Juvenile exposure and adult risk assessment with single versus repeated exposure of melphalan in the germ cells of male SD rat: Deciphering the molecular mechanisms

Melphalan significantly contributes to the increase in childhood cancer survival rate. It acts as a gonadotoxic agent and leads to testes damage, dysbalance in gonadal hormones, and impairment in the germ cell proliferation. Therefore, it might be a potent threat to male fertility in individuals who have undergone melphalan treatment during childhood cancer. However, the molecular mechanisms of melphalan-induced gonadal damage are not yet fully explored and they need to be investigated to determine the benefit-risk profile. In the present study, juvenile male SD rats were subjected to single and intermittent cycles of melphalan exposure in a dose-dependent (0.375, 0.75 and 1.5 mg/kg) manner. Methods of end-points evaluations were quantification of micronuclei formation in peripheral blood, sperm count, sperm motility and head morphology, sperm and testicular DNA damage, histological studies in testes, oxidative/nitrosative stress parameters. A single cycle of exposure at high dose (1.5 mg/kg) produced significant effect on micronuclei formation only after the first week of exposure, whereas failed to produce significant effect at the end of the sixth week. Intermittent cycles of exposure at the dose of 1.5 mg/kg produced significant alterations in all the parameters (micronuclei in peripheral blood, testes and epididymides weight and length, MDA, GSH and nitrite levels, sperm count and motility, sperm head morphology, testicular and sperm DNA damage, protein expression in testes and histological parameters). So, time of exposure as well as the amount of exposure (total dosage administered) is critical in determining the magnitude of the damage in germ cell risk assessment.

Melphalan induces cardiotoxicity through oxidative stress in cardiomyocytes derived from human induced pluripotent stem cells

BACKGROUND

Treatment-induced cardiotoxicity is a leading noncancer-related cause of acute and late onset morbidity and mortality in cancer patients on antineoplastic drugs such as melphalan-increasing clinical case reports have documented that it could induce cardiotoxicity including severe arrhythmias and heart failure. As the mechanism by which melphalan impairs cardiac cells remains poorly understood, here, we aimed to use cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CMs) to investigate the cellular and molecular mechanisms of melphalan-induced cardiotoxicity.

METHODS

hiPSC-CMs were generated and treated with clinically relevant doses of melphalan. To characterize melphalan-induced cardiotoxicity, cell viability and apoptosis were quantified at various treatment durations. Ca2+ transient and contractility analyses were used to examine the alterations of hiPSC-CM function. Proteomic analysis, reactive oxygen species detection, and RNA-Sequencing were conducted to investigate underlying mechanisms.

RESULTS

Melphalan treatment of hiPSC-CMs induced oxidative stress, caused Ca2+ handling defects and dysfunctional contractility, altered global transcriptomic and proteomic profiles, and resulted in apoptosis and cell death. The antioxidant N-acetyl-L-cysteine attenuated these genomic, cellular, and functional alterations. In addition, several other signaling pathways including the p53 and transforming growth factor-β signaling pathways were also implicated in melphalan-induced cardiotoxicity according to the proteomic and transcriptomic analyses.

CONCLUSIONS

Melphalan induces cardiotoxicity through the oxidative stress pathway. This study provides a unique resource of the global transcriptomic and proteomic datasets for melphalan-induced cardiotoxicity and can potentially open up new clinical mechanism-based targets to prevent and treat melphalan-induced cardiotoxicity.

Iron Overload Exacerbates Busulfan-Melphalan Toxicity Through a Pharmacodynamic Interaction in Mice

PURPOSE

Busulfan-melphalan high-dose chemotherapy followed by autologous stem cell transplantation is an essential consolidation treatment of high-risk neuroblastoma in children. Main treatment limitation is hepatic veno-occlusive disease, the most severe and frequent extra-hematological toxicity. This life threatening toxicity has been related to a drug interaction between busulfan and melphalan which might be increased by prior disturbance of iron homeostasis, i.e. an increased plasma ferritin level.

METHODS

We performed an experimental study of busulfan and melphalan pharmacodynamic and pharmacokinetics in iron overloaded mice.

RESULTS

Iron excess dramatically increased the toxicity of melphalan or busulfan melphalan combination in mice but it did not modify the clearance of either busulfan or melphalan. We show that prior busulfan treatment impairs the clearance of melphalan. This clearance alteration was exacerbated in iron overloaded mice demonstrating a pharmacokinetic interaction. Additionally, iron overload increased melphalan toxicity without altering its pharmacokinetics, suggesting a pharmacodynamic interaction between iron and melphalan. Based on iron homeostasis disturbance, we postulated that prior induction of ferritin, through Nrf2 activation after oxidative stress, may be associated with the alteration of melphalan metabolism.

CONCLUSION

Iron overload increases melphalan and busulfan-melphalan toxicity through a pharmacodynamic interaction and reveals a pharmacokinetic drug interaction between busulfan and melphalan.

[Melphalan pharmacokinetics during isolated limb regional perfusion in patients with skin melanoma and soft tissue sarcoma]

The study of pharmacokinetics of melphalan in the perfusate and blood plasma during isolated limb regional perfusion (ILRP) was carried out in patients with melanoma (n=21) and soft tissue sarcoma (n = 24). Melphalan was administered as 10 mg/l for a lower extremity and 13 mg/l for a upper extremity. Quantification of melphalan in perfusate and blood samples was performed by means of liquid chromatography/tandem mass spectrometry. 30 samples of the perfusate and 27 venous blood samples were analyzed. During the first 5 minutes of ILRP concentration of melphalan in the perfusate decreased to 13.2% of the initial value, and by the end of perfusion (60 minutes) it was 3.3%. The amount of melphalan in the blood plasma of the patients by the end of ILRP wasn't higher than 1.6% from the administered dose. That demonstrates minor systemic absorption of the drug during ILRP. Moreover melphalan concentration in the blood plasma during the perfusion was in average 0.015-0.223 mg/l which is significantly lower compared to the blood plasma concentrations after intravenous administration of melphalan. Thus ILRP procedure provided 97% of the melphalan dose accumulation in the soft tissues of a limb and in tumor tissues. Also pharmacokinetic advantage of melphalan over systemic administration of the drug was shown.

Separation and identification of trinucleotide-melphalan adducts from enzymatically digested DNA using HPLC-ESI-MS

Melphalan is a bifunctional alkylating agent that covalently binds to the nucleophilic sites present in DNA. In this study we investigated oligonucleotides prepared enzymatically from DNA modified with melphalan. Calf thymus DNA was incubated in-vitro with melphalan and the resulting modifications were enzymatically cleaved by means of benzonase and nuclease S1. Efficient sample preconcentration was achieved by solid-phase extraction, in which phenyl phase cartridges resulted in better recovery of the modified species than C(18). The applied enzymatic digestion time resulted in production of trinucleotide adducts which were efficiently separated and detected by use of reversed-phase HPLC coupled to an ion-trap mass spectrometer with electrospray ionization. It was assumed that melphalan could act as both a monofunctional and bifunctional alkylating agent. Mono-alkylated adducts were much more abundant, however, and the alkylation site was located on the nucleobases. On the other hand, we unequivocally identified cross-link formation in DNA, even though at low abundance and only a few adduct types were detected.

Flow cytometry analysis of single-strand DNA damage in neuroblastoma cell lines using the F7-26 monoclonal antibody

The F7-26 monoclonal antibody (Mab) has been reported to be specific for single-strand DNA damage (ssDNA) and to also identify cells in apoptosis. We carriedout studies to determine if F7-26 binding measured by flow cytometry was able to specifically identify exogenous ssDNA as opposed to DNA damage from apoptosis. Neuroblastoma cells were treated with melphalan (L-PAM), fenretinide, 4-hydroperoxycyclophosphamide (4-HC)+/-pan-caspase inhibitor BOC-d-fmk, topotecan or with 10Gy gamma radiation+/-hydrogen peroxide (H2O2) and fixed immediately postradiation. Cytotoxicity was measured by DIMSCAN digital imaging fluorescence assay. The degree of ssDNA damage was analyzed by flow cytometry using Mab F7-26, with DNA visualized by propidium iodide counterstaining. Flow cytometry was used to measure apoptosis detected by terminal deoxynucleotidyltransferase (TUNEL) assay and reactive oxygen species (ROS) by carboxy-dichlorofluorescein diacetate. Irradiated and immediately fixed neuroblastoma cells showed increased ssDNA, but not apoptosis by TUNEL (TUNEL-negative). 4-HC or L-PAM+/-BOC-d-fmk increased ssDNA (F7-26-positive), but BOC-d-fmk prevented TUNEL staining. Fenretinide increased apoptosis by TUNEL but not ssDNA damage detected with F7-26. Enhanced ssDNA in neuroblastoma cells treated with radiation+H2O2 was associated with increased ROS. Topotecan increased both ssDNA and cytotoxicity in 4-HC-treated cells. These data demonstrate that Mab F7-26 recognized ssDNA due to exogenous DNA damage, rather than apoptosis. This assay should be useful to characterize the mechanism of action of antineoplastic drugs.

