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2003;2:409-417.Mol Cancer TherPaul J. Shami, Joseph E. Saavedra, Lai Y. Wang, et al.

1Antineoplastic ActivityPotentNitric Oxide Donor of the Diazeniumdiolate Class with

Transferase-activatedS-JS-K, a Glutathione/Glutathione

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JS-K, a Glutathione/Glutathione S-Transferase-activatedNitric Oxide Donor of the Diazeniumdiolate Class withPotent Antineoplastic Activity1

Paul J. Shami,2 Joseph E. Saavedra, Lai Y. Wang,Challice L. Bonifant, Bhalchandra A. Diwan,Shivendra V. Singh, Yijun Gu, Stephen D. Fox,Gregory S. Buzard, Michael L. Citro,David J. Waterhouse, Keith M. Davies, Xinhua Ji,and Larry K. Keefer

Division of Medical Oncology, Department of Internal Medicine,University of Utah and Salt Lake City Veterans Administration MedicalCenters, Salt Lake City, Utah 84148 [P. J. S., L. Y. W.]; Basic ResearchProgram [J. E. S., B. A. D., G. S. B., M. L. C.] and Analytical ChemistryLaboratory [S. D. F.], Science Applications International Corporation-Frederick, National Cancer Institute at Frederick, Frederick, Maryland21702; Department of Pharmacology, University of Pittsburgh CancerInstitute, Pittsburgh, Pennsylvania 15213 [S. V. S., Y. G.];Macromolecular Crystallography Laboratory, National Cancer Instituteat Frederick, Frederick, Maryland 21702 [Y. G., X. J.]; Department ofChemistry, George Mason University, Fairfax, Virginia 22030 [K. M. D.];and Laboratory of Comparative Carcinogenesis, National CancerInstitute at Frederick, Frederick, Maryland 21702 [C. L. B., D. J. W.,L. K. K.]

Abstract

We have previously shown that nitric oxide (NO)

inhibits growth and induces differentiation and

apoptosis in acute myeloid leukemia cells, with the HL-

60 human myeloid leukemia line being particularly

sensitive to NO-mediated cytolysis. With the goal

of identifying a prodrug that can target NO to theleukemia cells without inducing NO-mediated systemic

hypotension, we have screened a series of O2-aryl

diazeniumdiolates designed to be stable at

physiological pH but to release NO upon reaction

with glutathione. O2-(2,4-Dinitrophenyl) 1-[(4-

ethoxycarbonyl)piperazin-1-yl]diazen-1-ium-1,2-diolate

(JS-K) proved to be the most active antiproliferative

agent among those tested in HL-60 cells, with an IC50of 0.20.5 M. After 5 days of exposure to 0.5 M JS-K,

HL-60 cells had differentiated and acquired some of

the phenotypic features of normal monocytes. One- to

2-day treatment with JS-K at concentrations of 0.51

M resulted in apoptosis induction in a concentration-and caspase-dependent manner. JS-K also inhibited

the growth of solid tumor cell lines but to a lesser

extent than HL-60 cells. JS-K was administered i.v. to

nonobese diabetic-severe combined immune deficient

mice at doses of up to 4 mol/kg without inducing

significant hypotension. The growth of s.c. implanted

HL-60 cells was reduced by50% when the mice

received i.v. injections three times/week with 4 mol/

kg boluses of JS-K. Histological examination of tumor

explants from JS-K-treated animals revealed extensive

necrosis. Similar results were seen with s.c. human

prostate cancer (PPC-1) xenografts. Our data indicate

that JS-K is a promising lead compound for the

possible development of a novel class of antineoplastic

agents.

Introduction

AML3 is a life-threatening disease with an annual incidence

of 2.25 per 100,000 (1). Despite therapy with different classes

of chemotherapeutic agents, including anthracyclines, 1--

D-arabinofuranosylcytosine, etoposide, all-trans-retinoic

acid, as well as high-dose therapy with stem cell rescue, the

overall 5-year disease-free survival remains 30%. This il-

lustrates the need for agents with novel mechanisms of

action.This study is based on the premise that the unusual sen-

sitivity of AML cells to the cytotoxic effects of NO can be

exploited for improved therapy of this malignancy. We pre-

viously showed that NO-releasing drugs of the diazenium-

diolate class induce apoptosis in the HL-60 human myeloid

leukemia cell line at relatively low concentrations (for exam-

ple, an IC50 of 0.006 mM for a cell-permeant prodrug that is

activated for NO release by intracellular esterases; Ref. 2).

This is in marked contrast to, for example, normal vascular

smooth muscle cells, which experience cytostasis without

toxicity in the continuous presence of 0.20.5 mM (Z)-1-[2-

(2-Aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-

diolate (half-life for NO release of 20 h) for 6 days (3), and to

primary hepatocytes, which can actually be protected from

apoptosis by exposure to a diazeniumdiolate that is acti-

vated for NO release by cytochrome P450 (4).

