The cell membrane and cell signals: new targets for novel anticancer drugs

Cytotoxicity of sphingoid marine compound analogs in mono- and multilayered solid tumor cell cultures

A subset of four synthetic sphingoid marine compound analogs was chosen from a preliminary in vitro cytotoxicity study for further analysis. The selected analogs were initially screened in monolayer cultures for their anticancer potential against a panel of eight human tumor cell lines, ovarian, colon and lung cancer, squamous cell carcinoma and leukemia producing IC50 values ranging from 1.5 to 6.9 microM. In a secondary screening, the sphingoid analogs were evaluated against multilayered postconfluent cultures of A2780 ovarian cancer and WiDr colon cancer cells. In this model, compounds 5 and 8 were the most active derivatives showing EC50 values in the range 25-32 microM. The performance of 5 and 8 against both cell lines was not dependent on the cell culture model as shown with resistance factor values in the range 8-12. Cell cycle studies in HL60 leukemia cells showed an arrest in G(0)/G1 at a low drug concentration (3 microM) but accumulation in S phase at a high drug concentration (9 microM). It can be concluded that the analogs showed a cell line independent activity, with an apparent selectivity against cells grown in more physiological three-dimensional condition compared to standard anticancer drugs.

Phase I and pharmacokinetic study of the cytotoxic ether lipid ilmofosine administered by weekly two-hour infusion in patients with advanced solid tumors

PURPOSE

A Phase I trial was performed to determine the dose-limiting toxicity and maximum tolerated dose, and to describe the pharmacokinetics of the alkyl-lysophospholipid, ilmofosine, when administered as a weekly 2-h infusion in patients with solid tumors.

EXPERIMENTAL DESIGN

Thirty-nine patients were entered into a trial of ilmofosine administered weekly for 4 weeks followed by a 2-week rest period. Dose escalation occurred in 10 levels from 12 to 650 mg/m(2).

RESULTS

Thirty-six patients were evaluable for toxicity. The median number of cycles per patient was 1 (range, 1-4). Dose-limiting gastrointestinal toxicity occurred at 650 mg/m(2) with grade 3 nausea in two patients and grade 3 vomiting and diarrhea in one patient. Grade 2 diarrhea was observed in four of six patients treated at 550 mg/m(2). In addition, two patients treated at 550 mg/m(2) and two patients treated at 650 mg/m(2) experienced a decline in performance status of two or more levels that was determined to be due to treatment. There were no tumor responses. Stabilization of disease for at least 8 weeks occurred in six patients. Plasma concentrations of ilmofosine and its sulfoxide metabolite were evaluated by high-pressure liquid chromatography. The elimination of both compounds was biexponential with terminal half-lives of approximately 40 h for ilmofosine and 48 h for the sulfoxide. The area under the concentration-time curve was dose-proportional for each compound, and there was no evidence of saturable kinetics.

CONCLUSIONS

The dose-limiting toxicity of ilmofosine is gastrointestinal and the recommended dose for Phase II trials is 450 mg/m(2) as a 2-h weekly infusion. The relatively long half-life of ilmofosine and its active metabolite support the use of this intermittent schedule.

Protein kinase C isoform expression and activity alter paclitaxel resistance in vitro

OBJECTIVE

The aim of this study was to assess the relationship of protein kinase C (PKC) isoform expression and functional activity to the development of multidrug resistance in gynecologic malignancies.

METHODS

Paclitaxel-resistant subclones (T30 and T30-Res) of the Mes-sa human uterine sarcoma cell line were selected through exposure to paclitaxel in vitro. Indices of relative drug resistance were determined by the MTT (3-[4,5-dimethyl-2-thiazolyl]-2, 5-diphenyl-2H-tetrazolium bromide) assay. Differences in the expression pattern of PKC isoforms were assessed by Western blot of cell lysates. Finally, the influence of PKC activity (i.e., translocation to the plasma membrane, confirmed by Western blot of plasma membrane bound protein) on resistance to paclitaxel was examined with the MTT assay in cells preincubated with PMA.