Proline prodrug of melphalan, prophalan-l, demonstrates high therapeutic index in a murine melanoma model

The therapeutic efficacy of prophalan-L, the L-proline prodrug of melphalan that demonstrated prolidase-dependent bioactivation to melphalan, was examined in vivo in a mouse melanoma model. Prophalan-L exhibited 2- to 2.5-fold higher hydrolytic and cytotoxic activity than prophalan-D, the D-analog, in B16-F10 murine melanoma cells in vitro. Prophalan-L cytotoxicity in B16-F10 cells was lower (GI50=221 microM) than that of melphalan (GI50=173 microM). The tumor growth profiles in C57BL/6J mice injected with B16-F10 cells and treated with melphalan (5.5 microg/g i.p.) and equimolar concentrations of the prodrugs demonstrated significant difference between the control (buffered saline) and melphalan or prophalan-L but no significant difference between control and prophalan-D or between melphalan and prophalan-L. Prophalan-L was significantly less toxic than melphalan, while no significant difference was observed in toxicity, measured as percent weight loss, between the prodrugs and saline control. Tumor reduction efficacy at high doses (12 microg/g i.p.) was similar for melphalan and prophalan-L; however, fatal toxicity was associated with melphalan while prophalan-L exhibited significantly lower systemic toxicity. An excellent correlation between GI50 and tumor reduction efficacy was observed for the tested drugs (r2=0.95). Prophalan-L thus demonstrates higher therapeutic index than melphalan in the murine melanoma model.

Proline Prodrug of Melphalan Targeted to Prolidase, a Prodrug Activating Enzyme Overexpressed in Melanoma

PURPOSE

To determine the bioactivation and uptake of prolidase-targeted proline prodrugs of melphalan in six cancer cell lines with variable prolidase expression and to evaluate prolidase-dependence of prodrug cytotoxicity in the cell lines compared to that of the parent drug, melphalan.

MATERIALS AND METHODS

Hydrolysis, cell uptake, and cell proliferation studies of melphalan and the L: - and D: -proline prodrugs of melphalan, prophalan-L: and prophalan-D: , respectively, were conducted in the cancer cell lines using established procedures.

RESULTS

The bioactivation of prophalan-L: in the cancer cell lines exhibited high correlation with their prolidase expression levels (r (2) = 0.86). There were no significant differences in uptake of melphalan and its prodrugs. The cytotoxicity of prophalan-L: (GI(50)) in cancer cells also showed high correlation with prolidase expression (r (2) = 0.88), while prophalan-D: was ineffective at comparable concentrations. A prolidase targeting index (ratio of melphalan to prophalan-L: cytotoxicity normalized to their uptake) was computed and showed high correlation with prolidase expression (r (2) = 0.82).

CONCLUSIONS

The data corroborates the specificity of prophalan-L: activation by prolidase as well as prolidase-targeted cytotoxicity of prophalan-L: in cancer cell lines. Hence, prophalan-L: , a stable prodrug of melphalan, exhibits potential for efficiently targeting melanoma with reduced systemic toxicity.

Characterization of a CC49-based single-chain fragment-beta-lactamase fusion protein for antibody-directed enzyme prodrug therapy (ADEPT)

CC49 is a clinically validated antibody with specificity for TAG-72, a carbohydrate epitope that is overexpressed and exposed on the cell surface in a large fraction of solid malignancies. We constructed a single-chain fragment (scFv) based on CC49 and fused it to beta-lactamase (BLA). Following optimization of the scFv domain by combinatorial consensus mutagenesis (CCM) for increased expression and stability, we characterized the protein variant for binding, in vivo pharmacokinetics (PK), and antitumor efficacy. The fusion protein TAB2.5 possessed a similar binding specificity relative to the parent antibody CC49. TAB2.5 also showed prolonged retention (T(1/2) = 36.9 h) in tumor-bearing mice with tumor/plasma ratios of up to 1000. Preliminary evaluation of TAB2.5, in combination with a novel prodrug, GC-Mel, resulted in significant efficacy in a colorectal xenograft tumor model and supports the utility of the protein as an agent for tumor-selective prodrug activation.

Metabolism of melphalan by rat liver microsomal glutathione S-transferase

One of the major problems in the treatment of human cancer is the phenomenon of drug resistance. Increased glutathione (gamma-glutamylcysteinylglycine, GSH) conjugation (inactivation) due to elevated level of cytosolic glutathione S-transferase (GST) is believed to be an important mechanism in tumor cell resistance. However, the potential involvement of microsomal GST in the establishment of acquired drug resistance (ADR) remains uncertain. In our experiments, a combination of liquid chromatography/electrospray ionization/mass spectrometry (LC/ESI/MS) was employed for structural characterization of the resulting conjugates between GSH and melphalan, one of the alkylating agents. The spontaneous reaction of 1mM melphalan with 5mM GSH at 37 degrees C in aqueous phosphate buffer for 1h gave primarily the monoglutathionyl and diglutathionyl melphalan derivatives, with small amounts of mono- and dihydroxy melphalan derivatives. We demonstrated that rat liver microsomal GST presented a strong catalytic effect on the reaction as determined by the increase of monoglutathionyl and diglutathionyl melphalan derivatives and the decrease of melphalan. We showed that microsomal GST was activated by melphalan in a concentration- and time-dependent manner. Microsomal GST which was stimulated approximately 1.5-fold with melphalan had a stronger catalytic effect. Thus microsomal GST may play a potential role in the metabolism of melphalan in biological membranes, and in the development of ADR.

The role of hyperthermia in regional alkylating agent chemotherapy

The role of hyperthermia during regional alkylating agent chemotherapy is controversial. The aim of this study was to determine the exact contribution of hyperthermia to tumor response during isolated limb infusion with l-phenylalanine mustard. Rats bearing rodent fibrosarcoma on the hindlimb underwent isolated limb infusion with saline, saline plus heat, l-phenylalanine mustard, l-phenylalanine mustard under conditions of normothermia, or l-phenylalanine mustard plus hyperthermia. Heat was administered locally using an in-line hot water circulation loop. Treatment with l-phenylalanine mustard at a concentration of 15 or 50 micrograms/mL was ineffective at producing tumor growth delay (P = 0.24 and 0.41, respectively). Furthermore, thermal enhancement of l-phenylalanine mustard activity was not seen at 15 micrograms/mL. However, administration of high-dose l-phenylalanine mustard, 50 micrograms/mL, with increasing amounts of heat yielded increasing tumor growth delay, increased regressions, and decreased proliferative index. Although l-phenylalanine mustard infusion under normothermia yielded a tumor growth delay of 7.1 days, combination l-phenylalanine mustard + hyperthermia treatment produced tumor growth delay of 27.0 days (P < 0.01; with two of five animals showing a complete response). Four hours after isolated limb infusion, 50.9% of cells in tumor treated with l-phenylalanine mustard + hyperthermia experienced apoptosis, whereas only 18.1, 16, and 4.4% of cells underwent apoptosis after treatment with l-phenylalanine mustard, saline + hyperthermia, or saline. The mean concentration of l-phenylalanine mustard within tumor relative to perfusate following isolated limb infusion was found to be similar among all groups at 0.023, 0.025, and 0.032 in animals undergoing isolated limb infusion with l-phenylalanine mustard, l-phenylalanine mustard + normothermia, and l-phenylalanine mustard + hyperthermia, respectively. These data indicate a synergistic cytotoxic effect of l-phenylalanine mustard + hyperthermia in isolated limb infusion, which is not attributable to enhanced tumor drug uptake.

Prolidase, a Potential Enzyme Target for Melanoma: Design of Proline-Containing Dipeptide-like Prodrugs

Bioinformatics tools such as Perl, Visual Basic, Cluster, and TreeView were used to analyze public gene expression databases in order to identify potential enzyme targets for prodrug strategies. The analyses indicated that prolidase might be a desirable enzyme target based on its differential expression in melanoma cancer cell lines and its high substrate specificity for dipeptides containing proline at the carboxy terminus. RT-PCR expression of prolidase and hydrolytic activity against N-glycyl-l-proline (GLY-PRO), a standard substrate of prolidase, determined in tumor cell lines, exhibited a high correlation (r(2) = 0.95). These results suggest the possibility of targeting prolidase with prodrugs of anticancer agents for enhanced selectivity. The feasibility of such a scenario was tested by (a) synthesizing prodrugs of melphalan that comprised linkage of the carboxy terminus of the l-phenylalanine moiety of melphalan to the N-terminus of l and d stereoisomers of proline and (b) determining their bioconversion and antiproliferative activities in SK-MEL-5 cells, a melanoma cancer cell line with high expression levels of prolidase. The results of hydrolysis studies of the l- and d-proline prodrugs of melphalan, designated as prophalan-l and prophalan-d, respectively, indicated a approximately 7-fold higher rate of activation of prophalan-l compared to prophalan-d in SK-MEL-5 cell homogenates. Prophalan-l exhibited cytotoxicity (GI(50) = 74.8 microM) comparable to that of melphalan (GI(50) = 57.0 microM) in SK-MEL-5 cells while prophalan-d was ineffective, suggesting that prolidase-specific activation to the parent drug may be essential for cytotoxic action. Thus, melphalan prodrugs such as prophalan-l that are cleavable by prolidase offer the potential for enhanced selectivity by facilitating cytotoxic activity only in cells overexpressing prolidase.