Received 11/25/02; revised 1/30/03; accepted 2/12/03.The costs of publication of this article were defrayed in part by thepayment of page charges. This article must therefore be hereby markedadvertisement in accordance with 18 U.S.C. Section 1734 solely to indi-cate this fact.1 This work is supported by a Translational Research Award from theLeukemia and Lymphoma Society (to P. J. S.). This project was alsosupported, in part, through National Cancer Institute Contract No. NO1-CO-12400 (J. E. S., B. A. D., S. D. F., G. S. B., M. L. C.), and NationalInstitute of Environmental Health Sciences Grant ES09140 (to S. V. S.).2 To whom requests for reprints should be addressed, at SLC VA MedicalCenter, Box 151M, 500 Foothill Boulevard, Salt Lake City, UT 84148.Phone: (801) 582-1565; Fax: (801) 583-9624; E-mail: [email protected].

utah.edu.

3 The abbreviations used are: AML, acute myeloid leukemia; BSO, buthi-onine sulfoximine; CDNB, 1-chloro-2,4-dinitrobenzene; DNP-SG, S-(2,4-dinitrophenyl)glutathione; GSH, glutathione; GST, glutathione S-transfer-ase; GSTCD, 1-( S-glutathionyl)-2,4,6-trinitrocyclohexadienate anion;HPLC, high performance liquid chromatography; JS-K, O2-(2,4-dinitro-phenyl) 1-[(4-ethoxycarbonyl)piperazin-1-yl]diazen-1-ium-1,2-diolate; MS,mass spectrometry; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetra-zolium bromide; NAC, N-acetyl-L-cysteine; NO, nitric oxide; NOD-SCID,nonobese diabetic-severe combined immune deficient; NSE, nonspecificesterase; xGSTY1-1, class Y (where Y is: A, ; M, ; P, ) GST of subunit

type 1 from x (where x is: h, human; r, rat).

409Vol. 2, 409417, April 2003 Molecular Cancer Therapeutics

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One might, therefore, expect NO-based therapies to ex-

hibit some selectivity for the NO-sensitive leukemia cells

relative to normal tissues. With this in mind, we have

screened a library of arylated diazeniumdiolates that were

designed to be activated for NO release by reaction with

cellular thiols such as GSH (5) with or without catalysis by

GST, the major isoforms (, , ) of which are expressed in

75, 55, and 95% of AML cases, respectively (6). We report

here on the in vitro and in vivo antineoplastic activity of the

most potent growth inhibitor of this family tested to date,JS-K (structure in Fig. 1).

Materials and Methods

Chemicals. JS-K (7) and 4-carbethoxy-PIPERAZI/NO (Ref.

8; structures shown in Fig. 1) were synthesized as described

previously. S-(2,4-Dinitrophenyl)glutathione was prepared by

the method of Mancini et al. (9). The pan-caspase inhibitor

Z-VAD-FMK was from Biomol (Plymouth Meeting, PA).

Daunorubicin and etoposide were from Dr. Grayden Harker

(University of Utah). HPLC grade solvents were purchased

from VWR Scientific Co. (South Plainfield, NJ). All other

chemicals were from Sigma (St. Louis, MO) unless otherwise

noted.

Chromatography and MS. HPLC separations were car-

ried out with a Phenomenex Luna 5 C18(2) column (Tor-

rance, CA) using a gradient of acetonitrile:water (each con-

taining 0.1% formic acid) at a flow rate of 1 ml/min

(percentage of acetonitrile: 25% for 0 5 min followed by a

linear program to 70% at 15 min).HPLC-MS studies were performed on an Agilent Capillary

Series 1100 LC/MSD Ion Trap mass spectrometer with elec-

trospray ionization in the positive ion mode. Separations

were effected as above, except that the flow rate was 15

l/min, and the gradient was 10% acetonitrile for 0 5 min

followed by a linear program to 70% at 15 min.

Molecular Modeling. CDNB is a model substrate used

extensively for monitoring activity of GST (10). It has been

shown that GSTs with a higher specific activity toward CDNB

stabilize the Meisenheimer complex (Fig. 1) at the transition

state better than the isoforms with poor catalytic activity for

CDNB-GSH conjugation (5). Crystal structures of a transition

state analogue, GSTCD

, in complex with rGSTM1-1, a class rat GST isoform (11), and with hGSTP1-1 (isoleucine-

104, alanine-113 variant), a class human GST isoform (12),

have been reported and provided the foundation for model-

ing the transition state of GST-catalyzed GSH conjugation of

JS-K. The initial model of the Meisenheimer complex of GSH

and JS-K for hGSTM1-1 and that for hGSTP1-1 built based

on the crystal structures of GSTCD in rGSTM1-1 (11) and

that in hGSTP1-1 (12), respectively. The models were subject

to geometry optimization using the conjugate gradient

method of Powell (13) and docked into the active sites of

ligand-free hGSTM1-1 and hGSTP1-1 built based on the

rGSTM1-1 (11) and hGSTP1-1 (12) structures, respectively.

