In both tumors,

only the MTD was active, and after the end of therapy both

tumors showed regrowth. This means that dosing for more



278 Flavopiridol Preclinical Prostate Cancer Activity

than 2 weeks is needed to control tumor growth for a longer

time. Nevertheless, the antitumor activity is remarkable be-

cause clinical standard cytotoxic agents effect no tumor re-

missions in our prostate cancer models.

Furthermore, the current cytotoxie chemotherapy has not

been effective in the treatment of metastatie prostate cancer

(42-44). Thus, the development of new therapeutic agents pref-

erably with novel mechanisms of action is desirable. These

studies, therefore, delineate a basis for the early assessment of

flavopiridol in patients with prostate carcinoma. Further studies

are necessary to understand the cellular basis for prostate-

selective antiproliferative activity seen in vitro and to outline the

action of flavopiridol in concert with conventional ehemother-

apeutic agents.

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1997;3:273-279. Clin Cancer Res M Drees, W A Dengler, T Roth, et al. activity in vivo for prostate carcinoma cells.Flavopiridol (L86-8275): selective antitumor activity in vitro and

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