Implications of enzymatic, acidic and thermal hydrolysis of DNA on the occurrence of cross-linked melphalan DNA adducts

Calf thymus DNA was treated with melphalan, a nitrogen mustard, and the formation of melphalan cross-linked DNA adducts was investigated. These cross-linked adducts could not be detected either in the enzymatically or in the thermally generated DNA hydrolysates. However, a search for DNA cross-linked adducts in the hydrolysates obtained under acidic conditions revealed the presence of different types of cross-links, mainly containing an adenine moiety. These results are very important because they show that the detection of cross-links is dependent on the hydrolytic procedure used and that these cross-linked adducts are formed under totally different reaction conditions from those in in vivo situations. This can explain the very low abundance or even the absence of cross-linked adducts in nitrogen mustard treated animals. The generally accepted theory that the anti-cancer activity of bifunctional mustards such as melphalan is due to cross-linking of DNA strands remains therefore from our point of view questionable.

Caspase-11 is not necessary for chemotherapy-induced intestinal mucositis

Mucositis is a common, dose-limiting toxicity associated with drug and radiation therapy for cancer. The ulcerative lesions of mucositis serve as systemic portals of entry for the micro-organisms that inhabit the mucosa of the gastrointestinal tract and the oral cavity, often leading to systemic infection. The pathogenesis of mucositis is complex, and consists of varying, sequential interactions between pro-inflammatory cytokines, transcription factors, and pro-apoptotic pathways of the mucosal epithelium and the cells and tissues within the submucosa. A possible mechanism for mucositis injury is the activation of caspases, a family of cysteine proteases. Caspase-11, one of 14 members of this enzymatic family, was studied to determine its role in the development of intestinal mucositis after exposure to melphalan in caspase-11 wild-type (+/+) and knockout (-/-) mice. Immunoblots demonstrated the activation of caspase-11 in duodenal and jejunal samples 24 and 48 h after melphalan administration. No significant differences in the level of intestinal cell death or macrophage infiltration, as measured by TUNEL staining and immunohistochemistry, were present between wildtype (+/+) and knockout (-/-) mice. These findings suggest that while caspase-11 activation occurs in response to melphalan, it does not have a primary role in the pathogenesis of intestinal mucositis.

p53 Elevation in Relation to Levels and Cytotoxicity of Mono- and Bifunctional Melphalan-DNA Adducts

We tested the hypothesis that bifunctional DNA adducts formed by a nitrogen mustard-based anticancer drug were more efficient than monofunctional adducts at causing elevation of p53, consistent with the difference in cytotoxicity. Human leukemia cell line ML-1 was exposed for 1 h to melphalan or its monofunctional derivative monohydroxymelphalan. Levels of DNA adducts, measured by specific immunoassay, were linearly related to the concentration of alkylating agent. Monohydroxymelphalan formed twice as many adducts as did equal concentrations of melphalan. After the removal of the alkylating agent, adduct levels were maintained or increased slightly up to 8 h and then decreased by 27 to 44% by 24 h. Alkaline elution analyses confirmed the absence of detectable DNA interstrand cross-links in cells exposed to monohydroxymelphalan. DNA single-strand breaks were detected after monohydroxymelphalan but not after melphalan. Levels of p53 were quantified by sensitive fluorogenic enzyme-linked immunosorbent assay at intervals up to 24 h after exposure of cells to various concentrations of melphalan and monohydroxymelphalan. The level of initially formed DNA adducts needed to cause elevation of p53 from a baseline level of 0.5 ng/mg total protein to 2 ng/mg was 5- to 8-fold higher for monohydroxymelphalan than melphalan. The concentrations of melphalan and monohydroxymelphalan (+/-S.D.) causing 50% growth inhibition were 1.2 +/- 0.4 and 28.1 +/- 1.6 microg/ml, respectively, a 23-fold difference. The adduct levels induced by these exposures were 9.3 and 420 nmol/g DNA for melphalan and monohydroxymelphalan, respectively, a 45-fold difference, which is considerably greater than the difference in efficacy at elevating p53.

Engineering a Thermostable Human Prolyl Endopeptidase for Antibody-Directed Enzyme Prodrug Therapy

We present a new antibody-directed enzyme prodrug therapy strategy (ADEPT) based on a post-proline cleaving endopeptidase and prodrugs, in which cytotoxic moieties are linked to a proline-containing peptide. Human prolyl endopeptidase was expressed in Escherichia coli and purified to homogeneity. The enzyme was active in buffer and in human serum but was rapidly thermally inactivated by incubation at 37 degrees C, thus preventing applications in vivo. While prolyl endopeptidase display on filamentous phage abolished viral infectivity and prevented directed evolution strategies based on phage display, we robotically screened 10752 individual colonies of mutant enzymes using a fluorogenic assay to improve enzyme stability. A single amino acid mutation (Glu289 --> Gly) improved protein stability, resulting in a half-life of 16 h at 37 degrees C in phosphate buffer. Two prodrugs were synthesized, in which an N-protected glycine-proline dipeptide was covalently coupled to doxorubicin and melphalan. (Benzyloxycarbonyl)glycylprolylmelphalan, but not the more sterically hindered doxorubicin prodrug, could be efficiently activated by prolyl endopeptidase [specific activity = 813.3 nmol min(-1) (mg of enzyme)(-1) at 25 degrees C]. The melphalan prodrug was essentially nontoxic to CHO, F9 teratocarcinoma, MCF7 breast adenocarcinoma, and p3U1 mouse myeloma cells up to millimolar concentrations, while prodrug incubation with the engineered prolyl endopeptidase mutant led to a cell killing profile superimposable to the one of melphalan. The prolyl endopeptidase mutant was then chemically coupled to the human antibody L19, specific to the EDB domain of fibronectin, a marker of angiogenesis. The resulting immunoconjugate retains antigen binding and enzymatic activity, thus opening the way to anticancer ADEPT applications.

Antitumor activity of the alkylating oligopeptides J1 (L-melphalanyl-p-L-fluorophenylalanine ethyl ester) and P2 (L-prolyl-m-L-sarcolysyl-p-L-fluorophenylalanine ethyl ester): comparison with melphalan

Peptichemio, a mixture of six short oligopeptides all comprising the alkylating amino acid m-L-sarcolysin, has shown clinical activity in several malignancies. Previous studies have suggested that activity mainly resides in one of the peptides, P2 (L-prolyl-m-L-sarcolysyl-p-L-fluorophenylalanine ethyl ester). In the present study the in vitro activity of P2 was further investigated and compared to melphalan and the novel alkylating dipeptide J1 (L-melphalanyl-p-L-fluorophenylalanine ethyl ester), which is structurally related to P2 and melphalan. Cytotoxic activity was studied using patient tumor cells in a non-clonogenic cytotoxicity assay, whereas cellular response, and kinetics thereof, were studied in the lymphoma cell line U-937 GTB. Cellular metabolism was studied using microphysiometry, kinetic effects on macromolecular synthesis by radiolabeled substrate incorporation and, finally, the microculture kinetic assay of apoptosis was used to monitor morphologic changes following drug exposure. The assays compared P2 favorably with melphalan. Interestingly J1 was even more cytotoxic, and produced more pronounced effects in the kinetic assays for macromolecular synthesis, metabolic activity and apoptosis. The results indicate that the delivery properties of J1 are improved compared to those of melphalan and P2.

Proline analogue of melphalan as a prodrug susceptible to the action of prolidase in breast cancer MDA-MB 231 cells

Proline analogue of melphalan (Mel-pro) was synthesized as a prodrug susceptible to the action of ubiquitously distributed, cytosolic imidodipeptidase-prolidase [E.C.3.4.13.9]. Conjugation of melphalan (Mel) with proline (Pro) through imido-bond resulted in formation of a good substrate for prolidase. Cytosolic location of prolidase in neoplastic cell suggests that proline analogue of melphalan (Mel-pro) may serve as a prolidase convertible prodrug. We have compared several aspects of pharmacologic actions of Mel and Mel-pro in estrogen-independent breast cancer MDA-MB 231 cells. It has been found that Mel-pro is more effectively transported into the MDA-MB 231 cells, evokes higher cytotoxicity, similar inhibitory effect on DNA synthesis, lower inhibitory effect on collagen biosynthesis and reduces IGF-I receptor and MAPkinase expression in MDA-MB 231 cells, compared to Mel. The results suggest that targeting of prolidase as a Mel-pro-converting enzyme may serve as a potential strategy in pharmacotherapy of breast cancer.

Fludarabine uptake mechanisms in B-cell chronic lymphocytic leukemia

Nucleoside derivatives are currently used in the treatment of hematologic malignancies. Although intracellular events involved in the pharmacologic action of these compounds have been extensively studied, the role of plasma membrane transporters in nucleoside-derived drug bioavailability and action in leukemia cells has not been comprehensively addressed. We have monitored the amounts of mRNA for the 5 nucleoside transporter isoforms cloned so far (CNT1, CNT2, CNT3, ENT1, and ENT2) in several human cell types and in normal human leukocytes. We then examined the expression patterns of these plasma membrane proteins in patients with chronic lymphocytic leukemia (CLL) and correlated them with in vitro fludarabine cytotoxicity. Despite a huge individual variability in the mRNA amounts for every transporter gene expressed in CLL cells (CNT2, CNT3, ENT1, and ENT2), no relationship between mRNA levels and in vitro fludarabine cytotoxicity was observed. Fludarabine accumulation in CLL cells was mostly, if not exclusively, mediated by ENT-type transporters whose biologic activity was clearly correlated with fludarabine cytotoxicity, which reveals a role of ENT-mediated uptake in drug responsiveness in patients with CLL.