The geometry of the protein-Meisenheimer complexes wasthen optimized, and the energy was minimized (13). Both

complexes were built in dimeric form considering the fact

that the biologically active forms of GSTs are dimeric pro-

teins and that the glutathionyl moiety of GSH interacts with

the side chains from both subunits (14 17). The Engh and

Huber (18) geometric parameters were used as the basis of

the force field. No crystal structure of class GST-bound

GSTCD is available. The Meisenheimer complex for class

GST was therefore built by modifying those for rGSTM1-1

and hGSTP1-1 based on the structures of hGSTA1-1-bound

S-benzyl-GSH (19) and the GSH conjugate of ethacrynic acid

(20). After energy minimization, the Meisenheimer complex

for hGSTA1-1 was docked in the active center of the enzyme,

and the protein-Meisenheimer complex was subject to en-

ergy minimization as described above. The molecular mod-

eling studies were carried out with program suites O (21) and

X-PLOR (22).

Determination of Specific Activity of Human GSTs to-

ward JS-K. Purified preparations of recombinant hG-

STA1-1, hGSTM1-1, and hGSTP1-1 were obtained from

Panvera (Madison, WI). The activity of human GST toward

CDNB was determined, as described by Habig et al. (10),

before activity measurements with JS-K to ensure that the

enzyme preparations were catalytically active. For activity

measurement toward JS-K, the reaction mixture in a final

volume of 1 ml contained 100 mM potassium phosphate

buffer (pH 6.5), 1 mM GSH, 0.045 mM JS-K, and an appro-

Fig. 1. Structures of JS-K, our control arylating agent CDNB, and theircommon GSH conjugate (DNP-SG). The structures of the Meisenheimercomplexespresumed to be intermediates in the two conjugation reactionsare also shown. 4-Carbethoxy-PIPERAZI/NO (the coproduct of the GSH/

JS-K reaction) generates NO spontaneously at physiological pH asshown; its half-life for NO release at 37C in 0.1 M phosphate buffer (pH7.4) was found to be 6.0 min (k 1.9 103 s1).

410 Antitumor Effects of JS-K, a Nitric Oxide Prodrug



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priate amount of human GST isoenzyme protein. The reac-

tion was started by the addition of JS-K, and the rate of

reaction was monitored by measuring decrease in absorb-

ance of JS-K at 298 nm because of its reaction with GSH.

The specific activity toward JS-K was calculated using an

extinction coefficient of 18 mM1

cm1

at 298 nm.Measurement of NO Release. Chemiluminescence de-

tection and quantification of NO evolving from the reactions

of JS-K were conducted using an NO-specific Thermal En-

ergy Analyzer (Model 502A; Thermedics, Analytical Instru-

ment Division, Waltham, MA) essentially as described previ-

ously (23). Briefly, pH 7.4 phosphate buffer containing 1 mM

GSH was sparged with inert gas until a steady detector

response was established. Where indicated, GSTs were

added to a final concentration of 1.67 g of enzyme/ml. The

NO release profile was followed at 37C for 45 min after

injecting JS-K at a final concentration of 133 nM to start the

reaction. The resulting curve was integrated to quantify the

amount of NO released/mol of compound.Cell Lines and Culture Conditions. HL-60, DLD1, and

U937 cells were from American Type Culture Collection (Ma-

nassas, VA). Meth A cells were from Dr. Wolfram Samlowski

(University of Utah). The PPC-1 cell line was provided by Dr.

Graeme Bolger (University of Alabama). For the cell growth

and apoptosis experiments, cells were cultured at a density

of 150,000 cells/ml in RPMI 1640 with 10% fetal bovine

serum at 37C in a 5% CO2-humidified atmosphere. Agents

were added at the indicated concentrations 24 h after culture

initiation. At the indicated time intervals, cells were harvested

and washed twice in PBS before processing for analysis of

growth, differentiation, and apoptosis.

Cell Growth, Differentiation, and Apoptosis Assays.

The number of viable cells was determined using the MTT

assay according to the manufacturers protocol (Promega,

Madison, WI) or using a Coulter counter. Differentiation was

evaluated using Wright and NSE staining of cells collected on

microscope slides by cytospin as described previously (24).