RESULTS

The indices of relative paclitaxel resistance of Mes-sa, Mes-sa-T30, and Mes-sa-T30-Res were 1-, 5-, and 11-fold, respectively. Five (alpha, gamma, iota, lambda, and mu) of the 11 known PKC isoforms were detected in all cell lysates. Only PKC-alpha and PKC-gamma expression increased with increasing indices of paclitaxel resistance. Interestingly, PMA induction of PKC activity reversed resistance to paclitaxel in all cell lines by 2- to 3-fold, and this reversal of drug resistance was associated with a time-dependent translocation of PKC-alpha and PKC-gamma to the plasma membrane compartment.

CONCLUSIONS

Increased expression of only the PKC-alpha and PKC-gamma isoforms correlates with increasing levels of paclitaxel resistance in Mes-sa cells in this in vitro experimental model. However, increased functional activity of these and other PKC isoforms leads to reversal in paclitaxel resistance. Therefore, PKC activating mechanisms normally present in primary tumor cells may be compromised in drug-resistant clones.

Modulation of resistance to ara-C by bryostatin in fresh blast cells from patients with AML

Bryostatin has shown promise both as a cytotoxic agent and more recently as a modulator of 1-beta-D-arabinofuranosylcytosine (ara-C) resistance. This compound is currently in phase I and II trials as a single agent. We have used the 3-4,5-dimethylthiazol-2,5-diphenyltetrazolium bromide (MTT) assay as a means of investigating the direct effects of bryostatin and the effects of co-incubating this agent with ara-C on fresh blast cells from 53 patients with acute myeloid leukaemia (AML) and myelodysplastic syndrome (MDS). Additional studies evaluated the levels of accumulation and retention of 1-beta-D-arabinofuranosylcytosine 5'-triphosphate (ara-CTP) in cells exposed to ara-C with and without bryostatin. Cells were exposed to bryostatin at a range of concentrations (0.1-100 nM) for 48 h and at 1 nM for both modulation studies and assessment of ara-CTP production. We found bryostatin to be cytotoxic in 18/58 (31%) tests whilst potentiation of formazan production in the MTT assay was seen in 21/58 (36%) patients. On co-incubation with bryostatin, 16/58 (27%) tests showed increased cytotoxicity to ara-C. Furthermore, there was a significant increase in the accumulation of ara-CTP on co-incubation with bryostatin (p = 0.0401). We found patients with in vitro resistance were more likely to become sensitised following exposure to bryostatin (p < 0.01). This study has emphasised the need to optimise treatment regimens for individual patients using this approach.

Moving the Frontiers of Cancer Chemotherapy for Solid Tumors by Changing the Scope of Drug Development

Our increasing knowledge on the pathogenetic mechanisms involved in the growth of cancer cells and on the molecular basis of cancer has influenced the development of anticancer agents. The realization that new targets should be evaluated for anti-cancer drug treatment has a.o. led to the introduction of the taxoids and topoisomerase I inhibitors. Attacking the known targets in a more sophisticated way led to the development of drugs with increased target specificity like Tomudex and Gemcitabine. Finally, using old drugs more efficiently by using pharmacokinetic and pharmacodynamic ananlysis hold a promise for the near future.

The prognostic significance of breast tumour microcalcifications

The prognostic significance of breast tumour microcalcifications has shown, so far, contradictory results. We compared two groups of breast cancer patients: in the first group, the patients showed mammographic and histological microcalcifications, whereas the second group did not. The prognostic indices for both groups included the size of the primary tumour, the number of positive axillary nodes, the histological type, the grade, the hormone-receptor status and the menopausal status. An increased number of cases with both oestrogen and progesterone receptor-positive tumours was found in the group with microcalcifications, which is a favorable prognostic sign for these patients. For the remaining prognostic indices there was no difference between the two groups.