Covalent binding of nitrogen mustards to the cysteine-34 residue in human serum albumin

Covalent binding of various clinically important nitrogen mustards to the cysteine-34 residue of human serum albumin, in vitro and in vivo, is demonstrated. A rapid method for detection of these adducts is presented, based on liquid chromatography-tandem mass spectrometry analysis of the adducted tripeptide Cys*-Pro-Phe after digestion of the protein with Pronase.

On the release and half-life of S100B protein in the peripheral blood of melanoma patients

The aim of the present work was to investigate the origin and half-life of endogenous S100B protein reported by many investigators as a useful melanoma serum marker. Within cells, S100B protein exists in homo- or heterodimer form containing mainly Ca(++), having a substantial fraction bound to membranes. As such, S100B is believed to be involved in the regulation of cytoskeleton. Also, a role in the cell cycle progression has been suggested. Although S100B appears having important intracellular functions, proofs of its secretion, at least at concentrations such as the ones measured in melanoma patients, are still lacking. Consistent with this view is the fact that immunohistology for S100 protein reported by numerous authors clearly indicate an exclusive intracellular staining. For these reasons, it was of a major interest to investigate how and when S100B is shed to the blood. Knowing that significant S100B levels are seen only in stage IV patients, we hypothesized that cell death may be the major source of circulating S100B protein in these patients. This hypothesis was studied in an in vitro model simulating cell death and in vivo in melanoma and other cancer patients undergoing highly cytotoxic regional immunochemotherapy using isolated limb perfusion with tumor necrosis factor and melphalan, as well as in tumor exudates and pleural fluids. Our results strongly suggest melanoma and endothelial cell death and subsequent continuous drainage to the blood as the major mechanism behind S100B release to the blood circulation. We estimated the endogenous S100B protein half-life to be about 30 min.

Role of multidrug resistance protein 1 (MRP1) and glutathione S-transferase A1-1 in alkylating agent resistance. Kinetics of glutathione conjugate formation and efflux govern differential cellular sensitivity to chlorambucil versus melphalan toxicity

We investigated the role of phase II (conjugation) and phase III (efflux) detoxification of the anticancer drugs melphalan (MLP) and chlorambucil (CHB). Although both drugs are substrates of Alpha-class glutathione S-transferases (GST) and the monoglutathionyl conjugates formed in these enzymatic reactions are transported by MRP1, we found that GSTA1-1 and MRP1 acted in synergy to confer resistance to CHB but not to MLP (Morrow, C. S., Smitherman, P. K., Diah, S. K., Schneider, E., and Townsend, A. J. (1998) J. Biol. Chem. 273, 20114-20120). To explain this selectivity of MRP1/GST-mediated resistance, we report results of side-by-side experiments comparing the kinetics of MLP- versus CHB-glutathione conjugate: formation, product inhibition of GSTA1-1 catalysis, and transport by MRP1. The monoglutathionyl conjugate of CHB, CHB-SG, is a very strong competitive inhibitor of GSTA1-1 (K(i) 0.14 microM) that is >30-fold more potent than that of the corresponding conjugate of MLP, MLP-SG (K(i) 4.7 microM). The efficiency of GSTA1-1-mediated monoglutathionyl conjugate formation is more than 4-fold higher for CHB than MLP. Lastly, both CHB-SG and MLP-SG are efficiently transported by MRP1 with similar V(max) although the K(m) for CHB-SG (0.37 microm) is significantly lower than for MLP-SG (1.1 microM). These results indicate that MRP1 is required for GSTA1-1-mediated resistance to CHB in order to relieve potent product inhibition of the enzyme by intracellular CHB-SG formed. The kinetic properties of MRP1 are well suited to eliminate CHB-SG at pharmacologically relevant concentrations. For MLP detoxification, where product inhibition of GSTA1-1 is less important, GSTA1-1 does not confer resistance because of the relatively poorer catalytic efficiency of MLP-SG formation. Similar analyses can be useful for predicting the pharmacological and toxicological consequences of MRP and GST expression on cellular sensitivity to various other electrophilic xenobiotics.

[Chemobiokinetics of sarcolysin and its peptides with glutaminic acid]

A 14C study of chemobiokinetics of sarcolysin and its peptides of glutaminic acid, dosage and routes of administration was conducted in intact rats and those bearing Walker's carcinoma. Similar in shape for peptides, kinetic curves differed from those found for sarcolysin. The rates of absorption and excretion of sarcolysin peptides in intraperitoneal and, particularly, oral administration were lower than those of sarcolysin. Tumor appeared to play a role in a higher rate of peptide excretion. While sarcolysin and its peptides distribution in organs and tissues was generally identical, time of peak radioactive concentration build-up was different. Time needed for accumulation and excretion of peptides from tumor was much longer than from other organs or tissues. Sarcolysin went chiefly to urine while peptides--to faeces.

A bioreversible prodrug approach designed to shift mechanism of brain uptake for amino-acid-containing anticancer agents

By derivatization at the N-terminus of amino acid-based anticancer agents (e.g. melphalan and acivicin) to form a drug delivery system (TDDS), we demonstrate a change in the mechanism of brain uptake from the large neutral amino acid transporter (LAT) pathway to passive. An in situ rat brain perfusion technique was used to determine the brain capillary permeability-surface area (PA) product for [(14)C]L-Leu as control (5.18 +/- 0.32 x 10(-2) mL/s/g), which was inhibited competitively (to 7-18% of control) by an excess concentration of the amino-acid-containing anticancer agents, acivicin and melphalan. However, TDDS did not compete for LAT-mediated brain uptake of the radiotracer [(14)C]L-Leu. Brain uptake of TDDS was determined after in situ brain perfusion followed by RP-HPLC along with LC-MS/MS detection of the analytes in brain samples. The PA product for CH(3)-TDDS containing melphalan (5.09 +/- 2.0 x 10(-2) mL/s/g) shows that these agents rapidly cross the blood-brain barrier. Furthermore, competition studies of CH(3)-TDDS with [(3)H]verapamil suggest that the TDDS interacts significantly with the multidrug resistant efflux system (P-glycoprotein) at the blood-brain barrier. Therefore, TDDS were shown to lack LAT-mediated brain uptake. The drug delivery systems, however, showed uptake predominantly via the passive route along with recognition by the multidrug resistant efflux protein at the cerebrovasculature.

Antibody recognition of melphalan adducts characterized using immobilized DNA: enhanced alkylation of G-Rich regions in cells compared to in vitro

The bifunctional alkylating agent, melphalan, forms adducts on DNA that are recognized by two previously described monoclonal antibodies, MP5/73 and Amp4/42. Immunoreactivity to MP5/73 was lost when alkylated DNA was exposed to alkaline pH, while Amp4/42 only recognized the structures formed after the alkali treatment. Competitive enzyme-linked immunoadsorbent assays (ELISAs) indicated that in 0.01 and 0.1 M NaOH, loss of immunoreactivity to MP5/73 occurred with half-lives that were at least 2-fold longer than half-lives for gain of immunoreactivity to Amp4/42. This supported previously published evidence that Amp4/42 did not simply recognize all the products formed by alkali treatment of adducts that were initially recognized by MP5/73. Adducts recognized by MP5/73 on RNA were considerably more stable at 100 degrees C and pH 7 than adducts on DNA. This was consistent with the hypothesis that immunorecognition involved N7 guanine adducts and ruled out the involvement of phosphotriesters in immunoreactivity. Synthetic oligodeoxyribonucleotides, covalently immobilized onto 96-well plates, were reacted with melphalan and incubated for various periods with alkali, and then the levels of adducts recognized by each antibody in replicate wells were assayed by a direct binding ELISA. Adducts formed on oligodeoxyguanylic acid were recognized very weakly by Amp4/42, unlike other DNA sequences that were tested. Retention of immobilized DNA during alkali treatment was confirmed by immunoassay of cisplatin adducts. Poor recognition by Amp4/42 of adducts in oligodeoxyguanylic acid was confirmed by a competitive ELISA. Amp4/42, unlike MP5/73, efficiently recognized adducts resulting from alkylation of DNA with chlorambucil. It is concluded that the two antibodies recognized melphalan adducts in different DNA sequence environments and that this explains (a) the different alkali stability of immunoreactive adducts and (b) previous results which showed that, in DNA from melphalan-treated cells, adducts recognized by Amp4/42 formed a smaller proportion of total adducts compared to DNA alkylated in vitro. The results presented here indicate that this was caused by a marked cellular influence on the overall sequence-dependent pattern of DNA alkylation or repair.

