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Clinical Trials |
Department of Medicine, Roswell Park Cancer Institute, Buffalo, New York 14263
| ABSTRACT |
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| INTRODUCTION |
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Several studies have shown a relationship between GSH levels and drug resistance (3, 4, 5) . In addition to detoxification, GSH may contribute to resistance indirectly and may enhance the repair of DNA damage caused by anticancer drugs (6, 7, 8, 9) . Increased efflux of GSH-Pt conjugates may contribute to decreased cellular cisplatin accumulation (10 , 11) . The multidrug-resistance-associated protein is either a GSH S-conjugate carrier (12) or requires GSH to pump the drugs out of cells (13) . Recent studies indicate that GSH may be involved in Bcl-2 function (14) .
Data from many experimental models suggest that depletion of cellular
GSH can improve chemotherapy efficacy. Inhibition of the key
biosynthetic enzyme
-glutamylcysteine synthetase by BSO depleted GSH
and enhanced activity for several Pt and alkylating agents in drug
resistance models (15, 16, 17)
. In Phase I trials, a
significant reduction in GSH levels was seen in the PBLs of patients
receiving BSO (18, 19, 20)
.
A metabolite of ifosfamide, chloroacetaldehyde, has been shown to deplete GSH in P388 cells (21) . In one patient, ifosfamide infusion (5 g/m2/8 h) given with an equal dose of mesna depleted GSH in the PBLs to 30% of baseline in 4 h (21) . In another clinical study (22) , a 5-day continuous infusion of 2.43.2 g/m2/day ifosfamide given with mesna (80% of the ifosfamide dose for 5 days) depleted total GSH, total cysteine, and total homocysteine in plasma. The GSH levels in cells were not measured in this study, but a depletion of cellular GSH was predicted from the findings of thiols in plasma. On the basis of this study and on the basis of an earlier study in which mesna alone was given to healthy volunteers (23) , it has been suggested that the effect on thiols in plasma is due to mesna. In a more recent study, Malik et al. (24) reported a maximal 2693% decline of GSH in PBLs on the 3rd day of a 6-day continuous infusion of ifosfamide (1 g/m2/day) given with mesna at the same dose.
Because mesna is an approved cytoprotective agent and is normally given with the anticancer agent ifosfamide, we evaluated the usefulness of the ifosfamide/mesna combination as a possible means of depleting GSH in leukocytes and on GSH and GSH precursors (cysteine and homocysteine) in plasma. The results of a Phase I study to determine the feasibility of GSH depletion by escalating doses of ifosfamide/mesna are reported here. Because there are extensive data that GSH depletion can help overcome resistance to Pt antitumor agents, carboplatin was included at a low-target area under the curve immediately after ifosfamide to take advantage of the expected nadir levels of GSH. The toxicity and response data from the clinical trial will be presented elsewhere.6
| MATERIALS AND METHODS |
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Thiols were measured in 24 patients. Plasma for thiol measurements was
collected before treatment and at 1, 3, 6, 24, 25, 28, and 48 h
after the start of the ifosfamide/mesna infusion. Pooled urine was
collected before the treatment and again at 24 and 48 h after the
beginning of ifosfamide/mesna infusion. The blood and urine were
brought to the laboratory on ice. Plasma and cells were collected
immediately. The WBCs were collected by dextran sedimentation. In three
patients, both WBCs and PBLs were collected for comparison of GSH
levels. PBLs were collected by Ficoll-Hypaque gradient centrifugation.
Plasma was aliquoted into 0.5 ml volumes and stored frozen at -20°C.
Plasma samples stored for GSH measurements received 50 µl of
L-serine/sodium borate to inhibit
-glutamyl
transpeptidase activity, which would otherwise destroy GSH
(26)
. Cells were stored frozen in 5% sulfosalicylic acid
to prevent oxidation of GSH (27)
.
Thiol Measurements.