Apoptosis was assayed by flow cytometry and by determin-

ing DNA fragmentation using agarose gel electrophoresis as

described previously (25). For the flow cytometry assay, we

used the propidium iodide staining method of Nicoletti et al.

(26).

In Vivo Studies of JS-K. NOD/SCID mice were bred and

maintained at the Huntsman Cancer Institute at the Univer-

sity of Utah. Experiments were performed on male or female

mice 6 8 weeks of age at the Animal Care Facility of the Salt

Lake City Veterans Administration Medical Center after ap-

proval by the Institutional Animal Care and Use Committees.

We measured systolic blood pressure on unanesthetized

NOD/SCID mice using an occluding tail cuff and a pulse

transducer connected to a blood pressure transducer/mon-

itor from World Precision Instruments (Sarasota, FL). Signals

from the blood pressure monitor and pulse transducer were

transmitted to a MacLab2 data acquisition device (pur-

chased from Stoelting, Wood Dale, IL) that feeds directly into

a Macintosh computer. The recorded data were analyzed

using the Chart data analysis software purchased from

Stoelting. Measurements were done in triplicate at each time

point.

To study the in vivo antineoplastic potency of JS-K, NOD/

SCID mice received injections in the flanks s.c. with HL-60 or

PPC-1 cells (2.5 106 cells/flank). When s.c. tumors were

palpable, treatment with JS-K or an equal volume of vehicle

(20% DMSO in PBS) was started using the indicated doses

and route. Tumor size was measured daily or every other dayusing a Vernier caliper. Tumor volume was calculated using

the formula: width length [(width length)/2] 0.5236.

Fifteen to 20 days after tumor cell implantation, animals were

sacrificed by CO2

inhalation, and tumors were collected for

histochemical analysis.

Histological Analysis of Tumors. At the completion of the

experiments, animals were sacrificed, and s.c. tumors were

dissected out, fixed in 10% formaldehyde, and imbedded in

paraffin. Four-m sections were cut and stained with H&E.

Calculations and Statistical Analysis. Results are ex-

pressed as averages of multiple experiments with SE. SE

was calculated as the SD of different measurements divided

by the square root of the number of measurements. Differ-ences were considered statistically significant if the P was

0.05 as calculated using the t test.

Results

Reactivity of JS-K with GSH. The diazeniumdiolate ion has

been judged to resemble chloride as a leaving group in SNAr

reactions (7). Because CDNB (structure shown in Fig. 1) is

known to react with GSH, we anticipated that JS-K would be

similarly converted to DNP-SG (structure also shown in Fig.

1). This was confirmed by HPLC-MS; as shown in Fig. 2A, an

85% conversion of JS-K to DNP-SG occurred within a 30-

min incubation period at 37C in pH 7.4 phosphate buffer.

Similar results were seen in RPMI 1640 cell culture. Pseudo-first-order kinetic plots for the reaction of GSH with JS-K in

0.1 M phosphate buffer (pH 7.4) were obtained with GSH (15

mM) in large excess of the substrate. Excellent first-order

behavior was observed over several half-lives and measured

first-order rate constants showed a linear dependence on

(GSH; Fig. 2C). The slope and y intercept of the line yielded

values for the second-order [k2 (1.02 0.04) M1 s1] and

first-order [k1 (4 12) 10

5 s1] rate constants for the

reactions of JS-K with GSH and water, respectively, at 37C

in 0.1 M phosphate buffer, pH 7.4. The UV spectral changes

accompanying the reaction in a second cell culture medium

(DMEM) are shown in Fig. 2B.

Hydrolysis of JS-K. JS-K proved resistant to simple hy-

drolysis under these conditions, as reflected in the near-zero

y intercept of Fig. 2C. The value of k1

obtained from the

intercept is in statistical agreement with the rate constant for

JS-K hydrolysis (1 106 s1) measured separately in the

absence of GSH. The small amount of the hydrolysis product

(2,4-dinitrophenol) seen in the chromatogram of Fig. 2A was

apparently formed in the DMSO stock solution, which in this

case had been stored in the refrigerator with intermittent use

during several weeks. Hydrolysis was much more facile at pH

12, which was expected for a compound type designed to be

activated for NO release by nucleophilic attack.

Solubility Limits. JS-K showed an interesting but poten-

tially nettlesome tendency to remain supersaturated in aque-

ous solutions, only to separate from the aqueous phase at a

411Molecular Cancer Therapeutics



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time course that was difficult to predict or control. For ex-

ample, our first attempts to follow the hydrolysis of 50 M

JS-K spectrophotometrically sometimes showed little

change, as expected from Fig. 2C, but at other times re-

vealed a variable and sometimes rapid rate of absorbance

loss without production of new maxima. The problem couldbe overcome by cleaning the cuvette with nitric acid before

filling it with buffer then adding the DMSO/JS-K stock solu-

tion, in which case absorbance changed little with time.