Down-regulation of CD98 in melphalan-resistant myeloma cells with reduced drug uptake

Although melphalan has been used as a therapeutic reagent for multiple myeloma, many patients become refractory. To elucidate the mechanism of resistance to melphalan, we generated a melphalan-resistant myeloma cell line, KHM-11(EMS), by treating a parental line, KHM-11, with a mutagen, ethylmethanesulfonate. KHM-11(EMS) is 55 times more resistant to melphalan. gamma-Glutamylcysteine synthetase, P-glycoprotein, multidrug-resistance-associated protein, lung-resistance-related protein and the Bcl-2 family of proteins were not responsible for the drug resistance in KHM-11(EMS). Intracellular incorporation of melphalan to myeloma cells was determined by using [(14)C]-labeled melphalan. Accumulation of melphalan in KHM-11(EMS) was 43% of KHM-11, while the efflux rates were comparable in both cell lines. The uptake of melphalan was inhibited by the addition of L-phenylalanine, indicating that melphalan is incorporated through the L-phenylalanine transporter as reported previously. Expression of CD98, which was recently cloned as an L-phenylalanine transporter, was 6-fold decreased in KHM-11(EMS), suggesting that CD98 may be correlated with the incorporation of melphalan. CD98 expression and incorporation of melphalan were analyzed in fresh purified myeloma cells from 5 patients. All myeloma cells from 4 cases expressed CD98 at a high level and incorporated melphalan. However, tumor cells from 1 case expressed CD98 at low levels and did not incorporate melphalan. Taken together, reduced melphalan uptake could be responsible for the drug resistance in KHM-11(EMS), and down-regulation of CD98 may be related to this phenomenon. Further investigation of the correlation between impaired drug uptake and down-regulation of CD98 in myeloma cells should be important to understand the mechanism of resistance to melphalan.

Analysis of melphalan adducts of 2'-deoxynucleotides in calf thymus DNA hydrolysates by capillary high-performance liquid chromatography-electrospray tandem mass spectrometry

Melphalan is a bifunctional alkylating agent that covalently binds with intracellular nucleophilic sites. A methodology using electrospray mass spectrometry was developed to detect and identify DNA adducts. Alkylation sites within a particular nucleotide were examined using electrospray tandem mass spectrometry hyphenated to capillary liquid chromatography in combination with a column switching system. In the reaction mixtures resulting from the interaction of 2'-deoxynucleotides and melphalan several base-aklylated adducts were found. In the case of 2'-deoxyadenosine monophosphate, thymidine monophosphate and 2'-deoxyguanosine phosphate alkylation was observed in the mononucleotide reaction mixtures but not in the DNA-hydrolysates. Calf thymus DNA was reacted in vitro with melphalan. The DNA pellet was isolated and enzymatically hydrolyzed with the aid of Nuclease P1. In this hydrolysate both mono-alkylated 2'-deoxynucleotides and dinucleotides were found. The most important adduct found was identified as the N-7 alklylated dGMP adduct. The alkylated dinucleotides were identified as a pdApdT/melphalan and pdGpdC/melphalan the latter being the most important.

Rapid development of glutathione-S-transferase-dependent drug resistance in vitro and its prevention by ethacrynic acid

Exposure of A2780 human ovarian tumor cells to a low concentration of melphalan in vitro for 7 days resulted in the development of melphalan resistance. This resistance was not a stable characteristic of the cells since it was lost after 2 weeks in culture in the absence of drug. The melphalan-resistant cells exhibited significant cross-resistance to cisplatin but only minor cross-resistance to doxorubicin. The resistant cells had elevated levels of glutathione-S-transferase activity and mRNA. Exposure of the cells to the ethacrynic acid resulted in a decrease in enzyme activity as well as a reversal of their drug-resistant phenotype, indicating that the enzyme is involved in the resistance. When ethacrynic acid was present during the 7-day exposure of the cells to melphalan, the development of drug resistance was prevented. This system may serve as a useful preliminary step in screening for agents which can prevent the development of chemotherapy-induced drug resistance in human cancer.

A monofunctional derivative of melphalan: preparation, DNA alkylation products, and determination of the specificity of monoclonal antibodies that recognize melphalan-DNA adducts

Bifunctional alkylating agents, such as those based on nitrogen mustard, form important parts of many anti-cancer chemotherapy protocols and are responsible for increased incidences of secondary tumors in successfully treated patients. These drugs generally form a majority of monofunctional DNA adducts, although the bifunctional adducts appear to be necessary for their powerful cytotoxic and antitumor effects. The relative importance of bifunctional as opposed to monofunctional adducts in the varied biological consequences of drug exposure has not been studied in detail, particularly in relation to the role and specificity of biochemical responses to therapy-related DNA damage. A simple method is described for the preparation of useful quantities of a pure monofunctional derivative of the nitrogen mustard-based drug melphalan. Monohydroxymelphalan was prepared by partial hydrolysis, purified by reversed phase chromatography, and characterized by MS, NMR, and HPLC. Contamination with melphalan was </=0.2%. The heat labile DNA base adducts formed by monohydroxymelphalan were shown to contain undetectable levels of cross-linked species. The ratio of adenine to guanine adducts was 0.62, similar to the equivalent ratio for melphalan. The sequence-dependent pattern of alkylation of purified DNA was indistinguishable from that of melphalan, but required a higher dose to achieve comparable extents of reaction. The specificities of two monoclonal antibodies that recognize melphalan-DNA adducts were investigated using DNA alkylated with [3H]monohydroxymelphalan. Adducts on this DNA showed similar immunoreactivities to adducts formed by melphalan. This shows clearly that neither antibody was specific for cross-linked adducts and that it is therefore possible to quantify adducts formed by both monohydroxymelphalan and melphalan with high sensitivities. The availability of monohydroxymelphalan in addition to melphalan, together with sensitive immunoassays for adducts on extracted DNA and in individual cells, constitutes a useful system for investigating cellular responses to the DNA modifications formed by a clinically relevant drug.

Development and activities of a new melphalan prodrug designed for tumor-selective activation

The synthesis of C-Mel, a cephalosporin carbamate derivative of the clinically used alkylating agent melphalan, is described. C-Mel was designed as an anticancer nitrogen mustard prodrug that releases melphalan upon tumor-specific activation by targeted beta-lactamase (bL). The Km and kcat values for bL hydrolysis of C-Mel were 218 microM and 980 s(-1), respectively. In vitro cytotoxicity assays with 3677 human melanoma cells demonstrated that C-Mel was 40-fold less toxic than melphalan and was activated in an immunologically specific manner by L49-sFv-bL, a recombinant fusion protein that binds to the melanotransferrin antigen on melanomas and on some carcinomas. L49-sFv-bL in combination with C-Mel led to regressions and cures of established subcutaneous 3677 tumors in nude mice. The effects were significantly greater than those of melphalan, which did not result in any long-term regressions in this tumor model. The therapeutic effects were comparable to those obtained in mice treated with the previously described L49-sFv-bL/7-(4-carboxybutanamido)-cephalosporin mustard (CCM) combination. However, C-Mel may be more attractive than CCM for clinical development since the released drug is clinically approved.

Monofunctional adenine N-3 adducts of melphalan: occurrence at a mutational hotspot sequence and resistance to removal by AlkA protein

Previous work showed that a CTAAA sequence in the supF gene of the shuttle plasmid pZ189 was a hotspot for mutagenesis by the aromatic nitrogen mustards melphalan and chlorambucil, and indirect evidence suggested adenine N-3 adducts as premutagenic lesions. In order to characterize the adducts formed at this sequence more directly, a substrate was prepared in which the three adjacent adenines in the CTAAA sequence were 3H-labeled. Following treatment of this substrate with [14C]melphalan, thermolabile adducts were depurinated and analyzed by HPLC. Only a single peak bearing both 3H and 14C label was detected and it coeluted with the single major adduct formed by the reaction of melphalan with free adenine base. Various spectrometric analyses of this species were all consistent with its identification as a monofunctional adenine N-3 adduct of melphalan. There was no evidence for any bifunctional adducts involving the labeled adenines. There was little if any release of the adenine N-3 adduct of melphalan by Escherichia coli AlkA protein, under conditions where 3-methyladenine was quantitatively released. The results support the proposal that monofunctional adenine N-3 adducts are intermediates in the generation of A.T-->T.A and A.T-->C.G transversions by aromatic nitrogen mustards.

Multidrug resistance protein-mediated transport of chlorambucil and melphalan conjugated to glutathione

The human multidrug resistance protein (MRP1) confers resistance of cells to a number of different cytostatic drugs and functions as an export pump for glutathione S-conjugates, glucuronides and other amphiphilic anions. The present study details for the first time MRP1-mediated ATP-dependent transport of various glutathione S-conjugates of the bifunctional alkylating agents chlorambucil and melphalan. In membrane vesicles prepared from cells expressing recombinant MRP1, the conjugates were transported at rates in the following order: monoglutathionyl chlorambucil > bisglutathionyl chlorambucil > monohydroxy monoglutathionyl chlorambucil and monoglutathionyl melphalan > monohydroxy monoglutathionyl melphalan. In addition, we show that membranes from chlorambucil-resistant GST-alpha-overexpressing CHO cells as well as from their parental cells express the hamster homologue of MRP1. With both CHO cell membrane preparations, we observed ATP-dependent transport of monoglutathionyl chlorambucil and of leukotriene C4, a glutathione S-conjugate and high-affinity substrate of MRP1. The transport rates measured in the resistant cells were only two- to three-fold higher than those measured in the control cells. These results together with cytotoxicity assays comparing MRP1-overexpressing cell pairs with the CHO cell pair indicate that, although MRP1-mediated transport is active, it may not be the rate-limiting step in chlorambucil resistance in these cell lines.