The following thiols were measured in plasma: total cysteine, total
homocysteine, total GSH and total mesna. In WBC free and total GSH were
measured. In urine total cysteine was measured. "Total"
refers to the free sulfhydryl form plus mixed disulfides of each
of the thiols.
Reverse-phase HPLC with fluorescence detection was used to quantitate all of the thiols after derivatization with monobromobimane (thiolyte), as described previously (28 , 29) . The derivatization of thiols in plasma, urine, and 5% sulfosalicylic acid extracts of cells was carried out for 20 min in the dark at pH 8.0 (30) . Mixed disulfides were reduced to the free form by incubation with DTT before derivatization with thiolyte (27) . The derivatization reaction was stopped by the addition of 50% sulfosalicylic acid, and after removing the precipitated protein by centrifugation, the derivatized thiols in the supernatant were subjected to HPLC. Quantitation was based on identically prepared analytical standards. The completeness of the reduction of disulfides to the free form by DTT was ensured by control experiments, and the entire HPLC procedure for the measurement of thiols in the clinical specimens was validated as described previously (29) . The concentration of thiols was expressed on a per milliliter basis for plasma and on a per milligram of protein basis for the cells. Protein was measured by the Lowry assay (31) .
Measurement of Chloroacetaldehyde in Plasma.
Chloroacetaldehyde in plasma was measured by reverse-phase HPLC with UV
detection after derivatizating chloroacetaldehyde in
deproteinized (perchloric acid) extracts of plasma with thiourea
to 2-aminothiazole (32)
. The aminothiazole was isolated by
solid-phase extraction (Bond-Elut Scx; Analytichem) and subjected to
HPLC. Quantitation was based on identically derivatized
chloroacetaldehyde analytical standards. For the measurements of
chloroacetaldehyde, blood samples were stabilized with formaldehyde
(32)
.
Reducing Property of Mesna.
The reducing ability of mesna was evaluated after incubating mesna with
cystine or homocystine in PBS for 1 h at 37°C. The free cysteine
and homocysteine resulting from the incubation were quantitated by
thiolyte derivatization and HPLC, as described above. The mesna
concentrations (5, 25, and 50 µg/ml) chosen were arbitrary to
represent the low, mid-level, and high concentrations observed for
mesna in plasma at different doses. The concentrations of the thiol
disulfides used in these experiments reflect the physiological
concentrations. DTT (2.5 mM), a typical reducing agent, was
used for comparison.
Statistical Evaluation.
All statistical analyses were carried out using the computer program
Epistat. The decline of thiols in plasma from pretreatment to the nadir
level was evaluated using a paired t test. The correlation
between the dose of ifosfamide/mesna and the extent of the decline in
thiols and of recovery at 48 h were evaluated using linear
regression analysis with calculation of Pearsons correlation
coefficient. The correlation between the total mesna levels in plasma
and those of cysteine or homocysteine in plasma was also evaluated
using linear regression analysis.
| RESULTS |
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Total homocysteine in plasma was measured at doses of 4, 6, 7, and 8
g/m2 (Fig. 2)
. The general profile of decline in
the total homocysteine in plasma in these patients was similar to that
of cysteine and was related to the ifosfamide/mesna dose
(r = 0.9; P = 2 x
10-5). At the highest dose of 8
g/m2, the levels fell below detection limits (<1
µM) by 24 h in two of four patients. The
one patient treated at 7 g/m2/day showed
depletion to 0.5% of the pretreatment level. The mean recovery at
48 h in these patients was 63 ± 17%, 60 ±
25%, and 29 ± 16% at the doses of 4, 6, and 8
g/m2/day. The patient treated at 7
g/m2/day showed a recovery only to 12% of the
pretreatment level. The recovery of homocysteine levels was inversely
correlated to the dose (r = 0.58; P =
3.8 x 10-2).