However, rinsing this solution out with distilled water fol-

lowed by refilling the cuvette with buffer and adding the

stock solution often led to shrinking absorbance with con-

comitant appearance of cloudiness in the cuvette; dissolu-

tion of the insoluble material, after isolating it by centrifuga-

tion, showed that the resulting solid (dissolved in acetonitrile)

was essentially pure JS-K in an amount equivalent to the

quantity lost from the supernatant. Quantification of the JS-K

in the buffer by HPLC and UV spectrophotometry after all

spectral changes had stopped indicated that JS-Ks solubil-ity limit in 0.1 M phosphate buffer at 37C and pH 7.4 con-

taining 1% DMSO was 10 M. The relative insolubility of

JS-K in aqueous media should be taken into account during

any experimental work involving dilution of organic JS-K

solutions with aqueous media because the observed tend-

ency toward supersaturation can lead to JS-K concentra-

tions that are much lower or higher than expected and thus

to erroneous results.

Catalysis of the GSH/JS-K Reaction and NO Release

by GST. The reaction of GSH with CDNB (Fig. 1) is catalyzed

by several classes of human GSTs, and, thus, this electro-

philic substrate is often used for quantifying their activity.

Given the similarity of diazeniumdiolate ions to chloride as a

leaving group in SNAr reactions (7), we expected JS-K also to

undergo GST-catalyzed conjugation with GSH. To gain in-

sights into the effect the obvious steric differences between

chloride and the diazeniumdiolate ion shown in Fig. 1 may

imply, we modeled the accommodation of the Meisenheimer

complex of JS-K in the active sites of the three major classes

of human GSTs, i.e., hGSTA1-1, hGSTM1-1, and hGSTP1-1.

As illustrated in Fig. 3, AC, both hGSTA1-1 and hGSTM1-1

classes of GSTs accommodate the Meisenheimer complex

very well, but hGSTP1-1 appears to have serious steric con-

flicts with the diazeniumdiolate moiety of the transition state

complex. On the basis of molecular modeling, we predicted

that hGSTA1-1 and hGSTM1-1 should be more effective

than hGSTP1-1 for catalyzing the GSH conjugation of JS-K.These predictions were confirmed by determining the activ-

ities of recombinant hGSTA1-1, hGSTM1-1, and hGSTP1-1

preparations toward JS-K. The data are summarized in Table 1.

Specific activities of GSTs toward CDNB were determined be-

fore activity measurement with JS-K to ensure that the enzyme

preparations were catalytically active. The specific activities of

the GSTs toward CDNB were comparable with the values pub-

lished in the literature (Ref. 27; Table 1). In agreement with our

max 354 nm, rather than DNP-SG, max 340 nm. C, increases in JS-Kconsumption rate at an initial concentration of 50 M in 0.1 M phosphate

(pH 7.4) at 37C as a function of increasing GSH concentration.

Fig. 2. Reactions of JS-K with thiols. A, HPLC traces of a reactionmixture containing 50 M JS-K and 500 M GSH at 37C in 0.1 M phos-phate (pH 7.4) at an observing wavelength of 300 nm immediately aftermixing (top) and 30 min later (bottom). The structures of JS-K and its GSHconjugate DNP-SG (extinction coefficients at 300 nm of 16 and 3.7 m M1

cm1, respectively) are shown in Fig. 1; 2,4-DNP is the JS-K hydrolysisproduct, 2,4-dinitrophenol, and DMSO is the cosolvent. B, UV spectralchanges during 30 min at 37C of a 50 M JS-K solution in serum-freeDMEMin the absence (left) and presence (right) of 1 mM GSH. Note that in this

cystine-containing medium, the product is S-(2,4-dinitrophenyl)cysteine,




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prediction, hGSTP1-1 was much less active than hGSTA1-1 or

hGSTM1-1 for GSH conjugation of JS-K.

That these reactions led to NO generation as predicted

was demonstrated by purging gases from the solution as

they formed into an NO-specific chemiluminescence detec-

tor. The results are shown in Fig. 3D. Consistent with JS-Ks

resistance to hydrolysis but its reactivity toward GSH as

noted above, no NO was detected until 1 mM GSH was

added to the pH 7.4 phosphate/0.67 M JS-K solution. In the

absence of enzyme, NO release began immediately upon

adding GSH, increasing in rate until plateauing at 8 min, and

integrating to a total of 1.1 mol of NO/mol of JS-K within 43

min of mixing. The hGSTP1-1 isoform catalyzed this reac-

tion, but only weakly; that this was not attributable to a

deleterious effect of JS-K exposure on the enzymes activity

was demonstrated in its unfettered ability, in the presence of

up to 80 M JS-K, to catalyze CDNBs conjugation with GSH.