Tumor cell heterogeneity: impact on mechanisms of therapeutic drug resistance

PURPOSE

The aim of these studies was to determine whether chemotherapy-resistant tumor cell sublines derived from a single starting cell population with identical treatment protocols, have the same mechanism of resistance.

METHODS AND MATERIALS

Twelve cyclophosphamide-resistant sublines were derived from KHT-iv murine sarcoma cells by repeated exposures to 2, 4, or 8 microg/ml doses of 4-hydroperoxycyclophosphamide (4-OOHCP). To investigate possible mechanisms of resistance, glutathione (GSH) levels, glutathione S-transferase (GST) activity, and aldehyde dehydrogenase (ALDH) activity were determined. In addition, studies with the GSH depletor buthionine sulfoximine (BSO) and the ALDH inhibitor diethylamino-benzaldehyde (DEAB) were undertaken.

RESULTS

Resistant factors to 4-OOHCP, assessed at 10% clonogenic cell survival, ranged from 1.5-7.0 for the various cell lines. Crossresistance to melphalan and adriamycin also were commonly observed. Increased GSH levels, GST activity and ALDH activity were detected in the sublines but not all exhibited the same pattern of biochemical alterations. The response to GSH and ALDH inhibitors also varied among the sublines; the resistance being reversible in some cell lines but not others.

CONCLUSION

The present results indicate that when resistant sublines are derived simultaneously from the same starting cell population, the observed mechanisms of resistance may not be the same in each of the variants. These findings support the hypothesis that preexisting cellular heterogeneity may affect mechanisms of acquired resistance.

Modulation of melphalan resistance in glioma cells with a peripheral benzodiazepine receptor ligand-melphalan conjugate

Peripheral benzodiazepine receptors (PBRs) are located on the outer membrane of mitochondria, and their density is increased in brain tumors. Thus, they may serve as a unique intracellular and selective target for antineoplastic agents. A PBR ligand-melphalan conjugate (PBR-MEL) was synthesized and evaluated for cytotoxicity and affinity for PBRs. PBR-MEL (9) (i.e., 670 amu) was synthesized by coupling of two key intermediates: 4-[bis(2-chloroethyl)-amino]-L-phenylalanine ethyl ester trifluoroacetate (6) and 1-(3'-carboxylpropyl)-7-chloro-1,3- dihydro-5-phenyl-2H-1,4-benzodiazepin-2-one (8). On the basis of receptor-binding displacement assays in rat brain and glioma cells, 9 had appreciable binding affinity and displaced a prototypical PBR ligand, Ro 5-4864, with IC50 values between 289 and 390 nM. 9 displayed differential cytotoxicity to a variety of rat and human brain tumor cell lines. In some of the cell lines tested including rat and human melphalan-resistant cell lines, 9 demonstrated appreciable cytotoxicity with IC50 values in the micromolar range, lower than that of melphalan alone. The enhanced activity of 9 may reflect increased membrane permeability, increased intracellular retention, or modulation of melphalan's mechanisms of resistance. The combined data support additional studies to determine how 9 may modulate melphalan resistance, its mechanisms of action, and if target selectivity can be achieved in in vivo glioma models.

Derivatives of melphalan designed to enhance drug accumulation in cancer cells

The objective of this study was to develop chemical strategies to improve the uptake and accumulation of melphalan (L-Mel and D-Mel), a cytotoxic agent, into cancer cells. Dipeptides synthesized from L- (or D-) Mel and L-glutamic acid (L-Glu) or L-valine (L-Val) and their methyl or ethyl esters (all compounds were trifluoroacetic acid salts) were evaluated for cytotoxicity and cellular uptake using Caco-2 cells, a human colon carcinoma cell line, and RT-2 cells, a rat brain glioma cell line. Treatment of Caco-2 cells with L-Mel or D-Mel (0.5 mg/ml equivalent of melphalan) for 48 h resulted in approximately 50% cell survival. Treatment of the Caco-2 cells with dipeptide derivatives of L-Mel (or D-Mel) (11c-d, 12c-d and 13) caused similar cytotoxicity effects (approximately 50-70% of cell survival). When the cytotoxicities of the esters of L-Mel, D-Mel and their dipeptide derivatives (11a-b, 12a-b and 14) in Caco-2 cells were determined, less than 10% cell survival was observed. Similar results were observed in RT-2 cells. When the cellular uptake properties of these compounds were determined in Caco-2 cell monolayers, L-Glu-L-Mel (12c), L-Glu-D-Mel (12d), and L-Mel-L-Glu (11c) generated slightly lower intracellular levels of L-Mel or D-Mel than when the cell monolayer was treated with the amino acids (L-Mel or D-Mel). In Caco-2 cells treated with 11c, 12c or 12d, low levels of the dipeptides were also detected. Caco-2 cell monolayers treated with D-Mel-L-Glu (11d) or D-Mel-L-Val (13) showed very low levels of the amino acids (L-Mel or D-Mel), but generally higher levels of the dipeptides. In contrast to the amino acids (L-Mel, D-Mel) or the dipeptide derivatives (11c-d, 12c-d and 13), the ester derivatives of the amino acids [L-Mel(OEt), D-Mel(OEt)] or the dipeptides (11a-b, 12a-b and 14) produced 5-20 times higher intracellular concentrations of potentially cytotoxic metabolites (e.g., L-Mel, D-Mel, Mel-containing dipeptides or Mel-containing dipeptide monoesters). L-Mel(OEt), D-Mel(OEt), L-Glu(OEt)-L-Mel(OEt) (12a), L-Glu(OEt)-D-Mel(OEt) (12b), and L-Mel-L-Glu(OEt)2 (11a) accumulated mainly as either L-Mel or D-Mel, and the percentages of L-Mel or D-Mel were 99%, 99%, 90%, 75% and 98% of the total intracellular concentration of potentially cytotoxic agents, respectively. D-Mel-L-Glu(OEt)2 (11b) accumulated as its monoester (> 95%) and D-Mel-L-Val(OMe) (14) accumulated as its dipeptide metabolite (> 98%). Inclusion of Gly-Pro, carnosine, L-Phe or L-Glu did not inhibit uptake of the dipeptide derivatives of L-Mel (or D-Mel) or their esters. These results suggest that the cellular uptake of the dipeptide derivatives of melphalan and their esters is probably via passive diffusion rather than being facilitated by an amino acid transporter or a di/tripeptide transporter. The higher intracellular levels of cytotoxic agents generated from the ester derivatives of the amino acids and the dipeptides are probably due to their higher lipophilicity and the overall neutral charge of the esters and subsequent intracellular formation of the more polar amino acids (L- or D-Mel) and/or Mel-containing dipeptides. Finally, these studies suggest that dipeptides of D-Mel [11b, 11d, 13] have inherent cytotoxicity properties.

Nitric oxide enhancement of melphalan-induced cytotoxicity

The effects of the diatomic radical, nitric oxide (NO), on melphalan-induced cytotoxicity in Chinese hamster V79 and human MCF-7 breast cancer cells were studied using clonogenic assays. NO delivered by the NO-releasing agent (C2H5)2N[N(O)NO]- Na+ (DEA/NO; 1 mM) resulted in enhancement of melphalan-mediated toxicity in Chinese hamster V79 lung fibroblasts and human breast cancer (MCF-7) cells by 3.6- and 4.3-fold, respectively, at the IC50 level. Nitrite/nitrate and diethylamine, the ultimate end products of DEA/NO decomposition, had little effect on melphalan cytotoxicity, which suggests that NO was responsible for the sensitization. Whereas maximal sensitization of melphalan cytotoxicity by DEA/NO was observed for simultaneous exposure of DEA/NO and melphalan, cells pretreated with DEA/NO were sensitized to melphalan for several hours after NO exposure. Reversing the order of treatment also resulted in a time-dependent enhancement in melphalan cytotoxicity. To explore possible mechanisms of NO enhancement of melphalan cytotoxicity, the effects of DEA/NO on three factors that might influence melphalan toxicity were examined, namely NO-mediated cell cycle perturbations, intracellular glutathione (GSH) levels and melphalan uptake. NO pretreatment resulted in a delayed entry into S phase and a G2/M block for both V79 and MCF-7 cells; however, cell cycle redistribution for V79 cells occurred after the cells returned to a level of cell survival, consistent with treatment with melphalan alone. After 15 min exposure of V79 cells to DEA/NO (1 mM), GSH levels were reduced to 40% of control values; however, GSH levels recovered fully after 1 h and were elevated 2 h after DEA/NO incubation. In contrast, DEA/NO (1 mM) incubation did not reduce GSH levels significantly in MCF-7 cells (approximately 10%). Melphalan uptake was increased by 33% after DEA/NO exposure in V79 cells. From these results enhancement of melphalan cytotoxicity mediated by NO appears to be complex and may involve several pathways, including possibly alteration of the repair of melphalan-induced lesions. Our observations may give insights for improving tumour kill with melphalan using either exogenous or possibly endogenous sources of NO.