Phase I trial of ilmofosine as a 24 hour infusion weekly

Ilmofosine, an ether lipid derivative of lysophosphatidylcholine has antineoplastic activity in vitro and in vivo. Maximum efficacy in preclinical models is associated with prolonged exposure to the drug. In a Phase I trial of a weekly 2 hour infusion schedule of ilmofosine, a syndrome of lethargy, diminished performance status, and mild hepatotoxicity was dose-limiting at 550 mg/m2. To avoid the higher drug concentrations associated with a brief infusion, a Phase I study of a weekly 24 hour infusional schedule was undertaken in an attempt to maximize dose-intensity. Doses were escalated from 550 to 800 mg/m2. Toxicities included nausea, anorexia, fatigue, and minor elevations of liver function tests. The dose limiting toxicity at 800 mg/m2 was a syndrome of severe abdominal pain. No neutropenia or thrombocytopenia was observed except in one patient who was found to have a myelodysplastic syndrome, thought not to be related to drug therapy. The more prolonged infusion schedule of ilmofosine did not result in a substantial increase in the tolerable dose.

The synergistic and antagonistic effects of cytotoxic and biological agents on the in vitro antitumour effects of suramin

Suramin has shown antitumour activity in vitro and in vivo. At plasma levels higher than 200 microM there is, however, excessive toxicity. We have, therefore, attempted to improve the antitumour effects of suramin in vitro by combining it with several other antitumour agents. The MCF-7 mammary carcinoma and PC3 prostate cancer cell lines were exposed continuously to suramin and the other agents for 6 days. The sulphorhodamine B (SRB) assay was used for the assessment of growth inhibition. The dose-response interactions were evaluated using the median-effect analysis with the Chou and Talalay computer programme. In the MCF-7 cell line, the combination of suramin plus doxorubicin (DXR), cisplatin (CDDP), 5-fluorouracil (5-FU) or tumour necrosis factor (TNF) resulted in synergistic growth inhibition, whilst its combination with miltefosine (HPC) was antagonistic. In the PC-3 cell line, suramin plus CDDP or TNF was synergistic, whilst its combination with DXR, 5-FU and HPC was antagonistic. All tested combinations with interferon-alpha (IFN-alpha), interferon-gamma (IFN-gamma) and with the combination of both IFN-alpha+IFN-gamma were not synergistic. The synergistic effect of suramin with DXR was schedule dependent. Pretreatment (addition of DXR on day 1 and suramin on days 2-5) was additive at the IC50 level, in both cell lines. Addition of DXR at day 5 was more effective than simultaneous exposure. We found a synergistic effect for the combination of suramin with CDDP and TNF in both cell lines. In addition the combination with DXR and 5-FU was synergistic in MCF-7. Sequential administration of DXR-suramin or suramin-DXR increased the growth inhibition.

The role of intracellular free calcium mobilization in the mechanism of action of antitumour ether lipids SRI 62-834 and ET18-OMe

Membrane-active antitumour ether lipids such as ET18-OMe (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine) and SRI 62-834 ((+-)-2-(Hydroxy[tetrahydro-2-(octadecyloxy) methylfuran-2-yl] methoxyl phosphinyloxy)-N,N,N-trimethylethaniminium hydroxide) are selectively cytotoxic to tumour cells in vitro. Their precise mechanisms of action are unclear, but they are known to have effects on cell membranes and cell signalling. A previous report suggested that ether lipids cause a biphasic sustained rise in intracellular free calcium [Lazenby et al., Cancer Res 50: 3327-3330, 1990]. We show here that the second phase is an experimental artefact due to cell membrane permeabilization by ether lipids in serum-free buffers. In serum-free medium, the membrane toxicity of antitumour ether lipids was increased 50-60 fold, when compared to medium containing 10% serum. Membrane disruption was neither dependent on extracellular calcium, nor modulated by preloading cells with the calcium chelators bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or 2-[[2-[bis(carboxymethyl)amino]-5-methylphenoxy]methyl]-6- methoxy-8-[bis(carboxymethyl)amino]quinoline. This indicates that the mechanism of membrane damage by ether lipids does not involve changes in calcium homeostasis. Using indo-1 and fura-2 as calcium probes, we established that lower concentrations of antitumour ether lipids do elicit a genuine monophasic and transient rise in intracellular free calcium, predominantly mobilized from internal stores. This acute calcium agonist activity of ether lipids is distinct from the inhibitory effects on cell signalling reported previously after more prolonged exposure. It appears that the calcium elevation induced by antitumour ether lipids is unlikely to be instrumental in their selective and potent antitumour activity.