The hGSTA1-1 and hGSTM1-1 isoforms proved much supe-rior to hGSTP1-1 as catalysts for JS-K conjugation. The data

of Fig. 3D on NO release are thus consistent with the

conclusion from the JS-K consumption studies of Table 1

that JS-K is metabolized much better by hGSTA1-1 and

hGSTM1-1 than by hGSTP1-1.

Growth Inhibitory Properties of JS-K. Fig. 4A shows a

comparison between the growth inhibitory ability of JS-K and

those of the chemotherapeutic agents daunorubicin and eto-

poside in our HL-60 assay system. The IC50

s of JS-K, dauno-

rubicin, and etoposide were 0.5, 0.01, and 0.3 M, respec-

tively. CDNB, a compound with the same aryl ring as JS-K

that does not release NO (Fig. 1), inhibited the in vitro growth

of HL-60 cells but at much higher concentrations, with anIC

50estimated at 6.7 M (data not shown). JS-K also inhib-

ited the growth of U937 (monocytic leukemia) cells with an

IC50 of 0.3 M (data not shown). Solid tumor cell growth was

also inhibited by JS-K, although to a lesser extent than

leukemia cells (Fig. 5); the IC50

s for the three lines we tested,

PPC-1, DLD-1, and Meth A, were an order of magnitude

greater than those for the two leukemia lines.

To determine whether modulation of the GST pathway

affects JS-Ks antineoplastic properties, we performed ex-

periments using NAC or BSO. NAC increases intracellular

GSH levels, whereas BSO inhibits its synthesis (28). Treat-

ment of HL-60 cells with NAC (0.3 0.5 mM) or BSO (0.2 0.3

mM) did not significantly affect cell growth. Pretreatment of

Fig. 3. Catalysis of JS-Ks NO release by GST. AC illustrate the accommodationof the Meisenheimer complex of JS-K in the active center of (A) hGSTA1-1, (B)hGSTM1-1, and (C) hGSTP1-1 as predicted by the molecular modeling studies. Theactive centers of the GSTs are shown as surface representations and the Meisen-heimer complexes as ball-and-stick models with atomic color scheme (carbon, gray;nitrogen, blue; oxygen, red; and sulfur, yellow). The illustration was prepared usingGrasp (29) and Raster3D (30). D, chemiluminescence traces showing the time courseof NO release from 0.67 M JS-K at 37C in 0.1 M phosphate (pH 7.4) containing 1.0mM GSH both alone and with 1.67 g/ml hGSTA1-1, hGSTM11, or hGSTP11.

Table 1 Specific activities (mol min1 mg1) of GST-catalyzed

glutathionylation of CDNB and JS-K in 0.10 M phosphate buffer at pH6.5 and 25Ca

Enzyme CDNB JS-K

hGSTA1-1 34 3.7

hGSTM1-1 200 14.9

hGSTP1-1 (I104, A113) 95 0.15

a The concentrations of CDNB and JS-K were 1.0 and 0.045 mM, respec-tively. The GSH concentration was 1.0 mM for both substrates. The proteinconcentrations were 0.025, 0.01, and 0.215 mg/ml for hGSTA1-1,hGSTM1-1, and hGSTP1-1, respectively.




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the cells for 2 6 h with NAC prevented the JS-K-induced

growth inhibition. Pretreatment of HL-60 cells with BSO for

2 6 h did not prevent the JS-K-induced growth inhibition,

whereas pretreatment of the cells with BSO for 24 h en-

hanced it (data not shown).

Induction of Leukemia Cell Apoptosis by JS-K. Be-

cause we had previously shown that spontaneous and

esterase-activated NO generators induce apoptosis in leu-

kemia cells (2, 25), we sought to determine whether JS-K had

a similar effect. Three days after addition of JS-K at concen-

trations of 0.5 and 1 M, the percentage of apoptotic HL-60

cells increased from 7 to 27 and 43%, respectively (Fig. 4B).

The flow cytometry experiments were confirmed with DNA

laddering assays (Fig. 4C). The pan-caspase inhibitor C-

VAD-FMK prevented the JS-K-induced growth inhibitory and

apoptotic effects (Fig. 4D).

Effect of JS-K on Leukemia Cell Differentiation. We

have previously shown that NO induces HL-60 cells to dif-

ferentiate along the monocytic phenotype (24). We therefore

determined whether JS-K induced differentiation as well.