Lack of glutathione conjugation of melphalan in the isolated in situ liver perfusion in humans

Tumor cell resistance against melphalan (LPAM) has been associated with increased cellular reduced glutathione (GSH) levels and glutathione S-transferase activity. Therefore, GSH conjugation of LPAM has been hypothesized to be a key factor in tumor cell resistance. In the present study, we evaluated GSH conjugation of LPAM by the perfused liver in patients with colorectal cancer metastases undergoing a Phase II study of isolated liver perfusion as well as in the rat. To evaluate whether LPAM-GSH conjugates were synthesized in the rat in vivo, LPAM was infused i.v. at a rate of 2.0 micromol/kg/min. In bile samples obtained during the infusion, two major GSH conjugates were identified by mass spectrometry: mono-hydroxy-mono-GSH-LPAM and di-GSH-LPAM. The maximum biliary excretion rate of these two conjugates accounted for only 1.3% of the LPAM infusion rate. In bile or perfusate samples from patients treated for 60 min initially with 0.3 mM LPAM in the perfusion medium via isolated liver perfusion (200 mg LPAM in approximately 2 liters perfusion medium), none of the above-mentioned conjugates were detected. When comparable rat liver perfusions were performed initially with 66 microM or 0.66 mM LPAM in the perfusion medium, bile samples did contain GSH-LPAM conjugates; the cumulative biliary excretion of the two conjugates amounted to 0.4 and 0.2% of the LPAM dose, respectively. These data suggest that both in rats and humans, hepatic GSH conjugation plays a very minor (if any) role in the elimination of LPAM and, therefore, that modulation of GSH levels is unlikely to affect the rate of elimination of this drug.

Enhanced host cell reactivation capacity and expression of DNA repair genes in human breast cancer cells resistant to bi-functional alkylating agents

Human breast carcinoma (MCF7-MLNr) cells resistant to the bifunctional drugs L-phenylalanine mustard (L-PAM, 5-fold resistance), mechlorethamine (9-fold), cisplatin (3-fold), and BCNU (3-fold) were used to investigate the role of DNA repair in the development of resistance to alkylating agents. We have previously shown that neither L-PAM transport and metabolism nor glutathione-associated enzymes were altered in MCF7-MLNr cells, compared to the sensitive cells MCF7-WT. This study shows that treatment of pRSV-CAT plasmid with L-PAM at concentrations up to 1 microM proportionally inhibit the expression of chloramphenicol acetyl transferase (CAT) activity, while higher concentrations abolished CAT activity. pRSV-CAT reactivation was significantly increased when plasmid was transfected into MCF7-MLNr cells, compared to MCF7-WT cells. This indicates that resistant cells have more efficient capacity to recognize and repair L-PAM induced DNA damage. The mRNA expression of DNA nucleotide excision repair genes ERCC1, XPD (ERCC2), XPB (ERCC3), and polymerase beta was found to be similar in both the MCF7-WT and MCF7-MLNr cells. Western blot analysis also reveals no difference in the expression of ERCC1, AP endonuclease, poly (ADP-ribose) polymerase, and alkyl-N-purine-DNA glycosylase proteins. The lack of correlation between enhanced host cell reactivation capacity in resistant cells, and the expression of these specific DNA repair genes suggests that proteins encoded by these genes are not rate limiting steps for resistance to bi-functional alkylating drugs in human breast cancer cells.

Products from alkylation of DNA in cells by melphalan: human soft tissue sarcoma cell line RD and Escherichia coli WP2

Alkylation of DNA was studied after treatment with [3H]melphalan (phenylalanine mustard; 1-2 microM) using a human tumour cell line, RD, in culture, or Escherichia coli (WP2 or WP2-uvrA strains) in growth medium. After 6 h at 37 degrees C, treated cells were isolated and re-suspended in fresh growth media. Samples were taken at times up to 48 h for isolation of DNA, and in some cases also RNA and protein (which were found to be alkylated to about the same extent as DNA). Alkylated DNA was analysed as previously described (M.R. Osborne and P.D. Lawley, Chem.-Biol. Interact 89 (1993) 49-60). The four principal products, mono-7-alkylguanine (G-M-OH); mono-3-alkyladenine (A-M-OH); and the cross-linked products G-M-G and A-M-G, were identified in DNA from melphalan treated cells, and quantitatively determined. In each case, alkylation of cellular macromolecules was maximal after about 6 h. In DNA of the human tumour cell line, the relative amounts of adenine products decreased with time, most markedly with A-M-OH to 42% of the 2-h value after 48 h. In DNA of both bacterial strains, A-M-OH was virtually undetectable even at early times. Comparisons between the time course of relative decreases in amounts of these alkylpurine products and the corresponding values for alkylated DNA in vitro suggest that the adenine products are subject to removal by repair enzyme action in E. coli of either strain.

A monoclonal antibody that recognizes alkali-stabilized melphalan-DNA adducts and its application in immunofluorescence microscopy

Monoclonal antibodies were produced that recognized alkali-stabilized modifications of DNA formed by the anticancer drug melphalan in order to permit measurement of melphalan-DNA adducts in individual cells by immunofluorescent staining. Antibody Amp4/42 did not bind to alkali-treated control DNA or to DNA that had been alkylated with melphalan but not exposed to alkali. In a competitive enzyme-linked immunoadsorbent assay using DNA that had been reacted with radioactive melphalan in simple solution a 50% reduction in assay signal was caused by approximately 100 fmol total melphalan-DNA adducts/assay well. This sensitivity was only slightly influenced by heat denaturation of the DNA before alkylation or by the frequency of alkylated sites on DNA. The heat stability of the adducts recognized by Amp4/42 was greatly increased by the alkali-induced change which, in 0.1 M NaOH at 37 degrees C, was complete by 30 min. Amp4/42 appears to recognize a ring-opened structure resulting from alkaline hydrolysis of 7-alkyldeoxyguanosine. Melphalan-DNA adducts formed in mammalian cells showed an alkali-induced increase in immunoreactivity which occurred at a similar rate to that seen in DNA that had been alkylated in simple solution, but their maximum overall immunoreactivity was approximately 10-fold lower. This indicated that in cells the adducts recognized by Amp4/42 were formed or persisted as a smaller proportion of total adducts compared with alkylation of pre-purified DNA in simple solution. This antibody permitted immunofluorescent detection of melphalan-DNA adducts in single cells.

The effect of hyperthermia and verapamil on melphalan cytotoxicity and transport in multidrug-resistant Chinese hamster ovary cells

The effect of both hyperthermia and verapamil on cytotoxicity and transport of melphalan was studied in a pleiotropic drug-resistant Chinese hamster ovary cell line (CHRC5) and in the drug-sensitive parent line (AuxB1). The CHRC5 cell line was selected for resistance to colchicine but is also cross-resistant to other drugs including melphalan. Verapamil (10 microM) increased melphalan cytotoxicity in drug-resistant cells but not in drug-sensitive cells. Hyperthermia (40 to 45 degrees C) increased melphalan cytotoxicity in both cell lines. In drug-resistant but not drug-sensitive cells, melphalan cytotoxicity was increased further when verapamil was combined with hyperthermia (40 to 45 degrees C). The increased cytotoxicity caused by verapamil in drug-resistant cells was accompanied by alterations in membrane permeability to melphalan. The cellular uptake of melphalan after 15 min increased in the presence of verapamil (7 to 30 microM) at 37 and 42 degrees C. When verapamil (10 microM) was present, the rate of efflux of melphalan from CHRC5 cells decreased by almost 40% at 37 degrees C. The rate of efflux was increased at 42 degrees C relative to 37 degrees C, but with verapamil the rate decreased to that obtained at 37 degrees C in CHRC5 cells. In drug-sensitive cells, verapamil (< or = 50 microM) did not affect either uptake or efflux of melphalan. These findings suggest that verapamil could be beneficial by increasing the effectiveness of melphalan in the elimination of multidrug-resistant cells. The combination of hyperthermia and verapamil could be especially advantageous by increasing melphalan cytotoxicity in a localized target region.

High-dose intravenous melphalan: a review

PURPOSE

To review the clinical pharmacology and clinical trials that have used intravenous (IV) high-dose melphalan (HDM).

METHODS

We reviewed the mechanism of action, clinical pharmacology, and clinical studies of HDM with and without autologous bone marrow support (ABMT) or peripheral-blood progenitor cells (PBPCs) in the following disease areas: myeloma, ovarian cancer, malignant lymphoma, breast cancer, neuroblastoma, Ewing's sarcoma, and acute leukemia.