In contrast to cysteine and homocysteine, the total GSH levels
measured in plasma showed two different patterns (Fig. 3)
. In 15 of 24
patients, there was a moderate depletion of GSH in plasma (Fig. 3
,
pattern A). In these patients, no clear relation was
observed between the drug dose and the decline in plasma GSH. The nadir
GSH levels in this group ranged from 22% to 67% of pretreatment
levels (mean, 53 ± 16%) and were significantly different from
the pretreatment levels (P = 1.7 x
10-6). Unlike the levels of cysteine and
homocysteine, GSH levels started to recover soon after the end of
ifosfamide infusion. In 9 of 24 patients, the GSH levels showed some
random fluctuation, with a suggestion of minor increases (Fig. 3
,
pattern B).
GSH in WBCs.
Free GSH constituted >90% of the total GSH in WBCs. The levels of
free GSH in WBCs are shown in Fig. 4
.
Total GSH in WBCs had an identical profile (data not shown). In
contrast to the observed depletion in thiols in plasma, no decline in
GSH was observed in these cells, even at the highest dose. In three
patients, GSH in PBLs was also measured, and the profile was found to
be similar to that in WBCs, with no decline in the levels (data not
shown).
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Reducing Ability of Mesna.
At concentrations of 5, 25, and 50 µg/ml, mesna produced free
cysteine and homocysteine when incubated with cystine or homocystine
(Fig. 8)
. No free GSH could be recovered
from GS-SG under the same conditions.
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| DISCUSSION |
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-glutamylcysteine synthetase and depleted GSH in PBLs in clinical
trials (18, 19, 20
, 33)
. However, BSO is not yet widely
available for clinical investigations. Limited clinical studies suggest
that ifosfamide/mesna can deplete GSH and its precursors in plasma
(22)
and GSH in PBLs (21
, 24) . The aim of the
current study was to confirm that ifosfamide/mesna can act as a
potential alternative to BSO. Carboplatin was given immediately
after ifosfamide, based on the enhanced cytotoxicity seen after GSH
depletion in in vitro studies. In our trial, ifosfamide/mesna treatment produced a dose-dependent depletion of total plasma cysteine and homocysteine in all patients. At the highest dose levels (7 and 8 g/m2), these thiols declined to 05% of the pretreatment levels in most patients. The rate of recovery of these thiols was also dose dependent. Total plasma cysteine and homocysteine nadir occurred at 24 h after treatment, and minimal levels were maintained throughout mesna infusion.
Our in vitro data show that mesna can reduce cystine and homocystine to cysteine and homocysteine. The total cysteine and homocysteine levels in plasma were inversely related to mesna levels. These results suggest that mesna reduces mixed disulfides to free forms, which are readily cleared by renal excretion, reactions with mesna/ifosfamide metabolites, or tissue uptake (22 , 23) . The increased urinary total cysteine excretion seen during and after ifosfamide/mesna treatment supports this hypothesis.
In contrast, plasma GSH showed only a modest decline in only 60% of
the patients, with no clear dose-response relationship. The GSH nadir
occurred at 24 h after treatment, followed by immediate recovery.
These data, plus the observation that mesna could not reduce
GS-SG to GSH in vitro, suggest that GSH depletion may
depend mostly on direct reactions with ifosfamide metabolites. This is
possible because
65% of total plasma GSH is in the free form as
compared with <5% of total cysteine and homocysteine
(22)
. Reactions between GSH and ifosfamide metabolites are
shown to be catalyzed by GST-
(34
, 35)
. Therefore, the
modest depletion of GSH may reflect levels of GST-
in plasma and
interindividual variation in the GST-
genotype/phenotype (36
, 37)
. This hypothesis does not preclude GSH from also reacting
with mesna.
Peak chloroacetaldehyde levels in plasma ranged from 1035 µM, well below the 1 mM concentration that significantly depleted GSH in P388 cells in vitro (21) . Chloroacetaldehyde levels did not correlate with plasma GSH depletion.