Cytotoxic effects of anticancer agents on subconfluent and multilayered postconfluent cultures

The cytotoxic effects of conventional (doxorubicin, 5-fluorouracil, cisplatin) and investigational (2',2'-difluorodeoxycytidine, hexadecylphosphocholine, EO9, rhizoxin) anticancer drugs were studied in subconfluent and multilayered postconfluent cultures of human colon and ovarian carcinoma cell lines. Chemosensitivity was assessed 4 days after a 24-h drug exposure with the sulphorhodamine B assay. Except for rhizoxin, all drugs tested yielded an EC50 (drug concentration producing absorbance readings 50% lower than those of non-treated wells) in postconfluent cultures that were higher than an EC50 obtained with subconfluent cultures. Compared with subconfluent cultures, postconfluent cultures showed decreased cellular nucleotide concentrations and ATP/ADP ratios, in addition to an increased percentage of G0/G1 cells. The activity of DT-diaphorase, a reductase involved in the bioactivation of EO9, was similar in sub- and postconfluent cultures. These results indicate similarity of the postconfluent model presented with those obtained with in vivo models and more complex in vitro techniques.

The Na+/H+ antiporter in oncology in the light of the spontaneous regression of cancer and cell metabolism

Multiple metabolic and biochemical interrelationships, as well as the most recent views on mechanisms of malignant cell growth, proliferation, and oncogen activity mediated by the Na+/H+ antiporter, can be integrated from the unitarian point of view of the dynamics of the hydrogen ion to parallel pH-related mechanisms involved in the Spontaneous Regression (SR) of some malignant tumors. Also, pH-related growth inhibitors of the amiloride series are considered as possible agents to be used in the adjuvant and co-adjuvant treatment of some human tumors as well as in the control of the metastatic process and in overcoming cancer multidrug resistance (MDR).

Doxorubicin weekly low dose administration: in vitro cytotoxicity generated by the typical pharmacokinetic profile

The cytotoxic effects of prolonged exposure to low concentrations of doxorubicin or a doxorubicin bolus were examined in vitro on four human breast cancer cell lines to simulate the plasma concentration profile of weekly low-dose (WLD) doxorubicin in breast cancer patients. Cells were exposed to doxorubicin for various prolonged times (24, 72, 120 and 192 h) and with different drug concentrations (5, 10, 20, 50 and 80 nmol/l). In a series of parallel experiments, cell lines were placed in contact with the drug for short periods (1 h) before prolonged exposure to doxorubicin; the concentrations of these pulses were 150, 250 and 350 nmol/l. A constant decrease in tritiated thymidine incorporation was noted as a function of the drug concentration and the duration of the cell contact with the drug. Interestingly the lowest concentrations (5-10 nmol/l) produced marked cytotoxic effects. For equivalent concentration x time values, experiments including doxorubicin pulses resulted in greater cytotoxicity than continuous exposure alone, in a dose-related manner. This finding was related to differences in intracellular doxorubicin concentrations. Results suggest that the rather empirically designed WLD doxorubicin schedule can generate greater cytotoxic effects than continuous doxorubicin administration alone.