RESULTS

HDM has a distribution half-life (t1/2 alpha) of 5 to 15 minutes and an elimination half-life (t1/2 beta) of 17 to 75 minutes at doses of 140 to 180 mg/m2, with significant intrapatient variability. At these doses, a wide range of areas under the concentration/time curve (AUC) have been reported, ie, 146 to 1,515 mg/min/mL. HDM has significant clinical activity in patients with multiple myeloma in relapse or when used as consolidative therapy in relapsed ovarian cancer, relapsed Hodgkin's disease, breast cancer, and relapsed neuroblastoma. Additional studies are required to determine the activity of HDM in Ewing's sarcoma or acute leukemia. Toxicities of HDM include myelosuppression, moderate nausea and vomiting, moderate to severe mucositis and diarrhea, and, infrequently, hepatic venoocclusive disease.

CONCLUSION

HDM has become an established and effective salvage regimen for children with relapsed neuroblastoma, as well as an effective consolidative treatment for children with high-risk disease (stage IV). HDM is emerging as an active and effective mode of treatment in patients with stage II and III myeloma. The favorable toxicity profile of HDM and the availability of PBPCs allows for repetitive therapy.

Characterization of cross-resistance to methotrexate in a human breast cancer cell line selected for resistance to melphalan

We have demonstrated previously decreased melphalan accumulation in a human breast cancer cell line selected for resistance to melphalan (MelR MCF-7). Cross-resistance studies of MelR MCF-7 cells revealed that this cell line was 6.7-fold cross-resistant to methotrexate, but only 2-fold resistant to trimetrexate. Methotrexate transport studies in MelR MCF-7 cells showed a 2-fold decrease in initial methotrexate uptake and a 2-fold decrease in the Vmax for methotrexate uptake in the resistant cells. Methotrexate resistance in MelR MCF-7 cells was also associated with a decrease in non-effluxable methotrexate following incubation with radiolabeled drug for 24 hr. Characterization of intracellular methotrexate after accumulation for 24 hr demonstrated decreased levels of free methotrexate in MelR MCF-7 cells. Analysis of methotrexate polyglutamate formation in MelR MCF-7 cells indicated that the decrease in non-effluxable, non-protein-bound methotrexate was associated with a 3-fold decrease in higher order methotrexate polyglutamate formation. No difference was noted in folylpolyglutamate synthetase activity between the resistant and parental cell lines. Therefore, the observed decrease in methotrexate polyglutamate formation in MelR MCF-7 cells appeared to result from decreased availability of substrate. There was no evidence of any alteration in the amount of the catalytic activity of dihydrofolate reductase in MelR MCF-7 cells compared with parental MCF-7 (WT MCF-7) cells; moreover, the binding affinity of dihydrofolate reductase for methotrexate and the percentage of protein-bound methotrexate were similar in both cell lines. In addition, the total amounts of thymidylate synthase protein and thymidylate synthase catalytic activity in MelR MCF-7 cells were unchanged. Thus, acquired methotrexate resistance in MCF-7 cells selected for resistance to melphalan appears to result from down-regulation of methotrexate uptake.

Covalent sequestration of melphalan by metallothionein and selective alkylation of cysteines

Rabbit liver metallothionein-2 is shown to form covalent bonds with the anticancer agent melphalan, in support of the hypothesis that covalent sequestration by metallothionein constitutes one mechanism for the cross-resistance acquired by cancer patients to therapeutic alkylating agents. Among 20 cysteines in the 2-domain protein, 89% of the first alkylation reaction occurs with 2 that cochelate a zinc cation in the carboxy domain. Computer-supported docking studies indicate a favorable binding site for melphalan near these cysteine sulfhydryl groups. Although folded metallothionein-2 is resistant to trypsin cleavage, alkylation by 1 mol of melphalan allows cleavage by trypsin between the two globular domains.

The cytotoxicity of melphalan and its relationship to pH, hypoxia and drug uptake

In the present in vitro studies we examined the effect of hypoxia and acidic pH, two important consequences of reduced blood flow in vivo, on the cytotoxicity of melphalan treatment in Chinese hamster V79-WNRE and SiHa human tumor cells. Cells were exposed to various concentrations of melphalan for 1 hr at 37 degrees C under oxic or hypoxic conditions; pH 6.6 or 7.4, and cell survival was measured. The cytotoxicity of melphalan was potentiated by both low pH and hypoxia, in both cell lines. The overall potentiation, expressed as an enhancement ratio (ER), from both hypoxia and low pH was 3.5 in V79-WNRE and 2.9 in SiHa cells. The potentiation of cell killing produced by hypoxia alone (ERHyp) ranged from 1.4 to 1.9, and was greater in V79-WNRE than in SiHa cells. The potentiation from low pH (ERpH) was approximately 2 in both cell lines. HPLC analysis showed substantial intracellular accumulation of melphalan in both cell lines. Hypoxia and reduced pH further enhanced uptake of melphalan but this was not sufficient by itself to account for the increased potentiation of cytotoxicity observed under those conditions.

Melphalan uptake, hyperthermic synergism and drug resistance in a human cell culture model for the isolated limb perfusion of melanoma

Isolated limb perfusion with melphalan is a long-standing treatment for melanoma but the clinical conditions have not been subjected to a systematic evaluation. In order to establish optimal conditions for perfusion, three human melanoma cell lines were cultured with melphalan in vitro under conditions comparable to in vivo therapy. The most important findings were that: (a) 41.5 degrees C was synergistic for melphalan killing of three human melanoma cell lines; (b) prolonging the treatment time beyond 1 h had little additional toxicity; and (c) varying the initial pH of the culture medium had no effect. After 1 h of treatment, cells accumulated more melphalan at 41.5 degrees C than at 37 degrees C, relative to the extracellular concentration. A cell line (MM418) derived from a primary tumour was the most resistant of the three lines; pigmented or non-pigmented sublines were equally resistant. The A2058 line showed the lowest level of synergism with hyperthermia, and displayed a marked plateau at 10% of controls in the dose-response for survival, yet no melphalan-resistant subpopulation could be isolated. The implications of this work are that (a) enhanced cellular uptake of melphalan may account for hyperthermic synergism of melphalan; (b) varying conditions other than treatment time will be necessary to deal with the variation in resistance between tumours; and (c) repeated cycles of treatment may be needed for phenotypes such as A2058 where melphalan resistance appears to be based on an epigenetic mechanism.

Kinetic analysis of the reaction of melphalan with water, phosphate, and glutathione

The reaction kinetics of the hydrolysis, phosphatolysis, glutathionyl conjugation, and alpha-glutathione-S-transferase (GST)-catalyzed glutathione conjugation of [3H-ring]melphalan were investigated at pH 6.5 and 7.4. The distribution of products relative to the initial parent compound radioactivity over time was measured by HPLC and analyzed by nonlinear regression techniques using a system of rate and distribution equations that describe the complete precursor-product pathways applicable to each reaction condition. The kinetic parameters calculated in the analysis were the first- and second-order rate constants of formation and the product ratios of the aziridinium intermediates. The second-order rate constants were normalized to those obtained for the hydroxylation reactions to yield relative rate constants. The first-order rate constants of aziridinium ion formation from melphalan and from all the monosubstituted melphalan species, except chloro, hydroxyl melphalan, were similar under all reaction conditions. The relative second-order rate constant for nonenzymatic glutathionylation of the aziridinium intermediate was 7 times larger at pH 7.4 than at pH 6.5. GST was found to react only with the aziridinium intermediate formed from melphalan and to dissociate slowly from the resultant GST-product complex (dissociation half-life, 1 hr at pH 7.4 and 3.5 hr at pH 6.5). The kinetic parameter estimates found in this study can be used to make preliminary calculations of the impact that cellular phosphate, glutathione, and GST concentrations will have on the intracellular detoxication of melphalan.

Expression of a rat glutathione-S-transferase complementary DNA in rat mammary carcinoma cells: impact upon alkylator-induced toxicity

The role of glutathione-S-transferase (GST) in alkylator drug resistance has been studied in MatB rat mammary carcinoma cells. A series of GST transfectant cell lines was established by using an expression vector containing the complementary DNA for the rat GST Yc gene under regulation of the SV40 early region promoter and the antibiotic resistance plasmid pSV2neo. Transfectant cell lines expressing up to 4-fold higher total GST activity than in the parental wild type cell line were identified. Southern blot analysis confirmed a DNA fragment corresponding in size to the transfected GST Yc complementary DNA. Wild type MatB cells contain very low levels of Yc protein, whereas the Yc+ clones showed greatly increased amounts of the Yc subunit. The effect of increased GST Yc activity on the sensitivity of the transfected clones to various cytotoxic agents was assessed by using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell survival assay. The clones expressing recombinant GST Yc were more resistant to melphalan (6- to 12-fold), mechlorethamine (10- to 16-fold), and chlorambucil (7- to 30-fold). In late passage populations of the GST Yc+ clones that had been grown over a period of 14 months under continuous selection in G418, GST activity was decreased and it was paralleled by a decrease in Yc protein. These late passage clones with diminished GST Yc content also demonstrate a partial reversion toward the wild type phenotype as determined by cytotoxicity assays using melphalan, mustargen, and chlorambucil. Interstrand DNA cross-links induced by mechlorethamine were significantly lower at 0, 2, and 20 h posttreatment in one of the GST Yc+ clones when compared to wild type MatB cells. These studies indicate that GST Yc overexpression can confer resistance to alkylating agents and that this correlates with inhibition of DNA cross-link formation.