HL-60 cells were treated with JS-K at a concentration of 0.5

M f or 35 days. Wright stain revealed morphological

changes consistent with a monocytic phenotype, namely

development of folded nuclei, large cytoplasms, and cyto-

plasmic vacuoles (Fig. 6). NSE (an enzyme specific to the

monocytic lineage) staining showed that JS-K increased the

percentage of HL-60 cells expressing NSE from 1 to 40%

(Fig. 6).

Fig. 4. Antiproliferative and proapoptotic effects of JS-K on HL-60 leukemia cells in vitro. A, comparison of the antiproliferative effects of JS-K with thoseof other chemotherapeutic agents. Cells were cultured with JS-K, daunorubicin, or etoposide for 3 days at the indicated concentrations. Cell viability wasdetermined using the MTT assay (averages and SE of three separate experiments). B, apoptosis induction by JS-K. Cells were cultured with JS-K at theindicated concentrations. At 72 h, the percentage of apoptotic cells was determined by flow cytometry (see text; averages and SE of three separateexperiments). C, DNA laddering in HL-60 cells resulting from 3-day exposure to 1 M JS-K (gel representative of three different experiments; MWM,molecular weight marker). D, reversal by the caspase inhibitor C-VAD-FMK of JS-Ks cytostatic and proapoptotic effects on HL-60 cells. Cells were culturedwith JS-K (0.75 M), C-VAD-FMK (50 M), or the combination for 3 days. Cell viability and apoptosis were determined using the MTT assay and flowcytometry, respectively (averages and SE of three separate experiments).




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In Vivo Effects of JS-K. Before determining in vivo an-

tineoplastic activity of JS-K, we next sought to identify a

dose that would not induce significant hypotension in NOD/

SCID mice. Using i.v. administration, we were able to esca-

late the dose of JS-K up to 4 mol/kg without observing

significant hypotension, whereas doses of 5 mol/kg or

higher had a notable hypotensive effect (data not shown).

Using estimates of the mouse blood volume, 4 mol/kg of

JS-K would be expected to yield peak blood levels of 17

M, which is far above its in vitro IC50

.

NOD/SCID mice were then implanted s.c. in the flanks with2.5 106 HL-60 cells/flank. When the tumors were palpable,

treatment was started with JS-K administered i.v. via the tail

vein three times/week at a dose of 4 mol/kg. Control mice

received an equal volume of vehicle through the same route

and according to the same schedule. JS-K treatment in-

duced a significant inhibition of in vivo leukemia cell growth

(Fig. 7A). Sixteen days after starting therapy, the average

tumor volumes in control and JS-K-treated mice were 8.34

0.72 and 3.13 1.14 cm3 (P 0.039), respectively, reflecting

a 50% reduction in tumor volume in treated mice. Histo-

logical analysis of HL-60 cell tumors obtained from vehicle-

treated mice revealed a uniform population of densely

packed myeloblasts. The cells were highly invasive, pene-

trating the surrounding tissues, and showing high mitotic

activity. Necrosis was minimal. On the other hand, histolog-

ical analysis revealed extensive (50%) cell necrosis in

HL-60 cell tumors obtained from the JS-K-treated mice as

compared with 10% in controls (Fig. 7B).

To determine whether JS-K inhibits thein vivo growth of solid

tumor cells, NOD/SCID mice were implanted with 2.5 106

PPC-1 (prostate carcinoma) cells and treated with 4 mol/kg

JS-K or an equal volume of vehicle i.v. three times/week. Similar

to the observation with HL-60 cells, JS-K treatment inhibited

the growth of PPC-1 cells in vivo. Nineteen days after start of

therapy, s.c. tumor implant volumes were 0.368 0.082 and

0.107 0.053 cm3 (P 0.0073) in vehicle and JS-K-treated

animals, respectively (Fig. 7C). Similar to HL-60 cells, PPC-1

cells were highly aggressive and invaded the surrounding tis-

sues. Histological analysis revealed extensive tumor necrosis in

implants obtained from JS-K-treated animals (Fig. 7D).

Discussion

The results suggest that JS-K might serve as a promising

lead compound in the search for new classes of antineoplas-

tic agents with novel mechanisms of action. In a test of its

antiproliferative effect on the HL-60 human myeloid leukemia

cell line, its consistently submicromolar IC50 (three separatedeterminations of 0.2, 0.5, and 0.5 M) compared favorably

with those of the clinical antileukemics daunorubicin (0.01

M) and etoposide (0.3 M). Chemical characterization of

JS-K showed that it resists hydrolysis in the absence of

strong nucleophiles but generates copious NO upon reaction

with GSH. The experiments additionally confirmed predic-

tions based on molecular models of transition state geometry

that two of three major human isoforms of GST could

strongly catalyze NO release from JS-K.