Although significant depletion of GSH precursors was apparent in
plasma, no depletion of GSH in was seen in WBCs, contrary to previous
reports (24)
. This unexpected result was not an artifact
of differing GSH assay methodologies: an enzymatic method
(24)
and our HPLC assay yielded similar results when
tested in parallel in a nonselected subset of patients (data not
shown). These observations suggest that: (a) mesna does not
enter WBCs because this could deplete GSH by mesna-GSH conjugation;
(b) free cysteine and homocysteine from reduction of mixed
disulfides may enter WBCs and support new GSH synthesis
(23)
; and (c) GST-
activity in WBCs may not
be high enough to deplete GSH through conjugation to ifosfamide
metabolites.
Our results suggest that prolonged infusion of mesna alone could potentially deplete cellular GSH by limiting precursors for GSH synthesis. If nadir levels of total plasma cysteine and homocysteine were maintained for a prolonged time, substrates for GSH synthesis would not be available. Along these lines, it is interesting to note that the results from one study with a more prolonged infusion schedule (a 6-day continuous ifosfamide/mesna infusion) than ours decreased PBL GSH levels by 2593% in 9 of 14 patients with advanced ovarian cancer (24) .
Clinical toxicity and response data from our study will be published separately.6 However, a brief comment on thiol depletion and clinical end points is appropriate. The maximum tolerated dose for ifosfamide was 6 g/m2; neutropenia and thrombocytopenia were dose-limiting. Cysteine and homocysteine depletion increased with ifosfamide/mesna dose; therefore, it is difficult to separate thiol effects from dose effects. Similarly, no relationship between GSH levels and toxicity was apparent. Only one objective response was observed (a partial response was seen in a patient with advanced colorectal cancer who was treated at the 8 g/m2 dose level). Interestingly, this patient had the greatest plasma GSH depletion of all (78%, from baseline); he did not experience qualitatively or quantitatively greater toxicity than other patients at the same dose level. Our observation suggests that further examination of the relationships between plasma GSH depletion, response to chemotherapy, and toxicity is warranted.
In conclusion, our data suggest the following: (a) mesna significantly depletes cysteine and homocysteine in plasma by reducing mixed disulfides to free forms, which are then cleared; (b) depletion of plasma GSH may be due to direct reactions between free GSH and ifosfamide metabolites and/or mesna; and (c) the lack of effect on WBC GSH may be related to the inability of mesna to enter these cells and to an inadequate duration of infusion. The individual effects of ifosfamide and mesna on plasma thiol modulation are unclear, and this relationship could be clarified by a randomized cross-over trial comparing the effects of a continuous infusion of mesna versus ifosfamide/mesna on thiol levels. In light of the extensive preclinical data implicating GSH in resistance to chemotherapy, it seems reasonable to perform additional investigations of clinically available, well-tolerated potential biochemical modulators.
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| FOOTNOTES |
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1 Supported by National Cancer Institute Grant
CA-62509. ![]()
2 To whom requests for reprints should be
addressed, at Department of Medicine, Roswell Park Cancer Institute,
Elm and Carlton Streets, Buffalo, NY 14263. Phone: (716)
845-3287; Fax: (716) 845-1659; E-mail: lakshmi.pendyala{at}roswellpark.org ![]()
3 Present address: Divisions of Medical Sciences
and Population Sciences, Fox Chase Cancer Center, Philadelphia, PA
19111. ![]()
4 Present address: Department of Medicine,
Dartmouth-Hitchcock Medical Center, Lebanon, NH 03756. ![]()
5 The abbreviations used are: GSH, glutathione;
BSO, buthionine sulfoximine; PBL, peripheral blood lymphocyte; HPLC,
high-performance liquid chromatography; Pt, platinum; GST glutathione
S-transferase; AUC, area under the curve; GS-SG,
glutathione disulfide. ![]()
6 R. P. Perez, L. Perdyala, G. Schwartz, N.
Meropol, and P. J. Creaven. A Phase I clinical trial of
ifosfamide/Mesna/carboplatin, manuscript in preparation. ![]()
Received 8/13/99; revised 12/14/99; accepted 12/20/99.
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