Consistent with our hypothesis that such an NO prodrug

might direct its toxic action selectively to the NO-sensitive

malignant cells while sparing less sensitive normal tissues,

JS-K injected i.v. three times/week inhibited the growth of

s.c. tumors formed from HL-60 xenografts in NOD-SCID

mice by approximately one-half. This suggests that JS-K

may serve as a useful lead for developing therapies against

leukemia.

As far as mechanism of action is concerned, the in vitro

experiments showed all of the characteristics of previous

studies with structurally different NO donors (including in-

duction of both apoptosis and differentiation in HL-60 cells)

and were thus consistent with NO-induced cytolysis. How-

ever, it is important to recognize that other pathways may

also be operative. For example, arylation of thiol groups in

critical protein residues could be contributing to the toxicity;

however, this may be a minor effect, as CDNB (which trans-

fers the same 2,4-dinitrophenyl group to cellular nucleophiles

Fig. 6. Effect of JS-K on differentiation of leukemia cells. HL-60 cellswere cultured with (right) and without (left) 0.5 M JS-K for a period of 5days. Differentiation was assessed morphologically (Wright stain, top) and

by determining NSE (bottom) expression (photomicrographs of one ex-periment representative of three).

Fig. 5. Inhibitory effect of JS-K on three different solid tumor cell lines.PPC-1 (prostate), DLD-1 (colon), and Meth A (mammary) cells were cul-tured with JS-K at the indicated concentrations for 3 days. Cell viabilitywas determined using the MTT assay (averages and SE of three separate

experiments).




9/10

as JS-K but without generating any NO) was an order of

magnitude less potent in the HL-60 screen. Whether there is

synergy between the growth inhibitory effects of NO and

arylation remains to be determined. Transcarbamoylation is

also a possible route to toxicity. The finding that N-acetyl-L-

cysteine prevented the growth inhibitory effects of JS-K

while BSO enhanced them suggests that one or more reac-

tive nitrogen/oxygen species (NO and/or the product of NO s

reaction with other reactive oxygen species) play a key role

in effecting the antineoplastic activity of JS-K.

As to the speci fic molecular targets that may be in-

volved, exposure to JS-K can in principle render any crit-

ical thiol group nucleophilically unreactive by S-nitrosation

or S-(2,4-dinitrophenyl)ation. It is likely that the cysteine-

containing caspases are directly or indirectly among the

mechanistically important targets of JS-Ks action be-

cause the nonspecific caspase inhibitor C-VAD-FMK re-

duced JS-Ks cytostatic and apoptotic effects on HL-60

cells. Other pathways are possible, though, and a fuller

explanation of the drugs antineoplastic activity must

await the outcome of future hypothesis testing.

Whatever the mechanism(s) that may be involved, the

present results support the choice of JS-K as a worthy

lead compound for additional drug discovery efforts. The

significant in vivo activity demonstrated in Fig. 7 was seen

in first generation experiments that could hardly be viewed

as optimized; the dose regimen was chosen based on

preliminary studies of bolus sizes that could be adminis-

tered without inducing systemic hypotension or other

manifestations of toxicity. It is entirely possible that a

continuous administration schedule would be much more

effective. Structural modification aimed at improving the

problematic solubility of this compound could beneficially

affect its absorption, distribution, and transport proper-

ties. With substantial efforts underway in other laborato-

ries4 as well as our own aimed at further characterizing

and refining the chemotherapeutic potential of this inter-

esting lead, we are hopeful that our ultimate goal of intro-

4 L. Jia; K. Tew and V. Findlay; J. Liu and M. Waalkes, personal commu-

nications.

Fig. 7. Effects of i.v. JS-K on growth of both human leukemia and solid tumor xenografts in NOD-SCID mice. A, growth rate of tumors produced when2.5 106 HL-60 cells were implanted s.c. in NOD/SCID mice [3 animals each for JS-K and vehicle treatments]. When tumors became palpable, JS-K wasadministered i.v. at a dose of 4 mol/kg three times/week. Tumor volumes were measured at regular intervals. At the time of experiment termination, tumor

implants were dissected out and analyzed histologically. Differences between vehicle and JS-K values from days 9 16 were significant at the P 0.05 level.B, JS-K induced extensive necrosis in these tumors (photomicrograph from 1 animal representative of 3; H&E stain, 100 magnification). C, growth rateof tumors produced when 2.5 106 human prostate carcinoma (PPC-1) cells were implanted s.c. in NOD/SCID mice (5 animals each for JS-K and vehicletreatments) and followed as in A above. Differences between vehicle and JS-K values were significant at the P 0.05 level for all time points after day 1.D, JS-K induced extensive necrosis in these tumors (photomicrograph from 1 animal representative of 5; H&E stain,100 magnification).




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ducing new and improved treatments for AML and other

malignant diseases will be significantly advanced.

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